Mineralogical characteristics of a Fe-Ni-Co-Cu sulphide mineralization in Sebuku Island, Indonesia.

Critical raw materials (e.g., Co, Ni, Pt, Sc…) are very important resources for high technology applications (e.g., renewable energies, modern electronic devices, e-mobility, medical). Most of these minor and trace elements are produced as by-products of base metal mining. Magmatic Ni-Cu sulphide ore deposits are of great economic relevance worldwide. The BMBF-funded project “StratOre” (FKZ 033R278) aims to study the high-tech element potential of newly found sulphide deposits in ophiolite sequences in Indonesia, a country, which is well known for its large number of mineral deposits. Here we present the first results of our study addressing the nature of the Sebuku Island Fe-Ni-Co-Cu sulphide ore and the distribution of base and precious metals in different minerals.

Sebuku Island (SE-Kalimantan) is part of the Cretaceous accretionary Meratus collision complex and represents an obducted ophiolite at the convergent boundary of Sundaland in the NW and the compressing Paternoster microcontinent in the SE. This ophiolite complex consists of mafic-ultramafic layers and rocks of the upper mantle of Permian to Jurassic age. Cretaceous intrusions, meta-sediments and mafic-intermediate intrusions are overlain by Tertiary and Quaternary sediments. Serpentinization started prior to obduction and became affected by cretaceous magmatism and alteration. One of the Early Cretaceous intrusions is supposed to represent a mantle wedge and is marked by high-temperature serpentine (Sulistyohariyanto and Soesilo, 2017, Cahyadi et al., 2017, Imani et al., 2020).

Massive Fe-Ni laterite (mined since 2000 by PT SILO) is overlying a recently observed Fe-Ni-Co-Cu sulphide mineralization, which is hosted by high-temperature serpentinite in the cumulus zone of the ultramafic ophiolite complex. Our sample material is taken from 14 drill cores within depths of 22 m to 109 m. The studied mineralization occurs as massive, erratic and disseminated sulphides. Formation of the ore body is probably resulted from subsequent magmatic and hydrothermal processes. The base metal sulphide-ore mineralogy is represented mainly by pentlandite, pyrrhotite, pyrite, chalcopyrite and cobaltite; a large number of further sulphides and arsenides are observed as well. We highlight the extraordinary enrichment of Co in Fe-S, Fe-Ni-S and Ni-S minerals, as well as the extraordinary enrichment of Ni in pyrrhotite and pyrite. Platinum-group elements are not enriched in any of the ore types.