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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
Matese Mts. and Caserta District Karst Bauxites (Campania Region, Southern Italy): Insights on Geochemistry, Paleoclimate, Paleoenvironment, and Parental Affinity
Full text linkIn the Campania region (Southern Italy), in the Matese Mts. (Albian to Turonian/Coniacian) and Caserta district (Albian to Cenomanian), two karst bauxite deposits outcrop, consisting of flat lenses over shallow karst carbonate. Although the mineralogy and geochemistry of Campania bauxite deposits have been widely studied in recent years, new major and trace elements relationships were provided to highlight paleoclimatic and paleoenvironmental conditions that occurred during their formation. The purpose of this research is to provide for the first time information on the paleoclimatic and paleoenvironmental conditions that affected the bauxites of Campania. These deposits formed during different periods since the Matese deposit formed during intense weathering processes with more abundant precipitation while the Caserta district deposit experienced a more long-lasting exposure event. During the formation of the studied bauxites, the drier conditions favored the replacement of kaolinite by boehmite. R-mode factor analysis showed geochemical affinity among Al2O3, TiO2, and Nb. REEs minerals are mainly associated with the bauxite matrix while Zr, Hf, and V were mainly concentrated in detrital minerals during the later stages of bauxitization. Parental affinity indices (Eu/Eu* vs. Sm/Nd; Eu/Eu* vs. TiO2/Al2O3) assessed the origin of the protolith of the Campania bauxites by rejecting the hypothesis of the dissolution of the bedrock carbonate. The results confirmed the eolian transport of parental material with an Upper Continental Crust and an intermediate to mafic magmatic composition
Geochemistry 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
Matera “foggials” (Southern Apennines): Multi-analytical characterization of the natural coatings of local grain storage structures
Full text linkThis paper focuses on a multi-analytical characterization of Matera (Basilicata
region, southern Italy) foggials used to collect and store grain commodities to
define their uses and the mineralogical, petrographic and geochemical features.
The old district of Matera (Sassi) was recognized as a UNESCO heritage site since
1993. Fifteen samples of rock fragments (generally limestone or calcarenites)
and coatings were collected from 4 different sites in the centre of Matera.
Petrographic observation with optical microscope and mineralogical analysis
showed that the mineralogical assemblage mainly consists of calcite and quartz
and subordinately, Fe-hydroxides. The most abundant major oxides are CaO,
Al2O3 and SiO2 while among the trace elements, the most enriched are Pb, Sr, Cr,
Ba and V. The REEs pattern shows a general flat distribution with no enrichment
or depletion of LREEs and HREEs. A negative correlation between CaO and
∑REEs content suggests that REEs has no affinity for carbonate phases. The multi
analytical characterization let us to hypothesize that Apulia region, in particular
the Apulian “red earth” from Murge area, could be among the areas of origin of
the materials used to produce the Matera foggials coating
GEOCHEMISTRY OF THE APULIAN KARST BAUXITES (SOUTHERN ITALY): CHEMICAL FRACTIONATION AND PARENTAL AFFINITIES
No full textThis 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 fractionation during the evolution of the bauxite.
The studied karst bauxite deposit exhibits an ooidal texture, is mineralogically homogeneous, 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, TiO2, Al2O3, and Fe2O3,
and analyses of element mobility 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 of >2. 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 precipitated. This cycle was repeated several times in the Apulian
karst bauxite in response to Eh decreases under alkaline conditions, promoted by fluctuations in the
groundwater level.Finally, we used the value of the Eu anomaly to discuss the parental affinity or
protolith of the bauxite. 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
Geochemistry and Geometrical Features of the Upper Cretaceous Vitulano Para-Autochthonous Karst Bauxites (Campania Region, Southern Italy): Constraints on Genesis and Deposition
Full text linkIn the Vitulano area, Upper Cretaceous bauxite bodies fill small depressions and karst cavities within Cretaceous shallow-water limestones. These bauxites were studied to understand the processes that led to their formation. Geochemical, mineralogical, and petrographic analyses were carried out on the bauxite samples, together with image analysis providing geometric parameters. The texture of Vitulano bauxite consists of ooids and sub-circular aggregates dispersed in a predominantly Ca-rich matrix. Ooids are generally formed by a single large core, often surrounded by an alternation of different aggregates of boehmite and Al-hematite reflecting different climate periods. The composition is dominated by the major elements Al2O3 and CaO with lower concentrations of Fe2O3 and SiO2. Boehmite, calcite, hematite, and anatase are the main mineralogical phases identified. Image analysis provided values of fractal dimension D that gives information on carbonate platform exposure times since it is linked to long-lasting sub-aerial events and diffusion-limited cluster aggregation processes. The tectonic evolution of the area played an important role in the genesis of the Vitulano bauxites since it favored the erosion, transport, and re-deposition of pre-existing bauxite material from the surrounding Campania bauxites. Based on this hypothesis, Vitulano bauxites are defined and classified as para-autochthonous, and this was supported by Eu/Eu* vs. Sm/Nd and Eu/Eu* vs. TiO2 Al2O3 indices displaying a similarity between Vitulano and the other Campania bauxites
