Quantitative mineralogical characterization of karst bauxite deposits in the southern Apennines, Italy.
Economic Geology, 108 (2013), 813-833.
1Dipartimento Scienze della Terra, dell’Ambiente e delle Risorse, Università degli Studi di Napoli Federico II, Via Mezzocannone 8, 80134 Napoli, Italy
2Camborne School of Mines, University of Exeter, Cornwall Campus, Penryn, Cornwall TR10 9EZ, United Kingdom
The southern Italian karst bauxite deposits occur at a Late Cretaceous hiatus on a Bahamian-type carbonate platform succession cropping out in the southern Apennines, and are mainly located in the Abruzzi Mountains, in Apulia (Gargano peninsula and Murge area), and in Campania. The bauxites of southern Italy are presently uneconomic, but can be considered as a model analogue for economic karst bauxite ores. The texture of the bauxite deposits is mainly oolitic to pisolitic but can also be arenitic-conglomeratic, suggesting reworking of evolved lateritic soils. The minerals boehmite and hematite are more abundant in the ooids, whereas kaolinite is found at high levels in the groundmass, and titanium oxides are also present. Furthermore, small amounts of rare earth elements have been detected in most of the bauxite occurrences. Scanning electron microscopy with energy dispersive spectrometry (SEM-EDS) and geochemical analyses on several of the deposits confirmed the diagenetic texture of the bauxite, and provided an estimate of the mineralogy of the different components and their chemical composition. The mineralogical and petrographic characteristics of bauxite are crucial for the Bayer process and can influence its economic evaluation: both the particle size distribution in the ore and the presence of reactive silica phases associated with Ti and Fe oxides can reduce the efficiency of processing. Quantitative analyses combining different methods (X-ray diffraction [XRD]-Rietveld quantitative phase analysis [QPA] and QEMSCAN®) have been carried out on a limited number of samples from two of the bauxite districts of the Campania region: the Matese Mountains and the Caserta province. QEMSCAN® analysis allowed a more rapid quantification of mineralogy (including trace detrital phases) and assessment of the individual textural characteristics of the bauxite lithotypes, showing a detailed image of the distribution of economic and noneconomic minerals and their intergrowths. This methodology can augment or replace other time-consuming quantitative phase analyses for mineralogical studies of bauxites, provided that the species identification protocol (SIP) database has been carefully validated by preliminary use of XRD and SEM-EDS. Both XRD (QPA) and QEMSCAN® analytical techniques can be complementary for bauxite ore evaluation, and a very powerful tool for exploitation and mineral processing.