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653

TheC anadian M ineralo gi s t Vol. 37, pp.653-663(1999)

HYDROTHERMALREE.RICH FROM THE PILANESBERG COMPLEX, SOUTHAFRICA

GEMA RIBEIRO OLIVO$ AND ANTHONY E. WILLIAMS-JONES

Department of Earth and Planetary Sciences,McGill University, 3450 University Street, Montreal, H3A 2A7,

Ansrnecr

The PilanesbergComplex, in , one ofthe world's largest alkaline complexes, contains large resourcesofZt and the rare-earth elements (REE). Eudialyte is the main carrier ofZr, and contains appreciable concentrationsofREE. It is particu- Iarly abundantin the green (2070by volume). It forms complexly zoned poikilitic domains that encloseaegirine. , , nepheline, , pectolite, , and an unidentified Na-Zr silicate. These domains mantle corroded silicates and replace outer zones of microcline euhedra. The eudialyte is partially replaced by fergusonite-(Y) and britholite. Compositionally, the eudialyte at Pilanesbergis unusual; it contains the highest concentration of Nb (up to 3.8 wt% M2O5) and the lowest concentrationsof Na (<11 .4 wt%oNa2O) and Fe (< 0 4 w t%oFeO) reportedin the literature. In addition, it has one of the highest contents of REE (up to'1.6 wt%oREE2O3, mainly Ce and La and traces of Sm and Nd) and Mn (up to 7 5 wtTo) reported in the literature. On the basis of 78 anions, its formula is (Na1230.REE13scaose &20)>lare (Cas+zMnose)>e(Mnzg6Fe00a)>3 (Zr266M62sHf6s6)13(Nbs66Si62s Taooo)>r Sizsr+ Or+ (OHrorCloqzFoo)22.2H2O. On the basis of textural relationships, the Pilanesberg eudialyte is interpreted to be hydrothermal. It seems to have formed from an orthomagmatic Na-M-REE-C1-F- bearing hydrothermal fluid that exsolved from an agpaitic syenitic magma. In this system, zirconium was probably remobilized from magmatic zircon as a F-complex (e.g., ZrF62-), and REE were introduced as Cl- and F-complexes. The incorporation of the REE in eudialyte probably occurred in responseto the reduced Cl- and F-activities that accompaniedprecipitation of sodalite and the unidentified (F-bearing) Na-Zr silicate.

Keynvords:eudialyte, , Pilanesberg,a.lkaline complex, hydrothermal, rare-earth elements, South Africa.

Sotrnaerru,

Le complexe alcalin de Pilanesberg,en Afrique du Sud, un des plus gros au monde, contient des ressourcesimportantes de Zr et de terres rares. L'eudialyte est le porteur principal du Zr, et contient aussi des teneurs importantes de terres rares. Ce min6ral est particulibrement abondant dans I'unit6 de sydnite ndph6linique verte (20Voen volume). Il forme des domaines poec6litiques zon6s de fagon complexe qui englobent aegyrine, albite, microcline, n6phdline, zircon, pectolite, sodalite, et un silicate Na-Zr non identifi6. Ces domaines recouvrent des silicates corrodds et remplacent les bordures des crisraux idiomorphes de microcline. L'eudialyte est remplac6e en partie par la fergusonite-(Y) et la britholite. Par sa composition, I'eudialyte h Pilanesbergn'est pas courarte. Elle possEdela concentration la plus 6lev6e en Nb qui soit (usqu'h 3.8 wtTo Nb2O5),et des teneurs en Na (<1 1.4 wtTo Na2O) et Fe (<0.4 wtqo FeO) qui sont les plus faibles dans la littemtufe. De plus, les teneurs en tenes rares (ZR) Q]usq]u'd7.6Vo ZRzO:, surtout Ce et La et des traces de Sm et de Nd) et Mn (usqu'h 7.5 wtva MnO) sont parmi les plus 61ev6esque I'on connaisse.Sur une base de 78 anions, la formule chimique est (Na123oREE1 3eCao se K0 20)>1a78 (Ca5 a2Mns 5s)26 (Mn2 e6Fe66a)23 (Zr2 6d{bs 2sHf6s6)1,3 (Nb6 66Sie2sTao oe)>t Si25 6a O7a(OH1 61Cls e2Fs e7)12.2H2O. Selon les critdres texturaux, nous croyons qu'il s'agit d'une origine hydrothermale. L'eudyalite semble s'Otreform6e d partir de la phase fluide orthomagmatique, porteuse de Na-Nb-REE-CI-F, issue du magma sy6nitique agpaltique. Dans ce systbme, le zirconium a probablement 6t6 remobilis6 par dissolution du zircon magmatique sous fonne de complexe fluorl (e.g., ZtF62-), etles terres rares ont 6t6 inftoduites sous forme de complexes chlor6 ou fluor6. L'incorporation des terres rares dans I'eudialyte r6sulterait d'une rdduction de 1'activit6 de Cl et de F qui accompagnala pr6cipitation de la sodalite et du silicate fluor6 d Na-Zr non identifi6.

