Late Permian kuroko-type mineralization in the Bully Hill area of the East Shasta district formed along two horizons within the Bully Hill rhyolite. In the Bully Hill, Rising Star, and Copper city mine areas, syngenetic mineralization is underlain by alteration zones characterized by addition of SiO₂ and K₂O, loss of NaCO, CaO, and Fe₂O₃* , and the assemblage quartz + sericite + pyrite. Quartz precipitation in the upper portion of the zones led to cyclic sealing of fluid flow, rupturing of the seal, quartz supers aturation, focused fluid discharge, and formation of sulfide mounds on the seafloor. Syngenetic mineralization occurred in several stages: (1) deposition of fine-grained banded sulfides and gangue; (2a) recrystallization of Stage 1 minerals, and void-filling by pyrite + weakly chalcopyrite-diseased sphalerite ± barite; (2b) deposition of coarsely-crystalline pyrite + heavily chalcopyrite-diseased sphalerite; (3a) void-filling and replacement by galena + chalcopyrite + tetrahedrite-tennantite; (3b) replacement by chalcopyrite; and (4) replacement by barite. ΣSO₄⁼ and H₂S contents of hydrothermal fluids decreased from Stages 2a to 3a. δ³⁴S values of sulfides increased successively from Stages 1 (-15.8 per mil) to 3a (3.0 per mil), reflecting variable inputs of biogenic sulfur and deep-seated fluid evolution. δ³⁴S values of syngenetic barites range from 12.4 to 22.5 per mil, reflecting cold seawater and hydrothermal sulfur sources, respectively. δ³⁴S values of epigenetic pyrite and barite within the alteration zones range from -0.7 to 2.7 per mil and 12.4 to 24.1 per mil, respectively. Primary and pseudosecondary fluid inclusions in epigenetic minerals have filling temperatures of 240 to 260 °C and salinities of 3 to 12.3 equivalent weight percent NaCl. Lower limits of fluid salinities and δ³⁴S values of barite indicate that seawater was a primary component of the hydrothermal fluid, and that seawater also mixed with discharging fluids on the seafloor. High fluid salinities, the lack of a silica source, and the close link of magmatism with mineralization indicate that magmatic fluids may have been an important component of the hydrothermal fluids.

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