Platinum-Group Mineral Characterization in Concentrates from High-Grade PGE Al-rich Chromitites of Korydallos Area in the Pindos Ophiolite Complex (NW Greece)

The Pindos ophiolite complex, located in the north-western part of continental Greece, hosts various podiform chromite deposits generally characterized by low platinum-group element (PGE) grades. However, a few locally enriched in PPGE + Au (up to 29.3 ppm) chromitites of refractory type are also present, mainly in the area of Korydallos (south-eastern Pindos). The present data reveal that this enrichment is strongly dependant on chromian spinel chemistry and base metal sulfide and/or base metal alloy (BMS and BMA, respectively) content in chromitites. Consequently, we used super-panning to recover PGM from the Al-rich chromitites of the Korydallos area. The concentrate of the composite chromitite sample contained 159 PGM grains, including, in decreasing order of abundance, the following major PGM phases: Pd-Cu alloys (commonly non-stoichiometric, although a few Pd-Cu alloys respond to the chemical formula PdCu₄), Pd-bearing tetra-auricupride [(Au,Pd)Cu], nielsenite (PdCu₃), sperrylite (PtAs₂), skaergaardite (PdCu), Pd-bearing auricupride [(Au,Pd)Cu₃], Pt and Pd oxides, Pt-Fe-Ni alloys, hollingworthite (RhAsS) and Pt-Cu alloys. Isomertieite (Pd₁₁Sb₂As₂), zvyagintsevite (Pd₃Pb), native Au, keithconnite (Pd₂₀Te₇), naldrettite (Pd₂Sb) and Rh-bearing bismuthotelluride (RhBiTe, probably the Rh analogue of michenerite) constitute minor phases. The bulk of PGE-mineralization is dominated by PGM grains that range in size from 5 to 10 µm. The vast majority of the recovered PPGM are associated with secondary BMS and BMA, thus confirming that a sulphur-bearing melt played a very important role in scavenging the PGE + Au content of the silicate magma from which chromian spinel had already started to crystallize. The implemented technique has led to the recovery of more, as well as noble, PGM grains than the in situ mineralogical examination of single chromitite samples. Although, the majority of the PGM occur as free particles and in situ textural information is lost, single grain textural evidence is observed. In summary, this research provides information on the particles, grain size and associations of PGM, which are critical with respect to the petrogenesis and mineral processing.     

Natural radioactivity distribution and gamma radiation exposure of beach sands from Sithonia Peninsula

This study aims to evaluate the activity concentrations of ²³⁸U, ²²⁶Ra, ²³²Th, ²²⁸Th and ⁴⁰K along the beaches of Sithonia Peninsula which are adjacent to the rock-types of the Sithonia Plutonic Complex. These range from 6–673, 5–767, 5–1750, 6–1760 and 185–875 Bq/kg respectively. The (% wt.) heavy magnetic (HM) fraction (epidote, allanite, hornblende, biotite and garnet), the heavy non-magnetic (HNM) fraction (monazite, zircon, titanite and apatite) and the total heavy (TH) fraction, were correlated with the concentrations of the measured radionuclides in the bulk samples. The HNM fraction seems to control the activity concentrations of ²³⁸U in all samples, while the HM fraction, at least for the heavy mineral rich samples bearing high amounts of epidote crystals with allanite cores, controls their ²³²Th content. The measured radionuclides in beach sands were normalized to the respective values measured in the granitic rocks, which are their most probable parent rocks, in order to provide data on their enrichment or depletion. The annual effective dose varies from 0.013 to 0.688 mSv y^?1 for local people working on the beach, while for tourists the annual external effective dose ranges between 0.003 and 0.165 mSv y^?1.     

Alteration of chromitites from the Voidolakkos and Xerolivado mines,Vourinos ophiolite complex,Greece: implications for deformation‐induced metamorphism

