The area around Kwinitsa is dominantly underlain by grey biotite plus or minus hornblende gneiss, amphibolite and minor sillimanite plus or minus garnet gneiss. Two sillimanite occurrences, noted on Geological Survey of Canada Map 3-1965, are located on the north side of the Skeena River, one opposite the mouth of Feak Creek, and the other north of Snag Point. These occurrences are part of a 20 kilometre zone which strikes northwest.

The gneisses are part of the Paleozoic(?) Central Gneiss Complex of the Prince Rupert-Skeena map areas.

Locally, 1.0 kilometre east of Kwinitsa, along Highway 16, excellent exposures of garnet-sillimanite-quartz-feldspar gneisses contain between 5 to 30 per cent garnet and 5 to 30 per cent silliman- ite. The sillimanite is generally present in densely felted layers ranging from 0.2 to 2.5 centimetres in thickness (Geological Survey of Canada Memoir 394). Similar mineralization occurs around Khatada Lake to the south (refer to Khatada Lake 103I 220).