carbonatite-hosted niobium deposit at aley, northern british columbia

Ore Geology Reviews 64 (2015) 642–666

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Carbonatite-hosted niobium deposit at Aley, northern British Columbia(Canada): Mineralogy, geochemistry and petrogenesis

Anton R. Chakhmouradian a,⁎, Ekaterina P. Reguir a, Ryan D. Kressall b, Jeremy Crozier c, Laura K. Pisiak d,Ravinder Sidhu a, Panseok Yang a

a Department of Geological Sciences, University of Manitoba, 125 Dysart Road, Winnipeg, Manitoba R3T 2N2, Canadab Department of Earth Sciences, Dalhousie University, 1459 Oxford Street, Halifax, Nova Scotia B3H 4R2, Canadac Hunter Dickinson Inc., 15th Floor, 1040 W. Georgia Street, Vancouver, British Columbia V6E 4H8, Canadad School of Earth and Ocean Sciences, University of Victoria, 3800 Finnerty Road, Victoria, British Columbia V8P 5C2, Canada

⁎ Corresponding author.E-mail address: [email protected] (A.R. Cha

http://dx.doi.org/10.1016/j.oregeorev.2014.04.0200169-1368/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 4 October 2013Received in revised form 9 April 2014Accepted 24 April 2014Available online 5 May 2014

Keywords:Aleyniobium depositCarbonatiteMetasomatic rocksPyrochlore-group mineralsColumbite-group mineralsFersmiteTrace-element compositionStable-isotope composition

The Aley Nb deposit in northern British Columbia, Canada, is hosted by metamorphosed calcite and dolomitecarbonatites of anorogenic affinity emplaced in Lower Paleozoic sedimentary carbonate rocks in the Devonian.Primary Nb mineralization consists of pyrochlore (commonly comprising a U–Ta-rich and F-poor core) andferrocolumbite developed as discrete crystals and replacement products after the pyrochlore. These phases andassociated heavy minerals (apatite ± magnetite ± zircon ± baddeleyite) precipitated early in the magmatichistory and probably formed laterally extensive cumulate layers up to at least 1.5 m in thickness. Fractionationof copious amounts of pyrochlore is reflected in the chemical composition of the carbonatites and their constituentminerals, which show large variations in Nb/Ta value, but a near-chondritic Zr/Hf ratio. Alkali-rich metasomaticrocks (in particular, fenites and glimmerites) associated with the carbonatites are barren; the bulk of Nb inthese rocks is contained in rutile, phlogopite and, to a much lesser extent, amphibole. When the passive marginof North America became the zone of plate convergence in the Cretaceous, the host carbonatites were stronglydeformed, which is manifested in structures and textures indicative of grain comminution, ductile flow, foldingand, locally, brecciation. The structure and continuity of the cumulate units enriched in Nb minerals wereprofoundly affected by these processes. Interaction of the carbonatites with crustal fluids of complex chemistryresulted in extensive dolomitization, replacement of the pyrochlore and ferrocolumbite by fersmite, anddevelopment of hydrothermal parageneses consistent with the lower greenschist-facies conditions. At theselate evolutionary stages, Nbwas mobilized only to a very limited extent and sequestered in a variety of minerals(fersmite, euxenite, Mg-rich ferrocolumbite and Nb-bearing rutile) typically occurring as scarce minute crystalsassociated with hydrothermal dolomite, quartz and chlorite. Progressive enrichment of the deformed dolomitecarbonatites in heavy C and O isotopes relative to primary calcite, coupled with changes in the trace-elementcomposition of Nb phases, indicate that the fluids were equilibrated with the wall-rock sedimentary rockshosting the Aley deposit and were capable of transporting F−, (PO4)

3−, U, Th and rare-earth elements, but notNb.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Niobium is a relatively rare lithophile transition metal, whoseaverage abundance in the crust is 8 ppm, i.e. about 540 times lowerthan that of Ti and 16 times lower than that of Zr (Rudnick and Gao,2003), the two elementsmost readily substituted by Nb in crystal struc-tures. In the crust, Nb is concentrated to potentially mineable levels incarbonate and alkali-silicate igneous rocks and their weathering prod-ucts. For example, calcite and dolomite carbonatites contain, on average,~340 and 260 ppm, respectively (Chakhmouradian, 2006), whereas

khmouradian).

peralkaline granites commonly contain 100–200 ppm and, in somecases, up to 2500 ppm Nb (Costi et al., 2009; Mungall and Martin,1996). Most of the globally produced Nb (80–90%) is used asferroniobium alloys in the manufacturing of high-strength low-alloy(HSLA) steels (Birat et al., 2012; Patel and Khul'ka, 2001). Even insmall concentrations, this element greatly improves the yield strength,weldability, corrosion and heat resistance of steel, and retards recrystal-lization of austenite (Patel and Khul'ka, 2001; Rittmann and Freier,2001). The use of HSLA steels in the automotive, construction and pipeindustries allows to reduce the weight of a manufactured product, ex-tend its operating range and lifespan, and to reduce transportationand exploitation costs. Owing to this increasing importance of Nb inmetallurgy, its global production has quadrupled since 2000. Because

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demand for this metal in China and other developing economies withlow current Nb consumption per tonne of steel (b40 g) is projected torise, but the supply market is essentially controlled by Brazil (N90%since 2000: IBRAM, 2012; Moreno, 2011), it is potentially vulnerableto export restrictions like those that stirred up the rare-earth marketin 2010. A single producer, Companhia Brasileira de Metalurgia eMineração (Table 1) sells nearly twice as much ferroniobium and otherNb products as all of its competitors combined. This is why Nb hasbeen designated as a “critical” or “strategic” metal in many countries(McGroarthy and Wirtz, 2012; Moss et al., 2011), most of which nowactively work toward securing their share in the Brazilian productionthrough investments (JOGMEC, 2011; MercoPress, 2011), or seek theirown sources of this element independent of Brazil (e.g., Dzhumayloand Skorobogat'ko, 2013; Pistilli, 2012).

The bulk of global Nb production (according to some estimates, asmuch as 98%) comes from pyrochlore, a family of complex Nb–Ta–Tioxides capable of accommodating a variety of large cations, such asNa, Ca, Sr, Th, U and rare-earth elements, REE (e.g., Chakhmouradianand Williams, 2004; see also Section 5). Although pyrochlores occur ina wide range of alkali igneous andmetasomatic rocks, only carbonatitesor products of their weathering have demonstrated economic viabilityas a source of this type of ore (Table 1). A small share of Nb productionis derived from other minerals and rock types, chiefly as byproducts inlarge-scale mining operations. The most important examples includecolumbite-group minerals [(Fe,Mn)(Nb,Ta)2O6] from granites andpegmatites of Africa and Brazil (Bastos Neto et al., 2009; Melcher et al.,2014–in this issue), aeschynite (REENbTiO6) and fergusonite (REENbO4)at Bayan Obo, China (Smith and Spratt, 2012; Smith et al., 2014–in thisissue), and loparite [(Na,REE,Ca,Sr,Th)(Ti,Nb,Ta)O3] from peralkalinefeldspathoid rocks at Lovozero, Russia (Kogarko et al., 2002; Mitchelland Chakhmouradian, 1996). The majority of currently active Nbexploration projects around the world focus their efforts on fresh orweathered carbonatites as their primary target; some key examplesare listed in Table 1. The Aley carbonatite complex in northern BritishColumbia, Canada, has been the site of intermittent exploration activi-ties since the early 1980s. These efforts have resulted in the delineationof a NI 43-101 compliant resource of 285.8 million tonnes (measured+indicated) averaging 0.37 wt.% Nb2O5 plus 144 million tonnes(inferred) averaging 0.32 wt.% Nb2O5 at a cut-off grade of 0.2 wt.%(Simpson, 2012).

The primary objective of the present work is to characterize thenature, compositional variation and distribution of Nb mineralizationat Aley, and interpret these data in the context of carbonatite petrogen-esis and regional geology. To our knowledge, this work is the first com-prehensive analysis of Nb distribution in a lithologically complexigneous deposit, which includes not only the principal Nb hosts butalso minerals containing trace levels of this element. Implications ofthese new findings for mineral exploration are also discussed.

2. Geological setting and history of exploration

The Aley niobium deposit (Lat. ~56°27′ N, Long. ~123°45′ W; NTSsheet 94B-5) is situated in the Muskwa Ranges of the northern RockyMountains some 140 km NNW of the town of Mackenzie in BritishColumbia, Canada. Tectonically, this part of the Canadian Cordillera isinterpreted as the Foreland fold-and-thrust belt (FB in Fig. 1), whichrepresents the deformed and metamorphosed thick (3–5 km) marginof a supracrustal wedge covering the Precambrian basement rocks ofthe North American craton (Price, 1994 and references therein). Thebelt comprises Neoproterozoic continental sediments, Paleozoiccarbonate platform and associated miogeoclinal and basin series, andMesozoic–Tertiary foredeep sediments. The conventional interpretationis that this sequence was deposited from the Neoproterozoic to Jurassicon the passive margin of the North American protocontinent and in theadjacent ocean basin, and underwent a foreland basin stage, which hasbeen linked to accretion of allochthonous oceanic terranes west of the

area beginning in the Late Jurassic (Price, 1994). The growth of the FBthrough compression, crustal thickening, uplift, erosion and depositionof the foredeep sequence ceased in the Eocene due to the onset oforogenic extension in the central Cordillera, which heralded a transitionto the present-day tectonic regime (Ewing, 1980; Price, 1986). Towardthe Pacific Coast, the FB is bordered by theOmineca(-Purcell) crystallinebelt (OB in Fig. 1), which comprises Mesozoic metamorphic corecomplexes exhumed during the Eocene extension (Brown and Read,1983; Parrish et al., 1988; Plint et al., 1992).

The geology of the Aley area was initially mapped by Irish (1970),but igneous carbonate rocks were not recognized there until a detailedexploration work by Cominco Ltd in 1980–1986 (Pride, 1987). Theserockswere overlooked during the regional 1:250,000mappingprobablybecause they intruded a carbonate sedimentary sequence andunderwent strong deformation during the Rocky Mountain orogeny(see below). The carbonatites are emplaced into the Late Cambrian–Early Ordovician Kechika Formation (Fig. 1), consisting of dolostones,interbedded limestones, marls, siltstones and shales, and overlainunconformably by the Late Ordovician Skoki Formation, whichcomprises two massive dolostone units separated from each other byvolumetrically minor submarine pyroclastic deposits and basalt flows(Mäder, 1986; McLeish, 2013). This predominantly carbonate sequencehas been interpreted in terms of Early Paleozoic marine transgressionsalong the continentalmargin; for further details and stratigraphic corre-lations, see Kent (1994).

Carbonatites outcrop predominantly on the southern slope of anarcuate ridge south of the Aley Creek as a roughly triangular bodyin plan view measuring ~7 km2 in area at the current erosion level(Fig. 1). The bulk of the intrusion is composed of texturally diversedolomite carbonatites; calcite carbonatites are volumetrically minorand were interpreted by Mäder (1986) to be localized in the narrowperipheral parts of the body (see Section 4 for further details). Thecarbonatites are encircled by a thick (up to 1 km inwidth) zone of com-petent dark-green to bluish grey silicate rocks collectively described byprevious workers as “amphibolites” (Pride, 1987) or “syenites” (Mäder,1986). A few hundred meters SW of the intrusive contact, the SkokiFormation hosts the Ospika pipe, a small diatreme-like body comprisingclast- and macrocryst-rich ultramafic lamprophyres best classified asaillikite (Pell, 1994). Thin (typically, b1.5m inwidth) dikes of ultramaficlamprophyre and dolomite carbonatite crosscut the fenites and Kechikadolostones W, NW and N of the complex and, locally, carbonatites in itsperipheral part. Careful petrographic analysis of the exocontact dikesshows that they represent ex situ bodies mobilized tectonically bycompression-induced ductile flow (e.g., Fig. 2a).

The first reasonably detailed petrographic and mineralogical studyof the Aley complex was done by Mäder (1986), who confirmed theidentity of the major Nb carriers in the carbonatites (fersmite,pyrochlore and columbite), and interpreted the textural characteristicsof these rocks as primary magmatic. On the basis of previously pub-lished and her own data, Pell (1994) interpreted the Aley carbonatitein the structural context of the FB as a cylindrical intrusion emplacedinto the miogeoclinal sedimentary sequence near the edge of the conti-nental shelf in the Devonian (339–349 ± 12 Ma), i.e. long before theonset of the Laramide orogeny in the Cretaceous. Elsewhere in the FB,carbonatites and related igneous ormetasomatic rocks occur in a similargeodynamic setting at the Rock Canyon Creek, Ice River, BearpawRidge and Wicheeda Lake (Dalsin et al., 2014–in this issue; Heaman,2009; Pell, 1994). West of the FB in the Omineca belt, Devonian toLower Carboniferous (330–365 Ma) carbonatites and alkali syenitesare known at the Three Valley Gap, Frenchman Cap Dome–MountGrace area, Trident Mountain–Blue River area, Lonnie and Vergil claims(Millonig et al., 2012; Parrish, 1995; Pell, 1994; Rukhlov and Bell, 2010;Fig. 1).

In 2007, the Aley exploration claims were acquired by TasekoMinesLtd. (Chung and Crozier, 2008), that initiated a detailed re-investigationof the deposit involving mapping, drilling and sampling. Between 2007

Table 1Major niobium deposits in carbonatites and genetically related rocks worldwide, including former and current producers.

Deposit, location, country Current developeror license holder;years active

Tectonic setting(or host structure)

Emplacem.age

Host rock(other associated rocks)

Major ore minerals(other minerals ofpotential interest)

Economic parameters

Neske-vara area,Vuoriyarvi, northernKarelia, Russia

Central Kola Expedition Kontozero rift in theFennoscandian Shield

369–383 Ma Carbonatites andphoscorites (olivinites,clinopyroxenites, ijolites,melteigites, feldspathoidsyenites)

Zirconolite,pyrochlore

1.6 Mt at 0.53% Nb2O5,0.017% Ta2O5; 0.2%Nb2O5 cut-off

Vishnevogorskoye(Vishnevye Gory),Kaslinskiy Dist.,Chelyabinsk Region,Urals, Russia

State reserve; mined in1943–1993 byVishnevogorsky GOK

East–Uralian zone(riftedmargin of the EastEuropean platform)

440–446 Ma,reset at 360–320 and 260–240 Ma

Nepheline syenites andcalcite carbonatites(albitites, fenites and otheralkali metasomatic rocks)

Pyrochlore 3.77 Mt at 0.15% Nb2O5

(lower grades reportedby Frolov et al., 2003)

Tomtor(skoye), OlenyokUlus, Republic of Sakha(Yakutia), Siberia,Russia

Alrosaa Udzhinskiy aulacogen(paleorift), Siberiancraton (NE part)

Neoprot.?UpperPermian?

