The Mile 397 occurrence is situated around a small creek gully, a few hundred metres northeast of the Alaska Highway, about 9 kilometres west-northwest of Summit Lake (Minister of Mines Annual Report 1960; Open File 1992-16, Figure 20).
The occurrence is in a region of folding and thrust faulting in Paleozoic sedimentary rocks just north of the Tuchodi Anticline, a major structure in the Muskwa Ranges of the Northern Rocky Mountains, involving rocks belonging to Ancestral North America (Geological Survey of Canada Map 1713A). Around the Alaska Highway northeast of Summit Lake, the southwest-dipping limb of a subsidiary anticline has been partly detached and displaced on a reverse fault (Geological Survey of Canada Map 1343A). This several-kilometres wide fold limb or thrust panel consists mainly of Devonian rocks, including the Middle Devonian Dunedin Formation which hosts the Mile 397 occurrence (Open File 1992-16). The formation generally consists of dark grey to black, fetid, well bedded limestone, calcarenite and dolostone (Geological Survey of Canada Memoir 373). In this area, it strikes approximately 315 degrees and dips 35 degrees southwest, although small folds are present.
The barite forms an irregular vein-like mass that extends up the steep, north side of the creek, from its bed to the lip of the canyon 60 metres above it (Minister of Mines Annual Report 1960). For the lower 30 metres, the 'vein' strikes 335 degrees and dips 60 degrees west, cutting through the more gently dipping limestone. From there on upwards, the vein's dip decreases and it becomes almost parallel with the limestone's bedding. Some post-mineralization faulting is indicated by slickensides on the vein's upper surface. The vein is 6 metres wide at the creek, and widens to 30 metres halfway up, near the change in dip; at the top it is 20 metres wide. In plan, the exposure is roughly 60 metres long.
The lower surface of the vein is very irregular, with several apophyses of barite projecting into the limestone wallrock, some for a length of 25 metres or more. There is some brecciation along the vein margin, with inclusions of limestone in the barite. Considerable replacement of limestone by barite is indicated in this zone.
The barite is inhomogeneous. Part is massive, part is coarsely crystalline, and at the creek it is extremely friable. It is also present as smaller veins, as solution cavity fillings, and as rosettes or blades in the host limestone (Open File 1992-16, page 40). The main impurities that are visible are limestone, coarse white calcite, and minor purple fluorite (Open File 1992-16). An 18-metre chip sample, taken near the northern end of the exposure, was analysed at 55.67 per cent barium; the specific gravity of the sample was 4.36 (Minister of Mines Annual Report 1960). A sample of purple fluorite was analysed at 73.64 per cent CaF2 (Open File 1992-16, page 35).
Small scattered lenses and veinlets of barite, fluorite and coarse white calcite are present in the area surrounding the main showing: on the slopes for a few kilometres along strike to the northwest and southeast, and up stream to the northeast for a few hundred metres (Minister of Mines Annual Report 1960). However, it does not appear that the deposit is significantly larger than the main vein.
