spodumene – petalite – eucryptite: mutual ...pegmatites minéralisées avec les granites à deux...

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The Canadian MineralogistVol. 39, pp. 729-746 (2001)

SPODUMENE – PETALITE – EUCRYPTITE: MUTUAL RELATIONSHIPSAND PATTERN OF ALTERATION IN Li-RICH APLITE–PEGMATITE DYKES

FROM NORTHERN PORTUGAL

BERNARD CHAROY§

Ecole Normale Supérieure de Géologie, CRPG–CNRS, BP 20, F-54501 Vandoeuvre-lès-Nancy Cedex, France

FERNANDO NORONHA§ AND ALEXANDRE LIMA§

Centro de Geologia, Faculdade de Ciências da Universidade do Porto, 4050 Porto, Portugal

ABSTRACT

The co-occurrence, at the thin section scale, of the three anhydrous Li-aluminosilicates spodumene, petalite and eucryptite, isnot a common feature. Such an association occurs in some aplite–pegmatite dykes of the Covas de Barroso district, northernPortugal. We describe their mutual relationships: where spodumene is an early magmatic phase, petalite precipitates directly froma late orthomagmatic fluid, the spodumene remaining metastably, whereas eucryptite is obviously hydrothermal and secondary.Replacement of the primary Li-minerals is the rule, but is diversified and selective: spodumene is mainly replaced by albite andmuscovite, petalite by K-feldspar and eucryptite. Muscovite is widespread, and the late hydrothermal paragenesis is dominatedby quartz associated with various mixtures of phosphates. The assemblage eucryptite, K-feldspar and albite is obviously inchemical disequilibrium with the early phases. The compositional evolution of the hydrothermal fluids (Na, K, Li) is tentativelybracketed. There is disequilibrium among thermodynamically conflicting mineral phases in terms of pressure, temperature andfluid:rock ratio, in a mobile and partly open system. A genetic affiliation of the Li-enriched pegmatites with the nearby two-micagranites is difficult to prove. The tectonically controled intrusion of biotite granites to the east of the pegmatite belt would seemto be responsible of the drop in pressure that accounts for the spodumene–petalite transition and the development of the wide-spread replacement by feldspars, not present in the western part of the belt.

Keywords: granitic pegmatite, spodumene, petalite, eucryptite, replacement by feldspars, Covas de Barroso, Portugal.

SOMMAIRE

La cohabitation, à l’échelle de la lame mince, des trois phases lithinifères spodumène, pétalite et eucryptite n’est pas uneobservation très fréquente. Un tel assemblage s’observe dans quelques associations aplite–pegmatite de la région de Covas deBarroso, dans le nord du Portugal. Nous décrivons leurs relations mutuelles. Si le spodumène est manifestement une phasemagmatique précoce, la pétalite précipite à partir d’un fluide orthomagmatique, expulsé lors d’une chute de pression, le spodumènedemeurant métastable. L’eucryptite est franchement hydrothermale et secondaire. Le remplacement des phases lithinifèresprécoces est la règle, et il est selectif: le spodumène essentiellement par l’albite et la muscovite, la pétalite par le feldspathpotassique et l’eucryptite. La muscovite secondaire est omniprésente, et le quartz domine la paragenèse hydrothermale tardive, enassociation avec différents phosphates. L’eucryptite et les deux feldspaths sont de toute évidence en déséquilibre chimique avecles phases lithinifères plus précoces. L’évolution de la composition (en termes de Na, K et Li) des fluides hydrothermaux estapprochée. Nos observations démontrent le large déséquilibre entre des phases thermodynamiquement contrastées en termes depression, température et rapport fluide : roche, dans un système mobile et partiellement ouvert. Si la filiation génétique despegmatites minéralisées avec les granites à deux micas voisins ne peut être affirmée, l’intrusion tectoniquement controlée degranites à biotite à l’est du champ pegmatitique serait responsable de la chute de pression justifiant la transition spodumène–pétalite et le déclenchement d’un remplacement par les feldspaths, absents dans la partie ouest du champ filonien.

Mots-clés: pegmatite granitique, spodumène, pétalite, eucryptite, remplacement par les feldspaths, Covas de Barroso, Portugal.

§ E-mail addresses: [email protected], [email protected], [email protected]

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730 THE CANADIAN MINERALOGIST

INTRODUCTION

Granitic pegmatites of the rare-element class consti-tute only a very minor percentage (less than 2%) of thenumerous dykes present in a regional pegmatite popu-lation (Stewart 1978, Černý 1993). Li-rich graniticpegmatites are affiliated with the LCT (i.e., Li–Cs–Ta)family ( Černý 1982, 1991a) within this class. Silicates(spodumene, petalite, lepidolite and, less commonly,eucryptite) and phosphates (mainly of the amblygonite–montebrasite and lithiophylite–triphylite series) are themain carriers of Li. Depending on pressure, both spo-dumene and petalite have been experimentally shownto be stable liquidus phases (Fenn 1986). Spatial andtemporal relationships between these phases have beententatively established by Heinrich (1948), Cameron etal. (1949), Ginsburg & Gushchina (1954), Stewart(1978), Norton (1983) and Ginsburg (1984). Followingthe experimental work of Stewart (1978), London(1984) has successfully demonstrated the use of a Li-aluminosilicate grid to constrain the P–T path of crys-tallization for such Li-rich pegmatites. However,mineral replacements affecting primary Li-rich phasesare widespread in the presence of fluids released late inthe crystallization ( Černý 1972, London & Burt 1982a).Alteration of petalite and spodumene to a mixture ofeucryptite, albite, K-feldspar and white mica is the nor-mal consequence of re-equilibration in a restricted sys-tem (London & Burt 1982a, Wood & Williams-Jones1993). “Pegmatite stews in its own juice” (Jahns 1982).

