Zebra-dolomite, tectonic deformation and hydrothermal fluid-flow: lessons learned from Asón Valley hydrothermal dolomites (northern Spain)

The knowledge of the three-dimensional geometry and spatial organization of structural traps has been one of the most powerful tools in hydrocarbon exploration for many years. Nevertheless, these models of structure geometry and organization do not consider the interaction of fluids and structures during deformation when the structure is being initiated and amplified. However, recent advances in the understanding of the role of fluid pressure in the initiation of faults and folds and the subsequent influence of these structures on the migration and concentration of fluids, has opened a new chapter in the role of structural geology in reservoir characterization. In this sense, the dolomitization model proposed for the Asón Valley hydrothermal dolomites contributes to ascertain the role of tectonic deformation and fluid pressure in the initiation of faults and folds and the subsequent influence of these structures on the migration and concentration of dolomitizing fluids. Zebra-dolomites have become a key feature to understand these processes.Zebra-dolomites are banded textures made up of mm to cm thick layers of alternating darker fine-grained dolomite and light coarse-grained dolomite. Its origin has been traditionally suggested to be purely diagenetic and closely related to MVT ores. However, a hydrothermal origin and its relationship to strain deformation in the precursor carbonate has been proposed in the last years. Superb outcrops in the Asón Valley area (northern Spain) have provided key examples to understand the origin of this dolomite texture and its role in the dolomitization process. The dolomites of the Asón Valley correspond to fracture-related hydrothermal dolomite bodies that spread out from dominantly NE-SW trending Ramales fault and are hosted in Albian shallow platform carbonates. The fracture network is related to local tectonic structures such as folds, diapirs and faults. Although they were controlled by regional NE-SW transtensional stress, each one of these local structures (i.e. Pondra Diapir, Ranero overstep, Pozalagua fault, etc.) was able to create local stress-states and fracturing patterns that had a major impact on fluid-flow and dolomitization processes.Many authors have discussed the migration of fluids along major faults during and immediately after reshear. By contrast, the migration of fluids in association with the development of folds has received comparatively little attention in research literature. Both cases controlled the Asón Valley HDT formation. The proposed dolomitization model considers faulting and buckling and related strain partitioning as the main controls in fluid-flow and dolomitization process and zebra-dolomite units were formed in main shear stress zones and acted as main fluid-flow conduits during the dolomitization process.