Fluid inclusion, H–O isotope and Pb–Pb age constraints on the genesis of the Yongping copper deposit, South China

Abstract

The Yongping copper deposit is located in the Gandongbei metallogenic belt, South China. Stratiform-like copper ore bodies are hosted in limestones of the Upper Carboniferous Outangdi Formation. These ore bodies were previously believed to be products of the Carboniferous sedimentary exhalative (Sedex) mineralization, but they also appear to be related to the Late Mesozoic intermediate-acidic stocks at Yongping. The ore minerals are pyrite, chalcopyrite, with minor magnetite, sphalerite and galena. The gangue minerals are quartz, sericite, garnet, diopside, scapolite, wollastonite, tremolite, epidote, calcite and chlorite. Four stages of alteration and mineralization can be observed. Garnet, diopside and scapolite were formed in the earliest prograde skarn stage. The pre-ore barren quartz–pyrite veins that accompany tremolite, epidote, magnetite and chlorite, were subsequently formed due to the destruction of the earlier skarn. The stratiform-like copper ore bodies, with minor quartz–polymetallic veins, were formed in the main mineralization stage. The post-ore stage is characterized by late quartz–calcite±pyrite veins. Fluid inclusions in quartz from the pre-ore barren quartz–pyrite veins, stratiform-like copper ore bodies, and post-ore quartz–calcite±pyrite veins were studied. Fluid inclusion petrography shows that two-phase liquid-rich (type I), two-phase vapor-rich (type II), and halite-bearing (type III) fluid inclusions are present in these three generations of hydrothermal quartz. Type II and type III inclusions occur only in quartz from the stratiform-like copper ore bodies, whereas type I inclusions occur in all stages of hydrothermal quartz. Type I inclusions in pre-ore quartz–pyrite veins have homogenization temperatures of 300–398°C and salinities of 3.6–8.1wt.% NaCl equivalent. Type II and coexisting type III inclusions in the stratiform-like copper ore bodies share similar homogenization temperatures of 233–324°C and two contrasting salinity groupings of 0.4–3.2wt.% and 28.2–39.0wt.% NaCl equivalent, respectively. The coexistence of type II and type III inclusions and their similar homogenization temperatures but contrasting salinities suggest that fluid boiling processes occurred. Type I inclusions in the post-ore quartz–calcite veins have homogenization temperatures of 122–250°C and salinities of 0.2–6.8wt.% NaCl equivalent. The hydrogen and oxygen isotopic compositions of hydrothermal quartz indicate that the mineralizing fluids gradually evolved from a magmatic to meteoric source. The Pb–Pb dating of pyrite from stratiform-like copper ore bodies suggests that mineralization took place in the Jurassic. The skarn alteration, inferred boiling fluid inclusion assemblages, stable isotopic features of magmatic fluids, and Jurassic ages imply that the Yongping copper deposit is a Jurassic skarn copper deposit rather than a Carboniferous Sedex deposit.