massive dolomitization of authorsuregina.ca/~chiguox/s/2010 lavoie et al aapg htd.pdf · 2010. 4....

AUTHORS

D. Lavoie � Geological Survey of Canada-Quebec, 490 de la Couronne, Quebec City,QC G1K 9A9, Canada; [email protected]

Denis Lavoie has been a research scientist for theGeological Survey of Canada since 1989. Hereceived his B.Sc.A. degree from Laval University,Canada, in 1982 and his Ph.D. in geology fromthe same university in 1988. His research focuseson basin evolution and evaluation of hydrocar-bon systems of Paleozoic basins in eastern andnorthern Canada, with a personal interest incarbonate reservoir diagenesis.

G. Chi � Department of Geology, Universityof Regina, Regina, SK S4S 0A2, Canada;[email protected]

Guoxiang Chi joined the University of Regina

Massive dolomitization ofa pinnacle reef in the LowerDevonianWest Point Formation(Gaspé Peninsula, Quebec):An extreme case of hydrothermaldolomitization throughfault-focused circulation ofmagmatic fluidsD. Lavoie, G. Chi, M. Urbatsch, and W. J. Davis

in 2002 and is an associate professor with theDepartment of Geology. He received his B.Sc.degree from Fuzhou University, China, in 1983and his Ph.D. in mineral resources from theUniversity of Quebec at Chicoutimi in 1992. Hisresearch focuses on geochemistry and hydro-dynamics of geologic fluids with fluid inclusions,stable isotopes, and numerical modeling offluid flow as the main tools.

M. Urbatsch � Department of Geology,University of Regina, Regina, SK S4S 0A2,Canada; [email protected]

Misty L. Urbatsch received a B.Sc. degree witha double degree in geology and geography fromthe University of Regina in 2008. She was hiredby Cameco Australia Pty. Ltd. in 2008 as an ex-ploration geologist and is currently working inAustralia exploring for unconformity style andsandstone-hosted uranium deposits.

W. J. Davis � Geological Survey of Canada-Ottawa, 601 Booth Street, Ottawa, Ontario K1A0E8, Canada; [email protected]

Bill Davis is a research scientist and the headof the geochronology laboratory at the GeologicalSurvey of Canada. He holds a B.Sc. degree ingeology from McGill University, an M.Sc. degree

ABSTRACT

Devonian pinnacle reefs of theWest Point Formation inGaspéPeninsula (eastern Canada) were built on paleotectonic highsin a foreland basin.Of the nine pinnacles known in outcrop, oneis dolomitized and occurs at the junction of two Acadian faults.

The petrography of the dolomitized facies has revealed thepresence of three dolomite phases and one late calcite cement.A first dolomite phase of small crystals is volumetricallyminor;the following dolomite phase dominates and consists of centi-meter-size replacive saddle dolomite crystals that contain fluidinclusionswith homogenization temperatures ranging between301 and 382°C. The third dolomite consists of millimeter- tocentimeter-size saddle dolomite crystals that fill late fractures;this phase is characterized by lower temperature fluid inclusions(159–171°C). A lower temperature calcite phase (107–123°C)fills some voids. Fluid inclusions are saline (8.7 to 13.3 wt.%NaClequiv). The dolomite and calcite phases are characterizedby very negative d18OVPDB (Vienna Peedee belemnite) values(between −19 and −14‰) and negative d13CVPDB values (be-tween −8 and −1‰). The replacive saddle dolomite phase orig-inated from a fluid with very positive d18OVSMOW (Vienna

from the University of Auckland, and a Ph.D.in geochemistry from Memorial University. Hisresearch involves the application of radiogenicisotopes and geochronology to the understand-ing of tectonic processes.Copyright ©2010. The American Association of Petroleum Geologists. All rights reserved.

Manuscript received May 2, 2009; provisional acceptance July 20, 2009; revised manuscript receivedAugust 21, 2009; final acceptance September 8, 2009.DOI:10.1306/09080909083

AAPG Bulletin, v. 94, no. 4 (April 2010), pp. 513–531 513

ACKNOWLEDGEMENTS

This article is dedicated to the memory of Pierre-André Bourque. His 30 years of dedicated andmeticulous work on carbonates on the GaspéPeninsula has led to our current understandingof facies architecture and evolution. Thanksare expressed to our colleagues working in theDelta Isotope and Geochronology laboratoriesof the Geological Survey of Canada. This articlebenefited from an early review by NicolasPinet and from diligent and detailed reviewsby Langhorne “Taury” Smith, Jeff Lonnee, andan anonymous reviewer. This is Geological Surveyof Canada contribution 20090075.The AAPG Editor thanks the following editorsfor their work on this article: Jeff Lonnee,Langhorne B. Smith, and an anonymous reviewer.

514 Hydrothermal Dolomitization of Lower Dev

standard mean ocean water) values (+8‰), whereas the fol-lowing dolomite and calcite precipitated from fluids with lowerd18OVSMOW values (+3.4 and +4.5‰). We propose that fault-focused circulation ofmagmatic fluids is responsible for the veryhigh-temperaturemassive dolomite replacement of the calcitehost, and high-temperature burial fluids later used fractures tocirculate in the dolomitized host to precipitate late dolomiteand calcite. Regionally, hydrocarbon migration is recognized atthe time of late calcite cementation.

INTRODUCTION

Hydrothermal dolomites are the primary exploration targetsfor conventional hydrocarbon reservoirs in the Paleozoic rocksof eastern North America (Davies and Smith, 2006; Smith,2006). Significant volumes of oil and gas have been producedfrom this somewhat process-controversial type of dolomite(Machel and Lonnee, 2002) in the Ordovician rocks of the Mi-chigan, Illinois, and Appalachian basins (Davies and Smith,2006). In eastern Canada, potential and demonstrated hydro-thermal dolomite reservoirs are recognized in passive marginand foreland basin Paleozoic carbonates, including (1) theLower Ordovician of western Newfoundland (Cooper et al.,2001; Azmy et al., 2008, 2009; Conliffe et al., 2009) and of An-ticosti Island (Lavoie et al., 2005; Lavoie and Chi, 2009); (2) theMiddle–Upper Ordovician of southern Quebec (Lavoie et al.,2009a), including Anticosti Island (Lavoie and Chi, 2009);(3) the Lower Silurian of Gaspé and northern New Brunswick(Lavoie and Morin, 2004; Lavoie and Chi, 2006, 2009); and(4) the Lower Devonian of Gaspé (Lavoie et al., 2001,2009a). The Jurassic carbonate reservoir rocks that host theDeep Panuke field offshore Nova Scotia are also interpreted tohave been formed by this process (Wierzbicki et al., 2006).

