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Petrography and geochemistry of early-stage, ®ne-and medium-crystalline dolomites in the Middle DevonianPresqu'ile Barrier at Pine Point, Canada

HAIRUO QINGDepartment of Geology, Royal Holloway, University of London, Egham Surrey, TW20 0EX , UK(E-mail: [email protected])

ABSTRACT

The petrography and geochemistry of ®ne- and medium-crystalline dolomites of the

Middle Devonian Presqu'ile barrier at Pine Point (Western Canada Sedimentary Basin)

are different from those of previously published coarse-crystalline and saddle

dolomites that are associated with late-stage hydrothermal ¯uids. Fine-crystalline

dolomite consists of subhedral to euhedral crystals, ranging from 5 to 25 lm (mean

8 lm). The dolomite interbedded with evaporitic anhydrites that occur in the back-

barrier facies in the Elk Point Basin. Fine-crystalline dolomite has d18O values

between )1á6 to ±3á8& PDB and 87Sr/86Sr ratios from 0á7079±0á7081, consistent with

derivation from Middle Devonian seawater. Its Sr concentrations (55±225 p.p.m.,

mean 105 p.p.m.) follow a similar trend to modern Little Bahama seawater dolomites.

Its rare earth element (REE) patterns are similar to those of the limestone precursors.

These data suggest that this ®ne-crystalline dolomite formed from Middle Devonian

seawater at or just below the sea ¯oor.

Medium-crystalline dolomite in the Presqu'ile barrier is composed of anhedral to

subhedral crystals (150±250 lm, mean 200 lm), some of which have clear rims toward

the pore centres. This dolomite occurs mostly in the southern lower part of the barrier.

Medium-crystalline dolomite has d18O values between )3á7 to )9á4& PDB (mean

)5á9& PDB) and 87Sr/86Sr ratios from 0á7081±0á7087 (mean 0á7084); Sr concentrations

from 30 to 79 p.p.m. (mean 50 p.p.m.) and Mn content from 50 to 253 p.p.m. (mean

161 p.p.m.); and negative Ce anomalies compared with those of marine limestones.

The medium-crystalline dolomite may have formed either (1) during shallow burial at

slightly elevated temperatures (35±40 °C) from ¯uids derived from burial compaction,

or, more likely (2) soon after deposition of the precursor sediments by Middle

Devonian seawater derived from the Elk Point Basin.

These results indicate that dolomitization in the Middle Devonian Presqu'ile barrier

occurred in at least two stages during evolution of the Western Canada Sedimentary

Basin. The geochemistry of earlier formed dolomites may have been modi®ed if the

earlier formed dolomites were porous and permeable and water/rock ratios were large

during neomorphism.

INTRODUCTION

Dolomites in the Middle Devonian (Givetian)Presqu'ile barrier host one of the world's largest

Mississippi Valley-type (MVT) deposits (Rhodeset al., 1984). In the subsurface of the North-westTerritories and north-eastern British Columbia,these dolomites are also important hydrocarbon

Sedimentology (1998) 45, 433±446

Ó 1998 International Association of Sedimentologists 433

reservoir rocks (e.g. Collins & Lake, 1989). Un-derstanding the origin and distribution of differ-ent dolomites in the Presqu'ile barrier is thereforeof great economic importance for both mining andoil industries. As massive replacement dolomitescommonly occur in the Western Canada Sedi-mentary Basin and some of these are importanthydrocarbon reservoirs (Morrow, 1982; Machel &Mountjoy, 1987), a better understanding of do-lomitization will be useful to predict the spatialdistribution of dolomite reservoirs elsewhere inthe basin.

Dolomite has been interpreted to have formedin many different environments by various ¯uids(Land, 1985; Machel & Mountjoy, 1986; Hardie,1987). These include: (1) evaporitic marine brinesin sabkha environments (e.g. Adams & Rhodes,1960; Patterson & Kinsman, 1982); (2) freshwater/seawater in mixing zones (e.g. Badiozamani,1978; Humphrey, 1988; Humphrey & Quinn,1989); (3) normal seawater in subtidal environ-ments (e.g. Saller, 1984); and (4) various ¯uids insubsurface environments (e.g. Mattes & Mountjoy,1980; Aulstead & Spencer, 1985; Machel & Mount-joy, 1987; Aulstead et al., 1988; Qing & Mountjoy,1989; Machel & Anderson, 1989; Mountjoy &Amthor, 1994). Dolomites formed in differentdiagenetic environments can be recognized bytheir petrography, spatial distribution, geochem-ical and isotopic signatures due to variations in¯uid-driving mechanisms, temperature andchemical compositions of the dolomitizing ¯uids(Land, 1985; Machel & Mountjoy, 1986; Hardie,1987).

