Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central : II. Thermal modelling and implications for the mecanisms of uranium mineralization

S. Scaillet and Michel Cuney and Christian Le Carlier de Veslud and Alain Cheilletz and Jean-Jacques Royer. ( 1996 )
in: Geochimica et Cosmochimica Acta, 60:23 (4673--4688)

Abstract

A two-dimensional (2-D) thermal modelling of the Saint Sylvestre—western Marche leucogranite complex (northwestern Limousin, French Massif Central, FMC) was conducted to help constrain the cooling history and the uranium mineralization postdating the time of intrusion by 40–50 m.y. Numerical simulation of the post-emplacement cooling of the complex (<700°C) indicates that the pluton thermally equilibrated with its country rocks (at around 360–400°C and a mean depth of 10.5 km) by conduction in as little as 4 m.y. after its intrusion age (324 ± 4 Ma). Integration of the regional Full-size image (<1 K) muscovite data in the 2-D model with an assumed universal Ar closure temperature of 325 ± 25°C reveals several sub-stages in the subsolidus cooling of the complex. The cooling pattern regionally defined by the muscovite data indicates that cooling was driven by the extensional exhumation and erosion of the thickened crust (+ intrusion) at a mean denudation rate of 0.3 mm y−1 Complete cooling below 325°C ended at 301 Ma via a transient faster denudation rate of 1.5 mm y−1 at the Westphalian-Stephanian boundary. The genetic relationships between the fluid circulations, the mineralization, and the cooling history of the pluton are discussed with particular emphasis on the tectonic process driving exhumation (extension). The initiation of the regional uplift at ∼320 Ma triggered at depth the circulation of in situ derived low-density aqueous fluids that reacted with the granite to form large vertical dissolution conduits (episyenites) characterized by the strong leaching of SiO2. The hydrothermal alteration was further enhanced during uplift by the structurally focused throughput of large volumes of aqueous fluids along brittle faults cutting across the laccolith. These conduits acted more than 20–30 m.y. after the trap formation as preferential channelways for the U-ore deposition at 270–280 Ma, due to sustained hydrothermal circulation adjacent to the high-heat producing terminal injections emplaced along the vertical fault zones and the metasomatic columns

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    BibTeX Reference

    @ARTICLE{,
        author = { Scaillet, S. and Cuney, Michel and Le Carlier de Veslud, Christian and Cheilletz, Alain and Royer, Jean-Jacques },
         title = { Cooling pattern and mineralization history of the Saint Sylvestre and western Marche leucogranite pluton, French Massif Central : II. Thermal modelling and implications for the mecanisms of uranium mineralization },
       journal = { Geochimica et Cosmochimica Acta },
        volume = { 60 },
        number = { 23 },
       chapter = { 0 },
          year = { 1996 },
         pages = { 4673--4688 },
      abstract = { A two-dimensional (2-D) thermal modelling of the Saint Sylvestre—western Marche leucogranite complex (northwestern Limousin, French Massif Central, FMC) was conducted to help constrain the cooling history and the uranium mineralization postdating the time of intrusion by 40–50 m.y. Numerical simulation of the post-emplacement cooling of the complex (<700°C) indicates that the pluton thermally equilibrated with its country rocks (at around 360–400°C and a mean depth of 10.5 km) by conduction in as little as 4 m.y. after its intrusion age (324 ± 4 Ma). Integration of the regional Full-size image (<1 K) muscovite data in the 2-D model with an assumed universal Ar closure temperature of 325 ± 25°C reveals several sub-stages in the subsolidus cooling of the complex. The cooling pattern regionally defined by the muscovite data indicates that cooling was driven by the extensional exhumation and erosion of the thickened crust (+ intrusion) at a mean denudation rate of 0.3 mm y−1 Complete cooling below 325°C ended at 301 Ma via a transient faster denudation rate of 1.5 mm y−1 at the Westphalian-Stephanian boundary.
    
    The genetic relationships between the fluid circulations, the mineralization, and the cooling history of the pluton are discussed with particular emphasis on the tectonic process driving exhumation (extension). The initiation of the regional uplift at ∼320 Ma triggered at depth the circulation of in situ derived low-density aqueous fluids that reacted with the granite to form large vertical dissolution conduits (episyenites) characterized by the strong leaching of SiO2. The hydrothermal alteration was further enhanced during uplift by the structurally focused throughput of large volumes of aqueous fluids along brittle faults cutting across the laccolith. These conduits acted more than 20–30 m.y. after the trap formation as preferential channelways for the U-ore deposition at 270–280 Ma, due to sustained hydrothermal circulation adjacent to the high-heat producing terminal injections emplaced along the vertical fault zones and the metasomatic columns }
    }