New insights into the mineralogy and geochemistry of sb ores from Greece

Antimony is a common metalloid occurring in the form of Sb-sulfides and sulfosalts, in various base and noble metal deposits. It is also present in corresponding metallurgical products (concentrates) and, although antimony has been considered a penalty element in the past, recently it has gained interest due to its classification as a critical raw material (CRM) by the European Union (EU). In the frame of the present paper, representative ore samples from the main Sb-bearing deposits of Greece (Kilkis prefecture, Chalkidiki prefecture (Kassandra Mines), and Chios Isl.) have been investigated. According to optical microscopy and electron probe microanalysis (EPMA) data, the Greek ores contain stibnite (Sb2 S3), boulangerite (Pb5 Sb4 S11), bournonite (PbCuSbS3), bertherite (FeSbS4), and valentinite (Sb2 O3). Bulk analyses by inductively coupled plasma mass spectrometry (ICP-MS) confirmed, for the first time published, the presence of a significant Hg content in the Kilkis Sb-ore. Furthermore, Kassandra Mines ores are found to contain remarkable amounts of Bi, As, Sn, Tl, and Se (excluding Ag, which is a bonus element). The above findings could contribute to potential future exploration and exploitation of Sb ores in Greece

The Rare Earth Elements Potential of Greek Bauxite Active Mines in the Light of a Sustainable REE Demand

More recent data of Greek bauxites from the Parnassos-Ghiona active mines prove that rare earth elements (REEs) occur mainly in the form of authigenic/diagenetic LREE3+ carbonate and phosphate minerals (bastnäsite/parisite group and florencite). Bulk geochemistry of representative samples, from underground mines and open pits, showed an increased content in LREE (ΣLREE—from La to Gd—varying between 106 and 913 ppm; avg. ΣLREE = 321 ppm; n = 17), and lower HREE (ΣHREE—from Tb to Lu including Y—varying between 45 and 179 ppm; avg. ΣHREE = 95; n = 17). The overall REE concentration (ΣREE + Y+Sc) varies from 192 to 1109 ppm (avg. 463 ppm; n = 17). The most abundant REE is Ce (min: 67 ppm; max: 655 ppm; avg. 193 ppm; n = 17), exhibiting in general a positive geochemical anomaly (avg. Ce/Ce* -CeA-: 2.6), identical to the case of marine Fe–Mn-crust and terrestrial desert vanish, implying also that Ce4+ may exist either in REE-oxides and/or epigenetically sorbed in Fe-oxides. On the other hand, Nd content, which is more interesting for the industry, is lower (avg. 41 ppm; n = 17). The concentration of REEs is much higher in Fe-rich (red) bauxite, compared to Fe-depleted (white) bauxite (avg. ΣREE + Y+Sc = 569 ppm and 268 ppm, respectively). The new data presented herein show a rather lower REE potential of Parnassos-Ghiona bauxites, compared to previous literature, but similar values compared to karst-type bauxites of the globe. Although their REE concentration is higher compared to that of various geochemical reference materials (i.e., positive REE geochemical anomalies in comparison with chondrites, UCC, PAAS, NASC, and ES), it is vitally lower compared to REE resources being mined, such as REE–Fe–Nb–Th deposits. A trend similar to REE geochemical trend also stands for most of the trace elements that are present in Greek bauxites—mainly HFSE—except for LILE. Besides, Greek bauxite metallurgical residue’s (red mud) REE content seems to be remarkably increased by almost two times compared to that of the Parnassos-Ghiona bauxite parent material. Scandium is another critical element. In the studied bauxites, it varies from 29 to 73 ppm (avg. 47 ppm; n = 17); it is typical for the Mediterranean and EU bauxites and laterites, but much lower compared to the exploitable Australian laterites. The Fe-rich samples contain higher concentrations of Sc compared to Fe-depleted (avg. 54 and 33 ppm, respectively). This means that common red Greek bauxite is rather more exploitable, with regard to Sc (and the rest REE), but not the white one (which is of high quality in terms of Al). Bulk geochemistry indicates that Sc is correlated to Fe but not to Zr, while microscale observations demonstrated the presence of fine-grained scandian-zircons. This is in line with a very recent study proving that Sc is mainly present in Fe-oxide minerals (mainly hematite and goethite) and zircons. Bulk geochemical Fe–Pb and Fe–As positive correlations are also verified among the associated trace elements. Finally, the investigation of the REE vertical distribution in a characteristic deposit of the B3 horizon (i.e., Pera Lakkos mine case study), showed that the REE concentration is increased in the Fe-rich domain (lying above the footwall limestone), as well as in the coal layer interstratified between the Fe-depleted domain and the hanging wall limestone. However, it is revealed that the Ce/Ce* (CeA) is increased in the coal layer and is raised to the uppermost Fe-depleted domain, but not the lowermost Fe-rich bauxite domain. This might be attributed to the Ce3+ ↔ Ce4+ and the LREE re-mobilization during the supergene/epigenetic processes. © 2018, The Minerals, Metals & Materials Society

