Mineralogy and geochemistry of bauxites from Parnassos - Ghiona mines and the impact on the origin of the deposits

Abstract

Karst-type bauxite deposits have been exploited in central Greece, in the area oflegendary Parnassos Mountain near the Oracle of Delphi, since 20’s. The existence of bauxitein Parnassos-Ghiona was announced around 1922, giving possibilities of developing analuminium industry in Greece. The Al-ores are hosted within Mesozoic carbonate formationsof the Parnassos-Ghiona geotectonic zone, which is characterized by nearly continuoussedimentation of epicontinental reef-like carbonates from the Upper Triassic to the UpperCretaceous. The exploitation is currently performed by three Greek mining companies(“Aluminium S.A.”, “S&B Industrial Minerals S.A.” and “ELMIN Hellenic Mining EnterprisesS.A.”), whereas there is also an Al industrial plant installed at the coastal zone of the Corinthgulf, which is the largest vertically integrated Al producer in the E.U.. Numerous geologicalstudies have been published about the deposits, whereas their main mineralogical andgeochemical characteristics have been described by several authors using variousconventional microscopic and analytical methods, such as optical microscopy, SEM-EPMA,XRD, XRF, Fire-Assay/AAS/ICP and TGA/DTA.The main purpose of the present dissertation was to combine -for first time in theliterature with regard to karst-type bauxites of Greece and the globe- diffraction (PXRD),analytical in bulk and microscale (ICP-MS/OES, LA-ICP-MS), thermal (TGA/DTG, DSC),spectroscopic in bulk and microscale (FTIR, 57Fe Mössbauer, high-resolution gamma-ray,Laser μ-Raman, SR μ-XRF, (μ)-XANES/(μ)-EXAFS), microscopic (optical, SEM-EDS/WDS), andnanoscopic (FEG TEM-EDS & EELS) techniques, together with magnetic susceptibilitymeasurements, for the detailed mineralogical and geochemical characterization and thestudy of mineral nanoparticles and nanominerals in typical Fe-rich (low grade, red-brown)and Fe-depleted (high grade, white-grey, Al-rich: Al2O3 ca. 80 wt.%) from active mines of theParnassos-Ghiona area. Nanoscopy, related to nanogeoscience, is expected to play a crucialrole in the exploration and exploitation of basic, noble and strategic metal resources. Theabove novel mineralogical and geochemical data were used to illustrate new insights intothe origin of the deposits. Moreover, STEM-HAADF and SF-ICP-MS techniques wereadditionally applied for characterization and leaching experiments with respect to bauxitemetallurgical residues (so-called “red mud”), in order to provide with additional data which are interesting not only to the mining but also to the metallurgical industry andenvironmental technology. In addition, stable isotope measurements (δ18O and δ13C) wereused in the case of hanging wall and footwall limestones of Pera Lakkos mine, to concludeabout the mineralogy and geochemistry of the intercalated bauxites.Bauxite samples were collected in collaboration with “Aluminium S.A.”, “S&B IndustrialMinerals S.A.” and “ELMIN Hellenic Mining Enterprises S.A.” from currently active mines inthe Parnassos-Ghiona area. The final composite samples concerned the Fe-rich and the Fedepletedbauxites. The PXRD investigation confirmed the presence of AlOOH polymorphs(diaspore and/or boehmite), TiO2 polymorphs (anatase and rutile), and Feoxides/oxyhydroxides,as the major mineral components. The laser μ-Raman results showedthat the different bauxite types (diasporic or boehmitic) can easily be identified regardless ofsample type by recording spectra in the low-wavenumber region (250 cm-1 – 600 cm-1)where distinct bands of the natural AlOOH polymorphs are easily discernible (448 cm-1 fordiaspore and 362 cm-1 for boehmite). The thermal analyses and FTIR measurements, alsocontributed to the elucidation of the volatile constituents (predominantly hydroxyls). Thebulk geochemical analyses revealed that, except for major elements (mainly Al and Fe), thestudied industrial bauxites contain an exceptional variety of trace elements, with remarkablepositive geochemical anomalies with respect to High-Field Strength Elements (HFSE), RareEarth Elements (REE), actinide elements (Th, U), as well as in most of compatible elements.Especial emphasis was given to Pera Lakkos mine where a case geochemical study