755

The C anadian M ine ralo g i s tVol. 31, pp. 755-7 66 (1993)

CARBON AND OXVGEN ISOTOPE GEOCHEMISTRY OF CALCITEFROM THE JUBILEEZn-Pb DEPOSIT, CAPE BRETON ISLAND

JOHN P. ARMSTRONG AND FRED J. LONGSTAFFEDepanment of Eanh Scientes, University of Westem Ontario, Inndon, Ontarto N6A 587

FRANCES J. TIEINDepartrnent of Geology and Geophysics, University of Calgary, Calgary, Albena T2N I N4

ABSTRACT

The Jubilee Zn-Pb deposig Nova Scotia. is one of many similar prospects hosted within Early Carboniferous limestones inthe Maritime region of eastem Canada At Jubilee, laminated limestone is overlain by transitional, interstratified limestone andanhydrite, with massive anhydrite capping the sequence. Mineralization is stratabound and hosted within the Pembroke brec-ci4 which probably formed as a result of dissolution of anhydrite interbeds within carbonate host-rock. Mineralized-matrixbreccia is the main host of the ore, and contains pyrite, marcasite, sphalerite, galena and barite, in addition to calcite. Liquidhydrocarbons and solid bitumen are abundant, particulmly within sulfide-rich domains, and also occrr as inclusions withinsphalerite and barite. Carbon and oxygen isotope values have been obtained for five varieties of calcite from tle Jubileedeposil (1) nnely laminated, unmineralized host limestone (pelsparite), 6180 between +26.3 and+27,1%o SMOW, 6r3Cbetween +0.4 and +2,2VooPDB, (2) early diagenetic calcite, 6180 = +29,1Vao,6t3C = +l.AVoo, (3) micritic to sparry calcite(replacement of anhydrite?), locally associated with minor, disserninated sulfides, 6180 betrreen +24.3 and +25.5%o,613Cbetween -24.0 and-22.8Voo, (4) main ore-stage, breccia-cementing calcite,6180 between +23.9 and+24.1Vao,613C between4 .l and, JA.9Voo, and (5) post-mineralization, drusy calcite, 6180 between +21.6 and +22.5%o, 6r3C between -23.4 and-23.2%o. Using tle isotopic data for calcite from Groups 3 and 4, and previously reported results on fluid inclusions(90-150"C), an average 6180 value of +9 t 3%a wx calculated for the mineralizing fluid. This value is typical of an evolvedbasinal brine, as has been suggested for other deposits in the area. However, the 6l3C values (-27.1 to -22.87oo) of calcite fromGroups 3, 4 and 5 are much lower than ore-stage calcite from the other deposits in the region (-10 to +3%oo). Anhydrite dissolu-tion, association of hydrocarbons with sulfides, and very low 613C values for most secondary calcite at Jubilee togettrer suggestcoupled oxidation of hydrocarbons, reduction of sulfate and precipitation of calcite at the site of mineralization. Contributionofinorganic carbon (e.9., from dissolution ofhost fimestone) was very limited during calcite precipitation associated with min-eralization.

Keywords: Jubilee Zn-Pb deposit, stable isotopes, hydrocarbons, brines, genesis, Nova Scotia.

Sovn,Ianr

Le gisement dZn-Pb de Jubilee, en Nouvelle-fuosse, est un parmi plusieun indices semblables ayant comme h6te les cal-caires carbonifbres prdcoces des provinces maritimes de I'Est du Canada A Jubile€, les calcaires lamin6s sont recouverts d'unes6quence fansitionnelle interstratifi6e de calcaire et d'anhydrite, et ensuite d'une anhydrite massive. Ia min6ralisation, qui estconforme i la stratigraphie, se trouve dans la brdche de Pembroke, qui serait due h la dissolution des interstratificationsd'anhydrite dans la sfuuence de carbonates. Une brbche i matrice min6ralis6e, roche-hdte de la plupan du minerai, contientpyrite, marcasite, sphal6rite, gal0ne et barytine, en plus de la calcite. Irs hydrocarbures liquides et le bitumen solide y sontabondants, surtout dans les amas de sulfures, et sont de plus inclus dans la sphal6rite et la barytine. Nous avons d6termin6 lesvaleurs isotopiques du carbone et de l'oxygdne pour cinq varidtds de calcite: l) roche-h6te non mindralis6e, tr laminations fines(pelsparite): 6180 entre +26,3 et+27,lVoo SMOW, 613C entre +0.4 et2.2%oo PDB,2) calcite diag6n6tique pr6coce, 6180+29.lVoo,6t3C l.4Voo,3) calcite micritique I transparente (due i la dissolution de 1'anhydrite?), localement associ6e d de faiblesconcetrfations de sulfures diss6min6s: 6180 errtr.e +24.3 et25.57oo,613C entre -24.0 et-22.8%o,4) stade principal de formationdu minerai, ciment calcitique de la brbche: 6180 entre +23,9 et +24J%o, 6l3C entre -27 .1 et -24.9Vo0, et 5) calcite tardive parrappoft a h min6ralisation: 6180 entre +21.6 et+22.5/oo,613C entre -23.4 et23.2Voo. D'aprbs les donn6es isotopiques pour lacalcite des groupes 3 et 4, et les r6sultats d'6tudes ant6riewes des inclusions fluides (9G-150'C), nous pr6conisons une valeurmoyenne de9 + 3Voo pour le rapport 6180 du fluide responsable de la min6ralisation. Cette valeur est typique des saumures6volu6es des bassins s6dimentaires, et des autres gisements de cette r6gion. Toutefois, le rapport 6l3C de la calcite des groupes3 , 4 et 5 (entre -27. I et -22,8%o) est beaucoup plus faible que dans la calcite typique des assemblages min6ralis6s des autresgisements de la rdgion (entre -10 et +3%o). La dissolution de fadydrite, I'association des hydrocarbures avec les sulfures, etles valeurs trbs faibles du rapport 6l3C pour la plupart de la calcite secondaire d Jubilee t6moigneraient d'un couplage de l'oxy-dation des hydrocarbures, la r6duction du sulfate, et la pr6cipitation de la calcite au cours de tra min6ralisation. La contributionde carbone inorganique, d0 par exemple d la dissolution du calcaire encaissant" aurait 6t6 trds limit6e pendant la formation de lacalcite associ6e i la min6ralisation.