Chromitites, associated with upper mantle spinel peridotites from the Voidolakkos and Xerolivado districts, located in the Vourinos ophiolite complex, northwestern continental Greece, were re‐investigated with respect to their structural and textural mode of occurrence, as well as their compositional signatures. They include variably deformed banded and podiform chromitite bodies made up of massive, semi‐massive, nodular, anti‐nodular, schlieren and disseminated chromian spinel. Chromitites have suffered intense shearing that was more severe in all but disseminated textured ore. Deformation has partly produced elongation of chromian spinel nodules and widespread protocataclastic zones within chromitites. The examined deposits are composed of magnesiochromite that shows a quite restricted range of Cr# [Cr/(Cr + Al)] values varying between 0.76 and 0.83, whereas Mg# [Mg/(Mg + Fe²⁺)] ranges from 0.55 to 0.67 accompanied by relatively low content in TiO₂ (<0.15 wt.% on average). Compositional data indicate that these high‐Cr chromitite bodies crystallized from melts of boninitic affinities that have been compositionally modified after reaction with depleted harzburgite, followed by interaction with relatively undifferentiated low SiO₂ melts within an intertwined system of dunite channels in the mantle wedge below the fore‐arc region of a supra‐subduction zone (SSZ). Magnesiochromite displays limited textural modification, being scarcely transformed to an opaque spinel phase along grain boundaries and fracture walls. The opaque spinel phase is characterized by elevated Cr# (0.76–0.97), relatively low Mg# (0.33–0.63) and low Fe³⁺# (≤0.14) and corresponds to ferrian chromite. Microscopic studies revealed that ferrian chromite is paragenetically associated with clinochlore even in unaltered chromitite specimens and the degree of serpentinization does not correlate with the frequency and abundance of alteration effects on magnesiochromite. Therefore, it is supported that regional metamorphism prior to serpentinization was responsible for the formation of the ferrian chromite–clinochlore association. In addition, magnesiochromite alteration was systematically recorded only in variably sheared chromitites displaying protocataclastic brecciation, thus it can be claimed that metamorphism was mainly governed by deformation mechanisms, which took place during the transition from ductile to semi‐brittle conditions. Ferrian chromite can be locally erratically enriched in MnO and ZnO, which is attributed to a Mn‐ and Zn‐bearing, slightly acid post‐magmatic fluid that invaded the chromitites across weakness zones. Overall, the data suggest that after magnesiochromite equilibration with the intergranular olivine, both phases were partly replaced by ferrian chromite and clinochlore, respectively, during a brief, fluid assisted episode of retrogade metamorphism that took place within a temperature interval between 700 and 300 °C, before ocean‐floor alteration. Copyright © 2014 John Wiley & Sons, Ltd.     

Polycrystalline apatite synthesized by hydrothermal replacement of calcium carbonates

Aragonite and calcite single crystals can be readily transformed into polycrystalline hydroxyapatite pseudomorphs by hydrothermal treatment in a (NH₄)₂HPO₄ solution. Scanning electron microscopy of the reaction products showed that the transformation of aragonite to apatite is characterised by the formation of a sharp interface between the two phases and by the development of intracrystalline porosity in the hydroxyapatite phase. In addition, electron backscattered diffraction (EBSD) imaging showed that the c-axis of apatite is predominantly oriented perpendicular to the reaction front with no crystallographic relationship to the aragonite lattice. However, the Ca isotopic composition of the parent aragonite, measured by thermal ionization mass spectrometry was inherited by the apatite product.Hydrothermal experiments conducted with use of phosphate solutions prepared with water enriched in ¹⁸O (97%) further revealed that the ¹⁸O from the solution is incorporated in the product apatite, as measured by micro-Raman spectroscopy. Monitoring the distribution of ¹⁸O with Raman spectroscopy was possible because the incorporation of ¹⁸O in the PO₄ group of apatite generates four new Raman bands at 945.8, 932, 919.7 and 908.8 cm⁻¹, in addition to the ν₁(PO₄) symmetric stretching band of apatite located at 962 cm⁻¹, which can be assigned to four ¹⁸O-bearing PO₄ species. The relative intensities of these bands reflect the ¹⁸O content in the PO₄ group of the apatite product. By using equilibrated and non-equilibrated solutions, with respect to the ¹⁸O distribution between aqueous phosphate and water, we could show that the concentration of ¹⁸O in the apatite product is linked to the degree of ¹⁸O equilibration in the solution. The textural and chemical observations are indicative of a coupled mechanism of aragonite dissolution and apatite precipitation taking place at a moving reaction interface.     

