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Ni-laterites from the Dominican Republic host rare but extremely platinum-group element (PGE)-rich chromitites (up to 17.5 ppm) without economic significance. These chromitites occur either included in saprolite (beneath the Mg discontinuity) or as 'floating chromitites' within limonite (above the Mg discontinuity). Both chromitite types have similar iridium-group PGE (IPGE)-enriched chondrite normalized patterns; however, chromitites included in limonite show a pronounced positive Pt anomaly. Investigation of heavy mineral concentrates, obtained via hydroseparation techniques, led to the discovery of multistage PGE grains: (i) Os-Ru-Fe-(Ir) grains of porous appearance are overgrown by (ii) Ni-Fe-Ir and Ir-Fe-Ni-(Pt) phases which are overgrown by (iii) Pt-Ir-Fe-Ni mineral phases. Whereas Irdominated overgrowths prevail in chromitites from the saprolite, Pt-dominated overgrowths are observed within floating chromitites. The following formation model for multistage PGE grains is discussed: (i) hypogene platinum-group minerals (PGM) (e.g. laurite) are transformed to secondary PGM by desulphurization during serpentinization; (ii) at the stages of serpentinization and/or at the early stages of lateritization, Ir is mobilized and recrystallizes on porous surfaces of secondary PGM (serving as a natural catalyst) and (iii) at the late stages of lateritization, biogenic mediated neoformation (and accumulation) of Pt-Ir-Fe-Ni nanoparticles occurs. The evidence presented in this work demonstrates that in situ growth of Pt-Ir-Fe-Ni alloy nuggets of isometric symmetry is possible within Nilaterites from the Dominican Republic.

Bauxite deposits, traditionally the main source of aluminum, have been recently targeted for their remarkable contents in rare earth elements (REE). With ƩREE (lanthanoids + Sc + Y) concentrations systematically higher than ~ 1400 ppm (av. = 1530 ppm), the Las Mercedes karstic bauxites in the Dominican Republic rank as one of the REE-richest deposits of its style.The bauxitic ore in the Las Mercedes deposit is mostly unlithified and has a homogeneous-massive lithostructure, with only local cross-stratification and graded bedding. The dominant arenito-roundgrained texture is composed of bauxite particles and subordinate ooids, pisoids and carbonate clasts. Mineralogically, the bauxite ore is composed mostly of gibbsite and lesser amounts of kaolinite, hematite, boehmite, anatase, goethite, chromian spinel and zircon. Identified REE-minerals include cerianite and monazite-Ce, whose composition accounts for the steady enrichment in lightrelative to medium-and heavy-REE of the studied bauxites.Considering the paleo-geomorphology of the study area, we propose that bauxites in the Las Mercedes deposit are the product of the erosion and deposition of lithified bauxites located at higher elevations in the Bahoruco ranges. Based on the available data, we suggest a mixed lithological source for the bauxite deposits at the district scale: bedrock carbonates and an igneous source of likely mafic composition.