(Iraduit par la R6daction)

Mots-c16s: gisement de Zn-Pb, Jubilee, isotopes stables, hydrocarbules, saumures, genbse, Nouvelle-Ecosse.

756 THE CANADIAN MINERALOGIST

Iurr.oouc"noN

Carbon and oxygen isotope data are presented herefor secondary calcite from the Jubilee Zn-Pb deposit,Nova Scotia (5.2Vo Zn, L.4Vo Pb;Hern et al. l99l).This deposit, located on the Iona Peninsula of CapeBreton Island. is one of manv Zn-Pb or Pb-Zn. ' ' ' :prospects within the Early Carboniferous (Visean-aged,34&-336 Ma) limestones of the Maritime regionin eastern Canada (Fig. 1). These occurrences haveattracted much attention because of their variablestyles of mineralization, their association with evapor-ite sequences, and their apparent genetic differencesfrom more typical Mississippi-Valley-type deposits.Recently, it has been suggested that some of the min-eralization was associated with deep-water vent com-munities (vonBitter et a/ 1990).

Stable isotope data have been reported for similarcarbonates from the Gays River, Pembroke andSmithfield Pb-Zn deposits (Fig. 1) in the Maritimes(Akande & Zentilli 1984, Ravenhurst et al. 198'7,1989). These authon concluded that (i) the mineraliz-ing fluids were hot, basinal brines, (ii) the metals were

derived from adjacent clastic rocks of basement prove-nance, (iii) Visean carbonate was the dominant sourcefor carbon and sulfur. but sulfir also was derived fromthe voluminous Mississippian evaporites within tlesection, and (iv) sulfate was canied by the fluids andreduced at the site of mineralization. Mineralizationlikely occurred circa 330 to 300 Ma, as indicated byresults of fission track. K-Ar and Rb-Sr datins(Ravenhurst et aL.7989, Arne et al. L990).

Evidence for chemosynthetic communities (e.9.,fossil tubes, serpulid and spirorbid wonns, conularids,brachiopods, bryozoa and ostracods) at hydrothermalvents associated with carbonate mounds has beenreported by Schenk et al. (1990) and von Bitter et al.(1990); the associated sulfide mineralization has beeninterpreted to result from hydrothermal venting duringmound formation. Such mounds have been reported inparts of Macumber Formation of the lower WindsorGroup in Nova Scotia and New Brunswick, theCodroy Group in Quebec and Newfoundland, and theParleeville Formation in New Brunswick (von Bitter& Plint-Geberl 1982, Dix & James 1988, 1989,McCutcheon 1988, von Bitter et al. L990).

LATt cARBol{lttR0us . PtRfitAilPtoou, cuutmuID, ilv[nsDAlj AltD $ils(l ct(lunF]ffiffiEit

[l#ffiffiii conglomerote, sondsfone, mudstone, cool

UTE TAUITS

TARIY GRBOI{IFTROUSlvilrDsoR 0n0uPI hoiite, onhydrito, rimestono,I oororo[oi slloslono, nuoslono

DEVoNIAil - (ARB0lilttR0utP +. ,..<l r0nrl0n01!10. sondslon8.P:;.'51 mudstone, voltoolr roclr

HADRYNIAH . MIDDIT DTI,|)IIIAIIE;ffiETEl molomorphk, Inlrutlyo, onr0slvoFr*.}u.;l.r$l ond sedlirentory rockt

Boyorry ef : : , i ;

ftc. 1. General geology and location of the Jubilee (I), Smithfield (S), Pembroke (P) and Gays River (G) deposits (afterBoehner er aL 1989).

'II{E JUBflFF ZrPb DEPOSIT 757

At Jubilee, no fossil tubes have yet been identified.We present evidence here that (i) supports a basinalorigin for a rather low-temperature (-100-150"C)metal-bearing brine, and (ii) stongly favors an organicsource of carbon (hydrocarbons) for secondary calcitethat crystallized as one product of. in situ reduction ofsulfate during mineralization.