Three examples of Portuguese Li-rich aplite–pegma-tite dykes from a larger population of pegmatite bodieswere selected for this study, in order to constrain theconditions of the magmatic stage and the steps of hy-drothermal re-equilibration. We seek to establish how,when and under what conditions were these Li-alumi-nosilicates formed and altered. The purpose of this studyis: 1) to establish the process of aplite–pegmatite for-mation, 2) to make comprehensive statements about thesteps in the development and evolution of thesepegmatites, 3) to summarize the mineralogical featuresof the different Li-aluminosilicate phases and to estab-lish their chronological relations, 4) to determine thenature of their replacement assemblages, and 5) toevaluate changes in the chemistry of the hydrothermalfluids with time and with falling temperature and pres-sure.

GEOLOGICAL SETTING

The Covas de Barroso district (CDB for short), AltoTâmega, northern Portugal, contains a large populationof several dozens of aplite–pegmatite dykes (Fig. 1).These bodies are hosted by low- to medium-grademetasedimentary rocks of Silurian age of the MiddleGalicia Tras-os-Montes geotectonic zone (Ribeiro et al.1979). Three phases of Hercynian deformation (D1 to

D3), leading to three superimposed schistosities (S1 toS3), have been recognized (Noronha et al. 1981). Sev-eral types of granite bodies are present in the vicinity ofthe pegmatite belt (Fig. 1). They differ in mineralogy(biotite or two-mica) and timing (deformed orundeformed) with regard to these episodes of deforma-tion. On the basis of their macroscopic fabric due todeformation by post-consolidation stress, the aplite–pegmatite dykes are considered older than the youngestunfoliated, post-tectonic, post-D3 biotite granites(Ferreira et al. 1987). The common pegmatites scatteredthroughout the two-mica granites (interior pegmatites)will not be considered in this study.

Two types of aplite–pegmatite dykes, both cross-cutting the metasediments, are encountered in the CDBdistrict. The first type is represented by countless, thin(meter-size, on average), mainly aplitic dykes and veins,which contain low-grade (

SPODUMENE – PETALITE – EUCRYPTITE RELATIONSHIPS 731

forcible emplacement of the dykes (Brisbin 1986). Flatenclaves (up to half a meter in diameter) of micaceousschists structurally similar to those of the footwall areoccasionally enclosed in the pegmatite bodies, com-monly with a reaction rim of recrystallized biotite. Thereis no visible metasomatic alteration of the walls.

Only a few of these large aplite–pegmatite dykescontain spodumene (Charoy et al. 1992), but a broaderdetailed field-based investigation is in progress to es-tablish the distribution of spodumene. Among the bod-ies of Li-enriched granitic pegmatites alreadyrecognized in the CDB district, three of them (Alijó:ALJ, Veral and Adagoi: ADG; Fig. 1) seem to encom-pass much of the variations in mineralogy of the Li-richspecies, mineral sequence and alteration patterns; theywere chosen for a trench and drilling program by IGM(Instituto Geológico e Mineiro, Portugal). Our detailedpetrographic analysis was conducted on numeroussamples from outcrops and drill cores taken from thesethree occurrences.

THE LI-BEARING ALUMINOSILICATES

Field aspects

The assemblage of rock-forming minerals in theseLi-rich aplite–pegmatites seems fairly simple and gra-nitic in bulk composition. At the outcrop scale, the peg-matitic assemblage consists of: 1) blocky euhedralphenocrysts of feldspar, mostly K-feldspar (as much as20 cm long), randomly distributed, but locally in scat-tered clusters or pods; the K-feldspar does not form agraphic texture, as is commonly encountered in marginsof numerous granitic pegmatites. 2) Spodumene formsisolated laths or rectangular, radiate or shapeless aggre-gates. 3) Quartz is found in minor rounded blebs or inintergranular distribution. 4) Muscovite, in centimetricflakes, is subordinate. All are mixed with a minor sac-charoidal aplitic matrix with albite, quartz and musco-vite. Spodumene is homogeneously distributed within

FIG. 1. Geological sketch-map of the Covas de Barroso (CDB) district in northern Portugal, showing the pegmatite belt and thevarious marginal intrusions of granite. The Li-rich aplite–pegmatite dykes are shown in bold. 1) Syntectonic two-mica gran-ites; 2) post-tectonic (post-D3) biotite granites. Insert: regional geology for location of the Régua–Verin fault system.

