81.Full

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IntroductionZIRCONIUM is an incompatible trace element and occurs inmost igneous rocks at parts per million concentrations as theaccessory mineral, zircon. However, in ultra-alkaline com-plexes (e.g., Ilímaussaq: Gerasimovsky, 1969; Strange Lake:Miller, 1986; Thor Lake: Trueman et al., 1988; Lovozero:Kogarko, 1990), it is locally present in percentage level con-centrations and may be accompanied by quantities of theheavy rare earth elements (HREE) sufficient to form a po-tentially exploitable resource. In most of these complexes, thezirconium occurs mainly as complex zirconosilicate minerals,such as elpidite (Na2ZrSi6O15�3H2O), catapleiite (Na2ZrSi3O9.2H2O), wadeite (K2ZrSi3O9), eudialyte (Na15Ca6(Fe2+,Mn2+)3Zr3(Si25O73)(O,OH,H2O)3(OH,Cl)2), vlasovite (Na2ZrSi4O11), and gittinsite. (CaZrSi2O7), but at Thor Lake, theprincipal zirconium mineral is zircon.

There is a broad consensus that ultra-alkaline igneous rocksare products of differentiation of partial melts from an unde-pleted, metasomatically enriched mantle (e.g., Marks et al.,2003; Halama et al., 2004). However, there is little agreement

over why the rare earth elements (REE) concentrate in theseintrusions, why HREE can be enriched relative to light rareearth elements (LREE), and whether the deposits are purelymagmatic or partly the product of hydrothermal processes(Boily and Williams-Jones, 1994; Schmitt et al., 2002; Markset al., 2003; Salvi and Williams-Jones, 2005). Although thereasons for the very high concentrations of Zr, Nb, andHREE in ultra-alkaline igneous systems are still poorly un-derstood, experiments by Watson (1979) and Linnen andKeppler (2002) have shown that zircon solubility in magmasincreases with increasing concentration of alkalis, and in per-alkaline melts can exceed 3 wt % ZrO2. Studies of melt inclu-sions have yielded similar results, showing that agpaitic mag-mas can have concentrations of Zr, Nb, and REE that, insome cases, are orders of magnitude higher than those inother types of magma. For example, Kovalenko et al. (1995)reported concentrations of Zr, Nb, and REE from rare metalperalkaline granites of the Khadlzan-Buregtey GranitoidComplex of 2.65, 0.58, and 0.62 wt %, respectively.

In several peralkaline complexes, fractional crystallization hasserved to further increase concentrations of the above ele-ments over those originally present in the melt. For example,

Controls on the Concentration of Zirconium, Niobium, and the Rare Earth Elements in the Thor Lake Rare Metal Deposit, Northwest Territories, Canada

EMMA R. SHEARD,1,† ANTHONY E. WILLIAMS-JONES,1 MARTIN HEILIGMANN,1 CHRIS PEDERSON,2 AND DAVID L. TRUEMAN2

1 Earth and Planetary Sciences, McGill University, 3450 University Street, Montreal, Quebec, Canada H3A 2A72 Avalon Rare Metals Inc., 130 Adelaide Street West, Suite 1901, Toronto, Ontario, Canada M5H 3P5

AbstractThe Thor Lake rare metal (Zr, Nb, REE, Ta, Be, Ga) deposits in Canada’s Northwest Territories represent

one of the largest resources of zirconium, niobium, and the heavy rare earth elements (HREE) in the world.Much of the potentially economic mineralization was concentrated by magmatic processes. However, there isalso evidence of autometasomatic processes and remobilization of Zr and REE by hydrothermal fluids.

The deposits are situated at the southern edge of the Slave province of the Canadian Shield, within the 2094to 2185 Ma alkaline to peralkaline Blachford Lake Intrusive Complex. A layered alkaline suite dominated byaegirine nepheline syenite occurs in the center of this suite of rocks and is considered to represent the youngestphase of the complex.

Much of the rare metal mineralization occurs in two subhorizontal tabular layers, which form upper andlower zones of the Nechalacho deposit (formerly the Lake zone), and in which Zr is hosted primarily by zircon,Nb primarily by ferrocolumbite and fergusonite-(Y), and HREE by fergusonite-(Y) and zircon. The LREE arepresent mainly in monazite-(Ce), allanite-(Ce), bastnäsite-(Ce), parisite-(Ce), and synchysite-(Ce). Much of theHREE mineralization in the lower mineralized zone occurs in secondary zircon, which forms small (10−30 µm)anhedral grains in pseudomorphs after probable eudialyte. In the upper zone, zircon is a magmatic cumulatemineral, which was replaced locally by secondary REE-bearing minerals. Element distribution maps of zirconcrystals in the upper zone indicate that the HREE were mobilized from the cores and locally precipitated asfergusonite-(Y) along microfractures. The light rare earth elements (LREE) were also mobilized locally fromboth primary zircon and inferred primary eudialyte. The occurrence of zircon in fractures, wrapped aroundbrecciated K-feldspar fragments, and as a secondary phase in pseudomorphs are evidence of its hydrothermalorigin and/or of remobilization of primary zirconium.

A model is proposed in which injection of separate pulses of miaskitic and agpaitic magma resulted in thecrystallization of an upper zone rich in zircon and a lower zone rich in eudialyte. Primary eudialyte was lateraltered in situ to zircon-fergusonite-(Y)-bastnäsite-(Ce)-parisite-(Ce)-synchysite-(Ce)-allanite-(Ce)-albite-quartz-biotite-fluorite-kutnahorite-hematite−bearing pseudomorphs by an inferred fluorine-enriched mag-matic hydrothermal fluid. Zirconium, niobium, and REE in both the upper and lower zones were subsequentlymobilized during multiple metasomatic events, which, for the most part, served to further enrich the primarylayers in REE (albitization generally dispersed REE and high field strength elements (HFSE)) and creatednew secondary REE-bearing phases.

† Corresponding author: e-mail, [email protected]

©2012 Society of Economic Geologists, Inc.Economic Geology, v. 107, pp. 81–104

Submitted: March 19, 2010Accepted: July 5, 2011

at both Ilímaussaq and Lovozero, REE and high fieldstrength elements (HFSE) were enriched locally by satura-tion of minerals like eudialyte and loparite ((LREE,Na,Ca)2(Ti,Nb)2O6) in the magma and their gravitational settlingto form layers in which these minerals are dominant. There isalso evidence that in some of these complexes autometaso-matic processes have further concentrated these elements.For example, REE and the HFSE in the Tamazeght Com-plex, Morocco, are hosted in secondary phases such as calciccatapleiite ((Ca,Na)ZrSi13O9.2H2O) and rinkite ((Ca,LREE)4

Na(Na,Ca)2Ti(Si12O7)2F2(O,F)2), which replaced primary zir-con and eudialyte (Salvi et al., 2000). In the most evolved lu-javrites at Ilímaussaq (Sørensen, 1997), there are hydrother-mal veins of steenstrupine (Na14LREE6Mn++Mn+++Fe++2(Zr,Th)(Si6O18)2(PO4)7.3(H2O)) and at Strange Lake, Quebec/Labrador, bulk-rock chemical analyses of fresh and alteredrocks point to hydrothermal enrichment of the HFSE by asmuch as 25% (Salvi and Williams-Jones, 1996).

The Thor Lake Nechalacho deposit is an ideal setting inwhich to investigate the relative importance of magmatic andhydrothermal processes involved in producing unusual en-richments of Zr, Nb, and REE. The bulk of the rare metalmineralization is hosted in zircon, which contains the highestconcentrations of REE ever reported in the literature. Thereare cumulate textures involving both zircon and inferred eu-dialyte, there is extensive hydrothermal alteration, and thereis strong evidence for the remobilization of REE, Zr, and Nbfrom primary zircon and inferred primary eudialyte.

Exploration of the Thor Lake deposits began in 1970, andafter years of intermittent exploration activity, five zones ofrare metal mineralization were delineated, namely the R, S,T, Fluorite, and Lake zones (Trueman et al., 1988). By far thelargest of these is the Lake zone, recently renamed theNechalacho deposit, which drilling has demonstrated to bepresent in the subsurface over an area of at least 2 km2. TheNechalacho deposit has been the focus of recent explorationby Avalon Rare Metals Inc. (Avalon) for Zr, Nb, and REEmineralization and is the subject of the present study. Thismineralization occurs in two zones, an upper zone with an in-dicated resource of 30.64 million metric tons (Mt) and a totalREE concentration of 1.48 wt %, and a lower zone with an in-dicated resource of 57.49 Mt and a total REE concentrationof 1.56 wt % (Table 1). Only a limited number of papers havebeen published on the Thor Lake deposits (Davidson, 1981;Cerny and Trueman, 1985; Trueman et al., 1988; Smith et al.,1991; Pinckston and Smith, 1995). However, an importantconclusion of all of these studies is that Thor Lake differsfrom other REE deposits in that hydrothermal alterationobliterated most of the original igneous textures such that

even the nature of the host rocks could not be properly dis-cerned. As a result of recent drilling by Avalon, we are nowable to partly decipher the prealteration history of the de-posit. The purpose of our study was to use the textural rela-tionships and composition of zircon and other rare metal min-erals in the Nechalacho deposit to investigate the role ofprimary magmatic, autometasomatic, and late hydrothermalprocesses in their crystallization, and determine the extent towhich some or all of the REE and HFSE were remobilized.In doing so, this paper not only describes an important exam-ple of a newly recognized large REE- and HFSE-enrichedlayered alkaline complex but proposes a model that explainsthe genesis of the Nechalacho deposit and can be applied tothe study of similar deposits elsewhere.

Geologic Setting

Regional geology

The Thor Lake rare metal (Zr, Nb, REE, Ta, Be, and Ga)deposits are located approximately 100 km southeast of Yel-lowknife, Northwest Territories (Fig. 1) at the southern edgeof the Slave province of the Canadian Shield in alkaline toperalkaline rocks of the Blachford Lake Intrusive Complex(Davidson, 1978; Trueman et al., 1988; Sinclair et al., 1992).According to Hoffman (1978), at ~ 2.1 Ga, the Slave Cratonrested over hot spots at Great Slave and Coronation Gulf. Aseries of rifts developed, some in the Wopmay Orogen (ca.1900 Ma) and the others along the eastern arm of Great SlaveLake (the Athapuscow Aulacogen), which coincides with theGreat Slave Fault zone, a major strike-slip shear zone with apostulated displacement of 500 km (Davidson, 1978; Bowringet al., 1984). It is here, just prior to rifting, that a series of al-kaline to peralkaline plutons were intruded to form theBlachford Lake Intrusive Complex.

