Fluid evolution and halogen fractionation in orogenic belts: A comparative fluid inclusion appraisal in the Eastern Pyrenees

Abstract

The physicochemical signatures of fluid-rock interaction, recorded in fluid inclusions, represent fundamental proxies for understanding the interplay between rock deformation, fluid migration, and ore deposit formation in the Earth's crust. Although fluid-rock interaction can take place simultaneously or sequentially both in the basement and the cover of collisional orogens, these two scenarios are generally investigated separately, thus precluding an integrated understanding of the processes involved. Here, we present a comprehensive study of the fluid evolution in the basement and cover rocks of the Pyrenees (SW Europe) by means of Geographic Information System (GIS)-assisted petrography, microthermometry, Electron Backscatter Diffraction (EBSD), Raman micro-spectroscopy, and Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) microanalysis of individual fluid inclusions. The investigated fluids are sampled from giant quartz veins, which are tens of meters wide and up to several kilometres long. Two-phase aqueous fluid inclusions hosted in the basement (Canigó and Cap de Creus massifs) and cover rocks (Roc de Frausa massif) show significant variations regarding their homogenization temperature (T_h), salinity, and chemical composition. Basement-hosted H₂O-NaCl-CaCl₂ fluids have T_h and salinity ranging between 190 and 220 °C and 12–16 wt% equivalent (eq.) NaCl in the Canigó massif, and between 190 and 260 °C and 16–20 wt% eq. NaCl in the Cap de Creus massif. Conversely, T_h of 180–240 °C and salinity of 4–7 wt% eq. NaCl were obtained for H₂O-NaCl cover rock-hosted fluids in the Roc de Frausa massif. Fluid salinity, cation concentrations, and halogen ratios suggest that basement-hosted fluids represent residual basinal brines that originated from seawater which underwent variable degrees of evaporation and organic matter interaction. In contrast, cover rock-hosted fluids represent seawater-like precursor fluids that record strong organic matter interaction without significant evaporative halogen fractionation. The data suggest that neither basement- nor cover-hosted fluids were released at depth from magmatic or metamorphic fluids, which agrees with geological constraints of the investigated areas. However, fluids from both basement and cover rocks infiltrated deep into the subsurface, as revealed by their variable metal and halogen concentration suggesting compositionally stratified fluid compositions at depth. Our data and conclusions differ from fluid inclusion studies reported for other giant quartz vein systems worldwide, suggesting that different fluid origins and evolution histories may drive the formation of these large quartz accumulations. Moreover, the large variations of the Br/Cl, I/Cl, and Br/I ratios show that halogen ratios should be used with caution: they are excellent tracers of the fluid evolution history, but may not provide a straightforward fluid origin classification.