Mineralogical and Geochemical Characteristics of the Gudang Handak Vein Group in the Pongkor Au‐Ag Deposit, West Java, Indonesia

The Gudang Handak vein group of the Pongkor epithermal Au‐Ag deposit in West Java comprises eight quartz‐calcite veins that are hosted by volcanic rocks of the Bayah Dome Complex. This study aims to elucidate the mineralogical and geochemical characteristics of the GH‐B and GH‐C veins of the Gudang Handak vein group. Studied parts of the GH‐B vein and the GH‐C vein have Au grade from 0.31 to 56.0 g/t (ave. 9.67 g/t Au) and Ag grade from 0.02 to 655 g/t Ag (ave. 133 g/t Ag), and Au grade from 0.27 to 22.7 g/t (ave. 6.49 g/t) and Ag grade from 0.02 to 455 g/t (ave. 94.9 g/t), respectively. The major gangue minerals are quartz and calcite with local adularia. Quartz occurs with granular, zonal, comb, pseudo‐bladed, pseudo‐acicular, microcrystalline, mosaic, ghost‐sphere, flamboyant, and feathery textures or forms. Calcite occurs as bladed, acicular, and granular forms. Adularia occurs as rhombic and sub‐rhombic forms. Ore minerals include electrum, acanthite‐aguilarite‐naumannite solid solution, sphalerite, pyrite, pyrargyrite, chalcopyrite, and polybasite, and are hosted by the microcrystalline quartz and mosaic quartz, granular calcite, and rhombic adularia. The microcrystalline quartz associated with the ore minerals in both the GH‐B and GH‐C veins exhibits short‐lived blue cathodoluminescence with a spectral peak at 380–390 nm. This CL characteristic is attributed to Al 3+ , where a couple substitution of Si 4+ by Al 3+ and Na + is organized in addition to excess Al. Quartz in the veins with relatively high Au grade tends to have elevated Al, Na, K, and Rb contents, where the Al and Na contents demonstrate the most continuous positive correlation to the Au grade of the veins. The microthermometry of the fluid inclusions in quartz, calcite, and adularia revealed the formation temperatures of 190°C–220°C and salinities of 0.1–3.5 wt% NaCl eq. A part of the GH‐B vein at 435 masl was formed by a boiling fluid at about 210°C–220°C or a little higher than that, and at depth estimated to be ca. 200–215 m below the paleo‐water table. The other studied portions of the GH‐B vein at 450 masl and 460 masl and the GH‐C vein at 490 masl were plotted close to the boiling curve. One of the principal mechanisms of the gold deposition in the Gudang Handak vein group was likely the fluid boiling. The electrum tarnish method indicates log f S 2 (in atm) ranging from −11.6 to −13.8 and −12.3 to −13.6 for the GH‐B vein at 450 masl and the GH‐C vein at 490 masl, respectively, and log f Se 2 (in atm) ranging from −16.7 to −14.7 for the GH‐B vein. The presence of aguilarite‐naumannite solid‐solution suggests that the Gudang Handak vein group at 435–490 masl formed under relatively higher f O 2 conditions compared to those of other parts of the deposit.