Copper, lead, zinc and cobalt mineralization in the English ... - CiteSeerX

J. geol. Soc. London, Vol. 139, 1982, pp. 569-579, 3 figs, 1 table. Printed in Northern Ireland

Copper, lead, zincand cobalt mineralization in the EnglishLake District: classification, conditions of formation and genesis C. J. Stanley & D. J. Vaughan SUMMARY: The copper, lead, zinc, and minorcobalt mineralization of the English Lake District has been classified on the basis of mineralogy, spatial distribution and geological setting of the veins, and age of mineralization. The two major types of mineralization are: (1) 'chalcopyrite-pyrite-arsenopyrite', of LowerDevonian(Caledonian) age, and (2) 'galenasphalerite' of earlyCarboniferousage. When theformer is closely associated with granite intrusions, more complex mineral assemblages are found involving tungsten or molybdenumbearingphases.Also distinguished from these two major types are later assemblages which probably result fromtheir alteration. A later (Upper Carboniferous-Permian)barytemineralization is also present. Minorgraphite and stibnitedeposits, and the major haematite mineralization found in the area, are incorporated into the classification but are not discussed, since they have been dealt with by other authors. Using data from fluid inclusions andexperimental data on particularmineral assemblages occurring in the major types of mineralization, estimates have been made of the temperatures and compositions of the ore-forming fluids. The Lower Devonian veins formed at temperatures in the range of350-200°C with typical US, values of 10-l"-lO-'h atmand aO, of -10-z510-45 atm or even less. The solutions were probablybrines of low tomoderate salinity (-5-10 equiv.wt% NaC1). The LowerCarboniferous veins formedattemperatures in the region of 110-130°C from highly saline brines (-23 equiv. wt% NaCl). Theories regarding the origins of the mineralization are also discussed. The distribution of Lower Devonian veins shows a clear relationship to the underlying composite granite batholith; the LowerCarboniferousgalena-sphalerite veins show no such relationship,buttensional tectonic activity in the early Carboniferous may have provided a mechanism for the opening of fissures. The source of the mineralizing fluids forming the Lower Devonian veins may have beenthe Borrowdale Volcanic Group, with contributionsfrom fluids associated with the granites, which are also likely to have provided asource of heat. The high salinities of the fluids producing the Lower Carboniferous veins suggest that seawater may have been involved in leaching the metals from basement rocks.

Copper, lead, zinc, and cobalt mineralization occurs in theLake District as vein material infilling fissures which cut the Lower Palaeozoic strata, and in fractures and joints in the Lower Devonian Skiddaw and Shap granites(Fig. 1).Upper Palaeozoic andlater sediments, and the Ordovician granitic rocks of the region (Rundle 1979) are generally poorly mineralized except for the haematite deposits in the W of the area. The geological setting for the mineralization has been discussed by Tneson (1977), Firman (1978), and in other chapters in the book edited by Moseley (1978). The mining history of the area hasbeendocumented by Postlethwaite (1913), Eastwood (1921), Dewey & Eastwood (1925), and Shaw (1970). There isnow renewed commercial interest in Lake District mineralization aftermorethan a decade of inactivity. However,atthe time of writing, Carrock tungsten mine has closed after working for a few years. Processing of lead, zinc, and baryte ore has commenced at Force Crag Mine, near Keswick, and it is likely that active mining will resume shortly. The authors and their collaborators have been engaged in mineralogical investigations of the Lake District copper,lead, zinc, and cobaltoccurrences, the

detailed results of which are reported elsewhere (Stanley 1979; Stanley & Criddle 1979; Ixer et al. 1979; Stanley & Vaughan 1980, 1981, & in press). The objectives of this paperareto use thesedetailed mineralogical data, together with the data available on the geological setting and age relations, to propose a classification of the mineralization and discuss the conditions of formation and the origins of the different kinds or 'types' of mineralization in the Lake District. Baryte, graphite, tungsten minerals, molybdenite, and haematiteconcentrations also occur in the area and are included in the classification, although not discussed in detail.

Classification of the mineralization A classification of Lake District mineralization is proposed, based onthe following criteria: (1) the mineralogy of the veins; (2) the spatial distribution of the veins and their geological setting; (3) the age relationships of the veins, based on field relations and radiometric age determinations.This classification is

001~7649/82/0900-0569$02.0001982 The Geological Society

570

C.J . Stanley & D.J. Vaughan

FIG.1. Sketch map of the geology of the Lake District, showing the distribution of the major non-ferrous mines and mineralization localities.

presented in Table 1, andincludes all types of mineralization occurring in the area. The small deposit of graphite with associated sulphides which occurs at Borrowdale (see Fig. 1 for this and other localities mentioned in the text), is possibly the oldestmineralizationin thearea.It has been classified by different authors either as being pre-Bala in age (Strens 1962, 1965) on the basis of structural evidence and model Pb ages (Moorbath 1962), or Devonian (Ineson & Mitchell 1975) onthe basis of K-Ar data. Devonian Mineralization of probable Lower (Caledonian) age constitutes some of the most important and widely developed deposits. Many of the veins are characterized by the presence of chalcopyrite, pyrite, and arsenopyrite, and are referred to as ‘chalcopyrite-pyrite-arsenopyrite type’ mineralization. The

veins generally have an E-W orientation although, in the Coniston district, the trend is NW-SE, and in the Caldbeck Fells area it is N-S. Other minerals (in addition to the common gangue phases) in this type of deposit include native bismuth, bismuthinite, and pyrrhotine. Whenmineralization of this type is closely associated with granite intrusions, more complex mineral assemblages are found. Wolframite, scheelite, and molybdenite occur at Carrock mine, in the aureole of the Skiddaw Granite; andmolybdenite, without significant tungstates is associated with the Shap ‘granite’ (adamellite). These special cases have been describedindetailelsewhere(Russell 1925; Grantham 1928; Hitchen 1934; Firman 1957; Shepherd et aE. 1976; Beddoe-Stephens & Fortey 1981). The unusual occurrence of cobalt mineralization at Scar Crag (Ixer et al. 1979) will be discussed in the following section.

