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 theirhost 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 also have contributed to PGE enrichment.
Mechanisms governing hydrothermal tungsten precipitation, including cooling, boiling, fluid mixing and fluid-rock interaction, represent longstanding controversial issues in economic geology. Herein, six genetically distinct titanite types from the Xiaoyao skarn-type tungsten deposit were investigated to reconstruct the complete magmatic-hydrothermal evolutionary history of the deposit. These types include primary magmatic titanite (Ttn-M) in fresh granodiorite; altered granitic titanite (Ttn-A) in altered granodiorite; as well as prograde skarn titanite (Ttn-P), early oxide-stage titanite (Ttn-O1), late oxide-stage titanite (Ttn-O2), and carbonate–sulfide-stage titanite (Ttn-C) hosted in skarn. In situ EPMA and LA-ICP-MS analyses reveal systematic compositional variations across all titanite populations. Ttn-M exhibits elevated W concentrations compared with that of W-barren granites, indicative of early pre-enrichment in the parental magma before hydrothermal exsolution. Ttn-A, Ttn-P and Ttn-O1 feature extremely high W, reflecting robust tungsten migration from residual magma into hydrothermal fluids and subsequent transport toward skarn. Abrupt W depletion in Ttn-O2 corresponds to large-scale scheelite precipitation. Consistent Y/Ho and Sr compositions, together with variable REE patterns, strongly favor fluid boiling—rather than fluid mixing, simple cooling, or fluid-rock reaction—as the dominant ore-forming trigger. This process elevates fluid pH and promotes the depolymerization of aqueous polytungstate species, thereby facilitating intensive scheelite precipitation. Titanite geochemical data also record a low-F ore-forming system, which inhibits magmatic differentiation and restricts the total tungsten endowment of the deposit. These findings demonstrate that multistage titanite geochemistry profiling can effectively trace tungsten migration pathways and precisely pinpoint the key trigger mechanisms for tungsten precipitation in skarn systems.
