The Tam showing lies within the Quesnellia Terrane, near its eastern boundary with the Barkerville Terrane. The geology of this region comprises dominantly black phyllite with interbedded deformed mafic volcanic rocks, considered to be of Upper Triassic age. These rocks are separated from the Barkerville Terrane to the east by the Eureka Thrust.

The primary lithology in the area of the showing is black slate and phyllite, interbedded with variable amounts of fine grained tuff. Quartz veins (+/- minor carbonate) occur predominantly within the argillaceous metasediments. The mineralization consists of argenti- ferous galena and pyrite with associated silver values.