THE PETROLOGY OF WALL-ROCK ALTERATION ZONES IN PORPHYRY CU-MO-AU DEPOSITS: CONSTRAINTS ON THE TIMING, TEMPERATURE, AND PRESSURE CONDITIONS OF SULFIDE ORE FORMATION

The pressure-temperature (P-T) stability of silicate wall-rock alteration mineral assemblages when combined with magmatic-hydrothermal P-T fluid flow paths provide high T estimates for Cu, Mo, and Au sulfide ore deposition. Such fluids escape from magmas at lithostatic pressures, and rapidly depressurize along adiabatic paths to hydrostatic conditions at ~500°C.

Proffett (2009) noted that shallow magmas produce abundant A quartz veins with Cu-Fe sulfides at ~2-4 km depth within zones of K-silicate alteration (K-feldspar + biotite), whereas deeper magmas produce early halos with K-feldspar-muscovite-biotite-±andalusite and Cu-Fe sulfide enclosed within broader K-silicate zones. The fluids that form deeper early halos are higher pressure, which enhances the stability of muscovite + andalusite relative to K-feldspar; the three phase assemblage is stable from 500 to 580°C with increased pressure. Early halos contain only sparse quartz vein fill, because the P-T path of fluid ascent traverses the retrograde solubility zone of quartz. Along both deep and shallow P-T ascent fluid paths, chalcopyrite solubility decreases sharply and ~80% of Cu likely precipitates between ~550 and 350°C. These estimates are consistent with K-silicate or early halo assemblages, and the interpretation of observed bornite-chalcopyrite intergrowths as cooling exsolution textures from a single parent Cu-Fe sulfide.

In the final stages of fluid flow, the magmatic source of fluids commonly is deeper than in earlier stages, and fluids amounts diminish and are lower temperature. D veins in many cases post-date several porphyry intrusions, each cut by early halos or A veins containing Cu-Fe sulfides. The D veins contain pyrite ± chalcopyrite ± quartz and have sericite±chlorite selvages that form at 450°C to 300°C, the lower limit of muscovite stability. D veins are quartz-poor in both early halo and A vein environments, attesting to P-T ascent paths with retrograde silica solubility. These D veins contain chalcopyrite where they cut older early halo and A veins containing Cu-Fe sulfides, but are commonly Cu-poor lateral to the K-silicate zone. This relationship suggests that much chalcopyrite in D veins is remobilized from earlier ores rather than introduced by the causative magmatic-hydrothermal fluids.