Fluid inclusion, stable and radiogenic isotope, and geochronological investigation of the polymetallic “five-element” vein deposit at the Eldorado Mine, Port Radium, Northwest Territories, Canada

Abstract

Uranium-rich, polymetallic “five-element” (Ni-Co-As-Bi-Ag) veins at the Eldorado Mine were investigated using complementary microanalytical techniques with the goal to improve understanding of the origins of fluids and metals, and timing of vein formation. Fluid inclusion assemblages record overall variability in mineralization P-T conditions from 100–300 °C, and up to 135 MPa. Mineralizing fluids carry the chemical signature of a highly saline brine, derived from evaporated, modified seawater: ~20–35 wt.% NaCl + CaCl₂ equivalent bulk salinity, and molar Cl/Br of ~230 during arsenide-stage mineralization and ~125 during sulphide-stage mineralization. The basinal brine was likely introduced from the 1740–1270 Ma Hornby Bay Basin. Arsenide mineralization is constrained at 1442 ± 36 Ma by U-Pb geochronology of coeval hydrothermal xenotime. ⁸⁷Sr/⁸⁶Sr values of 0.7046–0.7239 for vein carbonates conforms to a similar range of values in host rocks (ca. 1440 Ma), suggesting significant fluid-rock interaction and alteration of wall-rock.