Contributions to Geology 17.2

Introduction to the second uranium issue and some suggestions for prospecting

ROBERT S. HOUSTON Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
85-88

Keywords
Tertiary, uranium, exploration, deposits

Abstract
Interest in uranium geology and uranium exploration has been cyclic, but not from natural causes as is the case for many geologic phenomena. Instead, it is influenced by war, threats of war, shortages, temporary surpluses, poor planning, and fear of environmental hazards. Whether long-term needs for nuclear reactors will mean a continued high price for uranium remains to be seen, but current prices stimulate interest in exploration and it seems an appropriate time after nearly 10 years to publish a second uranium issue.

The first uranium issue of this journal was published in 1969 as part of a field conference held by the Society of Economic Geologists, and was edited by R. B. Parker and E. N. Harshman. This first issue, now out of print, concentrated on uranium deposits in sedimentary rocks with particular reference to roll-type deposits of the Wyoming Tertiary basins. Since the discovery of uranium in Tertiary basins in the early 1950s by a United States Geological Survey party under the direction of J. D. Love, the roll-type uranium deposits in these basins have been the basis of the Wyoming uranium industry. Although the genesis of these roll-type deposits is still debated and studied, they are well-enough understood to conduct sound exploration programs (Bailey, 1965). These exploration programs have been remarkably successful in the past and will most likely produce new deposits for the future, but it seems worthwhile to consider some uranium exploration frontiers while we continue to search for roll-type deposits.

Early Proterozoic tectonics of the central Rocky Mountains, North America

F. ALLAN HILLS U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225 ROBERT S. HOUSTON Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
89-110

Keywords
metamorphism, Proterozoic, tectonics, Rocky Mountains, subduction, continental, Medicine Bow, Front Range

Abstract
Archean ( > 2500 Ma old, formerly Precambrian W) granitic gneiss of the Wyoming Province is separated from lower Proterozoic (1600 to 2500 Ma old, also called Precambrian X) plagioclase and hornblende gneiss of the Central United States Province by a shear zone that ranges in width from several hundred meters to approximately 7 km.

Immediately north of the shear zone, lower Proterozoic ensialic metasedimentary rocks of greenschist and amphibolite facies are preserved. These were metamorphosed between 1620 and 1750 Ma ago. Calc-alkalic igneous rocks younger than 2400 Ma old are almost unknown north of the shear zone, and kyanite is widespread within approximately 10 km of the shear zone on its north side.

South of the shear zone, lower Proterozoic gneisses of apparently ensimatic origin are intruded by numerous plutons of gabbroic and of calc-alkalic affinity. Pre-orogenic or synorogenic granitic rocks yield Rb-Sr whole-rock ages of 1725 to 1665 Ma. Sillimanite rather than kyanite is widespread immediately south of the boundary, but andalusite and cordierite occur extensively farther south in the Front Range, Colorado. The shear zone cuts 1725-Ma-old granite in the Medicine Bow Mountains, Wyoming, and is cut by 1635-Ma-old post-orogenic red granite in the Sierra Madre, Wyoming. The apparently anorogenic anorthosite-syenite near Laramie, Wyoming, and the Sherman and related granites were intruded mainly south of the boundary zone approximately 1440 Ma to 1385 Ma ago.

The timing of metamorphism and plutonism, the disposition of calc-alkalic plutons and of ensialic and ensimatic lower Proterozoic metasedimentary rocks, and a possibly paired metamorphic belt appear to be compatible with a plate tectonics model in which an Atlantic-type continental margin developed during early Proterozoic time on the southern edge of a craton consisting of Archean rocks. The Atlantic-type margin collided with and was partially subducted beneath a volcanic arc approximately 1725 to 1635 Ma ago. The hypothesized subduction zone dipped toward the southeast.

Stratigraphy of the Phantom Lake Metamorphic Suite and Deep Lake Group and a review of the Precambrian tectonic history of the Medicine Bow Mountains

KARL E. KARLSTROM and ROBERT S. HOUSTON Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
111-134

Keywords
Precambrian, tectonic, Medicine Bow, metasedimentary, unconformities, Phantom Lake, Deep Lake, Libby Creek

Abstract
Proterozoic metasedimentary rocks (2500-1700 m.y. old) in the Medicine Bow Mountains of southeastern Wyoming are divided into three successions which are separated by unconformities: (1) the Phantom Lake Metamorphic Suite (oldest); (2) Deep Lake Group; and (3) Libby Creek Group (youngest).