Lateritic profile withreduced horizons overcarbonatites (ultramaficrocks, melteigites, ijolites,nepheline syenites, syenites,apatite–microcline–biotiterocks)

Secondarypyrochlore, minorferrocolumbite andrutile (REEphosphatesb)

1.18 Mtc at 6.71% Nb2O5,10.12% REO; 3.5% Nb2O5

cut-off

Chuktukon(skoye),Boguchanskiy Dist.,Krasnoyaskiy Kray,Siberia, Russia

KrasGeoResurs Chadobets dome (Spart), Irkineyevskiyaulacogen, Siberian cra-ton (SW part)

Lower Triassic Lateritic profile withreduced horizons overcarbonatites (alkali picrites)

Primary andsecondarypyrochlore (REEphosphatesb,cerianite, bastnäsite)

6.64 Mt at 0.60% Nb2O5,7.32% REO; 3.0% REOcut-off; 101.09 Mt at1.18% Nb2O5, 2.96% REO;0.40% Nb2O5 cut-off

Tatarskoye, MotyginskiyDist., KrasnoyaskiyKray, Siberia, Russia

Stal'mag/Severostal';minedin 2000–2010, ~300 t FeNbreported at peak production

Tatarsko-Ishimbinskayafault zone, Enisey Range(accretionary belt)

Neoprot. Lateritic and saproliticprofiles over dolomite andcalcite carbonatites (alkalimetasomatites)

Primary andsecondary pyrochlore,minor ferrocolumbite(apatite)

2.75 Mtd at 0.69%Nb2O5;7.81 Mt at 8.71% P2O5

Beloziminskoye (BelayaZima), Tulunskiy Dist.,Irkutsk Region, Siberia,Russia

N/A; formerly OrlovskiyGOK; pilot plant active in1984–1986

Uriksko-Iyskiy graben,Siberian craton (south-ern part)

Neoprot. Saprolitic profile over calciteand ankerite carbonatites(melteigites, ijolites,nepheline syenites)

Pyrochlore,ferrocolumbite(apatite)

182.22 Mt at 0.50 wt.%Nb2O5, 0.014 wt.%Ta2O5, 13.6 wt.% P2O5

Bol'shetagninskoye(Bol'shaya Tagna),Tulunskiy Dstr., IrkutskRegion, Siberia, Russia

N/A Uriksko-Iyskiy graben,Siberian craton (south-ern part)

Neoprot. Metasomatic microclinitesand glimmerites (ijolites,nepheline and alkali syenites,carbonatites)

Pyrochlore 66.58 Mt at 1.02%Nb2O5; 0.3% Nb2O5 cut-off

Mabounié, Moyen-Ogooué, Gabon

Compagnie Minière del'Ogooué (COMILOG)/ERAMET

? 660 Ma Lateritic profile overcarbonatites (syenites,lamprophyres, fenites)

Primary andsecondarypyrochlore (apatite)

14 Mt at 1.7% Nb2O5;360 Mt at 1.02% Nb2O5

and 24% P2O5

Mrima Hill, Jombo-Mrima alkaline com-plex, Kwale Dist.,southern Kenya

Cortec Mining Kenya Ltd./Pacific Wildcat ResourcesCorp.

Unnamed failed rift arm Post-Karoo(Jurassic)

Lateritic profile over calcitecarbonatites (agglomerate,melteigites,nepheline syenites,fenites, lamprophyres)

Secondarypyrochlore

105.3 Mt at 0.65%Nb2O5; 0.2% Nb2O5 cut-off

Lueshe, Rutshuru Dist.,North Kivu, DemocraticRepublic of the Congo

Disputed between SociétéMinière du Kivu (Somikivu)and Krall Metal Congo;mined in 1982–1993, 2000–2003e, 1346 t concentratereported at peak production

Western branch of theEast African rift

Neoprot.?Cambrian?

Lateritic profile over calcitecarbonatites (dolomitecarbonatites,clinopyroxenites, cancrinitesyenites, fenites)

Secondarypyrochlore

30–33 Mt (unverified)at 1.34% Nb2O5

St. Lawrence Columbiummine, Oka complex,Deux-Montagnes, Qué-bec, Canada

Niocan Inc.; mined in 1961–1976 by St. LawrenceColumbium and MetalsCorporation

Ottawa-Bonnècheregraben

~130 Ma Calcite carbonatites(jacupirangites, ijolites,okaites, melilitolites)

Pyrochlore 10.63 Mt at 0.68%Nb2O5

f; 0.40% Nb2O5

cut-off

Niobec, Saint-Honorécomplex, Fjord-du-Saguenay, Québec,Canada

IAMGOLD;mined since 1976,4700 t Nb in 2012

Saguenay graben, part ofthe St. Lawrence riftsystem

~650 Ma Dolomite carbonatites(calcite carbonatites,feldspathoid syenites,syenites, ijolites, fenites)

Pyrochlore, minorcolumbite

458.11 Mtg at 0.42%Nb2O5; 0.20% Nb2O5 cut-off

Aley, Muskwa Ranges,British Columbia,Canada

Taseko Mines Ltd. Foreland belt (riftedmargin of NorthAmerica)

~370 Ma Dolomite and calcitecarbonatites (fenites,glimmerites, lamprophyres)

Fersmite, primarypyrochlore,ferrocolumbite(apatite)

285.8 Mt at 0.37%Nb2O5; 0.20% Nb2O5 cut-off

Elk Creek, Johnson/Pawnee Co's, Nebraska,USA

NioCorp Developments Ltd.(formerly Quantum)

Nemaha uplift boundary(rejuvenatedMidcontinent rift?)

460–540 Ma Dolomite carbonatite(syenites)

Pyrochlore 19.32 Mt at 0.67%Nb2O5; 0.40% Nb2O5 cut-off

Barreiro, Araxá, MinasGerais, Brazil

Companhia Brasileira deMetalurgia e Mineração(CBMM); mined since 1965,accounts for 65–70% of globalNb production

“125° AZ lineament”,Alto Paranaíba upliftseparating the Paranáand São Francisco basins(plume-related?)

77–98 Ma Lateritic profile overdolomite and calcitecarbonatites; primarycarbonatites (glimmerite,fenite)

Primary andsecondarypyrochlore(monazite, apatite)

Residual: 440 Mt at 2.5%Nb2O5; primary: 940 Mtat 1.6% Nb2O5

Boa Vista and otherprojects, Catalão I andII, Goiás, Brazil

Anglo American Nióbio Bra-sil Ltda; mined since 1973,~4400 t Nb in 2012

“125° AZ lineament”,Alto Paranaíba upliftseparating the Paranáand São Francisco basins(plume-related?)

83–85 Ma Lateritic profile overcarbonatites andphoscorites (dunites,clinopyroxenites,glimmerites, fenites)

Primary andsecondarypyrochlore (apatite,monazite,vermiculite)

Residual: 3.4 Mt at 1.22%Nb2O5; primary: 51.1 Mtat 1.07%Nb2O5; 0.5–0.7%Nb2O5 cut-off

644 A.R. Chakhmouradian et al. / Ore Geology Reviews 64 (2015) 642–666

Table 1 (continued)

Deposit, location, country Current developeror license holder;years active

Tectonic setting(or host structure)

Emplacem.age

Host rock(other associated rocks)

Major ore minerals(other minerals ofpotential interest)

Economic parameters

(Morro dos) Seis Lagos,Amazonas, Brazil

Companhia de Pesquisa deRecursos Minerais (CPRM)

Takutu Lineament(graben) related toreactivation of theAmazon craton

Mesozoic(100–180 Ma?)h

Lateritic profile overcarbonatites (?) (sideritecarbonatites)

Nb-rich rutile andbrookitei (monazite,barite, hollandite)

2897.9 Mt at 2.81%Nb2O5

Crown deposit, MountWeld, WesternAustralia, Australia

Lynas Corp. Laverton tectonic zone,Yilgarn craton

2020–2060 Ma

Lateritic profile over calciteand dolomite carbonatites(glimmerites)

Secondarypyrochlore (REEphosphatesb)

37.7 Mt at 1.07% Nb2O5,0.024% Ta2O5, 1.16%REO,0.09% Y2O3

(This table was compiled using the following sources, listed alphabetically: Afanasyev, 2011; Anglo American, 2013; Biondi, 2005; Bogdanov et al., 1998; Bulakh et al., 2004; Bush, 2009;Clow et al., 2011; Cordeiro et al., 2011; Cox and Wilton, 2006; Daigle, 2011; Dzhumaylo and Skorobogat'ko, 2013; Farmer et al., 2013; Foster and Harrison, 2000; Freyssinet et al., 2005;Frolov et al., 2003; Giovannini, 2013; Hoatson, 2011; Horkel et al., 1984; Issa Filho et al., 1984; Johnson and Tegera, 2005; Kukharenko et al., 1965; Lapin and Tolstov, 1995; Lima andBezerra, 1991; Lomaev and Serdyuk, 2011; Nedosekova, 2007; Nedosekova et al., 2013; Niocan, 2011; Pell, 1994; Pozharitskaya et al., 2006; Puchkov, 2000; Simpson, 2012; Tolstovand Tyan, 1999; A.V. Tolstov, pers. commun; White, 1975; Woolley, 2001.)

a ICT Group recently announced their intent to develop the deposit in partnership with state-owned Rostec to potentially begin production of REE by 2018 (Dzhumaylo andSkorobogat'ko, 2013).

b REE phosphates in laterites are typically represented by secondary monazite (LREEPO4; LREE = La…Eu) with lesser amounts of xenotime (YPO4); florencite [LREEAl3(PO4)2(OH)6]and other members of the crandallite group are common in saprolitic profiles; small concentrations of churchite (YPO4·2H2O), rhabdophane (LREEPO4·H2O) and related phosphateswere identified in some deposits.

c Burannyi area only; two other areas containing a high-grade REE resource (Northern and Southern) have been identified (A.V. Tolstov, pers. commun.).d According to Frolov et al. (2003); an unverified figure of 2.43 Mt is cited in more recent online reports.e Recent illegal production was alleged by human right organizations.f Recently explored zone S-60 only (Niocan, 2011).g Whittle open pit option only (Clow et al., 2011).h Based on the ages of the broadly contemporaneous Takutu mafic rocks and Catrimani alkaline complex (Lima and Bezerra, 1991).i Identified as anatase in some sources.


and 2011, 104 holes totaling some 23 kmof corewere drilled in the fourprincipal zones of the deposit identified by Cominco (Central, Saddle,Bear and East), although the primary focus of this program was on theCentral zone. These activities also involved a structural analysis of theAley carbonatites and their country rocks by McLeish (2013), who chal-lenged the previous interpretations and proposed that the carbonatites

Fig. 1. The location and schematic geological map of the Aley carbonatite complex after Mäcarbonatites in the endocontact zone of the complex according to Mäder (1986) is shown by thThe numbered localities in the regional map show the location of alkaline rocks and carbonatWicheeda Lake (4), Verity–Blue River area (5), Trident Mountain (6), Mount Grace–Perry Rive

were emplaced as a sill contemporaneously with a Late-Devoniancrustal shortening tectonic event (D1) attributable to theAntler orogeny.He also revised lower-age constraints on the timing of magmatism atAley, obtaining a U–Pb age of 366 ± 2 Ma for titanite from the Ospikapipe. This value is in accord with our high-resolution ion-microprobeU–Pb data for baddeleyite (372 ± 8 Ma; work in progress). In the past

der (1986), Pell (1994) and McLeish (2013), with modifications. The extent of calcitee grey dashed line; the dash-dotted contour delineates the Central zone of the Nb deposit.ites (after Pell, 1994), including: Kechika River (1), Manson Creek (2), Mount Bisson (3),r (7), Three Valley Gap (8), Ice River (9), and Rock Canyon Creek (10).

Fig. 2. Selected structural and textural characteristics of the Aley carbonatites: (a) an ex situ dike of folded dolomite carbonatite emplaced in exocontact fenites; (b) aweathered surface offoliated carbonatite showing layers enriched in apatite; (c) drill core showing a transition from barren carbonatite (beige) to an oxide-rich cumulate layer in the center; (d) microphoto-graph (cross-polarized light, XPL) of strongly foliated dolomite carbonatite; (e) isoclinally folded dolomite carbonatite with numerous apatite-rich layers (white); (f) matrix-supportedcarbonatite breccia; (g) drill core composed of “orthocarbonatitic marbles” produced by ductile flow.


four years, efforts of our grouphave focused on detailed characterizationof the mode of occurrence, chemical composition and paragenetic rela-tionships of Nb minerals in the Aley deposit. The results of this workare presented below.

3. Research material and methodology

We sampled every accessible outcrop within the project area andinspected most of the core obtained during the 2007, 2010 and 2011drilling programs (stored at Williams Lake and Mackenzie, BritishColumbia), as well as most of the material from the 1985 and 1986Cominco programs still available at a core shed near the deposit. Intotal, 179 samples of carbonatites and associated rocks, covering theentire petrographic spectrum observed in outcrop and drill core, wereselected for study; 106 of these samples were observed to containminerals enriched in Nb and were hence selected for a more detailed

examination. The sampling locations, rock types and Nb-bearingminerals identified in this material using a variety of techniques arelisted in Appendix 1.

The samples were first examined using optical microscopy in trans-mitted polarized and reflected light to identify their major constituentminerals and textural characteristics. Because the spatial resolution ofoptical microscopy is limited and certain minerals could not be identi-fied reliably on the basis of their optical properties due to their smallgrain size or intimate association with other minerals, the initial petro-graphic analysis was followed up by a detailed study using back-scattered electron (BSE) imaging coupled with energy-dispersiveX-ray spectrometry (EDS). Raman microspectroscopy was used inthose cases where unambiguous identification could not be made onthe basis of optical examination and EDS data (e.g., rutile vs. other TiO2

polymorphs). Raman spectra were recorded using a HORIBA ScientificLabRAM ARAMIS instrument equipped with a 460-mm focal length

image of Fig.�2


spectrometer, multichannel electronically cooled charge-coupled devicedetector, motorized x–y–z stage and solid-state 532-nm laser (mpc6000by Laser Quantum)with a nominal output power of 50mW. The instru-ment was operated in confocal mode; an Olympus microscope coupledto the spectrometer was used to focus the laser beam on the samplesurface and collect the generated Raman signal. The spectra werecollected with a diffraction grating of 1800 grooves/mm; other instru-mental parameters (data-collection times, slit width, etc.) were opti-mized by performing multiple measurements on the same area. Thespectrometer was calibrated using a synthetic Si standard.

The major-element compositions of all constituent minerals weredetermined by wavelength-dispersive X-ray spectrometry (WDS)using a Cameca SX 100 electronmicroprobe operated at an acceleratingvoltage of 15 kVand a beamcurrent of 10 nA (20 nA for euxenite). Otherinstrumental conditions (including the choice of standards, electronbeam diameter and counting statistics) were optimized for each groupof minerals (carbonates, ferromagnesian silicates, niobates, phosphates,sulfides) individually to yield the best results (Appendix 2). The rawWDS data were reduced using the ZAF correction procedure. In total,about 1100 WDS analyses (including ~180 of Nb host phases) wereobtained.

