Diagenetic barite and sphalerite in middle Mesozoic sandstones, Scotian Basin, as tracers for basin hydrology

Cementation of sandstone by minor late barite and sphalerite is widespread in the Scotian Basin at burial depths >2 km (>1 mi), providing information on fluid flow in the basin. The texture and geochemistry of these minerals were analyzed by scanning electron microscopy and electron microprobe on samples from conventional core. Barite and sphalerite postdate silica and carbonate cementation, occurring in veins or occupying secondary porosity. They occur with diagenetic chlorite, kaolinite, pyrite, titania minerals, kutnohorite, and Mn-siderite. This study relates barite and sphalerite to the salt-tectonic evolution of the basin, based on seismic interpretation, and the thermal history of the basin, based on fluid inclusion studies. Barite is readily transported in basinal fluids >100°C (212°F), yet is consistently a very late diagenetic mineral. This implies that the source of Ba is because of the late diagenetic breakdown of K-feldspars at 2–3 km (1–2 mi) depth, which is confirmed by covariation of Ba and Rb in sandstones. Sulfur isotope data suggest that the was derived from Argo Formation evaporites that include 1%–7% anhydrite. Sphalerite is mobile only in saline formation water >140°C (>284°F) and requires long-distance transport through sandstones with Zn-rich Fe-Ti oxides. Active detachment faults on salt welds provide potential pathways and a source of salt for migrating formation water. The particularities of source and transport of both barite and sphalerite allow the pathways of basinal fluids and their relationship to active salt tectonics to be inferred, providing indirect dating of the later stages of diagenetic paragenesis corresponding to times of hydrocarbon charge.