Granitoid Metasomatism and Giant Quartz Vein Formation by Mineral Replacement: Insights from the Canigó Massif, Eastern Pyrenees

Metasomatism is a ubiquitous process in the Earth’s crust, exerting major controls on fluid, heat and mass transfer and rock deformation, and is commonly constituted by mineral replacement reactions. Different types of metasomatism may coexist and/or successively conceal each other in a given area. Deciphering the geochemical behaviour, regional extent and mineralogical changes of multi-stage metasomatism can be difficult because of the overprinting of successive events and their frequent relationship with deformation. Here, we investigate granitoid metasomatism, namely silicification, feldspathisation and sericitisation, in the Variscan basement rocks of the Canigó Massif (Eastern Pyrenees, SW Europe), which is spatially related to Giant Quartz Veins (GQVs) tens of metres wide and several kilometres long. Unaltered and altered granitic orthogneisses derived from Ordovician intrusives and late-Variscan granitoids, as well as GQV occurrences, are studied across scales through structural and textural characterisation, whole-rock geochemistry and Electron Backscatter Diffraction (EBSD). Geochemical analyses are further compared with a new database including more than 600 unaltered granite and orthogneiss samples from the Pyrenees and the Catalan Coastal Ranges (SW European Variscan Belt). Results show that silicification, the dominant metasomatic process, was related to regional-scale shear zones and contributed to form GQVs through mineral replacement. This is confirmed at the macro- (km), meso- (m–cm) and micro-scale (μm) by relict fabrics, mineral phases and structural features of the precursor rocks within veins, by a progressive depletion of all major and trace elements, except silica, in rocks sampled along decreasing distances from GQV outcrops, and by the localisation of mylonitic deformation along GQVs. Feldspathisation and sericitisation are, in contrast, restricted to specific sectors and exposed as albitite, trondhjemite and pale green mica-rich outcrops. It is suggested that most of the exposed areas of the studied GQVs are, accordingly, not veins sensu stricto but metasomatic products where the original fabrics and features of precursor rocks were overprinted during coupled deformation and Si-metasomatism. Results presented here have major implications for the scale and geochemical behaviour of multi-metasomatic events, as well as on the kinetics of mineral replacement processes leading to changes in the physicochemical properties of crustal rocks.