The Midcontinent of the United StatesPermissive Terrane for an 

Olympic Dam-Type Deposit? 

U.S. GEOLOGICAL SURVEY BULLETIN 1932 
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The Midcontinent of the United StatesPermissive Terrane for an 

Olympic Dam-Type Deposit? 

Edited by WALDEN P. PRATT and P.K. SIMS 

Midcontinent Strategic and Critical Minerals Project 1988 

Workshop Report 

U.S. GEOLOGICAL SURVEY BULLETIN 1932 
DEPARTMENT OF THE INTERIOR 

MANUEL LUJAN, JR., Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Any use of trade, product, or firm names in this publication is for descriptive 

purposes only and does not imply endorsement by the U.S. Government. 

UNITED STATES GOVERNMENT PRINTING OFFICE: 1990 

For sale by the 

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Library of Congress Cataloging-In-Publication Data 

Midcontinent Strategic and Critical Minerals Project (U.S.). Workshop (1988: 

Denver, Colo.) 

The midcontinent of the United States-permissive terrane for an 

Olympic-Dam type deposit ; Midcontinent Strategic and Critical Minerals 

Project 1988 Workshop report I edited by Walden P. Pratt and P.K. Sims. 

p. em. - (U.S. Geological Survey bulletin ; 1932) 

Includes bibliographical references. 

Supt. of Docs. no.: I 19.3: 1932 

1. Ore-deposits-United States-Congresses. 2. Strategic materialsUnited States-Congresses. I. Pratt, Walden P. II. Sims, P.K. (Paul Kibler), 

1918- . Ill. Title. IV. Series. 

OE75.B9 no. 1932 

[TN23] 

557.3 s-dc20 

[553'.0973] 9Q-3716 

CIP 
CONTENTS 

Introduction, by Walden P. Pratt and P. K. Sims 1 

Part 1 

Petrogenesis and tectonic setting of Middle Proterozoic iron oxide-rich ore 

deposits-An ore deposit model for Olympic Dam-type mineralization, by 

Steven A. Hauck 4 

Geologic setting and ages of Proterozoic anorogenic rhyolite-granite terranes 

in the central United States, by P.K. Sims 40 

General features of the St. Francois and Spavinaw granite-rhyolite terranes 

and the Precambrian metallogenic region of southeast Missouri, by Eva 

B. Kisvarsanyi 48 

Progress toward an occurrence model for Proterozoic iron oxide deposits-A 

comparison between the ore provinces of South Australia and southeast 

Missouri, by Marco T. Einaudi and Naomi Oreskes 58 

The geophysical signature of Olympic Dam and Midcontinent (U.S.A.) 

analogs [abstract], by William J. Hinze 70 

Part 2 

Midcontinent Strategic and Critical Minerals Project-Proposed 

Midcontinent Olympic Dam study 71 

Part 3 

Methodology for analysis of the Proterozoic metallogenic province of 

southeast Missouri-A blueprint for State and Federal cooperation (a.k.a. 

the "Olympic Dam Project") [abstract], by Eva B. Kisvarsanyi and 

Charles E. Robertson 76 

Geologic mapping and evaluation of the Pea Ridge iron ore mine 

(Washington County, Missouri) for rare-earth element and precious 

metal potential-A progress report, by Mark Alan Marikos, Laurence M. 

Nuelle, and Cheryl M. Seeger 77 

FIGURES 

1. Diagram showing tectonic evolution of iron oxide-rich ore deposits 5 

2-3. Maps showing: 

2. Middle Proterozoic iron oxide-rich districts known to contain about 1 

billion tons of ore 6 

3. Tectonic and generalized geologic features of the Gawler craton, 

South Australia 8 

4. Diagrams showing tectonic evolution of the Gawler craton, South Australia, 

including the Stuart Shelf and Mt. Woods Inlier 10 

5. Map showing paleomagnetic reconstruction of Australia during the Middle 

Proterozoic 14 

6. Chart showing generalized stratigraphy of Lower and Middle Proterozoic 

rocks, Kiruna area, northern Sweden 16 

7-8. Maps showing: 

7. Geology of the Kiruna iron district, Sweden 17 

Contents Ill 
8. Distribution and age of basement rocks and anorogenic (rapakivi-type) 

granite-rhyolite complexes of the Midcontinent region of the United 

States 20 

9. Chart showing geologic history of the Butler Hill-Taum Sauk caldera area, 

St. Francois Mountains, southeast Missouri 22 

10-13. Diagrams showing: 

10. Si02 versus K2 0 + Na2 0 for Middle Proterozoic anorogenic volcanicplutonic complexes 26 

11. Si02 versus Zr(l'i02 for Middle Proterozoic anorogenic complexes in 

Missouri and South Australia 27 

12. Model for evolution of Middle Proterozoic iron oxide-rich ore 

deposits 30 

13. Postulated Middle Proterozoic differentiation sequence to produce 

iron oxide-rich ore deposits 31 

14-19. Maps showing: 

14. Precambrian basement terranes in the northern Midcontinent region 

of the United States 42 

15. Major Proterozoic terranes in the Midcontinent region of the United 

States 48 

16. Geology of the St. Francois terrane, southeast Missouri, as 

interpreted from outcrop, drill-hole, and aeromagnetic data 50 

17. Ring structures, alkaline-intermediate rocks, and mineralization in 

the St. Francois terrane, southeast Missouri 52 

18. Major iron occurrences on the Stuart Shelf of South Australia 61 

19. Missouri iron deposits 61 

20. Drawing showing hypothetical example of an underground traverse 

map 79 

TABLES 

1. Major characteristics of Middle Proterozoic rapakivi granite-anorthosite 

massifs 25 

2. Chemical analyses of representative samples of rhyolite and granite, Proterozoic anorogenic terranes, central United States 44 

IV Contents 
Introduction 

By Walden P. Pratt and P.K. Sims 

The Olympic Dam (Roxby Downs) deposit in 

South Australia is one of the world's largest ore deposits. 

Primarily a hydrothermal iron oxide deposit, its 

approximately 2 billion metric tons (tonnes) of rock 

contain an estimated 32 million tonnes of copper, 1.2 

million tonnes of uranium oxide, 1.2 million kg of gold, 

and significant concentrations of rare-earth elements 

(REE) and silver (Roberts and Hudson, 1983; Olympic 

Dam Marketing Pty. Ltd., 1987). The host rocks are 

multistage breccias that contain a large component of 

granitic and some possible felsic debris in a hydrothermal, iron oxide-dominated matrix. On the basis of drilling 

prior to mine development these rocks were thought to 

be conglomerates in a graben, but recent work disputes 

the existence of the graben and indicates that the host 

rock is a large, branching breccia body that includes 

significant bodies of internal sediment (Olympic Dam 

Marketing Pty. Ltd., 1987; Einaudi and Oreskes, this 

volume). The deposit was discovered in 1975, at a depth 

of 350 m, after drilling of a stratigraphic test hole sited on 

coincident magnetic and gravity highs at their 

intersection with a major airphoto lineament; 

paradoxically, the orebody thus far drilled out accounts 

for the large gravity anomaly but not the observed 

magnetic one, which is attributed to a large, as yet 

undiscovered, magnetic body at greater depth. 

Hauck and Kendall (1984) and Meyer (1988) 

proposed that the Olympic Dam deposit can be 

considered the type example of an Olympic Dam "class" 

of ore deposits characterized by iron-rich copper-golduranium-REB-phosphorus-fluorine ores in potassiumrich granites and equivalent porphyries. In their view, 

other deposits in this class include Kiruna (Sweden), 

Bayan Obo (Inner Mongolia), and Pea Ridge (Missouri). 

Is the Pea Ridge deposit really of the Olympic Dam 

type? And could there be more of these giant deposits in 

the United States? Recently, there has been growing 

recognition that the Precambrian basement in the MidManuscript approved for publication January 22, 1990. 

continent region of the United States, especially the St. 

Francois and Spavinaw Proterozoic anorogenic granitic 

terranes in and adjacent to southern Missouri, may have 

high potential for an Olympic Dam-type deposit (Hauck 

and Kendall, 1984; Sims and others, 1988a, b). Indeed, in 

1981 UMETCO, a subsidiary of Union Carbide 

Corporation, began an exploration project in search of 

such a deposit in southwestern Missouri, on the basis of 

gravity and aeromagnetic anomalies, but stopped the 

project when metal prices dropped in 1982, after drilling 

two unsuccessful basement holes in Polk and Webster 

Counties (E.W. Kendall, oral commun., February 3, 

1988). With the broader potential in mind, we convened 

a workshop in Denver, Colo., February 2-3, 1988, under 

the auspices of the U.S. Geological Survey's (USGS) 

Midcontinent Strategic and Critical Minerals Project, 

with a dual purpose: (1) to review current data and 

hypotheses on the type deposit, on the permissive Midcontinent terranes, and on Union Carbide's exploration 

program, and (2) to design plans for an interdisciplinary 

research project to try to identify potential Olympic Dam 

target regions in the Midcontinent. Thirty-five people 

attended the workshop, 25 from the USGS and 10 having 

diverse affiliations but including direct and indirect ties 

to Olympic Dam, the Union Carbide program, and the 

Missouri and Kansas State geological surveys. Ten 

people spoke on a variety of topics ranging from general 

(tectonic setting) to detailed (hematite breccias), from 

geologic to geophysical modeling, and from Olympic 

Dam to the Midcontinent to Kiruna, Sweden (see 

program following references). The participants then 

divided into three working groups and wrote 

specifications for various tasks under the general 

headings of regional compilations and field studies, 

topical studies, and physicochemical processes of ore 

deposition in Olympic Dam-type deposits in the Midcontinent. Later, with the assistance of the working group 

leaders, we integrated these specifications into a single 

proposal. In the interest of public service, we are happy 

to share the details of this proposal with the exploration 

community for whatever use it may be, in whole or in 

part, as a possible plan of attack on a problem of great 

interest. 

Introduction 1 
This volume includes four papers from the 

workshop (three complete and one abstract) and one 

modified paper, as well as the integrated project 

proposal. An epilogue contains two short papers 

constituting an update on one aspect of the project 

proposal-mapping of the possibly analogous Pea Ridge 

iron ore deposit of southeast Missouri. 

Many of the participants expressed the feeling that 

the workshop was an appropriate tribute to the late 

Charles Meyer, formerly of the Anaconda Company 

(1940-53) and the University of California at Berkeley 

(1953-80) (Hunt, 1988). Meyer's enthusiastic 

recognition of the potential for Olympic Dam-type 

deposits in the Midcontinent, as communicated to several 

of the participants, was a key motivation that led 

eventually to this meeting, and but for his untimely death 

in November 1987 he would have taken part in it himself. 

REFERENCES CITED 

Hauck, S.A., and Kendall, E.W., 1984, Comparison of Middle 

Proterozoic iron oxide rich ore deposits, mid-continent 

U.S.A., South Australia, Sweden, and the People's 

Republic of China: Thirtieth Annual Institute on Lake 

Superior Geology, Wausau, Wise., Abstracts, p. 17-18. 

Hunt, J:P., 1988, Memorial to Charles Meyer (1915-1987): 

Economic Geology, v. 83, no. 1, p. 227-228. 

Meyer, Charles, 1988, Proterozoic ore-forming habitats-Tectonic and chemical transitions, in Kisvarsanyi, G., and 

Grant, S.K., eds., North American conference on tectonic 

control of ore deposits: University of Missouri-Rolla, 

1987, Proceedings, p. 217-235. 

Olympic Dam Marketing Pty. Ltd., 1987, Olympic Dam: Adelaide, Roxby Mining Corp. Pty. Ltd. and BP Australia 

Ltd., issue 4, 17 p. 

Roberts, D.E., and Hudson, G.R.T., 1983, The Olympic Dam 

copper-uranium-gold deposit, Roxby Downs, South 

Australia: Economic Geology, v. 78, no. 5, p. 799-822. 

Sims, P.K., Kisvarsanyi, E.B., and Morey, G.B., 1988a, Geology 

and metallogeny of Archean and Proterozoic basement 

terranes in the northern Midcontinent, U.S.A.-An 

overview: U.S. Geological Survey Bulletin 1815, 51 p. 

___ 1988b, The Precambrian basement of the northern 

Midcontinent, U.S.A.-A major frontier for mineral 

exploration, in Kisvarsanyi, G., and Grant, S.K., eds., 

North American Conference on Tectonic Control of Ore 

Deposits: University of Missouri-Rolla, 1987, 

Proceedings, p. 236-244. 

2 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
USGS Midcontinent Olympic Dam Workshop- Program 

Tuesday, February 2, 1988 

PART I-SCHEDULED SPEAKERS 

8:00 Welcoming remarks, David A. Lindsey, Chief, Branch of Central Mineral 

Resources, USGS 

8:05 Miscellaneous announcements and introductions, Walden P. Pratt, USGS 

8:15 Tectonic setting, origin, and evolution of Middle Proterozoic iron oxide-rich ore 

deposits, Steven A. Hauck, University of Minnesota-Duluth 

8:45 Breccias at the Olympic Dam mine, Tommy B. Thompson, Colorado 

State University 

9:15 Origin of hematite breccias at the Olympic Dam ore deposit, Naomi Oreskes, 

Stanford University and Western Mining Corporation 

10:00 Break 

10:30 The geophysical signature of Olympic Dam and Midcontinent U.S.A. analogs, 

William J. Hinze, Purdue University 

11:00 Usefulness of regional geologic information in developing an exploration model: a 

case history, Ernest W. Kendall, Union Carbide Corporation 

11 :30 Midcontinent Proterozoic terranes permissive for Olympic Dam-type mineralization, 

Paul K Sims, USGS 

Noon Lunch break 

1 :00 General features of the St. Francois and Spavinaw granitic terranes, Eva B. 

Kisvarsanyi, Missouri Geological Survey 

PART II-INFORMAL COMMUNICATIONS 

An outsider's visit to Olympic Dam, Warren I. Finch, USGS 

Sulfide mineralization at the Olympic Dam mine, Naomi Oreskes, Stanford University and 

Western Mining Corporation 

Breccias from the Missouri iron deposits, Timothy S. Hayes, USGS 

Some observations of the Kiruna district, Sweden, Murray W. Hitzman, Chevron 

Resources Company 

Introduction 3 
PART 1 

Petrogenesis and Tectonic Setting of 

Middle Proterozoic Iron Oxide-Rich Ore DepositsAn Ore Deposit Model for Olympic Dam-Type 

Mineralization 

By Steven A. Hauck1 

Abstract 

Late Early Proterozoic to Middle Proterozoic (1.8-1.0 

Ga) iron oxide-rich deposits are a family (magmaticexhalative) of ore deposits transitional between Archean to 

Early Proterozoic banded iron formations and Paleozoic iron 

oxide deposits. The Middle Proterozoic was a transitional 

tectonic period in which older Archean to Early Proterozoic 

shields became stabilized. Cratonic stabilization occurred 

after an Early Proterozoic orogeny and is represented by an 

anorogenic assemblage consisting of extensive, subalkaline 

felsic volcanic rocks, rapakivi granite-anorthosite massifs, 

continental sedimentary rocks, and minor alkalic mafic to 

intermediate volcanic rocks and characterized by vertical 

tectonic style. The anorogenic assemblage originated by 

underplating of the crust by a mantle hot spot or plume line. 

Iron oxide--rich ore deposits are associated with this 

anorogenic assemblage and are related to tensional faults 

and caldera collapse structures and alkaline intermediate 

volcanic rocks. 

The iron oxide-rich deposits are composed of 

magnetite and (or) hematite, as well as economically 

important, major to accessory amounts of Ti, P, U, light 

rare-earth elements (REE), base and precious metals, Ba, 

and F. Quartz, fluorapatite, and amphibole are the primary 

gangue minerals. 

Liquid immiscibility of an alkaline magma produces an 

iron-rich fraction that differentiates to form iron oxide-rich ore 

deposits. Assimilation of older banded iron formation by 

ascending magmas may contribute to the iron content of the 

magmas. The ability of liquid immiscibility to produce an iron 

oxide-rich phase depends on the composition (P and Ti) of 

1

UMETCO Minerals Corporation, P.O. Box 1029, Grand 

Junction, Colorado 81502. Current address: Natural Resources 

Research Institute, University of Minnesota, Duluth, 3151 Miller 

Trunk Highway, Duluth, Minnesota 55811. 

the original magma, temperature and pressure, and the 

oxygen fugacity of the magma. The final composition of the 

iron oxide-rich deposit depends on (1) when and where in 

the crust the iron-rich fraction is released, (2) the oxygen 

fugacity, and (3) the degree of differentiation. Examples of 

various types of iron oxide-rich deposits include: 

1. Lower crust-Fa-Ti-P deposits associated with 

anorthosites; 

2. Subvolcanic-Fe± P± Cu-precious metals deposits 

in Missouri, U.S.A.; Olympic Dam, South Australia; and 

Kiruna, Sweden; and 

3. Exhalative-Olympic Dam (Greenfield) Formation, 

South Australia; Missouri, U.S.A.; Kiruna, Sweden; and 

Sayan Obo Fa-light REE deposit, Inner Mongolia, People's 

Republic of China. 

A very low grade metamorphic or hydrothermal event 

commonly followed thickening of the crust by this Precambrian anorogenic assemblage. After this period of 

cratonic stabilization, intrusion of carbonatites, kimberlites, or 

alkaline complexes into younger sedimentary rocks was 

followed by an abortive rifting event. 

INTRODUCTION 

The Middle Proterozoic was a period of 

transitional tectonism during which the Archean greenstone belt tectonic style slowly evolved into the presentday "Wilson Cycle" plate tectonic style. (Note: Middle 

Proterozoic will be used in this paper, for brevity, to 

include the time period 1.8-1.0 Ga.) The Middle 

Proterozoic is dominated, for the most part, by three 

major types of lithologic terranes: 

1. Anorthosite massif-rapakivi granite/granulite 

facies terranes, such as the Adirondacks, New York; 

Labrador-Quebec, Canada; and Musgrave Block, South 

Australia. 

4 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
2. Extensive bimodal, felsic-dominated volcanic 

terranes and associated clastic-filled tensional basins, 

such as the Gawler Volcanics, South Australia; Norbotten County, northern Sweden; and Missouri, U.S.A. 

3. Large, clastic-filled tensional sedimentary 

basins and minor, associated bimodal, felsic-dominated 

volcanic and volcaniclastic rocks such as the Wernecke 

Mountains, Yukon, Canada; McArthur River Basin, 

Northern Territory, Australia; Athabasca and Hornby 

Bay basins, Canada; and Belt and Purcell Supergroups, 

U.S .A. and Canada. 

In all three types of terranes, plutonic and volcanic 

rocks, if present, were passively emplaced and represent 

a period of an orogenic activity. The dominant tectonic 

activity that formed the sedimentary basins was 

extensional. These terranes are at the margins of or 

within older shields that have had a complex history of 

deformation (Rogers and others, 1984). They define a 

period of global cratonic stabilization that is 

characterized by crustal thickening resulting from passive 

emplacement of igneous rocks, clastic sedimentation, 

and extensional tectonism. This cratonic stabilization is a 

transitional phase between the Archean and Phanerozoic 

plate tectonic styles (fig. 1 ). 

Middle Proterozoic iron oxide-rich ore deposits 

characterize the first two terranes. Ore deposits 

characteristic of the third terrane can be divided into two 

groups: (1) Pb-Zn-Ag and Cu±Co mineral deposits 

associated with nearshore to deeper basin sedimentary 

and associated volcanic rocks, such as McArthur River 

and Mt. Isa, Australia, and Belt and Purcell Supergroups 

(Troy, Blackhawk and Sullivan mines), and (2) large 

unconformity-type uranium deposits associated with the 

Athabasca and Kimbolgie basins. This report addresses 

only the relationships between the first two terranes and 

their associated iron oxide-rich ore deposits. 

Many authors (Gole and Klein, 1981; Meyer, 1981, 

1985; Gross, 1983; Rundquist, 1984a, b) have noted that 

the large banded iron formations (bif) of both Lake 

Superior and Algoma types were not formed after about 

1.8 Ga. The Algoma-type bif developed in deep water in 

tectonically active areas, whereas the Lake Superior-type 

INCREASING TONS OF IRON PER UNIT TIME (worldwide) 

L 

4 

SUBAQUEOUS 

Banded iron formation 

I I 

i 

J 

. 

§ 

~ 

SUBAQUEOUS/SUBAERIALISUBVOLCANIC 

..ci .cu>:>: _gc <.1> I :a: tfi.Czz ... 

~ e·~ i-a· i ~~ .. :!J c i!l!;i~ dl i:!! :!~1 -s ·l~ fi~ ~~ 6.!!~ 

IL

l I-!~ 01: sf~ .; o.;dl .9!f 

. -~ 

7~--------- ~ 

Deep-water and 7- -Sandy, oolitic, shallow inland sea } ~ 

7- - -shallow-volcanic environments 7 ___ ~ 

7- Chemical sediment -7 - § 

shallow sea ;f 

Fe (Cu± Co± Au) skarns Contact metasomatic skarna 7 

Layered igneous 

complexes 

Anorthosite 

massifs 

7-c:::>-7-c::::>-?-c:::>-7-7-c:>-7- 7- c:::>- 7--c=:> -7-7-7- c:::::> - 7 

Messina Stfllwat>Jr Great Dyke Bushveld Mu8k Ox Duluth Mlnn.JRa9gedy Mtns., Okla. Dufek Ant 

7 Anorthite ndesine _ _ _ _ _ _ _ 7-c:::>- ?-

Fe-Ti-apatite deposits Tin granites, Nigeria 

~ _ ~~k=m~_Oce==a=ni~c~cr~u=st:al~t=hi~ck:n:es~s~:::::;;;~~~~!~~~~~~~~~~S;km~ 7- - --= = == 7 Gabbro/diorite/syenite alkaline complexes 

7 Carbonatites 

7-•.,:: Kimberlites 

____ ...::S....:.:k~m:..__ __ ---=Co~n~t!!.!in~en~t~al~c~ru~s~ta!.!...l t!!.!h~ick~n~es~s~------------=2=5--=30=km:.:..:-;)la~ 

+----Thin, bouyant-----

Greenstone belts 

3 

Rigidity of continental crust > Plastic--------~ Rigid ___ ____,.,. 

Wilson-cycle tectonics 

CRATONIC STABILIZATION 

2 0 

BILLION YEARS 

Figure 1. Tectonic evolution of iron oxide-rich ore deposits. This study and compiled from many sources. 

Ore Deposit Model for Olympic Dam-Type Mineralization 5 
bif developed in relatively shallow water on stable 

continental platforms and marginal cratonic basins (Kimberley, 1978; Gross, 1980, 1983). Both types of bif 

probably have hydrothermal exhalative origins (Cameron, 1983; Gross, 1983) and formed under tensional 

tectonic conditions. According to Meyer (1981, 1985) 

and Gross (1983), bifs are absent from the Middle 

Proterozoic but reappear in the Late Proterozoic and 

Phanerozoic, primarily as smaller Algoma-type, deepwater deposits. 

Many other types of ore deposits, such as Archean 

porphyry copper and Archean gold (Homestake, Kalgoorlie, Witwatersrand), are absent in the geologic 

record from about 1.8 Ga to the Late Proterozoic 

(Hutchinson, 1981; Meyer, 1981, 1985; Rundquist, 

1984a, b). The only exceptions are large Pb-Zn-Ag 

deposits such as McArthur River, Australia; 

unconformity uranium deposits; Cu-Ag± Co deposits 

such as Troy·Blackhawk, Montana; iron oxide-rich Kiruna and Missouri ore deposits (fig. 1); and iron oxide 

deposits in the Adirondacks of New York (McLelland, 

1986). The Middle Proterozoic iron oxide-rich deposits 

contain 20-70 percent Fe, form single deposits or groups 

of deposits containing greater than 1 billion tons of ore, 

and represent a new family of ore deposits (Hauck and 

Kendall, 1984). In the iron ores, Cu, Au, U, Ag, light 

rare-earth elements (REB), P, and other trace elements 

can be economically or geochemically significant. 

Fe, light rare-earth elements 

1 billion tons 

BAYAN 080 

.t D ~~ ~ 

(l 

/Pt:(} 

As will be described, these deposits originated as 

magmatic segregation deposits that evolved into hydrothermal exhalative iron oxide-rich ore deposits. They may 

be genetically related to the Fe-Ti phosphate deposits 

associated with anorthosite massifs. 

Figure 2 shows the location of four of the largest 

known Middle Proterozoic iron oxide-rich ore districts. 

The following discussion concentrates on the geologic 

and tectonic setting of the host rocks and the ore deposits 

and the evolution of this family of deposits. 

This report is a partially updated version of a 

report written in 1984 for UMETCO Minerals Company, 

a subsidiary of Union Carbide Corporation. It is the 

result of a collective effort by myself, Dr. W J. Hinze, Dr. 

E.W. Kendall, and Dr. S.S. Adams, during 1978-1982, to 

develop a mineral exploration model for Olympic Damtype ore deposits, principally for the United States; 

however, I take full responsibility for the contents and 

conclusions of this paper. 

Acknowledgments.-Permission to publish this 

report has been graciously given by UMETCO Minerals 

Company. The report could not have been written 

without the support of Union Carbide's exploration 

management, in particular, D. Mathias, J.S. Hollingsworth, and E.W. Kendall. Useful discussions with other 

staff geologists during 1979-1982 are gratefully 

acknowledged. The translation of three Chinese papers 

on Bayan Obo was graciously provided by Cai Baochang 

~{}as(! ~ OLYMPIC DAM, S. AUSTRALIA 

_ Cu, U, Au, Ag, Fe± light rare-earth elements, Co 

I 2 billion tons 

I ~ 

\3 (} 

Figure 2. Middle Proterozoic iron oxide-rich districts known to contain about 1 billion tons of ore. 

6 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
of The Geological Survey of Liaoning, Xinchengzi, Shenyang, Liaoning, People's Republic of China. His efforts 

have added to my knowledge of this important ore 

deposit. I thank Mr. J. Suthard for introducing me to Mr. 

Cai and his capabilities. Final appreciation goes to W.P. 

Pratt (USGS) for providing me the opportunity to 

present the ideas in this paper and to P .K. Sims (USGS) 

for his comments and review of the paper's contents. 

GEOLOGY AND TECTONIC SETTING OF 

ORE DEPOSITS 

South Australia 

Regional Geology 

The Olympic Dam Cu-Au-U-Fe deposit is within 

Middle Proterozoic rocks on the Stuart Shelf of the 

Gawler craton (fig. 3). The Gawler craton consists of a 

core of relics of Late Archean to Early Proterozoic 

(2.5-2.3 Ga) gneisses (fig. 3) (Cooper and others, 1976; 

Branch, 1978; Daly and others, 1979; Fanning and 

others, 1979, 1981; Daly, 1981) and peripheral and 

overlying Early Proterozoic (2.1-1.8 Ga) metasedimentary rocks including Lake Superior-type banded 

iron formation (fig. 4) (Miles, 1954; Furber and Cook, 

1975; Parker, 1978; Lemon, 1979; Ramsay and Oliver, 

1979). Similar metasedimentary rocks and iron 

formations are to the northwest in the Tarcoola region 

(figs. 3, 4) (Daly, Webb, and Whitehead, 1978; Daly, 

Benbow, and Blissett, 1979; Daly, 1981) and to the 

northeast in the Mount Woods Inlier (figs. 3, 4) (Flint 

and Benbow, 1977; Benbow and Flint, 1979). These 

Early Proterozoic metasedimentary rocks were deposited 

unconformably on the Late Archean basement and, 

particularly in the northeastern Eyre Peninsula, were 

affected by the Kimban orogeny (Parker and Lemon, 

1982), an event that consisted mainly of amphibolitefacies metamorphism and deformation between about 

1.82 and 1.58 Ga. Syn- and post-tectonic granites were 

emplaced during the orogeny. 

Prior to the last phase of deformation ( -1.6 Ga) of 

the Kimban orogeny, clastic and volcaniclastic rocks of 

the Middle Proterozoic Moonabie Formation and 

bimodal rocks of the McGregor Volcanics were 

deposited in a local fault-bounded graben or grabens 

along the eastern side of the craton (fig. 4) (Crawford 

and Forbes, 1969; Nixon, 1975; Giles and others, 1979; 

Parker and Lemon, 1982). The McGregor Volcanics are 

dominantly felsic but contain minor basalt; andesite is 

absent. According to Giles and others (1979), the basalts 

evolved from melting of the mantle, whereas the dacites 

and rhyolites originated from melting of a dry, basic 

granulitic lower crustal source. Giles and others (1979) 

suggested that mantle diapirism was the heat source. 

The Wandearah Metasiltstone overlies the Moonabie Formation and McGregor Volcanics (fig. 4); it 

consists of a volcaniclastic sandstone, underlain in turn by 

glauconitic(?) and dolomitic siltstone, pink feldspar tuff, 

sandstone, a basaltic lava flow about 10 m thick, and a 

dolomitic siltstone conglomerate (Blissett, 1977). The 

formation is overlain by an unaltered basaltic flow 

(Roopena? Volcanics) that is geochemically similar to 

the mafic rocks of the McGregor Volcanics and to the 

Gawler Range Volcanics (Giles and Teale, 1979a). 

Metasiltstone beneath this flow has been altered to 

graphitic phyllite, and at one place the metasiltstone has 

been brecciated and extensively altered to hematite 

(Mason and others, 1978). 

The Corunna Conglomerate overlies the Wandearah Metasiltstone; it consists of continentally derived 

sedimentary rocks that were deposited in grabens (fig. 4) 

(Rutland and others, 1981). Flint and Parker (1981) 

correlated similar unmetamorphosed sedimentary rocks 

of the Blue Range Beds with the Corunna Conglomerate 

(fig. 3). These beds are primarily nonmarine sandstone 

and conglomerate equivalent to the basal part of the 

Corunna unit and are associated with faulting. 

The Tarcoola Formation (fig. 3) (Daly, 1981) in the 

central part of the craton (figs. 3, 4) may be correlative 

with the Corunna Conglomerate. The lower part of the 

Tarcoola Formation consists of continentally derived 

quartz-rich sedimentary rocks. The basal conglomerate 

consists of earlier bif fragments in a matrix of a fine 

hematite and traces of copper carbonate (Daly, 1981). 

The basal sequence has been intruded and metamorphosed by a younger, Hiltaba-type postorogenic 

granite. The entire basal sequence is 200 m thick and was 

deposited in relatively shallow water. Overlying the basal 

sequence is 400 m of finely laminated green claystone. 

Daly (1981) believed the Tarcoola Formation is 1,511 

million years old based on an age date from an interbedded tuffaceous rhyolite. Thus, these rocks are time 

equivalent with the Corunna Conglomerate and Gawler 

Range Volcanics and are therefore possibly correlative 

with the Olympic Dam Cu-U-Au-Ag-Fe deposit (fig. 4) 

(Webb and others, 1986; Fanning and others, 1988), 

especially because they are iron rich and copper 

mineralized. 

The Gawler Range Volcanics were erupted at 

1,516-1,631 Ma (Cooper and others, 1985), simultaneous with deposition of the Corunna Conglomerate. 

Recent U-Pb dating by Fanning and others (1988), gives 

an age of 1,592±2 Ma for the Gawler Range Volcanics. 

Rb-Sr ages of 1,500-1,530 Ma indicate later resetting of 

the Rb-Sr isotopic systems (Blissett and Radke, 1979; 

Ore Deposit Model for Olympic Dam-Type Mineralization 7 
W.A. Officer Basin 

Eucla 

INDIAN OCEAN 

Eyre 

200 400 KILOMETERS 

EXPlANATION D P ... mmn-Triaosk ba.;n 

Adelaidean rocks 

Tollu \Oicanics 

Gawler Range Volcanics 

Middle and Early Proterozoic rocks 

Local Archean relics 

Abbreviations 

MWI Mt. Woods inlier 

C Chandabooka caldera and 

Chitanilga Volcanic 

Complex region 

OD Olympic Dam 

T Tarcoola 

THZ Torrens hinge zone 

WT Woodroffe thrust fault 

---------- Craton margin 

a. 

VIC. 

Figure 3 (above and facing page). Tectonic and generalized geologic maps of the Gawler craton, South Australia. Modified 

from Flint and Parker (1981) and Rutland and others (1981). 

8 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
+ + + + 

+ + + 

+ + 

50 100 KILOMETERS 

EXPlANATION 

~ Adelaidean and Paleozoic sedimentary rocks 

Unconformity ca. 1,100 Ma 

+ + 

+ + + + ., ~ ... 

~ ...... -·~ 

... .... 

Posttectonic granite 

Gawler Range Volcanics 

Middle Proterozoic sedimentary rocks (Blue 

Range Beds and Corunna Conglomerate) 

Unconformity ca. 1.550 Ma 

Mylonite zone 

Syntectonic granite 

Lincoln Complex 

Hutchison Group, BIF shown by dotted line 

Unconformity ca. 2,100 Ma 

1::\~\l\l\\\\~l\:~~~\:\~ \::\\\~:~ Sleaford Complex 

---- Major fault 

Ore Deposit Model for Olympic Dam-Type Mineralization 9 
GAWLER CRATON 

AGE OROGENIC (includes Stuart Shelf) 

(Ma) EVENT DEPOSITIONAL MAGMATIC EVENTS METAMORPHISM 

CYCLES AND DEFORMATION 

u 

0 

N 

0 

(/) 

w - ~ 

-

t- Uplift 

u 

0 

N 

0 

w 

.....1 

~ DELAMERIAN 

OROGENY --?--?--?- Rifting-formation 

500- of Officer Basin 

Hawker 

Group 

equivalents 

- --?--?--?-

Wilpena 

Group 

u --?--?--?-

0 

N Umberatana '----, 

0 - Group /\. 

a: 

w --?--?--?- I0 

a: 

a.. 

w - Burra 

~ Group .....1 

(/) 

--?--?--?- u - z 

0 

Iu 

w 

I-

.....1 

u 1,000- <( 

0 z 

0 N U5 0 

a: Bedd Volcanics z w f.- w ~--- (1,076 ± 34 Ma) I- I ot - X 

a:~ w a..-

w MUSGRAVIAN .....1 

0 

0 OROGENY ~ -

t 

? 

- -?-?-?-?-

Figure 4 (above and facing page). Tectonic evolution of the Gawler craton, South Australia, including the Stuart Shelf 

and Mt. Woods Inlier. Modified from Compston and Arriens (1968), Thomson (1970), Harrington and others (1973), 

Cooper and others (1976), Flint and Benbow (1977), Giles (1977), Glen and others (1977), Webb and Thomson (1977), 

Mason and others (1978), Parker {1978), Webb and Horr {1978), Benbow and Flint (1979), Blissett and Radke {1979), 

Daly and others {1979), Fanning and others (1979, 1981, 1983, 1988), Giles and others (1979), Mortimer and others 

{1979), Webb {1979), Webb and Coats {1980), Daly (1981), Flint and Parker {1981), McWilliams (1981), Preiss and Forbes 

{1981), Rutland and others {1981), Parker and Lemon {1982), Cooper and others {1985), and Webb and others {1986). 

10 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
GAWLER CRATON 

AGE OROGENIC (includes Stuart Shelf) 

(Ma) EVENT DEPOSITIONAL METAMORPHISM 

CYCLES MAGMATIC EVENTS AND DEFORMATION 

- r---1-1-1-1-

u I-- Roopena Volcanics 

0 

7 Block faulting/ N 

0 

a: - I graben rrmation w _ Pandurra ~-o-e 1-------- Formation a: co o..E- WARTAKAN I-- Gawler Range 

w ? CJ4 Local cross EVENT Volcanics rhyolite ....I 

0 1,500-1-------- dikes (1,457 ± 22 Ma) 1 _j_ folding and 

0 fracturing and 

~ block faulting 

- 1-- Bl ue R B d ange e s [ J·f Hiltaba Granites (1,576 . Mal T Open ~~ and Corunna ~ Gawler Range Volcamcs 03 folding M2 ~ 

Conglomerate .2 (1, a. 

