He-Ar Isotopes and Trace Gas Compositions of Fluid Inclusions in Massive Sulphides from the Yushui Copper-Polymetallic Deposit, South China: Metallogenic Implications

The Yushui ore deposit, located in the middle section of the Yong'an-Meixian Hercynian depression, is a medium-sized Cu-polymetallic massive sulphide deposit in Eastern Guangdong Province, South China. This deposit is characterized by unusually high copper grade (up to 50-60 wt. % Cu). Other metallic elements, such as lead, zinc and silver, are also economically important in the Yushui ore bodies. The aim of this study was to apply N-2-Ar-He systematics, together with organic gases (light-hydrocarbon tracers), to constrain the origin and evolution of ore-forming fluids. The helium-argon isotopes and trace gas compositions of fluid inclusions trapped within metal sulphide minerals were measured for a number of bonanza ores from the Yushui deposit. The noble gas concentrations in the studied samples vary over one to two orders of magnitude (He-4: 2.27-160.00 x 10(-5) cm(3) STP g(-1); He-3: 0.53-34.88 x 10(-12) cm(3) STP g(-1); Ar-40: 6.28-37.82 x 10(-7) cm(3) STP g(-1); Ar-36: 1.25-10.40 x 10(-9) cm(3) STP g(-1)). Our data show a narrow range of He-3/He-4 ratios from 0.006 to 0.056 R-a (0.026 R-a on average, n = 8), which are considerably lower than the modern atmospheric end-member value; whereas the Ar-40/Ar-36 ratios (ranging from 333.76 to 501.68, with an average of 397.53) are significantly greater than that of air-saturated water. Most of the bornite samples have somewhat higher He-3/He-4 ratios of trapped fluids when compared to chalcopyrite. Overall, these He-Ar results are well within the range of crustal reservoir, thus implying a predominantly crustal source (originated from Caledonian basement) for ore-forming solutions, with little contribution from mantle-derived fluids. Analysis of the N-2-Ar-He composition in Cu-rich sulphides indicates that the Yushui ore-forming fluids were probably derived from formation water (or basinal hot brines). Moreover, organic gas species identified in sulphide-hosted fluid inclusions are mainly composed of C-1-C-4 alkanes, while the concentrations of unsaturated olefins and aromatic hydrocarbons are very low. In particular, most chalcopyrite samples with relatively low (HeHe)-He-3-He-/4 ratios (0.006-0.016 R-a) and Ar-40*/He-4 values (0.0002-0.0012) are generally characterized by very high CO2/CH4 ratios (60-102). All these suggest that main-stage Cu-Ag metallogenic processes might have not been affected by high-temperature magmatic activities or superimposed by strong metamorphic overprinting, although some chalcopyrite-rich ores appear to be influenced by later stage hydrothermal processes. In summary, neither magmatic input nor convecting seawater has played an important role in the formation of Yushui copper-polymetallic deposit. The massive sulphide ore bodies were products of water-rock interaction between metal-bearing basinal brines and the host sedimentary strata.