23
45 The Canadian M ineralo g i.s t Vol. 33, pp. 445467 (1995) LASER.ABLAflON ICP-MSMICRO.ANALYSIS OF CALCITE CEMENT FROM A MISSISSIPPI-VALLEY-TVPE Zn-Pb DEPOSI NOVASGOTIA: DRAMATIC VARIABILITY IN REE CONTENT ON MACRO. AND MICRO.SCALES DANIELJ. KONTAK Nwa ScotiaDepartnent of Naural Resources, P.O. Box 698,Hohfox Nova ScotiaB3J 279 SMONJACKSON Departnwnt of Eanh Sciences and Cente for Eafih Resoarces Research Memorinl Universityof Newfowdland St. John's, Nettound.land AIB il$ AssrRAcr Gays River is a Mississippi-Valley-type (MVD Zn-Pb deposit in Nova Scoti4 hosted by Visean-agedolomitized carbonates; the mineralization,fomed at ca. 3W Ma" is due to infiltration and replacement of the host-rock dolostone by heated(to 250oC),saline (ca.25 wt.Vo equivalent NaCl) brines. Isotopic data (Sr, C, O, Pb) indicate that the mineralizing fluids equilibrated with rocksof the Meguma Group,eitherasdetritus within the Devono-Carboniferous Horton Groupor in the underlyingbasement. The proportionof the REE of five different typesof syn- to post-mineralization calcite cement hasbeen determined vrd solutionchemisty anda laser-ablation microanalysis (LAM) - inductively coupled plasna- nuus spectrometry (ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm anddetermination of zoningprofiles by either vertical ablation or lateral taverses. The I-AM results indicate a large variation i^>REE (fron 1000to <1 times chondrite) and degree of fractionation,thus confirming the solution data and eliminating the possibility of putative conta- mination. Conelation of cathodoluminescence microscopyand LAM data indicate a systematic temporaldecrease n >REE and degree of fractionation and a concomitant increase in Mn content of calcite cement with time. The dataare explained by a combination of changing fluid:rock ratio andclosed-syst€m fractionation of theREE Thus,the elevated REE content$ of calcite areinheritedfrom the infiltrating fluids that equilibrated with Meguma Group tletritusin the Horlon Group,whereas relatively depleted REE valuesreflect buffering by the wallrock Closed-system fractionationof the REE can account for the variable (Lall-u)" values tlat characterize calcite with lower REE contents. Keywords: calcite, rare+arth elements, laser-ablationmicroanalysis, inductively coupled plasma - mass spectrometry (ICP-MS), GaysRiver, Mississippi-Valley-type mineralizatiotr, Nova Scoda- SotvrMens Le gisementde Gays River (Nouvefe-Ecosse) constitue un exemple de min6ralisationI Zn-Pb du typ 'Wississippi Valley'. ks roches h6tes sontdess€quences h carbonate d'Age vis6en. La mindralisation seserait form6eil y a environ300 Ma par infiltration et rempliacement desdolomiesencaissantes par dessaumures (environ 25Vo deNaCl ou fouivalent) chauff6es (usqu'i X0'C). ks donn6es isotopiques (Sr, C, O, Pb) montrent que les fluides min6ralisateurs ont atteintl'fouilibre avecles roches du Groupe de Megum4 soit sous forme de ddtrinrs dans les rochqs d6vono-carbonifdres du Groupe de Horton, soit avec le socle sous-jacenlNous avonsanalysd les cinq diff6rentstypes de cirnent calcitique syn- ou post-min6ralisation pour leur tenew en 6l6ments du groupe des tenesrares(7R) par traitement de solutions ou par microqnalyse par ablation avec rayon laser (LAM) et plasma avec couplage inductif et spectom6trie demasse QCP-MS). l,a technique LAM-ICP-MS permet uneanalyse ponctuellesur une 6chelle de 40 pm, et donc la ddtermination de profils de composition, soit par ablation verticale ou par traverses lat6rales. Les r6sultats de I'analyse par LAM indiquentune grande variation en teneurs totalesdesterresrares(entre 1000 et <1 fois les teneurs chondritiques) et en degr6 de fractionnement, confirmantainsi les donndes obtenues sur solutions et dliminant du mdme coup la possibittf d'unecontamination possible. Une corr6lation entreles observations nicroscopiques en cathodoluminescence et les r6sultats obtenus par LAM indiquent une diminution syst6matique des concentradons des terres rares et du degr6 de fractionnement et une augmentation dans la teneur en Mn de la calcite dans les gdnerations de plus en plus tardives. Nouspensons queles donndes illustrent des changements dans le rapport du volumede fluide i quantitd deroche, aussi bien que le fractionnement desterresrarcs en systime ferm6. C'estdonc dire que les tenews en terresraresde la calcite sont h6rit6es de la phase fluide qui s'est infiltr6e et qui avait atteint 1'6quilibre avecle ddtritusprovenant du Groupede Meguma et faisantpartie du Groupe de Horton, tandisque les teneurs tras faibles r6sultent du tamponnage d0 aux roches encaissantes. Le fractionnement des terres rares en systbme ferm6pourraitbien rendre compte des valeurs variables de (LalLu)N qui caract6risent la calcite i faible teneur en tsrre$ rares. (Iraduit par la R6daction) Mots-cVs: calcite,terres rares, microanalyse par ablationau laser,analyse par plasme avecc.ouplage inductif et specfom6trie de masse (ICP-MS), Gays Riveq mindralisation de type'Mississippi Valley", Nouvelle-Ecosse.

LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

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Page 1: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

45

The C anadian M ine ralo g i.s tVol. 33, pp. 445467 (1995)

LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE CEMENTFROM A MISSISSIPPI-VALLEY-TVPE Zn-Pb DEPOSI NOVA SGOTIA:

DRAMATIC VARIABILITY IN REE CONTENT ON MACRO. AND MICRO.SCALES

DANIELJ. KONTAK

Nwa Scotia Departnent of Naural Resources, P.O. Box 698, Hohfox Nova Scotia B3J 279

SMONJACKSON

Departnwnt of Eanh Sciences and Cente for Eafih Resoarces Research Memorinl University of NewfowdlandSt. John's, Nettound.land AIB il$

AssrRAcr

Gays River is a Mississippi-Valley-type (MVD Zn-Pb deposit in Nova Scoti4 hosted by Visean-age dolomitizedcarbonates; the mineralization, fomed at ca. 3W Ma" is due to infiltration and replacement of the host-rock dolostone byheated (to 250oC), saline (ca.25 wt.Vo equivalent NaCl) brines. Isotopic data (Sr, C, O, Pb) indicate that the mineralizingfluids equilibrated with rocks of the Meguma Group, either as detritus within the Devono-Carboniferous Horton Group or in theunderlying basement. The proportion of the REE of five different types of syn- to post-mineralization calcite cement has beendetermined vrd solution chemisty and a laser-ablation microanalysis (LAM) - inductively coupled plasna- nuus spectrometry(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profilesby either vertical ablation or lateral taverses. The I-AM results indicate a large variation i^>REE (fron 1000 to <1 timeschondrite) and degree of fractionation, thus confirming the solution data and eliminating the possibility of putative conta-mination. Conelation of cathodoluminescence microscopy and LAM data indicate a systematic temporal decrease n >REEand degree of fractionation and a concomitant increase in Mn content of calcite cement with time. The data are explained by acombination of changing fluid:rock ratio and closed-syst€m fractionation of the REE Thus, the elevated REE content$ of calciteare inherited from the infiltrating fluids that equilibrated with Meguma Group tletritus in the Horlon Group, whereas relativelydepleted REE values reflect buffering by the wallrock Closed-system fractionation of the REE can account for the variable(Lall-u)" values tlat characterize calcite with lower REE contents.

