Geology and exploration of porphyry copper deposits in North Sulawesi, Indonesia

Abstract

Newly discovered porphyry copper deposits in North Sulawesi (Celebes) are located in two principal districts, each with several centers of porphyry copper mineralization. The deposits were discovered by reconnaissance stream silt geochemistry, which detected anomalies of the order of 500 to 1,000 ppm Cu, against a background of 100 ppm Cu, with anomalous trains extending up to 3 km downstream. Subsequent follow-up revealed stream silt anomalies as high as 1.5 percent Cu. Ore targets were developed by detailed stream silt and rock geochemistry together with geologic mapping of lithology, alteration, sulfide distribution, and quartz veining. Gold proved superior to copper, lead, zinc, and molybdenum in the identification of centers of strongest hypogene mineralization, especially in the case of leached cappings, where a strong gold anomaly accompanied by a weak copper anomaly indicates high potential for supergene-enriched ore at depth.In the economically more important district (Tombuililato) high-level quartz diorite porphyry stocks intrude a middle to upper Tertiary island-arc sequence, consisting principally of andesite and rhyolite. Supergene-enriched mineralization is developed in some of the stocks, which are characterized by intense alteration and generally moderate sulfide contents (3-5 volume percent). Ore-grade hypogene mineralization is restricted to the porphyry and to a narrow andesitic wall-rock contact zone. These copper centers occur within a broad zone (5X15 km) of pyritic mineralization, which is best developed in the Tertiary rhyolitic rocks. In the other district (Tapadaa), copper mineralization occurs in complex dioritic plutons whose sulfide contents, copper grades, and degree of enrichment are generally lower than at Tombuililato.Hydrothermal alteration is characterized by the widespread occurrence of strong advanced argillic alteration and the presence of albite or montmorillonite, rather than potash feldspar, in the ore zone. Advanced argillic mineral assemblages comprise various combinations of quartz, andalusite, corundum, diaspore, pyrophyllite, alunite, kaolinite, and specularite. The highest copper grades are generally accompanied by sericite-albite or sericite-montmorillonite alteration, but biotite, chlorite, magnetite, and some advanced argillic minerals are present at least locally in the ore zone. Copper centers in the Tombuililato district occur within a broad zone of advanced argillic-altered rock which approximately coincides with the pyritic sulfide system. Drilling has shown that ore-grade mineralization underlies some areas of advanced argillic alteration at less than 100 m depth. In some cases this represents a change from advanced argillic to clay-sericite alteration with depth, but ore-grade hypogene copper mineralization does occur with advanced argillic alteration of quartz-diaspore-pyrophyllite type. In this case the total sulfide content is commonly high (3-10 volume percent) and the sulfide assemblage is characteristically pyrite-bornite-chalcopyrite.Exceptional abundance of advanced argillic alteration in North Sulawesi is thought to relate to the presence of albite or montmorillonite instead of potash feldspar, to the dioritic nature of the host rocks, and to unusually acidic ore fluids. Mineralogical data suggest that copper mineralization and alteration in the Tombuililato district may have occurred at relatively low temperatures (350 degrees -400 degrees C). The Tapadaa deposits are thought to be root zones of high-level stocks whose eroded parts were like the deposits now exposed at Tombuililato.