Journal of the Geological Society, London, Vol. 145, 1988, pp. 523-540, 17 figs, 5 tables. Printed in Northern Ireland

The aureole: a zone of metasomatism and source of ore metals in the English

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- izedSkiddaw 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- 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 sedimentary rocks were a source of ore metals in the Lake District.

The Crummock Water aureole (Fig. 1) is an elongate zone Kirkstile Formation, is composed of dark grey siltstone of bleached and indurated siltstone and mudstone within the and mudstone with sporadic thin sandstone beds. Skiddaw Group; a thick sequence of generally dark grey South of the Flags Formation mudstone, siltstone and sandstone of Tremadoc, Arenig and appears to die out, althoughsandstone 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 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 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 aprolonged, 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

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-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, anddrew 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

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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 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 [NY238 1821 and Little Town and between Fig. 3. Recumbent F1 fold with cross-cutting joints and lines of and (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 farther south on Lad Hows Beck [NY 1642111 light grey spotted siltstone with [NY 171 1921 and [NY 164 1841, where disseminated pyrite inside the bleached zone loses its spots they are refolded by F3 minorfolds producing complex 40 m from the northern boundary, but does not darken until structures. Although the main S3 cleavage is almost absent a 4 m boundaryzone wherethe colour change is rapid. as a penetrative fabric within the bleached rocks, it occurs as Around Murton the boundary is indistinct because of a close-spaced (5-50 mm apart), E-W-trending joint-set hematite staining andthe prevalence of sandstone. The which is superimposed across the D1 minor folds (Fig. 3) southern boundary is similarly gradational from Egremont and is axial planar to F3 folds. Bothmajor and minor toEnnerdale Bridge, but from nearthe north end of tourmaline veins and associated wall-rock alteration occur EnnerdaleWater eastwards to Causey Pike most of the predominantlyalong this S3 joint system (Fig. 4). southern margin is formed by a northerly dipping reverse Widely-spaced joints with little alteration and with very few fault. Along part of itslength the fault is marked by a tourmaline veins occur orientated approximately N-S (Fig. massive quartz vein with minor veins in the hanging-wall. 4). Bedding-parallel kink bands affecting the S3 joints (Fig. Between Rannerdale Knotts [NY1630 18061 and a 5) and open, sideways-closing minor folds of tourmaline gradational boundary is preserved just south of the fault. Within the bleached zone, siltstone and mudstone are the most prominent lithologies. The only fossils recorded are twograptolites fromnear the southern margin on [NY 180 1901. One (registered no. RX 1650) is a broadform of Didymograptus defrexus (Elles and Wood) fromnear the summit [NY 179718941, and this indicates a correlation with the low Arenig L). defrexus Zone, asdeveloped, for example, at Scawgill Quarry [NY 1772581. Theother graptolite, a specimen of Etagraptus tenuissimus (Hams andThomas) from a scree south-west of the summit [NY 1766 18721, suggests a similar horizon. These graptolites indicate a stratigraphic: level near the junction between the Hope Beck Slate and Loweswater Flags formations. Jeans (1974) mapped the rocks here as an arenaceous formation, although thereare only a few per U cent of sandstones within the recrystallized siltstone JOINTS n=298 sequence. Small areas of altered Loweswater Flags Formation occur only on the northern flank of Grasmoor and in the vicinity of Murton Fell. Fossils from beds just north of the bleached zone include representatives of the Zsograptus gibberulus Zone south of Carling Knott [NY1198 19861, and the D. hirundo Zoneat [around NY 212 2151. These suggest that the bulk of the bleached rocks belong to the Kirkstile Slate Formation (Fig.

2). TOURMALINEVEINS n=85 In the Grasmoor area the bleached rocks are folded into a major, composite, F2-3 anticline with an evenly dipping fig. 4. Orientation of joints (A) and tourmaline veins(B) in the southern limb on which few F3 minor folds are developed. Grasmoor-Low Bank area.

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veins are correlated with F4 folds outside the bleached zone minor albite, pyrite,traces of resistate minerals including although S4 is not present. Tourmaline veining is therefore tourmaline and zircon, and carbonaceouspigmentation. considered to post-date M and pre-date F4. Mica-foliation is bedding-parallel or oblique to bedding The area of bleached rocks (Fig. 1) shows no obvious owing to rotation into the penetrative S3 cleavage. Opaque relationship to any exposed intrusion. The southern margin matter tends tobe concentrated on S4. is close but tangential to the contact of the Skiddaw Group Bleached rocks also consist largely of quartz, muscovite, with the Ennerdale Granophyrecomponent of the batholith paragonite and chlorite, but the carbonaceous pigmentation (Fig. 1). A group of highly altered minor basaltic dykes and is lost and the pyrite content is very low; flakes of muscovite sills occurs close to thesouthern boundaryin the Scale and chlorite of detrital origin are preservedwhere Knott, Low Bank and Whiteless Pike [NY180 1901 areas. recrystallization is notadvanced. A bedding-parallel The dykes pre-date F3 and areprobably related to either the lepidoblastic texture of fine white mica is frequently present Eycott or Borrowdale volcanic episodes (Jeans 1974). except where the quartz content is appreciable or where Further dykes, ranging from acid to basic in composition, spotting is well developed. Many bleached pelitic rocks crop out close to the western limits of the bleached zone containspots up to 2 mm across which consist of near Egremont.Some of these are associated with the fine-grained chlorite orhave a core of sericitic material Ennerdale Granophyre (Eastwood et al. 1931), whilst others inside a chloritic margin. Locally, there is a more pervasive may be related to Ordovician volcanism and post-cleavage segregation into contrasting chloritic and micaceous areas igneous events. along sinuous, interlocking boundaries. Discrete porphyro- blastic muscovite and chlorite patches up to 0.2mm across Tourmaline and quartz veiniig may be present, and minute (<0.02mm) rutile crystals are widespread. Rare cross-cutting planes of oblique mica At least three generations of veining are presentin the orientation, which appear not to penetrate chloritic spots, bleached rocks. The earliest, described by Fortey & Cooper suggest mimetic overgrowth of S3 cleavage. (1986), exploited E-W (S3) joints and consists of sub-vertical veins of tourmalinized and silicified rock centred Table L Representative microprobe analyses on relatively thinquartz-tourmaline veins. These include of mica, chlorite and ‘biotite’ locally brecciated veins containingdisrupted fragments of tourmalinized rock set in a quartz-tourmaline matrix. The 1 4*2* 3 secondgeneration contain chlorite, muscovite, quartz, tourmaline, sodic-plagioclase, apatiteand rare allanite. SiO, 46.26 43.35 47.51 47.52 These veinlets pervaded the bleached zone and cut the early A1203 36.23 34.03 38.65 33.09 tourmaline veins. Chlorite-rich veinlets of this generation TiO, bd 0.43 bd 0.71 display cross-fibre structure and radiating sheaves of chlorite FeO‘ 1.42 6.12 1.13 2.63 flakes which grew along the vein walls beforequartz MnO bd bd bd bd occupied the remaining space. Preservation of such textures MgO 1.84 2.61 bd 1.54 indicateschlorite growth during dilation of the host CaO bd bd bd bd fractures, and an absence of any significant compression or Na,O bd bd 2.68 bd shearing since. The third generation comprises vuggy quartz K20 8.69 7.75 5.58 9.47 veins with a cross-fibre structure andnetworks of locally Total 94.44 94.29 95.55 94.96 pyritic quartz veinlets. 5 6 7 8 SiO, 24.50 24.68 34.99 36.07 Petrography and mineralogy A1203 24.57 24.11 23.65 28.37 TiO, bd bd 1.85 0.44 Skiddaw Group rocks north and south of the bleached zone FeO‘ 31.68 32.02 19.89 17.96 consist largely of quartz, muscovite, paragonite,chlorite, MnO 0.44 1.61 0.36 0.44 MgO 7.94 7.22 4.88 4.12 Na,O bd bd 0.35 0.89 K20 bd bd 2.31 3.45 Total 89.13 89.64 88.28 91.76

