Controls of copper mineralization at Coniston, English Lake District

Abstract

Analysis of the fracture pattern in the Coniston area of the Lake District indicates that copper mineralization is localized in a series of fractures produced during the main phase of deformation affecting the Borrowdale Volcanic Series. Three events are recognized on the basis of field and textural evidence obtained from polished sections: an early haematite mineralization, correlated with the low grade regional metamorphism affecting the rocks; the main phase of sulphide mineralization, with zoning, which is correlated with a granite intrusion at depth; and a late renewed movement on the veins, with carbonate mineralization, believed to be related to uplift of the granite. The distribution of copper deposits in the Lake District as a whole suggests that they are localized in and immediately adjacent to the Borrowdale Volcanic Series, in structurally favourable positions above the flanks and ridges in the underlying granite.

The geochemistry, metasomatism and petrogenesis of the granites of the English Lake District

“…The Lake District contains an important orefield with numerous mineral veins of hydrothermal origin (Stanley & Vaughan 1982;Dagger 1977;Shepherd et al 1976). Shepherd et al postulated, on the basis of isotopic and fluid inclusion evidence, that the tungsten ores of the province were deposited soon after the consolidation of the granites from saline solutions of mixed magmatic and circulating ground water origin, and Firman (1978) suggested a similar origin for the other vein systems in Lower Palaeozoic rocks.…”

Pre-Acadian copper mineralization in the English Lake District

“…Where arsenopyrite has overgrown siltstone fragments, it has clearly deflected cleavage seams and shows some brittle fracture with quartz in pressure fringes. Dagger (1977) suggested that at Coniston the introduction of original hematite, now replaced by magnetite, could have followed closely the formation of the fractures that are now occupied by the veins and that may have occurred soon after the regional metamorphism that accompanied folding of the volcanic rocks. An early date for this mineralization was thus implied.…”

“…A relatively shallow silicic igneous body was considered by Lee (1989) as one of several possible explanations for the residual gravity low in the Coniston area. The apparent spatial association of the copper-bearing veins with the Lake District batholith, which for many years was considered also to be Devonian (Firman, 1978b), led Dagger (1977) and Firman (1978a) to conclude that both were related. A link with the Lake District batholith was also preferred by Stanley & Vaughan (1982a).…”

“…However, the Rb-Sr isochron ages of around 430 Ma (Silurian: Llandovery) obtained by Rundle (1979) for the Eskdale and Ennerdale intrusions made a direct association between mineralization and magmatism appear unlikely because of the apparently long time span between the end of volcanism and inferred vein formation in Early Devonian times. Dagger (1977) showed that the metalliferous veins at Coniston fill northwest-trending fractures, and that prominent northeast-trending strike faults were generally unmineralized. Though Wheatley (1971) and Dagger (1977) concluded that the fault veins formed during the Acadian Orogeny, our work (British Geological Survey, 1996;Millward et al in press) indicates that at least some of the faults that are now mineralized were active during volcanism.…”

“…Dagger (1977) showed that the metalliferous veins at Coniston fill northwest-trending fractures, and that prominent northeast-trending strike faults were generally unmineralized. Though Wheatley (1971) and Dagger (1977) concluded that the fault veins formed during the Acadian Orogeny, our work (British Geological Survey, 1996;Millward et al in press) indicates that at least some of the faults that are now mineralized were active during volcanism. Thicknesses of constituent units within the Paddy End Member thicken abruptly to the northeast across the fault veins at Paddy End.…”