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This study provides a comprehensive literature review of the distribution, the platinum- group elements (PGE) composition, and mineral chemistry of chromitites associated with Mesozoic Tethyan ophiolites in the Mediterranean Basin. These suites outcrop in the northern Italian Apennines, the Balkans, Turkey, and Cyprus. Most chromitites occur in depleted mantle tectonites, with fewer found in the mantle-transition zone (MTZ) and supra-Moho cumulates. Based on their Cr# = (Cr/(Cr + Al)) values, chromitites are primarily classified as high-Cr, with a subordinate presence of high-Al chromitites. Occasionally, high-Al and high-Cr chromitites co-exist within the same ophiolite complex. High-Cr chromitites are formed in supra-subduction zone (SSZ) environments, where depleted mantle interacts with high-Mg boninitic melts. Conversely, high-Al chromitites are typically associated with extensional tectonic regimes and more fertile peridotites. The co-existence of high-Al and high-Cr chromitites within the same ophiolite is attributed to tectonic movements and separate magma intrusions from variably depleted mantle sources, such as mid-ocean ridge basalts (MORB) and back-arc basin basalts. These chromitites formed in different geodynamic settings during the transition of the oceanic lithosphere from a mid-ocean ridge (MOR) to a supra-subduction zone (SSZ) regime or, alternatively, within an SSZ during the differentiation of a single boninitic magma batch. Distinct bimodal distribution and vertical zoning were observed: high-Cr chromitites formed in the deep mantle, while Al-rich counterparts formed at shallower depths near the MTZ. Only a few of the aforementioned chromitites, particularly the high-Cr ones, are enriched in the refractory IPGE (iridium-group PGE: Os, Ir, Ru) relative to PPGE (palladium-group PGE: Rh, Pt, Pd), with an average PPGE/IPGE ratio of 0.66, resulting in well-defined negative slopes in PGE patterns. The IPGE enrichment is attributed to their compatible geochemical behavior during significant degrees of partial melting (up to 30%) of the host mantle. It is suggested that the boninitic melt, which crystallized the high-Cr chromitites, was enriched in IPGE during melt-rock reactions with the mantle source, thus enriching the chromitites in IPGE as well. High-Al chromitites generally exhibit high PPGE/IPGE ratios, up to 3.14, and strongly fractionated chondrite-normalized PGE patterns with positive slopes and significant enrichments in Pt and Pd. The PPGE enrichment in high-Al chromitites is attributed to the lower degree of partial melting of their mantle source and crystallization from a MOR-type melt, which contains fewer IPGE than the boninitic melt above. High-Al chromitites forming at higher stratigraphic levels in the host ophiolite likely derive from progressively evolving parental magma. Thus, the PPGE enrichment in high-Al chromitites is attributed to crystal fractionation processes that consumed part of the IPGE during the early precipitation of co-existing high-Cr chromitites in the deep mantle. Only a few high-Al chromitites show PPGE enrichment due to local sulfur saturation and the potential formation of an immiscible sulfide liquid, which could concentrate the remaining PPGE in the ore-forming system. [ABSTRACT FROM AUTHOR]

The research interest for many authors has been focused on the origin, recovery, and exploration of critical metals, including platinum-group elements (PGEs), with the aim of finding new potential sources. Many giant porphyry Cu deposits are well known around the Pacific Rim, in the Balkan–Carpathian system, Himalayas, China, and Malaysia. However, only certain porphyry Cu-Au deposits are characterized by the presence of significant Pd and Pt contents (up to 20 ppm). This contribution provides new analytical data on porphyry-Cu-Au±Pd±Pt deposits from the Chalkidiki Peninsula and an overview of the existing geochemical characteristics of selected porphyry-Cu deposits worldwide in order to define significant differences between PGE-fertile and PGE-poor porphyry-Cu intrusions. The larger Mg, Cr, Ni, Co, and Re contents and smaller LILE elements (Ba and Sr) in fertile porphyry-Cu-Au-(PGE) reflect the larger contribution from the mantle to the parent magmas. In contrast, the smaller Mg, Cr, Ni, Co, and Re contents and larger Ba and Sr in PGE-poor porphyry-Cu-Mo deposits from the Chalkidiki Peninsula (Vathi, Pontokerasia, and Gerakario) and Russia–Mongolia suggest the presence of parent magmas with a more crustal contribution. Although there is an overlap in the plots of those elements, probably due to the evolution of the ore-forming system, consideration of the maximum contents of Mg, Cr, Ni, and Co is proposed. Magnetite which separated from the mineralized Skouries porphyry of Greece showed small negative Eu anomalies (Eu/Eu* ≥ 0.55), reflecting a relatively high oxidation state during the cooling of the ore-forming system. The relatively high, up to 6 ppm (Pd+Pt), and low Cr content towards the transition from the porphyry to epithermal environment, coupled with the occurrence of Pd, Te, and Se minerals (merenskyite, clausthalite), and tetrahedrite–tennantite in fertile porphyry Cu deposits (Elatsite deposit, Bulgaria), reflect a highly fractionated ore-forming system. Thus, in addition to the crustal and mantle recycling, metasomatism, high oxidation state, and abundant magmatic water, other factors required for the origin of fertile porphyry-Cu deposits are the critical degree of mantle melting to release Pt and Pd in the ore-forming fluids and the degree of fractionation, as reflected in the mineral chemistry and geochemical data. [ABSTRACT FROM AUTHOR]

