Abstract

Previous studies have suggested that pyrite may armor potassium-bearing mineral inclusions from alteration-induced argon loss, thus providing improved 40Ar/39Ar age results. To test this hypothesis, matrix muscovite and pyrite crystals hosting muscovite inclusions were selected from two variably overprinted Yilgarn gold deposits (Western Australia) and analyzed by combined single crystal 40Ar/39Ar laser probe and “in vacuo” crushing methods. Pyrite grains from the ca. 2.60 Ga Mount Charlotte gold deposit exhibit evidence of minor argon loss, but give concordant high-temperature ages averaging 2594 ± 8 Ma (2σ), indistinguishable from previous age estimates. Matrix mica from the ca. 2.63 Ga Kanowna Belle gold deposit yielded a discordant age spectrum with all ages younger than 2.5 Ga, indicating substantial 40Ar* loss related to Middle Proterozoic thermal overprinting. Crushing and low-temperature heating experiments on pyrite crystals from Kanowna Belle produced anomalously old apparent ages (up to 7.0 Ga); however, in contrast to the matrix mica, total-gas pyrite ages generally approach or overlap the inferred time of gold mineralization. This behavior is attributed to diffusion of 40Ar* to internal muscovite-pyrite grain boundaries in response to the Middle Proterozoic thermal event, with limited external loss of argon. It is concluded that pyrite acts as a partially closed system for argon, but that the 40Ar/39Ar pyrite dating method has the potential to see through later thermal and/or alteration events and resolve controversial aspects of ore deposit geochronology.

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