Abstract
Analyses of He and Ar in ancient mineralising fluids, using fluid inclusions in pyrite as 'bottles' for the palaeofluids, can be used to trace atmospheric (meteoric), crustal and mantle contributions to the fluid and heat budget of extinct hydrothermal systems. Crushing pyrite grains in vacuo is an effective method of releasing the trapped fluids, which can be analysed using conventional noble gas techniques. We present analyses of noble gases from three gold deposits from the same deep-seated fault in the Ailaoshan Gold Province, China. The results demonstrate the palaeofluids were a mixture between a mantle-derived, magmatic fluid and two different groundwaters. Stable isotope compositions are consistent with a predominantly magmatic origin for the fluids and do not identify any meteoric component: it is the specific isotopic and abundance characteristics of He and Ar that allow unequivocal identification of both a mantle-derived component and a meteoric component to these fluids. It can also be shown that direct injection of high enthalpy, magmatic fluids occurred at one of the deposits, whereas heat was advected into the hydrothermal cell via low enthalpy fluids at the other two deposits.
Published Version
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