APPLIED GEOLOGY IN THE DISCOVERY OF THE SPOKANE MOUNTAIN URANIUM DEPOSIT, WASHINGTON

In 1975, Western Nuclear, Inc. discovered a blind uranium deposit on the north flank of Spokane Mountain approximately 38 miles (61 km) northwest of Spokane, Washington, after extensive geologic, geophysical, and geochemical programs had been conducted over a five-year period to locate the target area. The Spokane Mountain deposit was discovered in a northeast (Midnite) geophysical trend defined by airborne magnetic and reconnaissance induced polarization surveys. Regional and detailed soil and stream sediment geochemical programs helped narrow the target area. The deposit averages 8 ft (2m) thick with a grade of 0.25 weight percent eU3O8*, lies at 150 to 500 ft (46 to 152 m) depths, has a linear trend of N 70°-80° E with a 1,000-ft (305 m) minimum length and 150-ft (46 m) average width. Pitchblende in veinlets or as coatings on fractures is associated with iron sulfides and is located mainly in the unoxidized zone of the graphitic phyllite and chloritic schists of the Precambrian Togo Formation within 100 ft (30 m) of its contact writh the intrusive Cretaceous Loon Lake Porphyritic Quartz Monzonite. Preliminary studies indicate the deposit may have been of penesyngenetic( ?) and hydro-thermal origins with remobilization and late-stage supergene enrichment and is related to regional and local structural features. Although the Spokane Mountain deposit has many of the same characteristics of those of the Midnite mine, it differs in the following features: (1) the mineralized body strikes east-northeast; (2) mapable carbonate beds are absent; (3) mineralization penetrates the quartz monzonite; (4) the ore seems to have some stratigraphic control; (5) most of the uranium mineralization is reduced and below the present water table; and (6) ore grades and thicknesses appear to be lower than that of the Midnite but have wider distribution. © 1978 Society of Economic Geologists, Inc.