Mineralogy, fluid inclusions, and S–Pb isotope geochemistry study of the Tuboh Pb–Zn–Ag polymetallic deposit, Lubuklinggau, Sumatra, Indonesia

Abstract

The Tuboh Pb–Zn–Ag polymetallic deposit is located in Lubuklinggau, Sumatra, Indonesia. The ore bodies occur within the contact zone between the Eocene Jangkat quartz monzonite and limestone of the Jurassic–lower Cretaceous Rawas Formation and are controlled by skarn and NE–SW faults. In order to reveal the relationship between mineralization and magmatism, a comprehensive study was conducted that included detailed field observation, petrographic study, fluid inclusions, and S–Pb isotope analysis. The results include the following findings. 1) Four mineralization stages were recognized. The prograde stage is characterized by garnet and wollastonite. The retrograde stage is dominated by magnetite, hematite, chlorite, quartz, with minor sulfides. The quartz-sulfide stage is enriched in sphalerite, galena, chalcopyrite, pyrite, pyrrhotite and quartz. The quartz–calcite stage is typified by quartz and calcite. 2) Electron probe microanalysis (EPMA) results revealed that garnet in the Tuboh deposit is rich in andradite (Adr37-99Grs1-59), indicating skarn was formed under relatively oxidized conditions. The temperature and S fugacity for sphalerite crystallization were 270 to 392 °C, and 10−9.7 to 10−7.1 bars, respectively. The decreasing trend between S fugacity with temperature suggests that ore precipitation occurred in a progressive reducing environment. EPMA analyses demonstrate that the Ag is primarily hosted by galena, which is present as solid solution with Bi via coupled substitution of Pb in the galena. 3) Fluid inclusions trapped in garnet indicates that the ore-forming fluid was high in temperature with high to moderate salinity, at 380–450 °C, 9.21–21.4 wt% NaCl equivalent, respectively, which indicates ore-forming fluid may derive from magmatic fluids. Fluid inclusions from quartz suggest ore precipitation from cooler and moderate- to low-salinity fluids, at 337–420 °C and 6.45–11.1 wt% NaCl equivalent, respectively, which is in agreement with the sphalerite crystallization temperature. The fluid inclusions in the quartz and calcite reveal that ore-forming fluid from the post-ore stage was moderate to low in both temperature and salinity, at 151–266 °C and 0.35–7.45 wt% NaCl equivalent, respectively. The positive correlation between homogenization temperature and salinity indicates that the ore-forming fluid mixed with meteoric water. 4) The δ34S values of sulfides range from −2.44 to 2.52‰, suggesting a magmatic origin of S. The Pb isotopic ratios of the galena are very uniform and have low radiogenic Pb, at 206Pb/204Pb = 18.342–18.370, 207Pb/204Pb = 15.576–15.599, and 208Pb/204Pb = 38.369–38.430. This indicates that the metals originated from the Eocene Jangkat quartz monzonite. Finally, in combination of available data suggest that the Tuboh deposit is a typical skarn-type Pb–Zn–Ag polymetallic deposit, which is the result of the Eocene quartz monzonite intruded the Jurassic–Cretaceous Rawas Formation, and ore precipitation is the consequent of the mixture of ore-forming fluid with meteoric water and decreases in S fugacity.