Geology and Metallogeny of 

Archean and Proterozoic Basement Terranes 

in the Northern Midcontinent, U.S.A.-

An Overview 

Prepared in cooperation with the 

Missouri Geological Survey and 

Minnesota Geological Survey 

Geology and Metallogeny of 

Archean and Proterozoic Basement Terranes 

in the Northern Midcontinent, U.S.A.-

An Overview 

By P.K. SIMS, U.S. Geological Survey, and 

EVA B. KISVARSANYI, Missouri Geological Survey, and 

G.B. MOREY, Minnesota Geological Survey 

Prepared in cooperation with the 

Missouri Geological Survey and Minnesota Geological Survey 

A description of major known and potential mineral resources 

U.S. GEOLOGICAL SURVEY BULLETIN 1815 

CONTRIBUTION TO THE GEOLOGY OF THE MIDCONTINENT REGION 
DEPARTMENT OF THE INTERIOR 

DONALD PAUL HODEL, Secretary 

U.S. GEOLOGICAL SURVEY 

Dallas l. Peck, Director 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON:1987 

For sale by the 

Books and Open-File Reports Section 

U.S. Geological Survey 

Federal Center 

Box 25425 

Denver, CO 80225 

Library of Congress Cataloging-in-Publication Data 

Sims, P. K. (Paul Kibler), 1918-

Geology and metallogeny of Archean and Proterozoic basement 

terranes in the northern midcontinent, U.S.A.-an overview. 

(Contribution to the geology of the midcontinent region) (U.S. 

Geological Survey bulletin ; 1815) 

"Prepared in cooperation with the Missouri Geological Survey and 

Minnesota Geological Survey." 

Bibliography: p.42 

Supt. of Docs. no.: I 19.3:1815 

1. Geology, Stratigraphic-Precambrian. 2. Geology-Middle West. 

3. Ore-deposits-Middle West. I. Kisvarsanyi, Eva B. II. Morey, G. B. 

Ill. Missouri. Geological Survey. IV. Minnesota Geological Survey. V. 

Title. VI. Series. VII. Series: U.S. Geological Survey bulletin ; 1815. 

QE75.B9 no. 1815 557.3 s 87-600326 

[QE653] [551.7'1'0977] 
CONTENTS 

Abstract 1 

Introduction 2 

Acknowledgments 2 

Tectono-stratigraphic framework 2 

Sequential development of terranes 4 

Archean gneiss terrane 7 

Late Archean greenstone-granite terrane 9 

Early Proterozoic Wisconsin magmatic terrane and associated epicratonic 

rocks of the Penokean orogen 10 

Early Proterozoic rhyolite-granite terrane of southern Wisconsin 11 

Quartzite of Baraboo interval 11 

Early Proterozoic metamorphic and granitoid rocks of the Central Plains 

orogen 12 

Middle Proterozoic anorthosite and rapakivi granite of Transcontinental 

anorogenic province 14 

Middle Proterozoic St. Francois granite-rhyolite terrane 14 

Middle Proterozoic Spavinaw granite-rhyolite: terrane 17 

Middle(?) Proterozoic sedimentary rocks 17 

Middle Proterozoic Midcontinent rift system 18 

Origin and age of transcurrent faults 19 

Summary of tectonic evolution 22 

Metallogeny 24 

Archean gneiss terrane 24 

Late Archean greenstone-granite terrane 26 

Massive sulfide deposits 26 

Gold deposits 26 

Early Proterozoic Penokean orogen 26 

Massive sulfide deposits 27 

Other deposits 27 

Early Proterozoic rhyolite-granite terrane of southern Wisconsin 27 

Early Proterozoic Central. Plains orogen 27 

Metallic sulfide deposits associated with metamorphic rocks 27 

Metallic deposits associated with layered mafic complexes 28 

Middle Proterozoic St. Francois granite-rhyolite terrane 28 

Iron and copper-iron deposits 30 

Manganese deposits 30 

Vein deposits of tin-tungsten-silver-lead-antimony 33 

Uranium-bearing granitic rocks as a possible heat source for 

Mississippi Valley-type lead-zinc deposits 33 

Middle Proterozoic Spavinaw granite-rhyolitt~ terrane 33 

Middle Proterozoic Midcontinent rift system 34 

Stratiform copper deposits 34 

Redbed copper deposits 34 

Magmatic copper deposits 35 

Petroleum occurrences 35 

Hydrogen and nitrogen occurrences 36 

Speculative metallogeny 36 

Possible Olympic Dam-type deposits 37 

Possible unconformity-type uranium deposits in Quartzite of the Baraboo 

interval 40 

Summary and conclusions 41 

References cited 42 

Contents Ill 
FIGURES 

1. Map showing area covered by new basement map of the northern 

midcontinent 2 

2. Generalized Precambrian basement map of the northern midcontinent 3 

3. Generalized tectonic map of Precambrian basement rocks of the northern 

midcontinent 5 

4. Paleogeologic maps of Archean terranes in the Great Lakes region 7 

5. Paleogeologic maps showing successive stages of development of Early 

Proterozoic Penokean orogen 8 

6. Paleogeologic maps showing inferred development of the Early Proterozoic 

Central Plains orogen 9 

7. Gravity anomaly map of part of north-central United States 13 

8. Integrated surface and subsurface geologic map of the St. Francois terrane, 

southeastern Missouri 16 

9. Major known and inferred transcurrent faults in north-central United 

States 20 

10. Map showing the Vermilion fault system of dextral transcurrent faults and 

related sinistral faults, Vermilion district, northern Minnesota 21 

11. Generalized structure contour map of the Precambrian basement surface of 

the northern midcontinent 25 

12. High magnetic anomalies in Missouri and their sources 29 

13. Relationship of ring structures, trachytic ring intrusions, and ore deposits in 

the St. Francois terrane, southeastern Missouri 31 

14. Alkali-silica diagram showing selected volcanic rocks from the St. Francois 

terrane 32 

15. Iron-silica diagram showing selected volcanic rocks from the St. Francois 

terrane 32 

16. Ranges of selected trace elements from the trachyte suite of the St. Francois 

terrane compared to world averages of syenite 32 

17. Geologic map of the Precambrian basement rocks in southwestern Missouri 

and adjacent areas 39 

TABLE 

1. Major crust-forming events and associated metalliferous deposits in the 

northern midcontinent 6 

IV Contents 
Geology and Metallogeny of 

Archean and Proterozoic Basement Terranes 

in the Northern Midcontinent, LJ.S.A.-

An Overview 

.By P.K. Sims, Eva B. Kisvarsanyi 1, and G.B. Morey2 

Abstract 

The exposed and buried basement in the northern midcontinent (lat 36°-46° N., long 88°-100° W.) is a collage of 

tectono-stratigraphic terranes that range in age from Archean 

to Middle Proterozoic. Eight major terranes have been identified and delineated. From oldest to youngest, these are (1) 

Archean gneiss terrane (age, 2.6-3.6 Ga); (2) Late Archean 

greenstone-granite terrane (age, 2.6-2.7 Ga); (3) Early Proterozoic Wisconsin magmatic terrane (age, 1.83-1.89 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, 1.63-1.8 Ga); (6) Middle Proterozoic St. Francois graniterhyolite terrane (age, 1.48 Ga); (7) Middle Proterozoic Spavinaw 

granite-rhyolite terrane (age, 1.35-1.4 Ga); and (8) Middle Proterozoic Midcontinent rift system (age, 1.0-1.2 Ga). Other 

coherent rock units include quartzite of the Baraboo interval 

(age, "'1.63-1.76 Ga) and plutons of anorthosite and 

mesozonal 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 entirely or mostly oceanic-arc complexes that were accreted to 

the North American continent; whereas most other terranes 

are continental crustal segments that evolved in extensional 

environments. 

The northern midcontinent is traversed by numerous 

northwest-trending tectonic zones which are interpreted as 

shears and are characterized by cataclastic zones in the basement, by aligned granite and mafic intrusions of Middle Proterozoic age and, commonly, by faulting and folding in 

overlying Paleozoic rocks. Movements on the shears are poorly 

known, but the shears are believed to belong to a family of 

regional transcurrent dextral faults known from exposed areas 

in the Lake Superior region and adjacent Canada. They are 

potential sites for epigenetic ore deposits. 

The northern midcontinent has a high potential for undiscovered mineral resources because many of the terranes are 

favorable for ore generation. Exposed parts of the region-the 

Lake Superior region in the north and the Southeast Missouri 

district in the southeast-are major mineral-producing areas that 

1

Missouri Geological Survey. 2Minnesota Geological Survey. 

together have yielded ores, mainly of iron and copper, valued 

at several billion dollars. A further positive factor has been the 

relatively recent discoveries of as yet unmined deposits containing large zinc-copper and copper-nickel, cobalt-platinum 

resources in the Lake Superior region. Another positive aspect 

for exploration is the relatively shallow depth of much of the 

basement; in about two-thirds of the northern midcontinent 

area, the basement is less than 3,000 feet (91 0 meters) below 

the surface. 

Four major geologic terranes known to contain substantial mineral resources extend into the subsurface. The first, the 

Late Archean greenstone-granite terrane in the buried basement of the Dakotas, could host massive sulfide deposits about 

5 million tons in size, stratiform gold deposits, Algoma-type ironformatiion deposits, and possibly other types of deposits. Probably, felsic volcanic rocks of tholeiitic affinity would be the most 

favorable host rocks. 

The second, the Wisconsin magmatic terrane, contains 

at least four volcanic-hosted massive sulfide deposits offuture 

economic importance, the largest of which (Crandon) contains 

about 60 million tons of copper-zinc ore. Other known deposits 

are smaller, being about 5 million tons or less. Gold is another 

potential commodity. 

The third, the St. Francois granite-rhyolite terrane, hosts 

large deposits of iron and copper-iron and apparently smaller 

deposits of tin, tungsten, silver, lead, and antimony. 

The fourth terrane, rocks of the Midcontinent rift system, 

hosts world-class stratiform volcanic-hosted and sedimentaryhosted copper deposits and the very large, unmined gabbrohosted copper-nickel deposits of the Middle Proterozoic Duluth 

Complex. Iron oxide-rich mafic intrusive rocks could host 

platinum-group metals. In recent years, the sedimentary rocks 

have been targeted as potential sources of petroleum. 

Other terranes, by analogy with similar rocks elsewhere 

in the world known to contain valuable ore deposits, are potentially ore bearing. The quartzite of the Baraboo interval possibly 

could host unconformity-type uranium deposits. Also, rocks 

of the Central Plains orogen could host base-metal sulfide and 

tungsten deposits. Contrary to current exploration philosophies, 

the Archean gneiss terrane possibly could host base-metal 

sulfidE! and precious metal deposits, inasmuch as it probably 

has in part a greenstone-belt protolith. 

The Middle Proterozoic anorogenic terranes in the 

southE~rn part of the midcontinent-the St. Francois and 

Spavinaw granite-rhyolite terranes-provide a broad tectonic 

environment similar to that of the world-class Olympic Dam 

deposit in southern Australia, and they deserve consideration 

for exploration for such a deposit. 

Abstract 
INTRODUCTION 

A new Precambrian basement map of the northern 

midcontinent (lat 36°-46°N., long 88°-100°W.; Sims, 

1985, in press) provides a geologic framework within 

which known and potential mineral resources in the 

region can be discussed. Within the map area (fig. 1), 

significant known mineral deposits are confined to a part 

of Wisconsin within the Lake Superior region and to the 

St. Francois Mountains in southeastern Missouri. 

I I 210 100 KR.OMETIRI 

Figure 1. Map showing area covered by new basement map 

of the northern midcontinent by Sims (1985, in press). 

In order to provide a broader base for discussing 

the metallogeny of the region, we have extended the area 

of concern northward to the United States-Canada border 

and westward to about long 105°W. (fig. 2), because this 

area encompasses nearly all the important past and present metal-mining districts and potential ore deposits in 

the Lake Superior region and northern midcontinent. 

Although the map covers the Black Hills uplift in South 

Dakota-within the Trans-Hudson orogen-this area will 

not be discussed specifically in this report. 

Guild (1971, 1972) has shown that metallogeny (the 

study of the genesis of ore deposits in their total geologic 

environment) is a valuable approach to exploration and 

that a broad-scale relationship exists between types of ore 

and elements of global plate tectonics, at least in postEocene time. Albers (1981) successfully extended this approach to include the older Phanerozoic rocks in California. Sims (1987) attempted to show that metallogeny is 

a powerful tool also in understanding the distribution of 

mineral deposits in the Precambrian tectono-stratigraphic 

terranes of the Great Lakes region, and that it can be applied to other~ similar terranes. This report attempts to 

utilize this method of investigation to estimate the mineral 

potential of the buried Precambrian basement of the interior platform. 

The Precambrian basement map of the northern 

midcontinent region (scale 1: 1 ,000,000; Sims, 1985, in 

press) was compiled from 1 :500,000-scale maps submitted 

by the respective state geological surveys showing basement drill holes, lithotypes or preliminary geologic map 

units, and basement topography, contoured at 200-ft intervals. In compiling the map, available regional aeromagnetic (Zietz, 1982; Zietz and others, 1984; Burchett, 

1985) and gravity anomaly (Hildenbrand and others, 

1982) maps were utilized to define insofar as possible the 

extent and outlines of related rock bodies; all available 

isotopic age data were utilized (Marvin, 1987). The map 

is a revision of the earlier basement rock map for the 

northern midcontinent region (Bayley and Muehl berger, 

1968). 

The term "anorogenic" is used herein for granitoid 

and volcanic rocks that were generated along rift zones 

and within stable continental blocks and were emplaced 

during periods of negligible crustal strain. The granitoid 

rocks have been termed A-type (Loiselle and Wones, 

1979; Collins and others, 1982), because they are somewhat alkaline, anorogenic, and anhydrous, to distinguish 

them from I- and S-type (Chappell and White, 1974) 

orogenic granitoids. Anorogenic granitoids have low 

CaO, Al20 3 , and MgO, high K20/Na20, and greater 

than 60 percent Si02• They show definite iron enrichment compared to orogenic rock associations (Martin and 

Piwinskii, 1972), and they are relatively enriched in the 

incompatible trace elements. In the region covered by this 

report, the term anorogenic has been used previously by 

Kisvarsanyi (1980) to describe the alkali-feldspar-rich 

rocks of the St. Francois granite-rhyolite terrane, by 

Anderson (1983) to describe the granitoid rocks in the 

Transcontinental anorogenic province, and by Smith 

(1983) to describe the post-Penokean granite and rhyolite 

from southern Wisconsin. 

Throughout this report isotopic ages are generally 

reported with the abbreviation Ga (Giga-annum; 109 

years), but precise ages are reported with the abbreviation Ma (Mega-annum; 106 years) as for example 

3,545 ±45 Ma. 

Acknowledgments 

R.E. Denison provided descriptions of core samples 

from two wells in Fulton and Randolph Counties in 

northeastern Arkansas, which were recently obtained by 

the Arkansas Geological Commission. We benefited from 

numerous discussions with Geza Kisvarsanyi regarding 

the geology and mineral resources of Missouri and adjacent areas. Warren Day and Richard Grauch provided 

constructive reviews that materially improved the 

manuscript. 

TECTONO-STRATIGRAPHIC FRAMEWORK 

The tectono-stratigraphic framework of the Precambrian basement rocks in the northern midcontinent 

2 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
I I I I 

0 1 00 200 300 400 

MIDDLE PROTEROZOIC (900-1600 Ma) 

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 ( 1600-2500 Ma) 

Quartzite of "Baraboo interval" 

Metamorphic and granitoid rocks of Central Plains orogen 

Rhyolite and granite (--1.76 Ga) 

/I\/ I\ G . d . d k . 1 -:;-;:;... ramte an assoc1ate roc s (age uncertam) 

~t~.t.::n~: Granite (-1.8 Ga 1 

V}/::·~ Wisconsin magmatic terrane of Penokean orogen 

Rocks of Trans-Hudson orogen 

Epicratonic sedimentary and volcanic rocks ( ....... 1.9-2.1 Ga) 

ARCHEAN (2500 Ma and older) 

Not 

mapped 

Greenstone -granite terrane of Superior craton (-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 

BH Black Hills uplift 

Car Cambridge arch 

Cb Cheyenne belt 

CKu Central Kansas uplift 

GL TZ Great Lakes tectonic zone 

Mgi Missouri gravity low 

Nf Niagara fault 

SF St. Francois Mountains 

Figure 2. Generalized Precambrian basement map of the northern midcontinent. 

Tectono-Stratigraphic Framework 3 
can be inferred from the exposed rocks in the Lake Superior region and in the St. Francois Mountains of southeastern Missouri, from extrapolation of exposed terranes 

into the subsurface, and from basement drill-hole data 

together with pertinent aeromagnetic and gravity data. 

U-Pb zircon age data, obtained by M.E. Bickford, W.R. 

Van Schmus, and students (compiled by Marvin, 1987), 

provide a necessary time frame for the buried basement 

rocks. 

The combined data show that the exposed and 

buried rocks in the midcontinent region are a tectonic 

assemblage of anastomosing Early Proterozoic orogenic 

belts that border and marginally affect the southern part 

of the Archean Superior craton, Early Proterozoic intracratonic igneous and sedimentary rocks, Middle Proterozoic anorogenic intrusions and associated volcanic 

rocks, and a Middle Proterozoic intracratonic rift (fig. 

3). The rocks record several major crust-forming events, 

local reactivation of the Archean gneisses in the Penokean 

orogen, and the erosion and deposition of epicratonic successions. The major tectono-stratigraphic units for the 

broader region are shown on figures 2 and 3 and are listed 

in table 1. 

Sequential Development of Terranes 

Nine coherent episodes of major crust generation 

have been distinguished by geochronology in the northern 

midcontinent. The nominal ages of these events are 

3.6-2.6 Ga, 2.7-2.6 Ga, rv2.1-1.9 Ga, 1.89-1.83 Ga, 1.76 

Ga, 1.8-1.63 Ga, 1.48 Ga, 1.4-1.35 Ga, and 1.2-1.0 Ga. 

More local rock-forming events included deposition of 

quartzite of the Baraboo interval (1"\.11.76-1.63 Ga; Dott, 

1983) in the northern part of the region, intrusion of anorthosite and rapakivi granite within the Transcontinental 

anorogenic province (rv1.5-1.4 Ga; Silver and others, 

1977), and deposition of local(?) clastic sedimentary rocks 

in one or more basins in and adjacent to southwestern 

Missouri in Middle(?) Proterozoic time. 

The development of successive crustal segments can 

be discussed with respect to a series of simplified regional 

paleogeologic maps. In preparing the maps, the rocks 

were restored to their approximate position prior to the 

aborted rifting (Midcontinent rift system) in Keweenawan 

time (rv 1.1 Ga) (Sims, 1976b ); their geographic positions 

are based on their present-day orientations and relative 

positions because their locations on the earth in preKeweenawan time are not known. 

The oldest crustal segment, the Archean gneiss terrane (2.6-3.6 Ga) is inferred to have extended from the 

Lake Superior region eastward into the Lake Huron region (fig. 4A). Its southern and western extents are not 

known. The gneiss represents a remnant of a large sialic 

protocontinent. 

During a major tectonothermal event in the Late 

Archean (2.6-2. 7 Ga), large quantities of subaqueous 

volcanic rocks, derivative sedimentary rocks, and granitoid bodies were generated to the north of the older Archean protocontinent (fig. 4B). This crustal segment 

(greenstone-granite terrane), which formed within a time 

span of 50-100 m.y., was sutured to the older gneiss terrane near the end of the Archean. 

After stabilization and strengthening of the crust 

(rv2.5 Ga), an asymmetrical sequence of sediments 

(1"\.11.9-2.1 Ga) accumulated in the Lake Superior region 

(fig. 5A). The rocks were deposited in a large basin 

(Animikie) over the boundary (GL TZ; Great Lakes tectonic zone) between the two Archean terranes prior to 

continental breakup (rv1.9 Ga). Following breakup of the 

North American continent-and the development of 

oceanic crust, the remnants of which have been 

destroyed-a complex island arc system (Wisconsin 

magmatic terrane; 1.83-1.89 Ga) was formed on the south 

(present-day coordinates). Collision of this arc complex 

with the Archean craton and its partial cover of Early 

Proterozoic epicratonic rocks, about 1.85 b.y. ago, was 

responsible for the Penokean orogeny in the Great Lakes 

region (fig. 5B). 

Rifting apparently occurred in the northern part of 

the region shortly after cessation of the Penokean orogeny 

and was accompanied by the outpouring of subaerial, 

anorogenic rhyolite and the intrusion of cogenetic epizonal granite (1. 76 Ga) in southern Wisconsin and adjacent areas. The basement associated with these rocks 

probably is mainly older Early Proterozoic rocks of the 

Wisconsin magmatic terrane. Apparently, deposition of 

mature quartzite of the Baraboo interval began soon after 

the 1. 76-Ga magmatism in a dominantly fluvial depositional environment. Remnants of quartzite are present 

today over wide areas in Wisconsin and southern Minnesota and South Dakota and are known to be present, 

at least locally, in eastern Iowa (Sims, 1985). 

The last major orogenic event in north-central 

United States was breakup of the North American continent along a west-trending axis (fig. 6A), which 

truncated generally north-trending Archean and Early 

Proterozoic terranes to the north. Breakup was followed 

by the presumed development and eventual consumption 

of new oceanic crust and by the formation of ocean arc 

systems to the south. Collision of this arc complex with 

the older continent (fig. 6B) produced the Central Plains 

orogen (1.63-1.8 Ga). This orogen is interpreted to be 

continuous with the metamorphic foldbelts exposed in 

mountain ranges in Colorado and adjacent states (Sims 

and Peterman, 1986), and thus is a major segment of 

dominantly juvenile material that was accreted to the 

North American continent in the interval 1.6-1.8 Ga. 

