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 ton nes 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 i n Indonesia, proven within a short period (1988-1991). Ar-40/Ar-39 dating o n adularia samples give an age of 2.05 +/- 0.05 Ma. The deposit is of the l ow-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 produce d pyroclastic flows and accretionary lapilli with rare intercalations of ep iclastic rocks. This volcanic unit unconformably overlies Miocene subaqueou s volcanic andesitic rocks with interbedded epiclastic rocks. The mineraliz ed bodies are thick (average 4.2 m). steeply dipping, quartz-carbonate-adul aria veins with a very low sulfide content (<0.5 wt.%). Their genesis is re lated to an extensional episode within a tectonic corridor showing NW-SE an d 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 d ominant quartz and calcite and minor kutnahorite, rhodochrosite, and rhodon ite (CQ facies), (2) a network of banded quartz and former carbonate transf ormed into manganese oxides through supergene alteration (MOQ facies), (3) banded opaline milky quartz (BOQ facies), and (4) grey, locally banded, sul fide-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) clearl y indicate a dilational context, which is common in low-sulfidation epither mal 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 GS Q facies. The sulfides are dominated by pyrite, accompanied by common acant hite-aguilarite, polybasite-pearceite and electrum in which the gold conten t ranges from 48 to 74 wt.%. Sphalerite, galena, chalcopyrite and hessite a re fairly rare, although present within the CQ facies. The fluid inclusions of the four facies show homogenization temperatures ranging from 150 to 35 2 degrees C, indicating boiling of a hydrothermal fluid with an initial tem perature of around 205 degrees C; no marked difference is seen in the GSQ f acies, which has the highest gold content. Salinities are low, generally be low 1 wt.% eq. NaCl. Lead isotope compositions of the associated volcanic r ocks and the mineralization are very similar,Pb-206/Pb-204 between 18.706 a nd 18.814 and between 18.744 and 18.801 respectively, demonstrating a genet ic link between the Pliocene volcanism and the auriferous hydrothermal acti vity. The isotopic signature suggests that the source of the mineralization and associated volcanic rocks is an underlying ancient continental crust t hat melted and remobilized during the Pliocene volcanic and hydrothermal ev ents. These conclusions seem applicable to the entire Bayah Dome. The exist ence of both a tectonic corridor and a caldera favoured channelling of the hydrothermal fluids and the deposition of primary ore in the veins. Late in tense weathering of the ore deposit, to depths of 250 m below the surface, has given rise to manganese oxide layers, limonite zones, and silver micron uggets within the veins, as well as to gold enrichment.