Hydrothermal Dolomites in the Early Albian (Cretaceous) Platform Carbonates (NW Spain): Nature and Origin of Dolomites and Dolomitising Fluids

Hydrothermal Dolomites in the Early Albian (Cretaceous) Platform Carbonates (NW Spain): Nature and Origin of Dolomites and Dolomitising Fluids - This study documents the temporal and lateral variation in petrographic and geochemical signatures of fault-related dolomite bodies in the Ranero and El-Moro areas (Karrantza valley, Cantabrian mountains; NW Spain). These dolomite bodies are hosted in Albian carbonates, which were deposited in the Basque-Cantabrian Basin as a result of an intense rift-related subsidence with associated faulting along various orientations. Fluid circulations generated replacive and cement dolomites, paragenetically followed by various calcite cements. Petrography, mineralogical and geochemical investigations (XRD, ICP, XRF, stable and Sr isotopes) helped in distinguishing hydrothermal stages. Two major dolomite facies were observed according to their Fe-content. Early dolomites are ferroan and replace limestone more significantly than the later, non ferroan dolomites. Dolomites are generally stoichiometric (49.76 to 51.59 M% CaCO3) and exhibit a broad range of depleted delta O-18 values (-18.7 to -10.5 parts per thousand V-PDB), which may indicate multiphase dolomitisation and/or different degrees of recrystallisation. Decreasing delta O-18 values correlate with decreasing Fe content in dolomites. In the Ranero area, dolomites show less slightly depleted delta C-13 values (-0.15 to +2.13 parts per thousand V-PDB) relative to the host limestone delta C-13 signature, while these values are substantially more depleted in El-Moro area (down to -2.18 parts per thousand V-PDB). Hydrothermal calcites predating dolomitisation show less depleted delta O-18 values (-14.15 to -12.1 parts per thousand V-PDB) than post-dolomitisation calcite (-18.1 parts per thousand V-PDB). Sr isotope data suggest that the fluids interacted with siliciclastic lithologies (sandstone, shale). The dolomite fabric is variably altered through dedolomitisation and cataclastic deformation. Dolomitisation occurred in at least two main episodes. A first episode of pervasive ferroan dolomitisation probably resulted from compactional dewatering of basinal fluids from the nearby Basque trough and hydrodynamic fluid flow along the faults/fractures in the Albian carbonate platform. Subsequently, a second episode of very hot and localised dolomitisation may be related to igneous activity and convective flow.