16:30 〜 16:45
[HRE13-09] Understanding porphyry copper-gold deposits along eastern Sunda magmatic arc: characteristics, conditions of ore-formation and implications for future exploration
キーワード：porphyry, geochemistry, ore forming condition, Randu Kuning, Tasikmadu, eastern Sunda magmatic arc
The 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, two of which are the newly-discovered Randu-Kuning and Tasikmadu porphyry Cu-Au prospects in Java Island. The present study investigates the geochemical characteristics of the Randu Kuning prospect, and elucidates some diagnostic characteristics and conditions of ore-formation of the Tasikmadu prospect.
Randu Kuning. The study is focused on the whole-rock and mineral geochemistry and mineral geothermobarometry, emphasizing 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 with the younger deposits, suggesting the same magma source, which is mantle-derived oxidized I-type magma. The 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 fugacity inferred from biotite and amphibole chemistry indicates 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.
Tasikmadu. Various studies have been done on the Tasikmadu prospect including fieldwork and numerous laboratory analyses for a suite of representative samples were performed, including petrography, ore microscopy, XRD, EPMA, SEM-CL, FA-AAS, ICP-MS/AES, fluid inclusion petrography and microthermometry, and sulfur isotope analysis. The study area is composed of three diorite porphyries, i.e., fine-grained, medium-grained, and coarse-grained diorite porphyry, respectively. The intrusions are calc-alkaline with a high Sr/Y value, which is similar to many ore-bearing intrusions in the eastern Sunda arc. Ore mineralization occurs in quartz veins and veinlet stockwork, which is centered in the potassic zone associated with the fine-grained diorite porphyry. Ore minerals are dominantly chalcopyrite and bornite occurring in A- and B-veins, which cut earlier barren (EB)- and M-veins. Bulk rock analysis showed that the mineralization core has an average grade of 0.63 wt% Cu and 0.25 g/t Au, respectively. Outwardly, the potassic zone changes to the propylitic zone, which still bears copper in the quartz and pyrite veinlets, although the grade is low. Late chlorite-sericite and local argillic zones superimpose the earlier alteration zones. Late pyritic D-veins with a relatively lower copper content compared with the earlier veins. Fluid inclusion microthermometry revealed that the A- and B-veins in the potassic zone formed at 380-700 ºC by hypersaline fluids. The temperature temporally and spatially decreased, i.e., in the propylitic zone, the quartz veinlets show temperatures of 260-400 ºC, whereas the late chlorite-sericite zone formed at 190-320 ºC. Pressure calculation suggested that hypogene mineralization occurred at 1.1-1.2 km below the paleosurface, indicating a relatively shallow depth for a porphyry deposit. Nevertheless, the δ34S value of sulfides ranges from -2.0 to -0.1 ‰, inferring a magmatic origin. The Randu Kuning and Tasikmadu prospects share some similarities compared with other porphyry deposits worldwide, but it also reveals unique characteristics that differ from others, which implies certain ore-forming conditions. This may imply that the discoveries of new other porphyry prospects along eastern Sunda magmatic arc is still open.
Randu Kuning. The study is focused on the whole-rock and mineral geochemistry and mineral geothermobarometry, emphasizing 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 with the younger deposits, suggesting the same magma source, which is mantle-derived oxidized I-type magma. The 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 fugacity inferred from biotite and amphibole chemistry indicates 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.
Tasikmadu. Various studies have been done on the Tasikmadu prospect including fieldwork and numerous laboratory analyses for a suite of representative samples were performed, including petrography, ore microscopy, XRD, EPMA, SEM-CL, FA-AAS, ICP-MS/AES, fluid inclusion petrography and microthermometry, and sulfur isotope analysis. The study area is composed of three diorite porphyries, i.e., fine-grained, medium-grained, and coarse-grained diorite porphyry, respectively. The intrusions are calc-alkaline with a high Sr/Y value, which is similar to many ore-bearing intrusions in the eastern Sunda arc. Ore mineralization occurs in quartz veins and veinlet stockwork, which is centered in the potassic zone associated with the fine-grained diorite porphyry. Ore minerals are dominantly chalcopyrite and bornite occurring in A- and B-veins, which cut earlier barren (EB)- and M-veins. Bulk rock analysis showed that the mineralization core has an average grade of 0.63 wt% Cu and 0.25 g/t Au, respectively. Outwardly, the potassic zone changes to the propylitic zone, which still bears copper in the quartz and pyrite veinlets, although the grade is low. Late chlorite-sericite and local argillic zones superimpose the earlier alteration zones. Late pyritic D-veins with a relatively lower copper content compared with the earlier veins. Fluid inclusion microthermometry revealed that the A- and B-veins in the potassic zone formed at 380-700 ºC by hypersaline fluids. The temperature temporally and spatially decreased, i.e., in the propylitic zone, the quartz veinlets show temperatures of 260-400 ºC, whereas the late chlorite-sericite zone formed at 190-320 ºC. Pressure calculation suggested that hypogene mineralization occurred at 1.1-1.2 km below the paleosurface, indicating a relatively shallow depth for a porphyry deposit. Nevertheless, the δ34S value of sulfides ranges from -2.0 to -0.1 ‰, inferring a magmatic origin. The Randu Kuning and Tasikmadu prospects share some similarities compared with other porphyry deposits worldwide, but it also reveals unique characteristics that differ from others, which implies certain ore-forming conditions. This may imply that the discoveries of new other porphyry prospects along eastern Sunda magmatic arc is still open.