Uranium Deposits in Proterozoic Quartz-pebble Conglomerates (Report of the Working Group on Uranium Geology) | IAEA

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IAEA-TECDOC-427

URANIUM DEPOSITS IN PROTEROZOIC

QUARTZ-PEBBLE CONGLOMERATES

REPORT OF THE WORKING GROUP ON URANIUM GEOLOGY ORGANIZED BY THE

INTERNATIONAL ATOMIC ENERGY AGENCY

A TECHNICAL DOCUMENT ISSUED BY THE

INTERNATIONAL ATOMIC ENERGY AGENCY, VIENNA, 1987

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URANIUM DEPOSITS IN PROTEROZOIC QUARTZ-PEBBLE CONGLOMERATES IAEA, VIENNA, 1987

IAEA-TECDOC-427 Printed by the IAEA in Austria

September 1987

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PLEASE BE AWARE THAT

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WERE ORIGINALLY BLANK

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The IAEA does not normally maintain stocks of reports in this series.

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FOREWORD

The surge activity in exploration and research regarding uranium deposits which ended in the early 1980's added significantly to our

knowledge of uranium geology and the nature of uranium deposits. Much of the information that has been developed by government and industry

programmes has not been widely available and in many cases has not had

the benefit of systematic gathering, organization and publication. With

the reduced uranium exploration and research efforts there is a danger that much of the knowledge will be lost. In an effort to gather together the most important information on the types of uranium deposits, a series

of reports has been prepared each covering a specific type of deposit.

These reports are a product of the Agency's Working Group on Uranium Geology. This group gathered and exchanged information on key questions of uranium geology and co-ordinated investigations on important

geological questions.

The reports have been developed and through a series of projects.

The topics and the project leaders are listed below. This volume is the

last to be prepared in this series of Tec Docs.

Proterozoic Unconformity and Stratabound IAEA-TEC-DOC-315 1984 Uranium Deposits

- John Ferguson -

Surficial Uranium Deposits IAEA-TEC-DOC-322 1984 - Dennis Toens -

Sedimentary Basins and Sandstone- IAEA-TEC-DOC-328 1985 type Deposits

- Warren Finch -

Vein-type Uranium Deposits IAEA-TEC-DOC-361 1985 - Helmut Fuchs -

Uranium Deposits in Proterozoic (This volume)

Quartz-Pebble Conglomerates

- Desmond Pretorius -

The success of the projects is due to the dedication and efforts of the project leaders and their organizations, and the active

participation and contribution of world experts on the types of deposits involved. The Agency wishes to extend its thanks to all involved in the

project for their efforts. The reports constitute an important addition

to the literature on uranium geology. They have had an enthusiastic reception by the member states of the Agency and the uranium community worldwide.

Special thanks are extended to Desmond Pretorius of the Economic Geology Research Unit of the University of the Witwatersrand, who

organized and guided this project on Uranium Deposits in Proterozoic Quartz Pebble-Conglomerates, and to his colleagues and their

organizations, for their efforts and support in the preparation of this volume.

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EDITORIAL NOTE

In preparing this material for the press, staff of the International Atomic Energy Agency have mounted and paginated the original manuscripts as submitted by the authors and given some attention to the presentation.

The views expressed in the papers, the statements made and the general style adopted are the responsibility of the named authors. The views do not necessarily reflect those of the govern- ments of the Member States or organizations under whose auspices the manuscripts were produced.

The use in this book of particular designations of countries or territories does not imply any judgement by the publisher, the IAEA, as to the legal status of such countries or territories, of

their authorities and institutions or of the delimitation of their boundaries.

The mention of specific companies or of their products or brand names does not imply any endorsement or recommendation on the part of the IAEA.

Authors are themselves responsible for obtaining the necessary permission to reproduce copyright material from other sources.

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FAVORABILITY OF PRECAMBRIAN QUARTZ-PEBBLE CONGLOMERATES IN THE UNITED STATES

AS URANIUM HOSTS

J.R. ANDERSON, C.S. GOODKNIGHT, J.M. SEWELL, J.K. RILEY

Bendix Field Engineering Corporation, Grand Junction, Colorado,

United States of America Abstract

Precambrian quartz-pebble conglomerates in numerous areas in the United States were examined in detail under the U.S. Department of Energy's (DOE) National Uranium Resource Evaluation (NURE) program. The two most significantly mineralized areas are in Wyoming and are the subject of a separate paper in this volume. In this report, summary information is

presented on eight other areas in South Dakota, Montana, Colorado, Arizona,

California, Michigan, and Wisconsin.

Models were developed around the generally recognized favorability criteria for economic deposits in Precambrian quartz-pebble conglomerates.

The basic assumptions are that these conglomerates were deposited on Archean cratons at least 2.0 b.y. ago as fluvial oligomictic detritus that included pyrite and uraninite. Uranium in these conglomerates has not been depleted by

subsequent deformation or metamorphism.

Comparisons of features of the conglomerates in the United States with

the economic deposit model indicates those in the U.S. greater than 2.0 b.y.

old tend to be so metamorphosed and structurally deformed that survival of original detrital uraninite or related uranium accumulations is doubtful. No

detrital uraninite was recognized in these older conglomerates or in several younger ones. Thorium and uranium are in thorium-rich heavy mineral placer

concentrations typical of conglomerates of any age.

The knowledge gained from these investigations support the validity of

the time-space restriction observed in the economic deposits elsewhere in the

world. Since exposures of Precambrian rocks in the United States are limited, more favorable environments may exist in the subsurface. As geologic know-

ledge expands, target areas for exploration may be definable.

INTRODUCTION

As part of the DOE's NURE program, studies were initiated in the mid- 1970's to evaluate geologic environments in the United States in which significant uranium deposits had not yet been discovered but which host important

deposits elsewhere in the world. This report summarizes the results of

investigations to determine favorability for uranium deposits in one of these

environments, namely Precambrian quartz-pebble conglomerates. It is adapted

from a publication by Anderson and others [1] that is a review of the work of

numerous DOE-funded studies by selected authorities that are referenced in the text. These studies were preceded by geologic reconnaissance and sampling in

the 1960's in most of the quartz-pebble conglomerate areas of interest as part

of a regional analysis of radioélément distributions in Precambrian rocks of the United States [2].

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The methods used in the studies for this report included geologic map- ping, radiometric surveying, examining and describing outcrops, sampling for geochemical and pétrographie analyses, and interpreting results in terms of favorability for uranium. Drilling was conducted in two areas to provide information on the subsurface extent and uranium content of conglomerate host rocks.

Precambrian uraniferous quartz-pebble conglomerate is a specific type of placer uranium deposit with geologic characteristics whose presence defines a favorable environment. For instance, establishing the presence or clear indication of the former presence of uraninite as the chief uranium mineral is critical in evaluating the favorability of the environment of deposition. A clear distinction must be made between oligomictic, uraniferous quartz-pebble conglomerates that contain economic uraninite paleoplacer deposits, and the uneconomic monazite- and zircon-rich placers [3] that also, and far more commonly, occur in conglomeratic rocks.

