The Crummock Water aureole: a zone of metasomatism and source of ore metals in the English Lake District
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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.Keywords:
Metasomatism
Lake district
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Metasomatism
Tourmaline
Protolith
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Xenolith
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Magmatic volatiles are critically important in the petrogenesis of igneous rocks but their inherent transience hampers the identification of their role in magmatic and metasomatic processes. For example, while the role of magmatic volatiles in porphyry copper systems is relatively well understood, the behavior of volatiles and metasomatic fluids in mantle and crustal magmatic-hydrothermal systems remains a work in progress. Alkaline-carbonatite complexes usually originate from the mantle and typically host REE deposits which are thought to be almost always of a hydrothermal origin. The question thus arises as to the origin of the REE and fluids and if the alkaline and carbonatite magmas are accompanied by fluids from the mantle up and through the crust. This study presents the results of whole rock trace elements and Sm–Nd isotopes analyses of clinopyroxenites, melteigites, ijolites, melanosyenites, leucosyenites, granites, silicocarbonatites, magnesiocarbonatites, calciocarbonatites, ferrocarbonatites and polygenic breccias from the 1894 Ma Paleoproterozoic alkaline-carbonatite complex of Montviel, Abitibi, Canada. The metasomatic rocks range in REE concentrations from 100 ppm to 3.54 wt.% and show εNd values ranging from + 6.8 to − 7.2. Based on these results it is demonstrated that volatile-saturated magmas at Montviel were injected through 4 distinct mantle pulses which evolved by fractional crystallization, mixing of depleted mantle and crustal fluids and metasomatism.
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