Maria Economou-Eliopoulos

Platinum-group element distribution in chromite ores from ophiolite complexes: implications for their exploration

Abstract Compilation of some new data on ophiolites for Greece and Yugoslavia, and published data from previous studies, indicate that platinum-group element (PGE) and gold concentrations in chromite ores are generally low, ranging from less than 100 ppb to a few hundred ppb. However, samples from several ophiolite complexes exhibit an enrichment (of a few ppm) (a) only in Os, Ir and Ru,(b) only in Pt and/or Pd or (c) in all PGE. This enrichment (up to 10s ppm) is mainly related with chromitites hosted in supra-Moho dunites and dunites of the uppermost stratigraphic levels of the mantle sequence and it seems to be local, independent of the chromitite major element composition and the chromite potential of the ophiolite complexes. The contents of PGE combined with less chalcophile elements (Ni, Co, Cu), the ratios of incompatible/compatible elements, and PGE-patterns provide evidence for discrimination between chromitites derived from primitive magmas and those derived from partially fractionated magmas, although they have a similar major element composition. Thus, they can be used for a stratigraphic orientation in the mantle sequence, and therefore for exploration targets. Moreover, PGE data offer valuable information for the evaluation of the chromite potential in ophiolite complexes. The most promising ophiolites seem to be those which apart from the petrological and geochemical characteristics indicating extensive degree of partial melting in the mantle source contain only one chromite ore type (the other type being only in small proportion) of limited compositional variation, in both major elements and PGE, low ratios of Pd Ir , while PGE-enriched chromitites in the mantle sequence are only occasionally present. In contrast, ophiolites which contain both high-Cr and -Al chromitites, and where their chalcophile element data implies relatively extensive fractionation trend are not good exploration targets for chromite ores, although they are related with a SSZ environment.

Geochemistry of chromitites and host rocks from the Pindos ophiolite complex, northwestern Greece

Abstract Representative chromite samples of schlieren and massive ore types from various localities of the Pindos ophiolite complex were analyzed for platinum-group elements (PGE), S, Ni, Co and Cu, and mineral chemistry. Host dunites and harzburgite were also analyzed for the chalcophile elements. The chromite ores show a wide compositional variation. The Cr/(Cr + Al) ratio ranges from 0.47 to 0.83 while Mg/(Mg +Fe 2+ ) ranges between 0.42 and 0.73, but two populations can be distinguished (the high-Cr and high-Al types), which show a more restricted variation. Ranges of PGE contents (in ppb) are: Os = 8−150, Ir = 4−320, Ru = 15−550, Rh = 2−82, Pt = 3−150 and Pd = 1.5−20. Gold ranges between 1 and 13 ppb. The PGE and less chalcophile elements (Ni, Co and Cu), the incompatible/compatible element ratios, the PGE/S ratio and PGE patterns provide valuable information for the discrimination of chromite ores with a similar major-element composition. A fractionation trend in both chromitites and associated dunites can be used for stratigraphic orientation in the mantle sequences of ophiolite complexes and evaluation of their chromite potential.

Palladium, platinum and gold concentration in porphyry copper systems of Greece and their genetic significance

