Luciano Cortesogno*, Laura Gaggero, Gabriella Lucchetti and Roberto Cabella ^{Compositions and miscibility gap in Na-Ca clinopyroxenes through high-pressure metamorphism}

ABSTRACT - The Ligurian Alps expose several Alpine and pre-Variscan tectono-metamorphic units which record pressures in the range 0.9-1.7 GPa and temperatures of 300°- 750°C. Sodic and sodic-calcic clinopyroxenes were analysed in rocks of basic (basalt, gabbro and Fe-Ti oxide-gabbro), intermediate (Fe-Ti oxide-diorite, diorite) and acidic (rhyolite, rhyodacite) compositions and in meta-arenite to quartzschist metasediments. The control of metamorphic parameters, protolith and site heritage on clinopyroxene composition was investigated evidencing i) strong dependence of clinopyroxene - amphibole equilibria on Mg# and low Mg/Ca in the rock for different metamorphic conditions; ii) dehydration reactions occurring during the development of eclogitic assemblages; iii) a largely incomplete re-equilibration under lower metamorphic conditions. The resulting compositional zoning depends on the site heritage, protolith textural features, deformation intensity and element diffusivity.
At the lowest temperatures, jadeites and aegirines or aegirine-augites commonly coexist. The presence of the compositional gap also between grains tightly in contact suggests the existence of a solvus. The miscibility gap extending from the Jd-Aeg to the Jd-Quad join below 400°C, is narrowed for increasing temperatures. Since about 400°C, omphacite compositions become stable, and the miscibility gap still exists between omphacite and omphacite-rich jadeite, and reduces for increasing aegirine contents.
For temperatures exceeding 450°C, compositional gaps between jadeite and omphacite are recorded. A compositional gap between omphacite and calcic pyroxene is likely, and tends to reduce for T > 700°C.

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Silvia Serranti*, Vincenzo Ferrini, Umberto Masi, Massimo Nicoletti and Louis N. Conde Geochemical features of the massive sulfide (Cu) metamorphosed deposit of Arinteiro (Galicia, Spain) and genetic implications

ABSTRACT - Major and trace element contents in sulfide ores, gangue minerals and rock samples from the Arinteiro deposits hosted in metaophiolites of western Galicia were determined. S isotope ratios of selected sulfides were also analysed. Polymetallic orebodies, represented by abundant pyrrhotite, subordinate chalcopyrite and sphalerite, and rare pyrite, underwent metamorphic recrystallization, probably during the last stage of the Hercynian orogenesis as suggested by the K/Ar radiometric ages ascertained on host rocks (261 - 275 ± 20 Ma). Recrystallization took place at about 400°C and 5.5 kbar, log a_S2 was generally -7 atm or lower, log a_O2 was about -30 atm, and pH ranged mainly from 4.4 to 4.9.
The Co/Ni values and Se contents of the sulfides are consistent with the inferred volcano-sedimentary origin of the deposit, and d³⁴S (about 0 ‰) indicates that the source of S was magmatic.
Comparisons of the Arinteiro deposit with similar deposits of corresponding origin and metamorphic rank shows the typical characteristic of the former - dominant pyrrhotite and rare pyrite - indicating the greatly reduced conditions of hydrothermal fluids, due to interaction with organic matter.

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Paola Di Leo*, Salvatore Ivo Giano and Marcello Schiattarella Volcaniclastic layers in upper Triassic-Jurassic deep-sea sediments from the Lucanian Apennine, southern Italy: mineralogy, geochemistry, and palaeotectonic implications

ABSTRACT - In complex mountain chains such as the southern Apennines, analysis of ancient volcanics may provide new information on reliable palaeotectonic scenarios. In the lower part of the Scisti silicei Formation (upper Triassic-Jurassic, Lagonegro units) outcropping in the Agri Valley, Lucanian Apennine, volcaniclastic layers were found interbedded with pelagic sediments, showing facies changes over a narrow area. These layers are prevalently composed of quartz and feldspar. Compared with the UCC (Upper Continental Crust), the volcaniclastic beds are depleted in both large-ion lithophile (Rb, Sr) and high field strength (Ti, Nb, Hf, Zr, Th, RE) elements, and are strongly enriched in first-row transition elements (Mn, Ni, Cu) and Ce. Their volcaniclastic origin is clearly evidenced by the discrimination diagram based on first-row transition elements V, Cr and Ni and immobile elements Zr and Ti. The source rocks of these volcaniclastic layers were probably of dacitic-rhyolitic to rhyolitic composition, as suggested by chemical evidence. The chemical analyses of unaltered plagioclase, indicating albitic composition, are consistent with such a source. The distribution and concentration of REE are typical of andesitic magmas. The REE chondrite-normalized patterns of the sampled layers are similar to reported REE patterns of arc-related rhyolitic ignimbrites. The Ti-Y-Zr ternary plot and Y vs Nb and Y+Nb vs Rb discrimination diagrams proposed for felsics also indicate the affinity of the volcaniclastic layers with arc-related rhyolitic tephra in subacqueous environments. This suggests a more complex palaeogeographic scenario than the classic view, according to which the Lagonegro deep-sea basin was generated in an extensional regime and bordered by normal faults during the entire Mesozoic tectono-sedimentary evolution. A continental transform zone able to produce local transpressional structures and to emulate the magmatic context of collisional margins may be envisaged as partly responsible for the basin evolution in a general context of extensional tectonics. At shallower structural level, the transform fault system may have controlled the Triassic-Jurassic facies pattern of the Lagonegro basin whereas, at depth, such anisotropy probably influenced the crustal and magmatic evolution of this part of the Adria palaeomargin.