Cu, Pb, Zn &

CO mineralization, English Lake District

571

TABLE1: ClassiJcation of Lake District mineral deposits AGE

~~

I

CHARACTERISTIC MINERALS

REFERENCE

~

EXAMPLES

I

Jurassic c.190

-

1R0

Ma

Permo-Triassic ?

Rasta11 (1942) Davidson R Thompson (1948) Ineson R Mitchell ( 1 9 7 4 )

(alteration in the Caldbeck Fells PYROMORPHITE - COPPER SCWHIDES f mimetite f linarite f anglesite ite C psilomelane f smithsonite C other species

Roughtongill. Driggith, Potts GhyII. MALACHITE Drygill, and others f cerussmany area)

Shepherd (1973) HEMATITE f quartz + calcite f dolomite Rose R Dunham f haryte C fluorite

West Cumhria hematite deposits

(19771

-

U.Carh.(Stephanian) Strens (1962) to Permian Cough (1963) c.290 - 260 Ma Stanley (1979) (mostly along E-W normal faults)

BARYTE f quartz f carbonates f galena

Potts Ghyll, Sandheds, Ruthwaite, Force Crag ?, Brundholme ? , and others

Lower Carboniferous Stanley (1979) Stanley R c.360 - 330 Ma (mostlv alonn nor- Vaunhan (1981) mal faults with N-S NW-SE, R NE-SW directions)

QUARTZ f CHALCEDONY t. BARYTE f GALENA f SPHALERITE f chalcopyrite f tetrahedrite f native antimonv C antimonv SulDhosa~ts f calcite C dolomite f fluorite

Greenside, Hartsop Hall, Myers Head, Eagle Crag, Helvellyn, Tilberthwaite PbZn. Barrow. Force Crap. Goldscooe Ph. Yewthwaitr, Brandlehow, Old Brandlehow, Thornthwaite, Threlkeld, Brundholme, Driggith, Roughtongill, Carrock E-W,

Uncertain

QUARTZ

"

1

"l

.

.

2nd o t h e r s

Postlethwaite

-

Rohin Hood (Bassenthwaite)St Sunda) Crag

STIBNITE

(1913)

Davidson R Thompson (1948) Uncertain

Stanley R Criddle (1979)

Lower Devonian Kitchen (1934) c.390 - 370 Ma Shepherd et a l . (1976) (occupying normal faults or. less commonly, thrust - - and tear faults associated with theRussell (1925) Caledonian orogeny) Firman (1957)

-

(oxidation assemblages of earlier primary mineralization) BORNITE - COPPER SULPHIDES - MALACHITE chalcopyrite f tennantite f hematite f wittichenite f sphalerite

Pave York (Greenburn), Birk Fell (Greenburn). Seathwaite veins, Dale Head South and others

QUARTZ - MUSCOVITE - APATITE - WOLFRAMITE - Carrock (associated wlth the Skiddaw SCHEELITE - ARSENOPYRITE - PYRITE Granite) PYRRHOTIME - chalcopyrite - bismuthinlte -native bismuth - bismuth sulphotellurides - molyhdenite - sphalerite - carbonates

- -

- - -

- -

- -

_ -

- - - -

-

- - - -

-

QUARTZ - FLUORITE - PYRITE - MOLYBDENITE - Shap (associated with theShap Granite) BISMUTHINITE - native bismuth - pyrrhotine chalcopyrite - magnetite

_ _ _ _ _ _ _ _ - _ _ _ - - - - - _ - - - - - - Ixer et a l . (1979)

QUARTZ - APATITE - CHLORITE - ARSENOPYRITE Scar Crag CO-Fe SULPHARSENIDES - pyrite - native bismuth - bismuthinite - tourmaline

_ _ _ _ _ _ _ _ - _ _ _ _ - - - - - - - - - - - Stanley & Vaughan (1980, 1982)

Uncertain (pre-Bala ? ) (Devonian ? )

Strens (1962,

QUARTZ - CHLORITE - DOLQMITE - ARSENOPYRITE - PYRITE - CHALCOPYRITE f magnetite f pyrrhotine C native bismuth f hismuthinite f bismuth sulphoselenides and sulphotellurides t. sphalerite t galena k Co/Ni minerals GRAPHITE

-

pyrite

-

chalcopyrite

Ulpha, Coniston veins (South, Bonser, Drygill, Paddy End), Long Crag, Greenburr veins, T~lberthwaiteCu veins, Dale Head North, Goldscope Cu, Castle Nook, Copper Plate, Thrrlspot, Birkside Gill, Wanthwaite, Haweswater veins, Potts Ghyll Cu, Carrock End, and others Borrowdale

1965)

Ineson & Mitchell (1975)

The chalcopyrite-pyrite-arsenopyritetype mineralization commonly contains minor amounts of sphalerite and galena. This sphalerite invariably has a significant (>5 mole % FeS) iron content, and the galena contains only rare inclusions of bismuth sulphosalts, features which usually enable these two minerals to be distinguished fromthe sphalerite and galena in the later lead-zinc veins described below. Oxidation assemblages have been produced by the

alteration of the chalcopyrite-pyrite-arsenopyrite type veins, and are of uncertain age. Minor occurrences of & Thompson stibnite at StSundayCrag(Davidson 1948) and Robin Hood, Bassenthwaite (Postlethwaite 1913), are also of uncertain age and both have been placed between the early copper mineralization and the lead-zinc mineralization in Table 1. This is because of the absence of antimony minerals associated with the earlymineralization(antimony sulphosalts re-