The Phantom Lake Metamorphic Suite (> 3 km thick) is divided into a lower part containing dominantly metavolcanic rock and an upper part containing dominantly micaceous quartzite. The Deep Lake Group (3.3 km thick) contains quartzite, quartz-pebble conglomerate, glacial(?) paraconglomerate, marble, and phyllite and is subdivided into six formations.

The Phantom Lake Suite and Deep Lake Group were deposited during several sedimentary cycles of marine transgression and regression. Deposition of the Phantom Lake Suite involved subaerial volcanism. Deposition of the Deep Lake Group involved three cycles. The first cycle is a fining-upwards fluvial sequence. The second and third cycles began with glacial(?) deposition of paraconglomerates followed by deposition of marine and then fluvial sediments. Much of the sediment of the upper Phantom Lake Suite and Deep Lake Group was derived from a northeast source.

Radioactive, quartz-pebble conglomerates occur in the upper Phantom Lake Suite and lower Deep Lake Group. In outcrop, these conglomerates contain up to 141 ppm uranium and 916 ppm thorium, and have been strongly leached.

The Precambrian tectonic history of the Medicine Bow Mountains involved at least seven episodes of deformation. Three of these involve Archean rocks and probably took place more than 2500 m.y. ago. The other four involve Proterozoic metasedimentary rocks. The first Proterozoic deformation (D1) began during deposition of the upper Deep Lake Group and produced gently plunging, east-west trending folds. D2 began after deposition of the Libby Creek Group and produced gently plunging, northeast trending folds with steep limbs. D3 marked a change from northwest-southeast compression to northwest-southeast extension, and included an episode of intrusion of gabbroic magma and an episode of folding which produced open, northwest-southeast trending folds. D4 involved a rotation of all pre-existing structural elements about a sub-vertical fold axis.

Stratigraphy and structure of the lower part of the Precambrian Libby Creek Group, central Medicine Bow Mountains, Wyoming

RAY LANTHIER Groth Minerals Corporation, P. O. Box 2138, Steamboat Springs, Colorado 80477

Pages
135-148

Keywords
Libby Creek, Precambrian, Medicine Bow, Wyoming, Proterozoic, Headquarters, glacial

Abstract
The Libby Creek Group is the youngest group of the Early Proterozoic metasediments located in the Medicine Bow Mountains of southeastern Wyoming. The lower 3 km of the group consists of three formations. The Headquarters Formation (650 m thick) was deposited unconformably on units of the upper Deep Lake Group in a glacial-marine environment, and consists of a basal diamictite, a quartzite containing lenticular bodies of diamictites and phyllites, and a phyllite which grades upsection into quartzite of the Heart Formation. The Heart Formation (670 m thick) represents a change from glacial-marine to marine deposition, and is primarily quartzitic with a discontinuous phyllite unit near the center of the formation. The Medicine Peak Quartzite (1700 m thick) was deposited on a slowly subsiding, marine shelf, and consists of coarse-grained quartzite containing quartz pebble layers.

The rocks of the Libby Creek Group have been subjected to three deformational episodes. The first episode rotated bedding to nearly vertical attitudes around a northeast-trending fold axis, and the resulting flexural slip produced two major high-angle reverse faults. The second deformational episode produced folds with moderately plunging southeast trending fold axes and transverse faults, some of which were injected with mafic magma. The third deformational episode rotated the folds of the first two episodes around a nearly vertical fold axis.

