Abstract

Epithermal lead-zinc vein-type deposits of the Toyoha mine, Hokkaido, Japan, occur in the Green Tuff region of Miocene age. The veins are divided into two groups, those of E-W and NW-SE systems, which have different structural and mineral features. The Soya vein, representative of the NW-SE system, is characterized by the common occurrence of banding and symmetrical structures in the vein, which are useful to establish the succession of mineralization. Stages of mineralization recognized in the vein are (1) sulfide stage which is composed of sphalerite-pyrite-quartz with minor hematite, sphalerite-galena-pyrite, sphalerite-pyrite, and pyrite-pyrrhotite subbands; (2) carbonate-pyrite-clay stage which is composed of carbonate-pyrite-montmorillonite and carbonate-pyrite-alabandite subbands; and 3) carbonate stage which is composed of manganocalcite-pyrite-quartz and calcite-quartz subbands from outside (earlier) to inner side (later).About 20 primary minerals occur in the deposits. The probable secondary minerals (kaolinite, goethite, caryopilite, native silver, and szmikite) are also widely distributed.Composition of sphalerite coexisting with iron minerals such as hematite, pyrite, and pyrrhotite was analyzed by electron microprobe analyzer. The FeS of the sphalerite increases with successive mineralizations. Compositional zoning in a sphalerite grain was also studied. Compositional zoning of manganese content in a sphalerite grain enclosed by manganese minerals (rhodochrosite or alabandite) shows a parabola-like profile.Although compositional zoning in sphalerite and wide occurrence of secondary minerals suggest that postdepositional processes have modified the original assemblage and chemical composition of minerals in the deposits to some extent, the following features are clarified for Toyoha mineralization on the basis of detailed mineralogical and geochemical studies: (1) wide variation of oxygen fugacity throughout the ore formation and (2) reducing processes in the sulfide stage.Those features are compared and discussed with studies previously reported for other types of deposits.

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