Mineralogical and Geochemical Comparison between Non- and Weakly- Metamorphosed Besshi-type Deposits and Ridge Sulfide Deposits

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Electron microprobe analyzes of sphalerites from Besshi-type deposits, mainly at Besshi and its vicinity, Hitachi, and Shimokawa, revealed that the Mn/Zn and Co/Zn atomic ratios may have markedly increased during contact metamorphism, while the Cd/Zn ratios remained unchanged.Except for Shimokawa, the Mn/Zn and Co/Zn ratios of sphalerite precipitated from solutions are found to be both mainly lower than 2 x 10^<-3>. At Shimokawa, the Mn/Zn and Co/Zn ratios are highly variable, and high Co/Zn ratios occur in sphalerite of Cu-rich ores. The Cd/Zn ratios are mostly within 2 x 10^<-3> at Shimokawa and Hitachi. The ratios display a wide variation within 8 x 10^<-3> in the deposits at Besshi and its vicinity.Based on the equilibrium fractionation model between hydrothermal solution and sphalerite, the SIGMAm_<Cd>/SIGMAm_<Zn> and SIGMAm_<Mn>/SIGMAm_<Zn> ratios in the initial solutions responsible for the mineralizations at Besshi and its vicinity are estimated to be 2^-3 x 10^<-3> and lower than 20^-30 at 300-350ﾟC,respectively ; they are quite similar to the ratios of high-temperature solutions at EPR 21ﾟN.Low SIGMAm_<Cd>/SIGMAm_<Zn> and SIGMAm_<Co>/SIGMAm_<Zn> ratios characterize the solutions for the deposits at Hitachi. At Shimokawa, the hydrothermal solution related with Cu-mineralization was high in the SIGMAm_<Co>/SIGMAm_<Zn> ratio and contributed to the formation of Co-rich sphalerite in the Cu-rich ores.These data coupled with available ones including ore geology and sulfur isotope ratios suggest that the deposits at Shimokawa formed on sediment-covered ridge, and those at Besshi and its vicinity on sediment-free ridge far-off from land. The deposits at Hitachi and Yanahara may be genetically related with felsic volcanism. Lead-rich ore mineralogy of the sulfide deposit at Mariana Trough may be explained by the genetical connection with sialic crust.