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Talco em carbonatos

by G.J. Simandl1 and S. Paradisl2

1 British Columbia Geological Survey, Victoria, B.C., Canada
2Geological Survey of Canada, Pacific Geoscience Centre, Sidney, B.C., Canada


REf: talco, carbonato, sedimentar, dolomito, tremolita, serpentina

Simandl, G.J and Paradis, S. (1999): Carbonate-hosted talc; in Selected British Columbia Mineral Deposit Profiles, Volume 3, Industrial Minerals, G.J. Simandl, Z.D. Hora and D.V. Lefebure, Editors, British Columbia Ministry of Energy and Mines.


SYNONYMS: "Dolomite-hosted" talc deposits.

COMMODITIES: Talc and/or tremolite. Some of the commercial products derived from carbonate-hosted deposits and marketed as talc, contain over 50% tremolite.

EXAMPLES (British Columbia - Canada/International): Gold Dollar  Red Mountain, Saddle Occurrences ; Henderson Talc Deposit (Ontario, Canada), Treasure mine (Montana, USA), Gouverneur Talc (New York State, USA) and Trimouns deposit (France).


CAPSULE DESCRIPTION: Most of the economic carbonate-hosted deposits are lenticular or sheet-like bodies and are concordant with surrounding dolomitic marbles, siliceous dolomitic marbles, dolomites, schists and phyllites. The massive or schistose ore consists mainly of talc ± dolomite, ± tremolite, ± calcite, ± magnesite, ± chlorite, ± serpentine, ± phlogopite.

TECTONIC SETTING: Protolith deposited mainly in pericratonic environments; in most cases the talc formed later within metamorphic, fold or thrust belts.

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING:  Dolostones, dolomitic marbles or magnesite beds metamorphosed to greenschist facies or lower amphibolite facies represent a typical host environment. Upper amphibolite-grade marbles, where talc would not normally be stable, may contain retrograde talc zones.

AGE OF MINERALIZATION:  Mainly Precambrian to Early Paleozoic but may be younger. In most cases syn- or post-metamorphic.

HOST/ASSOCIATED ROCK TYPES: Dolomitic marbles and dolomites are the typical host, however some of the deposits are hosted by magnesite or mica schists. Phyllites, chlorite or mica schists, paragneiss and intrusive and metavolcanic rocks may be present adjacent to, or in the proximity of the talc deposits. Deposits may be crosscut by minor intrusions, such as diabase dikes.

DEPOSIT FORM: In most cases, podiform or deformed, sheet-like bodies oriented subparallel to the compositional layering within marbles and to geologic contacts. They commonly pinch and swell. Typical dimensions would be 2 to 20 m thick and tens to hundreds of m along strike and dip. Where fluids were the principal source of heat and/or silica, breccia zones and irregular deposits may occur near fault intersections.

TEXTURE/STRUCTURE:  Ore varies from fine-grained, massive or layered talc to coarse talc schists. Pseudomorphs of talc after tremolite are common in deposits that formed after the peak of metamorphism.

ORE [Principal and subordinate]:  Talc and tremolite (in some ores and commercial products tremolite is a principal constituent).

GANGUE MINERALOGY [Principal and subordinate]:  Dolomite, ± tremolite, ± calcite, ± magnesite, ± chlorite, ± serpentine, and ± phlogopite may be principal gangue minerals. Pyrite, ± graphite, ± mica, ± dravite, and ± anorthite are common accessory impurities.

ALTERATION MINERALOGY: In some deposits at least a portion of talc is believed to have formed by retrograde reactions from tremolite. In some cases, there is a replacement of biotite by chlorite and feldspar by sericite or chlorite in the host rock.

WEATHERING: Talc-bearing zones may form ridges where chemical processes dominate and topographic lows where physical weathering and/or glaciation are most important.

ORE CONTROLS: The main controls are the presence of dolomite or magnesite protolith, availability of silica and favourable metamorphic/metasomatic conditions. Talc deposits hosted by carbonate rocks may be divided into several subtypes according to the source of silica and geological setting:

a) contacts between carbonates, usually dolomitic marbles, and silica-bearing rocks, such as biotite-quartz-feldspar gneisses, schists, cherts and quartzites;
b) horizons or lenses of siliceous dolomite or magnesite protolith;
c) crests of folds, breccia zones, faults, and intersections of fault systems that permit circulation of metasomatic fluids carrying silica within dolomite or magnesite host; and
d) carbonates within the contact metamorphic aureole of intrusions, where silica has been derived from adjacent host rock.

