The siliciclastic Paleoproterozoic Nonacho Basin and underlying crystalline basement rocks host numerous historic mineral occurrences (e.g., Cu, Au, Ag, U). To date, no comprehensive research has been conducted on the Nonacho Basin in order to understand its metallogeny and hydrothermal history, or to assess its potential for economic mineral deposits. This study takes a holistic approach, encompassing the entire hydrothermal system expressed throughout the basin, as opposed to isolated mineral occurrences, to understand the metallogeny and elucidate the mineral potential of the region. Through combined field, petrographic and microanalytical work, the research will i) characterize the mineral occurrences, including mineralogy, host rock alteration, and structural controls, ii) constrain the timing and paragenesis, iii) elucidate the physico-chemical conditions during each mineralizing event, and iv) classify the mineral occurrences.
Field observations broadly discriminate at least four hydrothermal events characterized by mineralogically and temporally distinct assemblages: i) pink to reddish feldspar-quartz±magnetite, ii) epidote ± hematite, iii) black chlorite ± U, Au, Ag, Cu-sulphides, specular to black hematite, and muscovite, and iv) quartz ± Cu, Zn, Pb, sulfides, Ag, barite, carbonate, and fluorite. These assemblages are ubiquitous throughout the Nonacho Basin, crosscutting all lithologies including basinal siliciclastics, and basement granitoid, gabbro and gneiss. The location of mineralization is structurally controlled by deformation zones, likely initiated as mylonites and overprinted by brittle deformation during later hydrothermal activity.
Feldspar-quartz±magnetite mineralization occurs commonly as veins (up to 5 cm thick) or broad zones of replacement that extend over a few meters. Pink to red feldspar alteration, and more rarely biotite, along shear zones suggest widespread potassic alteration during early, high-temperature hydrothermal activity and ductile deformation. Broad zones of fine-grained light green epidote alteration are common in areas of mineralization, replacing hydrothermal feldspar as well as host rock minerals (clay minerals, feldspar). The presence of epidote in the basin rocks suggests the introduction of Ca by fluids. The Ca may be sourced from plagioclase of the basement gabbro. It is unlikely that Ca originated from basin carbonate as that lithology is not dolomitized. Epidote alteration is commonly crosscut by veinlets of coarse-grained black chlorite. This black chlorite is associated with U mineralization, chalcopyrite, bornite and specular hematite near the MacInnis Lake area of the southwestern part of the basin. Historical drillcore assays indicate anomalous Au (up to 0.6 ppm) and Ag (up to 2.5 ppm). Quartz stockwork is characterized by massive, coxcomb and vuggy quartz, which crosscuts the earlier styles of mineralization. It is expressed as multiple generations of orthogonal quartz veins, suggesting multi-episodic hydrothermal activity at shallow levels. In the Crest and Salkeld lake areas, quartz veins are associated with carbonization and fluorite, and late base metal sulfide mineralization.
While it is too early in the research to classify the mineral occurrences, it is noted that the system (and parts there of) share similarities with IOCG, unconformity-type U, and various base metal vein deposits. Future work will refine the metallogeny of the Nonacho Basin.