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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.
IDENTIFICATION
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).
GEOLOGICAL
CHARACTERISTICS
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 .
EXPLORATION GUIDES
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.
ECONOMIC FACTORS
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.
REFERENCES
Andrews, P.R.A. (1994):
The Beneficiation of Canadian Talc and Pyrophyllite Ores: a Review of
Processing Studies at CANMET; Canadian Institute of Mining and Metallurgy
Bulletin, Volume 87, No.984, pages 64-68.
Anonymous (1993): The
Economics of Talc and Pyrophyllite; 7th Edition; Roskill
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Geology of Industrial Rocks and Minerals; Dover Publications Inc, New
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Geology of Several Talc Occurrences in Middle Cambrian Dolomites, Southern
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Geology of Talc and Chlorite Deposits in Montana. Proceedings of the 27th
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(1980): The Mineralogy and Origin of the Talc Deposits near
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York Talc; in Characteristics of Mineral Deposit Occurrences, R.L.
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Markets - A World of Regional Diversity; Industrial Minerals, May
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Geology and Geochemistry of Talc Deposits in Madoc Area, Ontario;
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