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GENESIS OF URANIUM- AND GOLD-BEARING 

PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES
w

to
A2 Polished specimens of quartz-pebble conglomerate uranium ore. 

Both actual size.

A. AB Reef, Denison Mine, Elliot Lake, Ontario. (Samples courtesy of V. Ruzicka, photography by David M. Watson, both Geological Survey 

of 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 of

minute 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. ARMSTRONG

GEOLOGICAL 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 INTERIOR 

JAMES 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 distance 

down 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 and

microbiological 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. Moura 1 .

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 of

atmospheric 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, and

Myers. 

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 of 

energy-source materials. Two responses to this new 

awareness were the acceleration of the search for alternate energy sources and the substitution of abundant 

materials for scarce materials; in addition, scarce 

materials were channeled into more beneficial uses and 

were reserved to uses for which no acceptable substitute 

was 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 that 

appeared 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 U3 08 rose. Increased price for U3 08 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 the 

world began to look at their uranium resources. Because 

of 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 U3 08 for 

the 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 to 

depend on imports or new discoveries. As the cost of 

generating electricity is relatively insensitive to the cost 

of U3 08 , continued high prices for secure supplies of 

uranium seemed assured.

Such was the climate of the uranium market in late 

1974. 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 exclusively as a byproduct of gold mining. The rising price 

of 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 not 

previously recovered uranium to install new uraniumrecovery plants. At that time about 35 percent of the 

known world uranium resources was contained in 

quartz-pebble conglomerates in South Africa and 

Canada.

In late 1974 the United States started to take a hard 

look at the adequacy of its uranium supplies in the light 

of the cumulative need for uranium to meet the forecast 

demands for electricity to be generated by nuclear reactors. At this same time a report on South African goldand uranium-bearing quartz-pebble conglomerates that 

was 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 the 

occurrence of favorable geologic environments in the 

Lake Superior-Upper Michigan area and elsewhere, and 

because 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 a 

basis 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 and 

Canada; contributions were also to be made by others in 

the United States, Brazil, the United Kingdom, Germany, 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 also 

planned 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 not 

time to discuss these other deposits.

Selection of the panelists and their subjects for the 

workshop would have been a formidable task had it not 

been for the help of friends and colleagues and their 

knowledge 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 Appraisal Section, Economic Geology Subdivision, 

Geological Survey of Canada. Dr. John M. A. Forman, 

Mineral Resources Director, NUCLEBRAS, assisted in 

selecting 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 the 

workshop 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 was 

changed to October 1975 to coincide with a time when 

Dr. Pretorius would be in the United States on other 

matters. As it later developed, he had to cancel all plans 

of visiting the United States at that time. The workshop 

did benefit, however, from his advice in selecting the 

panelists.

Originally it was anticipated that the workshop would 

be an in-house study to be held in Reston, Va., but in the 

early stages of organization it was realized that all 

geologists in the Branch of Uranium and Thorium 

Resources, based in Denver, Colo., should attend, and 

that industry would also be interested in attending a 

meeting 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 Branch 

of 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 was 

held at the excellent conference facilities at the Green 

Center, Colorado School of Mines, Golden, Colo., on October 13-15, 1975, and was attended by about 250 

specialists from government, academe, and industry; 

about 125 companies connected with the uranium industry were represented. Following presentations by 

the speakers, discussion was among the speakers and 

the other invited participants. If time permitted, discussion 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 to 

the reader to decide. Most papers presupposed a

familiarity with the subject matter; the reader who 

regrets the lack of an extensive review paper for the 

Rand is referred to D. A. Pretorius, 1974, The Nature of 

the Witwatersrand Gold-Uranium Deposits: University 

of 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 think 

that much of its editorial system, for example, the 

Geologic Names Committee, could or should be used for 

authors 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 internal 

consistency. Subject matter, data, reasoning, and the 

like, were not changed from what the author submitted 

for publication. The reader will be interested to note the 

lack 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 of 

the discussion was lost because of technical failures in 

tape recording, but it is estimated that more than 90 

percent of the pertinent discussion was recovered and 

appears 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 U3 08

1 t U3 08 = 0.7693 metric ton U

A large part of the success and smooth running of the 

workshop 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 meetings 

and housing and recorded the proceedings. Special 

credit must go to Carolyn Willis for phonetically 

transcribing 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 most 

indebted to the panelists for their willingness to take
INTRODUCTION XI

part in the workshop and to prepare their papers for 

publication, and we are greatly indebted to the companies, governments, and other organizations who made 

possible 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

one was not submitted until April 1979. Most papers 

submitted after the workshop differ somewhat from the 

authors' original presentations; in some instances this 

difference 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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL 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

ILLUSTRATIONS

Page

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

TABLES

TABLE 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 CONGLOMERATES

INVESTIGATION OF WITWATERSRAND 

URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES

IN 1944-45

By WESTON BOURRET *

ABSTRACT

The paper discusses the results - both short- and long-term - of a 

1944 study undertaken by a team from the Manhattan Project (supported by the governments of Great Britain and Canada) to assess the 

potential for uranium occurrence in the Witwatersrand mining region 

of South Africa.

Details are given on the methods used by the Manhattan team and 

the results of the sampling study and survey that the team conducted 

in South Africa. The findings for 23 mines that were surveyed in the 

Witwatersrand are documented. These mines were the major producing mines in the Witwatersrand region.

The prior research that led the Manhattan Project administrators to 

concentrate 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 operation 

in 1952 to 1975.

INVESTIGATION OF WITWATERSRAND

URANIUM-BEARING QUARTZ-PEBBLE

CONGLOMERATES IN 1944-45 2

The presence of uraninite on the Central Rand was 

first reported in a heavy-mineral concentrate recovered 

from City Deep mines in October 1923 by R. A. Cooper. 

It should be mentioned, however, that Fred 

Wartenweiler, consulting metallurgist for the Johannesburg Consolidated Investment Group, was indirectly 

responsible for this discovery, in that he initiated a 

change 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 to 

sampling 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 results 

of 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 the 

recovery of coarse gold and heavy minerals. This change 

made possible Cooper's discovery.

Because of the abundance of pyrite and a variety of 

other heavies characteristic of placer deposits, the 

volume of concentrate caught on the corduroy far exceeded the capacity of the amalgamation plant to 

recover a high percentage of free gold. As a consequence, 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 riffles 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 to 

their composition and mineral content led Fred 

Wartenweiler to ask his mill superintendent, R. A. 

Cooper, to examine and identify the chemical and 

mineral content of the heavy fraction. The result of this 

research is the now-famous paper of Cooper's that appeared in the Journal of the Chemical, Metallurgical and 

Mining 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 concentrates collected from corduroy tables, representing 

300,000 tons3 of mine-run ore. The total weight of the

3 [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 in 

use. 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 made 

in 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; it 

assayed as follows:

Assay of radioactive fraction

Uranium Oxide (U3 08) __________________ 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, the 

uraninite itself being black. * * * As uraninite is distinctly softer than 

pyrites it is, of course, possible that a considerable amount may have 

been lost by slimming. The quantity recovered certainly is only a 

homeopathic 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 purpose 

of discovering and evaluating uranium deposits believed 

to hold economic potential for production. The governments of Great Britain and Canada cooperated in this effort during and following World War II.

Consistent with this objective, a literature research 

unit was established in 1943 in New York and 

Washington and at the Massachusetts Institute of 

Technology, under the direction of Dr. Joseph Sinclair, 

to identify potential uranium occurrences outside the 

United States that appeared to justify follow-up examination 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 from 

some 300,000 t of ore, the decision was made to send a 

field team under the direction of Weston Bourret to 

South 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 the 

Manhattan Project investigation, of course, was to examine and sample as many of the major operating mines 

along the East, Central, and West Rand as time permitted-specifically, those with gravity circuits incorporated in the flow scheme capable of recovering some 

uraninite.

Upon arrival in Johannesburg in May 1944, Bourret, 

assisted by Frank West, established contact with the 

Witwatersrand Chamber of Mines and consulting metallurgists of the major mining houses. Despite stringent 

wartime secrecy, the cooperation of the mining industry, South African government, and Chamber of 

Mines was essential, of course, to gaining access to the 

larger mines and getting the job done. Their superb 

cooperation is gratefully acknowledged, for without the 

help of so many metallurgists, engineers, and mine 

managers, successful negotiations and preparation for

South African uranium production by 1952 could have 

been delayed by several years. Although space limitations do not permit identifying every individual, the 

complete cooperation of consulting metallurgists and 

managers 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, and 

samples of concentrated products from gravity circuits, 

similar to those shown in figure 1, were taken for 

analysis and mineralogic study. In 1944 the most 

westerly operating mine on the Rand was Blyvooruitzicht, a new and high-grade mine that had just come into 

production. Unusually encouraging sample results from 

the 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 mineralogist 

of the Manhattan Project, from high-carbon ores of the 

Carbon Leader Reef. This footwall carbon seam is subsequently 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 team 

accomplished 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 product 

samples.

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, mineralization, 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 were 

prepared from sieved samples and mineral determinations were made.

The field team was equipped with a Herbach and 

Rademan 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 of 

concentrate samples. It was entirely inadequate, 

however, for testing ores containing only a few hundred 

parts per million. A number of samples were sent to the 

laboratories of the Manhattan Project in the United 

States for check assay. (This transfer was accomplished 

through diplomatic air pouch of the U.S. State Department.) The time interval of from two to three weeks in 

receiving results enabled confirmation of assays made in 

the field with the gamma ray counter.

The mines visited and sampled during the investigation are shown in the following list:

West Central Rand East

West Rand Consolidated 

Randfontein Estates 

Blyvooruitzicht 

Klerksdorp 

Dominion Reefs

Durban-Roodepoort Deep 

Consolidated Main 

Crown Mines 

Robinson Deep 

City Deep 

Nourse Mines 

Geldenhuis Deep 

Simmer and Jack 

Rose Deep

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 the 

U308 content of the heavy-mineral fractions recovered 

from the reconcentrated corduroy riffle tables is 

generally less than 1.0 percent, and the higher grade

ores, such as those from the Carbon Leader and Main 

Reef, contain far more uranium than gold on a weight 

ratio 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 capability 

of producing more than 500 t of gold annually. 

Therefore, it may be conservatively concluded (assuming 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 concentrate per year.

Upon returning to the United States in early 1945, the 

author prepared a summary report describing sampling 

methods, losses, and conclusions for submission to the 

directors of the Manhattan Project. Late in 1945, Dr. 

D'Arcy George and Frank West, together with other 

members 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 to 

continue the investigation.

TABLE 1. -Description and analysis of samples

[June-October 1944]

Sample

6915

6907

6931

6927

6935

6936

6937

6932

6912

6920

6923

6913

6922

6924

6925

6916

6917

6918

6919

6906

6934

6933

6926

Mine

Randfontein Estates

West Rand Consolidated

Durban-Roodeport

Consolidated Main Reef

Crown Mines "A" Mill

Crown Mines "B" Mill

Crown Mines "C" Mill

Robinson Deep

City Deep

Nourse Mines

Geldenhuis

Simmer and Jack

Rose Deep

East Rand Proprietary

ERP-Angelo Mill

Modderfontein New North Mill

Modderfontein New South Mill

Modderfontein B

Modderfontein New East Mill

Government areas

Government areas

Vogelstruisbult (East Rand)

Blyvooruitzicht (West Witwatersrand 

areas).

Sample description

Chiefly pyrite; concentrate from

Wilfley tables following removal 

of osmiridium streak (see sample 

6905). 

Daily composite concentrate, sized to

70 percent - 200 mesh; 82 percent 

U30a values in -200 mesh fractionfrom corduroy riffles (see sample 6930). 

Composite of weekly gravity concentrate; contains 3-4 oz Au per ton. 

About equal amounts of quartz and pyrite

More quartz than in sample 6935

Weighted average of the three Crown

mine samples is 9.0 Ib U3O8 per ton; 75 

percent quartz and 25 percent pyrite.

Before removal of osmiridium; pyritic concentrate with quartz. 

Mostly pyrite and quartz

trate not removed (see sample 6921). 

Contains osmiridium (see sample 6914 for

uranium osmiridium concentrate).

removal of osmiridium. 

Chiefly quartz and pyrite (Cason Mill)

Osmiridium concentrate removed

Daily composite heavy-mineral concentrate before osmiridium cut; essentially 

pyrite and pyrrhotite. 

Daily composite heavy-mineral concentrate before osmiridium cut; essentially 

pyrite and pyrrhotite. 

Daily concentrate composite sample

Daily concentrate composite sample

Daily composite after removing

osmiridium. 

Composite concentrate representing six

week milling run. 

Composite concentrate

Weekly composite containing osmiridium, 

visible uraninite, and thucnolite.

Reef

Main and Johnston

Main, Johnston, and Bird

Main

Main and South

Main and South

Main and South

South and Main Leader

South and Main Leader

South and Main Leader

Main and South Leader

South and Main Leader

Main Leader

Main Leader

Upper Leaders and Black

Upper Leaders and Black

Main Leader

Main Leader

Leaders, Black, and Kimberley

Upper Leaders and Kimberley

Main Leader

Carbon Leader ___________________

Tons milled 1 per month

340,000

240,000

200,000

210,000

105,000

75,000

110,000

100,000

95,000

80,000

65,000

150,000

70,000

105,000

80,000

72,000

90,000

70,000

130,000

210,000

220,000

75,000

12,000

Percent U-A 1 

(chem.)

0.04

.08

'.03

.09

.53

.48

.32

1.98

.71

.85

.09

1.30

1 1.08

.12

.12

.11

.03

.02

.03

.13

.10

1.74

1.20

Pounds U3O8 

per ton

0.80

1.60

.60

1.80

10.6

9.6

6.4

39.6

14.2

17.0

1.80

26.0

21.6

2.4

2.4

2.20

.60

.40

.60

2.60

2.0

34.8

24.0

1 Radiometric equivalent.
WITWATERSRAND URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES IN 1944-45 A5

TABLE 2. -Description and analysis of osmiridium and miscellaneous samples

[June-October 1944]

Sample Mine Sample description Reef Percent U30a Pounds U308 

(chem.) per ton

5977

5978

5979

Osmiridium gravity concentrates

6905

6914

6921

6929

6930

6938

Randfontein Estates

Simmer and Jack

Geldenhuis Deep

Consolidated Main Reef

Durban Roodeport

Robinson Deep

Gold-pyrite-osmiridium concentrate with

chromite, 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, with 

osmiridium, gold, mill iron, and pyrite.

Main and Johnston

Main and South

Main and South

Leader Middle South

Main and Leader _ _

Leader Middle South

1.01

3.28

.81

1.52

.24

63.8

20.2

75.6

16.2

30.4

4.8

1,276.0

Miscellaneous and hand specimens

6928

6939

Consolidated Main Reef

Daggafontein

Wilfley table tails after dressing corduroy

concentrates; essentially quartz. 

Heavy-mineral accumulation from ballmill

Leader Middle South _ _

Kimberly

nil 1

14.5 290.0

Vogelstruisbult

Do _______.

Randfontein Estates

liners; gift of Dr. E. Mendelssohn; 30-40

percent visible uraninite; also chromite,

rutile, and iron. 

Hand specimens of high-grade gold ore; high Black __.

.pyrite content.

Hand specimen of Kimberley Reef _____ Kimberley. 

Sample of Main Reef gold ore ________ Main __.

.335

.018 

.05

6.70

.36 

.12

1 Radiometric equivalent.

Following Drs. Bain and Davidson's conferences with 

the South African government, Dr. L. T. Nel of the 

South African Geological Survey was placed in charge of 

a team of geologists and metallurgists organized to expand the sampling of reefs reported to contain significant uranium. These locations included Blyvooruitzicht, 

Vogelstruisbult, Western Reefs, and East Daggafontein. Moreover, in 1946 a special uranium research committee was organized to recommend metallurgical 

research on the recovery of uranium by flotation, 

leaching, and alternative methods. This team consisted 

of T. K. Prentice, Fred Wartenweiler, and 0. A. E. 

Jackson. About the same time, a research mineralogist, 

Dr. W. R. Liebenberg, was appointed to study and 

report on the heavy minerals contained in the Witwatersrand conglomerate ores. With the assistance of a consulting metallurgist Robert R. Porter, the first test pilot 

plant was erected at Blyvooruitzicht and Western Reefs 

in 1949 to work out a flow sheet based on sulfuric-acid 

leaching for the recovery of uranium.

In 1951 a small amount of yellow cake was produced 

by the pilot plant at the Blyvooruitzicht mine; commercial production on the Rand started in 1952. Since that 

time, South Africa is credited with the production of 

more than 82,000 metric tons of uranium oxide (from 

more than 20 mines) in the form of yellow cake concentrates valued at more than $2 billion. The record of this 

production by years is set forth in figure 2.

According to the annual reports of the Chamber of 

Mines of South Africa, the average recovery from approximately 300 million metric tons of ore treated from

1952 through 1975 was 0.271 kg U308 per metric ton, or 

0.54 Ib U308 per short ton. Since discovery of the Rand 

in 1886, more than 3.3 billion metric tons of ore have 

been treated in cyanide mills, from which more than 

32,000 metric tons of gold bullion have been recovered. 

The tailings dumps and slime dams resulting from 

milling over 3 billion metric tons are intact around the 

north rim of the Witwatersrand Basin. Although the 

uranium content of these dumps is low, the uranium contained in them reaches an impressive figure, probably 

approaching 500 million pounds of U3 08 . Possibly the 

day is not too far off when it will become economically 

feasible to re-treat some of the old tailings, assuming a 

price, say, of $40 to $50 per pound U308 . In any event, 

the future of the Rand as one of the world's major 

uranium producers is secure.

SELECTED REFERENCES

Associated Scientific and Technical Societies of South Africa, 1957, 

Uranium in South Africa, 1946-1956 [A joint symposium arranged 

by * * * five constituent societies]: Johannesburg, v. 1, 546 p.

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; Reply to discussion, v. 24, no. 11, p. 

264-266, 1924.

Du Toit, A. L., and others, 1929, Kimberley-Johannesburg: International Geological Congress, 15th, South Africa, 1929, Guide A. 6, 

p. 1-34.

Fisher, M. S., 1939, Notes on the gold, pyrite, and carbon in the Rand 

banket: Institution of Mining and Metallurgy Bulletin no. 414, 36 

P-
A6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 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 and 

their 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 exploited 

auriferous 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: Institution 

of 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 Proceedings, 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 and 

other features of the rock: Geological Society of South Africa 

Transactions, v. 23, p. 64-97.

Prentice, T. K., 1940, Precious metal constituents of Witwatersrand 

ores: 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 of 

South 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 this 

historical review. I just want to mention to the audience what the situation is at the present. We have 

fifty gold mines in production at the moment and of 

these there are seven that have extraction units for 

uranium. There are ten uranium plants. We think that 

with the demand building up and some of our extraction plants which are moth-balled being prepared for 

production, we could probably eventually double the 

number 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 factors influencing production. I was very interested to 

hear the figure of 10,000 tons of U308 per year as the 

production capability of the Rand. I think that level of 

production probably could be reached provided that all 

economic factors were favorable. At the moment we 

certainly are far away from that situation. I think it is 

generally known that labor costs have increased to a 

very great extent and will certainly affect production
WITWATERSRAND URANIUM-BEARING QUARTZ-PEBBLE CONGLOMERATES IN 1944-45 A7

capability. Another important item is, of course, the 

gold price, which has not been very consistent. This inconsistency makes the decision-makers uneasy about 

installing expensive extraction plants. Another 

unknown 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 of 

work on the Rand since the days of the early investigations. Obviously, when I mention a figure of 10,000 

tons, I simply cite that as an ultimate productive 

capability, assuming that the economic factors were 

reasonably favorable.

Also I should mention in passing that there are slime 

dams and dumps, as you well know, from which 

uranium has not been recovered. In a sense, uranium 

has been stock piled in these enormous dumps and 

they are available for reprocessing in the event that 

the 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 of 

ore is processed annually, and at least ninety-nine and 

a 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 the 

amount 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 shale

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. U3 08 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 the 

United States Government in 1944 to undertake an investigation of 

the uranium mineralisation on the Witwatersrand as a part of the effort 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 Constituents of Rand Concentrates". These findings attracted little attention 

at the time but were recalled when the Manhattan Project was under 

way.

Dr. Bourret concluded that the Witwatersrand was potentially one 

of the world's most important and lowest cost producers of uranium 

and he estimated possible output at between 5,000 and 7,000 tons of 

uranium 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 undertaken 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. Distinguished 

contributions to the establishment of South Africa's great uranium industry 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 producers.

GENERAL

The Precambrian Development of 

an Oxygenic Atmosphere

By J. WILLIAM SCHOPF

GENESIS OF URANIUM AND GOLD BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-B

CONTENTS

	Page

Abstract ________________________________________________ Bl

Introduction _______________________________________________ 1

Biochemical considerations _____________________________________ 2

Relevance of model studies of the origin of life __________________________ 5

Geologic and paleobiologic considerations _____________________________ 5

Summary _______________________________________________ 7

Acknowledgments __________________________________________ 9

References cited ___________________________________________ 9

Discussion _______________________________________________ 10

in

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

THE 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 more 

qualitative than has perhaps been generally appreciated, a broad range 

of 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, with 

geological (and mineralogical) data suggesting a plausible date for the 

transition 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 by 

many workers. Diagenetic oxidation, to yield such sediments as banded iron-formations and red beds, may thus have occurred on the 

primitive 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 activity 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 the 

past decade, there has been a marked increase of interest throughout the world in the paleobiology of the 

Precambrian. Long-standing questions regarding the 

nature, antiquity, and even the existence of Precambrian life, questions dating from the mid-nineteenth century and the writings of Charles Darwin (1859), have at 

last begun to be answered. The question now is not 

whether evidence of Precambrian life exists, but rather, 

what does it say—what can it tell us of the timing and 

nature of early evolutionary advances and, especially, of 

the interactions between the evolving Precambrian 

biota 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 Precambrian 

atmospheric evolution or, more precisely, the timing and 

nature 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 sort 

of "back-up system" to aerobic metabolism in times of 

special stress. Among the various tissues of higher 

mammals, for example, the probably most complex—the 

specialized nerve tissue that forms most of the human 

brain—inevitably dies if it is deprived of oxygen for 

more than about four minutes. Such dependence on free 

oxygen, while differing in degree from species to 

species, is exhibited by virtually all other megascopic 

organisms 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 find 

exceptions to this general rule of oxygen dependence; 

certain protists and other intestinal parasites no doubt 

qualify (although all such forms appear to be evolutionary 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 that 

enable such mammals to maintain oxygen supplies to the 

brain at the expense of other, less oxygen-demanding, 

tissues). Free atmospheric oxygen plays a necessary and 

integral 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 an 

appreciable segment of geologic time, a segment encompassing 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

the 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) have 

been tolerated. The anaerobes of the present ecosystem, 

the majority of which appear to be evolutionary 

derivatives of this ancient, pre-aerobic biota, hark back 

to this early anoxic environment, as do the anaerobic 

metabolic pathways that still function (albeit in some 

cases only under special circumstances) in extant plants 

and animals.

These views, interpretations of the early history of life 

that have become widely accepted in recent years, have 

important ramifications for an understanding of 

geological 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—do 

available data require that such a change in atmospheric 

oxygen content actually occurred during the Precambrian? If so, what do they suggest regarding the timing 

of 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 function?), 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 these 

questions tends to be circumstantial rather than direct, 

and the answers generated, qualitative rather than 

quantitative. Still, while the various lines of relevant 

evidence (coming from rather disparate branches of the 

natural sciences) may individually seem less than fully 

compelling, they each are at least consistent with—and 

they commonly seem to point to—the occurrence of just 

such a transition, with several types of data suggesting 

that 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.

BIOCHEMICAL CONSIDERATIONS

Of the several lines of evidence that seem to bear on 

the question at hand, studies of the biochemical 

characteristics of modern organisms, coupled with an 

understanding of phylogenetic relationships and of the 

temporal distribution of appropriate types of fossils, are 

among the most powerful. While necessarily indirect—this line of evidence being based on the 

reasonable but unproved premise that the metabolic 

characteristics of fossil organisms can be accurately inferred from their morphology (by analogy with related 

modern organisms) or from their particular level of 

cellular or organismal complexity— this approach seems

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 the 

availability of free oxygen for various biosynthetic 

pathways and for aerobic metabolism. Indeed, such 

"argument by analogy," reinforced by an understanding 

of relevant evolutionary lineages, forms the basis of a 

vast majority of all paleoecologic and phylogenetic inference. One "knows" for rather similar reasons and 

with similar degrees of certainty that Paleozoic 

trilobites were both mobile and oxygen dependent.

Let us now apply this biochemical approach to the 

question 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 at 

least as long as such metazoans have been extant, by 

current estimates beginning some 700 m.y. ago (near 

the 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 applicable, and with equal force, to the occurrence of 

microscopic eukaryotes as well, for even the most 

primitive of such organisms, unicellular microscopic 

algae (whether rhodophytes or, as according to some 

phylogenies, chlorophytes), require molecular oxygen 

for intracellular synthesis of such compounds as sterols 

and polyunsaturated fatty acids, for use in aerobic 

respiration, and possibly also for that most 

characteristic of eukaryotic traits, eukaryotic (i.e., 

mitotic) cell division (Amoore, 1961a, 1961b). As is true 

at the multicellular level of eukaryotic organization, 

unicellular eukaryotes are aerobic, oxygen-dependent 

"biological machines"; the occurrence of such cells in the 

fossil record would thus seem indicative of the presence 

of biologically usable concentrations of free atmospheric 

oxygen. Although there is some uncertainty regarding 

the time of origin of eukaryotes, the most recent, 

thorough compilation of data bearing on the subject suggests that the lineage had probably become established 

by about 1,400 m.y. ago (Schopf and Oehler, 1976); on 

this basis it can be inferred that appreciable quantities of 

atmospheric oxygen were present at least as early as the 

mid-Proterozoic.

Regardless of the time of origin of the eukaryotic cell, 

however—whether somewhat earlier or substantially
THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE B3

later than the date noted above (and both possibilities 

have been suggested in the recent literature)—it is apparent that eukaryotes were preceded by an entirely 

prokaryotic biota, an ecosystem composed solely of 

forms 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 appear certain to have encompassed a broad range of 

physiological types, a spectrum including strict 

anaerobes, facultative anaerobes, microaerophiles, and 

obligate 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 supporting this conclusion are many and varied, but perhaps the 

most instructive are those derived from biochemical 

studies of modern blue-green algae, prokaryotes of the 

kind that were apparently responsible for deposition of 

most types of Precambrian stromatolites (laminated, 

organo-sedimentary structures that are abundant and 

widespread 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 of 

phycobiliproteins and chlorophyll-a), they exhibit a 

number of biochemical characteristics that suggest that 

they 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 a 

previously established anaerobic pathway during an early stage in blue-green-algae evolution (and certainly 

prior to the origin of eukaryotes, since they share this 

same oxygen-requiring biosynthetic system). Similarly, 

some (but not all) blue-green algae are capable of 

oxygen-requiring sterol synthesis, again involving a 

biosynthetic step (the cyclization of squalene) that was 

apparently "added on" to an earlier established 

anaerobic pathway that produces the necessary precursor. 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 involved in this energy-yielding process (the so-called "citric 

acid" or "Krebs" cycle) have been superimposed 

upon - "added on" to- a shorter and completely anaerobic pathway, one that is less efficient and that yields far 

less cellular energy per molecule of substrate metabolized than its aerobic derivative. As a rule, although such 

oxygen-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., they 

flourish 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 for 

which relevant data are available, virtually all "tend to 

prefer" (i.e., grow most rapidly within, whether in the 

laboratory or the field) microaerophilic rather than fully 

aerobic environments—optimum growth rates are commonly observed in the presence of about 10 percent free 

oxygen, 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 the 

filamentous cyanophyte Oscillatoria limnetica (Oren 

and 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, and 

thus not producing oxygen as a byproduct of photosynthesis), but if placed under oxygenic conditions, they can 

"switch on" their more normal aerobic apparatus (one 

using H20 as the hydrogen source, which thus results in 

the 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 increase 

in 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 the 

proper oxidation level for its incorporation into amino 

acids (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 the 

photosynthetic apparatus involved in production of free 

oxygen) 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 is 

relatively 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 conditions, 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 concentrations increased (owing principally to the photosynthetic 

activities of the blue-green algae themselves!), specialized, thick-walled heterocysts developed as a means of 

providing (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; Haystead 

and others, 1970) indicate that the enzyme-complex is 

partially deactivated (60 to 80 percent inhibition) by concentrations of ambient oxygen of about one percent and 

is 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 level 

of ambient oxygen content, and since heterocyst-like 

cells 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-Precambrian 

time.

Finally, a word should be said regarding the evolutionary precursors of blue-green algae, the photosynthetic bacteria. Although several groups of such 

photobacteria occur in the extant biota, differentiated 

one from the another principally by the nature of their 

pigmentation (and thus their capability to absorb light 

energy in various portions of the spectrum), the important point is that, without exception, the process of 

photosynthesis in such microbes is obligately anaerobic. 

Such organisms are incapable of using water as a 

hydrogen 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 of

bacterial 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 require 

anaerobic 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 actually 

occurred during the Precambrian, then it would seem 

reasonable 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 the 

development of aerobic photoautotrophs (precursors of 

present-day blue-green algae), conditions at the interface began to change and, by the ecological Principle of 

Competitive 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 of 

the mat community, an environment that many such 

microorganisms inhabit to the present day.

In summary, diverse types of biochemical data seem to 

indicate the following:

1. Appreciable quantities of atmospheric oxygen were 

available prior to the development both of invertebrate metazoans (about 700 m.y. ago) and of 

unicellular eukaryotes (about 1,400 m.y. ago).

2. Prokaryotes, such as blue-green algae, evolved in 

response to (and, as is discussed below, were the 

oxygen-producers responsible for) a transition from 

essentially anoxic to oxygenic conditions during the 

Precambrian.

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 conclusion 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 in 

geologic time.
THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE B5

RELEVANCE OF MODEL STUDIES OF THE 

ORIGIN OF LIFE

Since the early 1950's and the initial studies of Garrison and others (1951) and Miller (1953), literally hundreds of "early Earth experiments"—studies 

demonstrating the non-biologic formation of organic 

compounds under conditions simulating those believed 

to be present on the primitive Earth—have been carried 

out in scores of laboratories throughout the world. The 

vast majority of these have been gas-phase syntheses, 

experiments carried out both under highly reducing conditions (using a starting mixture of H2 , CH4 , NH3 and 

H20 vapor; for example, Miller, 1953) and under less 

reducing conditions (using a starting mixture of CO or 

C02 plus N2 , H2 and H20 vapor; for example, Abelson 

and Hoering, 1967) and by using a rather wide assortment of plausible energy sources (such as spark 

discharge, ultraviolet light, and ionizing radiation). 

Compounds so synthesized include amino acids, 

hydrocarbons, fatty acids, nitrogen-containing 

heterocyclics, sugars, and many other types of small 

molecules. The important point about these experiments 

is that in all such studies for which relevant data are 

available, such gas-phase syntheses have been reported 

to be inhibited by "small concentrations" or by even 

"traces" of free oxygen (see, for example, Abelson and 

Hoering, 1967).

Recently it has been demonstrated that a second type 

of abiotic synthesis, one involving the photocatalytic 

production of organic compounds on mineral surfaces 

(from CO or CH2 and traces of H2 vapor), is relatively 

unaffected by the presence of uncombined 02 (Hubbard 

and others, 1973). However, the yields of these apparently oxygen-insensitive, surface-dependent syntheses are quite low and, because of their inhibition by 

even small amounts of liquid water—the syntheses having been carried out under conditions designed to 

simulate those of the arid Martian regolith—they appear 

to be of "questionable importance on the primitive 

Earth" (Hubbard and others, 1975).

Thus, as currently understood, the nonbiologic synthesis of small organic compounds on the primitive 

Earth, the first step in the long chain of events that led 

ultimately to the formation of the earliest forms of life, 

seems to have required the near-absence of free oxygen. 

Of those synthetic reactions for which the mechanisms 

have been studied in detail, several require the presence 

of two specific, highly reactive intermediates: formaldehyde (e.g., an intermediate in the synthesis of 

sugars and of amino acids) and hydrogen cyanide (involved in the production of amino acids and of such 

nitrogen-containing heterocyclics as purines and 

pyrimidines). This latter compound is of special interest 

since, like carbon monoxide and hydrogen sulfide (starting materials commonly used in such syntheses),

hydrogen cyanide is lethal, even if present in only 

minute amounts, to many extant forms of life. From the 

perspective of the present, it thus seems puzzling (and, 

indeed, at first glance seems inexplicable) that living 

systems could have originated in a milieu that was in 

fact poisonous to the process of living. Such, of course, 

was not the case—both HCN and CO are lethal only to 

aerobic forms of life (since they react with, and deactivate, cytochromes involved in aerobic respiration), and 

H2S is a compound both used as a reactant by (e.g., 

photobacteria) and produced as a metabolic product of 

(e.g., sulfate-reducing bacteria) numerous extant 

anaerobic microbes. Thus, the inferred involvement of 

these compounds in the origin of life—like the results of 

virtually all other experimental work that has been done 

in this field—is consistent with, and seems supportive of, 

the concept that the origin of life on Earth could have 

occurred only under essentially anoxic conditions.

The application of results of these "early Earth experiments" to an understanding of the primitive atmosphere is, of course, highly model dependent. 

Knowledge of the origin of living systems is far from 

complete, and even if the Oparin-Haldane model is accepted without reservation, virtually no data are 

available to indicate, quantitatively, the degree to which 

abiotic syntheses are inhibited (but perhaps not completely terminated) by various low concentrations of ambient oxygen. Still the overall model seems too sensible 

to be seriously questioned with the data on hand—the 

"early Earth experiments" seem to require essentially 

anoxic conditions; the abiotic organic matter of interstellar dust clouds and that occurring in carbonaceous meteorites seem certain to have been synthesized in the absence of free oxygen; and the morphologically and biochemically least complex, and thus 

presumably most primitive, of known living organismssimple, heterotrophic or C02-using bacteria (namely, the 

clostridia and the methanogens; Broda, 1975)—are all 

obligate anaerobes. The conclusion seems difficult to 

escape: early in the Precambrian, at the time when life 

arose on Earth, the Earth's environment must have 

been virtually devoid of free atmospheric oxygen.

GEOLOGIC AND PALEOBIOLOGIC 

CONSIDERATIONS

The foregoing discussion has dealt almost exclusively 

with biochemical and organic chemical evidence relating 

to the development of an oxygenic environment. These 

matters have been here treated in some detail, principally because they have been largely overlooked in much of 

the other geologic literature on the subject and because, 

taken together, they appear to me to constitute perhaps 

the single most telling line of evidence establishing that 

a transition in atmospheric oxygen content actually occurred in the geologic past.
B6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

The timing of this transition, however, is another 

question altogether, one that is best addressed by referral to the geologic-paleobiologic record. As is noted 

above, available paleobiologic data seem to indicate that 

the transition occurred earlier than about 1,400 m.y. 

ago—the suggested time of origin of the eukaryotic cell 

type—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, much 

has 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 and 

speculation for several decades, a discussion most 

recently revived in a flurry of stimulating papers by 

Preston Cloud (1976) and by Eric Dimroth and his 

associates (Dimroth and Kimberley, 1976). Indeed, the 

geologic literature is replete with discussions of this 

general problem area, and other than to expand and update 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 the 

matter (but I do so only briefly, and rather reluctantly, 

for it seems to me that what is really needed are more 

data, not more verbiage).

Numerous workers have read the geologic record as 

indicating that a transition in atmospheric oxygen occurred roughly 2,000 m.y. ago. The principal lines of 

evidence 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. in 

age;

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 of 

billions of tons of oxidized iron—occur in strata approximately 2,000 m.y. in age but are extremely 

rare in appreciably younger terranes; the concept 

here is that these deposits might evidence the 

anoxic-oxygenic transition since their deposition is 

thought 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 David 

Grandstaff elsewhere in this volume (chap. C). He gives 

an excellent summary of his studies of the kinetics of oxidation 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 much 

of the recent literature on this problem, its significance 

now seems open to question since

1. the temporal distribution of these deposits is relatively 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 susceptible to geological recycling and are thus preserved 

only exceptionally in very ancient terranes (consequently, the present absence of red beds older than 

2,000 m.y., even if firmly established, would not 

necessarily indicate that such deposits were 

originally lacking);

3. the Viking explorations of Mars have shown that red 

beds need not necessarily reflect the occurrence of 

an "oxygenic" atmosphere, at least as the term is 

normally used—the Martian atmosphere contains 

only trace amounts of free oxygen (about 0.13 percent, a value less than that required by terrestrial 

organisms for aerobic metabolism), all of which is 

apparently of inorganic, photolytic origin, an atmosphere that has nevertheless produced a vast 

"red bed" on the planet's surface; and

4. an increasing body of geologic data (e.g., Dimroth, 

1975; Dimroth and Kimberley, 1976) seems to indicate that free oxygen (perhaps at a low p02 , with 

the oxygen having been produced by photolysis of 

atmospheric water vapor) was available for subaqueous 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 biological 

origin, 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 formed by reaction of iron with photolytically-produced ox-
THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE B7

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 the 

presence of water, of solar energy, and of an exosphere 

temperature sufficient to insure loss of hydrogen from 

the gravitational field of the Earth—conditions that all 

seem certain to have been satisfied since very early in 

Earth history), then such oxygen migh be expected to 

have reacted to produce banded iron-formation even at a 

low, nearly "anoxic" level of atmospheric p02 . In contrast to continental red beds, the resulting subaqueously 

deposited iron-formation might be rather readily 

preserved in the rock record. It is conceivable, 

therefore, that although deposition of Archean banded 

iron-formations and of other oxidized facies of similar 

age (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 had 

become stabilized (for example, at a low, photolyticallyregulated level) or whether it might have been increasing over time (as would be required for such units to 

reflect 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 existence 

of 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 measured 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 the 

enormous 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 rate 

of 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-Precambrian 

stromatolites (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 of 

the major phase of banded iron-formation deposition, 

raises the intriguing possibility that this phase of diversification 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 the 

geologic result of a major event in biologic history, the 

origin and early evolution of oxygen-producing and 

oxygen-utilizing microorganisms. And, as several 

workers have previously suggested (for example, Garrels and others, 1973; Cloud, 1974), the cessation of this 

major 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" of 

unoxidized iron and this, together with the saturation of 

other oxygen sinks (e.g., the conversion of H2S to 

sulfate), led to the establishment of a stable oxygenic environment.

SUMMARY

Current concepts regarding the origin of life on Earth, 

concepts supported by numerous laboratory studies of 

the nonbiologic synthesis of organic compounds under 

simulated "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 the 

present 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 other 

relevant biochemical characteristics of invertebrate 

metazoans, primitive unicellular eukaryotes, and prokaryotic blue-green algae and bacteria demonstrates 

that there is a strong correlation between degree of 

organizational 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 and 

methanogenic bacteria, were obligate anaerobes 

that developed during an early, essentially anoxic 

stage in atmospheric evolution.

2. The earliest photoautotrophs (cf. extant photobacteria), which similarly evolved prior to the 

development 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 of 

various 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 were 

benthonic and nonmobile, would have been hard 

pressed 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 totally 

prokaryotic biota diversified to give rise to 

facultative aerobes, microaerophiles, and obligate 

aerobes; biologically mediated, anaerobic-aerobic 

geochemical cycles of various elements (such as 

carbon, nitrogen, sulfur, and iron) became 

established; and photoautotroph-dominated, benthonic biological communities became stratified into an uppermost oxygenic zone (inhabited by 

aerobic photoautotrophs and other oxygen-tolerant 

prokaryotes) and an underlying anoxic zone (inhabited by facultative or obligate anaerobes, and 

including 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 interface, 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 ecosystem 

and, through their photosynthetic activity (and the 

subsequent burial of the photosyntheticallyproduced organic matter) caused an irreversible 

rise in atmospheric oxygen content.

4. By the time of origin of unicellular eukaryotes, the 

Precambrian 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 in 

modern prokaryotes and eukaryotes, Schwartz and Dayhoff (1978) have constructed a composite "evolutionary tree," which they interpret as indicating that aerobic respiration evolved 

in 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 of 

ultraviolet-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 by 

extant 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 their 

derivation from oxygen-dependent precursors and 

indicating that the Earth's atmosphere has been fully oxygenic since their origin during late Precambrian time.

This evolutionary sequence, apparently indicating 

that the transition in atmospheric oxygen content occurred earlier than about 1,400 m.y. ago (the suggested 

time of origin of the eukaryotic cell type), during the 

mid-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, this 

workshop, 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, free 

oxygen is produced readily (albeit slowly) in upper 

reaches of the atmosphere by ultraviolet-induced 

photolysis of water vapor and the accompanying loss of 

hydrogen from the Earth's gravitational field. This 

mechanism 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 of 

such sediments as banded iron-formations, during the 

Archean. Studies of this inorganic process indicate, 

however, that it would have been insufficient to have 

produced the large amounts of oxygen sequestered in 

oxide-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 by 

any known mechanism other than aerobic (e.g., bluegreen algal) photosynthesis. It is thus perhaps significant that calcareous stromatolites apparently produced 

by 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 relationship 

may be merely fortuitous, perhaps a result of rare 

preservation of older stromatolotic facies, this seemingly 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 interpreted as being of photobacterial origin (produced by
THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE B9

anaerobic photoautotrophs), and the apparent timelag 

between the origin of oxygen-producing photoautotrophs and the cessation of the major phase of banded 

iron-formation deposition (about 1,800 m.y ago) could be 

interpreted as encompassing the time required for oxidation of previously unoxidized inorganic substrates 

and the buildup of a stable, oxygenic atmosphere.

It is important to note that several aspects of this 

transition 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., apparently between 2,200 and 1,800 m.y. ago), the data do 

not indicate whether the transition occurred slowly as a 

more-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 the 

transition was sporadic, with atmospheric oxygen content fluctuating widely as a result of variable and essentially uncoupled rates of oxygen production (via 

photosynthesis and photolysis) and oxygen consumption 

(via oxidation of inorganic substrates and, assuming the 

presence of aerobic respiring microorganisms, via 

biological recycling of photosynthetically produced 

organic matter). Moreover, the ranges of atmospheric 

p02 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 attained during the mid-Precambrian), and direct 

evidence of several possible ramifications of the transition, such as a suggested episode of extinction of 

oxygen-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" for 

its time of occurrence of about 2,000 m.y. ago, additional 

and 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 necessarily 

reflect 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.

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 evolution 

of oxygen in the terrestrial atmosphere: Journal of Geophysical 

Research, 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 sedimentary 

rocks: Geologisches Rundschau 64, p. 751-767.

Dimroth, Erich, 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, p. 1161-1185.

Garrels, R. M., Perry, E. A., Jr., and Mackenzie, F. T., 1973, Genesis 

of 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., and 

Calvin, 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 and 

water: Involvement of surfaces in the formation and stabilization 

of 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 oxygenic 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 assuming 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 good 

data 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 high 

ultraviolet 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 hear 

later, there is evidence for some free oxygen as early as 3.76 billion years ago, that is the presence of 

oxidized iron minerals in the Isua (Greenland) ironformation. Geologists maintain the iron minerals are 

primary sedimentary syngenetic. If that is the case, 

there was oxygen, and it is my hunch that there was 

always some small amount of oxygen in the Earth's 

atmosphere as a product of photodissociation of 

water 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 the 

organisms occur in re-entrants in an erosion surface. These organisms are sitting there on an erosion surface not underneath the water, only a couple 

of 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 probable 

that 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 of 

Harvard 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 the 

material studied by Dr. Hallbauer—my comments 

can be based only on what I have read of Dr. 

Hallbauer's work and on the photographs and 

photomicrographs 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 free 

oxygen available during the time of deposition of the 

Witwatersrand, since it would be difficult to envision fungi either originating or surviving under 

essentially anoxic conditions, and this would set the 

transition to an oxygenic environment appreciably 

earlier than the 2,000 m.y. ago that I have suggested 

as being most plausible. However, the 

photomicrographs published by Dr. Hallbauer lead 

me to suspect that the structures he has discovered 

have probably been misinterpreted—specifically, 

they have a branch pattern that appears to me to be 

quite unlike that known to occur in any modern 

fiingi, and, in fact, in any modern thallophytes of 

which 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 of 

origin of oxygenic conditions. I hate to disagree, but 

it seems to me that it just doesn't fit in with 

everything else that is known in the Pre-Cambrian. 

On the other hand, that doesn't mean that it is 

wrong; it just means that it doesn't fit in and maybe 

you're looking at a different facies. If what you are 

saying 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 must 

have been appreciable quantities of atmospheric oxygen, above IVa percent of present atmospheric 

levels. This seems to me not to fit in with a lot of 

other data, but on the other hand, if they are fungi, 

it's of very considerable interest obviously.

Kalliokoski, Michigan Technical University: There are a 

series 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 be 

chased 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 hydrogen 

sulfide produced by sulfate-reducing bacteria can 

combine with iron and you can get pyrite
THE PRECAMBRIAN DEVELOPMENT OF AN OXYGENIC ATMOSPHERE Bll

precipitated. On the other hand, Bob Sweeney has 

shown one can get similar structures without the intervention of biologic activity. It is well established 

that pyrite can infill cells, and not only of 

microscopic algae but of forams and a variety of 

other 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 properly interpreted as Precambrian fossils or fossils 

generally.

Skinner: I don't think this is going to be answered by 

anybody 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 not 

see 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 from 

the very start. Now we must say from almost the 

very start because we did not include the Greenland 

material. 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 doing 

mass 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 to 

argue that all of the organic carbon that is sequestered in the Earth's crust could come from 

photosynthesis and therefore that all of the oxygen 

in the Earth's crust and in the Earth's atmosphere 

and oceans could also come from photosynthesis.

Mass balance calculations seem to be within an 

order of magnitude and seem to make good sense. It 

turns out also that if you look at those numbers, and 

I've just redone that whole business again, the new 

data 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 from 

green-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 all 

that reliable, you're allowed at least up to half the 

organic matter in the Earth's crust from other than 

photosynthesis. And if there was a large amount of 

abiotic synthesis going on or there was a fixation by 

organisms 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 other 

hypothesis that says it all comes from oxygenproducing photosynthesis.

James Trow, Michigan State University: Two short 

questions. First, do you know the age when a lot of 

these carbon beds show up, and how far down in the 

stratigraphic 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 is 

organic matter in those rocks which are reliably 

dated at older than 3.76 b.y. With regard to sulfates 

and sulfur deposits, I do not know the distribution of 

those in the Precambrian.

Uraninite Oxidation and the 

Precambrian Atmosphere

By D. E. GRANDSTAFF

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL 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

ILLUSTRATIONS

FIGURES 1-6. Graphs showing:

1. Schematic representation of the relative rate of dissolution of uraninite during the sedimentary cycle ___—__ C2

2. Residence time of particles in a soil profile, assuming a steady-state soil profile___——_————————————— 7

3. Concentration of dissolved oxygen in water and alluvial sediments of Bugaboo Creek, British Columbia, Canada —— 9

4. Maximum oxygen pressures allowing survival of uraninite as a detrital mineral, assuming significant dissolution

occurs only during transport and burial ______________________—————————_——————— 11

5. Maximum oxygen pressures allowing the survival of uraninite as a detrital mineral during the sedimentary cycle__ 11

6. Models for the evolution of oxygen in the Earth's atmosphere through geologic time _________________ 12

TABLE

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

Page

C3

in

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

URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE

By D. E. GRANDSTAFF1

ABSTRACT

Although uraninite is thermodynamically unstable at oxygen 

pressures 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 uraninite 

that many geologists believe is detrital (Davidson, 1965). 

Of these, the Witwatersrand Triad of South Africa and 

the Elliot Lake Group-Huronian Supergroup of Canada 

are the best known and documented occurrences. 

Although C. F. Davidson (1960, 1965) and others have 

supported a hydrothermal origin for the Witwatersrand 

and 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 abundant 

in or absent from the Precambrian atmosphere (Ramdohr, 1958). Holland (1962) calculated that uraninite 

would be thermodynamically stable only at oxygen 

pressures 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 consumed 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 that 

some free oxygen was present in the mid-Precambrian 

atmosphere.

It is well known that minerals may exist for appreciable periods out of equilibrium with their environments. Magnetite, for example, should be oxidized to 

hematite or some other ferric compound under Earth 

surface conditions. Magnetite survives as a common 

detrital mineral because its oxidation rate is quite slow 

under oxidizing surface conditions. Although the oxidation 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 indicate 

that some disequilibrium may prevail. Estimates of the 

amount of disequilibrium have not been made, however, 

because of lack of knowledge of the kinetics and rate of 

uraninite dissolution and the rate of geological processes (Rutten, 1971).

The purpose of this paper is to use experimental data 

on the kinetics of uraninite dissolution in order to make 

a 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 and 

other similar deposits.

EXPERIMENTAL DATA

The oxidation of uraninite (represented by U02) in 

water containing carbonate species at intermediate pH 

probably takes place according to the ideal formula

U02 + V2 02 + 2HCO-3 + H20 = U02(C03)2(H20)2-2 (1)

Cl
C2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Experimental data and discussion of the kinetics of 

uraninite dissolution have been reported in detail 

elsewhere (Grandstaff, 1973), and only a brief summary 

is given here. The dissolution of uraninite is represented 

by the following kinetic equation

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 factors 

of 5 and 35 have been found in a New Jersey 

river water and in coastal sea water.

FACTORS IN THE SURVIVAL OF PRECAMBRIAN 

URANINITE

It can be seen from the experimental data that the survival of uraninite as a detrital mineral is influenced by 

environmental and physiographic factors, such as pH, 

temperature, presence of organic species, rates and 

time required for erosion, transport, deposition, and 

burial; 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 dissolution 

of uraninite during the sedimentary cycle is shown in 

figure 1. Uraninite grains deep in the rock probably are 

not subject to significant attack by oxidation. Oxidation 

of organic matter, sulfides, and iron-bearing silicates, as 

well as of uraninite, depletes the ground water of oxygen. However, as erosion proceeds, the uraninite is 

brought closer to the surface, and the content of oxygen 

in the water and the rate of dissolution increase. At the 

surface the rate of dissolution is defined by atmospheric 

and environmental conditions. Dissolution of uraninite 

is quite rapid during periods of residence in acidic soil 

profiles. During transportation the rate of dissolution is 

somewhat slower due to the higher alkalinity and higher 

pH generally encountered in alluvial sediments and 

river waters. Finally, after deposition, the rate of 

dissolution decreases as the grain is buried and the concentration of oxygen in the ground water decreases.

<

DC

O 

CO 

CO

Q

Erosion Transport -*4* Burial

TIME

FIGURE 1.—Schematic representation of the relative rate of dissolution 

of 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 diminution 

of uraninite with time by chemical corrosion 

(-d(uran)/cfa) is equal to a rate function R[f\, given by 

equation (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 the 

summation 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)

i

in which Rt is the average rate of dissolution in, and tt is 

the duration of, the ith period. If the rate term is written 

as an explicit function of atmospheric oxygen, for example, the summation terms will have the form

PofR! t,

i

If the oxygen concentration is constant during some 

geological process, it, and other such constant terms,
URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE C3

may be separated from the summation or integral to 

produce

t (7)

This equation may be rearranged to evaluate for P0z, 

for example

Calculation of the maximum concentration of oxygen 

in the Precambrian atmosphere that would have permitted the survival of detrital uraninite and the deposition 

of the uraniferous conglomerates requires that as many 

of the environmental, mineralogical, and atmospheric 

variables as possible should be fixed and limits placed on 

the others.

COMPOSITION

The composition of the uraninite is quite an important 

variable in its oxidation kinetics. Unfortunately, the 

composition and the rate of oxidation measured on the 

uraninite as it exists today are probably not representative of the composition and the rate of oxidation of the 

uraninite at the time of weathering, transport, and 

deposition, more than 2 b.y. (billion years) ago. Some 

estimate of the composition of the uraninite at the time 

of sedimentation must be made. Several analyses of 

uraninite from the Witwatersrand and Elliot Lake 

deposits are given in table 1.

TABLE 1. Chemical compositions and NOC (nonuranium cation) 

values of some uraninite samples from the Witwatersrand and Elliot 

Lake

Blyvooruitzicht 

(1)

U02

U03 _

U308

Th02

PbO

REE

Zr02 + Ti02 __ 

Si02

MgO

CaO

Fe2 03

A12 03

S

H20* __________ 

Sn02 __________ 

Other

Total

NOC: 

Minimum 

Maximum ____

74.10 

2.70 

16.70 

1.00 

1.40 

1.78 

.16 

.81 

.84 

1.33

——

100.82

.13 

.19

Sub Nigel 

(1)

66.65 

1.63 

19.44 

2.13 

1.04 

2.07 

.30 

1.54 

1.81 

.99

____

97.60

.17 

.23

Dominion Reef 

(2)

17.10 

46.19

6.52 

14.85 

3.43 

.50 

1.43 

.11 

1.30 

1.12 

.08 

1.26 

2.50 

1.43 

.62

98.44

.21 

.24

Nordic 

(3)

22.7 

42.6

6.1 

22.6 

4.0 

.0 

.0 

.0 

.7 

.2 

.0 

.9

99.8

.15 

.17

References: (1) Liebenberg (1957), (2) Hiemstra (1968), (3) Grandstaff (1973).

Unfortunately, the uraninite analyses from 

Liebenberg (1957) and Hiemstra (1968) were made on 

uraninite concentrates by wet-chemical methods.

Cassiterite, monazite, leucoxene, hydrated iron oxides, 

quartz, and sericite were probably present as impurities 

(Liebenberg, 1957; Hiemstra, 1968). Therefore, the 

analytical results for Si02 , A1203 , Fe2 03 , Sn02 , H2 0+ , S, 

Zr02 , and Ti02 reported in these analyses were excluded 

as present primarily in impurities. The remaining 

elements were recalculated to 100 percent. Much of the 

lead was present as galena inclusions in the uraninite 

rather than in the uraninite lattice. However, it was included in the elements retained because the lead undoubtedly had its origin in the radioactive decay of 

uranium and thorium.

A first approximation of the composition of the 

uraninite can be made if the lead present in the chemical 

analyses is reconverted to uranium and thorium. The 

analysis calculated will approximate the composition of 

the uraninite at the time of crystallization if primary 

lead were negligible. Then, knowing or assuming an age 

for the uraninite at the time of erosion, the amount of 

lead created by radiogenic decay during that period may 

be calculated. This composition will approximate the 

composition of the uraninite at the time of erosion and 

deposition if there have been no major postdepositional 

changes in the composition of the uraninite other than 

that of radioactive decay of uranium and thorium.

The age of crystallization of uraninite from the Dominion Reef is approximately 3,100 ±100 m.y. (million 

years) (Nicolaysen and others, 1962). An extrusive in the 

Dominion Reef has been dated at 2,800 ±60 m.y. 

(Niekerk and Burger, 1969). Therefore, the age of the 

uraninite at the time of deposition of the Dominion Reef 

must have been approximately 300 ± 120 m.y. The Witwatersrand System overlying the Dominion Reef 

System must be somewhat younger. In the absence of 

dates on the time of sedimentation of the Witwatersrand System, the age of this system can only be specified 

as between 2,800 m.y., the age of the underlying Dominion Reef System, and 2,300 m.y., the age of the overlying Ventersdorp System (Niekerk and Burger, 1966). 

Assuming the uraninite from the Witwatersrand 

System had an age of crystallization of 3,100 m.y., the 

uraninite probably had an age of 550 ± 250 m.y. at the 

time of deposition of the Witwatersrand System.

If the age of crystallization of the Elliot Lake 

uraninite is approximately 2,500 m.y. and the age of the 

Matinenda Formation is greater than 2,288 ±87 m.y. 

(Roscoe, 1969), the age of the Elliot Lake uraninite at 

the time of deposition was approximately 150 ± 150 m.y.

The results of the calculations are shown in table 1. 

Values of NOC for the samples lie between 0.13 and 

0.24. Variation of NOC in each sample is a result of the 

uncertainty in the age of the uraninite at the time of erosion. Because of the number of operations and assumptions involved in the calculations of NOC at the time of
C4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

sedimentation and the possibility that the samples from 

which 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, the 

uraninite 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 dissolution rate and the oxygen pressures calculated by about a 

factor 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 the 

absence of unweathered uraninite from the source areas 

for comparison, it is not possible to assess directly possible mineralogical changes in grain size, shape, and composition that might have occurred as a result of chemical 

corrosion 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 Witwatersrand 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 the 

measurements were made on polished sections, the actual dimensions of the uraninite grains must be 

somewhat larger. Although uraninite grains from 

pegmatites may be quite large, the small size variations 

of uraninite grains in the Precambrian deposits and, 

particularly, the absence of any larger grains in the Witwatersrand and Elliot Lake deposits suggest that the 

uraninite 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, these 

euhedral 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 of 

existing uraninite samples from the Witwatersrand and 

Elliot 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 in

shape between circles and squares, implying that the 

particle 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 of 

rounding 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 have 

undergone 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 have 

undergone extensive modification.

The literature on interpretation of the rounding of the 

uraninite is quite extensive (Hiemstra, 1968). 

Liebenberg (1957) believed that the grains had a waterworn appearance. Hiemstra (1968) found that uraninite 

from 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) and 

Davidson (1958) believed that the uraninite had too high 

a 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 uraninite 

from the Witwatersrand or Ventersdorp Systems.

This apparent variation in rounding may be due to 

physical characteristics of the grains, normal distribution of mechanical processes, reworking, and variation 

of 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 the 

amount of material removed by dissolution is much less 

than that removed by mechanical abrasion, the shape of 

the 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 by 

mechanical 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 of 

chemical 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 C5

than the average Dominion Reef uraninite. If the two 

uraninite samples were exposed to oxidizing conditions 

for approximately the same length of time, about fifteen 

times 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 appreciable 

chemical corrosion and rounding of the Blyvooruitzicht 

uraninite. The Dominion Reef uraninite, however, having 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 appears 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 greater 

than those expected from purely mechanical attrition. 

This does not necessarily indicate that they were 

deposited colloidally, but that they have undergone a 

high degree of chemical corrosion in addition to physical 

attrition.

To estimate the composition of the Precambrian atmosphere, some estimate of the degree of chemical 

rounding should be made. Unfortunately, accurate 

estimates of the amount of material removed from the 

uraninite grains by chemical corrosion cannot be made 

because 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. This 

factor of 50-percent destruction of the original grain by 

chemical 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 probably the only organisms present during the early and 

middle Precambrian (see, however, Hallbauer, this 

volume, chap. M). These organisms were undoubtedly

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 ultraviolet radiation than has been proposed, or if oxygen 

pressures were higher than proposed, it is possible that 

microorganisms such as algae or bacteria did form a 

ground cover during the Precambrian. Even if ultraviolet 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 the 

rate of uraninite dissolution (Grandstaff, 1973). The 

organic matter may bond to the surface of the uraninite, 

blocking oxidation sites. Chave (1965), Chave and 

Schamlz (1966), and Suess (1970) found that polar 

organic 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 matter 

even at very low concentrations of dissolved organic 

matter. Suess (1970) estimated that ocean water contains approximately 1,000 times as much organic matter 

as necessary to saturate the calcite surfaces. The 

distribution of organic matter between the surface of 

uraninite 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 water 

probably influence the degree of retardation of the 

dissolution rate (Grandstaff, 1973), but the type and concentration of organic matter in Precambrian river and 

ocean 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 water 

of the Molluca River in New Jersey was decreased by 

about 5 times. In the absence of knowledge of Precambrian retardation factors, the retardation factors of 5 

and 35 times found in modern rivers and the ocean have 

been used as a first approximation.
C6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

ATMOSPHERIC CARBON DIOXIDE PRESSURE

It does not appear possible to specify the pressure of 

carbon dioxide in the Precambrian atmosphere with any 

certainty. The carbon dioxide pressure in the at 

mosphere appears to be controlled by equilibrium between magnesium carbonate and magnesium silicate 

systems, by rates of input of C02 into the atmosphere by 

volcanic 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 the 

survival 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 probably not the major factor, since much greater reduction 

of the oxidation rate is probably required to permit the 

survival of uraninite to the extent observed in Precambrian 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 detrital 

uraninite.

Tilloids have been described in association with the 

Witwatersrand (Wiebols, 1955) and Elliot Lake deposits 

(Casshyup, 1969). The relative chemical and 

mineralogical maturity of the Witwatersrand and Elliot 

Lake rocks suggests, however, that most of the 

sediments were probably not of glacial origin. 

Therefore, glacial action seems unable to account for the 

survival of the Precambrian uraninite.

If, however, a glacial origin for part of the Witwatersrand and Elliot Lake Sequences is accepted, then at 

least some of the radioactive quartz-pebble conglomerate 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 rate 

by less than a factor of two.

WEATHERING AND DENUDATION

Actively eroded uplands, even in the tropics, yield 

fresh mineral grains of relatively unstable silicates and 

oxides mixed with products of chemical weathering (Van 

Houten, 1972). Such grains apparently are derived from 

scouring of bedrock by rivers, and the grains are mechanically released from the rock without undergoing 

significant chemical weathering.

Brock and Pretorius (1964) suggested that the source 

areas surrounding the Witwatersrand were undergoing 

uplift along basin margin faults. Frarey and Roscoe 

(1970) and Pienaar (1963) have suggested that the 

deposition of the conglomerates containing the 

uraninite in Elliot Lake occurred after uplift of the 

source area. Under such conditions rivers in the source 

area would be rejuvenated, moving old, mature detritus 

from the source area and downcutting into fresh rock. 

Under these conditions uraninite could be scoured from 

bedrock by rivers without significant chemical attack.

Glaciers may also deliver virtually unweathered grains 

to rivers, where they may be mixed with chemically 

weathered minerals. Although glacial derivation of the 

uraninite 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 of 

denudation in large midlatitude basins is related to the 

mean 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 have 

been 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 C7

several tens of centimeters per thousand years. On 

mountainous slopes and in the absence of vascular 

plants to hold soil formed, soil and leached zones could 

have been fairly thin. Under these conditions, the 

diagram indicates that particles would probably reside 

in soil zones for periods on the order of 500 to 10,000 

years.

TRANSPORT

The source areas of the uraninite in Elliot Lake and 

the Witwatersrand have never been discovered. 

Although extensive nonmarine deposits may occasionally be found hundreds of kilometers from their source 

areas (Stanley and Wayne, 1972), if a molasse model is 

used (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 Witwatersrand 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

- 2

t

LLJ 

Q

O 

N

(D 0 

O

\\ <£>.

\

1 0

-1

LOG DENUDATION RATE, IN CENTIMETERS 

PER 103 YEARS

FIGURE 2.—Residence time of particles in a soil profile, assuming a 

steady-state soil profile. The range of most geological conditions is 

shown by the shaded area; residence times are given by diagonal 

lines.

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 Witwatersrand and Elliot Lake have diameters on the order 

of 0.1 mm. These grains are hydraulically equivalent to 

quartz 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 in 

alluvium for considerable periods of time by shifts of the 

river channel. This increases the residence time of the 

particle in the river system.

For periods of transport, assuming steady state, an 

approximate 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 the 

discharge of bedload sediments, p is the density of the 

sediments, approximately 2.6 g cm~3 (Judson and Ritter, 

1964), W is the width of the channel or portion of the 

channel, and D is the depth of sediment being 

transported.

It appears quite difficult to estimate accurately the 

depth of sediment being transported by the river. In 

some cases this depth may be estimated by using the 

height of sand waves or sand bars developed by grain 

movement.

A sand wave (also called a sand dune or bar) is a ridge 

that is formed on the bed of a river by movement of bed 

material and that is usually approximately normal to the 

direction of flow (Lane, 1947). Movement of the bedload 

takes place when grains are removed from the dune in 

regions 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 of 

variation 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 the 

sand waves are approximated as triangular solids, the 

equivalent 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 CONGLOMERATES

The 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 the 

Mississippi River is approximately 7-10 percent of the 

suspended load. Therefore, the bedload discharge is 

about 3xl013 g yr1 . If the channel width of the 

Mississippi River is approximately 1 km (Carey and 

Keller, 1957) and the amount of sediment being moved is 

equivalent to 5 m depth, assuming steady state, the rate 

of 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 approximately 1 km yr'1 .

These figures are only approximate since probably not 

all of the width of the channel is active in transport of 

material at any one time and the depth of sediment 

transported may also be variable.

By examination of the sediment type in Elliot Lake 

and the Witwatersrand it may be possible to estimate 

the rate of bedload transport in these Precambrian 

rivers.

The diameter of pebbles in the central Witwatersrand 

generally ranges from 25 to 75 mm and may be as great 

as 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 ability 

of a river to carry material of this size indicates that the 

river 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 assumed, 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 is 

assumed. However, the calculation indicates rates of 

transport of bedload material similar to those of present 

rivers.

These calculations indicate that during periods when 

the bedload is continuously entrained it may be 

transported 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 order 

of 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 the 

river system. In order to estimate the average transport 

rate of particles in such cases, the entire width and 

depth 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 maximum of approximately 200 km at Helena, Ark., to a 

minimum 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 from 

equation (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 Precambrian 

rivers 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 times 

of 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 pore 

water 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 the 

sediments to the river and to the water table, the 

permeability, the content of organic matter, and the 

mineralogy. 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 at 

present, 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 exposed to oxidation may be approximately equal to the 

time during which the particle is actually in sediments 

being moved by the river. In the second case, the particle is exposed to oxidation for the entire period it is in
URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE C9

transit 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 during 

transport. In the extreme cases in which a sediment particle was continuously entrained by the river or in a river 

surrounded by anoxic sediments, periods in oxidizing environments might have been as short as 100 years. At 

the other extreme, in which the particle remained in oxidizing zones during transit, periods of tens of thousands 

of years might have been reached.

DEPOSITION AND BURIAL

The exact mode, or modes, of deposition of the placers 

is still uncertain (Brock and Pretorius, 1964; Frarey and 

Roscoe, 1970). The two major models for deposition of 

the placers are those of terrestrial fluvial deposition and 

marine littoral deposition. The fluvial model appears to 

be well supported for the major uraniferous conglomerates in Elliot Lake (Pienaar, 1963; Frarey and Roscoe, 

1970). However, no single model appears able to explain 

the deposits of the Witwatersrand. Brock and Pretorius 

(1964) have suggested that several mechanisms may 

have been responsible for deposition.

Pienaar (1963) suggested that the gravels constituting 

the main Elliot Lake uraniferous conglomerates were 

probably laid down during periodic increases in the current velocity of the streams. Under such conditions 

oligomictic gravels would be deposited on the bottoms of 

wide stream channels as gravel bars or lag gravels. He 

attributed the interlacing of lenses of varying grain size 

to periodic fluctuations in the current velocity, as well as 

to differences in the current velocity in different parts of 

the channel.

n n

i 

1.0

I I I 

0.2 0.4 0.6 0.8

FRACTION OF SATURATION 

EXPLANATION

[] Water 

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.

The intimate intercalation of the uraninite-bearing 

conglomerates with generally uneconomic conglomerates 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 have 

been undertaken (Ingle, 1966), but periods of milling and 

concentration in beach deposits are largely unknown. It 

appears, however, that concentrations of heavy minerals may be formed within a few years.

Even if the uraninite were delivered to the basin of 

deposition virtually unweathered, it might still be 

destroyed if oxidizing conditions prevailed in the sediment (Walker, 1967). There has been long discussion of 

the origin of pyrite in both the Witwatersrand and the 

Elliot Lake deposits (Ramdohr, 1958; Liebenberg, 1957; 

Davidson, 1960, 1965; Schidlowski and Trurnit, 1966; 

Roscoe, 1969; and others). Part of the pyrite appears to 

be detrital. However, observations of pyrite overgrowths and framboidal pyrite indicate that some of the 

pyrite is of postdepositional, epigenetic origin. Ramdohr 

(1958) suggested that pyritization occurred shortly after 

deposition. He suggested that some of the concretionary 

pyrite, which is probably secondary in origin, may have 

undergone some transportation. This would indicate 

either erosion from an older formation or, more probably, erosion from alluvial sediments caused by shifts in 

the position of the stream channel. This suggests that 

pyritization occurred close to the surface and that 

ground waters in some sediments surrounding the river 

or beach contained so little free oxygen that pyrite could 

be precipitated. Therefore, it may be postulated that the 

sedimentary environment became reducing near the 

surface. 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 lagoonal 

facies of the placers might have been anoxic, even very 

near the surface (Saager, 1970).

Rates of deposition in molasse settings are fairly 

rapid. Rates are estimated at between 15 cm/1,000 years 

and 5 m/1,000 years (Fischer, 1969). Rates of deposition 

in the Siwalik Formation, one used by Reinike (1930) as 

a model for Witwatersrand sedimentation, may have 

been as high as 50-100 cm/1,000 years (Gansser, 1964). 

Therefore, grains may have been removed from strongly 

oxidizing environments within a few thousand years.

NUMERICAL CALCULATIONS

The experimentally derived reaction-rate equations (2 

and 8) were solved for the maximum oxygen pressure
CIO GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

that would allow survival of uraninite as a detrital 

mineral.

For purposes of the calculations the following values 

or ranges of values were used:

Specific surface area (SS) __________90 cm2 g"1

Organic Retardation Factor (RF) ____5-35

Nonuranium cation content (NOC) ___0.16 (mole fraction)

Carbon dioxide pressure _______5-100 PAL (0.0016-0.032 atm)

Temperature (T) _________________15°C

Alkalinity of river waters (A) ____1 x 10"3 Molar 

Residence time in soil profiles (TE) ___500-10,000 years 

Residence time in oxidizing en- 1,000-100,000 years

vironments during transit and

burial (TV). 

Fraction of original grain destroyed by 0.5 chemical corrosion (DF).

The hydrogen ion concentration in solution was 

calculated assuming that all activity coefficients were 

unity, that carbonic acid was the only major source of 

hydrogen ion, and that the solution was open to the atmosphere. This may lead to an underestimation of the 

acidity of ground and river waters if oxidation of sulfides contributed significant amounts of sulfuric acid or 

the decay and metabolism of organisms contributed 

organic acids or excess carbon dioxide to the water. 

Since pyrite apparently survived as a detrital mineral, 

the rate of oxidation of sulfides must have been quite 

slow. This suggests that addition of sulfuric acid to 

ground and river water may have been minor.

The hydrogen ion concentration in erosional zones 

was calculated assuming negligible alkalinity by the 

equation

[H1 = (K1BPC022)1/2 (10)

in which B is the Henry's law coefficient and KI is the 

first dissociation constant of carbonic acid (BKi = 

1.6xlO-8 Matm-1 atl5°C).

The hydrogen ion concentration in river and alluvial 

waters was calculated from the equation

(11)

(A)

in which (A) is the bicarbonate alkalinity 

(A) = (HC(V) & (Na+) + (K+) + 2(Ca+2)

+ 2(Mg+2)-(Cl-)-2(S04-2) .(12) 

(Broecker and Oversby, 1971).

The alkalinity of present average river water is 

approximately 1 x 10"3 M (Livingstone, 1966). The rate 

of chemical weathering appears to have been relatively 

constant over geologic time (Garrels and Mackenzie, 

1971). If the runoff from continents during the midPrecambrian was similar to the present amount, the 

alkalinity of Precambrian rivers may have been similar 

to that of present rivers. If this is correct, the average

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 water 

is unlikely to have been less than 7 since the early 

Precambrian, even under very high atmospheric carbon 

] dioxide pressures.

Assuming carbonate equilibria largely control solution 

pH, it can be shown that at pH greater than 6.3 the 

dissolution rate of uraninite in systems open to the 

atmosphere is independent of alkalinity or hydrogen-ion 

concentrations (Grandstaff, 1973). Therefore, small 

variations in Precambrian river- or ocean-water chemistry may not be significant in influencing the rate of 

uraninite dissolution. (Other details of the calculations 

are given in Grandstaff, 1973.) Results of the rate 

calculations are shown in figures 4 and 5.

Figure 4 is constructed under the assumption that oxidation during weathering is minor and that chemical 

corrosion occurs only during transportation and burial. 

This would occur if the uraninite were derived by scouring from bedrock or glacial weathering. Although glacial 

erosion seems unlikely in the case of the Witwatersrand 

and Elliot Lake deposits, the figure suggests that 

uraninite of the Blyvooruitzicht type could survive for 

some time as a detrital mineral if it were derived from 

glacial weathering, even under present atmospheric 

conditions. This is supported by observations in 

Pakistan, Canada, and Alaska of detrital uraninite that 

is apparently derived from glacial sources. The chemical 

composition and resistance to chemical corrosion of the 

uraninite in these recent placers is not known. The persistence of uraninite in the modern environment may be 

due not only to cold temperatures and glacial weathering but also to high resistance to chemical corrosion.

Figure 5 shows the maximum atmospheric oxygen 

pressure allowing the survival of uraninite as a detrital 

mineral under various periods of exposure to oxidizing 

conditions during weathering, transportation, and 

burial. TE is the residence time in soil profiles. TT is the 

residence time in oxidizing zones during transport and 

burial. Because the rate of dissolution during weathering is more rapid than that during transport or burial, 

the maximum pressures allowed are largely influenced 

by residence periods in soil profiles except under conditions in which the time required for transport and burial 

is much longer than the weathering period (TT> TE). 

The oxygen pressures allowing uraninite survival 

decrease with increasing periods of exposure. For the 

most likely conditions, oxygen pressures of 10~2 -10~6 

PAL are indicated.

These calculated pressures are maximum pressures. It 

is likely that the actual atmospheric oxygen pressure 

was somewhat lower. Also, these calculations do not
URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE Cll

-2 -3 -4 

LOG Po2 (PAL)

-6

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 are 

shown by the shaded area. RF, organic retardation factor; PAL, 

present atmospheric level; PCCV Par^ial pressure of carbon dioxide in 

atmosphere.

-2

-3

CD 

O

-5

-6

-7

. 7"f=500 years, flf=35, CO2 =5 PAL

/•f-103,/?f=35, C02 =5 PAL 

7"f=1Q3, RF=b, CO2 =5 PAL

7"£=103,flf=5, C02 =10 PAL 

7"f=1Q3, flf=35, CO2 =100 PAL

• TE= 1Q3, RF = 5, CO2 = 100 PAL 

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

-1 0 

LOG

take into account other possible mechanisms by which 

uraninite might be attacked and dissolved, such as by 

uranium-oxidizing bacteria or by formation of oxygen or 

other oxidative species by hydrolysis of water by radiation. If uranium-oxidizing bacteria or other oxidizing 

agents were present in the ancient environment, oxygen 

pressures required for survival of uraninite may have 

been much lower.

IMPLICATIONS FOR THE PRECAMBRIAN 

ATMOSPHERE

Present data suggest that the earliest terrestrial atmosphere was composed of reduced species, hydrogen 

and methane with nitrogen and water vapor (Holland, 

1962). Then, at some time before 3,500 m.y. ago, this atmosphere was converted into one containing nitrogen, 

argon, and carbon dioxide, possibly by the disappearance of free iron from the upper mantle and consequent volcanic exhalation of less-reduced species. Finally, at some more recent time, the photodissociation of 

water vapor with hydrogen escape and the aerobic 

photosynthesis of plants with burial of carbonaceous 

matter converted the atmosphere into one containing 

oxygen as a major component.

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 

progressively increasing from aerobic photosynthesis since the appearance and evolution of bluegreen algae.

2. The atmospheric oxygen remained low (less than 

0.001 atm) until the late Precambrian and early 

Phanerozoic time. Berkner and Marshall (1965, 

1966) proposed that this concentration of oxygen 

was reached as a result of photodissociation of 

water vapor. They ascribed the appearance of 

metazoa at the beginning of the Cambrian to the 

rise of oxygen pressure to the Pasteur point 

(approximately 0.01 PAL). They believed that the 

appearance of fossil land biota at the beginning of 

the Devonian could be correlated with a rise in atmospheric oxygen pressure to approximately 0.1
C12 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

PAL, a level providing an ozone layer capable of 

shielding 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 concentration (Cloud, 1968). Cloud ascribed the disappearance of banded iron-formations and the appearance of red beds about 2,000 m.y. ago to a rise 

in atmospheric oxygen at that time.

4. The atmospheric oxygen increased rapidly near 

the beginning of the Precambrian to a level near 

that 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 oxygen in the Precambrian atmosphere between 2 and 3 

b.y. ago was probably less than approximately 10"2 to 

10'6 PAL. This appears to rule out the progressive accumulation model (model 1) and the model of early high 

oxygen levels (model 4).

Because of the possible presence of photosynthetic 

organisms 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 the 

oxygen 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 oxygen in the Earth's atmosphere produced by photodissociation 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

n

-1

-2 

-3 

-4

-5

-6

——— i —————— I —————— i —————— i —————— r-i

• Model 4 ^ ———————————— —— — --X-- , 

//' /Model 3/

I ' ».',y <r:?f.-":-"?± 1 / 

I / S: S<B :5::ss::!S-i:?S / /

I ?• J:??:? SiSiSiJS / /

• ,

grant

Ahnerl 

rel 

Joi

ox^ 

At

pla

43210 

n

AGE, IN AEONS

FIGURE 6.—Models for the evolution of oxygen in the Earth's atmosphere through geologic time. (See text for further description of 

models.) Shaded area shows most probable ranges of oxygen 

pressure and time for formation of quartz-pebble uranium deposits.

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 was 

above 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 schematically 

correct. Low levels of oxygen, less than 10~2 to 10~6 

PAL, were present in the early and middle Precambrian 

atmosphere. 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 greater 

than approximately 0.1 PAL.

ACKNOWLEDGMENTS

This work was undertaken as part of a doctoral dissertation 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 work 

was supported by NDEA grants under Title IV and 

grants from Princeton University.

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DISCUSSION

[Hallbauer?]: Is there anything known about why some 

of the recent rivers carry uraninite, for instance, the 

Indus River in India?

Grandstaff: There are several possibilities here. First of 

all, that the uraninite could be quite highly thorian or 

have a high content of rare earths, which would tend 

to stabilize it against oxidation. So these would oxidize

very, very slowly, much more slowly than the Blind 

River or Witwatersrand uraninite. Secondly, the 

uraninite 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 this 

is about the same for other occurrences. Of course, in 

England the uraninite, pitchblende really, was derived 

from mine tailings, so didn't undergo any particular 

oxidation 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 contain 

thorian 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. What 

comparative conclusions, if any, do you draw about 

the atmospheres for these two occurrences?

Grandstaff: As was pointed out, thorium is not the whole 

story; 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 be 

very cerian rich. His X-ray data suggested it has a 

very small unit cell, much smaller than U02 . So I think 

you need a complete chemical analysis. Other than 

that, I'd suggest again that the very cold temperatures 

in 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 more 

resistant 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 the 

present. 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 have 

been.

Cannon: Have you considered what effect either ozone 

or monatomic oxygen might have on these rates? Both 

of these might be important species in a primitive atmosphere and should react more rapidly than 02 does.

Grandstaff: Yes, that's correct. However, if the oxygen 

pressure is less than 0.01 PAL or so, the ozone pressure would have been only a very, very small fraction 

of that. I didn't consider that it would be important 

because of its relatively low probable concentration, 

even though it is somewhat more reactive.

Roscoe: I wonder if the molasse model you have of erosion and deposition of the Huronian sands and gravels
URANINITE OXIDATION AND THE PRECAMBRIAN ATMOSPHERE C15

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 we 

have 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, we 

have very coarse sediments, so what we are seeing is 

only the coarsest fraction of the granitic rocks that 

have 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 appreciable length of time to have developed this whole 

sedimentary system. Moreover, the source area must 

have 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 the 

molasse type of deposition would be an exaggeration 

of what must have prevailed at that time.

Grandstaff: I agree there has been a large pass-through 

of 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 weathered minerals. They are submature rather than completely mature sediments, so I would say that the 

molasse model is not too bad.

[Schopf?:] I notice that your data are inconsistent with 

the Brinkmann calculation. It would be instructive to 

me if you could discuss what's wrong with the Brinkmann calculation and why it doesn't fit your experimental data.

Grandstaff: Brinkmann looks at the reaction water 

vapor plus a photon yielding hydrogen plus oxygen, 

and he calculated that oxygen pressure could have 

been brought up, at a maximum, to about one quarter 

of our present oxygen pressure. Brinkmann makes 

two assumptions which may not be entirely correct. 

The first assumption is that the quantum yield of the 

above 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 well 

known. It appears to be very highly dependent on 

pressures, on other species, and varies between about 

one and one millionth. So, depending on quantum 

yield, oxygen pressures which could be created might 

vary between a quarter and a quarter of a millionth of 

the present. The second thing that Brinkmann suggests is that all this hydrogen escapes; that is, that the 

escape 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

about one thousandth to one hundredth; that is, only 

about one in a thousand to one in a hundred of the 

hydrogen ions actually leaves the atmosphere. In my 

opinion his escape efficiency is unreasonably high, and 

I 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 of 

perhaps a hundred-thousandth. So I'd say that, rather 

than the quarter that he suggests, 10"3 to 10~6 is more 

reasonable for the maximum amount of oxygen which 

could be produced by photodissociation.

