Abstract

Hydrothermal alteration and associated lead-zinc-silver sulfide mineralization in the Hahns Peak quartz latite stock, Routt County, Colorado, form a zoned pattern which was identified by detailed geologic mapping plus petrographic and X-ray studies of clays, micas, and feldspars in surface and subsurface samples. Routine analysis of structural state and composition of alkali feldspars was an integral part of the X-ray studies.The two stages of alteration and mineralization identified within the stock are: pervasive albitization with minor deposition of disseminated pyrite and argillic and sericitic alteration strongly localized along fractures and breccia zones accompanied by deposition of pyrite, sphalerite, chalcopyrite, galena, and argentiferous tetrahedrite. The mineral assemblage of albite-orthoclase-quartz-sericite-pyrite indicated albitic alteration probably formed in the temperature range 400 degrees to 500 degrees C and the assemblage of sericite-dickite-montmorillonite-quartz-pyrite indicates that the sericitic and argillic alteration formed in the range of 250 degrees to 350 degrees C. Zones of advanced argillic and sericitic alteration crosscut the pervasive albitic alteration and grade outward into pervasive intermediate argillic alteration in less fractured rock.The mineralogic changes observed in the pervasive alteration zones are interpreted to be the result of gradual changes in solution composition as the hydrothermal solutions ascended in the stock. The mineralogic changes in the advanced argillic and sericitic zones are probably the result of more drastic changes in the solution composition caused by rapid escape of fluids due to release of pressure from structural readjustments in the stock. The changes responsible for sulfide deposition in the advanced argillic and sericitic zones probably were decreasing temperature, increasing pH of the solution, and increasing cation/H (super +) activity ratios in solution.

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