The Italian Northern Apennines contain several Fe-Cu-Zn-bearing, Cyprus-type volcanogenic massive sulfide (VMS) deposits, which significantly contribute to the Cu resources of Italy. The massive sulfide lenses and related stockwork mineralizations are hosted by several levels (from basalt to serpentinite) of the unmetamorphosed ophiolitic series; therefore, this region offers perfect locations to study the ore-forming hydrothermal system in detail. A combination of fluid inclusion microthermometry, Raman spectroscopy, electron probe analyses (chlorite thermometry) and stable and noble gas isotope geochemistry was used to determine the fluid source of the VMS system at Bargone, Boccassuolo, Campegli, Casali–Monte Loreto, Corchia, Reppia and Vigonzano. This question of the fluid source is the focus of modern VMS research worldwide, as it has a direct influence on the metal content of the deposit. The obtained temperature and compositional data are both in the typical range of VMS systems and basically suggest evolved seawater origin for the mineralizing fluid. Modification of seawater was most commonly due to fluid–rock interaction processes, which happened during long-lasting circulation in the crust. The role of a small amount of magmatic fluid input was traced only at the lower block of Boccassuolo, which may be responsible for its higher ore grade. This fluid origin model is evidenced by O, H and C stable isotopic as well as He, Ne and Ar noble gas isotopic values. [ABSTRACT FROM AUTHOR]

The crystal chemistry of some Cr-spinels from the lherzolite body of the Ronda peridotite in southern Spain has been investigated. Cell edge spans between 8.1692(2) and 8.2367(1) [Angstrom], while the oxygen positional parameter u ranges between 0.26306(7) and 0.26351 (7). By using the Princivalle thermometer (1999), an intracrystalline closure temperature between 640 and 840[degrees]C has been calculated. The higher temperatures are very close to the intercrystalline temperatures based on the olivine-spinel thermometer calculated by Woodland et al. (2006) for the Ronda orogenic lherzolites suggesting that the intracrystalline closure occurred soon after the intercrystalline closure. By comparison with Cr-spinels from lherzolite mantle xenoliths, it should be noted that: (1) the oxygen positional parameter can be linearly related to the intracrystalline temperature for both mantle peridotite and mantle xenolith Cr-spinels; (2) the intracrystalline closure temperature is reached faster and is higher in Cr-spinels in mantle xenoliths; and (3) Cr content is linearly related to u in mantle peridotite, but not in mantle xenoliths, suggesting the u value in Cr-spinels from mantle peridotite is driven solely by the chemistry of the spinels. Keywords: Cr-spinel, crystal chemistry, intracrystalline closure temperature, mantle peridotite, Ronda (Spain) DOI: 10.2138/am.2010.3545

