Mantle, crustal and atmospheric noble gases in ailaoshan gold deposits, Yunnan Province, China

Abstract

Helium and argon isotopic and elemental compositions of fluids released by crushing pyrite grains are reported from the Ailaoshan Gold Province, China. The province is a series of gold deposits hosted on a 200-km segment of a major normal fault formed during Eocene extension. The deposits have been well characterised using conventional geochemical and microthermometric techniques, the results of which are consistent with a predominantly high temperature “magmatic” fluid present in the ore minerals. In contrast, analyses of noble gases from three gold deposits demonstrate the palaeofluids were a mixture between a mantle-derived, magmatic fluid and two different low temperature fluids similar to modern groundwaters. The specific isotopic and abundance characteristics of helium and argon allow unequivocal identification of both a mantle-derived component and a surface-derived component to these fluids. The magmatic fluid dominates deposits from the northern end of the fault. The second groundwater is present only in deposits in the south of the province. By assuming fluid inclusion homogenisation temperatures in coexisting quartz are representative of fluid temperature, 3He/heat ratios can be estimated. The 3He/heat ratios vary systematically along the fault, from ≈ 30 × 10 −12 cm 3 STP J −1 in the northern deposits to <0.6 × 10 −12 cm 3 STP J −1 in the south, reflecting a major change in heat and fluid flow. Noble gas and heat compositions are consistent with direct injection of magmatic volatiles into hydrothermal fluids at the northern end of the fault. However, the flow regime clearly changed along the fault. There appears to be little or no direct input of magmatic volatiles in the south. It is likely that heat was introduced into the hydrothermal system by diffusion into low enthalpy, high He-concentration groundwater.