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©2006 Society of Economic Geologists, Inc. Economic Geology, v. 101, pp. 431–451
Middle Cambrian Brine Seeps on the Kicking Horse Rim and Their Relationship to Talc and Magnesite Mineralization and Associated Dolomitization, British Columbia, Canada WAYNE G. POWELL,† Department of Geology, Brooklyn College CUNY, 2900 Bedford Avenue, Brooklyn, New York 11210, and Department of Earth and Planetary Science, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024
PAUL A. JOHNSTON, Department of Earth Sciences, Mount Royal College, Calgary, Alberta, Canada T3E 6K6, and Royal Tyrrell Museum of Palaeontology, P.O. Box 7500, Drumheller, Alberta, Canada T0J 0Y0
CHRISTOPHER J. COLLOM, Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4 AND
KIMBERLEY J. JOHNSTON
Palaeontographica Canadiana, c/o Department of Earth Sciences, Mount Royal College, Calgary, Alberta, Canada T3E 6K6
Abstract Middle Cambrian basinal strata along the Kicking Horse Rim, including the Burgess Shale Formation, contain lenses of black, thin-bedded, noncalcareous, geochemically anomalous chloritic rock. The lenses have extremely high MgO concentrations (up to 31 wt %), or less commonly are enriched in Ba (up to 1,680 ppm), which contrasts with surrounding strata where MgO and Ba content are low. Location in paleotopographic lows, proximity to ancient submarine Escarpments, association with syneresis cracks, distinct major element patterns, high fluorine contents, and geochemical signatures indicative of deposition in oxygen-deficient environments, are interpreted as evidence that the lenses formed by precipitation from dense brines that seeped onto the sea floor through the adjacent platform. The strata that precipitated from the magnesian brines have been metamorphosed to lower greenschist facies and currently consist of clinochlore and minor quartz. Based upon the bulk-rock composition, the protolith of these seep-related strata is inferred to have been composed of Mg smectite, with lesser kaolinite, and minor brucite. Middle Cambrian carbonates along the Kicking Horse Rim also host ore suites of magnesian minerals, including talc and magnesite, which are interpreted to have formed from similar Mg-rich brines, as suggested by their compositional and mineralogical similarities and their spatial and temporal association. The existence of Mg brines also may account for early dolomitization of the platform margin during the Middle Cambrian.
Introduction THE MIDDLE Cambrian carbonate strata of the Rocky Mountains of southern British Columbia (Fig. 1A) host a diverse suite of mineral deposits including past-producing Pb-Zn mines, talc deposits, and a currently operating magnesite mine (Dawson et al., 1991). Each of the major orebodies occurs at or near the Cathedral Escarpment, which marks the steeply dipping contact between shale-dominated slope strata and platformal carbonate rocks. The similarity of position, of mineral assemblages, and of geochemical characteristics among Pb-Zn, talc, and magnesite deposits, and their association with regional dolostone, have led several researchers to suggest that they are cogenetic (Simandl et al., 1991; Nesbitt and Muehlenbachs, 1994; Nesbitt and Prochaska, 1998; Symons et al., 1998). However, there is disagreement as to the timing of ore and dolomite formation, the nature of the associated fluids, and the cause of regional fluid flow. Both syngenetic and epigenetic models † Corresponding
author: e-mail,
[email protected]
have been proposed, with inferred ages of mineralization including Cambrian, Silurian-Devonian, and Cretaceous. New evidence is presented here regarding a regionally extensive brine flow events in the Middle Cambrian. Field relationships and geochemical data indicate that extensive syndepositional Mg-rich brines were focused along submarine Escarpments at carbonate platform margins during the Cambrian (Delamaran and Marjuman stages). Also, new petrological data and field relationships are evidence that the talc, magnesite and dolomite mineralization at both the Cathedral and Eldon Escarpments is related to these magnesian brines. Regional geology The Middle Cambrian strata of southeastern British Columbia, which are schematically represented in Figure 2, are underlain by thick rift-related sequences. The Windermere Supergroup accumulated during Neoproterozoic extension (780–570 Ma). In the southern and central Rocky Mountains, the Windermere Supergroup is represented by the Miette Group, which is composed predominantly of deep-water
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FIG. 1. A. Location of study sites. B. Study sites in Yoho National Park. C. Study sites in the Kootenay National Park area. BNP = Banff National Park, KNP = Kootenay National Park, MAPP = Mount Assiniboine Provincial Park. Contour interval = 500'.
