Origin of the ore-forming fluids and metals of the Hetai goldfield in Guangdong Province of South China: Constraints from C-H-O-S-Pb-He-Ar isotopes

Abstract

The Hetai goldfield, located in the southern segment of the Qinzhou Bay-Hangzhou Bay Juncture Orogenic Belt (QHJB), is one of the largest gold mining districts in South China. Although the goldfield shares some similarities in ore geology and geochemistry to orogenic gold deposits, there has been controversy on the origin of the ore-forming fluids and metals. An integrated analysis on C-H-O-S-Pb-He-Ar isotopes has been carried out to trace the origins of the ore fluids and metals, thereby to evaluate the genesis of the Hetai goldfield. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions in sulfides from the Hetai ores exhibit a range of 0.27–0.62Ra and 932.4–6324.2, with the 4He and 40Ar concentrations varying from 1.99 to 96.80 (10−7cm3 STP/g) and 0.51–7.75 (10−7cm3 STP/g), respectively. This suggests a dominant crust-derived fluid mixed with significant mantle-derived fluids for the Hetai gold mineralization. The calculated δ18OH2O-VSMOW and measured δDH2O-VSMOW values of fluid inclusions in gold-bearing quartz range from 0.5 to 10.4‰ and from −83.1 to −52.4‰, respectively. The δ13CPDB and δ18OPDB of calcite range from −3.53 to −1.91‰ and −21.21 to −19.67‰, respectively, suggesting a deep-seated source of the C-bearing portion of the fluid. The δ34SCDT values of sulfides intergrown with native gold range from −4.0 to −1.3‰, with an average of −2.8‰, indicating that the majority of the sulfur came from granitic magma by partial melting of the host rocks, but likely with minor input from the mantle. The Pb isotopic components of sulfides from the Hetai ores have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.044–19.404, 15.568–15.793, and 38.426–39.597, respectively, indicating a main upper continental crust source of ore metals, but probably with minor contribution from the mantle. Linked to the ore geology and regional tectonomagmatic events, the integration of the isotopic compositions obtained in this study with previously published fluid inclusion and geochronologic data suggests that the ore fluids and metals may have undergone two stages of evolution, i.e. from the Indosinian metamorphic water-dominated fluids mixed with minor magmatic waters, to the Yanshanian magmatic water-dominated fluids mixed with mantle-derived fluids and meteoric waters.