wasperformed, including the surrounding coal and limestones. Among all trace elementsstudied, Th is stated to be important, related to mining, metallurgical and environmentalissues. According to XRF and ICP-MS analyses Th, is relatively increased in Fe-depletedsamples containing up to 62.75 ppm Th corresponding to 220 Bq/kg due to 228Ac (232Thseries),whereas Fe-rich samples are less Th-radioactive (up to 58.25 ppm Th, 180 Bq/kg dueto 228Ac). SEM-EDS indicated the presence of Ti-Fe-containing phases (e.g., ilmenite: FeTiO3),chromites and besides LREE-minerals (mostly bastnäsite/parisite-group) and zircon (ZrSiO4)hosting a part of the bulk Th. The presence of Th in diaspore and in Ti-containing phases (notdetected by SEM-EDS as in the case of REE-minerals and zircon) was investigated, intodistinct pisoliths of Fe-depleted bauxite, using μ-XRF and μ-XAFS. XAFS spectra of Th saltsand Th-containing reference materials were obtained as well. Accordingly, it was revealed, for the first time in the literature, that Ti-phases, and particularly anatase, host significantamounts of Th. This novel conclusion was complementary supported by LA-ICP-MS analysesindicating an average of 73 ppm Th in anatase grains together with abundant Nb (3356ppm), Ta (247 ppm) and U (33 ppm). The Th LIII-edge XAFS spectra as compared to referencematerials, gave also evidence that Th4+ may not replace Ti4+ in distorted [TiO6] fundamentaloctahedral units of anatase and ilmenite lattice (CN=6). The occupation of eitherextraframework sites of higher coordination (CN=6.9 or even CN=7.4), according to EXAFSsignals evaluation, or of defected/vacant ()sites is more probable. This is likely explainedby the difficulty of Th4+ to replace directly Ti4+ in [6]-coordinated (octahedral) sites due tothe large difference in the relevant ionic radii (0.940 Å and 0.605 Å, respectively).The discovery of Th-hosting anatase in microscale into karst-type bauxite, allowed furtherinvestigation concerning the study of mineral nanoparticles and nanominerals in Fe-depleted(high grade) samples, by means of a combined utilization of diffraction, spectroscopic,microscopic, and nanoscopic techniques. Initial characterization using SEM-EDS/WDS provedthe presence of Fe-Cr-Ti-containing diaspore, anatase and minor rutile, together with tracesof zircon, chrome-spinel and REE fluorocarbonates. The subsequent study by means of 57FeMössbauer, in correlation with magnetic susceptibility vs temperature measurements, andcomplementary Synchrotron-based spectroscopic techniques in microscale (μ-XRF and μ-XANES/-EXAFS), indicated that Fe3+, in contrast to [6]Cr3+, is not exclusively a component ofthe structure of α-AlOOH (diaspore). According to the Cr K-edge EXAFS spectral results, theinteratomic distance between the central atom of Cr and the first neighbor of O of the firstshell is calculated at 1.965 Å. The nanoscopic study, using FEG TEM-EDS & EELS, revealedTiO2 polymorph mineral nanoparticles, particularly rounded anatase nanocrystals dispersedinto the diaspore matrix (in areas appearing phase-homogeneous in microscale), as well asindividual needle-shaped rutile. Additionally, it was proved that, except Fe3+ substituting Al3+in the structure of diaspore, a percentage of the metal exists also in the form of peculiar Fe3+nanominerals (probably maghemite-type phases or most likely semi-amorphous/disorderedor even completely amorphous Fe phases in nanoscale) that are between 25 and 45 nm insize. Thus, diaspore in the studied karst-type bauxite concerns in fact a Fe-Cr-AlOOH low-T(sedimentary) phase, which was demonstrated for first time in the literature. On the otherhand, the occluded Ti mineral nanoparticles and Fe nanominerals, revealed by HRTEM, were hitherto unknown not only for the allochthonous karst-type bauxite deposits of centralGreece, but also for the overall bauxite deposit groups worldwide. Thus, the presentdissertation gives strong evidence for the importance of nano-mineralogy and -geochemistryin the characterization of Al-ores, and in general of metal oxide/hydroxide ore deposits, ashas also been proved in the case of metals in sulfide ores. Mineral nanoparticles andnanominerals, related to nanogeoscience issues, seem to be the final frontier of oremineralogy and geochemistry being playing a vital role in