Poynton U3A Geology Group Visit to Magpie Mine, May 2015

Lead in the South

Lead (Pb) is a natural metallic element. It is normally only found as the mineral ore, galena (lead sulphide (PbS)). Other common minerals in the South Pennine Orefield are fluorspar or fluorite (calcium fluoride (CaF₂)), barite (barium sulphate (Ba SO₄)) and calcite (calcium carbonate (CaCO₃)). Smaller quantities of Sphalerite (zinc sulphide (ZnS)) and chalcopyrite (copper-iron sulphide (Cu,FeS₂) – the copper ore at the Ecton mines) have also been mined.

The South Pennine Orefield extends across the East and North Eastern side of the limestone shelf of the (and underneath the adjacent shales and millstone grits). Lead is no longer mined there, but there are around 2000 named lead veins known from past mining activities. Some 10,000 mines were worked in these veins.

Lead veins in the South Pennine Orefield

Source: Origins of the Peak district Orefield, David G Quick, UK Journal of Mines and Minerals No 13

Before the beginning of lead mining it is estimated there were 4 million tonnes of galena in the Orefield.

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Poynton U3A Geology Group Visit to Magpie Mine, May 2015

Galena is found as a crystalline mineral deposit in the faults and fractures of the limestone. These faults and fractures predated the deposition of the minerals. They were reactivated and widened by the stretching of the crust around 290-270 ma which allowed hot, saline and slightly oily mineralised fluids to penetrate the limestone. The veins are found in rakes and scrins, and pipes and flats.

The origin of the minerals lay in the Namurian shales which underlay the millstone grits in the depositional basins of the Gainsborough Trough and the Edale Gulf to the North and East of the limestone shelf, and the Widmerpool Gulf to the South East.

Source: Origins of the Peak district Orefield, David G Quick, UK Journal of Mines and Minerals No 13

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Poynton U3A Geology Group Visit to Magpie Mine, May 2015

These basins were filled with muddy sediments as the deltas of the late advanced from the North East and the South. The shales in the basins had unusually high concentrations of fluorine, barium, lead, zinc and uranium. Highly saline fluids (ten times more salty than today’s seawater) trapped in the sediments picked up minute quantities of these metallic elements. As the shales were buried deeper by overlying deposits, the fluids were forced out into the adjacent limestone shelf which protruded up into the layers of shale. The top of the limestone shelf was at this time buried under 2 km of Namurian shales and millstone grits and Measure deposits. The shelf was also being uplifted towards the North East, which created a hydraulic gradient causing the fluids to flow to the South West. At this depth the fluid would have been about 130⁰ centigrade. It has been estimated that around 2000 km³ flowed out of 500 km³ of Namurian source rocks over 0.7 million years at a rate of 1 metre a year. These fluids contained about 20 ppm of lead. As the mineral-bearing fluids moved through the limestone they met cooler oxygen-bearing waters (which also had some sulphurous compounds). This caused the minerals to precipitate as crystals growing inwards from the walls of fissures and caves.

The uplift of the limestone shelf eventually led to the erosion of the , the millstone grits and the shales, exposing lead veins in the limestone at the surface.

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