Indonesia Mineral Deposits : Low-sulphidation epithermal Au deposits February 8, 2010

Low-sulphidation /adularia-sericite epithermal Au–( Ag) deposits are widespread in Indonesia, and those of vein type in Sumatra–West Java and Central and East Kalimantan predominate. Lebong Donok in Sumatra is a medium-sized (41.5 tonnes Au) bonanza Au deposit with textural and mineralogical similarities to the much larger Hishikari deposit in Kyushu, Japan ( Van Leeuwen, 1994). The recently discovered Gunung Pongkor deposit, West Java, is substantially larger ( 102 tonnes Au), but also comprises classical sulphide-and base metal-poor, low-sulphidation veins (Basuki et al., 1994 ). The low sulphide content ( < 1 vol.%) contrasts with those of the base metal-rich epithermal veins that are more common elsewhere in West Java (Marcoux and Mil6si, 1994) and in most other parts of Indonesia. Vein breccias characterize many of the low-sulphidation districts, most spectac­ularly at Lebong Tandai, Sumatra (Jobson et al., 1994) and Cirotan, West Java (Marcoux et al., 1993) .

The only low-sulphidation epithermal deposit which approaches giant status ( defined here as > 200 tonnes Au) is Kelian in East Kalimantan. However, this bulk-mined deposit is related to intrusive rocks, is rich in base metals and yields fluid inclusion temperatures

( up to 330°C) and salinities ( > 10 wt.% NaC1 equiv.) somewhat higher than typical for epithermal deposits ( Van Leeuwen et al., 1990). Consequently, a deeper level of formation, at least 900 m based on fluid inclusion geobarometry, was proposed by Van Leeuwen et al. ( 1990). Based on examination of recent mine exposures, this writer interprets the sedimen­tary rock-charged Muddy breccia at Kelian as a series of diatremes related to felsic plug-domes, and perceives similarities with the diatreme-hosted Au deposit at Montana Tunnels, Montana, U.S.A., also interpreted to have formed in the deep-epithermal environment (Sillitoe et al., 1985). Both Kelian and Montana Tunnels are rich in Zn, Pb and manganoan carbonates but lack appreciable quartz.

Most of the low-sulphidation epithermal deposits and prospects in Indonesia are associ­ated, at least spatially, with andesitic—dacitic volcanic rocks, which contrast with the felsic dome complex hosting the disseminated Au mineralization at Gunung Pani in western North Sulawesi (Kavalieris et al., 1990). The dome complex is part of a late Miocene—Pliocene felsic volcanic suite which is broadly coeval with the belt of intrusive rocks containing the Malala Mo deposit ( see above; Kavalieris et al., 1992). The presence of both epithermal Au and porphyry Mo mineralization in association with the same magmatic suite may suggest that the dome-hosted Au was concentrated in the shallow parts of a concealed porphyry Mo system (T. van Leeuwen, written commun., 1992) if it is accepted that certain epithermal precious-metal deposits, some containing Mo, represent the tops of porphyry Mo systems ( Sillitoe, 1992).

Furthermore, the enrichment of W ( as wolframite), Sn ( as cassiterite and Te-canfieldite ) and Ag ( including uytenbogaardite) in the low-sulphidation epithermal vein Au—Ag deposit at Cirotan, West Java ( Marcoux et al., 1993), is attributed by Marcoux and Milesi ( 1994) to its association with Pliocene dacitic magmatism shown, using Pb-isotopic data, to be of crustal origin. Similar Sn and Ag enrichment, as stannite, canfieldite and Ag sulphosalts, is also documented for the Mangani Au—Ag vein in Sumatra ( Kieft and Oen, 1974). Such lithophile-element enrichment in precious-metal deposits related to magmatism of crustal parentage recalls the epithermal, Ag-rich ( but Au-poor) tops to the Sn- and base metal-bearing vein and stockwork systems of the Bolivian Sn—Ag belt ( Sillitoe, 1992).