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Ministry of Energy Mines and Responsible for Core Review

N - Carbonatites

(Example Deposits)

BC Profile # Deposit Type Approximate Synonyms USGS Model #
N01 Carbonatite-hosted deposits - - 10
N02 Kimberlite-hosted diamonds Diamond pipes 12
N03 Lamproite-hosted diamonds - - 12
 

CARBONATITE-ASSOCIATED DEPOSITS: MAGMATIC, REPLACEMENT AND RESIDUAL


N01
by T.C. Birkett1 and G.J. Simandl2

1SOQUEM, Sainte-Foy, Quebec, Canada
2British Columbia Geological Survey, Victoria, British Columbia, Canada

 

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 (see also M08, this volume), 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 (REE, niobium, 094B 027), St. Honoré (niobium, Quebec, Canada), Mountain Pass (REE, California, USA), Palabora (apatite, South Africa).


Replacement/Veins:  Rock Canyon Creek (Fluorite, REE, 082JSW018), 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 (C01, C02) 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 (I11). 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; MINFILE

 

(Aley) 094B 027 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

 

Harben, P.W. and Bates, R.L. (1990):  Phosphate Rock; in Industrial Minerals Geology and World Deposits, Industrial Minerals Division, Metal Bulletin Plc., London, pages 190-204.

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.

Korinek, G.J. (1997): Niobium, Metals and Minerals, Annual Review, Mining Journal Ltd., page 63.

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.

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KIMBERLITE-HOSTED DIAMONDS


N02
by Jennifer Pell
Consulting Geologist

 

Pell, J. (1998): Kimberlite-hosted Diamonds, in Geological Fieldwork 1997, British Columbia Ministry of Employment and Investment, Paper 1998-1, pages 24L-1 to 24L-4.

 

IDENTIFICATION

 

SYNONYMS: Diamond-bearing kimberlite pipes, diamond pipes, group 1 kimberlites.

 

COMMODITIES (BYPRODUCTS): Diamonds (some gemstones produced in Russia from pyrope garnets and olivine).

 

EXAMPLES (British Columbia - Canada/International): No B.C. deposits, see comments below for prospects; Koala, Panda, Sable, Fox and Misery (Northwest Territories, Canada), Mir, International, Udachnaya, Aikhal and Yubilenaya (Sakha, Russia), Kimberly, Premier and Venetia (South Africa), Orapa and Jwaneng (Botswana), River Ranch (Zimbabwe).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Diamonds in kimberlites occur as sparse xenocrysts and within diamondiferous xenoliths hosted by intrusives emplaced as subvertical pipes or resedimented volcaniclastic and pyroclastic rocks deposited in craters. Kimberlites are volatile-rich, potassic ultrabasic rocks with macrocrysts (and sometimes megacrysts and xenoliths) set in a fine grained matrix. Economic concentrations of diamonds occur in approximately 1% of the kimberlites throughout the world.

 

TECTONIC SETTING: Predominantly regions underlain by stable Archean cratons.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: The kimberlites rise quickly from the mantle and are emplaced as multi-stage, high-level diatremes, tuff-cones and rings, hypabyssal dikes and sills.

 

AGE OF MINERALIZATION: Any age except Archean for host intrusions. Economic deposits occur in kimberlites from Proterozoic to Tertiary in age. The diamonds vary from early Archean to as young as 990 Ma.

 

HOST/ASSOCIATED ROCK TYPES: The kimberlite host rocks are small hypabyssal intrusions which grade upwards into diatreme breccias near surface and pyroclastic rocks in the crater facies at surface. Kimberlites are volatile-rich, potassic ultrabasic rocks that commonly exhibit a distinctive inequigranular texture resulting from the presence of macrocrysts (and sometimes megacrysts and xenoliths) set in a fine grained matrix. The megacryst and macrocryst assemblage in kimberlites includes anhedral crystals of olivine, magnesian ilmenite, pyrope garnet, phlogopite, Ti-poor chromite, diopside and enstatite. Some of these phases may be xenocrystic in origin. Matrix minerals include microphenocrysts of olivine and one or more of: monticellite, perovskite, spinel, phlogopite, apatite, and primary carbonate and serpentine. Kimberlites crosscut all types of rocks.