The Presqu'ile barrier in the Western CanadaSedimentary Basin offers a good opportunity tostudy various massive replacement dolomites.Four types of dolomites are identi®ed in thePresqu'ile barrier (Qing, 1991; Qing & Mountjoy,1990). In paragenetic sequence, they are ®ne-,medium-, and coarse-crystalline replacement do-lomites, and saddle dolomite cements. `Fine-crystalline dolomite' of previous studies (Jackson& Beales, 1967; Skall, 1975; Kyle, 1981, 1983;Krebs & Macqueen, 1984; Rhodes et al., 1984) ishere divided into ®ne- and medium-crystallinedolomites based on their different petrographictextures, lithofacies associations, and geochemis-try (Qing, 1991). Because the coarse-crystallineand saddle dolomites have been discussed inprevious publications (Qing & Mountjoy, 1992,1994a,b,c), this paper focuses on the petrographyand geochemistry of ®ne- and medium-crystallinedolomites, and compares them with late-stagecoarse-crystalline and saddle dolomites.

GEOLOGICAL SETTING

The Middle Devonian Presqu'ile barrier is locatedin the northern part of the Western CanadaSedimentary Basin (Fig. 1). It is a linear carbonatereef complex at least 400 km long with a widthranging from 20 km to 100 km. It extends fromoutcrops in the Northwest Territories near PinePoint into the subsurface of north-eastern BritishColumbia (Fig. 1). The thickness of the Presqu'ilebarrier is about 200 m in the Pine Point area. Thedevelopment of the Presqu'ile barrier restrictedseawater circulation in the Elk Point Basin duringMiddle Devonian time. As a result, evaporitesand carbonates were deposited south of thebarrier in the Elk Point Basin, whereas normalmarine sediments were deposited north of thebarrier (Fig. 1). Pine Point is located towards theeast end of the barrier (Fig. 1), where variousdolomite rocks host more than 80 individualMVT ore bodies.

A major basement fault, the McDonald Fault,occurs in the Pine Point area (Rhodes et al., 1984;Krebs & Macqueen, 1984) (Fig. 1). Based on aero-magnetic data, the McDonald Fault can be tracedfrom the Canadian Shield into the subsurface of theWestern Canada Sedimentary Basin (Jones, 1980;Ross et al., 1991) (Fig. 1). The McDonald Fault mayhave been reactivated during Middle Devonian orpost Devonian times and played a role in conduct-ing the diagenetic ¯uids for late stage dolomitizat-ion and mineralization (e.g. Campbell, 1966; Skall,1975; Krebs & Macqueen, 1984), although there isno de®nitive evidence to support this. The role ofthe McDonald Fault during Middle Devonian timewas questioned by Rhodes et al. (1984) becausestrata immediately beneath the barrier are `ofuniform thickness without any signs of verticaldisplacement'.

The regional geology at Pine Point and diage-netic processes have been described in details byJackson & Beales (1967), Skall (1975), Kyle (1981,1983), Krebs & Macqueen (1984), Rhodes et al.(1984), Qing (1991), Qing & Mountjoy (1990,1994a,b,c). The following is a brief summary ofthe stratigraphy and diagenesis of the Presqu'ilebarrier. The Keg River Formation represents aregional shallow-water platform that underliesthe barrier (Fig. 2). It consists of grey-browndolostones with abundant crinoid and brachio-pod fragments. The carbonate barrier itself com-prises the Pine Point and overlying SulphurPoint Formations (Fig. 2). Both were originallydeposited as marine limestones that since havebeen variably dolomitized to medium- and

434 H. Qing

Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 433±446

coarse-crystalline dolomites. The Pine Point andSulphur Point Formations grade southward into®ne-crystalline dolomite and anhydrite of theMuskeg Formation, deposited in the Elk PointBasin. To the north of the barrier marine shalesand carbonates of the Buffalo River and WindyPoint Formations inter®nger with and onlap thenorth ¯ank of the Presqu'ile barrier. The Pres-qu'ile barrier was subaerially exposed shortlyafter it developed, forming the sub-Watt Moun-tain unconformity. This unconformity is devel-oped over a wide area in the northern part of theWestern Canadian Basin (Meijer-Drees, 1988).The unconformity is overlain by the Watt Moun-tain Formation, which consists primarily ofgreen, silty, pyritic shales, locally alternatingwith nodular and argillaceous limestones ordolostones, limestone breccia, and minoramounts of anhydrite (Rhodes et al., 1984).

The diagenetic features in the carbonate rocksof the Presqu'ile barrier (Fig. 3) have been inter-preted to have occurred in submarine, subaerialand subsurface environments (Qing, 1991; Qing &Mountjoy, 1990). Submarine diagenetic featuresinclude: micrite envelopes, micrite, syntaxialcement, microspar cement, ®brous cement, and®ne crystalline dolomite. Subaerial diagenesisformed minor, localized pendant cements, anddissolution and brecciation (Qing & Mountjoy,1994b). The most signi®cant alteration, however,was produced by basinal and hydrothermal ¯uidsduring burial (Krebs & Macqueen, 1984; Qing,1991; Qing & Mountjoy, 1990, 1992, 1994a,b,c).Subsurface diagenesis resulted in blocky sparrycalcite cement, compaction and stylolitization,coarse-crystalline and saddle dolomites, whitecalcite, sulphide mineralization, and late-stagecalcite, ¯uorite, and bitumen.

