ARTICLE

Evolution of a Carboniferous carbonate-hosted sphaleritebreccia deposit, Isle of Man

Kevin L. Shelton & Justin M. Beasley & Jay M. Gregg &

Martin S. Appold & Stephen F. Crowley &

James P. Hendry & Ian D. Somerville

Received: 7 October 2010 /Accepted: 19 April 2011 /Published online: 6 May 2011# Springer-Verlag 2011

Abstract A newly discovered, extensive sphalerite-bearing breccia (~7.5 wt.% Zn) is hosted in dolomitisedCarboniferous limestones overlying Ordovician–Silurianmetasedimentary rocks on the Isle of Man. Althoughbase metal sulphide deposits have been mined histori-cally on the island, they are nearly all quartz veindeposits in the metamorphic basement. This study

investigates the origin of the unusual sphalerite brecciaand its relationship to basement-hosted deposits,through a combination of petrographic, cathodolumines-cence, fluid inclusion, stable isotope and hydrogeologicmodelling techniques. Breccia mineralisation comprisesfour stages, marked by episodes of structural deforma-tion and abrupt changes in fluid temperature andchemistry. In stage I, high-temperature (Th>300°C),high-salinity (20–45 wt.% equiv. NaCl) fluid of likelybasement origin deposited a discontinuous quartz vein.This vein was subsequently dismembered during a majorbrecciation event. Stages II–IV are dominated by open-space filling sphalerite, quartz and dolomite, respectively.Fluid inclusions in these minerals record temperatures of~105–180°C and salinities of ~15–20 wt.% equiv. NaCl.The δ34S values of sphalerite (6.5–6.9‰ Vienna-CanyonDiablo troilite) are nearly identical to those of oresulphides from mines in the Lower Palaeozoic metamor-phic rocks. The δ18O values for quartz and dolomiteindicate two main fluid sources in the breccia’s hydro-thermal system, local Carboniferous-hosted brines (~0.5–6.0‰ Vienna standard mean ocean water) and basement-involved fluids (~5.5–11.5‰). Ore sulphide deposition inthe breccia is compatible with the introduction andcooling of a hot, basement-derived fluid that interactedwith local sedimentary brines.

Keywords Sphalerite . Breccia deposit . Carboniferous . Isleof Man . Geochemistry . Dolomite

Introduction

During investigations of dolomitisation in Lower Car-boniferous sedimentary rocks of the Isle of Man (IOM),

Editorial handling: F. Tornos

Electronic supplementary material The online version of this article(doi:10.1007/s00126-011-0358-3) contains supplementary material,which is available to authorized users.

K. L. Shelton (*) : J. M. Beasley :M. S. AppoldDepartment of Geological Sciences, University of Missouri,Columbia, MO 65211, USAe-mail: SheltonKL@missouri.edu

J. M. GreggBoone Pickens School of Geology, Oklahoma State University,Stillwater, OK 74074, USA

S. F. CrowleyDepartment of Earth and Ocean Sciences,University of Liverpool,Liverpool L69 3GP, UK

J. P. HendrySchool of Earth and Environmental Sciences,University of Portsmouth,Portsmouth PO1 3QL, UK

I. D. SomervilleSchool of Geological Sciences, University College Dublin,Belfield,Dublin 4, Ireland

Present Address:J. M. BeasleyCH2M Hill, 8501 West Higgins Rd. Suite 300,Chicago, IL 60631, USA

Miner Deposita (2011) 46:859–880DOI 10.1007/s00126-011-0358-3

an extensive (140 m strike length, 1–10 m width)sphalerite-bearing breccia was found in the upper partof the Balladoole Formation. No significant base metalsulphide deposits have been described previously fromCarboniferous rocks on the IOM (Lamplugh 1903;Mackay and Schnellmann 1963). Historical accounts ofmining on the IOM concentrate on base metal sulphidedeposits occurring as epigenetic quartz–carbonate veins inLower Palaeozoic metasedimentary rocks (Fig. 1). TheIOM was a leading producer of base metals from theseveins during the late nineteenth and early twentiethcentury and produced ~268,000 tonnes of lead and256,000 tonnes of zinc (Garrad et al. 1972; Colman andCooper 2000). This study investigates the origin of theunusual sphalerite breccia and its relationship to basement-hosted deposits.

Geologic background

The geology of the IOM consists of Lower Palaeozoicmetamorphic basement rocks that are overlain unconform-ably by Carboniferous sedimentary rocks and Quaternaryglacial sediments (Fig. 1). Syn- to post-tectonic Caledonianigneous rocks intrude these basement rocks and crop outlocally across the island. Paleogene dykes traverse theisland and cut both the Carboniferous and basement rocks.

Lower Palaeozoic geology

Lower Palaeozoic metasedimentary basement rocks of theIOM are exposed over much of the island, and theirgeology is summarised by Lamplugh (1903), Gillott (1956),Simpson (1963a), Woodcock et al. (1999) and Chadwick et

Fig. 1 Location and generalizedgeologic map showing theLower Palaeozoic geology,Quaternary geology and areas ofhistorically mined base metalveins of the Isle of Man. TheBalladoole breccia zone isdenoted with an arrow. Modi-fied after the Isle of Man solidand drift geologic map, BritishGeological Survey (2001)

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al. (2001). They consist of the Ordovician Manx Group andthe Silurian Dalby Group. The Dalby Group is a minorconstituent and is exposed only along the western shorenear Peel (Ford et al. 1999; Chadwick et al. 2001).

Manx Group basement rocks were deposited along thecontinental margin of Avalonia in the Iapetus Ocean andconsist of metamorphosed mudstones, shales and sand-stones broadly equivalent to the Skiddaw Group ofnorthwest England and the Ribband Group of southeasternIreland (Ford et al. 1999; Stone et al. 1999; McConnell etal. 1999; Chadwick et al. 2001). The Manx Group exhibitslargely anchizonal to greenschist grade metamorphism,overprinted in central and eastern parts of the island bycontact metamorphism associated with granitic plutons(Roberts et al. 1990).

Progressive deformation during the Ordovician toDevonian developed a broad northeast–southwest trend-ing syncline across much of the island, striking parallelto the Iapetus Suture to the north. The Isle of ManSyncline is overprinted by episodes of polyphase defor-mation including a series of folds (F1–F3) and cleavages(S1–S3; Simpson 1964). Faulting during the polyphasedeformation resulted in subsequent formation of largequartz veins that are thought to be synchronous with the F2folding event (Simpson 1963b).

A series of syntectonic granitic rocks and other minorintrusions crop out locally across the island (Simpson 1965;Cornwell 1972). The timing of their emplacement is poorlyconstrained but is thought to have accompanied Caledoniandeformation. Age determinations are available only for theFoxdale granite. K–Ar dates range from 322±5 to 381±7 Maand whole rock Rb–Sr dates are 383±11 Ma (Brown et al.

1968; Ineson and Mitchell 1979; Chadwick et al. 2001),suggesting a possible Acadian orogenic event.

Carboniferous sedimentary geology

The Carboniferous sedimentary rocks of the IOM restunconformably upon Lower Palaeozoic metasedimentarybasement and are exposed only in the south of the Island nearCastletown (Lamplugh 1903; Dickson et al. 1987; Fig. 2).These rocks consist of a basal conglomerate overlain by acarbonate ramp and platform sequence. Hemipelagic sedi-mentary rocks, carbonate turbidites and volcanic rocks capthe Carboniferous succession, which attains a total thicknessof 415 m (Fig. 3; Chadwick et al. 2001).

The exposure of Carboniferous rocks is bounded on thewest by a northeast–southwest trending fault that juxta-poses Carboniferous rocks against the Manx Group(Fig. 2). To the north and east, the boundary of theCarboniferous rocks is exposed at the unconformablecontact of the Manx Group and the overlying LangnessConglomerate.

Early Carboniferous (Mississippian) sedimentationresulted from rapid crustal extension that led to thedevelopment of localised fault-bounded basins. This wasfollowed by regional subsidence during the Namurian andWestphalian (Pennsylvanian), which led to sedimentaryonlap of the structural highs (Chadwick et al. 2001).

Carboniferous stratigraphy

The stratigraphy of the Carboniferous rocks has beendescribed by Cumming (1846), Lamplugh (1903), Lewis

Fig. 2 Geologic map showing the bedrock and structural geology ofthe southern portion of the Isle of Man. Field areas sampled in thisstudy (Derbyhaven Formation, Balladoole Formation, Bowland Shaleand the Bradda and Foxdale areas of the Manx Group) are outlined.

The area of the sphalerite-bearing breccia zone is denoted with anarrow. Modified after the Isle of Man solid and drift geologic map,British Geological Survey (2001)

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(1930) and Dickson et al. (1987). Chadwick et al. (2001)modified the nomenclature and formation relationships ofDickson et al. (1987) using a British Geological Surveyscheme that relates closely to the sedimentary classificationof the Craven Basin of northern England, and thisclassification is used in our study (Fig. 3).

The basal Carboniferous unit is the Langness Conglomer-ate Formation, which consists of polymict conglomerate,breccia and poorly sorted sandstones deposited in an alluvialand fluvial setting (Lamplugh 1903; Dickson et al. 1987).

These overlie slates of the Manx Group across an irregularunconformity with up to 3-m scale relief. Quartz veins occurlocally, are continuous across the contact and rarely hostminor Cu sulphide mineralisation (Chadwick et al. 2001).

