The Tuya River Tertiary coal basin is located between the communities of Dease Lake and Telegraph Creek. The basin straddles the drainage of Tuya River and its tributaries, Little Tuya River and Mansfield Creek. Limits of the basin are poorly defined and in places it is overlain by recent volcanic rocks. However, it is estimated that the basin covers approximately 150 square kilometres and contains over 600 million tonnes of high-volatile B bituminous coal; a sizeable coalbed methane resource up to 0.04 Tcf (trillion cubic feet) may also exist (Fieldwork 1990, page 419).
The coal basin is bounded on the north by mafic rocks, possibly part of the Miocene to Pliocene Level Mountain Group. The eastern and western boundaries are probably fault controlled, with pre-Tertiary rocks to the east and younger volcanic rocks to the west. The southern boundary is arbitrarily defined by thick postglacial drift and absence of outcrop. Basement is composed of deformed Paleozoic and Mesozoic strata. Palynology dates the coal-bearing rocks as not younger than early Eocene and not older than Paleocene. They may be equivalent to the Tango Formation of the Sustut Group (Fieldwork 1990, page 420). Geological Survey of Canada Open File 2779 indicates bedrock to be Eocene Tanzilla Canyon Formation.
Sediments within the basin are generally coarse grained and poorly consolidated. In order of decreasing abundance, rock types are: sandstone, conglomerate and mudstone. The sandstones are medium to coarse grained, orange weathering and greyish when fresh. They contain numerous pebble grit bands and coal fragments. Conglomerates contain rounded volcanic and chert clasts ranging in size from granules to boulders, with pebbles predominating. They are yellow to orange weathering and form cliffs along the banks of the Little Tuya River. The mudstones are brown, sideritic and soft, and generally contain fine silty laminations. Vesicular basalts and diabases crop out in the basin.
Rocks structurally low in the succession in Mansfield Creek are mudstones, sandstones and a diabase sill, whereas rocks low in the succession in Tuya River are sandstones. Generally, rocks high in the successsion are conglomerates with volcanic clasts or basalt flows. Coal seams appear restricted to a zone fairly low in the succession.
The coal basin is an open, northerly plunging syncline, complicated by smaller scale faults and folds. The Tuya River coal basin was drilled and mapped in detail in the period 1979 to 1980 when interest in coal was high. Petro Canada mapped and drilled the western half (Thundercloud) of the basin and Esso Minerals Canada mapped the eastern half (see Tuya River, 104J 044).
In outcrop the coal is blocky, well banded and usually clean. It is often harder than the enclosing poorly consolidated sandstones. Seams vary in thickness up to 20 metres. Mudstone bands are common in the coal seams; bentonite layers are also conspicuous. The coal seams do not form part of fining-upwards sequences, and hangingwall and footwall contacts are sharp, with no particular enclosing rock type predominating. The coal is vitrain rich and contains an unusually high percentage of resin; some bands contain up to five per cent resin blebs ranging up to 5 millimetres in diameter. In places, the vitrain bands have a waxy lustre and conchoidal fracture which forms a distinctive eyed pattern on the fracture surfaces.
In the Thundercloud occurrence area, up to seven seams consisting of sub-bituminous B to high-volatile bituminous C rank coal are present. The seams are numbered 1 to 7 from stratigraphically lower to higher. Seam 1 varies from 3.61 to 5.03 metres thick and contains numerous carbonaceous claystone bands. Seam 2 ranges from 1.24 to 1.55 metres thick; seams 3 to 7 are best developed in the north of the property. They are complex composite seams which contain numerous bands and partings of shale and carbonaceous shale of up to approximately 50 per cent. Interseam intervals vary from 5 to 28 metres. The seams are commonly repeated and intersected by faults.
The coal analysed on average 11.14 per cent moisture, 24.25 per cent ash, 29.09 per cent volatile matter, 34.30 per cent fixed carbon, 0.46 per cent sulphur and has a calorific value of 4299 calories per gram.
A potential of 200 million tonnes of surface mineable coal was outlined in the western half of the basin to a depth of 500 metres (Fieldwork 1990, page 419).
In 2012 and 2013 Tuya Energy Incorporated conducted an airborne geophysical survey over the Tuya River project, covering the Thundercloud and Tuya River (104J 044) MINFILE occurrences. In March of 2015 Colonial Coal International Corporation acquired Tuya Energy Incorporated and the Tuya River Coal Project.