The abundances of selected trace elements weremeasured by laser-ablation inductively-coupled-plasma mass-spectrometry (LA-ICP-MS)using a 213-nm Nd-YAG Merchantek laser connected to a ThermoFinnigan Element 2 sector-field mass-spectrometer. The trace-elementcompositions were determined in polished thick sections (~100 μm inthickness), usingmatching BSE, transmitted- and reflected-light imagesof the areas analyzed byWDS to accurately position a laser beam on thesample and ensure that only inclusion-free areas unaffected by fractur-ing or alteration were ablated. All samples were checked under thepolarizing microscope after the analysis to verify that laser-ablation pitswere confined to the areas of interest and did not penetrate inclusionsor adjacent material. Because a volume of material is sampled duringablation, subsurface compositional heterogeneities (e.g., inclusions)may affect the quality of measurement and potentially producespurious data. To ensure that the acquired LA-ICP-MS data were notaffected by such compositional heterogeneities, each element signalwas checked for consistency. In total, ca. 320 LA-ICP-MS analyseswere acquired, of which about 16% were considered problematic anddiscarded. All analyses were performed using a beam size of 25–40 μm(adjusted depending on the size and homogeneity of individual mineralgrains) in Ar and He atmospheres, at a fluence of 4.1–6.2 J/cm2 andrepetition rate of 5–10 Hz. Other relevant LA-ICP-MS settings are sum-marized in Appendix 3.

Representative whole-rock samples were crushed to 80% passing a100-mesh sieve, pulverized to N85% passing a 200-mesh sieve and ho-mogenized. The homogenized samples were analyzed for majorelements by X-ray fluorescence and for trace elements by inductively-coupled-plasma mass-spectrometry following four-acid digestion (fornon-refractory elements) and Li2B4O7 fusion for REE and other refractoryelements. The analyses were performed at Inspectorate, an ISO-certifiedBureau Veritas company.

Monomineralic samples of calcite and dolomite were hand-pickedunder a binocular microscope only from the samples characterizedpetrographically. The samples were digested in H3PO4 for 2 to 3 h togenerate CO2 gas, in which C and O isotope ratios were measuredusing a Finnigan GasBench II connected to a Thermo Finnigan Delta VPlus isotope-ratio mass-spectrometer (IRMS). The sample gases weretransferred to the IRMS through two water traps and a gas chromato-graph. Calibrationwas performed by analyzing two international calcitestandards (NBS-18 and NBS-19) at the beginning, middle and end ofeach sample run. A calibration linewas calculated by least squares linearregression using the known andmeasured isotope values of the calibra-tion standards. For quality check, calibrated internal calcite standardCHI (δ13CV-PDB =−8.01‰; δ18OV-SMOW = 18.88‰) and dolomite stan-dard Tytyri (δ13CV-PDB= 0.78‰; δ18OV-SMOW= 23.62‰)were analyzed

simultaneously with the samples. Multiple analyses of these standards(187 and 250, respectively) yielded results within the standard devia-tion from the reference values (±0.10‰ for δ13C and±0.15‰ for δ18O).

4. Petrography

The concentrically zoned cylindrical structure of theAley carbonatitebody proposed by Mäder (1986; see Section 2) is overly simplistic.Relations between dolomite and calcite carbonatites are too complexand the petrographic diversity of both rock types, expressed on the out-crop and hand-specimen scales, is too extreme to bemapped accuratelyat that scale. The overwhelming majority of the Aley carbonatites showpervasive evidence of postmagmatic (nota bene!) deformation, includ-ing grain-boundary migration, comminution, inequigranular foliatedand swirly textures, alignment of elongate carbonate grains, apatite,amphibole and phlogopite crystals, folding and brecciation (someexamples are shown in Fig. 2). Cumulate rocks enriched in mineralsunsusceptible to ductile deformation (mostly, apatite and magnetite)contain relict grains of mechanically twinned calcite, dolomite andphlogopite with bent cleavage planes. Based on their appearance inhand specimen and thin section, some samples are best described asorthocarbonatitic marbles produced by ductile flow (e.g., Fig. 2g). Inour opinion, none of the aforementioned textures can be interpretedas primary magmatic (cf. Mäder, 1986). Those samples that lack anyevidence of deformation (e.g., Fig. 3a)were clearly recrystallized duringhydrothermal reworking (see below).

Mineralogically, dolomite carbonatites are the prevalent rock type,but textural and mineralogical variations within this group of rocksare significant and undoubtedly reflect a multiplicity of origins. Themost common type of carbonatite is a fine-grained conspicuouslyfoliated variety containing variable proportions of fluorapatite(hereafter referred to simply as apatite). These rocks typically exhibitan inequigranular banded texture due to the concentrated occurrenceof resorbed elongate apatite crystals, oxide minerals and very fine-grained dolomite as discontinuous layers, laminae and lenses intercalatedwith coarser-grained areas (Fig. 2b–e). Less common are massive,strongly sheared (schistose), equigranular and brecciated varieties(Fig. 3a, b). Medium- to coarse-grained areas commonly comprisezoned Sr-rich crystals (up to 1.0 wt.% SrO) with numerous minute(≤15 μm across) inclusions of calcite and burbankite [(Na,Ca)3(Sr,Ca,REE,Ba)3(CO3)5]. Some carbonatites are fresh and contain unalteredpyrite; other samples are strongly oxidized to chocolate-brown partial-ly decalcifiedmaterial (Fig. 3c) containing goethite pseudomorphs afterpyrite and goethite–calcite aggregates replacing dolomite.

In addition to anhedral grains with straight or serrated boundariesand ranging in size from a few tens of micrometers to several millime-ters (Figs. 2d, 3a, b), dolomite occurs abundantly in calcite carbonatitesand other rocks as fine-grained aggregates replacing calcite, veinletsand as a minor to major constituent of pseudomorphs after magnetiteandphlogopite (Fig. 3d–f).Most dolomite carbonatites are recrystallized(Fig. 3a) and crosscut by numerous fractures filled with hydrothermalrhombohedral dolomite, quartz, chlorite (Fig. 3g), muscovite, monazite,REE fluorocarbonates and, less commonly, rutile, albite, ankerite, calcite,late-stage apatite, barite, celestine, or strontianite. In addition to theaforementioned minerals, complex Nb oxides (see Section 5), zircon(as sub- to euhedral crystals associated with apatite), thorite (asinclusions in the Nb oxides), REE fluorocarbonates (as syntactic inter-growths), chalcopyrite and pyrrhotite (both as inclusions in, and inter-growths with, pyrite) were identified. Brown decalcified varieties ofthese rocks lack any recognizable primary textures or mineralogy(with the exception of apatite; Fig. 3c) and contain a high proportionof siderite developed as pseudomorphs after magnetite and liningdissolution cavities. We interpret this material as displaced (locally, byas much as 130 m) blocks of weathered carbonatite.

Calcite carbonatites are much less common than the dolomitecarbonatites, and occur predominantly in the peripheral part of the

Fig. 3. Selected textural characteristics of the Aley carbonatites: (a) brecciated massive fine-grained dolomite carbonatite (mDc) crosscut by veins comprising coarser-grained recrystal-lized dolomite carbonatite (rDc); (b) massive dolomite carbonatite in plane-polarized light (PPL); (c) weathered carbonatite composed of apatite clumps (Ap) and extensively oxidizedand decomposed dolomite (Dol), depth 134 m; (d) partially dolomitized carbonatite composed predominantly of calcite (Cal), magnetite (Mgt) and apatite (XPL); (e) partiallydolomitized calcite carbonatite with accessory sodic amphibole (Amp) in XPL; (f) extensively dolomitized carbonatite containing dolomite pseudomorphs (Dol ps) after phlogopiteand magnetite (PPL); (g) XPL photomicrograph of rhombohedral dolomite and quartz (Qtz) developed in a fracture in dolomite carbonatite; Chl = chlorite.


intrusion (Fig. 1), but also as variably dolomitized relict blocks (zones?)in its core. These rocks typically show a saccharoidal weakly foliated toswirly texture and contain a large assortment of minor and accessoryminerals, including (listed approximately in order of decreasingabundance): apatite, phlogopite (ranging to fluorophlogopite in a fewsamples), chlorite, magnetite, pyrite and products of its oxidation, com-plex Nb oxides (see Section 5), sodic amphiboles (for details, see Reguiret al., 2012), aegirine, quartz, ilmenite, zircon, pyrrhotite, monazite,baddeleyite, thorianite, barite and rutile. Fluorite, allanite, pentlandite,bradleyite [Na3Mg(PO4)(CO3)] and eitelite [Na2Mg(CO3)2] were identi-fied in one sample each. In common with burbankite, the latter twominerals occur as primary inclusions in early-crystallized minerals,attesting to high Na contents in their parentalmagma. In some samples,phlogopite, magnetite, amphiboles and aegirine gain the status ofrock-forming constituents (Fig. 4a–c). Most calcite carbonatite samplesexamined in the present work are dolomitized to variable degrees,from precipitation of minor dolomite along grain boundaries and frac-tures to pervasive replacement of calcite and other primary mineralsby dolomite, chlorite and associated secondary phases. Advanced

dolomitization obliterates primary petrographic features and producesmicrocrystalline pseudomorphs whose precursor mineral cannot beidentified with certainty (e.g., Fig. 3f). Although the bulk of dolomiteat Aley developed metasomatically at the expense of calcite andother primary magmatic minerals, or precipitated in fractures fromhydrothermal fluids, there is also no doubt that some dolomite(e.g., resorbed relicts in calcite, or zoned crystals with burbankiteinclusions) is igneous in origin. Primary (calcite-)dolomite carbonatitesare characterized by the presence of fresh sodic amphibole, aegirine,phlogopite, or pyrochlore, which appear not to occur in pervasivelydolomitized rocks.

Throughout the intrusion, carbonatites host numerous partiallydigested xenoliths of silicate rocks, most of which are fine-grained andhave a massive nondescript appearance (Fig. 4d). For explorationpurposes, these rocks were collectively referred to as “fenites” becauseof their superficial similarity to the fenites bordering the carbonatitecore. However, our petrographic work identified a variety of rocktypes, most of which are undoubtedly metasomatic in origin or, at thevery least, represent products of extensive recrystallization and thermal

image of Fig.�3

Fig. 4. Representativemineralogical characteristics of the Aley rocks: (a) early calcite carbonatite containing abundant fresh phlogopite (Phl), magnetite and apatite (XPL); (b) early calcitecarbonatite containing sodic amphibole, pyrochlore (Pcl) andphlogopite (PPL); (c) partially dolomitized calcite carbonatite enriched in ferrocolumbite (Cb), zircon (Zrn) and chlorite (Chl)pseudomorphs after phlogopite (XPL); (d) dolomite carbonatite (Dc) containing numerous fenite xenoliths (Fn) and cross-cut by hydrothermal quartz veinlets; (e) typical fenite in PPLshowing albite laths (Ab) and finer-grained acicular amphibole; (f) typical glimmerite composed of interlocking phlogopite crystals and accessory pyrochlore (XPL).


metamorphism. They derive from a variety of protoliths, which cannotbe identified with certainty or correlated to any of the rocks exposedin the vicinity of the Aley complex. The twomost commonmetasomaticrocks that can be readily linked to carbonatite magmatism are fenitesand glimmerites. The former are massive to brecciated (predominantlymatrix-supported) amphibole-rich rocks containing a variable propor-tion of albite, quartz, potassium feldspar, calcic to sodic clinopyroxeneand phlogopite (Fig. 4d, e). The abundance of amphibole and feldsparin these rocks led to their misinterpretation in the earlier literatureas “amphibolites” and “syenites” (see Section 2). Glimmerites arephlogopite-rich rocks (Fig. 4f) containing subordinate proportions ofdolomite, calcite, sodic amphibole or apatite, and developed at theexpense of fenites or another mafic precursor. The association of fenitesand glimmerites with primary dolomite and calcite (deposited in thatorder) suggests that carbonatitic magmas were responsible for themetasomatism that produced both of these rock types. A smaller pro-portion of silicate xenoliths are composed of albitite, muscovite–albiteand quartz-chlorite rocks, and could represent metasomatized igneousunits consanguineous with the host carbonatites, or simply reworkedfragments ofmetamorphic basement rocks. Further discussion of thissubject is beyond the scope of the present work.

5. Niobiummineralization: paragenetic and compositional variations

At Aley, both disseminated mineralization and locally concentratedoccurrences of Nb minerals were recognized. The latter form fine- tocoarse-grained layers and lenses ranging from a few mm to 1.5 m inwidth and enriched (locally to 3–5 vol.%) in euhedral pyrochlore orferrocolumbite (both commonly replaced by fersmite) associated withsub- to euhedral apatite, magnetite, zircon and baddeleyite crystals.Phlogopite and carbonates are typical interstitial phases. Theferrocolumbite–magnetite layers are more readily recognizable in out-crop and drill core owing to their dark color and coarse size ofmagnetitecrystals. The pyrochlore-rich layers are not nearly as rich in magnetite,and tend to be thinner and not as laterally continuous as theferrocolumbite-rich units. The most efficient way of tracking the lattertype of mineralization is with a UV lamp, revealing the conspicuousluminescence of apatite (lilac) and zircon (yellow), which otherwiseare difficult to identify on the core surface. Carbonatite adjacent to thelayers and lenses enriched in heavy minerals often contains intact andfragmented crystals of these minerals interspersed with finer-graineddolomite and undoubtedly generated by shearing and disaggregationof the cumulate material. With one exception (see below), the

image of Fig.�4


identification of Nb phases described in this section was confirmedby Raman microspectroscopy using published data for syntheticisostructural compounds as a reference.

5.1. Pyrochlore-group minerals

The earliest Nb mineral to form is pyrochlore, which occurs aseuhedral cubo-octahedral and octahedral crystals ranging from paleyellow to dark brown in color and from a few tens of μm to 4mmacross

Fig. 5. Pyrochlore (Pcl) occurrence, morphology and zoning; all except (a) are BSE images. (a)apatite inclusions (BSE image); (c) euhedral crystals of strontiopyrochlore inweathered carbondolomite (d) and dolomite–calcite carbonatites (Brt = barite); (g) irregularly shaped cluster o(h) zoned pyrochlore from glimmerite.

(Figs. 4b, 5a). Fresh pyrochlore is common in calcite carbonatitesexposed in the peripheral parts of the Aley complex and occurringlocally at the contact with fenitic xenoliths in the central part, but wasalso observed in primary dolomite carbonatites. This mineral is com-monly associated with apatite and contains euhedral inclusions of thelatter oriented at random with respect to the external foliation(Fig. 5b), i.e. further supporting our interpretation that foliation in theAley rocks is a postemplacement deformational, rather than a primarymagmatic, feature.

