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.