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Multiple Stages of Calcitization/Dedolomitization in the Fault-related Dolomite (Upper Albain), NW Spain: Petrographic, Geochemical and Isotopic Evidences

Mumtaz Shah and Waqar Ahmed

Petrographic, geochemical analyses and isotopic signatures show multiple events of calcitization associated with fault-related dolomite bodies in the Karrantza Valley, NW Spain). These dolomite bodies are hosted by Albian slope to platform carbonates, which were deposited in the Basque-Cantabrian Basin. Besides telogenetic calcite, hydrothermal calcite cements pre- and postdate dolomitization. Mineralogical and geochemical investigations (XRD, ICP-MS/OES, XRF, stable and Sr isotopes) helped in distinguishing these various calcite phases. Calcites exhibit a broad range of depleted δ18O values (–21.5 to –06.5‰ V-PDB), which advocate for multiphase calcitization/dedolomitisation at various temperatures. Furthermore, fluid inclusion analyses confirmed above results as calcite post-dating dolomite shows high Th (190-200°C) as compared to calcite formed before dolomitisation (Th= 120-140°C). Although, most of the dedolomitisation resulted in occluding the intercrystalline pore spaces in dolomite but some calcite phases showed unstable behaviour and underwent dissolution, which resulted in the increase in porous nature of the host rock. Fluid drives are not well constrained by our data, but both fluid types are focused along major faults that cross cut the platform edge and are associated with diapir tectonics.

AAPG Search and Discovery Article #90161©2013 AAPG European Regional Conference, Barcelona, Spain, 8-10 April 2013