The Duncan (No. 3) zone is located at 640 metres elevation above sea level on the east shore of Duncan Lake in the Slocan Mining Division. The Duncan Dam, completed in 1967, raised the lake level and flooded a good portion of the property.

Regionally, the area lies within the Kootenay Arc near the margins of the Ancestral North American Terrane. The Kootenay Arc is a curving belt of highly deformed metasedimentary and metavolcanic rocks which includes the Upper Proterozoic Horsethief Creek Group, the Eocambrian Hamill Group, the Lower Cambrian Badshot Formation, and the lower Paleozoic Lardeau Group. The volcano-sedimentary sequence is intruded by numerous Ordovician, Devonian and Mississippian granitoid plutons. The rocks have undergone regional metamorphism to middle or upper greenschist facies (Paper 1993-1).

The No. 3 zone is in grey, massive, banded or flecked marble of the Badshot Formation which overlies the Hamill Group and is characterized by cliff-forming, white to medium grey, commonly laminated marble or dolomitic marble. The marble horizons are tens of metres thick and usually separated by grey, locally calcareous schist. The marble is overlain by a thick succession of fine grained, dark grey and green schists of the Index Formation (Lardeau Group).

The sulphides are fine-grained sphalerite, galena and pyrite in thin tightly folded layers. Mineralization occurs as lenticular masses along the crests and troughs of small folds which plunge 10 to 15 degrees northwest (Bulletin 49).