GBot ocv oF THE IIBILEE DBPosrr

The Jubilee deposit was first explored in the early1930s. Further exploration during the late 1930s andmid-1940s failed to delineate a substantial orebody

(I{etn et al. l99l). The most recent efforts in explo-ration consisted of extensive diamond drilling duringthe 1970s by the Texam Joint Venture, and byFalconbridge in 1990. The deposit contains >0.5 mil-lion metic tonnes of 6.67o Ztu-Pb.

The Jubilee deposit is hosted within EarlyCarboniferous (Visean-aged, 348-336 Ma), laminaledlimestones of the Macumber Formation, in the basalportion of the Windsor Group (Ravenhurst et al. 1989,Hein et al. L99l). The Macumber Formation has asharp disconformable contact with the underlyingTournaisian-aged (360-350 Ma) Horton Group(Hamblin 1988, L992; Ravenhurst et al. 1989). In the

cs"rc'si{

JUBil.tE

r r-.-9un

WEIffiE[mm

0

- 5 0

100

150

\

\ .'

lnoAp rAUrTl4J'59'00'

Anhydrile

Irllsfllon zone

llmestono hrertlc

lomlnolsd llnorlons

(onglomorolo

> 3.5X Pb - Znovor 2.5 nelre:

0verburdon

60'57',30"

--- Foult

a Dlonond drlll hole| 000

60.56'30'

t00 Eolroi

Fro. 2. Surface geology and cross-section of the Jubilee tleposit (modified after Hein et al. l99l). Samples were analyzed from

cores ATG 51-78. ATG 19-77 and ATG 26-.77 .

758 TIIE CANADIAN MINERALOGIST

vicinity of the Jubilee deposit, the Horton Group iscommonly recognized as the oouppero, Horton orAinslie Formation (Hamblin 1988); drill-core intersec-tions consist of ungraded to massive conglomerate,with pebbly sandstone (Hein er al. 799L). The top ofthe Macumber Formation consists of interstratifiedlimestone and anhydriie, capped by massive anhydriie(Fre.2).

Sedimentary features within the MacumberFormation suggest that the limestone was depositedbelow wave base and mav have been resertimented bvslumping, debris flows or turbidity curents (Hein eral. l99l). The finely laminated limestone containssome beds with high concentrations of peloids, bio-clastic debris (molluscan and bryozoan shell frag-ments), and very fine-grained, disseminated pyrite.Pyrite-marcasite sulfi.dization along bedding planes isrestricted to more micritic and sparry laminae in theIimestone.

The Pembroke breccia is the major host of the oreat the Jubilee deposit, and lies structurally above, orreplaces, the Macumber Formation (tlein er at. 199D.The breccia has been interpreted as a karstic collapse-breccia, an evaporite-solution breccia, an intraclaststorm deposit, and the product of hydrofracturingrelated to nrineralization (Clifton 1962, Schenk 1984,Ravenhurst et al. 1989). According to Hein et aL(1991), the brecciation ar Jubilee mostly resulted fromdissolution 6f anhydrite laminae. Since remnants ofadydrite have not been observed in the breccia" thisinterpretation requires that dissolution or replacementof alhydrite was complete. Localized occurrences ofkarstic collapse-breccia were also noted by Hein et al.(1991), particularly in the northwestern portion of thestudy area (drill-core ATG-5 L--7 8, Fig. 2).

Two main breccia rock-types have been defined atJubilee by Hein et al. (1991): "rock-matrix'o brecciaand "mineralized-matrix" breccia. o'Rock-matrix',breccia contains clast-supported, chaotically distrib-uted, centimeter-sized, subrounded to angular frag-ments of the Macumber Formation. Most clasts retaintheir original laminated texture, and have not been sie-nificantly recrystallized. According to Hein et il.(1991), the clasts have fallen into space created by dis-solution of evaporite interbeds and interlaminae.Matrix material consists most commonlv of finelvcomminuted rock fragments derived from th!Macumber Formation.

The rock-matrix breccia is generally befter devel-oped in cores that contain some mineralization. Evenmore highly mineralized portions of the deposit, repre-sented by most of the material described in this paper,consist of oomineralized-matrix" breccia. In the sam-ples studied here, the rock fragments have been exten-sively recrystallized, and commonly contain finelydisseminated sulfides. Anhydrite is absent. Truematrix material was not observed. Instead" the recrvs-tallized fragments are cemented by equang sparry cal-

cite or fibrous and locally radiaxial-fibrous calcite(0.05 to I mm), together with sulfides.

Circular structures (on the millimeter scale) also arepresent in some breccia samples. Some of these fea-tures arise from concentric alteration of inclusion-poorand slightly finer-grained, inclusion-rich aggregates ofcalcite crystals. Others result from filling of vugs byequigranular, fibrous calcite capped by sulfides (see"Mineralizationo'). No textural evidence was obtainedto suggest that ttrese features originated as biogeniconcohtes or were related to the formation of bacterialmats associated with deep-water vents.

The Jubilee deposit is cut by a series of NNW-trending fault zones, of which the Jubilee and RoadFaults are the most significant (Fig. 2). These faultsbound a NW-frending horst. Interfingering of HortonGroup and Macumber Formation lithologies, and dra-matic changes in bed thickness and facies within bothcarbonate and clastic rocls near the Jubilee Fault, sug-ge$t that synsedimentary movement occurred duringdeposition of the uppermost Horton Group and contin-ued during formation of the Macumber Formationlimestones. The Jubilee Fault remained active duringdeposition of the interlaminated limestones and anhy-drite, but was inactive during deposition of the thickanhydrite that caps the sequence (tlern et al. I99l).

