Abstract
Application of the "time and stratabound model" for the origin of quartz-pebble conglomerates led to the discovery in 1975 of uranium, thorium and gold bearing conglomerates in the Late Archean and Early Proterozoic Rocks of southern Wyoming, U. S. A. The principal host rocks, in the Medicine Bow and Sierra Madre mountains, west of Laramie, are in early Proterozoic miogeoclinal metasediments. The rocks were deposited in a rifted passive margin and preserved where thick deltaic successions were buried by overriding thrusts. The most promising
deposits are in the Early Proterozoic Magnolia formation. The principal uranium minerals are uranothorite and coffinite. The conglomerates are at the base of a sedimentary sequence lying unconformably on older rocks and have well rounded, moderatedly sorted and tightly packed pebbles, dominantly of quartz. The matrix is composed of quartz, feldspars and phyllosilicates (muscovite, chlorite and biotite) formed from
metamorphism of an argilliceous matrix. The mineralogy present supports the concept that uranium rich rocks did not develop until late in the Archean.
The potential area underlain by quartz-pebble conglomerates is very large. The uranium grades in the Medicine Bow area increase from SW to NE, which is promising, but the conglomerate becomes deeply buried.
Resources in the M e d i c i n e Bow mountains are estimated from geologic considerations at more than 10,000 tonnes of uranium.
INTRODUCTION
Since 1975 a cooperative effort between the United States Geological Survey, United States Department of Energy, Geological Survey of Wyoming, the Geology Depart-ment, University of Wyoming, and various industry groups has resulted in the discovery of a series of uranium-thorium- and gold-bearing conglomerates in Late Archean
and Early Proterozoic metasedimentary rocks of southern Wyoming. The mineral
de-posits were found by applying the time and strata bound model for the origin of uranium-bearing quartz-pebble conglomerates to favorable rock types within a geologic terrane known from prior regional mapping (Houston and others, 1977).
No mineral deposits have been discovered that are of current (1983) economic interest, but preliminary resource estimates determined by the Kriging method of ore reserve estimation and an ore-grade cutoff of 100 ppm indicate that over 3A18 tons of uranium and over 1996 tons of thorium are present in the Medicine Bow Moun-tains and that over 440 tons of uranium and 6350 tons of thorium are present in the Sierra Madre (Borgman and others, 1981). Geologic reasoning suggests much larger reserves of uranium in the Medicine Bow Mountains perhaps more than 10,000 tons (Borgman and others, 1981).
Sampling has been inadequate to determine gold resources. High grade uranium deposits (deposits over 2 meters thick and averaging over 1000 ppm uranium) have not been detected by work to date but local beds of uranium-bearing conglomerate contain as much as 1380 ppm uranium over a thickness of 0.65 meters. These projects involved geologic mapping at scales from 1/6000 to 1/50,000, detailed sampling, and the evaluation of 48 diamond drill holes, but the area is too large to fully establish the economic potential with present information.
LOCATION AND GENERAL GEOLOGY
There are three main uplifts exposed in southeastern Wyoming which have rocks of Precambrian age in their core (fig. 1). The two westernmost ranges, the Medicine Bow Mountains and Sierra Madre contain Early Proterozoic metasedimentary rocks which are raiogeoclinal and are the hosts of radioactive quartz-pebble conglomerate.
The boundary between the Archean Wyoming Province and Proterozoic rocks of Colorado is exposed in southeastern Wyoming (fig. 1) and is a zone of sheared rocks that strikes generally northeast. This boundary has been referred to as the Cheyenne Belt (Houston and others, 1979) and may be part of a suture where island arcs that developed in what is now Colorado were attached to the Arche^n continent that con-stituted The Wyoming Province (Hills and Houston, 1979). North of the Cheyenne Belt Archean basement gneiss and granite is overlain unconformably or is in fault contact with metasedimentary and metavolcanic rocks that range in age from Late Archean to Early Proterozoic. Early Proterozoic metasedimentary rocks are found within 30 kilometers of this boundary, but have not been identified in the central
EXPLANATION
3 SHERMAN GRANITE (1400 m y) I LARAMIE ANORTHOSITE
| SYENITE (1435my)
-j SYNTECTONIC GRANITOID (1700 m y)
• QUARTZ DIORITE (1780my) a VOLCANOGENIC GNEISS
* * = MIGMATITE
| MAFIC INTRUSIONS
UPPER LIBBY CREEK GROUP LOWER LIBBY CREEK GROUP DEEP LAKE GROUP
ARCHEAN SUPRACRUSTAL ROCKS ARCHEAN GNEISS AND GRANITE 'THRUST FAULT
"SHEAR ZONE
SIERRA MADRE
106'
UJ
o
LARAMIE MTS.