bd,below detection; FeO‘ total iron as FeO. * Analysesinclude contributions from adjacent grains. 1,2 & 3 mica from bleached specimens (1 is a detrital flake); 4 mica from outside the bleached zone; 5 chlorite in bleached rock; 6 chlorite from outside the bleached zone; 7 & 8 ‘biotite’ in bleached rock. Energy-dispersive analyses by BGS Cam- bridge Instruments Microscan V microprobe with Canberra detector: 15 kV beam poten- tial, 0.2 nA specimen current, 100 S counting Fig. 5. Kink band forming sigmoidally folded cleavage cutting time,raw data corrected by aniterative bedding (faint low-angle fabric), Grasmoor [NY1649 20141. programme after Ware (1980).

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IIIIIIIII -A 2.0 - rnuscovlte 2.0 -

- (Na + K) - Pyrophyllite - muscovite - variationline - -

W -

0

1 .o IIIInIIII1111 Illllllllll 1 .Q 14.0 13.0 13.0 14.0 14.0 Total catdons catlons Total Fig. 6. Plot of total alkalis against total cations measuredin analyses of white mica in pelitic rocks from(A) outside and (B)inside the bleached zone,on a basis of 22 oxygen equivalents. Diamonds indicate detrital flakes; open circles fine matrix mica.The inset diagram shows the relationshipto theoretical mineral compositions.

Evidence of an earlier contact metamorphism is provided Embleton [NY 204430761, and 56 rocks from Hope Beck by rare angular chlorite patches which suggest replacement betweenNY 16842385 and NY 18422234. The bleached of andalusiteporphyroblasts. Traces of andalusite in pale rock sample comprises 24 rocks collected along the spotsand chlorite pseudomorphs possibly after cordierite, Hindscarth to PeelWyke traverse, which crosses the were also reported by Jeans (1974). bleached zone between Third Gill and Force Crag (Fig. l), Microprobe analyses of fine-grained white mica and and 60 rocks from the Rannerdale-Grasmoor area. chlorite(Table l), were affected by interference from Analysed specimens consisted of c. 2 kg of fresh rock adjacent grains. The effect of this interference is evident in chips with no visible vein material. B,Be and Li were Fig. 6, where it can be seen that most white mica analyses determined on all samples by inductively coupled argon contain excess cations (Feand Mg). Nevertheless, the plasma spectrometry (analysts: B. Tait & L. Auk, BGS) and analyses suggest that mica flakes of detrital origin are closer the remaining elements reported by X-ray fluorescence in composition to theoretical alkali-saturated mica than the spectrometry(XRFS) using fused glass beadsfor major coexisting metamorphic mica, andthat this distinction is elements and pressed powder pellets for trace constituents lessened in the bleached zone by the growth of a less (analysts: P. K. Harvey & B.P. Atkin, University of alkali-deficient mica (Fig. 6). The leastcontaminated Nottingham). chlorite analyses plot as ripidolite on Hey's (1954) chemical Traverseplots (Fig. 7), scatterplots (Fig. 8) and classification (ifall iron is treated asferrous) and do not non-parametric statistical methods(Table 2) were used to indicate any major difference between chlorites from within compare the composition of bleached and unbleached and outside the bleached zone. Analyses of a fine-grained, Skiddaw Group rocks. Many elements (Al, Be, Ba, CO, Cr, pale brown, weakly pleochroic mineral in bleached pelitic Ga, La, Mg, Na, Nb, P, Sr, Ti, U, V, Y and Zr) show no rocks from Grasmoor (Table 1, Nos 7 & 8) reveal a high overall differences in concentrationrelated to bleaching, titanium content which, with its optical appearance, although the results for Na and Uin particular show so indicates thatthe mineral is eitheran intermixture of much scatter (coefficient of variation >M) that any small chloriteand biotite, or biotite from which potassium has changes caused by bleaching would not be evident. Be and been lost. Zr concentrations are closely related to mudstone- sandstone variation and significant statistical differences in theirconcentrations in the normaland bleached groups Chemical changes in the bleached rocks (Table 2) are caused by the paucity of sandstones in the To determine whether the recrystallization and bleaching bleached zone. was essentially isochemical, chemical analyses of 84 rocks The concentration of other elementsdetermined is from the bleached zone were compared with those of257 distinctly modified in the bleached zone. As, B, K and Rb unbleached rocks collected from all three formations in the contents are much higher, whilst L01 (loss on ignition), Ni, Skiddaw Group.The sample of unbleached lithologies S and Zn values are lower (Table 2, Fig. 7). There are also comprises 201 rocks collected at c. 70 m intervals along a irregular decreases in Fe and Mn and increases in Ca which traverse across the Skiddaw Group outcrop from Hindscarth are statistically highly significant, though less obvious on Edge,Honister [NY 2188 15471 to PeelWyke, west of traverse plots (Table 2, Fig. 7). TheSi0,vs A1,03 plot

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Fig. 7. Traverse plots of rock analyses across the Crummock Water bleached zone along the line shown in Fig. 1, from Knott Rig (KR), through Third Gill (TG), Wandope (W), (CH) and Force Crag (FC) to Grizedale Pike (GP). Median levels of element concentrations in the bleached zone are denoted by a long-pecked line and in unbleached rocks by a short-pecked line. The limits of the bleached zone are denoted by vertical pecked lines.