592 Ma) ~Shearing ""-I-- ~ ... 

Wandearah ~ ._CI 

- 02 1soclinal 8. KIM BAN Metasiltstone 6 IJ j_folding !i - OROGENY 7 McGregor Volcanics 

1-- Moonabie - Moonta Porphyry Q) ·- -J:. 

Formation (1,737 ± 5 Ma) [_ H - c-Myola Volcanics (1,719 ± 4 Ma) _j 

Yadnarie Schist'"' ~ Donington Granitoid Suite 

I Cook Gap (Kirton Point 1,818 ± 13 Ma) 

c. Schist - :::3 a. ~ Upper part of ::::l 

E Middleback E 

u :::3 Quartzite (!) en 0 g Lower part of c N 0 

en 0 2,000- .g Middleback :c a: 

w =c Quartzite £ ~ "0 ::::l 

0 ~ Katunga ::r: a: I Dolomite 0.. 

~ \I 

- Warrow Quartzite a: -? 1 <( 

w 

-

-

c- Whidbey Granite Q) 

(2,350 ± 28 Ma) ·5~ 

SLEAFORDIAN (/) ·o - 1- c.~ 

OROGENY -II' 0 c ~ ~ 

CAR NOT I-- Kiana Granite cS(!) 

GNEISSES (2,440 ±58 Ma) 

2,500- --1--1-

CHRISTIE 

ARCHEAN GNEISS 

\V 

Ore Deposit Model for Olympic Dam-Type Mineralization 11 
Webb, 1979; Fanning and others, 1988). The Gawler 

Range Volcanics are flat lying, cover an area of 25,000 

km2

, and are primarily felsic (Branch, 1978; Blissett and 

Radke, 1979). Giles and Teale (1979b, 1981) reported 

geochemically similar volcanic rocks associated with 

some interbedded sedimentary rocks from the eastern 

side of the Adelaidean geosyncline southeast of the Mt. 

Painter Inlier. These volcanic rocks are geochemically 

similar to the Pepegoona Porphyry (1,309±129 Ma), 

which is exposed within the Mt. Painter Inlier (Coats and 

Blissett, 1971; Giles and Teale, 1979b). Thus, Gawler 

Range Volcanics or similar rocks probably occupy a 

much larger area than proposed by Branch (1978) and 

others. 

The Gawler Range Volcanics consist of felsicdominated flow rocks and ash-flow and air-fall tuffs 

(Crawford, 1963; Blissett, 1975; Turner, 1975; Giles, 

1977, 1979, 1988; Branch, 1978; Blissett and Radke, 

1979) that presumably originated from partial melting of 

a basic granulitic lower crustal source (Branch, 1978; 

Giles, 1979). Primitive potassic andesites and basalts 

associated with the Chandabooka Caldera of the Chitanilga Volcanic Complex (fig. 3) possibly originated from 

a relatively shallow (<50 km), large ion lithophile 

element-enriched zone of the mantle (Branch, 1978; 

Giles, 1979, 1988). Post-tectonic, S-type granites (Hiltaba; 1,576 Ma), which are probably comagmatic with the 

Gawler Range Volcanics (Blissett, 1975; Branch, 1978; 

Blissett and Radke, 1979; Cooper and others, 1985), 

intrude the Gawler Range Volcanics. Minor amounts of 

Cu, Au, Sn, and F and high radioactivity are associated 

with these granites (Crawford, 1963; Branch, 1978; Daly, 

1981). Webb (1979) indicated that some mafic intrusions 

also were emplaced early in this postorogenic episode. 

Post-Hiltaba felsic and mafic dikes (1,457 Ma) define the 

end of this volcanic-plutonic episode (Branch, 1978; 

Daly, 1981; Parker and Lemon, 1982). 

The Wartakan event from 1,500 to 1,425 Ma (fig. 

4) was the last tectonic event on the Gawler craton 

(Thomson, 1970; Parker and Lemon, 1982; Fanning and 

others, 1988). It was minor in significance and may be 

related to emplacement of Hiltaba-type postorogenic 

granites; it was followed by emplacement of 1,457-

million-year-old rhyolite dikes. At the end of this event 

cratonization of the Gawler craton was complete (Webb, 

1979; Parker and Lemon, 1982). The last significant 

event on the stable craton after about 1.4 Ga and 

deposition of the basal units of the Adelaidean geosyncline at 1,076 Ma was emplacement of pre-Adelaidean 

dolerite (Compston and Arriens, 1968; Mason and 

others, 1978; Giles and Teale, 1979a; Webb, 1979; 

Roberts and Hudson, 1983). 

The Torrens hinge zone (THZ) (fig. 3) separates 

the Gawler craton from the Adelaidean geosyncline on 

the east. It is a complex fault zone that separates the 

more deformed and thicker sedimentary rocks of the 

geosyncline from the thinner, relatively undeformed 

Middle Proterozoic and Adelaidean sedimentary rocks of 

the Stuart Shelf (Thomson, 1970; Preiss and Forbes, 

1981; Rutland and others, 1981 ). At least during Burra 

Group sedimentation (fig. 4), the THZ was a zone of 

normal faulting (Lambert and others, 1987). 

On the Stuart Shelf, the Pandurra Formation 

unconformably overlies the Roopena Volcanics. These 

red sandstones and arkoses were deposited in fluvial 

environments on downfaulted basement blocks and on 

the crystalline basement (Mason and others, 1978; Rutland and others, 1981). Equivalents to the Pandurra 

Formation have not yet been identified within the Adelaidean geosyncline. Rb-Sr dating of shale and siltstone 

from the Pandurra yield an age of 1,424±51 Ma, which 

may represent the age of deposition (Fanning and others, 

1983); however, this age is in conflict with a 1,317±30-

Ma age (Compston and others, 1966) for the underlying 

Roopena Volcanics. Because the Pandurra Formation 

unconformably overlies Olympic Dam-type Cu-U-Au 

mineralization at the Acropolis prospect west of Olympic 

Dam, the age of the formation places a minimum 

constraint on the age of this mineralization (Fanning and 

others, 1983). 

Eruption of the continental Roopena Volcanics 

(eastern Gawler craton) and deposition of associated red 

metasiltstone occurred about 1,317 Ma. Because the 

Roopena Volcanics are unconformably overlain by the 

approximately 1.4-billion-year-old Pandurra Formation, 

there is either a discrepancy in the age dating or 

previously undocumented tectonism (thrusting?). 

Overlying the Roopena Volcanics is a hematitized felsic 

porphyry that petrographically resembles felsic volcanic 

rocks of the McGregor and Gawler Range Volcanics. 

Giles and Teale (1979a) suggested that the Roopena 

Volcanics formed in the same tectonic environment as 

the bimodal McGregor and Gawler Range Volcanics; 

however, the Roopena Volcanics unconformably overlie 

the Moonabie Formation and the rhyolite of the McGregor Volcanics (Mason and others, 1978). Therefore, the 

exact stratigraphic position of the Roopena Volcanics is 

still in question, but their geochemistry and petrography 

support an origin in a tectonic environment similar to 

that of the overlying and underlying volcanic rocks. 

Unconformably overlying the Pandurra Formation 

are the mafic Beda Volcanics and associated arkosic 

sandstone and conglomerate of the Backy Point Beds 

(Mason and others, 1978). The Beda Volcanics differ 

geochemically from the Roopena Volcanics, the 

Roopena having higher Al2 0 3 , P2 0 5 , Ce, Y, and Zr 

(Giles and Teale, 1979a). The Beda Volcanics have been 

dated at 1,076±34 Ma (Webb and Horr, 1978; Webb and 

Coats, 1980). Giles and Teale (1979a) determined from 

geochemical studies that the Beda Volcanics and 

12 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
stratigraphically related Depot Creek Volcanics formed 

in a tensional tectonic environment in which there was 

active faulting; that is, the THZ. Basaltic doleritic dikes 

cut the Pandurra Formation and have been interpreted 

as roots of feeders to the Beda Volcanics (Mason and 

others, 1978). These dikes are associated with regional 

northwest-trending linear magnetic anomalies on the 

Stuart Shelf (Mason and others, 1978; Anderson, 1980). 

At Olympic Dam, an unaltered dolerite dike cuts the 

orebody but not the unconformably overlying Tregolana 

Shale of the Late Proterozoic Wilpena Group (Roberts 

and Hudson, 1983). If the 1.3-Ga age of Compston and 

others (1966) for the Roopena is correct, the age of the 

Olympic Dam deposit is constrained between approximately 1.3 and 1.1 Ga. On the other hand, if the 1.4-Ga 

age for the Pandurra Formation is correct, then 1.4 Ga is 

a minimum age for the deposit. 

The remainder of the stratigraphic succession in 

the Adelaidean geosyncline includes a 10-km-thick 

sequence of Late Proterozoic and Cambrian sedimentary 

rocks. These strata have been intruded by alkaline rocks 

and were folded during the Delamerian orogeny, 

beginning about 500 Ma. This orogeny may have 

originated by the initiation of rifting in the Officer Basin 

(fig. 3), which separates the Musgrave block from the 

Gawler craton (McWilliams, 1981). 

Preiss and Forbes (1981) suggested that Late Adelaidean sedimentation in the Adelaidean geosyncline and 

in the Amadeus Basin is sufficiently similar to warrant 

lithostratigraphic correlations between the two basins. If 

the paleomagnetic reconstruction for 1,400 Ma (fig. 5) 

and the correlation of Adelaidean sedimentary rocks 

between the two basins are correct, there should also be 

some correlation between the geology of the Musgrave 

block and the Gawler block. The Musgrave block consists 

primarily of 1.4-1.2-billion-year-old, quartzofeldspathic 

granulite-facies metamorphic rocks that have been 

intruded by 1.1-1.0-billion-year-old gabbros, dolerites, 

granites (some rhyolite), ultramafic complexes, and 

anorthosites (Nesbitt and Talbot, 1966; Arriens and 

Lambert, 1969; Gray and Oversby, 1972; Gray, 1977, 

1978; Gray and Compston, 1978; Gray and Goode, 1981; 

Rutland, 1981). The precursors of the granulite-facies 

rocks were felsic (?) and mafic volcanic rocks, ferruginous quartzite or iron formation, and pelite and marble 

(Gray, 1977; Moore and Goode, 1978) that were 

deposited about 1.6 Ga (Arriens and Lambert, 1969; 

Gray, 1978; Gray and Compston, 1978). This 

supracrustal succession was subsequently deformed four 

or five times (Collerson and others, 1972). Foliated and 

nonfoliated sets of dolerite dikes cut this supracrustal 

assemblage (Collerson and others, 1972). Rutland 

(1981) suggested that an earlier metamorphic event at 

1.8 Ga (Kimban orogeny?) also affected the Musgrave 

block. 

If the paleomagnetic reconstruction of McWilliams 

(1981) (fig. 5) is correct, the supracrustal succession of 

the Musgrave block could be equivalent to the Moonabie 

Formation or Corunna Conglomerate or Tarcoola 

Formation. Thus, the mafic, ultramafic, and anorthositic 

intrusive rocks of the Musgrave block would be 

equivalent in age and would also be possible source rocks 

for the Beda Volcanics (see fig. 4). Note also that the 

paleomagnetic reconstruction (fig. 5) correlates the THZ 

with the Woodroffe thrust. 

The last major intrusive event in South Australia 

was the emplacement, between the Early Permian and 

the Cenozoic, of one known carbonatite and several 

kimberlites (Moore, 1973; Ferguson, 1980; Jaques and 

others, 1985) into Adelaidean rocks in the southern part 

of the geocline. 

The development of grabens containing primarily 

quartz-rich sedimentary rocks and bimodal volcanic 

suites from the time of the McGregor Volcanics through 

the early development of the Adelaidean geosyncline 

indicates that extensional tectonism was the dominant 

tectonic process during this period. Olympic Dam and 

other iron-rich deposits are therefore part of this tectonic 

process. Equivalent bimodal volcanic rocks (Giles and 

Teale, 1979b) on the east side of the Adelaidean geosyncline suggest a similar tectonic process. 

Olympic Dam Cu-U-Au-Fe Deposit 

According to Roberts and Hudson (1983), the 

Olympic Dam Cu-U-Au-Fe deposit is a stratabound 

sedimentary-hosted ore deposit within unmetamorphosed, mineralized, monomict and polymict, 

sedimentary breccias or rudites. Recent work (Einaudi 

and Oreskes, this volume) shows, however, that the 

deposit is primarily in an extensive breccia pipe (granite 

hosted) that has a minor subaqueous expression of iron 

formation, felsic volcanic rocks, conglomerate, breccia, 

and hematitic siltstone (Greenfield Formation of 

Roberts and Hudson, 1983). The deposit is expressed 

geophysically by coincident magnetic and gravity 

anomalies. The gravity anomaly is attributed to the 

orebody, whereas the magnetic anomaly is unexplained 

but considered probably to overlie a large magnetic body 

at depth. 

The Olympic Dam deposit contains 2 billion metric 

tons of ore averaging 1.6 percent Cu, 0.64 kg/metric ton 

U3 0 8 , 0.6 glmetric ton Au, 3.0 gl metric ton Ag, and 

15-25 percent iron oxide. The discordancy of the 

geologic units and the quantity of breccia suggest that 

multiple hydrothermal events and hydraulic brecciation 

formed the deposit (Lambert and others, 1987). The 

mineralogy of the deposit is suggestive of an alkaline 

igneous source. O'Driscoll (1985) showed that 

Ore Deposit Model for Olympic Dam-Type Mineralization 13 
EXPlANATION 

<1,000 Ma Proterozoic platform cover D Phanerozoic platform cover 

1,400-1,000 Ma platform cover ® Olympic Dam, Roxby Downs 

1,400-1,000 Ma orogenic reactivation M P Mt. Painter 

:l:l:l:l:l:l:l:l:l:l:l:l:l:l > 1

•

400 Ma rocks 

MW Mt. Woods Inlier 

Figure 5. Paleomagnetic reconstruction of Australia during the Middle Proterozoic (1 ,400 Ma). Modified from McWilliams 

(1981) and Rutland (1981). 

14 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
emplacement of the Olympic Dam deposit was related to 

regional lineaments and suggested that lineament 

mapping (gravity, magnetic, and so on) was an important 

tool for locating the deposit. 

Two types of copper-mineralized rocks are present 

in the deposit (Roberts and Hudson, 1983): (1) stratabound bornite-chalcopyrite-pyrite and (2) lenses and 

crosscutting veins of chalcopyrite-bornite. Uranium, 

gold, and light REE are found with both types of 

mineralized rocks. Gangue minerals of the first ore type 

include hematite, quartz, sericite, and fluorite± 

carbonate±barite±rutile. Gangue minerals associated 

with the second type are hematite, fluorite, quartz, sericite, and chlorite. The sulfide minerals generally are 

disseminated in the matrix with hematite but are also 

present as massive sulfide clasts. The copper-mineralized 

rocks are vertically zoned from sulfur-rich, copper-poor 

minerals-pyrite-chalcopyrite-at the base of the 

deposit to sulfur-poor copper-rich minerals-bornitechalcocite-at the top. There is no apparent lateral 

zonation. 

Preliminary sulfur isotope data suggest that there is 

a magmatic sulfur component in the Olympic Dam 

mineralization (Lambert and others, 1987). The same 

type of alteration and mineralization at Mt. Gunson, 

south of Olympic Dam, exhibits similar isotope values 

associated with hydraulic fracturing (Lambert and 

others, 1987) that suggest the mineralization was not 

confined to a single area on the Gawler craton. 

Hematite is the major matrix mineral but is also 

present as clasts or in veins. Zones of massive hematite 

and hematite-rich breccia have "sedimentary" textures 

indicative of simultaneous, rhythmic and sequential 

deposition. Pisoliths and ooliths of hematite-fluoritesulfides, uranium-sulfides, siderite-hematite-sulfides, 

siderite, and siderite-chlorite also are within the stratabound mineralized rocks. Hematite rarely is present as 

pseudomorphs after magnetite or as inclusions within 

hematite. Magnetite is present as inclusions in siderite. 

Uranium minerals include coffinite, uraninite, and 

minor brannerite. Both uraninite and coffinite occur with 

the copper-mineralized rocks and with sericite, hematite, 

fluorite, and chlorite. Free gold occurs with both varieties 

of copper-mineralized rocks. Light REE are found in 

bastnaesite and florencite and with copper sulfide 

minerals, hematite, sericite, fluorite, and uranium 

minerals in the matrix. 

Hematite, sericite, and chlorite alteration 

accompanies mineralization. Silica and carbonate 

alteration are present locally. The diabase is the only 

unaltered rock within the deposit. There is no visible 

evidence of supergene enrichment or leaching. 

Webb and others (1986) correlated the Wandearah 

Metasiltstone with the Greenfield Formation at Olympic 

Dam (Roberts and Hudson, 1983). Giles (1988), 

however, considered the igneous activity and rifting 

associated with the development of the Gawler Range 

Volcanics as probably directly responsible for the 

formation of Olympic Dam. Such an association places 

formation of Olympic Dam between 1,615 and 1,570 Ma. 

Fanning and others (1988) showed the Wandearah Metasiltstone as overlying the Moonabie Formation and 

McGregor Volcanics. U-Pb dates on the McGregor 

Volcanics (Fanning and others, 1988) of 1,737±5 Ma 

place a minimum age for the formation of Olympic Dam 

deposit. 

Sweden 

Regional Geology 

Sweden is in the western part of the Baltic Shield, 

which is composed of Archean nuclei and surrounding 

Proterozoic rocks. In Sweden, Archean rocks crop out 

only in the area north and east of Kiruna. The Archean 

rocks and overlying deformed supracrustal metasedimentary and metavolcanic rocks extend southward 

from the outcrop area within the Karelian schist belt 

(Karelides) to the Skellefte province or district. The 

Skellefte province and areas to the south (Svecofennian 

complex or Svecofennides) consist of mafic to felsic 

metavolcanic rocks, metagraywacke, and abundant 

granitic rocks that formed 1.9-1.7 Ga (Patchett and 

others, 1987). The Svecofennides developed as a 

succession of island arcs, as the sites of subduction 

moved south and successive extinct arcs were accreted 

onto the Archean craton (Park, 1985). In the west, 

Archean and Early Proterozoic terranes are cut by a 

major belt of felsic volcanic and granitic rocks, including 

rapakivi granites that have Rb-Sr whole-rock ages of 

1.6-1.7 Ga (Patchett and others, 1987). 

Lower Proterozoic rocks in northern Sweden 

consist of the Greenstone Group (mafic flow rocks) and 

Kilavaara Quartzite Group (fig. 6). The Greenstone 

Group contains iron formations and associated limestone 

and graphitic sedimentary rocks. The mafic flow rocks 

are subalkaline in composition and oceanic in character. 

The iron formations are associated with carbonate rocks, 

phyllites, and mafic volcanic rocks (Lundberg, 1967; 

Lindroos, 1974). Witschard (1980) proposed that the 

Greenstone Group formed by incipient rifting during a 

regional tensional event. 

The Early Proterozoic ended with the Svecokarelian orogeny, which included intrusion of prekinematic 

(1.98-1.84 Ga) and synkinematic (1.78 Ga) granitic rocks 

(Lundqvist, 1979; Moorman and others, 1982). 

According to Moorman and others (1982), regional 

amphibolite-facies metamorphism culminated about 

Ore Deposit Model for Olympic Dam-Type Mineralization 15 
u 

6 UJN -.Jo Oa: 

Ow 

~~--0 

a: 

a_ 

u 

6 a:N 

UJQ 

sa: 

ow _jb a: 

a_ 

Vakko Formation 

Maattavaara 

Quartzite Group 

Porphyry 

Group 

Conglomerate of 

Kurravaara 

Kilavaara 

Quartzite Group 

Greenstone 

Group 

.;;;;;;.;;::::-~~~:.~Kiirunaavaara, 

1\ 1\ 1\ 1\ 

1\ 1\ 1\ 

1\ 1\ 1\ 1\ 

1\ 1\ 1\ 1\ 1\ 1\ 

1\ 1\ 1\ 1\ 1\ 1\ 

ooaoooc::;, 

Luossavaara, 

and Luossajarvi 

ores 

Figure 6. Generalized stratigraphy of Lower and Middle 

Proterozoic rocks, Kiruna area, northern Sweden. Modified 

from Parak (1975a) and Lundberg and Smellie (1979). 

1.82 Ga in the Bergslagen region in south-central 

Sweden. The metamorphism generally was accompanied 

by folding. 

The post-Svecokarelian Middle Proterozoic 

successions are anorogenic. In northern Sweden, a basal 

conglomerate and (or) felsic porphyries (1.9 Ga; Skiold 

and Cliff, 1984) of the Porphyry Group overlie Early 

Proterozoic Greenstone Group and associated rocks. 

The dominant felsic porphyries are subalkaline in 

composition, whereas the less prominent intermediate to 

mafic porphyries (for example, magnetite trachytes of 

Lundberg and Smellie, 1979) are alkaline (Lundberg and 

Smellie, 1979; Witschard, 1980; Frietsch, 1984). 

Conformably overlying the Porphyry Group are 

sandstones and related rocks of the Maattavaara Quartzite Group (known as the upper part of the Hauki 

Formation in the Kiruna area). The Middle Proterozoic 

iron oxide-phosphate and copper ores are within the 

Porphyry Group and the upper part of the Hauki 

Formation. 

Potassic granites, syenites, and rapakivi granites 

that range in age from 1,660 to 1,300 million years old 

intrude the Middle Proterozoic volcanic rocks. Many of 

the rapakivi granite bodies (massifs) are associated 

closely with anorthosite, gabbro, and syenite (Kornfalt, 

1976). Kornfalt proposed that intrusion of mantlederived mafic rocks into the lower crust provided the heat 

source to produce the rapakivi granites; intrusion of the 

rapakivi massifs occurred in a relatively stable platform 

and was accompanied by intense block movements. 

Metamorphism and folding postdate Middle 

Proterozoic anorogenic volcanism that Wilson and Akerblom (1982) relate to magmatic activity of an "adinotype" orogeny, especially from 1,750 to 1,500 Ma, and to 

tensional tectonism, particularly in northern Sweden, at 

1,550 Ma. Recent U-Pb dating by Skiold and Cliff (1984) 

indicates that felsic rocks of the Porphyry Group are 1.9 

billion years old; thus, the 1.5-Ga Rb-Sr age is the age of 

intrusion of the potassic granites and gabbros. Witschard 

(1980) stated that formation of the Porphyry Group of 

northern Sweden was related to tensional tectonism. The 

Middle Proterozoic granites are S-type granites, formed 

by partial melting of older crustal material, whereas 

granites in central and southern Sweden are more I-type 

suites, derived from mantle material (Wilson and Akerblom, 1982). Mafic dikes and continentally derived 

sedimentary rocks that formed about 1,300-1,200 Ma 

suggest continued uplift and extension. 

Late Proterozoic deposition was followed by 

deformation and metamorphism (Sveconorwegian orogeny/Grenville?) related to continental collision about 

1,050 Ma (Wilson and Akerblom, 1982). The last major 

tectonic event was formation of the Oslo graben during 

the Permian. Alkalin~ complexes, carbonatites, and kimberlites were intruded into the Archean and younger 

rocks between 1,700 and 281 Ma (Luridqvist, 1979). 

Kiru na District 

Types of Ores 

The Kiruna district in northern Sweden contains 

3.4 billion tons of iron oxide-apatite ore averaging 50-60 

percent Fe and 0.5-5 percent P (Frietsch, 1980). The ore 

contains less than 1 percent Ti and generally less than 0.1 

percentS and Mn (Grip, 1979). The main ore minerals 

are magnetite and hematite; the apatite is generally 

fluorapatite (Frietsch, 1974b ). Other accessory minerals 

include actinolite-tremolite, diopside, pyrite, chalcopyrite, bornite, and calcite. The orebodies are elongate 

and tabular and generally conformable with the regional 

stratigraphy. 

16 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
.............................. 

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 

0 2 KILOMETERS 

EXPLANATION 

Quartzitic sandstone Conglomerate of Kurravaara 

Lower part of Hauki Formation 

Rich in hematite l Greenstone of Kiruna 

GrG~~~;ne 1--'!-r-ll!--";r--i'-;:-;"-rl Sedimentary rocks in greenstone of Kiruna 

Rhyolite porphyry Mafic volcanic rocks 

Trachyte porphyry Apatite iron ore 

Trachyte Not outcropping 

Figure 7. Geology of the Kiruna iron district. Map modified from Frietsch (1982); terminology adapted from Lundberg and 

Smellie (1979) and Lundqvist (1979). 

The district contains three types of iron oxide-apatite 

orebodies (Geijer and Odman, 1974; Parak, 1975a, b; 

Frietsch, 1978). 

1. Kinma-type ores are primarily magnetiteapatite ores associated with minor hematite and actinolite. The irregular, elongate, disc-shaped orebodies 

contain varying amounts of red breccia in the hanging 

wall or footwall. Magnetite trachyte porphyry or trachyte 

porphyry is nearby or in the footwall. The Kiirunavaara 

(fig. 7) orebody is an example of this ore type. 

2. Per Geijer-type ores are stratigraphically higher 

than Kiruna-type ores. They are characterized by hematite and apatite and associated minor magnetite, quartz, 

and calcite. The orebodies are tabular and massive and 

lie on rhyolite. They contain more phosphorus (2-5 

percent) than those of the Kiruna type. Ore breccia is 

minor, but dikes or veins of hematite, hematite-apatite, 

or apatite are prominent in the footwall. As with wall 

Ore Deposit Model for Olympic Dam-Type Mineralization 17 
rocks of the Kiruna-type ores, the wall rocks have 

undergone silica and sericite alteration. The Rektorn and 

Henry deposits are examples of this type (fig. 7). 

3. Hauki-type ores are generally massive to 

banded hematite and contain variable quantities of 

quartz, sericite, calcite, barite, and tourmaline. They are 

flat lying and are at the stratigraphically highest level in 

the lower part of the Hauki Formation. 

Stratigraphy 

In the Kiruna area, the oldest units are mafic 

volcanic rocks (2.2 Ga; Skiold, 1986), sedimentary rocks, 

and iron formations of the Early Proterozoic Greenstone 

Group. The conglomerate of Kurravaara unconformably 

overlies the Greenstone Group. It is a polymict 

conglomerate containing pebbles of magnetite trachyte 

porphyry, lime~tone, quartz, banded sedimentary rocks, 

jaspilite, and red felsite (Parak, 1975b ). 

Volcanic rocks of the 1.9-billion-year-old Porphyry 

Group overlie the conglomerate of Kurravaara. Trachyte, trachyte porphyry, and magnetite trachyte 

porphyry (syenite, syenite porphyry, and magnetite 

syenite porphyry of Parak, 1975b; trachyte terminology 

after Lundberg and Smellie, 1979) overlie the 

conglomerate and form the basal part of the Porphyry 

Group. Geijer and Odman (1974) interpreted these units 

as caldera fill. The lower contact with the conglomerate 

and the upper contact with the overlying rhyolite 

porphyry are generally sharp. At the Ekstromsberg 

deposit 30 km west-southwest of Kiruna, the magnetite 

trachyte porphyry and the trachyte porphyry are 

intercalated with the rhyolite porphyry and are elongate 

parallel with the orebodies (Frietsch, 1974a). The gray 

trachyte is composed of plagioclase phenocrysts and 

quartz-rich amygdules in a matrix of quartz and feldspar. 

Amygdules and aggregates of augite, actinolite, 

magnetite, sphene, apatite, biotite, and chlorite may also 

be present. Pyrite, chalcopyrite, epidote, and zircon are 

the main accessory minerals. Around the orebodies, the 

trachytes are magnetite trachytes and the amygdules are 

aggregates of magnetite and actinolite (Lundberg and 

Smellie, 1979). Lundberg and Smellie (1979) noted that 

at the Mertainen deposit, 30 km southeast of Kiruna, 

magnetite-rich globular structures are throughout the 

trachyte both adjacent to and some distance from the 

orebody. They interpreted these structures as indicating 

immiscible separation, aided by a high volatile content of 

an iron-rich melt from the trachyte, to form the orebodies at Painirova and Mertainen. The immiscible 

separation forms the footwall ore breccia and the orebody. Geochemically, the trachyte lavas originated in an 

alkaline source region. 

At Painirova and elsewhere in the Kiruna District, 

the trachytic flow rocks are locally overlain by volcanic 

conglomerates, basalts, and tuffaceous sedimentary 

rocks (fig. 6) (Parak, 1975a, b; Lundberg and Smellic, 

1979; Smellie, 1980). In places the volcanic 

conglomerates have a magnetite matrix. Geijer and 

Odman (1974) attributed these conglomerates to 

deposition in local stream channels. 

Extensive subalkaline rhyolite porphyries form the 

next higher major unit and generally lie directly on the 

trachyte flow rocks. The rhyolite porphyries (quartzbearing porphyry of Parak, 1975a, b; terminology after 

Lundberg and Smellie, 1979) have a micropoikilitic 

texture resulting from devitrification. The rhyolites 

probably originated as ignimbrites (Offerberg, 1967; 

Lundberg and Smellie, 1979) and consist of quartz, 

microcline, and plagioclase and accessory magnetite and 

apatite± zircon± fluorite± calcite± tourmaline (Parak, 

1975b ). Phenocrysts are perthite and rarely quartz. The 

porphyries are generally red. Beds of agglomerates, 

conglomerates, lenses of iron ore, and one known bed of 

anhydrite are within the rhyolite porphyries (Parak, 

1975b). 

The lower part of the Hauki Formation overlies the 

rhyolite porphyry in the Kiruna area. The basal unit of 

the Hauki forms the hanging wall of the Per Geijer ores 

and consists of feldspar spherulites in a matrix of quartz. 

Geijer and Odman (1974) considered the unit to be 

silicified obsidian that was affected by emplacement of 

the Per Geijer ores. Parak (1975a, b), on the other hand, 

believed the unit is clastic because it is crossbedded and 

contains thin lenses of hematite and magnetite. Parak 

(1975b) noted that this unit also contains patches of an 

unknown radioactive mineral. 

Above the basal unit is the trachyte porphyry of the 

Hauki, a fine-grained gray rock consisting of albite, 

quartz, muscovite, sericite, biotite, hematite, and 

magnetite. Accessory minerals include orthite, zircon, 

tourmaline, apatite, pyrite, chalcopyrite, and bornite 

(Parak, 1975b ). Overlying this unit are sericite quartzite 

and sericite schist that consist of quartz, sericite, hematite, and potassium feldspar. Accessory minerals are 

calcite, tourmaline, bornite, and ankerite, as well as rare 

titanite, fluorite, orthite, and zircon. The banded hematite ores are within the lower part of the Hauki 

Formation. These ores are always stratigraphically higher 

than the Per Geijer ores. Similar rock types are present in 

the Ekstromsberg region. The youngest unit in the Kiruna region consists of conformable quartz sandstones, 

graywackes, shales, and conglomerates of the Vakko 

Formation. Potassic granites and some gabbros were 

intruded into these sequences about 1,525 Ma. Some 

deformation and local metamorphism and hydrothermal 

activity (with scapolite) accompanied intrusion of the 

granites. 

18 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Origin of Ores 

Frietsch (1970, 1974a, 1978, 1982, 1984) and Geijer and Odman (1974) believed that ores of the Kiruna 

region originated by magmatic differentiation and are 

contemporaneous with the enclosing volcanic rocks. Nystrom (1985) and Gilmour (1985) proposed that the iron 

ores have carbonatitic affinities. On the other hand, 

Parak (1973, 1975a, b, 1984, 1985) proposed a 

sedimentary-exhalative origin for the ores; chemical and 

mechanical sedimentation in a volcanic environment 

accounts for the textures observed within the ore 

deposits. Lundberg and Smellie (1979) and Smellie 

(1980) proposed that: 

1. Iron-rich material was assimilated by 

ascending trachytic lavas. 

2. Extreme magmatic differentiation and 

immiscibility coupled with a high volatile content 

produced the iron oxide-apatite ores. 

3. The iron ores represent every variation of 

origin, from magmatic deep-seated intrusive to shallowlevel intrusive to volcanic extrusive to volcanic 

sedimentary-exhalative. 

Wright (1986) suggested that although the Per 

Geijer ores may be exhalative in origin, the Kiruna ores 

were extruded as subaerial volcanic lava flows. 

Furthermore, he stated (p. 193) that "There is no need to 

seek a common origin for all the iron ores in the Kiruna 

district." Frietsch (1984) suggested that the iron deposits 

are associated with extensive faults that have been 

repeatedly reactivated. He stated that the Kiruna iron 

ores are associated with alkaline magmatism and formed 

during rifting. 

Other Ore Deposits of the Kiruna District 

Other iron ore deposits in the Kiruna district 

contain significant base and precious metals. Some of the 

iron ores in the Nautanen region southeast of Kiruna 

contain significant copper. At Tjarrojakka, 50 km westsouthwest of Kiruna, chalcopyrite, bornite, and 

magnetite are within tuffaceous zones in the wall rocks of 

the iron oxide-apatite deposit and within the iron deposit 

itself (Frietsch, 1980). Grip (1979) indicated that copper 

mineralization impregnates the ore breccia of the deposit 

and is accompanied by younger scapolite alteration. 

Copper deposits at the Tjarrojakka iron deposit probably 

are stratiform, and the ore reserves are 3.2 million tons at 

0.4 percent Cu, or 13 million tons at 0.2 percent Cu 

(Parak, 1985). Both gold and silver are locally 

concentrated. At the Gruvberget magnetite-hematiteapatite deposit, 3 km west of Svappavaara (fig. 6), 

chalcopyrite and bornite, which were previously mined, 

are found with scapolite in the northern and central parts 

of the deposit (Grip, 1979). The country rock is an 

altered trachyte porphyry. 

Finally, at Aitik, southeast of Kiruna, a 

disseminated Cu-Au-Ag deposit of 300 million tons 

contains an average of 0.4 percent Cu, 0.3 glton Au, and 

5 glton Ag (Zweifel, 1976; Frietsch, 1980). The host 

rocks are metasedimentary and possibly equivalent to the 

conglomerate of Kurravaara. Ore minerals are chalcopyrite, pyrite, magnetite, and pyrrhotite, and gangue 

minerals are quartz, barite, calcite, and fluorite. The host 

rocks have been metasomatically altered to tourmaline, 

sericite, and scapolite (Zweifel, 1976; Frietsch, 1980). 

Midcontinent Region, United States 

Regional Geology 

Proterozoic rocks of the Midcontinent region of 

the United States crop out in the Lake Superior area and 

in southeast Missouri. Proterozoic rocks between these 

two regions underlie a Paleozoic cover (Sims, 1985). 

Data on the basement rocks of the United States (fig. 8) 

are mostly derived from drill core and cuttings (Kisvarsanyi, 1980, 1981; Bickford, Sides, and Cullers, 1981; 

Van Schmus and Bickford, 1981; Anderson, 1983; 

Thomas and others, 1984; Sims, 1985; Bickford and 

others, 1986; Sims and Peterman, 1986). 

