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  • Author: Yang Yang x
  • Geosciences x
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Perovskite, reaction product of a harzburgite with Jurassic– Cretaceous accretionary wedge fluids (Western Carpathians, Slovakia): evidence from the whole-rock and mineral trace element data

Abstract

Perovskite (Prv) was discovered in an abyssal harzburgite from a “mélange” type blueschist-bearing accretionary wedge of the Western Carpathians (Meliata Unit, Slovakia). Perovskite-1 formation in serpentinized orthopyroxene may be simplified by the mass-balance reaction: Ca2Si2O6 (Ca-pyroxene-member)+2Fe2TiO4 (ulvöspinel molecule in spinel)+2H2O+O2=2CaTiO3 (Prv)+2SiO2+4FeOOH (goethite). Perovskite-2 occurs in a chlorite-rich blackwall zone separating serpentinite and rodingite veins, and in rodingite veins alone. The bulk-rock trace-element patterns suggest negligible differences from visually and microscopically less (“core”) to strongly serpentinized harzburgite due to serpentinization and rodingitization: an enrichment in LREE(La,Ce), Cs, ±Ba, U, Nb, Pb, As, Sb, ±Nd and Li in comparison with HREE, Rb and Sr. The U/Pb perovskite ages at ~135 Ma are interpreted to record the interaction of metamorphic fluids with harzburgite blocks in the Neotethyan Meliatic accretionary wedge. Our LA-ICP-MS mineral study provides a complex view on trace element behaviour during the two stages of rodingitization connected with Prv genesis. The positive anomalies of Cs, U, Ta, Pb, As, Sb, Pr and Nd in Cpx, Opx and Ol are combined with the negative anomalies of Rb, Ba, Th, Nb and Sr in these minerals. The similar positive anomalies of Cs, U, Ta, ±Be, As, Sb found in typical serpentinization and rodingitization minerals, with variable contents of La, Ce and Nd, and negative anomalies of Rb, Ba, Th, Nb and Sr suggest involvement of crustal fluids during MP-LP/LT accretionary wedge metamorphism. LA-ICP-MS study revealed strong depletion in LREE from Prv-1 to Prv-2, and a typically negative Eu (and Ti) anomaly for Prv-1, while a positive Eu (and Ti) anomaly for Prv-2. Our multi-element diagram depicts enrichment in U, Nb, La, Ce, As, Sb, Pr, Nd and decreased Rb, Ba, Th, Ta, Pb, Sr, Zr in both Prv generations. In general, both Prv generations are very close to the end-member composition. In spite of low concentrations of isomorphic constituents, Prv-1 and Prv-2 display the A(La,Ce)3++B(Fe,Cr)3+=ACa2++BTi4+ heterovalent couple substitution. A decrease of ferric iron in Prv-2 indicates increasing reduction conditions during rodingitization.

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Application of geostatistical analyst methods in discovering concealed gold and pathfinder elements as geochemical anomalies related to ore mineralisation

Abstract

The study area in the West Junggar Basin is known to be rich in hydrothermal gold deposits and occurrences, even though there has been minimum exploration in the area. It is here hypothesised that this area could host more gold deposits if mineral exploration methods were to be reinforced. This research is aimed at identifying geochemical anomalies of Au, and determining possible factors and conditions which facilitate the formation of anomalies by referring to As and Hg as gold pathfinders. Geostatistical analyst techniques have been applied to 9,852 stream sediments and bedrock data collected on a total surface of 1,280 km2 of West Junggar, Xinjiang (northwest China). The kriging interpolation and quantile-quantile plot methods, combined with statistical methods, successfully identified both Au and its pathfinders’ anomalies. In the present study, median was considered as background values (10.2 ppm for As, 9.13 ppb for Hg and 2.5 ppb for Au), whereas the 95th percentile were threshold values (28.03 ppm for As, 16.71 ppb for Hg and 8.2 ppb for Au) and values greater than thresholds are geochemical anomalies. Moreover, the high concentrations of these three discovered elements are caused primarily by hydrothermal ore mineralisation and are found to be controlled mainly by the Hatu and Sartohay faults of a northeast-southwesterly direction as well as their related secondary faults of variable orientation, which facilitate the easy flow of hydrothermal fluids towards the surface resulting in the formation of geochemical anomalies. Most of anomalies concentration of Au are found near the mining sites, which indicates that the formation of new Au anomalies is influenced by current or previous mining sites through geological or weathering processes. In addition, the low concentration of gold and its pathfinders found far from active gold mine or faults indicates that those anomalies are formed due to primary dispersion of hosting rock.

Open access
Soft-sediment deformation structures in cores from lacustrine slurry deposits of the Late Triassic Yanchang Fm. (central China)

Abstract

The fine-grained autochthonous sedimentation in the deep part of a Late Triassic lake was frequently interrupted by gravity-induced mass flows. Some of these mass flows were so rich in water that they must have represented slurries. This can be deduced from the soft-sediment deformation structures that abound in cores from these lacustrine deposits which constitute the Yanchang Fm., which is present in the Ordos Basin (central China).

The flows and the resulting SSDS were probably triggered by earthquakes, volcanic eruptions, shear stress of gravity flows, and/or the sudden release of overburden-induced excess pore-fluid pressure. The tectonically active setting, the depositional slope and the high sedimentation rate facilitated the development of soft-sediment deformations, which consist mainly of load casts and associated structures such as pseudonodules and flame structures. Sediments with such deformations were occasionally eroded by slurries and became embedded in their deposits.

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