shows a distinct shift inthe fields occupied by the two sample K and Rb and decreases in CO,Li, Ni, Pb, S, Zn and L01 populations (Fig. S), which is interpreted as indicating a net are evident, coincident with bleaching (Fig. 9). gain of silica by the bleached rocks. Several other elements Total organic carbon was determined on six bleached (CO,Cu, Li, Mg, Laand Pb), some of which show no and six unbleached rocks by evolved gas analysis (analyst: significant net differences in concentration between the two A. J. Bloodworth,BGS). The results indicate that carbon groups, display much greater variationin the bleached concentrations in bleached rocks (range: less than 0.039%) rocks, suggesting at least local redistribution within the are much lower than in the normal Skiddaw Group rocks bleached zone. (range: 0.039-0.59%), supporting the conclusion that Though most chalcophile elements (CO, Cu, Fe, Mn, Ni, bleaching results from loss of carbon. Pb, S, Zn) display patterns which indicategenerala Bi, Sb, Sn and W were determined by XRFS on 47 of lowering of the background within the bleached zone they the rocks collected across the southern margin of the also show localized increases to concentrations higher than bleached zone. Concentrations of these elements were low any found in the unbleached rocks (e.g. Zn, Fig. 7). This (W < 10 ppm, Sb < 9 ppm, Sn < 6 ppm) and in the case of Bi reflects the paucity of sulphides in most bleached rocks and all below the detection limit (3 ppm). Non-parametric the presence locally of minorsulphide mineralization statistical testsindicated no difference between the associated with late quartz veining. It suggests that, with the distribution or medians of the bleached (n = 23) and apparent exception of As, chalcophile elements were unbleached (n = 24) samplepopulations atthe 99% depletedduring the main bleaching event andenriched confidence level,though high coefficinets of variation, locally during subsequent vein mineralization. caused at least in part by closeness to the detection limits, Chemicalchanges associated with bleaching are clearly may hide small differences between the two groups. demonstratedonthe traverse crossing the southern Cl and F were determined colorimetrically (analyst: R. boundary of the bleached zone in Third Gill [NY 183 1901. Fuge, University CollegeAberystwyth) on 10 consecutive Herethe junction is notfaulted andthe rocks are of a traversesamples crossing the bleached zoneboundary in relatively uniform lithology. Strong net increases in As, B, ThirdGill. The five bleached samples contain appreciably

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210 TT: enriched in the bleached rocks, but strongly depleted in the tourmaline veins. 180 .. Element associations differ in the bleached and 150 unbleached rocks. Thelatter show a close correlation between a wide range of elements (Al, B, Ba, Be, Ce, Cr, pprn l20 Ga, K, La, Li, Nb, Ni, Rb, Sr, Th, Ti andV), interpreted as B an original association in detrital aluminosilicates and heavy 90 minerals preservedthrough diagenetic and burial meta- 60 morphism. Other small element groupings reflect the presence of minorsulphide-, phosphate- and carbonate- 30 bearing rocks. In bleached rocks the majorelement

0 association is split into two distinct groups (Ca, CO, Cr, Li, ,0-\ LoI, Mg, Ni and V; Al, Ba, Be, Ga, K, Rb, Th, and Ti) 900 / \ which are believed to reflect the recrystallization and segregation of chlorite and muscovite. A third grouping of 750 Cu, Fe, Mn, S and Zn is related to variation produced by 600 local sulphide mineral concentrations.

450 Rb-Sr dating 300 Seven samples of bleached rocks collected between Rannerdale Farm and Grasmoor, together with eight similar 150 samples collected at intervals along Croasdale Beck have 150 been analysed for Rb/Sr and87Sr/86Sr ratios (Table 3). As is indicated by the high value for the MSWD of 21.2, the data 120 do not all lie onthe best-fit line within the limits of yrn 90 experimental error when plotted on a conventional isochron diagram (Fig. 11). Nevertheless, the data points have a wide 60 and even distribution and a reasonable linear correlation, so the calculated age of 395 f 12 Ma is considered to be a good 30 estimate of the average isotopic agerecorded in these samples. This is borneout by the progressive removal of outliers (Table 4) which systematically reduces the MSWD, \ without significantly changing the slope of the line, until the \ \ remaining nine points define an isochron (MSWD = 2.9) 70 giving an age of 401 f 3 Ma (Fig. ll), whichis not %SiO, significantly different, but more precise, than that given by all the samples and must be recording the last isotopic 60 homogenization in these rocks. As detrital mica is preserved locally, the scatter on the regression line incorporating all the samples is attributedto variableincomplete re- 5c crystallization and isotopic resetting within the bleached rocks.

%AI203 No Rb-Sr data are available for Skiddaw Group rocks from outside the bleached zone, but K-Ar and Rb-Sr results Fig. 8. Plots of the A1,0, vs SO,, V, Ba and B content of bleached forother Lake District rocks show that many have rocks superimposed onthe fields occupied by 257 unbleached preserved ages considerably older than400Ma (Rundle Skiddaw Group rocks (denoted by pecked lines). 1979), indicating that the 400Ma date cannot be attributed to a regional metamorphic event. The age of 401 f 3 Ma correlates well with K-Ar ages of 399 f 9 Ma and 397 f 7 Ma higher F (X = 741 ppm, U = 81 ppm) than the unbleached for the Skiddaw (Shepherd et al. 1976) and Shap (Wadge et al. 1978) granites, respectively, and suggests thatthe rocks (X = 574 ppm, U = 81 pprn). In contrast Cl is strongly Crummock Wateraureole is associated with this early depleted in the bleached (X = 100 ppm, U = 40 ppm) com- Devonian magmatism, rather than theEnnerdale Gra- pared with the unbleached rocks (X = 289 ppm, (J = 131 ppm; Fig. 9). nophyre (420 f4Ma) or the Eskdale Granite (429 f4Ma; Comparisons of the composition of the bleached rocks Rundle 1979) events. and replacement tourmaline veins (Fortey & Cooper 1986) with respect to normalSkiddaw Group rocks are summarized in Fig. 10. For mostelements thepattern of Gravity modelling and image processing enrichment and depletion in the veins is similar to but more Bott (1974), in a study of his gravity survey data from the extreme thanthat in the bleached rocks. The notable Lake District, notedthat achange of gradient onthe differences are (i) that Na, Sr, REE and Y,which show no Bouguer anomaly map about 3 km north-west of the overall net change in the bleached rocks, are strongly EnnerdaleGranophyre coincided with an occurrence of depleted intourmaline veins and (ii) As, K andRb are spotted . His two-dimensional interpretation showed

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Table 2. Summary of analytical results for normal and bleached Skiddaw Group rocks