572

C. J . Stanley & D.J. Vuughun

ported from the Carrock mine area by Kingsbury & Hartley 1956, are probablyfrom the Carrock EastWest lead-zinc vein rather than the tungstenveins), andthe presence of antimony minerals at an early stage in the parageneticsequence in many of the lead-zinc veins. The lead-zinc veins which occur in many parts of the Lake District constitute another major type, and are termed 'galena-sphaleritetype'mineralization. The veins occuralongnormalfaults with N-S, NW-SE, and NE-SW directions. In addition to galena (which contains inclusions such as native antimony, antimony sulphosalts, andtetrahedrite), iron-poor sphalerite, baryte, and chalcopyrite also occur. Ineson & Mitchell (1974) suggested an early Carboniferous age, c. 330360 Ma, on the basis of K-Ar dating of the associated hydrothermal clay minerals in wall-rocks adjacent to the ores. There is no mineralogical evidence or structural support for theNamurianIWestphalian K-Ar agesfor the mineralizationat Brundholme,Greensides,and Threlkeld. As noted by Halliday (1977a), Ineson (1980), and Rundle (1981), theK-Ar ages obtainedfromillite-chlorite samples from wall-rocks should be treated with caution and should generally be interpreted as minimum ages for ore deposition. A 'baryte type' of mineralization,laterthan the 'galena-sphaleritetype'has also beendistinguished, with quartz, carbonates, and galena as the only associated phases. The veins are mostly along E-W normal faults, and occur in the N and central Lake District at Potts Ghyll, Sandbeds,and elsewhere.Baryte also occurs in joints in CarboniferousLimestone in the Caldbeck area, in the N of the Lake District. Radiometricevidence (K-Ar) hasbeenemployed to suggest an Upper Carboniferous (Stephanian) to Permian age,i.e. c. 260-290Ma (Ineson & Mitchell 1974). The haematite mineralization of West Cumbria has been discussed in detail by Strens (1962), Gough (1963), Shepherd (1973), Rose & Dunham (1977), and Dunham et al. (1978) and will not be considered here. It has been included in Table 1 and the assemblages may includeminor baryte, dolomite, andfluorite, in addition to haematite, calcite, and quartz. The age is considered to bePermo-Triassic(Shepherd 1973) or post-Triassic (Strens 1962; Gough 1963; Dunham et al. 1978), and maywell representthe last episode of primary mineralization in the Lake District. Assemblages of secondary lead and copper minerals from veins in the Caldbeck Fells area, in the N of the Lake District, probably relate to a widespread phase of alteration of the primary ore mineral assemblages andthe host rocks which hasbeen dated by K-Ar studies of wall-rock clay minerals as Jurassic, c. 190180 Ma (Ineson & Mitchell 1974). An alternative interpretation of these assemblages could be as nearsurfaceprimarymineralization, involving interaction of mineralizing fluids and oxidizing groundwater.

Mineralization conditions: compositions andtemperatures of the ore-forming fluids Using data from the study of fluid inclusions and, in certain cases, experimental data relating to the thermal stability of particular mineral assemblages, estimates have been made of the temperatures of the mineralizing fluids. Fluid inclusion studies have also provided data on the salinities of these fluids, and the coexisting mineral assemblages have been employed to suggest limiting values forthe activities of certain components at the time of ore deposition, particularly those of sulphur and oxygen. Chalcopyrite-pyrite-arsenopyrite type mineralization Changes in the composition and temperatures of the mineralizing fluids duringdeposition of the Lower Devonianmineralization have beenestimated using data from co-existing mineral assemblages, together with Kretschmar & Scott's (1976) arsenopyrite geothermometer; these results are summarized in Fig. 2. Interpretation of the arsenopyrite and cobalt-iron sulpharsenide mineralization at Scar Crag (Ixer et al. 1979) suggests that the early sulpharsenide minerals in the assemblage formed at temperatures of 350-400"C, with a sulphur activity of about 10-l" atm (Fig. 2; A). Later minerals could have formed at lower temperatures (-300°C) and with as2 -10-11 to 10-12 atm (Fig. 2; B). Mineralization in the Bonser vein at Coniston, and in the veins at Dale Head North and Wanthwaite, is more typical of the Lower Devonian deposits than is the Scar Cragoccurrence. IntheDaleHead North vein (Stanley & Vaughan 1980), early pyrite with cobaltiferouszones is interpreted as having been deposited at temperatures of 300-350°C with as2 10-"' atm (Fig. 2; C). Arsenopyrite was depoUS>- 10-13 to sited at 295275°C with 10-4 atm (Fig. 2; D , E). Bismuthinite, monoclinic pyrrhotine, chalcopyrite,andsphalerite were formedat lower temperatures of240-250°C (as2 10-l4 atm)andlatemarcasite,pyrite,and galena at atm; Fig. 2; F). -235°C (as2> In the southern Wanthwaite vein (Stanley 1979), the coexistence of hexagonal pyrrhotine having NFeS0.965 (where N is the mole fraction of FeS) and arsenopyrite with a composition of 32.9 atom % As was used as the basis for fixing the early mineralization at the intersection of the two relevant sulphidation curves at 310°C and asz - 10-13.5 atm (Fig. 2; G). Cooling could have led to the deposition of later arsenopyrite with a lower arsenic content (31.9 atom % As)at -290°C and as:, - 10-l' atm (Fig. 2; H), and of chalcopyrite,