HARTVILLE UPLIFT :
MEDICINE BOW MTS.
WYOMING PROVINCE
INDEXM MAP
NORTH PARK RANGE
STEAMBOAT SPRINGS
FRONT RANGE
0 10 20 30 40 50
Figure 1 - Index map showing generalized Precambrian geology of southeastern Wyoming and northern Colorado. Colorado geology modified from Tweto (1979).
part of the Wyoming Province. This Early Proterozoic miogeoclinal succession is interpreted as having been deposited along a rifted, passive (Atlantic-type) margin and is preserved near the margin where thick deltaic successions were buried by overriding thrusts during collision (Karlstrom, Flurkey, and Houston, 1984).
South of the Cheyenne Belt there are no rocks of Archean age. Proterozoic rocks are about 1800 m.y. old (Premo and Van Schmus, 1982) and consist of metavolcanic and metasedimentary rocks of island arc affinity (Hills and Houston, 1979; Houston, Schmidt and Lane, 1984).
The Archean basement of central and southeastern Wyoming consists of a granite-gneiss terrane and greenstone belts cut by Late Archean granite. The granite-gneiss terrane and greenstone belts are approximately 3000 m.y. old (Peterman and Hildreth, 1978) and there are, at least, two Late Archean episodes of granite formation, one about 2700 m.y. and another about 2500 m.y. (Peterman and Hildreth, 1978). The 2500
m.y. granites are especially significant from the viewpoint of uranium mineralization since they contain anomalous thorium and uranium and have been considered by some geologists and geochemists as the prime source of uranium in younger rocks of Wyoming (Houston, 1979; Stuckless, 1979). The Archean basement of the Medicine Bow Mountains and Sierra Madre is largely quartzofeldspathic gneiss, but small areas underlain by amphibolite and hornblende gneiss in the northern Sierra Madre and northeastern-most Medicine Bow Mountains may be parts of more extensive greenstone belts buried by Phanerozoic sedimentary rocks. Small Late Archean granites representative of both the 2700 m.y. suite and the 2500 ra.y. suite are exposed in the Sierra Madre, but only the 2500 m.y. granite is found in the Archean basement of the Medicine Bow Mountains.
Three metasedimentary and metavolcanic rock successions are exposed in the Sierra Madre and Medicine Bow Mountains that are approximately 15000 meters thick (table 1). These rock successions are the Late Archean Phantom Lake Metamorphic Suite, and the Early Proterozoic Deep Lake and Libby Creek Groups. The Late Archean Phantom Lake Metamorphic Suite is a rock succession having characteristics inter-mediate between a greenstone belt and typical Early Proterozoic miogeoclinal succes-sions, and is a mixture of volcanic rocks, volcanoclastic metasedimentary rocks, quartzite, phyllite, slate, and conglomerate. The stratigraphy of the Phantom Lake succession is not well-established because of poor preservation of primary structure and multideformation hence the designation Suite (Henderson, Caldwell,
Table 1 - Comparative stratigraphy of metasedimentary rocks in the Sierra Madre and Medicine Bow Mountains, Wyoming.
