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The Crummock Water Aureole: a Zone of Metasomatism and Source of Ore Metals in the English Lake District

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The Crummock Water Aureole: a Zone of Metasomatism and Source of Ore Metals in the English Lake District Journal of the Geological Society, London, Vol. 145, 1988, pp. 523-540, 17 figs, 5 tables. Printed in Northern Ireland The Crummock Water aureole: a zone of metasomatism and source of ore metals in the English Lake District D. C. COOPER,' M. K. LEE,'N. J. FORTEY ,l A. H. COOPER,'C. C. RUNDLE,3 B. C. WEBB2 & P. M. ALLEN' British Geological Survey, Keyworth, Nottingham NG12 5GG, UK 'British Geological Survey, Winhior Court, Windsor Terrace, Newcastle upon Tyne NE4 2HE, UK 3NERC Isotope Geology Centre, 64 Grays Inn Road, London WClX 8NG, UK Abstract: The Crummock Water aureole, an ENE-trending elongate zone of bleached and recrystall- ized Skiddaw Group rocks, 24 kmin length and up to 3 kmwide, is azone 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,O and C. Carbon loss is responsible for the bleaching. There are smaller and morelocalized net losses of Cu,Fe, Li andMn, and gains of Ca, F and Si, whilst CO, Pband REE areat leastlocally redistributed.Many chalcophile elements show evidence of initialwidespread depletion and sub- sequent local enrichment. The mineralogy of the rocks is little affected by the geochemical changes. Like their counterparts outside of the bleached zone, the metasomatized rocksconsist essentiallyof quartz, chlorite,muscovite, paragonite and rutile. Small aggregates and porphyroblasts of white mica and chlorite are developed. The metasomatism,which was accompanied by tourmalineveining, is superimposed on acontact 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 occurredat 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, highlyevolved granitic body intruded along thenorthern margin of amajor granitic-granodioritic component of the Lake District batholith. The bleached zone is associated with amajor lineament, which may reflect basement control on thelocation and form of theburied 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 Crummock Water aureole (Fig. 1) is an elongate zone Kirkstile Slate Formation, is composed of dark grey siltstone of bleached and indurated siltstone and mudstone within the and mudstone with sporadic thin greywacke sandstone beds. Skiddaw Group; a thick sequence of generally dark grey South of Grasmoor the Loweswater Flags Formation mudstone, siltstone and sandstone of Tremadoc, Arenig and appears to die out, although sandstone of broadly similar earlyLlanvirn age (Jackson 1978; Molyneux & Rushton age occurs in the Robson area (Fig. 1).The lithological 1985). The sequence is generally considered torepresent similarity of theHope Beck Slate and Kirkstile Slate fore-arcsedimentation close tothe southern margin of formationsmeans that where the Loweswater Flags Iapetus (e.g. Mitchell 1984). The base of the group is not Formation is absentthey are separable only on palaeon- seen. Theupper part is contemporaneous with volcanic tological criteriaand hence are probablyuntenable as rocks of the Eycott Group and bothare overlain by the formations over a wider area. calc-alkaline Borrowdale Volcanic Group (Fig. 2). This Three important phases of deformation (Dl-3) affect the succession is intruded by theLake District batholith, Skiddaw Group (Simpson 1967; Soper & Moseley 1978). D1 components of which range from Ordovician to Lower is represented by slump folds, both minor syn-sedimentary Devonian (Rundle 1979; Firman & Lee 1986). The Lower slump folds and major, penecontemporaneous gravity slide Palaeozoic rocks weresubjected to Caledoniantectonic structures (Fl) are present (Webb & Cooper 1988). D2 was eventsand regional metamorphismreached anchizone a prolonged, probably discontinuous, period of uplift that (prehnite-pumpellyite facies) conditions (Oliver et al. 1984). may have commenced in thelate Llanvirn. It was Remapping has confirmed Jackson's (1978) stratigraphy responsible forthe initiation of the main Lake District for the Skiddaw Group in the area north of Grasmoor (Fig. anticline (Downie & Soper 1972), the associated local 1) where three formations are recognizable. The oldest, the unconformity atthe base of the overlying Borrowdale Hope Beck Slate Formation (Fig. 2), comprises bluish-black Volcanic Groupand the major, pre-Ashgill unconformity and grey silty mudstone with sporadic0.2-10m thick above the volcanic group. Minor folds related