The origin of NO3− and N2 in deep subsurface fracture water of South Africa

Deep (>0.8km depth) fracture water with residence time estimates on the order of several Ma from the Witwatersrand Basin, South Africa contains up to 40μM of NO3−, up to 50mM N2 (90 times air saturation at surface) and 1 to ~400μM NH3/NH4+. To determine whether the oxidized N species were introduced by mining activity, by recharge of paleometeoric water, or by subsurface geochemical processes, we undertook N and O isotopic analyses of N species from fracture water, mining water, pore water, fluid inclusion leachate and whole rock cores.