POST-GENETIC EVOLUTION OF OPHIOLITE-HOSTED CR-PGE ORES

Chromitite layers, pods and lenses within ophiolite ultramafic rocks are major industrial chromium sources and contain sub-economic Platinum Group Elements (PGE) enrichments. PGE and chromium are considered Critical Raw Materials (CRM) by the European Union, due to their employment in a wide range of sectors. The criticality of CRM is related to their importance for the economy, and to the risk associated with their supply. The study of altered chromitites can give us important insights on their evolution, as well as on the behavior of precious metals in contact with altering fluids. In the present work, the three main post-magmatic processes affecting podiform chromitites have been addressed: high-T subsolidus re-equilibration of olivine and spinel, low-T circulation of fluids at high (chloritization) and low (serpentinization) fS2 and fO2 respectively. Olivine and chromite diffusivity patterns, developed during subsolidus re-equilibration due to Mg-Fe exchange reactions, are useful tools for the reconstruction of the thermal history down to 650 °C. In the present work we developed a new approach using an exponential function to model XMg variations with the distance from the grain boundary. The new approach produced reliable primary and re-equilibrated XMg values, then used to infer re-equilibrated and primary temperatures, as well as to estimate the cooling rates of the olivine- chromite-bearing rocks. The approach, developed starting from the case study of Finero subcontinental mantle, has been employed with good results also for two partially serpentinized ophiolite chromitites, Iballe and Nea Roda. Low-T circulating fluids at high fS2 and fO2 conditions can alter primary mineralogical assemblages and induce crystallization of secondary minerals. The high sulfur and oxygen fugacities favor the replacement of primary silicates by chlorite, and the formation of ferrian chromite. Chloritized chromitites from Gomati and Nea Roda (Greece) have been selected to study the remobilization of elements during the circulation of such fluids. Primary Base Metal Minerals (BMM) are altered and replaced by secondary ones, but no alloys are generated, as they are not stable under these conditions. Platinum Group Minerals (PGM) are not affected by the circulating fluids, and retain their magmatic imprint, as also confirmed by primary mantle-normalized PGE patterns, with a Ru peak. Low-T circulating fluids at low fS2 and fO2 conditions are the most common within ophiolite chromitites, and are responsible for the major altering process of this kind of deposit: serpentinization. Serpentinized chromitites from Skyros (Greece) and Abdasht-Soghan (Iran) have been selected to study the remobilization of elements during the circulation of such fluids. The low sulfur and oxygen fugacities favor the replacement of primary silicates by serpentine, and the formation of magnetite. A distinctive feature of serpentinization is the replacement of primary BMM with base metal alloys, as well as with secondary sulfides. PGM are also highly affected by the serpentinizing conditions, and tend to lose sulfur and be transformed into alloys. This generates a remobilization of PGE into the circulating fluids. The remobilization, estimated within the Abdasht-Soghan complex through a mass balance calculation on a partially desulfurized laurite grain, occurred at two different scales. Os was the first element to exit laurite lattice to form Os-rich alloys. However, the mass balance calculation showed that the PGM mostly lost Ru. This implies a second, larger scale, remobilization where Ru acted as the most soluble PGE in the desulfurizing fluids. The mobility of PGE into the fluids hence follows the order: Ru>>Os>>Ir. This is confirmed by PGE patterns, which present a double peak, for Ru and Os. Post-magmatic processes affect the economic assessment of chromite ore deposits. In this study we focused on the quality parameters of a chromite sand for the foundry industry. Foundry chromite sands are the most valuable chromite commodity, and are currently not produced within the EU. The best chromite foundry sands are from layered intrusions, with South Africa as the main producer. In order to find alternative sources of this commodity, we tested the behavior of two industrial ophiolite chromite sands from Iran and Greece, enriched through shaking tables and spirals. Ophiolite chromite sands do not meet the most critical quality parameter, the Acid Demand value, that depends on the reactivity of the silicate impurities in an acid environment. The presence of serpentine, a highly reactive mineral widespread in ophiolite chromitites, lowers the performance of the sand. On the other hand, olivine, the second most common silicate mineral in podiform chromitites, shows low reactivity. The production of chromite foundry sands starting from ophiolite chromites is possible, but only for those where the silicate impurities assemblage is dominated by olivine, with very low or negligible amounts of serpentine. Two examples of ophiolite chromitites with a low degree of serpentinization within the EU are Aetoraches and Rizo deposits, in Greece. Their chromite sands exceed the Acid Demand thresholds by a low amount, and an affordable degree of purification could lower the ADT values and make them suitable for the foundry market.