 

DEPOSIT FORM: Kimberlites commonly occur in steep-sided, downward tapering, cone-shaped diatremes which may have complex root zones with multiple dikes and "blows". Diatreme contacts are sharp. Surface exposures of diamond-bearing pipes range from less than 2 up to 146 hectares (Mwadui). In some diatremes the associated crater and tuff ring may be preserved. Kimberlite craters and tuff cones may also form without associated diatremes (e.g. Saskatchewan); the bedded units can be shallowly-dipping. Hypabyssal kimberlites commonly form dikes and sills.

 

TEXTURE/STRUCTURE: Diamonds occur as discrete grains of xenocrystic origin and tend to be randomly distributed within kimberlite diatremes. In complex root zones and multiphase intrusions, each phase is characterized by unique diamond content (e.g. Wesselton, South Africa). Some crater-facies kimberlites are enriched in diamonds relative to their associated diatreme (e.g. Mwadui, Tanzania) due to winnowing of fines. Kimberlite dikes may display a dominant linear trend which is parallel to joints, dikes or other structures.

 

ORE MINERALOGY: Diamond.

 

GANGUE MINERALOGY (Principal and subordinate): Olivine, phlogopite, pyrope and eclogitic garnet, chrome diopside, magnesian ilmenite, enstatite, chromite, carbonate, serpentine; monticellite, perovskite, spinel, apatite. Magma contaminated by crustal xenoliths can crystallize minerals that are atypical of kimberlites.

 

ALTERATION MINERALOGY: Serpentinization in many deposits; silicification or bleaching along contacts. Secondary calcite, quartz and zeolites can occur on fractures. Diamonds can undergo graphitization or resorption.

 

WEATHERING: In tropical climates, kimberlite weathers quite readily and deeply to "yellowground" which is predominantly comprised of clays. In temperate climates, weathering is less pronounced, but clays are still the predominant weathering product. Diatreme and crater facies tend to form topographic depressions while hypabyssal dikes may be more resistant.

 

ORE CONTROLS: Kimberlites typically occur in fields comprising up to 100 individual intrusions which often group in clusters. Each field can exhibit considerable diversity with respect to the petrology, mineralogy, mantle xenolith and diamond content of individual kimberlites. Economically diamondiferous and barren kimberlites can occur in close proximity. Controls on the differences in diamond content between kimberlites are not completely understood. They may be due to: depths of origin of the kimberlite magmas (above or below the diamond stability field); differences in the diamond content of the mantle sampled by the kimberlitic magma; degree of resorption of diamonds during transport; flow differentiation, batch mixing or, some combination of these factors.

 

GENETIC MODEL: Kimberlites form from a small amount of partial melting in the asthenospheric mantle at depths generally in excess of 150 km. The magma ascends rapidly to the surface, entraining fragments of the mantle and crust, en route. Macroscopic diamonds do not crystallize from the kimberlitic magma. They are derived from harzburgitic peridotites and eclogites within regions of the sub-cratonic lithospheric mantle where the pressure, temperature and oxygen fugacity allow them to form. If a kimberlite magma passes through diamondiferous portions of the mantle, it may sample and bring diamonds to the surface provided they are not resorbed during ascent. The rapid degassing of carbon dioxide from the magma near surface produce fluidized intrusive breccias (diatremes) and explosive volcanic eruptions.

 

ASSOCIATED DEPOSIT TYPES: Diamonds can be concentrated by weathering to produce residual concentrations or within placer deposits (C01, C02, C03). Lamproite-hosted diamond deposits (N03) form in a similar manner, but the magmas may be of different origin.

