Mineralogy of the Panulcillo skarn copper deposit, Coquimbo Region, Chile

The Panulcillo skarn copper deposit developed in an Early Cretaceous (Valan ginian) volcano-sedimentary sequence intruded by a Cretaceous diorite. The exoskarn zone of the deposit is composed of garnet skarn comprising grandit e garnet (Ad(12.9-47.4)) and phlogopite with lesser amounts of diopside (Di (89.6-91.6)), clintonite, pargasitic hornblende, actinolite, pumpellyite, c hlorite, quartz and calcite, exhibiting characteristic Mg-Al-rich assemblag es. The principal ores consist of chalcopyrite, monoclinic pyrrhotite, spha lerite, pyrite, and magnetite with small amounts of galena, and locally sec ondary marcasite, hematite and covellite, generally filling interstices of the garnet skarn. Chalcopyrite shows characteristic polysynthetic twinning, normally including fine star-like crystals of sphalerite as exsolution pro ducts. Sphalerite contains 14.4 - 22.5 mol% FeS and 1.6 - 9.2 mol% MnS, and is associated intimately with chalcopyrite. it is highly likely that such Fe-rich sphalerite has coexisted primarily with hexagonal pyrrhotite under low-sulfidation conditions at higher temperatures than 275 - 254 degreesC, below which monoclinic pyrrhotite is stable.