Mechanisms controlling the localisation of fault‐controlled hydrothermal dolomitisation, Derbyshire Platform, UK

Abstract

The Derbyshire Platform is a Mississippian aged flat-topped, steep sided platform that forms the westernmost expression of the Derbyshire-East Midlands Platform. On the south-east platform margin, 60 km 2 of Visean limestone has been dolomitised, forming two distinct bodies. One of these bodies forms along a major NW-SE trending basement fault and smaller, associated, N-S trending faults and fractures. This study uses outcrop, petrographic and geochemical analysis to better constrain the timing and mechanism for this fault-controlled dolomitisation. Field relationships demonstrate dolomitisation was multi-phase and initiated after the main phase of matrix pore-occluding calcite cementation on the Derbyshire Platform and terminated prior to the main phase of mineralisation. Fluids are interpreted to have fluxed from adjacent basins, primarily along strike-slip crustal faults that were reactivated during basin inversion at the onset of the Variscan Orogeny. Fluid supply was episodic and progressively confined to fractures as matrix porosity became occluded. The study demonstrates the complex interplay between basin kinematics, host rock permeability and timing of fluid supply through seismic valving along faults that connect the carbonate platform to basin compartments. This ultimately controlled the position of dolomite geobodies along faults and provides a record of fluid flow during the transition from thermal subsidence to post-rift basin inversion. The findings have implications for the exploration of both minerals and hydrocarbon within dolomitised host rocks and can inform studies of fluid transfer and reaction on carbonate platforms within the burial realm.

Dolomitisation of carbonate platform margins by fault‐controlled geothermal convection: Insights from coupling stratigraphic and reactive transport models

“…Model results broadly concur with field and petrographic data from the study area, which indicates dolomitisation prior to stylolitisation along the platform margin and E–W trending faults that are interpreted to have formed during basin extension (Breislin et al, 2020, 2022). Phase D1 dolomite was described from EC5 and EC6 strata within boreholes, mines and outcrops, with an irregular and diffuse lower dolomite–limestone contact at ca 200 m beneath the surface (Ford, 2002) within EC5 strata.…”

“…Isotopic and trace element signatures are consistent with dolomitisation from evolved, basinal brines at >80°C (Breislin et al, 2022). Phase D3 is much lower in volume (<10% dolostone volume) and forms replacive dolomite halos adjacent to N–S trending faults, while phases D4 and D5 each comprise <5% of the dolostone volume and occur as fracture filling dolomite cements, including saddle dolomite (Breislin, 2018; Breislin et al, 2022). These observations support suggestions from earlier modelling (Frazer et al, 2014) that tectonically induced release of over‐pressured basinal fluids can drive Mississippi Valley Type mineralisation and contribute to associated dolomitisation, but the observed dolomite volumes require significant dolomitisation prior to basin dewatering.…”

“…The large body on the southern platform margin mostly comprises D1 dolomite which is euhedral to subhedral with cloudy cores and clear rims and dull orange cathodoluminescence (Breislin et al, 2020). The earliest D1 phase, which is the focus of this study, forms up to 50% of the dolostone volume on the Derbyshire Platform and also occurs along E–W trending faults (Breislin et al, 2020, 2022). It is crosscut by stylolites and exhibits isotopic signatures and single‐phase fluid inclusions consistent with formation from a relatively low temperature (<50°C), sea water‐dominated fluid (Breislin et al, 2020).…”

Carbonate diagenesis: A celebration of the work of John Anthony Dawson (Tony) Dickson

“…This first phase of dolomite formed a template for later phases of dolomitisation on the same platform. The effects of this later phase, producing fault‐controlled dolomite bodies, is documented further by Breislin et al (2023). This paper, documents dolomitisation by hot, saline brines sourced from the adjacent hangingwall basin, related primarily to strike‐slip crustal faults and reactivated during basin inversion at the onset of the Variscan Orogeny.…”

“…Their model also evaluates the contribution of incorporating stratigraphic growth and fault propagation. They compare these results with the earliest phase of dolomitisation found in the Derbyshire Platform of northern England, a Mississippian aged flat-topped, steep sided platform that underwent a polyphased dolomitisation, described by Breislin et al (2023). This first phase of dolomite formed a template for later phases of dolomitisation on the same platform.…”