About 100 m.y. after cessation of the Central Plains 

orogeny, diachronous rhyolitic volcanism and associated 

4 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
BH 

Cb 

GLTZ 

Mgl 

Nf 

SF 

0 100 200 300 MILES 

I I I I I I I I I 

0 100 200 300 400 500 KILOMETERS 

FAULTS 

High -angle, relative movement not known 

High -angle reverse fault; bars on downthrown side 

Strike-slip, showing relative horizontal movement 

STRUCTURAL TREND LINES 

Proterozoic rocks 

Archean rocks 

Black Hills 

Cheyenne belt 

Great Lakes tectonic zone 

Missouri gravity low 

Niagara fault 

St. Francois Mountains 

Figure 3. Generalized tectonic map of Precambrian basement rocks of the northern 

midcontinent. Stipple pattern denotes intracratonic igneous and sedimentary rocks. 

Tectono-Stratigraphic Framework 5 
0'1 

> 

~ :-

~ :I 

AI 

:I 

Q. 

., 

a 

it 

a 

N 

0 

;:;· 

= AI 

~ 

3 

t1) 

a 

..... 

~ 

D1 

:I 

t1) 

~ 

z 

0 

::::5. 

:-

~ :I 

~ 

c.: 

t"l 

0 

a 

:;· 

t1) 

~ 

c: 

ill 

~ 

Table 1. Major crust-forming events and associated metalliferous deposits in the northern midcontinent 

Age 

(Ga) 

Terrane 

2.6-3.6 Archean gneiss terrane 

2.6-2.7 Late Archean greenstone-granite 

terrane. 

-1.9-2.1 Early Proterozoic 

epicratonic sedimentary rocks of 

Penokean orogen. 

1.83-1.89 Early Proterozoic 

Wisconsin magmatic 

terrane of Penokean 

orogen. 

1.76 Early Proterozoic 

rhyolite-granite 

terrane of 

southern Wisconsin. 

1.63-1.8 Early Proterozoic 

metamorphic and 

granitoid rocks of 

Central Plains 

orogen. 

1.48 Middle Proterozoic 

St. Francois 

granite-rhyolite 

terrane. 

1.35-1.4 Middle Proterozoic 

Spavinaw graniterhyolite terrane. 

1.0-1.2 Middle Proterozoic 

Midcontinent rift 

system. 

Local rock 

forming events 

Quartzite of 

Baraboo 

interval 

~1.63-

1.76 Ga). 

Intrusive rocks of 

Transcontinental 

anorogenic province (-1.4-1.5 

Ga). 

Tectonic 

setting 

Uncertain 

Oceanic arc 

Rift to shelf to 

deep-water 

environment on 

passive margin. 

Oceanic arc 

Extension 

Oceanic arc 

Extension 

Extension 

Clastic sedimentary Wrench basin(?) 

rocks. 

Extension (intracontinental 

rift). 

Description 

Amphibolite-bearing migmatitic gneisses, 

younger gneisses and schists of 

greenstone affinity, and 2.6-Ga granite • 

Tholeiitic basalt, basalt, komatiite, 

calc-alkaline basaltic andesite, 

andesite, and dacite; distinctly 

bimodal. Tonalite to granite intrusive 

rocks. Sutured to gneiss terrane in 

Late Archean. 

Bimodal basalt-rhyolite volcanic rocks 

and intercalated clastic and chemical 

sedimentary rocks. 

Tholeiitic basalt and calc-alkaline 

andesite to rhyolite extrusive rocks and 

calc-alkaline gneiss and tonalite to 

granite. Sutured to continental margin 

-1.85 Ga. 

Anorogenic rhyolite and cogenetic epizonal 

granite; locally overlain by fluvial 

quartzite of the Baraboo interval. 

Gneisses and schists and synkinematic 

granitoids extensively intruded by 

younger (1.35-1.48 Ga) granite. 

Ash-flow tuff of rhyolite composition and 

lesser trachyte and associated granitic 

rocks related to cauldrons and calderas. 

Rhyolite and associated granite 

Conglomerate and iron-rich clastic rocks 

Aluminum-rich olivine tholeiite, high-iron 

tholeiite and rhyolite; tholeiitic 

intrusions. 

Known mineral 

deposits 

Algoma-type iron-formation 

Lake Superior-type ironformation. 

Carbonate-hosted stratiform 

copper deposits. 

Volcanic-hosted massive 

sulfide deposits. 

Massive and disseminated 

zinc-copper deposits. 

Disseminated tungsten-copper 

deposits. 

Volcanic- and intrusionhosted iron and copperiron deposits. 

Volcanic-hosted manganese 

deposits. 

Vein deposits of tin, tung-, 

sten, silver, lead, and 

antimony. 

Volcanic- and sedimentaryhosted copper deposits. 

Gabbro-hosted copper-nickelcobalt deposits. 

Probable and 

speculative resources 

Volcanic-hosted massive 

sulfide deposits. 

Volcanic-hosted gold deposits. 

Volcanic-hosted massive 

sulfide deposits. 

Volcanic-hosted gold deposits 

Gabbro-hosted nickel-copper 

deposits. 

Volcanic-hosted gold 

deposits. 

Olympic-Dam-type iron-copperuranium-gold deposits. 

Gabbro-hosted iron-nickelcopper-cobalt deposits. 

Gabbro-ultramafic-hosted 

platinum-chromium-titanium 

deposits. 

Olympic Dam-type iron-copperuranium-gold deposits. 

Gabbro-hosted iron-nickelcopper-cobalt deposits. 

Olympic Dam-type iron-copperuranium-gold deposits. 

Olympic Dam-type iron-copperuranium-gold deposits. 

Redbed copper deposits 
150 

I 

I 

300 MILES 

I 

150 300 450 KILOMETERS 

EXPLANATION 

Archean 

Greenstone-granite terrane 

Gneiss terrane 

Known or inferred terrane boundary 

Figure 4. Paleogeographic maps of Archean terranes in the 

Great Lakes region. Modified from Sims (1987). A, Archean 

gneiss protocontinent (2.6-3.6 Ga). 8, Archean craton formed 

by suturing of Late Archean greenstone-granite (2.6-2.7 Ga) 

and Middle and Late Archean gneiss terranes. 

co genetic plutonism occurred in the southern part of the 

metamorphic-granitoid terrane of the Central Plains 

orogen, apparently as a consequence of rifting on a large 

scale. Magmatism in the St. Francois granite-rhyolite terrane (1.48 Ga) was followed by magmatism in the 

Spavinaw granite-rhyolite terrane (1.35-1.4 Ga). Small 

to large intrusions of alkali feldspar-rich granitic rocks 

(1"1..11.4-1.5 Ga) were emplaced at about the same time in 

a wide belt that transects the northern midcontinent and 

which has been termed the Transcontinental anorogenic 

province (Silver and others, 1977; Anderson, 1983). The 

rhyolite and associated granite in the St. Francois and 

Spavinaw terranes have chemical and mineralogic compositions similar to the plutonic rocks of the Transcontinental anorogenic province and perhaps are related 

petrogenetically. 

Subsequent to, or perhaps partly contemporaneous 

with, the later stages of the granite-rhyolite magmatism, 

clastic sedimentary rocks were deposited within and adjacent to a northwest-trending basin (or basins) floored 

by anorogenic granite. The extent of these rocks is not 

known, but remnants of them have been penetrated by 

drilling in southwestern Missouri and adjacent parts of 

Kansas and Arkansas (fig. 17). 

The youngest crust-forming event in the northern 

midcontinent was the magmatism and subsequent sedimentation within the Midcontinent rift system (fig. 5C), 

which aborted before achieving significant crustal separation. Dominantly mafic volcanic rocks and related 

hypabyssal intrusions of apparent mantle derivation are 

intercalated with lesser clastic sedimentary rocks within 

the rift system. Local uplifts, such as the Goodman swell 

(1"1..11.1 Ga; Peterman and Sims, 1986), accompanied the 

rifting, as indicated by Rb-Sr isotopic ages of biotite in 

rocks of Archean and Early Proterozoic ages. 

Archean Gneiss Terrane 

Rocks of the Archean gneiss terrane are exposed 

in the Minnesota River Valley in southwestern and eastcentral Minnesota and northern Wisconsin and Michigan. 

In Wisconsin and Michigan they mainly compose the 

cores of mantled gneiss domes or uplifted fault blocks 

(Morey and others, 1982) that formed as a result of reactivation during the Early Proterozoic Penokean orogeny. 

The gneiss terrane has been encountered by drilling in 

southwestern Minnesota, and it apparently extends 

beneath the Early Proterozoic Sioux Quartzite of the 

Baraboo interval (Sims, 1985). It appears to be terminated 

southward in northeastern Nebraska and southeastern 

South Dakota by the crosscutting Central Plains orogen 

(fig. 2). 

The gneiss terrane in the Minnesota River Valley 

composes a grossly conformable sequence of interlayered 

migmatitic gneisses that is a few thousand meters thick 

(Grant, 1972). It is intruded by younger granitic and 

pegmatitic rocks. The gneisses are folded on easttrending, gently plunging axes and have mineral assemblages characteristic of upper amphibolite- and granulitefacies metamorphism. 

In the central part of the valley, the gneisses are 

metamorphosed to granulite grade and are assigned to 

the Early and Middle Archean Montevideo Gneiss of 

Lund (1956). The Montevideo Gneiss consists of a gray, 

foliated, granodioritic to locally tonalitic paleosome that 

is cut both concordantly and discordantly by layers and 

veins of a red granitic neosome. The gray granodioritic 

paleosome yields a Rb-Sr whole-rock age of 3,680±70 

Ma, whereas the red granitic neosome yields an age of 

3,045 ± 32 Ma (Goldich and others, 1980). 

Tectono-Stratigraphic Framework 7 
0 150 300 MILES 

I I I 

I I I 

0 150 300 450 KILOMETERS 

EXPIANA TION 

Middle Proterozoic 

t:.":·:_.'::::·::_:::·:.\1 Midcontinent rift system 

Early Proterozoic 

Wisconsin magmatic terrane 

Marquette Range Supergroup and correlative rock; 

Archean basement exposed locally 

Late Archean 

Greenstone-granite terrane 

Late to Early Archean 

Gneiss terrane 

---- Contact-Approximately located; dashed where now 

covered by water; queried where uncertain 

---- Fault-Dashed where approximate; queried where uncertain 

~ Direction of tectonic transport 

_.:::.:GL:;.:.TZ:;;......_ Great Lakes tectonic zone; boundary between Archean 

terranes; extension beneath Marquette Range 

Supergroup shown as dashed line 

Figure 5. Paleogeologic maps showing successive stages of development of Early Proterozoic Penokean orogen. Modified from 

Sims (1987). A, Early Proterozoic epicratonic sequence ('''1.9-2.1 Ga), showing inferred outline of depositional basin. 8, Arccontinent collision ('''1 .85 Ga) which resulted in Penokean orogeny. C, Midcontinent rift system (1.0-1.2 Ga). 

In the southern part of the valley the gneisses are 

metamorphosed to the amphibolite grade. Three quasistratigraphic units that have been delineated are characterized from bottom to top by abundant, common, and 

rare rafts of amphibolite, respectively. The middle unit 

in this succession is assigned to the Archean Morton 

Gneiss of Lund (1956). As in the central part of the valley, 

the quartzofeldspathic gneisses are migmatized. However, 

the paleosome is compositionally layered and dominantly consists of tonalitic gneiss with lesser but locally significant quantities of granodioritic material. Rb-Sr and U-Pb 

isotopic data suggest that the tonalitic and granodioritic 

gneisses and the associated amphibolite lenses are 3.5 Ga 

or more old (Goldich and Wooden, 1980). Zircon dating 

by ion microprobe methods similarly indicates an age of 

3,535 ±45 Ma (Williams and others, 1984). The neosome, 

which is granite and locally pegmatite, formed during two 

later episodes. A deformed granite was emplaced 

3,043 ±26 Ma, and a younger, largely undeformed granite 

was emplaced 2,555 ±55 Ma (Goldich and Wooden, 

1980). 

Various rock units of supracrustal origin structurally overlie the amphibolite-bearing migmatitic gneisses. 

Although the supracrustal sequences contain discontinuous layers, lenses, and boudins of amphibolite, they 

lack any evidence of migmatization. The unmigmatized 

rocks include hornblende-pyroxene gneiss and garnetbiotite gneiss in the central part of the valley and several 

kinds of biotite gneiss that contain garnet, anthophyllite 

and cordierite or sillimanite and K-feldspar in the southern part. A metagabbro intrudes the hornblende-pyroxene 

and garnet-biotite gneisses in the central part of the valley. 

The supracrustal sequences and the metagabbro record 

a metamorphic event or events in the interval between 

2,600 and 2,900 m.y. ago (Wilson and Murthy, 1976), 

but presumably are themselves much older. 

The gneissic rocks are intruded by several granitic 

units of batholitic dimensions, including the Late 

8 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
Archean Sacred Heart Granite, which is a mediumgrained, generally homogeneous to weakly foliated rock 

that yields a Pb-Pb age of 2,605 ± 6 Ma, which Doe and 

Delevaux (1980) interpret as the time of emplacement. 

The Sacred Heart Granite is thus late tectonic or possibly 

posttectonic. 

46° 

46° 

I OCEANIC --~ 

co, 

'NB 

~------- "'.a 

100 

I I I 

0 100 200 

"') Rifted conti1nent~ll­ l margin ("'1.8 Ga I 

200 300 MILES 

1

1 1 1

1 

300 400 600 KILOMETERS 

100 200 300 MILES 

I I I I I I I I 

1 00 200 300 400 600 KILOMETERS 

" '( 

....... 

~ : : : : ~ 

~ ~:: :-, 

NF 

EXPLANATION 

Early Proterozoic 

Central Plains orogen 

Trans -Hudson orogen 

Wisconsin magmatic terrane ( Penokean orogen I 

Epicratonic rocks ( Penokean orogen I 

Archean 

Superior Archean craton 

Wyoming Archean craton 

Contact 

Fault, queried where uncertain 

Niagara fault 

SL T Storm Lake trend 

? extent uncertain 

/ Direction of tectonic transport 

NOTE: Early Proterozoic intracratonic rocks are omitted for simplicity 

Figure 6. Paleogeologic maps showing inferred development of the Early Proterozoic Central Plains orogen (presentday coordinates). A, Postbreakup stage, resulting from rifting of the continental margin ("'.~1.8 Ga). 8, Arc-continent 

collision (""1.65 Ga). 

Gneisses of comparable age and complexity are exposed (Watersmeet dome; Sims and others, 1984) .in 

northern Michigan. Approximately 3.56 Gain age, they 

are locally overlain by younger (2.64 Ga) metavolcanic 

rocks and are intruded by 2.6-Ga leucogranite (Peterman 

and others, 1985). In contrast to the Archean gneisses in 

the Minnesota River Valley, these gneisses were partly 

reactivated during the Penokean orogeny, and Rb-Sr 

whole-rock and mineral systems in the Watersmeet dome 

were reset 1,800-1,750 m.y. ago. 

Late Archean Greenstone-Granite Terrane 

The next younger crustal entity, the greenstonegranite terrane (Morey and Sims, 1976; Sims, 1976a), 

forms the southernmost part of the Superior province of 

Canada and the United States. This segment of the crust 

stabilized by the end of the Archean (2.5 Ga). It is separated from the Archean gneiss terrane to the south by 

a discontinuity termed the Great Lakes tectonic zone 

(Sims and others, 1980), which has been interpreted by 

Gibbs and others (1984) as a paleosuture. The greenstonegranite terrane can be traced by its characteristic eastnortheast-trending magnetic (Zietz, 1982) and gravity 

(Hildenbrand and others, 1982) anomalies from outcrops 

in northern Minnesota westward to central North Dakota 

Tectono-Stratigraphic Framework 9 
and South Dakota, where it is truncated by a northtrending belt of anomalies colinear with the SuperiorChurchill province boundary in Canada (Green and 

others, 1985). The Churchill province in this region has 

been termed the Trans-Hudson orogen by Hoffman 

(1981). 

The greenstone-granite terrane generally consists of 

low-grade volcanogenic rocks and intrusive plutonic 

rocks, but it can be subdivided further into volcanic and 

metasedimentary gneiss units (subprovinces; Goodwin, 

1978). Typically, the volcanic subprovinces are made up 

of several individual greenstone belts. In the United 

States, no evidence has been found of older continental 

crust on which the supracrustal rocks of the greenstone 

belt association have been deposited (Sims and Peterman, 

1981), and the geochemically "primitive" nature of the 

rocks has been interpreted as indicating direct additions 

of mantle-derived material to the crust (J ahn and Murthy, 

1975; Arth and Hanson, 1975). The mafic rocks are 

dominantly tholeiitic basalt but include komatiite (Schulz, 

1980). The less common felsic volcanic rocks, at least 

locally, have both tholeiitic and calc-alkaline affinities 

(Campbell and others, 1982). The granitoid rocks are 

mainly calc-alkaline, but relatively late tectonic and posttectonic rocks typically are alkaline (Arth and Hanson, 

1975; Day and Weiblen, 1986). 

Early Proterozoic Wisconsin Magmatic Terrane 

and Associated Epicratonic Rocks of the 

Penokean Orogen 

The Early Proterozoic Penokean orogen, as defined 

in the Lake Superior region (Cannon, 1973; Sims and 

Peterman, 1983), has a distinct tectono-stratigraphic 

zonation. Early Proterozoic epicratonic sedimentary and 

volcanic rocks, which overlie Archean basement on the 

north, are separated from a volcanic-plutonic (Wisconsin magmatic) terrane (Van Schmus, 1980) on the south 

by a major ductile shear zone (Niagara fault; Sims and 

others, 1985; Sedlock and Larue, 1985). The shear zone 

is interpreted as a suture formed at rv1.85 Ga when the 

magmatic (island) arc collided with the continental margin 

and its superjacent cover. This collision triggered the 

Penokean orogeny. 

The magmatic arc (Wisconsin magmatic terrane) is 

inferred from sparse drill-hole data and aeromagnetic and 

gravity signatures to extend in the subsurface into southern Minnesota, then through northwestern Iowa into 

northeastern Nebraska, where it is truncated by the Central Plains orogen (Sims, 1985). The epicratonic succession, on the other hand, probably terminates westward 

in central Minnesota (Southwick and Chandler, 1983; 

(fig. 5), apparently because of a northwest-trending, 

transformlike fault (Sims, 1987), although remnants of 

volcanic-sedimentary rocks of possible Early Proterozoic 

age extend discontinuously into southern Minnesota 

(Sims, 1985). 

The epicratonic rocks are a southward-thickening 

sequence of clastic and chemical sedimentary rocks as well 

as bimodal volcanic rocks and are assigned to the Early 

Proterozoic Marquette Range Supergroup in Michigan 

(Cannon and Gair, 1970) and the Early Proterozoic Mille 

Lacs and Animikie Groups in Minnesota (Morey, 1983a). 

The sequence includes the vast Lake Superior-type ironformations of the region. In Minnesota, Morey has divided the Mille Lacs and Animikie Groups into five 

depositional phases. The first two phases constitute an 

early rift to miogeoclinal succession of predominantly 

quartz rich rocks and lesser iron-formation and interlayered volcanic rocks. The third phase was a shelf 

sequence, including the major Lake Superior-type ironformations, whereas the fourth phase forms a transitional 

succession marked by rapid subsidence of the shelf and 

deposition of black carbonaceous muds. The fifth phase 

is a southward-thickening eugeoclinal succession of 

epiclastic sediments and bimodal volcanic rocks; the 

sediments are primarily graywacke deposited by 

southward-flowing turbidity currents. The bimodal 

volcanic rocks have chemical affinities with continental 

tholeiites, such as the volcanics in the Midcontinent rift 

system. It is now generally agreed that the Early Proterozoic epicratonic rocks were deposited in a basin 

(Animikie) on the passive rifted margin of the Superior 

Archean craton (Schulz and others, in press). 

The Wisconsin magmatic terrane is lithologically 

distinct from the Early Proterozoic margin accumulation 

in that it consists mainly of volcanic rocks-basalt and 

lesser rhyolite and andesite-and plutonic rocks, which 

range in composition from quartz diorite to granite 

(Schulz, 1984). An Archean basement is present in central Wisconsin but apparently is lacking in other parts 

of the terrane, although isotopic lead data (Afifi and 

others, 1984) and Nd-Sm model age data (Nelson and 

DePaolo, 1985) suggest a possible Archean component 

in the crust in the northern part of the magmatic terrane. 

At any particular locality in the magmatic terrane 

two or more volcanic successions commonly are present 

(Sims and others, 1985; LaBerge and Myers, 1984), but 

regional correlations have not been made. In the northern 

part (Ladysmith-Pembine belt), an Early Proterozoic 

migmatitic gneiss underlies dominant basalt, andesite, 

and rhyolite successions (Sims and others, 1985). Both 

the gneiss and the volcanic rocks are intruded by bodies 

of quartz diorite to granite. In the northeastern part of 

this terrane, an older succession of volcanic rocks that 

are composed of tholeiitic basalt and basaltic andesite is 

overlain by a calc-alkaline suite which ranges in composition from andesite to rhyolite (Sims and others, 1987). 

In the Monico area, near the Pelican River massive sulfide 

10 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
deposit, the volcanic rocks compose a bimodal suite of 

high-aluminum basalt to low-silica andesite pillowed 

flows and dacite to rhyolite tuffs and porphyries. Detailed 

U-Pb zircon isotopic dates from rocks in the northeastern 

part of the magmatic terrane indicate that accumulation 

of the gneisses and volcanic rocks, deformation and 

metamorphism, and emplacement of the granitoid 

plutons spanned a relatively short time interval, 

1,865-1,835 Ma (Sims and others, 1985). 

Early Proterozoic Rhyolite-Granite Terrane of 

Southern Wisconsin 

An extensive terrane of "'1. 76-Ga rhyolite and 

coeval epizonal granite is known in south-central Wisconsin from exposures in river valleys and from drill holes 

(Smith, 1983). The terrane is characterized magnetically 

by conspicuous northeast-trending, linear, positive 

anomalies, which reflect the substantial accessory 

magnetite content of the rocks. The northern edge of the 

terrane is marked by prominent east-northeast-trending 

magnetic lineaments (faults?), which separate it from 

Archean rocks to the north (Sims, 1985). The terrane is 

inferred to extend westward from known areas in Wisconsin to the Midcontinent rift system, where it is truncated. 

