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Mineralogy, mineral chemistry, and paragenesis of gold, silver, and base-metal ores of the North Amethyst vein system, San Juan Mountains, Mineral County, Colorado

January 1, 1994

Gold-rich adularia-sericite-type mineralization occurs near the southern margin of the San Luis caldera, at the intersection of the Equity fault and the northern extension of the Amethyst fault system. Mineralized rock is confined primarily to steeply dipping structures in silicified rhyolite and dacite. Intense sericitic alteration occurs at higher levels in the vein system, and wall rock adjacent to some veins is bleached. The ores are multiply brecciated, and vein filling locally shows sedimentary textures.

Textural, mineralogical, and chemical criteria indicate that there are at least two partially coextensive associations of mineral assemblages separated by periods of brecciation and sedimentation. An older gold-bearing association consists of two fine-grained ore stages, both of which contain electrum, uytenbogaardtite, tetrahedrite, silver sulfosalts, silver sulfides and base-metal sulfides, and a manganese-rich stage containing the assemblages (1) manganese silicate + manganese carbonate minerals + quartz and (2) magnetite + hematite + pyrite + quartz. A younger crosscutting association contains calcite, adularia, fluorite, and quartz, plus the assemblages (1) coarse-grained basemetal sulfides and (2) hematite + chlorite + quartz. Quartz, manganese-rich calcite, and trace pyrite line late-stage vugs.

Mineralogic, lead-isotopic, and fluid-inclusion characteristics of the younger association are similar to those of ores of the southern and central parts of the Creede mining district. In contrast, the gold and manganese-silicate assemblages of the older association are rare to absent in the southern and central parts of the district. The local and early occurrence of the manganese and gold assemblages may indicate that they formed in a small hydrothermal cell that predated the extensive hydrothermal system from which ores of the central and southern parts of the Creede district are proposed to have been deposited (Bethke, 1988). If similar early-stage cells were present in the southern and central parts of the district, they may have been replaced or overprinted by later assemblages, and they may remain to be discovered. In the latter case, mineral assemblages that formed at early stages in the paragenesis hold the most promise for gold exploration.