Hydrothermal garnet in porphyry copper related skarn deposits, Ali-Abad, Yazd Province, Iran

Abstract

The Ali-Abad porphyry copper deposit is located in the Central Iranian magmatic belt. As a result of subduction magmatism, Oligo-Miocene leucogranitoid rocks have intruded into conglomerates of lower Cretaceous age. Due to contact metamorphism the following mineral assemblage has occurred in the altered conglomerate: Garnet (andradite-grossular) + epidote+ quartz + calcite + pyrite Skarnification have only occurred in reactive carbonate clasts of the conglomerates. Chemical composition of garnets and classic discrimination scheme imply the porphyry copper related skarn-type mineralization. Garnets are characterized by abrupt chemical zoning area and thin bands. REE pattern of skarn garnet and host granite have similar trends, so it is concluded that garnets are hydrothermal in origin. The garnet mineralization episode occurred during skarnification, giving hydrothermal fluid temperatures of 422-472 ºC, likely this episode corresponds to the fluid boiling and high fO2 of garnet forming processes in the skarn system. Probably, the temporal decrease of hydrothermal fluid pressure led to rapid growths of andradite zones i.e., extensive hydrofracturing, porosity of conglomerate could be the dominant conditions which led to boiling of hydrothermal fluids and precipitation of andradite garnet.