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American Mineralogist, Volume 71, pages 396405, 1986

Formation of -rich gem pocketsin miarolitic

D.Lvm LoNoox School of Geology and Geophysics,University of Oklahoma, Norman, Oklahoma 73019

AssrRAcr Analysis of fluid inclusions in ,, tourmaline, and from miarolitic pegmatitesofAfghanistan, coupledwith aluminosilicate stability relations and with previous studiesfrom gem pegmatitesof SanDiego County, , indicate that pocket development in tourmaline-rich, miarolitic rare-element pegmatites occurs between ap- proximately 475 and, 425C and between 2800 and 2400 bars. This range of P and ?r is comparableto the conditions of late-stagecrystallization in geochemicallysimilar massive (nonmiarolitic) pegmatites.Whether gem pockets form may be dependentlargely on the timing of tourmaline crystallization. Formation of tourmaline removes an alkali borate component from residual pegmatitic melt, with the consequentdeposition of other alkali aluminosilicate and -forming and exsolution of copius amounts of HrO. If tourmaline crystallization is inhibited until the late stagesof consolidation, the large quantities of HrO that are liberated may form pegmatitic pockets.

IxrnonucrroN Gpor,ocy Much of the present understandingof pegmatite evo- The geology of miarolitic rare-element pegmatites of lution is basedon observationsfrom subhorizontal, lay- this type is suftciently similar worldwide that the typical ered,gem-bearing miarolitic rare-elementpegmatites (e.g., mineralogical and zonal characteristicscan be summa- Jahnsand Tuttle, I 963; Jahnsand Burnham, I 969; Jahns, rized in the diagram that servesas the logo for this issue 1979,1982).Pegmatitesofthistypearetypicallyenriched (Jahnsand Burnham, 1969; Ford, 1976, 1977)(Fig. 1). in rare alkalis (Li, Rb, Cs); in Be, Sn, and Ta; and notably The pegmatites generally display a layered rather than in tourmaline. They are chemically similar to more mas- concentric zonation, with predominantly banded albitic sive rare-element pegmatites such as the Tanco, Mani- on the footwalls, coarse microcline-quartz pe1- toba, and Harding, New , deposits.The best-stud- matite on the hangingwalls, and interior units that consist ied miarolitic rare-elementpegmatites are located in San principally of coarse-grainedcleavelandite, tourmaline, Diego County, California (e.g.,Jahns and Wright, l95l; microcline, qvartz, and fine-grained . More exotic Jahnsand Tuttle, 1963;Foord, 1976, 1977;Jahns, 1979; accessoriessuch as beryl, spodumene,Nb-Ta-Sn-(+3i; Taylor et al., I 979), but thesedeposits are typical of mia- , and phosphatesare concentratedin the quartz- or rolitic pegmatiteselsewhere that are lesswell known (e.g., cleavelandite-rich interior zones. Trace-element distri- in the Hindu Kush of and :Rossov- butions (especiallyof Li, Rb, and Cs) are generally con- skii, 1977, l98l; Rossovskiiand Shmakin, 1978; Ros- sistentwith a model of inward sequentialcrystallization, sovskii et al., 1975, 1978). In this paper, the P-T condi- although Jahns (1982) recognizedthat some mineralogi- tions and fluid evolution of rare-element miarolitic cally distinct internal units may crystallizesimultaneously pegmatitesare evaluatedin the light of recent fluid-inclu- (cf. Walker, 1985; Jollif et al., 1986). sion studiesand phase-equilibrium experiments,because Most miarolitic gem-bearingpegmatites contain spodu- deposits of this type have had such a profound effect on menein assemblagewith quartz in massive,primary units models for internal evolution of pegmatitesystems (Jahns and as pocket constituents (e.g., the Afghanistan Hindu and Burnham, 1969;Burnham and Nekvasil, 1986).This Kush: Rossovskii, 198l; Rossovskii et al., 1975; San Die- investigation of miarolitic pegmatites draws heavily on go County, California: Jahns, 1949; the Rumford-Newry two previous studies:experimental calibration of the lith- area, : Cameron et al., 1954; and , iumaluminosilicatephasediagram(London,1984a),and : Cassedanneand Cassedanne,l98l). This obser- a study of fluid inclusions in the Tanco pegmatite, Man- vation constrains the P-T conditions of primary - itoba (London, 1986).These works provide the frame- lizationandpocketformationinmiaroliticpegmatitesto work by which previous investigations of miarolitic peg- lie within the stability of the assemblagespodumene matites can be reevaluated,and they serve as the basis + quartz (London, 1984a).The spodumenewithin these for analysisof new data from miarolitic pegmatitesin the pegmatitesis nearly pure LiAlSirOu (e.g., seeanalysis in Hindu Kush of Afghanistan. Table I of London, 1984a, and analysesof spodumene 0003{04x/86/0304{396$02.00 396 LONDON: TOURMALINE-RICH GEM POCKETS 397 from other massiveand miarolitic pegmatitesin Table 5 Tr/ oflondon and Burt, 1982), so that the stability field for spodumeneis not displaced to higher temperaturesand r;t"' lower pressuresby extensivesolid solution (cf. Appleman and Stewart, 1968; London, 1984a).The experimental work on which the lithium aluminosilicate phasediagram (London, 1984a)is basedutilized natural pocket spodu- mene from the Hindu Kush pegmatites(London, 1984a, Table l); thus, this diagram is directly applicable to nat- equilibria. Note also that the applicability of the lithium aluminosilicate phasediagram dependsonly on the presenceof a lithium aluminosilicate phase, not on its abundance.

