Zirconosilicate Phase Relations in the Strange Lake (Lac Brisson)

American Mineralogist, Volume 80, pages 1031-1040, 1995

Zirconosilicate phaserelations in the Strange Lake (Lac Brisson) pluton, Quebec-Labrador,Canada

SrrrlNo SALvr,* ANrHorw E. Wrlr.HvrsJoNns Department of Earth and Planetary Sciences,McGill University, 3450 University Street, Montreal, Quebec }J3A 247, Canada

Ansrucr Petrographicobservations ofsubsolvus granitesat StrangeLake indicate that the sodium zirconosilicate elpidite crystallized under magmatic conditions, but that the calcium zir- conosilicatesarmstrongite and gittinsite are secondary.This interpretation is consistent with the extensive solid solution displayed by elpidite and the restricted compositions of armstrongite and gittinsite. Both calcium zirconosilicate minerals show textural evidence of having replaced elpidite, and in the case of gittinsite, with major volume loss. In the near-surfaceenvironment, gittinsite plus quartz fill the volume formerly occupied by elpidite. At gfeater depth, gittinsite and armstrongite partially replaced elpidite but are not accompaniedby quartz, and abundant pore spaceis observedwhere gittinsite is the principal secondaryphase. Below 70 m elpidite is generallyunaltered. Replacementof elpidite by armstrongite is interpreted to have been a result ofthe cation-exchangereaction NarZrSiuO,r.3HrO + Ca2+: CaZrSiuO,r.3HrO + 2Na* elpidite amstrongite in which volume is nearly conserved, and replacement by gittinsite is thought to have resulted from the reaction NarZrSiuO,r.3HrO+ Ca2++ 5HrO: CaZrSirO,+ 2Na+ + 4H4SiO? elpidite gittinsit€ which is accompaniedby a 650/ovolume reductron. An alteration model is proposedin which external Ca-rich, quartz-undersaturatedfluids dissolved elpidite and replacedit with gittinsite, where a"o,ogwas buffered mainly by the fluid (high water-rock ratio), and with armstrongite plus gittinsite, or armstrongite alone, where 4"o.,ogwas buffered to higher values by the rock (low water-rock ratio). The formation of gittinsite createdextensive pore spacethat was subsequentlyfilled, in the upper part of the pluton, when the fluid became saturated with quartz as its temperature de- creasedduring the final stagesofalteration.

INrnooucrroN curs as one or more of a large family of alkali and al- In most igneous rocks, Zr is an incompatible trace kaline-earth zirconosilicate minerals. The most impor- (NarZrSi.Oe-2H2O), element present in amounts ranging from < 10 to several tant of these are catapleiite dalyite (KrZrSi.O,,), (NarZrSiuO,,.3HrO), hundred parts per million and is accommodated as the elpidite eudialyte (OH,Cl)r], accessorymineral zircon. However, in peralkaline ig- [Nao(Ca,Ce)r(Fe2+,Mn,Y)ZrSirOr, vlasovite (NarZrSioO,,), (KrZrSirOn), neous rocks its concentration is commonly orders of and wadeite any of which principal partic- magnitude higher and may be locally sufficient to con- may be the zirconosilicate mineral in a (Vlasov stitutea potentially exploitabledeposit (e.g., Ilimaussaq: ular setting, e.g., eudialyte at Lovozero et al., 1966), catapleiite at Mont-Saint-Hilaire (Horv6th and Gerasimovsky,1 9 69 ; Lovozero:Kogarko, 1990; Strange (Fleet Lake: Miller, 1986;Kipawa: Allan, 1992;andThorLake: Gault, 1990),and vlasovite on AscensionIsland Trueman et al., 1988).T};.eZr mineralogyof peralkaline and Cann. 1967). rocks is also unusual. Although most peralkaline rocks Little is known about the factors controlling the distri- contain some zircon, the bulk of the Zr commonly oc- bution of zirconosilicate minerals, and there is little documentation of their parageneses.The only reversed ex- perimental study that provides data on their stability at geologically meaningful conditions is that of Currie and * Presentaddress: Laboratoire de G6ochimie-CNRS, Univer- sitePaul Sabatier,38 rue desTrente-Six Ponts, Toulouse F-3 1400, Z,aleskt(1985) on the dehydration reaction ofelpidite to France. vlasovite and quartz. Marr and Wood (1992) constructed 0003-o04x/94l09l 0- I 03I $02.00 103I r032 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS

|TRANGELAKE

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- >,fl S----Ir5

ffi E#

il1G,l+ I' ++ x@N*J : +++ + -:@ , ,, ++ + - lrsq, +++ ru €'# ,,+++ + '6 E +l +++++ + ELSONIANINTRUSION +l Fl quarlz monzonite +l APHEBIANBASEMENT +- p-1 amphiboliteand | -r metagabbro + + + + + |ll metadiorite f r--al qu2fir.Jeldspathic L-l & graphiticgneiss ffi EZI calc-srrrcategnetss

