Basinal fluid flow models related to Zn-Pb mineralization in the southern margin of the Maritimes Basin, Eastern Canada

Abstract

The carbonate-hosted Zn-Pb deposits in the Lower Windsor Group, Nova Scotia, are located along the southern margin of the Maritimes basin. Previous studies suggest that the deposits formed from basinal brines expelled from the basal part of the Maritimes basin (Horton Group), possibly related to tectonic events in late stages of the basin history, but it remains unclear where the specific source regions were and which mechanism is responsible for fluid expulsion from the source regions to the sites of mineralization. This paper presents two numerical models to address these problems. The first model, based on a stratigraphic profile ranging from the center to the southern margin of the Maritimes basin, simulates the distribution and evolution of fluid overpressures due to sediment compaction. The second model simulates the temperature distribution in a marginal sub-basin under the assumption that fluid flow was driven by topographic relief related to an uplift (highlands) proximal to the ore deposits.Modeling of fluid overpressure evolution indicates that, if the Horton Group sediments were laterally confined by basement highs or faults, fluid pressures approaching or exceeding loading pressures would be easily built up within the Horton Group after deposition of the overlying Windsor evaporites. Strong overpressures in the Horton Group rocks are predicted not only in the central part of the Maritimes basin but also in shallower sub-basins close to the sites of mineralization. In contrast, if the Horton Group rocks are assumed to be laterally continuous across the Maritimes basin, fluid pressures in the Horton Group would remain near hydrostatic and strong overpressures would be built up only within the evaporite layer. In both cases, the geothermal gradients would not be significantly disturbed by the sediment compaction-driven fluid flow.Modeling of topography-driven flow indicates that the geothermal gradients are only slightly disturbed if rock permeabilities are inherited from the compaction 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 at the site of mineralization could be increased relative to the background temperature, but under steady-state conditions it could not have reached the ca. 250 degrees C indicated by fluid inclusions in the deposits. Such high temperatures could be reached transiently if the conduits and basal aquifer had permeabilities higher than about 0.1 D.The numerical modeling results suggest that sediment compaction-driven fluid flow could not have been responsible for mineralization because it cannot satisfy the thermal conditions at the deposits. Topography-driven 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 overpressure development, we favor a model in which the main-stage ore-forming fluids were derived from the basal part of individual sub-basins proximal to the deposits and were driven by sudden release of overpressures, probably triggered by tectonic events. Topography-driven flow may have been dominant in the postore stage, i.e., after the dissipation of overpressures. This model agrees with other geochemical studies that indicate separate source regions for different deposits, high fluid flow rates, and involvement of lower salinity fluids after the main-stage mineralization.