The role of microorganisms on the mineralogical and geochemical characteristics of the Parnassos-Ghiona bauxite deposits, Greece

The Parnassos-Ghiona bauxite deposits of Greece are hosted within carbonate rocks and have been formed during different geological ages. The most economically important deposits occur in the B3 bauxite horizon, which is developed over long distance as a continuous layer of 1-10 m in thickness, within Cretaceous limestones. Due to intense tectonics, a significant (approximately 30 vol. %) bauxite ores along and near their contact with faults show a brittle deformation and change in the color from red to black-gray, in a distance of tens of meter. Commonly gray to whitish bauxites are aluminum-enriched (> 65 wt.% Al2O3) and iron depleted. The most salient feature of bauxite ores of black-gray, grayish-brown or yellowish color along faults is the abundance of secondary pyrite of varying size (from less than 1 mu m to a few hundreds mu m) and form, which is closely associated with organic matter. Frambroidal pyrite and pyrite veins crosscutting sulfide-rich zones are occasionally present. Dismembered pyrrhotite and pentlandite crystals are only occasionally present. Pyrite is Co, Ni free, while Se varies from 0.7 to 0.9 wt.% Se. Pyrrhotite is Ni-bearing (up to 0.5 wt.% Ni) and pentlandite contains cobalt (average 1.5 wt.% Co). Textural relationships between Fe-sulfides/Fe-oxides, and the total organic carbon (TOC) content, ranging from 0.06 wt.% in red-color bauxite ores to 2.78 wt.% in gray-color ores, is consistent with a biological activity. Bacteria may play a significant role in the mineralogical and chemical composition of laterite ores, and in leaching and beneficiation of low-grade ores. Due to the behaviour of ferric iron as an electron acceptor for energy metabolism in a wide spectrum of bacteria the mobility of iron in the bauxite ores may be the result of an effective leaching by percolating pore waters, depending mostly on the pH-Eh conditions. (c) 2006 Elsevier B.V. All rights reserved.