 

COMMENTS: In British Columbia the Cross kimberlite diatreme and adjacent Ram diatremes (MINFILE # - 082JSE019) are found near Elkford, east of the Rocky Mountain Trench. Several diamond fragments and one diamond are reported from the Ram pipes.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Kimberlites commonly have high Ti, Cr, Ni, Mg, Ba and Nb values in overlying residual soils. However, caution must be exercised as other alkaline rocks can give similar geochemical signatures. Mineral chemistry is used extensively to help determine whether the kimberlite source is diamondiferous or barren (see other exploration guides). Diamond-bearing kimberlites can contain high-Cr, low-Ca pyrope garnets (G10 garnets), sodium-enriched eclogitic garnets, high chrome chromites with moderate to high Mg contents and magnesian ilmenites.

 

GEOPHYSICAL SIGNATURE: Geophysical techniques are used to locate kimberlites, but give no indication as to their diamond content. Ground and airborne magnetometer surveys are commonly used; kimberlites can show as either magnetic highs or lows. In equatorial regions the anomalies are characterized by a magnetic dipolar signature in contrast to the "bulls-eye" pattern in higher latitudes. Some kimberlites, however, have no magnetic contrast with surrounding rocks. Some pipes can be detected using electrical methods (EM, VLF, resistivity) in airborne or ground surveys. These techniques are particularly useful where the weathered, clay-rich, upper portions of pipes are developed and preserved since they are conductive and may contrast sufficiently with the host rocks to be detected. Ground based gravity surveys can be useful in detecting kimberlites that have no other geophysical signature and in delineating pipes. Deeply weathered kimberlites or those with a thick sequence of crater sediments generally give negative responses and where fresh kimberlite is found at surface, a positive gravity anomaly may be obtained.

 

OTHER EXPLORATION GUIDES: Indicator minerals are used extensively in the search for kimberlites and are one of the most important tools, other than bulk sampling, to assess the diamond content of a particular pipe. Pyrope and eclogitic garnet, chrome diopside, picroilmenite, chromite and, to a lesser extent, olivine in surficial materials (tills, stream sediments, loam, etc.) indicate a kimberlitic source. Diamonds are also usually indicative of a kimberlitic or lamproitic source; however, due to their extremely low concentration in the source, they are rarely encountered in surficial sediments. Weathered kimberlite produces a local variation in soil type that can be reflected in vegetation.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: When assessing diamond deposits, grade, tonnage and the average value ($/carat) of the diamonds must be considered.. Diamonds, unlike commodities such as gold, do not have a set value. They can be worth from a few $/carat to thousands of $/carat depending on their quality (evaluated on the size, colour and clarity of the stone). Also, the diamond business is very secretive and it is often difficult to acquire accurate data on producing mines. Some deposits have higher grades at surface due to residual concentration. Some estimates for African producers is as follows:

 

Pipe Tonnage (Mt) Grade (carats*/100 tonne)
Orapa 117.8 68
Jwaneng 44.3 140
Venetia 66 120
Premier 339 40

 

 

 

 

 

 

 

 

 

* one carat of diamonds weighs 0.2 grams

 

ECONOMIC LIMITATIONS: Most kimberlites are mined initially as open pit operations; therefore, stripping ratios are an important aspect of economic assessments. Serpentinized and altered kimberlites are more friable and easier to process.

 

END USES: Gemstones; industrial uses such as abrasives.

 

IMPORTANCE: In terms of number of producers and value of production, kimberlites are the most important primary source of diamonds. Synthetic diamonds have become increasingly important as alternate source for abrasives.

 

SELECTED BIBLIOGRAPHY

 

Atkinson, W.J. (1988): Diamond Exploration Philosophy, Practice, and Promises: a Review; in Proceedings of the Fourth International Kimberlite Conference, Kimberlites and related rocks, V.2, Their Mantle/crust Setting, Diamonds and Diamond Exploration, J. Ross, editor, Geological Society of Australia, Special Publication 14, pages 1075-1107.

Cox, D.P. (1986): Descriptive Model of Diamond Pipes; in Mineral Deposit Models, Cox, D.P. and Singer, D.A., Editors (1986), U.S. Geological Survey, Bulletin 1693, 379 pages.

Fipke, C.E., Gurney, J.J. and Moore, R.O. (1995): Diamond Exploration Techniques Emphasizing Indicator Mineral Geochemistry and Canadian Examples; Geological Survey of Canada, Bulletin 423, 86 pages.

