Nature of magma and ore-forming conditions at the Randu Kuning porphyry Cu-Au deposit, Indonesia: a comparative study with other Cu-Au deposits in the region

ABSTRACTThe subduction-related Sunda-Banda arc hosts several giant and major Cu–Au–Mo porphyry and epithermal Au systems, making it one of the most important magmatic-related metallogenic belts in Indonesia. They are accompanied by a number of smaller systems, one of which is the recently discovered Randu-Kuning porphyry Cu-Au deposit on Java Island. The present study investigates the geochemical characteristics of this deposit through whole-rock and mineral geochemistry and mineral geothermobarometry, focusing on three sequential dioritic intrusions that make up the host igneous complex, two barren and the third mineralized. Moreover, a comparative analysis is performed between other porphyry deposits in the Sunda-Banda arc and with Grasberg in Papua. This study shows that the older porphyry deposits have mostly similar geochemical characteristics to the younger deposits, suggesting the same magma source, which is mantle-derived oxidized I-type magma. Amphibole geothermobarometry shows that the formation of syn-ore intrusion in the older porphyry was higher in temperature and pressure than those at the younger porphyry. The halogen and oxygen fugacities inferred from biotite and amphibole chemistry indicate an oxidized, water- and halogen-rich melt. Our study shows that combining amphibole and biotite chemistry and whole-rock geochemical data is useful to fingerprint the magmatic-hydrothermal processes, especially where multiple magmatic phases occur. This research also provides valuable insights into the magmatic-hydrothermal physiochemical conditions during the formation of the mineralized system.KEYWORDS: Biotiteamphibolemineral chemistryporphyrySunda arcRandu Kuning AcknowledgmentsThe authors would like to thank the management of PT Alexis Perdana Mineral (subsidiary of Far East Gold) for the permission and support during fieldwork activities. Mineral chemistry analysis using a JEOL JXA-8900 R EPMA was carried out at the Department of Mineralogy and Economic Geology, RWTH Aachen University; therefore, our sincere gratitude goes to Prof. F. Michael Meyer and Roman Klinghardt for their technical support and valuable assistance during laboratory works. Whole-rock geochemical data in this study were part of the PhD thesis from the fourth author at Universitas Gadjah Mada under first author supervision which was funded by the BPP-DN scholarship and PPPI (Sandwich-like) programmes. The second author is currently a PhD student at the University of Edinburgh supported by a NERC Doctoral Training Partnership grant (NE/S007407/1). Comments and suggestions from editor, Majid Ghaderi, and anonymous reviewer are greatly appreciated.Disclosure statementNo potential conflict of interest was reported by the author(s).Supplementary materialSupplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2283739Additional informationFundingThis study is made possible through financial support of the Academic Excellence Improvement Program, Universitas Gadjah Mada, with grant number of 7725/UN1.P.II/Dit-Lit/PT.01.03/2023.