Multiple fluid flow events from salt‐related rifting to basin inversion (Upper Pedraforca thrust sheet, SE Pyrenees)

Abstract

AbstractFluid systems in inverted rifted margins are challenging to interpret because fractures formed before compression were often reactivated acting as fluid pathways as new ones formed. Deciphering the fracture and fluid flow history in such complex settings has key implications for the prediction of the distribution of mineral resources. As an example, we reconstruct the fluid flow evolution of a portion of the inverted Pyrenean rift, the Upper Pedraforca thrust sheet, from the Mesozoic extension to the Alpine orogeny. We combine structural analysis and petrographic, geochemical and geochronological data obtained from 87 samples of fracture‐filling carbonate cements. During the Late Jurassic‐Early Cretaceous, low‐temperature seawater produced dolomitization of Jurassic and Lower Cretaceous limestones in an extensional setting. During the Early Cretaceous salt‐related extension, formation waters, probably evolved seawater or fluids that interacted with Triassic evaporites, at temperatures from 125 to 149°C migrated through fractures. The formation of breccias within post‐salt rocks in primary weld zones facilitated the upward migration of formation waters that interacted with pre‐salt rocks with high 87Sr/86Sr ratios. Formation waters at temperatures of 80°C migrated during the emplacement of the Upper Pedraforca thrust sheet in the Late Cretaceous‐Palaeocene. These fluids interacted with Upper Cretaceous Carbonates and/or Triassic evaporites. In contrast, the influence of meteoric fluids increased in shallower positions due to the exhumation of the SE Pyrenees during the Eocene‐Oligocene. Coevally, hot dolomitizing fluids migrated along diapir walls during the formation of secondary welds. Supergene ores documented in diapiric areas worldwide related to meteoric fluids, and the similar meteoric percolation occurred in the Upper Pedraforca thrust sheet, suggest that supergene mineralization could be found in the Pyrenees. Brecciation of rocks in primary weld zones and evaporite detachments, where mineralization accumulate in fractures from basement‐derived fluids, also suggests that similar ores could be found in the Pyrenees.