Thorium, uranium and rare earth mineralization in rocks of the Ugakhan gold deposit, Bodaibo ore region (Irkutsk oblast)

The paper reports on the results of studies of ore-bearing rocks of the Ugakhan gold deposit (Bodaybo district): metasandstones, metasiltstones and carbonaceous shales. The rocks consist of quartz, feldspar (albite, orthoclase), Fe-Mg chlorite, mica (muscovite, sericite) and carbonates (calcite, dolomite, anker-ite) and accessory titanite, rutile, tourmaline, zircon and apatite. All rocks contain fragments of microfossils exhibiting striking concentric zonation with alternated dark (carbonaceous matter) and light (carbonate-mica material) layers. In a range from metasandstones to carbonaceous shales, the rocks exhibit an increase in mica amount and the content (up to 3%) of carbonaceous matter, as well as the formation of regeneration rims around relict tourmaline and zircon. The REE mineralization includes silicates (REE-bearing epidote, thorite), fuorocarbonates (bastnesite) and phosphates (monazite, xenotime, ankylite), which are closely related to U minerals (uraninite, cofnite). Bastnesite, ankylite and thorite formed due to the decomposition of earlier REE-bearing epidote, whereas monazite and xenotime are the products of decomposition of apatite. Uraninite formed during lithifcation of matrix of carbon-bearing rocks and is replaced by cofnite. The thermal analysis of carbonaceous matter and the formation temperature of chlorite calculated using chlorite geothermometer (296–371 °С) indicate the transformation of rocks under conditions of sericite-chlorite subfacies of greenschist facies of metamorphism.

Low-Temperature-Low-Pressure Mineral Paragenesis of the Lower-Middle Jurassic Volcanics in the Eastern Pontides, NE Turkey

<p>The Lower-Middle Jurassic volcanic rocks in the eastern Pontides were formed in a subduction zone under the extensional tectonic regime. These volcanic rocks were experienced seawater alteration during forming. They also were exposed to the burial metamorphism under the Cretaceous and Eocene aged formations. In addition, the Cretaceous and Eocene granitoids cut these volcanic rocks in some places and metamorphosed them. In this study, the mineralogical changes of the volcanic rocks they have experienced since their formation were examined.</p><p>Plagioclase (An<sub>>42</sub>) + augite/diopside (En<sub>38-52</sub>Wo<sub>25-46 </sub>Fs<sub>7-25</sub>) + Fe-Ti oxides (Fe<sup>+3</sup>/(Fe<sup>+3</sup>+Fe<sup>+2</sup>) > 0.80) ± magnesiohornblende (Mg/(Mg+Fe<sup>+2</sup>) > 0.92) are the main rock-forming minerals in these volcanic rocks. Mineralogical traces of seawater alteration are mostly masked by subsequent geological events. However, Na-enrichment of the plagioclases, increased <sup>87</sup>/<sup>86</sup>Sr<sub>(i)</sub> isotope ratios (0.70462 to 0.70611) and some clay minerals, laumontit,  analsime minerals, which are observed in the XRD peaks of some samples, refer to the alteration of the seawater. The pumpelyite (Fe<sup>+2</sup>/Fe<sup>+2</sup>+Mg = 0.60-0.90), chlorite (Fe<sub>total</sub>/Fe<sup>+2</sup> + Mg = 0.15-0.95), sphene, calcite, dolomite and secondary quartz minerals were formed during burial metamorphism. The Fe-Ti oxides reached the chemical re-equilibrium under the new P-T conditions (magnetite Fe<sup>+3</sup>/(Fe<sup>+3</sup>+Fe<sup>+2</sup>) = 0.40-0.62; ilmenite Fe<sup>+3</sup>/(Fe<sup>+3</sup>+Fe<sup>+2</sup>) = 0.01-0.20). Epidote (Fe<sup>+2</sup>/(Fe<sup>+2</sup>+Mg) = 0.75-0.95) accompany the mineral paragenesis in some areas affected by Upper Cretaceous and Eocene granitoids.</p><p>Temperature estimations using the chlorite geothermometer and the phase relationships on the P-T diagrams show that the volcanics were heated up above 200°C in the buried areas where the granitoids were not effective. The temperatures were above 250°C in the areas where the magmatic rocks were effective. Taking into consideration the thickness of the formations that overlie the Jurassic volcanics, it can be suggested that the pressure affecting the Jurassic volcanics reached up to 1.5 kilobars.</p><p>Acknowledgement</p><p>This work was financially supported by Scientific and Research Projects Unit of Karadeniz Technical University with grant # 8920.</p>

Factors Controlling Hydrothermal Nickel and Cobalt Mineralization—Some Suggestions from Historical Ore Deposits in Italy

We compare three poorly known, historical Ni–Co-bearing hydrothermal deposits in different geological settings in Italy: The Ni–Co–As–Sb–Au-bearing Arburese vein system (SW Sardinia), the Co–Ni–As-rich Usseglio vein system (Piedmont), and the small Cu–Ag–Co–Ni–Pb–Te–Se stockwork at Piazza (Liguria). These deposits share various (mineralogical, chemical, thermal, and stable isotopic) similarities to the Five Element Vein-type ores but only the first two were economic for Co–Ni. The Sardinian Ni-rich veins occur in Paleozoic basement near two Variscan plutons. Like the Co-rich Usseglio vein system, the uneconomic Piazza deposit is hosted in an ophiolite setting anomalous for Co. The Sardinian and Usseglio deposits share a polyphasic assemblage with Ni–Co–As–Sb–Bi followed by Ag-base metal sulfides, in siderite-rich gangue, whereas Piazza shows As-free, Ag–Pb–Te–Se-bearing Co–Ni–Cu sulfides, in prehnite–chlorite gangue. Fluid inclusions indicated Co–Ni arsenide precipitation at ≈170 °C for Usseglio, whereas for the Sardinian system late sulfide deposition occurred within the 52–126 °C range. Ore fluids in both systems are NaCl-CaCl2-bearing basinal brines. The chlorite geothermometer at Piazza provides the range of 200–280 °C for ore deposition from CO2-poor fluids. Enrichments in Se and negative δ13C in carbonates suggest interaction with carbonaceous shales. These deposits involve issues about source rocks, controls on Co/Ni and possible role of arsenic and carbonate components towards economic mineralization.

Genesis and transformation of dickite in Permo-Triassic sediments (Betic Cordilleras, Spain)

In the Maláiguide Complex (Betic Cordilleras, Spain), dickite is widely developed in clastic Permo-Triassic sequences. The lateral extent stretches at least 300 km along the Betic range, whilst vertical extent is variable and appears limited to the lowest 20–150 m of these sequences. Textural, chemical and crystallochemical characteristics of the dickite, illite/mica and chlorite in dickite-bearing rocks and in the overlying (Permo-Triassic) and underlying (Carboniferous) rocks have been investigated to determine the approximate conditions in which dickite has developed. Using the chlorite geothermometer (based on A1IV contents), temperatures of 146±28°C and 169±12°C have been deduced for two Permo-Triassic members, and 305±12°C for Carboniferous. The Si content in illites has been used as a geobarometer, and pressures of 4.8±2 kbar have been estimated in Carboniferous rocks and tentative pressures of 2.7±3 and 2.1±2 kbar in Permo-Triassic members. Chemical evolution of phyllosilicates is accompanied by increasing illite crystallinity and % 2M1 polytype.