Abstract
The Abruzzi bauxite district includes the deposits located on the Campo Felice plateau and those of the Monti d’Ocre, which had been mined in the first part of the 20th century. Bauxite is of the karst type, with textures ranging between oolitic and oolitic-conglomeratic, the latter suggesting a partial reworking of evolved lateritic soils. The high contents of Al2O3 and Fe2O3 (average values 53.76 and 21.76 wt %, respectively) are associated with the presence of boehmite, hematite, and minor goethite. SiO2 and TiO2 have average values of 7.79 and 2.75 wt %, corresponding to the presence of kaolinite, anatase and rutile. Among the minor so-called “bauxitophile” elements V, Co, Ni, Cr and Zr, the most enriched is Cr, with an average value of 0.07 wt %. Nickel has an average value of 210.83 ppm. Vanadium shows an average value of 266.57 ppm, whereas the average Co concentration is 35.89 ppm. The total rare earth elements (REE) concentration in the sampled bauxite sites is variable between ca. 700 and 550 ppm. Among REEs, the most abundant element is Ce, with Ce anomalies commonly associated with authigenic REE-fluoro-carbonates, probably produced after the REEs remobilization from primary detrital minerals and their precipitation in neo-formed phases during the bauxitization process. Scandium and Ga occur in small amounts (57 and 60 ppm, respectively), but geochemical proxies of their remobilization and uptake in neo-formed minerals (Fe- and Al-(hydr)oxides, respectively) have been observed. The mean Eu/Eu* and Al2O3/TiO2 ratios and the Ni-Cr contents of the Abruzzi bauxites suggest that the parent rock of these deposits was a material of acid affinity, likely corresponding to volcanic tephra or eolic loess-type sands.
Highlights
Bauxites are economic concentrations of aluminum, developed from the weathering of alumosilicate-rich parent rocks
The ore textures of the Abruzzi bauxites that vary between oolitic and oolitic-conglomeratic in all deposits, the presence of several grains/pebbles of older bauxite, as well as the angular fragments of hematite-goethite among the ooids, as already suggested by [7,29], corroborate the fact that the bauxite horizons currently in place are derived from the erosion and re-sedimentation of older bauxites and possibly from eroded duricrusts, formed on the exposed carbonate platform
Other minor constituents occur both as fine-grained detrital minerals and as neo-formed phases (REE-fluoro-carbonates)
Summary
Bauxites are economic concentrations of aluminum, developed from the weathering of alumosilicate-rich parent rocks. These residual deposits are mainly formed under humid tropical to sub-tropical climates, with rainfalls in excess of 1.2 m and annual mean temperatures higher than 22 ◦ C [1]. Aluminum in bauxites is known to be precipitated in form of gibbsite [Al(OH)3 ]. Bauxites that were subjected to several diagenetic stages consist mainly of boehmite [γ-AlO(OH)], which is less hydrated than gibbsite, and diaspore [α-AlO(OH)]. As of January 2016, the worldwide bauxite reserves stood at 27.5 billion tonnes. Global bauxite production was estimated at 279.7 million tonnes in 2015, down 0.3% year-over-year, attributed to a production decline during the year in Brazil, Indonesia and China [2].
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