Compilation of new and published data on precious metal and associated trace-element contents in mineralized porphyry copper intrusions of Greece indicates that the Pd content in the potassic–propylitic zone of the Skouries deposit is relatively high, ranging between 60 and 200 ppb (average 110 ppb), whereas in the porphyry copper intrusions of Gerakario and Pontokerasia, it is very low (3 ppb Pd). The oxidized zone of Fissoka (OP-65) exhibits low Pd and Cu contents, but higher Au, Te, As, Pb and Zn contents. The Pt content is much lower than Pd in the porphyry copper intrusions studied. The Pd/Pt ratio in the Skouries deposit ranges from 9 to 60, while it is much lower (<2) in the mineralized samples from Fissoka, Gerakario and Pontokerasia. A decrease of the Pd content and Pd/Pt ratio with increasing Te, As, Pb and Zn content may indicate that the Pd distribution in porphyry copper systems is effected by the evolution of the magma-hydrothermal system. Also, mineralogical and geochemical data on the Skouries deposit, coupled with textural relations between base metal sulphides, PGM and Au–Ag tellurides, indicate that the main Pd-bearing mineral, merenskyite, is associated with the vein-type chalcopyrite or bornite, and was deposited during the main stage of mineralization. The high magmatic oxidation state and the magmatic features of the hydrothermal system (limited participation of meteoric portion) in the Skouries intrusion may be a strong control of its Pd potential. Assuming that Pd is mainly associated with chalcopyrite in the porphyry copper deposits, the calculated Pd (from measured Pd) contents in chalcopyrite (average 3300 ppb Pd) are relatively high, and comparable to that in the chalcopyrite concentrate made after the processing of large composite drill-hole samples (2400 ppb Pd to 21 wt.% Cu). It is considered to be an encouraging economic factor for Pd as a by-product, probably along with Au, although the main product is Cu.

Apatite and Mn, Zn, Co-enriched chromite in Ni-laterites of northern Greece and their genetic significance

The SEM-microprobe investigation of pseudo-autochthonous Fe–Ni deposits from the areas east Vermion, Edessa and Olympos, northern Greece, reveals the presence of apatite, and a significant Mn, Zn and Co enrichment in chromite, up to 14.0, 3.3 and 4.1 wt.%, respectively, due to the substitution for Mg2+ and Fe2+ by Mn, Zn and Co in the chromite lattice. Despite the significant Mn, Zn and Co contents (whole ore analysis) of some Ni-laterite deposits of the Balkan peninsula, Mn, Zn and Co enrichment in chromite is restricted to certain deposits, and its level seems to be a characteristic feature of each deposit. This may indicate that the Mn±Zn±Co enrichment has taken place after the re-deposition of the weathered material rather than during serpentinization and weathering of parent rocks. Mn±Zn±Co enrichment is commonly higher in the ferrian-chromite zone of chromite grains, and in high-Cr than high-Al chromite. The availability of Mn2+±Zn2+±Co2+ in solution during the diagenesis–metadiagenesis stage, due probably to the decomposition of organic matter, and the Eh–pH conditions may be major controlling factors for the incorporation of Mn, Zn or Co in the lattice of chromite. Apatite occurs as fine grained crystals (<10 to 50 μm) dispersed throughout the matrix of the ore and as inclusions within zoned crystals of chromite, between chromite core and magnetite rim. It is associated with Fe-oxides (goethite, hematite, magnetite), chamosite, and quartz. A salient feature of the apatite is its poikiloblastic texture (with inclusions of quartz and iron oxides). The apatite crystals exhibit both plastic and brittle deformation textures. The composition of apatite is homogeneous, either in different apatite types or within crystals themselves. Apatite is Cl-free and corresponds to the fluor-hydroxyl-apatite type with the following average composition: 51.35 wt.% CaO, 2.80 wt.% FeO, 41.4 wt.% P2O5 and 5 wt.% (F+H2O). The inclusions of apatite between chromite core and magnetite rim and the inclusions of magnetite and quartz dispersed throughout apatite suggest that these minerals crystallized at the same time. The association of apatite with Fe-chlorite and magnetite points to their formation during the diagenesis–metadiagenesis stage, while the gradual dissolution of apatite crystals dispersed throughout the matrix suggests that it was unstable during a subsequent stage of its formation.

Palladium, platinum and gold distribution in porphyry Cu±Mo deposits of Russia and Mongolia

