MAGNESITE, TALC, AND PB-ZN MINERALIZATION ASSOCIATED WITH MIDDLE CAMBRIAN BRINE SEEPS ALONG THE KICKING HORSE RIM, SOUTHEASTERN BRITISH COLUMBIA

The Kicking Horse Rim is a linear, fault-controlled, paleotopographic high over which the seaward edge of platformal strata repeatedly developed during the Middle Cambrian. Shale-dominated slope deposits along the Kicking Horse Rim, including the Burgess Shale Formation, contain lenses of black, thin-bedded, non-calcareous, geochemically-anomalous rock composed of >95% F-bearing clinochlore. The lenses show extremely high MgO concentrations (up to 31 wt%). Near the Monarch Pb-Zn mine on Mt Stephen, these rocks are commonly enriched in Pb (up to 247ppm) compared with nearby shale (<12 ppm Pb). The chloritic strata are thinly bedded, and are localized in pods and lenses within paleotopographic lows adjacent to the Cathedral and Eldon escarpments. Syneresis cracks commonly occur in the beds immediately below the chloritic pods. Fossils of low-diversity faunal assemblages are abundant along the rims of the anomalous rocks, whereas fossils of animals are very rare within the chloritic beds themselves. Location in paleotopographic lows, proximity to ancient submarine escarpments, association with syneresis cracks, unique major-element patterns, high F contents, and oxygen-deficient geochemical signatures, indicate that the lenses probably represent precipitates from dense brines that seeped onto the seafloor from adjacent platformal carbonates. Dolomitized platformal and slope carbonate strata along the Kicking Horse Rim host a suite of magnesian ore deposits, including talc occurrences and magnesite deposits. These deposits are commonly considered to be cogenetic with economic Pb-Zn deposits in the region. The spatial association and compositional similarity between ore bodies and exhalites, temporal similarity between exhalites and ore host rocks, and similarity of unusual mineralogical features such as disseminated quartz euhedra in dolostones suggest that flow of Mg-rich brines during the Middle Cambrian produced the talc and magnesite deposits of the region.