Arsenotučekite, Ni18Sb3AsS16, is a new mineral discovered in the abandoned chromium mine of Tsangli, located in the eastern portion of the Othrys ophiolite complex, central Greece. Tsangli is one of the largest chromite deposit at which chromite was mined since 1870. The Tsangli chromitite occurs as lenticular and irregular bodies. The studied chromitites are hosted in a strongly serpentinized mantle peridotite. Arsenotučekite forms anhedral to subhedral grains that vary in size between 5 μm up to 100 μm, and occurs as single phase grains or is associated with pentlandite, breithauptite, gersdorffite and chlorite. It is brittle and has a metallic luster. In plane-polarized light, it is creamy-yellow, the bireflectance is barely perceptible and the pleochroism is weak. In crossed polarized reflected light, the anisotropic rotation tints vary from pale blue to brown. Internal reflections were not observed. Reflectance values of arsenotučekite in air (Ro, Re′ in %) are: 41.8–46.4 at 470 nm, 47.2–50.6 at 546 nm, 49.4–52.3 at 589 nm, and 51.3–53.2 at 650 nm. The empirical formula of arsenotučekite, based on 38 atoms per formula unit, and according to the structural results, is (Ni16.19Co1.01Fe0.83)Σ18.03Sb3(As0.67Sb0.32)Σ0.99S15.98. The mass density is 6.477 g·cm−3. The simplified chemical formula is (Ni,Co,Fe)18Sb3(As,Sb)S16. The mineral is tetragonal and belongs to space group I4/mmm, with a = 9.7856(3) Å, c = 10.7582(6) Å, V = 1030.2(6) Å3 and Z = 2. The structure is layered (stacking along the c-axis) and is dominated by three different Ni-coordination polyhedral, one octahedral and two cubic. The arsenotučekite structure can be considered as a superstructure of tučekite resulting from the ordering of Sb and As. The name of the new mineral species indicates the As-dominant of tučekite. Arsenotučekite occurs as rims partly replacing pentlandite and irregularly developed grains. Furthermore, it is locally associated with chlorite. These observations suggest that it was likely precipitated at relatively low temperatures during: 1) the late hydrothermal stages of the ore-forming process by reaction of Sb- and As-bearing solutions with magmatic sulfides such as pentlandite, or 2) during the serpentinization of the host peridotite. The mineral and its name have been approved by the Commission of New Minerals, Nomenclature, and Classification of the International Mineralogical Association (number 2019–135). [ABSTRACT FROM AUTHOR]

Thalhammerite, Pd9Ag2Bi2S4, is a new sulphide discovered in galena-pyrite-chalcopyrite and millerite-bornite-chalcopyrite vein-disseminated ores from the Komsomolsky mine of the Talnakh and Oktyabrsk deposits, Noril'sk region, Russia. It forms tiny inclusions (from a few μm up to about 40-50 μm) intergrown in galena, chalcopyrite, and also in bornite. Thalhammerite is brittle and has a metallic lustre. In plane-polarized light, thalhammerite is light yellow with weak bireflectance, weak pleochroism, in shades of slightly yellowish brown and weak anisotropy; it exhibits no internal reflections. Reflectance values of thalhammerite in air (R1, R2 in %) are: 41.9/43.0 at 470 nm, 43.9/45.1 at 546 nm, 44.9/46.1 at 589 nm, and 46.3/47.5 at 650 nm. Three spot analyses of thalhammerite give an average composition: Pd 52.61, Bi 22.21, Pb 3.92, Ag 14.37, S 7.69, and Se 0.10, total 100.90 wt %, corresponding to the empirical formula Pd8.46Ag2.28(Bi1.82Pb0.32)Σ2.14(S4.10Se0.02)Σ4.12 based on 17 atoms; the average of five analyses on synthetic thalhammerite is: Pd 55.10, Bi 24.99, Ag 12.75, and S 7.46, total 100.30 wt %, corresponding to Pd8.91Ag2.03Bi2.06S4.00. The density, calculated on the basis of the empirical formula, is 9.72 g/cm³. The mineral is tetragonal, space group I4/mmm, with a 8.0266(2), c 9.1531(2) Å, V 589.70(2) Å3 and Z = 2. The crystal structure was solved and refined from the single-crystal X-ray-diffraction data of synthetic Pd9Ag2Bi2S4. Thalhammerite has no exact structural analogues known in the mineral system; chemically, it is close to coldwellite (Pd3Ag2S) and kravtsovite (PdAg2S). The strongest lines in the X-ray powder diffraction pattern of synthetic thalhammerite [d in Å (I) (hkl)] are: 3.3428(24)(211), 2.8393(46)(220), 2.5685(21)(301), 2.4122(100)(222), 2.3245(61)(123), 2.2873(48)(004), 2.2201(29)(132), 2.0072(40)(400), 1.7481(23)(332), and 1.5085(30)(404). The mineral honours Associate Professor Oskar Thalhammer of the University of Leoben, Austria. [ABSTRACT FROM AUTHOR]