Fig. 1. Location and simplified geologicalmap of the StrangeLake pluton (modified after Salvi and Williams-Jones, 1992). theoretical P-T diagrams for sodium and calcium zir- the calcium zirconosilicate armstrongite, which they conosilicate minerals using the results of Currie and Za- claimed occursboth as phenocrystsand as a replacement leski (1985),molar volume data, and the few constraints of elpidite. available from natural systems.Further advancesin our We reexaminethe genesisof elpidite, armstrongite,and understandingof Zr mineral petrogenesiswill require ba- gittinsite at StrangeLake, in light of textural and mineral sic thermodynamic data for the major phasesand careful chemical data collected from a representativediamond- documentation of field occurrences.The latter is the ob- drill hole locatednear the zone of economicinterest. These jective of the present contribution. data, in conjunction with mineral stability relationships In the Strange Lake pluton, Zr occurs mainly in the and published fluid-inclusion data (Salvi and Williams- form of the zirconosilicateminerals elpidite, armstrongite Jones, 1990),support a model involving low-temperature (CaZrSiuO,r.3HrO),and gittinsite (CaZrSirO,). These metasomaticreplacement of elpidite by armstrongite and minerals vary greatly in their relative proportions, and, gittinsite. locally, any one of them may be the principal or sole Zr phase.The highest concentrationsof Zr are in rocks con- Gnor,ocrct sETTrNc taining only gittinsite. Salvi and Williams-Jones (1990) The StrangeLake pluton (l 189 + 32 Ma, Pillet et al., observed that gittinsite plus quartz form pseudomorphs 1989), located in the easternRae province of the Cana- after an unidentified phase;on the basis of the morphol- dian Shield, is a peralkaline granite that intruded Aphe- ogy of the pseudomorphs and on fluid-inclusion evi- bian metamorphic rocks (1.8 Ga) and an Elsonian quartz dence,they suggestedthat a Ca-bearingfluid at -200 "C monzonite (1.4 Ga) (Fig. l). The intrusion is a subverti- caused the metasomatic transformation of the sodium cal, cylindrical body approximately 6 km in diameter and zirconosilicate elpidite to gittinsite + qvartz. Birkett et representsthe latest tectonic event in the area. Large roof al. (1992) subsequentlyproposed that gittinsite replaced pendantsare commonly observed(Fig. l), suggestingthat SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS r033

the presenterosional surfaceis close to the apical part of the pluton. On the basis of its peralkalinity, mode of em- placement, and age, the pluton may represent the Lab- rador extension ofthe Gardar anorogenic igneous event (Currie, 1985;Pillet er al., 1989). 18.0 Severalconcentric intrusive pulsesof peralkaline granite formed the Strange Lake pluton, representedby hypersolvusgranite, transsolvusgranite, and more evolved, 16.0 volatile-saturated subsolvusgranites (Nassif and Martin, l99l) (Fig. l). All phasesare F-rich and conrainprimary fluorite. They are mainly leucocratic, fine to medium 14.0 grained, and quartz saturated. Pegmatites are common and concentrated mainly in the subsolvus granite. An outwardly dipping ring fracture, marked by a heteroge- L 12.0 neous breccia in a fluorite-filled matrix, surrounds the N pluton. E The hypersolvusgranite consistslargely of phenocrysts o. 10.0 of quartz and perthite, and interstitial arfvedsonite, o. whereas the subsolvus granite contains two alkali feld- o spars(albite and microcline), and arfvedsonitecommonly o 8.0 occurs as phenocrysts.Transsolvus granite is transitional c) to the two other tlpes of granite, i.e., it contains perthite x phenocrystsin addition to albite and microcline. Arfved- 6.0 sonite in this unit occurs predominantly as fine-grained anhedra,imparting a melanocraticappearance to the rock. The hypersolvusgranite contains up to 50/oof zirconosil- 4.0 icate minerals, and the subsolvusgranite and pegmatites contain up to 300/oof these minerals. Elpidite is the principal Zr mineral in the hypersolvus granite, whereas el- 2.0 pidite, armstrongite, and gittinsite are the dominant Zr minerals in the subsolvus granite and pegmatites. kss common zirconosilicates at Strange Lake include cata- 0.0 pleiite, calcium catapleiite,dalyite, hilairite (NarZrSi.On. (t) (t) a(nu) 3H,O), vlasovite, and zircon (cf. Birkett et al., 1992). -s !F) ct) cn a Subsolvus granites commonly display hematitic alter- ,-:. A ation and, where altered, have higher Zr contents than & fresh hypersolvus or subsolvus granites (Fig. 2). Hema- Fig.2. A histogram showing the averagewhole-rock content tization was associatedwith the development of second- of Zr in the principal varieties of granite at StrangeLake. Ab- : : : ary high-field-strength element (HFSE) minerals, many breviations: hs hypersolvus,ts transsolvus,f-ss fresh sub- : : of which are Ca-bearing. Some of the most intense he- solvus, a-ss altered subsolvus,and s-ss strongly altered subsolvus.Standard deviations are I.9, 2.4, 1.3,2.3, and 0.3 x I 000 matization is in the center of the complex, where a sub- ppm, respectively. economic ore zone has been delineated containing some 30 million tonnesgrading 3.25o/oZrOr, 1.30/o REE oxides, 0.660/oY,Or,0.560/o NbrOr, and 0.12o/oBeO (Iron Ore Williams-Jon es, 199 2). Signifi cantly, this alteration is most Company of Canada, unpublished data). These granites strongly developed as halos around pegmatites that con- have lower alkali contents, higher Fer*/Fe2+, and higher tain primary high-temperature fluid inclusions rich in Ca contents than fresh or less-altered subsolvus rocks NaCl and without detectable Ca. (Miller, 1986;Salvi and Williams-Jones,1990; Boily and Williams-Jones,1994). Salvi and Williams-Jones(1990) Gporocy oF DDH SL-182 provided fluid-inclusion evidence of the interaction of The diamond-drill hole that is described in detail be- ore-zonerocks with a low-temperature,CaClr-dominated low is one of the deeper(88 m) of some 35 drill holes brine. The high 6'80 ofthese rocks suggeststhat the fluid that were logged by the authors and is located approxi- was externally derived (cf. Boily and Williams-Jones, mately 700 m southeastof the main exploration trench 1994). A second episode of alteration is representedby (Fig. l). This hole provides a representativecross section the replacement of arfvedsonite by aegirine. This event of the alteration and intersects several varieties of sub- is not well constrained temporally but has been shown to solvus granite, as well as several pegmatites(Fig. 3). All be limited spatially to the subsolvusgranite and is thought rock units contain one or more of the zirconosilicatemin- to representan earlier, high-temperatureevent (Salvi and erals elpidite, armstrongite, and gittinsite. 1034 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS

