Some secondary ore formation features of the Sar-Cheshmeh porphyry copper-molybdenum deposit, Kerman, Iran

Abstract

New secondary mineral features and the control of secondary molybdenum enrichment have now been recognized at the Sar-Cheshmeh copper deposit. In part of the deposit, copper is locally concentrated within a subvolcanic unit — the so-called Late Fine Porphyry in amounts that seem too great to have come from the primary phase of the same unit. Clay minerals are believed to have influenced the deposition of copper by through-flowing oxidized zone solutions within this otherwise subeconomic unit. These solutions have caused argillic alteration and copper concentration within the plagioclase phenocrysts. Fractures are filled with chrysocolla, but little chrysocolla was observed within the altered plagioclase phenocrysts. The scattered layers of sedimentary “unroofing breccia” — horizontal layers of Quaternary age accumulated within the depressions at the top of the deposit — contain minor amounts of native copper. The breccia consists of angular to subangular clasts of local derivation cemented by a matrix of hematite, limonite, geothite, and small rock fragments. Native copper, the only copper mineral associated with the breccia, mainly adhers to the cavity walls in the cementing materials. The amount of molybdenum within the leached capping of the Sar-Cheshmeh deposit is generally controlled by the amount of pyrite. Abundant pyrite has caused the retention of molybdenum within the leached capping of the pyritic halo as a result of the more acidic solution during weathering, in contrast to the leached zone of the pyrite-poor stock, where molybdenum is partially mobile.