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Carbonatites: related ore deposits, resources,footprint, and exploration methods

George J. Simandl & Suzanne Paradis

To cite this article: George J. Simandl & Suzanne Paradis (2018) Carbonatites: related oredeposits, resources, footprint, and exploration methods, Applied Earth Science, 127:4, 123-152,DOI: 10.1080/25726838.2018.1516935

To link to this article: https://doi.org/10.1080/25726838.2018.1516935

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Carbonatites: related ore deposits, resources, footprint, and explorationmethodsGeorge J. Simandla,b and Suzanne Paradisc

aBritish Columbia Geological Survey, Victoria, BC, Canada; bSchool of Earth and Ocean Sciences, University of Victoria, Victoria, BC, Canada;cNatural Resources Canada NRCan, Geological Survey of Canada, Sidney, BC, Canada

ABSTRACTMost carbonatites were emplaced in continental extensional settings and range in age fromArchean to recent. They commonly coexist with alkaline silicate igneous rocks, formingalkaline-carbonatite complexes, but some occur as isolated pipes, sills, dikes, plugs, lavaflows, and pyroclastic blankets. Incorporating cone sheets, ring dikes, radial dikes, andfenitisation-type halos into an exploration model and recognising associated alkaline silicateigneous rocks increases the footprint of the target. Undeformed complexes have circular,ring, or crescent-shaped aeromagnetic and radiometric signatures. Carbonatites can beeffectively detected by soil, till, and stream-sediment geochemical surveys, as well asbiogeochemical and indicator mineral surveys Carbonatites and alkaline-carbonatitecomplexes are the main sources of rare earth elements (REE) and Nb, and host significantdeposits of apatite, vermiculite, Cu, Ti, fluorite, Th, U, natural zirconia, and Fe. Nine per centof carbonatites and alkaline-carbonatite complexes contain active or historic mines, makingthem outstanding multi-commodity exploration targets.

ARTICLE HISTORYReceived 2 March 2018Revised 3 August 2018Accepted 20 August 2018

KEYWORDSCarbonatite; economicpotential; explorationmethods; REE; niobium;grade and tonnage

Introduction

Mineralised carbonatites and alkaline-carbonatitecomplexes are highly sought after, multi-commodity,but poorly understood exploration targets (Verwoerd1986; Mariano 1989a, 1989b; Richardson and Birkett1996a, 1996b; Pell 1996; Birkett and Simandl 1999).They are the main source of niobium (Nb; Mackayand Simandl 2014a; Simandl et al. 2018) and rareearth elements (REE; Simandl 2014; Verplank et al.2016), which are considered critical metals for keyeconomic sectors in industrialised countries (EuropeanCommission 2017), and have become popular explora-tion targets for mining companies worldwide. Alka-line-carbonatite complexes are significant sources ofCu, apatite, fluorite, vermiculite, and other commod-ities. Most modern studies on carbonatites addresstheir origin and aim to improve our understanding ofthe Earth’s mantle (Bell and Tilton 2001, 2002; Belland Simonetti 2010; Rukhlov et al. 2015).

Carbonatites and alkaline-carbonatite complexeswere considered controversial ever since A.G. Högbombegan detailed geological work at the Alnö Island alka-line-carbonatite complex in 1889. The history of carbo-natite research up to 1966 is well summarised in the 1stand 2nd editions of the widely available benchmarkpublication entitled ‘The Geology of Carbonatites’ byHeinrich (1980). A number of additional reviews have

been performed since that time, including the book edi-ted by K. Bell (1989) entitled ‘Carbonatites: Genesis andEvolution’. However, the most important review froman exploration geologist’s point of view is probablythe compilation of Wooley and Kjarsgaard (2008a)entitled ‘Carbonatite occurrences of the word; mapand database’, available free of charge for downloadfrom the Geological Survey of Canada website. Thehighly descriptive and factual nature of this publicationavoids traps and controversies associated with the gen-esis of carbonatites, and takes into consideration someof the key suggestions of Mitchell (2005) who attemptsto improve currently accepted classification of Le Maî-tre (2002) by making it more relevant to geologicalmappers and exploration geologists. The descriptiveapproach, in combination with the availability of dataas spreadsheets, will make the Wooley and Kjarsgaard(2008a) compilation a valuable source of informationfor years to come, since spreadsheets can be easilyupdated and customised by the user.

Herein, we review the definition and classification ofcarbonatites and summarize information pertinent forcarbonatite exploration such as tectonic setting, mor-phology, carbonatite-related alkali silicate rock associ-ations, accompanying alteration, and temporaldistributions. We also discuss carbonatites and alka-line-carbonatite complexes in terms of resources,

© 2018 The Author(s). Published by Informa UK Limited, trading as Taylor & Francis GroupThis is an Open Access article distributed under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, orbuilt upon in any way.

CONTACT George J. Simandl george.simandl@gov.bc.ca British Columbia Geological Survey, PO Box 933 Stn Prov Govt, Victoria, BC, Canada V8W9N3; School of Earth and Ocean Sciences, University of Victoria, Bob Wright Centre A405, 3800 Finnerty Road, Victoria, BC, Canada, V8P 5C2

APPLIED EARTH SCIENCE (TRANS. INST. MIN. METALL. B)2018, VOL. 127, NO. 4, 123–152https://doi.org/10.1080/25726838.2018.1516935

exploration methods, and prospectivity. The four mainobjectives of this paper are to (i) alert the explorationcommunity to the multi-commodity explorationpotential of carbonatites and alkaline-carbonatite com-plexes, (ii) highlight the relevance of tectonic setting,carbonatite-related alkali metasomatism (fenitisation-type metasomatism), and alkaline-carbonatite complexmorphology to the exploration geologist, (iii) presentrelevant advances in exploration methods, and (iv)bridge the gap separating carbonatite-related academicresearchers and mineral explorers.

Definitions and classifications

Carbonatites are defined by the International Union ofGeological Sciences (IUGS) as igneous rocks contain-ing more than 50%modal primary carbonates (Le Mai-tre 2002). Depending on the predominant carbonatemineral, a carbonatite is referred to as a ‘calcite carbo-natite’, ‘dolomite carbonatite’ (Figure 1), or ‘ferrocar-bonatite’, where the main carbonate is iron-rich. Ifmore than one carbonate mineral is present, thecarbonates are named in order of increasing modalconcentrations. For example, a ‘calcite-dolomite carbo-natite’ is composed predominately of dolomite. If non-essential minerals (e.g. biotite) are present, this can bereflected in the name as ‘biotite-calcite carbonatite’.

Where the modal classification cannot be applied,the IUGS chemical classification may be used(Figure 2(a)). This classification, based on wt.% ratios,

subdivides carbonatites into calciocarbonatites, magne-siocarbonatites, and ferrocarbonatites. For calciocarbo-natites, the ratio of CaO/(CaO +MgO + FeO + Fe2O3

+MnO) is greater than 0.8. The remaining carbonatitesare subdivided into magnesiocarbonatite [MgO > (FeO+ Fe2O3 +MnO)] and ferrocarbonatite [MgO < (FeO+ Fe2O3 +MnO)] (Woolley and Kempe 1989; Le Mai-tre 2002). If the SiO2 content of the rock exceeds 20%, itis referred to as silicocarbonatite. When the IUGSchemical classification is used, iron content in sul-phides and oxides should be excluded from consider-ation, otherwise, magnetite and hematite-richcalciocarbonates or magnesiocarbonatites may be erro-neously classified as ferrocarbonatites (Gittins andHarmer 1997). A natrocarbonatite is a special varietyof carbonatite consisting mainly of Na–K–Ca carbon-ates, such as nyerereite [(Na,K)2Ca(CO3)2] and gregor-yite [(Na,K,Cax)2–x(CO3)], known from Ol DoinyoLengai volcano (Tanzania).

A refinement to the IUGS chemical classificationbased on molar proportions, proposed by Gittins andHarmer (1997), introduces the term ‘ferruginous cal-ciocarbonatites’ (Figure 2(b)). The boundary separ-ating calciocarbonatites from magnesiocarbonatitesand ‘ferruginous calciocarbonatites’ is set at 0.75,above which carbonatites contain more than 50% cal-cite on a molar basis. Although not universallyaccepted, Gittins and Harmer’s classification is com-monly used in studies of carbonatite-hosted ore depos-its (e.g. Trofanenko et al. 2016).

A mineralogical-genetic classification of carbona-tites was proposed by Mitchell (2005). His benchmarkpaper points to pitfalls of the IUGS classification, andsubdivides carbonatites into ‘primary carbonatites’and ‘carbothermal residua’. The introduction of theterm ‘carbothermal residua’ is significant as it alertsmantle specialists to the fact that not all rocks cur-rently called ‘carbonatites’ by academicians satisfyIUGS definition. In their well-known and widelyreferenced compilation, Woolley and Kjarsgaard(2008a) have taken into consideration suggestionsproposed by Mitchell (2005), but instead of using ‘pri-mary’ and ‘carbothermal residua’ they adopted theterms ‘magmatic’ and ‘carbohydrothermal’. The rec-ognition of ‘carbohydrothermal carbonatites’ by aca-demia is beneficial to the exploration community asit eliminates a deep historic divide between research-ers with an interest in the Earth’s mantle studyingigneous carbonatites sensu stricto (as defined byIUGS), and those that study or explore for carbona-tite-related ore deposits. The term ‘carbohydrothermalcarbonatite’ is defined by Woolley and Kjarsgaard(2008b) as carbonatite which precipitated at subsoli-dus temperatures from a mixed CO2–H2O fluid thatcan be either CO2-rich (i.e. carbothermal), or H2O-rich (i.e. hydrothermal). The carbohydrothermalaspect of some carbonatite phases is in line with

Figure 1. Upper Fir dolomite carbonatite, British Columbia,Canada; slightly weathered surface; dolomite-brown, Na-amphibole – green, apatite – white. Coin for scale is 2 cm indiameter.

124 G. J. SIMANDL AND S. PARADIS

number of recent studies highlighting the role offluids in formation of carbonatite-related REE depos-its, such as at Bayan Obo Fe-REE-Nb deposit, InnerMongolia (Smith et al. 2015; Lai et al. 2016) andWicheeda Lake carbonatite-related REE mineralis-ation, Canada (Trofanenko et al. 2016), and withstudies that provide constraints on transportation ofREE by hydrothermal fluids and their deposition(e.g. Migdisov et al. 2016). Similarly, it may play animportant role in the genesis of other carbonatite-related deposit types (e.g. apatite ores at Seligdar apa-tite deposit, Russia; Prokopyev et al. 2017).

