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. Fluorspar mineralisation is found mostly on the eastern side of the dome.  The vein structures extend into the shales and gritstones, but these rocks form a barren cap to the mineralisation and extend 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 environment. Fluoritisation of limestone in fractured or well-jointed zones has also given rise to replacement deposits.  Limestone contains interbedded impervious volcanic rocks; occurring as tuffs and lavas and is locally called toadstone.

The toadstones can attenuate or even cut off the mineralisation, and in fact the thicker units form a lower boundary to 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 deposits 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, based on blended ore supplies from both underground and surface deposits. Open Pit sites generally last no longer than five years whereas underground developments, with significantly more associated infrastructure, last considerably longer. 

As a result of the limited size of open pits, an operator is frequently required to seek planning permissions for new sites and success is dependent on holding 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 put back to original ground level.  Occasionally it is desirable to leave some vestige or remnant of the mining operation and sites e.g shallow cliff features. Internationally regarded external consultants maintain restoration standards through liaisons with Glebe Mines.