Abstract

Analyses of six drill cores from and surrounding the North Canning uranium deposit in the Copper Mountain uranium district, Wyoming, suggest that a low-temperature hydrothermal system may have transported and precipitated uranium. Uranium is found in highly altered breccia zones and fractures in Archean granite and in overlying Tertiary sediment. Na montmorillonite invariably forms the matrix around stable and secondary K-feldspar and quartz in the breccias. Uraninite and amorphous pitchblende occur as disseminated fracture fillings and as encrustations on lithic fragments in the breccias. Hematite staining commonly occurs around uranium mineralization, and pyrite is disseminated in fractures and is intimately associated with uraninite. In the granite host rocks, sericite replaces plagioclase, epidote replaces plagioclase and biotite, and chlorite sporadically replaces biotite. Homogenization temperatures of fluid inclusions in quartz microveinlets average 136 degrees C.Anomalous uranium concentrations occur in the Copper Mountain granite (5 ppm regionally, but more than 20 ppm in a central core). Isotope systematics show 30 to 50 percent uranium losses in Tertiary time, from which preloss values greater than 35 ppm are calculated. The heat-generating capacity of this granite, using the back-calculated U value and averages of 28 ppm Th and 4.5 percent K 2 O is 11.2 mu W/m 3 . Steady state conductive temperatures greater than 150 degrees C would have been generated under 4.5 km of Cambrian to Cretaceous sediments. Laramide faulting opened the heated rock to convective water circulation, allowing silicate alteration, mobilization of uranium within the granite, and then deposition of uranium in the breccia zones.Activity diagrams show solution reaction paths associated with alteration and stability relationships for uraninite. Unaltered K-feldspar and plagioclase altered to sericite and Na montmorillonite suggest constant a (sub K (super +) ) /a (sub H (super +) ) and increasing a (sub Na (super +) ) /a (sub H (super +) ) ratios. The assemblage uraninite-pyrite-hematite is stable within f (sub o 2 ) values of 10 (super -44) to 10 (super -35) and f (sub s 2 ) values of 10 (super -20) to 10 (super -6 5) atm at an assumed P (sub CO 2 ) = 10 (super -2) atm and Sigma U = 10 ppb in the hydrothermal fluid. Oxidation of magnitite and sulfide solution species to hematite and hydrothermal pyrite caused reduction and precipitation of uranium from solution.

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