Canadian Mineralogist Vol. 16, pp.325-333 (1978) THE TANCO PEGMATITEAT BERNICLAKE, MANITOBA X. POLLUCITE* P. dNNNf ANDF. M. SIMPSONT Department ol Earth Sciences,University ol Manitoba, l(innipeg, Manitoba RJT 2N2 ABSTRACT grosso modo du dee!6 de substitution (Cs,Na). Les amas de pollucite sont recoup6spar des filons de Pollucite forms large lenticular bodies and pods quartz, microcline non-perthitique(A 0.92), albite in the upp€r intermediate zone of the Tanco peg- ordonn6e(/ 1.10') Oord6e localementtl'analcime matite, in association with quafiz, microcline c6sique) et muscovite lithique violac6e QMt) ave* perthite, petalite (spodumene * quartz) and ambly- l6pidolite 1M. Des entrelac,sde filonnets tardifs gonite. It was formed as a late constituent of contiennent de la muscovito lithique blanche (2MJ primary zonal crystallization and shows no evidence et un peu de spodumdne,suivis d'adulaire d6sor- of metasomatic oriein. The composition of pollu- donn6e.Le dernier stade d'alt6ration est repr6sent6 cite, established chemically or estimated optically par de la kaolinite verdatre et de la montmorillonite on 148 samples, is centred at 32 wt,Vo CszO (ap- (avec ou sans calcite). nroximately PollrgAnalrr). Some pollucite bodies tend to be enriched in Na along their margins. Cs Clraduit par la R6daction) and Na are the only major alka.lis; KRb and Li are subordinate. Negative Cs/Na and positive Rb/Tl correlations are the only well-defined trends among INrnoouctroN the alkalis. Refractive indices can be roughly correlated with tle (Cs, Na) substitution. pollucite Pollucite was discovered in the Tanco peg- bodies are veined by quartz, mostly non-perthitic matite in 1960 when Dr. R. W. Hutchinson microcline (A 0.92), low albite (/ 1.10.) locally noticed peculiar physical characteristicsof what rimmed by cesian analcime, and purple lithian had been previously logged as a quartz-feldspar muscovite (2M) with lepidolite (lM), A late net- intergrowth. Subsequent laboratory examination work of braided veinlets carries white lithian mus- by Dr. W. Moorhouse led to its correct identifi- covita (ZMr) and some spodumene, followed by cation 1,967).A substantial pollucite sanidine-type adularia. Greenish kaolinite and @rohberg body in pegmatite established shortly montmorillonite (-r calcite) formed last in the the was alteration sequenc€. afterwards. Recent underground excavations in the western part of the pegmalile have estab- lished the shape and size of several other bodies Sorrarrtarns detected earlier by drilling. The total reserve of pollucite in the deposit exceeds350,000 tons La pollucite se pr6sente en amas lenticulaires averaging 23.3 wt.Vo CszO; this representsthe dans la pegmatite zone moyenne sup6rieure de la largest accumulation of this mineral known in Tanco, accomDagnde de quartz, microcline perthi- tique, p6talite (spodumdne quartz) et ambligonite. the world today. + (1961) Elle s'est form6e tardivement lors de la cristallisa- With the exception of Nickel's de- tion zonaire primaire et ne montre nul indice d'ori- scription, there are no mineralogical data on the gine m6tasomatique. Sa composition, 6tablie par Tanco pollucite available in the literature, and analyse chimique ou d6duite des propri6t6s optiques its paragenetic position in the pegmatite is (en de 148 6chantillons, se situe verc 32Vo poids) rather poorly understood ,(Hutchinson 1959, de CsrO, i peu prds Poll6Analrr. Certains amas Wright 1963, Crouse & Cernf 1972). Also, pollucite de ont tendance i s'enrichir en sodium, mining and exploration at Tanco offer a rrnique bordure. en K, Rb et Li sont nettement subordon- opportunity to study compositional and altera- n6s i (Cs Na); les corr6lations Cs/Na (n6gative) * patterns pollucite three dimen- et Rb/Tl (nositive) sont les seules nettes pour les tion of bodies in m6taux alcalins. ks indices de r6fraction d6pendent sions; all extant descriptions of pollucite from individual localities are based on examination of one or two specimens.These considerations *Publication No. 26, Centre for Precambrian initiated the present study; it is based partly Studies. on a thesis by Simpson (1974), partly on the tPresent address: 75 Linden Avenue, Winnipeg, investigations of the first author before and Manitoba R3K 0M7. since the completion of this thesis. 