The origin of polymetallic Ni-Co-As-Bi-Sb(-Ag-U) veins in the East Arm Basin and southern Slave Province, Northwest Territories

Abstract

Polymetallic, “five-metals association (FMA)”-type Ni-Co-As-Bi-Sb (-Ag-U) hydrothermal vein deposits occur in the vicinity of the East Arm Basin, Great Slave Lake, at Blanchet Island and Copper Pass, Northwest Territories of Canada (Great Slave Lake East Arm basin), the locations of this study. Integration of bulk and microanalytical methods (e.g., cathodoluminescence (CL), scanning electron microscopy (SEM), fluid inclusion microthermometry, secondary ion mass spectrometry (SIMS), and laser ablation inductively coupled mass spectrometry (LA-ICPMS)) was focused on these systems with the goal of updating the current model for this deposit style. The vein systems in the East Arm Basin occur over 55 km, and share similar microthermometric, isotopic, ore tenor and paragenetic characteristics. Fluid inclusion results indicates salinity (~18-23 wt% CaCl₂ equiv.) and minimum entrapment T (~170-250°C) varied little with time during vein formation. An entrapment window between ~145 and 225°C and 0.5 to 0.95 kbar is estimated for coeval brine and carbonic inclusions. Fluid δ¹⁸O values increased by as much as 10 ‰ at the onset of mineralization. Quartz-carbonate hosted fluid inclusions in mineralized veins are enriched in Ca-Sr-Ba-Mg-Mn-Pb-Zn but contain only low-ppm concentrations of ore metals. Primary coeval bitumen inclusions, in contrast, are significantly enriched in U-Ni-Co-Bi-Ag-Sb-As-Mo-Cu. Integration of all data types strongly suggests that the precipitation of metals and bitumen was triggered by isothermal mixing of ¹⁸O-rich basinal brines and metal-rich bitumen particles or oil droplets; oxidation of this hydrocarbon phase resulted in metal precipitation. The basinal brines and associated metal-rich bitumen are thought to be derived from formly overlying outliers of the Athabasca -Hornby Bay -Thelon sedimentary basins that now outcrop ~300-1000 km away from the study areas. This work provides a direct confirmation for the involvement of basinal ore fluids, supporting recent models (i.e fluid mixing) and demonstrating hydrocarbons participated as metal transport agents.