Keywords: calcite, rare+arth elements, laser-ablation microanalysis, inductively coupled plasma - mass spectrometry(ICP-MS), Gays River, Mississippi-Valley-type mineralizatiotr, Nova Scoda-

SotvrMens

Le gisement de Gays River (Nouvefe-Ecosse) constitue un exemple de min6ralisation I Zn-Pb du typ 'Wississippi

Valley'. ks roches h6tes sont des s€quences h carbonate d'Age vis6en. La mindralisation se serait form6e il y a environ 300 Mapar infiltration et rempliacement des dolomies encaissantes par des saumures (environ 25Vo deNaCl ou fouivalent) chauff6es(usqu'i X0'C). ks donn6es isotopiques (Sr, C, O, Pb) montrent que les fluides min6ralisateurs ont atteint l'fouilibre avec lesroches du Groupe de Megum4 soit sous forme de ddtrinrs dans les rochqs d6vono-carbonifdres du Groupe de Horton, soit avecle socle sous-jacenl Nous avons analysd les cinq diff6rents types de cirnent calcitique syn- ou post-min6ralisation pour leurtenew en 6l6ments du groupe des tenes rares (7R) par traitement de solutions ou par microqnalyse par ablation avec rayon laser(LAM) et plasma avec couplage inductif et spectom6trie de masse QCP-MS). l,a technique LAM-ICP-MS permet une analyseponctuelle sur une 6chelle de 40 pm, et donc la ddtermination de profils de composition, soit par ablation verticale ou partraverses lat6rales. Les r6sultats de I'analyse par LAM indiquent une grande variation en teneurs totales des terres rares (entre1000 et <1 fois les teneurs chondritiques) et en degr6 de fractionnement, confirmant ainsi les donndes obtenues sur solutions etdliminant du mdme coup la possibittf d'une contamination possible. Une corr6lation entre les observations nicroscopiques encathodoluminescence et les r6sultats obtenus par LAM indiquent une diminution syst6matique des concentradons des terresrares et du degr6 de fractionnement et une augmentation dans la teneur en Mn de la calcite dans les gdnerations de plus en plustardives. Nous pensons que les donndes illustrent des changements dans le rapport du volume de fluide i quantitd de roche, aussibien que le fractionnement des terres rarcs en systime ferm6. C'est donc dire que les tenews en terres rares de la calcite sonth6rit6es de la phase fluide qui s'est infiltr6e et qui avait atteint 1'6quilibre avec le ddtritus provenant du Groupe de Meguma etfaisant partie du Groupe de Horton, tandis que les teneurs tras faibles r6sultent du tamponnage d0 aux roches encaissantes. Lefractionnement des terres rares en systbme ferm6 pourrait bien rendre compte des valeurs variables de (LalLu)N qui caract6risentla calcite i faible teneur en tsrre$ rares.

(Iraduit par la R6daction)

Mots-cVs: calcite, terres rares, microanalyse par ablation au laser, analyse par plasme avec c.ouplage inductif et specfom6triede masse (ICP-MS), Gays Riveq mindralisation de type'Mississippi Valley", Nouvelle-Ecosse.

Page 2: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

446 TIIE CANADIAN MINERALOGIST

Ixrr.ooucnox AND NATTIRE or rru PnosLEM

The geochemical signature of hydrothennal oredeposits is generally examined via the study of fluidinclusions (Roedder 1984) or of emanating fluids in thecase of active geothermal systems Qvlichard et al.1983). Unfortunately, standard methods of fluid-

inclusion analysis only provide information for themost concentated species (e.g., Na" C4 K) unless verysensitive tecbniques are used @nnkin et al. 1993).lnorder to circumvent this problem, recent work hasfocused on the fface-element chemistry of hydro-thermal minslals, and particularly their REE concenta-tions. With such data it is possible to make inferences

*ffiPb-Zn. . . .ABorite......Ofluorite... l

UPPER CARBONIFEROUSPICTOU GROUP

l-Fl sondstone, siltstone,

CANSO GROUPlE sondstone, siltstone,

Goys RiverDepos i t

LOWER CARBONIFEROUSWINDSOR GROUP

shole, cool p sondstone, siltstone, corbonotes, evoporites

TM

Frc. 1. (A) Location of the study area in eastern Canada Thearea outlined locates Figure lB in southern Nova Scotia-@) Simplified geological map of a part of southern NovaScotia showing the Carboniferous basins and their geolo-gy, wifh some of the mineral (Pb, Zn, 84 F) occurrencesand tleposits indicated. (C) Sratigraphic relations of theWindsor Group in the Musquodoboit and Shubenacadiesub-basins (from Giles & Boebner 1982).

Page 3: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

LAM_ICFMS ANALYSIS OF CAIfITE. GAYS RIVER MVT DMOSIT" N.S. 447

with respect to source reservoirs in equilibrium withthe mineralizing fluids (Miiller & Morteani 1983).Recent studies include: (1) carbonate phases in vein-type base- and precious-metal deposits @l6ller et aI.1979, L984, Kontak et al. L99l) and Mississippi-Valley-type OfifD PFZI deposits (Graf 1983, 1984,Grant & Bliss 1983); (2) fluorite from a variety ofdeposit types (Marchand et al. 1976, Collins & SnongL985, L992, J €brak et al. L985, Constantopoulos 1988,Hein et al. 1990; (3) tourmaline (Kng et al. 1988) and(4) scheelite from lode-gold deposits (Arlgln et aL1987). In such studies, it is difftcult to assess the influ-ence of various physical and chemical parameters onthe distribution coefficients (Marchand et al. 1976,Morgan & Wandless 1980, Raimbault 1985, Michard1989). The present data for both experimental andnatural systems, are considered insufficieng and morework is needed to determine the potential use of tace-element chemistry of hydrothermal minerals in ore-deposit studies.

In this paper, results of laser-ablation ICP-MSanalysis of calcite cement in an MVT deposit arereported. heliminary solution chemistry (CP-MS) ofcalcite separates indicated exteme variation n REEchemistry for calcite of similm paragenesis and habit(Kontak 1992U b, Kontak & Jackson 1993). Theseresults contast markedly with systematic geochemicalpatterns determined for calcite cement from otherMVT deposits (Graf 1983, 1984) and carbonate stra-h (e.9., Moldovanyi et al. L99O). In addition todocumenting the nature of the REE variation withindifferent types of calcite at Gays River, this study indi-cates the general application of the laser technique to:(1) determine K, values 1s1 mineral phases, (2) docu-

ment chemical homogeneity or heterogeneity inmineral phases, (3) determine the influence of variousparameters on trace-element partitioning @eeder &Grams 1987, Paquette & Reeder 1990, Paterson &Stephens 1992, Mdller et al. l99l), and (4) evaluatetrace elements ns tracers in hydrothermal systems withimplications for source reservoirs.