Cubo-octahedral crystal in calcite carbonatite; (b) euhedral unzoned crystal with multipleatite; (d–f) strongly zoned crystals with a U–Ta-rich and F-poor core from primary calcite–f pyrochlore crystals with U–Ta-rich and F-poor core from phlogopite–amphibole fenite;

image of Fig.�5


In general, the composition of pyrochlore-group minerals isextremely variable owing to the flexibility of their crystal structureand a potentially large selection of cations that can occupy the eight-fold-coordinated A and octahedrally coordinated B sites in thatstructure. In carbonatites and associated silicate-rich rocks such asphoscorites, the compositional variation of these minerals can beexpressed by the general formula [A](Ca,Na,U,Th,REE,Ba,Sr)2 − x

[B] (Nb,Ti,Ta,Zr,Fe)2 O6(OH,F)1 − y, where cations are listed approximately inorder of decreasing importance (Bambi et al., 2012; Chakhmouradianand Williams, 2004; Chakhmouradian and Zaitsev, 1999; Cordeiroet al., 2011; Hodgson and Le Bas, 1992; Hogarth, 1989; Hogarth et al.,2000; Lee et al., 2006; Torró et al., 2012; Zurevinski and Mitchell,2004). Hydrothermal and supergene alteration produces hydratedcation- and anion-deficient pyrochlores (potentially,with x approaching2 and y up to 1; Ercit et al., 1995) depleted in Na and F, but typicallyshowing elevated levels of Sr, Ba, Pb, REE or K relative to their magmaticprecursor (Chakhmouradian and Mitchell, 1998; Cordeiro et al., 2011;Lapin and Tolstov, 1995; Torró et al., 2012; Wall et al., 1996). Notuncommonly, these late-stage pyrochlores are also enriched in Si,but its structural role remains uncertain; their generalized formulacan be written as [A](Ba,Sr,REE,Pb,K,Ca,U,Th)Σ ≪ 2

[B] (Nb,Ti,Ta,Zr,Fe,Si)2(O,OH)6(OH,F)Σ ≪ 1 ⋅ zH2O.

Some pyrochlore crystals examined in the present work show onlysubtle compositional variation and no discernible zoning in BSE images(Fig. 5b), but the majority of samples contain strongly zoned grains(Fig. 5d–f) with a core depleted in Nb and F, but enriched in U and Taby a factor of 2–4 (locally, by one order of magnitude) with respect tothe rim (Fig. 6a, b; Table 2). The maximum Ta2O5 and UO2 contents(14.9 and 20.0 wt.%, respectively) were recorded in primary dolomitecarbonatite AMX1A. Titanium and Th levels are also elevated in somesamples, but do not correlate with the zoning pattern shown inFig. 5d–f. Most examined pyrochlores contain only a small A-site cationdeficiency (x≤ 0.3), indicating that theywere not affected by subsoliduscation leaching. One exception is weathered calcite carbonatite2007-009-37.9 (Fig. 5c), which contains Ca- and Na-deficient Ba-richstrontiopyrochlore (x = 1.3) similar to those described by Wall et al.(1996) and Chakhmouradian and Mitchell (1998).

Fig. 6. Compositional variation of pyrochlore expressed in atoms p

Variations in trace-element content, including the median andpercentile values, are given in Table 3. Note that, because LA-ICP-MS isessentially a “mini-bulk” technique and the size of U–Ta-rich cores isquite small, the reported variations in these elements are not asextensive as those determined with a 5-μm beam by WDS (Table 2).The greatest benefit of LA-ICP-MS for the current study is its capabilityto quantify the entire range of REE, which provide important petroge-netic information (Fig. 7). The chondrite-normalized REE patterns ofthe examined pyrochlores have an overall negative slope due to enrich-ment in light REE (LREE = La…Eu) and one-half of the 12 samplesanalyzed by LA-ICP-MS (i.e., 32 analyses) show a clear positive Euanomaly (Eu/Eu* = 1.1–7.2), which is observed in both U–Ta-richand “normal” compositions. In addition, ~75% of the data also show adistinctly subchondritic Y/Ho ratio (on average, 15.0 ± 4.2), whichtranslates into a pronounced negative Y anomaly in the median andlower-percentile patterns (Fig. 7a). The only three samples that give anear-chondritic Y/Ho ratio (on average, 30.6 ± 4.8) are CS2B, CM1Aand AMX3F, all of which represent incipient stages of pyrochlorereplacement by fersmite in dolomitized calcite carbonatites. Thus, weinterpret the absence of Y anomaly in these samples as the result ofhydrothermal overprint. The Nb/Ta and Th/U ratios are very variable,ranging from 1.8 and 0.1, respectively, in samples with a U–Ta-richcore to ~200 and 100, respectively, in homogeneous crystals (basedon the combined WDS and LA-ICP-MS data). The Zr/Hf ratio is muchmore consistent and approaches the chondritic value (36 ± 17).

In addition to the carbonatites, sub- to anhedral grains of pyrochloremeasuring 0.1–0.3 mm across were observed in one sample of fenite(AMX1B) and one sample of glimmerite (AM1A; Figs. 4f, 5g, h). Theformer contains a significant proportion of phlogopite (20–25%)replacing amphibole (60–65%) at the contact with dolomite–calciteveinlets (10–15%), whereas AM1A is mineralogically similar, butshows an inverse phlogopite-to-amphibole ratio. Based on the availabletextural evidence, there seems no doubt that sample AMX1B manifestsan incipient, and AM1A amore advanced, stage of “phlogopitization” bycarbonatite melts (fluids?). Pyrochlore in these rocks contains U + Taand exhibits the same type of zoning as described above (Fig. 6;Table 2).

er formula unit (apfu): (a) U vs. Ta; (b) F vs. Ta; (c) Th vs. Ta.

image of Fig.�6

Table 2Representative major-element compositions of pyrochlores from the Aley carbonatite complex.

Fenite Glimmerite Calcite carbonatite Cal-Dol crb Cal-Dol crb Dolomite carbonatites

wt.% 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Na2O 6.03 5.98 5.33 6.54 4.98 5.36 5.00 5.59 2.79 4.05 4.77 5.64 6.27 7.20 7.64 7.33 7.72 0.09CaO 13.23 15.03 10.90 13.08 12.83 12.70 14.82 15.54 15.74 16.07 16.84 8.99 10.27 12.22 12.86 13.24 14.43 2.06MnO n.d. n.d. n.d. n.d. 0.14 0.14 0.06 0.05 0.43 0.27 0.14 n.d. n.d. n.d. n.d. n.d. n.d. n.d.SrO 0.72 0.83 0.35 0.75 0.21 0.35 0.21 0.45 0.68 0.69 1.01 0.11 0.12 1.29 0.21 0.33 2.12 13.62La2O3 0.17 0.45 0.26 0.50 1.00 0.41 0.88 0.79 0.30 0.44 0.72 0.12 0.21 0.21 0.22 0.52 0.08 0.08Ce2O3 0.84 2.10 0.79 1.94 3.35 2.11 3.09 3.05 1.11 1.84 2.12 0.49 0.83 0.62 0.92 1.85 0.35 0.12Pr2O3 n.d. 0.29 n.d. 0.19 0.30 0.31 0.35 0.34 0.29 n.d. 0.31 n.d. n.d. n.d. 0.16 n.d. n.d. n.d.Nd2O3 0.17 0.63 0.41 0.49 0.55 0.54 0.82 0.50 n.d. 0.42 0.45 0.18 n.d. 0.18 0.19 0.49 0.20 0.11ThO2 2.43 3.91 3.70 4.31 7.60 11.06 6.24 2.70 8.57 4.03 0.26 2.76 2.43 0.32 1.30 3.97 0.10 n.d.UO2 7.81 n.d. 19.19 5.17 2.18 0.63 0.12 0.08 1.03 0.52 0.34 20.02 16.17 5.48 5.17 n.d. n.d. 0.94Fe2O3 0.10 0.12 0.18 0.08 0.52 0.27 0.28 0.24 1.63 0.53 0.56 1.71 0.60 0.11 0.26 0.07 0.09 0.52TiO2 4.83 5.16 11.45 6.39 6.69 6.53 6.46 5.06 2.81 3.74 3.06 7.55 6.83 2.58 2.98 4.19 2.04 2.92ZrO2 0.82 0.50 0.11 0.12 0.64 0.19 0.54 0.63 0.78 0.15 0.10 2.52 1.14 0.22 0.37 0.89 0.92 n.d.Nb2O5 52.29 61.29 40.39 55.64 48.03 52.60 56.48 61.37 53.91 60.99 65.38 31.19 39.95 57.59 60.04 61.60 67.16 68.77Ta2O5 7.73 1.12 4.82 1.16 7.56 3.94 1.66 0.84 8.72 3.98 3.08 14.93 11.46 7.38 4.43 2.45 0.14 1.25F 1.83 2.34 0.91 2.20 1.55 1.67 2.32 2.59 1.04 1.62 2.59 1.22 1.77 2.55 2.87 3.45 4.03 0.58-O = F2 0.77 0.99 0.38 0.93 0.65 0.70 0.98 1.09 0.44 0.68 1.09 0.51 0.75 1.07 1.21 1.45 1.70 0.24Total 98.23 98.76 98.41 97.63 97.48 98.11 98.35 98.73 99.39 98.66 100.64 96.92 97.30 97.04 98.25 98.93 98.28* 94.83**

Atoms per formula unit calculated on the basis of two B-site cationsNa 0.783 0.720 0.729 0.834 0.654 0.692 0.619 0.672 0.355 0.492 0.558 0.830 0.890 0.925 0.957 0.884 0.906 0.010Ca 0.950 0.999 0.823 0.922 0.932 0.906 1.014 1.032 1.107 1.078 1.088 0.731 0.806 0.868 0.890 0.883 0.936 0.130Mn – – – – 0.008 0.008 0.003 0.003 0.024 0.014 0.007 – – – – – – –

Sr 0.028 0.030 0.014 0.029 0.008 0.014 0.008 0.016 0.026 0.025 0.035 0.005 0.005 0.050 0.008 0.012 0.074 0.464La 0.004 0.010 0.007 0.012 0.025 0.010 0.021 0.018 0.007 0.010 0.016 0.003 0.006 0.005 0.005 0.012 0.002 0.002Ce 0.021 0.048 0.020 0.047 0.083 0.051 0.072 0.069 0.027 0.042 0.047 0.014 0.022 0.015 0.022 0.042 0.008 0.003Pr – 0.007 – 0.005 0.007 0.008 0.008 0.008 0.007 – 0.007 – – 0.004 – – – –

Nd 0.004 0.014 0.010 0.012 0.013 0.013 0.019 0.011 – 0.009 0.010 0.005 – 0.004 0.004 0.011 0.004 0.002Th 0.037 0.055 0.059 0.065 0.117 0.167 0.091 0.038 0.128 0.057 0.004 0.048 0.040 0.005 0.019 0.056 0.001 –

U 0.116 – 0.301 0.076 0.033 0.009 0.002 0.001 0.015 0.007 0.005 0.338 0.263 0.081 0.074 – – 0.012ΣA 1.943 1.883 1.963 2.002 1.880 1.878 1.857 1.868 1.696 1.734 1.777 1.974 2.032 1.957 1.979 1.900 1.931 0.715**Fe3+ 0.005 0.006 0.009 0.004 0.027 0.013 0.013 0.011 0.081 0.025 0.025 0.098 0.033 0.006 0.012 0.003 0.004 0.023Ti 0.243 0.241 0.607 0.316 0.341 0.327 0.310 0.236 0.139 0.176 0.139 0.431 0.376 0.129 0.145 0.196 0.093 0.129Zr 0.027 0.015 0.004 0.004 0.021 0.006 0.017 0.019 0.025 0.005 0.003 0.093 0.041 0.007 0.012 0.027 0.027 –

Nb 1.584 1.719 1.288 1.655 1.472 1.583 1.631 1.720 1.599 1.726 1.782 1.070 1.322 1.725 1.753 1.733 1.837 1.828Ta 0.141 0.019 0.092 0.021 0.139 0.071 0.029 0.014 0.156 0.068 0.051 0.308 0.228 0.133 0.078 0.041 0.002 0.020ΣB 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000 2.000* 2.000F 0.388 0.459 0.203 0.458 0.332 0.351 0.469 0.508 0.216 0.321 0.494 0.293 0.410 0.535 0.586 0.679 0.771 0.108

n.d.=not detected. * Total also includes 0.60 wt.% SiO2 (0.036 apfu Si). ** Total also includes 4.01 wt.% BaO (0.092 apfu Ba). Analyses: 1 AMX1B core; 2 AMX1B rim; 3 AM1A core; 4 AM1Arim; 5 CS2D core; 6–8 CS2D rim; 9 PH1E core; 10–11 PH1E rim; 12–14 AMX1A core; 15–17 AMX1A rim; 18 2007-009-37-9, secondary strontiopyrochlore.


5.2. Ferrocolumbite

Ferrocolumbite (also incorrectly referred in some sources as“niobite”), whose formula can be generalized as [A](Fe,Mn,Mg)[B](Nb,Ti,Ta)2O6, occurs in the Aley carbonatites as a primary phase, productof pyrochlore alteration and scarce discrete crystals in late-stage para-geneses. The former type is represented by euhedral black (opaque inthin section) platy oikocrysts with a pseudohexagonal habit andranging from a few tens of μm to 1.5 mm across. It has a submetallicluster and is visually very similar to magnetite, but has slightly weakerreflectance than the latter and contains abundant inclusions of carbon-ate minerals, apatite, ilmenite, zircon, monazite and thorite, which giveferrocolumbite a speckled appearance (Fig. 8a–e). Reliable identifica-tion of this mineral in hand specimen is problematic, given its opacityand intimate association with magnetite (Fig. 8a). Other mineralscommonly found in the same assemblage are apatite, phlogopite (orits alteration products), zircon and baddeleyite. In a few samples ofprimary (calcite-)dolomite carbonatite, ferrocolumbite is associatedwith, but crystallized somewhat later than, pyrochlore, as indicatedby their textural relations (Fig. 8f–h). Partial pseudomorphs offerrocolumbite after pyrochlore are rare and invariably also containfersmite (see below) developed later than, and at the expense of, theother two minerals. Late-stage discrete crystals of ferrocolumbite areplaty, devoid of any inclusions and often assembled into radiatingor subparallel clusters embedded in rhombohedral dolomite andassociated with rutile (Fig. 8i).

Most of the Aley columbite approaches the theoretical compositionFeNb2O6; it is poor inMg andMn (≤1.2 and 1.6 wt.% respective oxides),but shows appreciable variation with respect to Ti (0.9–4.6 wt.% TiO2)and Ta (0.1–5.1 wt.% Ta2O5). A few samples exhibit subtle zoning inBSE images (Fig. 8e). Appreciable levels of Ca and Th in some crystals(Table 4) most likely result from contamination, given the intimateassociation of this mineral with pyrochlore, fersmite and thorite. Thelate-stage variety shows a greater compositional variation with respectto Mg, Mn and Ti (up to 5.8, 2.6 and 5.6 wt.% respective oxides) atconsistently low Ta contents (b0.4 wt.% Ta2O5). The most Mg-rich com-positions approach the ferrocolumbite–magnocolumbite borderline(Table 4).

