2009 Portland GSA Annual Meeting (18-21 October 2009)

Paper No. 6
Presentation Time: 9:00 AM-6:00 PM


SHAVER, Stephen A., Department of Forestry and Geology, Sewanee--University of the South, 735 University Avenue, Sewanee, TN 37383, MANSKE, Scott L., P.O. Box 649, Lakeside, OR 97449, CURRIE, Jack, KGHM International, General Borgoño 934, Of. 802, Antofagasta, Chile, FAHEY, Patrick L., Quadra Mining Ltd, 257 Corte San Marco, Palm Desert, CA 92260, MAYA, Jorge, Minera Quadra Chile Ltd, General Borgoño 934, Of. 802, Antofagasta, Chile and REYNOLDS, T. James, FLUID INC, 1401 Wewatta St, #PH3, Denver, CO 80202, sshaver@sewanee.edu

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 Th) 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 Th, 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.