The Forty Thieves vein is hosted in andesites of the Upper Triassic Pioneer Formation, Cadwallader Group which is closely associated with diorite of the Permian Bralorne Igneous Complex; the rocks interfinger complexly and the diorite is suggested to be replacing the andesite rather than intruding it. The indistinct boundary between andesite-diorite is intruded by a Late Tertiary (?) 30 metre wide dacite porphyry dyke. The western boundary of the andesite-diorite is a west-dipping fault contact with a narrow belt of serpentinized peridotite of the President Ultramafics (correlative with the Permian and older Shulaps Ultramafic Complex).

The vein follows the main structural trend, striking northwest and dipping northeast along a 600-metre-long fissure (reverse fault) through the andesite, diorite and dacite porphyry. The vein consists of long lenses of quartz with a width of about 1 metre averaged along the shear. Pyrite, tetrahedrite and lesser chalcopyrite occur as streaks and disseminations, with malachite and azurite staining. In other parts, cloudy grey quartz surrounds altered wallrock inclusions containing stockworks of ankerite, sericite, chlorite and pyrite. The average assay obtained from vein quartz assays less than 0.34 gram gold per tonne, with similar values for wallrock samples (Minister of Mines Annual Report 1946, page 111). The Forty Thieves vein is reported to be very similar to the Arizona vein located nearby (see 092JNE024).