The Panulcillo skarn copper deposit developed in an Early Cretaceous (Valanginian) volcano-sedimentary sequence intruded by a Cretaceous diorite. The exoskarn zone of the deposit is composed of garnet skarn comprising grandite garnet (Ad12.9-47.4) and phlogopite with lesser amounts of diopside (Di89.6-91.6), clintonite, pargasitic hornblende, actinolite, pumpellyite, chlorite, quartz and calcite, exhibiting characteristic Mg-Al-rich assemblages. The principal ores consist of chalcopyrite, monoclinic pyrrhotite, sphalerite, pyrite, and magnetite with small amounts of galena, and locally secondary 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 products. 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°C, below which monoclinic pyrrhotite is stable