Davidson (1972, 1978) subdivided the Blachford Lake In-trusive Complex into six units representing five distinct intru-sive events. These units are the Caribou Lake Gabbro, theWhiteman Lake Quartz Syenite, the Hearne Channel Gran-ite, the Mad Lake Granite, the Grace Lake Granite, and theThor Lake Syenite (Fig. 1). The youngest of the intrusiveevents produced the Grace Lake Granite and the Thor LakeSyenite, which have gradational contacts, and were thereforeinterpreted by Davidson (1982) to have been emplaced con-temporaneously. A suite of igneous rocks, which we interpretto represent a layered alkaline complex (see below), was sub-sequently intersected by drilling in the core of the Thor LakeSyenite and intrudes the latter. This suite hosts the bulk of therare metal mineralization at Thor Lake (the Nechalacho de-posit). Several of the above units have been dated by U-Pb

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TABLE 1. Provisional Concentrations of REE, Ta, Nb, Zr, Ga, and Hf Oxides for the Nechalacho Deposit (Avalon Rare Metals Inc. Resource Estimate, updated January 2011)1

Metric tons TREO HREO ZrO2 Nb2O5 Ga2O3 Ta2O5

(millions) (%) (%) (%) (%) (ppm) (ppm)

Lower zone (indicated) 57.49 1.56 0.33 2.99 0.40 133 396Upper zone (indicated) 30.64 1.48 0.15 2.1 0.31 173 192

Abbreviations: HREO = high REE2O3, TREO = total rare earth oxides1 Cutoff grade for TREO (%) is 1.6

geochronology. Bowring et al. (1984) obtained a zircon age forthe Hearne Channel Granite of 2175 ± 7 Ma and reported anunpublished zircon age (supplied by A. Davidson) for theWhiteman Lake Quartz Syenite of 2185 ± 5 Ma. These au-thors also reported a zircon age of 2094 ±10 Ma for a drill coresample interpreted to represent the Thor Lake Syenite. Sin-clair et al. (1994) subsequently dated the Grace Lake Graniteusing zircon and monazite, at 2176.2 ± 1.3 Ma, which is es-sentially the same as the age obtained for the Hearne Chan-nel Granite. They also reinterpreted the origin of the sampleof supposed Thor Lake syenite dated by Bowring et al. (1984)and, based on its description as a magnetic rock containing 10to 20% zircon euhedra and xenomorphic grains, concludedthat the sample was from the rock unit that we now refer toas the Nechalacho Layered Alkaline Suite. We concur withthis reinterpretation and believe that this sample is from theupper mineralized zone (see below). The age of 2094 ±10 Mapreviously attributed to the Thor Lake Syenite is thus consid-ered to represent the age of the Nechalacho Layered AlkalineSuite and/or the rare metal mineralization contained therein.

All of the above rock units were intruded into Archean micaschists of the Yellowknife Supergroup, which surrounds theBlachford Lake Intrusive Complex. To the south aregraywackes, shales, and carbonates of the Great Slave Super-group (Trueman et al., 1988), which are separated from theBlachford Lake Intrusive Complex by a postulated faultedcontact along the Hearne Channel of Great Slave Lake.

The Nechalacho Layered Alkaline Suite

The Nechalacho Layered Alkaline Suite consists of a se-quence of silica-undersaturated intrusive rocks that increasein alkalinity with depth. The suite is only exposed at surfacelocally in a small area between Long Lake and Thor Lake(Fig. 2), but based on drill intersections, it is postulated to dipbeneath the Thor Lake Syenite in all directions, forming adome-shaped body that at the current depth of drilling (~300m) has a diameter of >1.5 km; its depth extent is unknown. Incross section, the suite progresses downward from a roof zoneof sodalite cumulates, through coarse-grained to pegmatiticaegirine (±arfvedsonite) syenite and layers of cumulate zircon,

Zr, Nb, Y, & REE IN THE THOR LAKE RARE-METAL DEPOSIT, NWT, CANADA 83

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Yellowknife

Fort Resolution

Hay River

ThorLake

Great Slave Lake

N

100 km

NECHALACHODEPOSIT

THOR LAKE PROPERTY

5 km

T-ZONE

Hearne Channel

APHEBIAN

Great Slave Supergroup

Yellowknife Supergroup:Burwash Formation

Two-mica Granite

ARCHEAN

Caribou Lake Gabbro

Thor Lake Syenite

Grace Lake Granite

Hearne Channel, Mad Lake Granites

Whiteman Lake Quartz Syenite

BLACHFORD LAKE COMPLEX

Leucoferrodiorite

Compton Intrusions

FIG. 1. Geologic map of the Blachford Lake Intrusive Complex, showing the location of the Nechalacho deposit and Tzone within the Thor Lake rare metal deposit (modified after Davidson, 1982).

to foyaitic aegirine nepheline syenite with laterally continuouslayers of zircon-bearing pseudomorphs inferred to representeudialyte cumulates (see below). Pervasive metasomatic flu-ids have strongly altered the rocks in this upper sequence,largely replacing the original mineralogy. However, below theinferred eudialyte cumulates the primary mineralogy and tex-tures are increasingly preserved and relatively unaltered ae-girine- and nepheline-bearing syenites and sodalite foyaitesare observed.

The roof sodalite cumulates are strongly altered, medium-to coarse-grained rocks that display intrusive contacts withthe overlying Thor Lake Syenite. Hexagonal, primary sodalitephenocrysts have been completely altered to fine-grainedmuscovite, illite, and/or chlorite in most samples examined.Sodalite crystals display a distinctive orange fluorescenceunder UV illumination (Fig. 3A), which allows them to beeasily identified in drill core despite their extensive alterationand replacement. In less altered samples, crystals of alteredsodalite are enclosed in coarse anhedra of orthoclase and anunknown altered dark green mineral, which could have beenaegirine or arfvedsonite. Orthoclase and albite are the inter-stitial minerals and are commonly intergrown and display aperthitic exsolution texture.

A sharp contact, commonly obscured by the alterationoverprint, is observed between the roof sodalite cumulatesand strongly altered aegirine (±arfvedsonite) syenite belowthem. This medium- to coarse-grained rock unit is highlyheterogeneous in texture and is variably porphyritic and foy-aitic with common pegmatitic lenses. A ~15 to 30 m thickupper rare metal mineralized zone, comprising fine wavylaminations and/or ribbons of zircon crystals (Fig. 3B, C) and

macroscopically visible brick red bastnäsite-(Ce) with locallyintense biotite + magnetite and fluorite + albite alteration oc-curs within this unit. Zircon is also present in veins where it iswrapped around brecciated K-feldspar fragments. Both theupper and lower boundaries to this zone are gradational.

The underlying foyaitic aegirine nepheline syenites are char-acterized by trachytic flow textures (Fig. 3D) imparting a pro-nounced subhorizontal alignment of the phenocryst phases(aegirine and/or K-feldspar and/or nepheline). The foyaites arehighly heterogeneous, both mineralogically and texturally, dis-playing numerous cyclical changes in the dominant phenocrystphase and local interlayering of microsyenites and coarsergrained porphyritic syenites. This unit hosts the ~15 to 60 mthick lower mineralized zone, which is composed of denselypacked pseudomorphs (Fig. 3E) inferred to represent eudia-lyte cumulates (see below), and generally has a gradationalupper boundary and sharp lower contact (Fig. 3F). In somedrill holes, the two mineralized zones are separated by un-mineralized rocks, whereas, in other holes, the upper zonegrades directly into the lower zone.

Plots of REE abundance with depth (Fig. 4) indicate thatthe heavy REE are concentrated in the lower mineralizedzone, that intermediate REE (e.g., Gd, Dy) have no apparentpreference for upper or lower mineralized zones, and that thelight REE are concentrated in the upper mineralized zone.The ore mineralogy in each zone is broadly similar. Zirconiumis hosted mainly by zircon, niobium by ferrocolumbite and fer-gusonite-(Y) ((Ce,La,Nd,Y)NbO4), the HREE by fergusonite-(Y) and zircon, and the LREE by monazite-(Ce) ((Ce,La,Th,Nd,Y)PO4), allanite-(Ce), bastnäsite-(Ce) ((Ce,La,Y)F(CO3)),parisite-(Ce) (Ca(Ce,La)2(CO3)3F2) and synchysite-(Ce) (Ca

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N

500m

Sub-outcrop map (based on drill-hole data)

415,000m 416,000m 417,000m

6,888,000m

6,887,000m

6,886,000m

Cross section based on drill holes L09-154 and L09-139

L09-154 L09-139

230.45m

199.95m

LEGEND

Sodalite roof cumulate

Albitized aegirine nepheline syenite

Upper mineralized zone

Lower mineralized zone

Thor Lake Syenite(undifferentiated)

Nechalacho Layered Alkaline Suite

Blachford Lake Complex

Grace Lake Granite

Biotitized and iron-oxide altered(plus minor quartz) aegirine nepheline syenite

Location of cross section

FIG. 2. Suboutcrop geology map (based on drill hole data) and north-south geologic cross section through the Nechala-cho Layered Alkaline Suite, showing the distribution of the various geologic units. The cross section through drill holes L09-154 and L09-139 illustrates the relationship of the mineralized zones to the host rocks.


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1 cm

A B

C D

E F

G H

1cm

or

1 cm

neab

aeg

1 cm

or

zrn

zrn

200 µm

chl

bt

zrnaln

mnz

magbt

sdlchl

orzrn

bt

1 cm

1 cm 1 cm

FIG. 3. A. Photograph of roof sodalite (sdl) cumulate in drill core under UV illumination showing a distinctive orange fluo -rescence. B. Drill core from typical upper mineralized zone rocks showing wavy laminations and/or ribbons of tan-coloredzircon (zrn) perpendicular to core axis (vertical hole). C. SEM-BSE image of type 1a zircon (zrn) in undulating laminationswrapping around pseudomorphs of precursor aegirine (aeg) now filled by magnetite (mag) and biotite (bt). Within the lam-inations, zircon crystals are broken and locally replaced by allanite-(Ce) (darker gray; aln) and monazite-(Ce) (white; mnz).Biotite and magnetite are the darker gray and/or black phases surrounding zircon-rich layers. D. Drill core showing a tra-chytic flow texture defined by K-feldspar and pseudomorphs after eudialyte in a dark matrix of chlorite and sericite. E. Drillcore containing densely packed tan-colored pseudomorphs after primary eudialyte. F. Drill core showing the boundary be-tween a microlayer of hematized zircon (pseudomorphs after eudialyte?) below and a foyaitic cumulate layer above. G. Drillcore displaying a cumulate zone with fine subhorizontal internal magmatic layering. H. Photograph of aegirine nephelinesyenite in drill core with aegirine (aeg) and nepheline (ne) as phenocrysts in a groundmass of orthoclase (or).

(Y,Ce,La,Nd,Gd)[F(CO3)2]). Table 2 lists the names and com-positions of ore minerals identified in the deposit. In thelower zone, zircon and REE-bearing minerals are secondaryphases in pseudomorphs formed after the in situ dissolutionof inferred primary eudialyte (see below).

