Exploration for carbonate-hosted base-metal mineralisation near Ashbourne, Derbyshire

Exploration has been carried out for lead-zinc-baryte mineralisation in Carboniferous (Dinantian) limestones to the south of the main mining district of the Peak District. Previous evidence from regional gravity surveys suggested the existence here of a ridge of limestones, the Snelston ridge, largely concealed beneath a thin cover of Permo-Triassic rocks. The structural setting suggests that the area could lie near the projected margin of the Staffordshire stable shelf area and at the northern margin of the Needwood Permo-Triassic Basin. Evidence provided by old mining activity in two small limestone inliers on the Snelston ridge encouraged the present exploration for concealed mineralisation either at the unconformity between these rocks and the overlying PermeTriassic or, more importantly, hosted by limestones. Interpretation of the improved regional gravity data sets and detailed traverses led to better definition of the location of the concealed limestone surface. Additional evidence on the concealed structure was provided by a commercial seismic reflection profile which became available during the project, and which clearly indicated the fault-controlled nature of the ridge. The gravity interpretation, supported by evidence from resistivity soundings, led to the siting of a borehole. This proved the existence of a shallow (60 m) Dinantian limestone ridge but found little mineralisation at the unconformity, apart from minor barium enhancement. At the Limestone Hill inlier, previous mining history and limited evidence from exposures pointed to the existence of sulphide and baryte mineralisation in the exposed Chadian knoll-reef limestone. High barium, lead and zinc values were obtained from soils over the exposed limestone and geophysical surveys improved the interpretation of the near-surface geology. A borehole proved extensive replacement galena-sphalerite-baryte mineralisation with up to 10% Pb over an intersection of about 10 m near the top of the knoll-reef. The mineralisation is of replacive type in limestone which has been extensively dolomitised to leave a very cavernous rock. There is no evidence of fluorite, but baryte is common at the surface and occurs in the borehole core. Two further boreholes on the flanks of the knoll-reef showed that the mineralisation is associated with the dolomitised summit and does not extend laterally to any great distance. ’ The proven existence of the Snelston ridge provides a possible target in an otherwise unexplored area. A geophysical approach to further exploration would involve detailed gravity surveys and the application of methods capable of locating mineralisation at depths of 100 - 200 m.