In the Paleozoic succession of the Upper Ordovician–Middle Devonian Gaspé belt in eastern Canada, a carbonatereef-rimmedplatform succession known as theWest Point For-mation straddles the Silurian–Devonian boundary (Bourque,2001). The Upper Silurian West Point Formation is a kilometer-thick, reef-rimmed platform in which three distinct reef com-plexes are recognized (Bourque, 2001); the overlying LowerDevonianWest Point Formation is represented by isolated pin-nacle reefs that grewup in response to rapid, tectonically driven,sea level rise (Bourque, 2001). Previous diagenetic researchon the Lower Devonian West Point pinnacles concluded thatthe primary porosity remained open for a significant period,mostly based on the high homogenization temperatures of fluid

onian Pinnacle Reef in Eastern Canada

inclusions, assuming normal burial geothermal gra-dients (Bourque et al., 2001a).

This article presents the results of a detaileddiagenetic study of a recently discovered dolomi-tized pinnacle reef (Lavoie, 2005) and integratesthe petrographic and geochemical informationwith the field characteristics of the dolomite body.The proposed model of dolomitization from veryhigh-temperature fluids is developed in light ofthe current understanding of tectonostratigraphicevolution, magmatic history, and fluid circulationin the Gaspé belt (Pinet et al., 2008; Lavoie et al.,2009b). Magmatic fluids are sometimes invokedin the formation of some high-temperature, dolo-mite-hosted Mississippi Valley-type (MVT) leadand zinc deposits (Sangster, 1996); the significanceof such fluids is commonly not considered in theformation of the hydrothermal dolomite hydrocar-bon reservoir, although these specific reservoirs areassumed to be closely process associatedwithMVTand sedimentary exhalative deposits (Davies andSmith, 2006).Thepresented case fromeasternCan-ada is the only onewe are aware ofwith evidence forhydrocarbon charge after the very high-temperature,massive hydrothermal dolomitization of the lime-stone host.

GEOLOGICAL SETTING

In eastern Canada, the Gaspé belt encompasses astratigraphic package of sedimentary and volcanicunits that were deposited between the Late Ordo-vician Taconic orogeny and the Middle DevonianAcadian orogeny (Figure 1) (Bourque et al., 2001b).Locally, the Late Silurian Salinic orogeny left itsmark at various localities (Lavoie, 2008).

Sedimentary and volcanic units assigned to theGaspé belt are found in major tectonostratigraphicassemblages, including fromnorth to south (Figure 1):(1) the Connecticut Valley-Gaspé synclinorium(CVGS), which consists of Upper Ordovician toMiddle Devonian units; (2) the Aroostook-Percéanticlinorium (APA), which consists of Upper Or-dovician to Lower Silurian units; and (3) the Cha-leurs Bay synclinorium (CBS), which encompassesUpper Ordovician to Lower Devonian units

(Bourque et al., 2001b). The contact betweenthe Gaspé belt (CVGS) and the Taconic alloch-thons to the north is either a fault (the ShickshockSud fault, Figure 1) or a major unconformity. Thecontact between the CVGS-APA and betweenAPA-CBS is commonly faulted, whereas a majorunconformitymarks the contact between the threetectonostratigraphic domains of the Gaspé belt andCarboniferous rock units.

Stratigraphy, Sedimentology, Paleoenvironments

The stratigraphic succession of the Gaspé belt wasdeposited in response to second-order, transgressive-regressive relative sea level fluctuations. Three re-gressive events (R1 to R3) are documented and areseparated by two transgressive episodes (T1 andT2)(Figure 2). These phases are used to divide the suc-cession into three stratigraphic packages. The Up-per Ordovician–Lower Silurian R1 succession isfollowed by the Lower–Upper Silurian T1-R2 pack-age, and the Gaspé belt succession is completed byan uppermost Silurian–Middle Devonian T2-R3stratigraphic interval. Shallow-marine carbonates arefound at the end of the first twomajor second-ordershallowing phases; the Lower Silurian Sayabec andLa Vieille formations are present at the end of theR1 phase (Lavoie et al., 1992), whereas the UpperSilurian West Point Formation is found at the endof the R2 phase (Bourque et al., 1986, 2001b;Bourque, 2001).

After the sea level lowstand (end R2) that re-sulted in the Upper Silurian Salinic unconformityat many places in eastern Quebec, another region-ally significant carbonate succession was depos-ited during the early phases of the transgressiveT3 event (Bourque, 2001; Bourque et al., 2001b).As sea level rose and covered the Salinic-exposedareas, a laterally well-zoned carbonate platform wasestablished in the Gaspé belt, forming a 1000-km(620-mi)-long reef tract that extended from Gaspéto upstate New York in the Late Silurian (Bourque,2001) (Figure 3). The Pridolian West Point reefcomplex consists of a laterally well zoned plat-form with fore-reef, reef margin, lagoonal or back-reef facies, and supratidal coastal plain (Bourque,2001). The fact that the reef margin reached a total

Lavoie et al. 515

Figure 1. Simplified geological map of the Gaspé Peninsula. The solid star locates the dolomitized pinnacle reef; the open star is for the closest field-exposed, nondolomitized pinnaclereef. The map is modified from Bourque et al. (2001b).

516Hydrotherm

alDolomitization

ofLowerDevonianPinnacle

ReefinEastern

Canada

vertical thickness ofmore than 600m (2000 ft) dur-ing a short time interval within the Pridolian (theentire Pridolian is 2.7 m.y.; Gradstein et al., 2004)is a strong indication of significant tectonic sub-sidence and generation of accommodation space.A significant increase in the rate of sea level rise oc-curred in the Early Devonian (Bourque, 2001), andonly the reef margin was able to keep pace for sometime, resulting in the formation of isolated pinnaclereefs, surrounded by fine-grained clastic sedimentsdeposited below wave base (Lachambre, 1987;Bourque, 2001).

