Genesis of Uraniun1- and Gold-Bearing
Precan1brian Quartz-Pebble Congloinerates
Proceedings of a Workshop,
October 13-15, 197 5, Golden, Colorado

G E O L O G I C A L S U R V E Y P R O F E S S I O N A L P A P E R I I 6 I - A - BB

GENESIS OF URANIUM- AND GOLD-BEARING
PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATEStow

Polished specimens of quartz-pebble conglomerate uranium ore.
Both actual size.

A2

A. AB Reef, Denison Mine, Elliot Lake, Ontario. (Samples courtesy of V. Ruzicka, photography by David M. Watson, both Geological Surveyof Canada.)
1. Photograph, artificial light. Note distribution of quartz pebbles; fine-grained pyritiferous matrix concentrated in upper
two-thirds of photograph. The bottom of the photograph is the stratigraphic top of the specimen.
2. Radioluxograph* of Al. Radioactivity is mostly confined to the pyritiferous matrix. Large radioactive areas result from clusters ofminute detrital grains of radioactive minerals, mostly uraninite, not large detrital grains of radioactive minerals.
\
B. Carbon Leader Reef, about 1500 m depth, Blyvooruitzicht Gold Mine, Carletonville, Witwatersrand. (Sample courtesy of D. K. Hallbauer,
Chamber of Mines of South Africa ; radioluxograph by David M. Watson, Geological Survey of Canada.)
1. Photograph, artificial light. The band of thucholite is the stratigraphic base of the specimen; note gold in it and along its upper mar­gin. Compare quartz pebble size with Al. Note concentration of pyrite in matrix.
2. Radioluxograph* of Bl. Note that radioactivity is concentrated in the thucholite layer and in the pyritiferous matrix.
* Dooley, J. R., Jr., 1958. The radioluxograph: a fast simple type of autoradiograph in Proceedings of the Second United Nations International Conference on the Peaceful Uses of Atomic
Energy: v. 3, p. 550-553.

Genesis of Uranium- and Gold-Bearing
Precambrian Quartz-Pebble Conglomerates
Proceedings of a Workshop,
October 13-15, 1975, Golden, Colorado
Edited by FRANK C. ARMSTRONGGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-A-BB

UNITED STATES GOVERNMENT PRINTING OFFICE, W ASH IN GT O N : 1 9 8 1

UNITED STATES DEPARTMENT OF THE INTERIORJAMES G. WATT, Secretary
GEOLOGICAL SURVEY
Dallas L. Peck, Director

First printing 1981
Second printing 1982

For sale by the Superintendent of Documents, U.S. Government Printing Office

Washington, D.C. 20402

CONTENTS
Frontispiece
Introduction, by Frank C. Armstrong.
History:
A. Investigation of Witwatersrand uranium-bearing quartz-pebble conglomerates in 1944-45, by Weston Bourret.
Discussion.
General:B. The Precambrian development of an oxygenie atmosphere, by J. William Schopf.
Discussion.
C. Uraninite oxidation and the Precambrian atmosphere, by D. E. Grandstaff.
Discussion.
South Africa:
D. Uraniferous quartz-pebble conglomerates in South Africa, by J. W. von Backstrom.
Discussion.
E. Examples that illustrate sedimentological aspects of the Proterozoic placer model on the Kaap-Vaal craton, Witwatersrand, South
Africa, by W. E. L. Minter.
Discussion.
F. The Dominion Reef Group, Western Transvaal, South Africa, by J. W. von Backstrom.
G. The crossbedded nature of Proterozoic Witwatersrand placers in distal environments and a paleocurrent analysis of the Vaal Reef
placer, by W. E. L. Minter.
Discussion following papers by Drs. von Backstrom and Minter.
H. Primary dispersal patterns of uraninite in the Proterozoic Vaal Reef placer deposit, Witwatersrand, South Africa, by W. E. L. Minter.
Discussion.
I. Examples of possible movement of gold in solution in the Witwatersrand, Ventersdorp, and Transvaal systems, by H. C. M. Whiteside.
Discussion.
J. Microprobe analyses of uranium and thorium in uraninite from the Witwatersrand, South Africa, and Blind River, Ontario, Canada, by
D. E. Grandstaff.
Discussion.
K. Preliminary notes concerning the uranium-gold ratio and the gradient of heavy-mineral size distribution as factors of transport distancedown the paleoslope of the Proterozoic Steyn Reef placer deposit, Orange Free State Goldfield, Witwatersrand, South Africa, by
W. E. L. Minter.
L. Geochemical studies on the origin of the detrital pyrites in the conglomerates of the Witwatersrand Goldfields, South Africa, by Rudolf
Saager.
M. Geochemistry and morphology of mineral components from the fossil gold and uranium placers of the Witwatersrand, by D. K.
Hallbauer.
Discussion following papers by Drs. Minter, Saager, and Hallbauer
N. Uraniferous constituents of the Witwatersrand conglomerates: Ore-microscopic observations and implications for the Witwatersrand
metallogeny, by Manfred Schidlowski.
Discussion following papers by Drs. Schidlowski, Hallbauer, and Lois Nagy.
0. Some aspects of the development of the Vaal Reef uranium-gold carbon seams, Witwatersrand Sequence: Organic geochemical andmicrobiological considerations, by John E. Zumberge, Bartholomew Nagy1 , and Lois Anne Nagy1 .
P. The distribution and sedimentary arrangement of carbon in South African Proterozoic placer deposits, by W. E. L. Minter.
Discussion following papers by Drs. B. Nagy and Minter.
Q. Some aspects of Witwatersrand mineralization with special reference to uranium minerals, by Clive Errol Feather.
R. Uranium mineralization of the Witwatersrand and Dominion Reef Systems, by P. R. Simpson and J. F. W. Bowles.
S. Detrital uraninite and pyrite: Are they evidence for a reducing atmosphere? by P. R. Simpson and J. F. W. Bowles.
Discussion following papers by Drs. Feather, Sims, and Simpson and Bowles.
Brazil:T. Uranium in Precambrian Moeda Formation, Minas Gerais, Brazil, by Jose Nilson Villaca and Luciano A. M. Moura1 .
Discussion.
Canada:U. The Blind River uranium deposits: the ores and their setting, by James A. Robertson.
V. Some metallogenic features of the Huronian and post-Huronian Uraniferous conglomerates, by Vladimir Ruzicka.
W. Temporal and other factors affecting deposition of Uraniferous conglomerates, by Stuart M. Roscoe.
X. Genesis of Canadian Uraniferous quartz-pebble conglomerate ores, by H. W. Little.
Discussion following papers by Drs. Robertson, Ruzicka, Roscoe, and Little.
1 Speaker
VII

VIII

CONTENTS

United States:
Y. Thorianite from the Hogatza placer, north central Alaska, by Mortimer H. Staatz.
Z. Basal conglomerates and weathered zones in the Marquette Range Supergroup, Northern Peninsula of Michigan-Age, indications ofatmospheric oxygen, and uranium potential, by William F. Cannon.
AA. Genesis of uranium-gold pyritic conglomerates, by W. Bradley Myers Discussion following talks by Drs. Staatz, Cannon, andMyers.
Summation:
BB. Thoughts about uranium-bearing quartz-pebble conglomerates: A summary of ideas presented at the Workshop, by Brian J. Skinner.
Discussion.

INTRODUCTION
By FRANK C. ARMSTRONG
The shock of the Arab oil embargo in 1973 suddenly
made everyone conscious of the dependency of modern
industrial societies upon assured steady supplies ofenergy-source materials. Two responses to this newawareness were the acceleration of the search for alter­nate energy sources and the substitution of abundantmaterials for scarce materials; in addition, scarcematerials were channeled into more beneficial uses andwere reserved to uses for which no acceptable substitutewas known. Immediately, of course, a high-priority
target for change was the use of petroleum and natural
gas in base-load central electricity-generating stations.
The world had a developed expanding technology thatappeared made-to-order as a heat source for central
generating stations, namely, nuclear reactors. The pro­mise of nuclear reactors created a strong demand for
uranium, and consequently the price of U308 rose. In­
creased price for U308 opened geologic environments
for exploration that theretofore had not been considered
because they were not economically viable. Almost
simultaneously, the oil-deficient industrial nations of theworld began to look at their uranium resources. Becauseof the long lead times involved and because of the large
capital expenditures necessary to construct nuclear
reactors, attention focused on the adequacy of uranium
resources to supply the anticipated new nuclear reactors
for their entire useful lives of thirty to forty years. The
world cumulative need for uranium to supply U308 forthe lifetime needs of the anticipated nuclear expansion
proved to be very large indeed. Only a few countries
with large uranium reserves and small nuclear-reactor
programs had adequate supplies of known uranium
resources. The rest of the western world would have todepend on imports or new discoveries. As the cost ofgenerating electricity is relatively insensitive to the costof U308 , continued high prices for secure supplies ofuranium seemed assured.
Such was the climate of the uranium market in late1974. Exploration for deposits that previously were con­sidered low-grade was undertaken, known low-grade
deposits were re-examined, and consideration was given
to resumption of production from low-grade mines
where production had ceased, particularly in South
Africa, where uranium had been produced almost ex­
clusively as a byproduct of gold mining. The rising priceof gold and the prospect of a favorable uranium market
in late 1974 encouraged former South African uranium

producers to reactivate their long dormant extraction
circuits and also encouraged others who had notpreviously recovered uranium to install new uraniumrecovery plants. At that time about 35 percent of the
known world uranium resources was contained inquartz-pebble conglomerates in South Africa andCanada.
In late 1974 the United States started to take a hardlook at the adequacy of its uranium supplies in the lightof the cumulative need for uranium to meet the forecast
demands for electricity to be generated by nuclear reac­
tors. At this same time a report on South African goldand uranium-bearing quartz-pebble conglomerates thatwas in preparation by a U.S. Geological Survey scien­tist, W. Bradley Myers, came to the attention of the
Director of the Geological Survey, Vincent E.
McKelvey. Because no uranium-bearing quartz-pebble
deposits were known in the United States, despite theoccurrence of favorable geologic environments in theLake Superior-Upper Michigan area and elsewhere, andbecause of the importance of this type of deposit in near­by Ontario and in South Africa, the Director suggested
to the Chief Geologist that a workshop on uraniumbearing quartz-pebble conglomerates be sponsored by
the U.S. Geological Survey to broaden understanding of
the origin and distribution of deposits of this type as abasis for further exploration. The Chief Geologist asked
me to organize such a workshop and to compile the pro­ceedings and discussions as a Professional Paper.
The main burden of presentation at the workshop
necessarily fell on scientists from South Africa andCanada; contributions were also to be made by others in
the United States, Brazil, the United Kingdom, Ger­
many, and Australia. Unfortunately, Australia could not
be represented because the workshop was called on such
short notice that their personnel had other com­mitments that took priority. Originally it was alsoplanned to have the panel informally discuss the quartzpebble conglomerates in Ghana, Finland, and Russia. As
it turned out, however, except for the remarks made by
Dr. Ruzicka about two Russian deposits, there was nottime to discuss these other deposits.
Selection of the panelists and their subjects for theworkshop would have been a formidable task had it notbeen for the help of friends and colleagues and theirknowledge of up-to-date developments. The speakers on
the Rand were selected with the advice of Dr. Desmond
IX

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

A. Pretorius, Director, Economic Geology Research
Unit, University of Witwatersrand, and Dr. Johann W.
von Backstrom, Director, Geology, Atomic Energy
Board of South Africa. The Canadian panelists were in­vited following the suggestions of Dr. Reward W. Little,
who at that time was Head, Uranium Resource Ap­
praisal Section, Economic Geology Subdivision,
Geological Survey of Canada. Dr. John M. A. Forman,
Mineral Resources Director, NUCLEBRAS, assisted inselecting the Brazilian panelist. Dr. S. H. U. Bowie,
Director, Institute of Geological Sciences, London, sug­gested that Dr. Peter R. Simpson attend the workshop
in his stead. Numerous colleagues assisted in selecting
the other speakers.
To a student of Precambrian quartz-pebble con­glomerates it must seem strange that such a workshop
could be held without including Dr. Desmond A.
Pretorius. It was originally intended to hold theworkshop in the late spring of 1975, but it soon became
apparent that many people, including Dr. Pretorius,
could not attend at that time. Accordingly, the date waschanged to October 1975 to coincide with a time when
Dr. Pretorius would be in the United States on othermatters. As it later developed, he had to cancel all plansof visiting the United States at that time. The workshop
did benefit, however, from his advice in selecting thepanelists.
Originally it was anticipated that the workshop would
be an in-house study to be held in Reston, Va., but in theearly stages of organization it was realized that allgeologists in the Branch of Uranium and Thorium
Resources, based in Denver, Colo., should attend, andthat industry would also be interested in attending ameeting at which experts from around the world would
be speaking. As many companies have uraniumexploration offices in Denver, moving the workshop to
Denver would fulfill two needs: personnel of the Branchof Uranium and Thorium would be able to attend, and by
opening the workshop to all, the Geological Survey
would be fostering its continuing mutually beneficial
dialogue with industry. Accordingly, the workshop washeld at the excellent conference facilities at the Green
Center, Colorado School of Mines, Golden, Colo., on Oc­
tober 13-15, 1975, and was attended by about 250
specialists from government, academe, and industry;
about 125 companies connected with the uranium in­
dustry were represented. Following presentations by
the speakers, discussion was among the speakers andthe other invited participants. If time permitted, discus­
sion was then extended to the general audience.
The workshop was designed to examine genetic con­cepts and their possible bearing on exploration.
Whether or not it was successful in that respect is left tothe reader to decide. Most papers presupposed afamiliarity with the subject matter; the reader who
regrets the lack of an extensive review paper for theRand is referred to D. A. Pretorius, 1974, The Nature of
the Witwatersrand Gold-Uranium Deposits: Universityof Witwatersrand, Johannesburg, Economic Geology
Research Unit, Information Circular No. 86, 50 p.
Before the workshop was held, it was decided that the
papers for the Professional Paper would not be put
through the rigorous editing "mill" of the U.S.
Geological Survey. The Geological Survey did not thinkthat much of its editorial system, for example, theGeologic Names Committee, could or should be used forauthors almost all of whom were not Geological Survey
employees and the majority of whom came from foreign
lands and discussed geology in foreign lands. According­ly, most of the editing has been for clarity and internalconsistency. Subject matter, data, reasoning, and thelike, were not changed from what the author submitted
for publication. The reader will be interested to note thelack of agreement among the authors on many subjects.
If nothing else, the workshop pointed out additional
areas for investigation.
To preserve the international flavor of the meeting,slightly different English usage, spelling, and phrases,
such as "on the Rand," have not been changed; similarly,
to record the spontaneity of the meeting, the discussions
have been transcribed as faithfully as possible. Some ofthe discussion was lost because of technical failures intape recording, but it is estimated that more than 90
percent of the pertinent discussion was recovered andappears in the "Discussions."
All additions and comments by the editor are enclosed
in square brackets ([ ]).
Usage in this report is as follows:
1 short ton = 1 ton (abbreviated t)
1 metric ton = 1.1023 short ton
1 short ton = 0.9072 metric ton
1 metric ton uranium metal (U)=1.3 t U308
1 t U308 = 0.7693 metric ton U

A large part of the success and smooth running of theworkshop must be credited to the U.S. Geological
Survey's Center for Continuing Education in Denver,headed by the late Dr. Harley Barnes and ably assisted
by Helen E. Eichler, Cynthia Rathbun, and David L.
Macke. They made all arrangements for the meetingsand housing and recorded the proceedings. Special
credit must go to Carolyn Willis for phoneticallytranscribing the discussions. She spent many long hours
trying to understand unfamiliar accents pronouncing
unfamiliar place names and discussing esoteric subject
matter. Thanks are also due Drs. von Backstrom and
Little for presiding over the South African and Cana­dian parts of the workshop, respectively. We are mostindebted to the panelists for their willingness to take

INTRODUCTION

part in the workshop and to prepare their papers for
publication, and we are greatly indebted to the com­panies, governments, and other organizations who madepossible the participation of the panelists.
About one-third of the manuscripts were not received
by the editor until 1978, and the revised draft of the final

XI

one was not submitted until April 1979. Most papers
submitted after the workshop differ somewhat from theauthors' original presentations; in some instances thisdifference may make the discussion difficult to follow.
As can be seen from dates of references cited, many
authors have updated their papers past 1975.

HISTORYInvestigation of Witwatersrand
Uranium-Bearing Quartz-Pebble Conglomerates
in 1944-45
By WESTON BOURRET
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-A

CONTENTS
Abstract _________________________________________________ Al
Investigation of Witwatersrand uranium-bearing quartz-pebble conglomerates in 1944-45 __1
Selected references _________________________________________
5
Discussion ________________________________________________
6

ILLUSTRATIONSPage

FIGURE 1. Photograph showing corduroy table feed on deck of Wilfley table, Robinson Deep mine _______________________ A3
2. Graph showing uranium production from South Africa gold mines ____________________________________
6

TABLESTABLE 1. Description and analysis of samples ______________________________________________________ A4
2. Description and analysis of osmiridium and miscellaneous samples ___________________________________5
inGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QJJARTZ-PEBBLE CONGLOMERATESINVESTIGATION OF WITWATERSRAND
URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATESIN 1944-45
By WESTON BOURRET *ABSTRACTThe paper discusses the results - both short- and long-term - of a1944 study undertaken by a team from the Manhattan Project (sup­ported by the governments of Great Britain and Canada) to assess thepotential for uranium occurrence in the Witwatersrand mining regionof South Africa.
Details are given on the methods used by the Manhattan team andthe results of the sampling study and survey that the team conductedin South Africa. The findings for 23 mines that were surveyed in the
Witwatersrand are documented. These mines were the major produc­
ing mines in the Witwatersrand region.
The prior research that led the Manhattan Project administrators toconcentrate efforts on the Witwatersrand is described. The history of
uranium production in the Rand since the 1944 study is delineated.
Tables show the findings of the sampling study in terms of various
major mines. A graph is included to show the growth of uranium pro­duction in the Witwatersrand from the initiation of the first operationin 1952 to 1975.

INVESTIGATION OF WITWATERSRAND
URANIUM-BEARING QUARTZ-PEBBLECONGLOMERATES IN 1944-45 2

The presence of uraninite on the Central Rand was
first reported in a heavy-mineral concentrate recoveredfrom City Deep mines in October 1923 by R. A. Cooper.
It should be mentioned, however, that Fred
Wartenweiler, consulting metallurgist for the Johan­
nesburg Consolidated Investment Group, was indirectly
responsible for this discovery, in that he initiated achange in Rand milling practice from conventional
1 Consultant, Belvedere, Calif.; formerly Senior Geologist, Manhattan District Project, U.S.
Corps of Engineers
2 This summary records events initiated through the Manhattan Project in 1944 that led tosampling and preliminary evaluation of more than 20 producing Witwatersrand gold mines.
The work was made possible through the joint effort of the Rand Gold Mining Industry,
Manhattan Project engineers, and the South African Chamber of Mines. The complete resultsof the sampling study and survey are presented in the First Uranium Report Witwatersrand,South Africa, published in 1973. This document is in the libraries of the South African Atomic
Energy Board, the Chamber of Mines of South Africa, and numerous South African univer­sities, in the U.S. Geological Survey libraries at Reston, Va., and Menlo Park, Calif., and in
the Congressional Library, Washington, D.C.

amalgamation plates through the installation of cor­duroy riffle tables ahead of the cyanide circuit for therecovery of coarse gold and heavy minerals. This change
made possible Cooper's discovery.
Because of the abundance of pyrite and a variety ofother heavies characteristic of placer deposits, thevolume of concentrate caught on the corduroy far ex­
ceeded the capacity of the amalgamation plant torecover a high percentage of free gold. As a conse­
quence, a number of Wilfley-type tables were installed
to recover a middling product from the corduroy tables
with a specific gravity of about six or less.
Since Wartenweiler's decision to install corduroy rif­
fles did, in fact, result in improved recovery of free gold,
as well as osmiridium and placer heavies, his flow sheet
was quickly adopted by a major segment of the Rand
gold industry. As a natural consequence of reconcentrating the corduroy fraction over tables, curiosity as totheir composition and mineral content led Fred
Wartenweiler to ask his mill superintendent, R. A.
Cooper, to examine and identify the chemical andmineral content of the heavy fraction. The result of thisresearch is the now-famous paper of Cooper's that ap­
peared in the Journal of the Chemical, Metallurgical andMining Society, in October 1923, followed by a reply to
discussion in May 1924. In his reply, Cooper reported on
the radioactivity of a concentrate from the City Deep
Mine. This concentrate was recovered over a period of
about three months in the course of dressing the concen­
trates collected from corduroy tables, representing
300,000 tons3 of mine-run ore. The total weight of the3 [Originally all record-keeping on the Rand was in English units and the short ton (2,000 Ibs)
was used. When the work was done on which this report is based, the English system was inuse. Accordingly, unless otherwise specified, English units are used in this report. Recordkeeping on the Rand is now in metric units; the change from English to metric units was madein the mid-1960's. In Figure 2 all uranium production is shown in metric units.]

Al

A2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

final (reconcentrated) product was only 720 grams; itassayed as follows:
Assay of radioactive fraction
Uranium Oxide (U308) __________________ 44.0 percent.
Radium ——__——___________________ 127 mg per ton
Osmiridium -_______________________ 28.35 grams

Then, Cooper went on to say (p. 266):
The streak on the concentrating table * * * is brownish black, theuraninite itself being black. * * * As uraninite is distinctly softer thanpyrites it is, of course, possible that a considerable amount may havebeen lost by slimming. The quantity recovered certainly is only ahomeopathic trace, but that does not detract from its technical in­terest.

The uranium exploration unit of the Manhattan
District Project was established in 1942 for the purposeof discovering and evaluating uranium deposits believed
to hold economic potential for production. The govern­ments of Great Britain and Canada cooperated in this ef­
fort during and following World War II.
Consistent with this objective, a literature research
unit was established in 1943 in New York andWashington and at the Massachusetts Institute ofTechnology, under the direction of Dr. Joseph Sinclair,
to identify potential uranium occurrences outside theUnited States that appeared to justify follow-up ex­
amination by field geologists and mining engineers of
the Manhattan Project staff.
Early in 1944, Margaret Cooper of Dr. Sinclair's
group directed attention to R. A. Cooper's paper on the
Witwatersrand "heavies" research. Even though only
720 g of final radioactive product were isolated fromsome 300,000 t of ore, the decision was made to send afield team under the direction of Weston Bourret toSouth Africa to expand on and look deeper into the work
initiated by Wartenweiler and Cooper in 1923. Since
Cooper's work provided results from only one sample
from the City Deep Mine, the main objective of theManhattan Project investigation, of course, was to ex­
amine and sample as many of the major operating mines
along the East, Central, and West Rand as time permitted-specifically, those with gravity circuits incor­
porated in the flow scheme capable of recovering someuraninite.
Upon arrival in Johannesburg in May 1944, Bourret,assisted by Frank West, established contact with the
Witwatersrand Chamber of Mines and consulting metal­
lurgists of the major mining houses. Despite stringent
wartime secrecy, the cooperation of the mining in­dustry, South African government, and Chamber ofMines was essential, of course, to gaining access to thelarger mines and getting the job done. Their superb
cooperation is gratefully acknowledged, for without thehelp of so many metallurgists, engineers, and minemanagers, successful negotiations and preparation forSouth African uranium production by 1952 could havebeen delayed by several years. Although space limita­
tions do not permit identifying every individual, thecomplete cooperation of consulting metallurgists andmanagers of the major mining houses is gratefully
acknowledged, particularly the efforts of Fred
Wartenweiler, T. K. Prentice, R. A. Cooper, and E. T.
Pinkney.
In the course of the investigation, more than 20 Rand
mining operations (representing more than 50 percent
of total Rand gold production) were examined, andsamples of concentrated products from gravity circuits,similar to those shown in figure 1, were taken foranalysis and mineralogic study. In 1944 the mostwesterly operating mine on the Rand was Blyvooruitzicht, a new and high-grade mine that had just come into
production. Unusually encouraging sample results fromthe Blyvooruitzicht table concentrates immediately
focused attention on the importance of the Far West
Rand and continuity of major uranium and gold
mineralization to the west and south. It was at
Blyvooruitzicht also that the presence of thucholite was
first identified by Dr. D'Arcy George, chief mineralogistof the Manhattan Project, from high-carbon ores of the
Carbon Leader Reef. This footwall carbon seam is subse­
quently referred to in the literature as an algal mat.
During the three and one-half month period spent on
the Witwatersrand field investigation, the project teamaccomplished the following:
1. Visited more than 20 mining or milling operations
and obtained bulk samples of uraniferous gravitycircuit mineral concentrates. These samples includ­ed Wilfley tables, batea, and amalgamation barrel
fractions.
2. Examined and assayed more than 42 productsamples.
3. Examined deep underground stoping operations at
16 Rand mines.
4. Observed methods of recovering uranium-gold con­centrates and attempted estimates of slime losses
from various-sized fractions.
5. Discussed broad aspects of Rand geology, mineraliza­
tion, and distribution of gold with the consulting
geologists and mine operators.
6. Discussed recovery (and losses) of heavy-mineral con­centrates by means of corduroy and Wilfley tables
with consulting metallurgists.
7. Separated and prepared mineral fractions utilizing a
Haultain superpanner. Sized fractions wereprepared from sieved samples and mineral deter­
minations were made.
The field team was equipped with a Herbach andRademan type 215 gamma counter. This instrument

WITWATERSRAND URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES IN 1944-45

A3

FIGURE 1.- Corduroy table feed on deck of Wilfley table, Robinson Deep mine, a, Wide light-gray band: pyrite, quartz and zircon, b, Sinuous, nar­row black band: principally chromite and niccolite. c, Narrow, somewhat sinuous light-gray streak: coarse iron, d, Wide black band: uraninite.
e, Wide light- and medium-gray band: osmiridium with some gold.

A4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

proved adequate for approximating the U308 content ofconcentrate samples. It was entirely inadequate,
however, for testing ores containing only a few hundredparts per million. A number of samples were sent to thelaboratories of the Manhattan Project in the UnitedStates for check assay. (This transfer was accomplishedthrough diplomatic air pouch of the U.S. State Depart­ment.) The time interval of from two to three weeks inreceiving results enabled confirmation of assays made in
the field with the gamma ray counter.
The mines visited and sampled during the investiga­
tion are shown in the following list:
West
West Rand Consolidated
Randfontein Estates
Blyvooruitzicht
Klerksdorp
Dominion Reefs

Central Rand
Durban-Roodepoort Deep
Consolidated Main
Crown Mines
Robinson Deep
City Deep
Nourse Mines
Geldenhuis Deep
Simmer and Jack
Rose Deep

East
East Rand Proprietary
Van Ryn Mines
New Modderfontein
Modderfontein Gold
East Modderfontein
Government gold areas
East Geduld Daggafontein
Vogelstruisbult

The results of analyses of bulk samples taken at these
operations are set forth in tables 1 and 2.
East and West Rand sample results show that theU308 content of the heavy-mineral fractions recoveredfrom the reconcentrated corduroy riffle tables isgenerally less than 1.0 percent, and the higher gradeores, such as those from the Carbon Leader and Main
Reef, contain far more uranium than gold on a weightratio basis. The actual uranium-to-gold ratio of different
mines and different reefs varies over a wide range;
however, the ratio appears to be between 15 and 20
parts uranium to one part gold - and often even higher.
Based on the 1944 production figures for gold bullion, in­dications are that the Witwatersrand has the capabilityof producing more than 500 t of gold annually.
Therefore, it may be conservatively concluded (assum­
ing the installation of uranium plants) that the potential
for uranium should be about 15 to 20 times the gold
figure, or 7,500 to 10,0001 of U308 contained in concen­
trate per year.
Upon returning to the United States in early 1945, theauthor prepared a summary report describing sampling
methods, losses, and conclusions for submission to thedirectors of the Manhattan Project. Late in 1945, Dr.
D'Arcy George and Frank West, together with othermembers of the Manhattan Project staff in New York
City, cooperated in the preparation of the final report.
The findings and recommendations of that report forced
Dr. George Bain, consultant to the Manhattan Project,and Dr. Charles F. Davidson, senior geologist for the
British Atomic Energy Board, to visit South Africa tocontinue the investigation.

TABLE 1. -Description and analysis of samples[June-October 1944]
SampleMine

6915

Randfontein Estates

6907

West Rand Consolidated

6931

Durban-Roodeport

6927

Consolidated Main Reef

6935
6936
6937

Crown Mines "A" Mill
Crown Mines "B" Mill
Crown Mines "C" Mill

6932
6912

Robinson Deep
City Deep

6920
6923

Nourse Mines
Geldenhuis

6913

Simmer and Jack

6922

Rose Deep

6924
6925
6916
6917
6918
6919
6906
6934
6933
6926

Sample description
Chiefly pyrite; concentrate from
Wilfley tables following removalof osmiridium streak (see sample
6905).
Daily composite concentrate, sized to
70 percent - 200 mesh; 82 percent
U30a values in -200 mesh fraction-

1 Radiometric equivalent.

Main, Johnston, and Bird

Tons milled
per month
340,000

1Percent U-A 1Pounds U3O8
(chem.)
per ton
0.04
0.80

1.60

240,000.08

200,000'.03.60

210,000.09

1.80

105,000
75,000110,000

.53
.48
.32

10.6
9.6
6.4

100,000
95,000

1.98
.71

39.6
14.2

80,00065,000

.85
.09

17.0
1.80

150,000

1.30

26.0

70,000

1 1.08

21.6

Main Leader
Main Leader
Upper Leaders and Black

105,000
80,00072,000

.12
.12
.11

2.4
2.4
2.20

Upper Leaders and Black

90,000.03.60

Main Leader
Main Leader
Leaders, Black, and Kimberley

70,000130,000
210,000

.02
.03
.13

.40
.60
2.60

Upper Leaders and Kimberley

220,000.10

2.0

Main Leader
Carbon Leader ___________________

75,00012,000

1.74
1.20

34.8
24.0

from corduroy riffles (see sample 6930).
Main
Composite of weekly gravity concen­
trate; contains 3-4 oz Au per ton.
About equal amounts of quartz and pyrite Main and South
Main and South
More quartz than in sample 6935
Main and South
Weighted average of the three Crown
mine samples is 9.0 Ib U3O8 per ton; 75
percent quartz and 25 percent pyrite.
South and Main Leader
Before removal of osmiridium; pyritic con­ South and Main Leader
centrate with quartz.
Mostly pyrite and quartz
South and Main Leader
Main and South Leader
trate not removed (see sample 6921).
Contains osmiridium (see sample 6914 for South and Main Leader
uranium osmiridium concentrate).

removal of osmiridium.
Chiefly quartz and pyrite (Cason Mill)
Osmiridium concentrate removed
Daily composite heavy-mineral concen­
trate before osmiridium cut; essentiallypyrite and pyrrhotite.
Modderfontein New South Mill
Daily composite heavy-mineral concen­
trate before osmiridium cut; essentiallypyrite and pyrrhotite.
Modderfontein B
Daily concentrate composite sample
Modderfontein New East Mill
Daily concentrate composite sample
Government areas
Daily composite after removing
osmiridium.
Government areas
Composite concentrate representing six
week milling run.
Vogelstruisbult (East Rand)
Composite concentrate
Blyvooruitzicht (West Witwatersrand Weekly composite containing osmiridium,areas).
visible uraninite, and thucnolite.
East Rand Proprietary
ERP-Angelo Mill
Modderfontein New North Mill

Reef
Main and Johnston

A5

WITWATERSRAND URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES IN 1944-45
TABLE 2. -Description and analysis of osmiridium and miscellaneous samples[June-October 1944]
SampleMine

Sample description

Percent U30a(chem.)

Reef

Pounds U308
per tonOsmiridium gravity concentrates
6905

Randfontein Estates

6914
6921

Simmer and Jack
Geldenhuis Deep

6929

Consolidated Main Reef

6930

Durban Roodeport

6938

Robinson Deep

Gold-pyrite-osmiridium concentrate withchromite, zircon, rutile, and uraninite.
Wilfley table composite concentrate
Composite pyrite concentrate from
corduroy.
Composite from corduroy tables, with
pyrite.
Recovered over corduroy, pyrite, mill iron,and quartz.
Isolated uraninite streak cut from Wilfley
table; about 75 percent uraninite, withosmiridium, gold, mill iron, and pyrite.

Main and Johnston

1.01

20.2

Main and South
Main and South

3.28
.81

75.6
16.2

1.52

30.4

Leader Middle South
Main and Leader__

Leader Middle South

.24

4.8

63.8

1,276.0

Miscellaneous and hand specimens
6928

Consolidated Main Reef

6939

Daggafontein

5977

Vogelstruisbult

5978
5979

Do _______.
Randfontein Estates

Wilfley table tails after dressing corduroy
concentrates; essentially quartz.
Heavy-mineral accumulation from ballmill
liners; gift of Dr. E. Mendelssohn; 30-40
percent visible uraninite; also chromite,rutile, and iron.
Hand specimens of high-grade gold ore; high.pyrite content.
Hand specimen of Kimberley Reef _____
Sample of Main Reef gold ore ________

Leader Middle South

_

_

nil 1
14.5

290.0

Black __.

.335

6.70

Kimberley.
Main __.

.018
.05

.36
.12

Kimberly

1 Radiometric equivalent.

Following Drs. Bain and Davidson's conferences with 1952 through 1975 was 0.271 kg U308 per metric ton, or
the South African government, Dr. L. T. Nel of the 0.54 Ib U308 per short ton. Since discovery of the Rand
South African Geological Survey was placed in charge of in 1886, more than 3.3 billion metric tons of ore havea team of geologists and metallurgists organized to ex­ been treated in cyanide mills, from which more thanpand the sampling of reefs reported to contain signifi­ 32,000 metric tons of gold bullion have been recovered.
The tailings dumps and slime dams resulting fromcant uranium. These locations included Blyvooruitzicht,
millingover 3 billion metric tons are intact around the
Vogelstruisbult, Western Reefs, and East Daggafon­
north
rim
of the Witwatersrand Basin. Although thetein. Moreover, in 1946 a special uranium research com­uranium
contentof these dumps is low, the uranium con­mittee was organized to recommend metallurgicaltainedinthemreaches
an impressive figure, probably
research on the recovery of uranium by flotation,
approaching500
millionpounds of U308. Possibly theleaching, and alternative methods. This team consisted
day
is
not
too
far
off
when
it will become economicallyof T. K. Prentice, Fred Wartenweiler, and 0. A. E.
feasibletore-treatsomeof
the old tailings, assuming a
Jackson. About the same time, a research mineralogist,
price,
say,
of
$40
to
$50
per
pound U308 . In any event,Dr. W. R. Liebenberg, was appointed to study andthefuture
of
the
Rand
as
one of the world's majorreport on the heavy minerals contained in the Witwateruranium
producers
issecure.
srand conglomerate ores. With the assistance of a con­sulting metallurgist Robert R. Porter, the first test pilot
plant was erected at Blyvooruitzicht and Western Reefs
SELECTED REFERENCES
in 1949 to work out a flow sheet based on sulfuric-acidleaching for the recovery of uranium.
In 1951 a small amount of yellow cake was produced Associated Scientific and Technical Societies of South Africa, 1957,
Uranium in South Africa, 1946-1956 [A joint symposium arrangedby the pilot plant at the Blyvooruitzicht mine; commer­by * * * five constituent societies]: Johannesburg, v. 1, 546 p.
cial production on the Rand started in 1952. Since that Cooper,
R. A., 1923, Mineral constituents of Rand concentrates:
time, South Africa is credited with the production ofChemical, Metallurgical and Mining Society of South Africa Jour­
more than 82,000 metric tons of uranium oxide (fromnal, v. 24, no. 4, p. 90-95; Reply to discussion, v. 24, no. 11, p.
more than 20 mines) in the form of yellow cake concen­
264-266, 1924.
trates valued at more than $2 billion. The record of this Du Toit, A. L., and others, 1929, Kimberley-Johannesburg: Interna­
tional Geological Congress, 15th, South Africa, 1929, Guide A. 6,
production by years is set forth in figure 2.
p. 1-34.
According to the annual reports of the Chamber of Fisher, M. S., 1939, Notes on the gold, pyrite, and carbon in the Rand
Mines of South Africa, the average recovery from ap­
banket: Institution of Mining and Metallurgy Bulletin no. 414, 36
proximately 300 million metric tons of ore treated fromP-

A6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 2. - Uranium production from South African gold mines.

Graham, K. L., and Wartenweiler, F., 1924, A research to determine
the size of the gold particles in the Witwatersrand banket ore andtheir rate of solution in cyanide: Chemical, Metallurgical and Min­ing Society of South Africa Journal, v. 24, no. 12, p. 285-292.
Graton, L. C., 1930, Hydrothermal origin of the Rand gold deposits, pt
I, Testimony of the conglomerates: Economic Geology, v. 25, sup­plement to no. 3, 185 p.
Jones, G. C., 1937, Correlation and other aspects of the exploitedauriferous horizons on the Witwatersrand mining field [presiden­tial address]: Geological Society of South Africa Proceedings, v.
39, p. 23-61.
Mellor, E. T., 1916, The conglomerates of Witwatersrand: Institutionof Mining and Metallurgy Transactions, v. 25, p. 226-291.
Mendelssohn, E., and Marland, E. F., 1934, An occurrence of monazite
in the Sub Nigel Mine, Witwatersrand: Geological Society of South
Africa Transactions, v. 36, p. 113-115.
Nel, L. T., 1934, The Witwatersrand System outside the Rand
[presidential address]: Geological Society of South Africa Pro­
ceedings, v. 36, p. 23-48.
Pelletier, R. A., 1938, Contribution to the geology of the Far West
Rand: Geological Society of South Africa Transactions, v. 40, p.
127-162.
Pirow, H., 1921, Distribution of the pebbles in the Rand banket andother features of the rock: Geological Society of South Africa
Transactions, v. 23, p. 64-97.
Prentice, T. K., 1940, Precious metal constituents of Witwatersrandores: Associated Scientific and Technical Societies of South Africa,
Annual Proceedings.
Reinecke, Leopold, 1928, The location of payable ore bodies in the
gold-bearing reefs of the Witwatersrand: Geological Society ofSouth Africa Transactions, v. 30 [1927], p. 89-119.

Young, R. B., 1917, The banket; a study of the auriferous con­glomerates of the Witwatersrand and the associated rocks: Lon­don, Gurney and Jackson, 125 p.

DISCUSSION

von BoLckstrom: I must say that I really enjoyed thishistorical review. I just want to mention to the au­
dience what the situation is at the present. We havefifty gold mines in production at the moment and ofthese there are seven that have extraction units foruranium. There are ten uranium plants. We think thatwith the demand building up and some of our extrac­
tion plants which are moth-balled being prepared for
production, we could probably eventually double thenumber of mines that would be able to produce
uranium. It is very difficult to say at this stage what
the uranium production capability would be from the
Witwatersrand mines because there are so many fac­
tors influencing production. I was very interested tohear the figure of 10,000 tons of U308 per year as the
production capability of the Rand. I think that level ofproduction probably could be reached provided that all
economic factors were favorable. At the moment wecertainly are far away from that situation. I think it isgenerally known that labor costs have increased to avery great extent and will certainly affect productionWITWATERSRAND URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES IN 1944-45

capability. Another important item is, of course, thegold price, which has not been very consistent. This in­consistency makes the decision-makers uneasy about
installing expensive extraction plants. Anotherunknown economic factor, of course, is inflation.
Bourret: Dr. von Backstrom, I apologize for not
specifically including your name among those I men­tioned. At that time, of course, you were not working
on the Rand. Nevertheless, I should call attention to
the fact that you have done a tremendous amount ofwork on the Rand since the days of the early investiga­
tions. Obviously, when I mention a figure of 10,000
tons, I simply cite that as an ultimate productive
capability, assuming that the economic factors werereasonably favorable.
Also I should mention in passing that there are slime
dams and dumps, as you well know, from whichuranium has not been recovered. In a sense, uranium
has been stock piled in these enormous dumps andthey are available for reprocessing in the event thatthe price of uranium and the market were favorable.
Recovery of uranium from these dumps is a project for
the future. As in excess of one hundred million tons ofore is processed annually, and at least ninety-nine anda half percent of that material ends up in slime dams,
the dumps contain a large uranium resource.
Myers: You spoke of making tests on some of the central
Rand mines. Could you give a statement as to theamount of uraninite contained in crude ore?
Bourret: I would say that the ore averaged perhaps 0.3
pound of U308 per short ton.
Myers: This would be a reasonable average for the cen­tral Rand?
Bourret: Yes. Another way of checking this is to go to
the Annual Report of the Chamber of Mines, where
the grade is given in kilograms per metric ton treated.
Now if in some of these mines they had sorting belts, if
they hand sorted out some of the waste rock or shaleA7

or obviously barren material, then these figures would
be a little on the high side.
[In 1975 the average recovery from ore processed by
the seven uranium-producing mines on the Rand was
0.189 kg U308 per metric ton, that is, 0.378 Ibs. U308 per
short ton.][The following is quoted from Mining Survey (no. 74,
April 1974, p. 28), a magazine published quarterly by the
Chamber of Mines of South Africa, Johnannesburg.]
URANIUM REPORT
Publication in the United States of the "First Uranium Report - Witwatersrand, South Africa" has thrown fresh light on the events that
led to the establishment of South Africa's uranium industry.
The report is by Dr. Weston Bourret who was employed by theUnited States Government in 1944 to undertake an investigation of
the uranium mineralisation on the Witwatersrand as a part of the ef­
fort mounted by the "Manhattan Project" to develop the atomic bomb.
Dr. Bourret visited South Africa to carry out field work. The results of
his work could not be published previously for security reasons.
The presence of uraninite or other uranium minerals in the goldbearing conglomerates of the Witwatersrand had been first reported
in 1923 in a technical paper by R. A. Cooper entitled "Mineral Constit­
uents of Rand Concentrates". These findings attracted little attention
at the time but were recalled when the Manhattan Project was underway.
Dr. Bourret concluded that the Witwatersrand was potentially oneof the world's most important and lowest cost producers of uranium
and he estimated possible output at between 5,000 and 7,000 tons ofuranium oxide a year. Events were to prove him right: production in
the late 1950s ran in excess of 6,000 tons a year.
Dr. Bourret recommended that "a detailed investigation be under­
taken promptly with a view to recovering uranium and determining
the overall uranium potential of this very important field."
As a result follow-up investigations were initiated by the Manhattan
Project and the British Atomic Energy Commission. Distinguishedcontributions to the establishment of South Africa's great uranium in­
dustry were then made by Professor George Bain, Consulting
Geologist of the United States Atomic Energy Commission and Dr. C.
F. Davidson of the Geology Survey of Great Britain. Production began
in 1952 and South Africa remains today one of the top world pro­
ducers.

GENERAL

The Precambrian Development ofan Oxygenic Atmosphere
By J. WILLIAM SCHOPF
GENESIS OF URANIUM AND GOLD BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-B

CONTENTS
Page

Abstract ________________________________________________
Introduction _______________________________________________
Biochemical considerations _____________________________________
Relevance of model studies of the origin of life __________________________
Geologic and paleobiologic considerations _____________________________
Summary _______________________________________________
Acknowledgments __________________________________________
References cited ___________________________________________
Discussion _______________________________________________

Bl
1
2
5
5
7
9
9
10
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE

By J. WILLIAM SCHOPF 'ABSTRACT
It has become rather widely accepted in recent years that (1) during
the geologic past, the Earth's atmosphere evolved from an initial"anoxic" to a later "oxygenic" state; that (2) this change was a result of
the cumulative effects of "green plant" (i.e., oxygen-producing)
photosynthesis; and that (3) the transition occurred during Precambrian, and probably Early Proterozoic, time. Although evidence in sup­port of these views appears to be both more circumstantial and morequalitative than has perhaps been generally appreciated, a broad rangeof biological (and paleobiological, biochemical, and organic-chemical)
data seems to establish that such a transition did in fact occur during
the Precambrian as a result of blue-green algal photosynthesis, withgeological (and mineralogical) data suggesting a plausible date for thetransition of about 2,000 million years ago. Nevertheless, it has
become increasingly evident that the Earth's early atmosphere was
not totally devoid of free oxygen, as had apparently been assumed bymany workers. Diagenetic oxidation, to yield such sediments as band­
ed iron-formations and red beds, may thus have occurred on theprimitive Earth prior to the advent of oxygen-producing photosynthesizers; if so, the mere presence of such deposits should not be con­strued as necessarily evidencing the occurrence either of biological ac­
tivity or of an environment sufficiently "oxygenic" to have been
habitable by aerobic forms of life.

INTRODUCTION

During the past quarter century—since the discovery
in the early 1950's of the now-famous microfossil
assemblage of the mid-Precambrian Gunflint Cherts(Tyler and Barghoorn, 1954)—and especially within thepast decade, there has been a marked increase of in­terest throughout the world in the paleobiology of the
Precambrian. Long-standing questions regarding thenature, antiquity, and even the existence of Precam­
brian life, questions dating from the mid-nineteenth cen­tury and the writings of Charles Darwin (1859), have atlast begun to be answered. The question now is notwhether evidence of Precambrian life exists, but rather,what does it say—what can it tell us of the timing andnature of early evolutionary advances and, especially, ofthe interactions between the evolving Precambrianbiota and the developing early environment?

1 Department of Earth and Space Sciences and Institute of Geophysics and Planetary
Physics, University of California, Los Angeles, CA 90024

Probably foremost among all such bioticenvironmental problems is the matter of Precambrianatmospheric evolution or, more precisely, the timing andnature of the development of an "oxygen-rich" at­mosphere, one capable of supporting the type of aerobic
metabolism on which we and most other living systems
are dependent. In the present ecosystem, anaerobic
organisms of course also exist, but collectively they are
only a small fraction of the Earth's extant biomass.
Although anaerobic metabolic pathways occur in living
aerobic organisms, they tend to be of subsidiary impor­tance only, operating for rather limited periods as a sortof "back-up system" to aerobic metabolism in times of
special stress. Among the various tissues of higher
mammals, for example, the probably most complex—thespecialized nerve tissue that forms most of the humanbrain—inevitably dies if it is deprived of oxygen for
more than about four minutes. Such dependence on freeoxygen, while differing in degree from species tospecies, is exhibited by virtually all other megascopicorganisms as well, plants included (since they, too,
respire aerobically, using pathways that are essentially
identical with those of higher animals). Indeed, among
nonmicrobial forms of life, one is hard-pressed to findexceptions to this general rule of oxygen dependence;certain protists and other intestinal parasites no doubt
qualify (although all such forms appear to be evolu­
tionary derivatives of originally free-living, aerobic
respirers), but the list is short (aquatic mammals, which
can remain submerged for extended periods of time, are
not included—special mechanisms have evolved thatenable such mammals to maintain oxygen supplies to thebrain at the expense of other, less oxygen-demanding,tissues). Free atmospheric oxygen plays a necessary andintegral role in the extant ecosystem of the planet.
From the perspective of the present, therefore, it
seems rather remarkable to realize that dependence on
oxygen is by no means an inherent requirement for the
existence of life on Earth—indeed, during the past
decade it has become increasingly apparent that for anappreciable segment of geologic time, a segment encom­
passing a large portion of Precambrian Earth history
and as much as half of the total history of life on Earth,Bl

B2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

generally quite sound. Just as morphological comparison
with modern analogues can be used to infer that
trilobites were mobile rather than sessile and that sabertooth cats were carnivores rather than herbivores,
biochemical comparison with modern taxa can be used
to infer that both of these types of now-extinct
heterotrophs were obligate aerobes, dependent upon theavailability of free oxygen for various biosynthetic
pathways and for aerobic metabolism. Indeed, such"argument by analogy," reinforced by an understandingof relevant evolutionary lineages, forms the basis of avast majority of all paleoecologic and phylogenetic in­ference. One "knows" for rather similar reasons andwith similar degrees of certainty that Paleozoic
trilobites were both mobile and oxygen dependent.
Let us now apply this biochemical approach to thequestion of atmospheric evolution. Oxygen-requiring
biosynthetic pathways (e.g., of tyrosine, hydroxyproline,
collagen, steroids) and oxygen-dependent metabolism(namely, aerobic respiration) are characteristic of in­vertebrate metazoans; it thus seems evident that free at­mospheric oxygen must have been readily available atleast as long as such metazoans have been extant, by
current estimates beginning some 700 m.y. ago (nearthe beginning of the "Ediacaran" or "Terminal Riphean"phase of the late Precambrian). Although it is perhaps
not widely appreciated, this line of reasoning is not
limited to the occurrence of fossil megascopic
eukaryotes such as metazoans; indeed, it seems ap­
plicable, and with equal force, to the occurrence of
microscopic eukaryotes as well, for even the mostprimitive of such organisms, unicellular microscopic
algae (whether rhodophytes or, as according to somephylogenies, chlorophytes), require molecular oxygenfor intracellular synthesis of such compounds as sterols
and polyunsaturated fatty acids, for use in aerobic
respiration, and possibly also for that mostcharacteristic of eukaryotic traits, eukaryotic (i.e.,
mitotic) cell division (Amoore, 1961a, 1961b). As is trueat the multicellular level of eukaryotic organization,
unicellular eukaryotes are aerobic, oxygen-dependent
BIOCHEMICAL CONSIDERATIONS"biological machines"; the occurrence of such cells in the
Of the several lines of evidence that seem to bear on fossil record would thus seem indicative of the presence
the question at hand, studies of the biochemical of biologically usable concentrations of free atmosphericcharacteristics of modern organisms, coupled with an oxygen. Although there is some uncertainty regarding
understanding of phylogenetic relationships and of the the time of origin of eukaryotes, the most recent,temporal distribution of appropriate types of fossils, are thorough compilation of data bearing on the subject sug­
among the most powerful. While necessarily in­ gests that the lineage had probably become establisheddirect—this line of evidence being based on the by about 1,400 m.y. ago (Schopf and Oehler, 1976); on
reasonable but unproved premise that the metabolic this basis it can be inferred that appreciable quantities ofcharacteristics of fossil organisms can be accurately in­ atmospheric oxygen were present at least as early as theferred from their morphology (by analogy with related mid-Proterozoic.
modern organisms) or from their particular level ofRegardless of the time of origin of the eukaryotic cell,
cellular or organismal complexity— this approach seems however—whether somewhat earlier or substantiallythe planet's atmosphere was virtually devoid of free ox­ygen and the Earth's biota was solely anaerobic; free ox­ygen was neither used by such life forms nor, probably,
could its presence (in excess of some minimal level) havebeen tolerated. The anaerobes of the present ecosystem,
the majority of which appear to be evolutionary
derivatives of this ancient, pre-aerobic biota, hark backto this early anoxic environment, as do the anaerobic
metabolic pathways that still function (albeit in somecases only under special circumstances) in extant plants
and animals.
These views, interpretations of the early history of lifethat have become widely accepted in recent years, haveimportant ramifications for an understanding ofgeological and biological processes on the Precambrian
Earth. It is thus appropriate to inquire as to just how
convincing the evidence really is regarding such a sup­posed major change in atmospheric composition—doavailable data require that such a change in atmospheric
oxygen content actually occurred during the Precam­
brian? If so, what do they suggest regarding the timingof this event, the source of the atmospheric oxygen, the"morphology" of the transition (was it part of a longterm continuum, or did it occur as an abrupt step func­
tion?), and the impact of this rise in oxygen content on
the Earth's biota and its environment? As will be evident
from the following discussion, evidence bearing on thesequestions tends to be circumstantial rather than direct,
and the answers generated, qualitative rather thanquantitative. Still, while the various lines of relevant
evidence (coming from rather disparate branches of thenatural sciences) may individually seem less than fullycompelling, they each are at least consistent with—and
they commonly seem to point to—the occurrence of justsuch a transition, with several types of data suggestingthat this event probably occurred during the midPrecambrian, some 2,000 m.y. (million years) ago.
Although additional and more telling data are no doubt
needed, this most crucial problem in atmospheric evolu­tion appears to be rapidly nearing solution.

THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE

later than the date noted above (and both possibilities
have been suggested in the recent literature)—it is ap­
parent that eukaryotes were preceded by an entirely
prokaryotic biota, an ecosystem composed solely offorms that, were they alive today, would be classed
among the bacteria or the "cyanobacterial" blue-green
algae. These prokaryotes provide additional evidence
regarding the development of an oxygenic environment.
Like their modern counterparts, fossil prokaryotes ap­
pear certain to have encompassed a broad range ofphysiological types, a spectrum including strict
anaerobes, facultative anaerobes, microaerophiles, andobligate aerobes. Indeed, this striking versatility in ox­ygen relations is one (among many) of the factors that
separate the prokaryotic and eukaryotic lineages and is
one of the principal characteristics (again among many)
that have led many workers to conclude that pro­karyotes diversified during a time of changing oxygen
content in the primitive environment. The data support­
ing this conclusion are many and varied, but perhaps themost instructive are those derived from biochemical
studies of modern blue-green algae, prokaryotes of thekind that were apparently responsible for deposition ofmost types of Precambrian stromatolites (laminated,organo-sedimentary structures that are abundant andwidespread in Precambrian carbonate facies younger
than about 2,300 m.y. in age).
Although blue-green algae are oxygen-producing
photoautotrophs and although they can cope with the intracellular oxygen thus produced (i.e., they possess
oxygen-mediating enzyme systems) and can apparently
use such oxygen both metabolically (namely, in aerobic
respiration) and in what may be oxygen-requiring
biosyntheses (i.e., in pathways leading to formation ofphycobiliproteins and chlorophyll-a), they exhibit anumber of biochemical characteristics that suggest thatthey probably evolved under changing conditions of am­bient p02. For example, many (but possibly not all) bluegreen algae can use molecular oxygen to produce
polyunsaturated fatty acids (via "oxidative
desaturation"); the precursors of these fatty acids,
however, are produced via a totally anaerobic pathway(similar to that which occurs in obligately anaerobic
bacteria), and it has thus been inferred that the oxygenrequiring portion of this pathway was "added on" to apreviously established anaerobic pathway during an ear­ly stage in blue-green-algae evolution (and certainly
prior to the origin of eukaryotes, since they share thissame oxygen-requiring biosynthetic system). Similarly,
some (but not all) blue-green algae are capable ofoxygen-requiring sterol synthesis, again involving abiosynthetic step (the cyclization of squalene) that wasapparently "added on" to an earlier establishedanaerobic pathway that produces the necessary precur­B3sor. Moreover, as is noted above, blue-green algae also
use oxygen in aerobic respiration; and, as in apparently
all aerobic (e.g., eukaryotic) organisms, the steps involv­
ed in this energy-yielding process (the so-called "citricacid" or "Krebs" cycle) have been superimposed
upon - "added on" to- a shorter and completely anaero­
bic pathway, one that is less efficient and that yields far
less cellular energy per molecule of substrate metaboliz­
ed than its aerobic derivative. As a rule, although suchoxygen-related "add-ons" are exhibited by eukaryotes,
they may be present or absent in prokaryotes, such as
blue-green algae; this rather striking difference in ox­ygen relations presumably reflects the fact that pro­karyotes had become established prior to, and became
diversified during, a major change in the oxygen content
of the Earth's environment.
In this regard, it is of significance to note that
although blue-green algae are "good aerobes" (i.e., theyflourish in many fully oxygenic settings on the present
Earth), the group tends to be "less aerobic" than is
typical of modern eukaryotes. Of those blue-greens forwhich relevant data are available, virtually all "tend toprefer" (i.e., grow most rapidly within, whether in thelaboratory or the field) microaerophilic rather than fully
aerobic environments—optimum growth rates are com­monly observed in the presence of about 10 percent freeoxygen, in contrast with the 20 percent of the present
atmosphere. Moreover, although it has long been rec­ognized that some blue-green algae are capable of living
within and reproducing under totally anoxic conditions(both in the laboratory and in the field), it has recently
been discovered that several species, among them thefilamentous cyanophyte Oscillatoria limnetica (Orenand others, 1977), are capable of "switching on" or "turn­ing off' their aerobic cellular machinery as a function of
the environment in which they occur. Under anoxic con­ditions, these species can photosynthesize anaerobically(using H2S as a hydrogen source, like photobacteria, andthus not producing oxygen as a byproduct of photosyn­
thesis), but if placed under oxygenic conditions, they can"switch on" their more normal aerobic apparatus (oneusing H20 as the hydrogen source, which thus results inthe photosynthetic production of free oxygen).
During the past decade, it has become increasingly
well established that the "heterocyst," a particular cell
type that occurs in several orders of filamentous bluegreen algae, probably evolved in response to an increasein environmental oxygen content (Schopf, 1974, p. 29).
These specialized, thick-walled cells have been shown to
enclose the oxygen-sensitive nitrogenase enzyme
system, a system that "fixes" atmospheric nitrogen by
combining it with hydrogen and thus reduces it to theproper oxidation level for its incorporation into aminoacids (and a system that is entirely lacking among

B4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

modern eukaryotes—in legumes, for example; nitrogenfixation is carried out by prokaryotic symbionts rather
than the eukaryotic host). In addition, such cells are
known to lack "Photosystem II" (that portion of thephotosynthetic apparatus involved in production of freeoxygen) and to be capable of high levels of aerobic
respiration. Under present-day oxygenic conditions,heterocysts apparently serve to protect the nitrogenase
enzyme system from oxidation by ambient oxygen. They
are thick-walled, so diffusion of oxygen into them isrelatively slow; they lack Photosystem II, so oxygen is
not generated within them; and they carry out active
aerobic respiration, so the cell is scrubbed of any uncombined oxygen that may be present. Under anoxic condi­
tions, however, many species of extant blue-green algae(including some that lack heterocysts) have been shown
to be capable of fixing atmospheric N2. Thus, it seems
likely that the enzymes required for N2 fixation probably
first appeared at a time during which the environment
was essentially anoxic and that as oxygen concentra­
tions increased (owing principally to the photosynthetic
activities of the blue-green algae themselves!), specia­
lized, thick-walled heterocysts developed as a means ofproviding (on a localized, microscopic scale) the
anaerobic environment needed to protect the oxygenlabile nitrogenases and, thus, to ensure that N2 fixation
could take place. Interestingly, in vivo and in vitro
studies of nitrogenases from various sources (Stewart
and Lex, 1970; Stewart and Pearson, 1970; Haysteadand others, 1970) indicate that the enzyme-complex ispartially deactivated (60 to 80 percent inhibition) by con­centrations of ambient oxygen of about one percent andis completely deactived at a p02 of about 5 percent; read
literally, this would seem to suggest that heterocysts
may have first appeared with the attainment of this levelof ambient oxygen content, and since heterocyst-likecells are first known from the fossil record in blue-green
algalike filaments about 2,200 m.y. in age (Nagy, 1978),their presence may indicate that a p02 of perhaps 1 to 5
percent may have been attained by mid-Precambriantime.
Finally, a word should be said regarding the evolu­
tionary precursors of blue-green algae, the photosyn­
thetic bacteria. Although several groups of suchphotobacteria occur in the extant biota, differentiatedone from the another principally by the nature of their
pigmentation (and thus their capability to absorb lightenergy in various portions of the spectrum), the impor­tant point is that, without exception, the process ofphotosynthesis in such microbes is obligately anaerobic.
Such organisms are incapable of using water as ahydrogen source in their version of the photosynthetic
reaction (rather, they commonly use H2S, hydrogen gas,or hydrogen atoms derived from organic compounds);
thus, oxygen is never produced as a byproduct ofbacterial photosynthesis and, indeed, the process cannot
take place under aerobic or apparently even under
microaerophilic conditions. On the face of it, this seems
a paradox—photosynthesis is a light-requiring process
and the photobacteria are benthonic, commonly matbuilding, microorganisms; but although they requireanaerobic conditions in which to carry out bacterial
photosynthesis, the sediment-water interface, for which
they are otherwise well adapted (and at which they are
required to live if they are to absorb the maximum
available solar energy), is an oxygenic environment. It
would appear that to photosynthesize optimally,
photobacteria must inhabit an environment that pro­hibits their photosynthesis from occurring! Whether
stochastic or not, it seems literally incredible that evolu­tion could be this capricious. In fact, the seeming
paradox can be rather easily explained: If one assumes
that a transition in atmospheric-oxygen content actuallyoccurred during the Precambrian, then it would seemreasonable to suggest that primitive photobacteria(precursors of the present several strains) evolved prior
to the appearance of oxygenic conditions, at a time when
the sediment-water interface was still essentially anoxic(rather than being oxygenic, as it is at present). With thedevelopment of aerobic photoautotrophs (precursors ofpresent-day blue-green algae), conditions at the inter­face began to change and, by the ecological Principle ofCompetitive Exclusion, oxygen-producers came to sup­plant photobacteria in the upper reaches of bacterial
mat communities (since they produced a toxin, free ox­ygen, that during daylight hours served to inhibit
bacterial photosynthesis); the photobacteria thus
became adapted to lower, relatively anoxic reaches ofthe mat community, an environment that many suchmicroorganisms inhabit to the present day.
In summary, diverse types of biochemical data seem toindicate the following:
1. Appreciable quantities of atmospheric oxygen wereavailable prior to the development both of in­vertebrate metazoans (about 700 m.y. ago) and ofunicellular eukaryotes (about 1,400 m.y. ago).
2. Prokaryotes, such as blue-green algae, evolved in
response to (and, as is discussed below, were theoxygen-producers responsible for) a transition fromessentially anoxic to oxygenic conditions during thePrecambrian.
3. Earlier evolving, more primitive prokaryotes, such as
the photobacteria, were adapted to an anoxic,
rather than an oxygenic world.
If these evidences have been read correctly, the conclu­
sion seems inescapable that a transition in atmospheric
oxygen content must have occurred and have had a pro­found effect on the Earth's biota, relatively early ingeologic time.

THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERERELEVANCE OF MODEL STUDIES OF THE
ORIGIN OF LIFE

B5hydrogen cyanide is lethal, even if present in only
minute amounts, to many extant forms of life. From theperspectiveof the present, it thus seems puzzling (and,Since the early 1950's and the initial studies of Gar­
indeed,atfirstglance seems inexplicable) that living
rison and others (1951) and Miller (1953), literally hun­systemscould
have
originated in a milieu that was indreds of "early Earth experiments"—studies
factpoisonous
totheprocess of living. Such, of course,
demonstrating the non-biologic formation of organic
was
not
the
case—both
HCN and CO are lethal only tocompounds under conditions simulating those believed
aerobic
forms
of
life
(sincethey react with, and deac­
to be present on the primitive Earth—have been carriedtivate,cytochromes
involved
in aerobic respiration), andout in scores of laboratories throughout the world. The
H2S
isacompoundbothused
as a reactant by (e.g.,vast majority of these have been gas-phase syntheses,
photobacteria)
andproducedasa metabolic product ofexperiments carried out both under highly reducing con­(e.g.,
sulfate-reducing
bacteria)
numerous extant
ditions (using a starting mixture of H2 , CH4, NH3 andanaerobic
microbes.
Thus,
theinferred
involvement of
H20 vapor; for example, Miller, 1953) and under lessreducing conditions (using a starting mixture of CO or these compounds in the origin of life—like the results of
C02 plus N2, H2 and H20 vapor; for example, Abelson virtually all other experimental work that has been doneand Hoering, 1967) and by using a rather wide assort­ in this field—is consistent with, and seems supportive of,ment of plausible energy sources (such as spark the concept that the origin of life on Earth could havedischarge, ultraviolet light, and ionizing radiation). occurred only under essentially anoxic conditions.
The application of results of these "early Earth ex­
Compounds so synthesized include amino acids,
periments"
to an understanding of the primitive at­hydrocarbons, fatty acids, nitrogen-containingmosphereis,of course, highly model dependent.
heterocyclics, sugars, and many other types of small
Knowledgeoftheorigin of living systems is far frommolecules. The important point about these experimentscomplete,andeven
if the Oparin-Haldane model is ac­
is that in all such studies for which relevant data are
available, such gas-phase syntheses have been reported cepted without reservation, virtually no data areto be inhibited by "small concentrations" or by even available to indicate, quantitatively, the degree to which"traces" of free oxygen (see, for example, Abelson and abiotic syntheses are inhibited (but perhaps not com­pletely terminated) by various low concentrations of am­
Hoering, 1967).
bient
oxygen. Still the overall model seems too sensible
Recently it has been demonstrated that a second typetobeseriously questioned with the data on hand—theof abiotic synthesis, one involving the photocatalytic"earlyEarth experiments" seem to require essentially
production of organic compounds on mineral surfaces
anoxicconditions; the abiotic organic matter of in­(from CO or CH2 and traces of H2 vapor), is relatively
terstellar
dust clouds and that occurring in car­unaffected by the presence of uncombined 02 (Hubbard
bonaceous
meteorites seem certain to have been syn­and others, 1973). However, the yields of these ap­
thesizedinthe absence of free oxygen; and the mor­parently oxygen-insensitive, surface-dependent syn­phologicallyand biochemically least complex, and thus
theses are quite low and, because of their inhibition by
presumably
mostprimitive, of known living organismseven small amounts of liquid water—the syntheses hav­
simple,heterotrophic
or C02-using bacteria (namely, theing been carried out under conditions designed toclostridia
andthemethanogens;
Broda, 1975)—are all
simulate those of the arid Martian regolith—they appear
obligateanaerobes.
The
conclusion
seems difficult toto be of "questionable importance on the primitive
escape:
early
in
the
Precambrian,atthetime when life
Earth" (Hubbard and others, 1975).
aroseonEarth,
theEarth'senvironment
must haveThus, as currently understood, the nonbiologic syn­been
virtually
devoidoffreeatmospheric
oxygen.
thesis of small organic compounds on the primitive
Earth, the first step in the long chain of events that led
GEOLOGIC AND PALEOBIOLOGIC
ultimately to the formation of the earliest forms of life,
CONSIDERATIONS
seems to have required the near-absence of free oxygen.
Of those synthetic reactions for which the mechanisms
The foregoing discussion has dealt almost exclusively
have been studied in detail, several require the presence with biochemical and organic chemical evidence relatingof two specific, highly reactive intermediates: for­ to the development of an oxygenic environment. These
maldehyde (e.g., an intermediate in the synthesis of matters have been here treated in some detail, principal­
sugars and of amino acids) and hydrogen cyanide (in­ ly because they have been largely overlooked in much ofvolved in the production of amino acids and of such the other geologic literature on the subject and because,
nitrogen-containing heterocyclics as purines and taken together, they appear to me to constitute perhaps
pyrimidines). This latter compound is of special interest the single most telling line of evidence establishing thatsince, like carbon monoxide and hydrogen sulfide (star­ a transition in atmospheric oxygen content actually oc­ting materials commonly used in such syntheses), curred in the geologic past.

B6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESThe timing of this transition, however, is anotherquestion altogether, one that is best addressed by refer­
ral to the geologic-paleobiologic record. As is noted
above, available paleobiologic data seem to indicate thatthe transition occurred earlier than about 1,400 m.y.
ago—the suggested time of origin of the eukaryotic celltype—but how much earlier remains in question (and it
should not be overlooked that some workers have sug­gested a much later time for the origin of eukaryotes
than the date here preferred). To resolve this question,
one must turn to the Precambrian rock record and a con­sideration of the temporal distribution and possible
significance of various types of sedimentary, oxygensensitive mineral assemblages. Over the years, muchhas been written on this subject. Traditionally regarded
as intertwined with the classic question of the origin of
banded iron-formations, it has been a topic of debate andspeculation for several decades, a discussion mostrecently revived in a flurry of stimulating papers by
Preston Cloud (1976) and by Eric Dimroth and his
associates (Dimroth and Kimberley, 1976). Indeed, thegeologic literature is replete with discussions of thisgeneral problem area, and other than to expand and up­
date ideas I have previously expressed (Schopf, 1975,
221-222), I have little new to contribute. Nevertheless,as requested by the Convener of this symposium, I will
here dutifully summarize my current thoughts on thematter (but I do so only briefly, and rather reluctantly,
for it seems to me that what is really needed are moredata, not more verbiage).
Numerous workers have read the geologic record as
indicating that a transition in atmospheric oxygen oc­
curred roughly 2,000 m.y. ago. The principal lines ofevidence usually cited are as follows:
1. "Easily oxidized" detrital pyrite and uraninite are
known to occur together in placer deposits older,but not in those younger, than about 2,000 m.y. inage;
2. Red beds, interpreted as being indicative of subaerial
oxidation and thus of an "oxygenic" atmosphere,
are known to occur in sediments younger, but not in
those significantly older, than this age;
3. The bulk of banded iron-formations of the worlddeposits containing individual reserves of tens ofbillions of tons of oxidized iron—occur in strata ap­
proximately 2,000 m.y. in age but are extremelyrare in appreciably younger terranes; the concept
here is that these deposits might evidence theanoxic-oxygenic transition since their deposition isthought to require both a predominantly anoxic
water column (necessary for dispersal of dissolved
ferrous iron over basin-sized areas) and an at least
marginally oxygenic atmosphere (with the oxygen
diffusing into the upper reaches of the water col­umn and there reacting with dissolved iron to cause
precipitation of ferric and ferro-ferric oxides).
The first of these three types of evidence, that relating
to detrital uraninites, is discussed in detail by DavidGrandstaff elsewhere in this volume (chap. C). He gives
an excellent summary of his studies of the kinetics of ox­
idation and of deposition of the mineral species in ques­tion and of their apparent relevance to the problem at
hand.
Although the second line of evidence, that relating to
the distribution of red beds, is commonly cited in muchof the recent literature on this problem, its significancenow seems open to question since
1. the temporal distribution of these deposits is relative­
ly poorly known (compared, for example, with the
distribution of banded iron-formations; Goldich,1973);
2. terrigenous deposits, of which continental red beds
are one of many types, are generally very suscepti­
ble to geological recycling and are thus preserved
only exceptionally in very ancient terranes (conse­
quently, the present absence of red beds older than
2,000 m.y., even if firmly established, would notnecessarily indicate that such deposits wereoriginally lacking);
3. the Viking explorations of Mars have shown that red
beds need not necessarily reflect the occurrence ofan "oxygenic" atmosphere, at least as the term isnormally used—the Martian atmosphere contains
only trace amounts of free oxygen (about 0.13 per­cent, a value less than that required by terrestrialorganisms for aerobic metabolism), all of which isapparently of inorganic, photolytic origin, an at­mosphere that has nevertheless produced a vast
"red bed" on the planet's surface; and4. an increasing body of geologic data (e.g., Dimroth,
1975; Dimroth and Kimberley, 1976) seems to in­
dicate that free oxygen (perhaps at a low p02 , withthe oxygen having been produced by photolysis of
atmospheric water vapor) was available for suba­
queous oxidation during the Archean—if so,
subaerial oxidation to produce red beds might also
have occurred.
Thus, it would appear that red beds need not be in­dicative of a fully oxygenic atmosphere, that the oxygen
required for their formation need not be of biologicalorigin, and that red beds might reasonably be expected
to have been formed (and to have been subsequently
recycled) prior to 2,000 m.y. ago.
Finally, as a corollary to the foregoing remarks re­garding red beds, it seems likely that the presence of
banded iron-formations need not reflect a transitional
stage in atmospheric evolution. If red beds can be form­
ed by reaction of iron with photolytically-produced ox-

THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE

ygen (as appears to have occurred on Mars), and if UVinduced photolysis of water vapor occurred in the Archean atmosphere (an occurrence dependent only on thepresence of water, of solar energy, and of an exospheretemperature sufficient to insure loss of hydrogen fromthe gravitational field of the Earth—conditions that allseem certain to have been satisfied since very early in
Earth history), then such oxygen migh be expected tohave reacted to produce banded iron-formation even at alow, nearly "anoxic" level of atmospheric p02. In con­trast to continental red beds, the resulting subaqueously
deposited iron-formation might be rather readilypreserved in the rock record. It is conceivable,
therefore, that although deposition of Archean bandediron-formations and of other oxidized facies of similarage (e.g., Dimroth and Kimberley, 1976) may have in­volved reaction with free oxygen, ambient oxygen con­centrations could have been quite low. The mere
presence of such units provides little indication of the ac­tual p02 and no evidence as to whether the p02 hadbecome stabilized (for example, at a low, photolyticallyregulated level) or whether it might have been increas­
ing over time (as would be required for such units toreflect a transitional stage in atmospheric evolution).
However, the very large volume of banded ironformation that occurs throughout the World in strata
aged about 2,000 m.y. (Goldich, 1973), and thus the con­sumption and burial of the huge amounts of oxygen that
their deposition seems to reflect, requires the existenceof an oxygen source far more efficient than that sug­gested above for the scattered oxidized facies known
from Archean-age sediments. The key here is that meas­
ured or calculated rates of 02 production by inorganic
photolysis (e.g., Brinkmann, 1969) are simply too low,by more than an order of magnitude, to account for theenormous amounts of oxygen that were entombed dur­ing this phase of mid-Precambrian iron-ore deposition(see Holland, 1973). Indeed, only one plausible
mechanism capable of accounting for the required rateof oxygen production is known: aerobic (e.g., blue-green
algal) photosynthesis, followed by burial (to prevent
aerobic recycling) of the organic matter produced. That
aerobic photosynthesizers were extant by this time
seems evidenced (albeit indirectly) by the "evolutionary
trees" generated from studies of biochemical evolution(for example, Schwartz and Dayhoff, 1978) and is firmly
supported by the now well-known occurrence of bluegreen-algalike microfossils in mid-Precambrianstromatolites (Schopf, 1974, 1975; Nagy, 1978).
Moreover, the fact that such stromatolites are first
known to have become abundant and widespread about
2,300 m.y. ago (Schopf, 1975), just prior to the onset ofthe major phase of banded iron-formation deposition,raises the intriguing possibility that this phase of diver­B7sification and rapid spread of such structures may
reflect the origin and early diversification of
02-producing photosynthesizers. Thus, the midPrecambrian banded iron-formations may simply be thegeologic result of a major event in biologic history, theorigin and early evolution of oxygen-producing andoxygen-utilizing microorganisms. And, as several
workers have previously suggested (for example, Garrels and others, 1973; Cloud, 1974), the cessation of thismajor phase of iron-formation deposition, about 1,800
m.y. ago, may be a result of the essentially total con­sumption of accessible, easily oxidized ferrous
substrates—the oceans had become "swept free" ofunoxidized iron and this, together with the saturation ofother oxygen sinks (e.g., the conversion of H2S tosulfate), led to the establishment of a stable oxygenic en­
vironment.
SUMMARY

Current concepts regarding the origin of life on Earth,concepts supported by numerous laboratory studies of
the nonbiologic synthesis of organic compounds undersimulated "early Earth conditions," require that living
systems originated at a time when the Earth's at­mosphere was virtually devoid of free oxygen. The pres­ent atmosphere, however, is highly oxygenic, and thepresent ecosystem is based largely on oxygen-requiring
metabolism. At some point in the geologic past,
therefore, the Earth's atmosphere must have undergone
a transition from an early, essentially anoxic stage to its
present oxygenic state. Comparison of oxygen-related
biosynthetic pathways, metabolic capabilities, and otherrelevant biochemical characteristics of invertebratemetazoans, primitive unicellular eukaryotes, and prokaryotic blue-green algae and bacteria demonstrates
that there is a strong correlation between degree oforganizational complexity and biochemical dependence
on free oxygen. This correlation seems most logically in­terpreted as reflecting the evolution of these groups in
response to a rise in atmospheric oxygen content during
Precambrian time:
1. The most primitive forms of life, prokaryotes com­parable in many respects to extant clostridial andmethanogenic bacteria, were obligate anaerobesthat developed during an early, essentially anoxic
stage in atmospheric evolution.
2. The earliest photoautotrophs (cf. extant photobacteria), which similarly evolved prior to thedevelopment of oxygenic conditions, carried out
anaerobic photosynthesis at the anoxic sedimentwater interface; these microorganisms probably
formed laminated, matlike communities, stratified
as a function of the light-absorbing capabilities ofvarious components of the biocoenose (and they

B8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

thus may have produced calcareous, bacteriogenic
stromatolites).
3. The derivation from such photobacteria of oxygenproducing, aerobic photoautotrophs (cf. extant
blue-green algae) resulted in evasion of free oxygen
into the Precambrian environment and the begin­ning of a major change in the Earth's ecosystem.
Although many contemporary anaerobes, especial­ly those that were planktonic or those that werebenthonic and nonmobile, would have been hardpressed to cope with the newly abundant, highly
reactive, free oxygen (and the beginnings of oxygenic conditions may thus have resulted in an ear­ly, major episode of biotic extinction2), the totallyprokaryotic biota diversified to give rise tofacultative aerobes, microaerophiles, and obligate
aerobes; biologically mediated, anaerobic-aerobic
geochemical cycles of various elements (such ascarbon, nitrogen, sulfur, and iron) became
established; and photoautotroph-dominated, ben­thonic biological communities became stratified in­to an uppermost oxygenic zone (inhabited byaerobic photoautotrophs and other oxygen-tolerant
prokaryotes) and an underlying anoxic zone (in­habited by facultative or obligate anaerobes, andincluding photobacteria in its upper portions).
Thus, with the advent of aerobic photoautotrophs,
photobacteria were supplanted at the sedimentwater interface. Because of the absence of effective
competition for photosynthetic space at this inter­
face, the oxygen-producing, stromatolite-forming
photoautotrophs spread rapidly to occupy virtually
all usable, accessible niches; they thereby became
the dominating element of the Earth's ecosystemand, through their photosynthetic activity (and thesubsequent burial of the photosyntheticallyproduced organic matter) caused an irreversible
rise in atmospheric oxygen content.
4. By the time of origin of unicellular eukaryotes, thePrecambrian atmosphere had become oxygenic,stabilized at a p02 similar to (but perhaps somewhat
lower than) that of the present, and members of this
newly evolving lineage were "wholly dependent"(for their metabolism, for important biosynthetic
2 Recently, based on sequence data from six protein superfamilies (feredoxin, 5S ribosomal
RNA, c-type cytochromes, azurin-plastocyanin, flavodoxin, and rubredoxin) that occur inmodern prokaryotes and eukaryotes, Schwartz and Dayhoff (1978) have constructed a com­posite "evolutionary tree," which they interpret as indicating that aerobic respiration evolvedin prokaryotes prior to the advent of oxygen-producing photosynthesis. If their concept is cor­rect, the oxygen utilized by these earliest aerobes would have to have been a product ofultraviolet-induced photolysis of water vapor; the oxygen content of the environment in­habited by these aerobes would apparently have to have been on the order of 0.2 percent or
greater (the so-called "Pasteur Point," the approximate concentration of oxygen required byextant microbes for aerobic respiration); such organisms (which must have possessed oxygenmediating enzyme systems), even if planktonic or if benthonic and nonmobile, would have been
little affected in the episode of biotic extinction that may have accompanied the advent of
aerobic photoautotrophy.

capabilities and, possibly, for mitotic reproduction)on the availability of free 02 .
5. Like their unicellular progenitors, multicellular
eukaryotes (e.g., invertebrate metazoans) are fun­damentally aerobic, a characteristic reflecting theirderivation from oxygen-dependent precursors andindicating that the Earth's atmosphere has been ful­ly oxygenic since their origin during late Precam­
brian time.
This evolutionary sequence, apparently indicatingthat the transition in atmospheric oxygen content occur­
red earlier than about 1,400 m.y. ago (the suggested
time of origin of the eukaryotic cell type), during themid-Precambrian, is consistent with, and seems sup­ported by, available mineralogic and geologic data. The
occurrence of placer deposits containing easily oxidized
pyrite and uraninite (the well-known uraniferous gold
deposits of South Africa, Brazil, and southern Canada),in strata older, but not younger, than about 2,000 m.y.,suggests that the Earth's atmosphere may have been
essentially anoxic prior to this time (Grandstaff, thisworkshop, Chapter C). This is not to imply, however,that the atmosphere was absolutely devoid of uncombined oxygen prior to 2,000 m.y. ago. Even today, freeoxygen is produced readily (albeit slowly) in upper
reaches of the atmosphere by ultraviolet-induced
photolysis of water vapor and the accompanying loss ofhydrogen from the Earth's gravitational field. Thismechanism would have been operable since the forma­tion of the planet and the beginnings of its atmosphere,
and it thus seems reasonable to suggest that this may
have been the source of the free oxygen apparently re­quired for diagenetic oxidation, and the production ofsuch sediments as banded iron-formations, during theArchean. Studies of this inorganic process indicate,however, that it would have been insufficient to haveproduced the large amounts of oxygen sequestered inoxide-rich rocks formed during the major episode of
banded iron-formation deposition about 2,000 m.y. ago;
indeed, the amounts of oxygen required appear to be far
too great to have been produced in the time involved byany known mechanism other than aerobic (e.g., bluegreen algal) photosynthesis. It is thus perhaps signifi­
cant that calcareous stromatolites apparently producedby blue-green algae are first known to become abundant
and widespread in the geologic record in strata about
2,300 m.y. in age. Although this temporal relationshipmay be merely fortuitous, perhaps a result of rare
preservation of older stromatolotic facies, this seeming­
ly abrupt appearance of widespread stromatolites could
reflect the origin and early diversification of aerobic
photosynthesizers. If so, the few older stromatolites
now known would appear to be most plausibly inter­
preted as being of photobacterial origin (produced by

THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE

anaerobic photoautotrophs), and the apparent timelag
between the origin of oxygen-producing photoautotrophs and the cessation of the major phase of bandediron-formation deposition (about 1,800 m.y ago) could beinterpreted as encompassing the time required for ox­
idation of previously unoxidized inorganic substrates
and the buildup of a stable, oxygenic atmosphere.
It is important to note that several aspects of thistransition are as yet poorly defined. For example,
although available data seem to suggest that the transi­tion occurred at "approximately" 2,000 m.y. ago (i.e., ap­
parently between 2,200 and 1,800 m.y. ago), the data do
not indicate whether the transition occurred slowly as amore-or-less unbroken continuum, during this period;
whether it occurred in a single episode, as a relatively
abrupt step-function of short duration; or whether thetransition was sporadic, with atmospheric oxygen con­tent fluctuating widely as a result of variable and essen­tially uncoupled rates of oxygen production (viaphotosynthesis and photolysis) and oxygen consumption(via oxidation of inorganic substrates and, assuming thepresence of aerobic respiring microorganisms, via
biological recycling of photosynthetically produced
organic matter). Moreover, the ranges of atmosphericp02 prior to and after the event are as yet ill defined(although biochemical data seem to suggest that p02
levels on the order of 1 to 5 percent may have been at­
tained during the mid-Precambrian), and direct
evidence of several possible ramifications of the transi­tion, such as a suggested episode of extinction ofoxygen-intolerant anaerobes and the presumed inhibi­tion of abiotic synthesis as a result of increased p02, has
yet to be detected in the rock record. Thus, although it
seems evident that such a transition in atmospheric ox­ygen content actually did occur during the Precambrian,
and although available data suggest a "best guess" forits time of occurrence of about 2,000 m.y. ago, additionaland more telling data will be needed before the timing,
nature, and ecologic impact of this event can be fully
elucidated.
ACKNOWLEDGMENTS

Research leading to preparation of this paper has been
supported by NSF Grant DEB 77-03888 (Systematic
Biology Program), by NASA Grant NGR 05-007-407,
and by the National Science Foundation under the Alan
T. Waterman Award, Grant DEB 77-22518. Views here
expressed are those of the author and do not necessarilyreflect the views of the NSF.
REFERENCES CITED
Abelson, P. H., and Hoering, T. C., 1967, Irradiation of mixtures of
mixtures of CO, N2 , and H2 : Carnegie Institution of Washington
Year Book 65, 1965-66, p. 360-362.

B9

Amoore, J. E., 1961a, Arrest of mitosis in roots by oxygen-lack or
cyanide: Royal Society [London] Proceedings B154, p. 95-108.
———1961b, Dependence of mitosis and respiration in roots upon ox­ygen tension: Royald Society [London] Proceedings B154, p.
109-129.
Brinkmann, R. T., 1969, Dissociation of water vapor and the evolutionof oxygen in the terrestrial atmosphere: Journal of GeophysicalResearch, v. 74, p. 5355-5357.
Broda, E., 1975, The evolution of the bioenergetic processes: New
York, Pergamon, p. 211.
Cloud, Preston, 1974, Evolution of ecosystems: American Scientist, v
62, p. 54-66.
Darwin, Charles, 1859, The origin of species by means of natural selec­tion, or the preservation of favoured races in the struggle for life:
London, John Murray.
Dimroth, Erich, 1975, Paleo-environment of iron-rich sedimentaryrocks: Geologisches Rundschau 64, p. 751-767.
Dimroth, Erich, and Kimberley, M. M., 1976, Precambrian at­mospheric oxygen: Evidence in the sedimentary distributions ofcarbon, sulfur, uranium and iron: Canadian Journal of Earth
Sciences, v. 13, p. 1161-1185.
Garrels, R. M., Perry, E. A., Jr., and Mackenzie, F. T., 1973, Genesisof Precambrian iron-formations and the development of at­mospheric oxygen: Economic Geology, v. 68, p. 1173-1179.
Garrison, W. M., Morrison, D. C., Hamilton, J. G., Benson, A. A., andCalvin, M., 1951, Reduction of carbon dioxide in aqueous solutions
by ionizing radiation: Science, v. 114, p. 416-418.
Goldich, S. S., 1973, Ages of Precambrian banded iron-formations:
Economic Geology, v. 68, p. 1126-1134.
Haystead, A., Robinson, R., and Stewart, W. D. P., 1970, Nitrogenase
activity in extracts of heterocystous and nonheterocystous bluegreen algae: Archiv fur Mikrobiologie, v. 74, p. 235-243.
Holland, H. D., 1973, The oceans: A possible source of iron in ironformations: Economic Geology, v. 68, p. 1169-1172.
Hubbard, J. S., Hardy, J. P., Voecks, G. E., and Golub, E. E., 1973,
Photocatalytic synthesis of organic compounds from CO andwater: Involvement of surfaces in the formation and stabilizationof products: Journal of Molecular Evolution, v. 2, p. 149-166.
Hubbard, J. S., Voecks, G. E., Hobby, G. L., Ferris, J. P., Williams, E.
A., and Nicodem, D. W., 1975, Ultraviolet-gas phase and-photocatalytic synthesis from CO and NH3 : Journal of Molecular
Evolution, v. 5, p. 223-241.
Miller, S. L., 1953, Production of amino acids under possible primitive
Earth conditions: Science, v. 117, p. 528-530.
Nagy, L. A., 1978, New filamentous and cystous microfossils, 2,300
m.y. old, from the Transvaal sequence: Journal of Palentology, v.
52, p. 141-154.
Oren, A., Padan, E., and Avron, M., 1977, Quantum yields for ox­
ygenic and anoxygenic photosynthesis in the cyanobacterium
Oscillatoria limnetica: National Academy of Science Proceedings,
v. 74, p. 2152-2156.
Schopf, J. W., 1974, Paleobiology of the Precambrian: The age of bluegreen algae, in Dobzhansky, T., and others, eds., Evolutionary
Biology, v. 7: New York, Plenum, p. 1-43.
———1975, Precambrian paleobiology; problems and perspectives:
Annual Review of Earth and Planetary Science, v.3, p. 213-249.
Schopf, J. W., and Oehler, D. Z., 1976, How old are the eukaryotes?:
Science, v. 193, p. 47-49.
Schwartz, R. M., and Dayhoff, M. 0., 1978, Origins of prokaryotes,
eukaryotes, mitochondria, and chloroplasts: Science, v. 199, p.
395-403.
Stewart, W. D. P., and Lex, M., 1970, Nitrogenase activity in the bluegreen alga Plectonema boryanum strain 594: Archiv fur
Mikrobiologie, v. 73, p. 250-260.
Stewart, W. D. P., and Pearson, H. W., 1970, Effects of aerobic andBIO

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

anaerobic conditions of growth and metabolism of blue-green
algae: Royal Society (London) Proceedings B175, p. 293-311.
Tyler, S. A., and Barghoorn, E. S., 1954, Occurrence of structurally
preserved plants in pre-Cambrian rocks of the Canadian Shield:
Science, v. 119, p. 606-608.

DISCUSSION

Irving Breger, U.S.G.S., Reston, Va.: If you are assum­
ing that photosynthetic organisms existed before 1.4
billion years ago, what protective mechanism do you
assume against the lethal ultraviolet radiation?
Schopf: In the first place, there are very good data in­dicating the presence of planktonic organisms
earlier than 1.4 billion years ago. There are gooddata for planktonic organisms back to about 2.2 or
2.3 billion. These organisms are flushed up into the
top of the water column; if there were a highultraviolet flux they would get "fried." They
presumably weren't getting "fried," and one answer
is that like modern organisms, these organisms
must have had an effective DNA repair mechanisms
for UV damage. Secondly, as I think we will hearlater, there is evidence for some free oxygen as ear­ly as 3.76 billion years ago, that is the presence ofoxidized iron minerals in the Isua (Greenland) ironformation. Geologists maintain the iron minerals areprimary sedimentary syngenetic. If that is the case,there was oxygen, and it is my hunch that there wasalways some small amount of oxygen in the Earth's
atmosphere as a product of photodissociation ofwater vapor. This being the case, it seems to me pro­bable that there was always some degree of an ozone
shield. One other thing I might mention is a paper by
Preston Cloud and Gerry Licari, dedicated to John
Gruner, in the American Journal of Science about
three years ago, on filamentous microorganisms
presumed to be blue-green algae in the Pokegama
Quartzite, in which Cloud and Licari show that theorganisms occur in re-entrants in an erosion sur­face. These organisms are sitting there on an ero­sion surface not underneath the water, only a coupleof centimeters down. The Pokegama underlies the
Biwabik and the Gunflint and is probably around 2.2
billion years old. So again, it seems to me probablethat UV flux was not a major problem at that point
in Earth's history.
Hallbauer: You have talked about only very simple
microorganisms so far, but you didn't mention the
carbonaceous material in the Witwatersrand, where
there are branched filaments and septate filaments
and forms which could be called megascopic fossils.
And I might mention here that E. S. Barghoorn ofHarvard recognized these forms as megascopic
fossils. Where could they fit in? Apparently there

are different mechanisms in the evolution of life
which should fit the model.
Schopf: Unlike Professor Barghoorn, I have not seen thematerial studied by Dr. Hallbauer—my comments
can be based only on what I have read of Dr.
Hallbauer's work and on the photographs andphotomicrographs he has published. As you know,
he has interpreted these structures to be fossil
lichens, and as such, he has interpreted them to con­tain a fungal, and thus eukaryotic, component. If his
interpretation is correct, it would then follow that
there must have been appreciable quantities of freeoxygen available during the time of deposition of the
Witwatersrand, since it would be difficult to envi­
sion fungi either originating or surviving underessentially anoxic conditions, and this would set thetransition to an oxygenic environment appreciably
earlier than the 2,000 m.y. ago that I have suggested
as being most plausible. However, thephotomicrographs published by Dr. Hallbauer leadme to suspect that the structures he has discovered
have probably been misinterpreted—specifically,
they have a branch pattern that appears to me to bequite unlike that known to occur in any modern
fiingi, and, in fact, in any modern thallophytes ofwhich I am aware. I thus suspect that these struc­tures are most likely to be nonbiologic and, thus,that they have no obvious bearing on the time oforigin of oxygenic conditions. I hate to disagree, but
it seems to me that it just doesn't fit in witheverything else that is known in the Pre-Cambrian.
On the other hand, that doesn't mean that it iswrong; it just means that it doesn't fit in and maybe
you're looking at a different facies. If what you aresaying is correct, that these are lichens, 2.5 billion
years in age, and if as lichens they have a fungal
component, and if those fungi had the same
metabolic features as modern ones, then there musthave been appreciable quantities of atmospheric ox­ygen, above IVa percent of present atmosphericlevels. This seems to me not to fit in with a lot ofother data, but on the other hand, if they are fungi,
it's of very considerable interest obviously.
Kalliokoski, Michigan Technical University: There are aseries of things that are being called fossils—these
are little clusters of pyrite grains. They are very
common in the recent record, and they can bechased back into the Precambrian. The question is,
do you consider these to be fossils?
Schopf: If I may, without meaning to be facetious, yes
and no. Sedimentary pyrite in general is evidence of
biologic activity in the sense that the hydrogensulfide produced by sulfate-reducing bacteria can
combine with iron and you can get pyrite

Bll
Mass balance calculations seem to be within anorder of magnitude and seem to make good sense. Itturns out also that if you look at those numbers, andI've just redone that whole business again, the newdata give you a better hunch as to what the true
numbers might be. These data also allow you to have
half of the organic matter sequestered in the Earth's
crust from other than photosynthetic processes. I
think what the data say is that there is a big slop in
the data. If you want to argue that it is all fromgreen-plant photosynthesis, you can argue that way.
On the other hand, if you want to point out that
there is a big slop in the data, that they are not allthat reliable, you're allowed at least up to half theorganic matter in the Earth's crust from other thanphotosynthesis. And if there was a large amount of
abiotic synthesis going on or there was a fixation byorganisms that were photosynthetic but non-oxygen
producing, the mass balance equations allow you to
do that. They certainly do not rule it out, and it is as
logically consistent with the data as the otherhypothesis that says it all comes from oxygenproducing photosynthesis.
James Trow, Michigan State University: Two shortquestions. First, do you know the age when a lot ofthese carbon beds show up, and how far down in thestratigraphic section do those carbon-bearing beds
occur? Second, do you have evidence of any native
sulfur beds in these rocks?
Schopf: With regard to the first question, there certainly
are graphitic schists that go way back into the Archean, the oldest ones of which I am aware, highly
carbonaceous material, come from the Swaziland se­quence. I don't know of comparably highly car­bonaceous material from the Isua iron-formation
region, although there is graphite and there isorganic matter in those rocks which are reliably
dated at older than 3.76 b.y. With regard to sulfatesand sulfur deposits, I do not know the distribution of
those in the Precambrian.

THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE

precipitated. On the other hand, Bob Sweeney has
shown one can get similar structures without the in­tervention of biologic activity. It is well establishedthat pyrite can infill cells, and not only of
microscopic algae but of forams and a variety ofother organisms; even the epidermal cells of leaves
are sometimes infilled by pyrite. Here, apparently,
the organic matter is used by bacteria for local pro­duction of H2S, the hydrogen giving the reducing
power which reduces sulfate in a marine environ­ment and thus pyrite is precipitated within cells. In
that case they are fossils, but I would be very hesi­tant to say that all framboidal pyrite could be pro­
perly interpreted as Precambrian fossils or fossils
generally.
Skinner: I don't think this is going to be answered byanybody else, Bill, so I hope you'll have a shot at it. If
you do in fact have large amounts of abiologic car­bon back in the early Precambrian, should you notsee some progressive change in the carbon isotopic
fractionation in this material?
Schopf: That's a good question to ask Manfred
Schidlowski.
Schidlowski: Actually we had a look at the carbon
isotope composition of sedimentary carbonates fromthe very start. Now we must say from almost thevery start because we did not include the Greenlandmaterial. The delta C13 values of the carbonates
were practically constant, always near zero per mil.
Schopf: I would say one other thing. Since Bill Ruby's
paper in the early fifties, people have been doingmass balances on carbon and oxygen in the geologic
record and using the relation between them as in­dicating that it is a plausible and consistent model toargue that all of the organic carbon that is se­questered in the Earth's crust could come fromphotosynthesis and therefore that all of the oxygenin the Earth's crust and in the Earth's atmosphereand oceans could also come from photosynthesis.

Uraninite Oxidation and thePrecambrian Atmosphere
By D. E. GRANDSTAFF
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-C

CONTENTS

Abstract _____________________.
Introduction______________________________
Experimental data ________________________
Factors in the survival of Precambrian uraninite
Composition ____________________________
Size, shape, and rounding _________________
Precambrian organisms and organic matter ___Atmospheric carbon dioxide pressure________
Temperature ___________________________

Cl
1
1
2
3
4
5
6
6

Factors in the survival of Precambrian uraninite - Continued
Weathering and denudation ________________.
Transport _______________________——__
Deposition and burial ________________——__
Numerical calculations ___________—__———___
Implications for the Precambrian atmosphere ______________
Acknowledgments ____________________—___
References cited _________—__________________
Discussion ___________——___________________

C6
7
9
9
11
12
12
14

ILLUSTRATIONSFIGURES 1-6. Graphs showing:
1. Schematic representation of the relative rate of dissolution of uraninite during the sedimentary cycle ___—__
2. Residence time of particles in a soil profile, assuming a steady-state soil profile___——_—————————————
3. Concentration of dissolved oxygen in water and alluvial sediments of Bugaboo Creek, British Columbia, Canada ——4. Maximum oxygen pressures allowing survival of uraninite as a detrital mineral, assuming significant dissolution
occurs only during transport and burial ______________________—————————_———————
5. Maximum oxygen pressures allowing the survival of uraninite as a detrital mineral during the sedimentary cycle__
6. Models for the evolution of oxygen in the Earth's atmosphere through geologic time _________________

C2
7
9
11
11
12

TABLE
Page

TABLE 1. Chemical compositions and NOC (nonuranium cation) values of some uraninite from the Witwatersrand and Elliot Lake.

C3inGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESURANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE
By D. E. GRANDSTAFF1

ABSTRACT

Although uraninite is thermodynamically unstable at oxygenpressures greater than approximately 10'21 atmospheres, calculations
based on the dissolution kinetics suggest that uraninite from the Witwatersrand Sequence may have survived as a detrital mineral at ox­ygen pressures as high as 10~2 to 10~6 of the present atmospheric level(PAL). Therefore, the mid-Precambrian atmosphere may have contain­ed small amounts of free oxygen, produced by aerobic photosynthesis
and photodissocation of water vapor.

INTRODUCTION

Detrital uraninite is extremely rare in modern
sediments because it is rapidly destroyed by oxidation. A
search of black sands for uraninite during World War II
failed to reveal any uraninite in recent placers. In con­trast, several Precambrian sequences contain uraninitethat many geologists believe is detrital (Davidson, 1965).
Of these, the Witwatersrand Triad of South Africa andthe Elliot Lake Group-Huronian Supergroup of Canada
are the best known and documented occurrences.
Although C. F. Davidson (1960, 1965) and others havesupported a hydrothermal origin for the Witwatersrandand Elliot Lake ores, a detrital-placer origin now seems
well supported by the available evidence.
The occurrence of detrital uraninite has been cited as
evidence that oxygen probably was much less abundantin or absent from the Precambrian atmosphere (Ramdohr, 1958). Holland (1962) calculated that uraninite
would be thermodynamically stable only at oxygenpressures less than 10~21 atmosphere. However,photosynthesis by blue-green algae and the photodissociation of water vapor may have contributed
significant amounts of oxygen to the Precambrian at­mosphere (Berkner and Marshall, 1965; Brinkmann,1969). Although oxygen may have been rapidly consum­
ed by oxidation of reduced volcanic gases and iron and
1 Department of Geology, Temple University, Philadelphia, PA 19122

other reduced species in sediments, it seems likely thatsome free oxygen was present in the mid-Precambrianatmosphere.
It is well known that minerals may exist for appre­
ciable periods out of equilibrium with their environ­ments. Magnetite, for example, should be oxidized tohematite or some other ferric compound under Earthsurface conditions. Magnetite survives as a common
detrital mineral because its oxidation rate is quite slow
under oxidizing surface conditions. Although the oxida­
tion of uraninite is much faster, modern occurrences of
detrital uraninite have been reported at Nation River,
British Columbia (Steacy, 1953), Cornwall, England(Davidson and Cosgrove, 1955), Hogatza placer, Alaska(Staatz, this volume, chap. Y) and the Indus River,
Pakistan (Zeschke, 1959, 1961). These reports indicatethat some disequilibrium may prevail. Estimates of theamount of disequilibrium have not been made, however,
because of lack of knowledge of the kinetics and rate ofuraninite dissolution and the rate of geological pro­
cesses (Rutten, 1971).
The purpose of this paper is to use experimental dataon the kinetics of uraninite dissolution in order to makea more quantitative estimate of the upper limits of ox­ygen in the Precambrian atmosphere that would allow
the survival of uraninite as a detrital mineral during
periods of weathering, erosion, transportation, deposi­tion, and burial and the formation of the uraniferous
conglomerate of the Witwatersrand, Elliot Lake andother similar deposits.
EXPERIMENTAL DATA

The oxidation of uraninite (represented by U02) inwater containing carbonate species at intermediate pHprobably takes place according to the ideal formula
U02 + V202 + 2HCO-3 + H20 = U02(C03)2(H20)2-2 (1)Cl

C2GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Experimental data and discussion of the kinetics ofuraninite dissolution have been reported in detailelsewhere (Grandstaff, 1973), and only a brief summaryis given here. The dissolution of uraninite is represented
by the following kinetic equation

Transport -*4* Burial

Erosion<DC

R== 1020 2S (SS) (#F)-'(

NOC)

(2)

exp (-7045/r)(day-')in which^ = rate of the dissolution reaction, the diminution of
uraninite as a function of time ( - d(uran)/cft)&S= geometric specific surface area (cm2g"1)
M)C=nonuranium cation mole fraction
D.0. = dissolved oxygen concentration (ppm)£C02 = total dissolved carbonate concentration (Molar)
EC02 = [H2C03] + [HCCV1] + [C03-2]
T= absolute temperature (°K)
RF= organic retardation factor. Retardation factorsof 5 and 35 have been found in a New Jersey
river water and in coastal sea water.

FACTORS IN THE SURVIVAL OF PRECAMBRIANURANINITE

It can be seen from the experimental data that the sur­vival of uraninite as a detrital mineral is influenced byenvironmental and physiographic factors, such as pH,
temperature, presence of organic species, rates andtime required for erosion, transport, deposition, andburial; mineralogical factors, such as chemical composi­tion and grain surface area; and atmospheric factors,
such as oxygen and carbon dioxide partial pressures.
A schematic representation of the rate of dissolutionof uraninite during the sedimentary cycle is shown infigure 1. Uraninite grains deep in the rock probably are
not subject to significant attack by oxidation. Oxidationof organic matter, sulfides, and iron-bearing silicates, aswell as of uraninite, depletes the ground water of ox­
ygen. However, as erosion proceeds, the uraninite isbrought closer to the surface, and the content of oxygenin the water and the rate of dissolution increase. At thesurface the rate of dissolution is defined by atmosphericand environmental conditions. Dissolution of uraninite
is quite rapid during periods of residence in acidic soil
profiles. During transportation the rate of dissolution issomewhat slower due to the higher alkalinity and higher
pH generally encountered in alluvial sediments andriver waters. Finally, after deposition, the rate ofdissolution decreases as the grain is buried and the con­centration of oxygen in the ground water decreases.

O
CO
CO

Q

TIME
FIGURE 1.—Schematic representation of the relative rate of dissolutionof uraninite during the sedimentary cycle.

The experimentally derived reaction equation (2) may
be represented as
R=, NOC, RF,..., t)](3)

in which the instantaneous reaction rate, the diminutionof uraninite with time by chemical corrosion(-d(uran)/cfa) is equal to a rate function R[f\, given byequation (2), and itself a function of time.
Rearranging the equation-d(wcan)=R[f\dt(4)
and integrating
DF=\R[f\dt(5)
in which DF is the fraction of the original grain
destroyed by oxidation. To simplify calculations, and in
the absence of data on exact variations in environmental
conditions during the Precambrian sedimentary cycle,
the complex dissolution rate function shown in figure 1
was approximated by a summation. The elements of thesummation represented the rate of dissolution during
periods of weathering, transportation, and burial.
The rate equation may be written as a summation
DF='LRl!ti(6)iin which Rt is the average rate of dissolution in, and tt isthe duration of, the ith period. If the rate term is written
as an explicit function of atmospheric oxygen, for exam­
ple, the summation terms will have the form
PofR! t,i

If the oxygen concentration is constant during somegeological process, it, and other such constant terms,

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

C3

Cassiterite, monazite, leucoxene, hydrated iron oxides,
quartz, and sericite were probably present as impurities(Liebenberg, 1957; Hiemstra, 1968). Therefore, theanalytical results for Si02 , A1203, Fe203, Sn02, H20+, S,
This equation may be rearranged to evaluate for P0z, Zr02, and Ti02 reported in these analyses were excluded
for example
as present primarily in impurities. The remaining
elements were recalculated to 100 percent. Much of thelead was present as galena inclusions in the uraninite
Calculation of the maximum concentration of oxygen rather than in the uraninite lattice. However, it was in­in the Precambrian atmosphere that would have permit­ cluded in the elements retained because the lead un­
ted the survival of detrital uraninite and the deposition doubtedly had its origin in the radioactive decay ofof the uraniferous conglomerates requires that as many uranium and thorium.
of the environmental, mineralogical, and atmospheric
A first approximation of the composition of thevariables as possible should be fixed and limits placed on uraninite can be made if the lead present in the chemical
analyses is reconverted to uranium and thorium. The
the others.
analysis calculated will approximate the composition of
COMPOSITION
the uraninite at the time of crystallization if primary
The composition of the uraninite is quite an importantlead were negligible. Then, knowing or assuming an age
variable in its oxidation kinetics. Unfortunately, thefor the uraninite at the time of erosion, the amount ofcomposition and the rate of oxidation measured on thelead created by radiogenic decay during that period may
uraninite as it exists today are probably not represen­be calculated. This composition will approximate thetative of the composition and the rate of oxidation of thecomposition of the uraninite at the time of erosion anduraninite at the time of weathering, transport, anddeposition if there have been no major postdepositional
deposition, more than 2 b.y. (billion years) ago. Somechanges in the composition of the uraninite other thanestimate of the composition of the uraninite at the timethat of radioactive decay of uranium and thorium.
of sedimentation must be made. Several analyses of
The age of crystallization of uraninite from the Domi­
uraninite from the Witwatersrand and Elliot Lakenion Reef is approximately 3,100 ±100 m.y. (million
deposits are given in table 1.
years) (Nicolaysen and others, 1962). An extrusive in the
Dominion Reef has been dated at 2,800 ±60 m.y.
TABLE 1. Chemical compositions and NOC (nonuranium cation) (Niekerk and Burger, 1969). Therefore, the age of thevalues of some uraninite samples from the Witwatersrand and Elliot uraninite at the time of deposition of the Dominion Reef
Lakemust have been approximately 300 ± 120 m.y. The Wit­
watersrand System overlying the Dominion Reef
Blyvooruitzicht
Sub Nigel
Dominion Reef
Nordic
(1)
(1)
(2)
(3)
System must be somewhat younger. In the absence ofdates on the time of sedimentation of the Witwaters­
22.7
U02
17.10
42.6
46.19
U03 _
rand System, the age of this system can only be specified
U308
74.10
66.65
as between 2,800 m.y., the age of the underlying Domi­
Th02
6.1
2.70
1.63
6.52
PbO
16.70
19.44
14.85
22.6
nion
Reef System, and 2,300 m.y., the age of the overly­
REE
1.00
2.13
4.0
3.43
ing Ventersdorp System (Niekerk and Burger, 1966).
.0
Zr02 + Ti02 __
1.40
1.04
.50
Si02
2.07
1.43
.0
1.78
Assuming the uraninite from the Witwatersrand
.0
MgO
.16
.30
.11
System had an age of crystallization of 3,100 m.y., the
CaO
.81
1.54
1.30
.7
.2
.84
1.81
1.12
Fe2 03
uraniniteprobably had an age of 550 ± 250 m.y. at the.0
A12 03
1.33.99.08
time of deposition of the Witwatersrand System.
.9
S
1.26
2.50
H20* ______________
If the age of crystallization of the Elliot Lake
1.43
Sn02 __________——
uraninite
is approximately 2,500 m.y. and the age of the
Other
.62
Matinenda Formation is greater than 2,288 ±87 m.y.
Total
99.8
100.82
97.60
98.44(Roscoe, 1969), the age of the Elliot Lake uraninite at
NOC:
Minimum
.13
.21
.15
.17
the time of deposition was approximately 150 ± 150 m.y.
.17
Maximum ____
.19
.23
.24
The results of the calculations are shown in table 1.
Values of NOC for the samples lie between 0.13 andReferences: (1) Liebenberg (1957), (2) Hiemstra (1968), (3) Grandstaff (1973).
0.24. Variation of NOC in each sample is a result of the
Unfortunately, the uraninite analyses from uncertainty in the age of the uraninite at the time of ero­
Liebenberg (1957) and Hiemstra (1968) were made on sion. Because of the number of operations and assump­uraninite concentrates by wet-chemical methods. tions involved in the calculations of NOC at the time of
may be separated from the summation or integral toproducet(7)

C4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

sedimentation and the possibility that the samples fromwhich these analyses were made are heterogeneous(Grandstaff, 1975), these figures are only approximate.
The uraninite most sensitive to the Precambrian at­mosphere, that is, the fastest reacting uraninite, theuraninite having the lowest NOC content, is that from
Blyvooruitzicht. This uraninite has a probable range of
NOC from 0.13 to 0.19. An average value of 0.16 has
been used in calculations. Use of other permissible NOC
values in the range 0.13 to 0.19 would affect the dissolu­
tion rate and the oxygen pressures calculated by about afactor of three at the extremes.
SIZE, SHAPE, AND HOUNDING

The source areas of the Elliot Lake and the Witwatersrand uraninite have not been located. In theabsence of unweathered uraninite from the source areas
for comparison, it is not possible to assess directly possi­
ble mineralogical changes in grain size, shape, and com­position that might have occurred as a result of chemicalcorrosion or abrasion.
Liebenberg (1957) measured the size of uraninite
grains from the Witwatersrand in polished sections. He
found a very small range of size and determined that the
average diameter of the uraninite grains in the Wit­
watersrand Sequence is about 75 /* and that in the Domi­nion Reef and Ventersdorp Contact Reef it is about 100/*. Roscoe (1969) found that uraninite grains in Elliot
Lake are about 100 /* in diameter. Since themeasurements were made on polished sections, the ac­tual dimensions of the uraninite grains must be
somewhat larger. Although uraninite grains frompegmatites may be quite large, the small size variationsof uraninite grains in the Precambrian deposits and,particularly, the absence of any larger grains in the Wit­
watersrand and Elliot Lake deposits suggest that theuraninite grains were never much larger than they are
now. Furthermore, some of the grains are euhedral or
subhedral; they retain what appear to be original crystal
faces. Since physical attrition and chemical corrosion
would tend to destroy the planar crystal faces, theseeuhedral or subhedral crystals must be close to their
original size. This suggests that only relatively small
variations in the mineralogical parameters occurred.
Therefore, as a first approximation, these factors are
assumed constant and are evaluated by examination ofexisting uraninite samples from the Witwatersrand andElliot Lake.
Seventy-five micometers have been chosen as a lowaverage particle dimension. The exact specific surface
area cannot be calculated since this requires a precise
knowledge of the particle shape of each grain. The par­ticles as seen in polished section are intermediate inshape between circles and squares, implying that theparticle shape is between spheres and cubes, and a
specific surface area of 90 cm2g"1 has been estimated on
this basis.
An additional constraint is imposed by the degree ofrounding and the survival of the uraninite grains. The
degree of rounding of the grains varies markedly, even
within single samples. Some grains are angular or
subangular. The shape of these grains appears to haveundergone very little modification by mechanical abra­sion or chemical corrosion. Other grains are subrounded
to rounded - often referred to as "muffin shaped"(Schidlowski, 1966)-and their shape appears to haveundergone extensive modification.
The literature on interpretation of the rounding of theuraninite is quite extensive (Hiemstra, 1968).
Liebenberg (1957) believed that the grains had a waterworn appearance. Hiemstra (1968) found that uraninitefrom the Dominion Reef System had a mean roundness
value in accordance with those of monazite and zircon
grains in the same samples. However, Koen (1961) andDavidson (1958) believed that the uraninite had too higha degree of rounding to be consistent with a granitic or
pegmatitic origin.
The descriptions of Hiemstra (1968) and Liebenberg(1957) suggest that angular and subangular grains are
more common in the Dominion Reef than in the main
Witwatersrand System. Hiemstra (personal commun.,1973) believed that uraninite from the Dominion Reef
System had a higher degree of angularity than uraninitefrom the Witwatersrand or Ventersdorp Systems.
This apparent variation in rounding may be due tophysical characteristics of the grains, normal distribu­
tion of mechanical processes, reworking, and variationof the period of exposure of the uraninite to oxidizing
conditions. It seems possible, however, that some roun­ding is influenced by compositional variations.
Rounding of grains during weathering and erosion
may occur by two processes, (1) mechanical abrasion and(2) chemical corrosion and dissolution. Both processes
will be active, but one process may dominate. If theamount of material removed by dissolution is much lessthan that removed by mechanical abrasion, the shape ofthe grain will be consistent with mechanical abrasion. If,
however, the amount of material removed by dissolution
is of the same order as or greater than that removed bymechanical abrasion, the grain will be more highly
rounded than is indicated by a purely mechanical model.
At the time of erosion the uraninite from the different
ore reefs probably had quite different chemical composi­tions. This probably would have affected the rate ofchemical corrosion. For example, if extreme values of
NOC (table 1) are used, the average Blyvooruitzicht
uraninite may have dissolved about fifteen times faster

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

than the average Dominion Reef uraninite. If the two
uraninite samples were exposed to oxidizing conditions
for approximately the same length of time, about fifteentimes more material would be dissolved from the
Blyvooruitzicht uraninite. If oxygen pressures were fair­ly high and the absolute velocity of the dissolution reac­tion were fast, this could have resulted in appreciablechemical corrosion and rounding of the Blyvooruitzicht
uraninite. The Dominion Reef uraninite, however, hav­
ing a slower reaction rate resulting in less chemical cor­rosion would have higher angularity.
Therefore, highly resistant grains, having high NOC
content, may exhibit degrees of attrition and rounding
consistent with purely mechanical processes. This ap­
pears to be the case in the Dominion Reef and perhaps
the Elliot Lake uraninite grains. Less refractory grains
or grains exposed to oxygenated environments for long
periods of time would exhibit roundness values greaterthan those expected from purely mechanical attrition.
This does not necessarily indicate that they were
deposited colloidally, but that they have undergone ahigh degree of chemical corrosion in addition to physicalattrition.
To estimate the composition of the Precambrian at­mosphere, some estimate of the degree of chemicalrounding should be made. Unfortunately, accurate
estimates of the amount of material removed from theuraninite grains by chemical corrosion cannot be madebecause the initial size and shape of the grains and the
proportion of physical attrition to chemical corrosion are
not known. If, however, the particles are near their
original dimensions, an estimate of the maximum
degree of chemical corrosion may be made. The small
grain-size variation and the presence of euhedral
uraninite grains suggest that, in general, the grains
were not reduced appreciably in size by corrosion or at­trition. If a sphere is created from an originally cubic
particle with the same dimension or diameter, nearly 50
percent of the original particle must be removed. Thisfactor of 50-percent destruction of the original grain bychemical corrosion has been used in calculations. This
amount of corrosion is probably a maximum, since few
of the uraninite grains are spherical and since both
physical and chemical attrition are involved.
PRECAMBRIAN ORGANISMS AND ORGANIC MATTER

Procaryotic bacteria and blue-green algae were prob­
ably the only organisms present during the early andmiddle Precambrian (see, however, Hallbauer, thisvolume, chap. M). These organisms were undoubtedly

C5

present in the oceans, and the remains of some of them
may be preserved in marine and deltaic sediments of the
Witwatersrand and Elliot Lake. It is not known,
however, if the organisms were present on land.
Berkner and Marshall (1965) suggested that at low at­mospheric oxygen partial pressures the flux of solar
ultraviolet radiation may have been sufficiently intense
to kill any organisms exposed on land. However, the"lethal level" of ultraviolet radiation proposed by
Berkner and Marshall may be too low (Godward, 1962).
If Precambrian organisms had higher tolerance to ultra­
violet radiation than has been proposed, or if oxygenpressures were higher than proposed, it is possible thatmicroorganisms such as algae or bacteria did form aground cover during the Precambrian. Even if ultra­
violet radiation was at lethal levels, some organisms
may have lived in niches in rock or deep in soil zones(Fischer, 1965).
The presence of organic matter in water decreases therate of uraninite dissolution (Grandstaff, 1973). The
organic matter may bond to the surface of the uraninite,
blocking oxidation sites. Chave (1965), Chave andSchamlz (1966), and Suess (1970) found that polarorganic matter formed surface complexes on calcium
carbonate grains and that these complexes decreased
rates of nucleation, growth, and dissolution. Suess(1970) found that the surfaces on calcite and dolomite
grains became saturated with absorbed organic mattereven at very low concentrations of dissolved organic
matter. Suess (1970) estimated that ocean water con­tains approximately 1,000 times as much organic matteras necessary to saturate the calcite surfaces. The
distribution of organic matter between the surface ofuraninite and water is not known. It seems likely,
however, that Precambrian river and ocean water con­tained dissolved organic matter and that some organic
matter was absorbed on uraninite surfaces. This would
result in a decrease or retardation of the dissolution
rate.
The type and concentration of organic matter in waterprobably influence the degree of retardation of the
dissolution rate (Grandstaff, 1973), but the type and con­centration of organic matter in Precambrian river andocean water and the extent of possible retardation are
unknown. It has been found (Grandstaff, 1973) that
dissolution rates were decreased by a factor of 35 times
in New Jersey coastal sea water, while the rate in waterof the Molluca River in New Jersey was decreased byabout 5 times. In the absence of knowledge of Precam­
brian retardation factors, the retardation factors of 5
and 35 times found in modern rivers and the ocean havebeen used as a first approximation.

C6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESATMOSPHERIC CARBON DIOXIDE PRESSURE

It does not appear possible to specify the pressure ofcarbon dioxide in the Precambrian atmosphere with any
certainty. The carbon dioxide pressure in the atmosphere appears to be controlled by equilibrium be­
tween magnesium carbonate and magnesium silicate
systems, by rates of input of C02 into the atmosphere byvolcanic exhalation and biological respiration, and by
removal from the atmosphere by photosynthesis,
weathering, and precipitation (Holland, 1968).
Calculations suggest that the Precambrian carbon
dioxide pressure may have been as much as 10 to 100
times PAL (present atmospheric level) (Holland, 1968),
but Cawley, Burruss, and Holland (1969) have suggested
that the carbon dioxide pressure in the pre-Silurian at­mosphere may have been as low as 5 PAL. Therefore,
limits of 5 and 100 PAL have been used in the present
calculations.
TEMPERATURE

Temperature has been regarded as a factor in the sur­vival of detrital uraninite. Since reactions are generally
slower at lower temperatures, glacial or periglacial con­ditions have been cited as environments in which thesurvival of uraninite would be more probable. However,decrease in the temperature from 25°C to 0°C only
decreases the reaction rate by a factor of three.
Although this aids the survival of uraninite, it is prob­
ably not the major factor, since much greater reductionof the oxidation rate is probably required to permit thesurvival of uraninite to the extent observed in Precam­
brian sediments.
However, weathering in glacial or cold environments
is almost entirely mechanical, resulting in minimal
chemical attack on mineral grains. The liberation of
uraninite from the parent rock by glaciers is probably
responsible for the survival of the present Alaskan,
Canadian, and Pakistani occurrences of detritaluraninite.
Tilloids have been described in association with the
Witwatersrand (Wiebols, 1955) and Elliot Lake deposits(Casshyup, 1969). The relative chemical andmineralogical maturity of the Witwatersrand and Elliot
Lake rocks suggests, however, that most of thesediments were probably not of glacial origin.
Therefore, glacial action seems unable to account for thesurvival of the Precambrian uraninite.
If, however, a glacial origin for part of the Witwaters­
rand and Elliot Lake Sequences is accepted, then atleast some of the radioactive quartz-pebble conglom­
erate may have been deposited in a moderate or cold
climate, under periglacial or interglacial conditions. A

temperature of 15°C has been used in the calculations.
Variation in the chosen temperature by 10° in either
direction would produce a change in the dissolution rateby less than a factor of two.
WEATHERING AND DENUDATION

Actively eroded uplands, even in the tropics, yield
fresh mineral grains of relatively unstable silicates andoxides mixed with products of chemical weathering (Van
Houten, 1972). Such grains apparently are derived fromscouring of bedrock by rivers, and the grains are me­
chanically released from the rock without undergoingsignificant chemical weathering.
Brock and Pretorius (1964) suggested that the sourceareas surrounding the Witwatersrand were undergoing
uplift along basin margin faults. Frarey and Roscoe(1970) and Pienaar (1963) have suggested that thedeposition of the conglomerates containing theuraninite in Elliot Lake occurred after uplift of thesource area. Under such conditions rivers in the sourcearea would be rejuvenated, moving old, mature detritus
from the source area and downcutting into fresh rock.
Under these conditions uraninite could be scoured frombedrock by rivers without significant chemical attack.
Glaciers may also deliver virtually unweathered grains
to rivers, where they may be mixed with chemicallyweathered minerals. Although glacial derivation of theuraninite in Witwatersrand and Elliot Lake remains
possible, it appears unlikely.
If uraninite was not scoured from bedrock by rivers or
glaciers it must have been derived by way of soil pro­files.
Estimates of denudation rates range from 0.5
cm/1,000 years near Hudson Bay (Holmes, 1965) to as
much as 10 m/1,000 years in tectonically active areas(Gilluly, 1949). Ahnert (1970) found that the rate ofdenudation in large midlatitude basins is related to themean elevation, and he calculated rates of summit
denudation as high as 60 cm/1,000 years in the Alps.
Rates of denudation as high as 100 cm/1,000 years havebeen calculated for the Himalayas (Leopold and others,1964).
Soil profiles and leached zones vary widely in depth.
Under tropical climates in tectonically inactive areas
leached zones may reach 50 or 100 m in depth (Leopold
and others, 1964). On slopes and in cold climates the soil
zone may be only a thin film.
Figure 2 indicates probable residence times of grains
in a soil profile under various conditions of denudation
rate and soil-profile depth, assuming a steady-state soil
profile. In mountainous areas after tectonic rejuvena­tion, rates of denudation would have been fairly high,URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

several tens of centimeters per thousand years. On
mountainous slopes and in the absence of vascularplants to hold soil formed, soil and leached zones could
have been fairly thin. Under these conditions, thediagram indicates that particles would probably residein soil zones for periods on the order of 500 to 10,000
years.
TRANSPORTThe source areas of the uraninite in Elliot Lake andthe Witwatersrand have never been discovered.
Although extensive nonmarine deposits may occasional­
ly be found hundreds of kilometers from their sourceareas (Stanley and Wayne, 1972), if a molasse model isused (Reinike, 1930), the source area of the sediments
was probably within 100 km. Conglomerates in the Alps,
Siwaliks, and the Andes are probably deposited within
100 km of their source areas. Brock and Pretorius (1964)
have suggested that the source areas for the Witwaters­
rand lie immediately to the north, presently buried
under the Bushveld, to the west or northwest of
Klerksdorp, and to the south of the basin buried under
the Karroo.

DC
LLJ

LLJ

o

C7

Sediment grains in rivers may be carried at various
velocities, depending on the grain size, the velocity of
the flow, the bed configuration, and other factors(Leopold and others, 1964). Uraninite grains of the Wit­
watersrand and Elliot Lake have diameters on the orderof 0.1 mm. These grains are hydraulically equivalent toquartz grains of 0.3 to 0.4 mm diameter (Mclntyre,1959). Therefore, they probably formed part of the
bedload.
During transport down rivers, sediment grains may
undergo deposition and re-entrainment by the river
many times. In entrenched rivers, transport of grains
may be fairly continuous, but in rivers having broad
flood plains, a sediment grain may be abandoned inalluvium for considerable periods of time by shifts of theriver channel. This increases the residence time of theparticle in the river system.
For periods of transport, assuming steady state, anapproximate velocity of bedload transport may be
calculated by the equation

V= dM/dt
P WD(9)in which V is the velocity of transport, dM/dt is thedischarge of bedload sediments, p is the density of thesediments, approximately 2.6 g cm~3 (Judson and Ritter,1964), W is the width of the channel or portion of thechannel, and D is the depth of sediment beingtransported.
It appears quite difficult to estimate accurately thedepth of sediment being transported by the river. In
some cases this depth may be estimated by using theheight of sand waves or sand bars developed by grain
movement.

\\ <£>.

- 2

t
LLJ
Q

O
N

(D 0
O

\
1

0

-1

LOG DENUDATION RATE, IN CENTIMETERS
PER 103 YEARSFIGURE 2.—Residence time of particles in a soil profile, assuming a
steady-state soil profile. The range of most geological conditions isshown by the shaded area; residence times are given by diagonallines.

A sand wave (also called a sand dune or bar) is a ridgethat is formed on the bed of a river by movement of bed
material and that is usually approximately normal to thedirection of flow (Lane, 1947). Movement of the bedload
takes place when grains are removed from the dune inregions of high flow and redeposited in regions of lower
flow. The size of the dunes depends on the flow velocity
and the sediment size (Morisawa, 1968). The full scale ofvariation in size within the range of conditions in rivers
is not known. Dunes approximately 2 m high and 100 m
wide have been found in the Columbia River (Jordan,
1962), and dunes as much as 10 m high and 150 m wide,in the Mississippi River (Carey and Keller, 1957). If thesand waves are approximated as triangular solids, theequivalent depth of sediment moved in the sand waves is
one half of the wave height. Therefore, the depth of sedi­ment moved by the Mississippi River in sand waves is as
much as 5 m.

C8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESThe suspended-sediment discharge of the Mississippi
River is approximately 3 x 1014 g yr1 (Judson and Ritter,1964). Fisk (1944) estimated that the bedload of theMississippi River is approximately 7-10 percent of thesuspended load. Therefore, the bedload discharge isabout 3xl013 g yr1 . If the channel width of theMississippi River is approximately 1 km (Carey and
Keller, 1957) and the amount of sediment being moved isequivalent to 5 m depth, assuming steady state, the rateof transport calculated from equation (9) is approximate­ly 2.3 km yr1 .
Assuming that the bedload is equal to 10 percent of
the suspended load, data for the Columbia River (Judson
and Ritter, 1964; Jordan, 1962) suggest that sediment
bedload material is transported at a rate of approx­
imately 1 km yr'1.
These figures are only approximate since probably not
all of the width of the channel is active in transport ofmaterial at any one time and the depth of sedimenttransported may also be variable.
By examination of the sediment type in Elliot Lakeand the Witwatersrand it may be possible to estimate
the rate of bedload transport in these Precambrianrivers.
The diameter of pebbles in the central Witwatersrand
generally ranges from 25 to 75 mm and may be as greatas 150 mm (Pretorius, 1964). The size of pebbles in the
Elliot Lake ranges from 25 to 100 mm (Pienaar, 1963).
This indicates that, at least during floods, the rivers
were competent to move cobbles of this size. The abilityof a river to carry material of this size indicates that theriver had a flow velocity of approximtely 1-3 m s"1(Sundborg, 1956). From this flow rate and the empirical
relationship derived by Colby (1961), such a river would
have a bedload discharge of 0.3 mm sand-sized particles
on the order of 2 to 10 x 1010 g m'1 of channel width per
year. If a depth of 5 m of sediment in transport is assum­
ed, a transport rate of 1.5-7.5 km yr1 may be calculated
from equation (9). This calculation should only be taken
as indicating an order of magnitude, particularly since
the calculation of the flow rate is influenced by flood
conditions and the depth of sediment transported isassumed. However, the calculation indicates rates oftransport of bedload material similar to those of present
rivers.
These calculations indicate that during periods when
the bedload is continuously entrained it may betransported at rates on the order of 1 km yr1 . Under
these conditions, residence time of the grains in river
systems 100 km in length would have been on the orderof 100 years.
In alluvial rivers, however, with broad flood plains,sediment grains may be abandoned in alluvium for con­siderable periods of time by shifts in the river channel.
This increases the residence time of the particle in theriver system. In order to estimate the average transport
rate of particles in such cases, the entire width anddepth of sediments in the alluvium must be considered.
The Mississippi River is used as an example. The
alluvial valley of the Mississippi River is approximately
1,000 km long. The valley varies in width from a max­
imum of approximately 200 km at Helena, Ark., to aminimum of 40 km at Natchez, Miss. The depth of
sediments in the valley is approximately 30-40 m(Thornbury, 1966). Using an average value of 120 km
width, an average sediment depth of 35 m, a transport
rate of approximately 30 m yr1 may be calculated fromequation (9). Thus, the residence time of sediments in
the Mississippi River valley is approximately 35,000
years.
Data from Everitt (1968) indicate a transport rate of
approximately 16 m yr'1 for sediments in the valley of
the Little Missouri. The transport rate and residence
time of particles will undoubtedly vary from one river
system to another and within individual river systems.
It is not possible to make any calculations of the mean
residence time of uraninite grains in the Precambrianrivers of the Witwatersrand and Elliot Lake since vir­tually no trace of the rivers remains. If the results for
the Mississippi River or the Little Missouri River are
taken as order of magnitude estimates, residence timesof tens of thousands of years are suggested.
However, the degree of oxidation and dissolution may
not be proportional to the total time spent in the
alluvium. From figure 3 it can be seen that the porewater in sediments somewhat removed from Bugaboo
Creek was highly undersaturated with respect to at­mospheric oxygen. The degree of undersaturation, if
any occurs, is dependent on the relationship of thesediments to the river and to the water table, thepermeability, the content of organic matter, and themineralogy. In impermeable sediment containing oxidizable organic matter or minerals, the pore water may
become highly undersaturated with oxygen with respect
to the atmosphere. If the oxygen pressure in the
Precambrian atmosphere and waters was less than atpresent, completely anoxic conditions might have been
more easily achieved at that time. In this case, the rate
and the amount of oxidation may be negligible.
However, if the grain is deposited above the water table,
or in a zone of fluctuating water table, or in a relatively
permeable sediment, the degree of undersaturation may
be minor. In the first case, the period the particle is ex­
posed to oxidation may be approximately equal to thetime during which the particle is actually in sediments
being moved by the river. In the second case, the parti­
cle is exposed to oxidation for the entire period it is in

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

C9transit in the river system. The actual case must lie
somewhere between these two extremes.
Therefore, it seems virtually impossible to establish
with any degree of certainty the period of time uraninite
grains might have spent in oxidizing zones duringtransport. In the extreme cases in which a sediment par­ticle was continuously entrained by the river or in a riversurrounded by anoxic sediments, periods in oxidizing en­
vironments might have been as short as 100 years. At
the other extreme, in which the particle remained in ox­
idizing zones during transit, periods of tens of thousandsof years might have been reached.

The intimate intercalation of the uraninite-bearing
conglomerates with generally uneconomic conglom­
erates may indicate that placers were produced by very
local current and depositional variations, without
significant diminution of the sedimentation rate.
This should have applied to the fluvial deposits of the
Witwatersrand as well. However, conditions in marine
littoral or submarine deposits might have been quite dif­ferent. Tracer studies of movement of beach sand havebeen undertaken (Ingle, 1966), but periods of milling andconcentration in beach deposits are largely unknown. It
appears, however, that concentrations of heavy miner­
als may be formed within a few years.
Even if the uraninite were delivered to the basin of
DEPOSITION AND BURIAL
deposition virtually unweathered, it might still be
The exact mode, or modes, of deposition of the placers destroyed if oxidizing conditions prevailed in the sedi­
is still uncertain (Brock and Pretorius, 1964; Frarey andment (Walker, 1967). There has been long discussion of
Roscoe, 1970). The two major models for deposition of
the origin of pyrite in both the Witwatersrand and thethe placers are those of terrestrial fluvial deposition andElliot Lake deposits (Ramdohr, 1958; Liebenberg, 1957;
marine littoral deposition. The fluvial model appears toDavidson, 1960, 1965; Schidlowski and Trurnit, 1966;
be well supported for the major uraniferous conglom­
Roscoe, 1969; and others). Part of the pyrite appears toerates in Elliot Lake (Pienaar, 1963; Frarey and Roscoe,
be detrital. However, observations of pyrite over­1970). However, no single model appears able to explain
growths and framboidal pyrite indicate that some of thethe deposits of the Witwatersrand. Brock and Pretorius
pyrite is of postdepositional, epigenetic origin. Ramdohr(1964) have suggested that several mechanisms may(1958) suggested that pyritization occurred shortly after
have been responsible for deposition.
deposition. He suggested that some of the concretionary
Pienaar (1963) suggested that the gravels constituting
pyrite, which is probably secondary in origin, may havethe main Elliot Lake uraniferous conglomerates were
undergone some transportation. This would indicate
probably laid down during periodic increases in the cur­either erosion from an older formation or, more prob­
rent velocity of the streams. Under such conditions
ably, erosion from alluvial sediments caused by shifts inoligomictic gravels would be deposited on the bottoms of the position of the stream channel. This suggests thatwide stream channels as gravel bars or lag gravels. He
pyritization occurred close to the surface and that
attributed the interlacing of lenses of varying grain size
ground waters in some sediments surrounding the riverto periodic fluctuations in the current velocity, as well asor beach contained so little free oxygen that pyrite could
to differences in the current velocity in different parts ofbe precipitated. Therefore, it may be postulated that thethe channel.
sedimentary environment became reducing near thesurface. The fluvial or marine facies of the placers would
probably have been aerated in the top few centimeters
owing to the aeration of the river or ocean, but lagoonalfacies of the placers might have been anoxic, even very
near the surface (Saager, 1970).
nnRates of deposition in molasse settings are fairlyi
I
I
I
rapid.
Rates are estimated at between 15 cm/1,000 years
1.0
0.2
0.4
0.6
0.8
and 5 m/1,000 years (Fischer, 1969). Rates of deposition
FRACTION OF SATURATION
in the Siwalik Formation, one used by Reinike (1930) asa model for Witwatersrand sedimentation, may have
EXPLANATION
been as high as 50-100 cm/1,000 years (Gansser, 1964).
[] Water
Therefore, grains may have been removed from strongly
oxidizing environments within a few thousand years.
0 Sand bars
• Other alluvium
FIGURE 3.—Concentration of dissolved oxygen (as a fraction of satura­tion) in water and alluvial sediments of Bugaboo Creek, British Col­umbia, Canada.

NUMERICAL CALCULATIONSThe experimentally derived reaction-rate equations (2and 8) were solved for the maximum oxygen pressure

CIO

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

pH of Precambrian rivers would not have been lower
than about 6.3, even at carbon dioxide pressures as
great as 100 times the present level.
Holland (1972) has shown that the pH of ocean waterisunlikely to have been less than 7 since the earlySpecific surface area (SS) __________90 cm2 g"1
Precambrian,even under very high atmospheric carbon
Organic Retardation Factor (RF) ____5-35] dioxide pressures.
Nonuranium cation content (NOC) ___0.16 (mole fraction)
Carbon dioxide pressure _______5-100 PAL (0.0016-0.032 atm)
Assuming carbonate equilibria largely control solution
Temperature (T) _________________15°C
pH, it can be shown that at pH greater than 6.3 theAlkalinity of river waters (A) ____1 x 10"3 Molar
dissolution rate of uraninite in systems open to the
Residence time in soil profiles (TE) ___500-10,000 years
atmosphere is independent of alkalinity or hydrogen-ion
Residence time in oxidizing en1,000-100,000 years
concentrations (Grandstaff, 1973). Therefore, small
vironments during transit andburial (TV).
variations in Precambrian river- or ocean-water chemis­
Fraction of original grain destroyed by 0.5 chemical corrosion (DF).
try may not be significant in influencing the rate ofuraninite dissolution. (Other details of the calculations
The hydrogen ion concentration in solution was are given in Grandstaff, 1973.) Results of the rate
calculated assuming that all activity coefficients were calculations are shown in figures 4 and 5.
unity, that carbonic acid was the only major source of
Figure 4 is constructed under the assumption that oxi­hydrogen ion, and that the solution was open to the at­ dation during weathering is minor and that chemicalmosphere. This may lead to an underestimation of the corrosion occurs only during transportation and burial.
acidity of ground and river waters if oxidation of sul- This would occur if the uraninite were derived by scour­fides contributed significant amounts of sulfuric acid or ing from bedrock or glacial weathering. Although glacial
the decay and metabolism of organisms contributed erosion seems unlikely in the case of the Witwatersrandorganic acids or excess carbon dioxide to the water. and Elliot Lake deposits, the figure suggests that
Since pyrite apparently survived as a detrital mineral, uraninite of the Blyvooruitzicht type could survive forthe rate of oxidation of sulfides must have been quite some time as a detrital mineral if it were derived fromslow. This suggests that addition of sulfuric acid to glacial weathering, even under present atmosphericground and river water may have been minor.
conditions. This is supported by observations in
The hydrogen ion concentration in erosional zones Pakistan, Canada, and Alaska of detrital uraninite thatwas calculated assuming negligible alkalinity by the is apparently derived from glacial sources. The chemical
equationcomposition and resistance to chemical corrosion of the2)1/2
uraninitein these recent placers is not known. The per­(10)
[H1 = (K1BPC02
sistence of uraninite in the modern environment may bein which B is the Henry's law coefficient and KI is the due not only to cold temperatures and glacial weather­first dissociation constant of carbonic acid (BKi = ing but also to high resistance to chemical corrosion.
1.6xlO-8 Matm-1 atl5°C).
Figure 5 shows the maximum atmospheric oxygenpressure
allowing the survival of uraninite as a detrital
The hydrogen ion concentration in river and alluvial
mineralundervarious periods of exposure to oxidizing
waters was calculated from the equation
conditions during weathering, transportation, and(11) burial. TE is the residence time in soil profiles. TT is theresidence time in oxidizing zones during transport and(A)burial.
Because the rate of dissolution during weather­in which (A) is the bicarbonate alkalinity
ing is more rapid than that during transport or burial,(A) = (HC(V) & (Na+) + (K+) + 2(Ca+2)
the maximum pressures allowed are largely influenced+ 2(Mg+2)-(Cl-)-2(S04-2) .(12) by residence periods in soil profiles except under condi­(Broecker and Oversby, 1971).
tions in which the time required for transport and burial
The alkalinity of present average river water is is much longer than the weathering period (TT> TE).
approximately 1 x 10"3 M (Livingstone, 1966). The rate The oxygen pressures allowing uraninite survivalof chemical weathering appears to have been relatively decrease with increasing periods of exposure. For theconstant over geologic time (Garrels and Mackenzie, most likely conditions, oxygen pressures of 10~2 -10~61971). If the runoff from continents during the mid- PAL are indicated.
These calculated pressures are maximum pressures. It
Precambrian was similar to the present amount, thealkalinity of Precambrian rivers may have been similar is likely that the actual atmospheric oxygen pressure
to that of present rivers. If this is correct, the average was somewhat lower. Also, these calculations do notthat would allow survival of uraninite as a detrital
mineral.
For purposes of the calculations the following values
or ranges of values were used:

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

Cll-2 . 7"f=500 years, flf=35, CO2 =5 PAL/•f -103,/?f=35, C02 =5 PAL-3

7"f=1Q3, RF=b, CO2 =5 PAL
7"£ =103,flf=5, C02 =10 PAL
7"f=1Q3, flf=35, CO2 =100 PAL

CDO-5 • TE = 1Q3, RF = 5, CO2 = 100 PAL
-2

-3

-4

-6

LOG Po2 (PAL)

FIGURE 4.—Maximum oxygen pressures allowing survival of uraninite 1
as a detrital mineral, assuming that significant dissolution occurs
only during transport and burial. The most reasonable conditions areshown by the shaded area. RF, organic retardation factor; PAL,
present atmospheric level; PCCV Par^ial pressure of carbon dioxide inatmosphere.

-6

TE = 104, RF= 5, CO2 = 100 PAL

-7
-1

0
LOG

take into account other possible mechanisms by whichuraninite might be attacked and dissolved, such as byuranium-oxidizing bacteria or by formation of oxygen orother oxidative species by hydrolysis of water by radia­
tion. If uranium-oxidizing bacteria or other oxidizing
agents were present in the ancient environment, oxygenpressures required for survival of uraninite may havebeen much lower.

FIGURE 5.-Maximum oxygen pressures allowing the survival of
uraninite as a detrital mineral during the sedimentary cycle. TE,
residence time in soil profiles; TT, residence time in oxidizing zones
during transport and burial; RF, organic retardation factor; PAL,
present atmospheric level.

Four general models of oxygen increase have been
proposed (see Fischer, 1972). They are shown
schematically in figure 6 and are described as follows:
1. The oxygen content of the atmosphere has been
IMPLICATIONS FOR THE PRECAMBRIAN
progressively increasing from aerobic photosyn­
ATMOSPHERE
thesis since the appearance and evolution of bluegreen algae.
Present data suggest that the earliest terrestrial at­mosphere was composed of reduced species, hydrogen 2. The atmospheric oxygen remained low (less than
0.001 atm) until the late Precambrian and earlyand methane with nitrogen and water vapor (Holland,
Phanerozoic time. Berkner and Marshall (1965,1962). Then, at some time before 3,500 m.y. ago, this at­1966) proposed that this concentration of oxygenmosphere was converted into one containing nitrogen,was reached as a result of photodissociation ofargon, and carbon dioxide, possibly by the disap­
water vapor. They ascribed the appearance of
pearance of free iron from the upper mantle and conse­
metazoa at the beginning of the Cambrian to thequent volcanic exhalation of less-reduced species. Final­
rise of oxygen pressure to the Pasteur point
ly, at some more recent time, the photodissociation of(approximately 0.01 PAL). They believed that thewater vapor with hydrogen escape and the aerobic
appearance of fossil land biota at the beginning ofphotosynthesis of plants with burial of carbonaceous
the Devonian could be correlated with a rise in at­
matter converted the atmosphere into one containingmospheric oxygen pressure to approximately 0.1
oxygen as a major component.

C12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESPAL, a level providing an ozone layer capable ofshielding the Earth's surface from deadly
ultraviolet radiation.
3. The oxygen level remained low until about 2,000
m.y. ago and then rose toward its present concen­
tration (Cloud, 1968). Cloud ascribed the disap­
pearance of banded iron-formations and the ap­
pearance of red beds about 2,000 m.y. ago to a risein atmospheric oxygen at that time.
4. The atmospheric oxygen increased rapidly near
the beginning of the Precambrian to a level nearthat of the present time. This rapid rise was due
either to photodissociation of water vapor (Brinkmann, 1969) or to the evolution of photosynthetic
algae.
The present study indicates that the pressure of oxy­
gen in the Precambrian atmosphere between 2 and 3b.y. ago was probably less than approximately 10"2 to
10'6 PAL. This appears to rule out the progressive ac­
cumulation model (model 1) and the model of early highoxygen levels (model 4).
Because of the possible presence of photosyntheticorganisms as early as 3 b.y. ago, this low level of oxygen
implies that either the rate of photosynthetic oxygen
production was quite low, possibly as a result of low
nitrate nutrient concentrations (Holland, 1973), or theoxygen produced by photosynthesis or photodissociation
was rapidly consumed by oxidation of reduced volcanic
gases or reduced elements in sediments.
Brinkmann (1969) calculated that the content of oxy­
gen in the Earth's atmosphere produced by photodis­
sociation could have been as high as approximately 0.27
PAL, depending on the extent of oxidation of reduced
species. However, it appears possible that the quantum

yield and the escape efficiencies used in the calculations
may be too high (Venugopalan and Jones, 1968; Walker,
1974). Thus the oxygen pressure from photodissociation
calculated by Brinkman (1969) appears too high.
Probable eucaryotic organisms are present in strata
as old as 1.3 b.y. (Schopf, 1970). Eucaryotic organisms
are fundamentally aerobic. This suggests that the con­centration of oxygen in the atmosphere at that time wasabove the Pasteur point, approximately 0.01 PAL, un­less the eucaryotic organisms were restricted to special
oxygen oases (Fischer, 1965; Weyl, 1968). Certainly, by
the time of origin of metazoa, before the beginning of
the Cambrian, the level of atmospheric oxygen had
reached 0.01 PAL. By the beginning of the Cambrian,
the presence of carbonate skeletons and cartillagenous
connective tissue indicates that the oxygen content of
the atmosphere must have been more than 0.1 PAL(Towe, 1969).
This evidence suggests that model 3 is schematicallycorrect. Low levels of oxygen, less than 10~2 to 10~6
PAL, were present in the early and middle Precambrianatmosphere. Higher oxygen pressures, greater than
0.01 PAL, were reached by 1.3 b.y. At the beginning of
the Cambrian, the requirements of collagen (Towe,1969) indicate that the oxygen pressure was greaterthan approximately 0.1 PAL.
ACKNOWLEDGMENTS

This work was undertaken as part of a doctoral disser­
tation at Princeton University under the supervision of
H. D. Holland. I acknowledge many helpful discussions
with and comments from A. G. Fischer, R. B.
Hargraves, S. Judson, and F. B. Van Houten. The workwassupported by NDEA grants under Title IV and———i——————I——————i——————i——————r-igrantsgrant from Princeton University.

n • Model 4 ^———————————————--X-- ,
-1
//'

-2
-3

/Model 3/I 'I /».',y <r:?f.-":-"?±
S: S<B :5::ss::!S-i:?SI?• J:??:? SiSiSiJS

1
/

/

/
/

/

-4
-5
-6
•

,

43210
AGE, IN AEONSFIGURE 6.—Models for the evolution of oxygen in the Earth's atmos­
phere through geologic time. (See text for further description ofmodels.) Shaded area shows most probable ranges of oxygenpressure and time for formation of quartz-pebble uranium deposits.

REFERENCES CITED
Ahnert, Frank, 1970, Functional relationships between denudation,
Ahnerlrelrelief,and uplift in large midlatitude drainage basins: American
Journalof Science, v. 268, p. 243-263.
Joi
Berkner, L. V., and Marshall, L. C., 1965, On the origin and rise ofoxygenconcentration in the Earth's atmosphere: Journal of theox^
AtAtmospheric
Sciences, v. 22, p. 225-261.
1966, Limitation on oxygen concentration in a primitivepla
planetary
atmosphere: Journal of the Atmospheric Sciences, v. 23,np. 133-143.
Brinkmann, R. T., 1969, Dissociation of water vapor and the evolutionof oxygen in the terrestrial atmosphere: Journal of GeophysicalResearch, v. 74, p. 5355-5368.
Brock, B. B., and Pretorius, D. A., 1964, Rand basin sedimentationand tectonics, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa: Geological Society of South Africa, v.
1, p. 549-599.

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE
Broecker, W. S., and Oversby, V. M., 1971, Chemical equilibria in the
Earth: New York, McGraw-Hill Book Company, 318 p.
Carey, W. C., and Keller, M. D., 1957, Systematic changes in the bedsof alluvial rivers: American Society of Civil Engineers Pro­
ceedings, Journal of the Hydraulics Division, v. 83, no. HY4, Paper
1331, 24 p.
Casshyap, S. M., 1969, Petrology of the Bruce and Gowganda Forma­tions and its bearing on the evolution of Huronian sedimentation in
the Espanola-Willisville area, Ontario (Canada): Palaeogeography,
Palaeoclimatology, Palaeoecology, v. 6, no. 1, p. 5-36.
Cawley, J. L., Burruss, R. C., and Holland, H. D., 1969, Weathering incentral Iceland: an analog of pre-Silurian weathering: Science, v.
165, p. 391-392.
Chave, K. E., 1965, Carbonates: Association with organic matter insurface seawater: Science, v. 148, p. 1723-1724.
Chave, K. E., and Schamlz, R. F., 1966, Carbonate-seawater interac­tions: Geochimica et Cosmochimica Acta, v. 30, p. 1037-1048.
Cloud, Preston, 1968, Atmospheric and hydrospheric evolution on theprimitive Earth: Science, v. 160, p. 729-736.
Colby, B. R., 1961, Effect of depth of flow on discharge of bed
material: U.S. Geological Survey Water-Supply Paper 1498-D, 10
PDavidson, C. F., 1958, Discussion of The attrition of uraninite, by G.
M. Koen: Geological Society of South Africa Transactions, v. 61, p.
193-194.
———1960, The present state of the Witwatersrand controversy: Min­ing Magazine, v. 102, p. 84-95, 149-159, 222-229.
-1965, The mode of origin of banket orebodies: Institution of
Mining and Metallurgy Bulletin no. 700 (Transactions), v. 74, p.
319-338.
Davidson, C. F., and Cosgrove, M. E., 1955, On the impersistance ofuraninite as a detrital mineral: Great Britain Geological Survey
Bulletin no. 10, p. 74-80.
Everitt, B., 1968, use of the cottonwood in an investigation of the re­cent history of a flood plain: American Journal of Science, v. 226,
p. 417-439.
Fischer, A. G., 1965, Fossils, early life, and atmospheric history: Na­tional Academy of Sciences Proceedings, v. 53, p. 1205-1215.
———1969, Geological time-distance rates: The Bubnoff unit:
Geological Society of America Bulletin, v. 80, p. 549-552.
-1972, Atmosphere and the evolution of life: Main Currents inModern Thought, v. 28.
Fisk, H. N., 1944, Geological investigation of the alluvial valley of thelower Mississippi River: U.S. Army Corps of Engineers, Mississip­
pi River Commission, p. 11-16.
Frarey, M. J., and Roscoe, S. M., 1970, The Huronian Supergroupnorth of Lake Huron, in Baer, A. J., ed., Symposium on basins andgeosynclines of the Canadian Shield: Canada Geological Survey
Paper 70-40, 265 p.
Gansser, Augusto, 1964, Geology of the Himalayas: New York, Interscience, 289 p.
Garrels, R. M., and Mackenzie, F. T., 1971, Evolution of sedimentaryrocks: New York, W. W. North and Co., 397 p.
Gilluly, James, 1949, Distribution of mountain building in geologic
time: Geological Society of America Bulletin, v. 60, p. 80-97.
Godward, M. B. E., 1962, Invisible radiations, in Lewin, R. A., ed.,Physiology and biochemistry of algae: New York, Academic Press,
p. 551-556.
Grandstaff, D. E., 1973, Kinetics of uraninite oxidation: Implications
for the Precambrian atmosphere: Princeton University, unpublish­ed Ph.D. dissertation.
———1975, Microprobe analyses of uranium and thorium in uraninitefrom the Witwatersrand, South Africa, and Blind River, Ontario,
Canada: Geological Society of South Africa Transactions, v. 77, p.
291-294.

C13

Hiemstra, S. A., 1968, The mineralogy and petrology of theuraniferous conglomerate of the Dominion Reefs Mine, Klerkdorp
area: Geological Society of South Africa Transactions, v. 71, p.
1-66.
Holland, H. D., 1962, Model for the evolution of the Earth's atmos­
phere, in Petrologic Studies: A volume to honor A. F. Buddington:
Geological Society of America, p. 447-477.
———1968, The abundance of C02 in the Earth's atmosphere throughgeologic time, in Ahrens, L. H., ed., Origin and distribution of theelements: New York, Pergamon Press, p. 949-954.
———1972, The geologic history of sea water - an attempt to solve theproblem: Geochimica et Cosmochimica Acta, v. 36, p. 637-651.
-1973, Ocean water, nutrients, and atmospheric oxygen, in Pro­
ceedings of symposium on hydrogeochemistry andbiogeochemistry, v. 1: Washington, D.C., the Clarke Co., p. 68-81.
Holmes, Arthur, 1965, Principles of physical Geology (2nd ed.): New
York, Ronald Press, 1,288 p.
Ingle, J. C., 1966, The movement of beach sand: Amsterdam, Elsevier.
Jordan, G. E., 1962, Large submarine sand waves: Scien, v. 136, p.
839-848.
Judson, S., and Ritter, D. F., 1964, Rates of regional denudation in the
United States: Journal of Geophysical Research, v. 69, p.
3395-3401.
Koen, G. M., 1961, The genetic significance of the size distribution of
uraninite in Witwatersrand bankets [with discussion]: GeologicalSociety of South Africa Transactions, v. 64, p. 23-54.
Lane, E. W., 1947, Report of othe subcommission on sediment termi­
nology: American Geophysical Union, v. 28, p. 937-938.
Leopold, L. B., Wolman, M. G., and Miller, J. P., 1964, Fluvial pro­
cesses in geomorphology: San Francisco, W. H. Freeman andCompany, 522 p.
Liebenberg, W. R., 1957, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef and the Black Reef, in
Uranium in South Africa 1946-1956: Associated Scientific andTechnical Societies of South Africa, v. 1, p. 20-218.
Livingstone, D., 1966, Chemical composition of rivers and lakes: U.S.
Geological Survey Professional Paper 440-G.
Mclntyre, D., 1959, The hydraulic equivalence and size distribution ofsome mineral grains from a beach: Journal of Geology, v. 67, p.
278-301.
Morisawa, M., 1968, Streams, their dynamics and morphology: New
York, McGraw-Hill, 175 p.
Nicolaysen, L. 0., Burger, A. J., and Liebenberg, W. r., 1962,
Evidence for the extreme age of certain minerals from the Domi­nion Reef conglomerates and the underlying granite in the western
Transvaal: Geochimica et Cosmochimica Acta, v. 26, p. 15-24.
Niekerk, C. B. van, and Burger, A. J., [1966], The age of theVetersdorp System: South Africa Geological Survey Annals, v. 3,(1964), p. 75-86.
———1969, Lead-isotopic data relating to the age of the Dominion
Reef Lava: Geological Society of South Africa Transactions, v. 72,
p. 37-45.
Pienaar, P. J., 1963, Stratigraphy, petrology, and genesis of the Elliot
Group, Blind River, Ontario, including the uraniferous con­glomerate: Canada, Geological Survey Bulletin, no. 83, 140 p.
Pretorius, D. A., 1964, The geology of the South Rand Goldfield, inHaughton, S. H., ed., The geology of some ore deposits in Southern
Africa: Geological Society of South Africa, v. 1, p. 219-282.
Ramdohr, Paul, 1958, New observations on the ores of the Witwater­
srand in South Africa and their genetic significance: GeologicalSociety of South Africa Transactions, Annexure to v. 61, 50 p.
Reinike, Leopold, 1930, Origin of the Witwatersrand System:
Geological Society of South Africa Transactions, v. 33, p. 111-133.

C14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

very, very slowly, much more slowly than the BlindRiver or Witwatersrand uraninite. Secondly, theuraninite appears to be derived glacially without any
oxidation during the erosion process and in the case of
the Indus is flushed rather rapidly down to the Hazara
district, where it is currently being buried. I think thisis about the same for other occurrences. Of course, inEngland the uraninite, pitchblende really, was derived
from mine tailings, so didn't undergo any particularoxidation during its "erosion." I went up to Bugaboo
Creek, British Columbia. As I indicated, that's one
area in Canada which is supposed to have uraninite. I
didn't find any uraninite, unfortunately; it may still be
there, but if so, it is very rare.
Simpson: My question follows on from the previous
speaker. The Indus River gravels today containthorian uraninite with a thorium-to-uranium ratio of
1:4, very similar to the ratio in the Dominion Reef of
1:4 to 1:5. The question I want to ask is this. Whatcomparative conclusions, if any, do you draw about
the atmospheres for these two occurrences?
Grandstaff: As was pointed out, thorium is not the wholestory; you need a complete analysis of the uraninite.
Stacy, in some conversations I have had with him, sug­gested that the uraninite of the Bugaboos might bevery cerian rich. His X-ray data suggested it has avery small unit cell, much smaller than U02. So I think
you need a complete chemical analysis. Other thanthat, I'd suggest again that the very cold temperaturesin the glacial environment tend to allow the uraninite
to persist, at least for a time.
Simpson: Would you agree, then, that your data doesn't
say much about the atmosphere in Dominion Reef
times?
Grandstaff: Well, the Dominion Reefs uraninite is moreresistant to corrosion, and I have suggested about 15
to 20 times, which would have allowed oxygen pres­sures of, say, a tenth to one hundred-thousandth of thepresent. Obviously, since it's not the most sensitive
uraninite, it doesn't really play any part in saying what
oxygen pressures in the Precambrian might havebeen.
Cannon: Have you considered what effect either ozone
or monatomic oxygen might have on these rates? Bothof these might be important species in a primitive at­mosphere and should react more rapidly than 02 does.
DISCUSSIONGrandstaff: Yes, that's correct. However, if the oxygenpressure is less than 0.01 PAL or so, the ozone pres­[Hallbauer?]: Is there anything known about why some
sure would have been only a very, very small fractionof the recent rivers carry uraninite, for instance, theof that. I didn't consider that it would be importantIndus River in India?
because of its relatively low probable concentration,
Grandstaff: There are several possibilities here. First ofeven though it is somewhat more reactive.
all, that the uraninite could be quite highly thorian or
have a high content of rare earths, which would tend Roscoe: I wonder if the molasse model you have of ero­sion and deposition of the Huronian sands and gravels
to stabilize it against oxidation. So these would oxidize

Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
the Canadian Shield: Canada, Geological Survey Paper 68-40, 205
PRutten, M. G., 1971, The origin of life by natural causes: Amsterdam,
Elsevier Publishing Company, 420 p.
Saager, Rudolf, 1970, Structures in pyrite from the Basal Reef in the
Orange Free State goldfield: Transactions, Geological Society ofSouth Africa, v. 73, pt. 1, p. 29-46.
Schidlowski, M., 1966, Beitrage zur Kenntnis der radioactiven
Bestandteile der Witwatersrand-Konglomerate, I. Uranpecherz in
den Konglomeraten des Oranje-Freistaat-Goldfeldes: Neues
Jahrbuch fur Mineralogie Abhandlungen, v. 105, p. 183-202.
Schidlowski, M., andTrurnit, P., 1966, Drucklosungserscheinungen an
Gerollpyriten aus den Witwatersrand-Konglomeraten. Ein Beitragzur Frage des diagenetischen Verhaltens von Sulfiden:
Schweizerische Mineralogische und Petrographische Mitteilungen,v. 46, p. 337-351.
Schopf, J. W., 1970, Precambrian micro-organisms and revolutionary
events prior to the origin of vascular plants: Cambridge
Philosophical Society, Biological Reviews, v. 45, no. 3, p. 319-351.
Stanley, K. 0., and Wayne, W. J., 1972, Epeirogenic and climatic con­trols of early Pleistocene fluvial sediment dispersal in Nebraska:
Geological Society of America Bulletin, v. 83, p. 3675-3690.
Steacy, H. R., 1953, An occurrence of uraninite in a black sand:American Mineralogist, v. 38, p. 549-550.
Suess, E., 1970, Interaction of organic compounds with calcium car­bonate. I. Association phenomena and geochemical implications:Geochimica et Cosmochimica Acta, v. 34, p. 157-168.
Sundborg, A. 1956, The river Klaralven, a study of fluvial processes:
Geografiska Annaler, v. 38, p. 127-137.
Thornbury, W. D., 1966, Principles of geomorphology: New York,
Wiley, 618 p.
Towe, K. M., 1969, Oxygen collagen priority and the early metazoan
fossil record: National Academy of Sciences Proceedings, v. 65, p.
781-788.
Van Houten, F. B., 1972, Iron and clay in tropical savanna alluvium,
Northern Colombia: A contribution to the origin of red beds:
Geological Society of America Bulletin, v. 83, p. 2761-2772.
Venugopalan, M., and Jones, R. A., 1968, Chemistry of dissociatedwater vapor and related systems: New York, Interscience, 463 p.
Walker, James C. G., 1974, Stability of atmospheric oxygen: American
Journal of Science, v. 274, p. 193-214.
Weyl, P. K., 1968, Precambrian marine environment and the develop­
ment of life: Science, v. 161, p. 158-160.
Wiebols, J. H., 1955, A suggested glacial origin for the Witwatersrandconglomerates: Geological Society of South Africa Transactions, v.
64, p. 23-54.
Zeschke, G., 1959, Uraninit-Vorkommen in rezenten Schwermineralsanden des Industales: Neues Jahrbuch fur Mineralogie
Abhandlungen, v. 93, p. 240-256.
———1961, Transportation of uraninite in the Indus River, Pakistan:
Geological Society of South Africa Transactions, v. 63, p. 87.