(Traduit par la R6daction)

Mots-cl6s: eudialyte, sy6nite n6ph6linique, Pilanesberg,complexe alcalin, hydrothermal, terres rares, Afrique du Sud

! Present address:Department of Geological Sciences,Queen's University, Kingston, K7L 3N6, Canada. E-mail addres s: [email protected]

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INrnooucrroN feldspathoidalsyenite, which foms the core of the struc- ture. Green nepheline syenite postdates the red and Eudialyte is a rock-forming Na-Ca zirconosilicate white , as well as a cone sheet of that may contain appreciable concentrations of rare- tinguaite, and is intruded by tinguaite dykes. earth elements (REE) and Y. The name "eudialyte", Rocks of the Pilanesberg Complex are enriched in from the Greek words "eu" (welI) and "dialytos" (dis- fluorine, strontium, niobium, tantalum, zirconium, solved), emphasizesan important property of this min- hafnium, the rare-earthelements, thorium and uranium eral, namely the easewith which it can be dissolved in (Lurie 1986). Of the major rock-types, the green dilute acids. This propefiy makes it possible to easily nepheline syenite,which has also been referred to in the beneficiate eudialyte-bearing ores using conventional literature as green foyaite and Ledig foyaite, has the heap-leachtechniques (Mariano 1989). highest content of zirconium (up to 19,900 ppm) and Eudialyte commonly occurs in agpaitic syenitesand rare-eafih elements(up to 6,500 ppm) (Table 1). Its high . The syenite-hosted occurrences include the REE content has previously been attributed to the pres- Lovozero (Kola Peninsula,Russia), Ilimaussaq ence of britholite, (REE,Ca,Y)s(SiOa,PO+)(OH,F),and (), and Pilanesberg (South Africa) massifs products of its weathering. However, contrary to previ- (Kogarko 1987,Larsen & Sgrensen1987, Lurie 1986), ous reports, a large proportion of the REE seemsto be whereas eudialyte in has been described at As- hosted by eudialyte, which comprises up to 20Vaof the cension Island, along the Mid-Atlantic Ridge (Harris el rock by volume. al. 1982) and Straumsvola,in the Antarctica (Harris & Rickard 1987). Although many papers have been pub- Tse GnrsN Nspner-nve SveNn:n, lished on the mode of occurrence and chemistry of eudialyte, the genesisof this has received lim- The greennepheline syenite (Fig. 1) is coarsegrained ited attention. Most investigators,however, favor a and comprises thick layers composed of variable pro- magmatic origin (e.9., Portnov 7964, Kogarko et al. portions of (mainly ) and . 1982,Krigman et al.1983, Larsen & Sgrensen1987). In addition to aegirine,the rock also containsmicrocline, The Pilanesbergcomplex is an ideal setting in which albite, nepheline, sodalite, pectolite, zircon, eudialyte, to investigatethe processescontrolling the formation of britholite, fergusonite-(Y), and traces of an unknown eudialyte and the incorporation of REE in its structure. This intrusive complex is unusually rich in this mineral, notably in a green nepheline syenite unit, which con- tains up to 2OVoeudialyte by volume. Moreover, this TABLE I COMPOSITION OF TIIE GREEN NEPHELINE SI'EMTE. PILANESBERG

eudialyte contains one of the highest concentrationsof CENTRALZONE sotmfiRl-Imlczom* REE reported in the literature (cf. Deer et al. 1986, (t!j.p"F) WTNPNT roPRT zuroPST

Johnsen & Gault 1997).The purpose ofour study is to Sro! (s%) 5132* determine the textural relationships and composition of Aloo, lJ 55* Fq03 E 58' eudialyte in the green nepheline syenite, and to further IrhO I l8* Mco 06+ our understandingof the processesthat led to the crys- C.O z75r tallization ofREE-rich eudialyte in this agpaitic syen- N&o l0 76* &o 3 08| itic complex. Tio, 02q Pzo' 0 03* LOI 28 Trm Pu-aNssseRcCoMpLEx Tobl 9636 k (pPn) nn 1500 1000 1500 Ce 3216 2400 1600 2400 The PilanesbergComplex is one of the world's larg- h 26t Nd 815 est alkaline igneousbodies, covering an areaof570 km2; Sm 722 h 1601 it was emplaced approximately l25O + 50 Ma ago (K- G 909 fr 15t Ar age, Retief 1963), along the eroded contact between Dy %7 the granitic and mafic phases(norite) of the Bushveld Ho 186 & 598 Complex (Fig. 1). The rocks of the Pilanesberg com- Tm 961 n 572 plex, which are now partially unroofed, comprise pyro- Lu 775 clastic and lava-flow Tobl REE 656343 3900 2600 3900 sequencesand inwardly dipping h 19q6*-1$@0 4800 9600 7700 ring-dykes (except in the north, where they locally dip H 374 t13 155 r14 Y 564*-546 BO 190 170 outward), concentrically disposed around the core M 2t20 2700 tmo 1040 T. 165 95 63 76 (Lurie 1986). From the core to the outerring, the arcu- U 2t7 63445 ate units comprise red and white feldspathoidal syeni- fr 806 320 ll0 372 Rb 199 317 300 4m tes, green nepheline syenite, white feldspathoidal Sr 2113*-t97? 410 530 230 Bs l9*-B 0 80 550 syenite [refened to in Lurie (1986) as foyaite], tinguaite,

