Ore geology and fluid evolution of the giant Caixiashan carbonate-hosted Zn–Pb deposit in the Eastern Tianshan, NW China

Abstract

The Caixiashan giant carbonate-hosted Zn–Pb deposit (~131Mt@ 3.95% Zn+Pb) formed by replacement of dolomitized marble, with stratiform massive and breccia bodies is located near the base of the Proterozoic Kawabulake Group limestone and marble. It is one of the largest carbonated-hosted massive sulfides Zn–Pb ore deposits in Northwest China to have been discovered in recent years.Abundant pyrite occurs in dolomitized marble, along fractures in dolomitized clasts in the host rocks and filling cracks in the host rock. Locally, colloform or framboidal pyrites are observed in the early period and sometimes replaced by the later sphalerite. The sulfide assemblage of the main ore stage is characterized by massive or disseminated sphalerite and galena, with less pyrite than the earlier stage, and minor pyrrhotite. Galena occurs as small veins cutting the early-formed sphalerite. Dolomite and calcite are the main gangue minerals that co-precipitated with these sulfides. Tremolite and quartz alteration commonly overprints the orebodies. According to the crosscutting relationships and the different mineral associations within the host rocks and ore bodies, three stages are recognized at Caixiashan, i.e., syn-sedimentary pyrite (stage I), pyrite alteration, sphalerite–carbonate and galena–pyrite–carbonate (stage II-1, stage II-2 and stage II-3, respectively) and magmatic/metamorphic reworking (stage III).Calcite and quartz crystals are important host minerals among the three hypogene stages (stages I–III, although quartz mainly occurred in stage III). Stage I contains only aqueous inclusions (W-type), which were homogenized from 110 to 236°C (main range of 138–198°C and average at 168°C; main range=average±σ) and the salinities are from 0.5 to 16.5 (main range of 5.1–15.1 with average of 10.1) wt.% NaCl eqv. In the pyrite alteration of stage II-1 the W-type fluid inclusions homogenized from 175 to 260°C (main range of 210–260 with average of 235) and the salinities range from 8.5 to 22.4 (main range of 16.7–20.1 with average of 18.4) wt.% NaCl eqv. In the main Zn–Pb mineralization stage (stage II-2–3), four types of fluid inclusions were identified an aqueous phase (W-type), a pure carbon phase (PC-type), a carbon phase containing (C-type) and mineral bearing inclusions (S-type). The W-type fluid inclusions of stage II-2–3 homogenized at 210 to 370°C (main range of 253–323 and average at 270) and the salinities range from 5.9 to 23.1 (main range of 13.3–20.3 with average at 16.8) wt.% NaCl eqv.; C-type homogenized at 237°C to 371°C and the salinities range from 6.4–19.7wt.% NaCl eqv.; S-type fluid inclusions homogenized at 211 to 350°C and daughter minerals melted between 340 and 374°C during heating, indicating a salinity range of 42 to 44wt.% NaCl eqv. PC-type fluid inclusions with homogenization temperatures of CO2 phase show large variation from 7.4°C to 21.2°C. Laser Raman analyses show that CH4, CO2 and SO42− coexist in the main mineralization stage fluids. The magmatic/metamorphic reworking stage only contains W-type fluid inclusions which yield homogenized between 220 and 360°C (main range of 251–325 and average at 288), with salinities ranging from 1.7 to 23.0 (main range of 14.3–20.0 and average at 18.8) wt.% NaCl eqv.The textural features, mineral assemblages and fluid geochemistry suggest that the Zn–Pb ores were formed through hydrothermal convection of hot marine waters along the faults and fractures resulting in metal (Zn, Pb and Fe) enriched stratiform orebodies. Subsequent rapid precipitation of sulfides was triggered by sulfate (SO42−) thermal reduction with the CH4 preserved in sedimentary rocks and early stage I pyrite bodies. This process occurred at moderate temperatures (ca. 270°C). Higher-temperature magmatic hydrothermal alteration overprinted the orebodies, but only provided a minor contribution to the mineralization.