Time terminology used in this report is as follows:

middle Proterozoic 1.8 to 1.0 b.y.

early Proterozoic 2.5 to 1.8 b.y.

Archean older than 2.5 b.y.

CRITERIA AND MODELS

For economic concentrations of uranium to accumulate in quartz-pebble conglomerates, proper time, source-rock, depositional-environment, and preservation conditions must be met. These conditions have been incorporated in genetic models by Houston and Karlstrom [4] and Button and Adams [5], and are used in a report by Karlstrom and others [6, 7]. Houston and Karlstrom [4] researched the Precambrian conglomerate deposits of the world and the more promising Precambrian terranes in the United States, and developed a genetic model to use for comparitive purposes. Button and Adams [5] synthesized the data available for uraniferous quartz-pebble conglomerates and established recognition criteria useful in predicting the presence or absence of deposits.

They emphasized the weighting of criteria to provide a basis for comparisons, rankings, and decision making and ranked the two most promising U.S. areas, the Medicine Bow Mountains in Wyoming and the Black Hills in South Dakota, relative to the Elliot Lake uranium district. Karlstrom and others [6, 7]

provided a summarized comparison of southeastern Wyoming conglomerates with the Houston-Karlstrom genetic model.

The simplified listing of general criteria in Table 1 was applied for basic screening of all Precambrian quartz-pebble conglomerates now recognized in the United States. Information generally is not yet adequate for detailed screening. Comments regarding the selected recognition criteria in Table 1 follow.

Economically significant uraniferous conglomerates have formed through mechanical concentration in basins between 2.0 and 3.2 b.y. old on an Archean craton. This time-spece criterion is essential, but other criteria must be met [4, 5].

It is not essential that the presence of uraniferous granitic source rocks be confirmed, however, geologic inference of their existence as a source of detritus for conglomerates is essential.

The presence of truly oligomictic, fluvial conglomerates is crucial as it indicates reworking of the sediments and associated detrital uraninite essential for placer uranium concentration has occurred.

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TABLE 1. SELECTED RECOGNITION CRITERIA AND WHETHER THEY ARE MET IN THE AREAS/ENVIRONMENTS EVALUATED

Selected recognition criteria

Areas/environments evaluated

1 2 3 4 5 6 7 8

Deposition between 2.0 and 3.2 b.y. ago

on Archean craton + ? + - - Upper Archean granitic source rocks ? - + + ? Oligomictic conglomerates of fluvial origin - - - ? ? Radioactive, pyritiferous conglomerates + ? -

Negligible primary iron oxide minerals + + +

Detrital uraninite indicated - - - - - Metamorphism less than amphibolite grade

and gentle deformation + - - ? + Areas/environments evaluated:

1. Black Hills

2. Southwestern Montana 3. Dickinson Group

4. Marquette Range Supergroup 5. McCaslin Formation

6. Needle Mountains

7. Kingston Peak Formation 8. Central Arizona Arch

Explanation:

+ » Criterion met.

- = Criterion not met.

? = Insufficient or conflicting data; indeterminable.

The presence of radioactive pyritiferous conglomerate is a major indicator of favorability. Ruzicka [8] believed that sulfides act as agents for preserving detrital uranium minerals during transportation, deposition, and diagenesis. The known economic uranium deposits lie within broader anomalously radioactive zones that are clearly pyritiferous.

The presence of more than trace amounts of primary iron oxide minerals in conglomerate matrices is considered a "killer" by Button and Adams [5], and primary hematite colors in host quartzites are extremely discouraging.

The apparent absence of either uraninite or indications of the former presence of uraninite does not justify rejection of an area if other favorable characteristics are present. However, definite hydraulic equivalences have been established between uraninite, detrital pyrite, thorium-rich resistate minerals, and quartz (and gold if present).

Although substantial dissolution and precipitation of detrital uraninite has occurred in some deposits, the absence of uraninite or related uraniferous alteration products in the appropriate size ranges of detrital quartz must be considered discouraging in otherwise favorable conglomerates.

The precise metamorphic grade at which detrital uranium concentrations of economic promise are mobilized and lost is not known. All the known large deposits are in rocks of the greenschist faciès except the Jacobina, Brazil, deposits which are of the amphibolite faciès and are mined mainly for gold. In this report, the preservation of uraninite placers is considered to be dependent upon regional metamorphism being less than that of the amphibolite faciès. Further, gentle—never severe—regional deformation is a feature of known economic deposits.

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AREAS EVALUATED

Summaries of the geologic features and the relative favorability for uranium deposits in eight of the thirteen areas with Precambrian quartz-pebble conglomerates in the United States shown in Figure 1 are presented in the following sections. The conglomerates that have attracted the most attention are in the Medicine Bow and Sierria Madre Mountains of southeastern Wyoming, reviewed by Robert Houston in a separate paper in this volume .

BLACK HILLS

Upper Archean or lower Proterozoic rocks crop out in the Nemo, Bear Mountain, and Harney Peak (?) areas in the Black Hills, South Dakota (Fig. 2).

These rocks are part of the crystalline core complex, of the Black Hills, 1.7 b.y. to at least 2.5 b.y. old, which are exposed as a result of early Tertiary Laramide uplift. In the northern Black Hills, intrusives were emplaced contemporaneously with Laramide uplift.

The rocks in the Black Rills have been subjected to three to six periods of folding and faulting and probably two periods of metamorphism, resulting in a complex structural setting [10]. The metasediments in the Nemo area have been metamorphosed to the greenschist faciès near the boundary of the garnet and biotite zones (Fig. 2), and represent the lowest metamorphic grade in the Black Hills.

In the Nemo area, the rocks of interest consist of radioactive, pyritiferous, polymictic quartz-pebble conglomerates in the Tomahawk Tongue of the Boxelder Formation. The lower Proterozoic Boxelder rocks were deposited as alluvial-fan faciès on or stratigraphically near the Archean basement. The Boxelder Formation has been bracketed between 2.56 and 2.09 +_ 0.10 b.y. old and lies near the eastern margin of the Wyoming Archean Province as discussed by Hills and Houston [9] and Houston and Karlstrom [4].

In the Bear Mountain Dome area (Fig. 2), at the western margin of the Black Hills, the Precambrian metasedimentary rocks have not been studied in detail. In general, these rocks consist of a granitic-pegmatitic core 2.5 b.y. old [11]. These granitic rocks have intruded an older biotite schist and both are unconformably overlain by metaconglomerate, quartzite, mica schist, amphibolite schist, and dolomitic marble (10). The rocks are structurally complex and have been metamorphosed to the amphibolite faciès (staurolite zone, Fig. 2). No anomalous radioactivity has been reported from this area

[5].

For the Harney Peak area, in the central southern Black Hills (Fig. 2), Button and Adams [5] have suggested that a quartzite near an inferred domal structure in the Harney Peak Granite may be associated with Archean or lower Proterozoic metasedimentary rocks similar to those in the Nemo area. The rocks in this area have been metamorphosed to the upper amphibolite faciès

(sillimanite zone, Fig. 2).