MEDICINE BOIjJ M O U N T A I N S SIERRA MftDRE
l/olcanogenic Gneiss Volcanogenic Gneiss
fault System
F r p n r h Slate Towner Greenstone Nash Fork F o r m a t i o n
S l a u g h t e r h o u s e F o r m a t i o n
S u q a r l o a f Q u a r t z i t e A A Copperton F o r m a t i o n Lookout Schist
fledicine Peak Q u a r t z i t e
Heart F o r m a t i o n B o t t l e Creek F o r m a t i o n
Headquarters Formation Rock K n o l l F o r m a t i o n
Uaqner F o r m a t i o n
C a s c a d e Q u a c t z i t e Cascade Q u a r t z i t e C a m p b e l l Lake F o r m a t i o n
^t^^H?**»^l*^*~*S**~f^*r^~^t
L i n d s e y Q u a r t z i t e Singer P e a k F o r m a t i o n
flagnolia F o r m a t i o n
us^^~**s~^r~^r*^~*s~**r^,?^^-C o n i c a l Peak Q u a r t z i t e N a g n o l i a F o r m a t i o n
B r i d g e r Peak Q u a r t z i t e
C o L b e r q Metavolcaoics S i l v e r Lake M e t a v o l c a n i c s
Sou Q u a r t z i t e
Rock M o u n t a i n C o n g l o m e r a t e A r c h e a n
G r a n i t e ) 5tud Creek V o l c a n i c 1 as 11 es Jack Creek Quartzite
Deep Gulcn C o n g l o m e r a t e
—^——'••^-^-^^^^•N^^^^^^'^^X^^X'N^
U u l c a n Cltn. H e t a w o l c a n i c s Q u e r l a n d Creek Gneiss
and Harnson, 1980). The exact age of the Phantom Lake Metamorphic Suite is uncer-tain because none of the volcanic rocks have been dated, but the lower part of the succession is cut by Late Archean intrusions (Hedge in Houston and others [ 1984a]), and the upper part is cut by undated intrusions that are also thought to be Late Archean (Karlstrom and others, 1981). The Phantom Lake Metamorphic Suite is overlain unconformably by rocks of the Early Proterozoic Deep Lake Group
which consists of six formations from oldest, the Magnolia Formation, Lindsey Quart-zite, Campbell Lake Formation, Cascade QuartQuart-zite, Vagner Formation, and Rock Kttoll Formation. The lower Deep Lake Group is interpreted as fluvial whereas the Campbell Lake Formation and younger formations are considered marine and glaciomarine (Karl-strom and others, 1981; Houston and others, 1981). The rocks of the Deep Lake Group are in fault contact with metasedimentary rocks of the Libby Creek Group, which is divided into a lower and upper part. The lower Libby Creek Group consists of five formations, from oldest, The Headquarter Schist, The Heart Formation, The Medicine Peak Quartzite, the Lookout Schist and The Sugarloaf Quartzite (Houston and others, 1968). This lower part of the Libby Creek Group is interpreted as marine; the Headquarters Sehn st as glaciomarine and the overlying formations as part of a deltaic succession (Karlstrom, Flurkey, and Houston, 1981). The upper Libby Creek Group which is in fault contact with the lower Libby Creek Group con-sists of three formations from oldest, the Nash Fork Formation, the Towner Green-stone, and the French Slate. The upper Libby Creek Group is also viewed as marine but these beds are thought to have been deposited farther offshore than rocks of the lower Libby Creek Group.
There is no direct dating of rocks of the Libby Creek Group, but the lower Libby Creek Group succession is cut by a felsic intrusion (the Gaps Intrusion) that has been dated at approximately 2000 m.y. (Hedge in Houston and others, 1984a).
Metamorphic dates for units of both lower and upper Libby Creek Group are approximately 1700 m.y. (Hills and Houston, 1979). On geologic grounds the lower Libby Creek
Group has been correlated with the Huronian Supergroup of Canada (2400-2100 m.y.) and the upper Libby Creek Group with the Marquette Range Supergroup (2100-1900 m.y.) of the Lake Superior region (Houston and others, 1977).
The rocks of the Phantom Lake Metamorphic Suite are the most highly deformed and metamorphosed of the metasedimentary successions. In the Sierra Madre Phantom Lake rocks are in a series of overturned folds with axial planes dipping south and these folds are refolded about a new axis in the northwestern Sierra Madre.
Phantom Lake rocks are also intensely folded in the Medicine Bow Mountains where folds strike east northeast and are refolded about new axes in the northeastern Medicine Bow Mountains. Strike faults are common in the Phantom Lake in both the Sierra Madre and Medicine Bow; some fold-fault systems of the Sierra Madre are probably nappes although the amount of displacement in individual faults has not
been established. Phantom Lake rocks are amphibolite facies reaching upper amphi-bolite facies locally.
Rocks of the Deep Lake and Libby Creek groups are less deformed than the Phantom Lake. Deep Lake Group metasedimentary rocks are in broad open folds in the central Medicine Bow Mountains that become tighter and more closely appressed to the northeast. In the Sierra Madre the Deep Lake Group is more deformed than in the Medicine Bow and parts of the section are removed by strike faults. Libby Creek Group rocks of the Medicine Bow Mountains are in a northeast striking syncline which has its southeast limb largely removed by a major fault, the Müllen Creek-Nash Fork shear zone (Houston and McCallum, 1961). In the Sierra Madre much of the Libby Creek Group is removed by thrust or high angle reverse faults, but partial sections are preserved at the southern margin.
The metamorphic rank of rocks of the Deep Lake and Libby Creek Groups varies from green schist in the central Medicine Bow Mountains to amphibolite facies in other areas.
