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A fluid inclusion study of diagenetic fluids in Proterozoic and Paleozoic carbonate rocks, Victoria Island, NWT

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A fluid inclusion study of diagenetic fluids in Proterozoic and Paleozoic carbonate rocks, Victoria Island, NWT
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  A fluid inclusion study of diagenetic fluids in Proterozoicand Paleozoic carbonate rocks, Victoria Island, NWT J. MATHIEU, D. J. KONTAK AND E. C. TURNER  Department of Earth Sciences, Laurentian University, Sudbury, ON, Canada  ABSTRACT Despite the presence of known economic resources in Canada’s Arctic archipelago, Victoria Island remains under-studied. This study addresses the fluid history and economic potential of two major carbonate units on VictoriaIsland by integrating fluid inclusion microthermometry with SEM-EDS analysis of evaporate mounds. Threecements containing fluid inclusion assemblages (FIA) occur in the Neoproterozoic Wynniatt Formation: saddledolomite, brown dolomite and calcite, in paragenetic order. The two dolomite-hosted cements have averagehomogenisation temperatures ( T  h ) for FIAs ( n  =  3) of 108 ° C (saddle) and 101 and 116 ° C, but metastability pre-cluded determining salinities; most calcite-hosted fluid inclusions are too small and/or necked to obtain  T  h  values,but rare larger inclusions have salinities from 1.7 to 0.4 wt. % NaCl equiv. SEM-EDS analysis of evaporatemounds indicates the fluid changed from an early K-rich (saddle dolomite), to a later K  +  Na (brown dolomite),and finally Na-rich (calcite), which suggests mixing of two end-member fluids (i.e. Na-rich and K-rich). Dolostoneof the lower Paleozoic ‘Victoria Island formation’ contains two cements: early quartz and late dolomite. Quartz-hosted FIAs ( n  =  2) have an average  T  h  value of 126 ° C, and salinity of 23.2 wt. % NaCl equiv., whereas FIAs( n  =  3) in dolomite have average  T  h  values of 109, 116 and 124 ° C; metastability precluded determining salinity.Evaporate mound analysis for the cements indicates evolution from a Na-rich to a Na  +  K fluid through interac-tion with reservoir rocks. A reduced, metal-rich fluid was present during quartz precipitation, as implied by thepresence of pyrite framboids along growth zones and nanoparticles of barite and sulphide minerals (Zn, Cu andPb) in evacuated inclusions, which suggests the area may have potential to host base-metal mineralisation.Importantly, distinguishing different fluid compositions in both of the case studies would not have been possiblewithout evaporate mound analysis and therefore the results emphasise integrating this technique into diageneticstudies.Key words: carbonate diagenesis, evaporate mounds, fluid inclusions, Franklinian Basin, Shaler Supergroup,Victoria Island, Victoria Island formation, Wynniatt FormationReceived 22 January 2013; accepted 21 August 2013Corresponding author: Jordan Mathieu, Department of Earth Sciences, Laurentian University, Sudbury, ON, Can-ada P3E 2C6.Email: jy_mathieu@laurentian.ca. Tel: +1 705 675 1151 ext. 2267. Fax: +1 705 675 4898. Geofluids   (2013)  13 , 559–578 INTRODUCTION  Victoria Island is a large island in the Canadian Arcticarchipelago underlain by nearly undeformed Proterozoicand Paleozoic sedimentary rocks. Much of the islandremains geologically unmapped and economically unex-plored but is known to have bitumen showings andnative copper occurrences (Thorsteinsson & Tozer 1962;R. Rainbird, personal communication 2012). Islands else- where in the Canadian Arctic archipelago (Fig. 1) containoil and gas in Paleozoic and Mesozoic strata (Gentzis et al.  1996; Chen  et al.  2000; Obermajer  et al.  2010).Proterozoic strata on Baffin Island contain the Nanisivik Zn-Pb deposit (McNaughton & Smith 1986), and Paleozoicstrata east of Victoria Island contain