Michael Nwakanma MSc Thesis Abstract
The Moss Au deposit is an orogenic-style gold deposit hosted in felsic to intermediate rocks of the western Shebandowan Greenstone Belt, close to the terrane boundary between the Wawa-Abitibi terrane and the Quetico metasedimentary basin, ~120 km west of Thunder Bay. The deposit has an inferred mineral estimate of 140.07 Mt of ore averaging 1.09 g/t Au, which yields 4.91 Moz (Goldshore, 2024). The majority of the gold is hosted within diorite and dacite and is localized by shear zones and an array of quartz-carbonate-pyrite veins. The central-felsic metavolcanic belt of the Shebandowan Greenstone belt comprises felsic to intermediate units surrounded by late granitic intrusions, such as the Burchell Lake and Moss Lake stocks. This study focused on characterizing the alteration and mineralization at the Moss deposit and investigating any correlation between alteration and gold mineralization. A combination of petrography, geochronology, geochemistry, and mineral chemistry was used to achieve the objectives of this study.
Alteration occurs in different styles and intensities but generally comprises albite, biotite, sericite, chlorite, carbonate, and epidote alteration. Sulfide minerals are dominated by pyrite with minor chalcopyrite, sphalerite and molybdenite. Sulfide abundance is commonly 2 – 10% of the samples but can be up to 15% within sulfide-rich veins. Disseminated and vein-hosted pyrite are the two main textures in which pyrite occurs within the host rocks. A total of 12 vein types were observed, with quartz and carbonate being the most dominant veins occurring together in five of the vein types. Using the observed textural and crosscutting relationships of the alteration, sulfides, and veins, a paragenetic sequence was developed, highlighting the secondary processes associated with the formation of the Moss Lake deposit. Deformation textures were observed in early and late alteration phases, suggesting a long deformation history that was broadly coeval with mineralization.
Quartz-carbonate-pyrite ± sericite ± chlorite ± epidote veins are host to most of the observed gold occurrences, and are common within or in proximity to shear zones. Gold was rarely associated with disseminated pyrite away from veins. Gold grains occurred as inclusions in
pyrite, on the rims of pyrite grains and in the groundmass around pyrite grains within the host vein and are genetically related to pyrite.
A Re/Os age (2708 ± 12 Ma) from molybdenite from a Type 3 quartz-carbonate-pyrite vein is interpreted to be the age of mineralization. This age, when compared with the ages of the host rock from the Skimpole Lake area (2721 ± 4 Ma; Corfu, 1998), and the nearby Burchell Lake stock (2680 ± 3 Ma; Corfu, 1983), constrains the gold mineralizing event between 2725 Ma – 2694 Ma. The age of gold mineralization overlaps with the age of the Central Felsic Metavolcanic belt (CFB) and indicates that the mineralizing event is older than the nearby intrusions.
Hyperspectral data showed the presence of different species of white mica and chlorite. The white mica with a spectral range of 2208 – 2216 nm is associated with most of the high gold values and corresponds to white mica with a mixed phengite-muscovite composition. Chlorite with a spectral range between 2242 – 2249 nm is associated with gold-bearing samples.
Mineral chemistry of chlorite and white mica varies with proximity to the center of the deposit. For white mica, Mg was highest in the samples proximal to the center of the deposit and lowest in the more distal samples. For chlorite, Mg and Si are highest in proximal samples and lower in distal samples, whereas Fe and Al are lower in the proximal samples and higher in the distal samples. These compositional changes in white mica and chlorite composition could be linked to the Tschermak substitution reaction, where Al is replaced by Si in the tetrahedral sites, while Mg or Fe is incorporated into the octahedral sites. This reaction can be attributed to temperature changes linked to interaction with mineralizing fluids during deposit formation. In general, the alteration intensity varies across the deposit with no clear vector towards mineralization. However, the mineral chemistry and spectral features of white mica and chlorite show a trend that can be used as a vector to ore, if properly applied.