and transsolvus granite inclusions. Subhedral to -co crysts euhedral aegirine occurs locally. The lower contact between this and the porphyritic granite is gradational over a few meters, with the proportion of arfvedsonite phe- nocrystsdecreasing upwards and the zirconosilicateshav- T ing two habits (see below). The upper contact is more abmpt, although lacking a chilled margin. This and the absenceof other evidence of intrusive relationships sug- 30 ! gest that these two main rock types were roughly coeval. o The equigranulargranite is intersectedin other drill holes 40 oF and is exposedin the trench, west of DDH SL-182. It U' can therefore be deduced that this unit forms a subhori- meters in thicknessin the cen- 50 zontal sheet a few tens of tral portion of the pluton (cf. Miller, 1990; Birkett et al., 1992). 60 The granites are altered to varying degrees,except in the bottom l0 m of the hole where they are relatively 70 t fresh. Alteration is manifested as a replacement of arfvedsonite by aegirine + hematite, replacements involving the zirconosilicates(see below), hematite staining 80 (particularly near the top ofthe hole), red Fe3+-activated cathodoluminescenceof feldspars,appearance of second- 90 ary fluorite, and the development ofextensive secondary porosity. The strongest replacement of arfvedsonite by aegirineis observedin and adjacentto severalmeterwide 100 pegmatitesthat cut the equigranular granite.

F;-l over-burden ffi elpidite DrsrnrntmoN oF zrRcoNosrlrcATE MINERALS ag IN DDH SL182 g,,E pegmatite armstrongite Euhedral elpidite occurs as doubly terminated ortho- G-;-.: equigranulargranite ffi boat shape also ob- L ! (euhedral rhombic prisms, with the typical elpidite) gittinsite (1913). DorDhvritic sranite & servedin rocks from Narssdrssukby Goldschmidt LU iunh.,iralelpldite.l quaflz Euhedral armstrongite has a pseudo-orthorhombic habit W indistinguishable from that of elpidite. All observed oc- Fig. 3. A schematicrepresentation of the geology,the zir- currences of armstrongite show polysynthetic twinning. conosilicatemineral habitsand distribution,and the porosity Gittinsite occurs almost exclusively as narrow feathery profileof DDH SL-I82. Darkershading corresponds to higher crystals forming radiating aggregates. porosity. Elpidite, armstrongite,and gittinsite have distinctly dif- ferent distributions and modes of occurrence(Fig. 3)' In The top half and final 13 m of the hole are composed the porphyritic granite at the bottom ofthe hole, elpidite of fine- to medium-grained porphyritic granite containing is the principal zirconosilicatemineral, and its abundance phenocrystsof arfvedsonite.This unit contains inclusions rangesfrom a few percent to 20o/oof the rock by volume; oftranssolvus granite ranging from one to a few tens of armstrongite is absent, and gittinsite is rare. Elpidite oc- centimeters in diameter and constituting 2-5o/oby vol- curs as anhedra up to 3 mm in diameter interstitial to ume of the rock. feldspar and quartz grains (Fig. 4A) or as large aggregates In the central portion of the hole, the granite is also of small grains (<0.1 mm in diameter) also distributed fine to medium grained but lacks arfvedsonite pheno- interstitially to the main rock-forming minerals (Fie. aB).