Uranium-bearing francolites present in organic-rich limestones of NW Greece: a preliminary study using synchrotron radiation and fission track techniques

Synchrotron radiation techniques (μ-XRF and μ-XANES) were applied to the study of organic-rich phosphatized limestones of NW Greece (Epirus). The results revealed uranium accumulation in areas of the material containing, among others, carbonate apatite (francolite) and organic matter. The UL3-edge of μ-XANES spectra showed that uranium was present in tetravalent form. U-bearing francolite crystals were separated from the rock and characterized by Raman spectroscopy and microprobe. The analysis of the crystals also indicated the presence of sodium and sulfur. The uranium presence in the crystals was also visualized, after neutron irradiation and etching, by the observation of the fission tracks. © 2016, Akadémiai Kiadó, Budapest, Hungary

Arsenic distribution and speciation in the bauxitic Fe-Ni-laterite ore deposit of the Patitira mine, Lokris area (Greece)

Bauxitic Fe-Ni-laterite ore from the Patitira mine in Lokris area (Greece) contains assemblages of unusual As-bearing goethite-type phases in samples with significant organic matter. The material was initially characterized by bulk ICP-MS and SEM-EDS. Furthermore, the distribution and speciation of As were studied by Synchrotron Radiation (SR) spectroscopic techniques, for first time in the literature. The SR μ-XRF elemental maps and the As K-edge μ-XAFS spectra revealed that As is exclusively correlated to Fe, occurring as As5+ in the form of arsenate anions (AsO4 3−). However, the arsenate anions, being considered as sorbed species on goethite-type phases, exhibit -at molecular scale- a disordered structural environment, resembling locally to the configuration of such anions in natural hydrated Ni arsenates (annabergite) rather than to natural hydrated ferric arsenates (scorodite). The metalloid element in study, possibly derived from As-mineralizations or volcanic rocks, has been transferred in the environment of the laterite re-deposition, where arsenates could interact with the aforementioned Fe-phases. © 2018 Elsevier B.V

A structural approach to the genesis of chrome ores within the Vourinos ophiolite (Greece) : Significance of ductile and brittle deformation processes in the formation of economic ore bodies in oceanic upper mantle peridotites