and red syenite (Fig. 1). The oldest intrusive rock is the * oDIy ftsiotr ICP dalyEi!; th6 dher mabEe6 rere cmdeted Eing firsiotr ICP-MS bchniqw * red syenite,which was subsequentlyintruded by the red l-*iu (rseo)

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Frc. 1. Geological map of the Pilanesberg Complex; the inset shows its location in relation to the Bushveld complex, western Transvaal, South Africa (after Lurie 1986).

Na-Zr silicate. Aegirine occurs as prismatic crystals con, aegirine, and , and fills fractures in the interlayered with albite (Fig. 2A), or encloses micro- unknown Na-Zr silicate (Figs. 3E, F). cline and nepheline crystals. It is also commonly in- A paragenetic sequence for the minerals in the green cluded in felsic minerals. Nepheline and microcline nepheline syenite is shown in Figure 4, and a detailed form tabular poikilitic crystals up to one centimeter in description of textures involving eudialyte is presented diameter; they contain inclusions of aegirine and albite. in the next section. The mineral assemblageand tex- In addition, nephelinecontains inclusions of microcline, tural relationships suggestthat the green nepheline sy- and is commonly replaced by sodalite, clays (Fig. 2B) enite formed under subsolvus conditions (reflected by and eudialyte. Albite also occurs in aggregateswith fine- the assemblagenepheline-microcline-albite) and under- grained microcline. Both are locally corroded went extensive subsolidus re-equilibration in the pres- and replaced by sodalite and eudialyte (Figs. 3,A.,B). ence of Cl-bearing hydrous fluids (as indicated by the Pectolite is found among albite and aegirine crystals, in replacementof nepheline, microcline, and albite by so- mafic layers. Zircon forms bipyramidal zoned crystals dalite). up to 200 pm in diameter, and is commonly included in eudialyte (Figs. 3C, D). Fergusonite-(Y) occurs as ir- TexruREs INvoI-vtNc Euorer,vrB regular grains (up to 20 pm in diameter) that are inter- stitial to the silicates or fill cleavagesand fractures in Eudialyte typically forms pleochroic (pink), com- and occupy embayments in zircon. The margin of plexly zoned poikilitic domains up to a few centimeters fergusonite-(Y) grains is commonly corroded and in diameter, which contain aegirine, albite, microcline, coated by britholite, which also replaces eudialyte, zir- nepheline, zircon, pectolite, and the unidentified Na-Zr

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As noted above, the eudialyte is complexly zoned MAGMATIC lHypnorHEnver_ (Fig. 3F). In back-scatteredelectron images, this zon- ing is manifested by very irregular light zones (10- Aegirine 30 pm wide) and dark patches commonly narrower than Zirqn 10 p.m. This fact makes the analysis of the dark patches difficult. Sixty analyseswere done, most of them in the Midocline light zones; the results of representative analyses are Albite presentedin Table 2. Nepheline The Pilanesberg eudialyte sample, based on analy-