NormalSkiddaw Group rocks, n = 257 BleachedSkiddaw Group rocks, n = 84Kolmogrov- Mann- Smirnov Whitney Standard StandardD Test (Z) M edian Mean DeviationMedianMean StatisticDeviationMedianMin.Mean Max.Min. Max. Statistic

SiO, 57.08 59.39 6.49 81.66 45.56 58.07 58.07 3.84 69.10 43.61 0.147 0.50 TiO, 1.12 1.02 0.22 1.44 0.30 1.12 1.11 0.08 1.24 0.75 0.279 1.82 403 20.81 19.09 4.46 25.34 6.10 21.12 20.79 2.14 24.85 13.89 0.182 1.86 Fe,O,* 9.39 9.25 1.51 15.23 3.66 8.14 8.21 1.44 11.63 4.95 0.355 5.53 MnO 0.23 0.29 0.23 2.26 0.02 0.13 0.18 0.13 0.84 0.05 0.410 5.90 MgO 1.81 1.80 0.25 2.75 1.12 1.76 1.79 0.56 6.44 1.13 0.175 1.99 CaO 0.20 0.42 1.43 17.16 0.02 0.31 0.49 1.12 10.41 0.06 0.337 5.31 Na,O 0.85 0.96 0.38 2.17 <0.01 0.88 0.97 0.48 3.70 0.19 0.104 1.21 KZ0 2.99 2.79 0.89 5.39 0.41 3.48 3.47 0.60 5.28 0.70 0.434 7.15 P205 0.17 0.20 0.14 1.72 0.07 0.17 0.18 0.05 0.38 0.09 0.177 0.54 L01 4.36 4.29 1.17 14.77 2.09 3.93 4.04 1.01 12.05 2.84 0.375 4.93 Cr 116 106 28.2 157 28 118 120 25.9 337 76 0.235 1S5 V 136 124 29.9 169 38 137.5 137 17.5 262 93 0.234 1.27 Ni 53 51.3 12.0 75 10 42 43.8 13.1 86 17 0.420 5.43 CO 20 20.5 7.31 61 4 17 20.0 11.9 71 4 0.231 1.78 cu 28 28.7 12.3 123 6 22 24.4 17.6 93 2 0.315 3.67 Zn 94 95.4 28.1 331 25 37.5 55.4 46.2 245 9 0.632 9.29 S 25 490' 1574* 20000 bd bd - - 1400 bd 0.547 2.96 As 16 18.5; 13.0' 117 bd 27.5 30.2 19.6 101 bd 0.371 5.83 MO bd bd __ 7 bd bd bd - 4 bd - - B 66 64.1 34.1 141 10 104 124 88.9 583 10 0.553 9.09 Li 108 106 29.2 304 8 91.5 95.6 35.9 20 1 41 0.307 2.91 Pb 17 20.5 19.1 192 4 14 27.6 45.8 364 3 0.322 1.96 Rb 141 130 40.6 227 22 167 167 32.0 296 41 0.494 7.92 Th 12 11.9* 4.27' 22 bd 14 14.2* 4.05* 25 bd 0.217 4.24 U 3 2.5* 1.61' 8 bd 3 2.88' 1.S4 6 bd 0.219 1.87 Ba 657 607 175 943 75 66 1 653 86.3 817 179 0.187 0.97 Sr 110 106 36.6 206 7 103 102 33.1 230 37 0.139 1.50 Nb 21 19.5 4.3 27 5 21 21.3 2.35 25 8 0.214 2.41 La 42 40.8 13.1 76 4 42 45.6 20.4 130 8 0.187 1.43 Ce 88 86.0 26.8 172 16 99.5 97.6 28.8 203 26 0.210 3.17 Y 35 35.0 7.60 72 15 36 36.7 6.78 57 23 0.113 2.00 Ga 26 23.9 6.13 35 6 26 26.0 3.47 33 15 0.164 1.44 Be 2.7 2.54 0.88 4.2 0.5 3.05 3.04 0.61 4.7 1.4 0.253 4.48 Zr 184 183 33.7 312 83 198 197 38.3 282 80 0.267 3.56

Fe,O,*: Total reported as Fe,O,. L01 is loss on ignition at 950°C. * Approximate figures calculated using 0.5 X detection limit for results less than the detection limit. Underlined values of Kolmogorov- Smirnov and Mann-Whitney test statistics denote a highly significant difference between the two samples (>99.9% confidence level).

an underlying'granite' ridge which was assumed tobe the Skiddaw Groupoutcrop. The anomaly coincides with the northernmostcomponent of theLake Districtbatholith. A bleached zone fromaround Ennerdale Bridge to its eastern morerecent and detailed BGS regional gravity survey (Lee limit at CauseyPike, but clearly extendseastwards for at 1986) also shows a marked change of gradientin thesame least another 5 km until it is interrupted by the anomalies area. A three-dimensional interpretation based on these dueto the Threlkeld Micrograniteand Skiddaw Granite. data(Lee 1984,1986) suggested the presence of a small There is no clearanomaly over the bleached rocks tothe high-level intrusion tothe north-west of theEnnerdale south-west of Ennerdale Bridge. Granophyre, but the exact form and extentof the body were Densitymeasurements on samples of Skiddaw Group not resolved. rocks from both inside and outside the bleached zone were For thisstudy the regional gravity data was supple- made to establish whether any density variation due to the mented by additionalobservations and reprocessed using alteration was responsible for the gravity anomaly. The parameters designed to resolve minoranomalies in the results for 24 samplesfrom within the bleached zone, 33 Crummockwater area.The Bougueranomalies were samples from outside and 19 samples from the geochemical re-calculated using a density of 2.78 Mg m-3 (the density of traverse across the southern contact are summarized in Fig. the Skiddaw Group,Table 5) interpolatedonto a closely 13. Grain densities are 0.01-0.02Mg m-3 lower, and spaced (0.5 km) grid and contoured at 0.5 mGal intervals. porosity values about 1% lower within the bleached rocks. The resulting Bouguer anomaly map (Fig. 12) shows a clear Variation is greater within the bleached zone, but overall change of gradientextending for some 20 km across the there is little change across the contact and the effect of the

