Geologic and Geochronologic Constraints on the Timing of Mineralization at the Nanisivik Zinc-Lead Mississippi Valley-Type Deposit, Northern Baffin Island, Nunavut, Canada

Abstract

The Nanisivik mine is located on Borden Peninsula, northern Baffin Island, about 750 km north of the Arctic Circle. The deposit is hosted by Mesoproterozoic sedimentary rocks of the Bylot Supergroup that rest unconformably on the Mary River Group, an Archean to Proterozoic basement complex of granites and gneisses. The Mesoproterozoic strata have been folded into broad, open folds with north-south–trending axes. The sulfides at Nanisivik are hosted in petroliferous dolostones of the Society Cliffs Formation. A series of long-lived, east-west–trending, normal faults divide the Nanisivik area into a series of horst and grabens. All known sulfide bodies are associated with normal faults, particularly where they cut the fold axes of antiforms. The geometry of the main sulfide body indicates that sulfides were emplaced after tilting and folding of the host strata and that a horizontal feature such as a gas-water interface controlled the precipitation of the sulfides. The sulfides show no displacement across the Keystone fault, although the fault has displaced the host stratigraphy by about 150 m and has displaced the Mine dike, a Franklin-aged intrusion (~723 Ma), by about 40 m with steep oblique-slip movement. A progressive potassium feldspar alteration selvage, about 3 m wide, on the margin of the Mine dike has been recognized. Textural evidence and immobile element ratios indicate that this is an alteration product of the dike, interpreted to have formed by the hydrothermal event that emplaced the sulfides. Similar alteration assemblages are present throughout the mine and are interpreted to be altered siliciclastic sedimentary rocks. The 40 Ar- 39 Ar ages of orthoclase from these alteration assemblages indicate a Middle Ordovician (461 Ma) age of mineralization. Although the regional process that resulted in the formation of the Nanisivik deposit remains unclear, it is possible that topographically driven fluid may have resulted from localized Middle Ordovician uplift in the region, as seen at the Navy Board High. Basinal brines may have migrated along the basement contact within the overlying Mesoproterozoic siliciclastic sedimentary rocks, and local basement topography, such as the horst and graben structures, may have focused fluid flow into the Society Cliffs Formation at Nanisivik. Local structural and stratigraphic traps would ultimately have localized and controlled the precipitation of the sulfides.