Polymetallic, U-bearing “five-element”-type vein systems in the NWT show contrasting grade and tonnage characteristics from one deposit district to another. The results here conclude a 3-year study comparing sub-economic examples in the Great Slave Lake area that are hosted in Archean (Slave craton) and Paleoproterozoic rocks (East Arm basin) and in the Great Bear Lake area that are hosted in Paleoproterozoic rocks (Great Bear magmatic zone), including an historically world-class deposit at Port Radium (Eldorado: ~13 Moz Ag; 6000 t U3O8). Integration of a variety of microanalytical methods has focused on: (i) characterizing processes responsible for ore metal precipitation; (ii) the source of metals and fluids, and (iii) the timing of vein formation.
In basement (host) rocks, early, pre-mineralization fluids were heated basement brines (minimum ~150-200oC; rich in Ca-Na-Sr-Ba; ~20–30 wt% CaCl2 eq.). Mineralization formed when immiscible hydrocarbons and marine evaporite brines (rich in Ca-Na-Mg-Pb-Zn; 20–35 wt% CaCl2 eq.; very low Cl/Br ratio) were introduced along basement faults. A large shift in brine δ18OVSMOW by +10 ‰ suggests that basement faults were efficiently flushed by these sedimentary basin-derived fluids at the time of mineralization. Both basement- and sedimentary basin-derived brines were poor in ore metals, containing only sub- to low-ppm concentrations. In contrast, immiscible hydrocarbons, now entrapped in fluid inclusions, were metal-rich, containing 100s of ppm U-Ni-Co-Bi-Ag-Sb-As-Mo-Cu. Integration of all data types shows that metal precipitation was triggered by mixing of heated, oxidizing basement brine (metal-poor) with evaporated seawater and metal-rich oil at P < 1 kbar and ~200°C.
At the Eldorado deposit, a revised age for arsenide-stage mineralization is constrained to 1442 ± 36 Ma by U-Pb (SHRIMP) geochronology of coeval hydrothermal xenotime. During that time, the Dismal Lakes Group of the Hornby Bay Basin was deposited (~1438 ± 8 Ma); the presence of both evaporites and black shales in this succession implicate its role in the supply of evaporitic brines, hydrocarbons, and metals to polymetallic vein systems. More broadly, brines and metal-rich hydrocarbons were likely sourced from former overlying intracratonic basins (e.g., Hornby Bay, Thelon, Athabasca) that covered a vast area at the time of vein formation.
As an exploration tool, fluid inclusion chemistry allows the discrimination of small, sub-economic deposits that formed in deep basement rocks, (farther below overlying metal sources) from shallower, much larger economic deposits that formed immediately below mature basin sequences. High grade polymetallic “five-element” vein systems can be linked to the protracted metallogenic evolution of previously overlying intracratonic sedimentary basins, and the metal endowment of these vein systems may be entirely unrelated to granitic magmatism in the Great Bear magmatic zone, aside from these contributing heat to drive fluid circulation.