Indonesia Mineral Deposits : High-sulphidation Epithermal Au—(Cu) Deposits February 8, 2010

High-sulphidation / acid-sulphate epithermal prospects containing Au with enargite are present in various parts of Indonesia, but none is yet known to be large or high grade by world standards. Prospects include Motomboto in the Tombulilato district (Carlile et al., 1990; Perello, 1994), Miwah in Aceh province, northern Sumatra ( A. Williamson, personal commun., 1992) and possibly Binebase in Sangihe island ( Swift and Alwan, 1990), part of the North Sulawesi—eastern Mindanao arc. To these undoubted high-sulphidation systems may be added several extensive zones comprising massive silicification of advanced argillic affiliation which, to date, have not been shown to contain any Cu or Au mineralization ( e.g. Carlile and Mitchell, 1994).

Some of these high-sulphidation zones are observed to be the shallow parts of porphyry Cu systems, as at Motomboto (Perello, 1994), and others are suspected also to be parts of lithocaps above porphyry Cu mineralization by analogy with numerous similar examples

in the circum-Pacific region ( e.g. Sillitoe, 1989). However, the major zone of replacement and residual silica at Masupa Ria in Central Kalimantan ( Thompson et al., 1994) is asso­ciated most obviously with low-sulphidation epithermal Au veins rather than with enargite­Au mineralization, as are much less extensive but apparently similar outliers of siliceous rock in the Mount Muro ( Simmons and Browne, 1990) and Muyup ( Wake, 1991) low­sulphidation districts in the same andesitic arc. This relationship leads Thompson et al. (1994) to propose superimposition of low- onto high-sulphidation mineralization during evolution of a single hydrothermal system at Masupa Ria. The same superimposition is also observed in parts of the Baguio Au district in the Philippines ( Aoki et al., 1993). High-and low-sulphidation mineralization types, generally of unequal economic importance, are commonplace in the shallow parts of intrusion-centred systems ( e.g. Sillitoe, 1989, 1992) although, as in the Tombulilato district (Perello, 1994), they are generally separated spa­tially: with low-sulphidation veins around and locally beneath a high-sulphidation centre. Similarly, at Batu Hijau, a swarm of low sulphidation veins is present distally with respect to the porphyry Cu—Au deposit and its partly preserved lithocap ( Meldrum et al., 1994 ).