Fig. 4. Photomicrographsofzirconosilicate mineral textures: ondary zircon spherulesand growing in pore spaceadjacent to (A) (74.9 m depth) anhedral crystal of elpidite (elp) and @) ag- elpidite; (G) (13.5 m) aggregateof featherygittinsite crystalsplus gregateof small elpidite crystals interstitial to quartz (qtz) and quartz interstitial to quartz and feldspars;(H) (trench sample) feldspar(feld); (C) (54.9 m) euhedralcrystal of elpidite; (D) (53.0 gittinsite-quartz pseudomorph after a euhedralphase. Photos A, m) partial replacementof elpidite by polysynthetically twinned B, C, D, E, and H are in crossed polarized ligh! F and G in armstrongite (arm); (E) (59.4 m) gittinsite (git) rosettes,accom- plain polarized light. Photos A, B, C, E, and G are 2 mm across; panied by pore space,partly replacinglarge elpidite crystals;(F) D. F. and H are I mm across. (44.7 m) radiating gittinsite crystals attached to a string of sec- SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS 1035 1036 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS

TABLE1, Representativeand average composition of zirconosilicates

Averages Elpidite Elpidite Armstrongite Gittinsite n:46 n: 14 n :20

sio, 62.0 63.5 63.2 63.6 62.9 63.4 41.9 Tio, n.o. n.o. 0.20 n.d. 0.27 0.16 0.17 ZrOz 19.0 18.9 '17.9 162 17.1 19.2 36.9 FeO- n.d. n.d. n.o. 0.29 0.07 0.o7 0.43 Mgo 0.60 0.32 A't1 0.54 o.44 0.09 0.23 MnO n.o. n.d. n.d. n.o. 0.07 n.d. 0.74 CaO 0.19 n.d. 1.42 2.43 0.92 10.0 17.7 Naro 8.83 809 7.69 8.04 9.01 n.o. 0.13 K.o n.o. n.o. n.d. n.d. o.21 0.23 n.d. Nbros n.o. n.o. n.o. n.d. 0.46 0.35 0.60 CerO" n.d. nd. n.o. n.d. n.d. n.d. 0.85 Nd,o3 0.46 n.d. n.o. n.d. 0.13 0.10 o.21 Total 91.1 90.8 91.1 91 1 91.6 93.5 99.8 Note: n.d : not detected. Analyses are reported in weight percent. 'Total Fe as FeO.

Elpidite is also the dominant zirconosilicate mineral in Rocks from the exploration trench, similar in texture the equigranulargranite (middle of the hole), but it forms and mineral assemblageto the equigranular granite in euhedralcrystals (Fig. aC) up to 3 mm long, and its abun- DDH SL- l 82, as well as equigranular granite intersected dancevaries between l5 and 250loof the rock by volume. at high levels by other drill holes, also contain aggregates Many elpidite crystalsdisplay varying degreesof rim and of gittinsite plus quartz and lack porosity. However, in core replacementby armstrongite and gittinsite. The vol- contrast to the upper part of hole DDH SL-182, these ume proportion of elpidite replaced by these minerals aggregatesform pseudomorphs after a euhedral phase(Fig. averagesl5-20o/o and increasesupwards. Where replace- 4H), which we interpret to be elpidite on the basis of the ment involved armstrongite alone, volume was con- occurrenceof elpidite crystals with similar morphology served (Fig. aD). However, elpidite crystals replaced by in equigranulargranite in DDH SL- I 82. The volume pro- gittinsite or by armstrongite + gittinsite all contain pore portions of gittinsite and quartz in the pseudomorphs, space (Fig. 4E and 4F). Although armstrongite can be whether after euhedral or interstitial elpidite, vary from found in contact with both elpidite and gittinsite, the lat- 40 to 70o/oand 30 to 600/0,respectively. Interestingly, the ter minerals rarely occur in contact with each other. In gittinsite-quartz pseudomorphsin both equigranular and the transitional zone between the two granites, both in- porphyritic granite occur where hematite alteration is most terstitial and euhedral habits of elpidite are observed. rntense. Farther up the hole (15-46 m), in porphyritic granite, elpidite has a habit similar to that of elpidite in the deep- Geochemistry est part of the hole. However, in this interval it experi- Electron microprobe analyseswere performed with a enced extensive dissolution, leaving behind pore space Cameca CAMEBAX equipped with an EDS detector (up to l0o/o of the rock) and calcium zirconosilicates, (NORAN high-purity Ge detector, NORVAR window). largely gittinsite (Fig. aD. In caseswhere an entire pocket Analyses were conducted in EDS mode to permit use of of elpidite was replaced,the proportions of gittinsite and a low beam current (15 kV, 0.6 nA current, and 2000- pore space are about 40 and 600/0,respectively, of the 3500 counts), to minimize the volatility of light elements precursor elpidite. Armstrongite is a minor phase and such as Na (light-element volatility is the major source replacedeither coresor rims of elpidite crystals.Replace- of error in zirconosilicate mineral analysis; see Bonin, ment of elpidite is more extensive toward the top of the 1988; Birkett el al., 1992).Where possible,the beam was interval. defocused to a diameter of 5 pm to minimize further In the top 10-15 m of drill core, gittinsite is the sole light-element volatility. Data reduction was performed zirconosilicate mineral. It forms feathery radiating crys- with the PROZA version of the Ob,4 correction routine tals up to 0.5 mm in length embedded in tiny anhedral using internal standards(Bastin and Heijligers, l99l). quartz grains. Gittinsite and quartz form coherent agge- Compositions of eachzirconosilicate are listed in Table gatesin the groundmass,interstitial to other minerals (Fig. I and are plotted as atomic proportions of Na, Ca, and 4G). The sizes, shapes,and sharp boundaries of these Zr/(Zr + Si) in Figure 5. The data indicate that elpidite aggregatessuggest that they fill interstitial volumes for- contains up to 37 molo/oof armstrongite in solid solution. merly occupied by crystals of elpidite. In this interval, By contrast, most of the gittinsite and armstrongite sam- gittinsite makes up from 5 to 20o/oof the rock by volume. ples have compositions very close to those of the ideal The granite is almost free of porosity and displays a patchy end-member phases.Generally, cores of elpidite euhedra red coloration becauseof fine-grainedhematite in the git- are more calcic than the rims and fine-grained, late-in- tinsite-quartz aggregates. terstitial aggregatessuch as shown in Figure 48. SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS lo37

