Geology

The Derbyshire Dome, a Carboniferous limestone structure, covers an area of approx. 200 square miles and is flanked, except to the south, by overlying shales and grits. The Fluorspar mineralisation is mostly to be found on the eastern side of the dome, the vein stuctures not extending in to the shales and gritstones which form a barren cap to orebodies extending east beyond the limestone outcrop. The main deposits are of fissure vein type and are restricted stratigraphically to Visean limestones of the Carboniferous System in a structurally controlled enviroment. Fluoritisation of limestone in fractured or well jointed zones has also given rise to replacement deposits. The limestone contains interbedded impervious volcanic rocks occurring as tuffs and lavas and is called locally toadstone. The toadstones can attenuate or even cut off the mineralisation, and in fact the thicker units form a lower boundary to the mineral extraction making the workable depth of veins approx. 100mts. Vein widths can vary over relatively short distances from between 1.5mts to 12mts wide and are composed of varying proportions of fluorite, baryte, calcite and galena with the fissure-fill and replacement-type deposists ranging in size from 5,000 to 1,000,000 tonnes.

The Peak District is rich in the historic remains, stretching back to Roman times, of a vein minerals industry, which was for a time the most important mining area in the world. Lead mining ceased at around the end of the 19th century leaving thousands of tonnes of fluorspar in old mine dumps, this became the basis for the early spar industry and provided the raw materials for metallurgical fluxes for the steel industry in Sheffield and for exports world wide.

Cavendish Mill was commissioned in 1965 and the open pit reserve controlled by Glebe Mines is currently 5 years, which with an active rolling exploration programme is being added to on a continuous basis. Sites generally last no longer than 5 years and there can be 6 or more operations active at any one time. Consequently there is frequent need to seek planning permissions for new sites and success is dependent upon a good reputation as a responsible operator with high standards of restoration. In most cases full restoration, in consultation with the Peak District National Park Authority, is back to original ground level, though occasionally it is desirable to leave some vestige or remnant of the mining operation and sites often incorporate this in the final restoration as shallow cliff features which are common to the area. Restoration standards are maintained through liaison with internationally regarded external consultants.