Phosphide minerals, generally less than 20 μm in size, are mainly made of Ni-V-Co-Mo constituents, discovered in concentrates obtained from chromitite samples from the Agios Stefanos mine, Othrys ophiolite, Greece. Spinel from the chromitite is rich in Fe2O3 and very similar to chromites from podiform chromitites. Its Cr/(Cr+Al) ratios are lower than those of chromites with a boninitic affinity. On the basis of their chemical compositions, the following phosphide minerals have been identified: melliniite, nickelphosphide and two phases that can be classified as (i) either Ni-allabogdanite or Ni-barringerite, and (ii) either V-allabogdanite or V-barringerite. Under reflected-light microscope, all the inspected phosphide minerals display similar reflectance and a creamy-yellowish color, despite their different compositions. Melliniite is isotropic, while nickelphosphide shows a weak anisotropism. The Ni-allabogdanite or Ni-barringerite and V-allabogdanite or V-barringerite display a strong anisotropism. They occur associated with awaruite, pentlandite, native vanadium, vanadium sulfides, Mo-Ni-V-Co alloy (probably hexamolybdenum), Hg selenide (possible tiemmanite) and several minerals composed of Ni, As and Sb. A few of them were in contact with chromite, chlorite, quartz and glass. This mineralogical assemblage suggests that the discovered minerals are natural in origin and they do not represent an artefact related to the sample preparation. Furthermore, it is suggested that the investigated minerals have been crystallized in a local reducing geochemical environment. During the serpentinization of peridotites, reducing fluids containing dissolved H2 are released from the reduction of H2O. Therefore, these phosphides and the associated minerals may have precipitated during the serpentinization process at low temperature. Nickel and cobalt may have been released during alteration of olivine or they were originally hosted in magmatic sulfides that were altered during the serpentinization. Phosphorous may represent an alteration product of apatite and olivine. Vanadium was probably released during the alteration of the host chromite. We speculate that Mo was originally hosted in sulfides or oxides and after its remobilization it was incorporated into the crystal lattice of the Mo-rich phases. Alternatively, the high reducing minerals of the Othrys may have formed because of the interaction of their host rock with a lightning. The stoichiometry of the analyzed phosphide minerals indicates that some grains found in the Othrys chromitites may represent new mineral species, such as Ni-allabogdanite or Ni-barringerite, as well as V-allabogdanite or V-barringerite. However, their small size and complex micro-intergrowths with other minerals inhibit the elaboration of an X-ray diffraction study, which would unequivocally determine their nature. To the best of our knowledge, melliniite and nickelphosphide in the Othrys chromitite represent the first finding of these rare minerals in terrestrial samples. [ABSTRACT FROM AUTHOR]

Small grains, approximately 15 µm in size, of a Ni-phosphide, have been discovered in two chromite deposits associated with altered mantle peridotite: (i) in Bakanov Kluch, Alapaesk ophiolite, Russia and (ii) in Agios Dimitrios, Gerakini-Ormylia ophiolite, Greece. On the basis of the chromite composition, both the Bakanov Kluch and the Agios Dimitrios chromitites are classified as Cr-rich chromitites with a boninitic affinity. Under reflected-light optical microscope, the Ni-phosphides are yellow in colour and isotropic. Their calculated stoichiometry, based on the average of 20 electron microprobe analyses, approaches the ideal formula (Ni,Fe)5P. One essential condition for the formation of terrestrial minerals in the Ni-Fe-P system is the presence of a highly reducing local geochemical environment. Serpentinization of oceanic peridotites produces reducing fluids containing dissolved H2 resulting from the reduction of H2O. Therefore, we suggest that the (Ni,Fe)5P from the Alapaevsk and Gerakini-Ormylia were very likely formed after the precipitation of the host chromitites during the serpentinization process at low temperatures. Nickel may have been released during the alteration of olivine and phosphorous may represent an alteration product of apatite. The (Ni,Fe)5P grains analyzed in the present work contain the highest value of Ni and they are characterized by the highest Ni to P ratio reported so far from similar minerals. Their stoichiometry indicates that the (Ni,Fe)5P grains found in the Alapaevsk and Gerakini-Ormylia chromitites represent a new mineral species. However, their small size prevents the acquisition of crystallographic data to support this assumption. [ABSTRACT FROM AUTHOR]