Zr/(Zr+SI) to be affectedby the electron-beampower density. Spec- tra of armstrongite are dominated by a band at about 415 1 nm and by a hump between500 and 650 nm (Fig. 68). gittinsite The band at 4l 5 nm closely resemblesEu2* activation in strontianite, apatite, and synthetic fluorite described by elpidite Mariano and Ring (1975). We thereforeinterpret Eu2+to be the principal activator of luminescencein armstron- armstronqrte gite; other lanthanides may account for the hump at 500- 650 nm. has luminescence. Na 10 20 80 90 Ca Gittinsite a distinct bright orange This color is due to a very strong band centered at 600 Fig.5. Cationproportions in theStrange Lake zirconosilicate nm (Fig. 6C). A wide band is also present at abott 420 mineralscalculated on thebasis of sevenand 15atoms of O for nm but is partially eclipsedby the 600 nm peak. Mn ion anhydrousand hydrousspecies, respectively. Solid circles indi- (Mn'* ) activation in apatite, feldspar,and carbonateshas cateend-member compositions. been shown to produce emissions between 520 and 650 nm, depending on the Mn site conf,guration (Marshall, Cathodoluminescence 1988;Mason and Mariano, 1990).Microprobe analyses indicate that gittinsite contains up to 1.5 wtoloMnO, sug- Elpidite, armstrongite, and gittinsite display strong lu- gesting that Mn2+ may be the activator in this mineral. minescencewhen excited by an electron beam. Similar Activation at 420 nm is similar to the homologous emis- but weaker luminescenceis observed under ultraviolet sion in armstrongite and elpidite and is therefore attrib- illumination. This property allowed the distribution of uted to Eu2+. zirconosilicatesto be establishedin many drill holes. Cathodoluminescenceobservations were made using an ELM-3R Luminoscope(Herzog et al., 1970). Emission DrscussroN spectra were collected in the 350-850 nm wavelength The euhedral habit ofelpidite in the equigranulargran- range,with a grating-typeH-20 Jobin-Yvon spectrometer ite suggeststhat this mineral crystallized relatively early, equipped with an R-928 photomultiplier detector (Mari- whereasits occurrenceas interstitial anhedra in the por- ano and Ring, 1975). phyritic granite indicates relatively late-stagecrystalliza- Elpidite typically displays a greenish luminescence, tion. In both rock types, however, elpidite is clearly a which generally decays to a duller shade of blue-green magmaticmineral and was the first zirconosilicateto form. after 2-3 min of continued exposure to the electron beam. By contrast, the textures describedabove show that arm- To minimize this, spectrafor elpidite were collected with strongite and gittinsite occur only as secondary subsoli- a defocusedbeam, reduced power, and increased scan- dus minerals, largely as replacementsof elpidite. ning interval. Emission spectraof elpidite show well-de- The formation of armstrongite is readily explained by finedpeaks at425,485, 550, and 580nm (Fig.6A). From the cation-exchangereaction comparison with spectrain Marshall (1988), we interpret NarZrSiuO,r.3HrO * Ca2* the peaks at 485 and 580 nm to representactivation by elpidile Dy3+; the 425 and 550 nm peaksare tentatively assigned to activation by Euz+ and Tb3+. : CaZrSiuO,,.3HrO+ 2Na+. (l) Armstrongite luminesceshues of blue and doesnot seem amstrongrte

160 b30 .; 60 .= rtn o o = -2O o40 o80 o

o &ro cl zo &40

300 400 500 600 700 800 300 400 500 600 700 800 A Wavelength(nm) B Wavelength(nm) C Wavelength(nm)