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

would give perhaps a falsely short time of residence
over what the actual situation may be, because I don't
think the comparison to molasse is really valid here.
You've got extremely coarse, fairly mature sediments
throughout the geological column of up to 20,000 feet,interrupted only perhaps by glacial episodes where wehave immature sediments. In other words, over an im­mense period of time, this time must have been long if
you consider also that the elastics are very coarse, wehave very coarse sediments, so what we are seeing isonly the coarsest fraction of the granitic rocks thathave been eroded and only the quartz out of that, only
a few percent of the rocks piled up in this immense col­umn. The sediments, therefore, must have had a very
volumetrically large source area through a very appre­
ciable length of time to have developed this wholesedimentary system. Moreover, the source area musthave been soil mantled virtually continuously, and not
cut by canyons, steep slopes or so on; the slopes would
have to have been fairly moderate and I think themolasse type of deposition would be an exaggerationof what must have prevailed at that time.
Grandstaff: I agree there has been a large pass-throughof finer grained sediments. The sediments are not,perhaps, as mature as one might think since there is a
lot of evidence for feldspars and other easily weath­
ered minerals. They are submature rather than com­pletely mature sediments, so I would say that themolasse model is not too bad.
[Schopf?:] I notice that your data are inconsistent with
the Brinkmann calculation. It would be instructive tome if you could discuss what's wrong with the Brink­mann calculation and why it doesn't fit your ex­
perimental data.
Grandstaff: Brinkmann looks at the reaction water
vapor plus a photon yielding hydrogen plus oxygen,and he calculated that oxygen pressure could havebeen brought up, at a maximum, to about one quarterof our present oxygen pressure. Brinkmann makes
two assumptions which may not be entirely correct.
The first assumption is that the quantum yield of theabove reaction is unity; that is, every photon causes
the splitting of water and the creation of odd oxygen.
The quantum yield in this reaction is not very wellknown. It appears to be very highly dependent onpressures, on other species, and varies between about
one and one millionth. So, depending on quantum
yield, oxygen pressures which could be created mightvary between a quarter and a quarter of a millionth ofthe present. The second thing that Brinkmann sug­gests is that all this hydrogen escapes; that is, that theescape factor is very high. In our present atmosphere,
at present exospheric temperatures of around 1,000 to
1,500 degrees Kelvin, the escape efficiency is only

C15

about one thousandth to one hundredth; that is, only
about one in a thousand to one in a hundred of thehydrogen ions actually leaves the atmosphere. In my
opinion his escape efficiency is unreasonably high, andI think if you consider what the atmosphere might
have been like during the mid-Precambrian, par­ticularly with a lower solar constant, that is, lower
luminosity, you will agree that the escape efficiency
would have been quite low. Take Venus as an example.
Exospheric temperatures are only on the order of 600
to 800 degrees Kelvin; that would give an efficiency ofperhaps a hundred-thousandth. So I'd say that, rather
than the quarter that he suggests, 10"3 to 10~6 is morereasonable for the maximum amount of oxygen which
could be produced by photodissociation.
[Minter?]: You have taken samples from different en­
vironments and placed them in the Dominion Reef,
which is a very old deposit. I do think that the evolu­tion in the model that you are purporting to describe is
not as general as you imply and that probably theoldest deposits were certainly deposited on very long
low-gradient paleoslopes. Secondly, your samples
from Blyvooruitzicht are samples from a reef which
has certainly been recycled many times; that reef lies
on an angular unconformity and is certainly not a
primary reef.
Grandstaff: Again, it seems to me that there is very lit­
tle necessary relation between the slope on which thematerial is resting and the source area; that is, if you
do have uplift in the source area, you probably do in­deed have virtual peneplanation at the beginning of
this episode. I would say that peneplanation was
followed by uplifting in the source area, downcuttingby rivers, relatively rapid erosion, and flushing of thismaterial into the reef. Obviously I have tried to in­
dicate some range in permissible exposure times toallow for this very basic uncertainty in the rates. With
regard to recycling the sediments, this too would tend
to increase exposure periods. It is not really very clear
to me how long and how necessary this recycling isthat is commonly alluded to. How much recycling
lengthens the exposure period I just don't know;
recycling can happen very rapidly, particularly inoceanic milling and formation of black sands onbeaches. Also, the uraninite and the host sediment do
not necessarily have to come from the same area or
have been exposed to the environment for the same
period of time.
Myers: Could you explain your statement about organic
matter slowing the rate of uraninite oxidation?
Grandstaff: I believe that the polar organic matter lat­ches onto the positively and negatively charged sur­faces and in this case essentially blocks the oxygenfrom approaching the oxidation site, so that essential-

C16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

ly the organic matter completely covers the surface
carbon dioxide content, presumably we were dealing
and doesn't allow the oxygen to get in, slowing downwith much higher average surface temperatures than
the reaction rate. The same thing has been noted in
we see today.
calcite. You can acidify a solution containing calcite, Grandstaff: No, not necessarily.
and the reaction should go quite quickly, but it doesn't Skinner: At least five to ten degrees more. No?
because of the organic coating on the grains.
Myers: Do I understand that you actually observed this Grandstaff: Okay. The thermal balance of the Earth isstill very much up in the air, so to speak. Calculations
slowing of the rate?
assuming
a lower solar constant suggest the atmos­
Grandstaff: Yes.
pherictemperature
was actually lower than at pres­Myers: And you are attributing it to the organic matter?
ent.
RasoolandSchneiderfind that the effect of C02
Grandstaff: Yes.
pressureontemperature
is more logarithmic than
Skinner: I would like first to ask you, if you only havelinear,andthatdoubling
C02
pressure causes an in­carbon dioxide at one hundred times the present level,
creaseofroughly
1°C,
the
C02
pressure must be in­where was the rest of the carbon? It doesn't seem to becreased
ten
times
to
get
the
next
degree C, and so
in carbonates or in sediments if you make mass bal­forth.
Thisisone
mechanism
bywhich
the Precam­
ance for that period, despite Bill's [Schopf] 50 percent
brian
temperature
could
have
been
made
higher.
slop.
Opposing
this
is
the
fact
that
the
solar
constant
at thatGrandstaff: I have to disagree with you there. All we aretime
waslower,
perhaps
only
about
75
percentof thesaying is that you have carbonates and you have a re­present,
and
would
cause
temperaturestobelower.
action between the carbonate, calcite and the magnes­
Also, the rate of orogeny and rate of volcanism at thatium silicate, some sort of chlorite and dolomite, andtime, as suggested by Richard Armstrong, may havesilicic acid giving you C02 in solution, or in the atmos­
beenhigher because of the higher radioactive decay.
phere. Because of the absence of silicifying organisms
Intensevolcanism would have erupted much material
during the Precambrian, we believe that the C02
intotheatmosphere, aerosols, which would have tend­pressures must have been somewhat higher. Thised to depress the temperature. The effect of aerosols
doesn't have really anything to do with the mass
on atmospheric temperature seems to be much morebalance between carbonate and organic carbon.
pronounced
than the effect of carbon dioxide. I think
Skinner: It requires you to keep adding C02 from theratherthanfearing that the surface temperature
mantle or somewhere for that kind of model, or else
would
get
too
high, I think we have to fear that the
you have to have almost as much carbonate in the sec­
surfacetemperature
would get too low. This sort oftion as you now have.
thing
has
not
been
treatedin a quantitative fashion.
Grandstaff: You certainly have carbonates, primary
Skinner:Ifthesurfacetemperature
gets too low, thenlooking carbonates, all the way back to Onverwacht
youdepressthedissolution
rate
of
the
phosphides andtime.
ofcourse
you
know
they
are
all
gone.
The
surface tem­
Skinner: Do you have the same amount?
peraturecannotremain
too
low
for
very
long
without
Grandstaff: Well, this is certainly a big question. There
destroying
theonly
weathering
processes
you'vegot
is a problem here, of course, in that the carbonates
to
get
rid
of
phosphides.
tend to be preferentially dissolved by later weather­ing. Garrels and Mackenzie have shown that the turn­ Grandstaff: Well obviously, if you freeze the whole sur­over time is only a few hundred million years for car­face of the planet there is no life. But within the sort ofbonates as compared to the billion years or so for
permissible range of 1° up to 30°, using an activation
silicates, so that it's not very clear how much our pres­energy for the hydrolysis of feldspar of 8.7 kilocalories
ent exposures really reflect the true Precambrian car­per mole, as suggested by Helgeson, you get a varia­
bonate record.
tion in rate of weathering of only a factor of two. It is
Skinner: Could I pursue not the abundance of car­about the same for uraninite. It's hard to tell what webonates but the question surrounding carbon dioxide aare seeing in the Precambrian in regard to the maturi­
little further? Since yours and anybody else's modelty of Precambrian sediments and the effectiveness offor the early Precambrian atmosphere has a higher
Precambrian weathering.

SOUTH AFRICA

Uraniferous Quartz-Pebble Conglomerates
in South Africa
By J. W. VON BACKSTROM
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-D

CONTENTS
Page

Abstract ________________________________________________Uraniferous quartz-pebble conglomerates in South Africa _____________________
Discussion _______________________________________________Dl
1g

ILLUSTRATIONSFIGURES 1,2. Maps showing the Witwatersrand basin:
1. Surface geology __________________________________________________________
2. Distribution of the Witwatersrand System ___________________________________________
3. Generalised geological sections of the Upper and the Lower Witwatersrand _________________________—
4. Comparative sections of the Witwatersrand Supergroup, showing uranium-bearing horizons _____________————
in

D2
3
4
5

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESURANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA
By J. W. VON BACKSTROMI
ABSTRACTThe purpose of this paper is to give a short background statementsummarising data on the Dominion Reef Group, the WitwatersrandSupergroup, and the Ventersdorp Contact Reef, with particularreference to the close relationship of gold and uranium with sedimen­
tary features as well as the mineralisation, conditions of deposition,and the nature of the quartz-pebble conglomerates.

URANIFEROUS QUARTZ-PEBBLECONGLOMERATES IN SOUTH AFRICA

Important concentrations of uranium minerals occur
in auriferous conglomerates present in four successive
Precambrian stratigraphic units, namely, Dominion
Reef, Witwatersrand, Ventersdorp, and Transvaal, of
which the two first named are important because of
their relatively large uranium content. These four units
do not conform to one another; they cover some tens of
thousands of square kilometres in the Provinces of theTransvaal and the Orange Free State.
An outcrop map of the above-mentioned formations isshown in figure 1, whilst figure 2 illustrates the suboutcrop when all the younger formations are removed.
The Dominion Reef Group comprises a basal group of
conglomeratic, arkosic, and quartzlitic rocks up to 100 m
thick, covered by about 900 m of lava of andesitic or
acidic composition. The group of sedimentary rocks
forming the base is, from an economic point of view, themost important part of the Dominion Reef Group. It is
composed largely of the disintegration products of theBasement Granite. The sediments consist of mediumgrained to coarse-grained feldspathic, and somewhat
micaceous, sandstone or quartzite, in which there arelenticular bodies of grit and conglomerate.
Two conglomerate bands along or close to the base of
the arenaceous sediments have been worked or pros­
pected for gold and uranium; these are known as the Up­
per and Lower Reefs. They are separated by barren con­glomerate or by a layer of quartzite. The Lower Reef,
which occurs at the base of the conglomerate zone, oc­
cupies depressions in the erosion surface of the Base­ment Granite. It is lenticular and has been the main gold

1 Director, Geology, Atomic Energy Board, Republic of South Africa. Retired, March 1978.

carrier in the area. The Upper Reef occurs some 18 m or
more higher up in the succession of conglomeratic,arkosic, and quartzitic rock. The reef is a small pebble
conglomerate passing laterally into a thin layer of grit
containing scattered pebbles. It contains a relatively
high concentration of detrital monazite and cassiterite.
The reef, varying in thickness from 5 to 120 cm, isremarkably consistent and was nowhere absent oversome 900 m of underground development on strike.
Where it is thin, the concentration of uraninite may bevery high, with sections 2-5 cm thick assaying as high as
one percent uranium. This reef is the main carrier ofuranium and, apart from the minerals already men­tioned, carries minor amounts of detrital chromite,garnet, zircon, ilmenite, and columbite-tantalite.
In the Witwatersrand Supergroup, economically im­
portant uranium and gold-bearing conglomerate bands
occur on flat planes of intraformational diastems,
disconformities, and unconformities, where they are
known as reefs or bankets. Each of these conglomeratebands represents a deposit formed after a hiatus in theprocess of sedimentation. They obviously consist largelyof reworked and resorted detritus derived from the ero­sion of underlying pebble and quartzite beds. Channels
and hollows, filled with eroded detritus derived from theerosion of previously laid sediments and known as
Hybrid or Footwall Reefs, are of common occurrence
immediately below the surface of contemporaneous ero­sion, upon which the principal conglomerate ore bodies
were laid down. Fluvial structures are often clearly
revealed by the channel deposits, some of which are
more than 100 m deep. Pay streaks in the Footwall
Reefs are aligned parallel to the course of the winding
channel, and the higher values of gold and uranium,
where present, tend to be confined to the basal portionof the reefs.
The remarkably close relationship of the concentra­
tions of the gold and uraninite with sedimentaryfeatures strongly suggests that the gold and uraninite
were deposited at the same time as the pebbles andother detrital components of the conglomerate. The peb­bles show variations in their packing index. They areusually not close packed, and sometimes only isolated
pebbles are present in the matrix. In some areas pebbles
Dl

D2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES• ••••••• •••••• • >r• • • • • • • • •*• •*• • •*•*•*O O

oo
OO CWOO O O O O O O

oo o oooooooooo
O O OOOOOOOOOOOOOO+7 OOOOOOOOOOOOOOOOOO O OO O

oo

O

ooo o
ooooooooooooo
OOOOGOOOOOOO

oooooooooooo
oooooooooo
OOOOOOOO—_—_poooooooooooo— — ^Jo oooooooooo

O O O O CLO O O O^> O O O—:O O O O O O O//O

oo o o o oooooooo/ —.-. _. Lava with intercalations of quartzi
u u conglomerate, boulder beds, tuffs

100 KILOMETERSI

FIGURE 1.—The Witwatersrand basin, surface geology (from R. Borchers, 1964, Exploration of the Witwatersrand System and its extension, inHaughton, S. H., ed., The geology of some ore deposits in southern Africa: Geological Society of South Africa, v. 1, diagram I).

are well sorted, with the larger ones at or near the base,and are oriented with the longest axis parallel to thedirection of strike of the beds in which they occur.
The pebbles in the ore-bearing conglomerates arecomposed mainly of vein quartz, with occasional pebbles
of chert, quartzite, red jasper, quartz porphyry, and,more rarely, tourmalinized rocks, greenish slate, andschistose-type pebbles. The most abundant constituentof the matrix is quartz, which occurs together withphyllosilicates such as sericite and pyrophyllite and withlesser quantities of chlorite, chloritoid, and tourmaline.
Pyrite, in the form of rounded grains, crystals, and ir­
regular patches, is a ubiquitous and often abundant consituent. Other sulphides that may be present are pyrrhotite, arsenopyrite, galena, chalcopyrite, sphalerite,
marcasite, cobaltite, and linnaeite. Hydrocarbon is com­monly found as granules and less often as thin layers orbands. Hydrocarbon containing minute particules ofuraninite is also known as thucholite.
The conglomerates form only a minor part of theURANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA;:; + r :r;:; + : + + + r : + \/ : : : : + ; : + ; : + ^ ^ ** : * ^D3

i—T—rij. j_ jGramte

and schist

X
26°

-27*

]

"

+J

100 KILOMETERS

FIGURE 2.—The Witwatersrand basin, distribution of the Witwatersrand System (from R. Borchers, 1964, Exploration of the Witwatersrand
System and its extensions in Haughton, S. H., ed., The geology of some ore deposits in southern Africa: Geological Society of South Africa, v.
1, diagram II).

whole Witwatersrand Supergroup (fig. 3) and vary inabundance in the different regions. In the Central Wit­
watersrand, conglomerates form 600 m of the 7,500 m
of sediments (fig. 3) and are most abundant in the Upper
Division of the Witwatersrand Supergroup (figs. 3, 4).
Most of them contain both gold and uraninite, but not
more than a dozen are rich and persistent enough torepay the cost of mining and extraction. These few are
the conglomerates that have produced by far the largest
output of gold and uranium in South Africa. They are

generally remarkably regular in their mode of occur­
rence, and individual conglomerate bands persist overwide areas. Beds of auriferous conglomerate also occur
in the Lower Division of the WitwatersrandSupergroup, particularly in the Government Reef
Member, but, with the exception of those west of
Klerksdrop, they have not proved to be of economic im­
portance.
From the bottom upwards, the principal uraniumbearing conglomerates in the Upper Division of the Wit-

D4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

1
r—
^~-—
1
Z

rr^- °nd

if

i-.-~

!§=

fi
Quartzite————— ———

3E

K,£——'

ntersdorp Sedments

Shalesg

1
a
ERGROUP

___„„

UJ

£

Coronation Shales

0

3 i

BE:

WITWATERS
Lower Coronation Quartzite

SLTr^S"1"

o
z

———

__,4,

0

5

—————

UJ

|
Shale and Quartzite=IB

Middling Conglomerate
Lower Conglomerate——•—-

Shale BandsbS
Hospital Hill Quartzite

1

3

HOSPITAL

3H
Lower

Foolwoll
Water Tower Slates=--^- -

Continued above°~ ——FIGURE 3.-Generalised geological section of the Upper Witwatersrand, showing cover of Ventersdorp and Karoo Supergroups(Orange Free State), and of the Lower Witwatersrand (Central
Rand).

watersrand Supergroup are the following: the Carbon
Leader, belonging to the Main Reef Group of con­glomerates; the White, Monarch, and one or two otherconglomerate reefs of the Bird Reef Group in the West
Rand, and their equivalents the Vaal Reef in the
Klerksdorp Goldfield and the Basal and Leader Reefs in
the Orange Free State Goldfield; and the May Reef, andits equivalents in the Kimberly Reef Group of con­glomerates in the Far East Rand. Besides these, there
are other gold-bearing reefs from which uranium is be­ing, or may be, profitably extracted as a byproduct ofgold production, for example, the Commonage Reef on
the horizon of the Main Reef Group, some of theWestern or Elsburg Reefs in the Klerksdorp Goldfield,
and the A, B, and Rainbow Reefs in the KimberleyElsburg Groups in the Orange Free State Goldfield. The
only uranium produced so far from the Lower Wit­
watersrand Division comes from the Afrikaner and In­
ner Basin Reefs in the Government Reef Group west of
Klerksdorp (see fig. 4).
The uraninite and gold are more or less uniformly
distributed over wide, flat areas occupied by the con­glomerate sheets. As in the case of gold, uraninite cantherefore be spoken of as evenly distributed in a generalway, when the whole extent of the goldfields is con­sidered, especially when comparison is made with themuch greater variations in the distribution of values
shown by nearly all lode or hydrothermal deposits. The
richer gold and uraninite concentrations in an individual
banket sheet tend to collect in pay streaks, which areusually characterised by the presence of pebbles larger
than those in the rest of the banket and by a greater con­centration of other ore particles, some of which areclearly detrital. The economically important uraniumand gold-bearing conglomerates, which form relatively
thin, unbroken sheets of great length and breadth, even­tually, with increasing distance from the original
shoreline or source rocks, either break up into isolated
patches or lenses of conglomerate containing notableamounts of very finely divided gold separated by lean orbarren quartzite or shale, or they thin out into a single,
rich line of heavy ore minerals and granules of hydrocar­
bon, along which isolated clusters of pebbles or scat­
tered pebbles may occur. It is not unusual to find visible
gold associated with these pebbles. Still farther awayfrom the source rocks, the conglomerate lenses become
smaller and smaller, and spaced farther and farther
apart until, ultimately, the conglomerate has passed
laterally into quartzite. Concomitantly, gold anduranium values decline until only traces or no values areleft where quartzite occupies the horizons on which theconglomerate ore bodies occur near the source rocks.
Mineralisation is confined to the ore-bearing con­glomerate and seldom crosses the stratification into thearenaceous wall rock, except where it has obviously

D5

URANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA

cc

i
Q_
a O- i_

O CO

O |~5

ID o ca>

1

I/

Unmberconfoxrmleityy*

/
/
/
/

::j
\
1

\
l
1

CO LU

^

> •> 1 l|l 1, 1 1|ll'l|l1 1 IIIl'l, 1

£

*»—

1

1

I
.0 *-o
^

-E\
™>

QQ

.£

1
CD"
—3

^

/

i \

e

\

\X

1-

s§

§°>i

1—

1
o-i

g

dHOQSH31N3A

aaia-NMAi

dnoyD

NM01S3dd3P

S

8

*

ix
o\
°-

2

UJ DQ

Z)OD

CD

O

tg

^|
f
.o-ro co

2,2<3

3:

i

*l

1 1
1

|

|| \§

=

1

I

s

Of->•

S|

II
I:

CO

QC

jnoyD

d33y iN3IAINy3AOD

TIIH IVlldSOH

g

B-

CD
8

I

I

I

CO

CO

8
LO

8
CD
LO
LO

§

CD
CD
CD

J______I

8
_CO

§

§

I______L

r;i

0

11i
i11

QNVHSa31VMllM H3M01

QNVHSH31V/VUIM H3ddH

8

\

\T

CD

1

sdnoyD
Dynasi3-A3iy3aiAii)i

X

2 O

oa

e -g

I

ii!

ii II
II
ii
n 1 1 s 10

I IIill1 ! 1'! ifi^iijii: I , ; 'l : ; ,j|i! j!||l!$(
\
\
\
\
\
\
§•
\
\
, \

2

i M 1

£ >>> 'h'lhllllil,CD—3 _E <

1

t

-0

00

co <

E

=

^

+ +
+

+

+

D6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

been derived from the conglomerate ore body itself.
There is a close relation between the distribution ofvalues and purely sedimentary features. Richer concen­
trations are commonest near the base of the con­glomerate along well-defined shaly or schistose footwall
partings and in conglomerate less than 30 cm thick.
They also tend to be disposed in pay streaks that have
many of the characteristics of those found in presentday placers.
The uraninite is finely disseminated as minute,discrete particles in the quartzose matrix that fills theinterstices between the pebbles. Along with the fine par­ticles of gold and grains of other heavy minerals, thoseof uraninite are very often more highly concentrated
along the bottom of the conglomerate; in some cases,
however, the higher concentrations of these heavy
minerals are distributed parallel to the bedding andtowards the middle or top portions of the conglomerate.
The Ventersdorp Contact Reef and the Black Reef oc­
cur at the base of the Precambrian Ventersdorp andTransvaal Groups, respectively. The Ventersdorp
Group, predominantly a volcanic formation, overlies asurface of Witwatersrand beds that has been eroded
down to varying depths. Gently inclined or flat-lying
basal beds of the Transvaal Supergroup in turn rest witha pronounced unconformity on the tiltea and eroded
Witwatersrand and Ventersdorp rocks. Both the
Ventersdorp Contact Reef and the Black Reef occur as
lenticular bodies of conglomerate in depressions andchannels in the Witwatersrand beds and are largely
composed of the erosion products, including the heavy
concentrates, of the immediately underlying Witwaters­
rand rocks. Many of the quartz, chert, and quartzite peb­bles of these reefs came from the Witwatersrand con­glomerates and already had their round shape when
they were released as a result of the disintegration ofthese conglomerates.
The Ventersdorp Contact Reef appears to consist of
partly eluvial and partly alluvial material; a small admix­
ture of volcanic tuff accounts for its dark colour. The
presence of uraninite and gold in the Ventersdorp Con­tact Reef and in the Black Reef is related to the distribu­
tion of the mineralisation in the bedrock from whichmost of their components have been derived. Both these
reefs are pyritic, and they carry uraninite and gold in
those localities where they rest upon or occur in thevicinity of suboutcrops of the uranium- and gold-bearingore beds in the Upper Witwatersrand. Away from theseore beds, uraninite and gold have hitherto not beenfound in significant amounts in the Ventersdorp Contact
and Black Reefs. The composition of these con­glomerates resembles that of the Witwatersrand banket
in many respects and also shows mineralogical changes
as a result of low-grade regional metamorphism.

DISCUSSION

B. Nagy: Just a very minor point of semanitcs. I wonder
if you would object to using the words "organic
matter" instead of "hydrocarbons," because in the Wit­
watersrand Vaal Reef, which we are studying, the car­bonaceous matter is not hydrocarbons. Also, I
wonder, again from a point of semantics because thisis going to develop tomorrow, whether the term
"thucholite" should really be used for this car­bonaceous matter which actually may be determined
to be a polymerlike organic material. Now I want toemphasize that by polymer I do not mean polymeriza­tion of methane, but I mean a polymer like a protein,
macromolecules. Would it not perhaps be better not to
use thucholite, because really nobody knows what
thucholite is?
von Backstrom: Dr. Nagy, I take both your points. I
speak as a regional geologist.
Wilfried Meyer, Falconbridge Nickel Mines, Ltd.,
Canada: Dr. von Backstrom, where in thestratigraphic succession would you place any change
in the atmosphere from anoxygenic to oxygenic?
von Backstrom: I'm afraid I wouldn't think myself
capable of answering that question. I would have torefer it to colleagues who presented the previous
papers.
Meyer: Is there an answer from anybody else?
Grandstaff: I think we have pointed out already that theappearance of eukaryotic organisms, the disap­
pearance of banded iron-formations, the appearanceof red beds, all at around 1.5 to 2 billion years appears
to indicate that the oxygen pressure did increase inthat time interval.
Meyer: The point I'm trying to make is this: In Canada
we've got this very rapid change from pyrite-bearing
beds to hematite-bearing beds in the Huronian, andthis change has often been used as evidence forchange in the oxygen content of the atmosphere. If
you take absolute early age dating from the Wit­
watersrand, the corresponding change must, in myopinion, come somewhere in the Transvaal Group, oreven earlier than that, yet you have detrital uraninite
in the basal conglomerate there and in some other.
There seems to be an inconsistency.
Schopf: I'm aware of that work and I think it's very in­teresting. It has bothered me, however, that manygeologists seem to be looking at this in a rather
simplistic way. It seems to me that it is probably incor­
rect to think of the evolution of atmospheric oxygen as
being a nice smooth curve. It is often drawn that way.
But I would suppose that there must have been fluc­tuations in the period between 2.3 and 1.9 b.y., ups
and downs, that will be reflected in what will be inter­
preted perhaps as conflicting evidence: oxygenic con-

URANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA

D7

ditions and then maybe reducing conditions, then ox- Samuel Adams, Anaconda Co., Uranium Division, Salt
ygenic conditions, and so forth. It would seem to me,
Lake City, Utah: This goes back to the paper on the
however, that looking at the dynamic processes in­rates of oxidation of pitchblende or uraninite. You
volved, that's exactly what should be expected rather
mentioned, or some of the other speakers did, the in­than a simple non-all situation. I suspect that it's reallyfluence of thorium on the rates of that dissolution, butmore complicated and that there was a period of timeit seems to me that perhaps we did not pay enough at­
in which both obtained.
tention to the state of oxidation of the uraninite itself
Grandstaff: Also, when you're speaking about oxidation
at the initiation of those reactions. For example, if Iof hematite, hematite is stable above oxygen
understand your data correctly, the degree of oxida­pressures around 10 to the minus 70, so what is oxtion of the samples you used ranged from about 12
ydizing for hematite still may not be very oxidizing forpercent U03 up to about 60 percent. The sample whichuraninite. There's quite a range in which the two canshowed the highest rate of dissolution was the sample
quite happily co-exist.
with the smallest amount of U03 , it was the least ox­
Meyer: Yes, but all this still does not explain this time
idized.
change from pyrite to hematite, fluctuations or not, Grandstaff: That's not correct.
because somehow you don't repeat that change. At Adams: That isn't?
least the pyrite in detrital form is not repeated. Why Grandstaff: No.
not?
Adams: That was the 22.7 figure?
Bourret: May I suggest that in a very broad sense we are Grandstaff: If you look at the Witwatersrand and theoverlooking one important factor here and that is theBlind River uraninites, both being about 60 percent
tremendous changes in the weather cycle. Torrential
U03, they both oxidized very, very slowly. In fact, we
rainfall in high ancestral mountains flushed large
looked at this, and there did not seem to be any very
quantities of coarse debris into the basin followed bysimple relation between U2 - U03 ratios and rates of
periods of quiescense during which fine sediments
oxidation; they both oxidized very, very slowly.
were deposited and even erosional gaps developed. Adams: Right. The figures that I have for the percent
With repeated tectonic and weather changes, the cy­oxidation of the uranium in the Dominion Reef and thecle was repeated over and over again. So, in a sense,
Nordic sample are 46 and 42 percent, respectively. So
we're prone to oversimplify the cycle of change thatthose really still are not as oxidized as many of thecaused the working and reworking and deposition of
uraninite samples. And what I wonder is whether orcoarse material.
not such studies shouldn't look at two phenomena:
Paul Weis, U.S.G.S., Reston, Va.: You mentioned thatfirst, the dissolution of uranium from those pit­
the Witwatersrand conglomerates are deposited on
chblendes and, secondly, the oxidation of uranium
surfaces unconformably and that the unconformities
followed by the dissolution. So my question would be,
represent considerable periods of erosion or at least
have you looked at the dissolution of these samples in,nondeposition. Do you feel that the deposition of thesay, oxygen-poor solutions to see what the solubility
conglomerates on these unconformities represents
is, strictly as a function of, say, the carbonate content,
very long periods of time or relatively short periods ofwhich ought to be fairly predictable? The other thingtime of actual deposition?
that I wonder about is the distribution of the U02 andvon Backstrom: Well, I think both because there are
the U03 in the uraninite. In other words, it may not besome very major breaks. Although we considered theuniformly distributed so that even these ratios may
Witwatersrand Supergroup as one formation, it is ac­
not tell us very much about the behavior of yourtually a large number of formations with some very
samples until we know how the oxidation states are
distinct angular unconformities. Also, when you take
distributed within the grains; in other words, it may
these various formations, again you find minor uncon­
already be armored with U03.
formities, in this case down to very short periods Grandstaff: The samples I took were chemicallyhomogeneous, at least to the microprobe. Obviously
where you have channel fillings and this sort of thing.
Irving Breger, U.S.G.S., Reston, Va.: Dr. vonyou cannot get at oxidation states with a probe; you
Backstrom and the previous speaker and otherhave to do this by titration with ferric sulfate, or fer­rous sulfate, but in X-ray, using diffraction, there wasmembers of the panel, can you tell me, since I don't
no line broadening or any suggestion that theknow, is the uraninite always associated with organic
uraninite might have been heterogeneous with respectmatter or can you find it without organic matter?
to U02-U03. Further, we took big uraninite crystals,von Backstrom: Is it always associated with organic mat­ter? I don't think so.
took off the outside layer and any gangue minerals
Schidlowski: Definitely not; I'll come back to that in myand anything that didn't look like uraninite. Then wetalk tomorrow.
crushed them so that the surfaces were all reasonably

D8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

fresh, fresh as one can make them, at any rate
geologically. We looked at the dissolution reaction inwater in those experiments. In a solution without car­bonate a scale of schoepite or some hydrated from of
U03 forms on the surface of the fragment. Oxidation
probably forms U03 *2H20, then carbonate attacks
this armored surface and pulls off the hexavalenturanium exposing another fresh surface, so the two
actually go on in steady state with respect to one
another.
Adams: I would still submit that the heterogeneity of thesamples wouldn't be demonstrated even by breaking

off the outsides, and it might not be surprising that therates of dissolution would be very difficult to correlatewith chemical parameters. Another comment I'd liketo make just in passing is that commonly when we
speak of uraninite and try to understand it either inthe conglomerates or the veins, we are rather casual
about precisely what we are talking about, in par­ticular what is the oxidation state of the uranium? And
if anyone would like to pursue that subject, I think it
would add a great deal to at least nailing down thechemical parameters of the systems we are trying tounderstand.

Examples That Illustrate Sedimentological Aspects of
the Proterozoic Placer Model on
the Kaap-Vaal Craton, Witwatersrand, South Africa
By W. E. L. MINTER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ - PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

PAPER

1161-E

CONTENTS
Page

Abstract ________________________________________________
Introduction ______________________________________________The present model __________________________________________
Alluvial-fan placers ________________________________________
Fluvial-fan placers ________________________________________
Proximal braided-belt placers __________________________________
Distal braided-belt placers ____________________________________
Conclusions _______________________________________________
References cited ____________________________________________
Discussion _______________________________________________

E1
1
1
2
2
2
3
4
4
6

ILLUSTRATIONSPage

FIGURES 1-7. Photographs of:
1. Subangular boulders in Ventersdorp Contact Reef _____________________________________
2. A thick layer of proximal Ventersdorp Contact Reef _________________________________—_—
3. Ventersdorp Contact Reef conglomerate interbedded with black shale __________________________
4. Sections from the B Reef of a channel at Freddies gold mine, Orange Free State Goldfield, as an example of a prox­
imal braided placer deposit: edge of B Reef channel deposit; edge of B Reef gravel bar; thin B Reef gravel lag;heavy-mineral concentration in thin B Reef gravel lag _______________________________-__
5. Longitudinal view of Vaal Reef paleochannel _________________________________________
6. Transverse view of Vaal Reef paleochannel edge _____________________________________
7. A polished slab of Vaal Reef __________________________________________________
in

E2
3
3
4
5
5
5

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXAMPLES THAT ILLUSTRATE SEDIMENTOLOGICAL ASPECTS OF
THE PROTEROZOIC PLACER MODEL ON
THE KAAP-VAAL CRATON, WITWATERSRAND, SOUTH AFRICA
By W. E. L. MlNTER

ABSTRACTIn the fluvial-fan model of Proterozoic placers in South Africa, midfan and fanbase environments are illustrated by the Ventersdorp Con­tact Reef, proximal braided-belt environments by the B Reef, distal
braided-belt environments by the Vaal Reef, and deltaiclike en­
vironments by sheets, ribbons, and pods at the extremities of theSouth and Basal Reefs. The nature of distribution of gold and uranium
in these various environments has been evaluated and the knowledge
successfully applied by the author to the valuation and practical miningof the reefs since 1965.

INTRODUCTION

My contribution is essentially sedimentological. I havedivided the contribution into five short papers so thatthey can be inserted at opportune intervals between
other papers to remind us that these samples come fromrocks and that we ought to view the results and shape
our thinking in the light of the sedimentologicalframework from which the samples have been derived.
In the conceptual model of gold-bearing placers on the
Witwatersrand, described by Pretorius (1966), alluvial
fanhead, midfan, flanks and fanbase environments were
included. Pretorius has more recently (1974) reworded"alluvial" to read "fluvial," but the fan terminology re­mains the same. During recent years a considerable
amount of detailed sedimentological research on manyof these "reefs" has been accomplished. In particular,
results of research by Armstrong (1965), Knowles(1966), Sims (1969) and Minter (1972), presented intheses to the University of the Witwatersrand, have,confirmed the applicability of Pretorius' model and haveprovided data with which to identify the sedimentary en­
vironments that prevailed. These principles have been
extensively and successfully applied to the evaluationand exploitation of many different Witwatersrand reefs
1 Geology Department, Anglo American Corporation, P.O. Box 20, Welkom 9460, South
Africa(Minter, 1969, 1970), and the results are recorded inseveral confidential company reports prepared between
1965 and 1973.
THE PRESENT MODEL

The preservation potential of placers in proximal en­
vironments along the upwarping edge of the intracratonic Witwatersrand basin was low sedimentologically because of transgressions and low structural­ly because of subsequent peripheral faulting. Converse­ly, the preservation potential of placers in distal en­
vironments was high. The distal extremities of placer
deposits have not been accessible, however, because oftechnical difficulties associated with mining diminishinggold contents at increasing depths. The full sedimentary
sequence in the model of a Witwatersrand reef has con­sequently been constructed from the economic parts ofvarious reefs that have been geologically preserved andexploited.
In essence, the present model describes the dispersalof sediment from eroding auriferous provenance areas
down dendritic systems of tributary channels into braid­
ed belts and finally into deltaic-shaped sand sheets, rib­bons, and pods. Placer deposits were developed in allthese environments, and the rock records display
characteristics appropriate to the various positions
down the paleoslope.
Witwatersrand reefs have been broadly classified into
two different groups, the bankets and the carbon-seam
reefs. The bankets were deposited in proximal en­
vironments and are composed of pebble-supported con­glomerates with detrital heavy minerals dispersed
throughout their matrix. The carbon-seam reefs were
sandbodies containing a minor small-pebble population
distributed as single-layered lags, as scattered pebbly
El

E2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

quartzites, and, rarely, as pebble-supported con­glomerates. Detrital heavy minerals were concentrated
at the base of the unit; lesser amounts were concen­
trated on sedimentary partings within the reef unit. The
carbon-seam reefs were deposited in distal en­
vironments. In both reef groups, the reef unit is definedby the maturity of its sand-sized sediment that is cleaner
than that in the sediments either above or below the reef
unit.
ALLUVIAL-FAN PLACERS

Preserved gold-bearing alluvial-fan deposits appear to
be confined to the Ventersdorp Group, which was large­
ly shed off reworked Witwatersrand horsts anddeposited into structurally controlled yoked basins. The
most extensive example is the Ventersdorp Contact
Reef in the Far West Wits Goldfield near Carltonville.
This deposit, described by Knowles (1966), is of thebanket type. At its proximal end, on West Driefontein, it
is composed of a thick accumulation of very coarse,
somewhat angular clasts (figs. 1, 2) with a mean size inexcess of -4 phi units. The largest clasts reported are
up to 36 cm in diameter. At a distance of 2.5 km down
the paleoslope from this position, the mean pebble size
has decreased rapidly to less than -3 phi units. The
standard error of these mean size measures is 0.02 phiunits.
Although the maturity of the reef indicates a wet
alluvial-fan rather than a dry alluvial-fan environment,
black shale interbedded in the conglomerate could be in­terpreted as playa-lake muds (fig. 3). Terracing or
multilevel channelling is evident, but insufficient areallyspread data are available to outline details of channel
patterns.
As in all bankets, the detrital heavy-mineral concen­
tration is closely related to packing density. Maximum
concentrations occur in densely packed (peb.blesupported) conglomerate, whereas pebbly quartzites
and quartzites of the reef unit are poorly mineralized.
Similar but less extensive wet alluvial-fan deposits
were shed off the Witpoortjie horst in the West Rand.
At South Roodepoort (Minter, 1970), reworked material
was shed southeast off the Roodepoort Fault scarp,
which dips 45°. About 6 km of the midfan part of thedeposit has been preserved. A very narrow fan was shednorthwest off the Witpoortjie Fault scarp, which dips
85° (Minter, 1971, personal files).

conglomerate layers are interbedded with quartzites toform a clastic cone of sediment, which is over 30 m thickat its proximal end and thins laterally and downslope,
within the area of economic interest, to about 15 m.
Over the exposed downslope distance of 2 km, the meanof the ten largest pebbles changes from 40 mm to 27
mm. The reefs that have been mined in this deposit areof the banket type.
PROXIMAL BRAIDED-BELT PLACERS

The most commonly preserved Witwatersrand reefs
were deposited in braided-belt environments. In theirmore proximal position they occur as channels averag­ing 50-100 cm deep with a channel index of as much as500. The channels, as in the B Reef at Freddies in the
Orange Free State Goldfield, were eroded into theunderlying footwall sediments and are separated byminor interchannel topographic elevations. The chan-FLUVIAL-FAN PLACERS

The Livingstone Reefs described by Steyn (1963) at
West Rand Consolidated on the West Rand have a
fluvial-fan geometry. As many as twelve gold-bearing

FIGURE 1.-Large subangular clasts of the Ventersdorp Contact Reef
in a proximal, wet alluvial fan, position at West Driefontein gold
mine near Carletonville on the Far West Witwatersrand.

THE PROTEROZOIC PLACER MODEL, THE KAAP-VAAL CRATON, WITWATERSRAND, SOUTH AFRICA

FIGURE 2.—A thick layer of coarse, poorly sorted Ventersdorp Con­tact Reef at West Driefontein, where the deposit is considered by
Knowles (1966) to represent the base of an alluvial fan.

nels, defined by their fill of more mature sediment, havewell-defined edges (fig. 4A). They are either filled withlarge pebble-supported conglomerate containing
detrital heavy minerals throughout the matrix or withmature pebbly quartzite or quartzite, in which detrital
heavy minerals are less well concentrated. The con­glomeratic fills in some instances are gravel bars (fig.
4B). In shallower channels only thin lags occur (fig. 4C),
overlaid by argillaceous quartzite. These lags may,
however, contain very high concentrations of detrital
heavy minerals (fig. 4D).
DISTAL BRAIDED-BELT PLACERS

Braided belts in a more distal position anastomose toform an apparent sheet of placer sediment, for example,
the Vaal Reef (fig. 5; see Minter, 1972). The channelled
bedforms are still evident (fig. 6), however, and indicate
the central position of the channelways. Channels are as

E3

FIGURE 3.-Large-pebble Ventersdorp Contact Reef conglomerate interbedded with laminated black shale, which in turn is channelled byprograding conglomerate and buried beneath amygdaloidal lava.
Western Deep levels gold mine near Carletonville on the Far West
Witwatersrand.

much as 100 cm deep but generally are less than 50 cm
and have a channel index of as much as 3,000. The placer
sediment in this more distal position is no longer apebble-supported conglomerate reef (banket) but is apebbly quartzite carbon-seam type of reef. The preserva­
tion of detrital heavy-mineral concentration was notdependent on physical protection by pebble layers(Minter and Toens, 1970) but, in the prevailing lowerenergy environment, the heavy minerals were depositedon sedimentary partings in the reef, usually the bottom
contact (fig. 7).
In the very distal positions, as in some South Reef ex­
posures at West Rand Consolidated in the West Rand
Goldfield and in the most easterly borehole exposures of
the Basal Reef in the Orange Free State Goldfield, thereef unit becomes a thin sand layer with occasional grits
or small pebbles (- 2.5 phi units) and a thin detrital layerof heavy-mineral concentrates. The reef unit is patchily
distributed in very distal environments, possibly taking
the form of pods and ribbons in deltaic-shaped areas.

E4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Armstrong, G. C., 1965, A sedimentological study of the U.K. 9
Kimberley Reefof the East Rand: University of the Witwaters­
rand, Johannesburg, unpublished M.Sc. thesis, 65 p.

Knowles, A. G., 1966, A paleocurrent study of the Ventersdorp Con­tact Reef at Western Deep Levels Limited on the Far West Rand:
University of the Witwatersrand, unpublished M.Sc. thesis, 125 p.
Minter, W. E. L., 1969, Areal and local models of gold distribution in
the Vaal Reef at Stilfontein gold mine, Klerksdorp, produced bymoving average surfaces [abs.], in Colloquium on trend surface
analysis in economic geology: Geological Society of South AfricaWitwatersrand University Economic Geology Research Unit,
Johannesburg, p. 7-8.
———1970, Gold distribution related to the sedimentology of a
Precambrian Witwatersrand Conglomerate, South Africa, as
outlined by moving-average analysis: Economic Geology, v. 65, p.
963-969.

B

D

CONCLUSIONS

Detailed sedimentological data from Ventersdorp andWitwatersrand reefs support a fluvio-deltaic conceptualmodel in which a fan-shaped geometry of distribution
with fan-head, braided-belt, and deltaic-sheet en­
vironments occurs.
REFERENCES CITEDFIGURE 4.—Sections from the B Reef of a channel at Freddies gold mine, Orange Free State Goldfield, as an example of a proximal braided placer
deposit. A, The edge of a B Reef channel; the conglomeratic channel-fill thins out against the channel bank to the left of the hammer. B, Theedge of a gravel bar in the B Reef; the gravel bar feathers out across the flat channel floor, which is evident at a position halfway down thehammer handle. C, A thin gravel lag of B Reef beneath crossbedded argillaceous quartzite. D, Closeup of a thin basal gravel lag of B Reef;
this illustrates the dense concentration of coarse-grained nodular pyrite and other detrital heavy minerals.

THE PROTEROZOIC PLACER MODEL, THE KAAP-VAAL CRATON, WITWATERSRAND, SOUTH AFRICA————1972, The sedimentology of the Vaal Reef in the Klerksdorp
area: University of the Witwatersrand, unpublished Ph.D. thesis,
170 p.
Minter, W. E. L., and Toens, P. D., 1970, Experimental simulation ofgold deposition in gravel beds: Geological Society of South Africa
Transactions, v. 73, p. 89-98.
Pretorius, D. A., 1966, Conceptual geological models in the exploration
for gold mineralization in the Witwatersrand basin, in Symposium
on mathematical statistics and computer applications in ore valua­tion: South African Institute of Mining and Metallurgy, Special
Publication, p. 225-275; also available as Witwatersrand Universi­ty Economic Geology Research Unit Information Circular no. 33,
38 p.

E5

———1974, The nature of the Witwatersrand gold-uranium deposits:
Witwatersrand University Economic Geology Research Unit, In­formation Circular no. 86, 50 p.
Sims, J. F. M., 1969, The stratigraphy and paleocurrent history of theupper division of the Witwatersrand system on President Steynmine and adjacent areas in the Orange Free State Goldfield, withspecial reference to the origin of the auriferous reefs: University of
the Witwatersrand, Johannesburg, unpublished Ph.D. thesis, 181
PSteyn, L. S., 1963, The sedimentology and gold distribution pattern of
the Livingstone Reef on the West Rand: University of the Wit­
watersrand, M.Sc. thesis, 132 p.

FIGURE 5.—Longitudinal view of three trough-crossbedded cosets in apaleochannel of the Vaal Reef placer, which was deposited in a distal
braided-stream environment. Hartebeestfontein gold mine in
Klerksdorp Goldfield. The horizontal arm of the rule, to the left ofcenter in the photograph, is 10 cm in length.

CM

FIGURE 6.—Transverse view of the edge of a paleochannel that has
anastomosed with adjacent shallow channelways in the distal
braided Vaal Reef placer, Stilfontein gold mine, Klerksdorp
Goldfield.

FIGURE 7.—A polished slab of Vaal Reef, which is a pebbly quartzarenite and is classified as a carbon-seam type of reef. The dense con­centration of nodular pyrite that is evident is accompanied by other
detrital minerals. The basal centimeter of reef, 2 cm above the baseof the specimen, contains 750 ppm gold while the remainder of thereef contains 35 ppm gold.

E6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESDISCUSSION

Skinner: Would it be possible for you to help me clarify
my thinking and perhaps some of the others by draw­
ing a cross section through one of these fans andperhaps putting what you think might be water depths
on it?
Minter: Water depths? Well, we try to make calculations
in connection with water depths using Alien's formulain which he bases the calculation on the actual scale ofthe trough crossbedding; one estimates that the water
depth was about one-half to one meter.
Skinner: Throughout the entire fan?
Minter: Yes. Well, an extensive area of ten miles wide
by ten miles downslope, the water depth probably is
about a meter, with some areas actually being
subaerial in the higher positions. In the proximal en­
vironments I think that the waters are primarily in thechannel ways, but in the distal braided belts I think
there was almost a continuous sheet of water withvery little sediment sticking out. I would quote
perhaps the Athabasca River, where you have exten­
sive braided belts of sand almost covered by water andvery shallow water conditions. Do you still want adiagram?
Skinner: No, no, that's all right. Then would most of thesedimentation and working be done during intermit­
tent floods, or do you envisage a continuous water
flow, as in the Athabasca model?
Minter: Continuous water flow. I have divided my con­tribution into a number of papers and I will elaborateon that later, but certainly one has a very unimodaltransport direction. It's a fluvial deposit with constantreworking and the velocity is such that one has a pi
crossbedding developed. It's a trough crossbedding,which is a fairly high velocity crossbedding whichmoves along in lunate sand dunes or linguoid sanddunes and continuously destroys its form and reworks
the sediment, thereby producing a very concentrated
heavy-mineral concentration at the base.
B. Nagy: I would like to ask a question regarding the
Athabasca model. The Athabasca River has fluvial
plains, some of which are basically sand and some ofwhich are basically clay. All of them are rather heavily
impregnated with organic matter. This does not seemto be the case here.
Minter: It is the form I am really referring to,
B. Nagy: Apparently the history is different. I just want

to clarify in my mind that in the Athabasca model thefine-grain sediments are the ones which contain in­digenous life rather than developing some kind of car­bonaceous matter later on. Is that what you have inmind?
Minter: Yes, possibly. I think of the environment real­
ly being a coarse sand with not very much finematerial, certainly not in the basal placer lode. Thereis very little fine material; I think the fine silicate
percentage is about 16 percent.
Nagy: I was talking about the clay-size fraction.
Minter: I use that [the Athabasca] as an example be­cause we have mapped out channel ways and this is
the configuration they have.
Simpson: You didn't describe where the carbon was in
the carbon-seam reefs. Could you tell us?
Minter: I am going to do that later. Is that all right?
Skinner: If you look at the entire basin downslope fromthe fans do you find enough clastic material to accountfor all the rest of the detritus that would have beeneroded from the source areas? Is all the rest of it
there?
Minter: Yes, I think so.
Skinner: You don't have some prior sorting in otherpathways for the material before it gets into the Wits
Basin?
Minter: I think that the sand was perhaps held in valleys
and flushed out. A lot of concentration perhaps took
place in valleys in the drainage area. I don't think
there's an outlet to the Wits Basin. I think that all thematerial is in there. Does that answer your question?
Skinner: Yes, that is what I was curious about.
Minter: These deposits have been described as very ex­tensive deposits but in fact when one takes themapart, one finds that they are in effect coalescing
deposits, and this often accounts for the difference ingold concentration laterally.
Hallbauer: What significance do you attribute to the oc­
currence of ventifacts in these deposits?
Minter: I have found ventifacts in the deposits but they
aren't in situ. They are upside down, some have facets
on both sides, they are very small pebbles, and all haveobviously been recycled. I can show you an example
later in my next paper where the sedimentation prior
to the deposition of one of these reefs produced adeltaic flat upon which pebbles could, and I am surewould, have been faceted.

The Dominion Reef Group,
Western Transvaal, South Africa
By J. W. VON BACKSTROM
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-F

CONTENTS
Page

Abstract ________________________________________________
Preamble ________________________________________________
General geological setting ______________________________________
Geology _________________________________________________
Mineralogy _______________________________________________
Relation of ore deposits to structures _________________________________
Source of the uranium ________________________________________
Depositional environment ______________________________________
Epilogue _________________________________________________
Selected references __________________________________________

Fl
1
2
3
4
5
5
7
7
8

ILLUSTRATIONPage

FIGURE 1. Map showing major geological features of the study area _______________________________________F2

TABLESPage

TABLE 1, Average uranium and gold values ____________________________________________________
2. Uranium values of upper Witwatersrand sedimentary rocks _____________________________________

Fl
6
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHE DOMINION REEF GROUP, WESTERN TRANSVAAL,
SOUTH AFRICA
By J. W. VON BACKSTROM1

ABSTRACTThe Dominion Reef Group, disconformably overlain by the Witwatersrand Supergroup, outcrops in four separate areas in the Western
Transvaal, Republic of South Africa. It consists essentially of a basal
sedimentary formation as much as 90 m thick, which contains two impor­tant uranium-bearing quartz-pebble conglomerates known as the Upper
Reef and the Lower Reef; an intermediate formation of altered andesitic
lava flows as much as 650 m thick with some intercalated beds of tuff;
and an upper formation of altered cherty rhyolites and dacites with athickness of at least 1,500 m. The basal sedimentary formation of the
Dominion Reef Group represents the earliest sedimentation in the region(>2,800 million years) predating that of the Lower WitwatersrandGroup (>2,700 million years).
This paper is devoted mainly to a description of the auriferous "reefs,"
their depositional environment, pay streaks that are a feature of these"reefs," their mineralogy, and the highly mineralised, pebbly, silicified
sandstone known as the Pay Band present near the top of the Upper
Reef.
The relation of ore deposits to structures, and the possible source of
the uranium, are discussed. Reasons are given why a hydrothermal
origin for the presence of uranium in the "reefs" is thought to beuntenable.

PREAMBLE

The Dominion Reef Group finds its major development
south of the village of Hartebeestfontein and around thevillage of Ottosdal in the Western Transvaal (fig. 1).
The Dominion Reef mine, situated 16 km south ofHartebeestfontein, started its life as a gold producer in1888 but, owing to low gold grades, was subsequently
closed down. It was reopened in 1936 as a gold producer,
the bulk of production coming from the lower Reef, which
initially yielded 5 g per metric ton; this yield stood at lessthan 2 g per metric ton when the mine closed down in
1953. A vigorous search for uranium was initiated in
1958, when sampling of the slimes indicated averagevalues of more than 0.46 kg per metric ton, and the mineacquired a new lease of life in 1959, when uranium pro­duction commenced and the Upper Reef was mined ex1 Director: Geology, Atomic Energy Board, Republic of South Africa.

tensively for uranium. When, because of overproduction,
the mine was closed down in 1961, more than 1,500 shorttons of U308 and 13 million rands (R) worth of gold had
been produced.
A recent press release (Rapport, 10 June 1975) gave theaverage uranium and gold values of different sections of
the Dominion Reef deposit south of Hartebeestfontein asshown in table 1.
TABLE 1. -Average uranium and gold values for different sections of the
Dominion Reef deposit
Values per metric ton
Area

Uranium
(kg)

Gold
fe)

Afrikander Lease
Rietkuil Section (part of Afrikander Lease) _Rhenosterhoek
KCG (Klerksdorp Consolidated Goldfields) _Dominion Reef
Vlakfontein
OttosdalLucaskraal (south of Ottosdal)

0.25
1.25
.50
.89
.80
.50
.34
.15

2.5
.7
1.5
4.5
.4
Trace
Trace
Trace

Very limited drilling (13, 11, and 2 boreholes, respec­tively) was undertaken in the three areas named last on
table 1; the results have been known since 1972 (vonBackstrom, 1972).
Between 1966 and 1970, exploration by means of coredrilling of the uraniferous conglomerates present in thebasal sedimentary rocks of the Dominion Reef Group wasundertaken. Then and during earlier exploration, 230
boreholes, with a total of 31,980 m, were completed by
the mining industry. Boreholes range in depth from lessthan 100 m to more than 1,000 m. Large deposits ofuranium of low to medium grade have been proved.
Associated gold values are very low. There are in ex­
istence two developed mines, which have been closed
down, but a feasibility study is under way to reopen themand other possible uranium producers.
FlF2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXPLANATIONSANO AND SOIL COVER
VENTERSDORP SUPERGROUPWITWATERSRANO SUPERGROUPBASAL SEDIMENTARY FORMATIONANOESITIC AND ACID LAVASDOMINION REEF

GROUP

ARC HE AN GRANITE
MAJOR FAULTSKLERKSDORP CONSOLIDATEDGOLDFIELDS
^ 4

4

41

44

4

44-444444444

444
4 4^ 444
444
4
4444
4
4
44°l44-4444444444
444.444444
44444
4
444\4444444
444.4
444444444
444\444444444
4444444444
444\444444
44444444
444-1-44
OTTOSDAL ,,W->/ 4
4
4
4
4
AREA
+
~r
4444
V 4
DOMINION REEFS 4
/4
4444
4/44
AREA
f
4
444
4U444 EXTENSION+

444444444444444)4444444444444444

44444444f444444444444

21024

6

8

»

12

U

16

18

20 KILOMETERS

FIGURE 1.—Major geological features of study area.

km wide bounded by the large northeast-striking
Rietkuil and Hartebeestfontein faults.
South of Hartebeestfontein in the Western Transvaal, 2. The Dominion Reef area forms a quasi-synclinal
granite of the Archean Complex is unconformably
structure plunging to the southwest. The
overlain by quartzites and conglomerates, which
sediments crop out over a north-south strike
together form the basal section of the predominantly
distance of at least 10 km and continue westward
volcanic sequence of the Dominion Reef Group. Thesean additional 15 km.
rocks are in turn disconformably overlain by shales and 3. The Vlakfontein area is almost entirely covered byquartzites of the Lower Witwatersrand Group. The
lavas of the Ventersdorp Supergroup, so that only
lavas and volcano-sedimentary rocks of the Ventersdorp
two small outcrops of Dominion Reef rocks appear.
Supergroup cover the above-mentioned flat-lying or in­
The area is bounded by the Rietfontein andclined older rocks, which are eroded to varying
Vlakfontein faults. The latter fault has no surface
stratigraphic depths.
manifestation; its presence was proved by drilling.
Although part of a single depositionary unit, rocks of 4. The basal sediments in the Ottosdal area, although
the Dominion Reef crop out in four separate areas, eachbroken by a number of cross faults, can be followedof which can be considered a structural entity (fig. 1):
along the line of strike for 20 km north of thevillage before they are again covered by
1. The Afrikander Lease area constitutes a fault block 7
GENERAL GEOLOGICAL SETTING

THE DOMINION REEF GROUP, WESTERN TRANSVAAL, SOUTH AFRICA

Ventersdorp lavas. South of the village, the basal
sediments crop out south of the Strydpoort fault.
GEOLOGY

The Dominion Reef Group consists of three members:
1. a basal sedimentary formation as much as 90 m
thick, consisting of conglomeratic, arkosic, andquartzitic rocks with subordinate shale bands;
2. an intermediate formation as much as 650 m thick,consisting of altered andesitic lava flows with inter­calated beds of tuff; and3. altered cherty rhyelites and dacites at least 1,500 m
thick, with intercalated tuff bands (known as
Wonderstone near Ottosdal) and schists and minor
breccias at the top.
Only the basal sedimentary formation, which contains
uranium and gold, is described further.
The basal formation is of economic importancebecause its conglomerate beds are auriferous; it has
been exploited in the past for gold and uranium. Exten­
sive resources of uranium of good grade were proved,
but associated gold values are low and few developed
mines exist. The economic viability of such mines would
be dependent on a satisfactory and sustained uranium
price based on long-term contracts.
The basal sedimentary formation of the Dominion
Reef Group rests unconformably directly on the base­ment granite and represents the earliest sedimentationin the region (>2,800 m.y. (million years)), predatingthat of the Lower Witwatersrand Group (>2,700 m.y.).
The thickness of the basal formation varies from 3 to 50
m south of Hartebeestfontein to 90 m north of Ottosdal(fig.l).
Many deep boreholes have proven that intercalated
sediments, from 60 cm to 40 m thick, occur in the first
60 m of the overlying andesitic lava.
The basal sedimentary formation consists largely ofthe disintegration products of the basement granite.
The sediments consist of medium-grained to coarse­
grained feldspathic and somewhat micaceous sandstoneor quartzite, in which there are lenticular bodies of gritand conglomerate. Evidently, the micaceous or sericitic
material in the rocks was derived from the breakdown of
the original detrital grains of feldspar. A conglomerate
along or close to the base of the arenaceous sediments
and another higher up in the succession, both of whichhave been worked or prospected for gold and uranium,
are known as the Dominion Reefs. The thickest bodies ofconglomerate occupy depressions in the old granite
floor, whereas the conglomerates are absent on humps.
Usually the lowest bed of conglomerate is separated
from the underlying granite by a band of sheared
granite or by a variable thickness of transition beds com­prising one or more of the following rock types: arkose,

F3grit, and quartz-sericite schist. Where there is no basal
conglomerate, feldspathic sandstone often grades im­perceptibly into the granite below. In many prospecting
boreholes the contact between sandstone and granite
could be established only radiometrically.
Two main auriferous conglomerate zones known as"reefs," separated by barren conglomerate or quartzite,
are present in the basal formation, which can be divided
into (1) a lower quartzite and conglomerate member,from 0 to 40 m thick, containing the Lower Reef; and (2)an upper quartzite and conglomerate member, between
4 and 50 m thick, containing the Upper Reef.
The Lower Reef, a lenticular, well-developed largepebble conglomerate, was the main gold carrier in themined area. It occurs sporadically at the base of thelower member, where it occupies depressions in the ero­sion surface of the basement granite. It shows marked
variation in texture where it rests on irregular portionsof the pre-Dominion erosion surface and more uniformi­
ty where the basement is fairly even. On the whole, theconglomerate thickens and pinches in a manner reflect­
ing the surface on which it was deposited, and depositionand preservation were controlled by depressions in thefloor. The thickness ranges from 0 to more than 3 m,
and pebbles vary in diameter from less than 5 to morethan 10 cm. Near the base, the Lower Reef is often poor­ly packed and sorted and merges into a pebbly quartzite.
The packing index improves at the top, where there isoften the best concentration of heavy minerals, in­cluding uraninite. The quartzite is medium grained tocoarse grained and sericitic, the sericitic being derived
from the feldspar of original arkosic sediments.
There is no significant angular unconformity between
the upper and lower members and, apart from channel­
ling, there is little differential erosion of the lower
member.
The upper member is developed throughout almost
the entire area. South of Hartebeestfontein it ranges
from 4 to 30 m thick; north of Ottosdal it is a maximumof 75 m thick. The Upper Reef, taken as the basal bed of
the upper member, consists in fact of several con­glomerate bands, which vary from poor to good in pack­ing and sorting. The conglomerate bands may rest on
any unit of the lower member. Pebbles range in size
from 2 to 5 cm, are well rounded to subrounded, andconsist almost exclusively of quartz. They may repre­
sent reworked and concentrated detrital material de­
rived from the erosion of earlier conglomerate andarenaceous rocks. The Upper Reef normally ranges
from 0.15 to 3 m thick; in some places it is as much as 5m thick.
The conglomerates of both the Lower and Upper
Reefs closely resemble the Witwatersrand banket andare likewise composed of pebbles of vein quartz set in a

F4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

dark-grey, highly siliceous, fine-grained matrix. The
pebbles show variations in their packing index. They areusually not closely packed, and sometimes only isolated
pebbles are present in the fine-grained matrix. The peb­bles are often severely sheared and fissured and are also
replaced along their edges and along fissures by the sur­rounding matrix materials. In some areas pebbles arewell sorted, the larger ones being at or near the base,and are oriented with the largest axis parallel to thedirection of strike. Some are split in half along their cen­tral axis parallel to the dip and are slightly displaced.
The matrix is sporadically mineralised by pyrite, andveins of pyrite have also been observed in fissures in thepebbles. In weathered conglomerate the matrix is
stained by yellow-brown oxidation products of hydrated
iron oxide and leucoxene.
Pay streaks similar to those found in alluvial deposits
are a feature of these reefs, but unfortunately there isno general uniformity in grade, and the distribution of
the uranium minerals is neither characteristic nor asignificant feature everywhere. There appears to be aconnection, however, between the distribution of
detrital minerals like monazite and cassiterite and thatof uraninite in the pay streaks. The pay streaks appear
to have been formed by stream action and not by the in­troduction of lateral mineralising solutions. Features
such as the alteration of wall rocks or uranium
mineralisation crossing the stratification were notobserved, and cross-cutting quartz, calcite, or otherveins that might have acted as feeders are conspicuously
absent.
It would appear that conditions during and after
deposition of the sediments were such that they permit­
ted the formation of a thin upper reef or, in places, analmost purely black sand well endowed with a full rangeof heavy alluvial material such as uraninite, monazite,
cassiterite, zircon, and garnet. This reef is completely
different in appearance from the main conglomerate
body and is amazingly consistent, the average thickness
being about 13 cm.
Although this reef always carries uraninite, a very
wide range of values is encountered. The concentrations
occur in well-defined bodies of considerable width, withvery little internal low-grade ore. These so-called pay
streaks vary in width from 130 to more than 300 m, andno high values have been encountered outside theseshoots. Recognition of the above-mentioned feature isnaturally of extreme importance where mining opera­
tions are concerned. As far as could be established,these streaks all follow a northeast trend and represent
ancient shorelines or a channel system under deltaic
conditions.
The width and consistency of the streaks seem to in­
dicate continuity down to considerable depths and, in­deed, the auriferous shoot in the Lower Reef, which has
supplied Dominion Reef Mines with ore for 18 years,
was mined to a vertical depth of 800 m. As mentioned
above, the average thickness of the mineralised Upper
Reef is approximately 13 cm, and there are well-definedpartings above and below it. This allows the applicationof mining methods that, together with ore sorting,should permit a low-tonnage but high-grade mill feed.
An extraordinary concentration of heavy minerals oc­
curs in an argillaceous, gritty, pebbly, silicified sand­stone known as the Pay Band, near or at the top of theUpper Reef, from which it differs completely. It is
generally less than 0.3 m thick, but can reach 1 m inthickness. It extends from the eastern part of the
Afrikander Lease Area westward to the Hartebeestfontein fault along an east-west trending belt 300 to 900 m
wide. The concentration of uraninite is usually three tofour times that in the best zone of the Upper Reef con­glomerate. Where the band is thin (20-50 mm), concen­
trations of uraninite may be as high as 1 percent; other­wise the Pay Band is virtually all detrital monazite andcassiterite, with minor amounts of detrital chromite,garnet, zircon, ilmenite, and columbotantalite.
MINERALOGY

The mineralogy has been previously described inreports by Liebenberg (1955, 1956, 1958, 1960), Malan(1959), Ortlepp (1962), Taylor, Bowie, and Home (1962),
and especially Hiemstra (1968), who gave a detailed
description of the following minerals:
1. Radioactive minerals: thorian uraninite, thorite,
monazite, betafite, euxenite, leucoxene, and zircon
2. Sulphides: pyrite, marcasite, galena, chalcopyrite,
covellite, sphalerite, cobaltite, linnaeite,
arsenopyrite
3. Metal: gold
4. Silicate minerals: garnet, pyroxene, epidote, sphene,
biotite, chlorite, sericite.
. Uraninite, the principal radioactive mineral present,
occurs as oval and roundish detrital grains similar in all
respects to those in the Witwatersrand banket. Accor­
ding to Liebenberg (1955), the* * *galena is ubiquitous in the uraninite and much more common thanin the banket of the Witwatersrand. The detrital shape of the majorityof the grains has been retained, but some are reconstituted, and specks
of uraninite are redistributed throughout the quartz-chlorite-sericite
matrix immediately surrounding the uraninite. Minute specks of secon­
dary uraninite also accompany the partly reconstituted grains of
detrital uraninite. Contamination of uraninite by sulphides, other thangalena, and by gold is rare. The grains of uraninite are slightly larger
than those in the Witwatersrand banket and are also well graded.

Monazite is one of the most abundant heavy minerals
in the Upper Reef, and it is preferentially associatedwith uraninite. In the Lower Reef the concentration isTHE DOMINION REEF GROUP, WESTERN TRANSVAAL, SOUTH AFRICA

much lower. In thin sections, the monazite is to be seen
as small grains, many of them well rounded, of the orderof 0.1 mm in diameter and always traversed by cracks.
All samples tested contained some monazite; the ore
contains about 12 percent monazite.
RELATION OF ORE DEPOSITS TO STRUCTURES

The distribution of uraninite and gold in the con­glomerate is intimately related to certain lithologicaland structural features of sedimentary origin.
Mineralisation is confined to the ore-bearing con­glomerate and seldom ever crosses the stratification in­to the arenaceous wall rock, except where it has obvious­
ly been derived from the conglomerate ore body itself.
There is a close relation between the distribution ofvalues and purely sedimentary features. Richer concen­
trations are commonest near the base of the con­glomerate along well-defined shaly or schistose footwall
partings.
Both uraninite and gold can, in general, be said to be
more or less uniformly and regularly distributed in theconglomerate reefs when the whole extent of the deposit
is considered. However, when comparison is drawn withthe much greater variation in the distribution of values
shown by nearly all lode deposits, there are variations,
often marked, in uranium content within the limits ofeach locality (fig. 1). The richer uraninite concentrations
tend to collect in pay streaks, usually characterised by acloser packing of well-rounded and sorted pebbles com­posed mainly of quartz and by a greater concentration ofother heavy-mineral particles including those of detrital
origin. Except for local deviations, the pay streaks
generally run roughly parallel to the original shoreline.
Enrichment in gold and uraninite is usually more pro­nounced where the conglomerate reefs rest on intraformational disconformities. From the information
available, this would seem to be the case particularly
where these conglomerates occupy depressions orhollows in the floor of deposition, or where they lie
against slight barriers across the floor, which acted as
traps.
In some areas, high values are present in welldeveloped conglomerate filling depressions that are
relatively shallow and have gentle slopes. Where thedepth of a depression becomes excessive, and especiallywhere its marginal slope suddenly steepens, con­glomerate development in the centre of the depression ispoor compared with that along its margins. The
marginal portions of such depressions are characterised
by well-developed and closely packed conglomerate
generally containing high values of gold and uranium,
whereas the central portion of the depressions is usually
filled with relatively thin conglomerate bands separated
by barren medium-grained and gritty quartzite.

F5

The general distribution of payable concentrations ofgold and uranium in the conglomerate is parallel to theoriginal shoreline of the sedimentary basin as inferredfrom the spread of the coarser sediments. Richer con­centrations in the conglomerate tend to be disposed in
pay streaks, which have many of the characteristics of
those found in modern placers, and a similarity in modeof deposition is strongly indicated. The distal part of anore-bearing conglomerate sheet may break up into
isolated lenticular bodies, which diminish in size andnumber until ultimately they are spaced too far apart for
profitable mining. There is a decrease in pebble size andmetal content outwards from the point of entry into thedepositionary basin.
In summary, uranium, which is intimately related togold, occurs in bedded conglomerate and quartzite near
the margin of a continental structural basin that issituated well within the boundaries of what is regarded
as a uranium province. It is assumed that the uranium
and the gold were deposited together as an integral partof the sedimentary series, but after the conglomerate
was laid down, and that they were transported no very
great distance, by water, from an original source (still
unknown) to the north and west.
SOURCE OF THE URANIUM

There are two obvious possible primary sources of theuranium: (1) older rocks from whose destruction were
derived the materials that form the shale, quartzite, andconglomerate of the succession; and (2) fluids contain­
ing uranium that penetrated the sediments during or
after their deposition and from which the oxide was
precipitated.
The sedimentary rocks were deposited in a large intracratonic basin whose boundaries have not yet been
precisely defined. If we consider the Dominion Reef
Group, with its preponderance of lava over sediment, aspart of the Witwatersrand Supergroup, and if we accept
the correlations of both the partly exposed rocks of the
Godwan beds of the Uitkyk Formation and the Godwan
beds of the Derdepoort strip near the TransvaalBotswana border with the lower beds of the super­group, it is clear that this basin of deposition diminishedconsiderably as time progressed.
The material that was transported into the basin musthave been derived primarily from the exposed parts ofthe basement complex—from the sediments andmetamorphosed sediments of the Kheis and Swaziland
Supergroups (sensu lato) and from the magmatic rocks
intruded into them—mainly granites and pegmatites,
the so-called "Old Granite." In such a basin, coarser andheavier sediments would tend to have been deposited
close to the shores, the finer and lighter material in

F6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

deeper water. As the area covered by water shrank, the made." His information, as cited by Nel, is summarisedcoarser equivalents of the Dominion Reef, Hospital Hill, by the author in table 2.
Government Reef, and Jeppestown Groups would be ex­
The important, but at present unknown, factor con­posed and could have contributed detritus to the overly­ cerning these cores is the actual average uranium con­ing Main-Bird and Kimberly-Elsburg Groups. Moreover, tent of the sections which carry less than 0.003 percent.
there is abundant evidence in the areas that are being Simpson's borehole logs of radioactivity (1951) show
mined that fluctuations of water level within the basin adequately that the radioactive content of quartzitic
and local tectonic movements led to considerable members of the succession varies from layer to layer andreworking of already deposited material.
from hole to hole. A few random specimens have yield­
If the uranium now present in the conglomerates were ed, on assay, values of up to 0.001 percent U308, but it
syngenetic, it could have been carried into the basin would be hazardous to accept this, or any other figure,either in solid form or in solution, or both. But having ar­ as an average of all material carrying less than 0.003
rived there, it had to remain—subject to the natural percent in order to arrive at an average for the wholeprocesses of decay and, if in solution, of groundwater succession in a given area. A significant fact revealed by
movement.
these figures is that the important concentrations of
The source rocks of the Witwatersrand Supergroup uranium are to be found only in certain narrow zones,
and the Dominion Reef Group are known to be which usually form less than 5 percent of the whole Up­mineralised, and their mineralisation is almost wholly per Witwatersrand Group—the average usually being
pre-Witwatersrand in age. Part of the mineralisation is about 2 percent. The remainder is not barren anda uranium mineralisation. The question naturally arises, generally shows as "normal" on the radiometric logs,
therefore, whether the quantity of uranium present in which is higher than the "normal" of the igneousthese beds, and particularly in the beds above the Jep­ material that intrudes the sediments. But there is not, inpestown Formation, of which we have more detailed this 95 to 98 percent of the sediments, anything like theknowledge, is greater than could be accounted for on the concentration found in the other 2 to 5 percent. What isassumption of a syngenetic origin.
true and significant, however, is that the average
Any attempt to answer such a question involves the uranium content would appear to be much higher thanconsideration of a number of factors of which our that of the many billions of metric tons of granite in theknowledge is very scant. Among these factors are the world which, on average, contain 3.5 ppm uranium andtotal original extent of the sediments of the succession, which have been mooted as very low-grade potential
the degree to which earlier beds in it were eroded and in­ uranium resource rocks.
corporated into later beds, the average uranium content
With regard to the uranium content of the graniteof the source rocks, and the average uranium content of basement, the presence of the pre-Witwatersrandthe existing remnants of the system.
granitic rocks in an overturned sequence of strata at
Only for the last of these factors have we sufficient in­ Vredefort afforded a unique profile covering a con­formation available to permit a rough approximation to siderable vertical section of the upper and middle crust.
be made. This information is derivable from assays of Uranium determinations of the "normal" granite under­
core samples from comparatively few of the hundreds of taken by Hart, Nicolaysen, and Gale (in press) averaged
boreholes that have been drilled in the search for 2.45 ppm (range, 1.03-3.93 ppm) or 0.000245 percent.
auriferous reefs in the Upper Witwatersrand beds. Nel The outermost margins of the central core, however,(1958) states that he obtained "information on uranium showed much higher values, 20-30 ppm, or 0.002 anddistribution in a vertical direction" from unpublished 0.003 percent uranium. The uranium concentration
notes by S. H. Haughton, formerly in charge of the found in 2 to 5 percent of the sediments of theGeological Unit. Haughton reported that "physical Witwatersrand Supergroup is thus from 3 to 70 times
assays of cores from certain boreholes logged in the greater than that present in the underlying basement
Orange Free State and Klerksdorp Goldfields have been granite.
TABLE 2.— Uranium values of upper Witwatersrand sedimentary rocks
Area
Klerksdorp Townlands.
Potchefstroom
Ventersburg OFS_
1 Cutoff. 0.003 percent.

Formation

Core
(m)

Mineralized(percent)

Upper Witwatersrand, Gold Estates
Group, Vaal Reef, Ada May Reefs,
and Commonage Reefs.
Upper Witwatersrand _________
Upper Witwatersrand, Footwall
quartzite, and Basal Reef.

4,943

5.29

7,035
4,068

2.8
1.33

Number ofholes

Equivalent
percent
U3(V

.080
.170

THE DOMINION REEF GROUP, WESTERN TRANSVAAL, SOUTH AFRICADEPOSITIONAL ENVIRONMENT

Since Witwatersrand gold was first discovered in
1886, there has been no unanimity amongst geologists
as to the conditions under which Dominion Reef and
Witwatersrand sedimentary rocks were formed, as iswitnessed by the large number of papers propounding
the many different theories. The reader is referred tothe excellent recent review by Pretorius (1975) of thislarge volume of information, which he summarised asfollows:
A review has been undertaken of the very large volume of informa­tion that has been published in the 89 years since gold was first
discovered on the Witwatersrand.
The literature has been surveyed for opinions voiced and theories
propounded for the nature of the depository in which the sediments
were laid down and the environmental conditions under which thegold- and uranium-bearing reefs were formed. The review has not been
directed towards examining the arguments for and against the various
hypotheses that have been advanced concerning the source of themetals and the mineralising processes that emplaced them in the con­glomerates, quartzites, and carbon seams. It is considered that about
90 contributions to the literature can be ranked as having added
significantly to a better understanding of the depositional environ­ment. It is apparent that, in the early period of mining operations,speculation favoured the conglomerates to have been formed on abeach. As limited amounts of quantitative data become available,
preference was given to a deltaic and, later, a valley-flat environment.
With the advent of detailed sedimentological studies, conclusions
became far more objective, and systematic observation replaced sub­jective speculation. The result is that a fair degree of consensus now
prevails on deposition having taken place along the interface between
a fluvial system that brought the sediments and heavy minerals froman elevated source-area to the northwest and a lacustrine littoral
system that reworked the material and redistributed the finer
sediments along the shoreline. The goldfields were formed as fluvial
fans that built up at several points along the northwestern periphery of
the intermontane, intracratonic lake, or shallow-water inland sea.
Each fan was the result of sediment accumulation at the mouth of ariver, which discharged through a canyon and flowed across a relative­
ly narrow piedmont plain, before entering the basin. The review has
also covered ideas on the origin of the carbon that is present in thesediments, since it has now been shown to be an important en­
vironmental indicator. Evidence is strongly in favour of the material
representing the remains of algal colonies that flourished, at certain
times only, about the mouths of the rivers. There is a suggestion thatthe plantlike forms might even have been more akin to lichens. Other
ideas are subscribed to by many geologists, and there can be little
doubt that, when the centenary of the Witwatersrand goldfields iscelebrated in 1986, disputes will still be raging as to how and where theauriferous sediments formed.

Hallbauer (1975), with the aid of the necessary scien­tific instrumentation developed in recent years,
established that a well-differentiated plant life existed
during Witwatersrand times, including bacteria, algae,
fungi, and lichenlike plants.
Sharpe (1949) first put forward the idea that the car­bon found in the banket might be the remains of some ofthe earliest forms of life.
Liebenberg (1955) emphasized that thucholite is very
scarce in the Dominion Reef and could hardly have

F7

played any significant role in the deposition of theuranium and gold in these rocks.
EPILOGUE

With artists' licence, supporters of hydrothermal in­
troduction paint impressive and interesting pictures of
the amazing ability of mineralising solutions to movefrom intrusive igneous bodies deep into sedimentaryrocks, without leaving any trail, until the site of deposi­tion is reached, and then to select, in an uncanny man­ner, only certain beds in which to deposit their load ofuranium. Yet, as far as the distribution of uraninite andgold in the Witwatersrand Supergroup and contiguous
formations is concerned, there are many features whichhydrothermalists do not seem to have been able to ex­
plain satisfactorily—features, for example, such as theabsence of ore tracts between a hydrothermal source
and the distant sites of ore deposits, the more or lessuniform and consistent distribution over hundreds ofsquare kilometres of gold and uraninite in the economic
conglomerate reefs, the fact that the mineralisation
does not cross from these conglomerates into quartzite
partings and adjacent beds (including other con­glomerates), the lack of mineral zoning and alteration ofwall rocks—all of which features tend to refutehydrothermal lode deposits.
Notwithstanding detailed regional mapping, the vast
amount of deep and extensive mining from the West
Rand to the Orange Free State goldfields, andwidespread prospecting (including the large number ofprospecting boreholes, some of which are as much as
3,500 m deep), no intrusive bodies have been foundwhich could have acted as a source of radioactive
mineralisation solutions. But hydrothermalists con­fidently believe that there are such hidden or unknown
sources deep down in, or elsewhere in other rocks
beyond, the Witwatersrand Basin—no matter how far
away. In passing from these unknown sources, themineralising solutions have left no trace of their paths
through and across the succession of four supergroups(the Dominion Reef, Witwatersrand, Ventersdorp, andTransvaal) with a total thickness of at least 18,000 m(perhaps even much more), assuming, as postulated byhydrothermalists, that the mineralisation solutions wereintroduced in post-Transvaal time. No channels along
which these solutions could have been introduced havebeen found. Hydrothermalists therefore contend that allthese formations were simply permeated by themineralising solutions. In whichever way the solutions
were introduced—whether along channels or by permea­
tion—they completely disregarded favourable sites fordeposition of their load of minerals, for example,
permeable volcanic tuffs in the Witwatersrand andF8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Ventersdorp Supergroups, volcanic breccias in the lat­ter, and breccias in the Dolomite Formation of theTransvaal Supergroup. Instead, the solutions selected
the lenticular Black Reef and Ventersdorp Contact
Reef, as well as certain preferred conglomerate beds inthe Witwatersrand Supergroup, which form some 2 per­cent of the total number of conglomerates in the Upper
Division of this Supergroup. The mineralising solutions
also selectively mineralised the stratigraphic
equivalents of the ore reefs in areas where the con­glomerates are not continuous sheets but are lenticular
patches that become smaller and smaller and are spaced
many kilometres apart. The solutions even selectively
mineralised the lenticular conglomerates at the base of
the Dominion Reef Group.
The age of the mineralisation postulated by hydrothermalists, namely, post-Transvaal, is definitely suspect on
geological grounds. In barren Ventersdorp con­glomerate and volcanic breccia, well-mineralisedboulders of Witwatersrand rocks have been found,showing that the gold and uraninite were already in fully
consolidated and cemented conglomerate before
Ventersdorp time. The principal economic conglomerate
reefs in the Witwatersrand Supergroup were cut bystreams during intraformational sedimentary breaks.
Several exposures underground reveal islands or rem­nants of reef completely surrounded by impervious andbarren argillaceous channel fillings. The tenor of thereef in the islands is the same as that of the broken reefon opposite sides of the channels. Boulders of reefs cut
by transgressive stream channels have also been record­
ed in barren channel fillings. Such channel fillings,
which consist of conglomerate or quartzite, carry higher
or lower values than the principal reefs across which
they cut. Such occurrences can reasonably be inter­
preted as indicating that the gold and uraninite were
present in the conglomerates before they were cut bystream channels during intraformational breaks in
sedimentation.
SELECTED REFERENCES
Backstrom, J. W. von, 1952, The Dominion Reef and Witwatersrand
Systems between Wolmaranstad and Ottosdal, Transvaal:
Geological Society of South Africa Transactions v. 55, p. 53.
———1962, Die geologic van die gebied om Ottosdal, Transvaal [TheGeology of the Area around Ottosdal, Transvaal]: South Africa

Geological Survey, explanation of sheet 2625D and 2626C, 63 p.,English summary.
-1972, Some favourability criteria in the search for uranium inthe major sedimentary formations of South Africa: Atomic Energy
Board Report PEL-212, ISBN 0 86960 316 7.
Hallbauer, D. K., 1975, The plant origin of the Witwatersrand"carbon": Minerals Science and Engineering, v. 7, no. 2, p. 111-131.
Hart, R. J., Nicolaysen, L. 0., and Gale, N. H., in press, Radio-element
concentrations in the deep profile through Precambrian basement of
the Vredefort Structure: Journal of Geophysical Research.
Hiemstra, S. A., 1968, The mineralogy and petrology of theuraniferous conglomerate of the Dominion Reefs Mine, Klerksdorp
area: Geological Society of South Africa Transactions, v. 71, pt. 1, p.
1-65.
Koen, G. M., 1958, The attrition of uraninite [with discussion]:
Geological Society of South Africa Transactions, v. 61, p. 183-196.
———1961, The genetic significance of the size distribution of
uraninite in Witwatersrand bankets [with discussion]: GeologicalSociety of South Africa Transactions, v. 64, p. 23-54.
Liebenberg, W. R., 1955, The occurence and origin of gold and radioac­
tive minerals in the Witwatersrand System, the Dominion Reef, theVentersdorp Contact Reef and the Black Reef [with discussion]:
Geological Society of South Africa Transactions, v. 58 p. 101-254.
———1956, A mineralogical approach to the development of theuranium extraction processes practised on the Witwatersrand:South African Institute of Mining and Metallurgy Journal v. 57, p.
153-208.
-1958, The mode of occurence and theory or origin of theuranium minerals and gold in the Witwatersrand ores: U. N. Interna­
tional Conference on the Peaceful Uses of Atomic Energy, 2d,
Geneva, Switzerland, 1958, Proceedings v. 2, p. 379-387.
-1960, On the origin of uranium, gold and osmiridium in the con­glomerates of the Witwatersrand goldfields: Neues Jahrbuch fur
Mineralogie, Abhandlungen, v. 94 (Festband Paul Ramdohr), p.
831-867.
Malan, S. P., 1959, The petrology and mineralogy of the rocks of the
Dominion Reef System near Klerksdorp: Univ. of the Witwaters­
rand, Unpublished dissertation.
Nel, L. T., 1958, The occurence of uranium in the Union of South
Africa: U. N. International Cjonference on the Peaceful Uses ofAtomic Energy, 2d, Geneva, Switzerland, 1958, Proceedings v. 2.
Ortlepp, R. J., 1962, On the occurence of uranothorite in the Dominion
Reef Geological Society of South Africa Transactions, v. 55, pt. 1, p.
197-202.
Pretorius, D. A., 1975, The depositional environment of the
Witwatersrand goldfields: a chronological review of speculations andobservations: Minerals Science and Engineering, v. 7, p. 18-47.
Sharpe, J. W. N., 1949, The economic auriferous bandets of the Upper
Witwatersrand beds and their relationship to sedimentation
features: Geological Society of South Africa Transactions v. 52, p.
265-288.
Simpson, D. J., 1951, Some results of radiometric logging in theboreholes of the Orange Free State goldfields and neighboring areas:
Geological Society of South African Transactions, v. 54, p. 99-133.
Taylor, K., Bowie, S. H. U., and Home, J. E. T., 1962, Radioactive
minerals in the Dominion Reef: Mining Magazine, London, v. 107,
no. 6, p. 329-332.

The Crossbedded Nature of Proterozoic
Witwatersrand Placers in Distal Environments anda Paleocurrent Analysis of the Vaal Reef Placer
By W. E. L. MINTER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

PAPER

1161-G

CONTENTS
Page

Abstract ________________________________________________
Introduction _______________________________________________
Crossbedded internal structure ___________________________________The Vaal Reef _____________________________________________
Spatial distribution of detrital heavy minerals ___________________________
Conclusion _________________________________________________
References cited ____________________________________________
Discussion following papers by Drs. von Backstrom and Minter _________________

Gl
1
1
2
8
8
9
9

ILLUSTRATIONS
FlcuREsl-4. Photographs of:
1. Footwall quartzite at Free State Saaiplaas in the Orange Free State Goldfield, showing a longitudinal view of troughcrossbedded sets, the external geometry of each set, and the concentration of dark heavy detrital minerals on
the toes of foresets _____________________________________________________
2. Footwall quartzite at Free State Saaiplaas in the Orange Free State Goldfield, showing a transverse view of troughcrossbedding to illustrate the slight discordance of the curved foresets _______________________
3. Two oblique, transverse views of very large scale trough crossbedding in proximal Steyn Reef at Western Holdings
Mine, Orange Free State Goldfield ____________________________________,_______
4. Distal Carbon Leader Reef, Blyvooruitzicht, Far West Wits Goldfield, illustrating a longitudinal view of the lowest
coset of trough crossbedded quartzite with dark detrital heavy-mineral concentrations ______________
5. A paleodip section of the strata in the vicinity of the Vaal Reef, illustrating the angular unconformity that the reef wasdeposited on _____________________________________________________________
6-9. Plans showing:
6. Isopachs of the truncated MBS quartzite, which underlies the Vaal Reef unconformably ______________
7-9. Moving-average vectors measured from:
7. Trough crossbedded Vaal Reef __________________________________________
8. Trough crossbedded quartzite beneath the Vaal Reef ______________________________
9. Axes of trough crossbedded units in argillaceous quartzite overlying the Vaal Reef _____________
10. A schematic model of the sequence of events that resulted in deposition and preservation of the Vaal Reef _________
11, 12. Photographs showing:
11. A transverse view of trough crossbedded distal Steyn Reef, Free State Saaiplaas, Orange Free State Goldfield, inwhich the barely preserved remnants of many trough units are marked by dark heavy-mineral concentrates
at the base of the reef unit _______________________________________________
12. Multilayered distal Steyn Reef, in which is illustrated a sequence of predominant migration followed by accumula­tion and then migration again ______________________________________________
in

Gl
2
2
3
3
4
5
6
7
8

8
8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHE CROSSBEDDED NATURE OF PROTEROZOIC WITWATERSRAND
PLACERS IN DISTAL ENVIRONMENTS AND A PALEOCURRENTANALYSIS OF THE VAAL REEF PLACER
By W. E. L. MlNTER1ABSTRACTThe Proterozoic placers in South Africa that were deposited in distal
environments are referred to as carbon-seam reefs. They are pebbly
quartz-arenite units with a characteristic pi-crossbedded internal
geometry. Their external geometry, which is sheetlike, was producedby the braiding of shallow channels in a dendroidal belt. Low-variance,
unimodal, paleocurrent data from the Vaal Reef are used to illustrate atransgressing, braided, fluvial environment followed by longshore
drift in a deepening water environment. The total variance, evident in
detrital heavy-mineral concentrations of carbon-seam placers, isrelated to the internal geometry, external geometry, and areal position
within the deposit.

INTRODUCTION

I have introduced the conceptual model (this volume,
chap. E), and I think we are aware of the framework. In
this paper I want to make you aware of the processes
that actually took place and that produced heavymineral concentrates.
The term "reef as used in the Witwatersrand context
refers to a fossil placer deposit. During early mining ac­
tivities the reefs being mined were mostly "bankets,"
pebble-supported conglomerates, that were identified by
the pebbly layer. Most reefs being mined today are of
the carbon-seam type, in which pebbles are of lesssignificance. Nevertheless, the term "reef has been ap­
plied to the pebble layer often evident on the basal con­tact of carbon-seam reefs. This layer is, however, only apart of the whole reef.

and is closely related to the nature of the internal
geometry. Every carbon-seam reef examined by theauthor has been characterised by the presence of troughcrossbedding, more explicity defined as pi crossbedding(Alien, 1963).
Trough crossbedding is a structure that develops at ahigher flow velocity than that required for planarcrossbedding. Planar crossbedding represents theprimary structure that develops at the lower velocity
end of a range of crossbedded structures in which thelinear edge of the lee side of the sand dune becomes in­creasingly curved with increasing velocity. Troughcrossbedding is developed by a scour-and-fillmechanism, and most units owe their origin to themigration of trains of large-scale asymmetrical dunes
with pronouncedly curved crests. The coarse sanddeposited under this flow regime contains very little
muddy material (figs. 1, 2).

CROSSBEDDED INTERNAL STRUCTURE

The carbon-seam type reef is a siliceous quartzite(quartz-arenite) unit that is overlain and generally
underlain by quartzites that are more argillaceous. Thisis an important aspect of the identification of a reef unit

1 Geology Department, Anglo American Corporation, P.O. Box 20, Welkom 9460, South
AfricaFIGURE l.-A longitudinal view of trough crossbedded sets il­lustrating the external geometry of each set and the concentra­tion of dark heavy detrital minerals on the toes of foresets, whichmerge along the trough-shaped base of each set. Footwall quart­zite at Free State Saaiplaas in the Orange Free State Goldfield.
Gl

G2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 2.—A transverse view of trough crossbedding to il­lustrate the slight discordance of the curved foresets, which dip
in both directions, with the trough-shaped base of the sets. Footwall quartzite at Free State Saaiplaas in the Orange Free State
Goldfield.

It is evident from figure 3 that this mechanism also
operated in proximal environments where coarse­
grained sand containing large pebbles and coarse­
grained heavy minerals was transported as 1-m-highsand dunes. However, these are rarely observed in prox­
imal environments, probably because they aredestroyed. Trough crossbedding is much more common
in distal deposits like the Carbon Leader (fig. 4).
THE VAAL REEF

The Vaal Reef is an ancient placer that was deposited
2,500 million years ago over an area of 260 km2 on the
Kaap-Vaal Craton in the Western Transvaal (Minter,

1972, 1976). The placer, stratigraphically situated in theMain-Bird Series of the Witwatersrand System, lies
upon an extensive, regular, angular unconformity with apaleoslope truncation angle of between 1:120 and 1:350(fig. 5).
Mining, 2,000 m below surface, of the 1-m-thick sheetof sedimentary rock that contains the placer deposit has
provided superb exposures and a unique opportunity toanalyse Proterozoic paleocurrents in detail.
The placer sediment, which is a pi-crossbedded quartzarenite with a mean grain size of 2.54 phi units, wasdeposited in a dendroidal fluvial system of shallow,
braided channels (fig. 6). Measurements of 484 foresets
from longitudinal sections of the trough-shaped sets in­dicate a mean scale of 8 cm, a mean inclination of 18°, avectoral mean of 144° with a variance of 4,721 (fig. 7).
Analysis of 142 foresets from a local part of the deposit
indicates a variance of 2,209, which implies that the in­crease in variance obtained from more widespread data
is attributable to "between braided belt" variance. Areal
distributions of moving-average vectoral-means con­form closely with local directions of the braided
channels.
Examination of a quartz-arenite unit beneath theangular unconformity, upon which the placer lies, in­dicates that it is a lobate body that lies on a disconformity. Analysis of 480 foreset measurements in this picrossbedded unit indicates a mean scale of 8 cm, a mean
inclination of 23°, a vectoral mean of 150° with avariance of 2,484 (fig. 8). Analysis of 297 foresets fromthe same local part of the deposit selected before in­dicates a slightly reduced variance of 2,169. It is evident
that very little variance can be attributed to "between
braided belts" and that the unit is essentially a fluviodeltaic fan.

FIGURE 3.--Two oblique, transverse views of very large scale trough crossbedding in proximal Steyn Reef at Western Holdings
Orange Free State Goldfield.

PROTEROZOIC WITWATERSRAND PLACERS, PALEOCURRENT ANALYSIS OF THE VAAL REEF PLACERFIGURE 4.-Distal Carbon Leader Reef illustrating a longitudinalview of the lowest coset of trough crossbedded quartzite with dark
detrital heavy-mineral concentrations on foresets, bottomsets, andparticularly at the base of the unit at a position halfway down thehammer handle. Blyvooruitzicht, Far West Wits Goldfield.

G3

The sediment overlying the Vaal Reef placer is a picrossbedded argillaceous quartzite. Mining of the placer
unit has exposed the base of this overlying unit, on
which trough forms are evident. Analysis of 500
measurements of axes of these troughs indicates a mean
scale of 25 cm and a vectoral mean of 065° with avariance of 269 (fig. 9). Analysis of 193 measurementsfrom the same local part of the deposit selected before
indicates a slightly reduced variance of 256. The small
difference is attributed to a regional curve in the
paleocurrent direction from 045° to 085° as the
paleocurrent follows the strike of the angular unconfor­
mity, which describes an embayment.
Integration of these paleocurrent data from three
superimposed quartzite layers reveals deposition of afluvio-deltaic fan, transgressive truncation of the fluviodeltaic fan and deposition of a Proterozoic gold anduranium placer by braided channel processes, burial of
the placer beneath argillaceous quartzite, which wastransported and deposited by longshore paleocurrents(fig- 10).
The very low variance of paleocurrent vectors
measured from pi-crossbedded sets in this study ap-

METERS
-I50

•I20
90
•60
30
VAAL REEF TRUNCATION:S^^H^>M>M: >:M^:^S^M

-0
-30
1KM

60

FIGURE 5.-A paleodip section of the strata in the vicinity of the Vaal Reef, illustrating the angular unconformity that the reef was deposited
on.

G4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
26°45'

Isopach contours
in feetBraided cha

FIGURE 6.—Isopach plan of the truncated MB5 quartzite, which underlies the Vaal Reef unconformably. The strike of isopachs reflects a synclinal
axis. Final erosional etches on the paleosurface are evident as a braided channel pattern.

PROTEROZOIC WITWATERSRAND PLACERS, PALEOCURRENT ANALYSIS OF THE VAAL REEF PLACER
26°45'

EXPLANATION

Crossbedding direction

FIGURE 7.—Moving-average vectors measured from trough crossbedded Vaal Reef.

G5

G6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
26°45'

EXPLANATION
Crossbedding direction

FIGURE 8.—Moving-average vectors measured from trough crossbedded quartzite beneath the Vaal Reef.

PROTEROZOIC WITWATERSRAND PLACERS, PALEOCURRENT ANALYSIS OF THE VAAL REEF PLACER

G726°45'

EXPLANATION
Crossbedding direction

FIGURE 9.—Moving-average vectors measured from the axes of trough crossbedded units in argillaceous quartzite overlying the Vaal Reef.

G8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTECTONIC FRAMEWORK—SEDIMENTARY RESPONSE

PRE-MB5

Deltaic MBS Lobe
Filling Downwarp

concentration in the reef unit and an upper layer where
heavy minerals and small pebbles are concentrated (fig.
12). Although the reef unit is stratigraphically con­tinuous from one position to another, the internal struc­tures are not, because of their curved geometry and theassociated heterogeneous distribution of detrital heavy
minerals. The total concentration of detritals is related
to the amount of migration and is reflected by the
number of sets per unit of accumulation.
CONCLUSION

As a result of the heterogeneous spatial distribution ofheavy minerals, the gold and uranium contents from adV'! »•*«•-«

Subaqueous Channelling
Dendroidal Bellof Vaal Reel

Transgrefisive Offshore Overlapof Vaal Reef by MB4

FIGURE 10.- A schematic model of the sequence of events that resulted
in deposition and preservation of the Vaal Reef.

FIGURE 11. -A transverse view of trough crossbedded distal Steyn
Reef in which the barely preserved remnants of many trough units
are marked by dark heavy-mineral concentrates at the base of thereef unit. The overlying trough crossbedded sets that are better
preserved accumulated more rapidly with consequently lessreworking and lower concentration of detrital heavy minerals.
Free State Saaiplaas, Orange Free State Goldfield.

proaches ideal limits and is attributed to goodstratigraphic control of the data, the small size of thearea, and the excellent exposures.
SPATIAL DISTRIBUTION OF
DETRITAL HEAVY MINERALSThe scour-and-fill mechanism that transported placer
sediment effectively concentrated detrital heavy
minerals on the toes of foresets and the trough-shapedbases of sets (fig. 11). When the rate of migration ofthese sets exceeded accumulation, the detrital heavy
minerals became concentrated at the base of the bed in aheterogeneous arrangement. This is well illustrated by
figure 11, in which the barely preserved remnants ofmany trough units are marked by dark heavy-mineral
concentrates.
Rates of accumulation and migration fluctuate so thata sequence of predominant migration followed by ac­
cumulation and then migration again produces a basal

FIGURE 12.-Distal Steyn Reef, in which is illustrated a sequence ofpredominant migration followed by accumulation and then migra­
tion again, which has produced a basal concentrate in the reef unit
and an upper layer where heavy minerals and small pebbles areconcentrated.

PROTEROZOIC WITWATERSRAND PLACERS, PALEOCURRENT ANALYSIS OF THE VAAL REEF PLACER

G9paper that had to do with a horst shedding material on
jacent samples display a standard deviation of as much
both sides. Do you attribute the gravels to localized
as 300 percent, and a single sample is seldom represen­tative. This variance of mineral content is a result of thesources like that?
crossbedded geometry of distal placer sediment and may Minter: In those Ventersdorp deposits in which we havealluvial fans, which I used in the example for the left
be thought of as a first order of variance.
partof the conceptual model, I think that the Wit­
A second order of mineral concentration variance,watersrandhas been reworked and that all thewhich is displayed by stretches of different concentra­
material
inthe
Ventersdorp Contact Reef in thosetion ranges, is attributed to changes of hydraulic energy
alluvial
fans
camefrom the horsts.
level and subtle paleosurface topography. These
stretches, in the case of the Vaal Reef, are oriented Miles: So that you had a sort of tectonic center.
parallel to the braided channel pattern evident on the Minter: That's a tectonic center and is applicable to the
Vaal Reef paleosurface.
younger part of the Witwatersrand, in fact theA third order of mineral concentration variance oc­
Ventersdorp. We did not have that type of source, I
curs on an areal scale and is apparently related to thethink, in the older Witwatersrand.
overall geographic distribution of the deposit.
Miles: That is, the horst contained gravels which on ex­
posure were reworked and deposited in the
Ventersdorp?
REFERENCES CITED
Minter: That's right.
Alien, J. R. L., 1963, The classification of cross-stratified units: Miles: Thank you.
Sedimentology, v. 2, p. 93-114.
Minter: Just one other reply to that question. I think
Minter, W. E. L., 1972, The sedimentology of the Vaal Reef in the
Larry Schloss would call these younger deposits yoked
Klerkdsdorp area: University of the Witwatersrand, unpublisheddeposits, whereas the others are intracratonic
Ph.D. thesis submitted to the Faculty of Science, 170 p.
deposits in a very much gentler depression, as it were.
Minter, W. E. L., 1976, Detrital gold, uranium, and pyrite concentra­
tions related to sedimentology in the Precambrian Vaal Reef Larry Milliken, Public Service, Denver, Colo.: This
placer, Witwatersrand, South Africa: Economic Geology, v. 71, no.
perhaps should be directed to Dr. von Backstrom. You
1, p. 157-176.
mentioned the presence of several refractory minerals
that make up a fairly significant percentage of the
DISCUSSION FOLLOWING PAPERS BY
heavy-mineral suite. Has any effort been made to ex­
DRS. VON BACKSTROM AND MINTER
tract uranium from these, or is all your uranium takenfrom uraninite?
Bourret: May I first of all compliment you on an out­ von Backstrom: I wouldn't be able to answer this directly
standing paper. I've been in most of the mines you
myself, but I wonder if Dr. Whiteside, who is here,
mentioned and I appreciate particularly what you
could answer it. I know there was some uranium pro­have to say, and I think you've done a tremendous job.
duction from the Dominion Reef mine. Dr. Whiteside,
Myers: Larry [Minter], could you explain a little bit more
do you know what they did in connection with any
about the difference between the pi crossbedding andrefractory minerals?
the trough crossbedding?
Whiteside: As far as I know, all that happened was that
Minter: It's the same thing.
it messed up the mill circuit at the Western Reefs
Myers: All right.
Mine and that we eventually gave up mining the Domi­
Minter: It's just that John Alien has classified crossnion Reef for that reason. Material from the Vaal Reef
and material from the Dominion Reefs were an­
bedding into 15 varieties, the reason being that there
tipathetic, rather.
is a different mechanism for each variety. This is very
important and particularly in this case because one Milliken: Thank you. Is any work being done at this timeon the milling treatment of those refractories?
needs a reworking process to create heavy-mineral
concentrates and it's interesting to see that one has Whiteside: They are definitely working on the Dominion
this process in the reef. It has never been emphasized
Reef, and they will put up a separate plant for it, so
before, but it occurs in all the Witwatersrand reefs. It
that would take care of the refractories. I think ac­
also occurs in the Blind River deposits.
cording to Dr. [S.H.U.] Bowie, the uraninite is not atrue uraninite and he will concede that the uraninite inCharles Miles, Protocol Corp., Denver, Colo.: Dr. Minter,earlier we had a source of these gravels or placers sortthe Dominion Reef is detrital only because it isthorium-bearing uraninite.
of boxing the compass, and you have one in your firstPrimary Dispersal Patterns of Uraninite in
the Proterozoic Vaal Reef Placer Deposit,
Witwatersrand, South Africa
By W. E. L. MINTER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQU A RT Z - P E B B L E CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-H

CONTENTS
Page

Abstract _____________________________________________
Introduction ___________________________________________Uranium distribution _______________________________________
Conclusion_____________________________________________
References cited _________________________________________
Discussion _____________________________________________

HI
1
1
2
2
2

ILLUSTRATIONSPage

FIGURE 1. Isopach map of the Main Bird Series, showing the relative position of the Vaal Reef in the Witwatersrand basin _______
2. Location plan of the seven mining lease areas where the Vaal Reef is being exploited ______________________
3. Stratigraphic column of Upper Witwatersrand Group in the Klerksdorp area ________________________—
4. Isopach plan of the MBS footwall sediments that the Vaal Reef truncated ______________________—__—
5. Moving-average plan of uranium content prepared from more than 900,000 samples of Vaal Reef _______________
6. Moving-average isopach plan of the Vaal Reef prepared from more than 900,000 data points __________________
7. Stratigraphic section along Vaal Reef paleostrike, illustrating the stratigraphic relation between the Zandpan Marker andthe Vaal Reef __________________________________________________________
8. Isopach and paleocurrent plan of the sediment between the Vaal Reef and the Zandpan Marker, illustrating the Zandpan
Marker's onlap _________________________________________________________
9. Moving-average isopach plan of the Vaal Reef basal carbon seam measured at 499 locations __________________
10. Moving-average uranium-gold ratio plan calculated from moving averages of more than 900,000 samples of the Vaal Reef __

H2
3
4
5
6
7
8
9
10
11
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESPRIMARY DISPERSAL PATTERNS OF URANINITE IN THE PROTEROZOICVAAL REEF PLACER DEPOSIT, WITWATERSRAND, SOUTH AFRICA
By W. E. L. MINTER l
ABSTRACTThe Vaal Reef is a strata-bound uranium orebody. Uraninite,
together with nodular pyrite and gold, is a detrital heavy-mineral com­ponent of the sediment. Consequently, the areal distribution patternsof uranium content match the braided, southeasterly dispersal patternevident in the Vaal Reef. Deeper channelways contain more uraninite.

INTRODUCTION

Well, I've really primed you, and now I would like torelate the uranium distribution in the Vaal Reef to thesedimentary environment that I have painted.
The Vaal Reef is a distal Proterozoic placer depositthat is being mined for its gold, uranium, and pyrite con­tent in the Klerksdorp Goldfield (fig. 1), on sevenmining-lease areas (fig. 2). Stratigraphically situatednear the top of the Main Bird Series of the Upper Witwatersrand System (fig. 3), the Vaal Reef was a pebbly
sand sheet, 1 to 150 cm thick, that covered approximate­ly 260 km2 (100 mi2). It is the sedimentary product ofanastomosing braided river channels in a shallow fluviatile environment.
The ancient southeasterly dispersal pattern indicatedby primary crossbedding structures within the sandbody is substantiated by meandering braided channel
forms, evident at the base of the sand body, that eroded
into underlying sediments (fig. 4).
Nodular pyrite (30,000 ppm (parts per million)),uraninite (500 ppm), and gold (15 ppm) are detrital
heavy-mineral components of the Vaal Reef that wereconcentrated at the base of the sand body and on inter­
nal sedimentary partings. Fossil hydrocarbon, ranging
from scattered fine-grained specks to a seam 5 cms
thick, is generally evident on the basal contact and lesscommonly on sedimentary partings.
URANIUM DISTRIBUTION

Uraninite is distributed vertically throughout the Vaal
Reef, 20 percent of the total content being concentrated
1 Geology Department, Anglo American Corporation, P.O. Box 20, Welkom 9460, South
Africa.

near the base of the reef. Similar concentrations may
also occur on internal sedimentary partings.
The positively skewed log-normal frequency distribu­
tion of uranium contained in total Vaal Reef samples
was reduced by combining large numbers of samples
with a moving-average technique. More than 900,000
samples from the area were used to produce a mean­
ingful contour plan (fig. 5). The southeast channel fabricis well defined by the uranium distribution.
A contour plan prepared in a similar way using totalthickness data of the Vaal Reef (fig. 6) describes the ex­
ternal geometry of the sedimentary sheet. The
southeasterly trend and braided nature of the reef are
well defined. It is evident from a comparison of theuranium and the thickness contour plans that a sedimen­
tary relationship exists-a thicker reef contains moreuranium.
A Stratigraphically higher pebbly layer known as the
Zandpan Marker onlaps the Vaal Reef (fig. 7) from thewest, and its uranium content enhances that of the Vaal
Reef in the area of convergence (fig. 8).
A moving-average contour plan (fig. 9) of the basalhydrocarbon thickness, based on 499 samples, describes
an area of thick hydrocarbon along the western side ofthe deposit that is a mile wide and extends for at least
four miles down the paleoslope. This is interpreted ashaving been an area favourable for the growth andpreservation of organic plant material (Minter, 1972).
The uranium content in areas where thick hydrocarbon
seams are prevalent is of the same order as in areas tothe east where hydrocarbon is conspicuously sparse. The
ratio between uranium and gold, calculated from themoving-average data used for figure 5 ranged from 7 to
19 over most of the area (fig. 10), and the distribution
pattern does not appear to have significant relation to
the carbon-seam isopach (fig. 9). These data do not sup­port previous concepts that the carbon seam was pro­
duced from liquid hydrocarbon by irradiation fromuranium nor that the uranium was precipitated fromsolution by carbon. Had either effect predominated, theHi

H2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

26°45°FIGURE 1. -Isopach map of the Main Bird Series (modified from Brock and Pretorius, 1964, pi. 2) showing the relative position of the Vaal Reef inthe Witwatersrand basin. Isopach contours in feet, contour interval 1,000 ft.

uranium content would have been preferentially greaterin thick carbon-seam areas.
CONCLUSIONSUraninite is a detrital component of the Vaal Reef
placer. The total uranium content is related to thethickness of the Vaal Reef, and the areal distribution
patterns of uranium content and thickness are sym­pathetic.
REFERENCES CITED
Brock, B. B., and Pretorius, D. A., 1964, Rand basin sedimentationand tectonics, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa: Geological Society of South Africa, v.
1, p. 549-599.

Minter, W. E. L., 1972, The sedimentology of the Vaal Reef in the
Klerksdorp area; University of the Witwatersrand, unpublished
Ph.D. thesis submitted to the Faculty of Science, 170 p.

DISCUSSION[Hallbauerl]-: [An incompletely recorded statement wasmade to the effect that uranium occurs in two forms inthe Vaal Reef, as detrital uraninite and in the organic
matter.]Minter: That's another story which is coming later, but
certainly there are at least three positions where one
can find radioactive minerals in the reef. As I said, on­ly 20 percent is on that bottom contact and a lot of that
20 percent is probably in the carbon, some of it isperhaps in the structure itself, some as detrital grains

DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA

H3

26°45'

STILFONTEINZANDPANBUFFELSFONTEIN

ELLATONWESTERN

HARTEBEESTFONTEIN

REEFS
Y,

VAAL REEFS

FIGURE 2.—Location plan of the seven mining lease areas where the Vaal Reef is being exploited.

to be down the paleodip, and consequently we canwithin the carbon. Certainly I am implying that the 80
perhaps exercise grade control, predict grades, etc.
percent which is distributed through the reef andWith uranium it is quite easy because the concentra­
which isn't associated with carbon is presumablytion is relatively high. With gold it's more difficult, but
detrital.
that's certainly one of the economic applications weBourret: In all your sections you showed the mineraliza­
use.
tion of uranium pretty much terminating as you pro­ceed to the southeast corner of the slides. Is this a Bourret: This does not mean, then, that this is the end of
structural feature or property lines?
the economic mining depth?
Minter: Property lines. We've reached a depth ofsomething like 7,000 feet and we are still going. But I Minter: No. That big black line at the bottom, which
represents the Vaal River, seperates Vaal Reefs fromcan bring you up to date in a few years time. There's a
Vaal Reefs South, which is an extensive new minevery interesting point associated with the mineraliza­
lease area just to the south that presumably has a life
tion trends shown and that concerns application ofof 20 years. We keep these maps up to date on a com­
sedimentology to mining. We have very broad trends
puter program; data goes in monthly and we use it.
and we can project them downdip, which also happens

H4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESMETERS
2000

VENTERSDORP GROUPFORMATION N SUB GROUP ^llLA JL*•'.','> Elsburgi> Kimberley
ElsburgGold
Estates

1500

a.
O
a:
o

.<

•!rrir--r ^
[Bird
Reef

o

z
a:

^

Voal_
Reef
•

-

0

0

V)

„ .

1000

WITWATEF^ivingstane
Reef
00 0 « • •> Main Bird
a:
Ul
a.

500

a.

> Main Reef

0

i^J—

JEPPESTOWN SHALE

FIGURE 3. -Stratigraphic column of Upper Witwatersrand Groupin the Klerksdorp area.

DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA

H5

26°45'

Isopach contours
in feetBraided cha
bedforms

FIGURE 4. - Isopach plan of the MB5 footwall sediments that the Vaal Reef truncated. The braided channel pattern that is superimposed reflects
the final erosional etch on the paleosurface and the Vaal Reef distributary system.

H6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES_____________________________

02

26°45'

4KM

EXPLANATION
I Low content

a*K':-:i

FIGURE 5. - Moving-average plan of uranium content prepared from more than 900,000 samples of Vaal Reef.

DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA
26°45'

EXPLANATION< 13 CENTIMETERS

FIGURE 6. -Moving-average isopach plan of the Vaal Reef prepared from more than 900,000 data points.

H7

H8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESMETERS

20-10

ZANDPAN MARKER-0

VAAL REEF*

-10
-20
L 30

2KM

FIGURE 1. - Section along Vaal Reef paleostrike to illustrate the stratigraphic relation between the Zandpan Marker and the Vaal Reef.

DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA

H9

EXPLANATION

Paleocurrent
Isopach contours
in inches•*.v Contact area

FIGURE 8. -Isopach and paleocurrent plan of the sediment between the Vaal Reef and the Zandpan Marker, illustrating the Zandpan Marker's
onlap.

H10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
26°45'FIGURE 9. -Moving-average isopach plan of the basal carbon-seam measured at 499 locations.

DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA

Hll

26°45'

Ratio, in parts per million
> 19
7-19
<

7

FIGURE 10. -Moving-average uranium-gold ratio plan calculated from moving averages of more than 900,000 samples of the Vaal Reef.

Examples of Possible Movement ofGold in Solution in the
Witwatersrand, Ventersdorp, and Transvaal Systems
By H. C. M. WHITESIDEGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-1

CONTENTS

Page

Abstract ________________________________________________
Introduction _______________________________________________
Heavy metals in the Black Reef, East Rand ____________________________
Gold in an amygdale in Ventersdorp lava _______________________________
Recent alluvials associated with the Witwatersrand basin _____________________
Selected references ___________________________________________
Discussion _________________________________________________

II
1
1
2
3
3
4
ill

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXAMPLES OF POSSIBLE MOVEMENT OF GOLD IN SOLUTION IN THE
WITWATERSRAND, VENTERSDORP, AND TRANSVAAL SYSTEMS
By H. C. M. WHITESIDE lABSTRACT

Many South African geologists consider that gold especially, andprobably uraninite, as well as other heavy minerals such as zircon,
chromite, and diamond, are detrital in the Precambrian quartz-pebble
conglomerates of South Africa. It is suggested here that solution ofgold and its redeposition elsewhere may have played a part in its
enrichment or impoverishment. Two examples of possible solution ofgold are cited, and the lack of modern placers associated withauriferous Witwatersrand is briefly discussed.

INTRODUCTION

It is generally considered that gold, and probably
uranium as well, has been concentrated by hydraulicprocesses in the conglomerates of the Dominion Reef,
Witwatersrand, Ventersdorp, and Transvaal Systems.
(It is proposed to retain the old divisions into systems
and series as these are more familiar than the terms
supergroup, group, and the like). Practically all South
African geologists consider that gold and uranium, as
well as other heavy minerals such as zircon and chrom­ite, are detrital and deposited along with the pebbles andsand grains. The gold and uranium deposits of the Wit­
watersrand type would therefore be syngenetic.
This short paper is an attempt to show that South
African geologists may have reacted too strongly
against the hydrothermal theory, which postulated thatgold was introduced into the conglomerates by mineral­ized hot waters emanating from a deep-seated igneous
source. Using "hydrothermal" in its widest sense of"heated waters," it is suggested that warm groundwaters may have taken gold and uranium into solution
and modified their concentration and distribution. This
does not mean that gold and uranium are not predomi­
nantly detrital, merely that there may have been move­
ment of these metals in solution in the period between
their deposition and the consolidation of the sediments.
1 Consulting Geologist, Sandton, Transvaal, Republic of South Africa. Formerly Consulting
Geologist for Anglo American Corporation.

Two examples are described that are suggestive ofsolution of gold and, in one of these cases, redeposition
in a new environment. The lack of modern placers asso­
ciated with erosion of the auriferous Precambrian con­glomerates in and around the Witwatersrand basin isalso briefly discussed.
HEAVY METALS IN THE BLACK REEF,
EAST RAND

The Black Reef, a dark conglomerate whose colour is
attributed to disseminated particles of carbon, isdeveloped in places at the base of the Black Reef Series,
the lowest member of the Transvaal System. It contains
appreciable gold but not much uranium and has been
mined in several localities encompassing the northernpart of the Witwatersrand basin, usually close to or
overlying the edge of that basin. The majority of theorebodies are restricted in size, and mining was on asmall scale compared with those mines that worked the
Witwatersrand or Ventersdorp conglomerates. Fromdescriptions in the literature it would appear that mostof the Black Reef deposits were infillings of scour chan­nels, composed of coarse, fairly thick conglomerates ex­
tending for only a few hundred metres in width and200-300 m downdip, separated by barren areas in which
the conglomerates are thin or even absent.
It has been stated by several geologists that the Black
Reef has derived its mineral content through the erosionof the underlying rocks in the vicinity and subsequentconcentration of the heavy minerals by sedimentary pro­
cesses. This can be said to have been adequately demon­
strated in some instances, particularly regarding thetwo largest known deposits on Government Gold Mining
Areas and Randfontein Estates, where auriferous Wit­
watersrand conglomerates form the immediate footwallor are very close by. A rough analysis shows that theII12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

footwall of the Black Reef where it has been mined is asfollows, in percent:
Ventersdorp:
Lavas _______________________________
Sediments and tuffs _______________________
Witwatersrand:
Upper _______________________________
Lower _______________________________
Basement, essentially granite ___________________

47
13
20
7
13

This tabulation seems to show that auriferous Black
Reef may be deposited some distance away from theoriginal source of gold.
A peculiarity of the Black Reef is that it contains
osmiridium in greater than normal concentration, com­pared with Witwatersrand conglomerates, in at least
three mines. J. A. Papenfus (1964) has demonstrated
that at Government Gold Mining Areas in the East Rand
there is a relation between quantity of osmiridium
recovered and the proximity of the Black Reef, which
was being mined, to a particular conglomerate (U.K.9 c)in the Kimberley Series. Beds of the Kimberley Series
constitute the immediate footwall of the Black Reef, anda large channel, filled with Black Reef conglomerates
and quartzites, has been cut into the Kimberley for a
distance of some 5 km.
It is reasonable to suppose that, in this distance, theosmiridium in the Black Reef was derived from the Kim­
berley Series, but it is difficult to account for theosmiridium in areas where the footwall is granite or
Ventersdorp lavas and sediments. One explanation isthat heavy metals such as gold and osmiridium can betransported in their metallic state farther than is atpresent thought possible, although the fact that concen­
trations of gold are found only near the edge of theBlack Reef basin appears to contradict this. A second
explanation is that there is a local but concealed sourceof these metals.
More than 25,000,000 metric tons of Black Reef ore
was mined on Government Gold Mining Areas, in addi­
tion to very many millions of metric tons from the Main
Reef Leader, the main ore horizon, and a fair tonnage
from conglomerates in the Kimberley Series. Fourmetals and minerals were recovered-gold, silver,
osmiridium, and pyrite-and statistics were kept of all
recoveries. The following differences, compared with
the Main Reef Leader and the Black Reef, emerge.
Although it is not possible to quantify the following
ratios because ore was derived from three sources, it isbelieved that the subsequent statements are correct
because there were periods when either the Main Reef
Leader or the Black Reef contributed more than 80 per­cent of all ore milled. It is not possible to assess theKimberley Reef because it never contributed a high per­centage to the mix, according to the available informa­tion.

1. The gold-to-osmiridium ratio is lower in the Black
Reef than in the Main Reef Leader. There is not a
great difference between the specific gravities ofthese two minerals, depending on their composi­tion, so the ratio should not change appreciably.
2. The gold-to-silver ratio is lower in the Black Reef
than in the Main Reef Leader. It is generally ac­
cepted that the silver content of detrital gold isless than that of vein gold, some of the silver be­
ing dissolved during transportation. If the Black
Reef gold has been derived from older conglom­
erates the opposite should be the case-that is,
the gold-to-silver ratio should be higher.
3. The pyrite content of the Black Reef is considerably
higher than that of the Main Reef Leader. "Buck­shot" pyrite, rounded grains varying in size from
0.5 mm to more than 2 cm, is the form in which it
generally occurs, and these grains are considered
to be replacements of detrital ferruginous
material or chert pebbles.
Summing up these facts, it is suggested that:
1. Normal sedimentary processes concentrated pebbles
and heavy minerals such as gold, osmiridium, zir­con, and chromite from previously existingsources such as the Witwatersrand succession,
the Older Granite, and perhaps the Ventersdorp
sediments.
2. Sulphur-bearing waters were introduced after depo­
sition but before consolidation. The sulphur could
be derived from organic matter or residual fluids
from the Ventersdorp volcanicity. Iron-bearing
minerals such as magnetite and hematite, as wellas small banded ironstone pebbles, would bealtered to pyrite nodules.
3. If the gold in the Black Reef contains more silverthan normal it suggests that silver has been add­
ed or that gold has been leached. During deposi­tion by sedimentary processes gold and osmi­
ridium should have retained a ratio similar to thatof the source rock, in this case, Witwatersrandconglomerates and quartzites. This is not so
because the gold:osmiridium ratio is lower in theBlack Reef than in nearly all Witwatersrand con­glomerates. This must mean that either addi­
tional osmiridium was introduced or gold was
reduced in quantity. These conclusions suggestthat gold was leached penecontemporaneouslywith the deposition of the Black Reef.
GOLD IN AN AMYGDALE IN VENTERSDORP
LAVAThis note is merely to record that one grain of gold
was found in an amygdale in lava a few inches, no morethan 15 cm, above the contact of lava with the top of theMOVEMENT OF GOLD IN SOLUTION, THE WITWATERSRAND, VENTERSDORP, TRANSVAAL SYSTEMS

Ventersdorp Contact Reef. This occurrence was in corefrom a borehole drilled to the west of Western Deep
Levels Mine for the purpose of proving continuity of the
Ventersdorp Contact Reef.
As far as is known, this is the only recorded instanceof visible gold in Ventersdorp lava. Gold in small quan­tities has been found in cores that have intersected
sediments in the Ventersdorp System, but the gold is
not visible and has probably been brought in as detritalgrains. Occasionally, amygdales have been noted that
contain pyrrhotite and chalcopyrite.
It is considered that the gold in the amygdale wastaken into solution by heated waters from the Venters­
dorp lavas circulating through the Ventersdorp Contact
Reef, which presupposes that the conglomerate was not
fully consolidated at the time.
This is no proof that gold has been carried in solution
in large quantities or over great distances, but the obser­
vations cited above may support the modified placer
hypothesis, which contends that the gold in the Witwatersrand conglomerates is detrital but has been
modified by hot fluids and now often is indistinguisablefrom hydrothermal gold.
RECENT ALLUVIALS ASSOCIATED WITH THE
WITWATERSRAND BASIN

It has often been noted by geologists working on the
Witwatersrand that there is no large-scale developmentof auriferous alluvials related to present-day
topography.
There are numerous examples of erosion of auriferous
beds and redeposition of the various constituents, suchas pebbles and heavy minerals, throughout the succes­
sion from the Witwatersrand to the Black Reef Series.
The U.K.9 Kimberley Reef has been shown to containmore gold downstream from areas where the middle
Kimberley has been truncated. The Ventersdorp Con­tact Reef clearly derives some of its gold from erosion ofunderlying auriferous conglomerates of the Upper Wit­
watersrand. On the West Wits Line, at West Driefontein, and Western Deep Levels Mines the Contact Reef
is enriched in the vicinity of the Main, Bird, andKimberley groups of conglomerates where the lattercrop out below it. An example of concentration of heavy
metals and minerals in the Black Reef has been de­scribed above.
It has been suggested that glaciation during Dwyka
time dispersed gold and heavy minerals so that concen­
tration into alluvials was impossible. This may be true
for that particular period, but the Karoo System is
Permo-Carboniferous in age and there must have been a
considerable time interval after the PermoCarboniferous during which the Karoo was strippedfrom the Central Witwatersrand area and the Upper

13

Witwatersrand rocks with their auriferous con­glomerates were eroded, yet no gold placers are known
to be related to that period of erosion.
On average, the conglomerates carried much moregold at outcrop than in depth, and much of it was native,
suggesting secondary enrichment. Could it be that gold
went into solution and was dispersed and that this is thereason why no alluvials were formed?
SELECTED REFERENCES

At the meeting held in Golden, Colo., in October 1975,
several of the geologists who attended expressed theview that there was insufficient description of thegeneral background of the Witwatersrand System andother Precambrian Systems that are hosts to auriferous
and uraniferous quartz-pebble conglomerates.
The following papers are thought to cover the geologyof the Witwatersrand both in general and, to a certain
extent, in detail:
Antrobus, E. S. A., and Whiteside, H. C. M., 1964, The geology of cer­tain mines in the East Rand, in Haughton, S. H., ed., The geology
of some ore deposits in southern Africa: Geological Society ofSouth Africa, v. 1, p. 125-160.
Borchers, R., 1961, Exploration of the Witwatersrand System and its
extensions: Commonwealth Mining and Metallurgical Congress,
7th, South Africa, Transactions v.2, p. 489-512.
Collender, F. D., 1960, The Witwatersrand System in the Klerksdorp
area as revealed by diamond drilling: Geological Society of South
Africa Transactions, 63, p. 189-230.
Cousins, C. A., 1965, Disconformities in the Main Reef zone of the Wit­
watersrand System, and their bearing on reef correlation, withparticular reference to the East, Central and West Witwaters­
rand: Geological Society of South Africa Transactions 68, p.
121-142.
de Kock, W. P., 1964, The geology and economic significance of theWest Wits Line, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa: Geological Society of South Africa, v.
1, p. 323-386.
Hiemstra, S. A. 1968, The mineralogy and petrology of the uraniferous
conglomerate of the Dominion Reefs Mine, Klerksdorp area:
Geological Society of South Africa Transactions, v. 71, p. 1-65.
Koen, G. M., 1958, The attrition of uraninite: Geological Society ofSouth Africa Transactions, v. 61, p. 183-196.
Mellor, E. T., 1917, The geology of the Witwatersrand: South Africa
Geological Survey Special Publication 3.
Papenfus, J. A., 1964, The Black Reef Series within the Witwaters­
rand basin with special reference to its occurrence at Government
Gold Mining Areas, in Haughton, S. H., ed., The geology of someore deposits in southern Africa: Geological Society of South Africa,
v. 1, p. 191-218.
Toens, P. D., and Griffiths, G. H., 1964, The geology of the West Rand,
in Haughton, S. H., ed., The geology of some ore deposits insouthern Africa: Geological Society of South Africa, v. 1, p.
283-321.
Whiteside, H. C. M., 1950, The Kimberley-Elsburg Series in certain
mines in the Far East Rand: Geological Society of South Africa
Transactions, v. 53, p. 243-252.
Winter, H. de la R., 1964a, The geology of the northern section of theOrange Free State Goldfield, in Haughton, S. H., ed., The geology
of some ore deposits in southern Africa: Geological Society ofSouth Africa, v. 1, p. 417-448.

14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESWinter, H. de laR., 1964b, the geology of the Virginia section of theOrange Free State Goldfields, in Hqughton, S. H., ed., Thegeology of some ore deposits in southern Africa: Geological Societyof South Africa, v. 1, p. 507-548.

DISCUSSION[As an aside in his talk Dr. Whiteside gave the follow­
ing approximate dates for the Rand: Black Reef, 2,000
m.y.(million years); Ventersdorp, 2,300 m.y.; Witwatersrand, 2,500 m.y.; Dominion Reef, 2,800 m.y.]
B. Nagy: When you talk about the transport of gold, do
you mean gold in molecular solution, or do you mean
gold in colloidal suspension?
Whiteside: I cannot answer. I am a field geologist and I
don't know.
B. Nagy: We are working on the Vaal Reef area and wehave considered various kinds of transport, placer
gold transport, even solution with hydrogen cyanide,
which doesn't seem to work. On the other hand, there
are a number of possibilities. For example, hydrothermal gold can interact with various organic com­pounds. Small gold particles can be carried this way,
but such transport is not on a molecular level. Also it ispossible that at the site of deposition very small par­ticles of gold can be partially mobilized by the organic
compounds present, giving the impression of dissolu­
tion. What I am saying is that from our experience I
cannot see molecular solution of gold. Despite the dif­ficulties involved I can see, however, transport of goldin colloidal suspension or as very fine placer grains.
Did you make the comment that in the Transvaal the
carbon is graphite?
Whiteside: I said it is carbon, but some people have said
it is graphite. I don't think it is.
B. Nagy: We published a paper in Nature which cameout June 26, 1975, in which we described two cyclic
ethers from a sample of the Transvaal. Well, for thechemists' sake, we found two ethyls, two propyls,
three methyltetrahydrofuran, two propyltetrahydropyran. In addition, we have found by ionization mass
spectrometry at least five thousand different peaks ofmolecular ions of various compounds: oxygen,hydrogen, nitrogen, and hydrocarbon compounds
ranging from very low molecular weight to very highmolecular weight. We did not find any graphite what­
soever. We did find, however, a very complex organic
chemistry which to us is indicative of a biota that had
evolved a paleobiochemistry by Transvaal time.
Whiteside: Thank you.
Simpson: Would you regard uranium as possibly concen­
trated in carbon by a similar process to the one you
describe for gold?
Whiteside: Yes.
Skinner: I would like to comment on gold transporta­tion. In a very short, interesting paper a few years ago

in Economic Geology, Holgeson and Garrels, discusstransportation of gold and of silica in an ordinarybrine; transportation is small but significant. It is not
zero. If you have a brine and have gold present, thatbrine is going to be saturated with gold. It is impossi­
ble not to move some of it. If you had any kind of ther­mal gradients developed in these basins you must have
moved some gold around. It seems quite conceivable
to me that a perfectly normal, ordinary brine may in­deed be the cause of what you describe, and I rather
doubt that organic compounds have much to do withit.
Whiteside: Thank you.
Feather: Did you discover there were any secondary
minerals associated with that gold inclusion within the
amygdale? Were there any secondary sulfide orarsenide minerals present?
Whiteside: Quite honestly, I don't know. The
mineralogists told me they were so scared of wiping
the gold off when they wanted to do more polishing on
it, that they were not able to determine very much ex­
cept that it was gold. Sorry.
Feather: Do we still have the sample at the researchlaboratory?
Whiteside: Yes, that sample is in a core shed on Western
Deep Levels. I must say, I would like to know a little
bit more about that.
Feather: Yes, I would like to look up the sample and infact also analyze for the silver content of the gold.
That would be interesting, too.
Whiteside: Yes, it would be. I should think they could dothat with a microprobe.
Wilfried Meyers, Falconbridge Nickel Mines, Ltd.,
Canada: I'd like to ask you more about the absence ofmagnetite throughout the Witwatersrand. This issomething that apparently also holds true for theBlind River area and other pyritic conglomerates in
Canada and other parts of the world. What happened
to the magnetite? It must have been there originallywith a vast granite source terrane. Where has it allgone to?
Whiteside: Well, as I say, I think it has been converted
to sulfide.
Myers: Where did all the sulfur come from?
Whiteside: That's one of the problems.
Meyers: If you expect sulfur to move into every nook andcranny within that vast sedimentary pile to change
every grain of magnetite or hematite that may havebeen there originally, you must have had an absolute,complete pervasion by solutions, or however that
sulfur got in there. What I am trying to drive at is, if
you can do that, you can also bring in uranium andgold in solutions and not by detrital means.
Whiteside: Well, the one thing I can assure you is that

MOVEMENT OF GOLD IN SOLUTION, THE WITWATERSRAND, VENTERSDORP, TRANSVAAL SYSTEMS

15

there is no magnetite in the Witwatersrand. The only
and also an enormous amount in the Ventersdorp. Go­
place it has ever been identified has been in pebbles,ing further, you've got the black carbon of the
not in the matrix. Occasionally I believe that you do
dolomite, and then finally you've got the Bushveld
get small grains of magnetite in the iron-bearing rocks
Complex, which presumably could have brought inas pebbles. And another thing, incidentally, you get
quite a lot of sulfur.
very little ilmenite, but you do get a lot of leucoxene, Schidlowski: Just a very quick comment on the sulfur
and that's almost spherical. Say, I'm talking out ofbudget. If you convert part of the pyrite into pyrturn. I am not a mineralogist. Sorry, but I don't know
rhotite you get a sulfur surplus which can be used forwhere the sulfur comes from, but there are, as I say,sulfidation processes. In fact, conversion of pyrite into
various stages roughly in the Witwatersrand where
pyrrhotite has obviously occurred during the pseudoyou could bring in sulfur. We have volcanicity in thehydrothermal reconstitution of the reef; that has been
Dominion Reef, in the Witwatersrand in two places,
observed in several places.

Microprobe Analyses of
Uranium and Thorium in Uraninite fromthe Witwatersrand, South Africa, andBlind River, Ontario, Canada
By D. E. GRANDSTAFF
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1 1 6 1 -J

CONTENTS

Abstract _________________________________________________
Introduction _______________________________________________
Material _________________________________________________
Analyses _________________________________________________
Results __________________________________________________
Discussion ________________________________________________
Acknowledgments ____________________________________________
References cited ____________________________________________
Discussion ________________________________________________

Jl
1
1
1
1
2
4
4
4

ILLUSTRATIONSPage

FIGURES 1-4. Graphs of:
1. Representative step scans across three uraninite grains _________________________________
2. U02 and Th02 content of uraninite grains ________________________________________
3, 4. Plots of U02/Th02 against Th02 in uraninite grains from:
3. The Vaal Reef West mine, Witwatersrand, South Africa ____________________________
4. The Rio Algom Nordic Mine, Blind River, Ontario, Canada ___________________________

J2
3
3
3
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESMICROPROBE ANALYSES OF URANIUM AND THORIUM IN URANINITEFROM THE WITWATERSRAND, SOUTH AFRICA, AND BLIND RIVER,ONTARIO, CANADA
By D. E. GRANDSTAFF
ABSTRACTMicroprobe analyses of uranium and thorum in uraninite grains fromthe Witwatersrand, South Africa, and Blind River, Ontario, revealthat although individual grains are fairly homogeneous, theassemblage of grains is quite heterogeneous. This heterogeneity ap­
pears to favor genetic concepts advocating a detrital, placer origin for
the uraninite.

INTRODUCTION

The Canadian sample was uncommonly rich inuraninite, and a single polished surface exposed a large
number of uraninite grains. All the grains analyzed fromthis sample appeared on this polished surface and lay
within an approximately 4 cm2 area.
Because the ore grade of the Witwatersrand sample
was somewhat lower than that of the Blind River sam­ple, only a few uraninite grains were exposed on any
single polished surface. Therefore, a small cube approx­
imately 64 cm3 was crushed to between 80 and 200
mesh, approximately 60 to 200 ^. This is about the size
range of many of the uraninite grains (Liebenberg,
1955). The uraninite grains were separated by a method
involving two heavy liquids, bromoform and Clerici solu­
tion, and the Frantz isodynamic separator. A polishedsection grain mount was made of the separated grains.
Microscopic examination revealed that in some cases theuraninite grains were still in contact with matrix
material around a large portion of their perimeters.
These grains were probably nearly whole and therefore
were used in the examination for internal zoning.

The bulk chemical compositions of uraninite concen­
trates from the Witwatersrand basin of South Africa
and the Blind River-Elliot Lake area of Ontario,
Canada, have been reported by several investigators(Liebenberg, 1955; Hiemstra, 1968; Roscoe, 1979). The
results of these analyses reveal that the concentrates
from the Blind River area are quite similar in regard totheir uranium and thorium content. The U02/Th02ratioof the Blind River concentrates varies from approx­
imately 8 to 12 and averages about 10 (Roscoe, 1979). In
contrast, analyses of concentrates from the Witwaters­
rand basin reveal a wider range of composition. The
U02/Th02 ratio varies from approximately 10 to 40 as aresult of the variation in Th02 content, which ranges
ANALYSES
from 1.5 to 6.5 percent. However, very little has beenreported on the composition of individual uraninite
The analyses were made using an ARL-EMX2
grains (Schidlowski, 1966) or variations within or be­ microprobe. The beam diameter was approximately 1 ^,tween grains. This paper reports the results of analyses the sample current was 0.05 ^ A and the accelerating
made by electron microprobe of uranium and thorium in voltage was 15 kV. The data obtained were corrected forindividual uraninite grains.
drift (which was negligible), deadtime, and background.
Artificial U02 and Th02 crystals were used as standards.
MATERIAL

Two samples were examined in this study, a sample ofvery high grade uranium ore from the Rio Algom Nordic
Mine, Elliot Lake, Ontario, Canada, and a lower gradeore sample from the Vaal Reef West mine, Klerksdorp
area, Transvaal, South Africa, from the upper divisionof the Witwatersrand System.
Department of Geology, Temple University, Philadelphia, Pa. 19122.

RESULTS

Results of analyses for uranium and thorium areshown in figures 1-4.
Step scans for uranium and thorium were made acrossseveral grains. Representative results are shown in
2 Any use of trade names and trademarks in this publication is for descriptive purposes onlyand does not constitute endorsement by the U.S. Geological Survey.

Jl

J2

LU
O
CC

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

64

64

64

62

62

62

60

60

60

58

58

58

56

56

10

10

10

8

8

8

6

6

O

D

• ••
• • •••..• •/•••* •:
••

LU
LU

6

••

GRAIN 1GRAIN 2GRAIN 3

FIGURE 1.-Representative step scans across three uraninite grains. These step scans illustrate the lack of zoning within individual uraninite
grains. Note, however, the difference in U02 and Th02 contents among the grains.

figure 1. The step scans did not reveal any major zoning
within the grains examined. Other grains were exam­
ined for zoning by use of X-ray area displays for uranium
and thorium. The X-ray area displays did not reveal zon­ing in the grains examined, although this method is lesssensitive than is use of step scans.
Quantitative analyses of spots in the uraninite grains
are shown in figure 2. Quantitative analyses were madein areas that were optically free of inclusions.
The results show that although individual grains are
fairly homogeneous internally, the assemblage of grains
is quite heterogeneous. Uranium concentration,
calculated as U02 , varies from 58 to 69 percent in theBlind River sample and from 55 to 77 percent in the Witwatersrand sample. Thorium, calculated as Th02 ,ranged from 4 to 9 percent in the Blind River sample
and from 0.8 to 11 percent in the Witwatersrand sam­ple. The uranium and thoruim in both samples are ap­
proximately log normally distributed.
The U02/Th02 ratio as a function of Th02 concentra­tion is shown in figures 3 and 4. The figures show a
hyperbolic trend, which would be expected from varia­
tion of a minor or trace element (thorium) in a matrix ofa major, relatively invarient element (uranium). The

trend is particularly clear in the Witwatersrand sample,
owing to the wide variation in grain compositions, but isalso apparent in the Blind River sample.
It is possible that some scatter of the data may be dueto the presence of inclusions of galena or silicates in theregion impinged upon by the electron beam or to the ex­pulsion of differing amounts of radiogenic lead as galena
by the different uraninite grains. However, this probably
does not account for the major trend observed. If the in­clusions or exsolution products contain little or no
uranium or thorium, either mechanism should generateapparent uraninite compositions lying along a mixing
line having a constant U02/Th02 ratio equal to that of
the original grain. Such a trend would lie at a high angle
to the trend actually observed in figures 3 and 4 except
at very low U02/Th02 ratios. Therefore, thesemechanisms are not considered important in causing thescatter of data shown in figure 2.
DISCUSSION

The origin of the Blind River and Witwatersranduraniferous conglomeratic deposits has been extensively
discussed. A variety of stratigraphic, geochronological,

J3

URANIUM, THORIUM IN URANINITE, WITWATERSRAND, SOUTH AFRICA BLIND RIVER CANADA

78

90

o
o

o

80 -

74

°

o

°

0
0 0 o

0

IN
PERCENT

70

o

70

0

-

O o

_

0
O

66

-

o

°
0
0

..

0 62
D

o

O

o
°

°0
o
•

54

-

60

o
o
o

°

n
°

<?
•*>

«b

CM

r-n

x:

•1

°.

50

O

° •* *• °d?»»

58
°

•

CT?>

0 o

§

40

°0

o'

30

o

,°

o
o
o

0°

•

o

—1—1—1— J.

0°

1
6

,

1,1
8

20

10

Th0 2, IN PERCENT
FIGURE 2.-U02 and Th02 content of uraninite grains from the Rio
Algom Nordic Mine, Blind River, Ontario (closed circles), and fromthe Vaal Reef West Mine, Witwatersrand, South Africa (opencircles).

10

00°

468

geochemical, and sedimentological evidence has beenbrought to bear on the problem of origin. Most
geologists appear to favor the placer or modified placer
hypothesis, which states that the uraninite grains were
deposited syngenetically in the sediments as placer
origins.
The composition of the uraninite grains has major
bearing on the problem of genesis. Most discussion has
centered on the thorium content of the grains. Lowtemperature, hydrothermally derived uraninite contains
little, if any, thorium, while uraninite derived frompyrogenetic sources, from granites or pegmatites, may
contain appreciable thorium (Frondel, 1958; Ramdohr,
1969). Therefore, the high thorium content of many of
the grains examined in this study supports the
hypothesis that they are derived from granitic or
pegmatitic sources.
The great compositional variability of the grains fromwithin a small rock volume produces additional problems
for advocates of a hydrothermal origin of the uraninite
grains. Such a great compositional variability would re­quire that the grains grew in very specialized microenvironments and that the mineralizing fluid washeterogeneous over short distances. Such conditions areunlikely. Any movement of a heterogeneous mineraliz­ing fluid through the rock might be expected to produce

0 <p
10

12

Th02 , IN PERCENT
FIGURE 3. -Plot of U02/Th02 against Th02 in uraninite grains from the
Vaal Reef West mine, Witwatersrand, South Africa.

16
14
12

O

10

oo o

•bo-

D

8
6

0

4

6
8
ThO2, IN PERCENT

10

FIGURE 4. -Plot of UO2/Th02 against Th02 in uraninite grains from the
Rio Algom Nordic Mine, Blind River, Ontario.

J4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

zoning in the growing grains. However, no zoning was Fuller, A. 0., 1958, A contribution to the petrology of the Witwaters­
rand System: Geological Society of South Africa Transactions, v.
observed in this study. Nor does it seem likely that a61, p. 20-50.
later thermal event could have erased any zoning. The ———1960,
Distribution of Witwatersrand uraninite: Economic
Witwatersrand sediments are regionally metamor­
Geology, v. 55, p. 842-843.
phosed only to the greenschist facies (Fuller, 1958), Hiemstra, S. A., 1968, The mineralogy and petrology of theuraniferous conglomerate of the Dominion Reef mine, Klerksdorp
while the Blind River sediments are virtually
area: Geological Society of South Africa Transactions, v. 71, p.
unmetamorphosed (Roscoe, 1969). Therefore, the1-66.
chemical variability appears more consistent with the Liebenberg,
W. R., 1955, The occurence and origin of gold and radioac­
hypothesis that pyrogenetic uraninite grains of varied
tive minerals in the Witwatersrand System, the Dominion Reef,
compositions were gathered from different localities
the Ventersdorp Contact Reef, and the Black Reef [with discus­sion]: Geological Society of South Africa Transactions, p. 101-254.
during erosion and this chemically heterogeneous mix­
Ramdohr,Paul, 1969, The ore minerals and their intergrowths
ture was transported and deposited as a mechanical[translation of 3d ed.]: Oxford, Permagon Press, 1,174 p.
mixture.
Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
The difference in variability of the composition of thethe Canadian Shield: Canada Geological Survey Paper 68-40, 205
two samples may yield some evidence concerning theirPsource areas. The uranite from the Blind River sample Schidlowski, Manfred, 1966, Beitfage zur Kenntnis der radioaktiven
Bestandteile der Witwatersrand-Konglomerate, I., Uranpecherz inhas much smaller variability than the. Witwatersrandden Konglomeraten des Oranje-Freistaat-Goldfeldes: Neues
sample. This may indicate that the uraninite in the Blind
Jahrbuch fur Mineralogie Abhandlungen, v." 105, p. 183-202.
River sample was derived from a single relatively Viljoen, R. P., Saager, Rudolf, and Viljoen, M J., 1970, Some thoughts
homogeneous source area, whereas the uraninite in theon the origin and processes responsible for the concentration ofgold in the early Precambrian of Southern-Africa: Mineralium
Witwatersrand sample had multiple sources.
Deposita, v. 5, p.164-180.
Such possibilities have already been suggested. Fuller(1960) and Hiemstra (1968) have suggested that the sedi­ment may have been derived from different source
DISCUSSION
areas. Viljoen and others (1970) have argued for aheterogeneous source for the Witwatersrand basin Skinner: Perhaps this is not a fair question for you,uraninite in order to account for the variation of thorium
David, but one of the South Africans may step in. If
content between the Dominion Reef and Witwatersrandthe Witwatersrand can be considered to be a large
Series uraninites. Variations of U02 and Th02 insample of the detrital minerals from the crust and if
samples from many parts of the Witwatersrand basin
the crustal abundance figures for thorium andshould be investigated to study the possibility of multi­
uranium are in fact correct, showing that thorium isple or heterogeneous sources for the uraninite.
four or five times more abundant than uranium, whereis all the thorium?
ACKNOWLEDGEMENTS
Grandstaff: I don't think that, in fact, the Witwaters­
I would like to thank Dr. E. S. A. Antrobus and therand represents a major sample of the crust. If you
Anglo-American Mining Corp. and the Rio-Algom Min­
plot up analyses of pegmatitic uraninites from theing Corp. for providing the samples, and Mr. C. Kulik
various parts of the world, they scatter quite widely infor aid with the microprobe analysis. This work wasuranium-to-thorium ratio. I think we are really looking
undertaken as a part of a doctoral dissertation at
at an individual chance segregation of uranium overPrinceton University. The work was supported bythorium.
NDEA grants under Title IV, and by grants to the Skinner: Then would it be a correct conclusion to draw
Princeton University computer center.
that if there is another hidden pebble conglomerate
somewhere the chances are it is a thorium rich one,
I would also like to thank the Geological Society of
not a uranium rich one?
South Africa for permission to have this article
republished. It appeared first in the Transactions of the Grandstaff: Could well be, yes.
Geological Society of South Africa in 1975, v. 77, p. Robertson: In comment to Dr. Skinner's question,
perhaps one should point out that certainly in the291-294. The U.S. Geological Survey also thanks theBlind River ores monazite is a fairly significant com­
Geological Society of South Africa for permission toponent.
republish this paper.
Roscoe: You could add to that that we do get con­glomerates in the Blind River ores whch have very
REFERENCES CITED
high thorium-uranium ratios, 4 to 1, and they appear
to complement the high uraninite ones. The concept is,
Frondel, Clifford, 1958 [1959], Systematic mineralogy of uranium andthorium: U.S. Geological Survey Bulletin 1064, 44 p.
of course, that this is hydraulic differentiation.

URANIUM, THORIUM IN URANINITE, WITWATERSRAND, SOUTH AFRICA BLIND RIVER CANADAGrandstaff: Those high thorium concentrations thatyou pointed out are largely monazite-zircon concen­
trates rather than uraninite concentrates.
Simpson: Did you analyze any uraninite within carbon?
Our analyses show that within a given sample, thistype of uraninite is generally thorium poor andhomogeneous.
Grandstaff: I analyzed only a couple of grains fromcarbon leaders, and they contained several percent
thorium and were relatively homogeneous.

J5

Feather: I would like to comment on your question,Peter [Simpson]. I have analyzed a great manyuraninite grains, both the distinctly detrital grains,that is, grains not associated with carbon, and grains
within carbon. And I have found as wide a variation ofthorium content in uraninite grains included in carbon
as I have found in uraninite grains that were notassociated with the carbon.
Grandstaff: I analyzed one thucholite uraninite grain
which had about 8 to 10 percent thorium. They do getthat high.

Preliminary Notes Concerning the Uranium-Gold Ratio and theGradient of Heavy-Mineral Size Distribution as Factors ofTransport Distance down the Paleoslope of theProterozoic Steyn Reef Placer Deposit,
Orange Free State Goldfield, Wit water sr and, South Africa
By W. E. L. MINTER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONALPAPER 1161-K

CONTENTS
Page

Abstract ________________________________________________
Introduction _______________________________________________
Particle-size changes in the Steyn Reef _______________________________
Mineral concentration changes in the Steyn Reef _________________________
Conclusions ________________________________________________
References cited ____________________________________________

Kl
1
1
1
2
2

ILLUSTRATIONSPage

FIGURE 1. Photographs of samples of Steyn Reef at 5-kilometer intervals down its paleoslope ________________________
2. Graphic plot illustrating progressive changes of particle size and uranium-gold ratio down the Steyn Reef paleoslope _____

K2
3
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESPRELIMINARY NOTES CONCERNING THE URANIUM-GOLD RATIOAND THE GRADIENT OF HEAVY-MINERAL SIZE DISTRIBUTION AS
FACTORS OF TRANSPORT DISTANCE DOWN THE PALEOSLOPE OF
THE PROTEROZOIC STEYN REEF PLACER DEPOSIT,
ORANGE FREE STATE GOLDFIELD,WITWATERSRAND, SOUTH AFRICA
By W. E. L. MlNTER1ABSTRACTThe size decrease of quartz pebbles, pyrite nodules, and zircon
grains, evident from samples of Steyn Reef taken from various posi­tions down a paleoslope indicated by crossbedding data, confirms theirdetrital origin. An increase in the ratio of uranium to gold, which ap­
pears to be related to their original size-frequency distribution, also in­
dicates the paleoslope direction and effectively distinguishes between
banket and carbon-seam reefs.

INTRODUCTION

The hydraulic size equivalence between uraninite
grains and the sand-sized fraction of South African Proterozoic placer deposits, described by Koen (1961), wassubstantiated by Viljoen (1963), who found that theequivalence also applied to pyrite and zircon grains andto a number of other minerals. These were consequently
considered to have been detrital components of theplacers. Furthermore, Viljoen demonstrated that there
was a decreasing size gradient of these minerals down
the paleoslope of the placers he sampled.
A gradient of this nature depends, however, on thehydraulics of the sedimentological environment in which
the placer was deposited and on the part of the paleo­slope that has been preserved. For instance, the ex­ponential tail of grain-size decrease that may be ex­pected in a distal position on a paleoslope plus the transgressive process involved during deposition of the Vaal
Reef (Minter, 1972) resulted in a practically insignificant
size gradient.
In the Orange Free State Goldfield, the Steyn Reef
placer deposit, which is a regressive deposit, lies uponpractically the same paleosurface as the Basal Reef
1 Geology Department, Anglo American Corporation, P.O. Box 20, Welkom 9460, South
Africa.

(Sims, 1969) and has a sequence from proximal, largepebble conglomerate banket reef to distal, small-pebbly
sand carbon-seam reef, preserved down a paleoslope
distance that exceeds 15 km. This provides an oppor­
tunity to study size changes of detrital minerals over awide range of hydraulic energy levels.
PARTICLE-SIZE CHANGES IN THE STEYN REEF

The mean sizes of quartz pebbles, massive pyrite
nodules, and zircon grains were obtained by measuring
their apparent maximum axes, in phi units, from thesmoothed surfaces, cut normal to bedding, of samples
collected down 15 km of paleoslope. Over this distance,quartz pebbles grade from cobble to small-pebble size,
pyrite nodules grade from small-pebble to pinhead size
(fig. 1), and zircons grade from fine to very fine grained.
The change in zircon sizes is less conspicuous than for
either pebbles or pyrite (fig. 2), but it was noticeable thatzircon grains become much more abundant in distal
samples. However, all three parameters reflect the
paleoslope that has been well defined by abundant crossbedding data (Sims, 1969). Isopleths will be drawn whenmore data are available and after the area has been
structurally reconstructed.
MINERAL CONCENTRATION CHANGES IN THE
STEYN REEF

Abundant sampling data obtained from mining of the
Steyn Reef down the entire paleoslope indicate that theoverall gold content decreases while the overall uranium
content increases. This is illustrated in figure 2 by a plotof the ratio of uranium to gold, the concentration of bothhaving been measured in parts per million. Isopleths beKl

K2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

ing constructed substantiate the paleostrike obtained
from crossbedding data.
High uranium-gold ratios appear to characterize thesmall-pebble distal environments of all Witwatersrandcarbon-seam reefs, and low ratios, those of proximal,banket reefs (Minter, 1972). Zircon and uraninite areconsidered to have been derived from the same granitic
provenance area, and, consequently, their low abun­dance in proximal deposits may be linked to an inherent
sparsity of coarse grains in the original supply.
CONCLUSIONS

Size gradients of pebbles, pyrite nodules, and zircon
grains in the Steyn Reef placer indicate the dispersal
direction of the prevailing paleocurrents and the detritalnature of the mineralization.

REFERENCES CITEDFIGURE 1.— Photographs of samples of Steyn Reef taken at 5-km inter­vals down a paleoslope that was indicated by crossbedding data in
the Orange Free State Goldfield.

Koen, G. M., 1961, The genetic significance of the size distribution of
uraninite in Witwatersrand bankets: Geological Society of South
Africa Transactions, v. 64, p. 23-46.
Minter, W. E. L., 1972, The sedimentology of the Vaal Reef in the
Klerksdorp area: University of the Witwatersrand, unpublished
Ph.D. thesis, 170 p.
Sims, J. F. M., 1969, The stratigraphy and paleocurrent history of theupper division of the Witwatersrand system on President Steyn
Mine and adjacent areas in the Orange Free State Goldfield, withspecial reference to the origin of the auriferous reefs: University of
the Witwatersrand, unpublished Ph.D. thesis, 181 p.
Viljoen, R. P., 1963, Petrographic and mineragraphic aspects of theMain Reef and Main Reef Leader of the Main-Bird Series, Wit­
watersrand system: University of the Witwatersrand, unpublished
M. Sc. thesis, 193 p.

K3

URANIUM-GOLD RATIO, HEAVY-MINERAL DISTRIBUTION, STEYN REEF DEPOSIT, SOUTH AFRICA

a.
a.

,\o

-60-

30

o

5 25
cc.

M
05
UJ

-i
o

—i ~40
20 CD
en
ui

1

15

a.

ISI

z

cc

at

10 §-20

o

00

ui

10

10

IXI

ui
IS

UJ

o
O

20

ui
K
(T

820
s

>-

CL

30

30

10

15

KILOMETERS

FIGURK 2.-A graphic plot that illustrates changes related to particle size and uranium-gold ratio that took place during transport of the Steyn
Reef placer down 15 km of paleoslope.