The West Point pinnacle reefs have a verticalrelief of close to 300 m (1000 ft) and kilometer-wide bases. The internal architecture of these indi-vidual structures is complex, with stromatoporoid-coral-microbial bioerected zones with irregularly

distributed areas made up of reworked bioclasticsediments (Bourque, 2001) (Figure 3). For mostof the pinnacles, a conglomeratic fore-reef detritalbelt is developed around individual buildups. Thepinnacles were buried rapidly during the Lochkovianunder offshore fine-grained clastics of the IndianPoint Formation. From the current understandingof the stratigraphy of Silurian–Devonian succes-sions in northern Gaspé, the pinnacles were buriedunder a maximum of 5 km (3.1 mi) of Lower De-vonian synorogenic sediments (Bourque et al.,2001b; Lavoie, 2008).

Tectonic Setting of the Pinnacle Reefs

Based on field surveys,map relationships, and fromavailablepublicdomain seismic information,Bourque

Figure 2. Stratigraphicchart for the Upper Or-dovician to Middle Devo-nian succession formingthe Gaspé belt in western(Témiscouata), southern,and northern Gaspé. Thefirst two shallow-marinecarbonate intervals, out-lined by the gray pattern,occur at the end of majorsea level fall (R1 and R2).The Lower Devonian pin-nacle reefs (outline inblack) of the West PointFormation (WP) are onlyknown in northern Gaspéand formed at the onsetof a major relative sea levelrise (T2). The fine hatchedpattern is for the Salinicunconformity, whereas thelarge hatched pattern isfor the Taconic unconfor-mity. Details on the strati-graphy and large-scalesea level history of the areacan be found in Bourqueet al. (2001b) and Lavoie(2008). APL/AC = Anse-àPierre-Loiselle and AnseCascon formations.

Lavoie et al. 517

(2001) and Bourque et al. (2001b) argued that theisolated pinnacle reefs grew on paleotopographichighs at themargin of tilted tectonic blocks in north-ern Gaspé.

The initiation of movement along the faultsthat bound the tectonic blocks has been related tothe inception of the Acadian foreland basin in theGaspé belt (Bourque et al., 2001b) in response tothe early phase of tectonic indentation of exotic ter-ranes (Gander andDunnage) at the compositemar-gin (Pinet et al., 2008).The extensional reactivationof ancestral Taconic faults is interpreted on the basisof facies architecture and thickness variations, and

518 Hydrothermal Dolomitization of Lower Devonian Pinnacle

is demonstrated tohave startedduring the late EarlySilurian at the end of sedimentation of the peritidalcarbonate platform (Lavoie et al., 1992; Bourqueet al., 2001b; Lavoie and Morin, 2004; Lavoie andChi, 2009). The formation of small grabens affect-ing Lower Silurian carbonates is observed on flat-tened regional seismic lines (Desaulniers, 2005).

These reactivated faults (Figure 1) have a pro-tractedhistory of extensional tectonism that spannedthe late Early Silurian to the end of the Early Devo-nian (Lavoie, 1992). These faults delineated tilted(northward dipping) tectonic blocks (Roksandic andGranger, 1981); the elevated southern tip of these

Figure 3. Cross-sectional representation of the three reef complexes of the Upper Silurian West Point Formation (1 to 3) and the LowerDevonian pinnacle reefs of the West Point Formation (4). In some areas, the Salinic unconformity erodes deeply into the sub–UpperSilurian succession, whereas elsewhere, it is not developed. The pinnacle reefs are commonly located on paleotopographic highs left bySalinic erosion (Bourque, 2001). Both the Silurian and Devonian reefs are located at the margins of faulted and tilted tectonic blocks. Thesereefs and platforms are surrounded by fine-grained clastic sediments of the Indian Point Formation. The position of the massively dolomi-tized facies of one Lower Devonian pinnacle is shown by the box. Modified from Bourque et al. (2001b).

Reef in Eastern Canada

blocks has been hypothesized to represent the pre-ferred site for development of reef facies in the LateSilurian and Early Devonian (Bourque, 2001).

In northern Gaspé, Lower and Middle Devo-nian volcanic flows and deeply to shallowly buriedmagmatic intrusions with associated dyke and sillcomplexes are preferentially located in a structural-ly complex area close to the junction of the Shick-shockSudandRivièreMadeleine faults (Lachambre,1987; Doyon and Valiquette, 1991; D’Hulst, 2007;Pinet et al., 2008) (Figure 1). The geochemistry ofthe intrusive rocks suggests that they are mostlyof the anorogenic A type (Whalen and Gariépy,1986; Wallace et al., 1990).

Previous Diagenetic Studies

Prior to this study, the diagenetic evolution of threeout of nine field occurrences of Lower Devonianpinnacle reefs had been addressed in a regional ap-praisal of the diagenetic understanding of Gaspébelt carbonates (Bourque et al., 2001a; Lavoieand Bourque, 2001). Even if the evolution of LowerDevonian pinnacles was based on a large numberof petrographic thin sections and geochemicalanalyses, note that the studied pinnacles were alldevoid of dolomite. The paragenetic successionconsists of up to seven discrete phases of calcite ce-ments that range from interpreted marine to deep-burial fracture-fill types. From crosscutting rela-tionships and geochemistry, the dolomite-freepinnacle diagenesis was concluded to have oc-curred in the presence of three fluid systems withsignificant involvement of 18O-enriched exoticwaters at high temperature (Bourque et al., 2001a).

METHODS

During the summer of 2005, a massively dolomi-tized margin of a pinnacle reef was described andsampled for petrophysical and diagenetic study.Six dolomite samples were collected, and six pol-ished thin sections were prepared for conven-tional and cathodoluminescence (CL) petrogra-phy. Four double-polished and stained (Dickson,1965) thin sectionswere prepared for petrography

and fluid inclusion microthermometry. For thedouble-polished thin sections, care was taken sothat the part of the thin section that was used forCL examination would not be used for fluid inclu-sion studies. Operating conditions for the cold cath-ode luminoscope were 0.5 mA under 10–15 kV.

