並列タイトル等インドネシア，パプアにおける深部グラズベルグ斑岩システムの熱水変質および流体の進化
一般注記The giant Grasberg Cu-Au-(Mo) deposit is one of the world’s largest porphyry Cudeposit located in Indonesia, which hosts 32 million metric tons of copper and 96 millionounces of gold. The deposit is contained in two overlapping porphyry systems: GajahTidur Cu-Mo-(Au) and Main Grasberg Cu-Au. The Grasberg Block Cave reserve that liesbeneath the Grasberg open pit has recently been associated with the Gajah Tidur (GT)porphyry that is older than the shallower Main Grasberg (MG) porphyry system. Thisstudy is intended to confirm and delineate the existence of GT porphyry in the GrasbergCu-Au-(Mo) deposit, decipher the hydrothermal fluid evolution, and optimize the wholerock geochemistry as well as chemical variation of GT porphyry white micas to develop exploration vectoring tools for the deep-seated porphyry Cu deposits.Dating of hydrothermal biotite and muscovite by K-Ar method revealed that the GTporphyry system is ~300 k.y. older than the shallower MG porphyry system, whichformed at 3.1 ± 0.05 Ma. The age-depth relationships between the GT and MG porphyrysystems suggest there was an accumulation of volcanic rocks in between the two porphyry events, associated with growth of a ~0.5 to 1 km high stratovolcano prior the formation of MG porphyry system. The configuration of alteration patterns and associated mineralization, supported by hydrothermal biotite and muscovite ages, indicates that all white mica assemblages and more than 70% of Cu, Mo, and Au contained in the Grasberg Block Cave reserve are associated with the GT porphyry system.Sulfur isotope study shows that the evolution of GT hydrothermal fluid is initiatedby the formation of K-feldspar-biotite at a temperature as high as 590 °C. Cooling of thisfluid from ~460° to ~370 °C allowed the formation of white mica-dominated alteration,which is associated with various amounts of anhydrite, chlorite, chalcopyrite, and pyrite.Based on the anhydrite and chlorite abundances, this white mica assemblage can be subdivided into muscovite-anhydrite-chlorite and muscovite-chlorite-anhydrite, both ofwhich contain remnant hydrothermal biotite. The muscovite-quartz ± pyrophylliteassemblage associated with covellite-pyrite mineralization formed in the outer part of the muscovite-anhydrite-chlorite alteration zone at a lower temperature range (340-280 °C).Based on mineral chemistry and infrared spectroscopy analysis, the muscoviteanhydrite-chlorite white micas are characterized by high Na, Fe, Ti, and V concentrations,and mainly display short-wave infrared Al-OH absorption wavelengths of 2,203-2,208nm. The muscovite-chlorite-anhydrite white micas have distinctly higher Mg content than the other two GT white mica assemblages but similar Al-OH absorption wavelengths to the muscovite-anhydrite-chlorite white micas. The muscovite-quartz ± pyrophyllite white micas have low Na, Fe, Mg, and Ti, but relatively high Si, Al, and F, and Al-OH absorption wavelengths are largely shorter than 2,202 nm. The amounts of Si and Al contained in the GT white micas allow occupancy of other cations, particularly Fe and Mg in octahedral lattice sites and cause variations in their short-wave infrared Al-OH absorption wavelengths.Three decades of exploration and geologic studies of the giant Grasberg Cu-Au-(Mo)deposit provide information that is relevant to porphyry exploration globally. Theexistence of GT porphyry below the shallower MG porphyry system reveals that there ispotential for nearby porphyry deposits at depth beneath the already discovered porphyry Cu districts. Taking an example from the deep-seated GT porphyry system, the wholerock geochemical anomaly of F, V, W, and Sn, and the white mica short-wave infrared Al-OH wavelength lie in between ~2,200 and ~2,215 nm can be integrated with other geological and geophysical information to locate the centre of porphyry Cu systems.
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