Gsor.octcAr, Sgrrnlc

The Gays River Zn-Pb deposi! located in theMeguma Terrane of southern Nova Scoti4 is hosled byWindsor Group (Visean) carbonates deposited inseveral sub-basins of the larger Maritimes Basin ofeastem Canada (Giles & Boehner 1982; Fig. 1). Across-section of the deposit (Fig. 2) and stratigraphiccolumn (Frg. lC) suffrce to illustrate the salient geo-logical features. The carbonate buildup developed ona subsftate of Lower Paleozoic metaturbidites of theMeguma Group which, away from the deposit, areoverlain by terrigeuous clastic rocks of the LateDevonian - Early Carboniferous Horton Group(Frg. 1B).The basal unit ofthe buildup is a carbonatebreccia containing fragments of Meguma Grouplithologies in a carbonate matrix, whereas the main unitconsists of interba* and balk facies, the lat0er beingmollusc- and alga-rich. Onlapping the carbonate bankare evaporites, Locally, the carbonate--evaporite con-tact has been deeply incised and infilled by poorlyconsolidated sediments of Cretaceous and younger age.The laterally extensive equivalent of the Gays RiverFormation is the Macumber Formation, a generallythin (<l m) laminated carbonate rock. Carbonatelithologies within the Musquodoboit and Shubenacadie

Fto. 2. Schematic cross-section (NW-SE) throughthe Gays River depositarea, showing the majorrock types and distribu-tion of the mileralized7$e.

4!f;'7o o . o

r":; i1,/K2i.ISii.NuSN{iYii\1.,.

.'viltstotl N

) . r '1 ' ) . r ;

Low Grade Zinc(open Pit Potontial)

Brecc,ia

WESTUINEB CANNA UHITIEDS ch e m a tl c C r oss- Sectlon

GAYS RIVER Ph/ZN DEPOSIT

Page 4: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

M8 TIIE CANADIAN MINERAI'GIST

basins have been variably dolomitized yia constant-volume replacement during diagenesis (Savardl992ub).

Mineralization Q.5 Mt of. l5Vo combined Zn +Pb)consists of massive and disseminated ore and isconfined to the carbonate host with the massive orepreferentially located at tle carbonate-evaporitecontact (Fig. 2). The massive ore replaces the car-bonaie and consists of medium-grained galena andfine-grained (1G40 pm), Fe-poor, beige sphalerite. Incontrast, disseminated mineralization occupies primarypore-space and tends to be coarser grained. Traceamounts of Fe and Cu sulfides. fluorite and barite alsooccur.

The deposit represents - e;ample of MVTmineralization (Anderson & MacQueen 1982) in thatthe mineralization formed as a result of infiltration ofheated (to 250"C), saline (23-35 wt.7o equiv. NaCl)brines that replaced the host-rock carbonate (Akande &Zentilh 1984, Kontak 1992a" b). Mineralization forrnedat ca.300 Ma, some 30 Ma after deposition of the hostrocks @an et al.1993, Kontak et al. 1994).

The mineral paragenesis for the deposit @gs. 3, 4)is identical to that for carbonale samples 6f similar agswithin the Carboniferous basins of southem NovaScotia (Savard L992U b). In general, mineralizationpostdates dolomitization of the host rock and growth ofdolomite euhedra but predates calcite cementation.

Car,crre Types AND PanecrNssrs

There are five types of calcite cement recognized inthe deposit area: (1, 2) calcite infilling primary pore-space, either barren (Vb) or mineralized (Vm); (3)calcite cement in the mafix of the mineralized basal

PARAGENEStri, GAYS RIVER Zn.Pb DEP(XIT

LIMESTOM otes rge)

MARINE CBMENIATIONDOI.OMITra.-TIONDOIOMNBCEMN]TMICROFRACTIJRINOIBNECgA

[email protected] CSMEX{T

breccia (B); (4) calcite intergrown with massive ore(O); Oris type occupies pore-space in the galena-richpart of the massive ore; (5) calcite euhedra coatingfracture surfaces (D; this type does not coexist with Pbor Zn sulfides and represents the lalest stage of calcitedeposition.

The calcite types are poorly constrained parageneti-cally, as calcite usually postdates galena and sphalerite(Figs. 3, 4). Even in the case of Vb and Vm types, it isnot possible to tell if these groupings represent separatestages, as calcite invariably overgrows sphaleriteeuhedra lining these cavities, but not all cavities arelined with sphalerite (Figs. 4E versus 4F) even wherecavities are separated by only I or 2 cm.

ANALYTTcAL TEcHMeuEs

Major- and minor-element compositions were deter-mined with a JEOL 733 Superprobe equipped with anOxford Link E)(L energy-dispersion system @al-housie University), with the following operatingconditions: accelerating voltage 15 kV, sample current10 nA, beam diameter 1-3 pm, counting time20 seconds. Instrument calibration was done on puremetal standards. and data reduction ofraw counts wasdone using Link's ZAF matrix-correction program.

Trace-element contents were determined usinginductively coupled plasma - mass spectrometryQCP-MS) ylc solution chemistry or by laser-ablationmicroprobe (LAM). High-quatity sepruates of calcitewere preparedby pulverizing material to -35l+60 meshand hand picking with the help of a binocular micro-scope. Calcite separates represent a single clot ofcement infilling pore-space or coating a single fracture-surface. Calcite separates (ca. 100 mg) were dissolvedin 8 M HNO3, evaporated to dryness and taken to afinal volume witl 0.2 M HNO3, and the solutions wereanalyzed on an Sciex Elan model 250 ICP-MS. Thewallrock dolostone was analyzed in a similu manner,except that if the sample did not fully dissolve in HNO,alone, a second attack with a 2:1 ratio of 8 M HNO, +6 N HCI was performed, after which HCI was removedwith two HNO3 evaporations before taking to a finalvolume. Further details of the analytical technique,precision and accuracy are glven it Jenter et al.(1990).

Microanalysis of calcite, fluorite and dolostone wasdone on thick (150 pm), doubly polished sections@gs. 4A, D) or hand-picked grains @ig. 48) using aNd-YAG laser (Jackson et al. 1992, Longeich et al.1993, Jenner et al. L993). All samples were photo-gfaphed prior to analysis, and examination of samplesafter each analysis allowed location of the ablationpit on &e photograph (Figs. 4A, D). Because ofthe low absorptivity and poor ablation qualitiesof calcite at the laser's fundamental wavelength of1064 nm, ablation was done using the frequency-quadrupled W beam (7u 266 nm); UV radiation is

FLUORIItsBARTTts

IIYDROCARBONS

r lME r+

FIc. 3. Simplified mineral paragenesis for the Gays RiverZn-Pb deposit Note that the calcite cement overlapsmuch of the sulfide paragenesis and is poorly constrainedv/ith respect to the mineralization.

Page 5: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

I.AM-ICFMS ANALYSIS OF CArcITE. GAYS RIVER MVT DEPOSIT. N.S.

FIc. 4. Photomicrographs of various host-rocks, cements and mineratization at the Gays River Zn-Pb deposir (A) Calcitecenent (Vb) in dolostone with tbree laser-ablation pits illustating the scale of analysis possible using the LAM tecbnique.@) Mount of hand-picked chips of calcite cement fre6 4 singls pore-space; this material wa$ used for LAM analysis.(C) Calcite cement (Vb) infilling primary pore-space in dolostone. Note the presence of fine-grained euhedra of dolomite(dog-tooth variety) lining the cavities and predating calcite cement. (D) Calcite cement containing three laser-ablation pitswith adjacent wallrock dolostone. (E) Sample of dolostone illustrating the following mineral paragenesis: (1) marine cement;(2) pervasive dolomitization of rock; (3) cementation by fine-grained euhedra of dolomite; (4) sphalerite minemlisatisa(i.e., cement); (5) late-calcite cement infilling remaining pore-space. Note that in some places one passes immediately fromstage 3 to 5. @ Sample of dolostone illustrating again tle mineral paragenesis as in Figure 4E, but here with galenamineralization. Note the well-develo@ nufine cement stage, absence of sphalerite and presence of large euhedra ofgalena that overlap the calcite cement stage.

