THE ALTERATION AND MINERALIZATION PARAGENESIS OF PORPHYRY-SKARN DEPOSITS IN KUCING LIAR-GAJAH TIDUR GRASBERG AREA, ERTSBERG DISTRICT, MIMIKA REGENCY, CENTRAL PAPUA PROVINCE

• Main Author: Auliya' Azhar Ramadhan, Irsyad
• Format: Final Project
• Language: Indonesia
• Subjects: Geologi, hidrologi & meteorologi
• Online Access: https://digilib.itb.ac.id/gdl/view/69581
• Institution: Institut Teknologi Bandung

The Kucing Liar Skarn is one of the Cu and Au prospect deposit areas in the Grasberg Igneous Rocks Complex, Ertsberg Mining District. Beneath it was a new intrusion, named the Gajah Tidur Prospect, that cut into previous Grasberg intrusions. It is necessary to study the distribution zone and the alterationmineralization stages to determine the area's prospects. Therefore, this study aims to determine the paragenesis of alteration and mineralization based on the alteration minerals assemblages present. The research location is in the Ertsberg Mining District, Mimika Regency, Central Papua Province. This area is located at coordinates 733500 mT-735200 mT and 9549800 mU-9551000 mU, UTM Zone WGS 53 S. The study was conducted by analyzing three drilled wells, namely wells RA12-01, MA80-53, and IRS-01, at the elevation below 3100 masl and more than 800 m from the surface. Macroscopic, microscopic observations (thin and polished), along with quantitative X-Ray Diffraction (XRD) analysis and geochemical assays were carried out to determine the lithology, alteration zone, and mineralization zone of the study area. Based on this analysis, the research area is composed of seven rock units. Sedimentary rocks that undergo contact metamorphism with intrusions formed metamorphic rock units which from oldest to youngest include Metasandstone Unit, Hornfels Unit, and Marble Unit. The intrusions in the study area consisted of four stages. They are Porphyry Microdiorite Unit, Monzodiorite Unit, Quartz Monzonite Unit, and Porphyry Diorite Unit. Hydrothermal alteration in the study area has a low to total intensity with a porphyry-skarn type of deposit. The sequence of alteration paragenesis started from stage 1 intrusion (Porphyry Microdiorite Unit) which formed contact metamorphism with the intruded sedimentary rocks formations. Metasomatism in the wall rocks occurred to form exoskarn prograde metasomatism zone (Garnet-Quartz-Diopside Zone) and continued to form retrogradation metasomatism zones (Phlogopite-Talk and Anhydrite-Garnet Overprint Zones; Actinolite-Phlogopit-Talk Zones). Stage 2 intrusion (Monzodiorite Unit) was present to form porphyry alteration zones of potassic (Quartz-Biotite ± Anhydrite), propylitic (Chlorite-Epidote and Biotite-Anhydrite Overprint Zones), phyllic (Quartz-Sericite ± Muscovite ± Anhydrite) and retrograde metasomatism zone of exoskarn (Magnetite-Anhdirite-Quartz Zone). Then, Stage 3 intrusion (Quartz Monzonite Unit) formed a phyllic overprinted potassic alteration zone (Overprint of Sericite-Calcite ± Anhydrite and Biotite ± Anhydrite) and a Quartz Veins Zone in the Metasandstone Unit. The decreasing temperature of the hydrothermal fluid caused the presence of argillic (Smectite ± Kaolinite) alteration in the shallower area, overprinting the earlier phyllic zone. Stage 4 intrusion (Diorite Porphyry Unit) was formed and released hydrothermal fluid forming a propylitic alteration zone (Chlorite-Calcite ± Epidote Zone). There are six mineralization zones in the study area, namely the Pyrite Zone, Pyrite- Covellite Zone, Chalcopyrite-Pyrite ± Covellite Zone, Pyrite-Sphalerite-Covellite Zone, Magnetite Zone, and Magnetite-Pyrite-Chalcopyrite Zone. The mineralized zone is present in a correlation with the presence of an alteration zone of porphyry skarn deposits. The potential grades of Cu and Ag mineralization are associated with the Chalcopyrite-Pyrite ± Covellite Zone and the Magnetite-Pyrite- Chalcopyrite Zone. Au grades are low but at some points in the Magnetite-Pyrite- Chalcopyrite Zone Au>0.5 ppm is present.