Fig. 1. Simpli®ed regional geological map of the Western Canada Sedimentary Basin during the Middle Devonian.The Presqu'ile barrier outcrops in the Pine Point area west of the Canadian Shield. It extends westward into thesubsurface of Northwest Territories and north-eastern British Columbia where its present burial depths are 2000±2300 m. Fine-crystalline dolomites were also sampled from 4 wells west of Pine Point: the open square is CameronHills F-51; the open circle, Pan Am Cameron C-22; solid dot, Murphy Alexandra Fall J-26; and cross, Cameron A±05.The insert map shows Pine Point property with locations of open pits (solid dots) and three traverses (A±A¢, B±B¢,and C±C¢) that were studied for lithofacies and spatial distribution of dolomites. The stratigraphy and dolomitedistribution of A±A¢ section are shown in Fig. 2.

Petrography and geochemistry of early dolomites at Pine Point 435


METHODS

At Pine Point, a number of open pits and a seriesof drill holes from three representative traverseswere systematically studied and sampled, and thespatial distribution of the various dolomitesmapped (Figs 1 and 2). Core samples from fourwells from the downdip, subsurface portion of thePresqu'ile barrier, west of Pine Point were studiedand sampled (Fig. 1). Approximately 160 thinsections were studied and all were stained withalizarin-red S and potassium ferricyanide (Dick-son, 1965). 100 thin sections were also studiedusing cathodoluminescence (CL) and representa-tive samples were studied with a scanningelectron microscope (SEM).

For carbon and oxygen isotope analyses, pow-dered carbonate samples ranged from 0á2±0á5 mgwere drilled from polished slabs using 100 lmdiameter drills. These samples were roasted at380 °C under vacuum for 1 hr to remove organicmatter. Samples were then reacted with anhy-drous phosphoric acid at 55 °C for about 10 minfor calcite and about 1 hr for dolomite. This was

done in an on-line gas extraction system connect-ed to an inlet of a VG 602E mass spectrometer. Allthe analyses were corrected for 17O (and reported

Fig. 3. Diagenetic paragenesis of the Presqu'ile barrierdetermined from petrographic study of samples fromopen pits and cores. Abbreviation: cx. � crystalline.Modi®ed from Qing and Mountjoy (1990).

Fig. 2. Stratigraphy and dolomitedistribution of traverse A-A¢ inthe western part of Pine Pointproperty (see Fig. 1 for location)based on core examination. Thenumbers are borehole numbers ofPine Point Mines and the thinvertical lines are cored intervals.(A) Stratigraphy of Presqu'ilebarrier at Pine Point. (B) Gener-alized spatial distribution of dif-ferent types of dolomites.Modi®ed after Rhodes et al.(1984) and Qing and Mountjoy(1994a).

436 H. Qing


in parts per thousand (&) relative to the Pee DeeBelemnite (PDB) standard). Analytical precisionwas monitored through daily analysis of pow-dered carbonate standards and maintained tobetter than 0á1& (1 r) for both d18O and d13C.

For Sr isotope analyses, » 5±15 mg sampleswere crushed in a stainless steel mortar prior tochemical dissolution. The dolomite samples wereleached in 0á5 N HCl to extract Sr. Sr wasextracted from the leachate with 2á5 N HCl usingstandard cation exchange techniques. The sam-ples were loaded as SrCl2 on a single Ta ®lamentand were analysed using a VG 354 thermalionization mass spectrometer. An error of �0á0025% has been calculated for the data, whichis the average 2r error for the internal precision ofa single analysis. The average value of the NBS987 standard during the course of the study was0á71022. The isotopic ratios of samples werenormalized against this value (Mountjoy et al.,1992).

Major (Ca and Mg) and trace elements (Sr, Na,K, Mn, Fe, and Ba) were analysed using an atomicabsorption spectrometer (AAS). Powdered dolo-mite samples (0á5 g) were dissolved in 10 mL 8%HCl solutions, which were ®ltered with ashlesspaper and then analysed. The detection limitswere: Sr 6 p.p.m., Na 5 p.p.m., K 10 p.p.m., Mn 2p.p.m., Fe 6 p.p.m., and Ba 40 p.p.m. Thereproducibility for duplicates was better thanthree relative percentage. The stoichiometry ofdolomites is calculated from major element con-centrations.

REE concentrations were determined, using aninductively coupled plasma mass spectrometer(ICP-MS). Samples (0á1 g) were digested in 1 N

HNO3 and analysed by ICP-MS using the methodof standard additions to correct for matrix effects.The reproducibility is within � 2%. The detec-tion limits and reagent blanks are from 0á001p.p.m. for Yb to 0á026 p.p.m. for Sm. Identi®ca-tion of REE fractionation was made by normaliz-ing the sample REE concentration to averagechondritic meteorites, making the data compara-ble to previous studies on ancient dolomites (e.g.Graf, 1984; Banner et al., 1988a,b; Dorobek &Filby, 1988).

