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The Miocene Climatic Optimum (MCO) represents a global warm period from approximately 17.0 to 14.7 Ma, which interrupted a period of long-term Cenozoic cooling. Continental paleoclimate records for the MCO vary in quality and global coverage, and due to the unique formation conditions for bauxites, they may serve as an excellent constraint of local climate during this period. In order to elucidate if the MCO promoted the establishment of a wet and warm climate allowing the formation of bauxites in southeastern mid- latitude Europe, we studied a section of previously undated massive karst bauxite (Crveni Klanac, CK) in central Dalmatia, Croatia, hosted in Upper Cretaceous limestones and overlain by Miocene Sinj Basin lacustrine deposits. Integrated mineralogical, morphological, and geochemical analyses indicate the predominant mineral phases of the homogenous bauxite matrix are authigenic, subhedral to euhedral kaolinite, and gibbsite. The in situ mineralization as a consequence of pedogenic processes indicates that the CK bauxites formed autochthonously. In situ U-Pb zircon ages of the lower, middle and upper stratigraphy of the CK bauxite are similar, dominated by Miocene and Oligocene ages, indicating they share similar protolith(s). Subsequent high-precision chemical abrasion-isotope dilution-thermal ionization mass spectrometry analyses indicate a maximum depositional age for bauxite parent material of 16.9576 ± 0.021 Ma. This maximum age of autochthonous bauxitization coincides with the onset of the MCO, and additional stratigraphic geochronological constraints limit the duration of bauxitization to less than approximately 700 ka. In order for insitu bauxitization to have occurred in southeastern parts of the mid-latitude continental European areas, paleoclimatic and paleoenvironmental conditions must have had mean annual temperature greater than 17–22°C and mean annual precipitation of more than 1100–1200 mm (literature data). Therefore, the CK autochtonous bauxites provide an independent climatic constraint, and serve as a novel example of using bauxite deposits for high precision paleoclimate constraints.

Bauxite is a residual rock, consisting mainly of a mixture of aluminium hydroxides, whose industrial significance is primarily due to its profitably exploitable alumina contents. In the last decades, bauxite, mainly karst bauxite, has been also considered as possible resource of a great number of economically interesting elements including some critical metals such as rare earth elements, Sc, Co, Ni, and Nb. In this study, we present results of univariate and multivariate (R-mode factor analysis) statistics performed on a large data set including chemical composition of the principal Cretaceous karst bauxites from central and southern Italy with the twofold aim to evaluate the inter-elemental relationship among major oxides and critical metals, and describe factors affecting their distribution. Univariate statistics reveal that some critical metals Co, Ni, and LREE, have a significant number of outliers. The Co-Ni relationship associated to the outliers dataset suggests that Co is likely hosted in Ni-rich phases whereas Ce accumulation in authigenic minerals, carbonate-fluoride and phosphate, is at the origin of LREE outliers. Multivariate R-mode statistics, applied to the outliers-devoid database, instead demonstrate that in absence of specific mineralization events, the distribution of most critical metals is controlled by Al-, Fe-, and Ti-oxi-hydroxides and, to a lesser extent, by detrital phases. Among the critical metals, Cr and Y are the exceptions. Their geochemical behaviour seems to be influenced primarily by their own chemical features that are responsible for the mobility of Cr during bauxitization and for the decoupling of Y from the REEs.

article i nfo This study focuses on a late Cretaceous karst bauxite deposit in the Murge area of the Apulia district, southern Italy. The first analysis of the vertical distributions of a wide range of elements (including REEs and selected trace elements) throughout the deposit was shown in order to identify the processes determining element frac- tionation during the evolution of the bauxite. The studied karst bauxite deposit exhibits an ooidal texture, is mineralogically homogeneous, and contains higher abundances of boehmite than of hematite, kaolinite, and anatase. The major element composition of the bauxite is dominated by elevated concentrations of SiO2 ,T iO2 ,A l2O3 ,a nd Fe2O3, and analyses of element mo- bility within the bauxite indicate that all elements except for Nb and Ni, and to a lesser extent Cr, are depleted relative to the immobile element Ti. R-mode factor analysis revealed a number of key findings: (i) some low- solubility elements (e.g., Zr, Th, Ti, V, Ga) were concentrated in detrital zircon and monazite (Zr, Th), in anatase (Ti, V), and possibly in boehmite and hematite (Ga) during the later stages of bauxitisation; (ii) Fe and Cr were concentrated during wet conditions, whereas Al and Co accumulated during dry conditions; (iii) distributions of the light REEs (LREEs) and the heavy REEs (HREEs) are controlled by the same processes, suggesting that little LREE/HREE fractionation occurred during bauxite formation; and (iv) the behaviour of cerium is different from that of the other REEs, and highly variable cerium anomalies are observed across the deposit, with three characteristic Ce/Ce* maxima with values ofN2. Parisite was the only authigenic cerium mineral detected during this study. Thus, we propose a three-step model to explain the distribution of Ce: cerianite is dissolved by cerium reduction, is transported by downward-moving meteoric water (per descensum), and finally parisite is precipi- tated. This cycle was repeated several times in the Apulian karst bauxite in response to Eh decreases under alka- line conditions, promoted by fluctuations in the groundwater level. Finally, we used the value of the Eu anomaly todiscuss theparental affinity or protolith of thebauxite. The value of the Eu anomaly (min. Eu/Eu* = 0.76, max. Eu/Eu* = 0.89) indicates that the bauxite was not derived from carbonates, but rather, that the majority of the bauxite was influenced by intermediate to mafic magmatic sources. Eu/Eu* vs Sm/Nd diagram suggests that the parental material for the bauxite was derived from a combination of a distant magmatic source and clastic material derived from a continental margin (northern Africa) to the south.

This paper examines the compositional features of some Fe and Mn accumulations of Sardinia in order to reconstruct the palaeoenvironmental conditions of ores formation, to better characterize the processes that lead to Fe and Mn minerals formation and to compare the Fe and Mn geochemical behavior within different depositional environments. The research focused on comparison between: 1) continental Fe oxyhydroxides concentrations, formed under oxic conditions on a palaeosurfaces of the Tacchi area, and the Uppermost Ordovician oolitic ironstones of shallow anoxic marine environment of the Nurra district; 2) marine supergene Mn ores of Logudoro Basin, hosted in a littoral conglomerate laying at the base of the Miocene transgressive calcareous sediments, and continental hydrothermal Mn mineralizations of San Pietro Island and Anglona district where they are the stockwork and veins filling within Miocene volcanic sequences. Chemical analyses (XRF, INAA and ICP-MS), XRD analysis, OM and SEM-EDS observations were performed on each samples. The analyses results suggest that although the Sardinian ores formed in different environments (marine vs continental) and under contrasting climatic (sub-glacial vs tropical) and environmental (supergene vs hydrothermal) conditions, they share similar mineralogical or geochemical features. Therefore, not necessarily dramatic palaeoenvironmental differences find a counterpart in geochemical features of the chemical sediments.

Nel presente contributo vengono descritte le associazioni vegetali riparie presenti nella zona studiata. L'indagine ha sviluppato anche aspetti di tipo biogeochimico, investigando i contenuti di alcuni elementi potenzialmente tossici nei tessuti di due specie vegetali riparie (Nasturtium officinale e Paspalum paspaloides).