The podiform chromitites in the Dagküplü and Kavak mines, Eskisehir ophiolite (NW-Turkey) : genetic implications of mineralogical and geochemical data

Mantle tectonites from Eskisehir (NW-Turkey) include high-Cr chromitites with limited variation of Cr#, ranging from 65 to 82. Mg# ratios are between 54 and 72 and chromite grains contain up to 3.71 wt% Fe2O3 and 0.30 wt% TiO2. PGE contents are variable and range from 109 to 533 pbb. Chondrite-normalized PGE patterns are flat from Os to Rh and negatively sloping from Rh to Pd. Total PGE contents and low Pd/Ir ratios (from 0.07 to 0.41) of chromitites are consistent with typical ophiolitic chromitites. Chromite grains contain a great number of solid inclusions. They comprise mainly of highly magnesian (Mg# 95-98) mafic silicates (olivine, amphibole and clinopyroxene) and base-metal sulfide inclusions of millerite (NiS), godlevskite (Ni7S6), bornite (C5FeS4) with minor Ni arsenides of maucherite (Ni11As8) and orcelite (Ni5-xAs2), and unnamed Cu2FeS3 phases. Heazlewoodite, awaruite, pyrite, and rare putoranite (Cu9Fe,Ni9S16) were also detected in the matrix of chromite as secondary minerals. Laurite [(Ru,Os)S2] was the only platinum-group minerals found as primary inclusions in chromite. They occur as euhedral to subhedral crystals trapped within chromite grains and are believed to have formed in the high temperature magmatic stage during chromite crystallization. Laurite has limited compositional variation, range between Ru0.94Os0.03Ir0.02S1.95 and Ru0.64Os0.21Ir0.10S1.85, and contain up to 1.96 at% Rh and 3.67 at% As. Close association of some laurite grains with amphibole and clinopyroxene indicates crystallization from alkali rich fluid bearing melt in the suprasubduction environment. The lack of any IPGE alloys, as well as the low Os-content of laurite, assumes that the melt from which chromite and laurite were crystallized had relatively high fS2 but never reached the fS2 to crystallize the erlichmanite. The presence of millerite, as primary inclusions in chromite, reflects the increasing fS2 during the chromite crystallization

Abiotic ammonium formation in the presence of Ni-Fe metals and alloys and its implications for the Hadean nitrogen cycle

Experiments with dinitrogen-, nitrite-, nitrate-containing solutions were conducted without headspace in Ti reactors (200°C), borosilicate septum bottles (70°C) and HDPE tubes (22°C) in the presence of Fe and Ni metal, awaruite (Ni80Fe20) and tetrataenite (Ni50Fe50). In general, metals used in this investigation were more reactive than alloys toward all investigated nitrogen species. Nitrite and nitrate were converted to ammonium more rapidly than dinitrogen, and the reduction process had a strong temperature dependence. We concluded from our experimental observations that Hadean submarine hydrothermal systems could have supplied significant quantities of ammonium for reactions that are generally associated with prebiotic synthesis, especially in localized environments. Several natural meteorites (octahedrites) were found to contain up to 22 ppm Ntot. While the oxidation state of N in the octahedrites was not determined, XPS analysis of metals and alloys used in the study shows that N is likely present as nitride (N3-). This observation may have implications toward the Hadean environment, since, terrestrial (e.g., oceanic) ammonium production may have been supplemented by reduced nitrogen delivered by metal-rich meteorites. This notion is based on the fact that nitrogen dissolves into metallic melts

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

In the serpentinized ophiolitic 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. © 1992 Springer-Verlag

Genesis and growth of internal microstructures of manganese nodule

20-24Microscopic and microprobe studies of manganese nodule from the Somali basin indicate that the reflectance of Fe-Mn oxyhydroxide varies with Mn and Fe concentrations. The non-oxyhydroxide components (detrital, biogenic debris, organic matter, etc.) appear to have a substantial control in the growth of different microstructures and show tendency to align their longest axes in the growth direction of structural units. This alignment appears to be due to the inward forces associated with surface tension of the 2 adjacent colloidal precipitate surfaces which have a tendency to grow into a spheroid, probably similar to phenomenon of drop formation. Detrital components within the nodule material identified include heavy titanium minerals, viz. titanomagnetite, ilmenite and rutile which may be the erosional products of precambrian granites of the Seychelles group of islands. The microprobe point analyses indicate that the studied nodule structures have accreted mainly by hydrogenous precipitation with < 15% detrital influence. The alkali and alkaline earth metals (Na, K, Ca and Ba) appear to exist in the Mn mineral phase as charge balancing cations or result from the adsorption of seawater cations on the large surface areas for delta-MnO2

Irarsite-hollingworthite solid-solution series and other associated Ru-, Os-, Ir-, and Rh-bearing PGM's from the Shetland ophiolite complex

Pure end and intermediate members of the irarsite-hollingworthite solid-solution series occur in the Shetland ophiolite complex. Hollingworthite frequently rims irarsite. Their compositions are unusually Pt poor, compared with analyses of these minerals from elswhere, suggesting the existence of a Pt-poor environment during their formation. Ir-Sb-S and Rh-Sb-S have been identified as inclusions within irarsite. Ir-Sb-S and Rh-Sb-S together with Rh-Ni-Sb are thought to be new platinum-group minerals (PGM's) in ophiolite complexes. Two types of laurite are present. An Os-rich (up to 22% Os) variety is entirely enclosed by chromite, whereas an Os-free variety is located in the silicate matrix interstitial to the chromite. Laurites in the rims of chromite grains are Os-free but contain tiny inclusions of native osmium. It is suggested that either the availability of Os decreased during crystallisation of the laurites or that Os has been removed from laurites not totally enclosed by chromite. In a few cases laurite is surrounded by a ruthenian pentlandite containing up to 12% Ru

Mineralogy of telluride-bearing epithermal ores in the Kassiteres-Sappes area, western Thrace, Greece

The Kassiteres-Sappes district represents a multi-centered, porphyry-epithermal system developed during the Oligocene to Miocene at a composite calc-alkaline to high-K calc-alkaline volcanic edifice. Precious and base metal mineralization postdates the emplacement of dacite and rhyolite porphyries and is partly superimposed on earlier microdiorite-related porphyry-style mineralization exposed at the Koryfes Hill prospect. A second mineralized porphyry-type system genetically related to a dacite porphyry body developed near the St Demetrios deposit. Tellurides occur mainly at the St Barbara prospect and the St Demetrios deposit. Based on petrographic, electron microprobe, and scanning electron microscope analyses, hessite, petzite, sylvanite, altaite, stützite and native tellurium occur in the St Barbara prospect. These tellurium-bearing minerals are hosted in intermediate-sulfidation type veins and accompanied by pyrite, chalcopyrite, tetrahedrite-group minerals, galena and native gold/ electrum. The St Demetrios mineralization includes hessite, altaite, stützite, and tetradymite in close spatial relation to a high-sulfidation assemblage composed of enargite, chalcopyrite, goldfieldite, and native gold. Tellurides were deposited at logfTe2 values of -8.5 to 7.1 and logfS2 values of - 10.7 to - 7.9 (275°C). The ore systems are characterized by Au, Ag, Te, Bi, and Mo, which suggests a magmatic contribution to the mineralizing fluids. Ore-forming components were likely derived from both the dacite and rhyolite porphyries. © Springer-Verlag 2006