Kirn [13] suggested several models characterizing the depositional environment of the metasediments in the Nemo area. These models consist of the following: (1) a lacustrine environment associated with alluvial fans, (2) a marginal-marine environment with prograding delta, and (3) a glaciofluvial environment. Figure 3 depicts Redden*s [14] interpretation of

the depositional and structural history of the Nemo area. The authors of this report believe that an alluvial-fan environment that grades basinward into marginal-marine or possibly braided-stream deposits best fits the radioactive

conglomerates in the Nemo area.

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Wyoming Archean f Province

Central Lorami«

EXPLANATION

Precambrian, uncertain 3 Pfoterozoic

Archean

T-^- Boundary of Archtan craton«

Figure 1. Generalized map of Archean and Proterozoic rocks in the conterminous United States showing specific areas with conglomeratic faciès.

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104*

« J E T6KI

T4N

T Z N

1 IS

T i S

EXPLANATION

For generalized l o c a t i o n s of rock types (See Table 5 for revised stratigraphy) •

[v'i;] Homey Peak Granite ( i 7 4 0 m y ) I__I Eugeosynclmal schists and phyllites I__j Quortzite undivided

Iron formation, schist, quortzite '.;.;',j Amprtibolite and metogobbro

Roberts Draw limestone and equiv Estes conglomerate and equivalents Nemo Group

Archean granite gneiss ( 2 5 b y )

— — —Metamorphic Isograds

0 5 10 15 20 krr

1 N

Bear

Mounfoin — Dôme

R9E 103-

Figure 2. Generalized geologic map of the Précambrien rocks in the Black Hills (modified from 4. 10, and 12).

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^.-V SRE ENWOOp TON6U

ULTRA- M A F I C TACONITE OLDER M E T A M O R P H I C ROCKS

T O M A H A W K TONGUE

B

B E N C H M A R K IRON FORMATION^

3 0 X E L D E R O U A R T Z I T E

E A S T

Figure 3. Diagrammatic sedimentation and structural sequence. Nemo area (from Redden, 14).

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The presence of quartzite pebbles containing rounded detrital quartz grains and blue quartz, probably vein quartz, indicates a sedimentary and an igneous source for the conglomerates. Therefore, the Nemo conglomerates are not truly oligomictic. The quartz-pebble conglomerates of the Elliot Lake and Witwatersrand deposits have been described as oligomictic by many workers [4].

Because detrital uraninite is associated with an igneous provenance, it is important that the conglomerates in question be dominated by clasts from an igneous source. This is not the case in the Nemo area, where, according to Redden1s [14] core descriptions, the quartzite pebbles constitute the majority of clasts in the drilled sections.

As mentioned before, the sediments in the Nemo area have been metamorphosed to the upper greenschist faciès, the lowest metamorphic grade in the Black Hills. Alteration products, such as fuchsite after detrital chromite and TiC>2 pseudomorphs after ilmenite(T), have been described by Redden [14]. However, the most important alteration mineral probably is uranothorite, tentatively identified petrographically by Kim [13] in a conglomerate matrix. The euhedral character of the uranothorite within the matrix of rounded detrital grains clearly indicates a postdepositional origin, possibly during metamorphism, for the uranothorite. Ortlepp [15] has described the in-situ alteration of uraninite to uranothorite in the Precambrian metasediments of the Dominion Reef area. This alteration process involves the replacement of thorium-rich uraninite by siliceous solutions and may involve the loss of some uranium. This process may have taken place in the Nemo area conglomerates. The combination of a poor sorting mechanism for concentrating detrital uraninite and postdepositional alteration of any detrial uraninite that was present in the Nemo quartz-pebble conglomerates could account for the low uranium content in Redden's core samples (Table 2).

This may also account for the difficulty that Kim and Redden had in identifying any uranium minerals.

TABLE 2. ANALYSES OF DRILL CORE FROM THE TOMAHAWK TONGUE CONGLOMERATE IN THE NEMO AREA, BLACK HILLS (FROM REDDEN, 13)

Hole number 2

2 2 3 3 3 3 3 4 4 4

Interval 397-400 404-415 421-430 248-254 255-259 336-353 435-442 543-548 235-246 320-335 339-343

Core length (ft)

3 11 9 6 4 17

7 5 11 15 4

Weighted Ü content (ppm)

55 55 59 122 40 40 54 65 50 76 100

Weighted Th content (ppm)

61 126 45 166 99 67 105 92 100 107 29

The Precambrian rocks of the Black Hills exhibit several general characteristics considered by Houston and Karlstrom [4] and Button and Adams [5]

to be favorable for uraniferous quartz-pebble conglomerates. These characteristics include: (1) an Archean craton tectonic setting, (2) an age between 2.1 and 2.5 b.y., and (3) a radioactive, pyritiferous clastic host rock. The results of field investigations in the Nemo area by Kim [13] and

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Redden [14] revealed several negative factors: (1) a proposed deposi.ti.onal environment that did not include the hydrologie regime necessary to concentrate placer uranium, (2) the existence of more than one provenance, (3) unfavorably high metamorphic grade, and (4) the general absence of uranium concentrations above 100 pptn. In the Bear Mountain and Harney Peak areas, the metamorphic grade is unfavorably high, reaching that of the amphibolite faciès

[10].

An unfavorable ranking of the Black Hills environments applies only to those rocks in the Nemo, Bear Mountain, and Harney Peak areas. Button and Adams [5] have suggested that other conglomerates may exist at a lower

stratigraphie level or to the northeast under Paleozoic cover.

SOUTHWESTERN MONTANA

Archean and Proterozoic supracrustal metasedimentary rocks crop out in southwestern Montana in the tobacco Root Mountains, southern Highland Mountains, Ruby Range, Greenhorn Range, northern part of the Gravelly Range, northern edge of the Madison Range, and several fault blocks along the Jefferson River (Fig. 4). These rocks were deposited near the northwestern margin of the Wyoming Archean Province craton (Fig. 1).

A generalized Precambrian stratigraphie and age sequence for southwestern Montana is given in Table 3. Quartz-pebble conglomerates, or even quartzites, are not abundant in these Archean and Proterozoic rocks. The LaHood Formation of the North Boulder Group, exposed in tectonic blocks along the Jefferson River, is part of the Belt Supergroup and contains coarse arkoses and conglomerates interpreted as having been deposited in a deltaic or braided-stream environment [16]. Rocks of the Cherry Creek Group consist mainly of quartzofeldspathic gneiss, but siliceous and dolomitic marbles, iron formations, calc-silicates, and quartzites also characterize the group.

Conglomeratic zones occur in the quartzites; however, they are poorly developed and, at best, may be classified as fine-pebble arkosic paraconglom- erates.

The deltaic or braided-stream sediments of the LaHood Formation, a part of the Belt Supergroup of middle Proterozic age, were deposited in an oxidizing environment [17]. Conglomerates of the LaHood Formation are assumed to be too young to host placer uranium deposits.

