Relationships between a porphyry Cu-Mo deposit, base and precious metal veins and Laramide intrusions, Mineral Park, Arizona

Abstract

In the Wallapai mining district, porphyry-style copper-molybdenum mineralization occurs within and near Laramide granitoid stocks at the center of an elongate zone of polymetallic quartz veins. The principal mineralized veins form a well-defined paragenetic sequence, with a simplified mineralogy progressing through: (1) anhydrite-molybdenite (AM) veins of quartz + K-feldspar + anhydrite + pyrite + biotite + molybdenite, (2) quartz-molybdenite (QM) veins with quartz + molybdenite + pyrite, (3) anhydrite-chalcopyrite (AC) veins containing quartz + K-feldspar + anhydrite + pyrite + magnetite + chalcopyrite + chlorite + rutile, (4) quartz-pyrite (QP) veins of quartz + sericite + pyrite, and quartz + pyrite + chalcopyrite + epidote + chlorite, and (5) polymetallic quartz (PMQ) veins, with a gangue assemblage of quartz + sericite + calcite + kaolinite(?) + epidote + chlorite, and a variety of base and precious metal minerals.The coexistence of vapor- and liquid-rich fluid inclusions in vein quartz shows that the molybdenite-bearing anhydrite and quartz veins formed at 360 degrees to 410 degrees C from boiling, low-salinity brines. Chalcopyrite deposition in the anhydrite-chalcopyrite veins was at 380 degrees to 420 degrees C from nonboiling brines averaging 20 equiv wt percent NaCl and 25 equiv wt percent KCl. The quartz-pyrite veins formed at 320 degrees to 350 degrees C from brines of 1 to 13 equiv wt percent NaCl. The polymetallic quartz veins formed from dilute brines of 1 to 7 equiv wt percent NaCl. Temperatures in the first stage of mineralization of the polymetallic-quartz veins abruptly increased to 400 degrees to 450 degrees C in and near Mineral Park, but declined outward to less than 300 degrees C on the district periphery. In subsequent stages, fluid temperatures decreased across the district to less than 200 degrees C. Pressures were 250 to 300 bars, probably in a hydrostatic pressure regime, during boiling related to molybdenite mineralization, but they may have increased to about 1,100 bars lithostatic pressure during chalcopyrite deposition in the anhydrite-chalcopyrite veins. A depth during mineralization of 3 to 4 km is suggested.Copper and molybdenum mineralization occurred in a lithocap environment above a progenitor intrusion but was not directly related to the exposed Ithaca Peak stocks. The evolution from hypersaline fluids in the anhydrite-chalcopyrite veins to low-salinity, lower temperature fluids in the quartz-pyrite veins suggests an influx of meteoric water into the waning porphyry-style hydrothermal system. Brines up to 100 degrees C hotter appeared during the formation of the paragenetically later polymetallic quartz veins and constituted a new, and much larger, hydrothermal system. The heat source, presumably an unexposed intrusion, was much larger than the porphyry-style system. The zonation in base metal ratios of polymetallic quartz veins is symmetrical with respect to the proposed heat source.