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FIG. 2. Generalized Lower and Middle Cambrian stratigraphic section for the Kicking Horse Rim with relative position of ore deposits, lenses of geochemically anomalous silicate rock (magnesian or barian), and associated lithologic features of quartz euhedra and carbonate mud mounds. Numbers correspond to specific ore deposits: 1 = Mount Brussilof mine, 2 = Red Mountain, 3 = Saddle, 4 = Silver Dollar, 5 = Monarch-Kicking Horse mine, 6 = Stephen Cirque, and 7 = Haiduk.
mass-flow deposits that formed along a rift margin (Slind and Perkins, 1966). The Miette Group is unconformably overlain by the Gog Group, which accumulated during a period of renewed rifting in the Early Cambrian (Bond and Kominz, 1984). The lower strata of the Gog Group are dominated by intertidal quartz arenites (now quartzites) with interbeds of siltstone, whereas the upper strata of the Gog Group are dominated by shallowwater, platformal, stromatolitic carbonate rocks (Fritz et al., 1991). Early Cambrian rifting was followed by Middle Cambrian regional subsidence (Bond and Kominz, 1984), during which the platform-basin transition repeatedly developed along a topographically high hinge zone that was probably controlled by underlying basement faults. This recurring linear paleotopographic feature is referred to as the Kicking Horse Rim (Aitken, 1971). To the west of the Kicking Horse Rim (oceanward), slope-facies shales and siltstones of the Middle Cambrian Naiset Formation unconformably overlie the Gog Group. Landward of the Kicking Horse Rim, the Mount Whyte Formation lies at the base of the Middle Cambrian section. It is composed of shale with oolitic and oncolitic limestone beds (Aitken, 1997). Along the Kicking Horse Rim itself, the Gog Group is overlain directly by carbonate rocks of the Cathedral Formation (Aitken, 1997).
At its distal margin, the Cathedral Formation consists of dolomitized pellet grainstone, cryptalgal laminate, and fenestral lime mudstone (Aitken, 1997). Thrombolitic facies are also common. East of the Kicking Horse Rim the Cathedral Formation is composed predominantly of burrow-mottled to massive lime mudstone (Aitken, 1997). The Cathedral Formation is intensely dolomitized, but much of the dolomite is fine grained and preserves primary features. At least two distinct generations of dolomite are recognized regionally along the Kicking Horse Rim: an early, fine-grained, brown or black pyritic dolostone is cut by white coarsely crystalline dolostone (Yao and Demicco, 1995). The edge of the Cathedral platform collapsed prior to the deposition of the overlying Stephen Shale formations and formed the submarine paleocliff known as the Cathedral Escarpment (Stewart et al., 1993). The Eldon Formation is a younger platformal carbonate unit that is similarly dolomitized along its distal margin. The dominant lithology of the Eldon Formation is peritidal, burrow-mottled lime mudstone (Aitken, 1997). However, along the Kicking Horse Rim fenestral lime mudstone, cryptalgal laminites, thrombolites, and stromatolites are also present (Aitken, 1997). The mid-Eldon Field Member comprises laterally variable, dark, recessive strata that occur proximal to the platform edge (Aitken, 1997). Like the Cathedral Formation, the platform front of the Eldon Formation collapsed to
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form the Eldon Escarpment (Stewart et al., 1993). Along the Eldon Escarpment, the slope deposits of the Tokumm unit are overlain by shales and ribbon limestones of the Vermilion subunit and, in turn, the locally fossiliferous, shaly Duchesnay unit, all within the Chancellor Group (Stewart, 1991). The Chancellor Group is a basinal succession that lies to the west of the Kicking Horse Rim and is dominated by argillaceous units, including the Burgess Shale Formation. The Chancellor Group also includes deep-water, ribbon carbonate slope deposits of the Takakkaw Tongue and Tokumm unit, which are correlative with platformal carbonate strata of the Cathedral and Eldon formations, respectively (Stewart, 