Integrated exploration success for gold at Wetar, Indonesia

Abstract

This paper describes the case history of a successful exploration programme for gold in Indonesia, initiated by CSR Minerals in 1984 and continued by Billiton Companies in Indonesia since 1988. Gold mineralization on Wetar island, Maluku, was first detected in 1986 as a stream sediment bulk leach extractable gold response of 10 ppb. Stream float mapping and sampling with systematic soil and rockchip sampling gave up to 13 g/t Au, with associated high Ag, Pb, As and Ba values at surface from W5 (Lerokis), one of seven promising initial anomalies. Detailed mapping and rockchip sampling, followed by scout diamond drilling, showed that most significant Au values were associated with barite-rich rocks in basinal structures. Systematic follow-up of the more promising anomalies began in 1987. The best results were obtained from W5 where a number of zones up to 200 × 500 m in size were outlined, with soil and rock values of 1 to 13 ppm Au. Ground geophysical surveys including electromagnetics (EM), induced polarization (IP) and resistivity gave several broad anomalies indicative of shallow conductors. Between 1987 and 1988, aerial reconnaissance was undertaken to locate other areas of visible alteration. This was integrated with Landsat thematic mapper data and defined seven red-stained anomalies in addition to W5, occurring within 5 km of the coast. Ground checks showed that only one of these anomalies, KK3 (Kali Kuning), contained significant surficial Au values. From 1989 to 1992, almost 20,000 m of diamond and reverse circulation (RC) drilling defined geological resources of 2.9 Mt at 3.5 g/t Au, 114 g/t Ag and 40% barite at Lerokis and 2.2 Mt at 5.5 g/t Au, 146 g/t Ag and 60% barite at Kali Kuning. Most other anomalies were then mapped and/or systematically sampled to delineate prospects outlined previously. Regional work is ongoing to define additional resources, using airphoto-based mapping and stream sediment sampling as the principal tools. Integrated exploration has discovered over 20 tonnes of near-surface Au on Wetar at a low cost. Mining operations began late in 1990 at Lerokis, and will begin in 1994 at Kali Kuning. This has formed the basis of an enterprise capable of giving direct employment to over 250 people for more than seven years and providing a return on exploration expenditure.