Fig. 6. Cathodoluminescenceemission spectra for zirconosilicate minerals (A) elpidite, @) armstrongite, and (C) gittinsite in subsolvusgranites from StrangeLake. Beam voltage: l0-15 keV; beam current: 0.5-1 mA; beam diameter: 5 mm. Scanningat 0.5 nm increments, stepping time of I s. Photomultiplier high-voltage: 950 V. 1038 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS

This reaction has a very small negativeAV" (- I I .0 I cm3/ The mineralogical relationships in DDH SL-182 occur mol; calculated from molar volume data in Marr and throughout altered parts ofthe pluton and suggestthat an Wood, 1992),which correspondsto 4.8o/ovolume reduc- episode of Ca metasomatism caused extensive replace- tion and is consistentwith the textural evidenceofessen- ment of elpidite by calcium zirconosilicatesto a depth of tially constant volume replacement of elpidite euhedra -70 m below the presenterosional surface, and that silica by armstrongite (Fig. 4D). was added in the form of quartz closer to the surface.On The replacement of elpidite by gittinsite can be ex- the basis of data from primary fluid inclusions in gittin- plained by the reaction site-quartz pseudomorphs, Salvi and Williams-Jones (1990) proposed that pseudomorphism was effected by NalrSiuO,r.3HrO + Ca2++ 5H2O Ca-rich brines at temperaturesof -200 'C. The replace- etpidite ment of magmatic elpidite by gittinsite or armstrongite : CaZrSirO, * 2Na+ + 4H4SiO9. Q) or both was probably caused by a similar fluid. Such gittinsite low-temperature formation of gittinsite and armstrongite explains their near-end-membercompositions. This con- Reaction2 has a largenegative AV.(- 149.1 cm3/mol or trasts with the extensive solid solution displayed by el- 650/ovolume reduction), thereby explaining the consid- pidite (cf. Fig. 5), which is interpreted to have crystallized erableporosity associatedwith replacementof elpidite by under magmatic conditions; magmatic conditions for el- gittinsite in the central part ofthe hole (Fig. 4E and 4F). pidite crystallization are also suggestedby zoned crystals In fact, the pore volume predicted by the AZ" of Reaction that, like plagioclasein igneoussystems, were initially Ca 2 compares closely with the observed porosity created rich. The activation of luminescence by Eu2+ in both where grains of elpidite have been entirely replaced by armstrongite and gittinsite but not elpidite (Fig. 6) is con- gittinsite: 60-700/o.The production of silicic acid in Re- sistent with Ca metasomatism, and the fact that, in its action 2 and the absenceof quartz accompanyinggittin- divalent state, Eu readily substitutesfor Ca. site indicates that the altering fluid was quartz undersat- The questions that now need to be addressed are urated. whether the Ca metasomatism was causedby orthomag- In the porphyritic granite in the top l0 m ofthe hole, matic fluids (Birkett ef al., 1992) or externally derived as well as in equigranulargranite at shallow levels in oth- formational waters (Salvi and Williams-Jones, 1990); er holes and exposed in the trench, elpidite is absent, whether or notZr was mobilized and concentratedby the gittinsite is the sole zirconosilicate, and there is no evi- metasomatism; why armstrongite formed at depth and dence of porosity. Instead, quartz fills the interstices gittinsite closer to the surface; why quartz addition was around gittinsite. The similarity in the sizes and shapes restricted to the near-surfaceenvironment; and ultimate- of the gittinsite-quartz aggregatesand the boat-shaped ly under what conditions and in responseto what factors pseudomorphsto elpidite aggregatesand euhedrain sim- alteration of elpidite to armstrongite and gittinsite oc- ilar rocks deeper in DDH SL-182 suggestthat gittinsite curred. and quartz formed at the expenseof precursor elpidite. The orthomagmatic fluid hypothesis can be ruled out This requires that gittinsite replacement of elpidite was by the evidencethat the fluids responsiblefor the deeper accompanied or followed by precipitation of additional alteration were qrtartz undersaturated.Ifthe altering fluid qtrartz. A reaction that might explain coprecipitation of were of magmatic origin it would have been in equilib- gittinsite and quartz is rium with the granite and therefore saturatedwith respect NarZrSiuO,r.3HrO + C3z+ to silica (any cooling would have caused deposition of elpidite quartz). Moreover, the average Ca concentration observed in altered subsolvusgranite (1.5 wt0/0vs. 0.5 wto/o : CaLtSi2Oj+ 2Na+ + 4SiO, + (3) 3H,O. in unaltered granites) cannot be explained by magmatic gittinsit€ quartz processesalone (cf. Nassif, 1993; Boily and Williams- However, AZ" for this reaction is -58.4 cm3/mol. This Jones,1994). represents a 25o/ovolume reduction, which should be The restriction of gittinsite mainly to the volume of the readily evident as pore space.The fact that there is no precursor phasemay indicate that Zr acted as an immo- unfilled volume in the spaceoccupied by gittinsite-quartz bile element during metasomatism.However, as reported aggregatesindicates that, if replacementwas according to above, the proportions of gittinsite to quartz in samples Reaction 3, there must have been precipitation of addi- from high levels are generally greater than the propor- tional quartz. An alternative explanation, supported by tions of gittinsite to pore space lower in the hole. Al- preservation of delicate rosettes of gittinsite, is that re- though the difference is small, it could be significant be- placement took place by Reaction 2 and quartz precipi- causeit may indicate that Zr was indeed remobilized by tated later. This implies that the metasomatic fluid was the metasomatic fluid, albeit to a limited degree. This initially undersaturatedwith respect to quartz, and that would be consistentwith the observedincrease of whole- atalater stage,silica solubility decreasedthereby causing rockZr in the strongly altered subsolvusgranites (Fig.2), precipitation of quartz in the pores at high levels in the i.e., in rocks containing the gittinsite-quartz pseudo- pluton. morphs. SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS 1039

