Chlorite chemistry, H-O-S-Pb isotopes and fluid characteristics of the Yuhai Cu-Mo Deposit in Eastern Tianshan: Implications for porphyry copper mineralization and exploration

Abstract

The Yuhai porphyry Cu-Mo deposit, located in the Eastern Tianshan metallogenic belt, northwest China, is a new discovery in the eastern section of Dananhu-Tousuquan arc, hosted mainly by quartz diorite and granodiorite intrusions. Based on alteration and mineralization assemblages as well as crosscutting relationships, three stages of hydrothermal activities have been identified: 1) early mineralization stage characterized by potassic alteration and minor Cu-Mo mineralization; 2) main mineralization stage with phyllic and propylitic alterations and extensive Cu-Mo precipitation; and 3) late mineralization stage forming barren quartz and minor calcite, chlorite, and epidote minerals. Sulfur isotope compositions of the Yuhai chalcopyrite, pyrite, bornite and molybdenite record a narrow range of −3.4 to +0.4 ‰, indicating a deep source of magmatic sulfur. Lead isotope data of sulfides from our previous study display consistent 208Pb/204Pb and 207Pb/204Pb ratios, but distinguishable 206Pb/204Pb ratios from the Yuhai intrusions analysed in this work. Combined sulfur and lead isotopic signatures, we suggest that the ore-forming components (metals and sulfur) were derived from a mantle-derived magmatic reservoir with some crustal components. The δ18O values of quartz, chlorite, calcite, and sericite samples separated from different mineralization stages range from 1.5 to 8.2 ‰ (n = 11), with calculated δ18OH2O values of −2.2 to 4.5 ‰, and the δD values of fluids range from −92.7 to −68.7 ‰ (n = 4), indicating the ore-forming fluids are composed of magmatic fluid and meteoric water. Mineral chemistry of chlorite, fluid inclusions microthermometry, and petrographic evidences reveal that the ore-forming fluids evolved from a high fO2 to relative weak oxidized condition with decreasing temperature, and copper was largely precipitated at temperatures of <320 °C in a weakly oxidized system during main stage. All these observations demonstrate that Yuhai was formed by hydrothermal fluids related to the diorite magma, and that the fO2 and temperature decreasing plays a critical role for abundant depositions of Cu and Mo. The Paleozoic diorite complex formed in a subduction setting is suggested as a significant exploration target for porphyry-style Cu mineralization in the Eastern Tianshan.