Magmatic arcs and associated gold and copper mineralization in Indonesia

Abstract

Gold mineralization in Indonesia formed in andesitic arcs which were active for intervals of between 3 and 20 My from the Cretaceous to Pliocene. Fifteen major arcs are identified with a total on land extent of over 15,000 kms. Known orebodies and major prospects are confined to six arcs within the mid-Tertiary to Pliocene age range. In Indonesia, these arcs total approximately 7,000 kms in length and contain combined historical production and current resources in excess of 2,500 tonnes of gold and 20 million tonnes of copper. Individual arcs or segments of arcs are characterized by specific types of mineralization reflecting both arc basement related to earlier collisions and reversals in tectonic polarity, and erosion level. Porphyry copper-gold, skarn, and high sulphidation enargite-gold deposits formed in both island arc and continental settings where intrusive stocks ascended to shallow depths near the base of a coeval volcanic sequence. Low sulphidation epithermal deposits, largely confined to the western Sunda-Banda and Central Kalimantan continental arcs, formed preferentially in the lower part of the volcanic pile and sometimes at the basement surface. The low sulphidation deposits may be underlain at depths of several kilometres by sills which provided heat for the hydrothermal systems; the deposits probably formed during late-stage extension on low-angle faults. Sediment-hosted mineralization developed where low sulphidation fluids encountered favourable calcareous marine sedimentary rocks at epithermal depths. The distinctive gold-silver-barite±base metal deposits of eastern Indonesia may have formed by venting of high sulphidation fluids into restricted shallow marine basins. As more information becomes available and understanding of arc systems increases, the proposed relationships of deposit type to upper basement and stage of arc evolution, and deposit abundance to erosion level, are likely to become increasingly useful to regional and district scale assessments of mineral potential, both in Indonesia and in arc environments elsewhere.