449

Page 6: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

450 TI{E CANADIAN MINERALOGIST

more strongly absorbe{ and results in superior charac-teristics of ablation in transition-element-poor minerals(Jenner et al. L993). LAM-ICP-MS procedures wereas described by Jenner et al. (1993). Calcium was usedas the intemal standard to correct for differences inablation yield between sample and calibration standardG\IBS 610 silicate glass). A slightly defocused beamwas used to produce ablation-pit diameters of ca.40 pn @gs. 44, D), thereby significantly improvingdetection limits compared to operation at optimumfocusing conditions (i.e., 1,0 gm). For each analysis,ablation was continued until the laser bored through themineral wafer (generally about 1G-20 to a maximum of40 seconds), as indicated by a sharp increase in Nafrom the underlying glass slide. The data were trans-mitted to a remote PC for processing using spreadsheetsoftware. From graphical examination of the signals,the presence of spikes potentially related to ablation ofmicroinclusions was assessed, and optimal backgroundand signal time-intervals for integration were selected.Concenftations were calculated using the algorithm ofJenner et al. (L993). Detection limits (3o) for mostelements showed considerable ranges (>10 fold varia-tion among samples). For the REEs, median values forthe detection limits were all close to 0.14 ppm,

corresponding to chondrite-normalized values rangingfrom approximately 0.5 for the light rare-earth ele-ments (I-REE') to 4.0 for the heavy rare-earth elements(HREE). Even after elimination of data below detec-tion limits, many low-abundance compositions haveextremely iregular chondrite-normalized patterns asa result of concentrations of some elements very closeto detection limits (or errots in calculation of thedetection limit inherent in techniques with near-zerobackground).

Complete analytical results are grven in Kontak(1992u in prep.) and are available from the seniorauthor upon request. The chondrite-normalizationvalues of Taylor & Mclennan (1985) are usedthroughout.

ANALYTICAL RFJULTS

oN DoLosroNE War:.nocr

Dolostone contains L6 to '24 wt.7o MgO, up to4.5 wt.Vo FeO and MnO. and 30 to 32 wt.Vo CaO(Iable 1, Fig. 5). Q6mparison of trace-element data(Sr, Ba Y, REE) from solution chemistry and LAManalyses (lable 2, Fig. 6) indicates similar results,which is remarkable given the contrast in sampling

Bo9ol brecclo colclls cemsnt AColclfo comeni (doldtonel oDolorlon€ ond dolomllo csmsni O

"/ "^-. '%h

"o o @A A

2

wT. % MgO6 1 2wT. % MgO

o

Fn

no

o q -N U- O o

ot colcite c€ment

ot r o

o

ot r o

S t r

o 4 a l 2 G 2 0 2 4

WT. % MgO4 € 1 2 1 6 Z A 2 4

wT. % MgO

Frc. 5. Binary plots illustrating major- and minor-element chemistry @MPA) for calcite cement (both in dolostone and basalbreccia host), dolomite cement and dolostone host-rock.

4,3

Fiold ol colclla csmont

tr

effi*o

o. ( , 3L

s "'u, . 2-3 r.s

I

54oS o o* o e

; 4 2

3€

4.5

4tr

o_ t r

trc\o B

n

o

o "r g -s " oi z3

3.9oE r= -* a o

s z3

t.5

I

o5

o - fo

Page 7: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

G9l-t0

Cak&B @ tu dolffi (Vb, VB ty!s) 6!€F Cn9l-10, ed.tr b h b6rl hEcda (B gp.).

volume of the two techniques. The elevated Bacontents for whole-rock material probably reflectstrace amounts of barite that were fiot encountered withLAM analysis.

The chondrite-normalized plots (CNP) for the REEdata of both barren and partially mineralized dolo-stones are typical of carbonate rocks in terrns ofZREE,degree offractionation l(Lall,un) in the range 3 to 10]and negative Eu anomalies (e.9., Jarvis et al. L975,Schieber 1988). The slight see-saw pattern for theLAM data (Frg. 68) indicates that concenfrationsapproach detection limits; in addition, there seems tobe a discernable difference tnthe LREE contents of thetwo samples analyzed, reflecting a difference inthe degree of fractionation of the REE. A single com-position of marine cement (see paragenesis in Fig. 3)also is shown in Figure 6B; such data could not beeasily obtained using conventional tecbniques. Theresults indicate that ttre marine cement has REE abun-dances identical to those in the dolostone, which showsthat the host rock buffered the fluids during diagenesis.This inheritance results from the fact that seawaterhas very low concentations of REE compared tocarbonates; hence even where there is a high fluid:rockratio" the wallrock will buffer the fluid chemisfi(Moldovanyi et al. 1990).

TABIT 2. AITIILYIICAL DATA FOB IXT.ffIOII& MAMBIAL DEMFfi1!INU'BY II\M ATi'D SOLUIION CIEMN'ITY, GAII IN'EB, NOVA SOOTIA

451

Lo Ce Pr Nd Sm Eu Gd Tb Dy Fb Er Tm yb Lu

Ftc. 6. Chondrite-normalized REC plots for dolostone host-rock (Gays River area and from within the regionalShubenacadie Basin), mineralized dolostone and massiveore. (A) Results for solution chemistry. (B) Results fromLAM analvses.

T-AM_ICP_MS ANALYS$ OF CALCTIE. GAYS RTVER MVT DMOSIT" N.S.

TABIJ 1. XETBESNTAIIVE A'PA DAf,A OTGAYS RII'Ets. N()yA SCOIIA

Q

30

n2 * %

Cf,9ll3 slc&.el&dol@t!ol@

d'916 dolffidde6tlol@€lc&!

&rl-3 dolMdol@6@Iqt9€lc&adol{'s@

051 2.9 6tj'o.s 23t 61.q'

tt8 1,9 $.8614.01 2.19 t1,16

ztg 0.t5 55.384,45 021 s35)21.93 0.t0 !{.t6o8 1.80 9_38

18.66 2.8 t6.10N.X t.91 56.1tr

0.60 2,40 @39o.fts 2zt Q.32

It.01 4,4 5E.60

0n 2,9 62-58031 0.86 59.64

st.86 0.62tr,94 0.t442]6 l.tJ3t.t6 2,9

rz66 0n3u3t 0.,4631.92 0,8s.ll 0.38

31.{0 t2l31.95 l.t85t.99 0.6058.79 0.6t31.t0 43t

5t.E5 0.64rE.19 0

€lcito€lcXB

4030

20

Q)=ooco

xoO lE

R€I.'2

@41.84A

GR.91€6

m9l-lzt

GR.9I91,

dol@dolMB

&l@dolMdol@ffi@

M

el@@

frfrlbrddol@

LAM 10.9 63 3.9LAM 15,1 10.4 63

rAM xi.I 63 6.6LAM 14.9 6J 43LAM t4.4 8,1 3.6LAM 4.0 U2 63

Sol@n 9,6 38.3 4,1

So@d loJ 46.9 52

Sold6 10.4 t 3.6 At 2.0 s.Et lJ

S O L U T I O N C H E M I S T R Y :

