The geochemical record of hydrothermal mineralization and tectonism inboard of the Appalachian Orogen: the Ottawa Embayment

The history of mineralization of the Ottawa Embayment includes migration of basin brines within the confines of an Early Paleozoic basin, and cross-stratal fracture flow linked to structural reactivation of an underlying Neoproterozoic failed-rift system. Paleoporosity consists of paleokarst (PAL) in Lower Ordovician dolostone (Beekmantown Group), and fractures (now veins) that cross-cut both Lower Paleozoic (PL) and Precambrian (PC) strata. A series of contrasting mineral assemblages, which define changing fluid compositions, fill paleokarst cavities and rare interconnected paleofractures (now veins). First, there is an early-PAL succession of quartz, saddle dolomite, then calcite, with accessory hydrocarbon as well as sulphide and sulphate minerals. The quartz defines migration of saline (>20 wt.%) silica brines and hydrocarbon that spans a temperature increase (defined by fluid inclusions) from <100 to <200 °C. This gradient is registered also by a change in state, from liquid to methane, of hydrocarbon trapped within the quartz crystals. The succession of saddle dolomite, then calcite associated with liquid hydrocarbon, marks an overall cooling to <110 °C. δ13C of the dolomite identifies strong sedimentary rock–water interaction, but a depleted 13C composition (∼−14‰) for the calcite illustrates that, with cooling, paleokarst cavities were sites of bacterially mediated sulphate reduction. Remaining paleokarst porosity is filled with a late-PAL mineral assemblage that contains low-temperature (mostly <60 °C) calcite with a varied assemblage of sulphide and sulphate minerals. However, a similar mineral assemblage occurs in regionally abundant PL calcite veins that, locally, visibly intersect paleokarst void space. Furthermore, PL veins are also connected to PC veins. Thus, the late-PAL period of mineralization is linked to regional fracture-controlled cross-stratal fluid migration, which locally intersected paleokarst. Isotope (S, O, C) geochemistry, mineral assemblages, and fluid inclusions from vein and late-PAL minerals identify mixing among four endmember solutions: evolved hydrothermal (<130 °C) basement-derived brines; cool (<60 °C) meteoric-derived fluids; dissolved Middle Ordovician evaporite; and residual basin brines. The age of mineralization awaits future analysis, but patterns and compositions of fluids in this interior basin are consistent with being far-field effects of Paleozoic and Mesozoic tectonism in the Appalachian orogen.