-

Cu, Pb, Zn & CO mineralization, English Lake District -6-7-

-9

573

0 Scar Crag

+* 0

Bonser vein, Coniston

-

-10-

I

Temperature

FIG.2 . US2-temperature diagram for some Lake District Lower Devonian veins (sulphidation.curves from Vaughan & Craig 1978; Barton & Skinner 1979). See text for lettering key.

sphalerite, galena, and pyrite at -230°C and aS2 atm (Fig. 2; J). The Bonser vein mineralization (Stanley & Vaughan, in press) is possibly the most complex of those studied; only in this vein did the assemblage permit reasonable estimates of oxygen activities to be made. Although it was not possible todetermine a 'fixed' point on the as2-temperature diagram (Fig. 2), a number of boundary conditions could be established. For example,the earlyore-mineral assemblage of magnetite(pseudomorphing haematite)and arsenopyrite (the latter with 34.1 atom % As) found in the Bonser vein could not haveformed below temperatures of -330°C in an aS2-buffered assemblage (Kretslater, marginal arsenopyrite chmar & Scott1976); (33.4 atom % As) could have formed above -290°C at aSz - l O - I 4 atm (Fig. 2; K, L). In the assemblage deposited during the main phase of chalcopyrite mineralization, native bismuth formed below its melt-

-

ing point of 271.5"C (a further lowering of this melting point by a few degrees can beconsidered if total pressure is taken into account; Klement etal. 1963); sphalerite (13.6-14.4 mole % FeS), which occurs as inclusions in monoclinic pyrrhotine, can only have formed in equilibrium with it at temperatures below 245-248°C (Scott & Kissin 1973); and late arsenopyrite (32.1-31.3 atom % As), which also occurs as inclusions in monoclinic pyrrhotine, may have formed under conditions shown in the area around M (Fig. 2) with T - 240°C and aSz - 1O-l6 atm if the assemblage was &buffered. The area around N (Fig. 2) at T 200°C and aS2 - 10-'' atm, incorporating the bismuthlbismuthinite sulphidation curve in the pyrite stability field, may represent the conditions of formation of the late assemblage of pyrite,bismuthinite, native bismuth, bismuth sulphoselenidesand sulphotellurides, galena, and cosalite in the Bonser vein. Stability fields for the Bonser vein assemblages at

-

574

C. J . Stanley & D.J . Vaughan

250°C and 350°C havebeenplotted onan aS2-aOz diagram (Stanley & Vaughan, in press). At 350°C, haematite is a stable phase only with aOz > 10-2s.4 atm; formagnetite to have pseudomorphed haematite, the aOz must have dropped below 10-25.4atmat 350°C or below 10-32.4atmat 250°C. Cooling of the mineralizing fluids andiora reduction in oxygen activity could have led to subsequent deposition under conditions ( a 0 2 - lOP3* to 10-44 atm) which incorporate late arsenopyrite (31.332.1 atom % As) sulphidation curves in the pyrrhotine stability field. These estimates of sulphur and oxygen activity for the LowerDevonian veins are in reasonableagreement with Holland’s (1965) ‘main line’ ore-fluid stability fields forthe commonly-observedhydrothermal minerals. Evidence from fluid inclusions in quartz coexisting with chalcopyrite, from Castle Nook (T. J. Shepherd, pers.comm.), indicates thatthe mineralizing fluids were low to moderately saline brines (-5-10 equiv. wt % NaCI). Corrected homogenization temperatures obtained were 255°C (assuming a purely hydrostatic pressure of 900 bars) or 345°C (assuming a purely an estimated lithostaticpressure of 2250 bars)for depth of cover of 9 km. These studies suggest that there was little regional variation in the conditions of formation of this type of mineralization over the LakeDistrict as a whole. Local variations are reflected in the varying mineralogy of certainveins,notably in deposits associated with the Lower DevoniangraniticintrusionsatCarrock (associated with the Skiddaw granite), and Shap (joint infillings in the Shap granite and aureole), and at Scar Crag, which lies above a supposed stock-like intrusion (Rose 1955). Elsewhere there is little mineralogical variation, except in the Coniston area, whereearly magnetitepseudomorphing haematite in the Bonser vein indicates initial aOzlaS2 ratios higher than those in other Lake District veins of similar age.

Galena-sphalerite type mineralization The study of fluid inclusions in fluorite and sphalerite of the Lower Carboniferous lead-zinc mineralization (Stanley 1979) shows that the mineralizing fluids were highly saline brines (-23 equiv. wt % NaCl), often with a complex chemistry (illustrated by the common occurrence of several daughter phases) and, onthe basis of the low final melting points of ice obtained, always containing significant CaCI2. Corrected homogenization temperatures lie between 110 and 130°C. The slightly higher temperatures obtained forthe Caldbeck Fells area in the N of theLake District may be dueto relatively lower overburden pressures compared with theareas SW and SE of Keswick. Results of this study are in agreement with

previous data (Smith 1973; T. J. Shepherd, pers. comm.). The lead-zinc veins generally have a simple primary mineralogy exceptwhere the earlier chalcopyritepyrite-arsenopyritetype mineralization is also represented in the assemblage (as in the Force Crag vein, in the Tilberthwaite Pb-Zn vein, and to a minor extent in the Driggith vein). Unlike the earlier Lower Devonian mineralization, the lead-zinc veins do not contain minerals which can be used to proposereasonable estimates of sulphur or oxygen activities.