@ MINERALIZEo DoLosToNE (n"5,- MASSIVE OREX DOLOSTONE (GAYS RIVER)A DOLOSTONE (REGIONAL BASINT

Lo Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm YbLu

(GR-9r-84A)

H l .or . r ALBAt.NT t

ANALYSTS

Page 8: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

452 TIIE CANADIAN MINERAI'GIST

o

ANar-yrrcer RFsuLTs oN CALcTE CetlGNr

Maj or- anL minor-elem.ent chernistry

Chemical compositions of calcite cement (Table 1,Fig. 5) indicate between 1 and 3 wt.7o MnO, with lesserconcentations of Mg (up to 2 wt.Vo MgO) and Fe (upta I wt.Vo FeO). There is a clear distinction between thecalcite cement in the basal-breccia host (B) andthe dolostone (Vb, Vm), the latter showing relativeemichmentinMn.

Trac e - element chernistry :bulk separates

The level of strontium varies from ca. 50 to250 ppm, with one sample at 500 ppm (FiC. 7); there isno consistent relationship between Sr content andcalcile type. Extreme emicbment (to >3000 ppm)occurs in tlree samples, but the high Ba contents ofthese samFles (ca. 200O ppm) suggests the presence

of barite as a contaminant. Although data wereacquired for additional tace-elements (Li, Be, Sc, V,Cr, Rb, Z, Nb, Mo, Cs, Hf, Ta, Tl, Bi, Th, lD,abundances were found to be consistently belowL-2ppm, the only exceptions being for Pb, Zn and Cu,which indicate trace amounts of sulfide (Kontak1992a).

The CNP for different types of calcite (Fig. 8) indi-cate large variations n>'REE and significant fraction-atton li.e., (La./Lu)"1, with the greatest variation fortypes Vb and Vm, both from the deposit area andregionally. Two patterns are distinguished: (1) anunfractionated CNP that is best represented by B and Otypes, and (2) a snongly fractionated CM most pre-valent in Vm and Vb types from the deposit area andregionally, although some unfractionated CNP occurfor Vm calcite. Other features include: (1) F-Opecalcite is intermediate between tlese two broadcategories (i.e., Vm and Vb); (2) B-type calsile showsa positive Ce-anomaly; (3) a negative Eu-anomalycharaclerizes all types of calcite. The variation of. REEdata for calcite types is not related to analyticalproblems or impurities, as replicate analyses indicatesimilar results. Whereas the variabiJity documentedhere contasts with results of previous REE studies ofcarbonates from MVI-type deposits (e.9., Graf 1983,1984, Grant & Bliss 1983), comparable heterogeneityis noted in carbonates from other hydrothermal oredeposits (Kontak a al. 1991, Kontak & Smith 1993,M6l7er et al.1979, L984).

Trace - element chemistry :las er- ablation microanaly sis ( I'AM )

Initial results of LAM analysis of calcite cement aresummarized in Figure 9. Three important observationsare noted: (1) spot analysis of a single grain (sampleGR-91-41, Fig. 9C) indicates large variability inX,REE, although the degree of fractionation (1e., CNP)recorded in different analyses is similar. Despite theapparent chemical heterogeneity of the sample,solution-chemis0ry data for the sample agree with theaverage of the five spot-analyses (Fig. 9C); (2) avertical profile tbrough a grain in the same samplediscussed previously (GR-91-41, Figs. 9A, B) indi-cates pronounced 2qning for the LREE (Ce, Nd), butrelatively little zonation for otler REE (Im, Eu) ortace elements CY, Sr); (3) where LAM data indicate ahomogeneous composition, the results agree with solu-tion chemisty for a bulk separate (sample GR-91-G,Fig. 9D). Collectively, these data thus offer an expla-nation for the large chemical variations recorded in thebulk analyses (Ftg. 8).

LAM analyses of calcite types were done on anadditional thirteen samples (representative samples inFigs. 10, Il, 13, 14) in order to fully characterize thenature and degree oftace-element variability in calcitecements, in an attempt to correlate this with their dif-

x E +x E +x E +

LAM ICP-MSx q + +

x x o t +x x o + + t rx X + v I + E n " l l ?X x + V V + tX O V 9 V + +

X X O V V V + +x O O v v A A v +x O o v v a a A t +X O + A V A A A V V + d vv t 9 a v a a a a g i V V V V

16o edo soo aoo 5oo 600Sr(ppm)

^ BuLx sEpaFATESto

aA A

v o n , 4 o l - R - ; lv E l o - o le t r l F - t lo e o + a l v , n - t r |a v v t r + e o l W - " 1

x x o + + v o l B _ r lt r A X X X i + O + q

o too 2@ 3@ 400 500 6@Sr(ppm)

Ftc. 7. Histogram plots of Sr contents (ppm) for calcitecements from Gays River determined by LAM-ICP-MSand bulk separales QCP-MS). Data are also summarizedin Table 5.

Page 9: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

FRACT1JRE C,qLCIIE

I.AM_ICP-MS ANALYS$ OF CArcTIE. GAYS RIVER MVT DEPOSIT" N.S.

SmEu Gd Ib Dy Ho Er TmYb Lu

453

.E

o

E

.E

a

oo

'c

o

0

E

o!

oo

oE

e' f

p

o

oo

e

o.E

5 roo

1 0 @Lo Ce Pr Nd SmEucdTb DyHo ErTmYb Lu SmEu Gd Ib Dy Ho Er TmYb Lu

LoC€PrNd SmEuCdTbDyHoE TmYbLu

Ftc. 8. Chondrite-normalizecl REE plots for calcite types from the Gays River deposit area and regional samples ftom thesurrounding sub-basins. All samples represent solution chemistry on bulk separates of calcite cement.

ferent habits. As expected from the preliminary results,the analyses indicate a large range in XREE and degreeof fractionation, although the least amount of variationis again noted for the B- and O-type calcite (Frg. 10).The LAM results also indicate that there is a largevariation in the degree of the Eu anomaly developed,rvith Eu/Bu* < 1.

Results of more detailed analyses for two samples ofVb-type calcite are presented in Figure 11 in order toassess infra- and intergrain variability. For sampleGR-91-84A (Fig. 11A), results of REE analyses againindicate both a homogeneous (1,2i 7 , 8, 9; 10, I l) andhighly variable (!o 5o 6) chemistry. Collectively, theCNP for these calcite 3amFles representing the sametype and apparenfly, the same paragenesis (see sketchin Fig. 11A) indicate two groupings, one with elevatedLREE ard,tREE emichmenl and the other with rela-tive depletion in XREE and an unfractionated CNP. Aswith the previous sample, data for GR-92 (Fig. 11B)indicate both homogeneity (6,7,2,3,5) and vmiability

(9, 10, I 1) on a inter- and intragrain scale with respectto KEE chemisry. This parcicular sample comes fromwithin the Musquodoboit basin @g. 1B) and wasanalyzed specifically for comparison to samples fromGays River. As with the bulk separates collectedregionally, this sample indicates the same degree ofvariability and emichment in >REE as observed forcalcite cements from the deposit.