Petrography, Geochemistry and Mineralogy of Serpentinite Rocks Exploited in the Ophiolite Units at the Calabria-Basilicata Boundary, Southern Apennine (Italy)
Full text linkA multi-analytical study on serpentinites in the ophiolite units (Calabria-Basilicata boundary, southern Apennines) was carried out on samples collected from a serpentinite quarry, locally called “Pietrapica”, which sitsin the Pollino UNESCO Global Geopark. Optical microscopy observations revealed the petrographic characteristics, ICP-MS was used to assess the chemical composition while EMPA mineral chemistry, Raman spectroscopy and X-Ray Powder Diffraction and were used altogether to trace the mineral composition of the rocks. Petrography revealed that serpentinites from Pietrapica quarry are essentially composed of serpentine group minerals, amphibole and carbonate minerals with lower abundances of talc and Cr-spinel. Raman spectroscopy and X-ray powder diffraction analysis clearly allowed to establish that carbonate minerals, serpentine and amphibole-like minerals, are the dominant phases, followed by 2:1 phyllosilicate. Electron probe microanalyses were carried out on different minerals in serpentinites samples including serpentine, amphibole, chlorite, clinopyroxene, magnetite, talc, quartz and titanite which are often associated with carbonate veins. Bulk geochemistry is dominated by major oxides SiO2, MgO and Fe2O3 while the most abundant trace elements are Ni and Cr. Chemical analysis showed that some heavy metals in the studied serpentinites such as Ni and Cr, are beyond the maximum admissible limits for Italian normative for public, private and residential green as well as for commercial and industrial use representing a potential environmental concern. Anyway, some of these heavy metals have been recently listed by Europe as critical raw materials and therefore, the Pietrapica abandoned quarry could represent a new resource considering their economic potentiality
X-ray diffraction and scanning electron microscopy quantitative mineralogical evaluation of the karst bauxite deposits in Campania, southern Italy
No full textBauxite resources in the world are estimated to be between 55 to 75 billion tons, occurring in Africa (32%),
Oceania (23%), South America and the Caribbean (21%), Asia (18%) and elsewhere (6%) (United States Geological Survey Minerals Yearbook, 2011). These resources are mostly used to produce alumina for metallurgical purposes. Approximately 70 to 80% of the world’s bauxite production is processed first into
alumina (Bayer process) (Hind et al., 1999), and then into aluminium by smelting and electrolysis (Hall– Herault process). The mineralogical variation in the composition of bauxite can affect the Bayer process, e.g. the effect of reactive silica, which contaminates the aluminium extraction process, and titanium minerals, which reduce the efficiency of the Bayer Process (Dudek et al., 2012; Smith, 2009). In this study, mineralogical characterization of some bauxite occurrences in Southern Italy has been carried out, including a comparison of results obtained from the quantitative analysis of the ore using the classic XRD-QPA (X-ray diffraction-quantitative phase analysis) method, and QEMSCAN (quantitative evaluation of minerals by scanning electron microscopy) automated technology, which represents a mineralogical investigation tool common-used in the bauxite industry (Goodall et al., 2005). The bauxite of Southern Italy that is presently uneconomic can be considered as a model analogue for economic karst bauxite ores. Several bauxite occurrences from two former mining districts in Southern Apennines (Matese Mts. and Caserta province) have been analyzed. XRD-QPA allowed an exact evaluation of the amounts of minerals occurring in the bauxite. The main Al-rich mineral is boehmite, whereas the most common silica-bearing phase is kaolinite. Hematite is the main Fe-bearing mineral. Goethite also occurs, as well as sporadic lepidocrocite. Anatase and rutile are quite ubiquitous. QEMSCAN was used in the fieldscan mode and allowed: (1) the identification and Abstracts classification of the mineralogical phases on the basis of their chemical composition, and (2) the abundance of these phases. Moreover, this technique has provided evidence of the textural characteristics of the ore that can be important in the Bayer process. In particular, it has been possible to determine that boehmite, which tends to be more concentrated in the ooids than in the matrix, contains variable amounts of Fe. Hematite occurs solely as detrital elements, kaolinite occurs mainly in the bauxite matrix and, Ti-oxides are generally associated with goethite and hematite. Comparison between the XRD quantitative mineralogical analyses and the corresponding analyses determined by QEMSCAN shows convergent results (Boni et al.). The XRD quantitative evaluation of bauxite phases gives useful constraints for the interpretation of QEMSCAN raw data, which may be very detailed but cannot separate polymorphous minerals that occur in bauxite. Furthermore, QEMSCAN quickly highlights the main textural characteristics of the bauxite, which can affect the Bayer process, but there are also some problems in identifying the composition of phases that have a grain size lower than the provided scanning resolution. However, by using both methods, and with the support of manual SEM (scanning electron microprobe), all the issues were isolated and resolved. For this reason the XRD-QPA and QEMSCAN techniques should be considered complementary for bauxite ore evaluation, and can provide a very powerful tool for their exploitation and mineral processing
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