Fluid inclusion systematics of the polymetallic (Co-Ni-As-Au ± Bi, Ag) veins of the Nictaux Falls Dam occurrence, Annapolis Valley, Nova Scotia

Abstract

This thesis is a fluid inclusion study on poorly understood cobalt-nickel-rich sulfarsenide (+Au,Ag,Bi) quartz veins hosted in the Silurian Kentville Formation metasediments, which outcrop at the Nictaux Falls Spillway, Annapolis Valley, Nova Scotia. Mineralization is constrained to i) early, laminated, quartz-sulfarsenide veins, and ii) sulfarsenide mineralized wall rock clasts in quartz breccia veins. Late quartz veins barren of mineralization crosscut mineralized veins. Similar barren veins are exposed in the other areas of the spillway, crosscutting the metasediments and the nearby outcropping Cloud Lake Pluton monzogranite of the South Mountain Batholith (SMB). Quartz-hosted fluid inclusions in all vein types (barren and mineralized, metasediment and granite hosted) are similar and classified into three types. Type-1 inclusions contain two-phases (L+V) and homogenize via vapour dissolution from 111.9 to 250°C (n=34). Type-2 inclusions contain three-phases (L+V+S) and homogenize at temperatures of 121.4°C to 250°C (n=18) via halite dissolution. Niether inclusions froze upon cooling to -180°C, suggesting high salinity and divalent cations (Ca²⁺). Although many of these inclusions did not homogenize by the set limit of 250°C, some appeared to be close to homogenization suggesting minimum entrapment temperature of 250°C. The type-2 microthermometry data indicates salinities of 29 to 35 wt% NaCl equivalent and minimum entrapment pressures of up to 3.0kbar. Despite cathodoluminescence imaging, type-1 and -2 inclusions are of indeterminate origin. Type-3 inclusions are monophase (L), and of secondary origin, and exhibit post entrapment modification. Raman spectroscopy indicate inclusions are water-dominated, with trace amounts of CH₄ in the mineralized zones. Ti-in-quartz thermometry provided an upper T constraint of 614°C. Decrepitate mound analysis determined that the solute composition of the fluids range from Na-rich to Ca-rich. The highly saline fluids are likely Na-rich marine brines from the Maritimes Basin that became Ca-rich through interaction with plagioclase of the metasedimentary rocks of the Meguma Terrane.