Microthermometric measurements were con-ducted using a Linkam heating-freezing stage cali-bratedwith synthetic fluid inclusions. The precisionofmeasurement is ±0.2°C formelting temperaturesand ±1°C for homogenization temperatures. Thefluid inclusion assemblage method (Goldstein andReynolds, 1994)was used to validate themicrother-mometric data. Salinities were calculated from ice-melting temperatures using a program by Chi andNi (2007), where the equation of Bodnar (1993)for the H2O-NaCl system was used.

Sampling of the carbonate material for oxygenand carbon stable and strontium radiogenic iso-tope analyses was done using a microdrill under abinocular microscope. The powder samples weremilled from polished parent block of the thin-sections, and integrity of the samples was assuredthroughCLexamination of themilled areas. Elevencarbonate powder samples were collected; theseconsisted of nine dolomite and two calcite cements.The carbonate powders were then treated and ana-lyzed at the Geological Survey of Canada Delta-Lab. Data are reported in the usual permil (‰)notation relative to the standard VPDB (ViennaPeedee belemnite) for carbon and oxygen. Preci-sion of the data is always better than ±0.1‰ forboth d18OVPDB and d13CVPDB.

Three subsets of the carbonate powders wereanalyzed for 87Sr/86Sr ratios. Samples of dolomitewere dissolved in dilute HCl and loaded on cationexchange columns (AG50X12) to separate Sr. Pu-rified samples were loaded on a single Re filamentwith aTaF5 emitter and run in static collectionmodeon a Tritonmass spectrometer in theGeological Sur-vey of Canada geochronology laboratory. Data werenormalized to a 86Sr/88Sr value of 0.1194. Analysesof the SRM987 gave a 87Sr/86Sr value of 0.710259 ±0.000004, and rock standard BCR-2 gave a valueof 0.705008 ± 0.00004 during the course of theanalyses. Replicate analyses were reproduced tobetter than 0.003%.

Lavoie et al. 519

Field Occurrence of the Dolomite Body

From fieldmapping (Bourque, 1977, 2001; Lacham-bre, 1987) and interpreted seismic (Bourque et al.,2006) and aeromagnetic (Pinet et al., 2008) data,the location and development of Lower Devonianpinnacle reefs have been shown to coincide withthe trace of Acadian dextral transpressive faults innorthern Gaspé (Malo and Béland, 1989; Malo,2001). Pinnacles are currently only known on ornear the Bassin Nord-Ouest, the Shickshock Sud,and the Rivière Madeleine faults, and an unnamed(subsurface) fault that parallels the Rivière Ma-deleine fault (Bourque, 2001; Pinet et al., 2008)(Figure 1). These pinnacles have been identifiedtentatively on seismic lines in the shallow subsurface(Bourque et al., 2006) although not a single ex-ploration well has ever targeted them. Of all theknown field localities that exposed pinnacle reefs,only the pinnacle at the locality known as the In-dice Barter (Lachambre, 1987) is dolomitized(Figure 1). The dolomitized pinnacle is located atthe intersection of two Acadian faults in northernGaspé, the Shickshock Sud and Rivière Madeleinefaults (Figure 1). Field mapping of the Silurian–Devonian succession in this area has documenteda large number of intrusives and dykes associatedwith the faults; the dolomitized pinnacle occursin a wide alteration zone with skarns and hornfels(Lachambre, 1987).

The exposed section of the dolomitized pin-nacle stretches for 300 m (1000 ft) along a riverbank and consists of massive, variably porous (1.5to 7.5%; Lavoie, 2009) dolostone (Figure 4A), withremnants of reef-derived limestones (Figure 4B).The northern exposed limit of the dolomitizedbody is intruded by a felsic dyke that postdatesthe dolomitization; this specific dyke is quartz andfeldspar-rich, granitic felsite (Lachambre, 1987). Tothe south, the contact with the nondolomitizedsection of the pinnacle is sharp; the nondolomi-tized limestone facies consists of carbonate rubblewith various early cemented stromatoporoid reefdetritus and rafts of crinoidal and metazoan facies(Figure 4C).

In the least dolomitized intervals, discrete struc-tural features indicative of transpressive motion


Figure 4. (A) Dolomitized section along the Ruisseau des Pé-kans in northern Gaspé. The section is shown by the solid star onFigure 1 and is stratigraphically located by the box on Figure 3.The lighter colored dolomite body is intruded by a felsic dyke(left side of the photograph). The visible zone of massive dolo-mite is 15 m (49 ft) wide by 300 m (984 ft) long. The 30-cm(12-in.)-long hammer (circle) is for scale. (B) Close-up view of do-lomitized stromatoporoid fragment. The coin is 2.5 cm (0.9 in.)wide. (C) Nondolomitized carbonate facies on the flank of thepinnacle reef; a 1-m (3-ft)-wide rounded bioclastic rudstone clastis circled; other rafted blocks are indicated by arrows. The con-tact between the dolomitized and nondolomitized carbonates isvery sharp and nondiffuse.


corroborate the interpreted kinematic of the Shick-shock Sud fault (Sacks et al., 2004). The sinistraland dextral movements on these structures led tosmall-scale openings that are the site of preferentialdolomitization (Figure 5A, B).

PETROGRAPHY OF THE DOLOMITIZEDPINNACLE REEF

The samples were collected in the massively dolo-mitized section of the pinnacle. Readers interestedin the detailed microfacies architecture and burialhistory of the nondolomitized pinnacles are re-

ferred to Bourque et al. (1986, 2001a). Petrographicexamination has resulted in the recognition of threephases of dolomite as well as a late calcite phase.

D1 Dolomite

The D1 dolomite phase is rare (volumetrically lessthan 1%) and consists of anhedral crystals thatrange in size from 0.005 to 0.05 mm (0.0002 to0.002 in.). The dolomite is ferroan in composi-tion and displays a pale orange color under CL.This fabric-retentive dolomite phase is found inassociation with zones containing later saddledolomite.

D2 Dolomite

The D2 dolomite is the most abundant phase inthe pinnacle reef, making up more than 90% ofthe total rock volume; it is represented by largecrystals that range from 0.5 to 8 mm (0.02 to0.3 in.) of iron-rich saddle dolomite (Figure 6A).This replacive dolomite is very dull luminescentto pale orange (Figure 6B). Under CL, the saddledolomite crystals are shown to be locally crosscutby abundant microfractures filled with calcitecement.