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Alessandro Sempio and Alessandro F. Gualtieri* Mineralogy of the Grès de Thiviers (northern Aquitaine, France)

ABSTRACT - - This paper reports the mineralogical characterisation of the Grès de Thiviers (GT), a unique natural pigment used in the industry of traditional tile ceramics. The only occurrence of this raw material, composed of quartz and goethite, is a small region in the SW of France were it is exploited and exported to the European and Far East markets. GT is mainly used as a bulk red pigment for white stoneware and its stability in firing is a direct consequence of the unique nature. In fact, despite the nature of the raw materials composing the ceramic mixture and their behaviour at high temperature, GT invariably yields a final red colour to the fired ceramic body. The stability stems from the fact that goethite (yellow colour) which later transforms into hematite (red colour) is the cement of quartz grains. Thus, in firing, because of the protective shield formed by the inert quartz grains, the newly-formed hematite crystals do not get in contact with other reacting phases such as the alkaline glass and retain the red colour (Sempio and Gualtieri, 2001).
The mineralogy of the GT deposits was described in the light of the peculiar genesis and chemical variability. In fact, GT occurrence is closely related to the karstic system permeating the late Cretaceous limestone in this region. According to the different formation environment it was possible to describe the four typical GT textures: laminar, oolitic, dendritic, and homeoblastic.
GT samples contain amorphous substance with composition [SiO₂+Fe₂O₃ or FeO(OH)] which is the colloidal precursor of the crystalline phases quartz and goethite.

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Alexander Portnov, Boris Gorobets, Alexander Rogozhin*, Alexander Bushev and Tatiana Kvitko Luminescent geochemical anomalies in the lithosphere and haloes of ore bodies

ABSTRACT - Haloes of luminescent minerals around kimberlite pipes, emerald-bearing bodies in metasomatites, gold-bearing bodies, mica pegmatite veins and mountain crystal veins were first revealed and studied. They are special cases of luminescent geochemical anomalies which occur in those parts of the Earth's crust where considerable interaction of rocks and abyssal fluids took place. The fluids leached trace elements, including the luminogenous transition metals of 3d-, 4f and some other groups from the country rocks, and then accumulated and deposited them. Accumulations of Mn, REE, U and other luminogenous elements form glow centers in secondary minerals. Outcrops of photoluminescent calcite, apatite, zircon, fluorite, cerussite, chlorargyrite and uranyl minerals in altered rocks may serve as indicators of mineral deposits. Photoluminescent grains of apatite having a violet-blue glow (Ce³⁺, Eu²⁺) detected by sand analysis, allow mapping of secondary mechanical haloes (scattering), helpful in searching for kimberlite pipes. Luminescence spectra of pathfinder minerals from the haloes are also presented, with the luminescent tracer elements identified in them.

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Vanni Moggi-Cecchi*, Curzio Cipriani, Paolo Rossi, Daniele Ceccato, Verardo Rudello and Hector Somacal Trace element contents and distribution maps of chalcopyrite: a micro-PIXE study

ABSTRACT - Micro-PIXE (Proton micro-beam Induced X-ray Emission) apparatus, with a beam scanning system, can provide information on both trace element contents and their distribution. This technique was applied to chalcopyrite samples from Italian and Slovak ore deposits. A 5 µm-diameter beam was used to obtain simultaneously trace element analysis with low detection limits and trace element distribution maps. The distribution of Zn appears to be related to sub-microscopic inclusions of sphalerite and kesterite, invisible in the corresponding SEM-BSE (Scanning Electron Microscope - Back-Scattered Electrons) and X-ray maps. In contrast, Ag and Se are homogeneously distributed, confirming the predicted tendency of these elements to replace Cu and S, respectively, in the structure of chalcopyrite.

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