Michael Nwakanma, MSc

The Moss Au deposit is an orogenic-style gold deposit hosted in dacite and diorites of the western Shebandowan greenstone belt, with an inferred mineral resource estimate of 140.07 Mt of ore averaging 1.09 g/t Au, yielding 4.91 Moz. Mineralization is concentrated within shear zones and an array of quartz-carbonate-pyrite veins. A Re/Os age (2708 ± 12 Ma) from molybdenite from a quartz-carbonate-pyrite ± molybdenum vein is taken to be the age of mineralization based on the relationship of gold with sulfides. This age, when compared with the ages of the host rock from Skimpole Lake area (2721 ± 4 Ma) and the nearby Burchell Lake stock (2680 ± 3 Ma), constrains the gold mineralizing event between 2725 Ma–2694 Ma.

Alteration at Moss occurs in variable intensities and 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% in sulfide-rich veins. Based on the observed textural and crosscutting relationships of the alteration, sulfides, and veins, a paragenetic sequence was developed; gold deposition is coeval with pyrite crystallization.

Alteration studies using hyperspectral Short Wave Infra-red (SWIR) data identified different species of white mica and chlorite at Moss. White mica with a spectral range of 2208–2216 nm is associated with high gold values and corresponds to a mixed phengite-muscovite composition. Chlorite with a spectral range between 2242–2249 nm is also associated with gold-bearing samples. The mineral chemistry of chlorite and white mica varies with proximity to the center of the deposit. This is characterized by an increase in Mg, Fe, Si, and a decrease in Al, in chlorite and white mica with increasing proximity to ore center. The variations in the Mg, Fe, Si, and Al contents are attributed to the Tschermak substitution reaction, where Al is replaced by Si in the tetrahedral sites, whereas Mg or Fe is incorporated into the octahedral sites; these variations can be used to distinguish hydrothermal and metamorphic chlorite and white mica species. The Tschermak reaction can be linked to temperature changes resulting from interaction with mineralizing fluids during deposit formation, hence can be used to track ore fluid pathways. Overall, mineral chemistry and spectral features of white mica and chlorite show a trend that can be used as a vector to ore.