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density variations within the batholith. In such cir- cumstances no model is unique, and the present interpretations are based on known geological constraints andthe simplest densitydistribution compatible with the available density measurements (Table 5). The models used (Figs 14 & 15) assume that the central part of the batholith consists of a granite (density 2.62Mg1r-~, based ondata from the Eskdale and Wasdaleoutcrops) with a body of granitic to granodioritic composition (assumed density 2.68Mg11-~) concealed on its northern side.These are flanked by intrusions of granodioritic composition (density 2.70Mg m-3, based dataon from the Eskdale Granodiorite). Three alternative interpretations are considered feasible for the Crummock Water anomaly itself. The first (Figs 14A & B; 15A & B) assumes an underlying intrusion of highly evolved granitic composition (assumed density 2.62Mg mP3). The relatively small size of the gravity anomaly precludes the presence of a large granitepluton and the anomaly is best explained in terms of a high-level intrusionemplaced along thenorthern margin of the granite-granodioriteblock. The intrusion is close tothe surfacebeneath section l and lies entirely beneaththe bleached rocks. Alongsection 2 the intrusionreaches to within a few hundred metres of the surface and lies mainly south of the bleachedzone. Although Figs 14B and 15B show a dyke-like form for thelower part of the intrusion the anomaly can equally well be interpreted without the thin feeder to thebase of the batholith. The anomaly can also be explained in terms of either a detached high level laccolith of granitic composition (Figs 14C & 15C) or a ridge on the northern margin of the concealed granite/granodiorite block (Figs 14D & 15D). All three interpretations assume the same basic structure for the rest of the batholith and the surrounding rocks. Although the fit tothe residual anomalies is acceptable along theentire length of both sections, the models were designed to investigate the Crummock Water anomaly and shouldnot be taken as definitive interpretations of the peripheral areas. Further variations on the interpretations shown in Figs 14 & 15 are possible, especially if a wider range of density values is allowed within the batholith. However, all the most plausible models have the following features in common: (i) any intrusion of granitic composition must have a relatively limited volume; (ii) granite or granodiorite reaches close to the surface beneath section 1 and lies at a depth of a few

...... S.. 1.. 1...... 1... hundred metres beneath section 2; (iii) the change of I 1 W KR Sample 266 292 gradient on the Bougueranomaly map which marksthe * No -Normal Bleached northern margin of the Crummock Water intrusion can be Fig. 9. Traverse plots across the southernmargin of the bleached traced for about 20 km and is possibly coincident with the zone from the western slopeof Knott Rig (KR) [NY 1917 18551 to ENE-trending, steeply dipping, northern margin of a major Wandope (W) along Third Gill (cf. Fig. 1). granite-granodioritic component of the batholith; and (iv) along section 2 the northern limit of the intrusion roughly underlies the southern margin of the bleached rocks. chemical changes and recrystallization can be discounted as Image processing techniques were used to examine the the primary cause of the gravity anomaly. regional setting of the gravity anomaly. Colour shaded-relief Interpretations of the gravity data along sections 1 and 2 images of the primary gravity field and the second vertical (Fig. 12) were used to establish the form of the intrusion derivative field are shown in Fig. 16. The primary field image beneath the bleached rocks and its relationship to the rest of (Fig. 16A) shows the known gravity lows (blue)over the the batholith. Preliminary modelling confirmed Bott’s main graniteoutcrops andthe concealedbatholith. The (1974,1978) observations that: (i) theEnnerdale Grano- shaded relief presentation emphasizes the change of phyre is best interpreted as a thin, low density body above gradient associated with the Crummock Water intrusion and the main batholith, and (ii) the strongly asymmetric shape of suggests thatthe elongate anomaly forms part of a more the gravity profiles is bestexplained in terms of lateral extensive gravity lineament(indicated by white arrows)

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O...*..O Bleached rocks A A Tourmaline veins

Fig. 10. Spidergram summarizing chemical changes in bleached rocks and tourmaline veins.

Table 3. Rb-Sr results for bleached rock samples extendingfrom St Bees Head in the west to the Vale of Eden in the east. The lineament shows up even more clearly Sample site: on the second derivative image (Fig. 16B), which emphasizes Gridreference ppm Rbppm Sr87Rb/86Sr 87Sr/s6Sr the gravity effects of high level intrusions. The Crummock (NW f5% f5% *OS% f0.003% Water gravity anomaly is responsible for the western part of the lineament, which to theeast follows a major topographic 1674 1900 139 60 6.7793 0.75418 low, passes through the Threlkeld Microgranite, and marks 1660 191499 183 5.4046 0.74656 thesouthern limit of magneticanomalies attributedto 1631 195073 191 7.7120 0.76019 Eycott Group lavas. 1662 1973 127 197 4.4696 0.74024 1673 1960 220 58 10.9966 0.77897 1686 194656 184 9.6712 0.77089 Formation of the aureole 1674 1926 125 164 3.8370 0.73793 0961 1772 101 218 6.2708 0.75049 The Crummock Water aureole, as defined by the area of 0961 1772 25 1 56 13.1138 0.78846 bleached rocks, representsan extensive zone of chemical 0963 1774 117 281 7.0064 0.75393 alteration superimposed on an ill-defined low-grade contact 0973 1786 101 227 6.5642 0.75336 metamorphism. Bleaching is attributed to loss of carbon and 0978 1794 134 154 3.3403 0.73516 may be caused by oxidation accompanying baking. 0981 1788 119 143 3.4883 0.73628 Conductive heating alone, however, is generally insufficient 0993 1790 129 142 3.1933 0.73390 to mobilize and expelmetals such as zinc' and ironfrom 1025 1796 147 160 3.1637 0.73326 shales (e.g. Craig 1979), and it is most probable that hydrothermal fluids channelled along cleavage-parallel joints Rb/Srratios and concentrations were determined by XRF and fractureswere responsible for the pervasive chemical spectrometry,and 87Sr/86Sr ratios by automated multi-collector alteration and associated veining. solid-source mass spectrometry. International standards analysed at The addition tothe bleached rocks of elements the same time gave Rb/Sr (atomic) = + for the NBS 8.22 0.04 (la) (B, K, Rb) typically enriched in the fluid phases of granitic 70a and 87Sr/86Sr= 0.710224 + O.ooOo18 for the NBS 987. Isochron regressioncalculations were made using aleast-squares fitting magmas (e.g. Charoy 1982) and in the upper levels of zoned programme with errors in both axes. Where the MSWD parameter silicic magma chambers (e.g. Hildreth 1981), together with a exceeds a value of 3 it is assumed that the scatter about the line is negative gravity anomaly, strongly suggest an evolved greaterthan can be attributed to analytical error alone and the granitic source. The geophysical constraints can be satisfied errors on the age and intercept (quoted as 2a) have been enhanced by anumber of intrusiveforms but the most plausible is by multiplying by the square root of the MSWD. All ages quoted in considered to bea sheet or dyke-like body with a highly this work have been calculated using the constants recommendedby evolved granitic composition (Figs 14B & 15B). Steiger & Jager (1977). The early Devonian (Harland et al. 1982) Rb-Sr age of

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I I I I I I ”Sri Table 5. Density values used in the gravity interpretation “Sr AGE 395+ 12Ma (2- sigma) ,,4” Intercept 0.71 600f0.00084 Assumed in MSW D 21.2 0.780 MSWD ,W‘ situ density Rock group Rock (Mg m-3) Source