As can be expected from the structural differences betweenpyrochlore and ferrocolumbite (e.g., Ercit et al., 1995; Tarantino et al.,2010), the latter contains much lower levels of large cations, such asSr, Pb, Th and U (Table 5). Similar to the pyrochlores, the Nb/Ta andTh/U ratios in the examined ferrocolumbite are highly variable (90–1700 and10–47),whereas the Zr/Hf ratio is essentially uniform (33±9).

5.3. Fersmite

Fersmite, [A](Ca,Th,REE)[B](Nb,Ti,Ta)2(O,OH)6, is by far the mostcommon Nb phase at Aley (65% of the samples; Appendix 1), occurringalmost exclusively in foliated fine-grained to inequigranular dolomitecarbonatites developed metasomatically at the expense of earlier-crys-tallized types. Several samples comprising both fresh and dolomitized

Table 3Trace-element composition of pyrochlore from the Aley carbonatite complex.

ppm Median (n = 55) P10 P25 P75 P90

Mn 565 96 338 956 2415Sr 2828 403 1639 5276 7384Ba 262 36 127 1092 3150Pb 987 493 634 1233 1667Sc 41 13 22 135 320Y 368 192 251 1058 3852La 1942 650 1072 2881 3840Ce 8464 3997 4770 11,741 14,313Pr 1040 452 699 1280 1561Nd 3955 1657 2578 4982 6639Sm 619 284 444 943 1449Eu 205 110 141 474 1005Gd 345 169 233 531 1238Tb 42 22 29 70 227Dy 180 95 123 276 829Ho 23 12.3 17.3 52 156Er 42 21 30 87 336Tm 4.7 2.3 3.2 10.0 41.0Yb 24 10.8 16.7 57 228Lu 1.7 0.8 1.2 6.1 26.0ΣLREE⁎ 17,065 7745 9946 21,248 28,376ΣHREE⁎⁎ 1013 533 712 2160 5089Zr 7645 2290 3940 16,857 25,219Hf 255 66 148 452 749Th 51,342 13,447 30,775 65,260 78,753U 16,410 2172 3521 44,500 107,950As 30 16.8 22 39 61Ta 35,809 5772 11,683 60,662 102,383

⁎ LREE = La…Eu.⁎⁎ HREE = Gd…Lu + Y.


rock contain pyrochlore or ferrocolumbite in unaltered areas of the sam-ple and fersmite in its metasomatized part. The greatest proportion offersmite is represented by pseudomorphs after pyrochlore andferrocolumbite of hexagonal shape (Fig. 9). The majority of these pseu-domorphs are composed entirely of fersmite and other alteration prod-ucts, and only a few contain relict fragments of the precursor mineral(Fig. 9a). In some cases, the shape of the precursor crystal is well-

Fig. 7. Chondrite-normalized diagram showing variations in REE contents in pyrochlore(a) and fersmite (b); only selected percentile and median values are shown.

preserved and the replaced mineral can be identified based on its hostparagenesis or inclusion pattern (Fig. 9b–d), but themajority of pseudo-morphs are cavernous or atoll-like and contain pockets infilled withsecondary minerals (Fig. 9e, f). In addition to apatite inherited from theprecursor phase, the pseudomorphs commonly enclose fine-grained ag-gregates of secondary chlorite, quartz, or dolomite associated with sub-to euhedral crystals of monazite (Fig. 9g). Muscovite, thorite and thoria-nite are less common, and goyazite [(Sr,Ca,Ba)Al3(PO4)2(OH)5⋅H2O] wasobserved in a single sample (AL-64a-10). The polyhedral shape of theprecursor crystal is not always preserved, which is particularly thecase where the rock was affected by fracturing and hydrothermalreworking (Fig. 9h).

The majority of fersmite pseudomorphs after pyrochlore arecomposed of radiating to subparallel clusters of light- to honey-yellowacicular crystals projecting into interior pockets or into the surroundingdolomite (Figs. 9f, 10a–d). Individual needles range from b10 to 150 μmin length at a width of 1–5 μm. Locally, this acicular fersmite grades intoa massive variety with indiscernible grain boundaries (Fig. 10e, f), orcoarser prismatic crystals in the center of a pseudomorph. Much lesscommon are pseudomorphs lacking a distinct polyhedral shape andcomposed predominantly of sector-zoned prismatic crystals measuringup to 200 × 50 μm in size (Fig. 10g, 10 h). Fersmite was also observed todevelop after primary ferrocolumbite and ferrocolumbite pseudo-morphs after pyrochlore (Fig. 10i, j).Manydolomite carbonatites affectedby hydrothermal recrystallization (~15% of the samples) containdiscrete acicular crystals of fersmite (and their aggregates) embeddedin late-stage chlorite or dolomite (Fig. 10k).

Variations in average atomic number (AZ) clearly visible in someBSE images of fersmite (Figs. 9h, 10e–h) arise predominantly from aheterogeneous distribution of Ca, Th, Nb and Ti (Table 6). Intragranularcompositional variations could be measured only for the prismaticvariety of fersmite, and show that the high-AZ zones contain up to7.2 wt.% ThO2 and 4.5 wt.% TiO2, but are depleted in Ca and Nb relativeto the low-AZ zones. There is a reasonable correlation between theseelements (Fig. 11a), suggesting that the principal substitution schemein the Aley fersmite is Ca2+ + 2(Nb,Ta)5+ = Th4+ + 2Ti4+. The Ulevels are generally low (b1400 ppm), whereas light lanthanides reachsignificant concentrations (7.0 wt.% LREE2O3) in some samples, butshow only a weak correlation with other major elements [Ca2+ +(Nb,Ta)5+ = REE3+ + Ti4+; R2 = 0.36], implying that a variety ofsubstitution mechanisms are involved in the incorporation of REE.Most of the structural formulae, recalculated on the basis of two B-sitecations, have a slight deficiency of total positive charge (on average,11.75 ± 0.18) relative to the ideal value of +12, implying partial occu-pancy of the anion site by (OH)− groups, which is supported by thepresence of O–H stretching Raman modes (3430–3630 cm−1) infersmite spectra. Our data also show that fersmite lacks detectable Naand F, i.e. the formula commonly given for this mineral in the literature(e.g., Mäder, 1986) should be revised.

Trace-element analyses were done only for fersmite pseudomorphscontaining visually continuous patches N40 μm across. Given theextreme textural complexity of the pseudomorphs and “mini-bulk”nature of LA-ICP-MS,we cannot rule out the presence of submicroscopicinclusions in the analyzed material, which possibly explains discrepan-cies in the maximum concentrations of some of the elements analyzedby both WDS and LA-ICP-MS (cf. REE, Zr and Ta values in Tables 6 and7). However, good consistency of certain element ratios across thedataset (see below) implies that the obtained trace-element data arereliable overall. The chondrite-normalized REE patterns are distinctlydifferent from those of pyrochlore (Fig. 7) owing to enrichment inmid-range REE and virtual absence of Eu anomalies (average Eu/Eu* =1.3 ± 0.2). Also in contrast to pyrochlore, the Y/Ho ratio is only slightlysubchondritic (23.8 ± 3.1). The Nb/Ta and Th/U ratios are extremelyvariable (25–3400 and 6–1040, respectively), whereas the Zr/Hf ratiois consistent and approaches the chondritic value (39 ± 11). The WDSanalyses give a comparable range of Nb/Ta values (9–1600). In terms

image of Fig.�7

Fig. 8. Ferrocolumbite (Cb) occurrence, morphology and zoning; BSE images. (a) Pseudohexagonal oikocrysts from a cumulate unit enriched in magnetite and apatite; (b) oikocrysts andapatite prisms enclosed in zircon; (c) ferrocolumbite in dolomitized calcite carbonatite; (d) fragmented oikocryst in dolomite–calcite carbonatite next to a disaggregated cumulate layer;(e) detail of (d) showing a weakly zoned ferrocolumbite oikocryst; (f) discrete ferrocolumbite and pyrochlore crystals from an apatite-rich lens; (g, h) ferrocolumbite mantling zonedpyrochlore; (i) a spray of late-stage Mg-rich ferrocolumbite associated with rutile (Rt).


of their Nb/Ta and Zr/Hf ratios, the compositions of fersmite overlapwith both pyrochlore and columbite data (Fig. 11b).

5.4. Euxenite

Euxenite, [A](REE,Ca,U,Th)[B](Nb,Ti,Ta)2O6, was identified in twosamples of hydrothermally reworked dolomite carbonatite (Appendix

1), where this mineral occurs as abundant subparallel and randomclusters of dark brown bladed crystals reaching 200 × 15 μm in size(Fig. 10 l). The crystals are embedded in chlorite; the same samplescontain fersmite pseudomorphs after pyrochlore. In contrast tofersmite, euxenite contains high levels of Y and heavy lanthanides(HREE), comparable levels of Th and U in the A site, as well as of Tiand Nb in the B site (Table 6). The mineral is metamict and does not

image of Fig.�8

Table 4Representative major-element compositions of ferrocolumbite from the Aley carbonatitecomplex.

wt.% 1 2 3 4 5 6 7 8 9

MgO n.d. n.d. 0.52 1.15 n.d. 0.09 0.05 n.d. 5.74CaO 0.44 0.27 0.26 0.22 2.12 0.35 0.34 1.79 0.09MnO 1.47 0.91 0.98 0.84 1.56 0.36 0.44 2.17 0.48FeO 20.25 20.52 19.10 17.29 17.85 20.61 20.92 16.31 12.35ThO2 0.58 0.36 n.d. 0.23 0.31 0.95 1.63 0.16 n.d.SiO2 0.15 0.11 n.d. 0.10 0.11 0.08 0.11 0.23 n.d.TiO2 3.85 4.02 3.57 3.55 4.55 0.92 0.97 1.33 3.03ZrO2 0.46 0.32 0.05 n.d. 0.42 2.49 2.07 0.12 n.d.Nb2O5 73.17 73.45 75.95 70.27 72.55 72.90 73.04 76.09 80.31Ta2O5 0.27 1.22 0.23 5.10 0.28 0.96 0.79 0.14 n.d.Total 100.64 101.18 100.66 98.75 99.75 99.72 100.36 98.64* 102.00

Atoms per formula unit calculated on the basis of six oxygen atomsMg – – 0.043 0.097 – 0.007 0.004 – 0.448Ca 0.026 0.016 0.015 0.013 0.126 0.021 0.021 0.108 0.005Mn 0.069 0.043 0.046 0.040 0.073 0.017 0.021 0.104 0.021Fe2+ 0.938 0.946 0.877 0.821 0.827 0.980 0.992 0.770 0.540Th 0.007 0.005 – 0.003 0.004 0.012 0.021 0.002 –

ΣA 1.040 1.010 0.981 0.974 1.030 1.037 1.059 0.988* 1.014Si 0.008 0.006 – 0.005 0.006 0.005 0.006 0.013 –

Ti 0.160 0.167 0.148 0.152 0.190 0.039 0.041 0.056 0.119Zr 0.012 0.009 0.001 – 0.011 0.069 0.057 0.003 –

Nb 1.832 1.831 1.885 1.804 1.817 1.874 1.872 1.942 1.899Ta 0.004 0.018 0.003 0.079 0.004 0.015 0.012 0.002 –

ΣB 2.016 2.031 2.037 2.040 2.028 2.002 1.988 2.016 2.018

n.d. = not detected. * Total also includes 0.30 wt.% UO2 (0.004 apfu U). Analyses: 1–3typical discrete crystals (PH1E, PH1E, 2007-006-37.6); 4 Mg–Ta-rich, AL-68b-10;5 Ca–Mn–Ti-rich, PH1E; 6 Zr-rich, AL-64b; 7 Th-rich, AL-64b; 8 U-rich, AMX2D; 9 lateMg-rich, VAR-09-01.


produce a recognizable Raman spectrum; however, the dearth of LREEand elevated U in its composition suggest that it is euxenite, ratherthan an aeschynite-type phase (e.g., Ercit, 2005; Ewing, 1976).

6. Other niobium hosts

To constrain the distribution of Nb in the Aley complex, we alsoanalyzed by LA-ICP-MS all major and accessory minerals from thecarbonatites and associated rocks. As can be expected, elevated concen-trations of Nb are found in Ti and Zr phases,which, for themost part, areconfined to the silicate lithologies (see below). Among rock-formingminerals, phlogopite, amphibole and magnetite are the only significantNb hosts; clinopyroxenes contain very low levels of this element(b20 ppm in ~80% of analyses), while carbonates and phosphates areessentially devoid of detectable Nb.

6.1. Titanium and zirconium minerals

A small proportion of Nb in the carbonatites is hosted in rutile,ilmenite, baddeleyite and zircon. The identity of rutile was confirmedby Raman microspectroscopy based on the presence of signals at 142–

Table 5Trace-element composition of ferrocolumbite from the Aley carbonatite complex.


Mn 4787 3439 4471 6060 7544Sr 87 41 53 147 216Pb 129 106 110 151 359Sc 35 9 15 197 264V 257 15 83 958 3127Zr 5723 3043 3981 18,054 19,504Hf 251 111 120 441 497Th 4153 2757 3465 6248 11,938U 224 70 126 338 1165As 23 0 11 28 33Ta 3100 295 304 4081 5051

143, 245–250, 440–445, 612–614 and ~810 cm−1 (Li et al., 2007);none of the other TiO2 polymorphswere detected. Themajority of rutileis present as extremely fine-grained reticulate aggregates probablydeveloped at the expense of ilmenite lamellae in pseudomorphs aftermagnetite (Fig. 12a). Discrete crystals of rutile b1 mm in length ofanhedral to euhedral habit (depending on their paragenesis) occur inappreciable concentrations (up to 4 vol.%) in somedolomite carbonatitesaffected by chloritization and silicification. The crystals are commonlyassembled in aggregates of different morphology, ranging from trellis-like to “cockscomb” to granular (Figs. 8i, 12b, c), and typically exhibitpatchy or speckled zoning due to extreme variations in Nb contentwithin the same crystal (1.9–19.4 wt.% Nb2O5 and 0–0.7 wt.% Ta2O5).The incorporation of Nb (±Ta) in the structure is compensated by bothFe2+ and Fe3+, as indicated by correlation diagrams (e.g., Fig. 12d); Mgand Mn are below their detections limits by WDS (b300 and 700 ppm,respectively).

In comparison with the carbonatites, rutile is more common inthe silicate rocks (Appendix 1), but shows similarly variable Nb at lowTa levels (0.8–24.6 and ≤0.15 wt.% respective oxides). The Ta concen-trations, determined by LA-ICP-MS in four samples from differentsilicate rocks, are consistent with the WDS data for the same samples(4.5–570 ppm); the Zr and Hf concentrations are consistently low(b500 and 15 ppm, respectively). The average Nb/Ta and Zr/Hf ratiosare 141±69and27±13, respectively. Because someof themetasomaticsilicate rocks (e.g., AM2C) contain ca. 1 vol.% of rutilewith ~9wt.%Nb2O5,this mineral undoubtedly accounts for the bulk of the whole-rock Nbbudget in these cases.

