THE SIERRA GORDA PORPHYRY CU-MO(AU) DEPOSIT, REGION II, NORTHERN CHILE, PART 1: ALTERATION, MINERALIZATION, AND FLUID INCLUSIONS

The Sierra Gorda porphyry Cu-Mo(Au) deposit is a measured and indicated resource of ~989 M tonnes @ 0.42% Cu, 0.027% Mo, and 0.069 g/t Au in two main mineralized centers: Catalina and the 281-Zone. Numerous 1-300m wide bodies of monzodiorite-to-granodiorite porphyry, as dikes (many with crenulate-qtz borders) or stocks and cupolas (with coarse-grained graphic groundmass), produced vein, disseminated, and lesser breccia-hosted Cu-Mo(Au) mineralization in both intrusions and andesitic wall rock. Hydrothermal breccias (bx) were permeable sites for sulfide deposition, but typically were not mineralizers. Both centers had a similar alteration/mineralization sequence: (1) early tourmaline (± qtz) as veins, flooding, vuggy bx, or qtz-feld-tourm pegmatite; (2) magnetite-albite (± qtz, Kspar) veins or flooding, (3) barren qtz veins, (4) secondary Kspar and/or biotite, as halos or flooding associated with veins, bx, or vein-dikes of qtz-(feldspar, moly) (± bio, wolframite, scheelite); (5) “chloritic” chalcopyrite-py veins or disseminations (± qtz, chl, sericite, anhydrite, specularite, bornite, gold), with halos of chl or chl-ser (± montmorillonite); (6) qtz-py-ser (± cpy) veins or bx with sericite halos; (7) late “base-metal veins” (281-Zone only) (qtz-py ± cpy ± galena ± sphalerite ± arsenopy) (± halos of ser or chl-ser), (8) late siderite (± dolomite) veins or bx. Microprobe data shows that chlorites were quite Fe-rich (~15-30 wt% FeO, “pycnochlorite”). Fluid inclusion reconnaissance study suggests that earliest magmatic/hydrothermal fluids (in tourm pegmatite and tourm-bx) were highly saline brines (35-45 wt% NaCl-eq., 375-425°C T_h) that fluctuated, at/near the ~400°C brittle/ductile transition, from lithostatic P to a hydrostatic P of ~200 bars (suggesting ~2km depth of formation). These brines locally flashed from liquid to vapor, producing vapor-halite inclusions and extremely vuggy tourmaline “flash breccias”. Chalcopyrite was deposited later, under hydrostatic P, from the lower-salinity “chloritic” fluids (300-375°C T_h, 4-15 wt% NaCl-eq). Limited data from late base metal (12 wt% NaCl-eq, 290-310°C) and qtz-py-sericite veins (5-6 wt% NaCl-eq, <220°C) suggest that these veins also formed hydrostatically from later lower-salinity fluids.