Birkett, T.C. and Simandl, G.J. (1999):
Carbonatite-associated Deposits: Magmatic, Replacement and Residual; 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.
and ultramafic carbonatite-hosted deposits.
Niobium, tantalum, REE, phosphate, vermiculite , Cu, Ti, Sr, fluorite, Th, U magnetite (hematite, Zr,
V, nickel sulphate, sulphuric acid, calcite for cement industry).
EXAMPLES (British Columbia (MINFILE
#) - Canada/International):
Magmatic: Aley St. Honoré (niobium, Quebec, Canada), Mountain
Pass (REE, California, USA), Palabora (apatite, South Africa).
Replacement/Veins: Rock Canyon Creek , Bayan Obo (REE, China), Amba Dongar (fluorite, India),
Fen (Fe, Norway), Palabora (Cu, vermiculite, apatite, South Africa).
Residual: Araxa, Catalao and Tapira (niobium,
phosphate, REE, Ti, Brazil), Cargill and Martison Lake (phosphates,
Carbonatites are igneous rocks with more than 50% modal carbonate minerals;
calcite, dolomite and Fe-carbonate varieties are recognized. Intrusive
carbonatites occur commonly within alkalic complexes or as isolated sills,
dikes, or small plugs that may not be associated with other alkaline rocks.
Carbonatites may also occur as lava flows and pyroclastic rocks. Only
intrusive carbonatites (in some cases further enriched by weathering) are
associated with mineralization in economic concentrations which occur as
primary igneous minerals, replacement deposits (intra-intrusive veins or
zones of small veins, extra-intrusive fenites or veins) or residual
weathering accumulations from either igneous or replacement protores.
Pyrochlore, apatite and rare earth-bearing minerals are typically the most
sought after mineral constituents, however, a wide variety of other
minerals including magnetite, fluorite, calcite, bornite, chalcopyrite and
vermiculite, occur in economic concentrations in at least one carbonatite
Carbonatites occur mainly in a continental environment; rarely in oceanic
environments (Canary Islands) and are generally related to large-scale,
intra-plate fractures, grabens or rifts that correlate with periods of
extension and may be associated with a broad zones of epeirogenic uplift.
DEPOSITIONAL ENVIRONMENT /
GEOLOGICAL SETTING: Carbonatites intrude all types of rocks and
are emplaced at a variety of depths.
AGE OF MINERALIZATION:
Carbonatite intrusions are early Precambrian to Recent in age; they appear
to be increasingly abundant with decreasing age. In British Columbia,
carbonatites are mostly upper Devonian, Mississippian or Eocambrian in
HOST/ASSOCIATED ROCK TYPES:
Host rocks are varied, including calcite carbonatite (sovite), dolomite
carbonatite (beforsite), ferroan or ankeritic calcite-rich carbonatite
(ferrocarbonatite), magnetite-olivine-apatite ± phlogopite rock,
nephelinite, syenite, pyroxenite, peridotite and phonolite. Carbonatite
lava flows and pyroclastic rocks are not known to contain economic
mineralization. Country rocks are of various types and metamorphic grades.
Carbonatites are small, pipe-like bodies, dikes, sills, small plugs or
irregular masses. The typical pipe-like bodies have subcircular or
elliptical cross sections and are up to 3-4 km in diameter. Magmatic
mineralization within pipe-like carbonatites is commonly found in
crescent-shaped and steeply-dipping zones. Metasomatic mineralization
occurs as irregular forms or veins. Residual and other weathering-related
deposits are controlled by topography, depth of weathering and drainage
minerals form pockets and fill fractures within ferrocarbonatite bodies.
Pyrochlore is disseminated; apatite can be disseminated to semi-massive;
bastnaesite occurs as disseminated to patchy accumulations; fluorite forms
as veins and masses; hematite is semi-massive disseminations; and
chalcopyrite and bornite are found in veinlets.
ORE MINERALOGY [Principal and
bastnaesite, pyrochlore, apatite, anatase, zircon, baddeleyite,
magnetite, monazite, parisite, fersmite.
fluorite, vermiculite, bornite, chalcopyrite and other sulphides,
anatase, pyrochlore and apatite, locally crandallite-group minerals
GANGUE MINERALOGY [Principal and
subordinate]: Calcite, dolomite, siderite, ferroan calcite,
ankerite, hematite, biotite, titanite, olivine, quartz.
ALTERATION MINERALOGY: A
fenitization halo (alkali metasomatized country rocks) commonly surrounds
carbonatite intrusions; alteration mineralogy depends largely on the
composition of the host rock. Typical minerals are sodic amphibole,
wollastonite, nepheline, mesoperthite, antiperthite, aegerine-augite, pale
brown biotite, phlogopite and albite. Most fenites are zones of
desilicification with addition of Fe3+, Na and K.
weather relatively easily and are commonly associated with topographic
lows. Weathering is an important factor for concentrating residual
pyrochlore or phosphate mineralization.
ORE CONTROLS: Intrusive
form and cooling history control primary igneous deposits (fractional
crystallization). Tectonic and local structural controls influence the
forms of metasomatic mineralization. The depth of weathering and drainage
patterns control residual pyrochlore and apatite deposits, and vermiculite
Worldwide, mineralization within carbonatites is syn- to post-intrusion
and commonly occurs in several types or stages:
1) REE-rich carbonatite and
ferrocarbonatite, magmatic magnetite, pyrochlore
2) Fluorite along fractures
3) Barite veins
4) U-Th minerals + silicification
5) calcite veining and reprecipitation of Fe oxides (hematite)
6) Intense weathering may take place at any later time.
