The Midnite mine is on the Spokane Indian Reservation, Stevens County, Wash. Geologic mapping and reconnaissance in the vicinity of the mine indicate metasedimentary rocks of probable Precambrian age have been intruded by two varieties of quartz monzonite of probable Cretaceous age. Porphyritic quartz monzonite underlies about three-fourths of the mapped area, and equigranular quartz monzonite underlies about one-thirtieth of the area. Metasedimentary rocks have been thermally metamorphosed in the vicinity of the intrusives. Uranium minerals are restricted to the vicinity of the contact between porphyritic quartz monzonite and schistose or spotted phyllite. The largest and richest uranium deposits are where the contact is relatively steeply dipping and highly irregular in detail. Uranium minerals are most abundant in brecciated and fractured or jointed schist but are also present along joints and grain boundaries in the adjacent quartz monzonite. Secondary uranium minerals comprise all known ore deposits; uraninite was found only in one diamond-drill core from the Midnite property. The origin of the deposits is not known, but several features suggest a hydrothermal origin.
