The Middle Permian Hongshanliang Manto-type copper deposit in the East Tianshan: Constraints from geology, geochronology, fluid inclusions and H–O–S isotopes

Abstract

The Hongshanliang copper deposit, hosted in the tuff of the Lower Carboniferous Yamansu Formation, is a typical copper deposit in the Aqishan–Yamansu belt, East Tianshan. Based on crosscutting relationships of veins, textural relationships and mineral assemblages, five alteration/mineralization stages at Hongshanliang have been established: chlorite–sulfide stage (Stage I), quartz–pyrite stage (Stage II), quartz–polymetallic sulfide stage (Stage III), late veins (Stage IV) and supergene process (Stage V).Stage I has mineral assemblage of chlorite–pyrite ± chalcopyrite ± quartz ± sericite, and is commonly characterized by euhedral pyrite with silicates (e.g., chlorite, quartz and sericite) and/or chalcopyrite as its pressure shadow, indicating an early mineralization event with apparent deformation. Stage II is characterized by quartz–pyrite ± chalcopyrite or locally quartz ± gypsum ± anhydrite ± pyrite veins cutting Stage I chlorite–pyrite. The main mineralization stage (Stage III) at Hongshanliang has typical mineral assemblages of quartz–chalcopyrite ± chlorite ± calcite ± sericite, quartz–chalcopyrite–sphalerite–galena ± pyrite and quartz–sphalerite–galena ± chlorite, with veins, veinlets, disseminations and local massive ore types, similar to Manto-type copper deposits regarding mineralization and ore structures. Detailed fluid inclusion study shows temperature of fluids decreased from Stage II (307–484 °C) through Stage III (peak at 160–180 °C, consistent with quartz–chlorite oxygen isotope geothermometer of 190 °C) to Stage IV (129–169 °C), with corresponding salinities of 2.7–26.2 wt% NaCl equiv. (peak at 8 wt%), 2.1–12.3 wt% NaCl equiv. (peak at 8 wt%) and 0.5–7.7 wt% NaCl equiv. (peaks at 4–6 wt%), respectively, indicating an evolved hydrothermal ore-forming system. Such fluid evolution can also be supported by H–O isotopes during water–rock reaction from Stage II (δ18Ofluid = 6.3–8.3‰ and δDfluid = −77‰ to −64‰) through Stage III (δ18Ofluid = − 1.7‰ to 7.0‰ and δDfluid = − 82‰ to − 58‰) to Stage IV (δ18Ofluid = − 2.5‰ to − 2.1‰ and δDfluid = − 77‰ to − 73‰) with a magmatic–hydrothermal origin. Furthermore, sulfur isotopes suggest that fluids of the two mineralization stages (Stage I and III) are predominantly magmatic–hydrothermal, with other influx contribution into the ore-forming system, i.e., minor Early Carboniferous seawater in Stage I (δ34Sfluid varying from − 3.5‰ to 6.1‰, with peak at 0‰) and minor organic materials in Stage III (δ34Sfluid varying from − 6.5‰ to 4.1‰, with peak at 0–1‰, and detection of CH4 and C2H6 from fluid inclusions).In combination of alteration, paragenesis, nature and source of ore-forming fluids and comparison with other typical deposits, we proposed that the Hongshanliang copper deposit underwent two mineralization events with the main mineralization similar to Manto-type copper deposits. 40Ar/39Ar dating of sericite from a massive chalcopyrite ore indicates the Hongshanliang main mineralization formed at 269.0 ± 0.4 Ma, generally coeval with regional K-feldspar granite emplacement (ca. 272 Ma). Integrating the regional tectonic setting, magmatism and metallogenesis of the Aqishan–Yamansu belt and the East Tianshan, we suggest that the Aqishan–Yamansu belt has potential of mineral prospecting for the Middle Permian Manto-type copper deposits.