to this phase sequences of greywacke sandstone. This is overlain by the have notbeen unequivocably identified in the Skiddaw Loweswater Flags Formationcomposed mainly of grey- Group, but have beendemonstrated in the overlying wacke sandstoneturbidites. The youngest strata,the Borrowdale Volcanic Group, wherethey arethe earliest 523 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/523/4889428/gsjgs.145.4.0523.pdf by guest on 26 September 2021 524 D. C. COOPER ET AL. -A '+ Egremont 'P Fig. 1. Location and extent of the Crummock Water aureole. The area containing bleached rocks is stippled, other ground occupied by Skiddaw Group rocks is blank. structures. D3 is the main deformation phase and is earlv Recognition of the aureole U Devonianin age. It produced the Caledonian folds and Thepale, indurated rocks about Crummock Water were cleavage (F3and S3 in the Skiddaw Group) present oncemapped asa distinct formation within the Skiddaw throughoutthe Lake District. Theseare deformed by Group called the Blakefell Mudstone(Dixon 1925; slightly later, sideways-closing minorfolds (F4) with an Eastwood et al. 1931), though it was recognized thatthe associated crenulation cleavage (S4). pale coloured rocks were associated locally with a thermal metamorphic aureole and tourmalinization, which might be ~ related to buried a granitic intrusion. Rose (1955) AGE + SERIES reinterpreted the pale coloured rocks as solely a product of Ma contact metamorphism and this view has been followed by 438 most recent authors (Jackson 1961, 1962, 1978; Jeans 1974). WINDERMERE ASHGILL GROUP The limits of the pale rocks in the Grasmoor area were mapped by Jeans (1974), who recorded traces of andalusite, 448 biotite and possible pseudomorphs after cordierite within CARADOC them, and drew attention to the apparent absence of any BORROWDALE mineral zonation. He considered the contact metamorphism 458 VOLCANIC to be pre-F3in age, possibly associated with a concealed GROUP LLANDEILO graniticintrusion linked tothe Skiddaw andEskdale granites, thenboth dated at 395 Ma (Miller 1961). The 468 murchisoni tourmaline veining was considered to belater than the LLANVIRN Mudstone Farrnal~on contactmetamorphism and post-F3 (Jeans 1974). More EYCOTT -?-?-- bifrdus recent dating, a revised geological timescale anda 478 Slate hirundo reassessment of all data led Rundle (1979) to assign the Skiddaw granite to the Lower Devonian (c. 395 Ma), but to ARENIG propose an Ordovician age (429 f4Ma) for the Eskdale 488 Granite.The age of the contactmetamorphism and its relationship theto tourmaline veining thus became TREMADOC uncertain. The Crummock Water aureole differs from most contact t The age of series boundaries are those glwen by metamorphic aureoles adjacent to exposed granites in that Harland and others (1982) extensive bleaching is developed instead of well developed Fig. 2. Ordovician lithostratigraphy in the English Lake District zones of new mineral growth. The work reported here was (adapted from Moseley 1984). instigated to determine the precise nature and extent of the Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/523/4889428/gsjgs.145.4.0523.pdf by guest on 26 September 2021 CR UM M OC K WATER AUREOLE: METASOMATISMAUREOLE:CRUMMOCKWATER 525 Crummock Water aureole and clarify its relationship to the granitic batholith and regional structure. The study forms part of the British Geological Survey’s (BGS) Lake District regional geological survey project. Extent, stratigaphy and structure of the aureole The Crummock Water aureole, as defined by the area of bleached rocks, is farmore extensive thanhitherto described (Fig. 1). At outcrop it is up to 3 km wide and 24 km long, extending from Causey Pike in the east to near Egremont in the west (Fig. 1). Around Ennerdale Bridge, however, the ground is entirely drift-covered and it is possible that the bleached rock west of here is separate from the main zone. To the south-east of the main zone patches of bleached rocks have been found on the ridge between Maiden Moor [NY238 1821 and Little Town and between Fig. 3. Recumbent F1 fold with cross-cutting joints and lines of Robinson and Hindscarth (Fig. 1). alteration in bleached siltstone and mudstone, Grasmoor [NY 1659 20491. Thenorthern boundary of the bleached rocks is commonly gradational. For example, near ForceCrag a gradualnorthwards darkening into unaltered rock takes Numerousrecumbent F0 and F1 minor folds (Fig. 3) are place over a distance of about 150 m. West of this at Liza obvious in this areaand
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