The Pembroke breccia is associated with a decreasein abundance of the intersfiatified limestone-anhydritesequence, an observation compatible with preferentialdissolution ofthe anhydrite interbeds, and the "reaccu-mulation" of undissolved limestone. Although thePembroke breccia is not a fault breccia, the unit isthickest and most heavily mineralized adjacent to theJubilee Fault. Hence, the location of the fault mayhave played a secondary role in the creation of tlebreccia. Hern et al. (1991) proposed that the JubileeFault controlled evolution of the Jubilee depositthroughout its history. Fluids venting along the faultduring sedimentation provided nutrients that led toenhanced accumulation of biogenic carbonate duringsedimentation; later, during diagenesis, the fault acledas a conduit (confined by the impermeable cap ofanhydrite) for formation waters, hydrocarbons andmineralizing fluids that entered the system.

Mxmal-zanoN

The mineralized-matrix breccia is the main host tothe ore. Besides calcite, it contains: (i) pyrite inter-grown with and replacing marcasite, (ii) zoned pyrite,(iii) honey-colored sphalerite that occurs as fine-grained disseminations within sparry calcite cementand recrystallized fragments Gigs. 3q b), and that ismost commonly associated with galena and baritealong fractures and in clots (Fig. 3c), and (iv) pydte -marcasite - galena intergrowths that cap vugJining,sparry to fibrous calcite. Fyrite can also occur as sub-hedral grains within recrystallized fragments of the

T'IIE JUBn FF Zr-Pb DEPOSIT

host rock. Barite is paragenetically late, and common- matter is present throughout the section, particularlyly occurs as a wispy filling of pore space, together within sulfide-rich domains. Hydrocarbon and bitu-with sphalerite and gAena, or in late veinlets. men inclusions are present in sphalerite and barite

Liquid hydrocarbons fill much of the remaining (Hen et aI. 1,99L), indicating that an organrc liquidporority, iniluaing joint planes, and solid bituminous entered the system at least as early as the begiming of

mineralization.Five varieties of calcite have been recognized in the

rhin s991ieas. From earliest to latest, these groups are:(1) unmineralized pelsparite (Fig. 4a), (2) early diage-netic calcite that comprises the circular microstruc-tures (Fig. 4b), (3) micritic to sparry calcite (pseu-dospar) (Fig. 4c), (4) f ibrous and locallyradiaxial-fibrous or sparry calcite that cements ttrebreccia, and is most strongly associated with sulfidemineralization (Figs. 3b, 4d, e), and (5) medium- orcoarse-grained, drusy calcite that postdates mineraliza-tion (Fig. 4f). Dolomite, in paticular the while sparryvariety typical of other Pb-Zn deposits in the regionand MississippiValley-type deposits in general, wasnot observed.

759

Fro. 3. a. Barite (brt) associated with sparry calcite (cal) and' sphalerite (sp), plane-polarized light. b. Sparry calcite

cement (Group 4) witl interlocking sphalerite (sp), plane-polarized light. c. Ore-stage sphalerite (sp) and galena(gn) with gangue comprised ofpyrite (py), barite and cal-cite; calcite and barite appear as the darkest gley tones inthis reflected-light photomicrograph. Scale bar in all pho-tomicrographs: 0.50 mm.

Ftc. 4. a. Unmineralized pelsparite with bioclastic debris(Group 1). Scale bar: 0.50 mm. b. Portion of an early dia-genetic, circular microstructure that contains Group-2 cal-cite. Scale bar: 0.50 mm. c. Pseudospar (Group 3) andassociated pyrite that probably replaced a ftagment oflimestone. Sparry calcite (Group 4) is present in the lowerright of the photomicrograph. Scale bar: 0.50 mm. d.Interlocking sparry calcite cement (Group 4) and brec-ciated Group-3 pseudospar. Scale bar: 0.50 mm. e.Pseudospar (Group 3) and sparry calcite cement (Group4). Scale bar:0.25 mm. f. Portion of a wg lined by late,drusy calcite (Group 5) formed within Group-3 calcite.Scale bar: 0.25 mm. All photomicrographs in plane-polar-ized light.

760 TITE CANADIAN MINERALOGIST

ANALY'ICAL MEnIoDs

The carbonate host-rock and the various groups ofsecondary calcite were sampled from thin sectionslabs using a high-speed precision drill equipped witheither a burr or diamond-tipped bit. The bit size (L ta2mm) was appropriate to prevent contaminationbetween different types of calcite during microsam-pling. Mineralogical purity of separates was checkedusing X-ray diffraction.

For isotopic analysis, 10-15 mg samples werereacted with orthophosphoric acid at25"C for twelvehours in evacuated tubes (McCrea 1950). The evolvedCO, was purified using cold traps at the appropriatetemperatures to eliminate possible contamination fromHrO and SO2. The carbon and oxygen isotope datawere obtained using an DAAC-upgraded VGMicromass 602C mass spectrometer, and are reportedin the usual b-notation relative to V-PDB (carbon) andV-SMOW (oxygen).

