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Pongkor, Cikidang, Ciawitali, Cirotan, Cikotok, Cipanglesseran |
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Java, Indonesia |
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Au Ag
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Super Porphyry Cu and Au
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The Pongkor low sulphidation epithermal gold deposit is located some 80 km southwest of Jakarta in Java, Indonesia. The mineralisation not previously known to either the local population or the Dutch was discovered in the late 1980's, with a feasibility in 1991 and production commencing in 1994.
(#Location: Pongkor - 6° 39' 41"S, 106° 33' 58"E).
Pongkor is the most significant of a cluster of similar deposits distributed around a volcano-tectonic feature, the Oligocene to Quaternary Bayah Dome, a rhyolite and calc-alkaline andesite complex with intercallations of Miocene limestone and sandstone which resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks. The central part of the Bayah dome is composed of andesitic-dacitic volcanic sequences of Oligocene-Miocene with Pliocene-Quaternary diorite to andesite intrusions. In the northern part of the dome, there is a large depression filled with dacitic ignimbrite and is intruded by resurgent andesitic-dacitic plugs. The western and eastern edges of the dome are composed of younger Pliocene to Quaternary age volcanic rocks. Epithermal mineralisation associated with volcanism in western Java include gold deposits within and on the flanks of the Bayah dome, exposed over an area of some 40 x 80 km. Two distinct styles of epithermal mineralisation are found in the area, both comprising gold-bearing quartz veins. They are referred to as 'Cirotan-' and 'Pongkor-type'. The Cirotan-type deposits, such as Cirotan, Cikotok and Cipanglesseran, are located in the central part of the Bayah dome, and comprise mineralised faults up to 30 m wide as at Cirotan, with ore shoots contained within a fracture system. The ore shoots contain abundant sulphide minerals (Marcoux and Milesi, 1994) and cockade breccia, with uncommon minerals such as uytenbogaardite and Zn-bearing greenockite (Marcoux et al., 1993; Milesi et al., 1994).
Gold deposits of the 'Pongkor-type', which include Pongkor, Ciawitali and Cikidang, are located in the northern and eastern part of the Bayah dome. They are characterised by a very low sulphide content (Milesi et al., 1999), and contain enhanced gold and manganese oxide (Basuki et al., 1994). Gold is present as electrum associated with manganese oxide in a milky quartz ±calcite gangue. Ore minerals are mainly electrum, mostly as free particles, acanthite, aguilarite associated with gold-bearing polybasite, and trace pyrite and sphalerite (Marcoux and Milesi, 1994).
Deposits in the Bayah Dome cluster (as quoted by Rosana and Matsueda, 2002) have been dated at: Cirotan - 1.7 ±0.1 Ma (Marcoux et al., 1993); Cipangleseran - 2.1 ±0.1 Ma; Ciawitali - 1.5 ±0.05 Ma (Marcoux and Milesi, 1994) by K/Ar; Ciguha vein of the Pongkor deposit - 2.05 ±0.05 Ma (Ar/Ar; Milesi et al., 1999); the Kubang Cicau vein also of the Pongkor deposit - 2.7 ±0.1 Ma (K/Ar; Kageyama, 1999) and Cikidang vein - 2.4 ±0.1 Ma (Rosana and Matsueda, 2002).
Pongkor
The Pongkor deposit comprises four main mineralised quartz veins located close to the northern internal rim of a volcano-tectonic feature, the Oligocene to Quaternary Bayah Dome, a rhyolite and calc-alkaline andesite complex with intercallations of Miocene limestone and sandstone which resulted from an explosive ignimbritic eruption that produced pyroclastic flows and accretionary lapilli with rare intercalations of epiclastic rocks. The host volcanic unit unconformably overlies Miocene subaqueous volcanic andesitic rocks with interbedded epiclastic rocks. The formation of the veins was related to an extensional episode of NW-SE and NNE-SSW conjugate strike-slip faults forming a tectonic corridor.
The four main known veins are, Pasir Jawa, Ciguha, Kubang Cicau and Ciurug, hosted by Plio-Quaternary volcanics. These veins have thicknesses that average from 1.5 to 8 m, locally up to 24 m, with strike lengths of 950 to 1500 m and vertical extents of 200 to 250 m to >300 m. They comprise a gangue of mainly quartz, adularia, Mn silicates and calcite.
