Pongkor (west Java, Indonesia): a Pliocene supergene-enriched epithermal Au-Ag-(Mn) deposit |
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| Authors: | J P Milési E Marcoux T Sitorus M Simandjuntak J Leroy L Bailly |
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| Institution: | (1) BRGM, 3, avenue Claude Guillemin BP 6009, 45060 Orléans cedex 2, France , FR;(2) P.T. Aneka Tambang (Persero), Unit Geologi, Jl Pemuda n°1, Jakarta 13210, Indonesia, ID;(3) UHP Nancy 1, LESH, BP 239, 54506 Vandoeuvre-lès-Nancy cedex, France, FR |
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| Abstract: | 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 |
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