The earliest stratigraphic occurrence of Carboniferouscarbonates on the IOM is the Derbyhaven Formation(Arundian), which is indicative of a transgressive carbonateramp succession. Basal oobioclastic grainstones (TurkeylandMember) pass into tempestite packstones with claystonepartings (Sandwick Member). Argillaceous-silty packstones(Skillicore Member) mark a change to regressive conditions(Dickson et al. 1987; Chadwick et al. 2001). Partial andselective dolomitisation of the Derbyhaven Formation can beseen within beds and along bedding planes.

The Knockrushen Formation (Holkerian) overlies theDerbyhaven Formation. It is composed of medium-beddedcarbonate wackestones—fine packstones with intercalatedblack claystones. The presence of rare, long wavelength ripplemarks in some packstones suggests deposition above stormwave base (Chadwick et al. 2001). Overlying the Knock-rushen Formation is the Hodderense Limestone Formation,which is composed of interbedded siliceous wackestones andblack shales deposited in a hemipelagic setting below stormwave base (Dickson et al. 1987). Together, these formationsindicate a deepening upwards succession.

The Balladoole Formation (Asbian) marks an abruptfacies change to a carbonate shelf setting that is believed tobe continuous laterally with the Urswick Limestone ofsouth Cumbria, England (Dickson et al. 1987). TheBalladoole Formation is composed of carbonate grainstoneand packstone and contains biohermal mud mounds similarto the platform-edge bioherms of Ireland, North Wales andnorthern England (Somerville et al. 1992, 1996; Warren etal. 1984; Chadwick et al. 2001).

Deposition of the Bowland Shale Formation wascontemporaneous with the Balladoole Formation (Fig. 3).The Scarlett Point Member lies at the base of the BowlandShale and is defined as the lowest black claystone overlyingthe Hodderense Limestone. Most of the Bowland Shale iscomposed of black, organic-rich hemipelagic claystone andsubordinate wackestone–packstone, but there are localiseddebris beds where it oversteps the Balladoole Formation.These debris beds contain calciturbidites and olistoliths ofbiohermal limestone, indicating deposition from gravityslides (Dickson et al. 1987; Quirk et al. 1990). Capping theBowland Shale are interbedded submarine volcaniclasticdebris flows, pillow lavas and hemipelagic claystones of theScarlett Volcanic Member (Chadwick et al. 2001).

Paleogene intrusions

Paleogene mafic and ultramafic dykes cut the Manx Groupand Carboniferous rocks of the IOM. They trend roughly

Fig. 3 Simplified stratigraphic column of the Carboniferous geologyalong the southern shore of the Isle of Man. Modified after Chadwicket al. (2001)

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west-northwest to east-southeast and impart only localthermal effects to the host rocks (Swift 1993). Theiremplacement is thought to relate to faulting caused by theformation of the North Atlantic Igneous Province and theIcelandic Plume (White 1988).

Lead–zinc–copper sulphide vein deposits

Manx Group veins

Lead–zinc–copper-bearing sulphide mineralisation ishosted in large, north–south and east–west-trendingepigenetic quartz and carbonate veins that cut themetasedimentary Manx basement rocks and are describedby Lamplugh (1903), Mackay and Schnellmann (1963),Simpson (1963a, b), Crowley et al. (1997), Ford (1999)and Chadwick et al. (2001). Base metal sulphide mineralspresent in these veins consist predominantly of galena,sphalerite and chalcopyrite. The galena is typicallyargentiferous (15–40 tr. oz. per tonne of lead), with thehighest recovery of silver of 400 tr. oz per tonne of leadfrom the Foxdale Mine (Lamplugh 1903).

General vein paragenesis consists of early quartz andpyrite followed by main-stage ore mineralisation ofgalena, sphalerite, chalcopyrite (± tetrahedrite) andgangue dolomite. The last recognised stage consists ofdolomite and calcite (Crowley et al. 1997). The ages ofvein mineralisation occurring in the Manx Group are notwell constrained. Parnell (1988) interpreted the presenceof uraniferous hydrocarbons in veins of the Laxey mine toindicate that veins formerly extended up into the organic-rich Carboniferous section and that the mineralisation wasno older than Carboniferous. K–Ar dates determined fromclay gouges and altered wall rocks form three maingroups: 310–320, ~250 and ~220 Ma (Ineson and Mitchell1979). Model lead isotope ages from galena at the Laxey,Foxdale and Bradda mines form two groups, 310–280 Maand 210–190 Ma (Crowley et al. 1997).

Minor veins in Carboniferous rocks

Lamplugh (1903), Mackay and Schnellmann (1963),Crowley et al. (1997) and Chadwick et al. (2001) notedthe presence of trace amounts of sphalerite, galena,chalcopyrite and pyrite occurring in Carboniferous sedimen-tary rocks. The main example of sulphide mineralisation ischalcopyrite in quartz veins in the Langness Conglomerate,where it was mined historically. Lamplugh (1903) alsoreported that sulphides were found in Carboniferous carbo-nates near Castletown, which were thought to be associatedwith Paleogene dykes. In July of 2010, we found minorsphalerite–galena veinlets in a 0.5-m-wide dolomite vein

crosscutting dolomitised host rock in Castletown Harbour.Crowley et al. (1997) referred to minor sulphide mineralisa-tion in veins associated with dolomitisation of the LowerCarboniferous limestones. However, there are no previousreferences to more extensive base metal sulphide mineralisa-tion in Carboniferous strata. A substantial sphalerite-richbreccia deposit from the upper Balladoole Formation isdescribed for the first time in this paper.

Carboniferous host rock alteration and veining

Balladoole Formation

Along the southern coast of the IOM, well-bedded lime-stones of the Balladoole Formation are extensively frac-tured and dolomitised. Within the zone of dolomitisation isan extensive breccia zone hosting abundant sphalerite andminor galena (Fig. 4). Brown replacement dolomitealteration creates a large halo around the breccia thatextends from near the contact of the Balladoole Formationand the Bowland Shale west-northwest to the BalladooleFault where massively dolomitised Balladoole Formation isjuxtaposed against more weakly altered KnockrushenFormation (Fig. 5a).

Randomly oriented dolomite veins fill the fracturenetwork of the host rock surrounding the breccia zone(Fig. 5b). These veins are characterised by two dolomitecement events (Fig. 5c) that are similar to early and latedolomite of the much less altered Derbyhaven Formation(Dickson and Coleman 1980; Beasley 2008). A detailedparagenetic history of the breccia zone is presented later.

Less intense alteration of the Balladoole Formation wasobserved approximately 1 km north of the breccia zone atthe Cross Welkin Hill quarry. There, large biohermalmounds are cut by small vertical dolomite veins up to2 cm in width. This dolomite is thought to be representativeof diagenesis distal from the effects of fault-relateddolomitisation.

Bowland Shale

Three generations of epigenetic carbonate veins are presentwithin the calcareous mudrocks of the Bowland Shale atPoyllvaaish, 0.8 km south-southeast of the breccia deposit.Dolomite is minor and forms veins <2 cm in width. Theseveins are characterised by nonplanar textures with frequentsmall, vug-filling, saddle-shaped crystals ~1 mm in size.

Subsequent ankerite veins are orientated east–west andvary in width from <1 to 20 cm. Ankerite also fills smallerextensional fractures and cements brecciated wall rockfragments in larger veins (Fig. 6a, b). It is typically salmonpink and coarsely crystalline, with saddle-shaped crystals

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up to 1 cm in size terminating into open spaces. The veinsinfrequently host trace amounts of sphalerite and chalco-pyrite (Fig. 6c). Paragenetically late milky white calciteveins obliquely cut and offset earlier ankerite veins(Fig. 6d). Euhedral calcite crystals up to 2 cm long and1 cm wide are also found in vugs within ankerite veins(Fig. 6e).

Paleogene mafic dykes (<2 m wide) were observed tocrosscut ankerite ± calcite veins and are unrelated to theseveins’ mineralising hydrothermal system. The dykes con-tain their own sets of fibrous calcite veins that are parallel

and perpendicular to dyke margins, which are unlike anyother calcite veins in the Bowland Shale.

Sphalerite-rich breccia zone

The mineralised breccia in the Balladoole Formation occupiesa north–south trending, wedge-shaped zone of 165×35 m inextent. Within it is a sphalerite-mineralised zone 140 m inlength and decreasing in width from 10 to 1 m towards thesouth (Fig. 4). The sphalerite zone has a grade of ~7.5 wt.%

Fig. 4 Geologic map of the Balladoole breccia zone and surroundingarea illustrated at lowest tide. The dolomite halo (grey) surrounds thebreccia zone and extends northwestwards to the Balladoole Fault.

Geological structures modified after the Isle of Man solid and driftgeologic map, British Geological Survey (2001)

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Zn (estimated from volume percent sphalerite in 22samples). Galena is present in trace amounts.

Paragenesis

The mineralised zone records a four-stage parageneticsequence punctuated by episodes of brecciation or fracturing.

The stages were recognised in crosscutting and texturalrelationships in 22 hand specimens and 12 polished thinsections. They are (I) early quartz pods, pre-brecciationsulphide deposition and pre- and syn-brecciation clastalteration; (II) sphalerite cement; (III) milky quartz rims onclasts and (IV) vug-filling dolomite and quartz (Fig. 7).

