El yacimiento tipo skarn de cobre de Panulcillo se desarrollo en una secuencia volcano-sedimentaria de edad cretacica inferior (Valanginiana), la que es intruida por un cuerpo cretacico de composicion dioritica. La zona del exoskarn caracteristicamente presenta una asociacion mineralogica rica en Mg y Al, y corresponde a un skarn de granate compuesto por granate tipo ugrandita (Ad12.9-47.4) y flogopita con candidades menores de diopsido (Di89.6-91.6), clintonita, hornblenda pargasitica, actinolita, pumpellita, clorita, cuarzo y calcita. Los minerales metalicos consisten mayoritariamente en calcopirita, pirrotina monoclinica, esfalerita, pirita, y magnetita con menos galena, localmente marcasita, hematita y covelina secundarias, generalmente rellenando los intersticios del skarn de granate. La calcopirita exhibe maclas polisinteticas caracteristicas, y normalmente contiene pequenos cristales de esfalerita en forma de estrellas como productos de exsolucion. La esfalerita contiene 14,4-22,5 % molar de FeS, 1,6-9,2 % molar de MnS, y esta estrechamente asociado a calcopirita. Es muy probable que este tipo de esfalerita rica en Fe originalmente haya coexistido con la pirrotina hexagonal en condiciones de baja sulfidacion y temperaturas superiores a 275-254°C, dado que a temperaturas inferiores, la fase estable corresponde a pirrotina monoclinica.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.