Analysis of Au content in sedimentary rocks around the Hishikari gold deposit, Japan

The basement rock of the Hishikari low sulfidation type epithermal gold deposit, Japan is the Cretaceous Shimanto Supergroup consisting of sandstone and shale. Rock samples of the Shimanto Supergroup were collected from the surface and drill cores and the Au contents were analyzed by ICP-MS. Since Au is commonly present in organic matter of sedimentary rocks, organic matter was decomposed by heating using a microwave oven. The Au content of the unaltered rock samples of the Shimanto Supergroup collected from the surface ranges from 1.9 ppb to 7.1 ppb in sandstone and 15.7 ppb in shale. The Au contents were relatively high, however, it may be necessary to analyze the same samples from leaching more than two times to determine more precisely their Au content because of the heterogeneous distribution of Au in the samples. In the Hishikari deposit, δ

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C value in calcite veins was lower than average crustal δ

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C value and this result suggests that carbon of low δC value in veins resulted from organic matter in the basement rock (Morishita 1993). Although Au in organic matter might be transported to the deposit by hydrothermal fluid in the Shimanto Supergroup, the result of rock samples containing organic matter collected below the deposit could not corroborate this notion.