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the pegmatitic differentiates throughout the dykes, butseems significantly enriched near the hanging wall (Fig.2). The bulk of the spodumene occurs as long blades ofeuhedral to subhedral crystals, pearly in color, up to 15cm long (mainly about 5 cm), in parallel growth (co-lumnar aggregates intermixed with quartz) or at random.Aggregates of smaller ragged crystals of spodumene areinterstitial between the feldspar megacrysts. Drill coreshave intersected a few definite layers (several centime-ters across) in which spodumene laths (up to 5 cm long),with a rough branching habit, would suggest a solid–liquid interface.

Petrographic description

K-feldspar phenocrysts are turbid and have no (orvery few) exsolution lamellae of albite, but they do con-tain albite laths from the matrix as inclusions. Thecloudy appearance is due to the abundance of micro-pores, as a fingerprint of hydrothermal alteration(Worden et al. 1990). They never display cross-hatchedtwinning, at least at the scale of optical microscopy. Thecore zone of most of the large crystals of primary albite

is decorated with small “droplets” of quartz. These earlyalbite crystals show bent twin lamellae and deforma-tion-induced twinning. Quartz commonly showsundulose extinction. Primary muscovite, scarce in dis-crete centimetric plates, seems at equilibrium with bothprimary feldspars. Most of the white mica occurs assmall secondary anhedral flakes growing at the expenseof the primary Li-aluminosilicate phases or along frac-tures.

Spodumene occurs as euhedral to subhedral crystals,with a predominant (100) and subordinate (110) devel-opment of the prism zone. It is mainly devoid of anymineral inclusions, suggesting that it precipitated quiteearly in the sequence. The simple twin (100) is com-mon. Also present are irregularly shaped poikilitic crys-tals containing sparse blebs of quartz. Petalite was onlyidentified in thin section. It follows spodumene as thestable Li-aluminosilicate (Fig. 3). Spodumene is in somecases penetrated, but not replaced by petalite alongcracks or twin planes. Petalite is partly altered alonggrain boundaries to a brownish aggregate of microcrys-tals of eucryptite (see below). At Veral, needles ofpetalite, together with euhedral quartz, fill angular voids

FIG. 2. Sketch of the macroscopic aspect of a strongly mineralized sample from Veral.

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among a meshwork of euhedral laths of spodumene(Fig. 4). Such a textural relationship shows that whenthe main generation of quartz precipitated, the mediumwas still saturated with Li–Al–silicate-forming compo-nents at P–T conditions in the field of stability ofpetalite. Spodumene and petalite develop a bright goldenyellow and a deep dark blue cathodoluminescence, re-spectively (Görz et al. 1970), whereas quartz remainsdark. Montebrasite, which seems interstitial, is sporadi-cally encountered as isolated, heavily altered grains withremnants of polysynthetic twinning.

The intimately mixed aplitic matrix may present apoorly developed banding defined by a rough variationin grain size, which signifies that the matrix is deposi-tional and not replacive in origin. Na-rich plagioclase isthe predominant (or only) feldspar present, with onlyoccasional tapered phenocrysts of turbid K-feldspar.Quartz is not a dominant phase. Muscovite, as isolatedflakes, together with rare grains of apatite, is subordinate.

Hydrothermal alteration

Widespread replacements show definite and consis-tently developed sequences, with selective corrosionleading up to complete replacement of spodumene by

albite (± muscovite), and of petalite by K-feldspar andeucryptite.

Albite is the major product of destabilization of spo-dumene. Remnants of spodumene in crystallographiccontinuity are embedded in medium-grained clear al-bite, which is surrounded by a fine-grained intergrowthof secondary K-feldspar + muscovite ± eucryptite(Fig. 5). Albite may accumulate, in association with sec-ondary K-feldspar, both with a branching habit. Part ofthis albite is subsequently altered to crystallographicallyoriented strings of white mica. In some cases, spo-dumene is invaded by radiating thin flakes of musco-vite. In one case (ADG), euhedral vuggy prisms ofspodumene in a large (decimetric) open cavity havebeen totally converted to a pale green, soft, waxypseudomorph of nearly pure fine-grained white mica.This material is very similar to that described as “rottenspodumene” by Graham (1975). Mica flakes occur op-tically parallel to the three orientations (100), (110)and (11̄0) of the original spodumene. Relations betweenboth minerals are structurally controlled by a topotacticmechanism. Li was nearly completely leached out dur-ing this replacement: 7.23 and 0.13 wt% Li2O remain inspodumene and pseudomorphic white mica, respec-tively.

FIG. 3. Back-scattered electron (BSE) image showing Z-contrast between spodumene laths (grey) coated by polycrystallinepetalite (light grey) against quartz (white) (A). Enlargement showing eucryptite in cracks cross-cutting petalite (B). Adagoipegmatite. Width of field of view: 2.4 mm in a, and 335 �m in b.

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Secondary K-feldspar is ubiquitous, in cross-cuttingveinlets, alone or together with albite, or grown at theexpense of petalite. It is invariably very turbid, and lacks

the occurrence of either distinctive grid twinning oralbite exsolution. It is very difficult to discriminate be-tween eucryptite and secondary K-feldspar under the

FIG. 4. BSE image showing Z-contrast between petalite nee-dles (grey), euhedral quartz (white), and a fine-grainedmixture of secondary K-feldspar (white) and eucryptite asinfill of an angular vug delimited by interlocking euhedrallaths of spodumene (A) or quartz (B) or both (C). Veralpegmatite. Width of field of view: 2.8 mm in A, 0.84 mmin B, and 1.2 mm in C.

