The Truroc deposit is located on the east bank of the Lussier River, 2.5 kilometres south of its confluence with Roam Creek, 26.5 kilometres southeast from the village of Canal Flats.
Gypsum in the Stanford Range occurs in rocks of Devonian age. The area is underlain by a sequence of evaporites and associated carbonate rocks of the Burnais Formation with an overlying limestone and shale sequence of the Harrogate Formation. More recent work proposed the term "Cedared Formation" for a sequence of dolomites, sandstones and limestones that is, in part, stratigraphically equivalent to the Burnais Formation. Much of the carbonate strata previously included in the Burnais Formation are now tentatively assigned to either the Cedared or Harrogate formations. The Harrogate Formation is the youngest Devonian unit.
Thin-bedded or laminated gypsum of the Burnais Formation is assumed to be in fault contact with the underlying Ordovician to Silurian Beaverfoot-Brisco Formation, or in conformable contact with the Cedared Formation, and overlain conformably by the black to dark grey limestone of the Harrogate Formation. The Beaverfoot-Brisco Formation is comprised of thin to medium-bedded light grey dolomite and limestone with characteristic ovular chert nodules and lenses in a carbonate matrix. The gypsum is of good quality ranging between 83 and 93 per cent gypsum. It varies in color from pale grey to grey, brownish grey and dark grey to black. Cream-colored laminae are also present.
The evaporite sequence has been folded into a series of northwest-plunging, 18 to 40 degrees, folds. Small scale faulting with minimal displacement is present west of the Elkhorn quarry.
In the Coyote Creek-Lussier River area, the Devonian sequence is overlain by a shale unit and carbonate strata of the Mississippian Banff Formation.
The southernmost exposures of gypsum in the Stanford Range occur in the Lussier River-Coyote Creek area. In the Lussier River valley, the majority of occurrences are located east of the river. Extensive and very thick overburden preclude tracing the gypsum over any significant distance. Where observed, the gypsum is steeply dipping to vertical. Faulting may have played an important role in the localization and preservation of these deposits. The dominant structural feature in the area is a north trending syncline with shallow dipping limbs. The axis of this syncline is located along the height of land separating the Lussier River and Coyote Creek. Gypsum is present along both limbs of the syncline (Open File 1991-15).
At the Truroc deposit, gypsum occurs on the east bank of the Lussier River, in steep bluffs, in excess of 30 metres high over a length of 200 metres. Sinkholes on both sides of the river suggest that gypsum may be present over a much larger area than observed in outcrop.
The gypsum is pale grey and thinly-bedded to laminar. Bedding and laminations are generally steeply dipping and very often severely contorted. Fault related breccia zones are observed in both outcrop and periodically in drill core. Native sulphur is present in trace amounts. Results of a drill program indicate that the gypsum is approximately 33 metres thick in its centre and 20 metres thick near its northern end.
Overlying the gypsum is black to dark grey limestone of the Harrogate Formation. To the south, the gypsum abuts against a calcareous tufa that is probably a facies equivalent. North of the gypsum, black aphanitic, steeply dipping limestone of the Cedared Formation occurs. The contact with the gypsum is not observed. Faulting is common and may mark the contact between the gypsum and this limestone.
Gypsum/anhydrite evaporite deposits commonly form in either standing bodies of water or within the vadose zone and upper phreatic zone on supratidal flats and desert playas. Characteristics of the former include laminated or bedded evaporites and soft sediment deformation with small faults. They are usually void of fossils. Based on these criteria it can be interpreted that gypsum deposits in the Stanford Range probably formed in a standing body of water. Water depths were probably shallow ranging from a few centimetres to a few metres. This is evidenced by the presence of cross- laminations, cut and fill structures and rip-up breccias. The presence of selenite is also indicative of a shallow water environment (Open File 1991-15).
A conservative reserve estimate suggests a potential for 20 million tonnes grading 85 to 90 per cent gypsum (Z.D. Hora, personal communication, 1991). No gypsum has been observed on the west side of the Lussier River at this locality. Drilling was unsuccessful in penetrating the overburden in this area. While the presence of sinkholes suggests that gypsum may be present at depth it is very doubtful that this gypsum, if present, can be exploited.
Westroc Industries holds the property.
