Chemical characterization and timing of uranium mineralization in the Nonacho Basin, NWTOnline pre-recorded
The intracratonic Paleoproterozoic Nonacho Basin, deposited on the western margin of the Rae craton, contains polymetallic (i.e. U, Cu, Fe, Pb, Zn, Ag) occurrences near the unconformable contact with crystalline basement rocks. Exploration began in the 1950s following detection of anomalous radioactivity, leading to preliminary investigations of numerous U occurrences.
Two distinct styles of U mineralization are associated with the Nonacho Basin: primary uraninite of the Hope occurrence (northern basin) is disseminated within hydrothermally altered quartz-biotite-albite rich deformation zones within the basement gneisses, whereas later chlorite-calcite ± quartz vein-hosted and disseminated secondary U-hydroxides occur in basement granitoids and basinal siliciclastics near MacInnis Lake (south-central basin). Although separated by over 50 km, the two areas are connected by a series of north-east trending fault structures.
At the Hope showing, milled albitized-oligoclase clasts are hosted in a hydrothermal quartz-biotite corridor. Coarse grained Th-rich uraninite occurs as overgrowths on and as intergrowths with metamict zircon and is characterized by four distinct U-Pb ages: (i) primary magmatic uraninite crystallization at 2784 ± 21.7 Ma, ii) isotopic resetting by magmatic heating at 2614 ± 10.5 Ma, iii) isotopic reworking at 2531 ± 17.4 Ma by the MacQuoid Orogen, and (iv) alteration by hydrothermal fluids at 1778 ± 14.4 Ma, which also precipitated biotite (1834 ± 1.2 Ma; Ar-Ar) and molybdenite (1794 ± 28 Ma; Re-Os). Molybdenite and chlorite locally overprint biotite, and are coeval with the hydrothermal resetting of uraninite. Local pseudomorphs of biotite by muscovite with thorite may indicate dissolution of early uraninite by low-T, acidic fluids.
MacInnis Lake showings contain fine-grained, Pb-poor, heterogeneous U-hydroxides (i.e., uranophane and coffinite). This mineralization is associated with chlorite-calcite ± quartz veins which post-date albitization and potassic alteration of host rocks. Two modes of occurrence have been recognized: (i) as overgrowths on earlier, partially dissolved bornite and chalcopyrite; and (ii) associated with hematite and rutile and pervasive sericitization of chlorite. Rare-earth element concentrations of U are high, up to 9.5 wt.% and have high HREE/LREE values which resemble REE signatures of intrusive U and IOCG-IOA style mineralization. Extensive albitization and localized potassic-iron alteration associated with remobilized vein-hosted U resembles albitite-hosted U (+hematite+chlorite+K-feldspar) of the Southern Breccia IOCG system in the Great-Bear Magmatic Zone. MacInnis Lake U may ultimately be derived from early U occurrences such as Hope, which were remobilized by fluids along regional-scale faults and precipitated on Fe-bearing sulfides and silicates, which acted as a redox trap for mineralization, in relatively high temperature (313-333°C) chlorite-calcite ± quartz veins.