Correlation betvv een compositions andc atha dolumine s c enc e in c alc it e

The calcite cements are strongly luminssssaf(Fig. l2), with a broad outer zone (i.e., late stage) ofuniform CL and a strongly zoned core (i.e., early stage)characterized by numerous bands of variable CL. TheCL results ale similar to those observed by Savard(L991, L992b) for calcite cements in the adjoiningsub-basins.

The locations of analyzed spots are indicated in the

MtNERAiJZm CALCTTE (WGS)

BARREN C,qLCm ryuGs)Lo csPrNd smEuCdIbPyHo ErTmYb L!

BRECCT (B) CALCm MASSfVE ORE-CALCM

Page 10: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

454 THE CANADIAN MINERALOGIST

u

f r r

1

ol

GR-91_G

(tr solution )s s & r y $ y Z . B a l ! G h X d h E ! C d T ! D r k & h h h f h U

+tM +tM +|f f+$N

Ftc. 9. Summary of the fi$t results of LAM analyses of calcite cements from Gays River, which illustrate the presence of bothhomogeneous and heterogeneous fiEE chemistry. (A, B) Vertical chemical profile tlrough calcite cement illustrating ZREEenrichment with depth within the grain. Note that there is paxallel behavior of all the ftEE, although best manifested by Ceand Nd because of their greater concentrations, whereas Sr concentration remains basically constant. (C, D) Comparison ofLAM and solution-chemistry results for calcite cements, illustrating homogeneous and hetercgeneous distributions oftheREE.

s 9 1 @ 1 1 0 U O m & F D 1 t 0 1 , ,sm ffi

c

Ei or2c

CL photographs in Figure 12; the trace element (Mn,Fe, Sr, Y, LaN) and CNP for six grains of F-type calciteexamined in detail are presetrted in Figures 13(GR-91-27) and 14 (cR-91-67). In sampleGR-91-67, tbree of the grains @, G, T) have similar,generally flat CNP arLd>REE, and the fourth grain (H)has a distinct.tREE-enriched pattern. Two grains withwell-developed zonation in luminescence @g. 12,

gtains E, G) have relative enrichment of REE and Y incore areas. In sample GR-91-27 (Ftg. 14), 6 similarpattem is developed with emichment of the XREE andLKEE fraclionation in grain cores. For grain B, a rim-to-rim profile (Frg. 14) indicates (1) a syslematicdecrease of LaN across the grain, (2) a symmetricalzonation of Y, and (3) an absence of zonation in Srand Fe.

FIc. 10. Representative LAM analyses for different calcits cements from Gays River and regional samples. Note in particularttre large range in EREE and the fractionation (1a., I*/Lu"). The results of solution chemistry on bulk separates from thesame samples are illushated where data are available.

GR-91 -41( " solution )

S r Y Z & b O P r M S F 8 s € d : ! D y E o & 1 6 f b b t U

-+

Page 11: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

GR-e1-3e (o) I ;l5 X6 A

7 vE O9 a

1 0 E

A < <I.AM_ICP-MS ANALYSIS OF CALCITE. GAYS RIVER MVT DEPOSIT. N.S.

1000

Lu

too

lir 100Eoz.oEo 1 0

lrJqJ

o 1

1000

0.1

1000

0.1

tooo

Eu

100

1 0

l lJ 100E.E.oz.oEo 1 0

prE

9( ) 1

0 .1

to

FJ IOOEe.oz.oEo l o

tIJhqo l

GR-s1-171 (Vm)

GR-e2-2 (vb)GR-964 (Vb,R)

l t l2 U3 o4 x5 a

LoCe Pr Nd SmEu @ TbDyHo Er TmYb LuLo Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm yb Luo.l

Page 12: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

456 THE CANADIAN MINERALOGIST

GR-e1-84A (Vb)

Lo Pr Sm Gd Dv Er YbCe Nd Eu Tb

'Ho Tm Lu

f-.--ooto"ron.

r--lcolciteL-l cemenl

. onolysis

1 000

@' lo0

1 0

L ! 1

Erlz.Ic) 1000

(JJ

( ) 1 0

1 000

1 0

1

Lo Pr Sm Gd Dv Er YbCe Nd Eu Tb

'Ho Tm Lu

5 m m

The main activator element causing luminescence inthe calcite is assumed to be Mtr, given that it is the onlyelement of sufficient abundance (0.5 to 1.5 wt.Vo Ml;Fig. 15); Fe conlents are sufficiently low that Fe is nota suppressor of CL (Mason 1987). There is a stongnegative correlation between ZREZ concenfration andzones of Mn-depletion (Frg. 15), and the generalenrichment of LREE in the cores of calcite cementscorrelates with depletion in Mn.

Comparison of solution chemistry versusI-A,M analyses

ln cases where data on solution chemisfi are

available, .REE concentrations have been compared toresuhs of LAM analyses (Figs. 9, 10, 11, 13, L4).Examination of our findings indicates that results forthe two techniques are only comparable in caseswhere LAM analyses indicate a relatively homo-geneous tace-element chemistry. In confrast whereLAM analyses indicate heterogeneity in REEabundance, the solution data deviate considerably interms of ZREE allrd fractionation. However, whereseveral separates of a single type of calcite have beenaaalyzed (Frg. 8), the range in results generallycoincide with the variation indicated from the LAMdata, Therefore, these results, together with the obser-vations from correlation of CL and LAM analvses.

Page 13: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

chemlstry

I-AM_ICP_MS ANALYSIS OF CAIfITts. GAYS RWER MVT DEPOSIT. N.S. 457

1 000

100

1 0

.I

1 000

100

1 0

'I

cR-e2 (w, R)

Lo Pr Sm cd Dy Er ybCo Nd Eu Tb -Ho Tm ru

e

indicate that use of results of CL should permit one topreselect before hand samples for bulk analysis versasLAM analysis.

The results of Sr analyses obtained from solutionchemistry and the LAM approach are compared inFigure 7. There is a good corelation of the results,although the modes occur at different concentrations,which may illustrate a sampling bias. However,whereas the deviation of a single samFle in thecase of the solution-chemisry data reflectscontamination, as discussed previously, suchvariations in the LAM analyses are considered toreflect inherent chemical heterogeneities within thecalcite.

LIFtozr

LrJF

J

O

Q.p€

Ftc. ll. Summary of l,AM analyses for some detailed studies of calcite cements intwo samples from Gays River. (A) sample GR-91-84a (Vb) from the tleposit area;@) sample GR-92 (Vb) from within the Shubenacadie basin.

f- dotostone

[--l cotcite cement

onolysis

2 m m

ANar,yncar Rrswrs oN FLUoRJTE Cmm,rr

A single sample of fluorite cement (GR-92-2) inter-grown with calcite (Vm) has been analyzed (Fig. 16).Of the three separate grains analyzed, two grains(analyses L,2, 3) are similsl h lsrms of both XREE andfractionation, with LREE depletion alld HREEenricbment. In contrast, the third grain (analyses 4, 5)is relatively enriched n >,REE and has the LREEenriched over the HREE; thts grain is also relativelyenriched in Sr (55 to 62 ppm) compared to the othergrains (14 ts 22 ppm). The variation in the REE notedhere is not unique for hydrothermal fluorite (Mtiller &Morteani 1983, Hein et al. L990, Constantopoulos

Page 14: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

TIIE CANADIAN MINERALOGIST

Ftc. 12. Cathodoluminescence photomicrographs of calcite cements (F-t'"e) witl the location of LAM analyses (REE andtrace-element data plotted in Figures 13 and 14). Note that there is a broa4 dull outer zorc and sfrongly zoned core areas inseveral grains of calcite.