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microscope, because both have a very turbid appear-ance, seem more or less contemporaneous, and gener-ally are closely associated. The identification ofeucryptite was very difficult. Eucryptite was firstly iden-tified by its pale pinkish cream fluorescence in short-wave ultraviolet light (Mrose 1953, Hurlbut 1962); theobservations were made with a microscope equippedwith a UV (about 365 nm) source (Fig. 6). At high mag-nification, fluorescent eucryptite presents a low degreeof crystallinity at a submicrometer scale. It is intimelymixed with amoeboid quartz, an Al-dominant phos-phate, apatite and Ab grains in an alveolar mush-likemixture. Because of the poor resolution at a magnifica-tion of �40, the UV fluorescence seems to be homoge-neous even though eucryptite is far from being thedominant phase in the mixture replacing petalite. Useof laser ablation – optical emission spectroscopy (Fabre2000) confirms the occurrence of Li in such a mixture.

A Raman investigation conducted on the three Li-aluminosilicates give very good conclusions for spo-dumene and petalite, but disappointing results, becauseof the strong fluorescence, for eucryptite (Fig. 7), eventhough, contrary to spodumene and petalite, eucryptite

does not present any visible cathodoluminescence emis-sion. Si/Al stoichiometry from EDS point-analyses con-firm the coexistence of the three aluminosilicates at thethin-section scale (Fig. 8).

A quite similar sequence of alteration, but at a largerscale, involving the assemblages eucryptite + albite ±K-feldspar and albite + muscovite, has been documentedfrom many Li-rich pegmatites elsewhere (London &Burt 1982a). These subsolidus assemblages correspondto a Na+-for-Li+ ion-exchange reaction [spodumene +Na+ = eucryptite + Ab + Li+], followed by influx of K+

(growth of microcline) and H+ metasomatism (growthof muscovite). Therefore, subsolidus conditions weresufficiently alkaline to stabilize eucryptite + Ab oreucryptite + K-feldspar, and then later, relatively acidto form secondary mica (Montaya & Hemley 1975).During these episodes of alteration, Li was removedfrom the primary pegmatite assemblage, partiallytrapped in secondary eucryptite or leached out of thesystem and expelled toward the host rocks. Silica is alsoleached out. A slight increase of Li and Si contents inadjacent micaceous schists confirms our inference.Quartz is largely dominant in the final assemblage, aseuhedral crystals or in aggregates with sutured joins andan undulose extinction.

Some of the samples studied are heavily altered, withonly small remnants of spodumene in a very fine-grained, spongy mush of secondary minerals. Thesesecondary phases, where phosphates of various compo-sitions dominate, have not been fully characterized be-cause of their small size and intimate intergrowth.

Tiny vugs are commonly encountered throughout theaplitic matrix. They probably result from the partial dis-solution of albite by a free fluid phase collected as dis-crete bubbles along grain boundaries. That fluid mainlyprecipitated quartz, but also K-feldspar and eucryptitelining these vugs. Red brown sphalerite, with sparseinclusions of galena, occurs sporadically in interstices orassociated with secondary quartz as an infilling of vugs.

A sequence of crystallization (Fig. 9) is tentativelyestablished from the mutual relations among all themajor and secondary mineral phases. The mineral se-quence depicted represents the most complete parage-netic succession, even though the relative importanceof one hydrothermal phase or the other differs widelyfrom one sample to another, even in a given aplite–peg-matite body.

DISCUSSION

Lithium, if present in a crystallizing system, does notbehave as a typical alkali element (Heier & Billings1970). It is not incorporated into the structure of thecommon felsic rock-forming minerals. Rather, the con-centration of Li builds up progressively in the evolvingclosed pegmatite system, and reaches a level sufficientto trigger the crystallization of a Li-bearing phase.Where F is available, Li-bearing micas may be formed.

FIG. 5. BSE image showing Z-contrast between remnants ofa spodumene crystal (black) replaced by albite (mediumgrey), blades of muscovite and a continuous rim of second-ary K-feldspar (both white) with subordinate eucryptite(also black) Adagoi pegmatite. Width of field of view:2.4 mm.

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So, it largely behaves incompatibly. The starting mate-rial has roughly a granitic composition. Thermal andcompositional inhomogeneities inside the crystallizingmelt are generally assumed in huge, zoned pegmatitebodies (Jahns 1982, Webber et al. 1999), and the de-gree of undercooling of the melt dominates the kineticsof crystal nucleation and growth (London 1992). Theoccurrence of cavities delineated by euhedral laths ofspodumene and of a few large vugs with free-standingeuhedral crystals of spodumene suggests the presenceof a separate fluid phase which, as experimentally dem-onstrated by London et al. (1989), will be only active atthe last stage of pegmatite consolidation. In the ADGpegmatite, spodumene laths are wholly jacketed by acontinuous thin coating of polycrystalline petalite incontact with quartz (Fig. 3). The stability field of themore siliceous petalite could be reached through a sud-den drop in confining pressure, leading to the exsolutionof a orthomagmatic hydrothermal fluid phase. An in-

