Distribution of High-Tech Critical Metals in soils from the Ronda Ultramafic Massif (SW Spain)

The Ronda ultramafic massif constitutes the Earth’s largest outcrop of peridotites (~ 300 km²) of the subcontinental lithospheric mantle (SCLM). This massif is located in the westernmost part of the Málaga province in SW Spain, and mainly consists of peridotites (lherzolites and harzburgites with lesser amounts of dunites) and mafic pyroxenite layers (usually <10 %). These rocks are arranged in a petrologic and geochemical zoning consisting, from the top to the bottom of the mantle section, of the following domains: (1) spinel (±garnet) tectonite corresponding to the exhumed SCLM roots, (2) granular peridotite formed by thermal erosion of pre-existing spinel (±garnet) tectonite due to upwelling of the asthenosphere during unroofing, and (3) plagioclase tectonite corresponding to shear zones originated shortly before or contemporaneously to the crustal emplacement. The rocks forming these three domains exhibit contrasting degrees of fertility in a wide suite of elements, offering an unequal opportunity to evaluate the impact that different bedrocks have for the distribution of High-Tech Critical Metals in soils.

We performed bulk-rock analyses of around 70 samples from 10 soil profiles above peridotites from the three aforementioned petrological domains. Regardless of the original bedrock, all the analyzed soils show a common trend of progressive enrichment of Fe₂O₃ and Al₂O₃ and depletion of MgO from bedrock to the atop of the profile. Minor elements such as MnO, TiO₂, Na₂O and K₂O overall increase from bottom to top, whereas SiO₂ remains generally unchanged. Interestingly, there is positive correlations between Fe₂O₃ and MnO as well as other transition metals such as Cr (up to ~9000 pm),  Co (up to 310 ppm), V (up to 181 ppm), Zn (up to 136 ppm) and Sc (up to 38 ppm). A general increasing of total REE (up to ~20 ppm) is also observed in most profiles with a significant enrichment of LREE over HREE.