For the benefit of exploration geologists and fieldmappers, in this paper, we use the term carbonatiteto broadly include both magmatic carbonatites andrelated carbohydrothermal phases, as it may be extre-mely difficult to distinguish between them on the out-crop or hand specimen scale without follow-uplaboratory studies. We use the term alkaline-carbona-tite complex to denote occurrences where magmaticand or carbohydrothermal carbonatites are spatiallyassociated with alkaline silicate rocks of igneous ormetasomatic origin.

In this review paper, the term ‘carbonatite-relatedore deposit’ is used in the broadest sense. It refers todeposits that are genetically or spatially associatedwith carbonatites or alkaline-carbonatite complexes.A carbonatite-related deposit could consist of unal-tered magmatic minerals; minerals that crystalisedfrom carbohydrothermal, hydrothermal, or meta-morphic fluids transporting metals; or minerals con-centrated by weathering or supergene enrichment,which commonly overly carbonatite protore. Conse-quently, a ‘carbonatite-related ore deposit’ could behosted by a carbonatite or alkaline-carbonatite com-plex, be situated at the contact between the intrusionand host rock, or be distal to the intrusion.

Origin of carbonatites

There are currently three main hypotheses explainingthe origin of carbonatite melts: (1) immiscible separ-ation of parental carbonated silicate magmas at crustalor mantle pressures (Kjarsgaard and Hamilton 1989;Lee and Wyllie 1998; Wyllie and Lee 1998); (2) crystalfractionation of parental carbonated silicate magmassuch as olivine melilitites or kamafugites (Veksler andLentz 2006); and (3) low-degree partial melting of carbo-nated mantle peridotite below 70 km depth (Wyllie andHuang 1976a, 1976b; Eggler 1978; Bell 1989; Dalton andPresnall 1998; Harmer and Gittins 1998; Bell and Rukh-lov 2004; Rukhlov et al. 2015). Hypotheses invoking orsupporting a possible derivation of carbonatites fromthe Earth’s crust (Lentz 1999; Ferrero et al. 2016), orfrom the Earth’s mantle with some crustal contribution(Cheng et al. 2017; Song et al. 2017), have also been pro-posed. Furthermore, a recent study based on boron iso-topes of carbonatites worldwide suggests that, althoughmost carbonatites may originate in the upper mantle,younger carbonatites (<300 My) probably involve atleast partial subducted crustal component (Hulett et al.2016). It is likely that not all carbonatite-formingmelts are of the same origin. However, regardless oftheir mode of formation, most researchers agree thatalkalis (Na and K) play an important role in the genesisof calcite and dolomite carbonatites, and ferrocarbona-tite intrusions.

The importance of alkalis in the genesis of carbona-tites is consistent with studies of low-temperature(<600°C) natrocarbonatitic lavas from Ol Doinyo Len-gai, Tanzania that contain 38–40 wt.% combined Na2Oand K2O, 4.5 wt.% F, 5.7 wt.% Cl, approximately15 wt.% Ca, and less than 1 wt.% combined Mg andFe (Keller and Krafft 1990; Keller and Zaitsev 2012).Petrographic and geochemical evidence from extrusivecarbonatites, such as Homa Mountain, Kenya (Clarke

Figure 2. Carbonatite classifications according to (a) IUGS based on wt.% (Le Maitre 2002) and (b) Gittins and Harmer (1997) basedon molar proportions. C/CMF is the molar ratio of CaO/[CaO + MgO + FeO* + MnO]; FeO* expressed as molar FeO if both FeO andFe2O3 are determined.

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and Roberts 1986); Tinderet, Kenya (Deans andRoberts 1984; Zaitsev et al. 2013); Kerimasi, Tanzania(Hay 1983; Zaitsev 2010; Guzmics et al. 2011); andKaluwe, Zambia (Ngwenya and Bailey 1990), as wellas evidence from intrusive carbonatites such as Bol’-shaya Tagna, Russia (Andreeva et al. 2006); Oka,Canada (Chen et al. 2013); Guli, Russia (Kogarkoet al. 1991); and Palabora, South Africa (Sharyginet al. 2011), suggests that calcite- and dolomite-richcarbonatites are residues or cumulates derived fromalkali-bearing (moderately alkaline) melts. Recently,‘halite intergrown with, and included in, dolomite, cal-cite, apatite, REE- fluorocarbonates, pyrochlore, fluor-ite, and phlogopite of a dolomitic carbonatite’ wasreported at the St.-Honoré alkaline-carbonatite com-plex (Canada) by Kamenetsky et al. (2015), furthersupporting the argument that parental magmas of pri-mary carbonatites are enriched in Na and Cl. A com-prehensive review of fluid and melt inclusion studiesof intrusive carbonatites by Veksler and Lentz (2006)further documents the alkaline nature of melts towhich intrusive carbonatites are related or fromwhich they originated. A hypothesis suggesting thatcalcite carbonatites evolve from natrocarbonatite par-ental magmas by loss of alkali due to fenitisation wasadvanced by Le Bas (1981) and Woolley (1982); how-ever, this hypothesis was challenged, if not discredited,by Twyman and Gittins (1987), Mitchell and Belton(2004), and Mitchell (2005).

A thermal barrier, as established by Cooper et al.(1975), was a major obstacle to most hypotheses propos-ing that natrocarbonatites evolved from moderatelyalkaline calcite carbonatites. However, recent exper-imental work suggests that natrocarbonatite lavas mayevolve from a moderately alkaline calcite carbonatiteprecursor (which is unmixed from nephelinites) by crys-tal fractionation of calcite and apatite (Weidendorferet al. 2017). This appears possible because of the abilityof halogens to suppress the calcite liquidus, therebyeliminating the thermal barrier (Weidendorfer et al.2017). There is no consensus regarding the origin ofmagnesium- and Fe-rich carbonatites. They are believedto either evolve from early calcite carbonatites by frac-tional crystallisation or be of hydrothermal origin(Thompson et al. 2002), essentially ‘carbohydrothermal’.

Exploring for carbonatites: where to lookand what to look for

Tectonic setting

Most carbonatites and alkaline-carbonatite complexesare emplaced in continental (88% cratonic, 10.5%non-cratonic) settings (Figures 3–4) in Archean andProterozoic rocks, or in Phanerozoic rocks underlainby a Precambrian basement (Sage and Watkinson1991; Woolley and Kjarsgaard 2008a; Pirajno 2015).

They form in extensional tectonic settings (Bailey1974, 1977, 1992), along major linear trends relatedto large-scale intra-plate fracture zones, in associationwith doming features (crustal arching), or in relationto slab windows in subducting plates (Duke 2009;Duke et al. 2014). The link between these tectonic fea-tures and intense magmatic activity means that manycarbonatites are also temporally and spatially relatedto ‘large igneous provinces’ (Pirajno 2000; Ernst andBell 2010). Some researchers (e.g. Nelson et al. 1988;Bell 2001, 2005; Pirajno 2015) consider mantle plumesessential to carbonatite genesis.

Carbonatites in orogenic settings are sometimesreferred to as ‘post-collisional’ (Chakhmouradianet al. 2008; Woodard and Hetherington 2014). This isan unfortunate term because carbonatites that arefound in orogenic settings may have been emplacedbefore a transition from extensional to compressionaltectonic regimes, or during post-orogenic extensionalrelaxation and collapse prior to dynamothermal meta-morphic climax. Examples of specific orogenic settingscontaining carbonatites are: (1) British Columbia Alka-line Province, Canada (Pell 1994, Millonig et al. 2012);(2) The Himalayan collision zone in western Sichuan,China (Hou et al. 2006, 2015); (3) Northwest Pakistan(Tilton et al. 1998); and (4) The Great Indian Protero-zoic Belt, India (Leelanandam et al. 2006). The Naantalicarbonatite in southwest Finland is also located in oro-genic setting (Woodard and Hetherington 2014).

Carbonatites are identified in three oceanic islandregions: (1) The Canary Islands; (2) The Cape VerdeIslands, and (3) The Kerguelen Islands; all of whichare located off the African continent (Figure 4; Woolleyand Kjarsgaard 2008a). However, it is possible that theseislands are underlain by remnants of continental litho-sphere stranded during drifting of the African plate(Bonadiman et al. 2005; Woolley and Bailey 2012).

Temporal patterns of carbonatite emplacement:can age be used to screen exploration targets?

Ages of carbonatite emplacement on a globalscaleRadiometric ages of cabonatites vary from Archean (e.g.Short Lake, Canada, 2684±170 Ma; Rukhlov and Bell

Figure 3. Tectonic setting of carbonatites and carbonatitecomplexes. Based on data from 527 carbonatites from Woolleyand Bailey (2012).

126 G. J. SIMANDL AND S. PARADIS

Figure 4. World map of carbonatite occurrences and their spatial relationship to Proterozoic and Archean rocks. Areas underlain by unsubdivided Proterozoic and Phanerozoic rock are not shown.Modified from Woolley and Kjarsgaard (2008a).

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2010) to very recent (e.g. Ol Doinyo Lengai, Tanzania,which is considered an active volcano). Frequencyplots of carbonatites with well-established ages indicatethat younger carbonatites are more abundant thanolder ones (Figure 5). This trend may be due to changesin tectonic activity during the Neoarchean Era that per-mitted emplacement of carbonatites. As favourable con-ditions became more widespread, carbonatites becamemore common (Woolley and Kjarsgaard 2008a). Thedifficulty to distinguish carbonatites from carbonate-rich metasediments in areas affected by a post-carbona-tite dynamothermal metamorphism may be anotherreason (Woolley and Kjarsgaard 2008a). Veizer (1992)proposed that the overall exponential decrease in theprobability of carbonatite preservation with increasingage of crustal segments is the main reason for thistrend. Variations in boron (B) concentrations inyoung (<300–400 Ma) carbonatites and their isotopiccomposition (δ11B) suggest that greater quantity of sub-ducted material may have reached plumes, andindirectly contributed increase in carbonatite activityduring that time period (Hulett et al. 2016).