Geochemical Studies on the Origin of
the Detrital Pyrites in the Conglomerates ofthe Witwatersrand Goldfields, South Africa
By RUDOLF SAAGER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

PAPER

1161-LCONTENTS
Page

Abstract _______________________________________________________

Introduction and geological setting _________________________________
Pyrite mineralogy ___________________________________________
Witwatersrand Basin _______________________________________
Archean greenstone terranes __________________________________
Sample preparation __________________________________________
Analytical procedure _________________________________________
Discussion of results __________________________________________
Factor analysis ____________________________________________
Lead-isotope investigation ______________________________________
Summary and conclusions ______________________________________
Acknowledgments __________________________________________
References cited ____________________________________________

LI

1
4
4
5
6
8
9
11
13
15
16
16

ILLUSTRATIONSFIGURES 1,2. Maps showing:
1. Main points of entry and general transport directions of sedimentary material into the Witwatersrand Basin ____
2. General geological setting of the Witwatersrand Basin and the major Archean greenstone belts of the Kaapvaal
Craton in the southern African Shield __________________________________________
3. Photomicrographs of ore samples showing examples of different pyrite structures in the Witwatersrand reefs _______
4. Electron-microprobe step-scanning profiles across a zoned pyrite from the Eagles Nest gold mine in the Barberton Moun­tain Land ______________________________________________________________
5. Graphs showing cumulative frequency distributions of trace-element values from the grouped pyrite samples and graphicalsolutions of Kolmogorov-Smirnov statistic ___________________________________________
6. Scatter diagram of the factor loading values ____________________________________________
7. Diagram showing 207Pb/204Pb versus 206Pb/204Pb __________________________________________

L2
3
6
9
10
13
14

TABLESPage

TABLE 1. The relative abundance of heavy minerals in the Witwatersrand sediments and their corresponding erosion units in themodel source area of the Barberton Mountain Land ______________________________________
2. Different pyrite structures in Witwatersrand reefs __________________________________________
3. Grouped results of trace-element analyses ______________________________________________
4. Results of Kolmogorov-Smirnov statistic _______________________________________________
5. Varimax factor score matrix ______________________________________________________
6. Partial Q-mode varimax factor matrix ________________________________________________
111

L4
5
9
11
12
12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESGEOCHEMICAL STUDIES ON THE ORIGIN OF THE DETRITAL PYRITESIN THE CONGLOMERATES OF THE WITWATERSRAND GOLDFIELDS,
SOUTH AFRICABy RUDOLF SAAGER1

troversy between the hydrothermalists and theplacerists (see for example Becker, 1909; Mellor, 1916;
Young, 1917; Graton, 1930; Liebenberg, 1955; Ramdohr, 1955; Davidson, 1957, 1960, 1961). The very com­
mon criterion, "Gold is where you find it," and the preoc­
cupation of the mine geologists on the Rand with thedaily problems of exploiting the precious metal also
hindered the search for the original source of the placer
minerals, in spite of its utmost importance to the case of
the placerists.
A great number of sedimentological, geological, andmineralogical studies carried out during the last twenty
years led to the general acceptance of the modified
placer theory as a metallogenetic concept for the Wit­
watersrand deposit. The concept implies that, after
their deposition, some of the detrital minerals, notably
gold and the sulfides, were reconstituted and lost theirwaterworn shapes as a result of the low-grade regional
metamorphic overprint of the embedding Witwaters­
rand and Dominion Reef sediments.
Today most investigators of the Witwatersranddeposit agree that the sediments and heavy minerals
originated essentially from a source area situated to the
INTRODUCTION AND GEOLOGICAL SETTING
north and west of the basin of deposition, with only
For many placer deposits the question of the original minor amounts of sedimentary material being intro­
source of the heavy minerals is an intriguing and often duced from the southeastern edge of the Witwatersrand
hotly debated subject of discussion. The solution of the Basin (Pretorius, 1974). Some of the evidence for thisproblem is not only of great academic interest but in conclusion is found in the conspicuous occurrence of all
many instances has proved to be of considerable the producing goldfields along the northwestern arc of
economic importance.
the Witwatersrand Basin, in the asymmetry of the gra­In the case of the early Precambrian fossil placers of dients of the paleoslopes, which are considerably steeper
the Witwatersrand Basin in South Africa, the problem on the northwestern side of the basin, and in the distinctof the provenance of the gold, uraninite, and other decrease of conglomerate thickness and pebble size in a
detrital conglomerate constituents has been over­ southerly to southeasterly direction. Pretorius (1966,
shadowed for a long time by the metallogenetic con­ 1974) convincingly demonstrated that each of the six
major goldfields represents a fluvial fan or fan delta (fig.
1). Furthermore, he concluded that the fluvial system
was short, the provenance area being close to the edge
ABSTRACTThe principal source area of the Witwatersrand sediments, including
heavy minerals such as gold, uraninite, and pyrite, lies to the west andnorth of the basin of deposition. Because this area is overlain byyounger rocks, the actual identification of the source area is virtually
impossible. On the basis of a hypothetical provenance model it has beensuggested earlier that the primary source of the sediments was in theArchean granite-greenstone terrane of the Kaapvaal Craton.
For the present study, 57 pyrite samples from primary gold depositsof the greenstone terrane (Swaziland Sequence) and from various gold
reefs of the Witwatersrand deposit have been investigated for theirtrace-element content (Co, Ni, Cu, Zn, Pb, Ag) in order to study thepossible presence of similar geochemical trends within both groups ofpyrite. The analytical data have been treated by statistical methods,
and the results indicate that both the primary and the detrital Wit­
watersrand pyrites, with respect to the investigated elements, weredrawn from the same population. They are both clearly different fromthe Witwatersrand pyrites formed in situ. Lead isotope investigations
carried out on sulfides from primary gold deposits of the Swaziland Se­
quence and on detrital and authigenic pyrites from the Witwatersranddeposit showed similar isotopic compositions of the lead in detrital
pyrite and in primary sulfides from the Swaziland Sequence. The
results of the present geochemical studies thus constitute strong
evidence that the Archean greenstone belts of the Kaapvaal Craton
were the most probable source of the detrital pyrites and also of thegold in the sediments of the Witwatersrand Basin.

Ll

L2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESMain point of entry of
sedimentary material
General transport direction
Witwatarsrand Basin
Subsidary basin
90 KILOMETERS

FIGURE 1.-Locations of main points of entry and general transport
directions of sedimentary material into the Witwatersrand Basin(modified from Brock and Pretorius, 1964, pi. 6).

of the basin of deposition. The entry points of sedimen­
tary material into the basin, corresponding to the apicesof the fans, thus must have remained relatively fixedduring a considerably long period of time-that is, from
Dominion Reef till Upper Witwatersrand times.
To pinpoint the actual provenance area of thesediments and the placer minerals and thus to locate theprimary gold and uranium-mineralized areas, however,is virtually impossible, as these areas are largely
covered by younger Proterozoic rocks, such as the
Ventersdorp lavas, the Transvaal sediments, and the
Bushveld Igneous Complex. The few windows in which
pre-Witwatersrand rocks are exposed, for example theJohannesburg Dome, Westerdam Dome, and Amalia
Dome, show only rock types similar to those found in theArchean basement of the eastern Transvaal Lowveld
but to no other pre-Witwatersrand sequences.
Viljoen and others (1970), after synthesizing recently
published data on the Witwatersrand Basin and on thebasement of the southern African Kaapvaal Craton, sug­gested that the primary source of the Witwatersrandgold and detrital sulfides should be sought in the Ar­
chean greenstone belts of the craton. As a hypotheticalmodel source terrane, they proposed the classic area of
the Barberton Mountain Land (fig. 2).
In the Kaapvaal Craton, the generally northeasttrending Archean greenstone assemblages are referred
to as the Swaziland Sequence. The lowermost membersof the Swaziland Sequence, which in the Barberton area
are fully preserved, constitute the Onverwacht Group,characterized by ultramafic, mafic, and acid volcanics(Viljoen and Viljoen, 1969; Anhaeusser, 1971). Theserocks are overlain by the argillaceous sediments of the
Fig Tree Group, which, in turn, is followed by the prin­cipally arenaceous sediments of the Hoodies Group,
forming the topmost members of the sequence. As withother greenstone belts in the Kaapvaal Craton, theBarberton Mountain Land is well mineralized andforms, next to the Witwatersrand Basin, the most im­
portant gold province in South Africa (fig. 2).
Most of the gold occurrences in the Barberton Moun­tain Land are located along a stratigraphic horizon inbasic volcanics at the base of the Upper Onverwacht
Group and along unconformities between the Upper
Onverwacht Group and the sedimentary groups of the
Swaziland Sequence. Viljoen and others (1969) showedthat the gold content of the ultramafic and mafic
magnesium-rich volcanics in the Barberton Mountain
Land ranges from 0.005 to 0.01 ppm (parts per million).
Gold contents of as much as 0.112 ppm have been ob­
served where the rocks have been weathered and re­cycled. The gold content in the accompanying intrusive
basement granites, on the other hand, is less than 0.005
ppm. From these geochemical trends and from various
geological and mineralogical observations, Viljoen andothers (1969) and Saager (1973a, b) concluded that thegold deposits in the greenstone belts are geneticallyrelated to the magnesium-rich mafic to ultramafic
volcanics and not to the late volatile phases of the in­truding granites. Descriptions of the various processes
that led to the formation of the gold deposits in thegreenstone belts of the Kaapvaal Craton are given by
Viljoen and others (1969) and Saager (1973b).
The suitability of the Barberton Mountain Land as amodel source terrane for the sedimentary filling of the
Witwatersrand Basin was demonstrated by Viljoen andothers (1970) owing to a remarkable correlation between
the expected heavy minerals derived from successive
erosional levels of such a terrane and the actual heavy
minerals encountered in the various horizons of the Wit­
watersrand sediments. The results of their study aresummarized in table 1. It demonstrates the relative
abundance of uraninite, monazite, cassiterite, andgarnet in the lower portions of the sedimentary succes­
sion, especially in the conglomerates of the Dominion
Reef Sequence. Chromite and the platinoids show a
distinct increase towards the topmost members of thesediments, and the gold, as well as the sulfides, reaches
a maximum in the Main Bird Series of the Upper Wit­
watersrand Sequence. This heavy-mineral distribution
indicates that, in the provenance area, the lower ero­sional level consisted mainly of mafic to ultramafic
rocks, whereas the higher levels of erosion consisted
more and more of granitic, pegmatite-rich rocks.

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA

EXPLANATIONGreenstone belts (Swaziland Sequence)Primary Gold Provinces >3300 m.y.
Dominion Reef and Witwatersrand Sequence
Placer Gold Province <2870 m.y.
Metamorphic belts
Granite-greenstone domes<3020 m.y.
B

Barberton Mountain Land

M
S
P

Murchison Range
Sutherland Range
Mount Mare\\/"Swazii I
S land fH
IndianOcean

FIGURE 2. -Part of the southern African Shield, showing general geological setting of the Witwatersrand Basin and the
major Archean greenstone belts of the Kaapvaal Craton. The locations of the major goldfields along the western andnorthern rim of the Witwatersrand Basin are marked with a heavy black line (modified from Viljoen and others, 1970,
fig. 1).

L3

L4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE I.-The relative abundance of some heavy minerals in the
rudaceous sediments of the Witwatersrand Basin and their cor­
responding erosion units in the model source area of the Barberton
Mountain Land
[Modified from Viljoen and others (1970, p. f 74). Relative abundance of heavy minerals (U,uraninite; G, garnet; M, monazite; C, cassiterite; Z, zircon; G, gold; S, sulfides; C, chromite,P, platinoids): T. not abundant; "ft. fairly abundant; TTT. very abundant]
Stratigraphic position__
in sediments of the Wit- U
watersrand Basin, in
Stratigraphic sequence

Kimberley-Elsburg
Series.

Main-Bird Series

Lower WitwatersrandSequence.

Dominion Reef
Sequence.

Detrital minerals__Main rock types in the
P model provenance terrane (Barberton Moun­tain Land), in reverse
Stratigraphic sequence

TT TT TT TTT TTT I. Ullrnmufv Unit (Lower
Oru-crwiichl Group):
Ultramafic to mafic
lavas.
Minor felsic tuffs
and porphyries. Lowvolatile tonalitic
granite.
TT TTT TTT TT T II. Mafic to Felsic Unil
{U/j/jer Ont'ent'iirht
Group):
Mafic, andesitic, andfelsic lavas.
Felsic to mafic pyroclasts.
Banded chert.
Low-volatile tonalitic
granite.
Some high-volatile
granite (root zone).
TT
T
T
f
III.
Argillaceous Unit
TT
(Fiy Tree Group):
Shales, greywackes,
banded ironstone.
Low-volatile tonalitic
granite.
High-volatile granite(root zone).
tt
T
T
T
IV.
Arenaceous Unit
TTT ttt TTT TTT(Moodifx Group):
Quartzites, conglomer­
ates.
High-volatile "hood"granite.
Pegmatites.

uraninite, chromite), in false footwalls and in footwallcontacts. Between 50 and 75 percent of the pyrites in
the reef horizons are rounded (fig. 3A). These grains
have been described by Becker (1897), who consideredthem to be of detrital origin.
Witwatersrand pyrites have been classified by Ramdohr (1955) and Saager (1970); the different pyrite struc­tures are summarized in table 2. In general, pyrite canbe differentiated into the three following groups:
1. Rounded allogenic pyrites form the bulk of therounded grains. They are exceptionallyunweathered, are generally less than 0.2 mm insize, and acquired their roundness by abrasion dur­ing sedimentary transport (fig. 3A-D). Besides theirwaterworn shapes, occasional primary inclusions ofsilver-rich gold particles, possessing a higher reflec­
tivity than the yellow (silver-depleted) reconstituted
gold in the conglomerate matrix, provide further
evidence for a detrital origin of these pyrite grains(see also Feather and Koen, 1975). Other primary
inclusions in the rounded allogenic pyrite
grains-pyrrhotite, pentlandite, chalcopyrite, and,less often, sphalerite - are essentially the same as in
the pyrite crystals of the primary gold mineraliza­
tions of the Archean greenstone belts.

2. Concretionary authigenic pyrites have diameters
several times larger than those of rounded allogenic
pyrites and form the largest proportion of the"buckshot pyrites," which often are as large as
hazelnuts. In spite of their generally loose struc­tures and delicate forms, the concretions have
The present geochemical investigation was under­
distinctly rounded outlines and are very welltaken to further test the source hypothesis of Viljoen
preserved(figs. 3E-I). On account of their loose
and others (1970). The study was restricted to pyrite
structures,long transportation distances seem tobecause it forms the most abundant ore mineral in thebeimprobable,
and Ramdohr (1955) and Saager
primary gold ores of the Archean greenstone belts and(1970)
havesuggestedthat the concretions werein the Witwatersrand conglomerates. It was carried out,
formedinsitu.
Skeletal-type
growth structures,first, by a trace-element investigation of pyrites fromsuchasdelicate
axial
crosses,havebeen found in
the two gold provinces and, second, by a comparison ofpracticallyallpyriteconcretions
andare charac­
the lead-isotope composition of Witwatersrand pyrites
teristic
features.
SaagerandMihalik(1967) ob­
with that of pyrites and lead-bearing sulfides associatedservedthattheconcretionary
pyriteaggregateswith various gold deposits in the Barberton Mountainvery often consist of an isotropic and an anisotropicLand.
variety, which differs markedly in colour, reflectivi­
ty, hardness, and trace content of nickel, cobalt,
PYRITE MINERALOGY
and arsenic. The pores in the marginal area of the
WITWATERSRAND BASIN
concretionary pyrite grains sometimes containnumerous fillings of gold, chalcopyrite, and galena.
In the Witwatersrand ores pyrite generally con­This indicates that the concretions acted as traps
stitutes more than 90 percent of the ore minerals pres­for metamorphically mobilized ore minerals, which,ent and appears to be the only macroscopically iden­
after deposition of the concretionary grains, in­tifiable ore mineral (Saager, 1970). It generally occurs
filtrated the pores where they now form inclusions.
disseminated throughout the entire width of individual
In rounded allogenic pyrite grains, inclusions ofore horizons but in many places is concentrated,galena are practically absent.
together with other detrital ore minerals (such as gold,

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA

L5

TABLE 2. -Different pyrite structures in Witwatersrand reefs[Modified from Saager, 1970, p. 32. Printed with the permission of the Geological Society of South Africa]
Morphology

Rounded

Structures

Compact or porous
Pseudomorphic ___Concretionary ____

Idiomorphic to hypidiomorphic _Nodules containing "mineralizedbacteria."
Compact or porous ___________
Compact or porous encrustions _Xenomorphic ___________

Veinlets and fracture filling.

Allogenic

Authigenic

Detrital pyrite and black sands
pyritized before their final
deposition.
Inhomogeneous black sands pyri­
tized before their final depo­
sition.

Black sands pyritized in situ.
Inhomogeneous black sands pyri­
tized in situ.
Generally formed in situ; very
rarely signs of short trans­
portation.
Formed in situ; indicating pres­ence of primitive life.
Formed in situ by reconstitutionof ferrous sulfide; rarely hydrothermal along dikes.
Formed in situ by reconstitution
during metamorphism; gene­rally overgrow older detrital
pyrites.
Formed in situ by reconstitution
during metamorphism.

pyrites and base-metal sulfides in the Witwaters­
Rounded pyrite nodules containing microspherirand ores (Saager, 1968).
cal structures of the "mineralized bacteria type" areclosely related to the concretionary pyrites. The
microspheres reach diameters of 10 to 15 m (micro­
ARCHEAN GREENSTONE TERRANES
meters) and, in rare cases, have been observed to be
as large as 45 m. In most cases, the structures are
In the primary gold mineralizations of the Archeanirregularly distributed in the nodules (figs. 3G, H).
Saager (1970) has suggested that these greenstone belts of the Kaapvaal Craton, pyrite is
microspheres may represent organic morphologies. generally the most abundant ore mineral and often
He concluded that the concretionary pyrites and forms the most important carrier of gold. In the goldrounded pyrite nodules, as products of precipita­ vein deposits, generally two generations of pyrite havetion, were formed in situ by the activity of been observed (Saager, 1973b). The older generation
anaerobic H 2S-generating, sulfate-reducing forms large, usually idiomorphic grains measuring as
microorganisms, which preferably existed in small much as 10 mm across. Inclusions of pyrrhotite andchalcopyrite are abundant; less common inclusions arepocketlike areas in the Witwatersrand Basin.
Reconstituted authigenic pyrites form idiomorphic to gold, magnetite, rutile, and sphalerite. The younger
hypidiomorphic grains or occur as late encrusta­ pyrite generation generally exhibits pentagonaltions on detrital components of the conglomerate, dodecahedral outlines, grain sizes being less than 100 /u..
particularly on pyrite (see figs. 3 J, K). The pyrite It generally surrounds the pyrite of the first generation
encrustations lead directly to the common stringer- and rarely contains inclusions of other ore minerals.
A noticeable feature of many pyrites from the green­
like pyrite veinlets that cut through detritalstonebelts is the common occurrence of a distinct zonal
minerals and form fracture fillings in otherstructurecaused by a variation in the trace-element con­minerals. This authigenic pyrite variety has been
tentofnickel,cobalt, and arsenic (fig. 3L; see also Vilformed by recrystallization during the metamor­joen
andothers,1969). Microprobe analyses ofphism of the Witwatersrand sediments (Saager,
brownish-coloured,
relatively
soft, weakly anisotropic1970).
pyritezones
yieldednickel,cobalt,and arsenic contents
In certain places dikes intrusive into the Wit­
of
as
much
as
1.75
percent,
0.17
percent,
and 0.24 per­watersrand rocks have influenced the content ofcent,respectively
(fig.
4).
For
more
detailed
descriptions
authigenic minerals and have led to the formationof enriched masses of pyrite, galena, chalcopyrite, of the mineralogy of the gold ores in the Archean green­and sphalerite. Such occurrences, however, are stone belts, the reader is referred to de Villiers (1957),
local and account only for a very small proportion of Schweigart and Liebenberg (1966), Viljoen and others
the authigenic idiomorphic to hypidiomorphic (1969), Saager (1973b) and Liebenberg (1973).

L6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 3. -Examples of different pyrite structures in the Witwatersrand reefs. A, Typical ore sample from the Basal Reef, showing abradedallogenic pyrite and reconstituted authigenic pyrite, which molds around the rounded grains (white). Note the detrital chromite in the centreof the photograph (dark grey). Free State Geduld Mines; reflected light, x 200. B, Concentrate of rounded allogenic pyrites from the Leader
Quartzite horizon. Welkom Gold Mines; reflected light, x 50. C, Rounded allogenic pyrite with "primary" inclusions of gangue (black) andgold (bright white, scratched). Free State Geduld Mines; reflected light, oil immersion, x 500. D, Two rounded allogenic pyrites in gangue(black) from the Basal Reef. Note the fracture fillings of reconstituted gold (bright white) and chalcopyrite (dark grey) in the shattered pyrite
grain on the left side of the photograph. Free State Geduld Mines; reflected light, oil immersion, x 500.

SAMPLE PREPARATION

Forty-four pyrite samples from 20 different primarygold deposits in the Barberton Mountain Land, the Murchison Range, and the Pietersburg Schistbelt, as well as
13 pyrite samples obtained from pay reefs of the Central
Rand, Klerksdorp, and Orange Free State Goldfields,
were used for the present investigation. Before concen­
tration, a polished section was made of each ore sample,
and the homogeneity and intergrowth relations of thepyrites were studied under the ore microscope.

Most of the ore specimens of primary gold deposits
showed large individual pyrite grains as much as 12.5mm in diameter. They were isolated from the sample
with the use of a dentist's drill, or, after gently crushingand cleaning, the grains were concentrated by handpicking under the binocular microscope.
The concentration of rounded allogenic Witwaters­
rand pyrites proved to be more complicated owing to thesmall grain sizes of the detrital pyrites (<0.5 mm) andthe omnipresence of various pyrite varieties. For theseDETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA

L7:»<fcaE

FIGURE 3. -Continued. E1, Ore sample from the Basal Reef, showing rounded allogenic pyrites (partly cataclastic) and a large porous pyrite con­cretion. Free State Geduld Mines; reflected light, x 60. F, A large, fractured, porous pyrite concretion. Free State Geduld Mines; reflected
light, x 100. G, "Mineralized bacteria" structures in a large authigenic pyrite concretion from the Basal Reef horizon. Free State Geduld
Mines; reflected light, oil immersion, x 500. H, Pyrite aggregate consisting of differently shaped microspherical structures. Note that along
the edge of the pyrite aggregate some of the globules are distinctly abraded, which suggests some transport of the aggregate. Free State
Geduld Mines, Basal Reef; reflected light, x 500.

reasons, only those samples could be used that had an
unusually high content of one of the various types ofpyrites. Suitable samples were so scarce that, althoughmore than 100 polished sections were investigated, only
5 were of samples that could be used for the concentra­tion of detrital pyrites. The selected samples werecrushed, sieved, and cleaned in a water stream. Follow­
ing this, the concentrates were obtained by heavy-liquid
separation and by handpicking the detrital grains underthe binocular microscope according to their distinct
morphological features.

The much larger grain sizes of the authigenic Witwatersrand pyrites, notably the concretionary buckshot
pyrites, facilitated concentration and allowed a greaternumber of samples to be investigated.
Following these concentration and separation proces­ses, all the pyrite samples were treated with 20 percent
HC1 and again cleaned ultrasonically in distilled water
until free of visible turbidity. Polished sections werethen made from about one-half of the concentrate, andtheir purity was examined under the ore microscope.
From these examinations, it was estimated that theL8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 3. -Continued. /, Photograph of a polished slab, showing the footwall contact of the B-Reef horizon with underlying ore-free shales. The
sample shows an unusually rich concentration of large concretionary buckshot pyrites. Free State Geduld Mines; x 5. J, Photograph of apolished slab from the Basal Reef horizon. This sample contains predominantly idiomorphic authigenic pyrites in a quartz-pebble con­glomerate. Free State Geduld Mines; x 5. K, Microphotograph of the authigenic pyrites shown in figure 3 J. Note the small detrital chromite
grains (dark grey), which are partly overgrown by the reconstituted pyrite formed in situ (white). Free State Geduld Mines; reflected light, oil
immersion, x 350. L, Zoned pyrite grains from a primary gold deposit in the Barberton Mountain Land. Sheba Queen Mine; reflected light,
oil immersion, x 500.

by an outside laboratory (Metallgesellschaft, Frankfurt)
using photometric and polarographic methods. The
limits of detection were 10 ppm for zinc and silver, 20
ppm for copper, cobalt, and nickel, and 50 ppm for lead.
The precisions (two standard deviations) range between
10 and 30 percent.
The selection of the elements under investigation was
ANALYTICAL PROCEDURE
based mainly on the following considerations:
The copper, zinc, lead, cobalt, nickel, and silver con­tents were determined with a Perkin Elmer Atomic Ab­ 1. Copper, zinc, and lead occur only in restrictedamounts ,as substitutions in the lattice of mostsorption Spectrophotometer at the University of Heidel­
natural pyrites (Boyle, 1965; Radcliffe and Meberg, and a number of control analyses were carried out
contamination of the pyrite concentrates by silicate
minerals or other sulfides was less than 5 percent by
volume and that, for the Witwatersrand samples, theseparation of the authigenic from the allogenic pyrite
variety was between 80 and 90 percent successful.

L9

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA

can assume that the nickel and cobalt values ob­
tained indicate the overall availability of these twoelements during the formation of the pyrites.
3. Silver occurs almost exclusively in solid solution withgold. Silver-bearing sulfides have been found only
as a few grains in a galena aggregate from the
Basal Reef in the Witwatersrand deposit (Saager,1968). The silver values, therefore, serve as ex­cellent indicators for the amount of gold present as
inclusions in the pyrite. Primary gold inclusions in
detrital Witwatersrand pyrites contain approxi­
mately 10 percent silver (Saager, 1969); and for thegold grains in primary ores of the Barberton Moun­tain Land, silver contents of 0.2-21.6 percent havebeen reported by Liebenberg (1973).
DISCUSSION OF RESULTS

According to the mode of formation and the origin of
the sampled pyrites, the geochemical data were divided
into the following three groups (see also table 3):
120
40
80
Group 1 (Gl), Authigenic Witwatersrand pyrites
MICROMETERS
Group 2 (G2), Allogenic Witwatersrand pyrites
FIGURE 4. -Electron-microprobe step-scanning profiles across a zoned Group 3 (G3), Greenstone-belt pyrites
pyrite from the Eagles Nest gold mine in the Barberton Mountain
The silver content of samples in Group 1 ranged fromLand.
0 to 390 ppm; in Group 2, from 0 to 10 ppm; and in Group
3, from 0 to 380 ppm. The silver content in most sam­Ween, 1969 and 1970; Scott and Barnes, 1972; and ples, however, lies below the limit of detection for theothers). The observed trace contents of these analytical method used (10 ppm), and consequently
elements thus indicate the extent to which the in­ silver is not considered in the statistical analysis.
vestigated pyrites were contaminated by micro­
For cobalt, nickel, copper, lead, zinc, and each of thescopic to submicroscopic inclusions of base-metal three groups of samples, the sample cumulative frequen­sulfides-for example chalcopyrite, sphalerite, and cy distribution was constructed (fig. 5). With the aid ofgalena, as the most common ones.
nonparametrie Kolmogorov-Smirnov statistic the
2. Nickel and cobalt, in contrast, enter the pyrite struc­ various cumulative frequency distributions were com­
ture with ease (Kullerud and others, 1962; Ram- pared, and, for each element, the null hypothesis was
dohr, 1969; and others), and since nickel-cobalt tested that the three sample frequency distributions (thesulfides occur rarely in the investigated ore samples three postulated pyrite groups) were drawn from(minute pentlandite exsolutions in pyrrhotite), one populations having the same frequency distribution.
TABLE 3.- Grouped results of trace-element analyses[x, mean; s, standard deviation; C, coefficient of variation]
Element
Group

Gl (8 samples), authigenic Witwatersrand pyrites:
x ______ ___________________'______________ppm__
s ________________________________________________________ppm__
C ______________________________________________________________
G2 (5 samples), allogenic Witwatersrand pyrites:
x ________________________________________________________ppm__
s ________________________________________________________ppm__
C ______________________________________________________________
G3 (44 samples), greenstone-belt pyrites:
x _______________________________________ppm__
s ________________________________________________________ppm__
C ______________________________________________________________

Co

Ni

Cu

Zn

Pb

1,301
932

1,347
490

251
109

56
30

963
1,209
1.07

216
88
198
195

.71

.41

.98

450
112

.36

.25

514
532
1.04

118
63

.43

.53

341
456
1.13

64
21

.54

.31

151
379
2.51

135
69
123
73

.51

.59

L10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

•100

100

61

373

685

145

30
110
190
PARTS PER MILLION

100

262

100

l-

993

1846
30
PARTS PER MILLION

110

190

FIGURE 5.-Cumulative frequency distributions of trace-element values from the grouped pyrite samples, and graphical solutions ofKolmogorov-Smirnov statistic. Gl, authigenic Witwatersrand pyrites; G2, allogenic Witwatersrand pyrites; G3, greenstone-belt pyrites(primary). dn, maximum vertical deviations between the sample frequencies: a, dG1_G2; m, dG2_G3; e, dG3_G1.

The Kolmogorov-Smirnov statistic (Smirnov, 1948;
Dixon and Massey, 1951; Miller and Kahn, 1965) wasused because no assumptions are needed about the nor­mality of the distributions and because it is not subject
to the very small sample limitation of chi-square. The
test is a graphic one, and the maximum vertical devia­
tion (dn) between the sample cumulative frequencies is
directly measured on the graph (see fig. 5). The obtained
dn values are then compared with values extracted fromtables, and the null hypothesis is either accepted or re­jected at a certain level of significance.
The results of the test are shown in table 4. At a 0.05
level of significance the measured maximum deviations(dn) do not in all cases exceed the values required to saythat the investigated sample groups were drawn fromdifferent populations. It follows that the proposedgrouping of the samples is not always geochemicallysignificant. This is particularly the case for the allogenic
Witwatersrand pyrites and the greenstone-belt pyrites(G2-G3). At a 95 percent level of probability-withrespect to all five investigated elements-the samples
were drawn from the same population. It thus can beassumed that both the allogenic Witwatersrand pyrites
and the pyrites from mineralizations of the greenstone
belts were formed in similar geological environments
under similar chemical and physical conditions. These
results support the source hypothesis of Viljoen andothers (1969) for an origin of the detrital Witwatersrand
pyrites from mineralizations in the Archean greenstoneterranes of the Kaapvaal Craton.
With respect to the other pairs of pyrite groups(G1-G2, Gl-G3)-that is, authigenic Witwatersrand
pyrites and allogenic Witwatersrand pyrites -on the one
side, and authigenic Witwatersrand pyrites andgreenstone-belt pyrites on the other side, the results are
not as distinct and uniform (fig. 5; table 4). For theelements cobalt, nickel, and lead the maximum devia­
tions (dn) exceed the required values, and the postulated

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLUFIELDS, SOUTH AFRICATABLE 4. -Results of the Kolmogorov-Smirnov statistics[Minimum deviation required at level: G1-G2, sign.0 05 =0.700; G2-G3, sign.0 05 = 0.580;
G1-G3, sign.o os = 0.590 (sign., significance)]

Element

G1-G2

Cobalt<4,_C2 = 1.000

Nickeldci-c2 = .875

CopperciGi_G2 = .625
H0:G1 = G2
c4i-c2 =.250
H0:G1 = G2
dc,-c2 = .875

Zinc
Lead

Pyrite groups
G2-G3
doi-n- 0.475
rloiCj2 = Cj3
Ho-tj2 = Cj3
ilolCji = CjO

G1-G3
dcl-c3= 0.955<£,-„=. 785
ilolCjl = CjO

dc2-C3 -.290
rloiCj2 = (joH0;G1 = G3

rlotCjS = Cj3

H0:G1*G3

sample groupings seem to be justified. The distributionof the elements copper and zinc, however, deviate very
little, and, at a 0.05 level, it can be assumed that the
samples were drawn from the same population. The pro­
posed grouping, with respect to copper and zinc, seems
to be invalid.
These contradictory results require further comment:
1. The copper and zinc trace contents of the pyrites, as
explained, have been caused by inclusions of basemetal sulfides and, thus, are an indication of thecontamination of the pyrite by other sulfides. Inclu­
sions of chalcopyrite and sphalerite have been
observed irregularly distributed in the pyrites of all
three postulated sample groups. They are primary
inclusions, which formed as mechanical admixtures(Ramdohr, 1969) during pyrite crystallization, or
are infiltrated secondary inclusions, which
migrated into the marginal areas of porous pyrites
during metamorphic periods (see also section on
pyrite mineralogy). Both types of inclusions are notcharacteristic of a specific pyrite group, and this isreflected in the Kolmogorov-Smirnov test.
2. Cobalt and nickel in pyrite show a different crystalchemical behaviour, occurring in solid solution. The
trace contents of these two elements are therefore
an indication of the availability of cobalt and nickel
during pyrite formation. It follows that these twoelements represent better indicators for the studyof the metallogenic origin of the investigatedpyrites.
Carstens (1941), Hegemann (1943), Cambel andJarkovsky (1967), and other workers have statedthat sedimentary and hydrothermal pyrites possess
different and characteristic cobalt and nickel
trends. Sedimentary pyrites have low Co/Ni ratios
and rather constant cobalt and nickel contents ofabout 10 ppm and 200 ppm, respectively. In con­trast, hydrothermal pyrites have more erratic andconsiderably larger cobalt and nickel contents(several thousand parts per million) and Co/Niratios that are greater than 0.1 and may be as highas 830 (Berg and Friedensburg, 1944). Results of

Lll

work by Hawley and Nichol (1961) indicate thatpyrites from magmatic ores commonly show higher
cobalt and nickel contents than do pyrites fromhydrothermal ores, and Wilson (1953), on the basisof relative availability, concluded that pyrite
derived from early magmatic melts would have ahigher nickel content, and probably a lower Co/Niratio, than would pyrite formed from late fluids.
Recent investigations (Tourtelot, 1964; Roscoe,
1965; Saager, 1965; Loftus-Hills, 1967; and others)suggest that the foregoing criteria are only valid ina generalized way and cannot be used to distinguishbetween different types of ore deposits. The studies
indicate, however, that in many cases pyrites from
specific occurrences or metallogenic provinces ex­hibit characteristic and persistent cobalt and nickel
trends, a conclusion that agrees with the present
study and that also explains why the elements
cobalt and nickel yielded the best results in the
Kolmogorov-Smirnov test.
The extremely high lead contents obtained (between
230 and 3,400 ppm) are significantly restricted tothe authigenic Witwatersrand pyrites (see table 3)
because the generally very porous pyrites quite
commonly have metamorphically infiltrated galena
inclusions in their marginal zones, and it can beassumed that the lead in these inclusions is ofradiogenic origin (Saager, 1970; Burger and others,
1962).
The allogenic Witwatersrand pyrites, which arevery compact (Saager, 1970) and practically free ofgalena inclusions of infiltrated radiogenic lead,
have lead contents of less than 240 ppm. Similar
low lead contents were found in the pyrites fromthe greenstone belts, which have been formed in ageological environment characteristically devoid of
lead-bearing minerals (Villiers, 1957; Saager andKoeppel, 1976). It is for these reasons that the leadvalues yielded the same statistical trend as
displayed by the cobalt and nickel data, in spite of
the fact that the crystal-chemical behaviour of leadis markedly different from that of nickel and cobalt.
FACTOR ANALYSIS

To further investigate the trace-element data of thepyrites and to test the results of the graphic
Kolmogorov-Smirnov statistic, a factor analysis of thedata was carried out. Factor analysis is a branch ofmultivariate statistics that recently has found increas­
ing application in various fields of the Earth sciences(Imbrie and van Andel, 1964; Nichol and others, 1969;
Saager and Sinclair, 1974; and others). The primary aimof the analysis is to investigate the intercorrelations

L12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

within a set of data and to find the smallest noncorrelated set of basic vectors that accounts for most of thevariance in the original set of data. Factor analysis,
therefore, tries to simplify a complex set of data by ex­pressing it in a minimal number of theoretical variables
or factors without significant loss of information. The
factors, which are linear combinations of the originalvariables, must be interpreted in terms of all available
geological and other information. For a detailed
mathematical elucidation of factor analysis and its pro­blems the reader is referred to the appropriate texts by
Spearman (1904), Kaiser (1958), and Harman (1960).
Q-mode factor analysis as employed in this study ex­
amines the variation from sample to sample, and its ob­
jective is to find groups of samples that are similar interms of their total composition. The observed varia­
tions from sample to sample are not independent but are
dependent upon underlying causes. If these causes are
the same for two samples, then the values measured inthese samples will be similar. The measure of similarity
used in the Q-mode analysis is the cosine "theta" of theangle between two sample vectors in w-dimensional
space. All the cosines obtained yield the cosine "theta" orsimilarity matrix. It is analysed mathematically, and theprincipal components or factors are extracted bysimultaneous consideration of all the sample similarities.
Once the minimum number of factors required has been
established, the factors are rotated according to specificmathematical criteria-in the present study, thevarimax rotation-to aid interpretation of geologicaldata.
The Q-mode factor analysis was carried out at theComputing Center of the University of Heidelberg, us­ing a modified FORTRAN program written by J. D. S.
Wilson, Department of Geology, University of British
Columbia. The results of the analysis showed that a
2-factor model accounts for 69.8 percent of the total
variance in the original pyrite data. The composition of
the two factors, or end members, can be roughly deter­
mined by scanning down the columns of the factor-scorematrix (table 5).
TABLE 5. - Varimax factor score matrix
Element

CobaltNickelCopperZinc
Lead

Factor 1

Factor 2- 1.0994-1.5330.2406.0034-1.1863-0.3326.1660-1.7544-1.3042- .2646

Factor 1 is essentially a cobalt-nickel-lead factor ac­
counting for 37.2 percent of the available information,and factor 2 is a copper-zinc factor accounting for 32.6
percent. This grouping into two factors corresponds

closely with the trend found by the KolmogorovSmirnov statistic (it shows that the behaviour of theelements cobalt, nickel, and lead is independent fromthat of the elements copper and zinc).
The factor loadings of each pyrite sample are the pro­jection of each factor axis on each sample vector or,
more simply, the composition of each sample withrespect to the extracted 2-factor model (table 6). The
factor loadings, as determined from the Q-mode factor
matrix, have been plotted on a scatter diagram (fig. 6).
The axes of the diagram represent the two factors, andthe units on the axes are given in factor loadings. They
vary between -1.0 and +1.0. The sum of the squared
factor loadings of each sample gives the communality.
This value indicates, for each sample, what proportion of
the variance has been explained by the factor modelused. The communality has a value of 1.0 if the par­ticular sample has been explained at 100 percent by the
extracted factors (table 6).
TABLE 6. -Partial Q-mode varimax factor matrix
Communality

Factor 1

Factor 2

B 24 _
SW8_
SW1_
EN4_
EN 85

0.7580
.9632
.9294
.3685
.8799

-0.5682.3311- .8541- .4252.4970-0.6594
.9239
.4472
.4333
.7955

ST 29dSG 29 _SB 191
528 ___
N03__

.6562
.8044
.2739
.8199
.6467

-.0182-.3615
.4096
.3324
.0690

.8099
.8205
.3257
.8423
.8012

25 ____
BR9 __
3.3 ___
3.4 ____
BS 184

.8154
.5274
.9028
.5997
.7320

.4806
.6046
-.9500-.7398-.8163.7645.4025-.0197-.2288-.25622HH__
SB 231
H231 __H 401 _
120 ___F III __
Variance, in percent ___Cumulative variance, inpercent ____________

.9046
.8854
.7985
.2412
.8199
.4721

- .9473
.9278
.7922
.2774
.3324
- .6855
37.1910- .0845-.1566-.4133-.4052.8423-.047232.600937.191069.7919

The scatter diagram (fig. 6) demonstrates theheterogeneous distribution of all the investigated traceelements in the greenstone-belt pyrites (see also table 3).
These pyrites originated from a large number ofgenetically different deposits that occurred in quite dif­ferent geological environments. The generally low
cobalt, nickel, and lead contents of these samples arereflected in the weak but distinct accumulation of data
points in the positive half of factor 1.
The allogenic Witwatersrand pyrites show a rather
similar, irregular distribution on the diagram. Only one

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA

L13

greenstone source terrane that, in its lower members, islargely built up of mafic-to-ultramafic rocks. The highlead contents can be explained by the radioactive decayof the uranium- and thorium-bearing placer minerals in
the Witwatersrand sediments.
Like the Kolmogorov-Smirnov test, the Q-mode factor
analysis shows that the allogenic Witwatersrand pyrites
and the greenstone-belt pyrites have geochemicalsimilarities that clearly set them apart from theauthigenic Witwatersrand pyrites. The investigation
also indicates that pyrites formed in a sedimentary en­
vironment are far more homogeneous in trace-element
contents than those originating from hydrothermal
deposits. This observation agrees with the generaltrends of the trace-element contents in sedimentary andhydrothermal pyrites.

£25

y BR9

u
.Fill

LEAD-ISOTOPE INVESTIGATION
-1
-1

0

+1

FACTOR 1 (Co, Ni, Pb)

EXPLANATION•
Authigenic Witwatersrand
pyrites®
Allogenic Witwatersrand
pyritesPrimary pyrites
FIGURE 6. -Scatter diagram of the factor loading values.

sample, F III, exhibits a certain affinity to the data
points of the authigenic Witwatersrand pyrites, possiblyreflecting an imperfect separation of the1 two pyrite
varieties during sample preparation.
The authigenic Witwatersrand pyrites, in contrast,
are far more homogeneous and exhibit a distinct cluster(fig. 6). With respect to the system investigated, thesesamples possess a very high degree of similarity. As all
authigenic Witwatersrand pyrites have comparatively
high cobalt, nickel, and lead contents, they all plot close
to the -1.0 value of factor 1. The high cobalt, nickel, andlead values can be regarded as an indication of the highavailability of these elements in the WitwatersrandBasin where authigenic pyrite formed, possibly as lateas the metamorphism of the sediments (Saager, 1970;
Koeppel and Saager, 1974). Relatively high cobalt andnickel contents in the Witwatersrand sedimentarymaterials are in good agreement with the source modelof Viljoen and others (1970), who postulated a granite-

As part of the same research project, various pyrite
and galena samples from Witwatersrand reefs and fromprimary gold deposits in the major greenstone belts of
the Kaapvaal Craton were investigated for their leadisotope composition. The results of the study have beenreported in detail in an earlier investigation (Koeppel
and Saager, 1974). They bear additional important infor­
mation on the provenance of the detrital pyrites in the
Witwatersrand sediments and are therefore summa­
rized below.
Uranium-thorium-lead geochronological dating ofvarious sequences of the Witwatersrand Basin has been
undertaken. It yielded for the Dominion Reef Sequence,at the base of the assemblage, a maximum sedimentary
age of 2,850 ± 55 m.y. (million years) (Allsopp, 1964) andfor the Ventersdorp Sequence, which directly overlies
the Witwatersrand Sequence, an age of 2,300 ± 100 m.y.
(Niekerk and Burger, 1966). Nicolaysen and others(1962), by means of uranium-lead investigations ofrounded uraninite and monazite grains and of total con­glomerate samples from the Dominion Reef sediments,
showed that these minerals have a crystallization age of
3,080 ± 100 m.y., which is consistent with the hypothesisof a detrital origin of the uraninite and monazite. The
uranium minerals apparently were reconstituted andunderwent a first complete lead loss 2,040 m.y. ago; asecond lead loss occurred in recent times (Wetherill,
1956; Nicolaysen and others, 1962). Lead-isotope studies
carried out on galena samples from various reefs of the
Witwatersrand deposits by Burger and others (1962)
yielded independent evidence for old detrital com­ponents approximately 3,000 m.y. old in the sedimen­
tary filling of the basin. The investigation furthermore
confirmed a period of lead loss 2,040 m.y. ago. This
alteration period coincides with the emplacement of the
Bushveld Granite at Houtbeck (Nicolaysen and others,

L14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES1958), representing a period of mild regional metamorphism of the Witwatersrand sediments (the solution
stage of the modified placer theory).
The results of the lead-isotope study of Koeppel andSaager (1974) are condensed on a lead-lead diagram (fig.
7). On this diagram a hypothetical field is delineated inwhich all the data points of the authigenic Witwaters­
rand minerals should plot. The construction of the field
necessitated the following assumptions:
1. The crystallization age of the uranium-bearing
minerals in the sediments is 3,100 m.y.
2. Only two periods of lead loss occurred, the first one
2,040 m.y. ago and the second one in recent times.
The upper boundary of the field is then given by thesecondary isochrone due to the first lead loss of the
3,100 m.y. old uranium-lead system. This isochrone hasa slope of 0.3834 and corresponds to the "main trend" ofBurger and others (1962). The lower boundary of the
field, owing to the second lead loss, starts at the lower
end of the upper boundary and has a slope of 0.124,
which is the present-day 207Pb/206Pb ratio of a uraniumlead system 2,040 m.y. old. It is important to note that17

all investigated authigenic Witwatersrand minerals of
Burger and others (1962) and of Koeppel and Saager(1974) did plot within the hypothetical field. Only two
data points, by the latter authors, are given because
most samples contain large amounts of radiogenic leads
and therefore plot outside of the diagram (fig. 7) on the
open side of the wedge-shaped field.
The field of data points of sulfides from primary goldores in the greenstone belts was experimentally deter­
mined (Saager and Koeppel, 1976). It does not complete­ly overlap the field of authigenic Witwatersrandminerals.
The results of sample 25 in this study are of special in­terest. No. 25 is a detrital-pyrite sample originatingfrom the Basal Reef horizon of the Free State Geduld
Mine in the Orange Free State Goldfield. It has a leadisotope composition that on the diagram (fig. 7) plots
outside the predicted field of authigenic minerals. Thus,
the lead-isotope composition of this sample concurs withthe rounded, waterworn shapes of the pyrites and, infact, confirms their detrital origin from a source outside
the Witwatersrand Basin.

s = 0.3834±.0017.
Best fit of galena data points fromthe Witwatersrand Sequence("main trend" Burger and others, 1962)*Pb

30BR, L0= 0.271 ±.003
E302F

16

15

s = 0.124±.004

3bv

Lower boundary for
authigenic minerals'Area of data points of sulfides
from primary gold deposits of
the Swaziland Sequence

14

3 Pb
'Pb
13

15

17

19

21

23

FIGURE 7. -Diagram showing 207Pb/204Pb versus 206Pb/204Pb. The field of authigenic Witwatersrand minerals is vertically hatched; thefield of sulfides from primary mineralizations in the Archean greenstone belts is diagonally hatched. Circles denote allogenic pyrites;squares, authigenic pyrites from the Witwatersrand sediments. L denotes an HC1 leach of the respective sample; s, slope of the line
(modified from Koeppel and Saager, 1974, p. 327). The following parameters were used for calculating the growth curve:(206Pb/204Pb)0 = 9.56; (207Pb/204Pb)o = 10.42; T0 = 4.55; 238U/204Pb = 9.0; X238 = l,537xlO- 10y" 1 ; X235 =9.722xl()- 1V 1 .

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA

L15

this pyrite variety from the other pyrites in­
The lead-isotope composition of sample 25 is identicalwith that observed in galena from the Rosetta Mine, avestigated and suggest a different mode of forma­
primary gold lode deposit located in Onverwacht Grouption, probably during sedimentation of Witwaters­
rand strata.
rocks of the Barberton Mountain Land. A remarkablelead line was also obtained from the three detrital-pyrite 3. Although comparatively few pyrite samples havebeen analyzed for their lead-isotope composition,
samples 25, 2, and 1 (sample 1 lies outside of the graph),
the results agree with the trends observed in thewhich were collected at the same locality in the Basal
trace-element investigations. The isotope study
Reef. The samples contain all the same first-stage leadprovides evidence that the detrital Witwatersrandindicative of an origin from a lead source similar to thatpyrites derive from a source area with a leadfound in the Archean greenstone terrane. The tieline of
isotope composition identical with that of thethe data points (fig. 7) has a slope of 0.271, which cor­greenstone belts of the Swaziland Sequence. The
responds with the present-day 207Pb/206Pb ratio of aobserved lead-isotope compositions, furthermore,uranium-lead system 3,360 m.y. old. Slopes yielding
indicate that the detrital Witwatersrand pyrites
such ages were commonly observed in pyrite samples
were formed much earlier than the Witwatersrandfrom greenstone belts by Saager and Koeppel (1976) andsediments.
also in whole rock samples from the Onverwacht Group(Sinha, 1972). Significantly, the age of 3,360 m.y. is 4. The provenance areas of the Witwatersrand
sediments, as demonstrated by sedimentological
identical with the minimum age of the volcanic rocks ofand geological evidence, lay to the north and west
the Upper Onverwacht Group of the Barberton Moun­and close to the rim of the basin. The provenancetain Land as reported by Niekerk and Burger (1969).
areas had far-reaching similarities and were of the
Samples 202f and 30 BR are also detrital pyrite
same age as the Barberton Mountain Land-asamples from the Klerksdorp and the Orange Free State
typical Archean granite-greenstone terrane-used
Goldfields, respectively. They exhibit lead-isotope com­
to explain the succession of the sediments and thepositions similar to those observed in sulfides fromheavy minerals in the Witwatersrand Basin.
primary gold mineralizations in the greenstone belts,
However, it must be noted that the Barberton
but also their data points fall into the predicted field of
Mountain Land, which lies some 300 km east of theauthigenic Witwatersrand minerals. However, it is con­
Witwatersrand deposits, can only be regarded as a
spicuous that the slopes of the tielines to the respectivemodel for the source terrane and that the actual
HC1 leaches of the two allogenic pyrite samples areprovenance areas lay closer to the basin but are
steeper than analogous tielines of the authigenic Wit­
now eroded away and (or) covered by younger
watersrand minerals. The hypothetical first-stage leadstrata.
of these detrital pyrites resembles that of galenas fromprimary deposits in the greenstone belts (fig. 7; see also 5. Pyrite and gold are intimately associated in the
primary and the fossil placer-gold province. It can
Ulrych and others, 1967).
be assumed that not only the detrital Witwaters­
rand pyrites but also the Witwatersrand gold haveoriginated from pyritic and often gold-rich deposits
SUMMARY AND CONCLUSIONS
in the greenstone belts of the Swaziland Sequence.
1. Compact, rounded pyrite grains from the Witwaters­
The gold deposits are genetically related to ultrarand sediments and pyrites from primary gold
mafic and mafic, often magnesium-rich, volcanics,
deposits in greenstone terranes of the Swaziland
which are exceptionally abundant in the Lower
Sequence exhibit identical heterogeneous traceOnverwacht Group at the base of the Swaziland Se­
element contents. At a high significance level their
quence. It is possible that the larger portion of thecobalt, nickel, and lead values were drawn fromdetrital sulfides came from the acid volcanics of the
populations having the same distribution. It follows
Upper Onverwacht Group.
that both pyrite varieties were formed under 6. The detrital uranium- and thorium-bearing minerals
similar chemical and physical conditions in similarin the Witwatersrand sediments have a crystalliza­
geological environments and that the compact,tion age of 3,040-3,160 m.y. This agrees well withrounded Witwatersrand pyrites are in fact of
the U-Pb and Rb-Sr ages reported for some of the
detrital origin.
granites in the basement of the Kaapvaal Craton.
2. The large, often extremely porous WitwatersrandBecause of the high thorium content of many
pyrites that were formed in the basin of deposition
uraninites in the Witwatersrand sediments, it haspossess remarkably homogeneous trace-element
been suggested that they are derived from avalues and conspicuously high lead, nickel, andgranitic or pegmatitic source. According to the pro­cobalt contents. These trends clearly differentiate
venance model such source rocks form the highest

L16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

erosional levels in a typical Archean granitegreenstone terrane. The model thus provides an ex­
planation of why no primary uranium mineraliza­tion has yet been found in the granite-greenstoneterranes of the Kaapvaal Craton.
7. Two geochemically and petrogenetically different
rock assemblages provided the sources for the gold
and uranium in the Witwatersrand sediments. Bothrock assemblages occupied different erosional
levels and, depending upon the actual prevailingdenudation and drainage conditions, varying
amounts of gold and uranium were deposited in thebasin. The expected trend from uranium-rich hori­zons at the bottom of the sedimentary sequence togold-rich horizons at its top is still observable.
Reworking of the sediments, weathering of materi­al, and various other sedimentary processes have
led to considerable deviations from this overall
trend.
ACKNOWLEDGMENTS

The writer is indebted to Drs. M. J. and R. P. Viljoen,
Johannesburg Consolidated Investment Co., who gave
advice and assistance during the field work of this in­vestigation. Prof. D. A. Pretorius and Dr. C. R.
Anhaeusser, of the Economic Geology Research Unit,
University of the Witwatersrand, Johannesburg, pro­vided many helpful suggestions. Special thanks are also
due to the Anglo American Corp. of South Africa for
helping the author to obtain samples from the
Witwatersrand gold reefs. The isotopic investigations
were carried out at the Laboratory for Isotope Geology
and Masspectrometry, Federal Institute of Technology,
Zurich, and a portion of the study was supported by theDeutsche Forschungsgemeinschaft (Grant Sa 210/1,2),
for which the author is grateful.
REFERENCES CITED
Allsopp, H. L., 1964, Rb-Sr ages from the Western Transvaal: Nature,
London, v. 204, p. 361-363.
Anhaeusser, C. R., 1971, Cyclic volcanicity and sedimentation in theevolutionary development of Archean greenstone belts of shieldareas; Geological Society of Australia Special Publications, v. 3, p.
57-70.
Becker, G. F., 1897, The Witwatersrand banket with notes on othergold-bearing pudding stones, in Mineral Resources of the UnitedStates, 1896: U.S. Geological Survey 18th Annual Report, pt. 5, p.
153-184.
———1909, Origin of the gold of the Rand Goldfields: Some featuresof the Rand banket: Economic Geology, v. 4, p. 373-384.
Berg, G., and Friedensburg, F., 1944, Nickel und Kobalt: Die
metallischen Rohstoffe, ihre Lagerungsverhaltnisse und ihre wirtschaftliche Bedeutung: Stuttgart, Ferdinand Enke Verlag, 280 p.
Boyle, R. W., 1965, Geology, geochemistry, and origin of the lead-zincsilver deposits of the Keno Hill-Galena Hill area, Yukon Territory:
Canada Geological Survey Bulletin 111, 302 p.

Brock, B. B., and Pretorius, D. A., 1964, Rand basin sedimentationand tectonics, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa: Geological Society of South Africa, v.
1, p. 549-599.
Burger, A. J., Nicolaysen, L. 0., and de Villiers, J. W. L., 1962, Lead
isotopic compositions of galenas from the Witwatersrand andOrange Free State, and their relation to the Witwatersrand andDominion Reef uraninites: Geochimica et Cosmochimica Acta, v.
26, p. 25-29.
Cambel, Bohuslar, and Jarkovsky, Jan, 1967, Geochemie der Pyrite
einiger Lager statten der Tschechoslowakei: Slovenska Akademia
Vied, Bratislava, 493 p. (including English summary).
Carstens, C. W., 1941, Ueber sedimentare Schwefelkiesvorkommen:
Norske videnskabels selskab, Trondheim, Forhandlinger, v. 14, p.
120-122.
Davidson, C. F., 1957, On the occurrence of uranium in ancient con­glomerates: Economic Geology, v. 52, p. 668-693.
———1960, The present state of the Witwatersrand controversy: Min­ing Magazine, v. 102, p. 84-95.
-1961, The Witwatersrand controversy: Mining Magazine, v.
105, p. 88-90.
Dixon, W. J., and Massey, F. J., 1951, Introduction to statisticalanalysis: New York, McGraw-Hill, 210 p.
Feather, C. E., and Koen, G. M., 1975, The mineralogy of the Wit­
watersrand reefs: Minerals Science and Engineering, v. 7, p.
189-224.
Graton, L. C., 1930, Hydrothermal origin of the Rand gold deposits;
Testimony of the conglomerates, part 1: Economic Geology, v. 25,
supplement to No. 3, 185 p.
Harman, H. H., 1960, Modern factor analysis: Chicago, University of
Chicago Press, 469 p.
Hawley, J. E., and Nichol, L, 1961, Trace elements in pyrite, pyrrhotite, and chalcopyrite of different ores: Economic Geology, v.
56, p. 467-487.
Hegemann, F., 1943, Die geochemische Bedeutung von Kobalt undNickel im Pyrit: Zeitschrift fur angewandte Mineralogie, v. 4, p.
12-239.
Imbrie, J., and van Andel, T. H., 1964, Factor analysis of heavymineral data: Geological Society of America Bulletin 11, p.
1131-1151.
Kaiser, H. F., 1958, The varimax criteria for analytical rotation in fac­
tor analysis: Psych., v. 23, p. 187-200.
Koeppel, V. H., and Saager, R., 1974, Lead-isotope evidence on the
detrital origin of Witwatersrand pyrites and its bearing on the pro­venance of the Witwatersrand gold: Economic Geology, v. 69, p.
318-331.
Kullerud, Gunnar, 1962, The Fe-Ni-S system. The Fe-Mo-S system.
The Cu-Ni-S system: Year Book 61,1961-62, Carnegie Institutionof Washington, p. 144-152.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef and the Black Reef:
Geological Society of South Africa Transactions, v. 58, p. 101-227.
———1973, Mineralogical features of gold ores in South Africa, in
Adamson, R. J., ed., Gold metallurgy in South Africa: Johan­
nesburg, Chamber of Mines of South Africa, p. 352-446.
Loftus-Hills, G., 1967, Cobalt and nickel in Tasmanian pyrites, in The
geology of Western Tasmania -a symposium: Hobart, Universityof Tasmania, 16 p.
Mellor, E. T., 1916, The conglomerates of the Witwatersrand [withdiscussion]: Institution of Mining and Metallurgy Transactions, v.
25, p. 226-348.
Miller, R. L., and Kahn, J. S., 1965, Statistical analysis in thegeological sciences: New York, John Wiley, 483 p.

UETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA
Nichol, lan, Garrett, R. G., and Webb, J. S., 1969, The role of somestatistical and mathematical models in the interpretation ofregional geochemical data: Economic Geology, v. 64, p. 204-220.
Nicolaysen, L. 0., de Villiers, J. W. L., Burger, A. J., and Strelow, F.
W. E., 1958, New measurements relating to the absolute age of
the Transvaal System and the Bushveld Igneous Complex:
Geological Society of South Africa Transactions v. 61, p. 137-163.
———Burger, A. J., and Liebenberg, W. R., 1962, Evidence for theextreme age of certain minerals from the Dominion Reef con­glomerates and the underlying granite in the Western Transvaal:
Geochimica et Cosmochimica Acta, v. 26, p. 15-24.
Niekerk, C. B. van, and Burger, A. J., [1966], The age of the
Ventersdorp System: South Africa Geological Survey, v. 3, (1964),
p. 75-86.
———1969, A note on the minimum age of the acid lava of the Onverwacht Series of the Swaziland System: Geological Society of South
Africa Transactions, v. 72, p. 9-21.
Pretorius, D. A., 1966, Conceptual geological models in the exploration
for gold mineralization in the Witwatersrand Basin, in Symposium
on mathematical statistics and computer application in ore valua­tion: South African Institute of Mining and Metallurgy, p.
255-266.
———1974 The nature of the Witwatersrand gold-uranium deposits:
Witwatersrand University Economic Geology Research Unit In­
formation Circular No. 86, 50 p.
Radcliffe, D., and McWeen, H. Y., 1969, Copper zoning in pyrite from
Cerro de Pasco, Peru: a discussion: American Mineralogist, v. 54,
p. 1216.
———1970, Copper zoning in pyrite from Cerro de Pasco, reply:
American Mineralogist, v. 55, p. 527.
Ramdohr, Paul, 1955, Neue Beobachtungen an Erzen des Witwatersrands in Sudafrika und ihre genetische Bedeutung: Deutsche
Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik
und allgemeine Naturwissenschaften, Abhandlungen Jahrgang
1954, no. 5, 43 p.
———1969, The ore minerals and their intergrowths (English transla­tion of 3d ed.): New York, Pergamon Press, 1,174 p.
Roscoe, S. M., 1965, Geochemical and isotopic studies, Noranda andMatagami areas: Canadian Institute of Mining and Metallurgy
Bulletin 58, p. 965-971.
Saager, Rudolf, 1965, Erzgeologische Untersuchungen an
kaledonischen Blei, Zink und Kupfer fuhrenden'
Kieslagerstatten im Nord-Rana-Distrikt, Nord-Norwegen: Zurich,
City-Verlag, 145 p.
———1968, Newly observed ore minerals from the Basal Reef in the
Orange Free State Goldfield in South Africa: Economic Geology,
v. 63, p. 116-123.
———1969, The relationship of silver and gold in the Basal Reef of the
Witwatersrand System, South Africa: Mineralium Deposita, v. 4,
No. 2, p. 93-113.
———1970, Structures in pyrite from the Basal Reef in the Orange
Free State Goldfield: Geological Society of South Africa Transac­
tions, v. 73, p. 29-46.
———1973a, Metallogenese prakambrischer Goldvorkommen in den
vulkano-sedimentaren Gesteinskomplexen (greenstone belts) derL17

Swaziland-Sequenz in Sudafrika: Geologische Rundschau, v. 62, p.
888-901 (includes English, French, Russian summaries).
-1973b, Geologische und geochemische Untersuchungen an
primaren und sekundaren Goldvorkommen im fruhen Prakambrium Sudafrikas: Ein Beitrag zur Deutung der primaren
Herkunft des Goldes in der Witwatersrand Lagerstatte: Universi­
ty of Heidelberg, unpublished D. Sc. thesis, 150 p.
Saager, R., and Koeppel, V. H., 1976, Lead isotopes and traceelements from sulfides of Archean greenstone belts in South
Africa; a contribution to the knowledge of the oldest known
mineralizations: Economic Geology, v. 71, p. 44-57.
Saager, R., and Mihalik, P., 1967, Two varieties of pyrites from the
Basal Reef of the Witwatersrand System: Economic Geology, v.
62, No. 5, p. 719-731.
Saager, R., and Sinclair, A. J., 1974, Factor analysis of stream sedi­ment geochemical data from the Mount Nansen area, Yukon Ter­
ritory, Canada: Mineralium Deposita, v. 9, p . 243-252.
Schweigart, H., and Liebenberg, W. R., 1966, Mineralogy andchemical behaviour of some refractory gold ores from the Barberton Mountain Land: National Institute for Metallurgy, Johan­
nesburg, Research Report, no. 8, 72 p.
Scott, S. D., and Barnes, H. L., 1972, Sphalerite geothermometry andgeobarometry: Economic Geology, v. 66, p. 653-669.
Sinha, A. K., 1972, U-Th-Pb-systematics and the age of the Onverwacht Series, South Africa: Earth and Planetary Science Letters,
v. 16, p. 219-227.
Smirnov, N., 1948, Table for estimating the goodness of fit of em­
pirical distributions: Annals of Mathematical Statistics, v. 19, p.
279-281.
Spearman, C., 1904, General intelligence, objectively determined andmeasured: American Journal of Psychology, v. 15, p. 201-293.
Tourtelot, H. A., 1964, Minor-element composition and organic-carbon
content of marine and nonmarine shales of Late Cretaceous age inthe western interior of the United States: Geochimica et
Cosmochimica Acta, v. 28, p. 1579-1604.
Ulrych, T. J., Burger, A. J., and Nicolaysen, L. 0., 1967, Least
radiogenic terrrestrial leads: Earth and Planetary Science Letters,
v. 2,p . 179-184.
Viljoen, R. P., and Viljoen, M. J., 1969, An introduction to the geologyof the Barberton granite-greenstone terrane: Geological Society ofSouth Africa, Special Publication 2, p. 9-27.
Viljoen, R. P., Saager, R., and Viljoen, J. J., 1969, Metallogenesis andore control in the Steynsdorp Goldfield, Barberton MountainLand, South Africa: Economic Geology, v. 64, p. 778-797.
-1970, Some thoughts on the origin and processes responsible forthe concentration of gold in the early Precambrian of Southern
Africa: Mineralium Deposita, v. 5, p. 164-180.
Villiers, J. E. de, 1957, The mineralogy of the Barberton gold deposits:
South Africa Geological Survey Bulletin 24, 60 p.
Wetherill, G. W., 1956, An introduction of the Rhodesia and
Witwatersrand age patterns; Geochimica et Cosmochimica Acta,v. 9, p. 220-292.
Wilson, H. D. B., 1953, Geology and geochemistry of base metal
deposits: Economic Geology, v. 48, p. 370-407.
Young, R. B., 1917, The banket of the South African goldfields: Lon­don, Gurney and Jackson, Ltd., 125 p.

Geochemistry and Morphology of
Mineral Components from theFossil Gold and Uranium Placers of the Witwatersrand
By D. K. HALLBAUER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-M

CONTENTS
Page

Abstract ______________________________
Introduction _________________________________________
Methods of study _____________________________________
The components of Witwatersrand conglomerates __________
Gold particles _____________________________________
Detrital gold _____________________________________
Biochemically concentrated gold _____________________
Recrystallised gold ________________________________
Detrital-gold intergrowths __________________________
Carbonaceous matter ______________________________

Ml

The components of Witwatersrand conglomerates - Continued
Uraninite ____________________________
Pyrite _______________________________
Other detrital heavy minerals ________________
Conclusions _______________—_———————————
References cited _______________———————————
Discussion _______________——————————————
Discussion following paper 1 by Dr. Hallbauer _—_———
Discussion following papers by Drs. Minter, Saager, andHallbauer (paper 2) _______________——————

M10
10
14
17
17
18
18
19

ILLUSTRATIONSPage

FIGURES 1-3. Photomicrographs showing:
1. The three major types of gold particles from Witwatersrand reefs ___________________________
2. Part of a slightly deformed crystalline gold particle ______________________________———__
3. Part of a gold particle extensively deformed after long transport _______________________—_—__
4. Graphs showing fineness of gold from some Witwatersrand conglomerates __________________________
5. Photograph of a carbonaceous layer, showing trapped detrital gold on the surface and filamentous gold within the car­bonaceous matter, Basal Reef, St. Helena gold mine _____________________________________
6. Photomicrographs showing intergrowths between gold and other minerals __________________________
7. X-ray radiograph of a carbonaceous layer with overlying quartzite, showing small gold filaments in the carbon andirregularly formed gold particles in the upper portion of the layer and in the quartzite __________________
8. Photomicrographs showing thin sections of carbonaceous matter ________________________________
9. Magnified X-ray radiograph of columnar carbonaceous matter _________________________________
10-12. Photomicrographs showing:
10. Structural details of columnar carbonaceous matter after oxidising of kerogen ___________________
11. Examples of biochemically concentrated gold extracted from carbonaceous matter _________________
12. Filaments from carbonaceous matter where bacterial or fungal structures have been replaced by sphalerite andgalena ___________________________________________________________
13. Graph showing stability of pyrite in atmospheres with various oxygen contents ________________________
14, 15. Photomicrographs showing:
14. Pyrite mud balls from different horizons in the Witwatersrand sequence _______________________
15. Detrital cobaltite, gersdorffite, and sphalerite ______________________________________
16, 17. Graphs showing:
16. Composition of arsenosulphides from various conglomerate horizons in the Witerwatersrand sequence _____
17. Rare-earth contents of zircons from three conglomerates in the Upper Witwatersrand _______________

M2
3
3
4
5
6
7
8
9
10
11
12
12
13
15
16
16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESGEOCHEMISTRY AND MORPHOLOGY OF MINERAL COMPONENTS FROM
THE FOSSIL GOLD AND URANIUM PLACERS OF THE WITWATERSRAND
By D. K. HALLBAUER
ABSTRACT

Using the hydrofluoric-acid leaching technique to obtain heavymineral grains intact from Witwatersrand conglomerates and thescanning electron microscope with microanalyzer attachment, thetechnique of morphological microscopy was applied to observe thethree dimensions of the grains and to study their composition.
In combination with stereo X-ray radiography of rock slabs and theapplication of various geochemical techniques, this afforded new infor­
mation on the processes that led to the formation of Precambrianplacer deposits.

INTRODUCTION

For close to 100 years the Witwatersrand deposits
have been studied by mineralogists and geologists,
helped by the ever-increasing sophistication of methods
and techniques. For some time now these deposits havebeen widely recognized as fossil placers. It can therefore
be assumed that the concentration of gold, uranium, andother heavy minerals took place in large palaeoecological systems in which the distribution of these minerals
was determined by the palaeoenvironment. The com­bination of modern and established mineralogical tech­niques can increase substantially the retrieval of infor­
mation on these environments that is contained in rockparameters and rock components. The recognition andunderstanding of the environment in which a fossil
placer was formed and of the relation between thevarious morphological forms and components of thedeposit present a potential approach for use to deter­mine economic horizons even if the economic minerals
are absent in a particular small sample.
In previous mineralogical studies of the various
minerals in such ores the classical methods of micro­
scopy, using thin sections and polished sections of rock,permitted observation of two dimensions of a mineral
grain. The study of rocks and ores by these methods wasdeveloped to a very advanced stage by greatmineralogists such as Paul Ramdohr (1975), and the
1 Geui-hermcal Division of the Mining Technology Laboratory, Chamber of Minos of South
Africa, Johannesburg.

knowledge of geological processes was enhanced greatly
by them. The development of the scanning electron
microscope with its high resolution (100A and better)and increased depth of focus, however, has opened up
the third dimension of mineral grains to examination by
microscopy. The use of the scanning electron
microscope in combination with hydrofluoric-acid diges­
tion method (Neuerburg, 1975), which produces concen­
trates of undamaged grains of many ore minerals, hasmade it possible in the past few years to advance further
the knowledge of geological processes that led to the for­mation of the Witwatersrand and other deposits.
METHODS OF STUDY

In order to be able to observe and study the mor­
phology and micromorphology of minerals it is essential
to free the mineral grains from the matrix rock. Depend­ing on the type of matrix, agents such as hydrochloric
acid, sulphuric acid, or hydrofluoric acid can be used todissolve the matrix and liberate the grains to be examin­ed. Softer, porous rocks can often be split into their com­ponents by the crystallization pressure of common alum(KA1(S04)2). Repeated soaking of the rock in a solutionof alum and slow drying causes the rock to disaggregate
into its components. After having been washed, themineral particles can be studied.
For the study of heavy minerals in Precambrian rocksof the Witwatersrand and similar deposits, the siliceous
matrix and the quartz pebbles were decomposed inhydrofluoric acid. This process requires about six weeks
of continuous acid treatment to liberate the heavy
minerals from about 100-200 g of uncrushed rock. Many
heavy minerals are insoluble in hydrofluoric acid and canbe recovered in their natural shape (Neuerburg, 1975).
Tests have shown so far that gold, pyrite, zircon, andchromite are not attacked or altered by hydrofluoric
acid, but that uraninite, apatite, monazite, andmagnetite are.
Ml

M2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESOf the major silicates present in the Witwatersrandrocks, only pyrophyllite is not dissolved in hydrofluoric
acid. Fine-grained pyrophyllite, if present in large
amounts, can be a disadvantage, but repeated, carefulwashing of the sample can remove most of it. The inert­
ness of pyrophyllite in hydrofluoric acid led, on the other
hand, to the discovery of pyrophyllite pebbles containing
fine-grained crystalline gold, providing an important
contribution to knowledge of the origin of the Wit­
watersrand deposits.
After being cleaned in water and acetone, the mineral

grains are ready for examination. Heavy fractions canbe obtained by heavy-liquid separation or by using the
micropanner (Muller, 1959), and individual grains are
selected for microscopy and mounted on an aluminium
holder to be placed in the scanning electron microscope,a Leitz AMR 1000. The mounting medium may be con­ducting silver paint or colloidal graphite for larger
grains or double-sided cellotape for small grains. Non­conducting specimens must be coated with carbon, gold,or palladium.
The scanning electron microscope is used widely in

FIGURE 1. Scanning electron photomicrographs of the three major types of gold particles
from Witwatersrand reefs. A, Detrital, flaky gold particle, indicating a medium to longtransport distance (40-60 km). B, Slightly distorted, detrital gold particle with partiallypreserved crystalline texture, indicating a transport distance of up to 30 km. C, Fila­mentous biochemical gold from the interior of carbonaceous matter. D, Filamentous gold
from the interior of carbonaceous matter, showing cell-like structures. E, Oriented
growth of secondary gold on pyrite. F, Recrystallised gold from small fissures in quart- zite.

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND

M3

biological, physical, and mineralogical research (Johan,
1974; Stoops, 1970; Metzger and others, 1977), as wellas in other fields. For the type of investigation describedhere it was greatly advantageous to have an attached
ORTEC microanalyser on the AMR 1000 to exploit theenergy-dispersive X-ray spectra of the particles examin­
ed and to obtain a semiquantitative analysis of thesamples and, in particular, to measure the fineness ofgold particles.

FIGURE 3.— Scanning electron photomicrograph of part of a gold parti­
cle from a Witwatersrand reef exhibiting a severely distorted sur­face typical for transport distances in excess of 30 km.

FIGURE 2.—Scanning electron photomicrograph of part of a crystalline
gold particle from a Witwatersrand reef showing abrasion anddistortion on protruding corners and edges, indicative of a shorttransport distance (up to 10 km).

THE COMPONENTS OF WITWATERSRANDCONGLOMERATESGOLD PARTICLESGold is present in the Witwatersrand reefs as discrete
particles of various shapes and sizes. From studies of themorphology of the individual gold particles it has beenpossible to classify the particles into three major groups:
detrital gold, biochemically concentrated gold, andrecrystallised gold (fig. 1).
DETRITAL GOLD

During transport of primary gold particles, their
typical crystalline microtexture (fig. 2) is deformed slow­
ly by repeated collisions with the bedload in the fluvial
system until the crystalline texture is destroyed andreplaced by a distorted and flaky microtexture (fig. 3).
In a comparison with gold from recent alluvial gold
deposits it was possible to show that the shapes of gold
particles undergo characteristic changes duringtransport down the slope of a placer deposit.

Thus the morphology of detrital gold should reflect theconditions of transport, distance of transport and, whenobserved on a lateral scale, the direction of transport(Hallbauer and Utter, 1977). From a large number ofobservations it was then concluded that the averagetransport distance for Witwatersrand gold was about 30
km.
All transitional shapes, ranging from crystalline tocompletely deformed, have been observed in gold par­ticles recovered from Witwatersrand deposits. It should
be possible, therefore, to reconstruct, from a few
strategically selected small samples of gold-bearing con­glomerate, the major outlines of a fossil placer, in­cluding the general transport direction.
In a study conducted along these lines, consideration
was given to the abundance of gold particles withcrystalline texture, and from this it was predicted that aseparate entry point existed in the northern part of theOrange Free State goldfields (Hallbauer and Utter,1977). This was confirmed by independent sedimentological investigations (Minter, 1977). Observations onsome of the other heavy minerals accompanying the gold
supported the findings of the gold-morphology studies.
It is of interest to note that gold particles recovered by
the hydrofluoric-acid leaching method from samples
originating from other fossil placers, such as the Domin­
ion Reef, Klerksdorp, and the Blind River Conglomerateof the Elliot Lake District, Canada, showed typicaldetrital features indicating a transport mechanism
similar to that identified for rocks of the Upper Wit­
watersrand Series.
The fineness-F = (Au x l,000)/(Au + Ag)-of Wit­
watersrand gold was previously known to vary between

M4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

different localities and stratigraphic horizons (Viljoen,
1971; Saager, 1973; Schidlowski, 1968a). No definite
trends, however, could be established, as the number ofgrains analysed was comparatively small. Analysis of alarger number of gold particles not only gave thestatistical distribution within a given sample but provid­
ed reliable information on lateral trends within a deposit
and differences between stratigraphic horizons.
Under recent transport conditions gold is rapidly
depleted of its silver content, owing to the oxidising con­ditions in most modern fluvial environments. It can beshown (Hallbauer and Utter, 1977) that the gold-silver
alloy is stable during fluvial transport in an oxygendeficient atmosphere, a condition commonly assumed
for the Precambrian.
The fineness of the gold in Witwatersrand deposits
should consequently reflect the fineness of the primarydeposits. It is therefore not surprising that many con­glomerate horizons have a characteristic fineness that
does not vary within one geological unit (Hallbauer, Ut­ter, and Hirdes, 1978; Utter, 1977). Facies changes,
reworking of older horizons, and other geologicalparameters can thus be "read off the fineness of gold
particles (Utter, 1977). Geochemical maps showing the
distribution of gold fineness can thus be used successful­ly to supplement sedimentological work.
Geological observations in the past have shown that
the Ventersdorp Contact Reef in the Klerksdorp
Goldfield truncates a number of older horizons. Conse­
quently, the fineness of gold particles from the
Ventersdorp Contact Reef showed a spread of finenessvalues from 750 to 960 (fig. 4), which reflects thefineness of gold from the truncated horizons (Utter,1977).
The bimodal distribution of fineness for the Vaal Reef
(fig. 4) can be correlated to the sedimentologicalevidence that predicted two facies on the Vaal Reef
horizon (Minter, 1976).
In contrast to these observations, the fineness of otherconglomerate units showed a remarkable unimodal
distribution, pointing to one source of gold for theseunits (fig. 4).
Grain-size determinations, which were carried out
simultaneously with the fineness measurements, show­
ed that no relationship exists between the size of gold
particles and the fineness.
BIOCHEMICALLY CONCENTRATED GOLDThe second type of gold, the biochemically concen­
trated gold, is associated genetically with layers of car­bonaceous matter. The association between high gold
values and carbonaceous matter in some Witwatersrand
reefs is well know. It has been possible in the past few
years to show that the carbon seams are the fossilized

20
10
0

< AO
<

30

(f) 20
I
°

10
0

o
o 4(

D

z 30H
LU

o 20
cr

y 10
o40302010700

800

900

1000

GOLD FINENESSFIGURE 4.—Fineness of gold from various reefs in the Witwatersrandarea. A, Vaal Reef, Klerksdorp Goldfield (Utter, 1978). B,
Ventersdorp Contact Reef, Klerksdorp Goldfield (Utter, 1978). C.
Basal Reef, St. Helena gold mine. D, Carbon Leader Reef, Blyvooruitzich gold mine. E, Ventersdorp Contact Reef, East Driefontein gold mine.

remains of Precambrian plants (Hallbauer and vanWarmelo, 1974; Hallbauer, 1975); the most common
plant represented by columnar carbonaceous material
was named Thuchomyces lichenoides (Hallbauer andothers, 1977). Colonies of these plants formed mats or
carpets, which trapped detrital gold by a corduroytablelike action (fig. 5); this gold was apparently thendissolved and redistributed either intracellularly or ex-

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND

M5

FIGURE 5.—Photograph of a carbonaceous layer, showing trapped detrital gold on the surface and filamentous gold within the carbonaceous
matter, Basal Reef, St. Helena gold mine.

tracellularly by the organisms. From this process there
originated the biochemical gold, which is present in the
carbonaceous material as small filaments, agglutinatedaggregates of filaments (fig. 1C, D) or irregularly formed,
often perforated, particles.
RECRYSTALLISED GOLDThe recrystallised gold apparently was remobilised
and transported for short distances of the order of a few
millimeters within the consolidated sediment and has
only limited, local significance. Transportation andmobilization could be attributed to local rises in thetemperatures caused by dyke intrusions, overlying
volcanic rock, or hydrothermal activity along faults andfracture planes.
Recrystallised gold generally occurs as crystalline ag­
gregates in small quartz veins or in thin fractures and asorientated growth on secondary pyrite (fig. IE, F).

DETRITAL GOLD INTERGROWTHSOf special interest in samples of Witwatersrand con­glomerates are detrital particles consisting of intergrowths of gold and other minerals and of pyrite andother detrital minerals. From their detrital nature,these can be regarded as primary intergrowths, andthey serve as indicators of the types of the provenancedeposits. In addition, a large concentration of these par­ticles can serve to indicate proximity to the entry point
to the basin.
In samples of Basal Reef, primary intergrowths ofsphalerite and gold and of sphalerite and chalcopyrite,
as well as the gold contents of sphalerite (about 250 ppm(parts per million)) and of chalcopyrite (about 6,000ppm), point to a strong genetic relation between gold,
sphalerite, and chalcopyrite. Typical of detrital goldsphalerite intergrowths (fig. 6C) are the etched surfaceof the sphalerite and the distorted and abraded edges ofM6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE GONGLOMERATES

45 fJLm

4 /Ltm

FIGURK 6.—Scanning electron photomicrographs of some intergrowths between gold andother primary minerals. A, Primary intergrowth of detrital pyrite and molybdenite
from a sample of Ventersdorp Contact Reef, East Driefontein gold mine. B, Surface
details of A, showing an aggregate of molybdenite flakes attached to the pyrite surface
but partially displaced from the original position (arrow). C, Detrital particle of goldsphalerite intergrowth from a sample of Basal Reef, St. Helena gold mine, Orange Free
State, with gold on the left partly surrounding a piece of sphalerite on the right. D,
Detail of C. showing the surface of the gold distorted along the edges during alluvial
transport. E, Side view of a slightly rounded particle consisting of an intergrowth ofgold and gersdorffite from a sample of B Reef, Loraine gold mine, Orange Free State.
F, Surface detail of E, showing the scratched and distorted gold surface with small
gersdorffite inclusions (arrows).

the gold (Fig. 6Z7). Primary intergrowths of gold andcobaltite-gersdorffite were observed in samples of B
Reef from the Loraine gold mine (fig. 6E, F). In thesesamples, detrital gold particles with low overall distor­
tion are a common feature; they are accompanied bypyrite and smaller amounts of molybdenite, tourmaline,
and other heavy minerals, as well as various amounts ofpyrophyllite in the form of small pebbles and fine­grained material.

In samples of Ventersdorp Contact Reef from theEast Driefontein gold mine, primary intergrowths ofpyrite and molybdenite (fig. GA, B), gold and bravoite,
and pyrophyllite pebbles and fine-grained gold wereobserved. As the detrital pyrite from this deposit con­tains small inclusions of gold (average gold content 250ppm) that can be regarded as primary, a common origin
can be assumed for gold, pyrite, molybdenite, bravoite,
and pyrophyllite. Pyrophyllite is regarded as theMINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND

hydrothermal alteration product of potassium feldspar(Deer, Howie, and Zussman, 1962) and is a major consti­
tuent of the sericite-quartz pyrite zone of porphyry-typeore deposits (Hollister, 1975). This suggests, therefore,that some primary deposits of the Witwatersrand gold
were perhaps similar to porphyry copper deposits or
mineralised acid volcanics and not to a greenstone ter­rain as found in the Barberton Mountain Land (Viljoen
and others, 1970). Concurrent geological considerations
led to the formation of a similar model supporting thefindings of the studies using scanning electron
microscopy (Button, A., University of the Witwater­
srand, Economic Geology Research Unit, personal commun., 1977).
A recent search in the vicinity of the Klerksdorp
Goldfield succeeded in finding small occurrences ofmineralised acid volcanics that are older than the Wit­
watersrand rocks and that contain minute quantities offree gold and are accompanied by pyrite, cobaltite andother minerals typical of Witwatersrand conglomerates.

M7

about 0.5 mm in diameter and as much as 10 mm long.
Between the columns and on top of the layer are heavy
minerals, including gold, pyrite, zircon, and others (fig.
7), which points to a mechanism of concentration similar
to a corduroy-table.

10mm

CARBONACEOUS MATTER

Prominent in a number of conglomerate horizons in
the Upper Witwatersrand and also locally in the Domi­nion Reef are layers of a coal-like material called "car­bon" or "thucholite." A recent investigation into its
organic geochemistry has shown that it is in fact a
kerogen similar to bituminous coal and considered to be
the result of polymerization of biochemicals fromdecayed primitive Precambrian micro-organisms(Zumberge and others, 1978). The majority of thepyrolysis breakdown products of this kerogen are alkylsubstituted aromatic hydrocarbons, low molecularweight aliphatic hydrocarbons, and some aromatic
sulphur and aliphatic oxygen compounds. The evolutionof the kerogen from the original biochemicals ofprimitive micro-organisms appears to have progressed
through free radical reactions influenced by a constantradiation from the accompanying finely dispersed
uraninite.
Although some of the structures of the carbon in the
carbon seams of the Witwatersrand deposits had been
observed in polished sections and in thin sections(Snyman, 1968), recognition of this carbon as thefossilized remains of Precambrian plants and the fullreconstruction of the internal structures of these plants
were made possible only by scanning electron
microscopy, while the spatial arrangement relative torock component was studied by X-ray radiography ofslabs of conglomerate.
Most of the smaller seams of carbonaceous matterprominently display a columnar structure. Stereo X-ray
radiographs in particular show that these small seams
consist of a carpetlike arrangement of vertical columns

FIGURE 7.—X-ray radiograph of a carbonaceous layer with overlying
quartzite showing small gold filaments (black) in the carbon and ir­
regularly formed gold particles (black) in the upper portion of thelayer and in the quartzite. Pyrite grains (P) can be distinguishedfrom the gold by their lighter grey colour. Middle Elsburg Reef,
Cook II gold mine.

Thin sections of such layers show finely laminatedquartz as filling between the columns, while coarse­
grained quartz and pebbles at the top form the transi­tion to the overlying conglomerate (fig. 8A). The finely
laminated quartz, which in places contains single
filaments of an obvious bioligical nature (fig. SB), can beinterpreted as silicious sediment deposited during quiet
periods.
Various sedimentological investigations andgeological observations point to an environment com­prising small stagnant pools, lagoons, wet sand, and clayon the lower slopes of wet alluvial fans or shallow wateralong the shore lines of an inland lake in which thisPrecambrian plant life flourished (Minter, 1976;
Pretorius, 1975).
The more-or-less constant shape of the columnar form
all over the Witwatersrand can be taken as an indicationthat it is the primary shape of a small plant or unit of aplant colony.
Some of the interior structure of individual columns isrevealed in X-ray radiographs taken of thin slices onhigh-resolution plates. These radiographs when magni­
fied show distinctly a vertical arrangement of filaments(fig. 9). Rather more pronounced is this on columns after
low-temperature ashing (fig. 10). Even with a group ofM8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESDFIGURE 8.—Photomicrographs A-D showing thin sections of carbonaceous matter. The
thin sections show the finely layered nature of quartz within the columnar carbon andgrainy texture of the detrital quartz on top of the carbon (D), as well as a filament of abiological nature protruding from the carbonaceous matter (C), are shown.
Photomicrograph E (a color reproduction of C) shows algal-like filaments of biologicalorigin protruding from carbonaceous matter. Carbon Leader Reef, Blyvooruitzicht gold

MINERAL COMPONENTS, FOSSIL GOLD AND CRANIUM PLACERS, WITWATERSRAND

M9

Additional support for the biological nature of the car­bonaceous or kerogen layers is obtained from gold fila­
ments within the columns. While most of the filamen­
tous structure in the columns consists of an almost con­stant mixture of uranium and lead, with small amountsof silicon, alumina, and iron, the number of gold
filaments varies from sample to sample, and gold
filaments are absent in some. Common to all gold con­tained within the kerogen is a shape composed of ir­
regular, often perforated, particles and filaments (fig.
1L4.-D) with a form very closely resembling inorganic
encrustations found on living micro-organisms(Hallbauer, 1975,1978). It is therefore assumed that the
Precambrian organisms in the Witwatersrand con­glomerates collected gold directly by biochemical pro­
cesses or indirectly from solutions and deposited thisgold extracellularly on the living or perhaps on the
decayed filaments.

FIGI-RK 8.—(See facing page for explanation.)

FIGURK 9.-Magnified X-ray radiograph of columnar carbonaceous
matter, taken on a high-resolution plate and magnified to show thefilamentous interior structure (white, rounded part is quartz). Car­bon Leader Reef, Blyvooruitzicht gold mine.

In addition to the biological structures mentioned,
filaments resembling actinomycetes or related orga­
columns, each individual is clearly defined by a thin, nisms have been observed in samples from the St.
membrane-like structure surrounding it. Although most Helena gold mine (Hallbauer, 1975; Hallbauer, Jahns,of the filaments when viewed in a light microscope do and Beltman, 1977). They appear to be replacements ofnot show biological features, occasional filaments with organisms by lead and zinc sulphide. Although thesetypical cell structures were observed (Hallbauer, 1975). filaments are well preserved (fig. 12A, B) and readily
This is somewhat surprising when considering the age of recognized by microbiologists as bacterial or fungal
these deposits (approx 2,700 m.y. (million years)). The filaments, they are a localized occurrence and appear to
low metamorphism (lower greenschist facies) and a be of a parasitical nature. Their acceptance or nonacceprelatively quiet tectonic history of the Witwatersrand tance does not influence the theory on the biologicalrocks, however, might be some influencing factors in the origin of the Witwatersrand carbonaceous matter orpreservation of these structures.
kerogen.

M10

GENESIS OP" URANIUM- AND GOLD-BE ARINGPRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

200pm
FIGURE 10.-Scanning electron photomicrograph showing structuraldetails of columnar carbonaceous matter after oxidizing of kerogenat 500°C. (Oxidizing in a plasma furnace (International Plasma
Corp.) at temperatures less than 100°C leads to the same type ofstructures.)
URANINITE

Uraninite was recognized early as a detrital compon­
ent of the Witwatersrand conglomerates (Liebenberg,
1955; Ramdohr, 1955). Of special interest, however, isthe association between uraninite and the carbonaceous
matter. Uraninite can be found as small (<100 ju,m), ir­
regular grains finely dispersed in carbonaceous matterand as rounded, detrital grains completely surrounded
or in various degrees intergrown with the latter. Often
the surrounding carbonaceous matter has the appear­
ance of a felt of small filaments (Schidlowski, 1968b).
Detailed observations on particles cleaned inhydrofluoric acid appear to indicate that micro­organisms attacked detrital uraninite and slowly
assimilated the mineral matter.
PYRITEThe detrital nature of most pyrites in the Witwaters­
rand conglomerates was recognized long ago frommacroscopic observations and studies of polished sec­
tions (Ramdohr, 1955; Liebenberg, 1955), and as manyas nine different types of pyrite identified by ore
microscopy have been described in detail (Schidlowski,
1970; Saager, 1973). These various types can, however,be classified into three major groups: (1) allogenic

detrital pyrite; (2) pyrite formed under sedimentary con­ditions shortly before deposition; and (3) authigenic
pyrite formed under various conditions in the solidified
sediment.
Allogenic detrital pyrite is present in samples of Wit­
watersrand and similar conglomerates as massive, nonporous particles that exhibit all stages of rounding, theresult of fluviatile transport. Typical of the Witwaters­
rand pyrites are inclusions of various minerals—such as
pyrrhotite, chalcopyrite, sphalerite, gold, and others(Utter, 1977)—that are similar to those found in primarydeposits. The allogenic detrital pyrite is, therefore, most
likely associated genetically with detrital gold.
Low chemical stability of pyrite in recent fluviatile
deposits has been taken to be an indication of an absenceof oxygen in the Precambrian atmosphere (Cloud, 1976;
Schidlowski and Junge, 1973). This is an assumption of
considerable importance to studies of the evolution oflife (Margulis and others, 1976). Although the presenceof detrital pyrite in recent rivers has been reported
repeatedly, little is known about periods of time forwhich pyrite can survive in a fluviatile environment
under the present oxygen partial pressure. Observations
on recent detrital pyrite from some Eastern Transvaal,
South Africa, rivers have revealed numerous orientatedetched pits on the surface of the pyrite grains. A similar
microsurface texture can be observed on some pyrite
particles from Witwatersrand reefs and from the similar
Blind River, Canada, conglomerates. If these textures
still represent those of the original particle surfaces, it
can be assumed that there was a low-level chemical at­tack on the surface during prolonged transport of theparticle and, therefore, that a low oxygen partialpressure existed in the Precambrian atmosphere.
Laboratory experiments were conducted during which
pyrite was tumbled in closed glass containers filled halfwith oxygen-free water and half with artificial at­mospheres or normal air. After 30 days of continuous
tumbling the S04 ~ 2 content of the water and the weightloss of the pyrite were determined. The results showthat the pyrite lost 0.01 percent of its weight by tum­bling in water under the present atmosphere. Thisweight loss was reduced to about 0.002 percent if the at­mosphere had an oxygen content of 2 percent, or one
tenth of the present (fig. 13). These results appear to in­
dicate that a low-level chemical attack on pyrite is possi­
ble under conditions assumed to have existed during thePrecambrian (Grandstaff, this volume, chap. C).
Within the group of sedimentary pyrite types, thepyrite "mud balls" are of particular importance as in­dicators of the nature of the palaeoenvironment. These
porous, rounded pyrite particles consist of fine-grained
pyrite aggregate and are known as "buckshot pyrites," aterm often used to describe all rounded pyrite grains;

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRANDMil

FIGURE 11.—Scanning electron photomicrographs showing examples of biochemically concentrated gold extracted from carbonaceous matter.

M12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 12.— Scanning electron photomicrographs showing filaments from carbonaceous matter where bacterial or fungal structures have been
replaced and preserved by sphalerite and galena. Basal Reef, St. Helena gold mine.

they can occur in sizes ranging from 0.1 to 10 mm indiameter and are indicative of economic horizons in the
Witwatersrand sequence. Polished sections of this typeof pyrite occasionally show cracks and indentations,which previously were regarded as having been causedby solution under pressure (Schidlowski and Trurnit,1966). However, three-dimensional examination of thesecracks on the intact grain after treatment with hydro­
fluoric acid suggested that they were caused byshrinkage and (or) plastic deformation of soft material(fig. 14). From this it was deduced that pyritic mud wasformed during Witwatersrand time and was depositedin pools and puddles of an ephemeral-stream system;
flooding after a dry period would then produce mud peb­bles and mud balls, as have been described for otherfluviatile environments (Ball, 1940; McKee and others,1967). It was further deduced that during a subsequent
dry period, shrinkage cracks as observed (Hallbauer,
1977) would form on the surfaces of the mud balls andoverlying pebbles would cause the formation of indenta­
tions in, and plastic deformation of, the mud pebbles.
The occurrence of pyrite mud balls in conjunction withventifacts in Witwatersrand conglomerates points to apalaeoenvironment characterised by desert conditions
interrupted by frequent flooding of the area. Stereo
X-ray radiographs of rock slabs showing mud flake
structures in pyrite layers support this interpretation(Hallbauer, 1974).
Pyrite mud balls generally contain small fragments ofmost other detrital minerals, as is shown by the study ofpolished sections and trace-element analyses (Utter,1977). Rounded pyrite particles with typical mud-ball

z
LU
o
DC
LU
CL

o

C/)
O5
C/)

E

16 -i

I

Oxidation of pyrite as S04 content of water

14 H

1 «-

10 2 -10 864 -10'r3_

2-—I———i———i———i——I———i———I———i———i———I
0

2
4
6
8
10 12
14
16
18
PERCENTAGE OF 02 IN ATMOSPHERE

20

FIGURE 13.—Stability of pyrite in atmospheres with various oxygen
contents.

structures have been observed in most Witwatersrandconglomerates, including those from the Vredefort area
and the Dominion Reef.
Authigenic pyrite in fossil placers was formed in therock during and after diagenesis and is easily recognizedby its crystalline morphology. It has been observed that

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND

100 Atm

10/am

FIGURE 14.—Scanning electron photomicrographs of pyrite mud balls. A, Pyrite mud balls
from an Elsburg Reef (EA 12), Loraine gold mine, Orange Free State, with wellpreserved indentation and system of cracks caused by plastic deformation by overlying
pebbles. B, Portion of the surface showing the fine-grained texture and small cracks, aswell as partial recrystallisation. C, Pyrite mud ball from the Vredefort area (Bird Series).
D, Surface details of C, showing the grainy texture. E, Pyrite mud ball from the Domin­
ion Reef showing the typical porous surface. F, Surface of E, showing the fine-grained
texture of irregularly formed grains.

M13

M14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

stratigraphic horizons in the Witwatersrand that con­tain large amounts of authigenic pyrite have, in most in­stances, subeconomic gold contents. This can be seen asa reflection of the original depositional environment,
which could be described as a low-energy environment
producing fine-grained arenaceous sediments withsulphide gels and mud, the precursors of the authigenic
pyrite. Only limited amounts of very fine grained gold
could be transported under such conditions.
The occasional occurrence of minute inclusions of goldin, as well as epitaxial growth of gold on, authigenic
pyrite points to a local redistribution of gold under con­ditions of elevated temperatures, which could result
from regional metamorphism or would exist in thevicinity of volcanic intrusions.
OTHER DETRITAL HEAVY MINERALSGold and pyrite normally are accompanied in the con­glomerates by varying amounts of other detrital heavy
minerals. Of economic importance in particular localities
are the platinum-group minerals and uraninite; thelesser-known minerals are chromite, zircon, cobaltite,
gersdorffite, arsenopyrite, rutile, molybdenite,
graphite, tourmaline, and others.
The detrital cobalt-nickel arsenosulphides and the
detrital sphalerite in Witwatersrand conglomerates areof special interest because they sometimes occur as
primary intergrowths with gold (fig. 6). Chemically,
they are even less stable than pyrite in a normal oxidiz­
ing fluviatile environment; thus their presence is an ad­
ditional indicator for a low oxygen content, if not an
absence of oxygen in the palaeoenvironment of Wit­
watersrand time. To date, all geochemical work on the
Witwatersrand and similar rocks elsewhere support thisconcept. Work on the distribution of rare-earth ele­ments in particular showed that only from about 1,950
m.y. B.P. (before present) on did the oxygen partialpressure approach the present level (Fryer, 1977), andthere appears to be no evidence for either a reducing
Precambrian atmosphere (Shimizu, 1976) or a highly ox­
idizing atmosphere, as postulated by several authors(Kimberley and Dimroth, 1976).
Except for a few obviously recrystallised gersdorffite
particles, most arsenosulphides encountered in HFtreated samples displayed a typical rounded, detritalmorphology (fig. 15).
The onset of recrystallisation observed on some grains
could easily be misleading when observed in polishedsections. There appears to be a tendency for incipient
recrystallisation to be more evident in the nickel-richmembers of this group.
There is a wide range in the compositions of the iron-,cobalt-, and nickel-arsenosulphide particles in Wit­

watersrand conglomerates. Within a certain reef hori­zon, however, the composition often remains fairly con­stant. As shown in figure 16, only particles from the
Basal Reef, St. Helena gold mine, vary greatly in theircompositions, whereas samples examined from theother localities have well-defined compositions.
Gold from the Basal Reef at the St. Helena Mine oc­
curs as two distinct varieties: coarse, crystalline, hardly
deformed particles; and flaky, extremely deformed gold
typical of a long transport distance. Seen together withthe variation in arsenosulphide composition, this could
be interpreted as indicating transport from two or more
sources to its present position.
A short transport distance for part of the heavymineral assemblage at St. Helena is supported by the oc­
currence of detrital sphalerite (fig. 15) and goldsphalerite intergrowths, as well as molybdenite andgraphite. The excellent cleavage and low hardness ofthese minerals make them good indicators of low-energy
environment and short distance of transport. A geo­chemical indication of a genetic relation between
sphalerite and gold is given by the gold content of
sphalerite grains, which averages about 200 ppm.
As in the case of pyrite, etch pits on the surface of
detrital sphalerite and arsenosulphide grains point to
the presence of small amounts of oxygen in the Precam­
brian atmosphere rather than to a complete lack of ox­
ygen.
Although it is a small part of the detrital heavymineral assemblage, chromite is a common constituentof all Witwatersrand conglomerates. It normally occurs
as well-rounded particles, often with a verrucose surface
structure, which could be of primary origin. In a study ofchromite composition in the Klerksdorp Goldfield (Ut­ter, 1977), no significant changes in the chemistry of
detrital chromite throughout the Upper Witwatersrand
could be observed, and the composition could best becompared with that of chromite from Archean green­stones.
The comparatively large amount of zinc observed inchromite particles from the Basal Reef appears to be a
local phenomenon.
It has been claimed in the past that a major portion of
the pyrite in Witwatersrand conglomerates was derived
from pyritization of "black sands," that is, frommagnetite and ilmenite (Liebenberg, 1955; Ramdohr,
1955). The evidence is based, however, on only a few
scarce occurrences of transitional stages, and consider­ing the well-documented detrital nature of most of thepyrite, ilmenite and magnetite can be considered to havebeen scarce in the primary deposits.
Well-rounded ilmenite particles without any sign ofpyritization have recently been observed in samples of
the Amazon Reef from the Vredefort area in the centre

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS. WITWATERSRAND

M15

30 /mm
FIGURE 15.—Scanning electron photomicrographs of detrital cobaltite, gersdorffite, and sphalerite. A, Partly recrystallised detrital grain of
gersdorffite from an Elsburg Reef (EA 12), Loraine gold mine, Orange Free State. B, Detrital particle of a nickel-bearing cobaltite from asample of B Reef, Loraine gold mine, Orange Free State. C, Rounded, obviously detrital particle of sphalerite with a slightly etched surface
from a sample of B Reef, Loraine gold mine, Orange Free State. D, Rounded, octahedral fragment of sphalerite from a sample of Carbon
Leader Reef, Western Deep Levels gold mine, Carletonville.

M16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Evander Goldfield, and the Kimberley Reef, East Rand
Goldfield, had almost equal trace-element compositions
as well as similar rare-earth patterns (fig. 17). The rareearth pattern for the Vaal Reef zircons, in contrast,
showed a less pronounced europium anomaly. Although
the results from more samples are still being awaited, it
appears that different source areas can be recognized
and geological units delineated by the trace-element
content of zircons in the heavy-mineral suite.

EXPLANATIONO

Kimberley Reef ,East Rand

0 Kimberley Reef, EvanderA

Vaal Reef

CoAsS

E 10
a
EXPLANATION

Basal Reef. St Helena Gold Mine
Carbon Leader Reef, Doornfontem Gold Mine
B-Reef, Loraine Gold Minei \xx

Detrital
GrainsBird Series, Vredefort Area
Recrystallized Gersdarffite, St. Helena Gold Mine

FIGURE 16.—Composition of arsenosulphides from various con­glomerate horizons in the Witwatersrand sequence.

of the Witwatersrand. Although the ilmenite wasaccompanied by detrital pyrite, no intergrowths of thetwo minerals were observed.
Another prominent constituent of most Witwaters­
rand conglomerates is zircon. It occurs as rounded,single crystals less than 100 jam across, and as fractured,
partly rounded remnants of larger crystals. Experi­ments have shown that hydrofluoric-acid treatment atroom temperature has no noticeable effect on zircons. A
comparison between the trace-element composition of
zircon concentrated from beach sand and the same
treated with hydrofluoric acid has shown no difference
between the two samples.
These samples and other concentrates of zircons fromvarious reef horizons were analysed by instrumental
neutron activation (Watterson, 1977). In all samples
analysed the zirconium-hafnium ratio was found torange between 43 and 48, which is similar to values
quoted in the literature for granites.
Zircons from one of the three horizons analysed, theVaal Reef, were found to contain anomalous amounts of
antimony and iron. Zircons from the Kimberley Reef,

La

Ce

Pr

Nd

Pm Sm

Eu

Gd

Tb

Dy

Ho

Er

i___i___i
Tm Yb LuRARE EARTH

FIGURE 17.—Rare-earth contents of zircons from three conglomerates
in the Upper Witwatersrand.

Although quartz is not part of the heavy-mineral suite,
it is nevertheless the major component of Witwaters­
rand conglomerates. Recently a research programme
has been initiated to study the trace-element content ofquartz and especially the rare-earth-element pattern by
instrumental neutron-activation analysis, as well as thedaughter minerals in fluid inclusions. The results so far
indicate that a classification of quartz pebbles can bemade using the above-mentioned criteria, and that more
information is to be gained on the origin of certain
quartz pebbles and their relation to gold mineralisation.
Various other heavy minerals, including the platinoid
minerals, tourmaline, and graphite (not actually a heavy
mineral), have been observed but not specially investi­
gated.

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND

CONCLUSIONSIn studies of the distribution of gold, uranium, andother heavy minerals in fossil placers the technique ofmorphological examination in the scanning electron mic­roscope opens up the third dimension to microscopy.
This supplements the classical two-dimensional micro­
scopy as a tool for geological investigation and makes
possible observations not previously attainable withearlier methods.
New investigative methods apply especially to thegeochemical studies of mineral grains made with themicroanalysing attachment of the scanning electron
microscope and with instrumental neutron-activation
analysis of single grains, as well as sample suites.
Fossilised organisms could consequently be recogniz­
ed from the morphology of their carbonaceous remains,
permitting their role in the concentration of gold anduranium to be explained.
Studies of the morphology of different types of gold,
pyrite, and other minerals have permitted conclusions
on the origin and distribution of economically importantminerals, as well as on the palaeoenvironment of Precambrian placers that were not possible with theclassical methods of microscopy.
REFERENCES CITED
Ball, H. S., 1940, Armoured mud balls—their origin, properties, androle in sedimentation: Journal of Geology, v. 48, p. 1-31.
Cloud, Preston, 1976, Major features of crustal evolution: GeologicalSociety of South Africa, Annexure to v. 79, Alex L. Du Toit
Memorial Lecture no. 14, 33 p.
Deer, W. A., Howie, R. A., and Zussman, J., 1962, Pyrophyllite, in
Rock-forming minerals; v. 3, Sheet silicates: London, Longmans,
p. 115-120.
Fryer, B. J., 1977, Rare-earth evidence in iron-formations of changing
Precambrian oxidation states: Geochimica et Cosmochimica Acta,v. 41, no. 3, p. 361-367.
Hallbauer, D. K., 1974, Nature of reefs: Chamber of Mines of South
Africa, Research Review 1974, p. 45-55.
———1975, The plant origin of the Witwatersrand "carbon": Minerals
Science and Engineering, v. 7, p. 111—131.
————1977, Morphology and Microtexture of minerals from fossil gold
and uranium placers: Chamber of Mines of South Africa, Research
Review 1976/77, p. 36-50.
-1978, Witwatersrand gold deposits—their genesis in the light ofmorphological studies: Gold Bulletin, v. 11, p. 18-23.
Hallbauer, D. K., Jahns, H. M., and Beltmann, H. A., 1977, Mor­phological and anatomical observations on some Precambrianplants from the Witwatersrand, South Africa: Geologische Rund­
schau, v. 66, p. 477-491.
Hallbauer, D. K., and Utter, T., 1977, Geochemical and morphologicalcharacteristics of gold particles from recent river deposits and thefossil placers of the Witwatersrand: Mineralium Deposita, v. 12, p.
293-306.
Hallbauer, D. K., Utter, T., and Hirdes, W., 1978, Size and composi­tion of gold, pyrite, and other sulphides in some Witwatersrandconglomerates: Coal, Gold and ^i >e Minerals of Southern Africa,
v. 26, p. 65-79.

M17

Hallbauer, D. K., and van Warmelo, K. T., 1974, Fossilized plants inthucholite from Precambrian rocks of the Witwatersrand, South
Africa: Precambrian Research, v. 1, p. 199-212.
Hollister, V. F., 1975, An appraisal of the nature and source of por­phyry copper deposits: Minerals Science and Engineering, v. 7, p.
225-233.
Johan, Om., 1974, Characterizing materials with the SEM today:
Research and Development, v. 25, p. 16-18.
Kimberley, M. M., and Dimroth, E., 1976, Basic similarity of Archaean
to subsequent atmospheric and hydrospheric compositions asevidenced in the distributions of sedimentary carbon, sulphur,uranium, and iron, in Windley, B. F., ed., The early history of the
Earth: London, John Wiley & sons, p. 579-585.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef, and the Black Reef:
Geological Society of South Africa Transactions, v. 58, p. 101-227.
Margulis, Lynn, Walker, J. C. G., and Rambler, Mitchell, 1976,
Reassessment of roles of oxygen and ultraviolet light in Precam­
brian evolution: Nature, v. 264, p. 620-624.
McKee, E. D., Crosby, E. J., and Berryhill, Hf L, Jr., 1967, Flooddeposits, Bijou Creek, Colorado, June 1965: Journal of Sedimen­
tary Petrology, v. 37, no. 3, p. 829-851.
Metzger, F. W., Kelly, W. C., Nesbitt, B. E., and Essene, E. J., 1977,
Scanning electron microscopy of daughter minerals in fluid inclu­
sions: Economic Geology, v. 72, no. 2, p. 141-152.
Minter, W. E. L., 1976, Detrital gold, uranium, and pyrite concentra­
tions related to sedimentology in the Precambrian Vaal Reef
placer, Witwatersrand, South Africa: Economic Geology, v. 71, no.
1, p. 157-176.
———1977, Areal size distribution of heavy minerals in the Proterozoic Basal Reef placer, Witwatersrand Supergroup: South
Africa, Proceedings Congress 77, Geological Society of South
Africa (in press).
Muller, L. D., 1959, The micropanner—an apparatus for the gravity
concentration of small quantitities of materials: Institution of Min­ing and Metallurgy Transactions, v. 68, 1958-59, v. 68, p. 1-7.
Neuerburg, G. J., 1975, A procedure, using hydrofluoric acid, for quan­titative mineral separations from silicate rocks: U. S. Geological
Survey Journal of Research, v. 3, no. 3, p. 377-378.
Pretorius, D. A., 1975, The depositional environment of the Wit­
watersrand goldfields: A chronological review of speculations andobservations: Minerals Science and Engineering, v. 7, no. 1, p.
18-47.
Ramdohr, Paul, 1955, Neue Beobachtungen an Erzen des Witwatersrandes in Sudafrica und ihre genetische Bedeutung: Deutsche
Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik
und allgemeine Naturwissenschaften, Abhandlungen Jahrgang
1954, no. 5, 43 p.
———1975, Die Erzmineralien und ihre Verwachsungen (4th ed.):Berlin Akademie-Verlag, 1,277 p.
Saager, R., 1973, Geologische und geochemische Untersuchungen an
primaren und sekundaren Goldvorkommen im friihen Prakambrium Sudafrikas: University of Heidelberg, unpublished D.Sc.
thesis, 150 p.
Schidlowski, M., 1968a, The gold fraction of the Witwatersrand con­glomerates from the Orange Free State Goldfield (South Africa):
Mineralium Deposita, v. 3, p. 344-363.
———1968b, Untersuchungen an kohliger Substanz aus dem
Prakambrium Sudafrikas: Umschau, no. 18, p. 566-567.
-1970, Untersuchungen zur Metallogenese im siid-westlichen
Witwatersrand-Becken: Geologisches Jahrbuch Beihefte, v. 85, 80
PSchidlowski, M., and Junge, Chr., 1973, Evolution der Erdatmosphare: Physikalische Blatter, v. 29, p. 203-212.

M18

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Schidlowski, M., and Trurnit, P., 1966, Drucklosungserscheinungen an
Gerollpyriten aus den Witwatersrand-Konglomeraten—Ein
Beitrag zur Frage des diagenetischen Verhaltens von Sulfiden:
Schweizerische Mineralogische und Petrographische Mitteilungen,v. 46, p. 337-351.
Shimizu, Mikio, 1976, Instability of a highly reducing atmosphere on
the primitive Earth: Precambrian Research, v. 3, p. 463-470.
Snyman, C. P., [1968?], Possible biogenetic structures in Witwatersrand thucholite: Geological Society of South Africa Transactions,v. 68 (1965), p. 225-235.
Stoops, G., 1970, Scanning electron microscopy applied to themicromorphological study of a laterite: Pedology, v. 20, p.
268-280.
Utter, T., 1977, Morphology and geochemistry of pyrites from theUpper Witwatersrand system of the Klerksdorp Goldfield:
Chamber of Mines of South Africa, Research Report no. 8/77, 38 p.
———1978, The origin of detrital chromites in the Klerksdorp
Goldfield, Witwatersrand, South Africa: Chamber of Mines ofSouth Africa Research Report no. 17/78, 15 p.
Viljoen, E. A., 1971, An electron-microprobe analysis of gold in the
Witwatersrand banket and in ores from the Barberton Mountain
Land: National Institute for Metallurgy, Johannesburg, Report no.
1361, 7 p.
Viljoen, R. P., Saager, R., and Viljoen, M. J., 1970, Some thoughts on
the origin and processes responsible for the concentration of goldin the early Precambrian of southern Africa: Mineralium Deposita,
v. 5, p. 164-180.
Watterson, J., 1977, The analysis of zircons from the Witerwatersrandreef by instrumental neutron analysis, in Annual Report to the
Chamber of Mines of South Africa: Witwatersrand University
Nuclear Physics Research Unit, Johannesburg, p, 11-17.
Zumberge, J. E., Sigleo, A. C., and Nagy, B., 1978, Molecular andelemental analyses of the carbonaceous matter in the gold anduranium bearing Vaal Reef carbon seams, Witwatersrand se­quence: Report to Chamber of Mines of South Africa, 53 p.

DISCUSSION[At the workshop Dr. Hallbauer presented three shortpapers, titled: (1) Stereo X-ray radiography andchromatographic painting techniques applied to thestudy of Witwatersrand conglomerates; (2) Morphology
of gold, pyrite, zircon, and chromite in the Witwater­
srand Series; (3) Genetic relation between organic
material and gold in Witwatersrand conglomerates. The
first one followed the presentation by Dr. Whiteside, thesecond one followed Dr. Saager's talk, and the third one
followed Dr. Schidlowski's paper.
Dr. Hallbauer has combined the three talks into asingle manuscript, chapter M of this report, whichsomewhat complicates orderly placement of the discus­
sion following each talk. The headings to the discussions
will aid the reader in following the different
discussions.]
DISCUSSION FOLLOWING PAPER 1 BY DR. HALLBAUER

Simpson: Your larger gold grains associated with thecarbonaceous material don't look like detrital grains.

Do you consider it possible, therefore, for the gold tohave recrystallised diagenetically on the carbonaceous
material which your beautiful photographs, if I may
say so, show so clearly as being relatively barren ingold?
Hallbauer: First of all, the large gold particles in the car­bonaceous material don't look detrital in thephotographs which I've shown, but they look detritalin the stereophotographs. And, apart from this, wehave dissolved many samples of conglomerate inhydrofluoric acid and paid special attention to theselarge gold particles, and I will show a few of themlater on and you can see quite clearly that they are
detrital.
Minter: On your first radiograph, on the right-hand sideon the foresets there were three or four gold particles.
You said there was no gold in the quartzite between
the pebble layers. We, on the contrary, have sliced
specimens and find that in the reef unit, which is thatmature layer of quartz arenite, between 8 and 18 per­cent of the total gold content is in the quartzite on
foreset layers. That's quite a high percentage of thetotal gold content and this is where one would expect
to find it as detrital particles.
Hallbauer: Yes, but on the other hand if you look at thecomparatively thin conglomerate, most of the gold iseither on the footwall contact or in the matrix sur­rounding the larger particles. In the photographs I've
shown you it's very difficult to distinguish between
pyrite and gold. You would have to look at the original
X-ray photographs where you can quite easily see thedifference, but you are quite right in so far as we've
found examples of gold distributed on foresets in thisreef and in the footwall rocks.
Skinner: Have you any specific reagents that you use foryour staining techniques with uraninite? Have you
developed any?
Hallbauer: We have done very little, but we've foundthat if you use nitric acid as a dissolving agent and use
potassium hydrate (KOH), then the uraninite shows up
as brown dots, that is, if you don't have too much iron.
You have to mask the iron before you do that. So quite
often, it might be three treatments, one after theother. And if you do it in quick succession, then thesolution which is in the pores has no time to disperse;
otherwise, you get blurred images of your grains. We
found with the contact method that you can detect
gold particles down to a diameter of 10 microns, but inthat case you have to expose for a longer time. The
small gold particles are surrounded by a small halo of
precipitate, but under the ordinary stereomicroscope
you can observe the small core within.

MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRANDDISCUSSION FOLLOWING PAPERS BY DRS. MINTER, SAAGER,
AND HALLBAUER (PAPER 2)

von Backstrom: Well, ladies and gentlemen, I think
you'll agree with me that we have listened to three ex­tremely interesting papers, and these are now open
for discussion.
If you would permit me to kick the ball off, I mustsay I am a bit confused. I think we have seen somevery strong evidence presented by Dr. Hallbauer thatthe distance of transport was very small, and this I
believe is also shown by one of the previous slides thatwas projected that was taken from work by Professor
Pretorius, and Dr. Saager also gave us some very in­teresting data showing that the provenance was quite
far away. The Barberton area from which the gold
could have been transported, I think, for those of you
who don't know South Africa, is of the order of about
300 miles from the central Rand, and is probably about
450 miles from the Orange Free State. Now, I wonder
what is the general view on this question today? Could
you give us the answer?
Saager: Yes. I think you should not look at the Barberton Mountain Land as the actual source area. It is amodel source area because we haven't got any
greenstone belts which we can actually look at closer
to the Witwatersrand Basin except the small
greenstone remnants in the Johannesburg dome, but
unfortunately we haven't found any sulfides in thegreenstone remnants. The small remnants preserved
are the very roots of the greenstone belts, and the rest
has been completely eroded away. I don't think the ac­tual source is the Barberton Mountain Land or any of
the greenstone belts in the eastern Transvaal, butsimilar greenstone belts, now covered by younger
rocks or eliminated by the Bushveld Complex, muchcloser to the basin.
Grandstaff: I have a question for Dr. Hallbauer. I am alittle concerned about possible alteration of the grainsurfaces during the HF treatment. Have you madeany SEM photos on grains prior to and after the HF
treatment to determine any possible alteration in thesurface characteristics?
Hallbauer: Yes, we examined some grains, particularly
zircon grains recovered from beach sands, before andafter hydrofluoric acid treatment, and there was nosign of any etching. More detailed investigations ofresistance to hydrofluoric acid treatment have been
done by Neuerburg from the U.S. Geological Survey
to develop this method of hydrofluoric acid treatment,
but as far as we are concerned, we haven't found any
signs of hydrofluoric acid attack, apart from acid at­tack on uraninite grains, of course. They preserve
their round shape, but they are completely altered to

M19hexafluoride. We found no evidence of acid attack on
pyrites.
Whiteside: May I make a couple of comments on theremarks that von Backstrom made just now. First ofall, Barberton is over to the northeast and I believe
everybody believes the transport direction is from thenorthwest or north; and secondly, there are remnantsof greenstone belts in the Vreysrus [?] dome, and there
was actually one small gold mine there.
Little: I'd like to ask Dr. Hallbauer if he has been able to
make any correlation of the degree of transport withthe paleogeography?
Hallbauer: [Hallbauer drew a sketch map of the Wit­
watersrand Basin and indicated that the gold particles
that showed signs of short transport were collected
near a source and gold particles that showed increased
distortion of their surfaces were collected farther
from a source. He said that that was the only informa­tion he had at that time, but that they are continuing
to collect data.]
Saager: We've been looking at some lead-isotopic com­position of nodular pyrites on which Dr. Hallbauer
identified mud cracks. Now the interesting result of
this investigation is somehow a puzzling result. It
shows that this pyrite probably has formed as early as
2,000 million years ago, which would have been during
the metamorphism of the sediments. Could you makesome remarks on that? We don't really understandthis age we have been getting.
Hallbauer: So far, our geochemical investigations haveproceeded only as far as the gold content of these
pyrites. We have found that the rounded crystalline
pyrite from one particular reef has an average gold
content of 10 ppm, which is more or less the amountthat can be accommodated in the crystal lattice. The
pyrite which could be typical for pyritic concretions
has the lowest gold content, average of V-h ppm. The
pyrite which we interpret as pyritic mud balls has gold
contents of something between 600 and 900 ppm, andwe selected these pyrites very carefully to make surethat no visible gold was attached to them. First of all,they were selected manually under thestereomicroscope and then the same fraction was
X-rayed on fine-grained film to make sure that nolarge gold particles were included within the pyrite.
Even so, the gold content was very high. The otherresults concerning the other trace elements—cobalt,
nickel and others—are not completed yet.
Saager: Wouldn't a high gold content be proof of a verylate formation of this mud cake, during probably thesolution stage of the modified placer theory?
Hallbauer: The possibility is that fine-grained gold wasincluded when the mud was still soft and pliable, or theM20

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

gold was transported into the porous pyrite and wasincluded in the conglomerate. We haven't yet madepolished sections and we can't answer this.
[Some discussion was not recorded while recording
tapes were changed.]
Myers: I think it's worth commenting on the fact thatthe osmiridium grains are recovered pnly from
amalgam barrel residues from the mill and so you can
get only one point per mine. Osmiridium occurs inparts per billion, so osmiridium grains are almost
never seen in polished-sections.
Bourret: In past years there was a substantial produc­tion of osmiridium; of course, that is not the case to­day. In view of the knowledge gained over the years, it
occurred to me that this might be a successful sourcearea to work on.
Whiteside: Can I just make a comment there? There wasa time, I think, when most of the production of
osmiridium came from the East Rand, and the ratio of
osmiridium to gold fell as you went across the Central
Rand until, I'm just quoting from memory, you foundthat on the West Rand or on the western side of theCentral Rand, it was about one part in six thousand.
The greatest production now comes from our latest
field, the Evander Goldfield, which is not so young
now. It lies to the east and is detached from the main
Witwatersrand Basin, and only the Kimberly Reef ismined. It thus looks as though there is a gradualreduction of osmiridium from east to west.
van Backstrom: I observed when I worked in the gold
mines that many of these reefs, especially at great
depth, are sheared and compacted. As a matter offact, I think you know that in reefs that have been
mined there can be, unless there is very strong sup­port, a plastic flow; the reefs start pushing out. Could
some of this movement in the reef itself be responsible
for the bending of gold pebbles?
Hallbauer: The structures you get on the gold particles
are so small that there is hardly any chance of move­
ment within the conglomerates. And apart from this,the structures are so very consistent with the struc­tures on nuggets that the movement within the confin­
ed spaces of the rock should produce a totally different
structure from the movement within loose pebbles.
Schopf: It would seem to me that these rocks have been
compacted during lithification and I wonder whether acertain amount of the markings you interpret as abra­sion could be introduced during the compaction of thesediment and therefore not necessarily be entirely in­troduced during transport?
Hallbauer: I am sure there are signs which could havebeen produced during the diagenesis and compactionof the sediments, but on the other hand, the surroun­ding quartz pebbles normally create a pressure-freearea within their matrix, and movement in that spaceshould be confined to only microns. I admit there is apossibility that these things could be mixed with move­
ment within the rock; it is very difficult to differen­
tiate between these.
Schopf: Have you generated a set of criteria that willallow you to differentiate between markings produced
at various times during transport, during sedimenta­
tion, during diagenesis, things of this sort?
Hallbauer: This is something we haven't done yet. Thereare examples, however, of gold particles which show
only small signs of distortion and others which show
signs of complete distortion. How to differentiate bet­
ween them will be shown in further work.
Simpson: Dr. Hallbauer, I would like to ask you withwhich detrital gold particles are you comparing yourmaterial, which you have alluded to several times inyour talk, because they don't compare at all to the typeof detrital gold particle that you see deposited byglacial action in, say for example, the strath of
Kildonan in Scotland?
Hallbauer: The gold particles we used came from rivers
in the Barberton area, and we assumed transportdistance from the last known outcrop of gold-bearing
rock to the place where we took our samples, about 30
to 50 km. In addition, we had samples of gold particles
from Blind River, but apart from that we haven't ex­
amined gold particles from other alluvial deposits yet.
Feather: I would like to ask Dr. Hallbauer, would you
care to comment on what proportion of the original
detrital gold was redistributed during subsequent
metamorphic activity in the reefs?
Hallbauer: That's a bit difficult to answer. In a large
number of samples we found signs of gold being
redistributed mostly as oriented growths on secon­
dary pyrites, but from rough judgment I would saythat this is an insignificant amount, probably less than
20 percent.
JamesB. Black, Atlas Minerals, Utah: I have a couple ofrather naive questions perhaps. To Dr. Saager: Could
you refer me to the method by which you independent­
ly establish the age of a lead loss? Some document that
I might be able to read that might detail your method?
Saager: There are two papers: One is by Berger, whichappeared in 1962 in Cosmochemica Acta, and theother one is by Nicolaysen, which is in the same
volume, also 1962.
Black: Thank you, that's fine. Then to Dr. Hallbauer:
Have you observed similar indications of transporta­tion on grains of uraninite?
Hallbauer: Unfortunately, most of the uraninite is at­tacked by hydrofluoric acid, and it would be extremely
difficult to say anything specific about the surface
structure of uraninit^ grains. Of course, you have toMINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND

M21

take into acccount the signs of acid attack, and it
which is about the size of a pea that shows seven faces.
might be better to say nothing at all. I think there is a
There is a geologist, Dr. Cecelia Valetsia [sp.?], whocertain amount of caution required in thesehas a gold crystal about three-quarters of an inch instatements.
diameter which has recrystallised in place. Why canBlack: How about attempting to get a photomicrograph
you be so sure that they recrystallised in place?
of a particle in place that has not been etched by
Because there are no hydraulic equivalents with thehydrofluoric acid?
gold. There are no gold veins in the vicinity; therefore,
Hallbauer: That we haven't attempted. I would say thatthe gold must have been transported a considerable
it is very difficult to find a grain of uraninite within the
distance. From this kind of evidence one can then con­matrix which has not been broken and where the sur­clude that the gold must have recrystallised. So, it isface has been kept in such a state that you can reallypeculiar that the gold in the Witwatersrand does notmake statements.
show recrystallisation. Now, there is a possibility that
Black: Thank you very much.
the chemical conditions were different in those days.
Bourret: I am sure that everyone is aware that there is
We know that uraninite was persistent; perhaps
an excellent paper by Liebenberg with dozens andbecause of similar conditions gold refused to migratedozens of photomicrographs of uraninite grains in reef
and therefore didn't recrystallize. But there is also theore, 50, 75, or 100 of them. They are all described andother fact that gold is exceedingly soft. Accordingly, I
they show clearly rounded detrital grains.
think that one would have to take particular care in
Hallbauer: If I may quickly comment, these detritalcollecting gold from in place or even collecting gold
grains from Liebenberg come from milled products.
from present-day placers to make sure that, if it has
They have been through the mill, and surface struc­crystallised in place, it doesn't become scratched be­tures are somewhat dubious.
tween the time it is dug out and the time that it is ex­
Myers: If I recall the literature correctly, heamined under the microscope.
[Liebenberg] specifically said that he dug some of James Trow, Michigan State: I have two questions. The
these [rounded uraninite grains] out of reef samples.
first for Dr. Minter: Would you care to speculate con­
Feather: May I comment on this. I have looked at a lot of
cerning the greater proportion of uranium compareduraninite grains recently. These are specifically expos­
to gold in the Canadian occurrences compared to thoseed on fresh fractures in the reefs, so that there is notin the Witwatersrand? Do you ascribe those dif­the problem that the grains might be rounded during
ferences to perhaps the Canadian occurrences being in
the milling process. I would say that invariably thea more distal portion of the fluvial system, or are thegrains contain at least one or two distinct crystal faces
differences more related to provenance, with greaterwith distinct corners and many grains are beautiful
greenstone surface exposure near the Rand compared
crystals. The evidence seems to suggest that they
to what we have in Canada?
have recrystallised in the reefs and that therefore any Minter: I think the reason is that one does have different
original features have been completely destroyed. The
provenance areas and different original amounts ofprevious idea that the grains were completely rounded
gold and uranium so that the ratio change has to be
I would contest, certainly from examining freshly ex­
based on each separate population. And, moreover,posed grains in the reefs. In my talk I'll show you someone finds that there is change going down the paleophotographs.
slope of a single reef. We have done this with a
Kalliokoski, Michigan Technological University: I would
number of reefs and find that there is a ratio change
like to make a comment about the nature of gold. It
down the paleoslope but that one can't compare one
seems that the gold in the Witwatersrand is in some
reef directly with another because on some paleorespects anomalous from more recent gold. I was in­slopes one can go from a coarse pyrite nodule to a fine
troduced to this subject by a gentleman at Stanford in
pyrite nodule in only 4 km, whereas in the Steyn Reef
the metallurgy department, a Colonel Crook, who
it takes 22 km. Obviously the gradients are different,
became experienced in studying gold. He was involved
the sizes of the deposits are different, and I think thatin some law cases where the question was whether thecertainly provenance areas supply different propor­
gold was Recent or Tertiary, and he was able totions. The deposit at Denison [Elliot Lake, Canada] isdemonstrate that Recent gold was quite irregular
not a distal deposit; it looks quite proximal from thewhereas Tertiary gold had recrystallised and had very
sizes of pebbles, et cetera, so therefore the original
definite crystal faces. A similar kind of recrystallised
uranium content must have been extremely high. I
gold has been found in some of the placers in Surinam,
have wondered whether the uraninite grains are ac­
and at a place in Venezuela called Icabaru you can findtually larger than those in the Witwatersrand, but I
octahedra. My wife has one that she forbids me to cut
have no data.

M22

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Trow: The second question is to Dr. Hallbauer: Have you
seen any octahedra of pyrite or any forms which mightsuggest that the pyrite is pseudomorphous aftermagnetite?
Hallbauer: No, I haven't observed any octahedra or any
signs of alteration from magnetite to pyrite.
Unidentified[Growick?]: A remark on that. I think thatRamdohr and several other authors have shown inclu­
sions of magnetite in pyrite pebbles, suggesting a
replacement of magnetite by pyrite, but all theseauthors could not give a clear indication where thisreplacement actually would have taken place. Has it
taken place in the basin our outside the basin? I do notknow.
Hans Adler, Energy Research and Development Admini­
stration: I would like to ask this question of Dr.
Minter. In regard to this 70/30 approximate ratio of
detrital to authigenic pyrite, is there any observablechange in this ratio going from the proximal to the
distal portions of the reefs, and also is there an obser­
vable difference in this ratio between the carbonbearing and the non-carbon-bearing reefs? Secondly,
has an attempt been made to determine differences inthe sulfur-isotope ratios of the authigenic and the
detrital pyrites?
Minter: I haven't observed any difference in that wild
estimate of the ratio going from proximal to distal
deposits, and the only places where I have seen a bigchange have been in the vicinity of dikes which havecooked up the reefs and remobilized the pyrite. In fact,in specimens of that nature one can't measure thepyrite nodules; they have been completely recrystallised. One other criterion in collecting samples was tomake sure that one collected pyrite from samples
taken some distance from any intrusives. I think I've
seen comments made on the sulfur-isotope ratios in
textbooks, and I think that somebody else is morequalified to comment on that.
Schidlowski: I think I'd better do that in the frame of mytalk.
Adler: That's fine. Could you comment on the ratio of
the two types of pyrites in the carbon and noncarbon
reefs? Is there any observable difference there?
Minter: No, none.
Adler: Thank you very much.
Weis, U.S. Geological Survey: This is for Dr. Minter.
You commented on the increase in the percentage ofzircon down the paleoslope and the change in the golduranium ratio and the increase in uranium content
down the paleoslope. Is there a change in the gold con­tent, and if so, what is it?
Minter: There is a small change in the gold content. It
decreases down the paleoslope slightly, but not nearly
as much as the increase in the uranium content. We in­tend to measure the size of the zircon grains and also
to analyse the zircon concentrations so that we hope
eventually to tie this up completely.

Uraniferous Constituents of
the Witwatersrand Conglomerates:
Ore-Microscopic Observations and
Implications for the Witwatersrand Metallogeny
By MANFRED SCHIDLOWSKI
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-N

CONTENTS
Page

Page

Abstract ______________________________
Introduction __________________.
Uraninite _____________________________
Occurrence within the conglomerates __.
Microscopic characteristics ___________
Chemistry _________________________
Origin of the uranium mineralization ___Brannerite ____________________________
Microscopic and chemical characteristics
Origin of the Witwatersrand brannerite _.

Nl
1
1
1
1
3
4
7
7
10

Carbonaceous matter ____________—_——_——————
Microscopic observations and their genetic significance __Characteristics of major carbon accumulations
Origin of columnar thucholite _____________—.
Source of the carbon constituents _____________.
Problems of uranium mineralization in Precambrian
conglomerates _________________————————.
Acknowledgments __________________——_——.
References cited ___________—_——————————Discussion following papers by Drs. Schidlowski, Hallbauer(paper 3), and Lois Nagy ___________________.

N10
10
14
14
19
19
22
22
24

ILLUSTRATIONSPage

FIGURES 1-3. Photomicrographs showing:
1. Detrital grains of uraninite from various uraniferous Witwatersrand conglomerates ______________—_
2. Hypidiomorphic uraninite particles from various Orange Free State gold conglomerates _________—_—_
3. Detrital grain of uraninite from Basal Reef, Orange Free State with minute specks and outgrowths of galena, andmicroscopic scanning pictures for uranium, thorium, and lead ______________________—_——__
4. Graph showing relation between roundness and average grain size in a Lake Erie beach sand and a Mississippi River sand_5, 6. Line drawings showing:
5. Muffin shaped quartz grains from "Ottawa Sand" facies of Ordivician St. Peter Sandstone, Ottawa, Illinois ____
6. Detrial monazite grains with typical muffin shapes from a recent West Australian beach placer ___________
7-13. Photomicrographs showing:
7. Two typical aggregates of colloform pitchblende from a vein-type uranium deposit, Sudetes Mountains, central
Europe ___________________________________________________________
8. Felts of needle- and lath-shaped brannerite crystals and "compact" brannerite aggregate from various Orange
Free State reefs ______________________________________________________
9. Pseudomorphs of brannerite after grains of detrial uraninite from Orange Free State reefs ____________
10. Microprobe scanning pictures for titanium, uranium, and thorium obtained from the muffin-shaped"uraninite ghost" shown in figure $A ___________________________________________
11. Two grains of uraninite from Elsburg Reefs, Orange Free States, with overgrowths of brannerite, and microprobe scanning pictures for titanium, uranium, and lead _______________________________
12. Grains of uraninite from various Orange Free State reefs, coated and veined or partially replaced by carbon­
aceous matter _________________________________________________________
13. Advanced stages of uraninite replacement by carbonaceous matter, in various Orange Free State con­glomerates microprobe scanning pictures for uranium _________________________________
14. Photographs showing microprobe scannings for uranium and thorium of the carbon-uraninite aggregate depicted
in figure 12D _____________________________________________________________
15. Photomicrograph and explanatory sketching of typical "carbon" footwall of a gold- and uranium-bearing conglomeratehorizon (Basal Reef, Loraine Gold Mines, Orange Free State), showing distribution of minerals _____________
16. Schematic block diagram of a "carbon reef characterized by a seam of fibrous thucholite at the base of a conglomerate
layer ________________________________________________________________
17-19. Photomicrographs showing:
17. Fibrous thucholite with characteristic mineral inclusions, from B Reef, Loraine Gold Mines, Orange Free State _18. Fibrous carbonaceous matter with inclusions of uraninite; flowage of carbonaceous matter; and scaly graphite(from various Orange Free State conglomerates mines _______________________________
19. Reconstruction of the lichenlike symbiont putatively responsible for the fibrous carbon aggregates _ ______
20. Graphs showing evolution of the Earth's^total budget of photosynthetic oxygen ________________________

N2
3
4
5
5
5
6
7
8
9
9
11
12
13
15
16
17
18
19
21
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESURANIFEROUS CONSTITUENTS OF THE WITWATERSRAND CONGLOMERATES:
ORE-MICROSCOPIC OBSERVATIONS AND IMPLICATIONS FOR THE
WITWATERSRAND METALLOGENY

By MANFRED SCHIDLOWSKI'
ABSTRACT

A review is given of the available ore-microscopic evidence pertain­
ing to the origin of the main uraniferous constituents of the banket.
Uraninite occurs principally in the footwall portions of the reefs asrounded or muffin-shaped grains some 100 micrometers in diameter,
which leave little doubt as to their waterworn, abraded nature. It canbe demonstrated by comparison with detrital monazite grains frompresent-day beach placers that the muffin shape displayed by individual
uraninite particles constitutes an unquestionable sedimentary abrasion
form. Brannerite appears to be an authigenic constituent of the reef,
most probably formed from uraninite and rutile as a result of
postdepositional "pseudohydrothermal" reconstitution. Branneritization of primary detrital uraninite grains has often preserved thecharacteristic abrasion forms of the uraninite parents (so-called"uraninite ghosts"). As for carbonaceous matter or "thucholite,"
microscopic evidence decidedly favors a genetic concept involving aradiolytic polymerization of originally mobile hydrocarbons (mostprobably indigenous to the rock) by pre-existing uraninite constituents,
with these latter subsequently subjected to progressive replacementby the ambient organics. Apart from a morphologic description of theprincipal "carbon" aggregates in the reef, a critical appraisal is given ofa recently proposed different concept for the origin of the organic
substances. Altogether, the mineragraphic evidence lends strong sup­port to a modified placer theory previously advanced for the origin of
the Witwatersrand metallogenic province as a whole.

INTRODUCTION

The rediscovery of uraninite in the Witwatersrandconglomerates, originally recorded by Cooper (1923),
during the World War II hunt for fissionable material
gave rise to a temporary revival in the 1950's of thelongstanding Witwatersrand controversy between the"placerist" and "hydrothermalist" schools (Davidson,
1953, 1957; Liebenberg, 1955; Ramdohr, 1955, 1958a,b). During these discussions, the crucial importance ofmineragraphic studies for the elucidation of the Wit­
watersrand metallogeny became apparent. Following
the pioneering work by Ramdohr and Liebenberg, animpressive array of detailed ore-microscopic investiga­
tions has appeared during the last two decades, fur­nishing prima facie evidence of a detrital origin for thebanket deposits (Keen, 1961; Schidlowski, 1966a, 1970a;
1 Max-Planck-Institut fur Chemie (Otto-Hahn-lnstitut), Saarstr. 23, D-65 Mainz, Federal
Republic of Germany.

Schidlowski and Trurnit, 1966; Hiemstra, 1968; Saager,1970). The following paper aims at briefly reviewing thebasic observational and microchemical evidence accruedin these publications to provide a synopsis of our present
knowledge of the principal radioactive constituents of
the reef.
URANINITE
OCCURRENCE WITHIN THE CONGLOMERATESBoth microscopic inspection and autoradiographs ofcomplete reef sections have invariably furnishedevidence that the bulk of the uraninite is mainly concen­
trated in the lower portions of the conglomerates, alongwith gold and other heavy minerals. The richest
assemblages of uraninite particles have always been
observed at the reef footwalls, the individual grains
sometimes perching like beads on a string immediately
at the footwall contacts. (All figure 1 photomicrographs
are of samples from such footwall seams.) Occurrencesof uraninite in the upper reef portions are relatively rare
and mostly confined to so-called "false footwalls" form­ing the base of a new depositional cycle within the same
conglomerate horizon. Pending the accumulation of con­tradictory evidence, this would identify the uraninite
fraction as a member of the original heavy suite of theconglomerates.
MICROSCOPIC CHARACTERISTICS

Both Ramdohr (1955) and Liebenberg (1955) haveidentified the prototype of the primary, unaltereduraninite grain within the Witwatersrand con­glomerates, which was subsequently recognized in all
further ore-microscopic surveys. In general, this pro­totype shows up as well-rounded particles some 80-150/*m in diameter (fig. 1). The ore microscopist will be par­ticularly struck by the ubiquitous occurrence of grains
exhibiting a typical "muffin shape," characterized by one
flat side, the opposite side forming a gentle bulge (fig.
\A, C, D, F, I, «/).
Nl

N2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 1. -Detrital grains of uraninite from various Witwatersrand conglomerates, mostly Basal and Elsburg Reefs, Orange Free State, many ofthem displaying excellent muffin shapes (A, C, D, J). Note dustlike inclusions of galena (white) in all grains, as well as concentrations of thismineral in cracks (B, C, H) and on octahedral cleavage planes (F). Parts of the galena have been "sweated out," forming outgrowths (A, B, C, D,H) or aureoles (G, J) around individual uraninite particles. Single grains are veined by pyrrhotite (K) intergrown with galena or overgrown by
authigenic pyrite (F, I) or sphalerite (D, left side). All photomicrographs taken in oil immersion.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

N3

In polished section, the gray groundmass of the grains
Apart from the typically well-rounded uraninite par­is always speckled with myriads of dustlike inclusions of ticles described and their cataclastic fragments,
galena (PbS), the radiogenic lead having been obviously hypidiomorphic grains have also been encountered (fig.
subjected to sulfurization during postdepositional 2). With the exception of single occurrences from which
reconstitution of the banket. (With both detrital and they have been reported in larger amounts—for exam­authigenic sulfides present in the reef in large quan­ ple, the Dominion Reef (Hiemstra, 1968}-these grainstities, this was actually to be expected.) Radiogenic are, however, relatively rare constituents of the banket.
galena often appears to be mobilized, forming crack fill­
CHEMISTRY
ings within (fig. IB, C, H, I, K) or aureoles around (fig.
IB, C, G, J) the individual uraninite grains, with small
X-ray diffraction data supplied by Liebenberg (1955)
amounts of the galena also dissipating into the sur­ have furnished proof that the microscopically identified
rounding groundmass. Apart from galena, reconstituted uraninite particles yield a powder diagram identical insulfides,—mostly pyrite, pyrrhotite, and chalcopyrite all respects with that of artificially prepared U02.
(fig. IK)— and also gold have been observed as crack fill­ Chemical analyses of uraninite concentrates from two
ings within uraninite particles, occasionally occupying different reefs, the Main Reef and the Carbon Leader,octahedral cleavage planes of the host mineral (figs. IF, have shown a Th02 content of 1.6 and 2.7 percent,25). Some grains are also transected by minute respectively (Liebenberg, 1955, p. 126). This general
stringers of carbonaceous matter (fig. 2A, C).
tenor was subsequently confirmed by a microchemical

FIGURE 2.—Hypidiomorphic uraninite particles from various Orange Free State gold conglomerates. Some grains(A, C) contain minute stringers of "carbon" showing up in different shades of gray due to pronounced reflection
pleochroism of the carbonaceous material. One particle (B) overgrown by authigenic pyrite is veined by gold,_theveins following, in part, the octahedral cleavage pattern. Note dustlike inclusions of galena (white) in all grains. All
micrographs taken in oil immersion.

N4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRLAN QUARTZ-PEBBLE CONGLOMERATES

analysis of an individual uraninite particle from the The claim advanced by Davidson (1957) in the WitBasal Reef, Orange Free State (fig. 3; Schidlowski, watersrand controversy, that abrasion is unlikely to
1966a, p. 190. For the uraninite fraction of the Domi­ have occurred in the small size class to which the bulk ofnion Reef, Hiemstra (1968) has reported an average the uraninite fraction belongs, does not stand up toTh02 content of about 6.5 percent. Recently, Grandstaff critical review. According to Correns (1960, p. 136), par­(1974) has submitted the results of a comprehensive ticles smaller than 0.1 mm in diameter may still be ex­microprobe investigation of uraninite from the Vaal cellently rounded in certain sedimentary environments.
Reef that showed the Th02 tenor of individual grains to Further, Pettijohn (1957, p. 63) regarded 0.1 mm as apossible lower limit for the rounding of sand grains. Ac­
lie mostly in the range of 2 to 7 percent.
cording to a graph by Pettijohn and Lundahl (1943), thequasi-linear
relation between the sphericity of grains
ORIGIN OF THE URANIUM MINERALIZATION
and their mean diameter still holds for the size class ofWith all information presently available on the about 100 /xm in the case of well-worked beach sands,
uraninite fraction of the banket, there can be no doubt whereas the respective function of a fluviatile sandthat this mineral is a detrital constituent of the rock. In suffers a breakdown at about 170 jum (fig. 4). However,particular, its distribution pattern within the con­ with the degree of abrasion, particularly of the smaller
glomerates is typical for a member of a suite of heavy size classes, largely dependent on the maturity of a sedi­
minerals, corresponding to that of gold and several ment, low sphericity values in immature detritals haveother heavy minerals. Further, the relation of the no significance at all for the issue under consideration.
economic mineralization in general to sedimentation
Moreover, if ordinary sand grains some 100 jum infeatures of the primary depositional environment long diameter still fall within the subrounded category, p
has been recognized (Reinecke, 1928; Sharpe, 1949).
> 0.25 (fig. 4), an abrasion of the considerably heavier
Prima facie evidence of a detrital origin of the uraninite particles (S.G. = 9.0-9.7) of the same size classuranium mineralization is provided by the clearly water- should present no problems at all. Actually, reasonably
worn, abraded nature of the individual uraninite grains. well-rounded uraninite grains as small as some 60 /*m in

PERCENT

10-

Th
20

—r~
30
30

40

—r~
50

—T—
60

—r~
70

DISTANCE, IN MICROMETERS

FIGURE 3. -Detrital grain of uraninite with minute specks and outgrowths of galena, and microprobe scanning pictures for uranium, thorium,and lead. Note higher thorium concentrations in central part of grain, shown also by quantitative step-scanning diagram traversing theparticle over a distance of some 7/tm. Average thorium is 2.42 percent; average Th02 is 2.75 percent. From Basal Reef, Orange Free
State; optical picture taken in oil immersion.

oooo

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY

N5

ooooo
OOOO

0.088

0.125
0.177
0.25
0.35
0.50
GRAIN SIZE, IN MILLIMETERS

FIGURE 4.-Relation between roundness and average grain size in a
Lake Erie beach sand (1) and a Mississippi River sand (2). Note thecontinuous decrease of sphericity values with decreasing grain size.
In the case of well-worked beach sands (1), the size fraction of about
0.1 mm falls within the "subrounded" category p between 0.25 and0.40 (from Pettijohn and Lundahl, 1943, fig. 6; published with per­mission of The Society of Economic Paleontologists and Minerologists, Journal of Sedimentary Petrology).

diameter (fig. LA, B) have been observed in the con­glomerates. According to the experience of this author,
this is about the smallest grain size to display abrasion
phenomena. Uraninite particles of smaller size are
either angular clastic debris or relict bodies of larger
grains that had suffered partial replacement by car­bonaceous matter (see fig. 13).
Among the various undoubted abrasion forms ob­
served, the muffin shape of a large number of uraninite
grains (fig. 1) is most conspicuous. This morphology isfairly widespread in practically all size classes of a large
variety of clastic rocks and probably constitutes one of
the most common abrasion forms known in pebble mor­
phology. Major quartz pebbles of this type have also
been described and illustrated from Witwatersrand con­glomerates (Du Toit, 1954, p. 100).
Comparison of the uraninite "muffins" with the shapesof other ancient and modern detrital clasts (fig. 5, 6) islikely to lend unequivocal support to a placer origin of
the uraninite fraction. The morphological analogies be­
tween the grain populations are actually so self-evident,
that any further detailed discussion seems superfluous.
It should be pointed out, however, that the excellently
rounded monazite muffins from a recent Australianbeach placer shown in figure 6 fall within a size range
similar to the Witwatersrand uraninites. This observa­
tion should definitely dispose of Davidson's (1957) ideathat grains of these dimensions are no longer susceptible

2 MILLIMETERS

FIGURE 5. -Muffin-shaped quartz grains from "Ottawa Sand" facies ofOrdovician St. Peter Sandstone, Ottawa, Illinois. Drawn from thinsection of grain mount.

ooo
oooo
0.1
I

0.2

i

0.3 MILLIMETERSI

FIGURE 6. -Detrital monazite grains with typical muffin shapes from a
recent West Australian beach placer, drawn from grain mount of amonazite concentrate.

to rounding. The obvious correspondence in grain mor­
phology between the Witwatersrand uraninites and theheavy-mineral fraction of a modern marine beach placeris suggestive of genetically related primary sedimentary
environments, thus vindicating previous interpretationsof the economic conglomerates as principally ancient lit­
toral placer deposits (Brock and Pretorius, 1964, p. 570
ff.). Incidentally, the ubiquitous detrital sulfide grains
within the Witwatersrand conglomerates, as well as

N6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

other members of the heavy fraction—for example,
chromite—exhibit similar morphological characteristics,
with muffin-shaped individuals abundant (Schidlowski,
1970a).
Brief reference should also be made in this context toDavidson's (1955, 1957) statement that the rounded ap­
pearance of the uraninite particles of the banket is thattypical of pitchblende from hydrothermal veins. Insteadof a lengthy discussion of this point, two micrographs ofcolloform, botryoidal pitchblende aggregates from agenuinely hydrothermal deposit are included in thispaper for the convenience of the reader not familiar with
textures of primary ore deposits (fig. 7). Comparison ofthese aggregates with the Witwatersrand uraninites
reveals a large array of fundamental distinctions; in par­ticular, the ovoid bodies do not display the faintest in­
dication of a muffin shape. Moreover, clustering and intergrowth of the ovoids is very common, whereas the
Witwatersrand uraninites are always individual,
discrete particles. Rare mention of uraninite "clusters"in the banket refers either to closely packed
assemblages of detrital grains (Liebenberg, 1955, pi. 19,
fig. 3) or to obviously reworked and redeposited
fragments from older reefs rich in uraninite particles(Koen, 1961, pi. 2). Furthermore, hydrothermal andother low-temperature pitchblende contains very little,

if any, thorium, the Th02 content always staying well
below 0.1 percent (Frondel, 1958; Ramdohr, 1969).
The relatively high Th02 and rare-earth-element con­tent of the Witwatersrand uraninites (Liebenberg, 1955,
p. 126) is, on the other hand, indicative of a hightemperature (pegmatitic) parent material and thus
decidely compatible with the concept of a detrital origin.
The wide scatter in the Th02 tenor, between 0.5 and 11
percent with 2-7 percent being most common, recently
reported by Grandstaff (1974) for individual U02 par­ticles of the same grain population from polished sec­
tions or hand specimens, provides additional proof of aheterogeneous sedimentary assemblage. The composi­tional variability of the grains would suggest a
polygenetic source, with each of the contributing areas
characterized by a definite thorium content of theuraninite as a function of different temperatures of for­mation of the parent pegmatite. As pointed out convinc­ingly by Grandstaff, an ,in-situ origin of the uraninite
particles would require that the mineralizing fluids were
heterogenous over extremely small distances, which (1)is unlikely and (2) should definitely produce zoning in thegrowing grains owing to the successive movement
through the rock of fluids of different composition.
Similar conclusions had been drawn by Ramdohr andothers (1965) from semiquantitative microprobe scan-

FIGURE 7,—Two typical aggregates of colloform pitchblende from a vein-type uranium deposit, Sudetes Mountains,
central Europe. Note that ovoid bodies on left picture, with concentric shells and radial cracks formed during the ag­
ing process of the parent gels, differ basically from the detrital uraninite particles of the Witwatersrand Right
photomicrograph depicts large botryoidal pitchblende aggregate. Oil immersion.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY

N7ning diagrams. The variable lead content of individual
uraninite grains noted during this investigation was in­
terpreted as reflecting different ages of formation of the
primary pegmatitic U02 material.

However, as demonstrated by Nuffield (1954) for theBlind River material, a brannerite phase is obtained
after momentary heating at red heat, a phenomenon
corroborated for material from both the Dominion Reef
(Taylor and others, 1962) and Witwatersrand reefs
BRANNERITE(Feather and Koen, 1975, p. 203).
In polished section, the mineral usually appears as
In contrast to the early recognition of brannerite2
felts of needle- or lath-shaped crystals (fig. 8A, B),(UTi206) as one of the principal uranium carriers in theBlind River conglomerates (Nuffield, 1954), the pioneer­ grading sometimes into irregular patches of massive
ing ore-microscopic studies of the Witwatersrand brannerite (fig. 8C). Both the needles and the compact
banket lack any specific references to this material. variety are speckled with minute inclusions of galena;
However, in 1957 Ramdohr, by comparing with Blind the larger ones sometimes display cubic outlines. The
River samples, was able to identify this phase in the massive variety of brannerite is often characterized by
Dominion Reef, expressing also the suspicion that the lath-shaped inclusions of pyrrhotite (fig. 8C) recognizedmaterial of his so-called "uranium ghosts" previously by Ramdohr (1958b) as being an aid in identifying bran­
described and at that time not yet identified might ac­ nerite in both Blind River and Witwatersrand samples.
tually represent brannerite. Similarly, Davidson (1957) Minute dustlike inclusions of anatase have been ob­
considered that the "uraniferous leucoxene" mentioned served and have been explained as the radioactive
by Liebenberg (1955) might be brannerite. Finally, Ram­ breakdown of uranium in the brannerite lattice, with thedohr (1958b) was able to definitely identify this mineral remaining Ti02 subsequently undergoing reconstitution
to anatase. Close association of brannerite with patches
in the Witwatersrand conglomerates.
of rutile is also common (fig. 8C). In places, massive
MICROSCOPIC AND CHEMICAL CHARACTERISTICS
brannerite forms overgrowths on detrital uraninite
The difficulties inherent in the identification of Wit­ grains and has a markedly lower reflectivity (see fig
watersrand brannerite are primarily due to (1) its occur­ 1L4, B) than the uraninite. Pending the accumulation ofrence as mostly tiny needle-shaped crystals at a confirmatory chemical evidence, it is suspected that the
micrometer scale, often intergrown in feltlike ag­ "secondary" uraninite described by Liebenberg (1955) isgregates, and (2) to its metamict state. It is optically largely identical with the compact variety of brannerite.
Peculiar brannerite aggregates are represented by theisotropic, and it does not yield the X-ray diffraction pat­so-called"uraninite ghosts" originally described by Ram­tern of brannerite. The lines for anatase, rutile, anddohr(1955).
Subsequent work (Ramdohr, 1958b;
galena usually show up on the diffractometer charts.
Schidlowski, 1966a) was able to show beyond doubt thatthese "ghosts" actually represented pseudomorphs of
2 [Brannerite is essentially a uranium titanate of variable composition that contains thorium
and minor amounts of calcium, cerium, ytrrium, niobium, and ferrous iron.]brannerite after detrital uraninite grains (fig. 9). As a

FIGURE 8.—Felts of needle- and lath-shaped brannerite crystals (A, B) and "compact" brannerite aggregate (Q from various Orange Free State
reefs. The compact variety (Q contains tiny needles of pyrrhotite and is largely surrounded by rutile. Oil immersion.

N8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 9. -Pseudomorphs of brannerite after grains of detrital uraninite, so-called "uraninite ghosts." In A, C, and E the abraded condition of theuraninite parents has been excellently preserved. With crossed nicols (D), the muffin shape of C is accentuated by abundant internal reflec­
tions of the brannerite. Other "ghosts"derive from hypidiomorphic U02 parents (B, F, G). The central parts of the pseudomorphs are leachedand the cavities partially invaded by authigenic chalcopyrite, pyrrhotite, and usually a little galena (A, B, E, G). Several grains are surround­
ed by aureoles of reconstituted sulfides, mostly lath-shaped pyrrhotite crystals (A, E, F) or pyrite (G). Note minute specks of galena in thebrannerite matrix of all aggregates. All examples from Orange Free State reefs. Oil immersion.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

N9rule, these "ghosts" excellently preserve the original pictures reflecting the distribution of the principal
forms of the parent U02 particles, and many of them ex­ elements in two samples of Witwatersrand brannerite
hibit distinct muffin shapes (fig. 9A, C). The central por­ are presented in figures 10 and 11.
Among the diagnostically valuable optical character­tions of the "ghosts" are usually leached, with theresulting cavities filled, in part, by reconstituted istics of brannerite listed by Ramdohr (1958b, 1969) andchalcopyrite, pyrrhotite, and galena (fig. 9A, B, E, G). by Ferris and Ruud (1971) is its lower reflectivity
Often the grains are also surrounded by aureoles of relative to uraninite (fig. 11) and to rutile (fig. 8Q. Whilethe compact variety of the mineral shows almost no in­
authigenic sulfides, most conspicuous in figure 9£", G.
X-ray spectrographic analyses usually show the ternal reflections, the latter are abundant in felted nests
presence of titanium, uranium, thorium, and lead along of needle-shaped crystals or "cloudy" branneritewith traces of iron and niobium. Microprobe scanning patches, particularly when observed in oil immersion

Tl
FIGURE 10. -Microprobe scanning pictures for titanium, uranium, and thorium obtained from the muffin-shaped "uraninite ghost"shown in figure 9A.

Ti
FIGURE 11. -Two grains of partially leached uraninite (A, B) with outgrowths of brannerite. Note distinctly lower reflectivity of brannerite ascompared to uraninite. Below, microprobe scanning pictures of B for titanium, uranium, and lead. The brannerite overgrowth stands out
prominently in the titanium scan. The rather low lead content of the brannerite indicates that this mineral is considerably younger than theneighboring uraninite. From Elsburg Reefs, Orange Free State. Optical pictures taken in oil immersion.

N10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES(fig. 9D). As a result of metamictization, the Witwatersrand species is practically isotropic, although an occa­
sional low birefringence has been reported for otherbrannerite occurrences.
Radioactive blasting and corrosion phenomena in thebanket caused by radioactive decay of brannerite havebeen recorded by Ramdohr (1958b) and the writer(Schidlowski, 1966c).
ORIGIN OF THE WITWATERSRAND BRANNERITE

According to the microscopic evidence summarized infigures 8, 9, and 11, there can be no doubt that bran­
nerite is an authigenic constituent of the Witwatersrandconglomerates. With a syngenetic origin excluded, it
must be a secondary mineral formed as a result of alarge-scale reconstitution of primary detrital minerals
during later processes of diagenesis and metamorphism.
Ramdohr (1957) was the first to assume that the bran­
nerite from the Blind River deposits had originated fromdetrital uraninite and rutile according to his so-called"Pronto reaction," U02 + 2Ti02 -UTi206, that was be­lieved to have taken place under "pseudohydrothermal"
conditions during the postdepositional history of theconglomerates. With uraninite and rutile present in the
Witwatersrand reefs in large quantities, the same reac­tion should have occurred there also, probably enhancedby the low-grade (greenschist-facies) metamorphism towhich the Witwatersrand region was subjected.
Besides, it is well known that several thermal events,
Ventersdorp volcanism, intrusion of the Bushveld Ig­
neous Complex, and emplacement of Karroo dolerites,
have left their imprint on the region as a whole and were
responsible for large-scale reconstitution of the detrital
sulfides with the formation in places of authigenic
mackinawite (Schidlowski and Ottemann, 1966).
However, as the Blind River province is virtuallyunmetamorphosed (Roscoe, 1969), the "Pronto reaction"
can be expected to proceed readily of its own accord in"normal" diagenetic environments. It has been shownthat brannerite can be synthesized easily from U02 andTi02 in the laboratory (Patchett and Nuffield, 1960).
With partial mobilization in the banket of bothuranium and titanium, obvious replacement phenomena,
as manifest in the formation of "uraninite ghosts," canbe readily accounted for. In "ghosts" U02 was largely
stationary and titania was the mobile phase, whereas
replacement of rutile by brannerite would require
mobilization of uraninite constituents. The ubiquitous
small brannerite specks interspersed in the con­glomerate matrix have certainly formed there in situ asa result of a random encounter of uranium- andtitanium-bearing mineralizing fluids. Details of the proc­
ess of brannerite formation in the reef have been
discussed elsewhere by the writer (Schidlowski, 1966a).

