AGE, ORIGIN, PETROLOGY AND PETROGRAPHY OF THE ERTSBERG DIORITE, WEST PAPUA, INDONESIA

The 2.5 km x 1 km outcropping body of the Ertsberg Diorite hosts several major copper-gold-bearing skarns in sedimentary rock along its margins and in roof pendants. U-Pb dates on zircons in holocrystalline, equigranular Ertsberg Diorite indicate a crystallization age of 2.97 ± 0.56 Ma. The Ertsberg Diorite ranges from monzodiorite to granodiorite composition and is cut by porphyry dike-like bodies of more evolved compositions that trend parallel to regional structures. The biotite-clinopyroxene assemblage characteristic of Ertsberg Diorite suggests depths less than 2 km, similar to present depth and depth estimates for the formation of the adjacent Grasberg intrusion 1 km to the NW. K-Ar dates on hydrothermal biotite (2.65 ± 0.12 Ma) and Re-Os dates on sulfides (2.56 ± 0.01 Ma) indicate mineralization at Ertsberg occurred soon after crystallization of the main igneous body, and shortly after mineralization at the Grasberg porphyry-diatreme center (2.98 ± 0.12 Ma)(Mathur et al, 2000). Although mineralization extends across similar vertical intervals in both systems and both systems intrude the same host rocks, Grasberg mineralization differs from Ertsberg mineralization in that Grasberg is characterized by much more intense late-stage hydrolytic alteration, finer-grained igneous phases, and diatreme brecciation. The similarity of the depths and timing of mineralization in the two centers suggests the differences in intrusive and mineralization styles may reflect differences in magma chemistry, structural preparation, and pre-intrusion characteristics of the wall rocks.