Geo- and thermochronology of the Ertsberg-Grasberg Cu-Au mining district, west New Guinea, Indonesia

The prolific Ertsberg-Grasberg Cu-Au mining district, located on the island of New Guinea in Indonesia, is host to the supergiant Grasberg porphyry copper deposit, and multiple giant skarns. The well-studied nature of the district provides geologic context for high resolution geochronology and thermochronology studies. The supergiant Grasberg porphyry copper deposit is hosted in the Grasberg Igneous Complex. Intrusions were dated using the novel zircon U/Pb depth profiling technique, and age results show the magmatic system was active from 3.6-3.1 Ma. Cu-Au mineralization initiated following intrusion of the MGI (3.22 ± 0.04 Ma) and predates the EKI (3.20 ± 0.04 Ma) and LKI (3.09 ± 0.05 Ma). Based on these cross-cutting relationships, the high grade core of the Grasberg deposit formed in less than 100 to 220 kyr. Age results for the Ertsberg pluton (31-2.8 Ma) and other minor intrusions shows that magmatism in the district took less than 1 myr. Zircon and apatite (U-Th)/He ages from a 2.2 km vertical profile in the Grasberg deposit record minimum cooling rates of 25°C/10 kyr near surface and 4°C/10 kyr at depth. These results indicate Grasberg ore formation occurred immediately following maar volcanism and was short-lived. Rapid cooling of surface samples precludes the presence of a 2 km volcanic edifice overlying the orebody. Rapid cooling at 2 km depth necessitates emplacement into cold country rock. As copper sulfide precipitation is temperature dependent, the tightness of isotherms in the ore zone contributes to the localization of copper mineralization into a small volume, resulting in an extraordinarily high ore grades. Garnet samples from the Big Gossan skarn were dated using the newly developed LA-ICP-MS garnet U/Pb chronometer. Age results show the skarn formed between 2.9–2.7 Ma, over a timespan of approximately 200 kyr. High U contents (10-100 ppm) and a consistent common Pb composition improve precision, and garnet ages agree with external age constraints. This study demonstrates that andradite garnet U/Pb chronometry can be a robust dating technique for constraining the timing and duration of skarn-forming hydrothermal systems.