Probably, similar rocks extend southward into northernmost Illinois (Sims, 1985), although these rocks have a 

different geophysical expression, which includes a conspicuous positive gravity anomaly, than the known 

1.76-Ga rhyolite-granite terrane (Hildenbrand and 

others, 1982). The positive gravity anomaly suggests that 

dense rocks such as basalt exist at shallow crustal depths. 

The rhyolite-granite terrane represents the oldest 

Proterozoic anorogenic magmatic activity in the Great 

Lakes area and probably is related to extensional tectonism. These rocks consist mainly of two mineralogically 

and chemically distinct rock suites (Smith, 1983): (1) a 

peraluminous suite of texturally variable ash-flow tuffs 

and related two-mica granites, and (2) a metaluminous 

suite containing quartz- and orthoclase-bearing rhyolites 

and related biotite-hornblende to biotite granites. The 

granitoid rocks are granophyric and apparently were 

intruded into their own volcanic cover. Younger, but undated intrusions form two additional suites: (1) the Baxter 

Hollow intrusive suite, which includes low silica, high 

strontium, and REE (rare-earth element)-depleted 

granites and chemically similar rhyolite dikes, and (2) the 

Denzer intrusive suite, which consists of a diorite intrusion and numerous tholeiitic basalt to andesite dikes. 

Rocks within the rhyolite-granite terrane are mildly metamorphosed (greenschist facies) and deformed 

along northeast-trending fold axes (Smith, 1978). Cataclastic textures are widespread. The folding occurred after 

deposition of the overlying quartzite of the ''Baraboo 

interval" and has been documented most thoroughly at 

Baraboo, Wis. (Dott and Dalziel, 1972). Rb-Sr wholerock isochron ages of about 1.615-1.64 Ga on the quartzite are thought to record the tectonothermal event that 

was responsible for the folding and metamorphism of the 

rhyolite and quartzite (Smith, 1983). 

Coeval (1. 76 Ga) granitic rocks intrude the older 

Early Proterozoic rocks of the Wisconsin magmatic terrane to the north in northern Wisconsin (Anderson and 

others, 1980; Sims, 1987). 

Quartzite of Baraboo Interval 

Dott (1983) proposed that the red quartzites south 

of Lake Superior be referred to as the ''Baraboo interval'' to distinguish them from younger quartz-rich rocks 

associated spatially with the Midcontinent rift system 

(Ojakangas and Morey, 1982), and this terminology is 

used here. 

The quartzites of the Early Proterozoic Baraboo 

interval include six major exposed units scattered over 

an area of 75,000 mil (194,000 km2

) south of the Great 

Lakes tectonic zone (Sims, 1985). In addition, buried 

quartzite has been penetrated by drilling in southeastern 

Wisconsin (Smith, 1978) and eastern Iowa (Anderson and 

Ludvigson, 1986). The major exposed bodies are the 

Sioux Quartzite, which composes the Sioux ridge in 

southeastern South Dakota and adjacent areas; the 

Baraboo Quartzite near Baraboo in southern Wisconsin; 

and the Barron Quartzite in Barron and adjacent counties, northwestern Wisconsin. All these quartzite bodies 

are presumed to be approximately the same age, that is, 

in the range of 1,600-1,700 Ma (Morey and Van Schmus, 

in press). 

The quartzites of the Baraboo interval mainly consist of tightly cemented, very mature quartz arenite that 

typically is red or pink to purple because of the presence 

of disseminated hematite. The interval also contains scattered beds of conglomerate, conglomeratic quartzite and, 

locally, red mudstone (pipestone). A kaolinitic regolith 

about 50 ft ( 15 m) thick underlies parts of the Sioux 

Quartzite (Southwick and Mossier, 1984), whereas 

volcanic rocks of rhyolitic composition underlie the 

Baraboo Quartzite. Mature basal conglomerates are 

known from several localities in the Sioux and a quartzite at McCaslin Mountains, in Oconto County, Wis. 

(Olson, 1984). The Baraboo Quartzite is overlain by a 

unique sequence, from bottom to top, of slate, dolomite, 

banded iron-formation, quartzite, and slate (Dalziel and 

Dott, 1970; Geiger, 1986). Except for the Waterloo 

Quartzite in southern Wisconsin, which contains intercalated beds of an aluminous pelitic schist having 

amphibolite-facies metamorphic mineral assemblages 

(Geiger and others, 1982), the quartzites have regional 

Tectono-Stratigraphic Framework 11 
greenschist-facies mineral assemblages. Both the Waterloo and Baraboo Quartzites are moderately to strongly 

deformed on approximately east-trending fold axes, 

whereas the Sioux and Barron Quartzites are only mildly deformed. 

The volcanic rocks beneath the Baraboo Quartzite 

have been dated at 1,760± 10 Ma (Van Schmus, 1978), 

which provides a maximum age for deposition. The upper age limit is not known directly, but is inferred to be 

approximately 1.63 Ga, which is the time of widespread 

Rb-Sr whole-rock and mineral resetting in much of 

Wisconsin and adjacent areas (see Peterman and others, 

1985, for summary). 

Facies and paleocurrent analyses as summarized by 

Ojakangas (in press) indicate that the quartz-rich detritus 

of the Baraboo interval was deposited dominantly by 

fluvial processes in a braided stream environment. Dott 

(1983) and Ojakangas and Weber (1984) suggested that 

the fluvial deposits of the Baraboo and Sioux were succeeded, at least locally, by a tidally influenced marine 

environment. Most of the detritus was probably of firstcycle origin, and was derived mainly from a hinterland 

to the north-a stable, intensively weathered and leached, 

peneplaned landmass. 

The quartzites of the Baraboo interval have been 

attributed to deposition along an east-trending passive 

continental margin which was later deformed by subduction (Dott, 1983; Anderson and Ludvigson, 1986); alternatively, Greenberg and Brown (1984) proposed 

sedimentation in an intracratonic setting with deformation due to anorogenic igneous activity and orogenic doming. Southwick and others (1986) have argued that there 

is no clear-cut evidence for marine sedimentation in the 

Sioux Quartzite. Because of this and because the sedimentation was associated with rhyolitic ignimbritic volcanism, 

Southwick and others (1986) have suggested that the 

Sioux was deposited in an intracratonic setting. Deposition probably occurred within a series of fault-bounded 

en-echelon basins that were oblique to the east-west 

passive continental margin proposed by Dott (1983) but 

more or less parallel to the trend of the Central Plains 

orogen. Therefore, the basins may have formed in 

response to tectonic activity along a plate margin which 

lay well to the south. 

Early Proterozoic Metamorphic and 

Granitoid Rocks of the Central Plains Orogen 

A northwest-trending belt of metamorphic and 

granitoid rocks that crosses Nebraska, northern Kansas, 

and Missouri was delineated during preparation of the 

basement map and named the Central Plains orogen 

(Sims, 1985). Later, Sims and Peterman (1986) suggested 

that the orogen correlates with the Early Proterozoic 

foldbelt exposed in the basement uplifts of Colorado and 

southern Wyoming, and accordingly that it is part of a 

wide, continuous foldbelt extending through northern 

Arizona and New Mexico into California (Silver and 

others, 1977). 

In the northern midcontinent, the Central Plains 

orogen is an arcuate belt at least 250 mi (400 km) wide 

(fig. 2) that is reflected by conspicuous linear magnetic 

(Zietz, 1982) and gravity (Hildenbrand and others, 1982; 

Guinness and others, 1982) anomalies. In Missouri, east 

of the Midcontinent rift system, northwest-trending 

magnetic (Zietz and others, 1984) and gravity (Hildenbrand and others, 1982; Arvidson and others, 1984) 

anomalies are pronounced. These anomalies reflect 

known or inferred lithologic and structural trends in the 

basement rocks (Kisvarsanyi, 1984). The dominant gravity feature, the Missouri gravity low (fig. 3, Mgl), has 

a maximum amplitude of ""40 mGal and has been modeled as a 4-6-km crustal excess at the Mohorovicic discontinuity (Arvidson and others, 1984). To the west of the 

Midcontinent rift system, in Kansas and Nebraska, the 

magnetic and gravity anomalies are somewhat more subdued but clearly indicate a sharp westward bend in the 

orogen at the Nebraska-South Dakota State line (fig. 7). 

The northern margin of the orogen, as shown on 

figure 7, is subparallel to anomalies to the south within 

the orogen. The northeast to east trend of the anomalies 

in eastern Colorado is subparallel to the structural trends 

of the buried basement rocks (Tweto, 1987). 

The ages of the rocks within the orogen were poorly 

constrained until recently because the only ages available 

were from highly disturbed Rb-Sr whole-rock and K-Ar 

mineral systems (Van Schmus and Bickford, 1981). More 

recent U-Pb zircon determinations on the metamorphic 

rocks and mesozonal granitoid rocks indicate a range in 

age from 1.63 to 1.8 Ga (Bickford, Harrower, and others, 

1981; Bickford and others, 1986). The older (>1.7 Ga) 

ages are from gneisses and granite in Kansas and 

Nebraska, whereas the younger ages are from granite and 

metarhyolite from Missouri. The ages are grossly 

equivalent to those on Early Proterozoic metamorphic 

and granitoid rocks exposed in the basement uplifts of 

Colorado and adjacent areas (Hedge and others, 1967; 

Peterman and others, 1968; Silver and Barker, 1968; 

Stern and others, 1971; Bickford and Boardman, 1984). 

Extensive petrographic data on drill-hole samples 

show that metavolcanic and metasedimentary rocks as 

well as gneissic granitoid rocks characterize the Central 

Plains orogen in Nebraska (Lidiak, 1972), Kansas 

(Bickford, Harrower, and others, 1981), and Missouri 

(Kisvarsanyi, 1974, 1984). The metamorphic rocks are 

mainly amphibolite facies but include greenschist- and 

granulite-facies assemblages as well as retrogressive 

assemblages. Textures range from granoblastic to 

cataclastic and mylonitic. The principal metamorphic 

12 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
0 150 300 450 KILOMETERS 

EXPLANATION 

~ Positive anomaly 

Itt: {d Negative anomaly 

-- Terrane boundary 

Figure 7. Gravity anomaly map of part of north-central United 

States. After Hildenbrand and others (1982). 

rocks are quartz-feldspar gneiss, biotite-, hornblende-, 

and quartz-muscovite schist, amphibolite, metarhyolite, 

marble, micaceous quartzite, and phyllite. The granitoid 

rocks range from quartz diorite through granodiorite to 

granite (Lidiak, 1972). Treves and Low (1985) stated that 

quartzite in western Nebraska contains sillimanite and 

muscovite and apparently lies on older granitic and 

metamorphic rocks. 

In central and western Missouri, where about 35 

holes have been cored in Precambrian rocks, several rock 

types of the orogen are associated with the northwesttrending Central Missouri ~opographic high (Kisvarsanyi, 

1974). The orogenic suite includes garnet-bearing quartzmicrocline gneiss, biotite schist, muscovite-talc schist, 

quartzite, forsterite marble, sillimanite-bearing quartzmicrocline gneiss, kyanite-bearing granite gneiss, 

metarhyolite, and amphibolite. 

The orogenic suite is intruded locally by layered 

mafic complexes of gabbro-norite composition, which are 

commonly associated with pronounced positive magnetic 

anomalies and appear to have been . emplaced along 

northwest-striking structural zones (Kisvarsanyi, 1984). 

Some are metamorphosed to metagabbro; most are pervasively intruded by 1.47-Ga granite (Bickford, Harrower, and others, 1981), which has resulted in strongly 

hybridized phases (epidiorite and migmatite). Sylvester 

(1984) suggested that some of the mafic complexes were 

formed under granulite-facies conditions in deep crustal 

levels and were tectonically uplifted to their present levels 

where they underwent retrograde metamorphism to produce the amphibolite-facies mineral assemblage that now 

exists. 

The rocks of the Central Plains orogen closely 

resemble the Early and Middle Proterozoic rocks in Colorado (Tweto, 1979) that include a diverse gneiss 

complex-rocks metamorphosed from a pre-1.7-Ga 

assemblage of volcanic, sedimentary, and intrusive igneous rocks-and granitoid rocks of the Routt and Berthoud Plutonic Suites (Tweto, 1977; Taylor and others, 

1984). The Early Proterozoic Routt Plutonic Suite is composed of 1.65-1. 76-Ga igneous rocks, which are mainly 

calc-alkaline granodiorite and quartz monzonite; the Middle Proterozoic Berthoud Plutonic Suite includes 

1.35-1.48-Ga granitic rocks (Peterman and others, 1968; 

Thomas and others, 1984), which mainly are peraluminous two-mica granites. The rocks of the Routt 

Plutonic Suite were emplaced late in the principal period 

of regional metamorphism and are generally foliated to 

intensely deformed, whereas the rocks of the Berthoud 

Plutonic Suite were emplaced after the main regional 

metamorphism and have a dominant primary flow 

structure. 

In Missouri, Kisvarsanyi (1984) has identified five 

principal northwest-trending tectonic zones on the basis 

of surface and subsurface geology and magnetic lineaments (Zietz and others, 1984; Sims, 1985). The tectonic 

zones are mainly basement faults (fig. 9); they coincide 

with major structures in the overlying Paleozoic strata. 

For example, the Bolivar-Mansfield tectonic zone is 

named after the Bolivar-Mansfield fault system, a zone 

of faults and folds mapped in the Paleozoic sedimentary 

rocks (McCracken, 1971). Basement tectonic zones similar to those in Missouri have been delineated in Kansas 

and Nebraska (Sims, 1985). These coincide with the Central Kansas uplift and the Cambridge and Chadron 

arches. Clearly, reactivation of the basement structures 

in the Paleozoic produced folds and faults in the overlying sedimentary rocks. 

A possible extension of the Central Plains orogen 

on the southern side of the Middle Proterozoic St. 

Francois granite-rhyolite terrane is suggested by core 

recently acquired by the Arkansas Geological Commission from a drill hole in Randolph County, northeastern 

Arkansas. About 6.5 ft (2 m) of muscovite-biotite 

quartzofeldspathic gneiss-probably a metamorphosed 

Tectono-Stratigraphic Framework 13 
argillaceous sandstone-was cored; the upper part is 

highly weathered (R.E. Denison, Mobil Research and 

Development Corp., written commun., 1986). 

Middle Proterozoic Anorthosite and Rapakivi 

Granite of Transcontinental Anorogenic Province 

A moderately well defined belt of anorogenic 

granitoid rocks (")1.4-1.5 Ga) that extends across North 

America from Labrador to southern California has been 

delineated in recent years (Silver and others, 1977; 

Denison and others, 1984). The rock types range from 

anorthosite, mangerite, and associated rapakivi granite, 

as in the Wolf River batholith in Wisconsin (Van Schmus 

and others, 1975), to two-mica granite, such as the Middle Proterozoic Graniteville Granite in southeastern 

Missouri (Kisvarsanyi, 1980) and the Middle Proterozoic 

Silver Plume Granite (Berthoud Plutonic Suite) and other 

similar granites in Colorado and elsewhere in the 

southwest (Anderson, 1983). The former are marginally 

metaluminous and generally contain biotite and (or) hornblende; the latter are marginally peraluminous and contain topaz as an accessory mineral. 

In the northern midcontinent, this group of 

anorogenic granitoid rocks is represented by the Wolf 

River batholith (Anderson, 1980; Anderson and Cullers, 

1978), the Red Willow batholith of southern Nebraska 

(Van Schmus and Bickford, 1981), and unnamed plutons 

in northern Illinois (Hoppe and others, 19&3) and adjacent Iowa. These rocks are similar petrochemically to 

epizonal granites in the region that have granophyric textures; the epizonal granites are coeval and cogenetic with 

rhyolites of the Middle Proterozoic St. Francois terrane 

(age, 1.48 Ga; Bickford, Harrower, and others, 1981) and 

Spavinaw terrane (age, 1.35-1.40 Ga; Thomas and others, 

1984). 

The anorogenic granites of the Transcontinental 

anorogenic province have distinctive mineralogies reflecting strongly differentiated potassic and iron-enriched 

compositions (Anderson, 1983). Alkali feldspars are 

predominant over plagioclase; mafic silicates are iron 

rich; and the accessory mineral suite includes ubiquitous 

fluorite as well as topaz, spinel, allanite, sphene, '!nd 

cassiterite (Kisvarsanyi, 1980). These characteristics clearly distinguish the anorogenic rocks from the older 

synorogenic, calc-alkaline plutons. Except for the Wolf 

River batholith, which is ilmenite-bearing, the anorogenic 

bodies are magnetite-bearing, and hence belong to the 

magnetite series of Ishihari (1977). Chemically, the 

anorogenic granites differ from other granitic suites in 

having high K20, K2

0/Na20, iron/magnesium, and 

fluorine and low CaO, MgO, and Al2

0 3 (Anderson, 

1983). The same major element chemistry is typical of 

the anorogenic plutons and associated rhyolite of the St. 

Francois terrane (Kisvarsanyi, 1972). Also, the anorogenic plutons of the Transcontinental anorogenic province are enriched in many LILE (large-ion lithophile 

elements), including rubidium, barium, gallium, yttrium, 

REE (except europium), zirconium, thorium, niobium, 

tin, beryllium, lithium, and uranium. In Missouri, many 

of these granites have been described as "tin granites" 

(Kisvarsanyi, 1980). 

Cullers and others ( 1981) have presented convincing arguments that most of the granite bodies of the 

Transcontinental anorogenic province have been derived 

from nonradiogenic crustal sources of Early Proterozoic 

age. For the Wolf River batholith, they have suggested 

that the anorthosite and (or) mangerite likely provided 

at least part of the heat source necessary for generation 

of the granitic melt. 

Because of the mineralogical, major element, and 

trace element similarities between rocks of the Transcontinental anorogenic province and rocks of the Middle Proterozoic epizonal terranes of the southern midcontinent, 

the former may merely correspond to deeper levels, now 

exposed by erosion, of the latter. In the epizonal terranes 

of the southern part of the area (fig. 2), unmetamorphosed , volcanic rocks are associated with co genetic 

granites. The chemically and mineralogically similar 

plutons of the Transcontinental anorogenic province, on 

the other hand, generally perforate older crust and are 

surrounded by older rocks; they appear to be "isolated" 

or circumscribed plutons, and their textural features 

reflect their emplacement at deeper crustal levels. We suggest that these mesozonal granitoids, which are coeval 

with Middle Proterozoic epizonal terranes, are possibly 

feeders or conduits for epizonal rocks that were removed 

by erosion before the onset of the Phanerozoic. Thus, 

the Transcontinental anorogenic province is possibly not 

a distinct tectonic and magmatic unit but merely a 

mesozonal equivalent of the epizonal terranes. 

Middle Proterozoic St. Francois 

Granite-Rhyolite Terrane 

The St. Francois terrane is one of two Middle Proterozoic epizonal granite-rhyolite terranes in the midcontinent, the other being the Spavinaw. About 900 km2 

(350 mF) of the terrane are exposed in the St. Francois 

Mountains, at the crest of the Ozark dome in southeastern 

Missouri (Tolman and Robertson, 1969; Anderson, 1970; 

Berry and Bickford, 1972; Kisvarsanyi, 1972; Pratt and 

others, 1979; Sides and others, 1981). Data from more 

than 500 drill holes indicate that the terrane extends outward from outcrop areas, below a cover of Paleozoic 

rocks; the terrane underlies most of southeastern Missouri 

(Kisvarsanyi, 1981) and has an inferred areal extent of 

at least 35,000 mi2 (90,000 km2). Its outer boundaries 

14 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
are not well defined because drill-hole data are sparse to 

the east toward the Illinois basin and to the south into 

the Mississippi embayment. However, Hoppe and others 

(1983) suggest that a similar granite-rhyolite terrane 

underlies most of Illinois, Indiana, and western Kentucky, 

as far east as the Grenville boundary, as shown on the 

regional map of Bickford and others (1986). U-Pb ages 

on zircons indicate a 1.48-Ga crystallization age for the 

rocks in the St. Francois terrane, but it does contain some 

younger (1.38 Ga) plutons (Bickford and Mose, 1975; 

Bickford, Harrower, and others, 1981). The terrane could 

extend as far west as the Decaturville cryptoexplosion 

structure (Offield and Pohn, 1979) and the Orla mafic 

complex (Kisvarsanyi, 1985), in central Missouri, as suggested by 1.47-Ga plutons which intrude the older metamorphic rocks in that area. 

The correlation of surface and subsurface geologic 

data and analysis of aeromagnetic maps in southeastern 

Missouri have shown that the St. Francois terrane apparently 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. 8; Sides and others, 1981; Kisvarsanyi, 1980, 1981). These volcano-tectonic 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 largely removed 

by pre-Paleozoic erosion, as much as 5,500 ft (1 ,680 m) 

of rhyolite ash-flow tuff are preserved locally. These 

rocks are thickest in the area of the Taum Sauk volcanotectonic depression (Anderson, 1970; Cordell, 1979) or 

caldera (Berry and Bickford, 1972) in the St. Francois 

Mountains. 

Most of the volcanic rocks are confined to a northwesttrending (fig. 8) topographic high that forms a buried 

ridge of knobs extending as far northwest as the Missouri 

River (see figure 3 in 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, the 

Grand River and Northeast Missouri tectonic zones 

(represented by faults of the same name on fig. 9), as 

defined by Kisvarsanyi (1984), suggests that their 

emplacement and distribution are controlled mainly by 

structures in the older terrane. Mesozonal granite of the 

same age as the St. Francois terrane (Bickford, Harrower, 

and others, 1981), which is part of the Transcontinental 

anorogenic province, has been identified along the northwestward extension of the St. Francois volcanic terrane, 

in north-central Missouri. 

