Petrography and Geochemistry of Early-Stage, ®Ne- and Medium-Crystalline Dolomites in the Middle Devonian Presqu'ile Barrier at Pine Point, Canada
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Sedimentology (1998) 45, 433±446 Petrography and geochemistry of early-stage, ®ne- and medium-crystalline dolomites in the Middle Devonian Presqu'ile Barrier at Pine Point, Canada HAIRUO QING Department 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 )1á6to±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 )3á7to)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 Mississippi Valley-type (MVT) deposits (Rhodes et al., 1984). In the subsurface of the North-west Dolomites in the Middle Devonian (Givetian) Territories and north-eastern British Columbia, Presqu'ile barrier host one of the world's largest these dolomites are also important hydrocarbon Ó 1998 International Association of Sedimentologists 433 434 H. Qing reservoir rocks (e.g. Collins & Lake, 1989). Un- GEOLOGICAL SETTING derstanding the origin and distribution of differ- ent dolomites in the Presqu'ile barrier is therefore The Middle Devonian Presqu'ile barrier is located of great economic importance for both mining and in the northern part of the Western Canada oil industries. As massive replacement dolomites Sedimentary Basin (Fig. 1). It is a linear carbonate commonly occur in the Western Canada Sedi- reef complex at least 400 km long with a width mentary Basin and some of these are important ranging from 20 km to 100 km. It extends from hydrocarbon reservoirs (Morrow, 1982; Machel & outcrops in the Northwest Territories near Pine Mountjoy, 1987), a better understanding of do- Point into the subsurface of north-eastern British lomitization will be useful to predict the spatial Columbia (Fig. 1). The thickness of the Presqu'ile distribution of dolomite reservoirs elsewhere in barrier is about 200 m in the Pine Point area. The the basin. development of the Presqu'ile barrier restricted Dolomite has been interpreted to have formed seawater circulation in the Elk Point Basin during in many different environments by various ¯uids Middle Devonian time. As a result, evaporites (Land, 1985; Machel & Mountjoy, 1986; Hardie, and carbonates were deposited south of the 1987). These include: (1) evaporitic marine brines barrier in the Elk Point Basin, whereas normal in sabkha environments (e.g. Adams & Rhodes, marine sediments were deposited north of the 1960; Patterson & Kinsman, 1982); (2) freshwater/ barrier (Fig. 1). Pine Point is located towards the seawater in mixing zones (e.g. Badiozamani, east end of the barrier (Fig. 1), where various 1978; Humphrey, 1988; Humphrey & Quinn, dolomite rocks host more than 80 individual 1989); (3) normal seawater in subtidal environ- MVT ore bodies. ments (e.g. Saller, 1984); and (4) various ¯uids in A major basement fault, the McDonald Fault, subsurface environments (e.g. Mattes & Mountjoy, occurs in the Pine Point area (Rhodes et al., 1984; 1980; Aulstead & Spencer, 1985; Machel & Mount- Krebs & Macqueen, 1984) (Fig. 1). Based on aero- joy, 1987; Aulstead et al., 1988; Qing & Mountjoy, magnetic data, the McDonald Fault can be traced 1989; Machel & Anderson, 1989; Mountjoy & from the Canadian Shield into the subsurface of the Amthor, 1994). Dolomites formed in different Western Canada Sedimentary Basin (Jones, 1980; diagenetic environments can be recognized by Ross et al., 1991) (Fig. 1). The McDonald Fault may their petrography, spatial distribution, geochem- have been reactivated during Middle Devonian or ical and isotopic signatures due to variations in post Devonian times and played a role in conduct- ¯uid-driving mechanisms, temperature and ing the diagenetic ¯uids for late stage dolomitizat- chemical compositions of the dolomitizing ¯uids ion and mineralization (e.g. Campbell, 1966; Skall, (Land, 1985; Machel & Mountjoy, 1986; Hardie, 1975; Krebs & Macqueen, 1984), although there is 1987). no de®nitive evidence to support this. The role of The Presqu'ile barrier in the Western Canada the McDonald Fault during Middle Devonian time Sedimentary Basin offers a good opportunity to was questioned by Rhodes et al. (1984) because study various massive replacement dolomites. strata immediately beneath the barrier are `of Four types of dolomites are identi®ed in the uniform thickness without any signs of vertical Presqu'ile barrier (Qing, 1991; Qing & Mountjoy, displacement'. 1990). In paragenetic sequence, they are ®ne-, The regional geology at Pine Point and diage- medium-, and coarse-crystalline replacement do- netic processes have been described in details by lomites, and saddle dolomite cements. `Fine- Jackson & Beales (1967), Skall (1975), Kyle (1981, crystalline dolomite' of previous studies (Jackson 1983), Krebs & Macqueen (1984), Rhodes et al. & Beales, 1967; Skall, 1975; Kyle, 1981, 1983; (1984), Qing (1991), Qing & Mountjoy (1990, Krebs & Macqueen, 1984; Rhodes et al., 1984) is 1994a,b,c). The following is a brief summary of here divided into ®ne- and medium-crystalline the stratigraphy and diagenesis of the Presqu'ile dolomites based on their different petrographic barrier. The Keg River Formation represents a textures, lithofacies associations, and geochemis- regional shallow-water platform that underlies try (Qing, 1991). Because the coarse-crystalline the barrier (Fig. 2). It consists of grey-brown and saddle dolomites have been discussed in dolostones with abundant crinoid and brachio- previous publications (Qing & Mountjoy, 1992, pod fragments. The carbonate barrier itself com- 1994a,b,c), this paper focuses on the petrography prises the Pine Point and overlying Sulphur and geochemistry of ®ne- and medium-crystalline Point Formations (Fig. 2). Both were originally dolomites, and compares them with late-stage deposited as marine limestones that since have coarse-crystalline and saddle dolomites. been variably dolomitized to medium- and Ó 1998 International Association of Sedimentologists, Sedimentology, 45, 433±446 Petrography and geochemistry of early dolomites at Pine Point 435 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 the subsurface 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 Cameron Hills 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 dolomite distribution of A±A¢ section are shown in Fig. 2. coarse-crystalline dolomites. The Pine Point and The diagenetic features in the carbonate rocks Sulphur Point Formations grade southward into of the Presqu'ile barrier (Fig. 3) have been inter- ®ne-crystalline dolomite and anhydrite of the preted to have occurred in submarine, subaerial Muskeg Formation, deposited in the Elk Point and subsurface environments (Qing, 1991; Qing & Basin. To the north of the barrier marine shales Mountjoy, 1990). Submarine diagenetic features and carbonates of the Buffalo River and Windy include: micrite envelopes, micrite, syntaxial Point Formations inter®nger with and onlap the cement, microspar cement, ®brous