The North Fork of Siwash Creek is underlain by Early and Middle Jurassic Ladner Group sediments east of the Hozameen fault, a major, steeply dipping, north-northwesterly trending fracture system which separates rocks of the Methow-Pasayten trough from members of the Permian to Jurassic Hozameen Complex to the west. 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. 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.
North-northwest striking, variably dipping Ladner Group argillite predominates in the area of the Rodd B occurrence, also referred to as the Spuz B-Rodd A. These rocks have been intruded by highly altered and sheared sills of felsic porphyry. Feldspar minerals within the intrusions have been altered to clay. A narrow fault slice of sheared carbonate rock containing minor mariposite-fuchsite mineralization (listwanite?) also occurs in the area.
Narrow veinlets of quartz with some scheelite cut the sills and occur along foliation planes in the sediments. Gold has reportedly been recovered from panned samples of intrusive rock and associated quartz veins. A one-metre wide sample taken near the contact of one sill assayed 1.99 grams per tonne gold (Assessment Report 6962).
Similarly altered and shattered medium-grained sills of probable granodioritic composition cut slates at the Spuz B South zone, situated approximately 400 metres to the southeast. A major fracture zone striking 067 degrees and dipping 73 degrees to the southeast cuts these rocks.
Quartz hosting associated scheelite mineralization occurs as fracture-fillings up to two centimetres wide and as an irregular network of veins both within the sills and adjacent slates. Gold has been recovered from panned samples of altered intrusive.
A third showing, known as the Quartz-Carbonate zone and situated approximately 500 metres northwest of the Rodd B occurrence, consists of disseminated sulphides in strongly sheared slates adjacent to another fault slice of fuchsite-bearing carbonate rock (listwanite). These rocks have been sheared by three closely spaced fractures splaying off the nearby Hozameen fault system.