Abstract

The Rainy Day mine, in the Circle Cliffs area, Utah, is developed on a long slender pod of U ore. Ore is localized in siltstone of the Moenkopi formation of Triassic age, on the S. edge of a channel about 3,300 ft. wide by 40 ft. deep that is filled with sandstone of the Shinarump member of the Chinle formation of Triassic age. Shale of the Chinle rests directly on siltstone of the Moenkopi on either side of the channel. The pod of ore is of moderately high grade, 1 and 1/2 by 4 ft. in cross section, and is continuous for a mined distance of more than 1,800 ft. Sphalerite, chalcopyrite, pyrite, marcasite, and galena are associated with a black U mineral, which is probably uraninite. Semiquantitative spectrographic analyses show that Pb, Cu, Ni, Co, Ag, Mo, Zn, Y, and Yb increase proportionately with U; the increase suggests that these metals were introduced by the ore-forming fluid. The deposit is on the E. flank of the NW.-trending Circle Cliffs anticline. No major faults are near the mine, nor were any through-going joints noted in the deposit. Efforts to correlate the localization of the deposit with minor structures were unsuccessful.

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