BASINAL FLUID-FLOW MODELS RELATED TO ZN-PB MINERALIZATION IN THE SOUTHERN MARGIN OF THE MARITIMES BASIN, EASTERN CANADA

The carbonate-hosted Zn-Pb deposits in the Lower Windsor Group, Nova S cotia, are located along the southern margin of the Maritimes basin. P revious studies suggest that the deposits formed from basinal brines e xpelled from the basal part of the Maritimes basin (Horton Group), pos sibly related to tectonic events in late stages of the basin history, but it remains unclear where the specific source regions were and whic h mechanism is responsible for fluid expulsion from the source regions to the sites of mineralization. This paper presents two numerical mod els to address these problems. The first model, based on a stratigraph ic profile ranging from the center to the southern margin of the Marit imes basin, simulates the distribution and evolution of fluid overpres sures due to sediment compaction. The second model simulates the tempe rature distribution in a marginal sub-basin under the assumption that fluid flow was driven by topographic relief related to an uplift (high lands) proximal to the ore deposits. Modeling of fluid overpressure ev olution indicates that, if the Horton Group sediments were laterally c onfined by basement highs or faults, fluid pressures approaching or ex ceeding lending pressures would be easily built up within the Horton G roup after deposition of the overlying Windsor evaporites. Strong over pressures in the Horton Group rocks are predicted not only in the cent ral part of the Maritimes basin but, also in shallower sub-basins clos e to the sites of mineralization. In contrast, if the Horton Group rod e are assumed to be laterally continuous across the Maritimes basin, f luid pressures in the Horton Group would remain near hydrostatic and s trong overpressures would be built up only within the evaporite layer. In both cases, the geothermal gradients would not be significantly di sturbed by the sediment compaction-driven fluid flow. Modeling of topo graphy-driven flow indicates that the geothermal gradients are only sl ightly disturbed If rock permeabilities are inherited from the compact ion model but could be strongly disturbed if higher permeabilities are assigned to the basal aquifer, and high-permeability zones (conduits) are assumed to cut through the evaporite layer and link the highlands (recharge area), basal aquifer, and discharge area. The temperature a t the site of mineralization could be increased relative to the backgr ound temperature, but under steady-state conditions it could not have reached the ca. 250 degrees C indicated by fluid inclusions in the dep osits. Such high temperatures could be reached transiently if the cond uits and basal aquifer had permeabilities higher than about 0.1 D. The numerical modeling results suggest that sediment compaction-driven fl uid flow could not have been responsible for mineralization because it cannot satisfy the thermal conditions at the deposits. Topography-dri ven flow may satisfy the thermal conditions at the deposits under the assumption of high-permeability conduits and a basal aquifer. Whether or not such conditions existed in the southern margin of the Maritimes basin needs further study. Based on the modeling results of fluid ove rpressure development, we favor a model in which the main-stage ore-fo rming fluids were derived from the basal part of individual sub-basins proximal to the deposits and were driven by sudden release of overpre ssures, probably triggered by tectonic events. Topography-driven flow may have been dominant in the postore stage, i.e., after the dissipati on of overpressures. This model agrees with other geochemical studies that indicate separate source regions for different deposits, high flu id flow rates, and involvement of lower salinity fluids after the main -stage mineralization.