SUB-BITUMINOUS COAL / A03

Cortesia del Gobierno de BRITISH COLUMBIA. Ministerio de Enegia y Minas

IDENTIFICATION

SYNONYM:

Steam coal, thermal coal, black lignite.

COMMODITIES (BYPRODUCTS):

Coal, coal liquids, (tar, gas).

EXAMPLES

(British Columbia - Canada/International): Princeton (092HSE089), Tulameen (092HSE209), Quesnel (093B036), Tuya River (104J044); Whitewood and Highvale mines (Alberta, Canada), Powder River Basin (USA).

GEOLOGICAL CHARACTERISTICS regrese arriba

CAPSULE DESCRIPTION:

Seams of black to brown coal hosted by clastic sedimentary rocks. The coal is banded dull and bright. Generally hard, sometimes the texture of the original vegetation is partially preserved.

TECTONIC SETTINGS:

Stable continental basins; shelves on the trailing edge of continents; foreland (molasse) basins; back-arc basins.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:

An area of slow sedimentation in fresh water with few or no marine incursions. Can be produced by fault blocks associated with strike-slip movement to limit sediment influx. Delta; shoreline swamp; raised swamp; lake; floating vegetation mats.

AGE OF MINERALIZATION:

Often Tertiary but can be older.

ASSOCIATED ROCK TYPES:

Sedimentary rocks exhibiting evidence of non-marine deposition. Carbonaceous mudstones, siltstones and sandstones are the most common, often with cross-stratification and other sedimentary structures formed in shallow water.

DEPOSIT FORM:

Coal seams generally conform with regional bedding; sometimes seams are deposited in areas of local subsidence, such as fault-controlled blocks or sink holes in karst topography, in which case deposits may be lens shaped. Occasionally seams can be thickened/deformed by surface slump, glacial drift or faulting. Seams may pinch out or split on a local or regional scale.

TEXTURE/STRUCTURE:

Sub-bituminous coal is usually composed mostly of clarain and vitrain. Footwall sediments are often penetrated by roots or weathered to clay (seatearth).

COAL SEAMS / ASSOCIATED MINERAL MATTER:

Sub-bituminous coal has Rmax values in the range of 0.4 to 0.6 %. In outcrop it can contain up to 30 % moisture. It usually contains a high proportion of vitrinite and lesser amounts of fusinite and liptinite. Mineral matter is in the coal as rock bands, as finely intermixed material of authogenic or detrital origin (inherent mineral matter) and as secondary material deposited in fractures and open spaces. Inherent mineral matter includes pyrite, siderite and kaolinite.

WEATHERING:

Weathering of sub-bituminous coal reduces the calorific value by oxidizing the carbon-hydrogen complexes. Minerals in the mineral matter will also oxidize. Pyrite oxidizes to sulphates. Secondary carbonates are formed.

ORE CONTROLS:

The regional geometry of the seam/seams is controlled by sedimentary features, such as the extent of the delta, trend of the shoreline, and trend of sand-filled river channels. Deformation (faulting and folding) is important in some deposits

ASSOCIATED DEPOSIT TYPES:

Lignite ( A02 ); bituminous coal ( A04 ), Shale-hosted Ni-Zn-Mo- PGE ( E16 ), Phosphate - upwelling type (F07).

COMMENTS:

Lignite has the lowest rank of all classes of coal (Rmax less than 0.4 %).

EXPLORATION GUIDES regrese arriba

GEOCHEMICAL SIGNATURE:

Geochemistry is generally not used as a prospecting tool for coal.

GEOPHYSICAL SIGNATURE:

Coal has a low density. Resistivity is variable to high. Surface techniques include direct-current profiling, refraction and reflection seismic, and gravity. Subsurface or bore-hole techniques include gamma logs, neutron logs, gamma-gamma density logs, sonic logs, resistivity logs and caliper logs.

OTHER EXPLORATION GUIDES:

Presence of: a down-slope coal bloom; coal spar; small oily seeps or methane escaping through the surrounding sediments. Zones where the coal outcrops have ignited and burnt to some depth underground.

ECONOMIC FACTORS regrese arriba

TYPICAL GRADE AND TONNAGE:

Gross heating value on an ash-free moist basis is 20 to 27 MJ/kg. Net useable heat will be lower because of the high moisture content and the presence of ash. Mine reserves range up to hundreds of millions of tonnes. The sub-bituminous coal resources of B.C. Tertiary coal basins commonly range up to 200 Mt (Hat Creek exceptional with 1000 Mt).

ECONOMIC LIMITATIONS:

: Coal is a bulk commodity which is expensive to transport. The moderate heating value and tendency for spontaneous combustion means that sub- bituminous coal is usually used locally for electrical power generation. The ratio of tonnage to useable heat is low so that there is a larger proportion of waste material (water, fly ash and slag) generated when burnt than for higher rank coals.

END USES:

Steam generation in turbines for electrical generation. Feed for liquefaction or gasification.

IMPORTANCE:

Approximately 8 to 10 Mt of sub-bituminous coal is required to generate 1 MW per year.

REFERENCES regrese arriba

  • Armstrong, W.M., Fyles, J.T., Guelke, C.B., Macgregor, E.R., Peel, A.L., Tompson, A.R. and Warren, I.H. (1976): Coal in British Columbia, A Technical Appraisal; B.C. Ministry of Energy, Mines and Petroleum Resources, Coal Task Force, 241 pages.

  • Cope, J.H.R., Duckworth, N.A., Duncan, S.V., Holtom, J.E.B., Leask, A.L., McDonald, K.A. and Woodman, S.P. (1983): Concise Guide to the World Coalfields; compiled by Data Bank Service, World Coal Resources and Reserves, IEA Coal Research.

  • Matheson, A. (1986) : Coal in British Columbia; B.C. Ministry of Energy, Mines and Petroleum Resources, Paper 1986-3, 169 pages.

  • Smith, G.G. (1989) : Coal Resources in Canada; Geological Survey of Canada, Paper 1989-4, 146 pages.


  • Cortesia del Gobierno de BRITISH COLUMBIA. Ministerio de Enegia y Minas
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