Burial diagenetic events, hydrocarbon emplacement and mineralisation in Dinantian limestones of northern England
Published mineralogical and diagenetic studies of the Derbyshire Platform focus upon the source of Mississippi - Valley type (MVT) mineralising fluids and hydrocarbon deposits or upon the diagenetic history of the area. This study aims to integrate these fields by developing a complete model for the geochemical evolution of the Derbyshire Platform and the surrounding basins, in line with the regional Variscan tectonic history of the area and the controls this imposed upon fluid flow. The study area is compared to diagenetic events on the southern margin of the Askrigg Platform and the adjacent Craven Basin which are thought to have been contemporaneous with events on the Derbyshire Platform. A sequence of late diagenetic calcite cements can be recognised on the Derbyshire Platform within vein systems and cross cutting relationships between veins and variations in cement type illustrate there were several phases of cementation. On the Askrigg Platform, burial calcite cements can be identified within veins and intergranular pores. On both the Derbyshire and the Askrigg Platforms, burial calcite cements were contemporaneous with hydrocarbon emplacement and MVT mineralisation. Compacting Dinantian - Namuiran shales in basins adjacent to the Derbyshire and Askrigg platforms offer the most likely source of fluids. Trace metals, hydrocarbons and fluids were progressively released from overpressured basins onto the platforms along fault systems. Fluids circulated on the platform within minor faults and fractures. Restricted volumes of fluid were released from the basins during the waning effects of Carboniferous extension, but the onset of Variscan compression in the mid-late Westphalian led to reactivation of fault systems and massive dewatering of the basins onto the platform. A model can therefore be established relating fluid flow to Variscan tectonic events in northern Britain.