[Minter?]: You have taken samples from different environments 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 the 

oldest 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 little necessary relation between the slope on which the 

material 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, downcutting 

by rivers, relatively rapid erosion, and flushing of this 

material into the reef. Obviously I have tried to indicate some range in permissible exposure times to 

allow 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 is 

that is commonly alluded to. How much recycling 

lengthens the exposure period I just don't know; 

recycling can happen very rapidly, particularly in 

oceanic milling and formation of black sands on 

beaches. 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 oxygen 

from 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 

and doesn't allow the oxygen to get in, slowing down 

the reaction rate. The same thing has been noted in 

calcite. You can acidify a solution containing calcite, 

and the reaction should go quite quickly, but it doesn't 

because of the organic coating on the grains.

Myers: Do I understand that you actually observed this 

slowing of the rate?

Grandstaff: Yes.

Myers: And you are attributing it to the organic matter?

Grandstaff: Yes.

Skinner: I would like first to ask you, if you only have 

carbon dioxide at one hundred times the present level, 

where was the rest of the carbon? It doesn't seem to be 

in carbonates or in sediments if you make mass balance for that period, despite Bill's [Schopf] 50 percent 

slop.

Grandstaff: I have to disagree with you there. All we are 

saying is that you have carbonates and you have a reaction between the carbonate, calcite and the magnesium silicate, some sort of chlorite and dolomite, and 

silicic acid giving you C02 in solution, or in the atmosphere. Because of the absence of silicifying organisms 

during the Precambrian, we believe that the C02 

pressures must have been somewhat higher. This 

doesn't have really anything to do with the mass 

balance between carbonate and organic carbon.

Skinner: It requires you to keep adding C02 from the 

mantle or somewhere for that kind of model, or else 

you have to have almost as much carbonate in the section as you now have.

Grandstaff: You certainly have carbonates, primary 

looking carbonates, all the way back to Onverwacht 

time.

Skinner: Do you have the same amount?

Grandstaff: Well, this is certainly a big question. There 

is a problem here, of course, in that the carbonates 

tend to be preferentially dissolved by later weathering. Garrels and Mackenzie have shown that the turnover time is only a few hundred million years for carbonates as compared to the billion years or so for 

silicates, so that it's not very clear how much our present exposures really reflect the true Precambrian carbonate record.

Skinner: Could I pursue not the abundance of carbonates but the question surrounding carbon dioxide a 

little further? Since yours and anybody else's model 

for the early Precambrian atmosphere has a higher

carbon dioxide content, presumably we were dealing 

with much higher average surface temperatures than 

we see today.

Grandstaff: No, not necessarily.

Skinner: At least five to ten degrees more. No?

Grandstaff: Okay. The thermal balance of the Earth is 

still very much up in the air, so to speak. Calculations 

assuming a lower solar constant suggest the atmospheric temperature was actually lower than at present. Rasool and Schneider find that the effect of C02 

pressure on temperature is more logarithmic than 

linear, and that doubling C02 pressure causes an increase of roughly 1°C, the C02 pressure must be increased ten times to get the next degree C, and so 

forth. This is one mechanism by which the Precambrian temperature could have been made higher. 

Opposing this is the fact that the solar constant at that 

time was lower, perhaps only about 75 percent of the 

present, and would cause temperatures to be lower. 

Also, the rate of orogeny and rate of volcanism at that 

time, as suggested by Richard Armstrong, may have 

been higher because of the higher radioactive decay. 

Intense volcanism would have erupted much material 

into the atmosphere, aerosols, which would have tended to depress the temperature. The effect of aerosols 

on atmospheric temperature seems to be much more 

pronounced than the effect of carbon dioxide. I think 

rather than fearing that the surface temperature 

would get too high, I think we have to fear that the 

surface temperature would get too low. This sort of 

thing has not been treated in a quantitative fashion.

Skinner: If the surface temperature gets too low, then 

you depress the dissolution rate of the phosphides and 

of course you know they are all gone. The surface temperature cannot remain too low for very long without 

destroying the only weathering processes you've got 

to get rid of phosphides.

Grandstaff: Well obviously, if you freeze the whole surface of the planet there is no life. But within the sort of 

permissible range of 1° up to 30°, using an activation 

energy for the hydrolysis of feldspar of 8.7 kilocalories 

per mole, as suggested by Helgeson, you get a variation in rate of weathering of only a factor of two. It is 

about the same for uraninite. It's hard to tell what we 

are seeing in the Precambrian in regard to the maturity of Precambrian sediments and the effectiveness of 

Precambrian weathering.
SOUTH AFRICA

Uraniferous Quartz-Pebble Conglomerates 

in South Africa

By J. W. VON BACKSTROM

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-D

CONTENTS

Page

Abstract ________________________________________________ Dl

Uraniferous quartz-pebble conglomerates in South Africa _____________________ 1

Discussion _______________________________________________ g

ILLUSTRATIONS

FIGURES 1,2. Maps showing the Witwatersrand basin:

1. Surface geology __________________________________________________________ D2

2. Distribution of the Witwatersrand System ___________________________________________ 3

3. Generalised geological sections of the Upper and the Lower Witwatersrand _________________________— 4

4. Comparative sections of the Witwatersrand Supergroup, showing uranium-bearing horizons _____________———— 5

in

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

URANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA

By J. W. VON BACKSTROMI

ABSTRACT

The purpose of this paper is to give a short background statement 

summarising data on the Dominion Reef Group, the Witwatersrand 

Supergroup, and the Ventersdorp Contact Reef, with particular 

reference to the close relationship of gold and uranium with sedimentary features as well as the mineralisation, conditions of deposition, 

and the nature of the quartz-pebble conglomerates.

URANIFEROUS QUARTZ-PEBBLE 

CONGLOMERATES 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 the 

Transvaal and the Orange Free State.

An outcrop map of the above-mentioned formations is 

shown 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, the 

most important part of the Dominion Reef Group. It is 

composed largely of the disintegration products of the 

Basement Granite. The sediments consist of mediumgrained to coarse-grained feldspathic, and somewhat 

micaceous, sandstone or quartzite, in which there are 

lenticular bodies of grit and conglomerate.

Two conglomerate bands along or close to the base of 

the arenaceous sediments have been worked or prospected for gold and uranium; these are known as the Upper 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, occupies 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, is 

remarkably consistent and was nowhere absent over 

some 900 m of underground development on strike. 

Where it is thin, the concentration of uraninite may be 

very high, with sections 2-5 cm thick assaying as high as 

one percent uranium. This reef is the main carrier of 

uranium and, apart from the minerals already mentioned, carries minor amounts of detrital chromite, 

garnet, zircon, ilmenite, and columbite-tantalite.

In the Witwatersrand Supergroup, economically important 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 conglomerate 

bands represents a deposit formed after a hiatus in the 

process of sedimentation. They obviously consist largely 

of reworked and resorted detritus derived from the erosion of underlying pebble and quartzite beds. Channels 

and hollows, filled with eroded detritus derived from the 

erosion 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 portion 

of the reefs.

The remarkably close relationship of the concentrations of the gold and uraninite with sedimentary 

features strongly suggests that the gold and uraninite 

were deposited at the same time as the pebbles and 

other detrital components of the conglomerate. The pebbles show variations in their packing index. They are 

usually 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

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FIGURE 1.—The Witwatersrand basin, surface geology (from R. Borchers, 1964, Exploration of the Witwatersrand System and its extension, in 

Haughton, 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 the 

direction of strike of the beds in which they occur.

The pebbles in the ore-bearing conglomerates are 

composed mainly of vein quartz, with occasional pebbles 

of chert, quartzite, red jasper, quartz porphyry, and, 

more rarely, tourmalinized rocks, greenish slate, and 

schistose-type pebbles. The most abundant constituent 

of the matrix is quartz, which occurs together with 

phyllosilicates such as sericite and pyrophyllite and with

lesser quantities of chlorite, chloritoid, and tourmaline. 

Pyrite, in the form of rounded grains, crystals, and irregular 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 or 

bands. Hydrocarbon containing minute particules of 

uraninite is also known as thucholite. 

The conglomerates form only a minor part of the
URANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA D3

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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 in 

abundance in the different regions. In the Central Witwatersrand, 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 to 

repay 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 occurrence, and individual conglomerate bands persist over 

wide areas. Beds of auriferous conglomerate also occur 

in the Lower Division of the Witwatersrand 

Supergroup, particularly in the Government Reef 

Member, but, with the exception of those west of 

Klerksdrop, they have not proved to be of economic importance.

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

ntersdorp Sedments

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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 other 

conglomerate 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, and 

its 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 of 

gold production, for example, the Commonage Reef on 

the horizon of the Main Reef Group, some of the 

Western 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 Witwatersrand Division comes from the Afrikaner and Inner 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 can 

therefore be spoken of as evenly distributed in a general 

way, when the whole extent of the goldfields is considered, especially when comparison is made with the 

much 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 are 

usually 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 are 

clearly 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 notable 

amounts of very finely divided gold separated by lean or 

barren quartzite or shale, or they thin out into a single, 

rich line of heavy ore minerals and granules of hydrocarbon, along which isolated clusters of pebbles or scattered pebbles may occur. It is not unusual to find visible 

gold associated with these pebbles. Still farther away 

from 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 and 

uranium values decline until only traces or no values are 

left where quartzite occupies the horizons on which the 

conglomerate ore bodies occur near the source rocks.

Mineralisation is confined to the ore-bearing conglomerate and seldom crosses the stratification into the 

arenaceous wall rock, except where it has obviously
URANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA D5

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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 of 

values and purely sedimentary features. Richer concentrations 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 the 

interstices between the pebbles. Along with the fine particles of gold and grains of other heavy minerals, those 

of 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 and 

towards the middle or top portions of the conglomerate.

The Ventersdorp Contact Reef and the Black Reef occur at the base of the Precambrian Ventersdorp and 

Transvaal Groups, respectively. The Ventersdorp 

Group, predominantly a volcanic formation, overlies a 

surface 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 with 

a 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 and 

channels in the Witwatersrand beds and are largely 

composed of the erosion products, including the heavy 

concentrates, of the immediately underlying Witwatersrand 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 of 

these conglomerates.

The Ventersdorp Contact Reef appears to consist of 

partly eluvial and partly alluvial material; a small admixture 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 distribution of the mineralisation in the bedrock from which 

most 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 the 

vicinity of suboutcrops of the uranium- and gold-bearing 

ore beds in the Upper Witwatersrand. Away from these 

ore beds, uraninite and gold have hitherto not been 

found 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 Witwatersrand Vaal Reef, which we are studying, the carbonaceous matter is not hydrocarbons. Also, I 

wonder, again from a point of semantics because this 

is 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 to 

emphasize 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 the 

stratigraphic 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 to 

refer 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 the 

appearance of eukaryotic organisms, the disappearance of banded iron-formations, the appearance 

of red beds, all at around 1.5 to 2 billion years appears 

to indicate that the oxygen pressure did increase in 

that 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, and 

this change has often been used as evidence for 

change in the oxygen content of the atmosphere. If 

you take absolute early age dating from the Witwatersrand, the corresponding change must, in my 

opinion, come somewhere in the Transvaal Group, or 

even 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 many 

geologists seem to be looking at this in a rather 

simplistic way. It seems to me that it is probably incorrect 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 interpreted perhaps as conflicting evidence: oxygenic con-
URANIFEROUS QUARTZ-PEBBLE CONGLOMERATES IN SOUTH AFRICA D7

ditions and then maybe reducing conditions, then oxygenic conditions, and so forth. It would seem to me, 

however, that looking at the dynamic processes involved, that's exactly what should be expected rather 

than a simple non-all situation. I suspect that it's really 

more complicated and that there was a period of time 

in which both obtained.

Grandstaff: Also, when you're speaking about oxidation 

of hematite, hematite is stable above oxygen 

pressures around 10 to the minus 70, so what is oxydizing for hematite still may not be very oxidizing for 

uraninite. There's quite a range in which the two can 

quite happily co-exist.

Meyer: Yes, but all this still does not explain this time 

change from pyrite to hematite, fluctuations or not, 

because somehow you don't repeat that change. At 

least the pyrite in detrital form is not repeated. Why 

not?

Bourret: May I suggest that in a very broad sense we are 

overlooking one important factor here and that is the 

tremendous changes in the weather cycle. Torrential 

rainfall in high ancestral mountains flushed large 

quantities of coarse debris into the basin followed by 

periods of quiescense during which fine sediments 

were deposited and even erosional gaps developed. 

With repeated tectonic and weather changes, the cycle was repeated over and over again. So, in a sense, 

we're prone to oversimplify the cycle of change that 

caused the working and reworking and deposition of 

coarse material.

Paul Weis, U.S.G.S., Reston, Va.: You mentioned that 

the Witwatersrand conglomerates are deposited on 

surfaces unconformably and that the unconformities 

represent considerable periods of erosion or at least 

nondeposition. Do you feel that the deposition of the 

conglomerates on these unconformities represents 

very long periods of time or relatively short periods of 

time of actual deposition?

von Backstrom: Well, I think both because there are 

some very major breaks. Although we considered the 

Witwatersrand Supergroup as one formation, it is actually a large number of formations with some very 

distinct angular unconformities. Also, when you take 

these various formations, again you find minor unconformities, in this case down to very short periods 

where you have channel fillings and this sort of thing.

Irving Breger, U.S.G.S., Reston, Va.: Dr. von 

Backstrom and the previous speaker and other 

members of the panel, can you tell me, since I don't 

know, is the uraninite always associated with organic 

matter or can you find it without organic matter?

von Backstrom: Is it always associated with organic matter? I don't think so.

Schidlowski: Definitely not; I'll come back to that in my 

talk tomorrow.

Samuel Adams, Anaconda Co., Uranium Division, Salt 

Lake City, Utah: This goes back to the paper on the 

rates of oxidation of pitchblende or uraninite. You 

mentioned, or some of the other speakers did, the influence of thorium on the rates of that dissolution, but 

it seems to me that perhaps we did not pay enough attention to the state of oxidation of the uraninite itself 

at the initiation of those reactions. For example, if I 

understand your data correctly, the degree of oxidation of the samples you used ranged from about 12 

percent U03 up to about 60 percent. The sample which 

showed the highest rate of dissolution was the sample 

with the smallest amount of U03 , it was the least oxidized.

Grandstaff: That's not correct.

Adams: That isn't?

Grandstaff: No.

Adams: That was the 22.7 figure?

Grandstaff: If you look at the Witwatersrand and the 

Blind River uraninites, both being about 60 percent 

U03 , they both oxidized very, very slowly. In fact, we 

looked at this, and there did not seem to be any very 

simple relation between U2 - U03 ratios and rates of 

oxidation; they both oxidized very, very slowly.

Adams: Right. The figures that I have for the percent 

oxidation of the uranium in the Dominion Reef and the 

Nordic sample are 46 and 42 percent, respectively. So 

those really still are not as oxidized as many of the 

uraninite samples. And what I wonder is whether or 

not such studies shouldn't look at two phenomena: 

first, the dissolution of uranium from those pitchblendes and, secondly, the oxidation of uranium 

followed by the dissolution. So my question would be, 

have you looked at the dissolution of these samples in, 

say, oxygen-poor solutions to see what the solubility 

is, strictly as a function of, say, the carbonate content, 

which ought to be fairly predictable? The other thing 

that I wonder about is the distribution of the U02 and 

the U03 in the uraninite. In other words, it may not be 

uniformly distributed so that even these ratios may 

not tell us very much about the behavior of your 

samples until we know how the oxidation states are 

distributed within the grains; in other words, it may 

already be armored with U03 .

Grandstaff: The samples I took were chemically 

homogeneous, at least to the microprobe. Obviously 

you cannot get at oxidation states with a probe; you 

have to do this by titration with ferric sulfate, or ferrous sulfate, but in X-ray, using diffraction, there was 

no line broadening or any suggestion that the 

uraninite might have been heterogeneous with respect 

to U02-U03 . Further, we took big uraninite crystals, 

took off the outside layer and any gangue minerals 

and anything that didn't look like uraninite. Then we 

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 in 

water 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 hexavalent 

uranium 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 the 

samples wouldn't be demonstrated even by breaking

off the outsides, and it might not be surprising that the 

rates of dissolution would be very difficult to correlate 

with chemical parameters. Another comment I'd like 

to make just in passing is that commonly when we 

speak of uraninite and try to understand it either in 

the 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 the 

chemical parameters of the systems we are trying to 

understand.
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 PRECAMBRIAN 

QUARTZ - PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-E

CONTENTS

Page

Abstract ________________________________________________ E1

Introduction ______________________________________________ 1

The present model __________________________________________ 1

Alluvial-fan placers ________________________________________ 2

Fluvial-fan placers ________________________________________ 2

Proximal braided-belt placers __________________________________ 2

Distal braided-belt placers ____________________________________ 3

Conclusions _______________________________________________ 4

References cited ____________________________________________ 4

Discussion _______________________________________________ 6

ILLUSTRATIONS

Page

FIGURES 1-7. Photographs of:

1. Subangular boulders in Ventersdorp Contact Reef _____________________________________ E2

2. A thick layer of proximal Ventersdorp Contact Reef _________________________________—_— 3

3. Ventersdorp Contact Reef conglomerate interbedded with black shale __________________________ 3

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: 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 _______________________________-__ 4

5. Longitudinal view of Vaal Reef paleochannel _________________________________________ 5

6. Transverse view of Vaal Reef paleochannel edge _____________________________________ 5

7. A polished slab of Vaal Reef __________________________________________________ 5

in

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

EXAMPLES THAT ILLUSTRATE SEDIMENTOLOGICAL ASPECTS OF

THE PROTEROZOIC PLACER MODEL ON 

THE KAAP-VAAL CRATON, WITWATERSRAND, SOUTH AFRICA

By W. E. L. MlNTER

ABSTRACT

In 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 environments by sheets, ribbons, and pods at the extremities of the 

South 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 mining 

of the reefs since 1965.

INTRODUCTION

My contribution is essentially sedimentological. I have 

divided the contribution into five short papers so that 

they can be inserted at opportune intervals between 

other papers to remind us that these samples come from 

rocks and that we ought to view the results and shape 

our thinking in the light of the sedimentological 

framework 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 many 

of these "reefs" has been accomplished. In particular, 

results of research by Armstrong (1965), Knowles 

(1966), Sims (1969) and Minter (1972), presented in 

theses to the University of the Witwatersrand, have, 

confirmed the applicability of Pretorius' model and have 

provided data with which to identify the sedimentary environments that prevailed. These principles have been 

extensively and successfully applied to the evaluation 

and 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 in 

several confidential company reports prepared between 

1965 and 1973.

THE PRESENT MODEL

The preservation potential of placers in proximal environments 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 environments was high. The distal extremities of placer 

deposits have not been accessible, however, because of 

technical difficulties associated with mining diminishing 

gold contents at increasing depths. The full sedimentary 

sequence in the model of a Witwatersrand reef has consequently been constructed from the economic parts of 

various reefs that have been geologically preserved and 

exploited.

In essence, the present model describes the dispersal 

of sediment from eroding auriferous provenance areas 

down dendritic systems of tributary channels into braided belts and finally into deltaic-shaped sand sheets, ribbons, and pods. Placer deposits were developed in all 

these 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 environments 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 concentrated on sedimentary partings within the reef unit. The 

carbon-seam reefs were deposited in distal environments. In both reef groups, the reef unit is defined 

by 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 largely shed off reworked Witwatersrand horsts and 

deposited 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 the 

banket 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 in 

excess 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 phi 

units.

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 areally 

spread data are available to outline details of channel 

patterns.

As in all bankets, the detrital heavy-mineral concentration 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 the 

deposit has been preserved. A very narrow fan was shed 

northwest off the Witpoortjie Fault scarp, which dips 

85° (Minter, 1971, personal files).

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

conglomerate layers are interbedded with quartzites to 

form a clastic cone of sediment, which is over 30 m thick 

at 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 mean 

of the ten largest pebbles changes from 40 mm to 27 

mm. The reefs that have been mined in this deposit are 

of the banket type.

PROXIMAL BRAIDED-BELT PLACERS

The most commonly preserved Witwatersrand reefs 

were deposited in braided-belt environments. In their 

more proximal position they occur as channels averaging 50-100 cm deep with a channel index of as much as 

500. The channels, as in the B Reef at Freddies in the 

Orange Free State Goldfield, were eroded into the 

underlying footwall sediments and are separated by 

minor interchannel topographic elevations. The chanFIGURE 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 E3

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, have 

well-defined edges (fig. 4A). They are either filled with 

large pebble-supported conglomerate containing 

detrital heavy minerals throughout the matrix or with 

mature 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 to 

form 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

FIGURE 3.-Large-pebble Ventersdorp Contact Reef conglomerate interbedded with laminated black shale, which in turn is channelled by 

prograding 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 a 

pebble-supported conglomerate reef (banket) but is a 

pebbly quartzite carbon-seam type of reef. The preservation of detrital heavy-mineral concentration was not 

dependent on physical protection by pebble layers 

(Minter and Toens, 1970) but, in the prevailing lowerenergy environment, the heavy minerals were deposited 

on sedimentary partings in the reef, usually the bottom 

contact (fig. 7).

In the very distal positions, as in some South Reef exposures 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, the 

reef unit becomes a thin sand layer with occasional grits 

or small pebbles (- 2.5 phi units) and a thin detrital layer 

of 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

CONCLUSIONS

Detailed sedimentological data from Ventersdorp and 

Witwatersrand reefs support a fluvio-deltaic conceptual 

model in which a fan-shaped geometry of distribution 

with fan-head, braided-belt, and deltaic-sheet environments occurs.

REFERENCES CITED

Armstrong, G. C., 1965, A sedimentological study of the U.K. 9 

Kimberley Reefof the East Rand: University of the Witwatersrand, 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 by 

moving 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

FIGURE 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, The 

edge 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 the 

hammer 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 E5

————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 of 

gold 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.

FIGURE 5.—Longitudinal view of three trough-crossbedded cosets in a 

paleochannel 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 of 

center in the photograph, is 10 cm in length.

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.

———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 the 

upper division of the Witwatersrand system on President Steyn 

mine and adjacent areas in the Orange Free State Goldfield, with 

special 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 Witwatersrand, M.Sc. thesis, 132 p.

CM

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 base 

of the specimen, contains 750 ppm gold while the remainder of the 

reef contains 35 ppm gold.
E6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

DISCUSSION

Skinner: Would it be possible for you to help me clarify 

my thinking and perhaps some of the others by drawing a cross section through one of these fans and 

perhaps 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 formula 

in which he bases the calculation on the actual scale of 

the 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 environments I think that the waters are primarily in the 

channel ways, but in the distal braided belts I think 

there was almost a continuous sheet of water with 

very little sediment sticking out. I would quote 

perhaps the Athabasca River, where you have extensive braided belts of sand almost covered by water and 

very shallow water conditions. Do you still want a 

diagram?

Skinner: No, no, that's all right. Then would most of the 

sedimentation and working be done during intermittent 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 elaborate 

on that later, but certainly one has a very unimodal 

transport direction. It's a fluvial deposit with constant 

reworking and the velocity is such that one has a pi 

crossbedding developed. It's a trough crossbedding, 

which is a fairly high velocity crossbedding which 

moves along in lunate sand dunes or linguoid sand 

dunes 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 of 

which are basically clay. All of them are rather heavily 

impregnated with organic matter. This does not seem 

to 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 the 

fine-grain sediments are the ones which contain indigenous life rather than developing some kind of carbonaceous matter later on. Is that what you have in 

mind?

Minter: Yes, possibly. I think of the environment really being a coarse sand with not very much fine 

material, certainly not in the basal placer lode. There 

is 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 from 

the fans do you find enough clastic material to account 

for all the rest of the detritus that would have been 

eroded 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 other 

pathways 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 the 

material 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 them 

apart, one finds that they are in effect coalescing 

deposits, and this often accounts for the difference in 

gold concentration laterally.

Hallbauer: What significance do you attribute to the occurrence 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 have 

obviously 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 a 

deltaic flat upon which pebbles could, and I am sure 

would, have been faceted.
The Dominion Reef Group, 

Western Transvaal, South Africa

By J. W. VON BACKSTROM

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-F

CONTENTS

	Page

Abstract ________________________________________________ Fl

Preamble ________________________________________________ 1

General geological setting ______________________________________ 2

Geology _________________________________________________ 3

Mineralogy _______________________________________________ 4

Relation of ore deposits to structures _________________________________ 5

Source of the uranium ________________________________________ 5

Depositional environment ______________________________________ 7

Epilogue _________________________________________________ 7

Selected references __________________________________________ 8

ILLUSTRATION

Page

FIGURE 1. Map showing major geological features of the study area _______________________________________ F2

TABLES

Page

TABLE 1, Average uranium and gold values ____________________________________________________ Fl 

2. Uranium values of upper Witwatersrand sedimentary rocks _____________________________________ 6

in

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

THE DOMINION REEF GROUP, WESTERN TRANSVAAL,

SOUTH AFRICA

By J. W. VON BACKSTROM1

ABSTRACT

The 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 a 

thickness 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 Witwatersrand 

Group (>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 be 

untenable.

PREAMBLE

The Dominion Reef Group finds its major development 

south of the village of Hartebeestfontein and around the 

village of Ottosdal in the Western Transvaal (fig. 1).

The Dominion Reef mine, situated 16 km south of 

Hartebeestfontein, started its life as a gold producer in 

1888 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 less 

than 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 average 

values of more than 0.46 kg per metric ton, and the mine 

acquired 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 short 

tons of U308 and 13 million rands (R) worth of gold had 

been produced.

A recent press release (Rapport, 10 June 1975) gave the 

average uranium and gold values of different sections of 

the Dominion Reef deposit south of Hartebeestfontein as 

shown in table 1.

TABLE 1. -Average uranium and gold values for different sections of the 

Dominion Reef deposit

Values per metric ton

Area

Afrikander Lease

Rietkuil Section (part of Afrikander Lease) _ 

Rhenosterhoek

KCG (Klerksdorp Consolidated Goldfields) _ 

Dominion Reef

Vlakfontein

Ottosdal

Lucaskraal (south of Ottosdal)

Uranium 

(kg)

0.25 

1.25 

.50 

.89 

.80 

.50 

.34 

.15

Gold 

fe)

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 (von 

Backstrom, 1972).

Between 1966 and 1970, exploration by means of core 

drilling of the uraniferous conglomerates present in the 

basal sedimentary rocks of the Dominion Reef Group was 

undertaken. 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 less 

than 100 m to more than 1,000 m. Large deposits of 

uranium of low to medium grade have been proved. 

Associated gold values are very low. There are in existence two developed mines, which have been closed 

down, but a feasibility study is under way to reopen them 

and other possible uranium producers.

Fl
F2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

EXPLANATION

SANO AND SOIL COVER 

VENTERSDORP SUPERGROUP 

WITWATERSRANO SUPERGROUP

BASAL SEDIMENTARY FORMATION 

ANOESITIC AND ACID LAVAS

DOMINION REEF GROUP 

ARC HE AN GRANITE

MAJOR FAULTS 

KLERKSDORP CONSOLIDATED 

GOLDFIELDS

^ 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\444 44 4444 

4444444444 

444\444444

44444444 

444

-1-44 

OTTOSDAL , , 

AREA + ~r 

4444

4444 4/44 AREA f 4 

444 4U444 EXTENSION +

444444 44444 

4444)44 44 

444444444444

W- ->/ 4 4 4 4 4

V 4 DOMINION REEFS 4 

/4

4444444

4f44444 

4444444

21024 6 8 » 12 U 16 18 20 KILOMETERS

FIGURE 1.—Major geological features of study area.

GENERAL GEOLOGICAL SETTING

South of Hartebeestfontein in the Western Transvaal, 

granite of the Archean Complex is unconformably 

overlain by quartzites and conglomerates, which 

together form the basal section of the predominantly 

volcanic sequence of the Dominion Reef Group. These 

rocks are in turn disconformably overlain by shales and 

quartzites of the Lower Witwatersrand Group. The 

lavas and volcano-sedimentary rocks of the Ventersdorp 

Supergroup cover the above-mentioned flat-lying or inclined older rocks, which are eroded to varying 

stratigraphic depths.

Although part of a single depositionary unit, rocks of 

the Dominion Reef crop out in four separate areas, each 

of which can be considered a structural entity (fig. 1):

1. The Afrikander Lease area constitutes a fault block 7

km wide bounded by the large northeast-striking 

Rietkuil and Hartebeestfontein faults.

2. The Dominion Reef area forms a quasi-synclinal 

structure plunging to the southwest. The 

sediments crop out over a north-south strike 

distance of at least 10 km and continue westward 

an additional 15 km.

3. The Vlakfontein area is almost entirely covered by 

lavas of the Ventersdorp Supergroup, so that only 

two small outcrops of Dominion Reef rocks appear. 

The area is bounded by the Rietfontein and 

Vlakfontein faults. The latter fault has no surface 

manifestation; its presence was proved by drilling.

4. The basal sediments in the Ottosdal area, although 

broken by a number of cross faults, can be followed 

along the line of strike for 20 km north of the 

village before they are again covered by
THE DOMINION REEF GROUP, WESTERN TRANSVAAL, SOUTH AFRICA F3

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, and 

quartzitic 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; and

3. 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 importance 

because its conglomerate beds are auriferous; it has 

been exploited in the past for gold and uranium. Extensive 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 sedimentation 

in the region (>2,800 m.y. (million years)), predating 

that 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 of 

the disintegration products of the basement granite. 

The sediments consist of medium-grained to coarsegrained feldspathic and somewhat micaceous sandstone 

or quartzite, in which there are lenticular bodies of grit 

and 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 which 

have been worked or prospected for gold and uranium, 

are known as the Dominion Reefs. The thickest bodies of 

conglomerate 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,

grit, 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 the 

mined area. It occurs sporadically at the base of the 

lower member, where it occupies depressions in the erosion surface of the basement granite. It shows marked 

variation in texture where it rests on irregular portions 

of the pre-Dominion erosion surface and more uniformity where the basement is fairly even. On the whole, the 

conglomerate thickens and pinches in a manner reflecting the surface on which it was deposited, and deposition 

and preservation were controlled by depressions in the 

floor. The thickness ranges from 0 to more than 3 m, 

and pebbles vary in diameter from less than 5 to more 

than 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 is 

often the best concentration of heavy minerals, including uraninite. The quartzite is medium grained to 

coarse 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 channelling, 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 maximum 

of 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, and 

consist almost exclusively of quartz. They may represent reworked and concentrated detrital material derived from the erosion of earlier conglomerate and 

arenaceous rocks. The Upper Reef normally ranges 

from 0.15 to 3 m thick; in some places it is as much as 5 

m thick.

The conglomerates of both the Lower and Upper 

Reefs closely resemble the Witwatersrand banket and 

are 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 are 

usually 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 are 

well sorted, the larger ones being at or near the base, 

and are oriented with the largest axis parallel to the 

direction 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, and 

veins of pyrite have also been observed in fissures in the 

pebbles. 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 is 

no general uniformity in grade, and the distribution of 

the uranium minerals is neither characteristic nor a 

significant feature everywhere. There appears to be a 

connection, however, between the distribution of 

detrital minerals like monazite and cassiterite and that 

of 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 not 

observed, and cross-cutting quartz, calcite, or other 

veins 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 permitted the formation of a thin upper reef or, in places, an 

almost purely black sand well endowed with a full range 

of 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, with 

very little internal low-grade ore. These so-called pay 

streaks vary in width from 130 to more than 300 m, and 

no high values have been encountered outside these 

shoots. Recognition of the above-mentioned feature is 

naturally of extreme importance where mining operations 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 indicate 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-defined 

partings above and below it. This allows the application 

of mining methods that, together with ore sorting, 

should permit a low-tonnage but high-grade mill feed.

An extraordinary concentration of heavy minerals occurs in an argillaceous, gritty, pebbly, silicified sandstone known as the Pay Band, near or at the top of the 

Upper Reef, from which it differs completely. It is 

generally less than 0.3 m thick, but can reach 1 m in 

thickness. 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 to 

four times that in the best zone of the Upper Reef conglomerate. Where the band is thin (20-50 mm), concentrations of uraninite may be as high as 1 percent; otherwise the Pay Band is virtually all detrital monazite and 

cassiterite, with minor amounts of detrital chromite, 

garnet, zircon, ilmenite, and columbotantalite.

MINERALOGY

The mineralogy has been previously described in 

reports 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. According to Liebenberg (1955), the

* * *galena is ubiquitous in the uraninite and much more common than 

in the banket of the Witwatersrand. The detrital shape of the majority 

of 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 secondary uraninite also accompany the partly reconstituted grains of 

detrital uraninite. Contamination of uraninite by sulphides, other than 

galena, 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 associated 

with uraninite. In the Lower Reef the concentration is
THE DOMINION REEF GROUP, WESTERN TRANSVAAL, SOUTH AFRICA F5

much lower. In thin sections, the monazite is to be seen 

as small grains, many of them well rounded, of the order 

of 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 lithological 

and 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 obviously been derived from the conglomerate ore body itself. 

There is a close relation between the distribution of 

values and purely sedimentary features. Richer concentrations 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 the 

conglomerate reefs when the whole extent of the deposit 

is considered. However, when comparison is drawn with 

the 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 of 

each locality (fig. 1). The richer uraninite concentrations 

tend to collect in pay streaks, usually characterised by a 

closer packing of well-rounded and sorted pebbles composed mainly of quartz and by a greater concentration of 

other 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 or 

hollows 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 the 

depth of a depression becomes excessive, and especially 

where its marginal slope suddenly steepens, conglomerate development in the centre of the depression is 

poor 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.

The general distribution of payable concentrations of 

gold and uranium in the conglomerate is parallel to the 

original shoreline of the sedimentary basin as inferred 

from 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 mode 

of deposition is strongly indicated. The distal part of an 

ore-bearing conglomerate sheet may break up into 

isolated lenticular bodies, which diminish in size and 

number until ultimately they are spaced too far apart for 

profitable mining. There is a decrease in pebble size and 

metal content outwards from the point of entry into the 

depositionary basin.

In summary, uranium, which is intimately related to 

gold, occurs in bedded conglomerate and quartzite near 

the margin of a continental structural basin that is 

situated 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 part 

of 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 the 

uranium: (1) older rocks from whose destruction were 

derived the materials that form the shale, quartzite, and 

conglomerate of the succession; and (2) fluids containing 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, as 

part 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 diminished 

considerably as time progressed.

The material that was transported into the basin must 

have been derived primarily from the exposed parts of 

the basement complex—from the sediments and 

metamorphosed 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 and 

heavier 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 

coarser equivalents of the Dominion Reef, Hospital Hill, 

Government Reef, and Jeppestown Groups would be exposed and could have contributed detritus to the overlying Main-Bird and Kimberly-Elsburg Groups. Moreover, 

there is abundant evidence in the areas that are being 

mined that fluctuations of water level within the basin 

and local tectonic movements led to considerable 

reworking of already deposited material.

If the uranium now present in the conglomerates were 

syngenetic, it could have been carried into the basin 

either in solid form or in solution, or both. But having arrived there, it had to remain—subject to the natural 

processes of decay and, if in solution, of groundwater 

movement.

The source rocks of the Witwatersrand Supergroup 

and the Dominion Reef Group are known to be 

mineralised, and their mineralisation is almost wholly 

pre-Witwatersrand in age. Part of the mineralisation is 

a uranium mineralisation. The question naturally arises, 

therefore, whether the quantity of uranium present in 

these beds, and particularly in the beds above the Jeppestown Formation, of which we have more detailed 

knowledge, is greater than could be accounted for on the 

assumption of a syngenetic origin.

Any attempt to answer such a question involves the 

consideration of a number of factors of which our 

knowledge is very scant. Among these factors are the 

total original extent of the sediments of the succession, 

the degree to which earlier beds in it were eroded and incorporated into later beds, the average uranium content 

of the source rocks, and the average uranium content of 

the existing remnants of the system.

Only for the last of these factors have we sufficient information available to permit a rough approximation to 

be made. This information is derivable from assays of 

core samples from comparatively few of the hundreds of 

boreholes that have been drilled in the search for 

auriferous reefs in the Upper Witwatersrand beds. Nel 

(1958) states that he obtained "information on uranium 

distribution in a vertical direction" from unpublished 

notes by S. H. Haughton, formerly in charge of the 

Geological Unit. Haughton reported that "physical 

assays of cores from certain boreholes logged in the 

Orange Free State and Klerksdorp Goldfields have been

made." His information, as cited by Nel, is summarised 

by the author in table 2.

The important, but at present unknown, factor concerning these cores is the actual average uranium content of the sections which carry less than 0.003 percent. 

Simpson's borehole logs of radioactivity (1951) show 

adequately that the radioactive content of quartzitic 

members of the succession varies from layer to layer and 

from hole to hole. A few random specimens have yielded, on assay, values of up to 0.001 percent U308 , but it 

would be hazardous to accept this, or any other figure, 

as an average of all material carrying less than 0.003 

percent in order to arrive at an average for the whole 

succession in a given area. A significant fact revealed by 

these figures is that the important concentrations of 

uranium are to be found only in certain narrow zones, 

which usually form less than 5 percent of the whole Upper Witwatersrand Group—the average usually being 

about 2 percent. The remainder is not barren and 

generally shows as "normal" on the radiometric logs, 

which is higher than the "normal" of the igneous 

material that intrudes the sediments. But there is not, in 

this 95 to 98 percent of the sediments, anything like the 

concentration found in the other 2 to 5 percent. What is 

true and significant, however, is that the average 

uranium content would appear to be much higher than 

that of the many billions of metric tons of granite in the 

world which, on average, contain 3.5 ppm uranium and 

which have been mooted as very low-grade potential 

uranium resource rocks.

With regard to the uranium content of the granite 

basement, the presence of the pre-Witwatersrand 

granitic rocks in an overturned sequence of strata at 

Vredefort afforded a unique profile covering a considerable vertical section of the upper and middle crust. 

Uranium determinations of the "normal" granite undertaken by Hart, Nicolaysen, and Gale (in press) averaged 

2.45 ppm (range, 1.03-3.93 ppm) or 0.000245 percent. 

The outermost margins of the central core, however, 

showed much higher values, 20-30 ppm, or 0.002 and 

0.003 percent uranium. The uranium concentration 

found in 2 to 5 percent of the sediments of the 

Witwatersrand Supergroup is thus from 3 to 70 times 

greater than that present in the underlying basement 

granite.

TABLE 2.— Uranium values of upper Witwatersrand sedimentary rocks

Area Formation Core 

(m)

Mineralized 

(percent)

Number of 

holes

Equivalent 

percent 

U3(V

Klerksdorp Townlands.

Potchefstroom 

Ventersburg OFS_

Upper Witwatersrand, Gold Estates 4,943 5.29

Group, Vaal Reef, Ada May Reefs,

and Commonage Reefs.

Upper Witwatersrand _________ 7,035 2.8 

Upper Witwatersrand, Footwall 4,068 1.33

quartzite, and Basal Reef.

.080

.170

1 Cutoff. 0.003 percent.
THE DOMINION REEF GROUP, WESTERN TRANSVAAL, SOUTH AFRICA F7

DEPOSITIONAL 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 is 

witnessed by the large number of papers propounding 

the many different theories. The reader is referred to 

the excellent recent review by Pretorius (1975) of this 

large volume of information, which he summarised as 

follows:

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 the 

gold- 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 the 

metals 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 a 

beach. 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 from 

an 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 a 

river, which discharged through a canyon and flowed across a relatively narrow piedmont plain, before entering the basin. The review has 

also covered ideas on the origin of the carbon that is present in the 

sediments, since it has now been shown to be an important environmental 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 that 

the 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 is 

celebrated in 1986, disputes will still be raging as to how and where the 

auriferous 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 of 

the earliest forms of life.

Liebenberg (1955) emphasized that thucholite is very 

scarce in the Dominion Reef and could hardly have

played any significant role in the deposition of the 

uranium and gold in these rocks.

EPILOGUE

With artists' licence, supporters of hydrothermal introduction paint impressive and interesting pictures of 

the amazing ability of mineralising solutions to move 

from intrusive igneous bodies deep into sedimentary 

rocks, 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 of 

uranium. Yet, as far as the distribution of uraninite and 

gold in the Witwatersrand Supergroup and contiguous 

formations is concerned, there are many features which 

hydrothermalists do not seem to have been able to explain satisfactorily—features, for example, such as the 

absence of ore tracts between a hydrothermal source 

and the distant sites of ore deposits, the more or less 

uniform and consistent distribution over hundreds of 

square 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 of 

wall rocks—all of which features tend to refute 

hydrothermal lode deposits.

Notwithstanding detailed regional mapping, the vast 

amount of deep and extensive mining from the West 

Rand to the Orange Free State goldfields, and 

widespread prospecting (including the large number of 

prospecting boreholes, some of which are as much as 

3,500 m deep), no intrusive bodies have been found 

which 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, the 

mineralising solutions have left no trace of their paths 

through and across the succession of four supergroups 

(the Dominion Reef, Witwatersrand, Ventersdorp, and 

Transvaal) with a total thickness of at least 18,000 m 

(perhaps even much more), assuming, as postulated by 

hydrothermalists, that the mineralisation solutions were 

introduced in post-Transvaal time. No channels along 

which these solutions could have been introduced have 

been found. Hydrothermalists therefore contend that all 

these formations were simply permeated by the 

mineralising solutions. In whichever way the solutions 

were introduced—whether along channels or by permeation—they completely disregarded favourable sites for 

deposition 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 the 

Transvaal Supergroup. Instead, the solutions selected 

the lenticular Black Reef and Ventersdorp Contact 

Reef, as well as certain preferred conglomerate beds in 

the 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-mineralised 

boulders 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 by 

streams during intraformational sedimentary breaks. 

Several exposures underground reveal islands or remnants of reef completely surrounded by impervious and 

barren argillaceous channel fillings. The tenor of the 

reef in the islands is the same as that of the broken reef 

on opposite sides of the channels. Boulders of reefs cut 

by transgressive stream channels have also been recorded 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 interpreted as indicating that the gold and uraninite were 

present in the conglomerates before they were cut by 

stream 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 [The 

Geology 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 in

the 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 the

uraniferous 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]: Geological 

Society of South Africa Transactions, v. 64, p. 23-54.

Liebenberg, W. R., 1955, The occurence and origin of gold and radioactive minerals in the Witwatersrand System, the Dominion Reef, the 

Ventersdorp 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 the 

uranium 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 the

uranium minerals and gold in the Witwatersrand ores: U. N. International 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 Witwatersrand, Unpublished dissertation.

Nel, L. T., 1958, The occurence of uranium in the Union of South 

Africa: U. N. International Cjonference on the Peaceful Uses of 

Atomic 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 and 

observations: 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 the 

boreholes 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 and 

a Paleocurrent Analysis of the Vaal Reef Placer

By W. E. L. MINTER

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-G

CONTENTS

	Page

Abstract ________________________________________________ Gl

Introduction _______________________________________________ 1

Crossbedded internal structure ___________________________________ 1

The Vaal Reef _____________________________________________ 2

Spatial distribution of detrital heavy minerals ___________________________ 8

Conclusion _________________________________________________ 8

References cited ____________________________________________ 9

Discussion following papers by Drs. von Backstrom and Minter _________________ 9

ILLUSTRATIONS

FlcuREsl-4. Photographs of:

1. Footwall quartzite at Free State Saaiplaas in the Orange Free State Goldfield, showing a longitudinal view of trough 

crossbedded sets, the external geometry of each set, and the concentration of dark heavy detrital minerals on 

the toes of foresets _____________________________________________________ Gl

2. Footwall quartzite at Free State Saaiplaas in the Orange Free State Goldfield, showing a transverse view of trough

crossbedding to illustrate the slight discordance of the curved foresets _______________________ 2

3. Two oblique, transverse views of very large scale trough crossbedding in proximal Steyn Reef at Western Holdings

Mine, Orange Free State Goldfield ____________________________________,_______ 2

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 ______________ 3 

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

deposited on _____________________________________________________________ 3 

6-9. Plans showing:

6. Isopachs of the truncated MBS quartzite, which underlies the Vaal Reef unconformably ______________ 4 

7-9. Moving-average vectors measured from:

7. Trough crossbedded Vaal Reef __________________________________________ 5

8. Trough crossbedded quartzite beneath the Vaal Reef ______________________________ 6

9. Axes of trough crossbedded units in argillaceous quartzite overlying the Vaal Reef _____________ 7 

10. A schematic model of the sequence of events that resulted in deposition and preservation of the Vaal Reef _________ 8 

11, 12. Photographs showing:

11. A transverse view of trough crossbedded distal Steyn Reef, Free State Saaiplaas, Orange Free State Goldfield, in 

which the barely preserved remnants of many trough units are marked by dark heavy-mineral concentrates 

at the base of the reef unit _______________________________________________ 8

12. Multilayered distal Steyn Reef, in which is illustrated a sequence of predominant migration followed by accumulation and then migration again ______________________________________________ 8

in

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

THE CROSSBEDDED NATURE OF PROTEROZOIC WITWATERSRAND

PLACERS IN DISTAL ENVIRONMENTS AND A PALEOCURRENT

ANALYSIS OF THE VAAL REEF PLACER

By W. E. L. MlNTER1

ABSTRACT

The 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 produced 

by the braiding of shallow channels in a dendroidal belt. Low-variance, 

unimodal, paleocurrent data from the Vaal Reef are used to illustrate a 

transgressing, 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, is 

related 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 activities 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 less 

significance. Nevertheless, the term "reef has been applied to the pebble layer often evident on the basal contact of carbon-seam reefs. This layer is, however, only a 

part of the whole reef.

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. This 

is an important aspect of the identification of a reef unit

1 Geology Department, Anglo American Corporation, P.O. Box 20, Welkom 9460, South 

Africa

and is closely related to the nature of the internal 

geometry. Every carbon-seam reef examined by the 

author has been characterised by the presence of trough 

crossbedding, more explicity defined as pi crossbedding 

(Alien, 1963).

Trough crossbedding is a structure that develops at a 

higher flow velocity than that required for planar 

crossbedding. Planar crossbedding represents the 

primary structure that develops at the lower velocity 

end of a range of crossbedded structures in which the 

linear edge of the lee side of the sand dune becomes increasingly curved with increasing velocity. Trough 

crossbedding is developed by a scour-and-fill 

mechanism, and most units owe their origin to the 

migration of trains of large-scale asymmetrical dunes 

with pronouncedly curved crests. The coarse sand 

deposited under this flow regime contains very little 

muddy material (figs. 1, 2).

FIGURE 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, which 

merge 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 CONGLOMERATES

FIGURE 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 coarsegrained sand containing large pebbles and coarsegrained heavy minerals was transported as 1-m-high 

sand dunes. However, these are rarely observed in proximal environments, probably because they are 

destroyed. 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 the 

Main-Bird Series of the Witwatersrand System, lies 

upon an extensive, regular, angular unconformity with a 

paleoslope truncation angle of between 1:120 and 1:350 

(fig. 5).

Mining, 2,000 m below surface, of the 1-m-thick sheet 

of sedimentary rock that contains the placer deposit has 

provided superb exposures and a unique opportunity to 

analyse Proterozoic paleocurrents in detail.

The placer sediment, which is a pi-crossbedded quartzarenite with a mean grain size of 2.54 phi units, was 

deposited 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°, a 

vectoral 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 the 

angular 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 a 

variance of 2,484 (fig. 8). Analysis of 297 foresets from 

the 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 PLACER G3

FIGURE 4.-Distal Carbon Leader Reef illustrating a longitudinal 

view of the lowest coset of trough crossbedded quartzite with dark 

detrital heavy-mineral concentrations on foresets, bottomsets, and 

particularly at the base of the unit at a position halfway down the 

hammer handle. Blyvooruitzicht, Far West Wits Goldfield.

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 a 

variance of 269 (fig. 9). Analysis of 193 measurements 

from 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 unconformity, which describes an embayment.

Integration of these paleocurrent data from three 

superimposed quartzite layers reveals deposition of a 

fluvio-deltaic fan, transgressive truncation of the fluviodeltaic fan and deposition of a Proterozoic gold and 

uranium placer by braided channel processes, burial of 

the placer beneath argillaceous quartzite, which was 

transported and deposited by longshore paleocurrents 

(fig- 10).

The very low variance of paleocurrent vectors 

measured from pi-crossbedded sets in this study apVAAL REEF TRUNCATION:S^^H^>M>M: >:M^:^S^M

1KM

METERS 

-I50

•I20 

90

•60 

30

-0

-30 

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 feet 

Braided 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 G5

26°45'

EXPLANATION

Crossbedding direction

FIGURE 7.—Moving-average vectors measured from trough crossbedded Vaal Reef.
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 G7

26°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 CONGLOMERATES

TECTONIC FRAMEWORK

— PRE-MB5

SEDIMENTARY RESPONSE

Deltaic MBS Lobe 

Filling Downwarp

Subaqueous Channelling

Dendroidal Bell 

of Vaal Reel

Transgrefisive Offshore Overlap 

of Vaal Reef by MB4

FIGURE 10.- A schematic model of the sequence of events that resulted 

in deposition and preservation of the Vaal Reef.

proaches ideal limits and is attributed to good 

stratigraphic control of the data, the small size of the 

area, and the excellent exposures.

SPATIAL DISTRIBUTION OF 

DETRITAL HEAVY MINERALS

The scour-and-fill mechanism that transported placer 

sediment effectively concentrated detrital heavy 

minerals on the toes of foresets and the trough-shaped 

bases of sets (fig. 11). When the rate of migration of 

these sets exceeded accumulation, the detrital heavy 

minerals became concentrated at the base of the bed in a 

heterogeneous arrangement. This is well illustrated by 

figure 11, in which the barely preserved remnants of 

many trough units are marked by dark heavy-mineral 

concentrates.

Rates of accumulation and migration fluctuate so that 

a sequence of predominant migration followed by accumulation and then migration again produces a basal

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 the 

associated 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 of 

heavy minerals, the gold and uranium contents from adV'! »•*«•-«

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 the 

reef unit. The overlying trough crossbedded sets that are better 

preserved accumulated more rapidly with consequently less 

reworking and lower concentration of detrital heavy minerals. 

Free State Saaiplaas, Orange Free State Goldfield.

FIGURE 12.-Distal Steyn Reef, in which is illustrated a sequence of 

predominant migration followed by accumulation and then migration again, which has produced a basal concentrate in the reef unit 

and an upper layer where heavy minerals and small pebbles are 

concentrated.
PROTEROZOIC WITWATERSRAND PLACERS, PALEOCURRENT ANALYSIS OF THE VAAL REEF PLACER G9

jacent samples display a standard deviation of as much 

as 300 percent, and a single sample is seldom representative. This variance of mineral content is a result of the 

crossbedded geometry of distal placer sediment and may 

be thought of as a first order of variance.

A second order of mineral concentration variance, 

which is displayed by stretches of different concentration ranges, is attributed to changes of hydraulic energy 

level and subtle paleosurface topography. These 

stretches, in the case of the Vaal Reef, are oriented 

parallel to the braided channel pattern evident on the 

Vaal Reef paleosurface.

A third order of mineral concentration variance occurs on an areal scale and is apparently related to the 

overall geographic distribution of the deposit.

REFERENCES CITED

Alien, J. R. L., 1963, The classification of cross-stratified units: 

Sedimentology, v. 2, p. 93-114.

Minter, W. E. L., 1972, The sedimentology of the Vaal Reef in the 

Klerkdsdorp area: University of the Witwatersrand, unpublished 

Ph.D. thesis submitted to the Faculty of Science, 170 p.

Minter, W. E. L., 1976, Detrital gold, uranium, and pyrite concentrations related to sedimentology in the Precambrian Vaal Reef 

placer, Witwatersrand, South Africa: Economic Geology, v. 71, no. 

1, p. 157-176.

DISCUSSION FOLLOWING PAPERS BY 

DRS. VON BACKSTROM AND MINTER

Bourret: May I first of all compliment you on an outstanding paper. I've been in most of the mines you 

mentioned and I appreciate particularly what you 

have to say, and I think you've done a tremendous job.

Myers: Larry [Minter], could you explain a little bit more 

about the difference between the pi crossbedding and 

the trough crossbedding?

Minter: It's the same thing.

Myers: All right.

Minter: It's just that John Alien has classified crossbedding into 15 varieties, the reason being that there 

is a different mechanism for each variety. This is very 

important and particularly in this case because one 

needs a reworking process to create heavy-mineral 

concentrates and it's interesting to see that one has 

this process in the reef. It has never been emphasized 

before, but it occurs in all the Witwatersrand reefs. It 

also occurs in the Blind River deposits.

Charles Miles, Protocol Corp., Denver, Colo.: Dr. Minter, 

earlier we had a source of these gravels or placers sort 

of boxing the compass, and you have one in your first

paper that had to do with a horst shedding material on 

both sides. Do you attribute the gravels to localized 

sources like that?

Minter: In those Ventersdorp deposits in which we have 

alluvial fans, which I used in the example for the left 

part of the conceptual model, I think that the Witwatersrand has been reworked and that all the 

material in the Ventersdorp Contact Reef in those 

alluvial fans came from the horsts.

Miles: So that you had a sort of tectonic center.

Minter: That's a tectonic center and is applicable to the 

younger part of the Witwatersrand, in fact the 

Ventersdorp. We did not have that type of source, I 

think, in the older Witwatersrand.

Miles: That is, the horst contained gravels which on exposure were reworked and deposited in the 

Ventersdorp?

Minter: That's right.

Miles: Thank you.

Minter: Just one other reply to that question. I think 

Larry Schloss would call these younger deposits yoked 

deposits, whereas the others are intracratonic 

deposits in a very much gentler depression, as it were.

Larry Milliken, Public Service, Denver, Colo.: This 

perhaps should be directed to Dr. von Backstrom. You 

mentioned the presence of several refractory minerals 

that make up a fairly significant percentage of the 

heavy-mineral suite. Has any effort been made to extract uranium from these, or is all your uranium taken 

from uraninite?

von Backstrom: I wouldn't be able to answer this directly 

myself, but I wonder if Dr. Whiteside, who is here, 

could answer it. I know there was some uranium production from the Dominion Reef mine. Dr. Whiteside, 

do you know what they did in connection with any 

refractory minerals?

Whiteside: As far as I know, all that happened was that 

it messed up the mill circuit at the Western Reefs 

Mine and that we eventually gave up mining the Dominion Reef for that reason. Material from the Vaal Reef 

and material from the Dominion Reefs were antipathetic, rather.

Milliken: Thank you. Is any work being done at this time 

on the milling treatment of those refractories?

Whiteside: They are definitely working on the Dominion 

Reef, and they will put up a separate plant for it, so 

that would take care of the refractories. I think according to Dr. [S.H.U.] Bowie, the uraninite is not a 

true uraninite and he will concede that the uraninite in 

the Dominion Reef is detrital only because it is 

thorium-bearing uraninite.

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 PRECAMBRIAN 

QU A RT Z - P E B B L E CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-H

CONTENTS

	Page

Abstract _____________________________________________ HI

Introduction ___________________________________________ 1

Uranium distribution _______________________________________ 1

Conclusion_____________________________________________ 2

References cited _________________________________________ 2

Discussion _____________________________________________ 2

ILLUSTRATIONS

Page

FIGURE 1. Isopach map of the Main Bird Series, showing the relative position of the Vaal Reef in the Witwatersrand basin _______ H2

2. Location plan of the seven mining lease areas where the Vaal Reef is being exploited ______________________ 3

3. Stratigraphic column of Upper Witwatersrand Group in the Klerksdorp area ________________________— 4

4. Isopach plan of the MBS footwall sediments that the Vaal Reef truncated ______________________—__— 5

5. Moving-average plan of uranium content prepared from more than 900,000 samples of Vaal Reef _______________ 6

6. Moving-average isopach plan of the Vaal Reef prepared from more than 900,000 data points __________________ 7

7. Stratigraphic section along Vaal Reef paleostrike, illustrating the stratigraphic relation between the Zandpan Marker and

the Vaal Reef __________________________________________________________ 8

8. Isopach and paleocurrent plan of the sediment between the Vaal Reef and the Zandpan Marker, illustrating the Zandpan

Marker's onlap _________________________________________________________ 9

9. Moving-average isopach plan of the Vaal Reef basal carbon seam measured at 499 locations __________________ 10

10. Moving-average uranium-gold ratio plan calculated from moving averages of more than 900,000 samples of the Vaal Reef __ 11

in

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

PRIMARY DISPERSAL PATTERNS OF URANINITE IN THE PROTEROZOIC 

VAAL REEF PLACER DEPOSIT, WITWATERSRAND, SOUTH AFRICA

By W. E. L. MINTER l

ABSTRACT

The 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 patterns 

of uranium content match the braided, southeasterly dispersal pattern 

evident in the Vaal Reef. Deeper channelways contain more uraninite.

INTRODUCTION

Well, I've really primed you, and now I would like to 

relate the uranium distribution in the Vaal Reef to the 

sedimentary environment that I have painted.

The Vaal Reef is a distal Proterozoic placer deposit 

that is being mined for its gold, uranium, and pyrite content in the Klerksdorp Goldfield (fig. 1), on seven 

mining-lease areas (fig. 2). Stratigraphically situated 

near 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 of 

anastomosing braided river channels in a shallow fluviatile environment.

The ancient southeasterly dispersal pattern indicated 

by primary crossbedding structures within the sand 

body 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 were 

concentrated at the base of the sand body and on internal sedimentary partings. Fossil hydrocarbon, ranging 

from scattered fine-grained specks to a seam 5 cms 

thick, is generally evident on the basal contact and less 

commonly 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 distribution 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 meaningful contour plan (fig. 5). The southeast channel fabric 

is 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 external geometry of the sedimentary sheet. The 

southeasterly trend and braided nature of the reef are 

well defined. It is evident from a comparison of the 

uranium and the thickness contour plans that a sedimentary relationship exists-a thicker reef contains more 

uranium.

A Stratigraphically higher pebbly layer known as the 

Zandpan Marker onlaps the Vaal Reef (fig. 7) from the 

west, 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 basal 

hydrocarbon thickness, based on 499 samples, describes 

an area of thick hydrocarbon along the western side of 

the deposit that is a mile wide and extends for at least 

four miles down the paleoslope. This is interpreted as 

having been an area favourable for the growth and 

preservation 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 to 

the east where hydrocarbon is conspicuously sparse. The 

ratio between uranium and gold, calculated from the 

moving-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 produced from liquid hydrocarbon by irradiation from 

uranium nor that the uranium was precipitated from 

solution by carbon. Had either effect predominated, the

Hi
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 in

the Witwatersrand basin. Isopach contours in feet, contour interval 1,000 ft.

uranium content would have been preferentially greater 

in thick carbon-seam areas.

CONCLUSIONS

Uraninite is a detrital component of the Vaal Reef 

placer. The total uranium content is related to the 

thickness 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 sedimentation 

and 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 was 

made to the effect that uranium occurs in two forms in 

the 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 is 

perhaps in the structure itself, some as detrital grains
DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA

26°45'

H3

STILFONTEIN

HARTEBEESTFONTEIN ZANDPAN

BUFFELSFONTEIN

ELLATON

WESTERN REEFS

Y, VAAL REEFS

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

within the carbon. Certainly I am implying that the 80 

percent which is distributed through the reef and 

which isn't associated with carbon is presumably 

detrital.

Bourret: In all your sections you showed the mineralization of uranium pretty much terminating as you proceed to the southeast corner of the slides. Is this a 

structural feature or property lines?

Minter: Property lines. We've reached a depth of 

something like 7,000 feet and we are still going. But I 

can bring you up to date in a few years time. There's a 

very interesting point associated with the mineralization trends shown and that concerns application of 

sedimentology to mining. We have very broad trends 

and we can project them downdip, which also happens

to be down the paleodip, and consequently we can 

perhaps exercise grade control, predict grades, etc. 

With uranium it is quite easy because the concentration is relatively high. With gold it's more difficult, but 

that's certainly one of the economic applications we 

use.

Bourret: This does not mean, then, that this is the end of 

the economic mining depth?

Minter: No. That big black line at the bottom, which 

represents the Vaal River, seperates Vaal Reefs from 

Vaal Reefs South, which is an extensive new mine 

lease area just to the south that presumably has a life 

of 20 years. We keep these maps up to date on a computer program; data goes in monthly and we use it.
H4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

METERS 

2000

1500

1000

500

0 iVoal_ 

Reef

llLA JL

*•'.','

•!rrir--r

• - 0 0 „ .

00 0 « • •

^J—

VENTERSDORP GROUP

i

^

FORMATION N 

> Elsburg

Gold

Estates

.<

[Bird

^

Reef

^ivingstane 

Reef

> Main Reef

SUB GROUP ^

> Kimberley 

Elsburg

> Main Bird

a.

O 

a: 

o

o

z

a:

V)

WITWATEF

a:

Ul

a.

a.

JEPPESTOWN SHALE

FIGURE 3. -Stratigraphic column of Upper Witwatersrand Group 

in the Klerksdorp area.
DISPERSAL PATTERNS OF URANINITE, VAAL REEF DEPOSIT, WITWATERSRAND, SOUTH AFRICA

26°45'

H5

Isopach contours

in feet 

Braided 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

_____________________________ 26°45'

0 2 4KM

EXPLANATION 

I Low content

*K':-:i a

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 H7

26°45'

EXPLANATION

< 13 CENTIMETERS

FIGURE 6. -Moving-average isopach plan of the Vaal Reef prepared from more than 900,000 data points.
H8 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

ZANDPAN MARKER 

VAAL REEF*

METERS

20

-10

-0

-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 of

Gold in Solution in the

Witwatersrand, Ventersdorp, and Transvaal Systems

By H. C. M. WHITESIDE

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-1

CONTENTS

Page

Abstract ________________________________________________ II

Introduction _______________________________________________ 1

Heavy metals in the Black Reef, East Rand ____________________________ 1

Gold in an amygdale in Ventersdorp lava _______________________________ 2

Recent alluvials associated with the Witwatersrand basin _____________________ 3

Selected references ___________________________________________ 3

Discussion _________________________________________________ 4

ill

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

EXAMPLES OF POSSIBLE MOVEMENT OF GOLD IN SOLUTION IN THE 

WITWATERSRAND, VENTERSDORP, AND TRANSVAAL SYSTEMS

By H. C. M. WHITESIDE l

ABSTRACT

Many South African geologists consider that gold especially, and 

probably 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 of

gold and its redeposition elsewhere may have played a part in its 

enrichment or impoverishment. Two examples of possible solution of 

gold are cited, and the lack of modern placers associated with 

auriferous Witwatersrand is briefly discussed.

INTRODUCTION

It is generally considered that gold, and probably 

uranium as well, has been concentrated by hydraulic 

processes 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 and 

sand grains. The gold and uranium deposits of the Witwatersrand 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 that 

gold 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 ground 

waters may have taken gold and uranium into solution 

and modified their concentration and distribution. This 

does not mean that gold and uranium are not predominantly detrital, merely that there may have been movement 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 of 

solution of gold and, in one of these cases, redeposition 

in a new environment. The lack of modern placers associated with erosion of the auriferous Precambrian conglomerates in and around the Witwatersrand basin is 

also 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, is 

developed 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 northern 

part of the Witwatersrand basin, usually close to or 

overlying the edge of that basin. The majority of the 

orebodies are restricted in size, and mining was on a 

small scale compared with those mines that worked the 

Witwatersrand or Ventersdorp conglomerates. From 

descriptions in the literature it would appear that most 

of the Black Reef deposits were infillings of scour channels, composed of coarse, fairly thick conglomerates extending for only a few hundred metres in width and 

200-300 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 erosion 

of the underlying rocks in the vicinity and subsequent 

concentration of the heavy minerals by sedimentary processes. This can be said to have been adequately demonstrated in some instances, particularly regarding the 

two largest known deposits on Government Gold Mining 

Areas and Randfontein Estates, where auriferous Witwatersrand conglomerates form the immediate footwall 

or are very close by. A rough analysis shows that the

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

footwall of the Black Reef where it has been mined is as 

follows, in percent:

Ventersdorp:

Lavas _______________________________ 47 

Sediments and tuffs _______________________ 13

Witwatersrand:

Upper _______________________________ 20 

Lower _______________________________ 7

Basement, essentially granite ___________________ 13

This tabulation seems to show that auriferous Black 

Reef may be deposited some distance away from the 

original 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, and 

a 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, the 

osmiridium in the Black Reef was derived from the Kimberley Series, but it is difficult to account for the 

osmiridium in areas where the footwall is granite or 

Ventersdorp lavas and sediments. One explanation is 

that heavy metals such as gold and osmiridium can be 

transported in their metallic state farther than is at 

present thought possible, although the fact that concentrations of gold are found only near the edge of the 

Black Reef basin appears to contradict this. A second 

explanation is that there is a local but concealed source 

of these metals.

More than 25,000,000 metric tons of Black Reef ore 

was mined on Government Gold Mining Areas, in addition 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. Four 

metals 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 is 

believed 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 the 

Kimberley 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 of 

these 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 accepted that the silver content of detrital gold is 

less than that of vein gold, some of the silver being dissolved during transportation. If the Black 

Reef gold has been derived from older conglomerates 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 existing 

sources such as the Witwatersrand succession, 

the Older Granite, and perhaps the Ventersdorp 

sediments.

2. Sulphur-bearing waters were introduced after deposition 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 well 

as small banded ironstone pebbles, would be 

altered to pyrite nodules.

3. If the gold in the Black Reef contains more silver 

than normal it suggests that silver has been added or that gold has been leached. During deposition by sedimentary processes gold and osmiridium should have retained a ratio similar to that 

of the source rock, in this case, Witwatersrand 

conglomerates and quartzites. This is not so 

because the gold:osmiridium ratio is lower in the 

Black Reef than in nearly all Witwatersrand conglomerates. This must mean that either additional osmiridium was introduced or gold was 

reduced in quantity. These conclusions suggest 

that gold was leached penecontemporaneously 

with the deposition of the Black Reef.

GOLD IN AN AMYGDALE IN VENTERSDORP 

LAVA

This note is merely to record that one grain of gold 

was found in an amygdale in lava a few inches, no more 

than 15 cm, above the contact of lava with the top of the
MOVEMENT OF GOLD IN SOLUTION, THE WITWATERSRAND, VENTERSDORP, TRANSVAAL SYSTEMS 13

Ventersdorp Contact Reef. This occurrence was in core 

from 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 instance 

of 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 detrital 

grains. Occasionally, amygdales have been noted that 

contain pyrrhotite and chalcopyrite.

It is considered that the gold in the amygdale was 

taken into solution by heated waters from the Ventersdorp 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 observations 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 indistinguisable 

from 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 development 

of auriferous alluvials related to present-day 

topography.

There are numerous examples of erosion of auriferous 

beds and redeposition of the various constituents, such 

as pebbles and heavy minerals, throughout the succession from the Witwatersrand to the Black Reef Series. 

The U.K.9 Kimberley Reef has been shown to contain 

more gold downstream from areas where the middle 

Kimberley has been truncated. The Ventersdorp Contact Reef clearly derives some of its gold from erosion of 

underlying auriferous conglomerates of the Upper Witwatersrand. 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, and 

Kimberley groups of conglomerates where the latter 

crop 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 concentration 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 stripped 

from the Central Witwatersrand area and the Upper

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 more 

gold 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 the 

reason why no alluvials were formed?

SELECTED REFERENCES

At the meeting held in Golden, Colo., in October 1975, 

several of the geologists who attended expressed the 

view that there was insufficient description of the 

general background of the Witwatersrand System and 

other Precambrian Systems that are hosts to auriferous 

and uraniferous quartz-pebble conglomerates.

The following papers are thought to cover the geology 

of 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 of 

South 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 Witwatersrand System, and their bearing on reef correlation, with 

particular reference to the East, Central and West Witwatersrand: Geological Society of South Africa Transactions 68, p. 

121-142.

de Kock, W. P., 1964, The geology and economic significance of the 

West 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 of 

South 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 Witwatersrand basin with special reference to its occurrence at Government 

Gold Mining Areas, in Haughton, S. H., ed., The geology of some 

ore 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 in 

southern 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 the 

Orange Free State Goldfield, in Haughton, S. H., ed., The geology 

of some ore deposits in southern Africa: Geological Society of 

South Africa, v. 1, p. 417-448.
14 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Winter, H. de laR., 1964b, the geology of the Virginia section of the 

Orange Free State Goldfields, in Hqughton, S. H., ed., The 

geology of some ore deposits in southern Africa: Geological Society 

of South Africa, v. 1, p. 507-548.

DISCUSSION

[As an aside in his talk Dr. Whiteside gave the following 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 we 

have 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 is 

possible that at the site of deposition very small particles of gold can be partially mobilized by the organic 

compounds present, giving the impression of dissolution. 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 gold 

in 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 came 

out June 26, 1975, in which we described two cyclic 

ethers from a sample of the Transvaal. Well, for the 

chemists' 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 of 

molecular ions of various compounds: oxygen, 

hydrogen, nitrogen, and hydrocarbon compounds 

ranging from very low molecular weight to very high 

molecular weight. We did not find any graphite whatsoever. 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 concentrated 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, discuss 

transportation of gold and of silica in an ordinary 

brine; transportation is small but significant. It is not 

zero. If you have a brine and have gold present, that 

brine is going to be saturated with gold. It is impossible 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 with 

it.

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 or 

arsenide 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 except that it was gold. Sorry.

Feather: Do we still have the sample at the research 

laboratory?

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 in 

fact also analyze for the silver content of the gold. 

That would be interesting, too.

Whiteside: Yes, it would be. I should think they could do 

that with a microprobe.

Wilfried Meyers, Falconbridge Nickel Mines, Ltd., 

Canada: I'd like to ask you more about the absence of 

magnetite throughout the Witwatersrand. This is 

something that apparently also holds true for the 

Blind River area and other pyritic conglomerates in 

Canada and other parts of the world. What happened 

to the magnetite? It must have been there originally 

with a vast granite source terrane. Where has it all 

gone 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 and 

cranny within that vast sedimentary pile to change 

every grain of magnetite or hematite that may have 

been 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 and 

gold 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 

place it has ever been identified has been in pebbles, 

not in the matrix. Occasionally I believe that you do 

get small grains of magnetite in the iron-bearing rocks 

as pebbles. And another thing, incidentally, you get 

very little ilmenite, but you do get a lot of leucoxene, 

and that's almost spherical. Say, I'm talking out of 

turn. I am not a mineralogist. Sorry, but I don't know 

where the sulfur comes from, but there are, as I say, 

various stages roughly in the Witwatersrand where 

you could bring in sulfur. We have volcanicity in the 

Dominion Reef, in the Witwatersrand in two places,

and also an enormous amount in the Ventersdorp. Going further, you've got the black carbon of the 

dolomite, and then finally you've got the Bushveld 

Complex, which presumably could have brought in 

quite a lot of sulfur.

Schidlowski: Just a very quick comment on the sulfur 

budget. If you convert part of the pyrite into pyrrhotite you get a sulfur surplus which can be used for 

sulfidation processes. In fact, conversion of pyrite into 

pyrrhotite has obviously occurred during the pseudohydrothermal reconstitution of the reef; that has been 

observed in several places.

Microprobe Analyses of 

Uranium and Thorium in Uraninite from 

the Witwatersrand, South Africa, and 

Blind River, Ontario, Canada

By D. E. GRANDSTAFF

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1 1 6 1 -J

CONTENTS

Abstract _________________________________________________ Jl

Introduction _______________________________________________ 1

Material _________________________________________________ 1

Analyses _________________________________________________ 1

Results __________________________________________________ 1

Discussion ________________________________________________ 2

Acknowledgments ____________________________________________ 4

References cited ____________________________________________ 4

Discussion ________________________________________________ 4

ILLUSTRATIONS

Page

FIGURES 1-4. Graphs of:

1. Representative step scans across three uraninite grains _________________________________ J2

2. U02 and Th02 content of uraninite grains ________________________________________ 3 

3, 4. Plots of U02 /Th02 against Th02 in uraninite grains from:

3. The Vaal Reef West mine, Witwatersrand, South Africa ____________________________ 3

4. The Rio Algom Nordic Mine, Blind River, Ontario, Canada ___________________________ 3

in

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

MICROPROBE ANALYSES OF URANIUM AND THORIUM IN URANINITE 

FROM THE WITWATERSRAND, SOUTH AFRICA, AND BLIND RIVER,

ONTARIO, CANADA

By D. E. GRANDSTAFF

ABSTRACT

Microprobe analyses of uranium and thorum in uraninite grains from 

the Witwatersrand, South Africa, and Blind River, Ontario, reveal 

that although individual grains are fairly homogeneous, the 

assemblage of grains is quite heterogeneous. This heterogeneity appears to favor genetic concepts advocating a detrital, placer origin for 

the uraninite.

INTRODUCTION

The bulk chemical compositions of uraninite concentrates 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 to 

their uranium and thorium content. The U02/Th02ratio 

of the Blind River concentrates varies from approximately 8 to 12 and averages about 10 (Roscoe, 1979). In 

contrast, analyses of concentrates from the Witwatersrand basin reveal a wider range of composition. The 

U02/Th02 ratio varies from approximately 10 to 40 as a 

result of the variation in Th02 content, which ranges 

from 1.5 to 6.5 percent. However, very little has been 

reported on the composition of individual uraninite 

grains (Schidlowski, 1966) or variations within or between grains. This paper reports the results of analyses 

made by electron microprobe of uranium and thorium in 

individual uraninite grains.

MATERIAL

Two samples were examined in this study, a sample of 

very high grade uranium ore from the Rio Algom Nordic 

Mine, Elliot Lake, Ontario, Canada, and a lower grade 

ore sample from the Vaal Reef West mine, Klerksdorp 

area, Transvaal, South Africa, from the upper division 

of the Witwatersrand System.

Department of Geology, Temple University, Philadelphia, Pa. 19122.

The Canadian sample was uncommonly rich in 

uraninite, and a single polished surface exposed a large 

number of uraninite grains. All the grains analyzed from 

this 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 approximately 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 solution, and the Frantz isodynamic separator. A polished 

section grain mount was made of the separated grains. 

Microscopic examination revealed that in some cases the 

uraninite 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.

ANALYSES

The analyses were made using an ARL-EMX2 

microprobe. The beam diameter was approximately 1 ^, 

the sample current was 0.05 ^ A and the accelerating 

voltage was 15 kV. The data obtained were corrected for 

drift (which was negligible), deadtime, and background. 

Artificial U02 and Th02 crystals were used as standards.

RESULTS

Results of analyses for uranium and thorium are 

shown in figures 1-4.

Step scans for uranium and thorium were made across 

several grains. Representative results are shown in

2 Any use of trade names and trademarks in this publication is for descriptive purposes only 

and does not constitute endorsement by the U.S. Geological Survey.

Jl
J2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

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LU 

LU

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10

8

6

GRAIN 2

10

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GRAIN 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 examined 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 less 

sensitive than is use of step scans.

Quantitative analyses of spots in the uraninite grains 

are shown in figure 2. Quantitative analyses were made 

in 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 the 

Blind 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 approximately 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 variation of a minor or trace element (thorium) in a matrix of 

a major, relatively invarient element (uranium). The

trend is particularly clear in the Witwatersrand sample, 

owing to the wide variation in grain compositions, but is 

also apparent in the Blind River sample.

It is possible that some scatter of the data may be due 

to the presence of inclusions of galena or silicates in the 

region 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 generate 

apparent 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, these 

mechanisms are not considered important in causing the 

scatter of data shown in figure 2.

DISCUSSION

The origin of the Blind River and Witwatersrand 

uraniferous conglomeratic deposits has been extensively 

discussed. A variety of stratigraphic, geochronological,
URANIUM, THORIUM IN URANINITE, WITWATERSRAND, SOUTH AFRICA BLIND RIVER CANADA J3

IN PERCENT

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Th02, IN PERCENT

FIGURE 2.-U02 and Th02 content of uraninite grains from the Rio 

Algom Nordic Mine, Blind River, Ontario (closed circles), and from 

the Vaal Reef West Mine, Witwatersrand, South Africa (open 

circles).

geochemical, and sedimentological evidence has been 

brought 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 from 

pyrogenetic 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 from 

within 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 was 

heterogeneous over short distances. Such conditions are 

unlikely. Any movement of a heterogeneous mineralizing fluid through the rock might be expected to produce

90

80 -

70

60

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§ 40

30

20

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Th02 , IN PERCENT

10 12

FIGURE 3. -Plot of U02/Th02 against Th02 in uraninite grains from the 

Vaal Reef West mine, Witwatersrand, South Africa.

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14

12

10

8

6

oo o

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ThO2 , IN PERCENT

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 

observed in this study. Nor does it seem likely that a 

later thermal event could have erased any zoning. The 

Witwatersrand sediments are regionally metamorphosed only to the greenschist facies (Fuller, 1958), 

while the Blind River sediments are virtually 

unmetamorphosed (Roscoe, 1969). Therefore, the 

chemical variability appears more consistent with the 

hypothesis that pyrogenetic uraninite grains of varied 

compositions were gathered from different localities 

during erosion and this chemically heterogeneous mixture was transported and deposited as a mechanical 

mixture.

The difference in variability of the composition of the 

two samples may yield some evidence concerning their 

source areas. The uranite from the Blind River sample 

has much smaller variability than the. Witwatersrand 

sample. This may indicate that the uraninite in the Blind 

River sample was derived from a single relatively 

homogeneous source area, whereas the uraninite in the 

Witwatersrand sample had multiple sources.

Such possibilities have already been suggested. Fuller 

(1960) and Hiemstra (1968) have suggested that the sediment may have been derived from different source 

areas. Viljoen and others (1970) have argued for a 

heterogeneous source for the Witwatersrand basin 

uraninite in order to account for the variation of thorium 

content between the Dominion Reef and Witwatersrand 

Series uraninites. Variations of U02 and Th02 in 

samples from many parts of the Witwatersrand basin 

should be investigated to study the possibility of multiple or heterogeneous sources for the uraninite.

ACKNOWLEDGEMENTS

I would like to thank Dr. E. S. A. Antrobus and the 

Anglo-American Mining Corp. and the Rio-Algom Mining Corp. for providing the samples, and Mr. C. Kulik 

for aid with the microprobe analysis. This work was 

undertaken as a part of a doctoral dissertation at 

Princeton University. The work was supported by 

NDEA grants under Title IV, and by grants to the 

Princeton University computer center.

I would also like to thank the Geological Society of 

South Africa for permission to have this article 

republished. It appeared first in the Transactions of the 

Geological Society of South Africa in 1975, v. 77, p. 

291-294. The U.S. Geological Survey also thanks the 

Geological Society of South Africa for permission to 

republish this paper.

REFERENCES CITED

Frondel, Clifford, 1958 [1959], Systematic mineralogy of uranium and 

thorium: U.S. Geological Survey Bulletin 1064, 44 p.

Fuller, A. 0., 1958, A contribution to the petrology of the Witwatersrand System: Geological Society of South Africa Transactions, v. 

61, p. 20-50.

———1960, Distribution of Witwatersrand uraninite: Economic 

Geology, v. 55, p. 842-843.

Hiemstra, S. A., 1968, The mineralogy and petrology of the 

uraniferous conglomerate of the Dominion Reef mine, Klerksdorp 

area: Geological Society of South Africa Transactions, v. 71, p. 

1-66.

Liebenberg, W. R., 1955, The occurence and origin of gold and radioactive minerals in the Witwatersrand System, the Dominion Reef, 

the Ventersdorp Contact Reef, and the Black Reef [with discussion]: Geological Society of South Africa Transactions, p. 101-254.

Ramdohr, Paul, 1969, The ore minerals and their intergrowths 

[translation of 3d ed.]: Oxford, Permagon Press, 1,174 p.

Roscoe, S. M., 1969, Huronian rocks and uraniferous conglomerates in 

the Canadian Shield: Canada Geological Survey Paper 68-40, 205

PSchidlowski, Manfred, 1966, Beitfage zur Kenntnis der radioaktiven 

Bestandteile der Witwatersrand-Konglomerate, I., Uranpecherz in 

den Konglomeraten des Oranje-Freistaat-Goldfeldes: Neues 

Jahrbuch fur Mineralogie Abhandlungen, v." 105, p. 183-202.

Viljoen, R. P., Saager, Rudolf, and Viljoen, M J., 1970, Some thoughts 

on the origin and processes responsible for the concentration of 

gold in the early Precambrian of Southern-Africa: Mineralium 

Deposita, v. 5, p.164-180.

DISCUSSION

Skinner: Perhaps this is not a fair question for you, 

David, but one of the South Africans may step in. If 

the Witwatersrand can be considered to be a large 

sample of the detrital minerals from the crust and if 

the crustal abundance figures for thorium and 

uranium are in fact correct, showing that thorium is 

four or five times more abundant than uranium, where 

is all the thorium?

Grandstaff: I don't think that, in fact, the Witwatersrand represents a major sample of the crust. If you 

plot up analyses of pegmatitic uraninites from the 

various parts of the world, they scatter quite widely in 

uranium-to-thorium ratio. I think we are really looking 

at an individual chance segregation of uranium over 

thorium.

Skinner: Then would it be a correct conclusion to draw 

that if there is another hidden pebble conglomerate 

somewhere the chances are it is a thorium rich one, 

not a uranium rich one?

Grandstaff: Could well be, yes.

Robertson: In comment to Dr. Skinner's question, 

perhaps one should point out that certainly in the 

Blind River ores monazite is a fairly significant component.

