The Crummock Water aureole: a zone of metasomatism and source of ore metals in the English Lake District

Abstract

The Crummock Water aureole, an ENE-trending elongate zone of bleached and recrystallized Skiddaw Group rocks, 24 km in length and up to 3 km wide, is a zone in which pervasive metasomatism has modified the composition of the dominantly siltstone and mudstone lithologies. The bleached rocks show a substantial net gain of As, B, K and Rb and loss of Cl, Ni, S, Zn, H 2 O and C. Carbon loss is responsible for the bleaching. There are smaller and more localized net losses of Cu, Fe, Li and Mn, and gains of Ca, F and Si, whilst Co, Pb and REE are at least locally redistributed. Many chalcophile elements show evidence of initial widespread depletion and subsequent local enrichment. The mineralogy of the rocks is little affected by the geochemical changes. Like their counterparts outside of the bleached zone, the metasomatized rocks consist essentially of quartz, chlorite, muscovite, paragonite and rutile. Small aggregates and porphyroblasts of white mica and chlorite are developed. The metasomatism, which was accompanied by tourmaline veining, is superimposed on a contact metamorphic event. It post-dates the main Caledonian cleavage but pre-dates late Caledonian minor folds. Rb-Sr whole rock isochrons suggest that the metasomatic event occurred at c. 400 Ma and was thus associated with the Lower Devonian Shap-Skiddaw granite magmatism and not the earlier Eskdale Granite or Ennerdale Granophyre magmatic events. Modelling of Bouguer anomalies indicates that geological and geochemical constraints are most simply satisfied if the metasomatism is attributed to a buried, elongate, highly evolved granitic body intruded along the northern margin of a major granitic-granodioritic component of the Lake District batholith. The bleached zone is associated with a major lineament, which may reflect basement control on the location and form of the buried intrusion. Loss of metals from the bleached rocks is related to penecontemporaneous and subsequent hydrothermal vein mineralization and demonstrates that Skiddaw Group sedimentary rocks were a source of ore metals in the Lake District.

The graphite deposit at Borrowdale (UK): A catastrophic mineralizing event associated with Ordovician magmatism

“…The values presented here compare to the range of 0.039--0.59% determined on a small number of typical Skiddaw Group rocks reported by Cooper et al (1988) and Bebout et al (1 999). It is worth noting that samples ground to <53 Iffil show significantly lower carbon contents (0.1 9-0.36%).…”

Pre-Acadian copper mineralization in the English Lake District

“…However, sulphur isotope studies (Lowry et al 1991) show that sulphides from the Bonsor vein contain isotopically heavy sulphur compared with inferred magmatic values from the Borrowdale Volcanic Group and the Eskdale and Ennerdale granites, but are consistent with derivation of the sulphur from the Skiddaw Group. Metals, as well as sulphur, may have been scavenged from the Skiddaw Group as suggested by Cooper et al (1988).…”

“…Firman, 1978a) to correlate the distribution of individual suites of deposits with structural features within or above the Lake District batholith, and to relate their emplacement to phases of activity within it. By demonstrating that the Skiddaw Group rocks of the Buttermere area were depleted of several elements during the development of the Crummock Water metasomatic aureole, Cooper et al (1988) inferred that these rocks were a potential source of some of the metals found within the Lake District veins. Isotopic studies by Lowry et al (1991) have shown that sulphur in sulphides in the copper and lead-zinc suites was probably derived from Skiddaw Group rocks, whereas magmatic sulphur is a major component of the mineralization associated with the Lower Devonian Shap and Skiddaw granites.…”

Boron metasomatism in the Brabant Massif (Belgium): Geochemical and petrographical evidence of Devonian tourmalinite pebbles

“…Fortey & Cooper 1986). Both of them are associated with Early Devonian intrusive activity Cooper et al 1988). Noteworthy are also some minor tourmaline mineralisations encountered in drill cores in East Anglia (Tim Pharaoh, pers.…”