crease of �SiO2 in this fluid would be likely (Stewart1978). Both quartz- and feldspar-making componentswill be transferred into this fluid, and constitute, togetherwith needle-like petalite, the infill of small scatteredvugs, as present in samples of the Veral body (Fig. 4).The mutual relationships spodumene–petalite (Fig. 3)suggest that petalite precipitates from a thin film alongthe spodumene–fluid (silica-rich) interface. The fluidacting as a source of components for the crystallizationof petalite was saturated with respect to this phase, andthe spodumene was preserved metastably from dissolu-tion. Stewart (1978) suggested that up to 0.64 wt%petalite can be dissolved in an aqueous vapour at 575°C,which is a normal consequence of closed-systemsubsolidus evolution of the pegmatite. The addition ofsilica in the fluid is also required to resaturate the rockwith respect to petalite. Secondary quartz precipitatesas vug infill or large interstitial aggregates (possiblypools of a silica-rich fluid).

FIG. 6. Aspects of luminescence of eucryptite with UV light (natural and UV light doublets; scale bar: 1 mm). A. Relictspodumene embedded in fine-grained eucryptite derived from petalite. B. Eucryptite in cracks of vuggy petalite. C. Mush ofeucryptite and K-feldspar around petalite needles in a void delimited by euhedral blades of spodumene (see Fig. 4A).

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The relatively small size of most of the aplite–peg-matite dykes points to a relatively rapid rate of coolingdown to the metamorphic isotherms. A model of vigor-ous convection between melt and fluid, as proposed byBurnham & Nekvasil (1986) to explain the segregationof some particular lithophile element in a pegmatiticsystem, will be unlikely because of the narrow geom-etry and structural homogeneity of these Portugueseaplite–pegmatite dykes. A thermal gradient inside thebody of pegmatite-forming melt, with Na preferentiallyenriched into the hotter (lower?) portions and K into thecooler (upper?) portions of the system, as suggested bythe experimental results of Orville (1963), the conclu-sions of Jahns & Burnham (1969), and many pegmatiteoccurrences worldwide (Webber et al. 1999), contra-dicts our field observations; rather, an aplitic (mainlyalbitic) matrix seems to cement the megacrystic part(mainly K-feldspar) of the pegmatite paragenesis.

Disequilibrium at the magmatic stage

The pegmatite-forming magma, whatever its status,achieves saturation in a Li-aluminosilicate in the stabil-

ity field of spodumene. This condition is reached late inthe main course of crystallization, after the precipita-tion of the feldspars. Petalite follows spodumene, butbecause of the lack of any apparent corrosion, spo-dumene remains (stably or metastably). The isochemicalreplacement of petalite by spodumene + 2 quartz (re-ferred as “squi”), which is commonly observed in manyLi-rich pegmatites (Hensen 1967, Černý & Ferguson1972, Rossovskiy & Matrosov 1974, Gomes & Nunes1990) has no relevance in our system. Chemical com-positions of reliable bulk samples from the three aplite–pegmatite dykes investigated are not available, butpartial analyses obtained from core sections at ALJ give1.55 wt% Li2O on average and as high as 2.15 wt%(Lima et al. 1997). Bulk compositions of three pegma-titic (coarse-grained) fractions obtained by channel-sam-pling across some spodumene-bearing aplite–pegmatitedykes from the western part of the CDB pegmatite beltare available for comparison (samples CHN in Table 1;Charoy et al. 1992). Petalite and hydrothermal alterationof feldspars are lacking in these occurrences. The Li2Ocontent varies from 1.35 up to 2.76 wt%. However, be-cause of the selective sampling, such high contents can-not be representative of the initial pegmatite-formingmelt as a whole. Stewart (1978) convincingly demon-strated, from experiments and reliable bulk composi-tions of pegmatites, that the Li content of many Li-richpegmatites, representative of the lowest-temperatureresidual melt, clusters closely around 1.5 wt% Li2O

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(corresponding to about 20% spodumene), with 2.2 wt%claimed to be the upper limit of Li2O that can accumu-late in a melt. The aplitic portions, intimately mixed andgenetically related with the pegmatitic fraction, havedrastically lower Li contents (0.10 wt% Li2O, on aver-age). Therefore, it is obvious than Li in the initial meltwas selectively partitioned into the pegmatitic, K-richdifferentiates, in contrast to the aplitic, Na-rich segre-gations. On the other hand, Li is absent in pegmatites(Pegm* in Table 1) encountered within the nearby two-mica granites.

Disequilibrium at the hydrothermal stage

Hydrothermal metasomatic replacement of pre-ex-isting minerals implies the attainment of fluid satura-tion. The sporadic development of alteration within agiven pegmatite, and even within a single thin section,signifies that these replacements are not produced byretrograde metamorphism but are indicators of changesin the chemistry of hydrothermal fluids. Coexistence of

heavily transformed and unreacted spodumene crystalsat the thin-section scale can be explained either by ki-netic barriers, inhomogeneity in fluid percolation, or bysealing of fractures and channelways due to the rela-tively larger molar volume of the alteration assemblage(London & Burt 1982a).