the Cornwallis Zn-Pbdistrict (Fig. 1), which includes the Polaris deposit (mined1976  –  2002, production approximately 20 million tonnes atapproximately 17% Zn + Pb; Dewing  et al.  2006, 2007a,b).Proterozoic and Paleozoic carbonate strata on VictoriaIsland contain multiple generations of cement that offer theopportunity to study their diagenetic history and may helpilluminate the area’s economic potential.This study describes and characterises the fluids that were present during precipitation of late diagenetic ©  2013 John Wiley & Sons LtdGeofluids (2013)  13 , 559–578 doi: 10.1111/gfl.12063  cements in the Neoproterozoic Wynniatt Formation andthe lower Paleozoic ‘Victoria Island formation’ on VictoriaIsland, using standard microthermometry and SEM-EDSon evaporate mounds (e.g. Haynes & Kesler 1987; Haynes et al.  1988). The nature and srcin of the fluids are inter-preted in the context of the burial history of the rocks, andimplications for base-metal and petroleum potential in thearea are discussed. The material used was collected as part SverdrupBasinBoothiaUplift Canada Basin Baffin Bay      N    W    T    N    U    N    A    V    U    T VictoriaIsland Prince of WalesBanksIsland Devon Island BaffinIsland Somerset Island  Axel Heiberg Melville   Corn-wallisBathurst Ellesmere Island  L      o     u       g     h      e     e     d        Duke of York Inlier  7 o 70 o 10011012013080 ooooo 7 o 75 o 75 o 75 o 7 o 70 o 80 o 4 km   Well (oil / gas / both):Past producing mine:  Ba r r o w S t ra i t Nanisivik MinePolarisMineBentHorn E.O. S  h e l   f   E    p i   c  r  a t  o n i   c   Fig. 2  Fig. 1.  Map of the Arctic archipelago highlighting features relevant to this study, modified after Dewing  et al.  (2007b). The Sverdrup and Canada basins andBoothia Uplift are shaded areas. The southern limit of the Ellesmerian fold-thrust belt (E.O.), the location of the lower Paleozoic transition from epicratonic(south and east) to continental shelf (north and west) settings of the Franklinian Basin, the locations of the Polaris and Nanisivik Zn deposits, and oil and gaswells, including Bent Horn, are also shown. ©  2013 John Wiley & Sons Ltd,  Geofluids  ,  13 , 559–578 560  J. MATHIEU  et al.  of a broad field programme focussed on the regional geol-ogy Victoria Island (Bedard  et al.  2012; Rayner & Rain-bird 2013; Rainbird  et al.  in press). GEOLOGICAL SETTING The regional geology of Victoria Island was established by Thorsteinsson & Tozer (1962), who provided the firstdescriptions of Proterozoic and Paleozoic strata. Moredetailed descriptions of the Proterozoic units were pro- vided by Young (1981), Rainbird (1991, 1992) and Rain-bird  et al.  (1994); the Paleozoic units were informally named and described by  Dewing  et al.  (2013).The Neoproterozoic Shaler Supergroup was deposited ina poorly understood epicratonic basin (Rainbird  et al. 1994; Long  et al.  2008). The upper part of the ShalerSupergroup includes, in decreasing age, the Minto Inlet, Wynniatt, Kilian and Kuujjua formations, which are over-lain by the Natkusiak Formation flood basalt (Rainbird1991), a volcanic unit associated with the approximately 723 Ma Franklin large igneous province (Heaman  et al. 1992). The Holman Island syncline and Walker Bay anti-cline (Fig. 2A) are evidence of later Neoproterozoic defor-mation; structural dip is no more than 10 ° .The Wynniatt Formation consists of shallow-marinelimestone and dolostone of the informal ‘lower’, ‘shale’,‘stromatolitic’ and ‘upper’ members (Rainbird 1991). Thisformation was locally karsted shortly before deposition of lower Paleozoic strata (Mathieu  et al.  2013). Evidence forhydrothermal brecciation and cementation is present in the Wynniatt formation (Mathieu  et al.  2013). The estimatedthickness of Proterozoic strata now overlying the WynniattFormation ranges from 0 m (at the unconformity site) toapproximately 2000 m (where the Natkusiak Formation is well preserved; Rainbird 1991).Latest Neoproterozoic to early Paleozoic rifting andtransgression resulted in the development of an epicratonicsea over most of Laurentia, which in arctic Canada resultedin the early to middle Paleozoic Franklinian Basin, a broad,stable, crudely north-facing, carbonate-dominated passive-margin and contiguous epicratonic environment (Fig. 1).Paleozoic strata that underlie most of Victoria Islandinclude Cambrian(?)