Skiddaw Group and 2.78 This work, see Fig. 0.760 13 basement Granite and 2.62 Eskdale Granite and Ennerdale 0.740 granophyre Granophyre (Bott 1974), supported by preliminary results for 59 BGS samples from the Eskdale Granite, Ennerdale 0.720 Granophyre and Wasdale Granite Granodiorite 2.70 Eskdale Granodiorite. Bott(1974) 3 I I I I I quoted values from two localities 2 4 6 8 10 12 of 2.69 and 2.73 Mg m-3, 8’Rb/86Sr respectively. Preliminary results from 23 BGS samplessuggest 2.70 Mg m-3, is an appropriate 87 I l l l l representative value S r/ 86Sr AGE 401f3Ma (2- sigma) Borrowdale 2.75 Bott (1974); Lee (1984) Intercept0.71589f0.00024 Volcanic Group MSW D 2.9 0.780 MSWD ,,+’ Lower 2.61 Bott (1974) quoted a value of Carboniferous 2.60 Mg m-3 for a typical Yoredale sequence. A greater 0.760 proportion of limestone would increase the average density. ,+,’ Overlying Namurian and 0.740 Westphalian rocks would reduce the overall density. A value of 2.61 Mg m-3 has been assumed 0.720 for the very limited areas of Carboniferous at the northern ends of sections 1 and 2 h U I I I 1 I Silurian 2.72 Representative value based on Bott 2 4 6 B 10 (1974) and Lee (1984) 87Rb/86Sr In situ density values are estimated fromavailable saturated and Fig. 11. Rb-Sr isochron diagrams for the bleached rocks based on grain density determinations. They represent the densityof the bulk of (a) 15 and (b) 9 samples. the rock group below the surface weathering layer.

401 f 3 Ma for the bleached rocks agrees with the petrographic and field evidence of a post F3 and pre F4 age forboth bleaching and tourmaline veining, and indicates that the sourceintrusion is associated with the Shap- The gravity anomaly assists in the definition of amajor Skiddaw magmatic event (Rundle 1979). lineament, and gravity modelling suggests that the northern The location andshape of the intrusion and bleached margin of the underlying intrusion is steeply dipping and can zone may be controlled by a major fracture in the basement. be interpreted as a fault (Figs 14 & 15). Also, the regional strike of the S3 cleavage becomes parallel to the long axis of the bleached zone in the Crummock Water area, suggesting Table 4. Calculated ages and errors for 9 to that it is associated with a zone of shear strain. A similar 15 bleached rock samples situation may exist to that found in the Irish Caledonides where a major c. 400Ma elongate intrusion associated with No. of Intercept a shear zone is located close to ovoid intrusions of similar pointsMSWD Agef2a f2a age (Hutton 1982). The mineralogy of the bleached rocks suggest that at the 15 21 15 395 f 12 0.7160f8 present level of erosion the stability field of biotite was 14 18 14 397f12 0.7159f8 approachedduring the metasomatic event, but that 13 14 13 397f 10 0.7160f7 andalusiteand cordierite were notstable. The lack of 12 10 12 39459 0.7163f6 mineralzonation within the bleached zone suggests only 11 7.1 39558 0.7163f5 10 4.7 399f7 0.7161 f5 limited temperature variation within it during the metaso- 9 2.9 9 401 f3 0.7159f2 matism, even though the transition to normal rocks is locally rapid. Such a pattern is consistent with the convective

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Fig. 12. Bouguer gravity anomaly map of the north-western Lake District based on the regional data supplemented by addi- tional observations (dots indicate station positions). Density for Bouguer corrections = 2.78 Mg m-3. Computer drawn contours generated by interpreting onto a 0.5 km grid. Heavy pecked line outlines the area of bleached rocks. Sections 1 and 2 extend beyond the map boundaries. Coded geological features are Skiddaw Granite (SK), Threlkeld Micro- granite (TH) and Ennerdale Granophyre (JW.

circulation of hydrothermal fluids ina buoyancy-driven pre-existing S3 fracture cleavage, faulting, the elongate form geothermal cell (Henley 1973; Norton & Knight 1977; of the intrusion, and a capping of more competent and less Moore 1982) and a model of this type, similar to that shown hydrous volcanic rock. Although no information is available in Fig. 17, is suggested to account for the formation of the for the bleached zone, stable isotope and fluid inclusion data bleached zone. Fluidcirculation andthe shape of the from LowerPalaeozoic rocks and quartz veins across the hydrothermal cell would have been constrained by the Lake District (Thomas et al. 1985) provide tacit support to this model in that they suggest the involvement of granite derived magmatic water in alteration of the rock pile. Normal Bleached Cations enriched in the bleached rocks (As, B, Ca, K, l Rb) are dominated by large, highly charged ions and ions capable of occupying alkali sites, whilst metals forming bivalent chloride complexes (CO, Cu, Fe, Mn, Ni, Zn) have ’I been removed. The contrast in behaviour of these groups of :L-0 elements suggests that fluorine-bearing acid fluids emanating from the granitereacted with Skiddaw Group rocks, 2.85 rT ! depositing large, highly charged cations in minerals such as muscovite and removing bivalent cationsaschloride complexes (c.f. Eugster 1985). Thepattern of vein mineralization suggests that on cooling thesecations were eventually preferentiallydeposited in tensionalzones and voids created by tectonicmovements, close to major lithological interfaces such as the granite/Skiddaw Group and Skiddaw Group/Borrowdale Volcanic Group boundaries.

X Gralndenslty Metallogenesis Data compiled by Stanley & Vaughan (1982) indicate that the first major episode of vein mineralization in the Lake District is of LowerDevonian age,and slightly younger Thlrd G111 I (370-390Ma) thanthe bleaching event. Elementsgained, Normal Sklddaw Group Bleached siltstones lost andredistributed within the bleached rocks include slltstones and mudstones and mudstones (33 samples) (24 samples) many of those concentratedinto mineral veins and the evidenceindicates thatthe two processes are related. Fig. W. Density measurements on Skiddaw Group rocks collected Elementsremoved from the bleached rocks in the largest on a traverse across the bleached zone boundary in Third Gill. quantities are Cl, Fe, Mn, S and Zn (Fig. 10, Table 2). The Sample spacings shown on the figure are not scale-true, spacings amounts of metal released may have been substantial, for vary between 50 and 100 m. example, if the bleached zone was originally 2 km wide and

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Fig. 16. Colour shaded-relief images of the gravity data. (A) Primary field (Bouguer anomaly minus a planar regional field), (B)second vertical derivative field calculatedon a 2 km grid according to the method of Elkins (1951). Arrows indicate the positionof the lineament associated with the Crummock Water intrusion. Codedgeological features areVale of Eden (VE), Skiddaw Granite (SK), Ennerdale Granophyre (EP) and Eskdale Granite (EG). Shadingachieved by treating data variation as topography illuminated from the north-west(to emphasize north-easterly trending features). The images were produced using the 1% image processing facilityat BGS, Keyworth.