From the microprobe scanning diagram for lead (fig.
11) of coexisting uraninite and brannerite it is obviousthat the brannerite, because of its sparse lead content, isconsiderably younger than the detrital U02 constituentof the reef, which conclusion is consistent with theassumed mode of brannerite formation. Possible conse­
quences of this type of radiogenic lead distribution for
the interpretation of the uranium-lead ages hitherto pro­
posed for the Witwatersrand have been pointed out
elsewhere (Ramdohr and others, 1965).
In discussing the origin of brannerite it should be
noted that rutile, as a necessary prerequisite for the"Pronto reaction," is fairly widespread in the Witwaters­
rand conglomerates (Ramdohr, 1955; Schidlowski,
1970a, p. 48 ff). Alteration of detrital ilmenite constit­
uents to rutile, as microscopically evident in many ex­cellent samples, should also have contributed extensive­
ly to the Ti02 budget of the banket. According to Ferris
and Ruud (1971), the brannerite of the Blind River prov­
ince has principally formed from detrital ilmenite thatwas diagenetically altered to rutile and anatase in thepresence of dissolved uranium.
CARBONACEOUS MATTERMICROSCOPIC OBSERVATIONS AND THEIR GENETICSIGNIFICANCE

The widespread occurrence of "carbon" constituents inthe Witwatersrand conglomerates was well known by
early workers (Young, 1917) and has received increased
attention since the discovery of the pronounced radioac­
tivity of the material. The application of reflected lightmicroscopy to the study of "thucholite"3 (Davidson andBowie, 1951; Ramdohr, 1955; Liebenberg, 1955;
Schidlowski, 1966a, 1966b) has shown it to be aheterogeneous mixture of "carbon" and uraninite withvarying amounts of practically all other minerals con­tained in the neighboring reef.
For a genetic interpretation of this peculiar carbonuraninite association, ore-microscopic observations haveturned out to be of prime importance. The initial stage inthe formation of such aggregates is represented byuraninite particles transected by stringers, and coatedby layers, of carbonaceous matter (fig. 2A, C; fig. 12A,
B.). Progressive replacement by carbon of the primary
U02 phase as a result of gradual enlargement of both thecarbon aureoles and the crack fillings is documented infigure 12C and D, in which the muffin shapes of theoriginal detrital grains are still easily recognizable. In an
advanced stage of replacement (fig. 13) the uraninite has
almost wholly disappeared except for small angular
3 The term "thucholite," coined from the chemical symbols of the main constituent elements(Th, U, C, H, and O) was originally introduced by Ellsworth (1928) for uraninite-bearing car­bonaceous materials. In the Witwatersrand literature it is larsHv used as a synonum for car­bonaceous matter.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY

Nil

In a remarkable consensus, the principal orerelict bodies floating in the groundmass of nodular car­bon aggregates. Specks of visible carbon thus originated microscopic investigators have explained these finding
have come to be known as "fly speck" carbon. Typical of in terms of an encounter of a pre-existing uraninite
the replacement process is a "drifting apart" of the phase and mobile, preferably gaseous, hydrocarbons.
resulting uraninite relicts, which, however, still fit each These hydrocarbons, while migrating through thestrata, were obviously polymerized by ionizing radiation
other excellently, like pieces of a puzzle (fig. 13D-G).

FIGURE 12.—Grains of uraninite coated and veined (A, B) or partially replaced (C, D) by carbonaceous matter. Because of its reflection
pleochroism the organic material shows up in different shades of gray. In the initial stages of replacement the pebble shape of the
primary uraninite grains is still excellently preserved (C, D). Remnants of uraninite floating in a "carbon" groundmass may bereadily recognized by their dustlike inclusions of radiogenic galena (white). Note loosely knit aureole of authigenic pyrite around
one of the major aggregates (D). Examples from various Orange Free State reefs. Oil immersion.

N12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 13. -Advanced stages of uraninite replacement by carbonaceous matter. The primary uraninite phase has largely disappeared except forsmall angular relict bodies. It is reasonable to assume that carbon nodules such as depicted in A derive from the replacement of single uraninite
grains. Replacement of chains of U02 particles would result in aggregates such as shown in B and C. Note characteristic "drifting apart" of in­dividual uraninite particles during progressive replacement (D-G) with galena (white) locally accumulating in the interstices. An enlargementof a particle (D) in the center of (C) is shown. Element scanning for uranium carried out on samples shown in F and G shows that the "carbon"
groundmass is virtually devoid of uranium. The uranium content of the material is thus confined to the interspersed uraninite particles. Ex­
amples taken from various Orange Free State conglomerates. All photomicrographs taken in oil immersion.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY

N13

emanating from uranium-bearing constituents of the ing of the chemical background of the replacement proc­
reef. With the process of polymerization entailing a ess, a final assessment of these matters must be post­transition into the solid state, the mobile organic poned.
Another mechanism for bringing about a uran­substances were fixed in close proximity to the sources
ium
carbon association that is not less importantof radiation, thus accounting for the carbonaceous
geochemicallythan the one described should be men­aureoles and crack fillings of many U02 particles. The
characteristic "drifting apart" of the uraninite tioned here. It is well known that hexavalent uranium offragments during replacement (fig. 13) may be conven­ the soluble uranyl complex [U6+ 02]2+ , which is often car­iently explained by a continuous supply of mobile ried by weathering solutions, may be precipitated asorganic material to the interface between the hydrocar­ U4+ 02 in the reducing environment of sedimentarybons already solidified and the remaining pieces of organic matter (McKelvey and others, 1955, p. 472).
This reaction, for the most part, is responsible for theuraninite.
This concept for the origin of the Witwatersrand conspicuous uranium concentrations in some coals,
thucholite is plausible because processes of radiolytic lignites, peats, and the like, which have been known forpolymerization of simple, mostly unsaturated, hydrocar­ a long time (Berthoud, 1875; Breger and others, 1955).
bons by a and 7-radiation have been simulated ex­ The economically important uranium province of theperimentally by various workers (Lind and Bardwell, Colorado Plateau (Rosenzweig and others, 1954;
1926; Lind, 1928; Colombo and others, 1964). Radiation Gruner, 1955) was formed mainly in this way. In thiseffects on the carbonaceous material can also be seen as area, fossil wood is commonly replaced by pitchblende,
haloes around many uraninite inclusions (figs. I3E, the low-temperature, Th02-poor cryptocrystalline varie­
ISA). More difficult to explain is the mechanism of ty of U02. In the Colorado Plateau, organic materials
uraninite replacement by carbon constituents as is seen concentrate uranium by providing a change in redox
under the microscope. Microprobe scanning pictures potential favorable for precipitation of the uranyl com­
(figs. I3F, G, 14) show the carbonaceous material to be plex carried in water, whereas in the Witwatersrandvirtually devoid of uranium. Accordingly, the replaced deposits migrating hydrocarbons are concentrated
U02 constituents are obviously not resorbed or around preexisting uranium-bearing minerals as a"digested" by the organic materials in some form such as result of radiolytic polymerization. Although both proc­
organo-metallic compounds. This does not mean, esses produce a close association of uranium and carbon
however, that such compounds may not have been in­ compounds, they achieve the association by completely
strumental in a transitory phase in the breakdown and different geochemical pathways.
Attempts to invoke a Colorado-type pedigree for themobilization of uraninite. Pending a better understand­U
FIGURE 14.—Microprobe scannings for uranium and thorium of the carbon-uraninite aggregate depicted infigure I2D. It is evident that the distribution pattern of uranium almost coincides with the areas oc­
cupied by uraninite. Note the increased thorium content of the uraninite particles in upper right corner.

N14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

uraninite constituents of the Rand conglomerates(Sharpe, 1949; Snyman, 1969; Reimer, 1975), in part
combined with the concept of subsequent reworking by
wave action of the U02-precipitates (Koen, 1961), may
safely be dismissed because of the high Th02 content in
Rand uraninite, which is not present in low-temperatureuraninite. Besides, it is well known that a large part of
the uraninite fraction of the banket is not associatedwith carbon at all. A crucial prerequisite for the forma­tion of Colorado-type deposits is, furthermore, an oxygenic weathering cycle to oxidize tetravalent uranium of
the primary deposits to the soluble hexavalent form,
which, in turn, is susceptible to subsequent precipitation
by reducing agents. With all evidence presently
available strongly favoring anoxygenic weathering on
the ancient continents prior to some 2xl09 years B.P.
(before present), this is a very unrealistic premise that,in the opinion of this writer, does not stand up to criticalexamination.
CHARACTERISTICS OF MAJOR CARBON ACCUMULATIONSWith the uraninite fraction mostly concentrated at thereef footwalls as part of the heavy suite, the bulk of theorganic materials were necessarily trapped at the very
base of the conglomerate horizons. When the uraninite
grains happened to lie close to each other, there would
result chainlike aggregates of carbon, as depicted infigure 13B and C. In this way, continuous bands of car­bonaceous matter may appear at the footwalls of reefs(fig. 15). As already noted by previous students, thesecarbon bands may vary in thickness from fractions of amillimeter to some centimeters, giving rise to a special
type of "carbon reef," the prototype of which isrepresented by the economically important "Carbon
Leader" of the West Rand. The carbonaceous materialof such bands is sometimes massive, but often displays acharacteristic fibrous texture perpendicular to the bed­ding, as noted by Liebenberg (1955, pi. 28). Figure 16
gives a schematic picture of a typical carbon reef with aseam of fibrous (or columnar) thucholite at its base.
Microscopic inspection of such aggregates(Schidlowski, 1966b) reveals that the fibrous carbon in
many places is heavily mineralized, as might be expected
inasmuch as it occupies those sites of the conglomerates
where the bulk of the economic minerals have been ac­
cumulated. The bands are particularly rich inredeposited gold (fig. 15); locally it preserves as casts theorthogonal pattern formed by the intracolumnar in­
terstices, and elsewhere it fills wedgelike cracks tran­
secting the individual fibres perpendicular to their longaxes (fig. 17A-C). With the exception of fragments ofthe primary U02 particles, which are spread throughout
the carbon matrix, all allogenic components are prac­
tically confined to the interstices between the fibres.

Rounded grains of pyrite, quartz, iridosmine, and otherminerals are engulfed by strands of carbon, and thisfeature accounts for a typical augen texture, with gold
and reconstituted sulfides accumulated in the pressure
shadows on either side of detrital minerals (fig. 17D, E).
Excellent flow lamination in carbonaceous material has
formed around inclusions of authigenic sphalerite par­ticles, with the latter often dragged apart("boudinaged") and elongated fragments tending to ar­range themselves parallel to the laminae (fig. I7F). The
marked plasticity of the carbonaceous material is also
demonstrated in figure 18C.
A section of fibrous carbon cut in a skew angle to thefibre direction is presented in figure 1&4, the ends of theindividual fibres showing up as lenticles often centeredby small uraninite inclusions. These inclusions havegiven rise to radioactive haloes within the surrounding
carbonaceous matter in which the reflectivity is marked­
ly increased. With crossed nicols (fig. 185), a certain
pregraphitization of the material is obvious, although it
has been demonstrated by various investigators that the
carbonaceous material is basically amorphous (Davidson
and Bowie, 1951; Liebenberg, 1955). Actually, graphite
is an extremely rare constituent of the banket (fig. 18D).
From the fabric of these fibrous aggregates it is evi­
dent that the carbonaceous matter in its present form isclearly younger than most minerals of the detrital suite.
On the other hand, there can be no doubt that the bulk of
the gold that also formed part of the original heavy frac­tion of the conglomerates has undergone large-scalereconstitution at a relatively late stage, being texturally
fully integrated in the kinematic pattern displayed by
the carbon aggregates.
ORIGIN OF COLUMNAR THUCHOLITE

In an attempt to account for columnar thucholite, thewriter (Schidlowski, 1966b), has drawn analogies to themode of origin of fibrous mineral aggregates grown atright angles to the bedding planes within intrastratal
partings. Such phenomena are well known fromminerals like gypsum, rock salt, calcite and manyothers. According to Miigge (1928), these aggregates
may be visualized as having resulted from the mutual
lateral impediment of minerals growing in the limited
space of a rock fissure, with crystal growth ultimately
possible only in the direction of the parting walls of thecrevice. Although applicable only with certain modifica­
tions to the Witwatersrand carbon, this model is likely
to give a satisfactory explanation for the occurrence ofthe columnar thucholite bands, which preferentiallycame to occupy so-called "footwall partings" at the baseof individual conglomerate horizons.
Recently, however, Hallbauer and van Warmelo(1974) have offered an alternative explanation, relating

FIGURE 15. -Typical "carbon" footwall of a gold- and uranium-bearing conglomerate horizon, showing distribution of minerals (Basal Reef, from Loraine Gold Mines, Orange
Free State). A, The carbonaceous material, gray displaying faint indications of a fibrous texture perpendicular to the bedding; at this low magnification (x 120), the angular bodies
of uraninite sprinkled through the carbon matrix cannot be recognized. Oil immersion. B, Explanatory sketch shows distribution of principal minerals: heavy lined, carbonaceous
matter; black, linnaeite; dense stipples, pyrite; wide stipples, quartz; white with scratches, gold (Au); strongly outlined, chromite (Cr).

B.

Q

i

N16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
Hanging-wall quartzite

Conglomerate
Fibrous "carbon"
Footwall quartzite

5 CENTIMETERS

FIGURE 16. -Schematic block diagram of a "carbon reef characterized
by a seam of fibrous thucholite at the base of a conglomerate layer.
The carbon fibres are approximately perpendicular to the bedding
planes. Note that the pebbles are relatively loosely packed within thequartzite matrix, as is often typical of this particular reef type.

these aggregates to colonies of a putative symbiotic
community assumed to consist of an algal partner and afungus. A reconstruction of Hallbauer (1975) of thisplant (fig. 19) does indeed suggest affinities to modernlichens. According to these authors, the carbon fibres
represent the fossil remains of the columnar isidia of alichenlike symbiont believed to have carpeted the floorof the primary depositional basin as extended colonies
and to have trapped gold and other detritals by a sort ofcorduroy-table action.
Contrary to the strong points raised by Hallbauer(1975) in support of this hypothesis, the student of the
Precambrian familiar with both the topic and the evolu­
tionary history of ancient life will have certain reserva­
tions as to the concept as a whole. These reservations
may be summarized as follows:
1. The fact that this putative symbiont appears justonce in the Earth's history must necessarily raisesuspicion, since lichens could be expected to standfor a rather "conservative" type of organism, whichshould have appeared repeatedly in a similar mor­
phological configuration in the paleontologicalrecord. This contrasts markedly with some organosedimentary structures attributable to algae, whichhave been found from almost the start of thesedimentary record until recent time.
2. Both the individual carbon fibres and their ag­
gregates do not display a morphological differentia­
tion suggestive of biological affinities when sub­jected to reflected-light microscopy (figs. 17,18). In
particular, the rheologic fabric of the fibrous seams(fig. YID-F) almost excludes the possibility that thefibres are structural remains of ancient plants.
Ultramicroscopic evidence claimed to favor abiologic nature of the columnar carbon appears tobe either inconclusive or largely overinterpreted.
There is, furthermore, no compelling evidence as
yet that the organic materials were primarily
responsible for the economic mineralization of thebanket. Notably, the textural relation between gold
and carbon as presently observed (fig. 17) is in­dicative of a diagenetic and (or) metamorphic
redistribution of the gold fraction within the carbon
bands but definitely not of a primary precipitationof the metal by biologic agents. In general, bothgold and uraninite are always found at the reef
footwalls, irrespective of the presence or absenceof carbon within a particular conglomerate horizon.
3. The putatively biogenic structures are preserved as
massive coaly matter. If these fibres really repre­
sent structurally preserved remains of a lichenlike
symbiont, the carbon content of the primary planttissue was certainly smaller by at least one or twoorders of magnitude than the carbon content of thepresent-day coaly layer. An original lichen carpet a
few centimeters thick undergoing coalification
would have been reduced to a carbon seam of
millimeter scale, with any fibrous texture largely
obliterated during the process of compaction. The
strongly negative 613C values of the carbonaceous
material, -30 permil and less relative to the Na­tional Bureau of Standards (NBS) isotope
reference Sample No. 20 (Schidlowski, 1968),
would furthermore suggest that the carbon ofthese layers derived, at least in part, from mobiliz­
ed fractions of sedimentary organic material inwhich the light isotope, 12C, had been increased as aresult of a "chromatographic" effect. Sedimentaryorganic materials normally display 613C values
close to - 25 permil (Degens, 1969; Eichmann and
Schidlowski, 1975). Hence the carbon compounds
accumulated in these bands are unlikely to repre­
sent isotopically undifferentiated, primary organic
matter.
4. The occurrence of relatively advanced organisms
such as lichens in the Witwatersrand System,

FIGURE 17.-Photomicrographs showing fibrous thucholite with characteristic mineral inclusions. A, Aggregate of columnar carbon; the in­dividual columns are clearly separated by intracolumnar interstices and crosscut by wedgelike drag fissures. With the exception of numerous
specks of galena within the second-from-left fibre, the aggregate is almost devoid of economic materials. B, C, Intracolumnar interstices filledwith gold (white, scratched) and chalcopyrite (light gray); the gold also occupies wedgelike drag fissures perpendicular to the fibrous texture.
D, E, Detrital pyrite grains in intracolumnar fissures; strands of carbon have flowed around the pyrite grains; gold (white, scratched)redeposited in the pressure shadows at each side of the pyrite inclusions. F, Sphalerite dragged apart ("boudinaged") in intracolumnar fissure.
Note flow lamination within thucholite, with an elongated sphalerite fragment in the upper right corner tending to arrange itself parallel tothe laminae. All examples from B Reef, Loraine Gold Mines, Orange Free State. Oil immersion.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

N17

N18

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES' h ;'VV

FIGURE 18.—Photomicrographs showing examples from various Orange Free State conglomerates. A, Fibrous carbon aggregate
cut at an angle to main fibre direction; the outlines of the fibres show as lenticles on the surface of the polished section.
Numerous relics of uraninite give rise to radioactive haloes within which the reflectivity of the carbonaceous material is marked­
ly increased. B, With crossed nicols, a certain "pregraphitization" of the material is optically evident. C, Demonstration of ex­treme mobility of the carbonaceous matter; taking advantage of a pre-existing cleavage fissure, the carbon has wedged apart a
detrital pyrite grain. D, Assemblage of scaly graphite in a quartz matrix. Oil immersion.

URAMFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

N19

SOURCE OF THE CARBON CONSTITUENTS

It is generally agreed by most students of the Rand
that the mobile precursors of the present hydrocarbons
were, in all likelihood, indigenous to the sequence, stem­
ming from the decomposition of contemporaneousorganic material buried in the sediments. A derivationfrom locally overlying coal measures of the Karroo
System, as envisaged by Davidson and Bowie (1951),
may be ruled out primarily because of the buoyancy of
migrating hydrocarbons. With the underlying basement
rocks constituting a very unlikely source, there is no
reason to seriously dispute the indigenous character ofthe carbonaceous substances.
Consistent with the assumed biogenicity of thematerial is its isotopic composition (Hoefs andSchidlowski, 1967; Hoering, 1967), the strongly
FIGURE 19.—Reconstruction of the lichenlike symbiont putatively negative 613C values with a minimum at -37.1 permil
responsible for the fibrous carbon aggregates at the base of reefs relative to the NBS standard suggesting genetic affinities(Hallbauer, 1975, fig. 35; published with permission of the National to oil and asphalt rather than to "normal" kerogens. The
Institute for Metallurgy, Minerals Science and Engineering). The
present carbon fibres are believed to represent the fossilized colum­ 6-values of carbon from Blind River fall within the samerange (Eichmann and Schidlowski, 1974). The reported
nar isidia of the original plant.
presence of a suite of amino acids (Prashnowsky and
Schidlowski, 1967) requires confirmation by moresophisticated techniques allowing a differentiation bet­
about 2.5 x 109 years B.P., is incompatible with the ween indigenous substances and possible contaminants.
currently established timetable of evolutionary A biogenic derivation of the carbon is also supported
diversification of ancient life. Although the timing by the occurrence within Witwatersrand rocks ofof the advent of eucaryotic organization is present­ microstructures suggestive of biologic affinities, as
ly under dispute (Knoll and Barghoorn, 1975), previously noted (Schidlowski, 1965, 1970b; Oberlies
there is general agreement that this quantum step and Prashnowsky, 1968; Pflug and others, 1969;
in evolutionary history cannot have substantially Hallbauer and van Warmelo, 1974). The most convinc­antedated the benchmark of some 1.5xl09 years ing evidence for biologic structures are myceliumlike
B.P. (Schopf, 1974). It might, of course, be argued filamentous aggregates described by Hallbauer and vanthat this benchmark is not yet the definite one, but, Warmelo from ashed residues of carbonaceous matter.
for a variety of reasons, a major shift towards an With such aggregates explained by their discoverers as
earlier age is unlikely to be expected. It seems fossil fungi (that is, eucaryotes) and their occurrence
justifiable, therefore, to state at this stage that the within the rock not yet confirmed by petrographic slides,
appearance some 2.5x 109 years ago of a symbiotic there were necessarily strong doubts as to the in­association involving a fungal (that is, eucaryotic) digenous character of these structures. However, accor­
partner is as improbable as the discovery of a ding to E. S. Barghoorn (personal commun., 1975), thedinosaur in the early Paleozoic.
indigenous character of these aggregates cannot be
In conclusion, the interpretation of the fibrous doubted, but their interpretation as fungi is incorrect. If
thucholite aggregates as fossilized plants must be rated a tentative interpretation of these remains as fossil
as almost certainly incorrect and, in the opinion of this bacteria (Actinomycetes) turns out to be correct, thesewriter, may safely be discarded. The same applies to forms should contribute substantially to the stock of ex­
metallogenic concepts based on a large-scale participa­ cellently preserved microfossils hitherto described fromtion of living organisms in the process of concentration Precambrian rocks.
of the principal pay metals, notably gold and uranium, inthe banket. Unless they are substantiated by clear-cut PROBLEMS OF URANIUM MINERALIZATION IN
PRECAMBRIAN CONGLOMERATES
paleontological or biogeochemical evidence, restraint
should be practiced in advocating such hypotheses, as
the economic mineralization of the Witwatersrand mayWith our present knowledge of the regional geology of
be reasonably explained in terms of "conventional" proc­ the Witwatersrand ore province (Brock and others,
esses operative in the formation of placer deposits.
1957; Brock and Pretorius, 1964) and with crucial

N20

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

sedimentologic and geochemical evidence having piled
up in support of a detrital origin of its economic con­glomerates, the hydrothermal genetic concept (Davidson, 1957, 1965) for uraninite in the banket may now besafely dismissed. Nevertheless, the occurrence ofuraninite constituents in a detrital assemblage raises
some questions, which are briefly addressed in thefollowing discussion.
It is well known that uraninite is thermodynamicallyunstable in the present weathering cycle (Davidson and
Cosgrove, 1955; Zeschke, 1959a; Dybek, 1962).
Tetravalent uranium in uraninite is readily oxidized tothe hexavalent state in oxygenic environments, thereby
giving rise to the soluble uranyl complex [U6+ 02]2 + ,which is subsequently carried away by weathering solu­
tions. Accordingly, uraninite is practically nonexistentin modern placers, the few noteworthy exceptions
described (Steacy, 1953; Zeschke, 1959b; Trushkova andKukharenko, 1961; Lang and others, 1962; Darnley,1962) being quantitatively unimportant and mostly
restricted to cold climates, where chemical weathering
is retarded as a result of low mean temperatures.
Moreover, with the exception of extremely old forma­tions such as the Dominion Reef, the Witwatersrand
reefs, and the conglomerates at Blind River and Serrade Jacobina, fossil uraninite placers are practicallyunknown throughout the sedimentary record. The oc­
currence, therefore, of uraninite in economic grades inlower and middle Precambrian detrital rocks mustnecessarily appear enigmatic.
Various efforts have been made to reconcile thepresence of detrital uraninite in these ancient forma­tions with the chemical instability of the mineral in theexogenous cycle. Ramdohr (1958a, b) conjectured that
the U02 constituents of the reef were still geologically
young and relatively refractory when subjected to thestress of weathering and transport, because their lat­tices had not yet been weakened by endogenic
a-bombardment. Wiebols (1955), remarking on the local
occurrence of tillitelike horizons within the Witwaters­
rand System, invoked the effects of a cold climate for thesurvival of the uraninite fraction. Several investigators
have, on the other hand, entertained the view that thepersistence of uraninite as a detrital mineral might in­
dicate an extremely low redox potential of the ancientenvironment—that is, an anoxy genie continental
weathering cycle (Louw, 1954; Liebenberg, 1955; Ram­dohr, 1958b; Schidlowski, 1966a).
The existence of a basically anoxygenic atmosphere
during the time of deposition of Precambrian uranium
conglomerates, T ^ 2.0 xlO9 years B.P. (fig. 20), islargely consistent with current concepts of atmosphericevolution (Berkner and Marshall, 1967; Cloud, 1968;
Schidlowski and others, 1975; Junge and others, 1975;

Schopf, this volume, chap. B). Although uraninite isthermodynamically stable only at a P02 ~ 10~21 atm,
Grandstaff (this volume, chap. C) has concluded that, atpartial pressures of oxygen between 10~2 and 10~6 of thepresent level, the oxidation rate may be sufficiently slow
for uraninite to survive under the set of environmental
and physiographic conditions that most probably
governed the deposition of the Witwatersrand rocks andrelated uranium-bearing conglomerates. The ubiquitouspresence of detrital pyrite in the reef could be likewise
ascribed to low environmental oxygen pressures, since
sulfides are also rapidly destroyed by oxidation.
Oxygen concentrations of the above range, particular­
ly those close to the so-called "Urey level" of 10~3 of thepresent partial pressure of oxygen, have long been
regarded as typical for the geological time span prior to2 x 109 years ago. The concept of an ancient anoxygenic
weathering cycle would therefore provide a general ex­
planation for the worldwide occurrence of pyriteuraninite placers during the early history of the Earthand is also consistent with an impressive array of otherrelevant evidence (fig. 20, lower part). At this stage, thepostulate of a substantially lower redox potential in theancient environment is probably the most reasonable
way to account for the origin of these deposits.
It might be argued, of course, that during processes of
rapid sedimentation any debris will stand a fair chanceof survival, irrespective of its thermodynamic proper­
ties. Hence, an ultimate acceptance of the "placerist"
concept for the uranium mineralization of Precambrian
conglomerates would not necessarily imply that the oc­
currence of uraninite among the detrital minerals is ac­
cepted as conclusive evidence of an anoxygenic con­tinental weathering cycle. Such reservations apparently
would be supported by the fact that the Witwatersrand
System exhibits, at least in part, obvious molasse
features, intercalated graywackes, unsorted boulder
beds, and the like, indicative of rapid burial ofweathered debris from the surrounding borderlands.
However, in the opinion of this writer, thesearguments may be severely weakened by the following
considerations: With the age of all known uraniumbearing conglomerates more than 2 x 109 years and themass half-age (life) for sedimentary rocks lying in therange of 500 to 600 x 106 years (Garrels and MacKenzie,
1971, p. 270), only between 6 and 12 percent of the total
sedimentary column deposited 2xl09 years ago can be
expected to have escaped subsequent destruction. For
sediments deposited during Witwatersrand time,
2.5x 109 years ago, the remaining proportion would ap­
proach 3 percent as a minimum. Within these scanty
relics of an originally huge sedimentary column, detritaluraninite is apparently well represented on a worldwide
scale, whereas all younger sediments, whose chances of

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY

N21

02 bound in Fe2 03(-39%)(02 )t =0.53eM tp (1-e^)= 1.16x10rV)

0 2 bound in(-56%)
Rise of 02 level inatmosphere-ocean system

0.2

02 in atmosphereocean system
(-5%)

\
t=0
4.5
^_...

1.0
4.0
i

i

Milestones of organic
evolution

2.0
3.0
11
A
(D

3.0
2.0
i

Occurrence of conti-

^
(D

i

4.0
1.0
——

A

/

4.5x109 y
COx10 9 y

AA——

^^^^^

nental 'red beds'
fVriirrpnrp

nf FP.^-^^^^^^^^^^

uu 2 piacers
oanaea ironstonest

START OF SEDIMENTARY

RECORD

FIGURE 20. -Evolution of the Earth's total budget of photosynthetic oxygen, based on the terrestrial 13C balance and current concepts for theaccumulation of the stationary sedimentary mass as a function of time (Schidlowski and others, 1975, fig. 14; published with permission of
Elsevier Scientific Publishing Co.). Stationary oxygen reservoirs existing at times t are expressed as fractions of the present reservoir (02)tp.
M^ is the present sedimentary mass with tp = 4.5 x 109 years. T is time before present in years. Column on right side shows partitioning of ox­ygen between the "bound" and "free" reservoirs in the present budget (Li, 1972), with molecular oxygen accounting for about 5 percent only.
Note disproportionation between bound and free reservoirs during the Precambrian and early Paleozoic, most probably due to a short circuitof the ancient oxygen cycle in the Precambrian seas. Tentative rise of free oxygen level in atmosphere-ocean system (black field) is inferredfrom paleontological and geological evidence shown. Milestones of organic evolution indicated are (1) appearance of oldest algal bioherms(photoautotrophic blue-green algae), (2) appearance of eucaryotic biota, (3) appearance of oldest eumetazoan faunas, (4) life conquering con­tinents (Upper Silurian); (5) appearance of exuberant continental floras (Upper Carboniferous).

preservation grow exponentially with decreasing
geological age, definitely lack these deposits. Because
the Phanerozoic record, which accounts for roughly 50
percent of the total sedimentary mass, has been exten­
sively examined by generations of field geologists andbecause prospecting for uranium has been carried on ata large scale for decades, it seems justified to conclude
that uraninite placers did not form during the later
history of the Earth. Likewise, pyrite conglomerates
have not been reported from younger geological forma­tions, whereas the association sulfide-uraninite ischaracteristic of Witwatersrand-type detrital golduranium deposits.

It is hard to conceive that geological environments
favorable for the formation of pyrite-uraninite placers
should not have existed repeatedly in the geologicalpast; notably, rapid burial of weathered debris has oc­
curred during numerous sedimentary cycles, as il­lustrated by the presence of numerous graywackes andother poorly sorted sediments in younger geological for­mations. Accordingly, it is very difficult to account forthe complete absence of pyrite-uraninite deposits fromyounger sediments without invoking changes in theredox potential of the environment. Confinement ofpyrite-uraninite placers to the early and middle Precam­
brian would certainly be too much of a coincidence if theN22

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESDu Toit, A. L., 1954, The Geology of South Africa: London, Oliver andBoyd, 463 p.
Dybeck, J., 1962, Zur Geochemie und Lagerstattenkunde des Urans:
Clausthaler Hefte zur Lagerstattenkunde etc., no. 1, Berlin, BornACKNOWLEDGMENTS
trager, 163 p.
Eichmann, R., and Schidlowski, M., 1974, Isotopic composition of car­Research leading to the preparation of this paper has
bonaceous matter from the Precambrian uranium deposits of the
been supported in various stages by the Deutsche
Blind River District, Canada: Naturwissenschaften, v. 61, p. 449.
-1975, Isotopic fractionation between coexisting organic carbonForschungsgemeinschaft (lately as part of SFB 73).
carbonate pairs in Precambrian sediments: Geochimica etMoreover, thanks are due to Paul Ramdohr and J. W.
Cosmochimica Acta, v. 39, p. 585-595.
Schopf for reading a preliminary draft of this paper or Ellsworth,
H. V., 1928, Thucholite, a remarkable primary carbon
parts thereof.
mineral from the vicinity of Parry Sound, Ontario: American
Mineralogist, v. 13, p. 419-439.
Feather, C. E., and Koen, G. M., 1975, The mineralogy of the
REFERENCES CITED
Witwatersrand reefs: Minerals Science and Engineering, v. 7, p.
Berkner, L. V., and Marshall, L. C., 1967, The rise of oxygen in the
189-224.
Earth's atmosphere, with notes on the Martian atmosphere: Ad­ Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: its occurrences andvances in Geophysics, v. 12, p. 309-331.
recognition by microprobe: Colorado School of Mines Quarterly, v.
Berthoud, E. L., 1875, On the occurrence of uranium, silver, iron, etc.,
66, no. 4, 35 p.
in the Tertiary formations of Colorado Territory: Academy of Na­ Frondel, Clifford, 1958 [1959], Systematic mineralogy of uranium andtional Sciences of Philadelphia Proceedings, v. 27, p. 363-365.
thorium: U.S. Geological Survey Bulletin 1064, 400 p.
Breger, I. A., Deul, M., and Rubenstein, S., 1955, Geochemistry and Garrels, R. M., and MacKenzie, F. Y., 1971, Evolution of sedimentarymineralogy of a uraniferous lignite: Economic Geology, v. 50, p.
rocks: New York, Norton, 397 p.
206-226.
Grandstaff, D. E., 1974, Microprobe analyses of uranium and thorium
Brock, B. B., Nel, L. T., and Visser, D. J. L., 1957, The geological back­in uraninite from the Witwatersrand, South Africa, and Blindground of the uranium industry, in Uranium in South Africa
River, Ontario, Canada: Geological Society of South Africa Trans­
1946-1956: Associated Scientific and Technical Societies of Southactions, v. 77, p. 291-294.
Africa, v. 1, p. 273-305.
Gruner, J. W., 1955, Concentration of uranium by carbon compounds:
Brock B. B., and Pretorius, D. A., 1964, Rand basin sedimentation andEconomic Geology, v. 50, p. 542-543.
tectonics, in Haughton, S. H., ed., The geology of some ore Hallbauer, D. K., 1975, The plant origin of the Witwatersranddeposits in southern Africa: Geological Society of South Africa, v.
"carbon": Minerals Science and Engineering, v. 7, p. 111-131.
1, p. 549-599.
Hallbauer, D. K., and van Warmelo, K. T., 1974, Fossilized plants inCloud, P. E., 1968, Atmospheric and hydrospheric evolution on thethucholite from Precambrian rocks of the Witwatersrand, Southprimitive Earth; Science, v. 160, p. 729-736.
Africa: Precambrian Research, v. 1, p. 199-212.
Colombo, U., Denti, E., and Sironi, G., 1964, A geochemical Hiemstra, S. A., 1968, The mineralogy and petrology of theinvestigation upon the effect of ionizing radiation on hydrocarbons:
uraniferous conglomerates of the Dominion Reefs mine,
Institute of Petroleum Journal, v. 50, p. 228-237.
Klerksdorp area: Geological Society of South Africa Transactions,
Cooper, R. A., 1923, Mineral constituents of Rand concentrates:
71, p. 1-65.
Chemical, Metallurgical, and Mining Society of South Africa Jour­ Hoefs, J., and Schidlowski, M., 1967, Carbon isotope composition ofnal, v. 24, p. 90-95.
carbonaceous matter from the Precambrian of the WitwatersrandCorrens, C. W., 1960, Die Sedimentgesteine, in Earth, T. W., Correns,
System: Science, v. 155, p. 1096-1097.
C. W., and Eskola, P., eds., Die Entstehung der Gesteine: Berlin, Hoering, T. C., 1967, The organic geochemistry of Precambrian rocks,
Springer, 422 p.
in Abelson, P, H., ed., Researches in Geochemistry: New York,
Darnley, A. G., 1962, Detrital uraninite from the Hunza River,Wiley, v. 2, p. 87-111.
Kashmir: Geological Survey of Great Britain, Atomic Energy Divi­ Junge, C. E., Schidlowski, M., Eichmann, R., and Pietrek, H., 1975,
sion, Age Determination Report 22, 4 p.
Model calculations for the terrestrial carbon cycle: Carbon isotope
Davidson, C. F., 1953, The gold-uranium ores of the Witwatersrand:geochemistry and evolution of photosynthetic oxygen: Journal of
Mining Magizine (London), v. 88, p. 73-85.
Geophysical Research, v. 80, p. 4542-4552.
———1955, Discussion on paper by W. R. Liebenberg: Geological Knoll, A., and Barghoorn, E. S., 1975, Precambrian eukaryotic organ­
Society of South Africa Transactions, v. 58, p. 232-236.
isms: A reassessment of the evidence: Science, v. 190, p. 52-54.
———1957, On the occurrence of uranium in ancient conglomerates: Koen, G. M., 1961, The genetic significance of the size distribution of
Economic Geology, v. 52, p. 668-693.
uraninite in Witwatersrand bankets: Geological Society of South-1965, The mode of origin of banket orebodies: Institute of Min­
Africa Transactions, v. 64, p. 23-46.
ing and Metallurgy Bulletin no. 700 (Transactions, v. 74), p. Lang, A. H., Griffith, J. W., and Steacy, H. R., 1962, Canadian deposits319-338.
of uranium and thorium: Canada Geological Survey Economic
Davidson, C. F., and Bowie, S. H. U., 1951, On thucholite and related
Geology Series no. 16, 90 p.
hydrocarbon-uraninite complexes, with a note on the origin of the Li, Y. H., 1972, Geochemical mass balance among lithosphere, hydro­
Witwatersrand gold ores: Great Britain Geological Survey Bulletin
sphere, and atmosphere: American Journal of Science, v. 272, p.
no. 3, p. 1-18.
119-137.
Davidson, C. F., and Cosgrove, M. E., 1955, On the impersistence of Liebenberg, W. R., 1955, The occurrence and origin of gold and radio­
active minerals in the Witwatersrand System, the Dominion Reef,
uraninite as a detrital mineral: Great Britain Geological Survey
the Ventersdorp Contact Reef, and the Black Reef: Geological
Bulletin no. 10, p. 74-80.
Society of South Africa Transactions, v. 58, p. 101-227.
Degens, E. T., 1969, Biogeochemistry of stable carbon isotopes, in
Eglinton, G., and Murphy, M. T. J., eds., Organic geochemistry: Lind, S. C., 1928, The chemical effects of alpha-particles and electrons:
New York, Reinhold, 130 p.
Berlin, Springer, p. 304-329.

formation of such deposits were governed exclusively bygeological factors.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY
Lind, S. C., and Bardwell, D. C., 1926, The chemical action of gaseousions produced by alpha-particles: IX, Saturated hydrocarbons:
American Chemical Society Journals, v. 48, p. 2335-2351.
Louw, J. D., 1954, Geological age determinations on Witwatersranduraninites using the lead-isotope method: Geological Society of theSouth Africa Transactions, v. 57, p. 209-230.
McKelvey, V. E., Everhart, D. L., and Garrels, R. M., 1955, Origin of
uranium deposits: Economic Geology, v. 50, p. 464-533.
Mugge, 0., 1928, Uber die Entstehung faseriger Minerale und ihrer
Aggregationsformen: Neues Jahrbuch fur Mineralogie, v. 58 (A),p. 303-348.
Nuffield, E. W., 1954, Brannerite from Ontario, Canada: American
Mineralogist, v. 39, p. 520-522.
Oberlies, F., and Prashnowsky, A. A., 1968, Biogeochemische undelektronenmikroskopische Untersuchung prakambrischer Gesteine: Naturwissenschaften, v. 55, p. 25-28.
Patchett, J. E., and Nuffield, E. W., 1960, The synthesis and crystal­lography of brannerite, pt. 10 o/Studies of radioactive compounds:
Canadian Mineralogist, v. 6, pt. 4, p. 483-490.
Pettijohn F. J., 1957, Sedimentary rocks: New York, Harper & Row,
718 p.
Pettijohn F. J., and Lundahl, A. C., 1943, Shape and roundness of
Lake Erie beach sands: Journal of Sedimentary Petrology, v. 13, p.
69-78.
Pflug, H. D., Meinel, W., Neumann, K. H., and Meinel, M., 1969, Entwicklungstendenzen des fruhen Lebens auf der Erde: Natur­wissenschaften, v. 56, p. 10-14.
Prashnowsky, A. A., and Schidlowski, M., 1967, Investigation of Precambrian thucholite: Nature, v. 216, p. 560-563.
Ramdohr, Paul, 1955, Neue Beobachtungen an Erzen des Witwatersrandes in Sudafrika und ihre genetische Bedeutung: Deutsche
Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik
und allgemeine Naturwissenschaften, Abhandlungen Jahrgang
1954, no. 5, 43 p., 33 pi.
———1957, Die "Pronto-Reaktion": Neues Jahrbuch fur Mineralogie,
Monatschefte, v. 1957, p. 217-222.
———1958a, New Observations on the ores of the Witwatersrand inSouth Africa and their genetic significance: Geological Society ofSouth Africa Transactions, annexure to v. 61, 50 p., 61 pi.
———1958b, Die Uran- und Goldlagerstatten Witwatersrand, BlindRiver District, Dominion Reef, Serra de Jacobina: Erzmikroskopische Untersuchungen und ein geologischer Vergleich:Deutsche Akademie der Wissenschaften zu Berlin, Klasse fiir
Chemie, Geologie und Biologie Abhandlungen, Jahrgang 1958, no.
3, 35 p., 19 pi.
-1969, The ore minerals and their intergrowths (English trans­
lation of 3d ed.): Oxford, Pergamon, 1,174 p.
Ramdohr, P., Ottemann, J., and Schidlowski, M., 1965, Mikroskop,
Mikrosonde und Deutung der Uran-Blei-Alter: Max-PlanckInstitut fur Kernphysik, Heidelberg, Schriftenreihe 1965/V/12, 13
p., 4 pi.
Reimer, T. 0., 1975, The age of the Witwatersrand System and othergold-uranium placers: Implications on the origin of the mineraliza­
tion: Neues Jahrbuch fur Minerologie Monatshefte v. 1975, p.
79-98.
Reinecke, Leopold, 1928, The location of payable ore bodies in the
gold-bearing reefs of the Witwatersrand: Geological Society ofSouth Africa Transactions, v. 30, 1927, p. 89-119.
Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
the Canadian Shield: Canada Geological Survey Paper 68-40, 205
PRosenzweig, A., Gruner, J. W., and Gardiner, L., 1954, Widespreadoccurrence and character of uraninite in the Triassic and Jurassic
sediments of the Colorado Plateau: Economic Geology, v. 49, p.
351-361.
Saager, R., 1970, Structures in pyrite from the Basal Reef in the

N23

Orange Free State Goldfield: Geological Society of South Africa
Transaction, v. 73, pt. 1., p. 29-48.
Schidlowski, Manfred, 1965, Probable life-forms from the Precambrian of the Witwatersrand System (South Africa): Nature, v. 205,
p. 895-896.
——1966a, Beitrage zur Kenntnis der radioaktiven Bestandteile der
Witwatersrand-Konglomerate I-III: Neues Jahrbuch fur Miner­
alogie, Abhandlungen v. 105/106, p. 183-202; 310-324; 55-71.
———1966b, Mineralbestand und Gefugebilder in Faseraggregaten
von kohliger Substanz ("Thucholith") aus den WitwatersrandKonglomeraten: Contributions to Mineralogy and Petrology, v. 12,
p. 365-380 (incl. English summary).
———1966c, Some observations on radioactive blasting haloes andradioactive corrosion phenomena in conglomerates from theOrange Free State Goldfield: Geological Society of South Africa
Transactions, v. 69, p. 155-159, 5 pi.
———1968, Critical remarks on a postulated genetic relationship be­
tween Precambrian thucholite and boghead coal, in Schenck, P.
A., and Havenaar, I., eds., Advances in organic geochemistry
1968; Oxford, Pergamon, p. 579-592.
———1970a, Untersuchungen zur Metallogenese im sudwestlichen
Witwatersrand-Becken (Oranje-Freistaat-Goldfeld, Sudafrika):
Geologisches Jahrbuch Beihefte, v. 85, 80 p., 23 pi.
-1970b, Elektronenoptische Identifizierung zellartiger Microstrukturen aus dem Prakambrium des Witwatersrand-System(>2.15 Mrd. Jahre): Palaontologische Zeitschrift, v. 44, p.
128-133, 3 pi.
Schidlowski, M., Eichmann, R., and Junge, C. E., 1975, Precambriansedimentary carbonates: Carbon and oxygen isotope geochemistry
and implications for the terrestrial oxygen budget: Precambrian
Research, v. 2, p. 1-69.
Schidlowski, M., and Ottemann, J., 1966, Mackinawite from the Wit­
watersrand conglomerates: American Mineralogist, v. 51, p.
1535-1541.
Schidlowski, M., and Trurnit, P., 1966, Drucklosungserscheinungen an
Gerollpyriten aus den Witwatersrand-Konglomeraten: Ein Beitragzur Frage des diagenetischen Verhaltens von Sulfiden:
Schweizerische Mineralogische und Petrographische Mitteilungen,v. 46, p. 337-351.
Schopf, J. W., 1974, The development and diversification of Precam­
brian life: Origins of life, v. 5, p. 119-135.
Sharpe, J. W. N., 1949, The economic auriferous bankets of the Upper
Witwatersrand beds and their relationship to sedimentation
features: Geological Society of South Africa Transactions, v. 52, p.
265-300.
Snyman, C. P., 1965, Possible biogenetic structures in Witwaters­
rand thucholite: Geological Society of South Africa Transactions,v. 68, p. 225-235.
Steacy, H. R., 1953, An occurrence of uraninite in a black sand:American Mineralogist, v. 38, p. 549-550.
Taylor, K., Bowie, S. H. U., and Home, J. E. T., 1962, Radioactive
minerals in the Dominion Reef: Mining Magazine (London), v. 107,
p. 329-332.
Trushkova, N. N., and Kukharenko, A. A., 1961, Atlas of placer
minerals: Moscow, State Publishing House of Scientific Literature,etc., 435 p.
Wiebols, J. H., 1955, A suggested glacial origin of the Witwatersrandconglomerates: Geological Society of South Africa Transactions, v.
58, p. 367-382.
Young, R. B., 1917, The banket of the South African goldfields: Lon­don, Gurney and Jackson, 125 p.
Zeschke, G., 1959a, Uber die Verwitterung und den Seifentransport
von naturlichen Uranoxyden: Aufschluss, v. 10, p. 129-134.
———1959b, Uraninit-Vorkommen in rezenten Schwermineralsanden des Industales: Neues Jahrbuch fiir Mineralogie,
Abhandlungen v. 93, p. 240-256.

N24

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESDISCUSSION FOLLOWING PAPERS BY DRS.
SCHIDLOWSKI, HALLBAUER (PAPER 34)
AND LOIS NAGY

[At the Workshop the paper by Zumberge, B. Nagy,
and L. Nagy was presented in two parts. Lois Nagy
spoke first and confined her remarks to the Precambrianmicropaleontology of the Vaal Reef. Bartholomew Nagy
followed and covered the rest of the subject matter.]Schopf: I would like to briefly address myself to the lasttwo papers in particular. It seems to me that those of
us who are concerned with the early history of life
have two primary tasks incumbent upon us if we arereporting objects that we interpret as Precambrianfossils. First, it must be established that these objects
are indigenous to the rock, that they are syngenetic
with deposition, that they are part of the rock and datefrom the time of original sedimentation. And second­ly, we must establish that these objects are of
biological origin; regardless of whether we interpretthem as fungi, algae, or what have you, there must bea set of criteria that allows us to say without doubtthat these are biologic objects, rather than aggrega­tions of organic matter of some other origin. Finally, I
would point out that, as I think is probably obvious tomost of us, this is a very difficult sort of game to play.
It is often, or at least in some cases, noi easy to knowwhether the structure you are looking at is biologicalin origin or not: it is usually somewhat easier toestablish that the object is indigenous to the sediment.
It should be said that this field is only about ten years
old, that there is very much to be learned, and thegeneralizations are not particularly well established.
So it is possible to have strange things turning up.
With regard to Dr. Hallbauer's paper, it seems to me
that he has established that there are fibrous ag­
gregates of organic matter within the Witwatersrandrocks. I think that is well established indeed. Personal­
ly, I do not believe that he has established at all thatthese structures are biological. Without getting into
the great details involved, it is my opinion that thebranching structure that he has shown differs from all
biological branching structures of which I am aware.
It is not a fungal type of branching structure, at least
those of which I am aware, and I have looked at quite anumber of modern fungi. These tubes that he has
shown are anastomosing, they are of variable
diameter, sometimes you have large ones, sometimes
you have small ones, sometimes you have multiple
branches, but, as many of you know, in general in
biological structures you get sort of a V-shaped
branch, and these tubes do not generally anastomoseback together again. On the slides that I saw, it seem4 See editors statement at start of Discussion, Chapter M.

ed to me that I could see several examples in which
there were multiple tubes coming off and then merg­ing back together. Well, maybe that's the way
primitive organisms were, but there is nothing likethat known today as far as I am aware. Secondly, I do
not understand at all, as Dr. Schidlowski pointed out,
how such a presumably delicate structure can bepreserved. These are not permineralized organisms, if
they are organisms. They are preserved in a sedimen­
tary environment, I guess mostly analagous to a coal,and in coals one does not see structures of this type
either. Even in coals that you know are of plant origin,
one doesn't get such delicate things as presumably
these organisms were. So it is my guess at this time,that although these are assuredly indigenous to a
Precambrian sediment, I believe that the case is very
far from established that they are biological.
With regard to the interesting things that Dr. [Lois]
Nagy has presented, I believe that neither thebiogenicity nor the Precambrian age of these struc­tures has been established. We see interesting par­ticles in scanning electron micrographs. The car­bonaceous nature of these structures is very wellestablished, but how do we know that they are ofPrecambrian age? This is a problem with looking at
microscopic objects with a scanning electron
microscope: it is a terrific tool for certain sorts ofstudies. But if you want to prove those objects are ofPrecambrian age, it seems to me that the best way to
study them is in thin sections. Furthermore, it seems tome that if they are biologic in origin they ought tohave the sort of regularity that one sees in modern
biologic systems. I would tend to have interpreted theobjects that Dr. Nagy showed as probably bits andpieces of organic material from the Witwatersrandrocks rather than being demonstrably remains of
micro-organisms. And I think this is also true about
Dr. Hallbauer's. In my opinion they seem to me to lackthe type of biologic regularity that one sees in biologic
systems. I am sorry to say all these things because I do
not appreciate standing up and pretending that I am
the great expert on all these types of things, and I
don't want to be impolite to either my colleagues or
anybody else. It's darned hard to be sure about these
sorts of things, and I think it behooves those of us whodeal with Precambrian fossils to nail the case down
just as firmly as we possibly can, to establish thatthese things are both Precambrian in age and of
biologic organization.
Hallbauer: I just want to point out that in scanningelectron photomicroscopy if objects overlie each other,
especially filaments, in the photograph it looks as if
they come together again. This I know is very difficult
to show in a set of photographs. In addition, I want toURANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

N25

say that to me there is no doubt that these things are
the kind that one sees in asbestos. And I would sug­of Precambrian origin. They are in the rocks, andgest that perhaps a line of research to follow on that
there is no reason to believe that they were introduced
would be single crystal X-ray crystallography and see
at a later stage. With regard to the other structures, I
if some of your filamentous structures are not in fact
am not a biologist, I can only say what other biologists
single crystals.
have told me with regard to the filamentous struc­ Hallbauer: Well, there again I can only repeat what I
tures. They have been identified as organisms by
was told by biologists. If you can show microscopic
Preston Cloud, although he had reservations and said,
preparations of these filaments to a mycologist and he"How did they get into the rock?" Some biologists sayrecognized them immediately as fungal structures,
they are biological structures, some others have reser­then there is nothing for the geologist to do.
vations, so it is very difficult to decide from my point Skinner: Would you permit me to tell a short story? Iof view as a mineralogist.
was working one time in Australia in some shallow
L. Nagy: I must say that this is one of the nice things
lakes, and it was in a region where a number of algal
about small scientific meetings, when you can getcolonies form little biscuit-shaped calcium carbonate
together with your colleagues and they sort ofgrowths. In the lake I was working in I saw something
disagree with you. This helps keep you on your toes,
that looked a little bit familiar. It was about the size ofand I think that what Bill [Schopf] said about my worka thumbnail and had a rounded shape and I thought,
is indeed quite true. One of the things that I should"Hello, here's another one of these things, but ahave pointed out is that I have looked at thin sections
different shape." So I gathered a bunch of them, tookof the carbonaceous matter, but unfortunately it is sothem back to the university, and gave them to thevery opaque you really can't see too much in it. So I've
biologist who looked at them and said, "Yes that's
got to scratch my head and go back to the drawing
true. That's just what they are. They are some kind ofboard to see how we can get petrographic thin sec­
algal growth. They are new, and we'd like to describe
tions that are made in such a way that we can see
them." They took about about a dozen of them andstructures in them. He had a very valid point. I havedissolved the calcium carbonate away, and thennever published a paper of any structures that havedescribed the gelatinous cellular structures that were
not also been found in thin sections. And this work willleft. They had even gone to the point of writing a
not be published until structures are indeed found in
paper and submitting it to the Royal Society of Southpetrographic thin sections.
Australia, at which it had been accepted. I mentioned
Robertson: I have a comment for Dr. Schidlowski withthis one day to Martin Glassner, who many of you
reference to the brannerite reaction. Ramdohr calledknow is a paleontologist, and he said, "What did thatthat the "Pronto reaction" from the Pronto mine inthing look like?" So I described it. He said, "Have you
Canada. That's the farthest south of the Elliot Lake
got any more?" And I pulled one out of the drawer,
deposits and is the one which would be more subject toand he said, "Well, I know what that is, that's alater metamorphic events. In the deposits farther
gastrolith from a crawfish, and the cormorants havenorth there are a number of uranium-bearing beds. In­
been eating those things." And true enough, I'd picked
formation is really rather scanty, but I think the in­these up underneath a log where the cormorants had
dication that we have is that the brannerite content
been sitting and passing them through their bodies
varies within the bed relative to uraninite and it varies
and dropping them into the lake. So I am not sure thatfrom bed to bed, so that where beds are stacked uponshowing it to a biologist always gives you a definitive
one another, we do not in fact find the sameanswer.
brannerite-uraninite ratio. This might suggest that Hallbauer: In that case, to what authority do you go?
some mechanism other than metamorphism was Schidlowski: Originally I wanted to briefly comment on
responsible.
Dieter's [Hallbauer] talk, but when Bill [Schopf] stood
Schidlowski: I would suggest that the amount of Ti02
up I got a little bit mixed up, so I propose to give mypresent is also one of the prerequisites. So metamor­
contribution as if I hadn't heard Bill's comment. I
phism is probably only one factor. That would be mywould have said, ... I would personally differentiate
answer to this.
clearly, and I am still convinced that the fibrous stuff isSkinner: I am not a biologist either, but I think that I
not biogenic, at least the pictures which I have seen
have seen Dr. Hallbauer's structures before. I have
have not yet convinced me. Whereas, in contrast to
seen them in coke. I would submit that perhaps much
Bill, I would have been inclined to acknowledge theof what he is seeing is an artifact of vesiculation. As
other stuff. I also remember that Pres Cloud when wefor the filamentous structures, I wonder if one should
met him—Bill, you were present in Leicester—that
not look for an origin for them in fibrous growths of
Pres said this is certainly biogenic stuff. And maybe

N26

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

you have not seen Dieter's last published pictures in Hallbauer: In some of Dr. Schidlowski's recent papers
that South African journal. Perhaps you should give it
he describes the occurrence of pristane and phytane asa look and think about it. But, anyhow, you under­
chlorophyll derivatives in the carbonaceous material.
stand more of that than I do, so it would be just a row
Now, how does he explain the presence of these things
between Pres Cloud and you.
in hydrocarbons which can be floated through the rock
Schopf: You know, this is getting to be a little bit like
and were polymerized by uranium?
what I refer to as anecdotal science. If I recall the con­ Schidlowski: Pyrolysis gas chromatography was done on
versation of which I believe you are speaking, Presthe carbonaceous matter, and we got a whole pat­
Cloud's point was that in Dr. Hallbauer's paper intern of hydrocarbons. We used mild pyrolysis, so wePrecambrian Research, Cloud interpreted the
got all this stuff which is volatile up to let us say
filaments shown in optical photomicrographs purport­
250°C. Among these graphs we got small peaks in theing to show cells to be certainly biological and certain­
place where one could expect phytane and pristane.
ly contamination.
We took these peaks as real, but we did not confirm
Schidlowski: Yes. He said they looked too modern, andthem with mass spectrometry, so I wouldn't put myhe suspected they were contaminations, but anyhow
head on the block for this. When we get a chance to go
they were biologic. We should perhaps split the prob­
deeper into the matter, we will certainly come nearer
lem in two portions. First, is that stuff biogenic, andto the truth. I think it is justified to record that at least
second, is it indigenous? I would yield to the first one,traces of these degradation products are actually pre­and I will concur with you that perhaps the second
sent. If that stuff is mobilized, it must have been taken
point needs to be proved.
along with the other hydrocarbons when theyMyers: I would simply like to ask how the temperatures
migrated through the sequence.
of oxidation were measured in Dr. Hallbauer's oxida­
Paul L. Weis, U.S.G.S., Reston, Va.: Dr. Schidlowski,
tion procedure.
one gets the impression in looking at your photomicro­
Hallbauer: There was no accurate measurement ofgraphs of the progressive decomposition, disintegra­
temperature. It was measured on [some?] elementtion, or disaggregation of uraninite grains that this isnearby. So it was plus or minus 500°C, say plus ora process that takes place over a considerable period
minus 10 to 20 degrees.
of time. The implication may be that this is not a synRuzicka: Dr. Schidlowski mentioned extinction ofgenetic effect, but something that takes place after­hydrocarbon within one or around one grain ofwards, in which case it answers at least part of youruraninite.
comment that, whether it is biogenic or not, it can't be
Schidlowski: What do you mean by "extinction"?
an original constituent of the rock. The other aspect of
Ruzicka: This is what I am going to ask you, because we
this question is, as the uraninite is attacked by theobserved that if you turn the table under crossedhydrocarbons, where does the excess uranium go?
nicols . . .
Schidlowski: Oh, you refer to the reflection pleochroism. Schidlowski: Into the brannerite, for instance, but here
Ruzicka: Yes. You get simultaneous extinction in large
reservation is justified, of course. If we admit that theareas. How do you explain it?
[carbonaceous] stuff has migrated, any one is justified
Schidlowski: I would say there must be a certain prein asking, "Where does it come from?" And thosegraphitization stage in which the carbonaceousfamiliar with the whole story know that Davidson
material tries to approach a certain order, finally, of
originally contended that the stuff came from overly­
course, aiming at graphite but not reaching it at theing Karroo coal measures. But hydrocarbons are light,
moment.
and because of that they do not sink into basins, andRuzicka: This is your explanation?
there are some other points against a source in the
Schidlowski: Yes. But I think there are other experts
coal measures. If you rule out the Karroo measures,
who have better explanations to offer, but I think as awe must look for sources of the carbon. We know what
first approximation I should be right. Actually, theis below and we also know what is above in the se­
stuff I showed you is not graphite at all. Many people
quence, and there are actually no strata which containhave found traces of graphite in the reefs and usuallymassive concentrations of reduced carbon in any
associate it with that carbonaceous stuff, but if you
modification. So I think, although we could not settle
would do some X-ray work you would not get graphite
the topic definitely, we can narrow it down and say
patterns. By the way, in reflected light microscopy it isthat, with 90 percent probability, the stuff is in­very easy to distinguish between carbon in the widest
digenous in the widest sense, although I must admit
sense and graphite in the proper sense, as anybody
there is a slight possibility still open for extraneous in­familiar with this type of work will admit.
troduction.

URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

Irving Breger, U.S.G.S., Reston, Va.: I'd like to make a
few comments for all that they are worth. The organic
matter associated with the actual uraninite grains andthe nature of that type of material is in question on thebasis of what was said here. First of all, what we havefound is that organic matter that has been subjected tolengthly irradiation by uranium and its daughter prod­
ucts tends not to go to graphite, but instead takes on adiamond-type structure, for the simple reason that thegraphite type lattice tends to buckle and you get amore diamond-like structure, which is harder. I sug­gest also that when we talk about organic matter andthis type of structure in the future, it would be very
nice to know that this organic matter contains morethan just carbon. Is there oxygen in it, too? How muchhydrogen? Just knowing that it contains carbon reallydoesn't give us much to work on.
The second thing I would like to say is really a sug­gestion for your consideration. Let's assume that wehave a sedimentary situation in which we have been
washing down, gold, uraninite, and other heavy
minerals, and we have a rather quiet situation inwhich we can also get perhaps some life forms
developing. Let's assume then that these life forms
were decomposing, because obviously they decom­
posed, too. If you are looking for a source of methane,
it must be there because we had reducing conditions. I
think we are all agreed that there was no oxygen
about at the time, so that if you are looking formethane to polymerize you don't have to go to the
Karroo Formation or anywhere else; it was in­digenous. Although I don't think that's the answer,
either. But let's assume that the uraninite and othermineral grains have now come down, they are buried
with this organic matter at the bottom of a shallowbasin, and the organic matter, being what it is, prob­
ably acidic in nature, tends to attack the mineral
grains, perhaps through crevices, and it seeps up into
them in such a way that it makes up those channels
that are backfilled that you have shown us in your ex­cellent pictures. What more does one need? Do you
have to go to very sophisticated explanations? It's thesame material, as far as I can tell.
There is another question I'd like to raise here withyou and that is: In some of the pictures you haveshown, you very clearly indicated that the organic
matter was highly metamorphosed in the sense it wasmuch more highly reflective than you would have ex­pected. Why wasn't the same organic matter in thecrevices of the actual uraninite grains just as equallyreflective as that that was not associated withuraninite grains?
Schidlowski: I think they are photographed in a posi­tion where the reflection pleochroism was responsible

N27

for a dark strip, so if you turned your microscopestage it would have cleared up.
Breger: I don't think so. I think you have picked aneasy answer out of it.
Another thing I would like to bring up is the time se­quence. What is the paragenesis of all this? Everybody
assumes that it's happening today, just this minute.
What is the time sequence in terms of the introductionof the uraninite grains into the sedimentary basin, thetime that it has taken for degradation of the uraninite
grains by the decomposing acidic organic matter sothat perhaps the grains are cracked and carbonaceous
material has moved up into and backfilled it? I'm notsure that this is a replacement in the truest sense of
the word.
Schidlowski: How would you account for that? Whatalternative would you offer?
Breger: Let's take a uraninite grain that is immersed ina collodial type organic goo, sitting at the bottom of abasin, and this uraninite grain has a few little
microcracks in it and attack occurs in this goo at thecracks and the organic matter seeps into the cracks
and breaks up the uraninite and perhaps dissolves it.
Schidlowski: Just in a mechanical way?
Breger: Mechanical and partially chemical, too.
Schidlowski: Why does the organic matter not pene­
trate into the cracks of other minerals, for instance,
the ubiquitous pyrite, the chromite, the zircon, and soon? I think this imposes certain limitations.
Breger: I think we are missing the chemistry. First ofall, I don't think organic matter attacks zircon.
Secondly, I don't think it attacks pyrite, but it does at­tack uraninite.
Schidlowski: It may invade in cracks in any othermaterial. So if it invades in cracks, if it were a purely
mechanical process, I would suggest that all minerals
were in equal position to get a little bit of that stuff.
Breger: But I didn't say purely mechanical; I saidmechanical and chemical. I said that the acidic en­
vironment will lead to an attack on the uraninite
grains.
Schidlowski: Actually, the point is that only in theuraninite particles do we have that stuff and no othercracks. So, okay, any other questions?
Breger: I don't know if there were so many questions
as statements, maybe not even of fact. I would also
like to suggest that if we had uranium in some of theseplants—well assume that they are plants for the
moment-it sure would mess up their life process, sothat all I can suggest here is that if the uranium did
get involved with these plants that it had to come after
they were dead. They served as a hollow zone in which
precipitation, either chemical or mechanical, could

N28

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

take place. I do want to say, though, that I thoughtthese two papers were just excellent.
Schidlowski: The mechanism which you alluded to now
is, I think, by far the most probably reason forbringing about a uranium-carbon association. It
worked, for instance, in the Colorado-type deposits,
but my personal opinion is that in the Wits deposits
things were a little bit different. So I would differen­
tiate between Colorado-type uranium-carbon associa­
tions and Witwatersrand type.
Breger: I wish you wouldn't even mention them in thesame breath.
Hallbauer: There is one remark that was made about
these structures being similar to coke. We havethought about this possibility, and we've done somework and we are convinced that coke looks totallydifferent. The pores in coke have a totally different
structure from the ones shown here. They are ir­
regular and you never have an isotropic structure as
the one shown. Then about the preservation of these,
let's call them organisms, there is no reason why theyshouldn't be preserved in such an excellent statebecause the Witwatersrand rocks have a history ofvery minor tectonic movements and most of the rocks
have not been folded. There is only very slightmetamorphism, and most of the organisms have beenpreserved in the matrix of the quartz-pebble con­glomerate, which acted as a shield, so there is no
reason why they should be squashed.
Whiteside: I would just like to make one comment,
and that is that there has been quite a lot of tectonism
in South Africa. In the Witwatersrand one gets faultsof displacements of two miles; around the Free State
Goldfield there are others probably with even more
displacement. There is a horizontal displacement I
know of of 3,000 feet in the East Rand, and so on. I
think the rocks have been subjected to quite a lot of
tectonism. And along the edge of the Free State basin
the lower portion of the upper Witwatersrand is over­turned.
Halbauer: What I wanted to say was that there wasvery little folding of the Witwatersrand strata, andDr. Whiteside is quite right in pointing out thesevarious structures. Indeed, in the vicinity of faults,
the carbonaceous material is completely powdered anddoes not show these structures. I have just selectedparts which have been preserved better than the otherones.
Graham Curtis, Gulf Mineral Resources: Dr. Hallbauer,
I understand when you processed your carbon you
heated it up to 500°C?
Hallbauer: Yes.
Curtis: Well, did you gather any of the gases or any ofthe materials that might have been driven off of the
carbon at that time?
Hallbauer: No.
Curtis: In coalification, either natural or in making coke,
there is always something left over to gather. It would
seem that collecting these residues would be impor­tant and might be a technique for determining
whether or not the carbonaceous material isbiological.
Hallbauer: That is a technique that is to some extent
used by Dr. Schidlowski's team; they heat the car­bonaceous material and recover the volatiles.
B. Nagy: The first paper after lunch, I presume, isgoing to be given by me, which will be a continuationof the last paper. I am going to report on the combined
pyrolsis, gas chromatography, and mass spectrometry of Vaal Reef samples heated to 450 °C in
10 ~ 6 torr vacuum.
Armstrong: I would like to ask about the radiation
damage to the carbon. Is there any technique tomeasure the amount of radiation to be expected over acertain period of time or a method for measuring theamount of radiation damage to carbon?
Schidlowski: To whom was the question addressed?
The only radiation damage I could think of is just theincrease in reflectivity. At the moment I can't recall
any other. Can you?
Armstrong: There was a slide I thought which showedsome radiation damage.
Schidlowski: Yes, you get cracking halos, but they area little bit different. Mostly they must be ascribed tozircons, because if zircons undergo metamictizationthat results in a volume increase, then all theneighboring stuff is blasted. As for radiation alone,some little blasting halos have been observed arounduraninite grains, but I think these phenomena are notso distinct.
Charles Miles, Protocol Corp., Denver, Colo.: For Dr.
Schidlowski. You studied uraninite with carbon. Was
there uraninite elsewhere, but nearby, in the samples
you studied that was not associated with carbon?
Schidlowski: Yes.
Miles: Is it the same size material?
Schidlowski: They are all about the same. I would say
between 80 microns and 160 with the average about
120. Fractured grains are, of course, smaller, and thegrains which I referred to as relict bodies within the
carbonaceous matter, they are between 40 and 50.
Sims: The aspect of preservation potential of thesecarbon seams seems to be bugging a lot of people. I'dlike to suggest that the process of sand-wave migra­
tion, accompanied perhaps by rapid silicification of thematrix of the sand, could in actual fact contribute toURANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY

the preservation and prevent subsequent destructionof fine filamentous, perhaps organic, productspreserved in these carbon seams. Perhaps thisthought is way out.
Joel Leventhal, U.S.G.S., Denver, Colo.: Since there
seems to be a lot of doubt about the biological natureof these things, I would like to throw a little bit of
doubt into the hydrocarbon situation. Livingorganisms in general don't show large amounts ofhydrocarbons, so I would say that's not a very goodsource for hydrocarbons in Dr. Schidlowski's presenta­
tion. Methane is the abundant hydrocarbon thatmicro-organisms produce. The problem is how do you
get enough of this gaseous methane to stick aroundlong enough to be polymerized? It's going to be inaqueous solution, and it's probably going to float out
again. So I think we may be back to the kind of thingthat Irving Brager is suggesting, that the stuff is justsetting there in a soup of carbon. The - 30 permil car­bon values don't sound particularly unusual; they are
not like the - 80 that's often quoted for natural gases.
Let me then ask a question: Is there carbonaceous
material without uranium? If so, perhaps the com­
parison of the carbonaceous material without theuranium compared to the carbonaceous material withthe uranium can be used as an index of the origin, or
the radiation damage, or the diagenesis, or any ofthese things.
Schidlowski: I have never seen carbonaceous matterwithout even at least faint relicts of uraninite, but I
recall that Ramdohr once mentioned that he had seensuch stuff but not in a proper carbon footwall of thereef but from somewhere higher in the reef. I am notsufficiently familiar with the occurrence to comment on
it. At least we cannot rule out that some carbonaceous
material occurs without uranium, so there must havebeen other fixation processes.
You also mentioned the isotopic composition of the
carbon. I think there is ample evidence by now thatundifferentiated organic matter on average yields
around - 25 permil. If we get values less than - 30, if I
remember correctly the Witwatersrand goes to - 37,

N29

then I think the likelihood is rather large that the car­bon is no longer undifferentiated and that the lightcarbon isotope has been increased by what I called a"chromatographic effect."Leventhal: I would say - 37 is definitely not light enoughto prove that it hasn't gone through some kind ofcracking rather than a biological process. I think most
petroleum people would say -37 just looks likemetamorphosed organic matter, not freshly depositedmaterial.
Schidlowski: Just to illustrate this point, we had a corefrom the Transvaal Dolomite, and there were carbon
partings in it. We sampled the carbon partings and therock between the partings, which contained about 0.5
percent organic matter. The finely dispersed car­bonaceous material within the rock yielded about - 26
permil, + 4, whereas from the carbon partings we got
values as low as -40 permil. We concluded that asecondary effect must have been responsible for thisaccumulation of carbon twelve. I think our conclusion
is fairly reasonable, at least in this case.
Hallbauer: I just want to mention that there arecarbonaceous seams without gold and with very low
uranium content. Normally they are of no interest to
the mining industry, and therefore they are quite easi­
ly overlooked.
Schidlowski: The example I was talking about was in
the Transvaal Dolomite, not in the Witwatersrand.
Ruzicka: We analyzed hydrocarbon in which were em­
bedded idiomorphic grains of uraninite, and the only
component we found in this hydrocarbon was sulfur,
about 24 percent sulfur but nothing else, not evensmall amounts of uranium or thorium.
Schidlowski: Elemental sulfur?
Ruzicka: Yes.
Schidlowski: At 25 percent it must have looked yellow
all over. That is in the Blind River carbon?
Ruzicka: Yes.
Schidlowski: Is somebody working on that at the
moment?
Ruzicka: Yes.

Some Aspects of the Development of the
Vaal Reef Uranium-Gold Carbon Seams,
Witwatersrand Sequence: Organic Geochemicaland Microbiological Considerations
By JOHN E. ZUMBERGE, BARTHOLOMEW NAGY, and LOIS ANNE NAGY
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-O

CONTENTS
Page

Abstract_________________________________________________
Introduction ______________________________________________
Materials and methods ________________________________________
Results_________________________________________________
Discussion _______________________________________________
Acknowledgments___________________________________________
References cited ____________________________________________

01
1
2
2
4
6
6

ILLUSTRATIONSFIGURE 1. Gas chromatogram of a Vaal Reef "carbon" sample ___________________________________________
2. Correlation between the ratio of dimethylnaphthalenes to other naphthalenes and uranium content ________________

Page

03
4

TABLE

TABLE 1. Mass spectral identifications of the major gas-chromatographic peaks _________________________________
in

Page

03

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESSOME ASPECTS OF THE DEVELOPMENT OF
THE VAAL REEF URANIUM-GOLD CARBON SEAMS, WITWATERSRAND SEQUENCE:
ORGANIC GEOCHEMICAL AND MICROBIOLOGICAL CONSIDERATIONS
By JOHN E. ZUMBERGE, 1 BARTHOLOMEW NAGY, and LOIS ANNE NAGY2
ABSTRACTThe solvent-insoluble organic matter (kerogen) that is the predomi­
nant syngenetic component of the Vaal Reef carbon seams was ana­lyzed by combined high-vacuum pyrolysis gas-chromatography mass
spectrpmetry, combined electron microprobe-scanning electron micro­
scopy and electron paramagnetic resonance spectroscopy. The
pyrolysis results indicate that the Vaal Reef "carbon" is a random
polymer-type matter consisting mainly of aromatic and short-chain
aliphatic hydrocarbons, as well as aromatic and other organic-sulfur
constituents. Scanning electron microscopy revealed coccoid andfilamentous microstructures in the carbonaceous seams. These struc­tures are conceivably of biological affinity and may be fossil blue-green
algae and (or) bacteria. Gold is often concentrated at the interfaces ofadmixed clay-carbon microenvironments. This implies physical andchemical interactions between clays and organic matter conducive tothe entrapment of detrital gold during deposition and the crystalliza­tion of colloidal/dissolved gold in the carbon seams. High concentra­
tions of organic free radicals (molecules with unpaired electrons) in the
carbonaceous matter, deduced from electron paramagnetic resonancespectroscopy, suggest that radiation emitted from uranium played aunique role in the reconstitution and polymerization of the progenitor
biochemicals.

INTRODUCTION

The Vaal Reef carbon seams are part of the extensive
gold-uranium deposits of the Witwatersrand Sequence,
which is between 2,500 and 2,750 m.y. (million years) old(Pretorius, 1974). These thin carbon seams, rich inuranium and gold, are commonly found along the basal
contact of the corresponding pebbly quartz-arenite bed.
The Vaal Reef stratiform deposit is as much as ~ 1.7 m
thick and covers an area of approximately 260 km2 . It ispart of the Main-Bird Series of the Upper Witwaters­
rand Sequence and is approximately 2,500 m.y. old. The
petrology of the Vaal Reef deposit, inclusive ofstratigraphical, structural, and mineralogical data, hasbeen studied in detail by Minter (1976). The samples
studied were obtained from the Klerksdorp area, which
Present address: Cities Service Co., Box 50408, Tulsa, OK 74150.
2 Laboratory of Organic Geochemistry, Department of Geosciences, The University of
Arizona, Tucson, AZ 85721.
'

is approximately 160 km southwest of Johannesburg,
South Africa.
Solvent-insoluble organic matter, which is calledkerogen, commonly occurs finely disseminated in Precambrian sedimentary rocks such as the Swaziland Se­
quence (Nagy and Nagy, 1969; Dungworth and
Schwartz, 1974), the Transvaal and Bulawayan stroma­
tolites in southern Africa (Zumberge and Nagy, 1975;
Nagy and others, 1977), and the McMinn shale,
Australia (Hoering and Abelson, 1965). It is generally
accepted that kerogen in Precambrian sedimentaryrocks evolved from the biochemical constituents ofbacteria and simple algae.
The origin of the Witwatersrand carbonaceous matterhas long been the subject of discussion. Yet, Gamier(1896) and Spilsbury (1908) had already considered thatthe "carbon" was derived from organisms (algae).
Sharpe (1949) concluded that the progenitors were most
likely bacteria and primitive algae along ancient
shorelines, intertidal zones, stream channels, andlagoonal deposits. This conclusion was based on studiesof the Main Reef, Main Reef Leader, Black Reef, andother reefs of the East Rand Goldfield. MacGregor(1953) considered that the Witwatersrand basin was sur­rounded by algal "peat bogs" and that reducing condi­
tions induced by decaying organic matter helped toprecipitate gold and uranium. Miholic (1954), Brock andPretorius (1964) and Snyman (1965) also proposed thatprimitive organisms, such as algae, gave rise to the car­bonaceous matter. Studies have been made of the
microbiology of modern algal mats (for example,
Golubic, 1976) and their organic chemistry, including
the newly formed kerogen, or protokerogen (for exam­
ple, Philp and Calvin, 1976). These modern analogs sup­port the contention that the carbon seams are ancientalgal and (or) bacterial mats and their rapidly evolved
protokerogen could well have played a significant role in
the accumulation of gold and uranium. On the basis ofthese considerations, it appears that the nature andhistory of the auriferous-uraniferous Witwatersrand
01

02

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

carbon seams cannot be fully understood without study­
ing the molecular composition and paleobiology of the"carbon."
MATERIALS AND METHODS

The molecular composition of seventeen Vaal Reef
"carbon" samples from the Klerksdorp area has been
determined. Before molecular analysis, the car­bonaceous matter was separated from the host rocks
with a solvent-cleaned scalpel. The carbonaceous
separates were powdered in acid-cleaned ball mills. To
exlude or minimize laboratory contamination, allglassware used during analysis was first cleaned in a
mixture of hot, concentrated 85-percent H2S04-15percent HN03; all solvents were freshly distilled. Pro­
cedure blanks (the entire analytical procedure withoutsamples) were repeatedly performed to search forlaboratory contaminations, but none were detected.
Next, the powdered Vaal Reef kerogen was con­secutively extracted by ultrasonication with benzene/
methanol (3:2 v/v), methanol, and water prior to analysis
by combined high-vacuum pyrolysis gas-chromatography mass spectrometry. This solvent extraction pro­
cess removes soluable organic compounds, which can bepostdepositional contaminations introduced into
sedimentary rocks and the insoluble polymeric kerogen(Abelson and Hare, 1968; Nagy, 1970; Smith and others,
1970; Sanyal and others, 1971). After drying, the decon­taminated samples were first degassed for ¥2 hour at
100 °C at ~ 10~6 torr vacuum in the pyrolysis apparatus
and subsequently were pyrolyzed at 450 °C for ¥2 houragain under 10~6 torr vacuum (Bandurski and Nagy,
1976). Pyrolysis was selected to degrade the solid
kerogen to low-molecular-weight fragments because it isa relatively simple and effective technique. Under op­
timum conditions pyrolysis liberates the components ofpolymers unaltered or only minimally altered by interand intramolecular rearrangements. Thermal energy isused to fragment the polymer, and high vacuum
minimizes the recombination and alteration of the
breakdown products. Pyrolysis under N2 or He at at­mospheric pressures may yield a variety of products thatare absent, as such, in the polymer. Degassing prior topyrolysis was necessary to remove traces of solventsand (or) contaminations that may have remained in thesamples after solvent extraction and drying. The
pyrolyzates (breakdown products) were directly in­troduced and separated on a polyphenyl ether (OS-138)
capillay gas-chromatographic column (46 m x 0.5 mm
int diam) in a Perkin-Elmer Model 226 gas
chromatograph, which was, in turn, connected to a
Hitachi RMU-6E mass spectrometer. Mass spec­
trometry was used for the identification of the pyrolysis
products. It needs to be stressed that although pyrolytic

conditions have been optimized (~10~6 vacuum, small
sample size), some of the pyrolyzates might have beensomewhat altered from the original kerogen com­ponents by thermal energy.
The electron paramagnetic resonance spectra ofpowdered Vaal Reef samples were obtained with a
Varian E-3 spectrometer at room temperature. The
microwave and modulation frequencies were set at
9.493 GHz and 100 Hz, respectively; the magnetic field
was 3,380 Gs, and the scan range was + 10 Gs. The scan
range was expanded to observe the presence of any in­organic free radicals. The organic free-radical abun­dances were determined by comparison with a standard(diphenylpicrylhydrazyl) of known free-radical content.
The scanning electron micrographs and electronmicroprobe analyses were obtained using an Applied
Research Laboratory Scanning Electron Microprobe
Quantometer. This is a single instrument designed toperform both as a scanning electron microscope and anelectron microprobe. By increasing the beam current
and changing other instrumental parameters, electronmicroprobe analyses can be performed on the very same
particles detected by scanning electron microscopy. This
instrument can detect light elements such as carbon,
nitrogen, and oxygen.
RESULTS

The pyrolysis results show that the Vaal Reef
"carbon," or, according to a more appropriate ter­minology, kerogen, is a random polymeric solid. It con­sists of clusters of condensed aromatic hydrocarbons, aswell as aromatic sulfur components connected by shortchain aliphatic hydrocarbon and alkyl sulfur bridges.
The polymeric structure is relatively tight and stablesince it is not broken up in vacuum below ~ 450°C. Notein this connection that pyrolysis at 300°C yielded
predominantly low-molecular-weight alkyl compounds.
A typical gas chromatogram showing the separated
pyrolysis breakdown products of one of the Vaal Reef
kerogen samples is shown in figure 1. The mass spectrometric identifications of peaks with numbers as­
signed on the gas chromatograph are listed in table 1.
The pyrolysis products of the other Vaal Reef kerogensamples were closely similar to the products listed intable 1. Alkyl substituted aromatic conponents and somearomatic sulfur compounds constitute the majority ofthe pyrolyzates listed in the table. There is no evidence
that the sulfur compounds are pyrolytic artifacts.
Pyrolysis of finely powdered pyrite with an aromatichydrocarbon yielded no organic-sulfur compounds underexperimental conditions identical with those used forthe kerogen pyrolysis. The few pyrolyzates that emerg­
ed from the gas-chromatographic column below 60 °C

DEVELOPMENT OF THE VAAL REEF URANIUM-GOLD CARBON SEAMS, WITWATERSRAND SEQUENCE

are not shown in figure 1 and table 1. These compounds
were mainly C2-C4 alkanes, alkenes and benzene, andminor quantities of low-molecular-weight alkyl sulfides,
acetaldehyde, acetone, and methanol. No nitrogencontaining organic compounds have been detected. It isnoteworthy that X-ray diffraction analysis revealed noevidence that these Vaal Reef "carbon" samples containcrystalline carbon—for example, graphite.
TABLE 1. Mass-spectral identifications of the major gas-chromatographic peaks[Peak numbers correspond to those in flig. 1)
PeakCompoundPeak

Compund

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17

toluene.
2-methylthiophene .
3-methylthiophene.
ethylbenzene.
p-xylene.
TO-xylene.
o-xylene.
n-propylbenzene.
methyfethylbenzene.
Do.
trimethylbenzene.
methylstyrene.
methylpropylbenzene.
d-benzene.
indene.
methylindane.
Do.

18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34

methylindene.
Do.
naphthalene.
benzothiophene.
methyl-1, 2-dihydronaphthalene.
2-methylnaphthalene.
1-methylnaphthalene.
ethylnaphthalene.
Do.
dimethylnaphthalene.
Do.
Do.
Do.
Do.
propylnaphthalene.
trimethylnaphthalene.
Do.

The results of the electron paramagnetic resonancespectroscopy studies on the Vaal Reef kerogen show an
unusually high concentration of organic free radicals

24

29
t

03

(3.4-5.5xl020 spins/gram carbon in the kerogen com­ponents; the g value was 2.0067). This free radical con­centration is greater by 1 to 2 orders of magnitude thanthe usual abundance of organic free radicals in coals (forexample, Austen and others, 1966). This excess of free
radicals may be attributed to the high abundance ofuranium in the Vaal Reef kerogen (as much as 10 per­cent by weight uranium). Radiation emitted by uranium
isotopes is known to affect organic matter, resulting, inpart, in free-radical production.
The uranium contents of ten separate Vaal Reef
kerogens show an inverse linear correlation with thequantities of dimethylnaphthalenes identified during
pyrolysis. With increasing uranium content, thedimethylnaphthalenes decreased in abundance relative
to the other naphthalenes. This correlation is shown infigure 2. Since it is more difficult to thermally cleave thearomatic carbon-carbon double bonds than the alkyl
bridges during pyrolysis, increasing abundances ofdimethylnaphthalenes may indicate somewhat less of anaromatic polymer matrix. Thus, the kerogen matrix is
less aromatic at relatively low uranium abundances thanat higher concentrations of this element. This is consis­
tent with the effect of prolonged radiation from uranium
on organic matter, first producing free radicals andsubsequently, aromatization (Huang and others, 1974).

23

27

2!,

30
3! i

25
r-26

(I

\m\ '

84

IN

68

76

isothermo! 195°

V

195°

52 Minutes

60

175°

155°

44

135° C

36
H———H

115°

95°

20

28
75°

DEGREES, CELSIUS
FIGURE 1.—Gas chromatogram of a Vaal Reef "carbon" sample showing separated breakdown products obtained by vacuum pyrolysis at 450°C.

04

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

s§
<
X

0.37
X

b

0.35
R= 0.8636
0.33

X

I 0.31

X

LU

I 0.29Q_>• 0.27
LU
Q

I

23456789

10

URANIUM, IN PERCENT
FIGURE 2.—Correlation between the ratio of dimethylnaphthalenes to other naphthalenes and uranium content. R is the coefficient of correlation.

The results of the electron microprobe analysis showthat the Vaal Reef carbon seams contain two finely ad­
mixed substances with different compositions. The firstcomponent contains mainly carbon and sulfur from the
kerogen; hydrogen cannot be detected by the electron
microprobe. The second substance consists mainly ofsilicon, aluminum, iron, and sulfur; the sulfur was
associated with iron. This second component is ap­
parently an iron-rich clay together with some pyrite.
The presence of clays was confirmed by the flakymineral morphology observed by the scanning electron
microscope and by X-ray diffraction. Gold was common­ly found between and near the interfaces of kerogen andclay clusters. It is interesting that one gold particle
within a clay cluster exhibited well-developed crystalline
morphology. The majority of the gold particles in bothkerogen and clay shows detrital morphologies. Wellpreserved microstructures of probable biological affinity
were also observed under the scanning electron

microscope (Nagy, 1975). These filamentous and coccoid
forms were found in powdered preparations and con­tained carbon.
DISCUSSION

The most likely source for the extensive Vaal Reef
kerogen deposits was ancient, primitive mat-building
procaryotes. It is not known if the Vaal Reef mats werebuilt by filamentous bacteria or blue-green algae, and it
is possible that it was a combination of both. The Vaal
Reef fossillike microstructures have so far been foundonly in the carbonaceous seams and not in the adjacentquartzite host rock. These microstructures are unornamented coccoid and tapered filamentous forms.
Modern algal mats consist of a tight, often weblike
matrix of filamentous microorganisms (together withcoccoid forms). Because there is no compelling evidence
that Precambrian mats had a different texture, it isDEVELOPMENT OF THE VAAL REEF URANIUM-GOLD CARBON SEAMS, WITWATERSRAND SEQUENCE

reasonable to assume that uranium, gold, and clays
became concentrated in the original Vaal Reef matsthrough sedimentary processes, as was suggested by,for example, Minter (1976).
It has been established that many species of matbuilding modern (extant) blue-green algae and bacteria
can live under anaerobic conditions. However, since
blue-green algae give off 02 as a result of photosyn­
thesis, it is not clear whether blue-green algal cells are
ever completely anaerobic (Brock, 1976). Photosynthetic
bacteria do not release 02 . A cross section of a mat fromthe Persian Gulf showed several distinctive layers 1 mm
thick caused by specialized microbial populations. The
brown surface layer consisted of organisms with abrown extracellular sheath pigment, which protected
the underlying layers from extensive solar radiation.
The transitional zone, where both aerobic and anaerobic
bacteria have been found, was pink. The bottom area
contained black iron-stained layers of mainly nonphotosynthetic anaerobic bacteria (Golubic, 1976). Thisillustrates the complexity of mat matrices.
Stromatolites are formed by mat-building blue-green
algae and bacteria, which can trap and bind sedimentary
particles. These sedimentary structures can form dome
or pancakelike structures from the blue-green algal
mats. However, in coastal marshes algal domes may
become partially buried, and thus their textures andfabric will be different from stromatolites formed inother environments (Monty and Hardie, 1976). The welldeveloped Belingwe (Bickle and others, 1975) and
Bulawayan (MacGregor, 1941) stromatolites (2.7 billion
years old; Wilson and others, 1978) are known from Zim­
babwe (Rhodesia). The Transvaal Sequencestromatolites in South Africa (for example, Young,1933) occur extensively and are also well developed.
Their age is ~2.3 billion years. These latter
stromatolites have yielded the first known evidence foralgal cell diversification (Nagy, 1974, 1978). Therefore,
it is somewhat surprising that there are no known
stromatolites associated with the extensive Vaal Reef
sediments and carbon seams. However, one of us (B.
Nagy) has seen an in situ, slightly domelike, semiconvex
structure in a carbonaceous seam on the wall of a Vaal
Reef mine tunnel at 2,600 m depth. It is interesting to
speculate whether this kerogen structure could havebecome a stromatolite by trapping more mineral par­ticles had the sedimentary and environmental conditions
been different before burial.
Current studies of algal mats from Baja California
provide some clues to the possible precursors ofkerogen. Philp and Calvin (1976) found that a
kerogenlike substance forms in algal-bacterial matsrelatively soon after burial. The chemical composition ofthis protokerogen is only partly known; it contains

05

aliphatic compounds, including alkanes. The major
cellular biochemical components of blue-green algae andbacteria are proteins, carbohydrates, and lipids. The
degradation products of these biochemicals can give rise
to a large number of organic compounds underanaerobic conditions. Some of these organic substances
could have played a role in the crystallization and ac­
cumulation of gold and (or) uraninite. The initialdegradation products of the algal and bacterial
biochemicals form a humic-type substance. This mattercan stabilize gold colloids in mats and thus preserve
them for crystallization during diagenesis. A similar
process was illustrated by Ling Ong and Swanson(1969).
These essentially nonaromatic biochemicals must havebeen also converted to the random aromatic polymer ofthe Vaal Reef kerogen, and, in the process, the proteins
lost their nitrogen content (Zumberge, 1976). One
should note that most common coals are also random
aromatic polymers; however, their origin can be traced
to higher plants, which contain an abundance of lignin (ahighly aromatic biopolymer), which did not evolve until
the Paleozoic.
Generally, exposure of organic chemicals to a, 13, and 7
irradiation tends to produce unsaturated molecules andsubsequent polymerization with the release of volatiles
such as H2 , CH4 , C02 , and NH3 (see Bovey, 1958.) For
example, when amino acids and proteins are irradiatedin the laboratory, they undergo deamination - that is,loss of NH3 (see Arena, 1971). Similarly, fatty acids
often decarboxylate - that is, lose C02 - when irradiated(Desrosier and Rosenstock, 1960). Thus, the reason why
nitrogen compounds were not detected among thepyrolysis products of the Vaal Reef kerogen may be
because the major nitrogen source, the algal andbacterial proteins, were deaminated by radiation fromuranium in the original mats and (or) subsequently in the(proto)kerogen. Older and younger Precambriankerogens of comparable biological origin releasedorganic nitrogen compounds upon pyrolysis (Nagy,
1976). These rocks, and their kerogen, however, lack theuranium found in the Vaal Reef kerogen. Variousorganic free-radical reactions were probably responsible
for the increasing aromaticity during the developmentof the Vaal Reef kerogen. The aromatic hydrocarbons
are more stable to irradiation than are saturated com­pounds- for example, alkanes - and this increase instability was a likely driving force to the final aromatization of the Vaal Reef kerogen. It should be noted,
however, that irradiation most likely was not the only
cause for the aromatization of the kerogen. Precam­
brian kerogens, particularly those in southern Africa,
are basically aromatic in composition, although to vary­ing extents. Temperature and time may also be major
factors causing aromatization.

06

GENESIS OF URANIUM AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

It appears that uranium has influenced to a con­siderable extent the diagenetic evolution of the Vaal
Reef carbonaceous matter by first aiding in the altera­tion of microbial biochemicals and then, through freeradical reactions, facilitating the evolution of the ran­
dom aromatic polymer. The Vaal Reef kerogen is thus
both diagenetically and chemically unique, first, because
it has been substantially affected by radiation during its
history and, second, because it lacks the common
oxygen- and nitrogen-containing kerogen components.
This study shows that, for the Witwatersrand carbon
seams and other ore deposits associated with car­bonaceous matter, organic geochemistry together withmicropalentology may help to elucidate certain prob­
lems in economic geology.
ACKNOWLEDGMENTS

The authors wish to thank Dr. W. E. L. Minter of theAnglo American Corp., Welkom, South Africa, for col­lecting the samples and explaining to one of us (B. Nagy)
the petrology and structure of the Vaal Reef bed in two
mines. Professor D. A. Pretorius of the Economic
Geology Research Unit at the University of the Wit­
watersrand, South Africa, provided continued advice.
Professor C. Steelink, The University of Arizona,assisted in obtaining the electron paramagnetic
resonance spectra, and Dr. Anne Sigleo, The Universityof Arizona (currently at the University of Maryland),determined the uranium abundances. Financial support
was provided by the Chamber of Mines of South Africa
through the Economic Geology Research Unit, Universi­
ty of the Witwatersrand, South Africa and by NASA
Grant NGR-03-0002-171.
REFERENCES CITED
Abelson, P. H., and Hare, P. E., 1968, Recent origin of amino acids in
the Gunflint chert: Geological Society of America Special Paper,
121, p. 2.
Arena, V., 1971, Ionizing radiation and life: St. Louis, C. V. Mosby Co.,543 p.
Austen, D. E. G., Ingram, D. J. E. Given, P. H., Binder, C.R., and Hill,
L. W., 1966, Electron spin resonance study of pure macerals, in
Gould, R. F., ed., Coal science: Advances in chemistry series 55,
Washington, D. C., A. C. S. Publications, p. 344-362.
Bandurski, E. L., and Nagy, Bartholomew, 1976, The polymerlike or­
ganic material in the Orgueil meteorite: Geochimica et
Cosmochimica Acta, v. 40, p. 1397-1406.
Bickle, J. J., Martin, A., and Nisbet, G. E., 1975, Basaltic andperidotitic komatiites and stromatolites above a basal unconformi­
ty in the Belingwe greenstone belt, Rhodesia: Earth and Planetary
Science Letters, v. 27, p. 155-162.
Bovey, F. A., 1958, The effects of ionizing radiation on natural andsynthetic high polymers: New York, Interscience Publishers, 287
PBrock, B. B., and Pretorius, D. A., 1964, Rand basin sedimentationand tectonics, in Haughton, S. H., ed., The geology of some ore

deposits in southern Africa: Geological Society of South Africa, v.
1, p. 549-599.
Brock, T. D., 1976, Environmental microbiology of living stromat­
olites, in Walter, M. R., ed., Stromatolites: Amsterdam, Elsevier,p. 141-148.
Desrosier, N. W., and Rosenstock, H. M., 1960, Radiation technology
in food, agriculture, and biology: Westport, Conn., AVI Publishing
Company, 401 p.
Dungworth, G., and Schwartz, A. W., 1974, Organic matter and traceelements in Precambrian rocks from South Africa: Chemical
Geology, v. 14, p. 167-173.
Gamier, J., 1896, Gold and diamonds in the Transvaal and the Cape:
Geological Society of South Africa Transactions, v. 2, p. 91-103,
109-120.
Golubic, S., 1976, Organisms that build stromatolites, in Walter, M.
R., ed., Stromatolites: Amsterdam, Elsevier, p. 113-126.
Hoering, T. C., and Abelson, P. H., 1965, Fatty acids from the oxida­
tion of kerogen: Carnegie Institution of Washington Year Book 64,
p. 218-223.
Huang, R. L., Goh, S. H., and Ong, S. H., 1974, The chemistry of free
radicals: London, Edward Arnold Publishers.
Ling Ong, H., and Swanson, V. E., 1969, Natural organic acids in thetransportation, deposition, and concentration of gold: Colorado
School of Mines Quarterly, v. 64, p. 395-425.
MacGregor, A. M., 1941, A Precambrian algal limestone in Southern
Rhodesia: Geological Society of South Africa Transactions, v. 43,
p. 9-16.
———1953, Discussion on "The gold-uranium ores of the Wit­
watersrand," by C. F. Davidson: Mining Magazine, v. 88, p.
281-282.
Miholic, S., 1954, Genesis of the Witwatersrand gold-uranium
deposits: Economic Geology, v. 49, p. 537-540.
Minter, W. E. L., 1976, Detrital gold, uranium, and pyrite concentra­
tions related to sedimentology in the Precambrian Vaal Reef
placer, Witwatersrand, South Africa: Economic Gology, v. 71, p.
157-176.
Monty, C. L. V., and Hardie, L. A., 1976, The geological significance ofthe freshwater blue-green algal calcareous marsh, in Walter, M.
R., ed., Stromatolites: Amsterdam, Elsevier, p. 448-447.
Nagy, Bartholomew, 1970, Porosity and permeability of the early Pre­cambrian Onverwacht chert: Origin of the hydrocarbon content;
Geochimica et Cosmochicica Acta, v. 34, p. 525-527.
Nagy, Bartholomew, 1976, Organic chemistry on the young Earth:
Evolutionary trends between 3,800 m.y. and 2,300 m.y. ago:
Naturwissenschaften, v. 63, p. 499-505.
Nagy, Bartholomew, and Nagy, L. A., 1969, Early PrecambrianOnverwacht microstructures: Possibly the oldest fossils on Earth?:Nature, v. 223, p. 1226-1229.
Nagy, Bartholomew, Nagy, L. A., Zumberge, J. E., Sklarew, D. S., andAnderson, P., 1977, Indications of a biological and biochemical
evolutionary trend during the Archean and early Proterozoic:
Precambrian Research, v. 5, p. 109-120.
Nagy, L. A., 1974, Transvaal stromatolite: First evidence for thediversification of cells about 2.2 x 109 years ago: Science, v. 183, p.
514-516.
———1975, Comparative micropaleontology of a Transvaal stromat­
olite (~2.3xl09 y. old) and a Witwatersrand carbon seam(~2.6x 109 y. old): Geological Society of America Abstracts with
Programs, Salt Lake City, Utah, v. 7, p. 1209-1210.
-1978, New filamentous and cystous microfossils, 2,300 m.y. old,from the Transvaal Sequence: Journal of Paleontology, v. 52, p.
141-154.
Philp, R. P., and Calvin, M., 1976, Kerogen structures in recentlydeposited algal mats at Laguna Mormona, Baja California: A
model system for the determination of kerogen structures in an-

DEVELOPMENT OF THE VAAL REEF URANIUM-GOLD CARBON SEAMS, WITWATERSRAND SEQUENCE
cient sediments, in Nriagu, J. D., ed., Environmental
biogeochemistry: Michigan, Ann Arbor, v. 1, p. 131-148.
Pretorius, D. A., 1974, The nature of the Witwatersrand golduranium deposits: Witwatersrand University, Economic Geology
Research Unit Information Circular No. 86, 50 p.
Sanyal, S. K., Kvenvolden, K. A., and Marsden, S. S., Jr., 1971,
Permeabilities of Precambrian Onverwacht cherts and other low
permeability rocks: Nature, v. 232, p. 325-327.
Sharpe, J. W. N., 1949, The economic auriferous bankets of the upper
Witwatersrand beds and their relationship to sedimentation
features: Geological Society of South Africa Transactions, v. 52, p.
265-288.
Smith, J. W., Schopf, J. W., and Kaplan, I. R., 1970, Extractable or­
ganic matter in Precambrian cherts: Geochimica et Cosmochimica
Acta, v. 34, p. 659-675.
Snyman, C. P., 1965, Possible biogenetic structures in Witwatersrand07

thucholite: Geological Society of South Africa Transactions, v. 68,
p. 225-235.
Spilsbury, E. G., 1908, Discussion on "The origin of the gold in the
Rand banket," by J. W. Gregory: Institution of Mining andMetallurgy, v. 17, p. 66-69.
Wilson, J. F., Bickle, M. J., Hawkesworth, C. J., Martin, A., Nisbet, E.
G., and Orpen, J. L., 1978, Granite-greenstone terrains of the
Rhodesian Archaean Craton: Nature, v. 271, p. 23-27.
Young, R. B., 1933, Conditions of deposition of the Dolomite Series:
Geological Society of South Africa Transactions, v. 36, p. 121-135.
Zumberge, J. E., 1976, The organic analysis and the development of
the Vaal Reef carbon seams of the Witwatersrand gold deposits:
University of Arizona, unpublished Ph. D. dissertation.
Zumberge, J. E., and Nagy, Bartholomew, 1975, Alkyl substituted
cyclic ethers in 2,300 Myr. old Transvaal algal stromatolite:
Nature, v. 255, p. 695-696.

The Distribution and
Sedimentary Arrangement of Carbon inSouth African Proterozoic Placer Deposits
By W. E. L. MINTER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-P

CONTENTS
Page

Abstract __________________________________________________
Introduction ________________________________________________The distribution of carbon ________________________________________
The arrangement of carbon ______________________________________
Conclusions ______________________________________________
Reference ________________________________________________
Discussion following papers by Drs. B. Nagy and Minter _____________________

pi
1
1
1
2
2
2

ILLUSTRATIONSPage

FIGURES 1-4. Photographs showing:
1. A thin discontinuous carbon seam on a rippled paleosurface sediment, B Reef, Freddies Mine, Orange Free State
Goldfield _______________________________________________________
2. A thick carbon seam with columnar structures, Vaal Reef, Vaal Reefs Mine, Klerksdorp Goldfield _________
3. A thin carbon seam at the base of a layer of distal Vaal Reef placer sediment, Stilfontein Mine, Klerksdorp Goldfield;a thin carbon seam on the Vaal Reef paleosurface and a second carbon seam at the base of a placer sandbody,
Stilfontein Mine, Klerksdorp Goldfield; a thin film of carbon attached to the surface of small pebbles, Basal Reef,
Western Holdings Mine, Orange Free State Goldfield; the curved bedform of a large trough-crossbedded unit
within the Steyn Reef, Saaiplaas Mine, Orange Free State Goldfield; carbon situated on two foreset planes ___4. Algae growing in pools filling trough-crossbedded hollows in halted sand dunes in a riverbed, White Umfolozi River,
Swaziland_______________________________________________________

PI
2

3
4
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHE DISTRIBUTION AND SEDIMENTARY ARRANGEMENT OF CARBON
IN SOUTH AFRICAN PROTEROZOIC PLACER DEPOSITS
By W. E. L. MINTER *ABSTRACT

Carbon, which occurs as grains, films, and thin seams in Witwatersrand Proterozoic placer deposits, is generally confined to carbon-seam
reefs that were deposited in distal environments. The distribution ofcarbon on paleosurfaces, on sedimentary accumulation surfaces likepebble layers, on trough-shaped bedforms of pi-crossbedded units andforesets, and on the winnowed top of placer sediments implies that its
growth took place contemporaneously with placer deposition in an
aquatic fluvial environment. The areal distribution of carbon seams in
distal environments is patchy, and its sparsity or total absence in someareas does not affect either the gold or the uranium content of theplacer. High gold and uranium contents that appear to be associatedwith carbon seams are at the base of the reef because that position
represents both the stable consolidated paleosurface upon which theplant material anchored itself and also the surface of bedload concen­
tration.

INTRODUCTION

The presence and nature of carbon that occurs in some
Witwatersrand placer deposits have caused much
speculation and encouraged investigations into its possi­
ble origin. It occurs in the form of fine grains, films, andseams, as much as 5 cm thick. Before the results of
microscopic and chemical investigations, based onrelatively few samples, are examined, it is important toconsider the distribution and arrangement of the carbon
relative to the sedimentary nature of the placer
deposits.

bodies and have consequently become known as carbonseam reefs. However, in the distal environments, thecarbon seams are neither uniformly distributed noralways present. For instance, an isopach map of the car­bon seam in the Vaal Reef, which was constructed fromapproximately 500 stations in the Klerksdorp Goldfield,
indicated that thicker carbon (fig. 2) was morewidespread along the western flank of the deposit(Minter, 1972).
THE ARRANGEMENT OF CARBON

In distal deposits the most common position occupied
by carbon is directly upon the paleosurface represented
by the surface contact of the placer footwall (fig. 3A).
The carbon is generally between 1 and 5 mm thick andexhibits a bladed or columnar structure at right angles

4

THE DISTRIBUTION OF CARBON

In banket reefs, where the placer sediment is com­posed of coarse conglomerate, carbon is rare andgenerally occurs as fine, sparsely scattered grains. In
shallow but proximal braided-channel deposits, like the
B Reef in the Orange Free State, fine grains withinlarge pebble lags are more abundant and thin patchy
seams occur (fig. 1). Carbon is very much more common
in distal sedimentary environments, where the placer
deposits were composed of small-pebble pebbly sand1 Geology Department, Anglo American Corporation, P.O. Box 20, Welkom 9460, South
AfricaFIGURE 1.—A thin discontinuous carbon seam on rippled paleosurface
sediment beneath a thin gravel lag of B Reef in proximal environ­ment. Freddies Mine in the Orange Free State Goldfield. (Scale is incentimeters.)
PI

P2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESCONCLUSIONSCM'FIGURE 2.—A thick carbon seam with columnar structures that
enclose sand grains, pebbles, and detrital minerals in distal en­
vironment. Western flank of Vaal Reef, Vaal Reefs Mine,
Klerksdorp Goldfield.

The distribution and arrangement of carbon in Witwatersrand placer deposits seems to be consistent withthat of an aquatic algallike plant. The sparsity of carbon
in proximal environments could have been a conse­
quence of the low preservation potential expected ofalgal mats under high velocity conditions. Thicker car­bon in the Vaal Reef along the western flank of thedeposit seems to have been similarly related to relative
hydraulic-energy levels.
During the deposition of placer sediment in distal en­
vironments the dominance of sediment migration fluc­tuated with sediment accumulation. During dominant
sediment migration, in the form of trains of small-scale
lunate sand dunes, small pebbles and detrital minerals
accumulated on a practically stable sediment surface
upon which an algal plant could also have thrived. These
conditions could account for the carbon on the paleosurface and overlying planes (figs. 3A and SB). During in­terruptions in flow, sand dunes may become stationary
and subaerial, as illustrated in figure 4. In this way,
algal material may grow on foresets and bottomsets andbe preserved in situ within the internal geometry of theplacer unit, as in figures 3D and 3E. Pebbles submergedin these environments would acquire an algal coating,which could be retained during subsequent transportand preserved, as shown in figure 3C. Carbon preservedon layers other than the paleosurface is seldom asso­
ciated with gold contents as high as found on the paleo­surface. This seems to be a factor of the lowest and op­
timum bedload position for gold.
REFERENCE
Minter, W. E. L., 1972, The sedimentology of the Vaal Reef in theto bedding. Basal lag concentrates of small pebbles andKlerksdorp area: University of the Witwatersrand, Johannesburg,
detrital heavy minerals occur on top of, and in some in­unpublished Ph.D. dissertation, 170 p.
stances are depressed or embedded in, the seam. Whenthese basal lags represent the pebbly base of a thicker
DISCUSSION FOLLOWING PAPERS BY
quartzite placer unit, an additional carbon seam has
DRS. B. NAGY AND MINTER
been seen to occur on top of the pebble layer (fig. 3E). In­
dividual pebbles coated with carbon have also been Hallbauer: I have a question for Nagy. How did you
determine the clay content in the carbonaceous
observed in the quartz-arenite matrix of the Basal Reefmaterial?
(fig. 3C), without an associated carbon seam.
Nagy:
You take a fraction from the total sample and go
Carbon seams also occur on the top surface of placerforthemajor X-ray peaks to get the area under theunits, as in distal parts of the Steyn Reef in the Orange
peaks
and
then correct for quartz and pyrite and other
Free State, and on primary structures within the reefmineralswhich
are present in large quantities. In
unit. These may be bedding planes of laminar layers,
otherwords,
theclay content which I presented was achannel surfaces, and trough-crossbedded foresets. In
clay
content
corrected
for the carbon, quartz, andfigure 3D, a trough-crossbedded unit in the Steyn Reef
pyrite.
contains carbon seams 5 mm thick on the bedform andon foresets (fig. 3E) of a trough-crossbedded unit that is Simpson: I would like to ask Dr. Minter whether hethinks the outer envelopes that he sees around pebbles
40 cm thick.

CARBON IN SOUTH AFRICAN PROTEROZOIC PLACER DEPOSITSP3

FIGURE 3.—A, A thin carbon seam at the base of a 5-cm-thick layer of
distal Vaal Reef placer sediment; Stilfontein Mine, Klerksdorp
Goldfield. B, A thin carbon seam on the Vaal Reef paleosurface
buried beneath a layer of small pebbles; a second carbon seam, whichis evident above the pebbly layer, lies at the base of 50 cm of placer
sandbody; Stilfontein Mine, Klerksdorp Goldfield. C, A thin film ofcarbon that is attached to the surface of small pebbles; Basal Reef,
Western Holdings Mine, Orange Free State Goldfield. D, The curved
bedform of a large trough-crossbedded unit within the Steyn Reef at
Saaiplaas Mine, Orange Free State Goldfield; a carbon seam is pre­sent on the surface of the bedform and on foresets. E, Carbon
situated on two foreset planes that merges with carbon on the bottomset; closeup of D.

P4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

could account for the carbonaceous material that Dr.
Schidlowski has described around uraninite?
Minter: I certainly think that it could account for that,
because the uraninite is also concentrated as a heavy
mineral on toe sets and would be incorporated andcould be wrapped up in the algal mat. I would also liketo draw your attention to a previous hypothesis, I
think suggested by Snyman or Liebenberg, that someof these uraninite grains might have actually decom­
posed to produce the carbon rather than carbon filter­
ing into the cracks; that the carbon represents decom­
position products. We hadn't mentioned that before
and that has been published.
Nagy: Now that the word algae has become respectableagain, I would like to make a comment. Algae that weknow of from modern blue-green algae, particularly
those which form stromatolites, secrete a large
amount of polysaccharides. They also decompose. So
wherever you have algae you won't have a lot of decay­
ing organic material which is going to be a green
mass. I think this is a very important consideration totake into account in evaluating the origin of the Vaal
Reef carbon.

FIGURE 4. - Halted sand dunes, which produce trough-crossbedded
units. Dark algal growth that is evident in the pool that fills one of
the troughs is growing on the base and on the leeside foreset plane;current is from right to left. White Umfolozi River, Swaziland.

Some Aspects of Witwatersrand Mineralization,
With Special Reference to Uranium Minerals
By CLIVE E. FEATHERGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

PAPER

1161-Q

CONTENTS

Abstract _______________________________
Introduction ________________________________________
Temperatures of metamorphic activity ___________________
Introduction - the modified placer theory ______________
Previous work ___________________________________The evidence of cobaltite and gersdorffite _____________
Optical characteristics of gersdorffite _________________Uranium-bearing minerals _____________________________Uraninite _______________________________________
Introduction __________________________________The UOJThO, ratio __________________________

Ql
1
1
1
2
2
8
9
10
10
10

Uranium-bearing minerals - Continued
Brannerite ___________________________
Introduction _________________________
Electron microprobe and X-ray diffraction investigation
Economic importance of brannerite ___________
U02 /Th02 ratio in the reefs as a whole ___________
Age of Witwatersrand uranium-bearing minerals _____
Paragenesis of gersdorffite and other secondary minerals ___General conclusions ________________________
Acknowledgements ________________________
References cited __________________________
Discussion ______________________________

Q12
12
13
15
19
19
20
20
20
22
23

ILLUSTRATIONSFIGURE 1. Photomicrographs showing typically rounded detrital cobaltite grains, from Cristaalkop Reef __________________
2. Electron-microprobe analyses of Witwatersrand cobaltite grains of detrital origin, plotted in the diagram FeAsS-CoAsSNiAsS______________._____________________________________________________
3. Scanning back-scattered electron ^Tfotomicrographs T)f~association oi mrthigenic gersdorffite^vith: goM, carbon, and otherminerals from Cristaalkop Reef ____________________________________________________
4. Electron microprobe analyses of authigenic sulpharsenide grains (mainly gersdorffite), plotted in the diagram FeAsS-CoAsSNiAsS, indicating temperatures of crystallization ___________________________________________
5. Sketch relating crystal structure ordering of authigenic gersdorffite with temperature for the system CoAsS-NiAsS _____
6. Scanning electron photomicrograph of a euhedral crystal of uraninite from Basal Reef _______________________
7. Diagram showing the variation of U02 /Th02 ratios for free uraninite and uraninite included in carbon _____________
8, 9. Scanning back-scattered electron photomicrographs showing:
8. Encrustation of brannerite on a partially altered uraninite grain being replaced by quartz _________________
9. Common occurrences of brannerite in uranium-extraction- plant residues from Vaal Reef __________________
10. Diagram showing variations in thorium content of brannerite ______________________________________
11. Scanning back-scattered electron and X-ray photomicrographs showing minute crystals of a calcium phosphate occuring as adiscrete phase in an aggregate particle of brannerite, quartz, and chloritoid, from Vaal Reef _________________

Q3
5
6
9
9
10
13
15
16
19
21

TABLESPage

TABLE 1. Chemical composition of sphalerite in the Witwatersrand reefs ____________________________________
2-6. Electron-microprobe analyses of:
2. Detrital cobaltite grains in some Witwatersrand reefs _____________________________________
3. Authigenic arsenosulphides of nickel and cobalt, mainly gersdorffite, in some Witwatersrand reefs ___________
4. Uraninite, in some Witwatersrand ores, not associated with carbon _______________________________
5. Uraninite inclusions in selected samples of uraniferous carbon __________________________________
6. Brannerite in some Witwatersrand reefs and recovery-plant samples ______________________________
7. X-ray powder-diffraction data for Witwatersrand brannerite, compared with previously published information on brannerite
from other sources ____________________________________________________________
8. Analyses of brannerite reported in the literature ____________________________________________
9. Comparison of uranium/thorium ratios of reef samples, analysed by X-ray fluorescence spectrometry, and the average ratiosof uraninite and (or) brannerite grains, analysed by electron microprobe _____________________________

Q2
4
7
11
12
14
17
18
19

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ PEBBLE CONGLOMERATESSOME ASPECTS OF WITWATERSRAND MINERALIZATION,
WITH SPECIAL REFERENCE TO URANIUM MINERALS
By CLIVE E. FEATHER1
ABSTRACT

Evidence is presented that suggests that temperatures of metamorphic activity in the vicinity of 400-600 °C have been achieved in the
Witwatersrand gold-bearing reefs, based on the chemical compositionof mixed crystals of gersdorffite and cobaltite that are undoubtedly ofauthigenic origin, being deposited at the same time as the gold was re­distributed and uraninite was probably recrystallized.
Sedimentological studies support the view that uraninite is of
detrital origin, but it has been suggested that the mineral resulted also
from precipitation from solution, since the "lichenlike plant," which isseen today as carbon ("thucholite") possibly also absorbed uranium
from solution during its growth. However, the U02/Th02 ratio varies
over a wide range in the grains from single reef samples, suggesting a
multiple granitic or pegmatitic detrital source.
All original features of the uraninite grains have been destroyed bysubsequent partial chemical dissolution and alteration to the uranium
titanate mineral brannerite (confirmed by electron microprobe and
X-ray diffraction studies).
Age determination by ion microprobe on minute volumes within themetamict crystals gives the same age (about 1,800 million years) for
both uraninite and brannerite, suggesting that recrystallization andalteration took place at the same time. This is younger than results ob­
tained on whole grains as determined by other investigators (about
2,000 million years), possibly because whole grains may be con­taminated with some radiogenic lead of a greater age, which occurs inpits and cracks in the grains and thus was not completely purged fromthe grains at the time of recrystallization.
Electron microprobe analyses of a large number of gersdorffite,
cobaltite, uraninite, and brannerite grains are presented.
Losses to uranium-extraction-plant residues are essentially due toenclosure of minute uraninite crystals in carbon and, especially in thewest of the Witwatersrand Basin, to enclosure of minute brannerite
crystals in acid-insoluble hosts, mainly phyllosilicates and leucoxene.

deposit. Our present knowledge of the ore mineralogy of
the reefs has been collated and reviewed by Feather andKoen (1975), who indicated certain areas where further
research is necessary.
With this in mind, in the present paper certain aspectsof mineralization are considered in detail; inter alia,
uranium mineralization and the temperature to which
the reefs were subjected during the major period ofmetamorphic activity that they underwent are dis­cussed.
Chemical compositions were determined with a JEOL
JXA-5A Electron Probe X-ray microanalyser using
pure metals, synthetic compounds, and analysedminerals as standards. Data refinement was achieved by
the computer program of Duncumb and Jones (1969).
Iron-filtered CoKa radiation was used for all 57.3 mm
Debye-Scherrer camera X-ray diffraction studies.
Samples of all the economically important reefs of the
Witwatersrand stratigraphic sequence were examined.
Polished sections of reef and heavy concentrates wereprepared. In addition, a large number of mine recoveryplant products were examined.
TEMPERATURES OF METAMORPHIC ACTIVITYINTRODUCTION—THE MODIFIED PLACER THEORY

It is not intended that the arguments that were put
forward for a hydrothermal (for example, Davidson,1960) or a placer origin for the mineralization of the con­INTRODUCTION
glomerates be discussed here. Recent evaluations by
In recent years, mainly as a result of the development Pretorius (1974a, b, 1975), among others, especiallyof sophisticated instrumentation such as the scanning with regard to the sedimentology of the Witwatersrand
electron microscope and electron microprobe, consid­ Basin, clearly support a placer origin for the mineraliza­
erable advances have been made in our understanding of tion of the reefs. In brief, it is believed that deposition
the processes responsible for the mineralization of the took place along the interface between a fluvial systemconglomerate reefs of the Precambrian Witwatersrand that brought sediments and heavy minerals from anelevated source area to the northwest of the basin and alacustrine littoral system that reworked the material
1 Senior Research Fellow, Anglo American Research Laboratories, P.O. Box 106, Crown
Mines, Transvaal, South Africa 2025
and redistributed the finer sediments along the shoreQi

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
Q2line of an ancient inland sea. The heavy minerals were TABLE 1.—Composition, in percent, of sphalerite in the Witwatersrand
reefs
concentrated by gravity settling and subsequent win­
nowing by wave and current action. Some investigators, [Names in parentheses are the abbreviated names of gold mines, as used in all the tables. The
such as Hallbauer (1975) and Riemer (1975), support the
numbers in parentheses refer to sample numbers of the Witwatersrand collection of the
X-ray Fluorescence Section, Anglo American Research Laboratories]
view that some of the gold and uranium was carried into
the basin in solution and deposited biochemically by an
Total
Fe
Grain
Sampleancient lichenlike life form that preferentially developed
33.0
98.5
64.3
1.2
SI
in quieter water conditions at the mouth of major rivers Vaal Reef (Vaal Reefs) (25) _
98.7
1.3
33.3
64.1
S2
99.8
1.3
33.1
65.4
S3
at the end of cycles of sedimentation.
1.1
32.6
98.5
64.8
S4
There is little doubt that some of these ore minerals,
99.6
65.4
1.2
33.0
S5
notably the gold, base-metal sulphides and arsenides,
98.8
1.1
32.9
64.8
S6
1.2
33.8 100.0
65.0
S7and uraninite, underwent subsequent alteration and
99.9
.9
33.0
S8
66.0
redistribution in situ in the reefs after burial during at
1.1
32.8
98.5
64.6
S9
least one major epoch of metamorphic activity. The date
1.2
33.1
99.2
64.9
Average ____________
7.3
33.0 101.0
S10
60.7
of this epoch is set by the age of the uranium-bearing Ventersdorp Contact Reef
(Western Deep Levels)
minerals, which would have been purged of their(32).
radiogenic lead at this time.
3.7
34.0 100.0
Basal Reef (Welkom) (184).
511
62.3
3.4
34.0 100.0512
62.6This is the modified placer theory, which enjoys a fair
32.6 100.0
3.8
513
63.6
degree of consensus, especially among those who are
99.9
3.5
33.5
514
62.9
99.8
62.2
3.9
33.7
515
familiar with the geology of the basin through mining
3.7
33.6 100.0
Average62.7
activities.
PREVIOUS WORK

Fuller (1958), using the method of Kullerud (1953), in
which the iron content of sphalerite is used as a geothermometer to determine the temperature at the time ofdeposition, obtained an average temperature of 450°C
for undoubtably authigenic sphalerite in samples fromthe Bird Reefs Group taken just south of Krugersdorp
on the Witpoortjie fault. Similarly, von Rahden (1964)
obtained a temperature of 400°C on a grain ofauthigenic sphalerite from the Bird Reefs Group at
Luipaardsvlei gold mine.
Barton and Kullerud (1958), however, point out that
this method gives only a minimum temperature of for­mation, and therefore the temperatures in the reefs may
have been still greater than those indicated. However,
the iron content has been shown to be not only depend­
ent upon temperature. This is borne out by the fact thatthe iron content of secondary sphalerite in the reefs
varies from 1 to 7 percent (table 1), even when second­ary pyrite and pyrrhotite are abundantly present.
The presence of various other secondary minerals in
the reefs has been cited as giving information as regards
temperature of formation. Pyrrhotite, which occurs
sporadically throughout the reefs, occurs in occasional
abundance in the vicinity of major dykes. Most in­vestigators believe it to have been derived from pyrrhotitization of primary pyrite. Exsolved pentlandite
lamellae are common, and both monoclinic and hex­
agonal pyrrhotite occur in the reefs. Both these facts areindicative of fairly high temperatures of pyrrhotitization(Ramdohr, 1955; von Rahden, 1964). Similarly, the.

presence of chloritoid replacing sericite in the matrix ofthe reefs has been considered by many (for example,
Ramdohr, 1955) to be an indicator of high temperatureand pressure conditions. Mackinawite, although very
rare, must have been crystallized at a minimum
temperature of 210-250°C, as cited by Schidlowski(1968). Graphite, occurring as bent and broken flakes,
apparently formed in situ as a result of graphitization ofcarbon (Saager, 1969; Schidlowski, 1967). X-ray diffractometric analyses of columnar and massive carbon,
believed to be the fossil remains of an ancient lichenlike
plant, frequently shows signs of incipient graphitization(Feather and Koen, 1975), indicating that fairly
widespread elevated temperatures were achieved dur­
ing metamorphism.
These observations are, however, of a qualitative
nature. Consequently, in the present investigation,
quantitative information was sought of a more suitablemineral assemblage, notably of the cobaltite andgersdorffite, which are widespread in the reefs.
THE EVIDENCE OF COBALTITE AND GERSDORFFITE

The cobaltite-gersdorffite grains are of two distinct
generations, allogenic and authigenic. The allogenic
grains, all with cobalt>nickel>iron—that is,cobaltite—are well rounded (fig. 1), may contain pyr­rhotite inclusions, and have undergone little or noalteration since burial in the reefs.

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALSQ3

FIGURE 1.—Typically rounded detrital cobaltite grains, two with somewhat rare pyrrhotite inclusions. This type of cobaltite is associated very
rarely with gold. Scanning secondary electron photomicrographs, x250. Cristaalkop Reef, Vaal Reefs Mining and Exploration Co., Ltd.

Electron-microprobe analyses of a large number ofgrains are plotted in the ternary diagram FeAsSCoAsS-NiAsS (table 2; fig. 2). Superimposed on thisdiagram are the chemical isotherms of mixed crystal for­mation of Klemm (1965). Different symbols are used to
identify each reef. The temperatures at which the grains
formed vary widely over the range of<300°C to about
600°C, and the grains may have been derived from one
or more possible hydrothermal deposits situated in the
primary source rocks to the northwest.
Further evidence may be derived from the crystal
structure of the cobaltite. X-ray diffraction analyses of alarge number of grains of varying composition con­sistently gave the Pca2i space group of the orthorhom-bic system. According to Giese and Kerr (1965) andGammon (1966), this is the low temperature ordered
crystal form of cobaltite, having crystallized attemperatures below about 800°C.
The second (authigenic) (Ni, Co) sulpharsenides (fig. 3)
are more variable in chemical composition (table 3; fig.
4). The majority have Ni>Co>Fe and thus may beclassified as gersdorffite. Indicated temperatures of for­mation of individual grains (table 3; fig. 4) vary within
the individual reef samples, and in the Witwatersrand
System as a whole, over the range of about 400-600°C.
Variation within reefs is explained by slow and zoned
crystal growth. Although not always present, the com­positions for a particular reef sample may be plotted

Q4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 2.—Electron-microprobe analyses, in percent, ofdetrital cobaltite grains in some Witwatersrand reefs[The numbers in parentheses refer to sample numbers of the Witwatersrand collection of the X-ray Fluorescence Section, Anglo-American Research Laboratories]
Atomic proportions (based on cations)
Grain

Ni

Co

Total

NiAsS

CoAsS

FeAsS

98.4
98.1
96.9

0.268
.091
.105

0.630
.811
.773

0.101
.098
.122

Vaal Reef, Vaal Reefs (North Section) (5)
4.6
44.5
20.3
101.7
3.3
44.2
20.1
101.7
3.5
43.1
19.8
100.1

0.138
.091
.223

0.734
.818
.680

0.129
.091
.097

Fe

As

S

Main Reef, South African Lands (150)
A63
A70
A75

8.2
2.8
3.4

19.3
24.9
25.0

A103
A104
A105

5.1
3.4
8.3

27.2
30.7
25.4

3.0
2.9
3.8

48.3
47.5
40.3

19.6
20.0
24.4

Vaal Reef, Vaal Reefs (186)Cl
C2C3
C4 ___
C5

2.0
9.5
12.1
8.9
8.3

31.9
19.8
17.0
21.3
19.2

2.0
4.5
6.9
6.1
6.1

45.5
45.1
44.2
44.3
45.2

18.9
19.2
19.8
19.3
19.7

100.3
98.1
100.0
101.2
98.5

0.056
.280
.334
.244
.246

0.887
.583
.469
.583
.567

0.058
.137
.197
.173
.187

C6
C7
C8
C9
CIO

15.0
10.5
7.2
7.2
9.4

14.6
21.5
25.9
25.3
22.7

6.6
3.9
2.9
3.7
3.5

44.1
44.8
45.0
44.6
44.2

18.9
18.6
18.8
19.1
19.3

99.2
99.3
99.8
99.9
99.1

.412
.292
.200
.199
.264

.400
.596
.717
.696
.635

.187
.112
.083
.105
.101

99.6
99.1
103.7

0.105
.099
.083

0.835
.802
.832

0.060
.099
.085

103.3
102.5
102.3
103.1
100.7
102.5
101.5

0.190
.338
.151
.222
.157
.161
.238

0.635
.525
.768
.624
.695
.760
.702

0.175
.137
.081
.154
.148
.079
.061

Leader Reef, Free State Geduld (200)
Alll
A112
A114

3.8
3.5
3.5

30.2
28.5
35.4

2.1
3.4
3.5

44.0
44.5
41.6

19.5
19.2
19.7

Cristaalkop Reef, Vaal Reefs (171)
A33
A37
A38
A41
A43
A46
A48

7.2
11.5
5.2
7.6
5.5
5.7
8.1

24.1
17.9
26.5
21.4
24.4
27.0
24.0

6.4
4.5
2.7
5.1
5.0
2.7
2.0

47.0
49.3
48.1
50.2
45.2
48.0
48.9

18.6
19.3
19.8
18.8
20.6
19.1
18.5

Carbon Leader, West Driefontein (167)
A12
A13
A14
A16
A17
A18

4.0
8.7
4.3
8.9
4.4
2.9

23.5
21.9
26.2
20.5
28.3
28.9

4.0
2.4
3.3
3.6
1.2
2.4

41.2
44.7
44.1
44.5
45.1
44.5

21.3
20.0
20.7
20.0
20.0
20.3

94.0
97.7
98.6
97.5
99.0
99.0

0.127
.263
.127
.269
.129
.085

0.742
.662
.772
.618
.826
.843

0.131
.075
.101
.113
.045
.073

A20
A21 _
A22
A23
A24
A28

5.0
8.1
10.3
9.7
1.6
8.5

30.2
26.8
23.0
17.7
32.8
19.2

1.9
2.8
4.5
7.0
2.6
6.7

44.8
46.0
46.3
43.0
43.2
44.0

19.7
19.7
19.5
20.6
20.3
18.7

101.6
103.4
103.6
98.0
100.5
97.1

.135
.215
.272
.281
.043
.246

.812
.709
.606
.511
.884
.554

.053
.077
.123
.209
.073
.200

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALSQ5TABLE 2.—Electron-microprobe analyses, in percent, ofdetrital cobaltite grains in some Witwatersrand reefs—Continued
Atomic proportions (based on cations)
Grain

Co

As

Total

NiAsS

CoAsS

FeAsS

Carbon Leader, Doornfontein (169)
A82
A85
A86
A87
A90

8.8
5.8
6.6
3.5
7.3

17.5
26.7
25.3
27.2
24.7

6.9
1.6
2.7
5.4
3.7

43.8
45.7
44.9
42.3
47.0

21.7
19.7
19.5
21.7
19.4

98.7
99.5
99.0
100.1
102.0

0.264
.170
.191
.097
.204

0.523
.781
.729
.749
.689

0.214
.049
.081
.154
.107

A93
A94
A95
A96
A97
A99

4.6
8.0
6.0
6.5
2.5
7.0

29.4
19.8
27.2
22.8
30.8
22.8

2.3
7.1
3.1
5.4
3.1
5.7

47.8
46.0
46.7
45.3
47.1
47.1

19.3
18.7
18.8
19.5
19.1
18.3

103.4
99.6
101.8
99.5
102.6
100.9

.167
.228
.165
.187
.069
.154

.808
.563
.747
.653
.844
.717

.065
.209
.088
.160
.088
.129

0.065
.017
.023
.284
.205
.062
.128

0.890
.928
.934
.581
.745
.876
.835

0.046
.055
.042
.135
.051
.062
.037

Ventersdorp Contact Reef, West Driefontein (168)
A53 ___________
A55 _
A56
A57
A58
A59
A51

2.2
.6
.8
9.8
7.1
2.2
4.3

30.4
33.1
32.1
20.1
25.9
31.0
28.1

FeAsS

45.4
44.5
47.2
46.4
46.8
47.2
45.8

1.5
1.9
1.4
4.5
1.7
2.1
1.2

EXPLANATION
Main Reef 150
Leader Reef 200
Vaal Reef 186
Vaal Reef 5
Cristaalkop Reef 171
Carbon Leader 167
Carbon Leader 169
Ventersdorp Contact
Reef 168

CoAsS

NiAsS

FIGURE 2.—Electron microprobe analyses of Witwatersrand cobaltite
grains of detrital origin plotted in the diagram FeAsS-CoAsS-NiAsS.
Superimposed on the diagram are the experimentally determinedchemical isotherms of mixed crystal formation of Klemm (1965), in­dicating that temperatures of crystallization in the source rocks
ranged from< 300°C to about 600°C. This wide range is indicated by
the reefs as a whole and even by individual reefs suggesting multiple
sources for detrital grains. Sample numbers in explanation from the
Witwatersrand Collection of the X-ray Fluorescence Section, Anglo
American Research Laboratories, South Africa.

20.0
21.4
19.7
19.8
19.4
19.6
19.8

99.5
101.5
101.2
100.6
100.9
102.1
99.2

along a line that extends toward pure NiAsS, shown by
dotted lines in figure 4. Zoned crystal growth is com­monly visible. For example, grain A 201 from the Basal
Reef has a core that is optically silver-white with agreyish tinge and a composition indicating atemperature of crystallization of 450°C. It is immediate­ly enclosed by a zone of higher nickel content, blueish incolour, which indicates a temperature of < 300°C. The
outermost zone is even more blueish, has a higher nickel
content, and thus was formed at an even lower
temperature.
The foregoing deductions are based on the premisethat cobalt, nickel, and iron were freely available in thereefs at the time of crystallization. The zoned structure
and crystallization trends (dotted lines in fig. 4) may also
be explained as due to depletion of cobalt and iron dur­
ing crystal growth. For this reason, in all cases, thetemperatures indicated must be regarded as minimum
temperatures of formation.
Additional information was sought in the crystal
structure of the authigenic gersdorffite. Both Pa3(pyrite) and P2x3 (ullmannite) space groups of the cubic
system are present. According to Bayliss and Stephenson (1967) and Bayliss (1969), these are the high and in­termediate temperature forms of gersdorffite and, as
may be deduced from figure 5, resulted from crystalliza­tion at temperatures in the range of 450-600°C. This isfurther confirmed by the fact that mixed crystals of theauthigenic sulpharsenide, with Co~Ni, gave the Pca2i
low temperature form of cobaltite, indicating that this

Q6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 3.—In contrast to detrital cobaltite, gersdorffite (G) occurs as an authigenic mineral, as irregular masses commonly intergrown with gold(A) and frequently associated with carbon (C). It may contain galena, chalcopyrite, and pyrrhotite, all these minerals being of common
paragenesis. These grains occur in the same reef sample as the cobaltite grains illustrated in figure 1. Scanning back-scattered electron
photomicrographs, x300. Cristaalkop Reef, Vaal Reefs Mining and Exploration Co., Ltd.

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALSQ7TABLE 3.— Electron microprobe analyses, in percent, of authigenic arsenosulphides of nickel, and cobalt, mainly gersdorjfite, in some Witwatersreefs[The numbers in parentheses refer to sample numbers of the Witwatersrand collection of the X-ray Fluorescence Section, Anglo-American Research Laboratories]
Atomic proportions (based on cations)_______________________
Grain

Total

Approximate
temperature (°C)

NiAsS

CoAsS

FeAsS

Grain

100.1
98.6
98.6

0.592
.698
.440

0.254
.233
.356

0.154
.069
.214

460
380
550

100.0
99.6
100.6
100.9
99.3
99.1
100.4
99.4
99.4

0.658
.718
.619
.606
.571
.673
.684
.583
.626

0.108
.131
.247
.214
.125
.212
.186
.232
.121

0.234
.151
.134
.180
.304
.114
.130
.185
.253

460
400
450
460
510
410
410
470
470

AverageforsampleVaal Reef, Vaal Reefs (North Section) (5)

A101
A102
A107

21.4
25.1
15.5

9.2
8.4
12.2

5.4
2.4
7.3

45.1
43.5
46.8

19.0
19.2
16.8

-460

Vaal Reef, Vaal Reefs (186)

N2
N3
N4
N5
N6
N8
N10
Nil
N12

23.0
25.6
23.0
22.3
19.6
23.7
25.0
20.5
21.7

3.8
4.7
9.2
7.9
4.3
7.5
6.8
8.2
4.2

7.9
5.2
4.8
6.4
10.1
3.9
4.6
6.3
8.5

45.0
44.7
44.4
44.6
45.5
45.5
45.0
45.6
45.8

20.8
19.4
19.2
19.7
19.8
18.5
19.0
18.8
19.2

——

450

Basal Reef, near dyke contact, Welkom (192)

Gl
G3
G4
G5
G6
G7

14.9
17.3
12.9
16.8
17.5
14.3

11.0
10.2
15.8
10.9
9.2
11.3

7.4
7.2
7.2
7.3
6.9
6.6

45.9
45.4
44.8
45.6
45.0
44.0

19.8
19.1
19.3
19.1
19.7
19.9

99.0
99.2
100.0
99.7
98.3
96.1

0.445
.495
.357
.477
.518
.441

0.327
.291
.437
.309
.271
.348

0.228
.213
.206.214
.211
.211

540
520
580
530
510
540

G8
G9
G10
Gil
G12
G13

13.4
10.2
10.7
17.3
17.8
17.4

13.9
13.1
18.3
10.2
9.6
10.3

6.9
6.7
6.2
7.9
7.8
7.7

45.7
45.5
45.6
44.9
45.9
46.0

18.2
20.0
19.3
19.9
19.7
19.5

98.1
95.5
100.1
100.2
100.8
100.9

.390
.338
.303
.485
.502
.488

.403
.433
.516
.286
.270
.288

.207
.229
.181
.229
.227
.223

570
590
600
530
520
530

G14
G16
G17
G18
G19
G20

15.0
17.8
18.1
15.9
18.6
18.5

10.6
10.3
9.6
11.1
9.4
10.1

6.8
7.8
7.2
7.5
7.3
8.5

45.8
44.6
44.7
46.0
44.0
42.8

19.0
20.0
19.7
19.3
19.4
19.8

97.2
100.5
99.3
99.8
98.7
99.7

.460
.493
.515
.458
.524
.495

.324
.284
.272
.319
.264
.270

.216
.223
.212
.223
.212
.235

540
520
510
540
505
520

99.4
99.3
100.5
100.7
103.1
102.6

0.600
.676
.425
.442
.544
.165

0.182
.126
.432
.382
.243
.686

0.219
.199
.143
.176
.213
.149

470
425
540
540
500
610

99.9
100.0
100.0

0.636
.856
.952

0.146
.068
.000

0.218
.076
.048

450
<300
<300

540

Leader Reef, Free State Geduld (200)

A113
A116
All?
A118
A119
A120

20.1
22.5
15.4
15.6
20.1
6.2

6.1
4.2
15.7
13.5
9.0
25.8

7.1
6.4
5.0
6.0
7.6
5.4

48.2
47.9
45.3
46.2
49.0
44.5

17.9
18.3
19.1
19.4
17.4
20.7

Zoned grain (A 201)

CenterIntermediate
Outside

23.5
30.1
34.6

5.4
2.4
<.l

7.8
2.6
1.7

42.9
46.2
43.7

20.3
18.7
20.0

——

510

Q8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 3. -Electron microprobe analyses, in percent, ofauthigenic arsenosulphides of nickel and cobalt, mainly gersdorffite, in some Witwatersrand reefs - Continued
Atomic proportions (based on cations)
Grain

Ni

Co

Total

Fe

Approximate
temperature (°C)

NiAsS

CoAsS

FeAsS

Grain

0.483
.404
.477
.605
.587
.440
.569
.663
.454

0.272
.438
.345
.142
.216
.427
.196
.122
.273

0.245
.158
.177
.253
.198
.133
.235
.214
.273

530
550
520
485
480
520
485
440
540

0.509
.502
.622

0.328
.378
.167

0.164
.121
.211

505
500
460

0.436
.526
.484
.544
.570
.481
.472

0.280
.442
.303
.299
.137
.302
.389

0.284
.032
.213
.158
.293
.217
.139

550
480
520
485
500
520
510

0.242
.091

0.156
.158

460
360

AverageforsampleCristaalkop Reef, Vaal Reefs (171)

A31
A36
A39
A40
A42
A44
A45
A47
A50

15.7
13.7
15.3
19.5
18.7
14.0
18.5
21.1
14.8

8.9
14.9
11.1
4.6
6.9
13.6
6.4
3.9
89

7.7
5.2
5.5
79
6.1
4.1
7.4
6.6
8.6

45.4
51.5
51.1
45.5
53.1
46.1
49.6
50.6
47.6

19 9
18.3
15.7

19 6

15.5
19 9
17.8
17 9

18.3

97.6
103.6
98 7
97.1
100.3
97.0
99.7
99.4
98.2

__

__
510

Carbon Leader, West Driefontein (167)

All
A15
A 19

16.7
16.7
20.1

10.8
12.6
5.4

5.2
3.9
6.6

45.8
44.9
47.5

19.0
19.7
17.6

97.5
97.8
97.2

490

Carbon Leader, Doornfontein (169)

A83
A84
A88
A89
A91
A98
A81

14.1
17.2
15.3
17.8
18.7
18.3
15.5

9.1
14.5
9.6
9.8
4.5
11.5
12.8

89
1.0
6.5
5.0
9.3
8.0
4.4

AC. q
45.0

47.8
47.2
46.7
48.1
45.1

20.0
20.1
17.5
19.0
19 4
18.8
20.7

98.0
97.8
96.7
98.8
98.6
104.7
98.5

__
__
__
__
510

Ventersdorp Contact Reef, West Driefontein (168)

A54
A50

20.4
25.5

8.2
3.1

5.1
5.2

46.0
46.8

cobaltite crystallized at temperatures of about 600 °C(see fig. 5).
To summarize, this study of the chemistry and crystal
structure of secondary gersdorffite-cobaltite has in­dicated that minimum temperatures in the range of
400-600°C, and probably closer to 600°C, have beenachieved in the Witwatersrand reefs during metamorphism of the reefs. Samples that were taken close to
dykes—for example, 192-Basal Reef (fig. 4)—indicatedonly slightly higher temperatures, about 500-600°C,
than did reef samples not associated with dykes—therest in figure 4, about 400-550°C. It appears, therefore,that these temperatures were widespread and not
localised to the vicinities of crosscutting dykes andrelated hydrothermal veins. However, a word of caution
should be noted here. The work of Klemm (1965) has notbeen independently and critically examined. In addition
to temperature, a variety of other factors might in­fluence the mixed crystal formation. However, the ex­cellent correlation with the temperatures indicated by

20.0
19.4

99.7
100.0

0.603
.751

410

the crystal structures does suggest that Klemm's
isotherms are applicable to the metamorphism of the
Witwatersrand System.
OPTICAL CHARACTERISTICS OF GERSDORFFITE

Witwatersrand gersdorffite is cubic, thus shows no
anisotropism or birefringence, and varies in colour fromsilver-white to white with a blueish tinge as nickel con­tent increases. The NiAsS end member is strongly
blueish and readily confused with skutterudite, which
has almost identical optical properties. There is addi­
tional chance of misidentification resulting fromdescriptions of gersdorffite in the literature. For exam­
ple, according to Uytenbogaardt and Burke (1971),
gersdorffite is white with a yellowish or pinkish tint.
In the past, a number of references to the presence ofskutterudite in the reefs have been made. Amongothers, Ramdohr (1955) found that the redistributed
gold was often associated with skutterudite, which is of

Q9

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS

about the same age, though sometimes distinctly
younger. Viljoen (1963) also noted the close associationof gold and skutterudite.
In the present investigation, skutterudite could not befound, despite an extensive search by electron
microprobe, and it is concluded that previous in­vestigators have confused gersdorffite with skut­
terudite. The descriptions that both these authors givefor skutterudite could apply equally to gersdorffite.

EXPLANATIONFeAsS* Leader Reef 200° Basal Reef 192" Vaal Reef 186
o Vaal Reef 5* Cristaalkop Reef 171* Carbon Leader 167« Carbon Leader 169* Ventersdorp Contact
Reef 168650

URANIUM-BEARING MINERALS

NiAsS

CoAsS

FIGURE 4.—Electron microprobe analyses of authigenic sulpharsenide
grains (mainly gersdorffite) plotted in the diagram FeAsS-CoAsSNiAsS. Superimposed on the diagram are the experimentally deter­
mined chemical isotherms (°C) of Klemm (1965), which indicate thelimits of mixed crystal growth. The analyses indicate that minimum
temperatures in the range of 400-600 °C were present in the reefs
during the period of metamorphism that remobilised the gold andcrystallized the secondary sulphides, sulpharsenides, and arsenides.
Sample numbers in the explanation from the Witwatersrand Collec­
tion of the X-ray Fluorescence Section, Anglo American ResearchLaboratories, South Africa.

800 r3/C

700
c/D
=>
c/D 600_i

A/r I'tffefo,

The important uranium-bearing minerals in the Wit­
watersrand and related reefs are uraninite and thealteration product of uraninite. Lesser uranium-bearing
minerals include zircon, monazite, columbite, betafite,uranothorite, euxenite, and xenotime.
In this section, the result of an extensive electronmicroprobe examination of these minerals is presented,
intending to complement the findings of previous in­vestigators, to present new information, and notably topresent quantitative microprobe analyses of the impor­tant uranium-bearing minerals.
Evidence is presented to confirm that the alteration
product of uraninite is brannerite. Particular attention
is drawn to this mineral, as it may be the main uraniumbearing mineral in the reefs, especially in the Vaal andBasal Reefs.

Disordered
Pa3

01

0

iscc. 5°° Ordered
LU

Q

400

£ 300

=>

Pea 2
200

100

CoAsS

NiAsS

FIGURE 5.—Sketch relating crystal structure ordering with temperature for the system CoAsS-NiAsS (atomic proportions) (after Bayliss, 1969). The
structures of authigenic sulpharsenides in the Witwatersrand indicate that temperatures in the range of 450-600 °C were attained duringmetamorphism of the reefs, in agreement with the temperatures of mixed crystal formation given in figure 4.

Q10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESURANINITEINTRODUCTION

Cooper (1923) was the first to record the presence ofuraninite in the reefs. Ramdohr (1955) and Liebenberg(1955) published the first detailed studies of the mineral,
which, in general, investigators have regarded as being
essentially of detrital origin. It invariably occurs ingreatest concentration close to the footwalls of thereefs, or on the false footwalls (Ramdohr, 1955) within
them, in apparent hydraulic equilibrium with otherheavy minerals that are definitely of detrital origin.
The grains have a limited size range. Liebenberg(1955) records that the average diameters range from
75-lOOjLtm. They contain abundant galena inclusions andmay enclose gold, gersdorffite, pyrrhotite, andchalcopyrite.
The grains are commonly rounded, tending to be ovalin shape (Liebenberg, 1955), and are surrounded byphyllosilicates, mainly sericite. Euhedral and subhedralgrains, however, are not uncommon. During the present
investigation, fractured sections of the reef (simply reef
samples that were broken open and the freshly exposed
surfaces studied) revealed the presence of uraninite
grains that were often bounded by some or all crystal
faces, as in figure 6. This might be expected from thefact that the grains were recrystallized and were purgedof radiogenic lead during metamorphism. Roundedgrains were invariably found to be accompanied bytraces of minute crystals of an alteration product.
It is suggested, therefore, that the rounding is due topartial chemical dissolution rather than detrital rounding.
Uraninite also occurs as minute anhedral andsubhedral inclusions in carbon. The carbon is believed tobe the fossil remains of an ancient lichenlike plant(Hallbauer, 1975). Thus two distinctly different genera­tions of uraninite, which will be referred to as free andincluded, respectively, occur in the reefs, but eachgeneration is remarkably uniform in texture throughout
the reefs.
THE UO2 /ThO2 RATIO

Whereas most investigators of the reefs support a
detrital origin for the uraninite not associated with car­bon, some, such as Koen (1961) and Riemer (1975), havesuggested that the uraninite may have precipitated fromsolution, aided by biochemical activity of the "lichen" orsimilar organisms.
Hallbauer and van Warmelo (1974) have pointed outthat the "lichen" has certainly absorbed both uranium
and gold, and other inorganic materials, during its

FIGURE 6.-Uraninite grains frequently display one or more crystal
faces, and euhedral grains like the one illustrated here, exposed on
the face of a fractured section of reef, are not as rare as believed byprevious investigators. Scanning secondary electron photo­
micrograph, x600. Basal Reef, President Brand Gold Mining Co.,Ltd.

growth. The question arises, however, as to whether the"lichen" "digested" detrital matter or whether it actuallyabsorbed these elements from solution. It is a common
observation that the carbon, the fossil form of the"lichen," replaces the uraninite grains partially or
almost wholly.
Recrystallization during metamorphic activity has
destroyed all the primary features of the uraninite. As
noted above, the present state of rounding of the grains
is probably due to partial dissolution rather than attri­tion, and evidence of crystallization as colloidal pitch­
blende—for example, radial textures and cracks—is alsoabsent.
During the progress of this investigation, Grandstaff(1974) published electron-microprobe analyses of freeuraninite grains in a sample of reef from Vaal Reefs
West mine. In common with the present author's find­ings at the time, Grandstaff found that although the in­dividual grains were fairly homogeneous, the overall
U02/Th02 ratio of grains varied vastly in the assemblageof uraninite grains in the reef sample. He concluded thatsuch heterogeneity favours a detrital, placer origin for
the uraninite, which has been derived from a number ofASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS

primary sources. He points out that the Witwatersrandgrains contain appreciable thorium (up to 10 percent
Th02), in common with a pyrogenetic (granitic or
pegmatitic) source, rather than low-temperature,hydrothermally derived uraninite, which is normallyvery low in thorium content.
In the present investigation, uraninite from a numberof reefs was analysed for uranium, thorium, lead, iron,calcium, and titanium by electron microprobe, using, asstandards, sintered U02 and thorium (prepared by the
Atomic Energy Board, Pelindaba), lead-metal, andanalysed hematite, wollastonite, and rutile. The results
are summarised in tables 4 and 5, and in figure 7 theU02/Th02 ratios of the grains are plotted against Th02
content. The data for free uraninite and uraninite in­cluded in carbon are plotted separately.

Qll

Within all the reefs examined great variation in the
U02/Th02 ratio occurs in the free uraninite, favouring,in each reef, a multiple primary source for the uraninite,in keeping with Grandstaff s (1974) findings. In addition,a similar variation occurs in the uraninite enclosed
within the carbon. Although it may be possible that the
U02/Th02 contents of the water that came into contact
with the "lichen" varied so vastly, it seems more likelythat the "lichen" "digested" detrital grains, hence thelarge and similar variation in U02/Th02 ratio.
The overall compositions of the free and included
uraninites are very similar—compare tables 4 and5—but before it could be proved that they are definitely
from a common source, variations in all elements withineach reef would have to be determined and the data
would have to be treated statistically.

TABLE 4.—Results, in percent, of the electron-microprobe analysis ofuraninite not associated with carbon in some Witwatersrand ores
[The numbers in parentheses refer to sample numbers of the Witwatersrand collection of the X-ray Fluorescence Section, Anglo-American Research Laboratories]
SourceGrain

Cristaalkop Reef (Vaal Reefs South) T21(171).
T22
T23
T24
T25
T27
T28
T29
T30
Average
Carbon Leader (Western Deep
T13
Levels) (135).
T14
T15
T16
T17
T18
T19
Average
Carbon Leader (West Driefontein) B43(167).
B44
B46 ____
B47
B48
B49
B52
Average
Main Reef (South Africa Lands)
B31(151).
B32
B36
B37 ____
Average
Basal Reef (Welkom) (184)
_U4
U5
U7
U8
U9
U10
Average
Overall Average

U02

Th02

Pb02

FeO

Ti02

CaO

Total

U02/Th02
ratio

65.8
66.5
70.5
62.4
63.0
66.6
66.3
63.5
65.7
65.6
69.6
66.4
63.8
62.6
67.1
69.2
67.9
66.7
61.1
71.3
68.2
70.0
68.2
67.3
67.6
68.1
68.7
69.1
67.3
63.5
67.2
68.2
70.5
70.1
66.6
72.5
64.8
68.8
67.2

5.3
6.1
1.7
6.1
8.0
3.2
1.4
3.9
10.2
5.1
2.7
2.5
2.0
7.0
5.2
2.1
7.2
4.1
5.4
2.6
4.3
2.1
3.5
6.4
9.2
4.2
3.3
2.1
1.5
2.9
2.5
6.3
3.3
5.2
4.5
2.6
3.3
4.2
3.9

23.3
21.9
23.8
24.7
23.0
23.3
28.0
28.5
18.9
23.9
26.1
27.8
30.3
24.5
21.1
28.0
18.3
25.2
27.9
24.2
28.2
23.6
24.7
21.6
22.8
24.7
14.7
21.0
24.4
17.3
19.4
19.6
16.8
24.8
25.2
26.1
23.7
22.7
23.6

0.8
.5
.5
.4
.5
1.0
.6
.6
.7
.6
0.2
.2
.2
.2
.2
.2
.2
.2
1.1
.6
.4
.4
.5
.4
.9
.6
2.3
1.1
.9
5.0
2.3
2.4
4.1
1.1
.9
1.6
5.2
2.6
1.0

<0.01
.02
.04
<.01
<.01
.04
.02
<.01
.08
.02
0.08
.12
.06
.04
.06
.10
.06
.07
0.25
.25
.10
.10
.12
.16
.14
.16
1.20
.50
.19
2.03
.98
<0.1
.2
.1
<.l
<.l
.2
.1
.16

0.6
.7
.8
1.1
1.0
.9
.9
.8
1.1
.9
1.0
.7
.7
.8
.7
.7
.6
.7
0.4
.6
.5
.6
.3
.5
.3
.5
0.4
.4
.3
.5
.4
0.4
.4
.5
.3
.5
.5
.4
.6

95.8
95.7
97.3
94.7
95.5
95.0
97.2
97.3
96.7
96.1
99.7
97.7
97.1
95.1
94.4
100.3
94.3
97.0
96.2
99.6
101.7
96.8
97.3
96.4
100.9
98.3
90.6
94.2
94.6
91.2
92.8
96.9
95.3
101.8
97.5
103.3
97.7
98.7
96.8

12.4
10.9
41.5
10.2
7.9
20.8
47.4
16.3
6.4
12.9
25.8
26.6
31.9
8.9
12.9
33.0
9.4
16.3
11.3
27.4
15.9
33.3
19.5
10.5
7.3
16.2
20.8
32.9
44.9
21.9
26.9
10.8
21.4
13.5
14.8
27.9
19.6
16.4
17.2

Q12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 5.—Electron-microprobe analysis, in percent, of uraninite inclusions in selected samples of uraniferous carbon[The numbers in parentheses refer to sample numbers of the Witwatersrand collection of the X-ray Fluorescence Section, Anglo-American Research Laboratories]
Source

Grain

U02

Th02

Pb02

FeO

Ti02

CaO

Total

U02/Th02
ratio

Vaal Reef (Vaal Reefs North) (25) __LI ____
66.0
2.4
28.0
0.1
0.1
0.2
96.8
27.5
L2 ____
74.0
3.2
18.0
.2
.2
.7
96.3
23.2
L6 ____
71.5
1.0
20.1
.2
.1
.6
93.5
71.5
L7 ____
76.6
2.5
13.6
.3
.1
.6
93.7
30.6
L8 ____
70.0
6.2
18.7
.3
.3
.5
96.0
11.3
L9 ____
68.1
2.7
24.2
.3
.1
.6
96.0
25.2
L10 ___
67.0_______4.4
22.2_____.2_______.3_______.7
94.8
15.2
Average ______________ __________70.5_____3.2____20.7______.2_______.2_______.6____95.4____22.0
Carbon Leader (Western Deep
Tl ____
69.7
2.6
19.7
0.4
0.3
1.0
93.7
26.8
Levels) (34).
T3 ____
66.4
.9
26.8
.3
.1
.5
95.0
73.8
T4____
65.2
7.8
18.8
.2
.4
.8
93.2
8.4
T5____
68.1
1.8
24.6
.3
.2
.7
95.7
37.8
T6____
70.0
1.5
23.4
.3
.3
1.0
96.5
46.7
T7____
65.8
4.4
22.9
.2
.2
.5
94.0
15.0
T8____
71.4
2.4
22.9
.2
.4
.7
98.0
29.8
T9____
69.1
3.7
18.7
.4
.2
.4
92.5
18.7
TIP ____
69.4_____4.7
17.7_____.1_____A________.4
92.7
14.8
Average ________________________68.3_____3.3
21.7_____.3________.3_______.7
94.6
20.7
Carbon Leader (Doornfontein)
B53 ___
71.2
1.3
24.7
0.6
0.2
0.5
98.5
54.8
(169).
B55 ___
71.2
1.3
24.6
.4
.3
.7
98.5
54.8
B56 ___
70.8
1.5
24.8
.6
.1
.6
98.4
47.2
B57 ___
73.0
3.2
19.1
.4
.6
.7
97.0
22.8
B58 ___
75.8
8.6
9.8
.6
.7
.8
96.3
8.8
B59 ___
69.0
8.3
19.5
.5
.3
.7
98.3
8.0
B60 ___
71.4
2.0
23.6
.3
.2
.6
98.1
35.7
B65 ___
69.7_____.4
26.1_____LI_______.1______.6
98.0
174.3
Average ________________________71.5_____3.3
21.5_____.6______.3_____.7
97.9
21.7
Basal Reef (Welkom) (184) ______U17 ___
63.8
1.7
23.9
0.4
<0.1
0.3
90.1
37.5
U20 ___
67.0
6.7
23.6
.3
<.l
.3
97.9
10.0
U23 ___
70.2
2.0
28.1
1.2
<.l
.4
101.9
35.1
U24 ___
68.6
2.2
29.8
.8
<.l
.7
102.1
31.2
U25 ___
68.6
2.5
30.4
1.3
<.l
.5
103.3
27.4
U26 ___
71.0
2.5
27.5
.9
<.l
.6
102.5
28.4
U30 ___
63.8
2.3
27.2
1.1
<.l
.5
94.9
27.7
U31 ___
71.6_____2.4
27.9______.9______<.!_____.3
103.1
29.8
Average ________________________68.1_____2.8
27.3________.9______<.l_____.5
99.6
24.3
21.7
Overall Average _______ ______
69.5
3.2
.2
.7
96.9
.5

Although the evidence in favour of a detrital source
for the uraninite appears to be most convincing, it is dif­ficult to explain why only the well-mineralized reefs con­tain the bulk of the uraninite. There are numerous con­glomerate and sandstone layers in the succession that
may contain other typical detrital grains, for example,
chromite and zircon, but that contain neither uraninite
nor gold. In addition, in the reefs, uraninite is much
more abundant than zircon by several tens of times. Thisis contrary to what might be expected from granitic or
pegmatitic source rocks, which are expected to havecontained more zircon than uraninite.
The mineralized reefs do contain one common ingre­
dient—that is, carbon—that is virtually absent in thebarren sediments. Despite the foregoing evidence itdoes seem possible that the ancient "lichen" played an
obscure role in aiding the concentration of both gold anduranium. However, it may be argued that the presenceof detrital uraninite was essential to the "lichen's" origin
and development.
BRANNERITEINTRODUCTION

As previously noted, the free uraninite grains are in­variably partially dissolved and altered. Both Ramdohr(1955) and Liebenberg (1955) noted the presence of analteration product. Liebenberg distinguished between
two products: (1) a common microcrystalline, relatively
homogeneous phase somewhat similar in appearance toparent uraninite, which he called "secondary uraninite,"and (2) uraniferous leucoxene. The "secondary
uraninite" forms encrustations (fig. 8) on or may partial-

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS

Q13

Tl

80

(174.3) B
+
70 -

**

468

468

ThO2, IN PERCENT

Th02, IN PERCENTA

B

EXPLANATION
Main Reef 151
Basal Reef 184
Vaal Reef 25
Cristaalkop Reef 171

12

e Carbon Leader 34
o Carbon Leader 135* Carbon Leader 167
x Carbon Leader 169

FIGURE 7.-Variation of U02/Th02 ratios. A, In free grains of uraninite. B, In uraninite associated with carbon. Grandstaff (1974) believed that suchvariation could result only if the uraninite were from multiple sources and suggested that it is of detrital origin, having been derived from anumber of granitic or pegmatitic bodies. The variation in carbon-included uraninite supports the theory that the "lichen," fossilised as carbon,"digested" detrital uraninite grains rather than absorbed these elements from solution. Sample numbers in the explanation, from the Witwatersrand collection of the X-ray Fluorescence Section, Anglo American Research Laboratories, South Africa.

ly or wholly replace grains. Ramdohr called this material"ghosts," since the outline of the parent grain is fre­quently discernible in the crystal orientation of thealtered material. Both "secondary uraninite" anduraniferous leucoxene may also be seen infiltratingneighbouring quartz, sericite, clay, and sulphide grains(fig. 9).
When it was established that the "secondary
uraninite" contained abundant titanium, it was
speculated (for example, Davidson, 1957; Davidson,
1960; Jacob, 1966; Schidlowski, 1966) that it might be
the uranium titanate mineral called brannerite, derived
from the combined reaction of uraninite and ilmenite ortitaniferous magnetite. Pyritization of ilmenites is abun­
dantly evident in the reefs, and titanium would havebeen freely available for reaction. Although Jacob (1966)

obtained a refractive-index value that was similar tothat of brannerite, attempts to identify positively themineral by X-ray diffraction failed, even when heating
experiments were carried out (von Rahden and
Hiemstra, 1967; Mihalik, 1968; von Rahden, 1970).
ELECTRONMICROPROBE

AND

X RAY

DIFFRACTIONINVESTIGATION

Using back-scattered electron images, the mineral ap­
pears to be homogeneous, despite abundant galena in­clusions. A number of grains were analysed for uranium,
thorium, lead, iron, titanium, calcium, and silicon, andthe results are cited in table 6. As in the uraninite
analyses (tables 4, 5), the elements are expressed as ox­
ides. The low totals probably result from metamictiza-

Q14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 6.—Results of electron-microprobe analysis, in percent, ofbrannerite in some Witwatersrand reefs and recovery-plant samples[Ideal U02 • 2Ti02 gives U02/Ti02 = 1.69 and U/Ti = 2.48. X-ray diffraction analysis of grain B6 is given in table 7. n.d., not determined. The numbers in parentheses refer to sample numbers of the
Witwatersrand collection of the X-ray Fluorescence Section, Anglo-American Research Laboratories]
Source

Grain

U02

Th02

Pb02

Ti02

FeO

CaO

Si02

Total

U02/Ti02
ratio

U/Ti
ratio

U02/Th02
ratio

A. Recovery-plant samples

Head Sample 1 (Vaal
Reefs).

