Geology, geochemistry and stable isotope studies of a porphyry molybdenum system, Ophir district, San Juan Mountains, Colorado
The Ophir district is a small base and precious metal mining district on the western edge of the middle Tertiary San Juan volcanic field of southwestern Colorado. Geological mapping has delineated a complex of propylitic and serlcitic hydrothermal alteration zones centred on a group of monzonitic quartz-feldspar porphyry intrusions, interpreted to be the surface expression of a shallow pluton. An average K-Ar age of 24.3 Ma for this intrusion suggests that it is a late phase of a large calc-alkaline batholith beneath the San Juan volcanic field. Propylitic zones, comprising an inner pyriticzone and an outer chloritic zone, are superimposed on an early regional propylitisation. Zones of intense sericitic alteration postdate propylitisation and are spatially related to an ENE-trending belt of small rhyolitic and breccia intrusions. K-Ar ages for sericites are indistinguishable from the quartz-feldspar porphyry ages. Numerous molybdenite showings occur in the sericitic and pyritic zones. A grid-based lithogeochemical survey reveals at least two stages of trace element dispersion. Copper and Mo enrichments are associated with the pyritic zone. Tungsten, Mo, Pb, As and Rb enrichment and Sr, Mn, Zn and Cu depletion are associated with the sericitic zones. The distribution of W, Mo, Pb and As suggests that they are vertically zoned above an ENE-trending mineralising source at depth. Stable isotope studies of whole-rock samples and mineral separates (biotite, sericite and quartz) indicate that 1) rocks of the regional propylitic zone have interacted with heated meteoric fluids, 2) further meteoric-hydrothermal activity accompanied pyritic and chloritic alteration, 3) sericite was deposited from a predominantly magmatic- hydrothermal fluid but subsequently interacted with meteoric fluids, 4) most base metal veins were deposited from predominantly meteoric fluids, 5) molybdenite vein hydrothermal fluid contained a significant magmatic component. The data suggests that 1) a meteoric-hydrothermal system developed in the environs of a quartz-feldspar porphyry pluton; local input of magmatic-hydrothermal fluids produced Cu-Mo-pyrite enrichment, 2) a major late stage input of more evolved magmatic-hydrothermal fluids, associated with emplacement of rhyolitic differentiates and breccia intrusions, produced sericitic alteration and associated geochemical anomalies, 3) the waning meteoric system collapsed back into the sericitic zones as the magmatic system faded. The present level of exposure intersects the base of the sericitic zone of a porphyry-style hydrothermal system. Since metal enrichments are weak, the potential for significant buried mineralisation appears to limited.