LIGNITE A02

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

IDENTIFICATION

SYNONYM:

Brown coal.

COMMODITIES (BYPRODUCTS): Coal, coal liquids, (tar, gas, leonardite).

EXAMPLES

(British Columbia - Canada/International): Hat Creek (092INW047); Skonun, Queen Charlotte Islands; Coal River (mapsheet 094M10W); Estevan (Saskatchewan); Texas (USA).

GEOLOGICAL CHARACTERISTICS
CAPSULE DESCRIPTION:

Seams of brown to black coal hosted by clastic sedimentary rocks. It can still contain some imprints of the original vegetation. Wet and dense with a dull lustre. Slacks (disintegrates) on exposure to air.

TECTONIC SETTINGS:

Stable continental basins; shelves on the trailing edge of continents; foreland (molasse) basins; back-arc basins; fault blocks, often associated with strike-slip movement to limit sediment influx.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:

An area of slow sedimentation, in fresh water, with few or no marine incursions. Delta; shoreline swamp; raised swamp; lake; floating vegetation mats.

AGE OF MINERALIZATION:

Quaternary; Tertiary; occasionally older.

ASSOCIATED ROCK TYPES:

Sedimentary rocks exhibiting evidence of fresh and or shallow water deposition; carbonaceous mudstones; siltstones and sandstones, often with cross-stratification and other sedimentary structures of shallow water origin.

DEPOSIT FORM:

Lignite 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 the regional scale.

TEXTURE/STRUCTURE:

Lignite retains a dull matted appearance and is composed mainly of the lithotype huminite. It is banded and jointed. Footwall sediments are often penetrated by roots or weathered to clay (seatearth).

COAL SEAMS / ASSOCIATED MINERAL MATTER:

Lignite is defined as coal with an Rmax value of less than 0.4 %. In outcrop it contains between 30 to 40 % moisture. It usually contains a high percentage of the maceral vitrinite and lower percentages of fusinite and liptinite. Mineral matter occurs in the lignite seams as 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. It may be dissimilar to that of the surrounding rocks.

WEATHERING:

Weathering of lignite reduces the calorific value by oxidizing the carbon- hydrogen complexes. Minerals such as pyrite oxidize to sulphates. Secondary carbonates are formed.

ORE CONTROLS:

The regional geometry of coal seams is controlled by sedimentary features such as extent of the delta, trend of the shoreline, and trend of sand-filled river channels. Subsequent deformation, such as faulting and folding, is important for higher rank coals.

ASSOCIATED DEPOSIT TYPES:

Peat (A01), sub-bituminous coal (A03), paleoplacers(C04).

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 lignite.

GEOPHYSICAL SIGNATURE:

Lignite has a low density. Resistivity is variable but can be low for lignite. Surface geophysical 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; nonmarine sediments; coal spar in the sediments; small oily seeps. Presence of lignite seams can also be detected by methane escaping through the surrounding sediments and burn zones where the lignite outcrop has burnt, baking the surrounding sediments.

ECONOMIC FACTORS regrese arriba
TYPICAL GRADE AND TONNAGE:

The heat value of lignite is low. Gross heating value on a moist ash-free basis is 15 to 20 MJ/kg. Net useable heat will be lower because of the high moisture content and included mineral matter. Mine reserves range from tens to hundreds of million tonnes.

ECONOMIC LIMITATIONS:

Lignite is a bulk commodity which is expensive to transport. The low heating value and tendency for spontaneous combustion usually restrict lignite to local uses. The ratio of tonnage to useable heat is low so that there is a large amount of waste material generated.

END USES:

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

IMPORTANCE:

Major source of fuel used for local electrical power generation. Approximately 10 to 20 Mt of lignite per year are required to support 1 MW of power generation capability.

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
Business contact: Commentaries, proposals or details for negotiation: Mr. Jorge Perazzo
51 1 2638212 (spanish please) 1—602-499 2708 (USA)
jperazzo@miningperu.com
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