The Cassiar Pit occurrence is exposed in the east wall of the Cassiar asbestos mine open pit (104P 005), above the serpentinite sliver hosting the asbestos orebody.

The showing consists of a small (less than 1 metre) lens of concordant, laminated, massive chalcopyrite-pyrite-sphalerite-quartz in intercalated green tuff and argillite of the Upper Paleozoic Sylvester Allochthon. This occurrence is classified as volcanogenic, syngenetic.

Siliceous horizons of possible exhalative origin occur near the tuff-argillite contact along strike to the north, while a pyritic quartz stockwork occurs to the east.