THE GENETIC MODEL OF WANAGON GOLD PROSPECT, TEMBAGAPURA DISTRICT (ERTSBERG DISTRICT), MIMIKA REGENCY, PAPUA

• Main Author: Hartiningsih, Endang
• Format: Dissertations
• Language: Indonesia
• Online Access: https://digilib.itb.ac.id/gdl/view/54428
• Institution: Institut Teknologi Bandung

The Wanagon Gold prospect is in the Ertsberg District, Papua Province, the eastern part of Indonesia. The prospect area predominantly underlay by Upper Mesozoic to Cenozoic sedimentary rocks intruded by the Diorite-Monzodiorite (Wanagon Sill) and Andesite intrusion (Wanagon Dike). The study by previous researchers at Wanagon Gold Prospect was based on pre-2005 exploration data and did not contain results from extensive additional drilling in 2007-2011. This research aims to construct the Genetic Model of Wanagon Gold Prospect based on the latest exploration data. The mineralogy determines using thin section and polish section analysis, a scanning electron microscope with an energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD), and Near-Infrared spectroscopy (NIR). The chemical composition of the rock was identified using an X-ray fluorescence spectrometer (XRF), while the element in pyrite determines by Niton XL3t. The geochemistry analysis consists of fluid inclusion analysis (petrography and microthermometry of fluid inclusion) and stable isotopes of ³?S. Four types of alteration comprise: (a) garnet-clinopyroxene-wollastonite ± olivine prograde skarn; (b) epidote-K-felspar-quartz-chlorite ± amphibole ± biotite retrograde skarn; (c) calcite-quartz ± kaolinite ± zeolite retrograde skarn, and (d) late-stage calcite-gypsum-smectite ± cristobalite ± tridymite. Skarn is the dominant alteration type, and locally the late-stage alteration present at the fault zone near the surface. Gold mineralization is hosted by Cretaceous Kembelangan Group and Tertiary New Guinea Limestone Group. The mineralization of gold is associated with retrograde alteration and locally with late-stage calcite-gypsum-smectite ± cristobalite ± tridymite. Gold present as inclusion in pyrite, and locally in sphalerite, galena, and gang minerals. There are three types of gold-bearing pyrite: (1) massive pyrite; (2) disseminated pyrite; (3) framboidal pyrite associated with clay minerals. Only massive pyrite contains visible gold-bearing minerals such as native gold, electrum, and gold-telluride minerals, while in non-massive pyrite, gold is only detected by the assay. Fluid inclusions study in quartz and sphalerite host minerals indicated the presence of isothermal mixing, surface fluid dilution (fluid mixing), and boiling. There is a pattern of decreasing temperature and salinity from the early (prograde) to late (retrograde) phases. Fluid inclusions in quartz host minerals comprise coexisting vapor-rich and multiphase types with homogenization temperatures > 400 °C and hypersaline composition (>25 wt % NaCl equiv.), indicating the fluids are of magmatic origin and related to prograde skarn formation (occurred at ~50 MPa with ± 5 km under hydrostatic condition). Two-phase liquid-rich inclusions in sphalerite with Th <400°C, and salinities <20 wt % NaCl equiv. are associated with retrograde skarn formation. Gold mineralization is associated with retrograde skarn formation that occurred at a pressure of ~10 to ~25 MPa that corresponds with a depth of < 3 km under hydrostatic condition. Based on the comparison of Co, Ni and As grade in massive pyrite and disseminated pyrite, it can be interpreted that the origin of mineralization fluid comes from fluid mixing with meteoric fluids dominated. This characteristic consistent with the value of ?³?S from massive pyrite that exhibits a short range of 0,56 ‰ to -3,3 ‰, close to the meteoric type of value of 0 ‰. The features of ?³?S of Wanagon Gold, primarily like Kucing Liar Skarn Deposit, as a distal skarn deposit that is typically found in the Cu-Au porphyry system, especially in Ertsberg District. According to the characteristic of alteration, mineralization, and geochemistry, the type of Wanagon Gold deposit is distal Gold Skarn, with locally overprint by Carlin-like Gold Deposit at near-surface adjacent the fault. The source of plutonism expected origin from depth more than 2 km that has not been reached by drilling. Gold deposits in the Wanagon Gold prospect, are genetically formed by the following stages: (1) faulting; (2) intrusion of Diorite-Monzodiorite ; (3) formation of prograde alteration in carbonate rocks and contact zone with Diorite-Monzodiorite intrusion rocks; (4) formation of retrograde alteration in the zone near the surface, by mixing with meteoric water; (5) formation of Gold Skarn mineralization; (6) reactivation of Wanagon Fault and North Wanagon Fault and Andesite Intrusion occurrences; (7) in the vicinity of the North Wanagon Fault zone, Andesite Intrusion has removed the previously formed gold mineralization zone; (8) reactivation of faults and formation of alteration and mineralization of Carlin-like gold deposit in dolomitic limestone adjacent the fault zone.