Structure and genesis of the South Pennine orefield
Mineralisation in the South Pennine MVT orefield (225km2) has resulted from the combined effects of basement structure, stress history and basin evolution during the Carboniferous. NW-SE-trending Caledonian thrusts in the basement beneath North Derbyshire were reactivated as normal growth faults during limestone sedimentation in the Dinantian. on a regional scale, a major low-angle detachment, dipping NE away from St. George's Land (20km to the SW of North Derbyshire), controlled basin development as far as the Askern-Spital High (100km to the NE). The North Derbyshire shelf developed on the up-dip crest of a NE-tilted half-graben, in the hanging wall of this detachment, directly above the zone of mantle upwelling. A similar structure also evolved in the Rotherham area, some 40km to the NE, where, it is inferred, another orefield exists in the subsurface. In North Derbyshire, the end of the Dinantian was a period of uplift and erosion. In the NE part of the shelf, dextral wrench faults developed above ENE-WSW-trending basement fractures. At the start of the Namurian the direction of extension rotated to NW-SE and by the end of the Westphalian the limestone was buried to a depth of about 2km (~130°C) due to the combined effects of thermal sag and sediment compaction. Mineralisation began in the early Stephanian, associated with a period of N-S extension. Fractures in the limestone formed during earlier tectonic events began to dilate, thus allowing inflow of acidic F-Ba-Pb-zn-S-enriched fluid expelled from overlying Namurian shales. This was replenished by meteoric water migrating westwards down-dip from the uplifted Askern-Spital-Nocton-Grantham High. Ore deposition occurred within the limestone as a result of increasing pH, Ca 2+ and SO42 in the orefluid due to wallrock dissolution and fluid mixing. Mineralisation probably continues in the subsurface for some distance to the SE of the South Pennine orefield in shallow-water limestones with a similar structural aspect to those exposed in North Derbyshire.