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, δ³⁴S_H2S 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.