Lake Superior Reg ion 

Archean rocks of the Superior province of the 

Canadian Shield extend into the Midcontinent region in 

Minnesota. The oldest Archean rocks (2.6-3.6 Ga) 

consist of high-grade gneisses in southern and central 

Minnesota and central Wisconsin (Van Schmus and 

Bickford, 1981). Younger Archean greenstone terranes 

(2.6-2.7 Ga) are north of this older Archean terrane and 

consist of subaqueous volcanic rocks, derivative 

sedimentary rocks, and granites (Sims and others, 1987). 

The Great Lakes tectonic zone (GLTZ) separates these 

two terranes in Minnesota. Over the GLTZ, Early 

Proterozoic sediments, including bif, collected in a large 

basin (Animikie) between about 1.9 and 2.1 Ga (Sims 

and others, 1987). According to Sims and others (1987), 

breakup of the North American continent occurred 

about 1.9 Ga. Following this breakup and the 

development of oceanic crust, a complex island arc 

system was formed 1.83-1.89 Ga. Collision of this island 

arc system with the Archean and Early Proterozoic rocks 

produced the Penokean orogeny in the western Great 

Lakes region and northwest Iowa. 

In the Wisconsin part of the Lake Superior region, 

the Penokean orogeny was followed by extrusion of 

extensive subalkaline rhyolite and emplacement of subalkaline epizonal granites and minor tholeiitic mafic 

Ore Deposit Model for Olympic Dam-Type Mineralization 19 
50° N._ 

• 

EXPLANATION 

Locations and age ranges 

of dated samples: 

1.41-1.49 Ga 

/!, 1.34-1.41 Ga 

0 1.03-1.09 Ga 

' \ 

\ 

\ 

1.6-1.7 Age range (Ga) of basement rocks in 

area outlined by dashed lines 

1.48 Average age (Ga) of anorogenic event, 

based on U-Pb dates 

Anorthosite 

Midcontinent rift zone 

~ 2.5 

/ 

., / 

/ 

0 200 400 600 BOO KILOMETERS 

I I I I I 

I 

/ 

/ 

I 

Figure 8. Distribution and age of basement rocks and anorogenic (rapakivi-type) granite-rhyolite complexes of the 

Midcontinent region of the United States. Modified from Anderson (1983), Hoppe and others (1983), and Thomas and others 

(1984). 

rocks at 1.76 Ga; deposition of fluvial sandstones and 

minor marine carbonate rocks and shales (the Baraboo 

interval); mild deformation and resetting of Rb-Sr 

systems by reheating at 1.63 Ga; and intrusion of the 

Wolf River rapakivi granite and associated anorthosite at 

about 1.5 Ga. These suites are the oldest anorogenic 

rhyolite and granite terranes. Smith (1978, 1983) 

recognized four chemically and mineralogically distinct 

anorogenic suites in the 1,760-million-year-old terrane in 

south-central Wisconsin: (1) peraluminous ash-flow 

tuffs, (2) metaluminous quartz- and orthoclase-bearing 

rhyolites and granophyric granites, (3) low-Si02 , highstrontium, and REE-depleted granites and rhyolite dikes, 

and ( 4) tholeiitic basaltic to andesitic intrusive rocks. The 

intrusive and extrusive metaluminous and peraluminous 

rocks originated by fusion of different lower crustal 

intermediate sources. The third suite formed by partial 

melting of an intermediate crustal source. Heat for the 

20 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
partial melting may have been provided by ascending 

mantle-derived tholeiitic magmas of the last suite (Smith, 

1983). 

Conformable with and locally unconformable with 

the 1.76-billion-year-old anorogenic granite-rhyolite 

suite are metasedimentary rocks of the Baraboo interval 

(1.76-1.63 Ga) (Dalziel and Dott, 1970; Dott, 1983; 

Greenberg and Brown, 1983, 1984). Rocks of the Baraboo interval are dominantly fluvial sandstones that were 

deposited on a stable craton (Dott, 1983; Greenberg and 

Brown, 1984). The area of fluvial deposition eventually 

was transgressed by a marine succession of carbonate 

rocks, shales, and iron formations that developed on a 

stable continental shelf (Dott, 1983). The iron formations 

are found with ferruginous quartzite (chert?) and ferruginous slate (Greenberg and Brown, 1983). Although the 

chert-iron-formation-shale sequences are near the top of 

the Baraboo interval succession, in many cases they rest 

directly on older basement without intervening fluvial 

quartzites (Greenberg and Brown, 1984). Rocks of the 

Baraboo interval may have been deposited in isolated 

basins, estuaries, highlands, and minor volcanic centers 

within a larger epicratonic basin (Greenberg and Brown, 

1984). The sequences may also be separated by major 

unconformities. About 1.63 Ga, there was a major 

resetting of Rb-Sr ages and minor deformation 

throughout the southern Lake Superior region (Van 

Schmus, Thurman, and Peterman, 1975; Sims and Peterman, 1980; Van Schmus and Bickford, 1981; Peterman 

and others, 1985). 

At 1.5 Ga, the Wolf River batholith and associated 

anorthosite were emplaced in central Wisconsin (Van 

Schmus, Medaris, and Banks, 1975). The batholith may 

be oogenetic with the anorthosite and was derived by 

partial melting of an intermediate to tonalitic crustal 

source (Anderson and Cullers, 1978; Anderson, 1980). 

Granitic rocks of similar age, composition, and origin are 

found in the St. Francois Mountains in Missouri and 

throughout the Midcontinent area (fig. 8) (Bickford, 

Sides, and Cullers, 1981; Cullers and others, 1981; 

Anderson, 1983). 

Missouri 

In southeast Missouri anorogenic rhyolites and 

epizonal granites (1.5 Ga) crop out in the St. Francois 

Mountains. These exposures are a small part of much 

larger anorogenic granite-rhyolite terranes in the Midcontinent subsurface (Bickford, Sides, and Cullers, 1981; 

Denison, 1981; Van Schmus and Bickford, 1981; 

Anderson, 1983; Hoppe and others, 1983; Thomas and 

others, 1984; Bickford and others, 1986). Sm-Nd studies 

(Nelson and DePaolo, 1982, 1985) on basement rocks in 

the Midcontinent indicate that these Middle Proterozoic 

rocks were formed rapidly by partial melting of 1.9-1.7-

billion-year-old crust. 

The rocks of the St. Francois Mountains consist 

primarily of undeformed rhyolites and granites (see 

Kisvarsanyi, this volume). Minor alkaline andesites and 

trachytic dikes and flows are contemporaneous with, but 

unrelated to, the rhyolites (Kisvarsanyi, 1981; Kisvarsanyi and Kisvarsanyi, 1981; Sides and Bickford, 1981; 

this study). Other mafic volcanic rocks, the "Silver Mines 

Mafic Group," are subalkaline and have both calcalkaline and tholeiitic affinities (Sylvester, 1984). Diabase 

dikes and gabbro bodies were emplaced late in the 

regional magmatic history (Amos and Desborough, 

1970; Sylvester, 1984). Several caldera complexes and 

associated ring plutons have been recognized both on the 

surface and in the subsurface (Kisvarsanyi, 1980, 1981; 

Bickford, Sides, and Cullers, 1981; Cullers and others, 

1981; Sides and others, 1981). The epizonal granitic 

plutons are penecontemporaneous with the rhyolites and 

represent frozen magma chambers of calderas (Sides and 

others, 1981). Like the 1.5-billion-year-old Wolf River 

batholith in central Wisconsin, the epizonal granites, ring 

plutons, and associated rhyolitic rocks are all products of 

partial melting of an intermediate (quartz diorite or 

graywacke), lower crustal source (Cullers and others, 

1981). Kisvarsanyi (1980) suggested that this magmatism 

resulted from intraplate hot-spot activity. The trachytic 

rocks, on the other hand, are related to tension fractures 

and caldera collapse structures (Kisvarsanyi and Kisvarsanyi, 1981; Kisvarsanyi, 1981; this study). Stromatolitic 

limestones, volcaniclastic rocks, hematitic iron 

formations, volcanic breccias, and minor volcanic rocks 

also are related to caldera collapse (Stinchcomb, 1976; 

Sides, 1981; Sides and others, 1981; this study). 

As in Wisconsin, Sweden, and South Australia, 

Rb-Sr ages of the Missouri rhyolites and granites were 

reset owing to a loss of radiogenic strontium about 

1.3-1.2 Ga (Bickford and Mose, 1975). Wenner and 

Taylor (1976) concluded from oxygen isotope studies 

that a regional hydrothermal event involving circulation 

of meteoric waters affected the St. Francois Mountain 

area. The event affected the felsic rocks and some of the 

mafic rocks; that is, it preceded emplacement of some 

mafic dikes but not others. The event was not related to 

emplacement of the felsic rocks but may be due to 

emplacement of Sylvester's (1984) "Skrainka Mafic 

Group." 

Following the granite-rhyolite magmatism at 

1.51-1.35 Ga, the Midcontinent rift system developed in 

the Midcontinent region (fig. 8). Continental tholeiitic 

mafic flow and intrusive rocks as well as minor felsic 

volcanic and sedimentary rocks compose the rift, an 

aborted structure. Southeast and east of the St. Francois 

Mountains is the late Precambrian and Paleozoic Reelfoot rift (Braile and others, 1982). 

Ore Deposit Model for Olympic Dam-Type Mineralization 21 
The youngest intrusive events in the region were 

emplacement of Paleozoic to Mesozoic carbonatitic and 

kimberlitic rocks (Zartman and others, 1967; Cannon 

and Mudrey, 1981; Basu and others, 1984). 

Iron Deposits of Missouri 

Field and drill-core studies indicate that the 

magnetite, magnetite-hematite, and hematite deposits 

intrude and are contemporaneous with the felsic volcanic 

sequence (fig. 9) (this study). Trachyte, magnetite trachyte, trachyandesite to basalt, and the iron 

mineralization originated from the same source (Kisvarsanyi, 1981; Kisvarsanyi and Kisvarsanyi, 1981; this 

study). Only four of the eight deposits (Pea Ridge, Pilot 

Knob subcrop, Pilot Knob outcrop, Iron Mountain) have 

been mined, although the other four (Bourbon, Kratz 

Springs, Boss-Bixby, and Camels Hump) have been 

drilled (Smith, 1968; Snyder, 1968; Kisvarsanyi, 1981). 

The deposits generally contain more than 40 percent Fe 

and may comprise several hundred million tons of ore. 

More than 200 million tons of ore are reported at the 

Bourbon deposit (Snyder, 1968), and more than 102 

million tons of ore reserves are reported to remain at the 

Pea Ridge mine after 20 years of mining (Skillings, 1982). 

These figures suggest that the iron ore reserves for the 

district may be as great as 1 billion tons. 

Magnetite±hematite is the major ore mineral in all of 

the deposits. Interpretations of the geology of the iron 

deposits by various authors suggest three different 

origins for the iron mineralization within the district: (1) 

intrusive magmatic, (2) late-stage hydrothermal, and (3) 

exhalative/fumarolic. 

The Boss-Bixby deposit is the only one of the eight 

deposits in which chalcopyrite and bornite are the main 

copper minerals (Smith, 1968). Accessory minerals 

include quartz, actinolite, calcite, fluorite, apatite, barite, 

pyrite, and chlorite. Brandom and others (1985) also 

found carrollite, cobaltian pyrite, and electrum and 

suggested that the deposit has many similarities to the 

Olympic Dam deposit on the basis of mineralogy, 

tectonic style, and paragenesis. 

The Pea Ridge orebody extends below the Precambrian erosion surface vertically for less than 1 km and 

is 183m wide (Emery, 1968). Trachytic rocks have been 

intersected in drill holes within 40 km of the mine and are 

projected, on the basis of other drill-hole and geophysical 

data, to form a broad half-arc around the deposit (Kisvarsanyi, 1981). It is not known whether trachytic rocks 

occur within the mine. As in some of the Kiruna orebodies, five distinct ore zones occur within the Pea Ridge 

orebody: (1) magnetite±apatite±monazite±fluorite± 

barite±calcite±quartz±pyrite, (2) specular hematite, 

(3) quartz-hematite ± fluorite ±barite ± molybdenite± 

tourmaline, ( 4) quartz-amphibole, and (5) hanging-wall 

DEPOSITION OF PALEOZOIC SEDIMENTARY ROCKS 

EROSION 

NORTHEAST FAULTING 

INTRUSION OF SHEPHERD MOUNTAIN GABBRO 

INTRUSION OF MUNGER GRANITE PORPHYRY-1.38 b.y. 

DEPOSITION 

Cope HoiiOIIV Formation 

Johnson Shut-ins Rhyolite TAUM 

Proffit Mountain Formation ~~~&RA Taum Sauk Rhyolite 

Royal Gorge Rhyolite 

Bell Mountain Rhyolite 

Wildcat Mountain Rhyolite 

Lindsey Mountain Rhyolite 

Ironton Rhyolite 

Buck Mountain Shut-ins Formation· andesite dikes(?) 

Pond Ridge Rhyolite 

Cedar Bluff Rhyolite 

Shepherd Mountain Rhyolite; folding(?) 

Unit6901 

Ironton Hollow Rhyolite 

Wolf Mountain Ignimbrite 

Tribby Breccia 

Iron Mountain Lake Felsite; Northwest faulting(?) 

EROSION 

CALDERA 

RESURGENCE 

(intrusion of 

ring plutons) 

CALDERA 

COLLAPSE 

INTRUSION-1.5 b.y. DEPOSITION ]BUTLER 

Butler Hill Granite Grassy Mountain Ignimbrite HILL 

Silvermine Granite Lake Killarney Formation CALDERA 

1

lnformal map unit of Berry (1976). 

Figure 9. Geologic history of the Butler Hiii-Taum Sauk 

caldera area, St. Francois Mountains, southeast Missouri. 

Compiled from Anderson (1976), Wracher (1976), Panna 

(1978), Sides (1981), Sides and others (1981), Panna and 

Hood (1983), and this study. 

ore breccia (Emery, 1968). The deposit also contains 

cobaltian pyrite (this study). 

According to Emery (1968), the orebody was 

forcefully injected into four volcanic porphyries. Zones 1, 

4, and 5 probably originated by forceful injection of an 

iron-rich magma, whereas zones 2 and 3 formed during 

late-stage hydrothermal activity (Emery, 1968). The 

contact between the magnetite and hematite zones is 

gradational. Similarities between trace mineral contents 

of the magnetite and hematite-quartz zones suggest that 

the hematite zone evolved from the magnetite zone by 

differentiation and an increase in oxygen fugacity. 

The Pilot Knob subcrop orebody is similar to the 

Pea Ridge orebody. It consists of two major types of ore: 

(1) coarse-grained, friable, massive magnetite± barite± 

calcite±quartz±orthoclase±molybdenite, and (2) finegrained, dense, disseminated magnetite± hematite (Murrie, 1973; Pinchock, 1975; Wracher, 1976; Michel and 

Pasteris, 1981; Panno and Hood, 1983). The contact 

between the two types of ore is gradational. 

The Pilot Knob outcrop orebody consists of finely 

banded silicic hematitic ores primarily composed of 

hematite, quartz, sericite, and minor barite (Anderson, 

1976). The fine layering is a mixture of hematite-quartzbarite intermixed with waterlaid air-fall tuffs and breccia 

(Anderson, 1976). The deposit contains relict raindrop 

impressions, soft sediment deformation, salt casts, crossbedding, and ripple marks. The ore contains as much as 

3 percent Ba (this study). 

22 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
The Pilot Knob subcrop ore deposit may have 

formed by forceful injection and replacement of the host 

rhyolitic ash flow by a magnetite-rich magma, followed by 

hematite-quartz-rich hydrothermal solution (Michel and 

Pasteris, 1981; Panno and Hood, 1983). Other studies 

(Anderson, 1976; this study) propose that the hematite 

was produced by fumarolic activity within a lacustrine 

environment. The sedimentary characteristics of the 

outcrop orebody suggest deposition in a lake in which the 

fluctuation of the water level was related to volcanism. 

The source of the hematite-quartz-barite solutions could 

be the same as that for the Pilot Knob subcrop orebody. 

The hot-spring origin for this deposit is further supported 

by interlayering of waterlaid tuffs, stromatolitic 

limestone, and fine-banded hematitic layers at the Cuthbertson Mountain manganese deposit south of Pilot 

Knob (Stinchcomb, 1976; this study). Other tuffaceous 

sedimentary rocks± hematite± copper± gold± silver are 

at Ketcherside Gap southwest of Cuthbertson Mountain 

(Stinchcomb, 1976; Sides, 1981; this study). Sides (1981) 

noted that all of these bedded volcaniclastic sedimentary 

rocks lie along or near proposed ring faults of a caldera. 

One possible interpretation of these data is that the 

sedimentary rocks originated as intracaldera lake 

sediments. Figure 9 illustrates the stratigraphic 

constraints placed on the origin of the iron deposits. If 

the outcrop orebody originated. from the same iron 

oxide-rich source, as proposed (Anderson, 1976; this 

study), then the iron deposits are related to caldera 

collapse. The Tribby Breccia and related units shown on 

figure 9 are interpreted as having originated during a 

caldera collapse phase (Sides and others, 1981). Kisvarsanyi and Kisvarsanyi (1981) observed that trachytic 

rocks and iron orebodies lie along or near centers of 

caldera ring complexes, and they suggested that 

emplacement of the orebodies was controlled by tension 

fractures associated with caldera collapse. Caldera 

resurgence probably sealed off the iron oxide source 

conduits. The Iron Mountain deposit also is associated 

with an andesite host rock and also is within the proposed 

ring dike of a caldera (Murphy and Ohle, 1968). 

Furthermore, hematitic alteration± fluorite as veins and 

zones around the iron oxide orebodies suggest that 

fluorite may have been a major volatile component. 

Inner Mongolia, People's 

Republic of China 

Regional Geology 

The Bayan Obo hematite-light REE deposit is on 

the northern edge of the Sino-Korean craton, which is 

composed of Archean greenstones and related rocks 

(Zhang and others, 1984). Lower Proterozoic rocks 

consist of the Wutai Group, a sequence of schists, 

gneisses, marbles, and ferruginous sedimentary rocks 

that were involved about 2.0 Ga in the Wutai orogeny. 

Intracratonic troughs (aulacogens) formed between 

1,950 and 800 Ma along the northern edge of the craton. 

Clastic rocks, magnesium -carbonate rocks, tillites, 

sedimentary iron formations, manganese-phosphorite 

beds, and local submarine volcanic rocks were deposited 

in the troughs. These units are overlain by schists, slates, 

phyllites, dolomites, and intermediate and felsic volcanic 

rocks of the Huto Group. The rocks were deformed by 

the Zhongtiao or Luliang orogeny about 1.8-1.7 Ga 

(Zhang and others, 1984), and cratonization is thought to 

have been completed about 1.5 Ga (He and others, 1983; 

Zhang and others, 1984). Kimberlites were emplaced 

into the Sino-Korean craton during the Paleozoic (Basu 

and others, 1984), Mesozoic, and Cenozoic (Zhang and 

Liu, 1983). 

Bayan Obo Iron Deposits 

The Bay an Obo iron oxide deposits consist of three 

hematite± magnetite ore bodies totalling 1 billion tons of 

ore averaging 30-32 percent Fe, 1-6 percent light REE, 

0.10 percent columbite, and 2 percent fluorite (Argall, 

1980). The iron orebodies are lenses and pods within 

dolomitic limestone; the hanging wall is slate. Felsic 

volcanic rocks (Li, 1983)-subaqueous lavas and tuffsof the Middle Proterozoic Bayan Obo Group are 

stratigraphically related to the dolomite. Hercynian 

granites and alkaline gabbros have been intruded into the 

Bayan Obo Group (Wang, 1980; Wang and Yang, 1983). 

The relationship between the orebodies and these 

intrusive rocks is not understood. Argall {1980) found 

sodic amphiboles and pyroxenes in the ore and suggested 

that the ore deposits are contact metasomatic. Wei and 

Shangguan {1983), on the other hand, using oxygen 

isotope studies of the magnetite and hematite from 

different types of ore, suggested that the ores have a 

variety of origins, ranging from sedimentary-exhalative to 

contact metasomatic. Wang {1981) suggested that the 

light REE mineralization formed under oxidizing 

conditions. Based on stable isotopes and geochemistry, 

Bai and Yuan (1983) suggested that the orebodies 

formed from an igneous carbonatitic-submarine volcanic 

source. The data are meager but suggest conditions of 

formation analogous to Olympic Dam (Greenfield 

Formation), Hauki hematites, and Pilot Knob outcrop 

orebodies; that is, formation by exhalation of iron-rich 

fluids into a marine or lacustrine environment during a 

period of felsic volcanism. 

Ore Deposit Model for Olympic Dam-Type Mineralization 23 
Other Iron Oxide-Rich Ore Deposits 

Other iron oxide-rich ore deposits that may belong 

to this class of ore deposits include the iron deposits of 

(1) the Adirondack Mountains and (2) the New JerseyNew York Highlands, and (3) the Tennant Creek ironcopper-gold deposits, Northern Territory, Australia. 

Adirondacks, New York 

McLelland (1986) concluded that anorthosite massifs and associated rocks of the Adirondacks represent a 

Middle Proterozoic anorogenic, bimodal caldera 

complex. Chemical trends of the anorthosite and felsic 

suites are similar to those of the Wolf River batholith of 

Wisconsin, the Ragunda Complex of Sweden, and the St. 

Francois Mountains of Missouri. McLelland (1986) also 

concluded that geochemical trends in the above areas are 

remarkably similar and further suggested that (1) the 

low-titanium iron deposits associated with the ash-flow 

tuffs and sediments, as for example, Benson mines 

(Crump and Beutner, 1968; Palmer, 1970; Sainey, 1973), 

were formed by exhalation of iron-rich fluids via hot 

springs and fumaroles, and (2) the iron deposits are 

remarkably similar to those in the St. Francois 

Mountains. The Adirondack low-titanium iron oxide 

deposits are anomalously high in Mn, Ba, P, and F 

(McLelland, 1985). Apatite is generally greater than 1 

percent but can be as high as 5-10 percent. Uranium can 

be present with magnetite throughout the felsic sequence 

(magnetite pegmatites, pegmatites, granites, syenites, 

and so forth) and also in the exhalative deposits (Nutt, 

1982). High-titanium iron deposits are also associated 

with the anorthosites, as for example at Sanford Lake, 

New York (Gross, 1968). 

New Jersey-New York Highlands 

The Precambrian geology of the New Jersey-New 

York Highlands is similar to that of the Adirondacks, as 

indicated by the rock types and high-grade metamorphism of the felsic volcanic (alaskites) and 

sedimentary rocks. Uranium, thorium, and REE are 

associated with the iron oxide deposits (Grauch, 1978; 

Kastelic, 1980; Baillieul and Indelicato, 1981; Gundersen, 1984), which have been studied by Offield 

(1967), Sims (1953, 1958), and Sims and Leonard (1952). 

Kastelic (1980), Grauch (1978), and Gundersen (1984) 

suggested that some of the iron oxide deposits are the 

distal facies of a subaqueous volcanogenic massive 

sulfide deposit. The enrichment of uranium, thorium, 

and REE in the iron oxide-rich deposits and the apparent 

presence of subaqueous conditions during their 

formation in the Late Proterozoic suggest similarity with 

iron oxide deposits of South Australia, Sweden, and 

Missouri. 

Tennant Creek, Northern Territory, Australia 

The copper-gold-bismuth-magnetite-hematite orebodies of the Tennant Creek district are in interbedded 

volcanic and sedimentary rocks of the early Middle 

Proterozoic (1,819-1,849 Ma) Warramunga Group 

(Crohn and Oldershaw, 1965; Black, 1984). The 

mineralization is believed to be contemporaneous with 

the main folding event (Black, 1984). The Warramunga 

Group consists mainly of shales, graywackes, and siltstones but also contains rhyolitic pyroclastic rocks and 

intrusive quartz-feldspar porphyries that are associated 

with the mineralization (Large, 1975; Black, 1984). 

Hematite- and magnetite-rich shales, siltstones, and iron 

formations are closely associated with the copper-goldbismuth mineralization (Large, 1975). The magnetitehematite mineralization is in lenticular, ellipsoidal, and 

pipelike bodies that crosscut sedimentary features 

(McNeil, 1966; Large, 1975). Although most studies 

indicate that the iron oxide-rich copper-gold-bismuth 

mineralization is epigenetic, the mineralization, regional 

geology, and host rocks could be interpreted as indicating 

a syngenetic iron-rich intrusive/exhalative system. 

RAPAKIVI GRANITEANORTHOSITE MASSIFS 

Rapakivi-type granites such as the Wolf River 

batholith in Wisconsin and the Butler Hill Granite in 

Missouri are widespread throughout the North 

American continent and were emplaced within the epizone (±5 km) (Anderson, 1983). At deeper crustal level~ 

(5-13 km), anorthosite is present with the rapakivi 

granite (Bridgewater and Windley, 1973; Emslie, 1978; 

Morse, 1982), and at still deeper crustal levels (23-27 

km), anorthosite is the main rock-forming unit (Morse, 

1982). A direct relationship between anorthosite and 

rapakivi granite has been proposed for the Wolf River 

batholith of Wisconsin and the Ragunda massif in 

Sweden (Kornfalt, 1976; Anderson, 1983): emplacement 

of the anorthosite provided a heat source for the melting 

of the lower crust and, therefore, the generation of the 

rapakivi granite suite (see fig. 13). Anorthosites may be 

present at depth in the St. Francois Mountains but not yet 

exposed by erosion (Anderson, 1983). An indirect · 

relationship between the two rock types has been shown 

in South Australia through paleomagnetic reconstruction 

(see fig. 5). Giles (1988), on the basis of petrochemical 

modeling of the Gawler Range Volcanics, suggested that 

these felsic volcanic rocks were generated under similar 

conditions and from a similar source area as other 

Middle Proterozoic anorogenic rapakivi granite suites. 

24 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Table 1. Major characteristics of Middle Proterozoic rapakivi granite-anorthosite massifs 

[From Bridgewater and Windley (1973), Bridgewater and others (1974), Emslie (1978, 1985), Wiebe (1980), Frost and Lindsley 

(1981), Philpotts (1981, 1982a, b, 1984), Condie (1982), Morse (1982), Valley and O'Neil (1982), Anderson (1983), Flower 

(1984), and Taylor and others (1984)] 

1. Passively emplaced in anorogenic terranes. 

2. Occur within or at margins of Early Proterozoic terranes that have previously undergone a major orogenic 

event. 

3 . Preceded or accompanied by amphibolite- or granulite-facies metamorphism in the lower crust. 

4. Associated with: 

a. Extensive subalkaline felsic volcanism. 

b. Coeval, local mafic to intermediate volcanic rocks from a mantle source; that is, alkaline volcanic 

rocks or intraplate continental tholeiite. 

c. Pre-existing zones of structural weakness. 

d. Graben formation, continental clastic sedimentation, and local marine transgressions. 

5 . Graben formation or caldera collapse structures accompany and postdate emplacement of rapakivi granite. 

6 . Anorthosite massifs, associated mafic rocks, and cogenetic rapakivi granite are intruded simultaneously or 

within 500 million years of each other. 

7 . Rapakivi granitic magmas are subalkalic to marginally peraluminous and are primary melts derived from 

fusion of the lower crust (27 -36 km) by emplacement of anorthosite and related intrusive rocks or other 

related heat sources: 

a. These relatively dry granitic magmas are distinctively potassium and iron enriched and contain 

ubiquitous accessory fluorite. 

b . They are emplaced in the epizone. 

8 . Anorthosite differentiation produced three immiscible liquids: 

a. A silica- and alkali-rich liquid to form quartz mangerite. 

b. An iron-rich melt to form ferrodiorite Uotunite). 

c. Initially dry anorthositic magma. 

9. Ferrodiorite magma differentiates to form another, even more iron-rich liquid that produces magnetiteilmenite-apatite ore deposits. Immiscibility is controlled by the phosphorus or titanium content of the 

melt or by pressure (depth). 

10. Anorthosite and related intrusive rocks, for example, ferrosyenite, can be intruded to depths of less than or 

equal to 10 km; that is, into the zone of meteoric water. 

11. Anorthosite massif-rapakivi granite terranes are related to a mantle plume line possibly associated with an 

abortive or incipient rifting event or with mafic magma ponding. 

Table 1 summarizes the primary characteristics of 

these rapakivi granite-anorthosite massif terranes. Of 

primary interest are the relationships between (1) 

extensive felsic volcanism and the rapakivi granites, (2) 

rapakivi granites and anorthosite massifs, and (3) iron 

oxide-titanium-phosphorus deposits associated with 

anorthosite massifs and volcanic- and volcanicsedimentary-hosted iron oxide deposits. The last 

relationship may be important to the origin of iron oxide 

deposits. Trachytic rocks associated with the subvolcanic- and volcanic-sedimentary-hosted iron oxide 

deposits are alkaline (see fig. 10) and therefore could be 

related to the anorthosites or to comagmatic, mantlederived mafic rocks. This relationship is important 

because it suggests a genetic link between the origin of 

the anorthosite-related iron oxide-apatite deposits and 

the subvolcanic- and volcanic-sedimentary-hosted iron 

oxide deposits. If, for example, as Valley and O'Neil 

(1982) and Frost and Lindsley (1981) have indicated, 

anorthosites and comagmatic mafic rocks can be 

emplaced at shallow depths ( < 10 km), then an iron 

oxide source would be available for the development of 

subvolcanic- volcanic-volcanic-sedimentary-hosted iron 

oxide ore deposits. 

Ore Deposit Model for Olympic Dam-Type Mineralization 25 
GEOCHEMISTRY 

Rock Geochemistry 

Whole-rock and trace element geochemistry can be 

used to determine the origin (mantle or partial melts of 

lower crust) of intrusive and extrusive igneous rocks. The 

geology of each area discussed previously is made up of 

anorogenic, coeval, bimodal volcanic and intrusive rocks 

and minor intercalated sedimentary rocks. Subalkaline 

felsic volcanic and intrusive rocks of a lower crustal 

source are the dominant rock types. Interlayered with 

and intrusive into the sub alkaline felsic volcanic rocks are 

subvolcanic to volcanic, alkaline mafic to intermediate 

trachytic rocks derived from a mantle source. Figure 10 

illustrates the bimodal nature of these rock suites 

(alkaline trachytic rocks and sub alkaline felsic rocks) for 

the Midcontinent (Wisconsin and Missouri), South 

Australia, Sweden, and the People's Republic of China. 

Recently, Giles (1988) presented new data for the 

Gawler Range Volcanics that exhibit similar geochemical 

results and conclusions (separate mantle and crustal 

sources for the mafic and felsic rocks, respectively). 

The quantity of mafic to intermediate alkaline 

rocks in these regions is minor. Bickford, Horrower, and 

others (1981) estimated that this rock suite makes up less 

than 10 percent of the rocks in the buried Precambrian 

basement, and this is certainly the case in the St. Francois 

Mountains of Missouri and in South Australia. In 

Sweden, on the other hand, magnetite trachytes and 

trachytes probably make up more than 5 percent of the 

total rock and possibly as much as 30-40 percent. This 

generalization is difficult to apply at Bayan Obo. 

All of these terranes have been subjected to a 

postanorogenic, low-grade metamorphic or hydrothermal event that has caused mobility of some elements, 

especially strontium and rubidium. Therefore, zirconium 

data were obtained from the literature and by analysis 

(this report) for rocks of both chemical suites from 

Missouri and South Australia. A plot of Si02 versus 

Zr/Ti02 (fig. 11) shows the dual nature of these suites; 

thus, the post-anorogenic event probably was not 

sufficiently strong enough to affect the less mobile 

elements. 

Petrochemical modeling by Giles (1988), using 

geochemical data for the Gawler Range Volcanics, 

indicates that (1) a refractory sialic residue of maficintermediate composition is a realistic source for the 

Gawler Range Volcanics, (2) low strontium isotope 

ratios for the volcanic rocks and postorogenic granites 

(Webb and others, 1986) are consistent with a lower 

crustal refractory sialic residue source, and (3) the mafic 

volcanic rock geochemical data indicate a primary mantle 

source, based on enriched light REE, Sr, Rb, Zr, and Pb 

tz 

w 

u 

0:: 

w 

0.. 

t- 10 I 

(!) 

w 

~ 

~ 

ci 5 tO 

z 

+ 

q. 

::c 

45 50 55 60 65 70 

Si02 I IN WEIGHT PERCENT 

EXPLANATION 

Q South Australia 

(80 analyses) 

~{]T£t ~iiis.!,alyses) 

C- "'- Sweden 

-- (245 analyses) 

* People's Republic of China 

(9 analyses) 

• Wisconsin, post-Penokean 

(35 analyses) 

Figure 10. Si02 versus K20 + Na20 diagram of Middle 

Proterozoic anorogenic volcanic-plutonic complexes. Data 

for trachytic rocks from Missouri and Sweden are in the 

alkaline field; data for the extensive felsic volcanic rock suites 

in each area are in the subalkaline field. Data from Missouri, 

Sweden, and the People's Republic of China are both 

regional and local (associated with the iron oxide deposits) in 

origin. From numerous sources. 

contents. Furthermore, modeling by Cullers and others 

(1981) indicates partial melting of an intermediate lower 

crustal source for the origin of the felsic volcanic and 

intrusive rocks in Missouri. The Middle Proterozoic 

anorogenic rapakivi granite suite has a "common geochemical fingerprint" of analogous high Fe20 3\ Ti02 , 

~0, REE, Zr, Nb, Y, Rb, and Ba contents as compared 

to Cenozoic calcalkaline volcanic rocks (Giles, 1988). It 

also has relatively low Al20 3, CaO, and Sr contents as 

compared with Cenozoic calcalkaline rocks having 

similar Si02 contents. Giles (1988) concluded that 

similar conditions and source regions were necessary for 

the generation of the felsic magmas. 

Mineral Chemistry 

The chemical composition of the various major and 

minor mineral phases in anorthositic and MissouriKiruna-type iron oxide deposits indicates a possible 

26 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
1-

~ 65~--------~~~~~~~~Yr~----~ u 

0: 

w 

CL 

~ 60~--------~~~~~~~=----=====~ (.9 

UJ 

~ 

~ 

N 

Q 

(/') 

0.01 

EXPlANATION 

t/7 _jl Missouri 

~ South Australia 

Figure 11. Si02 versus Zr/Ti02 diagram of Middle Proterozoic anorogenic complexes in Missouri and South Australia. 

Fields: sub-AB, subalkaline basalt; AB, alkali basalt, trachybasalt; B + TB + N, basanite, trachybasanite, nephelinite; A, 

andesite; RD + D, rhyodacite, dacite; R, rhyolite; Ph, phonolite; T, trachyte; C + P, comendite and pantellerite. From 

Floyd and Winchester (1978), Giles and Teale (1979a, b, 

1981), and this study. 

genetic link between the two types of ores. Trace element 

contents (Ti, V, Cr) of magnetite and hematite have been 

used to distinguish different genetic types of irontitanium oxide-apatite deposits (Kisvarsanyi and Proctor, 

1967; Frietsch, 1970, 1982; Loberg and Horndahl, 1983). 

Loberg and Horndahl (1983) proposed two types of iron 

deposits-orthomagmatic/exhalative and sedimentary 

bif. The first type can be subdivided into titaniferous 

(associated with gabbros and anorthosites) and nontitaniferous (Kiruna and Missouri). The orthomagmatic/exhalative group may indicate that a direct relationship 

exists between intrusive and exhalative iron±apatite 

deposits formed from the residual solutions of a 

magnetite magma (Smellie, 1980; Loberg and Horndahl, 

1983). Loberg and Horndahl (1983) and Frietsch 

(1970) also showed that the trace elements in the iron 

deposits of the older Greenstone Group in Sweden are 

similar to those in the Kiruna iron oxide-apatite deposits. 