Beneath the lower mineralized zone is a series of hetero-geneous, magmatically layered aegirine ± nepheline ± so-dalite syenites. These rocks are variably fine grained to peg-matitic and contain interlayers of microsyenite (Fig. 3G),foyaitic syenite, porphyritic aegirine nepheline syenite, andsodalite-rich syenite. In the dominant unit, the aegirinenepheline syenite, aegirine and nepheline form phenocrystsin a groundmass consisting dominantly of orthoclase withminor accessory phases and secondary alteration minerals(Fig. 3H). The nepheline phenocrysts form subhedraloikocrysts, up to 1 cm in diameter, and typically display apoikilitic texture produced by lath-shaped poikalites of ae-girine and albite, and anhedral poikalites of chlorite and zir-con. Aegirine phenocrysts display a range of shapes and sizes.They vary in length from <1 up to 8 mm and can be both un-zoned and rimmed by inclusion-free overgrowths. Most ae-girine crystals are fractured along cleavage planes with thefractures filled by biotite, albite, magnetite/hematite, andminor fluorite. The aegirine also hosts inclusions of allanite-(Ce) ((LREE,Ca,Y)2(Al, Fe3+)3(SiO4)3(OH)), zircon, and a Ca-Zr silicate (possibly gittinsite or vlasovite). Interstitial, bladedalbite is common in the groundmass between nepheline andaegirine crystals and locally replaces both. Sporadic, irregu-larly shaped patches of rare metal mineralization occur

throughout, and are dominated by zircon, but also containminor britholite-(Ce) (Ca2 [(Y,Ce)Ca]3[(OH,F)(SiO4,PO4)3]),allanite-(Ce), and thorite. Many of these patches, particularlyin the lower mineralized zone, have regular geometric shapeswith planar elements reminiscent of crystal faces, suggestingthat they represent pseudomorphs of an earlier mineral, in-ferred to be eudialyte. This interpretation is supported by theoccurrence of unaltered eudialyte intergrown with analcime(NaAl(Si2O6). (H2O)) and oneillite (Na15Ca3Mn3Fe2+3Zr3Nb(Si25O73)(O,OH,H2O)3(OH,Cl)2) in core from drill hole 85-L6at a depth of 349 m.

Hydrothermal AlterationThe rocks in the upper part of the layered alkaline suite

have been intensely altered by hydrothermal fluids, leavingonly relicts of their primary mineralogy. Hydrothermal alter-ation is pervasive and is not conformable to igneous layering.The principal alteration minerals are magnetite, hematite, bi-otite, chlorite, illite, muscovite, calcite, quartz, fluorite, andbladed albite (“cleavelandite”). Minor alteration minerals areankerite, siderite, kutnahorite (Ca(Mn,Mg,Fe)(CO3)2), scapo-lite, pyrite, and sphalerite.

Magnetite and biotite are the dominant alteration mineralsin the upper mineralized zone and were temporally and spa-tially associated with the formation of fergusonite-(Y), ferro-columbite, allanite-(Ce), monazite-(Ce), and bastnäsite-(Ce).These minerals replaced earlier minerals, such as aegirineand zircon, and filled fractures in primary K-feldspar. Locally,biotite replaced magnetite.

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Upper mineralized zone

Lower mineralized zone

4000

3000

2000

1000

0

80000

60000

40000

20000

0

40000

30000

20000

10000

0

3000

2000

1000

0

1500

1000

500

0

600

400

200

0

Gd 2

O3

La 2

O3

Dy 2

O3

Ce 2

O3

Yb 2

O3

Sm

2O3

0 50 100 150 0 50 100 150 0 50 100 150

0 50 100 150 0 50 100 150 0 50 100 150

FIG. 4. Plots of REE abundance (ppm) with depth (m) illustrating differences in the distribution of the REE content inthe upper and lower mineralized zones in drill hole L07-55, located within the North Tardiff zone (see Fig. 2).

Magnetite is also the dominant alteration mineral in thelower mineralized zone and biotite is much less abundantthan in the upper mineralized zone. Magnetite replaced thesecondary minerals in pseudomorphs after inferred eudialyteand is the dominant mineral in the fine-grained groundmasssurrounding the pseudomorphs, where it is accompanied byminor biotite and albite.

Below the lower mineralized zone, magnetite decreasessharply in abundance and hematite, becomes prevalent,forming rims around feldspar phenocrysts and locally mag-netite, commonly replacing these minerals completely. In thelayered aegirine ± nepheline ± sodalite syenites at depth, so-dalite is typically replaced by fine-grained muscovite, and ae-girine by magnetite or hematite.

Albitization is widespread but is most pervasive in theupper 50 m of the complex where a pegmatitic unit was re-placed almost entirely by blades of albite (“cleavelandite”),

displaying intense red cathodoluminescence (triggered byFe3+ activators). Albite having these characteristics is a com-mon product of alkali-metasomatism (Rae and Chambers,1988; McLemore and Modreski, 1990). Pegmatitic syeniteslower down in the complex have also been albitized and,where the albitization is less intense, remnant, partially al-tered K-feldspar megacrysts are observed. Vugs are commonin all the albitized pegmatites, suggesting that this alterationwas accompanied by a significant loss of volume; the vugshave been variably filled by late carbonates, clays, fluorite,and minor pyrite. In addition to affecting the pegmatites, al-bitization also locally overprinted magnetite- and biotite-al-tered aegirine ± arfvedsonite syenite and where these rockswere mineralized, destroyed zircon and the associated REE-bearing minerals.

Late-stage silicification (quartz), illitization, and carbonati-zation also affected the deposit. These alteration types, which


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TABLE 2. A List of Minerals (and their compositions) Found in the Nechalacho Deposit

Name Formula Name Formula

Silicates Halides

Aegirine NaFe3+(Si2O6) Fluorite CaF2

Aenigmatite Na2Fe2+5TiSi6O20

Albite Na(AlSi3O8) PhosphatesAllanite Ca(La,Y,Ce)(Al2Fe2+)Si3O12 Apatite Ca5(PO4)3(OH,F,Cl)Analcime NaAl(Si2O6).(H2O) Monazite (Ce,La,Th)PO4

Annite KFe2+3AlSi3O10(OH)1.5F0.5

Barkevikite Ca2(Fe,Mg,Al)-5(Si,Al)8O22(OH)2 CarbonatesBiotite K2(Mg,Fe2+)6-4(Fe3+,Al,Ti)0-2[Si6-5Al2-3O20](OH,F)4 Ankerite Ca(Mg,Fe2+,Mn)(CO3)2

Britholite Ca2[(Y,Ce)Ca]3[(OH,F)(SiO4,PO4)3] Bastnäsite (Ce,La,Y)F(CO3)Catapleiite Na2Zr(Si3O9).2H2O Calcite CaCO3

Cerianite (Ce4+,Th)O2 Dolomite CaMg(CO3)2

Chamosite (Fe2+,Mg)5Al[(OH)8AlSi3O10] Kutnahorite Ca(Mn,Mg,Fe)(CO3)2

Chlorite (Mg,Fe2+,Fe3+,Mn,Al)12[(Si,Al)8O20](OH)16 Lanthanite (Ce,La,Nd)2(CO3)38H2OEudialyte Na15Ca6(Fe2+,Mn2+)3Zr3(Si25O73)(O,OH,H2O)3(OH,Cl)2 Parisite Ca(Ce,La)2(CO3)3F2

Ferro-bustamite Ca(Fe2+,Ca,Mn2+)Si2O6 Siderite FeCO3

Synchysite Ca(Y,Ce,La,Nd,Gd)[F(CO3)2]Ferroricherite Na[Ca,Na][Fe2+5][(OH)2Si8O22] OxidesGittinsite CaZrSi2O7 Aeschynite (Ce,Nd,Y,Ca,Fe,Th)(Ti,Nb)2(O,OH)6

Lepidolite K(Li,Al)3(Si,Al)4O10(OH,F)2 Ashanite (Nb,Ta,U,Fe,Mn)4O8

Mesolite Na2Ca2(Al2Si2O10)3.8H2O Betafite (Ca,U)2(Ti,Nb)2O6(OH)Natrolite Na2(Al2Si3O10).2H2O Cassiterite SnO2

Oneillite Na15Ca3Mn3Fe2+3Zr3Nb(Si25O73)(O,OH,H2O)3(OH,Cl)2 Ceriopyrochlore (Ce,Ca,Y)2(Nb,Ta)2O6(OH,F)Orthoclase K(AlSi3O8) Columbite (Fe,Mn)(Nb,Ta)2O6

Pectolite NaCa2(HSi3O9) Columbo-tantalite (Fe,Mn)(Nb,Ta)2O6

Quartz SiO2 Fergusonite (Ce,La,Nd,Y)NbO4

Scapolite Na4[Cl(AlSi3O8)3] – Ca4[CO3(Al2Si2O8)3] Ferrocolumbite Fe++Nb2O6

Sericite K2Al4(Si6Al2O20)(OH,F)4 Hematite Fe2O3

Sodalite Na8(Al6Si6O24)Cl2 Ilmenite Fe2+TiO3

Thorite (Th,U)SiO4 Ixiolite (Ta,Nb,Sn,Fe,Mn)4O8

Topaz Al2SiO4(OH,F)2 Limonite FeO.OH.nH2OUranothorite (Th,U)SiO4 Magnetite Fe2+Fe23+O4

Willemite ZnSiO4 Nioboaeschynite (Y,Ca,Ce,Nd,Th)(Nb,Ta,Ta)2(O,OH)6

Wollastonite CaSiO3 Polycrase (Y,Ca,Ce,U,Th)(Ti,Nb,Ta)2O6

Zinnwaldite K(Li,Fe,Al)3(Si,Al)4O10(OH)F ZrSiO4 Samarskite (Y,Ce,U,Fe,Nb)(Nb,Ta,Ti)O4

Zircon ZrSiO4 Specularite Fe2O3

Titanomagnetite Fe(Fe,Ti)2O4

Sulfides Uraninite UO2

Barite BaSO4Yttrocolumbite (Y,U,Fe++)(Nb,Ta)O4

Chalcopyrite CuFeS2

Molybdenite MoS2

Pyrite FeS2

Pyrrhotite Fe7S8

Sphalerite ZnS

overprint all the earlier alteration types including albitization,were only developed locally and except, for carbonatization,do not appear to have been accompanied by any redistribu-tion of HREE. However, carbonatization (calcite, ankerite,siderite) may have been associated with REE fluoro-carbon-ate crystallization, as calcite, fluorite, and bastnäsite-(Ce)commonly occur together. This has been suggested by Coul-son and Chambers (1996) for the rocks of the North QôroqCentre in Greenland.

Distribution and Paragenesis of Rare Metal Minerals

HREE-bearing minerals

Zircon and fergusonite-(Y) are the two main hosts to heavyrare earth elements in the deposit; minor amounts of HREEare also present in ferrocolumbite. Three varieties of zirconare distinguishable microscopically.

Type 1 zircon is a tan-colored variety that occurs almost ex-clusively in the upper mineralized zone and consists pre-dominantly of euhedral, bipyramidal, zoned crystals (typi-cally, 30−60 µm and locally 100 µm in diameter), which havedistinct cores and rims and are locally brecciated. In mostcrystals, the cores are a dark brown color due to alteration byvery fine grained chlorite, quartz, samarskite-(Y), monazite-(Ce), and uranothorite. The cores are also the focus for frac-tures, which radiate outward and transect the translucentrims. Type 1 zircon occurs either as isolated crystals or is con-centrated in subhorizontal undulating laminations, whichwrap around pseudomorphs (now filled mainly by magnetiteand biotite), interpreted to represent aegirine phenocrysts(Fig. 3B). The laminations are parallel to the igneous layer-ing and locally form zones up to 30 cm thick, containing >30vol % zircon. Petrographically and particularly in backscat-tered electron imagery, the zircon in these laminations is ob-served to have been extensively replaced by allanite-(Ce) andto a much lesser extent by monazite-(Ce) and ferrocolumbite(Fig. 3C).

