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 d eposit in west Yunnan province, China. There is an excellent positive correlation between He-3/Ar-36 and Ar-40/Ar-36, and between Ar-40/He- 4 and He-3/He-4 in the inclusion-trapped fluids. This indicates that t he ore-forming fluid can be considered as a mixture of two endmember c omponents, a modified mantle fluid and crustal fluid. From the noble g as results and combined noble gas acid stable isotope behavior, the au thors conclude that the former is a high-temperature magmatic fluid di fferentiated from Machangqing alkali-porphyry and enriched in volatile s such as sulfur and carbon. The crustal fluid is likely a low-tempera ture meteoric fluid enriched in crustal radiogenic helium but with atm ospheric argon isotopic composition. The sulfur and carbon concentrati ons in the low temperature fluid are probably too low to produce a dis tinct C or S isotope signature: the mixing between high and low temper ature fluids can only be traced using noble gases. In addition, the po sitive relationship between He-3/He-4 and delta(34)S reflects the evol utionary physicochemical conditions of the mixed fluid. (C) 1998 Elsev ier Science B.V. All rights reserved.