Petolite ses of the light zones,is compositionally unusual com- pared with eudialyte from other localites (e.9.,Balashov Sodalite & Turanskaya 1960, Jones & Larsen 1989,Deer et al. Unlmom Na-Zr 1986,Mandarino & Anderson1989, Coulson & Cham- Eudiallte bers 1996, Coulson 1997, Johnsen & Gault 1997). It contains the highest concentrationof Nb (up to 3.8 wtTo Fergusonite-CO NbzOs) and the lowest concentrationsof Na (<11.4 wtTo Britholite Na2O) and Fe (<0.4 wt%oFeO) reported in the litera- ture. Furthermore, it has one of the highest contents of REE (tp to'7 .6 wt% REEzOz,mainly Ce, La, and traces of Nd and Sm) and Mn (up to 7.5 wtVa MnO) reported FIG.4. Parageneticsequence for the greennepheline syenite from Pilanesberg. in the literature. As is common to eudialyte from other localities, it contains Cl (up to 1.2 wt%o),and traces of F, Hi K, Al and Ta, and all the analysesyield low totals (up to 96 wtTo), which reflects the presenceof HzO and content (2 wt%oZrO2), this sample also has high con- OH in its structure (Coulson & Chambers 1996). The centrations of REE and Nb, 6500 ppm and2l2} ppm, levels of Y, P and Ti are lower than the detection limit respectively. of the EMP. As noted above, the dark zones were diffi- The agpaitic index of the green nepheline syenite, cult to analyze. Consequently results for them are less based on the above sample, is 1.35 (similar values for reliable than for the light zones. However, analysesof the index have been reported for Pilanesbergnepheline the widest dark patches suggestthat the dark zones are syenites by Kogarko et al. 1982). The replacement of richer in Ca, Si, Zr, and Fe, and poorer in RE4 M, and nepheline and feldspars by sodalite indicates that the Hf than the light zones (Table 2). syenite was altered by aqueous fluids enriched in Cl (Wellman 1970) and probably in Na. Consequently,the TUB SrorcmovrETRY oF Euonryrs agpaitic index reported may not reflect the original magmatic proportions of A1, K and Na. Nevertheless, Eudialyte is a cyclosilicate containing three- and the occurrence of magmatic aegirine is consistent with nine-fold rings of --oxygentetrahedra (Deer et al. a relatively high agpaitic index (Mitchell 1996). 1986).It is trigonal and crystallizesin the acentricspace- group R3m (Giuseppetti et al. 1971, Golyshev et al. CnerrlrcaL CollpostrroN oF TrrE EuDIALyTE I 97 1, Johnsen & Gault 1997) . Although the formula unit has usually been calculated on the basis of l9 The composition of the eudialyte was determined (O,OH,CI), there is someuncertainty about this conven- using a JEOL JXA-8900L automated wavelength-dis- tion, and the structural formula of eudialyte has been persion electron microprobe (EMP) at McGill Univer- variously given as: Narz (Ca, REE)6 (Fe2+,Fe3*,Mn, sity. Calibration for the analyseswas done using albite Mg! (Zr,Nb), 24 lS\O27-r(OH))14(Si3Oe)4 Cl., where (Na), diopside (Si, Ca, and Mg), (Al and K), 0. 1 < x < -O.9, | < y < 3, 0.7 < z < 1.4 (Giuseppetti et al. zircon (Zr and H0, (Fe), spessarrine(Mn), 1971),(Na,Ca, REE15 (Fez*,Mn) (Zr,Tl) t(Si3oe)21(OH, K2Ta2O6(Ta), Na2Nb2OoNb), TiO2 Gi), vanadinite Cl) (Deer et al. 1986), and Na15(Ca)6(Fe2',Mnz+) (Zr)3 (CD, CaF2 (F), and synthetic REE phosphatestandards (Si,M) (SizsOz:)(O,OH,H2O) (CI,OH)2 (Mandarino (La, Ce, Sm, Nd, and P), and Y3Fe5O12(Y). The operat- 1999). Johnsen & Gault (1997) recently completed an ing conditions were: accelerating voltage 15 kV and extensive review of the structure and chemical compo- beam current 30 nA. A l0-pm beam was usedfor analy- sition of eudialyte. On the basis of this review, and in sis, and the concentration of Na was measuredfirst to the absenceof specific structural information, they pro- minimize its loss through volatilization, although pose the best procedure for determining the stoichiom- Johnsen& Gault (1997) pointed out that Na volatiliza- etry of eudialyte is to adopt a basis of 78 anions, using tion and migration do not seem to be a major problem the general formula ABCDES|,O16 (Cl,OH,F)2, where in eudialyte analyzedunder similar conditions. x is 24 to 26, E = Zr + Hf (x 3), D = Nb +Ti (= 0.5),

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TABLE 2. COMPOSITION OF EIIDIALYTE IN A SAMPLE OF GREEN NEPHBLINE SYENITE FROM PILANESBERG

NazO SiOz ZtOz KrO CaO FeO NfnO IlfO2 Ce2O3 LarO3 Nd2O3 Su2O3 Nb2O5 Ta2O5 Total ffiOr