The mineralisation formed in a number of phases commencing with:
i). CQ facies - a stockwork of 1 to 10 cm veinlets of carbonate-quartz breccia with dominant quartz and calcite and minor kutnahorite, rhodochrosite, and rhodonite;
ii). MOQ facies - a network of banded quartz-adularia and manganese-silicates, mainly rhodonite and inesite, pseudomorphically replaced by rhodochrosite and kutnahorite, transformed into manganese oxides through supergene alteration;
iii). BOQ facies - milimetric to centimetric scale banded opaline milky microcrystalline quartz interbanded with 0.5 to 2 mm thick rhombic adularia crystals, with local rare sphalerite, chalcopyrite, pyrite and galena;
iv). GSQ facies - bonanza veins of grey, locally banded, quartz, with base metal sulphides (>0.3% Cu+Pb+Zn) and precious metals (100 to 400 g/t Au and 600 to 1600 g/t Ag) as veins cutting and locally brecciating the earlier mineralisation, and as discontinuous lenses several metres thick. Adularia was deposited contemporaneously with the quartz. The mineralogy and form of the veins (crustiform banding, vugs, collapse breccia) suggest they accompanied a dilational event.
Sulphide mineral abundances (dominated by pyrite, with associated acanthite-aguilarite, polybasite-pearceite) as well as gold and silver grades, increase steadily from stages 1 to 4, locally reaching 1 kg/t in the GSQ facies. The sulphides are also accompanied by electrum with gold contents of from 48 to 74 wt.%. Sphalerite, galena, chalcopyrite and hessite are fairly rare, although present within the CQ facies. Strongly developed late weathering which extends to depths of 250 m below the surface has produced manganese oxide layers, limonite zones, and silver micro-nuggets within the veins, as well as gold enrichment.
The original resource is quoted at 6 Mt @ 17 g/t Au, 162 g/t Ag (van Leeuwen, 1994).
and 5.97 Mt @ 16.4 g/t Au, 171.2 g/t Ag for 98 tonnes Au and 1026 tonnes Ag (Greffie, et al., 2002).
Cikidang
The Cikidang deposit is some ~10 km south of Pongkor and 90 km SW of Jakarta and was discovered in 1991 by Aneka Tambang. It lies on the eastern
flank of the Bayah dome and comprises four sub-parallel quartz-adularia-sericite(-calcite) veins that are rich in manganese oxide and limonite with very small amount of sulphides and a high concentration of gold and silver. These veins, veinlets and associated silicification are structurally controlled by a steeply dipping fault zone and are hosted within argillic altered volcaniclastic rocks (Rosana and Matsueda, 2002).
The host sequence comprises Early Miocene volcanic rocks consisting of lapilli tuff and breccia that belong to the Cimapag and Cikotok Formations (Sujatmiko and Santosa, 1992). The Early Oligocene Cikotok Formation is composed of volcanic breccia, lava, tuff and altered rocks that host quartz veins (Sukarna et al., 1994; Sukarna, 1999). These are overlain by Early Miocene Cimapag Formation sedimentary rocks which consist of claystone, limestone, sandstone and volcanic rocks. This formation is divided into three members; volcanic, claystone and limestone members. The volcanic member comprises polymictic breccia or conglomerate with intercalation of lapilli tuff, andesitic tuff and lava. The claystone member consists of claystone and sandstone. The limestone member is composed of limestone, marl and claystone. A Late Miocene andesite dyke cuts the prospect intruding into volcanic rocks. It is composed of hornblende, oligoclase and hypersthene with a plagioclase groundmass. Quaternary volcanic rocks cover most of the area, and are composed of volcanic breccia, lava, tuff and basalt. Two types of fault are developed around the Cikidang deposit, classified as pre- and post-mineralisation. The former trend north-south and dip to the west, and are recognised cross-cutting the volcanic rocks (Suwiyanto, 1988). They are characterised by the filling of mineralised quartz, clay, limonite and manganese oxide. The post-mineralization faults trend east-west and crosscuts the Cikidang mineralised veining, as shown by brecciation, slickensides and the associated joint pattern (Rosana and Matsueda, 2002).
The Cikidang system comprises four groups of mineralised quartz veins, named as the Barat, Tengah, Cikidang and Timur veins. The Cikidang vein trends roughly north-south and varies from 0.7 to 3 m in thickness, with a strike extent of up to 1000 m, dip 60 to 86°W. It is composed mainly of quartz, adularia and clay minerals, with variable calcite, manganese oxide and limonite and with very weak of sulphides, and contains colloform, comb, brecciated and massive structures. The quartz veins are fine-grained to chalcedonic, with colour variations from whitish brown, reddish brown to brown black, reflecting the dominant minerals. Gold and silver grades vary from trace to 74.9 g/t Au and 1.2 to 225.0 g/t Ag. Base metals (Cu, Zn and Pb) vary from trace to 0.02 ppm. As
the quartz vein pinches out, the mineralised vein changes to breccia with quartz cement (Rosana and Matsueda, 2002).
