Abstract
The Gunung Subang gold deposit is located in the Sunda arc volcanic belt in West Java, Indonesia. The ore bodies are associated with volcanic rocks from the Miocene Koleberes and Bentang Formation. To constrain the significance of the ore genesis and mineralization, this study was carried out with the aim of examining the deposit using methods such as detailed field observation, petrography, petrochemistry, zircon U-Pb dating, Hf isotope, fluid inclusion analysis, and sulfur and in-situ Pb isotope analyses. The results suggest the following. (1) The volcanic rocks in Gunung Subang belong to the basalt-andesite group, with 48.66–79.17% (average 61.04%) SiO2. The trace element patterns of the volcanic rocks exhibit relatively low HREE content, flat HREE patterns, and weak negative or positive Eu anomalies, which are similar to those of typical arc magmatic rocks. (2) Most zircons selected from the volcanic rocks are magmatic, and only a few are inherited. The magmatic zircons have a weighted mean U-Pb age of 17.0 ± 0.4 Ma, and the inherited zircons are from the Precambrian, dated at 887–2379 Ma. The magmatic zircons have εHf(t) values ranging from −1.24 to 13.59, indicating that the magmatism may be contaminated, predominantly with juvenile crust but also with a small percentage of ancient crust. (3) The sulfur isotopic compositions are fairly uniform, and the δ34S values of pyrite, galena, and sphalerite range from 0.53 to 2.97‰, suggesting a magmatic origin. (4) The Pb isotopic compositions of pyrite and galena are clustered, and some include relatively high amounts of radiogenic lead. The mean ratios of 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb are 18.59 ± 0.007, 15.65 ± 0.007 and 38.89 ± 0.017, respectively, indicating that the ore-forming material is highly related to Miocene to Pliocene volcanic rocks. (5) Fluid inclusions in the quartz from the ores are liquid and vapor-liquid phases, with components of the NaCl-H2O system with homogenization temperatures of 240–320 °C and salinities of 0.2–6.2 wt% NaCl. The coexistence of vapor and liquid-rich fluid inclusions suggests that boiling occurred throughout the deposition of the ore. (6) The results of Electron Probe Microanalysis (EPMA) indicate that pyrite is the major Au-bearing mineral. Combined with the observation of scanning electron microscope (SEM), gold is primarily present as Au-Ag-telluride minerals, and there may also be native gold (Au0). Finally, we consider the Gunung Subang is an epithermal gold deposit related to the Mid-Miocene magmatism.
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