Pongkor (west Java, Indonesia): a Pliocene supergene-enriched epithermal Au-Ag-(Mn) deposit

The Pongkor gold-silver epithermal deposit with reserves of at least 98 tonnes of gold and 1026 tonnes of silver, average grades 16.4 g/t Au and 171.2 g/t Ag is one of the most recent and largest gold and silver discoveries in Indonesia, proven within a short period (1988–1991). ⁴⁰Ar/³⁹Ar dating on adularia samples give an age of 2.05 ± 0.05 Ma. The deposit is of the low-sulfidation epithermal type and consists of four main mineralized quartz veins located close to the internal rim of a volcano-tectonic depression (caldera). This resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks. This volcanic unit unconformably overlies Miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks. The mineralized bodies are thick (average 4.2 m), steeply dipping, quartz-carbonate-adularia veins with a very low sulfide content (<0.5 wt.%). Their genesis is related to an extensional episode within a tectonic corridor showing NW-SE and NNE-SSW conjugate strike-slip faults, the major vein being located on the inner rim of the caldera. The vein fill reveals four successive stages of deposition marked by a specific facies: (1) carbonate-quartz breccia with dominant quartz and calcite and minor kutnahorite, rhodochrosite, and rhodonite (CQ facies), (2) a network of banded quartz and former carbonate transformed into manganese oxides through supergene alteration (MOQ facies), (3) banded opaline milky quartz (BOQ facies), and (4) grey, locally banded, sulfide-rich quartz breccia cutting all the other types (GSQ facies). Adularia was deposited at the same time as the quartz. The mineralogy and internal structures of the veins (crustiform banding, vugs, collapse breccia) clearly indicate a dilational context, which is common in low-sulfidation epithermal systems. Gold and silver grades, as well as sulfide mineral abundances, increase steadily through stages 1 to 4, locally reaching 1 kg/t in the GSQ facies. The sulfides are dominated by pyrite, accompanied by common acanthite-aguilarite, polybasite-pearceite and electrum in which the gold content ranges from 48 to 74 wt.%. Sphalerite, galena, chalcopyrite and hessite are fairly rare, although present within the CQ facies. The fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 382 °C, indicating boiling of a hydrothermal fluid with an initial temperature of around 205 °C; no marked difference is seen in the GSQ facies, which has the highest gold content. Salinities are low, generally below 1 wt.% eq. NaCl. Lead isotope compositions of the associated volcanic rocks and the mineralization are very similar, ²⁰⁶Pb/²⁰⁴Pb between 18.706 and 18.814␣and between 18.744 and 18.801 respectively, demonstrating a genetic link between the Pliocene volcanism and the auriferous hydrothermal activity. The isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust that melted and remobilized during the Pliocene volcanic and hydrothermal events. These conclusions seem applicable to the entire Bayah Dome. The existence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins. Late intense weathering of the ore deposit, to depths of 250 m below the surface, has given rise to manganese oxide layers, limonite zones, and silver micronuggets within the veins, as well as to gold enrichment. Received: 25 June 1997 / Accepted: 10 March 1998     

Fluid Inclusion, Rare-Earth Element and Stable Isotope Study of Carbonate Minerals from the Pongkor Epithermal Gold–Silver Deposit, West Java, Indonesia

The Pongkor gold–silver deposit is the largest low‐sulfidation epithermal precious metal deposit in Indonesia, and is of Pliocene age. The deposit consists of nine major subparallel quartz–adularia–carbonate veins with very low sulfide content. Vein infill records five paragenetic sequences, dominated by calcite in the early stage and quartz in the later stage of the hydrothermal evolution. Fluid inclusions in hydrothermal calcite and quartz of all stages indicate a temperature ranging from 180 to 220°C and a meteoric water origin (very low salinity close to 0 wt% NaCl equivalent). Carbon isotope data on calcite display a narrow range from ?6.5 to ?3.0‰δ¹³C. The oxygen isotope values have a wider range of +4.6 to +10.1‰δ¹⁸O. The broadly positive correlation of the δ¹³C versus δ¹⁸O plot suggests that the carbon species, which equilibrated during the formation of calcite, is dominated by H₂CO₃ not far from equilibrium with HCO₃^?. The abundance of rare earth and yttrium (REY) in carbonate samples is very low (>REY mostly <2 ppm). However, there is always a positive Eu anomaly, which indicates a deeper fluid reservoir at >250°C.     

印尼西爪哇Pongkor矿床:上新世浅成低温热液型Au-Ag-(Mn)矿床

Pongkor矿床属于浅成低温热液 Au- Ag矿床 ,金储量至少为 98t,平均品位为 16 .4× 10 -6;银储量10 2 6 t,平均品位 171.2× 10 -6.它是印尼新近发现的规模较大的 Au- Ag矿床 .冰长石 4 0 Ar/3 9Ar成矿年龄值为 2 .0 5± 0 .5Ma.矿床属于低硫化物浅成低温热液型 ,含有 4条主要的石英脉矿体 .石英脉主要位于火山构造凹陷 (破火山口 )的内缘 ,形成于爆破溶结凝灰岩的喷发 .该喷发产生了火山碎屑熔岩、火山砾及少量碎屑岩夹层 ,构成区内火山岩组合 ,不整合覆盖于中新世安山岩之上 .矿床矿体厚度较大 ,平均厚度为 4 .2 m,陡倾斜 ,由石英、方…