RADIOACTIVE QUARTZ PEBBLE CONGLOMERATE
Uranium-thorium-gold bearing quartz-pebble conglomerate is present in the Deep Gulch Conglomerate of the Jack Creek Quartzite, the lower division of the Late Archean Phantom Lake MetamorphicSuite, and the Conglomerate Member of the Magnolia Formation, the basal formation of the Early Proterozoic Deep Lake Group, in the Sierra Madre and Medicine Bow Mountains respectively. The best developed and most continuous beds of quartz-pebble conglomerate are in the basal Jack Creek Quartzite of the Carrico Ranch area of the northwest Sierra Madre, where radioactive conglomerate crops out in the overturned limb of a major fold for a distance of
13 kilometers (fig. 2). Unfortunately, the well-developed quartz-pebble conglomerates of the Late Archean. Jack Creek Quartzite are a thorium rather than a uranium resource with thorium to uranium ratios averaging 6.99. The chemical composition of primary thorium-uranium minerals in the Jack Creek Quartzite indicate that these minerals are mixtures of a thorium-bearing monazite and the thorium silicate huttonite.
These minerals along with monazite, zircon, and pyrite are the principal heavy minerals in the Jack Creek Quartzite. The thorium-uranium minerals of the Jack Creek Quartzite are believed to have been derived from 2800 m.y. or older Archean gneiss and granite of the Wyoming Province. Paleocurrent determinations in the Jack Creek Quartzite suggest a northerly source.
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In contrast to the Jack Creek Quartzite, the Early Proterozoic quartz-pebble conglomerate of the Magnolia Formation of the Deep Lake Group of the Medicine Bow Mountains contains minerals richer in uranium and has an average thorium to uranium ratio of 2.^9. The most promising deposits in the Magnolia Formation are in a three square mile area along Onemile Creek near the village of Arlington in the northeastern Medicine Bow Mountains. Here uranium-bearing quartz-pebble conglomerate is in the southwest plunging nose and in the complexly folded and overturned north-west limb of a major syncline (fig. 3). The principal uranium minerals in the Arlington locality are uranothorite and coffinite and uranium is also present in lesser amounts in monazite, zircon, and monazite-huttonite mixtures similar to the principal uranium-thorium-bearing minerals of the Sierra Madre. The heavy mineral suite is much more complex in the Magnolia Formation than in the Jack Creek Quartzite and contains minerals such as ilmenorutile and columbite that are almost certainly from a granitic source (Desborough and Sharp in Houston, Karlstrom, and Graff, 1979). Pyrite is the most abundant heavy mineral in both the Medicine Bow and Sierra Madre occurrences. No reliable paleocurrent measurements have been obtained in the quartz-pebble conglomerate beds of the Magnolia Formation but paleo-current determinations in overlying quartzites suggest a northeast to north source.
Inasmuch as the Magnolia Formation is believed to have been deposited after intrusion of uranium-rich Late Archean granites (2500-2600 b.y.) of the Wyoming Province, these granites of central and eastern Wyoming are considered to be the source of uranium-bearing minerals of the quartz-pebble conglomerate. The different source rocks of the Late Archean pebble conglomerate and Early Proterozoic quartz-pebble conglomerate are critical in that they determine the economic potential of the conglomerate.
In both the Sierra Madre and Medicine Bow Mountains radioactive quartz-pebble conglomerate is either at, or near, the base of a sedimentary succession that lies unconformably on older rocks. The quartz-pebble conglomerate is believed to have been deposited in braided streams and rivers that developed in the vicinity of tectonic (fault-controlled) highlands. Radioactive quartz-pebble conglomerate layers are individual beds or compound beds in coarse-grained quarlzite and they occur at different levels in quartzite units that are up to 800 meters thick (fig.4).
SCALE I 6000 0 5OO lOOOft
Figure 3 Geologic mop of the Onemile Creek area, northern Medicine Bow Mountains Compiled from surface and subsurface data See opposite page for cross sections
EXPLANATION
MAFIC INTRUSIVE ROCK Amphibol zed gabbro and pyro*emt
CASCADE Q U A R T Z I T E Pebbly q u o r t i . t e and non-radioactive
UNCONFORMIT r
I ——— . MuicovHk luborkos« ond quartz-pebble conglomérat«
LAJ UNIT 5 principal radioactive ion«* labeled 5o ond 5b [ 4 | UNIT4 SiaflfB chlonf« »cftr«f with paraconqloenerate lentet
l — - — i . Trough crossb«dd«d tu bo r k ose with ihm I 2 I ^ N ( T 2 conglomerate b«di
[ 1 J UNIT 1 Ar he« ic pa ro conglomerate and »uborkos*
MAP SYMBOLS UNCONFORMIT V
] qÇ [ Granite ana gran me g Fine qramed quorliir«
Mttabatall
I n f e r r e d faults, showing relative d i »place men I, dotted where invaded by igneous rocks - Overturned Synclme Y \ V Stretched pebble mineral a
•;,-5s7s mtnof fold am» lineohoni ' 5 * 2 * 5 Sinkt and dip ol bedding, i & ' overturned bedding, follati Over
LITHOLOCIC UNITS eheared and brecciateo rock phyllite ond ichitt coor»e-groi«ed quarlxite quartz-granule conglomerate rodwacfiv« quor'i-p«b»4» tonq porocongtomtrate
Drillhole« showing, bearing, plunge and t o t a l length Crotibed, ihowmg top CROSS SECTION SYMBOLS
unconformity bedding form lines T. A to word/aura y strike slip foul
Figure 3 - Geologic map of the Onemile Creek area, northern Medicine Bow Mountains.