A second tan-colored variety of zircon, which like type 1 zir-con is also restricted to the upper mineralized zone, forms clotsand irregular streaks, and occurs in veinlets (Fig. 5) that wraparound brecciated feldspar crystals. Its crystals are typicallybroken and form fragments in a matrix of biotite and mag-netite. For this reason and its chemical similarity to type 1 zir-con (discussed later), this second type is interpreted to repre-sent mechanically remobilized type 1 zircon. In order todistinguish this zircon from that described in the previousparagraph, it will be referred to henceforth as type 1b zirconand that of the previous paragraph as type 1a zircon.

Type 2 zircon comprises the bulk of the zircon in the lowermineralized zone, forming small (10−30 µm) anhedral grainsthat occur within pseudomorphs, which are present in layersvarying from tens of centimeters to several meters in thick-ness, and are interpreted to represent igneous cumulates.The pseudomorphs have two distinctive morphologies, rangefrom submillimeter to centimeter size, and contain no relictsof their precursor mineral(s). However, one of these pseudo-morph types (the overwhelmingly predominant type) has atabular habit very similar to that of eudialyte. These pseudo-morphs typically have subrounded shapes and contain the following mineral assemblage: zircon + fergusonite-(Y) +

bastnäsite-(Ce) + parisite-(Ce) + synchysite-(Ce) + allanite-(Ce) + albite + quartz + biotite + fluorite + kutnahorite ±minor hematite and rare sphalerite (Fig. 6A). The zirconwithin these pseudomorphs forms anhedral grains or largerclusters of grains (Fig. 6B), some of which are rimmed by anovergrowth of younger zircon. Pseudomorph cores are com-monly replaced entirely by hematite and more rarely mag-netite, leaving a rim of zircon inside the original outline of thepseudomorph. Over several meters both above and below thelower mineralized zone, the pseudomorphs have been par-tially to completely replaced by magnetite and/or hematite,indicating that the mineralized zone may have extendedmuch farther prior to alteration. Some pseudomorphs containspherical patches of quartz with radiating vermicular zircon,which are reminiscent of symplectic intergrowths (Fig. 6C).These pseudomorphs contain the same mineral assemblageas that shown in Figure 6A and are considered a variant of theoriginal alteration of eudialyte.

The second variety of pseudomorph, which is far less com-mon, has the shape of a doubly terminated prism and hasbeen replaced predominantly by zircon (Fig. 6D). This shapeis similar to that of elpidite, a common primary magmaticmineral in the Strange Lake Complex, Quebec/Labrador(Birkett et al., 1992; Salvi and Williams-Jones, 1995). Theproportion of zircon in these pseudomorphs is approximately80 vol %, which is much higher than would be expected forelpidite, suggesting that, if our interpretation of their precur-sors is correct, Na, silica, and H2O were removed during alteration.

Type 3 zircon is relatively uncommon, occurs in irregularpatches and in zones of brecciation mainly in the upper min-eralized zone, and is locally replaced by secondary albite. Therims on this zircon are narrow and characterized by unusuallyhigh birefringence, oscillatory zoning, and colloform textures(Fig. 7A). The rims and irregular patches within the cores canbe further distinguished by their response to UV illumination.Whereas type 1 and 2 zircons do not fluoresce or luminesce,type 3 zircon exhibits a blue to greenish-yellow fluorescence.Cathodoluminescence images reveal bright greenish-yellowluminescing patches within crystals of this zircon and paler

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FIG. 5. Photomicrograph in plane-polarized light of broken type 1b zircon

(zrn) crystals in a biotite (bt)-cemented vein through brecciated albite (ab).

blue zoned rims (Fig. 7B). This zoning is also evident inbackscattered electron imagery. The greenish-yellow emis-sion is attributed to Sm3+ or Dy3+ activation, as these REE areknown to produce a green emission. Similar luminescencehas been reported in zircon from the Khaldzan Buregte andTsakhir Nb-Zr-REE deposits in the Mongolian Altai (Kempeet al., 1999), where it was clearly demonstrated that the emis-sion was caused by Dy3+ activation.

Fergusonite-(Y) is present in both the upper and lowermineralized zones but is much more abundant in the latterwhere it typically comprises 1 to 5 vol % of the rock and is thedominant HREE-bearing mineral along with zircon. In theupper mineralized zone, fergusonite-(Y) occurs as small (<1mm) anhedral grains adjacent to primary zircon. In the lowermineralized zone, however, it forms small (<1 mm) anhedralgrains and more densely packed aggregates within thepseudomorphs after eudialyte where it is intimately associ-ated with zircon, which it also locally replaced (Fig. 8A). Fer-gusonite-(Y) also occurs outside the pseudomorphs as a re-placement product of columbite-(Mn).

Ferrocolumbite occurs as pinwheels with rounded black cen-ters and radiating spines up to 0.1 mm in diameter (Fig. 8B).

It also locally replaced zircon and fergusonite-(Y), in both theupper and lower mineralized zones. Columbite-(Mn) formssmall, isolated anhedral crystals and larger clusters of thesecrystals (up to 50 µm in diameter) adjacent to type 1 zirconand inferred eudialyte pseudomorphs (it also occurs withinsome pseudomorphs) and occurs as inclusions within biotite,chlorite, magnetite, and albite. These crystals commonly havebeen replaced by both fergusonite-(Y) (Fig. 8C) and bastnäsite-(Ce). The relationship of columbite-(Mn) to ferrocolumbite isunclear as these minerals are not observed together. How-ever, the fact that fergusonite-(Y) replaced columbite-(Mn)and was in turn replaced by ferrocolumbite indicates that for-mation of ferrocolumbite postdated that of columbite-(Mn).Lastly, samarskite-(Y) is present as anhedral inclusions in thecores of zircon crystals in the upper mineralized zone and alsoas larger grains (up to 30 µm diam) in biotite.

LREE-bearing minerals

REE fluoro-carbonate minerals, predominantly bastnäsite-(Ce), synchysite-(Ce), and minor parisite-(Ce), are most com-mon several tens of meters above the intervals containing thehighest concentrations of HREE in the upper mineralized


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D or mag

chl

zrn

50µm

200µm 1mm

FIG. 6. A. Photomicrograph in plane-polarized light of a single pseudomorph replaced by zircon (zrn) + fergusonite-(Y)(ferg) + allanite-(Ce) (aln) + parisite-(Ce)/synchysite-(Ce) + albite (ab) + quartz (qtz) + biotite (bt) + fluorite + calcite (cal).B. SEM-BSE image of anhedral to subrounded grains of zircon within a pseudomorph (white is fergusonite-(Y)). C. SEM-BSE image illustrating several pseudomorphs containing radially distributed vermicelli of zircon (pale gray) and interstitialquartz (black). D. Photomicrograph of a zircon pseudomorph after probable elpidite.

zone, where they are associated with fluorite and calcite andare important hosts of the LREE. Macroscopically, bastnäsite-(Ce) is brown-red to brick-red in color, reflecting alteration tohematite. It occurs with the other fluoro-carbonate mineralsas fine-grained disseminations interstitial to biotite, mag-netite, and zircon, and more rarely forms rims around zirconcrystals and replaced allanite-(Ce), occurring as streaks andirregular patches within this mineral (Fig. 8D). Synchysite-(Ce) and parisite-(Ce) are intimately intergrown with bothbastnäsite-(Ce) and allanite-(Ce) (Fig. 8E).

Allanite-(Ce) forms anhedral grains up to 1.5 mm in diam-eter and more massive clusters, which are commonly intersti-tial to zircon, and pseudomorphs after probable eudialyte. Itoccurs in both the upper and lower mineralized zones but ismuch more abundant in the upper zone. Typically red incolor, it displays a fiery orange luminescence under cathodo-luminescence excitation (Fig. 8F). Monazite-(Ce), which isfound mainly in the upper zone proximal to zones of HREEmineralization, occurs as elongate needles and/or rods or astiny crystals in zircon, K-feldspar, quartz, and biotite, havingreplaced the interiors of these minerals. Locally, monazite-(Ce) also replaced both bastnäsite-(Ce) and allanite-(Ce).

In the lower mineralized zone, the LREE occur predomi-nantly in bastnäsite group minerals and allanite-(Ce) and arefound both within and beyond the confines of the pseudo-morphs after probable eudialyte. There are a number of inti-mate intergrowths of these minerals and other HREE-bear-ing minerals within the pseudomorphs, namely betweenbastnäsite-(Ce)-parisite-(Ce)-synchysite-(Ce)-allanite(Ce)and fergusonite-(Y)-bastnäsite-(Ce)-gittinsite. The texturessuggest that these minerals most likely formed coevally.

Fluorite, sulfides, thorite, and apatite

Dark purple fluorite is common throughout the deposit andoccurs in many forms: infilling vugs in albitized pegmatites,together with bastnäsite-(Ce) and other LREE-rich minerals;as a local replacement of magnetite and biotite; in fractures inprimary K-feldspar; intergrown with illite in massive patchesfrom tens of centimeters to several meters in thickness; andintergrown with zircon and other REE-bearing minerals inzones of brecciation. Pyrite, chalcopyrite, sphalerite, andmolybdenite are locally abundant in the upper parts of thedeposit. In particular, pyrite occurs in calcite veins and com-monly as disseminations in magnetite-rich horizons where itlocally replaced magnetite. Thorite and other minor accessoryminerals occur as small (1−3 µm) anhedral grains in the coresof type 1 zircon crystals. Minor apatite, surrounded by albite,is associated with monazite-(Ce).

Summary

In summary, all the REE minerals described above locallyreplaced zircon, an observation that is significant and will bediscussed further. The simplified paragenesis of rare metalminerals and major alteration minerals in the upper mineral-ized zone is represented by the sequence: zircon → mag-netite-biotite-zircon-REE minerals → albite → fluorite-cal-cite-quartz-chlorite-hematite. A more detailed paragenesis isgiven in Figure 9. In the lower zone, all the REE mineralsformed synchronously by replacement of inferred eudialyteto create pseudomorphs.

Mineral ChemistryThe compositions of the REE-bearing minerals were de-

termined using wavelength-dispersive spectrometers on aJEOL JXA-8900L electron microprobe at McGill Universitywith a focused beam and an acceleration voltage of 20 kV, a30 nA beam current, a beam diameter of 3 µm for zircon, fer-gusonite-(Y), ferrocolumbite, and eudialyte; and a 10 µm beamdiameter for allanite-(Ce), monazite-(Ce), and bastnäsite-(Ce). Data reduction was performed using the ZAF correc-tion method. Counting times and standards used for each el-ement analyzed are reported in Table 3 and detection limitsare presented in Tables 4 and 5 with the relevant analyticaldata. Element X-ray maps were obtained using the same in-strument with the probe beam moving over a 900 × 900 pointgrid; counting intervals for X-rays at each spot were 60 msec.Brighter zones in the element maps reflect higher countrates. Many of the analyses of zircon crystals, particularly ofthe cores, reported low totals (e.g., type 1b core: 94.91 wt %).This is most likely due to the presence of H2O in the zirconstructure. Phosphorous contents are below detection and arenot reported.

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FIG. 7. A. Photomicrograph in plane-polarized light of type 3 zircon (zrn)

with colloform zoned rims, locally replaced by albite (ab) and biotite (bt). B.Cathodoluminescence image of the same zircon showing oscillatory zoning(blue fluorescence) and bright green-yellow fluorescent patches.