The understanding of ophiolites as hosts to economic\u2013scale chromitite ore deposits requires the integration of research relating their geochemical metallogenesis with studies of the mode and nature of their lithospheric-scale deformation. The Neotethyan Vourinos ophiolite in Northern Greece represents a well-preserved ophiolite complete with sheeted dikes, an in-situ Moho transition zone, and a nearly 8-km-thick upper mantle sequence. Chromitite bodies and sub-economic chrome ores occur throughout its ultramafic section, and above and below the petrological Moho; but, economic concentrations of chromitite are restricted to a region referred to as the \u201cmetalliferous zone\u201d within the upper mantle domain. Ductile deformation, which developed synchronously with mineral re-equilibration and late magmatic activity, re-concentrated chromite grains in mesoscopic fold noses and created massive-textured ore bodies. Re-equilibration of chromite and chromite\u2013olivine grains on microscopic\u2013scale during this ductile deformation produced geochemical patterns that are compatible with strain distribution. Whether re-equilibration varied with strain rate in the mantle lithosphere due to regional heterogeneous conditions, or whether the change in deformation\u2013induced modal distribution of the mineral aggregate more strongly affected the process of re-equilibration is yet to be determined. The richest ore bodies in Vourinos are represented by the schlieren ores of the Xerolivado Mine, and occur as extensive lenses of mylonitic chromitite, marking the highest degree of deformation within the complex. These textures display patterns of re-equilibration geochemical halos and transition from ductile into brittle deformation conditions

Geochemistry of coastal sands of Eastern Mediterranean: The case of Nisyros volcanic materials

Coastal sand samples collected from the northern part of Nisyros volcanic island (Dodecanese, Greece) were investigated for first time for their potential in strategic metals and compared with parental rocks of the island which are Quaternary volcanics with alternating lava flows, pyroclastic layers and lava domes and relevant materials located near granitoids of Northern Greece. The PXRD and SEM-EDS study of the sands revealed enhanced content of feldspars, Fe-Mn oxides, magnetite, tourmaline, pyroxenes, ilmenites, along with zircons, apatite and sulfide inclusions. The fresh hydrothermally deposited clayely material collected from the Nisyros caldera crater had a rather different mineralogical composition from the coastal one (alunite, anhydrite, opal-CT, quartz, kaolinite). UCC-normalized spidergrams indicated that the weathering processes contributed to accumulation of heavy minerals (mainly ilmenite), and strategic metals including V (1920 mg/kg) and Nb (245 mg/kg), in the coastal sand. The low REE concentration (ΣREE + Y = 240 mg/kg) could be attributed to the absence of REE-rich minerals. Moreover, the sands exhibit different geochemical patterns compared to the volcanic source rocks of the island, which are especially enriched in Large-Ion Lithophile Elements (LILE) and depleted in High Field Strength Elements (HFSE), such as Nb and Ta. On the other hand, the caldera material is enriched in volatile components, sulfur, chalcophile elements (Se, Bi, Hg, As, Pb) and Ba. Micro-XRF analyses of representative crystals showed that the high Nb content of the sands was associated with the Ti/Fe-rich phases (e.g. ilmenites). The geochemical composition of N Greece sands showed, because of their origin, enrichment not only in HFSE but also in REE. The study of the coastal heavy mineral sands originating from different geological environments of Greece provides information about the association of their mineral components with REE, other elements of economic interest (e.g. Co, Nb, Ta) and natural actinides. In addition, the study of the black sands of Nisyros island could be considered as a characteristic example of those from other parts of Hellenic Volcanic Arc (HVA) and other relevant Mediterranean regions. © 2017 Elsevier Gmb

New insights into nanomineralogy and geochemistry of Ni-laterite ores from central Greece (Larymna and Evia deposits)

Nickeliferous laterite ores from two typical central Greece deposits (Larymna and Evia), currently used in the LARCO GMMSA smelting plant to produce ferronickel, were characterized using a combination of diffraction, microscopic, and analytical techniques. X-ray diffraction patterns of various fractions, with emphasis to the clayey material (<2 μm), after glycolation and heating at 550 °C, indicated that both materials contain crystalline Fe3+ oxide (hematite) and chlorite-group phyllosilicates, whereas the Evia sample contains additional illite. Transmission electron microscopy investigations revealed that the LARCO laterite ores consist of complex nanoscale aggregates of the above-mentioned phases. Different Ni-bearing Mg-Fe-phyllosilicates (mainly chlorite-group minerals), occur in mixture with hematite. Nickel is present in all examined phases, and therefore the separation of pure Ni-phases, by physical or chemical methods, is practically unfeasible. Trace element bulk analyses showed that there no significant differences, with regarding to Ni content concentrations (ca. 0.6–0.7 wt.%), between the initial ore and the clay fraction, for both the Larymna and Evia laterites (ca. 30% and 26% wt.% enrichment respectively). However, the Larymna ore contains double quantities of Co and it is enriched in rare earths compared to Evia (ΣREE = 774 ppm and 76 ppm respectively), while Sc concentrations are comparable in both mining areas (64 ppm and 42 ppm respectively). Discrimination diagrams (e.g. Th/Sc vs. Zr/Sc and Ce/Ce* vs. Eu/Eu*) showed that LARCO laterite Ni-ores do not exclusively originate in ultrabasic -ophiolitic- rocks as previously considered. © 2019 Elsevier Gmb

