Fluid expulsion from overpressured basins: Implications for Pb–Zn mineralisation and dolomitisation of the East Midlands platform, northern England

Our understanding of burial diagenesis within carbonates is often limited by poor constraints on available fluid volumes and geochemistry. However, regional stratigraphic and burial history data are often readily available. Using these data to develop numerical models which couple sedimentological and hydrological basin evolution, we estimate the volumes and geochemistry of fluids that were available to drive dolomitisation and Pb-ore genesis within the Carboniferous, Derbyshire Platform of northern England. Current conceptual models of these processes invoke tectonic release of burial-induced overpressure developed within adjacent Dinantian basins as a drive for reactive fluid flow to the platform. Our simulations show that compaction-driven flow may lead to a supply of fluids that is more complex in its temporal evolution than may be expected. Spatial variations in the rate of fluid expulsion from different sediments lead to a staggered delivery of fluids from different sources. Rapid fluid expulsion within deeper sediments leads to a downwards-decreasing pressure gradient that subsequently draws down fluid from within overlying sediments. Thus, early fluid supply to the platform is sourced from the deep basins while later fluid supply descends from above the platform as well as from the sides. We suggest that such a flow development may have important implications for the relative timing and distribution of a sequence of diagenetic products within the platform. This hypothesis is tested using volume estimates from our simulations. We conclude that although this staggered fluid supply model may be applicable generically, it appears that it is only valid for explaining mineralisation in the Derbyshire Platform. Fluid volumes supplied to the platform are insufficient to explain dolomitisation. Our simulations are supported by a sensitivity analysis that identifies that compaction-driven flow in this system is strongly controlled by the rate of burial and sediment permeabilities within the compacting basins.