Some pre-existing iron-rich material may have been 

assimilated, or the deposits may have had a similar 

source or origin. 

Fluorapatite is a rare to minor component of both 

anorthositic- and Missouri-Kiruna-type iron oxide 

deposits. Frietsch (1974b) showed that apatite associated 

with the Kiruna deposits is fluorapatite. Fleischer (1983) 

determined, on the basis of REE contents of apatite from 

different rock-forming environments, that the Kirunatype ores had an intrusive-magmatic origin. Kolker 

(1982) noted that the Kiruna ores differ from 

anorthosite-related iron-titanium oxide-apatite ores in 

both composition and proportions of iron-titanium 

oxides and fluorapatite, but that the F:Cl ratio and REE 

composition of the fluorapatite in the two ore types are 

similar. Both types of apatites are also enriched in the 

light REE. Thus, Kolker (1982, p. 1156) stated 

"Assuming a common magmatic origin, a fundamental 

difference in the deposits may be attributed to the depth 

of emplacement***." 

Kolker (1982) and Philpotts (1967) found a 2:1 

ratio of iron-titanium oxides to fluorapatite in irontitanium oxide-apatite deposits associated with 

anorthosites. Kolker further noted that this ratio varies in 

the Kiruna ores because of the local variation in the 

amount of fluorapatite. In a study of trace elements in 

iron-titanium oxides in mafic and anorthositic rocks, 

Lister (1966) found that the titanium content decreases 

with progressive differentiation of a magma. If, as Kolker 

(1982) believes, the difference between the iron-titanium 

oxide-apatite ores of anorthosites and iron oxide-apatite 

ores of Kiruna is only depth of emplacement, then the 

variability of the iron-titanium oxide/apatite ratio and the 

lack of titanium in the iron oxides could be the result of 

differentiation of an iron-rich magma. 

DISCUSSION 

Most iron-titanium oxide-fluorapatite deposits are 

associated with anorthosite massifs (Philpotts, 1967, 

1981). Philpotts proposed, based on field and 

experimental data, that as crystallization of an 

anorthositic magma proceeds the residual liquid 

becomes enriched in silica, alkali elements, and iron. At 

some point the residual magma encounters a liquid 

immiscibility field and splits into two fractions-a silicaand alkali-rich liquid and an iron-rich liquid. The silicarich liquid crystallizes to form the quartz mangerite suite 

associated with anorthosites. The iron-rich fraction forms 

the ferrodiorites and iron-titanium oxide-fluorapatite 

deposits associated with anorthosites. 

Ore Deposit Model for Olympic Dam-Type Mineralization 27 
Feiss and others (1983) proposed that the 

Cranberry iron oxide deposits in western North Carolina 

formed in a similar manner. The deposits are REE 

enriched (Ussler, 1980) and consist of magnetite and 

clinopyroxene and minor apatite; however, because there 

are no known anorthosites in this area, they proposed 

that the iron oxide deposits originated from a ferrodioritic magma formed by liquid immiscibility from the 

crystallizing Bakersville Gabbro. Continued fractionation 

of the ferrodioritic magma then produced the Cranberry 

iron deposits. 

Philpotts (1982a, b) showed that liquid immiscibility can occur in sub alkaline ( calcalkaline, tholeiitic) 

and alkaline magmas. Because trachytic rocks associated 

with Middle Proterozoic iron oxide-rich ore deposits are 

alkaline in composition, anorthositic magmas or mantlederived alkaline gabbroic magmas are possible sources 

for these deposits. Van Marcke de Lummen (1985) also 

suggested that iron-titanium deposits can form through 

liquid immiscibility in a sub alkaline ( calcalkaline) 

magma. Therefore, the data of both Philpotts and Kolker 

suggest that the extensive, subalkaline felsic volcanic 

rocks in these provinces could also be a possible source 

of the metals in these deposits, given the right physicochemical conditions. 

The time at which liquid immiscibility occurs 

depends on the P2 0 5 and Ti02 contents of the 

fractionating melt and the oxygen fugacity of the melt. 

Experimental studies on mafic and felsic melts show that 

the liquid immiscibility gap expands-that is, extends the 

pressure range and lowers the temperature-with the 

addition of significant amounts ofP2 0 5 and Ti02 (Visser 

and Koster van Groos, 1979). Ryerson and Hess (1980) 

showed that the addition of phosphorus to a silicate melt 

will produce a silica-rich and a ferrobasaltic liquid. When 

liquid immiscibility occurs, phosphorus is strongly 

partitioned into the iron-rich fraction together with highcharge density cations-Ti, REE, Ta, Ca, Cr, Mn, Zr, 

Mg, Sr, and Ba (Ryerson and Hess, 1980; Watson, 1976; 

Naslund, 1983; Philpotts, 1984). Based on a study of 

iron-titanium oxide-fluorapatite deposits, Kolker (1982) 

suggested that sulfur is also partitioned to the iron-rich 

magma when immiscibility occurs. The partitioning of 

these elements to the iron-rich fraction would account 

for the presence of these elements in Middle Proterozoic 

iron oxide-rich ore deposits. Similarly, for the same 

reasons, base and precious metals and uranium, which 

are also high-charge density cations, would be partitioned 

to the iron-rich fraction. When an iron-rich magma 

enriched in these elements begins to crystallize (for 

example, apatite and ilmenite), specific elements would 

be concentrated further in the residual liquid together 

with silica. This late-stage liquid may be similar in 

composition to the quartz-hematite zones observed in the 

Missouri subsurface iron oxide deposits and to the exhalative iron oxide deposits such as Pilot Knob outcrop and 

Olympic Dam (Greenfield Formation). 

In a study of the effect of oxygen fugacity on liquid 

immiscibility in iron-bearing melts, Naslund (1983) 

stated that increasing oxygen fugacity increases the liquid 

immiscibility field. He proposed that the Kiruna iron ores 

formed by liquid-liquid exsolution from iron-rich melts 

under high oxygen fugacity and indicated that such high 

conditions in these magmas could develop through (1) 

reaction with oxidized wall rocks, (2) loss of hydrogen, or 

(3) absorption of ground water equilibrated with 

oxidized wall rocks or the atmosphere. A fourth 

possibility is the assimilation of previously existing iron 

oxide-rich ore deposits; that is, Early Proterozoic bif. 

This possibility has been suggested for the Kiruna 

deposits by Lundberg and Smellie (1979), Loberg and 

Horndahl (1983), and Smellie (1980). Assimilation of 

iron oxide-rich phases would tend to increase the iron 

content of the ascending magma. Once the magma 

reached the meteoric water zone, absorption of 

oxygenated ground water would further increase the 

oxygen fugacity of the magma. Valley and O'Neil (1982) 

and Frost and Lindsley (1981) showed that anorthosites 

and associated ferrodiorites and ferrosyenites can be 

emplaced at depths of less than 10 km; therefore, an 

iron-rich magma could be close to the surface in the zone 

of meteoric water and in an area of high f0 2 • A physicochemical study of restraints and conditions necessary 

for assimilation of lower crustal materials by ultramafic 

mantle-derived magmas led Sparks (1986) to suggest that 

high-temperature primitive magmas can assimilate 

substantial quantities (-50 percent) of lower crustal 

rock. He also suggested that anorthosites may be hybrids 

of lower crustal and primitive mantle melts; assimilation 

of bif or other iron oxide-rich rocks in the lower crust 

would alter the chemistry of ascending magmas. Sparks 

(1986) indicated that assimilation is very temperature 

dependent. The amount of crustal contamination of an 

ascending magma would decrease with decreasing 

temperature, and therefore the overall effect of 

assimilated material on the ascending magma 

composition would also decrease. 

Some iron oxide deposits, such as Boss-Bixby and 

Olympic Dam, have considerable associated base and 

precious metals. These metals can be concentrated 

through partitioning to the iron-rich magma and (or) 

through assimilation, depending on when assimilation 

occurs. In South Australia, the iron formation of the 

Middleback Subgroup could certainly underlie the 

Olympic Dam deposit inasmuch as bif is present along 

strike both north (in the Mt. Woods Inlier) and south of 

the deposit. Anderson (1980) interpreted aeromagnetic 

data in the Olympic Dam region to represent bif metasedimentary rocks of the Hutchison Group. This 

28 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
interpretation, if correct, could provide a possible source 

for the iron in the Olympic Dam deposit. Equivalent to 

the Middleback Subgroup in the Willyama Inlier (fig. 3, 

east side of Adelaide geosyncline) is the Thackaringa 

Group (Willis and others, 1983). The Cues and Himalaya 

Formations and the Rasp Ridge Gneiss of the Thackaringa Group also contain iron formation (magnetite). 

Associated with the iron formation are copper sulfide 

minerals (syngenetic) interlayered with massive, stratiform, and disseminated cobaltiferous pyrite in sodic 

plagioclase-quartz rocks (Willis and others, 1983), and 

minor Pb-Zn-Ag and barite. If similar rocks underlie 

Olympic Dam and if partial melting and assimilation 

associated with an ascending alkaline magma occurred, 

then an additional or primary source exists for these 

metals. 

Uranium is associated with apatite-bearing rocks in 

the Thomson Formation of the Early Proterozoic Animikie Group in Minnesota (McSwiggen and others, 1981 ). 

Similarly, copper sulfide deposits have been found in the 

Greenstone Group of northern Sweden. In both cases, 

similar Early Proterozoic rocks underlying the Middle 

Proterozoic iron oxide-rich orebodies could be sources 

for elements other than iron. 

Conversely, studies of REE in bif show that the 

heavy REE are concentrated in bif (Fryer, 1977). 

Therefore, the final iron oxide orebody should be 

enriched in heavy REE instead of in light REE as at 

Olympic Dam and Bayan Obo. Also, Loberg and Horndahl (1983) illustrated a distinct separation between 

sedimentary bif and magmatic iron deposits on the basis 

of major and trace elements. If bif contributed to the 

formation of the Middle Proterozoic iron oxide-rich 

deposits, then a continuum of trace element values 

should be present between the two end members, but this 

does not appear to be the case. 

A case for assimilation of an Early Proterozoic 

source can be made for only two of the four deposits-Kiruna and Olympic Dam. In the Midcontinent, 

orogenic rocks of the Central Plains orogen (1.7-1.63 

Ga) underlie, at least in part, the St. Francois terrane 

(Sims and Peterman, 1986); however, Early Proterozoic 

(1.9 Ga) rocks may underlie the Central Plains orogen 

rocks as inferred from Sm-Nd data (Nelson and DePaolo, 1985), and the composition of those rocks is inferred. 

The data available for Bayan Obo do not elucidate the 

presence or composition of any Early Proterozoic rocks, 

and the origin of the Middle Proterozoic iron oxide-rich 

deposits may be related to a magmatic source coupled 

with some contamination by older rocks. 

Another possible source for iron and other metals 

is Archean Algoma-type iron formations and other metalliferous deposits in the lower crust. These are generally 

very limited in lateral extent, however, and therefore are 

not likely candidates for an assimilated source. 

If the alkaline trachytic rocks observed in Missouri 

and Sweden represent an iron-rich immiscible fraction, 

then a silica-rich fraction should also be present. Philpotts (1982a, b) indicated that iron, calcium, and 

phosphorus partitioning to the iron-rich fraction is less 

pronounced in alkaline rocks such that some of these 

elements are partitioned to the silica-rich fraction. As 

figures 10 and 11 show, there is a wide range in the 

composition of the associated volcanic rocks. Some of the 

more felsic alkaline volcanic rocks probably represent the 

silica-rich immiscible fraction, which would contain more 

accessory magnetite, sphene, and apatite and be more 

syenitic. In the Midcontinent, this relationship is less 

obvious; however, Bickford, Horrower, and others 

(1981) and Yarger (1981) reported two 1.35-billionyear-old epizonal, magnetite-rich (2 weight percent) 

granites in the subsurface of east-central Kansas. 

According to Sims (1985), magnetite-bearing granitic 

rocks form many small plutons in both Kansas and 

Missouri. Granites and volcanic rocks similar to these 

could represent the "missing" silica-rich immiscible 

fraction. 

Early formed iron oxide-rich deposits could have 

higher contents of titanium and phosphorus in the form 

of ilmenite and apatite, assuming, of course, that the 

original magma-for example, a ferrodiorite magma 

associated with anorthosites-contained sufficient 

quantities of titanium and phosphorus before 

immiscibility occurred. As fractionation proceeds, 

titanium is removed through crystallization of ilmenite 

and phosphorus removed by precipitation of apatite 

(some uranium, light REE, and fluorine are also 

removed). Continued removal of these phases 

concentrates iron, silica, and barium, along with some 

fluorine, uranium, and light REE (if any remain) and 

base and precious metals. Textures at Olympic Dam 

suggest that sulfide minerals precipitated simultaneously 

with the iron minerals. This sequence of events is 

depicted graphically in figure 12. The final result of this 

differentiation sequence could be a low-temperature, 

quartz-rich, epithermal gold deposit. 

These iron oxide-rich ore deposits can originate 

through (1) differentiation and immiscibility of a mantlederived gabbro, as at Cranberry, North Carolina (Feiss 

and others, 1983), and Misi, Finland (Nuutilainen, 1968), 

(2) differentiation and immiscibility associated with the 

formation of a ferrodioritic magma from an anorthosite 

or related mantle-derived alkaline magma, or (3) partial 

melting and assimilation of older iron-rich rocks in 

combination with (1) or (2). Figure 13 illustrates on a 

hypothetical cross section of the crust the possible 

pathways in which Middle Proterozoic iron oxide-rich ore 

deposits could evolve. The diagram combines the 

observed presence of rocks of the rapakivi granite suite, 

anorthosite suite, and subalkaline rhyolitic and alkaline 

Ore Deposit Model for Olympic Dam-Type Mineralization 29 
GEOCHEMISTRY TECTONISM 

Resurgence/uplift 

Extension- graben 

formation, tectonic 

instability, caldera 

collapse 

Uplift extensiongrabens, intrusion 

Uplift, intrusion 

Partial melting under 

plating, minor upwarp 

MINERALOGY 

Hematite + 

fluorite-barite 

carbonate± 

quartz veins ± 

sulfide 

Hematite± 

sulfide 

Magnetitehematite± 

sulfide 

Magnetite± 

sulfide 

Magnetiteilmenite± 

trace sulfide 

GEOLOGY 

Minor mafic-felsic volcanism, twomica granites (?7), low-temperature 

hydrothermal activity, continental 

sedimentation 

Felsic (ash flow) volcanism, 

sedimentation, fumarollc 

activity (iron formations) 

Felsic volcanism, trachyte, ferrodiorite, diorite (alkaline?), biotite 

granite, intrusion (magnetite) 

breccias, sedimentation 

Gabbro (alkaline), biotite-amphibole 

granite, minor mafic volcanism(?) 

Anorthosite, gabbro, amphibole 

granite, charnockite, assimilation 

of iron formation(?) 

6 

6 

ORE DEPOSITS 

OLYMPIC DAM, S. AUSTRALIA; 

MISSOURI(?); CHINA(?) 

OLYMPIC DAM, S. AUSTRALIA; 

PILOT KNOB OUTCROP, 

MO.; HAUKI ORES, SWEDEN; 

BAYAN OBO, CHINA(?) 

OLYMPIC DAM, ACROPOLIS, 

S. AUSTRALIA; PILOT KNOB 

SUBSURFACE, MO.; REKTOR 

ORES, SWEDEN 

KIRUNA ORES, SWEDEN 

6 

MISI, Fl NLAND(?) 

D 

ADIRONDACKS, N.Y.; ALLARD 

LAKE, QUEBEC, CANADA 

D 

Figure 12. Model for evolution of Middle Proterozoic iron oxide-rich ore deposits. (The bars in the Geochemistry column are 

based on empirical observations of the mineralogy and geochemistry of the rocks from the literature and from field work.) 

30 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
trachytic volcanic rocks, and iron oxide-ri_ch sedi~ent~ry 

rocks. The presence of time-equivalent uon oxide-nch 

sedimentary rocks, for example, at Tarcoola (?) a?d 

Olympic Dam (Greenfield Formation), South Austraha, 

and Cuthbertson Mountain and Pilot Knob outcrop, 

Missouri, associated with iron oxide deposits suggests 

that these iron oxide-rich magmas could contribute to 

exhalative iron oxide-rich deposits. The Kiruna Mertainen and Painirova deposits indicate eruption of iron 

oxide-rich lavas. The intrusive nature of these iron oxiderich magmas, some having high volatile element contents, 

produces the intrusive breccias and massive 

magnetite±hematite orebodies seen at Olympic Dam, 

Kiruna, and Missouri (Lundberg and Smellie, 1979; 

Lambert and others, 1987). A mantle plume, plume line, 

or ponding of a mafic magma is the postulated heat 

source for the anorogenic event (Flower, 1984; Taylor 

and others, 1984; Emslie, 1985; Giles, 1988). 

Numerous authors have proposed that these 

Middle Proterozoic anorogenic rock suites were the 

result of an abortive rifting event (Emslie, 1978; Wiebe, 

1980; Morse, 1982; Anderson, 1983; McLelland, 1986; 

Giles, 1988; this study). Emslie (1985) and this study (fig. 

1) suggest that this anorogenic event was a transitional 

phase between Archean tectonism and Wilson Cycle 

tectonism. This passive tectonic event caused by underplating of the crust produced post -tectonic silicic 

magmas, uplift, vertical tectonic style erosion, and 

continental sedimentation, and therefore a thickening of 

the crust (Rogers and others, 1984). Crustal thickening is 

further enhanced by production of younger alkaline, 

carbonatitic, and kimberlitic magmas. Each of the 

stabilized crustal areas discussed above was also 

subjected to a younger abortive rifting event. This period 

of cratonic stabilization probably was a dominant process 

during the Middle Proterozoic and coincides with the 

break observed in the formation of many types of ore 

deposits (fig. 1) (Gole and Klein, 1981; Meyer, 1981, 

ANDESITE/ IRON OXIDE-RICH 

RHYOLITE BASALT SEDIMENTARY ROCKS 

Surface 

;;;> 

GABBRO Mantle 

t MANTLE PLUME 

Figure 13. Postulated Middle Proterozoic differentiation 

sequence to produce iron oxide-rich ore deposits. 

1985; Gross, 1983; Rundquist, 1984a, b). The Middle 

Proterozoic was, therefore, a period of profound tectonic 

change in the evolution of the Earth. 

A few Paleozoic iron oxide deposits have 

characteristics similar to the Middle Proterozoic 

Missouri-Kiruna-type iron oxide deposits. Iron oxideapatite deposits at Avnik, Turkey, and Bafq, Iran, are 

associated with subalkaline felsic volcanic rocks and 

mafic volcanic and intrusive rocks (Forster and Knittel, 

1979; Helvaci, 1984). Other characteristics of these 

deposits compare favorably with those of the MissouriKiruna type, and therefore these deposits ar~ classified 

on figure 1 with Missouri-Kiruna deposits. These 

deposits, although not anorogenic, illustrate that liquid 

immiscibility can also occur in subalkaline rocks (for 

example, Laco, Chile; Park, 1961). . . . 

The Mt. Painter Fe-U-F-REE±Ba deposit Withm 

the Mt. Painter Inlier of South Australia (fig. 3) consists 

of late Delamerian (Late Ordovician and Silurian), 

orogenic intrusive, granitic-hematitic:chloritic brecci~s 

contained within the Lower to Middle ProterozOic 

Radium Creek Metamorphics (Lambert and others, 

1982). Sulfur isotope values for pyrite imply that the 

deposit formed from magmatic fluids. Oxygen and 

carbon isotope data also support this origin, although 

some oxygen isotope data support mixing with meteoric 

waters. Lambert and others (1982, 1987) contended that 

the brecciation and related alteration were the result of 

hydraulic fracturing and hydrothermal activity from 

ascending granitic magmas. Fumarolic activity associated 

with this event is represented by banded quartz-hematite 

beds (Mt. Gee Beds). Many of the granitic complexes 

emplaced during the Delamerian orogeny are _alkali~e. 

The Mt. Painter deposit may also represent the uon-nch 

fraction of an alkaline magma that has undergone liquid 

immiscibility. 

Einaudi and Burt (1982) indicated that 

Fe±Cu±Co±Au contact metasomatic skarn deposits 

are present in oceanic island arcs or rifted continental 

margins associated with diorite and mafic volcanic rocks. 

These deposits and the younger iron oxide± apatite 

deposits shown on figure 1 (for example, Santorini 

Islands, Greece) are in extensional tectonic 

environments (Smith and Cronan, 1983; Bostrom and 

Widenfalk, 1984) and therefore may be similar to the 

Missouri-Kiruna type, which is envisioned as the 

product of extensional tectonism. There does not need to 

be a fundamental change in their mode of origin; that is, 

immiscibility of a mantle-produced magma. Whether the 

deposit was formed by submarine fumarolic act.ivity (the 

Santorini Islands), as a lava flow (El Laco, Chile; Park, 

1961), or as a contact metasomatic skarn (Gushan, 

People's Republic of China; Song and others, 1981 ), the 

source probably was alkaline mafic to intermediate 

intrusive rocks in which iron-rich elements were 

Ore Deposit Model for Olympic Dam-Type Mineralization 31 
concentrated by immiscibility and (or) differentiation. 

The thickness and tectonic stability of the crust in the 

area of ore formation are also primary controls on the 

formation of these types of deposits. 

The Middle Proterozoic is characterized as a 

period of cratonic stability and crustal thickening. Crustal 

thickening resulted from emplacement of post-tectonic, 

anorogenic rapakivi granite-anorthosite massifs, 

anorogenic volcanism, and continental sedimentation. 

Associated with these processes was minor, alkaline 

mafic to intermediate volcanism and plutonism. Major 

iron oxide-rich ore districts are directly associated with 

the alkaline volcanism. Emplacement of iron oxide-rich 

ore deposits is related to vertical and extensional tectonic 

features and (or) caldera collapse structures. 

The addition of new silica-rich material to the crust 

generally occurred at the edge of an Archean shield. Such 

areas had previously undergone deposition of Early 

Proterozoic continental shelf and island arc material, 

including banded iron formations, followed by a major 

orogeny. Underplating of the crust by a mantle plume or 

plume line caused (1) granulite-facies metamorphism, 

(2) fusion of the lower crust to produce rapakivi granite 

suites and then subalkaline felsic volcanic rocks as 

suggested by Giles (1988) for South Australia, and (3) 

emplacement of alkaline mafic rocks and anorthosite 

massifs. As the mafic magmas ascended through the 

crust, they underwent differentiation and (or) 

immiscibility. Assimilation and fusion of preexisting iron 

formation may have contributed to the iron content of 

the magma and increased the oxygen fugacity of the 

magma, particularly if the conclusions of Sparks (1986) 

concerning the formation of anorthosites from crustal 

contamination of a primitive mantle melt are valid. If 

immiscibility occurred, iron-rich and silicic- and alkalirich fractions formed. When and if immiscibility occurred 

depends on the (1) composition of the original melt 

(phosphorus and titanium contents), (2) pressure, (3) 

temperature, and ( 4) oxygen fugacity. When 

immiscibility occurred, Fe, Ca, Ti, P, Ba, REE, and other 

elements were partitioned to the iron-rich phase. The 

iron-rich magma then differentiated to form alkaline 

intermediate rocks and iron oxide-rich deposits. The 

silica-rich magma formed alkali-rich intrusive and extrusive rocks having abnormally high magnetite-apatitesphene contents. Experimental data indicate that an 

iron-rich melt could ongmate through liquid 

immiscibility from subalkaline magmas; however, field 

data, in the form of near-ore alkaline trachytic rocks, 

support an alkaline source. 

The iron oxide deposits range from subvolcanic/intrusive to extrusive/exhalative depending upon 

availability of structural conduits to the surface. The 

deposits are primarily magnetite and (or) hematite and 

contain major to accessory quantities of fluorapatite, 

barite, fluorite, sulfides, light REE, uranium, and 

precious metals. The final composition and location 

(surface versus subsurface) of the ore deposit depends 

on (1) when and where (pressure, temperature) in the 

crust liquid immiscibility occurred, (2) oxygen fugacity, 

(3) amount of differentiation after immiscible separation, (4) addition of meteoric components, and (5) 

structural constraints. The ore deposits individually, or as 

a group, generally contain more than 1 billion tons of ore. 

Following the emplacement of this anorogenic 

suite and associated iron oxide deposits, the rocks were 

subjected to a very low grade metamorphic or hydrothermal event that at least reset the Rb-Sr age dates. The 

region may have been subjected to uplift and erosion and 

deposition of younger sediments. Late-stage rifting and 

emplacement of kimberlites, carbonatites, and alkaline 

igneous complexes completed the cratonic history. 

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Ore Deposit Model for Olympic Dam-Type Mineralization 39 
Geologic Setting and Ages of Proterozoic Anorogenic 

Rhyolite-Granite Terranes in the Central United States 

ByP.K. Sims 

Abstract 

Four episodes of anorogenic magmatism have been 

recognized in the central part of the United States. From 

oldest to youngest, these are represented by the newly 

recognized 1 ,835-Ma granite-rhyolite assemblage in central 

Wisconsin, the 1, 760-Ma rhyolite-granite terrane in southern 

Wisconsin, the 1 ,480-Ma St. Francois granite-rhyolite terrane 

of southeast Missouri, and the 1,40Q--1,350 Ma Spavinaw 

granite-rhyolite terrane in northeast Oklahoma, southern 

Kansas, and adjacent Missouri. Successive episodes of 

magmatic activity become younger southward and overlie 

and intrude older orogenic provinces. The rhyolite-granite 

complexes of the two older episodes overlap rocks 

{1 ,89Q--1 ,840 Ma) of the Penokean orogen and those of the 

two younger ones, which are contiguous, overlap rocks 

{1 ,78Q--1 ,630 Ma) of the Central Plains orogen. Each of the 

terranes is characterized by rhyolite ash-flow tuff {ignimbrite) 

and anorogenic {A-type) granite. 

INTRODUCTION 

The purpose of this paper is to discuss the regional 

geologic framework of the known anorogenic graniterhyolite terranes in the northern Midcontinent of the 

United States, terranes presumed to be highly favorable 

as potential hosts for mineral deposits similar to that at 

Olympic Dam in South Australia (Roberts and Hudson, 

1983). 

The Olympic Dam deposit is essentially a hematiterich iron deposit that has economic concentrations of 

associated copper, uranium, gold, and light rare-earth 

elements (REE) (Roberts, 1986). It is spatially, and 

probably genetically, related to approximately 1.5-Ga 

anorogenic volcanic-plutonic rocks in the Gawler craton 

described by Branch (1978). In this respect, it has a 

setting similar to that of the Kiruna-type iron deposits in 

northern Sweden (Lundberg and Smellie, 1979) and in 

the St. Francois Mountains of southeast Missouri (Kisvarsanyi, 1981 ). All of these deposits contain vast 

tonnages of iron ore, anomalous to economic 

concentrations of base and precious metals, and 

anomalous amounts of barium, fluorine, uranium, 

thorium, and light REE. For these reasons, the Middle 

Proterozoic anorogenic St. Francois terrane, in 

particular, and other Proterozoic anorogenic terranes in 

the central United States are considered favorable as 

potential hosts for base- and precious-metal-bearing iron 

deposits similar to Olympic Dam. 

BASEMENT GEOLOGIC TERRANES 

The geologic framework of the basement rocks in 

the northern Midcontinent of the United States has been 

outlined previously (Sims and others, 1987) and is 

discussed only briefly here. 

Data on Precambrian basement rocks in the 

northern Midcontinent are inadequate to prepare a 

comprehensive geologic map that delineates individual 

lithologic units. Accordingly, in preparing the recent 

basement map (Sims, 1985, 1989), the Precambrian 

rocks were mapped in terms of tectonostratigraphic 

terranes, a concept similar to that riow being widely 

applied in the Cordillera of western North America. I use 

the term "terrane" for a geologic entity of regional extent 

that is characterized by a geologic history that differs 

from adjacent terranes; this definition departs from that 

used in the Cordillera (Jones and others, 1983) only in 

that a terrane need not be fault bounded. 

Eight major terranes have been identified and 

delineated in the northern Midcontinent (fig. 14). From 

oldest to youngest, these are (1) Archean gneiss terrane 

(age, 2.6-3.6 Ga) of Superior craton; (2) Late Archean 

greenstone-granite terrane (age, 2.6-2.75 Ga) of 

Superior craton; (3) Early Proterozoic Wisconsin 

magmatic terrane (age, 1.84-1.89 Ga) and associated 

epicratonic rocks (age, -1.9-2.1 Ga) of the Penokean 

orogen; ( 4) Early Proterozoic rhyolite-granite terrane of 

southern Wisconsin (age, 1.76 Ga); (5) Early Proterozoic 

metamorphic and granitoid rocks of the Central Plains 

orogen (age, 1.63-1.78 Ga); (6) Middle Proterozoic St. 

40 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Francois granite-rhyolite terrane (age 1.45-1.48 Ga); (7) 

Middle Proterozoic Spavinaw granite-rhyolite terrane 

(age, 1.35-1.40 Ga); and (8) Middle Proterozoic Midcontinent rift system (age, 1.0-1.2 Ga). Other coherent 

rock bodies of lesser areal extent include quartzites (age, 

-1.63-1.82 Ga) and the anorthosite and rapakivi granite 

of the Transcontinental anorogenic province (age, 

1.4-1.5 Ga). Three of the terranes-the Late Archean 

greenstone-granite terrane, the Wisconsin magmatic terrane, and the rocks of the Central Plains orogen-are 

oceanic-arc complexes, dominantly juvenile material, 

that were accreted to the North American continent; 

most other terranes are continental crust that evolved in 

extensional environments and were vertical additions to 

the crust. 

With respect to tectonic history, the Early Proterozoic Penokean orogen was the site of convergence along 

the southeast margin of the Archean Superior craton that 

culminated with the collision between the Wisconsin 

magmatic terrane and the craton at about 1,860 Ma 

(Sims and others, 1989). Later, rifting of the composite 

Archean-Early Proterozoic craton in what is now the 

central United States led to arc magmatism in the 

interval1,780-1,630 Ma and collision of the rocks of the 

Central Plains orogen with the composite continental 

crust (Sims and Peterman, 1986). 

ANOROGENIC GRANITERHYOLITE TERRANES 

Four distinct episodes of anorogenic magmatism 

have been recognized and partly delineated in the 

northern Midcontinent. From oldest to youngest, these 

are (1) 1,835-Ma granite-rhyolite in central Wisconsin, 

(2) 1,760-Ma rhyolite-granite in southern Wisconsin, (3) 

1,450-1,480-Ma granite-rhyolite in the St. Francois 

Mountains, southeast Missouri, and (4) 1,350-1,400-Ma 

granite-rhyolite in northeast Oklahoma and adjacent 

areas. Successive episodes of magmatic activity become 

younger southward. The two older periods of anorogenic 

magmatism, in Wisconsin, followed the Penokean orogeny (-1,860 Ma); the rhyolite-granite complexes 

intruded and overlapped rocks of the orogen. The two 

younger periods followed the Central Plains orogeny and 

stabilization of the crust ( -1,630 Ma); the rocks 

overlapped older rocks of the Central Plains orogen. 

The oldest (1,835 Ma) known anorogenic magmatism, recognized only recently (Sims, 1990) as a distinct 

magmatic interval, consists of two small areas of rhyolitic 

rocks, each about 25 km2 in area, and several plutons of 

comagmatic red alkali feldspar leucogranite that intrude 

older, Early Proterozoic arc-volcanic rocks (age, 

1,840-1,880 Ma) and Archean gneisses within an 

area of about 1,300 km2 (locality A, fig. 14). The rhyolitic 

rocks consist mainly of massive to flow-banded rhyolite 

that contains spherulites and lithophysae, felsic crystal 

and lithic tuff, volcanogenic graywacke (turbidites), and 

welded tuff. The granites contain abundant microcline 

microperthite, lesser weakly zoned sodic plagioclase, and 

sparse biotite and fluorite. Cataclastic textures are nearly 

ubiquitous, and metamorphism is weak. Both the rhyolite 

and the granite are extensively altered: biotite is mostly 

altered to hematite and chlorite or less commonly to 

epidote and sericite, and hematite typically forms 

inclusions in feldspar and dustlike fillings in cracks. 

The granite and rhyolite are peraluminous and 

have alkaline affinities, high FeO/FeO + MgO ( > 0.80), 

~0/Na 0 ( > 1.0), Ba/Sr, and KJRb ( > 300) ratios, and 

large negative europium anomalies (table 2). They are 

remarkably similar chemically to the rhyolite and granite 

of the 1,760-Ma age group in southern Wisconsin (table 

1) (Smith, 1978, 1983). Associated mineral deposits are 

unknown. 

The second oldest anorogenic magmatic terrane, 

the 1,760-Ma rhyolite-granite terrane of southern 

Wisconsin, is poorly exposed within an area of about 

10,000 km2 as isolated inliers in the Fox River valley and 

tributaries (locality B, fig. 14) (Smith, 1978, 1983). The 

rhyolite consists of interbedded ash-flow tuff and volcaniclastic sedimentary rocks of both peraluminous and 

metaluminous suites (Anderson and others, 1980). The 

associated cogenetic granites, which are particularly 

poorly exposed, are granophyric and possibly intruded 

their own volcanic cover. An undated diorite body and 

associated basaltic dikes are spatially associated with the 

rhyolite. The rhyolite is dominantly gray, but the granite 

at the village of Redgranite and elsewhere is dark red as 

a result of hematite and chlorite alteration of the sparse 

biotite and hematite filling of cracks in the rock. The 

rocks are mildly deformed on northeast-trending fold 

axes, have weak to moderate cataclastic textures, and are 

weakly metamorphosed (lower greenschist facies). 

Magnetite is sufficiently abundant as an accessory 

mineral to produce a modest aeromagnetic anomaly. 

Presumably, the rocks overlie and intrude older 

(1,890-1,830 Ma) rocks of the Penokean orogen. The 

peraluminous and metaluminous suites (table 1) are 

considered cogenetic and possibly comagmatic and have 

been interpreted as having been derived from partial 

melting of separate sources of intermediate composition 

(Smith, 1983). The diorite and associated basalt probably 

were derived by partial melting of a mantle source and 

should have provided the heat source for partial melting 

of crust. No known mineral deposits are associated with 

this anorogenic suite. 

The 1.45-1.48-Ga granite-rhyolite of the St. Francois terrane (Kisvarsanyi, 1981) contrasts with the older 

anorogenic terranes in that it occupies a vast area in the 

Anorogenic Rhyolite-Granite Terranes in the Central United States 41 
J+ + + + 

+ + + + I 

+ I + + + + + 

1 + + + + + 

+ + + + + + 

----- 0 500 KILOMETERS 

EXPlANATION 

MIDDlE PROTEROZOIC (1,600-900 Ma) 

+ + + _+ , ,.. ~ .., 1\ 

1 ~~ t"L 1 ~.lo 

Midcontinent rift system (1.0-1.2 Ga) 

Rhyolite and granite (1.35-1.48 Ga) 

Anorogenic anorthosite and 

rapakivi granite (-1.48 Ga) 

EARLY PROTEROZOIC (2,500-1,600 Ma) 

Quartzite of "Baraboo Interval" 

Metamorphic and granitoid rocks 

rocks of Central Plains orogen 

(/ \\ ~ II Rhyolite and granite (-1.76 Ga) 

I ~ I , 

- .._ I Granite and associated rocks 

(age uncertain) 

Granite ( -1.8 Ga) 

Wisconsin magmatic terrane of 

Penokean orogen 

Rocks of Trans-Hudson orogen 

Epicratonic sedimentary and 

volcanic rocks (-1.9 -2.1 Ga) 

ARCHEAN (2,500 Ma and older) 

~ 

~ Greenstone-granite terrane terrane of Superior craton 

GLTZ 

Cb 

BH 

Ca 

CKu 

Mgl 

Nf 

Car 

(-2.6-2.75 Ga) 

Gneiss of central Wisconsin 

(2.8-3.0 Ga) 

Gneiss of Wyoming craton 

Gneiss terrane (3.0-3.6 Ga); 

includes (-2.6 Ga) granite 

Limit of outcrop 

Fault or shear zone 

Great Lakes tectonic zone 

Cheyenne belt 

Black HiUs uplift 

Chadron arch 

Central Kansas uplift 

Missouri gravity low 

Niagara fault 

Cambridge arch 

Figure 14. Precambrian basement terranes in the northern Midcontinent region of 

the United States. Circled italic letters refer to locations discussed in text. Modified 

from Sims (1985). 