feathery gittinsite crystals embeddedin quartz, we prefer the latter explanation and propose that qvartz was pre- cipitated in responseto cooling of the fluids during the --) waning stagesof the hydrothermal system (Fig. 7C). frcsh rock AcxNowr-nocMENTs The research described in this paper was supported by NSERC strategic and operating grants awarded to A.E.W.-J. and forms part of a larger project involving S. Aja, M. Boily, R. Marr, R. Martin, G. Nassif, J. Roelofsen-Ahl,and S. Wood. Additional funding was provided by FCAR (Quebec) and a NATO travel grant We are indebted to the Iron Ore .el Company ofCanada for allowing accessto the StrangeLake property and Q'

RrrnnnNcrs crrED Allan, J.F. (1992) Geology and mineralization of the Kipawa yttrium- zirconium prospect,Quebec. Exploration and Mining Geology, 1, 283- 295. Bastin, G.F., and Heijligers, H.J.M. (1991) Quantitative electron probe Iog a :Hnstoo C microanalysisof ulraJight elements(oxygen-boron). In K.F.J. Hein- rich and D.E. Newbury, Eds.,Electron probe quantitation,p. 145-162. Fig. 7. Schematiclog af,,^,/a.^*vs. Plenum, New York dH.siogdiagrams showing Birkett, T.C., Miller, R.R., Roberts, A.C., and Mariano, A.N. (1992) Zir- the stabilityfields of the zirconosilicateminerals, well as as hy- conium-bearingminerals of the StrangeLake intrusive complex, Que- potheticalcompositions of theunaltered elpidite-bearing granite, bec-Labrador.Canadian Mineralogist, 30, 19 I -205. the hydrothermalfluid (A), and the alteredgittinsite-bearing Boily, M, and Williams-Jones, A.E. (1994) The role of magmatic and granite(C). Also shownare possible paths of metasomatictrans- hydrothermal processesin the chemical evolution of the Strange Lake formationof the rock during alteration(B). The shadedfields plutonic complex (Quebec-Labrador).Contributions to Mineralogy and representquartz saturationlevels in the fluid during metaso- Petrology, ll8,33-47 matism(A and B) and at lowertemperature (C). Bonin, B. (1988) Peralkaline granites in Corsica: Some petrological and geochemicalconstraints. Rendiconti della SocietdItaliana di Mineralo- gia e Petrologia,73, 119l-1194. Currie, K.L. (1985)An unusualperalkaline granite near Lac Brisson,Que- Further insight into this metasomatic processis gained bec-Labrador.Geological Survey ofCanada Report 85-lA, p. 73-90. by examining the stability fields of the zirconosilicate Currie, K.L., and Zaleski, E. (1985) The relative stability ofelpidite and minerals as a function of Na, Ca, and Si activity. Unfor- vlasovite: A P-I indicator for peralkaline rocks. Canadian Mineralo- 571-582. tunately, there few Elst,23, are thermodynamic data for the alkali Fleet, S.G., and Cann, J.R. (1967) Vlasovite: A secondoccurrence and a and alkaline-earth zirconosilicates (cf. Marr and Wood, triclinic to monoclinic inversion Mineralogical Mugazrne, 36, 233- 1992).Nonetheless, it is possibleto representphase-sta- 241 bility relationships schematically using the principles of Gerasimovsky,V.I. (1969) Geochemistryof the IlimaussaqAlkaline Mas- (South-West p. (in (not chemographic analysis 1966). In Figure 7 we pre- sif Greenland), 174 Nauka, Moskow Russian) Qnn, seen;extracted from Lithos, 26, 167, 1990). sent a schematiclog allo*/ec^,*vs. log a"..,ogdiagram for Goldschmidt, V. (1913) Atlas der Krystallformen, vol. 3. Carl Winters the systemNarO-CaO-ZrOr-SiOr-HrO and the phasesin UniversitZitsbuchhandlung,Heidelberg, Germany. the system observedin the altered rocks ofStrange Lake, Herzog,L.F., Marshall, D.J., and Babione,R.F. (1970) The luminoscope: plus vlasovite. Also shown on the diagram are possible A new instrument for studying the electron-stimulatedluminescence ofterrestrial, synthetic materials micro- locations ofthe boundary for quartz extra-terrestrial and under the saturation. scope.In J.N. Weber and E. White, Eds., Spacescience applications of We propose that, sometime after crystallization of at solid state luminescencephenomena, p. 79-98. MRL publication 70- Ieast the apical region of the intrusion, heated ground- l0l. StateCollege. Pennsylvania. waters in equilibrium with adjacent calc-silicategneisses Horv6th, L., and Gaulr, R.A. (1990) The mineralogy of Mont Saint-Hi- and gabbros (and therefore Ca-rich and, quartz undersat- laire, Quebec.Mineralogical Record, 21, 284-359. Kogarko, L.N. (1990) Ore-forming potential of alkaline magmas.Lithos, urated) entered the pluton (Fig. 7A). In the uppermost 26, 167-175. parts where water-rock ratios were high, the fluids dis- Mariano, A.N., and Ring, P.J. (1975) Europium-activated cathodolumi- solved elpidite, replacing it primarily with gittinsite (Fig. nescencein minerals. Geochimica et Cosmochimica Acta, 39, 649- 78, path I). Deeper in the pluton, fluid-rock ratios were 660. permitting Marr, R.A., and Wood, S.A. (1992) Preliminary petrogeneticgrids for lower, the rock to buffer 4ro",ooto higher val- sodium and calcium zirconosilicateminerals in felsic peralkalinerocks: ues, which were sufficient to stabilize gittinsite plus arm- The SiOlNarZrO, and SiOr-CaZrO, pseudobinarysystem. American strongite and locally armstrongite alone (Fig. 7B, path II). Mineralogist, 77, 810-820. The presenceof quartz with gittinsite near the surface Marshall, D.J. (1988) Cathodoluminescenceof geologicalmaterials, 146 p. requires either that P-Zconditions ofalteration changed Unwin-Hyman, Boston. Mason, R.A., and Mariano, A.N. (1990) Cathodoluminescenceactivation so as to permit saturation of quartz at lower aso5;6gor that in manganese-bearingand rare earth-bearing synthetic calcites. Chem- quartz was introduced later. In view ofthe textures, i.e., ical Geology,88, 191-206. 1040 SALVI AND WILLIAMS-JONES: ZIRCONOSILICATE PHASE RELATIONS

