The mineralogy, petrography, and paragenesis of the polymetallic (Co-Ni-As-Au) veins of the Nictaux Falls Dam occurrence, Annapolis Valley, Nova Scotia

Abstract

Cobalt occurrences in Nova Scotia are poorly understood and their association with five elements vein deposits is unclear. The Nictaux Falls polymetallic (Co-Ni-Au-Ag-Bi) vein occurrence is hosted in the spillway of the Nova Scotia Power Dam found along the Nictaux River, Annapolis County. The sulfarsenide mineralized quartz veins crosscut the Silurian-aged Kentville Formation metasediments, near its contact the Devono-Carboniferous South Mountain Batholith. Two main zones are identified, hosting two styles of mineralization: laminated sulfarsenide-quartz veins (MZ1) and quartz breccia veins containing sulfarsenide-mineralized wall rock clasts (MZ2). The laminated MZ1 veins are heterogeneous with euhedral sulfarsenides and coxcomb quartz occurring on opposite sides of the vein. Wall rock MZ2 clasts contain late net-texture sulfarsenides and quartz. Petrographic work identified a spatial relationship between sulfarsenides and wall rock gangue minerals (chlorite, biotite, rutile) in both styles. Both styles are characterized by early interstitial chlorite, biotite, and rutile, followed by rounded sulfide inclusions, and surrounded by compositionally zoned sulfarsenide (arsenopyrite, mantled by a secondary arsenopyrite, Fe-rich cobaltite, and Fe-Co-rich gersdorffite). Textural evidence using Micro-XRF imaging suggests the unidirectional zoning is a product of Rayleigh fractionation or a decrease in pH. Mafic intrusions present in the spillway and host metasediments were evaluated as potential metal sources. SEM-EDS analyses and S-isotope work tentatively suggest the metasediments are the most likely source of sulfur and metals for the NFDO. In situ SIMS work is required in order to identify possible event of fluid mixing between brines that equilibrated with the metasediments and marine brines of the Maritimes Basin. The NFDO comprises a unique mineral assemblage that could be an example of a five elements vein occurrence. Further studies of cobalt occurrences in the Meguma Terrane is required in order to understand cobalt metallogeny in Nova Scotia and possible five element vein occurrences in the province.