Hydrothermal evolution and ore genesis of the Beiya giant Au polymetallic deposit, western Yunnan, China: Evidence from fluid inclusions and H–O–S–Pb isotopes

Abstract

The Beiya porphyry–skarn gold-polymetallic deposit is one of the largest gold deposits in China, and also contains significant amounts of silver and base metals. The estimated reserve at the end of 2015 was 130million metric tonnes (Mt) of ores, grading 2.42g/t Au, 0.48wt.% Cu, 25.5wt.% Fe, 38.85g/t Ag, 1.24wt.% Pb, and 0.53wt.% Zn. The deposit is located in the Jinshajiang–Ailaoshan porphyry–skarn Cu–Mo–Au metallogenic belt in western Yunnan province, southwest China. Skarn alteration and mineralization are spatially and temporally associated with Eocene monzogranite porphyries, which were emplaced into Triassic carbonates. Field evidence and petrographic observations indicate there are four stages of hydrothermal activity, i.e. prograde (Stage I), iron oxide (Stage II), sulfide (Stage III) and carbonate stage (Stage IV). Stage I is characterized by the formation of garnet and pyroxene by high-temperature (398–560°C) hypersaline (48.81–67.24wt.% NaCl equiv.) hydrothermal fluids with H-O isotopic compositions that are similar to typical magmatic fluids. These fluids were most likely generated by the separation of brine from a silicate melt. In Stage II garnet and pyroxene was replaced by epidote, amphibole, chlorite, quartz and magnetite. Associated hydrothermal fluids are preserved in coexisting L-type and V-type fluid inclusions and are characterized by anomalously low δD values (approximately −80‰), and lower δ18O than Stage I fluids. The decrease in ore fluid δ18OH2O values with time coincided with marked decreases in fluid salinity and temperature. Based on the fluid inclusion and stable isotopic data, we suggest that the ore fluid evolved by boiling of the magmatic brine. Stage III is characterized by the development of pyrite, chalcopyrite, galena, and sphalerite. Fluids inclusions associated with Stage III display and continuous range of salinities, with the majority being low-salinity inclusions (1.28–3.54wt.% NaCl equiv.). These fluids yield lower δ18OH2O values (−0.85 to 5.52‰) and moderate δD values (−78.6 to −88.6‰). Altogether, these data indicate that Stage III fluids originated from the mixing of residual Stage II fluids with meteoric water. The sulfur isotope (δ34S) values of Stage III sulfides cover a narrow range of −4.2 to 1.2‰ (mean=−1.1‰), and boiling also occurred at low temperatures (165–347°C) during Stage III. In Stage IV, only L-type fluid inclusions are present, which yielded the lowest homogenization temperatures (157–272°C) and the lowest salinities (1.06–12.20wt.% NaCl equiv.). The H–O isotope data indicate that the ore-forming fluids were dominated by magmatic water in Stage I and Stage II, and that the magmatic water gradually mixed with circulating meteoric water during Stage III and Stage IV. This conclusion is further supported by the isotopic composition of lead of the ores, which is similar to that of the monzogranite porphyries, but differs significantly from Permian basalts and Triassic wall rocks.