SEDIMENTOLOGY OF THE LOWEST WINDSOR CARBONATE ROCKS - BASE-METAL HOSTS IN THE MARITIMES BASIN OF EASTERN CANADA

The Mississippian (Visean) Macumber and Gays River Formations of the W indsor Group in Nova Scotia are host to Pb-Zn-Ba (Cu-Ag) deposits. Tra nsport and deposition of base metals were possible because of an effic ient porosity-permeability system developed in both units. Depositiona l facies is a key parameter control ling the potential openness of car bonate units. The Macumber facies were deposited after the collapse of the Acadian orogen and ensuing tectonically driven sea-level rise. Th e Macumber Formation is divided into two regional lithosomes: a lower thin unit (< 2 m) of fine- and coarse-grained limestones deposited bel ow the fairweather wave base positioned on an outer shelf with local c hemosynthetic mounds, and an upper thicker unit (avg 10 m) of planar-b edded micritic limestones, most likely representing upper slope microb ial mats interbedded with deep marine sulfates. The permeability syste m was created by various brecciation events affecting the upper part o f the formation. The Gays River Formation, deposited upslope of the Ma cumber facies, is dominated by various organic mounds with intervening facies. The boundstone facies are indicative of below fairweather wav e base deposition, roughly at the limit of the photic zone. Primary gr owth porosity of the mounds provided the plumbing system later used by mineralizing fluids. These mounds also record the effects of a sea-le vel rise culminating in regional deposition of deep marine sulfates. O utside the eastern margin of the Laurentia craton, a similar Visean-ag ed mixed limestone sulfate succession is known from the France-Belgian basin in Baltica. Distribution of shelf and slope facies within the V isean mid-European ocean separating Laurentia, Baltica, and Gondwana s uggests that restricted oceanic circulation in a rapidly closing sea w as instrumental in the local occurrence of a salinity-stratified water mass and, hence, responsible for the presence of below wave base mari ne limestone and sulfate facies on the Laurentia and Baltica continent al margins.