Carbonatites with mineralogy similar to Ol DoinyoLengai natrocarbonatite may have been relatively com-mon in Earth’s history; however, because they convertrapidly into calcite carbonatites in the near surfaceenvironment (Zaitsev and Keller 2006), they are not pre-served in the geological record. Destruction of nyerereite(the main natrocarbonatite-forming mineral) in surfaceenvironments takes place over a few months or lessaccording to the following alteration sequence: nyerer-eite→ pirssonite→ calcite→ shortite (Zaitsev et al. 2008).

Woolley and Bailey (2012) covered the importanceof tectonic controls on carbonatite emplacement.Their study focused on six well documented carbona-tite provinces: (a) East Africa (Kenya, Uganda, Tanza-nia, and one occurrence in Zambia); (b) Namibia andAngola; (c) Eastern Russia (East Tuva, Enisei, EastSayan, Baikal, and Aldan); (d) Greenland; (e) Ontarioand southwest Quebec; and (f) Northern Europe,including the Kola Peninsula, northern Norway, Swe-den, and Finland. They recognised up to five episodesof carbonatite emplacement separated by hundreds ofmillions of years in some of these provinces. Theypointed out that within at least three of these carbona-tite provinces, including one comprising Ontario andsouthwest Quebec (Figure 6), plumes can be eliminatedas a mode of carbonatite emplacement because of thelack of relationship between the direction of the platemovement and the age of intrusions. Instead, carbona-tite magmatism appears to coincide with periods ofreactivation of faults and fissures associated withchanges in tectonic regimes (extensional period). Con-sider as an example the Ontario and Southwest Quebeccarbonatite province covered by Woolley and Bailey(2012). Although age dates from K-Ar on biotite andRb-Sr on whole rock may have been reset by mediumgrade metamorphic events, the U-Pb age dates on zir-con and baddeleyite corroborate the original data andconfirm that the timing of each carbonatite emplace-ment episode coincides closely with a period of relax-ation linked to a major orogeny (Figure 6).

Figure 6. Temporal relationship between intensive carbonatite magmatism and main orogenic events in Ontario, Canada andsouthwest Quebec, Canada. Potassium–Ar dates on biotite and Sr–Rb dates on whole rock from Woolley and Bailey (2012) andU–Pb dates on zircon and baddeleyite from Rukhlov and Bell (2010). Age estimates for Grenville, Trans Hudson, and Kenoran oro-genies from Tohver et al. (2006), Corrigan et al. (2009), and Moser et al. (2008), respectively.

Figure 5. The frequency of all known carbonatite occurrenceswith isotopic ages decreases with time. With the exception ofcarbonatites younger than 150 Ma, the frequency of minera-lised carbonatite occurrences shows a similar decrease. Figurebased on data from Woolley and Kjarsgaard (2008a), withminor updates.

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The frequency of mineralised carbonatite occur-rences follows a similar pattern to the frequency ofall carbonatites, except for the interval between 150and 0 Ma (Figure 5). Relatively shallow erosional levelsmay explain this divergence, with few exceptions, asmost carbonatite-related mineralisation is spatiallyrelated to intrusive carbonatite phases at depth,whereas their volcanic (near surface) equivalents arecommonly low grade or barren (unless subjected tosupergene enrichment). In view of the broad temporalrange of carbonatites on the world scale and the diver-gence of trends between non-mineralised and minera-lised carbonatites within the 150 to 0 Ma time interval,the age alone is of limited use for screening carbonatiteexploration targets.

Age relationships between individual barrenand mineralised carbonatite pulses on thecomplex-scale

Within the same alkaline-carbonatite complex, calcitecarbonatites are generally the oldest (Richardson andBirkett 1996a). Radiometric ages are seldom used sys-tematically to age date discrete intrusive phases of thesame alkaline-carbonatite complex; however, wherefield relationships are observable, calcite carbonatitesare cut by magnesian (dolomite carbonatite) pulses,which in turn predate more Fe±Mn-rich (ferrocarbo-natite) pulses as illustrated by the Chilva Island Com-plex (Garson and Smith 1958; Woolley 1982).However, in many field studies of cabonatite com-plexes, the cross-cutting relationships are either notdiscernable or only dolomite carbonatite or ferrocarbo-natite pulses were observed. Magmatic mineralisation(e.g. Nb, apatite, magnetite, and baddeleyite) is com-monly hosted by early to intermediate magmatic pulses(Richardson and Birkett 1996a). Such mineralisationmay be remobilised at later stages, possibly as a resultof post-magmatic overprint (e.g. Aley carbonatite, Brit-ish Columbia; Chakhmouradian et al. 2015). The REEmineralisation commonly post-dates Nb mineralis-ation (Richardson and Birkett 1996a), as exemplifiedby the dolomite carbonatite-hosted REE mineralisationat Wicheeda Lake deposit, British Columbia (Trofa-nenko et al. 2016), where the third generation dolomiteand REEmineralisation is interpreted as hydrothermal.Another example of where Nb mineralisation is post-dated by REE mineralisation is the St. Honoré alka-line-carbonatite complex, Quebec. Here, the principalhost to Nb mineralisation is a crescent-shaped, dolo-mite carbonatite cone sheet, surrounded in part by bar-ren calcite carbonatite; the late core of this complex isstrongly enriched in REE (Lafleur and Ayad 2012).

Although in most cases REE mineralisation is para-genetically late and appears hosted by carbonatites thatcould be considered carbohydrothermal, frequentlycited research of Mariano (1989a) suggests that, at

the Mountain Pass REE deposit (California), carbona-tite-hosted REE mineralisation is late magmatic basedlargely on textures. However, the lack of reported pyr-ochlore and columbite or their alteration products,which are common in typical magmatic carbonatites,and local enrichment in Ba suggests that by today’s ter-minology, the host to REE (bastnaesite-parisite) miner-alisation may be ‘carbohydrothermal’ in origin. Afollow-up study may be justified to confirm Mariano’s(1989a) original findings and eliminate incertitude.

Carbonatite-associated igneous rocks

Most intrusive carbonatite occurrences are part of alka-line-carbonatite complexes and are spatially tied to oneor more intrusive silicate rock groups including melili-tolites, ijolites, alkali gabbros, feldspathoid syenites,syenites, kimberlites, and lamprophyres or their volca-nic equivalents (Woolley and Kjarsgaard 2008a,2008b). Worldwide, only 24% of carbonatite rocksare not part of alkaline-carbonatite complexes (Wool-ley and Kjarsgaard 2008a).

Peridotites and pyroxenites, commonly found nearijolites and melilitolites, are not considered a distinctassociation because they are interpreted as cumulates(Woolley and Kjarsgaard 2008a). Nevertheless, theserocks are important from the exploration point ofview because they are commonly observed in deeplyeroded alkaline-carbonatite complexes, providingindirect information about the depth of erosion and,by extension, the mineralisation potential of the com-plex (Frolov 1971; Epshteyn and Kaban’kov 1984).

Phoscorites are magnetite, olivine, apatite rocksusually associated with carbonatites (Le Maitre 2002)and ultramafic rocks of alkaline-carbonatite complexes(Woolley and Kjarsgaard 2008b). In some cases, thereis gradation between ultramafic rocks and phoscorite(e.g. Yonghwa phoscorite-carbonatite pipe; Seo et al.2016). The definition presented by Le Maitre (2002),is very restrictive because olivine commonly retro-grades into pyroxene, amphibole, and serpentine. Amuch broader definition and classification of phoscor-ites are anchored in Russian literature (e.g. Yegorov1993; Krasnova et al. 2004) and proposes that phoscor-ite should be redefined as a ‘plutonic ultramafic rockcomprising magnetite, apatite, and one of the silicates,forsterite, diopside, or phlogopite’. According to Kras-nova et al. (2004), most of the 21 phoscorite (sensulato) occurrences that they are aware of are minera-lised. Should this broader definition be applied world-wide, the number of phoscorite occurrences wouldincrease substantially. For example, in British Colum-bia, the Aley carbonatite and several carbonatites inthe Blue River area would be considered to containlenses or fragments of phoscorite (Figure 7). This hasimportant implications from an exploration point ofview because, within several alkaline-carbonatite

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complexes, phoscorites are host to Nb, baddeleyite(natural zirconia), apatite and iron, with a potentialto recover scandium as a co-product (Rudashevskyet al. 2004; Ivanyuk et al. 2016; Kalashnikov et al. 2016).

Recognition of carbonatite-related intrusive silicaterocks is also important. In some cases, carbonatite rep-resents only a minor part of an alkaline-carbonatitecomplex. Consequently, recognising the spatial associ-ation of carbonatites to phoscorite and commonlyassociated intrusive silicate rocks substantially increasesthe size of an exploration target (Figure 8; Table 1).

Alkali metasomatism

Most intrusive carbonatites, alkaline-carbonatite com-plexes, and many agpaitic and miaskitic alkaline intru-sions are surrounded by country rock affected byintrusion-related metasomatism. Metasomatism isdefined as: ‘a solid state process by which the chemicalcomposition of a rock is altered in a pervasive mannerand which involves the introduction and/or removal ofchemical components as a result of the interaction ofthe rock with fluids’ (Zharikov et al. 2007). Fenitisationis defined as: ‘a metasomatic process leading to the for-mation of fenites’; where fenite is described as: ‘a high-temperature metasomatic rock characterised by thepresence of alkali feldspar, sodic amphibole and sodicpyroxene; nepheline, calcite and biotite/phlogopitemay also be present and typical accessories are titaniteand apatite’ (Zharikov et al. 2007). However, over theyears, geological vocabulary has evolved and today,most of carbonatite and alkaline-carbonatite com-plex-related alkali metasomatism is broadly referredto as fenitisation or fenitisation-type metasomatism(Eckermann 1948; Le Bas 1981; Woolley 1982; Moro-gan 1994; Martin 2006, Liu et al. 2018). Fenitisation-type metasomatism is used here because it represents

the compromise between those accepting the broaduse of the term fenitisation (sensu lato) and puristsinsisting on the use of the term fenitisation (sensustricto) as advocated by Zharikov et al. (2007). Feniti-sation-type metasomatism commonly consists of desi-lication accompanied by the addition of Na, K, Fe3+, ±Ca, ± Al to the host rock that surrounds carbonatites orcarbonatite-alkaline complexes (Figure 9; Morogan1994; Williams-Jones and Palmer 2002; Smith 2007;Le Bas 2008). Other elements that may be introducedinto country rock by fenitisation-type metasomatismare Ba, Nb, Sr, Sc, Rb, Zn, V (Le Bas 2008) and insome cases REE, and Nb. Such metasomatism maymanifest itself by the development of Na- and K-amphiboles, aegirine-augite, K-feldspar, albite, perthite,mesoperthite, antiperthite, nepheline, and pale brownmica, and albite (Figure 9). Minor and trace constituentscommonly observed in rocks affected by fenitisation-type metasomatism are carbonates, apatite, barite, rutile,magnetite, titanite, pyrochlore, monazite-(Ce), bastnae-site-(Ce), parisite-(Ce), and goyazite. Most of the silicaremoved from the country rock during desilicationmigrates towards the intrusive (Figure 9) and fromthat point of view, the process may be considered asbimetasomatic (forming endocontact and exocontactzones), as defined by Zharikov et al. (2007). Since1921, a large number of alkaline-carbonatite complex

Figure 7. Phoscorite lenses and irregular pods (sensu Krasnovaet al. 2004) consisting of magnetite with minor apatite, dolo-mite, and serpentine (possibly replacement of olivine) areenclosed in dolomite carbonatite, Aley carbonatite, BritishColumbia, Canada. Marker for scale measures 14 cm in length.