Roscoe: You could add to that that we do get conglomerates in the Blind River ores whch have very 

high thorium-uranium ratios, 4 to 1, and they appear 

to complement the high uraninite ones. The concept is, 

of course, that this is hydraulic differentiation.
URANIUM, THORIUM IN URANINITE, WITWATERSRAND, SOUTH AFRICA BLIND RIVER CANADA J5

Grandstaff: Those high thorium concentrations that 

you pointed out are largely monazite-zircon concentrates rather than uraninite concentrates.

Simpson: Did you analyze any uraninite within carbon? 

Our analyses show that within a given sample, this 

type of uraninite is generally thorium poor and 

homogeneous.

Grandstaff: I analyzed only a couple of grains from 

carbon leaders, and they contained several percent 

thorium and were relatively homogeneous.

Feather: I would like to comment on your question, 

Peter [Simpson]. I have analyzed a great many 

uraninite 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 of 

thorium content in uraninite grains included in carbon 

as I have found in uraninite grains that were not 

associated with the carbon.

Grandstaff: I analyzed one thucholite uraninite grain 

which had about 8 to 10 percent thorium. They do get 

that high.

Preliminary Notes Concerning the Uranium-Gold Ratio and 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, Wit water sr and, South Africa

By W. E. L. MINTER

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-K

CONTENTS

ILLUSTRATIONS

Page

Abstract ________________________________________________ Kl

Introduction _______________________________________________ 1

Particle-size changes in the Steyn Reef _______________________________ 1

Mineral concentration changes in the Steyn Reef _________________________ 1

Conclusions ________________________________________________ 2

References cited ____________________________________________ 2

Page

FIGURE 1. Photographs of samples of Steyn Reef at 5-kilometer intervals down its paleoslope ________________________ K2 

2. Graphic plot illustrating progressive changes of particle size and uranium-gold ratio down the Steyn Reef paleoslope _____ 3

in

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

PRELIMINARY NOTES CONCERNING THE URANIUM-GOLD RATIO

AND 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. MlNTER1

ABSTRACT

The 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 their 

detrital origin. An increase in the ratio of uranium to gold, which appears to be related to their original size-frequency distribution, also indicates 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), was 

substantiated by Viljoen (1963), who found that the 

equivalence also applied to pyrite and zircon grains and 

to a number of other minerals. These were consequently 

considered to have been detrital components of the 

placers. 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 the 

hydraulics 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 upon 

practically 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 opportunity to study size changes of detrital minerals over a 

wide 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 the 

smoothed 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 that 

zircon 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 when 

more 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 the 

overall gold content decreases while the overall uranium 

content increases. This is illustrated in figure 2 by a plot 

of the ratio of uranium to gold, the concentration of both 

having been measured in parts per million. Isopleths beKl
K2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 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.

ing constructed substantiate the paleostrike obtained 

from crossbedding data.

High uranium-gold ratios appear to characterize the 

small-pebble distal environments of all Witwatersrand 

carbon-seam reefs, and low ratios, those of proximal, 

banket reefs (Minter, 1972). Zircon and uraninite are 

considered 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 detrital 

nature of the mineralization.

REFERENCES CITED

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 the 

upper division of the Witwatersrand system on President Steyn 

Mine and adjacent areas in the Orange Free State Goldfield, with 

special 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 the 

Main Reef and Main Reef Leader of the Main-Bird Series, Witwatersrand system: University of the Witwatersrand, unpublished 

M. Sc. thesis, 193 p.
URANIUM-GOLD RATIO, HEAVY-MINERAL DISTRIBUTION, STEYN REEF DEPOSIT, SOUTH AFRICA K3

a. 30 

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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 of

the Witwatersrand Goldfields, South Africa

By RUDOLF SAAGER

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-L

CONTENTS

	Page

Abstract _______________________________________________________ LI

Introduction and geological setting _________________________________ 1

Pyrite mineralogy ___________________________________________ 4

Witwatersrand Basin _______________________________________ 4

Archean greenstone terranes __________________________________ 5

Sample preparation __________________________________________ 6

Analytical procedure _________________________________________ 8

Discussion of results __________________________________________ 9

Factor analysis ____________________________________________ 11

Lead-isotope investigation ______________________________________ 13

Summary and conclusions ______________________________________ 15

Acknowledgments __________________________________________ 16

References cited ____________________________________________ 16

ILLUSTRATIONS

FIGURES 1,2. Maps showing:

1. Main points of entry and general transport directions of sedimentary material into the Witwatersrand Basin ____ L2

2. General geological setting of the Witwatersrand Basin and the major Archean greenstone belts of the Kaapvaal

Craton in the southern African Shield __________________________________________ 3

3. Photomicrographs of ore samples showing examples of different pyrite structures in the Witwatersrand reefs _______ 6

4. Electron-microprobe step-scanning profiles across a zoned pyrite from the Eagles Nest gold mine in the Barberton Mountain Land ______________________________________________________________ 9

5. Graphs showing cumulative frequency distributions of trace-element values from the grouped pyrite samples and graphical

solutions of Kolmogorov-Smirnov statistic ___________________________________________ 10

6. Scatter diagram of the factor loading values ____________________________________________ 13

7. Diagram showing 207Pb/204Pb versus 206Pb/204Pb __________________________________________ 14

TABLES

Page

TABLE 1. The relative abundance of heavy minerals in the Witwatersrand sediments and their corresponding erosion units in the

model source area of the Barberton Mountain Land ______________________________________ L4

2. Different pyrite structures in Witwatersrand reefs __________________________________________ 5

3. Grouped results of trace-element analyses ______________________________________________ 9

4. Results of Kolmogorov-Smirnov statistic _______________________________________________ 11

5. Varimax factor score matrix ______________________________________________________ 12

6. Partial Q-mode varimax factor matrix ________________________________________________ 12

111

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

GEOCHEMICAL STUDIES ON THE ORIGIN OF THE DETRITAL PYRITES 

IN THE CONGLOMERATES OF THE WITWATERSRAND GOLDFIELDS,

SOUTH AFRICA

By RUDOLF SAAGER1

ABSTRACT

The principal source area of the Witwatersrand sediments, including 

heavy minerals such as gold, uraninite, and pyrite, lies to the west and 

north of the basin of deposition. Because this area is overlain by 

younger rocks, the actual identification of the source area is virtually 

impossible. On the basis of a hypothetical provenance model it has been 

suggested earlier that the primary source of the sediments was in the 

Archean granite-greenstone terrane of the Kaapvaal Craton.

For the present study, 57 pyrite samples from primary gold deposits 

of the greenstone terrane (Swaziland Sequence) and from various gold 

reefs of the Witwatersrand deposit have been investigated for their 

trace-element content (Co, Ni, Cu, Zn, Pb, Ag) in order to study the 

possible presence of similar geochemical trends within both groups of 

pyrite. The analytical data have been treated by statistical methods, 

and the results indicate that both the primary and the detrital Witwatersrand pyrites, with respect to the investigated elements, were 

drawn from the same population. They are both clearly different from 

the Witwatersrand pyrites formed in situ. Lead isotope investigations 

carried out on sulfides from primary gold deposits of the Swaziland Sequence and on detrital and authigenic pyrites from the Witwatersrand 

deposit 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 the 

gold in the sediments of the Witwatersrand Basin.

INTRODUCTION AND GEOLOGICAL SETTING

For many placer deposits the question of the original 

source of the heavy minerals is an intriguing and often 

hotly debated subject of discussion. The solution of the 

problem is not only of great academic interest but in 

many instances has proved to be of considerable 

economic importance.

In the case of the early Precambrian fossil placers of 

the Witwatersrand Basin in South Africa, the problem 

of the provenance of the gold, uraninite, and other 

detrital conglomerate constituents has been overshadowed for a long time by the metallogenetic controversy between the hydrothermalists and the 

placerists (see for example Becker, 1909; Mellor, 1916; 

Young, 1917; Graton, 1930; Liebenberg, 1955; Ramdohr, 1955; Davidson, 1957, 1960, 1961). The very common criterion, "Gold is where you find it," and the preoccupation of the mine geologists on the Rand with the 

daily 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, and 

mineralogical studies carried out during the last twenty 

years led to the general acceptance of the modified 

placer theory as a metallogenetic concept for the Witwatersrand deposit. The concept implies that, after 

their deposition, some of the detrital minerals, notably 

gold and the sulfides, were reconstituted and lost their 

waterworn shapes as a result of the low-grade regional 

metamorphic overprint of the embedding Witwatersrand and Dominion Reef sediments.

Today most investigators of the Witwatersrand 

deposit agree that the sediments and heavy minerals 

originated essentially from a source area situated to the 

north and west of the basin of deposition, with only 

minor amounts of sedimentary material being introduced from the southeastern edge of the Witwatersrand 

Basin (Pretorius, 1974). Some of the evidence for this 

conclusion is found in the conspicuous occurrence of all 

the producing goldfields along the northwestern arc of 

the Witwatersrand Basin, in the asymmetry of the gradients of the paleoslopes, which are considerably steeper 

on the northwestern side of the basin, and in the distinct 

decrease of conglomerate thickness and pebble size in a 

southerly to southeasterly direction. Pretorius (1966, 

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

Ll
Main 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).

L2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

characterized by ultramafic, mafic, and acid volcanics 

(Viljoen and Viljoen, 1969; Anhaeusser, 1971). These 

rocks 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 with 

other greenstone belts in the Kaapvaal Craton, the 

Barberton Mountain Land is well mineralized and 

forms, next to the Witwatersrand Basin, the most important gold province in South Africa (fig. 2).

Most of the gold occurrences in the Barberton Mountain Land are located along a stratigraphic horizon in 

basic 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) showed 

that 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 observed 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 and 

others (1969) and Saager (1973a, b) concluded that the 

gold deposits in the greenstone belts are genetically 

related 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 the 

greenstone belts of the Kaapvaal Craton are given by 

Viljoen and others (1969) and Saager (1973b).

The suitability of the Barberton Mountain Land as a 

model source terrane for the sedimentary filling of the 

Witwatersrand Basin was demonstrated by Viljoen and 

others (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 Witwatersrand sediments. The results of their study are 

summarized in table 1. It demonstrates the relative 

abundance of uraninite, monazite, cassiterite, and 

garnet in the lower portions of the sedimentary succession, especially in the conglomerates of the Dominion 

Reef Sequence. Chromite and the platinoids show a 

distinct increase towards the topmost members of the 

sediments, and the gold, as well as the sulfides, reaches 

a maximum in the Main Bird Series of the Upper Witwatersrand 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.

of the basin of deposition. The entry points of sedimentary material into the basin, corresponding to the apices 

of the fans, thus must have remained relatively fixed 

during a considerably long period of time-that is, from 

Dominion Reef till Upper Witwatersrand times.

To pinpoint the actual provenance area of the 

sediments and the placer minerals and thus to locate the 

primary 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 the 

Johannesburg Dome, Westerdam Dome, and Amalia 

Dome, show only rock types similar to those found in the 

Archean 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 the 

basement of the southern African Kaapvaal Craton, suggested that the primary source of the Witwatersrand 

gold and detrital sulfides should be sought in the Archean greenstone belts of the craton. As a hypothetical 

model 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 members 

of the Swaziland Sequence, which in the Barberton area 

are fully preserved, constitute the Onverwacht Group,
DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA L3

EXPLANATION

Greenstone 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 Murchison Range

S Sutherland Range

P Mount Mare

/"Swazii I 

S land fH

Indian 

Ocean

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 and 

northern rim of the Witwatersrand Basin are marked with a heavy black line (modified from Viljoen and others, 1970, 

fig. 1).
L4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

TABLE I.-The relative abundance of some heavy minerals in the 

rudaceous sediments of the Witwatersrand Basin and their corresponding 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

Detrital minerals __Main rock types in the 

P model provenance terrane (Barberton Mountain Land), in reverse 

Stratigraphic sequence

Kimberley-Elsburg 

Series.

Main-Bird Series

Lower Witwatersrand 

Sequence.

Dominion Reef 

Sequence.

TT

TTT ttt TTT TTT

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, and 

felsic lavas.

Felsic to mafic pyroclasts. 

Banded chert.

Low-volatile tonalitic 

granite.

Some high-volatile

granite (root zone). 

TT T T f III. Argillaceous Unit

(Fiy Tree Group):

Shales, greywackes, 

banded ironstone.

Low-volatile tonalitic 

granite.

High-volatile granite

(root zone). 

tt T T T IV. Arenaceous Unit

(Moodifx Group):

Quartzites, conglomerates.

High-volatile "hood" 

granite.

Pegmatites.

The present geochemical investigation was undertaken to further test the source hypothesis of Viljoen 

and others (1970). The study was restricted to pyrite 

because it forms the most abundant ore mineral in the 

primary gold ores of the Archean greenstone belts and 

in the Witwatersrand conglomerates. It was carried out, 

first, by a trace-element investigation of pyrites from 

the two gold provinces and, second, by a comparison of 

the lead-isotope composition of Witwatersrand pyrites 

with that of pyrites and lead-bearing sulfides associated 

with various gold deposits in the Barberton Mountain 

Land.

PYRITE MINERALOGY

WITWATERSRAND BASIN

In the Witwatersrand ores pyrite generally constitutes more than 90 percent of the ore minerals present and appears to be the only macroscopically identifiable ore mineral (Saager, 1970). It generally occurs 

disseminated throughout the entire width of individual 

ore horizons but in many places is concentrated, 

together with other detrital ore minerals (such as gold,

uraninite, chromite), in false footwalls and in footwall 

contacts. 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 considered 

them 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 can 

be differentiated into the three following groups:

1. Rounded allogenic pyrites form the bulk of the 

rounded grains. They are exceptionally 

unweathered, are generally less than 0.2 mm in 

size, and acquired their roundness by abrasion during sedimentary transport (fig. 3A-D). Besides their 

waterworn shapes, occasional primary inclusions of 

silver-rich gold particles, possessing a higher reflectivity 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 mineralizations 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 

distinctly rounded outlines and are very well 

preserved (figs. 3E-I). On account of their loose 

structures, long transportation distances seem to 

be improbable, and Ramdohr (1955) and Saager 

(1970) have suggested that the concretions were 

formed in situ. Skeletal-type growth structures, 

such as delicate axial crosses, have been found in 

practically all pyrite concretions and are characteristic features. Saager and Mihalik (1967) observed that the concretionary pyrite aggregates 

very often consist of an isotropic and an anisotropic 

variety, which differs markedly in colour, reflectivity, hardness, and trace content of nickel, cobalt, 

and arsenic. The pores in the marginal area of the 

concretionary pyrite grains sometimes contain 

numerous fillings of gold, chalcopyrite, and galena. 

This indicates that the concretions acted as traps 

for metamorphically mobilized ore minerals, which, 

after deposition of the concretionary grains, infiltrated the pores where they now form inclusions. 

In rounded allogenic pyrite grains, inclusions of 

galena are practically absent.
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 Structures Allogenic Authigenic

Rounded Compact or porous 

Pseudomorphic ___ 

Concretionary ____

Detrital pyrite and black sands 

pyritized before their final 

deposition.

Inhomogeneous black sands pyritized before their final deposition.

Idiomorphic to hypidiomorphic _

Nodules containing "mineralized

bacteria." 

Compact or porous ___________

Compact or porous encrustions _

Xenomorphic ___________ Veinlets and fracture filling.

Black sands pyritized in situ.

Inhomogeneous black sands pyritized in situ.

Generally formed in situ; very 

rarely signs of short transportation.

Formed in situ; indicating presence of primitive life.

Formed in situ by reconstitution 

of 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.

Rounded pyrite nodules containing microspherical structures of the "mineralized bacteria type" are 

closely related to the concretionary pyrites. The 

microspheres reach diameters of 10 to 15 m (micrometers) and, in rare cases, have been observed to be 

as large as 45 m. In most cases, the structures are 

irregularly distributed in the nodules (figs. 3G, H). 

Saager (1970) has suggested that these 

microspheres may represent organic morphologies. 

He concluded that the concretionary pyrites and 

rounded pyrite nodules, as products of precipitation, were formed in situ by the activity of 

anaerobic H2S-generating, sulfate-reducing 

microorganisms, which preferably existed in small 

pocketlike areas in the Witwatersrand Basin. 

Reconstituted authigenic pyrites form idiomorphic to 

hypidiomorphic grains or occur as late encrustations on detrital components of the conglomerate, 

particularly on pyrite (see figs. 3 J, K). The pyrite 

encrustations lead directly to the common stringerlike pyrite veinlets that cut through detrital 

minerals and form fracture fillings in other 

minerals. This authigenic pyrite variety has been 

formed by recrystallization during the metamorphism of the Witwatersrand sediments (Saager, 

1970).

In certain places dikes intrusive into the Witwatersrand rocks have influenced the content of 

authigenic minerals and have led to the formation 

of enriched masses of pyrite, galena, chalcopyrite, 

and sphalerite. Such occurrences, however, are 

local and account only for a very small proportion of 

the authigenic idiomorphic to hypidiomorphic

pyrites and base-metal sulfides in the Witwatersrand ores (Saager, 1968).

ARCHEAN GREENSTONE TERRANES

In the primary gold mineralizations of the Archean 

greenstone belts of the Kaapvaal Craton, pyrite is 

generally the most abundant ore mineral and often 

forms the most important carrier of gold. In the goldvein deposits, generally two generations of pyrite have 

been observed (Saager, 1973b). The older generation 

forms large, usually idiomorphic grains measuring as 

much as 10 mm across. Inclusions of pyrrhotite and 

chalcopyrite are abundant; less common inclusions are 

gold, magnetite, rutile, and sphalerite. The younger 

pyrite generation generally exhibits pentagonaldodecahedral outlines, grain sizes being less than 100 /u.. 

It generally surrounds the pyrite of the first generation 

and rarely contains inclusions of other ore minerals.

A noticeable feature of many pyrites from the greenstone belts is the common occurrence of a distinct zonal 

structure caused by a variation in the trace-element content of nickel, cobalt, and arsenic (fig. 3L; see also Viljoen and others, 1969). Microprobe analyses of 

brownish-coloured, relatively soft, weakly anisotropic 

pyrite zones yielded nickel, cobalt, and arsenic contents 

of as much as 1.75 percent, 0.17 percent, and 0.24 percent, respectively (fig. 4). For more detailed descriptions 

of the mineralogy of the gold ores in the Archean greenstone belts, the reader is referred to de Villiers (1957), 

Schweigart and Liebenberg (1966), Viljoen and others 

(1969), Saager (1973b) and Liebenberg (1973).
L6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 3. -Examples of different pyrite structures in the Witwatersrand reefs. A, Typical ore sample from the Basal Reef, showing abraded 

allogenic pyrite and reconstituted authigenic pyrite, which molds around the rounded grains (white). Note the detrital chromite in the centre 

of 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) and 

gold (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 primary 

gold 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 concentration, a polished section was made of each ore sample, 

and the homogeneity and intergrowth relations of the 

pyrites were studied under the ore microscope.

Most of the ore specimens of primary gold deposits 

showed large individual pyrite grains as much as 12.5 

mm in diameter. They were isolated from the sample 

with the use of a dentist's drill, or, after gently crushing 

and cleaning, the grains were concentrated by handpicking under the binocular microscope.

The concentration of rounded allogenic Witwatersrand pyrites proved to be more complicated owing to the 

small grain sizes of the detrital pyrites (<0.5 mm) and 

the omnipresence of various pyrite varieties. For these
DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA L7

:»<fca

E

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 of 

pyrites. Suitable samples were so scarce that, although 

more than 100 polished sections were investigated, only 

5 were of samples that could be used for the concentration of detrital pyrites. The selected samples were 

crushed, sieved, and cleaned in a water stream. Following this, the concentrates were obtained by heavy-liquid 

separation and by handpicking the detrital grains under 

the 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 greater 

number 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 were 

then made from about one-half of the concentrate, and 

their 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 CONGLOMERATES

FIGURE 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 a 

polished 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.

contamination of the pyrite concentrates by silicate 

minerals or other sulfides was less than 5 percent by 

volume and that, for the Witwatersrand samples, the 

separation of the authigenic from the allogenic pyrite 

variety was between 80 and 90 percent successful.

ANALYTICAL PROCEDURE

The copper, zinc, lead, cobalt, nickel, and silver contents were determined with a Perkin Elmer Atomic Absorption Spectrophotometer at the University of Heidelberg, and a number of control analyses were carried out

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 

based mainly on the following considerations:

1. Copper, zinc, and lead occur only in restricted 

amounts ,as substitutions in the lattice of most 

natural pyrites (Boyle, 1965; Radcliffe and Me-
DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA L9

40 80 

MICROMETERS

120

FIGURE 4. -Electron-microprobe step-scanning profiles across a zoned 

pyrite from the Eagles Nest gold mine in the Barberton Mountain 

Land.

Ween, 1969 and 1970; Scott and Barnes, 1972; and 

others). The observed trace contents of these 

elements thus indicate the extent to which the investigated pyrites were contaminated by microscopic to submicroscopic inclusions of base-metal 

sulfides-for example chalcopyrite, sphalerite, and 

galena, as the most common ones. 

2. Nickel and cobalt, in contrast, enter the pyrite structure with ease (Kullerud and others, 1962; Ramdohr, 1969; and others), and since nickel-cobalt 

sulfides occur rarely in the investigated ore samples 

(minute pentlandite exsolutions in pyrrhotite), one

can assume that the nickel and cobalt values obtained indicate the overall availability of these two 

elements during the formation of the pyrites. 

3. Silver occurs almost exclusively in solid solution with 

gold. 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 approximately 10 percent silver (Saager, 1969); and for the 

gold grains in primary ores of the Barberton Mountain Land, silver contents of 0.2-21.6 percent have 

been 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):

Group 1 (Gl), Authigenic Witwatersrand pyrites 

Group 2 (G2), Allogenic Witwatersrand pyrites 

Group 3 (G3), Greenstone-belt pyrites

The silver content of samples in Group 1 ranged from 

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 samples, however, lies below the limit of detection for the 

analytical method used (10 ppm), and consequently 

silver is not considered in the statistical analysis.

For cobalt, nickel, copper, lead, zinc, and each of the 

three groups of samples, the sample cumulative frequency distribution was constructed (fig. 5). With the aid of 

nonparametrie Kolmogorov-Smirnov statistic the 

various cumulative frequency distributions were compared, and, for each element, the null hypothesis was 

tested that the three sample frequency distributions (the 

three postulated pyrite groups) were drawn from 

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 Co Ni Cu Zn Pb

Gl (8 samples), authigenic Witwatersrand pyrites:

x ______ ___________________'______________ppm__ 1,301 1,347 251 56 963

s ________________________________________________________ppm__ 932 490 109 30 1,209

C ______________________________________________________________ .71 .36 .43 .54 1.07

G2 (5 samples), allogenic Witwatersrand pyrites:

x ________________________________________________________ppm__ 216 450 118 64 135

s ________________________________________________________ppm__ 88 112 63 21 69

C ______________________________________________________________ .41 .25 .53 .31 .51

G3 (44 samples), greenstone-belt pyrites:

x _______________________________________ppm__ 198 514 341 151 123

s ________________________________________________________ppm__ 195 532 456 379 73

C ______________________________________________________________ .98 1.04 1.13 2.51 .59
L10 

100

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

•100

61 373 685 30 110 190 

PARTS PER MILLION

145 262

100

l100

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 of 

Kolmogorov-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) was 

used 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 deviation (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 from 

tables, 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 say 

that the investigated sample groups were drawn from 

different populations. It follows that the proposed 

grouping of the samples is not always geochemically 

significant. This is particularly the case for the allogenic 

Witwatersrand pyrites and the greenstone-belt pyrites

(G2-G3). At a 95 percent level of probability-with 

respect to all five investigated elements-the samples 

were drawn from the same population. It thus can be 

assumed 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 and 

others (1969) for an origin of the detrital Witwatersrand 

pyrites from mineralizations in the Archean greenstone 

terranes 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 and 

greenstone-belt pyrites on the other side, the results are 

not as distinct and uniform (fig. 5; table 4). For the 

elements cobalt, nickel, and lead the maximum deviations (dn) exceed the required values, and the postulated
TABLE 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

Nickel dci-c2 = .875

Copper ciG i_G2 = .625

H0 :G1 = G2 

Zinc c4i-c2 =.250

H0:G1 = G2 

Lead dc,-c2 = .875

Pyrite groups

G2-G3

doi-n- 0.475

rloiCj2 = Cj3

Ho-tj2 = Cj3

ilolCji = CjO

dc2-C3 -.290

rloiCj2 = (jo

rlotCjS = Cj3

G1-G3

dcl-c3= 0.955

<£,-„=. 785

ilolCjl = CjO

H0;G1 = G3

H0 :G1*G3

DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLUFIELDS, SOUTH AFRICA Lll

work by Hawley and Nichol (1961) indicate that 

pyrites from magmatic ores commonly show higher 

cobalt and nickel contents than do pyrites from 

hydrothermal ores, and Wilson (1953), on the basis 

of relative availability, concluded that pyrite 

derived from early magmatic melts would have a 

higher nickel content, and probably a lower Co/Ni 

ratio, 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 in 

a generalized way and cannot be used to distinguish 

between 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 to 

the 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 be 

assumed that the lead in these inclusions is of 

radiogenic origin (Saager, 1970; Burger and others, 

1962).

The allogenic Witwatersrand pyrites, which are 

very compact (Saager, 1970) and practically free of 

galena inclusions of infiltrated radiogenic lead, 

have lead contents of less than 240 ppm. Similar 

low lead contents were found in the pyrites from 

the greenstone belts, which have been formed in a 

geological environment characteristically devoid of 

lead-bearing minerals (Villiers, 1957; Saager and 

Koeppel, 1976). It is for these reasons that the lead 

values yielded the same statistical trend as 

displayed by the cobalt and nickel data, in spite of 

the fact that the crystal-chemical behaviour of lead 

is markedly different from that of nickel and cobalt.

sample groupings seem to be justified. The distribution 

of 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 proposed 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 base 

metal sulfides and, thus, are an indication of the 

contamination of the pyrite by other sulfides. Inclusions 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 not 

characteristic of a specific pyrite group, and this is 

reflected 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 two 

elements represent better indicators for the study 

of the metallogenic origin of the investigated 

pyrites.

Carstens (1941), Hegemann (1943), Cambel and 

Jarkovsky (1967), and other workers have stated 

that 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 of 

about 10 ppm and 200 ppm, respectively. In contrast, hydrothermal pyrites have more erratic and 

considerably larger cobalt and nickel contents 

(several thousand parts per million) and Co/Ni 

ratios that are greater than 0.1 and may be as high 

as 830 (Berg and Friedensburg, 1944). Results of

FACTOR ANALYSIS

To further investigate the trace-element data of the 

pyrites and to test the results of the graphic 

Kolmogorov-Smirnov statistic, a factor analysis of the 

data was carried out. Factor analysis is a branch of 

multivariate statistics that recently has found increasing 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 aim 

of 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 the 

variance 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 original 

variables, 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 examines the variation from sample to sample, and its objective is to find groups of samples that are similar in 

terms of their total composition. The observed variations 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 in 

these samples will be similar. The measure of similarity 

used in the Q-mode analysis is the cosine "theta" of the 

angle between two sample vectors in w-dimensional 

space. All the cosines obtained yield the cosine "theta" or 

similarity matrix. It is analysed mathematically, and the 

principal components or factors are extracted by 

simultaneous consideration of all the sample similarities. 

Once the minimum number of factors required has been 

established, the factors are rotated according to specific 

mathematical criteria-in the present study, the 

varimax rotation-to aid interpretation of geological 

data.

The Q-mode factor analysis was carried out at the 

Computing 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 determined by scanning down the columns of the factor-score 

matrix (table 5).

TABLE 5. - Varimax factor score matrix

Element

Cobalt

Nickel

Copper

Zinc

Lead

Factor 1

- 1.0994

-1.5330

.2406

.0034

-1.1863

Factor 2

-0.3326

.1660

-1.7544

-1.3042

- .2646

Factor 1 is essentially a cobalt-nickel-lead factor accounting 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 the 

elements cobalt, nickel, and lead is independent from 

that 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 with 

respect 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, and 

the 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 model 

used. 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 -0.5682

.9632 .3311

.9294 - .8541

.3685 - .4252

.8799 .4970

ST 29d 

SG 29 _ 

SB 191

528 ___ 

N03__

25 ____ 

BR9 __

3.3 ___

3.4 ____ 

BS 184

2HH__ 

SB 231 

H231 __ 

H 401 _

120 ___ 

F III __

Variance, in percent ___

Cumulative variance, in

percent ____________

.6562

.8044

.2739

.8199

.6467

.8154

.5274

.9028

.5997

.7320

.9046

.8854

.7985

.2412

.8199

.4721

-.0182

-.3615 

.4096 

.3324 

.0690

.4806

.6046

-.9500

-.7398

-.8163

- .9473 

.9278 

.7922 

.2774 

.3324

- .6855

-0.6594 

.9239 

.4472 

.4333 

.7955

.8099

.8205

.3257

.8423

.8012

.7645

.4025

-.0197

-.2288

-.2562

- .0845

-.1566

-.4133

-.4052 

.8423

-.0472

37.1910

37.1910

32.6009

69.7919

The scatter diagram (fig. 6) demonstrates the 

heterogeneous distribution of all the investigated trace 

elements in the greenstone-belt pyrites (see also table 3). 

These pyrites originated from a large number of 

genetically different deposits that occurred in quite different geological environments. The generally low 

cobalt, nickel, and lead contents of these samples are 

reflected 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

u

-1

£25

yBR9

.Fill

-1 0 +1 

FACTOR 1 (Co, Ni, Pb)

EXPLANATION

•

Authigenic Witwatersrand 

pyrites

®

Allogenic Witwatersrand 

pyrites

Primary 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, possibly 

reflecting an imperfect separation of the 1 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, these 

samples 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, and 

lead values can be regarded as an indication of the high 

availability of these elements in the Witwatersrand 

Basin where authigenic pyrite formed, possibly as late 

as the metamorphism of the sediments (Saager, 1970; 

Koeppel and Saager, 1974). Relatively high cobalt and 

nickel contents in the Witwatersrand sedimentary 

materials are in good agreement with the source model 

of Viljoen and others (1970), who postulated a granitegreenstone source terrane that, in its lower members, is 

largely built up of mafic-to-ultramafic rocks. The high 

lead contents can be explained by the radioactive decay 

of 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 geochemical 

similarities that clearly set them apart from the 

authigenic Witwatersrand pyrites. The investigation 

also indicates that pyrites formed in a sedimentary environment are far more homogeneous in trace-element 

contents than those originating from hydrothermal 

deposits. This observation agrees with the general 

trends of the trace-element contents in sedimentary and 

hydrothermal pyrites.

LEAD-ISOTOPE INVESTIGATION

As part of the same research project, various pyrite 

and galena samples from Witwatersrand reefs and from 

primary gold deposits in the major greenstone belts of 

the Kaapvaal Craton were investigated for their leadisotope composition. The results of the study have been 

reported in detail in an earlier investigation (Koeppel 

and Saager, 1974). They bear additional important information on the provenance of the detrital pyrites in the 

Witwatersrand sediments and are therefore summarized below.

Uranium-thorium-lead geochronological dating of 

various 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) and 

for 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 of 

rounded 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 hypothesis 

of a detrital origin of the uraninite and monazite. The 

uranium minerals apparently were reconstituted and 

underwent a first complete lead loss 2,040 m.y. ago; a 

second 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 sedimentary 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 CONGLOMERATES

1958), 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 and 

Saager (1974) are condensed on a lead-lead diagram (fig. 

7). On this diagram a hypothetical field is delineated in 

which all the data points of the authigenic Witwatersrand 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 the 

secondary isochrone due to the first lead loss of the 

3,100 m.y. old uranium-lead system. This isochrone has 

a slope of 0.3834 and corresponds to the "main trend" of 

Burger 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 that

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 gold 

ores in the greenstone belts was experimentally determined (Saager and Koeppel, 1976). It does not completely overlap the field of authigenic Witwatersrand 

minerals.

The results of sample 25 in this study are of special interest. No. 25 is a detrital-pyrite sample originating 

from 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 with 

the rounded, waterworn shapes of the pyrites and, in 

fact, confirms their detrital origin from a source outside 

the Witwatersrand Basin.

17

16

15

14

*Pb

s = 0.3834±.0017. 

Best fit of galena data points from 

the Witwatersrand Sequence 

("main trend" Burger and others, 1962)

30BR, L

0

= 0.271 ±.003 

E302F

3bv

s = 0.124±.004 

Lower boundary for 

authigenic minerals

'Area of data points of sulfides 

from primary gold deposits of 

the Swaziland Sequence

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; the 

field 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()- 1V1 .
DETRITAL PYRITES IN CONGLOMERATES, THE WITWATERSRAND GOLDFIELDS, SOUTH AFRICA L15

The lead-isotope composition of sample 25 is identical 

with that observed in galena from the Rosetta Mine, a 

primary gold lode deposit located in Onverwacht Group 

rocks of the Barberton Mountain Land. A remarkable 

lead line was also obtained from the three detrital-pyrite 

samples 25, 2, and 1 (sample 1 lies outside of the graph), 

which were collected at the same locality in the Basal 

Reef. The samples contain all the same first-stage lead 

indicative of an origin from a lead source similar to that 

found in the Archean greenstone terrane. The tieline of 

the data points (fig. 7) has a slope of 0.271, which corresponds with the present-day 207Pb/206Pb ratio of a 

uranium-lead system 3,360 m.y. old. Slopes yielding 

such ages were commonly observed in pyrite samples 

from greenstone belts by Saager and Koeppel (1976) and 

also in whole rock samples from the Onverwacht Group 

(Sinha, 1972). Significantly, the age of 3,360 m.y. is 

identical with the minimum age of the volcanic rocks of 

the Upper Onverwacht Group of the Barberton Mountain Land as reported by Niekerk and Burger (1969).

Samples 202f and 30 BR are also detrital pyrite 

samples from the Klerksdorp and the Orange Free State 

Goldfields, respectively. They exhibit lead-isotope compositions similar to those observed in sulfides from 

primary gold mineralizations in the greenstone belts, 

but also their data points fall into the predicted field of 

authigenic Witwatersrand minerals. However, it is conspicuous that the slopes of the tielines to the respective 

HC1 leaches of the two allogenic pyrite samples are 

steeper than analogous tielines of the authigenic Witwatersrand minerals. The hypothetical first-stage lead 

of these detrital pyrites resembles that of galenas from 

primary deposits in the greenstone belts (fig. 7; see also 

Ulrych and others, 1967).

SUMMARY AND CONCLUSIONS

1. Compact, rounded pyrite grains from the Witwatersrand sediments and pyrites from primary gold 

deposits in greenstone terranes of the Swaziland 

Sequence exhibit identical heterogeneous traceelement contents. At a high significance level their 

cobalt, nickel, and lead values were drawn from 

populations having the same distribution. It follows 

that both pyrite varieties were formed under 

similar chemical and physical conditions in similar 

geological environments and that the compact, 

rounded Witwatersrand pyrites are in fact of 

detrital origin.

2. The large, often extremely porous Witwatersrand 

pyrites that were formed in the basin of deposition 

possess remarkably homogeneous trace-element 

values and conspicuously high lead, nickel, and 

cobalt contents. These trends clearly differentiate

this pyrite variety from the other pyrites investigated and suggest a different mode of formation, probably during sedimentation of Witwatersrand strata.

3. Although comparatively few pyrite samples have 

been analyzed for their lead-isotope composition, 

the results agree with the trends observed in the 

trace-element investigations. The isotope study 

provides evidence that the detrital Witwatersrand 

pyrites derive from a source area with a leadisotope composition identical with that of the 

greenstone belts of the Swaziland Sequence. The 

observed lead-isotope compositions, furthermore, 

indicate that the detrital Witwatersrand pyrites 

were formed much earlier than the Witwatersrand 

sediments.

4. The provenance areas of the Witwatersrand 

sediments, as demonstrated by sedimentological 

and geological evidence, lay to the north and west 

and close to the rim of the basin. The provenance 

areas had far-reaching similarities and were of the 

same age as the Barberton Mountain Land-a 

typical Archean granite-greenstone terrane-used 

to explain the succession of the sediments and the 

heavy minerals in the Witwatersrand Basin. 

However, it must be noted that the Barberton 

Mountain Land, which lies some 300 km east of the 

Witwatersrand deposits, can only be regarded as a 

model for the source terrane and that the actual 

provenance areas lay closer to the basin but are 

now eroded away and (or) covered by younger 

strata.

5. Pyrite and gold are intimately associated in the 

primary and the fossil placer-gold province. It can 

be assumed that not only the detrital Witwatersrand pyrites but also the Witwatersrand gold have 

originated from pyritic and often gold-rich deposits 

in the greenstone belts of the Swaziland Sequence. 

The gold deposits are genetically related to ultramafic and mafic, often magnesium-rich, volcanics, 

which are exceptionally abundant in the Lower 

Onverwacht Group at the base of the Swaziland Sequence. It is possible that the larger portion of the 

detrital sulfides came from the acid volcanics of the 

Upper Onverwacht Group.

6. The detrital uranium- and thorium-bearing minerals 

in the Witwatersrand sediments have a crystallization age of 3,040-3,160 m.y. This agrees well with 

the U-Pb and Rb-Sr ages reported for some of the 

granites in the basement of the Kaapvaal Craton. 

Because of the high thorium content of many 

uraninites in the Witwatersrand sediments, it has 

been suggested that they are derived from a 

granitic or pegmatitic source. According to the provenance 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 explanation of why no primary uranium mineralization has yet been found in the granite-greenstone 

terranes of the Kaapvaal Craton. 

7. Two geochemically and petrogenetically different 

rock assemblages provided the sources for the gold 

and uranium in the Witwatersrand sediments. Both 

rock assemblages occupied different erosional 

levels and, depending upon the actual prevailing 

denudation and drainage conditions, varying 

amounts of gold and uranium were deposited in the 

basin. The expected trend from uranium-rich horizons at the bottom of the sedimentary sequence to 

gold-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 the 

Deutsche Forschungsgemeinschaft (Grant Sa 210/1,2), 

for which the author is grateful.

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Geochemistry and Morphology of

Mineral Components from the

Fossil Gold and Uranium Placers of the Witwatersrand

By D. K. HALLBAUER

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-M

CONTENTS

Page

Abstract ______________________________ Ml 

Introduction _________________________________________

Methods of study _____________________________________

The components of Witwatersrand conglomerates __________

Gold particles _____________________________________

Detrital gold _____________________________________

Biochemically concentrated gold _____________________

Recrystallised gold ________________________________

Detrital-gold intergrowths __________________________

Carbonaceous matter ______________________________

The components of Witwatersrand conglomerates - Continued

Uraninite ____________________________ M10

Pyrite _______________________________ 10

Other detrital heavy minerals ________________ 14

Conclusions _______________—_——————————— 17

References cited _______________——————————— 17

Discussion _______________—————————————— 18

Discussion following paper 1 by Dr. Hallbauer _—_——— 18

Discussion following papers by Drs. Minter, Saager, and

Hallbauer (paper 2) _______________—————— 19

ILLUSTRATIONS

Page

FIGURES 1-3. Photomicrographs showing:

1. The three major types of gold particles from Witwatersrand reefs ___________________________ M2

2. Part of a slightly deformed crystalline gold particle ______________________________———__ 3

3. Part of a gold particle extensively deformed after long transport _______________________—_—__ 3

4. Graphs showing fineness of gold from some Witwatersrand conglomerates __________________________ 4

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 _____________________________________ 5

6. Photomicrographs showing intergrowths between gold and other minerals __________________________ 6

7. X-ray radiograph of a carbonaceous layer with overlying quartzite, showing small gold filaments in the carbon and

irregularly formed gold particles in the upper portion of the layer and in the quartzite __________________ 7

8. Photomicrographs showing thin sections of carbonaceous matter ________________________________ 8

9. Magnified X-ray radiograph of columnar carbonaceous matter _________________________________ 9 

10-12. Photomicrographs showing:

10. Structural details of columnar carbonaceous matter after oxidising of kerogen ___________________ 10

11. Examples of biochemically concentrated gold extracted from carbonaceous matter _________________ 11

12. Filaments from carbonaceous matter where bacterial or fungal structures have been replaced by sphalerite and

galena ___________________________________________________________ 12 

13. Graph showing stability of pyrite in atmospheres with various oxygen contents ________________________ 12 

14, 15. Photomicrographs showing:

14. Pyrite mud balls from different horizons in the Witwatersrand sequence _______________________ 13

15. Detrital cobaltite, gersdorffite, and sphalerite ______________________________________ 15 

16, 17. Graphs showing:

16. Composition of arsenosulphides from various conglomerate horizons in the Witerwatersrand sequence _____ 16

17. Rare-earth contents of zircons from three conglomerates in the Upper Witwatersrand _______________ 16

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

GEOCHEMISTRY 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 the 

scanning electron microscope with microanalyzer attachment, the 

technique of morphological microscopy was applied to observe the 

three dimensions of the grains and to study their composition.

In combination with stereo X-ray radiography of rock slabs and the 

application of various geochemical techniques, this afforded new information on the processes that led to the formation of Precambrian 

placer 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 have 

been widely recognized as fossil placers. It can therefore 

be assumed that the concentration of gold, uranium, and 

other 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 information on these environments that is contained in rock 

parameters and rock components. The recognition and 

understanding of the environment in which a fossil 

placer was formed and of the relation between the 

various morphological forms and components of the 

deposit 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 microscopy, 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 was 

developed to a very advanced stage by great 

mineralogists 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 digestion method (Neuerburg, 1975), which produces concentrates of undamaged grains of many ore minerals, has 

made 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 morphology 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 to 

dissolve 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 solution 

of alum and slow drying causes the rock to disaggregate 

into its components. After having been washed, the 

mineral particles can be studied.

For the study of heavy minerals in Precambrian rocks 

of the Witwatersrand and similar deposits, the siliceous 

matrix and the quartz pebbles were decomposed in 

hydrofluoric 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 can 

be recovered in their natural shape (Neuerburg, 1975). 

Tests have shown so far that gold, pyrite, zircon, and 

chromite are not attacked or altered by hydrofluoric 

acid, but that uraninite, apatite, monazite, and 

magnetite are.

Ml
M2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Of the major silicates present in the Witwatersrand 

rocks, only pyrophyllite is not dissolved in hydrofluoric 

acid. Fine-grained pyrophyllite, if present in large 

amounts, can be a disadvantage, but repeated, careful 

washing of the sample can remove most of it. The inertness 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 Witwatersrand deposits.

After being cleaned in water and acetone, the mineral

grains are ready for examination. Heavy fractions can 

be 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 long 

transport distance (40-60 km). B, Slightly distorted, detrital gold particle with partially 

preserved 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 well 

as in other fields. For the type of investigation described 

here it was greatly advantageous to have an attached 

ORTEC microanalyser on the AMR 1000 to exploit the 

energy-dispersive X-ray spectra of the particles examined and to obtain a semiquantitative analysis of the 

samples and, in particular, to measure the fineness of 

gold particles.

FIGURE 2.—Scanning electron photomicrograph of part of a crystalline 

gold particle from a Witwatersrand reef showing abrasion and 

distortion on protruding corners and edges, indicative of a short 

transport distance (up to 10 km).

THE COMPONENTS OF WITWATERSRAND 

CONGLOMERATES

GOLD PARTICLES

Gold is present in the Witwatersrand reefs as discrete 

particles of various shapes and sizes. From studies of the 

morphology of the individual gold particles it has been 

possible to classify the particles into three major groups: 

detrital gold, biochemically concentrated gold, and 

recrystallised gold (fig. 1).

DETRITAL GOLD

During transport of primary gold particles, their 

typical crystalline microtexture (fig. 2) is deformed slowly by repeated collisions with the bedload in the fluvial 

system until the crystalline texture is destroyed and 

replaced 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 during 

transport down the slope of a placer deposit.

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

Thus the morphology of detrital gold should reflect the 

conditions of transport, distance of transport and, when 

observed on a lateral scale, the direction of transport 

(Hallbauer and Utter, 1977). From a large number of 

observations it was then concluded that the average 

transport distance for Witwatersrand gold was about 30 

km.

All transitional shapes, ranging from crystalline to 

completely 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 with 

crystalline texture, and from this it was predicted that a 

separate entry point existed in the northern part of the 

Orange Free State goldfields (Hallbauer and Utter, 

1977). This was confirmed by independent sedimentological investigations (Minter, 1977). Observations on 

some 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 Dominion Reef, Klerksdorp, and the Blind River Conglomerate 

of the Elliot Lake District, Canada, showed typical 

detrital features indicating a transport mechanism 

similar to that identified for rocks of the Upper Witwatersrand Series.

The fineness-F = (Au x l,000)/(Au + Ag)-of Witwatersrand 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 of 

grains analysed was comparatively small. Analysis of a 

larger number of gold particles not only gave the 

statistical distribution within a given sample but provided 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 be 

shown (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 primary 

deposits. 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 geological 

parameters 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. Consequently, the fineness of gold particles from the 

Ventersdorp Contact Reef showed a spread of fineness 

values from 750 to 960 (fig. 4), which reflects the 

fineness of gold from the truncated horizons (Utter, 

1977).

The bimodal distribution of fineness for the Vaal Reef 

(fig. 4) can be correlated to the sedimentological 

evidence that predicted two facies on the Vaal Reef 

horizon (Minter, 1976).

In contrast to these observations, the fineness of other 

conglomerate units showed a remarkable unimodal 

distribution, pointing to one source of gold for these 

units (fig. 4).

Grain-size determinations, which were carried out 

simultaneously with the fineness measurements, showed that no relationship exists between the size of gold 

particles and the fineness.

BIOCHEMICALLY CONCENTRATED GOLD

The second type of gold, the biochemically concentrated 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( 

z 30H

LU

o 20 

cr

y 10

o40- 

30- 

20- 

10-

700

D

800 900 1000

GOLD FINENESS

FIGURE 4.—Fineness of gold from various reefs in the Witwatersrand 

area. 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 van 

Warmelo, 1974; Hallbauer, 1975); the most common 

plant represented by columnar carbonaceous material 

was named Thuchomyces lichenoides (Hallbauer and 

others, 1977). Colonies of these plants formed mats or 

carpets, which trapped detrital gold by a corduroytablelike action (fig. 5); this gold was apparently then 

dissolved 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, agglutinated 

aggregates of filaments (fig. 1C, D) or irregularly formed, 

often perforated, particles.

RECRYSTALLISED GOLD

The 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 and 

mobilization could be attributed to local rises in the 

temperatures caused by dyke intrusions, overlying 

volcanic rock, or hydrothermal activity along faults and 

fracture planes.

Recrystallised gold generally occurs as crystalline aggregates in small quartz veins or in thin fractures and as 

orientated growth on secondary pyrite (fig. IE, F).

DETRITAL GOLD INTERGROWTHS

Of special interest in samples of Witwatersrand conglomerates are detrital particles consisting of intergrowths of gold and other minerals and of pyrite and 

other detrital minerals. From their detrital nature, 

these can be regarded as primary intergrowths, and 

they serve as indicators of the types of the provenance 

deposits. 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 of 

sphalerite 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 6,000 

ppm), point to a strong genetic relation between gold, 

sphalerite, and chalcopyrite. Typical of detrital goldsphalerite intergrowths (fig. 6C) are the etched surface 

of 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 and 

other 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 of 

gold 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 and 

cobaltite-gersdorffite were observed in samples of B 

Reef from the Loraine gold mine (fig. 6E, F). In these 

samples, detrital gold particles with low overall distortion are a common feature; they are accompanied by 

pyrite and smaller amounts of molybdenite, tourmaline, 

and other heavy minerals, as well as various amounts of 

pyrophyllite in the form of small pebbles and finegrained material.

In samples of Ventersdorp Contact Reef from the 

East Driefontein gold mine, primary intergrowths of 

pyrite and molybdenite (fig. GA, B), gold and bravoite, 

and pyrophyllite pebbles and fine-grained gold were 

observed. As the detrital pyrite from this deposit contains small inclusions of gold (average gold content 250 

ppm) 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 M7

hydrothermal alteration product of potassium feldspar 

(Deer, Howie, and Zussman, 1962) and is a major constituent of the sericite-quartz pyrite zone of porphyry-type 

ore 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 the 

findings of the studies using scanning electron 

microscopy (Button, A., University of the Witwatersrand, Economic Geology Research Unit, personal commun., 1977).

A recent search in the vicinity of the Klerksdorp 

Goldfield succeeded in finding small occurrences of 

mineralised acid volcanics that are older than the Witwatersrand rocks and that contain minute quantities of 

free gold and are accompanied by pyrite, cobaltite and 

other minerals typical of Witwatersrand conglomerates.

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 from 

decayed primitive Precambrian micro-organisms 

(Zumberge and others, 1978). The majority of the 

pyrolysis breakdown products of this kerogen are alkylsubstituted aromatic hydrocarbons, low molecular 

weight aliphatic hydrocarbons, and some aromatic 

sulphur and aliphatic oxygen compounds. The evolution 

of the kerogen from the original biochemicals of 

primitive micro-organisms appears to have progressed 

through free radical reactions influenced by a constant 

radiation 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 the 

fossilized remains of Precambrian plants and the full 

reconstruction of the internal structures of these plants 

were made possible only by scanning electron 

microscopy, while the spatial arrangement relative to 

rock component was studied by X-ray radiography of 

slabs of conglomerate.

Most of the smaller seams of carbonaceous matter 

prominently display a columnar structure. Stereo X-ray 

radiographs in particular show that these small seams 

consist of a carpetlike arrangement of vertical columns

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

FIGURE 7.—X-ray radiograph of a carbonaceous layer with overlying 

quartzite showing small gold filaments (black) in the carbon and irregularly formed gold particles (black) in the upper portion of the 

layer and in the quartzite. Pyrite grains (P) can be distinguished 

from the gold by their lighter grey colour. Middle Elsburg Reef, 

Cook II gold mine.

Thin sections of such layers show finely laminated 

quartz as filling between the columns, while coarsegrained 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 be 

interpreted as silicious sediment deposited during quiet 

periods.

Various sedimentological investigations and 

geological observations point to an environment comprising small stagnant pools, lagoons, wet sand, and clay 

on the lower slopes of wet alluvial fans or shallow water 

along the shore lines of an inland lake in which this 

Precambrian 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 indication 

that it is the primary shape of a small plant or unit of a 

plant colony.

Some of the interior structure of individual columns is 

revealed in X-ray radiographs taken of thin slices on 

high-resolution plates. These radiographs when magnified 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 CONGLOMERATES

D

FIGURE 8.—Photomicrographs A-D showing thin sections of carbonaceous matter. The 

thin sections show the finely layered nature of quartz within the columnar carbon and 

grainy texture of the detrital quartz on top of the carbon (D), as well as a filament of a 

biological nature protruding from the carbonaceous matter (C), are shown. 

Photomicrograph E (a color reproduction of C) shows algal-like filaments of biological 

origin protruding from carbonaceous matter. Carbon Leader Reef, Blyvooruitzicht gold
MINERAL COMPONENTS, FOSSIL GOLD AND CRANIUM PLACERS, WITWATERSRAND M9

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

columns, each individual is clearly defined by a thin, 

membrane-like structure surrounding it. Although most 

of the filaments when viewed in a light microscope do 

not show biological features, occasional filaments with 

typical cell structures were observed (Hallbauer, 1975). 

This is somewhat surprising when considering the age of 

these deposits (approx 2,700 m.y. (million years)). The 

low metamorphism (lower greenschist facies) and a 

relatively quiet tectonic history of the Witwatersrand 

rocks, however, might be some influencing factors in the 

preservation of these structures.

Additional support for the biological nature of the carbonaceous or kerogen layers is obtained from gold filaments within the columns. While most of the filamentous structure in the columns consists of an almost constant mixture of uranium and lead, with small amounts 

of 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 irregular, 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 processes or indirectly from solutions and deposited this 

gold extracellularly on the living or perhaps on the 

decayed filaments.

FIGURK 9.-Magnified X-ray radiograph of columnar carbonaceous 

matter, taken on a high-resolution plate and magnified to show the 

filamentous 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 organisms have been observed in samples from the St. 

Helena gold mine (Hallbauer, 1975; Hallbauer, Jahns, 

and Beltman, 1977). They appear to be replacements of 

organisms by lead and zinc sulphide. Although these 

filaments are well preserved (fig. 12A, B) and readily 

recognized by microbiologists as bacterial or fungal 

filaments, they are a localized occurrence and appear to 

be of a parasitical nature. Their acceptance or nonacceptance does not influence the theory on the biological 

origin of the Witwatersrand carbonaceous matter or 

kerogen.
M10 GENESIS OP" URANIUM- AND GOLD-BE ARINGPRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

200pm

FIGURE 10.-Scanning electron photomicrograph showing structural 

details of columnar carbonaceous matter after oxidizing of kerogen 

at 500°C. (Oxidizing in a plasma furnace (International Plasma 

Corp.) at temperatures less than 100°C leads to the same type of 

structures.)

URANINITE

Uraninite was recognized early as a detrital component of the Witwatersrand conglomerates (Liebenberg, 

1955; Ramdohr, 1955). Of special interest, however, is 

the association between uraninite and the carbonaceous 

matter. Uraninite can be found as small (<100 ju,m), irregular grains finely dispersed in carbonaceous matter 

and as rounded, detrital grains completely surrounded 

or in various degrees intergrown with the latter. Often 

the surrounding carbonaceous matter has the appearance of a felt of small filaments (Schidlowski, 1968b). 

Detailed observations on particles cleaned in 

hydrofluoric acid appear to indicate that microorganisms attacked detrital uraninite and slowly 

assimilated the mineral matter.

PYRITE

The detrital nature of most pyrites in the Witwatersrand conglomerates was recognized long ago from 

macroscopic observations and studies of polished sections (Ramdohr, 1955; Liebenberg, 1955), and as many 

as 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 Witwatersrand and similar conglomerates as massive, nonporous particles that exhibit all stages of rounding, the 

result of fluviatile transport. Typical of the Witwatersrand pyrites are inclusions of various minerals—such as 

pyrrhotite, chalcopyrite, sphalerite, gold, and others 

(Utter, 1977)—that are similar to those found in primary 

deposits. 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 absence 

of oxygen in the Precambrian atmosphere (Cloud, 1976; 

Schidlowski and Junge, 1973). This is an assumption of 

considerable importance to studies of the evolution of 

life (Margulis and others, 1976). Although the presence 

of detrital pyrite in recent rivers has been reported 

repeatedly, little is known about periods of time for 

which 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 orientated 

etched 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 the 

particle and, therefore, that a low oxygen partial 

pressure existed in the Precambrian atmosphere.

Laboratory experiments were conducted during which 

pyrite was tumbled in closed glass containers filled half 

with 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 weight 

loss of the pyrite were determined. The results show 

that the pyrite lost 0.01 percent of its weight by tumbling in water under the present atmosphere. This 

weight 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 indicate that a low-level chemical attack on pyrite is possible under conditions assumed to have existed during the 

Precambrian (Grandstaff, this volume, chap. C).

Within the group of sedimentary pyrite types, the 

pyrite "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," a 

term often used to describe all rounded pyrite grains;
MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND Mil

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 CONGLOMERATES

FIGURE 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 in 

diameter and are indicative of economic horizons in the 

Witwatersrand sequence. Polished sections of this type 

of pyrite occasionally show cracks and indentations, 

which previously were regarded as having been caused 

by solution under pressure (Schidlowski and Trurnit,

1966). However, three-dimensional examination of these 

cracks on the intact grain after treatment with hydrofluoric acid suggested that they were caused by 

shrinkage and (or) plastic deformation of soft material 

(fig. 14). From this it was deduced that pyritic mud was 

formed during Witwatersrand time and was deposited 

in 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 other 

fluviatile 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 and 

overlying pebbles would cause the formation of indentations in, and plastic deformation of, the mud pebbles.

The occurrence of pyrite mud balls in conjunction with 

ventifacts in Witwatersrand conglomerates points to a 

palaeoenvironment 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 of 

most other detrital minerals, as is shown by the study of 

polished sections and trace-element analyses (Utter, 

1977). Rounded pyrite particles with typical mud-ball

z

LU

o

DC 

LU 

CL

C/)

o

C/) 

O5

E 

16 -i

I 14 H

1 «-

10 2 -10 -

8- 

6- 

4 -

10'r3_

2-

Oxidation of pyrite as S04 content of water

—I———i———i———i——I———i———I———i———i———I 

0 2 4 6 8 10 12 14 16 18 20

PERCENTAGE OF 02 IN ATMOSPHERE

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

contents.

structures have been observed in most Witwatersrand 

conglomerates, including those from the Vredefort area 

and the Dominion Reef.

Authigenic pyrite in fossil placers was formed in the 

rock during and after diagenesis and is easily recognized 

by its crystalline morphology. It has been observed that
MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND M13

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, as 

well 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 Dominion Reef showing the typical porous surface. F, Surface of E, showing the fine-grained 

texture of irregularly formed grains.
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 as 

a reflection of the original depositional environment, 

which could be described as a low-energy environment 

producing fine-grained arenaceous sediments with 

sulphide 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 gold 

in, 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 the 

vicinity of volcanic intrusions.

OTHER DETRITAL HEAVY MINERALS

Gold 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; the 

lesser-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 are 

of special interest because they sometimes occur as 

primary intergrowths with gold (fig. 6). Chemically, 

they are even less stable than pyrite in a normal oxidizing fluviatile environment; thus their presence is an additional indicator for a low oxygen content, if not an 

absence of oxygen in the palaeoenvironment of Witwatersrand time. To date, all geochemical work on the 

Witwatersrand and similar rocks elsewhere support this 

concept. 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 partial 

pressure approach the present level (Fryer, 1977), and 

there appears to be no evidence for either a reducing 

Precambrian atmosphere (Shimizu, 1976) or a highly oxidizing 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, detrital 

morphology (fig. 15).

The onset of recrystallisation observed on some grains 

could easily be misleading when observed in polished 

sections. There appears to be a tendency for incipient 

recrystallisation to be more evident in the nickel-rich 

members of this group.

There is a wide range in the compositions of the iron-, 

cobalt-, and nickel-arsenosulphide particles in Witwatersrand 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 their 

compositions, whereas samples examined from the 

other localities have well-defined compositions.

Gold from the Basal Reef at the St. Helena Mine occurs as two distinct varieties: coarse, crystalline, hardly 

deformed particles; and flaky, extremely deformed gold 

typical of a long transport distance. Seen together with 

the 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 occurrence of detrital sphalerite (fig. 15) and goldsphalerite intergrowths, as well as molybdenite and 

graphite. The excellent cleavage and low hardness of 

these 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 Precambrian atmosphere rather than to a complete lack of oxygen.

Although it is a small part of the detrital heavymineral assemblage, chromite is a common constituent 

of 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 of 

chromite 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 be 

compared with that of chromite from Archean greenstones.

The comparatively large amount of zinc observed in 

chromite 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, from 

magnetite 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 the 

pyrite, ilmenite and magnetite can be considered to have 

been scarce in the primary deposits.

Well-rounded ilmenite particles without any sign of 

pyritization 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 a 

sample 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

CoAsS

EXPLANATION

Basal Reef. St Helena Gold Mine 

Carbon Leader Reef, Doornfontem Gold Mine 

B-Reef, Loraine Gold Mine 

i \ Bird Series, Vredefort Area 

xx Recrystallized Gersdarffite, St. Helena Gold Mine

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

Detrital 

Grains

of the Witwatersrand. Although the ilmenite was 

accompanied by detrital pyrite, no intergrowths of the 

two minerals were observed.

Another prominent constituent of most Witwatersrand 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 at 

room 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 from 

various reef horizons were analysed by instrumental 

neutron activation (Watterson, 1977). In all samples 

analysed the zirconium-hafnium ratio was found to 

range between 43 and 48, which is similar to values 

quoted in the literature for granites.

Zircons from one of the three horizons analysed, the 

Vaal Reef, were found to contain anomalous amounts of 

antimony and iron. Zircons from the Kimberley Reef,

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.

E 10 

a

EXPLANATION

O Kimberley Reef , East Rand

0 Kimberley Reef, Evander 

A Vaal Reef

i___i___i

La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu 

RARE 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 Witwatersrand conglomerates. Recently a research programme 

has been initiated to study the trace-element content of 

quartz and especially the rare-earth-element pattern by 

instrumental neutron-activation analysis, as well as the 

daughter minerals in fluid inclusions. The results so far 

indicate that a classification of quartz pebbles can be 

made 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 investigated.
MINERAL COMPONENTS, FOSSIL GOLD AND URANIUM PLACERS, WITWATERSRAND M17

CONCLUSIONS

In studies of the distribution of gold, uranium, and 

other heavy minerals in fossil placers the technique of 

morphological examination in the scanning electron microscope opens up the third dimension to microscopy. 

This supplements the classical two-dimensional microscopy as a tool for geological investigation and makes 

possible observations not previously attainable with 

earlier methods.

New investigative methods apply especially to the 

geochemical studies of mineral grains made with the 

microanalysing 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 recognized from the morphology of their carbonaceous remains, 

permitting their role in the concentration of gold and 

uranium 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 important 

minerals, as well as on the palaeoenvironment of Precambrian placers that were not possible with the 

classical methods of microscopy.

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v. 26, p. 65-79.

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M18 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

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DISCUSSION

[At the workshop Dr. Hallbauer presented three short 

papers, titled: (1) Stereo X-ray radiography and 

chromatographic painting techniques applied to the 

study of Witwatersrand conglomerates; (2) Morphology 

of gold, pyrite, zircon, and chromite in the Witwatersrand Series; (3) Genetic relation between organic 

material and gold in Witwatersrand conglomerates. The 

first one followed the presentation by Dr. Whiteside, the 

second one followed Dr. Saager's talk, and the third one 

followed Dr. Schidlowski's paper.

Dr. Hallbauer has combined the three talks into a 

single manuscript, chapter M of this report, which 

somewhat complicates orderly placement of the discussion 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 the 

carbonaceous material don't look like detrital grains.

Do you consider it possible, therefore, for the gold to 

have recrystallised diagenetically on the carbonaceous 

material which your beautiful photographs, if I may 

say so, show so clearly as being relatively barren in 

gold?

Hallbauer: First of all, the large gold particles in the carbonaceous material don't look detrital in the 

photographs which I've shown, but they look detrital 

in the stereophotographs. And, apart from this, we 

have dissolved many samples of conglomerate in 

hydrofluoric acid and paid special attention to these 

large gold particles, and I will show a few of them 

later on and you can see quite clearly that they are 

detrital.

Minter: On your first radiograph, on the right-hand side 

on 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 that 

mature 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 the 

total 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 the 

comparatively thin conglomerate, most of the gold is 

either 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 the 

difference, but you are quite right in so far as we've 

found examples of gold distributed on foresets in this 

reef and in the footwall rocks.

Skinner: Have you any specific reagents that you use for 

your staining techniques with uraninite? Have you 

developed any?

Hallbauer: We have done very little, but we've found 

that 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 the 

other. And if you do it in quick succession, then the 

solution 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 in 

that 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, WITWATERSRAND M19

DISCUSSION 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 must 

say I am a bit confused. I think we have seen some 

very strong evidence presented by Dr. Hallbauer that 

the distance of transport was very small, and this I 

believe is also shown by one of the previous slides that 

was 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 a 

model 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 the 

greenstone 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, but 

similar greenstone belts, now covered by younger 

rocks or eliminated by the Bushveld Complex, much 

closer to the basin.

Grandstaff: I have a question for Dr. Hallbauer. I am a 

little concerned about possible alteration of the grain 

surfaces during the HF treatment. Have you made 

any SEM photos on grains prior to and after the HF 

treatment to determine any possible alteration in the 

surface characteristics?

Hallbauer: Yes, we examined some grains, particularly 

zircon grains recovered from beach sands, before and 

after hydrofluoric acid treatment, and there was no 

sign of any etching. More detailed investigations of 

resistance 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

hexafluoride. We found no evidence of acid attack on 

pyrites.

Whiteside: May I make a couple of comments on the 

remarks that von Backstrom made just now. First of 

all, Barberton is over to the northeast and I believe 

everybody believes the transport direction is from the 

northwest or north; and secondly, there are remnants 

of 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 with 

the paleogeography?

Hallbauer: [Hallbauer drew a sketch map of the Witwatersrand 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 make 

some remarks on that? We don't really understand 

this age we have been getting.

Hallbauer: So far, our geochemical investigations have 

proceeded 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 amount 

that 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, and 

we selected these pyrites very carefully to make sure 

that no visible gold was attached to them. First of all, 

they were selected manually under the 

stereomicroscope and then the same fraction was 

X-rayed on fine-grained film to make sure that no 

large gold particles were included within the pyrite. 

Even so, the gold content was very high. The other 

results concerning the other trace elements—cobalt, 

nickel and others—are not completed yet.

Saager: Wouldn't a high gold content be proof of a very 

late formation of this mud cake, during probably the 

solution stage of the modified placer theory?

Hallbauer: The possibility is that fine-grained gold was 

included 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 was 

included in the conglomerate. We haven't yet made 

polished 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 that 

the osmiridium grains are recovered pnly from 

amalgam barrel residues from the mill and so you can 

get only one point per mine. Osmiridium occurs in 

parts 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 source 

area to work on.

Whiteside: Can I just make a comment there? There was 

a 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 found 

that on the West Rand or on the western side of the 

Central 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 is 

mined. It thus looks as though there is a gradual 

reduction 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 of 

fact, 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 movement within the conglomerates. And apart from this, 

the structures are so very consistent with the structures on nuggets that the movement within the confined 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 a 

certain amount of the markings you interpret as abrasion could be introduced during the compaction of the 

sediment and therefore not necessarily be entirely introduced during transport?

Hallbauer: I am sure there are signs which could have 

been produced during the diagenesis and compaction 

of the sediments, but on the other hand, the surrounding quartz pebbles normally create a pressure-free

area within their matrix, and movement in that space 

should be confined to only microns. I admit there is a 

possibility that these things could be mixed with movement within the rock; it is very difficult to differentiate between these.

Schopf: Have you generated a set of criteria that will 

allow you to differentiate between markings produced 

at various times during transport, during sedimentation, during diagenesis, things of this sort?

Hallbauer: This is something we haven't done yet. There 

are examples, however, of gold particles which show 

only small signs of distortion and others which show 

signs of complete distortion. How to differentiate between them will be shown in further work.

Simpson: Dr. Hallbauer, I would like to ask you with 

which detrital gold particles are you comparing your 

material, which you have alluded to several times in 

your talk, because they don't compare at all to the type 

of detrital gold particle that you see deposited by 

glacial 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 transport 

distance 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 examined 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 secondary pyrites, but from rough judgment I would say 

that this is an insignificant amount, probably less than 

20 percent.

JamesB. Black, Atlas Minerals, Utah: I have a couple of 

rather naive questions perhaps. To Dr. Saager: Could 

you refer me to the method by which you independently 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, which 

appeared in 1962 in Cosmochemica Acta, and the 

other 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 

might be better to say nothing at all. I think there is a 

certain amount of caution required in these 

statements.

Black: How about attempting to get a photomicrograph 

of a particle in place that has not been etched by 

hydrofluoric acid?

Hallbauer: That we haven't attempted. I would say that 

it is very difficult to find a grain of uraninite within the 

matrix which has not been broken and where the surface has been kept in such a state that you can really 

make statements.

Black: Thank you very much.

Bourret: I am sure that everyone is aware that there is 

an excellent paper by Liebenberg with dozens and 

dozens of photomicrographs of uraninite grains in reef 

ore, 50, 75, or 100 of them. They are all described and 

they show clearly rounded detrital grains.

Hallbauer: If I may quickly comment, these detrital 

grains from Liebenberg come from milled products. 

They have been through the mill, and surface structures are somewhat dubious.

Myers: If I recall the literature correctly, he 

[Liebenberg] specifically said that he dug some of 

these [rounded uraninite grains] out of reef samples.

Feather: May I comment on this. I have looked at a lot of 

uraninite grains recently. These are specifically exposed on fresh fractures in the reefs, so that there is not 

the problem that the grains might be rounded during 

the milling process. I would say that invariably the 

grains contain at least one or two distinct crystal faces 

with distinct corners and many grains are beautiful 

crystals. The evidence seems to suggest that they 

have recrystallised in the reefs and that therefore any 

original features have been completely destroyed. The 

previous idea that the grains were completely rounded 

I would contest, certainly from examining freshly exposed grains in the reefs. In my talk I'll show you some 

photographs.

Kalliokoski, Michigan Technological University: I would 

like to make a comment about the nature of gold. It 

seems that the gold in the Witwatersrand is in some 

respects anomalous from more recent gold. I was introduced to this subject by a gentleman at Stanford in 

the metallurgy department, a Colonel Crook, who 

became experienced in studying gold. He was involved 

in some law cases where the question was whether the 

gold was Recent or Tertiary, and he was able to 

demonstrate that Recent gold was quite irregular 

whereas Tertiary gold had recrystallised and had very 

definite crystal faces. A similar kind of recrystallised 

gold has been found in some of the placers in Surinam, 

and at a place in Venezuela called Icabaru you can find 

octahedra. My wife has one that she forbids me to cut

which is about the size of a pea that shows seven faces. 

There is a geologist, Dr. Cecelia Valetsia [sp.?], who 

has a gold crystal about three-quarters of an inch in 

diameter which has recrystallised in place. Why can 

you be so sure that they recrystallised in place? 

Because there are no hydraulic equivalents with the 

gold. There are no gold veins in the vicinity; therefore, 

the gold must have been transported a considerable 

distance. From this kind of evidence one can then conclude that the gold must have recrystallised. So, it is 

peculiar that the gold in the Witwatersrand does not 

show recrystallisation. Now, there is a possibility that 

the chemical conditions were different in those days. 

We know that uraninite was persistent; perhaps 

because of similar conditions gold refused to migrate 

and therefore didn't recrystallize. But there is also the 

other fact that gold is exceedingly soft. Accordingly, I 

think that one would have to take particular care in 

collecting gold from in place or even collecting gold 

from present-day placers to make sure that, if it has 

crystallised in place, it doesn't become scratched between the time it is dug out and the time that it is examined under the microscope.

James Trow, Michigan State: I have two questions. The 

first for Dr. Minter: Would you care to speculate concerning the greater proportion of uranium compared 

to gold in the Canadian occurrences compared to those 

in the Witwatersrand? Do you ascribe those differences to perhaps the Canadian occurrences being in 

a more distal portion of the fluvial system, or are the 

differences more related to provenance, with greater 

greenstone surface exposure near the Rand compared 

to what we have in Canada?

Minter: I think the reason is that one does have different 

provenance areas and different original amounts of 

gold and uranium so that the ratio change has to be 

based on each separate population. And, moreover, 

one finds that there is change going down the paleoslope of a single reef. We have done this with a 

number of reefs and find that there is a ratio change 

down the paleoslope but that one can't compare one 

reef directly with another because on some paleoslopes one can go from a coarse pyrite nodule to a fine 

pyrite nodule in only 4 km, whereas in the Steyn Reef 

it takes 22 km. Obviously the gradients are different, 

the sizes of the deposits are different, and I think that 

certainly provenance areas supply different proportions. The deposit at Denison [Elliot Lake, Canada] is 

not a distal deposit; it looks quite proximal from the 

sizes of pebbles, et cetera, so therefore the original 

uranium content must have been extremely high. I 

have wondered whether the uraninite grains are actually larger than those in the Witwatersrand, but I 

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 might 

suggest that the pyrite is pseudomorphous after 

magnetite?

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 that 

Ramdohr and several other authors have shown inclusions of magnetite in pyrite pebbles, suggesting a 

replacement of magnetite by pyrite, but all these 

authors could not give a clear indication where this 

replacement actually would have taken place. Has it 

taken place in the basin our outside the basin? I do not 

know.

Hans Adler, Energy Research and Development Administration: 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 observable 

change in this ratio going from the proximal to the 

distal portions of the reefs, and also is there an observable difference in this ratio between the carbonbearing and the non-carbon-bearing reefs? Secondly, 

has an attempt been made to determine differences in 

the 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 big 

change have been in the vicinity of dikes which have

cooked up the reefs and remobilized the pyrite. In fact, 

in specimens of that nature one can't measure the 

pyrite nodules; they have been completely recrystallised. One other criterion in collecting samples was to 

make 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 more 

qualified to comment on that.

Schidlowski: I think I'd better do that in the frame of my 

talk.

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 of 

zircon 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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-N

CONTENTS

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 _.

Page

Nl 

1 

1 

1 

1

3

4

7

7

10

Page

Carbonaceous matter ____________—_——_—————— N10 

Microscopic observations and their genetic significance __ 10

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 ___________________.

14

14

19

19

22

22

24

ILLUSTRATIONS

Page

FIGURES 1-3. Photomicrographs showing:

1. Detrital grains of uraninite from various uraniferous Witwatersrand conglomerates ______________—_ N2

2. Hypidiomorphic uraninite particles from various Orange Free State gold conglomerates _________—_—_ 3

3. Detrital grain of uraninite from Basal Reef, Orange Free State with minute specks and outgrowths of galena, and

microscopic scanning pictures for uranium, thorium, and lead ______________________—_——__ 4 

4. Graph showing relation between roundness and average grain size in a Lake Erie beach sand and a Mississippi River sand_ 5 

5, 6. Line drawings showing:

5. Muffin shaped quartz grains from "Ottawa Sand" facies of Ordivician St. Peter Sandstone, Ottawa, Illinois ____ 5

6. Detrial monazite grains with typical muffin shapes from a recent West Australian beach placer ___________ 5 

7-13. Photomicrographs showing:

7. Two typical aggregates of colloform pitchblende from a vein-type uranium deposit, Sudetes Mountains, central

Europe ___________________________________________________________ 6

8. Felts of needle- and lath-shaped brannerite crystals and "compact" brannerite aggregate from various Orange

Free State reefs ______________________________________________________ 7

9. Pseudomorphs of brannerite after grains of detrial uraninite from Orange Free State reefs ____________ 8

10. Microprobe scanning pictures for titanium, uranium, and thorium obtained from the muffin-shaped

"uraninite ghost" shown in figure $A ___________________________________________ 9

11. Two grains of uraninite from Elsburg Reefs, Orange Free States, with overgrowths of brannerite, and microprobe scanning pictures for titanium, uranium, and lead _______________________________ 9

12. Grains of uraninite from various Orange Free State reefs, coated and veined or partially replaced by carbonaceous matter _________________________________________________________ 11

13. Advanced stages of uraninite replacement by carbonaceous matter, in various Orange Free State conglomerates microprobe scanning pictures for uranium _________________________________ 12

14. Photographs showing microprobe scannings for uranium and thorium of the carbon-uraninite aggregate depicted

in figure 12D _____________________________________________________________ 13

15. Photomicrograph and explanatory sketching of typical "carbon" footwall of a gold- and uranium-bearing conglomerate

horizon (Basal Reef, Loraine Gold Mines, Orange Free State), showing distribution of minerals _____________ 15

16. Schematic block diagram of a "carbon reef characterized by a seam of fibrous thucholite at the base of a conglomerate

layer ________________________________________________________________ 16 

17-19. Photomicrographs showing:

17. Fibrous thucholite with characteristic mineral inclusions, from B Reef, Loraine Gold Mines, Orange Free State _ 17

18. Fibrous carbonaceous matter with inclusions of uraninite; flowage of carbonaceous matter; and scaly graphite

(from various Orange Free State conglomerates mines _______________________________ 18

19. Reconstruction of the lichenlike symbiont putatively responsible for the fibrous carbon aggregates _ ______ 19 

20. Graphs showing evolution of the Earth's^total budget of photosynthetic oxygen ________________________ 21

in

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

URANIFEROUS 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 pertaining to the origin of the main uraniferous constituents of the banket. 

Uraninite occurs principally in the footwall portions of the reefs as 

rounded or muffin-shaped grains some 100 micrometers in diameter, 

which leave little doubt as to their waterworn, abraded nature. It can 

be demonstrated by comparison with detrital monazite grains from 

present-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 the 

characteristic abrasion forms of the uraninite parents (so-called 

"uraninite ghosts"). As for carbonaceous matter or "thucholite," 

microscopic evidence decidedly favors a genetic concept involving a 

radiolytic polymerization of originally mobile hydrocarbons (most 

probably indigenous to the rock) by pre-existing uraninite constituents, 

with these latter subsequently subjected to progressive replacement 

by the ambient organics. Apart from a morphologic description of the 

principal "carbon" aggregates in the reef, a critical appraisal is given of 

a 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 Witwatersrand 

conglomerates, 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 the 

longstanding Witwatersrand controversy between the 

"placerist" and "hydrothermalist" schools (Davidson, 

1953, 1957; Liebenberg, 1955; Ramdohr, 1955, 1958a, 

b). During these discussions, the crucial importance of 

mineragraphic studies for the elucidation of the Witwatersrand metallogeny became apparent. Following 

the pioneering work by Ramdohr and Liebenberg, an 

impressive array of detailed ore-microscopic investigations has appeared during the last two decades, furnishing prima facie evidence of a detrital origin for the 

banket 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 the 

basic observational and microchemical evidence accrued 

in these publications to provide a synopsis of our present 

knowledge of the principal radioactive constituents of 

the reef.

URANINITE 

OCCURRENCE WITHIN THE CONGLOMERATES

Both microscopic inspection and autoradiographs of 

complete reef sections have invariably furnished 

evidence that the bulk of the uraninite is mainly concentrated in the lower portions of the conglomerates, along 

with 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.) Occurrences 

of 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 the 

conglomerates.

MICROSCOPIC CHARACTERISTICS

Both Ramdohr (1955) and Liebenberg (1955) have 

identified the prototype of the primary, unaltered 

uraninite 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 CONGLOMERATES

FIGURE 1. -Detrital grains of uraninite from various Witwatersrand conglomerates, mostly Basal and Elsburg Reefs, Orange Free State, many of 

them displaying excellent muffin shapes (A, C, D, J). Note dustlike inclusions of galena (white) in all grains, as well as concentrations of this 

mineral 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 

is always speckled with myriads of dustlike inclusions of 

galena (PbS), the radiogenic lead having been obviously 

subjected to sulfurization during postdepositional 

reconstitution of the banket. (With both detrital and 

authigenic sulfides present in the reef in large quantities, this was actually to be expected.) Radiogenic 

galena often appears to be mobilized, forming crack fillings within (fig. IB, C, H, I, K) or aureoles around (fig. 

IB, C, G, J) the individual uraninite grains, with small 

amounts of the galena also dissipating into the surrounding groundmass. Apart from galena, reconstituted 

sulfides,—mostly pyrite, pyrrhotite, and chalcopyrite 

(fig. IK)—and also gold have been observed as crack fillings within uraninite particles, occasionally occupying 

octahedral cleavage planes of the host mineral (figs. IF, 

25). Some grains are also transected by minute 

stringers of carbonaceous matter (fig. 2A, C).

Apart from the typically well-rounded uraninite particles described and their cataclastic fragments, 

hypidiomorphic grains have also been encountered (fig. 

2). With the exception of single occurrences from which 

they have been reported in larger amounts—for example, the Dominion Reef (Hiemstra, 1968}-these grains 

are, however, relatively rare constituents of the banket.

CHEMISTRY

X-ray diffraction data supplied by Liebenberg (1955) 

have furnished proof that the microscopically identified 

uraninite particles yield a powder diagram identical in 

all respects with that of artificially prepared U02 . 

Chemical analyses of uraninite concentrates from two 

different reefs, the Main Reef and the Carbon Leader, 

have shown a Th02 content of 1.6 and 2.7 percent, 

respectively (Liebenberg, 1955, p. 126). This general 

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,_the 

veins 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 

Basal Reef, Orange Free State (fig. 3; Schidlowski, 

1966a, p. 190. For the uraninite fraction of the Dominion Reef, Hiemstra (1968) has reported an average 

Th02 content of about 6.5 percent. Recently, Grandstaff 

(1974) has submitted the results of a comprehensive 

microprobe investigation of uraninite from the Vaal 

Reef that showed the Th02 tenor of individual grains to 

lie mostly in the range of 2 to 7 percent.

ORIGIN OF THE URANIUM MINERALIZATION

With all information presently available on the 

uraninite fraction of the banket, there can be no doubt 

that this mineral is a detrital constituent of the rock. In 

particular, its distribution pattern within the conglomerates is typical for a member of a suite of heavy 

minerals, corresponding to that of gold and several 

other heavy minerals. Further, the relation of the 

economic mineralization in general to sedimentation 

features of the primary depositional environment long 

has been recognized (Reinecke, 1928; Sharpe, 1949).

Prima facie evidence of a detrital origin of the 

uranium mineralization is provided by the clearly waterworn, abraded nature of the individual uraninite grains.

The claim advanced by Davidson (1957) in the Witwatersrand controversy, that abrasion is unlikely to 

have occurred in the small size class to which the bulk of 

the uraninite fraction belongs, does not stand up to 

critical review. According to Correns (1960, p. 136), particles smaller than 0.1 mm in diameter may still be excellently rounded in certain sedimentary environments. 

Further, Pettijohn (1957, p. 63) regarded 0.1 mm as a 

possible lower limit for the rounding of sand grains. According to a graph by Pettijohn and Lundahl (1943), the 

quasi-linear relation between the sphericity of grains 

and their mean diameter still holds for the size class of 

about 100 /xm in the case of well-worked beach sands, 

whereas the respective function of a fluviatile sand 

suffers a breakdown at about 170 jum (fig. 4). However, 

with the degree of abrasion, particularly of the smaller 

size classes, largely dependent on the maturity of a sediment, low sphericity values in immature detritals have 

no significance at all for the issue under consideration. 