Griffin, W.L. and Ryan, C.G. (1995): Trace Elements in Indicator Minerals: Area Selection and Target Evaluation in Diamond Exploration; Journal of Geochemical Exploration, Volume 53, pages 311-337.

Gurney, J.J. (1989): Diamonds; in J. Ross, A.L. Jacques, J. Ferguson, D.H. Green, S.Y. O'Reilly, R.V. Danchin, and A.J.A. Janse, Editors, Kimberlites and Related Rocks, Proceedings of the Fourth International Kimberlite Conference, Geological Society of Australia, Special Publication Number 14, Volume 2, pages 935-965.

Haggerty, S.E. (1986): Diamond Genesis in a Multiply-constrained Model; Nature, Volume 320, pages 34-37.

Helmstaedt, H.H. (1995): "Primary" Diamond Deposits What Controls Their Size, Grade and Location?; in Giant Ore Deposits, B.H. Whiting, C.J. Hodgson and R. Mason, Editors, Society of Economic Geologists, Special Publication Number 2, pages 13-80.

Janse, A.J.A. and Sheahan, P.A. (1995): Catalogue of World Wide Diamond and Kimberlite Occurrences: a Selective Annotative Approach; Journal of Geochemical Exploration, Volume 53, pages 73-111.

Jennings, C.M.H. (1995): The Exploration Context for Diamonds; Journal of Geochemical Exploration, Volume 53, pages 113-124.

Kirkley, M.B, Gurney, J.G. and Levinson, A.A. (1991): Age, Origin and Emplacement of Diamonds: Scientific Advances of the Last Decade; Gems and Gemnology, volume 72, Number 1, pages 2-25.

Kjarsgaard, B.A. (1996): Kimberlite-hosted Diamond; 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 561-568.

Levinson, A.A., Gurney, J.G. and Kirkley, M.B. (1992): Diamond Sources and Production: Past, Present, and Future; Gems and Gemmology, Volume 28, Number 4, pages 234-254.

Macnae, J. (1995): Applications of Geophysics for the Detection and Exploration of Kimberlites and Lamproites; Journal of Geochemical Exploration, Volume 53, pages 213-243.

Michalski, T.C. and Modreski, P.J. (1991):  Descriptive Model of Diamond-bearing Kimberlite Pipes; in Some Industrial Mineral Deposit Models: Descriptive Deposit Models, editors, Orris, G.J and Bliss, J.D., U.S. Geological Survey, Open-File Report 91-11a, pages 1-4.

Mitchell, R.H. (1991): Kimberlites and Lamproites: Primary Sources of Diamond; Geoscience Canada, Volume 18, Number 1, pages 1-16.

Nixon, P.H. (1995): The Morphology and Nature of Primary Diamondiferous Occurrences; Journal of Geochemical Exploration, Volume 53, pages 41-71.

Pell, J.A. (1997): Kimberlites in the Slave Craton, Northwest Territories, Canada; Geoscience Canada, Volume 24, Number 2, pages 77-96.

Scott Smith, B.H. (1996): Kimberlites; in Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis and Economic Potential, R.H. Mitchell, editor, Mineralogical Association of Canada, Short Course 24, pages 217-243.

Scott Smith, B.H. (1992): Contrasting Kimberlites and Lamproites; Exploration and Mining Geology, Volume 1, Number 4, pages 371-381.

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LAMPROITE-HOSTED DIAMONDS


N03
by Jennifer Pell
Consulting Geologist

 

Pell, J. (1998): Lamproite-hosted Diamonds, in Geological Fieldwork 1997, British Columbia Ministry of Employment and Investment, Paper 1998-1, pages 24M-1 to 24M-4.

 

IDENTIFICATION

 

SYNONYMS: None.

 

COMMODITY: Diamonds.

 

EXAMPLES (British Columbia (MINFILE #) - Canada/International): No B.C. examples; Argyle, Ellendale (Western Australia), Prairie Creek (Crater of Diamonds, Arkansas, USA), Bobi (Côte d'Ivoire), Kapamba (Zambia), Majhgawan (India).

