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Giorgio Garuti, University of Leoben (Austria)
Joaquin Proenza, University of Barcelona (Spain)
Evgeny Pushkarev, Russian Academy of Sciences, Ural Branch (Russian Federation)
Ibrahim Uysal, Karadeniz Technical University (Turkey)
Federica Zaccarini, University of Leoben (Austria)
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Since the first discovery of laurite enclosed in chromite grains of the Kokkinorotsos chrome mine of Cyprus a large number of studies have been carried out indicating that ophiolitic chromitites contain a great variety of PGM that may become relevant petrogenetic indicators for their chromitite host. Ophiolitic chromitites from the Urals, the eastern Mediterranean Tethys and the Caribbean orogenic belts, exemplify the formation of chromite-PGM mineralization in different ages and geodynamic settings. The PGM population comprises two genetically different categories: the "primary" PGM, crystallized at high temperatures, and the "secondary" PGM, formed at relatively low temperatures as a result of PGE mobilization by the action of aqueous fluids. Both morphology and composition of the PGM grains provide a key to their genetic identification. Primary PGM are usually found as inclusions in fresh chromite. Chromitites in the deep mantle section of the ophiolites are dominated by Os-Ir-Ru alloys, laurite-erlichmanite and base-metal-Ir-Rh sulfides. These minerals crystallized at magmatic temperatures indicating a generally higher fS2 for chromitites from the Urals. The Os-Ir-Ru sulfarsenides and arsenides are particularly abundant in the chromitites of the Urals and locally in those of Tethyan and Caribbean ophiolites. Their appearance marks the increase of As activity in the melt, or indicates income of As-rich hydrothermal fluids at some post-magmatic stage. Major Pt-phases are sperrylite and Pt-Fe alloys. They occur in chromitites of the upper mantle section of Tethyan and Caribbean ophiolites (Albanides, Pindos, Loma Peguera, Potosi), and in chromitites of the transition zone (Troodos) or higher in the stratigraphy of supra-Moho cumulates (Nurali). Secondary PGM are usually located in altered gangue minerals (serpentine, chlorite, talc, Fe-hydroxides) or along cracks and fractures of chromite. They generally occur in chromitites which have undergone intensive oceanic metamorphism or long time exposure to supergene alteration in tropical climate. Secondary Ru-rich alloys containing substantial Fe and Ni have been described from altered chromitites all ophiolitic belts, and are believed to have derived from desulfuration of primary laurite under hydrothermal conditions. Laurite with almost pure RuS2 composition has been occasionally reported as a secondary PGM. Some chromitites contain secondary Pd-Cu alloys, Pd-Pb and Pd-Hg compounds crystallised from hydrothermal solutions. The occurrence of Ru-Ir-Fe oxides has been recently reported from several chromitites of all orogenic belts (Nurali, Ray-Iz, Vourinos, Othrys, Mugla, Loma Peguera), although they have not been characterised as new mineral species and still remain unnamed. These oxides are typical secondary PGM formed by weathering in the supergene environment. Textural relations suggest that they may originate by oxidation of a primary sulfide or alloy precursors.
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