Moreover, if ordinary sand grains some 100 jum in 

diameter still fall within the subrounded category, p 

> 0.25 (fig. 4), an abrasion of the considerably heavier 

uraninite particles (S.G. = 9.0-9.7) of the same size class 

should present no problems at all. Actually, reasonably 

well-rounded uraninite grains as small as some 60 /*m in

PERCENT

10-

Th

—r~ 

30

—r~ 

20 30 40 50

DISTANCE, IN MICROMETERS

—T— 

60

—r~ 

70

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 the 

particle 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.
URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY N5

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 the 

continuous 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 and 

0.40 (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 observed, the muffin shape of a large number of uraninite 

grains (fig. 1) is most conspicuous. This morphology is 

fairly 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 morphology. 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 shapes 

of other ancient and modern detrital clasts (fig. 5, 6) is 

likely to lend unequivocal support to a placer origin of 

the uraninite fraction. The morphological analogies between 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 Australian 

beach placer shown in figure 6 fall within a size range 

similar to the Witwatersrand uraninites. This observation should definitely dispose of Davidson's (1957) idea 

that grains of these dimensions are no longer susceptible

oooo

ooooo

OOOO

2 MILLIMETERS

FIGURE 5. -Muffin-shaped quartz grains from "Ottawa Sand" facies of 

Ordovician St. Peter Sandstone, Ottawa, Illinois. Drawn from thin 

section of grain mount.

ooo 

oooo

0.1 

I

0.2i

0.3 MILLIMETERS 

I

FIGURE 6. -Detrital monazite grains with typical muffin shapes from a 

recent West Australian beach placer, drawn from grain mount of a 

monazite concentrate.

to rounding. The obvious correspondence in grain morphology between the Witwatersrand uraninites and the 

heavy-mineral fraction of a modern marine beach placer 

is suggestive of genetically related primary sedimentary 

environments, thus vindicating previous interpretations 

of the economic conglomerates as principally ancient littoral 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 to 

Davidson's (1955, 1957) statement that the rounded appearance of the uraninite particles of the banket is that 

typical of pitchblende from hydrothermal veins. Instead 

of a lengthy discussion of this point, two micrographs of 

colloform, botryoidal pitchblende aggregates from a 

genuinely hydrothermal deposit are included in this 

paper for the convenience of the reader not familiar with 

textures of primary ore deposits (fig. 7). Comparison of 

these aggregates with the Witwatersrand uraninites 

reveals a large array of fundamental distinctions; in particular, the ovoid bodies do not display the faintest indication 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 and 

other 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 sections or hand specimens, provides additional proof of a 

heterogeneous 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 the 

uraninite 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 the 

growing grains owing to the successive movement 

through the rock of fluids of different composition. 

Similar conclusions had been drawn by Ramdohr and 

others (1965) from semiquantitative microprobe scanFIGURE 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 aging 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 N7

ning diagrams. The variable lead content of individual 

uraninite grains noted during this investigation was interpreted as reflecting different ages of formation of the 

primary pegmatitic U02 material.

BRANNERITE

In contrast to the early recognition of brannerite2 

(UTi2 06) as one of the principal uranium carriers in the 

Blind River conglomerates (Nuffield, 1954), the pioneering ore-microscopic studies of the Witwatersrand 

banket lack any specific references to this material. 

However, in 1957 Ramdohr, by comparing with Blind 

River samples, was able to identify this phase in the 

Dominion Reef, expressing also the suspicion that the 

material of his so-called "uranium ghosts" previously 

described and at that time not yet identified might actually represent brannerite. Similarly, Davidson (1957) 

considered that the "uraniferous leucoxene" mentioned 

by Liebenberg (1955) might be brannerite. Finally, Ramdohr (1958b) was able to definitely identify this mineral 

in the Witwatersrand conglomerates.

MICROSCOPIC AND CHEMICAL CHARACTERISTICS

The difficulties inherent in the identification of Witwatersrand brannerite are primarily due to (1) its occurrence as mostly tiny needle-shaped crystals at a 

micrometer scale, often intergrown in feltlike aggregates, and (2) to its metamict state. It is optically 

isotropic, and it does not yield the X-ray diffraction pattern of brannerite. The lines for anatase, rutile, and 

galena usually show up on the diffractometer charts.

2 [Brannerite is essentially a uranium titanate of variable composition that contains thorium 

and minor amounts of calcium, cerium, ytrrium, niobium, and ferrous iron.]

However, as demonstrated by Nuffield (1954) for the 

Blind 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 

(Feather and Koen, 1975, p. 203).

In polished section, the mineral usually appears as 

felts of needle- or lath-shaped crystals (fig. 8A, B), 

grading sometimes into irregular patches of massive 

brannerite (fig. 8C). Both the needles and the compact 

variety are speckled with minute inclusions of galena; 

the larger ones sometimes display cubic outlines. The 

massive variety of brannerite is often characterized by 

lath-shaped inclusions of pyrrhotite (fig. 8C) recognized 

by Ramdohr (1958b) as being an aid in identifying brannerite in both Blind River and Witwatersrand samples. 

Minute dustlike inclusions of anatase have been observed and have been explained as the radioactive 

breakdown of uranium in the brannerite lattice, with the 

remaining Ti02 subsequently undergoing reconstitution 

to anatase. Close association of brannerite with patches 

of rutile is also common (fig. 8C). In places, massive 

brannerite forms overgrowths on detrital uraninite 

grains and has a markedly lower reflectivity (see fig 

1L4, B) than the uraninite. Pending the accumulation of 

confirmatory chemical evidence, it is suspected that the 

"secondary" uraninite described by Liebenberg (1955) is 

largely identical with the compact variety of brannerite.

Peculiar brannerite aggregates are represented by the 

so-called "uraninite ghosts" originally described by Ramdohr (1955). Subsequent work (Ramdohr, 1958b; 

Schidlowski, 1966a) was able to show beyond doubt that 

these "ghosts" actually represented pseudomorphs of 

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 CONGLOMERATES

FIGURE 9. -Pseudomorphs of brannerite after grains of detrital uraninite, so-called "uraninite ghosts." In A, C, and E the abraded condition of the 

uraninite parents has been excellently preserved. With crossed nicols (D), the muffin shape of C is accentuated by abundant internal reflections of the brannerite. Other "ghosts"derive from hypidiomorphic U02 parents (B, F, G). The central parts of the pseudomorphs are leached 

and the cavities partially invaded by authigenic chalcopyrite, pyrrhotite, and usually a little galena (A, B, E, G). Several grains are surrounded by aureoles of reconstituted sulfides, mostly lath-shaped pyrrhotite crystals (A, E, F) or pyrite (G). Note minute specks of galena in the 

brannerite matrix of all aggregates. All examples from Orange Free State reefs. Oil immersion.
URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY N9

rule, these "ghosts" excellently preserve the original 

forms of the parent U02 particles, and many of them exhibit distinct muffin shapes (fig. 9A, C). The central portions of the "ghosts" are usually leached, with the 

resulting cavities filled, in part, by reconstituted 

chalcopyrite, pyrrhotite, and galena (fig. 9A, B, E, G). 

Often the grains are also surrounded by aureoles of 

authigenic sulfides, most conspicuous in figure 9£", G.

X-ray spectrographic analyses usually show the 

presence of titanium, uranium, thorium, and lead along 

with traces of iron and niobium. Microprobe scanning

pictures reflecting the distribution of the principal 

elements in two samples of Witwatersrand brannerite 

are presented in figures 10 and 11.

Among the diagnostically valuable optical characteristics of brannerite listed by Ramdohr (1958b, 1969) and 

by Ferris and Ruud (1971) is its lower reflectivity 

relative to uraninite (fig. 11) and to rutile (fig. 8Q. While 

the compact variety of the mineral shows almost no internal reflections, the latter are abundant in felted nests 

of needle-shaped crystals or "cloudy" brannerite 

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 as 

compared 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 the 

neighboring 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 occasional low birefringence has been reported for other 

brannerite occurrences.

Radioactive blasting and corrosion phenomena in the 

banket caused by radioactive decay of brannerite have 

been recorded by Ramdohr (1958b) and the writer 

(Schidlowski, 1966c).

ORIGIN OF THE WITWATERSRAND BRANNERITE

According to the microscopic evidence summarized in 

figures 8, 9, and 11, there can be no doubt that brannerite is an authigenic constituent of the Witwatersrand 

conglomerates. With a syngenetic origin excluded, it 

must be a secondary mineral formed as a result of a 

large-scale reconstitution of primary detrital minerals 

during later processes of diagenesis and metamorphism.

Ramdohr (1957) was the first to assume that the brannerite from the Blind River deposits had originated from 

detrital 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 the 

conglomerates. With uraninite and rutile present in the 

Witwatersrand reefs in large quantities, the same reaction should have occurred there also, probably enhanced 

by the low-grade (greenschist-facies) metamorphism to 

which the Witwatersrand region was subjected. 

Besides, it is well known that several thermal events, 

Ventersdorp volcanism, intrusion of the Bushveld Igneous 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 virtually 

unmetamorphosed (Roscoe, 1969), the "Pronto reaction" 

can be expected to proceed readily of its own accord in 

"normal" diagenetic environments. It has been shown 

that brannerite can be synthesized easily from U02 and 

Ti02 in the laboratory (Patchett and Nuffield, 1960).

With partial mobilization in the banket of both 

uranium and titanium, obvious replacement phenomena, 

as manifest in the formation of "uraninite ghosts," can 

be 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 as 

a result of a random encounter of uranium- and 

titanium-bearing mineralizing fluids. Details of the process 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 obvious 

that the brannerite, because of its sparse lead content, is 

considerably younger than the detrital U02 constituent 

of the reef, which conclusion is consistent with the 

assumed mode of brannerite formation. Possible consequences of this type of radiogenic lead distribution for 

the interpretation of the uranium-lead ages hitherto proposed 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 Witwatersrand conglomerates (Ramdohr, 1955; Schidlowski, 

1970a, p. 48 ff). Alteration of detrital ilmenite constituents to rutile, as microscopically evident in many excellent samples, should also have contributed extensively to the Ti02 budget of the banket. According to Ferris 

and Ruud (1971), the brannerite of the Blind River province has principally formed from detrital ilmenite that 

was diagenetically altered to rutile and anatase in the 

presence of dissolved uranium.

CARBONACEOUS MATTER

MICROSCOPIC OBSERVATIONS AND THEIR GENETIC 

SIGNIFICANCE

The widespread occurrence of "carbon" constituents in 

the Witwatersrand conglomerates was well known by 

early workers (Young, 1917) and has received increased 

attention since the discovery of the pronounced radioactivity of the material. The application of reflected light 

microscopy to the study of "thucholite"3 (Davidson and 

Bowie, 1951; Ramdohr, 1955; Liebenberg, 1955; 

Schidlowski, 1966a, 1966b) has shown it to be a 

heterogeneous mixture of "carbon" and uraninite with 

varying amounts of practically all other minerals contained in the neighboring reef.

For a genetic interpretation of this peculiar carbonuraninite association, ore-microscopic observations have 

turned out to be of prime importance. The initial stage in 

the formation of such aggregates is represented by 

uraninite particles transected by stringers, and coated 

by 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 the 

carbon aureoles and the crack fillings is documented in 

figure 12C and D, in which the muffin shapes of the 

original 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

relict bodies floating in the groundmass of nodular carbon aggregates. Specks of visible carbon thus originated 

have come to be known as "fly speck" carbon. Typical of 

the replacement process is a "drifting apart" of the 

resulting uraninite relicts, which, however, still fit each 

other excellently, like pieces of a puzzle (fig. 13D-G).

In a remarkable consensus, the principal oremicroscopic investigators have explained these finding 

in terms of an encounter of a pre-existing uraninite 

phase and mobile, preferably gaseous, hydrocarbons. 

These hydrocarbons, while migrating through the 

strata, were obviously polymerized by ionizing radiation

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 be 

readily 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 CONGLOMERATES

FIGURE 13. -Advanced stages of uraninite replacement by carbonaceous matter. The primary uraninite phase has largely disappeared except for 

small 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 enlargement 

of 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. Examples 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 

reef. With the process of polymerization entailing a 

transition into the solid state, the mobile organic 

substances were fixed in close proximity to the sources 

of radiation, thus accounting for the carbonaceous 

aureoles and crack fillings of many U02 particles. The 

characteristic "drifting apart" of the uraninite 

fragments during replacement (fig. 13) may be conveniently explained by a continuous supply of mobile 

organic material to the interface between the hydrocarbons already solidified and the remaining pieces of 

uraninite.

This concept for the origin of the Witwatersrand 

thucholite is plausible because processes of radiolytic 

polymerization of simple, mostly unsaturated, hydrocarbons by a and 7-radiation have been simulated experimentally by various workers (Lind and Bardwell, 

1926; Lind, 1928; Colombo and others, 1964). Radiation 

effects on the carbonaceous material can also be seen as 

haloes around many uraninite inclusions (figs. I3E, 

ISA). More difficult to explain is the mechanism of 

uraninite replacement by carbon constituents as is seen 

under the microscope. Microprobe scanning pictures 

(figs. I3F, G, 14) show the carbonaceous material to be 

virtually devoid of uranium. Accordingly, the replaced 

U02 constituents are obviously not resorbed or 

"digested" by the organic materials in some form such as 

organo-metallic compounds. This does not mean, 

however, that such compounds may not have been instrumental in a transitory phase in the breakdown and 

mobilization of uraninite. Pending a better understanding of the chemical background of the replacement process, a final assessment of these matters must be postponed.

Another mechanism for bringing about a uranium carbon association that is not less important 

geochemically than the one described should be mentioned here. It is well known that hexavalent uranium of 

the soluble uranyl complex [U6+ 02]2+ , which is often carried by weathering solutions, may be precipitated as 

U4+ 02 in the reducing environment of sedimentary 

organic matter (McKelvey and others, 1955, p. 472). 

This reaction, for the most part, is responsible for the 

conspicuous uranium concentrations in some coals, 

lignites, peats, and the like, which have been known for 

a long time (Berthoud, 1875; Breger and others, 1955). 

The economically important uranium province of the 

Colorado Plateau (Rosenzweig and others, 1954; 

Gruner, 1955) was formed mainly in this way. In this 

area, fossil wood is commonly replaced by pitchblende, 

the low-temperature, Th02-poor cryptocrystalline variety of U02 . In the Colorado Plateau, organic materials 

concentrate uranium by providing a change in redox 

potential favorable for precipitation of the uranyl complex carried in water, whereas in the Witwatersrand 

deposits migrating hydrocarbons are concentrated 

around preexisting uranium-bearing minerals as a 

result of radiolytic polymerization. Although both processes produce a close association of uranium and carbon 

compounds, they achieve the association by completely 

different geochemical pathways.

Attempts to invoke a Colorado-type pedigree for the

U

FIGURE 14.—Microprobe scannings for uranium and thorium of the carbon-uraninite aggregate depicted in 

figure I2D. It is evident that the distribution pattern of uranium almost coincides with the areas occupied 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-temperature 

uraninite. Besides, it is well known that a large part of 

the uraninite fraction of the banket is not associated 

with 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 critical 

examination.

CHARACTERISTICS OF MAJOR CARBON ACCUMULATIONS

With the uraninite fraction mostly concentrated at the 

reef footwalls as part of the heavy suite, the bulk of the 

organic 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 in 

figure 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, these 

carbon bands may vary in thickness from fractions of a 

millimeter to some centimeters, giving rise to a special 

type of "carbon reef," the prototype of which is 

represented by the economically important "Carbon 

Leader" of the West Rand. The carbonaceous material 

of such bands is sometimes massive, but often displays a 

characteristic 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 a 

seam 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 accumulated. The bands are particularly rich in 

redeposited gold (fig. 15); locally it preserves as casts the 

orthogonal pattern formed by the intracolumnar interstices, and elsewhere it fills wedgelike cracks transecting the individual fibres perpendicular to their long 

axes (fig. 17A-C). With the exception of fragments of 

the primary U02 particles, which are spread throughout 

the carbon matrix, all allogenic components are practically confined to the interstices between the fibres.

Rounded grains of pyrite, quartz, iridosmine, and other 

minerals are engulfed by strands of carbon, and this 

feature 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 the 

fibre direction is presented in figure 1&4, the ends of the 

individual fibres showing up as lenticles often centered 

by small uraninite inclusions. These inclusions have 

given rise to radioactive haloes within the surrounding 

carbonaceous matter in which the reflectivity is markedly 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 evident that the carbonaceous matter in its present form is 

clearly 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-scale 

reconstitution 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, the 

writer (Schidlowski, 1966b), has drawn analogies to the 

mode of origin of fibrous mineral aggregates grown at 

right angles to the bedding planes within intrastratal 

partings. Such phenomena are well known from 

minerals like gypsum, rock salt, calcite and many 

others. 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 the 

crevice. Although applicable only with certain modifications to the Witwatersrand carbon, this model is likely 

to give a satisfactory explanation for the occurrence of 

the columnar thucholite bands, which preferentially 

came to occupy so-called "footwall partings" at the base 

of individual conglomerate horizons.

Recently, however, Hallbauer and van Warmelo 

(1974) have offered an alternative explanation, relating
B.

i

Q

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).
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 the 

quartzite 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 a 

fungus. A reconstruction of Hallbauer (1975) of this 

plant (fig. 19) does indeed suggest affinities to modern 

lichens. According to these authors, the carbon fibres 

represent the fossil remains of the columnar isidia of a 

lichenlike symbiont believed to have carpeted the floor 

of the primary depositional basin as extended colonies 

and to have trapped gold and other detritals by a sort of 

corduroy-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 evolutionary history of ancient life will have certain reservations as to the concept as a whole. These reservations 

may be summarized as follows:

1. The fact that this putative symbiont appears just 

once in the Earth's history must necessarily raise 

suspicion, since lichens could be expected to stand 

for a rather "conservative" type of organism, which 

should have appeared repeatedly in a similar morphological configuration in the paleontological 

record. This contrasts markedly with some organosedimentary structures attributable to algae, which 

have been found from almost the start of the 

sedimentary record until recent time.

2. Both the individual carbon fibres and their aggregates do not display a morphological differentiation 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 the 

fibres are structural remains of ancient plants. 

Ultramicroscopic evidence claimed to favor a 

biologic nature of the columnar carbon appears to 

be 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 the 

banket. 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 precipitation 

of the metal by biologic agents. In general, both 

gold and uraninite are always found at the reef 

footwalls, irrespective of the presence or absence 

of carbon within a particular conglomerate horizon.

3. The putatively biogenic structures are preserved as 

massive coaly matter. If these fibres really represent structurally preserved remains of a lichenlike 

symbiont, the carbon content of the primary plant 

tissue was certainly smaller by at least one or two 

orders of magnitude than the carbon content of the 

present-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 of 

these layers derived, at least in part, from mobilized fractions of sedimentary organic material in 

which the light isotope, 12C, had been increased as a 

result of a "chromatographic" effect. Sedimentary 

organic 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 represent 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 filled 

with 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 to 

the 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 markedly 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

FIGURE 19.—Reconstruction of the lichenlike symbiont putatively 

responsible for the fibrous carbon aggregates at the base of reefs 

(Hallbauer, 1975, fig. 35; published with permission of the National 

Institute for Metallurgy, Minerals Science and Engineering). The 

present carbon fibres are believed to represent the fossilized columnar isidia of the original plant.

about 2.5 x 109 years B.P., is incompatible with the 

currently established timetable of evolutionary 

diversification of ancient life. Although the timing 

of the advent of eucaryotic organization is presently under dispute (Knoll and Barghoorn, 1975), 

there is general agreement that this quantum step 

in evolutionary history cannot have substantially 

antedated the benchmark of some 1.5xl09 years 

B.P. (Schopf, 1974). It might, of course, be argued 

that this benchmark is not yet the definite one, but, 

for a variety of reasons, a major shift towards an 

earlier age is unlikely to be expected. It seems 

justifiable, therefore, to state at this stage that the 

appearance some 2.5x 109 years ago of a symbiotic 

association involving a fungal (that is, eucaryotic) 

partner is as improbable as the discovery of a 

dinosaur in the early Paleozoic. 

In conclusion, the interpretation of the fibrous 

thucholite aggregates as fossilized plants must be rated 

as almost certainly incorrect and, in the opinion of this 

writer, may safely be discarded. The same applies to 

metallogenic concepts based on a large-scale participation of living organisms in the process of concentration 

of the principal pay metals, notably gold and uranium, in 

the banket. Unless they are substantiated by clear-cut 

paleontological or biogeochemical evidence, restraint 

should be practiced in advocating such hypotheses, as 

the economic mineralization of the Witwatersrand may 

be reasonably explained in terms of "conventional" processes operative in the formation of placer deposits.

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, stemming from the decomposition of contemporaneous 

organic material buried in the sediments. A derivation 

from 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 of 

the carbonaceous substances.

Consistent with the assumed biogenicity of the 

material is its isotopic composition (Hoefs and 

Schidlowski, 1967; Hoering, 1967), the strongly 

negative 613C values with a minimum at -37.1 permil 

relative to the NBS standard suggesting genetic affinities 

to oil and asphalt rather than to "normal" kerogens. The 

6-values of carbon from Blind River fall within the same 

range (Eichmann and Schidlowski, 1974). The reported 

presence of a suite of amino acids (Prashnowsky and 

Schidlowski, 1967) requires confirmation by more 

sophisticated techniques allowing a differentiation between indigenous substances and possible contaminants. 

A biogenic derivation of the carbon is also supported 

by the occurrence within Witwatersrand rocks of 

microstructures suggestive of biologic affinities, as 

previously noted (Schidlowski, 1965, 1970b; Oberlies 

and Prashnowsky, 1968; Pflug and others, 1969; 

Hallbauer and van Warmelo, 1974). The most convincing evidence for biologic structures are myceliumlike 

filamentous aggregates described by Hallbauer and van 

Warmelo from ashed residues of carbonaceous matter. 

With such aggregates explained by their discoverers as 

fossil fungi (that is, eucaryotes) and their occurrence 

within the rock not yet confirmed by petrographic slides, 

there were necessarily strong doubts as to the indigenous character of these structures. However, according to E. S. Barghoorn (personal commun., 1975), the 

indigenous character of these aggregates cannot be 

doubted, but their interpretation as fungi is incorrect. If 

a tentative interpretation of these remains as fossil 

bacteria (Actinomycetes) turns out to be correct, these 

forms should contribute substantially to the stock of excellently preserved microfossils hitherto described from 

Precambrian rocks.

PROBLEMS OF URANIUM MINERALIZATION IN 

PRECAMBRIAN CONGLOMERATES

With our present knowledge of the regional geology of 

the Witwatersrand ore province (Brock and others, 

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 be 

safely dismissed. Nevertheless, the occurrence of 

uraninite constituents in a detrital assemblage raises 

some questions, which are briefly addressed in the 

following discussion.

It is well known that uraninite is thermodynamically 

unstable in the present weathering cycle (Davidson and 

Cosgrove, 1955; Zeschke, 1959a; Dybek, 1962). 

Tetravalent uranium in uraninite is readily oxidized to 

the hexavalent state in oxygenic environments, thereby 

giving rise to the soluble uranyl complex [U6+ 02]2 + , 

which is subsequently carried away by weathering solutions. Accordingly, uraninite is practically nonexistent 

in modern placers, the few noteworthy exceptions 

described (Steacy, 1953; Zeschke, 1959b; Trushkova and 

Kukharenko, 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 Serra 

de Jacobina, fossil uraninite placers are practically 

unknown throughout the sedimentary record. The occurrence, therefore, of uraninite in economic grades in 

lower and middle Precambrian detrital rocks must 

necessarily appear enigmatic.

Various efforts have been made to reconcile the 

presence of detrital uraninite in these ancient formations with the chemical instability of the mineral in the 

exogenous cycle. Ramdohr (1958a, b) conjectured that 

the U02 constituents of the reef were still geologically 

young and relatively refractory when subjected to the 

stress 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 Witwatersrand System, invoked the effects of a cold climate for the 

survival of the uraninite fraction. Several investigators 

have, on the other hand, entertained the view that the 

persistence of uraninite as a detrital mineral might indicate an extremely low redox potential of the ancient 

environment—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), is 

largely consistent with current concepts of atmospheric 

evolution (Berkner and Marshall, 1967; Cloud, 1968; 

Schidlowski and others, 1975; Junge and others, 1975;

Schopf, this volume, chap. B). Although uraninite is 

thermodynamically stable only at a P02 ~ 10~21 atm, 

Grandstaff (this volume, chap. C) has concluded that, at 

partial pressures of oxygen between 10~2 and 10~6 of the 

present 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 and 

related uranium-bearing conglomerates. The ubiquitous 

presence 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, particularly those close to the so-called "Urey level" of 10~3 of the 

present partial pressure of oxygen, have long been 

regarded as typical for the geological time span prior to 

2 x 109 years ago. The concept of an ancient anoxygenic 

weathering cycle would therefore provide a general explanation for the worldwide occurrence of pyriteuraninite placers during the early history of the Earth 

and is also consistent with an impressive array of other 

relevant evidence (fig. 20, lower part). At this stage, the 

postulate of a substantially lower redox potential in the 

ancient 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 chance 

of survival, irrespective of its thermodynamic properties. Hence, an ultimate acceptance of the "placerist" 

concept for the uranium mineralization of Precambrian 

conglomerates would not necessarily imply that the occurrence of uraninite among the detrital minerals is accepted 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 of 

weathered debris from the surrounding borderlands.

However, in the opinion of this writer, these 

arguments may be severely weakened by the following 

considerations: With the age of all known uraniumbearing conglomerates more than 2 x 109 years and the 

mass half-age (life) for sedimentary rocks lying in the 

range 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 approach 3 percent as a minimum. Within these scanty 

relics of an originally huge sedimentary column, detrital 

uraninite is apparently well represented on a worldwide 

scale, whereas all younger sediments, whose chances of
URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WTTWATERSRAND METALLOGENY N21

0.2

(02 )t =0.53eM tp (1-e^)

= 1.16x10rV)

Rise of 02 level in 

atmosphere-ocean system

\

02 bound 

(-39%)

in Fe2 03

0 2 bound in 

(-56%)

02 in atmosphere

/

ocean system 

(-5%)

t=0 1.0 2.0 3.0 4.0

4.5 4.0 3.0 2.0 ^_... i i 11 i i

Milestones of organic A ^ 

evolution (D (D

Occurrence of conti- ^^^^^

nental 'red beds'

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

uu2 piacers

oanaea ironstones

1.0 C

—— A AA ——

4.5x109 y 

Ox109 y

t

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 the 

accumulation 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 circuit 

of the ancient oxygen cycle in the Precambrian seas. Tentative rise of free oxygen level in atmosphere-ocean system (black field) is inferred 

from 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 extensively examined by generations of field geologists and 

because prospecting for uranium has been carried on at 

a 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 is 

characteristic 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 geological 

past; notably, rapid burial of weathered debris has occurred during numerous sedimentary cycles, as illustrated by the presence of numerous graywackes and 

other poorly sorted sediments in younger geological formations. Accordingly, it is very difficult to account for 

the complete absence of pyrite-uraninite deposits from 

younger sediments without invoking changes in the 

redox potential of the environment. Confinement of 

pyrite-uraninite placers to the early and middle Precambrian would certainly be too much of a coincidence if the
N22 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

formation of such deposits were governed exclusively by 

geological factors.

ACKNOWLEDGMENTS

Research leading to the preparation of this paper has 

been supported in various stages by the Deutsche 

Forschungsgemeinschaft (lately as part of SFB 73). 

Moreover, thanks are due to Paul Ramdohr and J. W. 

Schopf for reading a preliminary draft of this paper or 

parts thereof.

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N24 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

DISCUSSION 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 Precambrian 

micropaleontology 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 last 

two 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 are 

reporting objects that we interpret as Precambrian 

fossils. 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 date 

from the time of original sedimentation. And secondly, we must establish that these objects are of 

biological origin; regardless of whether we interpret 

them as fungi, algae, or what have you, there must be 

a set of criteria that allows us to say without doubt 

that 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 to 

most of us, this is a very difficult sort of game to play. 

It is often, or at least in some cases, noi easy to know 

whether the structure you are looking at is biological 

in origin or not: it is usually somewhat easier to 

establish 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 the 

generalizations 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 aggregates of organic matter within the Witwatersrand 

rocks. I think that is well established indeed. Personally, I do not believe that he has established at all that 

these structures are biological. Without getting into 

the great details involved, it is my opinion that the 

branching 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 a 

number 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 anastomose 

back 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 like 

that 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 be 

preserved. These are not permineralized organisms, if 

they are organisms. They are preserved in a sedimentary 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 the 

biogenicity 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 well 

established, but how do we know that they are of 

Precambrian age? This is a problem with looking at 

microscopic objects with a scanning electron 

microscope: it is a terrific tool for certain sorts of 

studies. But if you want to prove those objects are of 

Precambrian age, it seems to me that the best way to 

study them is in thin sections. Furthermore, it seems to 

me that if they are biologic in origin they ought to 

have the sort of regularity that one sees in modern 

biologic systems. I would tend to have interpreted the 

objects that Dr. Nagy showed as probably bits and 

pieces of organic material from the Witwatersrand 

rocks 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 lack 

the 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 who 

deal with Precambrian fossils to nail the case down 

just as firmly as we possibly can, to establish that 

these things are both Precambrian in age and of 

biologic organization.

Hallbauer: I just want to point out that in scanning 

electron 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 

of Precambrian origin. They are in the rocks, and 

there is no reason to believe that they were introduced 

at a later stage. With regard to the other structures, I 

am not a biologist, I can only say what other biologists 

have told me with regard to the filamentous structures. They have been identified as organisms by 

Preston Cloud, although he had reservations and said, 

"How did they get into the rock?" Some biologists say 

they are biological structures, some others have reservations, so it is very difficult to decide from my point 

of view as a mineralogist.

L. Nagy: I must say that this is one of the nice things 

about small scientific meetings, when you can get 

together with your colleagues and they sort of 

disagree with you. This helps keep you on your toes, 

and I think that what Bill [Schopf] said about my work 

is indeed quite true. One of the things that I should 

have pointed out is that I have looked at thin sections 

of the carbonaceous matter, but unfortunately it is so 

very opaque you really can't see too much in it. So I've 

got to scratch my head and go back to the drawing 

board to see how we can get petrographic thin sections that are made in such a way that we can see 

structures in them. He had a very valid point. I have 

never published a paper of any structures that have 

not also been found in thin sections. And this work will 

not be published until structures are indeed found in 

petrographic thin sections.

Robertson: I have a comment for Dr. Schidlowski with 

reference to the brannerite reaction. Ramdohr called 

that the "Pronto reaction" from the Pronto mine in 

Canada. That's the farthest south of the Elliot Lake 

deposits and is the one which would be more subject to 

later metamorphic events. In the deposits farther 

north there are a number of uranium-bearing beds. Information is really rather scanty, but I think the indication that we have is that the brannerite content 

varies within the bed relative to uraninite and it varies 

from bed to bed, so that where beds are stacked upon 

one another, we do not in fact find the same 

brannerite-uraninite ratio. This might suggest that 

some mechanism other than metamorphism was 

responsible.

Schidlowski: I would suggest that the amount of Ti02 

present is also one of the prerequisites. So metamorphism is probably only one factor. That would be my 

answer to this.

Skinner: I am not a biologist either, but I think that I 

have seen Dr. Hallbauer's structures before. I have 

seen them in coke. I would submit that perhaps much 

of what he is seeing is an artifact of vesiculation. As 

for the filamentous structures, I wonder if one should 

not look for an origin for them in fibrous growths of

the kind that one sees in asbestos. And I would suggest that perhaps a line of research to follow on that 

would be single crystal X-ray crystallography and see 

if some of your filamentous structures are not in fact 

single crystals.

Hallbauer: Well, there again I can only repeat what I 

was told by biologists. If you can show microscopic 

preparations of these filaments to a mycologist and he 

recognized them immediately as fungal structures, 

then there is nothing for the geologist to do.

Skinner: Would you permit me to tell a short story? I 

was working one time in Australia in some shallow 

lakes, and it was in a region where a number of algal 

colonies form little biscuit-shaped calcium carbonate 

growths. In the lake I was working in I saw something 

that looked a little bit familiar. It was about the size of 

a thumbnail and had a rounded shape and I thought, 

"Hello, here's another one of these things, but a 

different shape." So I gathered a bunch of them, took 

them back to the university, and gave them to the 

biologist who looked at them and said, "Yes that's 

true. That's just what they are. They are some kind of 

algal growth. They are new, and we'd like to describe 

them." They took about about a dozen of them and 

dissolved the calcium carbonate away, and then 

described the gelatinous cellular structures that were 

left. They had even gone to the point of writing a 

paper and submitting it to the Royal Society of South 

Australia, at which it had been accepted. I mentioned 

this one day to Martin Glassner, who many of you 

know is a paleontologist, and he said, "What did that 

thing look like?" So I described it. He said, "Have you 

got any more?" And I pulled one out of the drawer, 

and he said, "Well, I know what that is, that's a 

gastrolith from a crawfish, and the cormorants have 

been eating those things." And true enough, I'd picked 

these up underneath a log where the cormorants had 

been sitting and passing them through their bodies 

and dropping them into the lake. So I am not sure that 

showing it to a biologist always gives you a definitive 

answer.

Hallbauer: In that case, to what authority do you go?

Schidlowski: Originally I wanted to briefly comment on 

Dieter's [Hallbauer] talk, but when Bill [Schopf] stood 

up I got a little bit mixed up, so I propose to give my 

contribution as if I hadn't heard Bill's comment. I 

would have said, ... I would personally differentiate 

clearly, and I am still convinced that the fibrous stuff is 

not biogenic, at least the pictures which I have seen 

have not yet convinced me. Whereas, in contrast to 

Bill, I would have been inclined to acknowledge the 

other stuff. I also remember that Pres Cloud when we 

met him—Bill, you were present in Leicester—that 

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 

that South African journal. Perhaps you should give it 

a look and think about it. But, anyhow, you understand more of that than I do, so it would be just a row 

between Pres Cloud and you.

Schopf: You know, this is getting to be a little bit like 

what I refer to as anecdotal science. If I recall the conversation of which I believe you are speaking, Pres 

Cloud's point was that in Dr. Hallbauer's paper in 

Precambrian Research, Cloud interpreted the 

filaments shown in optical photomicrographs purporting to show cells to be certainly biological and certainly contamination.

Schidlowski: Yes. He said they looked too modern, and 

he suspected they were contaminations, but anyhow 

they were biologic. We should perhaps split the problem in two portions. First, is that stuff biogenic, and 

second, is it indigenous? I would yield to the first one, 

and I will concur with you that perhaps the second 

point needs to be proved.

Myers: I would simply like to ask how the temperatures 

of oxidation were measured in Dr. Hallbauer's oxidation procedure.

Hallbauer: There was no accurate measurement of 

temperature. It was measured on [some?] element 

nearby. So it was plus or minus 500°C, say plus or 

minus 10 to 20 degrees.

Ruzicka: Dr. Schidlowski mentioned extinction of 

hydrocarbon within one or around one grain of 

uraninite.

Schidlowski: What do you mean by "extinction"?

Ruzicka: This is what I am going to ask you, because we 

observed that if you turn the table under crossed 

nicols . . .

Schidlowski: Oh, you refer to the reflection pleochroism.

Ruzicka: Yes. You get simultaneous extinction in large 

areas. How do you explain it?

Schidlowski: I would say there must be a certain pregraphitization stage in which the carbonaceous 

material tries to approach a certain order, finally, of 

course, aiming at graphite but not reaching it at the 

moment.

Ruzicka: This is your explanation?

Schidlowski: Yes. But I think there are other experts 

who have better explanations to offer, but I think as a 

first approximation I should be right. Actually, the 

stuff I showed you is not graphite at all. Many people 

have found traces of graphite in the reefs and usually 

associate it with that carbonaceous stuff, but if you 

would do some X-ray work you would not get graphite 

patterns. By the way, in reflected light microscopy it is 

very easy to distinguish between carbon in the widest 

sense and graphite in the proper sense, as anybody 

familiar with this type of work will admit.

Hallbauer: In some of Dr. Schidlowski's recent papers 

he describes the occurrence of pristane and phytane as 

chlorophyll derivatives in the carbonaceous material. 

Now, how does he explain the presence of these things 

in hydrocarbons which can be floated through the rock 

and were polymerized by uranium?

Schidlowski: Pyrolysis gas chromatography was done on 

the carbonaceous matter, and we got a whole pattern of hydrocarbons. We used mild pyrolysis, so we 

got all this stuff which is volatile up to let us say 

250°C. Among these graphs we got small peaks in the 

place where one could expect phytane and pristane. 

We took these peaks as real, but we did not confirm 

them with mass spectrometry, so I wouldn't put my 

head on the block for this. When we get a chance to go 

deeper into the matter, we will certainly come nearer 

to the truth. I think it is justified to record that at least 

traces of these degradation products are actually present. If that stuff is mobilized, it must have been taken 

along with the other hydrocarbons when they 

migrated through the sequence.

Paul L. Weis, U.S.G.S., Reston, Va.: Dr. Schidlowski, 

one gets the impression in looking at your photomicrographs of the progressive decomposition, disintegration, or disaggregation of uraninite grains that this is 

a process that takes place over a considerable period 

of time. The implication may be that this is not a syngenetic effect, but something that takes place afterwards, in which case it answers at least part of your 

comment that, whether it is biogenic or not, it can't be 

an original constituent of the rock. The other aspect of 

this question is, as the uraninite is attacked by the 

hydrocarbons, where does the excess uranium go?

Schidlowski: Into the brannerite, for instance, but here 

reservation is justified, of course. If we admit that the 

[carbonaceous] stuff has migrated, any one is justified 

in asking, "Where does it come from?" And those 

familiar with the whole story know that Davidson 

originally contended that the stuff came from overlying Karroo coal measures. But hydrocarbons are light, 

and because of that they do not sink into basins, and 

there are some other points against a source in the 

coal measures. If you rule out the Karroo measures, 

we must look for sources of the carbon. We know what 

is below and we also know what is above in the sequence, and there are actually no strata which contain 

massive concentrations of reduced carbon in any 

modification. So I think, although we could not settle 

the topic definitely, we can narrow it down and say 

that, with 90 percent probability, the stuff is indigenous in the widest sense, although I must admit 

there is a slight possibility still open for extraneous introduction.
URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY N27

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 and 

the nature of that type of material is in question on the 

basis of what was said here. First of all, what we have 

found is that organic matter that has been subjected to 

lengthly irradiation by uranium and its daughter products tends not to go to graphite, but instead takes on a 

diamond-type structure, for the simple reason that the 

graphite type lattice tends to buckle and you get a 

more diamond-like structure, which is harder. I suggest also that when we talk about organic matter and 

this type of structure in the future, it would be very 

nice to know that this organic matter contains more 

than just carbon. Is there oxygen in it, too? How much 

hydrogen? Just knowing that it contains carbon really 

doesn'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 we 

have a sedimentary situation in which we have been 

washing down, gold, uraninite, and other heavy 

minerals, and we have a rather quiet situation in 

which we can also get perhaps some life forms 

developing. Let's assume then that these life forms 

were decomposing, because obviously they decomposed, 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 for 

methane 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 other 

mineral grains have now come down, they are buried 

with this organic matter at the bottom of a shallow 

basin, and the organic matter, being what it is, probably 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 the 

same material, as far as I can tell.

There is another question I'd like to raise here with 

you and that is: In some of the pictures you have 

shown, you very clearly indicated that the organic 

matter was highly metamorphosed in the sense it was 

much more highly reflective than you would have expected. Why wasn't the same organic matter in the 

crevices of the actual uraninite grains just as equally 

reflective as that that was not associated with 

uraninite grains?

Schidlowski: I think they are photographed in a position where the reflection pleochroism was responsible

for a dark strip, so if you turned your microscope 

stage it would have cleared up.

Breger: I don't think so. I think you have picked an 

easy 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 introduction 

of the uraninite grains into the sedimentary basin, the 

time that it has taken for degradation of the uraninite 

grains by the decomposing acidic organic matter so 

that perhaps the grains are cracked and carbonaceous 

material has moved up into and backfilled it? I'm not 

sure that this is a replacement in the truest sense of 

the word.

Schidlowski: How would you account for that? What 

alternative would you offer?

Breger: Let's take a uraninite grain that is immersed in 

a collodial type organic goo, sitting at the bottom of a 

basin, and this uraninite grain has a few little 

microcracks in it and attack occurs in this goo at the 

cracks 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 penetrate into the cracks of other minerals, for instance, 

the ubiquitous pyrite, the chromite, the zircon, and so 

on? I think this imposes certain limitations.

Breger: I think we are missing the chemistry. First of 

all, 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 other 

material. 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 said 

mechanical and chemical. I said that the acidic environment will lead to an attack on the uraninite 

grains.

Schidlowski: Actually, the point is that only in the 

uraninite particles do we have that stuff and no other 

cracks. 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 these 

plants—well assume that they are plants for the 

moment-it sure would mess up their life process, so 

that 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 thought 

these two papers were just excellent.

Schidlowski: The mechanism which you alluded to now 

is, I think, by far the most probably reason for 

bringing 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 differentiate between Colorado-type uranium-carbon associations and Witwatersrand type.

Breger: I wish you wouldn't even mention them in the 

same breath.

Hallbauer: There is one remark that was made about 

these structures being similar to coke. We have 

thought about this possibility, and we've done some 

work and we are convinced that coke looks totally 

different. The pores in coke have a totally different 

structure from the ones shown here. They are irregular 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 they 

shouldn't be preserved in such an excellent state 

because the Witwatersrand rocks have a history of 

very minor tectonic movements and most of the rocks 

have not been folded. There is only very slight 

metamorphism, and most of the organisms have been 

preserved 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 faults 

of 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 was 

very little folding of the Witwatersrand strata, and 

Dr. Whiteside is quite right in pointing out these 

various structures. Indeed, in the vicinity of faults, 

the carbonaceous material is completely powdered and 

does not show these structures. I have just selected 

parts which have been preserved better than the other 

ones.

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 of

the 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 is 

biological.

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, is 

going to be given by me, which will be a continuation 

of 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 to 

measure the amount of radiation to be expected over a 

certain period of time or a method for measuring the 

amount of radiation damage to carbon?

Schidlowski: To whom was the question addressed? 

The only radiation damage I could think of is just the 

increase in reflectivity. At the moment I can't recall 

any other. Can you?

Armstrong: There was a slide I thought which showed 

some radiation damage.

Schidlowski: Yes, you get cracking halos, but they are 

a little bit different. Mostly they must be ascribed to 

zircons, because if zircons undergo metamictization 

that results in a volume increase, then all the 

neighboring stuff is blasted. As for radiation alone, 

some little blasting halos have been observed around 

uraninite grains, but I think these phenomena are not 

so 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 the 

grains which I referred to as relict bodies within the 

carbonaceous matter, they are between 40 and 50.

Sims: The aspect of preservation potential of these 

carbon seams seems to be bugging a lot of people. I'd 

like to suggest that the process of sand-wave migration, accompanied perhaps by rapid silicification of the 

matrix of the sand, could in actual fact contribute to
URANIFEROUS CONSTITUENTS, IMPLICATIONS FOR THE WITWATERSRAND METALLOGENY N29

the preservation and prevent subsequent destruction 

of fine filamentous, perhaps organic, products 

preserved in these carbon seams. Perhaps this 

thought is way out.

Joel Leventhal, U.S.G.S., Denver, Colo.: Since there 

seems to be a lot of doubt about the biological nature 

of these things, I would like to throw a little bit of 

doubt into the hydrocarbon situation. Living 

organisms in general don't show large amounts of 

hydrocarbons, so I would say that's not a very good 

source for hydrocarbons in Dr. Schidlowski's presentation. Methane is the abundant hydrocarbon that 

micro-organisms produce. The problem is how do you 

get enough of this gaseous methane to stick around 

long enough to be polymerized? It's going to be in 

aqueous solution, and it's probably going to float out 

again. So I think we may be back to the kind of thing 

that Irving Brager is suggesting, that the stuff is just 

setting 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 comparison of the carbonaceous material without the 

uranium compared to the carbonaceous material with 

the uranium can be used as an index of the origin, or 

the radiation damage, or the diagenesis, or any of 

these things.

Schidlowski: I have never seen carbonaceous matter 

without even at least faint relicts of uraninite, but I 

recall that Ramdohr once mentioned that he had seen 

such stuff but not in a proper carbon footwall of the 

reef but from somewhere higher in the reef. I am not 

sufficiently familiar with the occurrence to comment on 

it. At least we cannot rule out that some carbonaceous 

material occurs without uranium, so there must have 

been other fixation processes.

You also mentioned the isotopic composition of the 

carbon. I think there is ample evidence by now that 

undifferentiated organic matter on average yields 

around - 25 permil. If we get values less than - 30, if I 

remember correctly the Witwatersrand goes to - 37,

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 enough 

to prove that it hasn't gone through some kind of 

cracking rather than a biological process. I think most 

petroleum people would say -37 just looks like 

metamorphosed organic matter, not freshly deposited 

material.

Schidlowski: Just to illustrate this point, we had a core 

from the Transvaal Dolomite, and there were carbon 

partings in it. We sampled the carbon partings and the 

rock 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 a 

secondary effect must have been responsible for this 

accumulation of carbon twelve. I think our conclusion 

is fairly reasonable, at least in this case.

Hallbauer: I just want to mention that there are 

carbonaceous seams without gold and with very low 

uranium content. Normally they are of no interest to 

the mining industry, and therefore they are quite easily overlooked.

Schidlowski: The example I was talking about was in 

the Transvaal Dolomite, not in the Witwatersrand.

Ruzicka: We analyzed hydrocarbon in which were embedded idiomorphic grains of uraninite, and the only 

component we found in this hydrocarbon was sulfur, 

about 24 percent sulfur but nothing else, not even 

small 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 Geochemical 

and Microbiological Considerations

By JOHN E. ZUMBERGE, BARTHOLOMEW NAGY, and LOIS ANNE NAGY

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-O

CONTENTS

	Page

Abstract_________________________________________________ 01

Introduction ______________________________________________ 1

Materials and methods ________________________________________ 2

Results_________________________________________________ 2

Discussion _______________________________________________ 4

Acknowledgments___________________________________________ 6

References cited ____________________________________________ 6

ILLUSTRATIONS

Page

FIGURE 1. Gas chromatogram of a Vaal Reef "carbon" sample ___________________________________________ 03 

2. Correlation between the ratio of dimethylnaphthalenes to other naphthalenes and uranium content ________________ 4

TABLE

Page

TABLE 1. Mass spectral identifications of the major gas-chromatographic peaks _________________________________ 03

in

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

SOME 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

ABSTRACT

The solvent-insoluble organic matter (kerogen) that is the predominant syngenetic component of the Vaal Reef carbon seams was analyzed by combined high-vacuum pyrolysis gas-chromatography mass 

spectrpmetry, combined electron microprobe-scanning electron microscopy 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 and 

filamentous 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 of 

admixed clay-carbon microenvironments. This implies physical and 

chemical interactions between clays and organic matter conducive to 

the entrapment of detrital gold during deposition and the crystallization of colloidal/dissolved gold in the carbon seams. High concentrations of organic free radicals (molecules with unpaired electrons) in the 

carbonaceous matter, deduced from electron paramagnetic resonance 

spectroscopy, suggest that radiation emitted from uranium played a 

unique 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 in 

uranium 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 is 

part of the Main-Bird Series of the Upper Witwatersrand Sequence and is approximately 2,500 m.y. old. The 

petrology of the Vaal Reef deposit, inclusive of 

stratigraphical, structural, and mineralogical data, has 

been 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 called 

kerogen, commonly occurs finely disseminated in Precambrian sedimentary rocks such as the Swaziland Sequence (Nagy and Nagy, 1969; Dungworth and 

Schwartz, 1974), the Transvaal and Bulawayan stromatolites 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 sedimentary 

rocks evolved from the biochemical constituents of 

bacteria and simple algae.

The origin of the Witwatersrand carbonaceous matter 

has long been the subject of discussion. Yet, Gamier 

(1896) and Spilsbury (1908) had already considered that 

the "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, and 

lagoonal deposits. This conclusion was based on studies 

of the Main Reef, Main Reef Leader, Black Reef, and 

other reefs of the East Rand Goldfield. MacGregor 

(1953) considered that the Witwatersrand basin was surrounded by algal "peat bogs" and that reducing conditions induced by decaying organic matter helped to 

precipitate gold and uranium. Miholic (1954), Brock and 

Pretorius (1964) and Snyman (1965) also proposed that 

primitive 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 example, Philp and Calvin, 1976). These modern analogs support the contention that the carbon seams are ancient 

algal 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 of 

these considerations, it appears that the nature and 

history of the auriferous-uraniferous Witwatersrand

01
02 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

carbon seams cannot be fully understood without studying 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, all 

glassware used during analysis was first cleaned in a 

mixture of hot, concentrated 85-percent H2S04 -15- 

percent HN03 ; all solvents were freshly distilled. Procedure blanks (the entire analytical procedure without 

samples) were repeatedly performed to search for 

laboratory 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 process removes soluable organic compounds, which can be 

postdepositional 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 hour 

again under 10~6 torr vacuum (Bandurski and Nagy, 

1976). Pyrolysis was selected to degrade the solid 

kerogen to low-molecular-weight fragments because it is 

a relatively simple and effective technique. Under optimum conditions pyrolysis liberates the components of 

polymers unaltered or only minimally altered by interand intramolecular rearrangements. Thermal energy is 

used 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 that 

are absent, as such, in the polymer. Degassing prior to 

pyrolysis was necessary to remove traces of solvents 

and (or) contaminations that may have remained in the 

samples 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 spectrometry 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 been 

somewhat altered from the original kerogen components by thermal energy.

The electron paramagnetic resonance spectra of 

powdered 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 to 

perform both as a scanning electron microscope and an 

electron 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, as 

well as aromatic sulfur components connected by shortchain aliphatic hydrocarbon and alkyl sulfur bridges. 

The polymeric structure is relatively tight and stable 

since it is not broken up in vacuum below ~ 450°C. Note 

in 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 assigned on the gas chromatograph are listed in table 1. 

The pyrolysis products of the other Vaal Reef kerogen 

samples were closely similar to the products listed in 

table 1. Alkyl substituted aromatic conponents and some 

aromatic sulfur compounds constitute the majority of 

the pyrolyzates listed in the table. There is no evidence 

that the sulfur compounds are pyrolytic artifacts. 

Pyrolysis of finely powdered pyrite with an aromatic 

hydrocarbon yielded no organic-sulfur compounds under 

experimental conditions identical with those used for 

the kerogen pyrolysis. The few pyrolyzates that emerged from the gas-chromatographic column below 60 °C
DEVELOPMENT OF THE VAAL REEF URANIUM-GOLD CARBON SEAMS, WITWATERSRAND SEQUENCE 03

are not shown in figure 1 and table 1. These compounds 

were mainly C2-C4 alkanes, alkenes and benzene, and 

minor quantities of low-molecular-weight alkyl sulfides, 

acetaldehyde, acetone, and methanol. No nitrogencontaining organic compounds have been detected. It is 

noteworthy that X-ray diffraction analysis revealed no 

evidence that these Vaal Reef "carbon" samples contain 

crystalline carbon—for example, graphite.

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

[Peak numbers correspond to those in flig. 1)

Peak

1

2

3

4

5

6 

7 

8 

9

10

11

12

13

14

15 

16

17

Compound

toluene.

2-methylthiophene .

3-methylthiophene.

ethylbenzene.

p-xylene.

TO-xylene. 

o-xylene. 

n-propylbenzene. 

methyfethylbenzene.

Do.

trimethylbenzene.

methylstyrene.

methylpropylbenzene.

d-benzene.

indene. 

methylindane.

Do.

Peak

18

19

20

21

22

23 

24 

25 

26

27

28

29

30

31

32 

33

34

Compund

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

(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 than 

the usual abundance of organic free radicals in coals (for 

example, Austen and others, 1966). This excess of free 

radicals may be attributed to the high abundance of 

uranium 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, in 

part, in free-radical production.

The uranium contents of ten separate Vaal Reef 

kerogens show an inverse linear correlation with the 

quantities of dimethylnaphthalenes identified during 

pyrolysis. With increasing uranium content, the 

dimethylnaphthalenes decreased in abundance relative 

to the other naphthalenes. This correlation is shown in 

figure 2. Since it is more difficult to thermally cleave the 

aromatic carbon-carbon double bonds than the alkyl 

bridges during pyrolysis, increasing abundances of 

dimethylnaphthalenes may indicate somewhat less of an 

aromatic polymer matrix. Thus, the kerogen matrix is 

less aromatic at relatively low uranium abundances than 

at higher concentrations of this element. This is consistent with the effect of prolonged radiation from uranium 

on organic matter, first producing free radicals and 

subsequently, aromatization (Huang and others, 1974).

29

t

2!,

30 

3! i

\m\ '

24

27

25 

r-26

(I

IN

23

V

84 76 68 60 52 Minutes 44 36 H———H 28 20

95° 75° isothermo! 195° 195° 175° 155° 135° C 115°

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

§< 0.37 

X

0.35

0.33

0.31

b

I

LU

I 0.29 

Q_

>• 0.27 

LU

Q

X

R= 0.8636

X

X

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 show 

that the Vaal Reef carbon seams contain two finely admixed substances with different compositions. The first 

component contains mainly carbon and sulfur from the 

kerogen; hydrogen cannot be detected by the electron 

microprobe. The second substance consists mainly of 

silicon, aluminum, iron, and sulfur; the sulfur was 

associated with iron. This second component is apparently an iron-rich clay together with some pyrite. 

The presence of clays was confirmed by the flaky 

mineral morphology observed by the scanning electron 

microscope and by X-ray diffraction. Gold was commonly found between and near the interfaces of kerogen and 

clay clusters. It is interesting that one gold particle 

within a clay cluster exhibited well-developed crystalline 

morphology. The majority of the gold particles in both 

kerogen 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 were 

built 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 found 

only in the carbonaceous seams and not in the adjacent 

quartzite 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 with 

coccoid 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 05

reasonable to assume that uranium, gold, and clays 

became concentrated in the original Vaal Reef mats 

through 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 photosynthesis, 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 from 

the Persian Gulf showed several distinctive layers 1 mm 

thick caused by specialized microbial populations. The 

brown surface layer consisted of organisms with a 

brown 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). This 

illustrates 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 and 

fabric will be different from stromatolites formed in 

other 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 Zimbabwe (Rhodesia). The Transvaal Sequence 

stromatolites 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 for 

algal 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 have 

become 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 of 

kerogen. Philp and Calvin (1976) found that a 

kerogenlike substance forms in algal-bacterial mats 

relatively soon after burial. The chemical composition of 

this protokerogen is only partly known; it contains

aliphatic compounds, including alkanes. The major 

cellular biochemical components of blue-green algae and 

bacteria are proteins, carbohydrates, and lipids. The 

degradation products of these biochemicals can give rise 

to a large number of organic compounds under 

anaerobic conditions. Some of these organic substances 

could have played a role in the crystallization and accumulation of gold and (or) uraninite. The initial 

degradation products of the algal and bacterial 

biochemicals form a humic-type substance. This matter 

can 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 have 

been also converted to the random aromatic polymer of 

the 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 (a 

highly 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 and 

subsequent polymerization with the release of volatiles 

such as H2 , CH4 , C02 , and NH3 (see Bovey, 1958.) For 

example, when amino acids and proteins are irradiated 

in 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 the 

pyrolysis products of the Vaal Reef kerogen may be 

because the major nitrogen source, the algal and 

bacterial proteins, were deaminated by radiation from 

uranium in the original mats and (or) subsequently in the 

(proto)kerogen. Older and younger Precambrian 

kerogens of comparable biological origin released 

organic nitrogen compounds upon pyrolysis (Nagy, 

1976). These rocks, and their kerogen, however, lack the 

uranium found in the Vaal Reef kerogen. Various 

organic free-radical reactions were probably responsible 

for the increasing aromaticity during the development 

of the Vaal Reef kerogen. The aromatic hydrocarbons 

are more stable to irradiation than are saturated compounds- for example, alkanes - and this increase in 

stability 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. Precambrian 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 random 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 with 

micropalentology may help to elucidate certain problems in economic geology.

ACKNOWLEDGMENTS

The authors wish to thank Dr. W. E. L. Minter of the 

Anglo 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 Witwatersrand, 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 University 

of 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, University of the Witwatersrand, South Africa and by NASA 

Grant NGR-03-0002-171.

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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 organic 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 and 

peridotitic komatiites and stromatolites above a basal unconformity 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 and 

synthetic high polymers: New York, Interscience Publishers, 287 

PBrock, B. B., and Pretorius, D. A., 1964, Rand basin sedimentation 

and 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 stromatolites, 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 trace

elements 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 oxidation 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 the

transportation, 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 Witwatersrand," 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 concentrations 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 of 

the 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 Precambrian 

Onverwacht microstructures: Possibly the oldest fossils on Earth?: 

Nature, v. 223, p. 1226-1229.

Nagy, Bartholomew, Nagy, L. A., Zumberge, J. E., Sklarew, D. S., and 

Anderson, 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 the 

diversification of cells about 2.2 x 109 years ago: Science, v. 183, p. 

514-516.

———1975, Comparative micropaleontology of a Transvaal stromatolite (~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 07

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 organic matter in Precambrian cherts: Geochimica et Cosmochimica 

Acta, v. 34, p. 659-675.

Snyman, C. P., 1965, Possible biogenetic structures in Witwatersrand

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 and

Metallurgy, 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 in

South African Proterozoic Placer Deposits

By W. E. L. MINTER

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-P

CONTENTS

	Page

Abstract __________________________________________________ pi

Introduction ________________________________________________ 1

The distribution of carbon ________________________________________ 1

The arrangement of carbon ______________________________________ 1

Conclusions ______________________________________________ 2

Reference ________________________________________________ 2

Discussion following papers by Drs. B. Nagy and Minter _____________________ 2

ILLUSTRATIONS

Page

FIGURES 1-4. Photographs showing:

1. A thin discontinuous carbon seam on a rippled paleosurface sediment, B Reef, Freddies Mine, Orange Free State

Goldfield _______________________________________________________ PI

2. A thick carbon seam with columnar structures, Vaal Reef, Vaal Reefs Mine, Klerksdorp Goldfield _________ 2

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 ___ 3

4. Algae growing in pools filling trough-crossbedded hollows in halted sand dunes in a riverbed, White Umfolozi River,

Swaziland_______________________________________________________ 4

in

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

THE 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 of 

carbon on paleosurfaces, on sedimentary accumulation surfaces like 

pebble layers, on trough-shaped bedforms of pi-crossbedded units and 

foresets, 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 some 

areas does not affect either the gold or the uranium content of the 

placer. High gold and uranium contents that appear to be associated 

with carbon seams are at the base of the reef because that position 

represents both the stable consolidated paleosurface upon which the 

plant material anchored itself and also the surface of bedload concentration.

INTRODUCTION

The presence and nature of carbon that occurs in some 

Witwatersrand placer deposits have caused much 

speculation and encouraged investigations into its possible origin. It occurs in the form of fine grains, films, and 

seams, as much as 5 cm thick. Before the results of 

microscopic and chemical investigations, based on 

relatively few samples, are examined, it is important to 

consider the distribution and arrangement of the carbon 

relative to the sedimentary nature of the placer 

deposits.

THE DISTRIBUTION OF CARBON

In banket reefs, where the placer sediment is composed of coarse conglomerate, carbon is rare and 

generally occurs as fine, sparsely scattered grains. In 

shallow but proximal braided-channel deposits, like the 

B Reef in the Orange Free State, fine grains within 

large 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 

Africa

bodies and have consequently become known as carbonseam reefs. However, in the distal environments, the 

carbon seams are neither uniformly distributed nor 

always present. For instance, an isopach map of the carbon seam in the Vaal Reef, which was constructed from 

approximately 500 stations in the Klerksdorp Goldfield, 

indicated that thicker carbon (fig. 2) was more 

widespread 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 and 

exhibits a bladed or columnar structure at right angles

4

FIGURE 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 in 

centimeters.)

PI
P2

CM'

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

CONCLUSIONS

The distribution and arrangement of carbon in Witwatersrand placer deposits seems to be consistent with 

that of an aquatic algallike plant. The sparsity of carbon 

in proximal environments could have been a consequence of the low preservation potential expected of 

algal mats under high velocity conditions. Thicker carbon in the Vaal Reef along the western flank of the 

deposit seems to have been similarly related to relative 

hydraulic-energy levels.

During the deposition of placer sediment in distal environments 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 and 

be preserved in situ within the internal geometry of the 

placer unit, as in figures 3D and 3E. Pebbles submerged 

in these environments would acquire an algal coating, 

which could be retained during subsequent transport 

and preserved, as shown in figure 3C. Carbon preserved 

on layers other than the paleosurface is seldom associated with gold contents as high as found on the paleosurface. This seems to be a factor of the lowest and optimum bedload position for gold. FIGURE 2.—A thick carbon seam with columnar structures that 

enclose sand grains, pebbles, and detrital minerals in distal environment. Western flank of Vaal Reef, Vaal Reefs Mine, 

Klerksdorp Goldfield.

to bedding. Basal lag concentrates of small pebbles and 

detrital heavy minerals occur on top of, and in some instances are depressed or embedded in, the seam. When 

these basal lags represent the pebbly base of a thicker 

quartzite placer unit, an additional carbon seam has 

been seen to occur on top of the pebble layer (fig. 3E). Individual pebbles coated with carbon have also been 

observed in the quartz-arenite matrix of the Basal Reef 

(fig. 3C), without an associated carbon seam.

Carbon seams also occur on the top surface of placer 

units, as in distal parts of the Steyn Reef in the Orange 

Free State, and on primary structures within the reef 

unit. These may be bedding planes of laminar layers, 

channel surfaces, and trough-crossbedded foresets. In 

figure 3D, a trough-crossbedded unit in the Steyn Reef 

contains carbon seams 5 mm thick on the bedform and 

on foresets (fig. 3E) of a trough-crossbedded unit that is 

40 cm thick.

REFERENCE

Minter, W. E. L., 1972, The sedimentology of the Vaal Reef in the 

Klerksdorp area: University of the Witwatersrand, Johannesburg, 

unpublished Ph.D. dissertation, 170 p.

DISCUSSION FOLLOWING PAPERS BY 

DRS. B. NAGY AND MINTER

Hallbauer: I have a question for Nagy. How did you 

determine the clay content in the carbonaceous 

material?

Nagy: You take a fraction from the total sample and go 

for the major X-ray peaks to get the area under the 

peaks and then correct for quartz and pyrite and other 

minerals which are present in large quantities. In 

other words, the clay content which I presented was a 

clay content corrected for the carbon, quartz, and 

pyrite.

Simpson: I would like to ask Dr. Minter whether he 

thinks the outer envelopes that he sees around pebbles
CARBON IN SOUTH AFRICAN PROTEROZOIC PLACER DEPOSITS P3

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, which 

is evident above the pebbly layer, lies at the base of 50 cm of placer 

sandbody; Stilfontein Mine, Klerksdorp Goldfield. C, A thin film of 

carbon 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 and 

could be wrapped up in the algal mat. I would also like 

to draw your attention to a previous hypothesis, I 

think suggested by Snyman or Liebenberg, that some 

of these uraninite grains might have actually decomposed to produce the carbon rather than carbon filtering into the cracks; that the carbon represents decomposition products. We hadn't mentioned that before 

and that has been published.

Nagy: Now that the word algae has become respectable 

again, I would like to make a comment. Algae that we 

know 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 decaying organic material which is going to be a green 

mass. I think this is a very important consideration to 

take 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. FEATHER

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-Q

CONTENTS

Abstract _______________________________ Ql

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 __________________________

1

1

1

2

2

8

9

10

10

10

Uranium-bearing minerals - Continued

Brannerite ___________________________ Q12

Introduction _________________________ 12

Electron microprobe and X-ray diffraction investigation 13

Economic importance of brannerite ___________ 15

U02 /Th02 ratio in the reefs as a whole ___________ 19

Age of Witwatersrand uranium-bearing minerals _____ 19

Paragenesis of gersdorffite and other secondary minerals ___ 20

General conclusions ________________________ 20

Acknowledgements ________________________ 20

References cited __________________________ 22

Discussion ______________________________ 23

ILLUSTRATIONS

FIGURE 1. Photomicrographs showing typically rounded detrital cobaltite grains, from Cristaalkop Reef __________________ Q3

2. Electron-microprobe analyses of Witwatersrand cobaltite grains of detrital origin, plotted in the diagram FeAsS-CoAsSNiAsS______________._____________________________________________________ 5

3. Scanning back-scattered electron ^Tfotomicrographs T)f~association oi mrthigenic gersdorffite^vith: goM, carbon, and other

minerals from Cristaalkop Reef ____________________________________________________ 6

4. Electron microprobe analyses of authigenic sulpharsenide grains (mainly gersdorffite), plotted in the diagram FeAsS-CoAsSNiAsS, indicating temperatures of crystallization ___________________________________________ 9

5. Sketch relating crystal structure ordering of authigenic gersdorffite with temperature for the system CoAsS-NiAsS _____ 9

6. Scanning electron photomicrograph of a euhedral crystal of uraninite from Basal Reef _______________________ 10

7. Diagram showing the variation of U02 /Th02 ratios for free uraninite and uraninite included in carbon _____________ 13 

8, 9. Scanning back-scattered electron photomicrographs showing:

8. Encrustation of brannerite on a partially altered uraninite grain being replaced by quartz _________________ 15

9. Common occurrences of brannerite in uranium-extraction- plant residues from Vaal Reef __________________ 16

10. Diagram showing variations in thorium content of brannerite ______________________________________ 19

11. Scanning back-scattered electron and X-ray photomicrographs showing minute crystals of a calcium phosphate occuring as a

discrete phase in an aggregate particle of brannerite, quartz, and chloritoid, from Vaal Reef _________________ 21

TABLES

Page

TABLE 1. Chemical composition of sphalerite in the Witwatersrand reefs ____________________________________ Q2 

2-6. Electron-microprobe analyses of:

2. Detrital cobaltite grains in some Witwatersrand reefs _____________________________________ 4

3. Authigenic arsenosulphides of nickel and cobalt, mainly gersdorffite, in some Witwatersrand reefs ___________ 7

4. Uraninite, in some Witwatersrand ores, not associated with carbon _______________________________ 11

5. Uraninite inclusions in selected samples of uraniferous carbon __________________________________ 12

6. Brannerite in some Witwatersrand reefs and recovery-plant samples ______________________________ 14

7. X-ray powder-diffraction data for Witwatersrand brannerite, compared with previously published information on brannerite

from other sources ____________________________________________________________ 17

8. Analyses of brannerite reported in the literature ____________________________________________ 18

9. Comparison of uranium/thorium ratios of reef samples, analysed by X-ray fluorescence spectrometry, and the average ratios

of uraninite and (or) brannerite grains, analysed by electron microprobe _____________________________ 19

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

SOME 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 composition 

of mixed crystals of gersdorffite and cobaltite that are undoubtedly of 

authigenic 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 is 

seen 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 by 

subsequent 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 the 

metamict crystals gives the same age (about 1,800 million years) for 

both uraninite and brannerite, suggesting that recrystallization and 

alteration took place at the same time. This is younger than results obtained 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 in 

pits and cracks in the grains and thus was not completely purged from 

the 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 to 

enclosure of minute uraninite crystals in carbon and, especially in the 

west of the Witwatersrand Basin, to enclosure of minute brannerite 

crystals in acid-insoluble hosts, mainly phyllosilicates and leucoxene.

INTRODUCTION

In recent years, mainly as a result of the development 

of sophisticated instrumentation such as the scanning 

electron microscope and electron microprobe, considerable advances have been made in our understanding of 

the processes responsible for the mineralization of the 

conglomerate reefs of the Precambrian Witwatersrand

1 Senior Research Fellow, Anglo American Research Laboratories, P.O. Box 106, Crown 

Mines, Transvaal, South Africa 2025

deposit. Our present knowledge of the ore mineralogy of 

the reefs has been collated and reviewed by Feather and 

Koen (1975), who indicated certain areas where further 

research is necessary.

With this in mind, in the present paper certain aspects 

of mineralization are considered in detail; inter alia, 

uranium mineralization and the temperature to which 

the reefs were subjected during the major period of 

metamorphic 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 analysed 

minerals 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 were 

prepared. In addition, a large number of mine recoveryplant products were examined.

TEMPERATURES OF METAMORPHIC ACTIVITY 

INTRODUCTION—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 conglomerates be discussed here. Recent evaluations by 

Pretorius (1974a, b, 1975), among others, especially 

with regard to the sedimentology of the Witwatersrand 

Basin, clearly support a placer origin for the mineralization of the reefs. In brief, it is believed that deposition 

took place along the interface between a fluvial system 

that brought sediments and heavy minerals from an 

elevated source area to the northwest of the basin and a 

lacustrine littoral system that reworked the material 

and redistributed the finer sediments along the shoreQi
Q2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

line of an ancient inland sea. The heavy minerals were 

concentrated by gravity settling and subsequent winnowing by wave and current action. Some investigators, 

such as Hallbauer (1975) and Riemer (1975), support the 

view that some of the gold and uranium was carried into 

the basin in solution and deposited biochemically by an 

ancient lichenlike life form that preferentially developed 

in quieter water conditions at the mouth of major rivers 

at the end of cycles of sedimentation.

There is little doubt that some of these ore minerals, 

notably the gold, base-metal sulphides and arsenides, 

and uraninite, underwent subsequent alteration and 

redistribution in situ in the reefs after burial during at 

least one major epoch of metamorphic activity. The date 

of this epoch is set by the age of the uranium-bearing 

minerals, which would have been purged of their 

radiogenic lead at this time.

This is the modified placer theory, which enjoys a fair 

degree of consensus, especially among those who are 

familiar with the geology of the basin through mining 

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 of 

deposition, obtained an average temperature of 450°C 

for undoubtably authigenic sphalerite in samples from 

the 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 of 

authigenic 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 dependent upon temperature. This is borne out by the fact that 

the 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 hexagonal pyrrhotite occur in the reefs. Both these facts are 

indicative of fairly high temperatures of pyrrhotitization 

(Ramdohr, 1955; von Rahden, 1964). Similarly, the.

TABLE 1.—Composition, in percent, of sphalerite in the Witwatersrand

reefs

[Names in parentheses are the abbreviated names of gold mines, as used in all the tables. The 

numbers in parentheses refer to sample numbers of the Witwatersrand collection of the 

X-ray Fluorescence Section, Anglo American Research Laboratories]

Sample Grain Fe Total

Vaal Reef (Vaal Reefs) (25) _ SI

S2

S3

S4

S5

S6

S7

S8

S9

64.3

64.1

65.4

64.8

65.4

64.8

65.0

66.0

64.6

1.2

1.3

1.3

1.1

1.2

1.1

1.2

.9

1.1

33.0

33.3

33.1

32.6

33.0

32.9

33.8

33.0

32.8

98.5

98.7

99.8

98.5

99.6

98.8

100.0

99.9

98.5

Average ____________

Ventersdorp Contact Reef

(Western Deep Levels)

(32). 

Basal Reef (Welkom) (184).

Average

64.9 1.2 33.1 99.2

S10 60.7 7.3 33.0 101.0

511

512

513

514

515

62.3

62.6

63.6

62.9

62.2

3.7 

3.4 

3.8 

3.5 

3.9

34.0

34.0

32.6

33.5

33.7

100.0

100.0

100.0

99.9

99.8

62.7 3.7 33.6 100.0

presence of chloritoid replacing sericite in the matrix of 

the reefs has been considered by many (for example, 

Ramdohr, 1955) to be an indicator of high temperature 

and 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 of 

carbon (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 during metamorphism.

These observations are, however, of a qualitative 

nature. Consequently, in the present investigation, 

quantitative information was sought of a more suitable 

mineral assemblage, notably of the cobaltite and 

gersdorffite, 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 no 

alteration since burial in the reefs.
ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS Q3

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 of 

grains are plotted in the ternary diagram FeAsSCoAsS-NiAsS (table 2; fig. 2). Superimposed on this 

diagram 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 a 

large number of grains of varying composition consistently gave the Pca2i space group of the orthorhombic system. According to Giese and Kerr (1965) and 

Gammon (1966), this is the low temperature ordered 

crystal form of cobaltite, having crystallized at 

temperatures 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 be 

classified 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 CONGLOMERATES 

TABLE 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]

Grain

Atomic proportions (based on cations)

Ni Co Fe As S Total NiAsS CoAsS FeAsS

Main Reef, South African Lands (150)

A63

A70

A75

8.2 

2.8 

3.4

19.3 

24.9 

25.0

3.0 48.3 19.6 98.4 

2.9 47.5 20.0 98.1 

3.8 40.3 24.4 96.9

0.268 

.091 

.105

0.630 

.811 

.773

0.101 

.098 

.122

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

A103

A104

A105

5.1

3.4 

8.3

27.2 

30.7 

25.4

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

Vaal Reef, Vaal Reefs (186)

Cl

C2

C3

C4 ___ 

C5

C6

C7

C8

C9

CIO

2.0 

9.5 

12.1 

8.9 

8.3

15.0 

10.5

7.2 

7.2 

9.4

31.9 

19.8 

17.0 

21.3 

19.2

14.6 

21.5 

25.9 

25.3 

22.7

2.0 45.5 18.9 100.3 

4.5 45.1 19.2 98.1 

6.9 44.2 19.8 100.0 

6.1 44.3 19.3 101.2 

6.1 45.2 19.7 98.5

6.6 44.1 18.9 99.2 

3.9 44.8 18.6 99.3 

2.9 45.0 18.8 99.8 

3.7 44.6 19.1 99.9 

3.5 44.2 19.3 99.1

0.056 

.280 

.334 

.244 

.246

.412 

.292 

.200 

.199 

.264

0.887 

.583 

.469 

.583 

.567

.400 

.596 

.717 

.696 

.635

0.058 

.137 

.197 

.173 

.187

.187 

.112 

.083 

.105 

.101

Leader Reef, Free State Geduld (200)

Alll

A112 

A114

3.8 

3.5 

3.5

30.2 

28.5 

35.4

2.1 44.0 19.5 99.6 

3.4 44.5 19.2 99.1 

3.5 41.6 19.7 103.7

0.105 

.099 

.083

0.835 

.802 

.832

0.060 

.099 

.085

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 47.0 18.6 103.3 

4.5 49.3 19.3 102.5 

2.7 48.1 19.8 102.3 

5.1 50.2 18.8 103.1 

5.0 45.2 20.6 100.7 

2.7 48.0 19.1 102.5 

2.0 48.9 18.5 101.5

0.190 

.338 

.151 

.222 

.157 

.161 

.238

0.635 

.525 

.768 

.624 

.695 

.760 

.702

0.175 

.137 

.081 

.154 

.148 

.079 

.061

Carbon Leader, West Driefontein (167)

A12

A13

A14

A16

A17

A18

A20

A21 _

A22

A23

A24

A28

4.0 

8.7 

4.3 

8.9 

4.4 

2.9

5.0 

8.1 

10.3 

9.7 

1.6 

8.5

23.5 

21.9 

26.2 

20.5 

28.3 

28.9

30.2 

26.8 

23.0 

17.7 

32.8 

19.2

4.0 41.2 21.3 94.0 

2.4 44.7 20.0 97.7 

3.3 44.1 20.7 98.6 

3.6 44.5 20.0 97.5 

1.2 45.1 20.0 99.0 

2.4 44.5 20.3 99.0

1.9 44.8 19.7 101.6 

2.8 46.0 19.7 103.4 

4.5 46.3 19.5 103.6 

7.0 43.0 20.6 98.0 

2.6 43.2 20.3 100.5 

6.7 44.0 18.7 97.1

0.127 

.263 

.127 

.269 

.129 

.085

.135 

.215 

.272 

.281 

.043 

.246

0.742 

.662 

.772 

.618 

.826 

.843

.812 

.709 

.606 

.511

.884 

.554

0.131 

.075 

.101 

.113 

.045 

.073

.053 

.077 

.123 

.209 

.073 

.200
ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS Q5

TABLE 2.—Electron-microprobe analyses, in percent, ofdetrital cobaltite grains in some Witwatersrand reefs—Continued

Grain Co

Atomic proportions (based on cations)

As Total NiAsS CoAsS FeAsS

Carbon Leader, Doornfontein (169)

A82

A85

A86

A87

A90

A93

A94

A95

A96

A97

A99

8.8

5.8

6.6

3.5

7.3

4.6

8.0

6.0

6.5

2.5

7.0

17.5

26.7

25.3

27.2

24.7

29.4

19.8

27.2

22.8

30.8

22.8

6.9

1.6

2.7

5.4

3.7

2.3

7.1

3.1

5.4

3.1

5.7

43.8

45.7

44.9

42.3

47.0

47.8

46.0

46.7

45.3

47.1

47.1

21.7

19.7

19.5

21.7

19.4

19.3

18.7

18.8

19.5

19.1

18.3

98.7

99.5

99.0

100.1

102.0

103.4

99.6

101.8

99.5

102.6

100.9

0.264

.170

.191

.097

.204

.167

.228

.165

.187

.069

.154

0.523

.781

.729

.749

.689

.808

.563

.747

.653

.844

.717

0.214

.049

.081

.154

.107

.065

.209

.088

.160

.088

.129

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

1.5

1.9

1.4

4.5

1.7

2.1

1.2

45.4

44.5

47.2

46.4

46.8

47.2

45.8

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

0.065

.017

.023

.284

.205

.062

.128

0.890

.928

.934

.581

.745

.876

.835

0.046

.055

.042

.135

.051

.062

.037

FeAsS 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 determined 

chemical 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.

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 a 

greyish tinge and a composition indicating a 

temperature of crystallization of 450°C. It is immediately enclosed by a zone of higher nickel content, blueish in 

colour, 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 premise 

that cobalt, nickel, and iron were freely available in the 

reefs 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 during crystal growth. For this reason, in all cases, the 

temperatures 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 is 

further confirmed by the fact that mixed crystals of the 

authigenic 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 CONGLOMERATES

FIGURE 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 MINERALS Q7

TABLE 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]

Grain

Atomic proportions (based on cations) Approximate 

_______________________ temperature (°C)

Total Average 

NiAsS CoAsS FeAsS Grain for

sample

Vaal Reef

A101

A102

A107

21.4

25.1

15.5

9.2 

8.4 

12.2

5.4 

2.4 

7.3

, Vaal Reefs (North Section) (5)

45.1 

43.5 

46.8

Vaal Reef, Vaal

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

Basal Reef,

Gl

G3

G4

G5

G6

G7

G8

G9

G10

Gil

G12

G13

G14

G16

G17

G18

G19

G20

14.9

17.3

12.9

16.8

17.5

14.3

13.4

10.2

10.7

17.3

17.8

17.4

15.0

17.8

18.1

15.9

18.6

18.5

11.0 

10.2 

15.8 

10.9 

9.2 

11.3

13.9 

13.1 

18.3 

10.2 

9.6 

10.3

10.6 

10.3 

9.6 

11.1 

9.4 

10.1

7.4 

7.2 

7.2 

7.3 

6.9 

6.6

6.9 

6.7 

6.2 

7.9 

7.8 

7.7

6.8 

7.8 

7.2 

7.5 

7.3 

8.5

45.0 

44.7 

44.4 

44.6 

45.5 

45.5 

45.0 

45.6 

45.8

19.0 

19.2 

16.8

100.1 

98.6 

98.6

0.592 

.698 

.440

0.254 

.233 

.356

0.154 

.069 

.214

460 

380 

550

--

460

Reefs (186)

20.8 

19.4 

19.2 

19.7 

19.8 

18.5 

19.0 

18.8 

19.2

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

——

450

near dyke contact, Welkom (192)

45.9 

45.4 

44.8 

45.6 

45.0 

44.0

45.7 

45.5 

45.6 

44.9 

45.9 

46.0

45.8 

44.6 

44.7 

46.0 

44.0 

42.8

19.8 

19.1 

19.3 

19.1 

19.7 

19.9

18.2 

20.0 

19.3 

19.9 

19.7 

19.5

19.0 

20.0 

19.7 

19.3 

19.4 

19.8

Leader Reef, Free State Geduld

A113

A116

All?