 

GEOLOGICAL CHARACTERISTICS

 

CAPSULE DESCRIPTION: Diamonds occur as sparse xenocrysts and in mantle xenoliths within olivine lamproite pyroclastic rocks and dikes. Many deposits are found within funnel-shaped volcanic vents or craters. Lamproites are ultrapotassic mafic rocks characterized by the presence of olivine, leucite, richterite, diopside or sanidine.

 

TECTONIC SETTING: Most olivine lamproites are post-tectonic and occur close to the margins of Archean cratons, either within the craton or in adjacent accreted Proterozoic mobile belts.

 

DEPOSITIONAL ENVIRONMENT / GEOLOGICAL SETTING: Olivine lamproites are derived from metasomatized lithospheric mantle. They are generally emplaced in high-level, shallow "maar-type" craters crosscutting crustal rocks of all types.

 

AGE OF MINERALIZATION: Any age except Archean. Diamondiferous lamproites range from Proterozoic to Miocene in age.

 

HOST/ASSOCIATED ROCK TYPES: Olivine lamproite pyroclastic rocks and dikes commonly host mineralization while lava flows sampled to date are barren. Diamonds are rarely found in the magmatic equivalents. Lamproites are peralkaline and typically ultrapotassic (6 to 8% K2O). They are characterized by the presence of one or more of the following primary phenocryst and/or groundmass constituents: forsteritic olivine; Ti-rich, Al-poor phlogopite and tetraferriphlogopite; Fe-rich leucite; Ti, K-richterite; diopside; and Fe-rich sanidine. Minor and accessory phases include priderite, apatite, wadeite, perovskite, spinel, ilmenite, armalcolite, shcherbakovite and jeppeite. Glass and mantle derived xenocrysts of olivine, pyrope garnet and chromite may also be present.

 

DEPOSIT FORM: Most lamproites occur in craters which are irregular, asymmetric, and generally rather shallow (often the shape of a champagne glass), often less than 300 metres in depth. Crater diameters range from a few hundred metres to 1500 metres. Diamond concentrations vary between lamproite phases, and as such, ore zones will reflect the shape of the unit (can be pipes or funnel-shaped). The volcaniclastic rocks in many, but not all, lamproite craters are intruded by a magmatic phase that forms lava lakes or domes.

 

TEXTURE/STRUCTURE: Diamonds occur as discrete grains of xenocrystic origin that are sparsely and randomly distributed in the matrix of lamproites and some mantle xenoliths.

 

ORE MINERALOGY: Diamond.

 

GANGUE MINERALOGY (Principal and subordinate): Olivine, phlogopite, richterite, diopside, sanidine; priderite, wadeite, ilmenite, chromite, perovskite, spinel, apatite, pyrope garnet.

 

ALTERATION MINERALOGY: Alteration to talc carbonate sulphide or serpentine-septechlorite + magnetite has been described from Argyle (Jacques et al., 1986). According Scott Smith (1996), alteration to analcime, barite, quartz, zeolite, carbonate and other minerals may also occur. Diamonds can undergo graphitization or resorption.

 

WEATHERING: Clays, predominantly smectite, are the predominant weathering product of lamproites.

 

ORE CONTROLS: Lamproites are small-volume magmas which are confined to continental regions. There are relatively few lamproites known world wide, less than 20 geological provinces, of which only seven are diamondiferous. Only olivine lamproites are diamondiferous, other varieties, such as leucite lamproites presumably did not originate deep enough in the mantle to contain diamonds. Even within the olivine lamproites, few contain diamonds in economic concentrations. Controls on the differences in diamond content between intrusions are not completely understood. They may be due to: different depths of origin of the magmas (above or below the diamond stability field); differences in the diamond content of the mantle sampled by the lamproite magma; differences in degrees of resorption of diamonds during transport; or some combination of these factors.

