Geochemistry of the apulian allochthonous karst bauxite, Southern Italy: Distribution of critical elements and constraints on Late Cretaceous Peri-Tethyan palaeogeography

Abstract

A comprehensive study of pebbles from the 'Salento-type' allochthnous bauxite deposit (Otranto, southern Italy), originally derived from a pristine Campanian bauxite, has been performed for evaluating: 1) the chemical fractionation and inter-elemental relationships, especially for critical elements, 2) the climatic conditions that promoted bauxite formation, and 3) the provenance of the protolith(s) using zircon age data and conservative elemental proxies. The study confirms the capability of bauxite to concentrate many elements defined as critical by the European Union report on critical raw materials. Sc, Co, Ga, and especially Cr, are enriched when compared with the UCC composition and assuming Nb is immobile. Other critical elements such as the REEs, with the exception of La, are moderately depleted. R-mode factor analysis suggests that most of the variance in our chemical dataset is explained by a factor with significant weightings for TiO2, Al2O3, Fe2O3, Sc, V, Nb, REEs, Pb and Th. This arises from climate effects affecting the distribution of the more abundant oxides and some trace elements, including the critical metals Nb and REEs. The texture of the pebbles is typical for Apulian karst bauxites and consists of sub-spheroidal ooids composed of boehmite and dispersed in a fine-grained matrix. The growth of the ooids, which formed under dry climate, was described in terms of fractal geometry. The average fractal dimension value of the ooids in the pebbles is close to that of the diffusion-limited aggregation models suggesting the ooid growth can be modelled using a molecular diffusion pattern, based on Fick's first law. The calculated time required for growth of the boehmite concretions is ~45÷310ka. This finding is consistent with an intra-Campanian emersion event (74–76Ma) that occurred during a dry and warm climatic stage. Since most of the karst bauxites worldwide have an ooidic texture, evaluation of the composition of concretions and the time required for their growth represents a powerful tool in reconstructing the palaeoenvironment. The zircon grains collected from the pebbles of the Salento-type karst bauxite define several concordant age populations. The youngest cluster, Early Cretaceous in age (99÷127.5Ma), suggests that windborne particles from Cretaceous volcanics, possibly originating in the Carpatho-Balkan orogenic belts, provided material for further bauxitisation. The largest cluster (623÷689Ma) is of Neoproterozoic age, predominately from the Late Ediacaran and Cryogenian p.p.. The 900–540Ma Pan-African orogenic cycle was followed by continental-scale uplift and erosion, leading to the deposition of thick Cambrian–Ordovician siliciclastic sequences that represent the most widespread detrital sequence ever deposited on continental crust and that now cover large parts of North Africa. These Cambrian–Ordovician sandstones contain a large population of Neoproterozoic zircons of Cryogenian age. Neoproterozoic zircons also occur in the youngest (Silurian–Mesozoic) sandstones of the Saharan Metacraton. These sandstones also contain 1.0Ga detrital zircons, suggesting as the oldest zircons found in the Salento-type bauxite pebbles (866Ma and 941Ma in age) are younger representatives of the zircon cluster present in this sandstone unit.These zircon age determinations suggest that the source material for the Salento-type bauxite pebbles was a combination of magmatic material from a distant source and clastic material derived from a continental margin (North Africa). This result concurs with the Eu/Eu* vs. Sm/Nd binary diagram, on which bauxite pebbles fall close to a mixing curve with andesite and cratonic sandstone end-members. As our results indicate that material was sourced from the North African continental margin, we suggest that a continental bridge separated oceanic domains in the Late Cretaceous of the Peri-Tethyan domain.