The volcanic rocks of the St. Francois terrane are 

predominantly rhyolite ash-flow tuffs containing very 

high Si02 , K20/Na20, iron/magnesium, and fluorine, 

and low CaO, MgO, and Al20 3 (Kisvarsanyi, 1972; 

Bickford, Sides, and Cullers, 1981). They are characterized by perthitic alkali feldspar phenocrysts and iron-rich 

mafic minerals, including fayalite, ferrosilite, and ferrohastingsite. The mineralogy of the silicic volcanic rocks 

indicates that they have silica-oversaturated alkaline to 

peralkaline chemistry. Although some of the rocks are 

transitional to comendites, their agpaitic index (molecular 

ratio sodium +potassium/aluminum) is always less than 

one (Kisvarsanyi, 1981). Although the volcanic suite is 

bimodal because of the presence of minor basaltic flows 

interlayered with rhyolite in the Taum Sauk area (Blades 

and Bickford, 1976), intermediate and mafic rocks are 

rare. The intermediate rocks are chiefly trachyte and 

trachyandesite (Anderson, 1970; Kisvarsanyi, 1981). The 

volcanic suite is distinguished from those of calc-alkaline 

petrogenetic provinces by the absence of andesites. 

The granitic rocks of the St. Francois terrane have 

been classified into three distinct types based on composition and mode of occurrence (Kisvarsanyi, 1980, 1981): 

(1) subvolcanic massifs, (2) ring intrusions, and (3) central plutons. The subvolcanic massifs, which are the intrusive equivalents of the comagmatic rhyolites, are 

typical epizonal rocks having granophyric texture and perthitic alkali feldspar; biotite is the characteristic mafic 

mineral and magnetite is ubiquitous. Near the contact 

with the intruded rhyolites, the subvolcanic massifs consist almost entirely of fine-grained granophyre; at depth, 

they grade into medium- to coarse-grained rapakivi 

granite and thus form one part of the "anorogenic trinity" of Anderson (1983). The subvolcanic massifs are inferred to be the most widespread component of the St. 

Francois epizonal terrane (fig. 8). Wherever they are identified in drill holes, they are assumed to have once been 

covered by rhyolite that subsequently was stripped off. 

The ring intrusions are intermediate- to high-silica 

rocks whose emplacement is inferred to have been controlled by ring fractures related to caldera collapse and 

cauldron subsidence (fig. 8). 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 are typically highsilica, evolved LILE two-mica granites having distinctive 

accessory minerals and trace element suites (Kisvarsanyi, 

1980, 1981). The accessory minerals include abundant 

fluorite, topaz, apatite, spinel, allanite, sphene, and 

cassiterite. Because of the granites' relative enrichment 

in tin, lithium, beryllium, rubidium, barium, yttrium, 

niobium, uranium, thorium, and fluorine, they have been 

described as "tin granites" (Kisvarsanyi, 1981). They are 

inferred to have been emplaced in resurgent cauldrons 

and are circular to oval in plan; they have distinctive 

associated negative magnetic anomalies. 

Tectono-Stratigraphic Framework 15 
Steelville 0 

o Bonne Terre 

0 15 30 MILES 

0 15 30 45 KILOMETERS 

goo 

EXPLANATION 

lilliliilililii 

~ 

D 

Central 

pluton 

Ring 

intrusion 

Subvolcanic 

granite 

Volcanic 

terrane 

LOCATION MAP 

Figure 8. Integrated surface and subsurface geologic map of the St. Francois terrane, southeastern Missouri. Modified from 

Kisvarsanyi (1981 ). 

As discussed in the section on the Transcontinental 

anorogenic province, at least some of the 1.45-1.48-Ga 

granitoid plutons outside of the St. Francois Mountains 

possibly are deeper analogues of the epizonal St. Francois terrane. Both rapakivi massifs and "tin granite" 

plutons, which intruded and partly assimilated rocks of 

the older crustal terrane, have been identified in the 

buried basement. That unmetamorphosed volcanic rocks 

of Middle Proterozoic age are preserved at all in southeastern Missouri is fortuitous; these rocks must have been 

protected in low-lying depressions. The older crustal terrane, the Central Plains orogen, is inferred to underlie 

the younger epizonal terranes to the south. Neodymiumsamarium isotope data of Nelson and DePaolo (1985) imply a 1.8-2.0-Ga crustal source from which the epizonal 

rocks in the southern midcontinent were derived by partial melting. This is consistent with the earlier conclusion 

of Cullers and others ( 1981) that some of the rocks of 

the St. Francois terrane crystallized from primary crustal 

melts, and that most of the high-silica rocks crystallized 

from highly evolved magmas that were 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. A rift-related genesis for the terrane has 

been proposed by G. Kisvarsanyi (1975), an idea that was 

further developed by E.B. Kisvarsanyi (1980) as supportive trace element data (Viets and others, 1978) became 

available. The terrane has many of the petrological and 

geochemical attributes of Proterozoic rifts proposed for 

North America (Emslie, 1978; Anderson, 1983) and lacks 

only anorthosite massifs, which have not been encountered in drill holes. In contrast to the younger Keweenawan rift system, the 1.48-Ga tensional event(s) did 

not lead to extensive crustal separation and the emplacement of large volumes of basaltic magma in the rift(s). 

16 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
Middle Proterozoic 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 fro in 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 represented in surface expo.sures by a few small outcrops of micrographic granite 

porphyry near Spavinaw, in Mayes County, northeastern 

Oklahoma. More than 300 drill holes, however,.indicate 

that this terrane underlies most of northeastern Oklahoma, eastern Kansas, southwestern Missouri, and northern Arkansas (Denison, 1981, 1984; Kisvarsanyi, 1984; 

Thomas and others, 1984; Sims, 1985) and, thus, is an 

extensive Middle Proterozoic epizonal terrane. As used 

in this report, the Spavinaw terrane includes all the rocks 

of the Northeastern Oklahoma province (Washington 

County Volcanic Group, Spavinaw Granite Group, Osage 

County Microgranite), and the informal Central 

Oklahoma granite group mapped in the subsurface by 

Denison (1981). 

The volcanic rocks of the terrane (Washington 

County Volcanic Group of Denison, 1981) are dominantly rhyolite Oocally metamorphosed to hornfels) and minor 

dacite, trachyte, and andesite. Data from northeastern 

Oklahoma (Denison, 1981) suggest that rocks of intermediate composition are more widespread and (or) 

volumetrically more 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 virtually identical to those in 

the St. Francois terrane (Bickford, Harrower, and others, 

1981). 

The volcanic rocks of the Spavinaw terrane are on 

the northern and southern flanks of the broad, presediment Spavinaw arch (Denison, 1966), which extends from 

central Oklahoma east-northeastward 240 km (150 mi) 

into southwestern Missouri. The arch is underlain by 

micrographic granite porphyry and fine-grained granophyre of the Spavinaw Granite Group of Denison (1981). 

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 1.35-1.40-Ga rapakivi plutons (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. Some of the rapakivi 

granites in Missouri contain as much as 20 ppm tin, 

30 ppm niobium, and 5 ppm beryllium (E.L. Mosier, 

written commun., 1985) and have geochemical signatures 

similar to the "tin granites" identified in the St. Francois terrane (Kisvarsanyi, 1980). 

The rocks of the Spavinaw terrane are assumed to 

have formed in a tectonic regime similar to those of the 

St. Francois terrane. If a prerift, mantle-arching, crustalmelting, tensional tectonic environment applies to the St. 

Francois terrane, similar conditions and processes are 

likely to have produced the Spavinaw terrane. This implies that thermal anomalies in the mantle and attendant 

magma generation shifted southwestward with time. Although the 1.35-1.40-Ga Spavinaw rocks form a fairly 

well defined, apparently continuous terrane in the fourstate area of Kansas, Missouri, Oklahoma, and Arkansas, at least one 1.38-Ga pluton has been identified near 

the center of the exposed St. Francois terrane in southeastern Missouri (Bickford and Mose, 1975; Thomas and 

others, 1984). Other "exceptions" are likely to be present. They may signify that thermal activity was nearly 

continuous in the central midcontinent during the Middle Proterozoic, but that it culminated in different areas 

at different times. 

Middle(?) Proterozoic Sedimentary Rocks 

Clastic sedimentary rocks of low metamorphic 

grade have been penetrated by drilling at three localities 

in southwestern Missouri and adjacent parts of Kansas 

and Arkansas (fig. 17): (1) Vernon County, Mo., and adjacent Bourbon County, Kans., (2) Cedar County, Mo., 

and (3) Fulton County, Ark. The rocks apparently mainly 

occupy a graben between the Chesapeake and BolivarMansfield tectonic zones. The rocks at the first two 

localities were assigned to the X3m unit (Metamorphic 

rocks, undivided) on the open-file map (Sims, 1985), but 

they were designated Y s (Clastic sedimentary rocks) on 

the colored geologic map (Sims, in press) because of their 

distinctive lithology and low metamorphic grade. Subsequent to compilation of the basement map, the Arkansas Geological Commission obtained core from a drill 

hole in Fulton County, Ark., that penetrated sedimentary rocks (R.E. Denison, Mobil Research and Development Corp., written commun., 1986); the location of this 

hole is shown on figure 17. The rocks from the three 

localities appear to be parts of a distinctive sedimentary 

assemblage of presumed Middle Proterozoic age. 

In Vernon County, southwestern Missouri, a 

300-m-thick (984-ft-thick) succession of red clastics was 

described by Skillman (1948) as pre-Upper Cambrian. 

The rocks include feldspathic quartzite, argillite, and 

metaconglomerate containing rhyolite and granite clasts. 

The rocks are variably altered; albite and epidote are characteristic of the most intensely altered rocks, at deeper 

levels, whereas quartz and magnetite are characteristic 

Tectono-Stratigraphic Framework 17 
of the less intensely altered zones. Pyrite is conspicuous 

in argillite layers. Other alteration minerals are chlorite, 

sericite, calcite, and muscovite. The alteration minerals 

replace primary constituents, both as disseminations and 

as veinlets. Skillman (1948) recognized two stages of 

alteration: (1) extensive albitization resulting in almost 

complete local replacement of clastic sediments; and (2) 

later introduction of quartz, epidote, chlorite, muscovite, 

magnetite, calcite, and associated minerals. The sedimentary rocks lie on epizonal volcanic rocks of the Spavinaw 

terrane (Denison, 1966), which in turn appear to overlie 

rocks of the Central Plains orogen-indicated by a UPb zircon age of 1.65 Ga from a mylonitized granite core 

drilled within the graben (W.R. Van Schmus, University 

of Kansas, oral commun., 1986). 

In Cedar County, Mo., 80 ft (24m) of low rank 

argillite, meta-arkose, and metaconglomerate were 

penetrated in a drill hole near Stockton (Kisvarsanyi, 

1985). The fine-grained rocks are laminated and contain 

finely contorted layers of sericite and abundant granablastic magnetite and ilmenite. The metaconglomerate 

contains clasts of quartzite. The sedimentary rocks appear to overlie mesozonal granite of the Spavinaw terrane (age, 1.35-1.4 Ga). 

In Fulton County, Ark., 10ft (3m) of hornfelsic 

conglomerate were cored (R.E. Denison, Mobil Research 

and Development Corp., written commun., 1986). Clasts 

of both igneous (rhyolite, granite, and micrographic 

granite) and low-rank metasedimentary rocks occur in a 

sandy argillaceous matrix. The basement on which the 

rocks were deposited is unknown. These clastic rocks appear to represent remnants of a succession of pre-Late 

Cambrian age of unknown areal extent in the midcontinent region; other sedimentary rocks of Late Proterozoic age have been penetrated sporadically in southern 

Illinois, western Tennessee, and Kentucky (Denison and 

others, 1984), but their relationship to those described 

here is uncertain. 

The lithologies of the clastic rocks suggest that they 

were formed in local basins, possibly as a result of differential uplift caused by Middle Proterozoic reactivation of the Chesapeake and Bolivar-Mansfield basement 

tectonic (shear) zones (fig. 17). The tectonic instability 

probably occurred during or subsequent to the volcanism 

in the Spavinaw terrane (1.35-1.4 Ga). Pull-apart basins, 

comparable to the much younger Ridge basin in the San 

Andreas fault system in California (Crowell, 1974), are 

possible sites for deposition of the sediments. 

The alteration minerals that record the mild metamorphism of the sedimentary rocks probably formed from 

low-temperature, alkaline hydrothermal solutions that circulated through the clastic accumulation. The shear zones 

might be deeply penetrating structures that tapped warm 

alkaline waters. A regional metamorphism is not feasible 

as an explanation for the replacement minerals inasmuch 

as the rocks of the subjacent St. Francois and Spavinaw 

terranes are virtually unmetamorphosed. 

Middle Proterozoic Midcontinent Rift System 

The "Midcontinent rift system" is an informal term 

referring to the geological and geophysical record of intracontinental rifting in the North American continent 

during Middle Proterozoic time, about 1 ,200-1,000 Ma 

(Van Schmus and Hinze, 1985). Rifting was accompanied 

by the massive upwelling of mantle-derived magmas with 

the solidification of mafic plutonic rocks at depth and 

widespread volcanism and clastic sedimentation at the 

surface (fig. 2). 

Rift-related rocks are well developed in the Lake 

Superior region where a wide variety of mafic plutonic 

rocks, their coeval volcanic rocks, and derivative sedimentary rocks are exposed along both shores of Lake 

Superior. These rocks disappear beneath Paleozoic strata 

100 mi (160 km) south of the western end of Lake 

Superior, but they continue southward in the subsurface 

for more than 600 mi (1 ,000 km) as a belt of volcanic 

and sedimentary rocks some 25-50 mi (40-85 km) wide 

(King and Zietz, 1971). The mafic rocks yield the so-called 

''Midcontinent gravity high,'' one of the largest positive 

gravity anomalies in the United States. A related arm of 

the rift system has been identified by geophysical data 

at the eastern end of Lake Superior (Hinze and others, 

1975). This arm extends southeastward beneath Paleozoic 

strata of the Michigan basin to a point where it is apparently truncated by the Grenville front. 

The rift consists grossly of a medial horst of basaltic 

and lesser rhyolitic volcanic rocks. The horst is partly 

covered by epiclastic rocks, bounded by high-angle faults, 

and flanked by half -graben basins filled with epiclastic 

rocks. Two layered gabbroic bodies, the Duluth Complex in northeastern Minnesota and the Mellen Intrusive 

Complex in northwestern Wisconsin, flank the main rift 

in the exposed area. Other gabbroic bodies have been 

mapped in the subsurface of southeastern Minnesota and 

northeastern Iowa (Sims, 1985) from geophysical anomalies and a few drill-hole penetrations. 

The Keweenawan lavas exposed in the Lake Superior region constitute one of the world's major plateau 

or flood basalt provinces. The lavas and associated intrusions are typically tholeiitic in composition, as are other 

major plateau lavas, but they have a wide range of compositions (Green, 1982, 1983). Although dominated by 

olivine tholeiites and transitional basalts, the lavas also 

include large volumes of tholeiitic basaltic andesites and 

locally abundant rhyolites and icelandites. Green (1983, 

p. 419) stated that "The initial flows * * * are slightly 

more alkaline than succeeding eruptions, yet high in 

nickel and chromium as well as incompatible elements, 

18 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
but the most abundant types are (a) high-A! olivine 

tholeiites with variable iron/magnesium ratios and 

undepleted light rare-earth elements that show strong 

resemblance to some mid-ocean ridge basalts, and (b) 

high-iron transitional basalts or iron-titanium tholeiites 

which resemble the bulk of younger major continental 

tholeiitic provinces." Green (1983) prefers a mantlederivation for the rocks; they could have resulted from 

about 30 percent melting of mantle spinel lherzolite. 

Lead, strontium, and neodymium isotopic studies show 

the mantle source to be as old as 4 Ga (Leeman, 1977). 

The large relative volume of rhyolite seems too great to 

be accounted for by fractional crystallization of mantle 

melts (Green, 1983), but isotopic data imply little crustal 

contamination (Van Schmus and others, 1982). 

The epiclastic rocks in the Lake Superior region are 

of alluvial-fluvial and locally lacustrine origin. An older 

suite, which locally is intercalated with lava flows, consists of red shale and sandstone of lithic composition. 

These rocks were derived from sources within the rift and 

were deposited in several fault-bounded basins along the 

axis of the rift. In Wisconsin this sequence is called the 

Oronto Group (Middle Proterozoic); it is notable for a 

medial, dark, carbonaceous and pyritic shale and siltstone 

unit called the Nonesuch Formation (Daniels, 1982; 

Elmore, 1984). A younger suite, called the Bayfield 

Group (Middle Proterozoic) in Wisconsin, also is of 

alluvial-fluvial and locally lacustrine origin. It consists 

of red shale and sandstone of arkosic, feldspathic, and 

quartzose affinity. These rocks were derived from sources 

outside the rift and were deposited in large, half-graben 

basins along the flanks of the rift. 

In the Lake Superior region, the stratified rocks of 

volcanic and sedimentary origin have been assigned to 

the Keweenawan Supergroup, and this name also has 

been applied to similar rocks in other parts of the rift 

system. Because sedimentary rocks of both Oronto and 

Bayfield affinity have been recognized, these names also 

have been used in buried parts of the rift system, even 

though the various subsurface units are not continuous 

or necessarily correlative with those exposed in the Lake 

Superior region. 

Although an aborted intracratonic rift model is 

generally accepted for the rift system, structural details 

concerning the geometry of the rift and its consequent 

effect on volcanism and sedimentation are debatable (see 

Weiblen and Morey, 1980; Green, 1983, for discussion). 

Furthermore, although several attempts have been made 

to produce integrated models of relationships between the 

Midcontinent rift and deformation associated with the 

Grenville front (Gordon and Hempton, 1986), many 

details remain to be established. 

Recent modeling of gravity and aeromagnetic data 

in Minnesota and adjoining parts of Wisconsin (McSwiggen and others, 1986) has shown that the basic structure 

of the rift is more complex than the simple horst-graben 

relationship envisioned in many previous structural 

analyses (for example, Craddock and others, 1963; 

Mooney and others, 1970). Mafic volcanic rocks in the 

central part of the horst are as much as 9.3-12.5 mi (15-20 

km) thick, whereas the sedimentary rocks in the flanking basins are only about 2.5 mi (4 km) thick. Thus, this 

part of the Midcontinent rift system has a gross structural geometry similar to that observed in the Rio Grande 

rift in the southwestern United States (Cape and others, 

1983). Furthermore, modeling of seismic data from the 

southern (Kansas) part of the Midcontinent rift (Serpa 

and others, 1984) indicates that structural details in the 

upper part of the rift are markedly similar to those 

observed in the upper part of the Rio Grande rift. The 

seismic model takes into account the probable difference 

in the response of the upper and lower crust to extensional stresses, a factor that was not considered in earlier 

models of the rift system. 

ORIGIN AND AGE OF TRANSCURRENT FAULTS 

Northwest-trending faults are abundant in the basement rocks of north-central United States (fig. 9), and 

they appear to be part of a family of faults of this trend 

in the Superior craton and flanking Proterozoic terranes 

in Canada and the United States. The faults transect rocks 

ranging in age from Archean to Middle Proterozoic; those 

in Archean terranes have been shown to be dextral 

transcurrent faults (Sims, 1976a) that formed initially in 

Late Archean time and were reactivated at least in part 

in Proterozoic time. 

A classic example of a dextral transcurrent fault in 

an Archean terrane is the Vermilion fault system in northern Minnesota (fig. 10). This fault system consists of 

anastomosing fault strands, each of which shows dextral 

movement; the aggregate horizontal displacement on the 

fault system is 10.5-11.8 mi (17-19 km) (Sims, 1976a). 

Subsidiary complementary faults have sinistral movement, showing lateral offsets of as much as 4.3 mi (7 km) 

(Waasa fault). Faults of both sets are ductile shear zones 

characterized by mylonitization and locally by silicification. Hudleston and Southwick (1984) have suggested a 

mechanism for the origin of the dextral faults; they proposed that the Vermilion fault was formed during Late 

Archean time as part of a continuum of dextral shear of 

regional extent. Regional folding that produced easttrending structures was followed by later faulting, which 

was simply a more brittle expression of the shear regime. 

They attributed the deformation to transpression: oblique 

compression between two more rigid crustal blocks to the 

north and south. A similar structural regime occurred 

along the United States-Canada border (Day and Sims, 

1984; Poulson, 1983) and in areas northward to Hudson 

Bay (Card and Ciesielski, 1986). 

Origin and Age of Transcurrent Faults 19 
\ 

\ 

\~~ ---~oJ SD 1 

1.. 

I 

I 

goo 

\ 

r.·.·.·.•.•.•.•.•.•.'1l.4- _J~ 

'--~­

') IL 

40° 

0 100 KILOMETERS 

EXPLANATION 

1:::::::)\{J Midcontinent rift system 

LL....L...J Outline of Archean Superior craton 

FAULTS 

Vermilion 8 Central Kansas 

2 Quetico 9 Des Moines River 

3 Rainy Lake- 10 Northeast Missouri 

Seine River 

11 Grand River 

4 Reservation 

12 Central Missouri 

5 Cedar Creek 

13 Bolivar-Mansfield 

6 Chadron 

7 Cambridge 

14 Chesapeake 

Figure 9. Major known and inferred transcurrent faults in north-central United States. 

20 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
0 

I 

0 

I 

4 

4 

I 

I 

8 

I 

12 

8 

I 

I 

12 MILES 

I 

16 KILOMETERS 

Figure 10. Map showing the Vermilion fault system of dextral transcurrent faults and related sinistral faults, Vermilion 

district, northern Minnesota. Country rocks are Archean metamorphic and granitoid rocks, except as noted. Modified from 

Sims (1976a, fig. 4) with additions from Sims and Southwick (1985). 

Northwest-trending dextral faults also transect the 

Archean greenstone-granite terrane in northern Michigan 

and Wisconsin (Morey and others, 1982), the Archean 

gneiss terrane in southern Minnesota (Sims, 1985), and 

the Early Proterozoic Wisconsin magmatic terrane. 