Bl __
B3 __
B4 ____
B5 ___

Average
Head Sample 2 (Vaal
Reefs).

B6 __

40.1
37.0
42.2
26.6

3.1
4.5
4.6
2.3

0.3
1.0
8.1
.3

0.9

1.3
.8
1.6

36.5
42.6
33.9
45.1

0.6
.5
.6
.3

8.0
12.1
8.0
13.7

89.5
99.0
98.2
89.9

1.10
.87
1.24
.59

1.6
1.3
1.8
.9

12.9
8.2
9.2
11.6

36.5

3.6

2.4

1.2

39.5

0.5

10.5

94.2

0.92

1.4

10.1

33.6

5.1

14.1

1.0

34.8

0.4

6.4

95.4

0.97

1.5

6.6

B. Rock samples

Leader Reef (FreeState Geduld) (134).

B21 __
B22 __
B23 __
B24 __
B25 __
B26 __

Average
Vaal Reef 1 (Vaal
Reefs) (72).

B7 __B8 ____
BIO __
B14 __
B16 ___

Average
Vaal Reef 2 (Vaal
Reefs) (25).

Ul ____
U3 ____
U5 ____
U9 ____
U10 ___

Average
Carbon Leader(Western Deep
Levels) (74).

U15 ___
U18 ___
U19 __
U20 __

Average
Main Reef (South
Africa Lands) (151).

Average
Overall
Average

B27 __
B28 ___
B29 __
B33 ___
B35 ___

33.6
33.1
38.4
35.0
31.3
43.6

1.3
2.5
2.5
3.1
4.6
3.7

9.0
4.7
22.1
4.8
2.3
6.5

1.1
1.6
<.l

.7
1.0
3.5

45.2
46.2
32.6
25.0
31.7
11.5

0.2
.4
<.l
.6
.2
.6

4.7
6.9
1.0
9.6
17.3
11.3

95.1
95.4
96.6
78.8
88.4
80.7

0.74
.73
1.18
1.40
.99
3.79

1.1
1.1
1.8
2.1
1.5
5.8

25.8
13.2
15.4
11.3
6.8
11.8

35.8

3.0

8.2

1.3

32.0

0.3

8.5

89.1

1.12

1.7

11.9

32.5
27.2
31.9
26.4
25.2

3.4
2.0
.9
2.8
3.1

10.7
11.4
29.2
11.8
50.4

1.1
7.1
2.3
2.2
6.2

37.3
24.8
18.7
45.5
15.4

1.0
.5
1.1
.2
.3

5.7
11.7
7.8
8.7
4.7

91.7
84.7
91.9
97.6
102.3

0.87
1.10
1.71
.58
1.64

1.3
1.7
2.6
.9
2.5

9.6
13.6
35.4
9.4
8.1

28.6

2.4

22.7

3.8

28.3

0.6

7.7

94.1

1.01

1.5

11.9

26.2
39.9
38.3
34.4
31.2

0.9
3.2
1.8
5.4
3.0

17.0
37.8
32.9
10.9
8.8

14.1
11.3
14.3
9.1
14.8

30.5
21.7
27.0
24.7
25.7

0.2
.3
.5
.2
.2

n.d.
n.d.
n.d.
n.d.
n.d.

88.9
114.2
114.8
84.7
83.7

0.86
1.84
1.42
1.39
1.21

1.3
2.7
2.1
2.0
1.8

29.1
12.5
21.3
6.4
10.4

34.0

2.9

21.5

12.7

25.9

0.3

n.d.

97.3

1.31

2.0

11.7

43.0
45.9
47.1
43.9

3.8
1.7
2.8
.9

9.2
7.7
12.0
6.7

6.0
5.7
4.7
5.1

33.3
34.2
29.5
30.6

0.7
1.0
.4
.5

n.d.
n.d.
n.d.
n.d.

96.0
96.2
96.5
87.7

1.29
1.34
1.60
1.43

1.9
2.0
2.4
2.1

11.3
27.0
16.8
48.8

45.0

2.3

8.9

5.4

31.9

.7

n.d.

94.2

1.41

2.2

19.6

33.0
38.1
49.5
40.1
31.3

0.2
.9
2.8
.3
3.1

2.5
.9
17.6
<.l
<.l

0.6
.7
1.2
1.0
1.0

52.5
37.1
15.7
34.6
34.0

0.8
.9
.3
1.1
.4

3.7
5.6
5.4
3.5
12.3

93.3
84.2
92.5
80.6
82.1

0.63
1.03
1.33
1.16
.92

1.0
1.6
1.8
1.8
1.4

165.0
42.3
17.7
133.7
10.1

38.4

1.5

4.2

0.9

34.8

0.7

6.1

86.6

1.10

1.7

25.6

36.0

2.7

11.7

4.1

31.9

0.5

8.0

94.9

1.13

1.7

13.3

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALSFIGURE 8.—Encrustation of brannerite on a partially altereduraninite grain being replaced by quartz (black). Scanning backscattered electron photomicrograph, x 600. Vaal Reef, Vaal Reefs
Mining and Exploration Co., Ltd.

tion and voids in the crystal lattice structure andpossibly also from the fact that iron may be present as
Fe203 instead of FeO and lead may be present as asulphide rather than an oxide, because galena inclusions
are abundant, although visible inclusions were avoided
during analyses.
In table 6, the Si02 content of the grains varies from 1
to as much as 18 percent. It is probable that it is derived
from the abundant authigenic silica, which has invaded
all fractures and pores in the reefs, having entered thestructure of the uranium titanate either at the time offormation or later when the grains became metamict
and somewhat porous (in the same way, for example, as
sulphur entered the grains to form galena of radiogeniclead).
As in uraninite, the U02/Th02 ratio varies extensively
in the alteration product (fig. 10), helping to confirm its
derivation from the uraninite.
Selected grains were heated to 1,000°C for one hourand examined by X-ray diffraction analysis. The results
are given in table 7. Although the diffraction data may
partially fit a mixture of Pb304, quartz, and rutile or themineral may be contaminated with these three, there
are a number of important lines of these proposed im­
purities that are absent (italic in table 7). Ignoring thelines of an unknown impurity, the best fit is with thedata for brannerite given by Patchett and Nuffield(I960).2

Q15

It may be argued that a metamict microaggregate ofTi02 and U02 would, on heating, synthesize brannerite
and that the original mineral may not have been bran­
nerite at all. However, this is a problem inherent in all
studies of metamict minerals.
As Ferris and Ruud (1971) have pointed out, becauseof the uncertainty of heating experiments and X-ray dif­fraction analyses, structure analyses cannot be con­sidered essential for positive identification of metamict
minerals such as brannerite.
What we do know is that we have in the Witwatersrand a homogeneous mineral similar in appear­ance—medium grey, medium reflectance—to descrip­
tions of brannerite given in the literature (Pabst, 1954;
Patchett and Nuffield, 1960; Ferris and Ruud, 1971). It
is similar in composition to brannerite from othersources (table 8; Ferris and Ruud, 1971). For the overall
average in table 6, if the uranium content is increased towhat it would have been before radioactive decay, if thereasonable assumption is made that thorium, calcium,and iron substitute for uranium in the structure, and if
Si02 is ignored, the average normalised composition
would then be approximately 65 percent U02 and 35 per­cent Ti02, close to the ideal formula U02 -2Ti02 (62.8
percent U02 and 37.2 percent Ti02).
It is the opinion of the present author that theuranium titanate mineral in the Witwatersrand is in­deed brannerite.
The uraniferous leucoxene described first by
Liebenberg (1955) is found to be an admixture of leucox­
ene and varying amounts of tiny needles of brannerite(fig. 9, grain 5).
ECONOMIC IMPORTANCE OF BRANNERITE

Brannerite is most abundant in the reefs mined in theWest and Far West Witwatersrand, where it may be themost important uranium-bearing mineral, and is oflesser abundance in the Orange Free State. In the Mid­dle, East, and Far East Rand it is relatively rare.
According to Liebenberg (1955), referring to bran­
nerite as "secondary uraninite," its rate of dissolution in
the oxidising dilute sulphuric-acid leach solutions of theuranium-extraction plants is slightly greater than thatof uraninite. However, on examination of many leachresidues during the present investigation, in addition touraninite enclosed in carbon, brannerite was frequently
the most abundant uranium-bearing mineral in the planttailings. This is due to enclosure of the brannerite byminerals that are themselves insoluble. Phyllosilicates,
pyrite, quartz, and leucoxene are common refractory
2 Since this paper was submitted for publication, numerous DebyeSherrer diffraction patterns confirming brannerite have been obtained
from samples collected in several parts of the Witwatersrand.

Q16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALSQ17

TABLE 7. -Best obtained X-ray powder-diffraction data/or Witwatersrand brannerite, compared with previously published information on bran­
nerite from other sources[ASTM diffraction data for Pb304 , quartz, and rutile are listed for comparison, since the Witwatersrand data may be interpreted as a mixture of these three, although some important lines areabsent (italics) or may be contaminated with one or more of these materials (there is no fit with the powder diffraction data for Pb02 (ASTM 11-549), high temperature Pb02 (ASTM 22-389),
tridymite (ATSM 18-1170), and cristobalite (a- or/3-: ASTM 11-695 and 4-359)). In addition, the grain contains an unidentified substance, diffraction lines of which are given in paren­theses, d, lattice spacing, in angstroms; I, relative intensity; b, broad; f, faint; vf, very faint]hkl

Witwatersrandd(8.78)

001
201
200

(6.45)
Black
4.84
4.36
(3.78)

110
202
111
002
201
111
112
311
40 1(?)
203
312
310
400

——

I
6
1

Synthetic 1

Brannerite Blind River1d

Id

Quartz ASTM 5-490

Pb3 04 ASTM 8-19

I

d

I

d

———

——

A 9fi

QP:

100

3.343

100

3.28
3.11

7
19

——

——

2.903
2.787

48
45

2.632

30

2.444

2

2 4 ro

-| n

2.289
2.205

4
1

2.282

2.076

1

2 128
2.054

2.032
1.970

12
12

1.980

1.903
1.887
1.829

22
<1
<1

10
1

6.04
4.74
4.29

36
96
19

3.47
3.35

4
5

3.43
3.33

10
4

3.44
3.35

>100
>100

3.66

3

3.38

3.26
3.16
3.02

3
1
3

3.27

4

3.28

13

3.02

3

3.02

36

2.87
2.78
(2.69)
2.60
2.51
2.45

4
2
2
.5
4
4

2.92
2.76

5
5

2.90
2.77

36
33

2.51
2.46

4
4

2.53
2.47

31
33

2.33
2.24

1
2(b)

6
vf
vf

2.41
2.303
2.292
2.276
2.144
2.080

17
8
27
12
9
7

2.043
2.015
1.911
1.903
1.881

36
23
5
19
24

2.12
2.05

1
3

2.281
2.156
2.078

2.02

4

2.023

3

1.916
1.890
1.861

2
2
2

1.903
1.866

4
2

6.23

2 00^

—
—

1.778

vf

1.737

f

1.699
1.678

3
1

1.695
1.667

3
vf

1.633

1

1.621

4

(1.582)

1

I

1.798
1.749
1.732
1.723

4
8
5
3

1.709
1.675
1.642
1.630
1.609
1.597

20
5
7
19
18
6

1.569

7

————

1.755

30

1.70251.68971.64171.63022
2
8<1

1.5876

12

——

3.25

100

rii / o'y

50

12

2.297

8

9
10

—— —

6

— ——

c

:::
—

1 Q-t iy

E

d

11

3
8
2

l(b)

Rutile ASTM 21-1276

——
6.03
4.77
4.31

1.775

I

iy

1.801

<1

———

——

——

———

i £70

n

———
1.608

——
<1

—
___

1.6237
—— —

20

——

1.568

4

FIGURE 9.—Especially in the Vaal Reef in the Far West Witwatersrand, brannerite is an important, often the most important, uranium-bearing
mineral present in uranium-extraction-plant residues. Its presence is by virtue of its common enclosure, as minute crystals, in quartz (Qz),sericite (Sc), chlorite (Cl), and leucoxene (Lx), all of which are insoluble in oxidizing dilute sulphuric acid leach solution, thereby indirectly
rendering refractory a substantial proportion of the normally soluble brannerite present. Scanning back-scattered electron
photomicrographs, x480. Vaal Reef, Vaal Reefs Mining and Exploration Co., Ltd.

Q18

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 7. -Best obtained X-ray powder-diffraction data for Witwatersrand brannerite, compared with previously published information on branneritefrom other sources -Continued

theses, d, lattice spacing, in angstroms; I, relative intensity; b, broad; f, faint; vf, very faint]
WitwatersranddBrannerite Blind River 1

I

___
1

f^f^/1

1

I

d

i

——
——

——
——

1.561
1.558

6

Quartz ASTM 5-490dI

Rutile ASTM 21-1276
d

i

____

___

___

__

___

1.479710

1 I ZQS

10

Pb304 ASTM 8-19dISynthetic1

d

___

__

I rr r Qf\

r~7

1.51162—————
1.4687

——
4

———
———

——

1.494
——

2
——

——
1.482

——
vf

1.493
1.486

l(b)
3

__
——

__
——

__
——

1.458
1.441
——

vf
vf
——

1.450
1.431
____

4
2
___

1 /1^91

9

————

———

—————

———

1.4243

2

——
——

——
1.374
__
——

___
1
__
——

1.410
——
1.367
——
——

vf
——
2
——
——

1.415
1.384
1.376
——
——

2
3
4
——
——

——
1.3944
1.3728
——

——
<1
1
——

1.418
1.382
1.375
1.372

<1
7
11
9

__
——
——
____

——
——
——
___

1.34714— ——1.3598

20

__
——
——
——
——

__
——
——
——
——

__
——
——
——
——

1.3041

9

——
——
——
——

——
——
——
——

1.266
1.263
1.242
1.239

__
2
5
2
2

____
____
1.2441
——

___
___

——
——
——

__
——
——

__
——
___

——
——
——

——
——
——

1.234
1.218
1.200

5
3
3

____
____
1.2006

___
___
2

__
——

1

Q AQ

1 QlftQ

4

——
——
——
——
——

——
——
——
——
——

1.288
———
———
———
———

3
——
——
——
——

——
——
——

——
——
——

1.228
- ——
1.1997

2
——
5

4
——

1 Patchett and Nuffield (1960).

TABLE 8.— Some analyses of brannerite, in percent, reported in the literature[See Pabst (1954) and Frondel (1958)]
OriginKelly Gulch, Idaho
Cordoba, Spain
Crocker's Well, South Australia
Vosges, France
Mono County, Calif.

U02

Th02

41.93
49.79
30.05
41.65
38.41

4.10
1.20
12.81
.30
5.00

1 Totals are incomplete; refer to Frondel (1958) for complete analyses.

Pb02

ft 9

2.51
2 99

3.43

FeO
9 Q

Ti02
^Q ft

3.11

32.45

1

OQ

QP. 1 Q

5.4
2.4

35.2
32.9

CaO

Si02

9 Q

ft fi

2.74

2.7
2.8

.16

3.3
.5

Total 1

U02/Ti02
ratio

91.96

Q1 fi^

108
i ^

QO O1

QC

91.98
82.01

118

1.17

U/Ti
ratio

1 59

2
1
1.74
1.72
QfT
O/?

Q19

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS.

with that in the plant feed, is in any way more closely
associated with these phosphate minerals.

EXPLANATION

165 '°
t1
70~133.7***+*Main Reef 151
Leader Reef 134
Vaal Reef 72
Vaal Reef 25
Carbon Reef 74

UO2 /ThO2 RATIO IN THE REEFS AS A WHOLE

In table 9 a comparison is made of the overall
U02/Th02 contents of rock samples of the reefs, deter­
mined by X-ray fluorescence spectrometry, with theaverage obtained on uraninite and (or) brannerite grainsin the same reef samples by electron microprobe. The
ratio is always lower in the case of the overall rocksamples, which suggests that there are other thoriumbearing minerals in the reefs.

*
*
9

40

TABLE 9. - Comparison ofuran ium/thorium ratios of reef rock samples
as a whole, analysed by X-ray fluorescence spectrometry, and theaverage ratios of uraninite and (or) brannerite grains, analysed byelectron microprobe

#
•+•
*

*

Whole rock

**
*

*

+*
+»
* #

**
*

01

234

+

Sample

Reef

U02
(ppm)

Th02
(ppm)

U02/ThO,(ratio)U02 /Th02

25
27(72|
34
151
134
171
184

Vaal Reef
Vaal Reef
Carbon Leader
Main Reef
Leader Reef
Cristaalkop Reef
Basal Reef

1,125
442
914
19
4,892
5,243
4,040

199
166
61
5
570
468
1,530

5.7
2.7
15.0
3.8
8.6
11.2
2.6

17.7
11.9
20.7
26.2
11.9
12.9
20.9

56

Th02, IN PERCENT

FIGURE 10.—Variations in thorium content of brannerite. The
U02/Th02 ratio varies as widely in brannerite as it does in uraninite,which is to be expected, as it is the alteration product of uraninite.
Sample numbers in the explanation are from the Witwatersrand Col­lection of the X-Ray Fluorescence Section, Anglo American
Research Laboratories, South Africa.

Investigation shows that zircon, monazite, anduranothorite are present, although relatively rare, andall three contain varying and sometimes appreciable
amounts of thorium and relatively lesser uranium.
AGE OF WITWATERSRAND URANIUM-BEARING MINERALS

hosts (fig. 9). Finer milling of the ore to release the tiny
crystals of brannerite (generally <5/nm) would be im­practical. Only by dissolution of these refractory hostminerals will recovery efficiencies significantly improve.
It has been suggested by Laxen (1973) that (U, P) com­pounds, if present, may be relatively refractory and slow
to react with the leach solutions. Although Mihalik wasable to identify a (U, P) compound in the Dominion Reef,no similar compound could be found during the present
study of the reefs of the Witwatersrand andVentersdorp Systems.
In an intensive search for phosphorus compounds
associated with uranium, only monazite and a calcium
phosphate mineral, presumably apatite, could be found,occurring as minute crystals in the matrix of the con­glomerates and sometimes associated as discrete grains,with brannerite, as illustrated in figure 11. These
discrete phosphate grains are not expected to interferewith the dissolution of the brannerite. Also, there is noevidence that the brannerite in the residues, compared

Polished sections containing Witwatersranduraninite, brannerite, and zircon were sent to Drs.
Hinthorne and Anderson at the ARL Hasler ResearchCenter, Goleta, Calif., for age determination by ARL
IMMA Ion Microprobe. (For a description of the instru­
ment and technique see, for example, Andersen and
Hinthorne, 1972.) The results that they obtained are as
follows:
MineralUraninite (average)
Brannerite
ZirconPb207/Pb206
ratio

Age
(million years)

0.1078 + 0.0002.1103 ±0.007.2017

1,770 + 60
1,810
2,845

The age values were calculated using the newly
recommended decay constants of Tatsumoto and others(1973). The average value for uraninite (1,770 + 60 m.y.
(million years)) is somewhat lower than previously deter­
mined values of approximately 2,000 m.y. (Davidson,

Q20

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES1960). The difference might lie in the fact that the ion
beam sampled the lead within the crystal structure of
the uraninite, eliminating any contamination from theenvironment of the grains.
The fact that the brannerite and uraninite are prac­
tically of the same age (limits of accuracy overlap) may
indicate that the formation of brannerite by alteration of
uraninite coincided with the period when the remaining
uraninite recrystallized. This age, 1,800 m.y., agrees
with the last stages of emplacement of the Bushveld Ig­
neous Complex (Davidson, 1960), which thus may havebeen the source of the metamorphic activity.
The zircon age, 2,845 m.y., is somewhat older thanthat of the Witwatersrand System itself, approximately
2,500 m.y., and may be indicative of the age ofcrystallization or recrystallization of the source rocks.
PARAGENESIS OF GERSDORFFITE AND OTHERSECONDARY MINERALSIn geological terms, temperatures in the vicinity of
600 °C are not very high, and relatively low grade
metamorphism is expected to have occurred. Apartfrom very thorough cementing of the pebbles andmatrix, mainly through the redistribution of quartz, aproportion of, if not all of, the gold was redistributed,
virtually in situ, and gersdorffite and other secondary
minerals were formed from the alteration of pyrite andless stable detrital minerals that were present.
Gersdorffite is invariably associated with gold, as
noted by Saager (1969), with which it is commonly intergrown. So regular is this association that gersdorffite
may be used as an indicator of gold content. The associa­
tion is even closer than that of carbon and gold, wherecarbon may be present in abundance without the gold
content being sympathetically high. (In mining practice,carbon content is commonly used as an indicator of gold
content.)
Intergrowths of gersdorffite and gold were seen often
to contain chalcopyrite and, more rarely, pyrrhotite,sphalerite, tucekite (Ni9Sb2S8), pentlandite, stibiopallandinite, and sudburyite, all clearly secondary minerals of
the same generation. These integrowths are notably
xenomorphic, unlike secondary pyrite and galena of thesame age, which invariably show distinct cubic crystal
outlines.
At the same time, much rarer sperrylite, hollingworthite, ruthenarsenite, and iridarsenite are believed
to have formed from reconstitution of the remains ofhighly altered primary platinum group mineral grains.
Alteration of phyllosilicates in the matrix, pyritization ofiron oxides and ilmenite leading also to abundantneedles of rutile, recrystallization of uraninite andcrystallization of brannerite, and the growth of tour­
maline, bravoite, cubanite, proustite, stromeyerite, tennantite, and mackinawite, all very rare, are expected tohave taken place at this time.
GENERAL CONCLUSIONS

Investigation of the Witwatersrand cobaltitegersdorffite assemblage has revealed, on the basis of
crystal structure and chemistry, that temperatures inthe vicinity of 400-600 °C were achieved duringmetamorphism of the reefs.
It is apparent that the gold was remobilised and sec­
ondary base metal sulphides, sulpharsenides, andarsenides were formed during metamorphism. It ap­
pears that at the same time the uraninite wasrecrystallized and was partially altered to a uranium
titanate mineral, which is confirmed to be brannerite.
Age determinations by ion microprobe show that thelast period of recrystallization of uraninite, presumed tobe the same period as gave rise to the metamorphic ac­
tivity during which gersdorffite and related minerals
were formed, took place about 1,800 m.y. ago. This coin­cides with the last stages of emplacement of the
Bushveld Igneous Complex.
The variable U02/Th02 ratios in uraninite grains not
associated with carbon suggest that the grains are of
detrital origin, having been derived from a number of
granitic or pegmatitic sources. A similar variation in
uraninite included in carbon suggests that the ancient"lichen," which is fossilised as the carbon, "digested" thedetrital uraninite, rather than absorbed the U02 andTh02 from solution.
In addition to uraninite included in carbon, brannerite
enclosed by refractory silicate minerals is an importantsource of uranium losses to uranium extraction plant
residues.
ACKNOWLEDGEMENTS

The author is grateful to Anglo American ResearchLaboratories for having made this research project
possible, to all his colleagues in the various sections of
the Laboratories for their guidance and assistance, togold mines of Anglo American Corporation of SouthFIGURE 11. - Minute crystals of a calcium phosphate occuring as a discrete phase in an aggregate particle of brannerite, quartz, and chloritoid. No
uranium-phosphorus compound was detected in the reefs examined. Scanning back-scattered electron and X-ray photomicrographs, x 1,000.
Vaal Reef, Vaal Reefs Mining and Exploration Co., Ltd.

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS

Q21

Q22

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESAfrica, Ltd., and Gold Fields of South Africa, Ltd., forsamples, and to Applied Research Laboratories for theion-mocroprobe determination of the age of Witwatersrand uranium-bearing minerals.
Finally, the author wishes to pay tribute to Mr. E. J. J.
van Vuuren who, as the enthusiastic and inspiring coor­dinator of this project during his term of office at theLaboratories, was a fount of information by virtue of hislong experience on gold and uranium extraction plants.

REFERENCES CITED
Andersen, C. A., and Hinthorne, J. R., 1972, Ion microprobe mass
analyzer: Science, v. 175, p. 853-860.
Barton, P. B., Jr., and Kullerud, G., 1958, The Fe-Zn-S system, in An­
nual Report of the Director of the Geophysical Laboratory,
1957-58: Carnegie Institution of Washington, p. 227-229.
Bayliss, P., 1969, X-ray data, optical anisotropism, and thermal stabil­
ity of cobaltite, gersdorffite and ullmannite: Mineralogical
Magazine, v. 37, p. 26-33.
Bayliss, P., and Stephenson, N. C., 1967, The crystal structure of
gersdorffite: Mineralogical Magazine, v. 36, p. 38-42.
Cooper, R. A., 1923, Mineral constituents of Rand concentrates:
Chemical, Metallurgical and Mining Society of South Africa Jour­nal, v. 24, no. 4, p. 90-95.
Davidson, C. F., 1957, On the occurrence of uranium in ancient con­glomerates: Economic Geology, v. 52, p. 668-693.
———1960, The present state of the Witwatersrand controversy: Min­ing Magazine, v. 102, p. 84-95, 149-159, 222-229 (serialised).
Duncumb, P., and Jones, E. M., 1969, Electron probe microanalysis:
An easy-to-use computer program for correcting quantitative data:
Tube Investments Research Laboratories, Technical Report no.
260, 38 p.
Feather, C. E., and Keen, G. M., 1975, Mineralogy of the Witwaters­
rand reefs: Minerals Science and Engineering, v. 7, p. 189-224.
Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: its occurrences andrecognition by microprobe: Colorado School of Mines Quarterly, v.
66, no. 4, p. 1-35.
Frondel, Clifford, 1958 [1959], Systematic mineralogy of uranium andthorium: U.S. Geological Survey Bulletin 1064, 400 p.
Fuller, A. 0., 1958, Temperatures of formation of sphalerite from the
Bird Reef Group: Geological Society of South Africa Transactions,v. 61, p. 97-101.
Gammon, J. B., 1966, Some observations on minerals in the system
CoAsS-FeAsS: Norsk Geologisk Tidsskrift, v. 46, p. 405-426.
Giese, R. F., Jr., and Kerr, P. F., 1965, The crystal structures ofordered and disordered cobaltite: American Mineralogist, v. 50, p.
1002-1014.
Grandstaff, D. E., 1974, Microprobe analyses of uranium and thorium
in uraninite from the Witwatersrand, South Africa, and BlindRiver, Ontario, Canada: Geological Society of South Africa Trans­
actions, v. 77, p. 291-294.
Hallbauer, D. K., 1975, The plant origin of Witwatersrand "carbon":Minerals Science and Engineering, v. 7, p. 111-131.
Hallbauer, D. K., and van Warmelo, K. T., 1974, Fossilized plants inthucholite from Precambrian rocks in the Witwatersrand, South
Africa: Precambrian Research, v. 1, p. 199-212.
Jacob, R. E., 1966, The geology of the Vaal Reefs Mine: Rhodes Uni­
versity, Grahamstown, unpublished M. Sc. Thesis.
Klemm, D. D., 1965, Synthesen und Analysen in den Dreiecksdiagrammen FeAsS-CoAsS-NiAsS und FeS2-CoS2-NiS2 : Neues Jahrbuch
fur Mineralogie Abhandlungen, v. 103, p. 205-255.

Koen, G. M., 1961, The genetic significance of the size distribution of
uraninite in Witwatersrand bankets [with discussion]: GeologicalSociety of South Africa Transactions, v. 64, p. 23-54.
Kullerud, G., 1953, The FeS-ZnS system, a geological thermometer:
Norsk Geologisk Tidsskrift, v. 32, p. 61-147.
Laxen, P. A., 1973, A fundamental study of the dissolution in acid solu­
tions of uranium minerals from South African ores: National In­
stitute for Metallurgy, Johannesburg, Research Report 1550, 143
PLiebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef, and the Black Reef [withdiscussion]: Geological Society of South Africa Transactions, v. 58,
p. 101-254.
Mihalik, P., 1968, Uranium compounds in the Dominion Reefs, andtheir association with phosphorus compounds: National Institutefor Metallurgy, Johannesburg, Research Report 415, 8 p.
Pabst, A., 1954, Brannerite from California: American Mineralogist,
v. 59, p. 109-117.
Patchett, J. E., and Nuffield, E. W., 1960, The synthesis andcrystallography of brannerite, [pt.] 10 of Studies of radioactive
compounds: Canadian Mineralogist, v. 6, p. 483-490.
Pretorius, D. A., 1974a, The nature of the Witwatersrand golduranium deposits: Witwatersrand University Economic Geology
Research Unit Information Circular no. 86, 50 p.
————1974b, Gold in the Proterozoic sediments of South Africa:
Systems, paradigms, and models: Witwatersrand University
Economic Geology Research Unit Information Circular no. 87,
22 p.
-1975, The depositional environment of the Witwatersrand
goldfields: A chronological review of speculations and observa­
tions: Minerals Science and Engineering, v. 7, p. 18-47.
Ramdohr, Paul, 1955, Neue Beobachtungen an Erzen des Witwatersrands in Siidafrika und ihre genetische Bedeutung: Deutsche
Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik
und allgemeine Naturwissenschaften, Abhandlungen Jahrgang,
1954, no. 5, 43 p., English translation in Geological Society ofSouth Africa Transactions, Annexure to v. 61, 1958, 50 p.
Rahden, H. V. R. von, 1964, A detailed mineralogical investigation of
the Ventersdorp Contact Reef at Venterspost and Libanon gold
mines, West Rand, with special reference to the mode ofmineralization of this horizon: Witwatersrand University, Johan­
nesburg, unpublished M. Sc. Thesis.
————1970, Mineralogical and geochemical studies of some
Witwatersrand gold ores with special reference to the nature of
the phyllosilicates: Witwatersrand University, unpublished Ph. D.
Thesis.
Rahden, H. V. R. von, and Hiemstra, S. A., 1967, The mineralogy ofuranium in Western Area ores: National Institute for Metallurgy,
Johannesburg, Research Report 237, 6 p.
Riemer, J. 0., 1975, The age of the Witwatersrand system and othergold-uranium placers: Implications on the origin of the mineraliza­
tion: Neues Jahrbuch fur Mineralogie Monatschefte, v. 2. p. 79-98.
Saager, R., 1969, The relationship of silver and gold in the Basal Reefof the Witwatersrand System, South Africa: Mineralium Deposita,
v. 4, p. 93-113.
Schidlowski, M., 1966, Mineralbestand und Gefugebilder in Faseraggregaten von kohliger Substanz ('Thucholith') aus den
Witwatersrand-Konglomeraten: Contributions to Mineralogy andPetrology, v. 12, p. 365-380 (incl. English summary).
————1967, Note on graphite in the Witwatersrand conglomerates:
Geological Society of South Africa Transactions, v. 70, p. 65-66.
-1968, The gold fraction of the Witwatersrand conglomerates
from the Orange Free State Goldfield (South Africa): Mineralium
Deposita, v. 3, p. 344-363.
Tatsumoto, M., Knight, R. J., and Allegre, C. J., 1973, Time dif-

ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS
ferences in the formation of meteorites as determined from theratio of lead-207 to lead-206: Science, v. 180, p. 1279-1283.
Uytenbogaardt, W., and Burke, E. A. J., 1971, Tables for microscopic
determination of ore minerals (2d ed.): Amsterdam, Elsevier,
430 p.
Viljoen, R. P., 1963, Petrographic and mineragraphic aspects of theMain Reef and Main Reef Leader in the Main-Bird Series, Witwatersrand System: Witwatersrand University, Johannesburg,
unpublished M. Sc. Thesis.

DISCUSSION

Cannon: I would like to ask you about the metamorphic
temperatures that you quoted. They strike me as be­
ing quite high, and I wonder if you see this metamorphism in some of the more pelitic beds. You might
have garnet or even staurolite porphyroblasts inpelitic material at those temperatures. Do you findthose in the area?
Feather: No, they are extremely rare. Thesetemperatures do seem to be higher than I think mostpeople would like to see.
Skinner: I think the temperatures are too high because
the ways in which you estimated them are not reallyvalid ways to do it. The sphalerite temperature has fora long time now been shown to be invalid as atemperature measure. It is a function both of sulfur
pressure and of temperature. The old curve, that
presumably was used to get a minimum of 400
degrees, is itself wrong and has been replaced for
perhaps eight or nine years now by much more ac­
curate curves. The temperature that would be in­dicated if the sulfur pressure were controlled bypyrite, which one must assume it is, probably validly in
the Witwatersrand, is going to be somewhere between
zero and about 300 to 350 degrees, in that range. It's
not going to be up to 400 or 500 degrees. The second
way that you estimated temperature was with
gersdorffite, cobaltite, and arsenopyrite. That work is

Q23very suspect. The solvus determinations that you used
to base the estimates on are very suspect because of
extreme problems in kinetics in trying to gethomogenization in those materials. Using aqueous
synthesis, you can in fact make complete solid solution
all the way across from gersdorffite to cobaltite withup to about 15 percent iron. You can do that as low as
150°C, which suggests that you could form all of thoseat 150 degrees and perhaps lower. I think the caution
you threw out that you should await final decision un­
til somebody else has looked at the diagram is prob­
ably a very wise caution. I personally think that thatwork is of little or no value at all in estimating
temperature.
Feather: Thank you. I am very pleased for your com­ments. They certainly do throw out a word of verystrong caution.
Schidlowski: I address a question to the expert. Does theoccurrence of mackinawite narrow down things a little
bit?
Skinner: It depends in part on the composition ofmackinawite. If it were a very nickel-richmackinawite, you might be able to push thetemperature up to around 250 degrees. If it's just a
straight iron mackinawite, it probably is below 200
degrees for it to remain stable.
Schidlowski: Just another comment. I was particularly
happy to hear that the thorium tenor of the uraninite
fragments within the carbon have the same values as
the big grains. So I think that should dispose more orless definitely of all ideas that this uraninite has been
precipitated the cold way.
Simpson: Just want to comment on tho§e data. The
ranges are the same but the distribution of the data
within those ranges is different for the two en­
vironments.
Feather: I don't feel that we've analyzed sufficientgrains to statistically treat the data and to draw any
conclusions of that type.

Uranium Mineralization of the
Witwatersrand and Dominion Reef Systems
By P. R. SIMPSON and]. F. W. BOWLESGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-R

CONTENTS
Page

Abstract _____________________________________
Preface, by S. H. U. Bowie _______________________
Introduction __________________________________
Experimental methods __________________________
Mineralogical evidence __________________________
Pyrite ____________________________________Uraninite _________________________________
Grain size of uraninite and associated phases _____Uraniferous and auriferous carbonaceous material.
Churchite _________________________________

Rl
1
1
2
3
3
10
11
11
16

Mineralogical evidence— Continued
Origin of the carbonaceous material _____________
Indus River uraninite _____________________
Geochemical evidence ________—__———————————
Lead content of uraninite and pitchblende _________Uranium micromapping with thermal neutrons ______
Neutron-activation analysis of whole-rock samples ____
Bimodal nature of the uranium mineralization _________
Conclusions _____________________——__———
Acknowledgments _________________________
References cited __________________________

R17
17
19
19
20
22
22
23
23
24

ILLUSTRATIONSFIGURES 1, 2. Maps showing:
1. Sub-Ventersdorp geology of Witwatersrand Basin _____________________________________
2. Principal mining areas and sample localities of the Witwatersrand Basin _________________________
3-7. Photographs showing:
3. Banket ore sample, showing allogenic pyrite, chalcopyrite, and quartz, from Dominion Reef; concretionary pyrite ofspherical type, composed of alternate layers of pyrite and chalcopyrite, with a grain of gold, from West Driefontein, West Wits; authigenic reconstitued pyrite containing gold, from West Driefontein, West Wits; car­bonaceous matter showing coarse and fine-grained carbon, with gold, from Carbon Leader, West Wits _____
4. Electron-probe scanning images of allogenic pyrite containing galena and rimmed by uraninite and of localized
migration of radiogenic lead from uraninite to the rim of an adjacent allogenic pyrite, from Basal Reef, President
Brand, Orange Free State _________________________________________________
5. A uraniferous pyritic band consisting of concretionary and allogenic pyrite, from Basal Reef, President Brand,
Orange Free State ______________________________________________________
6. Uraniferous pyrite band consisting of mainly concretionary pyrite grains, from Basal Reef, President Brand, Orange
Free State __________________________________________________________
7. Electron-probe scanning images of a discrete galena grain occurring in a low-lead type pyrite concretion and ofgalena distribution in concretionary pyrite of high-lead type from Basal Reef, President Brand, Orange Free
State _____________________________________________________________
8-10. Graphs showing:
8. Cumulative-frequency diagrams for Th02 obtained by electron probe microanalysis of fine pitchblende grains in
granular "flyspeck" carbon, from Carbon Leader, West Wits; of coexisting granular allogenic uraninite and fine­grained pitchblende in columnar carbon, from Vaal Reefs, Klerksdorp; of allogenic uraninite from the Hunza
River, Kashmir, from Hazro, Pakistan, and from the Dominion Reef ________________________
9. A cumulative frequency diagram of U02/Th02 ratios for separate populations of uraninite and pitchblende from theCarbon Leader, the Vaal Reefs in the Witwatersrand, the Dominion Reef, the Indus River at Hazro, Pakistan,and the Hunza River, Kashmir ______________________________________________
10. Standard error of the mean size for coexisting quartz, pyrite, and uraninite plotted on the phi scale against specificgravity, for Bramley high grade, Dominion Reef ___________________________________
in

R2
3

5
6
7
8
9

10
11
12

IV

CONTENTSPage

FIGURES 11-16. Electron-probe images showing:
11. Granular carbonaceous material containing fine-grained particles of pitchblende and gold in a carbonaceous
matrix, from Carbon Leader, West Wits _______________________________________
12. Columnar carbon, showing zones of coarse fragmented allogenic uraninite and fine-grained disseminated pitch­
blende in a carbonaceous matrix, from Vaal Reefs, Klerksdorp __________________________
13. Columnar carbon (part of fig. 12) _____________________________________________
14. Columnar carbonaceous material, showing cluster of coarse-grained allogenic uraninite crystals partly digested
by carbonaceous matter, from Vaal Reefs, Klerksdorp _______________________________
15. Uraninite within carbonaceus material and ThOi content, from Vaal Reefs, Klerksdorp ______________
16. Electron-probe scanning images of probable churchite and the distribution of heavy rare-earth elements in prob­
able churchite ______________________________________________________
17. Photographs showing uraninite grains from the Hunza River, Kashmir, and an example of an altered uraninite grain __18. Cumulative frequency diagram showing the PbO content of uraninite and pitchblende from Dominion Reef, Witwatersrand, and Indus River sediments __________________________________________________
19. Photomicrographs showing a uraniferous pyrite band consisting principally of allogenic pyrite with composite lenticulargrains of quartz, kaolinite, and pyrophyllite and quartz, pyrophyllite, and kyanite, from Basal Reef, President Brand,
Orange Free State ________________________________________________________

R13
14
14
15
16
18
19
20
21

TABLESPage

TABLE 1. Comparison of heavy rare-earth distribution in xenotime, churchite, and yttrium phosphate phase relative to gadolinium ____
2. Neutron activation analyses of whole-rock samples ____________________________________________

R17
24

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

By P. R. SlMPSON and J. F. W. BOWLES 1

ABSTRACTPREFACE

Uranium-bearing minerals in the Witwatersrand and Dominion Reef
sediments have been studied by ore microscopic, electron microprobe,
fission track, and neutron activation analytical methods to determine
the controls of uranium mineralization. In the Dominion Reef, which
represents a high-energy banket type of depositional environment,
allogenic thorian uraninite occurs in hydraulic equivalence withallogenic pyrite, quartz, and possibly also gold in the sediments thathave uranium-thorium ratios between 3.1 and 5.6, indicating substan­tial amounts of thorium-rich resistate minerals.
The Witwatersrand sediments have uranium-thorium ratios ranging
between 7.1 and 19.6, indicating lesser amounts of resistates, which isconsistent with the lower-energy depositional environment. The prox­
imal or nearshore deposits are of banket type but are distinguishedfrom the Dominion Reef by the abundance of concretionary pyrite
formed within the basin and the presence of carbonaceous matter. The
distal deposits formed at greater distance from the shoreline containdecaying organic material, which has precipitated both uranium andgold from solution. Subsequent metamorphism has resulted in the for­mation of carbonaceous material bearing a finely disseminated lowthorium pitchblende and a fine dissemination of gold associated withsulphides and arsenides.
Further evidence of the existence of uranium in solution is to befound in the banket deposits. In this case fine disseminations ofuranium (>500 parts per million) occur in clay minerals within concre­
tionary pyrite nodules and in lenticles formed of clay minerals in Wit­
watersrand banket deposits. They represent reduction-deposition of
the soluble uranyl ion below the sediment-water interface where condi­
tions are reducing.
Allogenic thorian uraninite from the prebent-day Indus River has atexture, composition, and association with gold and pyrite similar toallogenic uraninite in the Witwatersrand and Dominion Reef Systems.
Thorian uraninite is a stable phase over large distances in this river.
Hence it would appear quite unnecessary to postulate a reducing at­mosphere for the transportation of detrital uraninite. Moreover theretention of sulphate and uranyl ions in solution in the model proposed
here suggests that the atmosphere was oxidizing at the time of deposi­tion. This conclusion indicates the likely occurrence in younger
sediments of mineralization of this type provided the necessarygeological criteria are met.

By S. H. U. BOWIE, F.R.S.

The problem of the origin of the mineralization of the
Witwatersrand and Dominion Reef has been one of thegreat controversies of the past 50 years. Any new at­
tempt to solve it requires the application of the most upto-date mineralogical techniques. Although unequivocable results have not yet been obtained, important new
information on the bimodal nature of the uranium
mineralization has been established.
The significance of this study, which illustrates the ap­
plication of techniques that will become more common­
place as the 21st century approaches, lies in providing
essential information on the origin of uranium in quartzpebble conglomerates. More than 35 percent of known
uranium reserves occur in such rocks, particularly in
Canada and South Africa but also, as recently reported,
in Algeria. Previously many geologists have held thaturanium can only be enriched to economic grades inPrecambrian rocks if anoxygenic atmospheric condi­
tions prevailed at the time of sedimentation. However,
the remarkable similarity between the subeconomic con­centrations of thorian uraninite in the present-day Indus
Valley and that of the Dominion Reef and Witwaters­
rand Systems, as well as other evidence, invalidates
any such concept.
Further work is required to complete the story of the
Witwatersrand mineralization, and it is hoped thatmultidisciplinary investigations will continue. The full
resolution of the genesis of the mineralization could
result in new discoveries that are essential to theavailability of uranium for future energy needs.
INTRODUCTION

1 Institute of Geological Sciences, Geochemical Division, 64-78 Gray's Inn Road, London
WC1X 8NG

Uranium mineralization in the Witwatersrand andDominion Reef sediments is presently and has been,
Rl

R2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

since its discovery by Cooper in 1923, the subject of
debate and speculation as to its origin.
Mineralization has been described variously as the
product of hydrothermal processes by Graton (1930) andsedimentary processes by Ramdohr (1958) and others.
The detrital hypothesis for the origin of uraninite has
been taken to account for all primary mineralization in
the basin by some workers. This idea has been further
developed to the point where a reducing atmosphere
was invoked as an essential prerequisite to the transpor­
tation of detrital uraninite grains (Liebenberg, 1955;
Grandstaff, 1975). This has led to the notion thaturaniferous conglomerates are thereby restricted bythese atmospheric conditions to the Archaean or earlyProterozoic (Robertson, 1974).
The purpose of this study is to re-evaluate the uranium
mineralization using mineralogical methods since theimplications of the earlier work for the further discoveryof uraniferous conglomerates would appear to be unduly
restrictive.
All specimens available to the authors were sectioned
for study to obtain as broad a view of the principal typesof mineralization as possible, though it is appreciatedthat the coverage of this basin is by no means as com­plete as could be wished. The outline geology, principal
mining areas, and section numbers of specimens studied
are shown in figures 1 and 2.

In pursuing this investigation, two questions havebeen uppermost in our minds: firstly, does the formationof these and similar deposits elsewhere require a reduc­ing atmosphere or merely reducing conditions of deposi­tion beneath the sediment/water interface; and second­ly, what are the respective roles of uranium as detritalgrains of uraninite and as uranyl ions in solution.
Answers to these two questions will have considerable
relevance to the understanding of the mode of formationand estimating the potential for new discoveries of thistype of mineralization.
EXPERIMENTAL METHODS

The details of the complex mineralization and the
distribution of uranium were elucidated by ore micro­
scopy, electron probe microanalysis, and fission track
registration. The techniques of the first have been
recently described by Bowie and others (1975), and thelast is discussed in the section on uranium micromapping.
The electron-probe analyses were undertaken with aCambridge Microscan-5 instrument fitted with a Can­berra energy dispersive analytical system. The latter
was used for the analysis of titanium, manganese, iron,
lead, thorium, uranium, and yttrium, whilst calcium andcerium were analysed simultaneously by the Bragg spec-

OHANNESBURG

EXPLANATION

Upper Witwatersrand
System (U.W.S.)
Lower Witwatersrand
SystemORANGE
FREE
STATE

\

*

Dominion Reef
System50km- — Subcrop of U.W.S.
Cape'
Town

D

Principal mining
areas

FIGURE 1.—Map of the Witwatersrand Basin illustrating sub-Ventersdorp geology and showing principal mining
areas.

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS,- JOHANNESBURG —R3^SPRINGS
2109 - 2110

WEST DRIEFONTEIN

1940-1967. US 5AWESTERN DEEP', LEVELS{ 'DOMINION

J \ REEFS^72083-2088.2092

/'"

WEST
WITS

DAGGAFONTEIN

VAAL REEFSDOMINION REEFS

1997-2028.2101, US1273C.
UG5538. UG9824

AFRIKANDER
REEF
2081

EXPLANATION

Principal mining
areas

FREDDIES REEF
2093, 2102- 2107

ORANGE
FREE
Location
STATE/PRESIDENT
BRAND2098 I
South Africa
PRESIDENT STEYN\? -2095-2096

Mining properties
with sample numbers

FREE STATE SAAIPLAS
2094,2097''^'

VIRGINIA REEF

2052-2053,2058-2059

50km

\

Town
FIGURE 2.—Map of the Witwatersrand Basin illustrating principal mining areas and sample localities. The numbers insmall print refer to polished thin sections. Polished sections from the reference collection are prefixed UG or US.

trometers of the microprobe. X-rays from each speci­
men were counted for 250 s at a low beam current oflxlO~9 A, which conditions were suitable for bothanalytical systems. Data were reduced by a PDF 11 com­puter, which calculates concentrations of the elements
present at one analytical point while the next is being
analysed. This technique enables an analysis for nineelements to be produced every 4-5 min. Pure metals
were used as standards for manganese, iron, thorium,and uranium; a rare-earth glass REE-3, kindly provided
by the University of Oregon (Drake and Weill, 1972), forcalcium, yttrium, and cerium; and chemically analysedrutile and galena, for titanium and lead, respectively.
The analytical methods for wavelength-dispersive analy­
sis described by Bowles (1975) were used for calcium andcerium. In another paper, Bowles (in preparation)
describes the quantitative analytical programme for theenergy-dispersive equipment and compares selected
results with those obtained with the Bragg spec­
trometers. The results are closely comparable for all
elements with the exception of uranium, for which theenergy-dispersive results are consistently 3 percent
lower than the Bragg spectrometer results. Investiga­
tion has shown that as between the pure metal and theoxide there is a change in the ratio of the Lal to La2 in­tensities. The lower resolution of the energy-dispersiveequipment records both X-ray emissions as a single

peak, so that a variation in the ratio does not affect theresults. The analyses presented here for uranium andthorium are those obtained using the energy-dispersiveequipment. The measured concentrations were cor­rected using the quantitative correction programmedevised by Mason and others (1969).
MINERALOGICAL EVIDENCEPYRITE

Pyrite is the principal opaque mineral of the Wit­
watersrand and Dominion Reef Systems. In hand speci­
men rounded grains of pyrite, commonly known as"buckshot" (Feather and Koen, 1975), display clearly
defined sedimentary features with well-developed bed­
ding and crossbedding in layers also containing sandsized grains of quartz. In the pebble-size conglomerate,
pyrite occurs as a discrete disseminated phase together
with other sand-size constituents, principally quartz, inthe matrix. Radioactivity is closely associated with
pyritic zones in the hand specimen, and those were
therefore selected for more detailed investigation. In­
deed, the presence of pyrite in the hand specimen is oneof the clearest indications of the likely presence of
anomalous radioactivity in the suite of specimens
studied.

R4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Ramdohr (1958), Saager (1970), and Koppel andSaager (1974) distinguished three types of pyrite in the
Witwatersrand ore: (I) allogenic, (II) concretionary
authigenic, and (III) reconstituted authigenic. Type I
pyrite has rounded and abraded outlines; it is completely
fresh, is uniformly compact and homogeneous, and has adistinctive range of inclusions (fig. 3A). There is no
doubt it is correctly described as allogenic. It is recog­
nized in this study as the dominant pyrite type of the
Dominion Reef System, whereas in the Witwatersrand
any one of the three types may be dominant. Pyrite ofthis type also occurs partially or completely enclosed
within very coarse quartz grains; it displays euhedralgrain boundaries towards the host quartz but has asubrounded and abraded margin where it forms the ex­terior of the composite grain (fig. 3A). Optical study andelectron-probe microanalysis indicate that galena is thecommonest phase included in allogenic pyrite. The exsolution blebs of galena are distinct from marginal intergrowths of pyrite and galena resulting from the migra­
tion of radiogenic lead from coexisting uraninite grains(fig. 4A,B).
Pyrite of type II is present in variable amounts in all
samples studied from the Witwatersrand System. It isreadily distinguished from type I by its porous natureand from type III by its rounded outline. The concre­
tions are generally coarser than coexisting allogenic
pyrite, ranging from granular to coarse sand size, al­though the individual pyrite crystallites that form the'concretions are often 10 jim in diameter or less.
However, in any one specimen the concretions areheterogeneous in nature with regard to shape, size, andcrystallite composition. Some concretions have spheri­
cally zoned alternating sulphide and silicate shells;
others exhibit a layered structure. Some are so dark andfine grained as to be indistinguishable from melnikovite
or colloform pyrite, which is thought to represent acrystallized FeS2 gel.
In the concretionary pyrite, the coarser crystallites,
which are better crystallized and brighter, are texturally
very similar to framboidal pyrite. The pore spaces be­
tween crystallites represent about 10 percent by volumeof the concretions, although zones occur where the py­rite content may be as low as 10 percent. Even within
one concretion the texture may be very variable, rang­
ing from fine-grained disseminated pyrite to a morecompact equigranular variety without pore spaces be­
tween crystallites. Saager and Mihalik (1967) proposedthat these concretions formed in situ, since they claimed
the "extremely delicate nature of the concretions ex­
clude any long transport distance." However, it should
be noted that the crystallites forming the concretions
are in most cases interconnecting, thereby providing arigid skeleton, and the pore spaces between crystallites

are filled by a matrix comprising quartz and a 7 A clay
mineral (unidentified), which provides additional
strength to the concretions. Most of these composite
structures have survived later metamorphism and defor­
mation relatively unaltered. Moreover, coexisting detrital quartz granules show appreciable pressure solution
effects along their margins, though both allogenic andconcretionary grains occurring in close proximity are
underformed, testifying to their relative rigidity andstrength.
In our view the heterogeneity of the concretionary
pyrite in any hand specimen and the evidence of somerounded granules that have apparently been fractured
and abraded before deposition indicate that most of theconcretionary pyrite has been transported to its present
location and did not form in situ (fig. 5). The variabilityof this pyrite in any particular sample is considered toreflect a source area and selective effects of transporta­tion processes. The intimately intergrown nature of thesulphide and silicate phases in the concretions is mostprobably a primary feature inherited from the sourcearea and is not due to subsequent introduction of silica,
since the regular fine-scale laminations observed insome cases preclude the possibility of later introduction(fig. 6). Hallbauer (1975) has described subspherical con­cretions of this type extracted for study by scanningelectron microscopy. However, there are rare occur­
rences of zoned spherical concretions. The example
shown (fig. 31?) has a thin inner zone of chalcopyrite withrare grains of gold and a siliceous rim partially over­grown by pyrite. The spherical outline is clearly a pri­mary feature, and deductions about the extent to whichtransport may or may not have occurred cannot ingeneral be made from a study of external form alone.
Electron probe scans of the concretionary pyrite in
one specimen (2463A) indicate that the lead content,
represented by galena, varies widely in amount and dis­tribution from concretion to concretion. In some (fig.
7A) galena is rare. Others have a high content of dissem­
inated galena in 10 /im grains uniformly distributed
throughout the concretion (fig. IE), and some have fineveinlets of galena probably representing later introduc­tion of lead.
Pyrite of type III occurs as euhedral grains in the con­glomerate matrix or forms overgrowths on detrital com­ponents. It is clearly authigenic in origin. Koeppel andSaager (1974) report enrichments of chalcopyrite,
sphalerite, and galena associated with the developmentof this pyrite type and also remark that the presence ofdykes intrusive into the Witwatersrand rocks has in­fluenced its development. However, these authors at­tribute the bulk of this reconstituted pyrite to regional
metamorphism. The presence of grains of gold, in some

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMSFIGURE 3.—A, Banket ore sample showing allogenic pyrite (Py), chalcopyrite (Cp), and quartz (Qtz); Dominion Reef, (US 1273C), reflected
light, air. B, Concretionary pyrite of spherical type composed of alternate layers of pyrite (Py) and chalcopyrite (Cp), with a grain ofgold (Au). Silicates occur in the body of the concretion and particularly in the rim, as well as in the porous matrix (black). West
Driefontein, West Wits (PTS 1966); reflected light, oil. C, Authigenic reconstituted pyrite (Py) containing gold (Au). West Driefontein, West Wits (PTS 1958); reflected light, oil. D, Carbonaceous matter showing coarse (cgC) and fine-grained (fgC) carbon, withgold (Au). Carbon Leader, West Wits (US 5); reflected light, oil.

R5

R6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
C

QJ

ll
s «

.2 TD

"rt "8

c o
g|
« £

.-§&
•S 'oT'I ^URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMSR7

FIGURE 5.-A uraniferous pyritic band consisting of larger concretionary and smaller allogenic pyrite (white). A, Reflected light. B, Correspond­
ing Lexan print. Uranium is enriched in some concretionary pyrite grains but not in others (see Lexan print). One concretionary pyrite grain(X) has been fractured and further worn before deposition. Uranium present in the matrix is accounted for by allogenic grains of uraninite (Y)and interstitial clay minerals. Basal Reef, President Brand, Orange Free State (PTS 2463B).

R8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 6.—Uraniferotls pyrite band consisting of mainly concretionary pyrite grains (white), which are aheterogeneous assemblage with regard to uranium enrichment (see Lexan print and compare grain's
marked x). A, Reflected light. B, Corresponding Lexan print. Uranium present in the matrix is ac­
counted for by the presence of allogenic grains of uraninite (y) and interstitial clay minerals. Basal Reef,
President Brand, Orange Free State (PTS 2463A).

TMFl,"^

m.V******f~,-~ ..^^***••*.
-*'<&"^rJk^-.*> *-*\^FIGURE 7. -Electron-probe scanning images showing (A) a discrete galena (G) grain occurring in a low-lead type pyrite concretion (Py). Uranium is present in the interstitial clay phase(C); (J5) gelena (G) distribution in concretionary pyrite of high-lead type is shown for comparison with low-lead type; both from Basal Reef, President Brand, Orange Free State(PTS) 2463 A.

Pb*f.N<- ?-,' « i *• ^ r ^"afep-r^ XS^ii*-*-•** f*-» t^**'* ^J^* -%^"*d"» "*^V*'f~'??%%&&&£&.«-

V >}•*•."• ,-A• .XM^*;:<vj<...
./••'-';

3"*4

a

g
^

3

o

o

RIO

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

cases abundant, noted in pyrite of this type during thepresent study (fig. 3C% indicates that gold and pyrite
were locally remobilized and redeposited together.

US 5,
Pitchblende,,
23 analyses

URANINITE

Before describing the details concerning the analysedmaterial, it is necessary to comment on the use of theterms uraninite and pitchblende. Uraninite is the namefor the species as a whole, including all of its varieties(Frondel, 1958). Pitchblende is a variety of uraninite
distinguished primarily by its appearance. In the Witwatersrand all detailed work to date shows that ura­
ninite found there can be divided into two groups: (1) awell-crystallized variety with significant thorium con­tent and (2) a poorly crystallized variety, which occurs
only in association with organic matter and has a low
thorium content. As these tend to be found in different
environments it is important to distinguish between
them, so in this paper "uraninite" is restricted to thewell-crystallized variety and "pitchblende" to the other.
Uraninite occurs as discrete allogenic grains varying
in shape from euhedral to subrounded and roundedgrains, usually within a limited size range in the fine tovery fine sand sizes (0.06-0.25 mm), and is often frac­tured. It occurs in this form both in the Dominion Reef
and Witwatersrand Systems. Thorium content is thebest indicator of primary differences in populations ofuraninite grains since lead content within and between
grains may be quite variable owing to exsolution andmigration of radiogenic lead. This lead mobility has agreater effect on the uranium content than on thorium,
which has a more homogeneous distribution and is lessreadily displaced. It is possible to observe in some cases
the migrations and reaction of radiogenic lead withnearby allogenic sulphide to form a galena rim (fig. 4B).
the same section the close proximity of brannerite to
decaying uraninite, with nonuraniferous titanium min­erals at a greater distance, is indicative of local migra­
tion and reaction of uranium with titanium minerals inthe sediments.
Electron microprobe analyses show in this study thatthe Dominion Reef uraninite has a median Th02 content
of 6.6 percent with a range from 5.4 to 8.4 percent (fig.
8Q and a median U02/Th02 ratio of 8.5 with a range
from 6.4 to 10.9 (fig. 9).
In the Vaal Reef of the Witwatersrand System, allo­
genic uraninite analysed in a columnar carbonaceousband (PTS 2034) has a median Th02 content of 3.5 per­cent with a range from 0.9 to 9.1 (fig. SB) and a median
U02/Th02 ratio of 19 with a range of 8.3 to 48 (fig. 9).
The separate populations of Th02 in uraninite grainsfrom the Dominion and Vaal Reefs were tested by analy­
sis of variance methods and found to be significantly dif-2034,
Pitchblende,
21 analyses
2034,
Allogenic uraninite,
92 analyses

LJJ
O

B

cc
LJJ
Q_

YP 19 A,
Hazro, Pakistan,.,
19 analyses

12

YP 10 A,
Hazro, Pakistani
29 analyses

UG 9814,
Dominion Reef,
11 analyses

NCL 1115,
Hunza, Kashmir,
33 analyses

1

50

c

99

CUMULATIVE PERCENT
FIGURE 8.—Cumulative-frequency diagrams for Th02 obtained by elec­tron probe microanalysis. A, Mass percent Th02 of fine pitchblende
grains in granular 'flyspeck' carbon, Carbon Leader, West Wits (US5). B, Mass percent Th02 of coexisting granular allogenic uraninite
and fine-grained pitchblende in columnar carbon, Vaal Reefs,
Klerksdorp (PTS 2034). C, Mass percent Th02 of allogenic uraninite
for one sample from the Hunza River, Kashmir, a tributary of the In­
dus River, two samples from Hazro, Pakistan, on the Indus, and one
sample from the Dominion Reef.

ferent at the 0.001 level, but variations within these
populations are not significant. Significant positive cor­
relation between calcium and manganese and between
iron and manganese is common to both uraninite popula­tions.
The Th02 content, U02/Th02 ratio, and correlation be­
tween calcium and manganese and iron and manganeseprobably reflect conditions of crystallization in thesource area. The higher median U02/Th02 ratios for the
Vaal Reef uraninite and the lower thorium content com­pared with uraninite from the Dominion Reef indicate
some variation in the source area for the two horizons.

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS

Rllnevertheless appear too small for exact hydraulicequivalence. This point was noted by Coetzee (1965), but
no satisfactory explanation has yet been proposed.

80

60

URANIFEROUS AND AURIFEROUS CARBONACEOUS
MATERIAL

40
Pitchblende in
granular carboi
Witwateisrand

1

50

99

CUMULATIVE PERCENT

FIGURE 9.—U02/Th02 ratios for separate populations of uraninite andpitchblende plotted on a cumulative frequency diagram. The highest
median values are given by fine-grained pitchblende in granular car­bon from the Carbon Leader (US 5). Allogenic uraninite from the
Vaal Reefs in the Witwatersrand (PTS 2034) has a lower ratio, whichis still higher than that of uraninite from the Dominion Reef (UG9824). Detrital uraninite from the Indus River at Hazro (YP 10A and
YP 19A) and the Hunza River, Kashmir, a tributary of the Indus, isplotted here for comparison with the Dominion Reef and Witwaters­
rand samples as they occur at the present time. The Dominion Reef
samples have also been recalculated to take account of decay of
uranium and thorium since crystallization at -3,000 million years
ago, and this correction tends to reduce the small difference between
populations.

However, fluid-inclusion studies do not reveal any dif­ferences in the quartz pebbles (Shepherd, 1977).
GRAIN SIZE OF URANINITE AND ASSOCIATED PHASESA typical sample (UG 5538) from the Bramley sectionof the Dominion Reef Mine, Klerksdorp, was examined
in detail by grain-size analysis. Coexisting allogenic
grains of quartz, pyrite, uraninite, and gold were
measured by means of the Humphries eyepiece microm­
eter, which can be used to classify grains directly accor­
ding to the phi scale. Each population, except for gold,
consisted of a hundred grains measured along the Feretdiameter, which is the maximum grain length projected
onto a fixed line. The populations appear to be single
populations in each case resulting in linear cumulativefrequency curves. Sample means and standard devia­
tions were calculated in terms of phi units, from which
the standard error of the mean was obtained. This wastransferred to a chart containing graphical results
calculated for Stokes law and the square-root law com­bined (Tourtelot, 1968), which indicates that quartz,
pyrite, and uraninite closely approximate to a hydraulically equivalent assemblage (fig. 10). The gold grains
counted were too few to be statistically significant but

Uranium and gold in association with carbonaceous
material is one of the most important modes of occur­
rence of these metals in the Witwatersrand Basin, par­ticularly in view of the uniformly high grades of min­eralization encountered and the wide lateral extent ofsuch horizons as the Carbon Leader in the Far West
Rand (Davidson and Bowie, 1951; Bourret, 1973). Any
satisfactory explanation of the nature of this metal-richcarbonaceous matter should take account of the follow­
ing observations:
1. The Carbon Leader, which represents a most striking
enrichment of uranium and gold, forms a very
restricted horizon in the reef never greater than 13
mm thick (Davidson and Bowie, 1951).
2. The levels of both uranium and gold over a wide
lateral extent are consistently high.
3. The U02/Th02 ratios in the carbonaceous type ofmineralization are higher compared with thebanket type of both the Witwatersrand and Domin­
ion Reef Systems, and the absence of the car­bonaceous type of mineralization in the latter.
4. Mineralization is absent in certain occurrences of carbonecous material.
Some authorities consider the carbonaceous material
represents fossilized remains of Precambrian plants(Hallbauer, 1975). Whether such is its origin or not,there is little doubt that it has exercised a key role in theformation of these deposits.
The carbonaceous material comprises granular andcolumnar varieties, which have many features in com­
mon whilst differing in certain important aspects(Davidson and Bowie, 1951). According to the analysesof Davidson and Bowie this material contains 43 percent
carbon and 2 percent hydrogen, and they called thisphase hydrocarbon. However, we have not attempted todefine the organic constituents of all the carbonaceous
materials studied here and have therefore retained amore general nomenclature used by Vine and others(1958).
The granular or "fly speck" material (fig. 11) consistsof ovoid to rounded grains (100-400 /*m) with an in­
terstitial matrix of subrounded allogenic pyrite,
chromite, and concretionary pyrite (-50 /*m). The
granules are partially moulded around interstitial
grains, particularly where they lie along grain boun­daries between granules (fig. 11), but the carbonaceous
matter at no point either replaces or invades the opaque

R12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

-e- 2
cc
LU
LU

Q 4

2

5

10

20

RELATIVE DENSITYFIGURE 10.—Standard error of the mean sizefor coexisting quartz, pyrite, and uraninite
plotted on the phi scale against specific gravi­
ty. The light curves are solutions for Stokes'
law and the square-root law combined where
they overlap. The relative grain sizes of thesuite of minerals plotted in heavy type in­dicate that quartz, pyrite, and uraninite are
hydraulically equivalent. The gold is smaller
than would be expected for hydraulicequivalence, but the number of grains
measured is inadequate for full statisticaltreatment. Bramley high grade, Dominion
Reef (UG 5538). (Diagram after Tourtelot,
1968.)

minerals. The granules have mottled extinction undercrossed polars in reflected light and contain two main
phases of carbonaceous matter (fig. 3D), consisting ofrelatively coarse-grained smooth areas with undulose
extinction interspersed with fine-grained material hav­
ing a similar reflectance of 14.3 percent (Type I ofDavidson and Bowie, 1951, p. 6). The fine-grainedmaterial is veined along grain boundaries by a dendritic
system containing pitchblende, gold, pyrite, galena, andgersdorffite. The gold and gersdorffite are commonly intergrown, as are the gold and pitchblende. Galena tends
to form separate grains but is also intergrown with
pyrite. These textural and mineralogical relationships
have been confirmed by both scanning and quantitativeelectron-probe analysis. The pitchblende has a medianTh02 content of 2.0 percent with a range from 1.1 to 2.9
percent (fig. SA) and a median U02/Th02 ratio of 33 witha range from 25 to 69 (fig. 9). Analysis of variance showsthat this population is significantly lower in its Th02
content at the 0.001 level than are all the other uraninite
and pitchblende phases in the Witwatersrand and Do­
minion Reef. There is significant correlation between
uranium and calcium (0.01 level), uranium and cerium(0.01 level), and PbO and Th02 (0.001 level). However,there is no correlation between calcium and manganeseor between iron and manganese as for the allogenic
uraninite, pointing to a distinct origin for the pitch­
blende in the granular carbon, which probably developedin situ during diagenesis and metamorphism.
The carbonaceous seam of the Carbon Leader usuallyoccurs along the basal parting in quartz-pebble con­glomerate, and has a well-developed columnar structurenormal to the bedding. It is linked with, but different
from, the granular form described. It consists of thesame two carbon phases, occurring as lenticular to ovoid
bodies with their long axes normal to the bedding. In­
dividual bodies are cut by discontinuous vertical sutures,
which emphasize the overall columnar effect (fig. 12).
Scanning electron-probe studies demonstrate that thesesutures carry fine-grained pitchblende, brannerite, gold,
and other phases (fig. 13).
The inclusions within the columnar carbonaceous
material are similar mineralogically to those in the
granular material, with one notable exception, namely
the occurrence of coarse-grained uraninite, often
subrounded and partly replaced by coarse-grained car­bonaceous matter (fig. 14). This uraninite is concen­
trated along one margin of the carbon seam though not
restricted to that margin. Analyses of neighbouring
fragments show that they belong to groups within which
the Th02 content is similar but may differ very con­siderably from that in adjacent groups, indicating achemically heterogeneous assemblage of uraninite
crystals subsequently fragmented and replaced by car­bonaceous matter (fig. 15). The chemistry of the allo­
genic uraninite grains has been previously described andis shown in figures 8 and 9. Pitchblende, associated withpartially replaced allogenic uraninite has a much higher
thorium content than the pitchblende in the granularcarbonaceous matter; it has a median Th02 content of
6.2 percent with a range from 3.17 to 6.9 percent (fig.
85) and a median U02/Th02 ratio of 9.0 with a range
from 6.2 to 12.2. This population is significantly dif­ferent from the coexisting coarse-grained allogenic
uraninite (0.001 level) but not from the uraninite in the
Dominion Reef. It is possible that the high thorium con­tent is attributable to solution and redeposition of bothuranium and thorium from the allogenic uraninite un­dergoing attack by carbonaceous matter.

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMSFIGURE 11.—Electron-probe backscattered electron image of granular carbonaceous material containing fine­grained particles of pitchblende (Pi) and gold in a carbonaceous matrix (black). Allogenic grains of pyrite (Py)and chromite (Cr) occur interstitially to the granular carbon, but allogenic uraninite was not detected. Carbon
Leader, West Wits (US 5)

R13

R14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 12.—Electron-probe backscattered electron image of columnar
carbon showing zones of coarse fragmented allogenic uraninite (Ur)and fine-grained disseminated pitchblende (Pi) in a carbonaceous
matrix (black). Vertical lineations of a strongly reflecting phase be­
tween these zones consist of fine disseminations of gold associatedwith brannerite, pitchblende, and churchite. Vaal Reefs, Klerksdorp(PTS 2034).

FIGURE 13.—Electron-probe scanning images from two overlapping
areas of columnar carbon within inset on figure 12 showing backscat­
tered electron images and distribution of uranium and thorium cor­
responding to coarse-grained uraninite and fine-grained pitchblende.
The latter is intimately associated with gold and a titaniferous phase,probably brannerite, in a carbonaceous matrix (black). Vaal Reefs,
Klerksdorp (PTS 2034).

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS

R15

FIGURE 14.—Electron-probe backscattered electron image of columnar carbonaceous material showing clusters of coarse-grained allogenic
uraninite crystals (Ur) partly digested by carbonaceous matter (black) with lobate zones of fine-grained pitchblende (Pi) in a carbonaceous
matrix. Vaal Reefs, Klerksdorp (PTS 2031).

R16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESThe disseminated pitchblende in fine-grained colum­ rial is also a common site for gold overgrowths. All thesenar carbonaceous matter is optically and texturally very textures suggest that much of the gold, pitchblende,
similar to that in the granular variety and is also in­ churchite (see next section) and associated sulphides
timately associated with galena, gersdorffite, gold, and crystallized in their present position as a result of
pyrite, forming an interconnecting dendritic texture of diagenetic or later metamorphic processes.
microcrystals in the manner previously described.
CHURCHITE
The outer margin of the carbonaceous matter, where
it is in contact with allogenic pyrite, is a favoured site
for gold concentration in the form of fine platelets. In­
A tenuous phase surrounds the fine-grained pitch­ternal pore spaces within carbonaceous matter are also blende of the columnar carbon (fig. 16). It contains ma­
an important site for the formation of complex inter- jor yttrium and phosphorous with the heavy rare-earth
growths of uraniferous silicates, gold, and gersdorffite. elements gadolinium, terbium, dysprosium, erbium, andPartially replaced uraninite within carbonaceous mate­ ytterbium, corresponding to either xenotime (YP04 or

FIGURE 15.—A, Electron-probe backscattered electron image showing
uraninite (Ur) within carbonaceous material. The destruction oflarge uraninite grains by the carbonaceous material is shown by thecomplementary outlines of adjacent grains and in the reconstructionof original grain outlines. B, The Th02 content clearly demonstrates
similarity within but differences between grains outlined. Vaal
Reefs, Klerksdorp (PTS 2031).

R17

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS

churchite (YP04 • 2H20). Comparison of the rare-earth square metres. Treatment of the carbon revealed a
distribution (table 1) with that reported in the literature skeletal framework of uranium and thorium oxides andfor xenotime and churchite points to an identity with the of gold, and slow oxidation of the carbon at 500°C left alatter because of the distinct enrichment of the former heavy-metal fabric, which appears to correspond to thein dysprosium, erbium, and, particularly, ytterbium rela­ structures described in the present study. Since these
metals are most probably deposited in pore spaces in thetive to gadolinium.
This is particularly interesting in the present context, carbonaceous matter, it is unlikely that they represent
since xenotime is found principally in granitic pegma­ the actual skeleton of the "plant" or are a replacement oftites whereas churchite2 can be formed by a combination any fibrous structure as Hallbauer (1975) suggests.
of biochemical processes and diagenetic reactions close However, the observation that several fine-grained
to the surface (Milton and others, 1944). In the occur­ phases containing heavy metals, notably lead, uranium,
rence described by these authors, spherulites of church­ thorium, gold and r.e.e., combined with arsenic or sul­
ite are formed as a coating on siliceous limonite or black phur, are closely associated with possible stromatolitic
manganese oxide. They suggest that the heavy rare algal mats or related structures may have genetic
earths were concentrated biochemically in the leaves of significance for the origin of mineralization in the Wit­
certain deciduous trees. Stability of the forest floor per­ watersrand, which is discussed later.
mitted concentration in the humus layers, from which
INDUS RIVER URANINITE
the churchite coating on existing iron and manganeseoxides was derived diagenetically. It seems possible,
Samples of uraninite from the Indus River were
therefore, that in the Witwatersrand some combination
analysedby electron probe for comparison with the alloof biochemical and diagenetic processes may have led to
genicuraniniteof the Dominion Reef and Witwaters­
the formation of churchite around the fine pitchblende
rand
Systemsandwith a view to assessing the stability
grains in the columnar carbon.
of uraninite as a detrital mineral in recent alluvial
ORIGIN OF THE CARBONACEOUS MATERIAL
deposits. The samples had been collected from the Hunza
River, a tributary of the Indus River in Kashmir
Several authors have reported carbonaceous material(Darnley,1962), and from the Indus alluvium neardevoid of mineralization, so neither uranium nor gold
Hazro,
Pakistan(Miller, 1963).
performs an essential role in its formation. The colum­
The
Hunza
sampleconsists of a 99-percent pure con­
nar structures are developed normal to the bedding, as
centrateinwhichnearlyall the uraninite grains have atin stromatolites, and it is unlikely therefore that they
least
one
well-defined
crystalface and some show cubic
are metamorphic structures as suggested by Liebenberg
form
with
littleevidenceof
corrosion.
They are in the(1955). Hallbauer and van Warmelo (1974) later pro­
very
fine
to
fine
range
of
sand
size
(0.05-0.20mm) (fig.
posed that the structures are a primary feature of the17).
Pyriteispresent
intheconcentrateaseuhedralcarbonaceous matter. They advanced a theory, further
cubes,
freshanduntarnished,andasconcretions.
An ex­
elaborated in Hallbauer and others (1975) and Hallbauer
tensiveassemblageofradioactive
resistateminerals
and(1975), that the columnar carbonaceous material is thesulphides
has
also
been
reported(Darnley,1962).
fossilized residue of a Precambrian symbiotic associa­
The Hazro samples, which Miller (1963) had reportedtion (designated Thycomyces lichenoids) consisting of an
to contain both uraninite and gold, were also realgal partner and a fungal organism and occupying
examined, involving the preparation of concentrates by
areas between a few square centimetres and a few
vanner, superpanner, and heavy liquids. The final con­centrate contained—in addition to uraninite—pyrite,
2 Described as weinschenkite by Henrich (1935), Milton and others, (1944), and Pokrovskiy
and others, (1965) but shown to be churchite by Claringbull and Hey (1953).
arsenopyrite, uranothorite, and a grain of gold. The
TABLE 1. —Comparison of heavy rare-earth distribution in xenotime, churchite, and yttrium phosphate phase in Witwatersrand columnar carbon
relative to gadolinium

Yttrium phosphate phase in carbon1 ______
Xenotime2
Churchite3
Churchite4

Gd

Tb

Dy

1
1
1
1

0.49
.53

1.42
5.57
1

.93

1 Ratio of La X-ray peaks from present study.
2 From an analysis by Jefford (1962, table 2, no. 1), who quotes similar analyses for other specimens.
3 Ratio of La X-ray peaks on Bavarian churchite by Henrich (1935).
4 From the average of two analyses by Pokrovskiy and others (1965, table 2).

Ho

Er

0.72
.72
.16

1.19
5.41
1
.53

Tm

Yb

1.26
trace

0.88
13.03
trace
.60

Lu

1.07
.10

R18

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Tm

Yb

Lu

B
FIGURE 16.—A, Electron probe scanning images of phase "A", probably churchite, containing major yttrium and phosphorus, which
overgrows pitchblende (Pi); the example shown occurs in fibrous material within columnar carbon shown in figure 12. B, The distribu­
tion of heavy rare-earth elements in phase "A" relative to gadolinium is compared here with examples of churchite and xenotime fromthe literature. Vaal Reefs, Klerksdorp (PTS 2023).

URANIUM MINERALIZATION t 7 THE WITWATERSRAND AND DOMINION REEF SYSTEMS

sulphides were fresh, but many of the uraninite crystals,
with at least one crystal face, had a distinct outerweathered zone, though beneath this they were freshand homogenous.
The Th02 content of one sample from the Hunza andtwo from Hazro are shown in figure 8C. The three
populations exhibit no significant difference, indicating
constancy in uraninite composition between sites 300
km apart. The mean U02/Th02 ratio for the Indus
samples is shown in figure 9, to which has been added
the ratio for the Dominion Reef uraninite adjusted for
uranium and thorium decay to eliminate the effect of thegreater age. There is no significant difference between
the two uraninite populations after adjustment.
Viljoen (1963) regarded the lack of oxidation of
detrital pyrite in the Witwatersrand System as evidenceof an atmosphere free of oxygen, and Liebenberg (1955)
and Grandstaff (1975) considered a nonoxidizing atmos­
phere was necessary for uraninite transportation.
Neither of these conclusions is supported by theevidence from the present-day Indus River, in which, as

R19

we have seen, uraninite, gold, pyrite, and othersulphides form a stable assemblage over several hun­dred kilometres.
GEOCHEMICAL EVIDENCE
LEAD CONTENT OF URANINITE AND PITCHBLENDE

Cumulative frequency curves for the lead content of
selected uraninite and pitchblende phases are shown infigure 18. The data fall on four main distributions asfollows: (1) relatively young (approximately 90 million
years) low-lead allogenic uraninite from the Indus, (2)pitchblende in granular carbon from the Carbon Leader
in the Witwatersrand with intermediate lead content,(3) allogenic uraninite in the Witwatersrand and Domin­
ion Reef with high lead content, and (4) pitchblende withhigh lead content intimately associated with allogenic
uraninite in the Witwatersrand.
The intermediate lead content of the pitchblende sam­ple US 5 is consistent with the textural observation that
this phase is younger than the allogenic uraninite and is

FIGURE 17 -Indus River uraninite. A, Uraninite grains from the Hunza River, Kashmir, a tributary of the Indus River; the uraninite grains
usually show at least one crystalline face and several cubes are present. B, The grain shown in detail exhibits alteration along discrete zones
parallel to well-developed crystal faces. Most of the grains in the sample, however, are fresh and unaltered. No correlation between degree ofalteration and Th02 content has been detected.

R20

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESURANIUM MICROMAPPING WITH THERMAL NEUTRONS

2034,

Allogenic uraninite incolumnar carbon, ,Vaal Reefs,/''
92 analyses /'Pitchblende incolumnar carbon,
Vaal Reefs,
21 analyses

O
JD
Q.

UG 9814,
Uraninite,
Dominion Reef,
11 analyses

(J
CC
ff

10

US 5,
Pitchblende in
granular carbon.
Carbon Leader,
23 analyses
YP 19 A,
Uraninite,
Indus,
19 analyses

NCL 1115,
Uraninite,
Hunza, Kashmir,
33 analyses\\^...~£~~

99
CUMULATIVE PERCENT

FIGURE 18.—Cumulative frequency diagram of the PbO content of
uraninite and pitchblende from Dominion Reef, Witwatersrand, andIndus River sediments.

formed in situ from the diagenesis and metamorphism of
the carbonaceous host. The low thorium content of thisphase is indicative of a low-temperature solution (thori­
um being relatively insoluble), from which uranium was
precipitated by absorption on decaying organic matter
represented by the carbonaceous material in the section.
The highest lead contents are given by the allogenic
thorian uraninite phases, which are probably a reflectionof their greater age. Partial rejuvenation of the allo­
genic uraninite from the Witwatersrand resulted in theformation of a lead-rich pitchblende in sample 2034.
However, the evidence from sample US 5 indicates thatthe presence of allogenic uraninite is not essential forthe formation of younger pitchblende, since it may also
form directly from the metamorphism of uraniumenriched carbonaceous material. Younger ages ex­hibited by pitchblende in the absence of allogenic
uraninite should not be generally interpreted therefore
as rejuvenation of an older allogenic uraninite, since inthe example studied here, pitchblende represents direct
formation of this phase from a metal-enriched organic
substrate; allogenic uraninite was not detected.

The distribution of uranium in Witwatersrand andDominion Reef samples was studied by the fission-track
method using Lexan polycarbonate for the registrationof fission fragments from the induced fission of 235U bythermal neutrons, as described by Price and Walker(1963), Kleeman and Lovering (1967), and Bowie andothers (1973). The method is ideal for providing informa­tion on the location of uranium in banket-type occur­
rences. The radioactive bands of interest can usually be
included within the breadth of a single polished thin sec­tion and radiation doses can be adjusted so as to spanconcentration ranges. The samples studied here havebeen subjected to a dose of approximately 5 x 1016n cnr2
in order to obtain a lower detection limit of 0.01 ppm(parts per million) and track saturation at about 40 ppm.
This enables assocations of uranium with mineralogicalphases to be readily determined when the uranium isbelow the limits of detection by microprobe.
The results show that uranium is closely confined todiscrete horizons both in the banket-type and carbo­
naceous reefs and that it not only occurs as grains ofuraninite but also in a finely disseminated form. Thus
uranium is present in pyrite concretions; within len­ticular fragments comprising clay-size quartz, kaolinite,and pyrophyllite; as a fine disseminated coating on allo­
genic pyrite grains otherwise uranium free; and, rarely,
within dislocations and shears (figs. 5, 6, 19).
Both microprobe and fission-track studies show thepresence of uranium in the interstitial 7-A clay phase in
the pyrite concretions (fig. 7A) but fail to detect it in the
pyrite itself. A combination of these techniques showsthat the uranium content of individual concretions isvariable within a single reef over a distance of a few
millimetres. Whereas some are virtually uranium free,others are enriched in amounts readily detected by elec­tron microprobe (>500 ppm). Uranium enrichment oc­
curs only in the rims of some concretions, whereas inothers, particularly the finer grained melnikovitic types,
the enrichment usually occurs throughout the concre­tion. This association of uranium with pyrite is par­ticularly striking where it occurs surrounded by anessentially barren quartzitic groundmass, and in casessuch as this the evidence for a primary association of
uranium with the concretion is strong and probably
related to deposition of uranium during the formation of
the concretion. The uranium in the rims of concretions
was probably absorbed during transport or diagenetically prior to lithification.
Allogenic pyrite is also closely associated withuranium, but the uranium in the pyrite is below detec­tion limit, and in this case the uranium occurs in a secon­
dary clay mineral forming a marginal overgrowth to thepyrite. This association is particularly well developed in

R21

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS

ui^ 13*a '53 Q>™ > &"§ ^ rt•&; cs o>

11:1

c c

8 * Sb<u -^s4=

CO"C c giu
0'S -S o>•§ .s s s
T3

§ "S OR22

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

allogenic pyrite occurring in close proximity to a carbo­
naceous band rich in uranium and gold (PTS 1944, West
Driefontein). In this case the uranium was probably
deposited in situ and subsequently incorporated into theclay mineral overgrowth, which formed during diagenesis and metamorphism. This relationship between
allogenic pyrite and overgrowths of fine-grained clay
minerals and uranium has been recorded widely and is
an important control in the deposition of uranium in the
Witwatersrand System. The control is specific sinceuraniferous silicates overgrowing allogenic pyrite occur
dispersed through a barren quartzite matrix and arereadily identifiable and distinguishable from the tex­tures produced by shearing and later introduction of
uranium.
The association of uranium with lenticular to rounded
granules consisting of quartz, kaolinite, and pyrophyllite
has been recognized in many of the Witwatersrandbanket-type ores studied here. The uranium occurs dis­seminated throughout the granules and produces point
sources of fission tracks, which have not yet been cor­
related with any specific uranium mineral (fig. 19). In
some specimens this is the only uranium-bearing mode.
The occurrence of these enriched granules in a barren
groundmass indicates that uranium enrichment mostprobably occurred either before deposition or at thelatest during diagenesis. Larger rounded pebbles also
occur, some consisting of quartz, pyrophyllite, kyanite,and possible illite with secondary spherulitic structures.
In this case uranium occurs on what appears to be aleached and weathered margin and along a vermicularveinlet structure within the pebble. Enrichment inuranium during transport and diagenesis related toweathering of the pebble rim (fig. 19) seems to be themost likely explanation of this occurrence. Regional
metamorphism has tended to reduce the thickness of theuraniferous rim, which is best developed where
weathered pebbles are relatively unaltered.

for comparison with uranium and thorium. This gradational variation in the U/Th ratio is an approximate
measure of the depositional environment. The highenergy environment has a low ratio, and the low-energy
environment a high ratio, with continuous variation be­
tween the two.
BIMODAL NATURE OF THE URANIUMMINERALIZATIONThe occurrence of allogenic grains of uraninite in the
Dominion Reef (Taylor and others, 1962) and the Wit­
watersrand (Liebenberg, 1955; Grandstaff, 1975) is wellestablished. This phase occurs in the Dominion Reef inassociation with resistate phases such as garnet, monazite, cassiterite, zircon, allogenic sulphides, rareauthigenic sulphides, and gold. The similarity of thisassemblage with modern gravels on banks in the Indus
is notable, although uraninite in the Indus is at present
subeconomic (Miller, 1963). In the Witwatersrand Sys­
tem the characteristically lower content of allogenic
uraninite and rounded resistate phase, the relatively
greater importance of concretionary pyrite, and the ap­
pearance of carbonaceous matter as an important acces­sory or even dominant phase represent a markedly dif­ferent style of mineralization.
The general similarity of the median values of the
U02/Th02 ratios between 14 and 20 for allogenic
uraninite from the Indus Valley, Dominion Reef, andWitwatersrand is most significant. However, the pitch­
blende associated with granular carbonaceous material
in the Witwatersrand has a much higher median ratio of
about 32, and this is a distinct and separate phase ofmineralization differing from the allogenic uraninite
texturally, compositionally, and in its close association
with carbonaceous matter and gold.
The fission track studies have clearly indicated an
association of uranium in Witwatersrand sediment withclay minerals occurring individually or intergrown with,or
overgrowing, pyrite, and in many cases this associa­
NEUTRON-ACTIVATION ANALYSIS OF WHOLE-ROCKtion
has the appearance of a primary, essentially
SAMPLESsyngenetic, relationship unaffected by appreciable
Instrumental neutron-activation analysis of whole- uranium remobilization.
rock samples was undertaken as an adjunct to the minEvidence of this type from Witwatersrand sediments
eralogical studies utilizing a method developed by Plant provides support for the existence of some uranium inand others (1976).
solution in the basin during sedimentation and diagen­The results are given in table 2 and demonstrate the esis, as discussed by Pretorius (1975). The uranium
increase in U/Th ratios from the Dominion to the Vaal would be highly mobile as the soluble uranyl ion, U022+,
Reefs in the Witwatersrand Basin. Freddies Reef, East which would subsequently be absorbed, reduced, andGeduld, and Daggafontein are somewhat higher and precipitated by organic-rich muds probably aided by andsimilar to one another, whereas samples from West associated with the simultaneous reduction of sulphate
Driefontein, dominated by carbonaceous matter, have by bacteria, as proposed by Trudinger (1971):
the highest ratios. Twenty-four additional elements ob­
tained by this method on the same sample are presented

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMSThe hydrogen sulphide formed would subsequently reactwith other metals in solution such as iron and lead toform the authigenic intergrowths of pyrite and galena
with clay minerals, gold, and uranium observed in thesediments.
The most likely modes of introduction of uranium,
gold, and sulphate in solution are the complex fault
system of the Witwatersrand Basin and dissolution ofcoexisting detrital minerals. Some of the sulphate would
have been precipitated in close proximity to hot springsat the margin of the basin, but some gold and uranium
would have remained in solution, perhaps with the aid of
metal-organic complexes (Vine and others, 1958; Ling
Ong and Swanson, 1974; Boyle and others, 1975). The
gold and uranium in solution would have been carriedout into the basin to be precipitated together on the
distal portions of the alluvial fans by the action of decay­
ing organic matter.
This process accounts for the observed primary distri­bution of uranium and gold, other than the purely
detrital material, and explains the separation of
uranium and thorium in the progression from the higher
energy Dominion Reef to the lower energy Witwaters­
rand environment. Though relatively unimportant in thehigher energy banket type of mineralization, deposition
from solution through the agency of organic matter
would seem to be responsible for major economic con­centrations of both uranium and gold over large areas in
the Witwatersrand Basin in the lower energy Carbon
Leader type of mineralization.
CONCLUSIONSThe overall control of the uranium mineralization in
the Witwatersrand and Dominion Reef Systems is thepresence of a depositional basin overlying an Archaeancraton in which very early differentiation on the model
proposed by Fyfe (1970, 1973) and amplified by Moorbath (1975) enriched the upper crust in oxyphile metals
including uranium. This process has been considered byBowie (1970) as a fundamental control in the delineationof metallogenic provinces and hence in the distribution
on a global scale of economic concentrations of uranium.
In this basin, uranium was deposited in two ways, as
detrital allogenic uraninite and by precipitation fromsolution. The detrital components were concentrated in
the higher energy conglomerate environment of the
Dominion Reef and proximal parts of the Witwaters­
rand Basin. The solution components including uranium
and gold were precipitated under reducing conditions
through the agency of decaying organic matter in thelower energy or distal regions of the basin, giving risethrough diagenesis and metamorphism to such uranif-

R23

erous horizons as the Carbon Leader of the West Wits,
which are not directly related to known palaeogeographical channels. It can now be clearly demonstrated that
relatively small amounts of thorium increase the resis­
tance to attrition and oxidation to the extent that athorian uraninite with 1 percent or more thorium oxide
can survive as river detritus under present-day at­mospheric conditions. All of the uraninite grains fromthe Witwatersrand Basin considered to be of detrital
origin contain 1 percent thorium oxide or more, hence it
is unnecessary to postulate a reducing atmosphere; in­deed, the retention of sulphate and uranyl ions in solu­
tion would seem to require the existence of an oxidizing
atmosphere in the model proposed here for the deposi­tion of uranium from solution.
This conclusion therefore suggests that the occur­
rence of pebble conglomerates with detrital uraninite
and pitchblende deposited from solution may have amore widespread occurrence throughout geological timethan hitherto suspected, provided the necessarygeological criteria for their formation are met.
ACKNOWLEDGMENTS

The writers wish to acknowledge the assistance of I.
R. Bashan and D. A. Briggs in preparing heavy-mineral
concentrates from the alluvium samples from Hazro, In­
dus River. B. R. Skilton and J. A. T. Smellie preparedpolished thin sections, and M. J. Cope prepared Lexan
prints. D. Atkin provided X-ray diffraction identifica­tion. In particular we wish to acknowledge helpful
discussions with S. H. U. Bowie, A. G. Darnley, and J.
E. T. Home. We are indebted to A. G. Darnley and J. M.
Miller for the Indus River samples and to S. H. U.
Bowie, A. G. Darnley, and H. C. M. Whiteside forsamples from the Dominion Reef and Witwatersrand
sediments. C. Goode and J. Herrington of the Herald
Reactor Group, A. W. R. E., Aldermaston, are thanked
for the provision of whole-rock instrumental neutronactivation analyses and for irradiation of sectioned
material for induced fission-track analysis. Jane Plant isthanked for helpful discussions on solution chemistry,and R. J. Howarth, for advice on statistical methods.
This work forms part of the research programme of theInstitute of Geological Sciences and is published withthe approval of the Director.
The authors and the U.S. Geological Survey thank The
Royal Society of London for permission to republish thispaper. It first appeared in The Royal Society's
Philosophical Transactions [Phil. Trans. R. Soc. Lond.
A. 286, 527-548, (1977)]. Much of the material containedin this paper was presented orally by P. R. Simpson at
the workshop sponsored by the U.S. Geological Survey.

R24

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 2.—Neutron-activation analysis, in parts per million, except where stated otherwise
SamplelocationRocksampleDominion Reefs

5A:
2006/2133
BR8N2(B):2004/2134
BR6N4(A):2018/2135
BR7N1(B):1998/2136
UR21A:
2055/2121
UR3:
2034/2122
UR5:
2039/2123
UR16:
2049/2124
FW4:
2105/2137
FW2:
2103/2138
EG4:
2064/2139
EG18:
2078/2140
26:
1995/2128
27:
1996/2129
KE4/7:
1975/2130
K6F12/11:
1979/2131
KG6/10:1978/2132
8A:
1959/2125
4B:
1950/2126
5:
1951/2127

Vaal Reefs

Freddies Reef

East Geduld

Daggafontein

West Driefontein

Na

K

Sc

Cr

Fe1

Co

Zn

As

Rb

Ag

Cd

Sb

Cs

1 2.19< 2,000

38

<400

10.2

570

<300

3,800

<100

<40

<100

<28

<5

<200

6,600

24

249

7.31

260

<300

340

<200

<35

<100

<6

<5

<200

< 2,000

30

374

4.96

425

<300

3,100

<200

<40

<100

<10

<6

< 1,000< 2,000

32

297

7.63

420

<300

3,600

<200

<40

<100

19

<10

<800

< 2,000

10

1,400

6.56

260

344

605

<100

<20

<100

25

<5

< 1,000< 2,000

13

450

2.80

216

<350

1,200

<200

<40

<100

53

<6

980

< 2,000

7

890

4.89

229

<160

670

<200

<40

<100

27

7

990

< 2,000

8

315

1.65

120

<160

370

<100

<30

<100

21

5

430

<100

<15

<100

6

<5

52

< 2,000

4.2

850

1.77

108

440

66

< 2,000

4.6

760

3.17

123

354

620

<80

22

<100

10

<5

330

5,900

6.7

2,300

5.09

200

366

640

<100

<15

<100

15

<5

190

930

4.6

560

3.63

117

295

140

<100

<40

<100

<5

<5

<100

24

<100

<5

<3

94

< 2,000

6

152

6.76

312

239

336

256

< 2,000

18

1,200

7.67

219

<800

296

<100

<40

<100

<5

<3

220

< 2,000

3

115

.21

21

220

29

<50

<15

<100

<5

<3

86

< 2,000

8

244

1.19

43

818

134

<100

<15

<100

<5

<3

<70

<15

<100

6

<3

520

2,200

11

650

2.79

55

<100

120

387

< 2,000

7.5

1,040

4.93

140

360

450

<100

46

<100

9

<3

204

< 2,000

4

235

4.34

59

264

46

<50

39

<100

<5

<3

<400

< 2,000

4

84

2.38

14

463

60

<150

130

<100

7

<3

REFERENCES CITED
Bourret, W., 1973, First uranium report Witwatersrand, South Africa:
Utah International, Inc., San Francisco, 51 p.
Bowie, S. H. U., 1970 Some geological concepts for consideration in
the search for uranium provinces and major uranium deposits, inUranium exploration geology: International Atomic Energy Agen­cy, Proceedings of a panel, Vienna, 1970, p. 285-300.
Bowie, S. H. U., Simpson, P. R., and Atkin, D. A., 1975, Reflectancemeasurements in monochromatic light on the Bowie-Taylor suiteof 103 ore minerals: Fortschritte der Minerologie 52, p. 567-582.
Bowie, S. H. U., Simpson, P. R., and Rice, C. M., 1973, Application offission-track and neutron activation methods to geochemical ex­
ploration, in Geochemical Exploration 1972: Institution of Mining
and Metallurgy, London, pp. 359-372.
Bowles, J. F. W., 1975, An automatic data-handling system for quan­titative X-ray microprobe analysis: Institute of Geological
Sciences, report no. 75/9, 14 p.
Bowles, J. F. W., (in preparation), Quantitative energy dispersive
analysis of ore minerals.
Boyle, R. W., Alexander, W. M., and Aslin, G. E. M., 1975, Someobservations on the solubility of gold: Canada Geological Survey
Paper 75-24, 6 p.
Claringbull, G. F., and Hey, M. H., 1953, A re-examination ofchurchite: The Mineralogical Magazine, 30, p. 211-217.
Coetzee, F., 1965 Distribtuion and grain-size of gold, uraninite, pyriteand certain other heavy minerals in gold-bearing reefs of the Wit­
watersrand Basin: Geological Society of South Africa Transactions
68, p. 61-88.

Cooper, R. A., 1923, Mineral constituents of Rand concentrates:
Chemical, Metallurgical, and Mineralogical Society of South
Africa, 24, p. 90-95.
Darnley, A. G., 1962, Detrital uraninite from the Hunza River,
Kashmir: Great Britain Geological Survey, Atomic Energy Divi­sion, Age Determination Report no. 22, unpublished.
Davidson, C. F., and Bowie, S. H. U., 1951, On thucholite and relatedhydrocarbon-uraninite complexes: Geological Survey of Great Bri­
tain Bulletin 3, p. 1-18.
Drake, M. J., and Weill, D. F., 1972, New rare-earth element stand­ards for electron microprobe analysis: Chemical Geology, v. 10, p.
179-181.
Feather, C. E., and Koen, G. M., 1975, The mineralogy of the Wit­
watersrand reefs: Mineral Science Engineering, v. 7, p. 189-224.
Frondel, Clifford, 1958 [1959], Systematic mineralogy of uranium andthorium: U.S. Geological Survey Bulletin 1064, 400 p.
Fyfe, W. S., 1970 Some thoughts on granitic magmas, in Newall, G.,
and Rast, N., eds., Mechanisms of igneous intrusion: Geological
Journal, Special Issue no. 2, p. 201-216.
———1973, The granulite facies, partial melting and the Archaean
crust: Royal Society of London Philosophical Transactions, A.273,
p. 457-462.
Grandstaff, D. W., 1975, Microprobe analyses of uranium and thorium
in uraninite from the Witwatersrand, South Africa, and BlindRiver, Ontario, Canada: Geological Society of South Africa Tran­sactions 77, p. 291-294.
Graton, L. C., 1930, Hydrothermal origin of the Rand gold deposits,
pt. 1, Testimony of the conglomerates: Economic Geology, v. 25,
supp. to no. 3, 185 p.

R25

URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMSTABLE 2.—Neutron-activation analysis—Continued
SamplelocationRocksampleDominion Reefs

5A:
2006/2133
BR8N2(B):2004/2134
BR6N4(A):2018/2135
BR7N1(B):1998/2136
UR21A:
2055/2121
UR3:
2034/2122
UR5:
2039/2123
UR16:
2049/2124
FW4:
2105/2137
FW2:
2103/2138
EG4:
2064/2139
EG18:
2078/2140
26:
1995/2128
27:
1996/2129
KE4/7:
1975/2130
K6F12/11:
1979/2131
KG6/10:1978/2132
8A:
1959/2125
4B:
1950/2126
5:
1951/2127

Vaal Reefs

Freddies Reef

East Geduld

Daggafontein

West Driefontein _____

Ba

La

Ce

Nd

Sm

Eu

Tb

Lu

Hf

Ta

Au

Th

U

U/Th
ratio

< 500

1 1.3

1,200

1,000

445

<2

20

14

<10

410

3

809

3,900

4.8

< 1,000

906

1,200

850

285

12

16

10

15

240

9

620

2,300

3.7

< 1,000

1,400

2,200

910

335

12

17

11

19

450

2

753

2,300

3.1

< 1,000

1,100

1,800

1,270

600

20

15

18

18

450

5

970

5,400

5.6

< 1,000

260

700

1,500

450

20

8

13

22

8

154

477

4,500

9.4

< 1,000

475

900

2,800

850

37

24

22

30

<100

218

821

8,400

10.2

< 1,000

248

350

860

200

17

6

6

36

7

69

298

2,100

7.1

< 1,000

172

500

940

250

15

6

8

11

<5

214

273

2,700

9.9

<500

59

260

170

60

4.7

<5

<3

18

30

90

52

685

13.2

65

5.3

<5

<3

<20

30

115

80

765

9.6

17

6

118

40

510

12.8

<500

68

135

195

<500

41

<200

280

<10

3.6

5

<3

<500

57

80

73

12

1.2

2

<3

11

4

< 1,000

44

<100

227

75

1.5

<2

<1

4

15

173

< 1,000

42

<10

400

160

8

3

<3

8

20

132

<5

1.5

20

75

3.8

71

895

12.6

102

2,000

19.6

< 1,000

20

22

9

6

1

<2

<1

<2

<5

43

< 1,000

16

45

112

30

1.5

<2

<1

2

2

28

10

337

<300

22

55

43

20

2

<2

<1

2

30

11

1ft

180

12.0

< 1,000

47

<300

88

40

3

<2

<5

19

50

428

40

460

11.5

< 1,000

25

<10

340

100

6

3

<3

7

<5

221

75

1,400

18.7

< 1,000

69

<30

454

140

7

16

4

<5

<5

444

99

1,500

15.2

.4

1 Percent.

Hallbauer, D. K., 1975, The plant origin of the Witwatersrand carbon:
Minerals Science and Engineering, v. 7, p. 111-131.
Hallbauer, D. K., Jahns, H. M., and Beltmann, H. A., 1975, Mor­phological and anatomical observations on some Precambrianplants from the Witwatersrand, South Africa: Chamber of Minesof South Africa Resolution Report 49/75, 22 p.
Henrich, F., 1935, A mineral occurring in Germany with rare earths as
essential constituents: Journal of Prakt. Chemistry, v. 142, p. 1-5.
Jefford, G., 1962, Xenotime from Ray field, N. Nigeria: American
Mineralogist, v. 47, p. 1467-1473.
Koppel, V. H., and Saager, R., 1974, Lead isotope evidence on the
detrital origin of Witwatersrand pyrites and its bearing on the pro­venance of the Witwatersrand gold: Economic Geology, v. 69, p.
318-331.
Kleeman, J. D., and Lovering, J. F., 1967, Uranium distribution
studies by fission track registration in Lexan plastic prints: Atomic
Energy Australia, v. 10, p. 3-8.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef and the Black Reef:
Geological Society of South Africa Transactions, v. 58, p. 101-227.
Ling Ong, H., and Swanson, V. W., 1974, Natural organic acids in thetransportation deposition and concentration of gold: Colorado
School of Mines Quarterly, no. 69, p. 395-425.
Mason, P. K., Frost, M. T., and Reed, S. J. B., 1969, B.M.-I.C.-N.P.L.
Computer programs for calculating corrections in quantitative
X-ray microanalysis: National Physical Laboratory I.M.S. Reportno. 2, unpublished.
Miller, J. M., 1963, Uraninite-bearing placer deposits in the Indus

alluvium near Hazro, Pakistan: Great Britain Geological Survey,
Atomic Energy Division Report no. 254, unpublished.
Milton, C., Murata, K. J., and Knechtel, M. M., 1944, Weinschenkite,
Yttrium phosphate dihydrate, from Virginia: American
Mineralogist, v. 29, p. 92-107.
Moorbath, S., 1975, The geological significance of early Precambrianrocks: Geological Association [London] Proceedings, v. 86, pt. 3, p.
259-279.
Plant, J., Goode, C., and Herrington, T., 1976, An instrumental
neutron activation method for multi-element mapping: Journal of
Geochemical Exploration, v. 6, no. 3, p. 299-319
Pokrovskiy, P. V., Tormosova, G. F., and Kolenkoi, L. I., 1965,
Weinschenkite from the Central Urals: Dokt. Earth Science Sec­tion 162, p. 133-136 (Translated from Dok. Akad. Nauk SSSR 162,
p. 173-175).
Pretorius, D. A., 1975, The depositional environment of the Wit­
watersrand gold fields: A chronological review of speculations andobservations: Minerals Science and Engineering, v. 7, p. 18-47.
Price, P. B., and Walker, R. M., 1963, A simple method of measuring
low uranium concentrations in natural crystals: Applied Physics
Letter 2, p. 23-5.
Ramdohr, Paul, 1958, New observations on the ores of the Witwaters­
rand in South Africa and their genetic significance: GeologicalSociety of South Africa Transactions, Annexure to v. 61, p. 1-50.
Robertson, D. S., 1974, Basal Proterozoic units as fossil time markersand their use in uranium prospection, in Formation of uranium ore
deposits—Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens, Greece,

R26

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

6-10 May 1974: Vienna, International Atomic Energy Agency, p.
495-512.
Saager, R., 1970, Structures in pyrite from the Basal Reef in the
Orange Free State Goldfield: Geological Society of South Africa
Transactions, v. 73, p. 31-46.
Saager, R., and Mihalik, P., 1967, Two varieties or pyrite from the
Basal Reef of the Witwatersrand System: Economic Geology, v.
62, p. 719-731.
Shepherd, T. J., 1977, Fluid inclusion study of the Witwatersrand golduranium ores. Royal Society of London Philosophical Transactions,A.286, p. 549-565.
Taylor, K., Bowie, S. H. U., and Home, J. E. T., 1962, Radioactive
minerals in the Dominion Reef: The Mineralogical Magazine, v.
107, p. 329-332.

Tourtelot, H. A., 1968, The hydraulic equivalence of grains of quartz
and heavier minerals and implications for study of placers: U.S.
Geological Survey Professional Paper 594-F.
Trudinger, P. A., 1971, Microbes, metals and minerals: Minerals
Science and Engineering, v. 4, p. 13-25.
Viljoen, R. P., 1963, Petrographic and mineragraphic aspects of theMain Reef and Main Reef Leader on the Main-Bird Series, Wit­
watersrand System: Unpublished M. Sc. Thesis, University of the
Witwatersrand, Johannesburg.
Vince, J. D., Swanson, V. E., and Bell, K. G., 1958, The role of humic
acids in the geochemistry of uranium: Second U.N. International
Conference on Peaceful Uses of Atomic Energy, Proceedings 2, p.
187-191.

Detrital Uraninite and Pyrite:
Are They Evidence for a
Reducing Atmosphere?
By P. R. SIMPSON and]. F. W. BOWLESGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

PAPER

1161-S

CONTENTS
Page

Abstract _____________________________________________
Introduction ___________________________________________
Evidence from allogenic uraninite in the Witwatersrand and Indus River ___________
Evidence from theoretical models of uraninite stability _____________________
Evidence from carbonaceous matter in the Witwatersrand ___________________
Evidence from authigenic pitchblende and disseminated uranium in the Witwatersrand
System ____________________________________________
Evidence from the sulphur cycle in the Witwatersrand _____________________
Inorganic surface oxidation of pyrite _____________________________
Bacterial surface oxidation of pyrite _____________________________
Bacterial reduction of sulphate below the sediment/water interface _____________
Summary ________________________________________________
Conclusions ____________________________________________
References cited _________________________________________
Discussion following papers by Drs. Feather, Sims, and Simpson and Bowles _________

SI
1
2
2
4
4
6
6
6
6
6
7
7
8

ILLUSTRATIONPage

FIGURE 1. Photomicrographic mosaics showing columnar carbonaceous matter containing detrital and secondary pitchblende; massive
carbonaceous matter containing authigenic pitchblende _________________________________-_—S5

TABLESTABLE 1. Electron microprobe analyses of uraninite grains from Indus alluvium, Hazro, Pakistan _________________—_—_
2. Correlation matrix for the chemical composition of uraninite grains listed in table 1 ________________________
in

S3
4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESDETRITAL URANINITE AND PYRITE:ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?
By P. R. SlMPSON and J. F. W. BOWLES 1
ABSTRACT

Many occurrences of uraniferous quartz-pebble conglomerates fromwidely separated regions are known to date from the period 2.8-2.0
billion years ago. The deposits of the Witwatersrand System areamong the most important of this type, and the occurrence in them ofallogenic uraninite and pyrite has been widely used as evidence thatthe atmosphere during that period of the Earth's history was reducing
or anoxygenic.
Comparison of modern alluvial sediments of the Indus River withthose of the Dominion Reef and Witwatersrand Systems indicatesstable assemblages of detrital thorian uraninite and pyrite of similar
grain size and composition. Authigenic pitchblende and pyrite provide
indirect evidence of uranyl and sulphate ions coexisting with detrital
thorian uraninite and pyrite during the formation of Witwatersrand
sediments. It is suggested that this bimodal form of transport anddeposition can be most readily accounted for by a model in which thesulphate and uranyl ions exist in superficial, oxidised, active sediments
and river waters draining into the Witwatersrand Basin while detritalgrains of uraninite were contained within the bedload of streams in
reducing conditions below the sediment/water interface. The per­sistence of uraninite during burial and diagenesis would be favored bycatastrophic burial of the sediments in a subsiding basin. This would
prevent oxidation of the abundant organic matter, now represented bycarbonaceous material, to give carbonate and bicarbonate ions, which
form uranyl carbonate complexes that are highly soluble even in reduc­ing conditions.
The occurrence of Precambrian uraninite-bearing conglomerates istherefore controlled by geological and not atmospheric conditions. It issuggested that the deposits are the erosion products of the upper
layers of a primitive, layered Archaean crust containing granitic rocks
enriched in thorian uraninite. The period 2.8-2.0 billion years ago isthought to represent a unique phase of crustal evolution, when exten­
sive erosion of uranium-rich upper crust resulted in the deposition of
uraniferous sediments in enclosed basins.
INTRODUCTION

Many occurrences of uraniferous quartz-pebble con­glomerates from widely separated regions of the Earthsuch as Witwatersrand in Southern Africa and Elliot
Lake in North America are known to date from theperiod 2.8-2.0 b.y. (billion years) ago. The deposits of
the Witwatersrand are economically among the mostimportant of this type. Gold and uranium are contained
1 Institute of Geological Sciences, Geochemical Division, 64-78 Gray's Inn Road, London
WCIX 8NG.

in conglomerate horizons in the lower portion of the
Dominion Reef system and in conglomerate and car­bonaceous reefs within the upper portions of the Wit­
watersrand System. The uranium has two principal
modes of occurrence, consisting principally of uraninite
grains in conglomerate and uraninite and pitchblende incarbonaceous horizons. Fresh unoxidised pyrite is pre­sent as coarsely crystalline grains and as fine-grained
concretions in conglomerate. Gold is present as grains inconglomerate and intergrown with pitchblende in car­bonaceous horizons (Simpson and Bowles, 1977).
Uraninite, pyrite, and gold thus occur as both allogenic
and authigenic minerals in this deposit.
According to Koppel and Saager (1974), the Wit­
watersrand rocks are flanked by a number of domelike
small inliers of granite-greenstone basement, which
form the elliptical frame defining the depositional basinof the Dominion Reef and Witwatersrand sediments.
These inliers are considered to be the provenance area
for allogenic uraninite, gold, pyrite, and associatedheavy minerals found in the basin, and direct connec­
tions have been established for gold and pyrite. The
allogenic uraninite is similar in age to the granites (Rundie and Snelling, 1977).
The presence of allogenic uraninite and pyrite in the
Witwatersrand sediments has been widely used as oneof the principal items of geological evidence that the at­mosphere during the period 2.8-2.0 b.y. ago was reduc­ing or nonoxidising (Liebenberg, 1955; Ramdohr, 1955,
1958; Holland, 1962; Schidlowski, 1966, 1970;
Schidlowski and others, 1975). Robertson (1974) con­siders that the occurrence of uraninite-bearing quartzpebble conglomerates is restricted to a period in time
following the development of extensive acid crust but
prior to the evolution of an oxidising atmosphere. Thissuggestion depends heavily, however, on the belief thaturaniferous conglomerates provide evidence for a reduc­ing atmosphere in the first instance. There is no disputeas to the relative abundance of these deposits in theearly Precambrian, which contains all the presently
known occurrences of uraniferous conglomerate ofSIS2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESIn order to test this suggestion and to investigate thepossible role of other cations in stabilizing allogenic
uraninite, 33 grains of uraninite from the Indus at Hazro
were analysed on an electron microprobe using both
energy-dispersive and wavelength-dispersive tech­niques. The microprobe analyses are presented in table
1. A correlation matrix for the electron-microprobe data
is given in table 2, which indicates very significant
negative correlation between U02 and Y203, U02 andCe203, Ti02 and Th02, Ti02 and Ce203 . Significant
positive correlations exist between U02 and Ti02, Th02
and Ce203, CaO and Ce203, Y203 and Fe203 . PbO andMnO do not correlate with any other oxides. The minor
element correlations suggest that the grains represent asingle population in the bedrock and are probably not
derived from multiple unrelated phases of mineralisa­
tion in which correlation would be less likely. The lack ofcorrelation of PbO is accounted for by radiogenic origin
and consequent uneven distribution within uraninite
grains.
Comparison of Th02 content with size, shape, roundness, degree of alteration, and sphericity of theuraninite grains suggests that there is no correlation
between the principal compositional variable, Th02
(1.20-12.08 percent), and the sedimentary features ofuraninite grains that are likely to have resulted fromweathering, transportation, and diagenesis. It is notknown, however, whether uraninite containing less than
1.2 percent Th02 is either relatively soluble or isunavailable in the source regions. From the textural in­
EVIDENCE FROM ALLOGENIC URANINITE IN
formation, uraninite containing as little as 1.2 percent
THE WITWATERSRAND AND INDUS RIVER
Th02 seems as likely to survive in the Indus as uraninite
The close similarity in size (0.05-0.25 mm) and com­ containing ten times as much. It is therefore suggested
positional range of allogenic uraninite from the Domi­ that in this population the compositional variation
nion Reef, Witwatersrand, Indus River alluvium near within the range studied has no measurable effect on
Hazro, Pakistan, and alluvium in the Hunza River, survival. On the other hand, uraninite grains collected
Kashmir, which is a tributary of the Indus, has been as a concentrate far from their primary sources willreported previously (Simpson and Bowles, 1977). The have encountered variable degrees of weathering, which
Th02 content and U02/Th02 ratio of uraninite from the differ for each grain during several cycles of erosion and
Dominion Reef correspond closely to those for uraninite deposition during downstream progression. The varia­
from Hazro, and the Th02 content and U02/Th02 ratio tion in attrition suffered by grains during transportof uraninite from the Vaal Reef correspond closely to therefore is the main factor controlling variation in
those for uraninite from the Hunza River. Calculation of grain characteristics and is unrelated to grain composi­
the uranium and thorium content of the Precambrian tion within the compositional range studied.
uraninite to account for radiogenic decay of uranium
and thorium since crystallization about 3.0 b.y. ago im­ EVIDENCE FROM THEORETICAL MODELS OF
proves this correlation. The uranium and thorium con­
URANINITE STABILITY
tent of the uraninite populations from the Indus and its
Errors in the assumptions used in designing models oftributaries span the full range of uraninite compositionsof the Dominion Reef and Witwatersrand. It has the natural environment are likely to lead to false
previously been suggested that since thorianite is resis­ estimates of uraninite stability. For example, Grandtant to oxidation and exists as a detrital mineral staff (1973) has calculated that for uraninite from
(Frondel, 1958), the thorium content of uraninite may Blyvooruitzicht in the Witwatersrand to survive erosion,increase its stability as a detrital mineral (Davidson, transportation, deposition, and burial, "oxygen levels
1960).
must have been less than approximately 10~ 2 to 10~ 6
economic grade. However, the new mineralogicalevidence for the origins of uraninite and pyrite is not inagreement with anoxygenic or reducing atmosphericconditions 2.8-2.0 b.y. ago. Thus an alternative reason
is necessary to explain the concentration of uraniferous
conglomerates in this period of the Earth's history.
Uraninite-, pyrite- and gold-bearing allogenic oremineral assemblages are known to occur in modern
sediments at several localities. For example, in the In­
dus River, where the concentrations are presently
regarded as subeconomic (Miller, 1963; Darnley, 1962),
uraninite is present in adequate quantity for study; andit can be deomnstrated that it is sufficiently stable underpresent-day atmospheric conditions to survive exhuma­
tion, surface weathering, and transport in the river
system over considerable distances (Simpson andBowles, 1977). Allogenic uraninite is also apparently
stable in other recent sediments. For example, Steacy(1953) has reported uraninite of unknown thorium con­tent in a black sand with notable development of cube
faces and brilliant lustre, and Coppens and Mayanda(1969) have also reported uraninite in beach sand de­
rived from the erosion products of the Quiberon granite
nearby. The purpose of this contribution therefore is to
re-evaluate selected aspects of uraninite and pyrite
distribution in Precambrian and comparable modern
sediments in relation to atmospheric and hydrosphericevolution.

DETRITAL URANINITE AND PYRITE: ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?

S3

TABLE 1. -Electron microprobe analyses, in percent, of 33 detrital uraninite grains from a sample of Indus alluvium, near Hazro, Pakistan[These grains were analysed by a combination of wavelength- and energy-dispersive techniques utilising correction procedures described by Mason, Frost, and Reed (1969) and to be described byBowles in a report in preparation. The samples were collected by Miller (1963) and further described by Simpson and Bowles (1977)]

Analysis
No.
1

2
3
4
5
6
7
8
9
10
11
12

Ti02

MnO

Fe203

PbO

Th02

U02

Y203

CaO

.28
.24
.30
.26
.24
.25

.08
.10
.04
.09

.17
~\9

.66
1.25
1.32
.50
.75
i ^i

1.20
1.27
1.38
2.19
2.27
2.35

99.89
98.40
98.54
97.95

.10
.14
.05
.13
.05
.15

.02
.02
.02
.02
.02
.02

.29
.28

.07

2.78
2.89
3.16
3.19
3.69

98 44
97.13

.21
.10

.29
.47
1.31
.35
.30
.43

96.57
95.35
98.31
96.57

.14
.10
.20
.72
.05
.19

.02
.02
.02
.08
.04
.24

94

.26
.23
.24

09
.07

.08
.04
.05
.08
.04

.14
.13
.18
19

.14
09
.18
9~\

9 7Q

97 71
96.63

Ce203

___

___

.03
.03
.03

Total

102.40101.54
101 79101.27101.31100.90102.20100.96101.45100.21102.44101.50

13
14
15
16
17
18

.27
.21
.22
.28
.24

91

.07
.06
.07
.09
.08
.06

.17
.16
.20
.19
.24
.17

.60
.39
1.43
1.34
1.15
1.19

3.82
3.92
3.93
4.09
6.32
6.43

96.89
96.01
93.44
91.91
87.87
89.80

.11
.46
.59
1.16
1.65
.88

.03
.05
.04
.03
.04
.02

.03
.10
.04
.07
.05

101.90101.35100.01
99.07
97.70
98.84

19
20
21
22
23
24

.16
.20
.19
.22
.17
.16

.06
.07
.04
.04
.07
09

19
.28
.17
.17
19
.13

.59
.85
1.20
.24
.52
.52

6.68
7.06
7.11
7.24
8.24
8.40

88.91
82.53
88.78
89.14
89.97
90.67

1.32
2.52
1.39
1.65
.99

.11
.08
.02
.04
.02
.40

.10
.07
.09
.01
.02
.23

99.05
93.66
98.99
98.75
100.19100.60

25
26
27
28
29
30

.12
.20
.23
.20
.13
.14

.10
.07
.07
.05
.10
.10

.26
.16
.26
.14
.27
.16

1.06
1.10
.57
1.06
.59
.41

8.65
8.69
8.70
9.00
9.05
9.08

86.84
88.42
88.53
84.68
85.58
88.81

1.40
.62
.59
1.61
1.58
.14

.02
.09
.03
.11
.06
.31

.08
.05
.10
.14
.13
.32

98.53
99.40
99.08
96.99
97.49
99.47

31
32
33

.08
09

19

.04
.10
.07

.14
.13
.15

.37
1.17
.59

9.20
10.69
12.08

90.36
86.77
83.75

.05
.05
.53

.02
.34
.06

.04
.42
.24

100.41
99.75
97.56

that of the present level in the mid-Precambrian." Thisconclusion is evidently incorrect since the presence of
detrital uraninite of similar composition in the Indus
River alluvium at the present time requires no special at­mospheric conditions.
When detrital uraninite grains enter a river systemthey are unlikely to suffer either oxidation below the
sediment/water interface, where conditions are general­ly reducing, or chemical attack and dissolution, provided
carbonate and bicarbonate ions are lacking (Garrels andChrist, 1965). In these conditions the grain may survive
for long periods and be transported over long distances
if periods of transport are short relative to total
residence time, thus reducing direct contact with oxi­
dised river water.

The greatest problem in any model of detritaluraninite stability arises with the need to deposit
relatively fresh and untarnished euhedral grains into ariver system since the greatest exposure to oxidation is
likely to occur at this point as the grains come in directcontact with the atmosphere. In rapidly exhumed,
eroded, and glaciated mountainous terrain this is pro­bably easier to achieve than in a more mature drainage
system, and these conditions help to account for thepresence of detrital uraninite in the Indus alluvium andprobably also by analogy in the Witwatersrand. In
Grandstaffs (1973) model the assumptions used toestimate the degree of exposure to oxidation at different
stages in the erosion cycle probably bear little relation to
the conditions in the present day Indus River basin or
the Witwatersrand.

S4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 2. -Correlation matrix for the chemical composition of 33 detrital uraninite grains determined by electron-probe microanalysis andshown in table 1[The uraninite grains were obtained from a sample of Indus alluvium near Hazro, Pakistan, collected by Miller (1963) and further described by Simpson and Bowles (1977)]

Tin

MnO
Fe20 3
PbO
Th02
U02
vo
CaO
Ce203

TiO,

MnO

1.000
-.341
-.124

1.000

01 ^

-.805
697

-.205
-.439
-.752

190
.043

.082
-.061
.111
.149
.374

Fe20,

1.000

AC'7

.238
.413
.586

- ?47

.007

EVIDENCE FROM CARBONACEOUS MATTER IN
THE WITWATERSRAND

Evidence has previously been given (Pretorius, 1975;
Hallbauer, 1975; Simpson and Bowles, 1977) that the
carbonaceous matter in the Witwatersrand represents
organic plant remains (fig. LA, B). These remains most
likely represent the metamorphosed and fossilised rem­nants of stromatolitic algal mats (Mendelsohn, 1976).
The Carbon Leader in the Upper Witwatersrand
System is an example of this type of formation covering
several hundred square miles of the West Witwater­
srand (Bourret, 1973). It therefore provides goodevidence for the establishment of photosynthesis andgeneration of atmospheric oxygen in sufficient quantity
to provide a shield from ultraviolet radiation, which
would otherwise destroy plant life (Fyfe, 1974).
The presence of atmospheric oxygen is also supported
by carbon-isotope evidence from Precambrianlimestones and dolomites, from which it has been con­cluded that close to 80 percent of the amount of oxygen
contained in the present oxygen budget should havebeen released prior to 3.0 b.y. ago (Schidlowski andothers, 1975).
EVIDENCE FROM AUTHIGENIC PITCHBLENDEAND DISSEMINATED URANIUM IN THE
WITWATERSRAND SYSTEM

The existence of fine-grained low-thorium pitchblendein carbonaceous material in the Witwatersrand probably
results in part from deposition of uranium in solution on­
to organic matter. Lower grade concentrations ofuranium have also been identified, by fission track
methods, in association with phyllosilicates, which arefinely intergrown with authigenic pyrite and also form
lenticular mudstone pellets. These occurrences are also

PbO

Th02

UO,

Y203

1.000
.090
.103
.156
-.070
.036

1.000
-.910
.406
.349
.715

1.000
.711
.216
.602

____
1.000
-.204
.025

CaO

Ce,03

____
____

____
____

1.000
.691

1.000

considered to represent deposition of uranium fromsolution (Simpson and Bowles, 1977). This evidence in­dicates that detrital uraninite in sediments upstream
underwent partial oxidation to form the soluble uranyl.
ion, which was then reprecipitated downstream below
the sediment/water interface in association withorganic-rich mud, authigenic pyrite, and decayingorganic remains of stromatolitic algal mats (fig. IB).
These reactions require oxidising atmospheric condi­
tions in order to function in the manner suggested, ex­
cept where the concentration of bicarbonate is high,since the uranium-carbonate complex ions can exist inaqueous solution under reducing alkaline conditions(Garrels and Christ, 1965). However, no carbonate has
been described from the sediments of the Witwater­
srand succession.
The evidence from the stromatolitic algal mats is in­dicative of a sufficient supply of atmospheric C02 to pro­
mote supratidal growth, and the reactions described inthis paper assume that the atmosphere was well mixed
and that water washed detrital grains into the site ofdeposition. The discussion on the sulphur cycle, in thenext section, indicates that pH conditions in themineralising surface water solutions were probably
neutral to acidic rather than alkaline as a result of theactivity of bacteria catalysing the production of relative­
ly large amounts of sulphate from allogenic pyrite.
Under these conditions the uranyl ion andorganometallic complexes were probably the principal
uranium species in solution during deposition of thesediments. The lack of carbonate during diagenesis isprobably attributable to rapid burial, which prevented
reaction between organic-rich sediments and the coex­
isting oxidising atmosphere. The nature and thickness (8km) of the Witwatersrand sediments and the preserva­
tion of large amounts of carbonaceous matter are goodevidence for rapid burial.

DETRITAL URANINITE AND PYRITE: ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?

S5

Qtz

FIGURE 1. - Photographic mosaics obtained by use of computer-contolled stepping motors, showing macrostructure of specimens. Carbonaceous
matter representing fossilized stromatolitic algal mats contains allogenic uraninite associated with authigenic pitchblende, intergrown withfinely divided authigenic gold. These relations are most clearly observed in the back-scattered electron image of the electron microprobe,
which exploits the difference in mean atomic weight between carbonaceous matter on the one hand and gold, uraninite, and pitchblende on
the other.
A, Polished thin section no. 2034, Vaal Reef. This is a section of an 8-mm thick horizon of columnar carbonaceous material (black) from aquartz-pebble conglomerate. Some quartz pebbles (Qtz) in the conglomerate layers at the upper and lower margins of the figure containeuhedral pyrite (Py). Detrital grains of uraninite (Ur) and quartz (Qtz) are overgrown or entrapped by the mat (black). Uranium and gold arealso deposited diagenetically as pitchblende (Pi) and authigenic gold (Au) and together with churchite (Ch) these authigenic phases outline thevertical columnar structure of the carbonaceous matter (black) (Simpson and Bowles, 1977).
B, Polished thin section no. 1952, West Driefontein. Massive carbonaceous matter in thin section (black) contains uniformly dispersed fine­grained authigenic pitchblende (Pi) and coarser grained authigenic gold (Au), which are often intergrown, with minor amounts of muchcoarser allogenic quartz (Qtz) and pyrite (Py). No heavy-metal detrital phases are present; hence the low-thorium pitchblende and gold repre­
sent deposition of uranium and gold from solution onto stromatolitic algal mats (black), followed by recrystallisation during diagenesis andmetamorphism. These processes have also remobilised lead of probably radiogenic origin into cracks where it is present as galena (g).

S6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEVIDENCE FROM THE SULPHUR CYCLE IN THE
WITWATERSRAND

started the oxidation of pyrite cycles through reactions(1) and (2).

The existence of abundant authigenic pyritic concre­
BACTERIAL SURFACE OXIDATION OF PYRITE
tions coexisting with allogenic pyrite grains (Simpson
and Bowles, 1977, figs. 6, 7) indicates that reactions ofAt the present time pyrite oxidation rates are greatly
the following type have probably occurred during forma­ accelerated by the activities of acidophilic iron- andtion of the sediments:
sulphur-oxidising bacteria (Silverman, 1972). Several
1. Partial oxidation of detrital allogenic pyrite to bacteria may be involved, but the most common andgenerate sulphate in solution
most effective are Ferrobacillus ferrooxidans, which ox­
2. Deposition of authigenic pyrite concretions and idises pyrite by direct attack, and Thiobacillus ferroox­
overgrowths in reducing environments below the idans, which are thought to catalyse the oxidation of
sediment/water interface, a reaction that takes Fe2+ to Fe3+ in the rate controlling reaction (1).
place in unconsolidated silts and muds now Microbial activity by these and other bacteria isrepresented by interstitial silicate phases in the predominant only under ideal conditions of low pH, a
concretions
temperature of about 30°C, and the presence of oxygen,
3. Reworking of these sediments and downstream iron, sulphur, and nutrients (N, C02), according to Hoag
transport of secondary authigenic pyrite washed and Webber (1976).
out of the unconsolidated muddy sediment andunoxidised allogenic pyrite, which were then
BACTERIAL REDUCTION OF SULPHATE BELOW THE
redeposited together.
SEDIMENT/WATER INTERFACE
All three stages were repeated in whole or in part until
final burial and diagenesis in subsiding portions of theBacteria also take part in reducing reactions, especial­basin, where pyrite and uraninite were maintained in a ly those belonging to the genera Desulfovibrio andreducing environment below the sediment/water inter­ Desulfotomaculum-, they are anaerobic and reduce
face and out of contact with the oxidising atmosphere. It sulphate to H2S as follows:is important to note here that fresh authigenic andSOI ~ + 8e + 10H +-*H2S + 4H20.
allogenic pyrite are also identified in samples from theIndus River by Simpson and Bowles (1977).
Iron in solution would form pyrite by reaction with
The reactions described are probably due to combined H2S (Trudinger, 1971). The abundant authigenic pyrite
inorganic and bacterial surface oxidation of pyrite to concretions from the Witwatersrand were presumably
form sulphate, associated with bacterial reduction of formed as the result of bacterial activity of this type insulphate below the sediment/water interface. These organic-rich muddy sediments (Saager, 1970).
processes are summarised in more detail in the following
In summary the evidence from the sulphur cycle
sections.
shows that the occurrence of pyrite in the Witwater­
srand is more readily explained by assuming oxidisingrather than reducing atmospheric conditions during
INORGANIC SURFACE OXIDATION OF PYRITE
deposition, probably in association with active bacterial
Inorganic oxidation of pyrite under surface conditions processes aiding dissolution and reprecipitation of
in equilibrium with atmospheric oxygen has been pyrite.
described by Stumm and Morgan (1970) and can be sum­
SUMMARY
marised as follows:
Initial step:
The evidence considered in this study indicates thatFeS2 + 7/2
2H
the terrestrial atmosphere at the time of formation of
Witwatersrand sediments was not detectably different
Rate controlling step:from that of the present day. The reasons for the earlier(1) suggestion that the atmosphere must be reducing orFe3+ +3H20 =nonoxidising to account for the presence of uraninite
and pyrite in the Witwatersrand are probably due to
FeS2 + 14Fe3+ + 8H20 = 15Fe2+ + 2SO(2) overestimation of the extent of atmospheric exposure of
The slowest reaction or rate controlling step is the ox­ these phases. Uraninite and pyrite stabilities and their
idation of Fe2 + ; the fastest reaction is the oxidation of environmental significance must be considered in rela­pyrite by Fe3+ ion. Therefore, once the reaction is tion to their geological context if the deductions made

DETRITAL URANINITE AND PYRITE: ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?

are to be realistic. The coexisting phases and style ofmineralisation in both ancient and modern river systems
are best explained by oxidising atmospheric conditions
and reducing conditions below the sediment/water inter­face.
Comparison of modern alluvial sediments of the Indus
River with those of the Dominion Reef and Witwatersrand Systems has shown that stable assemblages of
detrital thorian uraninite and pyrite of similar grain sizeand composition can be found in both ancient andmodern sediments. Authigenic minerals in the Witwatersrand provide indirect evidence of uranyl andsulphate ions coexisting with detrital thorian uraninite
and pyrite during formation of the Witwatersrand
sediments. It is suggested that this bimodal form oftransport and desposition can be most readily accounted
for by a model in which sulphate and uranyl ions exist insuperficial, oxidised, active sediments and river waters
draining into the Witwatersrand Basin while detritalgrains of uraninite were contained within the bedload of
streams in reducing conditions below the
sediment/water interface. The persistence of uraninite
during burial and diagenesis would be favoured bycatastrophic burial of the sediments in a subsiding basin.
This would prevent oxidation of the abundant organic
matter, now represented by the carbonaceous material,
to give carbonate and bicarbonate ions, which form
uranyl carbonate complexes and which are highly solu­
ble even in reducing conditions.
CONCLUSIONS

The great preponderance of economic uraniferous
conglomerates in the time span 2.8-2.0 b.y. ago is con­trolled by geological conditions and not by atmosphericfactors. This close association between uraniferous con­glomerate and Precambrian crust has previously been
pointed out by Bowie (1970), who considers this to be asignificant factor and guide for prospecting.
Accordiing to Moorbath (1975, 1976), continental
growth occurred on a major scale during the earlyPrecambrian. Juvenile plutonic material crystallised
slowly at depth and differentiated to form a compositionally layered lower crust of the type described by
Fyfe (1970,1973), Brown and Fyfe (1970), Heier (1973a,b), and Smith (1976), in which granitic components andincompatible elements migrate upwards to form calcalkaline plutonic rocks, leaving behind a thick residue of
depleted granulite facies rocks at greater depth. Highly
complex gradation crustal layering results. The period
2.8-2.0 b.y. ago therefore represents a unique phase ofcrustal evolution, when extensive erosion of uraniumrich upper crust resulted in the deposition of uraniferous
sediments in enclosed basins, under atmospheric condi­
tions similar to those of the present day.

S7

This conclusion is compatible with the findings ofother studies, notably those of Dimroth and Kimberley(1976). From physical considerations, Shimizu (1976)
concluded that an initial highly reducing atmosphere on
the Earth's surface would be highly unstable. Hencefrom physical and geological considerations theevidence indicates that the atmosphere in the period
2.8-2.0 b.y. ago was oxidising.
REFERENCES CITED
Bourret, W., 1973, First uranium report—Witwatersrand, South
Africa: San Francisco, Utah International, Inc., 51 p.
Bowie, S. H. U., 1970, Some geological concepts for consideration in
the search for uranium provinces and major uranium deposits, inUranium exploration geology: International Atomic Energy Agen­cy, Proceedings of a Panel, Vienna 1970, p. 285-300.
Brown, G. C., and Fyfe, W. S., 1970, The production of granitic melts
during ultrametamorphism: Contributions to Mineralogy andPetrology, v. 28, p. 310-318.
Coppens, Rene, and Mayanda, Marcel, 1969, Sur la presenced'uraninite dans le sable de la plage de Quiberon (Morbihan):Academic des Sciences, Comptes Rendus, Serie D., v. 268, no. 7, p.
1016-1018.
Darnley, A. G., 1962, Detrital uraninite from the Hunza River,
Kashmir: Great Britain Geological Survey Atomic Energy Divi­sion, Age Determination Report 22, unpublished.
Davidson, C. F., 1960, The present state of the Witwatersrand con­troversy: Mining Magazine, v. 102, p. 84-95, 149-159, 222-229.
Dimroth, E., and Kimberley, M. M., 1976, Precambrian atmospheric
oxygen; evidence in the sedimentary distributions of carbon,
sulfur, uranium and iron: Canadian Journal of Earth Sciences, v.
13, no. 9, p. 1161-1185.
Frondel, Clifford, 1958 [1959], Systematic mineralogy of uranium
and thorium: U.S. Geological Survey Bulletin 1064, 400 p.
Fyfe, W. S., 1970, Some thoughts on granitic magmas, in Newall, G.,
and Rast, N., eds., Mechanisms of igneous intrusions: Geological
Journal, Special Issue no. 2, p. 201-216.
———1973, The granulite facies, partial melting and the Archaean
crust: Royal Society of London Philosophical Transactions, A.273,
p. 457-462.
-1974, Geochemistry: Oxford University Press, 107 p.
Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals andequilibria: New York, Harper and Row, 450 p.
Grandstaff, D. E., 1973, Kinetics of uraninite oxidation: Implications
for the Precambrian atmosphere: Princeton University, unpublish­ed Ph.D. dissertation, 153 p.
Hallbauer, D. K., 1975, The plant origin of the Witwatersrand carbon:
Minerals Science and Engineering, v. 7, p. 111-131.
Heier, K. S., 1973a, Geochemistry of granulite facies rocks andproblems of their origin: Royal Society of London PhilosophicalTransactions, A.273, p. 429-442.
———1973b, A model for the composition of the deep continental
crust: Fortschritte der Mineralogie, v. 50, p. 174-187.
Hoag, R. B., Jr., and Webber, G. R., 1976, Significance for mineral ex­
ploration of sulphate concentrations in groundwaters: Canadian
Institute of Mining and Metallurgy, CIM Bulletin, v. 69, no. 776, p.
86-91.
Holland, H. D., 1962, Model for the evolution of the Earth's at­mosphere, in Petrologic studies; a volume to honour A. F. Buddington: Geological Society of America, p, 447-477.
Koppel, V. H., and Saager, R., 1974, Lead-isotope evidence on the
detrital origin of Witwatersrand pyrites and its bearing on theS8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

provenance of the Witwatersrand gold: Economic Geology, v. 69,
p. 318-331.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand Systems, the Dominion
Reef, the Ventersdorp Contact Reef and the Black Reef:
Geological Society of South Africa Transactions, v. 58, p. 101-227.
Mason, P. K., Frost, M. T., and Reed, S. J. B., 1969, B.M.-I.C.-N.P.L.,
Computer programs for calculating corrections in quantitative
X-ray microanalysis: National Physical Laboratory, I.M.S., Reportno. 2, unpublished.
Mendelsohn, F., 1976, Mineral deposits associated with stromatolites,in Walter, M. R., ed., Stromatolites: Developments in sedimentology 20: Amsterdam, Elsevier, p. 645-662.
Miller, J. M., 1963, Uraninite-bearing placer deposits in the Indus
alluvium near Hazro, Pakistan: Great Britain Geological Survey,
Atomic Energy Division Report no. 254, unpublished.
Moorbath, S., 1975, The geological significance of early Precambrianrocks: Geological Association [London] Proceedings, v. 86, pt. 3, p.
259-279.
———1976, Age and isotope constraints for the evolution of Archaean
crust, in Windley, B. F., ed., The early history of the Earth: New
York, John Wiley, p. 351-360.
Pretorius, D. A., 1975, The depositional environment of the Witwater­
srand gold fields: A chronological review of speculations and obser­
vations: Minerals Science and Engineering, v. 7, p. 18-47.
Ramdohr, Paul, 1955, Neue Beobachtungen an Erzen des Witwatersrands in Sudafrika und ihre genetische Bedeutung: Deutsche
Akademie der Wissenschaften zu Berlin, Klasse fur Mathematik
und allgemeine Naturwissenschaften, Abhandlungen Jahrgang
1954, no. 5, 43 p.
———1958, New observations on the ores of the Witwatersrand inSouth Africa and their genetic significance: Geological Society ofSouth Africa Transactions, Annexure to v. 61, p. 1-50.
Robertson, D. S., 1974, Basal Proterozoic units as fossil time markers
and their use in uranium prospection, in Formation of uranium ore
deposits—Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens, Greece,6-10 May 1974: Vienna, International Atomic Energy Agency, p.
495-512.
Rundle, C. C., and Snelling, N. J., 1977, The geochronology ofuraniferous minerals in the Witwatersrand Triad; an interpreta­tion of new and existing U:Pb age data on rocks and minerals fromthe Dominion Reef, Witwatersrand and Ventersdorp Supergroups:
Royal Society of London Philosophical Transactions, Series A., v.
286, no. 1336, p. 567-583.
Saager, R., 1970, Structures in pyrite from the Basal Reef in the
Orange Free State Goldfield: Geological Society of South Africa
Transactions, v. 73, p. 29-46.
Schidlowski, M., 1966, Beitrage zur Kenntnis der radioactiven
Bestandteile der Witwatersrand Konglomerate I-III: Neues
Jahrbuch fur Mineralogie, Abhandlungen 105/106, p. 183-202,
310-324, 55-71.
———1970, Untersuchungen zur Metallogenese im siidwestlichen
Witwatersrand-Becken (Oranje-Freistaat-Goldfeld, Sudafrika):
Geologische Jahrbuch Beihefte 85:80, 46 p.
Schidlowski, M., Eichmann, R., and Junge, C. E., 1975, Precambriansedimentary carbonates: Carbon and oxygen isotope geochemistry
and implications for the terrestrial oxygen budget: Precambrian
Research, v. 2, p. 1-69.
Shimizu, Mikio, 1976, Instability of a highly reducing atmosphere on
the primitive Earth: Precambrian Research, v. 3, p. 463-470.
Silverman, M. P., 1972, Sulphide mineral oxidation—microbial, in
Fairbridge, R. W., ed., The encyclopedia of geochemistry and en­
vironmental sciences: New York, Van Nostrand Reinhold, p.
1132-1134.

Simpson, P. R., and Bowles, J. F. W., 1977, Uranium mineralisation of
the Witwatersrand and Dominion Reef Systems: Royal Society of
London Philosphical Transactions, A.286, p. 527-547.
Smith, J. V., 1976, Development of the Earth-Moon system with im­
plications for the geology of the early Earth, in Windley, B. F.,ed., The early history of the Earth: London, Wiley-Interscience, p.
3-19.
Steacy, H. R., 1953, An occurrence of uraninite in a black sand:American Mineralogist, v. 38, p. 549-550.
Stumm, W., and Morgan, J. J., 1970, Aquatic chemistry: New York,
Wiley-Interscience, 583 p.
Trudinger, P. A., 1971, Microbes, metals and minerals: Minerals
Science and Engineering, v. 4, p. 13-25.

DISCUSSION FOLLOWING PAPERS BY
DRS. FEATHER, SIMS, AND SIMPSON AND
BOWLESWhiteside: May I just make a comment on not these twolast papers but the one previous to that by Mr.
Feather. He mentioned that the uraninite and brannerite were probably about 1,800 million years old.
Now, the Witwatersrand itself has been dated be­
tween 2,300 and 2,800, around about 2,500, whichmeans that there has been presumably some metamorphic event which has been stamped on theseuraninites. I believe that 1,800 million years is the best
age for the Bushveld Complex. Could there be any
connection, as a heat source especially?
Feather: I wanted actually to point that out in the courseof my talk. That I had in fact noted that the Bushveld
Igneous Complex was of the same age.
There is just something that I would like to comment
on from Peter Simpson's paper. I am very interested
in the uranium which is distributed in minerals otherthan the uraninite and brannerite and zircon and soon. We ran into a strange situation in analysis foruranium before X-ray fluorescence was used and thatwas the chemical dissolution of uranium in the samples
using nitric acid. We found that using a sample con­taining about 400 parts per million we were leaving
behind about 10 parts per million after leaching with
hot nitric acid for some time. In a search on concen­
trates which I derived from the residue material I had
great difficulty finding the uranium. Now you havepossibly pointed to the areas where the uranium ac­tually did exist, but of course the nitric acid would
have dissolved certain things such as the pyrite. Are
you suggesting that the uranium occurs actually in the
pyrite in those concretionary pyrite nodules, or are
you stating that the uraninite occurs in some in­terstitial phase or in the holes in those concretionary
bodies?
Simpson: I think you've put your finger right on it there.
It's very difficult to demonstrate this on a slide
because the resolution isn't good enough, but if youDETRITAL URANINITE AND PYRITE: ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?

would like to have a look at the sections under the
microscope and compare the Lexans directly with thesections, I think you will see for yourself that theuranium is not in the pyrite. It is related to the in­terstitial matrix, which I'll have to confess I haven't
put on the probe yet and haven't yet identified, but it
looks similar to the phyllosilicates in that lenticularbody which I described and out of which I got
pyrophyllite, kaolinite, and quartz. They are verysmall, and there is an exact correlation of the fission
tracks in the Lexan with the interstitial phase and notwith the pyrite. It isn't in the pyrite phase, although
the spherical grains are in the pyrite.
Saager: I think I would agree that the uranium is in thatinterstitial phase. During our lead-isotopic
measurements we did some uranium determinations. I
do not agree with your idea that there is a lot of
uranium around; the amounts we did find were be­
tween 15 and 70 ppm uranium in the buckshot pyrite. I
think this is a very small amount of uranium.
Simpson: The sections shown there have very few
detrital grains in them. They were picked out to work
on because they were radioactive; we couldn't under­
stand why they were radioactive because we couldn't
see any detrital grains. I think you'll find that in toto
there is a fair amount of uranium. There is a lot moreuranium in the phyllosilicates than in the buckshot
pyrite, actually, and that phyllosilicate phase seems to
be a great deal richer. You must remember that theuranium in the pyrite which is porous is diluted by
nonuraniferous pyrite, and the actual point where theuranium occurs is very much richer. If you've got an
overall value of 70 ppm, that's for the pyrites andphyllosilicate combined, and if you recognize that thephyllosilicates may be only 20 percent of the total
buckshot pyrite, you have got to multiply by a factor offive to get the actual concentration at that point,haven't you? Because the pyrite itself is barren. So it's
going to be 350 ppm for the phyllosilicate.
Saager: That's, of course, taking the higher value. I
think more of our values were closer to 15 or even 20.
Simpson: The Lexan track registration is saturated at
40 ppm, and all the tracks I showed you, where
uranium does occur, are well saturated. So theuranium content would be 40 ppm or more for the con­ditions of radiation employed for those particular sec­
tions. I have to admit, however, that we haven't done
this quantitatively, We've only done it qualitatively up
to this stage, but we could make much more detailed
quantitative assessments if this was worthwhile doing.
Or you could do it if you wanted to.
Skinner: I have two questions, Peter [Simpson]. First ofall. would you hazard a guess, or if you have ameasurement would you give us your estimate of how

S9much of the uranium in the Witwatersrand is in these
phyllosilictes?
Simpson: I wouldn't like to be quantitative about this,but I would say that a great deal more of it is there
than anybody has yet ever mentioned or possibly evenappreciated.
Skinner: Are you talking about 90 percent of it?
Simpson: No. I think a great deal of it is in the car­bonaceous material. There is a great deal of it indiscrete detrital phases. Just hazarding a guess, it
might be as high as 30 percent, but I haven't done it
quantitatively yet.
Skinner: Okay. My second question arises from an
observation I made from the slides that you showed
us. All of your pyrites are shattered, broken by brittle
fracture. I noticed the same things in Dr.
Schidlowski's excellent slides. I wonder if this is a
uniform feature of the Witwatersrand. Pyrite is astrong mineral, as you know, and requires quite astress before it's going to break. If the pyrite is all
shattered throughout the Witwatersrand, then it has
undergone considerably more deformation than I
would have thought from Dr. Hallbauer's discussion of
the carbon this morning.
Simpson: I would agree with that. In general, my ex­perience is that pyrite is shattered in many, manycases. I don't know the exact structural locations of
the samples I am working on, and it's very difficult tointerpret that type of information other than to makethe observation that, yes, in many cases the pyrite is
shattered.
Skinner: Would one of the others know?
Saager: I think that the shattering is actually a factor of
the size of the pyrite; very small, compact detrital
pyrites are definitely not shattered. The stuff we havebeen working on for the lead-isotope study wespecifically collected only pyrite that was not shat­
tered at all. These were very small detrital compact
pyrites, so I think it actually depends upon the size ofthe pyrite.
Feather: Peter, two questions: What is the possibility
that the uranium in the phyllosilicates may actually bepresent as very tiny needles of brannerite? Have you
been able to examine them in the scanning electronmicroscope?
Simpson: No, we haven't examined them by any in­strumental method other than by this method. The
trouble arises that when you put a thing in a reactor,
you are then very reluctant to put it in your probe fordanger of contamination of the probe, so you have to
make up one section to go in the probe, one section to
go in the reactor. We still are at an early stage in ourwork, and we haven't yet probed the sections cor­
responding to those that have been in the reactor. So I

S10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

don't have that information. Optically, I cannot see
any opaque phases in the phyllosilicate-uranhim
association. There is nothing in the rock which is
opaque. There is nothing there to lead one to suspect,optically or mineralogically, that there is any reason
for uranium to be there. But this is something that wehave come across over and over again in our uraniumprospecting activities in Britain, where time and timeagain geologists have sent in samples which they claim
to be radioactive and there are no visible uranium
minerals to be seen, no secondaries even. That is why
we adopted this technique as a routine procedure
because it does at least show you where the uranium is
at low levels when it is associated with phases you
don't recognize as uranium minerals. For example,
hematite or fine-grained limonite can contain up to athousand ppm uranium without showing any visible
signs and that that is the case. Having learned that,
we now use this technique routinely. So I don't see any
brannerite, but you are welcome to look at the slides
and examine them yourself if you wish.
Feather: I do find your observations very interesting. If
there was uranium in solution and if you assume that
it was in solution at the time that the reefs were laid
down, it doesn't seem unlikely that by an ion-exchange
reaction clay minerals could have absorbed uranium.
Another question that I would like to ask you iswhether it is not possible that uranium was absorbedby these minerals, not at the time of deposition, butduring some subsequent metamorphic epoch when
there was a lot of uranium redistributed in the reefs inthe form of brannerite, because brannerite occurs not
only in the way that we have seen as ghosts in theoriginal site of the uraninite grain, but it often
migrates considerable distances away from where theparent obviously was.
Simpson: Well, this is a very good question, and it's apoint that obviously one bore in mind in studying theserocks. The impression you get is that the uranium, inthis loosely bound form, is very closely confined to thesite of original deposition of phyllosilicates and pyrite.
It doesn't deviate even 1 or 2 mm away from thesevery fine layers of pyrite, and one would suspect that
if it had been introduced later at least some of theuranium would have found its way out of these layers.
The fact that it hasn't done so, of course, isn't ab­solutely conclusive. I mean, it is possible, I suppose,that these, for some reason or another we don't
understand, could have just been the pathways along
which uranium came. What I would encourage you to
do is to look at these things for yourself and find out towhat extent you can find evidence either way. The
time of introduction of the uranium is a bit of an open
question at the present, but because it's confined inthese very narrow layers I am inclined to suspect it's a

primary feature. And it's not surprising, after all,because this is exactly what happens in stream
sediments today.
There is one other point I should mention. If thisuranium was introduced later, this would require ox­
idizing solutions, no question about that. If the solu­
tions were oxidizing then you would expect to seesome effect on the pyrite. You would expect the pyrite
to be oxidized. The pyrite is not oxidized; it is not af­
fected in any way at all by the presence of thisuranium. It seems to be quite happy there, andtherefore I have concluded on balance that thisuranium was fixed at or around the time of depositionof the phases with which it is associated.
Feather: Well, I might differ with you there, but I don't
think we should go into it now.
Roscoe: Have you tried these same techniques with
Dominion Reef material?
Simpson: Unfortunately, we haven't got around to that
yet, and we don't have any appreciable quantity ofsamples. If you would like to send us a few samples totry it on * * *.
Roscoe: Well, I have no Dominion Reef samples.
Simpson: Sorry, did you say Dominion? Oh, I beg yourpardon. I mistook the question. I immediately assum­
ed you were talking about Canada. Dominion Reef
samples. Yes, we have tried this with Dominion Reef
samples, and we have found that the track distribution
corresponds to the sites of detrital minerals.
Roscoe: Well, of course the Elliot Lake ores are more
like the Dominion Reef type and the bulk of theuranium is in discrete heavy grains. I have never seen
an alpha track plate of any Elliot Lake ore that didn't
have a great deal of uranium in matrix material in thefine-grain muscovitic matrix, which usually has
smeared out leucoxene in it, too. There always aretracks in there. Your procedure, of course, is so sen­sitive that you do swamp any uranium-bearing
materials so you are exaggerating the amount in thematrix in this case. I think you may see the same typeof thing in the Dominion Reef and Elliot Lake ores,
maybe not to the same extent.
Schidlowski: I principally agree with most of yourpoints. I am just a little bit suspicious about yoursecondary uraninite, those tiny specks associated withgold. I am thinking of possible criteria to narrow these
down. My question goes rather to Dave [Grandstaff]
than to you. Dave, if this was secondary uraninite,shouldn't the thorium tenor be considerably lower?
Grandstaff: Yes, I think that this low-temperature
aqueous uraninite should have low thorium values. I
think this would be a good distinguishing criterion to
distinguish the detrital from the replacement secon­
dary uraninite. The uraninite should be probed before
it can be stated conclusively which it is.

DETRITAL URANINITE AND PYRITE: ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?

