1984 Volume 34 Issue 188 Pages 401-418
Small stock (13 km2) named as Hobenzan pluton of middle Cretaceous age is located geologically in W-province of Sanyo Belt but is associated with many vein-type copper deposits, which are characterized by the presence of tourmaline and polymetallic ore mineral assemblages including As, Co, Bi and W. The stock is composed of biotite-hornblende granodiorite and granite (12 km2 in exposed area) and some biotite aplitic granite (1km2) and trace of quartz monzodiorite. The granodiorite and granite have magnetic susceptibility of magnetite-series values in general but are characterized by low range as 100-565×10-6 emu/g and also by local ilmenite-series values, on which local reduction due to admixing of sedimentary wall rocks can be considered. The Hobenzan granitoids are after all an independent, intermediate-series between typical magnetite-series and ilmenite-series granitoids. The aplitic granite may be in situ differentiates of the original granodioritic magmas.
Chemically, these granitoids are also intermediate, because the major components are similar to those of the average composition of Japanese granitoids, except for K2O which is distinctly higher than the average composition. Among trace elements, Cl and S contents show characteristics of magnetite series but Rb, Sr, Pb and Li contents are more akin to those of average ilmenite-series granitoids in Japan. Yet, no strong concentration of Rb, Pb, Li and Sn is observed in well differentiated facies of the pluton. Sulfur isotope ratios of rock and ore sulfur mostly fall between +2.2 and +0.6%s δ84S(CDT), indicating magmatic source for the ore sulfur.
The Hobenzan granitoids may have originated in lower continental crust materials that have a moderate Fe2O3/FeO ratio and high contents of K2O, Cl, S, Cu and B. Such a crust may be expected originally in the Asiatic continent. Chlorine may have played most important role to concentrate the ore metals in and around the Hobenzan pluton.
