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Abstract

The Bau mining district, on the island of Borneo in the southwestern Pacific, has produced gold (45.5 tonnes [t] or 1.46 Moz), antimony (83,000 tons), and mercury (1,100 t or 32,000 flasks) from calcic skarn, calcite-quartz veins, and sedimentary rock-hosted replacement deposits that are concentrically arranged around microgranodiorite intrusions with Cu-(Mo) quartz stockwork mineralization. Ores are exceptionally enriched in arsenic. Oxidized disseminated replacement ores, which are chemically, texturally, and isotopically similar to Carlin-type gold deposits in northern Nevada, have contributed the majority of the gold production of the district. The Tai Parit mine, the largest in the district, has produced about 22 t (0.7 Moz) Au at an average grade of about 7.5 g/t (0.22 oz/t) Au. Ores were mainly treated by cyanidation. The concentric zonation pattern led previous workers to propose that these and other Carlin-style gold deposits are distal manifestations of magmatic-hydrothermal systems. This investigation presents new fluid-inclusion, isotopic, and mineralogical data in the context of previously obtained geological, chemical, and other information that advance our understanding of this district, enabling comparisons with Carlin-type deposits in Nevada and distal disseminated deposits elsewhere in the world.

Bau is situated on the western end of the Eocene to Miocene Central Kalimantan magmatic arc. A new K-Ar date on hydrothermal sericite of 10.4 ± 0.3 Ma from a stock with gold-bearing calcic skarns is within the age range of nearby intrusions dated at 11.6 to 9.3 Ma that form part of a NNE-trending, adakitic, magmatic belt. The subvolcanic intrusions and gold deposits are localized by NNE-striking normal faults that transect marine calcareous rocks of the Upper Jurassic Bau Formation and siliciclastic rocks of the Lower Cretaceous Pedawan Formation that are exposed along the axis of the NE-trending Bau anticline. Gold ore is best developed at the intersection of the Krian fault and the contact between these two formations.

The outward zonation from wollastonite-bearing skarn, through calcite-quartz veins, to decalcified and silicified limestone and clastic rock is indicative of decarbonation and Si metasomatism of limestone as hydrothermal fluids cooled. The predominance of sericite over kaolinite shows that fluids were near neutral to moderately acidic and contained a significant amount of potassium. The spatial distribution and paragenetic sequence of native antimony, aurostibite, stibnite, sphalerite (1–7 wt % Fe), pyrrhotite, pyrite, arsenopyrite, native arsenic, and realgar is evidence for cooling and desulfidation of ore fluids.

In sedimentary rock-hosted replacement deposits, mass loss due to carbonate dissolution is shown by enrichment of Ti and Al and depletion of Ca, Mg, and Sr. The strong introduction of Si, Fe, Mn, Zn, Pb, and Ag together with Au, As, and Sb is suggestive of cooling and mixing of saline and H2S-bearing fluids. Laser ablation-inductively coupled plasma-mass spectrometry analyses show that most of the Au resides in arsenopyrite and that Cu and Te are present in Sb and As minerals.

Cooling and decompression are shown by hypersaline fluid inclusions (25–38 wt % NaCl equiv) in Cu (Mo) stockworks and calcic skarn, which were trapped between 500° and 240°C and 400 and 30 bar, while low-salinity fluid inclusions (0–6 wt % NaCl equiv) in vein and Carlin-style deposits were trapped between 350° to 100°C and 200 and 1 bar. Fluid inclusions of intermediate salinity are indicative of fluid mixing. The maximum pressure corresponds to depths of 1.6 (lithostatic) to 4 km (hydrostatic).

The H, O, and C isotope compositions of sericite, wollastonite, quartz, calcite, and inclusion fluids strongly suggest that each deposit type formed from magmatic fluids that were shifted to lower δD values by magma degassing and higher δ18O and δ13C values by exchange with marine limestone. Only fluid inclusion water extracted from late drusy quartz is shifted toward, and late calcite plots on, the meteoric water line. The isotopic composition of S in pyrrhotite and pyrite is magmatic, whereas S in Te, Sb, and As minerals was derived from country rock. The data show how readily hydrothermal fluids of magmatic origin can be modified by reaction with wall rock, mixing with other fluids, and selective loss of lighter components.

In comparison to Nevada’s Carlin-type gold deposits, the Carlin-style gold deposits in the Bau district are smaller and more structurally controlled, have zonation in mineralogy and geochemistry indicative of steep thermal and chemical gradients around exposed porphyry intrusions, and formed from less acidic fluids by cooling and fluid mixing. In addition, Au resides in arsenopyrite, ore has more introduced Fe, Mn, Zn, Pb, Ag, Sb, and As and less Tl and Hg, and there is clear isotopic evidence for magmatic H2O, CO2, and H2S. The genetic links between magmatism and distal disseminated gold mineralization at Bau are a significant contribution to a growing body of evidence that Au and related trace elements in many Carlin-style gold deposits may be derived from magmas.

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