Early cretaceous ophiolites of the Yarlung Zangbo Suture Zone: insights from dolerites and peridotites from the Baer upper mantle suite,SW Tibet (China)

The Baer ophiolitic massif is located in the northern sub-belt of the western segment of the Yarlung Zangbo Suture Zone (YZSZ) and mainly consists of a lherzolite-dominant mantle suite, dolerite intrusions and limited crustal outcrops. The dolerites show sub-ophitic texture and light rare earth element-depleted chondrite-normalized rare earth element patterns similar to normal-mid-ocean ridge basalts (N-MORB); though, they display enrichments in fluid-mobile elements (Rb, Ba, and Sr) and marked depletions in Th and Nb. The U–Pb ages of several magmatic zircon grains recovered from two dolerite samples indicate that the intrusion of the dikes into the Baer lherzolitic mantle occurred at 125.6–126.3 Ma, consistent with the widespread mafic magmatism between 120 and 130 Ma in the Yarlung Zangbo ophiolites. The dolerites have slightly more radiogenic ⁸⁷Sr/⁸⁶Sr ratios (0.7043–0.7054) in comparison to N-MORB, whereas they show ¹⁴³Nd/¹⁴⁴Nd values (0.513067–0.513114) similar to N-MORB and high zircon Hf-isotope compositions. They have a limited range of Nd-isotope (ε_Nd(t) values: +8.2 to +9.1) and juvenile Hf-isotope compositions (ε_Hf(t) values: +8.4 to +14.2 and +10.0 to +15.1) indicating derivation from mantle melts. The moderate spread in the ε_Hf (t) values of zircons indicates derivation of the dolerites parental magma from a weakly contaminated spinel-bearing mantle source. This is also corroborated by the geochemical signatures of the Baer dolerites (enrichment in LILE and depletion in HFSE) suggesting minor slab input to the mantle source of the dike-filling melt. We suggest that the genesis of the dolerite dike-forming melt happened at a stage of subduction initiation in a sub-oceanic mantle domain mildly affected by fluids emanating from the downgoing slab. Our data combined with literature data allow us to presume that the intrusion of the dolerites into the Baer mantle corresponds to an early phase of subduction initiation beneath a developing forearc basin.     

An (in‐)coherent metamorphic evolution of high‐P eclogites and their host rocks in the Chinese southwest Tianshan?

In the (ultra‐)high‐P–low‐T metamorphic terrane of the Chinese South Tianshan, discontinuous mafic blocks and boudins (former upper oceanic crust) are now embedded in voluminous (mainly metasedimentary) host rocks. Two different models were proposed and relate the occurrence of both high‐P and ultra‐high‐P mafic and metasedimentary rocks to either (i) a tectonic mélange style exhumation, with no exhumation of coherent units, but different lithologies derived from different depths juxtaposed and intermingled during exhumation in the subduction channel, or (ii) the evolution of two coherent metamorphic belts: one with high‐P and the other with ultra‐high‐P conditions. In contrast to most previous studies in the Chinese South Tianshan which focused either on single eclogites or metasedimentary rocks (assumed as representative), this study concentrates on the systematic investigation of both mafic boudins and their immediate sedimentary host rocks, because the investigation of both lithologies and the comparison of their metamorphic evolution is crucial to reconstruct the geodynamical context of the whole (ultra‐)high‐P–low‐T metamorphic complex. Several sample pairs consisting of both lithologies were geochemically investigated and their respective metamorphic evolution was reconstructed using geothermobarometry and thermodynamic modelling. The latter approach considers changes in the mineral assemblage during the metamorphic evolution, as well as changes in mineral composition, which may help to determine the metamorphic history of a rock despite the preservation of critical mineral assemblages. All samples experienced a clockwise P–T path with overall maximum P–T conditions of 540–550 °C and 1.9–2.25 GPa for the host rocks, and 555–575 °C and 2.2–2.5 GPa for the eclogites. Peak‐metamorphic temperatures of ~525–540 °C of the metasedimentary host rocks were also confirmed by Raman spectroscopy of carbonaceous material. Results from thermobarometry and thermodynamic modelling are consistent with the observation that none of the samples contains mineral relicts indicating UHP conditions (like coesite in garnet) and neither conventional thermobarometry, nor thermodynamical modelling resulted in P–T conditions in the stability field of coesite. Thus, no evidence of ultra‐high‐P conditions was found. Given that the whole sampled river valley lies within the proposed ‘ultra‐high‐P sub‐belt’ and considering former studies, which showed that at other places within this ‘unit’ both ultra‐high‐P and high‐P rocks are now juxtaposed on a small scale, the formation of the whole (ultra‐)high‐P–low‐T metamorphic belt in the Chinese South Tianshan as a tectonic mélange style exhumation is more convincing than the formation and juxtaposition of two coherent metamorphic units with high‐P and ultra‐high‐P conditions respectively.     