Similar to rutile, ilmenite is common as lamellae in magnetite incarbonatites and was observed as discrete platy crystals b0.3 mm inlength only in two calcite carbonatite samples (CS2A andCS2E). The lim-ited LA-ICP-MS data available for these samples give 120–17,600 ppmNb and 38–1020 ppm Ta. Ilmenite is less common than rutile inthe fenites and contains Nb and Ta levels comparable to those in thecarbonatite samples.

Titanite is rare in the fenites andwas not observed in the carbonatites.Textural evidence suggests that some of the rutile and ilmenite in themetasomatic silicate rocks (e.g., AMX3C and AMX3D) developed atthe expense of titanite, possibly due to its reaction with carbonatite-derived fluids. The limited LA-ICP-MS data give 1700–2150 ppm Nband ~50 ppm Ta.

Zircon is relatively common in carbonatites (especially in cumulateunits) as euhedral crystals up to 1 mm associated with ferrocolumbite,pyrochlore, apatite and magnetite (Figs. 4c, 8b), but was not observedin any of the silicate rocks. The Nb content of zircon is variable, butlow (9–108 ppm); 75% of the LA-ICP-MS data give b50 ppm Nb. TheNb/Ta ratio is extremely variable (7–126), whereas the Zr/Hf ratio isconsistent and invariably superchondritic (88 ± 13). Baddeleyite isfound in relatively few samples (Appendix 1) as acicular crystals up to0.2 mm in length. In some cases, the mineral is enveloped in zircon,indicating an increase in silica activity early in the evolution of its hostrock. Baddeleyite crystals are typically too small for trace-elementanalysis; limited LA-ICP-MS data for sample CD3D give 2300–9200 ppm Nb and 1000–3700 ppm Ta at Nb/Ta = 2.3–2.5 (Zr/Hf =78–83). These values are typical for primary baddeleyite fromcarbonatites (Chakhmouradian, 2006).

6.2. Ferromagnesian rock-forming minerals

Magnetite is an important constituent of some cumulate rocks,where its content locally reaches 60 vol.%. Our trace-element dataindicate that its Nb and Ta abundances do not exceed 309 and128 ppm, respectively, with 70% of analyses showing b75 ppm Nb and6 ppm Ta (Nb/Ta = 6.3 ± 4.5). These results are consistent with verylimited published LA-ICP-MS data on material from other localities(Reguir et al., 2008). Simple calculations show that, even at the maxi-mum Nb content and highest recorded magnetite content in the rock

Fig. 9. Fersmite (Frs) occurrence and morphology; BSE images. (a) Incomplete pseudomorph of fersmite after zoned pyrochlore (note that the U–Ta-rich core was not affected byfersmitization); (b) fersmite pseudomorph after oscillatory zoned pyrochlore with a ferrocolumbite mantle (similar to that shown in Fig. 8 h; Bd = baddeleyite); (c, d) complete pseu-domorphs after pyrochlore and ferrocolumbite, respectively (note characteristic crystal habits, intimate association with apatite and inclusion patterns in both images); (e) atoll-textured pseudomorphs after pyrochlore; (f) atoll-textured pseudomorph after an unknown mineral infilled with muscovite (Ms); (g) atoll-textured pseudomorph after an unknownmineral infilled with monazite (Mnz); (h) pseudomorph after an unknown mineral with fersmite crystals projecting into the surrounding late-stage dolomite.


(300 ppm and 60 vol.%, respectively), this mineral will contributeonly a small fraction of the whole-rock Nb budget (i.e., 2% at 1.5 vol.%of ferrocolumbite in the same assemblage, and 1% at 3.0 vol.% offerrocolumbite).

An appreciable proportion of Nb in the fenites and glimmerites ishosted inphlogopite and sodic(-calcic) amphiboles. Because carbonatiteshave a much lower modal content of ferromagnesian silicates(e.g., Fig. 4a–c), their relative contribution to their whole-rock Nb

budget is far less significant. The Nb and Ta contents of both phlogopiteand amphiboles are comparable in the silicate and carbonate rocks.

Phlogopite and fluorphlogopite contain up to 660 ppm Nb and45 ppm Ta, but 96% of the LA-ICP-MS analyses have b430 ppm Nb andb34 ppm Ta, forming a fairly symmetrical distribution histogram witha median at 252 ppm Nb (Fig. 13a) and 6.6 ppm Ta (not shown). TheNb values are typical of carbonatitic micas (Reguir et al., 2009), whereasthe Ta values are noticeably higher in the Aley samples. The Nb/Ta ratios

image of Fig.�9


are highly variable in both carbonatites and silicate rocks, ranging fromsubchondritic values (4–5) in somedolomite carbonatites and fenites to590 in calcite carbonatite AMX2D. The Zr/Hf ratios are much less vari-able, approaching the chondritic value in both carbonatites (36 ± 26)and metasomatic silicate rocks (31 ± 18).

Amphiboles contain up to 216ppmNband 2.4 ppmTa, although 94%of the analyses show ≤150 ppm Nb and 1.3 ppm Ta (Fig. 13b). Thisrange and the median values (34 ppm Nb and 0.3 ppm Ta) are wellwithin the range identified for carbonatitic amphiboles by Reguir et al.(2012). In common with the phlogopites, the Nb/Ta ratios are highlyvariable, ranging from near-chondritic values to as much as 1090 (!)in calcite carbonatite AMX2D; the Zr/Hf values are consistently chon-dritic (38 ± 16 in the carbonatites and 39 ± 16 in the silicate rocks).

7. Whole-rock geochemistry of the carbonatites

The least variable major components in the whole-rock analysesare those concentrated in the rock-forming carbonates (CaO, MnO andSr in the calcite carbonatites; MgO, CaO and MnO in the dolomitecarbonatites). Variations in themodal proportion of phlogopite, amphi-bole and apatite in these rocks are reflected in large (one order of

Fig. 10. Morphology and zoning of fersmite (Frs) and euxenite (Eux); all except (a) and (b)(b) acicular crystals projecting into a cavity inside a pseudomorph (image in secondary electron(e) complexly textured pseudomorph after an unknownmineral consisting of acicular andmaswith thorianite (Th); (h) poorly preserved pseudomorph composed of sector-zoned prismatic ccomplete replacement of ferrocolumbite by prismatic fersmite; (k) discrete acicular crystals ofstage chlorite.

magnitude or more) variations in the content of SiO2, Al2O3, Na2O,K2O and P2O5 (Table 8). The two groups of carbonatites exhibit similartrace-element characteristics, including overlapping ranges of trace-element abundances and normalized patterns of trace-element distri-bution (Table 8, Fig. 14a). Overall, the calcite carbonatites are enrichedin K, Rb, Sr and Ba, but show somewhat lower Th, U, Nb and Ta levelsin comparison with the majority of the dolomite carbonatites. Theavailable WDS and LA-ICP-MS data suggest that the enrichment of theformer rocks in K and Rb stems from the greater modal abundance ofphlogopite (8.5–9.7 wt.% K2O; 30–440 ppm Rb), whereas the differencein Sr and Ba budget can be explained by the overall higher levels of theseelements in the rock-forming calcite (5400–10,200 Sr, 420–1200 Ba) incomparisonwith dolomite (23–8500 and 3–140 ppm, respectively). Thegenerally higher Th, U, Nb and Ta contents in the dolomite carbonatites(Fig. 14a) are related to the greater proportion of complex Nb oxides(mostly, fersmite) in these rocks.

Mineralized varieties of carbonatites (i.e., those containing N0.5wt.%Nb2O5 and abundant Nb phases readily detectable in thin section) aregeochemically similar to the rest of the samples (Table 8). However, inaddition to their enrichment in Nb and in some cases, Ta, most of themineralized rocks tend to show higher levels of HREE (Fig. 14b). Their

are BSE images. (a) Acicular crystals projecting into dolomite (dark-field illumination);s); (c–d) pseudomorphs after unknownminerals composed of radiating acicular crystals;sive fersmite andmonazite; (f) detail of (e); (g) sector-zoned prismatic crystals associatedrystals associated with monazite; (i) partial pseudomorph after ferrocolumbite; (j) nearlyfersmite in late-stage chlorite; (l) discrete acicular crystals of euxenite embedded in late-

image of Fig.�10

Fig. 10 (continued).


primitive-mantle-normalized trace-element patterns exhibit a promi-nent trough at U owing to their generally higher Th/U ratios (7–306)in comparison with the overwhelming majority of low-Nb samples(0.3–169). The much higher P2O5, Zr and Hf contents in calcitecarbonatite AL-64b-10 stem from the abundance of apatite and zirconin this sample (Fig. 8b). The average P2O5, Zr and Hf values arealso higher in fersmite-rich dolomite carbonatites relative to theirnon-mineralized counterparts (Table 8), although thehighestmeasuredcontents of these elements (11.6 wt.%, 179 and 3.4 ppm, respectively)were actually observed in sampleswith fairly lowNb levels (b800 ppm).

All examined rock types exhibit strongenrichment in LREE relative toHREE and a prominent negative Sc anomaly (Fig. 14). The chondrite-normalized La/Yb ratios overlap among the four sample groups, rangingfrom 25 to 100 in 94% of the samples. The Y/Ho ratio is strikingly consis-tent and approaches the chondritic value (26 ± 3 for all 56 samples).The Zr/Hf ratios show more variation (Table 8), but cluster close tothe chondritic value (40 ± 14 for all samples) and averages measuredfor the Nb phases and ferromagnesian silicates (see Sections 5 and 6).The Nb/Ta and Th/U ratios are extremely variable in and betweenthe two major carbonatite types. There is no meaningful correlationbetween either Nb and Ta, or Th and U (R2 ≪ 0.1), whereas Zr andHf values form a positive covariation trend with R2 = 0.93.

The fenites and glimmerites contain very low Nb levels (b1300; onaverage, 726 ± 327 ppm) and are generally devoid of minerals amena-ble to Nb recovery. The bulk of thewhole-rock Nb budget in these rocks

is accounted for by rutile, phlogopite and, to a lesser extent, amphiboles(see Sections 5 and6). Representative analyses of themetasomatic unitsare provided in Appendix 4.

8. Stable-isotope composition of the carbonatites

The C–O isotope compositions of carbonates from the calcite anddolomite carbonatites are expressed in the conventional δ notation inFig. 15. Primary calcite gives a range of δ13CV-PDB and δ18OV-SMOW valuestypical of igneous carbonates, i.e.−5 to−6 and 8 to 11‰, respectively(e.g., Demény et al., 2004; Taylor et al., 1967). Carbonate samples from afew primary dolomite carbonatites show a similar range of composi-tions, whereas the majority of dolomite carbonatites are enriched in18O and, to a lesser extent, 13C. Dolomite carbonatites occurring asdikes in the carbonate sedimentary rocks of the Kechika formationshow the highest δ13CV-PDB and δ18OV-SMOW ratios (−2 to −2.5 and15 to 16‰, respectively). Although the stable-isotope variation ofthe Kechika and other wall-rock units is unknown, it is reasonableto assume that it is similar to other Cambrian sedimentary rocks,i.e. δ13CV-PDB N −2‰ and δ18OV-SMOW = 21–24‰ (Veizer et al., 1999).The average values, calculated from 150 analyses provided by Losh(1997), Naipauer et al. (2005) and Sial et al. (2008) for Lower Paleozoiccarbonate units making up the foreland of the North and SouthAmerican Cordillera, are δ13CV-PDB = −0.9 ± 1.5‰ and δ18OV-SMOW =22.2 ± 3.5‰, i.e. within the range identified by Veizer et al. (1999)

image of Fig.�10

Table 6Representative major-element compositions of fersmite (1–7) and euxenite (8) fromdolomite carbonatites, Aley complex.

wt.% 1 2 3 4 5 6 7 8

MgO 0.20 n.d. n.d. n.d. n.d. 0.06 n.d. NaO 0.23CaO 15.07 16.62 16.20 15.85 14.33 13.67 15.77 CaO 2.80Y2O3 n.d. n.d. n.d. 0.63 n.d. n.d. n.d. Y2O3 14.69La2O3 n.d. n.d. n.d. n.d. 0.24 n.d. 0.19 Ce2O3 0.17Ce2O3 0.46 n.d. n.d. 0.12 2.59 n.d. 0.96 Gd2O3 2.84Pr2O3 0.15 n.d. n.d. n.d. 0.46 n.d. n.d. Dy2O3 3.18Nd2O3 0.96 n.d. n.d. 0.37 2.70 n.d. 0.25 Ho2O3 0.09Sm2O3 0.73 n.d. n.d. n.d. 0.41 n.d. n.d. Er2O3 1.71ThO2 2.22 0.38 n.d. 0.14 0.53 7.20 2.52 Yb2O3 0.73UO2 n.d. n.d. n.d. 0.21 n.d. n.d. n.d. Lu2O3 0.41

ThO2 3.87Fe2O3 0.17 n.d. 0.15 0.06 0.04 0.15 0.53 UO2 4.51SiO2 0.40 n.d. n.d. 0.07 n.d. 0.25 0.15 Fe2O3 0.42TiO2 3.90 0.46 1.03 1.44 4.47 2.27 2.49 SiO2 0.17ZrO2 n.d. n.d. n.d. n.d. n.d. 0.76 0.57 TiO2 20.02Nb2O5 72.99 80.63 73.33 75.61 73.55 72.39 75.14 Nb2O5 35.45Ta2O5 1.27 2.06 7.12 5.79 0.33 0.57 0.39 Ta2O5 7.03Total 98.64* 100.15 97.83 100.29 99.65 97.32 98.96 Total 98.32

Atoms per formula unit calculated on the basis of two B-site cationsCa 0.871 0.953 0.965 0.919 0.836 0.827 0.919 Na 0.027Y – – – 0.018 – – – Ca 0.179La – – – – 0.005 – 0.004 Y 0.467Ce 0.009 – – 0.002 0.052 – 0.019 Ce 0.004Pr 0.003 – – – 0.009 – – Gd 0.056Nd 0.019 – – 0.007 0.052 – 0.005 Dy 0.061Sm 0.014 – – – 0.008 – – Ho 0.002Th 0.027 0.005 – 0.002 0.007 0.093 0.031 Er 0.032U – – – 0.003 – – – Yb 0.013ΣA 0.948* 0.958 0.965 0.951 0.969 0.920 0.978 Lu 0.007

Th 0.053Mg 0.016 – – – – 0.005 – U 0.060Fe 0.007 – 0.006 0.002 0.002 0.006 0.022 ΣA 0.961Si 0.021 – – 0.004 – 0.014 0.008Ti 0.158 0.018 0.043 0.059 0.183 0.096 0.102 Fe 0.019Zr – – – – – 0.021 0.015 Si 0.010Nb 1.779 1.952 1.843 1.850 1.810 1.849 1.847 Ti 0.900Ta 0.019 0.030 0.108 0.085 0.005 0.009 0.006 Nb 0.957ΣB 2.000 2.000 2.000 2.000 2.000 2.000 2.000 Ta 0.114

ΣB 2.000

n.d. = not detected. * Total also includes 0.12 wt.% MnO (0.005 apfu Mn). Analyses: 12007-011-18; 2 2006-006-77.7; 3 AL-72; 4 AMX3B; 5 2010-034; 6 2007-005-39.1; 7AMX2D; 8 AMX3B.