RBsurrs

The stable isotope results for calcite are listed inTable I and illustrated in Figure 5. Three samples offinely laminated pelsparite (Group l) from outside theore zone have 6180 values of +26.3 ta +27.1%o (mean+26.8Voo), and 613C values of +0.4 fo +2.2%o (mean+l.3Voo), and lie in the middle of the range for "unal-tered" Mississippian marine carl;onates (Keith &Weber 1964, Wadleigh &Yorz;er 1992).

6lrs samFle of Group-2 calcite (circular microstruc-tures) has values of 6180 (+29.1%o) and 6l3C (+l.4Voo)similar to the carbonate host-rock Because the Group-

2 calcite is banded on the <0.5 mm scale (Fig. 3b), themicrosample analyzed contains several laminae. Asecond sample has a similar 6180 value (+26.3%o),buta much lower 613C value (-I2.5%o). Re-examinationof this sample showed that the Group-2 microstructurehad been brecciated, and partially ffilled by sparrycalcite from Group 4.

The micritic to spatry calcite (Group 3) has valuesof 6160 (+24.3 to *25.5Voo, mean +24.7%o) and 613C(-24.0 to -22.8Voo,mean -23.3%o) that cluster togethertightly. The 613C values are much lower than thoseobtained for other deposits from this region @g. 5).The Group-4 calcite cement, which is most stronglyassociated with sulfide mineralization, has similarvalues of 6180 (+23.9 to +24.7%0, mean +24.2%oo) toGroup 3, but the 6l3C values (-27.1 ta -24.9%oo, mean-26.2Voo) are lower. Carefirl microsampling ensuredthat the difference in isotopic composition betweencalcite from Groups 3 and 4 is real. The late-stage,drusy calcite (Group 5) has values of 6180 (+21.6%o)and 6r3C (-23.2%0) that plot at the end of the generaltrend of increasing 613C with decreasing 6180 exhi-bited by Group4 calcite (Fig. 5). Calcite inrergrownwith barite in one sample (J151-90A) also containspyrite, and traces of sphalerite, but texturally it is mostclosely associated with Group 5. Its values of 61EO(+22.5Voo) and 6r3C (-23.47oo) support this interpreta-tion.

Dscussrow

Pb-Zn mineralization within the extensive EarlyCarboniferous limestones extends from central NovaScotia northeastward to Newfoundland, and represents

TABLEl. OXYGENANDCATAoNISOTTOPB@MPOSnONS OFCA!trIB

S@pb 'Mhsatqg D6crtdor GNp drc96o atetu

J1S/L90A €L tr qtz wJ154-908 eltrritziriJ1s+90c @ttr.iqii

nmincll?.d pElspafte I 26.9 22rmh4f iad EhDsiE 1 3 (l4,mimfi4diEldEito 1 n,l L4

@lydiagffiticebne hw cElsdurt$ 2 A.I X.4

@lydiage@dc@hehmtricdqos0rEt6+spanyelolte 2.4 263 -125

spatyabibm6lsparyebile @tspaltyebiE @!Ear'alcite@tspsry @lcite @@1fibous b EdiEisl-fibmB @lctbfflma to ndiryial.fibou elc&efibl@ to radiryist-fihw alcie

drEv @loitevug:htng sporry elctte

J152.908 qllrpy

J152-90A @La!y

J15740A el,rqEJ157.90D @LtdlzJ15090 @lJ153-90A @1, rqtz

J15?-90B etJ157-90C @lJ15940A eln59.CIB eL trqEJ159-9rc @L rdtzJ151-908-i @lJ151-98-n €lJ151-908-iii ql

ATG-51-78 elJ151-SA €l,py,b!|sp

u.a .233255 -?4.0u3 -23.Oa2 -2,"a

8,9 -U.9.6 -269

4.7 -25.6u.6 -27.ru.7 -263B A - 2 44.4 -2622A,O -2Ss

21.6 -42n5 -4.45?

Grcup 1: mimllzed lircbB GruD 2 dia@etic qlcitq Grcup 3r 'systalllEd"timeclm h beia; Gm4: orcjseecicile: Gmm5i oolonru-lhiuelcit&E E trceB @lE 6lciq qe. ruita w = wie bn .'farir: so j8Dhaktritg,b - -'},fir@logr of Ei@ilrptoi*ed ftir'boilipic imtFis, odh;mii€d by X-Ey difradi@

TIIE JUBT.F:8 Zr-Pb DEPOSIT

carbon

",*[!]iliJ**' -(.dffi6 o*'

Pembroke ./ x{&UllMffiXW

761

-5

-10

-15

-20

-25

ore-stage calcite

Smithfield-+>

a significant, post-Acadian (circa 300 Ma) metallo-genic event or events (Ravenhurst et aI. 1989, Atne etal. 1,990, Savard 1991). The deposits formed withintransgtessive deep or shallow marine limestones thatdisconformably or unconformably overlie continental-ly derived clastic rocks. The mineralogy of the ore andgangue is simple, with little variation among deposits.Galena and sphalerite are tle main ore minerals. Thegangue consists of pyrite, marcasite, calcite, barite,anhydrite, selenite and fluorite.