 

GENETIC MODEL: Lamproites form from a small amount of partial melting in metasomatized lithospheric mantle at depths generally in excess of 150 km (i.e., within or beneath the diamond stability field). The magma ascends rapidly to the surface, entraining fragments of the mantle and crust en route. Diamonds do not crystallize from the lamproite magma. They are derived from harzburgitic peridotites and eclogites within regions of the sub-cratonic lithospheric mantle where the pressure, temperature and oxygen fugacity allow them to form in situ. If a lamproite magma passes through diamondiferous portions of the mantle, it may sample them and bring diamonds to the surface provided they are not resorbed during ascent.

 

ASSOCIATED DEPOSIT TYPES: Diamonds can be concentrated by weathering to produce residual concentrations or by erosion and transport to create placer deposits (C01, C02, C03). Kimberlite-hosted diamond deposits (N02) form in a similar manner, but the magmas may be of different origin.

 

EXPLORATION GUIDES

 

GEOCHEMICAL SIGNATURE: Lamproites can have associated Ni, Co, Ba and Nb anomalies in overlying residual soils. However, these may be restricted in extent since lamproites weather readily and commonly occur in depressions and dispersion is limited. Caution must be exercised as other alkaline rocks can give similar geochemical signatures.

 

GEOPHYSICAL SIGNATURE: Geophysical techniques are used to locate lamproites, but give no indication as to their diamond content. Ground and airborne magnetometer surveys are commonly used; weathered or crater-facies lamproites commonly form negative magnetic anomalies or dipole anomalies. Some lamproites, however, have no magnetic contrast with surrounding rocks. Various electrical methods (EM, VLF, resistivity) in airborne or ground surveys are excellent tools for detecting lamproites, given the correct weathering environment and contrasts with country rocks. In general, clays, particularly smectite, produced during the weathering of lamproites are conductive; and hence, produce strong negative resistivity anomalies.

 

OTHER EXPLORATION GUIDES: Heavy indicator minerals are used in the search for diamondiferous lamproites, although they are usually not as abundant as with kimberlites. Commonly, chromite is the most useful heavy indicator because it is the most common species and has distinctive chemistry. To a lesser extent, diamond, pyrope and eclogitic garnet, chrome spinel, Ti-rich phlogopite, K-Ti-richterite, low-Al diopside, forsterite and perovskite can be used as lamproite indicator minerals. Priderite, wadeite and shcherbakovite are also highly diagnostic of lamproites, although very rare.

 

ECONOMIC FACTORS

 

TYPICAL GRADE AND TONNAGE: When assessing diamond deposits, grade, tonnage and the average value ($/carat) of the diamonds must be considered. Diamonds, unlike commodities such as gold, do not have a set value. They can be worth from a few to thousands of $/carat depending on their quality (evaluated on the size, colour and clarity of the stone). Argyle is currently the only major lamproite-hosted diamond mine. It contains at least 75 million tonnes, grading between 6 and 7 carats of diamonds per tonne (1.2 to 1.4 grams/tonne). The Prairie Creek mine produced approximately 100 000 carats and graded 0.13 c/t. Typical reported grades for diamond-bearing lamproites of <0.01 to .3 carats per tonne are not economic (Kjarsgaard, 1995). The average value of the diamonds at Argyle is approximately $US 7/carat; therefore, the average value of a tonne of ore is approximately $US 45.50 and the value of total reserves in the ground is in excess of $US 3.4 billion.

 

END USES: Gemstones; industrial uses such as abrasives.

 

IMPORTANCE: Olivine lamproites have only been recognized as diamond host rocks for approximately the last 20 years as they were previously classified as kimberlites based solely on the presence of diamonds. Most diamonds are still produced from kimberlites; however, the Argyle pipe produces more carats per annum (approximately 38,000 in 1995), by far, than any other single primary diamond source. Approximately 5% of the diamonds are good quality gemstones.

 

SELECTED BIBLIOGRAPHY

 

Atkinson, W.J. (1988): Diamond Exploration Philosophy, Practice, and Promises: a Review; in Proceedings of the Fourth International Kimberlite Conference, Kimberlites and Related Rocks, Volume 2, Their Mantle/Crust Setting, Diamonds and Diamond Exploration, J. Ross, Editor, Geological Society of Australia, Special Publication 14, pages 1075-1107.

