Chromian spinel composition and the platinum-group minerals of the PGE-rich Loma Peguera chromitites, Loma Caribe Peridotite, Dominican Republic

The Loma Peguera chromitites are located in the central part of Loma Caribe peridotite in the Cordillera Central of Dominican Republic. The chromitite bodies are hosted in typical mantle peridotites, have small size (less than a few meters across) and show massive textures. Primary chromite composition is Cr-rich (0.74 < Cr# < 0.78) and exhibits systematically high Ti (average value: 0.84 wt% TiO2) and Fe3+ (average value: 7.82 wt% Fe2O3). The total PGE concentrations vary from 1.82 to 2.04 ppm, and show an enrichment in Os + Ir + Ru relative to Rh + Pd + Pt. Among the latter group, a positive anomaly of Pt (249-269 ppb) is present as a result of the appearance of Pt-bearing alloys. The PGM assemblage comprises Ru, Os, It and Pt-rich phases, including laurite, irarsite, unknown Ru3As2, Ru-Os-Ir-Fe oxides, unknown Pt-Ir-Ni-Fe and Ru-Os-Ir-Pt-Fe-Ni alloys, native Ru and Ru-rich Ni-Fe alloy. The Loma Peguera chromitites are rather unusual in chemical composition compared with those commonly occurring in the Moho transition zone or from mantle harzburgites in ophiolites, but bear some similarity to chromitites from Uralian-Alaska-type complex. The Al2O3 and TiO2 contents of the calculated composition of the melt in equilibrium with the Loma Peguera chromitites are similar to those from island-arc magmas and to some oceanic plateau basalts.