42 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Midcontinent region and apparently is of somewhat more 

diverse composition. Bickford and others (1986) 

assigned these rocks to their "Eastern granite-rhyolite 

province," which extends from Missouri into Ohio; more 

recently, von Breemen and Davidson (1988) showed 

from isotopic ages that the rocks apparently extend into 

the Grenville province immediately east of Lake Huron. 

In the type area, the St. Francois Mountains (locality C, 

fig. 14) and adjacent covered areas in southeast Missouri 

known through drilling, this terrane is characterized by 

inferred volcanotectonic features including ring 

complexes, cauldron subsidence structures with ring 

volcanoes and ring plutons, and resurgent calderas with 

central plutons (Pratt and others, 1979; Kisvarsanyi, 

1980, 1981; Sides and others, 1981), thus establishing a 

tectonic environment comparable to that of some of the 

classic ring complexes of the world, such as the Mesozoic 

ring complexes of northern Nigeria (Bowden and Turner, 

1974). In the St. Francois terrane, the dominant rocks are 

silicic volcanic rocks, mainly ash-flow tuff, and 

comagmatic subvolcanic granophyric and perthitic alkali 

feldspar granite (Bickford and others, 1981). Ring 

intrusions are trachyte to trachyandesite and related 

porphyritic low-silica intrusive rocks. Central plutons are 

high-silica, evolved large-ion lithophile element, twomica ("tin") granite, inferred to have been emplaced in 

resurgent cauldrons (Kisvarsanyi, 1981). The chemistry 

of the rocks is summarized in Sims and others (1987) and 

discussed in more detail by Kisvarsanyi (1972), Bickford 

and others (1981), and Cullers and others (1981). 

Average analyses of four major rock types are listed in 

table 2. The famed magnetite-hematite-apatite deposits 

of the area are spatially associated with the trachytic suite 

of Kisvarsanyi (1981), whereas Sn-W-Ag-Pb-Sb vein 

deposits are possibly genetically related to the so-called 

"tin granites." 

Regional, almost pervasive alteration of the 

volcanic and granitic rocks in the St. Francois terrane was 

recognized by Anderson (1970). Typically, alkali feldspar 

is altered to patchy or veined, highly turbid, red microperthite, and plagioclase is albitized. Primary mafic 

minerals are changed to magnetite, hematite, chlorite, 

epidote, and calcite. The resulting mineral assemblages 

are characteristic of propylitic alteration. Also, secondary 

potassium feldspar is commonly concentrated adjacent to 

fractures or mafic dikes. Anderson (1970) noted that the 

intensity of the alteration-mainly shown by increasing 

redness of the feldspars-increases from northeast to 

southwest in the exposed parts of the St. Francois 

Mountains. 

Wenner and Taylor (1976) showed by regional 

variations in 18 0 in the exposed terrane that 18 0 is 

enriched from northeast to southwest, in the same 

direction as the increasing redness of the feldspars-that 

is, increasing intensity of hematite alteration. They also 

noted that some of the mafic rocks (mainly dikes) that 

intrude the rhyolites and granites "have suffered very 

little 18 0 enrichment compared to the nearby rhyolites 

that they intrude" (p. 1590), and they concluded (p. 

1591) "that the high-180 alteration event must have 

peaked in intensity before certain other dikes and sills 

(mafic intrusions) were intruded." The age of the 

widespread hydrothermal meteoric alteration is 

uncertain inasmuch as none of the unaltered mafic dikes 

has been isotopically dated. Wenner and Taylor (1976) 

stated that "it seems certain that the pervasive high-18 0 

hydrothermal alteration event occurred later than 1,400 

m.y. ago" (p. 1593), inasmuch as an altered body of this 

age (Munger Granite Porphyry) in the southwestern part 

of the region has a U-Pb zircon age of 1,400 Ma 

(Bickford and Mose, 1975). They further concluded that 

the alteration could have occurred 1,100-1,200 m.y. ago, 

a time of widespread Rb-Sr and K-Ar reset ages. 

Unpublished lead isotope data reported by Pratt and 

others (1979, p. 5), however, indicate that the mafic rocks 

are evidently about the same age as other igneous rocks 

(1,400-1,500 Ma) of the region; accordingly, the hydrothermal alteration must have followed shortly after 

crystallization of the rhyolite and granite. 

The fourth and youngest of the granite-rhyolite 

terranes, the Spavinaw (Thomas and others, 1984), 

composes the "Western granite-rhyolite province" of 

Bickford and others (1986) and occupies an area 

comparable to St. Francois terrane. It is known mainly 

from the subsurface in Oklahoma (Denison, 1981; Denison and others, 1984). Apparently, the volcanic and 

granitic rocks are grossly equivalent lithologically to the 

rhyolite and subvolcanic granite of the St. Francois 

terrane, but data on the chemistry of the rocks are 

meager (Bickford and others, 1981). 

Sims and others (1987) suggested that the granitic 

plutons of the Middle Proterozoic Transcontinental 

anorogenic province (Silver and others, 1977; Anderson, 

1983) correspond to deeper levels, now exposed by 

erosion, of the St. Francois and Spavinaw granite-rhyolite 

terranes and thus possibly are the mesozonal equivalent 

of the epizonal terranes. Within the area of concern to 

this report, four bodies are known, the largest of which is 

the Wolf River batholith in Wisconsin (Anderson and 

Cullers, 1978; Anderson, 1980) (fig. 1). These plutons 

are chemically similar to the rocks in the granite-rhyolite 

terranes in that they are highly silicic, have high Fe/Mg 

and F and low Ca, Mg, and Sr, and are enriched in 

large-ion lithophile elements (K, Rb, Ba, REE, and U). 

Anderson (1980) estimated that the granites in the Wolf 

River batholith crystallized at depths less than 3.8 km. In 

contrast to most other anorogenic plutons in the 

Anorogenic Rhyolite-Granite Terranes In the Central United States 43 
Table 2. Chemical analyses of representative samples of rhyolite and granite, Proterozoic anorogenic terranes, central 

United States 

[Leaders ( --) indicate not determined; blank indicates sample not analyzed for element] 

1 ,760-Ma 

1 ,835-Ma age groue age groue St. Francois terrane 

Number 2 3 4 5 6 7 8 9 10 11 12 13 

MAJOR ELEMENT OXIDES (weight percent) 

Si02 75.6 75.8 75.9 75.7 71.8 76.14 68.62 75.77 76.34 75.06 

Al203 11.8 11.9 11.9 12.2 14.23 11.79 14.40 12.36 11.40 11.48 

Fe203 2.19 2.26 1.95 1.12 1.47 1.10 3.31 1.35 2.18 2.51 

FeO 1.87 0.98 0.88 

MgO 0.13 <0.10 0.11 0.03 0.29 0.09 0.93 0.15 0.05 0.05 

CaO 0.66 0.65 0.53 0.40 1.23 0.45 1.79 0.49 0.25 0.22 

Na20 3.76 3.81 3.79 3.70 3.98 3.16 4.03 3.44 3.67 2.54 

K20 4.26 4.18 4.53 4.61 4.30 5.65 3.86 4.80 4.58 5.70 

H2o+ 0.22 1.12 0.58 

H2- 0.06 0.01 

Ti02 0.15 0.16 0.12 0.07 0.27 0.24 0.44 0.17 0.18 0.21 

MnO <0.02 <0.02 0.2 0.2 0.10 0.02 0.08 0.04 0.06 

P205 <0.05 <0.05 <0.05 0.04 0.09 0.01 

I.DI 0.60 0.33 0.35 1.68 2.15 3.39 

DESCRIPTIONS AND LOCAUI'lES 

1,835-Ma age group 

Major oxide analyses by X-ray fluorescence: analysts, J. Taggart, A. Bartell, and D. Siems. Minor elements, Ba through Ce, by X-ray 

fluorescence analysis: analyst, John Jackson. Minor elements, Sc through Lu, by instrumental neutron activation analysis: analyst, J. Mee. 

1. Granophyric granite, Cary Mound, Wood County, Wisconsin. Sample W299 collected by K.J. Schulz. Lat 44°31' N.,long 90°15' W. 

2. Spherulitic rhyolite, Cary Mound, Wood County, Wisconsin. Sample W298-2 collected by K.J. Schulz. Lat 44°31' N.,long 90°16' W. 

3. Alkali feldspar granite, quarry northeast of Cary Mound. Sample W849 collected by K.J. Schulz. Lat 44°32' N., long 90° 10' W. 

4. Red leucogranite, Cold Springs granite quarry, northeast of Wausau, Marathon County, Wisconsin. Sample 23 reported in LaBerge and 

Myers (1983). Analyzed at Pennsylvania State University. 

1,760-Ma age group 

Source: Smith (1983). Major oxide analyses by wet chemical methods. Minor element analyses (Rb, Sr, Pb, Zn) by atomic absorption 

spectrometry. All other minor element analyses by optical emission spectrography. 

5. Peraluminous rhyolite, average of 10 samples. 

6. Granite at Redgranite, Wisconsin. 

St. Francois terrane 

Samples 7-10 reported in Bickford and others (1981); samples 11-13 reported in Cullers and others (1981). 

7. "Silvermine granite"; medium grained; average of 5 samples. 

8. "Butler Hill granite"; average of 44 samples. 

9. "Grassy Mountain ignimbrite"; average of 15 samples. 

10. "Taum Sauk rhyolite"; average of 6 samples. 

11. "Silvermine granite." 

12. "Butler Hill granite." 

13. "Grassy Mountain ignimbrite." 

Transcontinental province, which belong to the 

magnetite series, the Wolf River rocks belong to the 

ilmenite series of Ishihara (1977). A large anorogenic 

body penetrated by drilling in Stevenson County, Illinois 

(locality D, fig. 14), is altered near the unconformity 

beneath Paleozoic sedimentary rocks and is enriched in 180 and in many incompatible elements relative to 

deeper samples (Kyser, 1986). Kyser interpreted the 

alteration to have resulted from interaction with 

formation waters typical of the Illinois Basin at 

temperatures greater than 300 °C. 

POSSIBLE SIGNIFICANCE OF 

HYDROTHERMAL ALTERATION 

Propylitic alteration indicated by reddened feldspars and the replacement of mafic silicate minerals in 

the rhyolite-granite complexes characterizes the 1,835-

Ma group and the 1,480-Ma St. Francois terrane, as well 

as a part of the 1,760-Ma rhyolite-granite terrane. For 

the 1,835-Ma group, the hematitic alteration seems to be 

confined to the rocks of this age group, suggesting that it 

is a late-stage hydrothermal alteration related to this 

44 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Table 2. Continued 

Number 2 3 4 5 6 7 8 9 10 11 12 13 

MINOR ELEMENTS ~arts ~r million) 

Ba 1,390 1,660 1,236 661 390 

Rb 70 55 94 125 202 

Sr 41 73 32 126 21 

ZI. 395 415 267 224 590 

y 54 54 54 

Nb 27 28 28 

Ni <5 <5 <5 

Cr <20 <20 <20 18 8 

Cu <5 <5 <5 

Zn 35 47 45 

La 85 91 83 47.3 98 

Ce 165 182 161 90.0 225.2 

Sc 1.045 1.21 0.78 8.76 3.12 

Cr <2 2.0 <4 

Co 0.68 0.41 0.44 5.81 0.81 

Ni <21 <13 18 

Zn 19 33.8 28 

As >2 <Z <2 

Sb 0.1 0.082 0.56 

Rb 78.5 61.8 105 

Sr 59 74 36 

Ba 1,470 1,760 1,310 

Cs 0.203 0.126 0.32 

ZI. 434 421 333 

Hf 10.4 10.05 9.69 4.91 11.85 

Ta 2.18 2.23 2.48 

Th 15.6 15.19 16.7 13.79 24.1 

u 3.66 3.67 4.29 

Au <5 <7 <7 

La 100.7 111 81.9 47.3 98.5 

Ce 194 204 159 90.0 225.2 

Nd 73.9 80.9 62.1 37.8 74.5 

Sm 14.5 15.02 12.84 6.7 13.8 

Eu 2.19 2.49 1.61 1.8 0.7 

Tb 1.69 1.71 1.61 0.9 2.7 

Yb 6.48 6.35 6.76 3.6 9.1 

Lu 0.926 0.907 0.942 0.5 1.4 

Rb/Sr 1.71 0.75 2.94 0.99 9.62 

Ba!Sr 33.9 227 38.6 5.24 18.6 

particular magmatic system. In the 1,480-Ma St. Francois 

terrane, Wenner and Taylor (1976) showed that the 

propylitic alteration resulted from a regional, high-180, 

hydrothermal-meteoric alteration event that transgressed intrusive bodies and in part at least preceded 

crystallization of mafic dikes that cut the rhyolite-granite 

complexes. Probably the alteration event occurred 

1,400-1,500 Ma. Mafic dikes of probable similar age cut 

the Pea Ridge iron deposit (Emery, 1968) and other iron 

bodies in the area, and accordingly the regional alteration 

could be approximately the same age as the iron 

mineralization, in which case both could be genetically 

related, the regional alteration being a lower 

temperature halo around the higher temperature 

replacement iron deposits. The demonstration that the 

hydrothermal fluids causing the regional alteration 

940 389 200 271 

119 224 163 229 

198 56 12 35 

8.3 4.2 4.1 

15.4 17.9 

6.8 22.6 10.9 

30.4 54.9 33.98 

60.5 117 66.4 

6.10 9.4 7.2 

1.44 0.59 0.63 

1.26 1.67 1.78 

4.90 7.82 7.86 

0.63 1.23 1.02 

0.60 4.0 13.58 6.54 

4.75 6.95 16.67 7.74 

originated as meteoric surface waters (Wenner and 

Taylor, 1976) could explain the close association of 

hematite and magnetite in several of the iron deposits in 

the St. Francois terrane. Perhaps the hematite bodies are 

a later replacement of primary magnetite bodies, which 

developed from more oxidized meteoric waters. 

DISCUSSION AND SUMMARY 

The anorogenic magmatism in the Midcontinent 

occurred in areas that already had undergone orogeny, 

and it now is distinguished as A-type (Loiselle and 

Wones, 1979) and "within plate" (Pearce and others, 

1984) magmatism and contrasts with orogenic 

Anorogenic Rhyolite-Granite Terranes in the Central United States 45 
assemblages both petrographically and chemically. The 

anorogenic bodies are dominantly magnetite bearing 

(Anderson, 1983) and belong to the magnetite series of 

Ishihara (1977). It is generally agreed from chemical 

modeling (Cullers and others, 1981; Smith, 1983) that 

the anorogenic magmas were derived by partial melting 

of older crust, and this origin is consistent with 

neodymium data that indicate mantle separation ages for 

the rocks, regardless of crystallization age, in the range 

1,700-1,900 Ma (Nelson and DePaolo, 1985). 

The 1,760-Ma granite-rhyolite magmatism in 

southern Wisconsin overlapped in time the convergent 

orogenic activity related to the Central Plains orogen and 

its westward analogues (Bickford and others, 1986). 

Possibly, this magmatism in Wisconsin reflects extension 

that accompanied the accretion of rocks of the Central 

Plain orogen, similar to that described by Karig (1971) 

from the Mariana Islands in the South Pacific. 

Widespread anorogenic magmatism did not begin, 

however, until1,450-1,480 Ma, about 100 m.y. after the 

North American continent had stabilized. It was followed 

about 100 m.y. later by the 1,350-1,400-Ma anorogenic 

activity in the Western granite-rhyolite province (Bickford and others, 1986). These magmatic events took 

place along and near the Middle Proterozoic continental 

margin, almost certainly as a result of regional extension, 

but the underlying cause of extension remains equivocal. 

The tectonic environment of Middle Proterozoic 

anorogenic magmatic events seems particularly 

analogous to that in South Australia at the time the 

Olympic Dam deposit was formed (Roberts and Hudson, 

1983), and these anorogenic terranes in the Midcontinent 

region are favorable for the occurrence of deposits 

similar to Olympic Dam. 

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46 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
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igneous province, the St. Francois Mountains, Missouri 

(Contribution to Precambrian Geology 4): Missouri 

Geological Survey Report of Investigations 51, 103 p. 

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exposed Precambrian rocks, Rolla 1 o x 2° quadrangle, 

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region, U.S.A.: U.S. Geological Survey Miscellaneous 

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and metallogeny of Archean and Proterozoic basement 

terranes in the northern midcontinent, U.S.A.-An 

overview: U.S. Geological Survey Bulletin 1815, 51 p. 

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ed., Early Proterozoic geology of the Great Lakes region: 

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plutonic rocks in the buried Proterozoic of the Midcontinent and in the Wet Mountains, Colorado: 

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p. 1150-1157. 

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of Proterozoic terranes of mid-continental North 

America: Geological Society of America Bulletin, v. 100, 

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Missouri: Geological Society of America Bulletin, v. 87, 

p. 1587-1598. 

Anorogenic Rhyolite-Granite Terranes in the Central United States 47 
General Features of the St. Francois and Spavinaw 

Granite-Rhyolite Terranes and the Precambrian 

Metallogenic Region of Southeast Missouri 

By Eva B. Kisvarsanyi1 

Abstract 

The Midcontinent region of the United States is 

underlain by extensive anorogenic granite-rhyolite terranes 

that share a number of common features with the Proterozoic 

terrane in South Australia that hosts the giant Olympic Dam 

copper-uranium-gold deposit. These terranes have similar 

ages (1.4-1.7 Ga}, geologic settings (rifted continental crust}, 

magmatic associations (silicic-alkalic igneous rock suites}, 

and metal suites dominated by iron. The best described and 

most complexly mineralized of the Midcontinent terranes, the 

1.45-1.48-Ga St. Francois terrane in southeastern Missouri, 

is known from surface exposures as well as more than 500 

core holes, and contains 6 major and more than 30 smaller 

orebodies. The Spavinaw terrane is 100 m.y. younger than 

the St. Francois and underlies adjoining parts of Kansas, 

Oklahoma, Missouri, and Arkansas; it is known mostly from 

drill holes and is not a mineral producer. Both the St. Francois 

and Spavinaw terranes are characterized by epizonal granite 

and rhyolite dominated by potassium feldspar; they contain 

iron-rich mafic minerals and are enriched in the lithophile 

elements (Li, Be, Y, Nb, Zr, U, Th, F, and rare-earth 

elements}. The analogy between the St. Francois and South 

Australian terranes is further strengthened by similarities of 

their respective styles of mineralization. The similarities 

between the St. Francois and Spavinaw terranes and the 

basement terrane in South Australia single them out in the 

Midcontinent as excellent targets for exploration for ore 

deposits that could be favorably compared with the Olympic 

Dam deposit. 

INTRODUCTION 

The St. Francois and Spavinaw terranes of the 

Midcontinent of the United States are Middle Proterozoic epizonal terranes that have well-constrained 

crystallization ages (Bickford and others, 1981; Thomas 

1

Missouri Geological Sutvey, P.O. Box 250, Rolla, Missouri, 

65401. 

0 100 200 300 KILOMETERS 

Figure 15. Major Proterozoic terranes in the Midcontinent 

region of the United States. CNARS, Keweenawan basalts 

and clastic rocks associated with the central North American 

rift system. Modified from Sims (1989}. 

and others, 1984). The terranes are similar to each other 

in composition and mode of emplacement, but they 

formed approximately 100 m.y. apart. The St. Francois 

terrane is o.lder (1.45-1.48 Ga), is exposed in its type 

locality in the St. Francois Mountains, and underlies 

Paleozoic strata throughout most of southeast Missouri. 

The younger Spavinaw terrane (1.35-1.40 Ga) crops out 

near Spavinaw in northeast Oklahoma and underlies 

adjoining parts of Kansas, Oklahoma, Missouri, and 

Arkansas (fig. 15). Recent geochronologic data indicate 

that isolated plutons of Spavinaw age are present within 

the St. Francois terrane in southeast Missouri (Latham 

and others, 1988). 

The epizonal terranes are composed dominantly of 

anorogenic (alkaline and anhydrous) granitic rocks and 

associated rhyolites. Their areal distribution overlaps the 

south side of the Early Proterozoic Central Plains oro48 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
gen, a terrane of metasedimentary and metaigneous 

rocks (Sims and Peterman, 1986). The contact between 

the orogenic and anorogenic terranes is covered by 

Paleozoic strata. This contact is believed to be an 

erosional contact resulting from uplift that resulted in 

removal of the cover of epizonal rocks from the orogenic 

terrane in central Missouri (Sims and others, 1987). 

Mesozonal granitic rocks of both St. Francois and Spavinaw age locally perforate the Central Plains orogenic 

terrane and represent the preserved roots of the epizonal 

terranes (Sims and others, 1987). 

The contact between the St. Francois and Spavinaw 

terranes is obscure and in fact may not be one of sharp 

demarcation. Scattered younger ages in southeastern 

Missouri suggest that the Spavinaw thermal event was 

widespread and may have overlapped areally into the St. 

Francois terrane. 

The general petrologic, geochemical, and tectonic 

characteristics of the St. Francois and Spavinaw terranes 

are broadly similar to those of the Middle Proterozoic 

rocks that host the giant Olympic Dam copper-uraniumgold deposit at Roxby Downs, South Australia. 

Furthermore, the St. Francois terrane is a metallogenic 

province containing six major volcanic-hosted magnetitehematite deposits, one copper-iron deposit, and more 

than thirty smaller deposits of iron, manganese, and 

copper. One group of subeconomic deposits in the Silver 

Mine district is similar to the high-temperature tintungsten-silver-lead veins at Cornwall (Lowell, 1976). 

Sims and others (1987) suggested that the geology of the 

Middle Proterozoic anorogenic terranes in the Midcontinent is permissive for the occurrence of one or more 

deposits of the Olympic Dam type. The purpose of this 

paper, therefore, is to review the general characteristics 

of the St. Francois and Spavinaw terranes as determined 

from outcrop and subsurface data and to briefly describe 

their ore deposits. 

Acknowledgments.-The manuscript was critically 

reviewed by W.P. Pratt and Warren Day; originally 

typeset by Kathryn Adamick; and originally illustrated by 

Susan C. Dunn. 

St. Francois Granite-Rhyolite Terrane 

The St. Francois terrane is exposed in a 900-km2 

region in the St. Francois Mountains, at the crest of the 

Ozark dome in southeast Missouri, where it has been 

studied extensively (Tolman and Robertson, 1969; 

Anderson, 1970; Berry and Bickford, 1972; Kisvarsanyi, 

1972; Pratt and others, 1979; Sides and others, 1981). 

Here, the St. Francois terrane includes the St. Francois 

Mountains Volcanic Supergroup and the St. Francois 

Mountains Intrusive Suite. Data from more than 500 

drill holes indicate that the terrane extends below the 

cover of Paleozoic strata and underlies most of 

southeast Missouri (Kisvarsanyi, 1981), having an 

inferred areal extent of at least 100,000 km2

• Its 

boundaries are not well defined because drill-hole data 

are scarce to the east toward the Illinois Basin and to the 

south into the Mississippi embayment; however, Hoppe 

and others (1983) have suggested that a similar graniterhyolite terrane underlies most of Illinois, Indiana, and 

western Kentucky, as far east as the Grenville boundary. 

U /Pb ages on zircons indicate a 1.48-Ga crystallizati?n 

age for the St. Francois terrane, but the terrane contams 

some younger (1.38 Ga) plutons (Bickford and others, 

1981; Latham and others, 1988). The terrane may extend 

as far west as the Decaturville cryptoexplosion structure 

(Offield and Pohn, 1979) and the mafic complex at Orla 

(Kisvarsanyi, 1988) in central Missouri, as suggested ~y 

1.47-Ga plutons that intrude the older metamorphic 

rocks in that area. 

Correlation of surface and subsurface geologic 

data and analysis of aeromagnetic maps in southeast 

Missouri show that the St. Francois terrane consists of 

more than a dozen overlapping ring complexes, cauldron 

subsidence structures with ring volcanoes and ring plutons, and resurgent calderas with central plutons (fig. 1~) 

(Kisvarsanyi, 1980, 1981). These volcanotectomc 

features are comparable to some of the classic ring 

complexes of the world, such as the "younger" granites in 

Nigeria and Glen Coe in Scotland. Although the volcanic 

superstructure of the St. Francois terrane has been 

mostly removed by pre-Paleozoic erosion, as much as 

1,700 m of rhyolite ash-flow tuffs is preserved locally. The 

thickest succession of volcanic rocks has been mapped 

from outcrops in the St. Francois Mountains, in the area 

of the Taum Sauk volcanotectonic depression 

(Anderson, 1970; Cordell, 1979) or caldera (Berry and 

Bickford, 1972). The bulk of the volcanic rocks are 

confined to a northwest-trending belt (fig. 16), a topographic high having its peak in the outcrop area, that 

forms a buried ridge of knobs extending as far northwest 

as the Missouri River (see fig. 3, Denison and others, 

1984; also refer to Kisvarsanyi, 1974). The fact that the 

volcanic rocks are constrained by two major tectonic 

zones associated with the Early Proterozoic Central 

Plains orogen and the Grand River and Northeast 

Missouri tectonic zones, as defined by Kisvarsanyi 

(1984), suggests that their emplacement and distribution 

are controlled by structures in the older terrane. Mesozonal granite of the Transcontinental anorogenic 

province (Sims and others, 1987), and of the same age as 

the St. Francois terrane (Bickford and others, 1981), has 

been identified along the projected northwestward 

extension of the St. Francois volcanic terrane in northcentral Missouri. This lends additional support to the 

postulated tectonic control of the older terrane on the 

emplacement of the younger. 

Granite-Rhyolite Terranes and the Precambrian Metallogenic Region of Southeast Missouri 49 
0 10 20 30 40 50 KILOMETERS 

I I I I I I 

EXPlANATION 

Subvolcanic granite 

Figure 16. Geology of the St. Francois terrane, southeast 

Missouri, as interpreted from outcrop, drill-hole, and aeromagnetic data. Modified from Kisvarsanyi (1981). 

Volcanic rocks of the St. Francois terrane are 

predominantly rhyolite ash-flow tuffs containing very 

high Si02 , K2 0/Na2 0 ratios, Fe/Mg ratios, and F 

abundances and low CaO, MgO, and Al20 3 abundances 

(Kisvarsanyi, 1972). They are characterized by perthitic 

alkali feldspar phenocrysts and iron-rich mafic minerals, 

including fayalite, ferrosilite, and ferrohastingsite. 

Although some of the rocks are transitional to comendites, their agpaitic index is always less than 1 (Kisvarsanyi, 1981). Intermediate and mafic rocks are notably 

rare and of small volume, although the volcanic suite is 

identified as bimodal because of the presence of minor 

basaltic flow rocks interlayered with rhyolite in the Taum 

Sauk area (Blades and Bickford, 1976). The intermediate 

rocks are chiefly trachyte and trachyandesite (Anderson, 

1970; Kisvarsanyi, 1981). In general, the suite is 

distinguished from those of calc-alkaline petrogenetic 

provinces of subduction zones and compressional 

tectonic regimes by the absence of andesites and by 

alkali-rich chemistry. 

Granitic rocks of the St. Francois terrane have been 

classified into three distinct types on the basis of 

composition and mode of occurrence (Kisvarsanyi, 1980, 

1981): (1) subvolcanic granite massifs, (2) ring 

intrusions, and (3) central plutons. The subvolcanic 

granite massifs are comagmatic with the rhyolites and 

represent their intrusive equivalents. They are typical 

epizonal rocks having granophyric textures and 

containing perthitic alkali feldspar; biotite is the 

characteristic mafic mineral and magnetite is ubiquitous. 

Near their contact with the intruded rhyolites, the subvolcanic granite massifs consist almost entirely of finegrained granophyre; at depth, they grade into medium- to 

coarse-grained rapakivi granite and thus form one part of 

the "anorogenic trinity" discussed by Anderson (1983). 

The subvolcanic granite massifs are inferred to be the 

most widespread component of the St. Francois epizonal 

terrane (fig. 16). Wherever they are identified in drill 

holes, they are assumed to have been once covered by 

rhyolite that was stripped off during prolonged prePaleozoic erosion. 

The ring intrusions are intermediate- to high-silica 

rocks whose emplacement was controlled by ring 

fractures related to caldera collapse and cauldron 

subsidence (fig. 16). The suite of rocks associated with 

the ring structures ranges from trachyandesite through 

trachyte and syenite to amphibole-biotite granite; 

porphyritic textures are common. Trace element data 

indicate that the ring intrusions crystallized from magmas 

derived through partial melting of older crustal rocks 

(Cullers and others, 1981). 

The central plutons of the terrane typically are 

high-silica, two-mica granites having distinctive accessory 

minerals and trace element suites (Kisvarsanyi, 1980, 

1981). Accessory minerals include abundant fluorite, 

topaz, apatite, spinel, allanite, sphene, and cassiterite. 

Because of their relative enrichment in Sn, Li, Be, Rb, 

Ba, Y, Nb, U, Th, and F, the granites have been described 

as "tin granites" (Kisvarsanyi, 1980). They are inferred to 

have been emplaced in resurgent cauldrons, are circular 

to oval in plan, and are identified in the subsurface by 

distinctive negative magnetic anomalies. 

Some and perhaps all of the 1.45- to 1.48-Ga 

granitic plutons of the Transcontinental anorogenic 

province possibly represent deeper analogues of the 

epizonal St. Francois terrane (Sims and others, 1987). 

Both the rapakivi massifs and tin granite plutons, 

intruding and partly assimilating rocks of the older 

crustal terrane, have been identified in the buried 

basement. It is fortuitous that unmetamorphosed 

volcanic rocks of Middle Proterozoic age are preserved at 

all in southeast Missouri because these rocks must 

have been protected in low-lying depressions, possibly rift 

valleys, whereas the surrounding regions were stripped of 

their supracrustal cover. 

The older crustal terrane of the Central Plains 

orogen is inferred to underlie the younger epizonal 

terranes southward. Sm/Nd isotope data (Nelson and 

DePaolo, 1985) imply a 1.8- to 2.0-Ga crustal source 

from which the epizonal rocks in the southern Midcontinent were derived by partial melting. This is consistent 

with earlier data (Cullers and others, 1981), which 

indicate that at least some of the rocks of the St. Francois 

50 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
terrane crystallized from primary crustal melts, and that 

most of the high-silica rocks crystallized from highly 

evolved magmas derived through differentiation of the 

primary magmas. 

The rocks and associated ore deposits of the St. 

Francois terrane have features indicative of an 

extensional tectonic regime that existed within or near 

the margin of the Proterozoic North American continent. 

A rift-related genesis for the terrane was proposed by G. 

Kisvarsanyi (1975), an idea further developed by E.B. 

Kisvarsanyi (1980) as supporting trace element data 

became available. The terrane has many of the 

petrological and geochemical attributes of Proterozoic 

rifts and related extensional structures proposed for 

North America (Emslie, 1978; Anderson, 1983), except 

for anorthosite massifs, which have not been encountered 

in drill holes. 

In terms of one of the proposed models for the 

evolution of new continental crust dominated by granitic 

rocks (Bowden and Turner, 1974), peraluminous and 

peralkaline igneous suites are derived through mantle 

pluming, crustal arching, and partial melting of a granodioritic lower crust and a granitic upper crust. This model 

does not require the presence of large volumes of basaltic 

rocks in association with the extensional structures 

because basaltic volcanism is not a major component of 

the crust-forming process. In the St. Francois terrane, the 

absence of large volumes of basaltic rocks precludes 

development of the type of rifting that led to extensive 

crustal separation; however, postorogenic mantle arching 

and attendant crustal melting is postulated to have led to 

the development of extensional structures where new 

sialic crust was formed. In contrast to the younger, 

Keweenawan rift system, the 1.48-Ga tensional event did 

not lead to crustal separation and the emplacement of 

large volumes of basaltic magma. 

Spavinaw Granite-Rhyolite Terrane 

The rock association identified as the Spavinaw 

terrane is similar in composition and tectonic setting to 

the St. Francois terrane but is approximately 100 m.y. 

younger. Zircons from granite and rhyolite in Missouri, 

Kansas, and Oklahoma yield U/Pb ages between 1.35 and 

1.40 Ga (Thomas and others, 1984). 

The Spavinaw terrane is present in a few small 

outcrops of micrographic granite porphyry near Spavinaw, in Mayes County, northeast Oklahoma. Data 

from more than 300 drill holes indicate, however, that the 

terrane underlies most of northeast Oklahoma, 

eastern Kansas, southwest Missouri, and northern 

Arkansas (Denison, 1981, 1984; Kisvarsanyi, 1984; 

Thomas and others, 1984) and thus is an extensive 

Middle Proterozoic epizonal terrane in the southern part 

of the Midcontinent. As used in this report, the Spavinaw 

terrane includes all of the rocks of the northeastern 

Oklahoma province (Washington Volcanic Group, Spavinaw Granite Group, Osage Micro granite; all of Denison, 

1981) and the Central Oklahoma Granite Group mapped 

in the subsurface by Denison (1981). 

Volcanic rocks of the Spavinaw terrane 

(Washington Volcanic Group) are dominantly rhyolite, 

locally converted to metarhyolite (hornfels) by thermal 

metamorphism, and lesser dacite, trachyte, and andesite. 

Published chemical analyses of various rock units from 

selected drill holes in northeast 0 klahoma suggest 

that rocks of intermediate composition are more 

widespread and (or) volumetrically abundant than in the 

St. Francois terrane (although this observation may be 

more apparent than real due to sampling bias). The 

mineralogical and chemical composition of rhyolites and 

ash-flow tuffs in the Spavinaw terrane are identical to 

those in the St. Francois terrane. 

The volcanic rocks are on the northern and 

southern flanks of the broad, pre-sediment Spavinaw 

arch (Denison, 1966), which extends from central 

Oklahoma east-northeastward 240 km into southwest 

Missouri. The arch is underlain by micrographic granite 

porphyry and fine-grained granophyre of the Spavinaw 

Granite Group. These granites are petrographically 

similar to the subvolcanic granite massifs of the St. 

Francois terrane and, by analogy with the St. Francois 

terrane, are interpreted here as the comagmatic, 

intrusive phase of the volcanic rocks. Several rapakivi 

plutons (1.35-1.40 Ga) (Thomas and others, 1984) that 

perforate the older metamorphic terrane of the Central 

Plains orogen in eastern Kansas and western Missouri 

probably correspond to deeper levels of the subvolcanic 

granites within the Transcontinental anorogenic 

province. Some of the rapakivi granite bodies in Missouri 

contain as much as 20 ppm Sn, 30 ppm Nb, and 5 ppm Be 

(E.L. Mosier, written commun., 1985) and have geochemical signatures similar to the tin granites 

identified in the St. Francois terrane (Kisvarsanyi, 1980). 