D3 Dolomite

The D3 dolomite is restricted to fractures cut-ting through the pinnacle and makes up rough-ly 5% of the total rock volume. It consists of fer-roan saddle dolomite cement in large crystalsfrom 0.2 to 5 mm (0.008 to 0.2 in.) (Figure 6C).This dolomite cement is dull luminescent andalso locally crosscut by hairline microfractures(Figure 6D).

C1 Calcite

The C1 calcite phase (approximately 5% of totalrock volume) fills large fractures aswell asmicrofrac-tures cutting through the massive dolomite ground-mass and consists of anhedral ferroan crystals thatrange in size from 0.05 to 4 mm (0.002 to 0.2 in.).They are orange to yellow luminescent under CL(Figure 6E).

Figure 5. (A) Centimeter-size releasing zones that mimic pullaparts. The sinistral movement is shown by the white arrows. Thevoid is the site of preferential dolomitization (shown by blackarrows). The coin is 2 cm (0.7 in.) wide. (B) Dextral strike-slip de-formation (shown by white arrows) in limestone with the open-ings being the site of preferential fluid circulation and dolomiti-zation. The pen is 15 cm (6 in.) long.

Lavoie et al. 521

MICROTHERMOMETRIC RESULTS

Fluid inclusions in the dolomite and calcite werestudied to estimate the carbonate formation tem-peratures and parent fluid compositions. Fluidinclusion measurements of the dolomites (D2and D3) and calcite (C1) were taken from iso-lated inclusions and from clusters. The fluid inclu-sions are small (average of 4.8, 4.7, and 4.6 mmfor D2, D3, and C1 phases, respectively), all ofwhichwere two phases (liquid and vapor); the fluidinclusions are interpreted to be pristine and of pri-mary origin (Table 1, Figure 7). Homogenizationtemperatures (Th) range between 301 to 382°C(average of 352°C, N = 36) for the D2 dolomite,159 to 171°C (average of 165°C, N = 9) for theD3 dolomite, and 107 to 123°C (average of 115°C,N = 9) for the C1 calcite. Final ice-melting tem-


peratures (Tm) range from −13.4 to −4.3°C (average−9.5°C), −9.2 to −7.3°C (average −8.1°C), and −8to −2.9°C (average −5.7°C) for D2, D3, and C1phases, respectively. The average Tm values trans-late to salinities of 13.3 (D2), 11.8 (D3), and 8.7 (C1)wt.% NaClequiv.

OXYGEN, CARBON, ANDSTRONTIUM ISOTOPES

Oxygen and carbon isotope ratios weremeasured inD2 andD3dolomites and in theC1 calcite (Table 2,Figure 8). The D2 dolomite has d18OVPDB valuesthat range from −19.1 to −15.8‰ (average −17.2‰,N=7) and d13CVPDB values from−7.9 to−1.4‰ (av-erage −3.1‰). The D3 dolomite yielded d18OVPDB

values of −16.8 and −16.6‰ and d13CVPDB values

Figure 6. Photomicrographsof the Lower Devonian pinnaclereef. (A) Plane-polarized viewof the massive replacive saddleD2 dolomite phase. The crys-tals contain abundant inclusionsand have spectacular curvedfaces. (B) Cathodoluminescenceview of panel A, showing thedull luminescence of the D2 do-lomite. (C) Plane-polarized viewof a fracture-filling saddle dolo-mite (D3); the fracture wall isoutlined by the white dashed line.The fracture cuts through theD2 dolomite. Late calcite cement(C1) fills the remaining fracturespace and microfractures inthe D2 dolomite. (D) Cathodo-luminescence view of panel C,showing the very dull lumines-cence of the D3 dolomite, theslightly more luminescent D2dolomite, and the brightly lumi-nescent C1 calcite. (E) Cathodo-luminescence view of a latefracture filled with a luminescentC1 calcite phase; the fracturecuts through the late D3 fracture-fill saddle dolomite.


of −4.7 and −0.9‰. Finally, the C1 calcite gaved18OVPDB values of −16.5 and −14.1‰ andd13CVPDB values of −3.3 and −2.2‰.

Three analyses of 87Sr/86Sr ratios in the D2dolomite yielded values of 0.7084, 0.7092 and0.7092 (Table 2, Figure 9).

DISCUSSION

Paragenetic Succession and Timing

The dolomitizedWest Point pinnacle is character-ized by a limited number of preserved diageneticelements; the nearby nondolomitized pinnaclesare characterized by up to seven discrete carbon-ate cement phases that record initial sea floor toshallow-marine phreatic diagenetic zones downto deep burial (Bourque et al., 2001a). Themassivedolomitization recorded in the studied pinnacle hasmost likely overprinted previous diagenetic elements,leaving a limited diagenetic record. The major perva-sive dolomitization (D2) is time wise, separated fromthe followingD3 andC1 phases that are developedin fractures that cut through the D2 dolomite.

The bulk of dolomitization in the pinnacle oc-curred at the time of displacement along the Shick-shock Sud fault, as suggested by the preferential do-lomitization of centimeter-size releasing zones thatmimic pull aparts (Figure 5). From the paleotectonicreconstruction of Malo (2001) and Pinet et al.(2008), movement along the Shickshock Sudfault started initially as extensional in the late EarlySilurian and lasted up to the climax of Acadian de-formation in the Middle Devonian when trans-pressive conditions prevailed. The transition fromdominantly extensional to transpressive settingspossibly occurred at the onset of sedimentation ofthe Lower Devonian (Emsian) Gaspé sandstones(Bourque et al., 2001b; Malo, 2001; Pinet et al.,2008).At that time, theLowerDevonian (Lochkovian)West Point pinnacles were buried under 1 to 1.5 km(0.6 to 0.9 mi) of Lower Devonian strata (IndianPoint Formation and Upper Gaspé Limestones;Bourque et al., 2001b).