Page 15: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

LAM_ICFMS ANALYSIS OF CALCIIE. GAYS RIVER MVT DEPOSIT. N.S. 459

1988), although a more homogeneous REE chemistryis normally the case (Marchand et al. 1"976; J€&ak etal. 1985, Strong et al. L984,M6ary et al. L985).

DrscussroN

The data presented here for hydrothermal calcite andfluorite from the Gays River Zn-Pb deposit documentthe occurrence of dramatic chemical heterogeneity forthe REE on both the macro- (i.e., deposit) and micro-scale (i.e., <100 p.m in single grains). Such hetero-geneity has previously been documented for hypogenephases from mineral deposits, for example tourmaline(Kng et aI. L988), calcite (M0ller et aI. 1979, 1984)and fluorite (Collins & Strong 1985, 1992), but theconfirmation of this variability by spot analyses ofindividuat grains using a laser-ablation technique pro-vides corroborating evidence that variation in CNP by1000 times does occur. Thus, the variability docu-mented above is not related to potential contaminationby micro-inclusions, a caveat that has been a problemwith analyses of phases prepared using standardtechniques of mineral separation [e.9., Mchael (1988)yar.r,rr Mahood & Hildreth (1983)1.

An assessment of REE variability in calcite cem.entsat Gays River

The reasons for the REE variability in differentfypes of calcite from the Gays River deposit mayinclude: (1) variations in physicochemical parameters(T, P, solution chemisty), (2) wallrock reactions, (3)kinetics of minepl grov/th, and (4) chemical fractiona-tion of the fluid. Morse (1985) and Reeder & Grams(1987) have reviewed several of tlese factors as theyrelale to gfowth of calcite. Fluid-inclusion studies(Kontak 1992a) ndrcate that, during the growth ofcalcite, temperatures varied fuom ca. 100 to 250"C,and salinities were 23-35 equiv'. wt.7o NaCl.Unforhrnately, it is not possible to correlate betweenREE chemistry and temperature for the generations ofcalcite. However, this has been done for the St.Lawrence fluorite deposit in southeastem Newfound-land, where Collins & Snong (1985, L992) have shownthat fluorite precipitation was accompanied by a dropin temperature (ca. 400 to 100"C) and change insalinity (ca. 25 to 0 wlVo equiv. NaCl). Analysis ofthe fluorite indicates a systematic decrease n >REE(100 to 1 times chondrite), (LalLu)N and Sr content(80 to 10 ppm). This conffasts with i?EE variations atGays River of up to 1000 times chondrite withinindividual samples that show little conelation withdepletion or enrichment in Sr. A similar map.itude ofvariation in CNP of REE to that observed at Gays Riveroccurr in paragenetically similar calcite from base-metal deposits (Mdller et al. L984) characterized byonly a small range in temperatures of fluid-inclusionhomogenization. Thus, the large range it REE

variation at Gays River is not unique and need not berelated to fluctuations in physicochemical parameterssuch as temperature or solute chemistry.

Wallrock alteration can result in significant changesboth in the chemistry of the precursor and hydro-thermal fluid. However, alteration in the depositconsists only of replacement of the dolostone host andminimal modification of Meguma Group lithologiesforming the local basement (Kontak 1992a). Thus,wallrock alteration is not considered a viable means ofgenerating the changes in RZE chemistry of the calcite.

Paterson & Stephens (1992)have documented largevariations in trace-element abundance in sector-zonedgrains of titanite, which the authors relate to rate ofgrowth. In studies pertaining to calcite growtl @eeder& hosky 1986, Reeder & Grams 1987, Paquette &Reeder 1990), it has been shown that compositional-sector zoning is an important factor for frace-elementvariation within single grains. However, the variation(to L007o) documented for Mg, Mn and Sr does notapproach that observed for the REE in calcite cementfrom Gays River. More importantly, the REE variationdoes not correlate with Sr, suggesting, therefore, thatthese elements are geochemically decoupled.

Kinetic factors of possible importance with respectto ftace-element partitioning in calcite are rates of fluidflow (Miiller et al. I99l) and precipitation (Lorens1981, Lahann & Siebert 1982). In the first case, varia-tions by a factor of three in elemenVCa ratios can beaccounted for by changes in fluid-flow rates givenisothermal and isobaric conditions. In terms of pre-cipitation rates under equilibrium conditions, cationcongentation was found to be proportioml to the rateof precipitation from saturated solutions, with Srhaving a trend opposite to that of Cd, Co and Mn. TheCL studies (Ftg. 12) indicate strong zonation and,hence, nonequilibrium growth during calcite precipita-ticin. The narrow range in Sr content for calcite cementfrom Gays River suggests similar conditions duringprecipitation. Thus, whereas these kinetic factors mayhave been a factor, it seems that they cannot accountfor the magnitude of kEE variation observed in thecalcite fractions.

The potential for chemical fractionation of the fluidphase, specifically for the REE, is worth considering,as experimental studies indicate that the HREE fotmstronger carbonate complexes than the LREE@umonceau et al. L978, Cantrell & Byme L987, l-r,e& Byrne 1993). Thus, simple precipitation of calcitewill deplete the fluid n IREE and emich it lt HREE;this is the basis of the models proposed by Milllet et aI.(L979, 1984) in their studies of REE abundances incalcite from mineralized districts of the HarzMountains, Germany. In this model, growth of calcitein a closed system would favor I'REE depletion andHKEE enicbment of the fluid; thus later-stage calcitewill reflect this change in fluid chemistry. At GaysRiver, there is a tend of decreasing IREE and LaN

Page 16: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

GR-g1-67(F)

ErEzErErEs

o+tl

x (rim)

<-'.t;j*ii:io

460 TIIE CANADIAN MINERALOGIST

100

1 0 0

l n n

Gr . (core)G z oG s +Gr o (rim)

LJF8 1 0ozoIotrJF6 1J

(J

LuEuEuLo

t r !1 0

1 0 0

Tr .T z oT r o

Ravenhurst et al. L987, Kontak et aI. L993, Savud1992u b). These data show a continuum from aCarboniferous marine component having a 613C'FB of*SVoo, a 6tto.ro* of +25Voo and a 87SrF6Sr of 0.708 toa 6r3cor'" between 0 and -:7Voo, a 6180r*olu between+L2 and +16%o, and, a 87SrF6Sr of. 0.712, the lattervalues having been obtained for calcite cements mostclosely relaled to mineralization (i.e., O-, B- and Vm-types). This variation is explained in terms of increas-

LuEuLoLuEuLo

FIc. 13. l,AM data for calcite cement (F-type) from sample GR-91-67. (A) Summary ofdetailed LAM analyses for tbree grains showing the chondrite-normalized REE plotsfor three grains; location of the ablation pits are shown in Figure 12. Note that there isa general increase in the REE from the margin to the core of the grains. The resultslabeled T represent separate grains with I or 2 analyses. @) Sunmary of the face-element data for the grains analyzed plotted as function of their relative positions. -+

toward the margin of crystals @igs. 13, l4), i.e., in thelater-stage growth zones.