The available data, particularly the lack of aeromagnetic 

maps at 1:62,500 scale, do not permit delineation of ring 

complexes in the Spavinaw terrane as has been done in 

the St. Francois terrane. 

The Spavinaw and the St. Francois rocks are 

essentially anorogenic epizonal granites and rhyolites 

that have not been regionally metamorphosed; thus, it is 

assumed that each terrane formed in comparable 

tectonic regimes by similar crustal processes. If a pre-rift, 

mantle arching, crustal melting, tensional tectonic 

environment applies to the St. Francois terrane, similar 

conditions and processes are likely to have produced the 

Spavinaw terrane. It is possible that the thermal 

anomalies in the mantle and attendant magma 

generation shifted southwestward with time from the St. 

Granite-Rhyolite Terranes and the Precambrian Metallogenic Region of Southeast Missouri 51 
Francois to the Spavinaw terrane. Although the 1.35- to 

1.40-Ga Spavinaw terrane forms a fairly well defined, 

apparently continuous terrane in the four-State area of 

Kansas, Missouri, Oklahoma, and Arkansas, some 1.38-

Ga plutons have been identified within the St. Francois 

terrane in southeastern Missouri (Thomas and others, 

1984; Bickford and Mose, 1975; Latham and others, 

1988), and other "exceptions" are likely to be present. 

These may indicate that thermal activity was almost 

continuous in the southern Midcontinent during the 

Middle Proterozoic but culminated in different areas at 

different times. 

ORE DEPOSITS OF THE 

ST. FRANCOIS TERRANE 

In terms of past production and future potential, 

the St. Francois terrane is one of the most important 

Precambrian metallogenic regions of the Midcontinent. 

It has been a continuous source of iron ore since 1815, 

with a cumulative production valued at more than $750 

million and estimated reserves valued at $600 million in 

several undeveloped deposits. The terrane is also known 

for minor deposits of manganese and tin-tungsten-silverlead veins (Silver Mine district). Furthermore, the terrane may have an enormous future potential for complex 

Olympic Dam-type deposits of copper, uranium, REE, 

and gold. 

Iron Oxide-Rich Deposits 

More than 30 iron ore deposits are known in the 

Precambrian rocks of southeast Missouri and 

constitute an iron metallogenic province of major 

significance. The majority of the deposits are in the 

exposed Precambrian rocks of the St. Francois 

Mountains and include the historically important Iron 

Mountain and Pilot Knob (upper orebody) deposits (fig. 

17), which supplied iron for the Union cause during the 

Civil War. The six major deposits-Kratz Spring, 

Bourbon, Pea Ridge, Camels Hump, Boss (a copper-iron 

orebody of complex mineralogy), and Pilot Knob (lower 

orebody)-are in buried Precambrian rocks and were 

discovered by drilling on positive magnetic anomalies. 

Only the Pea Ridge and Pilot Knob (lower orebody) 

deposits have been developed; the Pilot Knob mine was 

shut down permanently in 1980, after 12 years of 

operation during which it produced 20 million tons of ore 

averaging between 35 and 40 percent Fe. The Pea Ridge 

mine is the only currently active iron mine in the district; 

except for a temporary shutdown in 1977-79, it has been 

in production since 1964. Ore grade at Pea Ridge is 

higher than at Pilot Knob, averaging between 53 and 57 

percent Fe. Other areas in the St. Francois terrane where 

drilling has disclosed significant iron mineralization 

include the Lake Spring, Missionary Ridge, and Floyd 

Tower magnetic anomalies (fig. 17). 

The Precambrian iron ore deposits of southeast 

Missouri have been classified as "Kiruna type" by Kisvarsanyi and Proctor (1967) because of many similarities 

to the Precambrian, volcanic-hosted, magnetite-apatite 

deposits in the Kiruna district of Norbotten, northern 

Sweden. The Missouri deposits, however, display a wide 

variety of features ranging from magmatic injection 

through hydrothermal veins and disseminations and 

contact metasomatic replacements to volcanic exhalative 

impregnations. Because of the wide range of features, 

various theories have been advocated for the genesis of 

the orebodies (see Emery, 1968, for Pea Ridge; Murphy 

., Kratz Spring ' -·-

Bourbon~ .~: ::··::: 

0

\ >'>,.}~ ;!' -~ Steelvllle 1 :::;=::F·I~yd \ 0 Potosi I Bonne Terre I , / 0 

, Jf) Tower~ ..... _"" 1,-", 

"~- ~Missionary 1 ·:=:=:=:=:=· Camels Hump _ ~idge .... \ 1 

/ ~: / "-"' I .:· I 

=:·· Lake ~· - ' 1 P'l · l Iron Mountain :: • .···=·· , 1 ot I • · Sprmg .. ...- 1 'Knob"-•·1

• \..Ironton e I 

· Boss " ~"oe' 

(~--,~;,:J~ ~~:;~:.~' \ I I ,_,... / \ 

, _.., ~ I I I ..,.. _ ......... _ ..... "" I I 

--.... ' \ 

\ 

1 Eminence\ 

I 0 \ 

' ./ 

-- . - _.,. 

/ ' • Greenville I \ _.. -, 

A A 1/-' \ I \ 

' AA ,\ I 

\ I / 0 ' 

' I I \ ,_,... I I 

37° '-/ 

' / .... _ .... 

0 10 20 30 40 50 KILOMETERS 

I I I I I I 

EXPlANATION 

Mineralization ./ 

' Outline of inferred 

' - _ .... central pluton • Fe 

+ Cu 

A Mn • Trachytic ring intrusion 

• Sn-W-Ag-PbSb-As 

Figure 17. Ring structures, alkaline-intermediate rocks, 

and mineralization in the St. Francois terrane, southeast 

Missouri. 

52 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
and Ohle, 1968, for Iron Mountain; Kisvarsanyi and 

Proctor, 1967, for Bourbon, Boss, and the small surface 

iron orebodies; Wracher, 1976, and Panno and Hood, 

1983, for the Pilot Knob lower orebody; Anderson, 1976, 

and Ryan, 1981, for the Pilot Knob upper orebody; and 

Snyder, 1969, for a general review of all the Precambrian 

iron deposits of th~ terrane). 

The principal ore minerals are magnetite and 

hematite; the Boss deposit contains appreciable amounts 

of copper sulfide minerals. The major deposits have a 

complex gangue mineral assemblage that includes 

garnet, quartz, calcite, actinolite, apatite, dolomite, 

fluorite, pyrite, and barite. At the Pea Ridge mine, a 

phosphate (apatite) concentrate has been recovered as a 

byproduct and used as a fertilizer; reportedly, an REE 

concentrate consisting chiefly of monazite has also been 

recovered. 

The Boss deposit displays by far the most complex 

ore mineralogy. It was explored during the 1950's and 

1960's by intensive drilling that produced approximately 

100,000 ft of Precambrian core. Kisvarsanyi and Smith 

(1988) reported chalcopyrite, bornite, pyrite, molybdenite, cobaltite, sphalerite, galena, chalcocite, and covellite 

in the ore in addition to iron oxide minerals, ilmenite, and 

rutile. They also reported potassium feldspar, garnet, 

biotite, muscovite (sericite), actinolite, epidote, chlorite, 

calcite, fluorite, and gypsum, but no apatite, and 

classified the mineralization as a high-temperature 

contact metasomatic deposit. Hagni and Brandom 

(1988) reported electrum, consisting of about 90 percent 

Au and 10 percent Ag, and an unspecified telluride 

mineral in the Boss ore. 

The orebodies display a variety of shapes and 

emplacement characteristics. There are tabular, dikelike, 

disc-shaped, cone-sheetlike, stratiform, and irregular 

orebodies known among them. Many of the orebodies 

are steeply inclined and were emplaced as massive 

fracture fillings. A significant component of most of the 

major deposits, notably Pea Ridge, Pilot Knob (lower 

orebody), and Iron Mountain, is breccia ore 

characterized by angular volcanic-rock clasts of diverse 

sizes cemented by the ore minerals. The orebodies seem 

to be spatially associated with Precambrian volcanotectonic features, such as calderas and ring-fracture 

systems in the St. Francois terrane (fig. 17). Fracturing 

and massive brecciation of the volcanic rocks resulting 

from cauldron subsidence and caldera collapse probably 

controlled emplacement of the deposits and provided 

favorable sites for ore deposition (Kisvarsanyi and Kisvarsanyi, 1981). 

A distinctive suite of alkaline-intermediate rocks, 

consisting of syenite, trachyte, magnetite trachyte, and 

trachyandesite (the trachyte suite of Kisvarsanyi, 1981 ), 

has been recognized in association with many of the 

deposits. This suite of rocks typically forms ring 

intrusions in the St. Francois terrane. Crosscutting 

relationships observed in the mines and in drill cores 

indicate that the timing of the mineralization is related to 

that of the ring intrusions. A common source, an 

alkaline-intermediate magma, for both the trachyte suite 

of rocks and the iron ores has been postulated (Kisvarsanyi and Kisvarsanyi, 1981 ). 

Manganese Deposits 

Several small manganese prospects in rhyolite are 

known in the southern part of the St. Francois Mountains 

(fig. 17). These deposits are of historical interest, most of 

them having produced only a few tens of tons of ore in the 

late 1800's (Grawe, 1943). The ore consists of braunite, 

pyrolusite, psilomelane, and wad. It occurs as veins in 

rhyolite, as breccia and fracture fillings, and as 

replacements in bedded air-fall tuffs. Secondary 

enrichment during weathering locally concentrated 

manganese and iron oxides in residual clays above the 

Precambrian-Paleozoic erosional nonconformity. 

The most productive of the old mines was the 

Cuthbertson or Marble Creek mine, which produced 

about 3,000 short tons of manganese ores and 

concentrates, containing more than 50 percent Mn, 

during intermittent mining between 1872 and 1958 

(Dorr, 1967). The mineralization is of the stratiform type 

in water laid, bedded tuffs. Inter bedding of the manganiferous tuffs with thin beds of travertine suggests that the 

deposit formed in a crater lake-type environment by 

fumarolic and hot spring activity. Evidence for some 

hydrothermal activity is also indicated by veinlets of 

barite and fluorite that cut the limestone beds (Kisvarsanyi and Kisvarsanyi, 1977). Similar deposits could have 

formed near the top of the volcanic pile throughout the 

St. Francois terrane, and some of them may be preserved 

in the buried part of the terrane. 

The Silver Mine District 

Deposits in the Silver Mine district of the exposed 

St. Francois terrane represent the only known hightemperature pneumatolytic-hydrothermal (xenothermal) 

ore-mineral association in the Proterozoic of the 

southern Midcontinent (fig. 17). The Silvermine Granite, 

one of the ring intrusions in the terrane, is cut by 

polymetallic quartz veins that have accompanying 

extensive greisenization of the wall rock. The mineral 

suite includes argentiferous galena, wolframite, arsenopyrite, sphalerite, cassiterite, chalcopyrite, covellite, 

hematite, stolzite, and scheelite-similar to the deposits 

of Cornwall and Saxony (Tolman, 1933; Lowell, 1976). 

Hagni (1984) identified argentiferous tennantite, antiGranite-Rhyolite Terranes and the Precambrian Metallogenic Region of Southeast Missouri 53 
monpearcite, and berryite in the main sulfide stage of 

mineralization and concluded that the base metal sulfide 

minerals and the silver minerals were deposited under 

epithermal conditions subsequent to greisenization and 

deposition of the quartz vein-type minerals. The ore 

solutions are assumed to have been derived from a tin 

granite central pluton at depth. The mineralization postdates alkali-olivine basalt and tholeiitic olivine-basalt 

dikes that cut both the Silvermine Granite and rhyolite 

ash-flow tuffs in the area. 

The deposits were mined for silver from 1877 to 

1894 and produced an estimated 3,000 ounces, as well as 

some 50 tons of lead. Between 1916 and 1946, an 

estimated 120 short tons of tungsten concentrates was 

produced by high grading the old dumps and mining in 

shallow surface diggings. Mining operations at the dozen 

or so small mines and prospects did not extend below 

about 200 ft from the surface, and the vertical extent of 

the ore systems in the district has not been explored or 

defined. The coincidence of the mineralization with a 

ring-fracture system (fig. 17) suggests structural control 

for the emplacement of the deposits. Similar deposits 

may be associated with buried ring complexes elsewhere 

in the region. The Silver Mine-type deposits are 

considered to be part of an important nonferrous 

metallogenic epoch that was related to later stages of 

Precambrian igneous activity, possibly, to resurgence in 

some calderas. 

The Olympic Dam Analogy 

The discovery in 1975 of the Olympic Dam deposit 

at Roxby Downs, South Australia, and subsequent 

reports about its apparently unique mineralization and 

giant size inspired the development of new exploration 

concepts pertaining to iron oxide-rich Middle Proterozoic deposits in the Midcontinent of the United States. 

Hauck and Kendall (1984; Hauck, this volume) 

developed an exploration model for an Olympic Damtype deposit in the region and suggested a genetic 

similarity between the Precambrian iron ores of the 

Kiruna district, southeast Missouri, and South Australia. 

Several exploration companies apparently developed 

similar models at about the same time, and at least 10 

exploration holes have been drilled in southern Missouri 

with the elusive Olympic Dam-type deposit as the target. 

This exploration activity was abruptly terminated in mid1983 when the U.S. minerals industry suffered a major 

setback. 

The fundamental analogies between the regional 

geologic settings of the Olympic Dam deposit (Roberts 

and Hudson, 1983; Hauck, this volume) and the Precambrian metallogenic region of southeast Missouri are: 

1. Stable cratonic region; 

2. Extensional tectonic regime; 

3. Long, linear, fault -controlled tectonic elements; 

4. Local accumulation of clastic sediments and 

breccias; 

5. Similar age of the host rocks (about 1.5 Ga); and 

6. Similar petrology and geochemistry of the host 

rocks (alkaline, an orogenic, bimodal volcanic-plutonic 

suite, and enrichment in iron, REE, phosphorus, 

fluorine, and other incompatible elements). 

Sims and others (1987) noted these analogies and 

concluded that the anorogenic terranes of the Precambrian basement in the Midcontinent are generally 

favorable for the occurrence of Olympic Dam-type 

deposits. They attributed particular importance to the 

local accumulation of clastic rocks in fault-controlled 

basins, as in the graben defined by the Chesapeake and 

Bolivar-Mansfield tectonic zones in southwest 

Missouri (Kisvarsanyi, 1984), because the Olympic Dam 

mineralization reportedly was emplaced within a thick 

sequence of coarse clastic sedimentary rocks and granitic 

breccias (Roberts and Hudson, 1983). 

CONCLUSIONS AND 

RECOMMENDATIONS 

As results of ongoing research emerge on the 

Olympic Dam deposit itself (Einaudi and Oreskes, this 

volume) and as specific comparisons continue to be 

made with the Midcontinent deposits, the analogies 

between the Olympic Dam and Midcontinent deposits 

appear to be more numerous, and the potential for an 

Olympic Dam-type deposit in the Midcontinent is greatly 

enhanced. The discovery of gold and telluride minerals in 

the Boss copper-iron deposit (Hagni and Brandom, 

1988) increases the similarities between the respective 

mineralizations. In the Pea Ridge orebody, a thoriumREB vein, abundant copper sulfide minerals with gold, 

and anomalous concentrations of tin have recently been 

recognized (Pea Ridge Iron Ore Company, oral commun., 1988). In view of the new data, a reexamination of 

all the iron oxide-rich deposits of the St. Francois terrane 

is warranted. A comprehensive and coordinated research 

effort should undertake the study of the metallogenesis 

of all known orebodies, including those of the Silver Mine 

district, in order to understand the complex magmatic, 

hydrothermal, tectonic, and sedimentary processes that 

produced the diverse deposits of the region. · 

A comprehensive research effort directed at the St. 

Francois metallogenic terrane could conceivably answer 

some unresolved questions about the Olympic Dam 

deposit itself. It should address some of the following 

questions: 

1. Are Olympic Dam breccias analogous to the 

volcanic breccias associated with the Missouri iron ores? 

54 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
2. Is the Olympic Dam deposit similar to the 

hematite-rich breccias and bedded iron ores in the St. 

Francois terrane, and is it therefore a more evolved, 

higher (crustal) level deposit in a similar, complex, 

evolving, magmatic and hydrothermal ore system? 

3. Is the magnetic anomaly at Olympic Dam the 

expression of a Pea Ridge-type orebody below the level 

of the hematite-rich deposit? 

4. Are the REE, gold, silver, tin, copper, 

uranium, and thorium enrichments in the ores the 

imprint of a later, Silver Mine-type metallogenic process 

(the "transgressive mineralization" of Roberts and Hudson, 1983)? 

5. What is the source and derivation of the iron, 

the all-important and pervasive component of both the 

Proterozoic host rocks and ores? 

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56 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
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Madison County, Missouri: Missouri Bureau of Geology 

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Granite-Rhyolite Terranes and the Precambrian Metallogenic Region of Southeast Missouri 57 
Progress Toward An Occurrence Model for 

Proterozoic Iron Oxide Deposits-A Comparison 

Between the Ore Provinces of South Australia and 

Southeast Missouri 

By Marco T. EinaudP and Naomi Oreskes2 

Abstract 

The Stuart Shelf region of South Australia and the St. 

Francois Mountains of southeast Missouri host iron-rich 

mineral deposits in alkalic igneous settings of midProterozoic age. These deposits have been known 

historically as sources of iron ore, but they may also contain 

economic concentrations of copper, uranium, gold, silver, 

and (or) rare-earth elements. The largest of these are the 

Olympic Dam deposit in Australia and the Pea Ridge deposit 

in Missouri. Striking similarities exist both between the 

Olympic Dam and Pea Ridge deposits and between the 

general features of the two or~ provinces. In addition to a 

broad similarity of age and regional setting, the deposits 

display similarities in large-scale wall-rock alteration and 

replacement, open-space veining and hydrothermal brecciation, abundant carbonate, barite, and (or) fluorite gangue 

minerals, and variable but locally significant abundances of 

copper-iron sulfide minerals. Variation among deposits of 

one province is as great as variation between deposits of 

different provinces and may be attributed to differences in 

wall-rock composition, structural control, depth of formation, 

and (or) metal content of local basement. We conclude that it 

is useful and appropriate to think of Olympic Dam and Pea 

Ridge and, by extension, other deposits in these regions, as 

belonging to the same class of mineral deposit, and that the 

observed diversity reflects the same fundamental process 

operating in different local environments. 

There is general agreement on the tectonic setting of 

these deposits, but considerable controversy as to the nature 

of the ore fluid, mechanisms of ore emplacement, and 

interpretation of elemental abundances. Most interpretations 

center on either (1) hydrothermal alteration and replacement 

1

Department of Applied Earth Science, Stanford University, 

Stanford, California 94305. 

2

Department of Geology, Stanford University, Stanford, 

California 94305. Current address: Department of Earth Sciences, 

Dartmouth College, Hanover, New Hampshire 03755. 

or (2) immiscible iron oxide magma injection. Although these 

two hypotheses are not mutually exclusive, in many cases 

these contrasting interpretations are based on the same 

outcrop. At both Olympic Dam and Pea Ridge, the field 

relations and ore textures do not require that an iron oxide 

magma was involved at any stage of the mineralizing process. In contrast, many aspects of the field relations, ore 

textures, mineralogy, and wall-rock alteration require that an 

aqueous hydrothermal fluid was involved at some stage. 

Furthermore, evidence from Olympic Dam suggests that 

fluids were circulated in a major, disruptive hydrothermal 

system, capable of mobilizing a variety of elements including 

rare-earth elements and physically transporting rock 

fragments. The conflict between these various 

interpretations, and their implications for understanding 

Proterozoic tectonic evolution, make these deposits an 

important topic for future study. 

INTRODUCTION 

In this paper we compare the iron provinces of the 

Stuart Shelf region, South Australia, and the St. Francois 

Mountains, southeast Missouri. The mineral deposits in 

these areas, including the world-class Olympic Dam 

copper, uranium, gold, silver, and rare-earth element 

(REE) deposit in South Australia, are iron-rich rocks 

that are found in alkalic igneous settings of midProterozoic age (Meyer, 1988a). Analogous types of 

deposits are found on other continents, including Europe 

(Kiruna, Sweden) and Asia (Bayan Obo, Inner Mongolia, People's Republic of China). The origin of these 

deposits and their classification into deposit types have 

been the subject of much speculation in the recent 

literature. For this reason, we felt that it would be useful 

to provide a summary based on our personal 

observations, recent unpublished field studies, and 

publications that are of limited circulation and therefore 

not generally available. 

58 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Although emphasis is placed on the comparison between 

the Olympic Dam deposit and the Pea Ridge deposit of 

southeast Missouri, a broader perspective on the scale of 

entire metallogenic provinces, which takes into account 

several deposits, is necessary in order to avoid overly 

provincial conclusions. 

We conclude that striking similarities exist at all 

scales, and that the geological characteristics of both 

provinces and their mineral deposits reflect similar petrotectonic histories and ore-forming processes. The same 

features (such as metal suite and deposit morphology) 

are not shared by all iron deposits in both provinces, but 

one would not expect this to be the case. Diversity 

reflects variations on the same fundamental ore-forming 

process operating in different local environments. It is 

this variability and the links between deposit types that 

invites study. 

One goal would be to list and describe essential 

features of these deposits on the basis of comparison, but 

this goal remains to be achieved and depends on further 

study. We hope that the present report contributes to a 

better definition of research directions to be pursued in 

both provinces as we move toward a genetic 

understanding. 

Acknowledgments.-We thank the following 

persons for stimulating discussion of the ideas presented 

in this paper: Steve Hauck, Timothy Hayes, Murray 

Ritzman, Eva Kisvarsanyi, Mark Marikos, Nigel Maund, 

Wally Pratt, Paul Sims, and last, but certainly not least, 

the late Charles Meyer. We also thank Warren Day and 

Paul Sims for helpful reviews. This study was partly 

funded by National Science Foundation Grant EAR 

8618432 to Einaudi and by the U.S. Geological Survey. 

THE CONTEXT: ANALOGIES BETWEEN 

OLYMPIC DAM AND OTHER DEPOSITS 

The size and economic significance of the Olympic 

Dam Cu-U-Au-Ag deposit have led to considerable 

discussion regarding its ongm and inspired 

reexamination of similar deposits. Roberts and Hudson 

(1983) concluded that Olympic Dam is a new type of 

stratabound sediment-hosted ore deposit having features 

that can be explained by low-temperature syngenetic or 

diagenetic processes. However, a number of authors (for 

example, N.G. Maund, B.P. Minerals, oral commun., 

1982; Goodell and Keller, 1984; Hauck and Kendall, 

1984; Brandom and others, 1985; Meyer, 1988a, b) noted 

similarities between Olympic Dam and the magnetiteapatite deposits of Kiruna, Sweden, the southeast 

Missouri iron province, and the iron-REE deposit of 

Bayan Obo, Inner Mongolia. Other workers (Bell, 1982; 

Youles, 1984) have drawn analogies between Olympic 

Dam and the hydrothermal Fe-Cu-U-REE breccias of 

Mount Painter, South Australia, and the Wernecke 

Mountains, Yukon. 

Hauck and Kendall (1984) and Meyer (1988a) 

summarized the features of a subset of the deposits listed 

above as: "iron deposits * * * in oxidized alkalic igneous 

settings of mid-Proterozoic age" (Meyer, 1988a, p. 13). 

Meyer divided the subset into two deposit types: 

"Kiruna-type Iron Ores" (Kiruna and southeast 

Missouri), and "Olympic Dam-Type Cu-U-Au Ores" 

(Olympic Dam, Mt. Painter, and Bayan Obo ). 

Presumably, Meyer did not treat all of these deposits as 

one type because of the differences in relative metal 

content (the Kiruna and Missouri deposits have much 

lower uranium and REE contents) and in gangue 

mineralogy (the Kiruna and Missouri deposits contain 

less carbonate minerals). Considering these differences 

to be only variations on a theme, the common features 

for the subset as a whole has the following characteristics 

(based on Hauck and Kendall, 1984; Meyer, 1988a): 

1. Subvolcanic environment in rifted continental 

crust setting is associated with 1.7-1.1-Ga alkalic rock 

suite. 

2. Ores are associated with an anorogenic, 

dominantly felsic, alkalic rock suite ranging from "red" 

granite and rapakivi granite to mangerite and charnockite and various volcanic equivalents. 

3. Igneous rocks contain abundant iron oxide 

minerals as accessories, and alkali feldspars are brick red 

due to exsolved hematite. 

4. Mineral deposits commonly exceed 1 billion 

tons of greater than 20 percent Fe (as magnetite and (or) 

hematite) and contain various lithophile elements such as 

uranium and REE, fluorine, and phosphorus. 

5. Sulfide minerals may also be present, 

especially those of copper, and these commonly are 

intimately associated with iron oxide minerals. 

6. Carbonate minerals, fluorite, and barite are 

common but not ubiquitous. 

COMPARISON OF STUART SHELF AND 

SOUTHEAST MISSOURI IRON 

PROVINCES 

"It is a well-known fact that the pre-Cambrian iron 

mineralization in southeastern Missouri exhibits 

considerable local diversity in mode of occurrence and 

mineral assemblage" (Meyer, 1939, p. 128). This 

statement also applies to the iron deposits of the Stuart 

Shelf region. The differences in characteristic features, 

combined with differences in interpretation of the same 

field relations, have led to considerable controversy on 

genetic hypotheses. The summary of key features that 

Occurrence Model for Proterozoic Iron Oxide Deposits 59 
follows proceeds from regional to mineralogical scales; 

we conclude with a discussion of genetic hypotheses. 

Age 

Mineral deposits in the two provinces formed at 

virtually the same time in the mid-Proterozoic. The age 

of Olympic Dam deposit is less than 1,600 Ma (U-Pb 

zircon age of granite host rock; Mortimer and others, 

1988) and greater than 680 Ma (Rb-Sr age of cover 

sediments; Webb and others, 1983). Unpublished 

isotopic age data on sericite and pitchblende (L.B. 

Gustafson, oral commun., 1979; N.N.A. Trueman, oral 

commun., 1985), relative lead and strontium isotope 

closure dates for apatite and biotite (Mortimer and 

others, 1988), and geologic relations (see later) suggest 

that the hydrothermal activity that produced Olympic 

Dam could be 100--200 m.y. younger than the granite 

host (Oreskes and Einaudi, 1990). Such an age would 

overlap with 1,525-1,450-million-year silicic volcanic and 

plutonic rocks exposed in the Gawler Ranges to the 

southwest (Webb and others, 1986) and thought to 

extend under the Stuart Shelf (see later). 

Most workers in the southeast Missouri iron 

province have concluded that the mineral deposits of the 

province are coeval with some of the igneous activity in 

the St. Francois terrane (Singewald and Milton, 1929; 

Meyer, 1939; Kisvarsanyi and Proctor, 1967), which has 

been dated by U-Pb methods on zircon at 1,480 Ma, with 

some late plutons at 1,380 Ma (Bickford and Mose, 1975; 

Bickford and others, 1981). 

Regional Setting 

The two iron provinces share many features in their 

regional setting, including an association with silicicalkalic volcanism and plutonism in a cratonic rift setting. 

In South Australia, the deposits are in the unexposed, 

pre-Adelaidean basement of the Stuart Shelf. 

Exploratory drilling shows that the basement consists of 

gneiss, calcareous and hematitic metasedimentary 

rocks, and deformed granite (Paterson, 1986a; Parker, 

1987) that are intruded by undeformed Middle Proterozoic granite that hosts the ores at Olympic Dam. 

Although volcanic rocks are a minor component at 

Olympic Dam, felsic to intermediate ash-flow tuffs and 

flow rocks and granitic rocks are major components of 

the wall rocks in other deposits of the Stuart Shelf, such 

as Acropolis (Paterson, 1986a, b). The Stuart Shelf is 

bounded to the south and west by the Gawler craton, ~a 

province of Archean to Early Proterozoic gneiss 

complexes and metasedimentary rocks that was intruded 

by syntectonic (Kimban orogeny) granites at 1,800--1,600 

Ma (Webb and others, 1986). The Kimban orogeny was 

followed by anorogenic silicic volcanism (Gawler Range 

Volcanics) at 1,525-1,490 Ma and the emplacement of 

high-level, alkali-rich granitic plutons (Hiltaba Suite) at 

1,500--1,450 Ma (Rutland and others, 1980; Cooper and 

others, 1985; Webb and others, 1986). It is now generally 

recognized that Proterozoic basement rocks of the Stuart 

Shelf are broadly correlative with rocks of the Gawler 

craton (Roberts and Hudson, 1983; Paterson, 1986a; 

Parker, 1987; Mortimer and others, 1988). Silicic volcanism and plutonism at about 1,525-1,450 Ma occurred 

in a broad, east-northeast trending, intracratonic belt 

from the Gawler Ranges Province to the Stuart Shelf and 

possibly extended (under the present Adelaidean cover) 

as far east as the Mount Painter region. Northwesttrending photolineaments may reflect extensional grabens (O'Driscoll, 1982; Roberts and Hudson, 1983), 

superimposed on the east-northeast-trending granite 

terrane at about 1,400 Ma, that controlled the disposition 

of continental red-beds (Pandurra Formation) and 

younger dolerite dike swarms and intrusions (Parker, 

1987). 

Iron deposits of southeast Missouri are found in a 

Middle Proterozoic continental setting within the St. 

Francois terrane, which consists predominantly of alkalic 

rhyolite and associated granite and lesser amounts of 

basalt, trachyte, and trachyandesite (Kisvarsanyi, 1981). 

Emplacement of igneous rocks was probably controlled 

by northwest-trending basement structures of Early 

Proterozoic age (Sims and others, 1987). The terrane is 

thought to consist of numerous ring dike complexes and 

resurgent calderas (Kisvarsanyi, 1980, 1981; Sides and 

others, 1981), and a continental rift setting has been 

proposed (Kisvarsanyi, 1975; Kisvarsanyi, 1980). 

Province Dimensions and 

Clustering of Deposits 

The linear array of deposits and the dimensions of 

the two provinces are strikingly similar. Both ore provinces are related to an igneous terrane that measures on 

the order of tens of thousands of square kilometers, and 

both contain a linear clustering of numerous individual 

deposits. 

The Stuart Shelf province contains six major known 

iron occurrences (Olympic Dam, Acropolis, Wirrda 

Well, Oak Dam, Dromedary Dam, and Bopeechee) (fig. 

18); numerous small magnetic anomalies have not been 

drilled (Paterson, 1986a). The deposits define a province 

that is elongate north-northwest for a distance of 110 km, 

from Bopeechee on the north to Oak Dam on the south; 

the province is 35 km wide, from Acropolis on the west to 

Wirrda Well on the east. The deposits are 15-30 km 

apart. 

60 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
136° E. 137° E. 138° E. 

30° ~----------------~----------------~ 

S. ,,,Bopeechee 'I II 

Olympic 

"Dam 

RD1~~· Wirrda 

~Well Acropolis,----, I I 

' I 

, ' I I 

', ....... / 

Dromedary ~:· Dam Oak ~--Dam I I ,_, 

0 50 KILOMETERS 

I I I I I I 

Figure 18. Major iron occurrences (delineated by dashed 

lines) on the Stuart Shelf of South Australia. 

The southeast Missouri province contains 6 major 

known iron deposits and 24 others of lesser known 

importance (Sims and others, 1987). Districts and 

deposits define a province that is elongate northnorthwest for a distance of 125 km, from Kratz Spring on 

the north to Greenville on the south; the province is 46 

km wide, from Boss on the west to Iron Mountain on the 

east (fig. 19) (Kisvarsanyi, 1988, fig. 5). The deposits are 

10-30 km apart. 

Magmatic Affiliation 

A spatial correlation between deposits and 

anorogenic silicic-alkalic igneous rock suites is 

recognized in both provinces, although the magmatic 

histories are complex and the genetic relationships 

between mineral deposits and a given magma type or 

magmatic episode remain to be demonstrated, 

particularly in the Stuart Shelf. Meyer (1988a, b) 

suggested that a regional genetic connection exists 

between Proterozoic iron deposits worldwide, crustal 

melts that included banded iron formation in their source 

regions, and carbonatites, although direct evidence of 

carbonatites is lacking on a local scale. 

Iron deposits on the Stuart Shelf are within a wide 

textural variety of granitic and volcanic rocks. The granite 

of Olympic Dam, host to the orebody, is a pink to 

~----------~9~1o~w~·--------------------~9~~W I 

Kratz Spring 

INDEX MAP OF MISSOURI 

• 

Bourbon • • Pea Ridge u 

38" r-- 0 Steelville 

N. 

• Floyd Tower 

0 Potosl 0 Bonne Terre 

37° 

N. 

~ Lake 

Spring 

Missionary Ridge • 

• Camels Hump 

• Boss 

oEminence 

I 

0 10 

I I 

• •Iron Mountain 

• 

lrontf• ·~ Pilot Knob . 

• • 0 Greenville 

20 30 40 50 KILOMETERS 

I I I I 

-

Figure 19. Missouri iron deposits (solid circles). Modified 

from Kisvarsanyi (this volume, fig. 17). 

yellowish-green, coarse- to medium-grained, equigranular biotite granite. It has characteristics of alkalic 

granites (Roberts and Hudson, 1983). Mortimer and 

others (1988) classified it as a quartz syenite but may 

have in part studied altered granite. Similar granites and 

granitic gneisses are wall rocks in other deposits of the 

Stuart Shelf (Paterson, 1986a). At the Acropolis deposit 

diorite, potassium-rich granite, gneissic granite, and 

alkali quartz syenite are present (and all are altered and 

(or) mineralized). A thick sequence of volcanic rocks is 

preserved at Acropolis; extensive hematization and chloritization mask the original rock types, but they are 

porphyritic and probably are felsic to intermediate flow 

rocks and ash-flow tuffs (Paterson, 1986b ). 

The granite of Olympic Dam is too coarse grained 

to represent a subvolcanic intrusion, and, inasmuch as all 

the evidence points to a near-surface origin for the ore 

deposit (Oreskes and Einaudi, 1990; Reeve and others, 

1990), we do not believe that there is a close temporal 

and genetic relation between ore-forming hydrothermal 

activity and the granite of Olympic Dam. Local felsic 

porphyry dikes, altered and dismembered by faults and 

breccia columns, may be an expression of the volcanic 

link, but nothing is known about the original composition 

of the volcanic rock. Undated diorite plugs and dolerite 

dikes cut the mineral deposits (Roberts and Hudson, 

1983; Paterson, 1986a; Reeve and others, 1990) and 

therefore are younger. Roberts and Hudson (1983) 

suggested that there may be a link between the high 

Occurrence Model for Proterozoic Iron Oxide Deposits 61 
contents of potassium, REE, barium, and fluorine in the 

Olympic Dam deposit and unidentified alkaline igneous 

activity in the region. 

The St. Francois terrane is mostly composed of 

silicic-alkalic igneous rocks containing very high 

abundances of fluorine and Si02 , high K2 0/Na2 0 and 

Fe/Mg ratios, and low abundances of CaO, MgO, and 

Al2 0 3 • The rocks are characterized by alkali feldspar 

phenocrysts and iron-rich mafic minerals (see summary, 

Sims and others, 1987). Volcanic rocks consist chiefly of 

rhyolite ash-flow tuffs and porphyritic flows; these are the 

host rocks for the majority of iron deposits. Comagmatic 

intrusions include fine-grained granophyre and coarsegrained rapakivi granite. In addition to the rhyolite host 

rocks, some deposits are hosted in other igneous rock 

types as well: at Boss-Bixby a syenite stock is mineralized 

(Snyder, 1968; Kisvarsanyi and Smith, 1988), and at Iron 

Mountain much of the ore is in an andesite flow (Murphy 

and Ohle, 1968). The iron ores postdate the alkali 

rhyolites and related intrusive rocks described above and 

predate high-silica, two-mica "tin granites" (Kisvarsanyi 

and Kisvarsanyi, 1977; Kisvarsanyi, 1981). At the Pea 

Ridge mine, younger mafic dikes and aplitic quartz-alkali 

f~ldspar dikes cut the magnetite ore breccias (Emery, 

1968); recent mapping indicates that muscovite-biotite 

granite pegmatites and quartz-cassiterite veins, possibly 

related to the two-mica granites, also cut the orebody (L. 

