Re–Os dating of sulphides from the Yushui Cu-polymetallic deposit in eastern Guangdong Province, South China

• Yushui is one of the richest copper deposits in China, and is characterised by high-grade copper in a small area.
• Re–Os isotope dating chalcopyrite and bornite ores for the Yushui deposit, yielded a Hercynian forming age of Yushui (308 ± 15 Ma), which is close to the age of the host strata.
• Quite high γOs value (+ 1349) suggest that the Yushui may be a sedimentary exhalative massive sulphide deposit.

The Yushui Cu-polymetallic deposit, which is associated with Ag, Pb, and Zn, is located in the middle part of the Yongan–Meixian Late Paleozoic Hercynian depression. It was discovered in eastern Guangdong Province in the late 1980s and is one of the richest copper deposits in China with high-grade copper averaging 3.25% and locally reaching 50–60%. The main ore body is located along the unconformity between the Upper Carboniferous Hutian Group limestone and the Lower Carboniferous Zhongxin Formation quartz sandstone with a bedded and lenticular morphology. The ores exhibit massive textures dominated by chalcopyrite, bornite, chalcocite, pyrite, sphalerite, galena, and a trace amount of argentite. Although researchers began studying the Yushui deposit in the early 1990s, the ore genesis remains controversial because of the lack of precise mineralisation age constraints. In this study, direct Re–Os dating of Cu sulphides aided in facilitating a better understanding of the timing of formation of the Yushui deposit. This study is the first attempt to use the Re–Os isotopic system for directly dating chalcopyrite and bornite ores for the Yushui deposit. The contents of Re, common Os, 187Re and 187Os in nine sulphides are 1.68–219.35 ppb, 0.003–0.427 ppb, 1.05–137.31 ppb, and 0.045–0.734 ppb, respectively. The isotope data yielded an isochron age of 308 ± 15 Ma (mean square weighted deviates = 2.4) using the 87Re/188Os–187Os/188Os plot, which is interpreted to represent the age of formation for these sulphides, suggesting that the mineralisation age of the Yushui deposit is close to the age of the host rocks. The 187Os/188Os initial value obtained from the Re–Os isochron is 1.81 ± 0.34, which corresponds to the γOs value of + 1349. This value indicates that the ore-forming materials were derived from the crust without mixing with materials from the mantle, and that the Yushui massive sulphide deposit may be of sedimentary exhalative origin.