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FIG. 8. A. SEM-BSE image illustrating fergusonite-(Y) (white; ferg) that locally replaced zircon (zrn). B. Photomicrographin plane-polarized light of zircon, monazite-(Ce) (mnz), and ferrocolumbite (pinwheels with black rounded centers and ra-diating spines) surrounded by quartz (qtz). Ferrocolumbite (fc) that replaced zircon can be seen on the right-hand side ofthe photograph. C. SEM-BSE image showing columbite-(Mn) (col) that was replaced by fergusonite-(Y) and bastnäsite-(Ce)(bast). D. SEM-BSE image of bastnäsite-(Ce) (bast) after allanite-(Ce) (aln). E. SEM-BSE image of allanite-(Ce) intergrownwith parisite-(Ce) (par) on the rim of a pseudomorph after probable eudialyte in the lower mineralized zone. F. Cathodolu-minescence image of fiery orange allanite-(Ce) between zircon crystals.

REE mineral chemistry

The REE are present in significant concentrations in zir-con, fergusonite-(Y), allanite-(Ce), monazite-(Ce), and bast-näsite-(Ce). The concentration of total REO (as REE2O3) intype 1 zircon crystals is highly variable, ranging from <1 to6.45 wt %, and the distribution varies from rim to core withsignificant zoning apparent in backscattered electron images.Electron microprobe X-ray element maps were produced inorder to observe the chemical nature of the zonation andchanges in the distribution of Zr, Nb, Y, Ce, Nd, Gd, Hf, Th,Si, Fe, Mg, K, and Ca within the structure of the four typesof zircon and within the pseudomorphs. The images were ac-quired by scanning the beam across the sample in a grid. Thesignals collected at each point in the grid were plotted withpixel brightness as a function of signal intensity (i.e., higherintensities were plotted as brighter colors). The above ele-ments were selected as they are the major elements in zircon,markers for the light, middle, and heavy REE or help identifyalteration minerals present in the altered cores.

Most of the type 1a and b zircon crystals that were mappeddisplay a central Y (up to ~3 wt % Y2O3) and Gd-rich, Nd and

Hf-poor core and an outer Y-poor, Nd- and Hf-rich rim, withsharp boundaries between the core and rim. This suggeststhat the rims were overgrown on the cores. LREE, repre-sented by La2O3 and Ce2O3, occur in concentrations up to0.05 and 0.50 wt % in the rims but only 0.02 and 0.35 wt % inthe cores, respectively. The corresponding concentrations ofmiddle REE (MREE), represented by Sm2O3, Gd2O3, andDy2O3, are up to 0.98, 0.88, and 0.55 wt % in the rims and1.18, 1.14, and 1.04 wt % in the cores, respectively. HREE,represented by Yb2O3, occur in concentrations up to 0.43 wt% in the rims and 0.72 wt % in the cores. In general, it is theMREE, namely Sm, Gd, and Dy, which are most enriched intype 1 zircon relative to the other REE. An important featureof these crystals is that the cores have been preferentially al-tered and yttrium was mobilized along radiating fractures tothe rims and beyond (Figs. 10, 11). This alteration likely rep-resents an atoll texture, in which the core of the crystal wasless stable than the rim and therefore more susceptible to al-teration. The occurrence of rare crystals of monazite-(Ce),samarskite-(Y), and thorite in some zircon cores can be ex-plained by calling upon the radiating fractures to act as path-ways for hydrothermal fluids into the grains. Transects from

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FIG. 9. Paragenetic sequence for the major REE-bearing and alteration minerals in the Nechalacho deposit. Line widthsindicate relative abundances of phases. Uncertainty in the precise placement of a mineral within the paragenesis due to alack of textural evidence is indicated by ‘?’.

Magmatic HydrothermalREE-BEARING MINERALS

Upper zone:

ZirconFergusonite-(Y)Columbite-(Mn)FerrocolumbiteAllanite-(Ce)Monazite-(Ce)Parisite-(Ce)Synchysite-(Ce)Bastnaesite-(Ce)

Lower zone:

EudialyteZirconFergusonite-(Y)Allanite-(Ce)Parisite-(Ce)Synchysite-(Ce)Bastnaesite-(Ce)

MAJOR ALTERATIONMINERALS

MagnetiteBiotiteHematiteChlorite ?Quartz ?AlbiteFluorite ?Calcite

rim to rim across individual crystals show that concentrationsof thorium are elevated in the altered cores (Fig. 12), sug-gesting that metamictization may have played an importantrole in promoting core alteration.

Electron microprobe element maps of the pseudomorphsafter inferred eudialyte (Fig. 13) show that Zr, Y, and Nb areconfined within the crystal boundaries, suggesting that theseelements were only remobilized on a scale of tens of microns.By contrast, Ca and LREE such as Ce are observed beyondthe crystal boundaries and were thus remobilized on a largerscale.

Table 4 reports the compositions of the different zircontypes. Overall, the compositions of type 1a and b zircon crys-tals are fairly similar (Fig. 14A, B). The rims, in particular,have very similar compositions, but their cores differ some-what in that the cores of type 1b zircon are slightly more en-riched in HREE than those of type 1a. Type 2 zircon has a dis-tinctive chondrite-normalized REE profile characterized by astrong depletion in the LREE and enrichment in the HREE(Fig. 14C) relative to type 1 zircon. For example, the averageconcentration of Y2O3 in type 2 zircon is 2.71 wt %, two tothree times the concentration in type 1 zircon. Concentrationsof Yb2O3 and Er2O3 are distributed similarly to those of Y2O3.Type 3 zircon is enriched in HREE in the narrow oscillatoryzoned rims and is depleted in all REE in the cores (Fig. 14D).The compositions of the rims are similar to those of type 1aand b zircon.

Table 5 reports the compositions of the other major REE-bearing mineral phases in the Nechalacho deposit. Fergu-sonite-(Y) is preferentially enriched in the middle and heavy


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TABLE 3. Counting Times and Standards Used in Electron Microprobe Analyses

Element Counting time (sec) Standard

La 150 MAC-LaY 50 MAC-YNb 100 Na2Nb2O6

Eu 150 MAC-EuCe 60 MAC-CeF 100 CaF2

Hf 40 ZirconDy 60 MAC-DyEr 60 MAC-ErSi 20 ZirconU 100 UO2

Ca 20 DiopsideGd 60 MAC-GdFe 20 gar1Al 20 gar1Pr 60 MAC-PrTh 100 ThO2

Sm 60 MAC-SmNd 60 MAC-NdZr 20 ZirconYb 70 MAC-YbNa 20 Na2Nb2O6

Tb 60 MAC-TbTi 20 TiO2

Mg 20 SpinelMn 20 SpessartineP 20 Ba-FeldK 20 OrthoclaseTa 60 MAC-Nd

TABLE 4. Average Compositions (wt %) of Rims and Cores of the Different Zircon Types Analyzed Using the Electron Microprobe (the ± figures report the standard deviations for each set of analyses)

Type 1a Type 1a Type 1b Type 1b Type 2 Type 2 Type 3 Type 3core (n = 18) rim (n = 18) core (n = 24) rim (n = 36) core (n = 49) rim (n = 9) core (n = 7) rim (n = 7)

ZrO2 60.48 ± 1.27 60.69 ± 0.55 59.01 ± 1.61 60.60 ± 1.47 55.15 ± 4.70 57.98 ± 1.72 64.76 ± 23.71 60.81 ± 22.61SiO2 29.46 ± 1.13 29.25 ± 0.42 27.11 ± 1.12 28.36 ± 0.82 32.07 ± 5.87 28.01 ± 0.43 30.90 ± 10.99 29.00 ± 10.85HfO2 1.38 ± 0.50 1.09 ± 0.18 1.16 ± 0.42 1.21 ± 0.19 1.22 ± 0.19 1.06 ± 0.11 1.56 ± 0.56 1.29 ± 0.47FeO 1.01 ± 0.44 0.57 ± 0.29 0.72 ± 0.51 0.33 ± 0.17 1.90 ± 1.20 0.61 ± 0.26 0.42 ± 0.34 0.52 ± 0.20Al2O3 0.25 ± 0.10 0.08 ± 0.03 0.16 ± 0.13 0.10 ± 0.05 0.36 ± 0.26 0.15 ± 0.05 0.10 ± 0.09 0.21 ± 0.09UO2 b.d. b.d. b.d. b.d. 0.04 ± 0.03 b.d. b.d. b.d.ThO2 0.05 ± 0.05 0.19 ± 0.10 0.10 ± 0.09 0.07 ± 0.06 0.17 ± 0.13 0.20 ± 0.08 b.d. 0.05 ± 0.04CaO 0.32 ± 0.29 0.13 ± 0.05 0.21 ± 0.14 0.25 ± 0.06 0.19 ± 0.12 0.14 ± 0.04 0.28 ± 0.23 0.25 ± 0.13F 0.66 ± 0.37 0.70 ± 0.20 0.67 ± 0.40 0.62 ± 0.29 0.15 ± 0.12 0.99 ± 0.18 0.16 ± 0.08 0.69 ± 0.23Na2O 0.20 ± 0.09 0.08 ± 0.04 0.06 ± 0.01 b.d. 0.21 ± 0.13 0.19 ± 0.06 0.06 ± 0.05 0.31 ± 0.13Y2O3 0.81 ± 0.43 0.66 ± 0.27 1.70 ± 0.06 0.36 ± 0.22 2.71 ± 1.22 3.63 ± 2.31 b.d. 0.63 ± 0.54Nb2O5 1.16 ± 0.71 2.62 ± 0.49 1.32 ± 0.66 3.31 ± 1.28 0.71 ± 0.57 2.07 ± 0.70 0.44 ± 0.34 1.75 ± 0.61La2O3 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.Ce2O3 0.21 ± 0.13 0.28 ± 0.10 0.15 ± 0.14 0.37 ± 0.09 0.06 ± 0.04 0.12 ± 0.06 b.d. 0.30 ± 0.13Pr2O3 b.d. 0.16 ± 0.06 b.d. 0.15 ± 0.09 b.d. b.d. b.d. b.d.Nd2O3 0.53 ± 0.39 1.17 ± 0.26 0.44 ± 0.40 1.18 ± 0.30 0.11 ± 0.04 0.42 ± 0.34 b.d. 0.92 ± 0.32Sm2O3 0.44 ± 0.32 0.52 ± 0.18 0.46 ± 0.35 0.52 ± 0.23 b.d. 0.31 ± 0.24 b.d. 0.29 ± 0.11Eu2O3 0.11 ± 0.07 0.10 ± 0.04 0.10 ± 0.09 b.d. b.d. b.d. b.d. b.d.Gd2O3 0.56 ± 0.42 0.41 ± 0.17 0.84 ± 0.67 0.36 ± 0.23 0.27 ± 0.14 0.70 ± 0.15 b.d. 0.29 ± 0.24Dy2O3 0.28 ± 0.18 0.19 ± 0.07 0.46 ± 0.37 0.11 ± 0.07 0.35 ± 0.19 0.79 ± 0.46 b.d. 0.29 ± 0.19Er2O3 0.07 ± 0.02 0.09 ± 0.04 0.15 ± 0.13 b.d. 0.28 ± 0.12 0.30 ± 0.22 b.d. 0.08 ± 0.08Tb2O3 b.d. b.d. b.d. b.d. b.d. b.d. b.d. b.d.Yb2O3 b.d. 0.10 ± 0.09 0.11 ± 0.09 b.d. 0.23 ± 0.07 0.15 ± 0.10 b.d. b.d.

