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Iron and manganese accumulations in Sardinia: the role of supergenic and hydrothermal processes
Full text linkThis 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
GENESIS OF autochthonous AND allochthonous aPULIAN KARST BAUXITES (SOUTHERN ITALY): CLIMATE CONSTRAINTS
No full textThe Apulian Carbonate Platform (ACP) in southern Italy has experienced several episodes of subaerial exposure, which were in some cases associated with the formation of karst bauxite deposits. The ACP contains both autochthonous canyon-like bauxite and allochthonous Salento-type bauxite, with the latter having been derived from a weathered and eroded pristine bauxite deposit. The remnants of this pristine bauxite are preserved as transported pebbles embedded in a clayey matrix.
The autochthonous bauxite and the pebbles of the allochthonous bauxite have the same texture of sub-spheroidal components (ooids) dispersed in a fine-grained matrix. The fractal dimension of ooids from both deposits is very similar and corresponds to the growth of "aggregates" under a diffusion-controlled process.
The ooids of the autochthonous bauxite have a different composition to those in the pebbles of the allochthonous bauxite, because they formed under different climatic conditions. During the Turonian, autochthonous bauxite ooids formed in alternating wet tropical conditions (promoting Al-hematite formation) and drier conditions (favouring boehmite stability). In the allochthonous pebbles, ooids formed mainly in a dry climate, promoting the formation of large boehmite cores. The ooids/matrix ratio and the geometrical features of the ooids reflect these climate differences.
The differences in composition and age (post-Turonian) of the Salento-type bauxite bedrock suggest that the pristine bauxite that produced the Salento-type pebbles was different in composition to and younger in age than the canyon-like bauxite. The latter probably formed during a middle Campanian emersion event (evidenced by large karstic cavities), which is correlated with the subaerial exposure of karst recorded on the Adriatic island of Brač
How elements, including heavy metals, behave during weathering in tropical climate? The case of Cretaceous and Messinian palaeosoils of Sardinia, western Mediterranean (Italy)
No full textGeochemistry and mineralogy of karst bauxites: the case of the Spinazzola deposit (Murge, southern Italy)
No full textThere 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
Ce anomalies and trace element distribution in Sardinian lithiophorite-rich Mn concretions
No full textSupergene concentrations of lithiophorite found in alluvial conglomerates and residual sandy clay deposits of Messinian age in northern Sardinia, were investigated for minero-chemical composition and fractionation processes. The Mn concentration occurs as cement in conglomerate, coatings on pebbles and as concretions in clays. In the mineralisation, some transition metals including Ni, Cu, and Zn along with Ba, U, Pb, and REEs, are enriched relative to the composition of the upper continental crust. Zn and Ni co-vary with Mn, suggesting that they are hosted in lithiophorite. Also the REEs, with the exception of Ce, co-vary with Mn. Ce, due to its redox chemistry, fractionates relative to the other REEs and precipitates as cerianite during Mn4+ reduction. Mn mineralisation has a different Ce signature as expressed by the Ce anomaly, which varies between 0.22 and 11.75. A two-stage model was proposed for explaining the Ce anomaly fluctuations. Precipitation of the samples with positive Ce anomalies occurred in the first stage from organic matter-free fluids which favored the oxidation of cerium on the Mn phase surface; in the second stage, instead, Ce-depleted solutions, resulting from the first stage, were responsible for the precipitation of samples with negative Ce anomalies
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