New insights into the mineralogy and geochemistry of sb ores from Greece

Antimony is a common metalloid occurring in the form of Sb-sulfides and sulfosalts, in various base and noble metal deposits. It is also present in corresponding metallurgical products (concentrates) and, although antimony has been considered a penalty element in the past, recently it has gained interest due to its classification as a critical raw material (CRM) by the European Union (EU). In the frame of the present paper, representative ore samples from the main Sb-bearing deposits of Greece (Kilkis prefecture, Chalkidiki prefecture (Kassandra Mines), and Chios Isl.) have been investigated. According to optical microscopy and electron probe microanalysis (EPMA) data, the Greek ores contain stibnite (Sb2 S3), boulangerite (Pb5 Sb4 S11), bournonite (PbCuSbS3), bertherite (FeSbS4), and valentinite (Sb2 O3). Bulk analyses by inductively coupled plasma mass spectrometry (ICP-MS) confirmed, for the first time published, the presence of a significant Hg content in the Kilkis Sb-ore. Furthermore, Kassandra Mines ores are found to contain remarkable amounts of Bi, As, Sn, Tl, and Se (excluding Ag, which is a bonus element). The above findings could contribute to potential future exploration and exploitation of Sb ores in Greece. © 2020 by the authors. Licensee MDPI, Basel, Switzerland

Metastable Iron (Mono)sulfides in the Shallow-Sea Hydrothermal Sediments of Milos, Greece

Metastable iron sulfides are involved in a series of biotic and abiotic processes in the marine environment, including the mineralization of organic matter. However, naturally occurring metastable iron (mono)sulfide minerals are rarely reported in marine sediments, and current information about their formation and characteristics comes from synthetic sulfides. Here, we studied sulfur speciation and mineralogy in a submarine surface core (0-22 cm depth) from an active, shallow-sea hydrothermal system (Milos, Greece) that is dominated by sulfur-metabolizing microorganisms. Geochemical analysis results showed S-Fe-As enrichment in the bottom layers of the core, which were further characterized using a suite of techniques. Powder X-ray diffraction and Synchrotron-based μ-X-ray diffraction did not show crystalline Fe-S compounds whereas scanning electron microscopy and Synchrotron-based X-ray fluorescence mapping indicated the presence of Fe-S(-As) phases and sulfur particles. Sulfur microspeciation by X-ray absorption near-edge structure spectroscopy showed a mixture of oxidation states, including organic sulfur species, indicative of active sulfur biogeochemical cycling. Ultimately, transmission electron microscopy was used for the identification of the Fe-S mineral assemblage in the samples that included arsenic-bearing pyrite and the metastable mackinawite, monoclinic pyrrhotite and greigite, alongside elemental sulfur nanoparticles. Previous studies on the mineralogy of Milos hydrothermal sediments omitted the presence of metastable iron sulfides, that were up to now known to form in marine sediments from estuaries and anoxic/euxinic basins. Our results highlight that the use of standard microscopic, spectroscopic and diffraction techniques may overlook the presence of metastable iron sulfides in natural samples. Considering that metastable iron sulfides are implicated in critical biogeochemical processes for the marine ecosystems, their role in sulfur, iron, and carbon cycling in modern and ancient marine sediments might be underrated. © 2022 American Chemical Society. All rights reserved