Petrography, zircon U-Pb geochronology and geochemistry of the Ertsberg Pluton, Ertsberg-Grasberg Mining District, Papua, Indonesia

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2020-06-19

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Makis, Jacob Paul

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Abstract

The Ertsberg Pluton is the largest (>15 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 four large skarns and a localized zone of classic porphyry style mineralization. Overall the intrusion has long been described as a diorite that is relatively homogeneous and resulted from the emplacement of a single batch of magma that underwent fractionation. In this study, two NE-SW trending, nearly 1200 m long cores were sampled for petrographic, geochemical, and geochronologic study. More than 90% Ertsberg pluton samples from these cores are best classified monzonite or monzodiorite (± quartz). Five types of magmatic rocks were identified in the two cores and included a medium-grained intermediate phase, a fine-grained intermediate phase, a dark mafic phase, a light phase, and an aplitic phase. Numerous internal contacts exist within the cores, most of which are gradational. Features such as plagioclase sieve cores and sieve rings, as well as albitic cores surrounded by anorthitic rims indicated an intermediate composition magma chamber was recharged and well mixed by hotter, mafic magmas. Major and trace element geochemistry was combined with LA-ICPMS zircon U-Pb crystallization ages to constrain the emplacement and evolution of the composite pluton. A suite of 63 samples were analyzed, 27 of which are from two cores sampled in this study. Out of the entire suite, 31 samples have zircon U-Pb crystallization ages and 31 samples have Nd and Sr isotopic measurements. When samples are plotted on Harker (SiO₂) and Fenner (MgO) diagrams, fractional crystallization trends are apparent. However, when the geochemistry is paired with zircon U-Pb crystallization ages, the oldest samples are the most felsic and the youngest samples are the most mafic. Trace element compositions mimic this relationship with the youngest samples having high concentrations of Sc, V, Ni and Cr. Furthermore, the Nd and Sr isotopic data define a mixing line. When the compositions are plotted against U-Pb ages, εNd increased and ⁸⁷Sr/⁸⁶Sr decreased through time. These trends are opposite to those expected for a single batch of magma that underwent fractional crystallization. The formation of the Ertsberg Pluton is explained by incremental emplacement of an original intermediate composition magma that was mixed with increasing proportions of recharging mafic magma. Space for the pluton was likely created by a combination of magma chamber floor subsidence (> 2 km) and by minor amounts of roof lifting (< 1 km) which were accommodated by movement along near-vertical faults near the margins. The bulk of the Ertsberg Pluton was constructed by the amalgamation of dikes/sills emplaced into a hot crystal-rich mush

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