The occurrence of an unusual galena-rich replacement flat with a well developed supergene assemblage, from the South Pennine Orefield, has been investigated. The reason for a galena-rich flat at this location has not been ascertained but the form of the deposit demonstrates a combination of lithological and structural controls. The main factors in producing a sulphate-dominant supergene assemblage have been the presence of a quartz and fluorite replacement halo to prevent buffering of weakly acidic meteoric solutions, an absence of primary calcite mineralization and the downward, within-the-vein, as opposed to in-limestone, flow of meteoric water. The deposit has initially evolved more like a ‘silicate rock-hosted’ than a carbonate-hosted deposit. The coarse anglesite crystal size, and its isolation from the associated galena, indicates unusually high and prolonged mobility of lead due to the long term stability of the supergene system during the presence of the Namurian shale cover. The deposit reverted to a more typical cerussite stability field on removal of the shale cover as limestone-equilibrated fluids, rich in dissolved carbonate, entered the flat.

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