Geology, mineralogy, ore fluid characteristics, and 40Ar/39Ar geochronology of the Kahang Cu-(Mo) porphyry deposit, Urumieh-Dokhtar Magmatic Arc, Central Iran

Abstract

The Kahang Cu-Mo porphyry deposit is located in the middle part of the Urumieh-Dokhtar Magmatic Arc, Central Iran, and is hosted by middle to late Miocene (~14 Ma) porphyritic quartz-diorite and granodiorite stocks intruded into the Eocene tuff, basaltic andesite, and andesitic flow sequences. The Miocene intrusions are calc-alkaline in composition and reflect arc-type to collisional environments (Nb/Y ~ 0.6) based on Nb-Y, Sr/Y-Y, and La/Yb-Yb discrimination diagrams.Copper mineralization occurs within the quartz diorite and granodiorite porphyries and volcanic rock sequences, which have been extensively altered to a central potassic zone (biotite-rich-K-metasomatism), a medial quartz-sericite-pyrite (phyllic) zone, an outer argillic alteration zone, and a peripheral propylitic alteration zone. Hypogene mineralization predominantly occurs within the central potassic zone within veins and breccias. The grade of the mineralization (avgs. 0.6% Cu with 120 million metric tons (Mt) ore reserve and decreases outward from the potassic zone, through a transitional potassic-phyllic zone, to the phyllic zone. Supergene mineralization (5 Mt at 0.3% Cu) that is partially developed in upper parts of the system forms <10 million metric tons (Mt) of the total resource of the Kahang deposit.Three stages of mineralization are recognized based on vein geometry, mineral assemblages, and ore textures. Early-stage mineralization is characterized by the occurrence of quartz-magnetite-chalcopyrite ± molybdenite-pyrite assemblages in veins, veinlets, and breccias with biotite-K-feldspar envelopes. Main-stage Cu- (Mo) mineralization is represented by quartz-chalcopyrite ± molybdenite ± pyrite-gold assemblages in disseminations within biotite-K-feldspar-sericite-altered rocks, and is responsible for the bulk of economic Cu-(Mo) mineralization. Late-stage mineralization, characterized by quartz-pyrite-sphalerite-galena assemblages in veinlets, voids, and fractures with kaolinite-illite envelopes occurs in upper parts of the deposit, and overprints previous alteration and mineralization.Four main types of fluid inclusion are distinguished in quartz from quartz-sulfide veins and veinlets: type I vapor-rich (V) two-phase inclusions with vapor/liquid ratios of 60 to 90; type II liquid-rich (L-V) inclusions with vapor/liquid ratios of < 50; type IIIa and IIIb (L-V-S) hypersaline inclusions with vapor/liquid ratios of 45 to 85 and solid phase of < 30; and type IV (L-V-S-H) multiphase inclusions. Ore fluid salinities during the main-stage mineralization were approximately 35 wt% NaCl equiv at temperatures between 220 and 450 °C and pressures between 500 and 1000 bar, corresponding to depths between 1 and 3 km.The calculated δ18Ofluid and δDfluid values for quartz-biotite veins associated with potassic alteration range between 9.7 and 3.1‰ and − 52 to − 49.8‰, respectively, consistent with precipitation from a high-temperature primary magmatic fluid, which gradually mixed with meteoric fluids in later stages. The δ34Ssulfide isotope compositions of stages II and III-chalcopyrite and pyrite range from 3.6 to 8.6‰ consistent with deposition from an oxidized, sulfate-dominant, high-temperature magmatic-hydrothermal fluid.The 40Ar/39Ar ages on hydrothermal biotite from porphyry stocks and potassic alteration zones indicate that the hydrothermal activity and subsequent porphyry Cu- (Mo) mineralization occurred between 14.73 ± 0.05 and 14.87 ± 0.06 Ma.