Na-rioh bmils fl isht bmds) | 7027 4429 9.34 0.31 10.19 0 05 7.44 0 42 0 01 0.89 3 69 232 0.63 0 05 3.83 o.39 9392 669 2 7t.78 4479 9.28 0.22 70.06 0 05 7.L9 0.40 0 00 0.91 3 76 3.33 0.33 0 03 3.53 0 51 95.35 7 44 3 70.95 44.90 937 0n 1038 0 06 7 52 0.45 0.06 0 90 3.41 219 0 95 0.10 3.69 0 33 95.31 6.65 4 7097 44.97 948 037 rO.47 0 10 7 47 0.42 0 01 0.86 3.65 201 0.85 o.0z 3.68 0.33 95 42 6.52 511.20 4477 9.40 024 10.20 0.03 7 57 0.45 0.00 0.90 4 t9 2.55 0 85 0.03 3.77 0 25 96.08 7 61 6 10.39 44 85 9.63 0 31 10 14 o.07 7.30 0 41 0.07 0.96 3 46 2.32 0 44 o07 3.50 0 41 94.09 629 7 10.99 45 05 9.51 0:B 10.77 008 771 035 000 1.08 3.60 2.tl 0 58 0 05 333 036 94.34 634 811.07 4484 946 0.27 7072 0.07 7.29 038 0 10 0.87 3.32 2.49 0,37 0 07 3.47 048 9442 625 9 1736 44.87 9 s7 0.20 10 08 0 t2 6.97 0.24 0 t2 I 13 3.59 2.18 0.64 007 360 017 9454 648 10 1109 45.04 9 47 0.30 10.16 011 7.77 020 005 098 3.7t 2.27 0 52 0 01 3.59 0 47 9484 6.51 11 lt 09 45.00 9 42 0.21 10.03 005 715 033 0.00 114 3 85 2.51 0.58 0 05 3.67 036 95 20 7.00 t2 70.a7 4482 9 54 0.29 10.33 0 15 7.18 0 31 0.03 0.90 2.90 2.46 0.60 013 3.62 o.33 9425 610 t3 70 89 44.94 9.33 0 31 10.17 0.06 7.23 028 0.00 0 88 3 84 2.87 0.45 0.00 382 0.36 95 22 7.16 14 1l 14 44.88 9.42 0 25 10.00 0.01 7 20 032 0 0t 704 3a2 240 0.5r 0.00 3 43 o.4l 9462 673 t5 17.23 4500 9.54 0 28 10.11 0 09 7-26 0.26 0 03 1.00 3 45 2l4 0.71 005 358 o 37 94.85 6 35 t6 1095 45.37 9,42 0.27 7036 0 08 7.34 0.34 0-09 0.97 3.59 2.77 0.70 006 362 0.41 96.03 7.06 r7 7078 45.26 9.36 0 31 10.70 0.06 7.47 0.67 0 00 0 83 3 69 796 0.E5 0 13 379 0 28 95.95 6.62 18 10 99 45 30 9.46 0 28 10-19 0 08 7 n o32 002 094 3.43 272 0 66 0.08 339 0.32 94 59 6.29 t9 71,13 45 3t 9.28 032 lO34 0 09 7.28 036 0.00 0 85 3 08 2.51 0.55 o.o7 3 7l o.37 9504 6.21 20 11.11 45 15 9.53 0.27 1026 0 10 7.27 0 29 0.09 0 91 3.17 2.L4 0.4 0.05 3.65 0.36 94 50 5 82 27 tt30 4535 9.69 027 lOlT 0.05 7 24 0.38 0.00 0.89 339 2.65 0.55 0.00 3.77 0.3E 95.89 6 58 22 tt.l8 4551 9.51 029 10.27 o.o4 7 25 0 30 0 00 0.86 3 56 1.95 0.69 0 13 370 ot9 9522 533 zi 1124 4552 9.50 0.23 1043 0.12 7.74 0.40 0 07 r.05 3.57 2 44 0.68 010 366 036 9624 6.78

Ca-rioh banils (dark bands) U 10.70 ,16.10 l0 00 0 26 11.08 041 744 0.25 005 1.03 1.83 1.13 0.32 0-00 3.13 o.77 79 3 28 25 993 4602 9.41 028 11.43 o37 7 38 0 15 002 t.o7 198 l7l 0.43 o.o7 3.18 0B 93.79 420 26 L062 46 01 9 66 0.30 11 15 0.36 7.24 0 22 td 1.10 1.87 1.39 0.33 0.06 3.59 0 10 93.86 3.65 27 1007 46.26 9.82 0.35 11.71 034 7 45 0.r7 0.06 0.99 1.90 1.37 0.44 0.00 3 14 013 94.27 3.71 2A 10.54 46.92 Ll45 037 11.54 o.42 695 0 28 0 08 1.01 7.24 069 025 0.06 2.92 029 9494 223 29 10.39 M.34 70.70 0 30 11 52 0.39 7 42 0.17 0.74 1.04 1.65 1.04 0.45 0.09 3 18 022 94.32 323 30 10.45 ,1683 9.91 0 29 17.64 0.56 7.53 0.23 ld 102 1.85 704 045 0.12 281 o31 94.99 3 45