1991). Where a megatruncation surface lies within the Takakkaw Tongue or the Tokumm lithologic units, we refer to the prefailure strata within these units as “lower” and the postfailure beds as “upper” with respect to the unconformity. Commonly the upper Takakkaw Tongue and upper Tokumm unit contain carbonate-dominated debris flow deposits. Sedimentary rocks along the Kicking Horse Rim experienced two periods of metamorphism (Powell, 2003). Peak temperatures during the first metamorphic event reached approximately 300° to 350°C within the lower greenschist facies, based upon vitrinite reflectance (Butterfield, 1996) and illite crystallinity indices (Powell, 2003), and was coeval with the development of a ubiquitous bedding-parallel foliation. The mineral assemblages and bedding-parallel foliation
associated with this Paleozoic burial metamorphism were overprinted by a regional retrograde metamorphic event during Mesozoic uplift and orogenesis (Powell, 2003). Ore deposits and dolomitization of the Kicking Horse Rim The Mount Brussilof deposit (Fig. 3) is the primary magnesite deposit in British Columbia, with proven and probable reserves in 1980 of 23.1 million metric tons (Mt) of ore grading over 93 percent magnesia in the calcined product (Simandl and Hancock, 1991). The orebody lies within the lower strata of the Cathedral Formation, approximately 100 m to the east of the Cathedral Escarpment. Other noneconomic magnesite occurrences exist along a 12-km zone, within 1 km of the Cathedral Escarpment (Simandl and Hancock, 1991; Fig. 3). All magnesite occurrences in the Mount Brussilof area are hosted by the Cathedral Formation but are not limited to a single stratigraphic horizon (Simandl et al., 1991). A small occurrence of massive magnesite was found during this study within the Eldon Formation at Haiduk Cirque North (Fig. 1C). This small pod of magnesite lies immediately adjacent to the Eldon Escarpment, reflecting relationships similar to the magnesite occurrences of the older Cathedral Formation. The Mount Brussilof ore consists of lenses, pods, and irregular masses of white and gray sparry magnesite that are cut by veins of dolomite, calcite, and pyrite. Minor mineral
FIG. 3. Location of occurrences and deposits of talc, magnesite, and Pb-Zn relative to the Cathedral Escarpment.
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components of the ore are dolomite, pyrite, tetrahedrite, malachite, phlogopite, talc, and coarse palygorskite (Simandl et al., 1991). Euhedral, needlelike quartz crystals, as well as radiating quartz rosettes are known from the magnesite ore and the peripheral dolomite (Simandl and Hancock, 1991). Fluid inclusions in magnesite are highly saline (18–25 wt % NaCl equiv; Nesbitt and Prochaska, 1998). Talc deposits occur within the Cathedral Formation and upper Takakkaw Tongue to the northwest of the Mount Brussilof deposit (Fig. 3; Benevenuto, 1993). Talc bodies also
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occur in the Field Member of the Eldon Formation and are described herein. The Red Mountain mine is the largest of the talc bodies of the Kicking Horse Rim, with an exposed area 260 m wide and a thickness of 30 m. The stratiform talc body is hosted by ribbon carbonates of the Takakkaw Tongue that lie immediately above quartzites of the Gog Group. Rhombohedral pseudomorphs of talc after dolomite (Fig. 4A) are common, and relict bedding on the scale of 2 to 5 cm is clearly evident in the talcose ore (Fig. 4B), indicating that the talc ore formed
FIG. 4. Photographs of features of talc deposits. A. Photomicrograph of light-colored talc ore from the Red Mountain mine, displaying a central rhombic talc crystal that is pseudomorphous after dolomite. B. Brecciated relict bedding in talc ore, Red Mountain mine. C. Chloritic talc pod at the upper contact of the Field Member in Stephen Cirque, together with a corresponding interpretive line drawing. D. Euhedral quartz crystals in dolostone directly above a talc pod, Stephen Cirque. E. Folded silicified dolostone directly above a talc pod, Stephen Cirque.