Several faults of Archean age in southwestern Minnesota and adjacent areas were reactivated in the Proterozoic. Southwick and Mossier (1984) have postulated 

that northwest-trending faults that border basins of Sioux 

Quartzite were active during sedimentation, as indicated 

by alternating thick coarsening- and fining-upward sequences. In northeastern Nebraska, Houser (in Houser 

and Gray, 1980) has shown that reactivation of a major 

northwest-trending fault (Reservation fault, fig. 9) continued, or was renewed, in the Paleozoic. This fault 

borders the Sioux Quartzite body on the southwest (Sims, 

1985); the Sioux is about 3,800 ft (1, 150 m) thick immediately northeast of it. Southwest of this fault, the 

basement is Precambrian crystalline rocks overlain directly by Ordovician strata, which indicates that the northeastern side was downdropped during or shortly after 

deposition of the Sioux. Stratigraphic patterns within the 

Paleozoic strata indicate that the fault acted as a hinge 

line· (southwestern side down) that controlled the position of Paleozoic shorelines. Paleozoic rocks are absent 

immediately northeast of the fault, whereas they thicken 

southwestward from it. Fault movement had ceased by 

Cretaceous time because there is no apparent vertical 

displacement of the base of Cretaceous strata on opposite 

sides of the fault. Today, the fault is the locus of abnormally high temperature ground water in the Upper 

Cretaceous Dakota Formation, the principal aquifer in 

the area. 

The kinematics of the faults to the south of those 

in outcrop areas, known only from the subsurface, is 

equivocal. However, the faults are tentatively included 

in the family of transcurrent faults, although they transect 

Proterozoic rocks and show no evidence of an Archean 

ancestry. These structures have been referred to, in Iowa, 

as "structural zones" (R.R. Anderson, Iowa Geological 

Survey, written commun., 1985) and, in Missouri, as 

"tectonic zones" (Kisvarsanyi, 1974, 1984). Although 

their origin is obscure, they have two things in common: 

they coincide with known structures in overlying 

Origin and Age of Transcurrent Faults 21 
Paleozoic rocks and they clearly were reactivated in 

Paleozoic time. 

A major northwest-trending fault in Montana and 

adjacent South Dakota (Peterman and Sims, in press), 

here termed the Cedar Creek fault (fig. 9), underlies the 

Cedar Creek or Glendive-Baker anticline (Gilles, 1952). 

This structure is expressed at the surface in Upper 

Cretaceous rocks as an asymmetrical anticline trending 

N. 30° W. and having a steeper southwestern limb. An 

isopach of the Phanerozoic rocks (Jensen and Mitchell, 

1972) indicates as much as 1,100 ft (330m) of vertical 

displacement of the basement surface across the anticline; 

the northeastern side moved upward. Gravity values 

decrease as much as 50-70 mGal across a steep gradient 

coincident with the axis of the anticline (Simpson and 

others, 1986), and magnetic values are markedly higher 

on the downthrown side as compared to the upthrown 

side (Zietz, 1982). The fault is interpreted (Peterman and 

Sims, in press) as the western boundary of the TransHudson orogen in Montana, with Archean rocks of the 

Wyoming craton on the western side. The fault is projected southeastward into Nebraska (fig. 9) on the basis 

of magnetic and gravity data. A subparallel fault, here 

termed the Chadron fault, coincides with the Chadron 

arch; the southwestern side of this fault also moved 

downward (Bayley and Muehl berger, 1968). A possible 

northwestern extension of this fault is the set of 

northwest-trending transcurrent faults in the Black Hills 

uplift (Redden and Norton, 1975). Faults that coincide 

with the Cambridge arch and the Central Kansas uplift, 

both Paleozoic structures, are present in Kansas to the 

southeast (Sims, 1985). The northwest-trending tectonic 

zones that have been recognized in Missouri (Kisvarsanyi, 

1984; Zietz and others, 1984) are characterized along their 

trends by faulting and folding in the Paleozoic rocks, Proterozoic igneous intrusions of both felsic and mafic rocks, 

and cataclastic granulation of the basement rocks. In central Missouri, two of the structures approximately bound 

the pronounced Missouri gravity low (Guinness and 

others, 1982; fig. 3). 

Although the age and origin of the faults in the central part of the midcontinent region are uncertain, it seems 

reasonable to interpret them as transcurrent structures 

kinematically related to those on the Superior craton and 

immediately adjacent Early Proterozoic terranes, as proposed earlier by Sims (1985). Some of the southern group 

of structures could be rift-related faults, however, as suggested by Guinness and others (1982). Regardless of their 

origin, the northwest-trending faults are fundamental 

crustal structures in the north-central United States that 

have profoundly affected the evolution of the Precambrian terranes and apparently also the overlying Paleozoic 

rocks. Within the Superior Archean craton (fig. 9), they 

originated in Archean time and, at least in part, were reactivated during Proterozoic time. In the Early Proterozoic 

terranes that partly surround the Superior craton, the 

faults necessarily are post-Archean inasmuch as none of 

these terranes are thought to have an Archean basement. 

This could be interpreted as indicating that dextral shear 

of subcontinental extent persisted intermittently in the 

Superior craton and flanking Early Proterozoic mobile 

belts from the Late Archean ("'2.6 Ga) to about 1.6 Ga. 

Possibly, these structures were utilized still later as transform faults during the Keweenawan rifting event, as suggested by the large lateral displacement of the central 

horst of the Midcontinent rift system in southern Minnesota (Sims, 1985). Finally, many of the northwesttrending basement structures were the sites of dominantly 

vertical movements in the Paleozoic, which produced 

mainly asymmetrical folds in the Paleozoic strata, such 

as the Cedar Creek anticline. 

SUMMARY OF TECTONIC EVOLUTION 

The early geologic history of the northern midcontinent spans an interval of more than 2,000 m.y. and 

records the fragmentation and reassembly of Archean and 

Early Proterozoic terranes of the North American continent and several Proterozoic anorogenic magmatic 

events. Much of the record is restricted to rocks in the 

buried basement-a record that is just beginning to be 

deciphered. 

The earliest event significant to the northern midcontinent was suturing of the two Archean crustal segments in the Lake Superior region-the gneiss terrane on 

the south and the greenstone-granite terrane on the north 

(fig. 4). The suture, termed the Great Lakes tectonic zone 

(GLTZ), trends generally northeastward across Minnesota and adjacent states in the Lake Superior region 

(Sims and others, 1980). Gibbs and others (1984) concluded that the suturing resulted from thrusting of the 

northern greenstone-granite terrane over the gneiss terrane, apparently in Late Archean time. 

Following stabilization of the Archean crust, Early 

Proterozoic epicratonic rocks were deposited in the 

Animikie basin (Minnesota and northern Michigan and 

Wisconsin; Morey, 1983a) during a period of extensional 

tectonism localized over and along the profound structural break (GL TZ) between the two Archean basement 

terranes (fig. 5A). These rocks are interpreted as recording the formation and evolution of a rifted continental 

margin (Schulz and others, 1987) along the southern edge 

of the Superior craton. Deposition began with sedimentation in elongate troughs and intervening platform areas 

(Larue and Sloss, 1980), then gave way to a broad shelf 

phase and finally to a flysch phase (Morey, 1983a). The 

flysch sequence is interpreted as deposits formed during 

rapid subsidence of the rifted margin following crustal 

breakup. The breakup of the continent led to ocean 

22 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
spreading, formation of volcanic-arc systems from about 

1.89 to 1.83 Ga (Wisconsin magmatic terrane), subduction along the southern rifted margin and, eventually, collision of the oceanic arcs with the passive margin ("Jl.85 

Ga; Penokean orogeny; fig. 5B). The juncture between 

the two contrasting terranes in Wisconsin is the Niagara 

fault, which is probably a high-angle thrust. A relatively 

small block of Archean gneissic crust that crops out in 

central Wisconsin (Sims, 1985) accompanied docking of 

the Early Proterozoic arc systems (Sims, 1985). 

At about the same time as the events that 

culminated in the Penokean orogeny, rifting of the 

western margin of the Superior craton occurred and 

resulted in new north-trending volcanic crust and eventual collision of volcanic arc systems with the continent 

at about 1.85 Ga (Lewry, 1981; figs. 2 and 3), to produce the Trans-Hudson orogen (Churchill province). 

Whether the Wyoming Archean craton was rifted from 

the Superior craton and subsequently returned or whether 

it is part of an exotic continental mass is not known. 

The Penokean orogeny and its predominantly calcalkalic magmatism was followed in the Lake Superior 

region by a 1. 76-Ga episode of an orogenic activity. 

Rhyolite ash-flow tuffs and cogenetic epizonal granite of 

metaluminous and peraluminous affinities that formed 

on older orogenic rocks in southern Wisconsin and adjacent areas (Smith, 1983) were derived by partial melting 

of older calc-alkaline rocks. The metaluminous and 

peraluminous affinity of these rocks is typical of the 

rhyolites and granites that form shortly after a major 

orogenic event (Rogers and Greenberg, 1981). 

In the Lake Superior region and apparently in areas 

at least as far south as Iowa (Sims, 1985), the 1. 76-Ga 

anorogenic magmatism was succeeded shortly(?) by 

deposition of mature quartzite that locally has thicknesses 

of as much as 5,000 ft (1,500 m). Apparently, the quartz 

sands were transported in generally southward flowing, 

braided river-alluvial plain environments and deposited 

in fault-bounded (rift?) basins, as proposed by Southwick 

and Mossier (1984) for the Sioux Quartzite in an intracratonic setting in southwestern Minnesota and adjacent areas. 

Both the 1. 76-Ga rhyolite-granite and the younger 

quartzite were mildly deformed and metamorphosed in 

southern Wisconsin at rvl.63 Ga (Van Schmus and 

Bickford, 1981; Peterman and others, 1985), whereas the 

Sioux Quartzite in the Sioux ridge area was virtually 

undeformed and unmetamorphosed. The cause of the 

1.63-Ga event, which has now been recognized over much 

of Wisconsin and Michigan, is not confidently known, 

but Dott (1983) has suggested that it might have been 

caused by continent-arc collision to the south of 

Wisconsin. 

Major orogenic activity, resulting in formation 

of the Central Plains orogen, took place virtually 

contemporaneously with the 1. 76-Ga anorogenic event 

and deposition of the succeeding quartzite. Judged from 

available U-Pb zircon data (Bickford and others, 1986), 

the orogenic activity spanned the interval from about 1.8 

to 1.63 Ga. By analogy with the presumed correlative 

Early Proterozoic foldbelt exposed in the basement uplifts 

of Colorado and adjacent states (Karlstrom and Houston, 

1984; Condie, 1982), we infer that the Central Plains 

orogen probably resulted from continent-arc collision. 

The volcanic rocks in the orogen that have been studied 

are dominantly calc-alkaline, and they have geochemical 

characteristics similar to those of modern convergentmargin volcanics (Condie and Shadel, 1984). We tentatively suggest that collision in the Central Plains orogen 

occurred in the interval 1.63-1.70 Ga, but it could have 

been a more complex event that involved successive accretion of volcanic arcs. In any case, this episode of 

magmatic activity, which presumably involved much of 

southwestern United States, represented a major episode 

of addition of new crust to the North American continent (see Bickford and others, 1986, for discussion). 

After a relatively quiescent interval of more than 

100 m.y. of which there is no known geologic record in 

the region, anorogenic magmatism occurred once again. 

Discrete, widely separated plutons, mainly of batholithic 

dimensions, were emplaced within the Transcontinental 

anorogenic province (Anderson, 1983). Anderson (1983) 

attributed the magmatism to local thermal doming in the 

mantle in a regional extensional setting. 

Approximately contemporaneously with, and possibly related to, the Transcontinental anorogenic 

magmatism, rhyolite and cogenetic epizonal granite 

(rvl.48 Ga) were formed in the St. Francois Mountains 

and vicinity in southeastern Missouri (Kisvarsanyi, 1981; 

Sims, 1985), in a postulated tensional tectonic regime. The 

rocks accumulated on and within an older crust, presumably mainly rocks of the Central Plains orogen, and 

like other anorogenic Precambrian rocks in the midcontinent, are interpreted to be the products of partial 

melting of the lower crust (Van Schmus and Bickford, 

1981; Nelson and DePaolo, 1985). This episode of 

anorogenic activity was followed by renewed graniterhyolite magmatism (1.35-1.4 Ga) in the Spavinaw terrane of Oklahoma, Kansas, and Missouri and areas to 

the south (Thomas and others, 1984). 

The nature and significance of the tectonic event 

responsible for accumulation of the Middle(?) Proterozoic sedimentary rocks in southwestern Missouri and adjacent areas in Kansas and Arkansas is speculative. 

Apparently, the sedimentary rocks reflect rather local 

tectonism-reactivation of major tectonic (shear) 

zones-although they could be part of a larger event that 

has not been recognized on a regional scale because of 

inadequate drill-hole coverage. 

The youngest major tectonic event in the region was 

Summary of Tectonic Evolution 23 
development of the Midcontinent rift system and its 

coeval igneous and sedimentary rocks (""'1.1 Ga). The rift 

transects and interrupts several older tectono-stratigraphic 

terranes, and it is widely recognized as a rift that aborted 

before an ocean basin was formed. The magmatic rocks 

have chemical characteristics of mantle-derived sources 

(Green, 1983; Naldrett, 1981). The rifting was accompanied by differential uplift that reset Rb-Sr biotite ages 

("" 1.14 Ga) over a large area, and which was greatest in 

northern Wisconsin (Peterman and others, 1985). The 

rapid uplift was accompanied by erosion, and detritus 

from the uplift presumably was shed into nearby tectonic 

basins, which have not been identified, if indeed they have 

survived. The uplift event may have affected areas south 

of Wisconsin also, because K-Ar biotite ages of 1'\.11.2 are 

common in Iowa and adjacent areas (Goldich and others, 

1966). 

Subsequent crustal movements in the northern midcontinent have been largely epeirogenic and mainly have 

involved basin development in the Paleozoic. During this 

tectonism, basement faults were commonly reactivated, 

with concomitant folding and faulting of the Paleozoic 

strata and the local movement of fluids, including oreforming fluids. 

METALLOGENY 

The Precambrian basement in the northern midcontinent represents a major frontier of future mineral exploration because of its high potential for suitable 

exploration targets. The Precambrian rocks exposed 

along the northern margin, in the Lake Superior region, 

have been one of the Nation's most important mineralproducing areas, especially of ores of iron and copper, 

and the Southeast Missouri district has been a large source 

of iron and a lesser source of other metals. The recent 

discovery of large resources of unmined material in the 

Lake Superior region (Sims, 1987) adds to the attractiveness of the midcontinent as a potential source of large 

mineral resources. 

Any synthesis of the metallogeny of the buried 

Precambrian basement in the midcontinent requires some 

assumptions, including (1) that lithologic associations of 

ore metals in specific exposed terranes also exist in the 

subsurface; and (2) that certain types of deposits known 

to be associated with specific tectonic environments can 

reasonably be inferred, by analogy, to exist in the buried 

basement. 

Several geologic terranes known to contain valuable 

mineral resources can confidently be extrapolated in the 

subsurface. Most notably, these include the Early Proterozoic Wisconsin magmatic terrane, the Middle Proterozoic St. Francois granite-rhyolite terrane, and the 

Middle Proterozoic Midcontinent rift system. Each of 

these crustal entities could contain substantial resources 

in concealed areas. The possible metallogeny of other exposed terranes not known to contain significant mineral 

deposits, such as the Middle Proterozoic quartzite of the 

Baraboo interval, can be inferred by analogy with terranes having comparable lithologies and tectonic settings 

elsewhere in the world. 

Because the broad tectonic environment of the 

anorogenic magmatic terranes in the southern part of the 

region is similar to that of the world-class Olympic Dam 

copper-uranium-gold deposit at Roxby Downs, South 

Australia, consideration is given to the possibilities of 

such a deposit in the midcontinent. 

The Precambrian basement in the midcontinent (lat 

36°-46° N., long 88°-100° W.) is largely covered by 

Phanerozoic platform sedimentary rocks, but in about 

two-thirds of the region it is easily within the range of 

the diamond drill and the depth at which mining can be 

carried out. As shown on figure 11, much of the basement surface is above an altitude of -2,000 ft (-610 m), 

relative to mean sea level, and thus is within depth limits 

of about two-thirds of a kilometer below the land surface. 

Archean Gneiss Terrane 

Exploration geologists have tended to shun highgrade metamorphic terranes in the region, and mineral 

deposits of economic size have not been found in the 

Archean gneiss terrane of the Lake Superior region. 

Elsewhere, however, huge sulfide deposits occur within 

strongly metamorphosed and deformed Precambrian 

gneisses, including the lead-zinc ores at Broken Hill, New 

South Wales (Johnson and Klingner, 1975), and the 

massive sulfide ores in the Namaqualand Metamorphic 

Complex, South Africa (Tankard and others, 1982). 

Sawkins (1984) has suggested that these deposits are 

highly metamorphosed equivalents of rift-related massive 

sulfide deposits. 

Reconstructions of various lithic and tectonic environments of the Archean gneiss terrane are conjectural 

because of the pervasive tectonic and metamorphic overprint. However, Goldich and others (1970) have emphasized that the gneisses are not protocrust but rather 

are rocks that evolved through processes similar to those 

recognized in many other gneisses of younger age. The 

garnet-biotite, sillimanite, and garnet-anthophyllitecordierite gneisses exposed in the Minnesota River Valley 

were interpreted by Grant ( 1972) as having been derived 

from sedimentary rocks, possibly graywacke and shale. 

Rare beds of intercalated quartzite (chert) and amphibolerich iron-formation further support this interpretation 

(Himmelberg and Phinney, 1967; Grant and Weiblen, 

1971; Grant, 1972). The amphibolite and hornblendepyroxene gneisses occur as layers or rafts in the 

24 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
ILLINOIS 

BASIN 

0 100 200 MILES 

~r----~,--_L'--,------~~ 0 100 200 300 KILOMETERS 

Area in which Precambrian surface is above -2000 ft 

EXPLANATION 

__ 

1 0 

_ Su uotu<e oontou" ( in h u nd<ed' o< teet l on the P .ec•m bti•n b"e m ent '"""e 

Datum is mean sea level. Contours below -4,000 ft not shown 

Figure 11. Generalized structure contour map of the Precambrian basement surface of the northern 

midcontinent. Modified from Sims (1985). 

quartzofeldspathic gneisses, implying that they too were 

originally layered rocks. Thus, Grant (1972) postulated 

an igneous origin for them, either as mafic volcanic rocks 

or as their hypabyssal equivalents. Evidence for formation of the mafic rocks during several different geologic 

episodes is indicated by amphibolites of both tholeiitic 

and komatiitic affinity (Wooden and others, 1980). 

The origin of the stratigraphically older migmatite 

gneisses is more equivocal, but from field relationships 

and geochemical criteria, Goldich and Wooden (1980) 

have concluded that these gneisses formed by the intrusion of coarse-grained tonalite into a volcanic pile of 

andesitic to dacitic flows and (or) pyroclastic material, 

which contained units of intercalated basalt. 

If the younger successions of the gneiss terrane in 

the Minnesota River Valley represent the metamorphosed 

Metallogeny 25 
derivatives of supracrustal sequences that include graywacke, shale, iron-formation, and mafic volcanic 

hypabyssal rocks-rocks typical of Late Archean 

greenstone-granite terranes-they deserve consideration 

as exploration targets because greenstone-granite terranes 

typically contain major mineral deposits. 

Late Archean Greenstone-Granite Terrane 

The counterpart of the Late Archean greenstonegranite terrane-the Superior province in Canadarecords the most varied and richest mineral production 

of any Archean terrane in the world (Franklin and Thorpe, 

1982). The principal types of ore deposits are volcanichosted massive sulfide deposits, volcanic- and intrusionhosted gold deposits, ultramafic- and gabbro-hosted 

nickel-copper deposits, and iron-formations of Algoma 

type. Of these several deposit types, only iron-formation 

has been mined extensively in the United States (Marsden, 

1968; Sims, 1972). However, the potential for the existence 

of economic massive sulfide and gold deposits is excellent, 

although exploration for such deposits in most of the 

northern midcontinent region is hindered by the cover of 

Paleozoic rocks and Pleistocene glacial deposits. 

Massive Sulfide Deposits 

Zinc-copper massive sulfide deposits of primitive 

type (Hutchinson, 1980) can be expected to be present 

in the volcanic subprovinces of the Late Archean 

greenstone-granite terrane. The most favorable host rocks 

are tholeiitic and calc-alkaline volcanic and volcaniclastic 

rocks (Hutchinson and others, 1971). Preliminary studies 

of REE data from Canada has suggested that rhyolitic 

rocks of tholeiitic affinity, which form the felsic members 

of bimodal volcanic sequences, are perhaps the prime 

hosts for zinc-copper-type massive sulfide deposits 

(Lesher and others, 1986). However, such felsic rocks 

have been reported in the United States only in the Rainy 

Lake area in Minnesota (Day, 1985). 

Past exploration in northern Minnesota has not encountered sulfide concentrations of probable economic 

significance, but thick zones of massive, submassive, and 

disseminated pyrite and (or) pyrrhotite have been 

penetrated, and thin, apparently discontinuous zones of 

zinc and copper sulfides have been encountered locally 

(Ojakangas and others, 1977). Several of the drilled iron 

sulfide bodies are thick sulfide-facies iron-formation; 

most other occurrences are in tuffaceous rocks of general 

dacitic composition. 

Gold Deposits 

Gold deposits in the Superior province of Canada 

occur in a wide variety of habitats (Franklin and Thorpe, 

1982), but most of the larger deposits are hosted by 

volcanic rocks in greenstone belts. These deposits are 

typically stratiform and are variously interpreted as being either syngenetic or epigenetic in origin, depending 

upon the convictions of the specific scientist. Geologists 

of the Ontario Geological Survey, for example, generally favor a structural control for gold deposits (see Colvine, 1983, for summary), including the large, recently 

discovered Hemlo deposit (Muir, 1983), whereas 

Cameron and Hattori ( 1985) have suggested that 

syngenetic processes were responsible for gold concentration at Hemlo. Cameron and Hattori stated that gold 

was deposited contemporaneously with pyrite and barite 

but recognized that some components of the ore were 

recrystallized and remobilized as a result of deformation 

and metamorphism. The role of syngenetic versus 

epigenetic processes in the formation of stratiform gold 

deposits has not been adequately defined and much further research remains to be done. 

