CARBONATITE-ASSOCIATED DEPOSITS: MAGMATIC, REPLACEMENT AND RESIDUAL
by T.C. Birkett¹ and G.J. Simandl²

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.

IDENTIFICATION

SYNONYMS:  Nephelinitic and ultramafic carbonatite-hosted deposits.

COMMODITIES (BYPRODUCTS):  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, Ontario, Canada).

GEOLOGICAL CHARACTERISTICS

CAPSULE DESCRIPTION: 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 complex.

TECTONIC SETTING:  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 age.

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.

DEPOSIT FORM:  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 development.

TEXTURE/STRUCTURE:  REE 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 subordinate]:

Magmatic:  bastnaesite, pyrochlore, apatite, anatase, zircon, baddeleyite, magnetite, monazite, parisite, fersmite.

Replacement/Veins:  fluorite, vermiculite, bornite, chalcopyrite and other sulphides, hematite.

Residual:  anatase, pyrochlore and apatite, locally crandallite-group minerals containing REE.

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.

WEATHERING:  Carbonatites 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 deposits.

GENETIC MODELS:  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.

COMMENTS: Carbonatites 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.

EXPLORATION GUIDES

GEOCHEMICAL SIGNATURE:  Resistant niobium or phosphate minerals in soils and stream sediments; F, Th and U in waters.

GEOPHYSICAL SIGNATURE: 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 carbonatites.

ECONOMIC FACTORS

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%.

ECONOMIC LIMITATIONS: 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 materials.

IMPORTANCE: Carbonatites 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 market.

REFERENCES

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Hõy, T. (1987): Geology of the Cottonbelt Lead-Zinc-magnetite Layer, carbonatites and Alkalic Rocks in the Mount Grace Area, French Cap Dome, Southeastern British Columbia; British Columbia Ministry of Energy, Mines and Petroleum Resources, Bulletin 80, 99 pages.

Korinek, G.J. (1997): Tantalum; Metals and Minerals, Annual Review, Mining Journal Ltd., pages 73-75.

Korinek, G.J. (1997): Rare Earths, Metals and Minerals, Annual Review, Mining Journal Ltd., pages 70-71.

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Le Bas, M.J., Sprio, B. and Xueming, Y. (1997): Origin, Carbon and Strontium Isotope Study of the Carbonatitic Dolomite Host of the Bayan Obo Fe-Nb-REE Deposit, Inner Mongolia, Northen China, Mineralogical Magazine, Volume 21, pages 531-541.

Mariano, A.N. (1989a): Nature of Economic Mineralization in Carbonatites and Related Rocks. in Carbonatites: Genesis and Evolution, K. Bell, Editor, Unwin Hyman, London, pages 149-176.

Mariano, A.N. (1989b): Economic Geology of Rare Earth Minerals. in Geochemistry and Mineralogy of Rare Earth Elements, B.R. Lipman and G.A. KcKay, Editors, Reviews in Mineralogy, Mineralogical Society of America, Volume 21, pages 303-337.

Pell, J. (1994): Carbonatites, Nepheline Syenites, Kimberlites and Related Rocks in British Columbia; British Columbia Ministry of Energy, Mines and Petroleum Resources, Bulletin 88, 133 pages.

Pell, J. (1996):  Mineral Deposits Associated with Carbonatites and Related Alkaline Igneous Rocks; in Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis and Economic Potential, Editor, R.H. Mitchell, Mineralogical Association of Canada, Short Course Volume 24, pages 271-310.

Richardson, D.G. and Birkett, T.C. (1996a):   Carbonatite-associated Deposits; in Geology of Canadian Mineral Deposit Types, O.R. Eckstrand, W.D. Sinclair and R.I. Thorpe, Editors, Geological Survey of Canada, Geology of Canada Number 8, pages 541-558.

Richardson, D.G. and Birkett, T.C. (1996b):  Residual Carbonatite-associated Deposits; in Geology of Canadian Mineral Deposit Types, O.R. Eckstrand, W.D. Sinclair and R.I. Thorpe, Editors, Geological Survey of Canada, Geology of Canada, Number 8, pages 108-119.