Stage I represents the alteration and mineralisation thatoccurred pre- to syn-brecciation. The earliest recognisedphase in stage I is large, discontinuous, milky white quartzpods that range in size from a few centimetres to 20 cm inwidth and ~10 cm to 0.5 m in length. These discontinuousand isolated pods are surrounded by deformed, brownreplacement dolomite (Fig. 8a).

Carbonate rock clasts that compose the bulk of the brecciaare angular and equant and range in diameter from <1 to~15 cm. They are cemented dominantly by stage IV dolomite.Varying degrees of alteration of the clasts have resulted inminor to complete dolomitisation (Fig. 8b–d).

Carbonate clasts range from grey dolomite clasts thatretain traces of original sedimentary structures to complete-ly recrystallised clasts with zebra-like textures of alternatinglight and dark brown dolomite with clear quartz. The lessaltered grey clasts preserve a polymodal, nonplanardolomite core surrounded by a rim of planar dolomitecement (dolomite classification of Sibley and Gregg 1987).Pyrite is present throughout the nonplanar dolomite asdisseminated, 1–5 μm, anhedral to subhedral masses.Carbonate clasts that have been more extensively alteredshow complete recrystallisation and planar-s textures.

Quartz–carbonate clasts are characterised by medium-grained (0.2–1.0 mm) clear quartz with trace inclusions ofanhedral chalcopyrite. Quartz overgrows small (0.3–1.2 mm) angular fragments of recrystallised dolomite thatcontain minor galena (Fig. 8b). The quartz exhibitsundulatory extinction, systematic increase in grain sizeaway from rock fragments, and is fractured and partiallybrecciated.

Galena in the breccia zone occurs as small veinlets thatmay have been parallel to original bedding prior tobrecciation and recrystallisation (Figs. 8b, c). The veinletsare characterised by small (0.5–1.0 mm) irregularly shapedcrystals that are arranged linearly and are associated withearly dolomite alteration. Galena also forms the matrix ofthe breccia along the periphery of the sphalerite-rich core ofthe breccia (Fig. 8d).

Stage II marks the beginning of open-space fillingmineralisation in the breccia zone. Sphalerite is the earliestphase and occurs commonly as millimetres to centimetressize, zoned, euhedral crystals surrounding the breccia clasts(Fig. 8b, c, f). It also forms small (<1 cm wide) veins andisolated crystals cemented by stage IV dolomite. Sphaleriteis fractured extensively and crosscut by minor late quartzand dolomite veinlets.

Fig. 5 Alteration of the Balladoole Formation: a photograph of theBalladoole Fault. The Knockrushen Formation (grey) is down-thrownagainst the Balladoole Formation (brown). b Dolomite veins (brown)filling a fractured network in the host rock (grey). c Doubly polishedfluid inclusion slide illustrating the early and late dolomite cementgenerations

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Sphalerite shows yellow brown to red brown growth zonesin transmitted light with a distinct dark red brown zone nearcrystal edges (Fig. 8b). Rare, small (≤50 μm) euhedralinclusions of galena are present in lighter-coloured,

intermediate zones of sphalerite crystals. Compositionsof sphalerite growth zones were determined for traversesacross two crystals using a scanning electron microscopeequipped with an energy dispersive spectrometer. OnlyZn, Fe, Cd and S were present in concentrations abovedetection levels. FeS contents are low (2.36–5.52 mol%)and do not correlate with the colour zones. CdS contentsare typically below detection in crystal cores but are 0.83–1.53 mol% in the outer, dark growth zones. High cadmiumcontents may indicate a link to basement metal sources, asmodern stream-estuary systems on the island that drainabandoned mining areas in the Manx Group also displayelevated levels of cadmium (Daka et al. 2003).

Stage III is composed of milky white quartz surroundingbreccia clasts. This ‘quartz rim’ is associated with an eventthat partially fractured and separated sphalerite from contactwith the clasts (Figs. 8b, c and 9a, b). This created small(<1–5 mm) open spaces that were filled with quartz. Thequartz forms subhedral to euhedral, medium-grained (0.2–

Fig. 7 Generalized paragenetic sequence of minerals from theBalladoole breccia zone

Fig. 6 Carbonate veins in theBowland Shale: a vein-fillingankerite cross cut by late calcite.b Ankerite-filled fracturesupporting brecciated wall rockmaterial. c Potassiumferricyanide-stained, ankerite–calcite vein containing sphaler-ite. d Late calcite vein crosscut-ting and offsetting ankerite vein.e Euhedral calcite crystals invug in an ankerite vein

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2.0 mm) crystals that radiate outwards from the clastsurface and that commonly contain fragments of earlierdolomite, galena and sphalerite. The quartz rim is itselffractured and exhibits undulatory extinction. Small frac-tures in sphalerite are also filled by this generation ofquartz, which occurs as fine-grained (50–100 μm) anhedralcrystals of milky quartz.

Stage IV consists dominantly of open-space fillingferroan-dolomite cement, subordinate quartz and minorpyrite. The pale brown dolomite overgrows sphalerite andquartz-rimmed clasts (Fig. 8b, e, f). The dolomite ischaracterised by medium-grained (1 to 2 mm) planarcrystals exhibiting sweeping extinction. Large voids arepresent in the breccia consisting of vuggy and channelpores that can be up to 2 cm in width and 8 cm in length.These voids contain 1–5 mm, euhedral saddle dolomitecrystals. Stage IV dolomite also occurs as veinlets thatcrosscut all previous stages of mineral growth.

Stage IV clear quartz fills vugs in the dolomite cement.Quartz also forms northwest–southeast trending veins thatcut the breccia zone. These veins are more resistant toerosion than the surrounding dolomite breccia zone andform ridges that are visible at low tide, which are 1.5 m in

height and 1 to 2 m in width containing 2–4 cm, euhedralquartz crystals. The orientation of these ridges is roughlyparallel to faults exposed in the nearby Bowland Shale andmay reflect a relationship to faulting (Fig. 4).

Supergene alteration observed in the breccia zoneconsists dominantly of hematite along dolomite grainboundaries. Hematite also forms anhedral masses fillingthe intercrystalline porosity of stage IV dolomite. Minorhematite occurs along microfractures in sphalerite.

Cathodoluminescence

Cathodoluminescence (CL) was used to compare thediagenetic phases from different field areas and stratigraph-ic units of the IOM. The use of CL permits a qualitativeassessment to determine whether or not there are geneticrelationships among diagenetic phases and to potentiallylink phases to mineralising fluid sources. Fifty-eightpolished thin sections from the sphalerite breccia zone,Carboniferous limestones and dolomites from coastalexposures in the south of the IOM and mineralised areaswithin the Manx Group basement rocks were analysed

Fig. 8 a Photograph of stage Iquartz pod surrounded by de-formed brown replacement do-lomite. b Doubly polished fluidinclusion section showing stageI carbonate clast and quartz–carbonate clasts surrounded bystage II open-space fillingsphalerite, stage III rim quartzand cemented by stage IV dolo-mite. c Doubly polished fluidinclusion section showing smallveinlets of recrystallised galenain stage I dolomitised clasts,cemented by stage II sphaleriteand stage III rim quartz. dGalena-cemented carbonateclast breccia from the peripheryof the breccia body. e Photo-graph illustrating the weatheredsurface of the sphalerite breccia.f Sphalerite-rimmed brecciaclasts cemented by stage IVdolomite

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using a CITL CL8200 MK5 CL system, operating from12.8 to 13.2 kV with a beam current of 270 μA and avacuum of 33.8 mbar.

Two types of dolomite CL behaviour were recognised inthis study. Replacement dolomite and early dolomitecement are characterised by red and purple mottled CLthat does not display zoning (Fig. 9a, b). This was observedin Bowland Shale dolomite veins, in stage I dolomite of theBalladoole breccia zone and in vein dolomite in the ManxGroup metamorphic rocks. In contrast, nonluminescent CLbehaviour was found in stage IV dolomite cement of theBalladoole breccia zone and in ankerite cement of theBowland Shale (Fig. 9c, d, i, j). Where supergene alterationis observed, the peripheries of dolomite and ankeritecrystals show a dark red-mottled CL. Sphalerite is non-luminescent with a thin bright red rim where sphalerite is incontact with stage IV dolomite (Fig. 9c, d). In areasaffected by supergene alteration, sphalerite shows brightpink and blue CL.

Four types of CL were observed in quartz in thebreccia zone of the Balladoole Formation and in veinquartz in the Manx Group: (1) indigo blue yellow zonedCL, (2) blue brown yellow zoned CL showing sectorzoning, (3) violet blue CL and (4) nonluminescent CL

(Fig. 9e–l). Indigo blue yellow zoned CL occurs only instage I quartz–carbonate clasts of the Balladoole Forma-tion’s breccia zone. This quartz is highly recrystallised andgrowth zones are observed only using CL. Zones alternatefrom yellow to dark indigo blue with decreasing widthtowards crystal edges (Fig. 9e, f). Blue brown yellowzoned CL was observed in the Balladoole Formation’sbreccia zone within stage I quartz pods (Fig. 9g, h) andstage IV open-space filling quartz. The blue brown yellowzoned CL grades to non-CL towards the edge of large,euhedral stage IV quartz crystals (Fig. 9i, j). Similar bluebrown yellow zoned CL behaviour of quartz was observedin sphalerite-bearing quartz veins in the Manx Groupmetamorphic rocks from the Bradda and Foxdale mineareas (Fig. 9k, l). Violet blue CL occurs only in the stageIII quartz rim of the Balladoole Formation’s breccia zone.Yellow streaks associated with fractures radiate inwardsfrom the mineral grains’ outer boundaries (Fig. 9a, b).Nonluminescent quartz was observed only in high-temperature (Th>300°C) quartz veins in the Manx Groupfrom the Foxdale and Bradda mine areas. Parageneticallylate calcite found throughout the Carboniferous carbonaterocks displays uniform bright yellow orange to yellow CLwith no zoning.