Origins of the mineralization Chalcopyrite-pyrite-arsenopyrite typemineralization

Wheatley (1971) proposed that felsic volcanic activity, exemplified by the Paddy End rhyolite in the Coniston area, implied that a magma of similar composition existed at depth. He suggested that, on cooling, this magma would have been a locus for circulating connate fluids which may have received additions fromjuvenilesources on migrating upwards within fault fissures that developed in response to the Caledonian orogeny. The main problem with this theory is the great length of time which would have elapsed from the period of felsic volcanic activity which formed the Paddy End rhyolite at c. 460 Ma, to the deposition of the mineralization in Lower Devonian times (c. 390 Ma). Most recent workers on Lake District mineralization have recognized that some relationship exists between the Lower Devonian mineralization and both the underlying compositegranitebatholithand the exposed Lower Devoniangraniticintrusions.Dagger (1977) suggested that the copper mineralization in the Lake District was controlled by the granite batholith, with mineralization occurring above the flanks of the batholith and close to ridges in the roof zone. In this model, the granite provided the heatsourcefor the mineralization, being responsible for the redistribution of traceamounts of copper and other ore-forming elements from the Borrowdale Volcanic Group. Firman (1978) also noted the close spatial relationship between the mineralization and the granite batholith. In the northern andcentral Lake District, the Lower Devonian (or ‘Caledonian’) mineral veins are generally near, and parallel to, ridges in the roof region of the underlying compositegranitebatholith (Fig. 3). Deposits in the southern Lake District, near Coniston, lie above the South Wall (Bott 1974) of the batholith, and could have formed along fractures opened up during the emplacement of the batholithafter the main period of Caledonian deformation. A structural relationship with the granite bodies is, therefore, very likely. However, the origin of the mineralizing fluids is

CU,Pb, Zn & CO mineralization, English Lake District

575

Permo-TriasSIc

./

Exposed granlte

- d ~ ~ : ~ ~ ~ e and r o u s

0

KEY

LowerDevonlan or'Caledonian'vem!

l a t e r veons

n

W C o n c c a l e d granite

,/Buried

FIG.3. Map of the Lake District illustrating the relationship batholith of Bott (1974).

less certain. Fluids derived from the dewatering of the Skiddaw Group or from Silurian strata seem unlikely agents for the mineralization, these having already lost waterat thepeak of metamorphism.Chalcopyritepyrite-arsenopyrite type mineralization in the Skiddaw Group occurs only near its junction with the Borrowdale Volcanic Group, and is absent from the Silurian strata. A likely source for the mineralizing fluids is the Borrowdale Volcanic Group. Elsewhere in the SouthernCaledonides,at Avoca, Eire,and Parys Mt, Anglesey, volcanogenic copper mineralization associated with felsic volcanic activity and local rifting was deposited in Lower Palaeozoic strata (Wheatley 1971; Thanasuthipitak 1974; Pointon 1979; Pointon & Ixer 1980). The mineralogy of these stratabound deposits is similar to that of many of the Lake District chalcopyrite-pyrite-arsenopyrite type deposits and it is possible that chemically-similar fluids existed as formation waters in the Borrowdale Volcanic Group (or in volcanic

gransterldge

of the major mineral veins to the composite granite

rocks of the Eycott Group in thenorthernLake District). Certain minerals (e.g. wolframite, scheelite, molybdenite, and apatite) are only found in veins close to, or cutting the Lower Devonian granite intrusions. It is reasonable to assume that many of theLake District veins of this age had a magmatic component, dependentontheir proximity to the granitic intrusions, in addition to a non-magmatic component. This would indicate a mixing of fluids derived from formation waters of the Borrowdale Volcanic Groupor volcanic rocks of the Eycott Group (possibly contributing Cu, Fe, Zn, Pb,Bi, As, S) with magmatic fluids associated with the Lower Devonian granites (possibly contributing W, MO, P, B, F, 0), or derived from leaching of the granites by circulating solutions. The granites would also have provided aheatsource for the circulation of these fluids. The observed mineralogical differences between

576

C. J . Stanley & D. J . Vaughan

many of the Lower Devonian veins can be explained by any one or more of the following: (a) variation in the ratio of magmatic to nonmagmatic fluids; ( b ) petrological and chemical differences between individual granitic intrusions forming the composite granite batholith; (c) petrological and chemical variations in the volcanic rocks of the Eycott and Borrowdale Volcanic Groups; ( d ) minor contributions (possibly CO, Fe, Ni, Mn, S, As) from other sources such asmeteoric water, formation waters derived from the Skiddaw Group, or associated with minor basic intrusive rocks; ( e ) deposition under slightly different pressuretemperature conditions. Galena-sphalerite type mineralization

The LowerCarboniferousgalena-sphaleritetype veins differ from the earlier chalcopyrite-pyritearsenopyritetype veins in showing no well-defined relationship to the granite batholith. The veins occur above the roof region and above the North and South Walls of the batholith (Bott 1974) and are generally N-S or NE-SW in direction, implying on overall E-W tension (see also Firman 1978). This isin agreement with the views of Russell (1976, 1978~) that,in early Carboniferoustimes, the area was subjected to terisional tectonic activity. The ForceCrag and Tilberthwaitelead-zinc veins show evidence of theearlier chalcopyrite-pyrite-arsenopyrite mineralization in addition to the galena-sphalerite mineralization. Firman (1978) proposed that uplift of theLake District towards the end of the Devonian was responsible for the opening of relatively shallow fissures in and above the central and eastern parts of the batholith. He maintained that these fissures providedchannels for mineralizing formation waters, depleted in Cu, Fe, deposition of the As, S and other elements (due to the earlier copper mineralization), and convectively recycled (although the depth involved may not have been adequate for convection). Deposits of possibly equivalent age (c. 360 Ma) and mineralogy but of proposedsyn-sedimentary or syndiagenetic origin (Russell 197%) are found in Carboniferouslimestones in Ireland. In Scotland, vein deposits similar to those of the Lake District occur in rocks of Lower Palaeozoic age at Tyndrum (Pattrick 1981) and Leadhills. For these Scottish and Irish deposits, Russell (19783) suggested that the metals and some of the sulphur could have come from Lower Palaeozoic geosynclinal sediments, and that the thermal energy driving the hydrothermal systems may have been residual heat from the Caledonian orogeny. Halliday (1977b) proposed that this Lower Carboniferous episode of metallogeny was related to widespread