PETROGRAPHY AND SPATIALDISTRIBUTION

Fine-crystalline dolomite

Fine-crystalline dolomite is found only in theback-barrier facies in the Muskeg Formation on

the south side of the Presqu'ile barrier based onthe core examination of three traverses fromeastern, central, and western part of Pine Pointproperty and of selected wells in the adjacentsubsurface (Figs 1 and 2). It is interbedded withevaporitic anhydrite in the Elk Point Basin. Fine-crystalline dolomite is either brown or grey inhand sample (Fig. 4a). Fossils and sedimentarystructures are well-preserved while fractures andvugs are usually ®lled with ®ne-crystalline anhy-drite (Fig. 4b).

Fine-crystalline dolomite consists of subhedralto euhedral crystals, ranging from 5 to 25 lm(mean 8 lm) with plane extinction (Fig. 4c).Crystal size gradually increases (to 100 lm) to-wards orebodies, suggesting possible neomorp-hism and/or recrystallization by later mineraliz-ing ¯uids. Under the SEM, ®ne-crystallinedolomite consists of euhedral crystals withsmooth and uncorroded crystal surfaces (Fig. 4d).Micro, intracrystalline pores (1±10 lm) are rareand porosity is low because of the interlockingtexture.

Medium-crystalline dolomite

Medium-crystalline dolomite is the most abund-ant type of dolomite in the Pine Point area. Itoccurs mainly in the southern lower part of thebarrier in the Pine Point Formation (Fig. 2b)which is in direct contact with evaporitic anhy-drite and ®ne-crystalline dolomite of the MuskegFormation in the Elk Point Basin. In the PinePoint area, there are two layers of porous sucrosicmedium-crystalline dolomite: one at the base ofthe Pine Point Formation, the B Spongy Memberof Rhodes et al. (1984); the other occurring justbelow the coarse-crystalline dolomite.

Medium-crystalline dolomite is medium todark brown. Fossil and sedimentary textures aregenerally recognizable (Fig. 5a), while dissolu-tion vugs and fractures are locally developed(Fig. 5b) and part ®lled by minor amounts of late-stage saddle dolomite or anhydrite cements.Medium-crystalline dolomite is composed ofanhedral to subhedral dolomite crystals (150±250 lm, mean 200 lm), with well de®ned crystalboundaries (Fig. 5c). In contrast to the coarse-crystalline dolomite that commonly displaysundulatory extinction (Qing & Mountjoy,1994a), these dolomites have plane extinction.Some medium-crystalline dolomite crystals haveclear rims toward the pore centres (Fig. 5c).Medium-crystalline dolomite is generally red/orange under CL, but bright orange blotches occur



along some crystal boundaries. Under SEM(Fig. 5), medium-crystalline dolomite consists ofanhedral±subhedral crystals with variableamounts of intra- and inter-crystalline porosity,suggesting possible dissolution by later diagenet-ic ¯uids.

GEOCHEMISTRY

Oxygen and carbon isotopes

Three ®ne-crystalline dolomites sampled fromthe Muskeg Formation in the subsurface of theElk Point Basin had d18O values from )1á6 to)3á8& PDB and d13C from 0á27±1á7& PDB(Fig. 6). However, ®ne-crystalline dolomite sam-pled around orebodies at Pine Point, hereaftertermed `altered, ®ne-crystalline dolomites' haslower d18O values ()6á7 to )7á9& PDB) (Fig. 6).Twenty-one medium-crystalline dolomite sam-ples yield d18O values from )3á7 to )9á4& PDB(mean )5á9& PDB) and d13C from 0á6±2á5& PDB.Their d18O values are distinctly more negativethan ®ne-crystalline dolomite (Fig. 6). Two bra-chiopod shell fragments from the Sulphur PointFormation have d18O values of )3á8 and )4á2&PDB and d13C of 1á8 and 1á9& PDB, respectively(Fig. 6).

Sr isotopes

The Sr isotope data reveal a general increase in87Sr/86Sr ratios from ®ne-crystalline to medium-crystalline dolomites (Fig. 7). Three ®ne-crystal-line dolomites yield 87Sr/86Sr ratios of 0á7079±0á7081 (Fig. 7). One sample of primary anhydriteassociated with these ®ne-crystalline dolomitehas a 87Sr/86Sr ratio of 0á7078. However, twoaltered ®ne-crystalline dolomites sampled adja-cent to orebodies at Pine Point have slightlyhigher 87Sr/86Sr ratios (0á7082 and 0á7083)(Fig. 7). One of three medium-crystalline dolo-mites has 87Sr/86Sr ratio of 0á7081, overlappingwith those of the ®ne-crystalline dolomite but theother two are more radiogenic (0á7085 and 0á7087,respectively) than those of ®ne-crystalline dolo-mites (Fig. 7).