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++++ ++++++ ...... +++++++++++ +++++++ +++++++++++ ++++++++++++

++++++++++++++

Fault?

D~ ,- Granlte mBorrowdale Volcanic Group Sklddaw Group, stlppling denotes mGranodiorite mbl eaching: lmes symbolise dommant Fig. 17. Schematic section to illustrate ". fractures andtourmalmite veins alteration and mineralization related to the \ Hypotheticalisotherms -.- Present land surface Crummock Water intrusion.

2 km thick and the Zn loss throughout was equal to that Although the substantial Ba-Pb-Zn deposits at Force Crag recorded (56 ppm; Table 2), then about 10 million tonnes of on the northern margin of the bleached zone (Fig. 1) are Zn were removed. It is therefore probable that the bleached believed to beCarboniferous or youngerin age, early rocks were a significant source of ore metals, although they quartz-pyrite-arsenopyrite-chalcopyrite-sphalerite mine- provide no evidence for the origin of the large tonnages of ralization reported by Freeman (1983) is most probably Ba, Cu and Pb extracted from LakeDistrict veins. another expression of the Lower Devonian event. Some of the metals released during the bleaching event H,O/CO, ratios in fluid inclusions from quartz veins at may have been redeposited to form ore deposits in or about Force Crag, Mosedale and resemble those for the bleached zone.Major vein mineralization is located peripheralparts of the Carrock Fell tungsten deposit close to the northern margin at Force Crag and minor ore (Shepherd & Waters 1984). No evidence for tungsten deposits havebeen worked ,at Scar Crag, enrichment or depletion was found, but scheelite is reported Buttermere, Loweswater and Mosedale. At Scar Crag (Fig. from this area (Kingsbury & Hartley 1957) and the 1) a CO-Fe-As-Bi bearing quartz vein also containing identification of a buriedgranitic intrusion and associated apatite and tourmaline was worked (Ixer et al. 1979). This is hydrothermal activity, supports the speculation of Shepherd a high temperature deposit (350-400°C) assigned tothe & Waters (1984) that tungsten mineralization may be 370-390Ma age group (Stanley & Vaughan 1982) which present at depth. Ixer et al. (1979) relate to a concealed 'Grasmoor granite'. The Crummock Water bleached rocks are therefore We concur with that view and suggest that the Scar Crag interpreted as overlying an emanative centre of granite- mineralization is directly linked to thebleaching event and is related mineralization. It produced early, high temperature a relatively large-scale example of the local chalcophile tourmaline veining with possible sub-surface tungsten elementenrichment noted on the geochemical traverses. mineralization. Hydrothermal alteration and bleaching of