In the United States, two gold deposits in the 

greenstone-granite terrane have been mined. The principal one, the Ropes mine in northern Michigan (Bornhorst and others, 1986), is active and has reported reserves 

of 20 million metric tons of ore containing 3.3 grams per 

ton gold and 22 grams per ton silver to the 900-ft level 

(Mudrey and Kalliokoski, in press). A second, minor 

deposit, the Little American mine at Rainy Lake, Minn., 

yielded a small amount of gold at about the turn of the 

century (Sims, 1972). Poulson (1983) has concluded from 

studies in the Wabigoon subprovince in an adjacent area 

in Canada, that gold-bearing quartz veins in the general 

Rainy Lake area occur in ductile shear zones and as dilations of regionally developed cleavage. These structures 

are related kinematically to the large, dextral, transcurrent faults in the region (Day and Sims, 1984), such as 

the Vermilion fault which was discussed in a preceding 

section of this report. Thus, recognition of the spatial 

relationship between gold and large dextral faults should 

be one objective of an exploration strategy for gold in 

this region. 

Early Proterozoic Penokean Orogen 

In the Lake Superior region, the Penokean orogen 

consists of the Early Proterozoic Wisconsin magmatic terrane, interpreted as an island-arc volcanic-plutonic complex (Sims and others, in press), and an Early Proterozoic 

epicratonic sedimentary-volcanic sequence, interpreted as 

a passive continental margin sequence (Schulz and others, 

in press). Inasmuch as the epicratonic sequence, which 

contains the large, valuable iron-formations of Minnesota, Michigan, and Wisconsin (Marsden, 1968), apparently effectively terminates to the south in central 

Minnesota (figs. 2, 5), the metallogeny of the epicratonic 

26 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
rocks is not discussed here. It should be mentioned, 

however, that in addition to the vast iron deposits a large, 

potentially valuable stratiform copper deposit in the Kona 

Dolomite (Chocolay Group of the Marquette Range 

Supergroup; Cannon and Gair, 1970) has been partly 

delineated by drilling. A resource in this deposit of at least 

100 million tons containing more than 0.5 percent copper and modest silver values has been reasonably proved 

(Brown, 1986). The copper occurs as disseminated chalcocite, bornite, and chalcopyrite, which show well-defined 

lateral zoning within originally pyritic sediments in 

quartzite and argillite units in the lower part of the Kona. 

Taylor (1972) interpreted the deposit as having formed 

in a sabkha environment. 

Massive Sulfide Deposits 

The Wisconsin magmatic terrane contains at least 

four known massive sulfide deposits of future economic 

significance (Mudrey and others, in press), which are scattered within an east-trending volcanic belt more than 

155 mi (250 km) long and less than 30 mi (50 km) wide 

(Sims, 1987). The actual length of the belt is not known 

because it passes under Paleozoic strata both to the east 

and west. The deposits belong to the primitive type of 

zinc-copper sulfide deposits of Hutchinson (1980), and 

accordingly they are grossly similar to the massive sulfide 

deposits in Archean greenstone belts within the Canadian 

Superior province (Franklin and Thorpe, 1982). The 

deposits were discovered by airborne electromagnetic 

methods and have been outlined by drilling (Mudrey and 

others, in press). 

The largest known massive sulfide body, near Crandon, Wis. (May and Schmidt, 1982), contains about 61 

million tons of ore, which contains 1 percent copper, 5.6 

percent zinc, 0.5 percent lead, 36.8 grams per metric ton 

of silver, and 1 gram per metric ton of gold. It consists 

of not only a steeply dipping tabular body, which is 

hosted by felsic to intermediate pyroclastic rocks, but also 

the underlying epigenetic stringer or feeder-pipe 

mineralization. Other known massive sulfide bodies are 

much smaller, being on the order of 5 million tons or less. 

Available chemical data are inadequate to characterize the volcanic rocks within the ore-bearing belt as 

a whole, but the volcanic rocks near Monico, Wis., in 

approximately the center of the ore-bearing belt, compose a bimodal suite whose end members are highalumina basalt and rhyolite (Sims and others, in press). 

This suite is compositionally similar to the bimodal calcalkaline volcanic rocks that host the Kuroko massive 

sulfide deposits of Japan (Dudas and others, 1983). 

Cathles and others (1983) have suggested that the Kurokotype deposits were formed in a failed rift, and, by 

analogy, the Early Proterozoic deposits in Wisconsin also 

could have formed in an aborted island-arc rift. Such a 

setting would be compatible with the general linear alignment of the known Wisconsin deposits. 

Sims (1985) interpreted the Wisconsin magmatic 

terrane as extending in the subsurface into southernmost 

Minnesota, northwestern Iowa, and northeastern 

Nebraska, on the western side of the Midcontinent rift 

system, as did Anderson and Black (1983). Accordingly, 

buried parts of the terrane could contain massive sulfide 

deposits of approximately the same tonnage and grade 

as those known in Wisconsin. The top of the basement 

in the terrane in Iowa and Minnesota is less than 0.35 mi 

(0.5 km) deep. 

Other Deposits 

A volcanic-hosted stratiform gold deposit of possible economic interest has been explored in central 

Wisconsin, east of Wausau, and anomalous gold values 

have been obtained elsewhere in this region from cherty 

rocks (G.L. LaBerge, oral commun., 1985). 

The Archean crustal block associated with the Early 

Proterozoic Wisconsin magmatic terrane, which is presumed to have been rafted in with the arc volcanics, contains an Algoma-type iron-formation (Jackson County 

iron mine) that has yielded a substantial amount of 

taconite ore (Sims, 1987). 

Early Proterozoic Rhyolite-Granite Terrane of 

Southern Wisconsin 

The rhyolite-granite terrane of southern Wisconsin has no reported metallic mineral occurrences, possibly 

in part because of its sparse exposure. Because of its 

similarity to the younger St. Francois granite-rhyolite terrane, however, it could contain the same types of mineral 

deposits as are known to occur in that terrane. 

Early Proterozoic Central Plains Orogen 

Inasmuch as rocks of the Central Plains orogen are 

not exposed anywhere in the midcontinent and drill cores 

apparently have not penetrated ore-bearing material, discussion of the metallogeny necessarily must be based 

mainly on analogy with the presumed correlative rocks 

exposed in the basement uplifts in Colorado and adjacent areas. Apparently younger, layered mafic complexes 

that intrude the metamorphic suite and have been drilled 

at several localities in Missouri are also discussed in this 

section. 

Metallic Sulfide Deposits Associated With Metamorphic Rocks 

In Colorado, two types of Precambrian metallic 

sulfide deposits have been recognized (Taylor and others, 

1983): (1) massive and disseminated sulfides, chiefly 

Metallogeny 27 
composed of zinc and copper but also containing 

gold, silver, and lead; and (2) disseminated copper in 

sulfides and tungsten in the mineral scheelite. The 

deposits are stratiform and were formed about 1.8 Ga, 

contemporaneously with their gneissic protolith, and 

were metamorphosed during Precambrian regional 

metamorphism. 

The Precambrian stratiform deposits of both 

copper-zinc and tungsten-copper types in Colorado occur in both metavolcanic and metasedimentary rocks. 

They tend to cluster spatially and follow specific stratigraphic zones, but no regional structural control on their 

distribution is evident (Taylor and others, 1984). In 

general, the most favorable host rocks are units having 

alternating feldspathic and calcium-magnesium-rich 

layers, but some deposits, particularly of tungsten-copper 

type, are contained in calc-silicate and hornblende gneiss. 

Quartzite is an unfavorable host rock. The deposits range 

in size from small pods to lenticular masses some tens 

of feet thick and as much as several thousand feet across. 

Shapes are complex and modified by folding. An idealized stratiform sulfide deposit consists of a central core 

rich in chalcopyrite, sphalerite, gahnite (zincian spinel), 

and galena that passes laterally into pyrite-rich material 

and, in the distal margins, to magnetite- and manganiferous garnet-bearing rocks (Taylor and others, 1984). The 

largest known stratiform sulfide deposit in Colorado, the 

Sedalia mine near Salida, produced about 90,000 tons of 

ore (chiefly copper and zinc sulfates) from the weathered, 

oxidized zone of a primary sulfide deposit (Sheridan 

and Raymond, 1984). The nearby Pecos mine, near 

Santa Fe, N. Mex., is a massive sulfide deposit of much 

larger size; it has been a major producer of zinc, lead, 

copper, gold, and silver. Tungsten stratiform deposits 

(Tweto, 1960) tend to occur as widely scattered, overall 

low grade concentrations of apparent minor economic 

significance. 

In the buried basement of the midcontinent region, 

areas of bimodal volcanism would be particularly favorable for the occurrence of stratiform sulfide deposits of 

the type known in Colorado and adjacent areas. Chromium, nickel, and cobalt geochemical signatures, which 

apparently result from the composition of mafic rock 

layers rather than from the deposits themselves, are associated with favorable gneiss bodies (Taylor and others, 

1984). Tungsten tends to be associated with both the zinccopper and tungsten-copper deposit types and could be 

a guide to favorable areas. Geophysical methods possibly 

could be utilized effectively to search for concealed 

deposits in areas of moderately thin Paleozoic cover. 

Metallic Deposits Associated With Layered Mafic Complexes 

Several layered mafic complexes in Missouri (fig. 

12), which produce pronounced magnetic anomalies, have 

been tested by exploratory drill holes because of their 

presumed potential economic importance. Such intrusions 

in cratonic settings elsewhere are major sources of 

iron-nickel-copper-cobalt ores and platinum-chromiumtitanium ores (Naldrett, 1981). They include such wellknown deposits as the Bush veld Complex in South Africa, 

the Stillwater Complex in Montana, and the NorilskTalnakh Complex in the U.S.S.R. 

In Missouri, one complex, the Orla pluton (loc. 15, 

fig. 12), has been tested by five core holes that penetrated 

a total of 1, 723 ft (525 m) of gabbroic rocks. The Orla 

pluton intrudes the older gneisses and schists of the Central Plains orogen. At its deepest drilled level, the complex is a fresh pyroxene gabbro composed of diallage, 

hypersthene, calcic plagioclase, trace olivine, much 

disseminated magnetite and chalcopyrite, and biotite. The 

gabbro is pervasively intruded by granitic dikes and veins. 

Zircons from one such vein yielded an age of 1.47 Ga 

(Bickford, Harrower, and others, 1981), which is the 

same age as the St. Francois granite-rhyolite terrane. The 

pervasive emplacement of granitic magma under mesozonal conditions produced extensive metasomatic alteration in the gabbro. Pyroxenes were uralitized, late biotite 

formed around earlier formed mafic minerals, and the 

gabbro was essentially converted to an epidiorite. 

By combining drill core information from the other 

mafic plutons shown on figure 12 with that from Orla, 

certain generalizations can be made about these layered 

complexes. They typically (1) have a differentiated cap 

of diorite and quartz diorite; (2) grade through gabbro, 

norite, and troctolite with depth; (3) locally contain anorthositic lenses; and (4) are cut by granite (Kisvarsanyi, 

1984). These rocks deserve further exploration even 

though they are covered by as much as 3,280 ft (1 ,000 

m) of Phanerozoic sedimentary rocks, because some 

bodies contain as much as 15 percent by volume of disseminated pyrrhotite, pentlandite, chalcopyrite, and 

magnetite. 

Middle Proterozoic St. Francois 

Granite-Rhyolite Terrane 

The St. Francois granite-rhyolite terrane has been 

by far the most important metal-producing area in the 

Proterozoic of the midcontinent. It has been a continuous 

source of iron ore since 1815 and constitutes an iron 

metallogenic province (Kisvarsanyi and Proctor, 1967; 

Snyder, 1969). Cumulative production from the Southeast Missouri district is valued at more than $750 million; 

the value of ore in the ground is estimated at $600 million 

(Kisvarsanyi, 1984). Minor deposits of manganese and 

polymetallic (tin-tungsten-silver-lead-antimony) quartz 

veins were mined in the St. Francois Mountains in the 

past. 

28 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
0 

0 

100 MILES 

100 KILOMETERS 

EXPLANATION 

--- Fault Tectonic zone 

MAGNETITE BODIES MAFIC PLUTONS 

Kratz Spring 9 St. Francisville 

2 Bourbon 10 Levasy 

3 Pea Ridge 11 Benton City 

4 Camels Hump 12 Wentzville 

5 Floyd Tower 13 Versailles 

6 Iron Mountain-Pilot Knob 14 Eldon 

7 Boss 15 Orla 

8 Lake Springs 16 Peace Valley_ 

17 Avon 

18 Bloomfield 

19 Malden 

goo 

Figure 12. High magnetic anomalies (black) in Missouri and their sources as inferred from drill holes and 

geophysical data. Modified from Kisvarsanyi (1984). 

Metallogeny 29 
Iron and Copper-Iron Deposits 

More than 30 iron and copper-iron deposits are 

known in the Southeast Missouri district; the majority 

of them are in the St. Francois Mountains. The six major deposits-Kratz Spring, Bourbon, Pea Ridge, Camels 

Hump, Boss (copper-iron), and Pilot Knob (lower ore 

body)-are in the buried part of the terrane and were 

discovered by the drilling of magnetic anomalies (fig. 12). 

Only two of the buried ore bodies, Pea Ridge and Pilot 

Knob, have been developed. The total yield from the Pea 

Ridge mine is more than 36 million tons of ore; proven 

reserves are about 100 million tons of ore containing 56 

percent iron (Kisvarsanyi, 1984). Production from the 

Pilot Knob lower ore body from 1968 to 1980 was 19 

million tons of ore grading at about 35 percent iron. 

The major deposits are of two general types: (1) 

iron (magnetite-hematite)-apatite plus rare earths and 

minor sulfides of iron and copper, and (2) copper 

(bornite-chalcopyrite )-iron (magnetite-hematite) with 

sulfides of cobalt and molybdenum, and gold. The 

deposits have been interpreted as magmatic injections at 

the Pea Ridge (Emery, 1968), Iron Mountain (Murphy 

and Ohle, 1968), Pilot Knob lower ore body (Wracher, 

1976), and Bourbon deposits (Kisvarsanyi and Proctor, 

1967; Snyder, 1969); hydrothermal veins and replacements at Cedar Hill, Shepherd Mountain, and other small 

near-surface deposits; and volcanic exhalative impregnations in bedded air-fall tuffs at the Pilot Knob upper ore 

body (Anderson, 1976). Recently, Panno and Hood 

(1983) interpreted both the subsurface and surface ore 

bodies at Pilot Knob to be replacement deposits. The Boss 

copper-iron ore body has been considered to be a contact metasomatic deposit (Kisvarsanyi and Proctor, 1967). 

The iron deposits consist of nontitaniferous 

magnetite and hematite ores that have relatively high 

alkali metal and rare-earth element contents. Apatite, 

monazite, fluorite, actinolite, garnet, and barite are characteristic associated minerals. Typically, ore magmas had 

associated hydrothermal and pegmatitic phases, and most 

deposits exhibit features characteristic of both magmatic 

and hydrothermal modes of origin. Ore bodies vary in 

size, shape, and emplacement characteristics; they are 

associated with silicic to intermediate rocks of the St. 

Francois granite-rhyolite terrane. However, the Boss 

deposit is in a syenite intrusion (Kisvarsanyi and Kisvarsanyi, 1977). Because of the similarity of the iron deposits 

to those at Kiruna, Sweden (Geijer and Odman, 1974; 

Frietsch, 1978; Wright, 1986), they have commonly been 

referred to as Kiruna type (Kisvarsanyi and Kisvarsany, 

1981). 

The iron and copper-iron deposits are considered 

to be genetically related to the trachyte suite (Kisvarsanyi, 

1981), which composes the inferred ring structures in the 

St. Francois terrane (figs. 8, 13). The suite includes 

syenite, trachyte, magnetite trachyte, trachyandesite, and 

trachybasalt. Magnetite is a primary, rock-forming constituent in the trachyte suite and composes as much as 

20 percent by volume of the magnetite trachytes. The 

trachytic rocks are spatially associated with distinctive, 

partial ring-shaped or arcuate positive magnetic anomalies (Zietz and others, 1984). Crosscutting relationships 

observed in drill core indicate that the iron and copperiron mineralization postdate the rhyolite and subvolcanic 

granite and predate the central pluton and diabase 

(Kisvarsanyi and Kisvarsanyi, 1981). This timing suggests 

that the mineralization is temporally as well as spatially 

related to the emplacement of the ring intrusions. 

Chemical analyses indicate that the trachyte suite 

is enriched in alkalies, iron, and volatiles. In an alkalisilica diagram (fig. 14), most of these rocks lie within the 

alkaline field, as defined by MacDonald (1968). Rhyolites 

of the terrane are mostly in the subalkaline field. In an 

iron-silica diagram (fig. 15), the trachyandesites show 

significant iron enrichment, the trachytes show somewhat 

lower iron contents, and both the trachytes and rhyolites 

show decreasing iron content with increasing silica. 

The trachyte suite is generally low in titanium, a 

feature also characteristic of the ores. Boron and fluorine 

contents are quite high (fig. 16). Phosphorous (P20 5) 

contents range from 0.3 to 0.4 weight percent (Kisvarsanyi, 1972, 1981). Purified magnetite from different ore 

bodies has trace element distribution patterns comparable 

to the trace element signature of the trachyte suite (Kisvarsanyi and Proctor, 1967). In contrast, magnetite from the 

central plutons and from diabase dikes in the St. Francois terrane has trace element patterns that differ from 

those of magnetite in the ore bodies. 

Kisvarsanyi and Kisvarsanyi ( 1981) have proposed 

that rather than being directly derived from the trachyte 

suite, the iron ores, as well as the associated trachytic 

rocks, have a common source in an alkalic-intermediate 

magma. Tension fractures associated with cauldron subsidence and caldera collapse, and the massive brecciation 

of the volcanic rocks of the terrane probably provided 

the most favorable sites for the emplacement of these 

magmas and ore solutions. 

Manganese Deposits 

Several small manganese prospects in rhyolite are 

kn-own in the southern part of the St. Francois Mountains (fig. 13). These deposits yielded 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 PrecambrianPaleozoic erosional nonconformity. 

30 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
BourbonO 

" \ 

eSteelville 

\ 

\ 

\ 

) 

~~ 

\ 

) 

\ ePotosi ) 

goo 

LOCATION MAP 

eBonne Terre 

'cd '-..........__ ~-. · > Q Camels Hump 

~----/ 

ST. FRANCOIS / 

/ ...--"""- .. ·. / 

( Q~l•on Mountain 

l Pilot Knob /:.) ( Q 

\ '-~·········<·.·.·.· : , .. : ....... QO, •o" Ironton 0 / } . . • > (]) [YD-./" 

C]) Silver Mine district 

/---" 

/ \ I ) 

( I 

\ / "-.. 

_ _.,./ 

0 ,.--........._ 

( "-.0 

\ \ \ } " '--/ / 

0 10 20 30 MILES 

0 10 20 30 40 50 KILOMETERS 

EXPLANATION 

MINERAL DEPOSITS 

Trachytic ring Intrusion 0 Iron 

Inferred ring structure 

~ Copper-Iron 

C]) Manganese 

O Ttn-tungsten-silver-lead-antlmony·arsenlc 

Figure 13. Relationship of ring structures, trachytic ring intrusions, and ore deposits in the St. Francois terrane, 

southeastern Missouri. Compiled by E.B. Kisvarsanyi (1986). 

Metallogeny 31 
+-' 

c: 

Q) 

0 

15 

Q5 10 c. 

:§ 

0 

-2 

+ 

0 N 

ctl 

z 

5 

/ 

•• 

• 

• 

/ 

/ 

/ 

/ 

/ 

• 

• 

• 

alkaHne . / 

• .AI /. 

/ 

/e 

/ 

.( . 

/ . 

subalkaline 

• 

Si02 (in percent) 

• 

• 

Figure 14. Alkali-silica diagram showing selected volcanic 

rocks from the St. Francois terrane: triangles are 

trachyandesites, circles are trachytes, and squares are 

rhyolites. Alkaline and subalkaline fields from MacDonald 

(1968). Source of chemical analyses in Kisvarsanyi (1972) 

and unpublished data in the files of the Missouri Geological 

Survey. 

30 

-;; 

c: 

Q) 

0 20 

Q5 

c. .. :§ ...... M 

0 

·~ .. N 

Q) 

u. 

+ 10 0 

Q) ••• u. • I • • • ·-: ... 

0 

40 50 60 70 

Si02 (in percent) 

• 

Figure 15. Iron-silica diagram showing selected volcanic 

rocks from the St. Francois terrane: triangles are 

trachyandesites, circles are trachytes, and squares are 

rhyolites. Source of chemical analyses in Kisvarsanyi (1972) 

and unpublished data in the files of the Missouri Geological 

Survey. 

5000 I2000 I1000 I500 I200 ~ 

E 

c. 100 - c. 

50 -

20 -

10-

5t--

2t- D 

-Sn 

EXPLANATION 

World average, syenite -

D 

(Turekian and Wedepohl~ 1961 l 

M;sso"d t"chytes 110 samplesi1J 

-

JJ _.______ 

-Be y B F 

Figure 16. Ranges of selected trace elements in parts per 

million from the trachyte suite ofthe St. Francois terrane compared to world averages of syenite from Turekian and 

Wedepohl (1961). Trace element data from Viets and others 

(1978). 

The most productive of the old mines produced 

about 3,000 short tons of manganese ores and concentrates, which contained more than 50 percent 

manganese, during intermittent mining between 1872 

and 1958 (Dorr, 1967). The deposit is stratiform in 

water-laid, bedded tuffs, which are interbedded with 

thin beds of travertine; this association suggests that the 

deposit formed in a crater lake 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 manganese 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. Craters, 

calderas, collapsed cauldrons, and ring structures would 

be the most favorable sites for this type of mineralization. The Precambrian-Paleozoic erosional nonconformity, especially where thick sections of basal 

clastics and residual clays are present, is considered 

to be a favorable horizon for secondary concentrations of both manganese and iron, and possibly also of 

copper. 

