The role of clay mineral reactions in supplying trace elements for diagenesis and mineralisation within the Lower Carboniferous Limestone of the Pennine Basin and North Wales Platform

    Research output: Contribution to conferenceOther

    Abstract

    The Derbyshire-East Midlands Platform and Askrigg Platform of northern England, and the North Wales Platform all host economic volumes of Pb-Zn-F-Ba-Cu mineralisation within Lower Carboniferous (Dinantian) platform limestones. This mineralisation is coeval with high volumes of calcite cementation, localised dolomitisation, and minor volumes of hydrocarbon. The tectono-stratigraphic framework for the region has been described in detail by Fraser and Gawthorpe (2003). Carbonate sedimentation took place in the Dinantian upon the footwalls of NE-SW and E-W trending Caledonian lineaments, which were reactivated by back-arc extension north of the Variscan Orogen. Within the hanging wall, more rapid subsidence created deep water basins that accumulated carbonate sediment shed from the platform margins and very fine grained siliciclastics. Progradation of turbidite-fronted siliciclastic systems in the Namurian led to deposition of a thick succession of organic-rich mudstones and shales within the basins as carbonate sedimentation waned. On the Derbyshire Platform, field, petrographical and geochemical fingerprinting of calcite cements that heal NE-SW and NW-SE trending fractures has revealed the intergrowth of calcite cements within cross-cutting fractures with increasing volumes of hydrocarbon, galena, barite, fluorite and sphalerite (Hollis and Walkden, 2002). Fluid inclusion and stable isotope data indicate precipitation of these cements took place from progressively hotter fluids, with the greatest volume of mineralisation associated with the penultimate, Zone 4C, calcite cement at temperatures up to 200oC. Building on a number of previous studies, Hollis and Walkden (2002) proposed that many of the metals for mineralisation were sourced from organic matter and clays within the mid-Carboniferous mudstones and shales within the adjacent basin. Evidence for this includes higher than average concentrations of Pb, Zn, F and Cu within Namurian shales within these basins. Fluid inclusion temperatures and stable isotope result data from calcite cements on the Derbyshire Platform suggest that dolomitisation and mineralisation was initiated as the basins entered the oil window, and clay transformations such as smectite to illite were initiated. The main stage of mineralisation appears to have been coincident with the onset of basin inversion in the late Carboniferous to Permian.In this context, a model is presented whereby trace elements for carbonate diagenetic reactions and mineralisation on the Derbyshire Platform were released from clay minerals, feldspar, mica and organic matter within the basin and transported as halogen-complexes and/or complexed within petroleum. Data from the Manx-Furness and Bowland Basins, adjacent to the North Wales and Askrigg Platforms is more sparse, but it is postulated that differences in sediment composition, permeability and burial history of the basins resulted in some fundamentally different patterns of diagenesis and mineralisation to the Derbyshire Platform.
    Original languageEnglish
    Publication statusPublished - Sept 2011
    EventFrontiers in Diagenesis: Clay and carbonate facies and their diagenetic pathways in reservoir rocks - University of Cambridge
    Duration: 14 Sept 201115 Dec 2011

    Conference

    ConferenceFrontiers in Diagenesis: Clay and carbonate facies and their diagenetic pathways in reservoir rocks
    CityUniversity of Cambridge
    Period14/09/1115/12/11

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