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Talc is a soft hydrated
magnesium silicate that occurs as fine platelets that are inert
and hydrophobic. Industrial talc is widely used in many applications,
most notably in the production of ceramics and refractories.
Consumers that require high quality talc include the paint, paper
and plastics industries, which collectively represent almost
60 % of talc consumption in North America.
Talc deposits are of hydrothermal origin. Tectonic activity plays
a major role in the formation of talc deposits by allowing fluids
to penetrate rocks, creating a micro-permeability that facilitates
reactions with the host rock.
Deposits can be subdivided into four types according to
the host rock:
Metamorphic rocks containing magnesian minerals;
Sedimentary rocks containing magnesian carbonates;
Ultramafic rocks;
Mafic rocks.
Talc deposits associated with metamorphic
rocks
Regional or contact metamorphism of siliceous or sandy dolomites
and talc-carbonate-bearing siliceous rocks produce dolomitic
marbles containing tremolite, actinolite or diopside. These rocks
can be transformed into steatite through interactions with silica-rich
fluids. Contact metamorphism of dolomites and dolomitic limestone
by granite or diabase intrusions may produce large bodies of
high grade talc. These masses may be several hundred metres long
by several metres wide. Hydrothermal fluid circulation may be
locally intense enough to alter granite and other siliceous rocks
into talc along the borders of the deposit. The most favorable
sites for talc formation are the contact zones with igneous and
sedimentary rocks, as well as zones marked by faulting and shearing.
Deposits of this type can be found in the United States, Canada
(Quebec, Ontario), Brazil, Italy, Slovakia, India, France, Australia
and China.
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).
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.
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 (E09) and deposits
such as Balmat, which is probably a metamorphosed sedex deposit
(E14).
Talc deposits associated with magnesian
carbonate rocks
Metasomatism or hydrothermal alteration of dolomitic rocks
produces talc through interaction with silica- and magnesium-bearing
fluids. The silica source may be sediments or hydrothermal solutions
derived from various aluminous silicate rocks (schists, micaschists,
pegmatites, granites) along strongly tectonized zones (faults,
shears, fractures). This process creates high quality talc that
is mainly found in veins that crosscut the dolomitic rocks. These
lenticular talc veins may reach several hundred metres in length
by several dozen metres in width. Dolomite, magnesite, chlorite
and quartz are the principal impurities. Talc deposits and talc-chlorite
schists are associated with dolomite, dolomitic limestone or
magnesian rocks. This deposit type has been found in the United
States, Germany, France, Austria, Russia, Finland, Australia,
India and China.
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).
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.
Talc deposits associated with ultramafic
rocks
Talc can be formed during serpentinization from ultramafic
rocks like peridotite. The process is followed by carbonatization
during which fluids containing more than 5 % CO2 are introduced
and a talc-carbonate rock is formed (i.e., talc-magnesite). This
rock may be further transformed into steatite through the interaction
with silica-bearing solutions. The talc rock may replace lenses
or large masses of serpentinite, but in most cases forms a layer
or crust around such masses. The talc lenses may measure several
hundred metres long by several hundred metres wide. The serpentinized
ultramafic rocks that host asbestos deposits may also contain
talc. These occurrences constitute the largest talc deposits
but can contain more impurities, most notably asbestos minerals.
Deposits associated with ultramafic rocks are found in the United
States, Canada (Quebec, Ontario), Russia and Norway.
Examples (British Columbia - Canada/International): Rawhide
, South Talc Lake Deposit , Gisby , J&J; Deloro magnesite-talc
deposit (Ontario, Canada), Luzcan mine of Thetford township and
Van Reet mine, Ponton township, (Quebec, Canada), Windham (Vermont,
USA), Lahnaslampi mine (Finland).
Capsule Description: Ultramafic-hosted talc-carbonate deposits
are located either along regional faults cutting ultramafic rocks
or at contacts between ultramafic rocks and siliceous country
rock. The ultramafic host rock is typically, but not necessarily
of ophiolitic affiliation. Deposits related to regional fault
systems cutting ultramafic host rock are commonly magnesite-rich.
Deposits located within sheets of serpentinized peridotite, found
along the periphery of ultramafic intrusions or near the borders
of tectonically transported peridotite slices are typically talc-rich.
Tectonic Settings: These deposits are found typically in obducted,
accreted or otherwise tectonically transported seafloor and ophiolite
slices or lenses and in ancient greenstone belts. However, serpentinized
ultramafic intrusions regardless of tectonic environment should
be considered as a favourable host.
Depositional Environment / Geological Setting: Faulted and
metasomatized ultramafic rocks and tectonically-transported serpentinites
in contact with siliceous rocks; the deposits are younger than
the ultramafic protolith.
Age Of Mineralization: Precambrian or younger. Post or syn-tectonic.
Host/Associated Rock Types: Talc-carbonate-bearing serpentinite,
steatite, talc schist, talc-magnesite-dolomite schist that may
contain serpentine/chlorite schist, dunite and serpentinite with
associated, commonly at least partially serpentinized gabbro,
pyroxenite, harzburgite and websterite or meta-komatiiate sills
and lavas. Because many of the talc-bearing rocks are allochthonous
there is a wide variety of associated lithologies.
Deposit Form: The fault-related deposits are irregular bodies
having their largest dimensions parallel to the faults. In some
cases only the hanging wall of the faults is mineralized. Small
ultramafic lenses are commonly entirely serpentinized, while
larger lenses consist of peridotite cores surrounded by serpentinite.
Steatite and talc schists are most likely to be found at the
contact of the serpentinite with siliceous rocks, however they
may also form tabular or irregular bodies.
Texture/Structure: Ore is massive or schistose, talc is fine
to coarse flakes.
Ore Mineralogy [Principal and subordinate]: Talc, magnesite,
rarely Ni-bearing minerals, such as pyrrhotite, pentlandite,
melnikovite and bravoite.
Gangue Mineralogy [Principal and subordinate]: Dolomite, serpentine,
chlorite, ankerite (Fe-rich dolomite), breunerite (Fe-rich magnesite),
olivine, magnetite, quartz, pyrite, asbestos, rutile, calcite,
chrome-mica.
Ore Controls: Primary control is the presence of a magnesium-rich
silicate rock to act as a source of magnesium. Permeable fault
zones or serpentinite-siliceous rock contacts control the sites
of talc formation.
Genetic Models: These deposits are commonly magnesite-rich
and are linked to CO2 and H2O metasomatism (carbonatization and
hydration) of ultramafic rocks by fluids following faults and
contacts. The following reactions 1, 2a and 2b and 3 illustrate
the concept:
1) 18 serpentine + magnetite + 30 CO2 à 9 Talc + 30
breunerite + 27 H2O + 1/2 O2
2a) 2 olivine + 1 CO2 + H2O à 1serpentine + 1 magnesite
2b) 2 serpentine + 3 CO2 à 3 magnesite + 1 talc + 3 H2O
3) 1 serpentine + 2 quartz à 1 talc + H2O
The talc formed during metasomatism and/or regional metamorphism.
Silica required for talc formation was derived from the country
rock.
Talc deposits associated with mafic rocks
Talc deposits associated with mafic rocks form in the same way
as those associated with ultramafic rocks. Talc can develop by
serpentinization of mafic rocks like gabbro. These deposits generally
produce low quality soapstone and are rarely economical to exploit.
This type of deposit can be found in Quebec.
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