In the only previous diagenetic study of theLowerDevonian pinnacle reefs in northernGaspé,

Bourque et al. (2001a) concluded, from the highhomogenization temperatures of the fluid inclu-sions in late calcite cements, that primary porosityremained open and available for fluid circulationfor a significant period. However, this contentionwas basedmostly on the assumption of normal geo-thermal gradients of 25°C/km and of parent fluidswith little evolved marine seawater isotopic ratios(d18OVSMOW [Vienna standard mean ocean water]of −5.1‰). Our interpretation of the geochemicaldata set differs significantly (see following), andthe geochemical data do not support the preserva-tion of significant primary pore space in the deepburial environment. Therefore, efficient fluid cir-culation would have had to rely on secondary frac-ture porosity, stressing the critical importance ofactive faulting for dolomitizing fluid circulation.

Fluid Geochemistry

A regional thermal maturation study (Roy, 2008)suggests that the Lower Devonian stratigraphicsuccessions in nearby areas of the northern Gaspébelt reached the dry gas zone (Ro of 3%); these pe-trographic reflectance data are consistent with thepresence of a 5-km (3.1mi) stratigraphic successionof Lower Devonian strata and a maximum geother-mal gradient of 30°C/km (Roy, 2008), all of whichwould generate a 180°C burial temperature (with a30°C seawater bottom temperature) in the LowerDevonian pinnacles. The high-temperature condi-tions indicated by fluid inclusions of the D2 dolo-mite could not have been generated through nor-mal burial.

In the studied pinnacle, the dolomite and cal-cite samples are characterized by very negatived18OVPDB ratios (as low as −19‰), the lowest val-ues ever reported in Paleozoic carbonates in easternCanada (Lavoie et al., 2009a). The fluid inclusionsindicate very high temperatures for replacementdolomite (up to 383°C). A study of Devonian-hosted base metal occurrences in northern Gaspépresents similar much depleted d18O ratios (aslow as −15‰, D’hulst, 2007) in quartz and dolo-mites that contain primary fluid inclusions withvery high homogenization temperatures (up to350°C). Paired d18OVPDB � T h values on a single

Lavoie et al. 523

Table 1. Fluid Inclusion Microthermometric Data from Carbonate Phases in Lower Devonian Pinnacle Reef

Sample

524

CarbonateHost Mineral

Hydrothermal Dolomi

Occurrence

tization of Lower De

Size (mm)

vonian Pinnacle R

Final MeltingTemperature (°C)

eef in Eastern Canada

HomogenizationTemperature (°C)

Salinity(wt.% NaCl)

80
D2 dolomite Isolated 3.5 −8.8 337 12.6 Isolated 4 −8.7 334.7 12.5 Cluster 4 −9.2 344.5 12.6 Isolated 4.5 −9.3 342.3 13.2 Isolated 5 −10.6 333.8 14.6 Isolated 4.5 −11 339.7 15.0 Cluster 5 −8.6 335.6 13.0 Cluster 6.5 −10.2 337.1 14.3 Isolated 6 −11.4 329.5 15.4 Isolated 8 −10.2 344.5 14.2 Cluster 4 −9.7 338.1 12.6 Isolated 4.5 −8.7 337.2 12.5 Isolated 4.5 −9.7 346.7 13.6 Isolated 5 −10.8 339.8 14.8 Cluster 5 −9.8 336.5 13.7 Isolated 8 −8.5 365 12.3
Average
5.1 −9.7 340.1 13.5
81
D2 dolomite Isolated 6 −10.6 301.5 14.6 Isolated 4 −10.6 367.3 14.6 Cluster 5 −8.8 370.1 12.9 Isolated 5 −11.6 372.3 15.6 Isolated 4.5 −9.6 370 13.5 Isolated 5 −10.4 365.4 14.4 Isolated 6 −11.7 382.6 15.7 Cluster 3.5 −9.8 354.7 12.6 Cluster 4.5 −11.6 362.8 15.0 Isolated 5.5 −13.4 368.6 17.3
Average
4.9 −10.8 361.5 14.6
82
D2 dolomite Isolated 4 −7.2 358.9 10.7 Isolated 4.5 −9.2 361 13.1 Cluster 4 −7.8 350.7 12.8 Isolated 4 −9.8 349.7 13.7 Isolated 4 −7.6 357.1 11.2 Isolated 4 −4.3 356 6.9 Isolated 4 −5.6 349.8 8.7 Cluster 4.5 −7.3 349.6 12.1 Isolated 4 −9.2 351.5 13.1 Isolated 4.5 −11.6 353.7 15.6
Average
4.15 −8 353.8 11.8
83
D3 dolomite Isolated 4 −8.7 170.5 12.5 Isolated 4 −9.2 167.1 13.1 Isolated 4.5 −8.7 163.2 12.5 Isolated 4 −8.2 161.8 11.9 Isolated 5.5 −7.8 166.7 11.5

dolomite or calcite cement sample (Table 3) can beused to calculate the d18OVSMOW signature of theparent fluid (O’Neil et al., 1969;Katz andMatthews,1977; Land, 1992; Zheng, 1999). The D2 dolomiteformed from a significantly 18O-enriched parentfluid with an average d18OVSMOW value of +8.3‰.The D3 dolomite appears to have originated froma different fluid with a d18OVSMOW value of +3.4‰,and the late calcite (C1) precipitated from a fluidwith a d18OVSMOW of +4.5‰. These d18OVSMOW

values could be explained by either a magmatic or abasinal fluid source or even by a mixture of the two.

The d13CVPDB ratios help to narrow down thepossibilities for sources for the late diageneticfluids. The Lower Devonian carbon isotopic ratiosof marine carbonates in the Gaspé belt are +1.5‰(Lavoie, 1993); these marine-derived values arereported in the pore and fracture-filling calcitephases of the nondolomitized pinnacles in north-ern Gaspé (Bourque et al., 2001a). The negatived13CVPDB ratios reported in this study are likely in-dicative of a nonmarine fluid, and the variabilityof the data suggests contamination from the ma-rine host rock that is replaced by the dolomite.

Table 1. Continued

Sample
CarbonateHost Mineral Occurrence Size (mm)
Final MeltingTemperature (°C)

HomogenizationTemperature (°C)

Lavoie et al.