32 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
Vein Deposits of Tin-Tungsten-Silver-Lead-Antimony 

Deposits in the Silver Mine district (fig. 13) of the 

exposed St. Francois terrane represent the only known 

high-temperature pneumatolytic-hydrothermal (xenothermal) ore-mineral association in the Proterozoic of the 

southern midcontinent. The Middle Proterozoic Silvermine Granite of Kisvarsanyi (1981), one of the ring intrusions in the terrane (figs. 8, 13), is cut by polymetallic 

quartz veins that have accompanying greisenization of 

the wallrock. The mineral suite includes argentiferous 

galena, wolframite, arsenopyrite, sphalerite, cassiterite, 

chalcopyrite, covellite, hematite, stolzite, and scheelite 

and is similar to the deposits of Cornwall and Saxony 

(Tolman, 1933; Lowell, 1976). Hagni (1984) identified argentiferous tennantite, antimonpearceite, and 

berryite in the main sulfide stage of mineralization, and 

concluded that the base-metal sulfides 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 mineral deposits postdate 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 from 1877 to 1894 and 

produced an estimated 3,000 oz of silver, as well as some 

50 tons of lead. Between 1916 and 1946, an estimated 120 

short tons of tungsten concentrates were 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 to more than about 

200 ft (61 m) 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. 13), however, suggests structural control for the emplacement of the deposits, and 

it is possible that similar xenothermal deposits are associated with buried ring complexes elsewhere in the St. 

Francois terrane. 

Uranium-bearing Granitic Rocks as a Possible Heat Source 

for Mississippi Valley-type Lead-Zinc Deposits 

The LILE-enriched central plutons ("tin granites"), 

such as the exposed Graniteville Granite of Kisvarsanyi 

(1981) in the St. Francois Mountains, contain as much 

as 18 ppm uranium and 43 ppm thorium, nearly three 

times the average of other granite types in the St. Francois terrane (Nash, 1977). The "tin granites" also are abnormally radioactive compared to other common rock 

types. Accordingly, they can be classified as HHP (highheat-production) granites. In addition to the exposed 

granite body, Kisvarsanyi (1981) has inferred the existence 

of eight such plutons in the buried segment of the St. 

Francois terrane (fig. 8). Possibly other, similar plutons 

exist but have not been penetrated by drilling. 

The HHP granite plutons possibly could have supplied some of the basinal heat that is indicated by 

measured fluid-inclusion temperatures of Mississippi 

Valley-type lead-zinc ore deposits in the midcontinent. 

A basinal brine theory of Mississippi Valley-type genesis 

generally has been favored by most workers since its proposal by White (1958). Recently, Bethke (1986) has 

presented convincing arguments for formation of the Upper Mississippi Valley district lead-zinc deposits by 

regional ground-water flow across the Illinois basin, 

which was initiated by uplift of the Pascola arch in postEarly Permian and pre-Late Cretaceous time. Among the 

constraints on this gravity-driven flow model is an adequate basinal heat flow sufficient to account for the 

150-200 OC temperatures commonly measured. Previously, abnormal heat flow in the southern part of the basin 

generally has been variously attributed to mafic dikes and 

sills of Permian age (Zartman and others, 1967), the 

Hicks dome cryptovolcanic structure of presumed Late 

Cretaceous age (McGinnis and others, 1976), and Mesozoic plutons inferred to exist from gravity and magnetic 

modeling by Schwalb (1982). The ages of these bodies 

are consistent with the presumed age range of the ore 

deposits. The HHP plutons of the St. Francois graniterhyolite terrane, which is inferred to extend eastward into 

southern Illinois (Sims, 1985; Bickford and others, 1986), 

also are apparently situated favorably to be possible heat 

sources. Radioactive decay of uranium could have 

resulted in sustained thermal anomalies in the basement 

near the plutons that would have been sufficient to appreciably heat basinal waters. 

Interestingly, the world-class Mississippi Valleytype lead-zinc deposits of the Southeast Missouri district 

have a close spatial relationship to the HHP plutons, as 

do the Tri-State zinc-lead deposits of Oklahoma, Kansas, and Missouri and the sphalerite deposits in northern 

Arkansas. We suggest that heat derived from the subjacent HHP plutons could have been a contributing factor 

to their genesis (see Leach and Rowan, 1986, for 

discussion). 

Middle Proterozoic Spavinaw 

Granite-Rhyolite Terrane 

The Spavinaw granite-rhyolite terrane has no 

historic record of mineral production. Petrographic, 

geochemical, and tectonic analogies with the St. Francois granite-rhyolite terrane suggest, however, that its 

metallogenesis could be similar to the St. Francois terrane and that the same types of mineral deposits could 

be expected in both. 

Metallogeny 33 
Middle Proterozoic Midcontinent Rift System 

Stratified rocks of the Keweenawan Supergroup in 

the Lake Superior region contain the world-famous 

volcanic- (White, 1968) and sedimentary-hosted (Ensign 

and others, 1968) copper deposits of the Keweenaw 

district, Michigan. A major copper resource also occurs 

in gabbroic rocks of the Duluth Complex in northern 

Minnesota (Listerud and Meineke, 1977). Although of 

marginal grade, the copper deposits of the Duluth Complex also contain appreciable quantities of nickel, cobalt, 

and possibly the platinum-group metals. These deposits 

are related to rifting processes and to associated mafic 

igneous activity involving mantle-derived materials. 

Therefore, it is reasonable to believe that similar deposits 

can occur elsewhere in the rift system. 

Stratiform Copper Deposits 

The volcanic- and sedimentary-hosted copper 

deposits of the Keweenaw district in Michigan contain 

disseminated mineral deposits. The deposits have distinctive mineralogical and zoning patterns, and the ores tend 

to have significant quantities of native copper or chalcocite and bornite, but only minor amounts of pyrite. Thus 

the ores tend to have large copper-iron ratios. These attributes are similar to those that characterize the so-called 

"rift-related stratiform copper deposits" of Sawkins 

(1984). Such deposits occur on all continents and range 

in age from 2,000 Ma to Miocene. However, a significant proportion of the deposits occurs in Middle to Late 

Proterozoic sequences (Sawkins, 1983), probably because 

of the widespread rifting that occurred about a billion 

years ago (Sawkins, 1976). 

The volcanic-hosted deposits consist predominantly of 

native copper that occurs primarily as open-space fillings 

and replacements in flow tops and conglomerate. The 

mineralization is controlled both by primary permeability and by tectonic fracturing. White (1968) proposed that 

the deposits were formed by heated, copper-bearing solutions of metamorphic origin that formed in deeper parts 

of the rift system where the rocks were compacted and 

metamorphosed. The ascending solutions deposited 

native copper in greatest abundance in localities close to 

the boundary between the epidote and pumpellyite 

metamorphic zones of Jolly (1974). This genetic model, 

however, is subject to further review. 

The sedimentary-hosted deposits occur in the Keweenawan Nonesuch Formation in dark laminated shale 

that contains abundant pyrite and organic material, including petroleum. In addition to copper, the deposits 

contain some silver (A. V. Heyl, oral commun., 1986). 

Although broadly stratiform, the deposits transgress lithic 

boundaries on a regional scale and consist predominantly of chalcocite that has replaced pyrite (Ensign and 

others, 1968). Brown (1971) has convincingly demonstrated that these deposits formed from copper-bearing 

solutions that flowed through parts of the Nonesuch, 

probably shortly after its deposition. White (1971) expanded on these observations and proposed a quantitative 

paleohydrologic model that involved the migration of 

copper-rich fluids from deeper parts of the rift system 

and the precipitation of copper at sites where the solutions encountered reducing environments. Weiblen and 

others (1978) and Morey and Weiblen (1978) proposed 

that the copper-charged solutions were produced by the 

diagenetic and low-grade metamorphic breakdown of 

copper-bearing basaltic detritus in the sedimentary sequences, rather than by leaching of the basalts themselves. 

In terms of exploration for additional deposits, four 

major conclusions can be drawn. First, stratiform copper deposits are a relatively common feature in sedimentary sequences associated with rift systems, regardless of 

age. These sedimentary sequences always contain at least 

some evidence of associated mafic magmatism. The riftrelated basalts seem to be the ultimate source of the 

metals. Volcanism can be either contemporaneous with 

sedimentation or just prior to it. Second, most copper 

deposits tend to occur within the first reduced sedimentary unit above an oxidized clastic sequence; that is, the 

deposits form from ascending solutions near sharp Eh 

boundaries in the sedimentary environments. Third, 

deposits tend to occur in areas of extensive faulting or 

in other permeable areas where the copper-bearing solutions can readily migrate from sources to traps. Fourth, 

the leaching of copper from the source rocks and migration of transporting fluid are promoted by enhanced levels 

of heat flow. 

Redbed Copper Deposits 

The redbeds of the Midcontinent rift system possibly could host redbed copper deposits, such as those 

found in the Middle Proterozoic Belt Supergroup in Montana (Lange and Sherry, 1983), the Permian Creta Formation in Oklahoma (Ripley and others, 1980), and in 

a variety of Precambrian, Permian, and Cretaceous redbed sequences in the Trans-Pecos region of Texas (Price 

and others, 1985). Although most of these known 

deposits are small, the ores are interesting because they 

typically contain appreciable amounts of silver. 

Redbed copper deposits differ from the stratiform 

copper deposits described above in that they form from 

descending ground waters that circulate through red (oxidized) sediments and carry copper leached from the host 

rock until a reducing zone is encountered. The copper 

is then precipitated either by reduction of sulfate or by 

reaction with iron sulfide to form chalcocite, chalcopyrite, cubanite, and related copper sulfides. Textural 

evidence from several deposits (Rose, 1976) indicates that 

34 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
the copper was precipitated after the sediments were 

deposited but before they were deeply buried. 

The key to formation of this kind of deposit is the 

development of chloride-bearing ground waters that 

greatly enhance the solubility of copper (Barnes and 

others, 1981). Such waters typically form in sabkha settings where evaporation can lead to ground waters with 

high salinities (Renfro, 1974) and possibly to anhydritecontaining evaporite deposits. 

Although neither evaporite deposits nor other 

evidence for sabkha environments have been recognized 

in the redbeds of the Midcontinent rift system, Daniels 

(1982) and Daniels and Elmore (1980) have described 

stromatolitic zones in the Oronto Group that apparently 

formed in an arid, periodically desiccated, shallow-water 

environment that was subjected to large variations in flow 

regime. High evaporation rates also are indicated by 

calcite pseudomorphs after gypsum. Similarly, Morey 

(1974) has described stromatolite-oolite-bearing limestone 

from Oronto-equivalent rocks in southeastern Minnesota 

and attributed their formation to organic activity and 

precipitation in a restricted lacustrine environment. Environments involving extreme chemical precipitation in 

restricted settings undoubtedly also occurred in other 

parts of the rift system. As with the stratiform copper 

deposits, the possibility of the presence of redbed copper deposits is further enhanced by the availability of 

copper-enriched detritus in the host rocks. 

Because this class of copper deposit for the most 

part lacks the detailed geologic, mineralogic, and 

geochemical studies needed to establish recognition 

criteria, any exploration effort in the rift system will require a great deal of drilling, even where overlying rocks 

are thin. Nevertheless, it may be possible to define the 

probable proximity of a deposit by using detailed 

geochemical studies of the paleohydrologic regime. For 

example, a recently completed hydrochemical study of 

ground water from the Midcontinent rift system in eastcentral Minnesota (Lively and Morey, 1985) has defined 

several major copper anomalies that could be related to 

either stratiform or redbed deposits in the subsurface. 

Magmatic Copper Deposits 

Gabbro-hosted copper deposits of the Duluth Complex constitute a large, untapped resource. Nickel, cobalt, 

and platinum-group elements are important potential 

byproducts of the copper ore. The copper- and nickelbearing sulfides, including chalcopyrite, cubanite, and 

pentlandite, as well as appreciable quantities of pyrrhotite, are dispersed widely in the lower part of the 

Duluth Complex where it is in contact with carbonaceous 

and pyritic rocks of Early Proterozoic age. Naldrett 

(1981) has assigned the deposits to his class of "intrusions feeding flood-basalt activity associated with 

intracontinental rift zones,'' and has suggested that the 

magma was derived directly from a mantle source. 

Although the magma was the source of the metals, the 

Early Proterozoic country rocks were the source of the 

suUur (Mainwaring and Naldrett, 1977; Weiblen and 

Morey, 1976; Ripley, 1981). Thus, the sulfides in the 

Duluth Complex formed by the coincidence of a metalrich magma being emplaced in a sulfur-rich country rock. 

As such, it is unlikely that similar copper-nickel deposits 

will be found elsewhere in the rift system. 

Several other aspects of the geology of the Duluth 

Complex may, however, be applicable to other parts of 

the rift system. Platinum-group elements in the Duluth 

Complex have relative proportions similar to those in 

other magmatic bodies where the elements are concentrated to ore grade (Naldrett and Duke, 1980). Ryan and 

W eiblen ( 1984) and Sabelin ( 1985) have shown that the 

platinum-group elements occur as discrete phases, either 

as native elements, as alloys of several native elements, 

or as several kinds of arsenides and sulfarsenides. These 

discrete phases occur as disseminated grains in gabbroic 

or troctolitic units dominated by titaniferous magnetite. 

Although the present discoveries are not economic, there 

is a growing belief that platinum-group metals could occur in economic concentrations in those parts of the 

Duluth Complex dominated by oxide-rich rocks. The significance of oxide-rich units as collectors for the 

platinum-group metals and the subsequent exsolution of 

these metals, either as alloys or in ways leading to combination with adjacent sulfides and arsenides to produce 

typical platinum-group minerals, is poorly documented 

(Naldrett and Watkinson, 1981). However, if association 

between oxide-rich rocks and platinum-group metals can 

be established, other oxide-rich plutonic rocks in the rift 

system, such as those in southeastern Minnesota and adjoining Iowa (Sims, 1985), may represent significant exploration targets for platinum deposits. 

Petroleum Occurrences 

For several years, an extraordinary search for 

petroleum has been underway within the sedimentary 

rocks of the Midcontinent rift system. The exploration 

effort stems in part from the fact that rift-related rocks 

of various ages hold approximately 10 percent of the 

world's discovered petroleum resources (Klemme, 1980). 

Because of its Precambrian age, the Midcontinent 

rift system would not normally be considered a potential 

source for oil or gas. However, it has been known for 

about 25 yr that the Nonesuch Formation contains both 

solid and liquid hydrocarbon material. The solid 

hydrocarbons consist in part of bacterial cells, algallike 

structures, and fungal hyphae (Moore and others, 1969). 

The liquid phase, which contains a full spectrum of 

hydrocarbons typical of crude oil, has been extensively 

Metallogeny 35 
described (Hoering, 1976; Hunt, 1979; Meinschein and 

others, 1964). However, the crude oil contains higher concentrations of alkanes and lower concentrations of 

aromatics than found in an average crude oil and 

resembles the paraffinic crude commonly associated with 

Paleozoic production in Pennsylvania (Barghoorn and 

others, 1965). There is little doubt that the oil is of organic 

origin (Moore and others, 1969), and that it was formed 

under mild catagenic conditions. Sulfide minerals that 

cannot exist above 95 °C are closely associated with the 

oil (Brown, 1971) and confirm a low thermal environment. It has been assumed that the Nonesuch oil is indigenous to the host rock (Eglinton and others, 1964) and 

that it is truly Precambrian in age. Structures and optical activities of alkanes in the Nonesuch oil are indicative 

of hydrocarbons that have been preserved for a long 

period of geologic time (Barghoorn and others, 1965). 

A Rb-Sr isochron age of 1,052 ± 5 Ma has been obtained 

from the shaly host rocks (Chaudhuri and Faure, 1967). 

More recently Ruiz and others (1984) obtained a Rb-Sr 

age of 1,047 ± 35 Ma from a calcite-filled vein in the formation. The presence of oil in vugs within the calcite veins 

implies that the oil was trapped at that time (Kelly and 

Nishioka, 1985). The fact that the oil is truly Precambrian in age has important implications for petroleum exploration elsewhere in the rift system. 

Because the Nonesuch has all the attributes of a 

hydrocarbon source rock, parts of northern Michigan and 

Wisconsin have been the center of considerable exploration interest (Dickas, 1984). Although obvious organicrich beds like those in the Nonesuch have not been 

recognized in any other part of the rift system, a stratigraphic sequence lithically similar to and presumably correlative with the Oronto Group has been recognized in 

southeastern Minnesota (Morey, 1977) and adjoining 

parts of Iowa (Yaghubpur, 1979). In Minnesota, this sequence contains dark shaly units at several stratigraphic 

levels, and it has been suggested that the shales contain 

organic material (Lee and Kerr, 1984) and thus have a 

potential for producing hydrocarbons. However, Hatch 

and Morey (1985) have shown that the extant organic 

material in Oronto-like rocks of southeastern Minnesota 

is thermally mature, which implies deeper burial or higher 

thermal gradients than the Nonesuch Formation. They 

concluded that any hydrocarbons generated were probably lost during subsequent erosion as the western flanking basin of the rift system was filled with Bayfield-like 

sedimentary rocks. 

Although Hatch and Morey ( 1985) have discounted 

the possibility of producing hydrocarbons from Orontolike material in southeastern Minnesota, the presence of 

an organic-rich shale in that area lends credibility to the 

expectation of organic-rich units in other parts of the rift 

system. The concept of thick sedimentary sequences containing organic-rich units in tectonic environments where 

structural and stratigraphic traps typically occur implies 

that both sources and reservoirs are amply developed 

within the Midcontinent rift system (Dickas, 1986). 

It should be emphasized that the copper and the 

hydrocarbons are similarly distributed in the Nonesuch 

Formation. Indeed, the hydrocarbons seem to have exerted a dominant control on localization of the copper 

mineralization (Kelly and Nishioka, 1985). If this analogy 

can be extended to other parts of the rift system, any exploration effort for one commodity will have a considerable impact on understanding how the other 

commodity is distributed. Hydrocarbon exploration is 

easier in deeply buried terranes than metal exploration, 

and hence the metal explorationists should follow the 

search for hydrocarbons in the rift system. 

Hydrogen and Nitrogen Occurrences 

Relatively high concentrations of hydrogen and 

nitrogen gas appear to occur in conjunction with the Midcontinent rift system in Kansas (Goebel and others, 1984) 

and adjoining parts of southwestern Iowa (Herman and 

others, 1984). The gases contain less than one percent 

other constituents, such as light hydrocarbons, helium, 

and carbon dioxide, and therefore have a unique composition. Field size has been estimated at 1.36 trillion 

cubic feet of hydrogen-the calculated energy equivalent 

of approximately 56 million barrels of high-gravity oil 

(AAPG Explorer, 1985, p. 17). Although completed in 

Mississippian (Kinderhookian) limestone, the producing 

wells are located near a fundamental fault that bounds 

the eastern side of the Midcontinent rift system, which 

was rejuvenated in Late Pennsylvanian time. The source 

of the gases is uncertain, but the general absence of 

hydrocarbons and carbon dioxide implies an abiogenic 

origin. The hydrogen in particular could have formed in 

several ways including: (1) serpentinization of ultramafic 

rocks which occur in the general vicinity of the drill holes; 

(2) mixing of waters of differing ionization potential; and 

(3) mantle outgassing along the deep bounding faults of 

the rift system. Although the validity of any or all of these 

possibilities has not been established, the presence of 

substantial quantities of hydrogen implies that the gas is 

being produced in major quantities at this time because 

the retention time for a fixed amount of the gas is roughly 

1,000,000 yr (Petroleum Information, 1984). The presence of trapped hydrogen over the rift system in Kansas 

also implies that commercial quantities of hydrogen could 

occur in Phanerozoic rocks that cover other parts of the 

rift system. 

SPECULATIVE METALLOGENY 

In addition to the metalliferous deposits known or 

inferred to be present in the northern midcontinent as 

discussed above, the region is possibly favorable for the 

occurrence of Olympic Dam-type deposits and for 

36 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
unconformity-type uranium deposits. These speculations 

are based mainly on analogies in regional geologic 

settings. 

Possible Olympic Dam-type Deposits 

The Olympic Dam deposit at Roxby Downs, South 

Australia, is a major resource of copper, uranium, gold, 

and silver. It contains at least 2,000 million metric tons 

of ore that has an average grade of 1.6 percent copper, 

G.06 percent uranium oxide (U30 8), 0.6 grams per metric 

ton of gold, and 3.5 grams per ton of silver (Roxby 

Management Services Proprietary, Ltd., 1986). The 

deposit also contains significant amounts of iron, lanthanum, and cerium. 

The Olympic Dam deposit is located within the 

Stuart shelf area of the Adelaide geosyncline, on the 

eastern flank of the Archean Gawler craton (Roberts and 

Hudson, 1983; Plumb, 1979). The basement to the flatlying Late Proterozoic (Adelaidean) to Cambrian 

sedimentary rocks in the geosyncline consists of rocks of 

the Gawler domain: (1) reworked Archean gneisses; (2) 

Early Proterozoic metasedimentary rocks-dominantly 

quartzite but including iron-formation; and (3) extensive, 

flat-lying, relatively undeformed, subaerial basalt-rhyolite 

bimodal volcanic rocks, which are dominated by felsic 

volcanic rocks-ignimbrites and lesser lavas and agglomerates (Branch, 1978). The felsic volcanic rocks lie 

within a basinlike feature surrounded by posttectonic 

granites; the volcanic and associated granitic rocks have 

Rb-Sr ages between 1.55 and 1.45 Ga (A.W. Webb, 1977, 

reported in Branch, 1978). Minor volcaniclastic sedimentary rocks, conglomerates, and sandstones are associated 

with the felsic volcanic rocks. 

