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Sulfur isotopic study of ore minerals and altered host rocks of Neogene-Quaternary overprinting hydrothermal systems in the Toyoha-Muine area, southwest Hokkaido, Japan

TORU SHIMIZU
Geochemical Journal, Vol. 46, No. 3, P. 179-191, 2012

ABSTRACT

Sulfur isotopes in vein sulfides (galena, sphalerite, and pyrite) and pyrite sulfur derived from overprinting hydrothermal systems in the altered volcanic host rocks of the Toyoha-Muine area are determined. Miocene and Pliocene-Pleistocene systems are found and are both characterized by Pb and Zn mineralization. The δ34S values for the vein sulfides in both systems show a similar range, i.e., 1.7~7.5‰ and 1.3~7.7‰ for the Miocene and Pliocene-Pleistocene systems, respectively. The δ34S values of pyrite sulfur in the Miocene altered rocks show a wider range (-4.0~10.5‰) than the contemporaneous ores, while Pliocene-Pleistocene values (0.8~6.3‰) show a narrow range, similar to the contemporaneous ores. Miocene ore deposits were formed from hydrothermal fluids under reduced conditions; aqueous sulfur in the solutions was predominated by dissolved H2S. From the δ34S of the Miocene sulfide minerals, δ34SH2S is calculated to be in the range 2.8~6.2‰, indicating that the sulfide sulfur was arc magmatic in origin. Although previous studies suggest that the source of sulfur and metals for the Pliocene-Pleistocene system can also be attributed to the contemporaneous arc magma, this study proposes another source. Combined with a previous study of tectonic setting, the common presence of Pb and Zn in the ores and the similar δ34S of the vein sulfides for distinct hydrothermal systems suggest that minor amounts of sulfide sulfur and base metals (Pb, Zn, and Fe) from the Pliocene-Pleistocene ores were introduced by dissolution and remobilization of sulfur and metals from the Miocene ores and altered rocks. The wide range of δ34S from pyrite sulfur in the Miocene altered rocks may be a result of the contribution of magmatic sulfur contaminated by reduced seawater sulfate; however, the narrow range of δ34S in the Pliocene-Pleistocene altered rocks is a result of the major contribution of magmatic sulfur and minor contribution of recycled sulfur from the Miocene ores and altered rocks, as in the case of the contemporaneous vein mineralization.

KEYWORDS

sulfur isotopes, overprinting hydrothermal systems, ore minerals, alteration, Toyoha-Muine

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