the exploration and exploitation ofbasic, noble and strategic metal resources.The findings about the mineralogy and geochemistry of the studied bauxites gave robustproof for the origin of the Panassos-Ghiona karst-type bauxites. The vital contribution of thepre-Cretaceous (Middle-Upper Jurassic) ophiolites of the Hellenides is more than obvious,due to the presence of detrital chromite grains, of possible zircon crystals from the basicmembers (gabbros) and the plagiogranites, and also of positive geochemical anomalies forrelevant compatible elements (Ti, Cr, V, Ni, Sc). This is in accordance to all previousobservations. It is evident that the enrichment in other trace elements, namely some HFSE(Zr, Ce) and actinides (U, Th), can hardly be attributed to contribution of ophiolites and mustbe derived from pre-existing acidic geological formations. Specific information wereobtained using discrimination geochemical diagrams based on ratios of “contrastingelements” (occurring either in acidic or mafic rocks), such as Th/Sc vs Zr/Sc, as well as on REEanomalies, i.e. Eu/Eu* and Ce/Ce*. Thus, it was stated evident that acidic igneous rocks,predominantly volcanic, also contributed, besides ophiolites, to the formation of ParnassosGhionabauxite deposits. It is proposed that Triassic volcanic rocks showing a variablecomposition, from sub-alkaline basalts to rhyolite, volcano-sedimentary complexes formedduring the rifting stage of Permo-Triassic age, and perhaps also Paleozoic igneous rocks(~300 Ma) occurring in the pre-Alpine basement of areas, such as the central Evia Island,have contributed to formation of later (Jurassic-Cretaceous) bauxites of Parnassos-Ghiona.Finally, bauxite refining solid wastes (red mud) from Greece were characterized using acombination of diffraction, microscopic, analytical, and spectroscopic techniques (XRD, SEMEDS,STEM-EDS/EELS, XRF, ICP-OES/MS, HR γ-ray Spectrometry, and XANES/EXAFS). The bulkXRD-detected crystalline phases concern hematite (α-Fe2O3), calcite (CaCO3), gibbsite & diaspore (AlOOH-polymorphs), Na-Ca-Al-silicate-carbonate & Ca-Al-hydroxysilicate phases(cancrinite- & “hydrogarnet”-type phases), quartz (SiO2), anatase (TiO2) andphyllosilicates/clays while the microscale study by SEM-EDS indicated a dominant “Al-Fe-CaTi-Si-Na-Crmatrix”. Bulk Fe K-edge XANES proved the abundance of Fe3+ and also revealedthe existence of minor Fe2+. Bulk analyses showed that except major Fe, Al, Ca, Si, Ti, Na andC (86.5 wt.%) and significant volatiles (LOI: 13.6 wt.%), the material contains Cr (2403 ppm),V (1081 ppm), Ni (902 ppm), As (164 ppm), Pb (120 ppm), as well as remarkable Th (111ppm). The latter, and also minor U (15 ppm), are responsible for radioactivity (352 and 134Bq Kg-1 for 232Th and 238U respectively) with total dose rate 285 nGy h-1. The radioactivity ofparent material (typical Greek bauxites) was also measured for comparison. Leachingexperiments, in conjunction with SF-ICP-MS, using Mediterranean seawater from Greece,indicated significant release of V, depending on solid/liquid ratio, and negligible release ofTh, and therefore of radioactivity, at least after 12 months leaching tests. However, almostall trace elements, including REE, are relatively mobile in concentrated acetic acid solutionimplying potential recovery technologies. Similar seawater- and acid-leaching were alsoapplied in industrial bauxite samples. Subsequent STEM-EDS/EELS study of leached red mudrevealed that the significant immobility of Th4+ is due to its incorporation into an insolublenano-perovskite phase with a major composition Ca0.8Na0.2TiO3. Various elementalimpurities, such as Ce, Nb, Zr, are detectable into the low-T & low-P nano-perovskite.Additionally, nanoscale -Th-free- insoluble Fe-oxides (nano-hematite/Ti-hematite, and nanomagnetite/Ti-magnetite),as well as nano-anatase, and nano-diaspore, were also found asconstituents of the “Al-Fe-Ca-Ti-Si-Na-Cr matrix”. Th LIII-edge EXAFS spectra showed, for firsttime in the literature, that Th4+, hosted in this novel Ca-Na-(Ce-Nb-Zr-Cr)-nano-perovskite ofred mud, occupies Ca2+ sites rather than Ti4+ sites. It is therefore, herein, stated that theabove nanophase is the reason of low Th release in acid medium, and subsequently of the Thimmobility into Greek red mud exposed in Mediterranean seawater.