Geochemistry and mineralogy of karst bauxites: the case of the Spinazzola deposit (Murge, southern Italy)

There is an increasing interest for bauxites, mostly due to the high content in some of them of selected elements such as Rare Earth Elements (REE), which are very important for new technological products. For this reason, also the study of bauxites like the uneconomic Italian deposits is important, because they are considered as “analogues” of other bauxites with an economic potential. Apulian karst bauxites mark a Late Cretaceous hiatus (Cenomanian-Turonian) in shallow water platform carbonates. One of the objects of our research is the Spinazzola bauxite deposit (Murge area), which consists of bauxite concentrations filling deep paleokarstic cavities. The stratigraphy of the Murgian area includes two principal limestone formations: Calcare di Bari (Valanginian-Cenomanian) at the bottom, and Calcare di Altamura (Coniacian-Campanian) at the top of the bauxite profile. The texture of this bauxite is mainly oolitic-arenitic. The analysis concerns a set of 25 samples collected in the Spinazzola deposit, along a 20mt profile. The whole profile includes two carbonate samples (hanging wall and footwall limestones) and one clay-rich sample, just below the hanging wall. This clay-rich sample possibly represents the protolith, from which the parent rock evolved into bauxite during middle-Cretaceous. X-ray diffraction shows that the mineralogy of the Spinazzola bauxite consists of boehmite, hematite, kaolinite, and minor anatase. The clay-rich sample is mostly made of kaolinite. The more abundant oxide is Al2O3, which reaches the highest value (almost 60 wt. %) at a depth of about 12 mt. The chemical and mineralogical patterns follow distinct trends in two distinct parts of the deposit. The SiO2 amount, for instance, is slightly higher in the lower part of the profile (average 15 wt. %), compared with the upper part where SiO2 does not reach 9 wt. % in average. The total amount of REE is higher in the lower part of the profile (570 ppm in average), relatively to the upper part (395 ppm in average). The mineralogical and chemical characteristics of the Spinazzola bauxite may suggest distinct climatic regimes during its formation. Further diagenetic evolution has been crucial in the distribution of the elements.