The first stage of subsolidus metasomatic alterationis usually alkaline (London & Burt 1982b, Burt & Lon-don 1982), followed by a weakly acid and potassic stage(“sericitic” alteration). Assuming a closed system, thetotal budget of Li in the fluid–mineral system should beconstant. More and more Li will go into solution as thereplacement of Li-bearing minerals by albite and K-feld-spar progresses (Wood & Williams-Jones 1993). There-fore, both Na/Li and K/Li in the fluid generally willdecrease with falling temperature. Replacement of spo-dumene (and eucryptite) by feldspars consumes silica,whereas the replacement of petalite liberates silica. Assuch, their replacement will not be contemporaneous butcomplementary:

FIG. 7. Raman spectra of the three lithium aluminosilicates. Spectrometer: Labram DILOR (ORIBA). Analytical conditions:excitation line: HeNe laser, 632.817 nm (15 mW); grating: 1800 lines; slit: 300 �m (G2R, UHP, Université de Nancy, Th.Lhomme, analyst).

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Spd + Na+/K+ + Qtz →Ab/Kf + Li+

Pet + Na+/K+ → Ab/Kf + Qtz +Li+

Ecr + Na+/K+ + 2Qtz → Ab/Kf + Li+

According to the reaction Spd → Ecr + Qtz, spodumene,eucryptite and quartz cannot be stable together, as isclear from the isobaric–isothermal �Na–Li–1 – �SiO2diagram of Burt & London (1982). Whether spodumenebreaks down to Ab or to Ecr + Ab depends primarily onthe activity of silica within and around the crystals ofspodumene (Rossovskiy 1971). Also, the formation ofpseudomorphic mica in spodumene is enhanced by thelocal lowering of silica activity, according to:

3 Spd + K+ + 2H+ → Ms + 3Qtz + 3Li+

However, the completeness of replacement of spo-dumene by white mica indicates that spodumene under-going alteration does not buffer the cation-exchangepotentials in the fluid, at least at that scale.

Gordiyenko et al. (1988) speculated about the chemi-cal conditions necessary for the equilibrium Spd–Ms–Ab–Kfs (400°C, 100 MPa) present as aggregates in themore differentiated parts of many rare-metal pegmatites.According to their findings, spodumene grew hydrother-mally from K-feldspar and was nearly always associ-ated with albite. Spodumene and muscovite are more orless exclusive. These authors used four variables to com-pute the stability fields of the different minerals:log[K+], log[Na+], log[Li+] and pH. They apparently

FIG. 8. BSE image illustrating the three coexisting Li aluminosilicates. At the top is a grain of euhedral marginal spodumene;medium gray: eucryptite as infill of fractures in petalite (white). Respective EDS spectra show the difference in Si/Al stoichi-ometry for the three phases. (Service commun de microanalyse, UHP Univ. de Nancy, A. Kohler, analyst). This microphoto-graph is an enlargement of Figure 6b.

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ignored the relevant work of London (1984) about Li-aluminosilicate stabilities: in fact, petalite should bestable in place of spodumene at their referred physicalconditions. Alkalinity of the medium, more than Li ac-tivity, should be of prime importance for the spo-dumene–muscovite competition, acidic for muscoviteand more alkaline (pH > 6) for spodumene. These re-sults contradict their analytical data from fluid inclu-sion leachates on quartz from quartz–spodumeneaggregates: Li is very minor, with Na/Li Ç8 to 17,whereas this ratio is found close to unity for comparable

experimental conditions at equilibrium (Sebastian &Lagache 1991, Lagache & Sebastian 1991).

Equilibrium boundaries for exchange reactions be-tween an alkaline solution, feldspars and Li-alumino-silicates were computed by Wood & Williams-Jones(1993) between 100 and 700°C at different pressures.Assuming equilibrium among Ecr, Qtz, Ab and Kfs, K/Li and Na/Li activity ratios can be estimated (Fig. 10):K/Li = 0.01 and Na/Li = 0.18 for 200°C and 100 MPa.With these conditions, a solution (taken to be ideal) with2 molal total alkalies in equilibrium with the above as-

FIG. 9. Interpretation of the sequence of crystallization. Thetime of eucryptite growth cannot be strictly defined, but itseems more or less contemporaneous with secondary tur-bid K-feldspar. Montebrasite is too scarce to be located inthe sequence, but is presumed to be late magmatic. Sph:sphalerite.

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semblage of minerals is predicted to have roughly thefollowing distribution of concentrations: 1.68 molal Li,0.30 molal Na and 0.02 molal K, also far from the ex-perimental results of Lagache & Sebastian (1991). Suchan assessment confirms the disequilibrium likely in thesystem, with Li going into solution at the expense of thecouple spodumene–petalite, whereas Na and K werecontinuously extracted from the solution to contributeto new feldspars. Fluids during such metasomatic pro-cesses are always out of equilibrium with their environ-ment (London 1990); their Li content will increasedramatically with decreasing temperature (Wood &Williams-Jones 1993), and such fluids will be expectedto invade the pegmatite envelope.

