Geochemical constraints on the petrogenesis of Cenozoic adakitic porphyries in West Papua, Indonesia: implications for the metallogeny of the Grasberg Cu-Au deposit and associated tectonic evolution

Abstract

The Cenozoic porphyries in the Grasberg area host a supergiant Cu–Au porphyry deposit in West Papua, Indonesia. These rocks display adakite-like geochemical features, including high Sr/Y and (La/Yb)N ratios, high Al₂O₃ and K₂O, low Y and Yb, and radiogenic Pb isotopes, distinguishing them from typical lower crustal melts. Integrated zircon U–Pb–Hf, whole-rock Sr–Nd–Pb isotopes, and trace element data indicate that three monzonite porphyries crystallized at 3.06 ± 0.19 Ma, 2.93 ± 0.16 Ma, and 2.86 ± 0.17 Ma. They show negative whole-rock εNd(t) (–9.8 to –8.5) and zircon εHf(t) (–20 to –1), high Ce⁴⁺/Ce³⁺ ratios (11.4–9056.7), and elevated oxygen fugacity, consistent with formation from oxidized, metal-fertile magmas.We propose that Grasberg porphyries formed by partial melting of an enriched, metasomatized lithospheric mantle mixed with melts from ancient lower continental crust. Mantle-derived magmas supplied heat, initial Cu–Au, high water content, and elevated fO₂, while crustal melts imparted adakitic signatures. Regional tectonics involved collisional delamination of the Australian continental lithosphere (starting ∼6 Ma), upwelling of hot asthenosphere, and decompression of stretched lithospheric mantle, promoting partial melting and magma mixing. Subsequent strike-slip faulting created pathways for shallow emplacement of adakitic monzonites, generating Cu–Au mineralization.High Ce⁴⁺/Ce³⁺ ratios and δ³⁴S values of pyrite and chalcopyrite indicate a magmatic source and strongly oxidized conditions, explaining the exceptional metal enrichment. This study establishes a geodynamic framework for the formation of igneous intrusions and the supergiant Grasberg Cu–Au deposit in a post-collisional, thin back-arc setting.