The Cosmo Mine property falls within the Archaean to Palaeo-Proterozoic Pine Creek Orogen, one of the major mineral provinces of Australia. The Pine Creek Orogen is a deformed and metamorphosed sedimentary basin up to 14km maximum thickness covering an area of approximately 66,000km2 and extending from Katherine in the south to Darwin in the north. It hosts significant mineral resources of gold, uranium and platinum group metals, as well as substantial base metals, silver, iron and tin-tantalum mineralization.
The Cosmo Mine mineralization occurs within the folded Koolpin Formation. This formation consists of interbedded siltstones, carbonaceous mudstones, banded ironstone, phyllites, dolerite sills and greywacke units. Mineralization at the Cosmo Gold Mine occurs within a marine siltstone package located between the Inner Zamu Dolerite sill and a +30m thick pyritic carbonaceous mudstone unit. Siltstones near the carbonaceous unit contact often contain nodular chert lenses, with the nodular units often interbedded with massive and banded siltstone units. The width of the gold hosting siltstones is 30 to 50m wide in the footwall of the F1 Fault and from several meters to 50+ meters wide in the hangingwall due to variably developed folding.
Gold mineralization is closely associated with arsenopyrite often seen within the nodular siltstone unit. The mineralization styles, both on the hangingwall and footwall of the major F1 Fault are very similar, with the main mineralization associated with, but not necessarily totally constrained within the nodular unit. The main sulfide minerals in the fresh rock are pyrite and arsenopyrite, with traces of sphalerite and chalcopyrite. Pyrrhotite occurs below depths of 300m and is predominantly seen in the carbonaceous unit.