volcanism during the evolution of shelf and geosynclinal seas in Western Europe. However, in the Lake District, the Lower Carboniferous (Courceyan; Butcher in Mitchell et al. 1978) Cockermouth Lavas are the only evidence of volcanic activity at this time. The fluids depositing the lead-zinc veins had considerably higher salinities (23-25 equiv. wt % NaCl) and lower temperatures (11@130"C) thanthose involved in the earlier, Lower Devonian, mineralization (S-l0 equiv. wt % NaCl and 20WOO"C). If the Lower Palaeozoic rockswere the sourcefor the lead-zinc mineralization, in addition to contributing to the earlier coppermineralization, then these differences in temperature and composition of the ore fluids must be explained.Sulphurisotopestudies on the Irish base metal deposits of Lower Carboniferous age (Coomer & Robinson 1976) have shown that Carboniferous seawater was involved in the mineralization. Hence, it is possible that seawater also contributed to the Lake District lead-zinc mineralization, being drawn inby convective flow generated by the still-cooling batholith. The Lower Palaeozoic sediments may not have been the only source of the metals; some may have been leached from the basement granites. The differences in temperature between the lead-zinc and the earlier copper mineralization could be explained in part by the lower geothermal gradients in the region resulting from the cooling of the basementgranites orthe erosion of much of the overlying strata subsequent to uplift, or be due to mixing with seawater as discussed above. Although the North Pennine lead-zinc deposits are of adifferentage and contain considerably more fluorite than those of theLake District,there are marked mineralogical similarities between them. Fluid inclusion studies on fluorites from the North Pennine ores (Sawkins 1966; Rogers 1978) show that the mineralizing fluids had high salinities (similar to those of the Lake District fluorites) and temperatures of 1S@200°C in the Alston Block and SO-130°C in the Askrigg Block. Temperatures obtained from fluid inclusion studies on Lake District fluorites (100-130°C) are similar to those of the Askrigg Block, although greater similarities are observed in the minor sulphide assemblages of the ores of the Alston Block area (Vaughan & Ixer 1980). However, the most striking similariities in terms of both mineralogy and age are with the mineralization in the Carboniferous limestones of Ireland.

Discussion The copper, lead, zinc, and cobalt veins of the Lake District can be classified mineralogically and on their geological relations and radiometric ages. Two major groups of vein deposits can be recognized: (1) Lower Devonian veins characteristically containing chalcopyrite-pyrite-arsenopyrite assemblages; (2) Lower

Cu, Pb, Zn & CO mineralization, English Lake District Carboniferous veins dominated by galena and sphalerite mineralization. The Lower Devonian mineralization apparently formedat temperatures in the range of2O0-35O0C, of themajor from fluids containingconcentrations ore-forming elements in accordance with the concentrations suggested for'main line' ore fluids. Typical as2 values appear to have been in the range of 1O-I' to atm and values of a 0 2 around 10-25 to 10-45 atm or even less. The solutions were probably brines of low to moderate salinity and appear to have been relatively uniform in character although subject to local variations, particularly when in close proximity to the Lower Devonian granites. These veins belong to a class of hydrothermal deposits typified by many base and precious metal veins in the Cordilleran region of North and South America. Here, as noted by Barton (1979), mineralization occurs in a relatively near-surface geological environment in which convection-driven meteoric water (with some magmatic contribution in certain cases) is circulating. The metal and sulphur may be of magmatic origin, or extractedfrom the mass of rock through which circulation is occurring. Deposition of the minerals takes place in regions of declining solubility, possibly resulting from cooling or reaction with a precipitant phase. The LowerCarboniferousgalena-sphaleritetype veins probablyformed attemperaturesaround 110130°C from highly saline brines. The mineralizing fluids were chemically similar to those which deposited lead-zinc veins in the North Pennine orefield, and in the Lower Carboniferous rocks of Ireland. Thus, for each of the major vein types, key aspects of the conditions of formation can be estimated. In the

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case of the Lower Devonian veins (chalcopyritepyrite-arsenopyrite type), a control on the distribution of the veins by the underlying composite granite batholith can also be clearly demonstrated.The galenasphalerite type veins show no such obvious controls on their geometric distribution, although tensional tectonic activity in the early Carboniferous may have provided the mechanism for the opening of fissures. For both major types of mineralization, the source of the mineralizing fluids remains speculative. In the case of the Lower Devonian veins, a possible major source would havebeen the Borrowdale Volcanic Group, but with contributions also from fluids associated with the Lower Devonian granites. The granites are likely to haveprovided the heatsource causing circulation of the mineralizing fluids whichmay also have derived elements such as cobalt from other minor basic source rocks. The galena-sphalerite type veins (Lower Carboniferous) also pose problems as regards the origin of the fluids andtheircontainedmetals, although the high salinities of these fluids lends support to the idea that seawater may have been involved in the leaching of metals from the basement rocks. A comprehensiveprogramme of stableisotope determinations would do much to further theunderstanding of the origin of the mineralizing fluids. ACKNOWLEDGMENTS. C. J. S . acknowledges the provision of an University of Aston research studentship, and the supervision of aspects of the work by Dr J. W. Gaskarth. We thank Alan Criddle, PeterEmbrey,DrR. J. Firman, Dr P. R. Ineson, Dr R. A. Ixer, Dr D. R. C. Kempe, and Dr M. J. Russell for their critical comments on the manuscript, althoughthe authorsare solely responsible for the views expressed.