Total 98.01 99.07 94.91 97.81 96.17 97.82 98.88 97.67

Notes: b.d. = below detection limit, which are as follows for oxides (wt %): Zr, 0.11; Si, 0.03; Hf, 0.10; Fe, 0.12; Al, 0.02; U, 0.04; Th, 0.04; Ca, 0.03; F,0.11; Na, 0.05; Y, 0.05; Nb, 0.06; La, 0.09; Ce, 0.04; Pr, 0.15; Nd, 0.11; Sm, 0.09; Eu, 0.09; Gd, 0.09; Dy, 0.11; Er, 0.05; Tb, 0.04, Yb, 0.06

REE, up to 7.5 wt % Dy2O3 and Gd2O3 (Fig. 14E), and is themain host of yttrium in the deposit (it contains ~25 wt %Y2O3). It also hosts significant concentrations of Ta2O5 (up to6 wt %; avg ~2.7 wt %), which has its highest bulk rock con-centration in the lower part of the lower mineralized zone (upto 0.12 wt %; avg ~0.06 wt %); this is where fergusonite-(Y) ismost abundant. The average stoichiometry of fergusonite-(Y)is (Y0.51Gd0.10Dy0.08Nd0.04Sm0.04Ta0.04)Nb1.00O4.

Ferrocolumbite has an average composition of (Fe2+0.92

Mn0.14)Ta0.11Nb1.80O6 and an Nb/Ta ratio very similar to thatof fergusonite-(Y) (16.4 vs. 25), consistent with the observa-tion that ferrocolumbite replaced fergusonite-(Y). Ferro-columbite hosts up to 7.95 wt % Ta2O5, 0.68 wt % Y2O3, andan average of ~68.5 wt % Nb2O5. Columbite-(Mn) is similarin composition to ferrocolumbite but has higher concentra-tions of Ta2O5 and MnO (up to 21.85 and 6.04 wt %, respec-tively) and lower concentrations of Nb2O5 and FeO (36.54and 9.11 wt %, respectively).

The unaltered eudialyte in drill core from hole 85-L6 hasan average composition of (Na11.96Ca1.32K0.33)(Ca4.19REE1.08

Y0.65Mn0.08)6(Mn2+2.98Fe2+

0.02)3Zr2.94Nb0.98(Si26.09O72) (O,OH,H2O)3

(OH,Cl0.38F0.45)Σ2, calculated using the method of Johnsen etal. (2003). This is close to the composition of the end mem-ber, kentbrooksite, in terms of Mn-REE-Nb-F, but in termsof Ca-Fe-Si-Cl is intermediate between kentbrooksite andeudialyte. In view of this, the mineral is referred to as eudia-lyte in this paper. This phase hosts up to 4.08 wt % Nb2O5,2.97 wt % Y2O3, and 5.97 wt % other REE oxides.

Monazite-(Ce), allanite-(Ce), and bastnäsite-(Ce) all dis-play similar chondrite-normalized REE profiles, being pref-erentially enriched in the LREE with a concave trend fromLa to Sm (Fig. 14F). The average compositions of these min-erals are (Ce0.47Nd0.22La0.19Pr0.06)P0.99O4, (Ca0.89Ce0.45La0.22

Nd0.15Pr0.05)(Al1.78Fe1.12)Si2.93O12 and (Ce0.38Nd0.19La0.17Pr0.05)F0.66C0.87O3, respectively. The HREE concentrations are gen-erally below the limit of detection of the electron microprobe.Monazite-(Ce) contains the highest concentrations of Ce2O3

(35 wt %) and Nd2O3 (15 wt %) of any of the REE-bearingminerals and rare zoned crystals have rims that are preferen-tially enriched in MREE and thorium. By contrast, allanite-(Ce) contains, on average, 7 wt % La2O3, 13 wt % Ce2O3, and4.5 wt % Nd2O3.

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TABLE 5. Average Compositions (wt %) of Fergusonite-(Y), Ferrocolumbite, Eudialyte, Monazite-(Ce), Allanite-(Ce), and Bastnäsite-(Ce) Analyzed Using the Electron Microprobe (the ± figures report the standard deviations for each set of analyses)

Fergusonite-(Y) Ferrocolumbite Eudialyte Monazite-(Ce) Allanite-(Ce) Bastnäsite-(Ce)(n = 42) (n = 12) (n = 21) (n = 47) (n = 33) (n = 26)

ZrO2 - - 10.73 ± 0.18 - - -SiO2 - - 46.68 ± 0.64 - 31.73 ± 0.58 -HfO2 0.20 ± 0.14 - 0.31 ± 0.05 - - -TiO2 - 1.96 ± 0.83 - - - -FeO 0.56 ± 0.36 19.01 ± 0.87 b.d. 0.09 ± 0.07 13.89 ± 1.46 b.d.Al2O3 - - - - 16.40 ± 1.86 -MgO - 0.22 ± 0.06 - - 0.37 ± 0.19 -MnO - 2.65 ± 1.00 6.42 ± 0.22 - 0.63 ± 0.25 -P2O5 - - - 29.80 ± 0.40 - -UO2 - 0.05 ± 0.02 - - - -ThO2 0.44 ± 0.39 b.d. - 1.51 ± 0.73 - -CaO 0.28 ± 0.16 0.06 ± 0.04 9.16 ± 0.20 0.23 ± 0.17 9.01 ± 0.78 0.14 ± 0.12F - - 0.21 ± 0.04 0.48 ± 0.07 - 7.76 ± 1.56Na2O - - 10.99 ± 0.38 - - -K2O - - 0.46 ± 0.03 - - -Y2O3 24.53 ± 2.13 0.30 ± 0.19 2.18 ± 0.50 0.66 ± 0.59 0.13 ± 0.10 0.31 ± 0.24Nb2O5 45.88 ± 1.61 70.64 ± 3.11 3.87 ± 0.24 - - -Ta2O5 2.57 ± 1.02 4.23 ± 3.44 - - - -SnO2 - 0.18 ± 0.10 - - - -La2O3 - - 1.34 ± 0.18 14.42 ± 1.49 6.53 ± 0.92 16.67 ± 2.02Ce2O3 0.47 ± 0.24 - 1.86 ± 0.41 31.99 ± 2.47 12.68 ± 1.23 35.31 ± 2.20Pr2O3 0.21 ± 0.19 - 0.20 ± 0.04 3.99 ± 0.40 1.48 ± 0.18 4.22 ± 0.48Nd2O3 2.56 ± 1.04 - 0.75 ± 0.20 14.26 ± 1.87 4.50 ± 0.50 15.24 ± 2.63Sm2O3 2.80 ± 1.06 b.d. 0.15 ± 0.05 1.62 ± 0.57 0.34 ± 0.16 1.38 ± 0.50Eu2O3 0.88 ± 0.31 - - 0.17 ± 0.13 0.16 ± 0.09 0.33 ± 0.24Gd2O3 6.76 ± 1.11 - 0.40 ± 0.05 0.95 ± 0.48 0.25 ± 0.14 0.87 ± 0.53Dy2O3 5.91 ± 1.16 - 0.37 ± 0.09 0.18 ± 0.15 - 0.10 ± 0.12Er2O3 2.06 ± 0.53 b.d. 0.20 ± 0.05 - - -Yb2O3 0.76 ± 0.31 - 0.12 ± 0.04 - - -

CO2 - - - - - 20.74* ± 2.04

Total 96.87 99.30 96.40 100.35 98.10 103.07

Notes: b.d. = below detection limit, which are as follows for oxides (wt %): Zr, 0.11; Si, 0.03; Hf, 0.10; Ti, 0.07; Fe, 0.04; Al, 0.02; Mg, 0.02; Mn, 0.03; P,0.08; U, 0.04; Th, 0.05; Ca, 0.02; F, 0.04; Na, 0.03; K, 0.03; Y, 0.03; Nb, 0.06; Ta, 0.14; Sn, 0.05; La, 0.10; Ce, 0.04; Pr, 0.05; Nd, 0.11; Sm, 0.07; Eu, 0.03; Gd,0.09; Dy, 0.09; Er, 0.05; Yb, 0.06

* calculated by stoichiometry, - = not analyzed

Discussion

Origin of the zircon

Although zirconium is widely believed to be immobile duringfluid-rock interaction and zircon is regarded by many as a strictlyigneous mineral, there have been a number of reports in the lit-erature of textures involving zircon consistent with its formationby hydrothermal processes (e.g., Rubin et al., 1993; Schaltegger,

2007; Anderson et al., 2008; Lichtervelde et al., 2009). For ex-ample, Rubin et al. (1989) reported Hf-enriched hydrothermalovergrowths on magmatic zircon from the Sierra Blanca Peaks,Texas, and similar overgrowths have since been reported for theBoggy Plain zoned pluton, Australia (Hoskin, 2005), and theMole Granite, Australia (Pettke et al., 2005). Kerrich and King(1993) also documented hydrothermal zircon with a “spongytexture” containing abundant inclusions of Au and fluid.


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BSE IMAGE NEODYMIUM

ZIRCONIUM YTTRIUM

HAFNIUM GADOLINIUM

100µm

BSE IMAGE NEODYMIUM

ZIRCONIUM YTTRIUM

HAFNIUM GADOLINIUM

100µm

FIG. 10. Electron microprobe element maps of type 1a zircon crystals, showing the distribution of Nd, Zr, Y, Hf, and Gd.The key features to note are the preferential enrichment of Nd, Zr, Hf, and Gd in the rims and Y in the cores. Yttrium is alsoobserved along fractures penetrating the rims.

In the Nechalacho deposit, there is evidence for the occur-rence of both magmatic and hydrothermal zircon. Based onthe nature of its distribution in wavy laminations and/or rib-bons, which parallel igneous layering and lithologic contacts,type 1 zircon (the main variety of zircon in the upper miner-alized zone) is interpreted to be a magmatic cumulate min-eral. It is, however, likely that the rims of type 1 zircon crys-tals represent overgrowths on primary cores which acted asnuclei for crystallization of later zircon and, it is thus possiblethat this later zircon deposited from a hydrothermal fluid. Itis also possible that the overgrowths simply represent a second stage of magmatic crystallization. Irrespective of whichinterpretation is correct, an important finding of our study isthat the zircon of the upper mineralized zone is dominantly ofmagmatic origin and therefore substantial proportions of theREE (particularly the HREE) were concentrated magmati-cally as a result of crystallization of zircon; the cores of type 1zircon are elevated with respect to the middle to heavy REE.

In contrast to type 1 zircon, type 2 zircon (the main varietyof zircon in the lower mineralized zone) is clearly hydrother-mal in origin. It formed in pseudomorphs of a mineral that weinterpret to be eudialyte and we consider it likely that this zir-con obtained its zirconium from the residues of the dissolu-tion of the eudialyte. There are several lines of evidence thatsupport this interpretation. First, the relative volume propor-tions of each of the secondary minerals (zircon + fergusonite-(Y) + bastnäsite-(Ce) + parisite-(Ce) + synchysite-(Ce) + allan-ite-(Ce) + albite + quartz + biotite + fluorite + kutnahorite ±minor hematite) are roughly consistent from one pseudomorphto another, suggesting that one or more elements (e.g., Zr, Y,Nb, Ce, Mn, Fe) were conserved. Second, the homogeneousdistribution of zircon in the pseudomorphs (Fig. 13) suggestsstrongly that Zr was conserved, and third, the volume pro-portion of zircon (~15%) is consistent with the proportion ofZr that would have been made available by dissolution of eu-dialyte (eudialyte contains ~11 mol % zirconium silicate). Fi-nally, we note that the ratio, TREE2O3/ZrO2 in eudyalite fromdrill hole 85-L6 is fairly similar to that for the bulk rock in thelower mineralized zone (0.71 vs. 0.55; Tables 1, 5), and that

the slightly lower ratio for the latter is consistent with thegreater mobility of the REE relative to Zr. Coulson (1997) re-ported a similar process of complete in situ breakdown andreplacement of primary eudialyte by new pseudomorphingphases in lujavrites from the North Qôroq Centre.