Magmatic mineralization may be linked
either to fractional crystallization or immiscibility of magmatic fluids.
Metasomatism and replacement are important Not all mineralization types
are associated with any individual carbonatite intrusion. In general, it
is believed that economic Nb, REE and primary magnetite deposits are
associated with transgressive (late) igneous phases, but understanding of
the majority of deposits is not advanced enough to propose any general
relationship of timing. mineralization at St. Honoré, for example, is
probably relatively early-formed.
ASSOCIATED DEPOSIT TYPES:
Nepheline syenite (R13) and nepheline syenite-related corundum deposits
and sodalite. REE and zircon placer deposits deposits can be derived from carbonatites. Wollastonite
occurrences are in some cases reported in association with carbonatites.
Fluorite deposits are known from the roof zones of carbonatite complexes.
Kimberlites and lamproites (common host-rocks for diamonds) may be along
the same tectonic features as carbonatites, but are not related to the
same magmatic event.
should be evaluated for a variety of the mineral substances as exemplified
by the exceptional Palabora carbonatite which provides phosphate (primary
and possibly hydrothermal), Cu (hydrothermal), vermiculite (weathering)
and also Zr, U and Th as byproducts. While extrusive carbonatite rocks are
known to contain anomalous REE values, for example the Mount Grace
pyroclastic carbonatite in British Columbia, they are not known to host
REE in economic concentrations.
Resistant niobium or phosphate minerals in soils and stream sediments; F,
Th and U in waters.
Magnetic and radiometric expressions and sometimes anomalous radon gas
concentrations furnish primary targets.
OTHER EXPLORATION GUIDES:
Carbonatites are commonly found over broad provinces, but individual
intrusions may be isolated. Fenitization increases the size of target in
regional exploration for carbonatite-hosted deposits. U-Th (radioactivity)
associated with fluorite and barite within carbonatites are considered as
indirect REE indicators. Annular topographic features can coincide with
TYPICAL GRADE AND TONNAGE:
Araxa deposit contains 300 million tonnes grading 3% Nb2O5;
Cargill deposit consists of 60 million tonnes at 20% P2O5;
Niobec deposit hosts 19 million tonnes grading 0.66 % Nb2O5;
Aley has extensive zones exceeding 0.66% Nb2O5
and locally exceeding 2%.
Competitive markets are established for most of the commodities associated
with carbonatites. In 1996, the world consumption was estimated at 22 700
tonnes of Nb2O5. Araxa mine, the largest single
source of Nb2O5 in 1996, produced 18 300 tonnes of
concentrate which was largely reduced into standard ferro-niobium.
Brazil’s second largest producer, Catalao, produced 3 600 tonnes of
ferroniobium The largest North American producer is Niobec Mine which
produced 3 322 tonnes of Nb2O5 which was also
reduced to ferro-niobium. At the end of 1996 standard grade ferro-niobium
sold at $US 15.2/kg, vacuum grade at $US 37.5/kg, nickel-niobium at US$
39.7 - 55.1/kg of contained niobium. Demand for REE in 1996 was estimated
at 65,000 tonnes/year contained rare earth oxides or US$ 650 million.
Currently China accounts for nearly half of the world production due
largely to heavy discounting, USA is the second largest producer.
Separated rare earths account for 30% of the market by volume but 75% by
value. Tantalum primary production for 1996 was estimated at 100.1 tonnes
of Ta2O5 contained in tantalum-bearing tin slags
(principally from smelters in Brazil, Thailand and Malaysia) and 426.0
tonnes of Ta2O5 in tantalite or other minerals.
END USES: Rare Earths
- mainly as a catalyst in oil refining, catalytic converters, glass
industry, coloring agents, fiber optics, TV tubes, permanent magnets, high
strength alloys and synthetic minerals for laser applications.
Phosphate: fertilizers, phosphorus, and phosphoric acid. Sr:
Color TV screens, pyrotechnics and magnets. Nb: carbon stabilizer
in stainless steel, niobium carbide used in cutting tools, Nb-containing
temperature-resistant steel used in turbines, Nb-base alloys in reactors,
super alloys for military and aerospace applications. Tantalum: in
corrosion-resistant alloys; implanted prosthesis; nuclear reactors and
electronic industry. Carbonates may be used in local portland
cement industries. Vermiculite is exfoliated and used in
agriculture, insulation, as lightweight aggregate, and other construction
are the main source of niobium and important sources of rare earth
elements, but have to compete for the market with placer deposits and
offshore placer deposits (Brazil, Australia, India, Sri-Lanka). They
compete with sedimentary phosphate deposits for a portion of the phosphate
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Korinek, G.J. (1997): Rare
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the Carbonatitic Dolomite Host of the Bayan Obo Fe-Nb-REE Deposit, Inner
Mongolia, Northen China, Mineralogical Magazine, Volume 21, pages
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Nature of Economic Mineralization in Carbonatites and Related Rocks. in
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London, pages 149-176.
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Carbonatites, Nepheline Syenites, Kimberlites and Related Rocks in British
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