Compiled from surface and subsurface data.
650O-Cross sections and drill sections f r o m the Onemile Creek area
130 i. ...
50
meters | f • s 0 5 10 IS 2025 Maximum Particle
Size (Mean) millimeters
biotite quartz plagioclose gneiss
° 5 10 is 20 25 Maximum Port.cle
Size (Mean) millimeters t e s s
-Figure 4. Grain size profile of the Deep Gulch Conglomerate
None of the formations of the Libby Creek Group contain significant uranium, and inasmuch as local beds of hematitic quartzite are in the French Slate and other formations of the Libby Creek Group, it seems probable that uraninite could not have been transported as a detrital mineral when beds of the Libby Creek Group were deposited.
PETROGRAPHY OF RADIOACTIVE BEDS
The most radioactive rocks in the Sierra Madre are subarkosic moscovitic quartz-pebble conglomerates of Unit 3 of the Deep Gulch Conglomerate (fig. 5) and the most radioactive rocks in the Medicine Bow Mountains are subarkosic, muscovitic small-pebble (quartz, granite, and quartzite) conglomerates of Unit 5 of the Magnolia Formation (fig. 3). Although these beds differ in age, the major constituents are basically the same (except granite fragments are rare in the Sierra Madre).
These constituents include quartz, rock fragments of granite and quartzite, K-feldspar, plagioclase, muscovite, chlorite, biotite, pyrite and a heavy mineral suite. Arkosic and subarkosic quartzites were originally bimodal, argillaceous sandstones and
quartz-pebble conglomerates were originally trimodal, argillaceous conglomerates.
Tables 2 and 3 show modal analyses of the sand and granule size fractions from the main radioactive units in the Medicine Bow Mountains and Sierra Madre.
Pebbles in the conglomerates are well rounded, generally moderately sorted, and tightly packed. The most radioactive conglomerates appear to be pebble-supported in both ranges, although stretching of pebbles in the Onemile Creek area of the Medicine Bow Mountains makes it difficult to decipher original packing densities.
Clasts in the Deep Gulch Conglomerate are entirely quartz and quartzite, with an average size range of 0.7 to 3.7 and maximum size of 5 cm. Clasts in the Magnolia Formation conglomerates are quartz, quartzite, and granite. They range in size
from granules to boulders 7.5 cm in diameter but are most commonly 1-3 cm in diameter.
Many of the conglomerates contain 100 percent quartz pebbles; others contain up to 20 percent quartzite and granite pebbles.
The matrix of the conglomerates is composed of quartz, feldspar, and
phyllosilicates. The phyllosolicates, muscovite, chlorite, and biotite are consid-ered to be raetamorphic minerals formed by recrystallization of an argillaceous matrix. Micas make up about 25 percent of the matrix of most conglomerates and some samples contain as much as 50 percent. The most radioactive conglomerates tend to be rich in muscovite and sericite, but poor in biotite and chlorite.
DESCRIPTION INTERPRETATION
Granute congkxnerQies with scour surfaces and ptonor crossbeds
Braid bar and channel accretion with planar crossbed representing upper flow regime conditions over bar tops
Planar crossbedded gronutar congtomerotes Transverse bar migration
Fmrtg-upward sequences of granular conglomérâtes
Vertical accretion of brad bars or channels
MotTii and clast supported higniy rodoactive quortz-pebble conglomérâtes, with planar crossbedded sandstone overlying conglomerates
Grovets represent coalescing compound bars and channel deposits, planar crossbedded sandstone represents upper flow regime condiNons. transverse bor migration, or foreset avalanche slopes of bars
Grovets represent coalescing compound bars and channel deposits, planar crossbedded sandstone represents upper flow regime condiNons. transverse bor migration, or foreset avalanche slopes of bars