Miller, R.R. (1986) Geology of the StrangeLake Alkalic Complex and the bec/Labrador: Evidence from fluid inclusions. Geochimica et Cosmo- associatedZr-Y-Be-REE mineralization Newfoundland Department chimica Acta. 54. 2403-2418. of Mines and Energy,Mineral Development Division Report 86-1, p. -(1992) Reduced orthomagmatic C-O-H-N-NaCI fluids in the 11-19. StrangeLake rare-metal granitic complex, Quebec/Labrador,Canada. -(1990) The StrangeLike pegmatite-aplitehosted rare metal de- EuropeanJournal of Mineralogy, 4, 11 55-l I 74. posit, Labrador. Neufoundland Department of Mines and Energy, Trueman,D.L., Pedersen,J.C., de St.Jorre, L., and Smith,D.G'W. (1988) GeologicalSurvey Branch, Report 90-1,p. l7l-182. The Thor Lake, N.W T , rare-metal deposits In R P. Taylor and D'F. Nassif, G.J (1993) The StrangeLake peralkaline complex, Qu6bec-Iab- Strong, Eds., Granite-related mineral deposits: Geology, petrogenesis rador: The hypersolvus-subsolvusgranite transition and feldspar min- and tectonic setting.Canadian Institute ofMining and Metallurgy, Spe- eralogy, 104 p. M.Sc thesis,McGill University, Montreal, Quebec. cial Volume 39, 279-284. Nassif, G.J., and Martin, R.F. (1991) The hypersolvusgranite-subsolvus Vlasov, K.A., Kuz'menko, M.2., and Es'kova, E.M. (1966) The Lovozero granite transition at Strange lake, Quebec-Labrador.Geological As- Alkali Massif, 627 p Oliver and Boyd, Edinburg. sociation ofCanada Abstracts with Program, 16, A89. Znn,E-An (1966)Construction ofpressuretemperature diagrams for mul- Pillet, D., Bonhomme,M.G, Duthou, J.L., and Shenevoy,M. (1989) ticomponent systemsafter the method of Schreinemakers:A geomet- Chronologie Rb/Sr et K.zAr du granite peralkalin du lac Brisson, Lab- rical approach U.S. Geological Survey Bulletin, 1225,1-56. rador central, Nouveau-Qu6bec.Canadian Journal of Earth Sciences, 26,328-332. Salvi, S., and Williams-Jones,A.E. (1990) The role of hydrothermal pro- M,,rNuscmrr RrcErvEDAucusr 12, 1994 cessesin the ganite-hosted Zr, Y, REE deposit at StrangeLake, Que- MeNuscnrrr AcCEPTEDMeY 31, 1995