Thompsons Mining District, Grand County, Utah, USAi

Regional Level Types
Thompsons Mining District	Mining District
Grand County	County
Utah	State
USA	Country

The Thompson (Yellow Cat) mining district is located on the crest and northeast limb of the Salt Valley anticline in central Grand County approximately 24 mi north of Moab. Although U-V are the most important commodities produced, the district also has minor Cu ±Ag and Mn prospects. The Thompson district was mined successively for radium (1911–23), V (1939–44), and U-V (post-1948) (Cannon, 1964). Total district metal production at modern metal prices is estimated at $39 million. The largest producers were the Parco, Blackstone Incline, and Ringtail–Mineral Treasure underground U-V mines.
The Thompson district is geologically situated in the Paradox Basin on the Colorado Plateau. The U-V deposits in the district occur as relatively continuous, tabular and roll-type orebodies and as isolated mineralized logs surrounded by a halo or aureole of lower grade ore. Tabular bodies are the most common, but roll-type deposits have also been exploited (Stokes and Mobley, 1954). Both tabular and roll-type orebodies occur together with roll-type orebodies grading into tabular bodies along bedding or unconformities. The size of individual orebodies ranges from a few to several thousand tons, but multiple bodies often cluster within an area of 400 to 800 ft long by 250 to 300 ft wide.
The major Thompson deposits are confined to the Salt Wash Member of the Upper Jurassic Morrison Formation. Overall the Salt Wash contains almost equal amounts of sandstone and mudstone/siltstone, but many of the uranium deposits are in the lowest sandstone horizon. Most deposits are in coarse- to medium-grained sandstone but are also present in siliceous pebble conglomerate in the uppermost sandstone-conglomerate horizons. Unoxidized deposits contain uraninite, coffinite, and montroseite associated with sulfides and selenides. The ore minerals occur as void fillings, as coatings on detrital grains, and as replacements of clay in thin seams, clay pebble conglomerates or galls, and as replacement of carbonaceous material.
Ore grades average about 0.25% U3O8 and 2.0% V2O5 (Cannon, 1964). Replacements of carbonaceous material may assay as high as 10% U3O8. The deposits contain anomalous amounts of Se, Mo, As, Ni, and Co. The association with anomalous selenium has been used successfully as a prospecting tool, because selenium indicator plants such as Astragalus occur over, or adjacent to, concealed deposits (Cannon, 1964). Most of the mines are now flooded.