Petrology and Fluid Inclusion Study of W+Sb±Au Quartz Vein Mineralization in Sebuku Island, South Kalimantan, Indonesia

Abstract

Sebuku Island in South Kalimantan province has hosted one of the largest Fe-Ni laterite deposits in Indonesia. Surface mapping has discovered new ore deposit types, notably quartz-bearing W+Sb±Au ore veins in the northern Sebuku. This study aims to characterize this newly-discovered deposit type and understand its origin. We present geological mapping, petrographic and ore microscopic observations, and data from XRD, bulk-ore major and trace element (XRF and ICPMS), as well as fluid inclusion microthermometric analyses. The results show that the host rocks are composed of metasandstone and metapelitic rocks. The quartz veins are associated with narrow alteration zones, comprising silicic, sericite and argillic types, which are strongly controlled by NE-trending dextral and SE-trending sinistral strike-slip faults. The veins trend approximately N120oE, and have massive, brecciated, drusy, comb and bladed textures. Ore minerals comprise mainly wolframite, stibnite, kermesite, tripuhyite, and minor arsenopyrite, pyrite and Fe-Ti oxides. The ores contain up to 958 ppm W and 1,220 ppm Sb, with wolframite and stibnite being the main ore minerals. Medium homogenization temperatures (Th) were found for the fluid inclusions, ranging from 238 to >350°C, which correspond to moderate fluid salinity of 1.4 to 5.4 wt.% NaCl eq. Based on those characteristics, the W+Sb±Au mineralization in Sebuku is similar to a mesothermal type