Fig. 4. Petrographic features of ®ne-crystalline dolo-mite. (A) Hand sample of ®ne-crystalline dolomite.Location: Muskeg Formation, Pine Point diamond drillhole 2162, 614 ft. Scale in cm. (B) Fine-crystalline do-lomite with anhydrite nodules and interlayer. Location:Muskeg Formation, Pine Point diamond drill hole2162, 645 ft. Scale in cm. (C) Thin section photomi-crograph of ®ne-crystalline dolomite with anhedraldolomite crystals ranging from 15 to 25 lm. Location:Muskeg Formation, well C-22, 3192 ft. Scale bar200 lm. (D) SEM photomicrograph of ®ne-crystallinedolomite. Euhedral dolomite crystals have smooth anduncorroded surface with rare micro intracrystal pores(arrow). Location: Muskeg Formation, Pine Point dia-mond drill hole 2162, 645 ft. Scale bar 10 lm.

438 H. Qing


Trace elements

Forty-®ve dolomite samples were analysed formajor (Ca and Mg) and trace elements (Sr, Na, K,Mn, Fe, and Ba). Only Sr and Mn displaycovariation (Fig. 8); other elements show no clearcovarying trends. Fine-crystalline dolomite has Srconcentrations from 55 to 225 p.p.m. (mean 105p.p.m.) and Mn content from 25 to 160 p.p.m.

(mean 86 p.p.m.) (Fig. 8). The Sr concentrationsin medium-crystalline dolomite range from 30 to80 p.p.m. (mean 50 p.p.m.) and Mn content from50 to 253 p.p.m. (mean 161 p.p.m.). Therefore,there is a general trend of decrease in Sr andincrease in Mn content from ®ne-crystalline tomedium-crystalline dolomites (Fig. 8).

Rare earth elements

Three ®ne-crystalline dolomites (Muskeg Forma-tion) are enriched in light REE but depleted in Eu,decreasing in concentration from La to Lu(Fig. 9a). The REE patterns of two of these samples

Fig. 6. Cross plot of d18O and d13C values for: (1) ®ne-crystalline dolomite (2) altered, ®ne-crystalline dolo-mite associated with mineralization, and (3) medium-crystalline dolomite. The possible d18O and d13C valuesof Middle Devonian seawater dolomite were calculatedassuming a temperature of 20 °C and d18O valuesMiddle Devonian seawater of )2á0 to )3& SMOW (seetext for discussion).

Fig. 5. Petrographic features of medium-crystallinedolomite. (A) Core sample of medium-crystalline do-lomite with well preserved dolomitized fossil frag-ments. Location: Pine Point Formation, Pine Pointdiamond drill hole 2813, 530 ft. Scale in cm. (B) Coresample of medium-crystalline dolomite with moldicand vuggy porosity. Location: Pine Point Formation,Pine Point diamond drill hole 2813, 870 ft. Scale in cm.(C) Thin section photomicrograph of medium-crystal-line dolomite. Some crystals have cloudy centres andclear rims, suggesting possible neomorphism or re-crystallization. Location: Pine Point Formation, PinePoint diamond drill hole 2813, 642 ft. Scale bar200 lm. (D) SEM photomicrograph of medium-crystal-line dolomite. Subhedral dolomite crystals have vari-able amounts of intracrystal pores (arrows). Location:Pine Point Formation, Pine Point diamond drill hole2813, 530 ft. Scale bar 100 lm.



are similar to those of marine limestone samplesfrom Pine Point (Fig. 9a), while the REE pattern ofthe third is similar to those of medium-crystallinedolomites, with a negative Ce anomaly (Fig. 9a).

REE patterns of ®ve medium-crystalline dolo-mite samples from the Pine Point Formation inthe Pine Point property differ from those of themarine limestone and most ®ne-crystalline dolo-mites. Relative to chondrite, the REE contentsdecrease from La to Sm, are nearly constant fromGd to Tm, and decrease again from Yb to Lu(Fig. 9b). Medium-crystalline dolomites also havenegative Ce anomalies compared with those ofmarine limestones (Fig. 9b).

DISCUSSION AND INTERPRETATION

The combined petrography, spatial distribution,diagenetic paragenesis, and geochemical datasuggest the following interpretation.

1 Fine-crystalline dolomites are restricted to theback-barrier facies and are interbedded withMuskeg evaporitic anhydrites of the Elk PointBasin. Because the Muskeg Formation evaporiteshave low porosity and permeability and areregarded as a regional aquiclude (e.g. Bachu,1997), large-scale ¯uid migration, a requirementfor massive dolomitization (e.g. Land, 1985),seems unlikely. This suggests that the ®ne-crys-talline dolomite formed penecontemporaneouslyat or just below the sea ¯oor from seawater beforelitho®cation of these evaporitic deposits.2 Because the Western Canada SedimentaryBasin was located within 30° of the palaeo-equator from Devonian to Triassic time (Habicht,1979), the average temperature for Devonianseawater was assumed to be 20 °C. Using d18Ovalues measured from brachiopod shells fromPine Point area ()3á85 and )4á21& PDB, Fig. 6)and a temperature of 20 °C, the calculated d18Ovalue of Middle Devonian seawater is about )2á0

Fig. 8. Cross plot of Sr and Mnconcentration for ®ne-crystallineand medium-crystalline dolomites.