325 326 THE CANADIAN IVIINERALOGIST Senapr.rxo ergo ExpSRIMBNTALMETHoDS The methods of optical study, density deter' mindies, chemical analyses,and- X-ray powder yielded Underground and drill-core sampling diffraction are as described bv Cernf & Sime 148 specimens of pollucite for laboratory study. son (1977), with the only exception that the An additional 30 specimens were collected to standards used 'for determination of sommon sllmins the alteration processes.The suite of constituents were heavily spiked with cesium, 148 samples was checked for refractive indices to balance the Cs content of pollucite and its (ru) and 41 specimens were then selected for effects on determination of other elements. partial chemical analyses. Three samples con- sidered representative of pollucite with low, OccunnnNcp IN THE PBoMATTTE average, and high z were chosen for a detailed parts of mineralogical study. Pollucite is restricted to the upper the pegmatite Pollucite from Tanco is generally homo' the eastern and western 1l6nks of to, the upper geneous on a small scale. Four randomly se- where it occurs within, or adjacent (5 1). This zone lected hand specimenswere checked fot n aI 3 intermediate zone in Fig. giant-size crystals to 5 spots on their surfaces. The ranges-of n consists mainly of quartz and -tn petalite to spodu- individual samples did not exceed 0.001' the of microcline perthite, altered (cl. Cerni & equivalent of about 0.6 v'rt.Vo C*O. The only mene * qaartz, and amblygoilte are the specimens veined by late Simpson L977, Table 1 for detailed mineralogy exceptions pods alteration products; these were excluded from of all zones). Pollucite forms numerous - and several the study. Fine veining by mica and spodu- and nodules I Io 2 m in diameter pollu-cite mene proved to be much more of an obstacle large lenticular bodies that constitute 180 75 x for obiaining reliable compositional data than zone (8). The largest of these, X !2y flank of the any inhomogeneity in the pollucite itself. Seven in size, is located in the eastern bodies are found of the 41 partial analyseshad to be discarded oeernatite. and three smaller positions are projected becauseof complex contamination of the sam- ioil" *"tt"tn part; their 1. ple, and 17 other analyseswere adjusted for ad- on the section plane in Figure simple smooth iri*t.rres of spodumene or muscovite, quanti- Masses of pollucite show are anhedral tatively determined by X-ray powder diffraction. boundaries against quartz, and LAKE LEVEL Frffi flflTtTflil| ffi @@a(a) m ffi |d@a(4, I T'Ttv @|@&6t [F,A Flo. 1. Longitudinal sectiotrthrough tle Tanco pecmatiq'.Most of tirc easxernpollucite body is exposedin this section; some of its boundaries shown 6ere and all ihe western pollucite bodres are projected on tlis section to show ttreir full extent. POLLUCITE AT BERNIC LAKB 327 Fro. 2, Typical yeining pattern in the eastern pollucite body, shown here separatedfrom blocky microcline perthite (upper left) by a dark selvage of quartz. Dark veinlets of quartz, feldspars and lepidolite dissect pollucite into polygonal blocks. Faintly visible system of subparallel whitish veinlets, gently sloping to the right, carries muscovite and spodumene.The hammerheadis 20 cm long. with respect to pdmary all other constituents of TABLEl. ATKALICoNTENTS AND n 0F THETANC0 P0LIUCI'IT zone (5). Pollucite may locally cement fractured or brecciated silicate assemblagesbut it is never Zone Sample Li20 K20 Rb20 observed # Na20 Cs20 Tl* n as a replacement. 'r.88 Veining and replacement by quartz, feldspars, R-H .09 .t2 .71 32.90 1.522 R-2-3 .06 'rt. tu .aa ,94 32.65 105 1,5224 micas and spodumene described later in-this R-3-4 ,ll .65 .09 ,79 34.10 1.524 article are widespread, though minor in volume R-4-2 no taEnq .89 3t.35 1, sl 98 R-2-6 .It t.od .J/ .86 3l .50 1.522 (Fig. 2). to These constitute the only major con- r- t-c '|I.64 .lt .97 33.04 1.5224 taminants of the bulk of the pollucite bodies; P-2-4 .ll 'I.63.87 . t5 .88 33.20 1.5225 P-4-6 .25 .t9 .74 32.00 1.5205 these were essentially meneminslslic before P-3-2 .08 1.72 .07 .72 3?.20 I.5198 P-4-4 .85 31.56 1.5i92 the onset of secondary processes. tq 834-r47. 5 r.84 .08 'I..80 31.52 1.5185 .20 ! 834-157.5 i.76 .t0 10 32.00 1.5205 CotrposltroNer, Vanresrlrry o 834-l87.5 .14 1.62'r .25 .59 3l.60 1.522 834-206.6 no .68 . 10 .73 32.40 1.5202 The variation of refractive index of pollucite M5-205.5 .23 I .46 .19 .82 32,26 1.5202 with the Cs/(Na*H:O) substitution has been M4-'165.5 'I.341.43.20 .80 3l.67 1.5206 M4-163.5 .l9 .82 32.03 I.5218 well established(e.g., Richmond & Gonyer 1938, **u39-95.8 'I .28 '1.96. 98 .17 .77 30.60 r00 1.517 Nel 1944, denf L974. Thus an optical survey UJY-OZ. ! ?l .18 .44 29.67 1.518 u39-75.5 1.73 .23 .86 31.27 r.5204 of 148 pollucite specimenswas undertaken as fB12-207.2 'I .29 .31 . l0 .68 32.57 80 '1.52041.5232 a first step in determining their compositional nB12-226.2.43 1.74 .16 .74 32.20 range. M20-179.5 .53 1.45 .24 .77 33.10 1.5238 The mean value of n is 1.5206, with a stand- M?0-181.2 .41 'IlEa?qe? 30.70 1.5202 -+- u-aul.5 .50 'I.45 .