P–T conditions

Petalite and spodumene are antithetic in terms of Pand T: the appearance of one in place of the other is nota function of composition of the medium, but mainlydepends on externally imposed P–T conditions, withpetalite as the high-T, low-P equivalent of spodumenein Li-rich pegmatites (Stewart 1978). Stability relationsamong the lithium aluminosilicates for quartz-saturatedbulk compositions are shown in the Li–Al silicate gridproposed by London (1984), with the moderate- to low-T portion of the system directly applicable to the petro-genesis of Li-rich pegmatites (Fig. 10). The shallow P–Tslope of the univariant reaction Pet → Spd signifies thatpetalite–spodumene relations are more dependent on Pthan on T. Spodumene will be stable only at P above170 MPa (the Qtz + Pet + Spd + Ecr invariant assem-blage), which precludes its appearance in shallow, low-P environments. At magmatic temperatures of 500 to700°C, pegmatites in which spodumene is the primaryLi-aluminosilicate are constrained to crystallizationpressures in the range of 300 to 400 MPa (London

1990). Under equilibrium conditions, petalite will bestable between 550–680°C at 200 MPa and 635–680°Cat 400 MPa. Below both minimum temperatures, petalitewill break down to the fine-grained Spd + Qtzintergrowth (squi). On the other hand, the stability fieldof Ecr + Qtz is limited to low P and T, below about320°C and 160 MPa (London 1984), and most of theeucryptite replacement after petalite (or spodumene)would be expected to occur during uplift at 100–200°C(Chakoumakos & Lumpkin 1990). Therefore, the min-eralogical evolution developed in our Portuguese aplite–pegmatite bodies suggests that primary crystallizationand subsequent retrograde alteration span a broad P–Tinterval, with retrograde alteration operating down tolow-P and low-T conditions (Fig. 10).

Fluid inclusions provide a means of evaluating mini-mum P–T conditions and fluid compositions duringconsolidation of pegmatites in recording the history oflate-magmatic fluids and their subsequent evolution.This is a significant focus of research for understandingpegmatite genesis. Results from the literature are strik-ingly variable, covering a wide range of P–T conditionsand compositions: from 260 to 350 MPa and from 420to 675°C (London 1985, Whitworth & Rankin 1989,Chakoumakos & Lumpkin 1990, Fuertes-Fuente &Martin-Izard 1998). A fluid-inclusion study (Doria etal. 1989) was performed on a few spodumene-bearingpegmatites from the CDB pegmatite belt; maximumconditions of crystallization of spodumene were con-strained to 250–300 MPa and 475–530°C. Fluid inclu-sions are very common in spodumene of the threeoccurrences studied, and are apparently primary, with acommon distribution parallel to the long axis of the hostmineral. They are mainly complex three-phase and CO2-bearing, with variable CO2:H2O ratios at room tempera-ture. Daughter minerals (carbonates) may be present(Lima 2000). The persistent occurrence of CO2 in flu-

FIG. 10. Log activity (Na or K/Li) versus temperature at 100 MPa; activities can be computed from the equilibrium albite/(K-feldspar) – eucryptite – quartz at 200°C (redrawn from Wood & Williams-Jones 1993).

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ids in many complex granitic pegmatites worldwidedemonstrates that CO2 plays a determining role in theirevolution (see literature above).

Temperature and pressure experienced during thepeak of metamorphism of the host rocks throughout theprovince (500°C, 300 to 350 MPa: 11 to 13 km depth,Guedes et al. 1997) provide an upper boundary for thepressure of emplacement and crystallization of the peg-matite-forming liquids. Muscovite can be stable belowH2O saturation of the melt; its occurrence does not pre-clude any H2O saturation through the exsolution of anaqueous fluid. Consequently, the main part of the aplite–pegmatite crystallizes in conditions of H2Oundersaturation, as recently demonstrated by London etal. (1989). As discussed above, fluid saturation wasreached once the main part of the pegmatite body hadalready crystallized. One may confidently infer that themain part of the exsolved aqueous fluid remained in thesystem. Pegmatite-induced alteration is not prevalent inthe micaceous schists around the bodies studied, andthere is apparently no mineralogical evidence of anymetasomatic alteration. Nevertheless, there is a signifi-cant chemical interaction between the proximal hostrocks and fluids derived from the pegmatites, becauseLi content (up to 1000 ppm Li, Lima et al. 1997) is threeto five times the background in adjacent schists. Fine-grained intergranular eucryptite (as revealed in a cur-sory UV investigation) could be responsible for thisexomorphic halo.

K-feldspar phenocrysts are well-ordered (low) mi-crocline. As described earlier, such feldspar is not cross-hatched, and exsolution lamellae of albite are rare orabsent in thin section. An X-ray powder-diffraction in-vestigation conducted on a dozen of alkali feldspar phe-nocrysts (3 to 5% mole Ab, reflecting quite a lowtemperature of final equilibration) shows that all arefully ordered microcline (obliquity ≈ 0.85 to 0.90; com-puted t1o + t1m = 1.00). The sodic plagioclase that co-exists with the microcline is close to the Ab end-member(An0–3 by electron-microprobe analysis). These nearlypure end-members (Table 2) are in equilibrium and con-sidered to have equilibrated at a subsolidus temperatureby fluid-mediated recrystallization. Twinning was oblit-erated at a quite low temperature, below the tempera-ture of the orthoclase–microcline transition ( Černý &Macek 1972).