-Silurian(?) carbonate and terrigenousclastic strata that were collectively known as map-unit 10(Thorsteinsson & Tozer 1962), but which have since beendivided into ten units (Dewing  et al.  2013). The Cambro-Ordovician ‘Victoria Island formation’ (formerly map-unit10b; Thorsteinsson & Tozer 1962) is a generally fabric-destructive, shallow-marine dolostone with intraclasts andlocally preserved microbial structures. This unit is, at leastin part, equivalent to the Franklin Mountain Formation onthe northern Canadian mainland, which has similar age, Ulukhaktok Kilometres W.A.H.S. Phanerozoic rocksNatkusiak Fm.Kilian Fm.Wynniatt Fm.Minto Inlet Fm.Reynolds Point Gp.Rae Gp. Legend Kuujjua Fm. (A) o 7130'N o 11600'W o 11500'W o 7115'N Cambrian clastic unit“Victoria Island fm.”PaleozoicNormal fault 048km Minto Inlet Fm.Lower carbonate mbr.Black shale mbr.Stromatolitic mbr.Upper carbonate mbr.Wynniatt Fm.Kilian Fm. Shaler Supergroup Natkusiak Fm. (B)  M i n t o  I n l e t iii Fig. 2.  (A) Bedrock geology of part of VictoriaIsland, after Thorsteinsson & Tozer (1962),highlighting Minto Inlier (coloured), Walker Bayanticline (W.A.) and the Holman Island syncline(H.S.). Black rectangle indicates area enlarged in(B). (B) Detailed geology of Minto Inlet (fromRainbird  et al.  in press) showing the locations of(i) Proterozoic and (ii) Paleozoic samplesaddressed by this study. ©  2013 John Wiley & Sons Ltd,  Geofluids  ,  13 , 559–578 Fluid evolution of Paleozoic cements  561  tectonic setting and geological characteristics (Norford &Macqueen 1975; Turner 2011). The age of the ‘VictoriaIsland formation’ is constrained by Middle Cambrian trilo-bites in the underlying unit and Early Ordovician con-odonts below the contact with the overlying unit (Dewing et al.  2013).The Boothia Uplift, a north-trending, 250 km-widestructural zone that extends northward from the exposedcraton on the mainland to Devon Island in the centralCanadian Arctic islands, formed as a result of far-field com-pressional forces from the Caledonian Orogeny to the eastin the late Silurian (Fig. 1; Miall 1986). Approximately 4  –  5 km of uplift occurred, resulting in north-trending faultsand folds, and deposition of clastic wedges on both sidesof the uplift (Okulitch  et al.  1991).The late Devonian Ellesmerian Orogeny produced asouth-east  –   vergent deformation front and a thick clastic wedge (Fig. 1; Embry 1991a). The Boothia Uplift acted asa local buttress against the deformation front (Okulitch et al.  1991): interaction with the south-directed orogenicstress reactivated faults associated with the Boothia Uplift(Turner & Dewing 2004; Dewing  et al.  2007a; Jober et al.  2007), and fluid movement through these faults wasresponsible for formation of Zn deposits in the CornwallisDistrict, including the Polaris Zn-Pb deposit (Dewing et al.  2007a). The Devonian clastic wedge is preserved onBanks (Thorsteinsson & Tozer 1962; Miall 1976), Melville(Harrison 1994) and Bathurst (Anglin & Harrison 1999)islands, but is absent on Victoria Island (Thorsteinsson &Tozer 1962). The proximity of these islands to VictoriaIsland allows for the assumption that this area was also partof the same fluvio-deltaic environment (Embry 1991a).The greatest preserved thickness of the clastic wedge(4 km) is on Banks Island, immediately west of VictoriaIsland (Miall 1976; Embry 1991a). Maturation of organicmaterial from several islands suggests