A118

A119

A120

Center

Intermediate

Outside

20.1

22.5

15.4

15.6

20.1

6.2

23.5

30.1

34.6

6.1 

4.2 

15.7 

13.5 

9.0 

25.8

5.4 

2.4 

<.l

7.1 

6.4 

5.0 

6.0 

7.6 

5.4

7.8 

2.6 

1.7

48.2 

47.9 

45.3 

46.2 

49.0 

44.5

Zoned grain

42.9 

46.2 

43.7

17.9 

18.3 

19.1 

19.4 

17.4 

20.7

(A 201)

20.3 

18.7 

20.0

99.0 

99.2 

100.0 

99.7 

98.3 

96.1

98.1 

95.5 

100.1 

100.2 

100.8 

100.9

97.2 

100.5 

99.3 

99.8 

98.7 

99.7

(200)

99.4 

99.3 

100.5 

100.7 

103.1 

102.6

99.9 

100.0 

100.0

0.445 

.495 

.357 

.477 

.518 

.441

.390 

.338 

.303

.485 

.502 

.488

.460 

.493

.515 

.458 

.524 

.495

0.600 

.676 

.425 

.442 

.544 

.165

0.636 

.856 

.952

0.327 

.291 

.437 

.309 

.271 

.348

.403 

.433

.516 

.286 

.270

.288

.324 

.284 

.272 

.319 

.264 

.270

0.182 

.126 

.432 

.382 

.243 

.686

0.146 

.068 

.000

0.228 

.213 

.206- 

.214 

.211 

.211

.207 

.229 

.181 

.229 

.227 

.223

.216 

.223 

.212 

.223 

.212 

.235

0.219 

.199 

.143 

.176 

.213 

.149

0.218 

.076 

.048

540 

520 

580 

530 

510 

540

570 

590 

600 

530 

520 

530

540 

520 

510 

540 

505 

520

470 

425 

540 

540 

500 

610

450 

<300 

<300

540

——

510
Q8 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

TABLE 3. -Electron microprobe analyses, in percent, ofauthigenic arsenosulphides of nickel and cobalt, mainly gersdorffite, in some Witwatersrand reefs - Continued

Grain

Atomic proportions (based on cations)

Ni Co Fe Total

NiAsS CoAsS FeAsS

Approximate 

temperature (°C)

Grain

Average

for 

sample

Cristaalkop 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

8 9

7.7

5.2

5.5

7 9

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

1 7 9

18.3

97.6

103.6

98 7

97.1

100.3

97.0

99.7

99.4

98.2

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

__

__

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

0.509

.502

.622

0.328

.378

.167

0.164

.121

.211

505

500

460 490

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

8 9

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

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

__

__

__

__

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

20.0

19.4

99.7

100.0

0.603

.751

0.242

.091

0.156

.158

460

360 410

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 been 

achieved in the Witwatersrand reefs during metamorphism of the reefs. Samples that were taken close to 

dykes—for example, 192-Basal Reef (fig. 4)—indicated 

only slightly higher temperatures, about 500-600°C, 

than did reef samples not associated with dykes—the 

rest in figure 4, about 400-550°C. It appears, therefore, 

that these temperatures were widespread and not 

localised to the vicinities of crosscutting dykes and 

related hydrothermal veins. However, a word of caution 

should be noted here. The work of Klemm (1965) has not 

been 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

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 from 

silver-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 additional chance of misidentification resulting from 

descriptions of gersdorffite in the literature. For example, 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 of 

skutterudite in the reefs have been made. Among 

others, Ramdohr (1955) found that the redistributed 

gold was often associated with skutterudite, which is of
ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS Q9

FeAsS EXPLANATION

* 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 168

650

CoAsS NiAsS

FIGURE 4.—Electron microprobe analyses of authigenic sulpharsenide 

grains (mainly gersdorffite) plotted in the diagram FeAsS-CoAsSNiAsS. Superimposed on the diagram are the experimentally determined chemical isotherms (°C) of Klemm (1965), which indicate the 

limits 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 and 

crystallized the secondary sulphides, sulpharsenides, and arsenides. 

Sample numbers in the explanation from the Witwatersrand Collection of the X-ray Fluorescence Section, Anglo American Research 

Laboratories, South Africa.

about the same age, though sometimes distinctly 

younger. Viljoen (1963) also noted the close association 

of gold and skutterudite.

In the present investigation, skutterudite could not be 

found, despite an extensive search by electron 

microprobe, and it is concluded that previous investigators have confused gersdorffite with skutterudite. The descriptions that both these authors give 

for skutterudite could apply equally to gersdorffite.

URANIUM-BEARING MINERALS

The important uranium-bearing minerals in the Witwatersrand and related reefs are uraninite and the 

alteration 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 to 

present 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 and 

Basal Reefs.

800 r700

600

c/D

=>

c/D_i

01

0

is 5°°

cc.

LU

Q 400

£ 300

=>

200

100

3/C

A/r

Disordered 

Pa3

I'tffefo,

Ordered

Pea 2

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 during 

metamorphism 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 CONGLOMERATES

URANINITE

INTRODUCTION

Cooper (1923) was the first to record the presence of 

uraninite 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 in 

greatest concentration close to the footwalls of the 

reefs, or on the false footwalls (Ramdohr, 1955) within 

them, in apparent hydraulic equilibrium with other 

heavy 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 and 

may enclose gold, gersdorffite, pyrrhotite, and 

chalcopyrite.

The grains are commonly rounded, tending to be oval 

in shape (Liebenberg, 1955), and are surrounded by 

phyllosilicates, mainly sericite. Euhedral and subhedral 

grains, 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 the 

fact that the grains were recrystallized and were purged 

of radiogenic lead during metamorphism. Rounded 

grains were invariably found to be accompanied by 

traces of minute crystals of an alteration product.

It is suggested, therefore, that the rounding is due to 

partial chemical dissolution rather than detrital rounding.

Uraninite also occurs as minute anhedral and 

subhedral inclusions in carbon. The carbon is believed to 

be the fossil remains of an ancient lichenlike plant 

(Hallbauer, 1975). Thus two distinctly different generations of uraninite, which will be referred to as free and 

included, respectively, occur in the reefs, but each 

generation 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), have 

suggested that the uraninite may have precipitated from 

solution, aided by biochemical activity of the "lichen" or 

similar organisms.

Hallbauer and van Warmelo (1974) have pointed out 

that 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 by 

previous investigators. Scanning secondary electron photomicrograph, 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 actually 

absorbed 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 pitchblende—for example, radial textures and cracks—is also 

absent.

During the progress of this investigation, Grandstaff 

(1974) published electron-microprobe analyses of free 

uraninite 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 assemblage 

of uraninite grains in the reef sample. He concluded that 

such 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 Qll

primary sources. He points out that the Witwatersrand 

grains 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 normally 

very low in thorium content.

In the present investigation, uraninite from a number 

of reefs was analysed for uranium, thorium, lead, iron, 

calcium, and titanium by electron microprobe, using, as 

standards, sintered U02 and thorium (prepared by the 

Atomic Energy Board, Pelindaba), lead-metal, and 

analysed hematite, wollastonite, and rutile. The results 

are summarised in tables 4 and 5, and in figure 7 the 

U02/Th02 ratios of the grains are plotted against Th02 

content. The data for free uraninite and uraninite included in carbon are plotted separately.

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 likely 

that the "lichen" "digested" detrital grains, hence the 

large and similar variation in U02/Th02 ratio.

The overall compositions of the free and included 

uraninites are very similar—compare tables 4 and 

5—but before it could be proved that they are definitely 

from a common source, variations in all elements within 

each 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]

Source

Cristaalkop Reef (Vaal Reefs South)

(171).

Average

Carbon Leader (Western Deep

Levels) (135).

Average

Carbon Leader (West Driefontein)

(167).

Average

Main Reef (South Africa Lands)

(151).

Average

Basal Reef (Welkom) (184)

Average

Overall Average

Grain

T21

T22

T23

T24

T25

T27

T28

T29

T30

T13

T14

T15

T16

T17

T18

T19

B43

B44

B46 ____ 

B47

B48

B49

B52

B31

B32

B36

B37 ____

_U4

U5

U7

U8

U9

U10

U02

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

Th02

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

Pb02

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

FeO

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

Ti02

<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

CaO

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

Total

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

U02/Th02 

ratio

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 CONGLOMERATES 

TABLE 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

.5 .2

CaO

.7

Total

96.9

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

Overall Average _______ ______ 69.5 3.2 21.7

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. This 

is contrary to what might be expected from granitic or 

pegmatitic source rocks, which are expected to have 

contained more zircon than uraninite.

The mineralized reefs do contain one common ingredient—that is, carbon—that is virtually absent in the 

barren sediments. Despite the foregoing evidence it 

does seem possible that the ancient "lichen" played an 

obscure role in aiding the concentration of both gold and

uranium. However, it may be argued that the presence 

of detrital uraninite was essential to the "lichen's" origin 

and development.

BRANNERITE

INTRODUCTION

As previously noted, the free uraninite grains are invariably partially dissolved and altered. Both Ramdohr 

(1955) and Liebenberg (1955) noted the presence of an 

alteration product. Liebenberg distinguished between 

two products: (1) a common microcrystalline, relatively 

homogeneous phase somewhat similar in appearance to 

parent 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

Tl

Q13

80

(174.3) B

+ 

70 -

**

468

ThO2, IN PERCENT

A

EXPLANATION

468

Th02, IN PERCENT

B

12

Main Reef 151 

Basal Reef 184 

Vaal Reef 25 

Cristaalkop Reef 171

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 such 

variation could result only if the uraninite were from multiple sources and suggested that it is of detrital origin, having been derived from a 

number 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 the 

altered material. Both "secondary uraninite" and 

uraniferous leucoxene may also be seen infiltrating 

neighbouring 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 or 

titaniferous magnetite. Pyritization of ilmenites is abundantly evident in the reefs, and titanium would have 

been freely available for reaction. Although Jacob (1966)

obtained a refractive-index value that was similar to 

that of brannerite, attempts to identify positively the 

mineral by X-ray diffraction failed, even when heating 

experiments were carried out (von Rahden and 

Hiemstra, 1967; Mihalik, 1968; von Rahden, 1970).

ELECTRON MICROPROBE AND X RAY DIFFRACTION 

INVESTIGATION

Using back-scattered electron images, the mineral appears to be homogeneous, despite abundant galena inclusions. A number of grains were analysed for uranium, 

thorium, lead, iron, titanium, calcium, and silicon, and 

the results are cited in table 6. As in the uraninite 

analyses (tables 4, 5), the elements are expressed as oxides. The low totals probably result from metamictiza-
Q14 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

TABLE 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 FeO Ti02 CaO Si02 Total U02/Ti02 

ratio

U/Ti U02/Th02 

ratio ratio

A. Recovery-plant samples

Head Sample 1 (Vaal 

Reefs).

Average

Head Sample 2 (Vaal 

Reefs).

Leader Reef (Free 

State Geduld) (134).

Average

Vaal Reef 1 (Vaal 

Reefs) (72).

Average

Vaal Reef 2 (Vaal 

Reefs) (25).

Average

Carbon Leader

(Western Deep 

Levels) (74).

Average

Main Reef (South 

Africa Lands) (151).

Average

Overall 

Average

Bl __ 

B3 __

B4 ____

B5 ___

B6 __

B21 __ 

B22 __ 

B23 __

B24 __

B25 __

B26 __

B7 __

B8 ____ 

BIO __

B14 __

B16 ___

Ul ____ 

U3 ____ 

U5 ____

U9 ____

U10 ___

U15 ___

U18 ___ 

U19 __ 

U20 __

B27 __

B28 ___ 

B29 __

B33 ___

B35 ___

40.1 

37.0

42.2

26.6

36.5

33.6

33.6 

33.1

38.4

35.0

31.3

43.6

35.8

32.5

27.2 

31.9

26.4

25.2

28.6

26.2 

39.9 

38.3

34.4

31.2

34.0

43.0

45.9 

47.1 

43.9

45.0

33.0 

38.1 

49.5

40.1

31.3

38.4

36.0

3.1 

4.5

4.6

2.3

3.6

5.1

1.3 

2.5 

2.5

3.1

4.6

3.7

3.0

3.4 

2.0 

.9

2.8

3.1

2.4

0.9 

3.2 

1.8

5.4

3.0

2.9

3.8

1.7 

2.8 

.9

2.3

0.2 

.9 

2.8

.3

3.1

1.5

2.7

0.3 

1.0

8.1

.3

2.4

14.1

B.

9.0 

4.7 

22.1

4.8

2.3

6.5

8.2

10.7 

11.4 

29.2

11.8

50.4

22.7

17.0 

37.8 

32.9

10.9

8.8

21.5

9.2

7.7 

12.0 

6.7

8.9

2.5 

.9 

17.6

<.l

<.l

4.2

11.7

0.9

1.3

.8

1.6

1.2

1.0

36.5

42.6

33.9

45.1

39.5

34.8

0.6 

.5

.6

.3

0.5

0.4

8.0 

12.1

8.0

13.7

10.5

6.4

89.5 

99.0

98.2

89.9

94.2

95.4

1.10 

.87

1.24

.59

0.92

0.97

1.6 12.9 

1.3 8.2

1.8 9.2

.9 11.6

1.4 10.1

1.5 6.6

Rock samples

1.1 

1.6 

<.l

.7

1.0

3.5

1.3

1.1 

7.1 

2.3

2.2

6.2

3.8

14.1 

11.3 

14.3

9.1

14.8

12.7

6.0

5.7

4.7 

5.1

5.4

0.6

.7 

1.2

1.0

1.0

0.9

4.1

45.2 

46.2 

32.6

25.0

31.7

11.5

32.0

37.3

24.8 

18.7

45.5

15.4

28.3

30.5

21.7 

27.0

24.7

25.7

25.9

33.3

34.2 

29.5 

30.6

31.9

52.5 

37.1

15.7

34.6

34.0

34.8

31.9

0.2 

.4 

<.l

.6

.2

.6

0.3

1.0 

.5 

1.1

.2

.3

0.6

0.2 

.3 

.5

.2

.2

0.3

0.7

1.0 

.4 

.5

.7

0.8 

.9 

.3

1.1

.4

0.7

0.5

4.7 

6.9 

1.0

9.6

17.3

11.3

8.5

5.7 

11.7 

7.8

8.7

4.7

7.7

n.d. 

n.d. 

n.d.

n.d.

n.d.

n.d.

n.d.

n.d. 

n.d. 

n.d.

n.d.

3.7 

5.6 

5.4

3.5

12.3

6.1

8.0

95.1

95.4 

96.6

78.8

88.4

80.7

89.1

91.7 

84.7 

91.9

97.6

102.3

94.1

88.9 

114.2 

114.8

84.7

83.7

97.3

96.0

96.2 

96.5 

87.7

94.2

93.3 

84.2 

92.5

80.6

82.1

86.6

94.9

0.74 

.73 

1.18

1.40

.99

3.79

1.12

0.87 

1.10 

1.71

.58

1.64

1.01

0.86 

1.84 

1.42

1.39

1.21

1.31

1.29

1.34 

1.60 

1.43

1.41

0.63 

1.03 

1.33

1.16

.92

1.10

1.13

1.1 25.8 

1.1 13.2 

1.8 15.4

2.1 11.3

1.5 6.8

5.8 11.8

1.7 11.9

1.3 9.6 

1.7 13.6 

2.6 35.4

.9 9.4

2.5 8.1

1.5 11.9

1.3 29.1 

2.7 12.5 

2.1 21.3

2.0 6.4

1.8 10.4

2.0 11.7

1.9 11.3

2.0 27.0 

2.4 16.8 

2.1 48.8

2.2 19.6

1.0 165.0 

1.6 42.3 

1.8 17.7

1.8 133.7

1.4 10.1

1.7 25.6

1.7 13.3
ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS Q15

FIGURE 8.—Encrustation of brannerite on a partially altered 

uraninite 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 and 

possibly also from the fact that iron may be present as 

Fe203 instead of FeO and lead may be present as a 

sulphide 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 the 

structure of the uranium titanate either at the time of 

formation 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 radiogenic 

lead).

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 hour 

and 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 the 

mineral may be contaminated with these three, there 

are a number of important lines of these proposed impurities that are absent (italic in table 7). Ignoring the 

lines of an unknown impurity, the best fit is with the 

data for brannerite given by Patchett and Nuffield 

(I960).2

It may be argued that a metamict microaggregate of 

Ti02 and U02 would, on heating, synthesize brannerite 

and that the original mineral may not have been brannerite at all. However, this is a problem inherent in all 

studies of metamict minerals.

As Ferris and Ruud (1971) have pointed out, because 

of 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 descriptions of brannerite given in the literature (Pabst, 1954; 

Patchett and Nuffield, 1960; Ferris and Ruud, 1971). It 

is similar in composition to brannerite from other 

sources (table 8; Ferris and Ruud, 1971). For the overall 

average in table 6, if the uranium content is increased to 

what it would have been before radioactive decay, if the 

reasonable 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 the 

uranium titanate mineral in the Witwatersrand is indeed brannerite.

The uraniferous leucoxene described first by 

Liebenberg (1955) is found to be an admixture of leucoxene 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 the 

West and Far West Witwatersrand, where it may be the 

most important uranium-bearing mineral, and is of 

lesser abundance in the Orange Free State. In the Middle, East, and Far East Rand it is relatively rare.

According to Liebenberg (1955), referring to brannerite as "secondary uraninite," its rate of dissolution in 

the oxidising dilute sulphuric-acid leach solutions of the 

uranium-extraction plants is slightly greater than that 

of uraninite. However, on examination of many leach 

residues during the present investigation, in addition to 

uraninite enclosed in carbon, brannerite was frequently 

the most abundant uranium-bearing mineral in the plant 

tailings. This is due to enclosure of the brannerite by 

minerals 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 MINERALS Q17

TABLE 7. -Best obtained X-ray powder-diffraction data/or Witwatersrand brannerite, compared with previously published information on brannerite from other sources

[ASTM diffraction data for Pb3 04 , quartz, and rutile are listed for comparison, since the Witwatersrand data may be interpreted as a mixture of these three, although some important lines are 

absent (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

001 

201 

200

110 

202

111 

002

201 

111

112 

311

40 1(?) 

203 

312 

310 

400

——

E

—

—

Witwatersrand

d

(8.78)

(6.45) 

Black 

4.84 

4.36 

(3.78)

3.47 

3.35

3.26 

3.16 

3.02

2.87 

2.78 

(2.69) 

2.60 

2.51 

2.45

2.33

2.24

2.12 

2.05

2.02

1.916 

1.890 

1.861

1.775

1.699 

1.678

1.633

(1.582)

I

6

1

10 

1

4 

5

3 

1 

3

4 

2 

2 

.5 

4 

4

1 

2(b)

1 

3

4

2 

2 

2

l(b)

3 

1

1 

1

Brannerite Blind River1

d

6.03

4.77 

4.31

3.43 

3.33

3.27 

3.02

2.92 

2.76

2.51 

2.46

2.281 

2.156 

2.078

2.023

1.903 

1.866

1.778 

1.737

1.695 

1.667

1.621 

1.568

I

3

8 

2

10 

4

4 

3

5 

5

4

4

6

vf 

vf

3

4 

2

vf 

f

3

vf

4 

4

Synthetic 1

d

6.04 

4.74 

4.29

3.44 

3.35

3.28 

3.02

2.90

2.77

2.53 

2.47

2.41 

2.303 

2.292 

2.276 

2.144 

2.080

2.043 

2.015 

1.911 

1.903 

1.881

1.798 

1.749 

1.732 

1.723

1.709 

1.675 

1.642 

1.630 

1.609 

1.597

1.569

I

36 

96 

19

>100 

>100

13 

36

36 

33

31 

33

17 

8 

27 

12 

9 

7

36 

23 

5 

19 

24

4 

8 

5 

3

20 

5 

7 

19 

18 

6

7

Pb3 04 ASTM 8-19

d

6.23

3.66

3.38

3.28 

3.11

2.903

2.787

2.632

2.444

2.289 

2.205

2.076

2.032 

1.970

1.903 

1.887 

1.829

1.755

1.7025 

1.6897 

1.6417 

1.6302

1.5876

I

11

3

100

7 

19

48 

45

30

2

4 

1

1

12 

12

22 

<1 

<1

30

2 

2 

8 

<1

12

Quartz ASTM 5-490 Rutile ASTM 21-1276

d I d

——— —— ————

A 9fi QP:

3.343 100

—— —— 3.25

rii / o'y

2

4 ro -| n

2.282 12 2.297

2

00^ c

2 128 9

2.054 10 —— —

1.980 6 — ——

1 Q-t iy i y

1.801 <1 ———

—— —— ———

i £70 n

——— —— 1.6237 

1.608 <1 —— —

I

——

——

100

50

8

:::

—

—

___

20

——

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 CONGLOMERATES

TABLE 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]

Witwatersrand

d I

___

1 f^f^/1 1

1.494 2 —— —— ——

__ __ __

—— —— ——

__

—— —— ___

1.374 1 —— __ __

—— —— ——

__ __ __

—— —— ——

—— —— ——

—— —— ——

—— —— ——

—— __ __

—— —— ——

—— —— ___

1 Patchett and Nuffield (1960).

Brannerite Blind River 1 Synthetic1

d I d

—— —— 1.561

—— —— 1.558

—— —— 1.493

1.482 vf 1.486

1.458 vf 1.450

1.441 vf 1.431 —— —— ____

1.410 vf 1.415

—— —— 1.384

1.367 2 1.376 —— —— ——

—— —— ——

1

Q AQ

—— —— 1.266

—— —— 1.263

—— —— 1.242

—— —— 1.239

—— —— 1.234

—— —— 1.218

—— —— 1.200

i

6

l(b)

3

4

2

___

2

3

4——

——

__

2

5

2

2

5

3

3

Pb3 04 ASTM 8-19 Quartz ASTM 5-490

d I d I

___ __ ____ ___

I

rr r Qf\ r~7

1.5116 2 ———

—— —— ——— ___

1.4687 4 ———

1 /1^91 9

———— ——— ————— ———

—— —— 1.418 <1

1.3944 <1 1.382 7

1.3728 1 1.375 11

—— —— 1.372 9

1.3471 4 — ——

1 QlftQ 4

—— —— 1.288 3 —— —— ——— ——

—— —— ——— ——

—— —— ——— ——

—— —— ——— ——

—— —— 1.228 2 —— —— - —— ——

—— —— 1.1997 5

Rutile ASTM 21-1276

d

___

1.4797

1 I ZQS

1.4243

__

——

——

____

1.3598

1.3041

____

____

1.2441 ——

____

____

1.2006

i

__

10

10

2

——

——

——

___

20

9

___

___

4 ——

___

___

2

TABLE 8.— Some analyses of brannerite, in percent, reported in the literature

[See Pabst (1954) and Frondel (1958)]

Origin

Kelly Gulch, Idaho

Cordoba, Spain

Crocker's Well, South Australia

Vosges, France

Mono County, Calif.

U02 Th02

41.93 4.10

49.79 1.20

30.05 12.81

41.65 .30

38.41 5.00

Pb02

ft 9

2.51

2 99

3.43

FeO Ti02 CaO Si02 Total 1

9 Q ^Q ft 9 Q ft fi Q1 fi^

3.11 32.45 2.74 .16 91.96

1 OQ QP. 1 Q QO O1

5.4 35.2 2.7 3.3 91.98

2.4 32.9 2.8 .5 82.01

U02 /Ti02 

ratio

108

i ^

QC

118

1.17

U/Ti 

ratio

1 59

2

QfT

1

O/?

1.74

1.72

1 Totals are incomplete; refer to Frondel (1958) for complete analyses.
ASPECTS OF WITWATERSRAND MINERALIZATION, WITH REFERENCE TO URANIUM MINERALS Q19

70

9 40

. EXPLANATION

165 '° * Main Reef 151 

t * Leader Reef 134 

1 * Vaal Reef 72 

~133.7 + Vaal Reef 25 

* Carbon Reef 74

*

*

#

•+• 

* 

*

**

* * +*

+» **

* #

* +

with that in the plant feed, is in any way more closely 

associated with these phosphate minerals.

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, determined by X-ray fluorescence spectrometry, with the 

average obtained on uraninite and (or) brannerite grains 

in the same reef samples by electron microprobe. The 

ratio is always lower in the case of the overall rock 

samples, which suggests that there are other thoriumbearing minerals in the reefs.

TABLE 9. - Comparison ofuran ium/thorium ratios of reef rock samples 

as a whole, analysed by X-ray fluorescence spectrometry, and the 

average ratios of uraninite and (or) brannerite grains, analysed by 

electron microprobe

Whole rock

Sample Reef U02 Th02 U02/ThO, U02 /Th02 

(ppm) (ppm) (ratio)

25 Vaal Reef 1,125 199 5.7 17.7

27(72| Vaal Reef 442 166 2.7 11.9

34 Carbon Leader 914 61 15.0 20.7

151 Main Reef 19 5 3.8 26.2

134 Leader Reef 4,892 570 8.6 11.9

171 Cristaalkop Reef 5,243 468 11.2 12.9

184 Basal Reef 4,040 1,530 2.6 20.9

01 234 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.

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 host 

minerals 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 was 

able 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 and 

Ventersdorp 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 interfere 

with the dissolution of the brannerite. Also, there is no 

evidence that the brannerite in the residues, compared

Investigation shows that zircon, monazite, and 

uranothorite are present, although relatively rare, and 

all three contain varying and sometimes appreciable 

amounts of thorium and relatively lesser uranium.

AGE OF WITWATERSRAND URANIUM-BEARING MINERALS

Polished sections containing Witwatersrand 

uraninite, brannerite, and zircon were sent to Drs. 

Hinthorne and Anderson at the ARL Hasler Research 

Center, Goleta, Calif., for age determination by ARL 

IMMA Ion Microprobe. (For a description of the instrument and technique see, for example, Andersen and 

Hinthorne, 1972.) The results that they obtained are as 

follows:

Mineral

Uraninite (average)

Brannerite

Zircon

Pb207/Pb206 

ratio

0.1078 + 0.0002

.1103 ±0.007

.2017

Age 

(million years)

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 determined values of approximately 2,000 m.y. (Davidson,
Q20 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

1960). 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 the 

environment of the grains.

The fact that the brannerite and uraninite are practically 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 Igneous Complex (Davidson, 1960), which thus may have 

been the source of the metamorphic activity.

The zircon age, 2,845 m.y., is somewhat older than 

that of the Witwatersrand System itself, approximately 

2,500 m.y., and may be indicative of the age of 

crystallization or recrystallization of the source rocks.

PARAGENESIS OF GERSDORFFITE AND OTHER 

SECONDARY MINERALS

In geological terms, temperatures in the vicinity of 

600 °C are not very high, and relatively low grade 

metamorphism is expected to have occurred. Apart 

from very thorough cementing of the pebbles and 

matrix, mainly through the redistribution of quartz, a 

proportion 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 and 

less 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 association is even closer than that of carbon and gold, where 

carbon 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 the 

same 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 of

highly altered primary platinum group mineral grains. 

Alteration of phyllosilicates in the matrix, pyritization of 

iron oxides and ilmenite leading also to abundant 

needles of rutile, recrystallization of uraninite and 

crystallization of brannerite, and the growth of tourmaline, bravoite, cubanite, proustite, stromeyerite, tennantite, and mackinawite, all very rare, are expected to 

have 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 in 

the vicinity of 400-600 °C were achieved during 

metamorphism of the reefs.

It is apparent that the gold was remobilised and secondary base metal sulphides, sulpharsenides, and 

arsenides were formed during metamorphism. It appears that at the same time the uraninite was 

recrystallized and was partially altered to a uranium 

titanate mineral, which is confirmed to be brannerite.

Age determinations by ion microprobe show that the 

last period of recrystallization of uraninite, presumed to 

be the same period as gave rise to the metamorphic activity 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" the 

detrital uraninite, rather than absorbed the U02 and 

Th02 from solution.

In addition to uraninite included in carbon, brannerite 

enclosed by refractory silicate minerals is an important 

source of uranium losses to uranium extraction plant 

residues.

ACKNOWLEDGEMENTS

The author is grateful to Anglo American Research 

Laboratories for having made this research project 

possible, to all his colleagues in the various sections of 

the Laboratories for their guidance and assistance, to 

gold mines of Anglo American Corporation of South

FIGURE 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 CONGLOMERATES

Africa, Ltd., and Gold Fields of South Africa, Ltd., for 

samples, and to Applied Research Laboratories for the 

ion-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 the 

Laboratories, was a fount of information by virtue of his 

long experience on gold and uranium extraction plants.

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in uraninite from the Witwatersrand, South Africa, and Blind 

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thucholite 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 University, Grahamstown, unpublished M. Sc. Thesis.

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fur Mineralogie Abhandlungen, v. 103, p. 205-255.

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uraninite in Witwatersrand bankets [with discussion]: Geological 

Society of South Africa Transactions, v. 64, p. 23-54.

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Norsk Geologisk Tidsskrift, v. 32, p. 61-147.

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PLiebenberg, W. R., 1955, The occurrence and origin of gold and 

radioactive minerals in the Witwatersrand System, the Dominion 

Reef, the Ventersdorp Contact Reef, and the Black Reef [with 

discussion]: Geological Society of South Africa Transactions, v. 58, 

p. 101-254.

Mihalik, P., 1968, Uranium compounds in the Dominion Reefs, and 

their association with phosphorus compounds: National Institute 

for Metallurgy, Johannesburg, Research Report 415, 8 p.

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v. 59, p. 109-117.

Patchett, J. E., and Nuffield, E. W., 1960, The synthesis and 

crystallography 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 

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22 p. 

-1975, The depositional environment of the Witwatersrand

goldfields: A chronological review of speculations and observations: 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 of 

South 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 of 

mineralization of this horizon: Witwatersrand University, Johannesburg, 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 of 

uranium 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 other 

gold-uranium placers: Implications on the origin of the mineralization: Neues Jahrbuch fur Mineralogie Monatschefte, v. 2. p. 79-98.

Saager, R., 1969, The relationship of silver and gold in the Basal Reef 

of 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 and 

Petrology, 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 Q23

ferences in the formation of meteorites as determined from the

ratio 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 the

Main 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 being 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 in 

pelitic material at those temperatures. Do you find 

those in the area?

Feather: No, they are extremely rare. These 

temperatures do seem to be higher than I think most 

people would like to see.

Skinner: I think the temperatures are too high because 

the ways in which you estimated them are not really 

valid ways to do it. The sphalerite temperature has for 

a long time now been shown to be invalid as a 

temperature 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 accurate curves. The temperature that would be indicated if the sulfur pressure were controlled by 

pyrite, 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

very suspect. The solvus determinations that you used 

to base the estimates on are very suspect because of 

extreme problems in kinetics in trying to get 

homogenization in those materials. Using aqueous 

synthesis, you can in fact make complete solid solution 

all the way across from gersdorffite to cobaltite with 

up to about 15 percent iron. You can do that as low as 

150°C, which suggests that you could form all of those 

at 150 degrees and perhaps lower. I think the caution 

you threw out that you should await final decision until somebody else has looked at the diagram is probably a very wise caution. I personally think that that 

work 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 very 

strong caution.

Schidlowski: I address a question to the expert. Does the 

occurrence of mackinawite narrow down things a little 

bit?

Skinner: It depends in part on the composition of 

mackinawite. If it were a very nickel-rich 

mackinawite, you might be able to push the 

temperature 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 or 

less 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 environments.

Feather: I don't feel that we've analyzed sufficient 

grains 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. BOWLES

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-R

CONTENTS

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 _________________________________

Page

Rl 

1

1

2

3

3

10

11

11

16

Mineralogical evidence— Continued

Origin of the carbonaceous material _____________ R17

Indus River uraninite _____________________ 17

Geochemical evidence ________—__——————————— 19

Lead content of uraninite and pitchblende _________ 19

Uranium micromapping with thermal neutrons ______ 20

Neutron-activation analysis of whole-rock samples ____ 22

Bimodal nature of the uranium mineralization _________ 22

Conclusions _____________________——__——— 23

Acknowledgments _________________________ 23

References cited __________________________ 24

ILLUSTRATIONS

FIGURES 1, 2. Maps showing:

1. Sub-Ventersdorp geology of Witwatersrand Basin _____________________________________ R2

2. Principal mining areas and sample localities of the Witwatersrand Basin _________________________ 3 

3-7. Photographs showing:

3. Banket ore sample, showing allogenic pyrite, chalcopyrite, and quartz, from Dominion Reef; concretionary pyrite of 

spherical 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 _____ 5

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 _________________________________________________ 6

5. A uraniferous pyritic band consisting of concretionary and allogenic pyrite, from Basal Reef, President Brand,

Orange Free State ______________________________________________________ 7

6. Uraniferous pyrite band consisting of mainly concretionary pyrite grains, from Basal Reef, President Brand, Orange

Free State __________________________________________________________ 8

7. Electron-probe scanning images of a discrete galena grain occurring in a low-lead type pyrite concretion and of 

galena distribution in concretionary pyrite of high-lead type from Basal Reef, President Brand, Orange Free 

State _____________________________________________________________ 9 

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 ________________________ 10

9. A cumulative frequency diagram of U02/Th02 ratios for separate populations of uraninite and pitchblende from the 

Carbon Leader, the Vaal Reefs in the Witwatersrand, the Dominion Reef, the Indus River at Hazro, Pakistan, 

and the Hunza River, Kashmir ______________________________________________ 11

10. Standard error of the mean size for coexisting quartz, pyrite, and uraninite plotted on the phi scale against specific

gravity, for Bramley high grade, Dominion Reef ___________________________________ 12

in
IV CONTENTS

Page

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 _______________________________________ R13

12. Columnar carbon, showing zones of coarse fragmented allogenic uraninite and fine-grained disseminated pitchblende in a carbonaceous matrix, from Vaal Reefs, Klerksdorp __________________________ 14

13. Columnar carbon (part of fig. 12) _____________________________________________ 14

14. Columnar carbonaceous material, showing cluster of coarse-grained allogenic uraninite crystals partly digested

by carbonaceous matter, from Vaal Reefs, Klerksdorp _______________________________ 15

15. Uraninite within carbonaceus material and ThOi content, from Vaal Reefs, Klerksdorp ______________ 16

16. Electron-probe scanning images of probable churchite and the distribution of heavy rare-earth elements in probable churchite ______________________________________________________ 18

17. Photographs showing uraninite grains from the Hunza River, Kashmir, and an example of an altered uraninite grain __ 19

18. Cumulative frequency diagram showing the PbO content of uraninite and pitchblende from Dominion Reef, Witwatersrand, and Indus River sediments __________________________________________________ 20

19. Photomicrographs showing a uraniferous pyrite band consisting principally of allogenic pyrite with composite lenticular 

grains of quartz, kaolinite, and pyrophyllite and quartz, pyrophyllite, and kyanite, from Basal Reef, President Brand, 

Orange Free State ________________________________________________________ 21

TABLES

Page

TABLE 1. Comparison of heavy rare-earth distribution in xenotime, churchite, and yttrium phosphate phase relative to gadolinium ____ R17 

2. Neutron activation analyses of whole-rock samples ____________________________________________ 24
GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

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

ABSTRACT

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 with 

allogenic pyrite, quartz, and possibly also gold in the sediments that 

have 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 is 

consistent with the lower-energy depositional environment. The proximal or nearshore deposits are of banket type but are distinguished 

from 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 contain 

decaying organic material, which has precipitated both uranium and 

gold 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 with 

sulphides and arsenides.

Further evidence of the existence of uranium in solution is to be 

found in the banket deposits. In this case fine disseminations of 

uranium (>500 parts per million) occur in clay minerals within concretionary pyrite nodules and in lenticles formed of clay minerals in Witwatersrand banket deposits. They represent reduction-deposition of 

the soluble uranyl ion below the sediment-water interface where conditions are reducing.

Allogenic thorian uraninite from the prebent-day Indus River has a 

texture, composition, and association with gold and pyrite similar to 

allogenic 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 the 

retention 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 necessary 

geological criteria are met.

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

WC1X 8NG

PREFACE

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 the 

great controversies of the past 50 years. Any new attempt 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 application of techniques that will become more commonplace 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 that 

uranium can only be enriched to economic grades in 

Precambrian rocks if anoxygenic atmospheric conditions 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 Witwatersrand 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 that 

multidisciplinary investigations will continue. The full 

resolution of the genesis of the mineralization could 

result in new discoveries that are essential to the 

availability of uranium for future energy needs.

INTRODUCTION

Uranium mineralization in the Witwatersrand and 

Dominion 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) and 

sedimentary 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 transportation of detrital uraninite grains (Liebenberg, 1955; 

Grandstaff, 1975). This has led to the notion that 

uraniferous conglomerates are thereby restricted by 

these atmospheric conditions to the Archaean or early 

Proterozoic (Robertson, 1974).

The purpose of this study is to re-evaluate the uranium 

mineralization using mineralogical methods since the 

implications of the earlier work for the further discovery 

of 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 types 

of mineralization as possible, though it is appreciated 

that 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 have 

been uppermost in our minds: firstly, does the formation 

of 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 detrital 

grains of uraninite and as uranyl ions in solution. 

Answers to these two questions will have considerable 

relevance to the understanding of the mode of formation 

and estimating the potential for new discoveries of this 

type of mineralization.

EXPERIMENTAL METHODS

The details of the complex mineralization and the 

distribution of uranium were elucidated by ore microscopy, electron probe microanalysis, and fission track 

registration. The techniques of the first have been 

recently described by Bowie and others (1975), and the 

last is discussed in the section on uranium micromapping.

The electron-probe analyses were undertaken with a 

Cambridge 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 and 

cerium were analysed simultaneously by the Bragg specOHANNESBURG

Cape' 

Town

ORANGE \ 

FREE * 

STATE 50km

EXPLANATION

Upper Witwatersrand 

System (U.W.S.)

Lower Witwatersrand 

System

Dominion Reef 

System

- — Subcrop of U.W.S.

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 5

A WESTERN DEEP 

', LEVELS

{ 'DOMINION ^72083-2088.2092

J \ REEFS /'" VAAL REEFS

WEST

WITS DAGGAFONTEIN

FREDDIES REEF

2093, 2102- 2107

DOMINION REEFS

1997-2028.2101, US1273C. 

UG5538. UG9824

AFRIKANDER 

REEF

2081

Location

\? - South Africa

Town

ORANGE

FREE 

STATE

/PRESIDENT

BRAND2098 I 

PRESIDENT STEYN

2095-2096 \

FREE STATE SAAIPLAS

2094,2097 '' ^'

VIRGINIA REEF

2052-2053,2058-2059

EXPLANATION

Principal mining 

areas

Mining properties 

with sample numbers

50km

FIGURE 2.—Map of the Witwatersrand Basin illustrating principal mining areas and sample localities. The numbers in 

small 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 specimen were counted for 250 s at a low beam current of 

lxlO~9 A, which conditions were suitable for both 

analytical 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 nine 

elements 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), for 

calcium, yttrium, and cerium; and chemically analysed 

rutile and galena, for titanium and lead, respectively. 

The analytical methods for wavelength-dispersive analysis described by Bowles (1975) were used for calcium and 

cerium. In another paper, Bowles (in preparation) 

describes the quantitative analytical programme for the 

energy-dispersive equipment and compares selected 

results with those obtained with the Bragg spectrometers. The results are closely comparable for all 

elements with the exception of uranium, for which the 

energy-dispersive results are consistently 3 percent 

lower than the Bragg spectrometer results. Investigation has shown that as between the pure metal and the 

oxide there is a change in the ratio of the Lal to La2 intensities. The lower resolution of the energy-dispersive 

equipment records both X-ray emissions as a single

peak, so that a variation in the ratio does not affect the 

results. The analyses presented here for uranium and 

thorium are those obtained using the energy-dispersive 

equipment. The measured concentrations were corrected using the quantitative correction programme 

devised by Mason and others (1969).

MINERALOGICAL EVIDENCE

PYRITE

Pyrite is the principal opaque mineral of the Witwatersrand and Dominion Reef Systems. In hand specimen rounded grains of pyrite, commonly known as 

"buckshot" (Feather and Koen, 1975), display clearly 

defined sedimentary features with well-developed bedding 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, in 

the matrix. Radioactivity is closely associated with 

pyritic zones in the hand specimen, and those were 

therefore selected for more detailed investigation. Indeed, the presence of pyrite in the hand specimen is one 

of 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 and 

Saager (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 a 

distinctive range of inclusions (fig. 3A). There is no 

doubt it is correctly described as allogenic. It is recognized 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 of 

this type also occurs partially or completely enclosed 

within very coarse quartz grains; it displays euhedral 

grain boundaries towards the host quartz but has a 

subrounded and abraded margin where it forms the exterior of the composite grain (fig. 3A). Optical study and 

electron-probe microanalysis indicate that galena is the 

commonest phase included in allogenic pyrite. The exsolution blebs of galena are distinct from marginal intergrowths of pyrite and galena resulting from the migration 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 is 

readily distinguished from type I by its porous nature 

and from type III by its rounded outline. The concretions 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 are 

heterogeneous in nature with regard to shape, size, and 

crystallite composition. Some concretions have spherically zoned alternating sulphide and silicate shells; 

others exhibit a layered structure. Some are so dark and 

fine grained as to be indistinguishable from melnikovite 

or colloform pyrite, which is thought to represent a 

crystallized 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 between crystallites represent about 10 percent by volume 

of 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, ranging from fine-grained disseminated pyrite to a more 

compact equigranular variety without pore spaces between crystallites. Saager and Mihalik (1967) proposed 

that these concretions formed in situ, since they claimed 

the "extremely delicate nature of the concretions exclude any long transport distance." However, it should 

be noted that the crystallites forming the concretions 

are in most cases interconnecting, thereby providing a 

rigid 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 deformation relatively unaltered. Moreover, coexisting detrital quartz granules show appreciable pressure solution 

effects along their margins, though both allogenic and 

concretionary grains occurring in close proximity are 

underformed, testifying to their relative rigidity and 

strength.

In our view the heterogeneity of the concretionary 

pyrite in any hand specimen and the evidence of some 

rounded granules that have apparently been fractured 

and abraded before deposition indicate that most of the 

concretionary pyrite has been transported to its present 

location and did not form in situ (fig. 5). The variability 

of this pyrite in any particular sample is considered to 

reflect a source area and selective effects of transportation processes. The intimately intergrown nature of the 

sulphide and silicate phases in the concretions is most 

probably a primary feature inherited from the source 

area and is not due to subsequent introduction of silica, 

since the regular fine-scale laminations observed in 

some cases preclude the possibility of later introduction 

(fig. 6). Hallbauer (1975) has described subspherical concretions of this type extracted for study by scanning 

electron microscopy. However, there are rare occurrences of zoned spherical concretions. The example 

shown (fig. 31?) has a thin inner zone of chalcopyrite with 

rare 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 which 

transport may or may not have occurred cannot in 

general 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 disseminated galena in 10 /im grains uniformly distributed 

throughout the concretion (fig. IE), and some have fine 

veinlets 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 and 

Saager (1974) report enrichments of chalcopyrite, 

sphalerite, and galena associated with the development 

of this pyrite type and also remark that the presence of 

dykes 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 SYSTEMS R5

FIGURE 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 of 

gold (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, with 

gold (Au). Carbon Leader, West Wits (US 5); reflected light, oil.
R6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

C QJ

ll

s « .2 TD "rt "8

c o

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.-§& •S 'oT' 

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URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS R7

FIGURE 5.-A uraniferous pyritic band consisting of larger concretionary and smaller allogenic pyrite (white). A, Reflected light. B, Corresponding 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 CONGLOMERATES

FIGURE 6.—Uraniferotls pyrite band consisting of mainly concretionary pyrite grains (white), which are a 

heterogeneous 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 accounted for by the presence of allogenic grains of uraninite (y) and interstitial clay minerals. Basal Reef, 

President Brand, Orange Free State (PTS 2463A).
3"*4

.«- *' TMFl," f~ ^ m.V ****** f~, V >}•*•."• • .XM^* ; '<&"^rJk * -~ ***••*. ;:<vj<... ,-A ./••'-' -* ^-.*> -*\^ ..^^ '??%%&&&£& *f.N<- ?-,' « i *• ^ r ^"afep-r^ XS^ii*-* -•** f*-» t^**'* - ^J^* -%^"*d"» "*^V oao

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Pb

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 g 

(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.
RIO GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

cases abundant, noted in pyrite of this type during the 

present study (fig. 3C% indicates that gold and pyrite 

were locally remobilized and redeposited together.

URANINITE

Before describing the details concerning the analysed 

material, it is necessary to comment on the use of the 

terms uraninite and pitchblende. Uraninite is the name 

for 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 uraninite found there can be divided into two groups: (1) a 

well-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 the 

well-crystallized variety and "pitchblende" to the other.

Uraninite occurs as discrete allogenic grains varying 

in shape from euhedral to subrounded and rounded 

grains, usually within a limited size range in the fine to 

very 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 the 

best indicator of primary differences in populations of 

uraninite grains since lead content within and between 

grains may be quite variable owing to exsolution and 

migration of radiogenic lead. This lead mobility has a 

greater effect on the uranium content than on thorium, 

which has a more homogeneous distribution and is less 

readily displaced. It is possible to observe in some cases 

the migrations and reaction of radiogenic lead with 

nearby 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 migration and reaction of uranium with titanium minerals in 

the sediments.

Electron microprobe analyses show in this study that 

the 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, allogenic uraninite analysed in a columnar carbonaceous 

band (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 grains 

from the Dominion and Vaal Reefs were tested by analysis of variance methods and found to be significantly difUS 5, 

Pitchblende,, 

23 analyses

2034,

Pitchblende, 

21 analyses

2034,

Allogenic uraninite,

92 analyses

LJJ 

O

cc

LJJ 

Q_

B

12

YP 19 A, 

Hazro, Pakistan,., 

19 analyses

UG 9814, 

Dominion Reef, 

11 analyses

YP 10 A, 

Hazro, Pakistani 

29 analyses

NCL 1115, 

Hunza, Kashmir, 

33 analyses

1 50 99

c

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 (US 

5). 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 Indus 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 correlation between calcium and manganese and between 

iron and manganese is common to both uraninite populations.

The Th02 content, U02/Th02 ratio, and correlation between calcium and manganese and iron and manganese 

probably reflect conditions of crystallization in the 

source 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 Rll

80

60

40

Pitchblende in 

granular carboi 

Witwateisrand

1 50 99

CUMULATIVE PERCENT

FIGURE 9.—U02/Th02 ratios for separate populations of uraninite and 

pitchblende 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, which 

is still higher than that of uraninite from the Dominion Reef (UG 

9824). Detrital uraninite from the Indus River at Hazro (YP 10A and 

YP 19A) and the Hunza River, Kashmir, a tributary of the Indus, is 

plotted here for comparison with the Dominion Reef and Witwatersrand 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 PHASES

A typical sample (UG 5538) from the Bramley section 

of 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 micrometer, which can be used to classify grains directly according to the phi scale. Each population, except for gold, 

consisted of a hundred grains measured along the Feret 

diameter, 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 deviations were calculated in terms of phi units, from which 

the standard error of the mean was obtained. This was 

transferred 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

nevertheless appear too small for exact hydraulic 

equivalence. This point was noted by Coetzee (1965), but 

no satisfactory explanation has yet been proposed.

URANIFEROUS AND AURIFEROUS CARBONACEOUS 

MATERIAL

Uranium and gold in association with carbonaceous 

material is one of the most important modes of occurrence of these metals in the Witwatersrand Basin, particularly in view of the uniformly high grades of mineralization encountered and the wide lateral extent of 

such 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-rich 

carbonaceous matter should take account of the following 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 of 

mineralization are higher compared with the 

banket type of both the Witwatersrand and Dominion 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 the 

formation of these deposits.

The carbonaceous material comprises granular and 

columnar varieties, which have many features in common whilst differing in certain important aspects 

(Davidson and Bowie, 1951). According to the analyses 

of Davidson and Bowie this material contains 43 percent 

carbon and 2 percent hydrogen, and they called this 

phase hydrocarbon. However, we have not attempted to 

define the organic constituents of all the carbonaceous 

materials studied here and have therefore retained a 

more general nomenclature used by Vine and others 

(1958).

The granular or "fly speck" material (fig. 11) consists 

of ovoid to rounded grains (100-400 /*m) with an interstitial 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
-e- 2 

cc

LU 

LU

Q 4

R12 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Th02 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 with 

a range from 25 to 69 (fig. 9). Analysis of variance shows 

that 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 Dominion 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 manganese 

or between iron and manganese as for the allogenic 

uraninite, pointing to a distinct origin for the pitchblende in the granular carbon, which probably developed 

in situ during diagenesis and metamorphism.

The carbonaceous seam of the Carbon Leader usually 

occurs along the basal parting in quartz-pebble conglomerate, and has a well-developed columnar structure 

normal to the bedding. It is linked with, but different 

from, the granular form described. It consists of the 

same two carbon phases, occurring as lenticular to ovoid 

bodies with their long axes normal to the bedding. Individual bodies are cut by discontinuous vertical sutures, 

which emphasize the overall columnar effect (fig. 12). 

Scanning electron-probe studies demonstrate that these 

sutures 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 concentrated 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 a 

chemically heterogeneous assemblage of uraninite 

crystals subsequently fragmented and replaced by carbonaceous matter (fig. 15). The chemistry of the allogenic uraninite grains has been previously described and 

is shown in figures 8 and 9. Pitchblende, associated with 

partially replaced allogenic uraninite has a much higher 

thorium content than the pitchblende in the granular 

carbonaceous 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 both 

uranium and thorium from the allogenic uraninite undergoing attack by carbonaceous matter.

2 5 10 20

RELATIVE DENSITY

FIGURE 10.—Standard error of the mean size 

for coexisting quartz, pyrite, and uraninite 

plotted on the phi scale against specific gravity. The light curves are solutions for Stokes' 

law and the square-root law combined where 

they overlap. The relative grain sizes of the 

suite of minerals plotted in heavy type indicate that quartz, pyrite, and uraninite are 

hydraulically equivalent. The gold is smaller 

than would be expected for hydraulic 

equivalence, but the number of grains 

measured is inadequate for full statistical 

treatment. Bramley high grade, Dominion 

Reef (UG 5538). (Diagram after Tourtelot, 

1968.)

minerals. The granules have mottled extinction under 

crossed polars in reflected light and contain two main 

phases of carbonaceous matter (fig. 3D), consisting of 

relatively coarse-grained smooth areas with undulose 

extinction interspersed with fine-grained material having a similar reflectance of 14.3 percent (Type I of 

Davidson and Bowie, 1951, p. 6). The fine-grained 

material is veined along grain boundaries by a dendritic 

system containing pitchblende, gold, pyrite, galena, and 

gersdorffite. 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 quantitative 

electron-probe analysis. The pitchblende has a median
URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS R13

FIGURE 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)
R14 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 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 between these zones consist of fine disseminations of gold associated 

with 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 backscattered electron images and distribution of uranium and thorium corresponding 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 CONGLOMERATES

The disseminated pitchblende in fine-grained columnar carbonaceous matter is optically and texturally very 

similar to that in the granular variety and is also intimately associated with galena, gersdorffite, gold, and 

pyrite, forming an interconnecting dendritic texture of 

microcrystals in the manner previously described.

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. Internal pore spaces within carbonaceous matter are also 

an important site for the formation of complex intergrowths of uraniferous silicates, gold, and gersdorffite. 

Partially replaced uraninite within carbonaceous material is also a common site for gold overgrowths. All these 

textures suggest that much of the gold, pitchblende, 

churchite (see next section) and associated sulphides 

crystallized in their present position as a result of 

diagenetic or later metamorphic processes.

CHURCHITE

A tenuous phase surrounds the fine-grained pitchblende of the columnar carbon (fig. 16). It contains major yttrium and phosphorous with the heavy rare-earth 

elements gadolinium, terbium, dysprosium, erbium, and 

ytterbium, corresponding to either xenotime (YP04 or

FIGURE 15.—A, Electron-probe backscattered electron image showing 

uraninite (Ur) within carbonaceous material. The destruction of 

large uraninite grains by the carbonaceous material is shown by the 

complementary outlines of adjacent grains and in the reconstruction 

of original grain outlines. B, The Th02 content clearly demonstrates 

similarity within but differences between grains outlined. Vaal 

Reefs, Klerksdorp (PTS 2031).
URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS R17

churchite (YP04 • 2H20). Comparison of the rare-earth 

distribution (table 1) with that reported in the literature 

for xenotime and churchite points to an identity with the 

latter because of the distinct enrichment of the former 

in dysprosium, erbium, and, particularly, ytterbium relative to gadolinium.

This is particularly interesting in the present context, 

since xenotime is found principally in granitic pegmatites whereas churchite2 can be formed by a combination 

of biochemical processes and diagenetic reactions close 

to the surface (Milton and others, 1944). In the occurrence described by these authors, spherulites of churchite are formed as a coating on siliceous limonite or black 

manganese oxide. They suggest that the heavy rare 

earths were concentrated biochemically in the leaves of 

certain deciduous trees. Stability of the forest floor permitted concentration in the humus layers, from which 

the churchite coating on existing iron and manganese 

oxides was derived diagenetically. It seems possible, 

therefore, that in the Witwatersrand some combination 

of biochemical and diagenetic processes may have led to 

the formation of churchite around the fine pitchblende 

grains in the columnar carbon.

ORIGIN OF THE CARBONACEOUS MATERIAL

Several authors have reported carbonaceous material 

devoid of mineralization, so neither uranium nor gold 

performs an essential role in its formation. The columnar structures are developed normal to the bedding, as 

in stromatolites, and it is unlikely therefore that they 

are metamorphic structures as suggested by Liebenberg 

(1955). Hallbauer and van Warmelo (1974) later proposed that the structures are a primary feature of the 

carbonaceous matter. They advanced a theory, further 

elaborated in Hallbauer and others (1975) and Hallbauer 

(1975), that the columnar carbonaceous material is the 

fossilized residue of a Precambrian symbiotic association (designated Thycomyces lichenoids) consisting of an 

algal partner and a fungal organism and occupying 

areas between a few square centimetres and a few

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).

square metres. Treatment of the carbon revealed a 

skeletal framework of uranium and thorium oxides and 

of gold, and slow oxidation of the carbon at 500°C left a 

heavy-metal fabric, which appears to correspond to the 

structures described in the present study. Since these 

metals are most probably deposited in pore spaces in the 

carbonaceous matter, it is unlikely that they represent 

the actual skeleton of the "plant" or are a replacement of 

any fibrous structure as Hallbauer (1975) suggests. 

However, the observation that several fine-grained 

phases containing heavy metals, notably lead, uranium, 

thorium, gold and r.e.e., combined with arsenic or sulphur, are closely associated with possible stromatolitic 

algal mats or related structures may have genetic 

significance for the origin of mineralization in the Witwatersrand, which is discussed later.

INDUS RIVER URANINITE

Samples of uraninite from the Indus River were 

analysed by electron probe for comparison with the allogenic uraninite of the Dominion Reef and Witwatersrand Systems and with a view to assessing the stability 

of uraninite as a detrital mineral in recent alluvial 

deposits. The samples had been collected from the Hunza River, a tributary of the Indus River in Kashmir 

(Darnley, 1962), and from the Indus alluvium near 

Hazro, Pakistan (Miller, 1963).

The Hunza sample consists of a 99-percent pure concentrate in which nearly all the uraninite grains have at 

least one well-defined crystal face and some show cubic 

form with little evidence of corrosion. They are in the 

very fine to fine range of sand size (0.05-0.20 mm) (fig. 

17). Pyrite is present in the concentrate as euhedral 

cubes, fresh and untarnished, and as concretions. An extensive assemblage of radioactive resistate minerals and 

sulphides has also been reported (Darnley, 1962).

The Hazro samples, which Miller (1963) had reported 

to contain both uraninite and gold, were also reexamined, involving the preparation of concentrates by 

vanner, superpanner, and heavy liquids. The final concentrate contained—in addition to uraninite—pyrite, 

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

1 

1

1

1

Tb

0.49 

.53

Dy

1.42 

5.57

1

.93

Ho

0.72

.72

.16

Er

1.19 

5.41

1

.53

Tm

1.26

trace

Yb

0.88 

13.03

trace

.60

Lu

1.07

.10

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).
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 distribution of heavy rare-earth elements in phase "A" relative to gadolinium is compared here with examples of churchite and xenotime from 

the literature. Vaal Reefs, Klerksdorp (PTS 2023).
URANIUM MINERALIZATION t 7 THE WITWATERSRAND AND DOMINION REEF SYSTEMS R19

sulphides were fresh, but many of the uraninite crystals, 

with at least one crystal face, had a distinct outer 

weathered zone, though beneath this they were fresh 

and homogenous.

The Th02 content of one sample from the Hunza and 

two 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 the 

greater 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 evidence 

of an atmosphere free of oxygen, and Liebenberg (1955) 

and Grandstaff (1975) considered a nonoxidizing atmosphere was necessary for uraninite transportation. 

Neither of these conclusions is supported by the 

evidence from the present-day Indus River, in which, as

we have seen, uraninite, gold, pyrite, and other 

sulphides 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 in 

figure 18. The data fall on four main distributions as 

follows: (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 Dominion Reef with high lead content, and (4) pitchblende with 

high 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 of 

alteration and Th02 content has been detected.
R20 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

URANIUM MICROMAPPING WITH THERMAL NEUTRONS

O

JD 

Q.

(J 

CC

ff

10

2034,

Allogenic uraninite in 

columnar carbon, , 

Vaal Reefs, /'' 

92 analyses /'

UG 9814, 

Uraninite, 

Dominion Reef, 

11 analyses

US 5,

Pitchblende in 

granular carbon. 

Carbon Leader, 

23 analyses

Pitchblende in 

columnar carbon, 

Vaal Reefs, 

21 analyses

NCL 1115, 

Uraninite, 

Hunza, Kashmir, 

33 analyses

\ ...~£~~

YP 19 A,

Uraninite,

Indus,

19 analyses

\^

99

CUMULATIVE PERCENT

FIGURE 18.—Cumulative frequency diagram of the PbO content of 

uraninite and pitchblende from Dominion Reef, Witwatersrand, and 

Indus River sediments.

formed in situ from the diagenesis and metamorphism of 

the carbonaceous host. The low thorium content of this 

phase is indicative of a low-temperature solution (thorium 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 reflection 

of their greater age. Partial rejuvenation of the allogenic uraninite from the Witwatersrand resulted in the 

formation of a lead-rich pitchblende in sample 2034. 

However, the evidence from sample US 5 indicates that 

the presence of allogenic uraninite is not essential for 

the 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 in 

the 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 and 

Dominion Reef samples was studied by the fission-track 

method using Lexan polycarbonate for the registration 

of fission fragments from the induced fission of 235U by 

thermal neutrons, as described by Price and Walker 

(1963), Kleeman and Lovering (1967), and Bowie and 

others (1973). The method is ideal for providing information on the location of uranium in banket-type occurrences. 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 span 

concentration ranges. The samples studied here have 

been 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 mineralogical 

phases to be readily determined when the uranium is 

below the limits of detection by microprobe.

The results show that uranium is closely confined to 

discrete horizons both in the banket-type and carbonaceous reefs and that it not only occurs as grains of 

uraninite 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 allogenic pyrite grains otherwise uranium free; and, rarely, 

within dislocations and shears (figs. 5, 6, 19).

Both microprobe and fission-track studies show the 

presence 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 shows 

that the uranium content of individual concretions is 

variable 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 occurs only in the rims of some concretions, whereas in 

others, 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 an 

essentially barren quartzitic groundmass, and in cases 

such 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 with 

uranium, but the uranium in the pyrite is below detection limit, and in this case the uranium occurs in a secondary clay mineral forming a marginal overgrowth to the 

pyrite. This association is particularly well developed in
URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS R21

ui^ 13

*a '53 Q> TM > &

"§ ^ rt

•&; cs o>

11:1

c c

8 * Sb

<u s

4= -^ CO

"C c g

0

'S -S o> iu

•§ .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 carbonaceous band rich in uranium and gold (PTS 1944, West 

Driefontein). In this case the uranium was probably 

deposited in situ and subsequently incorporated into the 

clay 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 since 

uraniferous silicates overgrowing allogenic pyrite occur 

dispersed through a barren quartzite matrix and are 

readily 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 Witwatersrand 

banket-type ores studied here. The uranium occurs disseminated throughout the granules and produces point 

sources of fission tracks, which have not yet been correlated 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 most 

probably occurred either before deposition or at the 

latest 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 a 

leached and weathered margin and along a vermicular 

veinlet structure within the pebble. Enrichment in 

uranium during transport and diagenesis related to 

weathering of the pebble rim (fig. 19) seems to be the 

most likely explanation of this occurrence. Regional 

metamorphism has tended to reduce the thickness of the 

uraniferous rim, which is best developed where 

weathered pebbles are relatively unaltered.

NEUTRON-ACTIVATION ANALYSIS OF WHOLE-ROCK 

SAMPLES

Instrumental neutron-activation analysis of wholerock samples was undertaken as an adjunct to the mineralogical studies utilizing a method developed by Plant 

and others (1976).

The results are given in table 2 and demonstrate the 

increase in U/Th ratios from the Dominion to the Vaal 

Reefs in the Witwatersrand Basin. Freddies Reef, East 

Geduld, and Daggafontein are somewhat higher and 

similar to one another, whereas samples from West 

Driefontein, dominated by carbonaceous matter, have 

the highest ratios. Twenty-four additional elements obtained by this method on the same sample are presented

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 between the two.

BIMODAL NATURE OF THE URANIUM 

MINERALIZATION

The occurrence of allogenic grains of uraninite in the 

Dominion Reef (Taylor and others, 1962) and the Witwatersrand (Liebenberg, 1955; Grandstaff, 1975) is well 

established. This phase occurs in the Dominion Reef in 

association with resistate phases such as garnet, monazite, cassiterite, zircon, allogenic sulphides, rare 

authigenic sulphides, and gold. The similarity of this 

assemblage with modern gravels on banks in the Indus 

is notable, although uraninite in the Indus is at present 

subeconomic (Miller, 1963). In the Witwatersrand System the characteristically lower content of allogenic 

uraninite and rounded resistate phase, the relatively 

greater importance of concretionary pyrite, and the appearance 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, and 

Witwatersrand is most significant. However, the pitchblende 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 of 

mineralization 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 with 

clay minerals occurring individually or intergrown with, 

or overgrowing, pyrite, and in many cases this association has the appearance of a primary, essentially 

syngenetic, relationship unaffected by appreciable 

uranium remobilization.

Evidence of this type from Witwatersrand sediments 

provides support for the existence of some uranium in 

solution in the basin during sedimentation and diagenesis, as discussed by Pretorius (1975). The uranium 

would be highly mobile as the soluble uranyl ion, U02 2+ , 

which would subsequently be absorbed, reduced, and 

precipitated by organic-rich muds probably aided by and 

associated with the simultaneous reduction of sulphate 

by bacteria, as proposed by Trudinger (1971):
URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS R23

The hydrogen sulphide formed would subsequently react 

with other metals in solution such as iron and lead to 

form the authigenic intergrowths of pyrite and galena 

with clay minerals, gold, and uranium observed in the 

sediments.

The most likely modes of introduction of uranium, 

gold, and sulphate in solution are the complex fault 

system of the Witwatersrand Basin and dissolution of 

coexisting detrital minerals. Some of the sulphate would 

have been precipitated in close proximity to hot springs 

at 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 carried 

out into the basin to be precipitated together on the 

distal portions of the alluvial fans by the action of decaying 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 Witwatersrand environment. Though relatively unimportant in the 

higher 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.

CONCLUSIONS

The overall control of the uranium mineralization in 

the Witwatersrand and Dominion Reef Systems is the 

presence of a depositional basin overlying an Archaean 

craton 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 by 

Bowie (1970) as a fundamental control in the delineation 

of 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 from 

solution. The detrital components were concentrated in 

the higher energy conglomerate environment of the 

Dominion Reef and proximal parts of the Witwatersrand Basin. The solution components including uranium 

and gold were precipitated under reducing conditions 

through the agency of decaying organic matter in the 

lower energy or distal regions of the basin, giving rise 

through diagenesis and metamorphism to such uraniferous 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 resistance to attrition and oxidation to the extent that a 

thorian uraninite with 1 percent or more thorium oxide 

can survive as river detritus under present-day atmospheric conditions. All of the uraninite grains from 

the 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 solution 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 occurrence of pebble conglomerates with detrital uraninite 

and pitchblende deposited from solution may have a 

more widespread occurrence throughout geological time 

than hitherto suspected, provided the necessary 

geological 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, Indus River. B. R. Skilton and J. A. T. Smellie prepared 

polished 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 for 

samples 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 is 

thanked for helpful discussions on solution chemistry, 

and R. J. Howarth, for advice on statistical methods. 

This work forms part of the research programme of the 

Institute of Geological Sciences and is published with 

the approval of the Director.

The authors and the U.S. Geological Survey thank The 

Royal Society of London for permission to republish this 

paper. It first appeared in The Royal Society's 

Philosophical Transactions [Phil. Trans. R. Soc. Lond. 

A. 286, 527-548, (1977)]. Much of the material contained 

in 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 CONGLOMERATES

TABLE 2.—Neutron-activation analysis, in parts per million, except where stated otherwise

Sample 

location

Dominion Reefs

Vaal Reefs

Freddies Reef

East Geduld

Daggafontein

West Driefontein

Rock 

sample

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

Na

1 2.19

<200

<200

< 1,000

<800

< 1,000

980

990

52

66

330

190

94

256

220

86

520

387

204

<400

K

< 2,000

6,600

< 2,000

< 2,000

< 2,000

< 2,000

< 2,000

< 2,000

< 2,000

< 2,000

5,900

930

< 2,000

< 2,000

< 2,000

< 2,000

2,200

< 2,000

< 2,000

< 2,000

Sc

38

24

30

32

10

13

7

8

4.2

4.6

6.7

4.6

6

18

3

8

11

7.5

4

4

Cr

<400

249

374

297

1,400

450

890

315

850

760

2,300

560

152

1,200

115

244

650

1,040

235

84

Fe1

10.2

7.31

4.96

7.63

6.56

2.80

4.89

1.65

1.77

3.17

5.09

3.63

6.76

7.67

.21

1.19

2.79

4.93

4.34

2.38

Co

570

260

425

420

260

216

229

120

108

123

200

117

312

219

21

43

55

140

59

14

Zn

<300

<300

<300

<300

344

<350

<160

<160

440

354

366

295

239

<800

220

818

<100

360

264

463

As

3,800

340

3,100

3,600

605

1,200

670

370

430

620

640

140

336

296

29

134

120

450

46

60

Rb

<100

<200

<200

<200

<100

<200

<200

<100

<100

<80

<100

<100

<100

<100

<50

<100

<70

<100

<50

<150

Ag

<40

<35

<40

<40

<20

<40

<40

<30

<15

22

<15

<40

24

<40

<15

<15

<15

46

39

130

Cd

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

<100

Sb

<28

<6

<10

19

25

53

27

21

6

10

15

<5

<5

<5

<5

<5

6

9

<5

7

Cs

<5

<5

<6

<10

<5

<6

7

5

<5

<5

<5

<5

<3

<3

<3

<3

<3

<3

<3

<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, in 

Uranium 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, Reflectance 

measurements in monochromatic light on the Bowie-Taylor suite 

of 103 ore minerals: Fortschritte der Minerologie 52, p. 567-582.

Bowie, S. H. U., Simpson, P. R., and Rice, C. M., 1973, Application of 

fission-track and neutron activation methods to geochemical exploration, 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, Some 

observations on the solubility of gold: Canada Geological Survey 

Paper 75-24, 6 p.

Claringbull, G. F., and Hey, M. H., 1953, A re-examination of 

churchite: The Mineralogical Magazine, 30, p. 211-217.

Coetzee, F., 1965 Distribtuion and grain-size of gold, uraninite, pyrite 

and certain other heavy minerals in gold-bearing reefs of the Witwatersrand 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 related 

hydrocarbon-uraninite complexes: Geological Survey of Great Britain 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 Witwatersrand reefs: Mineral Science Engineering, v. 7, p. 189-224.

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 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 Blind 

River, 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.
URANIUM MINERALIZATION OF THE WITWATERSRAND AND DOMINION REEF SYSTEMS 

TABLE 2.—Neutron-activation analysis—Continued

R25

Sample 

location

Dominion Reefs

Vaal Reefs

Freddies Reef

East Geduld

Daggafontein

West Driefontein _____

Rock 

sample

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

Ba

< 500

< 1,000

< 1,000

< 1,000

< 1,000

< 1,000

< 1,000

< 1,000

<500

<500

<500

<500

< 1,000

< 1,000

< 1,000

< 1,000

<300

< 1,000

< 1,000

< 1,000

La

1 1.3

906

1,400

1,100

260

475

248

172

59

68

41

57

44

42

20

16

22

47

25

69

Ce

1,200

1,200

2,200

1,800

700

900

350

500

260

135

<200

80

<100

<10

22

45

55

<300

<10

<30

Nd

1,000

850

910

1,270

1,500

2,800

860

940

170

195

280

73

227

400

9

112

43

88

340

454

Sm

445

285

335

600

450

850

200

250

60

65

<10

12

75

160

6

30

20

40

100

140

Eu

<2

12

12

20

20

37

17

15

4.7

5.3

3.6

1.2

1.5

8

1

1.5

2

3

6

7

Tb

20

16

17

15

8

24

6

6

<5

<5

5

2

<2

3

<2

<2

<2

<2

3

16

Lu

14

10

11

18

13

22

6

8

<3

<3

<3

<3

<1

<3

<1

<1

<1

<5

<3

4

Hf

<10

15

19

18

22

30

36

11

18

<20

17

11

4

8

<2

2

2

19

7

<5

Ta

410

240

450

450

8

<100

7

<5

30

30

6

4

15

20

<5

2

30

50

<5

<5

Au

3

9

2

5

154

218

69

214

90

115

118

1.5

173

132

.4

28

11

428

221

444

Th

809

620

753

970

477

821

298

273

52

80

40

20

71

102

<5

10

1ft

40

75

99

U

3,900

2,300

2,300

5,400

4,500

8,400

2,100

2,700

685

765

510

75

895

2,000

43

337

180

460

1,400

1,500

U/Th 

ratio

4.8

3.7

3.1

5.6

9.4

10.2

7.1

9.9

13.2

9.6

12.8

3.8

12.6

19.6

12.0

11.5

18.7

15.2

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 Precambrian 

plants from the Witwatersrand, South Africa: Chamber of Mines 

of 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 and 

radioactive 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 the 

transportation 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. Report 

no. 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 Precambrian 

rocks: 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 Witwatersrand gold fields: A chronological review of speculations and 

observations: 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 Witwatersrand in South Africa and their genetic significance: Geological 

Society 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 International 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 the 

Main Reef and Main Reef Leader on the Main-Bird Series, Witwatersrand 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. BOWLES

GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-S

CONTENTS

Page

Abstract _____________________________________________ SI

Introduction ___________________________________________ 1

Evidence from allogenic uraninite in the Witwatersrand and Indus River ___________ 2

Evidence from theoretical models of uraninite stability _____________________ 2

Evidence from carbonaceous matter in the Witwatersrand ___________________ 4 

Evidence from authigenic pitchblende and disseminated uranium in the Witwatersrand

System ____________________________________________ 4

Evidence from the sulphur cycle in the Witwatersrand _____________________ 6

Inorganic surface oxidation of pyrite _____________________________ 6

Bacterial surface oxidation of pyrite _____________________________ 6

Bacterial reduction of sulphate below the sediment/water interface _____________ 6

Summary ________________________________________________ 6

Conclusions ____________________________________________ 7

References cited _________________________________________ 7

Discussion following papers by Drs. Feather, Sims, and Simpson and Bowles _________ 8

ILLUSTRATION

Page

FIGURE 1. Photomicrographic mosaics showing columnar carbonaceous matter containing detrital and secondary pitchblende; massive

carbonaceous matter containing authigenic pitchblende _________________________________-_— S5

TABLES

TABLE 1. Electron microprobe analyses of uraninite grains from Indus alluvium, Hazro, Pakistan _________________—_—_ S3 

2. Correlation matrix for the chemical composition of uraninite grains listed in table 1 ________________________ 4

in

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

DETRITAL URANINITE AND PYRITE: 

ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE?

By P. R. SlMPSON and J. F. W. BOWLES1

ABSTRACT

Many occurrences of uraniferous quartz-pebble conglomerates from 

widely separated regions are known to date from the period 2.8-2.0 

billion years ago. The deposits of the Witwatersrand System are 

among the most important of this type, and the occurrence in them of 

allogenic uraninite and pyrite has been widely used as evidence that 

the atmosphere during that period of the Earth's history was reducing 

or anoxygenic.

Comparison of modern alluvial sediments of the Indus River with 

those of the Dominion Reef and Witwatersrand Systems indicates 

stable 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 and 

deposition can be most readily accounted for by a model in which the 

sulphate and uranyl ions exist in superficial, oxidised, active sediments 

and river waters draining into the Witwatersrand Basin while detrital 

grains 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 by 

catastrophic burial of the sediments in a subsiding basin. This would 

prevent oxidation of the abundant organic matter, now represented by 

carbonaceous 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 is 

therefore controlled by geological and not atmospheric conditions. It is 

suggested 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 is 

thought to represent a unique phase of crustal evolution, when extensive 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 Earth 

such as Witwatersrand in Southern Africa and Elliot 

Lake in North America are known to date from the 

period 2.8-2.0 b.y. (billion years) ago. The deposits of 

the Witwatersrand are economically among the most 

important 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 Witwatersrand System. The uranium has two principal 

modes of occurrence, consisting principally of uraninite 

grains in conglomerate and uraninite and pitchblende in 

carbonaceous horizons. Fresh unoxidised pyrite is present as coarsely crystalline grains and as fine-grained 

concretions in conglomerate. Gold is present as grains in 

conglomerate 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 Witwatersrand rocks are flanked by a number of domelike 

small inliers of granite-greenstone basement, which 

form the elliptical frame defining the depositional basin 

of the Dominion Reef and Witwatersrand sediments. 

These inliers are considered to be the provenance area 

for allogenic uraninite, gold, pyrite, and associated 

heavy minerals found in the basin, and direct connections 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 one 

of 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. This 

suggestion depends heavily, however, on the belief that 

uraniferous conglomerates provide evidence for a reducing atmosphere in the first instance. There is no dispute 

as to the relative abundance of these deposits in the 

early Precambrian, which contains all the presently 

known occurrences of uraniferous conglomerate of

SI
S2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

economic grade. However, the new mineralogical 

evidence for the origins of uraninite and pyrite is not in 

agreement with anoxygenic or reducing atmospheric 

conditions 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 Indus River, where the concentrations are presently 

regarded as subeconomic (Miller, 1963; Darnley, 1962), 

uraninite is present in adequate quantity for study; and 

it can be deomnstrated that it is sufficiently stable under 

present-day atmospheric conditions to survive exhumation, surface weathering, and transport in the river 

system over considerable distances (Simpson and 

Bowles, 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 derived 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 hydrospheric 

evolution.

EVIDENCE FROM ALLOGENIC URANINITE IN 

THE WITWATERSRAND AND INDUS RIVER

The close similarity in size (0.05-0.25 mm) and compositional range of allogenic uraninite from the Dominion Reef, Witwatersrand, Indus River alluvium near 

Hazro, Pakistan, and alluvium in the Hunza River, 

Kashmir, which is a tributary of the Indus, has been 

reported previously (Simpson and Bowles, 1977). The 

Th02 content and U02/Th02 ratio of uraninite from the 

Dominion Reef correspond closely to those for uraninite 

from Hazro, and the Th02 content and U02/Th02 ratio 

of uraninite from the Vaal Reef correspond closely to 

those for uraninite from the Hunza River. Calculation of 

the uranium and thorium content of the Precambrian 

uraninite to account for radiogenic decay of uranium 

and thorium since crystallization about 3.0 b.y. ago improves this correlation. The uranium and thorium content of the uraninite populations from the Indus and its 

tributaries span the full range of uraninite compositions 

of the Dominion Reef and Witwatersrand. It has 

previously been suggested that since thorianite is resistant to oxidation and exists as a detrital mineral 

(Frondel, 1958), the thorium content of uraninite may 

increase its stability as a detrital mineral (Davidson, 

1960).

In order to test this suggestion and to investigate the 

possible 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 and 

Ce203 , Ti02 and Th02 , Ti02 and Ce2 03 . Significant 

positive correlations exist between U02 and Ti02 , Th02 

and Ce203 , CaO and Ce203 , Y2 03 and Fe2 03 . PbO and 

MnO do not correlate with any other oxides. The minor 

element correlations suggest that the grains represent a 

single population in the bedrock and are probably not 

derived from multiple unrelated phases of mineralisation in which correlation would be less likely. The lack of 

correlation 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 the 

uraninite grains suggests that there is no correlation 

between the principal compositional variable, Th02 

(1.20-12.08 percent), and the sedimentary features of 

uraninite grains that are likely to have resulted from 

weathering, transportation, and diagenesis. It is not 

known, however, whether uraninite containing less than 

1.2 percent Th02 is either relatively soluble or is 

unavailable in the source regions. From the textural information, uraninite containing as little as 1.2 percent 

Th02 seems as likely to survive in the Indus as uraninite 

containing ten times as much. It is therefore suggested 

that in this population the compositional variation 

within the range studied has no measurable effect on 

survival. On the other hand, uraninite grains collected 

as a concentrate far from their primary sources will 

have encountered variable degrees of weathering, which 

differ for each grain during several cycles of erosion and 

deposition during downstream progression. The variation in attrition suffered by grains during transport 

therefore is the main factor controlling variation in 

grain characteristics and is unrelated to grain composition within the compositional range studied.

EVIDENCE FROM THEORETICAL MODELS OF 

URANINITE STABILITY

Errors in the assumptions used in designing models of 

the natural environment are likely to lead to false 

estimates of uraninite stability. For example, Grandstaff (1973) has calculated that for uraninite from 

Blyvooruitzicht in the Witwatersrand to survive erosion, 

transportation, deposition, and burial, "oxygen levels 

must have been less than approximately 10~ 2 to 10~ 6
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 by 

Bowles 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

13

14

15

16

17

18

19

20

21

22

23

24

25

26

27

28

29

30

31

32

33

Ti02

.28

.24

.30

.26

.24

.25

.29

.28

94

.26

.23

.24

91

.27

.21

.22

.28

.24

.16

.20

.19

.22

.17

.16

.12

.20

.23

.20

.13

.14

19

.08

09

MnO

.08

.10

.04

.09

09

.07

.07

.08

.04

.05

.08

.04

.07

.06

.07

.09

.08

.06

.06

.07

.04

.04

.07

09

.10

.07

.07

.05

.10

.10

.04

.10

.07

Fe203

.17~\9

.14

.13

.18

19

.14

09

.189~\

.21

.10

.17

.16

.20

.19

.24

.17

19

.28

.17

.17

19

.13

.26

.16

.26

.14

.27

.16

.14

.13

.15

PbO

.66

1.25

1.32

.50

.75

i ^i

.29

.47

1.31

.35

.30

.43

.60

.39

1.43

1.34

1.15

1.19

.59

.85

1.20

.24

.52

.52

1.06

1.10

.57

1.06

.59

.41

.37

1.17

.59

Th02

1.20

1.27

1.38

2.19

2.27

2.35

2.78

9 7Q

2.89

3.16

3.19

3.69

3.82

3.92

3.93

4.09

6.32

6.43

6.68

7.06

7.11

7.24

8.24

8.40

8.65

8.69

8.70

9.00

9.05

9.08

9.20

10.69

12.08

U02

99.89

98.40

98.54

97.95

97 71

96.63

98 44

97.13

96.57

95.35

98.31

96.57

96.89

96.01

93.44

91.91

87.87

89.80

88.91

82.53

88.78

89.14

89.97

90.67

86.84

88.42

88.53

84.68

85.58

88.81

90.36

86.77

83.75

Y2 03

.10

.14

.05

.13

.05

.15

.14

.10

.20

.72

.05

.19

.11

.46

.59

1.16

1.65

.88

1.32

2.52

1.39

1.65

.99

1.40

.62

.59

1.61

1.58

.14

.05

.05

.53

CaO

.02

.02

.02

.02

.02

.02

.02

.02

.02

.08

.04

.24

.03

.05

.04

.03

.04

.02

.11

.08

.02

.04

.02

.40

.02

.09

.03

.11

.06

.31

.02

.34

.06

Ce2 03

___

___

.03

.03

.03

.03

.10

.04

.07

.05

.10

.07

.09

.01

.02

.23

.08

.05

.10

.14

.13

.32

.04

.42

.24

Total

102.40

101.54

101 79

101.27

101.31

100.90

102.20

100.96

101.45

100.21

102.44

101.50

101.90

101.35

100.01

99.07

97.70

98.84

99.05

93.66

98.99

98.75

100.19

100.60

98.53

99.40

99.08

96.99

97.49

99.47

100.41

99.75

97.56

that of the present level in the mid-Precambrian." This 

conclusion 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 system 

they 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 and 

Christ, 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 oxidised river water.

The greatest problem in any model of detrital 

uraninite stability arises with the need to deposit 

relatively fresh and untarnished euhedral grains into a 

river system since the greatest exposure to oxidation is 

likely to occur at this point as the grains come in direct 

contact 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 the 

presence of detrital uraninite in the Indus alluvium and 

probably also by analogy in the Witwatersrand. In 

Grandstaffs (1973) model the assumptions used to 

estimate 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 CONGLOMERATES

TABLE 2. -Correlation matrix for the chemical composition of 33 detrital uraninite grains determined by electron-probe microanalysis and

shown 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)]

TiO, MnO Fe20, PbO Th02 UO, Y2 03 CaO Ce,03

Tin

MnO

Fe203

PbO

Th02

U02

v o

CaO

Ce2 03

1.000

-.341

-.124

01 ^

-.805

697

-.205

-.439

-.752

1.000

190

.043

.082

-.061

.111

.149

.374

1.000

AC'7

.238

.413

.586

- ?47

.007

1.000

.090

.103

.156

-.070

.036

1.000

-.910

.406

.349

.715

1.000

.711

.216

.602

____

1.000

-.204

.025

____

____

1.000

.691

____

____

1.000

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 Witwatersrand (Bourret, 1973). It therefore provides good 

evidence for the establishment of photosynthesis and 

generation 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 Precambrian 

limestones 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 have 

been released prior to 3.0 b.y. ago (Schidlowski and 

others, 1975).