References BARTON,P. B. 1979. Physical-chemical conditions of ore deposition. Contribution to Nobel Symposium on Chemistry at HighPressure andTemperature. Karlskoga, Sweden. -& SKINNER, B.J. 1979. Sulphide mineral stabilities. In: BARNES,H. L. (ed.). Geochemistry of hydrofhermal ore deposits, 2nd ed., 278-403. Wiley & Sons, New York. BEDDOE-STEPHENS, B. & FORTEY,N. J. 1981. Columbite from the Carrock Fell tungsten deposit. Mineralog. Mag. London, 44, 217-23. Borr, M. H. P. 1974. The geological interpretation of a gravity survey of the English Lake District and the Vale of Eden. J . geol. Soc. London, 130, 309-31. COOMER,P. G . & ROBINSON,B.W. 1976. Sulphurand sulphate-oxygenisotopes andthe origin of the Silverminesdeposits, Ireland. Mineralium Deposita, 11, 15569. DAGGER, G. W. 1977. Controls of copper mineralization at Coniston, English Lake District. Geol. Mag. 144, 195202.

DAVIDSON, W. F. & THOMPSON, N. 1948. Some notes on the minerals of Cumberland and Westmorland. N. W . Naturalist, 23, 13654. DEWEY,H. & EASTWOOD,T. 1925. Copperores of the Midlands, Wales, the Lake District, and the Isle of Man. Spec. Rep. Miner. Resour. G.B. 30. DUNHAM,K. C., BEER, K. E., ELLIS, R. A., GALLACHER, M. J., N u n , M. J. C., & WEBB,B. C. 1978. United Kingdom. In: Mineral Deposits of Europe. 1, Northwest Europe. Inst. Min. Metall. and Mineral. Soc. London. EASTWOOD, T. 1921. The lead and zinc ores of the Lake District. Spec. Rep. Miner. Resour. G.B. 22. FIRMAN, R.J. 1957. Fissure metasomatism in volcanic rocks adjacent to the Shap Granite, Westmorland. Q . .l. geol. Soc. London, 113, 205-22. - 1978. Epigeneticmineralization. In MOSELEY,F. (ed.) The Geology of the Lake District. Occ. Publ. Yorkshire Geol. Soc. No. 3. GOUGH,D. 1963. Geology of the Greenside lead mine, Cumberland. Thesis, PhD, Univ. London (unpubl).

