The mineralogy of platinum group elements in ophiolite peridotite, New Caledonia
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Peridotite
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The article considers a chemical variation of accessory and ore-forming chrome spinels from the Kraka ultramafic massif at the different scales, from the deposit to the thin section. A correlation analysis of compositional and structural features of ultramafic rocks and ores was performed. The ultramafic rocks and chromitites in the studied massif show the distinct deformation structures and tectonite olivine fabric. A typical chemical gap (i.e. Cr#=Cr/(Cr+Al)) was observed between peridotite, on the one hand, and dunite and chromitite, on the other hand, on the scale of deposits and ore-bearing zones. The location and size of this gap depend on the type of deposit. The gap becomes wider from the disseminated tabular bodies to the typical podiform ones. It has been found that in the thin initial dunite veinlets in peridotite the chrome spinels chemistry changes gradually and there is no Cr# gap between peridotite and dunite. The dunite venlets show a strong olivine fabric, which is an evidence of their high-temperature plastic flow origin. It has been revealed that new chrome spinel grains previously formed as rods or needles and then coarsened. We explained this observation as the result of impurity segregation, coalescence and spheroidization induced by the plastic deformation of olivine. It is inferred that a solid crystal flow is the main requirement for the dunite and chromitite body formation in the Kraka ophiolite massif. In the solid stream, the mineral phase separation takes place. For example, olivine and orthopyroxene grains of parental peridotite separate from one another, and weaker (more mobile) olivine grains form dunite bodies in which chromitite appears as a result of impurity segregation.
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The Khanozai Ophiolite is an important fragment of Zhob valley ophiolite belt. This contains thick serpentinized mantle peridotite which consists of ultramafic tectonite and transition zone. Ultramafic tectonite comprises of largely harzburgite with subordinate dunite and lherzolite whereas transition zone is dominantly made up of dunite with subordinate harzburgite. Peridotite contains thick chromitite bodies. The chromitite of Khanozai Ophiolite is found as pods, lenses and layered shapes. These chromitites bodies occur in massive, disseminated and nodular forms. The chromitite for Khanozai Ophiolite is classified as high-Cr chromitite based on Cr# (aver. 0.72). Chromite grains in chromitite are largely uniform in composition but few grains show ferrit-chromite alteration along veins and grain margins. High Cr#, low TiO2 and chromite composition of high-Cr indicated that high-Cr chromite ore deposits in the mantle portion have been crystallized from parental melt of boninitic composition. Geochemical composition of chromite spinels in chromitites and peridotite suggests that the chromitite and peridotite of Khanozai Ophiolite was formed in suprasubduction zone environment.
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This paper describes the presence of platinum-group minerals in the Vasarakangas chromitite from the 1.97 Ga Outokumpu ophiolite complex in eastern Finland. The platinum-group mineral assemblages as well as the compositions of laurite, irarsite and orarsite, and also the chondrite-normalized platinumgroup element patterns are compatible with those described in younger Mesozoic ophiolitic chromitites. In contrast to these younger chromitites, where Irgroup minerals occur mainly as inclusions in chromite, the platinum-group minerals at Vasarakangas are principally found as enclosed minor grains in euhedral matrix gersdorffites. The PGE distribution between separated chromite and gersdorffite fractions is in accordance with the observed exceptional mode of PGM occurrence.
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Abstract Voluminous platinum‐group mineral (PGM) inclusions including erlichmanite (Os,Ru)S 2 , laurite (Ru,Os)S 2 , and irarsite (Ir,Os,Ru,Rh)AsS, as well as native osmium Os(Ir) and inclusions of base metal sulphides (BMS), including millerite (NiS), heazlewoodite (Ni 3 S 2 ), covellite (CuS) and digenite (Cu 3 S 2 ), accompanied by native iron, have been identified in chromitites of the Zedang ophiolite, Tibet. The PGMs occur as both inclusions in magnesiochromite grains and as small interstitial granules between them; most are less than 10 μm in size and vary in shape from euhedral to anhedral. They occur either as single or composite (biphase or polyphase) grains composed solely of PGM, or PGM associated with silicate grains. Os‐, Ir‐, and Ru‐rich PGMs are the common species and Pt‐, Pd‐, and Rh‐rich varieties have not been identified. Sulfur fugacity and temperature appear to be the main factors that controlled the PGE mineralogy during crystallization of the host chromitite in the upper mantle. If the activity of chalcogenides (such as S, and As) is low, PGE clusters will remain suspended in the silicate melt until they can coalesce to form alloys. Under appropriate conditions of f S 2 and f O 2 , PGE alloys might react with the melt to form sulfides‐sulfarsenides. Thus, we suggest that the Os, Ir and Ru metallic clusters and alloys in the Zedang chromitites crystallized first under high temperature and low f S 2 , followed by crystallization of sulphides of the laurite‐erlichmanite, solid‐solution series as the magma cooled and f S 2 increased. The abundance of primary BMS in the chromitites suggests that f S 2 reached relatively high values during the final stages of magnesiochromite crystallization. The diversity of the PGE minerals, in combination with differences in the petrological characteristics of the magnesiochromites, suggest different degrees of partial melting, perhaps at different depths in the mantle. The estimated parental magma composition suggests formation in a suprasubduction zone environment, perhaps in a forearc.
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Abstract The transition-zone dunites in the Coto and Acoje blocks of the Zambales Ophiolite Complex are host to chromitites, nickel sulfides, and platinum-group minerals. The dunites are intercalated with harzburgites-lherzolites, layered ultramafic cumulate rocks, and layered gabbros. Chromitite pods are rimmed by dunite aureoles. Based on field and geochemical evidence, transition-zone dunites of the Zambales Ophiolite Complex were formed predominantly by crystallization processes. Limited partial melting and melt-mantle interaction can explain the formation and occurrence of the other dunite types, especially the aureoles around chromitites. The geophysical and petrological Moho in the Zambales Ophiolite Complex are located in two distinct horizons.
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Xenolith
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