EVIDENCE FROM AUTHIGENIC PITCHBLENDE

AND DISSEMINATED URANIUM IN THE

WITWATERSRAND SYSTEM

The existence of fine-grained low-thorium pitchblende 

in carbonaceous material in the Witwatersrand probably 

results in part from deposition of uranium in solution onto organic matter. Lower grade concentrations of 

uranium have also been identified, by fission track 

methods, in association with phyllosilicates, which are 

finely intergrown with authigenic pyrite and also form 

lenticular mudstone pellets. These occurrences are also

considered to represent deposition of uranium from 

solution (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 with 

organic-rich mud, authigenic pyrite, and decaying 

organic remains of stromatolitic algal mats (fig. IB). 

These reactions require oxidising atmospheric conditions in order to function in the manner suggested, except where the concentration of bicarbonate is high, 

since the uranium-carbonate complex ions can exist in 

aqueous solution under reducing alkaline conditions 

(Garrels and Christ, 1965). However, no carbonate has 

been described from the sediments of the Witwatersrand succession.

The evidence from the stromatolitic algal mats is indicative of a sufficient supply of atmospheric C02 to promote supratidal growth, and the reactions described in 

this paper assume that the atmosphere was well mixed 

and that water washed detrital grains into the site of 

deposition. The discussion on the sulphur cycle, in the 

next section, indicates that pH conditions in the 

mineralising surface water solutions were probably 

neutral to acidic rather than alkaline as a result of the 

activity of bacteria catalysing the production of relatively large amounts of sulphate from allogenic pyrite.

Under these conditions the uranyl ion and 

organometallic complexes were probably the principal 

uranium species in solution during deposition of the 

sediments. The lack of carbonate during diagenesis is 

probably attributable to rapid burial, which prevented 

reaction between organic-rich sediments and the coexisting oxidising atmosphere. The nature and thickness (8 

km) of the Witwatersrand sediments and the preservation of large amounts of carbonaceous matter are good 

evidence 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 with 

finely 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 a 

quartz-pebble conglomerate. Some quartz pebbles (Qtz) in the conglomerate layers at the upper and lower margins of the figure contain 

euhedral pyrite (Py). Detrital grains of uraninite (Ur) and quartz (Qtz) are overgrown or entrapped by the mat (black). Uranium and gold are 

also deposited diagenetically as pitchblende (Pi) and authigenic gold (Au) and together with churchite (Ch) these authigenic phases outline the 

vertical 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 much 

coarser allogenic quartz (Qtz) and pyrite (Py). No heavy-metal detrital phases are present; hence the low-thorium pitchblende and gold represent deposition of uranium and gold from solution onto stromatolitic algal mats (black), followed by recrystallisation during diagenesis and 

metamorphism. 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 CONGLOMERATES

EVIDENCE FROM THE SULPHUR CYCLE IN THE 

WITWATERSRAND

The existence of abundant authigenic pyritic concretions coexisting with allogenic pyrite grains (Simpson 

and Bowles, 1977, figs. 6, 7) indicates that reactions of 

the following type have probably occurred during formation of the sediments:

1. Partial oxidation of detrital allogenic pyrite to 

generate sulphate in solution

2. Deposition of authigenic pyrite concretions and 

overgrowths in reducing environments below the 

sediment/water interface, a reaction that takes 

place in unconsolidated silts and muds now 

represented by interstitial silicate phases in the 

concretions

3. Reworking of these sediments and downstream 

transport of secondary authigenic pyrite washed 

out of the unconsolidated muddy sediment and 

unoxidised allogenic pyrite, which were then 

redeposited together.

All three stages were repeated in whole or in part until 

final burial and diagenesis in subsiding portions of the 

basin, where pyrite and uraninite were maintained in a 

reducing environment below the sediment/water interface and out of contact with the oxidising atmosphere. It 

is important to note here that fresh authigenic and 

allogenic pyrite are also identified in samples from the 

Indus River by Simpson and Bowles (1977).

The reactions described are probably due to combined 

inorganic and bacterial surface oxidation of pyrite to 

form sulphate, associated with bacterial reduction of 

sulphate below the sediment/water interface. These 

processes are summarised in more detail in the following 

sections.

INORGANIC SURFACE OXIDATION OF PYRITE

Inorganic oxidation of pyrite under surface conditions 

in equilibrium with atmospheric oxygen has been 

described by Stumm and Morgan (1970) and can be summarised as follows:

Initial step:

FeS2 + 7/2 2H

Rate controlling step:

Fe3+ +3H2 0 =

FeS2 + 14Fe3+ + 8H20 = 15Fe2+ + 2SO

(1)

(2)

The slowest reaction or rate controlling step is the oxidation of Fe2 + ; the fastest reaction is the oxidation of 

pyrite by Fe3+ ion. Therefore, once the reaction is

started the oxidation of pyrite cycles through reactions 

(1) and (2).

BACTERIAL SURFACE OXIDATION OF PYRITE

At the present time pyrite oxidation rates are greatly 

accelerated by the activities of acidophilic iron- and 

sulphur-oxidising bacteria (Silverman, 1972). Several 

bacteria may be involved, but the most common and 

most effective are Ferrobacillus ferrooxidans, which oxidises pyrite by direct attack, and Thiobacillus ferrooxidans, which are thought to catalyse the oxidation of 

Fe2+ to Fe3+ in the rate controlling reaction (1). 

Microbial activity by these and other bacteria is 

predominant only under ideal conditions of low pH, a 

temperature of about 30°C, and the presence of oxygen, 

iron, sulphur, and nutrients (N, C02), according to Hoag 

and Webber (1976).

BACTERIAL REDUCTION OF SULPHATE BELOW THE 

SEDIMENT/WATER INTERFACE

Bacteria also take part in reducing reactions, especially those belonging to the genera Desulfovibrio and 

Desulfotomaculum-, they are anaerobic and reduce 

sulphate to H2S as follows:

SOI ~ + 8e + 10H +-*H2S + 4H2 0.

Iron in solution would form pyrite by reaction with 

H2S (Trudinger, 1971). The abundant authigenic pyrite 

concretions from the Witwatersrand were presumably 

formed as the result of bacterial activity of this type in 

organic-rich muddy sediments (Saager, 1970).

In summary the evidence from the sulphur cycle 

shows that the occurrence of pyrite in the Witwatersrand is more readily explained by assuming oxidising 

rather than reducing atmospheric conditions during 

deposition, probably in association with active bacterial 

processes aiding dissolution and reprecipitation of 

pyrite.

SUMMARY

The evidence considered in this study indicates that 

the terrestrial atmosphere at the time of formation of 

Witwatersrand sediments was not detectably different 

from that of the present day. The reasons for the earlier 

suggestion that the atmosphere must be reducing or 

nonoxidising to account for the presence of uraninite 

and pyrite in the Witwatersrand are probably due to 

overestimation of the extent of atmospheric exposure of 

these phases. Uraninite and pyrite stabilities and their 

environmental significance must be considered in relation to their geological context if the deductions made
DETRITAL URANINITE AND PYRITE: ARE THEY EVIDENCE FOR A REDUCING ATMOSPHERE? S7

are to be realistic. The coexisting phases and style of 

mineralisation 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 size 

and composition can be found in both ancient and 

modern sediments. Authigenic minerals in the Witwatersrand provide indirect evidence of uranyl and 

sulphate ions coexisting with detrital thorian uraninite 

and pyrite during formation of the Witwatersrand 

sediments. It is suggested that this bimodal form of 

transport and desposition can be most readily accounted 

for by a model in which sulphate and uranyl ions exist in 

superficial, oxidised, active sediments and river waters 

draining into the Witwatersrand Basin while detrital 

grains 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 by 

catastrophic 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 soluble 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 atmospheric 

factors. This close association between uraniferous conglomerate and Precambrian crust has previously been 

pointed out by Bowie (1970), who considers this to be a 

significant factor and guide for prospecting.

Accordiing to Moorbath (1975, 1976), continental 

growth occurred on a major scale during the early 

Precambrian. 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 and 

incompatible 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 of 

crustal evolution, when extensive erosion of uraniumrich upper crust resulted in the deposition of uraniferous 

sediments in enclosed basins, under atmospheric conditions similar to those of the present day.

This conclusion is compatible with the findings of 

other 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. Hence 

from physical and geological considerations the 

evidence indicates that the atmosphere in the period 

2.8-2.0 b.y. ago was oxidising.

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Heier, K. S., 1973a, Geochemistry of granulite facies rocks and 

problems of their origin: Royal Society of London Philosophical 

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———1973b, A model for the composition of the deep continental 

crust: Fortschritte der Mineralogie, v. 50, p. 174-187.

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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 and

radioactive 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., Report

no. 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. 

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rocks: 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 Witwatersrand gold fields: A chronological review of speculations and observations: 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.

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South Africa and their genetic significance: Geological Society of 

South 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 

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6-10 May 1974: Vienna, International Atomic Energy Agency, p. 

495-512.

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uraniferous minerals in the Witwatersrand Triad; an interpretation of new and existing U:Pb age data on rocks and minerals from 

the 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, Precambrian 

sedimentary carbonates: Carbon and oxygen isotope geochemistry 

and implications for the terrestrial oxygen budget: Precambrian 

Research, v. 2, p. 1-69.

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the primitive Earth: Precambrian Research, v. 3, p. 463-470.

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Fairbridge, R. W., ed., The encyclopedia of geochemistry and environmental 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.

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ed., The early history of the Earth: London, Wiley-Interscience, p. 

3-19.

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DISCUSSION FOLLOWING PAPERS BY

DRS. FEATHER, SIMS, AND SIMPSON AND

BOWLES

Whiteside: May I just make a comment on not these two 

last 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 between 2,300 and 2,800, around about 2,500, which 

means that there has been presumably some metamorphic event which has been stamped on these 

uraninites. 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 course 

of 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 other 

than the uraninite and brannerite and zircon and so 

on. We ran into a strange situation in analysis for 

uranium before X-ray fluorescence was used and that 

was 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 concentrates which I derived from the residue material I had 

great difficulty finding the uranium. Now you have 

possibly 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? S9

would like to have a look at the sections under the 

microscope and compare the Lexans directly with the 

sections, I think you will see for yourself that the 

uranium 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 lenticular 

body which I described and out of which I got 

pyrophyllite, kaolinite, and quartz. They are very 

small, and there is an exact correlation of the fission 

tracks in the Lexan with the interstitial phase and not 

with 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 that 

interstitial 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 between 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 understand 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 more 

uranium in the phyllosilicates than in the buckshot 

pyrite, actually, and that phyllosilicate phase seems to 

be a great deal richer. You must remember that the 

uranium in the pyrite which is porous is diluted by 

nonuraniferous pyrite, and the actual point where the 

uranium occurs is very much richer. If you've got an 

overall value of 70 ppm, that's for the pyrites and 

phyllosilicate combined, and if you recognize that the 

phyllosilicates may be only 20 percent of the total 

buckshot pyrite, you have got to multiply by a factor of 

five 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 the 

uranium content would be 40 ppm or more for the conditions of radiation employed for those particular sections. 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 of 

all. would you hazard a guess, or if you have a 

measurement would you give us your estimate of how

much 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 even 

appreciated.

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 in 

discrete 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 a 

strong mineral, as you know, and requires quite a 

stress 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, many 

cases. I don't know the exact structural locations of 

the samples I am working on, and it's very difficult to 

interpret that type of information other than to make 

the 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 have 

been working on for the lead-isotope study we 

specifically collected only pyrite that was not shattered at all. These were very small detrital compact 

pyrites, so I think it actually depends upon the size of 

the pyrite.

Feather: Peter, two questions: What is the possibility 

that the uranium in the phyllosilicates may actually be 

present 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 for 

danger 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 our 

work, and we haven't yet probed the sections corresponding 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 we 

have come across over and over again in our uraniumprospecting activities in Britain, where time and time 

again 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 a 

thousand 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 is 

whether it is not possible that uranium was absorbed 

by these minerals, not at the time of deposition, but 

during some subsequent metamorphic epoch when 

there was a lot of uranium redistributed in the reefs in 

the form of brannerite, because brannerite occurs not 

only in the way that we have seen as ghosts in the 

original site of the uraninite grain, but it often 

migrates considerable distances away from where the 

parent obviously was.

Simpson: Well, this is a very good question, and it's a 

point that obviously one bore in mind in studying these 

rocks. The impression you get is that the uranium, in 

this loosely bound form, is very closely confined to the 

site of original deposition of phyllosilicates and pyrite. 

It doesn't deviate even 1 or 2 mm away from these 

very fine layers of pyrite, and one would suspect that 

if it had been introduced later at least some of the 

uranium 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 to 

what 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 in 

these 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 this 

uranium was introduced later, this would require oxidizing solutions, no question about that. If the solutions were oxidizing then you would expect to see 

some effect on the pyrite. You would expect the pyrite 

to be oxidized. The pyrite is not oxidized; it is not affected in any way at all by the presence of this 

uranium. It seems to be quite happy there, and 

therefore I have concluded on balance that this 

uranium was fixed at or around the time of deposition 

of 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 of 

samples. If you would like to send us a few samples to 

try it on * * *.

Roscoe: Well, I have no Dominion Reef samples.

Simpson: Sorry, did you say Dominion? Oh, I beg your 

pardon. I mistook the question. I immediately assumed 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 the 

uranium 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 the 

fine-grain muscovitic matrix, which usually has 

smeared out leucoxene in it, too. There always are 

tracks in there. Your procedure, of course, is so sensitive that you do swamp any uranium-bearing 

materials so you are exaggerating the amount in the 

matrix in this case. I think you may see the same type 

of thing in the Dominion Reef and Elliot Lake ores, 

maybe not to the same extent.

Schidlowski: I principally agree with most of your 

points. I am just a little bit suspicious about your 

secondary uraninite, those tiny specks associated with 

gold. 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 secondary 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? Sll

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 dissolution 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's

all. 

von Backstrom: I'd like to make a comment to keep the

record straight. I think most South African geologists 

would bear me out that the Witwatersrand 

Supergroup has been subjected to a considerable 

amount of tectonic disturbance, not only faulting but 

also very pronounced angular unconformities. There 

is also a certain amount of folding. I think it stands to 

reason 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 and 

created a space problem. It certainly had severe effects on the Transvaal System, which covers the 

Ventersdorp. So my proposition is that I think that we 

must get away from this idea that the Witwatersrand 

has not been severely disturbed technically. I think 

that the fact that most of the mines have one or two 

geologists stationed at each mine and a lot of their 

work concerns displacement of the reef bears this out. 

The other point that I'd like to make is that in the 

Western 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 of 

shearing 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 that 

many of the pyrite pebbles have also been shattered. 

Bourret: I would like to add one further comment to 

what Drs. Whiteside and von Backstrom have said in 

that we are carried away a little bit with an oversimplification of the tectonic picture. Dennis Pretorius 

continually brings out the fact that there are 

numerous sills and dikes and that 10 to 15 percent of

the basin actually is occupied by sills and dikes which 

have 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 in 

with 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 have 

taken place and are stamped in the record by these 

sills and dikes.

Armstrong: I have a question that I'd like to ask the 

South African contingent. If I understand the model 

in its broader aspects, the present thinking is that the 

basin was filled by coalescing fans that came from the 

north, 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 the 

gold if it's derivied from five separate source areas. 

Are there any maps showing the fineness distribution 

of 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 any 

of you help me on this?

Whiteside: As far as I remember, Hargraves did some 

work some time ago on the silver content. He tried to 

plot the silver content of various parts of the Witwatersrand and I don't think the variation was very 

great. He tried to equate the silver content of the gold 

with depth, and I am not sure that he proved or 

disproved his point. I think you will find that report in 

one 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 less 

simultaneously, applied microprobe techniques. 

Speaking for myself, I think I found on the average a 

silver content of roughly 10 percent. I think it was between 11 and 8. And I think Rudolf Saager found 

something 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 a 

greater variation than we did in the Basal Reef, and he 

had samples, I think, from about three or four of the 

goldfields.
S12 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Armstrong: I would think that you could sample the 

East Rand and the Central Rand and the far west and 

the 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 the 

trace 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 think 

that this type of study, if it hasn't been made, could 

lend great support to the detrital or nondetrital origin 

of Rand gold and might be an important feature to be 

studied. Is it that difficult to study?

Myers: Zemansky and Desboro wrote a paper a couple of 

years ago in which they show that gold subjected to 

the 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 a 

can 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 a 

definite increase of silver in the ore samples down 

slope, whereas the fineness of the actual gold remains 

more or less constant. So silver must be floating 

around somewhere else.

Gramdstaff: Dimock and Hargraves did a little bit of 

microprobe work for silver on the uraninites and 

found 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 distribution,] as would be expected in a typical alluvial deposit 

where more pronounced movement of water down the 

main channel would have tended to leach out greater 

amounts of silver." I'm sorry he doesn't go on and explain that further.

Skinner: It's a well-known fact that's been known for a 

long, long time that detrital gold grains have gold-rich 

rims 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 by 

hydrofluoric acid treatment, for silver and trace 

elements by neutron activation analysis, but so far we 

haven't anything to report.

Feather: Just one final comment about the detrital gold 

grains being depleted in silver around the rims. This is 

absent 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, but 

when we come to the Central Rand and the East 

Rand, the overall silver contents of the samples from 

different reefs vary vastly, suggesting multiple source 

for the gold in that northern area as opposed to the 

west and south area, but we haven't gone into this in 

any 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 of 

silver 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 of 

samples 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 high 

up in the sequence, right at the top of the Witwatersrand 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 the 

silver content appears to increase as you go up the 

geological 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 way 

that 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 possible. And then also, of course, the organizing committee that did such a vast amount of work and those 

behind 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 information on work that is being done in Brazil in prospecting for uranium-bearing conglomerates. After 

hearing 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 the 

same reef could be something like 200 to 300 percent 

different from the true mean. If our mean grade is 

almost 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 know 

that in Canada and in South Africa the deposits are 

well 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 that 

those data are usable in Canada where the stratigraphy is well established and one can know when he 

reaches the footwall. One need only back up and 

wedge the hole and drill more carefully. The potential 

of those areas is already known. But how does one do 

the same thing in new areas? I understand also that 

the 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 take 

models that have already been developed and to select 

those 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 carried 

out.

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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-T

CONTENTS

Page

Abstract ________________________________________________ Tl

Introduction ______________________________________________ 1

Transportation and communication ______________________________ 1

Climate ____________________________________________ 1

Topography ____________________________________________ 1

History ________________________________________________ 1

Stratigraphic section _________________________________________ 2

Areas being prospected _______________________,________________ 3

Distribution of conglomerates in Gandarela Syncline _____________________—— 3

Mineralization _____________________________________________ 3

Mineralogical association _____________________________________ 7

Working methods ___________________________________________ 10

Conclusions _______________________________________________ 10

References cited __________________________________________ 13

Discussion _______________________________________________ 13

ILLUSTRATIONS

Page

FIGURE 1. Map showing location and study area ________________________________________________ T2

2. Diagram showing stratigraphic distribution of radioactive anomalies _________________________________ 3

3. Map showing Precambrian rocks around Sao Francisco Craton ____________________________________ 5

4. Diagram showing paleocurrent directions in Jacobina area ______________________________________ 6 

5, 6. Isopach maps of:

5. Moeda Formation in Gandarela Syncline ____________________________________________ 8

6. Total conglomerate in Gandarela Syncline _________________________________________ 9 

7-10. Photographs showing:

7. Quartzite boulder, east of the Gandarela Syncline ________________________________________ 10

8. Polymictic conglomerate, east of the Gandarella Syncline ___________________________________ 10

9. Conglomerate with black quartz pebble, in the west of the Gandarella Syncline _____________________ 10

10. Conglomerate with iron staining due to pyrite weathering __________________________________ 10

11. Cross section of southeast Gandarela Syncline ___________________________________________ 11

12. Map showing Th02/U3 08 ratio distribution in basal conglomerate in southeast Gandarela Syncline _____________—— 12

13. Graph showing the relation between titanium oxide and uranium and thorium oxides ____________________——— 13

TABLES

TABLE 1. Stratigraphic section in the Quadrilatero Ferrifero, Minas Gerais, Brazil ___. 

2. Stratigraphic correlations of Robertson with Maquine Group added by Villaca.

Page

T4

7

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

URANIUM IN PRECAMBRIAN MOEDA FORMATION, 

MINAS GERAIS, BRAZIL

By JOSE NlLSON VlLLACA and LUCIANO A. M. MOURA1

ABSTRACT

The Quadrilatero Ferrifero with an area of about 7,000 km2 is 

located south of Belo Horizonte in Minas Gerais, Brazil. Precambrian 

metaconglomerates (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 for 

next 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, at 

least 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 between lat 19°45' and 20°30' S. and long 43°22'30" and 44°7'30" W. and 

lies near Belo Horizonte, Minas Gerais State, Brazil.

INTRODUCTION

The Quadrilatero Ferrifero (about 7,000 km2) is mostly 

included between lat 19°45' and 20°30' S. and long 

43°22'30" and 44°730" W. and lies near Belo Horizonte, 

Minas Gerais State, Brazil (fig. 1).

Uranium-bearing conglomerates in the Moeda Formation are known to crop out far beyond the limits of the 

Quadrilatero.

TRANSPORTATION AND COMMUNICATION

Some paved highways give access to the neighborhood 

of prospecting sites. Usually it is not necessary to travel 

more than 20 km from paved roads to reach the prospects.

A railroad network built for iron-ore transportation is 

in operation. Passenger transport within the region is 

by bus or car.

Frequent airplane service from Belo Horizonte is 

available to the main cities of Brazil.

1 NUCLEBRAS, Distrito Belo Horizonte, Rua Washington, 801-S10N, Belo Horizonte, M. 

G., Brazil.

Long-distance dialing telephone systems connects 

Belo Horizonte with other places of the country and 

abroad.

Electricity is available for mining operation, with 

powerlines near most of the sites of Quadrilatero Ferrifero.

CLIMATE

The 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, and 

the minimum is usually above 0°C. The mean annual 

temperature is slightly below 20°C. Mean annual rainfall varies from place to place, from somewhat less than 

1,500 mm to slightly over 2,000 mm.

TOPOGRAPHY

The topography is marked by high and rugged peaks, 

long chains of mountains, with some deep canyons and 

valleys. The maximum elevation in Quadrilatero 

Ferrifero is 2,100 m at the top of Serra do Caraca, and 

the minimum, about 600 m, where the Paraopeba River 

leaves the region.

HISTORY

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 chemical 

values increase with depth.

Tl
T2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

PITANGUI AREA

BELO 

HORIZON1E

GANDARELA 

SYNCLINE

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.

STRATIGRAPHIC SECTION

The stratigraphy used in the Quadrilatero Ferrifero is 

shown in table 1, which is taken from Dorr (1969, pi. 12),

Robertson (1974) correlated the Moeda Formation of 

the Carac.a Group with other uranium-bearing Precambrian conglomerates of the world as shown in table 2. 

The Maquine Group was added to the correlation by 

Villaga (1974) owing to its radioactive conglomerates.

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 Precambrian rocks that are potential hosts for uranium 

deposits.

The vectors on figure 3 show known paleocurrent 

directions in the area resulting from crossbedding 

measurements plotted on polar diagrams. In the 

western Quadrilatero Ferrifero as well as in the 

Pitangui area the sediments came from the Sao Francisco Craton. In the Gandarela Syncline, however, the 

source is located to the east (Lindsey, 1974).
URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL T3

RIO DAS VELHAS 

SERIES 37 PERCENT

MINAS SERIES 

30 PERCENT

JASIC ROCKS

PERCENT

ITACOLOMI

SERIES 

2 PERCENT

GRANITIC AND

GNEISSIC ROCKS

27 PERCENT

FIGURE 2.—Stratigraphic distribution of radioactive anomalies.

The paleocurrent directions in the Jacobina area (fig. 

4) also indicate that the source lies to the east (Bastos 

and Moura, 1974).

The conglomerate pebble sizes both in the Jacobina 

area and in the eastern part of the Gandarela Syncline 

indicate a short distance of transport.

Jacobina, in the northeast corner of figure 3, is being 

studied by Unigeo, whereas Quadrilatero Ferrifero and 

Pitangui are almost unknown geologically. Reconnaissance work is necessary in order to locate where 

conglomerates occur.

AREAS BEING PROSPECTED

NUCLEBRAS intends to continue work in the Moeda 

Formation at Pitangui and in the Ouro Fino Syncline 

during 1976.

Now the Company is studying the Gandarela structure 

in sites already drilled. A mapping program is underway 

in the Maquine Group, where some radioactive conglomerates crop out. Included in the program are detailed 

geological mapping of some selected areas, surface 

sampling, and re-evaluation of the previous results of 

drilling programs. At least two years will be necessary 

to finish the work presently scheduled.

DISTRIBUTION OF CONGLOMERATES IN

GANDARELA SYNCLINE

The maximum known thickness of the Moeda Formation is about 350 m in the southern part of the Gandarela

Syncline. Its thickness decreases northward and northwestward (fig. 5), and the total thickness of conglomerate in the Moeda also decreases northward and 

northwestward (fig. 6). Both maps are based on reconnaissance traverses made in 1974 along the entire 

length of the outcrop of Moeda Formation around the 

syncline and on data from drill holes.

The thick conglomerates in the southeast area are 

coarse conglomerates; some reefs contain boulders and 

cobbles (figs. 7, 8). This type of conglomerate is usually 

polymictic and immature, with cobbles of schist, indicating a short distance of transport.

Along the western side of the basin the conglomerates 

are thin, pebbles are rarely bigger than 3 cm in diameter 

(figs. 9, 10), and the channels pinch out quickly. The 

mean paleocurrent direction indicates a source area to 

the east. These conglomerates are a distal facies of the 

conglomerates that crop out to the east.

These data suggest that the conglomerates are present in subsurface over a large area of the syncline. If 

conglomerates are present, this is a favorable area in 

which to explore for uranium deposits.

MINERALIZATION

Both polymictic conglomerates and oligomictic quartzpebble conglomerates occur in the western part of the 

Gandarela Syncline.

Southeast of this syncline at least two channels have 

been found, one of which is in the basal Moeda Forma-
T4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

TABLE l.—Stratigraphic section in the Quadrildtero Ferrifero, Minos Gerais, Brazil

[Only Minas and Rio das Valhas Series]

AGE

PRECAMBRIAN

SERIES

MINAS

O LJJ 

E>

GROUP

PIRACICABA

ITABIRA

CARACA

TAMANDUA

MAQUINE

NOVA LIMA

FORMATION

SABARA

BARREIRO

TABOES

FECHO DO FUNIL

CERCADINHO

GANDARELA

CAUE

BATATAL

MOEDA

(UNNAMED)

CAMBOTAS

CASA FORTE

PALMITAL

(UNDIVIDED)

BASEMENT NOT

EXPOSED

SEDIMENTARY 

ENVIRONMENT

FLYSCH

STABLE SHELF

STABLE SHELF

PARALIC

MOLASSE

FLYSCH
URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL T5

H- + + + + + -H

+ + + + + -H

+ + + +SAO+

FRANCISCO +

+ + + + +

FIGURE 3.—Precambrian rocks (dots) around Sao Francisco Craton (crosses). Vectors show paleocurrent directions.
T6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

310

290

250

230

350C N 10o

330 30C

210 150C

50°

110°

130C

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

TABLE 2.—Stratigraphic correlations ofRobertson (1974) with Maquine Group added by Villaga (1974)

T7

ENVIRONMENTS

OXIDIZING

(Red rocks)

REDUCING

(Sedments with 

pyrite and 

uranium)

CANADA

ELLIOT LAKE

LORRAINE

FORMATION

ELLIOT 

GROUP

(U)

MONTGOMERY

HURWITZ

GROUP

SOUTH AFRICA

TRANSVAAL

SUPERGROUP

—————— 2200 M.Y. | ————————

MONTGOMERY 

LAKE 

SEQUENCE

(U+Au)

WITWATERSRAND 

SYSTEM

DOMINION REEF 

SYSTEM

(Au + U)

BRAZIL

QUADRILATERO 

FERRIFERO

ITABIRA

GROUP

MOEDA 

FORMATION

MAQUINE 

GROUP

(U+Au)

AUSTRALIA

NULLAGINE

HAMERSLEY

GROUP

GREEN HOLE 

FORMATION

(U)

tion. Uranium may be present with some gold, and some 

drill holes cut ore-grade material (at the present price of 

US 08).

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 done 

in 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, and 

radioactive minerals. Amorphous carbonaceous material 

similar to thucholite has been found.

The thorium content of the conglomerates is low, and 

the Th02/U308 ratio is variable. Great lateral variations 

of the ratio occur within the same conglomerate, and the 

ratio 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 the 

Th02/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'

20°07'30"

EXPLANATION

Strike and direction of dip of beds

—P- Overturned beds

———— Fault

——*— Thrust fault

100-- Structure contour,in meters.

Dashed where control is poor

15 KILOMETERS

FIGURE 5. Isopach map of Moeda Formation in the Gandarela Syncline.
URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL T9

37°30'

20°00'

20°07'30"

EXPLANATION

Strike and direction of dip of beds 

-P- Overturned beds 

Fault

Thrust fault 

700— Structure contour, in meters.

Dashed where control is poor

o

I i

15 KILOMETERS 

J

FIGURE 6.—Isopach map of total conglomerate in the Gandarela Syncline.
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 8. Polymictic conglomerate, cobble size, east of the Gandarela

Syncline.

WORKING METHODS

Due to the structural and stratigraphic complexity of 

the area, a geologic mapping program covering 52 km2 

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; 

about 260 km of geological traverses are planned for the 

Gandarela Syncline. Geologic mapping is being done at a 

scale of 1:2,500.

Trenches will be opened on the conglomerate zones for 

channel sampling. Mineralized conglomerates will be 

mapped at a scale of 1:500 and more detailed sampling 

done.

FIGURE 9.—Conglomerate with black quartz pebble, in the west of the 

Gandarela Syncline.

%

:J^

ar*" '^i^5^'^ 

ti5?^>^ f *%&#•& '-srf

0^.'^^- ^ '- T% r^^-->-^s -,

-^ ^-r 

^--^ • ^'•-'V* ^j-' ,?*.>

FIGURE 10.-Conglomerate with iron staining due to pyrite weathering.

Diamond-drill cores, after being logged and systematically sampled, will be correlated with the mapping data in order to evaluate areas and to guide additional exploration.

CONCLUSIONS

Many data are needed to estimate the potential of conglomerate zones in areas of strong tectonism.

Although high uranium grades have been found in 

some holes, following favorable host rocks requires a 

good knowledge of the tectonics and facies variations
URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL Til

w

Drill hide

Drillhole

100 200 300 METERS

EXPLANATION

Conglomerate

Quartzite

Basement

Fault showing relative 

direction of movement

FIGURE 11.—Cross section of southeast Gandarela Syncline.
T12 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

43°38'45"

20°08'45"

OUTCROP OF 

BASAL MOEDA FORMATION^

0 200 400 600 800 1000 KILOMETERS 

I i I I I I __I

FIGURE 12.—TMVUsOg ratio distribution in the basal conglomerate in the southeast Gandarela Syncline.
URANIUM IN THE PRECAMBRIAN MOEDA FORMATION, MINAS GERAIS, BRAZIL T13

1000

•z. 

o

^ 800

On 

Q

600

400

200

100 300 500 700 900 

TiO2, IN PARTS PER MILLION

1100

FIGURE 13.—Relationship between titanium oxide (Ti02) and uranium 

(U3 08) and thorium (Th02) oxides.

within the area so that the holes can be located in the 

most favourable sites.

Grade variations within ore horizons demand wedging 

of the drill holes to provide many intersection of the 

reefs, 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 occurrence of uranium deposits and as the result of more 

work 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 structural 

development 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 International 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).

DISCUSSION

Cannon: Do you have any radiometric data on what the 

age of the Moeda Formation is?

Moura: There are some strontium-rubidium measurements that give less than 1,800 million years, but we 

usually 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 the 

reducing environment of the Moeda Formation; the 

Caue 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 prospection.

Whiteside: If I may just comment a little bit. I was very 

lucky. I visited Belo Horizonte about 1971 when they 

first 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 resistant beds that crop out, mostly quartzites. They stick 

up 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 some 

of 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 occurs, 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 very 

much more I can say, but on the whole, I don't think I 

would have correlated what I saw in the Moeda with 

what I know of the Witwatersrand. (This is a very 

long-range correlation, of course, in spite of continental drift.) I don't think there would be anything in 

Brazil equivalent to the division of argillaceousarenaceous in the lower Witwatersrand and 

arenaceous-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 CONGLOMERATES

Moura: I haven't seen any, but mineralogical studies 

have found uraninite and brannerite, sometimes 

associated with pyrite and sometimes with what was 

called 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 of 

Belo Horizonte and Jacobina, and you said that in the 

intervening area there are rocks of the same age and 

lithology and that this intervening area would be a 

good one to prospect. If I recall correctly, rocks of that 

same 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 for 

prospecting, but we have only scattered data on the 

western 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's 

buried 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 we 

find another Witwatersrand or another Blind 

River?" - remains unanswered. We are two-thirds of 

the way through our time here and may I ask the remaining speakers to give some thought to this most 

important 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 was 

that he agrees with your opening remarks. How do we 

find 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 enormous, 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 spent 

almost two years down there mapping the Quadrilatero Ferrifero, and I can confirm that the stratigraphic and structural problems are complex. We'd 

really like to start mapping all over again because now 

we know where the outcrops are, and with a better 

knowledge of the stratigraphy we could do a better 

job. So I think I'll echo Luciano's [Moura] statement 

that 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 can 

start using microprobes and that sort of thing beneficially.

Armstong: From a few days inspection I can state that 

the structure is more complicated than was indicated 

on 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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-U

CONTENTS

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 _____

Page

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

ILLUSTRATIONS

Page

FIGURES 1,2. Maps showing:

1. Location of Blind River-Elliot Lake area ___________________________________________ U2

2. Generalized geology of the Blind River-Elliot Lake area; schematic cross section of the Elliot Lake area ______ 4

3. Diagram showing formation of regolith ________________________________________________ 6

4. Correlation chart showing lateral variation of the Huronian Supergroup, Elliot Lake ______________________ 7

5. Diagram showing alkali content of Huronian polymictic conglomerates ______________________________ 10

6. Graph illustrating silica/alumina, ferric iron/ferrous iron, and soda/potash ratios for Huronian argillaceous rocks ______ 12

7. Map and cross section showing the geology of uranium deposits in the Quirke Syncline _____________________ 14

8. Diagram showing the correlation of quartz-pebble conglomerate reefs, Quirke zone _______________________ 15

9. Cross section, New Quirke Mine ____________________________________________________ 15

TABLES

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 _____________________.

Page

U2 

3 

5

17

17

17

ill

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

THE BLIND RIVER URANIUM DEPOSITS: 

THE ORES AND THEIR SETTING1

By JAMES A. ROBERTSON2

ABSTRACT

In the Blind River area, Proterozoic clastic sedimentary and minor 

volcanic rocks (Huronian Supergroup) unconformably overlie and 

transgress 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 are 

recognized.

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/metric 

ton), but lower grade material can be mined with increasing price. 

Some thorium and rare earths have been marketed. The general 

absence of gold reflects lack in the source area.

The conglomerate beds lie in southeasterly striking zones controlled 

by basement topography down-sedimentation from radioactive Archean 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 for 

sulphur 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 conglomerate 

and siltstone found north of Sudbury carry minor uranium and locally 

gold values. Provenance, lack of oxygen, transportation and depositional processes, and no major modification by later events are the 

dominant 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 and 

role.

INTRODUCTION

Economic Blind River uraniferous pyritic oligomictic 

conglomerates were found in 1953; the presence of 

uraniferous conglomerate had been known since 1948. 

Since 1953, the activities of mining companies, of uni1 This paper was presented at the workshop by James A. Robertson. In 1976 it was published under the same title by the Ontario Division of Mines as Miscellaneous Paper 65, 45 

pages. It is reprinted here, with some minor changes, with the permission of the Director, Ontario Geological Survey, Ministry of Natural Resources.

2 Chief, Mineral Deposits Section, Ontario Geological Survey, Mineral Resources Group, 

Ministry of Natural Resources, 77 Grenville St., Queen's Park, Toronto, Ontario, Canada.

panics, of universities, and of both Canadian and Ontario government geologists have led to a large body of 

literature and other data. This paper is an attempt to 

summarize the present state of knowledge and to provide guidelines for the exploration geologist or the 

geologist 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 third 

a 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 the 

uranium mines, lies 20 miles northeast of Blind River.

The region lies on the boundary between the Southern 

and 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 both 

of early Proterozoic sedimentation and of the midProterozoic metamorphism.

Table 1 is a table of formations using the nomenclature recommended by the Federal-Provincial Committee on Huronian Stratigraphy (Robertson and others, 

1969), and table 2 permits comparison with schemes 

used in publications prior to 1969. Table 3 provides a 

synopsis of stratigraphy and its relation to mineralization.

The bedrock of the area falls into three broad units, 

the distribution of which is shown on figure 2. These are 

(1) the Archean basement, consisting of Keewatin-type 

greenstone, Algoman granite, and minor mafic intrusives, (2) the Huronian sedimentary and metasedimentary rocks, with local minor mafic volcanics, and (3)

ui
U2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 1.- Location of Blind River-Elliot Lake area.

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 and 

the 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 is 

the Flack Lake Fault, with upper Huronian rocks exposed on the downthrown (north) side; and in the center is 

the Chiblow Anticline, the south limb of which is 

repeated by a major east-striking fault-the Murray 

Fault. The axes of these folds strike slightly north of 

west 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 features 

generally plunge eastward, but to the west of the 

batholith 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 axis 

of the McGregor Bay Anticline. When traced eastward,

TABLE 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]

Unit Dominant lithology

Age 

(million 

years)

Phanerozoic 

Cenozoic 

Pleistocene and Recent

Paleozoic

Ordovician (20) 

Precambrian 

Proterozoic

Keweenawan Supergroup 

Sudbury dikes (19)

Sand, gravel, till 

Unconformity 

Limestone

Olivine diabase

Intrusive contact 

Mount Lake Dike Quartz diabase

Intrusive contact with Nipissing Diabase f1) 

Gabbro, granite 

Granite

Hudsonian 

Croker Island Complex

(18) 

Cutler Batholith (18)

Penokean 

Nipissing (17)

Huronian Supergroup 

Cobalt Group 

Bar River (16) 

Gordon Lake (15) 

Lorrain (14)

Gowganda (13)

Quirke Lake Group 

Serpent (12) 

Espanola(ll) 

Bruce (11)

Hough Lake Group 

Mississagi (10) 

Pecors (9) 

Ramsay Lake (8)

Elliot Lake Group2 

McKim (7) 

Matinenda (5)

Intrusive contact

Quartz diabase, diorite 

Intrusive contact

Quartzite

Siltstone, sandstone 

Quartzite, conglomerate

arkose 

Conglomerate, greywacke,

quartzite

1,225

1,445

1,750

2,155

Unconformity-disconformity

Quartzite

Limestone, siltstone 

Conglomerate

Local disconformity

Quartzite

Argillite

Conglomerate

Local disconformity

Argillite

Quartzite, with or without

uranium conglomerate 

Conglomerate 

Arkose, with or without

uranium-conglomerate

regolith

Archean 

Late Archean intrusives

Unconformity 

Diabase

Intrusive contact 

Kenoran (Algoman) (3) Granite

2,500 

2,500 +

Early Archean 

intrusives

Keewatin (1)

Intrusive contact 

Gabbro

Intrusive contact

Volcanic and sedimentary 

rocks

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.
THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING

TABLE 2. -Systems of Huronian stratigraphic nomenclature

U3

COLLINS (1925)

Cobalt iesSeries

0)

w

0)

o

D

m

| 8 £ ^-= 1 ^£

•o w

Upper White Q 1

Banded Cherty

Lorrain

Gowganda 

Serpent

Espanola

Bruce

Mississagi

^(Ramsay Lake) 

Granites

McKim

< 

Schist Complex

ROBERTSON (1967a)

a

D 

O

CD

15 

.a 

o

CJ

a

D 

O

CD

0)

o

D

m

Bar River

Gordon Lake

Lorrain

Gowganda 

Serpent

Espanola

Bruce

Upper Mississagi

Middle Mississagi argillite

Lower Missi 

Archean

Conglomerate

ssagi argillite

Quartzite 

Algoman

Keewatin

ROSCOE (1960)

a

D 

O

CD

"ro 

.a 

o 

CJ

03 o 

-* 0) 3

•- -* o

D «J 2

a -'CD

-c a

D) D

§2

ICD

*- Q. 

O 3 

— O

LU(5

Upper White Q 1

Banded Cherty

Lorrain

Gowganda 

Serpent

Espanola

Bruce

Mississagi

Pecors

Whiskey

Nordic

Matinenda

Archean

ROBERTSON AND OTHERS 

(1969)

a 

o 

CD

"ro 

.a 

o 

CJ

0) Q. •£ 8 = 

= 12 

a CD

S.S&

OJS 2 I-1 CD

CM

.2.8. £

= «J O UJ--5

Bar River

Gordon Lake

Lorrain

Gowganda 

Serpent

Espanola

Bruce

Mississagi

Pecors

Ramsay Lake

McKim

Matinenda 

Archean

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-vertical 

faults that strike either northwest or parallel to the axial 

planes of the folds are common. The fault pattern, jointing, slickensides on bedding planes, and drag folds suggest that north-south compression formed the folds. 

Several periods of intensity of deformation and 

metamorphism can be demonstrated.

ARCHEAN

KEEWATIN-TYPE ROCKS

Keewatin-type rocks are found locally in the Archean 

of 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 the 

chlorite 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 approximately 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 and 

slightly radioactive. Bodies of the second type are found 

in the Quirke Lake, Aubrey Falls-Ranger Lake, and 

Elliot Lake areas (fig. 2; Robertson, 1960, 1961, 1967, 

1968a).

It is of interest that in the Montreal River area north 

of 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 statistical 

analysis of crossbedding and size distribution of chert
U4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

EXPLANATION

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.-......... M i«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 SETTING U5 

TABLE 3.— Summary of Huronian stratigraphy in the Blind River-Elliot Lake area

Group Formation Lithology Thickness 

(in feet)

Depositional 

environment Source Mineralization

Cobalt _____Bar River Quartzite

Gordon Lake _ Siltstone,

sandstone.

_At least 1,000 

at Flack Lake; at 

least 4,000 at 

Willisville. 

1,000 at Flack 

Lake; 3,000 at 

Willisville.

Shallow water ___Source north, 

but currents 

variable.

Shallow water ___ ________

Lorrain

Gowganda ____ 

Quirke Lake Serpent

Espanola

Bruce

Hough Lake Mississagi

Pecors

Ramsay Lake

Elliot Lake McKim

Matinenda

Volcanic rocks

Quartzite, conglomerate, arkose. 

Conglomerate, greywacke, quartzite, 

siltstone. 

Quartzite

Limestone, dolostone, siltstone. 

Conglomerate

Quartzite

Argillite

Conglomerate

Argillite-greywacke. 

Quartzite, arkose,

conglomerate. 

Andesite-basalt (felsic volcanics).

2,000-6,000

500-4,200 

.0 1,100

0 1,500

.0-200

.0-3,000 +

.40-1,000 +

.5-200

0-2,500 +

0-700 +

Local piles

Shallow water

Glacial in north; 

glacial-marine in 

south.

Shallow water

Glacial shallow

water. 

Shallow water

Shallow water

Glacial shallow

water.

(turbidite). 

Shallow water

Subaerial

North-northwest

North-northwest ___ 

Northwest

Northwest

North?

West-northwest in

west; north in 

southeast. 

North-northwest

Northwest?

Northwest

Northwest

Flack Lake Fault;

Murray Fault.

Th-U in

north; Cu?

U trace in

Victoria Tp.; 

Cu in limestone 

againt 

diabase.

U near basement highs. 

Traces U near

basement 

high. 

Traces U

where unconformable 

on Matinenda Formation. 

Traces U near

basement 

highs. 

U-Th-rare

earths in conglomerate in 

basement lows.1 

U-Th in conglomerate 

interbeds.

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 Huronian 

lay 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 north 

and northwest of the Elliot Lake area contains 

anomalous amounts of uranium (Darnley and Grasty, 

1971; Richardson and others, 1975). In general, those 

areas 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 was 

fractured 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 weathered 

zones have been preserved under the Huronian-Archean 

unconformity and, particularly, over the granitic rocks. 

At distances of greater than 100 yards (90 m) from the 

contact 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 development 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 of 

sericite. Rarely, angular fragments of vein quartz and 

patches 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 of 

angular to subangular quartz fragments up to an inch 

across.

This transition zone is generally interpreted as a 

regolith or fossil weathered zone developed during the 

Archean-Proterozoic interval. Such weathered zones 

have been observed over both red and grey phase 

Algoman granites. Core sections also show the development of crumbly chloritic material between the 

greenstone and the overlying sedimentary rocks (Rice, 

1958). This has also been regarded as regolith. Both 

Pienaar (1963) and J. A. Robertson (1960 and later) have 

analysed samples of regolith. Figure 3, derived from 

their analyses, is a straight-line diagram illustrating 

development of regolith. The following features are apparent:

1. Silica is a constant, and there is a slight gain in 

alumina.

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 the 

plagioclases 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 has 

been converted to FeO and removed by leaching. Such 

reduction may have been owing to the exclusion of the 

iron from the atmosphere by overlying material or to an 

atmosphere deficient in oxygen.

Roscoe and Steacy (1958, p. 4-5) studied the distribution of uranium and thorium in two series of saprolite 

(regolith) samples from the Quirke Lake area of 

Township 144. They stated, "Both show uranium to be 

about one-third less in the most altered samples than in 

the freshest granite. One shows a proportional loss of 

thorium which is only slightly less than the loss of 

uranium. The other shows a net gain of thorium."

As regolithic material contributed to the formation of 

the uraniferous, pyritiferous, oligomictic conglomerates 

of 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 of 

uranium 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 of 

residual regolith under reducing conditions. Slight 

topographic variation was developed.

HURONIAN SUPERGROUP

The Huronian Supergroup unconformably overlies the 

Archean 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 recognition 

of the cyclical nature of much of the sequence and the 

presence 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 FORMATION

The Matinenda Formation consists of greenish arkose, 

with or without uraniferous quartz-pebble conglomerate
THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING U7

beds, followed by grey quartzite. Crossbedding and 

pebble-orientation studies by McDowell (1957, 1963) and 

Pienaar (1963) indicate that the currents flowed from 

the northwest but were markedly influenced by basement topography, which in turn reflects the Archean 

structure. The ore conglomerates occur mainly in 

valleys 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 the 

regional pattern are the effects of basement highs and 

lows. 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 not 

disrupted by basement highs, the ore beds at Pronto, the

Approximate Vertical Scale La Ctoche 

Lake

2000 4000 FEET

I i I i I i I

o 400 800 1200 METERS

Approximate Horizontal Scale

0 2 4 6 8 10 MILES

I I I I I I

GORDON 

LAKE /

10 15 KILOMETERS

Flack Lake 

JM1

LORRAIN

BAR 

RIVER

Tp.163 

AR1

Tp.lB 

AR 2

Aird

GOWGANOA

Ten Mile 

Lake 9ui,rke 

Lake

"T

NT Z _ 

5-2

CM4/

~f Spannorth D 0 H / A 

{

// X

SERPENT^ Denvic // 

. Lake//;

PA24 MISSISSAGI

_MATIN|NDA 

ARCHEAN

Basement Basement

ORIGINAL" HURONIAN BELT

Basement Basement Basement 

Law High Low

ESPANOLA

RAMSAY LAKE

Basement High

SOUTHERN 

HURONIAN BELT

Advancing Shore Line Advancing Shore Line Quirke 

ore-zone

Pardee 

ore-zone

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; those 

at Quirke are small- to medium-sized pebbles, well 

sorted and rounded. Similar relations are seen going up 

sequence at both Nordic and Quirke, and packing 

becomes less pronounced.

McKIM FORMATION

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 between 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 

repeated south of the Murray Fault. Near the fault, it is 

represented by schists of the almandine-staurolite 

grade, but farther south it is represented only by poorly 

exposed less-metamorphosed rocks. The unit south of 

the fault was formerly believed to be Archean in age 

(Collins, 1925, 1936).

VOLCANIC ROCKS

In the Blind River area (fig. 2) mafic volcanic rocks are 

found at Thessalon, Dollyberry Lake, near Nordic Mine, 

south of Pronto Mine, and in the vicinity of Massey. 

Thick volcanic assemblages have also been recognized 

northeast 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 and 

amygdules. Ropey texture and flow-top breccias are 

recognised, 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 the 

Archean rocks (Robertson, 1968b; Innes, 1975; Bennett, 

1975). Locally at Thessalon and Massey, but better 

developed at Sudbury, intermediate or felsic flows are 

found. So far, attempts to date these suggest a date of 

about 2,400 m.y., but a reliable isochron needs to be 

established (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.

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; these 

are probably part of the same igneous cycle as the 

volcanic rocks and have similar chemical affinities. No 

age dates have been attempted.

The known uranium ore bodies are marginal to the 

volcanic piles, which may have physically controlled the 

current depositing the uraniferous conglomerates in the 

same way as the topographic highs reflecting basement 

geology.

Emanations from the volcanism may also have contributed sulphur to convert detrital magnetite to pyrite 

in the sediments.

HOUGH LAKE GROUP

The Hough Lake Group comprises the Ramsay Lake 

Formation, polymictic paraconglomerate; the Pecors 

Formation, argillite; and the Mississagi Formation, 

quartzite.

RAMSEY LAKE FORMATION

The Ramsay Lake conglomerate consists of pebbles 

and cobbles of grey granite, quartz, and mafic igneous 

rocks 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 with 

dropped clasts suggests ice rather than mudflow as the 

transportation medium. A crude bedding, particularly in 

the south, the presence of mud cracks and ripple marks 

on 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 more 

potassic than the upper part, reflecting incorporation of 

material 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 and 

others, 1972). The Ramsay Lake Formation indicates 

that the general climate was cold and that the uranium 

mineralization was syngenetic.
THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING U9

PECORS FORMATION

The Ramsay Lake Formation is followed conformably 

by the Pecors Formation, which comprises a sequence of 

argillites in the Quirke Lake-Elliot Lake area, of 

argillites, siltstones, and quartzites on the south limb of 

the Chiblow Anticline, and of siltstones and argillaceous 

quartzite 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 a 

quartzite 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 are 

characteristic.

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 the 

flank 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).

MISSISSAGI FORMATION

The Mississagi Formation consists of greenish arkose 

on the north limb of the Quirke Syncline. Elsewhere it is 

normally made up of well-bedded grey quartzite, but adjacent 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 influence 

of basement topography was much diminished.

Along the north limit of the Quirke Syncline and again 

adjacent to the Chiblow-Cutler basement high the 

yellow-green arkosic phase is characterised by radioactivity. Uranium and thorium, as well as potassium, is 

revealed 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

siltstone, the Espanola Formation; and quartzite, the 

Serpent Formation.

BRUCE FORMATION

The Mississagi Formation is overlain disconformably 

by the Bruce Formation - a conglomerate that consists 

of 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 have 

been seen in varvite lenses. The conglomerate can be 

traced 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 are 

channels 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 siliceous 

and is characterised by more pyrite and the lime and 

soda contents are greater than in the Ramsay Lake Formation. Where metamorphosed, the pyrite is replaced 

by pyrrhotite, and the formation has a magnetic expression. The basal part of the formation contains reworked 

material from the underlying Mississagi Formation.

ESPANOLA FORMATION

In the Blind River area the Espanola Formation consists of three units: a lower unit, characterized by 

limestone, the Bruce limestone; a middle unit, 

characterized by mudstone and greywacke, the 

Espanola greywacke; and an upper unit having a 

marked development of ferruginous dolomite, the 

Espanola limestone. This three-fold division is not possible in the southeastern part of the area mapped, and the 

formation 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 upper 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 spectacular 

appearance. Where the unit is complete, the thickness is 

generally 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, the 

thickness of the siltstone member is 300-400 ft (90-120 

m) 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 of 

iron suggests that the environment contained some free 

oxygen.

SERPENT FORMATION

The 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 the 

Serpent 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 the 

north channel of Lake Huron at the south limit of the 

area 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 Lake 

area, consists of the Gowganda Formation and the Lorrain Formation, the Gordon Lake Formation, and the 

Bar River Formation.

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 Lake 

area, north of the Flack Lake Faults (Roberston, 1971b; 

Siemiatkowska and Guthrie, 1975), it rests on the 

Archean basement. Locally, the contact can be seen 

truncating the bedding of the underlying formations, 

and consolidated or partly consolidated fragments of the 

underlying 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, and 

argillite. These rock types are found throughout the sequence, although the lower part is characterized by

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 (900 

m) 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 fine 

grained, chloritic, and characterised by high soda content relative to the lower Huronian polymictic conglomerate (fig. 5).

The origin of the Gowganda Formation has been much 

discussed. Dense boulder conglomerates, quartzites, and 

argillites 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 to 

turbidity 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 environment, 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 the 

east mapped by Card (1967a-d) and Casshyap (1966, 

1968), a marine rather than a continental glacial environment is indicated (Lindsey, 1966). Some clasts are

CaO

Na Q 10 20 30 40 50 60 70 80 90 K Q

GOWGANDA • BRUCE • 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 

use of this feature as a criterion for glacial action (Lindsay and others, 1970) and suggests that the striae are 

due to postdepositional tectonism, as described by 

Bielenstein and Eisbacher (1969). Of particular interest 

is the presence of iron oxide in the Gowganda Formation. The arkosic quartzites are red to pink in colour, 

reflecting the preservation of hematitic dust in the 

feldspar and interstitial material. Some thicker beds of 

argillite or siltstone contain magnetite and zones of such 

material can be traced on regional magnetic maps. No 

significant uranium anomalies have been identified, but 

scattered thorium and potassium values (Richardson 

and others, 1975) reflect the granitic content of the conglomeratic lower portion of the formation. The lack of a 

preserved regolith at the Archean surface (Robertson 

1970c; Wood, 1970a) may indicate oxidizing rather than 

the reducing conditions that prevailed during the deposition of older lower Huronian sediments elsewhere in the 

district (Robertson, 1969a; Roscoe, 1969; Wood, 1970a).

LORRAIN FORMATION

The Lorrain Formation conformably overlies the 

Gowganda Formation. In the Flack Lake area several 

units have been mapped within the Lorrain Formation 

in the following ascending stratigraphic order: (1) pink 

ferruginous quartzite and minor siltstone, (2) coarsegrained green arkose, (3) pink coarse hematitic arkose 

with radioactive (thorium:uranium >10:1) quartz-pebble 

conglomerates (Robertson 1970c; Robertson and 

Johnson, 1970); (4) interbedded pink and buff quartzite 

and, in a few places, greenish quartzite (quartz-chertjasper pebble bands are characteristic of the upper part 

of this member), (5) massive white quartzite, with 

quartz-chert-jasper pebble bands in the lower part. 

Sericite, kaolinite, pyrophyllite, and diaspore are found 

in the nonfeldspathic beds of members (4) and (5) 

(Chandler and others, 1969) and indicate the onset of 

warm rather than frigid conditions (Wood, 1970a; 

Young, 1970). Crossbedding is common in the Lorrain 

Formation in the Flack Lake-Mount Lake-Rawhide 

Lake area but is variable; southwesterly current directions have been interpreted by Hadley (1969) and by the 

writer, although the source area was probably to the 

north. North of Bruce Mines, Hadley (1969) deduced 

southeasterly current directions. The five members can 

be traced throughout the Flack Lake-Mount LakeRawhide Lake area (Robertson, 1971b; Wood, 1975; 

Siemiatkowska and Guthrie, 1975). Their equivalents 

can also be recognized and traced in the La Cloche LakeWhitefish Falls area (Card, 1971a; Chandler, 1969), but 

the individual units are both finer grained and thicker, 

indicating deposition farther from source. They have

been strongly folded, foliated, and somewhat metamorphosed; the clay minerals are represented by kyanite 

and andalusite (Church, 1967; Chandler, 1969; Robertson, 1970b, 1976).

GORDON LAKE FORMATION

The Lorrain Formation is overlain, apparently conformably, by the Gordon Lake Formation, a 1,000-ft 

(300-m) sequence of well-bedded siltstone, argillite 

chertstone, and fine- to medium-grained sandstone. 

There are three members: (1) a lower member comprising reddish sandstone and siltstone with anhydrite and 

gypsum nodules (and salt casts?), (2) a middle member of 

dark green siltstone, argillite, and minor sandstone, and 

(3) an upper member of reddish siltstone, argillite, and 

chert (Eisbacher and Bielenstein, 1969; Bottrill, 1970; 

Robertson, 1969b, 1970c). The middle member tends to 

form a scarp in the Flack Lake area that corresponds to 

a moderate magnetic anomaly (Robertson, 1971b). Current ripples, microcrossbedding, slumpage structures, 

and desiccation cracks indicate deposition in very 

shallow water (Young, 1969; Robertson, 1969b, 1970c; 

Robertson and Johnson, 1969,1970; Wood, 1970a, b). In 

the core of the La Cloche Syncline a few hundred feet of 

siltstone and quartzite that have been correlated with 

the Gordon Lake Formation also coincide with a 

magnetic anomaly (Robertson, 1976).

BAR RIVER FORMATION

The Gordon Lake Formation is overlain by the Bar 

River Formation, which comprises at least 1,200-1,500 

ft (360-450 m) of massive to well-bedded orthoquartzite, 

generally crossbedded and ripple marked (Robertson, 

1969b; Robertson and Johnson, 1969; Woodward, 1970; 

Eisbacher and Bielenstein, 1969). Mudcracks and desiccation features are also found, particularly in finer 

grained sandstones and intercalated siltstone bands. 

One unit is characterized by thin bedding and ferruginous siltstone and quartzite. The iron has been partly redistributed, and small-scale solution depositional 

fronts adjacent to joints and fractures are common. 

Some ripple-marked surfaces at this horizon are covered 

with segmented, tapering, sinuous structures of possible 

organic origin (Hoffman, 1967; Young, 1967; 

Donaldson, 1967). Young (1967) and Hoffman (1967) 

favoured metazoans (worms) as the organism involved, 

but Donaldson (1967) suggested that the structure 

represents the infilling of desiccation cracks in algal 

mats. In the same area there are many rather similar 

structures that are undoubtedly desiccation features. 

The desiccation features of the Gordon Lake Formation 

(Young, 1969; Robertson, 1969b) are so similar to the 

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 in 

doubt. Wood (1970a, b) has recorded oolites in the ferruginous rocks, and their presence along with the many 

primary structures indicated deposition in very shallow 

water. In the ferruginous rocks Bottrill (1970, personal 

commun.) has observed pyrite and in polished section 

has identified the major iron mineral as maghemite 

rather than hematite. The conditions of deposition, 

sedimentation, climate, and state of atmospheric oxidation represented by the upper Huronian rocks of the 

Flack Lake are discussed in more detail by Wood (1971).

SUMMARY 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 the 

southeast. 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 of 

similar rock types are due to shifts in provenance and to 

the efficacy of weathering and winnowing processes. 

Radioactivity and heavy minerals are characteristic of 

quartz-pebble conglomerates in the fluviatile near-shore 

facies. In the lower Huronian the association is of 

uranium and sulphides, predominantly pyrite, and in the 

upper Huronian it is of thorium and iron oxides. In the 

middle Huronian conditions became favourable for 

preservation of iron oxides. Glacial deposits are 

characteristic 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 compositional 

ratios for Huronian argillaceous rocks are shown. Of 

particular interest in figure 5 is the variation in maturity 

of the paraconglomerate matrices, which decreases 

towards the Gowganda Formation, the marked increase 

in soda, and, in the lower part of the paraconglomerate 

sequences, the incorporation of potassic material from 

the underlying arenite. Figure 6 shows an increase in 

ferric/ferrous iron ratio in the argillaceous rocks, reflecting change in general oxidation conditions; an increase in the Si02/Al2 03 ratio, reflecting increased efficacy of chemical weathering in the upper Huronian; 

and variations in the Na2 0/K20 ratio, reflecting changes 

in provenance from a granitic to a granodioriticgreenstone terrane. The arenaceous rocks show similar 

changes in provenance and efficacy of chemical 

weathering. The role of oxidation is apparent in the frequency of red beds from the Gowganda Formation up8

SiO2/AI 2O3

''.. Na2O/K2O

Fe 2O3/FeO

f

Me Kirn Gowganda

Pecors Lorrain

Gordon 

Lake

FICUKE 6.—Silica/alumina, ferric iron/ferrous iron, and soda/potash 

ratios for Huronian argillaceous rocks.

wards contrasted to the total lack of red beds in the 

lower formations.

POST-HURONIAN EVENTS

Following the close of sedimentation, the area was 

subjected to a long history of structural deformation, igneous intrusion, and, in the Penokean fold belt, regional 

metamorphism, which locally attained amphibolite 

facies (Card, 1964; Card and others, 1972; Robertson, 

1972). These events do not concern the formation of the 

uranium 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 with 

dikes 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 that 

part of the Blind River area in which uranium ores are 

found is minimal. The lack of intense metamorphism 

may be regarded as a factor in the preservation of the 

ore bodies. Age data on minerals, however, reflect some 

resetting of the decay clocks at times corresponding to 

known peaks of regional metamorphism or thermal 

events rather than episodic introduction of more 

uranium (Roscoe, 1969).

THE URANIUM DEPOSITS

Uranium and thorium-uranium deposits are found in 

conglomerates at a number of localities in the Blind 

River-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 U13

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 and 

valleys over softer zones in the greenstone belt is clearly 

demonstrated (fig. 7). At Pronto, however, there is no 

clear relation to basement geology, which raises the 

possibility 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 (the 

largest 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 the 

Nordic 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 the 

unworked 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 truncated 

by an unconformity at the base of the Ramsay Lake conglomerate.

At Quirke No. 2 and Denison, the best ore development 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) thick 

separated by barren arkose 0.6-2.4 m (2-8 ft) thick. 

Throughout much of the Denison Mine, ore grade was 

sufficient to permit mining of both conglomerate beds 

and the intervening quartzite as one unit using largescale equipment and trackless haulage. Quirke No. 2 has 

been developed using conventional haulage. Other conglomerate beds 0.6-3 m (2-10 ft) thick separated by 

quartzite beds 3.6-6 m (12-20 ft) thick are known on 

both the Quirke and Denison properties, but these have 

not been mined. At Stanrock Mine another reef was 

found under the Denison Reef in the southeastern part 

of the mine. The en echelon pattern with the oldest reef 

to 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 width 

of 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 in 

the sequence, the Lacnor Reef, was mined where grade 

attained 2.0 Ibs U3 08 per short ton (1 kg per metric ton). 

In the eastern part of the mine a third reef, the Pardee

Reef, attains ore grade, 2.3 Ibs U3 08 per short ton (1.15 

kg per metric ton) over 5 ft (1.5 m). This reef extends 

eastward over a basement ridge into the Pardee and 

Pecors mineralized zones (fig. 7). These reefs extend 

down rake to the Stanleigh Mine. Operations were 

largely carried out over the Lacnor and Nordic Reefs. 

As with the Quirke zone, other reefs of conglomerate of 

submarginal 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 by 

drop in grade of the radioactive units. This is probably the typical boundary and the definition is arbitrary.

4. Removal by erosion subsequent to deposition, as for 

example 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 caused 

by 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, and 

sericite and have an average pyrite content of 15 percent. Monazite and zircon are characteristic heavy 

minerals. Brannerite and uraninite are found in the 

matrix. Thucholite is found locally and may line fissures 

in the ore beds. The ore minerals are brannerite, 

uraninite, and monazite (Robertson, 1960 and later; 

Roscoe, 1957 and later; Pienaar, 1963; Roscoe and 

Steacy, 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

6 KILOMETERS

A

METERS -BASE OF RAMSAY LAKE CONGLOMERATE

A

FEET 

0

300 H

SCHEMATIC CROSS - SECTION

EXPLANATION

PIH^Z- Matinenda Formation

Conglomerate

Huronian-Archean 

contact at surface

Greenstone

Iron-formation outcrop; 

magnetic anomaly

Uranium deposit

Granite

FIGURE 1. Uranium deposits in the Quirke Syncline.
THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING U15

Quirke 1 and 2 

(Rio Algom Mines)

Ramsay Lake Formation

Quartzite 

Upper —————

Quartzite 

A* (Quirke 1)

Quartzite 

B ————

Quartzite 

C-L ——— 

Quartzite

C* (Quirke 2) 

Quartzite

FWC (Scraps) 

Quartzite

Basement

Denison 

Denison Mines Ltd.

Ramsay Lake Formation

Quartzite

————— F

Stanrock 

Denison Mines Ltd.

Quartzite

E

Quartzite 

D

Quartzite 

Floater reef(s) 

Quartzite

Quartzite

A!_____

Interbedded quartzite

B* 

Quartzite

Scraps 

Quartzite

Quartzite

Stf 

Quartzite

"st* • bt2 

Quartzite

Basement

St3 

Quartzite

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 paper 

in this volume (chap. V).

Gummites (soddyite and uranophane) are rare, but 

Rice (1958) stated that they form a significant proportion of the ore mineralization at the Spanish American 

Mine. Uranothorite has also been identified (Roscoe and 

Steacy, 1958, p. 14) and Patchett (1959, 1960) identified 

coffinite in altered material from the Nordic Mine.

Pyrite is the commonest sulphide mineral; it usually 

constitutes 10-15 percent of the matrix, but it may rarely 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 leucoxene 

that he considered to have developed from ilmenite exsolved from the original magnetite. More recently, Bottrill (1971) has suggested that the sulphur required was 

derived 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 "U-Cg

ARCHEAN 

H Basement

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 pyrite 

of the ore beds and the pyrite found in other rocks of the 

district.

Other sulphide minerals found are pyrrhotite (occasional 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 and 

Metallurgy, 1957).

In addition, Milne (1959), in samples collected for Patchett (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 and 

are not necessarily present in all samples that have been 

studied.

There has been much discussion and description of the 

main 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 in 

Australia 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, first 

described from the area by Nuffield (1954). Although 

some 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 oxides, 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 the 

material is detrital. Ramdohr (1957) has suggested that 

the 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. Experimental efforts to reproduce this reaction indicate 

that it takes place at temperatures in excess of those to 

which 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 from 

the 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). These 

authors concluded that Blind River brannerite, and the 

brannerite or uraniferous leucoxene of the Rand, formed 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 approximately 1 mm across. Ramdohr (1958a, b) and D. S. 

Robertson (1962a, b, 1974) have described rounded 

grains. 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 and 

thorium content may indicate a detrital origin. Rice 

(1958), however, believed that uraninite formed as a 

result of leaching from brannerite by hydrothermal solutions emanating from diabase; diabase is more common 

in the Nordic Mine than elsewhere. Patchett (1960) also 

supports a secondary origin for uraninite and stated 

that 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 concept of uniformitarianism (actualism) as argument 

against the detrital origin of uraninite.

Roscoe (1959b) has described the monazites from the 

Blind River ores and has pointed out that monazite can 

contain considerable uranium and is, therefore, one of 

the ore minerals. Grains are normally rounded to 

subangular and less than 0.3 mm in diameter. Grey 

varieties 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 the 

basement. The lateral variation in the ore-mineral and 

uranium-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.
THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING U17

TABLE 4.— Uranium, thorium, and titanium contents, in percent, of Blind River brannerite

[From Robertson (1968a, p. 92)]

Mine

Others than Blind

River. 

Nordic

Do

Denison

Do

Quirke

Do

Do

Can-Met

Regional

Author

Thorp (1963)

. Patcheet (1960) _

do

. Roscoe (1959a) _

do

do

do

do

. Pienaar (1963) _

. Thorpe (1963)

Colour

. Reddish brown

. Black

do

Comments

Altered

altered.

U3 08

40-52

32.6

32.2

41

36

24

20

6

8.70

30

Th02

0.3 .15

2.6

1.8

6.1

1.8

1.7

2.2

1.2

1.14

2.3

Ti02

32-40

30.3

30.5

20

30

37

30

27

n.d.

30

U3 08 /Th02 

ratio

40-52

12.5

17.9

6.7

20.0

14

9.1

5.0

7.6

13

TABLE 5. - Uranium and thorium contents in Blind River uraninite

[From Robertson (1968a, p. 93)]

TABLE 6.—Analyses of Blind River monazites

[From Robertson (1968a, p. 93)]

Author U3 08 Th02 U3 08 /Th02

Denison ____ Pienaar (1963) ______ 48.4

Do ______ __do ___________ -39.0

Do ______ __do ___________ 55.5

Nordic ____ Patchett (1960)______ 55.25

Quirke ____ Wanless and Traill (1956) 51.7

Pronto ____ __do ___________ 57.0

Nordic ____ Roscoe (1959a) ______ 64

Denison ____ __do ___________ 60

Panel _____ Thorpe (1963) ______ * 29.6

Regional ____ _do 60

5.6

7.2

5.4

6.25

4.1

5.12

6.3

5.9

3.84

6

8.65

5.4

10.3

8.85

12.6

11.1

10.15

10.2

1 7.7

10

1 Contaminated?

Locally, individual conglomerate bands may assay as 

high as 20 Ibs or more U3 08 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, the 

lateral extension to the Quirke zone, and the Pardee 

zone, however, contain considerable amounts of 

material with average grades of 0.5-1.5 Ibs (0.25-0.75

kg).

The arkose interbedded with the ore conglomerate is 

generally greenish in colour and is crossbedded (Roscoe 

1966; Robertson, 1968a, b, 1975a). Normally this 

crossbedding is of the festoon type, indicative of a 

fluviatile environment.

AGNEW LAKE AREA

The deposits at Agnew Lake (Little and others, 1972), 

currently under development, comprise gritose 

oligomictic conglomerate interbedded with arkose in a 

quartzite sequence probably equivalent to the Matinenda Formation. The pebbles are smaller and more sparse 

than in the Blind River ore, and pyrite is less conspicuous than at Blind River-Elliot Lake. Thorium-toRescue (1969h) Thorpe (1963)

Colour 

Th02 

U3 08

U3 08 /ThO,

1

Orange 

5.9 

.31

.0525

2

Yellow 

3.9 

.95

.244

3

6.7 Grey ' 

1.65

.246

4

Grey 

5.9 

2.9

.492

Range

5.6~-~6~2 

1 -2

.18- .32

Average

5T6~

1.4

.25

uranium ratios are higher than at Blind River-Elliot 

Lake, reflecting the presence of uranothorite and 

monazite 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 metric 

tons) U3 08 in material grading 1.88 Ibs U3 08 per short 

ton (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 the 

regional geology. Lower Huronian rocks are intermittently exposed in the southern portion of the Cobalt 

Plate but are largely concealed by Nipissing diabase and 

upper Huronian rocks. The regional geology has been 

discussed by Thomson (1960) and by Meyn (1972). 

Precise stratigraphic correlation with Blind River is 

unclear, but rocks equivalent to the Mississagi Formation are present. At the base of these, wherever exposed, there are anomalous concentrations of uranium and, 

at some localities, gold in conglomerates and uranium in 

argillaceous quartzites.

Early descriptions of the conglomerates (Thomson, 

1960; Meyn, 1972; Robertson, 1968a) compared these 

rocks 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 ironformation sparsely distributed in an argillaceous quartzite matrix. Many of the so-called quartz pebbles are actually quartzite of unknown provenance or ironformation similar to that of nearby Archean greenstone 

belts. Outcrops of these rocks in Vogt Township and 

Turner Township reportedly contain trace amounts of 

gold (Robertson, 1968b).

These deposits are clearly derived from Archean terrane but are of a provenance different from those at 

Blind River and Elliot Lake. Although these rocks are 

perhaps slightly younger, they must be considered part 

of the same metallogenic province. Additional prospecting and exploration in the southern part of the Cobalt 

Embayment should yield more data of stratigraphic and, 

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 of 

thorium and yttrium, from material grading 2 Ibs U3 08 

per short ton (1 kg per metric ton). The maximum production was in 1959, when 12,150 tons (11,020 metric 

tons) U308 were produced. In 1973, two mines were 

operating: the Denison Mine produced 1,712 short tons 

(1,552 metric tons) U308 from ore grading 2.57 Ibs U3 08 

per short ton (1.28 kg per metric ton), and the New 

Quirke Mine produced 2,409 short tons (2,185 metric 

tons) 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 a 

developed ore body, but the decision to start production 

would require a suitable sales contract.

Ontario's current production is more than 4,000 short 

tons (3,600 metric tons) U3 08 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) U3 08 

per year, which can be obtained by expanding operating 

plants, reopening closed plants, and some construction 

of new plants.

RESERVES

It was estimated (Robertson, 1975a) from data 

available to the public in January 1973, that Ontario 

deposits, predominantly the sedimentary deposits of 

Blind River-Elliot Lake-Agnew Lake areas, contain approximately 200,000 short tons (180,000 metric tons) of 

recoverable 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) from 

material with a millhead grade of 1.4 Ibs U308 per short

ton (0.70 kg per metric ton) and a cutoff grade of 1 Ib 

U308 per short ton (0.5 kg per metric ton) over mining 

widths. The higher grade uranium reserves also contain 

at least 100,000 short tons (90,000 metric tons) of 

recoverable Th02 . The higher grade material constitutes 

some 75 percent of the free world's reserves recoverable 

at less than $10.00 U.S. (1973) per Ib U308 (Little, 1974; 

Williams and Little, 1973; Organization for Economic 

Co-operation and Development, 1973). Estimated additional ore (possible ore and prognosticated ore) in the 

localities of the Blind River-Elliot Lake-Agnew Lake 

areas may be as much again, but extensive and costly exploration must be undertaken before existence of this 

material can be confirmed and it can be considered part 

of the reserves.

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 a 

number 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 been 

much debated. The similarity of the deposits at Blind 

River 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), and 

Gross (1968). Davidson also mentions similar deposits in 

Australia and Russia. This characteristic assemblage 

and its distribution is a major theme of this volume. The 

relation to major unconformities, particularly those 

marking the Proterozoic-Archean boundary, has been 

emphasized (Derry, 1961; Davidson, 1965; Robertson, 

1960 and later).

In the Blind River camp this unconformity can be placed at 2,500 m.y. ago; it is clear that the conditions did 

not persist beyond a minimum of 2,155 m.y. ago and 

that the period 2,500-2,000 m.y. ago seems the most 

favourable.

Bateman (1955, p. 371), Joubin and James (1957), 

Davidson (1957, p. 668), and Heinrich (1958) have cited 

as 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 the 

characteristic minerals gold, brannerite, uraninite, and 

pyrite. 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 U19

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 and 

shown as such on the Lake Huron Sheet (Canada 

Geological Survey, 1933) has since been proved to be 

older than the Huronian (Robertson, 1972; Card and 

others, 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 River 

uranium and thorium) by groundwater

3. Prolonged series of intrastratal migration, allowing 

mineralized groundwater to sink to the lowest 

permeable horizons, the oligomictic conglomerate 

beds, 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 and 

others (1960). However, Roscoe (1969) showed that 

the isotopic compositions are best explained by 

resetting 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 a 

preference 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 of 

syngenetic (placer) origin but were modified by later 

events.

Derry (1960) has suggested a syngenetic origin for the 

uranium mineralization but has raised the possibility 

that the uranium was largely carried in solution and 

reprecipitated 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 and

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 are 

radioactive (Robertson, 1960, 1967).

The sericitic matrix of the ore-bearing conglomerates 

is similar to the sericitic paleosoil developed on the 

Archean-Huronian unconformity and was probably 

derived from that regolith; there is no reason to suppose 

that it was produced by the passa'ge of hydrothermal 

solutions. Uraniferous oligomictic pebble conglomerates 

and a green arkose sequence are characteristic of 

whichever part of the Huronian overlies the basement in 

the area of the north shore of Lake Huron, and these 

rocks show a progressive northward overlap. Significant thicknesses and grades have so far been found only 

in the Matinenda Formation.

Quartz veins and other evidence of intense hydrothermal activity are not conspicuous in the rocks of the area 

and bear no sympathetic relation to the uranium 

deposits. Where found, the associated mineralization is 

of copper and other sulphide minerals. Within the Blind 

River-Elliot Lake camp, there is no indication that 

either the Nipissing (formerly included with the Keweenawan) or the olivine-diabase intrusions are a possible source of major radioactive mineralization. There is, 

however, evidence that ores were subjected to intense 

local alteration and that all rocks suffered sulphide 

mineralization 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 of 

thickness 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) and 

Theis (1973) have added cogent additional support to the 

modified 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 and 

papers, 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) published by the Geological Association of Canada contain 

much 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 the 

Geological Branch of the Ontario Division of Mines, 

Ministry of Natural Resources, provided assistance in 

the preparation of the illustrative material, and Miss T. 

Abolins and Miss C. Elliott helped in the production of 

the manuscript. Permission to participate in the 

workshop and to publish this paper was given by the 

Director of the Geological Branch, Ontario Division of 

Mines.

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————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 Department of Mines Preliminary Geological Map P.377.

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————1967, Recent geological investigations in the Elliot Lake-Blind 

River uranium area, Ontario: Ontario Department of Mines 

Miscellaneous Paper 9, 31 p.

————1968a, Geology of Townships 149 and 150: Ontario Department 

of Mines Geological Report GR 57, 162 p.

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Ontario: Canadian Institute of Mining ami Metallurgy Transactions, v. 62, no. 686, p. 619-634.

————1969b, Township 157, district of Algoma: Ontario Department 

of 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 Department 

of 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 of 

Algoma: Ontario Department of Mines Preliminary Geological 

Map P.318.

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——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 Department of Mines Preliminary Geological Map P.377.

——1966b, Salter Township, district of Sudbury: Ontario Department of Mines Preliminary Geological Map P.378.

——1967, Recent geological investigations in the Elliot Lake-Blind 

River uranium area, Ontario: Ontario Department of Mines 

Miscellaneous Paper 9, 31 p.

——1968a, Geology of Townships 149 and 150: Ontario Department 

of 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 Transactions, v. 62, no. 686, p. 619-634.

——19691), Township 157, district of Algoma: Ontario Department 

of 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 of 

Mines 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, Blind 

River-Elliot Lake area: Ontario Department of Mines and Northern Affairs Miscellaneous Paper 45.

——1972, Granitic plutonic rocks of the Southern Province of the 

CanadianShield (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 of 

Algoma: Ontario Department of Mines Prelinimary Geological 

Map P.560.

————1970, Township IB, district of Algoma: Ontario Department of 

Mines and Northern Affairs Preliminary Geological Map, P.609, 

Geological Series.
THE BLIND RIVER URANIUM DEPOSITS: THE ORES AND THEIR SETTING U23

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 and 

associated 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.

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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 Uses 

of Atomic Energy, 2d, Geneva, Switzerland, Sept. 1958, Proceedings, v. 2, p. 475-483.

Siemiatkowska, K. M., and Guthrie, A. E., 1975, Kirkpatrick Lake 

Area, district of Algoma, in Milne, V. G., and others, eds., Summary 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 Department 

of Mines Geological Report 1.

Thorpe, R. I., 1963, The radioactive mineralogy of the ore conglomerate at Panel Mine, Blind River, Ontario: Queen's University, 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 

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Wanless, R. K., and Traill, R. T., 1956, Age of uraninites from Blind 

River, Ontario: Nature, v. 178, p. 249-250.

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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 Department 

of Mines Preliminary Geological Map P.468.

———1968b, Township Q, district of Algoma: Ontario Department of 

Mines Preliminary Geological Map P.474.

———1970a, Evidence for a tropical climate and oxygenic atmosphere 

in upper Huronian rocks of the Rawhide Lake-Flack Lake area, 

Ontario [abs.]: Institute on Lake Superior Geology, 16th, Thunder 

Bay, 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 rocks 

of 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, 

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-1975, Geology of the Rawhide Lake area, district of Algoma:

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Ontario—relevance to the paleogeography of the Lake Superior 

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no. 3, p. 565-568.

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Thunder Bay, Ontario, May 6-9, 1970, Technical Sessions, 

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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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-V

Published with permission of the 

Director, Regional and Economic Geology, 

Geological Survey of Canada

CONTENTS

Abstract ________________________________________________ VI

Introduction ______________________________________________ 1

Some features of the uranium mineralization at Elliot Lake, Canada ______________ 2 

Some features of the post-Huronian conglomerates in the Ural Mountains and Enisey Crest,

U.S.S.R. ______________________________________________ 4

Conclusions _______________________________________________ 7

References _______________________________________________ 7

ILLUSTRATIONS

FIGURE 1. Photomicrograph of a uranium-ore sample from Denison Mine, Elliot Lake ______________________________ V2

2. Photograph showing a sample of uranium ore from Denison Mine, Elliot Lake, containing pyrite, brannerite, monazite,

uraninite, and hydrocarbon ______________________________________________________ 3

3. Photomicrograph showing arrangement of detrital grains of uraninite embedded in hydrocarbon ________———————— 4

4. Photograph showing a quartz pebble depressing a uraninite-hydrocarbon-rich band, from Denison Mine, Elliot Lake _____ 4 

5-7. Photomicrographs of portions of uraninite-hydrocarbon bands shown on figure 4 _______________________—- 5, 6

in

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

SOME METALLOGENIC FEATURES OF THE 

HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATES

By VLADIMIR RuziCKA1

ABSTRACT

Two genetic types of uraniferous conglomerates can be distinguished: (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 type 

of mineralization can occur only in regions where above-normal concentrations of radioelements and uranium minerals were present in the 

source rocks.