Archean or lower Proterozoic conglomeratic rocks in southwestern Montana are known only in quartzites in the Cherry Creek Group. Some clast-bearing conglomeratic zones are present in the Cherry Creek Group but no clast- supported rocks have been identified. The Archean Cherry Creek Group rocks were deposited on the margin of the Wyoming Archean Province craton in a continental shelf or miogeosynclinal environment. The shallow-marine environment was characterized by widespread deltas that had frequent influxes of clastic sediments, shales, siltstones, and feldspathic sandstones, alternating with carbonate deposition [18]. Cohenour and Kopp [16] found no evidence that

the radioactive quartzites in the Cherry £reek Group were deposited in channels or other fluvial environments. They concluded that the most likely depositional environment for these middle Archean rocks was barrier bar.

James and Hedge [18] dated the formation of the Cherry Creek rocks at about 3.1 b.y. ago, based on regression of strontium-87-to-strontium-86 ratios to typical mantle values. Mineralogie and element studies of the quartzites of the Cherry Creek by Cohenour and Kopp [16] suggest that they had two source

areas: a granitic terrane that included pegmatites containing uranium and thorium and a gabbro-pyroxenite terrane containing titanomagnetite bodies.

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0\

46°-

HIGHLAND MOUNTAINS

TOBACCO ROOT MOUNTAINS

E X P L A N A T I O N

N o r t h Boulder Group Pony Group Dillon Granite Migmotite Cherry Creek Groi^p

P o n y - C h e r r y CreeK P r e - C h e r r y Creek

Basement

Undifferentiated

"... '': Outline of mountain ranges

0 4 8 1 2 1 6 M M »

i ]_____i__i

0 4 e 12 16 K i i o m t l m

GEOLOGY AFTER DEPARTMENT OF ENERGY-Dillon and Bo2emon quad- rangle mops

30 30 III⁰

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TABLE 3. GENERALIZED PRECAMBRIAN STRATIGRAPHIC SEQUENCE IN SOUTHWESTERN MONTANA (AFTER COHENOUR AND KOPP, 15)

Precambrian unit Description Location

North Boulder Group (1.2 to 1.55 b.y.)

Dillon Granité Gneiss (2.6 to 2.8 b.y.)

Pony Group

(2.8 to 3.1 b.y.)

Cherry Creek Group (3.1 ± b.y.)

Pré-Cherry Creek Group

(3.2 jf b.y.)

Basement (3.3 ± b.y.)

Commonly unmetamorphosed coarse- grained component of the Belt Supergroup. Includes the LaHood conglomerate, sandstone, black shales, and carbonate rocks.

A tabular body of granitic composition, largely concordant between Cherry Creek and Pre-Cherry Creek rocks, upper almandite- amphibolite and granulite rank (Harrovian).

A layered sequence of metamorphosed clastic and pyroclastic sedimentary rocks. Now primarily biotite and hornblende gneisses and

amphibolites. Upper almandite- amphibolite and granulite rank (Harrovian).

Characterized by siliceous,

dolomitic marbles, calc-silicates, quartzites, quartzofeldspathic gneisses, amphibolites, sillimanite schist, and iron formations. Upper almandite-amphibolite and granulite rank (Barrovian).

Biotite-garnet-sillimanite- quartzofeldspathic gneisses, hornblende gneisses, chlorite schists, amphibolites,

hornblendites, and migmatites.

Upper amphibolite and granulite rank (Barrovian).

Granite gneiss.

Northwestern Highland Mountains

Northern fringes of the Tobacco Root Mountains Bridger Range

Ruby Range

Tobacco Root Mountains(î)

Northern Tobacco Root Mountains

Highland Mountains (undifferentiated) Northern Madison

Range(?)

Central and southern Tobacco Root Mountains Highland Mountains

(undifferentiated) Ruby Range

Greenhorn Range Gravelly Range

Western Madison Range Ruby Range

Greenhorn Range(?) Tobacco Root

Mountains (?)

Ruby Range Greenhorn Range Madison RangeO)

Rocks composing the "basement" in this area, and the probable source for the Cherry Creek Group, consist of granite gneiss and are believed to be more than 3.3 b.y. old, earlier than the intrusion of potaesic granites in the late Archean, believed by Button and Adams [5] to be important sources of uranium.

The areas found by Cohenour and Kopp [16] to have the highest radioactivity are in the southern Tobacco Root Mountains. The high radioactivity, up to 10 to 30 times background radiation, is attributable to fine- to coarse- grained quartzites of the Cherry Creek Group [15]. Only traces of pyrite are present in the conglomeratic quartzites.

The accessory heavy minerals magnetite and ilmenite are present in trace amounts in the radioactive conglomeratic zones [16], No uraninite, gold, or thucolite has been found in the radioactive quartzitea. Cohenour and Ropp

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[16] believed that the anomalous radioactivity was attributable to cheralite, a variety of monazite. Cheralite is in rounded to subrounded, detrital grains that are stratabound and commonly associated with graded bedding or cross- bedded strata; the greatest concentration of this mineral, much less than 1Z of the rock is where the quartzites contain a few pebbles of quartz and in zones where iron oxide minerals occur.

The high degree of metamorphism and several periods of intense deformation have obscured the relationship of the radioactive quartzites to unconformitites within the Cherry Creek Group. The rocks underwent dynamothennal metamorphism, dominantly upper amphibolite grade, about 2.75 b.y. ago [19].

The Archean and Proterozoic metasedimentary rocks in southwestern Montana are considered unfavorable for uranium deposits in Precambrian quartz-pebble conglomerate. The braided-stream depositional environment of the LaHood Formation [16] was formed in an unfavorable oxidizing environment during the middle Proterozoic. Although the conglomeratic rocks of the Cherry Creek Group were deposited on an Archean craton margin and the rocks are old enough to have been deposited in an oxygen-poor environment, no clast-supported

"true" conglomerates are known, the rocks were deposited prior to intrusion of upper Archean potassic granites (sources of uranium), and they underwent upper amphibolite-grade metamorphism. The radioactive quartzites show no evidence of being deposited in a fluvial environment and their radioactivity is attributable to weak placer accumulations of cheralite, a member of the thorium-rich monazite group.

DICKINSON GROUP

Most of the Proterozoic Marquette Range Supergroup between Lake Superior and Lake Michigan is underlain by Archean gneiss, granite, and greenstone;

however, in Iron and Dickinson Counties, Michigan, the supergroup is underlain by a poorly exposed sedimentary-volcanic sequence, the Dickinson Group (Fig.

5)[20]. The group, about 3 to 4 km thick, overlies basement gneisses and is divided into three units which are, from oldest to youngest, the East Branch Arkose, the Solberg Schist, and the Six Mile Amphibolite. The Dickinson Group is probably late Archean because it unconformably underlies the Marquette Range Supergroup, and radiometric dates of associated rocks suggest an Archean age. Regional stratigraphie relationships are illustrated in Figures 6 and 7.

The East Branch Arkose is mainly arkose interbedded with conglomerate and volcanic flows and tuffs. The Solberg Schist is probably sedimentary and volcanic and includes biotite, hornblende, and quartz-mica schists with a thin

iron-formation member. The youngest unit, a hornblende-plagioclase basaltic rock, is the Six Mile Amphibolite. Other nearby rocks considered to be in the Dickinson Group are quartz-sericite-magnetite schists, felsites, and green- schists that were originally intermediate and silicic lavas and pyroclastics.