The Tengah, Timur and Barat veins also trend north-south and dip to the west, with thicknesses varying from from 0.1 to 2.0 m, except the Timur vein, which dips to the east and is around 50 m in length. Their mineral assemblages include quartz, manganese oxide, limonite and clay minerals with weak sulphides. Ore grades
vary from trace to 12 g/t Au, and 3 to 110 g/t Ag (Rosana and Matsueda, 2002).
The four main mineralised veins at Cikidang have similar vein fill emplaced in four temporal mineralising stages as indicated by crosscutting relationships. The abundance of ore and gangue minerals vary among between stage. Limonite and manganese oxide exist in every stages together with quartz. Most of the electrum and silver minerals are associated with limonite and pyrite or occur as discrete grains. The four main stages are (after Rosana and Matsueda, 2002):
i). Quartz-smectite-chlorite with rare sulphide minerals, occurring as very fine-grained pyrite with gold, limonite and manganese oxide and grades of 0.03 g/t Au and 4.5 g/t Ag.
ii). Quartz-sericite-limonite, with quartz showing colloform, banded and massive textures. Limonite is also abundant, together with montmorillonite, sericite and adularia. Ore minerals are more abundant than the previous stage and include very fine-grained pyrite, gold and silver minerals, with grades of 13 g/t Au and 38 g/t Ag.
iii). Quartz-adularia-manganese oxide, which consists of abundant adularia and manganese oxide, and small amount of montmorillonite, sericite and limonite. Quartz is characteristically has banded, colloform and comb textures, and is white-yellowish in colour. Ore minerals are dispersed, very fine grained pyrite, gold and silver minerals, with grades of 20 g/t Au and 171 g/t Ag.
iv). Massive quartz, which is whitish-yellowish in colour, compact and massive, with small amounts of limonite and manganese oxide. Ore minerals are much less abundant.
Adularia from the banded quartz-adularia in the Cikidang vein yielded an age of 2.4 ±0.1 Ma (K/Ar; Geochron Laboratories, USA; Rosana and Matsueda, 2002)
Reserves estimated by Snowden Mining Industry Consultants include Proved + Probable Ore Reserves of 189,110 t @ 14.3 g/t Au, 79.9 g/t Ag (Antam Resources Ltd.,
unpubl. data; quoted in Rosana and Matsueda, 2002).
The information in the Cikidang summary is taken from Rosana and Matsueda (2002).
The most recent source geological information used to prepare this decription was dated: 2002.
Record last updated: 27/1/2021
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Pongkor
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Selected References:
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Greffie C, Bailly L, Milesi J-P 2002 - Supergene alteration of primary ore assemblages from low-Sulfidation Au-Ag epithermal deposits at Pongkor, Indonesia and Nazareno, Peru: in Econ. Geol. v97 pp 561-571
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Milesi J P, Marcoux E, Sitorus T, Simandjuntak M, Leroy J, Bailly L 1998 - Pongkor (west Java, Indonesia): a Pliocene supergene-enriched epithermal Au-Ag-(Mn) deposit : in Mineralium Deposita v34 pp 131-149
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Prihatmoko, S. and Idrus, A., 2020 - Low-sulfidation epithermal gold deposits in Java, Indonesia: Characteristics and linkage to the volcano-tectonic setting: in Ore Geology Reviews v.121, doi.org/10.1016/j.oregeorev.2020.103490.
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Rosana, M.F. and Matsueda, H., 2002 - Cikidang Hydrothermal Gold Deposit in Western Java, Indonesia: in Resource Geology v.52, pp. 341-352.
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Warmada, I.W., Lehmann, B. and Simandjuntak, M., 2003 - Polymetallic sulfides and sulfosalts of the Pongkor epithermal gold-silver deposit, West Java, Indonesia: in The Canadian Mineralogist v.41, pp. 185-200.
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White, N.C., Leake, M.J., McCaughey, S.N. andd Parris, B.W., 1995 - Epithermal gold deposits of the southwest Pacific: in J. of Geochemical Exploration v.54, pp. 87-136.
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Yuningsih, E.T., Matsueda, H. and Rosana, M.F., 2014 - Epithermal Gold-Silver Deposits in Western Java, Indonesia: Gold-Silver Selenide-Telluride Mineralization: in Indonesian Journal on Geoscience, (Jurnal Geologi Indonesia), v.1, pp. 71-81.
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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