Abstract
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% CaCO<sub>3<sub/>) and exhibit a broad range of depleted δ<sup>18<sup/>O values (–18.7 to –10.5‰ V-PDB), which may indicate multiphase dolomitisation and/or different degrees of recrystallisation. Decreasing δ<sup>18<sup/>O values correlate with decreasing Fe content in dolomites. In the Ranero area, dolomites show less slightly depleted δ<sup>13<sup/>C values (–0.15 to +2.13‰ V-PDB) relative to the host limestone δ<sup>13<sup/>C signature, while these values are substantially more depleted in El-Moro area (down to –2.18‰ V-PDB). Hydrothermal calcites predating dolomitisation show less depleted δ<sup>18<sup/>O values (–14.15 to –12.1‰ V-PDB) than postdolomitisation calcite (–18.1‰ 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.
Highlights
Pervasive dolomitisation has been linked with various processes in diverse geological settings (Warren, 2000; Machel, 2004)
Dolomite bodies can be differentiated by their dark grey color as compared to off-white host limestone (Fig. 5a)
The present study only focuses on dolomitisation events in the host limestone, since the early limestone diagenesis has MO-I MO-II
Summary
Pervasive dolomitisation has been linked with various processes in diverse geological settings (Warren, 2000; Machel, 2004). Recent integrated studies suggest that hydrothermal dolomite exists in many forms and likely to be far more common than previously thought (Berger and Davies, 1999; Moore, 2001; Cantrell et al, 2004; Nader and Swennen, 2004; Davies and Smith, 2006; Nader et al, 2009; LopézHorgue et al, 2010; Ronchi et al, 2010; Conliffe et al, 2010). Besides a few cases of stratiform hydrothermal dolomites, these usually occur as fabric destructive, stratadiscordant cutting across host limestone and concentrated along major faults (Wilson et al, 1990; Nader et al, 2004; Gasparrini et al, 2006; Lopéz-Horgue et al, 2010; Shah et al, 2010; Swennen et al, in press). It is important to understand this particular dolomitisation process, which may be of major economic importance
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