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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^1,, Paul A. Johnston², Christopher J. Collom³ and Kimberley J. Johnston⁴

¹ 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
² 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
³ Department of Geology and Geophysics, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada T2N 1N4
⁴ Palaeontographica Canadiana, c/o Department of Earth Sciences, Mount Royal College, Calgary, Alberta, Canada T3E 6K6

Corresponding author: e-mail, wpowell{at}brooklyn.cuny.edu

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.

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