Structural controls on fluid trapping at granite roof (Boborás granite and associated Brués gold vein system, Galicia, NW Spain): A continuum from magmatic to solid state deformation.

Abstract : When evaluating structural controls of mineralizations spatially associated with magmatic intrusions, rheological evolution of the system and early magmatic structural heritage are either ignored, or impossible to trace (e.g. late solid state mylonitization ˇ E). We present a case-study of Late-Hercynian granite roof (Boborás granite) affected by a fracture network evolving from early granitic sills and dykes to late quartzfelspar veins followed by Au-As-bearing quartz veins (Brués gold vein system). At the granite contact, hosting micaschists present low-dip foliation. Early pegmatitic sills are crosscut by high-angle Boborás granitic sills and dykes which are frequently bordered by late Au-As-bearing quartz shear veins. Associated vertical ¸Sen echelonˇT tension gashes filled by Au-bearing arsenopyrite and quartz are also observed. All these structures are oriented E-W to NE-SW. Microtectonics investigations allow to determine that all these structures formed within a homogenous tectonic context characterized by north verging normal motion. Evidences are : i) top of granite, microgranite sills show a N-S submagmatic extension expressed by E-W trending microfractures of plagioclase crystals filled by magmatic quartz; ii) steped wall rock of high-angle microgranite dykes and sigmoidal sills are compatible with northward normal motion; iii) late Au-As-bearing quartz shear veins parallel to high-angle dykes are developed within north verging normal faults underlined by drag folds in micaschists. All the structures described above have been interpreted within a continuous long lived tectonic event associated with continuous magmatic-hydrothermal evolving system. A regional extensional context is suspected to prevail during the Brués vein system formation. The evolution of the subsequent deformation from submagmatic state through the ductile/brittle one until the brittle one agrees with the cooling of pluton in extensional context.