atomioproportions calculatedon a basis of of 7 8 anions

Na Si Ca Fe 1\[n Hf F Cl Ca La Nal Ta

Na-riohbmcls 1t.63 2585 2.66 0.23 6.37 0 02 3.68 0 07 0.01 0 88 0.79 050 013 0 01 l.0l 0.06 I25t 25.84 2.61 016 6.22 0.02 3 51 007 000 089 079 o77 0.07 0.01 092 0 08 t2.21 25.83 263 0.20 6.40 0 03 3.66 0.07 0.11 0 88 072 0 46 0.20 o.02 0.96 0 05 1221 25.81 265 0.22 6.44 0 05 3 63 0.O7 0 01 0.84 077 o 42 0.17 0.00 0.95 0 05 t2.47 2571 2.63 0.t7 62a 0 01 3.65 0.07 0.00 088 088 0.54 0.17 001 096 004 11.69 26.03 2.73 0.23 6.31 0.03 3 s9 0.07 0.13 0.95 0.73 0 50 0.09 0.01 0.92 0 06 12.32 26.06 268 077 6.30 0.04 3 48 0.06 0.00 1.06 0.76 0 45 0.72 0 01 0.87 0.06 12.42 25.95 267 0.20 6.n 0 03 3.57 0 06 0 18 0.8s O70 0.53 0.08 0 01 0.91 0 08 9 72.74 25 270 0.15 6 25 0 06 3.42 0 04 0.22 1.11 076 0.47 0.13 001 094 003 t0 72.39 2s 96 266 0.22 6 27 0 05 3.50 0 03 0.10 0.96 0.78 0.48 0.11 0 00 0 93 0.06 rr 1239 25.93 2.65 0.16 6 19 0.02 3 49 0.06 0 00 1 ll 0.81 053 0t2 0.01 0.96 0 06 12 t220 2594 2.69 0 2t 6 40 0.07 3.52 0 05 0.06 0 88 0.61 053 012 0.03 095 005 13 1216 2588 2.62 0.23 6.21 0.03 3.53 0 05 0.00 0.86 0.81 0.61 0.09 0.00 099 006 74 1250 25.98 266 0.18 620 0.00 3.53 0 05 o.l4 7.02 0.81 0.51 0.11 0.00 0.90 0.06 t5 12.55 25.93 268 0.20 6.25 0 04 3.54 0 04 0.05 0.98 073 0.46 0.15 0.01 093 006 t6 7273 2591 2.62 0 20 634 0.04 3.55 0 06 0.16 0 94 0.75 o.57 0.14 0 01 0 93 0.06 t7 1193 25.84 261 0n 6.54 0.03 3 61 0.11 0 00 0 80 0.77 0 41 0.17 0 03 0.98 0.04 78 rzn 26.09 2.66 0.27 6.29 0.04 3.53 0.05 0.04 0.92 0.72 0 45 0.14 0.02 088 005 t9 72.37 25.98 2 59 0.23 6.35 0.04 3.53 0 06 0 00 0.83 0.65 0.53 0 ll 0.01 0.96 0.06 20 72.40 26.00 2.68 0 19 633 0 05 3.51 0 05 0 16 0.89 0.61 0.46 0.09 0.01 0 95 0.06 27 1250 2587 270 0.20 622 0 03 3.50 0.06 0.00 0.86 0-7L 056 0ll 0.00 0.97 0 06 22 12.39 2602 2.65 0.27 6.29 0.O2 3.51 0.05 0.00 0.84 0.75 041 014 0 03 0.96 0.03 23 72.& 2592 2.64 0 77 6.36 0.06 3.45 0.O7 0.13 101 0.74 0 51 0.I4 o.02 0.94 0 06

Ca+ioh baryls 24 71.E3 2628 2.78 0.19 677 0.20 3 59 0.04 0 09 1.00 0.38 0 0.06 0 00 0.81 0.03 25 11.00 26.28 2.62 0.20 6.99 0.1E 3 57 0.02 0 04 1.03 0.41 0.36 0.09 0 01 0 82 0.04 26 1774 26.24 2.69 0 22 6.81 O.l7 3 50 0.04 0.00 I 06 0.39 0 29 0,o7 0 01 0.93 0.o2 27 t1.02 26.25 272 0.25 7 72 0.16 3.58 0 03 0.10 0 95 0,39 029 009 0.00 0 81 002 28 \1.45 26.28 3 t3 0.26 6 0.19 3.30 0 04 0.13 0.95 0.25 0.t4 0.05 0.01 0.74 0.04 29 7142 2624 2.79 0 21 6.99 0 19 3.56 0.03 025 100 034 0.22 0.09 o02 0 81 0.03 30 1139 2634 2.?2 0.21 7 01 0.27 3.59 0 04 0 00 0,98 0 38 o.2l 0 09 o.02 071 005