Liberation of uranium minerals, their transportation to the site of 

deposition, 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 and 

deposition was governed by hydraulic and hydrodynamic conditions.

The sedimentary syngenetic origin of uranium mineralization may be 

illustrated 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 by 

deposition of monazite and brannerite, and ends with deposition of 

fragmented fine grains of uraninite embedded in sulphur-rich 

hydrocarbon. The bed containing hydrocarbon and uraninite is locally 

depressed by impact of an exotic pebble of quartz. This situation apparently testifies to the fact that the carbonaceous material is a product of algal mats that existed in quiet times between cycles of sedimentation. 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 environment.

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 can 

also be syngenetic.

Transportation of uranium from the site of original liberation to the 

site of deposition was apparently mainly in solutions and in refractory 

minerals 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 in 

the 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 processes: adsorption, complexing, precipitation, or redox changes according 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 from 

the 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 of 

uranium were formulated, among which special attention was paid to those related to the genesis of uranium 

mineralization in quartz-pebble Huronian conglomerates.

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 the 

quartz-pebble conglomerates were derived from studies 

of the stratigraphic setting, sedimentary nature, 

mineral composition, and geochemical features of the 

Rand-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 in 

the 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 the 

mentioned 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 CONGLOMERATES

The 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 host 

for the uranium mineralization is quartz-pebble conglomerate; in addition, the Russian deposits are also in 

uraniferous polymictic conglomerates.

SOME FEATURES OF THE URANIUM 

MINERALIZATION AT ELLIOT LAKE, CANADA

The 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 of 

radioelements. 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 metasediments 

with banded iron-formations. The granitic rocks that occur in areas from which clastic material was supplied to 

the Huronian sediments locally contain concentrations 

of uranium exceeding several times normal clarkes for 

such rocks (Richardson and others, 1975; Roscoe and 

Steacy, 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 of 

an 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 the 

disintegration of the source rocks (Roscoe, 1969). 

Evidence for this postulate, among other features, is the 

presence of subrounded grains of pyrite and other oreforming minerals, which are apparently of detrital 

origin (fig. 1). Deficiency of free oxygen might also have 

been partially related to volcanic activity. It is the 

author'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 by 

hydraulic action of the streams.

Elliot Lake ore was deposited in sedimentation cycles. 

One such cycle is illustrated in figure 2. It starts with 

deposition of a coarse fraction of light minerals (quartz) 

and continues with deposition of a gradationally finer 

and 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, and 

brannerite. The cycle ends with deposition of 

microscopic fragmented grains of uraninite embedded in 

hydrocarbon (fig. 3) (Ruzicka and Steacy, 1976).

The upward fining of the mineral grains in these 

cycles is proportional to their absolute weight and inversely proportional to their density. Figure 4 shows a 

quartz pebble of the subsequent cycle depressing the 

uraninite-hydrocarbon-rich band of the previous cycle.

The deposition of uranium was governed by sedimentary processes. There is no doubt about the detrital 

nature 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 rounded 

grains. The uraninite contains several percent of 

thorium (Ruzicka and Steacy, 1976). This testifies to the 

origin 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 the 

vicinity of uraninite grains. Since this enrichment is a 

local process, involving distances of only a few 

millimetres, it can be postulated that the formation of 

these 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 additional uranium has been introduced into the sediment, 

such a process may be regarded as redistribution of 

syngenetic uranium.
METALLOGENIC FEATURES, HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATES V3

•MMMMMMI

A

FIGURE 2.—A sample of uranium ore from Denison Mine, Elliot Lake, Ontario; the portion within the arrow area contains bedded grains of 

quartz, pyrite, brannerite, monazite, uraninite, and hydrocarbon and represents one sedimentation cycle. The arrow points towards the top 

of the bed. UR-HC = uraninite-hydrocarbon-rich section; PY-BR-MO = pyrite-brannerite-monazite-rich section; QU = quartz-pebble-rich 

section.

The hydrocarbon in the Elliot Lake ores, variously 

described as "radioactive hydrocarbon," "uranium 

hydrocarbon," 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 interpreted 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 sedimentation." 

The hydrocarbon usually exhibits uniform extinction
V4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 3.—Photomicrograph showing arrangement of detrital grains 

of uraninite (Ur) embedded in hydrocarbon (He). Line A-B denotes 

the plane of bedding (x 163). Reflected light.

and a columnar structure under crossed nicols (figs. 5, 

6). The fibres (columns) are oriented normal to the bedding plane. This corresponds with the orientation of the 

columnar carbon in ores from the Orange Free State 

Goldfield (Basal Reef), as reported by McKinney (1964). 

Hallbauer (1975) postulated an organic origin of the columnar carbon. Some of the hydrocarbon heals fractures 

in uraninite (fig. 7). Uraninite embedded in the 

hydrocarbon can be interpreted as evidence of transportation of uraninite in organic-supported colloids. Furthermore, if one accepts that the organisms were also 

active in the provenance area of the detrital material, 

then the possibility that the biological agents participated in liberation of uranium minerals from source 

rocks cannot be excluded. Therefore, accepting Silverman and Minoz' (1970) conclusions on similar situations, 

one can postulate that the liberation of uraninite from 

the source rocks might also be caused by fungal attacks. 

The abundant pyrite is apparently partly of detrital 

and partly of epigenetic origin (Roscoe, 1969). In addition to pyrite derived from Archean rocks, the sulphursaturated fluids from contemporaneous volcanic activity 

apparently enabled crystallization of pyrite in or immediately adjacent to the volcanic centres. Such fluids 

may also have introduced pyrite into the transported 

detritus, allowed crystallization of pyrite in situ within 

the deposit, and affected the detrital minerals by 

sulphurization during transportation and deposition. 

Some sulphur could also have been derived from organic 

matter or from meteoric sulphate solutions (Ferris and 

Ruud, 1971). It is the author's opinion that some of the 

pyrite exhibiting abnormal magnetic susceptibility in the

FIGURE 4.—A quartz pebble depressing a uraninite-hydrocarbon-rich 

band. A polished section of the Elliot Lake ore, Denison Mine 

(Geological Survey of Canada Photograph 203006M).

Elliot Lake ore (Hartman and Wyman, 1970) originated 

by sulphurization of magnetite. Sulphurization is apparently the cause of abnormally high content of sulphur 

in the hydrocarbon. H.R. Steacy of the Geological 

Survey of Canada investigated (personal commun.) 

"thucholites" from several types of environments and 

found the highest sulphur concentrations (more than 20 

percent sulphur) only in Elliot Lake hydrocarbons. The 

sulphides acted also as agents for preserving detrital 

uranium minerals during transportation, deposition, 

and diagenesis.

These characteristics support the idea of a 

sedimentary-syngenetic origin for the Elliot Lake ores. 

However, more detailed studies are necessary to obtain 

a complete model of the physical, chemical, and organic 

mechanisms involved in formation of this type of 

deposit.

The author proposes a term for uranium deposits of 

this type as "lower Proterozoic oligomictic pyritic conglomerates with syngenetic mineralization."

SOME FEATURES OF THE POST-HURONIAN

CONGLOMERATES IN THE URAL MOUNTAINS

AND ENISEY CREST, U.S.S.R.

Auriferous and uraniferous conglomerates in the 

U.S.S.R. have been recently most comprehensively 

described by Krendelev (1974), Khvostova (1969), 

Oslopovskikh (1968) and Krendelev and Zlobin (1972).
METALLOGENIC FEATURES, HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATES

, .", ,. ?

UR 

. "V *• „. HC

UR

V5

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 and 

Energy 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 for 

Mineral and Energy Technology.)

Several deposits containing gold or uranium, such as 

Laba, Baley, Kara-Su, Khuzhur, Timan, Panimba, and 

Eastern Ural Mountain, have been described in the Russian literature. Among these deposits, the Eastern Ural 

Mountain deposit and the Panimba occurrences contain 

some uranium.

The Eastern Ural Mountain deposit occurs in the 

Lower 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-dacite 

group 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: the 

lower, 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 are 

mainly oligomictic and monomictic with local thick 

masses 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 CONGLOMERATES

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 the 

epidote-amphibolite state of metamorphism. The 

metamorphic events took place 239 + 9 million 

years ago (and are therefore younger than the host 

rocks).

7. Distribution of secondary rock-forming minerals, 

such as twinned microcline, albite, muscovite, and 

blue quartz, coincides apparently with the distribution 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 not 

proven.

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 occurrences are also known (Ruzicka, 1971).

The uraniferous and rare-earth-element-bearing conglomerate occurs in a limb of an anticline, the core of 

which 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 m 

thick and extends along strike for several kilometres. 

The most abundant minerals present are pyrite, pyrrhotite, marcasite, chalcopyrite, and sphalerite. Gold in 

individual samples grades 1.55 gram per ton; nickel, 

0.045 percent; and cobalt, 0.059 percent. Uranium and 

thorium are present, but their amounts have not been 

disclosed.

The genesis of the Panimba deposit has been interpreted by Krendelev (1974) and can be summarized as 

follows:

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 activity the rocks have been decarbonatized and 

altered by magnesian metamorphism, during which 

sulphur was mobilized from organic matter and 

redistributed.

3. Permeable rocks, such as conglomerates, served as 

channels for mineralizing fluids.

4. Mineralization is therefore apparently of epigenetic 

origin and not necessarily of syngeneticsedimentary (that is, placer) type.

FIGURE 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 in 

veinlets 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) correlated 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 as 

well as the older (foot-wall) and younger (hangingwall) rocks.

4. The host rocks and the underlying complexes, which 

contain intrusive and extrusive rocks, have been 

regionally metamorphosed. Metamorphic events
METALLOGENIC FEATURES, HURONIAN AND POST-HURONIAN URANIFEROUS CONGLOMERATES V7

The author of this paper proposed the definition of the 

Panimba and similar uranium conglomeratic deposits as 

"post-lower Proterozoic conglomerates with epigenetic 

mineralization."

CONCLUSIONS

Comparisons between the two genetic types of 

uraniferous conglomerates allows generalization of 

their basic features as follows:

A. Lower Proterozoic pyritic oligomictic conglomerates 

with syngenetic mineralization:

1. The source area contains rocks with abnormal 

clarkes of radioelements and with uraniumbearing minerals.

2. Liberation of the uranium minerals took place 

by physical, chemical, and possibly also 

biological mechanisms under oxygen-deficient 

conditions.

3. Transportation of the uranium minerals in the 

detritus occurred under oxygen-deficient conditions, namely in an oxygen-deficient atmosphere combined with volcanic and 

postvolcanic exhalation. Some uranium 

minerals were probably transported as 

suspensions in colloidal organic matter.

4. Concentration and deposition of uranium and 

accompanying minerals were governed by 

hydraulic action of streams. Sedimentation 

took place at a time when primitive organisms 

already existed.

5. Excessive sulphur, a product of the associated 

volcanic activity, caused sulphurization of 

some minerals and hydrocarbon, the latter 

being derived from organic and possibly also 

inorganic material.

6. Preservation of concentrations of uranium 

minerals was possible, with local redistribution of uranium due to the presence of reducing agents in the deposit and by burial of the 

mineralized beds.

B. Post-lower Proterozoic conglomerates with epigenetic mineralization:

1. Deposits occur in areas containing rocks with 

higher uranium clarkes. Deposition of 

uranium in some of the source rock units 

could have been originally syngenetic.

2. The first stage in redistribution of uranium 

from the source area apparently involved 

transport of uranium from source to the site 

of deposition in solutions or in refractory 

minerals. The process presumably resulted in 

a further local concentration of uranium in 

the environment.

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 

in the Earth's crust. During this process the 

conglomerates served as channels for 

penetration of the uranium-bearing fluids.

4. Deposition of uranium took place by one or 

more of the epigenetic processes: adsorption, 

complexing, precipitation, or redox changes 

according to solubility and stability degrees of 

uranium compounds.

5. Preservation of primary mineralization was 

possible beneath the zone of oxidation.

These conclusions fully support criteria on 

metallogeny of uranium as formulated by the symposium of the International Atomic Energy Agency 

(1974) on the formation of uranium deposits. Furthermore, they allow formulation of definitions 

distinguishing syngenetic and epigenetic mineralization 

in uraniferous conglomerates.

REFERENCES

Darnley, A. G., Charbonneau, B. W., and Richardson, L. A., 1977, 

Distribution of uranium in rocks as a guide to the recognition of 

uraniferous regions, in Recognition and evaluation of uraniferous 

areas: International Atomic Energy Agency, Vienna, 17-21 

November 1975, Proceedings, p. 55-83.

Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: Its occurrence and 

recognition by microprobe: Colorado School Mines Quarterly, v. 

66, no. 4, 35 p.

Hallbauer, D. K., 1975, The plant origin of the Witwatersrand 

"carbon": Minerals Science and Engineering, v. 7, no. 2, p. 

111-131.

Hartman, F. H., and Wyman, R. A., 1970, Concentration of uranium 

minerals from Canadian ores by magnetic means: Canada Department of Energy, Mines and Resources, Mines Branch Technical 

Bulletin TB 118, 36 p.

International Atomic Energy Agency, 1974, Formation of uranium ore 

deposits: Proceedings of a symposium held in Athens, Greece, 

6-10 May, 1974: Vienna, International Atomic Energy Agency.

Khvostova, V. A., 1969, 0 redkometalnoy mineralizacii v metamorficheskikh konglomeratakh odnogo iz rayonov Urala [Rare metal 

mineralization in metamorphosed conglomerates from one region 

in Ural Mountains], in Redkometalnost nekotorykh metamorficheskikh i granitizirovannykh osadochnykh komplexov: Nauka, 

p. 36-57.

Krendelev, F. P., 1974, Metallonosnye konglomeraty mira 

[Metalliferous conglomerates of the world]: Novosibirsk, Nauka, 

240 p.

Krendelev, F. P., and Zlobin, V. A., 1972, Kolchedannoe orudenenie v 

dokembriyskikh konglomeratakh Eniseyskogo kryazha [Massive 

sulphide ore-mineralization in Precambrian conglomerates of the 

Enisey Crest]: Akademiya Nauk SSSR Doklady, 1972, v. 205, no. 

6, p. 1428-1431.

McKinney, J. S., and others, 1964, Geology of the Anglo-American 

Group mines in the Welkom area, Orange Free State Goldfield, in
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 and 

observations: 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 Association

of 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 Uses 

of 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 sedimentary 

features 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 localization 

of 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 their 

geochemical 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 PRECAMBRIAN 

QUARTZ - PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-W

CONTENTS

	Page

Abstract ________________________________________________ Wl

Introduction _______________________________________________ 1

Banket-bearing successions _______________________________________ 2

Distribution and lithologies ____________________________________ 2

Correlations in North America __________________________________ 2

Age __________________________________________________ 4

Early Proterozoic successions lacking bankets ____________________________ 5

Depositional environments of bankets near Elliot Lake_______________________ 8

Lithostratigraphy of the Elliot Lake Group ___________________________ 8

Matinenda Formation _______________________________________ 8

Conglomeratic zones ________________________________________ 9

Dollyberry basalts _________________________________________ 11

Depositional history: A conceptual model ____________________________ 12

Comparison of older and younger terriginous sediments ______________________ 13

Regional tectonic settings of bankets _________________________________ 14

Discussion _________________________________________________ 15

References cited ____________________________________________ 16

ILLUSTRATIONS

Page

FIGURES 1, 2. Maps showing;

1. Occurrences of pyritic uraniferous quartz-pebble conglomerates in North America ____________________ W3

2. Geology of pyritic conglomerate beds in the Montgomery Lake Group, Keewatin District, Northwest Territory,

Canada; cross sections _____________________________________________________ 6

3. Line drawing and map showing statigraphic relations in Elliot Lake Group near Elliot Lake __________________ 9

4. Line drawing showing subsurface distribution of conglomeratic and volcanic strata in the Elliot Lake Group near Elliot

Lake ____________________________________________________________________ 10

TABLE

Page

TABLE 1. North American successions containing bankets _______________________________________________ W4

ill

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

TEMPORAL AND OTHER FACTORS AFFECTING DEPOSITION OF

URANIFEROUS CONGLOMERATES

By STUART M. ROSCOE 1

ABSTRACT

Uraniferous, 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 terrestrial 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 and 

chemically mature colluvium that covered Archean protocontinents 

weathered under an atmosphere that lacked free oxygen.

The initial entrapment of quartz-gravel-bearing grits in Elliot Lake 

Group, as well as subsequent deposition, deformation, and preservation 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 by 

deposition of coarse clastic wedges or alluvial fans atop soil-mantled 

Archean 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 tectonic 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 in 

North 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 of 

these. 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.

INTRODUCTION

The pyritic quartz-pebble conglomerates, or bankets, 

considered in this publication have been reported in 

fewer than a dozen separate districts in the world, yet 

they include the most extensive known deposits of

1 Consulting geologist, 111 Kamloops Avenue, Ottawa, Canada. Since May, 1976, Regional 

Metallogenist, Northwestern Canadian Shield, Geological Survey of Canada.

uranium and gold, aggregating very large proportions 

of the world's resources of these metals. This is 

remarkable, as other types of uranium and gold deposits 

are vastly more numerous and occur in geological situations 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 from 

weathering 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 in 

otherwise-similar younger sediments although the 

younger sediments are collectively many orders of 

magnitude more abundant than the scattered remnants 

of 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, these 

younger deposits have had different histories of 

weathering, 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 in 

iron and particularly ferric oxide to ones showing an upward enrichment in ferric oxide. This change preceded 

the first widespread depositions of dolomite and the 

most voluminous world-wide depositions of ferric iron in 

banded iron-formations. Many authorities interested in 

genesis of iron-formations (Bayley and James, 1973; 

Beukes, 1973), in atmospheric and biological evolution 

(Oparin, 1938; Rubey, 1955; Cloud, 1973; Holland, 1973; 

and others), and in uranium deposits (Roscoe, 1969)

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 the 

term "oxyatmoversion" for this change (Roscoe, 1973).

Skeptics hinge their doubts about this hypothesis on 

several incorrect ideas. Foremost amongst these is the 

belief 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 the 

presence of pyrite in lieu of hematite is in itself sufficient 

reason 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 in 

forms resembling detrital grains. This attack can be discounted, as it ignores other evidence such as hydraulic 

equivalent sizes of minerals in bankets, variations in 

compositions amongst associated uraninite and pyrite 

grains, penecontemporaneous displacements of some of 

these grains during compaction, the iron-depleted 

character 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 herein 

show that major conglomeratic zones near Elliot Lake 

were deposited subaerially in headward, largely 

distributary, sections of swift, necessarily aerated 

streams-a most inhospitable environment for accumulation of organic material. Pyritic conglomerates occur 

in a range of geological environments, as do black-sand 

placers. Differences in chemical and textural maturity of 

sediments, associations with shale, greywacke, or 

volcanics, 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 LITHOLOGIES

Uraniferous 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 have 

been summarized by Roscoe (1973, fig. 3), with available 

isotopic dates of basements, banket host strata, and 

of red beds, black sands, iron-formations, and unconformities. 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 the 

host successions. Volcanics form important parts of 

most successions and are present in all, with the possible 

exception of Jacobina, which contains serpentinite and 

amphibolite 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 contain 

bankets.

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 in 

District 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 correlative 

with the Gowganda Formation. Huston (1968), on the 

basis of such comparisons, suggested that economically 

significant paleoplacers might be found in the Deep 

Lake Formation. Young (1973) concluded that the upper 

groups in the three areas showed evidence of climatic 

changes so similar and uncommon that he believed they 

must 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 other 

minor 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 CONGLOMERATES W3

FIGURE 1.-Occurrences of pyritic uraniferous quartz-pebble conglomerates in North America. The distribution of Archean rocks at time of 

deposition 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 sediment 

transport.
W4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

TABLE I.-North American successions containing bankets

Un,l Thickness Lith( ,| OKV Unit Thickness Lithology Unit Thickness 

(kilometers) M (kilometers) ' (kilometer;( Lithology

Huronian Supergroup, Ontario Medicine Bow Mountains, Wyoming Padlei area, Keewatin District

Cobalt Croup

Bar River Formation _

Gordon Lake Formation

Lorrain Formation __

Gowganda Formation. _

Quirke Lake (Iroup _____

Serpent Formation __

Espanola Formal ion __

Bruce Formation

Hough Cti-oup

Mississagi Formation _

I'ecors Formation _____

Ramsey Lake Format ion

Flliot Lake Group

McKim Formation

Metmenda Format ion _

Various volcanic units _

Various arenite units _

4 -9 Libhy Group (Lower 3-7

part).

1 Orthoquartzite. Surgarloaf Formation _ ~1.5

.3 - .9 Varicolored, siliceous Lookout Formation __ ~.5

siltstone.

.9 -2.0 Orthoquartzite. Medicine Peak Forma- ~2.5

Subarkose. 1 tion.

.9 -1.5 Arkose, siltstone 1 . Heart Formation ___ ~1.0

Orthoconglomerate.

Paraconglomerate. Headquarters Formation ~1.0

..r> -1.5 Deep Lake Formation _ >!.()

(upper part)

.25- .9 Arkose.

Suharkose.

.2 - .5 Siltstone.

Limestone.

.05- .2 Paraconglomerate.

.3 -2.0 (central part)

.2 -1.2 Subarkose 1

.0:5- .9 Siltstone.

.01- .2 Paraconglomerale.

0 -2.5

0 -1.0 Siltstone. (lower part)

0 - .3 Suharkose 1 .

II -1.2 Basalt 1

0 - .5 Subarkose/

Hurwitz Group (lower 1.0 -3.5

part).

Orthoquartzite.

Kinga Formation;

Varicolored, siltstone. Whiterock Member__ .2 - .3

Quartzite'. Ma?use Member .3 -1.0

Arkose.'

Siltstone. Padeli Formation ___ .3 - .5

Paraconglomerate.

Montgomery Lake 0 -l.fi

Group.

Quartzite.

Pebbly quartzite 1 .

Limestone and.

Paraconglomeratt1 .

yuartzite.

Subarkose/

Siltstone.

Paraconglomerate.

BasaK.

Basalt conglomerate.

Paraconglomerate.

Siltstone.

Subarkose/

Fine-grained orthoquartzite, white.

pink.

Orthoquartzite, subarkose. '

Paraconglomerate.

Siltstone.

Quartzite.

Subarkose/

Siltstone.

Subarkose/

Para- and orthoconglomerate.

Quart/Me.

Subarkose/

Acid volcamcs.

much uranium have been weathered and leached from 

outcrops. 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, and 

then by another paraconglomerate unit with limestone 

and siltstone, and finally both are capped by a thick 

tillite-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 produced by effects of sequential changes in climate in combination with epeirogenic pulses, it seems most unlikely 

that they could have been formed at different times.

AGE

The banket-bearing lower parts of the Huronian succession (Elliot Lake and Hough Groups) were deposited 

during an interval that began more than 2.35 aeons ago 

(Fairbairn and others, 1965) and ended no less than

2.29±.09 aeons ago (Fairbairn and others, 1969) according 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 is 

considered 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-rich 

rhyolite as well, but to a lesser extent. The Huronian 

rocks 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 contains the oldest reported black sands.

Banket-bearing strata unconformably overlie Archean 

rocks 2.5-3.0 aeons old, excepting the vertically dipping 

metastrata at Sakami Lake whose relation to adjacent
FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES W5

rocks, which are generally believed to be 2.45-2.75 

aeons old, has not been described. The basement in 

South Africa is at least 0.1 aeon older than basements 

beneath the Huronian, Deep Lake, and Padlei successions. 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 the 

Montgomery 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 Australian 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 are 

certainly more than 1.8 aeons old (Ferreira, 1972) and 

are most likely older than nearby sequences containing 

extensive banded iron-formations like those in 

Australia, North America, and South Africa, which are 

about 2.0 aeons old.

EARLY PROTEROZOIC SUCCESSIONS LACKING BANKETS

The 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 of 

basal 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 metamorphosed 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 than 

hematite and contain pyritic-or "yellow sand"-heavymineral accumulations (bankets) rather than black 

sands, whereas the reverse is true of younger arenites. 

The only cause that has been suggested for this is a 

change 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 we 

must examine the important early Proterozoic successions 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 successions including dolomite, shale, banded iron-formation,

quartzite, and volcanics deposited and deformed more 

than 1.8 aeons ago. Clastics with red beds and black 

sands are present in many of these successions, generally in upper parts but in some cases in middle or basal sections. 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 other 

successions with iron-formations, which collectively contain the bulk of the world's iron resources. Some extensive Aphebian successions 2.3-1.8 aeons old, however, 

lack significant iron-formations. Among these are 

volcanic-sedimentary strata in Manitoba believed to be 

about 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 unfavourable environmental factors in many cases, but it is 

difficult to identify these. It is possible that some "barren" successions were deposited before or after an optimum time period for deposition of banded ironformation, perhaps 2.1-2.0 aeons ago, as suggested by 

isotopic dates on volcanics associated with major ironformations (for example, Hamersley). Wherever adequate geological or isotopic evidence is available, the 

relatively common Aphebian rocks are found to be 

significantly younger than the banket-bearing successions. Important examples are given in succeeding 

paragraphs.

Huronian strata at Sudbury were folded, metamorphosed, intruded by Murray-Creighton granite 2.2 aeons 

ago (1.39 decay constant, Gibbons and others, 1972) and 

deeply eroded prior to deposition of the Whitewater 

Group, 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 and 

granitic 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 correlative 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 in 

South Africa unconformably overlies the banket-bearing
W6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 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 CONGLOMERATES W7

1

CQ UJ<

(X

EXPLANATION 

FOR A AND B

Glacial deposits

HURWITZ GROUP

AMETO FORMATION—Slate 

KINGA FORMATION—Quartzite

PADLEI FORMATION—Polymict

conglomerate 

MONTGOMERY LAKE GROUP

Quartzite, minor siltstone, 

conglomerate

Quartzite, feldspathic grit, with 

layers of pyritic quartz-pebble 

conglomerate (-)

Polymict ortho- and paraconglomerate

Quartzite, quartzose, medium grained; 

basal tuffaceous unit (v)

Felsic tuff

EXPLANATION 

FOR C

Feldspathic quartzite 

and grit

Quartzite

Siltstone

Polymict conglomerate

Fine-grained quartzite 

and siltstone

Pyritic quartz-pebble 

conglomerate

Grit layers

3A3 Diamond-drill holes

After Bell 1970, Young 1970, and Roscoe 1971

Gabbro

Granodiorite, tonalite

2

&XMetasedimentary rocks, graywacke, 

slate, iron-formation

Mafic volcanic rocks

Acid volcanic rocks

Strike and dip 

Fault line 

Diamond-drill holes

FIGURE 2.-Continued.
W8 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

groups. In Australia the Hamersley Group overlies the 

Fortesque Group and contains volcanics dated at 2.0 

aeons (Trendall, 1969).

DEPOSITIONAL ENVIRONMENTS OF BANKETS 

NEAR ELLIOT LAKE

LITHOSTRATIGRAPHY OF THE ELLIOT LAKE GROUP

This volume provides an opportunity to outline some 

previously unpublished concepts about the depositional 

environments of uranium-ore-bearing units near Elliot 

Lake. Data from deep exploratory drill holes west of the 

mines, in particular, have shed new light on the history 

of deposition of the Elliot Lake Group, strengthening 

the conclusion that the rocks were deposited subaerially. 

The data cannot be outlined in detail here, nor is it possible to repeat stratigraphic and petrographic descriptions that may be found elsewhere. A brief outline of 

stratigraphic relations is presented, however, to set 

descriptions of relations in the Elliot Lake area in 

perspective.

The uranium ores and principal prospects in the Huronian belt are in the Matinenda Formation of the Elliot 

Lake Group (table 1); pyritic uraniferous conglomerate 

layers are also present, sparingly, in the Livingstone 

Creek Formation, which is in the lower part of the Elliot 

Lake Group west of Elliot Lake, and within sequences of 

basalt flows in the Elliot Lake Group. The Mississagi 

Formation of the overlying Hough Group also contains 

pyritic uraniferous pebbly layers. The two banketbearing groups extend through a distance of some 200 

miles (320 km) in the Sault Ste. Marie-Sudbury region, 

and their combined thickness is as great as 20,000 ft 

(6,100 m) in the southeastern part of the belt.

The Elliot Lake Group includes all strata that nonconformably overlie Archean rocks and underlie the 

regionally extant Ramsay Lake Formation, a paraconglomerate that forms the basal unit of the Hough Lake 

Group. Major units of the Elliot Lake Group - volcanic 

rocks, coarse-grained subarkose and argillaceous 

rocks - are lenticular. Many of the outcrop areas of the 

group are separated, one from the other, so that it is not 

possible to establish the sequence of deposition of all 

volcanics and arenites throughout the belt.

The McKim Formation, comprising fine-grained 

argillaceous quartzite, siltstone, and argillite, forms the 

uppermost unit of the Elliot Lake Group in the eastern 

part of the belt near Sudbury. Here it is several thousand feet thick, and it overlies several thousand feet of 

volcanic rocks, including the Stobie Formation-basalt 

with some intercalated sediments - and the less extensive Coppercliff Rhyolite Formation. Subarkose called 

the Matinenda Formation (Roscoe, 1957a, b) forms most 

or all of the group in some central parts of the belt near

Blind River, Elliot Lake, and Quirke Lake, but sequences in flanking areas include basalt flows. A tongue 

of McKim Formation extends westward into this area 

over some parts of the Matinenda Formation, and a 

wedge of Matinenda Formation extends easterly 

towards Sudbury between the volcanics and the McKim 

Formation. In the western part of the belt, basalt flows 

of the Thessalon Formation overlie subarkose of the Livingstone Creek Formation. The Elliot Lake rocks are 

bevelled by an erosion surface and are overlain by the 

Ramsay Lake Formation, which is considered to be a 

tillite.

MATINENDA FORMATION

In the Elliot Lake area the Matinenda Formation has 

been subdivided into three coarse-grained subarkose 

units, successively the Ryan, Stinson, and Manfred 

Members (Roscoe, 1969). A fourth lenticular subarkose 

unit, the Keelor Member, has been identified 15-20 

miles (23-32 km) west of Quirke Lake (figs. 3, 4). The 

Ryan, Manfred, and Keelor Members contain abundant 

pyritic, radioactive quartz-pebble beds and are gritty, 

variegated, greenish coloured, and pebbly throughout. 

The Stinson Member is pale grey, relatively well sorted, 

and contains conglomerate beds only at its base. These 

beds include polymict, as well as oligomict, orthoconglomerates and they contain little pyrite or radioactivity. Some of the conglomerates, evidently 

deposited as open-framework cobble gravels, contain 

authigenic quartz, carbonate, pyrrhotite, and pyrite. 

The subarkose units are built of crossbeds that filled 

troughs, generally 1-4 ft (0.3-1.2 m) wide. Trends of 

troughs and dips of foresets are unimodally southeast in 

the eastern part of the area but are more southerly in 

outcrops 15 miles (24 km) west of Elliot Lake.

Basalt flows in the Elliot Lake Group west of Quirke 

Lake have been called Dollyberry Lake volcanics 

(Robertson, 1969). They are believed to have been 

erupted after deposition of the Ryan Member but prior 

to deposition of other Matinenda members. If the 

general practice of assigning formational rank to 

volcanic units in the Elliot Lake Group, and of excluding 

nonarenaceous rocks from the Matinenda Formation, 

were to be followed, it would create some confusion in 

discussions. For this reason, the Dollyberry Lake 

volcanic unit is referred to herein as "Dollyberry basalt" 

and treated, informally, as a member of the Matinenda 

Formation. Two other significant units are present in 

the area, but neither is along the line of section shown in 

figure 3. One is a wedge of volcanic rocks overlain by the 

Ryan Member east of the Nordic ore zone. Robertson 

(1961) mapped this wedge as questionable Archean but 

later (Robertson, 1973) he suggested that it is probably 

part of the Elliot Lake Group. The other additional unit
FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES W9

EXPLANATION

-4— Depth (feetx 1000) 

to base of Ramsay 

Lake Formation Quirke Lake 

ore zone 

D

10

I

20 MILES

A

Manfred Member

Ramsay Lake Formation

Keelor Member

Archean

granitic rocks,

meta strata

Ryan Member

L-1000

0

I I 1 1

5

i 1

10 MILES

I

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 limb 

of 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 erratically 

uraniferous, poorly sorted, pyritic oligomict conglomerate and overlain by Dollyberry basalt.

CONGLOMERATIC ZONES

The uranium-bearing conglomeratic zones in the 

Ryan, 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 CONGLOMERATES

N

10 MILES

FIGURE 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 5a 

and 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 be 

traced great distances and define stratigraphic levels 

within the zones, perhaps better than the pebble-rich 

sections, 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 and 

foreset dips-southeasterly in the Ryan and Manfred 

Members. Cross laminae are commonly graded, with 

pebbles at their bases. In some cases, pebbles evidently 

rolled down successive foresets to form a basal zone of 

pebbles 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 cross 

laminae (Pienaar, 1963). It seems likely that these represent the lower parts of troughs whose upper, more 

arenaceous, parts were planed off. The richest reef sections, containing the greatest concentrations of pyrite 

and other heavy-mineral grains, probably represent successions of such remnants of basal pebble-rich parts of 

troughs.

Pebbles are generally smaller in successively higher 

reefs and in each reef become progressively smaller in 

the downstream direction. In reefs mined at Quirke 

Lake they decrease from 5 cm or more in the most proximal 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) in 

diameter are restricted to proximal conglomerates with 

pebbles larger than 3 cm. Other heavy minerals, mainly 

titaniferous grains, monazite, and zircon, constitute less 

than 5 percent of total heavy minerals in the most proximal, largest pebble conglomerates but increase in progressively more distal conglomerates to more than 15 

percent. Titaniferous grains are as much as 1.5 mm in 

diameter 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 in 

diameter, are most abundant in yet more distal, sub-
FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES Wll

marginal parts of the zone and in uppermost smallpebble conglomerate beds. Granules and smaller particles of quartz, potash feldspar, and mica in matrices of 

large-pebble, as well as small-pebble, conglomerates are 

notably poorly sorted in contrast to pebbles and heavy 

minerals.

The conglomerate zones represent sites of remarkably 

efficient hydrodynamic sorting, wherein the largest and 

heaviest 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 conglomerate lenses were likely formed in part from reworked 

detritus initially deposited upstream as pebbly grit, and 

much fine detritus was undoubtedly winnowed out of the 

conglomeratic zone as it was built up. Nevertheless, the 

conglomerates do not represent lag gravels in any strict 

sense. All size ranges of detritus were transported and 

deposited together. The tremendous overall abundance 

of pebbles and heavy minerals in the zones must have 

been produced by concentration of these materials 

throughout prolonged transport.

Proportions of gravel to sand carried into the depositional area fluctuated and diminished as the conglomeratic zones were built up. Initially, floods carrying 

abundant coarse gravel alternated with floods carrying 

only small pebbles in addition to abundant sand and 

finer 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 important 

parts of all zones explored by drill holes. The Nordic 

(Ryan) zone rests directly on basement rocks and appears to have been deposited along a valley controlled by 

basement topography and perhaps also by syndepositional faulting. The small Pronto deposit 15 miles (24 

km) south of Elliot Lake is probably a remnant of a 

similar "valley" deposit. The much larger Quirke Lake 

(Manfred) zone heads atop Archean rocks and the 

eastern 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 fans 

where 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

•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 indicated by foreset dips. It is bounded to the east by a 

paleoridge 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 the 

downstream 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 with 

sizes of pebbles in the Nordic zone as they do in the 

Quirke Lake zone, but the patterns of pebble-size 

changes 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 from 

the central axis of the zone. Ore throughout the length 

of the chutelike zone is like that found through a much 

broader, 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 in 

titania, 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 have 

changed 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 development of fault scarps, as well as the lava fields or 

volcanoes themselves, likely had important influences 

on drainage systems and on locations of sites of gravel 

deposition. The distribution of the lavas provides clues 

concerning topography both prior and subsequent to 

their 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 important 

bankets at Elliot Lake. Moreover, bankets were 

deposited in Hough Group rocks long after the last eruptions occurred in Huronian rocks.

The lavas are amygdaloidal, and closely spaced 

scoriaceous and brecciated zones suggest that many 

flows are thin. No pillows have been noted. The rocks 

are dark grey to dark green for the most part, but 

bleached and argillized zones are found in many 

scoriaceous and brecciated sections, presumably flow 

tops. This alteration is similar to that found in 

greenstone at the Archean-Huronian nonconformity 

and to that found rimming greenstone and basalt clasts 

within Huronian strata. It is considered to represent interflow 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 the 

Summers 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 the 

northernmost known limit of the Ryan Member and the 

Nordic 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 thin 

southward 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 and 

Keelor 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) of 

basalt breccia or sharp-stone conglomerate grade up into basalt- and quartz-cobble conglomerate. The conglomerate matrix contains quartz and carbonate, as well 

as granules and argillitic detritus. It also contains concentrations of pyrite or pyrrhotite with traces of 

chalcopyrite not accompanied by radioactive minerals. 

The sulphides are clearly authigenic and are most commonly found at the top of the conglomerate or in 

separate beds a few feet above it within Stinson 

subarkose. Many basalt clasts are variably argillized. In

places, conglomerate with intensely altered clasts was 

covered 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 oxide, is concentrated at these obviously weathered surfaces and that uranium is not concentrated along with 

sulphides, 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 and 

the paleosol. These sediments, and the thick Crazy Lake 

arkose, are likely remnants of alluvial deposits correlative 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 this 

member is not well defined; it is largely an erosional 

rather than a depositional limit. Evident post-Dollyberry 

erosion 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 an 

easterly-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, and 

volcanic 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 a 

valley 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 a 

chutelike central section of the valley. Subsequently, 

coarse sand and fine gravel were deposited over a 

broader area.

Uplift to the north cut off the supply of sand and 

resulted in scouring of Ryan sand and gravel from the
FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES W13

northern part of the valley. This was followed by eruptions 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 accompanied by washing, or continued washing, of 

petromict gravels from nearby Dollyberry and Archean 

uplands onto the scoured and locally channelled surface 

of the Ryan Member, but little gravel was carried from 

the main provenance. Deposits of coarse, sorted Stinson 

sands were built up, culminating in development of a 

smooth 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 spread 

out 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 were 

carried 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 sediments 

of the McKim Formation were deposited atop the Stinson Member where it overlies the Ryan Member. The 

lack of recognized Manfred sands in this area is 

noteworthy (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 sand 

and fine gravel may have been eroded off the Stinson 

surface here, or-least likely - Manfred subarkose with 

beds 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 rifted 

zone 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, and 

bevelled 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

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 with 

positive relief.

This conceptual model will no doubt require important 

modifications. Nevertheless, it serves to dramatize some 

critical limitations that can be imposed on choices of 

possible ways that the nonoxidized uranium deposits, 

sediments, and paleosols could have been deposited and 

modified. The depositional environment could not have 

been more different from environments of deposition of 

younger 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 continuing 

even after their final burial beneath channels of swiftly 

flowing streams. Any organisms that might have been 

present 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 oxygen from all aerated surface and ground waters. 

Neither were they formed in an extremely unusual tectonic environment that has not recurred on a significant 

scale since early Proterozoic time. Clearly, the 

widespread paleosols were formed and the sediments 

were deposited subaerially under an atmosphere that 

contained no significant amount of free oxygen.

COMPARISON OF OLDER AND YOUNGER 

TERRIGENOUS SEDIMENTS

Extreme proximal depositional environments like 

those 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 environments 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. Obviously, 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 not 

provided evidence to the contrary.

Weakly radioactive layers with small pebbles are 

found 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. Pebble beds atop the upward-coarsening transition zone between Pecors argillite and Mississagi subarkose may 

have been deposited in topset beds of a prograding 

delta. Conglomerate beds interlayered with Huronian 

basalt flows represent yet another depositional environment.

Other variations amongst banket associations include 

differences in composition of detritus other than pebbles. Sand in conglomerate matrices and in associated 

beds range from protoquartzite to arkose (for example, 

locally in Ryan Member) and to subgreywacke (for example, 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 terrestrial environments are characterized by red beds, and 

they contain detrital heavy-mineral suites dominated by 

iron oxides. Concentrations of heavy minerals are black 

sands 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 that 

have been subjected to all sorts and degrees of alterations and metamorphic processes. No explanation more 

plausible than oxyatmoversion has been offered for the 

time-bounded character of the two suites of terrestrial 

sedimentary rocks.

The concept that bankets are heavy-mineral concentrations 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 the 

bankets. 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 have 

originated as iron oxides and iron-titanium oxides in Archean rocks in addition to larger amounts that 

originated as pyrite, and some was formed authigenically within the bankets (Roscoe, 1969). The scarcity of iron 

oxides compared to pyrite in paleosols and the lack of 

coarsely bladed integrowths of pyrite and anatase in the 

bankets suggest that iron oxides were sulphidized in the 

soils and that ilmenite-bearing grains were altered 

before rather than after deposition of the bankets. The 

excess sulphur required for the pervasive sulphidization 

must have been of atmospheric origin and, unless 

unreasonably high concentrations of hydrogen sulphide 

are postulated in the paleoatmosphere, anaerobic 

bacteria must have played an important role in the 

transformations. Some of the uranium minerals also 

might have been formed originally in the soils, but this 

seems unlikely in the case of uraninite that contains 

thorium and rare earths in amounts similar to those in 

uraninites in igneous rocks.

REGIONAL TECTONIC SETTINGS OF BANKETS

The depositional and erosional features described 

above reflect tectonic movements in combination with 

climatic changes and volcanic eruptions. Matinenda 

alluvium was likely collected and transported southerly 

over an extensive land surface following, perhaps, a 

slight increase in regional slope. We see in the formation 

only the coarsest fraction of this weathered detritusmainly the largest quartz grains that were present in 

eroded Archean granitoid rocks. These, along with 

heavy minerals and pebbles of vein quartz, were 

deposited in some sort of tectonic trap likely formed 

along ruptures. The bulk of material eroded from the 

uplifted continental core, consisting of fine-grained 

siliceous, feldspathic and argillaceous detritus, and 

dissolved 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 attendant on glacial loadings and unloadings superimposed on more general uplift of a central part of the 

Archean craton. The uplifts may have initiated the 

glacierizations marked by tillites and by some polymict 

orthoconglomerate and arkose, but the preponderance 

of submature and even supermature elastics - mainly 

coarse 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 W15

These were likely deposited in large lakes and shortlived 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 well 

to 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 sediment 

transport 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 remaining areas where Archean rocks were metamorphosed or 

remetamorphosed about 1.8 aeons ago (Hudsonian and 

Penokean orogenies). Intervening areas were likely also 

cratonized in Archean time. Moreover, Archean 

basements to Huronian and Medicine Bow strata must 

have 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 represent rifts wherein some Aphebian rocks were deposited 

on oceanic rather than continental crust.

Crossbed dips, southeasterly in Huronian quartzites, 

southwesterly in Deep Lake quartzites and westerly in 

Montgomery Lake quartzites, suggest a drainage divide 

or uplift axis extending north- to northeastward through 

western Ontario. The North American banket-bearing 

belts trend northeast, excepting the western part of the 

arcuate 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 very 

much more extensive originally, and there were probably 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 were 

limited and submountainous as suggested here, differential uplift and erosion must have been very considerable 

and occurred through a prolonged period.

DISCUSSION

The proposition that bankets were deposited 

subaerially under an anoxic atmosphere is susceptible to 

the criticism that some of the evidence for this is

negative. For instance, the observation that banketbearing pyritic quartzites and black-sand-bearing red 

beds are mutually exclusive and separated in the geological 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 been 

shown to have diagnostic features shared by all the ancient bankets but not found in indisputably epigenetic 

deposits. Most allegedly banketlike occurrences of 

pyrite 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 metamorphosed" Permian conglomerates "locally mineralized 

with uraninite, pyrite, and other sulphides" and Witwatersrand ores. In this, he ignored the facts that ancient bankets are all found through great successions of 

strata, 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 to 

recognize as epigenetic.

Doubts about an early Proterozoic oxyatmoversion 

stem most commonly from similarities between Archean 

and younger chemical and biogenic sediments. Carbonaceous material in older rocks is considered evidence that 

oxygen 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 oxidizing 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 oxygen in the atmosphere must have been dependent on 

evolution of photosynthetic organisms, initially in an 

anoxic environment. Photosynthesis and oxidation of 

organic 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 produced, reactions can be summarized as

There is no requirement that the atmosphere contain oxygen, although if oxygen-tolerant organisms are involved, this would enhance the process.

Red beds and black sands in the Cobalt Group and 

other Paleoaphebian strata indicate that free oxygen 

began to accumulate in the atmosphere not synchronously 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 have 

considered the probable course of atmospheric evolution. It does not damage their hypotheses in any important way, but it may place constraints on models for 

deposition of the early Proterozoic iron-formations. 

Another constraint, dealt with most effectively by 

Holland (1973), is a requirement for an enormous supply 

of iron to some sites such as the Hamersley Basin. Taking some liberties, perhaps, in applying and extending 

the model he has suggested, we can postulate the sequence of postoxyatmoversion events outlined in the 

following paragraphs.

Prolonged deep preoxyatmoversion erosion of continents supplied abundant ferrous iron, along with other 

cations 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 to 

accumulate in the atmosphere. Deposition of thick, extensive iron-formations in special environments and 

more 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 past 

shallows with extremely high levels of photobiotic activity. 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 occurrences in Elliot Lake ores perhaps reflects some 

degree 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 to 

recognize underground. Globules and sooty incrustations 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 feet 

above 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 in 

course gravels in rapids.

Mineralographic evidence and other specific evidence 

for detrital origin of uraninite and pyrite grains in 

bankets 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 be 

generally regarded as the only direct evidence that the 

atmosphere was anoxic until the beginning of Proterozoic time, although other evidence has been 

presented (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 research 

leading to a better understanding and more precise 

dating of this transition.

REFERENCES CITED

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Institute on Lake Superior Geology, Abstracts and Field Guides, 

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Bayley, R. W., and James, H. L., 1973: Precambrian iron-formations 

of 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 banded 

iron-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 Association 

of 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 Supergroup 

north of Lake Huron, in Baer, A. J., ed., Symposium on basins and 

geosynclines 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, Genesis 

of Precambrian iron-formations and the development of atmospheric oxygen: Economic Geology, v. 68, p. 1173-1179.
FACTORS AFFECTING DEPOSITION OF URANIFEROUS CONGLOMERATES W17

Gibbins, W. A., Adams, C. J., and McNutt, R. H., 1972, Rb-Sr isotopic 

studies of the Murray Granite: Geological Association of Canada 

Special Paper 10, p. 61-66.

Hiemstra, S. A., 1968, The mineraology and petrology of the 

uraniferous 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 in 

that 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 of 

Technology, p. 129-139.

Liebenberg, W. R., 1955, The occurrence and origin of gold and 

radioactive minerals in the Witwatersrand System, the Dominion 

Reef, the Ventersdorp Contact Reef and the Black Reef [with 

discussion]: 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.

Ramdohr, Paul, 1958, New observations on the ores of the Witwatersrand in South Africa and their genetic significance: Geological 

Society 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 of 

Mines, Preliminary Geological Map P.561.

——-1971, Private report.

-1973, A review of recently acquired geological data, Blind

River-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, Blind 

River, 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 Association 

of Canada Special Paper 12, p. 31-47.

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early atmosphere: Geological Society of America Special Paper 62, 

p. 631.

Ruzicka, V., 1971, Geological comparison between East European and 

Canadian uranium deposits: Canada Geological Survey Paper 

70-48, 196 p.

Theis, N. J., 1976, Uranium-bearing and associated minerals in their 

geochemical and sedimentological context, Elliot Lake, Ontario: 

Queen's University, Kingston, Ontario, unpublished Ph.D. thesis.

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79, p. 1527-1544.

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Mines area, Ontario, Canada: Journal of Geology, v. 73, p. 

755-780.

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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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-X

CONTENTS

	Page

Origin of the ore ____________________________________________ XI

Provenance _____________________________________________ 2

Sedimentation __________________________________________ 3

Effect of lavas __________________________________________ 3

Anoxygenic atmosphere _____________________________________ 3

Mineralogy and zoning ______________________________________ 4

Climate _______________________________________________ 5

Papaskwasati Basin __________________________________________ 5

Sakami Lake ______________________________________________ 5

Montgomery Lake __________________________________________ 5

Deposits in post-early Aphebian quartz-pebble conglomerates __________________ 5

Effects of metamorphism on Huronian rocks in the Grenville Province______________ 9

References cited ____ _________________________________————__ 10

Discussion following papers by Drs. Robertson, Ruzicka, Roscoe, and Little __________ 10

ILLUSTRATIONS

FIGURES 1-5. Maps showing:

1. Location of uraniferous quartz-pebble conglomerates in Canada_______________________—___— X2

2. Basement topography beneath the Papaskwasati Formation _______________________________ 6

3. Isopachs on the basal quartz-pebble conglomerate of the Papaskwasati Formation __________________ 7

4. Distribution of Hurwitz Group and associated rocks ____________________________________ 8

5. Potential areas for uraniferous quartz-pebble conglomerate deposits __________________________ 9

in

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

GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE

CONGLOMERATE ORES

By H. W. LITTLE1

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 Aphebian 

conglomerates (fig. 1). The principal controversy was 

between the syngeneticists and the hydrothermalists, 

the latter being greatly in the minority. Their main 

arguments centered on permeability of the conglomerate, concentration of heavy minerals at the base 

of the conglomerate, rounding of uranium minerals, 

whether due to erosion or colloidal precipitation, and 

thorium: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 metasomatic 

effects are greatest in the Pronto and Can-Met Mines in 

the 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 mineralization. Heinrich (1975) further emphasized the effects of 

metasomatic alteration there by classifying the types of 

alteration and their effects upon the ore and associated 

rocks.

At the present time, those geologists who have worked most closely with the deposits seem to be unanimous 

in the opinion that the ores are syngenetic. Some geologists 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 this 

regard. 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 granitic or metamorphic terranes. It is therefore difficult to 

arrive at a conclusion other than that the Agnew Lake 

quartz-pebble conglomerate uranium ores are of placer 

origin, for the metamorphism there is dynamic and the 

granites are older than the conglomerate. Indeed, the 

unconformity 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 concentrated at the base of conglomerate layers, where one appeared to be pressed down by a quartz pebble into the 

argillite 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 and 

Ruud (1971), who supported Derry (1960). Ferris and 

Ruud did not believe that leached brannerite derived 

from crystalline brannerite would form an internal network 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 observations was reported by J. A. Robertson to Roscoe (1969) 

and later published by himself (Robertson 1970), that in 

places where the polymictic Ramsay Lake Conglomerate truncates the ore-bearing conglomerate it is 

uraniferous. D. S. Robertson and Douglas (1970) commented at greater length upon this feature, implying 

that the uranium content is found only down the 

paleoslope and comprises radioactive debris from the 

erosion 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 CONGLOMERATES

EXPLANATION

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 in 

that, 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 the 

sediments. Roscoe (1969) has suggested that erosion of a 

greenstone 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 the 

ore-bearing sediments, concluded that the uranium content of the quartz monzonite was above normal and that

the thorium :uranium ratios were similar to those in the 

ores. 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 that 

the area sampled is the same area of provenance that existed 2,500 m.y. (million years) ago; in the writer's opinion, the provenance area must have been farther northwest and somewhat higher than the present erosion surface.
GENESIS OF CANADIAN URANIFEROUS QUARTZ-PEBBLE CONGLOMERATE ORES X3

SEDIMENTATION

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 of 

the succession. At Agnew Lake a northerly retreating 

shoreline has not been demonstrated. During the early 

stages 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 conglomerate 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 northern 

part of the Huronian geosyncline, deposition was nonmarine or in shallow epicontinental seas, whereas to the 

south 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 the 

following terms: "It is suggested that (paleocurrent) 

data are best interpreted as evidence for eastbound 

longshore currents in this basin of deposition rather 

than as the product of river channels running into a 

basin. In this environment, lag deposits of scoured pebble and conglomerate beds would form over or on the 

southwestern and western flanks of stratigraphically 

higher structures." They go on to suggest that in prospecting for deeply buried uranium deposits the possibility of finding point-bar or depositional types of placers at 

ancient channel mouths should be considered.

With a view to establishing more coordinated data, the 

Uranium 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 produced to aid in the prediction of the location of undiscovered 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 the

Spragge area, and to the east in the Sudbury area. 

Roscoe suggested that the volcanics would have a 

marked 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) at 

several 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 and 

Christ (1965) from experimental data showed that 

magnetite derived from erosion of the Archean basement would have been converted to pyrite in the low Eh 

environment of an anoxygenic atmosphere and in a 

hydrogen-sulphide-rich environment. The discovery of 

the Dollyberry volcanics by J. A. Robertson (1969) in 

Township 157, west of the Quirke ore zone, interbedded 

with Matinenda strata, indicated a rich source of 

hydrogen sulphide. In a written communication, T. J. 

Bottrill suggested to the writer in 1970 that this could be 

a 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; in 

Papaswasati 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, deepening 

of 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 in 

Precambrian 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 appeared 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 the 

argument for an anoxygenic atmosphere during early 

Huronian time. Briefly, they state that it is generally 

believed that an anoxygenic atmosphere existed and 

that, according to Preston Cloud, there is reason to 

believe that life initially developed at this time. From a 

study 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 the 

presence of abundant pyrite in the conglomerate of the 

Matinenda Formation all indicate that nonoxidizing conditions existed. The presence of detrital uraninite and 

pyrite 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 and 

presence of detrital pyrite do not necessarily indicate an 

oxygen-free atmosphere, for in the Appalachians all preSilurian Paleozoic sediments are drab and detrital pyrite 

of 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), who 

pointed 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 from 

the land was lowered about 2,050 m.y. ago, for the first 

time 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, his 

time scale does not agree with that of other writers, for 

he 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 confirmation 

of 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 at 

2,600 m.y. (Rao, 1975). Pyrite is reported to be present, 

but the quantity is not given; other minerals present are 

chalcopyrite, 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, and 

suggested that shallow-water sedimentation was influenced by an anoxygenic atmosphere, whereas in

deeper waters biologic photosynthesis by anaerobic 

organisms may have been taking place. Myers contended that the preservation of gold in a fossil placer deposit, 

like uraninite, would require an atmosphere of low oxygen concentration, particularly where the particle size 

is less than 40 ju,m, which size particles are subject to 

rapid 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 most 

abundant sulphide, ranging from 5 to 10 percent of the 

ore. In the mines the abundance of pyrite is generally 

thought to be in direct proportion to the grade of the 

ore, but Roscoe stated that although pyrite is generally 

more abundant in highly radioactive conglomerates, 

there is no positive correlation between pyrite and 

uranium 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 such 

minerals have been identified. On the other hand, at 

Agnew Lake, in addition to pyrite, only uranothorite 

and monazite have been reported.

Studies of variations in the Th02 :U308 ratios have 

been 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 the 

ratios varied from 1:1 to 1:30; in another reef they 

ranged 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) instead 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 X5

CLIMATE

Roscoe (1969) suggested that another environmental 

factor in the deposition and preservation of uraniferous 

placers in the lower Huronian succession was that of 

climate. He gave evidence that glacial conditions 

prevailed not only during Gowganda time but also in 

earlier times, particularly during deposition of the Ramsay Lake and Bruce Formations. Other writers have 

supported a tilloid and glaciolacustrine origin for these 

formations. 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 a 

factor in the concentration of uranium, as the paper by 

Grandstaff 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 have 

gone on contemporaneously. Sediments, including a 

basal quartz-pebble conglomerate, were deposited in a 

downfaulted basin that has been little deformed since 

the time of deposition (fig. 2). There were, however, no 

lavas 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 in 

thickness 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 abnormally 

rich 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 U30 8 , 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 schist 

that probably are metamorphosed lava. The pebbles are 

all of quartz, and most are less than 1 in. (2.5 cm) in 

diameter but are much stretched along the direction of 

the strike of the beds. Pyrite is quite abundant in the

matrix, and is commonly in cubic form. In general, 

radioactivity seems roughly proportional to the pyrite 

content. Once again, the lavas seem a likely source for 

the 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 has 

remarked that the succession is remarkably similar to 

that 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 occurred 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 found 

there, perhaps reconstituted.

DEPOSITS IN POST-EARLY APHEBIAN 

QUARTZ-PEBBLE CONGLOMERATES

The uraniferous deposits in the Cambrian Lake area 

(fig. 1), northern Quebec, have been known for several 

years. Quartz-pebble conglomerate intercalated with 

arkosic rocks has been traced along the strike for 

several miles. This formation lies at the base of the 

Kaniapiskau Supergroup on the west side of the 

Labrador 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 Mackenzie 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 the 

south 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 upward and possibly outward from the contact to arkoses containing the 

odd 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 much 

as 10 feet across, that have been only slightly moved from their
X6 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

EXPLANATION

Approximate outline of basin

-1000-

Elevation 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

51°30"N

EXPLANATION

Approximate outline of basin

50

Isopachs of the basal conglomerate 

25-ft intervals

Diamond-drill hole (vertical)

FIGURE 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 turn 

grades 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 in 

several places along the west contact of the sediments, but in large 

part the relationships on the west contact so far mapped are obscured 

by intense shearing, crushing and brecciation related to faulting in the 

contact 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 of 

arkosic sand. Lenses and beds of arkose are common, and the transition to the overlying arkoses is marked simply by increasing amounts 

of arkose. The arkoses are buff, pink, or light grey rocks, which in 

places 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 very 

large boulders of white quartzite are common. This unit is overlain by 

arkose.

The Nonacho Group is regarded as Helikian in age and 

so is younger than other successions that contain

uraniferous 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 and 

are faintly radioactive. This is succeeded by a 

nonradioactive bed of conglomerate and some 200 ft (60 

m) of feldspathic quartzite. This, in turn, is overlain by 

paraconglomerate 40-50 ft (12-15 m) thick and a zone of 

interbedded quartz-pebble conglomerate and feldspathic 

quartzite some 30 ft (9 m) thick.

The feldspathic conglomerates are as much as 4 ft (1.2 

m) in thickness and contain magnetite and minor pyrite. 

This zone is overlain by greenish brown argillite that in 

places contains malachite. Uranium is most abundant in 

the zone of quartz-pebble conglomerates, and, from a 

number of samples, it has been said to average close to 1 

Ib per short ton (0.5 kg per metric ton) (0.05 percent 

U3 08) 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 the 

conglomerates, although both have been found in the 

radioactive 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 X9

has any mineralogical work been done on the 

uraniferous zone. But this deposit is of more than casual 

interest to this group because it is of much higher grade 

than quartz-pebble conglomerates that are younger than 

early Aphebian age elsewhere.

In summary, I would like to propose that in searching 

for 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 and 

replacement-type deposits do occur in such an environment.

From a consideration of the criteria outlined in this 

discussion a map of Canada has been drawn that indicates areas in which additional quartz-pebble uranium 

deposits may be discovered (fig. 5).

EFFECTS OF METAMORPHISM ON HURONIAN 

ROCKS IN THE GRENVILLE PROVINCE

Finally, we might consider the effects of high-grade 

metamorphism and ultrametamorphism. Within the 

assemblage of paragneisses, amphibolites, schists, 

migmatites, and granitic rocks that compose the Grenville Province and other metamorphic terranes, beds of 

quartzite and quartz-pebble conglomerate, some of 

which 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 completing 

the full cycle by forming material that is generally 

regarded as typical of the area of provenance of the conEXPLAN 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 the

Canadian 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 of 

ultrametamorphism 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 in 

metamorphic terranes, therefore, could be a useful 

guide in tracking down these possible byproducts of 

uraniferous quartz-pebble conglomerates.

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uranium deposits, in Formation of uranium ore 

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6-10 May, 1974: Vienna, International Atomic Energy Agency, p. 

625-635!

Bain, G. W., 1972, An oxygenated atmosphere for early life: International 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: 

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Ferris, C. S., and Ruud, C. 0., 1971, Brannerite: Its occurrences and 

recognition by microprobe: Colorado School of Mines Quarterly, v. 

66, no. 4, p. 21-35.

Frarey, M. J., and Roscoe, S. M., 1970, The Huronian Supergroup 

north 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 and 

equilibria: New York, Harper and Row.

Heinrich, E. W., 1975, Hydrothermal alteration of the Mississagi conglomeratic quartzite, Pronto Mine, Blind River, Ontario (abs.): Institute 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 and 

radioactive 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 and 

Metallurgy 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 the 

Rand Goldfield, South Africa: U.S. Geological Survey Open-File 

Report 238.

Pettijohn, F. J., 1970, The Canadian Shield, in Symposium on basins 

and 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 International 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 of 

Canada: 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 of 

Mines Preliminary Geologic Map P.561. 

-1970, Geology of the Spragge area: Ontario Department of

Mines 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 Uses 

of 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 history 

of the Earth: International Geological Congress, 24th, Resumes, 

no. 24, p. 264-269.

Wanless, R. K., and Eade, K. E., 1975, Geochronology of Archeanand 

Proterozoic 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 presentation. I was really puzzled to see that, for instance, the 

pyrite 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 Xll

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 explain the uraninite particles within the carbon as having precipitated from soluble uranium compounds carried into the basin?

Ruzicka: No,

Schidlowski: No?

Ruzicka: Because of the very high thorium content in 

these 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 that 

the association of carbonaceous matter with uranium 

minerals and gold is a very important association. I'd 

like 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 half 

billion 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 conditions, then the decomposition decay product which is 

going 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 columnar structure which surrounds the uranium grains? 

Could these not be simply polymerization products by 

radiation 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 and 

evidently it has moved.

Schidlowski: I would principally concur to this statement.

Nagy: In other words, you pretty much agree with what 

I am saying that the hydrocarbon could have come 

from biological material.

Schidlowski: Yes, the decomposition of organic material 

in the widest sense. So since we had only bacteria and 

blue-greens, these should have constituted the probable source.

Nagy: I'm glad you used that word "organic."

Grandstaff: Dr. Ruzicka, I wonder if you could clarify a 

point on the extinction of the organic matter. I was 

wondering if the extinction bends around the pebble, 

where the pebble passes down into the underlying 

layer? Bending or disruption of the extinction might 

suggest 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 specific 

locality beside the pebble?

Ruzicka: Yes. I mentioned that this sample was taken 

from 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 a 

necessary 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 above 

ores in the Mississagi Formation? I might add, too, 

that I have looked at Aphebian conglomerates near 

volcanic rocks, like at Flin Flon, Wilson Island 

[Nairn?], and there again they are hematitic and high 

thorium.

Ruzicka: I think you must have uraninite present and 

afterwards the conclusion is drawn that it is related to 

the volcanic activity. It is practically a deduction of the 

observation. 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 a 

polymerization product, then it should be plain 

hydrocarbon, without any inorganic inclusions, but all 

the carbonaceous material from Blind River, as well as 

from South Africa, includes filamentous inorganic 

material which cannot be explained by just 

polymerization. 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 constitute 

redeposited material in the widest sense, so they just 

occupied 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 carbonaceous 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 a 

diameter 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 occurs in voids near the Carbon Leader Reef where it 

was heated and was redeposited. That is real 

polymerized carbonaceous matter which does not contain any inorganic material whatsoever. You can heat 

it, 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 be 

able 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 questions 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 the 

stopes that I have seen, the texture of the carbonaceous material is variable inch by inch. You can 

see repeated gradations between fly-speck carbon and 

individual granules which presumably contain remnants of detrital uraninite. They get more and more 

concentrated, and finally the point is reached where 

you get a silicate-rich thucholite, mechanically incorporating a bunch of the matrix; then this goes in 

another half inch to beautifully columnar stuff, and 

then 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 the 

contact between the top of the Dollyberry basalt and 

the 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 the 

repeated energy events which caused recycling of the 

material 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 is 

the alteration and introduction of uranium remobilized 

from other portions of the conglomerate around certain diabase dikes. For example, in the vicinity of 

these dikes you find uranium enriched as much as ten

times, and uranium content decreases with the distance from the dikes. But not all of these dikes have 

such an effect. Second, I would say that the influence 

of faults, for example, is not much in the Elliot Lake 

deposit.

Robertson: I would like to comment on the last two questions, first of all, on the Dollyberry: The structures of 

the volcanics suggests that they were in fact probably 

subaerial rather than subaqueous. As far as breaks in 

the 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't 

seem to have that. We have small-scale crossbedding 

and so on, and obviously coalescence of beds, but we 

don't have the major reworking that you seem to have 

on the Rand. The other comment I would like to make 

is something that I was supposed to say and didn't. 

There is in fact very little gold in the Blind River 

deposits. I would suggest that this is because in the 

hinterland 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 than 

those at Blind River (we don't have a good correlation 

on them), there is a certain amount of gold along with 

uraninite in some beds. We don't really know whether 

it is free gold or whether it's gold in sulfides, and in 

that 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 such 

as 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 source 

area and the concentration effected during the transportation 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 of 

heavy 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 conglomerates in Ghana which more or less fit in with this suite 

of conglomerates that we've been discussing. The interstitial mineralization, as far as I know, is hematite 

and there is no radioactivity, there's just gold.

Staatz: Both in the South African talks and in the Canadian talks we have discussed a lot about having brannerite. Brannerite is an exceedingly rare mineral except in this type of deposit. Now my question, I think 

I'll give it to Vlad [Ruzicka] there, is whether he 

believes that brannerite is derived like most of the 

other minerals, that it is brought in like the pyrite and 

the uraninite, are brought in, or whether brannerite is 

indeed formed within the formation?

Ruzicka: In my talk I mentioned Theis, who studied this 

problem, and I also mentioned the "Pronto reaction" 

of Ramdohr. I would say the "brannerites" are 

uranium-rich titania aggregates, which are relatively 

common, but according to Stacey there are also a few 

grains of true brannerite.

Staatz: Yes, but are these grains of "true brannerite" or 

"true aggregates," are they brought in from the outside or are they derived later in the rock?

Ruzicka: In this case I would say the true brannerites 

are of detrital origin.

Nick Ferris, Urania Exploration, Golden, Colo.: I will 

attempt to elaborate on brannerite a little bit. At first 

at Blind River it was thought to be an entirely detrital 

grain; however, Paul Ramdohr in the mid-fifties noted 

the delicate complex texture inside the small, rounded 

grains and concluded that brannerite might be 

authigenic, having been formed from a pre-existing 

mineral. Because of his observations at the Pronto 

Mine at the southern part of the district, he suggested 

the so-called "Pronto reaction," in which titania 

migrates to uraninite and combines with it to form 

brannerite, a uranium titanate. Ramdohr also felt the 

same reaction was responsible for brannerite in the 

South African ores, although he referred to the 

mineral as "uraninite ghosts." This has also been mentioned by Schidlowski, in 19661 believe, with excellent 

photographs and scan photos from the microprobe. 

Schidlowski also subscribed, at that time at least, to 

the "Pronto reaction" as the explanation. The texture 

of brannerite from several Blind River localities has 

convinced me that brannerite from quartz-pebble conglomerates is an authigenic mineral originating as a 

replacement of ilmenite when ilmenite altered to rutile 

in the presence of uranium in solution. There is 

evidence for this in the texture if you compare the textures of the Blind River brannerites, the needle network and blades (Nick Theis has some excellent photos 

of this and has done a lot of work on it) with the textures you get when you do indeed alter beach sands or

rocks containing ilmenite to rutile. This texture is 

what's suggestive of this process, as well as the fact 

that you should have ilmenite at Blind River and you 

don't have any. So presumably it was there and 

something happened to it. What I am suggesting here 

is that the "Pronto reaction" really is not valid; in it 

titania from an undisclosed source goes to and in part 

replaces uraninite. I believe instead that uranium in 

solution from any of several possible sources, some of 

which were mentioned yesterday, goes to a titania 

mineral and alters it to this delicate needle network 

and mixture of various things in the small, rounded 

grains which we now call brannerite and is found at 

Blind River and perhaps also at the Rand. I conducted 

as many tests as I could on some other brannerites 

from around the world using the microprobe and 

observing them with the microscope as well. The types 

of brannerites that we've been talking about these 

past two and a half days are decidedly different texturally; they also are different compositionally 

because they show a large variation in the amount of 

uranium which occurs in them, not only in the entire 

grain but within those parts of the grain which can be 

analyzed by microprobe. I guess my main point here is 

that uranium migrated to a titanium mineral, I believe 

ilmenite, and formed uraniferous rutile, or 

uraniferous leucoxene as Von Rahden and Hiemstra 

have said, which can in fact become rich enough to 

grade compositionally to true brannerite, if there is 

such a thing, when the ilmenite has decomposed. This 

is the twist, then, on the "Pronto reaction." Now, this 

is also not the question that I raised my hand for. My 

other question I'll direct to any of the Canadian contingent. In the Blind River ores there is a greenishyellow sericitic matrix. I'd like to know the origin of 

this. I've had various responses to this question before: 

that it is metamorphosed in-filling clay, a volcanic 

material, or decomposed, redistributed and altered 

feldspar, but I'm still curious.

Roscoe: I am, too. I assume it was a clay, it was detrital, 

and that there was potash restoration in it. It does incorporate the finest detrital fraction within the conglomerate, and it must have been in-filling. I assume 

that there was alteration subsequently.

Meyer, Falconbridge Nickel Mines, Toronto, Canada: I'd 

like to make a comment first on Brad Myers' question, 

which has already been answered by Jim Robertson 

and Stu Roscoe. The volcanics at the base of the 

Huronian do seem to be subaerial, and in fact there 

seems to be a considerable unconformity, or certainly 

a period of erosion, between the volcanics and the 

basal Matinenda. Our drilling has shown this quite 

conclusively. When I say ours I mean Falconbridge 

Nickel Mines, back in the midsixties. So much for a
X14 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

comment. Second, the question, addressed to Dr. 

Ruzicka. Would it be possible that the uranium in the 

upper Proterozoic and Cambrian conglomerates of 

Russia is genetically more akin to the Colorado 

Plateau type rather than the Lower Proterozoic conglomerates in South Africa and Blind River? If your 

answer is "yes" on that, then what bearing does that 

have on exploration in sedimentary rocks in this vast 

time span between the two?

Ruzicka: I would say, and as I mentioned in my talk, the 

mineralization is more epigenetic than syngenetic, so 

it is more similar to the Colorado Plateau type than 

Elliot Lake type. You have many occurrences of 

uranium mineralization in young conglomerates. For

example, you have North Carolina, the Ningyo-toge 

deposit in Japan; you also have mineralization in the 

Permo-Triassic conglomerates of the west Carpathians, and so on.

Meyer: My question was, what bearing on sedimentary 

rocks between the lower Proterozoic and, say, the preDevonian? In other words, before you got any plant 

cover, which seems to have been important to deposition in post-Devonian rocks. I want to know specifically pre-Devonian but post-lower Proterozoic.

Ruzicka: You can have uranium mineralization in sandstone in that time span, but this type of mineralization 

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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-Y

ILLUSTRATIONS

FIGURE 1. Photograph showing cubes of thorianite from the Hogatza placer _________ Yl 

2. Map showing generalized geology in the vicinity of the Hogatza placer ______ 2

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

THORIANITE l FROM THE HOGATZA PLACER, ALASKA

By MORTIMER H. STAATZ

The Hogatza placer lies along Bear Creek, a tributary 

of 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 he 

was studying the precious and nonf errous metals of this 

area for the U.S. Geological Survey. In 1974 Miller and 

the 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 abnormally radioactive. Two-thirds of the heavies in this 

concentrate 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 isostructural 

and 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 the 

thorianite 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 contain 

47.3 percent Th02 and 36.8 percent U02 .

The thorianite in the Hogatza placer was derived from 

a gneissic monzonite (fig. 2) that forms a border facies of 

the otherwise rather massive granodiorite of the Zane 

Hills pluton. The border facies is 5 to 10 times as 

radioactive 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 the 

workshop.] 

* U.S. Geological Survey, Denver, Colo.

FIGURE 1. -Cubes of thorianite from the Hogatza placer.

ppm. The thorium content of these samples ranged from 

46 to 268 ppm and averaged 128 ppm. Concentrates of 

the heavy minerals from these rocks vary somewhat, but 

amphibole, pyroxene, magnetite, zircon, and sphene are 

abundant 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 the 

material 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 the 

glacier spread out, much of the glacial material does not 

appear to have been reworked.

Yl
Y2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Hogatza 

Placer

EXPLANATION

Alluvium

Gra nod io rite

01234

,

Gneissic Monzonite 

Andesite 

Path of glacier

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).

Uraninite 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 have 

to be extremely favorable or not for uraninite or 

thorianite to occur in modern placers is not known. The 

rarity of uraninite and thorianite in post-Precambrian 

placers may not be a function of its preservation but 

rather of its rarity in source rocks, such as granitic rocks 

and veins.

References cited:

Frondel, Clifford, 1958, Systematic mineralogy of uranium and 

thorium: U.S. Geological Survey Bulletin 1064, 400 p.

Miller, T. P., and Ferrians, 0. J., Jr., 1968, Suggested areas for prospecting 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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-Z

CONTENTS

Abstract ______________________________ Zl

Introduction ____________________________ 1

Geology of the Marquette Range Supergroup __________ 1

General _____________________________ 1

Chocolay Group ________________________ 2

Menominee Group _______________________ 3

Baraga Group __________________________ 4

Relation to the Huronian Supergroup ______________ 6

Similarities ___________________________ 7

Differences ___________________________ 8

Origin of the uraniferous quartz-pebble conglomerates and the

potential for their occurrence in Michigan _________ 8

Origin of Blind River District uranium ore _________ 8

Page

Origin of the uraniferous quartz-pebble conglomerates and the

potential for their occurrence in Michigan- Continued 

The problem of atmospheric oxygen _____________ Z9 

Evidence for an oxidizing atmosphere during deposition of theMarquette Range Supergroup ______ 9 

Evidence from iron-formation ___________ 9 

Evidence from weathered zones ___________ 10 

Significance of oxidized rocks ______________ 12 

Potential for uranium-bearing quartz-pebble conglomerate

in Marquette Range Supergroup ______________ 13 

Exploration for uraniferous quartz-pebble conglomerate in

Michigan—problems and discovery probabilities _______ 15 

References cited __________________________ 16

ILLUSTRATIONS

Page

FIGURE 1. Index map showing generalized distribution of rocks of the Marquette Range Supergroup and Huronian Supergroup _____ Z2

2. Schematic stratigraphic section showing unconformable relations between the Chocolay, Menominee, and Baraga Groups __ 3

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 

4-9. Photographs of

4. Basal clastic rocks of the Chocolay Group _____________________________________________ 5

5. Quartz-pebble conglomerate at base of Menominee Group ___________________________________ 6

6. Basal conglomerate of Baraga Group _______________________________________________ 6

7. Iron-formation of the Menominee Group _____________________________________________ 10

8. Even-bedded jaspilite, with layered jasper and interbeds of specular hematite _______________________ 11

9. Oxidized granitic rocks at base of Baraga group _________________________________________ 11

10. Map showing distribution of gamma-ray anomalies from airborne survey and known uranium concentrations __________ 14

11. Curves relating probability of discovering one deposit to number of holes drilled, in the central northern peninsula of Michigan 16

in

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

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 J

ABSTRACT

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 sense 

that they lie directly on Archean rocks, are of three ages, and crop out 

along a strike length of about 600 kilometers. The conglomerates have 

been considered possible hosts for uranium ore because of close 

geographic proximity, approximate age equivalence, and certain 

lithologic similarities to uraniferous quartz-pebble conglomerates in 

the Huronian of Canada. To date, no uranium deposits are known in 

conglomerates of the Marquette Range Supergroup, but extensive 

cover by glacial deposits is a severe hindrance to exploration. There is 

a substantial possibility that even large deposits would not be found in 

outcrop.

Radiometric dating suggests that the Marquette Range Supergroup 

is 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 the 

Marquette Range Supergroup are equivalent to the uppermost units of 

the 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 black 

slates 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 and 

very 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 the 

most efficient exploration tool.

1 U.S. Geological Survey, Reston, Va.

INTRODUCTION

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 of 

South 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 and 

geographic location, as well as in certain lithologic and 

stratigraphic features. This paper considers the 

possibility that deposits of uraniferous quartz-pebble 

conglomerate like those in the Huronian might be 

discovered in the Marquette Range Supergroup. This 

possibility 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 conditions prevailed during deposition of the Marquette 

Range Supergroup and in terms of local geologic 

features of the Marquette Range Supergroup.

GEOLOGY OF THE MARQUETTE RANGE 

SUPERGROUP

GENERAL

The Marquette Range Supergroup is a thick sequence 

of metasedimentary and metavolcanic rocks in the central and western parts of the upper peninsula of 

Michigan and northern Wisconsin (fig. 1). The age of the 

rocks is firmly bracketed between 2.6 b.y., the age of the 

basement 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 CONGLOMERATES

BLIND RIVER 

ilSTRlGT

AREA OF 

THIS REPORT

0 100 KILOMETERS i i i i i___

I \ i i I

0 100 MILES

EXPLANATION

MARQUETTE

RANGE 

SUPERGROUP

HURONIAN 

SUPERGROUP

OTHER

PRECAMBRIAN 

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 the 

basal 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 as 

shown 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 than 

those within the sedimentary sequence, the discussion of 

conglomerates in the Marquette Range Supergroup emphasizes 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 peninsula 

of Michigan and does not include correlative rocks in the 

Gogebic district of the western upper peninsula and 

possibly correlative rocks in northern Wisconsin.

CHOCOLAY GROUP

The 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 Dead 

River basin (Puffett, 1969; Clark and others, 1975) (fig.
BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN Z3

B

GRANITIC-GNEISS

FIGURE 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 Archean 

rocks and are believed to be of equivalent ages. Clasts 

are mostly granitic rocks, with other Archean rocks also 

present. The matrix generally is chloritic, and the rocks 

typically are poorly sorted (fig. 4). Their wide distribution 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 more 

widespread than indicated by their present degree of 

preservation. The glacial deposits grade upward into 

marine deposits of sericitic schist and orthoquartzite; 

where glacial deposits are absent the marine sequence 

lies directly on the Archean. In the Menominee district 

(Bayley and others, 1966) the lowest 30-60 m of the 

marine sequence are pale-green sericitic quartzite and 

schist that is overlain by white, gray, and pink vitreous 

quartzite. Some basal beds contain abundant magnetite, 

tourmaline, and some pyrite. In central Dickinson County (James and others, 1961) as much as 100 m of gray 

and green sericitic quartzite and quartzose schist (25 

percent sericite, 75 percent quartz) are at the base of the 

section and are overlain by vitreous quartzite containing 

zones of quartz-pebble conglomerate as much as 1 m 

thick. In the eastern Marquette Range (Gair and 

Thaden, 1968; Puffett, 1974) sericitic and feldspathic 

schist and quartzite underlie vitreous white, gray, and

pink 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 the 

group.

MENOMINEE GROUP

The Menominee Group is a marine sequence generally 

with basal quartzite that grades upward into finer 

grained clastic rocks that in turn grade upward into 

iron-formation. The group contains the major economic 

iron-formations of the area. Commonly the Menominee 

Group lies unconformably on the Chocolay Group, but in 

much 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 and 

others, 1975). There, the group contains a basal orthoquartzite as much as 150 m thick with conglomerate 

zones near the base. Most conglomerates bear pebbles of 

vein 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 CONGLOMERATES

EXPLANATION

DICKINSON GROUP

BASAL CLASTIC ROCKS OF: 

BARAGA GROUP

-;,- v::^.v;::;;: MENOMINEE GROUP 

CHOCOLAY GROUP

FAULT 

UNCONFORMITY

——————— CONTACT OF

UNDETERMINED 

NATURE

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, and 

volcanic 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 ironformation 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 reworking 

of iron-formation of the Menominee Group. Such 

rocks 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 Z5

2. Thin, locally derived lenses of conglomerate containing 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 Archean surface and are most likely wave-washed 

bedrock rubble.

Wedges as much as 100 m thick of polymictic conglomerate with clasts generally 1-10 cm in 

diameter but locally as much as about 1 m in 

diameter (fig. 6£). Clasts are mostly ironformation, vein quartz, and older sedimentary 

rocks. The matrix is quartzose and typically very 

hematitic. Four such wedges are known, all in the

i^^^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 FIGURE 4.-Basal clastic rocks of the Chocolay Group. A, Tillite com- 

~jp«^ - jp posed 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 CONGLOMERATES

FIGURE 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 the 

Marquette trough on the south. One wedge, in the 

eastern Marquette district, contains monazite-rich 

placers within quartz-pebble conglomerate; grades 

are locally as high as 110 Ib of monazite per short 

ton (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 to 

the depositional basin.

RELATION TO THE HURONIAN SUPERGROUP

The relation of the Marquette Range Supergroup to 

the Huronian Supergroup has been in dispute for nearly 

a century and is still not known with certainty in spite of

FIGURE 6.-Basal conglomerate of Baraga Group. A, Conglomerate 

composed 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 

is especially important in assessing the uranium potential of conglomerates in the Marquette Range 

Supergroup because of the large uranium deposits of the 

Blind River district in basal rocks of the Huronian.

