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 Recent 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 succession 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, 104J 043) of the basin and Esso Minerals Canada mapped the eastern half (this description).
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.
Coal seams are present in the lower and middle members at the Tuya River occurrence. The seams are thickest and contain the least numbers of partings in the lower member. Two main seams are present in the lower member; the lower seam ranges from 1.45 metres thick in the northwest corner of the area to 2 to 3 metres in the Mansfield Creek area. The seam contains a number of partings and a sideritic horizon in the basal partings in the northwest. The upper seam ranges from 1.5 to 4 to 5 metres in thickness and also contains a number of partings. The seams are separated by approximately 1.2 metres of mudstone. The coal is high-volatile bituminous C in rank.
The middle member contains several thin coal seams 0.1 to 1.0 metre thick, but generally averaging approximately 0.4 metre. The coal is interbedded with, and contains numerous partings of mudstone.
A single coal seam was observed in the upper member. The seam is 0.5 metre thick and occurs in a 1.1 metre section of coal, shaly coal and mudstone.
The coal on this property analysed 11.35 to 16.9 per cent moisture, 5.1 to 9.92 per cent ash, 28.36 to 35.6 per cent volatile matter, 42.4 to 49.22 per cent fixed carbon, and 0.9 to 1.15 per cent sulphur. The calorific value ranges from 9680 to 11,401 BTU per pound (as received).
In 2012 and 2013 Tuya Energy Incorporated conducted an airborne geophysical survey over the Tuya River project, covering the Thundercloud (104J 043) and Tuya River MINFILE occurrences. In March of 2015 Colonial Coal International Corporation acquired Tuya Energy Incorporated and the Tuya River Coal Project.
