The general location of the Chischa occurrence is the headwaters of Tetsa River in the Muskwa Ranges of the Northern Rocky Mountains, approximately 18 kilometres south-southeast of the settlement of Summit Lake on the Alaska Highway (Open File 1987-13, Map, locality M 39).

The occurrence is based on the presence of magnesite in the Middle Proterozoic Chischa Formation. This is the lowest unit of the Helikan Muskwa Assemblage, and forms the core of the Tuchodi Anticline, an open, northwest-trending fold which formed on a ramp of a major northeast-verging thrust (Geological Survey of Canada Map 1343A; Geological Society of America, Geology of North America, Volume G-2, pages 111, 639, 642). The rocks belong to Ancestral North America (Geological Survey of Canada Map 1713A).

The Chischa Formation outcrops extensively at lower elevations in the upper section of the Tetsa River. The base is not exposed; the known thickness is about 950 metres. The unit comprises pale grey and pastel, very fine grained dolostone, fine-grained orthoquartzite and siltstone (Geological Survey of Canada Memoir 373, Paper 67-68). Sedimentary structures indicate deposition in a shallow water environment.

Dolomitic subunits near the base of the formation are reported to contain magnesite (Open File 1987-13, page 22). These rocks are pink to white, fine to medium grained, and form resistant cliffs and ledges. R.T. Bell of the Geological Survey of Canada has examined both these rocks and Australian magnesite deposits, and considers them remarkably similar (1986: Personal Communication). There is conflicting evidence whether this kind of carbonate-hosted magnesite deposit is of sedimentary or replacement origin (Open File 1987-13, page 7).

There is a vague reference to magnesite in mineral exploration records in the region, but they deal with barite-lead-zinc mineralization in a belt of Devonian carbonate rocks about 15 kilometres to the southeast (Assessment Report 9202, page 12).