Fig. 9 Cross-polarized light and cathodoluminescence photomicro-graph pairs (left and right, respectively) showing a, b stage I, red-mottled CL, replacement dolomite (E.Dol) observed in the Balladoolebreccia clasts; stage II, non-CL sphalerite (Sph) and stage III, violetblue CL quartz rim (Qz-rim). c, d Contact of stage II, non-CLsphalerite (Sph) and stage IV, non-CL dolomite (L. Dol). e, f Stage I

quartz–carbonate clast showing indigo blue yellow CL zoning. g,h Blue brownish yellow CL of stage I quartz pod. i, j Contact of stageIV, non-CL dolomite (L.Dol), and stage IV, blue brownish yellow CL-zoned quartz (Qz). k, l Blue brownish yellow CL Manx group quartzassociated with non-CL sphalerite

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Fluid inclusion study

Forty samples of carbonate and quartz–carbonate veinsfrom all field localities were prepared as 100–150-μm-thickdoubly polished sections for fluid inclusion study. Micro-thermometric data were obtained using a Linkam THMSG600 conduction heating/freezing stage (Table 1). Temper-atures of total homogenization (Th) have errors of ±2°C,and temperatures of melting (Tm of ice, clathrate and CO2),solution (Ts of halite) and homogenization of CO2-richphases have errors of ±0.2°C, based on replicate measure-ments of standard fluid inclusions (Shelton and Orville1980). Compositions of the trapped fluids were determinedfrom thermometric data using the MacFlinCor software(Brown and Hagemann 1995) and compiled data of Kerrickand Jacobs (1981), Bodnar and Vityk (1994) and Thiery etal. (1994).

Distribution and compositional types of fluid inclusions

Fluid inclusions were found in dolomite, quartz andsphalerite from the Balladoole Formation’s breccia zone,in ankerite and calcite veins from the Bowland Shale and inquartz veins from the Manx Group metamorphic rocks. The

size of fluid inclusions ranges from <1–50 μm with mostinclusions <10 μm in size.

Three compositional types of fluid inclusions have beenidentified on the basis of their phase relationships at 20°Cand behaviour during cooling (Fig. 10a–d). In order ofdecreasing abundance, they are two-phase aqueous inclu-sions, three-phase H2O–CO2–NaCl inclusions and aqueousinclusions with halite daughter minerals. A total of 490two-phase aqueous, 15 CO2-bearing and six halite-bearinginclusions were analysed (Beasley 2008).

Aqueous inclusions were found in all samples examinedand contain two phases (liquid + vapour). Primaryinclusions are rare and are found typically in carbonatecements as rhombic-shaped inclusions along growth zonesof euhedral crystals and as isolated inclusions (Fig. 10b).Primary inclusions were found in stage I and stage IVdolomite from the Balladoole Formation’s breccia zone, inankerite and calcite vein cements from the Bowland Shaleand in vein quartz from the Manx Group localities of ourstudy.

Pseudosecondary (PS) and secondary inclusions aremore common owing to the multiple episodes of breccia-tion, fracturing and recrystallisation that have affected therocks. They are present along healed fractures or as

Table 1 Summary of data for primary and pseudosecondary fluid inclusions in Carboniferous carbonate rocks and Ordovician metamorphicbasement rocks, southern Isle of Man

Formation/locality Mineral Th (°C) Tm (°C) Salinity (wt.% equiv. NaCl)

Regional dolomite cements

Balladoole Fm. Early dolomite cement 86 to 129 (n=14) −23.1 to −5.6 (n=14) 23.3 to 8.6

Late dolomite cement 228 to 268 (n=2) −22.9 (n=2) 23.2

Derbyhaven Fm. Early dolomite cement 101 to 111 (n=4) −5.3 to −4.3 (n=2) 8.2 to 6.8

Late dolomite cement 119 to 268 (n=10) −25.1 to −13.8 (n=6) 24.2 to 17.6

Vein-filling mineralisation

Bowland Shale Ankerite 87 to 288 (n=64) −24.7 to −13.2 (n=46) 24.0 to 17.0

Ankerite w/sphalerite 114 to 142 (n=8) −22.8 to −13.2 (n=8) 23.2 to 16.1

Calcite 95 to 315 (n=17) −19.7 to −5.4 (n=6) 22.1 to 8.4

Balladoole Breccia Zone Stage I clast dolomite 87 to 158 (n=9) −19.9 to −16.3 (n=9) 22.3 to 19.7

Stage I clast quartz

Stage I quartz pod 294 to 445 (n=11) −23.1 to −16.1 (n=11) 23.3 to 19.5

325 to 467 (n=6) Halite-bearing (n=6) 36.1 to 46.0

Stage II sphalerite 115 to 230 (n=3) −13.0 to −11.2 (n=3) 16.9 to 15.2

Stage III quartz 92 to 169 (n=19) −22.6 to −9.3 (n=19) 23.1 to 13.2

Stage IV dolomite 90 to 109 (n=6) −15.8 to −7.9 (n=6) 19.3 to 11.6

Dolomite w/quartz 148 to 292 (n=8) −21.0 to −14.0 (n=6) 22.3 to 17.8

Stage IV quartz 180 to 324 (n=28) −21.9 to −2.4 (n=13) 22.8 to 3.9

Manx Slate/Bradda Non-CL quartz 243 to 355 (n=11) CO2-bearing (n=11) <4.3

bl-brn-yell CL quartz 71 to 132 (n=17) −20.8 to −3.8 (n=17) 22.9 to 6.1

Manx Slate/Foxdale Non-CL quartz 250 to 278 (n=3) CO2-bearing (n=3) <4.3

257 to 318 (n=3) −3.8 to −0.4 (n=3) 6.1 to 0.7

bl blue, brn brown, yell yellow

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irregular three-dimensional clusters in vein-filling ankerite,dolomite, quartz, sphalerite and calcite (Fig. 10a).

CO2-bearing inclusions consist of three phases (H2O-liquid+CO2-rich liquid+CO2-rich vapour) and were foundas PS inclusions in base metal sulphide-bearing quartzveins in the Manx Group at the Foxdale and Bradda mines(Fig. 10d). The volumetric percentages of the carboniccomponents (liquid + vapour CO2) at 20°C vary typicallyfrom 10% to 60%. These inclusions occur in three-dimensional clusters and are associated with aqueousinclusions in individual fluid inclusion assemblages.

Halite-bearing inclusions consist of three phases (liquid +vapour + halite) and were found exclusively in stage I quartzpods in the Balladoole Formation’s breccia zone (Fig. 10c).They are present as round to blocky shaped, PS inclusionsthat form trails and clusters. The vapour phase occupies 10–30 vol.% of individual inclusions.

Fluid inclusion microthermometric data

Regional dolomite in Carboniferous rocks

Fluid inclusions in Carboniferous carbonates distal from theBalladoole breccia deposit were analysed to provide abaseline for comparison to those associated with fault-related dolomitisation. Early dolomite cement in theDerbyhaven Formation contains primary two-phase aque-ous fluid inclusions with Th values of 101–111°C (Table 1).Their Tm (ice) values are −5.3°C and −4.3°C, correspondingto salinities of 8.2 to 6.8 wt.% equiv. NaCl. Late dolomitecement contains primary aqueous inclusions with Th values

of 119–268°C (avg.=192°C). Their Tm (ice) values are−25.1°C to −13.8°C, corresponding to salinities of 24.2 to17.6 wt.% equiv. NaCl. Early dolomite cement in theBalladoole Formation, 0.4 to 1.0 km away from the brecciazone, contains primary and PS two-phase aqueous fluidinclusions with Th values of 86–129°C (avg.=110°C). TheirTm (ice) values are −23.1°C to −5.6°C, corresponding tosalinities of 23.3 to 8.6 wt.% equiv. NaCl (avg.=20.1%).Their Te values are −36.1°C to −25.1°C. These valuesindicate complex brines that likely contain CaCl2 inaddition to NaCl (Crawford 1981; Zhang and Frantz1989). Late dolomite cement in these same rocks containsprimary aqueous inclusions with Th values of 228°C and268°C. Their Tm (ice) value is −22.9°C, corresponding to asalinity of 23.2 wt.% equiv. NaCl.

For comparison, mean vitrinite reflectance values forLower Carboniferous mudstones on the IOM are 2.5±0.1%(Crowley et al. 1997), corresponding to maximum rocktemperatures of ~240°C (Barker and Pawlewicz 1986).These values are thought to reflect maximum burial duringCarboniferous basin evolution, with the high levels ofmaturity recording elevated heat flow associated both withextensional basin evolution and heating from late Caledo-nian granite underlying the southern half of the island(Crowley et al. 1997).

