Sedimentology of the Lowest Windsor Carbonate Rocks: Base Metal Hosts in the Maritimes Basin of Eastern Canada*

The Mississippian (Visran) Macumber and Gays River Formations ofthe Windsor Group in Nova Scotia are host o Pb-Zn-Ba (Cu-Ag) deposits. Transport and deposition f base metals were possible because ofan efficient porosity-permeability system developed in both units. Depositional f cies is a key parameter control-ling the potential openness ofcarbonate units. The Macumber facies were deposited after the collapse of the Acadian orogen and ensuing tectonically driven sea-level rise. The Macumber Formation is divided into two regional lithosomes: a lower thin unit (<2 m) of fine- and coarse-grained limestones deposited below the fairweather wave base positioned on an outer shelf with local chemosynthetic mounds, and an upper thicker unit (avg 10 m) of planar-bedded micritic limestones, most likely representing upper slope microbial mats interbedded with deep marine sulfates. The permeability s stem was created by various brecciation events affecting the upper part of the formation. The Gays River Formation, deposited upslope of the Macumber facies, is dominated by various organic mounds with intervening facies. The boundstone facies are indicative ofbelow fairweather wave base deposi-tion, roughly at the limit of the photic zone. Primary growth porosity of the mounds provided the plumbing system later used by mineralizing fluids. These mounds also record the effects of a sea-level rise culminating in regional deposition ofdeep marine sulfates. Outside the eastern margin of the Laurentia craton, asimilar Visran-aged mixed limestone sulfate succession is known from the Franco-Belgian basin in Baltica. Distribution of shelf and slope facies within the Visran mid-European ocean separating Laurentia, Baltica, and Gondwana suggests that restricted oceanic circulation in a rapidly closing sea was instrumental in the local occurrence of a salinity-stratified water mass and, hence, responsible for the presence of below wave base marine limestone and sulfate facies on the Laurentia nd Baltica continental margins