The Dickinson Group sediments probably are late Archean and overlie an uncon formity above Archean cratonic rocks. Possible source rocks for detrital uranium minerals are present in the area [23, 24]. The East Branch Arkose may be a fluvial deposit and contains beds of conglomerate 3 to 10 m thick. These favorable factors are considered to be outweighed by the negative factors that follow. The conglomerates are polymictic and contain clasts of quartzite, granite gneiss, slate, schist, and vein quartz. The nature of the rocks indicates the conglomerates were deposited in a local basin, and little or no reworking and reconcentration of placer minerals took place. Pyritiferous radioactive conglomerates have not been reported. The rocks have been metamorphosed to various degrees but are commonly of the

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Superior Province

^SUPERIORKE

Hurpmon Supergroup

SudtJury.

Iliot r ARQUETTE RANGE

SUPERGROUP _

Minnesota River Valley

50»

44°

100 MILES

Post-Precambnan cover

EXPLANATION

fc j Huronion Supergroup (2500-2160 m y )

O

Archeon granite-greenstone (erroné (2950-2500my.) E%%! Lower Proterozoic (~l700m.y.) granitic rocks |p<| Archean gneissic terror«(»3000m y )

f^l Marquette Range Supergroup and equivalents(2IOO- .•;;. Boundary of s p e c i f,c area investigated in this report 1900 m y), black is banded iron formation

~ Figure 5. Generalized geologic map of Précambrien rocks in the Superior Province of the United States showing specific areas discussed in this report (modified from King, 21, and Sims, 22).

(22)

K»

O

EXPLANATION

(--- I Momly Holt

l-~-~-~l (including, turbidit»i) 7_~jCH Mainly lloti

•~»~ ~' (with »omt volcanici) Mainly Iron formation Mainly carbonaKt

. Mainly quartzltti

Pol«ovall«y fill formation»

(include* paraconglomerote) Volcanic formations

Archian grttniton« b*ltt Arehean gncliiic and

granit« DOMmint Formation boundary

MARQUETTE R A N G E SUPERGROUP AND EQUIVALENTS

OICKINSON GROUP

Figure 6. Schematic fence diagram (not to scale) illustrating regional stratigraphie relationships in the lower Proterozoic strata of the Lake Superior

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Idealized Stratigraphy for Early Proterozoic

Rocks in Michigan and

Ontario

Iron Formation- Volcaniclastic

Sequence (2, 100-1, 800 m.y.)

Aluminous Qtzite- Stromatolitic, Dolomite Sequence

(2,200-2,000 m y )

Radioactive Conglomerate-

Tillite Sequence (2,500-2,200 m y.)

Late Archean Volcano- Sedimentary

Sequence (>2,400 m.y )

Marquette Range Supergroup

and Dickinson

Group, Michigan

fW/WWWVW Paint - a River Gp

0 =

at ~

% „ Birag« , >

to A- f ——

• Menommee

• Group ;

3 ^^^- , - \ '

v 'y'tivirtAAAamÄ 5 C. Kona^E 2 '"Chocolay'^"

Group

w vvwWv\W

> 2,1 50 m y -

wwywvwvy

r Dickinson <• j

> Group * *

Huronian Supergroup.

Ontario

-> 1,900my

— 1.950 m y

VAAAAAAAAAAA/

:-i - C o b a l t Group .^

Gowganda

vyvryvyvyjA

Quirk« Lake

o ijjjiv:?^r

£ Hough Let«

*c yvCvA/OOvî^

| Elliot Lake o Group

^ *<Matmenda.

>TheMaian, ' Copper Cliff Stobie, Pater

• Livingalon«

".Creek F m e

VVVVVWVVVVVVVVVVVVVVVVVVVVVWVVirvVW\/\rVWvV\l Key lithologie* are

— 2 4OO (?) m y

•hown

|*^ '^ Volcanogenic rock* [; ) Aluminum-rich quartut«

jj^^J Uranrterou* quartz-pebble conglomerate ^'j\ Stromatolitic dolomite [*»*.] Oiamiclit« (ttlltte?) Iron formation

Figure 7. Marquette Range Supergroup, Dickinson Group, and Huronian Supergroup stratigraphy and correlations (adapted from Karlstrom and others. 6).

amphibolite faciès. Clasts in the conglomerate have been stretched, the ratio of long axis-to-ahort axis dimensions being 3 to 1. This metamorphism and strong deformation has destroyed all but the most r e f r a c t o r y thorium-uranium minerals.

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The Dickinson Group metasedimentary rocks indicate that the Lake Superior region is not totally devoid of such rocks within favorable Precambrian quartz-pebble conglomerate age constraints. Also, the source of the conglomerate is an Archean craton that includes uraniferous granitic rocks.

As for negative factors, the known conglomerates of the Dickinson Group are polymictic and were deposited in a local basin. Oligomictic, well-worked, fluvial conglomerates in a continental-shallow marine setting are not now indicated. Also, any original placer uranium oxide minerals may not have survived the severe metamorphism and deformation.

More favorable quartz-pebble conglomerate environments may be present elsewhere in Michigan under Paleozoic or other cover.

MARQUETTE RANGE SUPERGROUP

The Precambrian Marquette Range Supergroup is an accumulation of weakly to severely metamorphosed sedimentary and volcanic rocks more than 15 km thick [25]. The supergroup and equivalents overlie the Archean basement, which is a southern extension of the Superior Province of the Canadian Shield.

Uraniferous Euronian Supergroup rocks in the Elliot Lake district (Fig. 8) are associated with units that have been traced to within about 200 km of known Marquette Range Supergroup units.

The Marquette Range Supergroup is divided into four groups which are, from oldest to youngest, the Chocolay, Menominee, Baraga, and Paint River.

The two older groups each occur in widely separated districts, whereas the Baraga is more contiguous and the Paint River ie apparently restricted to a local basin. The generalized distribution and makeup of the groups are out- lined in Figure 6.

Conglomerates and quartzites that overlie Archean basement have a possible strike length of more than 500 km. Some specific areas of Marquette Range Supergroup rocks have been studied in detail but very little is known in most areas due to exensive, thick glacial debris; sparse drilling; and lack of an adequate geophysical data base. Correlations between separately mapped districts have in many places not been clearly resolved. The generalized schematic diagram (Fig. 6) illustrates regional stratigraphie relations, but whole groups may be absent in places. For example, recent drilling between the Menominee and Marquette Ranges in four separated basins did not encounter Chocolay Group or Menominee rocks between Baraga Group Michigamme slates and Archean basement.

The contact between the Marquette Range Supergroup and the Archean basement is a fault in many areas and is intensely sheared in some places. The nature of the contact is discussed in more detail by Cannon [26].

