P.T Conditions and Oxygen Fugacity of The Intrusion Emplacement at The Batu Hijau Porphyry Copper Gold Deposit, Sumbawa Island:A Constraint From Geothermobarometric Data

Abstract

Among the southeast Asian countries, Indonesia, nowadays, has a biggest resources and production of copper and gold. The majority of copper and gold resources and production is derived from porphyry copper-gold deposits, such as Grasberg in Irian Jaya and Batu Hijau in Sumbawa Island. To enhance an understanding on the genesis and characteristics of the porphyry deposits, it is crucial to estimate the physicochemical conditions, including temperature, pressure (depth) and oxygen fugacity (f02) of the ore-bearing intrusions during their emplacement and crystallisation. There are many ways to estimate the physicochemical conditions, such as isotope geochemistry, microthermometric analysis of fluid inclusion and geothermobarometric analysis. In this paper, the physicochemical conditions of the causative tonalite intrusions in the Batu Hijau deposit will be constrained from the geothermobarometric data, by applying the hornblende-plagioclase and magnetite-ilmenite thermometers and the Al-in-hornblende barometer. The thermobarometers indicate that the tonalite porphyries emplace at 764 ± 22°C and 1.5 ± 0.3 kbar and completely crystallised at â540°C. If the pressure is assumed to be lithostatic, it is interpreted that the hornblende and plagioclase phenocrysts crystallised at depths of about 5.5 km. Log f02 of the magma crystallisation ranges from -20 to -12 (between NNO and HEM-MAG buffers)