The colloform nature of type 3 zircon (Fig. 7A) providesstrong evidence that this variety of zircon is also hydrothermalin origin, but that in contrast to type 2 zircon, both Zr and Siwere supplied by the hydrothermal fluid. If this interpreta-tion is correct, then it follows that the rims of type 1 zircon arealso hydrothermal in origin as the distribution of the REE inthem is very similar to that of type 3 zircon.

In summary, our observations of the Nechalacho depositsuggest that zircon in the upper mineralized zone was domi-nantly magmatic and that zircon in the lower mineralizedzone formed by an in situ replacement of eudialyte, withouthydrothermal mobilization of Zr on a scale of more than mi-crons. However, there was more substantial mobilization ofZr in the upper mineralized zone and this produced colloformhydrothermal type 3 zircon and overgrowths on type 1 zircon.

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YTTRIUM

zrn

ferg

40µm

FIG. 11. Electron microprobe map for yttrium illustrating fractures alongwhich yttrium moved from zircon (zrn) cores to form adjacent fergusonite-(Y) (ferg).

FIG. 12. Transects from rim to rim across a single zircon crystal, illustrat-ing differences in the distribution of Th, Y, Gd, and Hf between the core(3−5) and rims (1−2 and 6−8). The Y axis is in wt %.

0.6

0.4

0.2

THORIUM0.0

8.0

6.0

4.0

2.0YTTRIUM0.0

1.5

1.0

0.5

GADOLINIUM0.0

1.5

1.0

0.5

HAFNIUM0.0

1 2 3 4 5 6 7 8

Magmatic concentration of Zr, Nb, and REE

Zr, Nb, and REE mineralization in the Nechalacho depositis confined to cumulate layers of zircon in the upper zone,with associated columbite-(Mn), and to layers of pseudo-morphs after cumulate eudialyte in the lower zone. The layersin both zones typically contain > 6 wt % ZrO2, up to 0.8 wt %Nb2O5, and >3 wt % total rare earth oxides (TREO), achieved

by a combination of crystal settling and compaction of discretezircon and eudialyte crystals and later alteration, remobiliza-tion, and concentration by magmatic hydrothermal fluids.

Most researchers agree that peralkaline melts are productsof low degrees of partial melting in an undepleted, metaso-matically enriched mantle (Marks et al., 2003; Halama et al.,2004) and that silica-undersaturated rocks originate from dif-ferentiation of these magmas under relatively dry and variable


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100µm

ZIRCONIUM NIOBIUM

CALCIUM CERIUM

150 µm

YTTRIUM BSE IMAGE

FIG. 13. Electron microprobe element maps of a pseudomorph of inferred eudialyte illustrating the intensities of Y, Zr,Nb, Ce, and Ca. The relative proportions of each of these elements within the outline of the original crystal are approximatelyequal to those of the primary eudialyte.

fO2 conditions. A key characteristic of such magmas is theirability to evolve to high Cl and F contents before exsolving anaqueous fluid phase (Kogarko, 1974; Markl et al., 2001), al-lowing for extreme fractionation to low temperature. It isknown that elevated concentrations of alkalis and F in thesemagmas increase the solubility of the REE and HFSE by pro-moting formation of alkali-silicate or alkali-fluoride com-plexes with them (Keppler, 1993; Peiffert et al., 1996; Linnen,1998; Marr et al., 1998; Baker et al., 2002), thereby providinga mechanism for transporting these elements. Keppler (1993)

examined the effect of fluorine on zircon solubility in felsicmelts and demonstrated a strong positive correlation betweensolubility and fluorine content in melts containing 0 to 6 wt %F. Concentrations of Zr, Nb, and REE can reach several thou-sands of parts per million in natural systems. For example,Olivo and Williams-Jones (1999) reported concentrations ofZr, Nb, and REE in nepheline syenite from the PilanesbergComplex, South Africa of 1.81, 0.21, and 0.71 wt %, respec-tively. In many cases, the REE and HFSE are further concen-trated by saturation of the magma with minerals like eudialyte

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La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu

B

Type 1b zircon - rim

Type 1b zircon - core


A

Type 1a zircon - rim

Type 1a zircon - core


D

Type 3 zircon - rim

E


Fergusonite-(Y)


C

Type 2 zircon - core

Type 2 zircon - rim

La Ce Pr Nd Pm Sm Eu Gd Tb Dy

F

Monazite-(Ce) Bastnäsite-(Ce) Allanite-(Ce)

100

1000

10000

100000

FIG. 14. Chondrite-normalized REE profiles (Boynton, 1984) for zircon, fergusonite-(Y), monazite-(Ce), bastnäsite-(Ce),and allanite-(Ce). The Y-axis in all graphs represents sample/REE chondrite (McDonough and Sun, 1995).

and their settling under gravity to form cumulate layers inwhich these minerals are dominant (Sørensen et al., 2006).However, to our knowledge, there have been no reports inthe literature of cumulate zircon.

The presence of both simple and complex zirconosilicates,namely primary zircon and eudialyte in the same system, israre but has been reported, for example, from the TamazeghtComplex, Morocco (Salvi et al., 2000; Marks et al., 2008). Inthis complex, eudialyte is the principal Zr-bearing mineral inagpaitic nepheline syentites and forms cumulates, whereaszircon is the main Zr-bearing mineral in miaskitic nephelinesyenites but is a comparatively minor phase and does notform cumulates. Another example of the same phenomenonis provided by the McGerrigle Mountains, Quebec, wherezircon is concentrated in miaskitic rocks (2.2 modal % in onesample) and eudialyte is restricted to agpaitic rocks (Wallaceet al., 1990).

The primary zirconium mineralogy is a function of the al-kalinity of the rocks, where zircon is dominant in miaskiticrocks, eudialyte in agpaitic rocks, and both zircon and eudia-lyte occur in rocks of intermediate composition. Indeed, theterm agpaitic, which was originally introduced to refer tosyenites with atomic (Na + K)/Al ratio > 1.2 (alkalinity index;Ussing, 1912), now has the added restriction that Zr and Timinerals should be in the form of complex minerals, such aseudialyte and rinkite rather than zircon or ilmenite (Sørensen,1997). The term miaskitic is reserved for nepheline syenitesin which Zr is in the form of zircon and for which the alkalin-ity index is generally, but not exclusively, less than unity.

We propose that the upper mineralized zone of the Necha-lacho deposit at Thor Lake, which contains zircon cumulates,represents less evolved miaskitic rocks, and that the eudialytecumulate-bearing lower mineralized zone represents moreevolved agpaitic rocks. This interpretation is supported bylarge numbers of bulk rock analyses of 1 to 2 m long intervalsof drill core from a representative selection of drill holes (Fig.15), which show that rocks from the upper mineralized zone

generally have alkalinity indices <1 (mean of 169 samples =0.89), whereas those from the lower mineralized zone gener-ally have alkalinity indices >1 (mean of 38 samples = 1.20).However, it must be emphasized that both zones were in-tensely altered and it is therefore likely that there was con-siderable mobilization of the alkalis.

The factors determining whether a magma crystallizes zir-con or more complex zirconosilicates are not well understood.Clearly and not surprisingly the formation of complex zir-conosilicate minerals is favored by high alkalinity as shown byfield observations that zircon occurrence seems to be re-stricted to miaskitic rocks. However, silica activity may alsoplay a role as suggested by the results of experiments by Marret al. (1998) with peralkaline melts, which showed that zirconwas the melt-saturated phase for SiO2 contents above 55 wt%, whereas for lower SiO2 contents wadeite, a potassium zir-conosilicate, was the melt-saturated phase. In the Nechalachodeposit, it may therefore not be a coincidence that rocks inthe zircon-rich upper mineralized zone have a higher contentof SiO2 than in the lower zone, where Zr is interpreted tohave been present dominantly as eudialyte (Fig. 16). Finally,there is evidence from field studies that temperature mayplay a role. For example, at Strange Lake, zircon was the ear-liest mineral to crystallize, followed by the sodium zirconosil-icates, vlasovite, and elpidite (Birkett et al., 1992).

Another important HREE-bearing mineral interpreted tobe magmatic is columbite-(Mn), which, because of its rela-tively high density, accumulated with zircon in the upper min-eralized zone, where it is most abundant, and with eudialytein the lower mineralized zone. This interpretation is based onits close spatial association with primary zircon and eudialyte,textural evidence of selective replacement of columbite-(Mn)by fergusonite-(Y), and the occurrence of columbite-(Mn) asinclusions in alteration minerals such as biotite, chlorite, mag-netite, and albite. However, columbite-(Mn) can also be sec-ondary as indicated by its presence in some pseudomorphsafter eudialyte.


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FIG. 15. Histograms of bulk-rock alkalinity indices ((Na + K) / Al) of 1 to 2 m long intervals of drill core taken from a rep-resentative selection of drill holes throughout the deposit: A. Upper mineralized zone. B. Lower mineralized zone. Thesehistograms show that rocks from the upper mineralized zone generally have alkalinity indices <1, whereas those from thelower mineralized zone generally have alkalinity indices >1.

60

50

40

30

20

10

0

16

14

12

10

8

6

4

2

0<0.80 0.81-0.85 0.86-0.90 0.91-0.95 0.96-1.00

(Na + K) / Al

1.00-1.10 1.11-1.20 1.21-1.30 1.31-1.40 >1.41

(Na + K) / Al

Hydrothermal Zr, Nb, and REE mobilization

In most igneous systems, Zr, Nb, and REE are immobileduring fluid-rock interaction, and for this reason are widelyused as petrogenetic indicators (Pupin, 1980; Hoskin andSchaltegger, 2003; Schaltegger et al., 2005). However, there isincreasing evidence in the literature that these elements canbe transported by hydrothermal fluids under the restrictivesets of conditions that commonly prevail in alkaline igneoussettings. It has also been shown that the hydrothermal dura-bility of many minerals enriched in HFSE is compromised byalpha-decay-induced damage (Pidgeon et al., 1966; Sinha etal., 1992; Geisler et al., 2001; Lumpkin, 2001). Our study hasprovided evidence of metamictization, which resulted in pref-erential alteration of the cores of type 1 zircon crystals andmobilization of HREE from these crystals to form mineralssuch as fergusonite-(Y), which only occurs adjacent to zircon.Similar alteration has been documented by Anderson et al.(2008) for zircon from the Georgeville Granite, Nova Scotia.In this latter study, Anderson et al. (2008) showed that REEin metamict zircon were redistributed during alteration bydiffusion and dissolution and reprecipitation along microfrac-tures on a scale of tens of micrometers.

