MINERALOGY AND ORIGIN OF COPPERGOLD BEARING SKARN WITHIN THE BATU HIJAU PORPHYRY DEPOSIT, SUMBAWA ISLAND, INDONESIA

Abstract

The aim of this study ist o emphasize on the origin of copper-gold bearing skarn mineralization at the Batu Hijau deposit which is located at the southwestern corner of Sumbawa Island, Indonesia. Although most skarn are derived from limestones, nolimestone is known in the Batu Hijau deposit. Ca-rich andesitic volcaniclastic host rocks favor skarn alteration within the Batu Hijau deposit. The t ype of skarn can be classified as calcic-exoskarn, and locally controlled by faults and fractures. Two major stages consisting of four sub-stages of skarn forming processes can be divided by the mineral assemblages of skarn as prograde and retrograde stages. The prograde skarn consists of clinopyroxene and garnet ± magnetite formed at the trapping temperature of 440°-480 °C with 34-38 wt% NaCl eq. while retrograde skarn alteration is dominated by Fe-rich minerals such as amphibole and epidote formed at the trapping temperature down to 340°-360°C with 4-8 wt% NaCl eq. Opaque minerals include chalcopyrite, pyrite, sphalertite, and minor galena and bismuth- telluride. Gold was precipitated in the retrograde stage associated with bismuthtelluride minerals. The sulfur isotope data of skarn ranges from +0.1 to +1.7‰ (sulfide), and porphyry systems range from 0.04 to1.4‰ and 10‰ to 15‰ (sulfide and sulfate respectively). According to the fluid inclusion and sulfur isotope data, the origin of skarn and porphyry system can be suggested to be that the magmatic origin. Furthermore, the sulfur isotope data of the deposit evidently shows that a porphyry- related skarn mineralization exhibiting transition from one style to the next can be relatively rapid. The result of this research has indicated that the range of porphyry-related deposits, skarn and porphyry systems can form during a single prolonged hydrothermal event.