Geochemistry and platinum-group mineralogy of chromitites and the limonite zone from the Santa Cruz Ni-laterite deposit, Acoje, Philippines

The Santa Cruz Ni-laterite deposit, developed over the upper mantle section of the Acoje Ophiolite Block, provides a natural setting to investigate platinum-group element (PGE) and platinum-group mineral (PGM) behavior during serpentinization and lateritization of chromitites and their host peridotites. We studied dunite-hosted chromitites, laterite-hosted chromitites, and samples from the limonite zone. Despite hydrothermal and supergene overprints, all chromitites retain a magmatic signature typical of high-Cr ophiolitic chromitites (Cr# > 0.6). Trace-element patterns and calculated parental melts confirm crystallization from boninite-like melts during subduction initiation in a fore-arc setting. Laterite-hosted chromitites are Ir-group PGE (IPGE: Os, Ir, Ru)-enriched and contain Ru-Os-Ir-rich PGM enclosed within, or along the margins of, chromite, recording magmatic precipitation before or during chromite crystallization. In contrast, dunite-hosted chromitites are Pd-group PGE (PPGE: Rh, Pt, Pd)-enriched and contain Pt-Pd-rich PGM associated with interstitial base-metal sulfides, reflecting immiscible sulfide melt(s) segregation after chromite crystallization. These observations suggest that dunite-hosted chromitites are not the bedrock source of laterite-hosted chromitites; rather, both are distinct occurrences that crystallized at different stages during boninitic-like melt evolution. Serpentinization locally modified exposed PGM and sulfides and promoted secondary PGM formation. Lateritization produced a positive Pt anomaly observed in the chondrite-normalized PGE patterns of the limonite-hosted chromitite and its limonitic host matrix, absent from the saprolite-hosted chromitite, and comparable to that observed in the limonite-hosted chromitites from the Falcondo Ni-laterite, Dominican Republic. Limonite samples show PPGE enrichment and contain up to 431 ppb total PGE. These features are interpreted to reflect residual accumulation of resistant Pt-bearing phases, although supergene PGM neoformation may have contributed to PGE enrichment.