Balladoole Formation’s breccia zone

Stage I dolomite in breccia clasts is similar to earlydolomite in the Balladoole Formation. The number ofprimary aqueous fluid inclusions analysed from stage Idolomite (replacement dolomite in breccia clasts) was

Fig. 10 Photomicrographsshowing fluid inclusion types. aAqueous inclusion in stage IIsphalerite. b Primary aqueousinclusion in stage IV dolomitecement in the Balladoole brecciazone. c Aqueous halite-bearinginclusion in stage I quartz pod. dThree-phase, CO2-bearinginclusions in quartz from theManx Group at Bradda (Fig. 1)

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limited (n=9) by the opaque nature of the dolomite. Thvalues range from 87°C to 158°C (avg. 117°C), and Tm (ice)

values range from −19.9°C to −16.3°C, corresponding tosalinities of 22.3 to 19.7 wt.% equiv. NaCl (Fig. 11b).Secondary, aqueous inclusions in replacement quartz from

stage I quartz–carbonate clasts (n=5) have Th values of244–266°C and Tm (ice) values of −15.0°C to −9.0°C,corresponding to 18.6 to 12.9 wt.% equiv. NaCl.

Stage I, highly deformed quartz pods contain two-phaseaqueous inclusions and halite-bearing inclusions. Thoughobviously necked or unusually shaped fluid inclusions wereavoided, deformation of fluid inclusions in stage I quartzpods may have resulted in some of the high Th valuesmeasured (Fig. 11b; Table 1).

PS aqueous inclusions have eutectic melting temperatures(Te) between −45.6°C and −25.2°C. These measurementsindicate complex brines that likely contain CaCl2 inaddition to NaCl (Crawford 1981; Zhang and Frantz1989). These inclusions have Tm (ice) values that rangefrom −23.1°C to −16.1°C, indicating salinities between23.3 and 19.5 wt.% equiv. NaCl (Bodnar and Vityk 1994).Their Th values are 294–445°C (Fig. 11b). Secondary, two-phase inclusions have Th values of 305–440°C (n=25).

Aqueous, halite-bearing inclusions from the same sampleform two groups based on total homogenization behaviour:homogenization by halite dissolution (Ts (halite) > Th (liquid)) (n=4) and homogenization by vapour disappearance (Ts (halite) <Th (liquid)) (n=2). Ts (halite) values range from 270°C to 389°C,corresponding to salinities of ~36–46 wt.% equiv. NaCl. Thvalues are 325–467°C.

Stage II sphalerite contains rare PS and more abundantsecondary aqueous fluid inclusions. PS inclusions areisolated and were found only in the outer, dark browngrowth zones. Secondary inclusions are present throughoutthe crystals, and their morphologies are controlled bycleavage. PS inclusions have Th values of 115–230°C andTm (ice) values of −13.0°C to −11.2°C, corresponding tosalinities of 16.9–15.2 wt.% equiv. NaCl. Most secondaryinclusions (22 of n=24) have Th values of 58–135°C andTm (ice) values of −25.3°C to −9.5°C, indicating salinitiesfrom 24.2 to 13.4 wt.% equiv. NaCl. Two secondaryinclusions have Th values of 176–201°C and identical Tm(ice) values of −1.5°C, indicating a salinity of 2.5 wt.%equiv. NaCl.

Stage III quartz rims contain PS aqueous inclusions thatare isolated and irregular, or distributed along discontinuoushealed fractures that radiate inwards from the crystalboundary. Secondary inclusions occur in healed fracturesthat are continuous across crystal boundaries. Th values ofPS inclusions are 92–169°C and their Tm (ice) values are−22.6°C to −9.3°C, corresponding to salinities of 23.1 to13.2 wt.% equiv. NaCl (Fig. 11b). Th values of secondaryinclusions (n=10) are 70–161°C, and their Tm (ice) valuesare −21.8°C to −7.3°C, corresponding to salinities of 21.7to 11.6 wt.% equiv. NaCl.

Stage IV dolomite cement within the breccia zone issimilar to late dolomite outside the breccia and containsprimary aqueous fluid inclusions that are rhombohedral and

Fig. 11 Homogenization temperature (Th) vs. salinity of primary andpseudosecondary fluid inclusions hosted in a veins in the BowlandShale, b regional dolomite (early and late) in the Balladoole andDerbyhaven formations and breccia-cementing phases in the Balla-doole Formation, c quartz veins in Manx Group rocks. Note thesimilarity of low-temperature fluids present in the Manx Groupbasement rocks to those in the Balladoole breccia zone

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are distributed parallel to growth zones of euhedral crystals.PS inclusions have more irregular shapes and occur alongcleavage and growth zones. These inclusions have distinctranges of Th values, related to the host dolomite’s proximityto stage IV quartz veins. Primary and PS inclusions indolomite that is distal to quartz veins (n=6) have Th valuesof 90–109°C and Tm (ice) values of −15.8°C to −7.9°C,corresponding to salinities of 19.3 to 11.6 wt.% equiv. NaCl.Those in dolomite cements associated with quartz veins havehigher Th values of 148–292°C (avg.=229°C; n=8) and Tm(ice) values of −21.0°C to −14.0°C (n=6), corresponding tosalinities of 22.3 to 17.8 wt.% equiv. NaCl (Fig. 13).

Most PS inclusions in stage IV quartz (n=25) have Thvalues of 180–324°C (avg. 233°C) and Tm (ice) values of−21.9°C to −8.8°C (n=10), corresponding to salinities of22.8 to 12.6 wt.% equiv. NaCl. Three PS inclusions have Thvalues of 200–324°C and Tm (ice) values of −5.3°C to −2.4°C,indicating lower salinities of 8.2 to 3.9 wt.% equiv. NaCl(Fig. 11b). Secondary inclusions (n=44) have Th values of73–333°C and Tm (ice) values of −23.9°C to −5.9°C,corresponding to salinities of 23.7 to 9.1 wt.% equiv. NaCl.

Veins in the Bowland Shale

Primary and PS inclusions in vein-filling ankerite have Thvalues of 87–288°C (avg.=170.0°C) and Tm (ice) values of−24.7°C to −13.2°C, corresponding to salinities of 24.0 to17.0 wt.% equiv. NaCl (Fig. 11a). Ankerite that is associatedwith minor sphalerite has inclusions with Th values of 114–142°C and Tm (ice) values of −22.8°C to −13.2°C. Secondaryinclusions (n=67) have Th values of 80–300°C and Tm (ice)

values from −23.2°C to −0.3°C, corresponding to salinitiesof 23.4 to 0.5 wt.% equiv. NaCl. Ninety percent of theseinclusions have salinities >11.3 wt.% equiv. NaCl.

Primary and PS fluid inclusions in calcite have Th valuesof 95–315°C and Tm (ice) values of −19.7°C to −5.4°C,corresponding to salinities of 22.1 to 8.4 wt.% equiv. NaCl.Secondary inclusions (n=35) have Th values of 71–285°C.Their Tm (ice) values form two groups, −22.2°C to −11.6°Cand −3.6°C to 0.0°C, corresponding to salinities of 22.9 to15.6 and 5.8 to 0.0 wt.% equiv. NaCl, respectively.

Quartz veins in the Manx Group

Non-CL quartz in ore-bearing veins from Foxdale andBradda contains PS CO2-bearing fluid inclusions with Thvalues of 250–278°C and 243–355°C, respectively. Tm(CO2)

values from Foxdale range from −58.2°C to −57.4°C,indicating CH4 contents of 0.03–0.05 mol% in the carbonicportion of the inclusions. Tm(CO2) values from Bradda are−57.4°C to −56.5°C and indicate purer CO2 (<0.03 mol%CH4). Th(CO2) values from Foxdale and Bradda range from21.5°C to 22.0°C and 22.4°C to 27.8°C, respectively. Their

Tm(clathrate) values range from 7.8°C to 17.1°C, reflectingsalinities <4.3 wt.% equiv. NaCl (Fig. 11c).

PS, two-phase, aqueous inclusions from Foxdale have Thvalues of 257–318°C and Tm (ice) values of −3.8°C to −0.4°C,corresponding to salinities of 6.1 to 0.7 wt.% equiv. NaCl.Secondary inclusions have Th values that range from 116°Cto 260°C. Tm (ice) values of these inclusions range widelyfrom −12.4°C to −0.2°C, corresponding to 16.3 to 0.3 wt.%equiv. NaCl.

Blue brown yellow CL-zoned quartz associated withdolomite from Bradda contains only two-phase aqueousinclusions. Primary and PS inclusions have Th values of71–132°C and Tm (ice) values of −20.8°C to −3.8°C,reflecting salinities of 22.9 to 6.1 wt.% equiv. NaCl(Fig. 11c). Secondary inclusions (n=7) have Th values of58–126°C and Tm (ice) values of −23.5°C to −1.2°C,corresponding to salinities of 23.2 to 2.0 wt.% equiv. NaCl.

Fluid inclusion summary and interpretation

Based on microthermometry and phase relations, fourdistinct fluid types were trapped in quartz, sphalerite andcarbonate veins in Carboniferous rocks and Manx Groupbasement rocks: (1) high-salinity brines with salinities of36–46 wt.% equiv. NaCl, (2) moderate-salinity brines (6–24 wt.% equiv. NaCl), (3) low-salinity aqueous fluids(<6 wt.% equiv. NaCl) and (4) CO2-bearing fluids.