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Skiddaw Group rocks was followed by local, high ELKINS,T. A. 1951. The second derivative method of gravity interpretation. temperature quartz-sulphide vein mineralization. Even- Geophysics, 16,29-50. tually, following a lengthy period of fluid circulation, EUGSTER, H.P. 1985. Granites and hydrothermalore deposits: a geochemical framework. Mineralogical Magazine, 49, 7-23. possibly sustained by a high heat-flow from the underlying FIRMAN,R. J. & LEE M. K. 1986. The age and structure of the concealed batholith, Ba-Pb-Zn mineralization was depositedin Lake District batholith and its probable influenceon subsequent response to cooling and. fracturingrelated to post- sedimentation,tectonics and mineralisation. In: NESBI~,R.W. & Caledoniantectonic events (Fig. 17). The development of NICHOL,I. (eds) Geology inthe real world-thc Kingsley Dunham volume. Institution of Mining and Metallurgy, 117-27. several such centres of mineralization above theLake FORTEY,N. J. & COOPER,D. C. 1986.Tourmalinisation in the Skiddaw District batholith,overlapping in time and space, would Grouparound Crummock Water, English Lake District. Mineralogical explain the observed relationships of the vein mineralization Magazine, 50, 17-26. to the batholith. FREEMAN,F. J. B. 1983. The geology and geochemistry of Force Crag Mine, . MSc. thesis, Camborne School of Mines. HARLAND,W. B., COX,A. V., LLEWELLYN,P. G., PICKTON,C. A. G., SMITH, Conclusions A. G. & WALTERS,R.1982. GeologicA Timescale. Cambridge University Press. We conclude that the Crummock Water aureole represents HENLEY,R. W. 1973. Some fluid dynamics and ore genesis. Tramactiom of anarea in which contactmetamorphism of the Skiddaw the Institution of Mining and Metallurgy (Section B: Applied Earth Group was followed by hydrothermal alteration, producing Science), 82, B1-8. HEY,M. H. 1954. A new review of the chlorites. Mineralogical Magazine, M, an elongatezone of bleached and indurated rocks cut by 277-92. toumaline veins. Both metamorphism and hydrothermal HILDRETH,W.1981. Gradients in silicic magma chambers: implications for alteration are related toan underlying elongate granitic silicic magmatism. Journal of Geophysical research, 86, 10153-92, intrusion c. 400 Ma in age whose position andform are HUTTON,D. H. W.1982. A tectonic model for the emplacement of the main Donegal Granite, NWIreland. Journal of the Geological Society, probably fault controlled. Bleaching and veining post-date London, l39,615-31. Caledonian F3 structures but pre-date F4. Bleaching, caused IXER, R. A., STANLEY, C.J. & VAUGHAN,D. J. 1979. Cobalt-, nickel-, and by loss of carbon, was accompanied by formation of a iron-bearingsulpharsenides from the North of England. Mineralogical lepidoblastic texture and millimetre scale chloritic spotting. Magazine, 43, 389-95. JACKSON,D. E. 1961.Stratigraphy ofthe Skiddaw Group between The bleached zone is interpreted as representing a section Buttermereand Mungrisdale, Cumberland. Geological Magazine, 98, throughpart of a fossil hydrothermal convection system, 515-28. instigated by granitic intrusion. This system added As, B, -1962. Graptolite zones in the Skiddaw Group in Cumberland, England. Ca, F, K, Rb and Si to the bleached rocks and removed Journal of Palaeontology, 36, 300-13. large quantities of Cl, Fe, Mn, S, Zn and watertogether - 1978. The Skiddaw Group. In: MOSELEY,F. (ed.) The geology of the Lake District. YorkshireGeological Society Occasional Publication, 3, with lesser quantities of other bivalent metals. These 79-98. materials, together with others derived from the intrusions JEANS,P. J. F. 1974. The structure, metamorphirm and strarigraphy of the and scavenged from the host rocks by circulating fluids Skiddaw Slates east of CrummockWater, Cumberland. PhDthesis, driven by heat from the batholith, represent the source of University of Birmingham. KINGSBURY,A. W. G. & HARTLEY,J. 1957.Childrenite from theLake metalsfound in theLake District vein-style mineral District, Cumberland. Mineralogical Magazine, 31, 498. deposits. LEE, M. K.1984. The three dimensional form of the Lake District granite batholith. Investigations of the geothermal potential of the U.K., British Geological Survey. The authors are particularly grateful to P. K. Harvey, B. P. Atkin, - 1986. A new gravity survey of the Lake District and three-dimensional B. A. R. Tait, L. Ault and R. Fuge for analytical data. A. W. A. modelof the granite batholith. Journal ofthe Geological Society, Rushtonprovided details of fossil occurrences in and around the London, 143,425-35. aureole.Colleagues and friends, too numerous to mention MILLER,J. A. 1961. The potassium-argon ages of the Skiddaw and Eskdale individually,helped bydiscussing and commenting on the Granites. Geophysical Journal, 6, 391-93. manuscript. J. Dunkelyand C. Simpson assisted. This paper is MITCHELL,A. H. G. 1984.The British Caledonides: interpretations from Cenozoic analogues. Geological Magazine, U1, 35-46. published by permission of the Director, British Geological Survey MOLYNEUX,S. G. & RUSHTON,A. W. A. 1985. Discovery of Tremadoc rocks (NERC). in the Lake District. Proceedings of the Yorkshire Geological Society, 45, 123-7. MOORE,J. McM.1982. Mineral zonation near the granitic batholiths of References south-west and northern England and some geothermal analogues. In: Ban, M. H. P. 1974. The geological interpretationof a gravity survey of the EVANS,A. M.(ed.) Metallization Associated with Acid Magmatism. English Lake District and the Vale of Eden. Journal of the Geological Wiley, Chichester, 229-41. Society, London, 130, 309-31. MOSELEY,F. 1984. Lower Palaeozoic lithostratigraphical classification in the - 1978.Deep structure. In: MOSELEY,F. (ed.) The geology ofthe Lake English Lake District. Geological Journal, 19, 239-47. District. Yorkshire Geological Society Occasional Publication, 3, 25-40. NORTON,D. & KNIGHT,J. 1977.Transport phenomena in hydrothermal CHAROY,B. 1982. Tourmalinisation in Cornwall, England. In: EVANS,A. M. systems: cooling plutons.American Journal of Science, 277, 937-81. (ed.) Mineralisation Associated with Acid Magmatism. Wiley, Chichester, OLIVER,G. J. H., SMELLIE,J. L., THOMAS,L. J., CASEY,D. M., KEMP,A. E. 63-83. S., EVANS,L. J., BALDWIN,J. R. & HEPWORTH,B. C. 1984. Early CRAIG,J. R. 1979. Geochemical aspects of the origins of ore deposits. In: Palaeozoicmetamorphic history of theMidland Valley, Southern SIEGEL,F. R.(ed.) Review of research on modemproblem in Uplands-LongfordDown Massif and the LakeDistrict, British Isles. geochemisny. UNESCO, 225-71. Transactiom of the Royal Society of Edinburgh, Earth Sciences, 75, DIXON,E. E. L. 1925. In: Summary of progress of the Geological Survey of 259-73. Great Britain for 1924. HMSO, London, 70-1. ROSE, W. C. C. 1955. The sequence and structure of the Skiddaw Slates in DOWNIE,C. & SOPER, N.J. 1972. Age of the Eycott Volcanic Group and its the Keswick-Buttermere area. Proceedings of the Geologists’ conformablerelationship to the Skiddaw Slates in the English Lake Association, London, 65, 403-6. District. Geological Magazine, 109, 259-68. RUNDLE,C. C. 1979. Ordovician intrusions in the English Lake District. EASTWOOD,T., DIXON, E. E. L., HOLLINGWORTH,S. E. & SMITH,B. 1931. Journal of the Geological Society, London, l36, 29-38. The geology of the Whitehaven and Workington disfrict. Memoirs of the SHEPHERD,T. J. & WATERS,P. 1984. Fluid inclusion gas studies, Carrock Fell GeologicalSurvey of Great Britain, explanation of Sheet28, HMSO, tungstendeposits, England: implications for regional exploration. London. Mineralium Deposita, 19, 304-14.

Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/145/4/523/4889428/gsjgs.145.4.0523.pdf by guest on 26 September 2021 540 D. C. COOPER ET AL.

-, BECKINSALE,R., RUNDLE, C. C. & DURHAM,J. 1976. Genesis of the convention in the use of decay constants in general cosmochronology. Carrock Fell tungstendeposits: fluid inclusion and isotopic study. Earth and Planetary Science Letters, 36, 359-62. Transactions of the Institution of Mining and Metallurgy (Section B: THOMAS,L. J., HARMON,R. S. & OLIVER,G. J. H. 1985.Stable isotope Applied Earth Science), 85, B63-73. composition of alterationfluids in low-grade Lower Palaeozoic rocks, SIMPSON,A. 1%7. The stratigraphy and tectonics of the Skiddaw Slates and English Lake District. Mineralogical Magazine, 49, 425-34. the relationship of the overlying Borrowdale Volcanic Series in part of WADGE,A. J., GALE,N. H., BECKINSALE,D. R. & RUNDLE, C.C. 1978. A the Lake District. Geological Journal, 5, 391-418. Rb-Sr isochron age for the Shap Granite. Proceedings of the Yorhhire SOPER, N. J. & MOSELEY,F. 1978.Structure. In: MOSELEY,F. (ed.) The Geological Society, 42, 297-305. geology of the Lake District. YorkshireGeological Society Occasional WARE,N. G.1980. Computer programs and calibration with the PIBS Publication, 3, 45-67. technique for quantitative electron probe analysis using a lithium drifted STANLEY,C. J. & VAUGHAN,D. J. 1982.Cooper, lead, zinc and cobalt silicon detector. Computers and Geosciences, 7 (1981), 167-84. mineralization in the English Lake District: classification, conditions of WEBB, B. C.& COOPER,A. H. 1988. Slump folds and gravity slide structures formation and genesis. Journal of the Geological Sociery, London, 139, ina Lower Palaeozoic marginal basin sequence (the Skiddaw Group) 569-79. NW England. Journal of Structural Geology (in press). STEIGER,R. H. & JAGER,E. 1977.Subcommission on geochronology:

Received 19 June 1987; revised typescript accepted 18 January 1988.

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