Abstract

The Jamestown district, Boulder County, Colorado, is a major producer of fluorspar; prior to 1940, gold, gold-telluride, and lead-silver ores were mined. Fluorite occurs as a primary mineral in phases of the composite sodic granite stock at Jamestown and in breccia zones, stockworks, and pipe-shaped bodies in and adjacent to the stock. Gold and telluride mineralization occurs with and without fluorite in veins peripheral to the stock. A great variety of fluid-inclusion types is present in the district, reflecting chemically and physically diverse fluids. Fluid inclusions in early-stage quartz from the fluorspar deposits have filling temperatures of 250 degrees to 375 degrees C, salinities of 20 to 30%, and boiling is indicated at many locations. Most inclusions associated with the main fluorite stage have salinities of at least 26% to more than 50% and filling temperatures in the range of 250 degrees to 350 degrees C. Probable cognate fluorite on the inclusion walls and as many as ten daughter minerals precipitated from some primary inclusions in fluorite attest to the presence of salt and CaF 2 -rich polycomponent fluorspar-depositing fluids. Carbon dioxide-rich liquids and vapors were present during and after the main period of fluorspar deposition.Gold-bearing veins contain fluid inclusions indicating that two fundamentally different types of fluids were present. Inclusions in pregold quartz and fluorite have filling temperatures which range up to 375 degrees C, and most have very high salinities comparable to those from the fluorspar deposits, suggesting that early quartz and fluorite of the gold-bearing veins was roughly contemporaneous with the main period of fluorspar deposition. Fluid inclusion and other thermometric data (Kelly and Goddard, 1969) suggest that gold and telluride deposition occurred at temperatures below 300 degrees C. Fluid inclusions most directly associated with gold deposition have filling temperatures in the range 205 degrees to 270 degrees C and have about 4% salinity. These data suggest that the gold-bearing fluids either evolved from the fluorite-bearing fluids or were later unrelated hydrothermal pulses introduced on reopened structures.Boiling occurred at many locations that were at relatively high elevations or adjacent to the sodic granite stock. From the interpretation that boiling occurred and from the heterogeneity of CO 2 contents, we deduce that fluid pressures were low and variable, commonly near 150 bars, but ranging to as much as approximately 500 bars when CO 2 pressures were high; these determinations are compatible with the depth of cover estimated from geomorphic reconstruction using the Flattop peneplain. The absence of strong thermal or salinity zonation of fluids relative to the outcropping stock suggests that fluids were emanating from a larger intrusive body at depth.

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