The relative distribution of critical (Sc, REE) and transition metals (Ni, Co, Cr, Mn, V) in some Ni-laterite deposits of New Caledonia

The European Union recently updated the list of raw materials considered as critical for its industry on the basis of their high economic importance and their relative supply risk. This list now defines 26 critical raw materials that include among others, rare earth elements (REE), scandium, vanadium and cobalt. Among the different primary resources of these critical metals, lateritic deposits are particularly targeted. Recent investigations have shown that the latter elements can be accumulated during weathering in consequence of residual and secondary enrichment. These environments are also characterized by significant concentrations of transition metals (e.g. Mn, Cr, Ni), which are not defined as critical to date but remain of primary economic importance. In this study, we investigate the potential enrichment of some critical (REE, Sc, V, Co) and base transition metals (Mn, Cr, Ni) in Ni-lateritic deposits of New Caledonia by combining mineralogical, bulk-rock, and in situ mineralogical and geochemical approaches. In addition, particular attention was paid to the development of a new standard compound, labelled StdGoe 1.1, to ensure accurate and reproducible analyzes of iron oxides by LA-ICP-MS. Based on this new specifically developed standard, the concentrations obtained for most elements present in iron oxides were shown to be significantly lower, by 20 to 50%, compared to concentrations obtained from calibration using the NIST SRM 610 standard. Such discrepancy is attributed to strong matrix contrasts between finely divided iron oxides particules and the silicate glass NIST standard, and highlight that previous concentrations measured by LA-ICP-MS in iron oxides and available to date in the literature must be considered with care. Based on this new development, our results show that critical and transition metals are concentrated in different horizons of the lateritic profile. Ni mainly concentrates in secondary Ni-bearing phyllosilicates in saproliths, while it is mainly hosted by goethite in limonite levels. Mn and Co both precipitate as Mn-oxides at the interface between saproliths and limonite facies, while they are sorbed into iron oxides (goethite and hematite) after dissolution of Mn-oxides in the upper levels of the profile. The main fraction of Cr and V is hosted by primary chromites, which are weathering-resistant relative to the other minerals. On the other hand, the Cr and V fraction released after pyroxene dissolution is integrated into