For many years the rocks of the Marquette Range 

Supergroup were called Huronian. Early workers suggested the correlation based on similarities in rock 

types, approximately equivalent age, and close 

geographic proximity. In 1958, James (1958) suggested 

that the rocks in Michgan be called Animikie Series 

because correlation with the Animikie Group in Minnesota seemed more certain than with the Huronian. In 

1970 (Cannon and Gair, 1970) the term "Animikie" was 

abandoned and the rocks were given the local name of 

Marquette Range Supergroup, primarily to avoid conflict with the stratigraphic code of the U.S. Geological 

Survey.

Over the past ten years radiometric age determinations have helped to define the absolute ages of the 

Huronian Supergroup and the Marquette Range Supergroup. Both sequences lie unconformably on Archean 

rocks at least 2.6 b.y. old. In Ontario, dikes and sills of 

Nipissing diabase were intruded after Huronian 

sedimentation and probably after early phases of 

regional deformation of the Huronian. An age of about 

2.15 b.y. is well established for the Nipissing (Van 

Schmus, 1965; Fairbairn and others, 1969) and seems a 

reliable minimum age for the Huronian. Fairbairn and 

others (1969) determined an age of 2.29 b.y. for argillite 

in the Gowganda Formation, which may be the time of 

deposition or diagenesis. Thus deposition of the Huronian Supergroup is rather reliably bracketed between 2.6 

and 2.15 b.y. ago.

In Michigan, the Marquette Range Supergroup was 

deformed and metamorphosed about 1.9 b.y. ago (Banks 

and Van Schmus, 1971; Goldich and others, 1961; 

Aldrich and others, 1965). Banks and Van Schmus 

(1971) report a zircon concordia age of 2.1 b.y. for "porphyritic red granite" that forms two small plutons surrounded by Archean rocks in central Dickinson County. 

These plutons were interpreted to have been emplaced 

prior to deposition of the Marquette Range Supergroup 

(James and others, 1961) on the basis of internal structures that were not believed consistent with postMarquette Range structures. Banks and Van Schmus 

therefore considered that 2.1 b.y. was a maximum age 

for the Marquette Range Supergroup, an age which, if 

correct, invalidates any correlation with the Huronian.

It is my opinion that enough uncertainties exist in the 

data that the radiometric age should not be considered a 

maximum age for the Marquette Range Supergroup for 

several reasons. First, the porphyritic red granite has 

had a complex postemplacement history, including 

granulation and recrystallization of minerals (James and

others, 1961), and the radiometric age may not be the 

time of emplacement. No intrusive rocks or thermal 

events of this age are known elsewhere in the area. Second, radiometric determinations on other rocks only a 

short distance from the porphyritic red granite yield 

spurious results and in places do not define an Rb-Sr 

isochron (Banks and Van Schmus, 1971), suggesting 

that isotopic systems have been disturbed, perhaps over 

a large area. Finally, the porphyritic red granite is not 

! proved to be older than the Marquette Range Superi group and is nowhere in contact with it; the proposed 

age relations are based solely on structural arguments. 

Thus, even if 2.1 b.y. is the age of emplacement of the 

porphyritic red granite, it cannot, with complete confidence, be considered to define the age of the Marquette Range Supergroup.

A single age on volcanic rocks in the Baraga Group is 

about 2 b.y. (Banks and Van Schmus, 1971). If this age is 

correct, the Baraga Group is younger than the Huronian, but correlation of the Menominee and Chocolay 

Groups with the Huronian is still possible. A correlation 

of the Gowganda Formation of the Huronian with basal 

Chocolay Group tillites of the Marquette Range Supergroup and of the Lorrain Quartzite with quartzites of 

the Chocolay Group has been suggested by Young 

(1973). This correlation is attractive in light of striking 

lithologic similarities of these parts of the section and of 

the relative "uniqueness" of continental glacial deposits 

in time, but it is weakened by the presence of other conglomerates in the Huronian, also believed to be of glacial 

origin (Young and Chandler, 1968; Roscoe, 1969; Frarey 

and Roscoe, 1970), which could also be correlated with 

the Chocolay Group.

In my opinion, the question of correlation cannot be 

answered adequately with existing data, and the possibility should be held open that part or all of the Marquette Range Supergroup is equivalent in age to the 

Huronian Supergroup. Surely, in evaluating the possibility of the occurrence of uraniferous quartz-pebble conglomerates in Michigan, the Marquette Range Supergroup should not be excluded solely on the basis of the 

scant radiometric data suggesting that it might be 

younger than rocks in the Huronian to which known occurrences of such deposits seem to be restricted.

In the following sections are listed some major 

similarities and differences between the Marquette 

Range Supergroup and Huronian Supergroup that illustrate the geologic grounds on which the question of 

correlation can be argued.

SIMILARITIES

1. Both supergroups are thick sedimentary accumulations deposited sometime during the interval 2.6 to 

1.9 b.y. ago.
Z8 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

2. Both contain relatively uncommon Precambrian rock 

types, 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), for 

which one age of 2 b.y. has been obtained (Banks 

and Van Schmus, 1971).

4. The two supergroups are in close geographic proximity 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 of 

the Marquette Range Supergroup can be traced 

eastward beneath Paleozoic cover for several tens 

of kilometers before being truncated by trends of 

younger Keweenawan rocks. The Keweenawan 

rocks are now believed by many to occupy an arcuate 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, the 

Marquette Range Supergroup was deposited somewhat north of its present location with respect to 

the Huronian and was, prior to Keweenawan rifting, even closer to the Huronian than at present. 

Taking these factors into consideration, it is likely 

that no more than 150 km separated the two supergroups prior to 1.1 b.y. ago. Considering the likelihood that the sedimentary rocks originally extended well beyond their present outcrop areas, it is not 

unlikely that both could have formed in the same 

sedimentary basin.

DIFFERENCES

1. The Marquette Range Supergroup contains thick 

deposits of iron-formation at several stratigraphic 

horizons, whereas iron-formation is virtually 

unknown in the Huronian.

2. Deposition of the Marquette Range Supergroup 

culminated in accumulation of a very thick sequence 

of eugeosynclinal deposits. A similar tectonic environment of sedimentation is not recognized in the 

Huronian.

ORIGIN OF URANIFEROUS QUARTZ-PEBBLE

CONGLOMERATES AND THE POTENTIAL FOR

THEIR OCCURRENCE IN MICHIGAN

ORIGIN 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 the 

origin of Blind River ore is one means of assessing the 

possibility that similar ore might be found in Michigan.

The mode of origin of Blind River uranium ore has 

been controversial, virtually since the deposits were 

discovered 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, opinion has been divided between two points of view, one 

that favors an epigenetic (hydrothermal or groundwater) origin for uranium mineralization and the invariably associated pyrite and a second that favors a 

syngenetic (clastic) origin for uranium minerals and 

pyrite. Among supporters of the latter view are included 

J. A. Robertson (1968), D. S. Robertson (1974), and 

Roscoe (1969, 1973), all of whom are intimately acquainted with Blind River deposits through many years 

of 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 such 

as uraninite and pyrite, unstable in an oxygen-rich atmosphere, to be eroded, transported, and accumulated 

into placers. They believe the atmosphere began to accumulate free oxygen shortly after deposition of the 

basal Huronian units, thus inhibiting further development of such placers. They based their arguments on the 

drab color and generally unoxidized nature of lower 

Huronian rocks, including the uraniferous conglomerates, and of underlying fossil soils, in contrast to more 

oxidized units in the upper parts of the Huronian and in 

other sequences believed to be slightly younger than the 

Huronian. D. S. Robertson described such sequences on 

a worldwide scale and suggested that the change in oxidation state of the sediments reflects a unique point in 

atmospheric evolution that can be used as a world-wide 

time 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 Z9

THE PROBLEM OF ATMOSPHERIC OXYGEN

The question of when free oxygen first appeared in the 

atmosphere 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 that 

time so that iron could be transported in the relatively 

soluble ferrous form. An oxygen partial pressure of 

about 10 ~ 70 atm is required to permit significant concentrations of dissolved ferrous iron in water. The presence 

of oxide-rich iron-formation throughout much of recorded geologic time prior to 2 b.y. ago requires a source to 

supply oxygen during deposition if ferric iron minerals 

were precipitated from dissolved ferrous iron. If the atmosphere were anoxygenic, some alternate source is 

needed, and Cloud (1965, 1973) has suggested that 

primitive photosynthetic microorganisms produced oxygen that was immediately consumed by ferrous iron in 

sea water. Thus, oxygen partial pressure was buffered 

at about 10- 70 atm as long as sufficient ferrous iron was 

in the oceans and iron-formation was precipitated in the 

process. 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 oxygen to begin to accumulate in the atmosphere.

Recently, Holland (1973) and Drever (1974) have proposed models that can account for iron-formations with 

an 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 accumulate 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 and 

precipitated iron minerals as the solubility of iron was 

diminished.

Although this model is not demonstrably the means by 

which iron-formation was deposited, it does, nevertheless, provide a plausible means of forming ironformation under an oxygen-rich atmosphere, and the 

presence of iron-formation cannot be taken as evidence 

of 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

along with more direct evidence from the uraniferous 

conglomerates themselves. If one accepts models for the 

origin 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 be 

taken to indicate that both pyrite and uraninite were 

stable under the atmosphere existing at the time when 

iron-formations were deposited.

If, on the other hand, one accepts a model such as 

Drever's, in which iron-formations are formed under a 

relatively oxygen-rich atmosphere, then the presence of 

iron-formation cannot be used to conclude that pyriteuraninite placers could be formed stably. Rather, it 

would be likely that both minerals would be unstable, at 

least 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 and 

weathered zones along unconformities, however, do contain evidence of an oxygen-rich atmosphere during at 

least part of the time of deposition of the supergroup 

and indirectly bear on the potential for pyrite-uraninite 

placers in Michigan.

EVIDENCE FROM IRON-FORMATION

The Negaunee Iron-formation, the principal ironformation of the Menominee Group in the Marquette 

district, contains textural and mineralogical features 

that 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 jasper 

that are interbedded with specular hematite (fig. 7A). 

The wavy bedding and oolitic character indicate a highenergy and presumably shallow-water environment of 

deposition. Other units contain more reduced 

assemblages that, where not strongly altered by 

metamorphism, include siderite, minnesotaite, and 

magnetite. They typically are rather even bedded, suggesting quiet water during deposition (fig. IE).
Z10 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

In 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 meter 

or less thick (fig. IE), and the abrupt change in both bedding characteristics and oxidation state is repeated 

many times across the outcrop. These lithologic changes 

must reflect slight variations in water depth or depth of 

wave 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 of 

oxygen to form the thoroughly oxidized wavy-bedded 

jaspilite was the atmosphere. The agitated water in 

which wavy-bedded jaspilite was deposited brought 

oxygen-rich water to the sediment-water interface, so 

that 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, iron 

carbonate, and iron silicates were therefore stable.

EVIDENCE FROM WEATHERED ZONES

Northern Michigan is an area of very poor bedrock exposure. Consequently, exposures of unconformities are 

rare. Rarer still are exposures showing effects of 

weathering in rocks beneath unconformities. Nevertheless, enough data have been gathered over many 

years of study to substantiate some rather firm conclusions.

The most extensive weathered zone known is 

developed beneath the unconformity separating the 

Menominee 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 by 

weathering 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 that

FIGURE 7. -iron-formation »f the Menominee Group. A, Wavy-bedded jaspilite was a primary facies of iron-formation and had

TMt 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

ocr ol specular hematite n (lighter ft layers). B, p Even bedded chertmagnetite-iron silicate iron-formation (units a) in contact with wavy

bedded jaspilite (units 6).
BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN

—• • - —; »'-_, • i—IK i

Zll

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, yet 

other evidence strongly suggests that some jaspilite has 

formed 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" of 

sideritic 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 to 

have formed by oxidative weathering as originally proposed 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 

in these rocks beneath the unconformity. At only two 

localities, both in the Dead River basin, have I found 

features that might be caused by oxidative weathering. 

At both localities, gray granitic gneiss beneath the unconformity has a thin pink zone developed on it immediately beneath the unconformity (fig. 9A). The zone, 

which varies from about 1 cm to several centimeters in 

thickness, may be caused by oxidation of the gray 

gneiss. The change in color is caused primarily by a color 

change in plagioclase. White to gray plagioclase, typical 

of most of the gray gneiss, is altered to pink and orange 

near the unconformity. In addition, mafic components, 

mostly biotite, apparently were leached from the pink 

zone. The basal sediments of the Baraga Group are black

FIGURE 9.-Oxidized granitic rocks at base of Baraga Group. A, Gray 

granitic gneiss (a) converted to pink granitic gneiss (b) beneath unconformable contact with black pyritic shale (c) at base of Baraga 

Group. B, Pebble of gray granitic gneiss (upper part of specimen) 

with rind of pink oxidized granite in black pyritic shale of basal 

Baraga Group.

pyritic shale that shows little or no oxidation, so the pink 

zone beneath the unconformity must have formed before 

sedimentation. A few pebbles of the gray gneiss in local 

conglomerate lenses have similar pink rinds (fig. 9E) 

that also were probably formed by oxidative weathering 

prior to sedimentation.

At the same localities, pockets of deeply weathered 

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 to 

limonite is not known but it may be a remnant of ironrich residual soil that would be expected to form under 

an oxidizing atmosphere.

Elsewhere, Archean rocks beneath the unconformity 

show no effects of weathering or oxidation. If 

weathered zones or residual soils had formed, they were 

apparently totally removed and reworked during 

Baraga Group transgression. Thus weathering and oxidation 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 hundreds 

of meters beneath the surface, whereas weathering of 

surrounding rocks was not deep enough for weathered 

zones to have survived Pleistocene glaciation.

In summary, two independent lines of evidence lead to 

the 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 atmosphere. Extensive iron-formations were deposited in at 

least four younger stratigraphic intervals of the Marquette Range Supergroup. From this I conclude that the 

presence of iron-formation cannot be used to infer an 

anoxygenic atmosphere and is therefore not a reliable 

guide in assessing the potential for placer accumulation 

of uraninite.

Data on weathering along unconformities at the base 

of 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 my 

knowledge, is beneath the Menominee Group in the 

western Marquette district. At two localities the basal 

conglomerate of the Menominee Group is separated 

from underlying Archean granitic gneiss by a zone of 

gray-green chloritic and sericitic material believed to be 

a 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 vein 

quartz, chert, and older sedimentary-rock fragments. 

The base of the conglomerate and upper parts of the 

regolith contain minor pyrite and magnetite. The rocks 

were metamorphosed to staurolite grade, and pyrite is 

generally recrystallized to well-formed cubes, but, rarely, rounded, possibly detrital grains are preserved. 

Although these rocks might be interpreted to have formed in an anoxygenic atmosphere, data are too sparse to 

justify 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 oxygen 

to oxidize a substantial amount of iron. A problem arises 

of how to interpret this in terms of oxygen partial 

pressure in the atmosphere and its bearing on the ability 

of uraninite to persist in a clastic sedimentary environment. The stability relations of the minerals involved are 

rather well known and provide a first step in solving this 

problem. Hematite becomes the stable iron oxide at vanishingly small oxygen partial pressures (~10~65 atm) and 

has a very broad stability field. Uraninite remains stable 

at 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, and 

the presence of hematite in a rock does not automatically preclude the presence of uraninite under equilibrium 

conditions.

The problem is complicated, however, by two considerations: (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 in 

placers 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 is 

necessary to consider some quantitative and kinetic 

aspects of oxidation as well as intensive variables that 

govern mineral stabilities. The formation of oxide facies 

iron-formation and of extensive oxidized zones along the 

post-Menominee Group unconformity requires that a 

large amount of oxygen be delivered to the site of oxidation 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 found 

that 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 and 

Blockley (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 capable 

of 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 atmosphere 

probably contained no more than a few tens of grams of 

oxygen. The entire atmospheric oxygen supply could oxidize literally no more than a handful of uraninite during 

oxidative weathering.

In my opinion, rocks such as those in the Menominee 

Group that appear to have been oxidized as a result of 

contact with the atmosphere must have formed under ar
BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN Z13

atmosphere with oxygen levels well in excess of minimum 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 by 

photosynthesis 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 oxidation 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 the 

original 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 of 

whether uraninite could persist to form placer deposits 

under such an atmosphere. Although uraninite may be 

well 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 any 

other metastable detrital mineral, depends on rates of 

weathering, erosion, and burial relative to the rate of 

breakdown. 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 conglomerates 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 the 

onset of oxidizing conditions. Rather, it seems more likely that such deposits would gradually become scarcer in 

younger rocks as oxygen levels increased, requiring progressively more favorable conditions to preserve 

uraninite 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 than 

during the Huronian if the Marquette Range Supergroup is partly younger than the Huronian, but their 

presence is certainly not precluded.

POTENTIAL FOR URANIUM-BEARING QUARTZ-PEBBLE

CONGLOMERATE IN THE MARQUETTE RANGE

SUPERGROUP

The foregoing arguments in favor of a strongly oxidizing atmosphere during deposition of at least part of the 

Marquette Range Supergroup suggests that the conditions may have been less favorable for formation of 

quartz-pebble uranium ore than they were during 

deposition of the Huronian. Nevertheless, our understanding of the requisite conditions for their formation 

and of the age relations between the Marquette Range 

Supergroup and Huronian are still uncertain enough so 

that 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 U3 08 

and 102 ppm Th02 (Malan and Sterling, 1969), and 

27 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 being supplied to the sedimentary basin (Vickers, 

1956b).

4. The presence of monazite-rich placers in parts of the 

basal Baraga Group (Vickers, 1956a) indicates that 

physical conditions of sedimentation were suitable 

to form significant placer accumulations of 

minerals that were originally disseminated in older 

rocks.

5. Fragmentary evidence suggests that basal sediments 

of the Chocolay and Menominee Groups are not 

strongly 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 and 

others, 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 Archean 

rocks that may have been reworked and incorporated 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 that 

thorium was not being preferentially retained over 

uranium 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 in 

many areas. The tectonic style of the area, which I 

have discussed in more detail elsewhere (Cannon, 

1973), consists basically of upfaulted blocks of Archean basement rocks with the sedimentary rocks

V \.~.

EXPLANATION

•

GAMMA-RAY ANOMALIES 

FROM AIRBORNE SURVEY

KNOWN URANIUM 

CONCENTRATION

D 10 20 MILES 

i ' '

i i 

D 10 20 KILOMETERS

FIGURE 10.-Map showing distribution of gamma-ray anomalies from airborne survey (Balsley and others, 1950) and known uranium concentrations (Vickers, 1956b).
BASAL CONGLOMERATES, WEATHERED ZONES, MARQUETTE RANGE SUPERGROUP, MICHIGAN Z15

of the Marquette Range Supergroup draped over 

the uplifts. The contact is intensely sheared in 

many places, and the lowest part of the section is 

eliminated by faulting. Thus, the basal units of 

principal interest for uranium exploration are not 

present in some areas. Because of poor exposure 

and the general conformity of bedding with the 

basal contact, even where the contact is a fault, it is 

commonly difficult to judge the extent of fault truncation of the base of the section. Figure 3 shows my 

best 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 exploration in the area, albeit most was not concerned with uranium exploration, no significant 

deposits have been found. Radioactivity of most 

conglomerates is only slightly higher than that of 

adjacent Archean rocks. Rarely, radioactivity 

reaches three times that of the adjacent Archean 

rocks. As discussed in more detail in the following 

section, this may not be an overriding negative factor. Because of the very limited exposures of 

bedrock there is a substantial probability that even 

a major district would not be discovered in outcrop.

EXPLORATION FOR URANIFEROUS

QUARTZ-PEBBLE CONGLOMERATE IN

MICHIGAN—PROBLEMS AND DISCOVERY

PROBABILITIES

In the previous discussions many similarities and differences between the Marquette Range Supergroup and 

the Huronian have been considered. One additional important difference is the amount of Pleistocene drift 

cover on bedrock. The Huronian rocks near Blind River 

are very well exposed because of thin drift cover; indications of several of the known ore deposits were seen initially 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 geophysical 

techniques might detect buried ore bodies, but I would 

like to illustrate here that blind diamond drilling might 

also be a valuable and relatively inexpensive technique. 

By blind drilling I mean that drill holes would not be 

located 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

the contact is rather accurately known from detailed 

mapping for most of that length. By analogy with Blind 

River, uranium ore is most likely to be in the lowest 50 m 

of the sedimentary section. Thus the total exploration 

target is relatively small and well defined. Maps of the 

Blind River district (J. A. Robertson, 1968) indicate that 

ore 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 randomly 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 of 

drill holes is a straight-line function between these two 

extremes. If more than one deposit exists in the area the 

probability of making a discovery increases exponentially with the number of deposits. Figure 11 shows curves 

relating number of drill holes to probability of making a 

single 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 of 

finding one deposit.

I would suggest, therefore, that blind diamond drilling 

is a realistic means of conducting a major exploration 

program in the area. Two hundred drill holes, each 

about 50 m long, would have a probability of about 0.5 of 

finding 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 commonly no better than 1 in 10.

Figure 11 can also be used to estimate the probability 

of 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 amount 

of 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 were 

in Michigan, chances of discovery of a single deposit in 

outcrop are only about 0.5 after all outcrops have been 

thoroughly explored. Because all outcrops have not been 

thoroughly explored to date, the probability that a 

deposit would be discovered with current knowledge is 

considerably 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 the 

area in a manner that is not greatly dependent on outcrop information, and they suggest that the failure to 

find deposits to date should not be viewed as a greatly 

discouraging factor in considering future exploration.

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50 100 150 200 250 300 

NUMBER OF DRILL HOLES

350 400

FIGURE 11. -Curves relating probability of discovering one deposit to 

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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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-AA

CONTENTS

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 _________

Transport in complexed solution 

Bacterial sulfur cycle ____________

Thucholite _____________________

Inverse relation of uranium and gold 

Genesis of gold in reefs ___________

Radiolysis of water ___________

Oxidizing reactions _______. 

Maturation of reefs __________

5

7

8

8

9

10

12

12

13

Page

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

ILLUSTRATIONS

Page

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 water

at 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 U3 08 /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 U3 08 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 U3 08 /Au ratio for individual

reefs in the Witwatersrand and for the composited Blind River-Elliot Lake district __________________ 20

TABLES

Page

TABLE 1. Ages of pyritic uranium-gold conglomerates ____________________________-_—_——————————————— AA1

2. Uranium and gold contents and the U3 08 /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 CONGLOMERATES

GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES

By W. BRADLEY MYERS l

ABSTRACT

The ancient pyritic ore conglomerates have a common origin best exemplified 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 not 

similar to modern gold placers. Placers result from the near incapacity 

of streams and currents to transport coarse gold. Placers as rich as 

Rand reef occur only in narrow paystreaks within 15 kilometers of a 

coarse-gold source.

The broad dispersion of gold in the reefs is due to solution transport 

of metal complexed as aurous sulfide, leached anoxygenically from 

crustal rocks, probably from sea-floor basalt, and precipitated by a 

slow 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 slowly dissolved uranium, precipitated gold, and oxygenated thucholite. 

These actions define a maturing process. A uraninite placer accumulating on an unconformity becomes progressively converted to a 

gold reef with little residual uraninite. The most mature reefs tend to 

grade toward the thucholite-seam type, very thin but exceedingly rich 

in gold. A combination of chemical attack and physical reworking accounts 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.

INTRODUCTION

Probably 75 percent of the world's gold resources and 

at least 50 percent of its uranium resources are contained 

in Precambrian pyritic conglomerates 2.0-3.0xl09 yr 

old (table 1). These conglomerates contain detrital 

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 conglomerates is crystalline, and in the conventional view it 

is assumed to have been initially detrital. It is deduced 

here that, on the contrary, the gold was transported as 

complexed ions in anoxygenic solution and was precipitated on long-lasting unconformities on the floors of 

shallow seas by a slow reaction driven by the radioactive 

decay of uranium.

U.S. Geological Survey, Reston, Va.

TABLE 1. -Ages, in billions (109) years, of pyritic uranium^, 

conglomerates

Locality Reference

Serra de Jacobina, Brazil _

Moeda conglomerate,

Brazil _______________

Blind River-Elliot Lake,

Canada _____________

Krivoi Rog Series,

Ukraine _____________

>2

>2

2.3 -2.5

Kursk Magnetic Anomaly

Series, Ukraine. 

Witwatersrand, South

Africa ______________ <2.3 -2.8

Pongola Basin, South 

Africa ______________

Cordani and others 

(1973).

Do.

Roscoe (1969, 1973).

Chukhrov and others 

(1970).

Do.

Nierkerk and Burger 

(1964, 1969).

<2.85-3.35 Beukes (1973).

I wish to thank the South African mining houses for 

their many courtesies and their unfailing hospitality during 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 

to the Elliot Lake district, Ontario. I am indebted to D. 

A. Singer and J. De Young of the U.S. Geological Survey 

for their help in statistical problems. This paper has 

benefited greatly from reviews by colleagues Warren 

Hamilton and Paul B. Barton, Jr.

GEOLOGIC SETTING OF THE WITWATERSRAND

REEFS

The Rand pyritic conglomerates (Witwatersrand, 

South Africa) support the world's largest gold production and have been extensively studied. Six or so major 

productive conglomerates and about twice as many 

minor ones are scattered through a cratonic section 9 

km thick (Dominion Reef, Witwatersrand, and Ventersdorp Lava Systems; Haughton, 1969; Pretorius, 1964).

AAl
AA2 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

The 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-documented pyritic reef (Haughton, 1969). The surmounting 

shaly Pretoria Series of the Transvaal System (2.0 x 109 

yr) contains hematite-chamosite oolites (James, 1966), in 

contrast to the more reduced iron-formation of the 

underlying sequences. The massive release of oxygen to 

the previously anoxygenic atmosphere by algal photosynthesis, 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 permitting development of reduced iron-formation (French, 

1973; Cloud, 1973; Garrels and others, 1973) or preservation 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 that 

lie conformably with the sections, are characteristically 

thinner 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; de 

Kock, 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 and 

others, 1964; Cox, 1967). At both Blind River-Elliot 

Lake and the Krivoi Rog localities the sericitization of 

greenstone 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 source 

is 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 were

likely authigenic, growing in voids between siliceous 

clasts.

The Witwatersrand System underwent low-grade 

burial metamorphism. Diagenetic reconstitution of the 

feldspar 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 feldspar 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 phyllosilicates 

have also recrystallized in the reefs from the supposed 

authigenic minerals described. Chloritoid has developed 

in the upper rocks of appropriate composition, and, exceptionally, kyanite (Liebenberg, 1955) has recrystallized from pyrophyllite and more aluminous minerals.

DETRITAL MINERALS UNSTABLE IN AN 

OXYGEN ATMOSPHERE

Pyrite is overwhelmingly the most abundant opaque 

mineral 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 detrital 

from 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 granular 

aggregates, replacements of various iron oxides, and 

porous concretionary nodules. All of these 

characteristically occur as water-worn remnants (Ramdohr, 1958; Saager, 1970; Liebenberg, 1960) (fig. 1). The 

rounded clasts of pyrite are in general hydraulic 

equivalence 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) hydraulically 

equivalent with detrital grains of chromite, zircon, and 

quartz, occurs sparingly in the matrix of all Witwatersrand reefs (Liebenberg, 1955, 1960; Ramdohr, 1958; 

Coetzee, 1965). The lead-isotope age of the uraninite is 

about 3.0 x 109 yr (Burger and others, 1962), older than 

the lead-isotope age of 2.8xl09 yr (Nierkerk and 

Burger, 1969) of the oldest reef series, further indicating a detrital origin. Electron microscope studies 

by Grandstaff (this volume, chap. J) and by Feather (this 

volume, chap. Q) show widely varying thorium contents 

for different grains in single samples, conclusively proving 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 CONGLOMERATES AA3

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 grains 

or pebbles in all reefs.

uraninite show derivation from a high-temperature 

granitic-pegmatitic source. A modern equivalent of the 

source rock may be certain Phanerozoic French granites 

where uraninite occurs as tiny accessory crystals within 

rock 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) contain 

improverished suites of the detrital minerals expectable 

from the underlying basements. The surviving minerals 

are those chemically and physically resistant; notable is 

the ultrastable mineral zircon. Waterworn minerals of 

the Rand reefs have high Vickers microhardness (Young 

and 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 and 

uraninite in an aqueous system (fig. 2) show that neither

of these minerals is stable in the present oxygenated atmosphere. This conclusion is upheld by the rarity of 

pyrite and the virtual absence of uraninite as detrital 

minerals in modern sediments. The diagrams indicate 

that in an anoxygenic environment abundant pyrite 

would tend to protect uraninite in the presence of weak 

oxidizing 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-sensitive 

minerals, 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-free 

conglomerate, also found in older Precambrian deposits, 

suggests that the pyrite-uraninite association is not 

simply that of two Eh-sensitive minerals unstable in an 

oxygenated atmosphere.
AA4 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

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FIGURE 2. - Eh-pH diagrams of the stability fields of pyrite and uraninite and the equilibrium distribution of 

soluble sulfur species in water at 25°C and one atmosphere total pressure. A, Equilibrium distribution of 

soluble sulfur species; the ratios of the sulfur species are unity at the field boundaries. B, Stability relations 

of iron oxides and sulfides at an activity of dissolved sulfur of 10" 5 . C, Stability relations of uraninite and 

uranium 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 from 

Garrels and Christ (1965) and Hostetler and Garrels (1962).
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA5

GOLD

The Witwatersrand gold is crystallized in irregular 

particles normally from 0.002 to 0.5 mm, exceptionally 

as 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). Summarizing 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 these 

figures illustrate the extremely small size of the minerals of economic 

importance in the reefs. No medium- to large-sized nuggets have been 

found during mining operations, and it can be concluded that, in regard 

to gold-particle size, the detrital deposits of the Witwatersrand are not 

comparable with modern-day placers.

The economy with which gold is won from gravels, the 

unfamiliar units in which it is reported, and the romance 

surrounding rich placer pockets combine to obscure the 

evident 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 (see 

following paragraphs). This paradox underscores the 

lack of quantitative analysis in the easy acceptance of a 

placer origin for the gold of pyritic conglomerates.

As part of the Heavy Metals program of the U.S. 

Geological Survey, modern studies have been conducted 

on the Tertiary gravels of the Sierra Nevada (Yeend, 

1974), stream placers in Oregon (Boggs and Baldwin, 

1970), and the beach placers at Nome, Alaska (Nelson 

and Hopkins, 1972). This work has reestablished the 

validity 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 to 

source (Emery and Noakes, 1968).

Economic gold placers result from the near-incapacity 

of 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).]

remotely approach the richness of representative Rand 

reef (7-21 ppm (Pretorius, 1974)) occur only in narrow 

paystreaks (Lindgren, 1911), which are generally within 

a few kilometers, at most, say, 15 km, of a coarse-gold 

source (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 and 

grade rapidly decrease downstream (Boggs and 

Baldwin, 1970), as area of the deposits increases, just as 

the yield of gold decreases exponentially away from the 

upper end of a dredge table (Janin, 1918, p. 182-184). 

But even the most extensive of modern gold placers, the 

dredge fields, are small (5-50 km (Lindgren, 1911)) compared with the reefs (as much as 400 km2 (Antrobus and 

Whiteside, 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 the 

reefs! In contrast to modern placers, the Main Reef 

Leader of the East Rand has been mined throughout a 

downcurrent distance of 30 km (Pretorius, 1974), with 

the gold content and the pebble diameter of the mature 

resistate assemblage paralleling a slow decrease in size 

of detrital zircons (1.4 percent/km (Viljoen, 1968)).

According to Pretorius (1974), 55 percent of the 

world'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 imaginable, so as to conform to the mined distribtion of the 

Main Reef Leader. For, regardless of the amount of gold 

available, the evidence points to a steeper grade and 

grain-size gradient in the down-current direction than is 

developed in the East Rand Basin.

Figure 3 shows that the grain size of the predicted 

gold 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 flattening. So do those of the U.S., dominantly Alaskan, 

placers (~1 mg) described by Tuck (1968). This indicates 

that resistance to transport, rather than some winnowing effect, is the prime factor in the formation of gold 

placers.

If the gold was carried in complexed solution, its broad 

dispersion 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

0.0

EXPLANATION

Natural gold particles from 6 Siberian 

placers

Settling velocity envelope, gold from 

12 Siberian placers

-20 Settling velocity curves of fabricated 

weighed gold spheres progressively 

flattened to discs

Sphere-0.002 mg

Settling velocity 

of average Rand 

hypothetical placer 

gold grains Flake-0.6 mg

CO

cc. 

O

80

0 

Sphere

10 20

FLATNESS FACTOR

30 40 

Disc

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 factor determined from equation ((length + width)/(2 thicknesses))-!.
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA7

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 of 

granitic rock minerals. Hydrogen-ion metasomatism 

(discussed in the section, Bacterial Sulfur Cycle) of the 

feldspathic 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-sized 

fragments of serpentinized ultramafic rock.

TRANSPORT IN COMPLEXED SOLUTION

The Rand gold is believed to have been transported in 

complexed solution (Myers, 1971). It is thought to have 

been 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 at 

room 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 complexes lie at the top of the pyrite field, straddling the 

dissolved 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 were 

transported as clasts and gold moved in complexed solution. 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 dominated 

by the bacterial sulfur cycle. Gold would have been 

deposited with Eh shifts in either direction; it is likely 

that an oxidizing reaction originally precipitated the 

gold of the reefs.

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 of 

dissolved 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).

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Eh of modern 

atmosphere

CD

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9

x

o

CD

z

CJ

Q 

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DC

6 

PH

B

10 12
AA8 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

BACTERIAL SULFUR CYCLE

The writer has proposed (Myers, 1970, 1971) that an 

anaerobic 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 ancient 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 is 

slight (Harrison and Thode, 1958; Perry and others, 

1971; Hoefs and others, 1968). The formation of new 

biogenically mediated pyrite was dependent on the 

relatively high activity of dissolved Fe+2 adjacent to 

original detrital pyrite and other iron minerals concentrated on the reef surface in the infralittoral zone.

The bacterial sulfur cycle must have produced significant amounts of H2S04 , which would have reacted immediately with the feldspathic debris in the sedimentary 

environment to produce authigenic clays and (or) 

chlorite (Helgeson and Mackenzie, 1970). Buffering by 

the original silicates would have kept the pH near 

neutral. The complete conversion of this debris in the 

upper productive part of the Witwatersrand system (see 

the section, Geologic Setting of the Witwatersrand

SUNLIGHT

pH

2CO2 +H2

FIGURE 5.-Modern anaerobic bacterial sulfur cycle. The 

cycle occurs in stagnant ponds on the underside of algal 

mats and at shallow depth in certain lakes under 

anaerobic conditions. Bacteriochlorophyll of the colored 

sulfur bacteria, with an absorption spectrum utilizing 

light energy not absorbed by algae, photosynthesizes 

organic matter from C02 . Sulfate-reducing bacteria consume organic matter with oxygen extracted from 

sulfate.

Reefs) is believed dependent on bacterially mediated 

hydrogen-ion metasomatism during deposition.

Oolitic textures in Rand concretionary pyrite have 

been interpreted as biogenic (Schweigart and Rahden, 

1965); indeed, much smaller microstructures in porous 

and concretionary reef pyrite have been considered to 

represent 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 locally 

contain 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 are 

minor reef accompaniments. The assemblage suggests 

that the ancient sulfide nodules were analogues of the 

porous, bacterially mediated (Ehrlich, 1971) manganese 

oxide nodules of the present seafloor, which are potential 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 of 

silver. Though available data are not conclusive, the 

weight of the evidence suggests that much of the silver 

of the reefs was once contained in the virtually ubiquitous pyrite.

THUCHOLITE

Uraniferous thucholite, a jet-black, brittle asphaltite, 

is closely associated with detrital uraninite in the ore 

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 

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 remains 

of 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 a 

millimeter to several centimeters in thickness (de Kock, 

1964).

Thucholite also replaces uraninite in Canadian 

Precambrian pegmatitites (Ellsworth, 1928) and in the 

Precambrian high-temperature complex-metal ore 

deposit of Boliden, Sweden (Grip and Odman, 1944). In 

these settings and in the Rand, the mineraloid is thought 

to have been formed by polymerization of fluid hydrocarbons by uraninite radiation (Liebenberg, 1955; 

Schidlowski, 1969). Experimental radiation of any type 

of n-paraffins of intermediate chain length has three 

products (Charlsby, 1954): (1) long-chain infusible 

molecules with lowered H/C ratios (graphite is formed
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA9

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 many 

of 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 by 

Whitehead (1954) suggest the uranium here believed 

originally present in the Rand reefs was adequate to 

produce the thucholite substrate-hydrocarbons from 

bacterial 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 in 

thucholite sheets. The 13C values in thucholite are typical 

of 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 latter 

from the Witwatersrand (Pierce and others, 1958).

Hallbauer (1975; this volume, chap. M) has proposed 

that the thucholite represents the remains of primitive 

lichen. The characteristic cross-fibered habit of the 

asphaltite is taken by him to represent fungal hyphae; he 

interprets 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 flattened to carbon films by rock compression. It is even 

more difficult to imagine why the delicate microscopic 

filamentous structures would be preserved after undergoing not only strong compression but also very high 

radiation dosages from the ubiquitous detrital uraninite 

and why these structures are seen only in the severely 

oxidized material. Hallbauer has acknowledged (oral 

commun., Oct. 14, 1975) the possibility that the 

thucholite 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 whose 

orientation was controlled by the original fibrous texture of the thucholite and which were preserved during 

the subsequent oxidation. According to this interpretation, the gold filaments figured by Hallbauer (1975) were 

drawn 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

of 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 site 

by a combination of ionizing radiation and the presence 

of dissolved uranium adjacent to detrital uraninite. (The 

oilfield uraniferous asphaltites of the Texas Panhandle 

formed from the abstraction of dissolved uranium by 

petroleum, according to Pierce and others (1964), and 

contain no particulate uraninite larger than 2 jum.) It is 

maintained here that the apparent "digestion" of detrital 

uraninite by thucholite is illusory. The detrital uraninite 

was instead dissolved along minute flaws and fractures 

by an aqueous oxidation (see the section, Genesis of Gold 

in 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 of 

the 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 finally 

immobilized 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 oxidation and solution. Elsewhere in the Rand, high-grade 

concentrations of uraninite are invariably thucholitized 

(Liebenberg, 1955). That such areas were nowhere 

sealed off by clay minerals or silica is probably owing to 

a side effect of the strong radioactivity (see the section, 

Genesis of Gold in Reefs, Oxidizing Reactions).

INVERSE RELATION OF URANIUM AND GOLD

This investigation has recognized that gold and 

uranium vary inversely in the pyritic conglomerates. 

The relation is held to be due to the progressive solution 

of 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 another 

explanation 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 a 

considerable 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, for 

reefs very rich in gold to have very high uranium/gold ratios. Such high 

ratios are found usually in reefs with low gold values and indicate a 

more 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 are 

listed for which production figures for both uranium and 

gold are available.

TABLE 2. — Uranium and gold contents, in parts per million, and the 

U308/Au ratios of pyritic conglomerates in South Africa, Canada, 

and Brazil

District, mine, or single reef U3 08 Au U3 0,/Au

1

2 

3

4

5

fi

7

8 

9 

10

11

12 

13 

14

15

16

17

18

19

20

21 

22

23

?,4

?,5

?,6

?,7

?,8

29 

30

31

3?,

33

34

35

36

37

Blind River-Elliot Lake district, Canada, 34 samples ' ____ 

Upper Monarch Reef, Zone 2, West Rand 

Consolidated Mine. 2 

Blind River-Elliot Lake district, Canada 3

Monarch Reef, West Rand Consolidated Mine * _________ 

Upper Monarch Reef, Zone 4, West Rand 

Consolidated Mine. 2 

Luipaardsvlie Mine, uranium section, 1956-19644 West Rand Consolidated Mine, uranium section, 

1959-1972.4 

East Champ d'Or Mine, uranium section, 1959-1964 4 ____ 

Randfontein Mine, uranium section, 1959-1964 4 

Randfontein Mine, SD 32 Shaft area 5 White Reef, West Rand Consolidated Mine 2

Randfontein Mine, Cooke Section,No. 3 Shaft area 5 _____ 

Randfontein Mine, Cooke Section, No. 2 Shaft area 5 _____ 

Vogelstruisbult Mine i - i

Virginia Mine 4 ' 8

Zandpan Mine 4 ' 8 Western Reef Mine 4 - 6

Daggafontein Mine 4 ' 6

Vaal Reefs Mine 4 > 8

Hartebeestfontein Mine 4 > 8

Randfontein Mine, Cooke Section, 

No. 1 Shaft area. 5 

Buffelsfontein Mine 4 ' 6

Ellaton Mine 4 - 6

Harmony Mine 4 > 8

Welkom Mine 4 ' 6

Freddies Consolidated Mine 4 ' 8

President Steyn Mine 4i6

Loraine Mine 4 - 6

Canavieras Mine, Serra de Jacobina, 

Brazil 7 . 

Doornfontein Mine 4 ' 6

Stilfontein Mine 4 - 6

Eastern section, Free State Geduld Mine 8

Blyvooruitzicht Mine 4 - 6 West Driefontein Mine 4 > 6

President Brand Mine 4 - 6

Central section, Free State Geduld Mine 8 Western section, Free State Geduld Mine 8

1,777 

1,331

1,400

1,250 

954

791

712

402.3 

578.2 

900

627

450 

400 

243

243.5

343

239

193

321

296.5

200 

262

153

217.5

142.5

132

138

117

150 

145

135

109.5

168

164.5

131

67

115

0.262 

.700

.857

1.720 

1.320

1.330

1.280

1.000 

2.040 

5.00

5.010

6.00 

7.00 

8.280

8.420

13.050

9.210

8.350

16.100

15.450

11.00 

14.580

8.600

12.960

10.610

11.330

12.000

10.720

15.430 

15.170

14.940

12.620

21.370

28.660

23.950

23.920

46.870

6,780 

1,900

1,750

727 

723

595

556

402 

283 

180

125

75.0 

57.1 

29.3

28.9

26.3

26.0

23.1

19.9

19.2

18.2 

18.0

17.8

16.8

13.4

11.7

11.5

10.9

9.7 

9.6

9.0

8.7

7.9

5.7

5.5

2.8

2.5

"3

0>&*

Immature

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 and 

Orange 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) represent all uranium ore milled and all gold ore milled from a 

single mine. In all such cases the uranium is a coproduct, 

and the amount of uranium ore milled is less than that of 

gold ore, ranging from 10 percent to 98 percent of the

tonnage 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. Evidently, if the uranium ore makes up but a small percentage of 

the gold ore, the uranium grade of the plotted point on 

figure 6 will be higher than the true average would have 

been for all gold ore. Yet the distribution of such points 

shows that this factor is not a dominant one in determining whether the mine in question lies above or below the 

calculated curve.

Most of the mine-wide averages include ore from 

several reefs. Though the grade and U308/Au ratio in 

such 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 tonnage 

of 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 be 

classed as inferior to those from single reefs, reef zones, 

or mine sections, the distribution of points around the 

calculated curve does not appear to support this conclusion.

The three points plotted for Canada and Brazil (solid 

triangles) do not represent formal averages of specific 

amounts of ore. One of the Blind River-Elliot Lake 

averages represents the arithmetic mean of all samples 

(34) for which combined uranium-gold assays were 

available, regardless of reef or location. The other point 

is 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 tons 

of 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 is 

read as a diagram of reef maturity, with the abscissa as 

a 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 detrital 

uraninite and of large uraninite crystals in a Canadian 

pegmatite (Liebenberg, 1955; Ramdohr, 1958; 

Schidlowski, 1969)-thucholitization apparently both 

digested uranium and expelled it from the system.

All gold-bearing Rand reefs contain thucholite, with 

which 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

1750

1500

1250

DC 

HI 

Q.

1000

750

500

250

20-

EXPLANATION

? 8 ratio on maturity curve

A Average ore for Canada and Brazil

IN SOUTH AFRICA:

t—(————1> Three annual plots of single mine sections (grade varies radially from origin). Tick indicates average ore

O Minewide average; all U ore milled, all 

Au ore milled

• Single reef or single reef zone

10 20 30 

Au, IN PARTS PER MILLION

40 50

FIGURE 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 and 

Gold. The curve is from the best-fit linear regression calculated by the method of least squares on the logarithms of the data 

points.
AA12 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

FIGURE 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. Horizontal 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 a 

property of the bituminous material itself; instead, it occurs because thucholite surrounds highly radioactive 

uraninite whose nuclear decay resulted in the precipitation of gold.

RADIOLYSIS OF WATER

The radioactive decay of detrital uraninite accumulating on the Rand reefs caused the radiolysis of 

water and the formation of free radicals according to the 

following reactions:

H20

H2 0 + eH20 + + H20

H20 + + 

OH" + 

H30+ +

H, and 

OH,

where w indicates ionizing radiation and • H and • OH 

denote charge-free free radicals (Dragonic and 

Dragonic, 1971, p. 10-11).

When water is irradiated with X-rays or y-rays in a 

completely water-filled vessel and the ionizing radiation 

is then removed, it is found that the free radicals recombine and only water remains. Contrastingly, if the same 

radiolysis takes place in the presence of a large volume 

held at a vacuum, there is a continuous evolution of free 

hydrogen (Alien, 1961, p. 24, 75). From this it is clear 

that under low-pressure conditions where hydrogen can 

diffuse 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 the 

fractionation would have been effective. A hydrogen 

pressure in the ancient atmosphere of 10~ 4 atm and a 

reef 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 the 

reef 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 even 

less. Thus, the calculation shows that diffusion of 

hydrogen would have been an effective fractionating 

mechanism.

OXIDIZING REACTIONS

Radiolysis fractionation would have resulted in an excess 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 reactions:

H2 0 —H • + -OH, f~i TT f~* TT • _i_ • r^TT

•"OHVCH4 , 

H2 , 

H2 02 (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 oxygenated 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 fractions 

of 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 by 

reactions involving water (Colombo and others, 1964). 

Water served as hydrocarbon oxidant because the bubble of reducing gases in the test vial did not equilibrate 

with oxidizing hydroxyl radicals in the subjacent 

hydrous mixture.

H • + • OH + • CH3

H • + • H

OH '• + • OH
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA13

Similar oxidizing reactions must have taken place during the formation of natural uraniferous petroleum 

asphaltites investigated by Pierce and others (1958, 

table 1). Plotting of their data on 23 asphaltites and one 

Rand thucholite shows a direct relation between the 

uranium content and oxygen content. This correlation is 

best displayed as an inverse relation between uranium 

and the C/0 ratio (fig. 8.).

Such reactions would have been very slow because the 

radioactive 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 the 

Phanerozoic 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 be 

evoked 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 the 

section, Thucholite). Lower-grade accumulations commonly contain unthucholitized grains, implying that 

these areas were sealed off by authigenic silica or clay 

minerals from chemical interaction. It seems likely that 

the high-grade concentrations gave off enough 

hydrogen that they were kept permeable to aqueous 

solutions.

MATURATION OF REEFS

Several processes working concurrently during the 

formation of the reefs control the inverse relation between uranium and gold:

1. Chemical reactions driven by the nuclear decay of 

uranium:

a. A rate of uranium solution that depended, via 

the uraninite-driven radiolysis of water, on 

the amount of uranium present, and hence decreased exponentially with time from the 

onset of thucholitization to the time of effective burial. (Radioactive decay is a first-order 

reaction - that is, the number of atoms that 

decompose in unit time will be proportional to 

the number present.)

b. A linear precipitation of gold, the rate determined at first by the concentration of the 

scarce aurous complex, later by the diminished availability of the precipitant, which in turn 

was controlled by the concentration of 

uraninite.

2. Concentration due to reworking:

a. The gold content increased exponentially with 

time because of concentration into an everthinning reef.

FIGURE 8. - Uranium content and C/0 ratio for 23 uraniferous petroleum 

asphaltites and a Rand thucholite. Data from Pierce and others (1958, 

table 1) recalculated to an ash- and sulfur-free basis. The curve is from 

the best-fit linear regression calculated by the method of least squares 

on the logarithms of the data points.

b. The rate of the chemically controlled exponential decrease in uranium was slowed by the opposed process of concentration.

Together, these actions define a maturing process. A 

uraninite placer, accumulating on an unconformity, 

became progressively converted to a gold reef with little 

residual uraninite. Immature uraninite-rich reefs tend 

to be relatively thick conglomerates with dispersed 

metal content (Roscoe, 1969; Pretorius, 1964). The most 

mature reefs tend to grade toward the thucholite-seam 

type, very thin but exceedingly rich in gold (Liebenberg, 

1955; de Kock, 1964). A combination of chemical attack 

and physical reworking accounts for the general thinness of mature reefs and for the detrital implication of 

such features as cylindrically rolled gold particles encased in pyrite (de Kock, 1964, p. 368). A maturing reef 

at any given moment is the sum of the chemical processes that make it progressively more uranium-poor 

and gold-rich and the reworking that redistributes the 

chemical products as it concentrates them by thinning 

the assemblage.

As is more fully explained in the section on Degrees of 

Association of Uranium and Gold, mature reefs (table 2) 

are those whose rates of gold precipitation are controlled by the amount of uraninite present-that is, 

those reefs in which the amount of uraninite is so
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 of 

gold deposition.

Hydrogen-ion metasomatism of feldspathic debris and 

the 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 is 

expectable that these elements should be present in the 

authigenic reef minerals. Gold is present both in the 

phyllosilicates 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 disseminated 

uranium (Roscoe, oral commun., October 14, 1975). 

These findings independently suggest that uranium was 

mobile 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 a 

linear time scale by assuming that the uranium loss with 

time maintained a constant half life (this is further explained in the section, Competency of Radiolysis). The 

gold content is assumed to have increased exponentially 

with time owing to reworking, which concentrates the 

continually 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 later 

stages of reef development. Figure 9 treats the initial 

uranium 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 that 

take 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 characteristic 

U308/Au ratio (see points 6, 8, and 9 on fig. 6). However, 

for several reasons adjacent small samples show wide 

and erratic variations in the ratio. As will be explained 

in a later section, in immature reefs the ratio is dependent 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 inTIME

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-rich 

areas. These immature patches skew the curve of figure 

6, reflecting residual uranium on the gold-rich arm, from 

the ideal hyperbolic form. Lastly, the contents of the 

maturing reefs were mixed and concentrated by 

repeated reworking.

Pyrite appears to decrease in abundance with increasing 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 de 

Jacobina gold ore (Bateman, 1958) and to about 3 weight 

percent 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 special 

case. At the Free State Geduld Mine (32, 36, 37 on table 

2) 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 this 

reef is a reworked remnant of a thicker, less mature 

reef, then the pyrite of the remnant, calculated back to 

about a meter thickness, turns out to be about one-third 

that 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 was 

scanty or absent, uraninite was completely dissolved by 

the oxidizing effects of radiolysis.
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA15

Paradoxically, uraninite is locally more abundant than 

pyrite in immature ore at the Dominion Reefs Mine 

(Hiemstra, 1968a, p. 5). However, the high thorium content of the Dominion Reef uraninite (Grandstaff, this 

volume, chap. J; Feather, this volume, chap. Q) renders 

the mineral resistant to oxidation and solution, so that 

the 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 gold 

in the Rand reefs also progressivley oxygenated 

thucholite.

The positive correlation coefficients between U308 and 

gold in the Rand ores (Saager and Esselaar, 1969) thus 

reflect not the detrital nature of both components, as 

these 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 to 

sedimentological factors primarily reflect such control 

of placer uraninite (d ~ 9) rather than placer gold 

(d~ 17V2 ).

12345 

LOGio Au CONTENT, IN PARTS PER MILLION

FIGURE 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 the 

best-fit linear regression calculated by the method of least squares.

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 the 

best-fit linear regression calculated by the method of least 

squares.

COMPETENCY OF RADIO LYSIS

The competency of the decay energy of uraninite as a 

driving 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 to 

support the oxidation reactions. Evidently the 

calculated 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 between supply and demand of oxidant would be reduced 

or erased if an estimate could be made for the 

mechanical abrasion of pyrite, which is not allowed for 

in the following discussion but which surely must have 

taken place.)

In the Witwatersrand Basin the productive reefs are 

scattered through a stratigraphic section with a maximum thickness of about 9 km, spanning a time interval 

of half billion years. In the Central Rand a stratigraphic 

thickness of 8.55 km (Pretorius, 1964) separates the 

volcanics 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 and 

Burger, 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 CONGLOMERATES

Therefore, 

(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 in 

the 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 the 

chemical half-life, or intensity of solution of detrital 

uraninite, during reef formation. In figure 6 it is seen 

that the Loraine point (28) projects along a maturity ray 

to intersect the normalized curve at a U3 08 concentration of 165 ppm. The U3 08 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 U3 08 , 16 times the concentration of 

the normalized Loraine point. The Loraine point then 

lies 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 the 

effect of thinning the reef during reworking (see 

paragraph 2b in the section, Maturation of Reefs). It can 

be shown that by assuming a concentration of 2, due to 

thinning of the reef in 500,000 years to one-half its 

former thickness, which seems reasonable, the true 

chemical half-life of uraninite is halved from the net or 

apparent rate: 500,000 yr/2 = 250,000 yr.

By comparing the chemical half-life of uraninite on the 

reef with the radioactive half-life of 238U and 235U, we can 

calculate the number of atoms of uranium chemically 

dissolved 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, and

for 235U,

7.13xlO8 28.52xl02 molecules U02 dissolved

0.25 x 106 per 235U nucleus decayed.

A large excess of pyrite was oxidized with the 

uraninite. The function of the pyrite as an oxidation buffer indicates that it would only have been oxidized to the 

minimum necessary to dissolve the sulfide:

UO, + 2 • OH -* U02(OH)2 

FeS2 + 6 • OH - Fe +2 + S2 03 - 2 + 3H2 0

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 originally 

prevailing were only 46.1 and 61.3, respectively. 

Assume that a pyrite-uraninite mix at an FeS2/U02 ratio 

of 61.3 was continually dissolved from the reef 

thoughout its history. In the earliest stage assume this 

to 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/U3 08 

ratio assumed may be combined (if the "U02 " of the 

chemical formula is equated with the U3 08 of mining 

statistics) to estimate the number of hydroxyl free 

radicals-needed to oxidize each molecule of uraninite and 

the accompanying pyrite:

Molecules 

oxidized

1 U02 _. 

61.3 FeS2

Total

Hydroxyl

free radicals

consumed

_______________________ 2

________________________ 368

________________________ 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 shorter 

radioactive 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

550,202

6,876,978

Total for natural uranium = 6.9xl06 hydroxyl free 

radicals produced for each nucleus decayed.

Dragonic and Dragonic (1971, p. 125) report a mean 

value of 4.1±0.5 water molecules decomposed per 100 

eV absorbed in the radiolysis of water. This value was 

averaged from "several hundred papers reporting data 

obtained under different experimental conditions."
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA17

Birch (1954, p. 151-152) lists the effective radioactivedecay energy of uranium as follows:

Isotope MeV per atom Cal per g-year

235U

47.4

45.2

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 radioactive 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 water 

molecules 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 H2 0 molecules decomposed per

uranium nucleus decayed.

ORANGE FREE STATE GOLDFIELD

In the Orange Free State Goldfield, uranium is 

relatively high, and gold low, on the margins of the field, 

where the unconformity beneath the productive Basal 

Reef is minimal (Winter, 1964a, b; McKinney and 

others, 1964; Minter, this volume, chap. K). Gold increases and uranium diminishes as the unconformity 

becomes more pronounced: the reef increased in maturity where the unconformity was of longer duration. 

Detrital grains of platinoids correspondingly are more 

abundant—0.008 ppm—and less mature—Os + Ir = 

66-71 percent (Cousins, 1973)— on the margins of the 

goldfield and less abundant—0.0025-0.004 ppm—and 

more mature—Os + Ir = 76-79 percent—where the unconformity is longest lived. The decrease in abundance 

is 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 origin 

of 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 expectable 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 environment is due to chemical stability and superior hardness; oxidizable osmium and iridium both diminish with 

maturity in modern placers accumulating in an oxidizing 

atmosphere, and platinum becomes increasingly dominant (Mertie, 1939, p. 135-137).

WHITE REEF

A series of as many as a dozen generally uranium-rich 

reefs scattered through 80 m of quartzite (Toens and 

Griffiths, 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 (U3 08/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 for 

both uranium and gold (Toens and Griffiths, 1964). 

Figure 12 shows that the U3 08/Au ratio varies progressively 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, suggests 

that 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 are 

available (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 REEFS

In the country lying between the West Rand and the 

West 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 sequence 

of three conglomeratic subzones spanning a 

stratigraphic 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 the 

mature 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)

19

18

29

Au 

(ppm)

11

7

6

U3 08 

(ppm)

200

400

450

U3 0,/Au 

(ratio)

18

57

75
AA18 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

In this 'segment, scattered patches of gold ore 

(with UaOg/Au ratios of 10-15) mined

West Rand Consolidated

2 MILES 

I

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 GOLD

In the early stages of reef formation (immature reef) 

uraninite was present in excess of the amount needed to 

precipitate the amount of gold available in complexed 

solution. In the later stages (mature reef) uraninite was 

so diminished in amount that the effects of its radioactive 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 a 

mature reef a perfect association between uranium and 

gold would be expectable. In immature reefs the degree 

of association or correlation would have been less than 

perfect.

Discussions of the reef maturation process have until 

now made the simplifiying assumption that the reef had

6810

White Reef-^

12.5

GOLD RICH

FIGURE 13.-Us08 /Au range of the White Reef, West Rand, and the 

Basal Reef, Orange Free State, compared with all reefs. (Curve from 

fig. 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 been 

highly variable from point to point. Figure 14 shows the 

effect of different original concentrations of detrital 

uraninite in a maturing reef. Each curve represents the 

decrease 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-
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA19

mined the rate of gold precipitation. This constant ratio 

defines maturity for a particular original concentration 

of 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 detrital 

uraninite occurring with significant frequency in the 

reef. 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 seem 

to have been achieved in the Witwatersrand, for another 

factor intervened that progressively diminished the 

degree of association of uranium and gold. This factor is

FIGURE 14. - Changes in the ratio \J3 0S present per unit of gold 

precipitated for different detrital uraninite concentrations in a reef.

the sealing-off of local patches of reef against chemical 

interaction (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 in 

a 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 coefficient of correlation r (also termed the product-moment 

correlation coefficient). The linear relation between two 

variables X and Y is calculated by the formula

where 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 samples 

of pyritic conglomerate of the widest range of maturity 

(table 4).

TABLE 4.— U3 08/Au ratios and the linear coefficient of correlation r 

between uranium and gold for sample suites from reefs in South 

Africa and Canada

Sample 

suite

Number of y 0 /Au 

samples ratjo 

(locality)

Source

1 , Elliot Lake district _ 34 6,779 0.13

Dominion Reef:

2 Dominion Reefs Mine _ 

White Reef: 

Randfontein Mine:

3 F __

20 477.2

24 261.1

4 E

5 G

6 D

7 C

8 B

9 A

Vaal Reef:

10 Zandpan Mine.

(14 level)

15 

(8 level)

25 

(6 level)

13 

(14 level)

20 

(8 level)

17 

(8 level)

11 

(3 level)

11 Hartebeestfontein

Mine 

Ventersdorp Contact Reef:

12 Venterspost Mine ____ 

Basal Reef:

13 Free State Geduld 

Mine.

44

43

56

109

150.7

128.4

122.1

112.3

81.30

61.44

23.31

17.36

6.62

2.78

.55 

.38

.89 

.36 

.63 

.91 

.80 

.85

.70 

.605

Roscoe (1969, 

Appendix table C).

Hiemstra (1968b, table V).

B.D. Stewart, Johannesburg 

Consolidated Investment 

Co., Ltd., written commun., Sept. 18, 1975. 

Do.

Do. 

Do. 

Do. 

Do. 

Do.

Hiemstra and Rahden 

(1969). 

Do.

Do.

Saager and Esselaar 

(1969).

1 Numbered points on figure 15.
AA20 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

I

00

OCO

=> 2

O

(5

O

\ I I

III 1

Immature 

reefs

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 U3 08 /Au ratio for individual reefs in the Witwatersrand and for the composited Blind 

River-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 foregoing 

analysis and clearly supports the outlined hypothesis of 

origin of the reefs. The relatively wide scatter of points 

from the immature reefs probably derived from the 

small sample sizes, although all but the Canadian sample 

are well above the limit for a five-percent level of 

significance. Because the Elliot Lake district sample is 

composited from several productive conglomerates 

rather than a single reef, it has been excluded from an 

alternate treatment of the immature reefs. Theoretical

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 the 

U308/Au ratio of the suite.

VOLCANIC INFLUENCE

BLIND RIVER-ELLIOT LAKE

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 upper 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 

H2 S is quickly oxidized to S04 - 2). The Blind River reefs, 

in contrast to those of the Rand, characteristically contain tightly packed, subangular clastic grains of 

uraninite in unthucholitized streaks and cross laminae. 

Thucholite is generally rare but locally abundant 

(Roscoe, 1969; Ruzicka, this volume, chap. V); the low 

content 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 and 

presumably 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 for 

which both gold and uranium assays are available 

(Roscoe, 1969, Appendix table C) is 0.26 ppm, twice the 

average yield of gold dredging (see the section, Gold, 

Placer Origin Unlikely).

VENTERSDORP CONTACT REEF

In the Rand, the contact between Witwatersrand 

strata and overlying Ventersdorp andesites is commonly 

an angular unconformity marked by the Ventersdorp 

Contact Reef, which, though not widely productive and 

unique in its explicit volcanic association, is a typical 

Rand reef (Haughton, 1969; de Kock, 1964). In and near
GENESIS OF URANIUM-GOLD PYRITIC CONGLOMERATES AA21

the Western Areas Mine, however, no unconformity is 

detectable, and a tuffaceous gradational zone as much as 

30 m thick, here believed the marine equivalent of early 

lavas, lies beneath andesite flows. Within the zone and 

as 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 the 

solubility of aurous thiocomplexes and, with unconformably older reefs as additional local gold sources (de 

Kock, 1964; Papenfus, 1964), would have increased their 

concentration as well and so increased the rate of gold 

precipitation in the reefs.

DISCUSSION

The traditional view that the gold of the pyritic conglomerates is placer has been generally refuted in the 

section, 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 the 

reefs. He has emphasized sedimentation on the branching distributaries of a large deltaic fan. Minter's 

model is silent as to the significance of the unconformities upon which the productive reefs lie. Deltas and 

fans, taken as a whole, are areas of uninterrupted 

deposition. Yet by contrast the Rand reefs, of delta-wide 

dimensions, are developed on through-going unconformities marked by the reworking of material eroded from 

the 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 less 

competent fine-grained detrital uraninite to the distal 

end of the fan. Gold would be preferentially deposited 

near 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 of 

oligomictic, cleanly washed reefs forming on unconformities 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 those 

of the other type" (oligomictic cleanly washed conglomerate). This initial formation of relatively uraniniterich, gold-poor conglomerate is in conflict with the 

mechanism of selectively bypassing uraninite down current, whereas the variable maturity model advanced 

here is in complete harmony with Winter's observations.

In the section on the Orange Free State Goldfield, an 

inverse relation between gold and detrital platinoid 

grains is described. If the Rand gold were detrital, a

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 much 

heavier than the near-spherical brittle platinoid particles 

(fig. 3). The average median diameter of the latter (58 

jum), as reported by Cousins (1973), is just that computed 

for hypothetical spherical particles with d~17.5 in 

hydraulic equivalence with the average Rand reef clasts 

(fig. 3). The hydraulic equivalence of platinoid grains 

and reef clasts, coupled with the inverse relation between platinoids and gold, strongly suggests that gold 

was not of placer origin but was transported in solution.

The thesis developed here has important implications 

in assessing the uranium reserves of the pyritic conglomerates, particularly those of the Witwatersrand, 

the mined representatives of which are very largely gold 

rich. If the gold-rich reefs are the products of longcontinued maturation on unconformities, then they 

should both grade into and be apposed by less mature 

uranium-rich reefs. On the ground that unconformities 

of lesser duration should be more numerous than those 

of 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 the 

section, Inverse Relation of Uranium and Gold), who 

noted that high ratios of U308/Au "indicate a more 

widespread 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 historical 

pattern of an industry devoted first and foremost to the 

mining 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 of 

the abundance was delayed until about a decade later.

The West Rand Goldfield leads in production from 

uranium-rich Witwatersrand reefs. Yet the recently 

published reserves of a new section of a single mine (see 

table 3) contain more uranium than the total mined 

through 1975 from the West Rand. It is significant that 

the uranium-rich reefs of the Cooke Section of the Randfontein Mine exploit a stratigraphic zone not yet mined 

elsewhere. It seems likely that the Witwatersrand will 

emerge as an even more massive reserve of uranium 

than has been heretofore recognized.

APPENDIX: DETERMINATION OF ORIGINAL 

URANIUM

The original amount of natural uranium and its 

isotopic distribution at 2.5xl09 yr ago may be determined from that remaining at present by use of tables of 

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 

the atomic ratios of radiogenic lead isotopes to the 

parent uranium isotopes. The original amount of 

uranium isotope is simply that remaining at present plus 

that 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 is 

taken as unity, the original amounts are

238U 5 U

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.

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DISCUSSION FOLLOWING TALKS BY 

DRS. STAATZ, CANNON, AND MYERS

Skinner: Brad [Myers], 111 confine myself to three 

points, 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 maintain 

that the Russian information on settling velocities is 

incorrectly applied by you to the grain size of the Witwatersrand gold. The second point concerns Eh-pH 

diagrams. How you draw those fields depends on the 

molalities 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 AA25

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 be 

correctly interpreted as the reason why waters have 

not flushed out the uranium and gold, not why they 

brought gold in. That is, you have shown a field where 

you have uraninite, gold, and pyrite stable, and the 

pyrite controls the system in part and doesn't let the 

gold and uranium be flushed out. The third point is one 

I don't know very much about and I would hope 

somebody in the audience might know a great deal 

more about it, but I would very much doubt that the 

radiation flux in these waters is anywhere near high 

enough for the radiolysis reaction to go on. I would imagine 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 and 

thought that it would go on fast enough. I have had 

criticism on this point before, and at this point I do not 

want to tie myself down on it.

Grandstaff: I'd suggest that the presence of uraninite in 

other old Precambrian rocks, pegmatites for example, 

would suggest that this process is not important in 

destroying the uraninite.

Myers: In the original description of thucholite, the 

author 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 Precambrian rocks may not have been subjected to the proper 

conditions for a sufficient length of time.

B. Nagy: Just a very brief comment. In your paper you 

have shown a normal alkane transformed by irradiation into what you call thucholite. In other words, the 

reaction says that thucholite is a polymerized 

hydrocarbon, obviously, basically aliphatic. Who 

analyzed 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; these 

are 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 to 

use thucholite because this is not the classical definition) from other materials also.

Myers: Wait, you say that the Rand carbonaceous 

material is not thucholite?

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" and 

what you call "thucholite," is something other than 

normal 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 he 

got very long chain hydrocarbons plus hydrogen plus 

methane, but this is done anhydrously. When you do it 

with water then you have radiolysis of water to contend with, and this is an oxygenating reaction. There 

have been experiments on crude oil irradiated with 

water, and you get an increase in the percentage of 

asphaltenes, and the reason this is oxidizing is because 

you form reducing gases in a bubble in a vial, and 

these 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 aromatic 

heteroatomic 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 going 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, assuming 

that 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 whole 

graph would be invalid. Say, for instance, if you had 

used the average stoping width of, say, 1 meter. Some 

mines might have all this as reef; other mines might 

have only 10 centimeters of reef, and in mining it is 

diluted 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 be 

somewhere up here, destroying the whole diagram.
AA26 GENESIS OF URANIUM- AND GOLD-BEARING PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES

Myers: The objection was made that I have used average 

values instead of individual values, and I can't see that 

that's a very serious objection. I used millions of tons 

of milled ore rather than individual reef samples, and 

it 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 have 

used 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 make 

those curves, and he used assays of reef. There certainly has to be a pretty good correlation between the 

reef 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 instance, 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 the 

whole lot are going through the mill, and every one of 

those 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 to 

vary from seven to over forty. Now, which one do you 

take?

Myers: What I took were reserve values given to me by 

Anglo-American for the average grade of U308 and 

gold for the eastern, central, and western sections of 

Free 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 from 

White Reef.

Minter: There happen to be two reefs there.

Myers: And remember we had three rather straight 

plots; those were year-by-year plots with the average. 

West Rand Consolidated is not on there. It's another 

spot, and it's another spot because they do mine many 

[reefs], and if you plot yearly figures, you don't get a 

straight line. You get a Brownian movement because 

of what you just said.

Minter: The White Reef doesn't carry very high uranium 

content; it's the Monarch Reef that carries the high 

uranium 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 southeast 

of the Witwatersrand. There are pyritic conglomerates in the Ukraine; we don't know how much 

uranium they contain, but they contain detrital 

uraninite, and they contain up to a gram or so of gold. 

In terms of crustal abundance, there's a fair amount of 

gold in Elliot Lake, and if we go down east of Sudbury 

to Lake Timagami you get a couple of parts per million 

of 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 PRECAMBRIAN 

QUARTZ-PEBBLE CONGLOMERATES

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1161-BB

CONTENTS

	Page

Introduction ______________________________________________ BB1

Uniqueness of the deposits _____________________________________ 1

Time of formation __________________________________________ 1

Sites of deposition __________________________________________ 2

Source areas _____________________________________________ 3

Diagenesis ______________________________________________ 4

Metamorphism and recrystallization ________________________________ 4

Discovering new deposits _____________________________________ 4

Discussion following Dr. Skinner's summation __________________________ 5

ILLUSTRATION

Page

FIGURE 1. Drawing showing possible placement in an idealized fluviatile system of five uraniferous detrital sediments___________ BBS

in

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

THOUGHTS ABOUT URANIUM-BEARING

QUARTZ-PEBBLE CONGLOMERATES: 

A SUMMARY OF IDEAS PRESENTED AT THE WORKSHOP

By BRIAN J. SKINNER l

INTRODUCTION

The reasons for my selection as the summarizer and 

final 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 found 

the talks to be extraordinarily provocative and my attendance 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. Therefore, I propose to comment on six or seven of the most 

interesting, 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 in 

solution, 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 DEPOSITS

The first point concerns the uniqueness of quartzpebble conglomerates. It struck me, as I listened to the 

presentations, that I had heard similar words 30 years 

ago, right after the Second World War, when one of the 

periodic debates about Precambrian iron-formations 

erupted. Similar debates about iron-formations had occurred 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 and 

faster 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 preponderance of information from the Witwatersrand and 

Blind River deposits. But at this conference you have 

discussed a number of other quartz-pebble conglomerates, and you have convinced me that Precambrian 

conglomerates suitable to be uranium hosts are probably 

much more common than now accepted. They are not 

highly unique and probably not due to a special set of 

local circumstances. This suggests there are many more 

Precambrian quartz-pebble conglomerates yet to be 

found. 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 return 

for another conference 30 years hence, I suggest we 

would be discussing three or more times the number of 

deposits we are discussing today.

TIME OF FORMATION

My second point concerns time of formation. It is a 

question that many of the speakers mentioned and many 

of you questioned from the floor. Quartz-pebble conglomerates can, of course, be found in geological sections 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 must 

have prevailed for a limited time or that some special 

conditions of surface waters and atmosphere existed for 

a 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 the 

eastern 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 a 

problem 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 change 

of tectonic patterns, which is distinguished by a new 

style of sedimentation occurring in cratonic basins. If 

similar basins and suitable source areas had existed at 

earlier times, still older quartz-pebble conglomerates 

could have formed. But they did not form because the 

basins and adjacent sediment areas did not have the 

right geometries.

What of the other end of the time bracket? The 

youngest deposits that seem to fit the features of 

uraniferous 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 of 

detrital grains of uraninite. The base of the Transvaal is 

about 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 feel 

that the argument for atmospheric control-a simple atmosphere composition control-was proven. I am not 

sure 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 atmosphere with a low partial pressure of oxygen might be erroneously interpreted to be a reducing atmosphere if a 

given oxidation reaction did not proceed. The lack of 

reaction may simply mean that reaction kinetics are too 

slow for oxidation to occur in the time available. David 

Grandstaff demonstrated that atmospheres with partial 

pressures 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 detrital

uraninite [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 the 

cratons relative to the poles when the basins formed and 

were filled by sediments? Was the appropriate style of 

sedimentation a feature of high latitudes and hence low 

temperatures? 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 oxygenated atmosphere with as much as 10- 4 atm of oxygen as 

far back as 3 b.y., and still had transport of detrital 

grains of pyrite and uraninite for long distances. We 

cannot, therefore, conclude that the atmosphere did exert 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, or 

in 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 the 

streams flowed into lakes or at least into bodies of water 

that were shallow and possibly intermittently wet and 

dry. 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 can 

be discerned and that the different deposits simply 

represent different portions of large drainage systems. 

When Larry Minter spoke he used a simple sketch to 

demonstrate where, in a long drainage system, the Witwatersrand sediments formed. He drew a stream 

system which rose in highlands, meandered across a 

flood 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 in 

figure 1. The Brazilian deposits seem to be proximal 

facies. What the basement relations are remains to be 

seen, and of course the entire section of the Brazilian 

sediments remains to be exposed. However, I suggest 

that the Brazilian sediments fit in as shown in figure 1, 

the Moeda overlapping with the Elliot Lake environment and the Jacobina overlapping with the Witwatersrand 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 

Canadian deposit with a lot of very well sorted material. 

It seems to include the distal ends of the fans and to 

reach beyond the ends of the fans.

Blind River 

(CANADA)

Moeda and 

Jacobina 

(BRAZIL)

Witwatersrand 

(SOUTH AFRICA)

Montgomery 

> Lake 

(CANADA)

FIGURE 1.-Possible placement in an idealized fluviatile system of five 

uraniferous detrital sediments from Canada, South Africa, and 

Brazil.

We might ask whether there are modern equivalents 

of the ancient drainage systems. I would draw an 

analogy with the drainage system that dispersed the Indonesian 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 must 

have been operating in the Witwatersrand and Blind 

River 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 occurring in places. I suggest there is a clue here for the

uranium explorationist. The specific gravity of uraninite 

is about 10.5; that of cassiterite, about 7; and that of ilmenite, about 4.7. Both cassiterite and ilmenite are 

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 and 

3 b.y. ago, it, too, could have been concentrated in offshore sands by marine currents. Perhaps we should not 

focus all of our attention on fluviatile systems..

SOURCE AREAS

My fourth point is that, although the sediment source 

areas 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 of 

uraninite. But there is an unresolved question about 

grain sizes. The observed sizes of detrital uraninite 

grains are larger than the grain sizes of primary 

uraninite grains in most gneisses and granites. So, we 

have to seek regions where metamorphism has sweated 

pegmatites and quartz veins out of host rocks and in the 

process grown relatively coarse grained uraninite. This 

leads to the conclusion that source areas must all be 

deeply eroded. Because most quartz-pebble conglomerates 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 the 

source areas. It seemed to me that the major role played 

by mechanical weathering was clearly established. The 

conclusion leads us back to the question of climate and 

the possibility that all of the uraniferous quartz-pebble 

conglomerates were influenced by glacial action. That 

is, we should consider the possibility that all of the 

source areas were high and that at least the initial rock 

breakage was mechanical and was due to frost action.

The possible significance of carbon from the sediment 

source area seemed to me to be a red herring. There is 

no compelling evidence to suggest that any carbon arrived 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 we 

are dealing with a new set of chemical questions, 

because subsurface waters have different compositions 

from surface waters. The surface waters could, as we 

have heard, have been oxygenated, and the transport 

and 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 to 

the surface waters.

I was impressed with data from the Witwatersrand 

suggesting that 30 percent of the sulfides were 

deposited after detrital deposition and that a significant 

fraction of the uranium has also been similarly deposited 

or redeposited. I suggest that the most likely time for 

this to happen was during diagenesis when the relatively 

porous sediments were bathed in waters that were at 

least 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 both 

oxidation states of uranium and, depending on the water 

composition, variable amounts of U+4 and U+4 would be 

present. This in turn implies that uranium could move 

around in subsurface waters, dissolving here, depositing 

there. There could be other driving forces besides state 

of oxidation of the water, of course. One important driving 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 out 

of hydraulic equilibrium with their enclosing sediments.

METAMORPHISM AND RECRYSTALLIZATION

All 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 be 

altered by chemical reactions and are susceptible to 

recrystallization, which causes changes in grain structure. The mineral on which most of us seem to have 

underestimated the influence of metamorphism is 

pyrite. 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 track

in his work on the Witwatersrand. Similar work is needed on other deposits because we do need to understand 

how 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 was 

that 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 we 

must 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 convinced 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 DEPOSITS

My 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 such 

deposits are greater than in other areas.

I would concentrate on two features in choosing the 

broad areas to search. First, you obviously have to 

choose cratonic areas, preferably cratons that have 

large 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 sections beneath the Lake Suprior-type iron-formations. In 

Brazil the Caue Itabirite overlies the conglomerates; in 

Witwatersrand, the Transvaal System. But what about 

the 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 involvement of climate, we should be thinking about 

paleogeography. If cold climate and mechanical 

weathering 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 of 

the pole. I would, of course, concentrate first on basins 

existing between 2 and 3 b.y. ago, but I would not limit 

the search to this specific time. Why? Simply because 

the apparent younger age cutoff is so uncertain. Mortimer Staatz' observations suggest that, given the right 

conditions of source area and weathering, detrital 

uraninite can be of any age. Perhaps we should look at 

areas affected by extensive glaciations of all ages, even 

the Permian, provided a reasonable source area and 

suitable sedimentary basin exists.

DISCUSSION FOLLOWING DR. SKINNER'S 

SUMMATION

Armstrong: Incidently, there are quartz-pebble conglomerates in Australia in the Hamersley area.

Skinner: That's the earliest prediction I've ever made 

that has paid off.

Breger: I'm delighted that you mentioned temperature, 

because we have found in the laboratory that both 

uraninite and pyrite are completely stable at room 

temperature even to atomic oxygen. But if you just 

raise the temperature a little bit, they will decompose. 

So apparently there is a limitation in temperature 

range and stability range even in the presence of oxygen that hasn't even been mentioned here.

Skinner: Maybe that's why there is so little organic matter left in these things; it was too cold for the 

organisms to grow anywhere else, except in those infrequent little puddles that were left to the open sun.

Roscoe: May I add a note of caution to this concept of 

temperature being so important, since Dr. Little attributed comments about cold temperature in the 

Elliot Lake area to me. In the Montgomery Lake area, 

where there is permafrost to great depth, the oxidation is to great depth, too. There is considerable oxidation to the deepest depth that has been drilled there, 

whereas at Elliot lake it's an eighth of an inch or so, 

except along fractures.

Skinner: You mean the paleoregolith at Montgomery 

Lake was several hundred feet?

Roscoe: No, on the present surface. Under the present 

climatic conditions with permafrost, the rock is deeply 

weathered and leached apparently, as compard to 

more southerly zones in the Canadian Shield. It's quite 

surprising.

Skinner: I would think that the analogy is perhaps not 

quite right, that one would need an elevated terrain 

and the rapid erosion that might come with permanent 

ice and seasonal thawing. The region that you are talking about is a highly glaciated one that once had 

glaciation but doesn't now.

Roscoe: No. The point I was trying to make was that, in 

this case at least, a cold climate has not protected 

pyrite and other easily oxidizable and leached 

material, particularly.

Myers: I'd like to comment. I thought that Irv Breger 

said that experiments showed that uraninite and 

pyrite were stable at room temperature in the 

presence of oxygen. Was that with water? And I want 

to go ahead and say that in the Rand both pyrite and 

uraninite now are not stable at room temperature, and 

with uraninite they have to be careful in even washing 

the. specimens off, if they are going to analyze them 

for uranium. Uraninite disappears in a matter of 

months. The other comment I would like to make, to 

Brian Skinner, is that you are quite right, it just happens that pyritic conglomerates with both gold and 

uranium happen to be already known in the Kursk 

Magnetic Anomaly Series and in the Krivoy Rog 

Series.

Skinner: Anybody else want to fulfill some other predictions?

PaulMartz, Gulf Mineral Resources Co., Denver, Colo.: 

Since you mentioned the Hamersley Basin I just 

thought that I'd point out an occurence of quartzpebble conglomerate there that I think may be of interest to you. It's on the Rocklea Dome, which is near 

Tom Price and Parrabardu. As I recall, the core of the 

dome is Archean Warrawoona Volcanics intruded by 

granites. The core is unconformably overlain by a sequence of Proterozoic quartz-pebble congolmerates 

and sands that looke very much like the pictures on 

the screen. Stratigraphically above that is the 

Fortescue Group of volcanics. At the top of the 

Fortescue are the Pyradie pyroclastics and above that, 

the Hamersley Group iron-formations. Unconformably overlying the dome is a thick sequence of quartzpebble conglomerates. However, I never did see any 

real coarse clasts like some shown in the illustrations; 

most of the material is much finer grained. The conglomerate unit may be 2,000 to 3,000 feet thick.

Skinner: Any uranium?

Martz: I wasn't looking for uranium at the time.

Hallbauer: I would just like to say that from 

paleomagnetic measurements a latitude of somewhere 

between 0 and 30 degrees was calculated for the 

Precambrian area for the southern part of Africa. 

That has been published.

Skinner: For the Witwatersrand period?

Hallbauer: Yes.

& U.S. GOVERNMENT PRINTING OFFICE: 1982-361-594/206