Salinity(wt.% NaCl)

Isolated
6 −7.3 163.8 10.9 Isolated 6 −8.1 164.6 11.8 Isolated 4 −7.3 158.5 10.9
Average
4.7 −8.1 164.8 11.8
83
C1 calcite Isolated 4.5 −2.9 114.7 4.8 Isolated 4.5 −6.5 107.3 9.9 Isolated 4 −4.6 108.7 7.3 Isolated 4.5 −4.1 118.4 6.6 Isolated 4 −6.8 112.4 10.2 Isolated 5 −5.2 118 8.1 Isolated 4.5 −5.9 113.7 9.1 Isolated 5 −7.1 123.4 10.6 Isolated 5 −8 118.9 11.7
Average
4.6 −5.7 115.1 8.7
Figure 7. Microthermometricresults from fluid inclusions in theLower Devonian pinnacle reef ofthe West Point Formation. Thethree diagenetic phases (D2, D3,and C1) are characterized byrelatively narrow and nonover-lapping homogenization tem-perature (Th) data with a signifi-cant decrease of Th throughtime (D2 > D3 > C1). Ice-meltingtemperature values (and sali-nities) mostly overlap with a trendof decreasing salinities throughtime. Data are in Table 1.

525

D2 DolomiteThe very negative d18OVPDB and very high Th offluid inclusions for the D2 dolomite are best ex-plained by the involvement of high-temperaturemagmatic fluids with d18OVSMOW values commonlyranging from +5 to +13‰ (Taylor, 1979, 1987;Whalen et al., 1996; Mach and Thompson, 1998).Deep basinal brines may also be 18O-enriched, butit is less common for them to have d18OVSMOW val-ues above +5‰ (Longstaffe, 1987). Similar veryhigh Th of fluid inclusions (over 300°C) in carbon-


ates dolomitized by a fluid with d18OVSMOW ratiosabove +5‰ are documented in the mining literature(Megaw et al., 1988; Mach and Thompson, 1998).Interestingly, a parent fluid with a d18OVSMOW ratioof +7‰ has been recorded by dolomite cement innearby Devonian base metal occurrences (D’hulst,2007).

Magmatic fluids are characterizedbyvarious car-bon species with d13CVPDB values commonly rang-ing from −5 to −3‰ (Taylor, 1987). Conversely,deep-seated basinal brines can be 13C depleted asa result of input HCO�

3 derived from organic mat-ter in the brine. However, given the anomalouslyhigh Th values, the magmatic hypothesis is moreplausible for the D2 dolomite. The 87Sr/86Sr ratiosof the D2 dolomite (Table 2, Figure 9) are eitherhigher or slightly lower than the assumed EarlyDevonian seawater ratio (0.7087; Denison et al.,1997). Assuming the D2 parent fluid is of mag-matic origin derived from local A-type magmas,which are significantly less radiogenic than coevalseawater (Whalen et al., 1996), the near seawatervalues are taken as the result of mixing with thepervasively replaced Lower Devonian carbonatesor more pronounced assimilation or exchange withrocks having an elevated radiogenic Sr source, suchas those found in the numerous clastic intervals be-neath the pinnacles (Lavoie et al., 2009a).

D3 DolomiteThe d18OVSMOW value for the fluid that precipitatedthe fracture-filling D3 dolomite is lower (+3.4‰)

Table 2. d18OVPDB, d13CVPDB, and87Sr/86Sr Ratios of Carbonate

Phases

Sample
CarbonateHost Mineral
d18OVPDB

(‰)
d13CVPDB(‰) 87Sr/86Sr
80
D2 dolomite −17.1 −1.4 81 D2 dolomite −15.8 −1.7 82 D2 dolomite −19.1 −7.9 83 D2 dolomite −16.7 −4.4 86 D2 dolomite −17.0 −1.6 0.708383 86 D2 dolomite −17.5 −2.5 0.709216 87 D2 dolomite −17.0 −2.1 0.709199
Average
−17.2 −3.1
83
D3 dolomite −16.6 −4.7 87 D3 dolomite −16.8 −0.9
Average
−16.7 −2.8
83
C1 calcite −16.5 −3.3 83 C1 calcite −14.1 −2.2
Average
−15.3 −2.7
Figure 8. The d18OVPDB versus d13CVPDBdiagram for the carbonate cements in thedolomitized Lower Devonian pinnacle reef ofthe West Point Formation. The two dolo-mite and the calcite phases are all char-acterized by strongly negative d18OVPDB andd13CVPDB ratios. M = Lower Devonian ma-rine field from Lavoie (1993). Data arein Table 2.


compared toD2. This coupledwith the significantlylower Th values of dolomite D3 could suggest thatdeep basinal brines were responsible for the precip-itation of the D3 dolomite. From field relation-ships, the fracture-filling D3 dolomite postdatesthe D2 dolomite. This indicates that some timeafter the pervasive dolomitization of the host pin-nacle, high-temperature basinal brines used Acadianfractures to circulate in the dolomitized pinnacle.

C1 CalciteThe last fluid event recorded by the dolomitizedpin-nacle precipitated calcite cement in fractures. Thefluid had a d18OVSMOW value of +4.5‰, which isconsistent with the signature of the high-temperaturebasinal brine responsible for the D3 dolomite andboth have similar negative d13CVPDB ratios. The closeto 50°C homogenization temperature differencefrom fluid inclusion between the D3 dolomite andC1 calcite could be related to progressive cooling ofthe geosystem after the thermal event. In a previousdiagenetic study, the late calcite fracture-fill cement

of the LacMadeleine pinnacle (5 km [3.1mi] to theeast; Bourque et al., 2001a) is petrographically sim-ilar to our C1 calcite phase and is characterizedby saline (18.6 wt.% NaClequiv) fluid inclusionswith very high Th (174 to 252°C), although nod18OVPDB data are available. The late calcite cementof the LacMadeleine pinnacle is interpreted as a deepburial cement precipitated by high-temperaturebrine (Bourque et al., 2001a).

