The Bigjon Lake area is underlain by Early and Middle Jurassic Ladner Group siltstone and argillite and Permian to Jurassic Hozameen Complex chert in contact along the Hozameen fault, a major, steeply dipping, north-northwest trending fracture system extending from northern Washington State to the Fraser River. Most of the mineral occurrences in the area lie east of but generally close to this fault, which encloses metaplutonic rocks of the Coquihalla serpentine belt between Mount Dewdney and Siwash Creek. A small mass of serpentinite has been mapped along the fault just north of and beneath Bigjon Lake. The Ladner Group and, to a lesser extent, Hozameen Complex rocks are cut by a variety of small intrusive bodies ranging in composition from gabbro through granodiorite to syenite.
The Monument vein system comprises four segments, fragmented by a set of north trending dextral crossfaults. The massive white quartz veining with minor feldspar and carbonate strikes north-northwest and dips vertically or steeply to the east at the surface. It is hosted by Ladner Group argillite and siltstone which have been silicified adjacent to the vein contact. Altered felsic sills, similar to those found at the Rodd B (092HNW053) to the south, occur in close association with the veining and may have been involved in the emplacement of the economic mineralization.
The two northern segments vary from one to two metres wide and total approximately 160 metres in aggregate length. East-west striking, vertical crossfractures spaced a few centimetres apart cut the vein. Scheelite reportedly occurs along the eastern contact of the southern segment.
Assays from five surface samples across an average width of 1.72 metres averaged 2.88 grams per tonne gold. The best drill intersection in this area assayed 15.43 grams per tonne gold across 3.1 metres (Assessment Report 6962).
A five-metre wide greywacke interbed in the slates west of the northern-most section of the vein hosts fine-grained disseminated pyrite, pyrrhotite and chalcopyrite. Narrow quartz stringers in this unit were observed locally. Quartz stringers with minor fuchsite have also been introduced along shear zones cutting slate parallel to and west of the main vein. Fine-grained pyrite and minor chalcopyrite are disseminated in the surrounding rocks.
The two southern segments of the Monument vein vary from 1.5 to 2.5 metres wide and are exposed nearly continuously for approximately 200 metres. Conspicuous vertical crossfractures cut the vein at right angles.
Visible gold, occurring as 0.5 to 2.0-millimetre nuggets in vugs in the vein and as thin flakes on shears in the slates adjacent to the vein, has been observed near the north end of the southern section. A 1.75-metre wide sample taken at the southern end of the southern vein segment assayed 22.6 grams per tonne gold (Assessment Report 6962). Pyrite and arsenopyrite associated with argillite xenoliths and minor galena, native copper and possible marcasite occur within the vein in this area.
