GEOLOGY, GEOCHEMISTRY AND FLUID INCLUSION STUDY OF THE BATURAPPE EPITHERMAL SILVER-BASE METAL PROSPECT, SOUTH SULAWESI, INDONESIA

The Baturappe epithermal silver-base metal prospect is situated in south of Sulawesi island, Indonesia. The prospect lies in the shoshonitic/alkaline southern arm of the Tertiary western Sulawesi plutono-volcanic arc. The Baturappe prospect is developed in the late Middle-Miocene Baturappe Volcanics which in the study area consists of respectively from the older to the younger: basaltic-andesitic lava, gabbroic-dioritic stock, and basaltic-andesitic dykes. Mineralizations (veins, sulphide stringer and disseminated sulphide) in the prospect mostly hosted in the basaltic-andesitic lava, including its volcanic breccia member. Two types of geological structure were developed in the prospect area, early radial volcanic structure and late tectonic-related northwest-southeast trend fault. The tectonic-related fault controlled the emplacement of significant epithermal base metal veins, whereas the radial structure mostly associated with thinner and barren quartz veins. The disseminated sulphide mineralization is controlled by the permeable pores of the volcanic breccia host. Petrochemical study revealed that the volcanics is potassic alkaline (shoshonitic) silica-undersaturated basaltic-andesitic in composition. The lava consists of trachy basalt, basaltic trachy-andesite, alkali basalt, tephrite, basanite, hawaiite, mugearite, and picrite. The composition of the volcanic-related intrusions generally correspond to those of their equivalent lava, mostly gabbroic, with a range from dominant alkali gabbro to less ultramafic, and andesite-trachy andesite to dacite. Behaviours of certain trace elements, particularly enrichments of Rb, Cs, Sr and LREE are consistent with the characteristics of the typical postcaldera K-rich volcanics. Interpretation of volcano-tectonic setting and magmatic evolution based on evaluation of major and trace elements suggested that the volcanics was generated in a within-plate (intraplate) environment, in extensional (rift) tectonic regime, which is not directly related to synchronous subduction event. The magma was originated from melting of previously metasomized upper mantle which is enriched in potassium and other incompatible elements. Hydrothermal alteration is zoned from distal to proximal: chlorite, epidotechlorite- calcite, and illite-quartz and quartz-carbonate zones. The chlorite zone is distributed at the periphery of the hydrothermal system. The epidote-chloritecalcite zone is a vein-related propylitic alteration which distributed in narrow zone around the veins, on the outer part of illite-quartz zone and quartz-carbonate zone. The illite-quartz zone (argillic zone) is developed within maximum 7 m from the structural-controlled veins at Baturappe and Bangkowa are