GENETIC MODEL:  Most carbonate-hosted talc deposits are believed to be formed by the reaction:

3 dolomite + 4 SiO2 + H2O = 1 talc + 3 calcite + 3 CO2

Silica may be provided either from adjacent quartz-bearing rocks, from silica layers within the carbonates, or by hydrothermal fluids. Absence of calcite in ores from several deposits indicates that talc may have formed in an open system environment and calcium was allowed to escape. The source of heat may be provided by regional metamorphism, contact metamorphism or by heat exchange from hydrothermal fluid. In environments where sedimentary-hosted magnesite deposits are known to occur, talc could have been produced by the reaction:

3 magnesite + 4 SiO2 + H2O = 1 talc + 3 CO2

In this second reaction calcite precipitation is not expected. This reaction takes place at lower temperature (given identical pressure and XCO2 conditions) than the dolomite reaction, therefore, magnesite may be almost completely converted to talc before dolomite starts to react.

Pseudomorphs of talc after tremolite and the presence of upper amphibolite grade, metamorphic assemblages in host rocks of some of the deposits indicate that talc post-dates the metamorphic peak and is probably of retrograde origin. Depending on the individual deposits, metamorphic or metasomatic (hydrothermal) characteristics may be predominant.

ASSOCIATED DEPOSIT TYPES: Chlorite deposits, marble (R04), high-calcium carbonate (filler-grade) and limestone (R09), dolostone (R10), sedimentary-hosted magnesite deposits and deposits such as Balmat, which is probably a metamorphosed sedex deposit .


GEOCHEMICAL SIGNATURE:  Systematic study of soils to identify anomalous concentrations of talc using the X-ray diffraction method has proven successful.

GEOPHYSICAL SIGNATURE:  Electromagnetic methods can be used to identify carbonate contacts with other lithologies or talc-related fault zones impregnated with water.

OTHER EXPLORATION GUIDES:  Talc in residual soils. Talc occurs within belts of dolomitic rocks in metamorphosed terranes or adjacent to intrusive rocks. Contacts with silica-bearing metasediments or intrusions are favourable loci for deposits.


TYPICAL GRADE AND TONNAGE:  Grade is highly variable. For example, New York state talc ores commonly contain over 50% tremolite.

ECONOMIC LIMITATIONS:  Major talc producing countries are China, USA, Finland, France, Brazil and Australia. Underground mining is economically feasible in case of high quality ores, but most mining is by open pit. Actinolite, tremolite and anthophyllite impurities are undesirable because of environmental restrictions on these minerals. The most common properties measured to determine possible applications for talc concentrates are: mineral composition, dry brightness (green filter), whiteness, specific gravity, oil absorption, pH, particle size distribution, tapped density, loose density, Hegman fineness and chemical composition including L.O.I.

END USES: In 1996, almost 1 million tonnes of talc valued at $US 100 million was sold or used in the USA. Talc is used in ceramics (28%), paint (18%), paper (17%), plastics (6%), roofing (11%) and cosmetics (4%). Insecticides, rubber refractories and other applications account for 16% (in USA). Cut or sawed blocks of fine-grained talc (steatite which is also used for carving) may sell for up to $US 2000.00 tonne. Paint and ceramic-grade talc is sold for $US 110.00 to 200.00/tonne, depending on the degree and method of processing. Some filler grades are sold at $US 600.00/tonne and cosmetic-grade talc and surface treated materials may sell for more than $US 2000.00/tonne.

IMPORTANCE:  Talc may be substituted by clay or pyrophyllite in ceramics; by high calcium carbonate and kaolin in some paper applications and by other fillers and reinforcing agents in plastics. Talc from carbonate-hosted deposits also has to compete with products derived from ultramafic-hosted talc deposits (M07) in a number of applications. In North America carbonate-hosted deposits supply mainly the ceramic, paint and, to some extent the plastic markets.


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Benvenuto, G. (1993):  Geology of Several Talc Occurrences in Middle Cambrian Dolomites, Southern Rocky Mountains, British Columbia; B.C. Ministry of Energy, Mines and Petroleum Resources, Geological Survey Branch, Geological Fieldwork, Paper 1993-1, pages 361-379.

Berg, R.B. (1991):  Geology of Talc and Chlorite Deposits in Montana. Proceedings of the 27th Forum on Geology of Industrial Minerals, Banff, Alberta; B.C. Ministry of Energy Mines and Petroleum Resources, Open File 1991-23, pages 81-92.

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DEPÓSITOS - 26/04/2004 18:25:00

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