Origin of the kEE variation of calcite cetnentsat Gays River

Potentially important in understanding REE valii-ability are the results of C, O and Sr isotopic analysisof all types of calcite cement (Akande & Znntilli L984,

Page 17: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

I.AM-ICFMS ANALYS$ OF CALCITE, GAYS RIVER MVT DEPOSIT, N.S.

GR-9r- 67 (F)

F E

461

6

s;3

t . 5

t .o

o.5

4000

Fa 2oooc\

300

E 2OO

ct. roo Sr ' \-=-- \ \ ,

Y.

80

40Loru

' \--+{

T1T2Tg E1-aE5 H1---+H4 G1-->G4

ing fluid:rock ratio during infiltration ofthe carbonate explained in part by a temporal change in fluid:rockbank by an extemal fluid; Ravenhurst et al. (1987), ratios based on the following: (1) the earliest stage ofKontak et al. (1993) and Savard (1992b) considered calcite growth, as represented by core regions, arethis extemal fluid to have equilibrated with rocls of fhe closest paragenetically to minsmllization, at which timeMeguma Group, either as detitus within the Horton fluid:rock ratios are infened to have been the highestGroup or the undedying basemenl. A similar conclu- (see above); these foactions are the most enriched insion is supported by Pb isotopic analysis of galena REE; Q) core regions of calcite grains are relatively@avenhurst et al. 1989, Kontak L992a). depleted in Mn compared to the margin of the grains,

T\e REE data for the calcite cements can be whichgrewunderlowerfluid:rockratios(i.e.,buffered

40

- 2 0trCICL

Mn

@

A.n/\/v

@

A

@@

Page 18: LASER.ABLAflON ICP-MS MICRO.ANALYSIS OF CALCITE …(ICP-MS) technique. The LAM-ICP-MS technique permis analysis on the scale of 40 pm and determination of zoning profiles by either

GR-91-27(F)

ArAzArA+

. (core)n

x

o (rim)

.<r SolutionChemistry

462 TIIE CANADIAN MINERAI.OGIST

g.' . (cor4C r oC r +C z oCs x (rim)

by wallrock). In this model, the external fluid rsinfened to have been enriched m REE, as reflected bythe data in Figures 13 and 14. However, interaction ofths mineralizing fluids with rocks of the MegumaGroup, which have CNP and ZREE typical of shales(1e., NASC of Gromet et al. 1984) will not generalesuch REE emichment. Such enrichment would occur,however, if an accessory phase within these metasedi-mentary rocks were to break down. Examination of theCNP of calcite with the highest REE indicate generally

1 0 0

1 0 0

T s .

unfractionated patlerns simils 16 those of apatite (e.9.,Hanson 1980), a phase present in Meguma Group rocksas a derital component.

The subsequent decrease in the REE content oflater-stage calcite cements can be explained by either (1)closed-system fractionation, as discussed by Mdller &Morteani (1983), or (2) decreasing fluid:rock ratiossuch that calcite inherits the REE signature of the dolo-stone wallrock (Frg. 6), which is relatively depleteds6mpared to paragenetically eadier calcite, or some

100

@ulF8 1 0oz-O

trJt-6 1J

O

1 0

1 0 0EuLoLuEu

1 n1 0

LO

Hc. 14. LAM data for calcite cement (F-type) from sample GR-9I-27. (A) Summary ofdetailed LAM analyses for tbree gains showing the chondrite-normalizecl REE plotsfor three grains; location ofthe ablation pits are shown in Figure 12. Note that there isa well-conelated insease in the LREE from the margin to the core of the grains. Theresults labeled T represent a single analysis of a grain. (B) Summary of the trace-element data for the grains analyzed plotted as function of their relative positions. -+

EuLoLuEu

Br .B z oB g +B + t rB s xB e .

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LAM_ICP_MS ANALYSIS OF CALCITE. GAYS RTVER MVT DEPOSIT. N.S.

GR-9 f -27 (F l

Sr

. v-

Y

40

T3 A 1 - + A 4 B t - + 8 6 C l - - + C g

463

@s|-I

t . 5

t .o

o.5

2000

tooo

200

loo

combination of these two mechanisms. The CNP forcalcile with low IREE has a variable (Lallu)r{ valueeither less than or greater tlan unity, suggesting there-fore that both of the aforementioned processes partici-pated, as neither one alone can account for variablefractionation.

Coxcl-ustoNs

Calcite cements from the Gays River Zn-Pb deposit

ECLCI

ECLCL

50

30

toECICL

60

20

show large ranges in both the abundance and fraction-ation of the REE. The variation was first determinedusing bulk methods, but use of a laser-ablation micro-probe (LAM) confirms that tlis variability is real andnot due to micro-inclusions. In addition. LAM ana-lysis and cathodoluminescence microscopy show asystematic decrease in both XREE and LaN with time,concurrent with an increase in the IvIn contents oflater-stage calcite. The data reflect the following processes:(1) infiltration of the Gays River bank by an external

Mn

a

@ @ @

Fe

LoN

\ ^v\*

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464 TIIE CANADIAN MINERALOGIST

N r.oots3

Flc. 15. Plot of wt.7o Mn versus La1,1 for calcite samplesGF.41-n and GR-91-67. Note the strong negative cor-relation between the two parameters and the fact that twosubparallel, but distinct trends are defined. The strong CLzonation in the core of the calcite grains @ig. 12) mainlyreflects increasing but fluctuating levels of Mn (theactivator element).

Lo Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tmyb Lu

Flc. 16. Chondrite-normalized REE plot of LAM resulsfor fluorite cement from the Gays River deposit. Note thatof the tlree grains analyzd there is an atrparent con-sistency within a grain, but among grains two distinct andmarkedly different pattems are developed.

fluid enriched n KEE due to breakdown of apatiteduring fluid migration through the Horton Group,which is dominated by Meguma Group deffitus (or theMeguna Group as basement proper), (2) decreasingfluid:rock ratio during calcite growth, and (3) closed-system fractionation of calcite. Whereas the firstprocess accounts for the enrichment of. REE (to 1000times chondrite), the second ard thfud can explain thedecrease in XREE and variable (Lallu\ values.

AcKNOWLEDGuvrB.lTs

Work at Gays River was financed through the NovaScotia Deparhent of Natural Resources and Canada -Nova Scotia Mineral Development Agreement(L99VL992 MDA). D. Kontak has enjoyed numerousdiscussions with D. Sangster and M. Savard withregards the genesis of the Gays River deposit andsincerely acknowledges their contribution to thisproject. Operation of tle analytical facilities atMemorial University is supported by NSERC tunding.Cathodoluminescence photographs were kindly sup-plied by R. Mason, Memorial Universify, and NSDNRstaff assisted with preparation of the paper.Constructive criticism by joumal referees and meticu-lous editing by F.C. Hawthorne and R.F. Martinresulled in substantial improvement; all are sincerelyacknowledged. This paper is published with thepermission of the Director of the Mineral ResourcesDivision, Nova Scotia Department of NaturalResources.

Rmm.sl,rcFs

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LASER ICP-MS ANALYSES

.i\j t t . . o GR-91-2? (F)' . \ - . cR -9 t -62 (F ,

a \ .

\ g .

ea \

\ _ d

nh/

g

Eo.uo

2 A 4 0 6 0 e oLoN

o:!tEoc

{ t oo:o5

trI

( FLUORITE )

S r Y| . 14.6 zlo2 +1 t9 .9 7 t .93 0l 22.5 tos4 xl 62.4 56.65 trJ 55.4 69.4

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