A review of the stratigraphy and uranium potential of Early Proterozoic (Precambrian X) metasediments in the Sierra Madre, Wyoming

PAUL GRAFF Department of Geology, The University of Wyoming, Laramie, Wyoming 82071

Pages
149-158

Keywords
Precambrian, metasedimentary, Proterozoic, Wyoming, Sierra Madre, uranium, Phantom Lake, Archean

Abstract
Precambrian metasedimentary rocks exposed in the Sierra Madre, Wyoming are divided into three groups: (1) layered amphibole gneisses, quartzites, and metavolcanic rocks of Archean age, which are infolded and interlayered with felsic gneisses; (2) the Early Proterozoic Phantom Lake Metamorphic Suite, which consists of metavolcanic rocks, graywackes, paraconglomerates, metadolomites, and mature clastic rocks. (This suite is not divided into formations but can be separated into three units representing major changes in depositional environment and source rocks. The rocks of these three units are of fluvial, volcanogenic, and mass-movement origins, and all three units appear to be interlayered. The depositional unit dominated by fluvial processes is separated into upper and lower sequences by a uraniferous quartz-pebble conglomerate); and (3) the Early Proterozoic Deep Lake Group, a sequence which represents a gradual but fluctuating change from fluvial to near-shore marine deposition. (This group is divided into seven formations, which are variously dominated by mature clastic rocks including uraniferous quartz-pebble conglomerates, quartzites, phyllites, paraconglomerates, quartzites, phyllites, quartz-pebble conglomerates, massive sheared quartzites, calcareous phyllite, and metadolomites.)

Correlation of the Sierra Madre metasedimentary sequences with those occurring in the Medicine Bow Mountains can be accomplished using the following criteria: (1) similar ages of deposition (Early Proterozoic) are suggested by radiometric ages determined for underlying Archean terranes (2450 m.y.) and for post-depositional granites (1800 m.y.); (2) similarities in lithologies are seen in equivalent formational assemblages; (3) uraniferous marker horizons occur in both mountain ranges; (4) cyclical sedimentary units are recognized in both areas; and (5) four major deformational events affected both areas in the same sequence and style.

Uranium occurrences are usually found in quartz- and chert-pebble conglomerate lenses within coarse-grained feldspathic quartzites located low in the Proterozoic section. Though detailed mineralogic and sedimentologic work has not been performed on these rocks, the mineralization appears to best fit the fossil placer model.

Uranium, thorium, and gold in the lower Proterozoic(?) Estes Conglomerate, Nemo District, Lawrence County, South Dakota

F. ALLAN HILLS U.S. Ceological Survey, Denver Federal Center, Denver, Colorado 80225

Pages
159-172

Keywords
uranium, Estes, Dakota, Proterozoic, Black Hills, pyrite

Abstract
The Estes Conglomerate, which is exposed in the Nemo District on the northeastern flank of the Black Hills, South Dakota, is inferred to be of early Proterozoic age (early Precambrian X or Paleoaphebian of Canada) and to be resting on granitic continental crust of late Archean age (late Precambrian W of former usage). The Estes contains beds of quartzite and quartz-pebble conglomerate (oligomictic conglomerate) with matrices of micaceous quartzite that locally contain 5 to 25 percent dispersed pyrite. Highly oxidized outcrop samples of the oligomictic conglomerate have anomalously high contents of both uranium (10 to 122 ppm) and thorium (20 to 970 ppm). Abundant old prospect pits and several abandoned mines attest to the widespread but minor gold content of the Estes Conglomerate. Fifty percent of the analyzed samples (13 of 26) of oligomictic conglomerate contain at least 0.05 ppm of gold, but the maximum value is only 1.39 ppm. The high thorium and gold values in the oligomictic conglomerate favor a placer mechanism for the concentration of radioactive minerals, and the thorium appears to eliminate the possibility that mineralization is the product of low-temperature epigenetic processes, such as reduction and fixation of uranium by pyrite.

Uranium in the Estes Conglomerate may be of an origin similar to the origin of the economically very important uranium deposits in the Matinenda Formation of the Elliot Lake District, Ontario. Because uranium is rapidly dissolved in acidic, oxygenated ground water (such as is present where pyrite is weathering), most of the uranium originally present in the analyzed samples has probably been leached out. Conglomerate located below the zone of weathering and oxidation has potential for economic uranium deposits.

