Bulletin of the Geological Society of Malaysia, Volume 9, Nov. 1977, pp. 31 – 50
U.S. Geological Survey, Menlo Park, California 94025, USA
Abstract: Although skarn composes a sizeable proportion of the altered and metalized parts of many epizonal systems, its implications for our understanding of porphyry copper genesis have been neglected. Skarns associated with porphyries apparently can form prior to, during, and after emplacement of related porphyritic granitic bodies. In the porphyry systems, alteration is extremely widespread, and fracture-controlled chemical changes may extend far beyond the skarn. Crystallization of garnet and pyroxene from non-boiling but saline fluids was followed in some skarns by deposition of subeconomic concentrations of copper and iron sulfides. Ore formed in these anhydrous skarns, however, only after repeated passage of Cu-bearing fluids, causing the subsequent, paragenetically complex crystallization of quartz and K-feldspar, amphibole, amphibole±epidote, biotite±amphibole, carbonate± chlorite-white micas and sulfides. In one deposit the fluids became progressively saline as the skarns evolved and boiled during the late stages. In many districts, the hydrous mineral assemblages parageneticalIy overlap the final emplacement of the nearby granitic bodies as the fluid flow was periodically disrupted while the rocks opened and closed to the surface. In contrast to the epizonal porphyry coppers, deep-seated skarns, typified by the Pine Creek tungsten deposit in east-central California, show a horizontal extent of skarn limited closely to rock near the granite-carbonate contact. Metasomatic effects beyond the skarn are absent, and hypogene ore formed penecontemporaneously with the skarm minerals, in contrast to the porphyry coppers.