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Garnierites and garnierites: textures, mineralogy and geochemistry of garnierites in the Falcondo Ni-laterite deposit, Dominican Republic.

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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% TiO 2) and Fe 3+ (average value: 7.82 wt% Fe 2 O 3). 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, Ir and Pt-rich phases, including laurite, irarsite, unknown Ru 3 As 2 , 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 Al 2 O 3 and TiO 2 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.

AB ST R ACT : New chemical and structural data on sepiolite-falcondoite in garnierite veins from the Falcondo Ni-laterite deposits, central Dominican Republic, are reported. Samples of Ni-sepiolitefalcondoite vary in colour from whitish green to green depending on the NiO content (wt.%) and the amount of silica present. The texture is normally schistose and friable but samples with considerable quartz and/or amorphous silica are compact and hard. Back-scattered electron images indicate that the samples are composed of at least three generations of Ni-sepiolite-falcondoite. The extreme refined cell parameters for Ni-sepiolite-falcondoite vary from 13.400(2), 27.006(4), 5.273(1) Å to 13.340(3), 27.001(6), 5.267(1) Å (space group Pncn). As the Ni content increases there is a small reduction in the a parameter. Chemical compositions determined by electron probe microanalysis cover a large interval of the Ni-sepiolite-falcondoite solid solution (Fal 3 and Fal 77 ). Individual samples show a considerable range in composition with the widest range determined in one sample from 4.63 to 22.40 wt.% NiO.