A SERPENTINITE “SMOKING GUN” IN THE BURGESS SHALE: EVIDENCE FOR DEEP-SOURCED MAGNESIUM-CHARGED HYDROTHERMAL BRINES AND MUD VOLCANISM THAT FUELLED THE BURGESS BIOSYSTEM

Re-examination of a clinochlore (Mg-chlorite)/dolomite-dominated breccia, collected in 2009 at the Fossil Gully Fault upslope from the Mt Stephen Trilobite Beds, revealed a population of exotic clasts that resembled serpentinite. The serpentinite has now been confirmed by X-ray diffraction, and geochemical and petrographic analyses as the lizardite member of the serpentine mineral series. Lizardite is a significant (~20-30%) clast type along with clinochlore (~75%) in the dolomite-cemented breccia. We have also detected disseminated lizardite elsewhere in the Burgess Shale including: fossil-bearing strata of the Campsite Cliff Shale Mbr at the Trilobite Beds; several horizons in the Burgess Shale section at Fossil Ridge; and, in muscovite-rich mud slurry deposits in the Raymond Quarry Shale Mbr(?) at a fossil site on the North Shoulder of Mt Stephen. The presence of serpentine in the Burgess Shale is a “smoking gun”, providing necessary and sufficient evidence for deep sourced, hydrothermal Mg brines and associated mud volcanism. Implications for paleoecologic interpretations of associated biota and for understanding the evolution of the Kicking Horse Rim are major. The brines produced a massive magnesium metasomatic feature here named the Kicking Horse Rim Magnesium Anomaly (~100 km long and up to 15 km wide). This anomaly includes the well known Kicking Horse and Monarch Pb-Zn-(Ag) deposits, the Mt. Brussilof magnesite deposit, numerous probably protrusive talc bodies (especially the cluster around Talc Lake), and the abundant widespread hydrothermal dolomites that have largely replaced the westerly extent of the middle Cambrian Eldon and Cathedral carbonate platforms. The above features are all ascribed to Mg-CO₂-Cl brines generated during mantle heating, dehydration, and steatization of serpentinite in the mid-to lower crust beneath the Kicking Horse fault system in middle Cambrian time. Brines and mud volcanism were likely major factors in the distribution and rapid burial of fossil assemblages in Middle Cambrian deposits along the Kicking Horse Rim. These observations provide impetus to investigate other Cambrian lagerstätten to determine if deep-sourced serpentinization-derived materials are present and, if so, determine their potential paleoecologic and evolutionary implications.