578

C . J. Stanley & D.J. Vaughan

SkiddawSlates in theKeswick-Buttermere area. Proc. GRANTHAM, D. R. 1928. The petrology of the Shap Granite. Proc. Geol. Assoc. London, 39, 299-331. Geol. Assoc. London, 65, 403-6. -& DUNHAM,K. C. 1977. Geology and hematite deposits HALLIDAY, A.N. 1977a. K-Ar dating of mineralization episodes-a discussion. Econ. Geol. 72, 870-1. of South Cumbria. I.G.S. Economic Memoir 58148. -19776. Isotope age studies of mineralization in Western RUNDLE,C.C. 1979. Ordovicianintrusions in the English Lake District. J . geol. Soc. London, 136, 29-38. Europe. Thesis, PhD, Univ. Newcastle (unpubl). HITCHEN, C.S. 1934. The Skiddaw Granite and its residual - 1981. Discussion on the age of mineralization at Parys Mountain, Anglesey. J . Geol. Soc. London, 138, 755-6. products. Q . J . geol. Soc. London, 90, 158-200. HOLLAND, H. D. 1965. Some applications of thermochemical RUSSELL,A. 1925. Anotice of the occurrence of native arsenic in Cornwall; of bismuthinite at Shap, Westmordatato problems of ore deposits. 11, Mineral assembland; and of smaltite and niccolite at Coniston,Lanlages and the composition of ore-forming fluids. Econ. cashire. Mineralog. Mug. London, 20, 299-304. Geol. 60, 1101-58. INESON, P. R.1977. Ores of the Northern Pennines, the Lake RUSSELL,M. J. 1976. Incipient plate separation and possible related mineralization in landsbordering theNorth District, and North Wales. Ch. 5 in WOLF, K. M. (ed.) Atlantic. Spec. Pap. Geol. Assoc. Canada, 14, 339-49. Handbook of stratabound and stratiform ore deposits, 5. - 1978a. Downward excavating hydrothermal cells and Elsevier, Amsterdam. - 1980. ConferenceReport.Dating mineralization. J . of an underlying Irish-type ore deposits:importance geol. Soc. London, 137, 101-2. Trans. Inst. Min. Metall. 87, thick Caledonianprism. -& MITCHELL, J. G. 1974. K-Ar isotopic age determinaB168-71. tions from some Lake Districtminerallocalities. Geol. -1978b. Mineralization in a fractured craton. In: BOWES, D.R. & LEAKE,B. E. (eds.). Crustal evolution in Mug. 111, 521-37. northwestern Britain and adjacent regions. See1 House -& - 1975. Potassium-argon ages from thegraphite Press, Liverpool. deposits and relatedrocks of Seathwaite, Cumbria. SAWKINS,F. J. 1966. Ore genesis in theNorthPennine Proc. Yorkshire Geol. Soc. 40, 413-8. IXER,R.A., STANLEY,C. J., & VAUGHAN,D. J. 1979. orefield in the light of fluid inclusion studies. Econ. Cobalt-, nickel-, and iron-bearingsulpharsenidesfrom Geol. 61, 385-401. thenorth of England. Mineralog. Mag. London, 43, Scorn, S. D. & KISSIN,S. A. 1973. Sphalerite composition in the Zn-Fe-S system below 300°C. Econ. Geol. 68,475-9. 389-95. KINGSBURY,A.W. G . & HARTLEY,J. 1956. Cosalite and SHAW, W. T. 1970. Mining in the Lake Counties. Dalesman other lead sulphosalts at Grainsgill, Carrock Fell. MinerPubl. Co., Clapham, Lancaster. 128 pp. SHEPHERD,T. J. 1973. Geochemical evidence for basement alog. Mug. London, 31, 298-300. KLEMENT,W., JAYARAMAN, A.,& KENNEDY,G.C. 1963. control of the West Cumberland hematite mineralization. Phasediagrams of arsenic,antimony, and bismuth at Thesis, PhD, Univ. Durham (unpubl). -, BECKINSALE, R., RUNDLE, C. C.,& DURHAM,J . 1976. pressures up to 70 kbar. Phys. Rev. 131, 632-7. KRETSCHMAR, V. & Scorn, S. D. 1976. Phaserelations Genesis of the Carrock Fell tungsten deposits: fluid involving arsenopyrite in the system Fe-As-S and their Trans. Inst. Min. Metall. inclusion andisotopicstudy. application. Can. Mineral. 14, 364-86. 85, B63-73. MITCHELL,M,,TAYLOR,B. J. & RAMSBOTTOM, W. H. C. SMITH,F. W. 1973. Fluid inclusion studieson fluorite from 1978. Carboniferous. In: MOSELEY.F. (ed.) The Geolothe North Wales orefield. Trans. Inst. Min. Metall. 82, B174-6. gy of the Lake District. OCC. Publ.Yorkshire geol. Soc. No. 3. STANLEY, C. J. 1979. Mineralogical studies of copper, lead, MOORBATH,S. 1962. Lead isotope abundance studies on zinc, and cobalt mineralization in the English Lake Dismineraloccurrences in the British Isles. Philos. Trans. trict. Thesis, PhD, Univ. Aston (unpubl). __ & CRIDDLE, A. J. 1979. Mineralization at Seathwaite R. Soc. London, A254, 295-360. MOSELEY,F. (ed.) 1978. The Geology of the Lake District. Tarn,nearConiston, English Lake District. The first OCC.Publ. Yorkshire geol. Soc. No. 3. occurrence of wittichenite in Great Britain. Mineralog. PATTRICK.R. A . D. 1981. The vein mineralization ofTynMug. London. 43, 10S7. drum, Scotland, and a study of substitution in tetrahed- - & VAUGHAN,D. J. 1980. Interpretative studies of rites. Thesis, PhD. Univ. Strathclyde (unpubl). copper mineralization to the south of Keswick, England. POINTON,C.R. 1979. Volcanogenic sulphide mineralization Trans. Inst. Min. Metall. 89, B25-30. at Avoca, Eire, Parys Mt., Anglesey, and in S. E. - & - 1981. Nativeantimony and bournoniteinterCanada-A comparative study. Thesis, PhD, Univ. growths in galena from the English Lake District. MinerAston (unpubl.) alog Mug. London, 44, 257-60. -& IXER,R. A. 1980. Parys Mountain mineral deposit, -& - (in press). Mineralization in the Bonser vein, Anglesey,Wales: geology and ore mineralogy. Tram. Coniston, English Lake District;mineral assemblages, Inst. Min. Metall. 89, B143-55. pareagenesis, and formation conditions. Mineralog. POSTLETHWAITE, J. 1913. Mines and mining in the (English) Mag. London. Lake District. 3rd. ed. W. H. Moss., Whitehaven. STRENS, R. G.J. 1962. The geology of the BorrowdaleRASTALL,R.H. 1942. Theore deposits of theSkiddaw Honister district (Cumberland) withspecialreference to District. Proc. Yorkshire geol. Soc. 24, 329-43. the mineralization. Thesis, PhD, Univ. Nottingham (unROGERS,P. J. 1978. Fluid inclusion studies on fluorite from publ) . - 1965. The graphitedeposit of Seathwaite in Borrowthe Askrigg Block, Trans. Inst. Min. Metall. 87. B125-31. ROSE, W. C - C . 1955. Thesequenceandstructure of thedale,Cumberland. Geol. Mag. 102, 393-406.

Cu, Pb, Zn &

CO mineralization, English Lake District

THANASUTHIPITAK, T. 1974. Relationship of mineralization to petrologyatParysMt.,Anglesey. Thesis, PhD, Univ. Aston (unpubl). VAUGHAN, D. J. & CRAIG,J . R. 1978. Mineralchemistry of metal sulphides. Cambridge University Press, 493 pp. -& IXER,R. A. 1980. Studies of sulphide mineralogy of

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North Pennine ores and its contribution to genetic models. Tram. Inst. Min. Metall. 89, B99-110. WHEATLEY,C. J . V. 1971. Aspects of metallogenesis within theSouthern Caledonides of Gt. Britain andIreland. Trans. Inst. Min. Metall. 80, B211-23.

Received 26 October 1981; revised typescript received 2 February 1982. C. J . STANLEY, Departmentof Mineralogy, British Museum (NaturalHistory), Cromwell Road, South Kensington, London SW7 5BD. D. J. VAUGHAN, Departmentof Geological Sciences, University of Aston, Gosta Green, Birmingham B4 7ET.