Table 7Trace-element composition of fersmite from the Aley carbonatite complex.


Mn 517 300 403 986 2767Sr 226 58 119 1474 2907Ba 636 121 205 3959 5621Pb 639 372 523 851 1063Sc 41 18 20 53 78V 101 47 63 124 161Y 6763 1764 4296 14,137 18,274La 342 215 266 491 680Ce 3131 1624 1768 4103 4645Pr 650 503 534 798 947Nd 3917 2563 3476 4519 5323Sm 1654 688 1253 2473 3192Eu 803 238 579 1247 1648Gd 1844 582 1201 3566 4557Tb 306 85 190 690 891Dy 1605 462 990 3920 4781Ho 260 78 159 680 795Er 554 175 331 1397 1536Tm 61 22 36 151 171Yb 322 122 185 722 886Lu 31 15 19 75 94ΣLREE 10,728 8555 9663 12,393 13,785ΣHREE 4885 1544 3141 11,306 13,605Zr 6375 663 3024 12,728 17,405Hf 231 16 67 304 469Th 17,246 9837 14,957 29,805 41,983U 512 19 77 1456 2410As 35 24 31 48 73Ta 1540 319 856 7995 9537


and appreciably different from the composition of primary carbonatesfrom the Aley complex. The gently sloping trend of carbonatite valuesis therefore interpreted to indicate isotopic re-equilibration of igneouscalcite and dolomite with crustal fluids enriched in CO2, i.e. circulated

Fig. 11. Compositional variation of fersmite. (a) Th + 2Ti vs. Ca+ 2(Nb,Ta), expressed in apfu.the Nb/Ta scale is logarithmic, but the Zr/Hf scale is not).

through, and equilibrated with, the dolostones. Although assimilationof carbonate wall-rocks would also produce a trend of heavy-isotopeenrichment (Demény et al., 2004),we can confidently rule this explana-tion out because (1) there is no structural or petrographic evidence formassive dolostone assimilation, which would be required to increasethe primary isotopic values by several per mil, and (2) both calciteand dolomite carbonatites would be expected to have been equallyaffected by assimilation, which is clearly not the case (Fig. 15).

9. Discussion and conclusions

9.1. Evolution of Nb mineralization in the Aley deposit

The available petrographic evidence indicates that the evolution ofNb mineralization at Aley commenced with the crystallization of U–Ta-rich pyrochlore in some primary (calcite-) dolomite carbonatites, as

(b) Zr/Hf vs. Nb/Ta; the composition of other Nb hosts is shown for comparison (note that

image of Fig.�11

Fig. 12. Morphology, zoning and composition of rutile (Rt) from carbonatites; (a–c) BSE images. (a) Rutile-quartz aggregate (pseudomorph after an unknown mineral?) associatedwith late-stage quartz, rhombohedral dolomite, monazite and chlorite; (b) a trellis-like cluster of euhedral rutile crystals; (c) anhedral rutile crystals developed interstitially with respectto apatite; light-colored areas in (b) and (c) correspond to Nb-rich compositions. (d) Correlation between the FeO and Nb2O5 contents (wt.%) in rutile (dashed line); solid lines indicatetwo principal substitution mechanisms, where the incorporation of Nb in the rutile structure is compensated by Fe2+ and Fe3+.


well as locally and on a far smaller scale, in silicate rocks metasomatizedby carbonatites. This stage was followed by the deposition of nearlystoichiometric pyrochlore either as discrete crystals (e.g., Figs. 4b, 5b),or as overgrowths on U–Ta-rich cores (Figs. 5d–f). This zoning pattern,common in primary magmatic pyrochlore (e.g., Chakhmouradian andWilliams, 2004; Lee et al., 2006), has been interpreted in terms ofdecreasing Ta availability and rising F activity in a carbonatitic melt(Kjarsgaard and Mitchell, 2008). Our data (Fig. 6b) are in accord withthis interpretation. Note also that apatite inclusions (2.9–4.3 wt.% F)are common in stoichiometric pyrochlore, but not U–Ta-rich cores(cf. Fig. 5b and d). The paragenetic association of U–Ta-rich pyrochlore(0.2–3.1 wt.% F) with phlogopite (0.9–6.9 wt.% F) and, in some cases,amphiboles (1.0–1.6 wt.% F) suggests that the increase in a(F–) couldbe related to the cessation of crystallization of the ferromagnesiansilicates early in the evolution of carbonatitic magma, and prior to thedeposition of low-U–Ta pyrochlore (1.8–5.4 wt.% F) and apatite. Of par-ticular note is the occurrence of cation-deficient strontiopyrochlore(Fig. 5c) in displaced blocks of apparently weathered carbonatite(Fig. 3c), implying that parts of the Aley deposit were exposed prior to

Fig. 13.Histogram of Nb distribution in rock-forming silicates in carbonaties andmetasomatic roand amphibole (b).

the Laramide deformation and developed a lateritic profile, most ofwhich was subsequently removed by erosion.

Ferrocolumbite is a late magmatic to postmagmatic phase formingdiscrete oikocrysts, as well as overgrowths on, and pseudomorphsafter, pyrochlores (Fig. 8). The replacement would clearly have toinvolve a reducing acidic fluid facilitating the removal of Na, Ca, OHand F and capable of transporting Fe2+:

NaCaNb2O6ðFyOH1−yÞ þ Fe2þ þ ð1−yÞHþ⇔FeNb2O6 þ Naþ

þ Ca2þ þ yF− þ ð1−yÞH2O ð1Þ

Mass-balance calculations show that in extreme cases (e.g., calcitecarbonatite 2007-006-37.6 containing 7270 ppm Nb), the replacementof all pyrochlore (69.9 wt.% Nb2O5; 5.2 wt.% F) by ferrocolumbitewould have resulted in the release of F amount equivalent to 1.6 g offluorite per 1 kg of rock. The paucity of fluorite in the examined rocks(Section 4) implies that the concentration of F− in the fluid remainedbelow the fluorite solubility curve (Tropper and Manning, 2007 andreferences therein). The extent of pyrochlore replacement by secondary

cks. The range of compositions shown by both rock types is similar for both phlogopite (a)

image of Fig.�12

image of Fig.�13

Table 8Whole-rock compositions of carbonatites from the Aley complex, British Columbia.

Calcite carbonatites (n = 5) Cal carbonatitewith Cb

Dolomite carbonatites (n = 44) Dol carbonatites with Frs (n = 6)

Average ESD Min Max Average ESD Min Max Average ESD Min Max

Oxide, wt.%SiO2 4.07 5.85 0.99 14.52 2.91 1.68 2.07 0.18 10.05 2.65 3.33 0.63 9.14TiO2 0.09 0.10 0.01 0.27 0.52 0.10 0.10 b0.01 0.37 0.11 0.09 0.04 0.25Al2O3 0.43 0.36 0.10 1.06 0.72 0.32 0.47 0.03 2.87 0.41 0.19 0.19 0.74Fe2O3 3.59 1.75 1.88 6.41 45.13 4.01 1.18 1.91 8.53 4.19 1.45 2.42 6.65MgO 3.10 2.45 1.40 7.40 2.04 17.60 2.21 6.65 20.07 16.81 0.81 15.84 18.17CaO 43.30 5.56 38.46 52.56 26.4 31.34 1.90 27.82 38.80 30.92 3.49 24.85 34.30MnO 0.34 0.11 0.20 0.48 0.25 0.47 0.21 0.21 1.17 0.35 0.16 0.22 0.65Na2O 0.27 0.36 0.10 0.91 0.09 0.04 0.04 0.02 0.30 0.06 0.05 0.03 0.17K2O 0.25 0.21 0.05 0.60 0.11 0.07 0.14 b0.01 0.81 0.08 0.04 0.01 0.14P2O5 3.90 3.17 0.31 7.92 12.92 3.50 2.37 0.07 11.49 5.52 1.84 3.49 8.17LOI 34.69 3.92 32.06 41.53 8.85 40.49 2.93 32.03 45.02 37.73 2.00 35.08 40.09

Element, ppmV 69.0 114.3 4.0 272.0 551 30.01 36.9 b1.0 177.0 41.0 35.6 10.0 100.0Cr 8.6 7.3 2.0 20.0 2 6.8 5.7 2.0 29.0 6.2 3.6 2.0 11.0Co 13.6 16.2 2.1 41.7 28.3 6.4 5.9 1.3 38.0 8.6 7.5 1.6 22.4Cu 20.2 16.3 3.8 46.0 178.9 22.5 18.0 3.2 80.1 65.9 34.6 6.6 108.9Zn 99.0 137.7 14.0 344.0 132 18.0 10.0 6.0 52.0 10.8 7.1 6.0 25.0Li 5.8 4.4 0.8 12.9 1.5 2.5 2.8 0.3 14.5 5.1 5.1 0.9 12.8Rb 7.4 4.9 2.2 14.9 3 1.3 2.7 b0.1 16.5 2.5 1.6 0.2 4.5Cs 0.3 0.2 b0.05 0.5 1.1 0.2 0.2 b0.05 0.7 0.2 0.1 0.1 0.2Sr 6491.1 1769.4 3634.7 7945.9 3004.3 1862.8 1490.2 261.4 5741.3 1389.1 1663.3 451.3 4727.0Ba 667.4 228.5 478.0 1054.0 203 53.5 94.1 11.0 636.0 41.2 18.3 25.0 69.0Pb 17.8 12.2 9.1 26.4 14.6 9.8 11.7 1.4 64.6 12.0 2.7 8.0 15.2Sc 4.1 4.6 0.6 12.2 2.4 6.3 3.2 2.2 16.6 5.3 1.1 4.6 7.6Y 89.0 11.7 76.0 98.2 85.5 58.9 27.4 22.1 157.5 94.6 30.3 46.3 125.8La 368.1 74.1 274.0 466.7 474.3 324.4 134.4 96.3 N1000 377.1 122.2 218.7 564.2Ce 748.7 175.1 515.5 961.2 N1000 593.2 189.6 215.8 N1000 705.6 163.1 468.3 N1000Pr 83.8 21.2 55.9 107.2 131.4 105.1 125.1 25.4 745.7 90.0 21.7 59.4 119.6Nd 308.4 83.2 203.2 398.1 499.9 255.8 87.8 95.2 N1000 334.1 67.7 238.6 422.6Sm 48.4 12.3 33.5 61.3 79.6 47.1 29.1 15.0 164.4 55.2 7.1 43.7 62.9Eu 13.6 3.5 9.4 17.3 20.5 11.6 7.2 3.4 46.4 17.0 3.6 11.7 22.0

Element, ppmGd 41.6 9.7 29.8 51.7 63.7 40.7 29.4 12.9 183.0 47.0 6.8 34.4 52.6Tb 3.9 0.7 3.2 4.7 5.5 3.2 1.6 1.1 9.7 4.7 1.1 2.9 5.6Dy 21.5 3.3 18.0 25.3 28.1 17.5 9.0 6.3 54.3 25.2 5.9 14.7 30.5Ho 3.2 0.4 2.7 3.6 3.7 2.4 1.4 0.8 8.1 3.7 1.1 1.8 4.8Er 7.9 1.1 6.3 8.8 8.1 5.8 3.3 1.8 17.8 8.9 2.8 3.8 11.9Tm 0.9 0.2 0.7 1.1 0.8 0.6 0.4 0.1 1.9 1.0 0.4 0.3 1.4Yb 5.9 1.0 4.3 7.1 4.6 4.0 2.2 1.0 9.9 5.9 2.0 2.1 7.7Lu 0.7 0.1 0.5 0.9 0.4 0.5 0.3 b0.1 1.2 0.7 0.2 0.2 0.9Zr 15.3 22.6 2.1 55.2 290.2 18.1 27.7 2.3 179.4 46.8 25.8 16.6 76.1Hf 0.88 0.77 b0.1 1.42 9.9 0.48 0.54 b0.10 3.39 1.28 0.66 0.5 1.90Th 22.2 8.6 11.0 29.0 408 81.4 94.1 6.0 564.0 285.7 235.7 65.0 611.0U 4.8 6.4 0 16.0 2 11.8 34.3 b1.0 191.0 8.2 7.4 1.0 20.0Nb 139.2 174.3 23.3 433.1 14,801 1198.2 1235.8 30.8 5473.0 7176.9 2481.2 4296.0 10,765.3Ta 5.02 7.54 b0.05 16.00 18.10 18.14 37.89 b0.05 213.82 36.92 42.34 2.90 114.41Mo 1.34 1.23 0.17 2.87 0.37 1.70 4.02 0.09 27.04 0.50 0.41 0.17 1.24

Selected element ratios (mass)Y/Ho 27.8 0.5 27.3 28.4 23.1 25.5 3.0 19.4 36.5 25.6 1.3 23.4 27.0(La/Yb)cn 43.8 9.8 32.7 54.3 71.4 63.7 32.0 26.1 167.7 49.6 23.9 24.8 91.9Zr/Hf 34.7 5.8 30.6 38.9 29.3 40.8 15.8 7.1 93.3 36.4 3.8 31.1 40.1Th/U 7.0 4.9 1.8 13.5 204.0 51.1 97.3 0.3 564.0 78.4 112.9 6.6 305.5Nb/Ta 254.7 233.6 10.3 510.0 817.7 336.9 401.3 3.5 2086.8 941.3 1429.5 52.9 3777.3Zr/Nb 0.137 0.118 0.023 0.336 0.020 0.040 0.061 0.002 0.255 0.006 0.003 0.003 0.011


ferrocolumbite is unknown because it was subsequently overprinted byfersmite (see below).

As described in Sections 5 and 6, pyrochlore, ferrocolumbite andassociated silicate minerals are characterized by a great variation intheir Nb/Ta values, which in some cases, exceeds two orders of magni-tude, at a relatively stable Zr/Hf ratio. These peculiar geochemicalcharacteristics are echoed in the whole-rock compositions (Table 8)and are best explained by early fractionation of a low-Nb/Ta mineralcapable of scavenging large amounts of Nb + Ta, but incapable ofchanging significantly the Zr/Hf ratio of the residual melt. This mineralis most likely the U–Ta-rich pyrochlore (Nb/Ta= 2–12), which appearsto have been a common early liquidus phase in the Aley carbonatites.Chakhmouradian and Williams (2004) showed that fractionation of

even small amounts of U–Ta-rich pyrochlore (on the order of 1 wt.%)is sufficient to raise the Nb/Ta value by an order of magnitude. The frac-tionated pyrochlore probably concentrated in cumulate layers similar tothose known at other Nb deposits (e.g., Mitchell, 2014–in this issue),which were subsequently disrupted by postmagmatic deformation.

