Proximal to distal geochemical variations of the Ruwai polymetallic skarn deposit, Central Borneo, Indonesia: Insights from sulfides chemistry and implications to exploration

Abstract

The Ruwai deposit is the largest polymetallic skarn deposit in Borneo and is located within the Schwaner Mountains Complex. The mineralization is hosted by metalimestone of the Jurassic Ketapang Complex and the causative intrusions belong to the Cretaceous Sukadana Granitoids. Despite its large resources, the geochemical characteristics of the Ruwai skarn are still not well known. Thus, this study provides the whole-rock geochemistry of the skarn, metalimestone and intrusions, as well as the chemical compositions of sulfides (i.e., pyrite, sphalerite, galena, and chalcopyrite). The occurrence of skarn mineralization can be divided into proximal (Gojo-Karim) and distal (Central Gossan-Southwest Gossan) zones. Furthermore, the mineralization stage can be divided into prograde, retrograde and supergene stages. The whole-rock geochemical analyses clearly show that the skarn is enriched in several metals such as Ag, Nb, Co, La, Cu, V, Zr and Ti, which are higher in intrusions compared to metalimestone, which implies that these metals are mostly derived from magma. On the other hand, the skarn is also enriched in As, Pb, and Se, elements that are higher in metalimestone than in the intrusions. Thus, it can be inferred that metalimestone provided significant amounts of these metals to the mineralizing fluid. The pyrite analyses show that Cu, Zn, Bi and Ag increase towards the distal zone, while Fe, Mn and Bi in sphalerite also increase towards the distal zones Copper and Sb in galena and Ag in chalcopyrite collected from the distal zone increase from Central Gossan to Southwest Gossan zones while Bi in both minerals decrease. These trends of trace metal contents in sulfides can be utilized as a vectoring tool to delineate the center of the hydrothermal system.