The petrology and paragenesis of fracture mineralization in the Sellafieldarea, west Cumbria

The petrology and paragenesis of fracture mineralization hosted in the Ordo vician, Dinantian (Carboniferous Limestone) and Permo-Triassic rocks of the Sellafield area were studied as part of the United Kingdom Nirex Limited p rogramme of site investigations around Sellafield in west Cumbria for a dee p repository for radioactive waste. This paper summarizes the petrological and mineralogical observations from 23 of the deep (up to 2 km) boreholes d rilled by Nirex. A paragenetic sequence of nine Mineralization Episodes (ME1 to ME9) was rec ognized, and has been interpreted in the context of the geological and hydr ogeological history of the Lake District massif and East Irish Sea Basin ma rgin areas. ME1 to ME3 produced silicate and sulphide-dominated Palaeozoic epithermal to mesothermal mineralization associated with pre-Acadian hydrot hermal circulation and late-Caledonian (mid-Devonian) intrusions. ME4 to ME 7 represent carbonate-sulphate-fluorite-hematite-dominated mineralization a ssociated with warm, complex, Na-Cl-Ca-SO4 brines expelled from the East Ir ish 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 dil ute, sulphate-poor groundwaters carrying barium, resulting in the precipita tion of barite. ME8 and ME9, by contrast, are closely related to the develo pment of the present-day groundwater system. ME8 involved telodiagenetic, s upergene 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 w ith ME8, but represents mineralization in the deeper, more-reducing groundw ater environment. This mineralization episode is dominated by calcite (some times associated with pyrite, marcasite, barite, anhydrite or gypsum) preci pitated from groundwaters of similar composition to those found in the Sell afield area at the present day. Observations suggest that ME9 mineralizatio n occurred during the Quaternary and is ongoing in the present groundwaters .