The Jubilee deposit can be usefully compared withthe Gays River, Pembroke and Smithfield Pb-Znaccumulations (Frg. 1). The Gays River deposit (-12million metric tonnes of 7Vo Pl>-Zn) is hosted withinthe flank of a dolomitized carbonate bank that uncon-formably overlies quartz metawackes of fte MegumaGroup, and is overlain and bordered by beds of anhy-drite. Ore zones at Gays River are dominated by strati-forrn, disseminated, and locally massive domains ofsulfide that cut lithological contacts and follow car-bonate textures as open-space fillings and sulfidereplacements (Akande &Tnntil\ 1984, Kontak 1992).The Pembroke Pb deposit is characterized by galenathat occurs in calcite veins, and by post-ore, vugJiningcalcite (Ravenhurst et al. L989). The Smithfielddeposit (0.5 million metric tonnes of 6Vo Pb-Zn) ismost similar to the Jubilee; it is a Pb-Zn-Cu-Baocqurence hosted within the brecciated limestone ofthe Lower Windsor Group that disconformably over-

FqAs

NO(a

a

calcite

15 2A 25

o18o %o sMowFlc. 5. Carbon and oxygen isotope compositions of Group-l to -5 calcite samples from

the lubilee deposit. Fields for host carbonate and ore-stage calcite from the GaysRiver. Pembroke and Smithfield deoosits also are shown (from data in Ravenhurst eral.1989).

fh+-30 t

0 10

lies the clastic rocks of the Horton Group @avenhurstet aL.7989).

Savard (1992) has suggested that diagenetic, pre-mineralization dolomite precipitated in the Gays RiverFormation from saline brines at 60-105"C. Fluio-inclusion data for ore-stage calcite at Gays River sug-gest that temperatures peaked at 767-178"C; lowertemperatures (1.32-165"C) were obtained for fluidinclusions from post-ore calcite, fluorite and barite(Akande & Zentilli 1984, Ravenhvst et al. 1989).Higher temperatures are indicated for the Pembrokedeposit (160-250'C) (Ravenhurst et al. 1989). AtSmithfield, crystallization of ore-stage calcite occurredat 140 to 172'C (Ravenhurst et al. 1989). In all cases,fluid-inclusion data indicate that the ore-forming flu-ids were saline (2 24 equivalent wt.7o NaCl) (Akande& Zentrlli 1984, Ravenhurst et al. 1989). The 6180values calculated for the ore-forming fluids (uncor-rected for salinity) using the temperature data andcompositions of the host calcite (after Friedman &O'Neil 1977) are typical of basinal brines: Gays River+3%o, Pembroke +lMoo,and Smithfield +8%o.

Only limited fluid-inclusion data are available forthe Jubilee deposit. Stewart (1978) reported tempera-tures in excess of 300'C for sphalerite and barite.Howevero Hern et al. (1991) argued that fluid/vaporratios for two-phase inclusions in sphalerite are muchmore consistent with those known for Gays River andPine Point, and indicate temperatures of crystallization

carbonate

AtrA+rx

Group 1Group 2Group 3Group 4Group 5Group 5?

762 TIIE CANADIAN MINERALOGIST

of approximately 150"C. Furthermoren their primaryfluid-inclusion measurements for late-stage barite atJubilee give an average temperature of homogeniza-tion of 89'C (range 82-98'C; Hein et al. L99L).If ore-stage calcite (av. 6180 +24.2Voo) crystallized between90'and 150'C, 6180 values (uncorrected for salinity)of +9 t 37oo can be calculated for the brine. Such val-ues are most like those obtained for the Smithfield andPembroke deposits, rather than Gays River. TheJubilee and Smithfield deposits, in particular, show thegreatest similarity, not only in temperature of ore for-mation, calculated composition of the fluid, and stableisotope composition of the ore-stage calcite Gtg. 5),but also in size (-0.5 million metric tonnes), grade(67o combined,Pb-Zn) and style of mineralization(i.e., hosted by limestone breccia).

Organic carbon and in situ reduction of sulfate

The available 613C data for the Gays River,Smithfield and Pembroke deposits (-10 < 613C <+37oo, Fig.5) contrast sharply with the much lowervalues (-27.1 to -22.8%o) obtained at Jubilee for cal-cite from Group 3, Group 4 (main ore-stage) andGroup 5 (post-ore). The former results have beeninterpreted to indicate variable addition of organic car-bon to the inorganic carbon involved in crystallizationof ore-stage calcite (Ravenhurst et al. 1989). However,only the Smithfield deposit shows a consistent shifttoward the strongly negative 6l3C values that charac-terize the Jubilee deposit, and that indicate emergenceof organic carbon as the dominant reservoir durinsformation of ore-stage calcite.

Low-613C values [ke those observed at Jubilee mayindicate that calcite formed by oxidation of methaneseeps on the sea floor. For example, Matsumoto(1990) reported values of -33.2 to -26.1.Va0 for calcitepresumed to have formed by microbial oxidation ofmethane at a hydrocarbon seep offshore of BaffinIsland. However, because ofthe very low 6l3C valuesof biogenic methane, carbonates resulting frommethane oxidation generally have much lower andcommonly more variable values: Gulf of Alaska,-53.2Voo (Barnes et al. 1982), off-shore New Jersey,40 to 49%o (Hathaway & Degens 1969), offshoreOregon and Washington, -66 to -22Voo (Ritger et al.1987), and the North Sea, -62 to -52%o (Hovland eral. L987). Bicarbonate produced during anaerobic oxi-dation of methane may have even lower values of 613C(by 2 to l{Voo) than the methane from which it formed(lVhiticar et al. 1986, Raiswell 1987).

