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[BPT05-P16] Pb isotope ratios of the Akeshi Au deposit, Kagoshima, Japan: Implication for gold mineralization
Keywords:Akeshi gold deposit, slab-fluid, Pb isotopic ratio, ore-forming fluid, hydrothermal deposits
In the present study, to detect direct information of source of metals contributing to the formation of epithermal gold deposits, we study Pb isotopic compositions of sulfide ores from the Hishikari and Akeshi gold deposits. The Hishikari and Akeshi gold deposits both in Kagoshima prefecture, Japan, are the typical of the Hokusatsu-type and Nansatsu-type gold deposits, respectively. Especially, the Hishikari deposit is known as one of the world's highest-grade gold deposits .
Analytical result shows that the ore samples have three trends centering the host rock in 206Pb/204Pb–207Pb/204Pb–208Pb/204Pb isotopic compositional space. Trend-1 constitutes a mixing trend between the host rock and the bed rock (Shimanto Supergroup), which is consistent with the previous ore-forming model. Trend-2 extends from host rock to an inferred composition of PHS-fluid (slab-derived fluid from Philippine Sea plate ). This trend implies that the slab-derived fluid contributes to mineralization of hydrothermal ore deposits as suggested recently . Trend-3 cannot be explained by any geochemical end-member considered here, so more detailed investigation is required. A relationship between Pb isotopic ratio and Au concentration suggests that both the bed rock and the PHS-fluid contribute to Au mineralization.
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