The mine stands at the collision of the Indo-Australian and the Pacific tectonic plates. Two distinct phases of intrusion have led to the production of nested coaxial porphyry ore bodies and sulphide rich skarn at the margins, while sedimentary strata includes Eocene clastic carbonate limestone with siltstones and sandstones near the base.The Dalam Diatreme (DD) forms the first intrusive stage, being highly fragmental and characterised by clasts and a matrix of dioritic composition.Mineralisation is largely disseminated and chalcopyrite dominant, having average grades of 1.2% copper and 0.5 g/t gold.
The second intrusive stage, the Main Grasberg Stock (MG), is composed of non-fragmental, porphyritic monzodiorites, forming a quartz-magnetite dilational stockwork with veinlet controlled copper-gold mineralisation. This is a high-grade resource, with averages of 1.5% copper and 2 g/t gold.There is also a third intrusive stage, associated with the South Kali Dykes, which was the final intrusion and the most weakly mineralised.
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