Simpson: I agree.
Grandstaff: The second point I'd like to make is on reac­tion kinetics of pyrite. The reaction kinetics of pyrite
are not known at this point, but you can't necessarily
assume that conditions under which uraninite would
break down, would be oxidized and dissolved, would
necessarily cause appreciable oxidation and dissolu­
tion of pyrite.
Simpson: There has been very little said about oxidizing
solutions in these rocks at all up to now, but there does
seem to be some evidence for them. The point is, if
there was going to be something to be seen in the rock,
I would expect to see some effect on the pyrite. That'sall.
von Backstrom: I'd like to make a comment to keep therecord straight. I think most South African geologists
would bear me out that the WitwatersrandSupergroup has been subjected to a considerable
amount of tectonic disturbance, not only faulting but
also very pronounced angular unconformities. Thereis also a certain amount of folding. I think it stands toreason with a very old formation like that, followed by
the Ventersdorp volcanicity (outside Klerksdorp,
boreholes of over 10,000 feet failed to drill through the
Ventersdorp into the Witwatersrand below), there
must have been an extreme disturbance in the floor of
the basin and very deep valleys developed. Then there
was the Bushveld Complex episode, which as you
know is an enormous complex, that was emplaced in
the immediate vicinity of the Witwatersrand andcreated a space problem. It certainly had severe ef­
fects on the Transvaal System, which covers the
Ventersdorp. So my proposition is that I think that wemust get away from this idea that the Witwatersrandhas not been severely disturbed technically. I thinkthat the fact that most of the mines have one or two
geologists stationed at each mine and a lot of theirwork concerns displacement of the reef bears this out.
The other point that I'd like to make is that in theWestern Transvaal, certainly in the Ottosdal area
where I worked and mapped, the pebbles in the lower
part of the Dominion Reef were badly disturbed. They
were shattered, and there is a terrific amount ofshearing and displacement within the conglomerates
within the reef, even a certain amount of streaking
out. This is further evidence of tectonic disturbance,and I think that this would account for the fact thatmany of the pyrite pebbles have also been shattered.
Bourret: I would like to add one further comment towhat Drs. Whiteside and von Backstrom have said inthat we are carried away a little bit with an over­
simplification of the tectonic picture. Dennis Pretorius
continually brings out the fact that there are
numerous sills and dikes and that 10 to 15 percent ofSll

the basin actually is occupied by sills and dikes whichhave followed the fault patterns. There are three
periods of igneous activity which are accounted for by
the sills and dikes. The first is the post-Ventersdorp,
then the post-Transvaal, and finally the post-Karroo
periods of intrusive activity, and of course these tie inwith the Bushveld Complex period of about 1,900
million years ago. I submit that not enough attention
has been given to the tectonic disturbances that havetaken place and are stamped in the record by thesesills and dikes.
Armstrong: I have a question that I'd like to ask theSouth African contingent. If I understand the modelin its broader aspects, the present thinking is that thebasin was filled by coalescing fans that came from thenorth, the northwest, and the southwest, and that
there were five or six separate source areas. No one
has touched on the fineness of the gold. For you nongold-miners, I am not talking about size, I'm talking
about the purity of the gold. I would think, to support
a placer genesis for the gold, that it would be highly
unlikely that there would be a uniform fineness of thegold if it's derivied from five separate source areas.
Are there any maps showing the fineness distributionof the gold of the entire Rand, the distribution of the
trace elements, the copper content, the silver content,or the ratios of different elements in the gold? Can anyof you help me on this?
Whiteside: As far as I remember, Hargraves did somework some time ago on the silver content. He tried toplot the silver content of various parts of the Wit­
watersrand and I don't think the variation was very
great. He tried to equate the silver content of the goldwith depth, and I am not sure that he proved ordisproved his point. I think you will find that report inone of the publications of the Economic Geology
Research Unit of Wits University, one of the very
early ones.
Schidlowski: As far as I remember, Hargraves worked
on bullion, whereas Rudolf Saager and I, more or lesssimultaneously, applied microprobe techniques.
Speaking for myself, I think I found on the average asilver content of roughly 10 percent. I think it was be­
tween 11 and 8. And I think Rudolf Saager foundsomething similar. Is that correct?
Saager: Yes.
Schidlowski: I worked only on Orange Free State reefs.
Saager: Unfortunately, I also worked only on Orange
Free State reefs, but I think there is a recent paper by
Liebenberg, and there's an internal report of the Na­tional Insitute of Metallurgy by [Viljoen?]. He found agreater variation than we did in the Basal Reef, and hehad samples, I think, from about three or four of thegoldfields.

S12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Armstrong: I would think that you could sample theEast Rand and the Central Rand and the far west andthe Orange Free State and that, if indeed you do have
five separate sources for the sediments and the gold, I
would think that the character of the gold would be
finger-printed by its trace-element content. A good
many years ago I worked in the Porcupine district,
Ontario, Canada, and there was a theft from the Dome
Mine. Half the brick that was stolen was recovered by
the gold squad, and they were able to identify it by thetrace elements in it, and there was no doubt about it.
The gold from each mine in the distict had distinctive
suites or amounts of trace elements in it. I would thinkthat this type of study, if it hasn't been made, could
lend great support to the detrital or nondetrital originof Rand gold and might be an important feature to bestudied. Is it that difficult to study?
Myers: Zemansky and Desboro wrote a paper a couple ofyears ago in which they show that gold subjected tothe temperatures that the Rand simply had to be sub­jected to will homogenize in a relatively short time
geologically. This complicates the problem. It is fur­ther complicated by the evident fact that there is silver
somewhere other than in the gold. It just makes it acan of worms.
Saager: We analyzed gold on the microprobe, and we
also had assays run on the same ore samples. We
found almost no variation of fineness of the actual
gold samples, but we found an increase of the silver
content of the total ore sample. Also, you find adefinite increase of silver in the ore samples downslope, whereas the fineness of the actual gold remains
more or less constant. So silver must be floatingaround somewhere else.
Gramdstaff: Dimock and Hargraves did a little bit of
microprobe work for silver on the uraninites andfound very low levels of silver in the uraninite. This
could be an additional source of silver.
Bourret: Going back to this very famous volume by
Pretorius, I'd like to read the comment he makes on
page 42 with respect to silver where he states, "The
percentage [of] silver in the [gold] bullion is lowest in
the center of the lobe [of the conglomerate distribu­
tion,] as would be expected in a typical alluvial deposit
where more pronounced movement of water down themain channel would have tended to leach out greater
amounts of silver." I'm sorry he doesn't go on and ex­
plain that further.
Skinner: It's a well-known fact that's been known for along, long time that detrital gold grains have gold-richrims and silver-rich cores. So there is nothing
mysterious about this.
HalU)auer: We have just started with analysis of

selected gold particles, freed from the rock byhydrofluoric acid treatment, for silver and traceelements by neutron activation analysis, but so far wehaven't anything to report.
Feather: Just one final comment about the detrital gold
grains being depleted in silver around the rims. This isabsent in the Witwatersrand; the grains are
remarkably homgenous. Also we've analyzed a large
number of gold grains, and just to follow on what
Rudolf [Saager] and Manfred [Schidlowski] have said,
we do find that in the Vaal and Basal Reef samples
there is very little variation in the silver content, butwhen we come to the Central Rand and the East
Rand, the overall silver contents of the samples fromdifferent reefs vary vastly, suggesting multiple source
for the gold in that northern area as opposed to thewest and south area, but we haven't gone into this inany detail yet.
Armstrong: Did you analyze the gold for copper?
Feather: No, nickel and iron, and I haven't drawn any
conclusions from those two elements.
Skinner: Brad Myers, I think, already gave the answer
to the reason why you wouldn't expect to see the rims
in the Witwatersrand. The Zamansky and Desboro
paper he referred to is a study on the diffusion rate ofsilver in gold and gold in silver, and at a temperature
like 150 or 200, you simply couldn't preserve a rim,
you would just homogenize the whole grain.
Whiteside: May I just say one last word. In the assay
offices on the East Rand, they used to deduct 10 per­cent from gold assays to allow for the silver content ofsamples from the Main Reef Leader, and 15 percent
for samples from the Kimberley Reef, which is quite
an appreciable difference. I have had gold assays
made of some of the very small and narrow reefs highup in the sequence, right at the top of the Witwater­
srand in the West Rand, that showed a silver content
of almost 50 percent. So there is quite a considerable
variation in the silver content of Rand gold, and thesilver content appears to increase as you go up thegeological column.
von Backstrom: Ladies and Gentlemen, it only remains
for me as convener of the South African contribution
to thank each and every one of you for your coopera­tion and the papers you have presented, and the waythat you have presented them, and also, the par­ticipants in the discussions. And on behalf of all of us,a special word of thanks to the United States
Geological Survey who made this wonderful trip possi­
ble. And then also, of course, the organizing commit­
tee that did such a vast amount of work and thosebehind the scenes, thanks ever so much. Also, all of
our American friends for your hospitality. Thank you.

BRAZIL

OPENING REMARKS

Moura: Ladies and gentlemen:
I'm glad to be again in this nice country of yours
attending this meeting. Unfortunately, Dr. Villaga
could not come to talk about uranium in con­glomerates of the Precambrian Moeda Formation in
Minas Gerais State. I'm going to try to do my best in
spite of my problems in understanding and talking
your language. First of all I'd like to thank you all on
my company's behalf and on my own for the invitation
to participate in this Workshop.
You are aware of the problems the world is facing
now due to the shortage of energy and the increasing
price of fuels. Of course, we realize that something has
to be done about it.
I had originally intended only to present some infor­
mation on work that is being done in Brazil in pros­
pecting for uranium-bearing conglomerates. Afterhearing the excellent papers that have been presented
during the last two days, however, I thought it would
be of interest to make a few brief introductory
remarks about the problems we are facing in Brazil,
where we are in the earliest stages of prospecting.
It seems to me that, because of the world energy
situation, new districts of uranium- and gold-bearing
conglomerates should be found as quickly as possible.
As a field geologist I have been wondering how could
the detailed studies that we have heard these last days
be used in finding new areas of interest. I have to con­fess that I did not succeed in getting any answer.
Maybe I didn't understand the papers; I don't know.
I did get some scattered hints, however, that may
make my work in Brazil become even harder. Dr.
Minter, for instance, pointed out that the mean gold

content in conglomerate from several cores of thesame reef could be something like 200 to 300 percent
different from the true mean. If our mean grade isalmost at the cutoff grade, how can we decide to put
down a shaft where the decision is based on drill hole
results in a completely new area? Of course, I knowthat in Canada and in South Africa the deposits arewell known and the models are already developed.
How can we recover thin layers of carbonaceous
material in quartzite without enough stratigraphic
control? How can we use mineralogical associations to
help in prospecting new areas? I understand thatthose data are usable in Canada where the stratig­
raphy is well established and one can know when hereaches the footwall. One need only back up andwedge the hole and drill more carefully. The potentialof those areas is already known. But how does one dothe same thing in new areas? I understand also thatthe mineralogical associations are different in Canada
from those of the Witwatersrand and that they depend
principally upon the rocks in the source areas. In
Brazil the Jacobina as well as the Quadrilatero Ferrifero conglomerates are different from those of Canada
and South Africa.
In the prospecting stage one hardly can afford
detailed research. I think it is more useful to takemodels that have already been developed and to selectthose features that better fit our own problems, trying
to use them as a first approach. Later on, when mines
are in operation, then detailed research can be carriedout.
As we are short of time, I think I'd better present
the paper.

Uranium in the Precambrian Moeda Formation,
Minas Gerais, Brazil
By JOSE NILSON VILLAQA and LUCIANO A. M. MOURA
GENESIS OF URANUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

PAPER

1161-T

CONTENTS
Page

Abstract ________________________________________________
Introduction ______________________________________________
Transportation and communication ______________________________
Climate ____________________________________________
Topography ____________________________________________
History ________________________________________________
Stratigraphic section _________________________________________
Areas being prospected _______________________,________________
Distribution of conglomerates in Gandarela Syncline _____________________——
Mineralization _____________________________________________
Mineralogical association _____________________________________
Working methods ___________________________________________
Conclusions _______________________________________________
References cited __________________________________________
Discussion _______________________________________________

Tl
1
1
1
1
1
2
3
3
3
7
10
10
13
13

ILLUSTRATIONSPage

FIGURE 1. Map showing location and study area ________________________________________________
2. Diagram showing stratigraphic distribution of radioactive anomalies _________________________________
3. Map showing Precambrian rocks around Sao Francisco Craton ____________________________________
4. Diagram showing paleocurrent directions in Jacobina area ______________________________________
5, 6. Isopach maps of:
5. Moeda Formation in Gandarela Syncline ____________________________________________
6. Total conglomerate in Gandarela Syncline _________________________________________
7-10. Photographs showing:
7. Quartzite boulder, east of the Gandarela Syncline ________________________________________
8. Polymictic conglomerate, east of the Gandarella Syncline ___________________________________
9. Conglomerate with black quartz pebble, in the west of the Gandarella Syncline _____________________
10. Conglomerate with iron staining due to pyrite weathering __________________________________
11. Cross section of southeast Gandarela Syncline ___________________________________________
12. Map showing Th02/U308 ratio distribution in basal conglomerate in southeast Gandarela Syncline _____________——
13. Graph showing the relation between titanium oxide and uranium and thorium oxides ____________________———

T2
3
5
6
8
9
10
10
10
10
11
12
13

TABLESPage

TABLE 1. Stratigraphic section in the Quadrilatero Ferrifero, Minas Gerais, Brazil ___.
2. Stratigraphic correlations of Robertson with Maquine Group added by Villaca.

T4
7

GENESIS OF URANIUM

AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESURANIUM IN PRECAMBRIAN MOEDA FORMATION,MINAS GERAIS, BRAZIL
By JOSE NlLSON VlLLACA and LUCIANO A. M. MOURA1

ABSTRACTThe Quadrilatero Ferrifero with an area of about 7,000 km2 islocated south of Belo Horizonte in Minas Gerais, Brazil. Precambrianmetaconglomerates (Proterozoic) of the Moeda Formation and Maquine Group are believed to be favorable host rocks for uranium
deposits. Some areas are now being studied or have work planned fornext year.
Drilling succeeded in detecting at least three channels in different
areas with ore-grade uranium-bearing oligomictic conglomerates.
Reserve calculations require additional detailed work in those areas.
Some models indicate that the sediments came from the Sao Fran­cisco Craton, but paleocurrent directions in the Gandarela Syncline, as
well as at Jacobina, indicate that the detritus came from the east, atleast at these sites. This means that other cratonic areas must exist to
the east of these outcrops.
The Quadrilatero Ferrifero (about 7,000 km2) is mostly included bet­
ween lat 19°45' and 20°30' S. and long 43°22'30" and 44°7'30" W. andlies near Belo Horizonte, Minas Gerais State, Brazil.

Long-distance dialing telephone systems connects
Belo Horizonte with other places of the country andabroad.
Electricity is available for mining operation, withpowerlines near most of the sites of Quadrilatero Ferri­fero.
CLIMATEThe climate in Quadrilatero Ferrifero, as well as in all
central Brazil, is semitropical. Extreme temperatures
are not known. The maximum is rarely over 30°C, andthe minimum is usually above 0°C. The mean annual
temperature is slightly below 20°C. Mean annual rain­
fall varies from place to place, from somewhat less than
1,500 mm to slightly over 2,000 mm.
TOPOGRAPHY

INTRODUCTION

The topography is marked by high and rugged peaks,
The Quadrilatero Ferrifero (about 7,000 km2) is mostly
longchains of mountains, with some deep canyons andincluded between lat 19°45' and 20°30' S. and long
valleys.
The maximum elevation in Quadrilatero
43°22'30" and 44°730" W. and lies near Belo Horizonte,
Ferriferois 2,100 m at the top of Serra do Caraca, andMinas Gerais State, Brazil (fig. 1).
theminimum,
about 600 m, where the Paraopeba River
Uranium-bearing conglomerates in the Moeda Forma­leavestheregion.
tion are known to crop out far beyond the limits of the
Quadrilatero.
HISTORY

TRANSPORTATION AND COMMUNICATION

Some paved highways give access to the neighborhoodof prospecting sites. Usually it is not necessary to travel
more than 20 km from paved roads to reach the pros­
pects.
A railroad network built for iron-ore transportation isin operation. Passenger transport within the region isby bus or car.
Frequent airplane service from Belo Horizonte isavailable to the main cities of Brazil.
1 NUCLEBRAS, Distrito Belo Horizonte, Rua Washington, 801-S10N, Belo Horizonte, M.
G., Brazil.

The first prospecting work for uranium in
Quadrilatero Ferrifero was done in 1971 by carborne
survey, when more than 100 radioactivity anomalies
were found. Of those anomalies, 27 percent were in
granitic and gneissic rocks, 37 percent in schists of the
Rio das Velhas Series, 30 percent in the Minas Series,
mostly in conglomerates, and 6 percent in other rocks(fig. 2).

Due to the deep weathering in the area, chemical
assays usually showed low uranium values in surface
samples of conglomerates, 20 to 30 ppm for anomalies
that give 2xl03 to 5xl03 cps (SPP-2). The chemicalvalues increase with depth.
Tl

T2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESPITANGUI AREA
BELO
HORIZON1E

GANDARELASYNCLINE

FIGURE 1.—Location and study area.

After preliminary mapping and radiometric studies,areas were selected for drilling. Encouraging results,
with some ore-grade intersections in drill holes, in­dicated the need of more detailed mapping for better
evaluation of the areas.

The correlation (table 2) gives 2,200 million years as
the age of the Moeda Formation, but rubidiumstrontium determinations indicate an age of about 1,350
million years. It is thought that the radiometric age
reflects later events rather than the age of deposition.
Around the Sao Francisco Craton (fig. 3) are PrecamSTRATIGRAPHIC SECTION
brian rocks that are potential hosts for uranium
deposits.
The stratigraphy used in the Quadrilatero Ferrifero is
The vectors on figure 3 show known paleocurrent
shown in table 1, which is taken from Dorr (1969, pi. 12), directions in the area resulting from crossbedding
Robertson (1974) correlated the Moeda Formation of measurements plotted on polar diagrams. In the
the Carac.a Group with other uranium-bearing Precam- western Quadrilatero Ferrifero as well as in thebrian conglomerates of the world as shown in table 2. Pitangui area the sediments came from the Sao Fran­The Maquine Group was added to the correlation by cisco Craton. In the Gandarela Syncline, however, theVillaga (1974) owing to its radioactive conglomerates.
source is located to the east (Lindsey, 1974).

URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL

T3

MINAS SERIES
30 PERCENTRIO DAS VELHASSERIES 37 PERCENTJASIC ROCKSPERCENTITACOLOMISERIES
2 PERCENTGRANITIC AND
GNEISSIC ROCKS
27 PERCENT

FIGURE 2.—Stratigraphic distribution of radioactive anomalies.

The paleocurrent directions in the Jacobina area (fig. Syncline. Its thickness decreases northward and north­4) also indicate that the source lies to the east (Bastos westward (fig. 5), and the total thickness of con­and Moura, 1974).
glomerate in the Moeda also decreases northward and
The conglomerate pebble sizes both in the Jacobina northwestward (fig. 6). Both maps are based on recon­area and in the eastern part of the Gandarela Syncline naissance traverses made in 1974 along the entire
indicate a short distance of transport.
length of the outcrop of Moeda Formation around theJacobina, in the northeast corner of figure 3, is being syncline and on data from drill holes.
studied by Unigeo, whereas Quadrilatero Ferrifero and
The thick conglomerates in the southeast area are
Pitangui are almost unknown geologically. Recon­ coarse conglomerates; some reefs contain boulders andnaissance work is necessary in order to locate where cobbles (figs. 7, 8). This type of conglomerate is usuallyconglomerates occur.
polymictic and immature, with cobbles of schist, in­dicating a short distance of transport.
AREAS BEING PROSPECTED
Along the western side of the basin the conglomerates
arethin, pebbles are rarely bigger than 3 cm in diameter
NUCLEBRAS intends to continue work in the Moeda(figs.
9, 10), and the channels pinch out quickly. The
Formation at Pitangui and in the Ouro Fino Syncline
mean
paleocurrent direction indicates a source area toduring 1976.
the
east.
These conglomerates are a distal facies of the
Now the Company is studying the Gandarela structure
conglomerates
that crop out to the east.
in sites already drilled. A mapping program is underway
These
data
suggestthat the conglomerates are pres­in the Maquine Group, where some radioactive conglom­
entinsubsurface
overa large area of the syncline. If
erates crop out. Included in the program are detailed
conglomerates
arepresent,
this is a favorable area ingeological mapping of some selected areas, surface
whichtoexploreforuranium
deposits.
sampling, and re-evaluation of the previous results ofdrilling programs. At least two years will be necessary
MINERALIZATION
to finish the work presently scheduled.
Both polymictic conglomerates and oligomictic quartzDISTRIBUTION OF CONGLOMERATES IN
pebble conglomerates occur in the western part of the
GANDARELA SYNCLINE
Gandarela Syncline.
Southeast of this syncline at least two channels have
The maximum known thickness of the Moeda Forma­tion is about 350 m in the southern part of the Gandarela been found, one of which is in the basal Moeda Forma-

T4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE l.—Stratigraphic section in the Quadrildtero Ferrifero, Minos Gerais, Brazil[Only Minas and Rio das Valhas Series]

AGE

SERIESGROUPFORMATION

SEDIMENTARYENVIRONMENTSABARA

FLYSCHBARREIROPIRACICABATABOES
FECHO DO FUNIL
CERCADINHOPRECAMBRIANSTABLE SHELFMINASGANDARELAITABIRACAUEBATATALCARACA
MOEDA(UNNAMED)STABLE SHELF

CAMBOTASPARALICTAMANDUACASA FORTE
MOLASSEMAQUINE
PALMITALO LJJE>NOVA LIMA(UNDIVIDED)BASEMENT NOTEXPOSEDFLYSCHURANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL

H-

+

+

+

+

+ -H

+

+

+

+

+

-H

+ + + +SAO+

FRANCISCO +

+

+

+

+

+

FIGURE 3.—Precambrian rocks (dots) around Sao Francisco Craton (crosses). Vectors show paleocurrent directions.

T5

T6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

350C
330

N

10o
30C

50°

310

290

110°

250

130C

230

210

150C

FIGURE 4—Paleocurrent directions in Jacobina area. Long vector indicates paleocurrent direction for the area; short vectors indicate mean
paleocurrent direction for each block.

URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL

T7

TABLE 2.—Stratigraphic correlations ofRobertson (1974) with Maquine Group added by Villaga (1974)

BRAZILCANADAENVIRONMENTS

SOUTH AFRICAQUADRILATERO

AUSTRALIA

NULLAGINEELLIOT LAKE

MONTGOMERYOXIDIZINGLORRAINE

HURWITZ

TRANSVAALITABIRAHAMERSLEY(Red rocks)

FORMATIONGROUPSUPERGROUP

GROUP

GROUP

——————FERRIFERO

2200 M.Y.

|————————WITWATERSRANDSYSTEM
REDUCING(Sedments with
pyrite anduranium)

ELLIOT
GROUP

(U)

MOEDA
FORMATION

MONTGOMERYGREEN HOLELAKE
SEQUENCE

DOMINION REEF
SYSTEM

MAQUINE
GROUP(U+Au)(Au + U)

(U+Au)

tion. Uranium may be present with some gold, and somedrill holes cut ore-grade material (at the present price ofUS08).
Thinning of conglomerates, imbricate thrust faults,
and an overturned fold are shown in the cross section(fig- 11).
The structural complexity of the area and the great
variation in grade require more detailed work to be donein order to delimit conglomerates and to evaluate
reserves.
MINERALOGICAL ASSOCIATION

The matrix of the conglomerate is composed
dominantly of fine quartz, pyrite, and rutile, with small
amounts of zircon, kyanite, muscovite, molybdenite, andradioactive minerals. Amorphous carbonaceous material
similar to thucholite has been found.

FORMATION(U)

The thorium content of the conglomerates is low, andthe Th02/U308 ratio is variable. Great lateral variationsof the ratio occur within the same conglomerate, and theratio varies from one conglomerate to another. Figure
12 illustrates this variation in the basal conglomerate of
the Moeda Formation in the southwest side of the Gandarela Syncline.
The relation between the U308/Ti02 and theTh02/Ti02 ratios in the basal conglomerate, based on
drill core assays, is shown in figure 13. The uranium con­tent increases where Ti02 exceeds 150 ppm, and it
decreases where Ti02 exceeds 600 ppm. The pronounced
decrease in the uranium content where the Ti02 reaches
350 ppm is related to insufficient data.
Insufficient gold analyses were available for valid con­clusions to be drawn about its relation to uranium,
thorium, and other elements present.

T8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
37°30'

20°00'

EXPLANATION
Strike and direction of dip of beds
—POverturned beds———— Fault——*— Thrust fault
100-- Structure contour,in meters.
Dashed where control is poor

20°07'30"

15 KILOMETERS

FIGURE 5.

Isopach map of Moeda Formation in the Gandarela Syncline.

URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL
37°30'

20°00'

EXPLANATION
Strike and direction of dip of beds
Overturned beds
Fault
Thrust fault
700— Structure contour, in meters.
Dashed where control is poor

-P-

20°07'30"oI

15 KILOMETERSi

J

FIGURE 6.—Isopach map of total conglomerate in the Gandarela Syncline.

T9

T10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES. fa i>~ B^V*-? **' >*•^SJs- i-^Jf •/ ^-^ s*.- A « *>£FIGURE 7.—Quartzite boulder, east of the Gandarela Syncline.

FIGURE 9.—Conglomerate with black quartz pebble, in the west of theGandarela Syncline.

:J^%

ar*" '^i^5^'^
ti5?^>^
f *%&#•& '-srf^T% r^^-->-^s-^ -,^-r0^.'^^- '-^'•-'V*^j-' ,?*.>^--^ •FIGURE 8. Polymictic conglomerate, cobble size, east of the Gandarela
Syncline.

WORKING METHODSFIGURE 10.-Conglomerate with iron staining due to pyrite weathering.

Diamond-drill cores, after being logged and sys­
tematicallysampled, will be correlated with the map­
Due to the structural and stratigraphic complexity of
ping
data
in
order to evaluate areas and to guide addi­
the area, a geologic mapping program covering 52 km2
tionalexploration.
of the Gandarela Syncline was approved and is being
carried out at the moment. East-west geologic traverses
200 m apart are being run from north-south base lines;
CONCLUSIONS
about 260 km of geological traverses are planned for theGandarela Syncline. Geologic mapping is being done at a
Many data are needed to estimate the potential of con­scale of 1:2,500.
Trenches will be opened on the conglomerate zones for glomerate zones in areas of strong tectonism.
Although high uranium grades have been found inchannel sampling. Mineralized conglomerates will bemapped at a scale of 1:500 and more detailed sampling some holes, following favorable host rocks requires agood knowledge of the tectonics and facies variations
done.

URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZILw
Drillhole

Drill hide

100

300 METERS

200

EXPLANATION

Conglomerate

Basement

Quartzite

Fault showing relative
direction of movement

FIGURE 11.—Cross section of southeast Gandarela Syncline.

Til

T12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
43°38'45"

20°08'45"

OUTCROP OF
BASAL MOEDA FORMATION^

0
I

i

200
I

400
I

600
I

800
I

1000 KILOMETERS__I

FIGURE 12.—TMVUsOg ratio distribution in the basal conglomerate in the southeast Gandarela Syncline.

URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZILDISCUSSION

1000

•z.
o
^

800

600

O
n

400

Q

200

100

T13

300

500

700

900

1100

TiO2, IN PARTS PER MILLIONFIGURE 13.—Relationship between titanium oxide (Ti02) and uranium(U308) and thorium (Th02) oxides.

within the area so that the holes can be located in themost favourable sites.
Grade variations within ore horizons demand wedging
of the drill holes to provide many intersection of thereefs, since a simple intersection is not able to collect a
representative sample of large areas.
The extensive Proterozoic rocks with lithology similar
to that of the Moeda Formation that crop out around
Sao Francisco craton are hosts suitable for the oc­
currence of uranium deposits and as the result of morework may become favorable exploration targets.
REFERENCES CITED
Bastos, A. C., and Moura, L. A. M., 1974, Relatorio de viagem a
Jacobina, Bahia: CNEN/DEM, unpublished report (ERBHO file).
Dorr, J. V. N., 2d, 1969, Physiographic, stratigraphic and structuraldevelopment of the Quadrilatero Ferrifero, Minas Gerais, Brazil:
U.S. Geological Survey Professional Paper 641-A, llOp.
Lindsey, D. A., 1974, Depositional environments and paleocurrent
directions in the Precambrian Moeda Formation, Minas Gerais,
Brazil; U.S. Geological Survey Open-File Report 75-264, 22p.
Robertson, D. S., 1974, Basal Proterozoic units as fossil time markers
and their use in uranium prospection, in Formation of uranium ore
deposits - Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens, Greece,6-10 May 1974: Vienna, International Atomic Energy Agency, p.
495-512.
Villaca, J. N., 1974, Relatorio anual do escritorio regional de Belo
Horizonte: CNEN/DEM unpublished report (ERBHO file).

Cannon: Do you have any radiometric data on what theage of the Moeda Formation is?
Moura: There are some strontium-rubidium measure­ments that give less than 1,800 million years, but weusually consider that these ages might reflect later
events and be due to metamorphism. We don't know
yet. We have red-bed formations right above thereducing environment of the Moeda Formation; theCaue Formation is itabirite iron ore. In other areas of
the world this change from reducing to oxidizing con­ditions is thought to have occured about 2,200 million
years ago.
Robertson: Along the same lines, do you have an age on
the Sao Francisco Craton?
Moura: No. We are just in the early stages of prospec­
tion.
Whiteside: If I may just comment a little bit. I was very
lucky. I visited Belo Horizonte about 1971 when theyfirst found uranium and I saw some of the outcrops.
They have great difficulties there because weathering
is extremely deep and it's only a few of the more resis­
tant beds that crop out, mostly quartzites. They stickup as hogback or sawtooth ridges. The few outcrops I
did see of conglomerate were small pebble, I would say
about a half an inch, pyritic, and looked very like someof the Witwatersrand conglomerates, but those did
not carry either gold or uranium. I think it's mostly
towards the base of the Moeda that mineralization oc­
curs, and I think the ones I saw must be somewhere
near the top. The only piece of real mineralization, if I
may say so, that I saw was a black mineral in a little
bit of core. They asked me what it was, and I said, "I
don't know." Actually, it was a piece of uraninite; it
was about 2 mm in length and 1 mm broad, and it
looked as though it was rounded. There is not verymuch more I can say, but on the whole, I don't think I
would have correlated what I saw in the Moeda withwhat I know of the Witwatersrand. (This is a very
long-range correlation, of course, in spite of continen­
tal drift.) I don't think there would be anything in
Brazil equivalent to the division of argillaceousarenaceous in the lower Witwatersrand andarenaceous-rudaceous in the upper Witwatersrand. I
never saw anything like that in Brazil.
Ruzika: Can you tell us something about the carbon con­tent in the Moeda?
Moura: Some analyses were made by Robertson and
Associates of carbon seams and things like that, that
Robertson suspected could be thucholite. In his report
he reported two or three samples as thucholite, but I
don't think that it has been established yet.
Skinner: With so much titanium around, did you see any
brannerite? Is there much brannerite in the depositT14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESMoura: I haven't seen any, but mineralogical studies
have found uraninite and brannerite, sometimes
associated with pyrite and sometimes with what wascalled thucholite, but we are not sure of the last one.
In the matrix is fine quartz, pyrite, and rutile. Fine
quartz grains and pyrite are the most abundant
minerals, and small amounts of zircon, kyanite,
muscovite, and molybdenite are also present in the
area.
Armstrong: On your map you showed the area south ofBelo Horizonte and Jacobina, and you said that in theintervening area there are rocks of the same age andlithology and that this intervening area would be agood one to prospect. If I recall correctly, rocks of thatsame age also extend an equal distance to the north­west along the western flank of the Sao Francisco
Craton. Is that an equally good prospecting area?
Moura: Yes, we think so. You saw the Sao Francisco
Craton and on both sides of it there were some data
points. We think that both areas are favorable forprospecting, but we have only scattered data on thewestern area. On the southwest side of the craton in
the Pitangui area we have found uranium- and goldbearing conglomerate, but we don't know if it will
make a mine.
Armstrong: Yes, but I really meant a long way north­west of the Pitangui area.
Moura: Yes, it continues over there. Sometimes it'sburied by younger rocks but it in appears some places.
Meyer, Falconbridge Nickel Mines, Ltd., Canada: May I
just echo some of Mr. Moura's words earlier in his
talk? The excellent presentations of the last two days
will give valuable direction to any geologist who has
an established ore body in the ground, but the basic

question in the minds of many of us here "How do wefind another Witwatersrand or another BlindRiver?" - remains unanswered. We are two-thirds ofthe way through our time here and may I ask the re­maining speakers to give some thought to this mostimportant question.
Moura: Excuse me, I didn't get the question he made.
Could you translate?
Armstrong: It doesn't require an answer. All he said wasthat he agrees with your opening remarks. How do wefind other deposits?
Thorman, U.S.G.S.: I would like to echo what Dr.
Whiteside said, probably a little louder, in that if you
haven't been there you can't appreciate the explora­tion problems. Weathering is intense, the area is enor­
mous, in fact it's huge, and the work they are doing
now is quite good in view of the manpower that's
available and the area that you have to cover. I spentalmost two years down there mapping the Quadrilatero Ferrifero, and I can confirm that the stratigraphic and structural problems are complex. We'dreally like to start mapping all over again because now
we know where the outcrops are, and with a betterknowledge of the stratigraphy we could do a betterjob. So I think I'll echo Luciano's [Moura] statementthat the microprobe and other things are great tools
once you know where you are geologically, but it takes
a tremendous amount of basic work before you canstart using microprobes and that sort of thing bene­
ficially.
Armstong: From a few days inspection I can state thatthe structure is more complicated than was indicatedon the cross sections. The thrust faults have been cut
by later high-angle faults so that the rocks are cut up
into fault-bounded blocks.

CANADAThe Blind River Uranium Deposits:
The Ores and Their Setting
By JAMES A. ROBERTSON
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-U

CONTENTS
Page

Abstract _______________________
Introduction ____________________
Regional geology _________________
Archean ____________________
Keewatin-type rocks _______
Algoman granitic rocks _____
Huronian Supergroup _________
Elliot Lake Group _________
Matinenda Formation _.
McKim Formation ______
Volcanic rocks _________
Hough Lake Group ________
Ramsay Lake Formation
Pecors Formation ______
Mississagi Formation ___Quirke Lake Group ________
Bruce Formation _______
Espanola Formation ____
Serpent Formation _____

Ul
1
1

3
3
3

6
6
6

9
9
9
9
9
10

Page

Regional Geology - Continued
Huronian Supergroup - Continued
Cobalt Group _______________________ U10
Gowganda Formation ________________
10
Lorrain Formation __________________
11
Gordon Lake Formation _______________
11
Bar River Formation _________________
11
Summary of Huronian __________________
12
Post-Huronian events _____________________
12
The uranium deposits _______________________
12
Distribution of uranium ore deposits - Blind River-Elliot
Lake ____________________________
13
Lithology and mineraology of uranium-ore deposits ____
13
Agnew Lake area ________________________
17
Cobalt embayment _______________________
17
Production ___________________________
18
Reserves _________________________—__
18
Origin of uranium deposits of Blind River type _______
18
Acknowledgments _________________________
19
Selected references _____________________—__
20

ILLUSTRATIONSPage

FIGURES 1,2. Maps showing:
1. Location of Blind River-Elliot Lake area ___________________________________________
2. Generalized geology of the Blind River-Elliot Lake area; schematic cross section of the Elliot Lake area ______
3. Diagram showing formation of regolith ________________________________________________
4. Correlation chart showing lateral variation of the Huronian Supergroup, Elliot Lake ______________________
5. Diagram showing alkali content of Huronian polymictic conglomerates ______________________________
6. Graph illustrating silica/alumina, ferric iron/ferrous iron, and soda/potash ratios for Huronian argillaceous rocks ______
7. Map and cross section showing the geology of uranium deposits in the Quirke Syncline _____________________
8. Diagram showing the correlation of quartz-pebble conglomerate reefs, Quirke zone _______________________
9. Cross section, New Quirke Mine ____________________________________________________

U2
4
6
7
10
12
14
15
15

TABLES
Page

U2
3
5
17
17
17

TABLE 1. Formations for the Blind River-Elliot Lake area _____________.
2. Systems of Huronian stratigraphic nomenclature ____________.
3. Summary of Huronian stratigraphy in the Blind River-Elliot Lake area
4. Uranium, thorium, and titanium contents of Blind River brannerite __.
5. Uranium and thorium contents in Blind River uraninite _________.
6. Analyses of Blind River monazites _____________________.
ill

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHE BLIND RIVER URANIUM DEPOSITS:
THE ORES AND THEIR SETTING1
By JAMES A. ROBERTSON2
ABSTRACTIn the Blind River area, Proterozoic clastic sedimentary and minor
volcanic rocks (Huronian Supergroup) unconformably overlie andtransgress northward over dominantly granitic Archean terrane(2,500 million years) and are intruded by Nipissing Diabase (2,150
million years). Later deformations and metamorphic events arerecognized.
The Matinenda Formation (basal Huronian) comprises northwardderived arkose, quartzite, and pyritic, uraniferous oligomictic con­glomerates, which contain 75 percent of Canada's uranium reserves.
Historic grades approximate 2 pounds U308/ton (1 kilogram/metricton), but lower grade material can be mined with increasing price.
Some thorium and rare earths have been marketed. The generalabsence of gold reflects lack in the source area.
The conglomerate beds lie in southeasterly striking zones controlled
by basement topography down-sedimentation from radioactive Ar­
chean granite. Distribution of monazite relative to uraninite and "brannerite" and the presence of uranium values in overlying polymictic con­glomerates, which truncate the ore beds, indicate that the mineraliza­tion is syngenetic, probably placer. The role of penecontemporaneous
mafic volcanics is problematical, but these could have been a source forsulphur in the pyrite.
Drab-coloured rocks, uranium and sulphide mineralization, and a
post-Archean regolith formed under reducing conditions all suggest a
reducing environment. Sedimentary features indicate deposition in
fast-flowing shallow water and possibly a cold climate. In the upper
Huronian (Lorrain Formation), a monazite and iron oxide assemblage
associated with red beds suggests a change to oxidizing conditions.
Similar deposits occur at Agnew Lake, and polymictic conglomerateand siltstone found north of Sudbury carry minor uranium and locallygold values. Provenance, lack of oxygen, transportation and depositional processes, and no major modification by later events are thedominant factors in the formation and preservation of the ore bodies.
Research is needed on mineraological relations, detailed sedimentology of the conglomerates, and the volcanic rocks, their nature androle.

panics, of universities, and of both Canadian and On­
tario government geologists have led to a large body ofliterature and other data. This paper is an attempt tosummarize the present state of knowledge and to pro­vide guidelines for the exploration geologist or thegeologist in the field who may find himself in favorable
terrane; it also serves as background for the more detail­ed papers that follow. The paper is in three sections:first a description of the regional geology, second a
description of the ore-bearing conglomerates, and thirda discussion of those factors of genetic significance.
REGIONAL GEOLOGY

Blind River (fig. 1) lies on the north shore of Lake
Huron, halfway between Sudbury and Sault Sainte
Marie. The town of Elliot Lake, built to service theuranium mines, lies 20 miles northeast of Blind River.
The region lies on the boundary between the Southernand Superior Province of the Canadian Shield. The
Superior Province (Goodwin and others, 1972) com­prises Archean rocks that were affected by the Kenoran
Orogeny (2,500 m.y. (million years)), and the Southern
Province (Card and other, 1972) includes Proterozoic
rocks affected by the Penokean Orogeny (1,750 m.y.).
The Blind River-Elliot Lake area lies at the margin bothof early Proterozoic sedimentation and of the midProterozoic metamorphism.
Table 1 is a table of formations using the nomencla­
ture recommended by the Federal-Provincial Commit­
tee on Huronian Stratigraphy (Robertson and others,
INTRODUCTION
1969), and table 2 permits comparison with schemes
usedin publications prior to 1969. Table 3 provides a
Economic Blind River uraniferous pyritic oligomictic
synopsisof stratigraphy and its relation to mineraliza­
conglomerates were found in 1953; the presence oftion.
uraniferous conglomerate had been known since 1948.
The bedrock of the area falls into three broad units,
Since 1953, the activities of mining companies, of unithe distribution of which is shown on figure 2. These are
1 This paper was presented at the workshop by James A. Robertson. In 1976 it was pub­(1) the Archean basement, consisting of Keewatin-typelished under the same title by the Ontario Division of Mines as Miscellaneous Paper 65, 45
greenstone, Algoman granite, and minor mafic inpages. It is reprinted here, with some minor changes, with the permission of the Director, On­tario Geological Survey, Ministry of Natural Resources.
trusives, (2) the Huronian sedimentary and metasedi2 Chief, Mineral Deposits Section, Ontario Geological Survey, Mineral Resources Group,
mentary rocks, with local minor mafic volcanics, and (3)Ministry of Natural Resources, 77 Grenville St., Queen's Park, Toronto, Ontario, Canada.

ui

U2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE 1. Formations for the Blind River-Elliot Lake area
[Codes keyed to fig. 2 are in parentheses. Geologic ages given are from a variety of sources]

Dominant lithology

Unit

Phanerozoic
Cenozoic
Pleistocene and Recent

Sand, gravel, till

Paleozoic
Ordovician (20)PrecambrianProterozoic
Keweenawan Supergroup
Sudbury dikes (19)

Unconformity
Limestone

Olivine diabase

Intrusive contact with Nipissing Diabase f1)
Hudsonian
Croker Island Complex
Gabbro, granite(18)
Cutler Batholith (18)GraniteQuartz diabase, diorite

Unconformity-disconformity

Hough Lake Group
Mississagi (10)Pecors (9)Ramsay Lake (8)Elliot Lake Group2
McKim (7)Matinenda (5)Quartzite
Limestone, siltstone
Conglomerate
Local disconformity
Quartzite
Argillite
Conglomerate
Local disconformity

ArcheanLate Archean intrusives
Kenoran (Algoman) (3)Keewatin (1)2,155Quartzite
Siltstone, sandstone
Quartzite, conglomeratearkoseConglomerate, greywacke,
quartzite

Gowganda (13)

Early Archean
intrusives

1,750

Intrusive contact

Huronian Supergroup
Cobalt GroupBar River (16)Gordon Lake (15)Lorrain (14)

Quirke Lake Group
Serpent (12)Espanola(ll)Bruce (11)1,445Intrusive contact

PenokeanNipissing (17)the post-Huronian intrusive rocks, comprising the
Nipissing diabase sills, diabase dikes, the Cutler Granite,
the Croker Island Complex, and olivine diabase believed
to be Keweenawan in age (only the Cutler Batholith andthe Croker Island Complex are shown in figure 2). The
structure is also illustrated in figure 2. In the northcentral zone lies the Quirke Syncline; farther north isthe Flack Lake Fault, with upper Huronian rocks expos­
ed on the downthrown (north) side; and in the center isthe Chiblow Anticline, the south limb of which isrepeated by a major east-striking fault-the Murray
Fault. The axes of these folds strike slightly north ofwest and plunge gently west, giving the sedimentary
units a reverse S-shaped outcrop. To the south of the
Murray Fault, east of Algoma, the folding and metamorphism are more intense and complex. To the east of the
Cutler Batholith, fold axes and other linear featuresgenerally plunge eastward, but to the west of thebatholith the linear structures plunge westward. Only
the most clearly defined folds are shown in figure 2. The
east end of the Cutler Batholith is controlled by the
Spanish Anticline (Robertson, 1966a, 1975a; Cannon,1967). The La Cloche Hills are defined by the Lorrain
quartzite, which forms the limbs of the La Cloche
Syncline. The Croker Island Complex lies near the axisof the McGregor Bay Anticline. When traced eastward,

1,225

Intrusive contact
Quartz diabase

Mount Lake Dike

FIGURE 1.- Location of Blind River-Elliot Lake area.

Age(millionyears)

Argillite
Quartzite, with or withouturanium conglomerate
Conglomerate
Arkose, with or withouturanium-conglomerate
regolith
Unconformity
Diabase

2,500

Intrusive contact
Granite2,500 +

Intrusive contact
GabbroIntrusive contact
Volcanic and sedimentaryrocks

1 Mount Lake Dike may be 1,795 m.y. old.
2 Volcanic rocks are found locally in the Elliot Lake Group (6); each occurrence has been
given its own name; age about 2,400 m.y.

U3

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTINGTABLE 2. -Systems of Huronian stratigraphic nomenclature

Upper White Q 1
Cobal
Seriesiest

Banded Cherty

a
D
O

CD
15
.a
o

LorrainGowganda

CJ

Serpent

w
D

m

Bruce

a
D
O

Mississagi

CD
0)

o
D

^(Ramsay Lake)1•o ^£w<

Upper White Q 1

Gordon Lake

CD

Banded Cherty

D
O

"ro
.a
o
CJ

LorrainGowganda•- -* o«J 2
aD -'CD

Upper Mississagi

-c a
D) D

Middle Mississagi argillite§2

Conglomerate
Lower Missi ssagi argillite
Quartzite

McKim

Bar River

a
o
CD
"ro
.a
o
CJ

LorrainGowganda

Gordon Lake
LorrainGowganda
Serpent

ICD

*- Q.
O 3
— O
LU(5

0)

Espanola
Bruce

Q.

•£ 8 == 12
a CD

PecorsS.S&
OJS 2
I-1 CD

Matinenda

Archean

Bruce

PecorsRamsay Lake

Whiskey
Nordic

Espanola

Mississagi

Mississagi

AlgomanArcheanSchist Complex

ROBERTSON AND OTHERS(1969)

Serpent03o-* 0) 3Bruce

mGranites

|8
£ ^-=

a

Espanola0)o

Bar River

Serpent

Espanola0)

ROSCOE (1960)

ROBERTSON (1967a)

COLLINS (1925)

CM.2.8. £= «J OUJ--5

McKim
Matinenda

Archean

Keewatin

1Q equals Quartzite
2Volcanic rocks are found locally in the Elliot Lake group.

the La Cloche Syncline and the McGregor Bay Anticline
are part of a series of folds trending easterly between
Espanola and the Grenville Front (Card, 1967a-d;
Young, 1966; Young and Church, 1966).
Bedding-plane slips, thrust faults, and near-verticalfaults that strike either northwest or parallel to the axial
planes of the folds are common. The fault pattern, joint­
ing, slickensides on bedding planes, and drag folds sug­gest that north-south compression formed the folds.
Several periods of intensity of deformation andmetamorphism can be demonstrated.
ARCHEANKEEWATIN-TYPE ROCKS

Keewatin-type rocks are found locally in the Archeanof the North Shore of Lake Huron and particularly
underlie the Huronian rocks near Elliot Lake. The rocktypes found include massive and pillow lavas, pyroclastic
rocks, and sedimentary rocks including lean ironformation. The strike is northwest, and dips are general­ly steep in the northeast. Metamorphism is of thechlorite grade, rising to the amphibolite grade in hybrid
zones close to contacts with the Algoman granite. The
general lack of greenstone in the cratonic block north of
the Huronian basin and a low frequency of known gold

occurrences in the area believed to be the source area
are mirrored in the lack of gold in the uraniferous con­glomerates.
ALGOMAN GRANITIC ROCKS

Granitic rocks of Algoman age (2,500 m.y. (Lowdon,
1960, 1961; Lowdon and others, 1962, 1963; Van
Schmus, 1965; Wanless and others, 1965)) underlie ap­
proximately half the area shown on figure 2. These
granitic rocks may be divided into two broad groups: (1)medium- to coarse-grained, gneissic to massive
granodiorite, generally grey to pink in colour with abun­dant inclusions of Keewatin-type rocks; and (2) massive
red quartz monzonite, generally without inclusions andslightly radioactive. Bodies of the second type are foundin the Quirke Lake, Aubrey Falls-Ranger Lake, andElliot Lake areas (fig. 2; Robertson, 1960, 1961, 1967,
1968a).
It is of interest that in the Montreal River area northof Sault Ste. Marie and eastwards as far as Aubrey Falls
there are occurrences of pitchblende coating joints in
the granitic rock or associated with the contact of late
Precambrian diabase and early Precambrian granite. In
the same area uraninite occurs in Archean pegmatite(Nuffield, 1955; Robertson, 1968b, 1975a). A statisticalanalysis of crossbedding and size distribution of chert

U4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXPLANATION
Post-Huronian granitic rocks
Huronian: sedimentary rocks;
volcanic rocks
Red-phase Algoman granitic rocks
Grey-phase Algoman granitic rocks£ \s

Anticline, syncline, with plunge

Archean metavolcanic belts
and Post Huronian mafic+ NTT /.•I•^>^I>iis^•!•I•,xl^tl•\;iOH*.*?Xl.•.•.•.•I•x^<tf/»^.•^^.y^^*«... .^...... .-fia...............v.-......... Mi«u«........x>. .... / f=r?7.. .Tm.rv.TMa^iwv.-.. .-Of... -.j-ar^.... ^ -PHJ, ^SSSIJ&:

Western limit o+area mapped ..
as Birch Laker:
Granite j.
i^^jff^^t-T'i^'i'i^'Jx.-iv

West Grant _,^?£ast Grant I.

FIGURE 2. - Generalized geologic map of the Blind River-Elliot Lake area, with schematic cross section of the Elliot Lake area (line A-B). Units on
the cross section are keyed to table 1.

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTINGU5

TABLE 3.— Summary of Huronian stratigraphy in the Blind River-Elliot Lake areaGroup

Formation

Cobalt _____Bar River

Quartzite

Gordon Lake _ Siltstone,sandstone.
Lorrain

Quirke Lake

Hough Lake

Elliot Lake

Thickness(in feet)

Lithology

Depositional
environment

SourceShallow water ___Source north,_At least 1,000
at Flack Lake; atbut currents
variable.
least 4,000 at
Willisville.
Shallow water ___ ________
1,000 at Flack
Lake; 3,000 at
Willisville.
North-northwest
Shallow water
2,000-6,000

Quartzite, conglo­
merate, arkose.
Gowganda ____ Conglomerate, grey- 500-4,200
wacke, quartzite,
siltstone.
Serpent
Quartzite.0 1,100
Espanola
Limestone, dolo0 1,500
stone, siltstone.

Glacial in north;glacial-marine insouth.
Shallow water

Bruce

Conglomerate.0-200Mississagi

Quartzite.0-3,000 +Glacial shallow
water.
Shallow water

PecorsArgillite.40-1,000 +Shallow water

Ramsay Lake

Conglomerate.5-200Glacial shallow
water.

McKim

Argillite-greywacke.

0-2,500 +Matinenda

Quartzite, arkose,
conglomerate.

0-700 +

Volcanic rocks

Andesite-basalt (fel- Local piles
sic volcanics).

(turbidite).

North-northwest ___Northwest
Northwest

North?
West-northwest in
west; north insoutheast.
North-northwest
Northwest?

Northwest

Shallow waterNorthwest

Subaerial

Flack Lake Fault;
Murray Fault.

Mineralization

Th-U innorth; Cu?

U trace inVictoria Tp.;
Cu in lime­stone
againtdiabase.
U near base­ment highs.
Traces U nearbasementhigh.
Traces U
where unconformableon Matin­enda Forma­tion.
Traces U nearbasement
highs.
U-Th-rareearths in con­glomerate inbasement
lows.1
U-Th in con­glomerateinterbeds.

All uranium deposits of commercial importance in the Blind River-Elliot Lake area are in the Matinenda Formation.

pebbles in the Mississagi Formation (McDowell, 1957)
indicated that the source area for the lower Huronianlay 130-250 miles (210-400 km) west-northwest of
Thessalon.
More recently airborne y-ray-spectrometer surveys
have confirmed that much of the granitic terrane northand northwest of the Elliot Lake area contains
anomalous amounts of uranium (Darnley and Grasty,
1971; Richardson and others, 1975). In general, thoseareas containing the massive red quartz monzonite ex­hibit anomalous uranium content.
The massive red granitic rocks of the Archean base­ment are held to be the source of the uranium found in
the Huronian conglomerate.
After the peak of the Kenoran Orogeny the area wasfractured and numerous diabase dikes were intruded.

The region was then subjected to a prolonged period of
weathering and peneplanation. During this period
hollows or valleys developed over the more susceptible
rocks, such as those composing the greenstone belts.
Throughout the Blind River area partially weatheredzones have been preserved under the Huronian-Archean
unconformity and, particularly, over the granitic rocks.
At distances of greater than 100 yards (90 m) from thecontact with the overlying Huronian, the granitic rocks
exposed at the surface are of the normal type. However,as the actual contact is approached, the plagioclase
feldspar grains become yellow, owing to the develop­
ment of sericite, and the ferromagnesian minerals are
no longer present; the granitic texture is preserved by
the quartz and microcline crystals. This material passes
into an unsorted aggregate of partly corroded quartz

U6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

and microcline grains in a yellow-green matrix ofsericite. Rarely, angular fragments of vein quartz andpatches of less-altered granite may be present. This
material is overlain by sorted sericitic arkose, in which
bedding and crossbedding are generally visible. In some
outcrops the actual contact may be marked by a band ofangular to subangular quartz fragments up to an inch
across.
This transition zone is generally interpreted as aregolith or fossil weathered zone developed during theArchean-Proterozoic interval. Such weathered zones
have been observed over both red and grey phase
Algoman granites. Core sections also show the develop­
ment of crumbly chloritic material between thegreenstone and the overlying sedimentary rocks (Rice,
1958). This has also been regarded as regolith. BothPienaar (1963) and J. A. Robertson (1960 and later) haveanalysed samples of regolith. Figure 3, derived fromtheir analyses, is a straight-line diagram illustrating
development of regolith. The following features are ap­
parent:1. Silica is a constant, and there is a slight gain inalumina.
2. Zirconia is constant, reflecting stability of zircon.
3. Total iron and both ferrous and ferric iron show
marked loss; ferric iron is lost more completely
than ferrous.
4. Magnesia and manganese have been partially lost.
5. Lime and strontium and soda have been markedly
reduced.
6. Potash, rubidium, and water ( + ) are strongly in­creased.
These observations reflect (1) the destruction of theplagioclases and the removal of the soluble constituents,(2) the stability of the potassic feldspars (microcline),and (3) the formation of hydrated clay minerals
represented by sericite. The trace elements follow the
major elements in pairs: Mn-Mg, Rb-K, and Sr-Ca.
Total iron has been lost, suggesting that Fe203 hasbeen converted to FeO and removed by leaching. Such
reduction may have been owing to the exclusion of theiron from the atmosphere by overlying material or to an
atmosphere deficient in oxygen.
Roscoe and Steacy (1958, p. 4-5) studied the distribu­
tion of uranium and thorium in two series of saprolite(regolith) samples from the Quirke Lake area ofTownship 144. They stated, "Both show uranium to beabout one-third less in the most altered samples than in
the freshest granite. One shows a proportional loss ofthorium which is only slightly less than the loss ofuranium. The other shows a net gain of thorium."As regolithic material contributed to the formation of
the uraniferous, pyritiferous, oligomictic conglomeratesof the basal Lower Huronian, the presence in the

GAIN

LOSS

Rb_
SrO
FIGURE 3. - Formation of regolith.

regolith of FeO (ferrous iron) and the persistence ofuranium normally soluble in an oxidizing environment
are of interest, whatever the cause of reduction.
The source area for the uranium-bearing rocks was
subjected to prolonged weathering with development ofresidual regolith under reducing conditions. Slight
topographic variation was developed.
HURONIAN SUPERGROUP

The Huronian Supergroup unconformably overlies theArchean rocks. The classical descriptions were given by
Logan (1863) and Collins (1925). Recent work has
resulted in several revisions of Huronian stratigraphy(Robertson and others, 1969), including the recognitionof the cyclical nature of much of the sequence and thepresence of mafic volcanic rocks near the base of the se­quence at many localities (Roscoe, 1969; Robertson,
1971b).
ELLIOT LAKE GROUP

The lowermost group of the Huronian is the Elliot
Lake Group, comprising the Matinenda and McKim For­mations and local mafic volcanic assemblages.
MATINENDA FORMATIONThe Matinenda Formation consists of greenish arkose,with or without uraniferous quartz-pebble conglomerate

U7

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING

beds, followed by grey quartzite. Crossbedding andpebble-orientation studies by McDowell (1957, 1963) andPienaar (1963) indicate that the currents flowed fromthe northwest but were markedly influenced by base­ment topography, which in turn reflects the Archean
structure. The ore conglomerates occur mainly invalleys in the basement surface (see fig. 7). The
thickness of the Matinenda Formation increases from 0
at the north shore of Quirke Lake through 600 ft (180 m)

near Elliot Lake to 700 ft (210 m) on the north side of the
Murray Fault at the Pronto Mine. Superimposed on theregional pattern are the effects of basement highs andlows. The formation has not been recognized south of
the Murray Fault east of Algoma, but it does lie under
Lake Huron west of Blind River. As northward overlap
is pronounced (fig. 4) the ore-beds at Nordic are older
than those at Quirke. If the northward overlap were notdisrupted by basement highs, the ore beds at Pronto, theApproximate Vertical Scale

La Ctoche
Lake

I
o

2000
I
i

i
400

I

i

800

4000 FEET
I
1200 METERSGORDON
LAKE/

Approximate Horizontal Scale
0

I

2

4

I

6

I

8

I

I

10

10 MILESI

15 KILOMETERSLORRAIN

Flack Lake
JM1
BAR
RIVER

Aird

Tp.163AR1
GOWGANOA

Ten Mile
Lake

ESPANOLA

9 ui,rke
Lake

NT

Tp.lB
AR 2

Z_
5-2

CM4/

"T
~f Spannorth D 0 H// X

SERPENT^/ A{

Denvic//.
Lake//;PA24

MISSISSAGIRAMSAY LAKE

_MATIN|NDA
ARCHEANORIGINAL" HURONIAN BELT
Basement
Advancing Shore Line

Basement
Advancing Shore Line

Basement
Law
Quirkeore-zoneBasementHigh

Basement
Low
Pardeeore-zoneBasement HighSOUTHERN
HURONIAN

BELT

FuiL'RK 4.—Lateral variation of the Huronian Supergroup, Elliot Lake area.

U8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

most southerly known occurrence, would be the oldest,but in the absence of a regional marker horizon this can­not be established. The quartz clasts at Pronto are cob­bles, partly sorted and moderately rounded; those at
Nordic are large pebbles, well sorted and rounded; thoseat Quirke are small- to medium-sized pebbles, wellsorted and rounded. Similar relations are seen going up
sequence at both Nordic and Quirke, and packing
becomes less pronounced.
McKIM FORMATION

Innes (1974, 1975) has studied the volcanic rocks at
Sudbury. These data and limited work on the rocks at
Massey (Robertson, 1976) and Dollyberry Lake (J. A.
Robertson, unpublished) show the rocks are subalkaline
tholeiites and host iron and copper sulphide. Between
Massey and Sudbury, layered gabbro-anorthosite in­trusive bodies are found cutting Archean rocks; theseare probably part of the same igneous cycle as thevolcanic rocks and have similar chemical affinities. No
age dates have been attempted.
The known uranium ore bodies are marginal to thevolcanic piles, which may have physically controlled thecurrent depositing the uraniferous conglomerates in thesame way as the topographic highs reflecting basement
geology.
Emanations from the volcanism may also have con­tributed sulphur to convert detrital magnetite to pyritein the sediments.

Argillaceous strata constituting the McKim Forma­tion thicken southeasterly and form the most important
formation of the Elliot Lake Group on the south limb of
the Chiblow Anticline north of the Murray Fault be­
tween Shedden Township and the east limit of the area
shown on figure 2 (Robertson, 1965a, 1966a, b, 1975a).
To the east of Algoma, this unit, but without any
underlying arkose or Archean basement rocks, is
HOUGH LAKE GROUP
repeated south of the Murray Fault. Near the fault, it isrepresented by schists of the almandine-staurolite
The Hough Lake Group comprises the Ramsay Lake
grade, but farther south it is represented only by poorly Formation, polymictic paraconglomerate; the Pecors
exposed less-metamorphosed rocks. The unit south of Formation, argillite; and the Mississagi Formation,
the fault was formerly believed to be Archean in age quartzite.
(Collins, 1925, 1936).
VOLCANIC ROCKSRAMSEY LAKE FORMATIONIn the Blind River area (fig. 2) mafic volcanic rocks arefound at Thessalon, Dollyberry Lake, near Nordic Mine,south of Pronto Mine, and in the vicinity of Massey.
Thick volcanic assemblages have also been recognizednortheast of Sault Ste. Marie (Duncan and Aberdeen
Townships) and between Agnew Lake and Sudbury.
Characteristically the rocks are massive and dark green
to black and only locally show feldspar phenocrysts andamygdules. Ropey texture and flow-top breccias arerecognised, and pillows are very rare or absent, sug­gesting a subaerial environment. Pillows have been
observed in the upper part of the Duncan sequence (Bennett, 1975) and in the Sudbury area (Innes, 1975), sug­gesting transition to submarine deposition. Locally,arkose and uranium-bearing pyritic quartz-pebble con­glomerate are found either interbedded in the fringe
areas of the piles or between the volcanic rocks and theArchean rocks (Robertson, 1968b; Innes, 1975; Bennett,
1975). Locally at Thessalon and Massey, but better
developed at Sudbury, intermediate or felsic flows arefound. So far, attempts to date these suggest a date ofabout 2,400 m.y., but a reliable isochron needs to beestablished (Fairbairn and others, 1969). J. A. Robertson (1969a) has pointed out the close spatial relations to
two major fault zones, the Murray and the Flack, which
are also zones of marked change in sedimentation.

The Ramsay Lake conglomerate consists of pebbles
and cobbles of grey granite, quartz, and mafic igneousrocks scattered in a dark grey greywacke-to-quartzite
matrix characterised by large grains of smoky quartz
and pyrite. The lithology indicates mass transportation,
and an association with graded greywackes withdropped clasts suggests ice rather than mudflow as thetransportation medium. A crude bedding, particularly inthe south, the presence of mud cracks and ripple markson the upper surface, and crossbedding in quartzite or
greywacke lenses indicate deposition in shallow water.
On the north shore of Quirke Lake the Ramsay Lake
Formation rests directly on basement (Robertson, 1961,
1968a), but in the Quirke, Denison, and Panel Mines it
truncates the uraniferous horizons in the Matinenda
Formation and is itself slightly radioactive. Elsewhere,
the Ramsey Lake Formation truncates arkose; the basal
part of the formation is both lighter in colour and morepotassic than the upper part, reflecting incorporation ofmaterial from the Matinenda Formation.
The formation thickens from 0-15 ft (0-4.5 m) at
northernmost outcrops to more than a thousand feet(300 meters) in the Espanola-Sudbury area (Card andothers, 1972). The Ramsay Lake Formation indicatesthat the general climate was cold and that the uranium
mineralization was syngenetic.

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTINGPECORS FORMATIONThe Ramsay Lake Formation is followed conformably
by the Pecors Formation, which comprises a sequence ofargillites in the Quirke Lake-Elliot Lake area, ofargillites, siltstones, and quartzites on the south limb of
the Chiblow Anticline, and of siltstones and argillaceousquartzite where found to the south of the Murray Fault.
The transition zone from the Ramsay Lake con­glomerate comprises argillite, which may include varvite with dropped clasts (Robertson, 1968a), but locally aquartzite bed may be present. The argillites are fre­quently ripple marked. On the south limb of the Chiblow
Anticline and south of the Murray Fault, ripple marks,
microcrossbedding, and slumpage structures arecharacteristic.
The thickness ranges from 100 ft (30 m) at Quirke
Lake (Robertson, 1961, 1968a) to about 1,000 ft (300 m)
south of the Murray Fault (Robertson, 1976), but the for­mation may be missing over well-developed basement
highs, and its thickness is markedly reduced over the
crest of the Chiblow Anticline. This reduction is in
sedimentation thickness and not due to folding. On theflank of a basement high running from Chiblow Lake to
the Cutler-Massey area some of the argillaceous beds of
the Pecors Formation are moderately radioactive(Robertson, 1975a, 1976, p. 46).

U9

siltstone, the Espanola Formation; and quartzite, theSerpent Formation.
BRUCE FORMATIONThe Mississagi Formation is overlain disconformably
by the Bruce Formation - a conglomerate that consistsof boulders of white granite and greenstone in a partly
sorted, slightly pyritic, siliceous greywacke matrix. The
Bruce Formation is probably a tillite that has been sub­jected to some sorting. Locally, dropped clasts havebeen seen in varvite lenses. The conglomerate can betraced throughout the entire district. There are marked
local variations in thickness, but to the north of the Mur­ray Fault the unit is generally less than 200 ft (60 m)
thick (fig. 4). South of the Murray Fault, the con­glomerate is represented by a few feet of conglomeratic
grit. It is missing in places, and elsewhere there arechannels of boulder conglomerate up to several hundred
feet in thickness.
The close similarity to the Ramsay Lake Formation
should be noted (table 1). The matrix is more siliceousand is characterised by more pyrite and the lime andsoda contents are greater than in the Ramsay Lake For­mation. Where metamorphosed, the pyrite is replacedby pyrrhotite, and the formation has a magnetic expres­sion. The basal part of the formation contains reworked
material from the underlying Mississagi Formation.

MISSISSAGI FORMATIONThe Mississagi Formation consists of greenish arkose
on the north limb of the Quirke Syncline. Elsewhere it isnormally made up of well-bedded grey quartzite, but ad­
jacent to basement highs the yellow-green colour and in­creased feldspar content are again characteristic.
Thicknesses range from 600 ft (180 m) at Quirke Lake to
1,500 ft (460 m) near Elliot Lake (Robertson, 1961,
1968a) and to 2,700 ft (820 m) on the south limb of the
Chiblow Anticline, as exposed both north and south of
the Murray Fault at Blind River (Robertson, 1964). Cur­rent direction was from the northwest, and the influenceof basement topography was much diminished.
Along the north limit of the Quirke Syncline and again
adjacent to the Chiblow-Cutler basement high theyellow-green arkosic phase is characterised by radioac­
tivity. Uranium and thorium, as well as potassium, isrevealed by y-ray spectrometry (Darnley and Grasty,1971). In the northern section thin pyritic quartz-pebble
bands, carrying sulphide and trace uranium and thorium
mineralization, are found (Robertson, 1963).
OJJIRKE LAKE GROUP

The Quirke Lake Group comprises polymictic
paraconglomerate, Bruce Formation; carbonate and

ESPANOLA FORMATIONIn the Blind River area the Espanola Formation con­sists of three units: a lower unit, characterized bylimestone, the Bruce limestone; a middle unit,characterized by mudstone and greywacke, theEspanola greywacke; and an upper unit having amarked development of ferruginous dolomite, theEspanola limestone. This three-fold division is not possi­
ble in the southeastern part of the area mapped, and theformation was not subdivided either north or south of
the Murray Fault (fig. 4; Robertson, 1965a, b, c,, 1966a;
Robertson and McCrindle, 1967). Near Espanola an up­
per crossbedded quartzite member is present (Card,
1976).
The limestone member consists of thinly interbedded
cream-coloured limestone and siltstone. Differential
weathering and drag folding give the rock a spectacularappearance. Where the unit is complete, the thickness isgenerally 100 ft (30 m).
The siltstone and dolomite members can be
distinguished from each other only by the brownweathering ferruginous dolomite bands in the latter.
Both members are characterized by intraformational
breccias, siltstone and conglomerate dikes, mudcracks,
and ripple marks. These features indicate shallow-water

U10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

deposition and tectonic disturbance. Occasional quartzite beds, which are more common to the northwest,
show crossbedding that is indicative of a northwesterly
derivation. Where complete, in the Quirke Syncline, thethickness of the siltstone member is 300-400 ft (90-120m) and that of the dolomite member is 150 ft (45 m).
This unit is of considerable interest, as it represents
an early carbonate. The presence of carbonate and ofiron suggests that the environment contained some freeoxygen.
SERPENT FORMATIONThe Espanola Formation is overlain by the Serpent
Formation, a white feldspathic quartzite, which is best
exposed in the northern and eastern sections of the
Quirke Syncline. The maximum known thickness of theSerpent Formation is 1,100 ft (330 m). Crossbedding,and lithology and thickness changes in individual
members, again indicate a derivation from the north­west. Ripple marks and mudcracks indicate shallowwater conditions. The unit is also exposed in Shedden
Township north of Walford Lake and on islands in thenorth channel of Lake Huron at the south limit of thearea mapped (fig. 2; Robertson, 1965a, b, c, 1966a,
1975b, d; Robertson and McCrindle, 1967).
The lateral variation in thickness of the lowermost for­mations of the Huronian is illustrated in figure 4.
COBALT GROUP

The Quirke Lake Group is overlain unconformably by
the Cobalt Group, which, in the Blind River-Elliot Lakearea, consists of the Gowganda Formation and the Lorrain Formation, the Gordon Lake Formation, and the
Bar River Formation.

boulder conglomerate in which red granitic clasts
predominate,and the upper is characterized by quartzite
and argillite. Within the Quirke Syncline, as mapped by
the writer, the Gowganda Formation is about 1,700 ft(510 m) thick (Robertson, 1963). To the west, in areas
mapped by Frarey (1959,1962), it is at least 3,000 ft (900m) thick. To the north near Mount Lake, the formation
lies on basement rocks and is less than 500 ft (150 m)
thick (Wood, 1968a, b, 1975). To the south in the La
Cloche Syncline, the Gowganda Formation is some
4,200 ft (1,260 m) thick (Robertson and McCrindle, 1967)
at La Cloche Lake and in the Willisville-Espanola area
(Casshyap, 1966,1968). The conglomerate matrix is finegrained, chloritic, and characterised by high soda con­tent relative to the lower Huronian polymictic con­glomerate (fig. 5).
The origin of the Gowganda Formation has been muchdiscussed. Dense boulder conglomerates, quartzites, andargillites were definitely water laid; varved con­glomerates and greywackes probably formed under con­ditions characterized by alternate freezing and thawing,
although some authorities would ascribe these rocks toturbidity currents; and sparse boulder conglomerates
with a disrupted greywacke matrix may be either tillites
or mudflow deposits. Ovenshine (1965a, b) has described
the structures of the Gowganda Formation of the Elliot
Lake-Blind River area and has endorsed a glacial en­
vironment, particularly on the basis of numerous
dropped clasts in varved greywackes. In the southeast
part of the area shown in figure 2 and in the areas to theeast mapped by Card (1967a-d) and Casshyap (1966,1968), a marine rather than a continental glacial en­
vironment is indicated (Lindsey, 1966). Some clasts are
CaO

GOWGANDA FORMATION

Within the map area (fig. 2), the Gowganda Formation
rests on all formations between the upper Mississagi and
the Serpent; in the Mount Lake and Kirkpatrick Lakearea, north of the Flack Lake Faults (Roberston, 1971b;
Siemiatkowska and Guthrie, 1975), it rests on theArchean basement. Locally, the contact can be seen
truncating the bedding of the underlying formations,and consolidated or partly consolidated fragments of theunderlying rocks are found in the lowermost beds of the
Gowganda Formation; elsewhere, the contact is a "softsediment" contact, and in yet other localities it is a knifesharp contact.
The Gowganda Formation is a heterogeneous
assemblage of conglomerate, greywacke, quartzite, andargillite. These rock types are found throughout the se­quence, although the lower part is characterized by

Na Q 1020

GOWGANDA •

30

40

50

60

BRUCE •

70

80

90

K Q

RAMSAY LAKE

FIGURE 5. - Alkali content, Huronian polymictic conglomerates.

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING

Ull

striated, but J. A. Robertson (1971a) has questioned the been strongly folded, foliated, and somewhat metamor­
use of this feature as a criterion for glacial action (Lind- phosed; the clay minerals are represented by kyanite
say and others, 1970) and suggests that the striae are and andalusite (Church, 1967; Chandler, 1969; Robertdue to postdepositional tectonism, as described by son, 1970b, 1976).
Bielenstein and Eisbacher (1969). Of particular interest
GORDON LAKE FORMATION
is the presence of iron oxide in the Gowganda Forma­
The Lorrain Formation is overlain, apparently con­tion. The arkosic quartzites are red to pink in colour,
reflecting the preservation of hematitic dust in the formably, by the Gordon Lake Formation, a 1,000-ft
feldspar and interstitial material. Some thicker beds of (300-m) sequence of well-bedded siltstone, argillite
argillite or siltstone contain magnetite and zones of such chertstone, and fine- to medium-grained sandstone.
material can be traced on regional magnetic maps. No There are three members: (1) a lower member compris­significant uranium anomalies have been identified, but ing reddish sandstone and siltstone with anhydrite andscattered thorium and potassium values (Richardson gypsum nodules (and salt casts?), (2) a middle member ofand others, 1975) reflect the granitic content of the con­ dark green siltstone, argillite, and minor sandstone, andglomeratic lower portion of the formation. The lack of a (3) an upper member of reddish siltstone, argillite, andpreserved regolith at the Archean surface (Robertson chert (Eisbacher and Bielenstein, 1969; Bottrill, 1970;
1970c; Wood, 1970a) may indicate oxidizing rather than Robertson, 1969b, 1970c). The middle member tends to
the reducing conditions that prevailed during the deposi­ form a scarp in the Flack Lake area that corresponds totion of older lower Huronian sediments elsewhere in the a moderate magnetic anomaly (Robertson, 1971b). Cur­
district (Robertson, 1969a; Roscoe, 1969; Wood, 1970a). rent ripples, microcrossbedding, slumpage structures,and desiccation cracks indicate deposition in very
shallow water (Young, 1969; Robertson, 1969b, 1970c;
LORRAIN FORMATION
Robertson and Johnson, 1969,1970; Wood, 1970a, b). In
The Lorrain Formation conformably overlies the the core of the La Cloche Syncline a few hundred feet of
Gowganda Formation. In the Flack Lake area several siltstone and quartzite that have been correlated withunits have been mapped within the Lorrain Formation the Gordon Lake Formation also coincide with ain the following ascending stratigraphic order: (1) pink magnetic anomaly (Robertson, 1976).
ferruginous quartzite and minor siltstone, (2) coarse­
grained green arkose, (3) pink coarse hematitic arkose
BAR RIVER FORMATION
with radioactive (thorium:uranium >10:1) quartz-pebble
The Gordon Lake Formation is overlain by the Bar
conglomerates (Robertson 1970c; Robertson andJohnson, 1970); (4) interbedded pink and buff quartzite River Formation, which comprises at least 1,200-1,500
and, in a few places, greenish quartzite (quartz-chert- ft (360-450 m) of massive to well-bedded orthoquartzite,
jasper pebble bands are characteristic of the upper part generally crossbedded and ripple marked (Robertson,of this member), (5) massive white quartzite, with 1969b; Robertson and Johnson, 1969; Woodward, 1970;
quartz-chert-jasper pebble bands in the lower part. Eisbacher and Bielenstein, 1969). Mudcracks and desic­
Sericite, kaolinite, pyrophyllite, and diaspore are found cation features are also found, particularly in finerin the nonfeldspathic beds of members (4) and (5) grained sandstones and intercalated siltstone bands.
(Chandler and others, 1969) and indicate the onset of One unit is characterized by thin bedding and fer­
warm rather than frigid conditions (Wood, 1970a; ruginous siltstone and quartzite. The iron has been part­
Young, 1970). Crossbedding is common in the Lorrain ly redistributed, and small-scale solution depositional
Formation in the Flack Lake-Mount Lake-Rawhide fronts adjacent to joints and fractures are common.
Lake area but is variable; southwesterly current direc­ Some ripple-marked surfaces at this horizon are coveredtions have been interpreted by Hadley (1969) and by the with segmented, tapering, sinuous structures of possible
writer, although the source area was probably to the organic origin (Hoffman, 1967; Young, 1967;
north. North of Bruce Mines, Hadley (1969) deduced Donaldson, 1967). Young (1967) and Hoffman (1967)
southeasterly current directions. The five members can favoured metazoans (worms) as the organism involved,
be traced throughout the Flack Lake-Mount Lake- but Donaldson (1967) suggested that the structure
Rawhide Lake area (Robertson, 1971b; Wood, 1975; represents the infilling of desiccation cracks in algal
Siemiatkowska and Guthrie, 1975). Their equivalents mats. In the same area there are many rather similar
can also be recognized and traced in the La Cloche Lake- structures that are undoubtedly desiccation features.
Whitefish Falls area (Card, 1971a; Chandler, 1969), but The desiccation features of the Gordon Lake Formation
the individual units are both finer grained and thicker, (Young, 1969; Robertson, 1969b) are so similar to theindicating deposition farther from source. They have most organic-looking structures of the Bar River For-U12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

mation that the organic nature of the latter remains indoubt. Wood (1970a, b) has recorded oolites in the fer­
ruginous rocks, and their presence along with the many
primary structures indicated deposition in very shallow
water. In the ferruginous rocks Bottrill (1970, personalcommun.) has observed pyrite and in polished section
has identified the major iron mineral as maghemiterather than hematite. The conditions of deposition,
sedimentation, climate, and state of atmospheric oxida­
tion represented by the upper Huronian rocks of the
Flack Lake are discussed in more detail by Wood (1971).

8

SiO2/AI 2O3''..
Fe 2O3/FeOf

Me Kirn

Gowganda
PecorsSUMMARY OF HURONIAN

The Huronian rocks of the area thus comprise thick se­quences of clastic sediments and minor tholeiitic basalts.
The sediments were derived from the adjacent Archean
Craton and were deposited as migrating diachronous
facies with cyclical rejuvenation. The depositional basin
extended east-west and deepened towards thesoutheast. The distribution of volcanic rocks and marked
changes in thickness and facies of sedimentary units
were controlled by hinge zones, which later became
regional fault zones. Minor changes in composition ofsimilar rock types are due to shifts in provenance and to
the efficacy of weathering and winnowing processes.
Radioactivity and heavy minerals are characteristic ofquartz-pebble conglomerates in the fluviatile near-shore
facies. In the lower Huronian the association is ofuranium and sulphides, predominantly pyrite, and in theupper Huronian it is of thorium and iron oxides. In the
middle Huronian conditions became favourable forpreservation of iron oxides. Glacial deposits arecharacteristic of the lower and middle Huronian, whilst
the uppermost Huronian was apparently deposited in a
dry and hot environment. Figure 5 illustrates the alkali
content of the matrices of the Huronian polymictic
paraconglomerates; in figure 6 three compositionalratios for Huronian argillaceous rocks are shown. Of
particular interest in figure 5 is the variation in maturityof the paraconglomerate matrices, which decreases
towards the Gowganda Formation, the marked increasein soda, and, in the lower part of the paraconglomeratesequences, the incorporation of potassic material fromthe underlying arenite. Figure 6 shows an increase inferric/ferrous iron ratio in the argillaceous rocks, re­flecting change in general oxidation conditions; an in­crease in the Si02/Al203 ratio, reflecting increased ef­
ficacy of chemical weathering in the upper Huronian;and variations in the Na20/K20 ratio, reflecting changes
in provenance from a granitic to a granodioriticgreenstone terrane. The arenaceous rocks show similarchanges in provenance and efficacy of chemical
weathering. The role of oxidation is apparent in the fre­quency of red beds from the Gowganda Formation up-

Na 2 O/K2O

Lorrain

GordonLake

FICUKE 6.—Silica/alumina, ferric iron/ferrous iron, and soda/potashratios for Huronian argillaceous rocks.

wards contrasted to the total lack of red beds in thelower formations.
POST-HURONIAN EVENTS

Following the close of sedimentation, the area was
subjected to a long history of structural deformation, ig­
neous intrusion, and, in the Penokean fold belt, regional
metamorphism, which locally attained amphibolitefacies (Card, 1964; Card and others, 1972; Robertson,1972). These events do not concern the formation of theuranium deposits. The oldest intrusion, the Nipissing
diabase, took place at 2,155+ m.y. ago (Van Schumus,
1965), and this serves to place a minimum age on the
Huronian sedimentary rocks. Alteration (albitization,
chloritization, and carbonatization) associated withdikes or sills has locally affected the uranium deposits,
which were clearly in place prior to the intrusion(Robertson, 1968a, 1970a),
In general, deformation and metamorphism of thatpart of the Blind River area in which uranium ores arefound is minimal. The lack of intense metamorphism
may be regarded as a factor in the preservation of theore bodies. Age data on minerals, however, reflect someresetting of the decay clocks at times corresponding toknown peaks of regional metamorphism or thermal
events rather than episodic introduction of moreuranium (Roscoe, 1969).
THE URANIUM DEPOSITSUranium and thorium-uranium deposits are found in
conglomerates at a number of localities in the BlindRiver-Elliot Lake and Agnew Lake areas, as well as
throughout the Huronian belt (Robertson, 1968a, 1975a;
Thomson, 1960).

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING
DISTRIBUTION OF URANIUM ORE DEPOSITS—BLIND
RIVER-ELLIOT LAKE

The workable uranium deposits of the Blind River
camp are found as quartz-pebble pyritic conglomerate
beds in zones controlled by basement topography (fig.
7). In the Quirke Syncline the relation of the uraniferous
conglomerates to granite-greenstone contact areas andvalleys over softer zones in the greenstone belt is clearly
demonstrated (fig. 7). At Pronto, however, there is noclear relation to basement geology, which raises thepossibility that there may be other economic uranium
deposits underlain by granite. It is clear, however, that
Pronto is located on the flank of a regional basement
high.
The ore zones strike northwest-southeast and are con­trolled by basement structures. The Quirke zone (thelargest in the area) is 32,000 ft (9,600 m) long and from
6,000 ft (1,800 m) to 9,000 ft (2,700 m) wide, and theNordic zone is 19,000 ft (5,700 m) long and from 4,400 ft(1,320 m) to 6,000 ft (1,800 m) wide (Robertson, 1967,
1968b). The Pronto deposit (Robertson, 1970a) and theunworked zones are of smaller dimensions.
At Quirke No. 1 the main ore zone is approximately 30
m (100 ft) above basement and is approximately 3.5 m(12 ft) thick. Toward the east the ore zone is truncatedby an unconformity at the base of the Ramsay Lake con­glomerate.
At Quirke No. 2 and Denison, the best ore develop­
ment is in the Denison Reef some 30 m (100 ft) below the
Quirke Reef. The Denison Reef normally comprises two
conglomerate zones each 1.8-3.6 m (6-12 ft) thickseparated by barren arkose 0.6-2.4 m (2-8 ft) thick.
Throughout much of the Denison Mine, ore grade wassufficient to permit mining of both conglomerate beds
and the intervening quartzite as one unit using largescale equipment and trackless haulage. Quirke No. 2 hasbeen developed using conventional haulage. Other con­glomerate beds 0.6-3 m (2-10 ft) thick separated byquartzite beds 3.6-6 m (12-20 ft) thick are known on
both the Quirke and Denison properties, but these havenot been mined. At Stanrock Mine another reef was
found under the Denison Reef in the southeastern partof the mine. The en echelon pattern with the oldest reefto the southeast conforms to the regional overlap pat­tern. The nomenclatures used in the various mines of
the Quirke zone and their stratigraphic relations are il­lustrated schematically in figure 8.
At the Nordic Mine the main ore bed comprised con­glomerate or conglomerate with quartzite over a widthof 3 m (10 ft) with a grade of 2.5 Ibs U308 per short ton(1.25 kg per metric ton). Locally another reef lower inthe sequence, the Lacnor Reef, was mined where grade
attained 2.0 Ibs U308 per short ton (1 kg per metric ton).
In the eastern part of the mine a third reef, the Pardee

U13

Reef, attains ore grade, 2.3 Ibs U308 per short ton (1.15kg per metric ton) over 5 ft (1.5 m). This reef extends
eastward over a basement ridge into the Pardee andPecors mineralized zones (fig. 7). These reefs extend
down rake to the Stanleigh Mine. Operations werelargely carried out over the Lacnor and Nordic Reefs.
As with the Quirke zone, other reefs of conglomerate ofsubmarginal grade or minor extent are known.
Five types of boundary to the ore beds are known:
1. Outcrop of the conglomerate bed.
2. Wedging due to contact with either regional or local
basement "highs". To the west of Quirke No. 1 the"basement" may be the edge of a pile of Huronian
volcanics rather than Archean.
3. Lateral thinning of conglomerate and thickening of
the intervening quartzite beds accompanied bydrop in grade of the radioactive units. This is prob­
ably the typical boundary and the definition is ar­bitrary.
4. Removal by erosion subsequent to deposition, as forexample in Quirke, Denison, and Panel, where
there is an unconformity at the base of the Ramsay
Lake conglomerate. Where material from con­glomerates of the Matinenda Formation has been
incorporated in the Ramsay Lake conglomerate,
the latter contains anomalous but minor amounts
of uranium radioactivity.
5. Fault contact. Locally, areas were not mined either
because of unfavourable mining conditions caused
by faults, extensive fracturing, or diabase dikes or
because of unfavourable milling conditions causedby albite, chlorite, or carbonate alteration.
Some of these relationships are illustrated in figure 9,a cross section through the New Quirke (Quirke No. 2)
Mine of Rio Algom Mines, Ltd.
LITHOLOGY AND MINERALOGY OF URANIUM ORE
DEPOSITS

The typical ore-bearing conglomerates of the Matinen­da Formation consist of well-rounded, well-sorted
quartz pebbles in a matrix of quartz, feldspar, andsericite and have an average pyrite content of 15 per­cent. Monazite and zircon are characteristic heavy
minerals. Brannerite and uraninite are found in thematrix. Thucholite is found locally and may line fissuresin the ore beds. The ore minerals are brannerite,uraninite, and monazite (Robertson, 1960 and later;
Roscoe, 1957 and later; Pienaar, 1963; Roscoe andSteacy, 1958).
Thucholite is present in the ore but is also in fractures
as a postore secondary mineral. Almost no work has
been done on thucholite at Blind River. The recent in­terest in hydrocarbon at the Rand has prompted Ruzicka

U14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

4 MILES
6KILOMETERSA

A
METERS

FEET
0-BASE OF RAMSAY LAKE CONGLOMERATE

300 H

SCHEMATIC CROSS - SECTION

EXPLANATIONPIH^Z- Matinenda Formation
Conglomerate
Huronian-Archeancontact at surface

Greenstone
Iron-formation outcrop;
magnetic anomaly
Uranium deposit

GraniteFIGURE 1. Uranium deposits in the Quirke Syncline.

U15

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING

Quirke 1 and 2(Rio Algom Mines)
Ramsay Lake Formation
Quartzite
Upper —————

DenisonDenison Mines Ltd.

Stanrock
Denison Mines Ltd.

Ramsay Lake Formation
Quartzite————— F

Quartzite
A* (Quirke 1)

Quartzite

Quartzite
B ————

Quartzite
D

Quartzite
C-L ———
Quartzite

Quartzite
Floater reef(s)QuartziteE

C* (Quirke 2)
Quartzite

Quartzite

Quartzite

A!_____
Interbedded quartzite

StfQuartzite

B*

FWC (Scraps)
Quartzite
Basement

Quartzite

"st*
• bt2
Quartzite

ScrapsQuartzite

St3
Quartzite

Basement

Basement

FIGURE 8.—Correlation and nomenclature of quartz-pebble conglomerate reefs, as used in the mines of the Quirke zone. Asterisks indicate beds
having supported production.

to initiate comparative studies on Blind River material;an initial progress report is included in Ruzicka's paperin this volume (chap. V).
Gummites (soddyite and uranophane) are rare, but
Rice (1958) stated that they form a significant propor­
tion of the ore mineralization at the Spanish American
Mine. Uranothorite has also been identified (Roscoe andSteacy, 1958, p. 14) and Patchett (1959, 1960) identified
coffinite in altered material from the Nordic Mine.
Pyrite is the commonest sulphide mineral; it usuallyconstitutes 10-15 percent of the matrix, but it may rare­
ly be as much as 30 percent. Pyrite is concentrated in
the matrix, and only rarely is there indication of replace­ment or fracture filling in the quartz pebbles. Individual
grains may be rounded ("buckshot" pyrite) or subhedral
to massive. R. G. Arnold (1954) suggested that the
pyrite formed by sulphidization of detrital magnetite,and he described grains showing cores rich in leucoxenethat he considered to have developed from ilmenite exsolved from the original magnetite. More recently, Bottrill (1971) has suggested that the sulphur required wasderived from the lower Huronian volcanic rocks.
However, no laboratory studies have been undertaken• Diabase
HURONIAN
M Gowganda
OH Serpent

HI Espanola• Bruce[7j Mississagi® Pecors— Ramsey Lake
M Matinenda
ARCHEANH Basement

"U-Cg

0

400 800 FEET

0

100 200 METERS

FIGURE 9.—Cross section, New Quirke mine.

to test this hypothesis. Pienaar (1963), in a traceelement study, could not distinguish between the pyriteof the ore beds and the pyrite found in other rocks of the
district.
Other sulphide minerals found are pyrrhotite (occa­
sional scalenohedral pyrrhotite crystals have been foundU16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

in vuggy cavities), chalcopyrite and minor arsenopyrite,
galena (probably radiogenic), cobaltite, and marcasite,molybdenite, and sphalerite as coatings in late fractures(often with thucholite). At Denison Mine, tourmaline
crystrals were observed in a late quartz veinlet. The
following detrital heavy minerals have also been iden­tified: Anatase, amphibole, barite, apatite, cassiterite,
chromite, diopside, garnet, epidote (allanite?), gold,
fluorite, hematite, ilmenite, magnetite, monazite, rutile,scheelite, sphene, spinel, tourmaline, xenotime,
yttrofluorite, and zircon, partly as malacon and more
rarely as cyrtolite (Abraham, 1953; Arnold, 1954;
Holmes, 1956; Milne, 1959; Patchett, 1960; Pienaar,
1958, 1963; Ramdohr, 1957, 1958a, b; Rice, 1959;
Roscoe, 1957; Roscoe and Steacy, 1958, p. 13-15;
Thorpe, 1963; Canadian Institute of Mining andMetallurgy, 1957).
In addition, Milne (1959), in samples collected for Pat­
chett (1960) at the Nordic Mine, identified chamosite,
greenalite, and grunerite. These iron silicates were
probably derived from the ridge of iron-formation form­ing the eastern boundary of the Nordic channel (fig. 7).
Although a wide variety of detrital minerals have been
identified and described by the authors listed, it should
be stressed that these occur in very small quantities andare not necessarily present in all samples that have beenstudied.
There has been much discussion and description of themain minerals in the ore, mainly from polished-section
work. The similarity to the gold- and uranium-bearing
bankets of the Witwatersrand (Ramdohr, 1958a, b),radioactive conglomerate at Jacobina in Brazil(Bateman, 1958; Gross, 1968), and some deposits inAustralia and Russia has been stressed in the literature(Davidson, 1957; Derry, 1960).
The major ore mineral at the Pronto Mine and in the
Quirke Mine A Reef (Theis, 1973) is brannerite, firstdescribed from the area by Nuffield (1954). Althoughsome relatively fresh material has been observed (Rice,
1958), brannerite is typically found as ovoid red-brown
to black grains in the metamict state showing bladed
rutile surrounded by a uranium oxide and rare-earth ox­
ides, the two-phase uranium-titanium compound of
Pienaar (1958, 1963). Pienaar (1958, 1963), Roscoe(1969), and Robertson and Steenland (1960) suggested
that the rounding is due to transportation and that thematerial is detrital. Ramdohr (1957) has suggested thatthe brannerite rather than decomposing is authigenic
and has proposed the "Pronto Reaction,"
U02 + 2-3Ti02->UTi02 _ 306 _8,
which he held took place during metamorphism. Ex­
perimental efforts to reproduce this reaction indicatethat it takes place at temperatures in excess of those towhich the rocks have been subjected (Gruner, 1959, p.

1315; Patchett, 1959). The brannerite generally contains
small inclusions of pyrrhotite and of radiogenic galena(Thorpe, 1963, p. 39). Table 4 (from Robertson, 1968b)
lists the published partial analyses of brannerite fromthe Blind River area along with the regional value
selected by Thorpe.
Blind River brannerite has been recently studied by
Ferris and Ruud (1971) and by Theis (1973). Theseauthors concluded that Blind River brannerite, and thebrannerite or uraniferous leucoxene of the Rand, form­
ed at low temperatures during diagenesis as a result of
uranium migrating to decomposing ilmenite.
Uraninite is the second most important mineral at
Pronto and apparently the most important in the Nordic
zone and in the C Reef at Quirke Mine (Theis, 1973).
Generally it occurs as black subhedral grains approx­
imately 1 mm across. Ramdohr (1958a, b) and D. S.
Robertson (1962a, b, 1974) have described roundedgrains. Derry (1960) and Thorpe (1963) have both
selected 6 percent as the regional value of Th02 content.
Table 5 (from Robertson, 1968b) summarizes the early
published data on Blind River uraninite, since confirmed
by Grandstaff (1974) and reported in this volume (chap.
J). This corresponds to the composition of pegmatitic
rather than hydrothermal uraninite. The rounding andthorium content may indicate a detrital origin. Rice
(1958), however, believed that uraninite formed as aresult of leaching from brannerite by hydrothermal solu­
tions emanating from diabase; diabase is more common
in the Nordic Mine than elsewhere. Patchett (1960) also
supports a secondary origin for uraninite and statedthat the cores are carbon-rich. Davidson (1957, 1965)
has used the general lack of uraninite in modern placers
and geochemical principles along with the Lyellian con­
cept of uniformitarianism (actualism) as argument
against the detrital origin of uraninite.
Roscoe (1959b) has described the monazites from theBlind River ores and has pointed out that monazite cancontain considerable uranium and is, therefore, one ofthe ore minerals. Grains are normally rounded tosubangular and less than 0.3 mm in diameter. Greyvarieties of monazite are strongly radioactive, may con­tain uranothorite or thorite (Roscoe, 1959b; Patchett,1959), and have pyrite inclusions. Table 6 (from Robertson, 1968b) gives Roscoe's (1959b) uranium-thorium
analyses of monazites and the values selected by Thorpe(1963). Roscoe has shown (1959a, b) that the uraniumthorium ratios in monazite are comparable to that of thebasement. The lateral variation in the ore-mineral anduranium-thorium ratios, as studied by Roscoe (1959a, b),
D. S. Robertson (1962, 1974), J. A. Robertson (1968a),and Thorpe (1963), are best explained by the relative
stability of monazite during transportation; monazite is
the chief radioactive mineral in the occurrences in the
Lorrain Formation.

U17

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTINGTABLE 4.— Uranium, thorium, and titanium contents, in percent, of Blind River brannerite[From Robertson (1968a, p. 92)]
Mine

Others than BlindRiver.
NordicDo
DenisonDo
QuirkeDo
Do
Can-MetRegional

AuthorColourComments

Thorp (1963).

Patcheet (1960) _.
do.

Roscoe (1959a)
do
do
do
do
Pienaar (1963)
Thorpe (1963).
.

Reddish brown

Alteredaltered.

_. Black
do
_

U308

Th02

Ti02

U308/Th02
ratio

40-52

0.3 .15

32-40

40-52

32.6
32.2

2.6
1.8

30.3
30.5

12.5
17.9

41
36
24
20
6
8.70
30

6.1
1.8
1.7
2.2
1.2
1.14
2.3

20
30
37
30
27
n.d.
30

6.7
20.0
14
9.1
5.0
7.6
13

TABLE 5. - Uranium and thorium contents in Blind River uraninite

TABLE 6.—Analyses of Blind River monazites[From Robertson (1968a, p. 93)]

[From Robertson (1968a, p. 93)]

AuthorDenison ____
Do ______
Do ______
Nordic ____
Quirke ____
Pronto ____
Nordic ____
Denison ____
Panel _____

U308

Pienaar (1963) ______ 48.4__do ___________ -39.0__do ___________ 55.5
Patchett (1960)______ 55.25
Wanless and Traill (1956) 51.7__do ___________ 57.0
Roscoe (1959a) ______ 64__do ___________ 60
Thorpe (1963) ______ * 29.6

Regional ____ _do

60

Th02

U3 08/Th02

5.6
7.2
5.4
6.25
4.1
5.12
6.3
5.9
3.84

8.65
5.4
10.3
8.85
12.6
11.1
10.15
10.2
1 7.7

6

10

1 Contaminated?

Locally, individual conglomerate bands may assay ashigh as 20 Ibs or more U308 per short ton (10 kg per
metric ton), but over mining widths of the order of 9-30
ft (3-9 m) the average grade of those beds that are, or
have been, mined is 2-3 Ibs U308 per short ton (1-1.5 kg
per metric ton). The fringe areas to mined beds, thelateral extension to the Quirke zone, and the Pardee
zone, however, contain considerable amounts ofmaterial with average grades of 0.5-1.5 Ibs (0.25-0.75kg).
The arkose interbedded with the ore conglomerate is
generally greenish in colour and is crossbedded (Roscoe
1966; Robertson, 1968a, b, 1975a). Normally thiscrossbedding is of the festoon type, indicative of afluviatile environment.
AGNEW LAKE AREA

The deposits at Agnew Lake (Little and others, 1972),
currently under development, comprise gritose
oligomictic conglomerate interbedded with arkose in aquartzite sequence probably equivalent to the Matinenda Formation. The pebbles are smaller and more sparsethan in the Blind River ore, and pyrite is less con­spicuous than at Blind River-Elliot Lake. Thorium-to-

ColourTh02
U3 08
U 308/ThO,

1
Orange
5.9
.31
.0525

Rescue (1969h)2
3
Yellow
Grey'
6.7
3.9
.95
1.65
.246
.244

4
Grey
5.9
2.9
.492

Thorpe (1963)
Range
Average
5.6~-~6~2

5T6~

1

1.4
.25

-2
.18- .32

uranium ratios are higher than at Blind River-Elliot
Lake, reflecting the presence of uranothorite andmonazite as the dominant radioactive minerals. PostHuronian deformation is considerably more intense,
resulting in steep dips and in development of cleavage
and stretched pebbles. Little and others (1972)
estimated the reserves at 8,000 short tons (7,250 metrictons) U308 in material grading 1.88 Ibs U308 per shortton (0.940 kg per metric ton).
COBALT EMBAYMENT

That part of southern Ontario lying north of Sudbury
is known as the Cobalt Embayment, it should be noted,
however, that neither thrust faulting nor plate tectonics
have been recognised as significant factors in theregional geology. Lower Huronian rocks are intermit­
tently exposed in the southern portion of the Cobalt
Plate but are largely concealed by Nipissing diabase andupper Huronian rocks. The regional geology has been
discussed by Thomson (1960) and by Meyn (1972).
Precise stratigraphic correlation with Blind River isunclear, but rocks equivalent to the Mississagi Forma­tion are present. At the base of these, wherever expos­
ed, there are anomalous concentrations of uranium and,at some localities, gold in conglomerates and uranium inargillaceous quartzites.
Early descriptions of the conglomerates (Thomson,
1960; Meyn, 1972; Robertson, 1968a) compared theserocks with the Blind River oligomictic conglomerates.
On closer examination, however, this comparison is in­valid. Many of the conglomerates are polymictic, con-

U18

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

taining fragments of granite, greenstone, and iron- ton (0.70 kg per metric ton) and a cutoff grade of 1 Ib
formation sparsely distributed in an argillaceous quart- U308 per short ton (0.5 kg per metric ton) over mining
zite matrix. Many of the so-called quartz pebbles are ac­ widths. The higher grade uranium reserves also containtually quartzite of unknown provenance or iron- at least 100,000 short tons (90,000 metric tons) offormation similar to that of nearby Archean greenstone recoverable Th02 . The higher grade material constitutes
belts. Outcrops of these rocks in Vogt Township and some 75 percent of the free world's reserves recoverable
Turner Township reportedly contain trace amounts of at less than $10.00 U.S. (1973) per Ib U308 (Little, 1974;
gold (Robertson, 1968b).
Williams and Little, 1973; Organization for Economic
These deposits are clearly derived from Archean ter- Co-operation and Development, 1973). Estimated addi­rane but are of a provenance different from those at tional ore (possible ore and prognosticated ore) in theBlind River and Elliot Lake. Although these rocks are localities of the Blind River-Elliot Lake-Agnew Lakeperhaps slightly younger, they must be considered part areas may be as much again, but extensive and costly ex­of the same metallogenic province. Additional prospec­ ploration must be undertaken before existence of thisting and exploration in the southern part of the Cobalt material can be confirmed and it can be considered part
Embayment should yield more data of stratigraphic and, of the reserves.
hopefully, of economic value.
PRODUCTION

Between 1955 and 1973 the Blind River-Elliot Lake
camp has produced, from 12 mines, approximately 1.5
billion dollars worth of uranium, and minor amounts ofthorium and yttrium, from material grading 2 Ibs U308
per short ton (1 kg per metric ton). The maximum pro­duction was in 1959, when 12,150 tons (11,020 metrictons) U308 were produced. In 1973, two mines wereoperating: the Denison Mine produced 1,712 short tons
(1,552 metric tons) U308 from ore grading 2.57 Ibs U308
per short ton (1.28 kg per metric ton), and the New
Quirke Mine produced 2,409 short tons (2,185 metrictons) U308from ore grading 3.4 Ibs U308 per short ton(1.7 kg per metric ton) (1973 Annual Reports for
Denison Mines Limited and Rio Algom Mines Limited).
The Agnew Lake Mine north of Espanola has adeveloped ore body, but the decision to start production
would require a suitable sales contract.
Ontario's current production is more than 4,000 shorttons (3,600 metric tons) U308 per year, but the known
deposits can support a production of between 11,000 and
14,000 short tons (10,000 and 12,700 metric tons) U308
per year, which can be obtained by expanding operating
plants, reopening closed plants, and some constructionof new plants.
RESERVES

It was estimated (Robertson, 1975a) from data
available to the public in January 1973, that Ontario
deposits, predominantly the sedimentary deposits ofBlind River-Elliot Lake-Agnew Lake areas, contain ap­
proximately 200,000 short tons (180,000 metric tons) ofrecoverable U308 from material with a millhead grade of
1.8 Ibs U308 per short ton (0.90 kg per metric ton) and an
additional 150,000 short tons (136,000 metric tons) frommaterial with a millhead grade of 1.4 Ibs U308 per short

ORIGIN OF THE URANIUM DEPOSITS OF BLIND
RIVER TYPE

The following discussion is largely taken from earlier
summaries by the author (Robertson, 1968a, 1969a).
Uranium and thorium mineralization occurs at anumber of localities throughout the world in quartzpebble conglomerates bearing appreciable pyrite and,especially at the Witwatersrand in South Africa, gold in
the matrix. The origin of these conglomerates has beenmuch debated. The similarity of the deposits at BlindRiver to one or another of several of the well-known
deposits at Witwatersrand, South Africa, and Jacobina,
Brazil, have been pointed out by Bateman (1958), Davidson (1957, 1965), Derry (1960, 1961), Joubin (1960), andGross (1968). Davidson also mentions similar deposits inAustralia and Russia. This characteristic assemblage
and its distribution is a major theme of this volume. The
relation to major unconformities, particularly thosemarking the Proterozoic-Archean boundary, has beenemphasized (Derry, 1961; Davidson, 1965; Robertson,
1960 and later).
In the Blind River camp this unconformity can be plac­
ed at 2,500 m.y. ago; it is clear that the conditions did
not persist beyond a minimum of 2,155 m.y. ago andthat the period 2,500-2,000 m.y. ago seems the mostfavourable.
Bateman (1955, p. 371), Joubin and James (1957),
Davidson (1957, p. 668), and Heinrich (1958) have citedas evidence of a hydrothermal origin the supposedly
high uranium-to-thorium ratios, the high titanium-toiron ratio, and the association of titanium, cobalt, nickel,
thorium, and uranium in a deposit carrying thecharacteristic minerals gold, brannerite, uraninite, andpyrite. Patchett (1960), after a detailed study of only
three samples from the Nordic Mine, regarded the ores
as epigenetic. Joubin (1954) suggested the "Keweenawan" diabase as a source but in a later paper(1960) admitted that mining evidence clearly indicatedTHE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING

that the diabase postdated the uranium mineralization.
Davidson (1957) suggested that the (supposedly) postHuronian granite lying to the southeast was the pro­bable source; however, much of the granite formerly
considered to be of possible post-Huronian age andshown as such on the Lake Huron Sheet (Canada
Geological Survey, 1933) has since been proved to beolder than the Huronian (Robertson, 1972; Card andothers, 1972). Only the Cutler granite, exposed south of
the Murray fault 20 miles (32 km) south of Elliot Lake, is
now considered to be of post-Huronian age (Robertson,
1964; Van Schmus, 1964b, 1965).
In 1965, Davidson (1965) returned to the question of
the origin of banket ore bodies and considerably
modified his earlier views. The revised hypothesis may
be summarized as follows:
1. Deposition of molasse-type sediments in deep basins
with conglomerate near the basal unconformity
2. Leaching of the metal content (for Blind Riveruranium and thorium) by groundwater
3. Prolonged series of intrastratal migration, allowing
mineralized groundwater to sink to the lowest
permeable horizons, the oligomictic conglomeratebeds, where the metals would be reprecipitated
4. The thermal energy for cycling groundwater would
be derived from postsedimentation intrusions. For
the Blind River area, the final cycle of groundwater
took place during the Hudsonian (Penokean)
orogeny, as evidenced by the 1,700-m.y. age for
uraninite and brannerite obtained by Mair andothers (1960). However, Roscoe (1969) showed thatthe isotopic compositions are best explained byresetting rather than introduction of new material.
Abraham (1953) and McDowell (1957, 1963) regarded
the ores as fossil placer deposits. Pienaar (1958, 1963)
and Roscoe (1957 and later) have also indicated apreference for a placer origin. D. S. Robertson (1962a,b) has also assembled many data, particularly on
uranium/thorium ratios, that is suggestive of a placer
origin. Holmes (1957) suggested that the ores were ofsyngenetic (placer) origin but were modified by later
events.
Derry (1960) has suggested a syngenetic origin for theuranium mineralization but has raised the possibility
that the uranium was largely carried in solution andreprecipitated in gravel banks by bacterial agencies.
Joubin (1960) has published similar ideas but has not in­cluded bacterial precipitation.
It may be noted that other conglomerates in the
district, for example, the Matinenda polymictic con­glomerate and the Ramsay Lake, Bruce, and Cobalt con­glomerates, do not carry markedly high uranium values,
although all, particularly the Bruce, carry pyrite andU19

pyrrhotite. An exception to this occurs when such a con­glomerate unconformably overlies the uranium-bearing
sequence or infringes on the basement. It should be
noted that the red granitic bodies in the basement areradioactive (Robertson, 1960, 1967).
The sericitic matrix of the ore-bearing conglomerates
is similar to the sericitic paleosoil developed on theArchean-Huronian unconformity and was probably
derived from that regolith; there is no reason to supposethat it was produced by the passa'ge of hydrothermal
solutions. Uraniferous oligomictic pebble conglomerates
and a green arkose sequence are characteristic ofwhichever part of the Huronian overlies the basement in
the area of the north shore of Lake Huron, and theserocks show a progressive northward overlap. Signifi­
cant thicknesses and grades have so far been found onlyin the Matinenda Formation.
Quartz veins and other evidence of intense hydrother­
mal activity are not conspicuous in the rocks of the areaand bear no sympathetic relation to the uranium
deposits. Where found, the associated mineralization isof copper and other sulphide minerals. Within the BlindRiver-Elliot Lake camp, there is no indication thateither the Nipissing (formerly included with the Keweenawan) or the olivine-diabase intrusions are a possi­
ble source of major radioactive mineralization. There is,however, evidence that ores were subjected to intense
local alteration and that all rocks suffered sulphidemineralization at the time of intrusion of the Nipissing
diabase and the time of regional folding. Agedetermination data are consistent with this concept.
The overall distribution of beds and the behaviour ofthickness and grades are more consistently explained by
the modified placer hypothesis, which the author has
supported (Robertson, 1960 and later). The recent
mineragraphic studies of Ferris and Ruud (1971) andTheis (1973) have added cogent additional support to themodified placer hypothesis.
The deposits were derived from Archean terrane to
the north and northwest, transported by rapidly moving
water, deposited in a near-shore fluviatile environment
under reducing conditions about 2,500 m.y. ago, and,later, subjected to diagenesis and to minor alteration
during subsequent intrusive metamorphic and tectonic
events.
ACKNOWLEDGMENTS

This review has drawn heavily on earlier reviews andpapers, particularly those of J. A. Robertson (1968 a, b,
1969a, b, 1972) and Roscoe (1969). Special Papers 11(Price and Douglas, 1972) and 12 (Young, 1973) publish­
ed by the Geological Association of Canada containmuch that is pertinent to Huronian geology and thus toU20

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

the uranium deposits. R. Balgalvis and J. Michalik of theGeological Branch of the Ontario Division of Mines,
Ministry of Natural Resources, provided assistance inthe preparation of the illustrative material, and Miss T.
Abolins and Miss C. Elliott helped in the production of
the manuscript. Permission to participate in theworkshop and to publish this paper was given by the
Director of the Geological Branch, Ontario Division of
Mines.
SELECTED REFERENCES
Abraham, E. M., 1953, Geology of parts of Long and Spragge
Townships of Blind River uranium area, district of Algoma: On­tario Department of Mines, Preliminary Report 1953-2, 10 p.,
map.
Arnold, R. G., 1954, A preliminary account of the mineralogy andgenesis of the uraniferous conglomerate of Blind River, Ontario:
University of Toronto, unpublished M. A. Sc. thesis.
Bateman, J. D., 1955, Recent uranium developments in Ontario:
Economic Geology v. 51, p. 361-372.
———1958, Uranium-bearing auriferous reefs at Jacobina, Brazil:
Economic Geology, v. 53, p. 417-425.
Bennett, Gerald, 1975, Jarvis Lake-Garden River area, district ofAlgoma, in Milne, V. G., and others, eds., Summary of field work,
1975, by the Geological Branch: Ontario Division of Mines,
Miscellaneous Paper 63, p. 80-83.
Bielenstein, H. V., and Eisbacher, G. H., 1969 Tectonic interpretationof elastic-strain-recovery measurements at Elliot Lake, Ontario:
Canada Department of Energy, Mines and Resources, Mines
Branch, Research Report R-210, 64 p.
Bottrill, T. J., 1970, Geology and genesis of uranium deposits in the
Huronian and associated geology, Blind River, Sudbury and
Gowganda areas, Ontario, in Report of activities: Canada
Geological Survey Paper 70-1, pt. A, April to Oct. 1969, p. 57-58.
———1971, Uraniferous conglomerates of the Canadian Shield, in
Report of activities: Canada Geological Survey, Paper 71-1, pt. A,
April to Oct. 1970, p. 77-83.
Canada Geological Survey, 1933, Lake Huron sheet, Ontario(Geological compilation by W. H. Collins): Canada Geological
Survey Map 155A (3d ed.), scale 1 inch to 8 miles.
Canadian Institute of Mining and Metallurgy, 1957, Mining,
metallurgy, and geology in the Algoma uranium area: Com­
monwealth Mining and Metallurgical Congress, 6th, 1957, Cana­dian Institute of Mining and Metallurgy.
Cannon, W. F., 1967, Plutonic evolution of the Cutler area, Ontario:
Syracuse University, New York, unpublished Ph. D. thesis.
Card, K. D., 1964, Metamorphism in the Agnew Lake area, Sudbury
district, Ontario, Canada: Geological Society of America Bulletin,v. 75, p. 1011-1030.
———1967a, Bay of Islands sheet (west half) Bay of Islands-McGregor
Bay area, districts of Sudbury and Manitoulin: Ontario Depart­
ment of Mines Preliminary Geological Map P.440.
———1967b, Bay of Islands sheet (east half), Bay of Islands-McGregor
Bay area, districts of Sudbury and Manitoulin: Ontario Depart­
ment of Mines Preliminary Geological Map P.441.
———1967c, McGregor Bay sheet (west half): Ontario Department ofMines Preliminary Geological Map P. 442———1967d, McGregor Bay sheet (east half): Ontario Department ofMines Preliminary Geological Map P.443.
———197 la, Panache Lake area, districts of Sudbury and Manitoulin:
Ontario Department of Mines and Northern Affairs, Preliminary
Map P.668, Geological Series.

——197 Ib, Panache Lake area (east part), Districts of Sudbury andManitoulin: Ontario Department of Mines and Northern Affairs,
Preliminary Map P.669, Geological Series.
-1976, Geology of the Espanola-Whitefish-Falls, District of Sud­bury, Ontario: Ontario Division of Mines Geoscience Report No.
131, 70 p.
Card, K. D., Church, W. R., Franklin, J. M., Frarey, M. J., Robertson,
J. A., West, G. F., and Young, G. M., 1972, The Southern Pro­
vince, in Price, R. A., and Douglas, R. J. W., eds. Variations in tec­
tonic styles in Canada: Geological Associatian of Canada Special
Paper 11, Montreal, Pierre Des Marais, p. 335-380.
Casshyap, S. M., 1966, Sedimentary petrology and stratigraphy of the
Huronian rocks south of Espanola, Ontario; University of Western
Ontario, unpublished Ph. D. thesis, 232 p.
———1968, Huronian stratigraphy and paleocurrent analysis in the
Espanola-Willisville area, Sudbury district, Ontario Canada: Jour­nal of Sedimentary Petrology, v. 38, no. 3, p. 920-942.
Chandler, F. W., 1969, Geology of the Huronian rocks of Harrow
Township and surrounding areas, North Shore of Lake Huron, On­tario: University of Western Ontario, London, Ontario, unpublish­ed Ph. D. thesis, 328 p.
Chandler, F. W., Young, G. M., and Wood, J., 1969, Diaspore in earlyProterozoic quartzites (Lorrainn Formation) of Ontario: Canadian
Journal of Earth Sciences, v. 6, No. 2, p. 337-340.
Church, W. R., 1967, The occurrence of kyanite, andalusite, andkaolinite in lower Proterozoic (Huronian) rocks of Ontario (abs.):
Geological Association of Canada Annual Meeting, Kingston,
1967, Abstracts of Technical Proceedings, Geological Associationof Canada Annual Meeting, Kingston, 1967, p. 14-15.
Collins, W. H., 1925, The North Shore of Lake Huron: Canada
Geological Survey Memoir 143, 160 p., maps.
———1936, The Sudbury series: Geological Society of America
Bulletin, v. 47, p. 1675-1690.
Darnley, A. G., and Grasty, R. L., 1971, Airborne radiometric surveyof the Elliot Lake area: Canada Geological Survey, Open File no.
75.
Davidson, C. F., 1957, On the occurrence of uranium in ancient con­glomerates: Economic Geology, v. 52, p. 668-693.
———1965, The mode of origin of banket orebodies: Institution of
Mining and Metallurgy Transactions, v. 74, pt. 6, p. 319-338.
Deny, D. R., 1960, Evidence of the origin of the Blind Riveruranium deposits: Economic Geology, v. 55, p. 906-927.
———1961, Economic aspects of Archean-Proterozoic boundaries:
Economic Geology v. 56, p. 635-647.
Donaldson, J. A., 1967, Precambrian vermiform structures: A new in­
terpretation: Canadian Journal of Earth Sciences, v. 4, no. 6, p.
1273-1276.
Eisbacher, G. H., and Bielenstein, H. V., 1969, The Flack Lake depres­sion, Elliot Lake area, Ontario, in Report of activities: Canada
Geological Survey, Paper 69-1, pt. B, Nov. 1968 to March 1969, p.
58-60.
Fairbairn, H. W., Hurley, P. M., Card, K. D., and Knight, C. J, 1969,
Correlation of radiometeric ages of Nipissing diabase and Huro­nian metasediments with Proterozoic orogenic events in Ontario:
Canadian Journal of Earth Sciences, v. 6, p. 489 -497.
Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: Its occurrences andrecognition by microprobe: Colorado School of Mines Quarterly, v.
66, no. 4, 35 p.
Frarey, M. J., 1959, Echo Lake, district of Algoma: Canada Geological
Survey Map no. 23-1959.
•1962, Bruce Mines, Ontario: Canada Geological Survey Map no.
32-1962.
Frarey, M. J., and Roscoe, S. M., 1970, The Huronian Supergroupnorth of Lake Huron, in Baer, A. J., ed., Symposium on basins andgeosynclines of the Canadian Shield: Canada Geological Survey
Paper 70-40, p. 143-157; also 3 responses on p. 158.

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING
Goodwin, A. M., Ambrose, J. W., Ayers, L. D., Clifford, P. M., Currie,
K. L., Ermanovics, I. M., Fahrig, W. F., Gibb, U. A., Hall, D. H.,
Innes, M. J. S., Irvine, T. N., MacLaren, A. S., Norris, A. W., andPettijohn, F. J., 1972, The Superior Province, in Price, R. A., andDouglas, R. J . W., eds., Variations in tectonics styles in Canada:
Geological Association of Canada Special Paper 11, Montreal,
Pierre Des Marais Inc., p. 527-623.
Grandstaff, D. K., 1974, Microprobe analysis of uranium and thorium
in uraninite from the Witwatersrand, South Africa, and BlindRiver, Ontario, Canada: Geological Society of South Africa Tran­sactions, v. 77, p. 291-294.
Gross, W. H., 1968, Evidence for a modified placer origin forauriferous conglomerates, Canavierras Mine, Jacobina, Brazil:
Economic Geology, v. 63., no. 3, p. 271-276.
Gruner, J. W., 1959, The decomposition of ilmenite: Economic
Geology, v. 54, p. 1315-1316.
Hadley, D. G., 1969, Depositional framework of the jasper-bearing
conglomerate in the Lorrain Formation, Ontario (abs.): GeologicalSociety of America, Annual Meeting, 82d, Atlantic City, N. J.,
Abstracts with programs for 1969, pt. 7, p. 87-88.
Heinrich, E. W., 1958, Mineralogy and geology of radioactive raw
materials: New York, McGraw-Hill.
Hoffman, H. J., 1967, Precambrian t'ossils(?) near Elliot Lake, Ontario:
Science, v. 156, no. 3774, p. 500-504.
Holmes, S. W., 1956, The Algom Mines--geology: Western Miner and
Oil Review, v. 29, no. 7, p. 66-68.
————1957, Pronto Mine, in Structural geology of Canadian ore
deposits: Canadian Institute of Mining and Metallurgy, Geology
Division, v. 2, p. 324-339.
Innes, D. G., 1974, Proterozoic volcanism and associated sulphidebearing metasedimentary rocks in the Sudbury area: Institute ofLake Superior Geology, Annual Meeting, 20th, Sault Ste. Marie,
May 1-5, 1974, Abstracts and Field Guides, p. 16.
————1975, McKim Township, district of Sudbury, in Milne, V. G.,
and others, eds. Summary of field work, 1975, by the Geological
Branch: Ontario Department of Mines Miscellaneous Paper 63, p.
100-103.
Joubin, F. R., 1954, Uranium deposits of the Algoma district, Ontario:
Canadian Institute of Mining and Metallurgy Transactions, v. 57,
p. 431-437.
————1960, Comments regarding the Blind River (Algoma) uraniumores and their origin: Economic Geology, v. ,55, p. 1751-1756.
Joubin, F. R., and James, I). H., 1957, Algoma uranium district, in
Structural geology of Canadian ore deposits: Canadian Institute of
Mining and Metallurgy, v. 2, p. 305-317.
Lindsay, J. F., Summerson, C. H., and Barrett, P. J., 1970, A long-axis
clast fabric comparison of the Squantum "Tillite", Massachusetts,and the Gowganda Formation, Ontario: Journal of SedimentaryPetrology, v. 40, no. 1, p. 475-479.
Lindsey, D. A., 1966, Sediment transport in a Precambrian ice
age—the Huronian Gowganda Formation: Science, v. 154, p.
1442-1443.
Little, H. W., 1974, Uranium deposits in Canada--their exploration
reserves and potential: Canadian Institute of Mining andMetallurgy Bulletin, v. 67, no. 743.
Little, H. W., Smith, E. E. N., Barnes, F. Q., and others, 1972,
Uranium deposits of Canada: International Geological Congress
Guidebook, no. 24, pt. C67, 64 p.
Logan, W. E., 1863, Report on the geology of Canada (chap. 4):
Canada Geological Survey, Report of Progress to 1863, p. 50-66.
Lowdon, J. A. compiler, 1960, Isotopic ages, rept. 1 of"Age determina­
tions by the Geological Survey of Canada: Canada Geological
Survey Paper 60-17, 51 p.
———1961, Isotopic ages, rept. 2 of Age determination by the
Geological Survey of Canada: Canada Geological Survey Paper
61-17, 127 )).

U21

Lowdon, J. A., Leech, G. B., Stockwell, C. H., and Wanless, R. K.,
1963, Age-determination and geological studies:: Canada
Geological Survey Paper 63-17, 140 p.
Lowdon, J. A., Stockwell, C. H., Tipper, H. W., and Wanless, R. K.,
1962, Age-determinations and geological studies: Canada
Geological Survey Paper 62-17, 140 p.
McDowell, J. P., 1957, The sedimentary petrology of the Mississagi
Quartzite in the Blind River area: Ontario Department of Mines,
Geological Circular no. 6, 31 p.
————1963, A paleocurrent study of the Mississagi Quartzite along theNorth Shore of Lake Huron: Johns Hopkins University, unpublish­ed Ph.D dissertation.
Mair, J. A., Maynes, A. D., Patchett, A. D., and Russell, R. D., 1960,
Isotopic evidence on the origin and age of the Blind River Uranium
Deposits: Journal of Geophysical Research, v. 65, no. 1, p. 341-348.
Meyn, H. D., 1972, The Proterozoic sedimentary rocks north andnortheast of Sudbury, Ontario, in Huronian stratigraphy andsedimentation: Geological Association of Canada Special Paper no.
12, p. 129-145.
Milne, V. G., 1959, The nonradioactive heavy minerals of theMississagi conglomerate, Blind River, Ontario: University ofToronto, Ontario, unpublished M. A. thesis.
Nuffield, E. W., 1954, Brannerite from Ontario, Canada: American
Mineralogist, v. 39, p. 520-522.
————1955, Geology of the Montreal River area: Ontario Departmentof Mines Annual Report v. 64, pt. 3, p. 1-30.
Organization for Economic Co-operation and Development, 1973,
Uranium: Resources, production and demand; a joint report by theOECD Nuclear Energy Agency and the International Atomic
Energy Agency: Paris, Organization for Economic Co-operation
and Development, 140 p.
Ovenshine, A. T., 1965a, Glacial interpretation of Precambrian
Gowganda Formation, north shore of Lake Huron, Canada, in
Abstracts for 1964: Geological Society of America, Special Paper
no. 82, p. 146.
————1965, Sedimentary structures in portions of the Gowganda For­mations, north shore of Lake Huron, Canada: University ofCalifornia, Los Angeles, unpublished Ph. D. thesis.
Patchett, J. E., 1959, Some contributions to the mineralogy of theBlind River ore conglomerate: Unpublished talk given to
Mineralogical Association of Canada.
————1960, A study of radioactive minerals of the uraniferous con­glomerate. Blind River area: University of Toronto, Ontario, un­published Ph. D. thesis.
Patchett, J. E., and Nuffield, E. W., 1960, The synthesis andcrystallography of brannerite, [pt.J 10 of Studies of radioactive
compounds: Canadian Mineralogist, v. 6, pt. 4, p. 483-490.
Pienaar, P. J., 1958, Stratigraphy, petrography, and genesis of the
Elliot Lake Group including the uraniferous conglomerates,
Quirke Lake Syncline, Blind River, Ontario: Queens University,
Kingston, Ontario, unpublished Ph.D. thesis.
————1963, Stratigraphy, petrology and genesis of the Elliot LakeGroup, Blind River, Ontario, including the uraniferous con­glomerate: Canada Geological Survey Bulletin 83.
Price, R. A., and Douglas, R. J. W., 1972, Nature and significance ofvariations in tectonic styles in Canada, /// Price, R. A., andDouglas, eds., Variations in tectonic styles in Canada: Geological
Association of Canada Special Paper no. 11, p. 626 -688.
Ramdohr, Paul, 1957, Die "Pronto-Reaktion": Neues Jahrbuch fur
Mineralogie, Monatshefte, Jahrgang 1957, v. 10 -11, p. 217-221.
————1958a, New observations on the ores of the Witwatersrand inSouth Africa and their genetic significance: Geological Society ofSouth Africa Transactions, Annexure to v. 61, 50 p.
————1958b, Die Uran- und Goldlagerstatten: Witwatersrand-- BlindRiver district—Dominion Reef-Serra de Jacobina; erz-

U22

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

miktroskopishe Untersuchungen und ein geologischer Vergleich:
Berlin Akademie-Verlag, 35 p.
Rice, R., 1958, Geology and ore deposits of the Elliot Lake district, On­tario: Emmanuel College, Cambridge, England, unpublished Ph.D.
thesis.
Richardson, K. A., Killeen, P. G., and Charbonneau, B. W., 1975,
Results of a reconnaissance type airborne gamma-ray spec­
trometer survey of the Blind River-Elliot Lake area: Canada
Geological Survey Paper 75-1, Part A, p. 133-135.
Robertson, D. S., 1962a, Thorium and uranium variations in the BlindRiver ores; Economic Geology v. 57, p. 1175-1184.
————1962b, Thorium and uranium variations in the Blind River
camp: Canadian Mining Journal, p. 58-65.
————1974, Basal Proterozoic units as fossil time markers and theiruse in uranium prospection, in Formation of uranium ore
deposits—Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens Greece,6-10 May, 1974: Vienna, International Atomic Energy Agency, p.
495-512.
Robertson, D. S., and Steenland, N. C., 1960, The Blind River uraniumores and their origin: Economic Geology v. 55, p. 659-694.
Robertson, J. A., 1960, The general geology of part of the Blind Riverarea, Ontario: Queen's University, Kingston, Ontario, unpublished
M.Sc. thesis, 433 p.
————1961, Geology of Townships 143 and 144: Ontario Departmentof Mines Geological Report GR 4.
————1963, Geology of Townships 155, 156, 161, 162, and parts of 167
and 168, district of Algoma: Ontario Department of Mines
Geological Report GR 13.
————1964, Geology of Scarfe, Mack, Cobden and Striker townships,
district of Algoma: Ontario Department of Mines Geological
Report GR 20.
————1965a, Shedden Township and part of I.R. no. 7, district ofAlgoma: Ontario Department of Mines Preliminary Geological
Map P.318.
————1965b, I.R. 7, East and Offshore Islands, district of Algoma: On­tario Department of Mines Preliminary Geological Map P.319.
————1965c, I.R. 5, West and Offshore Islands, district of Algoma:Ontario Department of Mines, Preliminary Geological Map P.320.
————1966a, Victoria Township, district of Algoma: Ontario Depart­
ment of Mines Preliminary Geological Map P.377.
————19661), Salter Township, district of Sudbury: Ontario Depart­
ment of Mines Preliminary Geological Map P.378.
————1967, Recent geological investigations in the Elliot Lake-BlindRiver uranium area, Ontario: Ontario Department of Mines
Miscellaneous Paper 9, 31 p.
————1968a, Geology of Townships 149 and 150: Ontario Departmentof Mines Geological Report GR 57, 162 p.
————19681), Uranium and thorium deposits of northern Ontario: On­tario Department of Mines, Mineral Resources Circular no. 9.
————1969a, Geology and uranium deposits of the Blind River area,
Ontario: Canadian Institute of Mining ami Metallurgy Transac­
tions, v. 62, no. 686, p. 619-634.
————1969b, Township 157, district of Algoma: Ontario Departmentof Mines, Preliminary Geological Map P.561.
————1970a, Geology of the Spragge area, district of Algoma: Ontario
Department of Mines Geological Report GR 76, 109 p.
———1961, Geology of Townships 143 and 144: Ontario Departmentof Mines Geological Report GR 4.
————1963, Geology of Townships 155, 156, 161, 162, and parts of 167
and 168, district of Algoma: Ontario Department of Mines
Geological Report GR 13.
———1964, Geology of Scarfe, Mack, Cobden and Striker townships,
district of Algoma: Ontario Department of Mines Geological
Report GR 20.

——1965a, Shedden Township and part of I.R. no. 7, district ofAlgoma: Ontario Department of Mines Preliminary Geological
Map P.318.
——19651), I.R. 7, East and Offshore Islands, district of Algoma: On­tario Department of Mines Preliminary Geological Map P.319.
——1965c, I.R. 5, West and Offshore Islands, district of Algoma:Ontario Department of Mines, Preliminary Geological Map P.320.
——1966a, Victoria Township, district of Algoma: Ontario Depart­
ment of Mines Preliminary Geological Map P.377.
——1966b, Salter Township, district of Sudbury: Ontario Depart­
ment of Mines Preliminary Geological Map P.378.
——1967, Recent geological investigations in the Elliot Lake-BlindRiver uranium area, Ontario: Ontario Department of Mines
Miscellaneous Paper 9, 31 p.
——1968a, Geology of Townships 149 and 150: Ontario Departmentof Mines Geological Report GR 57, 162 p.
——19681), Uranium and thorium deposits of northern Ontario: On­tario Department of Mines, Mineral Resources Circular no. 9.
——1969a, Geology and uranium deposits of the Blind River area,
Ontario: Canadian Institute of Mining and Metallurgy Transac­
tions, v. 62, no. 686, p. 619-634.
——19691), Township 157, district of Algoma: Ontario Departmentof Mines, Preliminary Geological Map P.561.
——1970a, Geology of the Spragge area, district of Algoma: Ontario
Department of Mines Geological Report GR 76, 109 p.
——19701), Geology of the Massey area, districts of Algoma,
Manitoulin and Sudbury: Ontario Department of Mines, OFR
5043, 2 folders; accompanied by 3 maps.
——1970c, Township 1A, district of Algoma: Ontario Department ofMines and Northern Affairs Preliminary Geologic Map P.610.
——197la, A long-axis clast fabric comparison of the Squantum"Tillite," Massachusetts, and the Gowganda Formation, On­tario—Discussion: Journal of Sedimentary Petrology v. 41, no. 2,
p. 606-608.
——1971b, A review of recently acquired geological data, BlindRiver-Elliot Lake area: Ontario Department of Mines and Nor­thern Affairs Miscellaneous Paper 45.
——1972, Granitic plutonic rocks of the Southern Province of theCanadianShield (abs.): Institute on Lake Superior Geology,
Abstracts and Field Guides, no. 18 [Paper no. 29].
——1975a, Uranium and Thorium deposits of Ontario, East Central
sheet, districts of Thunder Bay, Algoma, Cochrane, Sudbury,
Timiskaming and Nippissing: Ontario Division of Mines
Preliminary Map P.971, Mineral Deposits Series.
——19751), Victoria and Salter Townships, district of Sudbury: On­tario Division of Mines Map 2308.
——1975c, Part of Indian Reserve No. 5 and offshore islands,
districts of Algoma and Manitoulin: Ontario Division of Mines Map
2309.
-1976, Geology of the Massey area, districts of Algoma,
Manitoulin and Sudbury: Ontario Division of Mines Geoscience
Report GR 236, 130 p.
Robertson, J. A., Card, K. D., and Frarey, M. J., 1969, The FederalProvincial Committee on Huronian Stratigraphy progress report:
Ontario Department of Mines Miscellaneous Paper 31, 26 p.
Robertson, J. A., Frarey, M. J., and Card, K. D., 1969, The FederalProvincial Committee on Huronian Stratigraphy-- Progress
report: Canadian Journal of Earth Sciences, v. 6, p. 335-336.
Robertson, J. A., and Johnson, J. M., 1969, Township 163, district ofAlgoma: Ontario Department of Mines Prelinimary Geological
Map P.560.
————1970, Township IB, district of Algoma: Ontario Department ofMines and Northern Affairs Preliminary Geological Map, P.609,
Geological Series.

THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING
Robertson, J. A., and McCrindle, W. E., 1967, l.R. No. 5, Spanish
River Reserve (central and east portions) and adjacent islands,
district of Algoma: Ontario Department of Mines, Preliminary
Geological Map P.438.
Roscoe, S. M., 1957, Geology and uranium deposits, Quirke Lake-Elliot
Lake-Blind River area, Ontario: Canada Geological Survey Paper
56-7, 21 p.
————1959a, On thorium-uranium ratios in conglomerate andassociated rocks near Blind River, Ontario: Economic Geology, v.
54, p. 511-512.
————1959h, Monazite as an ore mineral in Elliot Lake uranium ores:
Canadian Mining Journal, July 1959, p. 65.
————1960, Huronian age rocks classified studied [abs.]: Northern
Miner, v. 46, no. 4, p. 20 (404).
____1966, Unexplored uranium and thorium resources of Canada:
Canada Geological Survey Paper 66-12, 11 p.
-1969, Huronian rocks and uraniferous conglomerates in the
Canadian Shield: Canada Geological Survey Paper 68-40, 205 p.
Roscoe, S. M., and Steacy, H. R., 1958, On the geology and radioactive
deposits of Blind River region, in United Nations, Survey of raw
material resources: International Conference on the Peaceful Usesof Atomic Energy, 2d, Geneva, Switzerland, Sept. 1958, Pro­
ceedings, v. 2, p. 475-483.
Siemiatkowska, K. M., and Guthrie, A. E., 1975, Kirkpatrick LakeArea, district of Algoma, in Milne, V. G., and others, eds., Sum­
mary of field work, 1975, by the Geological Branch: Ontario Divi­sion of Mines Miscellaneous Paper 63, p. 84-87.
Theis, N. J., 1973, Comparative mineralogy of the Quirke No. 1 and
New Quirke Mines, Rio Algom Mines Ltd., Elliot Lake, Ontario:
Queen's University, Kingston, Ontario, unpublished M. Sc. thesis.
Thomson, J. E., 1960, Uranium and thorium deposits at the base of the
Huronian System in the district of Sudbury: Ontario Departmentof Mines Geological Report 1.
Thorpe, R. I., 1963, The radioactive mineralogy of the ore con­glomerate at Panel Mine, Blind River, Ontario: Queen's Universi­
ty, Kingston, Ontario, unpublished M.Sc. thesis.
Van Schmus, W. R., 1964, The geochronology of the Blind River-Bruce
Mines area, Ontario, Canada: University of California, Los
Angeles, unpublished Ph.D. thesis.
———1965, The geochronology of the Blind River-Bruce Mines area,
Ontario, Canada: Journal of Geology, v. 73, p. 755-780.
Wanless, R. K., Stevens, R. D., Lachance, G. R., and Rimsaite, R. Y.
H., 1965, Age determinations and geological studies, pt 1, Isotopic
ages, Report 5: Canada Geological Survey Paper 64-17, 126 p.
Wanless, R. K., and Traill, R. T., 1956, Age of uraninites from BlindRiver, Ontario: Nature, v. 178, p. 249-250.

U23

Williams, R. M., and Little, H. W., 1973, Canadian uranium resources
and production capability: Canada Department of Energy, Mines
and Resources, Ottawa, Mineral Bulletin MR 140, 27 p.
Wood, J., 1968a, Township U, district of Algoma: Ontario Departmentof Mines Preliminary Geological Map P.468.
———1968b, Township Q, district of Algoma: Ontario Department ofMines Preliminary Geological Map P.474.
———1970a, Evidence for a tropical climate and oxygenic atmospherein upper Huronian rocks of the Rawhide Lake-Flack Lake area,
Ontario [abs.]: Institute on Lake Superior Geology, 16th, ThunderBay, Ontario, May 6-9, 1970, Technical Sessions, Abstracts, p.
45-46.
———1970b, Upper Huronian stratigraphy and sedimentation,
Rawhide Lake area, Ontario: University of Western Ontario, Lon­don, unpublished M.Sc. thesis.
———1971, Environments during deposition of upper Huronian rocksof the Rawhide Lake-Flack Lake area, Ontario [abs.]: Geological
Association of Canada-Mineralogical Association of Canada, Joint
Annual Meeting, May 13-15, 1971, Sudbury, Abstracts of Papers,
p. 74-75.
-1975, Geology of the Rawhide Lake area, district of Algoma:Ontario Division of Mines Geological Report GR 129, 67 p.
Woodward, N. B., 1970, A measured section and description of the Bar
River and Gordon Lake Formations along Highway 639, Township157, Ontario, Canada: Cornell University, Ithaca, New York, un­published Senior thesis (B.A.).
Young, G. M., 1966, Huronian stratigraphy of the McGregor Bay area,
Ontario—relevance to the paleogeography of the Lake Superior
region: Canadian Journal of Earth Sciences, v. 3, no. 2, p. 203-210.
———1967, Possible organic structures in early Proterozoic (Huro­nian) rocks of Ontario: Canadian Journal of Earth Sciences, v. 4,
no. 3, p. 565-568.
———1969, Inorganic origin of corrugated vermiform structures inthe Huronian Gordon Lake Formation near Flack Lake, Ontario:
Canadian Journal of Earth Sciences, v. 6, no. 4, pt. 1, p. 795-799.
-1970, Widespread occurrence of aluminous minerals in Aphebian quartzites [abs.]: Institute on Lake Superior Geology, 16th,
Thunder Bay, Ontario, May 6-9, 1970, Technical Sessions,
Abstracts, p. 47-48.
Young, G. M., ed., 1973, Huronian stratigraphy and sedimentation:
Geological Association of Canada Special Paper 12.
Young, G. M., and Church, W. R., 1966, The Huronian System in the
Sudbury district and adjoining areas of Ontario—a review:
Geological Association of Canada Proceedings, v. 17, p. 65-82.

Some Metallogenic Features of the
Huronian and Post-Huronian
Uraniferous Conglomerates
By VLADIMIR RUZICKA
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEYPROFESSIONAL

Published with permission of the
Director, Regional and Economic Geology,
Geological Survey of Canada

PAPER1161-VCONTENTS
Abstract ________________________________________________
Introduction ______________________________________________
Some features of the uranium mineralization at Elliot Lake, Canada ______________
Some features of the post-Huronian conglomerates in the Ural Mountains and Enisey Crest,
U.S.S.R. ______________________________________________
Conclusions _______________________________________________
References _______________________________________________

VI
1
2
4
7
7

ILLUSTRATIONSFIGURE 1. Photomicrograph of a uranium-ore sample from Denison Mine, Elliot Lake ______________________________
2. Photograph showing a sample of uranium ore from Denison Mine, Elliot Lake, containing pyrite, brannerite, monazite,uraninite, and hydrocarbon ______________________________________________________
3. Photomicrograph showing arrangement of detrital grains of uraninite embedded in hydrocarbon ________————————
4. Photograph showing a quartz pebble depressing a uraninite-hydrocarbon-rich band, from Denison Mine, Elliot Lake _____
5-7. Photomicrographs of portions of uraninite-hydrocarbon bands shown on figure 4 _______________________—in

V2
3
4
4
5, 6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESSOME METALLOGENIC FEATURES OF THE
HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATESBy VLADIMIR RuziCKA1
ABSTRACT

Two genetic types of uraniferous conglomerates can be distin­
guished: (1) those with syngenetic mineralization, and (2) those with
epigenetic mineralization.
Uraniferous conglomerates with syngenetic mineralization are
restricted to lower Proterozoic sediments, such as those in the Huronian Supergroup in Canada. It is postulated that deposits with this typeof mineralization can occur only in regions where above-normal con­centrations of radioelements and uranium minerals were present in thesource rocks.
Liberation of uranium minerals, their transportation to the site ofdeposition, and their deposition along with other heavy minerals took
place under oxygen-deficient conditions but at a time when primitive
organisms existed.
The depositional environment was near volcanic centres; excessive
sulphur, a product of volcanic and postvolcanic activity, apparently
caused sulphurization of some minerals and of hydrocarbon, the latter
derived from organic and possibly inorganic material.
Concentration of uranium minerals during transportation anddeposition was governed by hydraulic and hydrodynamic conditions.
The sedimentary syngenetic origin of uranium mineralization may beillustrated by a portion of an ore deposit that corresponds to a com­plete cycle of sedimentation. The cycle starts with deposition of coarse
grains of quartz, continues with the deposition of a mixture of finer
grains of quartz and pyrite with distinct graded bedding, is followed bydeposition of monazite and brannerite, and ends with deposition offragmented fine grains of uraninite embedded in sulphur-richhydrocarbon. The bed containing hydrocarbon and uraninite is locallydepressed by impact of an exotic pebble of quartz. This situation ap­
parently testifies to the fact that the carbonaceous material is a prod­
uct of algal mats that existed in quiet times between cycles of sedimen­
tation. The succeeding cycle again starts with the deposition of coarse
grains of quartz.
Preservation of uranium mineralization during and after diagenesis
was possible owing to reducing conditions of the depositional environ­
ment.
Uraniferous conglomerates with epigenetic mineralization are
localized in regions containing rocks with higher contents of uranium.
Primary concentration of uranium minerals in these source rocks canalso be syngenetic.
Transportation of uranium from the site of original liberation to thesite of deposition was apparently mainly in solutions and in refractoryminerals by hydraulic action. Mobilization of uranium from the source
rocks and its redistribution were governed by hydrodynamic gradients
related to erosional, tectonic, magmatic, or metamorphic processes inthe Earth's crust. The conglomerate beds apparently served as chan­nels for penetration of uranium-bearing fluids.
1 Geological Survey of Canada, 601 Booth Street, Ottawa, Canada, K1A OE8

Deposition of uranium took place by one or more epigenetic proc­
esses: adsorption, complexing, precipitation, or redox changes accord­
ing to the degrees of solubility and stability of uranium compounds.
The mineralization in this type of deposit has been preserved
beneath the zone of oxidation.
Evidence of these mentioned features is based upon studies on the
Huronian uraniferous conglomerate from Elliot Lake, Canada, and on
the late Precambrian and Paleozoic radioactive conglomerates fromthe eastern Ural Mountains and Enisey Crest region, U.S.S.R.

INTRODUCTION

At the symposium of the International Atomic Energy
Agency (1974) dealing with the formation of uranium
deposits, fundamental criteria on the metallogeny ofuranium were formulated, among which special atten­
tion was paid to those related to the genesis of uranium
mineralization in quartz-pebble Huronian conglom­
erates.
It was suggested that reports of younger con­glomerates of this type such as Cambrian conglomerate,
described in the Soviet literature, are in error.
Criteria related to some metallogenic features of thequartz-pebble conglomerates were derived from studiesof the stratigraphic setting, sedimentary nature,mineral composition, and geochemical features of theRand-Elliot Lake type conglomerate. No further
description of this type of mineralization was given in
the International Atomic Energy Agency report (1974).
The genesis of uranium mineralization in postHuronian conglomerates was discussed only briefly inthe subsequent discussion, which showed that this topic
needs more clarification and the deposit types require
more exact classification.
Therefore this study concentrates on metallogenic
features of the Huronian and post-Huronian con­glomerates in order to contribute to the solution to thementioned problems. The author's views on the long­standing question of a syngenetic versus epigenetic
origin of various kinds of conglomerate uranium
deposits are also presented.
VI

V2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESThe conclusions are based upon studies of the Huronian uraniferous conglomerates, Elliot Lake, Canada,
and of the late Precambrian and Paleozoic radioactive
conglomerates of the eastern Ural Mountains and the
Enisey Crest region, U.S.S.R. In all instances the hostfor the uranium mineralization is quartz-pebble con­glomerate; in addition, the Russian deposits are also inuraniferous polymictic conglomerates.
SOME FEATURES OF THE URANIUMMINERALIZATION AT ELLIOT LAKE, CANADAThe Geological Survey of Canada airborne gamma-ray
spectrometric map of the Elliot Lake-Blind River region(Richardson and others, 1975) shows that the Elliot
Lake and Blind River uranium deposits are localized
within a terrane exhibiting abnormal concentrations ofradioelements. The deposits are confined to lower Proterozoic sediments of the Huronian Supergroup whose
provenance was the pre-Huronian basement rocks:
granites, granite gneisses, and other felsic rocks con­taining enclaves of metavolcanics and metasedimentswith banded iron-formations. The granitic rocks that oc­
cur in areas from which clastic material was supplied to
the Huronian sediments locally contain concentrationsof uranium exceeding several times normal clarkes forsuch rocks (Richardson and others, 1975; Roscoe andSteacy, 1958). Samples of these rocks contain between 5
and 10 ppm (parts per million) eU (Darnley and others,1977). This is considered a fundamental metallogenic
feature and is a necessary condition for the formation ofan economic uranium deposit of this type.
The source rocks underwent intensive weathering
during pre-Huronian time. It is postulated that weather­ing and disintegration of the source rocks took place
under oxygen-deficient conditions (Roscoe, 1969, 1973).
It is also postulated that the transportation of the
detritus from the site of origin to the site of deposition
occurred under conditions similar to those for thedisintegration of the source rocks (Roscoe, 1969).
Evidence for this postulate, among other features, is thepresence of subrounded grains of pyrite and other oreforming minerals, which are apparently of detrital
origin (fig. 1). Deficiency of free oxygen might also havebeen partially related to volcanic activity. It is theauthor's opinion that both an oxygen-deficient at­mosphere and volcanic activity were necessary factors
in formation of the Elliot Lake deposits.
Concentration of uranium minerals was governed byhydraulic action of the streams.
Elliot Lake ore was deposited in sedimentation cycles.
One such cycle is illustrated in figure 2. It starts withdeposition of a coarse fraction of light minerals (quartz)
and continues with deposition of a gradationally finerand finer mixture of quartz and pyrite grains. Farther

FIGURE 1.—Photomicrograph of a uranium ore sample collected in
Denison Mine, Elliot Lake, containing rounded grains of pyrite (py)(x 3). Reflected light.

upward in the section the graded bedding encompasses
concentrations of finer grains of pyrite, monazite, andbrannerite. The cycle ends with deposition of
microscopic fragmented grains of uraninite embedded inhydrocarbon (fig. 3) (Ruzicka and Steacy, 1976).
The upward fining of the mineral grains in thesecycles is proportional to their absolute weight and in­versely proportional to their density. Figure 4 shows aquartz pebble of the subsequent cycle depressing theuraninite-hydrocarbon-rich band of the previous cycle.
The deposition of uranium was governed by sedimen­
tary processes. There is no doubt about the detritalnature of the quartz grains; the pyrite occurs as angular
to subrounded grains that show a decrease in size
toward the top of the bed; the brannerite is present
mainly as ragged grains, and the monazite, as roundedgrains. The uraninite contains several percent ofthorium (Ruzicka and Steacy, 1976). This testifies to theorigin of the uraninite in a high-temperature environ­ment.
Roscoe (1969) and Theis (1976) observed composite
grain aggregates containing titania and uranium that,
according to Theis, are enriched by uranium only in thevicinity of uraninite grains. Since this enrichment is alocal process, involving distances of only a few
millimetres, it can be postulated that the formation ofthese aggregates occurred in situ—that is, during
diagenesis. Thus, this phenomenon can be called"uranization." It apparently corresponds to Ramdor's"Pronto-reaction" (Ramdohr, 1958). Because no addi­
tional uranium has been introduced into the sediment,such a process may be regarded as redistribution ofsyngenetic uranium.

V3

METALLOGENIC FEATURES, HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATESA•MMMMMMIFIGURE 2.—A sample of uranium ore from Denison Mine, Elliot Lake, Ontario; the portion within the arrow area contains bedded grains ofquartz, pyrite, brannerite, monazite, uraninite, and hydrocarbon and represents one sedimentation cycle. The arrow points towards the topof the bed. UR-HC = uraninite-hydrocarbon-rich section; PY-BR-MO = pyrite-brannerite-monazite-rich section; QU = quartz-pebble-richsection.

The hydrocarbon in the Elliot Lake ores, variously
described as "radioactive hydrocarbon," "uraniumhydrocarbon," or more generally as "thucholite" might
be of biogenic origin (Ruzicka and Steacy, 1976). A
similar phenomenon has been studied by Hallbauer(1975) in the Witwatersrand uranium ores and inter­
preted as evidence for plant origin of the Witwatersrand"carbon." Pretorius (1975) interpreted the carbonaceous
material arranged in narrow layers in the gold-uranium
conglomeratic ores as a product of "algal or lichen col­onies preferentially developed about the mouths of the
major rivers and at the end of certain cycles of sedimen­
tation."The hydrocarbon usually exhibits uniform extinction

V4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 3.—Photomicrograph showing arrangement of detrital grainsof uraninite (Ur) embedded in hydrocarbon (He). Line A-B denotes
the plane of bedding (x 163). Reflected light.

4.—A quartz pebble depressing a uraninite-hydrocarbon-richand a columnar structure under crossed nicols (figs. 5, FIGURE
band. A polished section of the Elliot Lake ore, Denison Mine
6). The fibres (columns) are oriented normal to the bed­
(Geological Survey of Canada Photograph 203006M).
ding plane. This corresponds with the orientation of thecolumnar carbon in ores from the Orange Free State Elliot Lake ore (Hartman and Wyman, 1970) originatedGoldfield (Basal Reef), as reported by McKinney (1964). by sulphurization of magnetite. Sulphurization is ap­
Hallbauer (1975) postulated an organic origin of the col­ parently the cause of abnormally high content of sulphur
umnar carbon. Some of the hydrocarbon heals fractures in the hydrocarbon. H.R. Steacy of the Geologicalin uraninite (fig. 7). Uraninite embedded in theSurvey of Canada investigated (personal commun.)hydrocarbon can be interpreted as evidence of transpor­ "thucholites" from several types of environments andtation of uraninite in organic-supported colloids. Fur­ found the highest sulphur concentrations (more than 20
thermore, if one accepts that the organisms were also percent sulphur) only in Elliot Lake hydrocarbons. The
active in the provenance area of the detrital material, sulphides acted also as agents for preserving detritalthen the possibility that the biological agents par­ uranium minerals during transportation, deposition,
ticipated in liberation of uranium minerals from sourceand diagenesis.
rocks cannot be excluded. Therefore, accepting SilverThese characteristics support the idea of a
man and Minoz' (1970) conclusions on similar situations, sedimentary-syngenetic origin for the Elliot Lake ores.
one can postulate that the liberation of uraninite from However, more detailed studies are necessary to obtainthe source rocks might also be caused by fungal attacks. a complete model of the physical, chemical, and organic
The abundant pyrite is apparently partly of detrital mechanisms involved in formation of this type ofand partly of epigenetic origin (Roscoe, 1969). In addi­ deposit.
tion to pyrite derived from Archean rocks, the sulphurThe author proposes a term for uranium deposits ofsaturated fluids from contemporaneous volcanic activity this type as "lower Proterozoic oligomictic pyritic con­apparently enabled crystallization of pyrite in or im­ glomerates with syngenetic mineralization."
mediately adjacent to the volcanic centres. Such fluids
may also have introduced pyrite into the transported SOME FEATURES OF THE POST-HURONIAN
detritus, allowed crystallization of pyrite in situ within CONGLOMERATES IN THE URAL MOUNTAINS
the deposit, and affected the detrital minerals by
AND ENISEY CREST, U.S.S.R.
sulphurization during transportation and deposition.
Auriferous and uraniferous conglomerates in theSome sulphur could also have been derived from organic
matter or from meteoric sulphate solutions (Ferris and U.S.S.R. have been recently most comprehensively
Ruud, 1971). It is the author's opinion that some of the described by Krendelev (1974), Khvostova (1969),
pyrite exhibiting abnormal magnetic susceptibility in the Oslopovskikh (1968) and Krendelev and Zlobin (1972).

METALLOGENIC FEATURES, HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATES

,

.",

V5

,. ?
UR

. "V *•

„.

HC

UR
HC

FIGURE 5.—Photomicrographs of a portion of the uraninite-hydrocarbon band labeled UR-HC on figure 4 (UR=uraninite, HC= hydrocarbon).
A, under parallel nicols. B, under crossed nicols. Reflected light. (Identification and photos by S. Kaiman, Canada Centre for Mineral andEnergy Technology.)

FIGURE 6.—Photomicrographs of a portion of the uraninite-hydrocarbon band labeled UR-HC on figure 4 (UR=uraninite, HC = hydrocarbon,
M0 = monazite). A, under parallel nicols. B, under crossed nicols. Reflected light. (Identification and photo by S. Kaiman, Canada Centre forMineral and Energy Technology.)

Several deposits containing gold or uranium, such asLaba, Baley, Kara-Su, Khuzhur, Timan, Panimba, andEastern Ural Mountain, have been described in the Rus­
sian literature. Among these deposits, the Eastern Ural
Mountain deposit and the Panimba occurrences containsome uranium.
The Eastern Ural Mountain deposit occurs in theLower Cambrian. The ore-bearing horizon unconformably overlies Precambrian gneisses, which have been
metamorphosed to the garnet facies. The basement
rocks also comprise effusives of the rhyolite-dacitegroup and intrusives of the biotite-microcline granite
group. A regolith as much as 10 m thick is developed on
the basement rocks (Krendelev, 1974).

The Cambrian sediments that host the uranium
mineralization are usually classified into two groups: thelower, approximately 1,000 m thick, consisting of quartzites with intercalations of argillites and conglomerates,
and the upper, 500-1,000 m thick, consisting of argillites
with intercalations of quartzites and tuffs.
Uranium mineralization is confined mainly to basal
and intraformational conglomerates. The basal con­glomerates (0.3-1.0 m thick) are polymictic, carrying
dominant quartz pebbles and minor granite and quartzite pebbles. The intraformational conglomerates aremainly oligomictic and monomictic with local thickmasses ranging from 3.0 to 9.0 m. The monomictic por­tions of the conglomerates are, as a rule, more

V6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 7.—Photomicrograph of a portion of the uraninite-hydrocarbon
band labeled UR-HC on figure 4 (UR = uraninite, HC = hydrocarbon,
PO = pyrrhotite). Reflected light. (Identification and photo by S.
Kaiman, Canada Centre for Mineral and Energy Technology.)

mineralized than are the oligomictic parts. In both cases
uranium, thorium, rare-earth elements, niobium,
titanium, and zirconium mineralization is exclusively
confined to the matrix.
The heavy-mineral assemblage of the conglomerates
consists of zircon, malacon, thorite, uranothorite,
uranothorianite, allanite, ilmenite, rutile, brannerite(T),hematite, magnetite, pyrite, and other minerals, pyrite
being only a minor constituent. Pitchblende and gold
have not been reported. The uranium/thorium ratios
range generally from 1:1 to 1:18, and in those parts
directly overlying the basement granites, from 3:1 to
1:10. Concentrations of the ore minerals also occur inveinlets and in metasomatic disseminations. Tonnages
and grades of the deposit have not been disclosed.
The genesis of the Eastern Ural Mountain deposit has
been a subject of discussion. Shcherbin (1968a, b) cor­
related genesis of the deposit with the genesis of the"ancient" (that is, lower Proterozoic) uraniferous pyritic
quartz-pebble conglomerates and classified the deposit
as an "ancient placer" type.
On the other hand, Krendelev (1974) opposed Shcher­bin and summarized his own opinion as follows:
1. The mineralization is confined to lower Paleozoic
sediments (and in this respect differs from the
Elliot Lake-Rand type).
2. The area with the mineralization is spatially related
to a deep-seated fault.
3. Volcanic activity affected the ore-bearing beds aswell as the older (foot-wall) and younger (hangingwall) rocks.
4. The host rocks and the underlying complexes, whichcontain intrusive and extrusive rocks, have beenregionally metamorphosed. Metamorphic events

caused mobilization and redistribution of all rockforming components.
5. Locally, contact metamorphism took place.
6. Ore mineralization in the conglomerates, quartzites,
and acidic extrusives is apparently related to theepidote-amphibolite state of metamorphism. The
metamorphic events took place 239 + 9 million
years ago (and are therefore younger than the hostrocks).
7. Distribution of secondary rock-forming minerals,
such as twinned microcline, albite, muscovite, andblue quartz, coincides apparently with the distribu­
tion of the ore mineralization.
8. Because of these facts and certain typomorphic
features of the ore-forming minerals, the oreforming processes can be interpreted as epigenetic.
Thus the syngenetic theory, as formulated by
Shcherbin (1968b), should be considered as notproven.
The second deposit, Panimba, occurs within the cen­tral auriferous belt of the Enisey Crest in the Central
Asian part of the U.S.S.R., where other uranium occur­
rences are also known (Ruzicka, 1971).
The uraniferous and rare-earth-element-bearing con­glomerate occurs in a limb of an anticline, the core ofwhich consists of a sedimentary sequence of limy shales,
quartzites, and breccias. The limb contains beds of con­glomerates intercalated in quartzites and black shales.
The rocks are of upper Proterozoic age and are highly
metamorphosed.
The mineralized conglomerate is between 5 and 10 mthick and extends along strike for several kilometres.
The most abundant minerals present are pyrite, pyrrhotite, marcasite, chalcopyrite, and sphalerite. Gold inindividual samples grades 1.55 gram per ton; nickel,
0.045 percent; and cobalt, 0.059 percent. Uranium andthorium are present, but their amounts have not been
disclosed.
The genesis of the Panimba deposit has been inter­
preted by Krendelev (1974) and can be summarized asfollows:
1. The origin of the mineralization has been apparently
related to the magmatic processes of a granite
intrusion.
2. During the period of magmatic and postmagmatic ac­
tivity the rocks have been decarbonatized andaltered by magnesian metamorphism, during whichsulphur was mobilized from organic matter andredistributed.
3. Permeable rocks, such as conglomerates, served aschannels for mineralizing fluids.
4. Mineralization is therefore apparently of epigenetic
origin and not necessarily of syngeneticsedimentary (that is, placer) type.

