This thesis investigates the behavior of platinum-group elements (PGE) and the mineralogical evolution of platinum-group minerals (PGM) from hypogene to supergene environments, with particular emphasis on whether PGM can form in-situ during lateritization. The origin of PGM in supergene environments remains debated, with contrasting models involving i) the inheritance and mechanical liberation of hypogene PGM during weathering, versus ii) their in-situ formation following PGE mobilization and reprecipitation. At the Falcondo Ni-laterite deposit (Dominican Republic), Pt-Ir-Fe-Ni grains found in a highly weathered chromitite hosted in limonite and characterized by a positive Pt anomaly in chondrite-normalized PGE patterns have previously been interpreted as possible products of neoformation in the lateritic soil. These grains show complex porous textures and occur as accumulations of Pt-Ir-Fe-Ni nanoparticles within Fe-oxyhydroxides. Their occurrence in the upper part of the lateritic profile and their textural relationships with Fe-oxyhydroxides have been used to support PGE mobilization and in-situ PGM growth during lateritization. However, alternative interpretations involving alteration of pre-existing PGM, dissolution, short-distance remobilization, and reprecipitation have also been proposed. Hence, the origin of the Pt-Ir-Fe-Ni alloys remains unresolved.

To investigate this problem, three case studies representing different stages in the evolution of PGM from hypogene to supergene environments were examined: the PGE-rich chromitites of the Herbeira Massif in the Cabo Ortegal Complex (Spain), the Santa Cruz Ni-laterite deposit in Acoje (Philippines), and the Falcondo Ni-laterite deposit. The Herbeira chromitites provide a hypogene reference for the study of primary magmatic PGE concentration in ultramafic rocks and subsequent PGM modification by hydrothermal-related processes, whereas investigation of the Ni-laterites from Santa Cruz and Falcondo allows the study of the behavior of PGE and PGM during intense tropical weathering of ultramafic rocks.

At Herbeira, the PGE-rich chromitites are characterized by enrichment in Pd-group PGE (PPGE: Rh, Pt, and Pd) relative to Ir-group PGE (IPGE: Os, Ir, and Ru) and abundant PGM dominated by Rh-Ir-Pt-bearing arsenides and sulfarsenides, Pt-Ir-Pd-base-metal-bearing alloys, and Pt-Pd-bearing sulfides, commonly associated with base-metal sulfides. Their formation is related to magmatic processes, whereas subsequent post-magmatic processes related to the circulation of hydrated fluids during amphibolite-facies metamorphism that affected these rocks, promoted local PGE mobilization and the alteration of primary PGM into secondary PGM phases. This modification produced porous Pt-bearing alloys and complex PGM textures composed of PGE- and base-metal-bearing nanoparticles, interpreted to result from the mobilization, precipitation, and reorganization of PGE during interaction with hydrothermal fluids. These observations show that porous and complex PGM textures can also develop through hypogene processes. 

In the Santa Cruz Ni-laterite, the PGE and PGM assemblages found in chromitites record the progressive effects of magmatic, serpentinization-related, and lateritic processes. The primary PGM assemblage reflects a magmatic history broadly comparable to that recorded by the Herbeira chromitites. However, during lateritization, resistant PGM phases were preserved and residually concentrated at the upper part of the lateritic profile. This is accompanied by redistribution of PGE within the lateritic profile, including a positive Pt anomaly in chondrite-normalized PGE patterns observed in a limonite-hosted chromitite, interpreted mainly as the result of residual Pt enrichment rather than direct evidence of supergene PGM neoformation. However, the occurrence of a porous Ir-Fe-Ni-Pt grain enclosed in Fe-oxyhydroxides and characterized by the accumulations of PGE-bearing nanoparticles, show textural similarities to the Pt-Ir-Fe-Ni grains from Falcondo, suggesting that comparable processes, including neoformation (?), may have been involved in the genesis of these grains.

High-resolution observations of the Pt-Ir-Fe-Ni alloys from Falcondo reveal porous textures characterized by aligned nanostructures within a goethite/hematite matrix. These nanostructures occur in bundles with different orientations and form previously undescribed stitching textures, defining parallel and triangular patterns. The alloys exhibit a preferential stoichiometry of (Pt, Ir)_0.26Ni_0.29Fe_0.45 and are absent in pristine chromitites. The grains also contain elongated voids and Pt- and Ir-rich patches. Nanoscale observations show that the grains are compositionally heterogeneous polycrystalline aggregates composed of distinct Fe-Ni-rich, Ir-rich, and Pt-Ir-rich alloy domains, with d-spacings similar to cubic phases. Pt nanoparticles occur within goethite and hematite filling pores, suggesting late-stage Pt mobilization and precipitation in the lateritic environment. To evaluate the different possibilities for their genetic history, these observations were compared with PGM described in the literature from magmatic, hydrothermal-related, and supergene environments. Moreover, the unusual textures of the Pt-Ir-Fe-Ni alloys show textural similarities with Widmannstätten patterns found in Fe-Ni meteorites, without implying any genetic relationship. High-resolution textural and compositional observations of the Pt-Ir-Fe-Ni grains do not provide a definitive conclusion on their genesis but show that they cannot be readily explained by a single genetic process and instead suggest a complex, potentially multistage history involving hypogene precursors and subsequent supergene modification leading to PGM (trans-)formation. High-resolution isotopic and trace-element analyses may provide further constraints on the origin of the Pt-Ir-Fe-Ni alloys and help identify possible biogeochemical fingerprints associated with their formation.