PERMIAN HYDROTHERMAL DEPOSITS IN THE MINO TERRANE, CENTRAL JAPAN - IMPLICATIONS FOR HYDROTHERMAL PLUMES IN AN ANCIENT OCEAN-BASIN

Geochemical features of metalliferous deposits, cherts and basalts from the Early Permian Hashikadani Formation in the Mino Terrane, central Japan were studied. The metalliferous deposits occur as thin Fe-rich layers about 5 cm thick and Mn-rich crusts up to 30 cm thick in cherts overlying basalt. The Fe-rich layers contain large amounts of magnetite and are intercalated in the massive chert unit just above the basal basalt. The Mn-rich crusts occur in the bedded chert unit, which always overlies massive cherts, and their dominant Mn-mineral is braunite. These metalliferous deposits show highly fractionated MnO/Fe2O3* (total iron as Fe2O3) values and are depleted in such elements as Ni, Co, Cu and Zn; these features indicate their hydrothermal origin. The values of Fe2O3/FeO and MnO/Fe2O3* in samples abruptly increase above the lithologic boundary between the massive chert and the bedded chert units. This stratigraphic change of lithofacies and chemical characteristics can be attributed to the precipitation of Fe-oxides prior to Mn-oxides from a hydrothermal solution that was gradually diluted with seawater and became oxic. The formation process of the Permian metalliferous deposits from central Japan can be inferred by analogy with modern hydrothermal deposits recently discovered along active spreading centres.