Figure 8. The average surface area of carbonatites is approxi-mately 3 km2. In most cases, the carbonatites are surroundedby alkali silicate rocks, which are surrounded by a zone of feni-tisation. If an explorationist recognizes the carbonatite, relatedsilicate rocks (∼2 km2), and associated fenitised zone (∼5 km2),the target area increases to 10 km2. This figure is based on datafrom 26 carbonatite complexes, with surface areas varyingfrom 0.01 to 78.5 km2, listed in Table 1.

130 G. J. SIMANDL AND S. PARADIS

related zones affected by fenitisation-type metasoma-tism have been studied and reviewed in terms of theirmorphology, extent, texture, mineralogy (Garson 1962;Woolley 1969, 1982; Le Bas 2008), major and traceelement analysis of bulk rock (Kresten 1988; Morogan

1989, 1994), and mineral chemistry (Mian and Le Bas1986). Similar to metasomatic zones related to better-documented mineralising systems such as porphyrycopper (e.g. Sillitoe 2010; Hedenquist and Taran 2013)and massive sulphide deposits (e.g. Hannington,

Table 1. Surface areas of selected carbonatite complexes in terms of carbonatite rocks, alkaline silicate rocks, and fenites.Carbonatite Silicate rocks Fenitisation Other

ReferenceCarbonatitecomplex Total (km2) km2 % km2 % km2 % km2 %

1 Amba Dongar 6.7 3.2 47.7 2.6 39.4 0.7 10.3 0.2 2.5 William-Jones and Palmer (2002)2 Otjisazu 11.9 0.9 7.9 10.7 90.0 0.3 2.1 Gunthorpe and Buerger (1986)3 Araxá 21.2 7.0 33.0 5.4 25.4 8.8 42.6 Traversa et al. (2001); Issa

Filho et al. (1984)4 Panda Hill 9.8 1.9 19.3 7.9 80.4 0.03 0.3 Basu and Mayila (1986)5 Naantali 3.5 0.2 4.3 3.4 95.6 Woodard and Hölttä (2005)6 Fen 6.5 3.3 50.7 0.6 9.7 2.6 39.6 Kresten and Morogan (1986)7 Callander Bay 7.8 0.002 0.03 0.3 4.0 7.5 96.0 Currie and Ferguson (1971)8 Kangankkunde 2.9 0.2 6.2 2.7 93.8 Bowden (1985)9 Sokli 62.9 21.6 34.4 13.5 21.5 27.8 44.2 Sarapää et al. (2013)10 Okorusu 8.2 0.2 2.7 2.7 32.7 4.9 59.5 0.4 5.1 Pirajno (1994)11 Lueshe 4.4 2.8 64.6 0.8 17.9 0.8 17.5 Maravic and Morteani (1980)12 Sarfartôq 78.5 1.0 1.3 13.2 16.8 64.3 81.9 Secher and Larsen (1980)13 Pollen 1.3 0.2 17.4 0.8 56.0 0.4 26.6 Robins and Tysseland (1983)14 Virulundo Mountain 9.2 5.2 56.4 4.0 43.6 Torró et al. (2012)15 Alnö 6.1 0.4 6.3 3.7 61.5 2.0 32.2 Morogan and Lindblom (1995)16 Copperhead 0.01 0.00003 0.5 0.01 97.9 0.0001 1.7 Rugless and Pirajno (1996)17 Ipanema 9.3 0.0002 0.002 0.6 6.0 8.8 94.0 Guarino et al. (2012)18 Salitre I 43.3 2.4 5.5 31.1 71.7 8.3 19.1 1.6 3.7 Barbosa et al. (2012)19 Loe Shilman (Western) 0.2 0.08 56.1 0.1 43.9 Mian and Le Bas (1986)20 Oka 8.4 5.0 59.4 2.4 28.8 1.0 11.8 Lentz et al. (2006)21 Afrikanda 5.6 0.3 4.9 3.7 66.1 1.6 29.0 Wu et al. (2013)22 Newania 3.2 2.3 71.3 0.8 25.4 0.1 3.3 Schleicher et al. (1997)23 Barra do Itapirapuã 4.4 3.1 69.1 1.4 30.9 Andrade et al. (1999)24 Aley 7.0 4.8 68.3 0.006 0.08 2.2 31.6 McLeish et al. (2010)25 Siilinjärvi 14.8 1.2 8.11 13.59 91.9 Puustinen (1970)26 Qaqarssuk 11.8 3.5 29.8 8.3 70.2 Kunzendorf and Secher (1987)

Average 13.4 2.7 27.9 3.6 27.7 4.6 40.6 2.6 3.8

Note: Surface areas calculated from available geological maps in referenced documents.

Carbonatite

Exocontact zone UnaffectedUnaffected

Country Rock

3+FeNaSi

±Al

K±Ca

Na and K amphiboleAegirine-augiteK-feldsparMesoperthiteAntiperthiteAlbitePale brown mica±wollastonite±nepheline±minor constituents

No scale

Endocontact zone

Figure 9. Schematic representation of bi-metasomatic fenitisation-type interaction between carbonatite melt and related fluidswith country rock. Direction of migration of elements is indicated by arrows. Minerals commonly observed in country rock affectedby fenitisation-type metasomatism are listed.

APPLIED EARTH SCIENCE 131

2014), the extent and intensity of metasomatism relatedto carbonatites and alkaline-carbonatite complexesdepends on large number of parameters including (1)Chemical composition, temperature, and pH of thefluids; (2) Chemical and mineralogical composition ofcountry rock (protolith); (3) Permeability and porosityof the country rock; (4) Temperature gradient betweenthe source of fluids and the country rock, (5) Fluid/rock ratio; (6) Duration of fluid movement, and (7)Others parameters (Korzhinskii, 1970; Zharikov et al.2007; Elliot et al. 2018). From the geological mapper’sand exploration geologist’s point of view, the simplestand most efficient method to vector towards the sourceof fenitizing fluids is based on mineralogical obser-vations. Under ideal conditions, fenitisation-type meta-somatism and its intensity can be observed with thenaked eye (Figures 10 and 11), using a hand lens orpolarising microscope (Figure 12).

As with carbonatite-associated silicate rocks, recog-nising fenitisation-type metasomatic halos duringexploration significantly increases the footprint of thecarbonatite or alkaline-carbonatite complex (Figure8). The documented thickness of fenitisation-typemetasomatc zones varies from a few centimetres to sev-eral kilometres and their shape is influenced to a largeextent by the geometry of permeable zones open tometasomatic fluids.

Estimates of surface areas affected by fenitisationrelated to carbonatites or alkaline-carbonatite com-plexes based on maps in scientific and governmentpublications are contained in Table 1. This datademonstrates that in some cases the fenitisation-typemetasomatic aureole substantially increases the foot-print of the carbonatite or alkaline-carbonatite com-plex. For example, at Panda Hill, Naantali, CallanderBay, Kangankkunde, and Ipanema, fenitisation rep-resents more than 80% of the complex in terms of sur-face area.

For many alkaline-carbonatite complexes, it isdifficult or impossible to determine if fenitisation-type metasomatism is related directly to the carbonatiteor to the spatially associated ijolite rocks (Morogan1994). Where more than one source of magmatic orcarbohydrothermal fluids is involved, several fenitisa-tion-type metasomatic patterns may overlap, compli-cating the vectoring procedure. For the purpose ofthis paper, we adopt the classification developed byHeinrich (1985) and popularised by Le Bas (2008),which divides fenites into ‘sodic’, ‘potassic’, and‘sodic-potassic’ (an intermediate between the twoend-members). Sodic fenites are characterised by thepresence of Na-rich amphiboles and pyroxenes, andK- and Na-feldspars. In extreme cases of Na fenitisa-tion, albitite rock will develop (Denaeyer 1966; LeBas 2008). Within carbonatite complexes, sodic fenitesare believed to form earlier, at higher temperatures, andat deeper levels than potassic fenites (Le Bas 2008).Potassic fenites consist mainly of K-rich feldspars(orthoclase or microcline), and in some cases may con-tain low-alumina phlogopite or biotite. Potassic fenitesmost commonly develop adjacent to or above theupper levels of intrusive calcite and dolomite carbona-tites. These fenites plot in the leucite solidus field on thequartz-nepheline-kalsilite diagram (Le Bas 2008).Historically, it has been suggested that within alka-line-carbonatite complexes, early calcite carbonatitesare accompanied by strong Na-K fenitisation-typemetasomatism, and that there is no fenitisation-typemetasomatism related to ferrocarbonatites (Le Bas1987); however, there are a number of cases where fer-rocarbonatite-related fenitisation-type metasomatismhas been recognised, such as the Swartbooisdrift com-plex, Namibia (Drüppel et al. 2005) and the GiffordCreek ferrocarbonatite complex, Western Australia(Pirajno et al. 2014).

Figure 11. Green-blue rock consisting of Na-amphibole, Na-clinopyroxene, and feldspar produced by fenitisation-typemetasomatism observed in contact with carbonatite (pale tomedium brown, left upper corner of picture). The late centi-meter-thick white vein, cutting green-blue rock, consists of cal-cite; Aley Nb deposit, British Columbia, Canada.

