The presence of PGM associated with the podiform chromitites in the Jurassic–Cretaceous ophiolite of Sebuku Island (South Kalimantan, Indonesia) is reported for the first time. Two types of chromitite have been recognized; one with high-Cr composition (Cr/(Cr + Al) > 0.7) occurs in the deep mantle, the other, high-Al (Cr/(Cr + Al) < 0.6), is located close to the Moho transition zone. The TiO2-Al2O3 relations indicate affinity to IAT and MORB, for the high-Cr and high-Al chromitites, respectively. However, both are believed to have formed by mantle/melt reaction and differentiation of a magma characterized by an initial IAT composition related to an SSZ. Primary magmatic inclusions (<10 μm) of laurite characterized by Ru/Os chondritic ratio are the only PGM found in the high-Cr chromitites, indicating crystallization from undifferentiated magma, at low fS2 in the mantle. In contrast, the high-Al to chondrite, suggesting the increase of fS2 in the evolved melt. Besides laurite, the high-Al chromitite contains a complex assemblage of secondary PGM (Pt-Fe, garutiite, iridium, ruthenium–magnetite aggregates, zaccariniite and unnamed Ru and Mn oxides). These secondary PGM have an irregular shape and occur exclusively in the chlorite matrix sometimes associated with Mn-Ni-Fe-Cr hydroxides. They are interpreted to have formed by desulfuration of primary interstitial PGM sulfides or to have precipitated from secondary fluids during low T alteration. The relative abundance of PPGE in the high-Al chromitite is interpreted as a result of PGE fractionation during differentiation of the parent melt of the chromitites.

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