The lower Proterozoic Chocolay Group consists mainly of marine rocks with local conglomerates, but in some basal, small-basin or valley-fill deposits it consists of paraconglomerate, tillite, arkose, and argillite. These basal rocks may be related to Precambrian glaciation, although Bayley and others [27] questioned such a relation. In the Menominee Range, the basal Chocolay includes the Fern Creek Formation; in the Marquette area, the equivalent is the Enchantment Lake Formation and the Reany Creek Formation. All three have been tentatively correlated with the upper part of the Huronian Supergroup of Ontario, above the uraniferous conglomerates [28, 29, 30]. Locally, these basal formations are overlain by sericitic schists, orthoquartzites, and dolomites of the Mesnard, Sturgeon, and Sunday Quartzites. Minor argillite,

slate, and iron formation are also present in the Chocolay.

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\

E X P L A N A T I O N Marquette Range Supergroup Huronion Supergroup

Other Précambrien rock»

° °J Paleozoic r o c k s

to

OJ Figure 8 Geologic map of Précambrien rocks showing generalized limits of the Elliot Lake district and the Marquette Range Supergroup and Huroman Supergroup (adapted from King, 21).

(26)

The marine Menominee Group unconformably overlies the Chocolay or the Archean basement. The youngest rocks of the Menominee Group include the major

iron formations of Michigan.

The Baraga Group is a thick eugeosynclinal assemblage of graywacke, shale, iron formation, and volcanic rocks. In the Marquette area, the Good- rich Quartzite of the Baraga overlies Menominee Group iron formation. This quartzite has interlayered conglomerate beds that, grouped together, make up the most extensive basal conglomerate in the Marquette Range Supergroup. The youngest units of the Baraga are the thick slates and grayvackes of the Michigamme and Tyler Formations which cover large areas in northern Michigan.

The youngest member of the supergroup, the Paint River Group, is mostly graywacke and slate and some iron formation. The member is restricted to a relatively small basin south of the bulk of the metasedimentary rocks.

In general, the northern area of the supergroup is only weakly metamorphosed to the greenschist faciès. Much higher grade metamorphism, including that of the amphibolite faciès, has been described farther south or associated with metamorphic "nodes" [22, 31]. Deformation also increases to the south.

The Marquette Range Supergroup lies on an Archean craton, and Archean granitic and volcanic rocks that could have been a source for uranium in later sediments are described in numerous reports, including Vickers [23], Malan and Sterling [24], and Trow [32].

The youngest elements of the basal Archean rocks have been dated at 2.7 b.y. [22], and the maximum intensity of the Penokean orogeny has been dated at about 1.8 to 1.9 b.y. ago [33]. These ages bracket the supergroup. There are important questions regarding the maximum age of the supergroup that are not answered by available data, but it is indicated that the supergroup is primarily younger than about 2 b.y. Most geologists who have worked in the region believe that the rocks are largely younger than the Uuronian Supergroup in Ontario which hosts the Elliot Lake uranium deposits [34].

Exposures of unconformities are rare, and even rarer are exposures showing effects of weathering at the base of the Chocolay. Deeply weathered regoliths in Archean rocks that may have supplied detrital uraninite to basal rocks of the supergroup have not been reported. If the earliest Chocolay deposits are the result of continental glaciation, regoliths, weathered zones, and possible uraninite and thorium-uranium resistate minerals may have been removed by glaciation.

The subbasins below Chocolay marine units include polymictic conglomerates and graywacke conglomerates (the tillites). Clasts are mostly granitic, but clasts of other Archean rocks, including gneiss, greenstone, mafic schists, vein quartz, and iron formation, are also reported. The matrix has been described as commonly chloritic, and the rocks are typically poorly sorted. In general, it appears that all the lower supergroup conglomerates, including these younger than glacially related ones and below the major iron formations, are typically polymictic with granitic and feldspar clasts. Some conglomerates containing pebbles of vein quartz and chert also include clasts of carbonate rocks and granite in an arkosic matrix. No oligomictic pyrite- bearing conglomerates or widespread continental clastic units that might

contain them have been reported.

The only conglomerates other than those in the Chocolay Group considered to have any possible uranium potential are those of the Baraga Group. The conglomerates in the Goodrich Quartzite of the Baraga Group contain monazite-

(27)

rich placers within quartz-pebble conglomerate in the Palmer area [23], How- ever, the placers are not pyritiferous and no uranium minerals are reported.

There is abundant evidence that Baraga Group sediments formed after the atmosphere of Earth was strongly oxidizing.

The basal quartzite formations of supergroup equivalents in Wisconsin and Minnesota were also reviewed for indications of deposition before the advent of significant free atmospheric oxygen. Pink and red colors due to disseminated hematite are fairly common and indicate all associated conglomerates are unfavorably young. Also, possibly favorable fluvial environments are not reported for these quartzites, and nothing known indicates their presence.

The matrix of the conglomerates in the local subbasins of the Chocolay Group is commonly chloritic but varies from quartzitic to arkosic to high in graywacke. The main minerals include chlorite, sericite, carbonates, quartz, and feldspar. Some basal beds are high in accessory magnetite and other iron oxides. Basal quartzites associated with the conglomerates contain accessory magnetite, rutile, zircon, and epidote [27]. The magnetite content in the

basal conglomerates is discouraging with regard to the possible presence of detrial uraninite because typical uraniferous quartz-pebble conglomerates lack magnetite.

Déformâtional and metamorphic effects on supergroup rocks are extremely variable and, in many areas, probably include destruction of any placer uraninites that might have been present. Button and Adams [5] concluded that the northern, less deformed and metamorphosed half of the supergroup basin appears to be more favorable than the southern half for economic deposits in uraniferous conglomerates.

No significantly favorable environments in the Marquette Range Supergroup are indicated. The presence of Archean basement that could have provided uranium for later placer or nonplacer concentration justify continued interest in the Lake Superior region as geologic knowledge increases. In regard to Precambrian quartz-pebble conglomerates, the ages of deposition of the known rocks are considered too young, based on radiometric dating and substantiated by the presence of primary iron oxides. Also, the bulk of the rocks of the supergroup are marine, and depositional environments having favorable fluvial conglomerates and associated placer concentrations are not known in basal sediments. Extensive exploration programs will be required to fully evaluate the Marquette Range Supergroup.

MCCASLIN FORMATION

The McCaslin Formation in northeastern Wisconsin (Fig. 1) is included in a ridge 40 km long and 3 to 8 km wide. The formation consists of quartzites and basal quartz-pebble conglomerates. The metasediments are older than 1.5 b.y., which is the age of intruded granitic rocks of the Wolf River batholith.

Van Schmus [34] believed the formation to be 1.9 b.y. old.

The dominant structure in the area is the McCaslin syncline, the axis of which plunges 5° W. The younger granitic rocks crop out within and along the syncline. Rocks that are mapped as older than the metasediments include volcanics, granite, quartz diorite, and gneiss. The McCaslin Formation and other geologic units in the area are near the southern limit of the Superior Province, and possibly Archean rocks are exposed about 80 km northwest of the syncline. Details regarding the geology are included in theses by Mancuso [35, 36] and Motten [37].