that approximately 4 km of Devonian strata were eroded during the Ellesmeri-an Orogeny  (Dewing & Obermajer 2009). This thicknessis probably an overestimation, because vitrinite reflectancerecords the highest temperature reached, whether it wasfrom burial or some other cause. Jurassic extension onBanks Island probably caused a thermal pulse, which wouldhave resulted in vitrinite reflectance indicating deeper bur-ial than truly took place; this discrepancy is shown by vitri-nite reflectance data that do not agree with the sonic velocity of shale in the Muskox D-87 drill hole on BanksIsland (Dewing & Obermajer 2009). The maximum thick-ness of the Devonian clastic wedge is therefore assumed tohave been between 4 and 8 km, although sonic velocitiesimply that the thickness was probably 6  –  7 km.In the early Carboniferous, rifting and formation of theSverdrup Basin in the north-western Arctic islands (Fig. 1)marked the end of the Ellesmerian Orogeny (Davies &Nassichuk 1991), as indicated by extensional structuresunder the basin (Forsyth  et al.  1979). The southern limitof the Sverdrup Basin is on Melville Island. The basinaccumulated approximately 12 km of strata and became apredominantly deep-water basin (Embry 1991b). South- west of the Sverdrup Basin, Jurassic extension producedthe Canada Basin (part of the present Arctic Ocean) (Miall1979) with strata preserved on land in grabens on BanksIsland (Embry & Dixon 1992). Rifting in the Cretaceousemplaced flood basalts and sills in the Sverdrup Basin(Embry 1991b; Dewing  et al.  2007b).The Cretaceous to Oligocene Eurekan Orogeny wascaused by counter-clockwise rotation of Greenland withrespect to North America, which caused compression inthe northern Arctic islands and extension in the southernislands (Okulitch & Trettin 1991): folds and thrust faultson Ellesmere Island, extension in Baffin Bay and CanadaBasin, and normal faulting that produced islands and straitsin the southern Arctic islands are Eurekan features. Iso-static uplift associated with the opening of Baffin Bay con-tributed to the present-day geography of the Arctic islands,and shifted the main sedimentary depocentre to the pres-ent-day continental margin north-west of the Arctic islands(Trettin 1991). METHODS Samples of cement-bearing carbonate rock were collectedfrom the Neoproterozoic upper Wynniatt Formation andfrom the Cambro-Ordovician ‘Victoria Island formation’(Fig. 2B). Wynniatt Formation dolostone samples arefrom below a sub-Cambrian karst surface exposed at thehead of Minto Inlet (Mathieu  et al.  2013) and containthree late-stage cements in vugs and fractures (Fig. 3A,C).‘Victoria Island formation’ samples are from an area char-acterised by north-east-trending normal faults south of Minto Inlet; associated brecciation is limited to the imme-diate vicinity of present-day map traces of normal faults(Fig. 4A).Polished thin sections (30  l m thick) were made fromrock slabs and studied petrographically in both transmittedand reflected light using an Olympus BX-51 microscope. A single-wavelength ultraviolet (385 nm) light source wasused to identify the fluorescence of hydrocarbons in thesample from the ‘Victoria Island formation’. The fluores-cence colour was compared with the American PetroleumInstitute (API) gravity chart qualitatively.Fluid inclusion microthermometry using 100  l m thick,doubly polished thin sections was undertaken at LaurentianUniversity, Sudbury, Ontario, using a Linkham THMSG600heating  –  freezing stage with an automated controller unit andOlympus BX-51 microscope equipped with a Q-Imagingdigital capture system. The heating  –  freezing stage calibration was checked using synthetic fluid inclusions: CO 2 (  56.6 ° C), the freezing point of H 2 O (0 ° C) and critical ©  2013 John Wiley & Sons Ltd,  Geofluids  ,  13 , 559–578 562  J. MATHIEU  et al.  point of H 2 O (374 ° C). Inclusion salinities were