COMBINED MAJOR AND TRACE ELEMENT GEOCHEMISTRY AND ZIRCON U/PB GEOCHRONOLOGY OF THE ERTSBERG PLUTON, ERTSBERG-GRASBERG MINING DISTRICT, PAPUA, INDONESIA: MAGMA CHAMBER RECHARGE AND MIXING

The Ertsberg Pluton is the largest (>10 km³) and youngest (3.1-2.8 Ma) igneous body within the renowned Ertsberg-Grasberg mining district of Papua, Indonesia. It is associated with ore-grade Cu-Au mineralization in the form of three large skarns (Ertsberg/Gunung Bijih, Ertsberg East Skarn System: GBT/IOZ/DOZ/MLZ/DMLZ, and Dom) and a localized zone of classic porphyry style mineralization. Ertsberg rocks are clinopyroxene + titanite bearing and range in composition from quartz monzodiorite/monzodiorite to quartz monzonite/monzonite, but overall the intrusion is relatively homogeneous compared to others in the district. For this reason, it was thought that the bulk (>95% volume) of Ertsberg was emplaced as a single batch of magma that underwent fractionation. In this study, major and trace element geochemistry was combined with LA-ICP-MS zircon U/Pb ages to constrain the emplacement and evolution of Ertsberg in detail. A suite of 57 samples was analyzed, 29 of which have zircon U/Pb ages. Of these, 24 samples (six have zircon U/Pb ages) are from two NE-SW trending, near-horizontal drillholes, up to 1200 m long. When all samples are plotted on standard Harker diagrams for SiO₂ and MgO, apparent fractional crystallization trends are observed with SiO₂ ranging from 54-64 wt. % and MgO ranging from 1-3 wt. %. Samples with high SiO₂ or low MgO, have low Ca, Fe, Ti and P and high K. However, when major element geochemistry is compared with zircon U/Pb ages, the oldest samples are the most felsic and the youngest samples are the most mafic. Trace elements mimic this relationship with the youngest samples having high concentrations of Sc, V, Ni and Cr. These trends are opposite to those expected for a single batch of magma that underwent fractional crystallization. The geochemistry of the Ertsberg Pluton is explained by blending an original intermediate composition magma with increasing proportions of recharging mafic magma.