The Laramide orogeny caused asymmetric F2 folding of the Aleycarbonatites and their wall-rocks (McLeish, 2013), metamorphism andconspicuous textural changes within the intrusion, summarized inChapter 4. From an exploration standpoint, the most important ofthese changes was pressure-induced ductile flow of the carbonatites,which gave rise to ubiquitous foliation (e.g., Fig. 2b–e), accompaniedby redistribution of heavy minerals (apatite, pyrochlore, magnetite,ferrocolumbite, zircon, baddeleyite) within the deposit. The available

Fig. 14.Whole-rock trace-element compositions of non-mineralized (a) and mineralized (b) carbonatites from Aley normalized to the primitive-mantle composition of McDonough andSun (1995).


evidence suggests that, in common with other primary carbonatitedeposits, these minerals initially formed cumulate units, that werestretched out parallel to the flow-induced foliation, folded and locally

Fig. 15. Carbon-oxygen stable-isotope compositions of Aley carbonatites relative to thecomposition of mantle-derived igneous carbonates (Taylor et al., 1967) and Cambriansedimentary carbonates from the Cordillera (Losh, 1997; Naipauer et al., 2005; Sial et al.,2008) and worldwide (Veizer et al., 1999).

pulled apart to form boudinage and lenticular structures such as thoseshown in Fig. 2c. Notably, carbonate-poor rocks remained brittle duringthe deformation, as indicated by brecciation of the metasomatic silicateunits (Fig. 4d) and emplacement of ex situ carbonatite dikes into thewall-rock fenites (Fig. 2a). The limited occurrence of brecciatedcarbonatites within the deposit (Figs. 2f, 3a) indicates that the deforma-tion occurred largely below the depth of brittle–ductile transition forcarbonate rocks (N0.2 kbar), but well above the transition depth forfelsic silicate rocks (Mogi, 2007). In a compressional environment, thistransition occurs in carbonate rocks at different confining pressuresdepending on their grain size, porosity, content of non-carbonatematerial, availability of water, and temperature (e.g., De Bresser et al.,2005; Paterson and Wong, 2005). The ductile regime terminated withthe development of an extensive network of fractures that served aspassageways forfluids precipitating late-stage dolomite, chlorite, quartzand other hydrothermal minerals (Figs. 3g, 4d). It is not clear whetherthe replacement of igneous carbonates by dolomite preceded the hy-draulic fracturing, or the two processes were contemporaneous. Theformer scenario appearsmore likely, given the significant compositionaldifferences between the groundmass and late-stage rhombohedraldolomite (Reguir and Chakhmouradian, 2013).

The crystallization of fersmite can be clearly linked to pervasive do-lomitization of primary carbonatites during the metamorphic stage. Lo-cally, not only the primary carbonates, but also phlogopite and

image of Fig.�14

image of Fig.�15


magnetite were pseudomorphed by late-stage dolomite (±chlorite ±quartz). Apatite was also affected, but to a much lesser degree (workin progress). The source of fluids responsible for the dolomitization,and its timing relative to the deformation event remain to be ascertained.The textural (Figs. 9, 10) and chemical (Fig. 11b) data indicate that bothpyrochlore and ferrocolumbite underwent replacement by fersmite:

NaCaNb2O6ðFyOH1−yÞ þ ð1−yÞHþ⇔CaNb2O6 þ Naþ þ yF−

þ ð1−yÞH2O ð2Þ

FeNb2O6 þ Ca2þ⇔CaNb2O6 þ Fe2þ ð3Þ

The lack of any evidence of deformation in the pseudomorphs, par-ticularly given the brittleness of acicular fersmite crystals (Fig. 10a–d),implies that the replacement was post- or, at least, late synkinematic.Comparison of the trace-element characteristics of pyrochlore andfersmite indicates enrichment of HREE and, to a greater extent, Y inthe latter mineral relative to the precursor pyrochlore (Fig. 7). LightREE, Th and U were preferentially removed from the precursor phase,whereas Nb, Ta, Zr and Hf remained essentially immobile, maintainingthe primary Nb/Ta and Zr/Hf ratios. Niobium mineralization beyondthepseudomorphs is restricted to rare (andvolumetrically insignificant)acicular crystals of fersmite, ferrocolumbite and euxenite associatedwith the late-stage paragenesis (Figs. 8i, 10k, l). Scarce rutile, alsooccurring in this association (Fig. 12a–c), has a much lower Nb contentand contributes little to the Nb budget. Assuming nil mobility of high-field-strength elements (Nb, Zr and Ti) in the fluid responsible for thedolomitization of the Aley carbonatites, mass-balance calculationsbased on the median trace-element compositions (Tables 3 and 7)give consistently high enrichment factors (ke ≥ 4) for HREE, ke(Y) N ke(Ho), and show depletion (ke b 1) of La, Ce, Pr, Th and especially U infersmite (Fig. 16). A significant proportion of the remobilized LREEand Th was precipitated essentially in situ as monazite (e.g., Figs. 9g,10d–f) and fluorocarbonates, implying enrichment of the late-stagefluid in phosphate and F leached from the abundant primary apatite.In contrast to Th, U remained mobile because its concentrations in thelate-stage LREE phases do not exceed 25 ppm (cf. 400–2400 ppm Thin monazite and 3900–17,700 ppm in bastnäsite). The fate of the Uleached from pyrochlore is uncertain because only a small proportionof it was incorporated in euxenite and other late stage-phases. Thestable-isotope data (Section 8) suggest that the fluid equilibrated withthe wall-rock carbonate sedimentary rocks, producing the heavy-isotope enrichment trend shown in Fig. 15. Hence, we infer that thefluid was highly oxidized and chemically complex, probably containinguranyl, REE carbonate, phosphate and fluoride species.

Fig. 16. Changes in the distribution of LREE, HREE, Th and U inmetasomatically developedfersmite relative to its precursor pyrochlore calculated on the basis of zero gain/loss of Nb(white bars), Zr (grey bars) and Ti (black bars). Note strong depletion in U and moderateenrichment in Y and HREE in the fersmite.

The replacement of primary phlogopite by chlorite, and associationof quartz, chlorite, dolomite and, locally, muscovite and albite infracture-confined parageneses indicate that the waning stage ofmetamorphism occurred in a brittle regime under the greenschist-facies conditions essentially constrained by the chlorite and phlogopiteisograds at relatively high X(CO2), required to stabilize dolomite +quartz relative to calcite + amphibole (e.g., Bucher-Nurminen, 1982;Ferry, 1988; LeAnderson and Munoz, 1987). A close well-studied meta-morphic analogue to themetamorphosed Aley carbonatites is theNotchPeak contact aureole in Utah, where the phlogopite isogradwas locatedat T = 400–430 °C in a X(CO2) range of 0.3–0.7 (Cui et al., 2001 andreferences therein). However, in our case, it is difficult to constrain themetamorphic conditions precisely because K+was amobile componentwhose activity in the fluid is, by and large, unknown.

9.2. Comparison with other carbonatite-hosted Nb deposits

Carbonatite-hosted Nb deposits have been studied extensively sincethe seminal work of Russian geologists on the Kola Peninsula, Ural andSayan Mountains (Es'kova and Ganzeev, 1964; Gaidukova, 1960;Kukharenko et al., 1965; Zdorik, 1966; Zhabin and Gaidukova, 1962),most of which, unfortunately, is not readily accessible to the Westernreader. More recent publications in international journals, based ondetailed mineralogical and experimental studies utilizing electron-microprobe techniques, have addressed a number of important aspectsof Nb mineralization in carbonatites and related rocks, including thecompositional variation of pyrochlores and its relation to the evolutionof carbonatitic magma (Bambi et al., 2012; Chakhmouradian andWilliams, 2004; Kjarsgaard and Mitchell, 2008; Zurevinski and Mitchell,2004), phase relations between pyrochlores and perovskite-groupmin-erals (Jago and Gittins, 1993; Mitchell and Kjarsgaard, 2004; Kjarsgaardand Mitchell, 2008), and Nb partitioning between immisciblecarbonate and silicate melts (Suk, 2012; Veksler et al., 1998, 2012).The focus of the previous research has been largely on pyrochloresbecause these minerals are the principal Nb host in most of thecurrently or previously producing deposits associatedwith carbonatites(Table 1). Columbite-group minerals are much less common, but dooccur as a volumetrically important primary constituent, or a productof pyrochlore replacement in some reworked calcite and dolomitecarbonatites (e.g., James and McKie, 1958; Nedosekova, 2007; Sage,1991; Simandl et al., 2001).

The Aley deposit is unique in that fersmite, and not pyrochlore orferrocolumbite, is the principal ore mineral present in potentiallyeconomic concentrations. It is noteworthy that fersmite is exceedinglyrare in carbonatites, and is comparatively more common as a late-stage accessory phase in granitic pegmatites (for detailedbibliography, see Appendix 5). Minor quantities of this mineral havebeen reported from carbonatites in Ravalli County, Montana (Hessand Trumpour, 1959), at Mbeya in Tanzania (van der Veen, 1960),Lueshe in the Democratic Republic of Congo (Van Wambeke, 1965),East Sayan Mts. (Gaidukova and Sidorenko, 1983), Chernigov zone inUkraine (Kapustin, 1986; Povarennykh, 1985), Vishnevye Mts.(Lebedeva and Nedosekova, 1993), Biraya in Siberia (Chernikov et al.,1994), and Afrikanda in the Kola Peninsula (Chakhmouradian andZaitsev, 1999). None of these or other known occurrences approachthe Aley complex in scale or variety of compositional and texturaltypes of fersmite mineralization. At Lueshe (Table 1), fersmite devel-oped after both pyrochlore and ferrocolumbite, but the extent of itsdistribution within the deposit has not been studied (Van Wambeke,1965). In appreciable concentrations, fersmite occurs inmetasomatizeddolostones of the Bayan Obo Fe-REE and low-grade Nb (0.19 wt.%Nb2O5) deposit in China (Smith and Spratt, 2012), which is interpretedby some workers to have carbonatitic affinity (for details, see Smithet al., 2014–in this issue). At Bayan Obo, fersmite is extracted togetherwith aeschynite and other complex Nb oxides as a byproduct and

image of Fig.�16


concentrated using diphosphonic acid flotation for further Nb recovery(e.g., Ren et al., 2009).

Another important feature of the Aley deposit is the low mobilityof Nb during both the emplacement of primary carbonatites andtheir subsequent reworking by fluids. The Nb content of metasomaticxenoliths embedded in carbonatites averages 700 ppm (Section 7)and is even lower in exocontact fenites (~70 ppm; Mäder, 1986). Therestricted occurrence of pyrochlore in these rocks can be clearly linkedto “phlogopitization” by carbonatite-derived melts or fluids (seeSection 5). The bulk of the late-stage Nb mineralization is the productof in situ replacement of the primary Nb minerals. Newly-formedacicular crystals of fersmite, ferrocolumbite and euxenite do occur inthe hydrothermal paragenesis within the carbonatites, but are tooscarce to contribute significantly to thewhole-rock Nb budget. Unfortu-nately, the available published data onNbdistribution inmetasomatitesat other carbonatite complexes are grossly insufficient to say howcommon or extensive Nb dispersal halos typically are in these igneoussystems. Where the composition of the protolith is known and changesin Nb content across the fenite–carbonatite contact can be tracedsystematically, Nb enrichment in the metasomatic rock is restricted tothe exocontact or, in some cases, is not observed at all (Le Bas, 2008).This is in striking contrast to glimmerites at Qaqarssuk (Greenland)and microclinites at Bol'shetagninskoye (Russia), where pyrochloreconcentrations reach potentially economic levels, i.e. 0.8–1.0 wt.%Nb2O5 (Azarnova et al., 2010; Knudsen, 1991; Pozharitskaya et al.,2006). At the formerly producing Vishnevogorskoye deposit in Russia(Table 1), amphibole (±chlorite ± phlogopite) metasomatic rockshost fersmite and aeschynite, whereas pyrochlore is localized predom-inantly in the associated dolomite–calcite carbonatites (Nedosekova,2007).

The scale and character of mineralization at Aley is consistent withthe interpretation of Pell (1994) that this carbonatite complex wasemplaced in a rift setting, similar to that of the overwhelming majorityof carbonatite-related Nb deposits around the world (Table 1), and isdifficult to reconcile with the orogenic model proposed recently byMcLeish (2013). Post- and, in rare cases, synorogenic carbonatitemagmas are characteristically poor in Nb and Ta (Chakhmouradianet al., 2008; Chakhmouradian, 2006; Wang et al., 2001); these magmassometimes give rise tomineable rare-earth deposits (e.g., Castor, 2008),but lack Nb mineralization. Calcite carbonatites developed in these tec-tonic settings contain ≤130 ppm Nb and 3 ppm Ta at average concen-trations of 9 and 0.4 ppm, respectively (authors' unpublished data).The bulk of Nb in these rocks is distributed among accessory Ti phases(such as titanite and rutile), whereasU–Ta-rich pyrochlore is characteris-tically absent and low-Ta pyrochlore is observed in exceedingly smallconcentrations in only a few postorogenic carbonatites. These generalobservations also apply to the Mianning–Dechang carbonatite provincein China (Hou et al., 2006; Wang et al., 2001; Xu et al., 2003), whichMcLeish (2013) considers a modern analogue to Aley (p. 29). The folia-tion and other textural features, interpreted in the latter study as evi-dence of synkinematic emplacement and by Mäder (1986) as primarymagmatic, arose in the course of postmagmatic, metamorphic evolutionof the Aley complex and its re-equilibration with crustal fluids. Thus,we conclude that the Aley Nb deposit is associated with typicalanorogenic carbonatites whose primary igneous characteristics (bothmineralogical and textural) have been strongly affected and, to a greatextent, erased by deformation, metasomatism and other subsolidusprocesses during the Laramide orogeny. As we demonstrate in the pres-entwork, the effects of these processes on the nature and distribution ofmineralization cannot be neglected and must be taken into account inexploration projects targeting Nb and other rare-metal resources.

Acknowledgements

The present work would not have been possible without the gener-ous logistical and financial support of Taseko Mines Ltd. (Vancouver,

Canada), who also provided us with unlimited access to the depositand their field materials. The help of Rene Victorino, Mercedes Richand Anthony Leung with these materials is greatly appreciated. ARCalso wishes to thank Alexander Tolstov and Tony Mariano for sharingtheir data on various Nb deposits, as well as Duncan McLeish and JesseCollison for their companionship in the field and useful discussions.The manuscript has benefitted from constructive reviews by AnnaDoroshkevich and Simon Blancher. Jindra Kynicky is thanked for hisexpert editorial handling. The lab component of the present work wassupported by Taseko Mines, the Natural Sciences and EngineeringResearch Council of Canada (NSERC) and Canada Foundation forInnovation. The help of Misuk Yun with stable-isotope analysis isgratefully acknowledged. The NSERC and University of Manitoba arealso acknowledged for their support of EPR and LKP.

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carbonatite-hosted niobium deposit at aley, northern british columbia

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