Central City, Colorado; the upper part of an alkaline porphyry molybdenum system

The distribution of molybdenite, fluorite, rhodochrosite, enargite, and telluride minerals, together with anomalous quantities of Cu, Pb, Zn, and Mo in rocks, and variations in stable isotope ratios define a complex, zoned hydrothermal system in the Central City mining district. The molybdenite mineralization is centered on a Laramide quartz bostonite or alkali rhyolite breccia pipe that occurs at the intersection of the Dory Hill fault and the Idaho Springs-Ralston shear zone. Stream sediment geochemistry suggests that significant amounts of W may be present in the system. Uranium, base, and precious metal mineralization of a similar age, but with a distribution different from the molybdenite mineralization, is also present and is considered to be a part of the same hydrothermal system.Fluid inclusion data for the molybdenite and precious and base metal mineralization indicate that fluid inclusions associated with early molybdenite veins are of two types: CO ₂ rich and rare halite-bearing, with respective homogenization temperatures of 340 degrees to 420 degrees C and 240 degrees to 340 degrees C. Inclusions associated with precious metal-bearing pyrite and base metal veins are aqueous, two-phase types with homogenization temperatures in the range of 220 degrees to 380 degrees C. Those from late molybdenite veins are aqueous, two-phase inclusions with homogenization temperatures of 200 degrees to 280 degrees C. Salinities (as equiv. wt % NaCl) are between 34 to 42 for halite-bearing inclusions and 2 to 12 for all other types. No evidence of boiling has been found. Trapping pressures of CO ₂ -rich inclusions are 0.46 to 0.75 kb, which suggest an emplacement depth of about 2 km for the molybdenite mineralization.Respective quartz delta ¹⁸ O and fluid inclusion delta D values are: (1) early molybdenite veins: +9.3 to +14.3 per mil and -87 to -66 per mil; (2) precious metal-bearing pyrite and base metal veins: +8.4 to +15.9 per mil and -90 to -47 per mil; and (3) late molybdenite veins: +10.1 to +15.4 per mil and -77 to -69 per mil with associated fluorite having fluid inclusion delta D values of -70 to -57 per mil. A late telluride vein has a quartz delta ¹⁸ O value of +11.2 per mil and fluid inclusion delta D value of -86 per mil. These data suggest that all except the telluride ore fluids contained large proportions of a D-rich (magmatic) fluid. The distribution of fluid inclusion delta D values defines two distinct anomalies, each zoned with delta D enrichment occuring inward toward centers which correspond to the areas of known molybdenite occurrences and precious and base metal mineralization. Likewise, delta D values for sericites developed during early hydrothermal activity increase inward toward the quartz bostonite pipe, various intrusive breccias, and molybdenite mineralization in the district. There is no indication at Central City of negative delta ¹⁸ O anomalies in the Precambrian country rock within or peripheral to the centers of mineralization suggesting that ore fluids were thus essentially confined to discrete fractures.It is concluded that molybdenite and other mineralization in this classic gold mining district may be related and may represent the upper part of an alkaline porphyry molybdenum system.