Fig. 7. 87Sr/86Sr ratios of dolomitesand anhydrite in the Presqu'ile bar-rier. The best estimated 87Sr/86Srratio of Middle Devonian seawater,according to Burke et al. (1982),ranges from 0á7078±0á7081.

440 H. Qing


to )3& Standard Mean Ocean Water (SMOW),using Friedman & O'Neil (1977) equation. Thisvalue is comparable with the estimated d18Ovalue of )2& SMOW for Devonian seawaterbased on a larger sample set by Popp et al.(1986). The d18O values of dolomite precipitatedin bulk solution equilibrium with this MiddleDevonian seawater at a temperature of 20 °Cshould range from )1á2 and )2á2& PDB, calcu-lated using an equation of Land (1985). Theheaviest d18O of the unaltered ®ne-crystallinedolomite is )1á58& PDB, which falls within therange of calculated Middle Devonian seawaterdolomite (Fig. 6), suggesting that the dolomitizing

¯uids for ®ne-crystalline dolomites were proba-bly Middle Devonian seawater.3 Analysis of marine carbonates indicates thatthe 87Sr/86Sr ratio of oceans has varied systemat-ically throughout Phanerozoic time, but hasapparently been constant in the open ocean atany given time (Burke et al., 1982). As theisotopic fractionation of Sr during carbonateprecipitation is negligible, the Sr isotopic com-position of marine carbonates are assumed torepresent those of seawater at the time of depo-sition (Veizer, 1983). Therefore, unaltered MiddleDevonian marine carbonates should have a87Sr/86Sr ratio similar to Middle Devonian sea-water, which is estimated to be between 0á7078and 0á7081 (Fig. 7) (Burke et al., 1982). The87Sr/86Sr ratios of unaltered ®ne-crystalline dolo-mite range from 0á7079±0á7080. These values arewithin the range of the estimated 87Sr/86Sr ratio ofMiddle Devonian seawater (Fig. 7), also suggest-ing that the dolomitizing ¯uids were MiddleDevonian seawater.4 From a study of Miocene marine dolomitesfrom Little Bahama Bank, Vahrenkamp & Swart(1990) suggested that the amount of Sr in dolo-mite is related to both the Sr content of thedolomitizing ¯uids and the stoichiometry of thedolomite crystals. A dolomite with 50±51 mol %CaCO3 precipitating in equilibrium with seawatershould have a Sr content of about 50±70 p.p.m.,but a nonstoichiometric marine dolomite with 60mol percentage CaCO3 could have a Sr content ashigh as 250 p.p.m. The Sr concentrations of PinePoint ®ne-crystalline dolomite range from 55 to225 p.p.m., and follow the same trend as LittleBahama seawater dolomites (Fig. 10), suggestingthat ®ne-crystalline dolomite may have formedfrom ¯uids with Sr2+/Ca2+ ratios similar to thoseof modern seawater.5 The REE patterns of two of the three ®ne-crystalline dolomite samples are similar to thoseof the marine limestone samples (Fig. 9), suggest-ing that either the ®ne-crystalline dolomite mayhave formed from ¯uids with similar REE char-acteristics as marine limestones, and/or that theREE patterns of the precursor limestones were notsigni®cantly altered during/after dolomitization.

Thus, the ®ne-crystalline dolomite probablyformed from Middle Devonian seawater at or justbelow the sea ¯oor. Those sampled near the PinePoint orebodies, however, have been altered bylater diagenetic ¯uids during mineralization asindicated by their larger crystal size, lower d18Ovalues ()6á7 to )7á9& PDB), and slightly higher87Sr/86Sr ratios (0á7082±0á7084).

Fig. 9. (A) REE patterns of three ®ne-crystalline dolo-mites sampled from the Muskeg Formation in drillcores about 5 km south of the Presqu'ile barrier wherethe in¯uence of late-stage hydrothermal ¯uids is min-imal. (B) REE patterns of 5 medium-crystalline dolo-mites from the Pine Point Formation. Shaded area in(A) and (B) represents the REE patterns of marinelimestones from Presqu'ile barrier.



For the medium crystalline dolomite, the dataconstrain but do not prove an unequivocal origin.The medium-crystalline dolomite has d18O valueslower than calculated d18O values for dolomiteprecipitated in equilibrium with Middle Devon-ian seawater. The lower d18O values could haveoriginated from compaction ¯uids of MiddleDevonian seawater parentage during shallow tointermediate burial at slightly elevated tempera-tures. Such ¯uids would have d18O values similarto seawater, or slightly heavier as a result ofrecrystallization of the rock matrix in pore ¯uidsof seawater parentage (Machel & Anderson, 1989).Taking the highest d18O value of )3á7& PDB asthe possible original d18O signature of medium-crystalline dolomite and )2 to )3& SMOW asrepresentative of Middle Devonian seawater, thetemperature during dolomitization would beabout 35±40 °C (calculated using the equation ofLand, 1985). Using an average geothermalgradient of 30 °C km)1, the burial depth ofdolomitization would be 500±700 m. Based onthe reconstructed burial history of the Presqu'ilebarrier at the Pine Point (Mountjoy & Amthor,1994), medium-crystalline dolomite would haveoccurred during the late Devonian time. Thisinterpretation is also supported by the Sr isotopedata. Medium-crystalline dolomite has slightlyhigher 87Sr/86Sr ratios than Middle Devonianseawater (Fig. 7), suggesting burial dolomitizing¯uids with slightly radiogenic Sr isotopes.