calculatedusing final melting temperature of ice ( T  m [ice]) for aqueousinclusions (Bodnar 1993). Fluid inclusions were homogen-ised repeatedly to test consistency. The fluid inclusions thatfroze were also reheated multiple times to ensure accurateand precise measurements and salinities.Two procedures were used to prepare evaporate moundsfor subsequent analysis. For quartz cement, chips wereheated at a rate of 60 ° C min  1 to 350 ° C to inducedecrepitation of fluid inclusions; samples were kept at350 ° C for  <  2 min to obtain optimal amounts of evaporatemound residue produced from individual fluid inclusions(Haynes & Kesler 1987; Haynes  et al.  1988). The quartzchips were then adhered to a glass slide with carbon tapeand carbon-coated for SEM-EDS analysis. For carbonatecement, a Fisher Brand microscope cover glass was placedunder the chip and the same heating rate used; after cool-ing to  < 50 ° C, the chip was removed, leaving the debrisand evaporate mounds on the glass surface of the coverslip; this procedure precluded any influence of the carbon-ate mineral substrate during SEM-EDS analysis. The glasscover slips used in this experiment were not pure silica, butcontained, in addition to Si, consistent proportions of Na,K, Al, Ti and Zn, as determined from SEM-EDS analysis.Frequent analysis of the glass slide was carried out toensure both its compositional consistency and the accuracy of the testing. Final compositions of the evaporate mounds were calculated by subtracting the proportional amount of the contaminant (i.e. Na, K) in the glass based on theamount of Si detected in each analysis.Imaging and analysis of fluid inclusion evaporatemounds were undertaken at Laurentian University using aJEOL 6400 scanning electron microscope running with a voltage of 20 kV, with an INCA EDS detector and soft- ware. The minimum detection limit for the operating con-ditions of this study was about 0.5 wt. %. Na + K, the latterof which accounted for  > 25% of the totals on an oxygen-free basis (i.e. Na, K, Cl, Si), which ensured minimal errorsin estimating the Na:K ratios of the analysis. The apparentconcentration of an element can be exaggerated by   in situ  fractionation during mound precipitation (Haynes  et al. 1988; Kontak 2004). This potential problem was avoidedin two ways: (i) rastering over mounds that were of sufficient size ( > 10  l m) and (ii) conducting multiple analy- 5 μm10 μm10 μm   10 μm2 μm (E) (F)(G) iii 5 mm (A) iii 0.5 mm (B) iiiiii (C)(D)(H) (I) Wynniatt Fm.SDSDWynniatt Fm.Є sst 2 mm Fig. 3.  Wynniatt Formation cements and their fluid inclusions. (A) Cut rock slab showing thetypical appearance of three cements: (i) saddledolomite, (ii) brown dolomite and (iii) calcite. (B)Photomicrograph of cements (i) to (iii) in plane-polarised light. (C) Hand sample of WynniattFormation with Cambrian quartz sandstonefilling a karst void followed by saddle dolomite(SD) cement. (D) Photomicrograph in cross-polarised light of saddle dolomite (SD) cementsurrounding quartz sand grains (arrows) inferredto be derived from overlying Cambrian map-unit10a, which provides a maximum age for cementprecipitation. (E) Typical aqueous FIA in thesaddle dolomite in plane-polarised light showinguniform L-V ratios indicative of low-T inclusions.(F) A large fluid inclusion, part of a larger population, in calcite cement with a shapeindicative of necking (G) Crystallographic planedecorated by small ( < 1  –  3  l m) aqueous fluidinclusions; the small size of these inclusionsmade microthermometry difficult. (H) Back-scattered electron (BSE) image showing thetypical size and shape of evaporate moundsproduced from the saddle dolomite. (I) BSEimage of partly dendritic evaporate moundsproduced from the brown dolomite. ©  2013 John Wiley & Sons Ltd,  Geofluids  ,  13 , 559–578 Fluid evolution of Paleozoic cements  563
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