Mechanism of hydrothermal alteration of natural self-irradiated and synthetic crystalline titanate-based pyrochlore

Pyrochlore-based ceramics are considered as a potential host to immobilize high-level nuclear waste in a geological repository. For this application it is essential to investigate the fundamental mechanism of the equilibration of pyrochlore in aqueous solutions. We treated cuboids of a synthetic crystalline and a natural metamict, i.e., self-irradiation-damaged Ti-based pyrochlore in 1 ml of a 1 M HCl solution containing 43.5% ¹⁸O at 250 °C for 72 h. After the experiments the colours of the samples turned from black to almost white and their surfaces were covered by micron-sized crystallites. Both samples were mainly transformed into rutile with subordinate anatase and the reaction can be traced several tens of micrometers into the cuboids. In the case of the natural pyrochlore an additional phase (aeschynite [(REE)(Ti, Nb)₂(O, OH)₂]) appeared as crystals on the surface and within an up to 100 μm thick layer at the interface between TiO₂ and the unreacted pyrochlore. ¹⁸O is in both cases highly enriched in the reaction products with a sharp gradient (on a micrometer scale) to the unreacted pyrochlore. The replacement reaction retains even fine-scale morphological features typical for pseudomorphs. This observation along with the enrichment of ¹⁸O in the product phases and the textural features clearly show that the dissolution of pyrochlore is spatially coupled with the precipitation of stable (metastable) TiO₂ phases at a moving front. During such a coupled process dissolution at the proceeding front is the rate controlling step. The aeschynite phase at the interface is thought to be either an intermediate alteration product or crystallized from a fluid which is located at the interface between the TiO₂ polymorphs and the unreacted pyrochlore due to changing transport properties between the interface and the fluid with increasing thickness of the TiO₂ rim. We emphasize here that our results produced under relatively extreme batch-experimental conditions show similarities with nature as well as with results derived from experiments conducted under moderate conditions rather expected in a nuclear repository.     

Mineralogy,geochemistry and geotectonic significance of harzburgites from the southern Dramala upper mantle suite,Pindos ophiolite complex,NW Greece

The Dramala massif, located in the Dinarides–Hellenides orogenic belt, forms the mantle section of the Neotethyan Pindos ophiolite complex in NW continental Greece. Its southern domain is comprised mainly of voluminous harzburgite masses with variable clinopyroxene and olivine modal abundances, ranging from clinopyroxene‐bearing to typical and olivine‐rich harzburgites. The harzburgite varieties are characterized by elevated Cr# [Cr/(Cr + Al)] in Cr‐spinel (0.43–0.79), high forsterite (Fo) content in olivine (0.90–0.93), low Al₂O₃ content in clinopyroxene (≤1.77 wt.%) and poor whole‐rock abundances of Al₂O₃ (≤0.68 wt.%), CaO (≤0.68 wt.%), Sc (≤11 ppm) and REE, which are indicative of their refractory nature. In terms of fO₂ values, the southern Pindos harzburgites plot between the FMQ‐2 (Fayalite–Magnetite–Quartz) and FMQ + 2 buffers. Simple batch and fractional melting models are not sufficient to explain their depleted composition. Their Ni/Yb ratios vs. Yb bulk‐rock abundances can be reproduced by up to 22–31% closed‐system non‐modal dynamic melting of an assumed spinel‐bearing lherzolite source. Cr‐spinel chemistry data suggest that the southern Dramala harzburgites were formed in an oceanic centre and then were reworked in the mantle wedge above a subducted slab. Combined petrographic and compositional data indicate that the studied harzburgites interacted with arc‐derived tholeiitic melts. This interaction resulted in substantial olivine and minor Cr‐spinel addition to the studied harzburgites, thus enhancing their refractoriness. Cryptic metasomatism was plausibly responsible for the demolition of any strong geochemical signatures suggestive of a previous melting event in a spreading centre. Comparable observations from the neighbouring Vourinos suite imply that the southern Dramala harzburgites probably represent an arc/fore‐arc mantle region within the mutual Pindos–Vourinos, Mesohellenic lithospheric mantle. Copyright © 2014 John Wiley & Sons, Ltd.