The Chastot Creek showing is hosted by the Upper Mississippian to Permian Nahlin ultramafic body, part of the Cache Creek Complex. The Nahlin body is 100 kilometres long and up to 8 kilometres wide and is the largest alpine-type ultramafic in the Canadian Cordillera. On the southwest it is faulted against Lower Jurassic sedimentary rocks (Inklin Formation, Laberge Group, and Upper Triassic Stuhini Group volcanic rocks) and on the northeast against Upper Paleozoic rocks of the Cache Creek Complex. This ultramafic body consists of oceanic crustal ultramafic rocks consisting of: peridotite, dunite, pyroxenite, which is in general serpentinized, and locally includes pods of nephrite jade and small bodies of listwanite, rodingite and talc. The rocks weather uniformly reddish brown and are generally devoid of vegetation.

The ultramafic body largely consists of green to black peridotite with fine-grained partly serpentinized olivine, orthopyroxene, augite, and chrome spinel. The pyroxene forms discrete crystals and crystal clusters ranging from 0.3 to 1.3 centimetres across. The principal variation in the body is the degree of serpentinization which is most intense along contacts and sheared or brecciated zones.

Exposed contacts between the Nahlin ultramafic body and layered Jurassic and Triassic rocks are invariably marked by fault zones adjacent to which the peridotite has been sheared and serpentinized. The Nahlin fault, which bounds the southwestern margin of the body, comprises a subparallel network of anastomosing shear planes and fractures with steep north or vertical dips. The width and complexity of the Nahlin fault varies.

Adjacent to this major fault network the peridotite has been carbonatized producing bright orange weathering outcrops (listwanite) from a few metres to over 7 metres in width. Ankerite is the principal carbonate but veins of pure white, microgranular magnesite and coarsely crystalline dolomite are also present. The carbonitized zones are riddled with a network of thin chalcedony or opal stringers, as well as traces of bright green nickeliferous chlorite.

A nickel occurrence along Chastot Creek, as shown on GSC Map 1262A, hosts traces of millerite and veins of magnesite in shear zones along the margin of the Nahlin ultramafic body. The nickel-bearing zones are sheared, carbonated serpentinite, veined with a stockwork of chalcedony and opal stringers.