Interplay between fracturing and hydrothermal fluid flow in the asón valley hydrothermal dolomites (Basque-Cantabrian Basin, Spain)

In Asón Valley hydrothermal dolomite area (Basque-Cantabrian Basin, northern Spain), an overlapping stepover area between two major basement faults, the Cabuerniga and Ruahermosa transtensional faults, was the location for different scales and types of extensive fractures. This fracture mesh affected the Albian Ranero limestone and was formed in a dilational jog, a regional fluid throughflow area. It acted as pathway for overpressured fluids that controlled the dolomite mineralization in the Ranero massif area. The study of synchronous structural features, fluid flow channelling and dolomitization processes indicated their tectonic control. The fluid circulated and concentrated preferentially in more fractured areas with increased permeability, such as extensional chimneys, creating dolomite bodies. Repeated extensive tectonic activity enhanced fracture porosity, promoting overpressured fluid migration and cyclical dolomitization events. The studied fracture pattern suggests the presence of sinistral transtension during the formation of extensional joint-sets that channellized the mineralizing fluid-flow. A gradation in structural features and dolomite facies and textures is visible from the main Pozalagua fault dolomite body to peripheral dolomite bodies as the Breccia body and Ranero megajoints. These differences could reflect a proximal-distal trend from the main fluid-flow area along the Pozalagua fault to the Ranero megajoints. © The Geological Society of London 2012.