A fairly well constrained P–T path of crystallizationspanning from magmatic to subsolidus conditions forthese pegmatite bodies is shown in Figure 11.

The pegmatite field as a whole

Geological mapping of pegmatite belts worldwidehas, at least statistically, proven the existence of a re-gional compositional zoning, and the rare-elementpegmatites outcrop farthest from the supposed parentalgranites ( Černý 1991b, 1992, Norton & Redden 1990,Shearer et al 1992, Fuertes-Fuente & Martin-Izard

1998). Li-rich pegmatites are certainly more numerousthan known examples indicate in this Hercynian part ofPortugal. Other occurrences are already known west-ward (Gomes & Nunes 1990), but with quite differentphase relations: petalite is the early phase, and is af-fected by a subsequent “squi” breakdown, which signi-fies a different evolution in the pressure regime. Theapparent dispersion of the already recognized Li-bear-ing pegmatite dykes in the CDB district (Fig. 1) doesnot provide convincing field evidence of any geneticrelationship with the nearby, apparently more favorable,two-mica granites. The numerous pegmatitic differenti-ates within these two-mica granites (interior pegmatites)are Li-free (Pegm* in Table 1), as routinely documentedin most examples of pegmatite populations worldwide.However, there is a very different pattern among themineralized pegmatite bodies, according to their loca-

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tion within the CDB pegmatite belt. The western ones(CHN bodies) are free of petalite and alteration to feld-spars (Charoy et al. 1992), whereas the eastern ones (thisstudy) exibit a more complex evolution: petalite ispresent, and there is widespread alteration to feldspars.Such a contrasted behavior suggests some direct linkwith the intrusion of post-tectonic (post-pegmatite) bi-otite granites to the East. A drop in pressure (spodumeneto petalite) may be related to the system of NNE–SSW(Régua–Verin, inset in Fig. 1) sinistral faults developedclose to the eastern part of the belt (Guedes et al. 1997).This fault system permitted the ascent of the magma thatformed the biotite granite, with the consequent overprint

on the nearby older occurrences of pegmatite. It is notthe case in the western part of the belt.

CONCLUSIONS

Aplite–pegmatite dykes of granitic composition arenumerous in this Iberian part of the Hercynian orogeny,but only a very small percentage of them is Li-rich, withspodumene and, more rarely, petalite as primary phases.For the first time, the full sequence of the three Li-alumi-nosilicates spodumene, petalite and eucryptite, and theirrelative chronological relationships, are described indetail for some aplite–pegmatite dykes from northern

FIG. 11. Inferred pressure–temperature path (arrow) for the magmatic–hydrothermal crystallization of the aplite–pegmatitedykes. Features in the diagram: 1 the triple point for the aluminosilicates (Holdaway 1971), 2 the Li–Al silicate grid (London1984), 3 the liquidus to solidus interval for the Harding pegmatite (Jahns 1982), 4 the maximum conditions for the crystalli-zation of spodumene in the CDB pegmatite belt (Doria et al. 1989). The stippled area shows the probable P–T range forcrystallization of the spodumene + quartz assemblage in Li-enriched granitic pegmatites (London & Burt 1982b). Thehatchured area shows the peak conditions of regional metamorphism.

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Portugal. The magmatic paragenesis in the pegmatiticportions is assumed to be albite + K-feldspar + quartz(minor) + muscovite (accessory) + spodumene (with ahighly heterogeneous distribution), in contrast with thequite minor aplitic matrix, where albite is largely domi-nant and spodumene absent. Petalite follows spodumeneas a result of a sudden drop in confining pressure (fromlithostatic to hydrostatic?), probably contemporaneouslywith some Hercynian faulting and associated uplift ofthe chain, which generates a separate fluid phase. Itseems likely that petalite precipitated directly from thatfluid. Miarolitic cavities (as in Veral) constitute evi-dence of fluid saturation prior to the complete crystalli-zation of the pegmatite-forming melt. The hydrother-mal history of the pegmatite body began with thewidespread but variable metasomatic replacement of theformer Li-aluminosilicates, marginally or along cracks,mostly by albite, muscovite and K-feldspar, and by lateeucryptite selectively at the expense of petalite, togetherwith secondary phosphates. During this superimposedhydrothermal alteration, Li is progressively leached outof the system and dispersed into the surrounding metase-dimentary units. The P–T path suggested for the evolu-tion of these Portuguese Li-rich aplite–pegmatite bodiesseems to be atypical with regard to most of the descrip-tions in the literature, because spodumene remains stable(or metastable) against petalite, contrary to what is gen-erally encountered elsewhere.

ACKNOWLEDGEMENTS

P. Černý, R.F. Martin and an anonymous reviewerprovided constructive and helpful comments. It is a realpleasure to acknowledge the guidance they gave us.

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Received December 21, 1998, revised manuscript acceptedMay 7, 2001.

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spodumene – petalite – eucryptite: mutual ...pegmatites minéralisées avec les granites à deux...

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