Abstract
The Gas Hills uranium district is in central Wyoming along the southeastern margin of the Wind River basin. The initial discovery was made by Neil E. McNeice in the fall of 1953. Earliest development of the district's ore reserves was quite slow, but accelerated when the larger, more experienced mining firms became active in the area. Surface drilling proved to be the most effective tool for finding and developing the uranium ore reserves. New drilling techniques were needed to obtain samples of the below-water-table ores. Many methods were tried, but frozen core drilling and bucket augering proved to be the most reliable for obtaining accurate samples. Since the initial discovery, the Gas Hills uranium district has produced about 12% of the United States total production. The present land surface is characterized by barren, subdued, rolling hills. These are traversed locally by steeply dipping hogback ridges of older, more resistant rocks, which are the flanks of truncated, northward plunging folds formed by crustal disturbances prior to the deposition of the Wind River Formation. A steep erosional escarpment that rises abruptly above the north sloping basin floor bisects the region and divides the surface drainage between tributaries of the Wind River on the north and tributaries of the Sweetwater River on the south. Volcanism occurred during late Eocene time, as evidenced by relic vents at the southern end of the Rattlesnake Hills, and by local volcanic debris in the middle and upper Eocene rocks. Sedimentary rocks exposed in the Gas Hills uranium district include sandstone, limestone, dolomite, shale, and tuffaceous sandstone, mudstone, and shale. They range in age from Cambrian to Miocene and have a composite thickness of over 14,000 ft. The source beds for the uranium deposits are arkosic sandstones interstratified with lenticular mudstones and shales. Two distinct types of sandstone are present in the Wind River Formation. The youngest is yellowish-orange to yellowish-gray arkose, derived primarily from Precambrian gneissoid and granitoid rocks; it contains little clay, abundant calcium carbonate, and limonite cement, and is host for all uranium deposits of the district. The second type of sandstone is pale yellowish-gray to pale olive, derived from areas of schists of Precambrian age; it contains abundant clay matrix. There are four types of uranium deposits in the district, the most important being the solution-front deposits. They were formed along the margins of highly altered, tabular sand beds that are enclosed by overlying and underlying fine-grained siltstone, claystone, and carbonaceous mudstone beds. Solution fronts can be followed for long distances and individual ore bodies along them may reach thousands of feet in length. The solution fronts are ideally crescentic or C-shaped when viewed in cross section, with thin mineralization forming the tips of the crescents. The uranium minerals occur as earthy brown to black coating on, and interstitial fillings between, the quartz-sand grains. The primary uranium ore minerals are coffinite and uraninite. The three other types of deposits include transitional-bedded, oxidized, and residual-remnant deposits. Several quite large transitional bedded deposits have been mined, but the oxidized and residual-remnant deposits are commonly small and difficult to mine. Ground waters, trapped by the southward tilting of the Tertiary rocks during late Miocene time, became stagnant. These waters dissolved uranium and other elements from the enclosing rocks, and after erosion had exposed the highest beds of the Wind River Formation, the mineral-rich solutions gained egress from the enclosing sand aquifers on the north and the solution-front ore deposits began to form. End_of_Article - Last_Page 905------------
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