Nuelle, M. Marikos, and M. Einaudi, unpub. data, 1988). 

Kisvarsanyi (1981) proposed a genetic relation between 

the iron ores of southeast Missouri and a suite of 

subvolcanic ring intrusions of trachyte, syenite, and 

trachyandesite. Trace element data suggest that these 

rocks were derived from partial melts of older crustal 

rocks (Cullers and others, 1981). 

Morphology and Structural 

Control of Orebodies 

Except for the broad regional controls involving 

possible continental rifts or other forms of deep crustal 

structures, the structural control for the localization of 

individual deposits is poorly known in both provinces. 

Orebody morphologies may be controlled by local 

features of secondary rock permeability such as fault or 

shear zones and intrusive contacts (leading to steep, dikeor pipe-like morphology) and by zones of high primary 

rock permeability such as porous tuffs, poorly welded 

portions of ash-flow tuffs, and certain sedimentary strata 

(producing tabular-concordant morphology). 

Perhaps the best established structural control is in 

the Olympic Dam deposit. Here, we have shown 

(Oreskes and Einaudi, 1990) that the heterolithic 

hematite-granite breccia bodies, many of which are 

elongate and dikelike, strike north-northwest, parallel 

with regional photolinears. These breccias commonly 

display steeply dipping, wavy foliation broadly parallel 

with the orientation of the breccias. We interpret the 

overall deposit morphology and foliation to reflect 

structural control by preexisting, steep north-northweststriking faults that were probably active during formation 

of the breccias (Oreskes and Einaudi, 1990). Toward the 

margins of the hematitic breccias, discontinuous layers of 

crushed granite are interleaved with hematite-quartz 

microbreccia. Similar textures were described by Meyer 

(1939, p. 132) in the magnetite-hematite fissure veins on 

Shepherd Mountain, near Pilot Knob, southeast 

Missouri, as "flecks and stringers of red feldspathic 

material penetrate the granular ore in crudely parallel 

bands." The feldspathic material consists of "severely 

altered" and locally "fractured and twisted" coarsegrained feldspar. Meyer concluded that these layers are 

hydrothermal in origin and were subjected to differential 

stress after they formed. The fissure veins on Shepherd 

Mountain were thought to be representative of channelways for hydrothermal solutions that formed higher level 

stratabound replacement ores in favorable horizons such 

as the concordant hematite ores in bedded pyroclastic 

rocks at the summit of Pilot Knob (Meyer, 1939, 

p. 194-195). 

Snyder (1968) described the overall morphology of 

the Pea Ridge magnetite body as a discordant, vertical, 

dikelike mass, crescentic in plan view, with an 800 m 

strike length, 100m thickness, and vertical extent beyond 

the limit of drilling at 700 m. This view of the deposit 

morphology may change as we assess the extent of 

post -ore faulting and the timing of ore formation relative 

to the age of the enclosing volcanic rocks, which also dip 

steeply. If the magnetite orebody formed in a vertical 

attitude, it would have had to postdate significant tilting 

of the enclosing vokanic rocks. Other massive iron 

deposits of the Missouri province display variable 

morphology, including (1) dome-shaped shells having 

vertical limbs at Iron Mountain, interpreted as vaulted 

collapse structures (Murphy and Ohle, 1968); (2) 

irregular, steeply dipping bodies parallel with intrusive 

margins at Boss-Bixby (Snyder, 1968; Kisvarsanyi and 

Smith, 1988); and (3) tabular-concordant bodies in 

unwelded portions of ash-flow tuffs at the Pilot Knob 

subsurface deposit (Panno and Hood, 1983) and in 

well-bedded volcaniclastic breccias and sedimentary 

rocks at the Pilot Knob surface deposit (Meyer, 1939). 

Textures of Orebodies and 

Relation to Wall Rocks 

Massive iron oxide-bearing rocks, various types of 

breccias having an iron oxide matrix, and layering, 

foliation, or stratification are common features of iron 

62 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
ores in both provinces. Most of the textures can be 

ascribed to hydrothermal processes including large-scale 

wall-rock replacement, which in many cases preserves 

wall-rock textures; open-space veining accompanying 

fracturing; and hydrothermal brecciation accompanying 

massive iron oxide mineralization. Where original 

unfaulted contacts are preserved, some degree of 

gradation between massive iron oxide bodies and their 

enclosing wall rocks is observed; sharp contacts between 

the two rarely have been described except for open-space 

veins. This gradation is expressed in a variety of ways 

including increased density of veins or abundance of iron 

oxide disseminations or decreased abundance and size of 

wall-rock fragments toward the center of the deposit. 

The most striking feature of the breccias of 

Olympic Dam is lacking in the deposits that have been 

described in southeast Missouri: vast volumes of heterolithic, matrix-supported breccias consisting of 30-70 

percent iron oxide matrix and fragments generally less 

than 10 em in diameter. Also missing from the southeast 

Missouri deposits are examples of the iron oxide (quartz) 

micro breccia and local accretionary lapilli (related to 

fluidization?) that constitute the central core of the 

Olympic Dam deposit (Oreskes and Einaudi, 1990). 

These breccia types can be considered, however, to be 

the result of local variations in the intensity of the 

brecciation process; many other features of orebody 

textures are common to both provinces. These include 

finely laminated hematitic siltstones and hematitic 

volcaniclastic conglomerates at Olympic Dam (Oreskes 

and Einaudi, 1990), Oak Dam (Parker, 1987) and the 

Pilot Knob surface deposit (Meyer, 1939); magnetite 

veins and veinlets at Acropolis, Olympic Dam (Paterson, 

1986b; Oreskes and Einaudi, 1990), Pea Ridge, Pilot 

Knob, Boss Bixby, and Iron Mountain (Meyer, 1939; 

Murphy and Ohle, 1968), and iron oxide disseminations 

in associated igneous rocks on a broad scale. 

The hematitic siltstones at Olympic Dam and the 

Pilot Knob surface deposit are remarkably similar in 

appearance. At Olympic Dam, we found "hematite 

clasts" within these siltstones and believe that at least 

some detrital hematite component is present, although 

much of the fine-grained euhedral hematite dispersed 

through the beds may have been precipitated (or recrystallized?) in situ before the silts were lithified. At the 

Pilot Knob surface deposit, Meyer (1939) and Panno and 

Hood (1983) interpreted the textures as evidence of 

replacement of matrix siltstone and of selective 

replacement of porous volcanic clasts by iron oxide 

minerals. Anderson (1976) and Nold (1988), on the 

other hand, concluded that these clasts were mineralized 

before incorporation into the sedimentary breccia and 

that the bulk of evidence favors syngenetic deposition of 

hematite in a sedimentary environment. 

In contrast with the sedimentary rocks, evidence 

from igneous rocks for replacement by iron oxide 

minerals is widespread and relatively unambiguous. At 

Olympic Dam, we recognized both relict granite textures 

in the subsurface in hematite rock on scales of tens of 

meters and relict feldspar phenocrysts (pseudomorphed 

by hematite) in thin sections of even the most massive ore 

(Oreskes and Einaudi, 1990). At Pilot Knob, Panno and 

Hood (1983) described gradational contacts between ore 

and tuff; glass shards and pumice fragments in the ore 

convinced them that ore replaced tuff. At Iron Mountain, 

local gradational contacts between ore and andesite wall 

rocks were interpreted by Murphy and Ohle (1968) to be 

the result of replacement. 

At Pea Ridge, preliminary underground mapping 

reveals that at least some of the margins of the deposit 

are subparallel with compaction or flow foliation in the 

enclosing volcanic rocks. Conformity of ore with "flow 

structures" in the volcanic host rock was noted at other 

deposits in southeast Missouri, including Bourbon and 

Kratz Springs (Snyder, 1968). This parallelism is 

particularly evident at Pea Ridge in those areas where 

magnetite-hematite bodies are in unfaulted contact with 

their original volcanic wall rocks and where the process 

of wall-rock replacement can be seen through the 

progressive increase in abundance of veins and streaks of 

iron oxide minerals within the rock foliation. Wall-rock 

clasts are most abundant in the hematitic borders of the 

ore and are less common within the core of massive 

magnetite. The generally massive, textureless nature of 

the magnetite bodies at Pea Ridge is reminiscent of 

magnetite bodies at Acropolis, South Australia. 

The absence of relict igneous minerals in massive 

iron oxide bodies or in matrices of ore breccias is not 

conclusive evidence for the mechanism of ore 

emplacement. Such relations could indicate either that 

precursor rock or rock flour was totally replaced or that 

mineralization occurred in open spaces. Unambiguous 

evidence for the latter mechanism is provided by Murphy 

and Ohle (1968) in their description of banded ore veins 

at Iron Mountain; on the basis of these and other 

textures, Murphy and Ohle concluded that most of the 

ore formed in open spaces. 

Metal Suites 

The Olympic Dam deposit contains more than 2 

billion metric tons of rock having an average grade of 20 

percent Fe, 1.6 percent Cu, 0.5 kg!t uranium oxide, 0.5 glt 

Au, 3.5 glt Ag, and greater than 1,500 ppm REE. In 

addition to these elements, Olympic Dam is highly 

anomalous in Ba, F, S, co;, and P and is slightly 

anomalous in Co, Ni, Cr, As, and Te (Roberts and 

Hudson, 1983, 1984). Acropolis contains significant Cu, 

Occurrence Model for Proterozoic Iron Oxide Deposits 63 
U, and REE in massive magnetite bodies; one drill hole 

intersected 66 m of 0.7 percent Cu and 35 ppm U30s 

(Paterson, 1986b ). Sub economic U, Cu, and Au 

concentrations are associated with magnetite-hematite at 

Wirrda Well, and subeconomic uranium and REE 

concentrations are associated with hematite at Oak Dam 

(Parker, 1987). Based on metal grades, it has commonly 

been stated that drilling on the Stuart Shelf has not found 

another Olympic Dam; however, based on broader 

geologic relations as summarized above, numerous 

mineral deposits similar to Olympic Dam exist in the 

basement of the Stuart Shelf. Just as in the southeast 

Missouri province, these deposits exhibit extreme 

variations in their contents of metals other than iron. 

Some of these variations, particularly in copper and 

uranium, may be the result of supergene enrichment 

processes (Meyer, 1988a; Oreskes and Einaudi, 1990). 

Differences in metal contents also may reflect exposure 

of different levels in the original systems. These 

variations may be expressed in part by variations in the 

proportion of magnetite to hematite in the ores; the 

hematite-dominant ores of Olympic Dam could be the 

more near surface expression of magnetite-dominant 

ores such as those drilled at Acropolis. Variations in 

magnetite/hematite proportions as a function of depth 

have been recognized in the southeast Missouri ores 

(Snyder, 1968). 

The majority of known deposits in southeast 

Missouri lack the degree of enrichment in metals (other 

than iron) that is exhibited by Olympic Dam; they also 

lack a comparable enrichment in F, P, and C03 =; 

however, Olympic Dam so far is unique in the Stuart 

Shelf. The southeast Missouri deposits are slightly 

anomalous in phosphorus and fluorine and commonly 

are locally enriched in copper (Murphy and Ohle, 1968; 

Snyder, 1968); Boss-Bixby represents a sulfide-rich end 

member that has elevated concentrations of Cu, Co, Ni, 

Mo, Pb, and Zn as well as Fe (Kisvarsanyi and Proctor, 

1967; Kisvarsanyi and Smith, 1988). At Pea Ridge, local 

areas of highly anomalous REE oxides (as high as several 

percent in some samples) and gold (intercepts 3-4 ft long 

of 0.15-3.8 ppm Au) recently have been found on the 

outer fringe of the iron oxide bodies (L. Tucker, oral 

commun., 1988); these occurrences certainly help to 

strengthen the analogy between Olympic Dam and Pea 

Ridge. 

Gangue Minerals and their 

Paragenesis in Massive Ore 

The massive iron oxide ores in both provinces 

contain very sparse gangue minerals: gangue minerals 

are intergrown with iron oxide minerals, are clasts within 

breccias, and are veins that cut breccias. Chlorite and 

carbonate minerals are the most abundant and 

ubiquitous, and fluorite, apatite, barite, quartz, sericite, 

and feldspar are common in many deposits. The sulfide 

content is extremely variable among deposits. 

At Olympic Dam, sericite, chlorite, siderite, 

fluorite barite and monazite are characteristic of the 

main s;age or' brecciation and iron oxide deposition. 

Except for sericite and chlorite, these minerals continued 

to be deposited later in crosscutting veins. Relict igneous 

quartz is the most abundant gangue mineral in the 

breccia bodies and comprises from 10 to 40 percent of 

the rock. Hydrothermal quartz, on the other hand, is 

conspicuously absent from the main stage of Fe-Cu-REE 

mineralization (Oreskes and Einaudi, 1990) but is 

present in minor amounts in late-stage veins (Rob_erts 

and Hudson, 1983). Disseminated copper and uon 

sulfide minerals, which are not gangue minerals at 

Olympic Dam, constitute some 5-10 per~ent of the 

breccia bodies outside the "barren" hematite core and 

probably formed in the waning stages of hematite breccia 

formation. 

Breccias at Olympic Dam contain more sulfide and 

carbonate minerals and less hydrothermal quartz than 

most southeast Missouri deposits. At Pea Ridge, Emery 

(1968) reported that quartz is present in the magnetite 

ores as a fine-grained intergrowth with magnetite and in 

vugs accompanied by fluorite, barite, calcite, and hematite. Additional gangue minerals include apatite and 

pyrite. The hematite ores contain a similar suite of 

gangue minerals, but pyrite is absent (Emery, 1968). 

Recent mapping at Pea Ridge shows that barite locally is 

abundant in hematite zones and visible quartz is present 

dominantly in late veins associated with crosscutting 

granite pegmatites (L.M. Nuelle, M.A. Marikos, and 

M.A. Einaudi, unpub. data, 1988). 

Quartz also accompanied iron oxide deposition in 

the Pilot Knob area. In the veins of Shepherd Mountain, 

Singewald and Milton (1929) and Meyer (1939) reported 

a paragenetic sequence of early quartz-hematite, 

followed by quartz-chlorite-magnetite-pyrite, and late 

quartz-chlorite-orthoclase-hematite. Accessory gangue 

minerals in veins include apatite, epidote, zircon, and 

locally garnet. In contrast to the vein deposits, the bedded 

hematitic ores at Pilot Knob contain no pyrite, and 

sericite is present rather than orthoclase. Meyer (1939) 

concluded that the paragenetic sequence is early 

quartz-hematite, followed by quartz-sericite-magnetite, 

and late quartz-sericite-hematite. Accessory gangue 

minerals include tourmaline, sphene, and apatite. 

Iron Mountain is clearly anomalous in terms of 

gangue mineralogy: the deposit contains abunda~t 

andradite garnet (Murphy and Ohle, 1968). Thts 

anomaly may reflect wall-rock control in which andesite 

is a local source for calcium; some interaction between 

ore fluids and wall rocks is recognized in the alteration of 

64 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
andesite fragments and wall rock to epidote and chlorite. 

Garnet is present as pseudomorphs after early actinolite, 

in veins that cut hematite, and in the centers of hematite 

veinlets associated with quartz and calcite. Quartz is the 

most abundant gangue mineral overall, and calcite is 

almost as abundant; both probably formed late relative to 

iron oxide minerals and garnet. Accessory gangue 

minerals include apatite, dolomite, fluorite, barite, 

galena, pyrite, chalcopyrite, and bornite. 

Wall-Rock Alteration 

Variations in wall-rock alteration between deposits 

of the two provinces are poorly known but may depend at 

least in part on original wall-rock composition. In 

deposits where wall rocks are dominantly felsic, such as 

Olympic Dam (Roberts and Hudson, 1983), Pilot Knob 

(Meyer, 1939), and Pea Ridge (Emery, 1968), wall-rock 

alteration is dominated by potassic assemblages 

including alkali feldspar and sericite, and silicification 

commonly is present; in deposits where mafic and (or) 

intermediate rocks are present, such as at Iron Mountain 

(Murphy and Ohle, 1968), alteration is dominated by 

more calcic assemblages including actinolite and epidote. 

In this context, the widespread actinolite-matrix breccia 

and actinolized fragments in the "hanging wall" of Pea 

Ridge are anomalous because of the rhyolite wall rocks 

and may be the result of alteration of mafic dike 

fragments (Emery, 1968). Hematitic (or magnetitic) and 

chloritic alteration, particularly of mafic mineral grains in 

igneous rocks, is common in all deposits. 

Clearly defined wall-rock alteration halos on 

margins of fractures and veins in igneous rocks generally 

are absent at Olympic Dam but are characteristic of most 

of the deposits under consideration. At Olympic Dam, 

sericitic alteration of granite is widespread beyond the 

ore zones, but sericite gradually increases toward the 

hematite breccia complex. In our petrographic studies, 

we have documented a replacement sequence that 

involves feldspar-+sericite-+hematite (Oreskes and Einaudi, 1990). It is unknown whether the orange to brickred color of alkali feldspar in the granite of Olympic Dam 

represents (1) an alteration feature related to the oreforming events, (2) an earlier alteration feature 

unrelated to the ore-forming event, or (3) a primary 

feature of the granite. In Acropolis drill core, orange 

alkali feldspar is in magnetite veins with apatite and 

locally appears to be concentrated in volcanic rocks on 

the margins of massive magnetite bodies (personal 

observations, 1986); however, numerous barren 

intercepts also contain red granite and granitic gneiss. 

Brick-red granite is characteristic of the Stuart Shelf 

basement, as it is of other Proterozoic alkalic granite 

terranes, and its significance remains to be understood. 

At Pea Ridge, some types of wall-rock alteration do 

appear as halos on fractures and veins. Most striking is 

the orange alkali feldspar found as envelopes on 

fractures and on hematite veinlets in rhyolite; locally, 

these fractures probably were reopened during a later 

chlorite-forming event. The relation of this alkali feldspar 

to iron oxide mineralization is unclear and is further 

complicated by the presence of late pegmatites and 

quartz veins that also introduce alkali feldspar (L. M. 

Nuelle, M. A. Marikos, and M. Einaudi, unpub. data, 

1988). 

Genetic Hypotheses 

The diversity of genetic hypotheses centers chiefly 

on the nature of the ore fluid, the mechanisms of ore 

emplacement, and the interpretation of elemental 

abundances in ore rather than on the broader questions 

of tectonic setting and igneous heritage. 

Olympic Dam.-Roberts and Hudson (1983) 

concluded that Olympic Dam represents a very unusual 

example of a syngenetic/diagenetic, low-temperature 

hydrothermal deposit in a subaerial playa-lake-graben 

environment. Deposition of hematite, sulfide minerals, 

uranium, REE, fluorite, siderite, and barite was from 

low-temperature hydrothermal fluids linked to alkaline 

volcanic activity that infiltrated laterally from outside the 

known mineralized zone. Thinly bedded hematitic 

sedimentary rocks ("banded iron formation" of Roberts 

and Hudson) are considered to be of fumarolic-playalake origin. The source of copper, iron, and gold is 

thought to have been mafic igneous rocks in the 

basement. The syngenetic, sediment-hosted component 

of the interpretation for the orebody as a whole has been 

dropped as the result of further study, including fluid 

inclusion evidence that saline hydrothermal fluids were 

involved (Roberts and Hudson, 1984) and underground 

exposures that indicate many breccia bodies are steeply 

dipping and crosscutting (Oreskes and Einaudi, 1990; 

Reeve and others, 1990). 

Mt. Painter.-Similarities between Olympic Dam 

and the Mt. Painter uranium deposit, on the east side of 

the Adelaide geosyncline, were noted by Youles (1984), 

who emphasized a connection to anorogenic alkaline 

magmas. Mt. Painter was interpreted by Lambert and 

others (1982) as a high-temperature hydrothermal 

breccia deposit related to epizonal granitic plutonism; 

local venting of hydrothermal fluids formed laminated 

hematite-quartz beds. Isotopic data were used by Lambert and others to conclude that sulfur (and, indirectly, 

iron) had a magmatic source and that hydrothermal 

fluids represented a mixture of magmatic/metamorphic 

and meteoric water. Uranium and fluorine were thought 

to have been derived from older Proterozoic basement. 

Occurrence Model for Proterozoic Iron Oxide Deposits 65 
Pilot Knob.-A hydrothermal replacement origin 

for bedded ores in the volcaniclastic sediments of the 

Pilot Knob surface deposit was proposed by early 

workers (Crane, 1912; Geijer, 1915; Singewald and Milton, 1929); in particular, Singewald and Milton (1929) 

presented evidence that they felt invalidated the 

suggestion by Spurr (1927) that these ores were blacksand beach deposits. Meyer (1939) generated additional 

evidence for hydrothermal replacement and suggested 

that hematitic pebbles, which only superficially resemble 

iron oxide fragments, actually represent selective 

replacement of porous or easily replaced material. He 

concluded that all of the near-surface ores of the Pilot 

Knob area formed below the surface at depths no greater 

than several thousand feet from high-temperature hydrothermal fluids originating during the final stages of 

differentiation of granitic magma. The hydrothermal 

replacement hypothesis was reiterated more recently by 

Ridge (1972) and Panno and Hood (1983); however, 

Panno and Hood concluded, as did Murrie (1973) and 

Wracher (1976) before them, that basal magnetite breccia ores of the subsurface Pilot Knob orebody probably 

represent magmatic injections, although the evidence 

they present is not conclusive. The final chapter on Pilot 

Knob has not been written: Anderson (1975) and Nold 

(1988) reopened the possibility that some of the bedded 

ores at Pilot Knob are detrital accumulations modified by 

syngenetic precipitation of hematite from subaqueous 

hot springs. On the scale of the Pilot Knob system, these 

theories are not mutually exclusive: however, many of 

these different conclusions are based on the same 

outcrop. 

Pea Ridge and Iron Mountain.-The history of 

ideas regarding these two deposits would read very much 

like that summarized above for Pilot Knob, with the 

difference that the lack of bedded ores simplifies the 

theories to two end members: (1) high-temperature 

hydrothermal replacement (Singewald and Milton, 1929) 

and (2) iron oxide magma injection (Geijer, 1930; Ridge, 

1957; Amstutz, 1960; Kisvarsanyi and Proctor, 1967). 

Murphy and Ohle (1968) did not commit themselves on 

Iron Mountain; they simply pointed out that several 

features are consistent with a hydrothermal origin. 

Emery (1968) favored a modified iron oxide magma 

theory for Pea Ridge in which late-stage hydrothermal 

activity formed the hematite cap and the hematite-silica 

and quartz-amphibole alteration of volcanic rocks. 

CONCLUSIONS 

The Proterozoic iron provinces of South Australia 

and southeast Missouri share many common features at 

the regional, district, and orebody scales. The two provinces are analogous, and iron deposits within them 

represent similar types; that is, it is appropriate and 

useful to think of Olympic Dam and Pea Ridge, for 

example, as belonging to the same class of mineral 

deposit. Variations between deposits of one province are 

as great as variations between deposits of different provinces. These variations, as well as the essential common 

features, need to be better defined. At present, from an 

economic point of view, the question of commonality of 

massive iron oxide ores to all deposits is perhaps less 

interesting than the question of REE and gold 

enrichment in only a few. 

For the deposits summarized above, the field 

relations and textures of the ores do not require that an 

iron oxide magma was involved at any stage during 

mineralization. Hypotheses involving iron oxide magmas 

for the Missouri ores are based on nondefinitive and 

ambiguous field relations, on analogies with the 

controversial magnetite-apatite deposits of Kiruna, 

Sweden (Geijer, 1930; Freitsch, 1984; Parak, 1984; Gilmour, 1985; Wright, 1986), and on questionable analogy 

with the El Laco, Chile, magnetite flows (Park, 1961). 

Many of the field relations and the textures, minerals, 

and wall-rock alteration features of the deposits 

summarized above require that an aqueous fluid, and in 

most cases a hydrothermal fluid, was involved in some 

stages of the mineralization process. 

Where an aqueous fluid is thought to have been 

involved, diversity of opinion centers on the importance 

of syngenetic versus epigenetic origins for stratiform 

hematitic ores (for example, Olympic Dam, Pilot Knob 

surface). This controversy again is the result of 

ambiguous field relations, with different interpretations 

being based on the same exposure. It is identical in its 

origins to the controversy surrounding other stratiform 

ore occurrences, such as parts of volcanogenic massive 

sulfide, stratiform lead-zinc, and stratabound copper 

deposits. Because field and textural relations are 

ambiguous in many of these cases, the resolution of this 

question must be based additionally on fluid inclusion 

and isotopic· data that can resolve relative ages of rock 

types and ore and that can identify the physicochemical 

properties of, and sources of components in, the ore 

fluids. 

In conclusion, we suggest the following lines of 

inquiry for future work. 

1. Relation of iron-rich ores to wall rock. Detailed 

geochronological studies, especially attempts to date the 

ores themselves, will help to clarify which igneous 

rocks-if any of those presently exposed-are closely 

related temporally to formation of the ores. Detailed 

field mapping and petrographic study are prerequisites to 

identifying unequivocal hydrothermal phases for age 

dating. 

66 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
2. Temperature of deposition of ores. Currently 

available data from Olympic Dam suggest that maximum 

temperatures of deposition of hematitic ores were less 

than 350 oc (N. Oreskes, 1989, unpub. data). Are 

magnetite-rich ores deeper, higher temperature 

phenomena, possibly closer to magmatic sources, or do 

they reflect differences in the local environment into 

which ore fluids are emplaced? Detailed fluid inclusion 

studies of minerals intergrown with or otherwise clearly 

cogenetic with magnetite are needed to answer this 

question. 

3. REE transport. Evidence from Olympic Dam 

conclusively demonstrates that REE were mobilized as 

part of the hydrothermal system (Oreskes and Einaudi, 

1990). Therefore, the presence of anomalous REE 

concentrations in iron-rich ores cannot be used a priori 

as evidence of a magmatic origin via liquid immiscibility. 

Study of the distribution, abundance, and mineralogy of 

REE in several deposits of this type should shed light on 

mechanisms of hydrothermal REE transport. 

4. Source of metals. Lacking magmatic 

immiscibility as a direct cause of metal partitioning, the 

origin of metals in these deposits remains a major 

unanswered question. Study of minor element geochemistry and radiogenic isotope systematics will help to 

elucidate the role of local basement in supplying metals 

to these systems. 

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Occurrence Model for Proterozoic Iron Oxide Deposits 67 
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68 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
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Economic Geology, v. 79, p. 1941-1944. 

Occurrence Model for Proterozoic Iron Oxide Deposits 69 
The Geophysical Signature of Olympic Dam and 

Midcontinent (U.S.A.) Analogs 

By William J. Hinze1 

Abstract 

The Olympic Dam deposit was discovered on the basis 

of hypothesized geologic models utilizing geophysical 

methods and remote sensing. Geophysical techniques were 

particularly important in guiding the discovery drill hole 

because the deposit is completely hidden, being overlain by 

350m of younger flat-lying sedimentary rocks. The initial drill 

hole was located on isolated but coincident gravity and 

magnetic anomalies identified on regional anomaly maps. 

Only these regional data were available for analysis. They 

indicate that the Olympic Dam deposit lies within a 

northwest-striking assemblage of intense local anomalies 

with a regional magnetic and gravity minimum, which was 

interpreted to represent a downdropped block (of primarily 

volcanic and volcanogenic sedimentary rocks?) related to 

the Torrens fault and the Adelaidean rifting event. The ore 

deposit is associated with an approximately 1 ,000-nT 

magnetic anomaly and a 10 +-mGal gravity anomaly, both of 

which are near the northwest fault-bounded margin of the 

inferred downdropped block. The character of the anomalies 

is consistent with a northwest-striking minor waban, as was 

1

Department of Earth and Atmospheric Sciences, Purdue 

University, West Lafayette, Indiana 47907. 

suggested by early geologic interpretations that the host rock 

of the Olympic Dam deposit is a graben-fill sedimentary 

breccia. Modeling of the anomalies and results of wavenumber domain filtering show that the ore body, on the basis of 

existing data, can approximately satisfy the gravity anomaly 

but not the magnetic anomaly. The source of the Olympic 

Dam anomalies is much shallower than those of similar 

anomalies observed in the Acropolis and Stuart Creek anomalies-features that have affinities to the Olympic Dam 

anomaly. 

The Proterozoic history of the central United States has 

many similarities to that of south-central Australia, including 

the presence of magmatic iron oxide deposits. Regional 

geophysical studies in the Midcontinent of the United States 

show widespread development of rifts in the 1 ,500-Ma 

anorogenic granite-rhyolite terrane and numerous coincident 

local gravity and magnetic anomalies similar to those 

observed on the Stuart Shelf of Australia. These rifts may 

preserve associated sedimentary rocks, which could be the 

host for ores derived from magmatic hydrothermal fluids. 

Modeling of several of these coincident magnetic anomalies 

in Missouri, south of the St. Francois Mountains iron 

deposits, indicates sources similar to those modeled for 

Olympic Dam. However, other regions within the 1 ,500-Ma 

orogenic felsic terrane of central North America have similarly 

favorable geophysical and geological characteristics and 

should be investigated further. 

70 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
PART 2 

Midcontinent Strategic and Critical Minerals ProjectProposed Midcontinent Olympic Dam Study 

This document proposes an interdisciplinary study 

whose ultimate goal would be to identify areas in the 

Midcontinent of the United States that appear to be 

broadly favorable for the occurrence of Olympic Damtype ore deposits. 

JUSTIFICATION 

The Olympic Dam copper-gold-uranium deposit in 

South Australia is one of the world's largest ore deposits, 

consisting of an estimated 2 billion metric tons of rock 

with an average grade of 1.6 percent Cu (32 million tons), 

0.6 kg/ton uranium oxide (1.2 million tons), and 0.6 g!ton 

Au (1.2 million kg). It is buried at a depth of 350 m 

(1,150 ft) beneath the surface and was discovered in 1975 

by a prospecting drill hole sited on coincident gravity and 

magnetic highs and a major airphoto linear. The host 

rocks of the deposit are iron oxide-rich breccias of 

granitic and felsic volcanic rocks, interpreted by Western 

Mining Company as a breccia in a Middle Proterozoic 

anorogenic granitic terrane. 

A new compilation of data on the basement in the 

Midcontinent region indicates that parts of this region, 

especially the St. Francois and Spavinaw granitic terranes 

in and adjacent to southern Missouri, may have strong 

potential for Olympic Dam-type deposits. One major 

mining company (Union Carbide) began an exploration 

project for such a deposit in southwestern Missouri in 

1981, but abandoned it after drilling two unsuccessful 

holes, when metal prices declined sharply in 1982. At 

least one other major company has also mounted a 

preliminary exploration project in this region using a 

similar exploration model. 

The general objectives of the proposed study are to 

define a working descriptive model for Olympic Damtype deposits; to compile regional syntheses of the 

geology, geophysics, and geochemistry of the Midcontinent basement terranes; to investigate the geophysical 

setting and anomaly characteristics of known or inferred 

igneous rock units with special reference to anorogenic 

granites; to make detailed studies of selected iron-ore 

deposits in the Midcontinent that appear to be analogs of 

the Olympic Dam deposit; to use a variety of analytical 

and experimental techniques to determine the processes 

of formation of the Midcontinent iron-ore deposits; and 

to apply this new knowledge of processes to the regional 

syntheses in order to determine areas in the Midcontinent that are potentially favorable for Olympic Dam-type 

deposits. These objectives would be reached through 

concurrent and integrated lines of research defined 

below in terms of specific Tasks. 

This proposal is the result of a planning workshop 

held February 2-3, 1988, under the sponsorship of the 

U.S. Geological Survey Midcontinent Strategic and 

Critical Minerals project. A list of participants is 

attached. 

STATEMENT OF TASKS, 

OBJECTIVES, AND STRATEGIES 

Task 1: Definition of a Descriptive Model 

Objective 

To develop an up-to-date descriptive exploration 

model for Olympic Dam-type deposits that will be 

applicable to the Midcontinent. 

Strategy 

Review all available published and unpublished 

information on the Olympic Dam deposit; construct a 

geologic and geophysical working model; discuss it with 

any available persons who have direct knowledge of the 

Olympic Dam deposit; if possible, arrange a visit of at 

least several days duration to Olympic Dam to inspect the 

type deposit and discuss the model with the personnel 

who are most familiar with it. Modify the working model 

accordingly. Key questions to be answered include (1) 

Proposed Midcontinent Olympic Dam Study 71 
what are the essential features and processes of the 

Olympic Dam deposit that would be necessary to form 

the same type of deposit elsewhere, and (2) which of the 

Olympic Dam features and processes are variants that 

would not necessarily be associated with the same type of 

deposit elsewhere? In other words, what is the essential 

definition of the Olympic Dam "type"? 

Time Frame 

Year 1. 

Task 2: Regional Geophysical Synthesis 

Objective 

To compile and upgrade regional geophysical data 

sets of the Midcontinent to a level commensurate with 

the overall project objectives. 

Strategy 

a. Compile and index available data sets of gravity 

and magnetic surveys for the Midcontinent region. 

b. Index available remote-sensing data sets and 

compile scenes of "global" reflectance (MSS), thermal 

(HCMM), and thermal/reflectance (AVHRR) data and 

evaluate for lineament and feature analysis. 

c. Prepare derivative maps of geophysical data 

sets including those to be used in the regional geologic 

synthesis (Task 3). 

d. Compile (and where necessary perform) geophysical property (petrophysical) measurements of Precambrian rock samples for use in modeling of geophysical anomalies. 

e. Solicit gravity, magnetic, geoelectric, seismic, 

and borehole data of private companies, especially 

companies that have explored in southeast Missouri, 

as soon as possible. Inquiries should be addressed to 

Hanna Mining, Union Carbide, New Jersey Zinc, St. Joe 

Minerals, Cleveland Cliffs, National Lead, and others 

that may have privately acquired data for small or large 

areas of the St. Francois terrane. 

f. Upgrade aeromagnetic coverage of selected 

priority areas of the Midcontinent region to 1-km line 

spacing and publish these as maps and as digital data. All 

new magnetic surveys should include aeromagnetic gradiometric measurements. Priority regions include the St. 

Francois and Spavinaw terranes and southern Wisconsin. 

Gravity coverage of identified areas of high magnetic 

anomaly should be upgraded to a station spacing of 1 mi 

or closer as allowed by elevation control. 

Time Frame 

Items (a), (b), and (e) year 1; items (c) and (d) 

continuing; item (f), subsequently, subject to availability 

of funds. 

Task 3: Regional Geologic Synthesis 

Objective 

To upgrade the existing basement geologic map of 

the region, using all available drill-hole, geophysical, and 

remote sensing data, compiling data wherever possible in 

digital format. 

Strategy 

a. Compile a geologic map of the buried and 

exposed Precambrian basement surface of the southern 

Midcontinent region, St. Francois and Spavinaw terranes. 

