Tectonics and geology of porphyry Cu-Au deposits along the Eastern Sunda Magmatic Arc, Indonesia

The discovery of the world-class Cu-Au deposit at Tumpangpitu, East Java (1.9 Gt @ 0.45% Cu, 0.45 g/t Au), has reinforced the eastern Sunda arc as a significant metallogenic belt that is highly promising for the discovery of major porphyry deposits. The arc hosts three premier porphyry Cu-Au deposits at Batu Hijau, Elang, and Tumpangpitu (>300 t Au and >5 Mt Cu). These giant porphyry deposits are confined to the eastern segment of the eastern Sunda arc (eastern Java to Sumbawa), constructed on island-arc crust where the Roo Rise is being subducted. By contrast, major epithermal (low-sulfidation dominant) deposits associated with poorly endowed porphyry prospects occur along the western segment of the arc (western Java), developed on thick continental crust on the southern margin of Sundaland associated with subduction of typically thin Indian oceanic crust.

Porphyry Cu-Au deposits of the eastern Sunda arc are spatially associated with small, nested, dioritic to tonalitic intrusive complexes with low-K calc-alkaline to weakly alkaline signatures. Hypogene alteration and mineralization developed during three main temporally and spatially overlapping events, termed early, transitional, and late. The hypogene Cu-Au mineralized zone of the giant deposits forms a bell-shaped body of 0.3% Cu shell greater than 1 km in both diameter and vertical extent that is centered on multiphase, elongate, pencil-like tonalite intrusions, 200 to 1,200 m in diameter and >2 km vertical extent. Large (>20 km²) lithocaps associated with high-sulfidation epithermal systems and postporphyry mineralization diatreme breccias characterize productive porphyry districts at Batu Hijau, Elang, and Tumpangpitu. The known porphyry deposits with metal content exceeding 300 t Au and >5 Mt Cu all formed later than 5 Ma, suggesting an important change in the metallogeny of the arc at this time. These giant porphyry deposits formed relatively rapidly, typically over a period of 30 to 860 k.y. during the Pliocene-Pleistocene, between 5 and 2.5 Ma.

Porphyry deposits along the eastern Sunda arc display characteristic geologic features similar to porphyry deposits elsewhere. Significant differences include low-K systems, dominance of hydrothermal biotite over K-feldspar, early intense actinolite alteration associated with ore, widespread shreddy chlorite after secondary biotite, narrow quartz-sericite-pyrite alteration zones, lack of development of chalcopyrite veins, large overlying lithocaps, and abundant chalcocite-bornite-covellite veinlets associated with late overprinting epithermal systems. Hydrothermal oligoclase is also more abundant than K-feldspar in the central biotite-magnetite zones.