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C = Fe + Mn (< 3), B = Ca + REE + remaining of Mn Onrcnt or rHE ELIDIALYTE (= 6) anda = Na + K + Sr + Y (15-17). The difficulty in establishing a satisfactory method Kogwko et al. (1982) have reviewed the texturesand for calculating the structural formula of eudialyte is compositions of eudialyte-bearing rocks from various probably due to: (l) the presenceof extra ions and H2O alkaline complexes,including Lovozero, Ilimaussaqand in the large spacesin the structure, (2) extensive substi- Pilanesberg, and concluded that eudialyte is of mag- tution in the various sites (Johnsen& Gault 1997), (3) matic origin. They also conducted a limited number of the occurrenceof five or more Na sites(Johnsen & Gault experimentsdesigned to investigate the magmatic crys- 1997), and (4) variations in the number of anions tallization of eudialyte. On the basis of theseexperimen- (Johnsenet al.1997). tal results, they concluded that eudialyte can crystallize The empirical formula of the Pilanesbergeudialyte only from agpaitic melts. They also showedthat the tim- was calculated using the averagecomposition obtained ing of crystallization and hence the morphology of the from 23 analysesof the light zones (Table 2); as dis- eudialyte are strongly dependenton the zirconium con- cussedearlier, the dark zones are commonly finer than tent of the magma. In agpaitic nepheline syenites with the width of the electron-microprobe beam. Using the over I wtvo ZrO2, oudialyte crystallizes contemporane- method proposed by Johnsen & Gault (1997) and ously with the main rock-forming minerals, and is idio- Johnsen et al. (1998), the stoichiometry of the morphic. On the other hand, if the melt contains less Pilanesbergeudialyte (sample P6) is: (Na123&EErzg than I wt%oZrO2, eudialyte crystallizes later than the Cae3eK6 20)214 j8 (Ca5a2Mns ss)>6 (Mn2 e6Fe004)>3 main rock-forming minerals, and is poikilitic and (Zr2 66Nbs.2sHf6s6)13 (Nb6 665ie.2sTasq6)2 1 Si25.6aO7a xenomorphic. (OH1 61Clee2Fss7)22.2H2O.This formula is superfi- In the green syenite of the Pilanesberg complex, cially similar to that of kentbrooksite, the Mn-REE-Nb- which is agpaitic and contains more than 2 wt%oZrO2, F end-member of the eudialyte group of minerals eudialyte should be an early crystallizing idiomorphic (Johnsen et al. 1998). However, using the parameters phase, according to the findings of Kogarko et al. SiCaFeCl and MREEMnF defined by Johnsen& Gault (1982). On the contrary, as discussed above, textural (1997) and Johnsen et al. (1998), the Pilanesberg relationships suggestthat the eudialyte formed late, re- eudialyte is intermediate in SiCaFeCl between placing aegirine, zircon, both feldspars,the Na-Zr sili- kentbrooksite and eudialyte from the type locality cate, and pectolite. The occurrence of eudialyte in Kangerdluarsuk, Ilimaussaq (Johnsenet al. 1998),but fractures and cleavageswithin feldspars,nepheline, and enriched in NbREEMnF relative to an intermediate the Na-Zr silicate suggeststhat it is postmagmatic, i.e., member of the series (Fig. 5). In particular, the hydrothermal. Significantly, the early crystallization of Pilanesberg eudialyte contains concentrations of Ca aegirine, followed by feldspar and nepheline in these similar to that from Ilimaussaq and twice that of rocks, suggestscrystallization at H2O pressureshigher kentbrooksite.For this reason,the eudialyte-$oup min- than 500 bars (Kogarko & Romanchev 1976). More- eral at Pilanesberg is more appropriately referred to as over, the replacementof feldsparsand nepheline by so- eudialyte. dalite indicates that the green nepheline syenite has been metasomatized by Cl- and probably Na-rich aqueous fluids. It is therefore attractive to propose that the eudialyte in the green nepheline syenite crystallized from aqueousfluids exsolved from the magma. In addi- tion to Cl, these orthomagmatic brines also carried F, tro which is found in minor concentrationsin the unknown Na-Zr silicate, in eudialyte (- O.l wtvo), and in appre- ia ciable concentrationsin britholite Gl wtvo\. In proposing a hydrothermal origin for eudialyte, the +z first issue that needs to be addressedis the notion that z Zr is generally immobile during hydrothermal alteration. 0 Although it has been widely assumedin the literature property has been 28 30 32 34 36 that Zr is immobile, and indeed this used to monitor alteration, a number of recent studies Si-Cu+Fe+Cl have shown that Zr can be quite mobile under certain conditions. For example, Gier6 & Williams (1992) rc- ported presence zirconolite (CaZrTi2OT) in hy- FIc. 5. A plot of lSiCaFeCl versus >MREE'MnF in atoms the of per formula unit, apfu, comparing the composition of drothermal veins in dolomite adjacent to the Adamello eudialyte from Pilanesberg (diamond) with that of eudialyte batholith, . Vard & Williams-Jones (1993) docu- (E) from the type locality Kangerdluarsuk, Ilimaussaq, and mented the occurrence of hydrothermal weloganite kentbrooksite (K). Data and tie line from Johnsen& Gault [(Sr3Na2Zr(CQ)6.3H2O)] in vugs in sills in (1997) and Johnsenet al. (1998). the Francon quarry, Montreal, Quebec. F;ubin et al.

