Seven Mile Canyon Mining District, Grand County, Utah, USAi

Regional Level Types
Seven Mile Canyon Mining District	Mining District
Grand County	County
Utah	State
USA	Country

The Seven Mile Canyon district of the Green River mining area is located approximately 10 mi northwest of Moab in Grand County. Uranium was discovered in Seven Mile Canyon in 1948 and major mines were operated from the early 1950s to 1988. The district is a significant U ±V ±Cu producer. Total district metal production at modern metal prices is estimated at $47 million. The largest mines are the Bicentennial, Thornburg-Memorial, and Shinarump.
Seven Mile Canyon is situated in the Paradox Basin on the Colorado Plateau. The main U host is the Upper Triassic Chinle Formation. The mineralized zones are quasi-horizontal, but the zones cross lithologic boundaries and have little recognizable relationships to permeability. In fact, most of the ore in the Thornburg-Memorial and Bicentennial mines was in silty to clayey, fine-grained sandstone. Ore zones can be up to 18 ft thick, but most are much thinner. Ore zones are generally bleached. The ore occurs as discontinuous, single or multiple, tabular lenses or saucer-shaped zones of disseminated U mineralization in the lower 25 to 30 ft of the Chinle Formation (USGS Model 30c). Three mineralized horizons are present in the Shinarump No. 1 mine: (1) a basal siltstone with interbedded shale, sandstone, and conglomerate, 1 to 10 ft thick; (2) the upper part of an overlying gray-brown, 2- to 10-ft-thick limestone pebble conglomerate; and (3) within a carbonaceous siltstone 5 to 10 ft stratigraphically above the pebble conglomerate (Finch, 1954). Droullard and Jones (1955) reported that the basal ore zone was deposited in channels scoured into the underlying Moenkopi Formation and pebble conglomerate ore zones were moderately high-grade (>1.0% U3O8) and characterized by massive uraninite and its oxidation products. Most of the production from the Shinarump No. 1 was from the basal siltstone zone whereas most of the production from the Shinarump No. 3 was from the upper siltstone unit. Little has been written about the Thornburg-Memorial and Bicentennial mines, but most of the ore at these mines appears to have been in the same horizon as the Shinarump No. 3.
Ore minerals consist mostly of uraninite, chalcocite, and pyrite with subordinate chalcopyrite and bornite (Finch, 1954). The ore minerals occur as impregnations, replacements of carbonaceous material and as coatings on limestone clasts. The ore is reportedly high in carbonate and low in vanadium. The ratios of U3O8 to V2O5 range from 1:3 to 500:1. Few vanadium-bearing minerals are recognized and most of the vanadium is probably associated with clays (Droullard and Jones, 1955). Reported grades ranged from 0.18% to over 0.40% U3O8 and up to 0.13% V2O5.