Basin-Scale Mineral and Fluid Processes at a Mississippian Platform Margin

Abstract

Fault-controlled dolomite has been well studied and described in various localities in the Pennine Basin and North Wales. Fluid flow modelling 1 indicates sufficient fluid volumes for dolomitisation could have been supplied along faults from the juxtaposed basinal sediments, but geochemical (PHREEQ) models indicate insufficient Mg. Reactive transport models (RTM) show geothermal convection of seawater could have occurred but geochemical data is consistent with dolomitisation from evolved basinal brines that interacted with siliciclastic sediments and/or volcanics. What is investigated in this study is whether geothermal convection provided a precursor to dolomitisation by fluids expelled by basin dewatering, and if such a process is of global importance. The Derbyshire Platform is a Mississippian rimmed shelf, the westernmost expression of the East Midlands Platform. On the SE platform margin, 50km² of Visean limestones have been dolomitized, forming two major bodies associated with major NW–SE trending basement lineaments and volcanics. The onset of compressional tectonics associated with the Variscan Orogeny resulted in multiple phases of NW-SE and NE-SW trending fault/fracture controlled calcite cementation and Pb-Zn-F-Ba mineralization 2. This study uses outcrop and newly available core from the southern margin of the Mississippian Derbyshire-East Midlands Platform to better constrain the timing and mechanism for dolomitisation. Dolomitisation is usually fabric destructive with a range of textures that suggest multiple phases of fluid flux. Geochemical data indicates slightly modified seawater, with a contribution from hydrothermal fluids, was responsible for dolomitisation. New data based on a refined paragenesis is now being used to test the hypothesis that dolomitisation by geothermal convection of seawater during early burial was an important pre-requisite for later dolomitisation by hotter, more evolved basinal brines. Demonstration of a feedback mechanism between these processes has the potential to inform arguments that favour mass fluid transfer during burial diagenesis. 1. FRAZER, M. 2014. Unpublished PhD thesis, University of Manchester 2. HOLLIS, C., AND WALKDEN, G., 2002, V72, no.5, p700-710

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016