The REE patterns of medium-crystalline dolo-mite differ from those of the precursor limestones,suggesting a high water/rock ratio during do-lomitization (cf. Banner et al., 1988a,b). Thenegative Ce anomalies in the REE patterns ofmedium-crystalline dolomite compared withthose of precursor limestones (Fig. 9b) indicate arelative depletion of Ce in the dolomitizing ¯uids.The negative Ce anomalies commonly occur in

the natural oxidizing waters (e.g. in seawater),because the solubility of Ce4+ is low in theoxidizing environment and is removed preferen-tially with authigenic Fe-Mn oxides or scavengedby particulate matter (e.g. Elder®eld et al., 1981;DeBaar et al., 1985). The negative Ce anomalies inthe medium-crystalline dolomite therefore couldsuggest (1) an oxidizing condition during do-lomitization, or (2) removal of Ce from dolomiti-zing ¯uids in an oxidizing environment beforedolomitization. If later is the case, the medium-crystalline dolomite would have inherited thenegative Ce anomaly from dolomitizing ¯uidsregardless of whether dolomitization occurred inan oxidizing or reducing environment. Becausethe similarity between the smaller ionic radius ofMg and the ionic radii of the heavy REE (Graf,1984; Dorobek & Filby, 1988), dolomite should beenriched in heavy REE relative to calcites. Theenrichment of heavy REE in the medium crystal-line dolomite relative to the precursor limestonesmay therefore have resulted from the largerdistribution coef®cients of heavy REE for dolo-mite than calcite.

An alternative interpretation is that medium-crystalline dolomite formed soon after sedimen-tation of the Pine Point Formation by MiddleDevonian seawater derived from the Elk PointBasin (Skall, 1975; Kyle, 1981, 1983; Rhodes et al.,1984; Krebs & Macqueen, 1984). This interpretat-ion is based primarily on the spatial distributionand geochemical trends of medium-crystallinedolomite. Medium-crystalline dolomite occursmostly in the southern lower part of the Pres-qu'ile barrier in the Pine Point Formation(Fig. 2b). The absence of medium-crystallinedolomite in the upper part of the barrier doesnot appear to be due to lower porosity and per-meability because the upper part of the barrierwas intensively dolomitized to coarse-crystalline

Fig. 10. Cross plots of Sr (p.p.m.)and mol percentage MgCO3 for ®ne-crystalline and medium-crystallinedolomites. The trend line is for Mi-ocene marine dolomites from LittleBahama Bank (Vahrenkamp &Swart, 1990).

442 H. Qing


dolomite during later diagenesis (Qing & Mount-joy, 1994a). Thus, the restriction of medium-crystalline dolomite to the southern lower part ofthe barrier suggests dolomitizing ¯uids may have

been derived from the Elk Point Basin to thesouth of the barrier.

If medium-crystalline dolomite was formed byMiddle Devonian seawater, it should havegeochemical signatures of Middle Devonian sea-water, similar to the ®ne-crystalline dolomitediscussed above. However, its geochemistry isdifferent (lower d18O values, slightly higher87Sr/86Sr ratios, and relatively low Sr but highMn concentrations). This difference could havebeen caused by neomorphic alteration duringlater burial diagenesis. Dolomites can form as aCa-rich, poorly ordered metastable phase, whichwould experience crystal enlargement and chem-ical modi®cation during progressive stabilization(Land, 1985; Mazzullo, 1992). Unless earlierformed dolomites have a low porosity and per-meability, their initial geochemical properties canbe altered at elevated temperatures in the subsur-face (Land, 1985; Mazzullo, 1992). This could bethe case for the medium-crystalline dolomite atPine Point, which is generally porous and per-meable and regarded as part of a regional aquifer(Bachu, 1997). It occurs immediately beneath themassive coarse-crystalline dolomite and Pb-Znminerals that were related to the hydrothermal¯uids (Krebs & Macqueen, 1984; Rhodes et al.,1984; Qing & Mountjoy, 1994a,b). The medium-crystalline dolomite therefore could have beenneomorphosed by later hydrothermal ¯uids atelevated temperatures in the subsurface. Somemedium-crystalline dolomite crystals havecloudy centres and clear rims (Fig. 5) and somehave blotchy of bright orange (CL) suggesting

Fig. 11. The d18O pro®le of medium- and coarse-crys-talline dolomites from borehole 2813, Pine Point. Me-dium-crystalline dolomite closer to the medium/coarsecrystalline dolomite boundaries have lower d18O val-ues, suggesting that medium-crystalline dolomiteprobably have been modi®ed by later diagenetic ¯uidsthat precipitated coarse-crystalline dolomite.

