Ecton Copper Mines SSSI

Ecton Copper Mines: SK 099581

The site consists of a complex of individual mine workings and spoil dumps, some of which are connected underground and may have been excavated to depths as great as 400 m. The mines were worked for copper from the 16th Century, being abandoned late in the 19th Century. Large areas of the mines are now flooded and inaccessible.

Ecton Copper Mines consists of an array of veins developed predominantly within the Ecton Limestones of Lower Carboniferous age. These are concentrated within the structurally complex area of the Ecton Anticline where folding and faulting occurred during the Variscan Orogeny. Mineralisation occurred in NNW and ENE oriented fissures and fractures. However, the bulk of the mineralisation took place in a series of pipe-like deposits associated with the axial region of the anticline. The precise mechanism by which the mineralising fluids penetrated these rocks is not fully understood and a number of alternatives have been suggested.

The Mixon Limestones and Shales which rest on the Ecton Limestone formed a cap sealing the mineralising fluids into the Ecton Anticline. Fractures which had formed in the brittle Ecton Limestone provided conduits for the mineralising fluids and space for the precipitation of the ore minerals.

Mineralisation consists of an upper, lead rich zone, and a lower, copper rich zone. The zoning reflects fractionation processes that took place during the precipitation of minerals from the mineralising fluid. Primary copper minerals include chalcopyrite, chalcocite and bornite. Galena is the primary lead mineral and small quantities of sphalerite indicate the presence of zinc. Secondary copper minerals include malachite, azurite, cuprite and tenorite. Cerussite is a secondary lead mineral and smithsonite and hydrozincite, secondary minerals of zinc. Pyrite, arsenopyrite and celestine also occur while the main gangue minerals are calcite, barite and fluorite.

Isotope dates of 270–235 million years ago for the alteration of igneous rocks in the surrounding area have been taken to suggest that the mineralisation event was Permian in age. The isotopic composition of the sulphides present m the minerals indicate that the mineralising brines originated from shales of Carboniferous age which were deposited in the basins surrounding the Derbyshire High. These brines migrated onto the Derbyshire High both on convection currents driven by crustal heating and through gravity flow driven by tectonic inversion. For many of the mineralised bodies in Derbyshire, the brines are known to have originated from basins to the east of the Derbyshire High. The position of Ecton Copper Mines relative to the Derbyshire High, as well as the large concentrations of copper suggests that the mineralising brines originated from a different basin to those responsible for mineralisation in the rest of the Derbyshire Ore Field. Such brines may have been as far travelled as the Irish Sea Basin, and may have been injected several times.

Study of the paragenesis and morphology of the remaining mineral assemblages at Ecton Copper Mines is essential for an understanding of the Derbyshire Ore Field as a whole. Such studies also provide information related to the mineralising brines. The implications of such studies are far-reaching in terms of understanding metallogenic processes on the Derbyshire High, Cheshire Basin, and Irish Sea Basin.

See Also: Ecton Hill Field Studies Association

Source: SSSI citation, Natural England

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