Mixing of CO2 produced by (i) oxidation ofmethane and (ii) dissolution of host carbonate (613C =+2Voo) could produce the depletion in l3C at Jubilee,but only if an invariant and forhritously appropriateproportion of carbon were contributed from eachreservoir. A more appropriate source would be oxida-tion of liquid hydrocarbons, which typically have

average 6r3C values near -28%o (Stahl 1979). In ananalogous situation, 613C values as low as -26.6 to-25.8%o are known for calcite cements in the Trentonlimestone, northwestern Ohio, a unit that is a majorproducer of oil and gas, and also contains small occur-rences of Pb-Zn mineralization (Haefner et al. 1988).

At Jubilee, the physical evidence for dissolution ofpre-existing laminae of anhydrite plus the ubiquitousassociation of hydrocarbons with sulfides collectivelysuggest that the low 613C values of Groups 3,4 and 5calcite may have resulted from coupled reduction ofsulfate and oxidation of hydrocarbon at the site ofmineralization. This reaction probably occurred in afashion similar to that described bv Anderson &Garven (1987),

CaSOa + CHa = [r5 + CaCO3 + HrOo (1)

where CH. is used only in a general sense to representhydrocarbons. Whether the reaction at Jubilee wasmediated by microbial processes is not obvious fromthe data that we have collected..

Ghazban et al. (1990) invoked such a reaction toexplain 6l3C values as low as -L2%o for white sparrydolomite associated with mineralization at theNanisivik Pb-Zn deposits, Baffin Island. However,following the model of Anderson & Garven (1987),they noted that the carbon incorporated into the sparrydolomite was derived from two sources, (i) hydrocar-bons (6r3C = -28Voo) and (ii) host dolostone (613C =+2%o). Dissolution ofthe dolostone host proceeded asa consequence of acidity produced during the sulfate-reduction process:

CaMg(Co3)2 +2H,tCot=C*+ + Mg2*+4HCo3- (2)(host dolostone)

Hence the carbon-isotope composition of sparrydolomite that ultimately precipitated in this settingreflected the proportion of CO2 derived from eachsource plus the dolomite-CO, fractionation at the tem-perature(s) of crystallization. This model (in modifiedforrn) could be invoked to explain the carbon-isotoperesults for ore-stage calcite from the Smithfielddeposit.

However, the much lower 6l3C values of Groups 3,4 and 5 C27.7 to -22.87oo) from the Jubilee depositstrongly suggest that crystallization of this calciteoccurred within a largely closed system with respect toinorganic carbon. Carbon was derived almost entirelyfrom hydrocarbons with little input of inorganic car-bon from host limestones (av. 6l3C +l.4Voo). Whereascoupled oxidation of hydrocarbons and dissolution-replacement of sulfate laminae in the host rock mayhave been closely associated with formation of thelow-l3C calcite at Jubilee, it is not necessary or evenfeasible to invoke large-scale dissolution of the hostlimestone.

Anderson & Garven (1987) have noted that gypsumor ar:hydrite can be replaced directly by calcite at low

TIIE JUBILEE ZrPb DEPOSIT 763

temperatures (<200'C) provided that the sulfate issimultaneously reduced to HrS, and that the H2S is notprecipitated immediately by base metals. Such a sce-nario could explain the 613C values (av. -23.3%oo;Fig.5, Table 1) for the Group-3 calcite, most of whichcrystallized prior to the main stage of sulfide precipita-tion. Assuming a 613C value of about -28Voo for theCO2 produced from complete oxidation of the hydro-carbon, the Group-3 calcite results are consistent withthe equilibrium carbon-isotope fractionation betweencalcite and CO2at about 100'C (10001n4 = *5Voo:Dernes et al. 1974).

The shift to the still lower 613C values (by -2 to3%o: Fie.5) of the main ore-stage calcite (Group 4)may be an artifact of the small sample-population, butit may have further significance. Anderson & Garven(1987) noted that precipitation of metal (M) sulfides insuch deposits is accompanied by the liberation of H+,

ilIz* +HrS =2I]+ + MS, (3)

which, in turn, could cause dissolution of carbonatehost-rocks (and creation of solution-collapse breccias).At Jubilee, however, the very low 6l3C values ofGroup-4 calcite (av. -26.6Voa; Table l) probably ruleout substantial involvement of carbon from suchsources (a.g., Group-l or -2 limestone; 613C between+0.4 and +2.2%oo). Instead, release of H+ may havecaused species such as HCO; to become a dominant

product of oxidation of hydrocarbon in the ore fluids.Within the limits of current understanding @mljcb etal. 1970. Deines et al. 1974, Mook e/ al. L974,Robinson 1975), a carbon-isotope fractionation(1000lncr) of about +I to 37oo can be anticipatedbetween calcite and HCO; at temperatures typical ofore deposition at Jubilee. Ore-stage calcite (1.e., Group4), with a 613C value of about -26Voo, would be theexpected product of complete (-28Voo) oxidation ofhydrocarbons.