Bergman, S.C. (1987): Lamproites and other Potassium-rich Igneous Rocks: a Review of their Occurrence, Mineralogy and Geochemistry; in Alkaline Igneous Rocks, J.G. Fitton and B. Upton, Editors, Geological Society of London, Special Publication 30, pages 103-190.

Fipke, C.E., Gurney, J.J. and Moore, R.O. (1995): Diamond Exploration Techniques Emphasizing Indicator Mineral Geochemistry and Canadian Examples; Geological Survey of Canada, Bulletin 423, 86 pages.

Griffin, W.L. and Ryan, C.G. (1995): Trace Elements in Indicator Minerals: Area Selection and Target Evaluation in Diamond Exploration; Journal of Geochemical Exploration, Volume 53, pages 311-337.

Haggerty, S.E. (1986): Diamond Genesis in a Multiply-constrained Model; Nature, Volume 320, pages 34-37.

Helmstaedt, H.H. (1995): "Primary" Diamond Deposits What Controls their Size, Grade and Location?; in Giant Ore Deposits, B.H. Whiting, C.J. Hodgson and R. Mason, Editors, Society of Economic Geologists, Special Publication Number 2, pages 13-80.

Jacques, A.L., Boxer, G., Lucas, H. and Haggerty, S.E. (1986): Mineralogy and Petrology of the Argyle Lamproite Pipe, Western Australia; in Fourth International Kimberlite Conference, Perth, Western Autralia, Extended Abstracts, pages 48-50.

Jennings, C.M.H. (1995): The Exploration Context for Diamonds; Journal of Geochemical Exploration, Volume 53, pages 113-124.

Kjarsgaard, B.A. (1996): Lamproite-hosted Diamond; 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 568-572.

Levinson, A.A., Gurney, J.G. and Kirkley, M.B. (1992): Diamond Sources and Production: Past, Present, and Future; Gems and Gemmology, Volume 28, Number 4, pages 234-254.

Macnae, J. (1995): Applications of Geophysics for the Detection and Exploration of Kimberlites and Lamproites; Journal of Geochemical Exploration, Volume 53, pages 213-243.

Mitchell, R.H. (1991): Kimberlites and Lamproites: Primary Sources of Diamond; Geoscience Canada, Volume 18, Number 1, pages 1-16.

Mitchell, R.H. and Bergman, S.C. (1991): Petrology of Lamproites; Plenum Press, New York, 447 pages.

Nixon, P.H. (1995): The Morphology and Nature of Primary Diamondiferous Occurrences; Journal of Geochemical Exploration, Volume 53, pages 41-71.

Scott Smith, B.H. (1992): Contrasting Kimberlites and Lamproites; Exploration and Mining Geology, Volume 1, Number 4, pages 371-381.

Scott Smith, B.H. (1996): Lamproites; in Undersaturated Alkaline Rocks: Mineralogy, Petrogenesis and Economic Potential, R.H. Mitchell, Editor, Mineralogical Association of Canada, Short Course 24, pages 259-270.

Scott Smith, B.H. and Skinner, E.M.W. (1984): Diamondiferous Lamproites; Journal of Geology, Volume 92, pages 433-438.

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Examples of Industrial Rock Deposits


BC Profile # Global Examples B.C. Examples
N01 Palabora (South Africa), Oka (Québec), Mountain Pass (California) Aley, Mount Grace tuff
N02 Koala, Panda, Sable, Fox and Misery (Northwest Territories, Canada); Mir, International, Udachnaya, Aikhal and Yubilenaya (Sakha, Russia); Kimberly, Premier and Venetia (South Africa); Orapa and Jwaneng (Botswana); River Ranch (Zimbabwe)

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N03 Argyle, Allendale (Western Australia); Prairie Creek (Crater of Diamonds, Arkansas, USA); Bobi (Côte d'Ivoire); Kapamba (Zambia); Majhgawan (India) - -