Petrology and geochemistry of precious and base metal mineralization, North Amethyst vein system Mineral County, Colorado
Foley, Nora Katherine
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Gold, silver and base metal-rich minerals of the North Amethyst vein occupy fractures in 26 Ma volcanic rocks near the southern margin of the San Luis caldera, at the northern edge of the Creede district, San Juan Mountains, Colorado. The veins contain two associations of mineral assemblages that are separated locally by breccias and sediments. The earlier, fine-grained association consists of quartz, rhodonite, Mn-carbonates, hematite, magnetite, electrum, Au-Ag-sulfides, Ag-sulfosalts, and base metal sulfides. The second association consists of coarser-grained quartz, calcite, sericite, chlorite, hematite, adularia, fluorite, base metal sulfides, and Ag-tetrahedrite. The first association (fluid boiling at >350°C to 270°C, <2 wt% HaCl equiv.) has a calculated depth of >1000m; the second association (nonboiling, basemetal assemblages: 280°C to 220°C, 5-11 wt%; boiling, quartz + fluorite + calcite: 270-220°C, 0-4.5 wt%) may have formed at -500 meters. Deeper estimates from geologic reconstruction (-1500m) support minera1ogial evidence that CO₂ played a role in mineral deposition. The second association also may have formed much later in time than the Au-rich stages, perhaps after extensive erosion. Adularia of the second association has an age of 25.13 Ma (M. Lanphere, pers. commun., 1987). Paragenetically early Au-rich North Amethyst galenas have lead isotopic compositions that are less radiogenic than galenas that formed later, as part of the second association. Galenas from central and southern Creede veins overlap compositions of galenas from the later North Amethyst association. Alpha-Corsair vein galenas are isotopically similar to North Amethyst Au-stage galenas. All galenas are isotopically more radiogenic than host volcanics and probably derived their lead compositions from both Precambrian basement and Tertiary volcanics or their equivalents. The first association (Mn-Au) is absent in the southern and central Creede district, whereas, the second association correlates with assemblages of the southern and central district. The local and early occurrence of the Mn-Au assemblages may indicate that they formed in a localized hydrothermal system that predated the extensive hydrothermal system from which ores of the central and southern Creede district are proposed to have been deposited. Least radiogenic and paragenetically early mineral assemblages bold the most promise for gold exploration in the central San Juan area.
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