Stratabound Pb-Zn-Ba-F mineralisation in the Alston Block of the North Pennine Orefield (England) : origins and emplacement

The North Pennine Orefield Alston Block has produced approximately 4 Mt Pb, 0.3 Mt Zn, 2.1 Mt fluorite, 1.5 Mt barite, 1 Mt witherite plus a substantial amount of iron ore and copper ore from predominantly vein-hosted mineralisation in Carboniferous Limestones. However, a significant proportion of this production (c.20 per cent) came from stratabound deposits. Though much is known about the vein-mineralisation, the relationship between the veins and the stratabound mineralisation is not well understood. New petrographic, isotopic and fluid inclusion data derived from samples of stratabound mineralisation allow us to present a unified model that addresses the genesis of both the vein and stratabound styles of mineralisation. We recognise three episodes of mineralisation: (i) Dolomitisation and ankeritisation — in the vicinity of the stratabound mineralisation limestones are pervasively dolomitised/ankeritised with extensively developed vuggy porosity. This occurred in the presence of a high-salinity brine consistent with fluids derived from adjacent mud and shale-filled basins. (ii) Main stage fluorite–quartz–sulphide mineralisation — in addition to metasomatic limestone replacement, repeated brecciation, dissolution and hydrothermal karstification plus additional porosity modification and redistribution of carbonate occurred, with open space filled with fluorite, galena, sphalerite, quartz and barite. Microthermometric and microchemical fluid inclusion data provide direct evidence for mineralisation in response to mixing of a low-salinity sodic groundwater with a high-salinity, calcic, Fe-rich, metalliferous brine which had highly elevated metal contents relative to ‘normal’ high total dissolved sulphur (TDS) sedimentary brines. (iii) Late stage barite mineralisation — this is paragenetically late and appears to represent the waning stages or distal portions of the main hydrothermal circulation under cooler conditions.