The Middle Proterozoic rocks that host the Olympic Dam deposit occur within a northwest-trending 

graben about 13,000 ft (4,000 m) wide that is developed 

along a major northwest-trending tectonic zone of 

regional extent (O'Driscoll, 1982). The graben is floored 

by alkali-feldspar granite and contains narrower troughs 

filled with a thick sequence of coarse clastic sedimentary 

rocks. The basin fill includes distinctive hematite-rich 

breccias and thinly bedded iron-formation. Stratigraphic 

relationships indicate that volcanism was contemporaneous in the region with sedimentation; the ironformation is interpreted to have been deposited in a playa 

lake setting in which the iron was introduced from a 

fumarolic source (Roberts and Hudson, 1983). The 

precise relationship of these mineralized rocks to regional 

stratigraphic units is not known. The mineralized rocks 

do not crop out; they are unconformably overlain by a 

1, 150-ft (350-m)-thick cover of the Adelaidean sedimentary rocks, which completely obscure the underlying 

deposit. 

The Olympic Dam deposit is essentially a large 

stratabound hematite-rich iron deposit that contains large 

amounts of associated copper, uranium, gold, and light 

REE. The sulfide assemblages are similar to those 

associated with stratabound copper deposits. In addition 

to the stratabound material, much of the deposit consists 

of transgressive, discordant sulfide-bearing material and 

associated hematite. Uranium mineralization generally is 

closely associated with copper zones, whereas gold seems 

to be concentrated separately. Both the stratabound and 

the transgressive deposits are epigenetic and formed from 

hydrothermal solutions that contain sulfur of magmatic 

derivation (Roberts and Hudson, 1983, 1984). 

A general, preliminary model can be formulated for 

the Olympic Dam deposit. It is necessarily based largely 

on published descriptions of the deposit (Roberts and 

Hudson, 1983; Roxby Management Services Proprietary, 

Ltd., 1986) and on interpretation of the tectonic 

environment. 

1. The tectonic setting of the Olympic Dam deposit 

was an extensional regime within a stable cratonic region, 

apparently near the Early Proterozoic continental margin. 

The region (Gawler domain) had become a stable cratonic 

platform by about 1.6 Ga (Parker, 1983; Plumb, 1979). 

2. Following the culmination of cratonization, narrow, elongate, local(?) fault-controlled depositional 

basins were formed in and along major northwesttrending tectonic zones; coarse clastic sediments, mainly 

sedimentary breccias, accumulated in these basins. 

Volcanism accompanied sedimentation and possibly was 

bimodal because both mafic and felsic volcanic clasts are 

present in the breccias. Stratabound deposits of ferrous 

iron-bearing minerals were formed contemporaneously 

with sedimentation. This mineralization was followed by 

additional stratabound mineralization that contained 

hematite, copper sulfides, uranium, REE-bearing 

minerals, fluorite, and barite; this additional stage apparently was the main stage of copper-uranium 

mineralization. 

3. Recurrence of fault-related tectonism and hydrothermal activity led to renewed mineralization and 

reworking of preexisting mineralized rocks and to the formation of transgressive discordant ore bodies and 

polymict breccias. The tectonism was accompanied by 

emplacement of additional igneous material. 

4. The age of the mineralization can be constrained 

within broad limits. The lack of penetrative deformational structures in the ores and associated rocks indicates 

that the ores are younger than the regional rv 1. 6-Ga 

orogenic deformational event in the Gawler domain. The 

ores are older than the widespread, undated dolerite dikes 

in the region, which are virtually unaltered and do not 

intrude the Adelaidean cover sequence. Possibly, the 

mineralization was broadly coeval with the bimodal 

volcanism documented in the Gawler range, which 

Speculative Metallogeny 37 
occurred at "'1.5 Ga (A.W. Webb, as reported by 

Branch, 1978). 

5. An alkaline igneous (anorogenic) connection for 

the ores is strongly suggested by the (a) complex multielement association of iron, copper, uranium, precious 

metals, rare-earth elements, phosphorus, fluorine, and 

barium; (b) closely associated (spatially) bimodal volcanic 

rocks and alkali feldspar granite; and (c) moderately oxidized iron-rich ore-bearing solutions. 

6. Magmatic processes that possibly can account 

for the Olympic Dam deposit could have been similar to 

those proposed for the volcanogenic fluorite-hematite 

deposits at Vergenoeg in the Bushveld Complex, South 

Africa (Crocker, 1985). Crocker attributed the Vergenoeg 

ores to immiscibility between two magma fractions-a 

silica-sodium-rich melt and an actinolite-rich mafic 

melt-in a highly differentiated and fractionated 

Bushveld granitic melt. The immiscibility led to partitioning of iron oxide-fluoride-phosphate and REE cations 

into the mafic melt. Upward migration of the two immiscible fractions led eventually to violent explosion due 

to structural failure of the roof, with the consequent 

deposition of felsic (rhyolitic) pyroclastics followed by 

the expulsion of the mafic magma fraction, which was 

converted to magnetite-siderite-fluorite plus residue as a 

result of pressure release. Further separation of the 

magma fractions occurred as a consequence of immiscibility or fractional crystallization and yielded the different, spatially segregated iron and fluorite bodies. As 

at Olympic Dam, stratiform deposits are transected by 

mineralized, discordant breccia pipes. 

In conclusion, it seems probable that the Olympic 

Dam ores were formed in an extensional environment involving a large evolving, oxidized, iron-rich hydrothermal system associated with active anorogenic magmatism. 

The northern midcontinent has several major 

geologic features that are grossly similar to the setting 

in which the Olympic Dam deposit was formed, as 

enumerated below. 

(1) The geologic setting of the northern midcontinent region is broadly similar to that of the Gawler craton 

in South Australia. The region had stabilized by "'1.6 Ga 

(Sims and Peterman, 1986), following the Central Plains 

orogeny and its equivalent in the southwestern United 

States (Bickford and others, 1986). At about 1.5 Ga, approximately 100 m.y. after the culmination of cratonization of the crust, voluminous anorogenic magmatism 

occurred in a part of the region, which extended from 

Wisconsin (Anderson, 1983) through northern Illinois 

(Hoppe and others, 1983) into Missouri. This magmatism 

is recorded by the discrete rapakivi-type granite plutons 

and locally associated anorthosite bodies, such as 

the Wolf River batholith (Van Schmus and others, 

1975) in the Transcontinental anorogenic province, 

and by the anorogenic epizonal rocks of the St. Francois 

granite-rhyolite terrane (Bickford, Harrower, and others, 

1981). About 100 m.y.later, anorogenic magmatism comparable in all essential respects to that in the St. Francois terrane occurred in the area to the west and possibly 

overlapped the older granite-rhyolite terrane in southern 

Missouri (Sims, 1985). The younger anorogenic rocks 

(Spavinaw granite-rhyolite terrane) have crystallization 

ages in the range of 1.35-1.4 Ga (Thomas and others, 

1984). We interpret all the epizonal granite and rhyolite 

as having formed during region-wide extension. These 

rocks, as well as the deeper seated anorogenic intrusions 

of the Transcontinental anorogenic province, are considered to have been derived by partial melting of 

somewhat older crustal rocks (Cullers and others, 1981; 

Van Schmus and Bickford, 1981; Nelson and DePaolo, 

1985). 

2. Clastic sedimentary rocks that are inferred to 

have been deposited concurrently with or perhaps later 

than the 1.35-1.4-Ga granite-rhyolite magmatism of the 

Spavinaw terrane accumulated at least locally in the 

southern part of the northern midcontinent region, apparently in one or more fault-controlled basins along the 

major northwest-trending Chesapeake and BolivarMansfield tectonic zones (Sims, 1985). Drilling has 

penetrated sedimentary accumulations in Vernon County, Mo., and adjacent Bourbon County, Kans., Cedar 

County, Mo., and Fulton County, Ark. (fig. 17). The 

sedimentary rocks include conglomerates containing 

clasts of rhyolite and granite as well as argillite, quartzite, and arkose. The sedimentary rocks reflect local tectonic instability, probably resulting from reactivation of 

the northwest-trending faults. The rocks are variably 

altered to albite, epidote, chlorite, and calcite, and at one 

locality (Cedar County) are iron rich (mainly magnetite). 

Samples from this locality contain as much as 10 percent 

iron and anomalous concentrations of chromium (as 

much as 50 ppm), nickel (as much as 100 ppm), and 

cobalt (as much as 50 ppm). Such rocks are possibly 

analogous to the sedimentary rocks hosting the stratabound iron-copper-uranium deposits at Olympic Dam, 

but so far as is known, they lack detectable copper, 

uranium, and precious metals, which make up the ores 

at Olympic Dam. 

3. As discussed earlier, the anorogenic rocks of the 

St. Francois granite-rhyolite terrane are strongly differentiated and enriched in alkalies, iron, and volatiles; some 

are enriched in LILE, including rubidium, barium, 

gallium, yttrium, REE, zirconium, thorium, niobium, tin, 

lithium, and uranium. Major, high-grade iron deposits 

and local copper-iron deposits are genetically and spatially associated with the anorogenic suite and display a wide 

variety of ore types, including magmatic, hydrothermal, 

and exhalative deposits, and thus indicate the existence 

of evolved, oxidized magmatic-hydrothermal systems. 

Although these systems are dominated by iron, they 

38 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
Mesozonal granite 

17 • flows and tuffs "v2v t> <J Felsic (148 Ga) ~> • terrane · ., "' St. Francois . 

/1- 1 /I Epizonal gramte ...... y g-1 

/1~- ;/ PROTEROZOIC 

EARLY . 'd rocks of 

75 KILOMETERS 

t contact 

Approxima e thrown side 

High-angle fau It , bar an d ball on down 

lative movemen t not known 

Fault, ,. ffeetl on the level 

--10-- Structure Precambnan c~nto bedrock urs (In hundreds surface. oDatum is mean sea 

0 Drill hole to b a sement 

ammphic and !J'amtml63-1.8 Gal . 

M~entral Plains orogen \ . h estern Missou rl 

ks in sout w . n basement roc . (1984). f the Precambna b E B Kisvarsanyl 7 

Figure 1 Geologic map o contours y .. . Geology and structure Sims (1985). 

and adjacent areas. Modified from 

Specu I a tive Metallogeny 39 
contain anomalous concentrations of copper and have 

associated volatiles, including fluorine. Similar deposit 

types and highly differentiated anorogenic rocks can be 

expected to be present in the Spavinaw granite-rhyolite 

terrane because of the closely similar rock chemistries. 

Considering the large area covered by the Middle Proterozoic anorogenic terranes and their evolved magmatichydrothermal systems, it can reasonably be inferred that 

mineral deposits of Olympic Dam type could exist 

somewhere in these terranes. If the sedimentary rocks in 

southwestern Missouri and adjacent areas (fig. 17) are 

in part at least contemporaneous with anorogenic 

magmatism, ore-bearing fluids from the magmas could 

have been introduced into the sedimentary pile. 

Previously, Hauck and Kendall (1984) suggested 

that several areas in the central United States, from 

Missouri to the Great Lakes region, are favorable for the 

occurrence of iron oxide-rich deposits of the type present at Olympic Dam, Australia, Kiruna, Sweden, the St. 

Francois Mountains, and elsewhere. They proposed that 

this type of ore body composes a diverse family of 

economically significant, multicommodity, genetically 

related deposits that are intermediate in age between Early 

Proterozoic banded iron-formations and Phanerozoic 

ironstones. They emphasized, in the same way we do, that 

these deposits ( 1) occur within tensional basins on the 

margins of Archean shields, (2) were formed during a 

global period of cratonic stabilization, and (3) are 

genetically related to magmas derived from postorogenic 

lower crustal melting. 

Possible Unconformity-Type Uranium Deposits in 

Quartzite of the Baraboo Interval 

Quartzite of the Baraboo interval was deposited in 

an intracratonic setting within the northern part of the 

midcontinent region. The quartz arenites are dominantly fluvial deposits that lie unconformably on complex 

metamorphic terranes of older Archean and Early Proterozoic age. Because of the composition and tectonic environment of the quartz arenites, Ojakangas (1976) and 

Cheney (1981) have suggested that these rocks are 

favorable for the occurrence of possibly large 

unconformity-type uranium deposits. They based their 

suggestions on general criteria for unconformity-related 

vein-breccia uranium deposits, as defined by Mathews 

(1978). Considerable drilling in search of uranium during the past decade, especially in the Sioux Quartzite in 

southwestern Minnesota, has not encountered uranium 

concentrations, however. 

The type area of unconformity-type, or Athabascatype (Cheney, 1981), uranium deposits in North America 

is the Athabasca basin in northern Saskatchewan, 

Canada. The deposits are spatially associated with and 

presumably genetically related to a major regional unconformity between metamorphosed Archean and Early 

Proterozoic basement rocks and overlying unmetamorphosed red sandstone of the Middle Proterozoic Athabasca Formation. Pre-Athabasca weathering preceded 

deposition of the sandstone. Deposits that occur in the 

Pine Creek geosyncline in the Northern Territory in 

Australia (Ferguson and Goleby, 1980) are similar to 

those in Saskatchewan in terms of geologic setting and 

age, host rock alteration, mineralogy, geochemistry, and 

general spatial association with graphitic and carbonaceous tocks. The age of the mineralization in Saskatchewan is estimated to be "'900-1 ,250 Ma, that is, 

200-400 m.y. younger than the host rock (Athabasca Formation; Cumming and Rimsaite, 1979). The age of the 

uranh.1m mineralization is approximately coincident with 

the time of emplacement of a regional set of diabase dikes 

(Hoeve and others, 1980). 

The uranium deposits in the Athabasca area (Hoeve 

and others, 1980) occur in the sandstone immediately 

above the unconformity and in fractures in the basement 

rocks, as veins, breccia-fillings, and disseminated impregnations. A structural control is indicated by the occurrence of uranium minerals, mainly uraninite and 

coffinite, in fractures, some of which moved repeatedly 

during the mineralization. Associated alteration in the 

basement rocks, the pre-Athabasca weathered rocks, and 

the sandstone above the unconformity is recorded by 

abundant chlorite, sericite, and tourmaline, and evidence 

of magnesia metasomatism. Amorphous hydrocarbons 

are present in most deposits. The uranium deposits are 

uncommonly high in grade (as much as several tens of 

percent uranium), and the uranium commonly has 

associated nickel-, cobalt-, arsenic-, copper-, and zincbearing minerals, mainly as base-metal sulfides and 

sulfarsenides, which at places constitute ore minerals. 

Quartz, carbonate minerals, siderite, chlorite, sericite, 

and adularia are common gangue minerals. Pagel (1977) 

has reported that fluid inclusions in gangue quartz from 

one mine indicate a temperature of formation of 

160 ± 10 °C and a pressure of less than 1 kilo bar. 

Several models involving supergene, magmatic, and 

diagenetic-metamorphic waters, or combinations of each, 

have been proposed to account for the Athabasca 

unconformity-type uranium deposits. Of these, the 

diagenetic-hydrothermal model of Hoeve and Sibbald 

(1978), modified by Hoeve and others (1980), seems most 

applicable. This model relates uranium mineralization to 

diagenetic processes in the Athabasca Formation. Under 

conditions of deep burial, oxidizing, uraniferous saline 

waters in the Athabasca interacted with graphite-bearing 

rocks of the subjacent basement to yield reducing solutions containing carbon dioxide and methane. Mineralization resulted from interaction of the reducing solution 

and the oxidizing diagenetic solution that carried the ore 

components, in the vicinity of the unconformity. 

40 Archean and Proterozoic Basement Terranes, Northern Midcontinent, U.S.A. 
The quartz arenites of the northern midcontinent 

are similar in tectonic setting, age, and lithology to the 

Athabasca Formation in northern Saskatchewan. They 

were deposited on older Archean and Early Proterozoic 

metamorphic and granitoid rocks, and the region has 

been stable since deposition. The Sioux Quartzite, in particular, is virtually unmetamorphosed and only mildly 

deformed. The quartzites, however, almost certainly are 

older than the Athabasca inasmuch as one body (the 

McCaslin Quartzite in northern Wisconsin) is cut by an 

intrusive pluton related to the 1.48-Ga Wolf River 

b_atholith and all bodies are inferred to be at least 1.63 

Ga (Van Schmus and Bickford, 1981). The quartzites are 

mineralogically mature and consist mainly of framework 

quartz grains; conglomeratic beds are commonly present 

near the base; hematitic clayey matrix and beds of red 

argillite have been altered by diagenesis to sericite, 

muscovite, kaolinite, diaspore, and pyrophyllite (Morey, 

1983b). A weathered profile occurs beneath the Sioux 

Quartzite (Southwick and Mossier, 1984). 

Radon, helium, and dissolved U3

0 8 are anomalously concentrated in water wells near the basal contact 

of the Sioux Quartzite in the Cottonwood basin of southwestern Minnesota. Southwick and Lively (1984) noted 

that the anomalies could be related to a concealed 

uranium deposit, but suggested that the radon anomalies 

more likely indicate concentrations of radium rather than 

uranium. 

Other metallogenetic aspects of unconformity-type 

uranium deposits, as outlined by Hoeve and others 

(1980), are largely unknown, partly because of poor exposures in the midcontinent. Graphitic- or carbonaceousbearing basement rocks are not known in the vicinity of 

quartzite outcrops, and the chloritic alteration· that 

characterizes the unconformity-type uranium deposits has 

not been recognized. Regardless of these apparently 

negative aspects, we suggest that further exploration, including deep drilling into the basement, is warranted 

because of the possible potential for giant uranium 

deposits (Cheney, 1981). 

SUMMARY AND CONCLUSIONS 

The northern midcontinent is a critical region for 

understanding the construction of the North American 

Precambrian craton. It is a collage of tectonostratigraphic terranes ranging in age from Archean to 

Middle Proterozoic that record several major crustforming events and the erosion and deposition of platformal, epicratonic successions. Assembly of the craton 

was accomplished largely by 1.6 Ga, through lateral accretion of oceanic arc complexes to preexisting continental crust. Following construction of the composite craton, 

new additions to the crust were mainly through vertical 

addition of diachronous Middle Proterozoic (1.48 and 

1.35-1.4 Ga) subaerial rhyolite and cogenetic granite, 

which were derived from and deposited on older (1.8-1.6 

Ga) crustal rocks. A major, younger addition to the 

North American craton was the mafic magmatism and 

accompanying clastic sedimentation within the aborted 

Midcontinent rift system (1.2-1.1 Ga). 

The tectonic processes that led to the development 

of the Precambrian composite crust in the midcontinent 

produced a variety of ore-generating environments (table 

1 ). Oceanic-arc complexes-the Late Archean greenstonegranite terrane, the Early Proterozoic Wisconsin magmatic terrane of the Penokean orogen, and the Early 

Proterozoic rocks of the Central Plains orogen-contain 

known and potential stratabound zinc-copper massive 

sulfide deposits, gold deposits, and iron-formations of 

Algoma type. In addition, the Archean gneiss terrane, 

at least in part, may be a highly metamorphosed and tectonized greenstone belt; minable deposits have not been 

delineated within it, however. The volcanic-hosted 

massive sulfide deposits in the Early Proterozoic calcalkaline Wisconsin magmatic terrane have lithologic and 

chemical similarities to the Miocene Kuroko-type massive 

sulfide deposits and, by analogy, possibly formed in an 

arc-related rift within a convergent margin environment. 

The remaining terranes in the midcontinent are continental crustal segments that evolved in extensional environments during the Early and Middle Proterozoic. The 

Early Proterozoic sedimentary-bimodal volcanic Marquette Range Supergroup, which contains the large 

Superior-type iron deposits of the Lake Superior region, 

is interpreted as a passive margin sequence (Schulz and 

others, in press) that developed on the southern margin 

of the Superior Archean craton. The iron deposits do not 

extend into the subsurface, except possibly for small 

deposits in northwestern Iowa (Sims, 1985; Yaghubpur, 

1979). 

The 1. 76-Ga rhyolite-granite terrane of southern 

Wisconsin initiated a period of extensive alkali-rich 

anorogenic magmatism in the midcontinent region as a 

result of crustal melting that persisted intermittently until 

1.35 Ga. The 1.76-Ga anorogenic magmatism occurred 

virtually simultaneously with the orogenic magmatism in 

the Central Plains orogen and areas to the west (Bickford 

and Boardman, 1984), and perhaps reflects rifting related 

to the more southerly orogenic tectonism. Mineral 

deposits of potential economic value are not known to 

be associated with the 1. 76-Ga anorogenic magmatism, 

but its lithologic and chemical similarities to the younger 

ore-bearing St. Francois granite-rhyolite terrane are 

favorable for it to contain iron-rich bodies and possibly 

other types of mineral deposits. The 1.48-Ga anorogenic 

magmatism, represented by the anorthosite-rapakivi 

granite assemblages in the Transcontinental anorogenic 

province, such as the Wolf River batholith, and the 

Summary and Conclusions 41 
epizonal granite-rhyolite of the St. Francois terrane, are 

possibly manifestations of the exposure of different erosional levels, exposing on the one hand a predominant 

mesozonal igneous suite and on the other hand an 

epizonal igneous suite. This episode of anorogenic 

magmatism produced the valuable iron and copper-iron 

ores of the St. Francois terrane. It was followed about 

100 m.y.later by very similar granite-rhyolite anorogenic 

magmatism in the Spavinaw terrane to the south and 

west. 

The youngest event related to the prolonged extensional environment during the Proterozoic was development of the Midcontinent rift system, an aborted rift 

filled by mafic volcanics and hypabyssal intrusive rocks 

and derivative sedimentary rocks. It contains known, 

large rift-related stratiform copper deposits and magmatic 

copper-nickel and possible platinum-group elements; 

possibilities are excellent that similar deposits exist in the 

subsurface. 

The gross tectonic environment of the Middle Proterozoic anorogenic terranes in the southern part of the 

region is remarkably similar to that of the giant Olympic 

Dam deposit in South Australia (Roberts and Hudson, 

1983) and of other iron-rich deposits of comparable age 

(Hauck and Kendall, 1984). Accordingly this area merits 

consideration as an exploration target for such a deposit. 

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, U.S. GOVERNMENT PRINTING OFFICE: 1987-573-047166,015 REGION NO.8