Nature and Origin of Mineralizing Fluids in Hyperextensional Systems: The Case of Cretaceous Mg Metasomatism in the Pyrenees

International audienceDuring Albian time, the hyper-extension of the Pyrenean passive margin led to a hyper-thinning of the continental crust and the subsequent sub-continental mantle exhumation. The giant Trimouns talc-chlorite deposit represents the most prominent occurrence of Albian metasomatism in the Pyrenees, with the occurrence of the largest talc deposit worldwide. Consequently, this deposit located on a fault zone and a lithological contact represents one of the major drains at the Pyrenees scale and one of the best geological targets in order to decipher the origin(s) of the fluid(s) that circulated during this period. The talc-chlorite ore is characterized by the presence of brines trapped in dolomite, quartz and calcite fluid inclusions at the vicinity of the talc rich zone. Considered as responsible for the talc formation, these fluids may be interpreted in several ways: i) primary brines expelled from triassic evaporites, ii) secondary brines produced through halite leaching by diagenetic/metamorphic fluids, iii) brines derived from sea water serpentinization of mantle rocks. Stable isotope analyses (δ13C, δ18O, δD and δ37Cl) and Cl/Br ratio measurements in fluid inclusions and their host minerals were carried out in order to decipher the origin of the fluid(s) involved in the formation of the ore deposit. Data are consistent with the primary brines origin of the mineralizing fluid, which could have been expelled from Triassic levels. Other hypotheses have been tested, as for example the production of brines by seawater concentration during serpentinization. The geochemical proxies used in this study provide equivocal results. The first hypothesis is by far the most realistic one considering the numerous occurrences of Trias formation nearby, their deformation during extension and the drainage of the expulsed brines attested by fluid inclusions all around the deposit. The exhumation of the mantle is considered alternatively as a major source of heat and stress that favored brine migration along major shear zones. Our results fit well with brine circulation in a hyper-extensional geodynamic context, which relates the formation of the talc-chlorite ore, the thinning of the continental crust, and the exhumation of the sub-continental mantle, in accordance with by recent works