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(1993) have describedzircon- replacementbod- REE in hydrothermal fluids. However, at high tempera- ies in limestone next to bodies of subvolcanic rhyolite ture, high chlorinity, and low pH, conditions that may in . Unfortunately, the behavior of zirconium in be inferred for the hydrothermal fluids responsible for hydrothermal fluids is still poorly understood. To our eudialyte crystallization (see above), REE are com- knowledge, only one study has investigated high-tem- plexed predominantly by chloride and fluoride (Haas et perature complexation of Zr in aqueous fluids (Aja ef al.1995). al. 1995). Although this study was very preliminary, it On the basis of textural and chemical featuresof the did demonstrate that Zr forms stable comolexes with green nepheline syenite, and the thermodynamic stud- F, SO42-and OH- under hydrothermal conditions, and ies mentioned in this paper, we thus propose that the that appreciableZr can be mobilized, depending on the REE were transported mainly as chloro- and F-com- activities of these ligands. plexes in orthomagmatic fluids that metasomatizedthe In the absenceof detailed information on the chem- green syenite. We furthermore propose that the REE istry of the fluid phase affecting the green nepheline were incorporated in eudialyte as a result of the reduc- syenite at Pilanesberg,we are unable to evaluate quan- tion in Cl- and F-activity that accompaniedprecipita- titatively the relative importance of different Zr com- tion of sodalite and the unknown (F-bearing) Na-Zr plexes. However, given the likelihood that the silicate. hydrothermal fluids responsible for metal transport were orthomagmatic and F-bearing (up to O.l wtVo in the Coucr,usroNs unknown Na-Zr silicate and in eudialyte), we suggest that Zr could have been mobilized by zirconium-fluo- The eudialyte composition at Pilanesbergis unusual ride complexes such as (ZrF)2-. We also cannot dis- becauseof its high contentsof Nb (up to 3.8 wt%o),REE count the possibility of Zr transport by a hydroxy (up to 7.6 wt%o,mainly Ce, La, and traces of Nd and complex, e.g., Zr(OH)ao, which is the dominant com- Sm), and Mn (up to 7.5 wtVo), and its low contents of plex in the absenceof significant activities of compet- Na (up to ll.4 wt%o)and Fe (up to 0 .4 wt%). This chem- ing ligands suchas F (Ajaet a|.1995). istry reflects a hydrothermal origin for the mineral, On the basis of the textural relationships discussed which is supported by textural relationships that dem- earlier, we propose that the source of Zr in eudralyte onstratea secondaryor replacementorigin. We propose was zircon. In caseswhere eudialyte replaces zircon,Zr that the eudialyte crystallized from an orthomagmatic was clearly conserved. Elsewhere, Zr was evidently Na-Nb-REE-C1- and F-bearing hydrothermal fluid, mobilized, and we propose that this occurred as a result which patitioned from an agpaitic syenitic magma. We of the dissolution of zircon (zircon cyrstals are com- envisage that the Zr was locally remobilized from monly corroded, as described above) by fluorine-bear- magmatic zircon as a zirconium-fluoride complex ing o^rthomagmaticfluids, which transported the Zr as a (ZrF6z-), and that the REE were introduced by the ZrF6l- complex and redepositedit as eudialyte. orthomagmatic fluid as Cl- or F-complexes (or both). Incorporation of REE in eudialyte occurred becauseof ENnrcnunNr oF EuDrALyrE rN REE a reduction in Cl- and F-activities due to precipitation of sodalite and the unknown Na-Zr silicate. As discussed earlier, the eudialyte in our sample of AcrNowr,soceN,El.{Ts green nepheline syenite has one of the highest contents of REE reported in the literature. Only eudialyte from We thank R. Harmer for introducing AEWJ to the Mont Saint-Hilaire and Ascension Island have higher geology of the PilanesbergComplex and facilitating his concentrations of REE, up to 9.78 and 8.39 wt%o visit there, D. Palmer for assistingin the field work, G. REE2O3,respectively (Johnsen& Gault 1997,Hanis et Poirier and R. Mineau for valuable advice on conduct- al. 1982). By contrast, eudialyte in other major alkaline ing EMP and SEM analyses, respectively, and C. intrusions, e.g., the Lovozero and Ilimaussaq com- Normand for numerous stimulating discussions on the plexes, typically contains <3 wtVo REE2O3(Balashov mineralogy of alkaline complexes.The manuscriptben- & Turanskaya 1960, Deer et al. 1986). Significantly, efitted from reviews by O. Johnsen,R.F. Martin, and an eudialyte in these other complexes is interpreted to be anonymous reviewer. The research was supported by of magmatic origin (Kogarko et al. 1982). grants from the Natural Sciences and Engineering Re- Why is the eudialyte at Pilanesberg,which is clearly searchCouncil ofCanada and the Fonds pour la Forma- hydrothermal in origin, enriched in REE? In order to tion de Chercheurset l'Aide d la Rechercheof Quebec. understand the behavior of. REE in hydrothermal sys- tems, several researchershave investigated the stabili- RsrsnnNcm ties of aqueousREE complexes (e.g., Wood 1990, Haas et al. 1995, Gammons et al. 1996). These studies indi- tue, S.U.,WooD, S.A. & WIru*rs-JoNns,A. E. (1995):The cate^that a ^variety of -ligands,including F Cl-, OH-, aqueousgeochemistry of Zr andthe solubility of someZr- SOa'-. CO3r-, and POai-, are capableof complexing the bearingminerals. AppI. Geochem.1O, 603-620 -

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