The map should include all previously published data and 

should, especially, be updated and refined using newly 

available basement drill-hole data. The map should 

include depth-to-basement contours where the basement 

is covered and should show collar locations of drill holes 

examined where possible. The map should cover the area 

lat 35°-40° N., long 88°-100° W., at a scale of 1:500,000. 

b. Compile an annotated list of basementpenetrating drill holes and, where available, their borehole logs, and make the list available both in digital 

(floppy disk) and printed form. Tabulated data should 

include locations, collar elevations, formation tops, 

basement depths, Precambrian rock types penetrated, 

sample storage sites, and notation for any and all geochemical, petrologic, and isotopic work done on the 

Precambrian samples. All of the area between lat 3S0-46° 

N., long 88°-104° W., should be covered, and this new 

database should be merged with the similar database 

from the current study of the Trans-Hudson orogenic 

belt. 

c. Compile a lineament map of the area lat 

35°-46° N., long 88°-100° W., from remote sensing 

imagery of various types and available gravity and 

magnetic maps and make it available in both printed and 

digital forms. 

d. All of the preceding new data, including the 

geophysical data under Task 2, should be used in an 

attempt to produce an interpretive basement terrane 

map series of the St. Francois and Spavinaw terranes 

that might include such items as thickness modeling of 

the anorogenic igneous rocks, terrane boundary 

refinement, and structural geology of the buried 

basement. 

72 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Time Frame 

Year 1 and subsequent followup as new data 

become available. 

Task 4: Regional Geochemical Synthesis 

Objective 

To expand studies of the regional geochemistry of 

Midcontinent basement rocks and dispersion halos. 

Strategy 

. Make additional analyses of St. Francois and Spavmaw terrane basement samples, particularly analyses to 

refine knowledge of elements not well covered by the 

six-step semiquantitative spectrographic analytical 

methods used by R. Erickson and coworkers in the Rolla 

1° x 2° quadrangle UGSG CUSMAP work. The existing 

Paleozoic geochemical database should be critically 

reevaluated for clues to geochemical haloes in 

sedimentary rocks immediately overlying the Precambrian basement. Erickson's sample pulps and many 

new basement samples that expand areal coverage 

should be analyzed by methods such as X-ray 

~uorescenc~ and induction-coupled plasma spectroscopy 

m order to Improve understanding of possible pathfinder 

elements such as barium, phosphorus, and fluorine. 

Delayed neutron activation analysis of basement samples 

for uranium and thorium should be conducted. 

Phanerozoic-aquifer well waters should be investigated 

as a sampling medium in areas where covered deposits 

are known. All this work and previous basement-rock 

analyses should be compiled into a digital database that 

can be merged with the drill-hole penetration database of 

Task ~b. Additionally, special attention should be paid to 

chemistry of the Precambrian rocks immediately below 

the Paleozoic unconformity, which may represent a paleoweathering profile in which pathfinder elements could 

have been concentrated. 

Time Frame 

Years 1-2. 

Task 5: Detailed Midcontinent 

Site Studies 

Objective 

To document ~s thoroughly as possible the 

geolo~, geophysical signature, ore mineralogy, paragenesis, petrology, wall-rock alteration, and geochemistry of selected iron oxide deposits in the St. 

Francois terrane that might have formed from processes 

analogous to those that formed the Olympic Dam 

deposit. 

Strategy 

a. Map or log in detail the workings and (or) 

cores of the Pea Ridge mine, Boss-Bixby deposit, and 

Pilot Knob (surface) deposit, and sample them for geochemical and petrogenetic studies and measurement of 

geophysical properties. These deposit studies should be 

integrated with the regional systematic geochemical and 

isotopic studies and detailed geophysical profiles and 

with studies of St. Francois terrane magmatic evolution. 

Studies of physical and chemical processes of ore 

deposition should follow from these mapping and 

sampling efforts. Of immediate priority are (1) to initiate 

a ~apping and sampling project at Pea Ridge mine, 

whtch has, for economic reasons, an uncertain continued 

mine life, and (2) to secure the Boss core, carefully 

recovering all core possible from the core rack collapse in 

the storage building at Bourbon, Missouri. 

b. Make detailed magnetic and gravity surveys 

and additional geochemical and petrophysical analyses of 

basement drill-hole samples of the Salem-Lake Spring 

anomaly. 

. c. Make detailed gravity, magnetic, and, in places, 

audio-magneto-telluric (AMT) and other geoelectric 

profiles of selected geologic and geophysical features to 

search for critical anomaly characteristics that may 

discriminate mineral deposits from all other features. 

These should include profiles over anomalies known to 

be caused by intrusions. 

d. Seek (from both private and public sources) 

and study additional cores from the Iron Mountain and 

Pilot Knob areas and from other subsurface discoveries 

such as Camels Hump, Bourbon, and Kratz Spring. 

e. Prepare detailed derivative gravity, magnetic, 

and remote sensing (thematic mapper, SPOT, soyuzcarla, radar) maps of areas selected by geology and 

geophysics consensus as leading targets. 

Time Frame 

Item (a)-mine mapping and core logging-should 

begin as soon as possible and may last 1-3 years. The Pea 

Ridge mine may close in the near future, and the 

Boss-Bixby cores as currently stored are in precarious 

condition. (Mine mapping was actually begun by the 

Missouri Geological Survey in April 1988.) Item (b) 

year 1; item (c) year 1 or 2, whenever possible; item (d) 

years 2-4; item (e) years 4 and 5. 

Proposed Midcontinent Olympic Dam Study 73 
Task 6: Studies of Ore Genesis 

Objective 

To understand the major processes involved in ore 

deposition, including the following: 

a. Age of the mineral deposits-Establish when 

the deposits in the St. Francois Mountains were 

emplaced to determine if they are coeval with any 

magmatic system exposed or known in the St. Francois 

Mountains. 

b. Metal sources-Determine the processes 

needed to produce enrichments of iron, copper, REE, 

and uranium sufficient to form an ore deposit and the 

sources of these elements. 

c. Metal transport-Determine how the metals 

transported to the site of deposition (magmatic, hydrothermal, or supergene). 

d. Depositional mechanisms and environmentEvaluate temperature-pressure effects, mixing, 

boiling, oxidation, and pH and depositional mechanisms. 

e. Plumbing system -Study (1) origins of permeability-fracture versus lithologic-and effects of 

regional and local structures; (2) chemical solution paths 

(such as collapse brecciation); (3) explosive processesgaseous (such as diatreme), magmatic, phreatic, hydrofracturing. 

f. Postdepositional modification-Study effects 

of metamorphism, oxidation, metasomatism, tectonic 

modification. 

Strategy 

An understanding of the various parts of the system 

outlined above can be approached through detailed 

topical studies based on and conducted in conjunction 

with the extensive mine, district, and regional geological 

and geophysical investigations. Particular emphasis 

should be placed on the Pea Ridge and Boss-Bixby 

deposits, which represent the oxide-dominant and 

copper sulfide-rich end member deposits currently 

known in the region, respectively. The topical studies are 

generally interrelated and should not be viewed as 

separate entities. They include: 

a. Petrogenetic studies to define host rocks, 

possible source rocks, ore mineralogy, ore paragenesis, 

and wall-rock alteration. 

b. General geochemical studies. 

(1) Chronologie studies-includingAr-Ar, U-Pb, 

Rb-Sr and Nd-Sm isotopic systems-to determine the 

age of mineralization, alteration, host rocks, and thermal 

events (both local and regional). 

(2) Stable isotope (C, 0, S, H) geochemical 

investigations to gain information on the source of metals 

(also using Pb and Sr isotope systematics), the nature and 

path of the ore-forming fluids, and the physicochemical 

conditions during ore deposition and various periods of 

alteration. 

(3) Trace element studies of whole-rock systems 

and individual phases to yield data relevant to the same 

problems as the stable isotope investigations. 

c. Microthermometry, compositional, and quadrupole gas analysis of fluid inclusions to help identify 

the nature of the ore-forming fluids, transportation 

mechanism(s) of the various ore metals, and conditions 

required for ore deposition. 

d. Experimental studies of metal transport and 

complexing in fluorine-rich hydrothermal systems. 

e. Modeling (the development of dynamic 

multiple working hypotheses) of chemical and magmatic 

processes to provide constraints, directions, and tests for 

all the genetic studies. Types of required modeling 

exercises include but are not limited to the following two 

examples. 

(1) Path-calc type modeling (computer-based 

calculation of reaction paths) to provide information on 

which metals, as what species, can be transported in a 

model fluid through rocks of specific compositions and to 

predict what mineral phases will form as the result of 

changing physical or chemical environments. This 

exercise will begin simplistically and become more 

complex as data become available and as the needed 

thermodynamic data are obtained. 

(2) Modeling of magmatic systems that are most 

likely to be genetically related to the ore deposits (for 

example, host granitic rocks, magnetite trachyte, rhyolite, 

buried anorthosite) to establish the fundamental 

intensive and extensive parameters controlling their 

development. (For example, is there a unique source and 

(or) evolutionary path that an associated parental magma 

system must undergo in order to develop an Olympic 

Dam-type deposit?) In this modeling, an attempt should 

be made to quantitatively establish if any of these 

magmatic systems are capable of developing the iron-rich 

fluid phase or exsolved melt responsible for 

mineralization and, if so, to determine the factors 

controlling genesis of the fluid or melt. From these 

results, plausible mechanisms for the development of the 

iron-rich copper-REE-uranium-bearing ore-forming 

fluids can be elucidated. If a magmatic origin for the 

deposits is indicated, the petrogenetic criteria for their 

development should be defined and then integrated into 

an ore deposit model useful for exploration. 

Time Frame 

Item (e) should begin immediately and continue 

for the duration of the project. Items (a) through (d) 

should be phased in during year 1 and continued through 

to the end of the project.) 

74 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
List of participants, by group 

(Asterisk (*) indicates group leader.) 

Group 1: Compilations and Field Studies 

Timothy S. Hayes* 

Pieter Berendsen 

Lin Cordell 

Martin B. Goldhaber 

Tom Hildenbrand 

William Hinze 

Murray Hitzman 

Marty Kane 

Eva Kisvarsanyi 

Ernest L. Ohle 

Walden P. Pratt 

PaulK. Sims 

U.S. Geological Survey (USGS), Mineral Resources 

Kansas Geological Survey 

USGS, Geophysics 

USGS, Geochemistry 

USGS, Geophysics 

Chevron Resources 

USGS, Geophysics 

Purdue University 

Missouri Geological Survey 

University of Arizona 

USGS, Mineral Resources 

USGS, Mineral Resources 

Group 2: Processes and Mechanisms 

Richard I. Grauch* 

David Frishman 

Linda Gundersen 

Steven A. Hauck 

Ernest Kendall 

Peter J. Modreski 

Naomi Oreskes 

John Philpotts 

Geoffrey Plumlee 

Ian Ridley 

Lanier Rowan 

Robert 0. Rye 

Tommy Thompson 

USGS, Geochemistry 

USGS, Mineral Resources 

USGS, Sedimentary Processes 

University of Minnesota-Duluth; formerly Union 

Carbide Corporation 

Union Carbide Corporation 

USGS, Mineral Resources 

Stanford University 

USGS, Geochemistry 

USGS, Geochemistry 

USGS, Geochemistry 

USGS, Geochemistry 

USGS, Isotope Geology 

Colorado State University 

Group 3: Topical Studies 

Warren C. Day* 

Edward A. du Bray 

Ralph L. Erickson 

Ken Ludwig 

Klaus Schulz 

Holly J. Stein 

Alan Wallace 

USGS, Mineral Resources 

USGS, Mineral Resources 

USGS, Geochemistry 

USGS, Isotope Geology 

USGS, Mineral Resources 

USGS, Mineral Resources 

USGS, Mineral Resources 

Group 4: Modeling the Olympic Dam Deposit 

Warren I. Finch* USGS, Sedimentary Processes 

Proposed Midcontinent Olympic Dam Study 75 
PART3 

Methodology for Analysis of the Proterozoic 

Metallogenic Province of Southeast MissouriA Blueprint for State and Federal Cooperation 

(a.k.a. the "Olympic Dam Project") 

By Eva B. Kisvarsanyi 1 and Charles E. Robertson 1 

Abstract 

Southeast Missouri has 6 major (2G-200 million tons of 

ore) and 24 minor magnetite-apatite-hematite deposits that 

define a Proterozoic metallogenic district 125 km long and 50 

km wide, elongated north-northwest. The deposits are 

hosted by silicic volcanic rocks of the anorogenic St. Francois granite-rhyolite terrane, which is partly exposed in the St. 

Francois Mountains. Iron mining from shallow surface 

deposits commenced in 1815 and continued until 1966; 

since 1964, mining activity has shifted to deposits covered by 

400-500 m of Paleozoic strata. The petrotectonic setting of 

the southeast Missouri deposits is strikingly similar to that of 

the giant Olympic Dam Cu-Au-Ag-U-REE deposit, the largest 

among a number of lesser deposits on the Stuart Shelf in 

South Australia. Both the St. Francois and Stuart Shelf 

deposits belong to a special class of Proterozoic iron oxiderich deposits that share a number of common features and 

may result from similar ore-forming processes. 

The St. Francois terrane in southeast Missouri is by far 

the most accessible of the Proterozoic anorogenic terranes 

generally considered favorable for hosting Olympic Damtype deposits in the Midcontinent of the United States. In 

addition to its extensive outcrops in the St. Francois 

Mountains, more than 650 drill cores amounting to a 

cumulative total of 40,000 m are available for study. 

Furthermore, the Pea Ridge underground mine provides 

access to one of the major deposits in the northwestern part 

of the district. As a major component of its Midcontinent 

Strategic and Critical Minerals Project, the U.S. Geological 

Survey (USGS), in cooperation with the Missouri Geological 

Survey (MGS), has undertaken a 5-year multidisciplinary 

study to develop a descriptive model for Olympic Dam-type 

deposits and identify areas favorable for hosting such 

1

Missouri Geological Sutvey, P.O. Box 250, Rolla, Missouri 

65401. 

deposits in the region. The project is an extension of the 

regional geologic and metallogenic synthesis of the northern 

Midcontinent (Sims and others, 1987) by the USGS and a 

logical continuation of the Operation Basement program by 

the MGS. The cooperative research is planned to take 

advantage of the specific expertise and capabilities of both 

agencies. 

The principal responsibilities of the MGS include but 

are not limited to (1) compiling a digital data base for drill 

holes to the Precambrian basement and updating the 

existing basement map, (2) detailed mapping of accessible 

Pea Ridge mine workings, in cooperation with the Pea Ridge 

Iron Ore Company, and sampling for systematic 

petrographic, ore microscopic, and geochemical analysis 

(see Marikos and others, this volume), {3) detailed logging 

and sampling of core from major undeveloped deposits, 

including 30,000 m of exploration core from the Boss copperiron orebody, and {4) mapping accessible workings and 

sampling available core from inactive mines in the St. Francois Mountains. The USGS will be primarily responsible for 

{1) regional geophysical synthesis, {2) regional geochemical 

synthesis, and {3) studies of ore depositional processes. 

Both agencies will collaborate in specific site studies, as in 

the Pea Ridge mine mapping project, and in various topical 

studies on ore mineralogy and paragenesis, wall-rock 

alteration, and lineament and structural analysis and in 

development of a descriptive model. Status reports will be 

released as open-file maps and reports; formal reports will be 

published subsequently by either of the two agencies or in 

professional journals, as deemed appropriate. 

REFERENCE CITED 

Sims, P.K., Kisvarsanyi, E.B., and Morey, G.B., 1987, Geology 

and metallogeny of Archean and Proterozoic basement 

terranes in the northern midcontinent, U.S.A.-An 

overview: U.S. Geological Survey Bulletin 1815, 51 p. 

76 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
Geologic Mapping and Evaluation of the Pea Ridge 

Iron Ore Mine (Washington County, Missouri) for 

Rare-Earth Element and Precious Metal PotentialA Progress Report 

By Mark Alan Marikos1

• 2 , Laurence M. Nuelle1 , and Cheryl M. Seeger1 

Abstract 

Ongoing geologic mapping of the Pea Ridge deposit 

has revealed rock types and relations not previously 

documented in this or any other southeast Missouri Precambrian iron ore deposit. Certain features of the Pea Ridge 

deposit are similar to those of the Olympic Dam deposit of 

South Australia, including a breccia pipe complex 

instrumental in the transportation of iron, copper, rare-earth 

elements, thorium, uranium, and gold during a late-stage 

hydrothermal event. We believe that the Pea Ridge deposit 

represents a deeper, magnetite-rich variant of the hematitic 

Olympic Dam type and that study of the Pea Ridge deposit 

may reveal some of the processes responsible for 

emplacement of iron deposits of the Midcontinent United 

States and may lead to recovery of metals not currently being 

recovered from such deposits. 

INTRODUCTION 

As part of a cooperative effort by the U.S. 

Geological Survey and the Missouri Department of 

Natural Resources, Geological Survey Program, the Pea 

Ridge iron ore deposit is being evaluated as a possible 

variant of the Olympic Dam deposit of South Australia. 

The Pea Ridge Iron Ore Company has graciously 

granted access to the mine for geologic mapping and 

topical studies to determine ore genesis and to evaluate 

the deposit for mineral commodities not currently being 

recovered. 

1

Missouri Geological Survey, P.O. Box 250, Rolla, Mossouri, 

65401. 

2

Current address: Department of Earth and Space Sciences, 

3806 Geology Building-University of California at Los Angeles, Los 

Angeles, California 90024. 

The Pea Ridge deposit, long interpreted as a 

magmatic intrusion of magnetite into Middle Proterozoic 

intrusive porphyritic rocks, has been considered to 

consist of several distinct mineralogical zones (Emery, 

1968). Later work by James R. Husman (1989) in the 

early 1980's refined the understanding of the mineralogy, 

texture, and distribution of the zones and first 

documented the distribution patterns of rare-earth 

elements (REE) and gold. Husman also identified the 

host rock as volcanic, rather than plutonic. 

Our work to date has further refined the 

mineralogical zonation and delineated a breccia pipe 

zone, previously undescribed. The zonation is that of 

orebody scale or assemblage zoning. Our zonal 

terminology is subject to revision as the investigation 

proceeds. 

The Pea Ridge deposit is more complex than 

previously reported. The amphibole zone probably 

represents alteration of a pyroxene body that preceded 

magnetite emplacement (Marikos, in press). We now 

know that some magnetite was emplaced by hydrothermal wall-rock replacement or metasomatism. The core of 

the magnetite body, however, may have been emplaced 

by a much different mechanism such as magmatic 

injection or even by supercritical vapor-phase 

replacement of a breccia complex filled by a very active 

fluid. 

Acknowledgments.-We greatly appreciate the 

hospitality of the Pea Ridge Iron Ore Company. We 

thank Robert Z. Reed, President, and John Schoolcraft, 

Plant Superintendent, for making the mine available for 

our work and allowing access to company data. We thank 

Larry J. Tucker, Head Mine Engineer, and Don Roberts, 

Mine Superintendent, for their time in helping us 

become familiar with mine working procedures and the 

mine layout, and especially Larry Tucker, who has been 

actively sampling and delineating the REE distribution 

Geologic Mapping and Evaluation of the Pea Ridge Iron Ore Mine 77 
for the last several years and has shown us the work done 

by J. R. Husman as well as himself. We also appreciate 

the cooperation of all the miners. 

MAPPING METHOD 

A geological mapping program was designed to 

document systematically the geologic and mineralogical 

features of the deposit (Nuelle, Seeger, and Marikos, 

1990). The mapping method was mostly developed by the 

Anaconda Company for mapping porphyry copper 

deposits. The map scale of 1 in. = 20 ft reveals many 

features important to the understanding of the deposit, 

features that would be missed using a smaller scale. 

Geologic features along drift ribs are projected to a 

horizontal map plane about 5 ft above the floor. Twentyone color-coded pencils are used to represent 

mineralogical and structural features, thereby minimizing note writing and allowing the geologic and 

mineralogical features to be readily apparent from visual 

scanning. Rock features such as composition, structure, 

texture, and vein fillings are plotted on the "rock side" of 

the field map; alteration features are plotted on the "air 

side" (that is, the open-space area representing the mine 

workings) (fig. 20). 

The extent of mined-out areas precludes mapping 

the entire deposit. Instead, we map traverses that cross 

several geologic boundaries to document ore emplacement and other geologic events. Three levels will be 

mapped extensively to determine vertical variations in the 

deposit. Smaller areas on other levels containing 

significant exposures will be mapped to gain a better 

understanding of important geologic relationships. 

Because of the differences in map scale between 

company base maps and our maps and the number of 

color-coded pencils used, map reproduction is difficult. 

Difficulties, however, are greatly reduced with the use of 

personal computers. Company mine maps are digitized 

and st~red in data files using GSDRAW (a U.S. 

Geologtcal Survey computer drafting program). When a 

traverse is selected, the desired part of the mine map is 

reproduced at the required scale. This map then becomes 

the base map for the underground traverse. When 

underground traverses are complete, the information 

from all traverses on the same mine level are plotted on 

a posting sheet, which thus becomes a plan-view geologic 

map. The detailed compilation maps will b~ published 

initially as open-file maps by the Missouri Division of 

Geology and Land Survey. 

MINERALOGICAL ZONES 

To date we have defined six mineralogical 

assemblages, or zones; each zone is characterized by a 

distinct mineralogy (Nuelle, Marikos, and Seeger, 1990), 

which largely follows the zones described by Emery 

(1968). 

The amphibole zone (Emery's quartz-amphibole 

zone) is mostly amphibole and varying amounts of 

quartz, magnetite, and pyrite. Clinopyroxene morphology 

exhibited by amphibole (ferro actinolite) crystals suggests 

that the rock was initially a pyroxenite. The pyroxenite 

may have been a skarn related to initial stages of 

magnetite emplacement, a separate rock type (such as an 

intrusive body cutting the host rock), or a mineral 

aggregate emplaced cogenetically with magnetite 

emplacement. Much of the quartz in the amphibole zone 

probably was cogenetic with the original pyroxene-pyroxene crystals (now amphibole) terminate in quartz 

pods that at one time must have been voids-but some of 

the quartz pods are veined by the pyroxene. A second, 

later stage of quartz and pyrite cuts the amphibole zone. 

The magnetite zone (Emery's magnetite zone) 

contains more than 60 percent Fe, of which about 55 

percent is in magnetite. The magnetite is accompanied by 

varying amounts of pyrite, chalcopyrite, and apatite. The 

sulfide minerals probably are paragenetically later than 

the magnetite, and the apatite probably is mostly coeval 

with the magnetite. Although we have not yet mapped 

extensively in the magnetite zone, preliminary 

reconnaissance mapping shows textures that could be 

relict breccia textures. If the presence of a preexisting 

breccia is confirmed, magnetite replacement could have 

been by a normal hydrothermal process or by a process 

involving supercritical vapor-phase replacement by 

invasion of an aggressive fluid. 

The hematite zone (Emery's specular hematite and 

quartz-hematite zones) forms much of the outer 

periphery of the magnetite zone and represents a hydrothermally altered part of the main magnetite orebody. 

The zone is made up of dominantly specular hematite 

and minor magnetite, chlorite, barite, and calcite. Apatite 

is uncommon and occurs as corroded crystals; pyrite is 

generally absent. 

The brecciated wall-rock-magnetite zone (Emery's 

porphyry bteccia zone) consists of wall-rock breccias 

found discontinuously along margins of the magnetite 

body. Textures range from incipient crackle breccia at 

the margin to massive magnetite and only sparse wallrock fragments. Much of the breccia is pseudobreccia 

(replacement breccia), in which wall-rock fractures were 

infilled by magnetite and the wall rock adjacent to the · 

fractures was metasomatically replaced by magnetite. 

Advanced stages of this replacement process resulted in 

a magnetite matrix-supported breccia in which the wallrock fragments have not been greatly rotated or abraded. 

The breccia pipe zone, previously undescribed, 

consists of at least three breccia bodies containing 

varying amounts of potassium feldspar, barite, apatite, 

78 Midcontinent U.S.-Permisslve Terrane for Olympic Dam Deposit? 
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as shown here. Minimal field notes accompany the map. Rock features are plotted on the rock side of the drift, and alteration features are plotted on the 

air side . 
thorium, REE phosphate minerals, and rock-flour 

matrix. A breccia pipe complex cuts magnetite, amphibole rock, host rock, and mafic dikes on the footwall 

(north) side of the magnetite body and may extend more 

than 100 m north of the footwall. The soft rock-flourmatrix-supported breccias and some of the silicified 

breccias are easily mapped by using a gamma-ray 

scintillometer because of their high thorium content. 

Although all known pipes are near the footwall, the only 

accessible exposure of the hanging wall is cut by a felsic 

dike and slightly radioactive quartz veins; thus, pipes may 

also be present beyond the hanging wall, south of the 

magne.tite ore. The highest gold abundances are spatially 

associated with the breccia pipes. In addition, REE oxide 

contents are as high as and in excess of 10 weight percent. 

Lithic fragments in the breccia pipes include 

volcanic rock, magnetite, hematite, quartz-amphibole 

rock, felsic dike rock, mafic dike rock, and earlier 

silicified breccia. Well-rounded and angular fragments 

are in close pr~ximity to each other and vary in degree of 

alteration. Mineralogic alterations associated with pipe 

emplacement are magnetite to hematite, amphibole to 

chlorite (chamosite), and feldspar to apple-green 

muscovite. Cement mineralogy is complex and variable 

and includes combinations of barite, quartz, microcline, 

anhydrite, fluorite, chamosite, muscovite, calcite, and 

REE minerals. 

The character of the breccia pipe material varies 

from rock-flour-matrix-supported to silicified breccia. 

Most of the original textures in silicified rocks have been 

obliterated. Visible porosity in the breccia pipes ranges 

from almost zero in silicified rocks to approximately 50 

percent in some rock-flour-matrix-supported breccias. 

Crystal-lined cavity volume is from microscopic to more 

than a cubic meter. 

The silicified zonf! (in part, Emery's quartzamphibole and quartz-hematite zones) is in wall rock or 

in other assemblages that were silicified during a 

relatively late stage quartz-flooding event. The quartz 

flooding silicified areas of host rock and breccia pipes 

and obscured original textures. Areas of silicification, as 

well as other areas, are cut by quartz-feldspar veins that 

contain variable amounts of barite, fluorite, chalcopyrite, 

chlorite, monazite, rutile, xenotime, apple-green 

muscovite, biotite, and pyrite. These quartz veins cut all 

other zones and fill fractures, sigmoidal dilation voids, 

and breccia interstices. The veins are irregularly shaped, 

randomly oriented, of variable thickness, and commonly 

discontinuous. Where they cut magnetite, the magnetite 

can be altered to hematite as far as 2 m from the veins, or 

there may be no visible alteration envelope. Trace 

amounts of nickel, cobalt, lead, tellurium, and 

molybdenum minerals also are present in these veins. 

MAFIC AND FELSIC DIKES 

Postmagnetite mafic dikes intrude the magnetite 

zone and host rocks; they strike east-west, dip about 70° 

N. to vertical, and are from 2 em to more than 4 m thick. 

Some of these dikes were emplaced before the REBbearing breccia pipes because fragments of the dike 

rocks have been observed in the pipes. 

Felsic dikes, aplitic to pegmatitic in texture, cut all 

zones of mineralization in the orebody and thus were 

emplaced late. Fragments of felsic dikes have also been 

noted in breccia pipes. Some felsic dikes are cut by 

quartz-biotite-pyrite veins; these dikes were emplaced 

before and during breccia pipe formation and hydrothermal activity. On the 2275 level, magnetiteapatite-amphibole veins occupy fractures that have the 

same orientation as a nearby felsic dike. The dikes are 

orange, variably oriented, and from 2 em to 2 m thick. 

They contain alkali feldspar and quartz, and sparse 

biotite in some pegmatitic zones. The pegmatitic zones 

locally segregate into quartz-biotite-feldspar-pyritechalcopyrite veins that cut the dikes. Pyrite- and chloritefilled fractures with halos of feldspathic alteration 

surround the dikes where the dikes cut host rock. 

POSSIBLE INTRAMAAR SEDIMENT 

At various places in the mine, a laminated rock of 

alternating hematite and chlorite-feldspar-illite layers is 

present. In thin section most of the hematite shows 

euhedral magnetite morphology, which indicates that the 

hematite was altered from magnetite. This laminated 

rock occurs as discrete blocks of arenite that are oriented 

oblique to the northeasterly dip of the host rock. Many 

blocks have tight, recumbent folds in places. These 

blocks have been observed in areas adjacent to breccia 

pipes and in iron ore. Their fragmentation and 

orientation suggest that they have been transported, 

probably downward through breccia pipes. They are 

presumed to be sedimentary in origin because of the 

presence of ripple marks and mudcracks. They may have 

originated as an intramaar sediment and may indicate 

that an exhalative or sedimentary iron ore deposit once 

capped (or was at some distance above) the Pea Ridge 

deposit. 

CONCLUSION 

Whether or not the Pea Ridge deposit proves to be 

an Olympic Dam analog, the data gained thus far have 

revealed insights valuable to future resource assessments 

in the Midcontinent of the United States. We now know 

80 Midcontinent U.S.-Permissive Terrane for Olympic Dam Deposit? 
that many "new" types of deposits could exist in this 

region, and further study of this deposit will provide data 

useful in exploration. The Pea Ridge mine offers a 

unique window into this buried terrane and deserves 

further study. As of this writing, detailed mapping, 

petrographic studies, and geochemical investigations are 

continuing. Stable isotope, fluid inclusion, and geophysical studies are planned. 

REFERENCES CITED 

Emery, J.A., 1968, Geology of the Pea Ridge iron ore body, in 

Ridge, J.D., ed., Ore deposits of the United States, 

1933-1967 (Graton-Sales Volume, v. 1): American 

Institute of Mining, Metallurgical, and Petroleum 

Engineers, p. 359-369. 

Husman, J.R., 1989, Gold, rare earth element, and other 

potential by-products of the Pea Ridge iron ore mine, 

Washington County, Missouri: Missouri Department of 

Natural Resources, Division of Geology and Land 

Survey, Open-File Report OFR-89-78-MR, 18 p. 

Marikos, M.A., in press, The quartz-amphibole zone, Pea 

Ridge mine, Washington County, Missouri [abs.]: 

Missouri Academy of Science, Annual Meeting, 

Proceedings, 1989. 

Nuelle, L.M., Marikos, M.A., and Seeger, C.M., 1990, Geologic 

mapping and evaluation of the Pea Ridge iron ore mine 

(Washington County, Missouri) for rare-earth earth and 

precious metal potential, in Pratt, W.P., and Goldhaber, 

M.B., eds., U.S. Geological Survey-Missouri Geological 

Survey Symposium-Mineral-resource potential of the 

Midcontinent, Program and Abstracts, St. Louis, 

Missouri, April 11-12, 1989: U.S. Geological Survey 

Circular 1043, p. 20-21. 

Nuelle, L.M., Seeger, C.M., and Marikos, M.A., 1990, Geologic 

mapping project of the Pea Ridge iron ore deposit, 

Washington County, Missouri, in Pratt, W.P., and 

Goldhaber, M.B., eds., U.S. Geological Survey-Missouri 

Geological Survey Symposium-Mineral-resource 

potential of the Midcontinent, Program and Abstracts, 

St. Louis, Missouri, April 11-12, 1989: U.S. Geological 

Survey Circular 1043, p. 22. 

irU.S. GOVERNMENT PRINTING OFFICE: 1991-573-048/26017 Geologic Mapping and Evaluation of the Pea Ridge Iron Ore Mine 81 



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investigations in all phases of hydrology, including hydrogeology, 

availability of water, quality of water, and use of water. 

Cb'culars present administrative information or important scientific 

information of wide popular interest in a format designed for distribution 

at no cost to the public. Information is usually of short-term interest. 

Water-Resources Investigations Reports are papers of an interpretive nature made available to the public outside the formal USGS publications series. Copies are reproduced on request unlike formal USGS 

publications, and they are also available for public inspection at 

depositories indicated in USGS catalogs. 

Open-File Reports include unpublished manuscript reports, maps, 

and other material that are made available for public consultation at 

depositories. They are a nonpermanent form of publication that may be 

cited in other publications as sources of information. 

Maps 

Geologic Quadrangle Maps are multicolor geologic maps on 

topographic bases in 7 1/2- or 15-minute quadrangle formats (scales mainly 1:24,000 or 1 :62.500) showing bedrock. surficial, or engineering geology. Maps generally include brief texts; some maps include structure 

and columnar sections only. 

Geophysical Investigations Maps are on topographic or planimetric 

bases at various scales; they show results of surveys using geophysical 

techniques, such as gravity, magnetic, seismic, or radioactivity, which 

reflect subsurface structures that are of economic or geologic significance. 

Many maps include correlations with the geology. 

Miscellaneous Investigations Series Maps are on planimetric or 

topographic bases of regular and irregular areas at various scales; they 

present a wide variety of format and subject matter. The series also includes 7 1/2-minute quadrangle photo geologic maps on planimetric bases 

which show geology as interpreted from aerial photographs. Series also 

\ncludes maps of Mars and the Moon. 

Coal Investigations Maps are geologic maps on topographic or 

planimetric bases at various scales showing bedrock or surficial geology, stratigraphy, and structural relations in certain. coal-r~~urce ar~. 

Oil and Gas Investigations Charts show stratigraphic inforrnauon 

for certain oil and gas fields and other areas having petroleum potential. 

Miscellaneous Field Studies Maps are multicolor or black-andwhite maps on topographic or planimetric bases on quadrangle or irregular areas at various scales. Pre-1971 maps show bedrock geology 

in relation to specific mining or mineral-deposit problems; post-1971 

maps are primarily black-and-white maps on various subjects such as 

environmental studies or wilderness mineral investigations. 

Hydrologic Investigations Atlases are multicolored or black-andwhite maps on topographic or planimetric bases presenting a wide range 

of geohydrologic data of both regular and irregular areas; principal scale 

is 1:24,000 and regional studies are at 1:250,000 scale or smaller. 

Catalogs 

Permanent catalogs, as well as some others, giving comprehensive listings of U.S. Geological Survey publications are available under 

the conditions indicated below from the U.S. Geological Survey, Books 

and Open-File Reports Section, Federal Center, Box 25425, Denver, 

CO 80225. (See latest Price and Availability List) 

"Publkatlons of the Geological Survey, 1879-1961" may be purchased by mail and over the counter in paperbaek book form and as a 

set of microfiche. 

"Publications of the Geological Survey, 1962- 1970" may be purchased by mail and over the counter in paperback book form and as a 

set of microfiche. 

"Publications of the U.S. Geological Survey, 1971- 1981" may be 

purchased by mail and over the counter in paperback book form (two 

volumes, publications listing and index) and as a set of microfiche. 

Supplements for 1982, 1983, 1984, 1985,1986, and for subsequent 

years since the last permanent catalog may be purchased by mail and 

over the counter in paperback book form. 

State catalogs, "List of U.S. Geological Survey Geologic and 

Water-Supply Reports and Maps For (State)," may be purchased by mail 

and over the counter in paperback booklet form only 

"Price and Availability List of U.S. Geological Survey Publications," issued 8JU'lually, is available free of charge in paperback booklet form only. 

Selectedcoplesofamonthlycatalog"NewPublicationsoftheU.S. 

Geological Survey" available free of charge by mail or may be obtained 

over the counter in paperback booklet form only. Those wishing a free 

subscription to the monthly catalog "New Publications of the U.S. 

Geological Survey" should write to the U.S. Geological Survey, 582 

National Center, Reston, VA 22092. 

Note.--Prices of Government publications listed in older catalogs, 

8JU'louncements, and publications may be incorrect. Therefore, the 

prices charged may differ from the prices in catalogs, announcements, 

and publications.