Hydrodynamique des systèmes minéralisés péri-granitiques : étude du gisement à W-Sn-(Cu) de Panasqueira (Portugal)

Abstract : The vein and greisen Sn-W deposits are magmatic-hydrothermal systems that provide an important part of the world W production and represent an important source of Sn. The formation of these deposits involves continuum of magmatic-hydrothermal processes and implies the transfer and the focusing of a large amount of mineralizing fluids. This study aims to improve understanding of hydrodynamic and geological processes involved during the transport and the deposition of metals leading to the formation of these deposits. We have performed a complete study combining (i) field works (geological and structural studies), (ii) fluid flow reconstruction via the textural analysis of tourmaline growth bands, (iii) experimental determination of permeability changes during greisenization, and (iv) numerical modeling of peri-granitic fluid flow accounting for magmatic fluid production and dynamic permeability related to fluid-rock interactions. This methodology was applied in the case of the world-class W-Sn-(Cu) Panasqueira deposit, which represents a reference site to study magmatic-hydrothermal processes leading to the formation of large vein and greisen deposit. Our results demonstrate that the releasing and the expulsion of ore-bearing magmatic fluids triggered greisenization of the apical part of granite intrusion, which caused generation of porosity (~8.5%) and therefore a significant increase of permeability (from 10-20 to 10-17 m²) in massive greisen composing the granite’s roof. The development of this permeable pathway constitutes an important drain promoting the expulsion and the focusing of magmatic fluids produced during the crystallization of the underlying granite. This enhancement of magmatic fluids expulsion (i) promotes significantly fluid flux and transfer of metals, and (ii) the establishment of high fluid pressure conditions, which coupled with the regional compressive crustal regime, triggered the opening of mineralized veins above the granite roof. Finally, this study emphasizes that reactive hydrothermal fluids are able to generate their own pathways in initially impermeable rocks. This process represents an important mechanism to enhance fluid flow and promote the formation of large hydrothermal deposits.
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Gaëtan Launay. Hydrodynamique des systèmes minéralisés péri-granitiques : étude du gisement à W-Sn-(Cu) de Panasqueira (Portugal). Sciences de la Terre. Université d'Orléans, 2018. Français. ⟨tel-02101051⟩

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