METALLOGENIC FEATURES, HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATESThe author of this paper proposed the definition of the
Panimba and similar uranium conglomeratic deposits as"post-lower Proterozoic conglomerates with epigenetic
mineralization."V7

3. The critical stage in the epigenetic process in­volved mobilization of stage 2 uranium and its
redeposition, governed by hydrodynamic gra­dients, related to erosional, tectonic,
magmatic, or metamorphic processes active
CONCLUSIONS
in the Earth's crust. During this process theconglomerates served as channels for
Comparisons between the two genetic types ofpenetration of the uranium-bearing fluids.
uraniferous conglomerates allows generalization of
4. Deposition of uranium took place by one or
their basic features as follows:
more of the epigenetic processes: adsorption,
A. Lower Proterozoic pyritic oligomictic conglomerates
complexing, precipitation, or redox changes
with syngenetic mineralization:according to solubility and stability degrees of
1. The source area contains rocks with abnormal
uranium compounds.
clarkes of radioelements and with uranium5. Preservation of primary mineralization wasbearing minerals.
possible beneath the zone of oxidation.
2. Liberation of the uranium minerals took place
These conclusions fully support criteria onby physical, chemical, and possibly also metallogeny of uranium as formulated by the sym­
biological mechanisms under oxygen-deficient posium of the International Atomic Energy Agency
conditions.
(1974) on the formation of uranium deposits. Further­
3. Transportation of the uranium minerals in the more, they allow formulation of definitions
detritus occurred under oxygen-deficient con­ distinguishing syngenetic and epigenetic mineralization
ditions, namely in an oxygen-deficient at­
in uraniferous conglomerates.
mosphere combined with volcanic andpostvolcanic exhalation. Some uranium
REFERENCES
minerals were probably transported assuspensions in colloidal organic matter.
Darnley, A. G., Charbonneau, B. W., and Richardson, L. A., 1977,
4. Concentration and deposition of uranium andDistribution of uranium in rocks as a guide to the recognition ofaccompanying minerals were governed byuraniferous regions, in Recognition and evaluation of uraniferous
areas: International Atomic Energy Agency, Vienna, 17-21
hydraulic action of streams. Sedimentation
November 1975, Proceedings, p. 55-83.
took place at a time when primitive organisms
Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: Its occurrence andalready existed.
recognition by microprobe: Colorado School Mines Quarterly, v.
5. Excessive sulphur, a product of the associated66, no. 4, 35 p.
volcanic activity, caused sulphurization of Hallbauer, D. K., 1975, The plant origin of the Witwatersrand"carbon": Minerals Science and Engineering, v. 7, no. 2, p.
some minerals and hydrocarbon, the latter
111-131.
being derived from organic and possibly also
Hartman, F. H., and Wyman, R. A., 1970, Concentration of uranium
inorganic material.
minerals from Canadian ores by magnetic means: Canada Depart­
6. Preservation of concentrations of uranium
ment of Energy, Mines and Resources, Mines Branch Technicalminerals was possible, with local redistribu­
Bulletin TB 118, 36 p.
tion of uranium due to the presence of reduc­ International Atomic Energy Agency, 1974, Formation of uranium ore
deposits: Proceedings of a symposium held in Athens, Greece,
ing agents in the deposit and by burial of the
6-10 May, 1974: Vienna, International Atomic Energy Agency.
mineralized beds.
Khvostova, V. A., 1969, 0 redkometalnoy mineralizacii v metamorB. Post-lower Proterozoic conglomerates with epi­
ficheskikh konglomeratakh odnogo iz rayonov Urala [Rare metal
genetic mineralization:mineralization in metamorphosed conglomerates from one region
in Ural Mountains], in Redkometalnost nekotorykh metamor1. Deposits occur in areas containing rocks withficheskikh i granitizirovannykh osadochnykh komplexov: Nauka,
higher uranium clarkes. Deposition of
p. 36-57.
uranium in some of the source rock units Krendelev,
F. P., 1974, Metallonosnye konglomeraty mira
could have been originally syngenetic.
[Metalliferous conglomerates of the world]: Novosibirsk, Nauka,
2. The first stage in redistribution of uranium
240 p.
from the source area apparently involved Krendelev, F. P., and Zlobin, V. A., 1972, Kolchedannoe orudenenie v
dokembriyskikh konglomeratakh Eniseyskogo kryazha [Massive
transport of uranium from source to the sitesulphide ore-mineralization in Precambrian conglomerates of theof deposition in solutions or in refractory
Enisey Crest]: Akademiya Nauk SSSR Doklady, 1972, v. 205, no.
minerals. The process presumably resulted in6, p. 1428-1431.
a further local concentration of uranium in McKinney, J. S., and others, 1964, Geology of the Anglo-American
Group mines in the Welkom area, Orange Free State Goldfield, inthe environment.

V8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Haughton, S. H., ed., The geology of some ore deposits in southern
Africa: Geological Society of South Africa, v. 1, p. 451-506.
Oslopovskikh, V. N., 1968, Metodika poiskov drevnykh
metallonosnykh konglomeratov [Methodology of exploration for
ancient metalliferous conglomerates]: Vysshoye Uchebnoye
Zavedeniye, Izvestiya, Geologiya i Razvedka, no. 5, p. 72-81.
Pretorius, D. A., 1975, The depositional environment of the Witwatersrand goldfields: A chronological review of speculations andobservations: Minerals Science and Engineering, v. 7, no. 1, 1975,
p. 18-47.
Ramdohr, Paul, 1958, Die "Pronto-Reaktion": Neues Jahrbuch fur
Mineralogie, Monatshefte, Jahrgang 1957, v. 10-11, p. 217-222.
Richardson, K. A., Holman, P. B., and Elliott, B. E., 1975, Airborne
radiometric map, Blind River, Ontario, 41J; a set of maps
1:250,000 showing distribution of integral counts, uranium,
thorium and potassium and their ratios: Canada Geological Survey
Open File 262, May, 1975.
Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
the Canadian Shield: Canada Geological Survey Paper 68-40, 205
P———1973, The Huronian supergroup, a Paleoaphebian succession
showing evidence of atmospheric evolution, in Young, G. M., ed.,Huronian stratigraphy and sedimentation: Geological Associationof Canada Special Paper no. 12, p. 31-47.
Roscoe, S. M., and Steacy, H. R., 1958, On the geology and radioactive

deposits of Blind River Region, in United Nations, Survey of raw
material resources: International Conference on the Peaceful Usesof Atomic Energy, 2d, Geneva, Switzerland, 1-13 September
1958, Proceedings, v. 2, p. 475-483.
Ruzicka, Vladimir, 1971, Geological comparison between East Euro­pean and Canadian uranium deposits: Canada Geological Survey
Paper 70-48.
Ruzicka, Vladimir, and Steacy, H. R., 1976, Some sedimentaryfeatures of conglomeratic uranium ore from Elliot Lake, Ontario:
Canada Geological Survey Paper 76-1A, p. 343-346.
Shcherbin, S. S., 1968a, Rudnye formacii v drevnykh metamorfizovannykh grubooblomochnykh porodakh [Ore formations in ancient
metamorphosed rudites], in Geologiya i poleznye iskopaemye
Urala: Sverdlovsk.
———1968b, Geologicheskie usloviya formirovaniya i lokalizacii
radioactivnoredkometalnogo orudeneniya v drevnykh
konglomeratkh [Geologic conditions for formation and localizationof radioactive rare-earth ore mineralization in ancient con­glomerates], in Vol'fson, F. I., ed., Geologiya i voprosy genezisa
endogennykh uranovykh mestozozhdeniy: Moscow.
Silverman, M. P., and Minoz, E. F., 1970, Fungal attack on rock:
Solubilization and infrared spectra: Science, v. 169, p. 985-997.
Theis, N., 1976, Uranium-bearing and associated minerals in theirgeochemical and sedimentological context, Elliot Lake, Ontario:
Queen's University, unpublished Ph.D. thesis, 158 p.

Temporal and Other Factors Affecting
Deposition of Uraniferous Conglomerates
By STUART M. ROSCOE
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ - PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-W

CONTENTS
Page

Abstract ________________________________________________
Introduction _______________________________________________
Banket-bearing successions _______________________________________
Distribution and lithologies ____________________________________
Correlations in North America __________________________________
Age __________________________________________________
Early Proterozoic successions lacking bankets ____________________________
Depositional environments of bankets near Elliot Lake_______________________
Lithostratigraphy of the Elliot Lake Group ___________________________
Matinenda Formation _______________________________________
Conglomeratic zones ________________________________________
Dollyberry basalts _________________________________________
Depositional history: A conceptual model ____________________________
Comparison of older and younger terriginous sediments ______________________
Regional tectonic settings of bankets _________________________________
Discussion _________________________________________________
References cited ____________________________________________

Wl
1
2
2
2
4
5
8
8
8
9
11
12
13
14
15
16

ILLUSTRATIONSPage

FIGURES 1, 2. Maps showing;
1. Occurrences of pyritic uraniferous quartz-pebble conglomerates in North America ____________________
2. Geology of pyritic conglomerate beds in the Montgomery Lake Group, Keewatin District, Northwest Territory,
Canada; cross sections _____________________________________________________
3. Line drawing and map showing statigraphic relations in Elliot Lake Group near Elliot Lake __________________
4. Line drawing showing subsurface distribution of conglomeratic and volcanic strata in the Elliot Lake Group near Elliot
Lake ____________________________________________________________________

W3
6
9
10

TABLE
Page

TABLE 1. North American successions containing bankets _______________________________________________
ill

W4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTEMPORAL AND OTHER FACTORS AFFECTING DEPOSITION OF
URANIFEROUS CONGLOMERATESBy STUART M. ROSCOE 1

uranium and gold, aggregating very large proportionsof the world's resources of these metals. This isremarkable, as other types of uranium and gold deposits
are vastly more numerous and occur in geological situa­
tions that are widespread. Moreover, all known bankets,invariably containing a distinctive suite of enriched con­stituents including uranium and gold, are in successions
that are either reasonably reliably dated or interpretable
as more than about 2.3 aeons (109 years) old. They con­tain the coarsest fractions of detritus derived fromweathering of extensive granitoid terrains. Any such
sediments this old contain disseminated pyrite, and peb­bly layers within them contain coarser grains of pyrite
along with rutile, zircon, monazite, other undoubtedly
detrital heavy minerals, and gold and uranium minerals.
Pyrite has not been found to occur in this manner inotherwise-similar younger sediments although theyounger sediments are collectively many orders ofmagnitude more abundant than the scattered remnantsof pre-2.3 aeon quartzose sandstones.
Quartzose elastics deposited less than 2.3 aeons ago
contain fine-grained interstitial ferric oxide and heavy
detrital iron oxides in lieu of pyrite. Clearly, theseyounger deposits have had different histories ofweathering, sedimentation, diagenesis, metamorphism,or epigenetic modifications than have the older ones.
The change in sediment type is coincident with a change
in character of paleosols from pyritic ones depleted iniron and particularly ferric oxide to ones showing an up­
ward
enrichment in ferric oxide. This change preceded
INTRODUCTION
the first widespread depositions of dolomite and the
The pyritic quartz-pebble conglomerates, or bankets, most voluminous world-wide depositions of ferric iron inconsidered in this publication have been reported in banded iron-formations. Many authorities interested in
fewer than a dozen separate districts in the world, yet genesis of iron-formations (Bayley and James, 1973;
they include the most extensive known deposits of Beukes, 1973), in atmospheric and biological evolution(Oparin, 1938; Rubey, 1955; Cloud, 1973; Holland, 1973;
1 Consulting geologist, 111 Kamloops Avenue, Ottawa, Canada. Since May, 1976, Regionaland others), and in uranium deposits (Roscoe, 1969)
Metallogenist, Northwestern Canadian Shield, Geological Survey of Canada.
ABSTRACTUraniferous, auriferous, pyritic quartz-pebble conglomerates,
unknown in strata less than 2.3 billion years old, are considered preoxyatmoversion analogues of black-sand placers common in younger ter­
restrial sediments. Radioactive conglomerates in Huronian rocks near
Elliot Lake, and others, are within the most proximal facies of onceextensive sheets of terrigenous elastics derived from soil mantles andchemically mature colluvium that covered Archean protocontinents
weathered under an atmosphere that lacked free oxygen.
The initial entrapment of quartz-gravel-bearing grits in Elliot LakeGroup, as well as subsequent deposition, deformation, and preserva­
tion of Huronian rocks, was governed by the development of tectonically unstable conditions in an epicontinental sector of the Superior
Craton. Warping of the southerly inclined paleoslope, adjustments
along old fractures, and eruptions of basalt were accompanied bydeposition of coarse clastic wedges or alluvial fans atop soil-mantledArchean basement rocks, weathered Huronian basalt flows, and finer
grained sands that flanked or overlay other gritty conglomeratebearing lenses.
Successions resembling the Huronian Supergroup are found west of
Hudson Bay, in Wyoming, and in central Quebec, suggesting that tec­
tonic traps for coarse elastics with pyritic conglomerates developed ex­tensively around a more stable core of the Archean protocontinent
about 2.5 billion years ago. Comparable tectonic conditions developed
earlier in South Africa, where bankets occur in strata 2,8 to 2.3 billion
years old. Possibilities of discovering other occurrences of this type inNorth American rocks older than 2.6 billion years are remote, but one
occurrence in Quebec may be in this category. Favourable areas are
the Huronian Supergroup, other remnants of Paleoaphebian strata(2.6 to 2.3 billion years), and highly metamorphosed equivalents ofthese. Many of these remnants are buried under more extensively
preserved younger Proterozoic strata, but the Paleoaphebian rocks
should not be regarded as basal phases of the Proterozoic tectonostratigraphic units.

Wl

W2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

have considered that these phenomena reflect a change
in the Earth's atmosphere from one lacking free oxygen
to one containing free oxygen. I have suggested theterm "oxyatmoversion" for this change (Roscoe, 1973).
Skeptics hinge their doubts about this hypothesis on
several incorrect ideas. Foremost amongst these is thebelief that an atmosphere containing free oxygen is as
essential a condition for the chemical precipitation of
ferric iron in Archean iron-formations as it is for the sur­vival and deposition of detrital and authigenic hematite
in continental red beds (see Davidson, 1965). This belief
requires a further unwarranted assumption that thepresence of pyrite in lieu of hematite is in itself sufficientreason to suppose that banket-bearing successions were
deposited not subaerially under high-energy conditions
but in euxinic marine environments (see Pettijohn,1970). Alternatively, it has been supposed that the
pyritic rather than hematitic character of the strata
must be due to metamorphism or regionally pervasive
metasomatic processes. Other critics purport that the
hypothesized change in the atmosphere is based only on
inconclusive evidence that uraninite and pyrite occur informs resembling detrital grains. This attack can be dis­counted, as it ignores other evidence such as hydraulicequivalent sizes of minerals in bankets, variations incompositions amongst associated uraninite and pyrite
grains, penecontemporaneous displacements of some ofthese grains during compaction, the iron-depletedcharacter of paleosols, and the pyritic and nonhematitic
character of host strata evidently deposited subaerially.
Knowledge of environments of deposition of various
pyritic conglomerates is thus important not only as an
exploration guide but also as a critical factor in decipher­ing atmospheric evolution. New data presented hereinshow that major conglomeratic zones near Elliot Lakewere deposited subaerially in headward, largely
distributary, sections of swift, necessarily aerated
streams-a most inhospitable environment for accumu­
lation of organic material. Pyritic conglomerates occur
in a range of geological environments, as do black-sandplacers. Differences in chemical and textural maturity of
sediments, associations with shale, greywacke, orvolcanics, or differences in metamorphism have
nowhere been found to determine that pyrite rather
than iron oxides dominate the heavy-mineral or
diagenetic-mineral assemblages.
BANKET-BEARING SUCCESSIONS
DISTRIBUTION AND LITHOLOGIESUraniferous and auriferous pyritic conglomerates are
known in South Africa, in the Fortesque Group in
Western Australia, at Serra de Jacobina in Brazil, in the
Huronian Supergroup north of Lake Huron, at Sakami
Lake 90 miles (145 km) east of James Bay, and at Padlei(Montgomery Lake) 100 miles (160 km) west of Hudson
Bay. In addition, the Deep Lake Formation in the
Medicine Bow Mountains in southeast Wyoming con­tains limonitic, slightly radioactive, slightly auriferous
quartz-pebble layers that represent bankets in which
pyrite has been almost entirely destroyed by weathering
and uranium minerals leached in surface exposures. The
North American localities are shown in figure 1.
Stratigraphic data for a number of these areas havebeen summarized by Roscoe (1973, fig. 3), with available
isotopic dates of basements, banket host strata, andof red beds, black sands, iron-formations, and uncon­
formities. Some of this information showing common
mities. Some of this information showing common
features amongst banket-bearing successions is outlined
more specifically below.
The bankets occur in slightly feldspathic to highly
feldspathic argillaceous quartzites, which dominate thehost successions. Volcanics form important parts ofmost successions and are present in all, with the possible
exception of Jacobina, which contains serpentinite andamphibolite bodies. Serpentinite is also present at
Sakami Lake. All successions contain argillaceous units.
Tillites are present in the Huronian, Medicine Bow,
Padlei, and possibly the Witwatersrand sequences. The
Witwatersrand is particularly thick and varied. It is
underlain by the dominantly volcanic Dominion Reef
and overlain by Ventersdorp lavas; both containbankets.
CORRELATIONS IN NORTH AMERICA

The Huronian Supergroup (A), the Deep Lake Forma­tion and an overlying section of the Libby Group in
Wyoming (B), and the Montgomery Lake Group
together with the lower part of the Hurwitz Group inDistrict of Keewatin (C) are compared in table 1.
Blackwelder (1926) noted similarities between Medicine
Bow and Huronian rocks and, in particular, considered
the Headquarters Schist to be a tillite likely correlativewith the Gowganda Formation. Huston (1968), on thebasis of such comparisons, suggested that economicallysignificant paleoplacers might be found in the Deep
Lake Formation. Young (1973) concluded that the uppergroups in the three areas showed evidence of climaticchanges so similar and uncommon that he believed theymust be coeval. In 1973, I found that limonitic quartzpebble layers at several places and at different
stratigraphic positions within the Deep Lake Formation
contained uranium, thorium, gold, titanium, and otherminor constituents in amounts much like those in weakly
radioactive pyritic small pebble beds of the Mississagi
Formation but in amounts different from those in blacksand-bearing pebble beds. Thus the Deep Lake Forma­tion contains bankets, although pyrite and no doubt

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATESW3

FIGURE 1.-Occurrences of pyritic uraniferous quartz-pebble conglomerates in North America. The distribution of Archean rocks at time ofdeposition of earliest Proterozoic rocks is indicated by the vertical pattern on the Superior, Slave, and Wyoming provinces and on other areas
where rocks metamorphosed in Proterozoic time evidently included Archean basement. Most intervening blank areas were likely also part of
the Archean craton, with possible exceptions of areas containing Aphebian volcanic rocks (V). Arrows indicate directions of sedimenttransport.

W4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTABLE I.-North American successions containing bankets
Un,lThickness(kilometers)

Lith( ,| OKV
M

Unit

Huronian Supergroup, Ontario
Cobalt Croup
Bar River Formation _

4

-9

1

Orthoquartzite.

Thickness(kilometers)

Lithology'

Medicine Bow Mountains, Wyoming
Libhy Group (Lowerpart).
Surgarloaf Formation _

3-7
~1.5

Orthoquartzite.

Gordon Lake Formation.3 - .9

Varicolored, siliceous Lookout Formation __siltstone.

~.5

Varicolored, siltstone.

Lorrain Formation __.9 -2.0

Quartzite'.

.9 -1.5

Medicine Peak Formation.
Heart Formation ___~2.5

Gowganda Formation. _Orthoquartzite.
Subarkose. 1
Arkose, siltstone 1.
Orthoconglomerate.
Paraconglomerate.

~1.0

Arkose.'Headquarters Formation~1.0

Siltstone.
Paraconglomerate.

Deep Lake Formation _>!.()Quirke Lake (Iroup _____..r> -1.5

Serpent Formation __.25- .9

Espanola Formal ion __.2 - .5

Bruce Formation.05- .2

Hough Cti-oup

Arkose.
Suharkose.
Siltstone.
Limestone.
Paraconglomerate.

.3 -2.0.2 -1.2.0:5- .9

Subarkose 1
Siltstone.

Ramsey Lake Format ion.01- .2

Paraconglomerale.

0
0

-2.5
-1.0

Siltstone.

Metmenda Format ion _

0

- .3

Suharkose 1 .

Various volcanic units _Various arenite units _

II
0

-1.2
- .5

Basalt 1
Subarkose/

Thickness ; ((kilometer
Padlei area, Keewatin District

Hurwitz Group (lowerpart).

1.0 -3.5

Kinga Formation;
Whiterock Member__
Ma?use Member.2 - .3.3 -1.0

Padeli Formation ___Montgomery LakeGroup.
Quartzite.
Pebbly quartzite 1 .
Limestone and.
Paraconglomeratt1 .

Lithology.3 - .50Fine-grained orthoquartzite, white.
pink.
Orthoquartzite, subarkose. 'Paraconglomerate.
Siltstone.

-l.fi
Quartzite.
Subarkose/
Siltstone.
Subarkose/Para- and orthoconglomerate.

(central part)

Mississagi Formation _I'ecors Formation _____

Flliot Lake Group
McKim Formation(upper part)Unit(lower part)

yuartzite.
Subarkose/
Siltstone.
Paraconglomerate.

BasaK.
Basalt conglomerate.
Paraconglomerate.
Siltstone.
Subarkose/

Quart/Me.
Subarkose/Acid volcamcs.

2.29±.09 aeons ago (Fairbairn and others, 1969) accor­
ding to Rb:Sr isochron dates (1.39 decay constant) on
volcanics in the Elliot Lake Group and argillaceous
sediments in the stratigraphically higher Cobalt Group.
The 2.35-aeon date is on Coppercliff rhyolite and it isconsidered a minimum date, as several Rb:Sr isochrons
on basalts in the Elliot Lake Group yield poorly defined
dates ranging from 2.2 to 1.9 aeons (Knight,
1966)-doubtlessly variably reduced due to metamorphism that must have affected the more rubidium-richrhyolite as well, but to a lesser extent. The Huronianrocks are intruded by Nipissing diabase dated as 2.16
aeons old (Van Schmus, 1965; Fairbairn and others,1969) and by the Murray granite, 2.23 aeons old (Gibbins
and others, 1972). The 2.29-aeon date on Gowganda
argillite is especially important as the Gowganda For­mation contains the oldest reported red beds-hematitic
shales and arkoses-and it is overlain by Lorrain quartzite, which is also hematitic, not pyritic, and which con­AGE
tains the oldest reported black sands.
The banket-bearing lower parts of the Huronian suc­Banket-bearing strata unconformably overlie Archean
cession (Elliot Lake and Hough Groups) were deposited
during an interval that began more than 2.35 aeons ago rocks 2.5-3.0 aeons old, excepting the vertically dipping(Fairbairn and others, 1965) and ended no less than metastrata at Sakami Lake whose relation to adjacentmuch uranium have been weathered and leached fromoutcrops. Furthermore, Deep Lake lithologies are in­distinguishable from those in the Huronian sequence
beneath the Gowganda Formation, and they are also
stratigraphically ordered in the same sequence. The two
successions, dominated by various feldspathic quartzites, are both interrupted sequentially first by basalt,then by tillitelike paraconglomerate with argillite, andthen by another paraconglomerate unit with limestoneand siltstone, and finally both are capped by a thicktillite-bearing unit. No limestone unit has been found in
Montgomery Lake rocks (fig. 2), but otherwise these are
not much different.
Insofar as these very distinctive successions were pro­
duced by effects of sequential changes in climate in com­bination with epeirogenic pulses, it seems most unlikelythat they could have been formed at different times.

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES

rocks, which are generally believed to be 2.45-2.75
aeons old, has not been described. The basement inSouth Africa is at least 0.1 aeon older than basements
beneath the Huronian, Deep Lake, and Padlei succes­
sions. Bankets in the African successions were
deposited through an interval extending from 2.8 to 2.3
aeons (Niekerk and Burger, 1966, 1969). Volcanics
overlying bankets in the Fortesque Group of Australia
have been dated isotopically at approximately 2.2 aeons(Trendall, 1969).
Evidence that the Deep Lake Formation and theMontgomery Lake Group are correlative with the lower
groups of the Huronian Supergroup has been presented
above. If this is accepted, it can be concluded that all
North American as well as South African and Austral­
ian bankets are in strata more than 2.2-2.3 aeons old.
There is no reason to suppose that the remaining known
bankets-those in Brazil-are any younger. They arecertainly more than 1.8 aeons old (Ferreira, 1972) andare most likely older than nearby sequences containing
extensive banded iron-formations like those inAustralia, North America, and South Africa, which areabout 2.0 aeons old.
EARLY PROTEROZOIC SUCCESSIONS LACKING BANKETSThe Cobalt, lower Libby, and lower Hurwitz Groups
contain minor black sands and red beds but lack bankets
and pyrtic arenites. Otherwise, they are similar to lower
Huronian Groups and Deep Lake cycles, which consist ofbasal paraconglomerate units overlain by fine-grained
sediments and then by pyritic quartzites with bankets.
The difference cannot be attributed to metamorphism as
both upper and lower sequences have been metamor­
phosed together to greatly different extents in different
areas. Thus we have evidence in three widely separated
areas that coarse arenites deposited more than about 2.3
aeons ago contain interstitial pyrite rather thanhematite and contain pyritic-or "yellow sand"-heavymineral accumulations (bankets) rather than blacksands, whereas the reverse is true of younger arenites.
The only cause that has been suggested for this is achange from an atmosphere lacking stable free oxygen
to one containing significant amounts of oxygen(Roscoe, 1969, p. 83). Such a change should have been
worldwide and probably instantaneous in terms of
isotopically resolvable geological time. To test this wemust examine the important early Proterozoic succes­
sions that were deposited after the banket-bearing suc­cessions and that are preserved through much more ex­tensive areas.
The oldest post-Archean strata, apart from the few
known small remnants of 2.8-2.2-aeon banket-bearing
strata discussed above, are dominantly marine succes­
sions including dolomite, shale, banded iron-formation,

W5

quartzite, and volcanics deposited and deformed morethan 1.8 aeons ago. Clastics with red beds and blacksands are present in many of these successions, general­
ly in upper parts but in some cases in middle or basal sec­
tions. These rocks form some of the most extensive belts
of folded Precambrian rocks. They include the CircumUngava (Kaniapiskau, Mistassini, Belcher), Marquette
Range, Hamersley, Minas Gerais, Transvaal, and othersuccessions with iron-formations, which collectively con­tain the bulk of the world's iron resources. Some exten­
sive Aphebian successions 2.3-1.8 aeons old, however,lack significant iron-formations. Among these arevolcanic-sedimentary strata in Manitoba believed to beabout 1.85 aeons old, strata in the Coronation
Geosyncline in the northwesternmost part of the Cana­dian Shield, and other sequences in western Canada.
Lack of banded iron-formation is no doubt due to un­
favourable environmental factors in many cases, but it isdifficult to identify these. It is possible that some "bar­ren" successions were deposited before or after an op­
timum time period for deposition of banded ironformation, perhaps 2.1-2.0 aeons ago, as suggested byisotopic dates on volcanics associated with major ironformations (for example, Hamersley). Wherever ade­quate geological or isotopic evidence is available, therelatively common Aphebian rocks are found to besignificantly younger than the banket-bearing succes­
sions. Important examples are given in succeeding
paragraphs.
Huronian strata at Sudbury were folded, metamor­
phosed, intruded by Murray-Creighton granite 2.2 aeonsago (1.39 decay constant, Gibbons and others, 1972) anddeeply eroded prior to deposition of the WhitewaterGroup, which was intruded by the Sudbury Irruptive at
1.87 aeons (Krogh and Davis, 1974). In Michigan, 300
miles (480 km) west of Sudbury, the Marquette Range
Supergroup was deposited locally atop metamorphic andgranitic rocks about 2.1 aeons old, as well as on older
Archean rocks (Banks and Van Schmus, 1972). In cen­tral Quebec, 400 miles (640 km) northeast of Sudbury,
the Chibougamau Group, which is almost certainly cor­
relative with the Gowganda Formation, was faulted,
highly deformed, and eroded prior to deposition of the
Mistassini Group (Roscoe, 1969, p. 116, 117, fig. 9). The
upper part of the Libby Group in Wyoming contains
dolomite, black shale, shaly magnetite iron-formation,and volcanics, all similar to rocks in the Marquette
Range Supergroup. The metamorphosed banketbearing rocks at Sakami Lake are older than arkosic red
beds of the nearby, little-disturbed Sakami Formation,
which is probably correlative with red-bed- and blacksand-bearing elastics composing the lowest unit of the
Kaniapiskan Supergroup. The Transvaal System inSouth Africa unconformably overlies the banket-bearing

W6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 2.-Pyritic conglomerate beds in the Montgomery Lake Group, Keewatin District, N.W.T., Canada. A,
Geologic map. B, Cross section A-B. C, Cross section showing detail for diamond-drill holes 1, 2, and 3.

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATESEXPLANATIONFOR C

EXPLANATIONFOR A AND B

Feldspathic quartzite
and grit

Glacial deposits
HURWITZ GROUPAMETO FORMATION—Slate
KINGA FORMATION—Quartzite

Quartzite

Siltstone
PADLEI FORMATION—Polymict
conglomerate
MONTGOMERY LAKE GROUP

Polymict conglomerate

Quartzite, minor siltstone,
conglomerate

1CQ

UJ<
(X

Fine-grained quartzite
and siltstone

Quartzite, feldspathic grit, withlayers of pyritic quartz-pebble
conglomerate (-)

Pyritic quartz-pebble
conglomerate

Polymict ortho- and paraconglomerate
Grit layers
Quartzite, quartzose, medium grained;basal tuffaceous unit (v)3A3

Diamond-drill holes

After Bell 1970, Young 1970, and Roscoe 1971

Felsic tuff

GabbroGranodiorite, tonalite

2

&
X-

Metasedimentary rocks, graywacke,slate, iron-formation
Mafic volcanic rocks

Acid volcanic rocks
Strike and dip
Fault line
Diamond-drill holes
FIGURE 2.-Continued.

W7

W8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESBlind River, Elliot Lake, and Quirke Lake, but se­quences in flanking areas include basalt flows. A tongueof McKim Formation extends westward into this area
over some parts of the Matinenda Formation, and awedge
of Matinenda Formation extends easterly
DEPOSITIONAL ENVIRONMENTS OF BANKETS
towards
Sudbury between the volcanics and the McKim
NEAR ELLIOT LAKE
Formation.
In the western part of the belt, basalt flows
LITHOSTRATIGRAPHY OF THE ELLIOT LAKE GROUP
of the Thessalon Formation overlie subarkose of the Liv­
This volume provides an opportunity to outline some ingstone Creek Formation. The Elliot Lake rocks arepreviously unpublished concepts about the depositional bevelled by an erosion surface and are overlain by theenvironments of uranium-ore-bearing units near Elliot Ramsay Lake Formation, which is considered to be a
Lake. Data from deep exploratory drill holes west of the tillite.
mines, in particular, have shed new light on the history
MATINENDA FORMATION
of deposition of the Elliot Lake Group, strengthening
In the Elliot Lake area the Matinenda Formation has
the conclusion that the rocks were deposited subaerially.
The data cannot be outlined in detail here, nor is it possi­ been subdivided into three coarse-grained subarkose
ble to repeat stratigraphic and petrographic descrip­ units, successively the Ryan, Stinson, and Manfred
tions that may be found elsewhere. A brief outline of Members (Roscoe, 1969). A fourth lenticular subarkose
stratigraphic relations is presented, however, to set unit, the Keelor Member, has been identified 15-20
descriptions of relations in the Elliot Lake area in miles (23-32 km) west of Quirke Lake (figs. 3, 4). The
Ryan, Manfred, and Keelor Members contain abundant
perspective.
The uranium ores and principal prospects in the Huro- pyritic, radioactive quartz-pebble beds and are gritty,
nian belt are in the Matinenda Formation of the Elliot variegated, greenish coloured, and pebbly throughout.
Lake Group (table 1); pyritic uraniferous conglomerate The Stinson Member is pale grey, relatively well sorted,layers are also present, sparingly, in the Livingstone and contains conglomerate beds only at its base. These
Creek Formation, which is in the lower part of the Elliot beds include polymict, as well as oligomict, orthoconLake Group west of Elliot Lake, and within sequences of glomerates and they contain little pyrite or radio­
basalt flows in the Elliot Lake Group. The Mississagi activity. Some of the conglomerates, evidently
Formation of the overlying Hough Group also contains deposited as open-framework cobble gravels, containpyritic uraniferous pebbly layers. The two banket- authigenic quartz, carbonate, pyrrhotite, and pyrite.
bearing groups extend through a distance of some 200 The subarkose units are built of crossbeds that filled
miles (320 km) in the Sault Ste. Marie-Sudbury region, troughs, generally 1-4 ft (0.3-1.2 m) wide. Trends ofand their combined thickness is as great as 20,000 ft troughs and dips of foresets are unimodally southeast in
the eastern part of the area but are more southerly in(6,100 m) in the southeastern part of the belt.
The Elliot Lake Group includes all strata that noncon- outcrops 15 miles (24 km) west of Elliot Lake.
Basalt flows in the Elliot Lake Group west of Quirke
formably overlie Archean rocks and underlie theregionally extant Ramsay Lake Formation, a paracon- Lake have been called Dollyberry Lake volcanics
glomerate that forms the basal unit of the Hough Lake (Robertson, 1969). They are believed to have beenGroup. Major units of the Elliot Lake Group - volcanic erupted after deposition of the Ryan Member but priorrocks, coarse-grained subarkose and argillaceous to deposition of other Matinenda members. If therocks - are lenticular. Many of the outcrop areas of the general practice of assigning formational rank togroup are separated, one from the other, so that it is not volcanic units in the Elliot Lake Group, and of excluding
possible to establish the sequence of deposition of all nonarenaceous rocks from the Matinenda Formation,
were to be followed, it would create some confusion in
volcanics and arenites throughout the belt.
The McKim Formation, comprising fine-grained discussions. For this reason, the Dollyberry Lakeargillaceous quartzite, siltstone, and argillite, forms the volcanic unit is referred to herein as "Dollyberry basalt"
uppermost unit of the Elliot Lake Group in the eastern and treated, informally, as a member of the Matinenda
part of the belt near Sudbury. Here it is several thou­ Formation. Two other significant units are present insand feet thick, and it overlies several thousand feet of the area, but neither is along the line of section shown in
volcanic rocks, including the Stobie Formation-basalt figure 3. One is a wedge of volcanic rocks overlain by thewith some intercalated sediments - and the less exten­ Ryan Member east of the Nordic ore zone. Robertson
sive Coppercliff Rhyolite Formation. Subarkose called (1961) mapped this wedge as questionable Archean but
the Matinenda Formation (Roscoe, 1957a, b) forms most later (Robertson, 1973) he suggested that it is probablyor all of the group in some central parts of the belt near part of the Elliot Lake Group. The other additional unitgroups. In Australia the Hamersley Group overlies theFortesque Group and contains volcanics dated at 2.0
aeons (Trendall, 1969).

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATESEXPLANATION-4— Depth (feetx 1000)
to base of Ramsay
Lake Formation

W9

Quirke Lakeore zoneD

10I

20 MILESA

Manfred Member

Ramsay Lake Formation
Keelor Member
Archeangranitic rocks,
meta strata

Ryan Member

L-1000
0

I

I

1

1

5
1

i

10 MILESI

B
FIGURE 3. - Stratigraphic relations in Elliot Lake Group near Elliot Lake. A, Map showing outcrops of Elliot Lake Group and depth to the base of
the Ramsay Lake Formation. B, Schematic cross section near Elliot Lake showing relations among arenaceous members of the Matinenda
Formation, Dollyberry basalt, and McKim Formation in the Elliot Lake Group.

is a lens of poorly sorted arkosic rocks that outcrop
through a length of 5,000 ft (1,500 m) at Crazy Lake, 17
miles (27 km) west of Quirke Lake, along the north limbof the syncline. The Crazy Lake unit is truncated by the
Flack Lake Fault so its base is unknown, but it is as
much as 500 ft (150 m) thick. It is capped by erraticallyuraniferous, poorly sorted, pyritic oligomict con­glomerate and overlain by Dollyberry basalt.

CONGLOMERATIC ZONESThe uranium-bearing conglomeratic zones in theRyan, Manfred, and Keelor Members consist of quartzpebble conglomerate, pebbly grit, and coarse subarkose
interlensed through as much as 100 ft (30 m) of strata.
Pebble-rich sections, with lenses containing wellpacked, well-sorted pebbles, alternate with sections con-

W10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESN

10 MILESFIGURE 4.-Subsurface distribution of conglomeratic and volcanic strata in the Elliot Lake Group near Elliot Lake. Designations of units by
number, their stratigraphic positions, and depths to the base of the Ramsay Lake Formation are as shown in figure 3. Unit 4 is shown only
where it overlies Archean basement (1); elsewhere, it overlies unit 2 and southerly areas of unit 3, and it underlies southerly areas of units 5aand 5b. The latter units overlap unit 3. The most closely spaced dots indicate areas with the most abundant conglomerate beds. Arrows in­dicate paleocurrent directions. Contour-like lines of unit 3 approximate 500-ft (152-m) thickness intervals.

taining few pebbles. Some of the latter sections can betraced great distances and define stratigraphic levels
within the zones, perhaps better than the pebble-richsections, which locally are called "reefs." The reefs are
not layers but, rather, are concentrations of lenses con­taining well-packed pebbles. The conglomerate lenses
are crossbedded and are similar to troughs in pebblefree subarkose. They also have similar trends andforeset dips-southeasterly in the Ryan and Manfred
Members. Cross laminae are commonly graded, withpebbles at their bases. In some cases, pebbles evidently
rolled down successive foresets to form a basal zone ofpebbles imbricated downstream. Most of the longest
axes of pebbles are either normal or parallel to foreset
dips, indicating that many were "weathercocked,"rotated parallel to current flow, after they ceased roll­ing. Pebbles are similarly oriented where they are tight­ly packed in lenses that lack arenaceous or pyritic crosslaminae (Pienaar, 1963). It seems likely that these repre­
sent the lower parts of troughs whose upper, morearenaceous, parts were planed off. The richest reef sec­
tions, containing the greatest concentrations of pyrite
and other heavy-mineral grains, probably represent suc­cessions of such remnants of basal pebble-rich parts oftroughs.
Pebbles are generally smaller in successively higher
reefs and in each reef become progressively smaller inthe downstream direction. In reefs mined at Quirke
Lake they decrease from 5 cm or more in the most prox­
imal areas to 2 cm in distal areas less than 3 miles (5 km)
to the southeast. Pyrite grains show corresponding
decreases from about 1 mm or more to less than 0.3 mm.
Uraninite grains 0.05-0.20 mm (mostly 0.1 mm) indiameter are restricted to proximal conglomerates withpebbles larger than 3 cm. Other heavy minerals, mainly
titaniferous grains, monazite, and zircon, constitute lessthan 5 percent of total heavy minerals in the most prox­
imal, largest pebble conglomerates but increase in pro­
gressively more distal conglomerates to more than 15
percent. Titaniferous grains are as much as 1.5 mm indiameter in proximal conglomerates, less than 0.3 mm
in the most distal ones. Monazite grains, about 0.3 mm
in diameter, are most abundant, as abundant as titania
grains, in upper reefs and near the distal margin of the
Quirke Lake ore zone. Zircon grains, less than 0.3 mm indiameter, are most abundant in yet more distal, sub-

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES

marginal parts of the zone and in uppermost smallpebble conglomerate beds. Granules and smaller par­ticles of quartz, potash feldspar, and mica in matrices oflarge-pebble, as well as small-pebble, conglomerates arenotably poorly sorted in contrast to pebbles and heavy
minerals.
The conglomerate zones represent sites of remarkably
efficient hydrodynamic sorting, wherein the largest andheaviest mineral grains, including pyrite and uraninite,were concentrated in the most energetic environments
along with the largest quartz pebbles (Roscoe, 1969; N.
J. Theis, personal commun.). Individual rich conglom­
erate lenses were likely formed in part from reworked
detritus initially deposited upstream as pebbly grit, andmuch fine detritus was undoubtedly winnowed out of theconglomeratic zone as it was built up. Nevertheless, theconglomerates do not represent lag gravels in any strict
sense. All size ranges of detritus were transported anddeposited together. The tremendous overall abundanceof pebbles and heavy minerals in the zones must havebeen produced by concentration of these materials
throughout prolonged transport.
Proportions of gravel to sand carried into the depositional area fluctuated and diminished as the conglom­
eratic zones were built up. Initially, floods carrying
abundant coarse gravel alternated with floods carrying
only small pebbles in addition to abundant sand andfiner detritus. Stream competency waned as the zones
built up and finally only infrequent, exceptional floods
carried gravel onto upper parts of deposits as much as
300 ft (90 m) thick.
Two different types of conglomeratic zones can be dis­tinguished in the area, despite the fact that erosion in
Matinenda time, as well as later, has removed importantparts of all zones explored by drill holes. The Nordic(Ryan) zone rests directly on basement rocks and ap­
pears to have been deposited along a valley controlled bybasement topography and perhaps also by syndepositional faulting. The small Pronto deposit 15 miles (24km) south of Elliot Lake is probably a remnant of asimilar "valley" deposit. The much larger Quirke Lake
(Manfred) zone heads atop Archean rocks and theeastern margin of Dollyberry lava fields, extends
southward overstepping the northern limit of the Stinson Member, and spreads out and thins in a fanlike man­ner over the relatively smooth top surface of the Stinson. The conglomerate-rich Keelor Member has rela­tions similar to the Manfred Member at Quirke Lake. It
was evidently deposited as a series of coalescent fanswhere streams debouched from lava fields and Archean
uplands onto a more gently sloping surface of a Stinson
sand "plain," which was probably an older bajada
deposited by streams that did not carry gravel.
The Nordic zone, as outlined by the area mined within

Wll•two closely spaced reefs near its base, is about 4.5 miles(7.2 km) long and about 1 mile (1.6 km) wide, trending
southeast parallel to the paleocurrent direction in­
dicated by foreset dips. It is bounded to the east by apaleoridge formed by a band of Archean cherty ironformation. The main ore reefs abut against this paleorise. A stratigraphically higher reef with economic
potential, however, extends easterly over the higher
paleosurface and is overlain by subarkose containing
many thin subeconomic reefs. All of the conglomerate
beds feather out rapidly towards the west in thedownstream shadow of the "Summers High," a con­siderable hill of Archean granite that was not covered by
an appreciable thickness of Elliot Lake strata.
Sizes and proportions of heavy minerals vary withsizes of pebbles in the Nordic zone as they do in the
Quirke Lake zone, but the patterns of pebble-sizechanges within individual reefs in the two zones are
significantly different. There is little decrease in pebble
size from the most proximal to the most distal part of
the Nordic ore zone. Instead, pebble sizes and uraninite
abundances decrease laterally both east and west fromthe central axis of the zone. Ore throughout the lengthof the chutelike zone is like that found through a muchbroader, entirely proximal, part of the Quirke zone. The
eroded southerly extension of the zone, like its flanks,
must have included abundant smaller pebble con­glomerate with heavy-mineral suites relatively rich intitania, monazite, and zircon, similar to conglomerates
in the medial and distal parts of the Quirke Lake zone.
This suggests that a zone like the Nordic zone could havechanged downstream into one like the Quirke Lake
zone.
DOLLYBERRY BASALTS

The Dollyberry basalts require special consideration
because of their close spatial and temporal relation to
the uranium ore deposits near Elliot Lake. Rifting was
likely the main link between basalt eruptions and the
beginning of deposition of Huronian elastics. Tectonic
movements accompanying eruptions, including develop­
ment of fault scarps, as well as the lava fields or
volcanoes themselves, likely had important influenceson drainage systems and on locations of sites of gravel
deposition. The distribution of the lavas provides clues
concerning topography both prior and subsequent totheir eruption and thus contributes to interpretations
concerning locations and histories of gravel-bearing
paleostream systems. Some workers have suggested
that the uranium ores, or pyrite within them, might be
genetically and chemically related to contemporaneous
volcanism (Robertson, 1973; Ruzicka, this volume, chap.
V). This idea can be discounted for many reasons,

W12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

among them the evidence presented in following
paragraphs, that the Dollyberry eruptions were not con­temporaneous with deposition of any of the importantbankets at Elliot Lake. Moreover, bankets were
deposited in Hough Group rocks long after the last erup­
tions occurred in Huronian rocks.
The lavas are amygdaloidal, and closely spaced
scoriaceous and brecciated zones suggest that manyflows are thin. No pillows have been noted. The rocks
are dark grey to dark green for the most part, butbleached and argillized zones are found in manyscoriaceous and brecciated sections, presumably flow
tops. This alteration is similar to that found ingreenstone at the Archean-Huronian nonconformity
and to that found rimming greenstone and basalt clasts
within Huronian strata. It is considered to represent in­
terflow weathering of lava surfaces.
Dollyberry basalts outcrop along the north limb of the
Quirke Lake Syncline 4-15 miles (6.5-24 km) northwest
pf Quirke Lake. They overlie paleosol-mantled basement
rocks in this outcrop belt, except in its northwesternmost part, where they overlie more than 500 ft (150 m)
of Crazy Lake arkose and are truncated by the Flack
Lake Fault. The basalt pile is thicker, locally more than
1,000 ft (300 m) thick, along the outcrop belt than in sub­surface drill intersections to the south. A main lobe ex­tends 8 miles (13 km) south along the west flank of theSummers High, and a thinner lobe of flows extends
south along the east flank of this paleohigh (fig. 4). The
eastern lobe, as presently outlined, ends very near thenorthernmost known limit of the Ryan Member and theNordic ore zone. The lavas must have flowed southward
around the Summers High from eruptive centres near or
north of the Flack Lake Fault.
West of the Summers High, Dollyberry basalts thinsouthward at an average rate of more than 200 ft (60 m)
per mile. The surface of the lava fields, barring norther­ly tilting, must have had a correspondingly high slope
gradient toward the south when Stinson sands andKeelor gravels were deposited on their lower reaches. It
also had considerable local relief, as revealed by in­tersections in deep drill holes. In some of these,
Dollyberry lavas are overlain by Stinson subarkose con­taining few pebbles. In others, as much as 70 ft (21 m) ofbasalt breccia or sharp-stone conglomerate grade up in­
to basalt- and quartz-cobble conglomerate. The con­glomerate matrix contains quartz and carbonate, as wellas granules and argillitic detritus. It also contains con­centrations of pyrite or pyrrhotite with traces ofchalcopyrite not accompanied by radioactive minerals.
The sulphides are clearly authigenic and are most com­monly found at the top of the conglomerate or inseparate beds a few feet above it within Stinson
subarkose. Many basalt clasts are variably argillized. In

places, conglomerate with intensely altered clasts wascovered by a thin layer of green argillite, representing
redistributed residual clay, prior to deposition of Stinson
sand. It is noteworthy that iron sulphide, not ferric ox­
ide, is concentrated at these obviously weathered sur­faces and that uranium is not concentrated along withsulphides, as it is in bankets.
The basal conglomeratic section of the Stinson
Member contains abundant basalt clasts where it
overlies and channels the Ryan Member, as well as
where it overlies Dollyberry basalts. The abundant con­glomerates in the Ryan Member do not contain basalt
clasts, so the Ryan is considered older than the
Dollyberry basalts. Only a few holes have been drilled
through the basalts. Most of these passed directly into
paleosol-mantled granite, but several holes intersected a
few feet of sorted arkosic sediments with quartz clasts,
pyrite, and radioactive minerals between the basalt andthe paleosol. These sediments, and the thick Crazy Lakearkose, are likely remnants of alluvial deposits cor­
relative with the Ryan Member.
The eastern lava lobe is believed to have filled a broad
valley previously occupied by the stream system that
deposited the Ryan Member. The northern limit of thismember is not well defined; it is largely an erosionalrather than a depositional limit. Evident post-Dollyberryerosion may have obliterated evidence of more extensive
pre-Dollyberry erosion of the Ryan Member.
DEPOSITIONAL HISTORY: A CONCEPTUAL MODEL

Interlenticulated units forming the Elliot Lake Group
were deposited about 2.4 aeons ago on successive eroded
paleosurfaces that were shifted and warped in aneasterly-trending fissured zone within the Superior
Craton. The clastic sediments, largely clay and quartz
clasts, were shed southward from weathered crystalline
rocks in a more central part of the rising craton. The
coarsest detritus was entrapped in the rifted zone, andvolcanic rocks, principally basalts, were extruded along
fissures.
The Ryan Member, the oldest clastic unit in the Elliot
Lake mining area, was deposited by braided streams in avalley that trended and broadened southeastward. Early
Ryan deposition drew on a plentiful nearby quartzgravel supply that was then exhausted and not
replenished from distant sources for a very long time.
The coarsest gravels, containing abundant detrital
pyrite and uraninite, were deposited early along achutelike central section of the valley. Subsequently,
coarse sand and fine gravel were deposited over abroader area.
Uplift to the north cut off the supply of sand andresulted in scouring of Ryan sand and gravel from theFACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES

northern part of the valley. This was followed by erup­
tions of Dollyberry basalt 7-15 miles (11-24 km) north­west of the present position of Quirke Lake, producing a
volcanic edifice built up as much as 1,500 ft (450 m)
above its Archean floor. Lavas flowed southward onto
lower ground, filling the northern part of the Nordic
valley. The rough, locally steeply-sloping, lava surfaces
were weathered and eroded under anoxic conditions
during a period when little or no detritus was carried in­to the area.
Resumption of southerly detrital transport was ac­
companied by washing, or continued washing, ofpetromict gravels from nearby Dollyberry and Archean
uplands onto the scoured and locally channelled surfaceof the Ryan Member, but little gravel was carried fromthe main provenance. Deposits of coarse, sorted Stinson
sands were built up, culminating in development of asmooth surface sloping gently southward from bases at
steeper slopes and spurs of weathered Archean rocks
and Dollyberry lavas.
Deposition of the Manfred and Keelor Members began
abruptly when streams suddenly began to carry abun­dant quartz gravel in addition to finer detritus
southward over the Archean surface and the
topographically lower parts of Dollyberry lava fields.
Gravels were deposited where streams debouched from
confined watercourses at the piedmont angle and spreadout over the Stinson bajada. Distributary fans built up,out onto the sands and back onto the igneous rocks. The
finest gravel and lightest grains of heavy minerals werecarried to the most distal parts of fans. Slopes decreased
as the deposits aggraded. Stream competency waned,
and the proportion of sand to gravel in flood deposits in­creased.
Medium- to fine-grained argillaceous clastic sedimentsof the McKim Formation were deposited atop the Stin­son Member where it overlies the Ryan Member. The
lack of recognized Manfred sands in this area isnoteworthy (fig. 3). Manfred outwash may have been
diverted entirely to the southeast, so that no fine gravels
were deposited near the Nordic zone, or Manfred sandand fine gravel may have been eroded off the Stinson
surface here, or-least likely - Manfred subarkose withbeds of small-pebble conglomerate may grade abruptly
into much finer grained argillaceous quartzite of the
McKim Formation. The McKim Formation was likely
deposited partly in a flooded eastern section of the riftedzone near Sudbury, but a fluviatile depositional environ­ment seems more likely for the tongue at Elliot Lake.
The Elliot Lake strata, particularly those in proximal
localities, as at Quirke Lake, were tilted, warped, andbevelled prior to glaciation and deposition of Ramsay
Lake tillite, which forms the basal unit of the Hough
Group. It is noteworthy that the Ramsay Lake Forma­tion is thinner where it overlies the richest parts of the

W13

Quirke Lake and Nordic ore zones than it is in areas
flanking the conglomeratic zones. This is consistent with
the idea that the ore zones were parts of fans withpositive relief.
This conceptual model will no doubt require important
modifications. Nevertheless, it serves to dramatize somecritical limitations that can be imposed on choices ofpossible ways that the nonoxidized uranium deposits,
sediments, and paleosols could have been deposited andmodified. The depositional environment could not havebeen more different from environments of deposition ofyounger euxinic deposits. It was not at all different
physically from environments of deposition of some of
the most oxidized younger sediments. The sedimentary
particles were exposed to chemical attack by surface
and ground water saturated with atmospheric gases
throughout a history beginning with their births in welldrained soils lacking vegetative cover and continuingeven after their final burial beneath channels of swiftly
flowing streams. Any organisms that might have beenpresent would have been subject to similar attack. The
deposits were not laid down and buried near areas
where biotic or volcanic activity might have produced
vast quantities of reductants capable of scavenging ox­
ygen from all aerated surface and ground waters.
Neither were they formed in an extremely unusual tec­
tonic environment that has not recurred on a significant
scale since early Proterozoic time. Clearly, thewidespread paleosols were formed and the sediments
were deposited subaerially under an atmosphere that
contained no significant amount of free oxygen.
COMPARISON OF OLDER AND YOUNGERTERRIGENOUS SEDIMENTS

Extreme proximal depositional environments likethose of Elliot Lake ores were not an essential condition
for the formation of bankets. Witwatersrand ores,
stratigraphically far above basement, may have been
formed in more distal parts of fluvial systems.
Subeconomic bankets were evidently deposited in a
wider range of environments similar to various en­
vironments of deposition of black sands in hematitic
elastics less than 2.3 aeons old. Some examples of varied
pyritic suites are outlined in following paragraphs. Ob­
viously, it is not possible to prove that the pyritic suites
and their closest nonpyritic counterparts are mutually
exclusive and time bound, but extensive exploration of
terrigenous sediments throughout the world has notprovided evidence to the contrary.
Weakly radioactive layers with small pebbles arefound through several thousands of feet of Mississagi,
Deep Lake, and Montgomery Lake strata, so deposi­tional sites must have extended great distances
downstream from fall lines. Beds with quartz pebbles as

W14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

much as an inch or more in diameter in the Montgomery
Lake Group (fig. 2) are interlayered with coarse- to fine­grained quartzite and with siltstone. Some beds overlie
siltstone and form bases of upward-fining cycles likely
deposited in a relatively distal fluvial environment. Peb­
ble beds atop the upward-coarsening transition zone be­
tween Pecors argillite and Mississagi subarkose may
have been deposited in topset beds of a prograding
delta. Conglomerate beds interlayered with Huronianbasalt flows represent yet another depositional environ­
ment.
Other variations amongst banket associations include
differences in composition of detritus other than peb­bles. Sand in conglomerate matrices and in associatedbeds range from protoquartzite to arkose (for example,
locally in Ryan Member) and to subgreywacke (for ex­
ample, at Manfred-Ramsay Lake contacts and perhaps
in some Montgomery Lake beds). Although volcanics
are found in most host successions, their associations
with bankets range from intimate to remote. The Manfred and Keelor Lake conglomeratic zones were
deposited considerably later than were lavas in the
Elliot Lake district. Pyritic beds in the Mississagi For­mation, and through the greater parts of other host for­mations, were not coeval with any known volcanism.
Volcanic processes or rocks thus cannot be imagined
possible sources of sulphur, other banket constituents,
or pervasive reducing agents.
Younger quartzose arenites deposited in various ter­
restrial environments are characterized by red beds, andthey contain detrital heavy-mineral suites dominated byiron oxides. Concentrations of heavy minerals are blacksands rather than bankets. The difference cannot be at­tributed to differences in depositional environments or
to differences in postdepositional histories. Black-sandbearing successions, like older pyritic successions, were
deposited in a range of environments-alluvial fans,braided streams, flood plains, deltas, beaches,
volcanically active areas, and tectonically stable and tectonically unstable areas. They are found in rocks thathave been subjected to all sorts and degrees of altera­tions and metamorphic processes. No explanation moreplausible than oxyatmoversion has been offered for thetime-bounded character of the two suites of terrestrial
sedimentary rocks.
The concept that bankets are heavy-mineral concen­
trations deposited under an anoxic atmosphere provides
a rationale for the occurrence of abundant detrital
pyrite and locally abundant detrial uraninite within
them, but it does not automatically dispel all mysteries
concerning the origin of these and other minerals in thebankets. These problems do not fall within the scope of
this paper, but one important pheonomenon must be
mentioned briefly.

A considerable proportion of the pyrite must haveoriginated as iron oxides and iron-titanium oxides in Archean rocks in addition to larger amounts thatoriginated as pyrite, and some was formed authigenically within the bankets (Roscoe, 1969). The scarcity of ironoxides compared to pyrite in paleosols and the lack ofcoarsely bladed integrowths of pyrite and anatase in thebankets suggest that iron oxides were sulphidized in thesoils and that ilmenite-bearing grains were altered
before rather than after deposition of the bankets. The
excess sulphur required for the pervasive sulphidizationmust have been of atmospheric origin and, unlessunreasonably high concentrations of hydrogen sulphide
are postulated in the paleoatmosphere, anaerobic
bacteria must have played an important role in thetransformations. Some of the uranium minerals also
might have been formed originally in the soils, but thisseems unlikely in the case of uraninite that contains
thorium and rare earths in amounts similar to those inuraninites in igneous rocks.
REGIONAL TECTONIC SETTINGS OF BANKETS

The depositional and erosional features describedabove reflect tectonic movements in combination withclimatic changes and volcanic eruptions. Matinenda
alluvium was likely collected and transported southerly
over an extensive land surface following, perhaps, aslight increase in regional slope. We see in the formation
only the coarsest fraction of this weathered detritusmainly the largest quartz grains that were present ineroded Archean granitoid rocks. These, along withheavy minerals and pebbles of vein quartz, were
deposited in some sort of tectonic trap likely formed
along ruptures. The bulk of material eroded from theuplifted continental core, consisting of fine-grained
siliceous, feldspathic and argillaceous detritus, anddissolved salts, was carried farther south down the con­tinental paleoslope.
Cyclic similarities between Huronian Groups above
the Elliot Lake Group have been emphasized by Roscoe(1957a, b, 1969, 1973) and by Frarey and Roscoe (1970)
and have been attributed to epeirogenic movements at­
tendant on glacial loadings and unloadings super­
imposed on more general uplift of a central part of theArchean craton. The uplifts may have initiated theglacierizations marked by tillites and by some polymict
orthoconglomerate and arkose, but the preponderanceof submature and even supermature elastics - mainlycoarse quartzites-through most of the Huronian suc­cession militates against any possibility that moun­tainous terrains were carved in Archean rocks near the
Huronian belt. The tillites are overlain by fine-grained
sediments and, in the Quirke Lake Group, limestone.

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES

These were likely deposited in large lakes and short­
lived seas flooding the depressed craton. Subsequent re­bound resulted in resumption of deposition of course
elastics in the Huronian belt, with the bulk of detritus
and dissolved cations (of sodium, calcium, magnesium,iron) and silica carried farther down the continental
paleoslope. The arenites that dominate the Huronian
succession thicken southward through the Huronian
belt, so the Archean platform must have extended wellto the south into what is now the tectonically younger
Grenville Province (Frarey and Roscoe, 1970).
The distribution of Archean rocks in North America,
as well as banket localities and general sedimenttransport directions therein, is shown in figure 1. The
Superior, Slave, and Wyoming provinces, where rocks
were not significantly metamorphosed more recently
than 2.5 aeons ago, are not distinguished from remain­
ing areas where Archean rocks were metamorphosed orremetamorphosed about 1.8 aeons ago (Hudsonian andPenokean orogenies). Intervening areas were likely also
cratonized in Archean time. Moreover, Archean
basements to Huronian and Medicine Bow strata musthave once extended well south of Superior and Wyom­ing provinces. A few areas containing volcanic rocks
about 1.9 aeons old, designated V in figure 1, may repre­
sent rifts wherein some Aphebian rocks were depositedon oceanic rather than continental crust.
Crossbed dips, southeasterly in Huronian quartzites,
southwesterly in Deep Lake quartzites and westerly inMontgomery Lake quartzites, suggest a drainage divide
or uplift axis extending north- to northeastward throughwestern Ontario. The North American banket-bearing
belts trend northeast, excepting the western part of thearcuate Huronian belt. They are at an angle to the
hypothesized uplift axis, not peripherally disposed
around the uplift or around any likely cratonic block. It
is suggested, therefore, that they mark intracratonic
ruptures that were sites of vulcanism and of crustal flexturing that entrapped and eventually folded continental
sediments.
The known Paleoaphebian belts were no doubt verymuch more extensive originally, and there were prob­
ably others. Great volumes of Archean granitic rocks
must have been eroded to provide coarse quartz grains
in Huronian and other thick quartzite wedges (Frarey
and Roscoe, 1970, p. 154). If provenance areas werelimited and submountainous as suggested here, differen­tial uplift and erosion must have been very considerable
and occurred through a prolonged period.

W15

negative. For instance, the observation that banketbearing pyritic quartzites and black-sand-bearing red
beds are mutually exclusive and separated in the geo­
logical column by a time barrier at 2.3 aeons carries
risks that exceptions may be found. Some have in fact
been suggested (for example, Ruzicka, 1971, p. 148-158;
Bottrill, 1971, p. 82). However, none of these has beenshown to have diagnostic features shared by all the an­
cient bankets but not found in indisputably epigenetic
deposits. Most allegedly banketlike occurrences ofpyrite and uranium in younger rocks are irrelevant by
the advocate's own account. So that I may be absolved
from suspicion of attempting "proof by definition," let us
consider a comparison that C. F. Davidson (1965, p.
1198) made between locally "somewhat metamor­
phosed" Permian conglomerates "locally mineralizedwith uraninite, pyrite, and other sulphides" and Witwatersrand ores. In this, he ignored the facts that an­
cient bankets are all found through great successions ofstrata, not as local impregnations of pyrite in normally
nonpyritiferous beds, and that alleged younger analogs
are never extensive, never geochemically or mineralogically similar to bankets, and never difficult torecognize as epigenetic.
Doubts about an early Proterozoic oxyatmoversion
stem most commonly from similarities between Archean
and younger chemical and biogenic sediments. Carbona­
ceous material in older rocks is considered evidence thatoxygen was produced photosynthetically from carbon
dioxide. Sedimentary iron oxides indicate that ferrous
iron was oxidized to ferric iron in ancient seas, and rare
local occurrences of sulphates apparently required ox­
idizing conditions. The existence of these phenomena,
however, do not refute evidence that atmospheric gases
were incapable of oxidizing ancient surficial materials.
Anaerobes utilize oxidation-reduction reactions. As
outlined by Cloud (1973), the accumulation of free oxy­
gen in the atmosphere must have been dependent on
evolution of photosynthetic organisms, initially in an
anoxic environment. Photosynthesis and oxidation oforganic material can be represented by the reactions
C02^C +02 .
If ferrous iron is present, and if carbonaceous material
or iron oxide is buried or otherwise removed as pro­
duced, reactions can be summarized as

There is no requirement that the atmosphere contain ox­ygen, although if oxygen-tolerant organisms are involv­
ed, this would enhance the process.
DISCUSSION
Red beds and black sands in the Cobalt Group and
The proposition that bankets were deposited other Paleoaphebian strata indicate that free oxygen
subaerially under an anoxic atmosphere is susceptible to began to accumulate in the atmosphere not syn­the criticism that some of the evidence for this is chronously with but well in advance of deposition of theW16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

great circum-Ungava and Lake Superior ironformations. This is contrary to beliefs held by Cloud(1973), Garrels and others (1973), and others who haveconsidered the probable course of atmospheric evolu­tion. It does not damage their hypotheses in any impor­tant way, but it may place constraints on models fordeposition of the early Proterozoic iron-formations.
Another constraint, dealt with most effectively by
Holland (1973), is a requirement for an enormous supplyof iron to some sites such as the Hamersley Basin. Tak­
ing some liberties, perhaps, in applying and extending
the model he has suggested, we can postulate the se­quence of postoxyatmoversion events outlined in thefollowing paragraphs.
Prolonged deep preoxyatmoversion erosion of con­tinents supplied abundant ferrous iron, along with othercations and silica, to the ocean. Despite continual deposi­tion of much iron as sulphide, magnetite, carbonate,silicates, and some hematite, sea water below the photic
zone and an intermediate mixing zone remained a reser­voir of dissolved ferrous iron even after oxygen began toaccumulate in the atmosphere. Deposition of thick, ex­tensive iron-formations in special environments andmore widespread deposition of dispersed iron minerals
depleted the reservoir significantly during a com­paratively short time. The most favourable environment
for deposition of banded iron-formation would be
developed where a strong current of "reservoir water"
agitated by sea-floor volcanic eruptions flowed pastshallows with extremely high levels of photobiotic activi­
ty. A strait or seaway formed by rifting and marginal
flooding would be ideal. The Labrador Trough could
represent such a situation. None of the North American
belts with major iron-formations are definite
pericratonic orogens; all might be intracratonic.
The considerable interest shown in hydrocarbon oc­
currences in Elliot Lake ores perhaps reflects somedegree of suspicion that these represent a significant
part of the story, as they may in the Witwatersrand. We
have no data on abundance of thucholite in North
American bankets. Possibly it is not quite as rare in
Elliot Lake ores as I have imagined. Thin seams of black
thucholite in dark uraninite-rich layers are difficult torecognize underground. Globules and sooty incrusta­
tions of very weakly radioactive hydrocarbon are more
conspicuous but are far from common. These coat frac­tures and minerals in vuggy veinlets that are not
restricted to bankets; in fact, they have been intersected
in drill cores away from ore zones and hundreds of feetabove target strata. It seems most reasonable to sup­pose that scarce hydrocarbon found in ore is also
epigenetic. Certainly there would have been more
hospitable places for organic material to grow than incourse gravels in rapids.

Mineralographic evidence and other specific evidence
for detrital origin of uraninite and pyrite grains inbankets have not been reviewed in this paper because
these have been well documented elsewhere(Liebenberg, 1955, 1960; Ramdohr, 1958; Hiemstra,
1968; Roscoe, 1969). These observations seem to begenerally regarded as the only direct evidence that theatmosphere was anoxic until the beginning of Pro­
terozoic time, although other evidence has beenpresented (Roscoe, 1969). The timing of the oxyatmoversion is too important a matter to leave standing
on one foot (see Davidson, 1965, p. 1195). Perhaps
evidence given in this paper for a transition in weather­ing and subaerial sedimentation processes from anoxic
to oxidizing 2.3-2.2 aeons ago will encourage researchleading to a better understanding and more precise
dating of this transition.
REFERENCES CITED
Banks, P. 0., and Van Schmus, W. R., 1972, Chronology of Precambrian rocks of Iron and Dickinson Counties, Michigan, pt. II [abs.]:
Institute on Lake Superior Geology, Abstracts and Field Guides,
No. 18 [Paper 23].
Bayley, R. W., and James, H. L., 1973: Precambrian iron-formationsof the United States: Economic Geology, v. 68, p. 934-959.
Bell, R. T., 1970, Preliminary notes on the Hurwitz Group, Padlei
map-area, Northwest Territories: Geological Survey of Canada
paper 69-52.
Beukes, N. J., 1973, Precambrian iron-formations of southern Africa:
Economic Geology, v. 68, p. 960-1004.
Blackwelder, Elliot, 1926, Precambrian Geology of the Medicine Bow
Mountains: Geological Society of America Bulletin, v. 37, p.
615-658.
Bottrill, T. J., 1971, Uraniferous conglomerates of the Canadian
Shield, in Report of Activities, Part A, April to October 1970:
Canada Geological Survey Paper 71-1, pt. A, p. 77-83.
Cloud, Preston E., 1973, Paleoecological significance of the bandediron-formation: Economic Geology, v. 68, p. 1135-1142.
Davidson, C. F., 1965, Geochemical aspects of atmospheric evolution,
in Symposium on the evolution of the Earth's atmosphere: Na­tional Academy of Sciences Proceedings, v. 53, no. 6, p.
1194-1205.
Fairbairn, H. W., Hurley, P. M., Card, K. D., and Knight, C. J., 1969,
Correlation of radiometric ages of Nipissing diabase and Huronian
metasediments with Proterozoic orogenic events in Ontario: Cana­dian Journal of Earth Sciences, v. 6, p. 489-497.
Fairbairn, H. W., Hurley, P. M., and Pinson, W. H., 1965, Reexamination of Rb-Sr whole rock ages at Sudbury: Geological Associationof Canada Proceedings, v. 16, p. 95-101.
Ferreira, E. 0., 1972, Tectonic map of Brazil, explanatory note: Brazil
Departamento Nacional da Producjio Mineral, Boletim no. 1, 14 p.
Frarey, M. J., and Roscoe, S. M., 1970, The Huronian Supergroupnorth of Lake Huron, in Baer, A. J., ed., Symposium on basins andgeosynclines of the Canadian Shield: Canada Geological Survey
Paper 70-40, p. 143-157.
Garrels, R. M., Perry, E. A., Jr., and Mackenzie, F. T., 1973, Genesisof Precambrian iron-formations and the development of at­mospheric oxygen: Economic Geology, v. 68, p. 1173-1179.

FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES
Gibbins, W. A., Adams, C. J., and McNutt, R. H., 1972, Rb-Sr isotopic
studies of the Murray Granite: Geological Association of CanadaSpecial Paper 10, p. 61-66.
Hiemstra, S. A., 1968, The mineraology and petrology of theuraniferous conglomerates of the Dominion Reefs mine,
Klerksdorp area: Geological Society of South Africa Transactions,v. 71, pt. 1, p. 1-65.
Holland, H. D., 1973, The oceans: A possible source of iron in iron for­mations, in Precambrian iron-formations of the world: Economic
Geology, v. 68, p. 1169-1172.
Huston, R. S., 1968, A regional study of rocks of Precambrian age inthat part of the Medicine Bow Mountains lying in southeastern
Wyoming: Wyoming Geological Survey Memoir 1, 167 p.
Krogh, T. E., and Davis, G. L., 1974, The age of the Sudbury nickel irruptive: Carnegie Institution of Washington Year Book 73, p.
567-569.
Knight, C. J., 1966, A study of Rb-Sr whole rock ages of volcanics on
the north shore of Lake Huron, Ontario, Canada: U.S. Atomic
Energy Commission Report MIT 1381-14, 14th Annual Progress
Report for 1966, U.S.A.E.G., Massachusetts Institute ofTechnology, p. 129-139.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef and the Black Reef [withdiscussion]: Geological Society of South Africa Transactions, v. 58,
p. 101-254.
———1960, On the origin of uranium, gold, and osmiridium in the con­glomerates of the Witwatersrand goldfields: Neues Jahrbuch fur
Mineralogie Abhandlungen, v. 94, Festband Ramdohr, p. 831-866.
Niekerk, C. B. van, and Burger, A. J., [1966], The age of the
Ventersdorp System: South Africa Geological Survey Annals, v. 3
(1964), p. 75-86.
———1969, Lead-isotopes data relating to the age of the Dominion
Reef lava: Geological Society of South Africa Transactions, v. 72,
pt. 11, p. 37-45.
Oparin, A. L, 1938, The origin of life: London, MacMillan.
Pettijohn, F. J., 1970, The Canadian Shield-a status report, 1970, in
Baer, A. J., ed., Symposium on basins and gosynclines of the Cana­dian Shield: Canada Geological Survey Paper 70-40, p. 248.
Pienaar, P. J., 1963, Stratigraphy, petrology, and genesis of the Elliot
Group, Blind River, Ontario, including the uraniferous con­glomerates: Canada Geological Survey Bulletin 83, 140 p.

W17

Ramdohr, Paul, 1958, New observations on the ores of the Witwaters­
rand in South Africa and their genetic significance: GeologicalSociety of South Africa Transactions, Annexure to v. 61, 50 p.
Robertson, J. A., 1961, Geology of Townships 143 and 144: Ontario
Department of Mines, Geological Report no. 4.
———1969, Township 157, District of Algoma: Ontario Department ofMines, Preliminary Geological Map P.561.
——-1971, Private report.
-1973, A review of recently acquired geological data, BlindRiver-Elliot Lake area: Geological Association of Canada Special
Paper 12, p. 169-198.
Roscoe, S. M., 1957a, Geology and uranium deposits, Quirke LakeElliot Lake, Blind River, Ontario: Canada Geological Survey Paper
56-7, 21 p.
———1957b, Stratigraphy, Quirke Lake-Elliot Lake Sector, BlindRiver, Ontario: Royal Society of Canada, Special Publication 2, p.
54-58.
———1969, Huronian rocks and uraniferous conglomerates in the
Canadian Shield: Canada Geological Survey Paper 68-40, 205 p.
————1971, Private report.
-1973, The Huronian Supergroup, a Paleoaphebian succession
showing evidence of atmospheric evolution, in Young, G. M., ed.,Huronian stratigraphy and sedimentation: Geological Associationof Canada Special Paper 12, p. 31-47.
Rubey, W. W., 1955, Development of the hydrosphere and at­mosphere, with special reference to probably composition of theearly atmosphere: Geological Society of America Special Paper 62,
p. 631.
Ruzicka, V., 1971, Geological comparison between East European andCanadian uranium deposits: Canada Geological Survey Paper
70-48, 196 p.
Theis, N. J., 1976, Uranium-bearing and associated minerals in theirgeochemical and sedimentological context, Elliot Lake, Ontario:
Queen's University, Kingston, Ontario, unpublished Ph.D. thesis.
Trendall, A. F., 1969, Three great basins of Precambrian banded-ironformation deposition: Geological Society of America Bulletin, v.
79, p. 1527-1544.
Van Schmus, W. R., 1965, The geochronology of the Blind River-Bruce
Mines area, Ontario, Canada: Journal of Geology, v. 73, p.
755-780.
Young, G. M., 1970, Private report.
———1973, Tillites and aluminous quartzites as possible time markers
for Middle Precambrian (Aphebian) rocks of North America:
Geological Association of Canada Special Paper 12, p. 97-127.

Genesis of Canadian Uraniferous
Quartz-Pebble Conglomerate Ores
By H. W. LITTLE
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICALSURVEY PROFESSIONAL PAPER 1161-X

CONTENTS
Page

Origin of the ore ____________________________________________
Provenance _____________________________________________
Sedimentation __________________________________________
Effect of lavas __________________________________________
Anoxygenic atmosphere _____________________________________
Mineralogy and zoning ______________________________________
Climate _______________________________________________
Papaskwasati Basin __________________________________________
Sakami Lake ______________________________________________
Montgomery Lake __________________________________________
Deposits in post-early Aphebian quartz-pebble conglomerates __________________
Effects of metamorphism on Huronian rocks in the Grenville Province______________
References cited ____ _________________________________————__
Discussion following papers by Drs. Robertson, Ruzicka, Roscoe, and Little __________

XI
2
3
3
3
4
5
5
5
5
5
9
10
10

ILLUSTRATIONSFIGURES 1-5. Maps showing:
1. Location of uraniferous quartz-pebble conglomerates in Canada_______________________—___—
2. Basement topography beneath the Papaskwasati Formation _______________________________
3. Isopachs on the basal quartz-pebble conglomerate of the Papaskwasati Formation __________________
4. Distribution of Hurwitz Group and associated rocks ____________________________________
5. Potential areas for uraniferous quartz-pebble conglomerate deposits __________________________

X2
6
7
8
9
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESGENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLECONGLOMERATE ORES
By H. W. LITTLE 1

ORIGIN OF THE ORE

As in the Rand district of South Africa where con­troversy over the origin of the deposits raged violently
for some years, at Blind River controversy began almost
at the outset of production from the Early Aphebianconglomerates (fig. 1). The principal controversy was
between the syngeneticists and the hydrothermalists,
the latter being greatly in the minority. Their mainarguments centered on permeability of the con­glomerate, concentration of heavy minerals at the baseof the conglomerate, rounding of uranium minerals,
whether due to erosion or colloidal precipitation, andthorium:uranium ratios.
The primary dispute of syngenetic versus hydrothermal origin for the ore has continued for many years. J.
A. Robertson (1968) has pointed out that metasomaticeffects are greatest in the Pronto and Can-Met Mines inthe Elliot Lake area (fig. 1), where the hydrothermalists
have marshalled most of their supporting data. Derry(1960) believed that the albitic metasomatism at Pronto
is later than both radioactive and sulphide mineraliza­
tion. Heinrich (1975) further emphasized the effects ofmetasomatic alteration there by classifying the types ofalteration and their effects upon the ore and associatedrocks.
At the present time, those geologists who have work­
ed most closely with the deposits seem to be unanimousin the opinion that the ores are syngenetic. Some geolo­
gists believe that the ores may be entirely or partly
formed by chemical or biochemical precipitation, Derry(1960) being the initial proponent of the latter, which is
supported in part by Ruzicka (this volume, chap. W).
The relatively high thorium content of the uraninite,
however, weighs against this possibility. Grandstaff(this volume, chap. J) offered further evidence in thisregard. Moreover, at Agnew Lake (fig. 1) uranothorite
1 Consulting Geologist, 8111 Sanders Road, No. 61, Richmond, B. C. V7A 4L9, Canada.

is the predominant ore mineral, and it cannot have been
formed in this way; it is formed only in a hightemperature environment such as pegmatites and grani­
tic or metamorphic terranes. It is therefore difficult toarrive at a conclusion other than that the Agnew Lakequartz-pebble conglomerate uranium ores are of placer
origin, for the metamorphism there is dynamic and thegranites are older than the conglomerate. Indeed, theunconformity is folded by plastic deformation of the
granite, and the pebbles are greatly elongated.
Roscoe (1969) has demonstrated that some uraninite
grains are detrital, that they are commonly concentra­
ted at the base of conglomerate layers, where one ap­
peared to be pressed down by a quartz pebble into theargillite below, and that in places they show graded bed­ding, and Ruzicka has given strong supporting evidence(see chap. W). Roscoe's belief that at least some brannerite as well is detrital has been disputed by Ferris andRuud (1971), who supported Derry (1960). Ferris andRuud did not believe that leached brannerite derived
from crystalline brannerite would form an internal net­
work of needles and blades of anatase and rutile such as
is found in the Blind River and Elliot Lake ores. Fur­thermore, brannerite is relatively rare in hydrothermal
and pegmatitic occurrences that would be expected to be
the source of detrital brannerite.
It was noted long ago that ore is not concentrated
near faults or dykes. One of the most important observa­
tions was reported by J. A. Robertson to Roscoe (1969)and later published by himself (Robertson 1970), that inplaces where the polymictic Ramsay Lake Conglomer­
ate truncates the ore-bearing conglomerate it isuraniferous. D. S. Robertson and Douglas (1970) com­mented at greater length upon this feature, implyingthat the uranium content is found only down the
paleoslope and comprises radioactive debris from theerosion of the quartz-pebble conglomerate. This not only
proves a pre-Ramsay Lake age for the ore but is strong
evidence in favour of a syngenetic origin.
XI

X2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXPLANATION

Early Aphebian conglomerates (>2200 m.y.)
1. Blind River
2. Elliot Lake
3. Agnew Lake
4. Papaskwasati Basin
5. Sakami Lake
6. Padlei area
Late Aphebian or Helikian conglomerates (<2200 m.y.). .
7. Cambrian Lake•^ 'V;iu,^//FIGURE 1.- Location of uraniferous quartz-pebble conglomerates in Canada.

PROVENANCE

The question of provenance is to me a taxing one inthat, although the subarkoses that are intercalated with
the quartz-pebble conglomerate reefs are evidence for a
granitic provenance, granitic pebbles in the reefs are ex­tremely rare. Furthermore, quartz veins are not abun­dant in the granites northwest of the Elliot Lake area,
which paleocurrent data show to be the source of thesediments. Roscoe (1969) has suggested that erosion of agreenstone belt might have provided a prolific local
source for pebbles both of chert and of vein quartz.
Roscoe and Steacy (1958), from a sampling of the porphyritic, red Archean quartz monzonite northwest of theore-bearing sediments, concluded that the uranium con­tent of the quartz monzonite was above normal and thatthe thorium :uranium ratios were similar to those in theores. Subsequent testing of these granitic rocks by Bottrill (1970) with a portable gamma-ray spectrometer,
which enabled wider sampling, showed, as a result of 32
readings, that the uranium and thorium values are close
to the clarke values. The thoriunr.uranium ratio was 7:1,
dissimilar to that of 2:1 obtained by Roscoe and Steacy.
More recently, airborne gamma-ray spectrometry by the
Geological Survey of Canada northwest of the Quirke
ore zone has shown a relatively high uranium content in
the supposed source rocks. It is, however, unlikely thatthe area sampled is the same area of provenance that ex­
isted 2,500 m.y. (million years) ago; in the writer's opin­
ion, the provenance area must have been farther north­west and somewhat higher than the present erosion sur­face.

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORESSEDIMENTATION

Originally it was believed that the Matinenda Forma­tion was deposited in a deltaic environment, and later,that the strandline retreated northward during sedi­mentation, so that the more northerly reefs are stratigraphically higher. Beginning with Pienaar (1963), re­cent authors have emphasized a fluviatile origin, par­ticularly for the coarser elastics, but at the same time
have retained the concept of a marine origin for parts ofthe succession. At Agnew Lake a northerly retreating
shoreline has not been demonstrated. During the earlystages of exploration it was thought to be the case
(Robertson and Douglas, 1970) but subsequently, after
the structural picture had been largely resolved, it
became questionable whether more than one conglomer­
ate reef is there (Barnes, 1972).
There was some speculation whether the sedimenta­tion took place in a marine or nonmarine environment.
In a recent symposium on basins and geosynclines in the
Canadian Shield, Card, discussing a paper by Frarey
and Roscoe (1970), offered evidence that in the northernpart of the Huronian geosyncline, deposition was nonmarine or in shallow epicontinental seas, whereas to thesouth it was in a deep-water marine environment.
Frarey and Roscoe disputed this contention and main­tained that deposition of coarse elastics was entirely
fluviatile.
In recent years there has been a suggestion that there
is an east-west orientation within the ore zones of the
Quirke, Nordic, and Pronto Mines. This idea has been
expressed recently by Rupert and others (1972) in thefollowing terms: "It is suggested that (paleocurrent)
data are best interpreted as evidence for eastboundlongshore currents in this basin of deposition rather
than as the product of river channels running into abasin. In this environment, lag deposits of scoured peb­ble and conglomerate beds would form over or on thesouthwestern and western flanks of stratigraphicallyhigher structures." They go on to suggest that in pros­
pecting for deeply buried uranium deposits the possibili­
ty of finding point-bar or depositional types of placers at
ancient channel mouths should be considered.
With a view to establishing more coordinated data, theUranium Resource Evaluation Section of the Geological
Survey of Canada, in cooperation with the Ontario Divi­sion of Mines, has set up a computerization system
whereby paleogeographie and isopach maps can be pro­
duced to aid in the prediction of the location of un­
discovered orebodies.
EFFECT OF LAVAS

Roscoe (1969), and subsequently others, correlated his
Elliot Lake Group with mafic lava-bearing formations to
the west in the Thessalon area, to the south in theX3Spragge area, and to the east in the Sudbury area.
Roscoe suggested that the volcanics would have amarked physical effect on Matinenda sedimentation.
Subsequent to his report, mafic volcanic rocks interbedded with Matinenda sediments, as Roscoe had
predicted, were reported by J. A. Robertson (1969) atseveral places along the Quirke Syncline, particularly at
Dollyberry Lake, west of the Quirke ore zone.
The problem of lack of magnetite in the Aphebian
paleoplacers has been largely resolved. Garrels andChrist (1965) from experimental data showed thatmagnetite derived from erosion of the Archean base­ment would have been converted to pyrite in the low Ehenvironment of an anoxygenic atmosphere and in ahydrogen-sulphide-rich environment. The discovery of
the Dollyberry volcanics by J. A. Robertson (1969) in
Township 157, west of the Quirke ore zone, interbeddedwith Matinenda strata, indicated a rich source ofhydrogen sulphide. In a written communication, T. J.
Bottrill suggested to the writer in 1970 that this could bea source of the abundant pyrite in the uraniferous con­glomerate ores. Ruzicka (chap. W, this volume) has also
shown this as a likely source of pyrite and added further
evidence by showing, with H. R. Steacy, that thucholite
present in the ores is abnormally high in sulphur. In the
Sakami Lake deposits (fig. 1), where lavas were pro­bably present, sulphur is fairly abundant; inPapaswasati Basin, where no lavas are known, it is not.
The discovery of Huronian lavas in the Quirke
Syncline points up the need for careful reappraisal of
drill holes that were stopped in greenstone in the belief
that the Archean basement had been penetrated. In
some cases the greenstone may have been Huronian
lava or even sheared diabase; in either case, deepeningof the drill hole would be advisable, because quartzpebble conglomerate may be intersected at a lower
horizon.
ANOXYGENIC ATMOSPHERE

The concept of an anoxygenic atmosphere early inPrecambrian time was proposed at least as early as
1938, and in 1955 it was applied specifically to explain
the origin of uraniferous placers by Liebenberg (1955).
A number of papers supporting this theory have ap­
peared in print in recent years and many are reviewed
by Roscoe (1969) and Myers (1970). Frarey and Roseoe(1970) summarized the geological data that support theargument for an anoxygenic atmosphere during early
Huronian time. Briefly, they state that it is generally
believed that an anoxygenic atmosphere existed andthat, according to Preston Cloud, there is reason tobelieve that life initially developed at this time. From astudy of the paleosol existing on the Archean basement
it was shown that iron was lost rather than accumulated

X4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

and the ferric:ferrous ratio decreased rather than in­creased. Frarey and Roscoe believed that the drab col­our of most Huronian rocks, the lack of hematite below
the Cobalt Group, in which hematite is present, and thepresence of abundant pyrite in the conglomerate of theMatinenda Formation all indicate that nonoxidizing con­ditions existed. The presence of detrital uraninite andpyrite in the quartz-pebble conglomerates was cited as
further evidence of the lack of oxygen in the at­mosphere. Pettijohn (1970) objected to the concept but
solely on the grounds that absence of red colour andpresence of detrital pyrite do not necessarily indicate anoxygen-free atmosphere, for in the Appalachians all preSilurian Paleozoic sediments are drab and detrital pyriteof mine tailings is transported many miles downstream.
Further support for an anoxygenic atmosphere prior
to 2,200 m.y. ago has been offered by Bain (1972), whopointed out that triploblasts and diploblasts, which re­quired oxygen for their metabolism, first appeared 2,200
m.y. ago. He also contended that the pH of runoff fromthe land was lowered about 2,050 m.y. ago, for the firsttime below the uraninite solution value. Tugarinov(1972) agreed that such an atmosphere change took
place but that it occurred 3,000 m.y. ago. However, histime scale does not agree with that of other writers, forhe maintains that sedimentary iron ores were deposited
2,500 to 2,300 m.y. ago, followed immediately by
precipitation of carbonates.
More recent papers, based upon more isotopic age
determinations, have centered in upon an age of 2,200
m.y. ago as the time of the critical point of oxygen
enrichment of the atmosphere.
From India we have had recent apparent confirmationof this hypothesis in a recent announcement that quartzpebble conglomerate containing pitchblende, monazite,
and zircon and carrying interesting values in uranium
has been discovered in the Dharwar metamorphic com­plex, which was affected by metamorphism dated at2,600 m.y. (Rao, 1975). Pyrite is reported to be present,
but the quantity is not given; other minerals present arechalcopyrite, covellite, ilmenite, rutile, and magnetite.
On the other hand, many pyritiferous quartz-pebble
conglomerates of early Aphebian age are low or lacking
in uranium values, whereas one that is Helikian contains
interesting values. It will be described later.
W. B. Myers' paper (1970) dealt largely with the Witwatersrand but has application to the Blind River-Elliot
Lake deposits. He pointed out that in the Rand both
detrital and oolitic pyrite occur in shallow-water
sediments, together with the general absence of pyrite
from graded-bedded siltstones and shaly sediments, andsuggested that shallow-water sedimentation was in­fluenced by an anoxygenic atmosphere, whereas indeeper waters biologic photosynthesis by anaerobic
organisms may have been taking place. Myers contend­
ed that the preservation of gold in a fossil placer deposit,
like uraninite, would require an atmosphere of low ox­
ygen concentration, particularly where the particle size
is less than 40 ju,m, which size particles are subject torapid solution.
MINERALOGY AND ZONING

In many papers the mineralogy of the quartz-pebble
conglomerate ores has been described. One of the best
and most detailed is that by Roscoe (1969), who includes
all previous work in his discussion. Pyrite is the mostabundant sulphide, ranging from 5 to 10 percent of theore. In the mines the abundance of pyrite is generally
thought to be in direct proportion to the grade of theore, but Roscoe stated that although pyrite is generally
more abundant in highly radioactive conglomerates,
there is no positive correlation between pyrite anduranium content.
Sulphur isotope compositions have been determined
for a number of pyrite crystals from the ore con­glomerates. These show a S-range (Roscoe, 1969) from
0.0 to +2.7 per mil, which is comparable with the range
reported in Witwatersrand ores (Hoefs and others,1968), and strongly indicates a magmatic or hydrothermal origin rather than an origin by biochemical
precipitation. It does not, of course, preclude a detrital
origin.
In Blind River-Elliot Lake ores brannerite, uraninite,and uranoan monazite, in order of decreasing impor­tance, are the main ore minerals. Many silicates, oxides,
and other minerals have been reported in minor quantity
at Blind River and Elliot Lake; in all, some 44 suchminerals have been identified. On the other hand, atAgnew Lake, in addition to pyrite, only uranothorite
and monazite have been reported.
Studies of variations in the Th02:U308 ratios havebeen of interest with respect to zoning and its bearing
on origin of the ores. Data have been obtained from millheads, which represent very large samples, and drill­hole information, representing very small samples. In
one mine, records of Th02 :U308 ratios of carload lots
from all parts of the mine were plotted. In one reef theratios varied from 1:1 to 1:30; in another reef theyranged from 1:1 to 1:53. Furthermore, in the Quirke
property the Th02 :U308 ratio increases from northeast
to southwest (D. Sprague, oral communication, 1970) in­
stead of from northwest to southeast, as has been
claimed for the district in earlier papers. A reexamination of old data and the inclusion of new data is
recommended.

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORES
CLIMATE

Roscoe (1969) suggested that another environmental
factor in the deposition and preservation of uraniferous
placers in the lower Huronian succession was that ofclimate. He gave evidence that glacial conditions
prevailed not only during Gowganda time but also inearlier times, particularly during deposition of the Ram­say Lake and Bruce Formations. Other writers havesupported a tilloid and glaciolacustrine origin for theseformations. This thesis was further elaborated by
Frarey and Roscoe (1970), who indicate that evidence
for a glacial origin for Ramsay Lake and Bruce
paraconglomerates is more compelling than for the wellpublicized Gowganda. The effect of such temperature
conditions on the uraniferous conglomerate may be afactor in the concentration of uranium, as the paper byGrandstaff in this volume (chap. C) has shown.
PAPASKWASATI BASIN

In the Papaskwasati Basin, Quebec (fig. 1), sedimenta­tion similar to that in the Lower Huronian seems to havegone on contemporaneously. Sediments, including abasal quartz-pebble conglomerate, were deposited in a
downfaulted basin that has been little deformed since
the time of deposition (fig. 2). There were, however, nolavas interbedded with the basal part of the succession,and as an apparent consequence, pyrite is far from abun­dant in the quartz-pebble conglomerate, which ranges inthickness from a few to more than 200 ft (60 m) (fig. 3).
Fluvioglacial processes have not been identified here,
and perhaps this, too, prevented concentration, but
source rocks, which are to the east, are not abnormallyrich in uranium. Some 69 diamond-drill holes were
drilled to intersect the conglomerate, and much surface
trenching was done. However, uranium values ranged
from a trace to about 0.03 percent U308 , with a few local
assays somewhat higher. Results of exploration were
therefore not encouraging and no further work has been
done.
SAKAMI LAKE

D. S. Robertson (1974; Robertson and Lattanzi, 1974)
described an interesting occurrence of uraniferous
quartz-pebble conglomerate on the west side of Sakami
Lake (fig. 1), which is in Quebec, east of James Bay. A
thin succession of feldspathic quartzite, which has been
traced intermittently for about 5 miles (8 km), contains
at least three thin interbeds of quartz-pebble con­glomerate and some of amphibolite and chloritic schistthat probably are metamorphosed lava. The pebbles are
all of quartz, and most are less than 1 in. (2.5 cm) indiameter but are much stretched along the direction of
the strike of the beds. Pyrite is quite abundant in the

X5matrix, and is commonly in cubic form. In general,
radioactivity seems roughly proportional to the pyrite
content. Once again, the lavas seem a likely source forthe pyrite in the sediments.
D. S. Robertson has speculated that the age of those
deposits is Archean because they have been involved in
the Kenoran orogeny. On the other hand, he hasremarked that the succession is remarkably similar tothat of the Lower Huronian and has suggested that
sedimentological conditions rather than time limits set
by geochronology should govern the age classification.
The sedimentation at Sakami Lake seems to have occur­
red at about the same time as the Lower Witwatersrand—that is, prior to 2,500 m.y. The thoriumiuranium
ratio seems to be higher than in Elliot Lake ores but
lower than in those of Agnew Lake Mine.
MONTGOMERY LAKE

Uraniferous quartz-pebble conglomerates in the Hurwitz Group at Montgomery Lake, Padlei area (figs. 1, 4),have been covered in chapter W, this volume, by Roscoe
and need not be further discussed here. Metamorphic
equivalents of the Hurwitz Group extend for a great
distance to the south, and similar ones may be foundthere, perhaps reconstituted.
DEPOSITS IN POST-EARLY APHEBIANQUARTZ-PEBBLE CONGLOMERATESThe uraniferous deposits in the Cambrian Lake area
(fig. 1), northern Quebec, have been known for several
years. Quartz-pebble conglomerate intercalated witharkosic rocks has been traced along the strike forseveral miles. This formation lies at the base of the
Kaniapiskau Supergroup on the west side of theLabrador Trough and is of Aphebian age. It is regarded
as late Aphebian because the basal formation contains
hematitic beds. The heavy minerals identified in the con­glomerate are pyrite, monazite, and minor uraninite. In
this respect they resemble conglomerates of the Lorrain
Formation, which are relatively rich in thorium and low
in uranium.
The Nonacho Group (fig. 1) in the District of Macken­
zie has been described by McGlynn (1966, p. 32) as
follows:
The Nonacho Group comprises a conformable sequence of polymictic
conglomerates,.conglomeratic arkoses, arkoses, and shales. Along thesouth part of the east contact the basal units consist of a variable
thickness of sedimentary breccia that comprises angular boulders of
the underlying granitic rock in a matrix of arkose. This rock grades up­
ward and possibly outward from the contact to arkoses containing theodd angular boulder of granite gneiss and beds and lenses of
disintegrated granite. Between the breccia and basement is a zone con­sisting of granitic gneiss broken into large blocks, measuring as muchas 10 feet across, that have been only slightly moved from their

X6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXPLANATION

Approximate outline of basin-1000Elevation of basement (above sea level)
100-ft intervals

FIGURE 2.—Basement topography beneath the Papaskwasati Formation (compiled by R. A. Boulay).

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORES

X7

EXPLANATION

Approximate outline of basin
50
Isopachs of the basal conglomerate
25-ft intervals

Diamond-drill hole (vertical)

51°30"NFIGURE 3.—Isopach map showing the basal quartz-pebble conglomerate of the Papaskwasati Formation (compiled by R. A. Boulay).

X8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
104°

100=

96"

92°

EXPLANATION
Montgomery L«k* Group
Hurwitz Group
Probtbl* Hurwitz Group •quivilcnt
Post-Hurwit* Group "Ennadai Group

4. - Distribution of Hurwitz Group and associated rocks (from Wanless and Eade, 1975). Reproduced with permission of the National
Research Council of Canada (from the Canadian Journal of Earth Sciences, v. 12, p. 96).
original positions, separated by seams of muddy arkosic sand. This
rock grades to fractured granite with the odd sand seam that in turngrades to unfractured gneisses, migmatites, etc. Locally these basal
units are overlain by polymictic conglomerates rather than arkose. In
several places the contact zone is intensely sheared and the above rela­tionships are thereby obscured. A similar unconformity is observed inseveral places along the west contact of the sediments, but in largepart the relationships on the west contact so far mapped are obscured
by intense shearing, crushing and brecciation related to faulting in thecontact area. Observations made to date indicated that the sediments
were deposited on a basement of considerable relief.
In the southeast part of the area the basal arkoses are overlain con­formably by widely bedded polymictic conglomerate. This rock con­tains closely packed boulders ranging in size from fractions of inches to
2 feet, of granitic rocks, vein quartz, quartzite, basic and acidic
volcanic rocks, and fine-grained porphyritic rocks with a matrix ofarkosic sand. Lenses and beds of arkose are common, and the transi­tion to the overlying arkoses is marked simply by increasing amountsof arkose. The arkoses are buff, pink, or light grey rocks, which inplaces are conglomeratic in that they contain lenses and beds of con­glomerate, beds of conglomerate that are the thickness of one pebble,
and single pebbles or clusters of pebbles. Beds of red or green shale up
to 10 inches thick and beds of ill-defined zones of shale chip con­glomerate also occur. Crossbedding, parallel laminations, and ripple
laminations are found in some horizons in the arkoses. These arkoses
grade into a second band of polymictic conglomerate in which verylarge boulders of white quartzite are common. This unit is overlain byarkose.

The Nonacho Group is regarded as Helikian in age andso is younger than other successions that containuraniferous quartz-pebble conglomerates. The basal unit
is pink feldspathic quartzite, succeeded by a polymictic,
nonradioactive conglomerate. Above this is 200-300 ft(60-90 m) of white to greenish feldspathic quartzite con­taining thin beds that are rich in specular hematite andare faintly radioactive. This is succeeded by anonradioactive bed of conglomerate and some 200 ft (60m) of feldspathic quartzite. This, in turn, is overlain byparaconglomerate 40-50 ft (12-15 m) thick and a zone ofinterbedded quartz-pebble conglomerate and feldspathic
quartzite some 30 ft (9 m) thick.
The feldspathic conglomerates are as much as 4 ft (1.2m) in thickness and contain magnetite and minor pyrite.
This zone is overlain by greenish brown argillite that inplaces contains malachite. Uranium is most abundant in
the zone of quartz-pebble conglomerates, and, from anumber of samples, it has been said to average close to 1
Ib per short ton (0.5 kg per metric ton) (0.05 percent
U308) across 6 ft (1.9 m) and to extend several thousand
feet along the strike. The horizon has been traced inter­mittently for miles. No gold or tin is reported from theconglomerates, although both have been found in theradioactive hematitic beds.
Unfortunately, no samples of this material have as yet
been obtained by the Geological Survey of Canada nor

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORES

has any mineralogical work been done on theuraniferous zone. But this deposit is of more than casualinterest to this group because it is of much higher grade
than quartz-pebble conglomerates that are younger thanearly Aphebian age elsewhere.
In summary, I would like to propose that in searchingfor quartz-pebble conglomerate deposits, the best place
to look is in the lower part of early Proterozoic basins,
but it is unwise to ignore the younger basins, for viable
ores may be found there, and certainly vein andreplacement-type deposits do occur in such an environ­
ment.
From a consideration of the criteria outlined in this
discussion a map of Canada has been drawn that in­
dicates areas in which additional quartz-pebble uranium
deposits may be discovered (fig. 5).

X9

EFFECTS OF METAMORPHISM ON HURONIANROCKS IN THE GRENVILLE PROVINCE

Finally, we might consider the effects of high-grade
metamorphism and ultrametamorphism. Within theassemblage of paragneisses, amphibolites, schists,migmatites, and granitic rocks that compose the Grenville Province and other metamorphic terranes, beds ofquartzite and quartz-pebble conglomerate, some ofwhich are uraniferous, may be found from place to place.
In some of these metamorphic terranes pods of white
pegmatites containing biotite, magnetite, and uraninite
are abundant. These pegmatites may represent
remobilization and limited migration of uraniferous
quartz-pebble conglomerate deposits, thus completingthe full cycle by forming material that is generally
regarded as typical of the area of provenance of the con-

EXPLAN ATI 0 N
SOUTHERN PROVINCE
1. Elliot Lake-Agnew Lake area
SUPERIOR PROVINCE
2. Sudbury-Cobalt area
3. Papaskwasati basin
4. Otish Mountains
5. Sakami Formation
CHURCHILL PROVINCE
6. Hurwitz basin
7. Thelon basin
8. Nonacho basin
9. Sosan Formation
10. Chantrey basin
11. Equalulik basin
12. Chioak Formation
13. Kaniapiskau basin
14. Sims Formation
GRENVILLE PROVINCE
15. Wakeham basin
BEAR PROVINCE
16. Snare basin
17. Hornby Bay basin
18. Epworth basin
SLAVE PROVINCE
2
19. Goulburn basin
ARCTIC PLATFORM^
20. Glenely Formation
uj(Victoria and Banks Islands) -'FIGURE 5.-Potential areas for uraniferous quartz-pebble conglomerate deposits (from Little, 1974, fig. 3). Reproduced with permission of theCanadian Institute of Mining and Metallurgy.

X10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

glomerate deposits. The red intrusive pegmatites of the
Bancroft area may therefore be the product ofultrametamorphism of uraniferous quartz-pebble con­glomerate deposits, accompanied by soda and potash
metasomatism. Either process might produce the socalled porphyry uranium deposits of Frank Armstrong(1974). Careful working out of the stratigraphy inmetamorphic terranes, therefore, could be a useful
guide in tracking down these possible byproducts ofuraniferous quartz-pebble conglomerates.
REFERENCES CITED
Armstrong, F. C., 1974, Uranium resources of the future—"porphyry"uranium deposits, in Formation of uranium ore
deposits—Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens, Greece,6-10 May, 1974: Vienna, International Atomic Energy Agency, p.
625-635!
Bain, G. W., 1972, An oxygenated atmosphere for early life: Interna­
tional Geological Congress, 24th, Abstracts, p. 24.
Barnes, F. Q., 1972, Elliot Lake uranium district, in International
Geological Congress Guidebook, Field Excursion C67.
Bottrill, T. J., 1970, Uraniferous conglomerates of the Canadian
Shield, in Report of activities: Canada Geological Survey Paper
71-1, pt. A, p. 77-82.
Derry, D. R., 1960, Evidence of the origin of the Blind River deposits:
Economic Geology, v. 55, p. 906-927.
Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: Its occurrences andrecognition by microprobe: Colorado School of Mines Quarterly, v.
66, no. 4, p. 21-35.
Frarey, M. J., and Roscoe, S. M., 1970, The Huronian Supergroupnorth of Lake Huron, in Symposium on basins and geosynclines of
the Canadian Shield: Canada Geological Survey Paper 70-40, p.
143-156.
Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals andequilibria: New York, Harper and Row.
Heinrich, E. W., 1975, Hydrothermal alteration of the Mississagi con­glomeratic quartzite, Pronto Mine, Blind River, Ontario (abs.): In­
stitute of Lake Superior Geology, Abstracts and Field Guides, no.
21, p. 20.
Hoefs, J., Nielsen, H., and Schidlowski, M., 1968, Sulfur isotope abun­dances in pyrite from the Witwatersrand conglomerates:
Economic Geology, v. 63, no. 8, p. 975-977.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef, and the Black Reef:
Geological Society of South Africa Transactions, v. 58, p. 101-223.
Little, H. W., 1974, Uranium deposits in Canada—their exploration,
reserves, and potential: Canadian Institute of Mining andMetallurgy Bulletin, v. 67, no. 743, p. 155-163.
McGlynn, J. C., 1966, Thekulti Lake area, in Jenness, S. E., ed.,Report of activities, May to October 1965: Canada Geological
Survey Paper 66-1, p. 32-33.
Myers, W. B., 1970, An hypothesis of the chemical environment of theRand Goldfield, South Africa: U.S. Geological Survey Open-File
Report 238.
Pettijohn, F. J., 1970, The Canadian Shield, in Symposium on basinsand geosynclines of the Canadian Shield: Canada Geological
Survey Paper 70-40, p. 239-252.
Pienaar, P. J., 1963, Stratigraphy, petrology, and genesis of the Elliot

Group, Blind River, Ontario, including the uraniferous con­glomerate: Canada Geological Survey Bulletin 83.
Rao, B. V. R., 1975, Discovery of uraniferous Precambrian con­glomerate at Chikmagalur, Karnataka, India: Current Science, In­dia, v. 44, no. 5, p. 174-175.
Robertson, D. S., 1974, Basal Proterozoic units as fossil time markers
and their use in uranium prospection, in Formation of uranium ore
deposits— Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens, Greece,6-10 May 1974: Vienna, International Atomic Energy Agency, p.
495-512.
Robertson, D. S., and Douglas, R. F., 1970, Sedimentary uranium
deposits: Canadian Institute of Mining and Metallurgy Bulletin, v.
63, no. 697, p. 557-566.
Robertson, D. S., and Lattanzi, C. R., 1974, Uranium deposits ofCanada: Geoscience Canada, v. 1, no. 2, p. 8-19.
Robertson, J. A., 1968, Geology of Township 149 and Township 150:
Ontario Department of Mines Geological Report 57.
————1969, Township 157, District of Algoma: Ontario Department ofMines Preliminary Geologic Map P.561.
-1970, Geology of the Spragge area: Ontario Department ofMines Geological Report 76.
Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
the Canadian Shield: Canada Geological Survey Paper 68-40.
Roscoe, S. M., and Steacy, H. R., 1958, On the geology and radioactive
deposits of Blind River region, in United Nations, Survey of raw
material resources: International Conference on the Peaceful Usesof Atomic Energy, 2d, Geneva, Switzerland, 1-13 September
1958, Proceedings, v. 2, p. 473-483.
Rupert, R. J., Leahy, E. J., and Mirza, S., 1972, Subsurface
stratigraphy, Blind River-Elliot Lake Sheet, District of Algoma:Ontario Division of Mines Preliminary Map P.763.
Tugarinov, A. I., 1972, On the evolution of ore formation in the historyof the Earth: International Geological Congress, 24th, Resumes,
no. 24, p. 264-269.
Wanless, R. K., and Eade, K. E., 1975, Geochronology of ArcheanandProterozoic rocks in the southern District of Keewatin: Canadian
Journal of Earth Sciences, v. 12, no. 1, p. 95-114.

DISCUSSION FOLLOWING PAPERS BY DRS.
ROBERTSON, RUZICKA, ROSCOE, AND LITTLE

Schidlowski: Just a quick comment on the ore
miscroscopic slides shown during the last presenta­
tion. I was really puzzled to see that, for instance, thepyrite is in a virtually unabraided state. If I didn't
know the whole geological background story I would
not be prepared to admit that these are detrital
pyrites. What are the exact reasons for the extreme
immaturity of these detritals in the Blind River area?
Armstrong: To whom is this addressed?
Schidlowski: To all Canadians.
Roscoe: Well, I don't know which pyrite grains you're
speaking about there.
Schidlowski: Some of yours. I think you showed us one
slide, and they looked reasonably good. I even spotted
one single muffin.
Roscoe: Yes, there are some of them all right.
Schidlowski: But particularly the grains shown by Dr.
Ruzicka. I think they were in an unrounded state.

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORES

Roscoe: They may have been recrystallized. They may
have been overgrown.
Schidlowski: So they should have been buried very, very
quickly. Are these bands associated with lots of graywackes or subgraywackes? What is the exact nature of
the arenitic component of the sequence?
Ruzicka: It's a little bit arkosic.
Schidlowski: Okay, that would fit in with it. The next
part of my question goes to you specifically. Did I
understand you correctly that you would prefer to ex­
plain the uraninite particles within the carbon as hav­
ing precipitated from soluble uranium compounds car­ried into the basin?
Ruzicka: No,
Schidlowski: No?
Ruzicka: Because of the very high thorium content inthese uraninites.
Schidlowski: Okay, then I did not understand that too
well. Okay, that was all.
Nagy: I think that by now it seems to be rather clear thatthe association of carbonaceous matter with uranium
minerals and gold is a very important association. I'dlike to make a comment and ask a question of Drs.
Ruzicka and Schidlowski, and perhaps even Brian
Skinner would like to comment on this. Micropaleontologists generally believe that by two and a halfbillion years ago life was present on Earth only as very
simple, nonnucleated cells, bacteria, and blue-green
algae. Blue-green algae and bacteria, like all living
things, die. Now, if they die under anaerobic condi­
tions, then the decomposition decay product which isgoing to be formed is going to be methane. Now I
wonder, do we have to introduce any kind of fancy
theories about polymerization of abiological methane
or of very complex organisms recorded by the colum­
nar structure which surrounds the uranium grains?
Could these not be simply polymerization products byradiation from the uranium minerals of methane of
biological origin?
Ruzicka: I think this is quite possible. The hydrocarbon
occurs in several forms. The one I showed you reminds
me of algal matter or stromatolites, but quite often
you can find hydrocarbon in very young fractures andevidently it has moved.
Schidlowski: I would principally concur to this state­
ment.
Nagy: In other words, you pretty much agree with what
I am saying that the hydrocarbon could have comefrom biological material.
Schidlowski: Yes, the decomposition of organic material
in the widest sense. So since we had only bacteria andblue-greens, these should have constituted the prob­
able source.
Nagy: I'm glad you used that word "organic."

Xll

Grandstaff: Dr. Ruzicka, I wonder if you could clarify apoint on the extinction of the organic matter. I waswondering if the extinction bends around the pebble,
where the pebble passes down into the underlying
layer? Bending or disruption of the extinction mightsuggest whether the organic matter or the organiza­tion was present before or after compaction.
Ruzicka: It is a very good idea to observe this, but it
hasn't been done, unfortunately.
Myers: Dr. Ruzicka, I didn't quite get it clear, but did
that thucholite that you show come from the specificlocality beside the pebble?
Ruzicka: Yes. I mentioned that this sample was takenfrom the lower reef of the Denison Mine.
Roscoe: Dr. Ruzicka, I wonder if you would elaborate on
your proposal that eruption of volcanic rocks was anecessary part of the concentration of the pyritic con­glomerates. I think you probably said Elliot Lake ores,
but what is the distinction between ores and pyritic
conglomerates which occur stratigraphically far aboveores in the Mississagi Formation? I might add, too,that I have looked at Aphebian conglomerates nearvolcanic rocks, like at Flin Flon, Wilson Island[Nairn?], and there again they are hematitic and highthorium.
Ruzicka: I think you must have uraninite present andafterwards the conclusion is drawn that it is related to
the volcanic activity. It is practically a deduction of theobservation. It's a conclusion, it's a fact, it is there.
Hallbauer: I want to make a remark about this car­bonaceous material. If it is assumed that it is apolymerization product, then it should be plainhydrocarbon, without any inorganic inclusions, but all
the carbonaceous material from Blind River, as well asfrom South Africa, includes filamentous inorganic
material which cannot be explained by justpolymerization. I have shown a few examples from
Blind River where the filaments have all the same
diameter, all the same structure, so polymerization on­ly is not the answer. I think that it's possibly too easy
to say that paleobiologists have agreed that there
were only simple organisms in the Precambrian rocks;
therefore, nothing else may exist. I think this is a bit
too easy.
Schidlowski: May I just comment on that briefly. I would
say, Dieter [Hallbauer], that the reconstituted
minerals within the interfibral spaces just constituteredeposited material in the widest sense, so they justoccupied those places which were allowed by the car­bonaceous structures and they just occupied the little
spaces. That would be my explanation for that.
Hallbauer: What do you understand on the carbona­
ceous structure, first of all?

X12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

ScMdlowski: I just mean that the little space between
the individual carbon seams.
Hallbauer: No. The mineral matter is inside the car­bonaceous matter.
ScMdlowski: Yes, of course, the uraninite relics.
Hallbauer: No, it's even finer. The filaments have adiameter of about 1 or 2 microns, so how do they fit in­to this? I've got examples of real polymerized car­bonaceous matter from the Witwatersrand which oc­
curs in voids near the Carbon Leader Reef where it
was heated and was redeposited. That is realpolymerized carbonaceous matter which does not con­tain any inorganic material whatsoever. You can heatit, and it just is vaporized. This carbonaceous material
occurs as crusts on quartz crystals and others.
Armstrong: We've had a lot of discussion about this car­bonaceous material. I don't think we're going to beable to decide it. And time is running short. If there's
some new aspect to it that can be brought up for
discussion, fine, but maybe there are some other ques­
tions that people would like to ask.
Myers: I would like to make just one comment. If you go
and look in a stope of the Carbon Leader, at least thestopes that I have seen, the texture of the car­bonaceous material is variable inch by inch. You cansee repeated gradations between fly-speck carbon andindividual granules which presumably contain rem­nants of detrital uraninite. They get more and moreconcentrated, and finally the point is reached where
you get a silicate-rich thucholite, mechanically incor­
porating a bunch of the matrix; then this goes inanother half inch to beautifully columnar stuff, andthen in a few more inches you may have pebbles. All
you've got to do is go look in the stopes.
ScMdlowski: I would completely subscribe to that.
Myers: I would like to ask Stu Roscoe, has he seen thecontact between the top of the Dollyberry basalt andthe overlying gravels? Does it have a regolith on it?
Roscoe: Yes, there are regoliths on top of the Dollyberry;that's common.
Bourret: My question is directed principally to Dr.
Ruzicka. Yesterday, in discussing the Rand, we learned
and were told repeatedly about the tectonic activity, re­juvenation, and the raising of the source areas, and therepeated energy events which caused recycling of thematerial flowing into the Rand basin. My question is: Is
there any evidence in the Elliot Lake deposits that
there have been repeated tectonic events to supply
energy for reconstitution and reworking?
Ruzicka: I would say there are two such effects. One isthe alteration and introduction of uranium remobilized
from other portions of the conglomerate around cer­tain diabase dikes. For example, in the vicinity ofthese dikes you find uranium enriched as much as ten

times, and uranium content decreases with the dis­
tance from the dikes. But not all of these dikes havesuch an effect. Second, I would say that the influenceof faults, for example, is not much in the Elliot Lake
deposit.
Robertson: I would like to comment on the last two ques­
tions, first of all, on the Dollyberry: The structures ofthe volcanics suggests that they were in fact probably
subaerial rather than subaqueous. As far as breaks inthe conglomeratic sequence are concerned, we don't
have quartz-pebble conglomerate reworked into an
overlying quartz-pebble conglomerate, the situation
corresponding to, say, the Witwatersrand. We don'tseem to have that. We have small-scale crossbedding
and so on, and obviously coalescence of beds, but wedon't have the major reworking that you seem to haveon the Rand. The other comment I would like to makeis something that I was supposed to say and didn't.
There is in fact very little gold in the Blind Riverdeposits. I would suggest that this is because in thehinterland of the area I was describing and the area
shown in the gamma-spectrometry map, there is very
little greenstone. When you go east into the bottom
part of the Cobalt Plate, where the beds carrying
mineralization are probably somewhat younger thanthose at Blind River (we don't have a good correlationon them), there is a certain amount of gold along withuraninite in some beds. We don't really know whether
it is free gold or whether it's gold in sulfides, and inthat area the basement rocks contain much more in
the way of greenstone. I would relate the gold content
in conglomeratic ores in the Huronian to the pro­venance.
Roscoe: I'd like to add another postscript on what Jim[Robertson] said. I have seen a composite pebble suchas described in the Rand, a pebble of conglomerate
within conglomerate, in the Montgomery Lake area
but never in the Elliot Lake area.
Bourret: If the Blind River-Elliot Lake conglomerates
have not been reworked, very many times or perhaps
only slightly, then how do you account for the very
high uranium content compared to Witwatersrand
reefs, which have been reworked a great many times?
Ruzicka: I would say this is a concentration in the sourcearea and the concentration effected during the trans­
portation and deposition.
Saager: Dr. Ruzicka, do you find any cobalt and nickel
sulfides in the Blind River conglomerates, and do you
find any chromite in the Blind River?
Roscoe: Yes you do. You find millerite and arsenopyrite.
Ruzicka: But very little.
Roscoe: Chromite, cassiterite. There's a very rich suite ofheavy minerals, but quantitatively they are very small.
Whiteside: I'm sorry to jump up and down like this but

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORES

X13

just for the record, there are quartz-pebble conglome­rocks containing ilmenite to rutile. This texture is
rates in Ghana which more or less fit in with this suite
what's suggestive of this process, as well as the factof conglomerates that we've been discussing. The in­that you should have ilmenite at Blind River and you
terstitial mineralization, as far as I know, is hematite
don't have any. So presumably it was there andand there is no radioactivity, there's just gold.
something happened to it. What I am suggesting here
Staatz: Both in the South African talks and in the Cana­is that the "Pronto reaction" really is not valid; in it
dian talks we have discussed a lot about having brantitania from an undisclosed source goes to and in part
nerite. Brannerite is an exceedingly rare mineral ex­
replaces uraninite. I believe instead that uranium in
cept in this type of deposit. Now my question, I thinksolution from any of several possible sources, some of
I'll give it to Vlad [Ruzicka] there, is whether hewhich were mentioned yesterday, goes to a titania
believes that brannerite is derived like most of themineral and alters it to this delicate needle network
other minerals, that it is brought in like the pyrite andand mixture of various things in the small, rounded
the uraninite, are brought in, or whether brannerite is
grains which we now call brannerite and is found atindeed formed within the formation?
Blind River and perhaps also at the Rand. I conducted
Ruzicka: In my talk I mentioned Theis, who studied thisas many tests as I could on some other brannerites
problem, and I also mentioned the "Pronto reaction"from around the world using the microprobe andof Ramdohr. I would say the "brannerites" areobserving them with the microscope as well. The types
uranium-rich titania aggregates, which are relativelyof brannerites that we've been talking about thesecommon, but according to Stacey there are also a few
past two and a half days are decidedly different texgrains of true brannerite.
turally; they also are different compositionally
Staatz: Yes, but are these grains of "true brannerite" or
because they show a large variation in the amount of"true aggregates," are they brought in from the out­
uranium which occurs in them, not only in the entire
side or are they derived later in the rock?
grain but within those parts of the grain which can be
Ruzicka: In this case I would say the true brannerites
analyzed by microprobe. I guess my main point here isare of detrital origin.
that uranium migrated to a titanium mineral, I believe
Nick Ferris, Urania Exploration, Golden, Colo.: I will
ilmenite, and formed uraniferous rutile, or
attempt to elaborate on brannerite a little bit. At firsturaniferous leucoxene as Von Rahden and Hiemstra
at Blind River it was thought to be an entirely detrital
have said, which can in fact become rich enough to
grain; however, Paul Ramdohr in the mid-fifties noted
grade compositionally to true brannerite, if there is
the delicate complex texture inside the small, roundedsuch a thing, when the ilmenite has decomposed. Thisgrains and concluded that brannerite might beis the twist, then, on the "Pronto reaction." Now, thisauthigenic, having been formed from a pre-existingis also not the question that I raised my hand for. My
mineral. Because of his observations at the Pronto
other question I'll direct to any of the Canadian con­Mine at the southern part of the district, he suggested
tingent. In the Blind River ores there is a greenishthe so-called "Pronto reaction," in which titania
yellow sericitic matrix. I'd like to know the origin of
migrates to uraninite and combines with it to form
this. I've had various responses to this question before:brannerite, a uranium titanate. Ramdohr also felt thethat it is metamorphosed in-filling clay, a volcanic
same reaction was responsible for brannerite in thematerial, or decomposed, redistributed and altered
South African ores, although he referred to thefeldspar, but I'm still curious.
mineral as "uraninite ghosts." This has also been men­ Roscoe: I am, too. I assume it was a clay, it was detrital,tioned by Schidlowski, in 19661 believe, with excellent
and that there was potash restoration in it. It does in­photographs and scan photos from the microprobe.
corporate the finest detrital fraction within the con­
Schidlowski also subscribed, at that time at least, toglomerate, and it must have been in-filling. I assume
the "Pronto reaction" as the explanation. The texture
that there was alteration subsequently.
of brannerite from several Blind River localities has Meyer, Falconbridge Nickel Mines, Toronto, Canada: I'dconvinced me that brannerite from quartz-pebble con­like to make a comment first on Brad Myers' question,
glomerates is an authigenic mineral originating as awhich has already been answered by Jim Robertson
replacement of ilmenite when ilmenite altered to rutile
and Stu Roscoe. The volcanics at the base of thein the presence of uranium in solution. There is
Huronian do seem to be subaerial, and in fact there
evidence for this in the texture if you compare the tex­
seems to be a considerable unconformity, or certainly
tures of the Blind River brannerites, the needle net­a period of erosion, between the volcanics and thework and blades (Nick Theis has some excellent photos
basal Matinenda. Our drilling has shown this quiteof this and has done a lot of work on it) with the tex­
conclusively. When I say ours I mean Falconbridgetures you get when you do indeed alter beach sands or
Nickel Mines, back in the midsixties. So much for a

X14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

example, you have North Carolina, the Ningyo-togecomment. Second, the question, addressed to Dr.
deposit in Japan; you also have mineralization in the
Ruzicka. Would it be possible that the uranium in the
Permo-Triassic conglomerates of the west Car­
upper Proterozoic and Cambrian conglomerates ofpathians, and so on.
Russia is genetically more akin to the Colorado
Plateau type rather than the Lower Proterozoic con­glomerates in South Africa and Blind River? If your Meyer: My question was, what bearing on sedimentaryrocks between the lower Proterozoic and, say, the preanswer is "yes" on that, then what bearing does that
Devonian?
In other words, before you got any plant
have on exploration in sedimentary rocks in this vast
cover, which seems to have been important to deposi­time span between the two?
tion in post-Devonian rocks. I want to know specifical­
Ruzicka: I would say, and as I mentioned in my talk, thely pre-Devonian but post-lower Proterozoic.
mineralization is more epigenetic than syngenetic, so
it is more similar to the Colorado Plateau type than Ruzicka: You can have uranium mineralization in sand­Elliot Lake type. You have many occurrences ofstone in that time span, but this type of mineralization
uranium mineralization in young conglomerates. For
is different from the Elliot Lake type.

UNITED STATESThorianite from the
Hogatza Placer, Alaska
By MORTIMER H. STAATZ
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-Y

ILLUSTRATIONSFIGURE 1. Photograph showing cubes of thorianite from the Hogatza placer _________
2. Map showing generalized geology in the vicinity of the Hogatza placer ______Yl
2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHORIANITE l FROM THE HOGATZA PLACER, ALASKA
By MORTIMER H. STAATZThe Hogatza placer lies along Bear Creek, a tributaryof the Hogatza River, where it emerges from the Zane
Hills. This locality is about 240 km west of Fairbanks
and just east of the Seward Peninsula. A dredge has
mined gold from this placer since at least the early
1950's. Thorianite was found in a dredge sample col­lected by Thomas Miller during the mid-1960's, when hewas studying the precious and nonferrous metals of thisarea for the U.S. Geological Survey. In 1974 Miller andthe writer spent several days in the area collecting
samples of both placer material and the source rocks.
Concentrate from the sluice boxes on the dredge are ab­
normally radioactive. Two-thirds of the heavies in thisconcentrate are magnetite. The next most abundant
mineral is thorianite, which commonly occurs in small
black sand-size cubes (fig. 1). Other minerals identified,in approximate order of abundance, are zircon, sphene,
amphibole, spinel, garnet, ilmenite, hematite, gold,
epidote, and goethite. Thorianite is both isostructuraland isomorphous with uraninite, and a complete solid
solution exists between Th02 and U02 (Frondel, 1958, p.
47-49). The names thorianite and uraninite are
restricted to the halves of the series with Th f] U and
UD Th, respectively, in atomic percent. A sample of thethorianite from the Hogatza placer was analyzed by J.
N. Rosholt, Jr., of the U.S. Geological Survey in Denver,by the isotope-dilution method. It was found to contain47.3 percent Th02 and 36.8 percent U02 .
The thorianite in the Hogatza placer was derived froma gneissic monzonite (fig. 2) that forms a border facies ofthe otherwise rather massive granodiorite of the Zane
Hills pluton. The border facies is 5 to 10 times asradioactive as the main igneous mass. The uranium con­tent of 11 samples of the gneissic monzonite ranged
from 11 to 129 ppm (parts per million) and averaged 47
1 [This mineral prior to analysis was thought to be uraninite and was so reported at theworkshop.]* U.S. Geological Survey, Denver, Colo.

FIGURE 1. -Cubes of thorianite from the Hogatza placer.

ppm. The thorium content of these samples ranged from46 to 268 ppm and averaged 128 ppm. Concentrates of
the heavy minerals from these rocks vary somewhat, but
amphibole, pyroxene, magnetite, zircon, and sphene areabundant in most. Allanite and biotite occur in many
samples, and thorianite, garnet, pyrite, molybdenite,
muscovite, goethite, betafite, and thorite occur sparsely
in some samples. The thorianite occurs as small cubes
similar to those found in the Hogatza placer.
Much of the material in the Hogatza placer is glacial
till. During the Pleistocene a glacier came down Clear
Creek (fig. 2). Part of this glacier came over a saddle
separating this creek from the headwaters of Bear
Creek, scouring the dividing ridge of gneissic mon­zonite, and pushed down Bear Creek, where thematerial was deposited. The glacial till deposited along
Bear Creek has been reworked by the stream, but
beyond the eastern front of the Zane Hills where theglacier spread out, much of the glacial material does notappear to have been reworked.
Yl

Y2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESUraninite and thorianite are rare in modern placers.
The Hogatza placer is not unique, however, as uraninite
was reported in a black sand in British Columbia by Hal
Steacy (Steacy, 1953, p. 549). Whether conditions haveto be extremely favorable or not for uraninite orthorianite to occur in modern placers is not known. The
rarity of uraninite and thorianite in post-Precambrianplacers may not be a function of its preservation but
rather of its rarity in source rocks, such as granitic rocks
and veins.
References cited:

Hogatza
Placer
EXPLANATION
Alluvium

Gneissic Monzonite

Gra nod io rite

Andesite, Path of glacier01234

8 KILOMETERS

FIGURE 2.—Generalized geology in the vicinity of the Hogatza
placer. Derived from figures 3 and 4 of Miller and Ferrians (1968,
p. 7, 9).

Frondel, Clifford, 1958, Systematic mineralogy of uranium andthorium: U.S. Geological Survey Bulletin 1064, 400 p.
Miller, T. P., and Ferrians, 0. J., Jr., 1968, Suggested areas for pro­
specting in the central Koyukuk River region, Alaska: U.S.
Geological Survey Circular 570, 12 p.
Steacy, H. R., 1953, An occurrence of uraninite in a black sand:American Mineralogist, v. 38, nos. 5, 6, p. 549-550.

Basal Conglomerates and Weathered Zones in the
Marquette Range Supergroup, Northern Peninsula of MichiganAge, Indications of Atmospheric Oxygen, and Uranium Potential
By WILLIAM F. CANNON
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-Z

CONTENTS
Page

Abstract ______________________________
Introduction ____________________________
Geology of the Marquette Range Supergroup __________
General _____________________________
Chocolay Group ________________________
Menominee Group _______________________
Baraga Group __________________________
Relation to the Huronian Supergroup ______________
Similarities ___________________________
Differences ___________________________
Origin of the uraniferous quartz-pebble conglomerates and thepotential for their occurrence in Michigan _________
Origin of Blind River District uranium ore _________

Zl
1
1
1
2
3
4
6
7
8
8
8

Origin of the uraniferous quartz-pebble conglomerates and thepotential for their occurrence in Michigan- Continued
The problem of atmospheric oxygen _____________
Evidence for an oxidizing atmosphere during deposi­tion of theMarquette Range Supergroup ______
Evidence from iron-formation ___________
Evidence from weathered zones ___________
Significance of oxidized rocks ______________
Potential for uranium-bearing quartz-pebble conglomeratein Marquette Range Supergroup ______________
Exploration for uraniferous quartz-pebble conglomerate inMichigan—problems and discovery probabilities _______
References cited __________________________

Z9
9
9
10
12
13
15
16

ILLUSTRATIONSPage

FIGURE 1. Index map showing generalized distribution of rocks of the Marquette Range Supergroup and Huronian Supergroup _____
2. Schematic stratigraphic section showing unconformable relations between the Chocolay, Menominee, and Baraga Groups __3. Geologic map of the central part of the northern peninsula of Michigan showing the distribution of basal clastic rocks of the
Chocolay, Menominee, and Baraga Groups _____________________________________________
4-9. Photographs of
4. Basal clastic rocks of the Chocolay Group _____________________________________________
5. Quartz-pebble conglomerate at base of Menominee Group ___________________________________
6. Basal conglomerate of Baraga Group _______________________________________________
7. Iron-formation of the Menominee Group _____________________________________________
8. Even-bedded jaspilite, with layered jasper and interbeds of specular hematite _______________________
9. Oxidized granitic rocks at base of Baraga group _________________________________________
10. Map showing distribution of gamma-ray anomalies from airborne survey and known uranium concentrations __________
11. Curves relating probability of discovering one deposit to number of holes drilled, in the central northern peninsula of Michigan

Z2
3
4
5
6
6
10
11
11
14
16
in

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESBASAL CONGLOMERATES AND WEATHERED ZONES IN THE
MARQUETTE RANGE SUPERGROUP, NORTHERN PENINSULA OF MICHIGANAGE, INDICATIONS OF ATMOSPHERIC OXYGEN, AND URANIUM POTENTIAL

By WILLIAM F. CANNON J
ABSTRACTINTRODUCTION

The Marquette Range Supergroup is a thick accumulation of
sedimentary and volcanic rocks deposited between 2.6 and 1.9 billion
years ago. Conglomerates and quartzites that are basal in the sensethat they lie directly on Archean rocks, are of three ages, and crop out
along a strike length of about 600 kilometers. The conglomerates havebeen considered possible hosts for uranium ore because of close
geographic proximity, approximate age equivalence, and certain
lithologic similarities to uraniferous quartz-pebble conglomerates inthe Huronian of Canada. To date, no uranium deposits are known inconglomerates of the Marquette Range Supergroup, but extensive
cover by glacial deposits is a severe hindrance to exploration. There isa substantial possibility that even large deposits would not be found in
outcrop.
Radiometric dating suggests that the Marquette Range Supergroupis at least in part younger than the Huronion, but results are not con­clusive. The best guess as to correlation is that the lowest units of theMarquette Range Supergroup are equivalent to the uppermost units ofthe Huronian.
Iron-formations and weathered zones near the middle of the Mar­quette Range Supergroup contain indications of an oxygen-rich at­mosphere. Because uraninite is not stable in the presence of more than
traces of free oxygen, it may have broken down too rapidly to be con­centrated in placers during deposition of the Marquette Range
Supergroup, but the possibility of its metastable survival under an at­mosphere that was almost certainly less oxygen-rich than at present
seems quite possible.
A favorable uranium potential for conglomerates of the Marquette
Range Supergroup is indicated by the widespread presence of
anomalously uraniferous Archean basement rocks, abundant quartzpebble conglomerate in basal units, uranium concentrations in blackslates and iron-formations in slightly younger parts of the supergroup,and monazite-rich placer deposits in some of the quartz-pebble con­glomerates.
Exploration of the area is hindered by very extensive drift cover andvery few outcrops. Exploration programs should not be designed so as
to be dependent on outcrop data but rather should employ techniques
capable of detecting ore beneath several tens of feet of drift. Direct
sampling of bedrock by diamond drilling may, in the long run, be themost efficient exploration tool.

Vast deposits of uranium occur in fossil placer
deposits in conglomerates older than 2 b.y. (billion
years) in many parts of the world, the Witwatersrand ofSouth Africa and Huronian of Canada being the best
known and economically most important. The Mar­quette Range Supergroup in northern Michigan and nor­thern Wisconsin is similar to the Huronian in age andgeographic location, as well as in certain lithologic andstratigraphic features. This paper considers thepossibility that deposits of uraniferous quartz-pebble
conglomerate like those in the Huronian might bediscovered in the Marquette Range Supergroup. Thispossibility is assessed in terms of the possible correla­tion of the two sequences, the genesis of ores in the
Huronian, and the possibility that similar genetic condi­
tions prevailed during deposition of the Marquette
Range Supergroup and in terms of local geologic
features of the Marquette Range Supergroup.

1 U.S. Geological Survey, Reston, Va.

GEOLOGY OF THE MARQUETTE RANGE
SUPERGROUP
GENERAL

The Marquette Range Supergroup is a thick sequenceof metasedimentary and metavolcanic rocks in the cen­tral and western parts of the upper peninsula ofMichigan and northern Wisconsin (fig. 1). The age of therocks is firmly bracketed between 2.6 b.y., the age of thebasement on which they unconformably lie, and 1.9 b.y.,
the age of the Penokean orogeny during which they
were deformed and metamorphosed.
The supergroup was deposited unconformably on an
Archean granite-greenstone terrane. Little direct
evidence of the nature of the land surface is known
because of later deformation, but because a relatively
long time span separates the late Archean orogenic acZl

Z2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESBLIND RIVER
ilSTRlGT

AREA OFTHIS REPORT

0
i i i
I
\
0

i

i

100
KILOMETERSi___i
I
100 MILES

EXPLANATIONMARQUETTERANGESUPERGROUP

HURONIAN
SUPERGROUP

OTHERPRECAMBRIAN
rocks

PALEOZOIC
rocks

FIGURE 1.-Index map showing generalized distribution of rocks of the Marquette Range Supergroup and Huronian Supergroup.

tivity from the later sedimentation and because thebasal Marquette Range sediments are similar to blankettype clastic-carbonate sequences, the surface is pre­sumed to have been of low relief.
In addition to the unconformity at the base of the sec­tion, two major unconformities are within the section asshown in figure 2 and are used to separate groups.
Because of erosion of older sediments along these un­conformities, and perhaps because of onlap relations,the basal clastic units of three different ages lie on Archean basement at various places. Because uraniferous
conglomerates in the Huronian are confined to con­glomerates that lie on or near the Archean rather thanthose within the sedimentary sequence, the discussion of
conglomerates in the Marquette Range Supergroup em­
phasizes those that lie on or near Archean basement
rather than those within the section. The discussion is

also confined to the central part of the upper peninsulaof Michigan and does not include correlative rocks in the
Gogebic district of the western upper peninsula andpossibly correlative rocks in northern Wisconsin.
CHOCOLAY GROUPThe Chocolay Group, the oldest unit of the Marquette
Range Supergroup, is an orthoquartzite-carbonate se­quence deposited in a stable tectonic environment. In
general, the basal unit is marine quartzite with local con­glomerate beds, but tillites and waterlaid conglomerate,arkose, and argillite of glacial affinity are known in the
Menominee district (Pettijohn, 1943), the Calumet
trough (James and others, 1961), the eastern Marquette
district (Gair and Thaden, 1968; Gair, 1975), and DeadRiver basin (Puffett, 1969; Clark and others, 1975) (fig.

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN

Z3B

GRANITIC-GNEISSFIGURE 2.-Schematic stratigraphic section showing unconformable relations between the Chocolay, Menominee, and Baraga Groups.
Radiometric age determinations are discussed in the text.

3). The rocks of glacial affinity all lie directly on Archeanrocks and are believed to be of equivalent ages. Clasts
are mostly granitic rocks, with other Archean rocks alsopresent. The matrix generally is chloritic, and the rocks
typically are poorly sorted (fig. 4). Their wide distribu­
tion and the lack of high relief on the Archean surface
suggest that these rocks are a product of continental
glaciation and originally were probably much morewidespread than indicated by their present degree ofpreservation. The glacial deposits grade upward into
marine deposits of sericitic schist and orthoquartzite;
where glacial deposits are absent the marine sequencelies directly on the Archean. In the Menominee district(Bayley and others, 1966) the lowest 30-60 m of themarine sequence are pale-green sericitic quartzite andschist that is overlain by white, gray, and pink vitreousquartzite. Some basal beds contain abundant magnetite,
tourmaline, and some pyrite. In central Dickinson Coun­ty (James and others, 1961) as much as 100 m of grayand green sericitic quartzite and quartzose schist (25percent sericite, 75 percent quartz) are at the base of thesection and are overlain by vitreous quartzite containing
zones of quartz-pebble conglomerate as much as 1 m
thick. In the eastern Marquette Range (Gair andThaden, 1968; Puffett, 1974) sericitic and feldspathic
schist and quartzite underlie vitreous white, gray, andpink quartzite containing minor pebbly zones bearing
pebbles of vein quartz, chert, and quartzite. Paleocurrent measurements (Pettijohn, 1957) indicate a source
to the northwest for the marine clastic rocks of thegroup.
MENOMINEE GROUP

The Menominee Group is a marine sequence generallywith basal quartzite that grades upward into finer
grained clastic rocks that in turn grade upward into
iron-formation. The group contains the major economiciron-formations of the area. Commonly the Menominee
Group lies unconformably on the Chocolay Group, but inmuch of the western Marquette district it lies directly on
Archean rocks (Puffett, 1974; Cannon and Klasner,
1974, 1975a, b; Klasner and Cannon, 1975a, b; Clark andothers, 1975). There, the group contains a basal ortho­quartzite as much as 150 m thick with conglomeratezones near the base. Most conglomerates bear pebbles ofvein quartz and chert, but other rock types are also pre­sent, including carbonate rocks, older quartz-pebble con­glomerate, and granitic rocks. Clasts vary from rounded
to subrounded and range in diameter up to about 30 cm(fig. 5). Locally the conglomerate matrix contains small
amounts of pyrite, some of which is in rounded, possibly
detrital grains.

Z4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESEXPLANATIONDICKINSON GROUPBASAL CLASTIC ROCKS OF:BARAGA GROUP-;,- v::^.v;::;;: MENOMINEE GROUP
CHOCOLAY GROUPFAULTUNCONFORMITY——————— CONTACT OF
UNDETERMINEDNATURE

20 MILES
0

20 KILOMETERS

FIGURE 3. - Geologic map of the central part of the northern peninsula of Michigan showing the distribution of basal clastic rocks of the Chocolay,
Menominee, and Baraga Groups. Where no pattern is shown, basal rocks are volcanic, iron-formation, or dolomite.

BARAGA GROUP

The Baraga Group is a very thick accumulation of
eugeosynclinal graywacke, shale, iron-formation, andvolcanic rocks, but the basal part of the section is
generally a shallow-water transgressive sequence that
contains the most extensive basal conglomerates in the
Marquette Range Supergroup. Typically, coarse clastic
rocks at the base grade upward into slates and inter­calated iron-formation. Less commonly, slate and iron-formation lie directly on Archean rocks with only local
lenses of basal conglomerate.
Three types of conglomerate are distinguished:
1. Thin (3 m or less) oligomictic ferruginous chertpebble conglomerate derived mostly by reworkingof iron-formation of the Menominee Group. Suchrocks are most common where the Baraga Group
lies directly on the Menominee Group but are also
found locally lying on Archean rocks.

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN

2. Thin, locally derived lenses of conglomerate contain­
ing mostly fragments of immediately underlying
Archean rocks in a quartzose or chloritic matrix(fig. 6A). Some lenses are demonstrably related to
the flanks of local topographic highs on the Ar­
chean surface and are most likely wave-washedbedrock rubble.

Z5

Wedges as much as 100 m thick of polymictic con­glomerate with clasts generally 1-10 cm indiameter but locally as much as about 1 m indiameter (fig. 6£). Clasts are mostly ironformation, vein quartz, and older sedimentaryrocks. The matrix is quartzose and typically very
hematitic. Four such wedges are known, all in thei^^^M?^ ;*>'VtTV-' *rvi
•1^'3 •j!**'°r_.2?

' x".'W i «P*t>"..'«fcCk* X" ""•*Mil- '.si'... 1-'-t&iS«.A^»*T *^f. f? ».-•*•..+-&.

>•*,•»«.

vK^Ti.:'-*-tot. •»'-~A~jp«^ - jp*';"

FIGURE 4.-Basal clastic rocks of the Chocolay Group. A, Tillite composed of large clasts of granitic rock in chloritic matrix. B, Fluviatile
deposits of granitic pebble conglomerates interbedded with finer
grained arkose and argillite. C, Dropstone clast of granite in chloritic
argillite. D, Ripple-marked marine orthoquartzite.

Z6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 5. -Quartz-pebble conglomerate at base of Menominee Group.
Subangular to rounded pebbles of vein quartz are in arkosic matrix.
Magnet is about 4 cm in diameter.

Marquette district. Two are demonstrably positive
depositional forms, deltas or alluvial fans, rather
than channel deposits, and their spatial distribution
suggests that they were derived from an early
uplift of the Archean rocks that now bound theMarquette trough on the south. One wedge, in theeastern Marquette district, contains monazite-richplacers within quartz-pebble conglomerate; grades
are locally as high as 110 Ib of monazite per shortton (55 kg per metric ton)(Vickers, 1956a). All four
wedges lie unconformably on the Menominee
Group, but the abundance of vein-quartz pebbles in
the conglomerates indicates that Archean base­ment rocks were being eroded and transported tothe depositional basin.
RELATION TO THE HURONIAN SUPERGROUP

The relation of the Marquette Range Supergroup to
the Huronian Supergroup has been in dispute for nearlya century and is still not known with certainty in spite of

FIGURE 6.-Basal conglomerate of Baraga Group. A, Conglomeratecomposed of angular fragments derived from underlying granitic
gneiss and rounded fragments of vein quartz (lighter colored clasts).
Contact of conglomerate with gneiss (gr) shown by dotted line. B,
Conglomerate with fragments of iron-formation from underlying
Menominee Group and well-rounded vein quartz fragments derived
from a more distant Archean source area.

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN

numerous radiometric age determinations. This problem others, 1961), and the radiometric age may not be theis especially important in assessing the uranium poten­ time of emplacement. No intrusive rocks or thermal
tial of conglomerates in the Marquette Range events of this age are known elsewhere in the area. Sec­Supergroup because of the large uranium deposits of the ond, radiometric determinations on other rocks only ashort distance from the porphyritic red granite yield
Blind River district in basal rocks of the Huronian.
For many years the rocks of the Marquette Range spurious results and in places do not define an Rb-Sr
Supergroup were called Huronian. Early workers sug­ isochron (Banks and Van Schmus, 1971), suggestinggested the correlation based on similarities in rock that isotopic systems have been disturbed, perhaps overtypes, approximately equivalent age, and close a large area. Finally, the porphyritic red granite is notgeographic proximity. In 1958, James (1958) suggested ! proved to be older than the Marquette Range Superthat the rocks in Michgan be called Animikie Series i group and is nowhere in contact with it; the proposed
because correlation with the Animikie Group in Min­ age relations are based solely on structural arguments.
nesota seemed more certain than with the Huronian. In Thus, even if 2.1 b.y. is the age of emplacement of the1970 (Cannon and Gair, 1970) the term "Animikie" was porphyritic red granite, it cannot, with complete con­abandoned and the rocks were given the local name of fidence, be considered to define the age of the Mar­
Marquette Range Supergroup, primarily to avoid con­ quette Range Supergroup.
flict with the stratigraphic code of the U.S. Geological
A single age on volcanic rocks in the Baraga Group is
Survey.
about 2 b.y. (Banks and Van Schmus, 1971). If this age isOver the past ten years radiometric age determina­ correct, the Baraga Group is younger than the Huro­tions have helped to define the absolute ages of the nian, but correlation of the Menominee and Chocolay
Huronian Supergroup and the Marquette Range Super­ Groups with the Huronian is still possible. A correlationgroup. Both sequences lie unconformably on Archean of the Gowganda Formation of the Huronian with basal
rocks at least 2.6 b.y. old. In Ontario, dikes and sills of Chocolay Group tillites of the Marquette Range Super­
Nipissing diabase were intruded after Huronian group and of the Lorrain Quartzite with quartzites ofsedimentation and probably after early phases of the Chocolay Group has been suggested by Youngregional deformation of the Huronian. An age of about (1973). This correlation is attractive in light of striking
2.15 b.y. is well established for the Nipissing (Van lithologic similarities of these parts of the section and of
Schmus, 1965; Fairbairn and others, 1969) and seems a the relative "uniqueness" of continental glacial deposits
reliable minimum age for the Huronian. Fairbairn and in time, but it is weakened by the presence of other con­others (1969) determined an age of 2.29 b.y. for argillite glomerates in the Huronian, also believed to be of glacialin the Gowganda Formation, which may be the time of origin (Young and Chandler, 1968; Roscoe, 1969; Frarey
deposition or diagenesis. Thus deposition of the Huron­ and Roscoe, 1970), which could also be correlated with
ian Supergroup is rather reliably bracketed between 2.6 the Chocolay Group.
and 2.15 b.y. ago.
In my opinion, the question of correlation cannot be
In Michigan, the Marquette Range Supergroup was answered adequately with existing data, and the possi­
deformed and metamorphosed about 1.9 b.y. ago (Banks bility should be held open that part or all of the Mar­and Van Schmus, 1971; Goldich and others, 1961; quette Range Supergroup is equivalent in age to the
Aldrich and others, 1965). Banks and Van Schmus Huronian Supergroup. Surely, in evaluating the possi­(1971) report a zircon concordia age of 2.1 b.y. for "por- bility of the occurrence of uraniferous quartz-pebble con­phyritic red granite" that forms two small plutons sur­ glomerates in Michigan, the Marquette Range Super­rounded by Archean rocks in central Dickinson County. group should not be excluded solely on the basis of the
These plutons were interpreted to have been emplaced scant radiometric data suggesting that it might beprior to deposition of the Marquette Range Supergroup younger than rocks in the Huronian to which known oc­(James and others, 1961) on the basis of internal struc­ currences of such deposits seem to be restricted.
tures that were not believed consistent with postIn the following sections are listed some major
Marquette Range structures. Banks and Van Schmus similarities and differences between the Marquettetherefore considered that 2.1 b.y. was a maximum age Range Supergroup and Huronian Supergroup that il­for the Marquette Range Supergroup, an age which, if lustrate the geologic grounds on which the question ofcorrect, invalidates any correlation with the Huronian. correlation can be argued.
It is my opinion that enough uncertainties exist in thedata that the radiometric age should not be considered a
SIMILARITIES
maximum age for the Marquette Range Supergroup forseveral reasons. First, the porphyritic red granite has 1. Both supergroups are thick sedimentary accumula­
had a complex postemplacement history, including
tions deposited sometime during the interval 2.6 togranulation and recrystallization of minerals (James and1.9 b.y. ago.

Z8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

2. Both contain relatively uncommon Precambrian rocktypes, such as glacial deposits, thick orthoquartzites, and carbonate rocks.
3. Both were intruded by major systems of mafic dikes
and sills. The Huronian was intruded after sedi­mentation about 2.15 b.y. ago. The Marquette
Range Supergroup was probably intruded several
times, the major event being during volcanism dur­ing Baraga Group deposition (Cannon, 1973), forwhich one age of 2 b.y. has been obtained (Banksand Van Schmus, 1971).
4. The two supergroups are in close geographic prox­
imity and approximately on strike with each other(fig. 1). In outcrop they can be traced to within
about 250 km of each other, the intervening area
being covered by Paleozoic rocks or the waters of
Lake Superior. On aeromagnetic maps, trends ofthe Marquette Range Supergroup can be traced
eastward beneath Paleozoic cover for several tensof kilometers before being truncated by trends ofyounger Keweenawan rocks. The Keweenawanrocks are now believed by many to occupy an ar­
cuate rift with roughly the shape of a southward
opening U, the northern apex being approximately
in the center of Lake Superior. The interior por­tion, including the Marquette Rang 3 Supergroup,
is believed to have moved southward as the rift
opened about 1.1 b.y. ago. If this is correct, theMarquette Range Supergroup was deposited some­what north of its present location with respect tothe Huronian and was, prior to Keweenawan rif­ting, even closer to the Huronian than at present.
Taking these factors into consideration, it is likelythat no more than 150 km separated the two super­groups prior to 1.1 b.y. ago. Considering the likeli­
hood that the sedimentary rocks originally extend­
ed well beyond their present outcrop areas, it is notunlikely that both could have formed in the same
sedimentary basin.
DIFFERENCES

1. The Marquette Range Supergroup contains thickdeposits of iron-formation at several stratigraphic
horizons, whereas iron-formation is virtually
unknown in the Huronian.
2. Deposition of the Marquette Range Supergroupculminated in accumulation of a very thick sequenceof eugeosynclinal deposits. A similar tectonic en­
vironment of sedimentation is not recognized in the
Huronian.

ORIGIN OF URANIFEROUS QUARTZ-PEBBLECONGLOMERATES AND THE POTENTIAL FOR
THEIR OCCURRENCE IN MICHIGANORIGIN OF BLIND RIVER DISTRICT URANIUM ORE

Because of the many similarities between the Huro­nian rocks that contain the Blind River uranium deposits
and the Marquette Range Supergroup, considering theorigin of Blind River ore is one means of assessing thepossibility that similar ore might be found in Michigan.
The mode of origin of Blind River uranium ore has
been controversial, virtually since the deposits werediscovered more than 20 years ago. It is not my purpose
to discuss these arguments in detail. Many of my co­authors in this volume are much more qualified than I,
and the subject is thoroughly covered. In general, opin­
ion has been divided between two points of view, onethat favors an epigenetic (hydrothermal or groundwater) origin for uranium mineralization and the in­variably associated pyrite and a second that favors asyngenetic (clastic) origin for uranium minerals andpyrite. Among supporters of the latter view are included
J. A. Robertson (1968), D. S. Robertson (1974), andRoscoe (1969, 1973), all of whom are intimately ac­
quainted with Blind River deposits through many yearsof research. Judging from the most recent literature,
opinion is approaching a consensus in favor of a clastic
origin.
Recently, Roscoe (1969, 1973) and D. S. Robertson(1974) have expanded the theory of placer origin to in­clude implications on the evolution of the atmosphere. In
their view, the atmosphere contained no free oxygen
prior to about 2.2 b.y. ago, thus allowing minerals suchas uraninite and pyrite, unstable in an oxygen-rich at­mosphere, to be eroded, transported, and accumulated
into placers. They believe the atmosphere began to ac­
cumulate free oxygen shortly after deposition of thebasal Huronian units, thus inhibiting further develop­
ment of such placers. They based their arguments on the
drab color and generally unoxidized nature of lower
Huronian rocks, including the uraniferous conglom­
erates, and of underlying fossil soils, in contrast to moreoxidized units in the upper parts of the Huronian and inother sequences believed to be slightly younger than the
Huronian. D. S. Robertson described such sequences ona worldwide scale and suggested that the change in ox­
idation state of the sediments reflects a unique point in
atmospheric evolution that can be used as a world-widetime marker. In the view of Roscoe and D. S. Robertson,
uraniferous conglomerates must be restricted to rocks
older than the oldest rocks in the Marquette Range
Supergroup.

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN
THE PROBLEM OF ATMOSPHERIC OXYGEN

The question of when free oxygen first appeared in theatmosphere has now become a principal issue in con­sidering the origin and stratigraphic distribution of
uraniferous quartz-pebble conglomerates, and I would
like to discuss briefly some general points before
discussing evidence from the Marquette Range Super­group in more detail. Many researchers have concluded
that the presence of vast iron-formations about 2 b.y. old
indicates that the atmosphere was anoxygenic at thattime so that iron could be transported in the relatively
soluble ferrous form. An oxygen partial pressure ofabout 10 ~ 70 atm is required to permit significant concen­
trations of dissolved ferrous iron in water. The presenceof oxide-rich iron-formation throughout much of record­
ed geologic time prior to 2 b.y. ago requires a source tosupply oxygen during deposition if ferric iron minerals
were precipitated from dissolved ferrous iron. If the at­mosphere were anoxygenic, some alternate source isneeded, and Cloud (1965, 1973) has suggested thatprimitive photosynthetic microorganisms produced ox­ygen that was immediately consumed by ferrous iron in
sea water. Thus, oxygen partial pressure was bufferedat about 10- 70 atm as long as sufficient ferrous iron wasin the oceans and iron-formation was precipitated in theprocess. In Cloud's view, this process culminated about 2
b.y. ago in a great episode of iron-formation deposition,
consuming the supply of ferrous iron and allowing ox­
ygen to begin to accumulate in the atmosphere.
Recently, Holland (1973) and Drever (1974) have pro­
posed models that can account for iron-formations withan atmosphere containing substantial free oxygen.
Drever suggested that with an atmosphere containing
about one-fifth or less of present oxygen levels and with
sufficient biologic productivity to supply organic carbon,
ocean water below the thermocline would be anoxygenic
because of uptake of oxygen by free carbon. Iron in
detrital sediments deposited below the thermocline
would be partially dissolved to ferrous iron and ac­
cumulate to a concentration of about 10 ppm (parts per
million). As this water welled up into shallower levels, it
became oxidized by contact with the atmosphere andprecipitated iron minerals as the solubility of iron wasdiminished.
Although this model is not demonstrably the means bywhich iron-formation was deposited, it does, never­theless, provide a plausible means of forming ironformation under an oxygen-rich atmosphere, and thepresence of iron-formation cannot be taken as evidenceof an anoxygenic atmosphere with as complete con­fidence as has been done by many workers.
These arguments concerning iron-formation have im­portant bearing on the possible occurrence of uraninite
and pyrite as placer minerals and should be considered

Z9

along with more direct evidence from the uraniferous
conglomerates themselves. If one accepts models for theorigin of iron-formation requiring an anoxygenic at­mosphere, with oxygen partial pressures about 10~ 7U
atm, then one must also accept that uraninite and pyrite
are stable so long as iron-formations are being
deposited, because both minerals remain stable at ox­ygen pressures much greater than 10~ 7U. For instance,
Holland (1962) has calculated that uraninite remains
stable until oxygen partial pressure reaches about 10~ 21
atm. Thus, the presence of iron-formation could betaken to indicate that both pyrite and uraninite werestable under the atmosphere existing at the time wheniron-formations were deposited.
If, on the other hand, one accepts a model such as
Drever's, in which iron-formations are formed under arelatively oxygen-rich atmosphere, then the presence ofiron-formation cannot be used to conclude that pyriteuraninite placers could be formed stably. Rather, it
would be likely that both minerals would be unstable, atleast in a strict thermodynamic sense, under such an at­mosphere.
EVIDENCE FOR AN OXIDIZING ATMOSPHERE DURING
DEPOSITION OF THE MARQUETTE RANGE SUPERGROUP

Pyrite and uraninite have never been found in large
quantities in conglomerates in the Marquette Range
Supergroup, and there have been no studies bearing
directly on the stability of uraninite and pyrite during
deposition. Some features in both iron-formation andweathered zones along unconformities, however, do con­tain evidence of an oxygen-rich atmosphere during atleast part of the time of deposition of the supergroupand indirectly bear on the potential for pyrite-uraninite
placers in Michigan.
EVIDENCE FROM IRON-FORMATIONThe Negaunee Iron-formation, the principal ironformation of the Menominee Group in the Marquette
district, contains textural and mineralogical featuresthat suggest an oxygen-rich atmosphere during deposi­tion. The Negaunee, which contains many lithologic
types of iron-formation, includes persistent
stratigraphic units of wavy-bedded jaspilite containing
irregular, lenticular, commonly oolitic beds of jasperthat are interbedded with specular hematite (fig. 7A).
The wavy bedding and oolitic character indicate a highenergy and presumably shallow-water environment ofdeposition. Other units contain more reduced
assemblages that, where not strongly altered bymetamorphism, include siderite, minnesotaite, andmagnetite. They typically are rather even bedded, sug­gesting quiet water during deposition (fig. IE).

Z10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESIn much of the district the correlation of bedding
characteristics with oxidation state of iron is strikingly
consistent. Even-bedded iron-formation, presumably of
deep-water deposition, contains reduced mineral
assemblages, whereas wavy-bedded oolitic ironformation, of shallow-water deposition, generally con­tains thoroughly oxidized minerals. One outcrop shows
an interlayering of these lithologic types in beds a meteror less thick (fig. IE), and the abrupt change in both bed­
ding characteristics and oxidation state is repeatedmany times across the outcrop. These lithologic changes
must reflect slight variations in water depth or depth ofwave action, so that wavy-bedded hematitic ironformation was deposited above wave base, whereas
even-bedded, more reduced iron-formation was
deposited below wave base. It follows that the source ofoxygen to form the thoroughly oxidized wavy-bedded
jaspilite was the atmosphere. The agitated water inwhich wavy-bedded jaspilite was deposited brought
oxygen-rich water to the sediment-water interface, sothat all iron was oxidized to the ferric state. The quieter
water in which even-bedded iron-formation was
deposited did not maintain sufficient chemical com­munication with the atmosphere to provide enough ox­ygen to oxidize all available iron, and magnetite, ironcarbonate, and iron silicates were therefore stable.
EVIDENCE FROM WEATHERED ZONESFIGURE 7. -iron-formation »f the Menominee Group. A, Wavy-beddedocr
n (lighterft layers). B,
p Even bedded chertol specular hematite
magnetite-iron silicate iron-formation (units a) in contact with wavy

bedded jaspilite (units 6).

Northern Michigan is an area of very poor bedrock ex­posure. Consequently, exposures of unconformities are
rare. Rarer still are exposures showing effects ofweathering in rocks beneath unconformities. Never­theless, enough data have been gathered over many
years of study to substantiate some rather firm conclu­
sions.
The most extensive weathered zone known isdeveloped beneath the unconformity separating theMenominee and Baraga Groups and is best developed in
the Negaunee Iron-formation in the Marquette district(fig. 8). Van Rise and Leith (1911) proposed that jaspilite
near the top of the Negaunee Iron-formation formed byweathering of originally sideritic iron-formation. The
presence of fragments of jaspilite in basal con­glomerates of the overlying Baraga Group was taken as
evidence that weathering had taken place before Baraga
Group deposition. James (1954), who argued thatjaspilite was a primary facies of iron-formation and had™t formed by oxidation of sideritic iron-formation, cast
. .
TT J TT.
-, r . , ,
-,
-,
,1
•
doubt on Van Hlse and Leith s weathering hypothesis.