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The McCaslin sediments consist mainly of massive and red-gray quartzites and quartz-pebble conglomerates. A basal quartz-pebble conglomerate is overlain by a sequence of quartzites that in many places display cross-bedding and ripple marks. The quartzites are reddish in places due to disseminated hematite, which is the major accessory mineral in the conglomerate.

The McCaslin Formation may have been deposited on Archean cratonic rocks before intrusion of the granitic rocks of the Wolf River batholith. The unfavorably young age of 1.9 b.y. for deposition is strengthed by the presence of hematite in associated quartzites and as a major accessory in bedrock conglomerates. Deposition was probably marine and not fluvial [36], and there is no accessory pyrite.

It is possible that rocks of the "right" age, geologic setting, and characteristics may be present but have not been reported. As in the other Proterozoic quartzite-conglomerate occurrences in the Lake Superior region, no units are now recognized as older than 2 b.y.

NEEDLE MOUNTAINS

Precambrian quartz-pebble conglomerates in the Needle Mountains of southwestern Colorado occur in the Vallecito Conglomerate and the basal conglomerate of the Uncompahgre Formation (Fig. 9). The Vallecito Conglom- erate crops out in the southern part of the Needle Mountains and consists primarily of nonfoliated, cross-bedded, pebbly quartzite. These sediments were deposited in an alluvial-fan system [39] and were later subjected to regional metamorphism of amphibolite grade and deformed into large open folds.

The thick Uncompahgre Formation crops out in a strip across the northern and eastern parts of the Needle Mountains and consists of intercalated quartzites and pelitic units that have been isoclinally folded. The formation has a thin, discontinuous, basal conglomerate unit which is best exposed in

the western part of the outcrop area of the formation.

The Vallecito Conglomerate, Uncompahgre Formation, and other Precambrian formations mentioned in this section are fully decribed in a comprehensive overview of the Precambrian geology of the Needle Mountains by Barker [40].

Burns and others [39] proposed that the Vallecito and Uncompahgre were deposited on an erosion surface underlain by an older series of meta-

sedimentary and metaigneous rocks. Their interpretation of the stratigraphie order of some of the Precambrian rocks in the Needle Mountains, shown in Figure 10, is considerably different from that of previous workers.

The Needle Mountains are at least 400 km from the craton of the Wyoming Archean Province. It is unlikely that upper Archean granitic rocks from that craton contributed detritus to the Vallecito or Uncompahgre conglomerates.

Although no direct radiometric dating has been done for the Vallecito Conglomerate and Uncompahgre Formation, the age can be bracketed by using reliable rubidium-strontium dates determined by Bickford and others [41] for adjacent stratigraphie units. The dates of the underlying Bakers Bridge Granite and the overlying Eolus Granite (Fig. 10) indicate that the metasedimentary detrital rocks of the Vallecito and Uncompahgre were deposited between 1.46 and 1.75 b.y. ago, during the interval between the Boulder Creek and Silver Plume events of middle Proterozoic age. The Vallecito and Uncompahgre are, therefore, too young to have been deposited under reducing atmospheric conditions prevalent more than 2.0 b.y. ago.

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I0r°43' EXPLANATION

37«45'

37« 3O

c0 L 0 R A 0 0

N««dl. Mounloln«

_ HIUSOOLC CO ARCMULET» CO

Younger intrutive rock*

Includt Trlmttt Cranitt, EltCtra Lot

one Eotui

Older intrusive rocks tnclud* Ttnmilt and and Twilight

Vollecilo Conglomerate ^ K O Z 4 OUl ora.

12 Miltt

20 Kllom«t*rt

Contact Foult

Heavy-lined areas studied by Burns and o t h« r» (39)

Figure 9. Generalized geologic map of Précambrien rocks in the Needle Mountains (after Barker, 40).

The Vallecito Conglomerate was deposited in an alluvial-fan system and exhibits most characteristics of a braided-stream environment [39]. Conglom- erates of the Vallecito are not as mature texturally or mineralogically as those in productive conglomerates, but the pebble-conglomerate faciès is oligomictic and contains vein quartz that makes up an average of 64% of the pebbles. Most requisite sedimentologic characteristics for fossil placers are found in the Vallecito Conglomerate.

Clast-supported conglomerates are lenticular and rare in the basal Uncompahgre Formation. The basal Uncompahgre conglomerates contain less muscovite and sericite in their matrix than do the Vallecito conglomerates and were probably deposited in a fluvial or marginal-marine setting [39].

Pyrite in the Vallecito and Uncompahgre conglomerates is present only in trace amounts. No radioactive conglomerates were found by Burns and others [39] in either of the formations. The maximum value of uranium from analyses of Vallecito Conglomerate samples was 11 ppm and the maximum for Uncompahgre conglomerate samples was 5 ppm.

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Eolus Granite ~1 46 b y

Uncompahgre Formation

Vallecito Conglomerate Tenmile Granite and

Bakers Bridge Granite -1 75 by

Twilight Gneiss ~1 78 b y

Irvmg Formation >1 8 b y

Middle Proterozoic

Middle and lower Proterozoic

Figure 10. Generalized stratigraphie column and ages of some of the Précambrien rocks in the Needle Mountains (compiled from Burns and others. 39).

The iron oxide mineral content of the Vallecito and Uncompahgre conglomerates is obviously high as shown by commonly pink or red coloration at outcrops. Modal analyses of samples of the pebble-conglomerate faciès of the Vallecito show that they contain at least several percent iron oxide minerals (39). Banded iron formation, specular hematite, and jasper compose up to 30%

of the pebbles in both the Vallecito and Uncompahgre conglomerates, which indicates the presence of an oxidizing atmosphere before these conglomerates were deposited.

Regional metamorphism up to amphibolite grade affected the Vallecito Conglomerate and Uncompahgre Formation soon after deposition. This metamorphic event was accompanied by deformation and is referred to by Barker [42] as the Uncompahgran disturbance. The older Vallecito was deformed into large-scale folds and metamorphosed to the amphibolite faciès, but most rocks do not exhibit pronounced foliation. The Uncompahgran disturbance was more intense farther to the north where the younger Uncompahgre Formation was isoclinally folded and metamorphosed from the greenschist to the amphibolite faciès from west to east, respectively.

The Vallecito Conglomerate and the basal conglomerate of the Uncompahgre Formation are considered unfavorable for Precambrian quartz-pebble conglomerate uranium deposits. Although the Vallecito Conglomerate was deposited in a braided-stream environment, a favorable setting for uraniferous quartz-pebble conglomerates, it was far from an Archean cratonic source of uranium, it underwent amphibolite-grade metamorphism, and it was deposited during the middle Proterozoic in an oxygen-rich atmosphere. The lack of detrital pyrite and uranium oxide minerals and the high content of iron oxide minerals are characteristics of the Vallecito that indicate it is too young to host placer uranium deposits. The basal Uncompahgre conglomerates have similar unfavorable characteristics.

