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Les paléofluides de la fin de la collision varisque : Evolution pression-température-composition et rôle métallogénique. Une étude pluridisciplinaire des districts à Au d'Europe occidentale

Abstract : Fluid circulations associated with the formation of gold deposits in western Europe (Castromil in North west Iberia, Moulin de Cheni in the French Massif Central, and Petra-kova Hora in Bohemian Massif) have been studied in order to reconstruct the P-T evolution at the end of the variscan orogeny and to understand the mass transfer occurring during the associated metallogenesis crisis. A general model of formation of gold deposits in the variscan belt from western Europe has been established. Two major stages have been defined: -The mesozonal stage: The formation of the quartz structures occurs at temperatures in the range of 450-550°C at 8-12 km depth along 100 m to 10 km-scale faults which constituted draining zone for fluid circulations at that stage. Fluids are aqueous-carbonic with moderate salinity. CO2 is always the dominant species and CH4 content is controlled by the equilibrium between fluids and graphite. Isotopic data indicate that fluids lost their signature due to chemical interactions with the host metamorphic rocks at high temperature. -The epizonal stage: The uplift around 5-6km and decompression processes yield to change in the pressure regime (lithostatic to hydrostatic conditions). An important microfracturing allow the circulation of aqueous fluids having moderate salinity at temperature higher than 350°C either in the quartz structures and in the host rocks. These fluids are probably issued from aqueous-carbonic fluids having lost their gas compound during decompression events. Mixing process occurs with fluid of surficial origin having meteoric signatures and yie1ds to the destabilisation of gold complex at temperature around 250°C. Gold precipitation occurs preferentially within sulphides, which have previously been microfractured. Their surfaces are considered as efficient traps thanks to electrochemical phenomena. Mass transfer associated to the formation of gald deposits can be described at two different scales: 1) locally, Fe, Ca and K are mobilised from biotites and plagioclases from the host rocks. Important quantity of sulphides precipitated thanks to the Fe supply. 2) at the scale of the different lithologies for elements like As, S and Sb. The rocks hosting the mineralisations do not exhibit anomalous Au contents (except Paleozoic rocks of the Vila Pouca de Aguiar district in north-Portugal). However, absence of anomalous Au contents does nat preclude ore formation by remobilisation of gold from host rocks as gold is very soluble in fluids at high temperature (>400°C). Suitable conditions for gold scavenging occurred during the general decompression of the metamorphic unit (e.g. during the uplift) associated to intense brittle microfissuring. The increasing permeability induce large fluid percolations in host rocks allowing the gold remobilisation and the ore deposit formation.
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http://docnum.univ-lorraine.fr/prive/SCD_T_2001_0186_VALLANCE.pdf
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https://hal.univ-lorraine.fr/tel-01746853
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Submitted on : Thursday, March 29, 2018 - 10:46:08 AM
Last modification on : Monday, December 14, 2020 - 2:53:57 PM

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Jean Vallance. Les paléofluides de la fin de la collision varisque : Evolution pression-température-composition et rôle métallogénique. Une étude pluridisciplinaire des districts à Au d'Europe occidentale. Sciences de la Terre. Université Henri Poincaré - Nancy 1, 2001. Français. ⟨NNT : 2001NAN10186⟩. ⟨tel-01746853⟩

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