Dolomitization Model and Significance forHydrocarbon Prospectivity

Hydrocarbon exploration in Gaspé started almost150 yr ago with minimal success (Lavoie andBourque, 2001). Over the years, the primary tar-gets have been the Lower Devonian sandstones andlimestones that are at shallow depths. Most of thehistorical wells have produced variable, but alwayssmall, volumes of oil and/or gas, although recently,some economic production has been established(Galt gas and Haldimand oil fields; Lavoie et al.,2009a, b). Hydrothermal dolomitization of shal-low-marine Lower Silurian limestones in easternCanada has been documented (Lavoie and Morin,2004; Lavoie and Chi, 2006, 2009). The model ofearly shallow-burial dolomitization from high-temperature and saline fluids circulating along ac-tive faults in the basin agrees well with the classicalcases documented elsewhere (Davies and Smith,2006). The Lower Silurian dolomite bodies were,at least locally, charged by hydrocarbons (Lavoieand Chi, 2009; Lavoie et al., 2009a, b), but given

Figure 9. The 87Sr/86Sr curve for the Devonian (Denison et al.,1997). Three analyses for the D2 dolomite in the Lower DevonianPinnacle reef are shown by the black rectangle. Two of the do-lomites are slightly more radiogenic than the assumed seawatervalues (dashed black curve), and one value is less radiogenic.Data are in Table 2.

Table 3. d18OVSMOW Composition of Parent Fluids from Paired
Th and d18OVPDB of Cement Phase
Sample
Cement Th (°C) d18OVPDB
(‰) Mineral

Lavoie et al.

d18OVSMOW

(‰) Water

80
D2 340 −17.1 8.2 81 D2 361 −15.8 9.9 82 D2 357 −19.1 6.6
Average
8.2
83
D3 165 −16.6 3.4 83 C3 115 −14.1 4.5
527

the recent documentation of the dolomitizationprocess, exploration wells have yet to search forthese targets in the subsurface.

Evidence for hydrothermal dolomitization ofUpper Silurian to Lower Devonian reefs and car-bonate platforms of the West Point Formation islimited (Lavoie, 2005; Urbatsch, 2008; this study).However, the model of upward fluid migrationalong faults has already been proposed by Bourque(2001) in a synthesis of tectonosedimentary condi-tions for reef settlement in the Gaspé belt in theLate Silurian–Early Devonian. In northern Gaspé,the early movements along the east-west–trendingShickshock Sud and the northeast-southwest–trending Rivière Madeleine faults (Figure 1) havegenerated an extensional domain at their junction;a similar tectonic setting is common in major hydro-thermal dolomite fields around the world (Daviesand Smith, 2006). Note that Pinet et al (2008),using aeromagnetic data and surface geology, con-sidered this specific area as a preferred locus formagma ascent in northern Gaspé.

It is significant for the dolomitization modelthat other exposed pinnacles located at some dis-tance from this extensional domain are not dolo-mitized. The pinnacle closest to the dolomitizedone (Lac Madeleine buildup; 5 km [3.1 mi] tothe east) also recorded high thermal conditions(up to 252°C; Bourque et al., 2001a) but withoutdolomite. Obviously, if dolomitization had pro-ceeded from a regional, large-scale flow of high-temperature basinal brines, it would be highly prob-lematic to have one pinnacle massively dolomitizedand adjacent ones totally devoid of dolomite. More-over, if basinal brines used open fractures to mi-grate upward and to dolomitize pinnacles, thenall other pinnacles should record some dolomiteas they were built at the margin of faulted blocks;however, only the one at the Indice Barter is dolo-mitized and occurs in an area of demonstrated, sig-nificant deep- and shallow-seatedDevonianmagmaemplacement. Moreover, dolomitization fromcontact metasomatism with a nearby dyke is dis-carded as the dyke postdates the alteration of thelimestones, andLower Silurian to LowerDevoniancarbonates cut by other dykes in this area are dolo-mite free.


The dolomitizationmodel suggested herewouldrepresent an extreme case of hydrothermal dolo-mitization related to magma emplacement andfluid migration using active open faults. However,the dolomite body formed through this processwould form an exquisite porous hydrocarbon res-ervoir. Thismodel ismostly unknown in the hydro-carbon literature even if other cases of porous, veryhigh-temperature dolomite bodies associated withintrusive rocks are described in the mining litera-ture (Megaw et al., 1988; Beaty et al., 1990; Machand Thompson, 1998; and others).

Dolomitization is physically associated with aspecific tectonomagmatic corridor, although theeventual hydrocarbon charge remains problematic.In Gaspé, high-quality Ordovician hydrocarbonsource rocks outcrop at the periphery of the Silurian–Devonian basins, and fair-quality Lower Devoniansource rocks (post-West Point Formation) are rec-ognized within the stratigraphic succession (Roy,2008; Lavoie et al., 2009a, b). Early hydrocarboncharge from the Ordovician source rocks into theLower Silurian sandstones and dolomites has beendocumented (Lavoie and Chi, 2002; Lavoie andMorin, 2004; Lavoie et al., 2009a, b) although evi-dence for a significant charge in the LowerDevonianpinnacle is limited. However, light hydrocarbonsandoil-rich inclusions are locally abundant in the latecalcite phases of the nondolomitized pinnacles(Bourque et al., 2001a). This late charge event is sig-nificant because it appears to have occurred after themassive dolomitization of the pinnacle reef.

CONCLUSIONS

In northernGaspé, isolated Lower Devonian pinna-cles were formed at a time of rapidly rising sea leveland were preferentially developed on the elevatedfaulted margins of tilted tectonic blocks (Bourque,2001). An outcrop of amassively dolomitized LowerDevonian pinnacle reef of the West Point Formationhas been recognized in an area of significant magmaascent at the junction of two major faults: the Shick-shock Sud and Rivière Madeleine. This pinnacle reefis the only dolomitized example of exposed LowerDevonian pinnacles in the Gaspé area.


On the basis of local and regional field obser-vations, and from petrography and geochemistryof the carbonates, we propose that pervasive dolo-mitization proceeded fromhigh-temperaturemag-matic fluids forced upward during displacementalong the Shickshock Sud and Rivière Madeleinefaults. A first dolomitization event resulted in amassive dolostone body that was later crosscut byAcadian fractures that were cemented by a seconddolomite phase that originated from hot basinalbrines. Someof the fracture porosity and remainingsecondary pore space were eventually filled by cal-cite cement from hot but significantly cooler ba-sinal brines. In nearby pinnacles, late fracture-fillcalcite phases are host to abundant light hydrocar-bons and oil inclusions, suggesting a possible hydro-carbon charge event some time after the massivedolomitization.

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