The current study has provided evidence of some remobi-lization of Zr and HREE by hydrothermal fluids. In theupper mineralized zone, minor zircon (type 3) forms collo-form lamellae and overgrowths, and the HREE were mobi-lized out of the cores of type 1 zircon to form crystals of fer-gusonite-(Y) by replacement of nearby grains of primarymagmatic columbite-(Mn). The latter is also true to a lesserextent in the lower mineralized zone, where columbite-(Mn)adjacent to eudialyte pseudomorphs was replaced by fergu-sonite-(Y). In contrast to this limited mobility of Zr and theHREE, the LREE appear to have been mobilized on scalesranging from micrometers to meters. They are concentratedin the minerals, allanite-(Ce) and monazite-(Ce), which typ-ically occur interstitially to magmatic zircon and eudialyte,and as bastnäsite-(Ce), parisite-(Ce), and synchysite-(Ce),which commonly show no spatial association with primary zir-con and eudialyte pseudomorphs; in the upper mineralizedzone these minerals are most abundant meters above the in-tervals containing the highest concentrations of HREE.

As a result of recent experimental studies, we now havesome insights into the behavior of Zr in hydrothermal fluids(Migdisov and Williams-Jones, 2009) and a relatively goodunderstanding of the behavior of the REE in these fluids(Migdisov and Williams-Jones, 2002, 2007; Migdisov et al.,2009). The study of Zr shows that this element forms itsstrongest complexes with fluoride and that fluids with HFconcentrations on the order of 0.1 m can dissolve 10s ppm ofzircon at 200°C. Significantly higher concentrations of REEcan be dissolved by fluids with such HF concentrations. Fur-thermore, the LREE form considerably stronger complexeswith fluoride than the HREE and are therefore more easilymobilized. The REE also form relatively strong complexeswith chloride and, as for fluoride, the strongest complexes arewith the LREE. These studies satisfactorily explain the rela-tively limited mobility of Zr and the HREE, and the muchgreater mobility of the LREE in the Nechalacho deposit.

In light of the above discussion and the abundance of fluo-rite in the deposit, it seems plausible that the REE wereleached from primary minerals, namely zircon and eudialyte,by forming fluoride complexes, and then redeposited as fer-gusonite-(Y), allanite-(Ce), monazite-(Ce), bastnäsite-(Ce),parisite-(Ce), and synchysite-(Ce). In the case of fergusonite-(Y), allanite-(Ce), and monazite-(Ce), the mobilization, forthe most part, was on the scale of millimeters, and for fergu-sonite-(Y) was controlled by the occurrence of proximalcolumbite-(Mn). However, as noted above, the REE concen-trated in bastnäsite-(Ce), parisite-(Ce), and synchysite-(Ce)of the upper mineralized zone were mobilized on a scale ofmeters. By analogy with the model proposed for the deposi-tion of these minerals in the Strange Lake Pluton, Québec-Labrador (Salvi and Williams-Jones, 1990), and noting theirassociation with fluorite and calcite, we propose that theLREE were transported as fluoride complexes in magmatichydrothermal fluids, which subsequently mixed with calcium-and carbonate-bearing fluids near the top of the layered com-plex. These latter fluids were likely external and their mixingwith the magmatic hydrothermal fluids caused immediate de-position of fluorite, which is relatively insoluble in the pres-ence of Ca. This destabilized the REE-fluoride complexes,making the REE available for deposition as fluorocarbonateminerals.

Genetic model

The proposed geologic model for the Zr, Nb, and REE min-eralization in the Nechalacho deposit involves intrusion of analkaline, volatile-rich, aegirine nepheline syenite into the ThorLake Syenite, at the center of the Blachford Lake IntrusiveComplex. The alkaline nature of the rocks and their high con-centration of incompatible elements suggest a small degree ofpartial melting of upwelled mantle, possibly at depths of 60 to100 km (Sørensen, 1997; Martin, 2006). Repeated injectionsof magma, fractional crystallization, and convective overturnproduced a layered igneous body. As the convecting magmawent through cycles of saturation and undersaturation in aphase (i.e., aegirine, nepheline, K-feldspar, sodalite, zircon, oreudialyte), because of convective overturn and mixing of dif-ferent pulses of magma, layers were formed (Fig. 17). Themixing likely caused monomineralic crystallization of zirconor eudialyte by displacing the liquidus from a cotectic to a

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FIG. 16. Bulk-rock silica content (wt %) as a function of depth in drill holeL09-162. Samples represent 1 to 2 m long intervals of drill core.

0 50 100 150 200 (m)

SiO

2(w

t. %

)

80

70

60

50

40

30

20

10

0

field in which only a zirconosilicate mineral was stable. Theresult was a lower mineralized zone comprising multiple eu-dialyte cumulate layers and an upper zone comprising multi-ple zircon cumulate layers. We propose that the differencebetween these two zones was the result of the alkali contentof the magmas that formed them, i.e., miaskitic or less alka-line in the case of the upper mineralized zone and agpaitic ormore alkaline in the case of the lower mineralized zone.

Fluids of probable magmatic origin exsolved from aegirinenepheline syenite at depth were responsible for alteration ofthe primary eudialyte and zircon (autometasomatism). Thesemetasomatic fluids also caused biotitization, K-feldspathiza-tion, and replacement of primary minerals (mainly aegirine)

by magnetite or hematite as the fluids evolved composition-ally with evolution of the aegirine nepheline syenite magma.The eudialyte crystals were dissolved and completely re-placed by secondary phases within the original crystal bound-aries. Some elements, e.g., Zr, Nb, and the HREE, were onlyremobilized on a scale of microns or tens of microns, i.e., theyremained largely within the boundaries of the pseudomorphs,whereas other elements, e.g., Na, Ca, Cl, H2O, and LREE,were removed by the fluids exsolved from the lower, moreperalkaline part of the intrusion. Zirconium and REE weremobilized from type 1a and b metamict zircon cores in theupper mineralized zone along fractures to form secondary zir-con, fergusonite-(Y), allanite-(Ce), and monazite-(Ce). The


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FIG. 17. Cartoon illustrating the intrusion of aegirine nepheline syenite into the Thor Lake Syenite. Repeated injections

of magma, bottom upward crystallization due to gravity accumulation and convective overturn due to decreasing tempera-ture, produce a layered igneous body. Hydrothermal alteration of the upper and lower mineralized zones leads to remobi-lization of a variety of elements, including the LREE.

LREE were mobilized from both eudialyte and zircon as flu-oride complexes by magmatic hydrothermal fluids and de-posited distal to their hosts. In the upper parts of the com-plex, this occurred as a result of mixing with Ca-bearing fluidsof probable external origin, which caused precipitation of flu-orite, destablizing the REE-fluoride complexes. Later Na-rich fluids overprinted and locally replaced the bulk of theprimary mineralogy, including zircon and eudialyte, with al-bite (cleavelandite), mainly, but not exclusively, in the upperparts of the deposit.

Comparison to other deposits

A small number of HREE-enriched deposits occur in otherparts of the world. The Ilímaussaq layered alkaline complex inthe Gardar province of southern Greenland (Marks et al.,2004) is similar in many respects to the Nechalacho deposit,in geologic setting, size, and the preferential enrichment ofthe HREE. At Ilímaussaq, the HREE are concentrated in lu-javrites, which comprise cumulates of aegirine, arfvedsonite,and eudialyte (Larsen and Sørensen, 1987), whereas in theNechalacho deposit there are also cumulates of zircon, andeudialyte is pseudomorphous after a zircon-bearing mineralassemblage. To our knowledge, Nechalacho is the onlyHREE deposit in which zircon cumulates have been recog-nized. However, both zircon and eudialyte occur in theTamazeght alkaline complex in Morocco (Salvi et al., 2000)and, although zircon is not a cumulate mineral and is presentin relatively small proportions, it is hosted by miaskitic rocksas has been proposed for the Nechalacho deposit. TheMotzfeldt intrusive suite, also located in the Gardar provinceof southern Greenland (Schoenenberger and Markl, 2008),formed by successively intruding melt batches of varyingcomposition from a common magma source at depth as isproposed for the Nechalacho deposit. Five units crystallized amiaskitic mineral assemblage and a final magmatic batch anagpaitic assemblage. This complex is a type example of thetransition from miaskitic to agpaitic mineral assemblages.

Hydrothermal alteration is less significant in these othercomplexes than in the Nechalacho deposit, however, it is lo-cally important. For example, at Tamazeght, primary zirconand eudialyte have been partly or completely replaced by sec-ondary phases that are richer in F and Ca, such as cancrinite,calcic catapleiite, and rinkite, and primary eudialyte has beenpartially replaced by secondary eudialyte enriched in REEand HFSE (Schilling et al., 2009). In the Gardar province ofsouthern Greenland, the transition from an oxidized to a re-duced fluid was shown to be correlated with a change from amore reduced, Fe2+-bearing miaskitic mineral assemblage toa more oxidized, Fe3+-bearing agpaitic assemblage (Schoe-nenberger and Markl, 2008).

Although many similarities can be drawn between Ilímaus-saq, Motzfeldt, Tamazeght, and the Nechalacho deposit, theNechalacho deposit is unusual in its REE mineralogy and inhosting a potentially economic resource of the HREE.

ConclusionsThe Nechalacho deposit at Thor Lake contains extraordi-

narily high concentrations of Zr, Nb, and REE as zircon, fer-gusonite-(Y) and ferrocolumbite, bastnäsite-(Ce), allanite-(Ce), and monazite-(Ce). Upper and lower mineralized zones

have been defined, with the former representing discontinu-ous, heterogeneous zircon-rich cumulate layers and the lattermultiple cumulate eudialyte layers. Primary eudialyte hasbeen completely replaced by secondary phases, namely zir-con, fergusonite-(Y), bastnäsite-(Ce), parisite-(Ce), synchysite-(Ce), allanite-(Ce), albite, quartz, biotite, fluorite, kutnahorite,and minor hematite, which are pseudomorphous after theprecursor eudialyte, and primary zircon has been leached ofREEs. The model proposed to explain the origin of the min-eralization in the Nechalacho deposit calls on both magmaticand hydrothermal processes. The eudialyte and zircon cumu-late layers can be explained by the intrusion, fractional crys-tallization, and mixing of evolving alkaline to peralkaline mag-mas. Magmatically derived hydrothermal fluids altered thedistribution of REE within the upper and lower mineralizedzones, locally enriching and depleting them, and created a setof secondary minerals, which will be the main target of futureexploitation.

AcknowledgmentsE. Sheard gratefully acknowledges the support of a Natural

Sciences and Engineering Research Council (NSERC)Alexander Graham Bell postgraduate scholarship, a GE-OTOP (Quebec interuniversity network for advanced studiesand research in geoscience) postgraduate scholarship, and aMcGill Principal’s Graduate Fellowship. Financial supportfor the research was provided by a grant from Avalon RareMetals Inc. and a matching grant from the NSERC-CRDprogram to AEW-J. Avalon Rare Metals Inc. also provided ac-cess to the field area, drill core, geochemical data, and inter-nal reports. The research benefited from helpful discussionswith V.J. van Hinsberg, J.R. Clark, A. Migdisov, K. MacWilliam,and W. Mercer. L. Shi provided valuable analytical assistanceand V. Moeller provided several drill core photographs. Themanuscript was improved by the reviews of I. Coulson, D.Sinclair, and R. Taylor.

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