High-salinity brines were found only in quartz pods thatformed early in the Balladoole breccia zone’s mineralparagenesis (Fig. 7). If these enigmatic inclusions resultedfrom boiling of a more moderate-salinity fluid (withoutentrapment of the co-existing vapour phase), they wouldreflect pressures of <300 bars (Driesner and Heinrich 2007).This pressure would correspond to a depth of ~0.9 km,assuming purely lithostatic conditions, or ~3 km, if purelyhydrostatic.

Moderate-salinity fluids were found throughout theBalladoole Formation, including the breccia zone, and inthe Bowland Shale. Their range of salinities (6–24 wt.%equiv. NaCl) is observed in all generations of carbonate andquartz in the Carboniferous section (Beasley 2008).

Primary ore fluids responsible for sphalerite could not bedetermined due to the absence of primary fluid inclusions.PS and secondary inclusions that are present in sphaleriterecord a moderate-salinity fluid that is associated with latergenerations of carbonate and quartz mineralisation.

Th and Tm (ice) values of fluids in ankerite veins from theBowland Shale are similar to those for inclusions in stages II–IV minerals in the breccia zone of the Balladoole Formation(Fig. 11b). This similarity and the presence of trace sphaleritein ankerite veins (Fig. 6c) suggests that the Bowland Shalemay have acted as a confining unit overlying the Balladoole

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breccia zone and that the ankerite veins represent the escapeof ore-depositing fluids from the breccia zone.

Low-salinity fluid Low-salinity (<6 wt.% equiv. NaCl)fluids were found as secondary inclusions in ankerite andcalcite veins in the Bowland Shale and are also reportedfrom the Derbyhaven Formation (Beasley 2008). Theyrepresent a temporally distinct fluid not associated with ore-related dolomite or ankerite mineralisation.

CO2-bearing fluids These fluids were found only in non-luminescent quartz veins in the Manx Group (in ore-bearingveins at Foxdale and in massive unmineralised quartz veinsat Bradda). We interpret this fluid to be a metamorphic fluidpresent in the Manx Group that was not involved in thehydrothermal systems in the Carboniferous rocks.

Stable isotope studies

Samples of host rock limestone and dolomite and vein- andvug-filling carbonate and quartz were collected for stableisotope study. Oxygen isotope compositions of carbonatesand quartz, carbon isotope compositions of carbonates andsulphur isotope compositions of sulphides were measured.Quartz was reacted with ClF3 using a modification of theClayton and Mayeda (1963) method. Carbonates werereacted with phosphoric acid at 50°C using an automatedKiel device. Sulphides were prepared using techniquesdescribed by Grinenko (1962). Isotopic data are reported instandard δ notation relative to the Pee Dee belemnite (V-PDB) standard for C and O in carbonates, Vienna standardmean ocean water (V-SMOW) for O in quartz and CanyonDiablo troilite (V-CDT) for S (Beasley 2008). The standarderror of each analysis is ±0.2‰ for δ18O values of quartzand ±0.1‰ for δ13C and δ18O values of carbonates andδ34S values of sulphides (see Supplementary Material).

Carbonates

To provide a framework within which to interpret thehydrothermal overprint associated with the Balladoole brecciazone, regional limestone and early replacement dolomitesamples were collected for stable isotope analysis. Limestonesfrom the Derbyhaven and Balladoole formations have δ13Cvalues of −1.0‰ to 2.4‰ and δ18O values of −9.5‰ to−5.2‰ V-PDB (n=10). Replacement host dolomite of theBaladoole Formation has similar ranges to those of thelimestone (δ13C=0.2–0.9‰; δ18O=−9.0‰ to −6.2‰; n=8).The δ13C and δ18O values of host–rock calcite in theBowland Shale are 1.2–3.6‰ and −11.1‰ to −6.4‰,respectively (n=11; Fig. 12 a, b).

Early dolomite cement in the Balladoole Formation hasδ13C values of 0.2–0.7‰ and δ18O values of −9.2‰ to−5.1‰ (n=8) and overlaps the values for this formation’slimestone and host dolomite (Fig. 12b). Early dolomitecement in the Bowland Shale has δ13C values of −1.8‰ to3.2‰ and δ18O values of −13.0‰ to −8.5‰ (n=6), whichoverlaps and extends to lower values than those of this unit’shost rock calcite (Fig. 12a).

Late dolomite cements in the Balladoole Formation,including stage IV of the breccia zone, have δ13C values of−1.8‰ to 0.9‰ and δ18O values of −16.0‰ to −5.6‰ (n=9). They display a covarying pattern of decreasing δ13C andδ18O values (Fig. 12b). For comparison, dolomite cementsin ores from the Zn–Pb–Cu deposits at Bradda (Fig. 1),

Fig. 12 Plots for δ13C and δ18O values of carbonate minerals in a theBowland Shale Formation and b the Balladoole Formation (stage IVbreccia zone dolomites are included in the late dolomite cements). Asimilar paragenetic trend towards lower δ18O values with laterparagenetic stage is observed in the carbonate cements of eachformation. For comparison, dolomite cements in ores from the Zn–Pb–Cu deposits at Bradda (Fig. 1), hosted in metamorphic basement rocksof the Manx Group, are also shown. These values covary in a similarmanner and overlap those of dolomites from the Balladoole Formationbreccia zone, which may indicate that similar fluids were involved inore-depositing systems in both basement and overlying carbonaterocks

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hosted in metamorphic basement rocks of the Manx Group,have δ13C values of −3.4‰ to 0.2‰ and δ18O values of−15.7‰ to −9.0‰ (n=26). These values covary in a similarmanner and overlap those of dolomites from the BalladooleFormation breccia zone (Fig. 12b). This may indicate thatsimilar fluids were involved in ore-depositing systems inboth basement and overlying carbonate rocks.

Ankerite veins in the Bowland Shale have δ13C valuesof −2.5‰ to 2.2‰ and δ18O values of −18.7‰ to −11.4‰(n=47). The ranges of these δ18O values are 5‰ to 6‰lower than those of limestone and early diageneticcarbonates in this unit (Fig. 12a). Thicker ankerite veinscommonly display ~2–3‰ decreases of δ18O values, fromearlier vein edges (δ18O=−11.5‰ to −12.4‰) to later veincentres (δ18O=−13.3‰ to −14.9‰). Ankerite veinlets thatcrosscut these veins have δ18O values of −14.5‰ to−15.2‰. Trace sphalerite (Fig. 6c) is associated withankerite whose δ18O values are −13.7‰ to −14.5‰.

Calcite veins in the Derbyhaven, Balladoole and BowlandShale formations show large variations in δ13C (−5.8‰ to2.7‰) and δ18O (−17.5‰ to −6.4‰) values (n=34). Theδ13C and δ18O values of calcite vary significantly, and noregional trends were observed. Their large ranges of δ13Cand δ18O values likely indicate multiple sources of calcite-depositing fluids and for clarity they have been omitted fromFig. 12.

Quartz

Quartz was sampled from all stages of the Balladoolebreccia zone (stage I quartz pod and quartz–carbonate clast,stage III rim quartz and stage IV quartz vein). Forcomparison, quartz from veins in the Manx Groupmetasedimentary rocks was also analysed. The δ18O valueof stage I quartz in early pods is 12.5‰ V-SMOW. That ofquartz in an altered clast is 16.9‰. The δ18O value of stageIII rim quartz is 18.3‰ and that of stage IV vein quartz is19.7‰. Higher-temperature quartz (Th~275°C) from a non-CL ore-bearing vein at Foxdale in the Manx Group has a δ18Ovalue of 14.7‰. Lower-temperature quartz (Th~125°C)associated with dolomite in the Bradda deposits of the ManxGroup has a δ18O value of 19.8‰.

Sulphides

The δ34S values of five sulphide samples were determined(cores and edges of two zoned stage II sphalerites and onestage II galena). The δ34Ssphalerite values range from 6.5‰to 6.9‰ V-CDT with no systematic variation between coreand rim. The δ34Sgalena value is 2.9‰. The values forsphalerite are virtually identical to those measured byCrowley et al. (1997) for sphalerite from vein-hosted oredeposits in Manx Group metamorphic basement rocks at

Laxey, Foxdale and Bradda. This similarity is interpreted toreflect a common basement source of sulphur for the ManxGroup- and Carboniferous-hosted deposits (Fig. 13).

Calculated δ18Owater values

Data for dolomite and ankerite show similar trends ofdecreasing δ13C and δ18O values with decreasing parageneticage (Fig. 12a, b). Shifts in δ18O to more negative values arefound within the Balladoole Formation (from early dolomitecement, −5.1‰ to late dolomite cement, −16.0‰) and theBowland Shale (from early dolomite cement, −8.5‰ to lateankerite cement, −18.7‰). These variations in δ18O valuesare too large to reflect temperature variations of a singlefluid, but instead require geochemical difference in fluids,either in their sources or in their history of interaction withrocks and/or other fluids along their flow paths.