Certainly the features discussed in the previous section

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN—• • -—;»'-_, •Zll

i—IK i

FIGURE 8. - Even-bedded jaspilite with unformly layered jasper (darker
layers) and interbeds of specular hematite (lighter layers).

indicate that some jaspilite is a primary sediment, yetother evidence strongly suggests that some jaspilite hasformed by weathering and oxidation of sideritic ironformation. Gair (1975) describes exposures in the Cliff
Shaft Mine in the eastern part of the Marquette district,in which a thick jaspilite unit contains "islands" ofsideritic iron-formation and jaspilite grades on strike in­to sideritic iron-formation. These changes undoubtedly
occurred before regional deformation and metamorphism about 1.9 b.y. ago, and because such rocks are
restricted to a stratigraphic interval immediately below
the pre-Baraga Group unconformity, they seem likely tohave formed by oxidative weathering as originally pro­
posed by Van Hise and Leith.
In many places the Baraga Group lies directly on Archean granitic rocks or greenstone (fig. 3), and during
the past year I have searched for evidence of weathering FIGURE 9.-Oxidized granitic rocks at base of Baraga Group. A, Grayin these rocks beneath the unconformity. At only two
granitic gneiss (a) converted to pink granitic gneiss (b) beneath unlocalities, both in the Dead River basin, have I foundconformable contact with black pyritic shale (c) at base of Baraga
Group. B, Pebble of gray granitic gneiss (upper part of specimen)features that might be caused by oxidative weathering.
with rind of pink oxidized granite in black pyritic shale of basal
At both localities, gray granitic gneiss beneath the un­
Baraga Group.
conformity has a thin pink zone developed on it im­
mediately beneath the unconformity (fig. 9A). The zone,
which varies from about 1 cm to several centimeters in pyritic shale that shows little or no oxidation, so the pinkthickness, may be caused by oxidation of the gray zone beneath the unconformity must have formed before
gneiss. The change in color is caused primarily by a color sedimentation. A few pebbles of the gray gneiss in local
change in plagioclase. White to gray plagioclase, typical conglomerate lenses have similar pink rinds (fig. 9E)
of most of the gray gneiss, is altered to pink and orange that also were probably formed by oxidative weathering
near the unconformity. In addition, mafic components, prior to sedimentation.
mostly biotite, apparently were leached from the pink
At the same localities, pockets of deeply weatheredzone. The basal sediments of the Baraga Group are black limonitic material are along the unconformity. The

Z12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

nature of this material prior to recent oxidation tolimonite is not known but it may be a remnant of ironrich residual soil that would be expected to form underan oxidizing atmosphere.
Elsewhere, Archean rocks beneath the unconformity
show no effects of weathering or oxidation. If
weathered zones or residual soils had formed, they wereapparently totally removed and reworked during
Baraga Group transgression. Thus weathering and ox­
idation appear to have been more extensive in the ironformation, which is similar to the case prevalent today.
Weathering during late Precambrian or Phanerozoic
time has, in places, oxidized iron-formation for hundredsof meters beneath the surface, whereas weathering ofsurrounding rocks was not deep enough for weatheredzones to have survived Pleistocene glaciation.
In summary, two independent lines of evidence lead tothe conclusion that the atmosphere contained abundant
free oxygen at least as early as the close of Menominee
Group sedimentation, at least 2 b.y. ago. Iron-formation
was apparently readily formed under such an atmos­
phere. Extensive iron-formations were deposited in atleast four younger stratigraphic intervals of the Marquette Range Supergroup. From this I conclude that thepresence of iron-formation cannot be used to infer ananoxygenic atmosphere and is therefore not a reliable
guide in assessing the potential for placer accumulationof uraninite.
Data on weathering along unconformities at the baseof the Chocolay and Menominee Groups are very scant,but no evidence of oxidation along these unconformities
has been found. The only occurrence of a regolith, to myknowledge, is beneath the Menominee Group in thewestern Marquette district. At two localities the basal
conglomerate of the Menominee Group is separated
from underlying Archean granitic gneiss by a zone ofgray-green chloritic and sericitic material believed to bea regolith. The material forms a unit from 1 to about 3 m
thick that grades downward into granitic gneiss and lies
beneath conglomerate composed of pebbles of veinquartz, chert, and older sedimentary-rock fragments.
The base of the conglomerate and upper parts of theregolith contain minor pyrite and magnetite. The rocks
were metamorphosed to staurolite grade, and pyrite is
generally recrystallized to well-formed cubes, but, rare­
ly, rounded, possibly detrital grains are preserved.
Although these rocks might be interpreted to have form­
ed in an anoxygenic atmosphere, data are too sparse tojustify any firm conclusions.
SIGNIFICANCE OF OXIDIZED ROCKS

The two independent lines of evidence just discussed
indicate that by the close of deposition of the Menominee

Group the atmosphere contained sufficient free oxygento oxidize a substantial amount of iron. A problem arisesof how to interpret this in terms of oxygen partialpressure in the atmosphere and its bearing on the abilityof uraninite to persist in a clastic sedimentary environ­ment. The stability relations of the minerals involved arerather well known and provide a first step in solving thisproblem. Hematite becomes the stable iron oxide at vanishingly small oxygen partial pressures (~10~ 65 atm) andhas a very broad stability field. Uraninite remains stableat oxygen partial pressures up to about 10~21 atm(Holland, 1962) and has a large overlap with the stability
field of hematite. Thus, uraninite and hematite can be in
equilibrium over a wide range of partial pressures, andthe presence of hematite in a rock does not automatical­
ly preclude the presence of uraninite under equilibrium
conditions.
The problem is complicated, however, by two consid­
erations: (1) It is possible that the atmosphere contained
free oxygen far in excess of minimum amounts required
to stabilize hematite; (2) concentration of a mineral inplacers may reflect its ability to persist metastably
rather than its true thermodynamic stability.
Considering first the amount of free oxygen required
to form the oxidized rocks in the Menominee Group, it isnecessary to consider some quantitative and kinetic
aspects of oxidation as well as intensive variables thatgovern mineral stabilities. The formation of oxide facies
iron-formation and of extensive oxidized zones along thepost-Menominee Group unconformity requires that alarge amount of oxygen be delivered to the site of oxida­
tion in a geologically short period of time. The erosional
break between the Menominee and Baraga Groups can­not be a long time span, because Gair (1975) has foundthat the two groups are locally conformable and interbedded. The rate of sedimentation of oxide facies
iron-formation cannot be accurately estimated, but
assuming that it is not greatly dissimilar from rates
determined from varved layers in iron-formation in the
Hammersley Range, Australia, by Trendall andBlockley (1970), it, too, represents a short time period.
An atmosphere containing only the minimum oxygen
level to form hematite (~10- 70 atm) is for practical pur­poses anoxygenic and seems very unlikely to be capableof supplying sufficient oxygen in the required time span
to form the hematitic rocks. Even at the upper stability
limit of uraninite (~10~ 21 atm), the entire atmosphereprobably contained no more than a few tens of grams ofoxygen. The entire atmospheric oxygen supply could ox­
idize literally no more than a handful of uraninite duringoxidative weathering.
In my opinion, rocks such as those in the Menominee
Group that appear to have been oxidized as a result ofcontact with the atmosphere must have formed under ar

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN

atmosphere with oxygen levels well in excess of mini­
mum amounts required to stabilize hematite and oxidize
uraninite. If this were not the case and the atmosphere
contained only a few tens of grams of oxygen, that gas
would be so widely dispersed that even in a dynamic
system in which 02 is continually being supplied byphotosynthesis or photodissociation of water and con­sumed by oxidation of reduced compounds, it seems
unlikely that the large quantities of 02 that were
evidently supplied to weathered zones in the Marquette
Range Supergroup could be delivered to the site of ox­
idation in a short time span. The preserved portions of
the oxidized rocks contain many thousands of tons of 02
presumably derived from the atmosphere, and theoriginal zone of oxidized rocks may have contained
literally billions of tons of 02. The principal question is,could an atmosphere that at any one moment contained
only a fraction of a ton of 02 produce so much oxidation?
I believe the answer is no; the atmosphere probably con­tained a great deal more 02.
This, then, brings us to the second consideration ofwhether uraninite could persist to form placer deposits
under such an atmosphere. Although uraninite may bewell removed from its stability field, it might persist
metastably. Magnetite, for instance, has an even lower
oxygen tolerance than uraninite yet is a common detrital mineral even today as a result of its relatively slow
reaction rate. The preservation of uraninite, like anyother metastable detrital mineral, depends on rates ofweathering, erosion, and burial relative to the rate ofbreakdown. The rate of oxidative breakdown of uran­inite should be controlled by the partial pressure of ox­ygen and should therefore increase with time as the at­mosphere evolved from oxygen-free toward the present
oxygen levels. I would suggest, therefore, that the
restriction in time of uraniferous and pyritic conglom­
erates cannot be defined by an absolute time marker
about 2.2 b.y. ago, as has been suggested, particularly if
the first appearance of hematite is used to define theonset of oxidizing conditions. Rather, it seems more like­ly that such deposits would gradually become scarcer inyounger rocks as oxygen levels increased, requiring pro­
gressively more favorable conditions to preserveuraninite or requiring the occurrence of uranium in
more thorium-rich and hence more oxygen-tolerant
minerals. The presence of modern-day uraninite-bearing
placers in the Indus River (Miller, 1962; Zeshke, 1961)
supports this view.
In light of these considerations, uranium-bearing con­glomerates might be less likely to have formed during
deposition of the Marquette Range Supergroup thanduring the Huronian if the Marquette Range Super­group is partly younger than the Huronian, but theirpresence is certainly not precluded.

Z13

POTENTIAL FOR URANIUM-BEARING QUARTZ-PEBBLECONGLOMERATE IN THE MARQUETTE RANGE
SUPERGROUP

The foregoing arguments in favor of a strongly oxidiz­
ing atmosphere during deposition of at least part of theMarquette Range Supergroup suggests that the condi­
tions may have been less favorable for formation ofquartz-pebble uranium ore than they were during
deposition of the Huronian. Nevertheless, our under­
standing of the requisite conditions for their formationand of the age relations between the Marquette Range
Supergroup and Huronian are still uncertain enough sothat the uranium potential of quartz-pebble con­glomerates in the Marquette Range Supergroup is best
judged directly by considering features of the rocks
themselves.
A few positive and negative factors based on local
geologic features are discussed in the following
paragraphs.
Favorable factors include:
1. Lenses of quartz-pebble conglomerate are relatively
common near the base of the sedimentary section
and provide a large volume of potentially mineraliz­ed rock.
2. Archean granitic rocks in many areas of northern
Michigan are anomalously rich in uranium (fig. 10)
and could have provided a source for radioactive
minerals for overlying sediments. One bulk sample
of Archean granitic gneiss contains 15 ppm U308
and 102 ppm Th02 (Malan and Sterling, 1969), and27 samples collected throughout northern
Michigan average 16 ppm U308 and range from
3-40 ppm (Kinnaman and Illsley, 1962).
3. Uranium concentrations in slate and in ironformation in several areas within higher
stratigraphic units of the Marquette Range
Supergroup (fig. 10) indicate that uranium was be­
ing supplied to the sedimentary basin (Vickers,
1956b).
4. The presence of monazite-rich placers in parts of thebasal Baraga Group (Vickers, 1956a) indicates thatphysical conditions of sedimentation were suitable
to form significant placer accumulations ofminerals that were originally disseminated in older
rocks.
5. Fragmentary evidence suggests that basal sedimentsof the Chocolay and Menominee Groups are notstrongly oxidized. The only known regolith is drabcolored magnetite and pyrite-bearing rock. Pyrite,
although not necessarily detrital, is a minor but
widespread mineral in basal quartzites of both the
Chocolay and Menominee Groups (Tyler andothers, 1940).

Z14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

6. Glaciation at the beginning of Chocolay sedimentaion
should have provided a large volume of freshly
ground and, at least locally, uranium-rich Archeanrocks that may have been reworked and incor­
porated into overlying rocks, possibly with resul­tant concentration in placers. Where glacial
deposits lie on anomalously radioactive rocks, they,
too, are anomalously radioactive, locally somewhat
more radioactive than are the underlying gneisses.
7. Scant data from the Chocolay and Menominee
Groups indicate the Th/U ratios of the con­glomerates are low; most are in the range 2:1 to
4:1. These are comparable to or somewhat lower

than ratios from Archean rocks and indicate thatthorium was not being preferentially retained overuranium in the clastic sedimentary environment.
Ratios from the Baraga Group are much higher but
are strongly influenced by a relatively large
number of samples from monazite-rich placers.
Unfavorable factors include:
1. The contact between the Marquette Range
Supergroup and the Archean basement is a fault inmany areas. The tectonic style of the area, which I
have discussed in more detail elsewhere (Cannon,
1973), consists basically of upfaulted blocks of Ar­
chean basement rocks with the sedimentary rocks

EXPLANATION•GAMMA-RAY ANOMALIESFROM AIRBORNE SURVEYKNOWN URANIUM
CONCENTRATION

V

\.~.

D
i
i
D

i
10

10
'

20 MILES'
20 KILOMETERS

FIGURE 10.-Map showing distribution of gamma-ray anomalies from airborne survey (Balsley and others, 1950) and known uranium concentra­
tions (Vickers, 1956b).

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGANof the Marquette Range Supergroup draped overthe uplifts. The contact is intensely sheared inmany places, and the lowest part of the section iseliminated by faulting. Thus, the basal units ofprincipal interest for uranium exploration are notpresent in some areas. Because of poor exposureand the general conformity of bedding with thebasal contact, even where the contact is a fault, it iscommonly difficult to judge the extent of fault trun­
cation of the base of the section. Figure 3 shows mybest estimate of the nature of the Marquette Range
Supergroup-Archean contact and indicates which
segments of the contact are relatively undisturbed
unconformity and which are faults.
2. In spite of a great deal of geologic mapping and ex­
ploration in the area, albeit most was not concern­ed with uranium exploration, no significant
deposits have been found. Radioactivity of mostconglomerates is only slightly higher than that ofadjacent Archean rocks. Rarely, radioactivity
reaches three times that of the adjacent Archeanrocks. As discussed in more detail in the following
section, this may not be an overriding negative fac­
tor. Because of the very limited exposures ofbedrock there is a substantial probability that evena major district would not be discovered in outcrop.
EXPLORATION FOR URANIFEROUS
QUARTZ-PEBBLE CONGLOMERATE INMICHIGAN—PROBLEMS AND DISCOVERYPROBABILITIESIn the previous discussions many similarities and dif­ferences between the Marquette Range Supergroup andthe Huronian have been considered. One additional im­
portant difference is the amount of Pleistocene drift
cover on bedrock. The Huronian rocks near Blind Riverare very well exposed because of thin drift cover; indica­
tions of several of the known ore deposits were seen in­
itially in outcrop. In Michigan, on the other hand, glacial
deposits are thick and bedrock exposures are rare. Much
less than one percent of the outcrop belt of the basal
clastic units is exposed. Exploration of these units
therefore requires a technique capable of detecting
mineralized rock through drift cover generally several
meters thick and commonly several tens of meters thick.
A number of standard geochemical and geophysicaltechniques might detect buried ore bodies, but I would
like to illustrate here that blind diamond drilling mightalso be a valuable and relatively inexpensive technique.
By blind drilling I mean that drill holes would not belocated to explore known anomalies, but rather would
simply be located so as to intersect known basal clastic
zones. There are about 650 km of strike length of basal
clastic units lying on Archean basement. The location of

Z15

the contact is rather accurately known from detailed
mapping for most of that length. By analogy with BlindRiver, uranium ore is most likely to be in the lowest 50 mof the sedimentary section. Thus the total exploration
target is relatively small and well defined. Maps of theBlind River district (J. A. Robertson, 1968) indicate thatore bodies or mineralized rock typically have a strike
length of about 1.5 km. If deposits of that size were in
Michigan, a single deposit would occupy about one fourhundreths of the strike length of the outcrop belt to be
explored, and the probability of hitting it with a random­
ly placed drill hole would be one in 400. On the other
hand, to be certain of hitting it would require 400 drill
holes spaced about 1.5 km apart along the outcrop belt.
The discovery probability for any other small number ofdrill holes is a straight-line function between these two
extremes. If more than one deposit exists in the area theprobability of making a discovery increases exponential­
ly with the number of deposits. Figure 11 shows curves
relating number of drill holes to probability of making asingle discovery for cases in which 1, 2, 5, and 10
deposits exist. It shows, for instance, that if a major
district (10 deposits) comparable to Blind River exists in
Michigan, the probability of discovering one deposit
would be 0.5 with only 25 drill holes and 0.75 with 50
holes. If a much smaller district (2 deposits) were in
Michigan, 200 drill holes would have a 0.75 probability offinding one deposit.
I would suggest, therefore, that blind diamond drillingis a realistic means of conducting a major exploration
program in the area. Two hundred drill holes, eachabout 50 m long, would have a probability of about 0.5 offinding a single existing deposit, about 0.75 of finding
one of two existing deposits, and a much higher pro­bability of finding one of a larger number of deposits.
Such drilling is modest in comparison to petroleum
wildcat exploration, where expected success is common­
ly no better than 1 in 10.
Figure 11 can also be used to estimate the probabilityof discovery of a deposit in outcrop. Most of the strike
length of basal clastic units has been mapped in detail,and maps are available showing outcrop distribution.
Using these data, I have estimated that the total amountof outcrop of the lower 50 m of the section provides in­formation equivalent to about 25 diamond-drilling holes.
If a major district comparable in size to Blind River werein Michigan, chances of discovery of a single deposit inoutcrop are only about 0.5 after all outcrops have beenthoroughly explored. Because all outcrops have not beenthoroughly explored to date, the probability that adeposit would be discovered with current knowledge isconsiderably less than 0.5. If only one major deposit is in
Michigan, the probability of finding it in outcrop is very
low, only about 0.05.

Z16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

These relations illustrate the necessity to explore thearea in a manner that is not greatly dependent on out­
crop information, and they suggest that the failure tofind deposits to date should not be viewed as a greatly
discouraging factor in considering future exploration.
REFERENCES CITED
Aldrich, L. T., Davis, G. L., and James, H. L., 1965, Ages of minerals
from metamorphic and igneous rocks near Iron Mountain, Michi­
gan: Journal of Petrology, v. 6, p. 445-472.
Balsley, J. R., Jr., Davis, F. J., Nelson, R. A., Reinhardt, P. W., andStead, F. W., 1950, Airborne radioactivity survey of parts of Marquette, Dickinson, and Baraga Counties, Michigan: U.S. Geological
Survey [geophysical investigations map], scale 1:126,720.
Banks, P. 0., and Van Schmus, W. R., 1971 Chronology of Precambrian rocks of Iron and Dickinson Counties, Michigan (abs.): In­
stitute on Lake Superior Geology, 17th, Duluth, Minn., p. 9.
Bayley, R. W., Dutton, C. E., and Lamey, C. A., 1966, Geology of theMenominee Iron-bearing district, Dickinson County, Michigan, andFlorence and Marinette Counties, Wisconsin: U.S. Geological
Survey Professional Paper 513, 96 p.
Cannon, W. F., 1973 The Penokean Orogeny in northern Michigan, in
Young, G. M., ed., Huronian stratigraphy and sedimentation:
Geological Association of Canada Special Paper 12, p. 251-271.

50

100 150 200 250 300
NUMBER OF DRILL HOLES

350 400

FIGURE 11. -Curves relating probability of discovering one deposit tonumber of holes drilled for cases in which 1,2,5, and 10 undiscovered
deposits exist in the central northern peninsula of Michigan. Curves
are drawn on the assumption that 650 km of basal clastic rocks are in
the unexplored (drift-covered) outcrop belt and a single deposit
would extend for 1.5 km along the belt.

Cannon, W. F., and Gair, J. E., 1970, A revision of stratigraphic
nomenclature for middle Precambrian rocks in northern Michigan:
Geological Society of America Bulletin, v. 81, p. 2843-2846.
Cannon, W. F., and Klasner, J. S., 1974, Preliminary geologic map of
the Witch Lake 15-minute Quadrangle, Michigan: U.S. Geological
Survey open-file map.
————1975a, Preliminary geologic map of the southern part of the
Diorite 71/2-minute Quadrangle, Michigan: U.S. Geological Survey
open-file map.
————1975b, Preliminary geologic map of the southern part of theChampion 7V2-minute Quadrangle, Michigan: U.S. Geological
Survey open-file map.
Clark, L. D., Cannon, W. F., and Klasner, J. S., 1975, Bedrock geologic
map of the Negaunee SW Quadrangle, Marquette County,
Michigan: U.S. Geological Survey Geologic Quadrangle Map
GQ-1226.
Cloud, P. E., 1965, Significance of the Gunflint (Precambrian)microflora: Science, v. 148, p. 27-35.
————1973, Paleoecological significance of banded iron-formation:
Economic Geology, v. 68, p. 1135-1143.
Drever, J. L, 1974, Geochemical model for the origin of Precambrianbanded iron-formations: Geological Society of America Bulletin, v.
85, p. 1099-1106.
Fairbairn, H. W., Hurley, P. M., Card, K. D., and Knight, C. J., 1969,
Correlation of radiometric ages of Nipissing diabase and Huronian
metasediments with Proterozoic erogenic events in Ontario: Cana­dian Journal of Earth Sciences, v. 6, p. 489-497.
Frarey, M. J., and Roscoe, S. M., 1970, The Huronian Supergroupnorth of Lake Huron, in Baer, A. J., ed., Symposium on basins andgeosynclines of the Canadian shield: Canada Geological Survey
Paper 70-40, p.143-157.
Gair, J. E., 1975, Bedrock geology and ore deposits of the Palmer
Quadrangle, Marquette County, Michigan: U.S. Geological Survey
Professional Paper 769, 159 p.
Gair, J. E., and Thaden, R. E., 1968, Geology of the Marquette andSands Quadrangles, Marquette County, Michigan: U.S. Geological
Survey Professional Paper 397, 77 p.
Goldich, S. S., Nier, A. 0., Baadsaard, Halfdan, Hoffman, J. H., andKrueger, H. W., 1961, The precambrian geology andgeochronology of Minnesota: Minnesota Geological Survey
Bulletin 41, 193 p.
Holland, H. D., 1962, Model for the evaluation of the Earth's at­mosphere, in Petrologic studies, a volume to honor A. F. Buddington: Geological Society of America, p. 447-477.
____1973, The oceans: A possible source of iron in iron-formations:
Economic Geology, v. 68, p. 1169-1172.
James, H. L., 1954, Sedimentary facies of iron-formation: Economic
Geology, v. 49, p. 235-293.
————1958, Stratigraphy of pre-Keweenawan rocks in parts of nor­thern Michigan: U.S. Geological Survey Professional Paper 314-C,
p. 27-44.
James, H. L., Clark, L. D., Lamey, C. A., and Pettijohn, F. J., 1961,
Geology of central Dickinson County, Michigan: U.S. Geological
Survey Professional Paper 310, 176 p.
Kinnaman, R. L., and Illsley, C. T., 1962, Geochemical and geophysicalreconnaisance in northern Michigan and northeastern Wisconsin:
U.S. Atomic Energy Commission Report RME-1099, 135 p.
Klasner, J. S., and Cannon, W. F., 1975a, Preliminary geologic map of
the southern part of the Michigamme 7V2-minute Quadrangle,
Michigan: U.S. Geological Survey open-file map.
————1975b, Preliminary geologic map of the southern part of theThree Lakes 71/2-minute Quadrangle: U.S. Geological Survey openfile map.
Malan, R. C., and Sterling, D. A., 1969, A geologic study of uranium
resources in Precambrian rocks of the western United States: U.S.
Atomic Energy Commission Open-File Report AEC-RD-10, 25 p.

BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN
Miller, J. M., 1962, Aeroradiometric survey of Indus Valley aluminum:
Great Britain Geological Survey, Atomic Energy Divison Report
241, 13 p.
Pettijohn, F. J., 1943, Basal Huronian conglomerates of Menominee
and Calumet districts, Michigan: Journal of Geology, v. 51, p.
387-397.
———1957, Paleocurrents of Lake Superior Precambrian quartzites:
Geological Society of America Bulletin, v. 68, p. 469-480.
Puffett, W. P., 1969, The Reany Creek Formation, Marquette County,
Michigan: U.S. Geological Survey Bulletin 1274-F, p. F1-F25.
————1974, Geology of the Negaunee Quadrangle, Marquette County,
Michigan: U.S. Geological Survey Professional Paper 788, 53 p.
Robertson, D. S., 1974, Basal Proterozoic units as fossil time markers
and their use in uranium prospection, in Formation of uranium ore
deposits - Proceedings of a symposium * * * organized by the In­
ternational Atomic Energy Agency and held in Athens, Greece,6-10 May, 1974: Vienna, International Atomic Energy Agency, p.
495-512.
Robertson, J. A., 1968, Geology of Township 149 and Township 150:
Ontario Department of Mines Geologic Report 57, 162 p.
Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
the Canadian shield: Canada Geological Survey Paper 68-40, 205p.
———1973, The Huronian Supergroup, a Paleoaphebian succession
showing evidence of atmospheric evolution, in Young, G. M., ed.,Huronian stratigraphy and sedimentation: Geological Associationof Canada Special Paper 12, p. 31-47.
Trendall, A. F., and Blockley, J. G., 1970, The iron-formations of the

Z17

Hammersley Group, western Australia, with special reference tothe associated crocidolite: Western Australia Geological Survey
Bulletin 119, 353 p.
Tyler, S. A., Marsden, R. W., Grout, F. F., and Thiel, G. A., 1940,
Studies of the Lake Superior Precambrian by accessory mineral
methods: Geological Society of America Bulletin, v. 51, p.
1429-1538.
Van Hise, C. R., and Leith, C. K., 1911, The geology of the Lake
Superior region: U.S. Geological Survey Monograph 52, 641 p.
Van Schmus, W. R., 1965, The geochronology of the Blind River-Bmce
Mines area, Ontario, Canada: Journal of Geology, v. 73, p.
755-780.
Vickers, R. C., 1956a, Geology and monazite content of the Goodrichquartzite, Palmer area, Marquette County, Michigan: U.S.
Geological Survey Bulletin 1030-F, p. 171-185.
———1956b, Origin and occurrence of uranium in northern Michigan:
U.S. Geological Survey open-file report, 76 p.
Young, G. M., 1973, Tillites and aluminous quartzites as possible time
markers for middle Precambrian (Aphebian) rocks of North
America, in Young, G. M., ed., Huronian stratigraphy andsedimentation: Geological Association of Canada Special Paper 12,
p. 97-128.
Young, G. M., and Chandler, F. W., 1968, Possible glacial origin forthree Precambrian (Huronian) conglomerates, north shore of Lake
Huron (abs.): Institute on Lake Superior Geology, 14th, Superior,Wis., p. 42-43.
Zeshke, G., 1961, Prospecting for ore deposits by panning heavy
minerals from river sands: Economic Geology, v. 56, p. 1250-1257.

Genesis of Uranium-Gold
Pyritic Conglomerates
By W. BRADLEY MYERS
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-AA

CONTENTS
Page

Page

Abstract ______________________________ AA1
Introduction ____________________________
1
Geologic setting of the Witwatersrand reefs ___________
1
Detrital minerals unstable in an oxygen atmosphere ______
2
Gold _________________________________
5
Placer origin unlikely _________
5
Transport in complexed solution
7
Bacterial sulfur cycle ____________
8
Thucholite _____________________
8
Inverse relation of uranium and gold
9
Genesis of gold in reefs ___________
10
Radiolysis of water ___________
12
Oxidizing reactions _______.
12
Maturation of reefs __________
13

Genesis of gold in reefs-Continued
Competency of radiolysis ___________________ AA15
Orange Free State Goldfield _________________
17
White Reef ___________________________
17
Middle Elsburg Reefs _____________________
17
Degrees of association of uranium and gold _________
18
Volcanic influence _______________________
20
Blind River-Elliot Lake __________________
20
Ventersdorp Contact Reef ________________
20
Discussion _______________________________
21
Appendix: Determination of original uranium _________
21
References cited __________________________
22
Discussion following talks by Drs. Staatz, Cannon, and Myers _24

ILLUSTRATIONSPage

FIGURE 1. Photograph showing pyrite pebbles from the Basal Reef, Western Holdings Mine, Orange Free State ______________ AA3
2. Eh-pH diagrams of the stability fields of pyrite and uraninite and the equilibrium distribution of soluble sulfur species in waterat 25°C and one atmosphere total pressure _____________________________________________
4
3. Schematic diagram comparing settling velocities of natural gold particles from Siberian placers with similar fabricated gold
particles, and average Rand hypothetical placer gold grains ____________________________________
6
4. Eh-pH diagrams of the aqueous gold-sulfur system and its relation to the stability fields of pyrite and uraninite _________
7
5. Diagram of modern anaerobic bacterial sulfur cycle in stagnant ponds and shallow lakes ______________________
8
6. Graph showing inverse relation between uranium and gold content of reefs in South Africa, Canada, and Brazil _________
11
7. Block diagram showing gold encasing thucholite in the South Reef, West Rand ___________________________
12
8-11. Graphs showing:
8. Uranium content and carbon-oxygen ratio for 23 uraniferous petroleum asphaltites and a Rand thucholite ______
13
9. Rate of uranium loss and gold gain plotted against time for reefs in South Africa, Canada, and Brazil __________
14
10. Volatile content and gold content of 11 thucholites from Rand reefs ______________________________
15
11. Volatile content and oxygen content of 23 uraniferous petroleum asphaltites and one Rand thucholite _________
15
12. Sketch map showing distribution of U308/Au ratios in the White Reef, West Rand _________________________
18
13-15. Graphs showing:
13. U3 08/Au range of the White Reef, West Rand, and the Basal Reef, Orange Free State, compared with all reefs ____
18
14. Changes in the ratio U308 present per unit of gold precipitated for different detrital uraninite concentrations in a reef _19
15. The linear coefficient of correlation r between uranium and gold and the logarithm of the U308/Au ratio for individual
reefs in the Witwatersrand and for the composited Blind River-Elliot Lake district __________________
20

TABLESPage

TABLE 1. Ages of pyritic uranium-gold conglomerates ____________________________-_—_——————————————— AA1
2. Uranium and gold contents and the U308/Au ratios of pyritic conglomerates in South Africa, Canada, and Brazil —————————
10
3. Indicated ore, Cooke Section, Randfontein Mine ________________—_——————————————————————————
17
4. UjOg/Au ratios and the linear coefficient of correlation between uranium and gold for sample suites from reefs in South Africa
and Canada ___________________________________—_——————————————————————————
19
ill

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESGENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES
By W. BRADLEY MYERS l

ABSTRACTThe ancient pyritic ore conglomerates have a common origin best ex­
emplified by the Witwatersrand deposits. All contain detrital pyrite
and uraninite, which are unstable in modern oxygenated environments
and were deposited in a reducing atmosphere. The Rand reefs are notsimilar to modern gold placers. Placers result from the near incapacityof streams and currents to transport coarse gold. Placers as rich as
Rand reef occur only in narrow paystreaks within 15 kilometers of acoarse-gold source.
The broad dispersion of gold in the reefs is due to solution transportof metal complexed as aurous sulfide, leached anoxygenically fromcrustal rocks, probably from sea-floor basalt, and precipitated by aslow reaction driven by the radioactive decay of detrital uraninite.
Radiolysis of water on shallow marine uncomformities resulted in dif­fusion of hydrogen to the atmosphere and a slight excess of hydroxyl
free radical in the reef environment. The mild oxidizing tendency slow­
ly dissolved uranium, precipitated gold, and oxygenated thucholite.
These actions define a maturing process. A uraninite placer ac­
cumulating on an unconformity becomes progressively converted to agold reef with little residual uraninite. The most mature reefs tend tograde toward the thucholite-seam type, very thin but exceedingly richin gold. A combination of chemical attack and physical reworking ac­
counts for the general thinness of mature reefs.
Pyrite, like uraninite, decreases in abundance with increasing
maturity; "buffering" by pyrite moderated the oxidative depletion of
uranium. Where pyrite was scanty or absent, uraninite was completely
dissolved by the effects of radiolysis and no ore formed.

TABLE 1. -Ages, in billions (109) years, of pyritic uranium^,conglomerates
Locality

Serra de Jacobina, Brazil _Moeda conglomerate,
Brazil _______________
Blind River-Elliot Lake,
Canada _____________
Krivoi Rog Series,
Ukraine _____________

Reference>2>2

2.3 -2.5

Cordani and others(1973).
Do.
Roscoe (1969, 1973).
Chukhrov and others(1970).

Kursk Magnetic Anomaly
Series, Ukraine.
Witwatersrand, South
Africa ______________<2.3 -2.8

Pongola Basin, South
Africa ______________

Nierkerk and Burger(1964, 1969).

<2.85-3.35

Beukes (1973).

Do.

I wish to thank the South African mining houses fortheir many courtesies and their unfailing hospitality dur­
ing my stay there. Especial thanks are due to Dr. E. S.
A. Antrobus of Anglo American Corp. and Dr. D. A.
Pretorius, Witwatersrand University, who made many
arrangements for me. Dr. W. E. L. Minter kindly guided
me on numerous mine visits. The staffs of the Denison
and New Quirke Mines were very helpful during my visit
INTRODUCTION
to the Elliot Lake district, Ontario. I am indebted to D.
Probably 75 percent of the world's gold resources and A. Singer and J. De Young of the U.S. Geological Survey
at least 50 percent of its uranium resources are contained for their help in statistical problems. This paper hasin Precambrian pyritic conglomerates 2.0-3.0xl09 yr benefited greatly from reviews by colleagues Warren
old (table 1). These conglomerates contain detrital Hamilton and Paul B. Barton, Jr.
uraninite and pyrite, which are readily oxidized and are
unstable in modern oxygenated environments and were
deposited in a reducing atmosphere. Gold in the ore con­ GEOLOGIC SETTING OF THE WITWATERSRAND
glomerates is crystalline, and in the conventional view it
REEFS
is assumed to have been initially detrital. It is deduced
The Rand pyritic conglomerates (Witwatersrand,
here that, on the contrary, the gold was transported asSouth
Africa) support the world's largest gold produc­complexed ions in anoxygenic solution and was precip­tion
andhave been extensively studied. Six or so major
itated on long-lasting unconformities on the floors of
productive
conglomerates and about twice as many
shallow seas by a slow reaction driven by the radioactive
minor
ones
are scattered through a cratonic section 9
decay of uranium.
km thick (Dominion Reef, Witwatersrand, and Ventersdorp Lava Systems; Haughton, 1969; Pretorius, 1964).
U.S. Geological Survey, Reston, Va.
AAl

AA2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESThe sedimentary rocks are mostly feldspathic quartzite(Fuller, 1958), typically cross bedded; they also include
much iron-formation (Beukes, 1973) and barren con­glomerate but virtually no carbonate rocks; and they lie
on a dominantly granitic Archaean basement. The sec­tion is overlain unconformably by the stromatolitic
Dolomite Series of the Transvaal System, whose basal
carbonaceous beds contain the youngest well-docu­
mented pyritic reef (Haughton, 1969). The surmounting
shaly Pretoria Series of the Transvaal System (2.0 x 109yr) contains hematite-chamosite oolites (James, 1966), in
contrast to the more reduced iron-formation of theunderlying sequences. The massive release of oxygen to
the previously anoxygenic atmosphere by algal photo­
synthesis, such as that which here deposited 1,200 m of
Dolomite Series carbonate, may have raised the ambient
oxygen pressure to levels generally beyond that permit­
ting development of reduced iron-formation (French,
1973; Cloud, 1973; Garrels and others, 1973) or preser­
vation of detrital pyrite.
Rand and other pyritic conglomerates consist of wellrounded resistate clasts, mostly quartz and chert(Roscoe, 1969; Liebenberg, 1955). In the Rand the pro­ductive conglomerates, referred to by the mining term
"reefs," lie on unconformites (Haughton, 1969) within
the sequences and, unlike the barren conglomerates thatlie conformably with the sections, are characteristicallythinner than 1 m and may be only single layers of peb­bles (Pretorius, 1974). The clasts are typically small. The
average pebble diameter of the reefs that have yielded
the greater part of the Rand gold is between 1 and 5 cm
(Pretorius, 1964; Antrobus and Whiteside, 1964; deKock, 1964; Winter, 1964a, b; McKinney and others,1964).
Sericitic alteration characterizes weathering zones on
the eroded Archaean floors beneath the pyritic con­glomerate sequences of the Rand, the Pongola Basin(Mathews and Scharrer, 1968), Blind River-Elliot Lake
(Roscoe, 1969), the Krivoi Rog (Gershoyg and Kaplum,1970), and probably the Serra de Jacobina (Leo andothers, 1964; Cox, 1967). At both Blind River-Elliot
Lake and the Krivoi Rog localities the sericitization ofgreenstone indicates the marked addition of potassium.
A similar addition during modern seafloor alteration of
spreading-ridge basalts to potassic mixed-layer clay
(Hart, 1973) implies that the ancient pyritic con­glomerate sequences also were marine. A marine sourceis also implied for the authigenic gibbsite, kaolinite,
potassic mixed-layer clay and (or) chlorite (Helgeson and
Mackenzie, 1970) needed as the substrates of recrystallized phyllosilicates in the reef conglomerates. The
cleanly washed, resistate, heavy-mineral-rich matrix
could not have contained detrital clay; feldspar or mica
are scarcely more likely. Thus reef phyllosilicates werelikely authigenic, growing in voids between siliceous
clasts.
The Witwatersrand System underwent low-grade
burial metamorphism. Diagenetic reconstitution of thefeldspar in the productive upper section is much more
complete than that in the nearly barren lower section(Fuller, 1958; de Jager, 1964), and therefore some in­vestigators have interpreted it as the result of magmatism-related hydrothermal alteration. Detrital feld­
spar in the lower Witwatersrand has been only partly
sericitized, whereas that in the upper part has been vir­tually completely changed to muscovite, pyrophyllite,
and chlorite (Fuller, 1958). The same phyllosilicateshave also recrystallized in the reefs from the supposed
authigenic minerals described. Chloritoid has developedin the upper rocks of appropriate composition, and, ex­
ceptionally, kyanite (Liebenberg, 1955) has recrystal­
lized from pyrophyllite and more aluminous minerals.
DETRITAL MINERALS UNSTABLE IN ANOXYGEN ATMOSPHERE

Pyrite is overwhelmingly the most abundant opaquemineral in most of the Rand reefs (Saager, 1970) and,though widely variable in amount, might average 3 wt
percent (weight percent) of the mined gold ore(Pretorius, 1964). Part of the pyrite is clearly detritalfrom the very old crustal rocks and locally has a
207Pb/206Pb age of 3.36 xlO9 yr (Koppel and Saager,1974). Pyrite is present as single crystals and granularaggregates, replacements of various iron oxides, andporous concretionary nodules. All of thesecharacteristically occur as water-worn remnants (Ramdohr, 1958; Saager, 1970; Liebenberg, 1960) (fig. 1). The
rounded clasts of pyrite are in general hydraulicequivalence with undisputed clasts of other minerals of
various densities (Pretorius, 1974; Coetzee, 1965).
Crystalline uraninite, in closely sized and generally
well-rounded clastic grains (median grain size: avg,
0.103 mm; range, 0.083-0.136 mm) hydraulicallyequivalent with detrital grains of chromite, zircon, andquartz, occurs sparingly in the matrix of all Witwaters­
rand reefs (Liebenberg, 1955, 1960; Ramdohr, 1958;
Coetzee, 1965). The lead-isotope age of the uraninite isabout 3.0 x 109 yr (Burger and others, 1962), older thanthe lead-isotope age of 2.8xl09 yr (Nierkerk andBurger, 1969) of the oldest reef series, further in­dicating a detrital origin. Electron microscope studies
by Grandstaff (this volume, chap. J) and by Feather (thisvolume, chap. Q) show widely varying thorium contents
for different grains in single samples, conclusively prov­
ing detrital derivation from multiple sources.
Primary inclusions of zircon in uraninite (Hiemstra,
1968a) and the relatively high thorium contents of theGENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATESAA3

FIGURE 1. - Pyrite pebbles from the Basal Reef, Western Holdings Mine, Orange Free State. 1-cm scale (x 5.4). The shape of the large
pebble is controlled by layering in the pyrite; freshly mined pebbles are bright, untarnished. Pyrite occurs as rounded sand grainsor pebbles in all reefs.

uraninite show derivation from a high-temperature
granitic-pegmatitic source. A modern equivalent of thesource rock may be certain Phanerozoic French granites
where uraninite occurs as tiny accessory crystals withinrock mineral grains (Barbier, 1972).
The lack of Rand uraninite clasts larger than 0.4 mm(Liebenberg, 1955) suggests such a source for the reef
U02.
The reefs of the Blind River-Elliot Lake deposits(Roscoe, 1969) and the Rand (Liebenberg, 1955) containimproverished suites of the detrital minerals expectablefrom the underlying basements. The surviving minerals
are those chemically and physically resistant; notable isthe ultrastable mineral zircon. Waterworn minerals of
the Rand reefs have high Vickers microhardness (Youngand Millman, 1964; McLeod and Chamberlain, 1969); ex­cept for a few diamonds and rare corundum(Liebenberg, 1960; Raal, 1969), the hardest is pyrite.
Eh-pH diagrams of the stability fields of pyrite anduraninite in an aqueous system (fig. 2) show that neitherof these minerals is stable in the present oxygenated at­mosphere. This conclusion is upheld by the rarity ofpyrite and the virtual absence of uraninite as detrital
minerals in modern sediments. The diagrams indicatethat in an anoxygenic environment abundant pyrite
would tend to protect uraninite in the presence of weakoxidizing tendencies by "buffering" Eh.
Bezgubov and others (1970) found that there are two
types of Precambrian uraniferous quartz-pebble con­glomerates: (1) pyritic conglomerates containing
uraninite and (2) magnetic or martitic conglomerates
containing thorite, monazite, and zircon. The con­glomerates of the first type contain Eh-sensitiveminerals, in contrast to those of the second type, and are
confined to the older Precambrian. The lack of Ehsensitive uraninite from the second type, pyrite-freeconglomerate, also found in older Precambrian deposits,
suggests that the pyrite-uraninite association is not
simply that of two Eh-sensitive minerals unstable in anoxygenated atmosphere.

AA4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES-i- i.u

II

TX.

1

1

1

+0.8
rr

<"O >V>»^
Nw

tj

iS
o
in

+0.6

\

\v

+0.4 -

X

SO4 2aq

+0.2

C\

0.0

-

_^\^9

-0.2

\^
V.

1

-0.4 _

x xx

xi

^x v

-0.6
n
R
U.o

1
C)

1

1

1

246

8

10

1

10

12

10

12

PH

A
-M.U

I

IX

I

X.

B
I

I

+0.8

^^x;

+0.6
+0.4
+0.2

U+6

\\
u+4 N\^—

0.0
-0.2

^\

i

"^\

- ^^C^^X

^^^ \

-0.4

^\

\

-0.6

I

-0.8
C)

I

I

I

I

24

6

8

10

1:

0

24

6

PH

PH

C

D

8

FIGURE 2. - Eh-pH diagrams of the stability fields of pyrite and uraninite and the equilibrium distribution ofsoluble sulfur species in water at 25°C and one atmosphere total pressure. A, Equilibrium distribution ofsoluble sulfur species; the ratios of the sulfur species are unity at the field boundaries. B, Stability relationsof iron oxides and sulfides at an activity of dissolved sulfur of 10" 5 . C, Stability relations of uraninite anduranium ions wth PCO2 = 10~ 3-4 atm, the partial pressure in the modern atmosphere. D, Combined
diagram showing stability fields of pyrite and uraninite in relation to the triple point, (S04 ~ 2 aq) / (H2S aq |US' aq). Boundaries of solids, at activities of 10~ 6 of dissolved iron and uranium species. Adapted fromGarrels and Christ (1965) and Hostetler and Garrels (1962).

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATESGOLDThe Witwatersrand gold is crystallized in irregularparticles normally from 0.002 to 0.5 mm, exceptionallyas much as 1-2 mm, in length (Liebenberg, 1955;
Hallbauer and Joughin, 1973). Nuggets are lacking, and,save for rare late veinlets, the quartz and chert pebbles
are barren (Pretorius, 1974; Liebenberg, 1955). Though
the vast majority of particle shapes are not recognizably
detrital, a placer origin has been widely accepted
because the gold is syngenetic (Antrobus and Whiteside,1964) and because the bulk of the reef components are
generally accepted as waterworn.
PLACER ORIGIN UNLIKELY

But the ancient conglomerates simply are not com­parable to known gold placers (Joralemon, 1952). Sum­
marizing the Rand reefs, Pretorius (1974, p. 40) stated:
The average size of the gold grains is 0.035 mm; of the uraninite
grains, 0.065 mm; and of the platinoid grains, 0.055 mm. All thesefigures illustrate the extremely small size of the minerals of economic
importance in the reefs. No medium- to large-sized nuggets have beenfound during mining operations, and it can be concluded that, in regard
to gold-particle size, the detrital deposits of the Witwatersrand are notcomparable with modern-day placers.

The economy with which gold is won from gravels, theunfamiliar units in which it is reported, and the romancesurrounding rich placer pockets combine to obscure theevident fact that the Witwatersrand reefs are richer2
and far more extensive than any modern gold placers.
Even the notoriously gold-poor reefs of the Blind RiverElliot Lake district in Canada contain twice the gold
content yielded by average gold-dredge gravels (seefollowing paragraphs). This paradox underscores thelack of quantitative analysis in the easy acceptance of aplacer origin for the gold of pyritic conglomerates.
As part of the Heavy Metals program of the U.S.
Geological Survey, modern studies have been conductedon the Tertiary gravels of the Sierra Nevada (Yeend,1974), stream placers in Oregon (Boggs and Baldwin,1970), and the beach placers at Nome, Alaska (Nelsonand Hopkins, 1972). This work has reestablished thevalidity of the lessons of prior workers (Lindgren, 1911;
Janin, 1918). Placers do not depend fundamentally on
the functioning of some special winnowing process but
on a nearby source of coarse gold. The United Nations
Economic Commission for Asia and the Far East has
counselled that marine gold placers are inherently
unlikely because of the proximity of gold placers tosource (Emery and Noakes, 1968).
Economic gold placers result from the near-incapacityof streams and currrents to transport coarse gold (Tuck,
1968; Shilo and Shumilov, 1970). Such placers as* [In 1976 the average gold recovery from Rand ores was 0.2686 troy ounces per short ton(9.21 grams per metric ton) milled (Mining Survey, 1977).]

AA5

remotely approach the richness of representative Rand
reef (7-21 ppm (Pretorius, 1974)) occur only in narrow
paystreaks (Lindgren, 1911), which are generally withina few kilometers, at most, say, 15 km, of a coarse-goldsource (Nelson and Hopkins, 1972; Emery and Noakes,
1968). With few exceptions, rich gravel is bouldery,poorly sorted, and compositionally immature (Yeend,1974) and contains sporadic nuggets. Grain size andgrade rapidly decrease downstream (Boggs andBaldwin, 1970), as area of the deposits increases, just as
the yield of gold decreases exponentially away from theupper end of a dredge table (Janin, 1918, p. 182-184).
But even the most extensive of modern gold placers, thedredge fields, are small (5-50 km (Lindgren, 1911)) com­pared with the reefs (as much as 400 km2 (Antrobus andWhiteside, 1964)), whereas dredge-field average grade(0.10-0.13 ppm (Lindgren, 1911; Janin, 1918)), though
not directly comparable, is but one percent of that of thereefs! In contrast to modern placers, the Main Reef
Leader of the East Rand has been mined throughout adowncurrent distance of 30 km (Pretorius, 1974), withthe gold content and the pebble diameter of the mature
resistate assemblage paralleling a slow decrease in sizeof detrital zircons (1.4 percent/km (Viljoen, 1968)).
According to Pretorius (1974), 55 percent of theworld's total gold won throughout recorded history has
been mined from the Witwatersrand. If the Rand gold
were detrital, an unequaled concentration of giant
parent deposits, now vanished, must have closely
rimmed this local basin.
But what if such unequaled concentration of deposits
had rimmed the basin? Judging from modern gold
placers, it would not have been possible to transport
gold clasts in running water, in whatever amount imag­
inable, so as to conform to the mined distribtion of theMain Reef Leader. For, regardless of the amount of gold
available, the evidence points to a steeper grade andgrain-size gradient in the down-current direction than isdeveloped in the East Rand Basin.
Figure 3 shows that the grain size of the predictedgold particles of the average Rand reef lies well below
the lower limit of those in Siberian placers. In the
Siberian placers the minimun gold particles have a con­stant weight (~1.5 mg), regardless of the degree of flat­
tening. So do those of the U.S., dominantly Alaskan,placers (~1 mg) described by Tuck (1968). This indicatesthat resistance to transport, rather than some winnow­
ing effect, is the prime factor in the formation of gold
placers.
If the gold was carried in complexed solution, its broaddispersion in the Rand reefs becomes understandable.
The equally broad dispersion of detrital uraninite (d~9)
is, to begin with, much less a problem than for gold(d^> 17.5). Beyond that, the uniformly small size (average

AA6

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
0.6t

EXPLANATION
Natural gold particles from 6 Siberian
placers
Settling velocity envelope, gold from
12 Siberian placers-20 Settling velocity curves of fabricatedweighed gold spheres progressively
flattened to discs

CO
cc.
O

80

Settling velocity
of average Rand
hypothetical placer
gold grains

Sphere-0.002 mg

0.0

0

10

Flake-0.6 mg

20

30

40
Disc

SphereFLATNESS FACTOR

FIGURE 3.—Schematic comparison of settling velocities of natural gold particles from Siberian placers with similar
fabricated gold particles and average Rand hypothetical-placer gold grains. The settling velocity of the Rand
hypothetical-placer gold was determined from the average size of undoubted clastic mineral grains in the reefs by ex­perimental settling velocities from Gibbs and others (1971). The Rand clast sizes were averaged from data in
Liebenberg (1955) and Coetzee (1965). Graph modified from Shilo and Shumilov (1970, figs. 1, 2, table 1). Flatness fac­
tor determined from equation ((length + width)/(2 thicknesses))-!.

AA7

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

median grain size, 0.103 mm) of the uraninite from
granitic sources suggest that it may have been largely
transported to the reef site in sand-sized clasts ofgranitic rock minerals. Hydrogen-ion metasomatism(discussed in the section, Bacterial Sulfur Cycle) of thefeldspathic debris would have freed the tiny uraninite
crystals at the reef site and simultaneously obliterated
any evidence of this means of transport. In a similar
fashion the rare grains of detrital platinoids (c£~17.5)
found in the reefs (see section, Orange Free State
Goldfield) were probably transported in sand-sizedfragments of serpentinized ultramafic rock.

+ 1.0

I

+0.8
+0.6
+0.4
+0.2
0.0

TRANSPORT IN COMPLEXED SOLUTION

The Rand gold is believed to have been transported incomplexed solution (Myers, 1971). It is thought to havebeen leached anoxygenically from ordinary crustal
rocks, most likely from sea-floor basalt. The aurous
thiocomplexes (Krauskopf, 1951; Seward, 1973) are ob­vious gold solvents in an aqueous pyritic environment atroom temperature. The reactions for two of the more
likely complexes are:Au - HS"-^ AuS- + V2 H2,
Au + H2S + HS"-^ Au(HS)2 - + V2 H2.
In figure 4, Eh-pH diagrams of the stability field of the
AuS~ complex are compared with others of interest.
Other thiocomplexes have similar stability fields. At
near-neutral pH the narrow stability fields of the com­
plexes lie at the top of the pyrite field, straddling thedissolved sulfide-dissolved sulfate join. Figures 2 and 4
show that the association of pyrite, detrital uranimte,
and aurous thiocomplex constitutes a system that may
explain the Rand reefs. Pyrite and uraninite weretransported as clasts and gold moved in complexed solu­
tion. The gold complexes would have been stable either
in a system held generally at the upper limit of the pyrite
field by oxidizing tendencies or in a system dominatedby the bacterial sulfur cycle. Gold would have been
deposited with Eh shifts in either direction; it is likelythat an oxidizing reaction originally precipitated thegold of the reefs.

IX

-0.2
-0.4
-0.6
-0.8
6

10

12

PH

+ 1.0

Eh of modern
atmosphere

+0.8
+0.6
+0.4

CD

+0.2

9
x
o

Z
N

.c
LLJ

0.0
-0.2

CD

z
CJ

-0.4
FIGURE 4.-Eh-pH diagrams showing the AuS~ solubility
window in the aqueous Au-S system and its relation to
the stability fields of pyrite and uraninite. A, Stability
relations of gold and aurous sulfide complex in water at
25 °C and one atmosphere total pressure at an activity ofdissolved sulfur of 10 ~ 5 and the controlling equilibrium
distribution of soluble sulfur species (see fig. 2A). Boun­dary of field of AuS~ complex at an activity of 10 ~ 8
dissolved gold. B, Stability fields of aurous sulfide com­plex, pyrite (see fig. 25), and uraninite (see fig. 2C).
Adapted from Garrels and Christ (1965) and Hostetler
and Garrels (1962).

Q
LLJ
DC

-0.6
-0.8
0

6
PH

B

10

12

AA8

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESBACTERIAL SULFUR CYCLE

Reefs) is believed dependent on bacterially mediatedhydrogen-ion metasomatism during deposition.
Oolitic textures in Rand concretionary pyrite havebeen interpreted as biogenic (Schweigart and Rahden,1965); indeed, much smaller microstructures in porousand concretionary reef pyrite have been considered torepresent mineralized bacteria (Ramdohr, 1958; Saager,
1970; Schidlowski, 1966). Concretionary pyrite also con­tains significant nickel and cobalt (Saager and Esselaar,1969). Pyrite concentrates from the Vaal Reef locallycontain 1-3 percent nickel, the bulk of it in the crystal
lattice (F. H. S. Vermaas, unpublished data). Authigenetic cobalt, copper, and nickel ore minerals areminor reef accompaniments. The assemblage suggeststhat the ancient sulfide nodules were analogues of theporous, bacterially mediated (Ehrlich, 1971) manganeseoxide nodules of the present seafloor, which are poten­
tial nickel-copper-cobalt ores (Cronan and Tooms, 1969;
Tinsley, 1975).
Like the modern oxide nodules, the ancient pyrite con­cretions may also have contained significant amounts ofsilver. Though available data are not conclusive, theweight of the evidence suggests that much of the silverof the reefs was once contained in the virtually ubi­
quitous pyrite.

The writer has proposed (Myers, 1970, 1971) that ananaerobic bacterial sulfur cycle existed in the Rand reef
environment. Colored sulfur bacteria, primitive(Margulis, 1969) photosynthetic sulfide-oxidizing prokaryotes now symbiotic with sulfate-reducing bacteria
in restricted niches (Kondrat'eva, 1965; Takahasi and
Ichimura, 1970; Triiper and Genovese, 1968), would
have flourished widely in the photic zone in an oxygenfree system (fig. 5). As in figure 5, the postulated an­
cient bacterial cycle balanced organic matter and C02,
as well as dissolved sulfide and sulfate. At a near-neutral
pH, the cycle would have regenerated pyrite (fig. 2A, B,
D), generally keeping the concentration of soluble sulfur
species low (10 ~ 5 m) where isotopic fractionation isslight (Harrison and Thode, 1958; Perry and others,
1971; Hoefs and others, 1968). The formation of new
biogenically mediated pyrite was dependent on therelatively high activity of dissolved Fe+2 adjacent tooriginal detrital pyrite and other iron minerals concen­
trated on the reef surface in the infralittoral zone.
The bacterial sulfur cycle must have produced signifi­
cant amounts of H2S04, which would have reacted im­
mediately with the feldspathic debris in the sedimentary
environment to produce authigenic clays and (or)chlorite (Helgeson and Mackenzie, 1970). Buffering by
THUCHOLITE
the original silicates would have kept the pH near
neutral. The complete conversion of this debris in the
Uraniferous thucholite, a jet-black, brittle asphaltite,
upper productive part of the Witwatersrand system (see is closely associated with detrital uraninite in the ore
the section, Geologic Setting of the Witwatersrand conglomerates and must be related genetically to it.
Perhaps one-fourth of the total uranium in the Rand
reefs is disseminated in this oxygenated bitumen, which
SUNLIGHT
has variably replaced uraninite. Thucholite envelops
detrital uraninite grains and wedges them apart along
fractures. The asphaltite progressively separates
uraninite shards and digests them so that little remainsof the original grains (Liebenberg, 1955; Ramdohr,
1958; Schidlowski, 1969). Much of the Rand thucholite
occurs as tiny granules, "fly-speck carbon"; at many
localities granules grade by coalescence into beddingplane sheets of thucholite that vary from a fraction of amillimeter to several centimeters in thickness (de Kock,
1964).
pH
Thucholite also replaces uraninite in Canadian
2CO2 +H 2
Precambrian pegmatitites (Ellsworth, 1928) and in the
Precambrian high-temperature complex-metal ore
deposit of Boliden, Sweden (Grip and Odman, 1944). In
FIGURE 5.-Modern anaerobic bacterial sulfur cycle. The
these settings and in the Rand, the mineraloid is thought
cycle occurs in stagnant ponds on the underside of algalmats and at shallow depth in certain lakes underto have been formed by polymerization of fluid hydrocar­
anaerobic conditions. Bacteriochlorophyll of the coloredbons by uraninite radiation (Liebenberg, 1955;
sulfur bacteria, with an absorption spectrum utilizing
Schidlowski, 1969). Experimental radiation of any typelight energy not absorbed by algae, photosynthesizesof n-paraffins of intermediate chain length has threeorganic matter from C02 . Sulfate-reducing bacteria con­products (Charlsby, 1954): (1) long-chain infusible
sume organic matter with oxygen extracted fromsulfate.
molecules with lowered H/C ratios (graphite is formed

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

with very high radiation dosages), (2) gaseous shortchain paraffins with increased H/C ratios, particularly
methane, (3) free hydrogen. All three products are pre­sent in the Rand goldfields. Methane is present in manyof the mines, and gases containing as much as 77.5 per­cent H2 have been reported from several mines(Liebenberg, 1955). Thucholite locally contains graphite(Schidlowski, 1967).
Hydrocarbons could be formed by alpha-particle bom­bardment of the fatty acids present in organic sediments(Liebenberg, 1955; Whitehead, 1954). Yield estimates byWhitehead (1954) suggest the uranium here believed
originally present in the Rand reefs was adequate toproduce the thucholite substrate-hydrocarbons frombacterial organic matter.
The hypothesis that sheets of cross-fibered thucholite
represent Precambrian algal coal has been disproved by
Schidlowski (1969). The lenticles, which some in­vestigators thought to be compaction-flattened algal
bodies, are in fact oblique sections of cross-fibers inthucholite sheets. The 13C values in thucholite are typicalof petroleum, not of coal (Schidlowski, 1969; Eichmann
and Schidlowski, 1974). A petroleum origin is further in­dicated by the systematic relation between H/C ratios
and total radiation dosage in a series of uranium-bearing
petroleum asphaltites and thucholites, two of the latterfrom the Witwatersrand (Pierce and others, 1958).
Hallbauer (1975; this volume, chap. M) has proposedthat the thucholite represents the remains of primitive
lichen. The characteristic cross-fibered habit of theasphaltite is taken by him to represent fungal hyphae; heinterprets the "fly-speck carbon" granules as fruiting
bodies. After ashing the thucholite at 500°C, he finds
microscopic fillamentous structures, which he believes
are of plant origin. It is hard to see why the fragile
fungal structures he proposes would not have been flat­
tened to carbon films by rock compression. It is evenmore difficult to imagine why the delicate microscopic
filamentous structures would be preserved after under­
going not only strong compression but also very highradiation dosages from the ubiquitous detrital uraninite
and why these structures are seen only in the severely
oxidized material. Hallbauer has acknowledged (oralcommun., Oct. 14, 1975) the possibility that thethucholite might have been converted to coke in the ear­ly stages of the ashing treatment at 500°C. It seems
likely that the filaments are coke structures whoseorientation was controlled by the original fibrous tex­
ture of the thucholite and which were preserved during
the subsequent oxidation. According to this interpreta­tion, the gold filaments figured by Hallbauer (1975) weredrawn out by deformation of heat-softened gold at
500°C.
A modified paragenesis of thucholite is sketched here.
It is based on the thesis that a soluble organic substrate

AA9of thucholite was everywhere present in the waters of
the Rand reefs. It is believed to have been converted by
stepwise reactions directly to protothucholite on the siteby a combination of ionizing radiation and the presenceof dissolved uranium adjacent to detrital uraninite. (Theoilfield uraniferous asphaltites of the Texas Panhandle
formed from the abstraction of dissolved uranium by
petroleum, according to Pierce and others (1964), andcontain no particulate uraninite larger than 2 jum.) It is
maintained here that the apparent "digestion" of detritaluraninite by thucholite is illusory. The detrital uraninite
was instead dissolved along minute flaws and fracturesby an aqueous oxidation (see the section, Genesis of Goldin Reefs, Oxidizing Reactions)—the thucholite resulted
from solution of the uraninite, but did not cause it.
It seems likely that, for the most part, uraninite grains
were thucholitized while immobilized in the matrix ofthe reef. Coatings of protothucholite were probably
rubbed off most of the active grains of uraninite in inter­mittent motion on the reef surface. Those uraninite
grains that escaped thucholitization after being finallyimmobilized in the reef matrix are thought to have been
sealed off from effective contact with water and dis­solved organic matter by the development of authigenic
clays and (or) chlorite (see the section, Bacterial Sulfur
Cycle) and by the deposition of silica within the reef
(Liebeberg, 1955, p. 191). The abundant uraninite of the
Dominion Reef has not been thucholitized because its
high thorium content (Grandstaff, this volume, chap. J;
Feather, this volume, chap. Q) renders it resistant to ox­
idation and solution. Elsewhere in the Rand, high-gradeconcentrations of uraninite are invariably thucholitized(Liebenberg, 1955). That such areas were nowhere
sealed off by clay minerals or silica is probably owing toa side effect of the strong radioactivity (see the section,
Genesis of Gold in Reefs, Oxidizing Reactions).
INVERSE RELATION OF URANIUM AND GOLDThis investigation has recognized that gold anduranium vary inversely in the pyritic conglomerates.
The relation is held to be due to the progressive solutionof uraninite and precipitation of gold in the reef environ­ment. The uranium-gold ratio is considered to be an in­dex of maturity of individual conglomerates (for anotherexplanation of the varying uranium-gold ratios, see
Minter, this volume, chap. K).
Earlier, Winter had hinted broadly at the inverse rela­tion and its meaning in his descriptions of the Orange
Free State Goldfield (1964a, p. 436, 442; 1964b, p. 532,
545-546). Very recently, an instance of the inverse rela­tion was described without comment. Of the new Cooke
Section of the Randfontein Mine, Mining Magazine
wrote (1976, p. 263), "From north to south the gold value
generally decreases while the uranium value increases."AA10

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Most comprehensively, Cousins, who had access to aconsiderable body of assay results not available to me,
stated (1956, p. 106-108):
Uranium/gold ratios show only minor variations within a block of ore
and show regular variations from locality to locality * * * Very wide
variations in the ratio have been observed; the highest ratio being
3,000 times as great as that of the lowest. It appears rare, however, forreefs very rich in gold to have very high uranium/gold ratios. Such highratios are found usually in reefs with low gold values and indicate amore widespread distribution of uranium than gold.

The South African points shown on figure 6 and in
table 2 represent averages made up of very large quan­tities of mined ore or developed reserves. All mines arelisted for which production figures for both uranium andgold are available.
TABLE 2. — Uranium and gold contents, in parts per million, and theU308/Au ratios of pyritic conglomerates in South Africa, Canada,
and Brazil
District, mine, or single reef

U308

1 Blind River-Elliot Lake district, Canada, 34 samples ' ____ 1,777
2 Upper Monarch Reef, Zone 2, West Rand
1,331
Consolidated Mine. 2
3 Blind River-Elliot Lake district, Canada 3
1,400
4 Monarch Reef, West Rand Consolidated Mine * _________ 1,250
5 Upper Monarch Reef, Zone 4, West Rand
954
Consolidated Mine. 2
fi Luipaardsvlie Mine, uranium section, 1956-19644
791
712
7 West Rand Consolidated Mine, uranium section,
1959-1972.4
8 East Champ d'Or Mine, uranium section, 1959-1964 4 ____
402.3
9 Randfontein Mine, uranium section, 1959-1964 4 578.2
10 Randfontein Mine, SD 32 Shaft area 5
900
11 White Reef, West Rand Consolidated Mine 2627
12 Randfontein Mine, Cooke Section,No. 3 Shaft area 5 _____ 450
13 Randfontein Mine, Cooke Section, No. 2 Shaft area 5 _____ 400
14 Vogelstruisbult Mine i - i
243
15 Virginia Mine 4 ' 8
243.5
16 Zandpan Mine 4 ' 8
343
17 Western Reef Mine 4 - 6
239
18 Daggafontein Mine 4 ' 6
193
19 Vaal Reefs Mine 4 > 8
321
20 Hartebeestfontein Mine 4 > 8
296.5
21 Randfontein Mine, Cooke Section,
200
No. 1 Shaft area. 5
22 Buffelsfontein Mine 4 ' 6
262
23 Ellaton Mine 4 - 6
153?,4 Harmony Mine 4 > 8
217.5?,5 Welkom Mine 4 ' 6
142.5?,6 Freddies Consolidated Mine 4 ' 8
132
President
Steyn
Mine
4i6
?,7
138
?,8 Loraine Mine 4 - 6
117
29 Canavieras Mine, Serra de Jacobina,
150
Brazil 7 .
30 Doornfontein Mine 4 ' 6
145
31 Stilfontein Mine 4 - 6
135
3?, Eastern section, Free State Geduld Mine 8109.5
33 Blyvooruitzicht Mine 4 - 6
168
34 West Driefontein Mine 4 > 6
164.5
35 President Brand Mine 4 - 6
131
36 Central section, Free State Geduld Mine 867
37 Western section, Free State Geduld Mine 8115

AuU30,/Au

0.262 6,780.700 1,900.857 1,750
1.720
727
1.320
723

"3

1.330
1.280

595
556

Immature

1.000
2.040
5.00
5.010
6.00
7.00
8.280
8.420
13.0509.210
8.350
16.10015.450
11.00

402
283
180
125
75.0
57.1
29.3
28.9
26.3
26.0
23.1
19.9
19.2
18.2

14.5808.600
12.96010.61011.33012.00010.72015.430

18.0
17.8
16.8
13.4
11.7
11.5
10.9
9.7

15.17014.94012.62021.37028.66023.95023.92046.870

9.6
9.0
8.7
7.9
5.7
5.5
2.8
2.5

0>
&*

0>

o>

3

JS

^

1 Roscoe (1969, Appendix, table C).
1 London Mining Journal (1972).
3 Finch and others (1973).
4 Chamber of Mines of South Africa, 1967-1972, Annual Reports 78-83; Transvaal andOrange Free State Chamber of Mines, 1955-1966, Annual Reports 67-77.
5 Mining Magazine (1976).
6 Through 1969.
' Bateman (1958).
8 K. J. Bell (Ango American Corp. of S. Africa, Ltd, written commun., October 20, 1971);
1971 ore reserves.

The mine-wide averages (open circles on fig. 6) repre­
sent all uranium ore milled and all gold ore milled from asingle mine. In all such cases the uranium is a coproduct,
and the amount of uranium ore milled is less than that ofgold ore, ranging from 10 percent to 98 percent of thetonnage of the primary product. The amount of uranium
ore represented by individual points ranges from 1.8
million metric tons to 32.2 million metric tons. Evident­
ly, if the uranium ore makes up but a small percentage ofthe gold ore, the uranium grade of the plotted point on
figure 6 will be higher than the true average would havebeen for all gold ore. Yet the distribution of such points
shows that this factor is not a dominant one in determin­
ing whether the mine in question lies above or below thecalculated curve.
Most of the mine-wide averages include ore fromseveral reefs. Though the grade and U308/Au ratio insuch cases will differ from those of the component beds,
the composite average is as valid in determining the in­verse relation between uranium and gold as an average
made up of ore from a single reef.
The points for single reefs and single-reef zones (solid
circles) or annual plots of single mine sections (line plots)
represent gold and uranium values for a given tonnageof ore. The amounts of ore represented are comparable
to those of the mine-wide averages, commonly involving
many millions of metric tons. An inspection of figure 6
will show that although the mine-wide averages must beclassed as inferior to those from single reefs, reef zones,
or mine sections, the distribution of points around thecalculated curve does not appear to support this conclu­
sion.
The three points plotted for Canada and Brazil (solid
triangles) do not represent formal averages of specificamounts of ore. One of the Blind River-Elliot Lakeaverages represents the arithmetic mean of all samples(34) for which combined uranium-gold assays wereavailable, regardless of reef or location. The other pointis a published estimate of district-wide average grade.
The Brazilian point is a published estimate of average
grade based on the milling of a few thousand metric tonsof ore from a single stope.
GENESIS OF GOLD IN REEFS

The present thesis holds that the gold content of a reef
increases with the duration of the unconformity on
which the conglomerate was reworked. When figure 6 isread as a diagram of reef maturity, with the abscissa asa crude scale of age, both absolute and relative amounts
of uranium are seen to decrease with age. This accords
with descriptions of thucholitization of Rand detritaluraninite and of large uraninite crystals in a Canadianpegmatite (Liebenberg, 1955; Ramdohr, 1958;
Schidlowski, 1969)-thucholitization apparently bothdigested uranium and expelled it from the system.
All gold-bearing Rand reefs contain thucholite, withwhich a widely variable proportion of the gold is directly
associated (fig. 7) (Horwood, 1911; Liebenberg, 1955;
Hallbauer and Joughlin, 1973). Abundances of gold andGENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

AA11

2000

EXPLANATION
20-

1750

? 8 ratio on maturity curveA

Average ore for Canada and Brazil
IN SOUTH AFRICA:t—(————1> Three annual plots of single mine sec­
tions (grade varies radially from ori­
gin). Tick indicates average ore

1500

O

Minewide average; all U ore milled, all
Au ore milled•

Single reef or single reef zone

1250

DC
HI
Q.

1000

750

500

250

10

20

30

40

Au, IN PARTS PER MILLIONFIGURE 6. - Uranium and gold content of reefs in South Africa, Canada, and Brazil. Data points are listed in decreasing order of
U3 08/Au ratio (table 2). Within a single mine, ore grade fluctuations in a reef of nearly constant maturity give rise to the wide­ly recognized sympathetic variations in gold and uranium content (data points 6, 8, and 9). Among different mines on the same
reef, or among different reefs an inverse relationship generally exists between uranium and gold. The symbol r is the linear
coefficient of correlation, explained more fully in the section, Genesis of Gold in Reefs, Degrees of Association of Uranium andGold. The curve is from the best-fit linear regression calculated by the method of least squares on the logarithms of the data
points.

50

AA12

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESFIGURE 7.-Gold (black) with thucholite (stippled) and very minor
sulfide (S), probably pyrite, in quartzite gangue (blank). Gold encases
but does not crosscut thucholite granules, indicating that the gold
was introduced early, not after the formation of fractures. Horizon­
tal face of the block is parallel to reef, a single lamina on a minor un­conformity; South Reef, West Rand. Bar scales 1 mm long.

thucholite tend to vary together (de Kock, 1964), and by
far the richest gold ores are thucholite laminae such as
the Carbon Leader, only a few millimeters or cen­timeters thick but containing up to several weightpercent gold (Liebenberg, 1955; de Kock, 1964). The
notable association of gold with thucholite is not due to aproperty of the bituminous material itself; instead, it oc­
curs because thucholite surrounds highly radioactive
uraninite whose nuclear decay resulted in the precipita­tion of gold.
RADIOLYSIS OF WATER

The radioactive decay of detrital uraninite ac­
cumulating on the Rand reefs caused the radiolysis ofwater and the formation of free radicals according to thefollowing reactions:
H20 H20 + +H20 + e- OH" + H, andH20 + + H20 H30 + + OH,
where w indicates ionizing radiation and • H and • OH
denote charge-free free radicals (Dragonic andDragonic, 1971, p. 10-11).
When water is irradiated with X-rays or y-rays in acompletely water-filled vessel and the ionizing radiationis then removed, it is found that the free radicals recombine and only water remains. Contrastingly, if the sameradiolysis takes place in the presence of a large volume
held at a vacuum, there is a continuous evolution of freehydrogen (Alien, 1961, p. 24, 75). From this it is clearthat under low-pressure conditions where hydrogen candiffuse from the system, a fractionation of the reducing

and oxidizing free radicals will occur. Such a fractiona­tion must have occurred on the shallow unconformities
on which the reefs certainly accumulated, whether they
were marine, as favored here, lacustrine, or fluviatile. A
calculation was carried out to determine whether thefractionation would have been effective. A hydrogenpressure in the ancient atmosphere of 10~ 4 atm and areef 0.9 m thick with a uraninite content of 2,000 ppm(parts per million) U308 were assumed. The diffusion
system would have kept the hydrogen pressure of thereef at 10 ~ 3 atm with a reef depth of 20 m. As the reefs
probably accumulated at depths considerably less than
20 m, the effective reef pressure would have been evenless. Thus, the calculation shows that diffusion ofhydrogen would have been an effective fractionating
mechanism.
OXIDIZING REACTIONSRadiolysis fractionation would have resulted in an ex­
cess of hydroxyl free radicals in the reef environment. In
the presence of detrital uraninite, pyrite, aurous sulfide
complex, and hydrocarbons, hydroxyl free radicals
would have given rise to the following oxidizing reac­
tions:
H20 —H • + -OH,
f~i TT
f~* TT• _i_ • r^TT
H • + • OH + • CH3•"OHVCH4,
H • + • H H2,
OH '• + • OH H202 (liquid hydrogen
peroxide),
U02 + 2 • OH - U02(OH)2 ,U02 + H202 - U02(OH)2,
2AuS" + 6 • OH -* 2Au + S203" 2 +3H20, and(CB-, H2B_0, + ' OH - C^H^-OxOH (thucholite).
These oxiding reactions would have dissolved
uraninite, precipitated gold, and polymerized oxy­
genated thucholite from hydrocarbons.
Although hydrocarbons polymerized anhydrously by
nuclear reactors (Charlsby, 1954) grossly simulate
thucholite, the natural mineraloid is oxygenated(Ellsworth, 1928; Liebenberg, 1955; Pierce and others,
195&). Increases in the oxygenated asphaltite fractionsof crude oils have been recorded during experimental
aqueous irradiations that freed hydrogen and methane(Colombo and others, 1964). Part of the gas was
generated by anhydrous degradation reactions(Charlsby, 1954) within oil globules in an oil-water emul­sion, but part was produced within the same test vial byreactions involving water (Colombo and others, 1964).
Water served as hydrocarbon oxidant because the bub­ble of reducing gases in the test vial did not equilibratewith oxidizing hydroxyl radicals in the subjacenthydrous mixture.

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

Similar oxidizing reactions must have taken place dur­ing the formation of natural uraniferous petroleumasphaltites investigated by Pierce and others (1958,
table 1). Plotting of their data on 23 asphaltites and one
Rand thucholite shows a direct relation between theuranium content and oxygen content. This correlation isbest displayed as an inverse relation between uranium
and the C/0 ratio (fig. 8.).
Such reactions would have been very slow because theradioactive decay of natural uranium is itself a very slow
process. But the productive Rand reefs range in age
from 2.8 to 2.3x 109 yr, an interval nearly as long as thePhanerozoic eon. In the Witwatersrand Basin this inter­val of 500 million years was one of uncommon crustal
stability. The unconformities on which the reefs occur
seem likely to have endured for several million years.
Thus secular reactions that would have been grossly in­competent in many environments may plausibly beevoked to explain the relation shown by figure 6.
Locally, these slow reactions appear to have been
more brisk. High-grade concentrations of uraninite in
the Rand are almost invariably thucholitized (see thesection, Thucholite). Lower-grade accumulations com­monly contain unthucholitized grains, implying thatthese areas were sealed off by authigenic silica or clay
minerals from chemical interaction. It seems likely thatthe high-grade concentrations gave off enoughhydrogen that they were kept permeable to aqueous
solutions.

AA13

FIGURE 8. - Uranium content and C/0 ratio for 23 uraniferous petroleumasphaltites and a Rand thucholite. Data from Pierce and others (1958,
table 1) recalculated to an ash- and sulfur-free basis. The curve is fromthe best-fit linear regression calculated by the method of least squareson the logarithms of the data points.

b. The rate of the chemically controlled exponen­tial decrease in uranium was slowed by the op­
posed process of concentration.
Several processes working concurrently during theformation of the reefs control the inverse relation be­
Together, these actions define a maturing process. A
tween uranium and gold:
uraninite placer, accumulating on an unconformity,
1. Chemical reactions driven by the nuclear decay of became progressively converted to a gold reef with little
uranium:
residual uraninite. Immature uraninite-rich reefs tenda. A rate of uranium solution that depended, via to be relatively thick conglomerates with dispersed
the uraninite-driven radiolysis of water, on metal content (Roscoe, 1969; Pretorius, 1964). The mostthe amount of uranium present, and hence de­ mature reefs tend to grade toward the thucholite-seamcreased exponentially with time from the type, very thin but exceedingly rich in gold (Liebenberg,
onset of thucholitization to the time of effec­ 1955; de Kock, 1964). A combination of chemical attack
tive burial. (Radioactive decay is a first-order and physical reworking accounts for the general thin­reaction - that is, the number of atoms that ness of mature reefs and for the detrital implication of
decompose in unit time will be proportional to such features as cylindrically rolled gold particles en­
the number present.)
cased in pyrite (de Kock, 1964, p. 368). A maturing reefb. A linear precipitation of gold, the rate deter­ at any given moment is the sum of the chemical pro­mined at first by the concentration of the cesses that make it progressively more uranium-poor
scarce aurous complex, later by the diminish­ and gold-rich and the reworking that redistributes theed availability of the precipitant, which in turn chemical products as it concentrates them by thinningwas controlled by the concentration of the assemblage.
uraninite.
As is more fully explained in the section on Degrees of
2. Concentration due to reworking:
Association of Uranium and Gold, mature reefs (table 2)a. The gold content increased exponentially with are those whose rates of gold precipitation are con­time because of concentration into an ever- trolled by the amount of uraninite present-that is,thinning reef.
those reefs in which the amount of uraninite is so
MATURATION OF REEFSAA14

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

diminished that the supply of uranium, rather than the
supply of gold, controls the rate of precipitation of gold.
Immature reefs, by contrast, are those in which uranium
is in excess, and the supply of gold controls the rate ofgold deposition.
Hydrogen-ion metasomatism of feldspathic debris andthe formation of biogenic pyrite during the evolution of
the reef were described in the section, Bacterial Sulfur
Cycle. In the presence of dissolved gold and uranium it isexpectable that these elements should be present in theauthigenic reef minerals. Gold is present both in thephyllosilicates and in pyrite (Liebenberg, 1955; Ramdohr, 1958; de Kock, 1964), but the time of its deposition
is debatable. Recently P. R. Simpson, by fission-track
measurements, has found more than 40 ppm of uranium
disseminated in the phyllosilicates of the Rand reefs. He
believes this to have been absorbed at the time of deposi­tion (Simpson, oral commun., October 14, 1975).
Similarly, the sericitic matrix of Elliot Lake con­glomerates has been shown to contain disseminateduranium (Roscoe, oral commun., October 14, 1975).
These findings independently suggest that uranium wasmobile during deposition of the reefs.
The relation of uranium solution to gold precipitation
and accumulation may be visualized more readily on
figure 9. The assumption made earlier is restated here:
The curve traced by average ore grades on figure 6 may
be used to describe the maturation of the uranium-gold
reefs. In figure 9 the abscissa has been transformed to alinear time scale by assuming that the uranium loss withtime maintained a constant half life (this is further ex­
plained in the section, Competency of Radiolysis). The
gold content is assumed to have increased exponentiallywith time owing to reworking, which concentrates thecontinually precipitating gold into an ever-thinning reef.
The ratio of uranium loss to gold gain dropped con­tinuously, rapidly at first and ever slower in the laterstages of reef development. Figure 9 treats the initialuranium grade as constant, showing only a single curve,
in a comprehensive treatment differing grade would
generate a nested family of curves. Note that in addition
to indicating the effects of the chemical reactions thattake place during reef development, figure 9 is con­cerned with the grade of the metals as they become con­centrated in the thinning reef.
Cousins (1956, p. 106-108) recognized that large
samples of reefs from a single mine have a characteristicU308/Au ratio (see points 6, 8, and 9 on fig. 6). However,for several reasons adjacent small samples show wideand erratic variations in the ratio. As will be explainedin a later section, in immature reefs the ratio is depen­
dent on the uraninite concentration and thus changes
from point to point. Ratio fluctuations are also caused by
the sealing of small patches of reef against chemical in-

TIME
FIGURE 9. - Rate of uranium loss and gold gain of reefs in South Africa,
Canada, and Brazil. Rates are plotted against time.

teraction (see the section, Thucholite), leaving uraniniterich immature reef residual against more gold-richareas. These immature patches skew the curve of figure
6, reflecting residual uranium on the gold-rich arm, fromthe ideal hyperbolic form. Lastly, the contents of thematuring reefs were mixed and concentrated byrepeated reworking.
Pyrite appears to decrease in abundance with increas­
ing maturity, seemingly along a curve like that of figure
6. In the Elliot Lake uranium reefs pyrite averages
perhaps 10 weight percent (Griffith, 1967; Roscoe,
1969), dropping to 3-5 weight percent in the Serra deJacobina gold ore (Bateman, 1958) and to about 3 weightpercent in the average Rand conglomerate gold reef(Pretorius, 1964). In some of the thinnest, most mature
reefs pyrite is scarce (Pretorius, 1964) or virtually ab­sent (fig. 7). Some thin gold-rich reefs present a specialcase. At the Free State Geduld Mine (32, 36, 37 on table2) the reef, a few centimeters thick, is rich in pyrite(Saager and Esselaar, 1969) and is overlain by barren
quartzite very lean in pyrite. If one assumes that thisreef is a reworked remnant of a thicker, less mature
reef, then the pyrite of the remnant, calculated back toabout a meter thickness, turns out to be about one-thirdthat of the average Rand conglomerate gold reef.
Pyrite was depleted by the reaction that dispersed
uraninite; buffering by pyrite moderated the oxidative
dispersal of uranium. It follows that where pyrite wasscanty or absent, uraninite was completely dissolved by
the oxidizing effects of radiolysis.

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATESParadoxically, uraninite is locally more abundant thanpyrite in immature ore at the Dominion Reefs Mine
(Hiemstra, 1968a, p. 5). However, the high thorium con­tent of the Dominion Reef uraninite (Grandstaff, thisvolume, chap. J; Feather, this volume, chap. Q) renders
the mineral resistant to oxidation and solution, so thatthe association is not in conflict with the thesis advanced
here.
Thucholite composition varies with maturity. Eleven
Rand thucholites (de Kock, 1964) show a direct relation
between weight-percent volatiles and the logarithm of
the gold content (fig. 10). Organic analyses of 23
uraniferous petroleum asphaltites and one Rand
thucholite (Pierce and others, 1958) show a strong direct
correlation between volatile content and oxygen content(fig. 11). It follows that the gold-rich thucholites are
more oxygenated than the gold-poor ones. The reactions
that depleted uraninite and pyrite and precipitated goldin the Rand reefs also progressivley oxygenatedthucholite.
The positive correlation coefficients between U308 andgold in the Rand ores (Saager and Esselaar, 1969) thus
reflect not the detrital nature of both components, asthese authors (p. 450) have postulated, but rather the
dependence of the precipitated gold on the presence of
detrital uraninite. In a like manner, the many
demonstrations that gold in the Rand reefs responds tosedimentological factors primarily reflect such controlof placer uraninite (d ~ 9) rather than placer gold(d~ 17V2).

12345
LOGio Au CONTENT, IN PARTS PER MILLIONFIGURE 10.-Volatile content and logarithm of the gold
content of 11 thucholites from the Rand reefs. Data from de Kock(1964) recalculated to an ash- and sulfur-free basis. The line is thebest-fit linear regression calculated by the method of least squares.

AA15

10
20
30
VOLATILES, IN WEIGHT PERCENT

FIGURE 11.-Volatile content and oxygen content of 23
uraniferous petroleum asphaltites and one Rand thucholite.
Data from Pierce and others (1958, table 1). Analyses
presented on ash- and sulfur-included basis. The line is thebest-fit linear regression calculated by the method of leastsquares.

COMPETENCY OF RADIO LYSISThe competency of the decay energy of uraninite as adriving force of the oxidizing reactions is indicated in
the following sections. For each uranium nucleus
decayed, 3.8 x 106 hydroxyl free radicals are supplied by
the radiolysis of water, whereas 6.9x 106 are needed tosupport the oxidation reactions. Evidently thecalculated supply of oxidant falls short of a quantitative
confirmation of competency, but in view of the limited
data the fit is considered satisfactory, as supply and de­mand vary by less than a factor of two. (The gap be­
tween supply and demand of oxidant would be reduced
or erased if an estimate could be made for themechanical abrasion of pyrite, which is not allowed forin the following discussion but which surely must havetaken place.)
In the Witwatersrand Basin the productive reefs arescattered through a stratigraphic section with a max­
imum thickness of about 9 km, spanning a time intervalof half billion years. In the Central Rand a stratigraphic
thickness of 8.55 km (Pretorius, 1964) separates thevolcanics of the Dominion Reef Series (2.8xl09 yr(Niekerk and Burger, 1964)) from the quartz porphyry in
the Ventersdorp andesites (2.3 xlO9 yr (Niekerk andBurger, 1964)).
Total time __________________________ 500 x 106 yr
Estimate of time lost by unconformity ________—— 100 x 106 yr
Effective time _____________________—— 400 x 106 yr

AA16

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTherefore,(8.55km)/(400xl06 yr)= 21.38 m (70 ft) per million years.
At the Loraine Mine, Orange Free State, the Basal Reef
truncates a maximum of 140 ft (43 m) of footwall beds.
Therefore,
140/70 =2-million-year duration of Basal Reef unconformity.
Adoption of the Central Rand section, the thickest in
the basin, is a conservative procedure, for it results inthe minimum duration for an unconformity.
With an estimate of the length of time taken to form
the Basal Reef at Loraine, it is possible to estimate thechemical half-life, or intensity of solution of detritaluraninite, during reef formation. In figure 6 it is seenthat the Loraine point (28) projects along a maturity ray
to intersect the normalized curve at a U308 concentra­tion of 165 ppm. The U308 concentration at zero gold
concentration is indeterminate, for beyond the upper
limit of figure 6 the extrapolated curve becomes assymptotic to the zero gold ordinate. It is approximated here
at about 2,640 ppm U308, 16 times the concentration of
the normalized Loraine point. The Loraine point thenlies 4 half-lives (24 = 16) below the zero gold point. The
apparent length of the chemical half-life of uraninite
during reef formation is thus 2,000,000 yrs/4 = 500,000
yrs.
However, this estimate does not take into account theeffect of thinning the reef during reworking (seeparagraph 2b in the section, Maturation of Reefs). It canbe shown that by assuming a concentration of 2, due tothinning of the reef in 500,000 years to one-half its
former thickness, which seems reasonable, the truechemical half-life of uraninite is halved from the net orapparent rate: 500,000 yr/2 = 250,000 yr.
By comparing the chemical half-life of uraninite on thereef with the radioactive half-life of 238U and 235U, we cancalculate the number of atoms of uranium chemicallydissolved for each atom of uranium undergoing nuclear
decay:
radioactive half-life 238U
chemical half-life U02 and reef
_ number of molecules U02 dissolved on reef
238U nucleus decayed
14.51 xlO9 ^ 18.04xlO3 molecules U02 dissolved per
0.25x 106
238U nucleus decayed, andfor 235U,
7.13xlO8
28.52xl02 molecules U02 dissolved
0.25 x 106
per 235U nucleus decayed.
A large excess of pyrite was oxidized with theuraninite. The function of the pyrite as an oxidation buf­

fer indicates that it would only have been oxidized to theminimum necessary to dissolve the sulfide:
UO, + 2 • OH -* U02(OH)2
FeS2 + 6 • OH - Fe +2 + S203 - 2 + 3H20
From the relative amounts of pyrite and uraninite in an
immature and a highly mature reef (see section, Matura­tion of Reefs), the presumably steady-state relation
varies from an FeS2/U308 ratio of 71, to one of 95,
respectively. Because the amount of uraninite present
2.5 x 109 yr ago was about 155 percent of that remaining
today (see Appendix), the FeS2/U02 ratios originallyprevailing were only 46.1 and 61.3, respectively.
Assume that a pyrite-uraninite mix at an FeS2/U02 ratioof 61.3 was continually dissolved from the reef
thoughout its history. In the earliest stage assume thisto be balanced by an increment of biogenic pyrite
amounting to one-third that retained in the reef. This in­crement would dimish to zero in the most mature reef.
(This is a simplifying assumption purely for ease of
calculation.)
The oxidizing reactions presented and the FeS2/U308
ratio assumed may be combined (if the "U02" of thechemical formula is equated with the U308 of mining
statistics) to estimate the number of hydroxyl free
radicals-needed to oxidize each molecule of uraninite andthe accompanying pyrite:
Hydroxyl
free radicals
consumed

Molecules
oxidized

1 U02 _.
61.3 FeS2
Total_______________________________________________

2368________________________370

For each 238U and 235 U nucleus decayed, the minimum
number of hydroxyl free radicals that must have been
produced is as follows:
238U: 18.04 x!03 x 370 = 6,674,800,
235U: 28.52 x!02 x 370 = 1,055,240.
At 2.5xl09 years ago the 235U isotope, with shorterradioactive half-life than that of 238U, would have been
more abundant in a sample of uranium. It can be shown(see Appendix) that 235U would then have constituted
five percent of natural uranium:
238U: 6,674,800 x 94.786 percent
235U: 1,055,240 x 5.214 percent
Total ____________.

6,326,776
550,2026,876,978Total for natural uranium = 6.9xl06 hydroxyl free
radicals produced for each nucleus decayed.
Dragonic and Dragonic (1971, p. 125) report a meanvalue of 4.1±0.5 water molecules decomposed per 100
eV absorbed in the radiolysis of water. This value wasaveraged from "several hundred papers reporting dataobtained under different experimental conditions."

AA17

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

Birch (1954, p. 151-152) lists the effective radioactivedecay energy of uranium as follows:
Isotope

MeV per atom
47.4

235U

45.2

Cal per g-year

0.71
4.3

At 2.5 x 109 years ago, the mass of a sample of uranium
would have been more than 1 ¥2 times its present mass,
which has been depleted due to its continuing radioac­
tive decay to lead (see Appendix). The radioheat produc­tion of 238U then would have been 146.84 percent of its
present value; that of 235U, 48.93 percent of the present
heat production of 238U. The decomposition of watermolecules can now be calculated as follows:
238U: 4.Ix 47.4 x!04 x 146.84 percent=
2,853,689
235U: 4.Ix 45.2 x!04 x 48.93 percent=
906,771
Total __________________
3,760,460
Total = 3.8xl06 H20 molecules decomposed per
uranium nucleus decayed.
ORANGE FREE STATE GOLDFIELD

In the Orange Free State Goldfield, uranium isrelatively high, and gold low, on the margins of the field,
where the unconformity beneath the productive Basal
Reef is minimal (Winter, 1964a, b; McKinney andothers, 1964; Minter, this volume, chap. K). Gold in­creases and uranium diminishes as the unconformity
becomes more pronounced: the reef increased in maturi­
ty where the unconformity was of longer duration.
Detrital grains of platinoids correspondingly are moreabundant—0.008 ppm—and less mature—Os + Ir =66-71 percent (Cousins, 1973)— on the margins of thegoldfield and less abundant—0.0025-0.004 ppm—andmore mature—Os + Ir = 76-79 percent—where the un­conformity is longest lived. The decrease in abundanceis in part due to the electromotive replacement of lessnoble platinum-group alloys by complexed gold. Some of
the few platinoid grains met with in polished sections
are partially replaced by gold (Liebenberg, 1955; Ramdohr, 1958). The rare near-spherical gold grains adduced
by Liebenberg (1955) as evidence for the detrital originof the Rand gold are here interpreted as complete
replacements. (Figure 3 shows that thin flakes rather
than near-sperical grains of placer gold would be expec­
table in the reefs. In contrast, the hard, brittle platinoids
were not deformed into flakes.) The increase in osmium
and iridium content with maturity in the anoxygenic en­
vironment is due to chemical stability and superior hard­
ness; oxidizable osmium and iridium both diminish withmaturity in modern placers accumulating in an oxidizing
atmosphere, and platinum becomes increasingly domi­
nant (Mertie, 1939, p. 135-137).

WHITE REEF

A series of as many as a dozen generally uranium-richreefs scattered through 80 m of quartzite (Toens andGriffiths, 1964) occur locally in the West Rand but are
not as numerous elsewhere in the Witwatersrand Basin.
The local proliferation of the Bird Reefs suggests
strongly that they were, for the most part, short-lived
phenomena. One of these is the White Reef (fig. 12).
Small patches of mature White Reef (U308/Au, 10-15)
have been mined for gold in the northeastern part of the
field, but the greatest production has come from Randfontein, where much less mature reef (minimum
U308/Au, range 61-261) was mined in large quantity forboth uranium and gold (Toens and Griffiths, 1964).
Figure 12 shows that the U308/Au ratio varies pro­
gressively from point to point across the mined extent.
The small amount of mature gold reef mined, in contrast
to the extensive mining of less mature reef, suggeststhat maturation may have been degenerative. Figure 13
shows the range of uranium-gold ratios for the White
Reef and for the Basal Reef of the Orange Free State.
The combined range of the two reefs, independently
documented by figure 12 and by Minter (this volume,
chap. K), is the greater part of the range of all pyritic
conglomerate reefs for which uranium-gold ratios areavailable (fig. 6; table 2). The range of White Reef
uranium-gold ratios crosses the gradation between im­mature and mature reefs, as sketched in tables 2 and 4
and figure 15, and suggests that no fundamental dif­ference exists between the two types of reef.
MIDDLE ELSBURG REEFSIn the country lying between the West Rand and theWest Wits Line a zone of conglomerates called the Mid­dle Elsburg Reefs has recently been brought into pro­duction (Mining Magazine, 1976; Papenfus, 1964, p.
390). The mined reefs lie in the upper half of a sequenceof three conglomeratic subzones spanning astratigraphic thickness of as much as 120 m. The upper
conglomerate of the top group of reefs is the most per­sistently auriferous, whereas the conglomerates of the
middle group have the most consistent uranium content.
Table 3 shows that in going from the immature to themature end of the Cooke Section the gold content of 66
million metric tons of ore nearly doubles as the uranium
content drops by more than half.
TABLE 3. -Indicated ore, Cooke Section, Randfontein Mine
[Mining Magazine, 1976]
Area
No. 1 Shaft Area (North)

No. 2 Shaft Area (Central)

No. 3 Shaft Area (South)

Metric tons
(millions)

Au
(ppm)

U308
(ppm)

U30,/Au(ratio)

19
18
29

11
7
6

400
450

200

18
57
75

AA18

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
6810

In this 'segment, scattered patches of gold ore(with UaOg/Au ratios of 10-15) minedWest Rand Consolidated

2 MILESI
3 KILOMETERS

FIGURE 12. - Sketch map showing distribution of Us08/Au ratios in the
White Reef, West Rand. Randfontein Us08/Au data based on 125
assays provided by Johannesburg Consolidated Investment Group.
Other data are from Toens and Griffiths (1964).

DEGREES OF ASSOCIATION OF URANIUM AND GOLDIn the early stages of reef formation (immature reef)
uraninite was present in excess of the amount needed toprecipitate the amount of gold available in complexedsolution. In the later stages (mature reef) uraninite wasso diminished in amount that the effects of its radioac­
tive decay did not precipitate all available dissolved gold.
The amount of gold precipitated was thus controlled by
the limited amount of uraninite present. Under the lat­ter conditions a fixed amount of gold would have been
precipitated for each unit of uraninite present. Thus in amature reef a perfect association between uranium andgold would be expectable. In immature reefs the degreeof association or correlation would have been less thanperfect.
Discussions of the reef maturation process have until
now made the simplifiying assumption that the reef had

White Reef-^
12.5

GOLD RICH

FIGURE 13.-Us08/Au range of the White Reef, West Rand, and theBasal Reef, Orange Free State, compared with all reefs. (Curve fromfig. 6; specific values from tables 2 and 4 and fig. 12.)a uniform grade. In any real situation the original con­centration of the detrital uraninite would have beenhighly variable from point to point. Figure 14 shows theeffect of different original concentrations of detritaluraninite in a maturing reef. Each curve represents thedecrease with time of an arbitrary concentration of
detrital uraninite, the highest concentration being
represented by the steepest curve. The horizontal line
connecting the bottoms of the curves represents the con­stant ratio (U308 present per unit of gold precipitated)achieved as each successively higher concentration of
detrital uraninite diminished to the level where it deter-AA19

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

mined the rate of gold precipitation. This constant ratio
defines maturity for a particular original concentrationof detrital uraninite. At time t0 the ratios (U308 present
per unit of gold precipitated) would be markedly dif­ferent still. At time tn the degree of correlation between
uranium and gold in a suite of samples would be poorer
than in samples taken at time tl or t2 .
uranium and gold in a suite of samples would be poorer
than in samples taken at time ^ or t2.
Let us assume that the uppermost curve of figure 14
represents the highest original concentration of detritaluraninite occurring with significant frequency in thereef. Then the point just off the right-hand side of the
figure, where this curve intersects the maturity line,
represents the time at which the reef as a whole would
have achieved maturity and thus a perfect correlation
between uranium and gold.
As will be shown below, this condition does not seemto have been achieved in the Witwatersrand, for anotherfactor intervened that progressively diminished thedegree of association of uranium and gold. This factor isthe sealing-off of local patches of reef against chemicalinteraction (see the section, Maturation of Reefs), and it
would have become more widespread with time, halting
the maturation of the patches at intermediate levels. By
juxtaposing various U308/Au ratios the process would
have progressively reduced the degree of correlation
between the two metals.
This analysis predicts that the degree of correlation
between uranium and gold should rise from low values ina very immature reef to a maximum at intermediate
maturity and then fall to lower values at higher
maturities.
In this treatment the degree of correlation between
the two elements will be expressed by the linear coeffi­
cient of correlation r (also termed the product-moment
correlation coefficient). The linear relation between two
variables X and Y is calculated by the formulawhere X and Y represent (Wonnacott and Wonnacott,1970, p. 103-108) the arithmetic means of X and Y. The
quantity r varies from + 1 (perfect positive correlation)
through 0 (no correlation) to - 1 (perfect inverse correla­tion).
In figure 15, r for uranium and gold is plotted against
the logarithm of the U308/Au ratio for thirteen samplesof pyritic conglomerate of the widest range of maturity(table 4).
TABLE 4.— U308/Au ratios and the linear coefficient of correlation r
between uranium and gold for sample suites from reefs in South
Africa and Canada
Samplesuite

Number of y 0 /Au
samplesratjo(locality)
34

6,779

0.13

Roscoe (1969,
Appendix table C).

20

477.2

.55

Hiemstra (1968b, table V).

24(14 level)

261.1

.38

15
(8 level)25(6 level)
13(14 level)20(8 level)
17(8 level)
11(3 level)

150.7

.89

B.D. Stewart, Johannesburg
Consolidated Investment
Co., Ltd., written commun., Sept. 18, 1975.
Do.

1 , Elliot Lake district _Dominion Reef:
2 Dominion Reefs Mine _White Reef:
Randfontein Mine:
3 F __

4 E
5 G
6 D
7 C
8 B
9 A
Vaal Reef:
10 Zandpan Mine.

FIGURE 14. - Changes in the ratio \J30S present per unit of gold
precipitated for different detrital uraninite concentrations in a reef.

Source11 Hartebeestfontein
Mine
Ventersdorp Contact Reef:
12 Venterspost Mine ____
Basal Reef:
13 Free State Geduld
Mine.

128.4

.36

Do.

122.1

.63

Do.

112.3

.91

Do.

81.30

.80

Do.

61.44

.85

Do.

44

23.31

43

17.36

Hiemstra and Rahden(1969).
Do.

56

6.62

.70

109

2.78

.605

1 Numbered points on figure 15.

Do.
Saager and Esselaar(1969).

AA20

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

\

I

I

III

considerations, however, and figure 15 combine to sug­gest that the linear correlation coefficient r of a sample
suite of ore grade should not depend on whether the
samples eame from one or many reefs but only on theU308/Au ratio of the suite.

1

VOLCANIC INFLUENCE
BLIND RIVER-ELLIOT LAKE

Immature
reefs
I

00

OCO

=> 2
O

(5

O

Mature
reefs

.2

.4

.6

.8

1.0

FIGURE 15. -The linear coefficient of correlation r between uranium
and gold compared with the logarithm of the U308/Au ratio for in­dividual reefs in the Witwatersrand and for the composited BlindRiver-Elliot Lake District. The dashed-line slope excludes the
Canadian sample. (Data from table 4). The lines are the best-fit
linear regressions calculated by the method of least squares.

The samples fall into an immature group, for which r
increases with increasing maturity, and a mature group,
for which r decreases with further increases in maturity.
The variation of r is in accord with the foregoinganalysis and clearly supports the outlined hypothesis oforigin of the reefs. The relatively wide scatter of points
from the immature reefs probably derived from thesmall sample sizes, although all but the Canadian sample
are well above the limit for a five-percent level ofsignificance. Because the Elliot Lake district sample iscomposited from several productive conglomerates
rather than a single reef, it has been excluded from analternate treatment of the immature reefs. Theoretical

The Blind River-Elliot Lake uranium-bearing con­glomerates occur at or near the base of a Proterozoic
sedimentary sequence, which in the vicinity of the mines
lies with angular unconformity upon Archaean
greenstone (Roscoe, 1969), locally pyritized along its up­
per contact. The protecting pyrite in these reefs is abun­dant and coarsely angular. It may have been partly
derived from sulfidization of iron-rich Archaean
greenstone weathering nearby, for basal Proterozoic
volcanism within the district itself (Roscoe, this volume,
chap. W), and extending 30 km to the south (Roscoe,
1969), must have released abundant H2S into the anoxygenic environment (in modern environments volcanic
H2S is quickly oxidized to S04- 2). The Blind River reefs,
in contrast to those of the Rand, characteristically con­tain tightly packed, subangular clastic grains ofuraninite in unthucholitized streaks and cross laminae.
Thucholite is generally rare but locally abundant(Roscoe, 1969; Ruzicka, this volume, chap. V); the lowcontent of gold is attributed here to the brevity of
periods of reef development. Uraniferous pyritic con­glomerates extend eastward from Elliot Lake, and at
Lake Timagami, 200 km easterly, a more mature basal
conglomerate of similar age locally contains ten times
more gold (>2.7 ppm Au, <180 ppm U308; Thomson,1960). Here, instead of the iron-rich greenstone of the
Elliot Lake area, Archaean iron-formation lies unconformably below the mineralized conglomerate andpresumably contributed the iron for the pyrite.
The Blind River-Elliot Lake ores are commonly
regarded as containing insignificant amounts of gold.
Yet the average gold content for the 34 samples forwhich both gold and uranium assays are available(Roscoe, 1969, Appendix table C) is 0.26 ppm, twice theaverage yield of gold dredging (see the section, Gold,
Placer Origin Unlikely).
VENTERSDORP CONTACT REEF

In the Rand, the contact between Witwatersrandstrata and overlying Ventersdorp andesites is commonlyan angular unconformity marked by the Ventersdorp
Contact Reef, which, though not widely productive andunique in its explicit volcanic association, is a typical
Rand reef (Haughton, 1969; de Kock, 1964). In and near

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

the Western Areas Mine, however, no unconformity isdetectable, and a tuffaceous gradational zone as much as
30 m thick, here believed the marine equivalent of earlylavas, lies beneath andesite flows. Within the zone andas much as 100 m below, several thick, relatively poorly
sorted, coarse conglomerates (Papenfus, 1964), polymictic in part, are atypically rich in gold. A high H2S
pressure during volcanism would have increased thesolubility of aurous thiocomplexes and, with unconformably older reefs as additional local gold sources (deKock, 1964; Papenfus, 1964), would have increased theirconcentration as well and so increased the rate of gold
precipitation in the reefs.
DISCUSSION

AA21

direct relation between the two metallic particles would
be expectable, for they have similar densities (d~17.5).
This would follow even though the malleable gold would
expectably have been flaky and thus in particles muchheavier than the near-spherical brittle platinoid particles(fig. 3). The average median diameter of the latter (58jum), as reported by Cousins (1973), is just that computedfor hypothetical spherical particles with d~17.5 inhydraulic equivalence with the average Rand reef clasts(fig. 3). The hydraulic equivalence of platinoid grains
and reef clasts, coupled with the inverse relation bet­
ween platinoids and gold, strongly suggests that gold
was not of placer origin but was transported in solution.
The thesis developed here has important implicationsin assessing the uranium reserves of the pyritic con­glomerates, particularly those of the Witwatersrand,
the mined representatives of which are very largely goldrich. If the gold-rich reefs are the products of longcontinued maturation on unconformities, then theyshould both grade into and be apposed by less matureuranium-rich reefs. On the ground that unconformitiesof lesser duration should be more numerous than thoseof greater duration, less-mature uranium reefs should be
the more abundant. Indeed, as much is stated by
Cousins (1956, p. 106-108; quoted in this report in thesection, Inverse Relation of Uranium and Gold), who
noted that high ratios of U308/Au "indicate a morewidespread distribution of uranium than gold." Never­theless, the tonnage of uranium-rich reefs so far mined
constitutes only 1.25 percent of that of gold-rich reefs.
This paradox is held to be due simply to the historicalpattern of an industry devoted first and foremost to themining of gold.
Recognition of the presence of a significant amount of
uranium in the reefs did not come until World War II(Bourret, this volume, chap. A), and wide knowledge ofthe abundance was delayed until about a decade later.
The West Rand Goldfield leads in production fromuranium-rich Witwatersrand reefs. Yet the recently
published reserves of a new section of a single mine (seetable 3) contain more uranium than the total minedthrough 1975 from the West Rand. It is significant thatthe uranium-rich reefs of the Cooke Section of the Randfontein Mine exploit a stratigraphic zone not yet minedelsewhere. It seems likely that the Witwatersrand will
emerge as an even more massive reserve of uranium
than has been heretofore recognized.

The traditional view that the gold of the pyritic con­glomerates is placer has been generally refuted in thesection, Gold. It remains to comment on the detailed
mode of transportation of the allegedly placer gold.
Minter (this volume, chap. E) has advanced the latest
version of the fluviatile model of sedimentation of thereefs. He has emphasized sedimentation on the bran­ching distributaries of a large deltaic fan. Minter's
model is silent as to the significance of the unconfor­
mities upon which the productive reefs lie. Deltas andfans, taken as a whole, are areas of uninterrupted
deposition. Yet by contrast the Rand reefs, of delta-wide
dimensions, are developed on through-going unconfor­
mities marked by the reworking of material eroded fromthe underlying beds.
Minter (this volume, chap. K) would explain the in­verse relation between uranium and gold in the Orange
Free State Goldfield by the preferential transport of lesscompetent fine-grained detrital uraninite to the distal
end of the fan. Gold would be preferentially depositednear the head of the fan. Such a mechanism is appealing
but does not seem to fit field descriptions. Winter(1964a, p. 435-436, 446; 1964b, p. 530-532, 541,
545-546), describing this goldfield, built up a picture ofoligomictic, cleanly washed reefs forming on unconfor­
mities from underlying polymictic subgraywacke. He
wrote (p. 436): "Generally, the U308:Au ratio is con­siderably higher in the somewhat polymictic con­glomerates developed in the subgraywacke, even though
the absolute gold contents do not nearly approach thoseof the other type" (oligomictic cleanly washed con­glomerate). This initial formation of relatively uraniniterich, gold-poor conglomerate is in conflict with the
APPENDIX: DETERMINATION OF ORIGINAL
mechanism of selectively bypassing uraninite down cur­
URANIUMrent, whereas the variable maturity model advanced
here is in complete harmony with Winter's observations.
The original amount of natural uranium and its
In the section on the Orange Free State Goldfield, an isotopic distribution at 2.5xl09 yr ago may be deter­
inverse relation between gold and detrital platinoid mined from that remaining at present by use of tables ofgrains is described. If the Rand gold were detrital, a Steiff and others (1959). The tables, which were

AA22

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

computed for the calculation of lead-isotope ages, list Burger, A. J., Nicolaysen, L. 0., and de Villiers, J. W. L., 1962, Lead
isotopic compositions of galenas from the Witwatersrand andthe atomic ratios of radiogenic lead isotopes to theOrange Free State, and their relation to the Witwatersrand andparent uranium isotopes. The original amount of
Dominion Reef uraninites: Geochimica et Cosmochimica Acta. v.
uranium isotope is simply that remaining at present plus
26, p. 25-59.
Charlsby, A., 1954, The cross-linking and degradation of paraffinthat decayed to the equivalent lead isotope.
Atomic ratios of indicated isotopes at 2.5X109 yr[Stieff and others (1959, p. 34]
N206/N2 38

N207/N236

0.468478

10.36340

If the present amount of the parent uranium isotope istaken as unity, the original amounts are
238U

5U

1.468478

11.36340

The present 235U content of natural uranium = 0.711
percent.
11.363x0.711 percent = 8.079 percent,
238U = 146.8478 percent present natural uranium,
235U = 8.0794 percent present natural uranium,
Natural uranium at 2.5 x 109 yr = 154.9272 percent pre­sent natural uranium.
The isotopic composition of the natural uranium would
have been:
238U: 146.8478/154.9272 = 94.79 percent,
235U: 8.0794/154.9272 = 5.21 percent.
REFERENCES CITED
Alien, A. D., 1961, The radiation chemistry of water and aqueous solu­
tions: Princeton, D. Van Nostrand Co., 204 p.
Antrobus, E. S. A., and Whiteside, H. C. M., 1964, The geology of cer­tain mines in the East Rand, in Haughton, S. H., ed., The geology
of some ore deposits in southern Africa: Johannesburg, GeologicalSociety of South Africa, v. 1, p. 125-160.
Barbier, J., 1972, L'abondance des boxworks d'uraninite dans lesgranites: un guide strategique possible pour les gisements
d'uranium: [France] Bureau de Recherches Geologiques etMinieres Bulletin, ser. 2, sec. 11, no. 1, p. 1-9.
Bateman, J. D., 1958, Uranium-bearing auriferous reefs at Jacobina,
Brazil: Economic Geology, v. 53, no. 4, p. 417-425.
Beukes, N. J., 1973, Precambrian iron-formations of southern Africa:
Economic Geology, v. 68, no. 7, p. 960-1004.
Bezgubov, A. I., Dement'yev, P. K., Kruglova, V. G., and Modnikov, I.
S., 1970, Tipy drevnikh radioaktivnykh konglomeratov [Types ofradioactive paleoconglomerates], in Geokhimiya i mineralogiya
radioaktivnykh elementov Sibiri: Akademiya Nauk SSR Sibirskoye
Otdeleniye, Institut Geologii i Geofiziki, Novosibirsk, p. 156-164.
(Abstract in Chemical Abstracts, 1971, v. 74, no. 10, p. 44224, en­try 44228n).
Birch, F., 1954, Heat from radioactivity, in Faul, H., ed., Nuclear
geology: New York, John Wiley and Sons, p. 148-174.
Boggs, Sam, Jr., and Baldwin, E. W., 1970, Distribution of placer goldin the Sixes River, southwestern Oregon—a preliminary report:
U.S. Geological Survey Bulletin 1312-1, 27 p.

chains by high-energy radiation: Royal Society [of London] Pro­
ceedings, Series A., v. 222, p. 60-74.
Chukhrov, F. V., Vinogradov, V. I., and Ermilova, L. P., 1970, On theisotopic sulfur composition of some Precambrian strata:
Mineralium Deposita 5, 209-222
Cloud, Preston, 1973, Paleoecological significances of the banded ironformation: Economic Geology, v. 68, no. 7, p. 1135-1143.
Coetzee, F., 1965, Distribution and grain-size of gold, uraninite, pyrite,and certain other heavy minerals in gold-bearing reefs of the Wit­
watersrand Basin: Geological Society of South Africa Transac­
tions, v. 68, p. 61-88.
Colombo, Umberto, Denti, Ennio, and Sironi, Giuseppe, 1964, Ageochemical investigation upon the effects of ionizing radiation onhydrocarbons: Institute of Petroleum Journal, v. 50, p. 228-237.
Cordani, U. G., Amaral, G., and Kawashita, K., 1973, The Precambrianevolution of South America: Geologisches Rundschau, v. 62, p.
309-317.
Cousins, C. A., 1956, The value distribution of economic minerals withspecial reference to the Witwatersrand gold reefs: GeologicalSociety of South Africa Transactions, v. 59, p. 95-113.
___1973, Platinoids in the Witwatersrand System: South African
Institute of Mining and Metallurgy Journal, v. 73, no. 6, p.
184-199.
Cox, D. P., 1967, Regional environment of the Jacobina auriferous con­glomerate, Brazil: Economic Geology, v. 62, no. 6, p. 773-780.
Cronan, D. S., and Tooms, J. S., 1969, The geochemistry of manganese
nodules and associated pelagic deposits from the Pacific and Indian
Oceans: Deep-Sea Research, v. 16, p. 335-359.
de Jager, F. S. J., 1964, The Witwatersrand System in the SpringsNigel-Heidelberg sector of the East Rand Basin, in Haughton, S.
H., ed., The geology of some ore deposits in southern Africa:
Johnannesburg, Geological Society of South Africa, v. 1, p.
161-190.
de Kock, W. P., 1964, The geology and economic significance of theWest Wits Line, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa, Johannesburg, Geological Society ofSouth Africa, v. 1, p. 323-386.
Dragonic, I. G., and Dragonic, Z. D., 1971, The radiation chemistry ofwater: New York, Academic Press, Inc., 242 p.
Ehrlich, H. L., 1971, Bacteriology of manganese nodules: V, Effect ofhydrostatic pressure on bacterial oxidation of Mn11 and reductionof Mn02: Applied Microbiology, v. 21, p. 306-310.
Eichmann, R., and Schidlowski, M., 1974, Isotopic composition of car­bonaceous matter from the Precambrian uranium deposits of theBlind River district, Canada: Naturwissenschaften, v. 61, no. 10, p.
449.
Ellsworth, H. V., 1928, I. Thucholite, a remarkable primary carbon
mineral from the vicinity of Parry Sound, Ontario: American
Mineralogist, v. 13, p. 419-439.
Emery, K. 0., and Noakes, L. C., 1968, Economic placer deposits ofthe' continental shelf: United Nations Economic Commission for
Asia and the Far East, Committee for Coordination of Joint Pro­
specting for Mineral Resources in Asian Offshore Areas, Technical
Bulletin, v. 1, p. 95-111.
Finch, W. L, Butler, A. P., Armstrong, F. C., and Weissenborn, A. E.,
1973, Uranium, in Brobst, D. A., and Pratt, W. P., eds., UnitedStates mineral resources: U.S. Geological Survey Professional
Paper 820, p. 456-468.

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES
French, B. M., 1973, Mineral assemblages in diagenetic and low-grade
metamorphic iron-formation: Economic Geology, v. 68, no. 7, p.
1063-1074.
Fuller, A. 0., 1958, A contribution to the petrology of the Witwatersrand System: Geological Society of South Africa Transactions, v.
61, p. 19-50.
Garrels, R. M., and Christ, C. L., 1965, Solutions, minerals, andequilibria: New York, Harper and Row, 450 p.
Garrels, R. M., Perry, E. A., Jr., and Mackenzie, F. T., 1973, Genesisof Precambrian iron-formations and the development of at­mospheric oxygen: Economic Geology, v. 68, no. 7, p. 1173-1179.
Gershoyg, Y. G., and Kaplum, Y. Y., 1970, Old metamorphosed
weathering crust of igneous rocks of the Krivoi Rog Basin:
Academy of Sciences of the U.S.S.R. Doklady, Earth Science Sec­
tions, v. 195, November-December, p. 65-67.
Gibbs, R. J., Matthews, M. D., and Link, D. A., 1971, The relationshipbetween sphere size and settling velocity: Journal of Sedimentary
Petrology, v. 41, no. 1, p. 7-18.
Griffith, J. W., 1967, The uranium industry-its history, technology,
and prospects: Canada Department of Energy, Mines andResources, Mineral Resources Division Mineral Report 12, 335 p.
Grip, E., and Odman, 0. H., 1944, On thucholite and natural gas fromBoliden: Sveriges Geologiska Undersokning Arsbok, v. 38, ser. C,
no. 464, 14 p.
Hallbauer, D. K., 1975, The plant origin of the Witwatersrand"carbon": Minerals Science and Engineering, v. 7, no. 2, p.
111-131.
Hallbauer, D. K., and Joughin, H. C., 1973, The size distribution andmorphology of gold particles in Witwatersrand reefs and theircrushed products: South African Institute of Mining andMetallurgy Journal, v. 73, p. 395-405.
Harrison, A. G., and Thode, H. G., 1958, Mechanism of the bacterial
reduction of sulfate from isotope fractionation studies: Faraday
Society Transactions, v. 54, p. 84-92.
Hart, R. A., 1973, A model for chemical exchange in the basalt-sea
water system of oceanic layer II: Canadian Journal of Earth
Sciences, v. 10, no. 10, p. 799-816.
Haughton, S. H., 1969, Geological history of southern Africa:
Capetown, Geological Society of South Africa, 535 p.
Helgeson, H. C., and Mackenzie, F. T., 1970, Silicate-sea water
equilibria in the ocean system: Deep-Sea Research, v. 17, p.
877-892.
Hiemstra, S. A., 1968a, The mineralogy and petrology of theuraniferous conglomerate of the Dominion Reefs Mine, Klerksdorp
area: Geological Society of South Africa Transactions, v. 71, p.
1-65.
———1968b, The geochemistry of the uraniferous conglomerate of the
Dominion Reefs Mine, Klerksdorp area: Geological Society ofSouth Africa Transactions, v. 71, p. 67-100.
Hiemstra, S. A., and Rahden, H. V. R. von, 1969, The association ofgold and uranium with other minerals in different Witwatersrand
reefs: National Institute for Metallurgy, Johannesburg, unpublish­
ed report no. 837.
Hoefs, J., Nielsen, H., and Schidlowski, M., 1968, Sulfur-isotope abun­dances in pyrite from the Witwatersrand conglomerates:
Economic Geology, v. 63, no. 8, p. 975-977.
Horwood, C. B., 1911, The mode of occurrence and genesis of the car­bon in the Rand bankets: Geological Society of South Africa Tran­sactions, v. 13, p. 69-92.
Hostetler, P. B., and Garrels, R. M., 1962, Transportation andprecipitation of uranium and vanadium at low temperatures, withspecial reference to sandstone-type uranium deposits: Economic
Geology, v. 57, no. 2, p. 137-167.
James, H. L., 1966, Chemistry of the iron-rich sedimentary rocks: U.S.
Geological Survey Professional Paper 440-W, 60 p.

AA23

Janin, C., 1918, Gold dredging in the United States: U.S. Bureau of
Mines Bulletin 127, 226 p.
Joralemon, I. B., 1952, Age cannot wither, or varieties of geologicalexperience: Economic Geology, v. 47, no. 3, p. 243-259.
Kondrat'eva, E. N., 1965, Photosynthetic bacteria (Fotosinteziruyushchie bakterii), translated from Russian by Jean Salkind:
U.S. Department of Commerce National Technical Information
Service AEC-tr-6203/LK, 243 p.
Koppel, V. H., and Saager, R., 1974, Lead isotope evidence on the
detrital origin of Witwatersrand pyrites and its bearing on the pro­venance of the Witwatersrand gold: Economic Geology, v. 69, no.
3, p. 318-331.
Krauskopf, K. B., 1951, The solubility of gold: Economic Geology, v.
46, no. 8, p. 858-870.
Leo, G. W., Cox, D. P., and Carvalho, J. P. P. de, 1964, Geologia departe sul da Serra de Jacobina, Bahia, Brasil: [Brazil] Ministeriodas Minas e Energia, Departamento Nacional da Producao
Mineral, Boletin 209, 87 p.
Liebenberg, W. R., 1955, The occurrence and origin of gold andradioactive minerals in the Witwatersrand System, the Dominion
Reef, the Ventersdorp Contact Reef, and the Black Reef:
Geological Society of South Africa Transactions, v. 58, p. 101-227.
————1960, On the origin of uranium, gold, and osmiridium in the con­glomerates of the Witwatersrand goldfields: Neues Jahrbuch fur
Mineralogie Abhandlungen, v. 94, p. 831-867.
Lindgren, W., 1911, The Tertiary gravels of the Sierra Nevada ofCalifornia: U.S. Geological Survey Professional Paper 73, 226 p.
London Mining Journal, 1972: Supplement, January 28.
McKinney, J. S., and others, 1964, Geology of the Anglo-American
Group mines in the Welkom area, Orange Free State Goldfield, inHaughton, S. H., ed., The geology of some ore deposits in southern
Africa: Johannesburg, Geological Society of South Africa, v. 1, p.
451-506.
McLeod, C. R., and Chamberlain, J. A., 1969, Reflectivity and Vickers
microhardness of ore minerals—chart and tables: Canada
Geological Survey Paper 68-64.
Margulis, L., 1969, New phylogenies of the lower organisms: Possible
relation to organic deposits in Precambrian sediment: Journal of
Geology, v. 77, no. 5, p. 606-617.
Matthews, P. E., and Scharrer, R. H., 1968, A graded unconformity atthe base of the early Precambrian Pongola System: GeologicalSociety of South Africa Transactions, v. 71, p. 257-271.
Mertie, J. B., Jr., 1939, Platinum deposits of the Goodnews Bay
district, Alaska: U.S. Geological Survey Bulletin 910-B, p.
115-145.
Mining Magazine, 1976, Randfontein's Cooke Section—a new gold
mine for a famous old producer: Mining Magazine, v. 134, no. 4, p.
261-267.
Mining Survey, 1977, Analysis of working results: Mining Survey, v.
84, no. 1, p. 28.
Myers, W. B., 1970, An hypothesis of the chemical environment of theRand Goldfield, South Africa: U.S. Geological Survey Open-File
Report 1389, 5 p.
———1971, Precambrian pyritic gold- and uranium-bearing con­glomerates: Geological Society of America, Abstracts with Pro­
grams, v. 3, p. 656-657.
Nelson, C. H., and Hopkins, D. M., 1972, Sedimentary processes anddistribution of particulate gold in the Northern Bering Sea: U.S.
Geological Survey Professional Paper 689, 27 p.
Niekerk, C. B. van, and Burger, A. J., [1966], The age of the
Ventersdorp System: South Africa Geological Survey Annals, v. 3
(1964), p. 75-86.
———1969, Lead isotopic data relating to the age of the Dominion
Reef lava: Geological Society of South Africa Transactions, v. 72,
p. 37-45.

AA24

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Papenfus, J. A., 1964, The geology of the Western Areas Gold Mineand vicinity, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa: Johnannesburg, Geological Society ofSouth Africa, v. 1, p. 387-391.
Perry, E. C., Jr., Monster, J., and Reimer, T., 1971, Sulfur isotopes in
Swaziland System barites and the evolution of the Earth's at­mosphere: Science, v. 171, p. 1015-1016.
Pierce, A. P., Gott, G. B., and Mytton, J. W., 1964, Uranium andhelium in the Panhandle gas field, Texas, and adjacent areas: U.S.
Geological Survey Professional Paper 454-G, 57 p.
Pierce, A. P., Mytton, J. W., and Barnett, P. R., 1958, Geochemistry of
uranium in organic substances in petroliferous rocks, in United
Nations, Survey of raw material resources: International Con­
ference on the Peaceful Uses of Atomic Energy, 2d., Geneva, Sept.
1958, Proceedings, v. 2, p. 192-198.
Pretorius, D. A., 1964, The geology of the Central Rand Goldfield, inHaughton, S. H., ed., The geology of some ore deposits in southern
Africa: Johnannesburg, Geological Society of South Africa, v. 1, p.
63-108.
———1974, The nature of the Witwatersrand gold-uranium deposits:
Witwatersrand, University Economic Geology Research Unit In­
formation Circular 86, 50 p.
Raal, R. A., 1969, A study of some gold mine diamonds: American
Mineralogist, v. 54, no. 1, p. 292-296.
Rahden, H. V. R. von, and Urli, G. L. P., 1969, The association of gold
and uranium with other minerals in different Witwatersrand reefs:
Johannesburg, National Institute for Metallurgy Research Report
837, 25 p.
Ramdohr, Paul, 1958, New observations on the ores of the Witwaters­
rand in South Africa and their genetic significance: GeologicalSociety of South Africa Transactions, Annexure to v. 61, 50 p.
Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in
the Canadian Shield: Canada Geological Survey Paper 68-40, 205
P———1973, The Huronian Supergroup, a Paleoaphebian succesion
showing evidence of atmospheric evolution, in Young, G. M., ed.,Huronian stratigraphy and sedimentation: Geological Associationof Canada Special Paper 12, p. 31-47.
Saager. R., 1970, Structures in pyrite From the Basal Reef in the
Orange Free State Goldfield: Geological Society of South Africa
Transactions, v. 73, p. 29-46.
Saager, R., and Esselaar, P. A., 1969, Factor analysis of geochemical
data from the Basal Reef, Orange Free State Goldfield, South
Africa: Economic Geology, v. 64, no. 4, p. 445-451.
Schidlowski, M., 1966, Zellular strukturierte Elemente aus dem
Prakambrium des Witwatersrand-Systems (Siidafrika): Deutsche
Geologische Gesellschaft Zeitsschrift, v. 115, (1963), no. 2-3, p.
783-786.
———1967, Note on graphite in the Witwatersrand conglomerates:
Geological Society of South Africa Transactions, v. 70, p. 65-66.
-1969, Critical remarks on a postulated genetic relationship be­
tween Precambrian thucholite and boghead coal, in Advances inorganic geochemistry, 1968: Oxford, Pergamon Press, p. 579-592.
Schweigart, Hartmut, and Rahden, Herbert von, 1965, Oolithische
Strukturen in Pyriten des Ventersdorp-Contact-Reefs, Siidafrika:
Geologisches Rundschau, v. 54 (1964), no. 2, p. 1143-1148.
Seward, T. M., 1973, Thio complexes of gold and the transport of goldin hydrothermal ore solutions: Geochimica et Cosmochimica Acta,v. 73, p. 379-399.
Shilo, N. A., and Shumilov, Y. V., 1970, New experimental data on set­tling of gold particles in water: Academy of Sciences of •theU.S.S.R. Doklady, Earth Science Sections, v. 195, p. 184-187.
Stieff, L. R., Stern, T. W., Oshiro, S., and Senftle, F. E., 1959, Tables
for the calculation of lead-isotope ages: U.S. Geological Survey
Professional Paper 334-A, 40 p.

Takahashi, M., and Ichimura, S., 1970, Photosynthetic properties andgrowth of photosynthetic sulfur bacteria in lakes: Limnology andOceanography, v. 15, no. 6, p. 929-944.
Thomson, J. E., 1960, Uranium and thorium deposits at the base of the
Huronian System in the District of Sudbury: Ontario Departmentof Mines Geological Report 1, 40 p.
Tinsley, C. R., 1975, Economics of deep ocean resources-A questionof manganese or no-manganese: Mining Engineering, v. 27, no. 4,
p. 31-34.
___1975a, Deepsea Ventures' ocean claim-an analysis: Mining
Engineering, v. 27, no. 4, p. 51.
-1975b, Processing - no longer a problem: Mining Engineering,v. 27, no. 4, p. 53-55.
Toens, P. D., and Griffiths, G. H., 1964, The geology of the West Rand,
in Haughton, S. H., ed., The geology of some ore deposits insouthern Africa: Johnannesburg, Geological Society of South
Africa, v. 1, p. 283-321.
Triiper, H. G., and Genovese, S., 1968, Characterization of photosyn­
thetic sulfur bacteria causing red water in Lake Faro (Messina,
Sicily): Limnology and Oceanography, v. 13, no. 2, p. 225-232.
Tuck, R., 1968, Origin of the bedrock values of placer deposits:
Economic Geology, v. 63, no. 2, p. 191-193.
Viljoen, R. P., 1968, The quantitative mineralogical properties of theMain Reef and Main Reef Leader of the Witwatersrand System:
Witwatersrand University Economic Geology Research Unit In­
formation Circular 41, 63 p.
Whitehead, W. L., 1954, Hydrocarbons formed by the effects ofradioactivity and their role in the origin of petroleum, in Faul, H.,
ed., Nuclear geology: New York, John Wiley and Sons, p. 195-218.
Winter, H. de la R., 1964a, The geology of the northern section of theOrange Free State Goldfield, in Haughton, S. H., ed, The geology
of some ore deposits in southern Africa: Johannesburg, GeologicalSociety of South Africa, v. 1, p. 417-448.
————1964b, The geology of the Virginia section of the Orange Free
State Goldfields, in Haughton, S. H., ed., The geology of some ore
deposits in southern Africa: Johnannesburg, Geological Society ofSouth Africa, v. 1, p. 507-548.
Wonnacott, R. J., and Wonnacott, T. H., 1970, Econometrics: New
York, John Wiley and Sons, 445 p.
Yeend, W. E., 1974, Gold-bearing gravel of the ancestral Yuba River,
Sierra Nevada, California: U.S. Geological Survey Professional
Paper 722, 44 p.
Young, B. B., and Millman, A. P., 1964, Microhardness and deforma­tion characteristics of ore minerals: Institution of Mining andMetallurgy Transactions, v. 73, p. 437-466.

DISCUSSION FOLLOWING TALKS BY
DRS. STAATZ, CANNON, AND MYERS

Skinner: Brad [Myers], 111 confine myself to threepoints, three points that I don't think are stable points
on which to build your hypothesis. The first is settling
velocity. Dealing with the Witwatersrand, I think you
are dealing entirely with bedload and settling
velocities have little to do with it. So I would maintainthat the Russian information on settling velocities isincorrectly applied by you to the grain size of the Wit­
watersrand gold. The second point concerns Eh-pH
diagrams. How you draw those fields depends on themolalities of the various species in solution. You didn't
really give any justification for those you chose, and I

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

think you could draw those diagrams somewhat dif­ferently.
Myers: If you would like, I can give you the details. I
have a manuscript if you care to look at it.
Skinner: Even if we leave them the way you've drawn
them, I would maintain that the diagram should becorrectly interpreted as the reason why waters havenot flushed out the uranium and gold, not why theybrought gold in. That is, you have shown a field where
you have uraninite, gold, and pyrite stable, and thepyrite controls the system in part and doesn't let thegold and uranium be flushed out. The third point is one
I don't know very much about and I would hopesomebody in the audience might know a great deal
more about it, but I would very much doubt that theradiation flux in these waters is anywhere near highenough for the radiolysis reaction to go on. I would im­
agine that it is many orders of magnitude too low.
Myers: On the intensity of the radiolysis, it certainly
would go on. The point here is whether it would go on
fast enough. I did some preliminary calculations andthought that it would go on fast enough. I have had
criticism on this point before, and at this point I do notwant to tie myself down on it.
Grandstaff: I'd suggest that the presence of uraninite inother old Precambrian rocks, pegmatites for example,
would suggest that this process is not important indestroying the uraninite.
Myers: In the original description of thucholite, theauthor stressed that the thucholite replaced coarse
uraninite in the pegmatite. For the reaction that I
described to take place, near-surface low-pressure
conditions are needed. Uraninite in other Precam­
brian rocks may not have been subjected to the properconditions for a sufficient length of time.
B. Nagy: Just a very brief comment. In your paper you
have shown a normal alkane transformed by irradia­
tion into what you call thucholite. In other words, thereaction says that thucholite is a polymerizedhydrocarbon, obviously, basically aliphatic. Whoanalyzed the organic composition of the thucholite?
Myers: These are old analyses; they are in Liebenberg.
B. Nagy: How have the data been analyzed?
Myers: They have been analyzed by a relatively
primitive method, but they are oxygen containing.
Other thucholites have been analyzed by Pierce; theseare oxygen containing.
B. Nagy: Then the hydrocarbon transformed into
thucholite is not the parent material. In other words,
you are making the thucholite or whatever (I hate touse thucholite because this is not the classical defini­tion) from other materials also.
Myers: Wait, you say that the Rand carbonaceous
material is not thucholite?

AA25

B. Nagy: Well, I don't want to get embroiled; I talked
about this yesterday. This is just terminology. But
what I am saying is this, that I would imagine that if
people would accept your hypothesis that the starting
material, which I call "solid organic substance" andwhat you call "thucholite," is something other thannormal alkane hydrocarbons.
Myers: Of course it is. It's oxygenated, as my reaction
shows.
B. Nagy: But you didn't show the steps.
Myers: I can't show the steps. However, Charlesby has
irradiated polyethylene in a nuclear reactor, and hegot very long chain hydrocarbons plus hydrogen plus
methane, but this is done anhydrously. When you do itwith water then you have radiolysis of water to con­tend with, and this is an oxygenating reaction. There
have been experiments on crude oil irradiated withwater, and you get an increase in the percentage ofasphaltenes, and the reason this is oxidizing is because
you form reducing gases in a bubble in a vial, andthese do not equilibrate. These are very high radiation
rates, of course.
B. Nagy: You brought in asphaltenes. I talked yesterday
about asphaltenes, and all these things are aromaticheteroatomic compounds. They are not aliphatic com­pounds.
Myers: Which ones are you talking about?
B. Nagy: I'm talking about what you call thucholite. I'm
talking about asphaltenes.
Myers: What you've found was as the result of
pyrolysis?
B. Nagy: Yes.
Armstrong: I believe that you gentlemen should get
together. Dr. Hallbauer? Dr. Hallbauer, you're not go­
ing to talk about thucholite, are you?
Hallbauer: I would like to make a comment on one
diagram you used to support your hypothesis where
you plotted the gold in ppm against the uranium oxide.
In that diagram you used the milled ore, assumingthat the milled ore is the same as reef.
Myers: I'm aware of this and I explained it.
Hallbauer: Your scale at the bottom went to about 50
ppm, but if you use reef you get values of 1,000, 2,000,
sometimes 10,000 ppm, so these values would be way
out, they would be somewhere up here. So your wholegraph would be invalid. Say, for instance, if you had
used the average stoping width of, say, 1 meter. Somemines might have all this as reef; other mines might
have only 10 centimeters of reef, and in mining it isdiluted with base rock. If you draw this as milled ore,
you assume that it is all the same material. That is, it
is all reef. But it's not all reef, and these thin con­glomerates contain up to a few thousand ppm of gold.
I would say, therefore, that your gold values would besomewhere up here, destroying the whole diagram.

AA26

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESMyers: The objection was made that I have used averagevalues instead of individual values, and I can't see that
that's a very serious objection. I used millions of tonsof milled ore rather than individual reef samples, andit seems to me that this is the proper way of doing it.
There are better figures, but I don't have them.
Hallbauer: Well, the only difference is if you would haveused individual values for conglomerates then you
couldn't have drawn this diagram. The diagram is in­valid. You can't use it to support your hypothesis.
Myers: Why is it invalid?
Hallbauer: (Reply not recorded. Tape blank.)
Myers: Larry Minter supplied us with some very in­teresting curves, and he used 930,000 assays to makethose curves, and he used assays of reef. There cer­tainly has to be a pretty good correlation between thereef and the milled ore; it is not perfect, but there has
to be a pretty good correlation.
Armstrong: I don't think this is going to be settled.
Larry [Minter], do you want to say something?
Minter: I think you have some terrible data. For in­
stance, if you take the milled ore from Randfontein or
the West Rand Consolidated, they happen to be min­ing twenty different reefs at the same time, and thewhole lot are going through the mill, and every one ofthose reefs represents a different type of environ­ment. That's just one point. The other point is that on
FSG, for instance, the gold-uranium ratio happens tovary from seven to over forty. Now, which one do youtake?
Myers: What I took were reserve values given to me by
Anglo-American for the average grade of U308 andgold for the eastern, central, and western sections ofFree State Geduld.
Minter: I don't think that is really representative.

Myers: It's representative. It's the reserve data.
Minter: Yes, but it's mixed data.
Myers: It's all from Basal Reef.
Minter: No, it actually isn't.
Myers: It must be very close to all. As far as I know,
Randfontein uranium production is very largely fromWhite Reef.
Minter: There happen to be two reefs there.
Myers: And remember we had three rather straightplots; those were year-by-year plots with the average.
West Rand Consolidated is not on there. It's anotherspot, and it's another spot because they do mine many
[reefs], and if you plot yearly figures, you don't get astraight line. You get a Brownian movement becauseof what you just said.
Minter: The White Reef doesn't carry very high uranium
content; it's the Monarch Reef that carries the highuranium content.
Myers: Would you care to see the data sent me by JCI?
Minter: Sure. One other point is that we still haven't
really said why this unique process happened only
once and where all the gold came from and why we
happen to have got it all in the Witwatersrand? Has
this happened anywhere else?
Myers: It's not happened to the same extent. However,there are other gold-uranium conglomerates—the
Denny Dalton is one, in the Pongola Series southeastof the Witwatersrand. There are pyritic con­glomerates in the Ukraine; we don't know how muchuranium they contain, but they contain detritaluraninite, and they contain up to a gram or so of gold.
In terms of crustal abundance, there's a fair amount ofgold in Elliot Lake, and if we go down east of Sudbury
to Lake Timagami you get a couple of parts per millionof gold.

SUMMATION

Thoughts about Uranium-Bearing
Quartz-Pebble Conglomerates:
A Summary of Ideas Presented at the Workshop
By BRIAN J. SKINNER
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIANQUARTZ-PEBBLE CONGLOMERATESGEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-BB

CONTENTS
Page

Introduction ______________________________________________
Uniqueness of the deposits _____________________________________
Time of formation __________________________________________
Sites of deposition __________________________________________
Source areas _____________________________________________
Diagenesis ______________________________________________
Metamorphism and recrystallization ________________________________
Discovering new deposits _____________________________________
Discussion following Dr. Skinner's summation __________________________

BB1
1
1
2
3
4
4
4
5

ILLUSTRATIONPage

FIGURE 1. Drawing showing possible placement in an idealized fluviatile system of five uraniferous detrital sediments___________inBBSGENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATESTHOUGHTS ABOUT URANIUM-BEARINGQUARTZ-PEBBLE CONGLOMERATES:A SUMMARY OF IDEAS PRESENTED AT THE WORKSHOP
By BRIAN J. SKINNER l

INTRODUCTION

The reasons for my selection as the summarizer andfinal speaker at this intriguing conference are just as
obscure to me as they are to you. I suspect the main
reason is that I know little or nothing about quartzpebble conglomerates and their uranium contents, so I
can be expected to approach my task in an unbiased
way. Whatever the reason, let me remark that I foundthe talks to be extraordinarily provocative and my atten­
dance at the conference a great privilege.
I cannot hope to summarize in a few minutes all of the
fascinating discussions of the past three days. There­
fore, I propose to comment on six or seven of the mostinteresting, and to my mind most pertinent, topics
discussed. I will not include the debate of detrital versus
nondetrital origin for the uranium, gold, and other con­stituents among my topics because it seems clear to me
that even if some of the ore constituents were carried insolution, most of them were moved as detrital grains.
Some of the points I make will probably be self-evident
to those of you who have long acquaintances with uraniferous quartz-pebble conglomerates, but I believe the im­portance of the points makes them worth repeating.
UNIQUENESS OF THE DEPOSITSThe first point concerns the uniqueness of quartzpebble conglomerates. It struck me, as I listened to thepresentations, that I had heard similar words 30 years
ago, right after the Second World War, when one of theperiodic debates about Precambrian iron-formations
erupted. Similar debates about iron-formations had oc­
curred at earlier times, but the discussions were neces­sarily restricted because the formations seemed to be

1 Department of Geology and Geophysics, Yale University, New Haven, CT (few in number and to have formed because of special,
local geological circumstances. But easier travel andfaster communication made it apparent by the second
half of the 20th century that iron-formations of the Lake
Superior type are really quite widespread. Surprisingly,
large Precambrian iron-formations are still being dis­covered today-for example, the recently discovered
Nabberu Basin deposits in Western Australia. Further­more, it now is apparent that these iron-formations
were normal sediments in their day. Local and special
circumstances are not needed.
Our understanding of uranium-bearing quartz-pebble
conglomerates today is like our understanding of ironformations 30 years ago. It is biased by the prepon­
derance of information from the Witwatersrand andBlind River deposits. But at this conference you havediscussed a number of other quartz-pebble conglom­
erates, and you have convinced me that Precambrian
conglomerates suitable to be uranium hosts are probably
much more common than now accepted. They are nothighly unique and probably not due to a special set of
local circumstances. This suggests there are many more
Precambrian quartz-pebble conglomerates yet to befound. Whether they will prove to be mineralized, and if
mineralized, worth mining, remains to be seen. But as
far as uniqueness or nonuniqueness is concerned, I am
led to a conclusion of nonuniqueness. If we could returnfor another conference 30 years hence, I suggest wewould be discussing three or more times the number ofdeposits we are discussing today.
TIME OF FORMATIONMy second point concerns time of formation. It is aquestion that many of the speakers mentioned and manyof you questioned from the floor. Quartz-pebble con­glomerates can, of course, be found in geological sec­
tions of any age. The ones we are interested in have
BBl

BB2

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

special contents of uranium, pyrite, and gold, sug­gesting either that a special style of sedimentation musthave prevailed for a limited time or that some special
conditions of surface waters and atmosphere existed fora period of Earth's history. Regardless of the reasons,
there does indeed seem to be a specific time during
which conditions around the globe were generally con­ducive to formation of uraniferous quartz-pebble con­glomerates. Those few small deposits that fall outside
the specific time range (for example, those in theeastern Urals) seem to be due to special, local conditions
and to have somewhat different and distinct character­istics.
There are no known deposits older than 2.8, or at a
maximum, 3.0 b.y. (billion years). This may or may not
be a real time barrier. One could argue that it is simply aproblem of sampling. I maintain that this is not the case.
I am persuaded by the observations of Stuart Roscoe
that what we see in the record at 2.8-3.0 b.y. is a changeof tectonic patterns, which is distinguished by a newstyle of sedimentation occurring in cratonic basins. If
similar basins and suitable source areas had existed atearlier times, still older quartz-pebble conglomerates
could have formed. But they did not form because thebasins and adjacent sediment areas did not have theright geometries.
What of the other end of the time bracket? The
youngest deposits that seem to fit the features ofuraniferous quartz-pebble conglomerates are those at
the base of the Transvaal System, as discussed by Dr.
Whiteside. They were formed by reworking of older
sediments but must still have involved transport ofdetrital grains of uraninite. The base of the Transvaal isabout 2.0 b.y. old, which suggests that uraniferous
quartz-pebble conglomerates were limited to a 1-b.y.
time span.
The reason for a cutoff at 2 b.y. ago, if it was a distinct
cutoff, was not settled at this conference. I do not feelthat the argument for atmospheric control-a simple at­mosphere composition control-was proven. I am notsure we have even been thinking correctly about the at­mosphere. There was a lot of talk about an oxidizing at­mosphere versus a reducing atmosphere. But an atmos­
phere with a low partial pressure of oxygen might be er­roneously interpreted to be a reducing atmosphere if agiven oxidation reaction did not proceed. The lack ofreaction may simply mean that reaction kinetics are too
slow for oxidation to occur in the time available. DavidGrandstaff demonstrated that atmospheres with partialpressures as high as 10 ~ 4 atm of oxygen are compatible
with extensive transport of detrital uraninite and pyrite.
Temperature, he pointed out, played an even more im­portant role in reaction rates. That point was strongly
reinforced by Mort Staatz' report of modern detritaluraninite [thorianite] in a glacial stream in Alaska. Thus,
climate, and in particular the average temperatures,
may be a vitally important control in the formation of
uraniferous quartz-pebble conglomerates.
This leads me to an obvious question: Where were thecratons relative to the poles when the basins formed andwere filled by sediments? Was the appropriate style ofsedimentation a feature of high latitudes and hence lowtemperatures? Until that question is answered I believe
the question of a cutoff time must remain open. Present
evidence suggests we could very well have had an oxyge­
nated atmosphere with as much as 10- 4 atm of oxygen as
far back as 3 b.y., and still had transport of detritalgrains of pyrite and uraninite for long distances. We
cannot, therefore, conclude that the atmosphere did ex­
ert a control, and this in turn means we cannot conclude
that the 2 b.y. is a real time limit.
SITES OF DEPOSITION

The third point concerns the sedimentary environ­ment in which the uraniferous sediments were
deposited. Were they deposited in streams, in lakes, orin the ocean? The answer seems clear from the evidence
at Blind River, from Brazil, and above all from the Witwatersrand that we are dealing with sediments
deposited by surface-flowing streams and that thestreams flowed into lakes or at least into bodies of waterthat were shallow and possibly intermittently wet anddry. We are obviously dealing with fluviatile systems.
Think back about the geology of the different deposits
that were discussed. I suggest that a genetic linkage canbe discerned and that the different deposits simply
represent different portions of large drainage systems.
When Larry Minter spoke he used a simple sketch todemonstrate where, in a long drainage system, the Witwatersrand sediments formed. He drew a stream
system which rose in highlands, meandered across aflood plain, then cut across fans and formed a delta. In
figure 1, I have indicated the approximate depositional
environment for the Witwatersrand conglomerates. The
Blind River deposits are similar but with differences.
There are no big distal fans and the deposits sit in places
on regolith. I suggest that their depositional environ­ment, relative to the Witwatersrand, is as shown infigure 1. The Brazilian deposits seem to be proximal
facies. What the basement relations are remains to beseen, and of course the entire section of the Brazilian
sediments remains to be exposed. However, I suggestthat the Brazilian sediments fit in as shown in figure 1,
the Moeda overlapping with the Elliot Lake environ­ment and the Jacobina overlapping with the Witwater­
srand fans. For most of the other deposits mentioned
the geology was too sketchy to decide where they fit, but

URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES: A SUMMARY OF IDEAS

BBS

one that probably can be placed is Montgomery Lake, a uranium explorationist. The specific gravity of uraninite
Canadian deposit with a lot of very well sorted material. is about 10.5; that of cassiterite, about 7; and that of ilIt seems to include the distal ends of the fans and to menite, about 4.7. Both cassiterite and ilmenite arereach beyond the ends of the fans.
relatively resistant to chemical attack by modern
seawater and can be concentrated by marine currents. If
uraninite was stable in sea and lake water between 2 and3 b.y. ago, it, too, could have been concentrated in off­
shore sands by marine currents. Perhaps we should not
focus all of our attention on fluviatile systems..
SOURCE AREAS

Blind River(CANADA)

Moeda and
Jacobina(BRAZIL)

Witwatersrand(SOUTH AFRICA)>

Montgomery
Lake(CANADA)FIGURE 1.-Possible placement in an idealized fluviatile system of fiveuraniferous detrital sediments from Canada, South Africa, andBrazil.

We might ask whether there are modern equivalentsof the ancient drainage systems. I would draw ananalogy with the drainage system that dispersed the In­
donesian alluvial cassiterite during the Pleistocene. The
drainage system, now largely drowned because of rising
sea level, was at least as large as the systems that musthave been operating in the Witwatersrand and BlindRiver cases. The dispersal of cassiterite in the alluvial
deposits can be observed down through the stream
system, and a steady decrease in grain size is found.
Now marine reworking of the sediments is said to be oc­
curring in places. I suggest there is a clue here for the

My fourth point is that, although the sediment sourceareas seem to share many things in common, the areas
themselves are not unusual. The rock types common in
the 3-b.y.-old source areas are greenstones, high-grade
metamorphic rocks, and granites. The metamorphic
rocks and granites all contain small amounts ofuraninite. But there is an unresolved question about
grain sizes. The observed sizes of detrital uraninite
grains are larger than the grain sizes of primaryuraninite grains in most gneisses and granites. So, we
have to seek regions where metamorphism has sweatedpegmatites and quartz veins out of host rocks and in theprocess grown relatively coarse grained uraninite. This
leads to the conclusion that source areas must all be
deeply eroded. Because most quartz-pebble conglomer­
ates are near the bottom of the sediment piles, we must
conclude that the tectonic forces that produced the sedi­ment basins on the old cratonic blocks also formed
highlands by elevating formerly deeply eroded terrains.
Considerable discussion was devoted to the impor­tance of chemical versus mechanical weathering of thesource areas. It seemed to me that the major role playedby mechanical weathering was clearly established. The
conclusion leads us back to the question of climate andthe possibility that all of the uraniferous quartz-pebble
conglomerates were influenced by glacial action. Thatis, we should consider the possibility that all of thesource areas were high and that at least the initial rockbreakage was mechanical and was due to frost action.
The possible significance of carbon from the sedimentsource area seemed to me to be a red herring. There isno compelling evidence to suggest that any carbon arriv­
ed from the sediment source areas. At least a third of
the conference was spent discussing carbonaceous mat­ter, and I have to admit that I remained unconvinced of
its importance. The carbonaceous matter is present in
the sediments because the primitive plants from which it
was derived were happy in the shallow-water depositional environment. It is preserved because the environ­ment is one in which organic matter was rapidly buried.
I do not think the case for trapping detrital gold or
uraninite in algal mats is very convincing. I was in-BB4

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

trigued by the discussions that surrounded the impor­tance of carbonaceous matter in diagenetic processes,
however, and this leads to my fifth point.
DIAGENESIS

Once we start dealing with questions of diagenesis weare dealing with a new set of chemical questions,because subsurface waters have different compositions
from surface waters. The surface waters could, as wehave heard, have been oxygenated, and the transportand deposition of unstable uraninite and pyrite could
have been artifacts of slowed-down reaction kinetics.
The subsurface waters must have had many sinks for ox­ygen and as a result were probably reducing relative tothe surface waters.
I was impressed with data from the Witwatersrandsuggesting that 30 percent of the sulfides were
deposited after detrital deposition and that a significant
fraction of the uranium has also been similarly depositedor redeposited. I suggest that the most likely time forthis to happen was during diagenesis when the relatively
porous sediments were bathed in waters that were atleast slightly reducing and when primitive bacteria
were, in all probability, living off the buried car­bonaceous matter. The chemistry of the subsurface
waters deserves further discussion and investigation. I
suspect they were not too different from the subsurface
waters we would find in similar environments today. By
this I mean the water would allow the presence of bothoxidation states of uranium and, depending on the watercomposition, variable amounts of U+ 4 and U +4 would bepresent. This in turn implies that uranium could movearound in subsurface waters, dissolving here, depositing
there. There could be other driving forces besides stateof oxidation of the water, of course. One important driv­
ing force, for example, would be the tendency of small
grains to dissolve and be redeposited on larger grains.
Diagenetic dissolution and redistribution may possibly
be the reason that some detrital minerals seem to be outof hydraulic equilibrium with their enclosing sediments.
METAMORPHISM AND RECRYSTALLIZATIONAll of the uraniferous quartz-pebble conglomerates we
see today have been metamorphosed to some extent,
and, of course, all of the materials in the deposits can bealtered by chemical reactions and are susceptible torecrystallization, which causes changes in grain struc­ture. The mineral on which most of us seem to haveunderestimated the influence of metamorphism ispyrite. From the information presented at the con­ference I believe that pyrite is the mineral to study in­sofar as unravelling metamorphic effects is concerned.
Dr. Saager, in particular, seems to be on the right trackin his work on the Witwatersrand. Similar work is need­ed on other deposits because we do need to understandhow materials have been changed and moved around.
What happened to the carbon during metamorphism
and also during the earlier phase of diagenesis? Con­sidering the location of thucholite relative to uraninite,
it seems clear that some carbon compounds were mov­ing during diagenesis. It is not clear just what it wasthat moved, however, because the possibility of methane
being polymerized to form the compounds we see today
seems remote, despite suggestions to the contrary. But
even if carbon compounds did move during diagenesis,
they have been baked during metamorphism, and wemust understand the baking before we can be sure about
the ingredients.
The scanning electron microscope images presented
by Dr. Hallbauer are spectacular. But I was not convinc­
ed he was observing primary biogenic structures.
Rather, I suspect he is seeing the products of surface
reactions that occurred during diagenesis, or, more like­ly, during metamorphism. The internal carbon struc­tures he discovered are very likely biogenic in origin, but
my bet is that the extraordinary filamentous structures
are artifacts of metamorphism.
DISCOVERING NEW DEPOSITSMy seventh and final point is an answer to that fervent
plea, "Where do I find another uraniferous quartzpebble conglomerate?" If I knew, I would not be stan­ding here, I would be out staking it. But I do think we
can pick areas in which probabilities of finding suchdeposits are greater than in other areas.
I would concentrate on two features in choosing thebroad areas to search. First, you obviously have tochoose cratonic areas, preferably cratons that havelarge sedimentary basins formed between 2 and 3 b.y.
ago. The easy way to approach this is to consider the se­quence of sediments in the basins. A thick clastic section
is followed, usually unconformably, by a distinctive
chemical sediment we commonly call a banded ironformation. Use this as a guide. Look at the sediment sec­
tions beneath the Lake Suprior-type iron-formations. In
Brazil the Caue Itabirite overlies the conglomerates; in
Witwatersrand, the Transvaal System. But what aboutthe Labrador Trough, Venezuela, Mauretania, Liberia,and Hamersley and Nabberu Basins in Australia, Krivoy
Rog and the Kursk regions in Russia, the Indian ironformations and many others? They all deserve study.
The second obvious question to ask concerns
paleogeography. If there is any reality to the involve­
ment of climate, we should be thinking about
paleogeography. If cold climate and mechanicalweathering are important, then we should look at

URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES: A SUMMARY OF IDEAS

BBS

cratonic basins, of any age, formed within 30 degrees ofing about is a highly glaciated one that once had
the pole. I would, of course, concentrate first on basins
glaciation but doesn't now.
existing between 2 and 3 b.y. ago, but I would not limit Roscoe: No. The point I was trying to make was that, inthe search to this specific time. Why? Simply because
this case at least, a cold climate has not protected
the apparent younger age cutoff is so uncertain. Mor­pyrite and other easily oxidizable and leachedtimer Staatz' observations suggest that, given the rightmaterial, particularly.
conditions of source area and weathering, detrital Myers: I'd like to comment. I thought that Irv Breger
uraninite can be of any age. Perhaps we should look at
said that experiments showed that uraninite andareas affected by extensive glaciations of all ages, evenpyrite were stable at room temperature in thethe Permian, provided a reasonable source area andpresence of oxygen. Was that with water? And I wantsuitable sedimentary basin exists.
to go ahead and say that in the Rand both pyrite anduraninite now are not stable at room temperature, andwith uraninite they have to be careful in even washing
DISCUSSION FOLLOWING DR. SKINNER'S
the. specimens off, if they are going to analyze them
SUMMATION
for uranium. Uraninite disappears in a matter of
Armstrong: Incidently, there are quartz-pebble con­months. The other comment I would like to make, toglomerates in Australia in the Hamersley area.
Brian Skinner, is that you are quite right, it just hap­
Skinner: That's the earliest prediction I've ever madepens that pyritic conglomerates with both gold andthat has paid off.
uranium happen to be already known in the Kursk
Breger: I'm delighted that you mentioned temperature,
Magnetic Anomaly Series and in the Krivoy Rog
because we have found in the laboratory that bothSeries.
uraninite and pyrite are completely stable at room Skinner: Anybody else want to fulfill some other predic­
temperature even to atomic oxygen. But if you justtions?
raise the temperature a little bit, they will decompose. PaulMartz, Gulf Mineral Resources Co., Denver, Colo.:
So apparently there is a limitation in temperature
Since you mentioned the Hamersley Basin I just
range and stability range even in the presence of ox­
thought that I'd point out an occurence of quartzygen that hasn't even been mentioned here.
pebble conglomerate there that I think may be of in­
Skinner: Maybe that's why there is so little organic mat­terest to you. It's on the Rocklea Dome, which is nearter left in these things; it was too cold for the
Tom Price and Parrabardu. As I recall, the core of theorganisms to grow anywhere else, except in those in­dome is Archean Warrawoona Volcanics intruded byfrequent little puddles that were left to the open sun.
granites. The core is unconformably overlain by a se­
Roscoe: May I add a note of caution to this concept ofquence of Proterozoic quartz-pebble congolmerates
temperature being so important, since Dr. Little at­and sands that looke very much like the pictures on
tributed comments about cold temperature in thethe screen. Stratigraphically above that is the
Elliot Lake area to me. In the Montgomery Lake area,
Fortescue Group of volcanics. At the top of thewhere there is permafrost to great depth, the oxida­
Fortescue are the Pyradie pyroclastics and above that,tion is to great depth, too. There is considerable oxida­
the Hamersley Group iron-formations. Unconform­tion to the deepest depth that has been drilled there,
ably overlying the dome is a thick sequence of quartzwhereas at Elliot lake it's an eighth of an inch or so,
pebble conglomerates. However, I never did see any
except along fractures.
real coarse clasts like some shown in the illustrations;
Skinner: You mean the paleoregolith at Montgomery
most of the material is much finer grained. The con­
Lake was several hundred feet?
glomerate unit may be 2,000 to 3,000 feet thick.
Roscoe: No, on the present surface. Under the present Skinner: Any uranium?
climatic conditions with permafrost, the rock is deeply Martz: I wasn't looking for uranium at the time.
weathered and leached apparently, as compard to Hallbauer: I would just like to say that from
more southerly zones in the Canadian Shield. It's quite
paleomagnetic measurements a latitude of somewhere
surprising.
between 0 and 30 degrees was calculated for the
Skinner: I would think that the analogy is perhaps not
Precambrian area for the southern part of Africa.
quite right, that one would need an elevated terrain
That has been published.
and the rapid erosion that might come with permanent Skinner: For the Witwatersrand period?
ice and seasonal thawing. The region that you are talk­ Hallbauer: Yes.

& U.S. GOVERNMENT PRINTING OFFICE: 1982-361-594/206