40Ar/39Ar dating of pyrite from gold deposits in low grade terranes

Abstract

40Ar/39Ar dating of fine-grained white micas from gold deposits, formed under low-grade metamorphic conditions, is often complicated by multiple mica populations, recoil loss/redistribution of 39ArK and loss of 40Ar. Recent studies have suggested that pyrite armours enclosed mica inclusions against argon loss and, therefore, may provide improved age information in low grade terranes. This premise is tested by applying the 40Ar/39Ar pyrite dating method to three gold deposits in Victoria, namely Stawell, Fosterville and Walhalla. These analyses indicate that, although pyrite behaves as a partially closed system, the method is able to ?see through? younger hydrothermal overprints. At the same time, 40Ar/39Ar analyses of pyrite from the Fosterville gold mine failed to constrain the timing of gold deposition due to the encapsulation of detrital mica grains by the pyrite. These results emphasize the importance of petrological control on sample selection and combining 40Ar/39Ar analyses of pyrite and matrix mica.