Diagenetic conditions of fibrous calcite vein formation in black shales: petrographic, chemical and isotopic evidence

Antitaxial fibrous calcite veins (2-6 cm thick) outcrop parallel to bedding in the Bluefish Member of the Middle Devonian (Givetian) Hare Indian Formation, Norman Wells area, Northwest Territories, Canada. The Bluefish Member consists of dark brown to black laminated shales with total organic matter content ranging from 1.7-8.0 wt%. The pattern of incorporation of host-shale fragments and tiny inclusions into the fibrous calcite indicates repreated episodes of vein opening and sealing. The formation of finely crystalline non-stoichiometric Ca-rich dolomite disseminated in the shale inclusions occurred subsequent to the emplacement of fibrous calcite veins under elevated burial temperatures. -from Authors