Helium and Argon isotope systematics in fluid inclusions of Machangqing copper deposit in west Yunnan province, China

This paper reports the helium and argon isotope compositions of fluid inclusions hosted in pyrites from Machangqing alkali–porphyry copper deposit in west Yunnan province, China. There is an excellent positive correlation between 3He/36Ar and 40Ar/36Ar, and between 40Ar*/4He and 3He/4He in the inclusion-trapped fluids. This indicates that the ore-forming fluid can be considered as a mixture of two endmember components, a modified mantle fluid and crustal fluid. From the noble gas results and combined noble gas and stable isotope behavior, the authors conclude that the former is a high-temperature magmatic fluid differentiated from Machangqing alkali–porphyry and enriched in volatiles such as sulfur and carbon. The crustal fluid is likely a low-temperature meteoric fluid enriched in crustal radiogenic helium but with atmospheric argon isotopic composition. The sulfur and carbon concentrations in the low temperature fluid are probably too low to produce a distinct C or S isotope signature: the mixing between high and low temperature fluids can only be traced using noble gases. In addition, the positive relationship between 3He/4He and δ34S reflects the evolutionary physicochemical conditions of the mixed fluid.