The very low 613C values of the post-ore, Group-Scalcite (-23.4, -23.2%o) are very similar to thoseobtained for the Group-3 calcite. Whereas this stage ofcalcite precipitation also was dominated by carbonmostly derived from hydrocarbons, cessation of sul-fide precipitation may be reflected in the return toslightly higher values of 613C. In this case, however,other explanations rnay be more appropriate, since theGroup-5 calcite has significantly lower 6rEO values(+21.6, +22.5Voo) than either Group 3 or 4 (+23.7 to+25.5Voo) (Table 1), and plots on a trend that describesthe inverse relationship between the 613C and 6180values of Group-4 calcite @g. 6). A similar correla-tion also exists for Group-3 calcite (Fig. 6).

This behavior may indicate that the system was notentirely closed to carbon of inorganic origin. Theobserved trends on Figure 6 can be explained by inuo-duction of a small fraction of low-l8O water that car-

F.{

\aU(a

a

-22

-23

-24

-25

-26

-27

+

a Group 3+ Group 4I Group 5x Group 5?

+ +

?a2726252423222I-?3 -

zo

o 18o

7ao SMOW

Flc. 6. Carbon and oxygen isotope results for Groups-3, -4. and -5 calcite samples fromthe Jubilee deposit. Linear regression lines are shown for Groups-3 and -4 calcite.The two data points for Group 5 (post-ore, drusy calcite) plot at the low-rdO, high-"6end of the general trend observed for Group 4 (main ore-stage calcite).

7U TIIE CANADIAN MINEMLOGIST

ried carbon of inorganic origin (i.e., 613C = Moo). If so,the results for Group-S calcite suggest that externalconfibution of inorganic carbon (carried by surface-derived meteoric water), while remaining very small,increased from Group 4 to 5. Perhaps notably, theGroup-5 sample is from the drill-core (ATG-51-78)that intersected the portion of the Jubilee deposit inter-preted by Hern et al. (L991) to be a karstic collapse-breccia (Fig. 2). Alternatively, similar panerns forsiderite from the Keno Hill Ag-Pb-Zn district wereinterpreted to reflect consumption of water during pro-duction of CO2 and CHa from graphite in a closed sys-tem (Lynch et al. 1990). A modified version of thisscenario (involving liquid hydrocarbons rather thangraphite) might be possible at Jubilee, at least duringthe latter stages of calcite crystallization, once thelocal supply of sulfate (i.e., anhydrite) had beenexhausted.

Although insufficient data exist to distinguishamong these and other possibilities, we can note thatthe lower 6180 values for Group-5 calcite cannot beeasily explained simply as a temperature effect. Fluid-inclusion data for Jubilee (i.e.,for late barite) suggestthat temperafures were declining (rather than rising)following mineralization, similar to the patternobserved for post-ore calcite at Gays River, Pembrokeand Smirhfield deposits (Akande & Zentilli 1984,Ravenhurst et d/. 1987, 1989).

CoNclustoNs

The limited scope of this study does not permit pre-sentation of a comprehensive model for mineralizationat Jubilee. However, two significant comments con-ceming the mineralization process are possible. First"the 6180 values (+9 + 3%o) of the ore-forming fluid(s)are most characteristic of an evolved basinat brine,like that proposed for the Smithfield and Pembrokedeposits. Second, the 613C values (-27.1 to -2.8Voo)of secondary calcite (Groups 3, 4 and 5) most closelyassociated with mineralizing fluids are substantiallylower than those obtained for other Pb-Zn deoosits ofCarboniferous age in the Maritimes (-10 to +3%o).Evidence for dissolution of anhydrite, the ubiquitousassociation ofhydrocarbons with the sulfides, and thevery low 613C values of calcite can be linked in amodel of coupled oxidation of hydrocarbons andreduction of sulfate close to the site of mineralization.The secondary calcite forned in an almost completelyclosed system, in that the carbonate species required toprecipitate calcite were derived almost entirely fromhydrocarbons. Only limited input of inorganic carbon(a.9., dissolution of host limsslsne) is permitted. Inthis scenario, the Jubilee deposit represents the low-temperature, hydrocarbon-rich end-member in thespectrum of related styles of Pb-Zn mineralizationpresent in the Early Carboniferous Maritime lime-stones.

Acn{owLEDcEtvDNTs

Financial support for the analytical studiesdescrib€d here was provided by NSERC Grant 47387(to FJL). We are also grateful for the technical supportof Mr. P. Middlestead in the Stable Isotope Laboratoryat the University of Westem Ontario. The field pro-$amme during which samples were obtained was sup-ported by subconftacts with the Geological Survey ofCanada (to FIID, through Geomarine Associated Ltd.,Halifax, and Cuesta Research Ltd., Dartmouth, and isin part associated with the MDA Agreements betweenNova Scotia and the Federal Government. We thankH. Mumin, S. Araujo, K. Schrijver and an anonymousreviewer for their helpful comments on an earlier ver-sion of this manuscript.

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Received July 2, 1992, revised manuscript acceptedNovember 27. 1992.