Fig. 12. Cross plot of 87Sr/86Sr ra-tios and d18O values of variousdolomites associated with Pres-qu'ile barrier. The geochemicalsignatures of medium-crystallineand altered ®ne-crystalline dolo-mites can be interpreted as a mix-ture of two end members, onerepresents Devonian seawater sig-natures, and the other correspondsto hydrothermal ¯uids associatedwith late-stage dolomitization andmineralization.



neomorphism by later diagenetic ¯uids (cf. Ban-ner et al., 1988a,b).

Neomorphism is also indicated by the graduallowering of d18O values for medium-crystallinedolomites when approaching the medium/coarsecrystalline dolomite contact at borehole 2813from the Pine Point area (Fig. 11). The d18O valueof medium-crystalline dolomite remains almostconstant at about )4& PDB for samples from30 m below the medium/coarse crystalline dolo-mite contact. However, near the contact d18Ovalue of medium-crystalline dolomite displays agradual shift towards lower values (Fig. 11). Atthe medium/coarse crystalline dolomite contact,the d18O value of medium-crystalline dolomitereaches )7á6& PDB, a value very close to thatof the adjacent coarse-crystalline dolomite(Fig. 11). This suggests that d18O values of medi-um-crystalline dolomite may have been modi®edor reset by later diagenetic ¯uids that wereresponsible for the formation of coarse-crystallinedolomite.

Alteration and modi®cation of medium-crys-talline dolomite by later diagenetic ¯uids are alsosupported by cross plotting 87Sr/86Sr ratios andd18O values of various types of dolomites associ-ated with the Presqu'ile barrier (Fig. 12). Ageneral trend of progressively decrease in d18Owith a corresponding increase in 87Sr/86Sr ratiosfrom ®ne-crystalline, to medium-crystalline,coarse-crystalline and saddle dolomites occurs(Fig. 12). Two end members are evident in thesedolomites. One is the unaltered ®ne-crystallinedolomite, that has the lowest 87Sr/86Sr (0á7079)and the highest d18O values ()1á6 to )3á8& PDB),representing dolomitization by the Middle Dev-onian seawater. The other end member (coarse-crystalline and saddle dolomites) has the mostradiogenic 87Sr/86Sr ratios and lowest d18O valuesand characterizes dolomitization associated withlate-stage hydrothermal ¯uids (see Qing & Mount-joy, 1994a for details). Medium-crystalline dolo-mite and the altered ®ne-crystalline dolomitenear the orebodies have 87Sr/86Sr ratios andd18O values between these end members(Fig. 12) consistent with early dolomitization byMiddle Devonian seawater later modi®ed byhydrothermal ¯uids.

CONCLUSIONS

Fine-crystalline dolomite associated with theMiddle Devonian Presqu'ile barrier at Pine Point

is interpreted to have formed penecontemporane-ously at or just below the sea ¯oor from MiddleDevonian seawater, because: (1) it is interbeddedwith evaporitic anhydrites in the back-barrierfacies; (2) its oxygen isotopes and Sr isotopecompositions are similar to dolomites precipitat-ed from Middle Devonian seawater; (3) and its Srconcentration and mol percentage CaCO3 followsthe same trend as do Little Bahama Bank seawaterdolomites.

Medium-crystalline dolomite probably formedduring shallow burial by ¯uids derived fromcompaction based on its lower d18O valuesslightly radiogenic 87Sr/86Sr ratios relative to the®ne crystalline dolomite, and different REE pat-terns from the limestone precursors. An alternateinterpretation is that medium-crystalline dolo-mite formed soon after deposition by Devonianseawater from the adjacent Elk Point basin, butwas altered at higher temperatures by the laterdiagenetic ¯uids. This is supported by the re-striction of medium-crystalline dolomite to thesouthern lower part of the Presqu'ile barrier,which is in direct contact with evaporites in theElk Point Basin. If so, its geochemical signaturescan be interpreted as mixture of two end mem-bers: one from Devonian seawater; and the otherfrom hydrothermal ¯uids associated with late-stage dolomitization and mineralization.

This study indicates that dolomitization in theMiddle Devonian Presqu'ile barrier occurred in atleast two stages in different diagenetic environ-ments during evolution of the Western CanadaSedimentary Basin. The geochemistry of earlierformed dolomites could have been modi®ed ifthey were porous and permeable and water/rockratio was large during neomorphism.

ACKNOWLEDGMENTS

The geochemical data were gathered during myPhD research at McGill. I am greatly indebted toDr E. Mountjoy for his help throughout thisproject. Partial funding was provided by McGillUniversity, Geological Survey of Canada, aNSERC grant to E. Mountjoy, and Royal Hollow-ay, University of London. I am grateful to PinePoint Mines of Cominco Ltd. for information and®eld assistance. I appreciate discussions with J.Amthor, D. Bosence, R. Hesse, H. Machel, R.Macqueen, E. Mountjoy, and D. Sangster; andcomments by the Journal reviewers, A. Kendalland D. Morrow; and editor, J. Andrews.

444 H. Qing


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Manuscript received 27 January 1997; revision accept-ed 5 August 1997.

446 H. Qing


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