Mineralogical and geochemical characteristics of hydrothermal alteration of basalt in the Kuroko mine area, Japan: implications for the evolution of a Back Arc Basin hydrothermal system Original Research Article

Basalt in the Furutobe District of the Kuroko mine area in Japan is characterized by abundant chlorite and epidote. Fluid inclusion studies indicate that chlorite is formed at lower temperatures (230–250°C) than epidote (250–280°C). The seawater/basalt mass ratio for the early chlorite-rich alteration was high (max. 40), but that for the later alteration was low (0.1–1.8). The CaO, Na2O and SiO2 of the bulk rock correlate negatively with MgO, while FeO and Σ Fe correlate positively with MgO. These changes in the characteristic features of hydrothermal alteration from early to late are generally similar to those for a mid-ocean ridge geothermal system accompanying basalt alteration.