Geology, fluid inclusion and isotope geochemistry of the Hongyuan reworked sediment-hosted Zn–Pb deposit: Metallogenic implications for Zn–Pb deposits in the Eastern Tianshan, NW China

Abstract

The Hongyuan Zn–Pb deposit (ca. 150km southeast of Hami city, Xinjiang) is located in the Central Tianshan Terrane. Discovered in 2009, the deposit is mainly hosted by the Mesoproterozoic Kawabulake Group siliceous slate and carbonaceous marble.Based on the mineral assemblages and their megascopic/microscopic textural relationships, the Hongyuan Zn–Pb mineralization/alteration can be divided into: Sedimentary period (Stage I: Sedimentary), skarn period (Stages II to VI: Early skarn, late skarn, early sulfide, late sulfide and carbonate) and supergene period (Stage VII: Supergene stage).Three types of fluid inclusions (FIs), i.e., aqueous (W-type), CH4-bearing aqueous inclusion (W1-type) and solid-bearing (S-type) inclusions, were identified in the Stages IV to VI quartz, calcite and sphalerite: Stage IV contains W1- and W-type FIs, whereas Stage V contains mainly W-type and rare S-type FIs. Only W-type FIs occur at Stage VI. The Stage IV W-type FIs homogenized at 139–341°C (mainly at 220–240°C), with eutectic temperature of −55 to −30°C and salinities of 2.7–14.4wt.% NaCl eqv. The Stage IV W1-type FIs homogenized at 177–402°C (mainly at 230–380°C), with eutectic temperature of −78 to −55°C and salinities of 3.6–19.6wt.% NaCl eqv. The Stage V W-type FIs homogenized at 115–273°C (mainly at 190–220°C), with eutectic temperature of −61 to −42°C and salinities of 3.6–14.3wt.% NaCl eqv. The Stage VI W-type FIs homogenized at 90–206°C (mainly at 100–160°C), with eutectic temperature of −65 to −33°C and salinities of 3.9–13.1wt.% NaCl eqv. The ore-forming fluids consisted of an initial H2O–NaCl–CaCl2 fluid system, may have evolved from high-medium temperature and medium salinity to medium-low temperature and low salinity.The δ18OH2O and δDH2O of the Stage III fluids are 10.2‰ and −73.8‰, respectively. At Stage IV, the δ18OH2O of quartz ranges from −2.1‰ to 2.7‰; while the δ13CPDB and δ18OSMOW of calcite range from −1.6‰ to −0.2‰ and 3.5‰ to 8.0‰, respectively. At Stage V, the δ18OH2O and δDH2O of quartz range from −3.7‰ to 3.1‰ and −107‰ to −63‰, respectively, while the δ13CPDB and δ18OSMOW of calcite range from 0.1‰ to 0.6‰ and 11.8‰ to 13.5‰, respectively. At Stage VI, the δ18OH2O and δDH2O of quartz range from −8.1‰ to −7.8‰ and −111‰ to −96‰, respectively, while calcite has δ13CPDB and δ18OSMOW of −3.6‰ and 4.1‰, respectively. The H–O–C isotopic compositions at Hongyuan suggest that the ore-forming fluid sources may have changed from magmatic fluids to meteoric water. The sphalerite and pyrite δ34S ranges 16.05–16.80‰, indicating that the sulfur may have been mainly derived from the reduction of Precambrian sulfates. Meanwhile, the Hongyuan sphalerite and galena have Pb isotopic affinities of the ore-hosting Kawabulake Group, suggesting that the lead was mostly derived from the sedimentary host rocks and influenced by magmatic-hydrothermal fluids.We propose that the Hongyuan Zn–Pb mineralization may have initially formed by Mesoproterozoic sedimentary exhalative processes, and was strongly reworked by Late Carboniferous contact metasomatism, whose intensive skarn alteration may have transformed and upgraded the ores. This superimposing mineralization model may also be applicable to other newly-discovered Zn–Pb deposits in the Eastern Tianshan.