Umeå University's logo

The origin of platinum-group minerals (PGM) in the supergene environment remains debated, with contrasting models invoking (i) mechanical liberation of hypogene PGM during weathering versus (ii) in-situ neoformation of PGM. We address this debate using enigmatic Pt-Ir-Fe-Ni grains hosted in a chromitite body enclosed in limonite and provide new insights into their origin based on their morphologies, textures, structures, compositions, and mineralogical associations. The grains exhibit porous textures produced by accumulations of Pt-Ir-Fe-Ni nanoparticles arranged in aligned nanostructures within a goethite matrix. The aligned nanostructures occur in distinct bundles with three main orientations, forming previously not described stitching textures i.e. an interlacing of bundles, defining parallel and triangular patterns. The alloys are cubic, exhibit a preferential stoichiometry of (Pt, Ir)0.26Ni0.29Fe0.45 and are absent in pristine chromitites. In order to find pros and cons for their genetic history we compare our observations with globally occurring PGE alloys from magmatic, hydrothermal-related, and supergene environments. Moreover, we discuss the textural similarities with other naturally occurring materials such as martitization products and, without genetic implications, Fe–Ni meteorites showing Widmannstätten patterns. This contribution does not provide a final conclusion on how Pt-Ir-Fe-Ni alloys found in the limonite of a Ni-laterite form. Instead, we present high-resolution observations that can be used to speculate different scenarios for their possibly multi-stage genesis.