Rb–Sr geochronology and geochemistry of the Xiaotongchang basalt-hosted copper deposit in the Jinping area, SW China

Abstract

The Xiaotongchang basalt-hosted copper deposit in Jinping, southern Yunnan Province, SW China, is located in the southern part of the Emeishan large igneous province. Here, we use Rb–Sr, Re–Os and S–Pb–C–O isotope data to constrain the age and genesis of the deposit. Petrological investigations reveal two main stages of mineralization. The early-stage is characterized by disseminated or massive sulfides in an assemblage of pyrite + chalcopyrite + quartz + albite + calcite. Rb-Sr isotopic dating of early-stage chalcopyrite yields an age of 256 ± 9.4 Ma (MSWD = 5.4), which is slightly younger than the eruption age of the Jinping basalts. The δ34SCDT values of the sulfides range from +5.8‰ to +7.7‰, suggesting that the metals of the early-stage copper mineralization originated from the Emeishan basalts and that the ore-forming fluids were contaminated by crustal rocks. These units may indicate that the early-stage orebodies represents syngenetic hydrothermal mineralization related to the eruption of Emeishan flood basalts. The late-stage mineralization is characterized by a pyrite + chalcopyrite + bornite + chalcocite + quartz + calcite assemblage that is developed in veins or breccia. The Re-Os isotopic dating of late-stage chalcopyrite samples reveals that they formed at 240–231 Ma. The Pb isotopic compositions ((206Pb/204Pb)t = 17.521–19.227, (207Pb/204Pb)t = 15.480–15.760, and (208Pb/204Pb)t = 37.778–38.731; t = 235 Ma) show that the late-stage copper orebodies have a heterogeneous source and originated mainly from the crust. The late-stage sulfides have higher δ34SCDT values (+7.9‰ to +10.7‰) than the early-stage sulfides. The narrow range of sulfur isotopic compositions of the sulfides associated with both stages and the Pb isotopic compositions of late-stage chalcopyrite indicate that some of the metals associated with late-stage mineralization originated from the Jinping basalts. The C–O isotopic compositions of calcite samples show that the magmatic hydrothermal fluids in the late stage flowed through and were contaminated by the Maokou limestone. Therefore, we suggest that the late-stage orebodies represent magmatic hydrothermal mineralization associated with the Late Triassic rhyolite porphyry and that the tectono-magmatic hydrothermal fluid remobilized copper from the basalts. The copper deposits formed at favourable interfaces within the basalt stratigraphy or in favorable structural locations.