Supergene processes in galena-replacement mineralization from the Longstone Edge vein system, Derbyshire

The occurrence of an unusual galena-rich replacement flat with a well devel oped supergene assemblage, from the South Pennine Orefield, has been invest igated. The reason for a galena-rich flat at this location has not been asc ertained but the form of the deposit demonstrates a combination of litholog ical and structural controls. The main factors in producing a sulphate-domi nant supergene assemblage have been the presence of a quartz and fluorite r eplacement halo to prevent buffering of weakly acidic meteoric solutions, a n absence of primary calcite mineralization and the downward, within-the-ve in, as opposed to in-limestone, flow of meteoric water. The deposit has ini tially evolved more like a 'silicate rock-hosted' than a carbonate-hosted d eposit. The coarse anglesite crystal size, and its isolation from the assoc iated galena, indicates unusually high and prolonged mobility of lead due t o the long term stability of the supergene system during the presence of th e Namurian shale cover. The deposit reverted to a more typical cerussite st ability field on removal of the shale cover as limestone-equilibrated fluid s, rich in dissolved carbonate, entered the hat.