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KINGSTON PEAK FORMATION

Quartz-pebble conglomerates occur in the Mountain G i r l submember w i t h i n the Kingston Peak Formation of the Pahrump Group of upper Proterozoic sediments along the western f l a n k of the southern Panamint Range in southeastern C a l i f o r n i a (Figs. 11 and 12). The Pahrump Group unconformably overlies older Proterozoic metamorphic and igneous rocks which include a complex of augen gneiss, the World Beater Complex, that has anomalously high uranium and thorium content and is 1.8 b . y . o l d . The Kingston Peak Formation radioactive conglomerates are younger than 1.35 b . y . and may be less than 1.1 b . y . o l d . Rocks in the Kingston Peak Formation between the unconformity below the Pahrump Group and the radioactive conglomerates of the Mountain Girl submember include a high proportion of dolomites and other marine rocks. The s t r a t i g r a p h y is summarized in Figure 12. The rocks s t r a t i g r a p h i c a l l y between the conglomerate and the World Beater Complex are between 2 and 4 km thick.

Included in these rocks is the "favorable submember" which is basically a quartz-mica schist that hosts n o n d e t r i t a l uranium occurrences. Metamorphic grade is variable from place to place, amphibolite grade being the highest reported.

The Archean and lower Précambrien rocks of early workers have now been e s t a b l i s h e d as P r o t e r o z o i c . Basement rocks beneath the Pahrump Group were o r i g i n a l l y mainly volcanic and sedimentary units and are now orthogneisses and paragneisses with g r a n i t i c intrusions. The u r a n i f e r o u s World Beater Complex is not considered by Carlisle and others [43] to have been a possible source of uranium to the conglomerates.

The environment of the Mountain G i r l conglomerates is mainly f l u v i a l - d e l t a i c and braided stream. Anomalously high radioactivity occurs in broadly lenticular oligomictic q u a r t z - p e b b l e conglomerate layers from several c e n t i m e t e r s to l m thick. The highest uranium value reported is only 40 ppm.

According to C a r l i s l e and others l*»3] cerium contents are u n u s u a l l y high and a r e d i r e c t l y r e l a t e d t o u r a n i u m a n d t h o r i u m , w h i c h s u g g e s t s t o t h e m t h a t m o n a z i t e h o s t s a l l t h r e e e l e m e n t s a n d i s t h e s o u r c e o f r a d i o a c t i v i t y ,

t y p i c a l l y 5 to 10 times background.

Matrix m i n e r a l s in the radioactive conglomerates consist of q u a r t z (5551), microcline (20J), biotite (10%-30J), and opaques (51; mainly magnetite). It

i s n o t e w o r t h y t h a t v a r i a b l e a m o u n t s o f p l a g i o c l a s e h a v e b e e n r e p o r t e d , magnetite is present in more than trace amounts, and pyrite is absent.

The uranium occurrences in schists older than the conglomerates i n c l u d e b r a n n e r i t e . U r a n i u m e n r i c h m e n t p r o b a b l y i s d u e t o u r a n i u m t r a n s p o r t e d i n s o l u t i o n ; n o t h i n g suggests t h e presence o f d e t r i t a l u r a n i u m o x i d e s i n a n y s e d i m e n t s o f t h e K i n g s t o n Peak F o r m a t i o n . M e t a m o r p h i s m u p t o a m p h i b o l i t e g r a d e i s n o t c o n s i d e r e d b y C a r l i s l e a n d o t h e r s [ 4 3 ] t o h a v e m o b i l i z e d o r otherwise a f f e c t e d the uranium or thorium present in the Precambrian rocks.

Factors that g r e a t l y reduce the probability of occurrence of uraniferous q u a r t z - p e b b l e c o n g l o m e r a t e s a r e t h e absence o f a n A r c h e a n b a s e m e n t h a v i n g f a v o r a b l e source rocks and the young age of the known conglomerates and other Precambrian rocks. Other strongly negative factors include indications that conglomerate uranium contents and r e l a t i v e l y high r a d i o a c t i v i t y are due solely t o m o n a z i t e , t h a t p y r i t e i s a b s e n t i n c o n g l o m e r a t e m a t r i c e s , a n d t h a t m a g n e t i t e i s t h e m a i n h e a v y m i n e r a l . T h e o n l y f a v o r a b l e f a c t o r i s t h e presence of f l u v i a l conglomerates having characteristics of deposition in a braided-stream environment.

(32)

Oeoth v.

Volley?- XN

\ N

Death \

\ Valley

I X I N

EXPLANATION

L a t e r Précambrien s e d i m e n t a r y r o c k s Includes Pahrump Group E a r l i e r Precambnan c r y s t a l l i n e r o c k s

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) V a l l € y

Junction

\ ^^

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5 0 5

11 l M I I

15 Km

i_Si

(33)

PAHRUMP GROUP Noonday DolomiteKINGSTON PEAK FORMATION

Redlands Member Radcliffe Member Sentinel Peak Member

South Park Member Wildrose Submbr

Mountain Girl Submbr ' Middle Park Submbr Sourdough Limestone Mbr

SURPRISE MEMBER

V -Û

E

0)

5

CT) C

l/>o.

_5a E _*

Quartzue Submbr Argillaceous Submbr

<

^Submember^Favorable

<=^__Qolomitic Sbm Arkosic Submember

Beck Spring Dolomite

Crystal Spring Formation

World Beater Cmplx

Earlier Precambrian Quo Fldsp Gneiss

"Unit including quartz-pebble conglomerates

Figure 12. Stratigraphy of the Kingston Peak Formation and other units, including the position of the Mountain Girl submember (adapted from Carlisle and others, 43).

C E N T R A L A R I Z O N A ARCH

I n t h e e a s t e r n p a r t o f t h e c e n t r a l A r i z o n a A r c h ( F i g . 1 3 ) , a v a r i e t y o f P r o t e r o z o i c c o n g l o m e r a t e s o c c u r s i n t h r e e m a j o r u n c o n f o r m i t y - b o u n d e d rock g r o u p s . T h e g r o u p s a r e , f r o m o l d e s t t o y o u n g e s t , t h e A l d e r G r o u p , M a z a t z a l Group, and Apache Group. As shown by the stratigraphie column (Fig. HO, the conglomerates were deposited in two r e l a t i v e l y brief i n t e r v a l s from 1.711 to

1.65 b . y . ago and f r o m 1.35 to 1.25 b . y . ago. The two t i m e p e r i o d s are separated by the M a z a t z a l orogeny [ W ] . The rocks that predate the M a z a t z a l orogeny are g e n t l y to s e v e r e l y deformed, whereas those that postdate it are e s s e n t i a l l y f l a t lying and undeformed.

(34)

EXPLANATION

UPPER PROTEROZOIC SEDIMENTARY ROCKS

MIDDLE PROTEROZOIC SEDIMENTARY ROCKS

LOWER PROTEROZOIC METAMORPHIC AND PLUTONIC ROCKS (Includ«» torn» middl«

ProUrozolc plutonlc rock»)

Figure 13. Generalized extent of Précambrien rocks in Arizona (from Katliokoski and others, 48).

Uranium Deposits in Proterozoic Quartz-pebble Conglomerates (Report of the Working Group on Uranium Geology) | IAEA

IAEA-TECDOC-427