HRTEM-AEM-HAADF-STEM study of platinum-group elements within a mantle-derived Cr spinel (Lherz; North-Eastern Pyrenees, France)
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We investigate the origin of high platinum-group element (PGE) abundances associated with chromite-rich rocks by determining the relative partitioning of these elements between chromite- and sulfide-silicate liquids. Chromites were crystallized in the presence of immiscible sulfide and silicate melts in experiments at 1 GPa, producing a few, relatively large (20–50 μ ) crystals, which were analyzed by laser ablation ICP-MS. Our results show that the PGE inventory of chromite and silicate melt produced in experiments is dominated by sulfide and/or alloy micronuggets and that the intrinsic PGE content of these phases is low (sub-ppm), despite high concentrations in coexisting sulfide liquid (i.e., up to alloy saturation). Lower bounds on minimum sulfide-silicate melt PGE partition coefficients (D PGE ) calculated from this data are 0.4 to 10 × 10 4 , which are similar to values determined in previous studies, confirming the extreme fractionation of these elements into the sulfide phase. Rhenium, which was added to experiments in order to constrain Re-Os fractionation, is highly concentrated in sulfide liquid, present at low but uniform levels in silicate melt, and undetectable in chromite. Calculated sulfide-silicate melt D Re are 3.3 to 5.2 × 10 4 , and experiments yielded lower bounds for D Os /D Re of 3, indicating that sulfide-silicate melt equilibrium can fractionate Re from Os. Minimum sulfide melt-chromite partition coefficients are 1,000 or more, indicating that coexisting sulfide melt will be the dominant host for the PGE. Using this partitioning data, we have calculated the mass balance for Ir in chromite-sulfide mixtures and show that for rocks with greater than 200 ppm sulfur, less than 24 percent of the Ir will be in chromite, illustrating that chromite is not the significant PGE host even in low sulfur chromitites. In an experiment saturated in Ir-Re alloy, we measured a maximum iridium concentration in run-product chromites of 150 ppb, which, when combined with an estimate of the Ir activity in the coexisting alloy, yields a maximum Ir solubility of ∼210 ppb. We have found examples of chromitites with Ir contents exceeding this value, indicating that these samples have accumulated an additional PGE-bearing phase. Such results support the notion that interstitial sulfide liquid, or accessory minerals included at the magmatic stage (i.e., laurite, alloys), are most likely to be the dominant primary hosts for PGE in chromite-rich rocks.
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ABSTRACT The abundance of Ru in chromite has been suggested as an indicator of sulfide liquid saturation in komatiites. The komatiite magma-derived Archean Coobina intrusion is known to be barren in terms of sulfide mineralization. Therefore, the Coobina intrusion can serve as a useful case study to test the applicability of Ru abundance in chromite as a potential indicator for sulfide mineralization, as well as for better understanding the PGE-chromite association in general. The Coobina intrusion is a highly deformed layered intrusion interpreted to be a flared dike. It contains multiple massive chromitite seams that have been recently mined for metallurgical-grade chromite. In this study, 18 samples from chromitite seams throughout this intrusion are investigated for their whole-rock platinum group element (PGE) contents, which are compared to their chromite mineral chemistry (including PGE content), the platinum group mineral (PGM) mineralogy, and Re-Os isotope systematics. Each sample has a similar chromite major and minor element chemistry, but a unique trace element signature, even within the same seam. In general, there are higher concentrations of Ru (>300 ppb) within chromite in the southeast (toward the feeder dike) and lower concentrations (<50 ppb Ru) in the northwest. At a sample scale, Ru in the whole rock and Ru in solid solution in the chromite are inversely correlated, while Ir shows a positive correlation between the whole rock and chromite mineral chemistry, indicating differing partitioning behaviors within the iridium-group PGE (IPGE = Os, Ir, Ru). The inverse correlation between Ru in solid solution within chromite and Ru in whole-rock chromitite suggests that, for seams with high Ru in whole rock, Ru is occurring within separate PGM phases. This is supported by the observation that the samples with high whole-rock Ru also have a high number of visible metal alloy and/or PGM inclusions. Although these inclusions are not necessarily Ru-rich phases, their presence suggests that there is a preference for these samples to form nuggets, which may restrict Ru partitioning into the chromite crystal structure. We suggest that the low Ru values in the Coobina chromite are a result of transient sulfide saturation. The Re-Os isotopic composition of the Coobina chromitite is chondritic [γ187Os(3.189 Ga) = −0.63 ± 0.21] and is consistent with derivation of the Coobina parental magma from the convecting upper mantle source, providing evidence for the mantle origin of the Coobina PGE inventory. If using chromite as a detrital indicator mineral for magmatic sulfide exploration, it must be kept in mind that transient sulfide saturation within chromitite seams may give a false positive signature.
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