Figure 10. Nearly monomineralic, massive, K-feldspar-richfenite locally observed at the contact of Lonnie carbonatitewith metapelite country rock, British Columbia, Canada.

132 G. J. SIMANDL AND S. PARADIS

Morphology and geometry ofalkaline-carbonatite complexes

This section contrasts the morphologies of undeformedcarbonatites in unmetamorphosed intracratonic set-tings to those that are strongly deformed (pre-meta-morphic) in high grade metamorphic terranes. In thelatter case, there may be a need to use microscopy,geochemistry, stable isotopes, and rock associationsto distinguish between carbonatites and carbonatemetasediments.

Undeformed carbonatites in extensional settingsThe classic carbonatite model (Figure 13(a)) proposedby Garson and Smith (1958) was popularised by Hein-rich (1980) and Bowden (1985), and is still in use (e.g.Pirajno 2015). This model, based largely on field obser-vations from Chilva alkaline province in southernMalawi, fits many complexes from Eastern Africa andelsewhere. More recent models (Figure 13(b, c)) havebeen proposed by Le Bas (1977, 1987), and Sage andWatkinson (1991). The model of Le Bas (1987) displayswell the age relationships between lithological unitsand highlights fenitisation-type overprints (Figure13b). The model produced by Sage and Watkinson(1991) displays a limited number of lithologies (Figure8c) relative to the Garson and Smith (1958) model;however, it better depicts the relationship betweenthe volcanic edifice and crater facies. The erosionallevel shown in this model (Figure 13(c)) correspondsto the current level of exposure displayed by carbona-tites in southwest Quebec and Ontario, Canada.

No model depicts all of the possible rock associ-ations encountered in alkaline-carbonatite complexes,or is universally applicable. At deep erosion levels, car-bonatites are commonly spatially associated with ultra-mafic rocks. At moderate levels, they are spatiallyassociated with pyroxenites and jacupirangites, andwith ijolites and nepheline syenites at progressivelyshallower levels (Garson and Smith 1958).

Regardless of the model that we adopt, subvolcanic(hypabyssal) rocks in carbonatite complexes com-monly form radial dikes, cone sheets, and ring dikes(Figure 14). Examples include Salpeterkop structure,South Africa (Verwoerd 1990); Alnö, Sweden (Ecker-mann 1942, 1948, 1966; Kresten 1980); Gardiner com-plex, Greenland (Nielsen 1980); and Oka, Canada(Gold et al. 1967). These dikes and cone sheets mayconsist of nephelinites, alkali-rich mafic rocks, melilito-lites, phonolites, trachytes, lamprophyres, ijolites, orcarbonatites. The emplacement of a magma chamber(carbonatite or otherwise) and its related radial dikes,cone sheets, ring dikes, and doming is fundamentallyunderstood based on a combination of geologicalfield observations, laboratory experiments, andnumerical modelling (Koide and Bhattacharji 1975;Kresten 1980; Walter and Troll 2001; Klausen 2004;Bistacchi et al. 2012; Andersson et al. 2013). Whereanisotropic regional (tectonic) stress exceeds intru-sion-related stresses, steeply dipping or subverticaldike swarms are likely to form. Where local stressesrelated to a shallow magma chamber predominate,subvertical radial dikes; subvertical to outward dipping,semicircular to crescent shaped, nearly concentric ring

Figure 12. Variation in intensity of fenitisation-type metasomatism perpendicular to the strike of a 30 m thick carbonatite lens.Intensity of metasomatism decreases with increasing distance from calcite carbonatite contact, as recorded by disappearance ofaegirine and Na-amphibole over the distance of approximately 25 m. Lonnie carbonatite, British Columbia, Canada.

APPLIED EARTH SCIENCE 133

dikes; and inward dipping, circular, oval, or crescentshaped, concentric cone sheets are expected to form(Figure 14). These intrusion-related features can be

used to vector towards mineralisation. Radial dikesare expected to intersect at the vertical projection ofthe intrusive centre. Because ring dikes are subverticalto steeply dipping, the outermost of these roughlycoincide with the horizontal extent of the magmachamber. Cone sheets dip toward the intrusive centre.In most older models, the projections of the visible por-tions of cone sheets are interpreted to meet near asingle point coinciding with an explosion focus, suchas at Chilwa, Malawi (Garson and Smith 1958); Alnö,Sweden (Eckermann 1966); and Homa Mountain,Kenya (Le Bas 1977). In such models, the cone sheetsfarthest away from the magma chamber will have theshallowest dip; however, modelling by Bistacchi et al.(2012), shows that at least in some cases cone sheetsmay be coaxial with the same apical angle. At the levelswhere magma pressure exceeds lithostratigraphicpressure, zones of brecciation develop and domingtakes place. The volcanic edifices of carbonatites and

Figure 13. Morphology of carbonatite complexes as proposed by: (a) Garson and Smith (1958); (b) Le Bas (1987); and (c) Slightlymodified from Sage and Watkinson (1991) to show convex and concave nature of ring dikes and cone sheets, respectively. None ofthese models provides a complete picture; however, in combination these models provide a useful summary for explorationgeologists.

Magma chamber

Radial dikes

Conesheets

Volcanicneck

Ringdikes

Figure 14. Schematic three-dimensional presentation ofrelationship between magmatic chamber, ring dikes, radialdikes, cone sheets, and volcanic neck of a carbonatite complex.

134 G. J. SIMANDL AND S. PARADIS

related intrusions commonly consist of tuffs, agglomer-ates, and lava flows consisting of nephelinites, mellilite-bearing rocks, and, in some cases, phonolites and tra-chytes (Garson and Smith 1958).

Distinction between carbonatites andsedimentary carbonate rocks affected by highgrade metamorphism and tectonic activityDistinction between well exposed intrusive carbona-tites and carbonate rocks of sedimentary origin doesnot present a problem in unmetamorphosed intracon-tinental extensional settings because the morphology oflithological units, cross-cutting relationships (Figure 13(a, b, c); Figure 14), primary magmatic and carbohy-drothermal textures, and mineralogical compositionsare preserved (e.g. Barker 1993). Post-emplacementdeformation significantly changes the shape of a carbo-natite and related mineralised zones, and commonlyassociated metamorphism modifies the mineralogical,chemical, and textural characteristics of the ore andgangue minerals. This is especially true where dyna-mothermal metamorphism reaches upper amphiboliteor granulite facies, at which point many distinguishingfeatures are partially or completely obliterated. Examplesinclude Dahomeyide suture zone, West Africa (Attohet al. 2007); carbonatite-marble dikes of Abyan Province,Yemen Republic (Le Bas et al. 2004); metacarbonatesfrom Paleoproterozoic collision zone of the Borboremaprovince, NE Brazil (EJ dos Santos et al. 2013; RV Santoset al. 2013); and some carbonatites of the British Colum-bia alkaline province, Canada (Figure 15a, b; Kulla andHardy 2015). Both marbles derived from carbonatitesand those from sedimentary carbonates may appear

concordant with country rocks, be repeated by folding(Figure 16) or faulting, display metamorphic layering,and develop identical granoblastic, porphyroblastic, lepi-doblastic, and mylonitic textures.

Metacarbonates (marbles) of uncertain origin that arespatially related to alkaline silicate gneiss, blue–greenamphibolite (preserved fenitisation zone), or pyroxeniteshould be suspected to have originated from a carbonatiteprotolith. Shoshonitic igneous rocks or their meta-morphic equivalents commonly indicate a transition intectonic regimes involving block faulting and uplift withinisland arcs (Morrison 1980) and have been reported inassociation with post-collisional carbonatites by Houet al. (2006), Chakhmouradian et al. (2008), and Woo-dard and Hetherington (2014). The presence of blue–green Na- and K- amphiboles, perovskite, pyrochlore,

Figure 15. Morphology of the Upper Fir carbonatite, part of the Blue River carbonatite cluster, British Columbia, Canada shown in(a) vertical longitudinal section and (b) vertical cross-section perpendicular to longitudinal section [blue vertical line in (a)]. Meta-morphosed carbonatites commonly appear concordant with lithological contacts and metamorphic layering within the country rockand are repeated by folding (simplified from Kulla and Hardy 2015).

Figure 16. Folds within dolomite carbonatite highlighted bywhite apatite-rich layers; Aley Carbonatite, British Columbia,Canada.

APPLIED EARTH SCIENCE 135

columbite-(Fe), and fersmite in marble is evidence of acarbonatite origin. Le Bas et al. (2004) note that the pres-ence of aluminous minerals such as anorthite, scapolite,and ‘fassaitic’ pyroxene are (subsilicic aluminium ferriandiopside; Ca(Mg,Fe3+,Al)(Si,Al)2O6) indicative of a sedi-mentary origin; however, relying on these minerals tobe diagnostic of a non-carbonatite origin in metamor-phosed rocks may be misleading, as carbonatites canincorporate Al-rich country rock xenoliths or xenocrysts,and volcanic carbonatites may have been in contact withan Al-rich regolith or interbedded with Al-rich sedimentsprior to metamorphism.

If diagnostic textural and mineralogical criteria aremissing, geochemical analysis of REE, Nb, Sr, and Baconcentrations is recommended. Carbonatites arestrongly enriched in light rare earth elements (LREE)and are characterised by steep chondrite-normalisedREE patterns, without a Eu anomaly (Figure 17). The[Sr + Ba]-[REE + Y] discrimination diagram (Figure18) proposed by Samoilov (1991) can also be used to

discriminate between marble of sedimentary and car-bonatitic origins.

Isotopic composition can also be used to differen-tiate carbonatites from marbles of sedimentary origins.Ideally, magmatic carbonatites have well constrainedisotopic compositions and on δ18O and δ13C diagramsplot within the primary carbonatite fields (Figure 19(a)) defined by Taylor et al. (1967) and Keller andHoefs (1995), easily distinguishing them from carbon-ates of sedimentary origin. However, carbonatites thatare interpreted as primarily based on mineralogical cri-teria may plot outside of these fields, as illustrated onFigure 19(b) by the data from Indian carbonatitesfrom Ray and Ramesh (2006). The horizontal trend(no change in δ13C), indicates the effects of alterationby meteoric water, whereas an increase in both δ18Oand δ13C is commonly attributed to fractional crystal-lisation (Figure 19(a)). If a marble plots within or near

Figure 17. Chondrite-normalised REE plot of representativesamples from Aley cabonatite (Nb-deposit) and Wicheeda car-bonatite (REE-deposit), British Columbia, Canada, and field ofunmineralised carbonatites from Hornig-Kjarsgaard (1998).

