Hydrothermal iron oxide-Cu-Au (IOCG) mineralization at the Jalal-Abad deposit, northwestern Zarand, Iran

Abstract

The Jalal-Abad iron oxide ± Cu ± Au ± Bi ± Co deposit is hosted in Early Cambrian volcanosedimentary units (CVSU) of the Kashmar–Kerman zone, Central Iran. Magnetite is the main ore mineral at depth and is associated with Na-Ca rich alteration, dominated by actinolite. Magnetite compositions (EPMA) are very low in TiO2 (0.03–0.07 wt%), V2O3 (0.02–0.06 wt%) and CoO (0.01> to 0.42 wt%). At intermediate to shallow levels potassic alteration is associated with chalcopyrite, pyrite, arsenopyrite, cobaltite, bismuthinite and gold. The mineralization occurs in massive, breccia matrix, open space filling, disseminated and vein-like styles. Gold occurs as small inclusions (<50 µm) within chalcopyrite and pyrite. The δ34S values of pyrite at Jalal-Abad vary from 4.7 to 22.8% with an average of 12.7% (CDT) indicating more than one source of S, including evaporites, and minor mixing with magmatic sulfur. The δ18O values of magnetite are 1.0–6.4%, with an average of 3.4% consistent with precipitation from magmatic-hydrothermal fluids. The δ13C values of dolomites are between −5 and −11% (PDB), while the δ18O values varies from 13.6 to 14.3%, indicating hydrothermal dolomitization. Multiphase fluid inclusions in quartz from the iron ore stage (L + V + S) are abundant with homogenization temperatures of 220–480 °C. Salinities vary between 32 and 53 wt% NaCl equivalent. Laser-ablation inductively-coupled-plasma mass spectrometry (LA-ICP-MS) analyses performed on individual fluid inclusions hosted by quartz indicate significant concentrations of Fe and Cu, up to 16,076 and 3204 ppm with an average of 6914 and 792 ppm respectively. Ca/K values for fluid inclusions are 0.53–8.68 with an average of 1.75 indicating mixing of magmatic and non-magmatic fluids. Our proposed model for the Jalal-Abad deposit is that during the Early Cambrian gabbro-diorite magmas intruded the sedimentary sequence and provided magmatic constituents and a heat source for hydrothermal processes. Stable isotope, microthermometry and chemistry of fluid inclusions suggest that both magmatic and basinal fluids play a role in the mineralization.