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Personne :
Makvandi, Sheida

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Makvandi

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Sheida

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Université Laval. Département de géologie et de génie géologique

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ncf13701002

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  • PublicationAccès libre
    Trace element composition of scheelite in orogenic gold deposits
    (Société de géologie appliquée aux gîtes minéraux, 2019-09-11) Makvandi, Sheida; Sciuba, Marjorie; Beaudoin, Georges; Grzela, Donald
    Scheelite from 25 representative orogenic gold deposits from various geological settings was investigated by EPMA (electron probe micro-analyzer) and LA-ICP-MS (laser ablation-inductively coupled plasma-mass spectrometer) to establish discriminant geochemical features to constrain indicator mineral surveys for gold exploration. Scheelite from orogenic gold deposits displays five REE patterns including a bell-shaped pattern with a (i) positive or (ii) negative Eu anomaly; (iii) a flat pattern with a positive Eu anomaly and, less commonly, (iv) a LREE-enriched pattern, and (v) a HREE-enriched pattern. The REE patterns are interpreted to reflect the source of the auriferous hydrothermal fluids and, perhaps, co-precipitating mineral phases. Scheelite from deposits formed in rocks metamorphosed at upper greenschist to lower amphibolite facies have low contents in REE, Y, and Sr, and high contents in Mn, Nb, Ta, and V, compared to scheelite formed in rocks metamorphosed below the middle greenschist facies. Scheelite from deposits hosted in sedimentary rocks has high Sr, Pb, U, and Th, and low Na, REE, and Y, compared to that hosted in felsic to intermediate rocks. Statistical analysis including elemental plots and multivariate statistics with PLS-DA (partial least square-discriminant analysis) reveal that the metamorphic facies of the host rocks as well as the regional host rock composition exert a strong control on scheelite composition. This is a result of fluid-rock exchange during fluid flow to gold deposition site. PLS-DA and elemental ratio plots show that scheelite from orogenic gold deposits have distinct Sr, Mo, Eu, As, and Sr/Mo, but indistinguishable REE signatures, compared to scheelite from other deposit types.
  • PublicationAccès libre
    Chemical composition of tourmaline in orogenic gold deposits
    (Société de géologie appliquée aux gîtes minéraux, 2020-06-10) Makvandi, Sheida; Sciuba, Marjorie; Beaudoin, Georges
    Tourmaline from eighteen orogenic gold deposits and districts, hosted in varied country rocks and metamorphic facies, was investigated by EPMA (Electron Probe Micro-Analyzer) and LA-ICP-MS (Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry) to establish discriminant geochemical features to constrain indicator mineral surveys for gold exploration. Such tourmaline most commonly belongs to the alkali group, with a dravitic composition. LA-ICP-MS results were investigated with binary plots and PLS-DA (Partial Least Square-Discriminant Analysis). PLS-DA suggests that the major element composition of tourmaline from orogenic gold deposits is buffered by the hydrothermal fluid, whereas trace element composition is strongly controlled by the composition and the metamorphic facies of the country rocks. Contents of Sn, Ga, Ti, Rare Earth Elements (REE), Zr, Hf, Nb, Ta, Th and U vary with the metamorphic facies of the country rocks. Tourmaline from orogenic gold deposits has high contents of Sr, V, and Ni and low Li, Be, Ga, Sn, Nb, Ta, U, and Th compared to tourmaline from other deposit types and geological environments. Binary plots such as Sr/Li vs. V/Sn, Sr/Sn vs. V/Nb, Sr/Sn vs. Ni/Nb and Sr/Sn vs. V/Be, as well as PLS-DA, discriminate tourmaline from orogenic gold deposits from that of other settings. Binary plots highlight a transitional variation in the trace element composition of tourmaline from metamorphic, to magmatic-hydrothermal, to magmatic environments.