Figure 18. The [Sr + Ba] to [REE + Y] discriminative bivariateplot is one of the criteria recommended to distinguish carbona-tites from marbles derived from carbonate sedimentary rocks(Samoilov 1991).

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18VSMOW

31B

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European carbonatites(Bell and Simonetti 2010)

Greenland carbonatites(Bell and Simonetti 2010)

Indian carbonatites (Ray and Ramesh 2006)

North American carbonatites(Bell and Simonetti 2010)

African Carbonatites(Bell and Simonetti 2010; Reid and Cooper 1992;Horstmann and Verwoerd1997)

South American carbonatites(Bell and Simonetti 2010)

Kola Peninsula, Russia(Demeny et al. 2004)

Phanerozoic carbonates(Knauth and Kennedy 2009)

Phanerozoic carbonates in clastics(Knauth and Kennedy 2009)

Fractional crystallization

Fractional crystallization

Meteoric water alteration

Meteoric water alteration

Primary igneous carbonatites(Keller and Hoefs 1995)

Primary igneous carbonatites(Keller and Hoefs 1995)

Primary igneous carbonatites(Taylor et al. 1967)

Primary igneous carbonatites(Taylor et al. 1967)

Figure 19. δ18O and δ13C plots showing: (a) Composition ofprimary igneous carbonatites, fractional crystallisation andmeteoric water alteration trends, and composition of Phanero-zoic sedimentary carbonates and carbonates in clastic sedi-mentary rocks; and (b) Compositional fields of European,Greenland, South American, Indian, African, and Kola peninsulacarbonatites. Compositional fields corresponding to Africanand Indian carbonatites are elongated along the trend repre-senting alteration by interaction with meteoric water. TheKola Peninsula carbonatite field follows the trend representingfractional crystallisation. Chondrite value from McDonough andSun (1995).

136 G. J. SIMANDL AND S. PARADIS

the primary carbonatite fields (Figure 19(a, b)) or if thedata form a trend originating from them, then thismarble is likely derived from a carbonatite protolith,as is the case of several carbonatites metamorphosedto upper amphibolite facies in the British ColumbiaAlkaline province, Canada. If a marble has higherthan expected δ18O and plots along the fractional crys-tallisation trend, it could have been derived from (1) acarbonatite protolith that interacted with late carbohy-drothermal or metamorphic fluids; (2) metamorphismof carbonate-rich sedimentary rocks; or (3) meta-morphism of hydrothermal carbonate bodies. Dis-tinguishing metamorphosed carbonatites andcarbohydrothermal carbonates from metacarbonatesof sedimentary origin may be impossible if they plotwithin the Phanerozoic carbonates fields (e.g. Africanand Indian carbonatites; Figure 19(a, b)).

Carbonatite-related ore deposits

Carbonatites and alkaline-carbonatite complexes and,in some cases, associated fenitisation-type metasomaticzones and overlying regolith (including zones of super-gene enrichment) are favourable hosts of metallic andindustrial mineral deposits (Figure 20). Reviews andcompilations of carbonatite-related mineralisation areprovided by Deans (1966), Heinrich (1980), Mariano(1989a, b), Pell (1996), Richardson and Birkett(1996a, 1996b), Birkett and Simandl (1999), Woolleyand Kjarsgaard (2008a), Berger et al. (2009), Simandl(2014), and Mackay and Simandl (2014a, 2015).

Alkaline-carbonatite complex-related ore depositsrepresent large resources in terms of REE (Figures 21(a, b) and 22(a), e.g. Bayan Obo, China; Maoniuping,

China; Mountain Pass, USA; and Mount Weld, Austra-lia). Most of these deposits are strongly enriched inLREE and have a high LREE/REE(total) ratio relativeto peralkaline intrusion-related, and ion adsorptionclay deposits (Mariano 1989a, Simandl 2014; Verplanket al. 2016); however, they also contain significantresources of heavy rare earth elements (HREE; Figure22(b)). Under current market conditions, concentratesof REE-bearing minerals from typical alkaline-carbo-natite complex related deposits are the main sourceof LREE. The ion adsorption clay deposits are lowgrade relative to alkaline–carbonatite intrusion-related- and peralkaline intrusion-related depositsand can’t compete effectively against high-grade-carbo-natite-related deposits as a source of LREE (Figure 22(a, b)). However, because peralkaline complex-relatedREE deposits are metallurgically challenging (Verbaanet al. 2015), and carbonatite concentrates have a highLREE/REE(total) ratio, the REE-bearing ion adsorptiondeposits which benefit from simple and cost-effectivemetallurgy are currently the main source of HREE(Simandl 2014). With the exception of the LREE-enriched, loparite and eudialyte–bearing, nepheline-feldspar-aegirine pegmatite at Lovozero (Russia),there is currently no production of REE from peralka-line intrusion-hosted deposits.

Alkaline-carbonatite complex-related REE mineral-isation may consist of unweathered carbonatite rocksuch as at Mountain Pass, USA, (Castor 2008) andSt-Honoré, Canada, (Lafleur and Ayad 2012) or corre-sponds to overlying regolith such as at Mount Weld,Australia (Lottermoser 1990) and Araxá, Brazil (Neu-mann and Medeiros 2015). The REE mineralisationtends to be concentrated in late carbonatite pulses

Figure 20. Vertical section of a hypothetical carbonatite mineralising system displaying the relationship between metallic andindustrial mineral deposits relative to lithological units and geological contacts. The ‘distal’ carbo-hydrothermal fluid-related min-eralisation or hydrothermally remobilised mineralisation (away from alkaline-carbonatite complex) and residual deposits withinweathered crust above the carbonatite complex are also highlighted. Bi – biotite, Px- pyroxene. Modified from Laznicka (2006).

APPLIED EARTH SCIENCE 137

forming central breccia zones, ring dikes, or conesheets, and fracture and open space fillings withinthese features. The REE are probably concentrated incarbohydrothermal fluids and melts produced by frac-tional crystallisation from carbonatitic melts (Richard-son and Birkett 1996a; Song et al. 2016). In severalcases, the REE mineralisation is interpreted as distalto carbonatite or alkaline-carbonatite complex; how-ever, in most of these cases the relation of REE miner-alisation to the ultimate source of fluids from which itformed is not clear or is controversial (e.g. the famousBayan Obo REE-Nb-Fe deposit in Central Mongolia;Smith et al. 2015; Lai et al. 2016; Liu et al. 2018).Based on the close spatial relationship of many REEdeposits to faults, fractures, suture zones, breccias,and other paleo-permeable zones, in combinationwith paragenetic studies of ore minerals, and in agree-ment with supporting experimental evidence indicat-ing that REE, Nb, and Ta can be transported byhydrothermal or carbo-hydrothermal fluids (e.g. Wil-liams-Jones et al. 2012; Timofiev et al. 2015, 2017, Mig-disov et al. 2016; Trofanenko et al. 2016), it isreasonable to expect the presence of REE (± Nb) depos-its exploration targets distal to the alkaline-carbonatitecomplexes (Figure 20).

Alkaline-carbonatite complex-related deposits arealso the main source of Nb (Figures 21(c, d) and 23(a); e.g. Catalão, Brazil; Lueshe, Democratic Republicof Congo; and St. Honoré, Oka, and Aley, Canada) as

summarised by Verwoerd (1986), Mariano (1989b),Berger et al. (2009), Mackay and Simandl (2014a),and Simandl et al. (2018). The Nb mineralisationmay be part of the carbonatite rock unit (e.g. St-Hon-oré, Canada; Tremblay et al. 2015), carbonatite-associ-ated alkaline rock (e.g. Crevier deposit, Canada; Solgadiet al. 2015), fenitised zone, or overlying regolith (e.g.Catalão I, Brazil; Cordeiro et al. 2011; and Araxá, Bra-zil; Issa Filho et al. 2015). Although high Ta concen-trations are expected mainly in pegmatites andperalkaline intrusion-related deposits (Mackay andSimandl 2014a), some deposits related to alkaline-car-bonatite complexes have comparable Ta concen-trations to pegmatites and contain significantresources of tantalum (e.g. Upper Fir carbonatite andCrevier alkaline-carbonatite complex, Canada; Figure23(b)). Unfortunately, under current market con-ditions, low Ta2O5/[Ta2O5 + Nb2O5] ratio in concen-trates makes carbonatites unattractive for Taextraction using conventional technology (Simandlet al. 2018).

Vermiculite and phlogopite deposits are predomi-nantly hosted by mafic or ultramafic rocks of the alka-line-carbonatite complex (e.g. Northern pyroxenite atPalabora, South Africa; Heinrich 1970; Fourie and DeJager 1986) near the contacts of carbonatites withthese rocks, or within mafic country rocks (e.g.Upper Fir carbonatite, Canada; Simandl et al. 2010).Most carbonatite-related apatite deposits currently in

Figure 21. Examples of main REE and Nb minerals and their concentrates; (a) Mixture of monazite [(Ce,La,Nd,Th)PO4] and bastnae-site [(Ce, La)(CO3)F] forms orange-brown fracture fillings in dolomite carbonatite; pen for scale; Wicheeda Lake prospect, BritishColumbia, Canada. (b) Bastnaesite concentrate containing 60–65 wt.% rare earth element oxides (REO), Mountain Pass carbonatite,California, USA; scale in millimetres. (c) Pyrochlore crystal [(Na,Ca)2Nb2O6(OH,F)] from the Upper Fir carbonatite, British Columbia,Canada; scale in millimetres (d) Pyrochlore concentrate, Niobec Mine, Quebec, Canada; scale in millimetres.