The porphyry Cu–Mo deposits in the southern Siberian craton and Northern Mongolia including subduction-, collision-, and rift-related igneous series of early Paleozoic to Mesozoic age, are all characterized by varying crustal contribution to their parent magmas and the presence of explosive breccias. Precious metal and associated element contents are reported for mineralized samples, sulphide concentrates, and chalcopyrite–molybdenite flotation concentrates from porphyry Cu±Mo intrusions of Russia (Sora, Aksug and Zhireken) and Mongolia (Erdenetiun-Obo). Average PGE contents in rocks are 17 ppb Pd and 22 ppb Pt in the Aksug deposit, 13 ppb Pd and <10 ppb (detection limit) Pt in the Sora, 14 ppb Pd and 21 ppb Pt in the Erdenetuin-Obo and 18 ppb Pd and 28 ppb Pt in the Zhireken deposit. Average gold content in rocks is 61 ppb in the Aksug deposit, 17 ppb in the Sora, 21 ppb in the Erdenetuin-Obo, and 30 ppb in the Zhireken deposit. The highest average 2.3 ppm Ag was recorded in the rocks of the Sora deposit. In general, the precious metal distribution in the studied deposits has no apparent relationship with the alteration types. The molybdenum content in rocks ranges between <1 and 128 ppm Mo in the Aksug deposit, from <1 to 5400 ppm Mo in the Sora deposit, from 2 to 755 ppm Mo in the Erdenetuin-Obo deposit, and from 3 to 1530 ppm Mo in the Zhireken deposit. The average copper content in rocks is 1540 ppm in the Aksug deposit, 460 ppm in the Sora deposit, 1460 ppm in the Erdenetuin-Obo deposit, and 220 ppm in the Zhireken deposit. The Cu–Mo ratios are highest in the Aksug and Erdenetuin-Obo deposits and lowest in the Sora and Zhireken deposits. The correlation matrix for selected major and trace element data on mineralized samples and sulphide concentrates (34 samples) indicates that precious metals are associated with either chalcopyrite or molybdenite. Copper shows a strong positive correlation with Au and Pd (r≥+0.91) while Mo shows a strong positive correlation with Pt (r=+0.98). In addition, Mo exhibits a strong positive correlation with W (r=+0.94). An interelement positive correlation between precious metals is 0.57 (Ag–Au). The Pd and Pt contents of both chalcopyrite and molybdenite in flotation concentrates are low, varying from 9 to 83 ppb Pd and from <10 to 110 ppb Pt. The highest values recorded are 924 ppb Pd in sulphide concentrates from the Aksug deposit, and 684 ppb Pd and 299 ppb Pt in sulphide concentrate from breccia of the Zhireken deposit. The highest gold content, 5450 ppb Au, is in chalcopyrite flotation concentrate from the Aksug deposit, whereas in the majority of concentrates the gold content is in the order of a few hundreds parts per billion. The relatively low contents and limited variation of the precious metal contents in the individual deposits of Russia and Mongolia point to the essential role of the composition of the source and composition of parent magmas.

Platinum-group minerals and tetraauricupride in ophiolitic rocks of Skyros island, Greece

SummaryIn the serpentinizedophiolitic rocks from Skyros island, two distinct assemblages of base metal sulphides (BMS) and platinum-group minerals (PGM) occur. The first (early) generation is associated with chromitites which are enriched in platinum-group elements (PGE). The highest values were recorded in samples from Achladones (Ru 1210, Ir 780, Os 630, Rh 228, Pt 208, Pd 22; all values in ppb). Mineral inclusions in chromite consist of Ni-Fe sulphides and Os-rich laurite, and crystallized at high sulphur fugacity (fS2) during chromite formation. The second (late) generation is closely associated with Au-rich, PGE-poor magnetite ores which host a complex assemblage of inclusions consisting mainly of graphite, Cu-Fe- and pure Cu sulphides, sperrylite and tetraauricupride. Their accompanying hydrous silicates are Cl-bearing. It is assumed that this mineral assemblage was deposited by hydrothermal processes during serpentinization.ZusammenfassungIn den serpentinisierten Ophiolithen der Insel Skyros wurden zwei unterschiedliche Bildungsgenerationen von Sulfiden (BMS) und Platinmineralen (PGM) festgestellt. Die erste (frühere) Generation ist an Chromitite gebunden, die hohe Gehalte an Elementen der Platingruppe (PGE) aufweisen. Die höchsten PGE-Kontzentrationen wurden in den Proben der Lokalität Achladones gefunden (Ru 1210, Ir 780, Os 630, Rh 228, Pt 208, Pd 22; alle Gehalte in ppb). Die Einschlüsse in Chromit bestehen aus Ni-Fe Sulfiden und Os-reichem Laurit. Diese Minerale kristallisierten bei hoher Schwefelfugazität (fS2) während der Bildung der Chromite. Die zweite (spätere) Generation ist eng assoziiert mit Au-reichen und PGE-armen Magnetiten. Sie führen eine komplexe Einschluß-Paragenese bestehend aus Graphit, Cu-Fe- und reinen Cu Sulfiden sowie Sperrylith und Tetraauricuprid. Die begleitenden Hydrosilikate sind Cl-haltig. Die Bildung dieser Mineralparagenese wird durch hydrothermale Prozesse während der Serpentinisierung erklärt.