In order to assess the relative importance in space andtime of different fluid sources that may have beenresponsible for mineral precipitation (i.e. local brines inthe Carboniferous section vs. basement-derived fluids),ranges of equilibrium δ18Owater values were calculated forδ18O values of carbonates and quartz. Temperatures utilisedin the calculations were based on Th values of primary andPS fluid inclusions, where applicable. For samples in whichfluid inclusion data were unavailable, temperature estimatesrepresentative of that portion of the mineral paragenesiswere used. Although Th values have not been corrected forpressure effects, based on the similarity of Th values andmaximum temperatures estimated from vitrinite reflectance,

Fig. 13 Frequency diagram of δ34S values of sphalerite and galenafrom the Carboniferous-hosted, Balladoole breccia zone (this study)and mining localities in the Manx Group basement rocks (Crowley etal. 1997)

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the temperature correction is thought to be small (<20°C,Potter 1977). This small temperature variation does notsignificantly affect the calculated results or conclusionsdrawn from them. Oxygen isotope fractionation equationsused were the calcite–water equation of O’Neil et al.(1969), the dolomite–water equation of Friedman andO’Neil (1977), the quartz–water equation of Clayton et al.(1972) and the ankerite–water equation of Zheng (1999).

Balladoole Formation δ18Owater values

Calculated equilibrium δ18Owater values of fluids depositingstage I dolomite in breccia clasts and early dolomite cementrange from 1.3‰ to 6.0‰ V-SMOW (avg. 3.0‰, T=115°C).At this same temperature, water in equilibrium with the hostrock calcite of the Balladoole Formation would have aδ18Owater value of 5.8‰. We interpret the calculated watervalues to indicate that fluids responsible for early replace-ment dolomite and early dolomite cements were local(evolved seawater) brines that approached isotopic equilibri-um with the host rock.

The δ18Owater values calculated (at 300°C) for the stage Iquartz pod and for quartz alteration of breccia fragments are9.6‰ and 5.5‰, respectively. The stage III quartz rimyields a calculated δ18Owater value of 2.4 (at 150°C). At thissame temperature, water in equilibrium with the host rockcalcite of the Balladoole Formation would have a δ18Owater

value of 16.0‰.Late dolomite cement, including stage IV in the breccia

zone, displays a large range of δ18Odolomite values (−16.0‰to −5.6‰ V-PDB). Of these, higher-temperature dolomitecement yields calculated δ18Owater values of 3.0–14.3‰ V-SMOW (at 230°C), whereas lower-temperature dolomitecement yields δ18Owater values of 3.9–5.0‰ (at 110°C). Theδ18Owater values in equilibrium with host rock calcite atthese temperatures are 13.6‰ and 5.6‰, respectively. Thelarge range in δ18Owater values calculated for dolomite,compared to those for waters in equilibrium with the hostrock, suggests the presence of multiple fluids, at least oneof which approached isotopic equilibrium with the hostrock. Stage IV vein quartz yields a calculated δ18Owater

value of 8.7‰ (at 230°C).

Bowland Shale δ18Owater values

Early dolomite cement yields calculated δ18Owater values of−3.0‰ to 1.6‰ V-SMOW (avg. −0.2‰, T=110°C). Forcomparison, at this same temperature, water in equilibriumwith the calcite component of the Bowland Shale wouldhave δ18Owater values of 3.3–7.3‰ V-SMOW (avg. 4.8‰).

Ankerite veins in the Bowland Shale yield δ18Owater

values of −0.6‰ to 9.0‰ V-SMOW (T=170°C). At thissame temperature, water in equilibrium with the calcite

component of the Bowland Shale has δ18Owater values of8.3–12.3‰ V-SMOW. The large range in δ18Owater valuescalculated for dolomite and ankerite, compared to those forwaters in equilibrium with the host rock, suggests thepresence of multiple fluids, at least one of whichapproached isotopic equilibrium with the host rock.

Manx Group δ18Owater values

Higher-temperature (>270°C), non-CL, ore-associated veinquartz from the Foxdale mining area (Fig. 1) within theManx Group yields a calculated δ18Owater value of 5.7‰ V-SMOW. Lower-temperature (~125°C), blue brown yellowCL-zoned quartz associated with dolomite at Bradda yieldsa value of 1.4‰ V-SMOW.

Summary of δ18Owater values

Two main groups of δ18Owater values (V-SMOW) areassociated with quartz: (1) Values of 5.5–9.6‰ areassociated with higher-temperature (>230°C) stages I andIV quartz in the Balladoole breccia zone and in basement-hosted, ore-bearing quartz from Foxdale and (2) lowervalues (1.4‰ to 2.4‰) are associated with quartz fromstage III of the Balladoole breccia zone and lower-temperature (~125°C) vein quartz associated with dolomitefrom the Manx Group rocks at Bradda.

Calculated δ18Owater values indicate that at least twofluids affected the Balladoole breccia zone, the overlyingBowland Shale and the Manx Group basement rocks. Thefluids are interpreted to be lower-temperature brine withinthe Carboniferous carbonates and a higher-temperaturefluid of likely basement origin.

The large range of δ18Owater values of the breccia-cementing stage IV dolomite may reflect the renewedimportance of local Carboniferous-hosted brines as thehydrothermal system waned. Infrequently, late faults per-mitted sporadic introduction of basement-involved fluidsinto the cooling hydrothermal system, manifested by stageIV quartz veins.

Conceptual model

A conceptual model of dolomitisation and ore mineraldeposition was created for the Balladoole Formation’sbreccia zone by combining the results of the parageneticstudy, CL, fluid inclusion microthermometry and stableisotope geochemistry (Fig. 14). CL behaviour of quartz,temperature and salinity of its preserved fluids, δ13C andδ18O values of dolomite cements and δ34S values ofsphalerite indicate likely basement contributions to min-eralising fluids in the Balladoole breccia zone. Our

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conceptual model of fluids sourced from a Lower Palae-ozoic basement-equilibrated fluid reservoir infiltrating acarbonate sequence via faults is similar to modelsproposed for the Irish Zn–Pb orefield, though with acouple of key differences. Notably, the cooler fluidimplicated in the Irish-type deposits contributes most ofthe sulphide to the ores via bacterial sulphate reductionand is more saline than the basement-derived fluid(Wilkinson 2003; Wilkinson et al. 2005).

The mineralising and deformational events recognisedin the breccia zone are interpreted to consist of thefollowing: (1) Prior to large-scale deformation events,resident fluids in the carbonate section partially dolomi-tised the limestone. This dolomitisation event is observedthroughout the Carboniferous limestones in the southernIOM and in the breccia zone is preserved within clasts.The controls on this dolomitisation are the subject ofongoing investigations. (2) Quartz pods are thought to berelicts of an originally more continuous quartz vein thatwas dismembered during the major brecciation event.This quartz vein represents early faulting that permittedintroduction of basement-involved brines into the Car-boniferous section. Subsequent episodes of deformationand replacement dolomitisation focused on the faultedarea. (3) Minor galena mineralisation, parallel to bedding,resulted from fault access that allowed basement-derivedfluids to penetrate laterally into more permeable portionsof the host rock. (4) A major brecciation event increasedpermeability and porosity around the fault zone, enablingpulses of basement-derived fluids to deposit zonedsphalerite crystals as open-space fillings. (5) The relativelyimpermeable, overlying Bowland shale may have causedhydrothermal fluids to pond beneath it, resulting in the largelateral extent of hydrothermal replacement dolomite. Ankeriteveins in the Bowland Shale, containing trace amounts ofsphalerite, may reflect escape of these same ore fluids. (6) Asthe hydrothermal system waned, the relative importance of

local brines within the Carboniferous section re-emerged,resulting in late dolomite cementation.

Hydrogeologic model

A sensitivity analysis was conducted, employing a two-dimensional, finite element numerical model representativeof the conceptual model, in order to evaluate a basementcomponent of fluid flow utilising the fault as a fluidpathway (Fig. 15a). The goals of conducting the analysiswere (1) to constrain a minimum hydraulic conductivity offluids necessary to permit upwards fluid movement in thefault and penetration into units bounding the fault and (2) toreconstruct temperature variations imposed by an upwardsflow of higher-temperature fluids of basement origin.

Governing equations

The numerical model was performed using CPFLOW: afinite element, coupled variable-density groundwater flowand heat transport programme developed by Raffenspergerand Garven (1995). CPFLOW evaluates groundwater flowand heat transport based on stream function using a fluidmass conservation governing equation that couples theconservation of groundwater vorticity and conservation ofthermal energy. Heat transport is affected by both advectionand conduction of the matrix, whereas fluid viscosity anddensity are computed by iterations of groundwater flow andheat transport.

Hydrogeologic values of permeability, hydraulic conduc-tivity, thermal conductivity and porosity are not available forthe Carboniferous rocks and the Manx Group basement rocksof the IOM. Values utilised in this study were collected fromtwo sources. UK Nirex Limited (1996) published anassessment report regarding a deep subsurface nuclear wasterepository in Cumbria, England. This report was followed by

Fig. 14 Conceptual model ofthe interaction of basement-derived hydrothermal fluids andresident Carboniferous-hostedsedimentary brines. Arrows rep-resent directions of inferred fluidmigration. Darker shading indi-cates massively dolomitisedareas representing the dolomitehalo surrounding the brecciazone

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an overview of the hydrogeology of Sellafield, an areaapproximately 80- to 100-km east-northeast of the IOM(Chaplow 1996). In these reports, a range of modelled meanhydraulic conductivity values of Carboniferous limestoneswere determined to be 1.0×10−8 to 1.0×10−6 m/s. Thesevalues are the closest approximation of hydraulic valuesinferred for the Carboniferous rocks of the IOM.