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by replacement of carbonate beds. Clinochlore, amorphous carbonaceous material, and pyrite are common constituents of talc ore (Benevenuto, 1993). Fluorapatite is a trace component, and F is a minor component of both talc and chlorite, comprising approximately 0.3 wt percent of these minerals (Table 1). Talc bodies in the basal Cathedral Formation include the Saddle occurrences and the Silver Moon occurrence on Mount Whymper, 1.4 and 20 km northwest of the Red Mountain deposit, respectively. Like the Red Mountain and Gold Dollar deposits, the Saddle and Silver Moon occurrences are hosted by carbonate strata that were deposited directly upon quartzites of the Gog Group. However, these bodies occur several kilometers inboard of the Cathedral Escarpment, where the set of deposits are aligned parallel to the Escarpment (Fig. 3). Botryoidal talc occurs at the Silver Moon deposit. Quartz is a minor disseminated component of the Saddle occurrences. Quartz is more abundant at the Silver Moon deposit, where it occurs as quartz pods, quartz-dolomite pods within the talc bodies, and as prismatic euhedral crystals 1 to 20 mm in length; these quartz euhedra are associated with fractures in dolostone immediately above the ore (Benevenuto, 1993). Lead-zinc ore was extracted from The Monarch and Kicking Horse Pb-Zn mines at the Cathedral margin near Field, British Columbia (Fig. 3). Sphalerite and galena most commonly occur as coarse, disseminated grains in dolomitic breccias that are composed of gray dolomitic fragments in a black, graphitic dolomite matrix (Ney, 1954). Irregular veinlets of sphalerite and galena are common where the white dolomite is abundant and crosscuts the dark mineralized breccia (Ney, 1957). Yao and Demicco (1995) recognized two distinct types of dolomite along the Kicking Horse Rim. An early phase of dolomitization produced black, fine-grained, crystalline dolomite with associated disseminated pyrite. Dikes of white dolomite that contain fractures filled with megacrystalline
dolomite cut the black dolostone (Yao and Demicco, 1995). The younger white, sparry dolomite envelopes the magnesite, talc, and Pb-Zn deposits of the region. Pb-Zn ore is hosted by the older, dark dolomite. There is no consensus in the literature as to the age of, and fluid source for, Kicking Horse Rim ore deposits and their host dolostones. Although Fritz and Simandl (1993) suggested that a syngenetic, evaporite origin should be considered for talc and magnesite mineralization, most researchers agree that an epigenetic, replacement origin is most likely for these deposits (Nesbitt and Muelenbachs, 1994; Yao and Demicco, 1995; Nesbitt and Prochaska, 1998; Symmons et al., 1998). However, inferred ages of emplacement include Late Cretaceous based upon paleomagnetic characteristics (Symmons et al., 1998), mid-Paleozoic based upon the age of evaporites in the region as well as field relationships of carbonate clasts (Nesbitt and Muehlenbachs, 1994; Yao and Demicco, 1995; Nesbitt and Prochaska, 1998), and Middle Cambrian based upon field relationships and geochemical patterns in dolostones (Moore, 1994; Spencer and Hutcheon, 1999; Jeary, 2002). To test the hypothesis that the region was hydrothermally active during the Middle Cambrian and that this hydrothermal activity affected the shelf break environment, we undertook a field-based investigation of platform-hosted talc and magnesite orebodies, and basin-hosted fossil localities, along the Kicking Horse Rim. If a major fluid-flow event capable of producing extensive dolomitization and replacement-style mineralization of the platform front occurred in the Middle Cambrian, then evidence of these fluids should be recorded in the adjacent shale-dominated strata of the Chancellor basin. Field relationships of geochemically and mineralogically anomalous features noted in the Chancellor basin, including the Burgess Shale Formation, were compared with features associated with Kicking Horse Rim talc and magnesite bodies. The results of these field, geochemical, and petrological studies are presented herein.
TABLE 1. Composition of Sheet Silicates in Shale, Talc Ore, and Geochemically Anomalous Strata Rock type
Site
Mineral
SiO2 TiO2 Al2O3 FeO MnO MgO CaO Na2O K2O P2O5 BaO F Total (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %) (wt %)
Mg seep rock Mg seep rock Mg seep rock
Ogygopsis beds Ogygopsis beds Haiduk Cirque North
Chlorite Chlorite Chlorite
31.15 31.59 31.34
0.06 0.03 0.07
20.66 20.11 20.55
3.73 3.71 3.18
0.01 0.01 0.01
29.96 30.05 30.54
0.07 0.05 0.21
0.01
