Field observations of polymetallic vein occurrences (Ag and base metal - Cu, Pb, Zn) mineralization in the Nonacho Basin (Churchill Province) suggest that the mineralization style at Crest and Salkeld Lake are late-stage with respect to other (e.g., U, Fe) styles in the basin. Host rock lithologies consist of intensely folded and faulted metasedimentary rocks (clastic) and biotite and granitic gneiss with lesser felsic-intermediate intrusions. Stockwork- and shear-style mineralization occur along, and cross-cut, metaigneous-metasedimentary contacts. A variety of alteration styles affect the host rocks but predate mineralization including i) early quartz-feldspar ±magnetite veins, ii) epidote ± hematite as pervasive replacement and veins, iii) black chlorite that pseudomorphs biotite in gneiss, and iv) quartz stockwork ± muscovite, carbonate. These alteration assemblages are observed throughout the basin.
An initial goal is to provide a deposit classification (i.e., IOCG, porphyry, skarn, etc.). Petrographic analysis of samples shows at least three different mineralization assemblages comprising stockwork- or fracture-hosted styles. These include: (i) disseminated and fracture-hosted chalcopyrite within metasedimentary and gneissic host rocks; (ii) massive quartz-(carbonate) veins containing bornite-chalcopyrite vug infill; (iii) sulfide-quartz breccias containing angular to sub-rounded clasts of earlier quartz vein material surrounded in massive (in order of abundance from highest to lowest) galena-sphalerite (cadmium-rich)-bornite-chalcopyrite. Optical and scanning electron microscopy show that all sulfides are texturally late-stage relative to quartz-carbonate, and that where bornite is present, it replaces chalcopyrite. All mineralization styles are associated with early silicification-sericitization manifesting as a bleached appearance in the country rocks in vein halos, and a later carbonate-barite-fluorite alteration assemblage.
Preliminary observations on fluid inclusions show that quartz fragments host abundant secondary inclusion trails containing mixed liquid-vapour inclusions containing a low density vapour phase, consistent with either boiling and/or a very shallow level of formation.
The style of mineralization (quartz-carbonate-sulfide vein and breccia), composition of the mineralized assemblage (galena, sphalerite, chalcopyrite, bornite), and associated alteration styles (silicification, sericitization followed by carbonate, barite, fluorite) is reminiscent of both high and low temperature deposit styles. Possible comparable mineral deposit styles include: (i) base metal skarns (e.g., Whitehorse Copper Belt, YT; Gaspé, QC); (ii) manto-type carbonate replacement deposits occurring peripherally to known or suspected porphyry deposits (e.g., Butte and Leadville, USA; Prairie Creek, NWT) (iii) clastic metasediment vein-hosted base metal deposits (Keno Hill, YT; Coeur d’Alène, USA). Future work will comprehensively evaluate these mineralization styles as possible analogues for the Nonacho Basin occurrences and establish their relationship, if any, to earlier mineralization styles and potentially causative intrusions.