Zinc Contents of Mafic Micerals in Granitic Rocks, with Special Reference to Ore Chemistry

In order to know the reason why the Cu/Zn-Pb ratios of skarn-type deposits related with the ilmenite-series granitic rocks are markedly higher than those related with the magnetite-series, comparative mineralogical studies were carried out for the two types of granitic rocks in the Chugoku district. An attention was focused on the be-havior of Zn during the crystallization of granitic magma, because Zn contents of mafic minerals can be obtained by non-destructive electron microprobe analyses. Microscopic observation indicats that no sphalerite occurs in the magnetite-series granitic rocks, which im-plies that the magmas corresponding to the granitic rocks were undersaturated in respect to ZnS. Electron micro-probe analyses revealed that the Fe/(Fe+Mg) ratios of mafic minerals such as biotite and hornblende in the ilmen-ite-series granitic rocks are remarkably higher than those in the magnetite-series, and the Zn contents are posi-tively correlated with the Fe/(Fe+Mg) ratios in the ilmenite-series granitic rocks. Also the Zn/Fe rations seem to be slightly higher in the ilmenite-series granitic rocks. In contrast, the correlation between Zn contents and Fe/(Fe+Mg) ratios is ambiguous in magnetite-series granitic rocks. With a progress of crystallization differentiation, therefore, Zn may be removed more effectively in the il-menite-series granitic magma, and the contents may be growing scarce in the fractionated ilmenite-series granitic magma. On the other hand, magnetite-series granitic magma probably increases the Zn-contents in the advanced stage of the crystallization. The hydrothermal fluids genetically related to the fractionated magnetite-series grani-tic magma are likely to be enriched in Zn, being favorable for the formation of Zn-rich deposits.