138 G. J. SIMANDL AND S. PARADIS

production, such as Tapira, Brazil (Capponi et al.2009); Ipanema, Brazil (Born 1986); Catalão I, Brazil(Carvalho and Bressan 1986); and Matongo, Burundi(Decree et al. 2015) were enriched by weathering.Examples of exceptions are the Siilinjärvi mine, Fin-land (O’Brien et al. 2015), and Cajati mine, Jacupira-naga Complex, Brazil (Alves 2008). Copper, U, Th,and baddeleyite (natural zirconia) were produced fordecades from the Palabora carbonatite-phoscorite

complex in South Africa (Heinrich 1970; Clarke1981; Milani et al. 2017), but baddeleyite is currentlyproduced only from the Kovdor deposit in Russia(Dickson 2015).

Other materials produced from carbonatites orrelated rocks are: iron (e.g. Kovdor, Russia; Dickson2015; Bayan Obo, China; Smith et al. 2015; and Pala-bora, South Africa; Heinrich 1970); fluorite (e.g.Mato Preto, Brazil; Okorusu, Namibia; and Amba

Figure 22. Grade and tonnage of REE-bearing deposits associated with carbonatite complexes in terms of (a) Total REO (TREO) and(b) Heavy REO (HREO) relative to pegmatites, peralkaline complexes, and rare element granites. Updated from Simandl (2014).

APPLIED EARTH SCIENCE 139

Dongar, India; Hagni 1999); carbonates for lime andcement production (e.g. Tororo, Uganda and Xiluvo,Mozambique; van Straaten 2002; and Jacupiranga,Brazil; Alves 2008); and sodalite for use as dimen-sion, ornamental, and semi-precious stone (e.g.Swartboosdrift, Namibia; Menge 1986; and CerroSapo, Bolivia; Schultz et al. 2004). Some alkaline-car-bonatite complexes, such as Tapira, Brazil (Swane-poel 2014) and Powder Horn, USA (resource of350 million tonnes with an average grade of 11.5%TiO2; Van Gosen and Lowers 2007; Van Gosen2009), contain titanium minerals, and were con-sidered for future development. However, both pro-jects stalled due to difficulties in producing market-acceptable TiO2 feedstock using conventional oralternative technologies.

In summary, carbonatites and alkaline-carbonatitecomplexes should be considered exceptional multi-commodity targets. Several of them host large depositscontaining metals (Nb, Fe, Cu and a variety of noblemetals as by-products), industrial minerals (e.g. apatiteand vermiculite), construction materials required tobuild and maintain local infrastructure (crushed aggre-gate and dimension stone), and a variety of niche pro-ducts such as badelleyite and sodalite, which is used asornamental and semi-precious stone. Projects invol-ving niche products (e.g. badelleyite and sodalite) are

not ideal for large exploration and mining companies;however, such projects can sustain smaller commu-nities and offer value-adding opportunities and relatedemployment.

Exploration methods

Copper and Fe deposits within the Palabora alkaline-carbonatite complex were discovered and mined forcopper and iron by African tribes about 1300 A.D.(Heinrich 1970), long before Bose (1884) firstdescribed a carbonatite occurrence and before A.G.Högbom recognised the magmatic origin of carbonaterocks at the Alnö carbonatite complex in Sweden nearthe end of the nineteenth century (Heinrich 1980)using traditional prospecting. Worldwide, a significantnumber of carbonatites and related deposits were dis-covered after the Second World War by traditionalprospecting and during regional geological mapping.The rate of discovery increased significantly between1956 and 1966 due to technical improvements in air-borne radiometric and magnetic survey technology incombination with aerial photointerpretation. Thismid-century surge in the rate of discovery is apparentfrom the numbers of known carbonatites reported ina series of consecutive review studies: Pecora (1956),Heinrich (1958), Heinrich (1966) and Woolley and

Figure 23. Grade and tonnage of (a) Nb and (b) Ta deposits associated with carbonatite complexes relative to pegmatites, peralka-line complexes, and rare element granites. Diagonal lines indicate tonnage of contained Nb2O5 and Ta2O5. From Simandl et al.(2018). Most grade and tonnage references are available in Mackay and Simandl (2014a). Abbreviations: (HR) hard rock ore, (W)weathered ore, (HR+W) hard rock and weathered ore combined.

140 G. J. SIMANDL AND S. PARADIS

Kjarsgaard (2008a), listed 32, 60, 320, and 527 knowncarbonatite occurrences, respectively. Selectedexamples of the use of well-established methods (radio-metric and magnetic) and new promising explorationmethods are described below.

Alkaline-carbonatite complexes which have notbeen severely deformed by post-intrusive tectonicactivity have oval, circular, ring-shaped, and crescent-shaped configurations of diagnostic rock units withcontrasting geophysical properties. These characteristic

morphological features are commonly detected byground and airborne magnetic and radiometric sur-veys, and in some cases gravity surveys (Satterly1970; Thomas et al. 2011, 2016; Shives 2015). TheOka alkaline-carbonatite complex, located approxi-mately 15 km west of Montreal, Canada (Figure 24(a)), is a particularly good example of applying thesegeophysical methods (Thomas et al. 2011). It is of Cre-taceous age and is emplaced in the gneisses of theCanadian Shield exposed within an erosional window

Figure 24. Oka carbonatite complex, Quebec, Canada (a) Geological setting, (b) Internal compositional rings, (c) Bouger gravityanomaly, (d) Residual total magnetic field, (e) Equivalent thorium, and (f) First derivative of magnetic field. Modified from Thomaset al. (2016).

APPLIED EARTH SCIENCE 141

through flat lying Paleozoic sedimentary rock of theSt. Lawrence Platform (Figure 24(a)). The complexaligns roughly with Cretaceous Monteregian intrusions(e.g. St. Bruno). The complex is oval-shaped, approxi-mately 10 km in length, and consists of two distinctlobes and numerous concentric ring dikes (Figure 24(b); Gold et al. 1967). It hosted the St. LawrenceColumbium mine which operated from 1961 to 1977,and consisted of two open pits and underground work-ings. Significant Nb resources remain in the ground.The Bouguer gravity anomaly (Figure 24(c)), residualtotal magnetic field (Figure 24(d)), equivalent Th(Figure 24(e)), and the first vertical derivative of themagnetic field (Figure 24(f)) coincide closely with theoutline of the complex.

In some cases, glacial dispersal of ore mineralsdown-ice from the deposit enhances the size of boththe geophysical and geochemical exploration target.For example, the Allan Lake carbonatite in Ontario,Canada, which was discovered by airborne gamma-ray spectrometry (5 km line spacing), is a small unex-posed carbonatite body approximately 0.4 km2 inarea, not including the fenite zone. The discovery ofsuch a small intrusion was possible because carbona-tite-derived material was incorporated into till, increas-ing concentrations of Ba, Nb, Th, Ce, La, Zn, Mn, andFe 10 to 20 times, and Y, P, Cu, Pb, Mo, Co, and U 5 to10 times those of background levels. The initial Thanomaly detected by an airborne gamma-ray surveycoincided with the Th-rich till and large Th-bearingboulders that were dispersed down-ice for nearly5 km from the deposit (Ford et al. 1988).

Examples of other carbonatites with distinctive geo-physical signatures are the Elk Creek (Nebraska, USA)and Catalão I (Brazil) intrusive alkaline-carbonatitecomplexes. Elk Creek is a semi-circular (6×8 km)intrusive complex covered by a 200 m thick sequenceof sedimentary rocks. It hosts the largest known Nbresource in the United States and significant REE min-eralisation (Drenth 2014). This complex coincides witha roughly annular vertical gravity gradient high, a sub-dued central low, and a magnetic high surrounded bymagnetic lows (Drenth 2014). The highest Nb concen-trations are encountered within a dolomitic carbona-tite, and REE are concentrated in a barite-dolomitecarbonatite. Catalão I alkaline-carbonatite complexhosts important Nb and phosphate deposits, and hasbeen covered by radiometric, magnetic, and gravi-metric surveys. Inversions of gravity and magneticdata reveal very similar models in terms of volumeand shape of the complex (Mantovani et al. 2016),and highlight the value of integrated studies.

Remote sensing involves acquiring, processing, andinterpreting spectral and spatial data from space andairborne platforms resulting from the interactionbetween matter and electromagnetic energy (Sabins1997). In general, multi- and hyperspectral instruments

are able to map host rock mineralogy based on materialspecific absorptions by vibrational processes in theShort-Wave Infrared (SWIR) region and are promisingtechniques for delineating carbonatites and associatedREE mineralisation (e.g. Rowan et al. 1986; Boescheet al. 2015).

Limitations of using ASTER and similar multispec-tral instruments over the Mountain Pass REE deposit,California, are provided by Rowan and Mars (2003).Due to limited spectral resolution, major mineralogycan be mapped, but minor differences (type of carbon-ate, accessory minerals) cannot. Hyperspectral remotesensing methods proved useful in outlining alkaline-carbonatite complexes that are not heavily covered byvegetation or glacial overburden such as the Sarfartoqcarbonatite complex, southernWest Greenland (Bedini2009); Epembe, Namibia (Zimmermann et al. 2016);and the Khanneshin carbonatite, Afghanistan (Marsand Rowan 2011).

A review of the application of visible and shortwaveinfrared spectroscopy for the analysis of REE-bearingminerals is provided by Turner et al. (2015). TheREE minerals exhibit characteristic absorptions, pri-marily in the Visible to Near Infrared (VNIR) region.However, as these absorptions are caused by transi-tional processes in the atom, they are sharp and ahigh spectral resolution is required. Several laboratorystudies (e.g. Rowan et al. 1986; Turner et al. 2015;Neave et al. 2016) suggest that Nd is the key pathfinderelement when exploring directly for REE deposits.Neave et al. (2016) indicate that hyperspectral instru-ments such as the Airborne Visible-Imaging Spec-trometer (AVIRIS), HyMap, or EnvironmentalMapping and Analysis (EnMap) may provide the spec-tral resolution needed to detect targets containing morethan 30,000 ppm Nd if datasets are collected at metre-scale spatial resolution. However, since most carbona-tite-hosted mineralisation is rich in La and Ce relativeto Nd (Figure 17), and grades of carbonatite-relateddeposits do not normally exceed 10% REO(total) (Figure22a), future technological refinements are required tomake this approach adequate in exploration for REEdeposits.

Limitations due to high detection limits of the aboveremote sensing methods in terms of total REE and Ndmay be at least partially circumvented by taking advan-tage of recent advances in drone-borne technology.Booysen et al. (2017) were able to detect Nd absorp-tions and differentiate REE enriched dikes at LofdalFarm, Namibia, with a small snapshot hyperspectralcamera (attached to drone) from 40 m altitude.