Occurrence of apatite associated with magnetite in an ophiolite complex (Othrys), Greece

Abstract Small irregular to lens-like occurrences (maximum 0.5 × 1 m) of apatite associated with magnetite and silicates are present in the Agoriani area of the Othrys ophiolite complex, central Greece. The Ano Agoriani area is dominated by peridotite (plagioclase lherzolite) of the mantle sequence, intruded by irregular bodies, dikes, and veins of gabbro, as well as by dikes of pyroxenite and pegmatitic gabbro. Apatite occurs as large (up to 3 cm long) well-formed crystals associated with magnetite. The aggregates consist predominantly of apatite, or massive magnetite with subordinate amounts of apatite. Apatite may also be accompanied by silicate minerals, mainly chlorite and lesser amounts of serpentine, tremolite, and Ni-silicates (nepouite, pimelite), and by Ni-sulfides (pentlandite, violarite, heazlewoodite). The apatite-magnetite association from the Agoriani area differs from nelsonite hosted by anorthosite suites with respect to: (1) the host rock type (ophiolites); (2) the highly variable proportion between apatite-magnetite; (3) the large size (up to 3 cm) of the apatite crystals; (4) the lack of fluorine (<20 ppm F) in the apatite; (5) the presence of abundant liquid-rich, fluid inclusions in apatite, and (6) the lack of ilmenite. The high V content (700-1000 ppm) of the magnetite from Agoriani differs from that of disseminated Fe-Ti mineralization (Ti-magnetite, ilmenite) in the magmatic sequence of ophiolite complexes (mainly hosted in gabbronorites), as well as the pure, massive magnetite associated with Fe-Ni-Cu-Co sulfides found in shear zones in ophiolite complexes. The composition of the apatite (chlor-hydroxylapatite), the presence of abundant primary two-phase aqueous fluid inclusions in the apatite, and the composition of the associated magnetite and sulfides, suggest that a hydrothermal system played an essential role in the formation of these deposits.

Natural contamination by As and heavy metals in soil, their bio-accumulation and potential sources: the case of a travertine limestone quarry, Greece

The first mineralogical and geochemical investigation of the travertine limestone, soil and corresponding plants associated with the Neogene basin of Varnavas, NE Attica, revealed a significant enrichment in the metalloid As. The total concentrations of As ranged from 61 to 210 ppm in limestone and 33 to 430 ppm in the associated soil demonstrating a wide variation of values. Calcite is a common authigenic mineral within travertine limestone, forming fine uniform micritic aggregates, having As and Mg concentrations lower than detection limits of EDS analysis. Clastic dominated minerals are quartz (both fine- and coarse-grained), muscovite, clinochlore, illite, pyrite, galena, arsenides, rutile, sphene, zircon, REE-minerals and albite. Goethite and Fe-Mn-oxides occur between calcite grains. The presence of fossilized micro organisms, resembling foraminifera, in travertine limestone combined with hydrous Fe-Mn-oxides, suggests a possible marine transgression during the evolution of the basin.The As content in plants ranges from 1.1 to 28 ppm As in shoots, and 0.8 to 114 ppm As in roots. The translocation factor, which is defined as the ratio of metal concentration in the shoots to the roots, is relatively low (average 0.33%) suggesting that the internal transport of metals from the roots to shoots was restricted. The bioaccumulation factor, which is defined as the ratio of metal concentration in the plants to that in soil, exhibits a wide range from relatively low (5.2–9.0% for As, Fe, Cr, Ni and Pb), much higher (56–67% for Cu and Zn) and exceptionally high (160% for Mo). A significant correlation between the translocation factors for Fe and As may confirm that Fe-Mn oxides/hydroxides represent the major sorbing agents for As in soils. The presented data, due to As contamination in travertine limestone, soil and plants, suggest a potential environmental risk not only for that part of Greece but in general for similar depositional environments.

