Harshman reported in 1972 that the Shirley Basin area is in Albany, Carbon, Converse, and Natrona Counties, Wyo., about 35 airline miles south of Casper. It is accessible over two improved and many local dirt roads. The area may be divided into four geographic units or subareas: the Shirley Basin, Bates Hole, Bates Creek drainage area, and the Laramie Mountains. Perennial streams drain all four sub-areas, although summer flows are low.
Maximum relief is in Bates Hole, an erosional depression, where altitudes range from about 6,100 feet along Stinking Creek to about 8,000 feet on Chalk Mountain in the southwest corner. Altitudes in the Shirley Basin, the subarea of economic interest, range from about 6,900 feet along the Little
Medicine Bow River on the southeast to about 7,800 feet along the north rim of the basin.
The Shirley Basin contains one of the major uranium ore reserves in the United States, and as of late 1969 the deposits were being mined by Kerr McGee Corp., Petrotomics Co., and Utah Construction and Mining Co. Most of the ore is milled locally into yellowcake (U3O8) Rocks in the Shirley Basin area range in age from Precambrian to Quaternary. The Precambrian rocks crop out in the core of the Laramie Mountains and are part of an ancient terrane that was folded, metamorphosed, and subsequently invaded by granitic batholiths and mafic dikes. The intrusive activity was followed by a long period of erosion, and by the end of Precambrian time the region was reduced to a nearly
flat surface.
Throughout the Paleozoic Era and much of the Mesozoic, the area was part of a stable foreland that lay along the east side of the Cordilleran geosyncline. Epicontinental seas repeatedly transgressed the area from the west and northwest, and a thick series of marine, littoral, and continental sediments was deposited in the Shirley Basin area. The formations, from oldest to youngest, are:
1) the Madison Limestone of Mississippian age,
2) the Casper Formation of Pennsylvanian and Permian age,
3) the Goose Egg Formation of Permian and Triassic age,
4) the Red Peak, Alcova, and Jelm Formations of Triassic age, and
5) the Sundance and Morrison Formations of Jurassic age.
Near the end of the Jurassic Period the western shores of the epicontinental seas migrated eastward, and great amounts of clastic material, eroded from highlands to the west, were deposited along the shores of, or in, the Cretaceous seas. From oldest to youngest the rocks of Cretaceous age of the Shirley
Basin are:
1) the Cloverly Formation, principally of continental origin, and
2) the Thermopolis, Mowry, Frontier, Niobrara, and Steele Formations, principally of marine and deltaic origin.
The Laramide orogeny deformed the area at the close of the Cretaceous, and the seas withdrew for the last time. Erosion of the newly formed mountains began in the Paleocene and continues now. The clastic debris that had been eroded from the highlands to the east and west of the Shirley Basin was deposited on an erosional surface which was cut into Cretaceous and older rocks. Formations of Tertiary age present in the area are, from oldest to youngest: Fort Union(?), Wind River, Wagon Bed, White River (two members) , and Arikaree. Quaternary formations include several stages of stream gravels and alluvium.
The pre-Tertiary rocks have been folded and faulted on a minor scale. A syncline trends southeastward through the western part of the area. Dips of the rocks on the eastern limb of the syncline are 2°-12° SW., and on the western limb the dips are northeastward and somewhat flatter. The area was domed in late Tertiary time; exposed Tertiary rocks dip about 1° N. and have been broken by a few faults that show small displacements.
The ground-water table in the area lies from a few feet to 100 feet or more below the surface. Ground water is tributary to the surface drainage system and has a southerly gradient of 10-30 feet per mile. The principal anions in the ground water are sulfate and bicarbonate; radioelements include uranium, radium, and radon. Measurements of pH range from 6.6 to 8.3 and have a mean and median of about 7.7.
The uranium deposits in the Shirley Basin are of major significance and constitute about one-sixth of the Nation's ore reserves. They are in the Wind River Formation of early Eocene age in two thick sandstone intervals which are separated by siltstone and silty claystone beds. The deposits bound large tongues of altered sandstone, commonly in so-called roll forms at the margins and as tabular layers on the top and bottom surfaces, a spatial relationship useful as an exploration guide. Ore bodies range from a few hundred tons to a few hundred thousand tons and in ore grade, as mined, from 0.1
to 0.7 percent U3O8. High-grade ore may contain as much as 20 percent U3O8 .
The edges of the altered-sandstone tongues in the Shirley Basin area separate oxidized iron minerals within the tongues from reduced iron and uranium minerals in the material surrounding them. For this and other reasons, the ore-bearing solutions that produced this alteration are thought to have been oxidizing. The geochemistry of the ore and associated minerals and the arkosic nature of the ore-bearing sandstones suggest that these solutions were alkaline.
Hydrogen sulfide of biogenic origin is believed to have played an important part in forming the ore deposits, although precipitation of the ore minerals may have been caused by nonbiogenic reactions. Ore deposition probably took place at least 500 feet and perhaps as much as 1,500 feet below the ground surface. Uraninite is the only identified ore mineral; accessory minerals are abundant pyrite and minor amounts of marcasite, calcite, hematite, native (?) selenium, and an unidentified sulfate mineral. Altered sandstone contains goethite, limonite, and ferroselite. Uraninite and pyrite coat sand grains and fill interstices between sand grains; there is some replacement of the arkose by pyrite.
Many elements in the ore, and in altered and unaltered sandstone near ore, show a systematic distribution about the edge of the altered-sandstone tongue. These elements include uranium, iron, selenium, carbon, beryllium, sulfur, copper, and vanadium. The sequence of events that gave rise to the Shirley Basin
ore deposits is believed to have been as follows:
1) Downward percolation of vadose waters that leached uranium from weathered granite in nearby highlands and (or) from the tuffaceous constituents of rocks overlying the ore-bearing intervals,
2) lateral movement of this water through the most permeable beds of the Wind River Formation,
3) reaction of the oxidizing ore-bearing solutions with the rocks through which they moved, causing a lowering of the Eh and pH of the solutions and deposition of the ore and associated minerals, and
4) passage of the spent ore-bearing solutions beyond the zone of deposition and out of the basin.
The zone of ore deposition is considered to have been a dynamic feature, migrating basinward by oxidation and solution on the updip side of the ore body and reduction and deposition on the downdip side. A rather sharp drop in the pH of the ore-bearing solution probably occurred at the edge of the altered sandstone tongue when pyrite was oxidized by the ore-bearing solutions.
The age of the Shirley Basin uranium deposits is not well documented, but the best evidence suggests that it is about 18 million years.
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