Soil, till, and stream-sediment geochemical surveysare among geochemical methods used in explorationfor carbonatite and related deposits. Soil geochemistry,where applicable, is favoured for its ability to delineateor extend mineralised zones prior to extensive tren-ching or drilling programmes, as shown by the results

142 G. J. SIMANDL AND S. PARADIS

of surveys over the Upper Fir Carbonatite, BritishColumbia, Canada (Kulla and Hardy 2015). Typically,conventional sampling of the B soil horizon followedby an aggressive sample dissolution method (such asfusion with sodium peroxide, lithium meta-borate, orlithium tetra-borate) prior to inductively coupledplasma mass or emission spectroscopy (ICP-MS/ES)is recommended for analysis, especially if Nb, Ta,REE, and Zr are of interest, because fusion-ICP-MS/ESmeasures the near total concentration of an element inthe sample. Fusion-ICP-MS/ES analyses ensure thatNb, Ta, REE, and Zr present in weathering-resistantminerals are detected. Without total dissolution,anomalies corresponding to weathering-resistant min-erals dispersed by mechanical processes such as hydrau-lic and glacial transport (Winterburn 2015) are unlikelyto be detected. Additionally, ‘Mobile Metal Ion’ (MMI),designed for detecting mineral deposits below glacialoverburden (e.g. clay, till, and sand), is a promising car-bonatite exploration technique as illustrated by an orien-tation survey aiming to detect mineralisation over theFen alkaline-carbonatite complex, Norway (Lie andØstergaard 2011). This method is based on the premisethat metal ions released from blind oxidising mineralis-ation rise vertically due to capillary rise and evaporation,and are adsorbed into clay particles in the top 10–25 cmof the soil cover, allowing detection in soil samples byMMI technology (Mann 2007; Turner et al. 2007).

Biogeochemical exploration is another promisingapproach; however, carbonatite-related cases employingthis method are exceedingly rare in western scientificand government reports. Two examples that the authorsare familiar with are the orientation surveys over theAllan Lake carbonatite, Ontario, Canada (Ford et al.1988) and the Upper Fir carbonatite, British Columbia,Canada (Fajber et al. 2015). Barium, P, Sr, Co, and Cu inleaf tissue of sugar maple trees were analysed forpathfinder minerals at Allan Lake. Vegetation over thecarbonatite showed concentrations of 13 times the back-ground concentrations for Ba, and 21 times for Sr (Fordet al. 1988). Lanthanum, Ce, Pr, Nd, Sm, Dy, Fe, Nb, Ta,P, and Y were identified by Fajber et al. (2015) as pre-ferred pathfinder elements in spruce and fir twigs andneedles at the Upper Fir carbonatite. This study alsoshowed that, in contrast to the absence of Eu anomaliesin REE profiles of carbonatite rocks, the chondrite nor-malised REE profiles of vegetation and soil over the car-bonatite may display strong negative Eu anomalies(Fajber et al. 2015). Although Nb is considered relativelyimmobile in surface environments, Nb and U anomaliesin humus corresponding to major structural featureswithin the Sokli alkaline complex, Finland, were docu-mented by Vuotovesi et al. (1980).

Carbonatites and related mineralisation are knownto have detectable signatures in till (e.g. Allan Lake,Ontario; Ford et al. 1988), glaciofluvial sediments(e.g. Sokli carbonatite complex, Finland; Perttunen and

Vartiainen 1992), and stream-sediments (e.g. Aley car-bonatite, British Columbia, Canada; Mackay andSimandl 2014b). An important development in recentyears was the commercialisation of hand held XRFinstruments and analytical protocols permitting theanalysis of many key pathfinder elements such as La,Ce, Nd, Pr, Y, Nb, Th, Sr, and Ba in concentrationsrequired for carbonatite exploration (Simandl et al.2014a, 2014b, 2014c). Hand held XRF significantlyreduces the costs of lithogeochemical, soil, and stream-sediment sample analyses, and increases explorationefficiency by providing real-time data in the field.

Recent studies demonstrated that high density(ϱ > 3.9 g/cm3) Nb- and REE-bearing minerals com-monly present in carbonatites, such as pyrochlore,columbite-(Fe), fersmite, monazite, and REE-bearingfluorocarbonates, are easy to concentrate using shakingtables or other gravity-based separators. Most of theseminerals are not ideal for visual identification, and inmany cases occur in grain size fractions too small forhand-picking; however, they can be identified by oneof several automated measurement techniques basedon Scanning Electron Microscopy (SEM) and Energy-Dispersive X-ray Spectroscopy (EDS). Orientation sur-veys over three known carbonatite deposits that com-bine stream-sediment geochemistry and automatedquantitative scanning electron microscopy of streamsediments suggest that carbonatite-related Nb mineral-isation can be detected more than 11 km downstream(Mackay et al. 2015, 2016). This approach, in combi-nation with newly developed pyrochlore and colum-bite-Fe discrimination diagrams (Mackay andSimandl 2015) and use of the ‘direct indicator mineralconcept’ (Simandl et al. 2017) may popularize the useof indicator minerals for detection of carbonatitesand exploration for related Nb and REE deposits.Apatite may become particularly useful indicator min-eral (Mao et al. 2016; Chakhmouradian et al. 2017).

The ‘direct indicator mineral’ concept combines lowcost stream-sediment surveys and automated scanningelectron microscopy to replace traditionally expensiveindicator mineral surveys, eliminating the need forlarge samples and mineral hand-picking. When thisapproach is applied, only sediment samples anomalousin carbonatite pathfinder elements (detected usingpXRF), with or without gravity pre-concentrating,need to be analysed by automated quantitative scan-ning electron microscopy to determine from whichdeposit-type direct indicator minerals were derived.

Successful exploration and development ofcarbonatite-related deposits: what are theodds?

The exploration industry estimates that less than one in1000 discoveries become a profitable mine (Marshall2012; Preston 2015). According to the Ontario Mining

APPLIED EARTH SCIENCE 143

Association (2015), the odds are even lower: ‘Onlyabout one mineral exploration project in ten is takento the drill stage, and one drill programme in 1000finds a viable mineral deposit’. Data compiled byWoolley and Kjarsgaard (2008a) indicates thatapproximately 6% of the 527 reported carbonatitesand alkaline-carbonatite complexes host active mines,3% hold historic mines, and 11% contain an establishedmineral resource. These numbers suggest that carbona-tites have exceptional potential, and that a newly dis-covered alkaline-carbonatite complex has a 9 out of100 probability of hosting a mine.

Conclusion

Carbonatite-related deposits are the main sources ofNb and LREE; significant sources of baddeleyite, Fe,Cu, vermiculite, phlogopite, fluorite, apatite, calciumcarbonate, and sodalite; and have historically producedU and Th. Given that 9 out of 100 carbonatites andalkaline-carbonatite complexes contain currently pro-ducing or historic mines, carbonatite-related mineralis-ing systems represent outstanding multi-commodityexploration targets.

Knowledge of tectonic setting, typical rock associ-ations, and deposit morphology can assist exploration-ists seeking carbonatites and related deposits.Statistically, the most favourable exploration areas forcarbonatites and related deposits are intracratonic rift(extensional) settings; however, most outcropping, ornear-surface intrusive carbonatites and alkaline-carbo-natite complexes in these settings are likely already dis-covered. The search for carbonatites in orogenicsettings is not favoured by current statistics, butBayan Obo, the world’s largest REE-Fe-Nb deposit, isfound in such a setting. Strongly metamorphosed anddeformed carbonatites were historically difficult toidentify; consequently, most new intrusive carbonatitediscoveries will probably be in collisional settings.Identifying and incorporating carbonatite-related sili-cate igneous rocks and fenitisation-type metasomaticzones into carbonatite models significantly increasethe size of exploration targets. Under ideal conditions,the intensity of fenitisation-type metasomatism mayalso be used to vector toward the source of metasoma-tising fluids. Knowledge of carbonatite morphology(spatial relationships between ring dikes, radial dikes,and cone sheets) permits vectoring toward the intrusivecentre in undeformed geological settings; however,applications of these relationships in deformed terrainscan be problematic at best. Age alone is not a valid par-ameter of prospectivity; however, within the samecomplex, the latest carbonatite pulses are commonlyenriched in REE.

Although there is no universally applicable methodused in exploration for carbonatites and related min-eral deposits, on the regional or reconnaissance-scale,

airborne geophysical methods (mainly radiometricand magnetic) are proven to be able to locate and delin-eate carbonatites. Ground radiometric and magneticmethods are useful for follow-up surveys. A variety ofgeochemical (e.g. stream-sediment, soil, and till chem-istry; ‘direct indicator minerals’) and biochemicalmethods have been successfully used, tested, or custo-mised for exploration and detection of carbonatitesand related deposits.

Acknowledgements

This project was supported by the Targeted GeoscienceInitiatives 4 (2010–2015) and 5 (2016–2020), a NaturalResources Canada programme. The Specialty Metal com-ponents of these initiatives are collaborative efforts betweenthe Geological Survey of Canada and the British ColumbiaGeological Survey, Victoria, BC. Reviews and drafting byPearce Luck, Michaela Neetz, and Carlee Akam from theBritish Columbia Geological Survey were essential for com-pletion of this manuscript. Sections covering geochemical,geophysical and remote sensing exploration methods werereviewed for completeness by Ray Lett, Geochemical Con-sultant (Victoria), Michael Thomas from the Geological Sur-vey of Canada (Ottawa), and Robert Zimmermann fromHelmholtz Institute Freiberg for Resource Technology (Frei-berg, Germany). The section on genesis benefited from con-structive comments by Alexei Rukhlov, British ColumbiaGeological Survey, Victoria, BC, Canada. Encouragementfrom Alan Galley, Malleus Consulting Corp., Ottawa, ON.,Mike Villeneuve, Geological Survey of Canada (Ottawa,ON), and David Lefebure formerly of the British ColumbiaGeological Survey and currently with Geologic Ltd. (SaltSpring Island, BC, Canada) were essential for initiating andcompleting this document. The final version of this manu-script benefited from suggestions and comments of threeanonymous reviewers and handling by Simon Jowitt, co-edi-tor of Applied Earth Science.

Disclosure statement

No potential conflict of interest was reported by the authors.

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