Platinum-group elements and gold in komatiitic rocks from the Agrilia Formation, Othrys ophiolite complex, Greece

Highly magnesian lavas (32 wt.% MgO) from the Agrilia Formation, Othrys ophiolite complex, central Greece, were analyzed for major and trace elements. Average platinum-group elements (PGE) concentrations (all in ppb) are: Os 3.6, Ir 0.68, Ru 4, Pt 1.2, Pd 8.5, Rh < 0.5 and Au 19. Although the studied lavas from the Agrilia Formation have unusually high Mg, Cr and Ni contents, they differ from boninites in respect to the dominance of olivine phenocrysts, the lack of orthopyroxene and primary hornblende and the preferential association of chromite with groundmass. The higher Pd than Pt concentrations and the lack of detectable Rh appear to be characteristic of the Agrilia ultramafic lavas. The PGE concentrations, the relatively low values of Pd/Ir ratios (∼ 12), the chondrite-normalized PGE patterns — which are almost flat, with exception of a Pt anomaly — and the mineral chemistry of the studied lavas are comparable to typical komatiites, suggesting that these lavas may represent primitive magmas. The derived parental magma for the studied lavas (in equilibrium with Fo90.5) contains ∼ 17 wt.% MgO and is similar to those which gave rise to modern analogues of the Archean komatiites from Gorgona island, Colombia. However, the variation in Ti/V and Ti/Sc ratios in Agrilia lavas and the enrichment in some incompatible elements (Sr, Rb, Ba, La) may indicate a modification of the magma composition in the mantle source region by a subduction component. The occurrence of the Triassic komatiitic (and boninitic) lavas in combination with tholeiitic lavas with mid-ocean ridge basalt (MORB) affinities of the Othrys complex, may suggest changes in the geotectonic setting during the development of the Othrys complex from a fore-arc (formation of the highly magnesian lavas) to back-arc tectonic setting (formation of the main complex).

Spatial Evolution of the Chromium Contamination in Soils from the Assopos to Thiva Basin and C. Evia (Greece) and Potential Source(s): Anthropogenic versus Natural Processes

The investigation of the contamination in soil, plants and groundwater revealed a spatial evolution, with an increasing trend in the Cr, Fe, Ni, Mn and Co contents in soils from the Assopos to Thiva basin, followed by C. Evia and Ni-laterite deposits, suggesting that the latter and their parent ophiolites are a potential source for these metals. In contrast, the contamination in groundwater by Cr(VI), ranging from 2 to 360 μg/L Cr, and a varying degree of salinization is probably due to both human activities and natural processes. A diverse source for the contamination of soil and groundwater in the Assopos-Thiva basins is consistent with the increasing trend of the Mg/Si ratio and Cr(VI) concentration in water. The use of deep karst-type aquifer instead of the shallow-Neogene one may provide a solution to the crucial environmental problem. The selective extraction by EDTA and alkaline solution showed that Cr and Fe are less available than Mn. The Cr contents in plants range from <1 to tens of mg/kg, due probably to the high resistance of chromite. However, the average Crtotal contents in plants/crops are higher than normal or sufficient values, whilst Crtotal accumulation [(% metals in plants × 100)/metal in soil] and Cr(VI) accumulation are relatively low. There is a very good positive correlation between accumulation factors for Cr and Fe (R2 = 0.92), suggesting a similarity concerning their uptake.