The petrology and paragenesis of fracture mineralization in the Sellafield area, West Cumbria

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doi: 10.1144/pygs.52.2.215
Authors:Milodowski, A. E.; Gillespie, M. R.; Naden, J.; Fortey, N. J.; Shepherd, T. J.; Pearce, J. M.; Metcalfe, R.
Author Affiliations:Primary:
British Geological Survey, Keyworth, United Kingdom
Volume Title:Proceedings of the Yorkshire Geological Society
Source:Proceedings of the Yorkshire Geological Society, Vol.52(Part 2), p.215-241. Publisher: Yorkshire Geological Society, Leeds, United Kingdom. ISSN: 0044-0604
Publication Date:1998
Note:In English. 91 refs.; illus., incl. 3 tables, sketch map
Summary:The petrology and paragenesis of fracture mineralization hosted in the Ordovician, Dinantian (Carboniferous Limestone) and Permo-Triassic rocks of the Sellafield area were studied as part of the United Kingdom Nirex Limited programme of site investigations around Sellafield in west Cumbria for a deep repository for radioactive waste. This paper summarizes the petrological and mineralogical observations from 23 of the deep (up to 2 km) boreholes drilled by Nirex. A paragenetic sequence of nine Mineralization Episodes (ME1 to ME9) was recognized, and has been interpreted in the context of the geological and hydrogeological history of the Lake District massif and East Irish Sea Basin margin areas. ME1 to ME3 produced silicate and sulphide-dominated Palaeozoic epithermal to mesothermal mineralization associated with pre-Acadian hydrothermal circulation and late-Caledonian (mid-Devonian) intrusions. ME4 to ME7 represent carbonate-sulphate-fluorite-hematite-dominated mineralization associated with warm, complex, Na-Cl-Ca-SO4 brines expelled from the East Irish Sea Basin area during progressive burial of the thick Carboniferous and Permo-Triassic sedimentary sequence. Sulphate-carrying brines expelled to the basin margin in the Sellafield area mixed locally with cooler, more dilute, sulphate-poor groundwaters carrying barium, resulting in the precipitation of barite. ME8 and ME9, by contrast, are closely related to the development of the present-day groundwater system. ME8 involved telodiagenetic, supergene alteration and weathering by near surface oxidizing groundwaters. Isotopic evidence indicates that ME8 was probably initiated following Late Tertiary uplift and may still be ongoing in some areas. ME9 may be coeval with ME8, but represents mineralization in the deeper, more-reducing groundwater environment. This mineralization episode is dominated by calcite (sometimes associated with pyrite, marcasite, barite, anhydrite or gypsum) precipitated from groundwaters of similar composition to those found in the Sellafield area at the present day. Observations suggest that ME9 mineralization occurred during the Quaternary and is ongoing in the present groundwaters.
Subjects:Borrowdale Volcanic Group; Caradocian; Clastic rocks; Cores; Fractures; Hydrothermal alteration; Mesozoic; Metasomatism; Mineral assemblages; Mineralization; Ordovician; Paleozoic; Paragenesis; Petrology; Radioactive waste; Sandstone; Sedimentary rocks; Sherwood Sandstone; Site exploration; Triassic; Underground disposal; Upper Ordovician; Volcaniclastics; Waste disposal; Waste disposal sites; Cumbria England; England; Europe; Great Britain; Sellafield England; United Kingdom; Western Europe; Saint Bees Formation
Coordinates:N542400 N542400 W0033000 W0033000
Abstract Numbers:00M/2044
Record ID:1999026328
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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