Uranium and thorium concentrations in Precambrian granites as indicators of a uranium province in central Wyoming

JOHN S. STUCKLESS U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225

Pages
173-178

Keywords
thorium, uranium, Wyoming, Owl Creek, Laramie, granite

Abstract
Detailed studies of the granitic rocks from the Granite Mountains and preliminary studies of some granitic samples from the Owl Creek Mountains and Laramie Range suggest that the region of central Wyoming has been a uranium province since early Precambrian time. It is suggested that uranium deposits of several different types and with ages of early Precambrian to Holocene might reasonably be expected within and near this central Wyoming uranium province.

Thorium contents of Precambrian granites in this region are generally anomalously high as compared to contents cited as typical for granites. Uranium contents of surface samples are generally not anomalously high, but isotopic evidence shows that most samples have lost much uranium during the Cenozoic. Measured Th/U ratios for samples from the Granite Mountains are mostly greater than 5, but calculations based on 208Pb and 206Pb indicate that Th/U ratios would be less than 3 if uranium had not been lost. It is proposed that thorium content of country rocks may be better than uranium content as an indicator of an uranium province and that Th/U ratios may be a useful indicator of uranium loss.

Genesis of the Schwartzwalder uranium deposit, Jefferson County, Colorado

EDWARD J. YOUNG U.S. Geological Survey, Denver Federal Center, Denver, Colorado 80225

Pages
179-186

Keywords
uranium, Schwartzwalder, Colorado, hydrothermal, formation, mineralization

Abstract
A meteoric hydrothermal origin is proposed for the Schwartzwalder uranium deposit, which occurs in fractured Precambrian metamorphic rocks. Three factors that contributed to formation of the deposit are: (1) structures that provided conduits and open spaces for mineralization; (2) nearby sedimentary rocks for a source of uranium; and (3) a magmatic heat source. A meteoric hydrothermal origin seems likely because: (1) pitchblende in the deposit is rich in Mo but poor in Th and rare earths, which is typical of sedimentary pitchblende; (2) unit-cell edges of the pitchblende (uraninite) of about 5.42 Angstroms fit sandstone and vein pitchblendes rather than pegmatitic (magmatic) uraninite; and (3) muscovite in the pitchblende-veined pegmatite host rock retains a Precambrian age according to K-Ar age determinations, indicating a relatively low temperature for the ore solutions.

It appears that meteoric water, moving downdip along bedding planes in sedimentary strata that had been tilted during Laramide tectonism, leached uranium from the rocks and percolated into fractures and into the Golden fault zone. Subsequently, intrusion of the Ralston dike of mafic monzonite (61.9 + 2.5 m.y. ago) introduced magmatic heat in and along the Golden fault zone and initiated convective flow of ground water through that zone and through other deep faults such as the Rogers and Illinois. Parts of the Illinois fault and hanging wall faults west of the Illinois reopened, and the wall rocks were brecciated in response to shearing; these brecciated zones and openings served as "depositional traps" for the pitchblende about 60 m.y. ago. Published radiometric ages of the Ralston dike (an unlikely source of uranium) and of the uranium ore are consistent with this sequence of events.

Classification of uranium deposits

MILTON O. CHILDERS and ROBERT V. BAILEY Power Resources Corporation, Denver, Colorado 80222

Pages
187-199

Keywords
classification, uranium, structure, intrusive, deposits

Abstract
The classification of uranium deposits described in this report is tabulated as follows:

I. Strata-controlled
A. Sandstone-conglomerate hosts
1. Trend deposits
a. Mineralized trends parallel to trends of hosts (paleodrainage control)
b. Mineralized trends generally crossing paleodrainage patterns
2. Roll front deposits
a. Bifacies roll fronts with both oxidized and reduced facies
b. Monofacies roll fronts which are uniformly reducing
3. Stack deposits
4. Precambrian heavy mineral deposits
B. Carbonate hosts
1. Paleokarst deposits
2. Reef-trend deposits
3. Calcrete deposits
C. Lignite, black shale, and phosphatic rock hosts
1. Uraniferous lignites
a. Low-grade wide distribution in lignites marginal to oxidizing environments
b. Deposits (including high-grade) in lignites associated with permeable sandstones below a regional unconformity)
2. Uraniferous black shales
II. Structure- or fracture-controlled (vein- or similar-type deposits)
III. Intrusive-controlled