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Federica Zaccarini, University of Leoben (Austria)
Joaquin Proenza, University of Barcelona (Spain)
Nikolay Rudashevsky, Center for New Technologies (Russian Federation)
Louis Cabri, Cabri Consulting Inc. (Canada)
Giorgio Garuti, University of Leoben (Austria)
Vladimir Rudashevsky, Center for New Technologies (Russian Federation)
Joan Carles Melgarejo, University of Barcelona (Spain)
John Lewis, The George Washington University (United States)
Ronald Bakker, University of Leoben (Austria)
Longo Francisco , Falcondo XStrata Nickel (Dominican Republic)
Devaux Edwin, Falcondo XStrata Nickel (Dominican Republic)
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Platinum group element (PGE) rich chromitites (up to more than 2 ppm total PGE) from the Loma Peguera ophiolite (Dominican Republic) have been investigated using the innovative electric pulse disaggregation (EPD) and hydroseparation (HS) techniques with the aim of determining the mineralogical residence of the noble metals. These two techniques liberate the minerals of interest from their host rock and then concentrate the heavy minerals, including Platinum group minerals (PGM). The PGM have been analyzed by electron microprobe and by Raman spectroscopy. Confirming the results previously obtained using the traditional method of an in situ investigation, specific phases of all the PGE, except Pd, have been found. However, the concentrates provide a more representative assessment of PGM and more than 400 grains, ranging between 20 and 120 m in size, have been found. Excluding some grains of tetraferroplatinum and nickelferroplatinum, the majority of the discovered PGM, on the basis of their chemical composition, potentially represent new mineral species. Most of the new PGM (about 80%) consist of Ru,Os,Ir,Fe with composition similar to other Ru,Os,Ir,Fe oxygenated compounds reported from other ophiolitic chromitites. They also display similar optical properties, being white yellowish in color, less reflectant than other PGM and strongly anisotropic. Most of these PGM form single phase grains and show a very irregular shape. Based on the WDS spectra, some of the Ru,Os,Ir,Fe phases proved to contain oxygen, whereas others also contain Si and Mg, suggesting that some of them represent a mixture of PGM and silicate, possibly serpentine. The following other potentially new PGM have been also identified: Ir(Fe,Ni)3, (Ir,Pt)(Fe,Ni), (Ru,Pt)(Fe,Ni), (Fe,Ni,Ru,Os,Co)2S, and RhNiAs. With few exceptions, the PGE alloys and (Fe,Ni,Ru,Os,Co)2S form single phase grains. However, some of them are zoned, showing a rim enriched in oxygen. They are white in color and possibly isotropic. RhNiAs is yellowish and displays a strong anisotropy, from orange to blue green and always occurs with grains of Ir bearing alloys and (Fe,Ni,Ru,Os,Co)2S. RhNiAs and (Fe,Ni,Ru,Os,Co)2S display a very characteristic Raman spectrum. In contrast, the Raman spectra of the PGE alloys are flat. These different responses to Raman spectroscopy suggest the presence of a covalent and metallic bond in the As S bearing PGM and PGE alloys, respectively. One disadvantage of an investigation of EPD concentrates is that some in situ textural information is lost. However, on the basis of their morphology and some attached associated minerals, the presence of oxygen, chromite, and serpentine, we can argue that most of the discovered PGM are secondary in origin, i.e. formed at low temperature during some post magmatic processes.
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