Accounting for differences in lithologies and textures ofthe Carboniferous sedimentary rocks, the general modelledhydraulic conductivities from Nirex Limited (1996) andChaplow (1996) were varied up to one order of magnitudefor carbonate rocks, using typical carbonate sedimentaryrock hydraulic values from Freeze and Cherry (1979). Datawere not available for thermal conductivity of the Carbon-iferous or Manx Group rocks of the IOM. Values used forthe simulations were taken from general lithologic descrip-tions of Blackwell and Steele (1988).

Hydrostratigraphy

The hydrostratigraphy of the model allowed for a total ofnine stratigraphic units including a unit defined as a faultbreccia (Fig. 15b). These units correspond to the stratig-raphy defined by Chadwick et al. (2001) (Fig. 3). Toaccount for the vertical flow through the fault, theupwards component of permeability (kz) and hydraulicconductivity (Kz) are assumed to be greater than thehorizontal (Kx, kx) counterpart. All other hydrostrati-graphic units retain a Kx/Kz≥100.

Boundary conditions

Boundary conditions used for the simulation consist of aleft and right boundary of constant heads representing arelatively horizontal hydraulic gradient of −3 m/km acrossthe model. The upper boundary is considered to have a noflow component and a prescribed temperature of 100°C,representing a buried flow system. These boundaries areheld constant through all simulations of the sensitivityanalysis. The lower boundary conditions are the variablesthat have been modified between each simulation. Much ofthe lower boundary is considered no flow where the ManxGroup is in contact with the edge of the model. Where thefault intersects the edge of the model, an increasing rangeof prescribed flow was used starting with 0.1 m/year andthen increasing to 1 m/year. At increments of 1 m/year, theflow rate was increased to a maximum of 5 m/year to assessthe effect that flow through the fault imposes on the steady-state horizontal flow. Heat flow (65 Wm−1 year−1) alongthe lower boundary was held constant through the simu-lations, representing a moderate to moderately high crustalheat flow (Frederiksen et al. 2001).

A limitation of the CPFLOW programme is theassumption that salinity increases with depth. Based onfluid inclusion measurements, moderate-salinity brineswere present within the Carboniferous carbonates and inthe underlying Manx Group basement rocks. Thesalinity of the model is constrained to a small rangefrom 13.5 to 15.0 wt.% NaCl, thought to best approx-imate the system. Governing equations and the hydraulic

Fig. 15 a Finite element mesh generated for sensitivity analysis of theconceptual model shown in Fig. 14. b Hydrostratigraphy used for thesimulation. c Results of sensitivity analysis of basement flow rate of5 m/year showing that fluids migrating upwards through the faultimpose a temperature gradient that begins to change rapidly near thecontact of the Balladoole Formation and the organic-rich, low thermalconductivity Bowland Shale (z=350 to 450 m)

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conductivity and thermal conductivity values used forthe numerical simulation are found in Beasley (2008)and in Supplementary Material.

Simulation results

Results of the simulations indicate that (1) fluids from thefault can penetrate into the surrounding sedimentary rockswith increased flow rates, (2) the heat from basement fluidscan increase the temperature of upper carbonate rocks byadvection to levels observed in fluid inclusion Th valuesand (3) cooling of a hot basement-derived fluid is a viablemechanism for sphalerite precipitation.

For fluids to exit the fault, a minimum vertical hydraulicconductivity within the fault must be ≥1 m/year (Fig. 15c).At increased flow rates, the flow exiting the fault overprintsthe steady-state flow pattern induced by the hydraulicgradient. Flow rates >5 m/year tend to overcome the flowpatterns induced by the hydraulic gradient and result inflow moving horizontally away from the fault parallel tobedding. The upwards flow rates (≥1 m/year) show that theflow patterns in the conceptual model are plausible.

Temperature variations show dependency on verticalfluid flow through the fault. At low basement fluid flowrates, temperature variations appear largely controlled byconduction and show minor advective effects in unitswith high hydraulic conductivities. With higher flowrates (5 m/year), advection is the main cause of heattransport and can increase temperatures from 115°C to160°C within the Balladoole Formation where sphaleritemineralisation occurs (Fig. 15c).

The cooling (from 160°C to 115°C) of the basement-derived fluid that occurs as it migrates across the thermalgradient and outwards into the bounding sedimentary unitsis a plausible mechanism for sphalerite and quartz precip-itation in the breccia zone (Holland and Malinin 1979;Rimstidt 1997; Seward and Barnes 1997). Dilution of salinebasement-derived fluids through mixing with less salinebrines present within the host Carboniferous rocks wouldalso have lowered zinc solubility and contributed tosphalerite precipitation.

The flow dynamics observed in the simulation verify theflow patterns of the conceptual model. These flow patternscombined with the resulting temperature effects on theCarboniferous section indicate that a basement componentto flow is likely in the genesis of the BalladooleFormation’s breccia zone.

Conclusions

The thermal, chemical and fluid evolution of theCarboniferous-hosted, sphalerite-mineralised breccia zone

was deciphered by combining mineral paragenesis and CL,fluid inclusion microthermometry, stable isotope geochem-istry and hydrogeologic modelling. Conclusions that can bereached are:

1. Mineralisation of the breccia deposit is composed of fourdiscrete paragenetic stages, which are marked by episodesof structural deformation and abrupt changes in fluidtemperature and chemistry. Stage I preserves high-temperature (Th>300°C), high-salinity (20.0–45.0 wt.%equiv. NaCl) fluids associated with deposition of aquartz vein that may have penetrated the underlyingManx Group metasedimentary rocks. Pressures duringstage I quartz deposition are thought to be ~200 to 300bars, corresponding to a minimum depth of ~1 km,assuming lithostatic pressure conditions.

2. Based on mineral CL, fluid inclusion studies andstable isotope geochemistry, stage II–IV open-space-filling mineralisation of the breccia zone is thought tobe the result of multiple types of brines. Calculatedδ18Owater values, based on analyses of minerals in thebreccia, indicate two main fluid sources, localCarboniferous-hosted brines (~0.5–6.0‰ V-SMOW)and basement-derived fluids (~5.5–11.5‰ V-SMOW),whose relative importance varied in time. The fluidhistory of the breccia is compatible with the introduc-tion of a hot, basement-derived fluid that interactedwith local sedimentary brines. As the hydrothermalsystem waned, the relative importance of local brineswithin the Carboniferous section re-emerged, resultingin stage IV breccia-cementing dolomite. The δ34Svalues of the breccia’s sphalerite (6.5–6.9‰) arevirtually identical to those of ore sulphides from mineswithin metasedimentary rocks of the Manx Group,consistent with a basement origin for the metal-bearingfluid’s ore constituents.

3. A proposed scenario for sphalerite precipitation in thebreccia zone is cooling and dilution of upwards migrat-ing, ore-bearing fluids from the Manx Group basementrocks. Hydrologic simulations suggest that a moderate-temperature (~180°C), basement-derived fluid at upwardsflow rates of ≥1 m/year could have migrated verticallyalong the fault and penetrated into the surroundingsedimentary units. Advective heat transfer to the Carbon-iferous rocks would have created a thermal gradient,across which metal-bearing brines would have cooled.The overlying, organic-rich Bowland Shale would haveacted as an insulating unit, causing isotherms to be closelyspaced near the contact of the Balladoole Formation andBowland Shale (Fig. 15c), creating a zone suitable forsphalerite precipitation. Dilution of saline basement-derived fluids through mixing with less saline brinespresent within the host Carboniferous rocks would also

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have lowered zinc solubility and contributed to sphaler-ite precipitation. Warm fluids would have pondedbeneath the Bowland Shale, resulting in the wider lateralextent of the hydrothermal dolomite halo. Ankeriteveins, containing trace sphalerite, likely represent escap-ing ore fluids where the Bowland Shale confining unitwas breached by fractures.

4. The similarity of quartz, dolomite cements and fluids inthe Carboniferous carbonate-hosted breccia deposit tothose in basement-hosted sulphide ore deposits sug-gests that basement-hosted deposits also may beCarboniferous age or younger.

Implications for exploration

The Balladoole sphalerite-bearing breccia contains a dolo-mite matrix and is hosted within a massively dolomitisedcarbonate body. The lack of a weathering/erosional contrastbetween the breccia and the surrounding rock resulted inthe absence of any prominent surface expression of thebreccia. This may explain why the extent of sphaleritemineralisation in this portion of the island’s Carboniferousrocks was not recognised previously. It is possible thatother mineralised zones may exist.

Base metal sulphide mineralisation could be presentelsewhere in the Carboniferous section. Similar carbonatefabrics of the Balladoole, Knockrushen and Derbyhavenformations (Fig. 3) indicate that these units are all potentialhosts for ore mineralisation. Detailed studies of geologicstructures and host rock alteration would help to define thelateral and vertical extents of potential mineralising systemsthroughout the Carboniferous section and the underlyingbasement.

Acknowledgements We thank Chris Persellin for his assistance withCL microscopy. We are grateful to Juan Watterson for stimulatingdiscussions of Manx geology and loan of his copy of Mackay andSchnellmann’s (1963) report on Manx ore deposits. A constructivereview by Julian Menuge significantly improved the clarity of thiscontribution. Bill and Morag McIntosh of Roylin House, Port St. Maryare thanked for their hospitality during our several field seasons on theisland. Support for this project was from the Midas Ore Research Fundof the Department of Geological Sciences, University of Missouri.

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