Kur,.llu PEGMATTTEs,ArcHnNrsr.lN Miarolitic gem pegmatitesthat contain abundant spo- dumene, beryl, and tourmaline lie in the Hindu Kush of easternAfghanistan (Fig. 2), and they aregeologically sim- ilar to miarolitic pegmatiteselsewhere (Rossovskii, 198l; Rossovskiiet al.,1975,1978). The Afghanistanmiarolitic pegmatitesoccur mostly in the roof zonesof biotite-mus- covite .The peak metamorphic grade attained by surrounding metasedimentary rocks was medium-pres- sure amphibolite grade, as shown by -staurolite- -andalusiteassemblages in metapelites(Rossov- POCKET skii et al., 1978). ZONE Rossovskii et al. (1978) reported the following para- genesisof pocket minerals. Two generations of spodu- menewere noted: (l) coarse-grained(primary) spodumene embeddedin massive qvarlz, microcline, and cleavelan- dite and (2) kunzite (gem spodumene)in miarolitic pock- ets.Primary spodumeneand microcline servedas the sub- stratefor deposition of kunzite. Quartz, beryl, tourmaline, and other accessoryphases encrust kunzite and LAYERED were the latest-formed phases.Samples of spodumene, AND beryl, tourmaline, and quartz from the Kulam deposit NON.LAYERED were obtained from collections of the Smithsonian Insti- APLITIC tution, the American Museum of Natural History, and ZONE from private mineral dealersfor examination of fluid in- clusion properties and for comparison with the reported resultsof fluid- studiesfrom the San Diego peg- matites.Rossovskii (1977,l98l; Rossovskiiet al., 1978) presenteda partial analysis of fluid inclusions from this pegmatite field.

Properties of fluid inclusions Fluid inclusions are present in all phasesstudied and possessseveral uniform similarities and differences.In all samples,the abundanceoffluid inclusions decreasesfrom Scale: 1 cm on the ftgure equals about 8 cm M =microcline crystal base to tip. Solid inclusions with no associated fluids are abundant at the bases and outer margins of crystals. Equation (2) ofPotter et al. (l 978) was usedto calculate Fig. l. (A) Tourmaline-rich pocket from the Himalaya dike, salinities fluids. of aqueous of homogeneous SanDiego County, California. Dark prisms within the pocket are aqueousfluids were calculatedfrom equation (Al) of Bod- crystals of . The scalebar is l0 cm. (B) Schematiccross nar (1983). Isochoreswere constructedby comparison to section through the Himalaya dike, San Diego County, Califor- pertinent graphical data in Fisher (1976) and in Roedder nia. Both figuresreprinted from Foord (1977)by permission of and Bodnar (1980). the author. 398 LONDON: TOURMALINE-RICH GEM POCKETS

Fig. 2. Locationof the miarolitic pegmatites(star) of Af- ghanistan.The pegmatitesare located near Nuristan in the Lagh- manprovince; individual pegmatite groups are referred to by the locationnames Kulam, Mawi, Nilaw, and Kurgal.

Microthermometry was performed on a Linkam TH 600 programmableheating and freezingstage at the Uni- versity of Oklahoma. The Linkam stagewas calibrated with respectto melting points of 25 standard or spectro- scopic-gradecompounds overthe range-57"C to +307t. Dissolution of aluminosilicate minerals within inclusions was accomplishedby sealingpolished chips in Pt foil for runs at P: 2 kbar (to prevent decrepitationofinclusions). The progressof dissolution was monitored by examina- tion of the proportions of crystals to glassupon quench. Fig.3. Histogramsof microthermometricdata for inclusions (white (stippled quartz For daughter-mineralassemblages, minimum entrapment in spodumene bars),beryl bars), Olack- bars),and tourmaline(ruled bars)from the Kulam pegrnatites, temperaturesare defined by the run temperatures from Afghanistan.(A) Freezing-pointdepression ofaqueous tiquid (T- which quenchedproducts glassplus include only aqueous ice;bar graphssuperimposed, not stacked);(B) temperaturesof fluids. liquid-vaporhomogenization (4 LD for crystal-richinclusions Most fluid inclusions contain low-salinity aqueousfluids in spodumene,and (C) temperaturesof liquid-vaporhomoge- and no other detectablevolatile components (e.g.,COr). nization(f" LV) for crystal-richinclusions in beryl. On the basis of freezing-point depression (Z* ice), the salinites of the aqueous component in most inclusions virtually all inclusions (Figs. 4D, 4J). Some of the non- rangefrom 0.9 to 7.9 wto/o(mean : 3.7) NaCl equivalent silicatephases dissolve prior to decrepitationat 275-350{. (Fig. 3A). The low salinities preclude an accurate mea- The aluminosilicate minerals show no evidence of dis- surementofeutectics (first appearanceofliquid upon heat- solution up to decrepitation. Dissolution of aluminosili- ing a frozen inclusion). Samplesof spodumene,beryl, and cate minerals (as monitored by quenched runs from 2 quartz contain two or more generationsof fluid inclusions kbar; seeLondon, 1986, for methodology) begins at ap- (Fig. a). Most inclusionsin spodumeneand beryl are crys- proximately 375'C and is essentially complete in most tal-rich, whereasinclusions in quartz and tourmaline con- inclusions by 500'C. Thesecrystal-rich inclusions afe cut tain no solid phases. and modified by later generationsof inclusions with few Spodumeneand beryl. In spodumene and beryl, large to no crystalline solids but similar aqueous-fluidsalinities inclusions with slightly irregular negativecrystal forms of (Fie. 3). the host phasesare particularly crystal-rich (Fig. 4). Elec- As a whole, temperatures of liquid-vapor homogeni- tron energy-dispersive(EDS) analysesof opened inclu- zation show considerable scatter, but consistent trends sions reveal a fairly consistent assemblageof quartz, al- emergewhen inclusion populations are sortedon the basis bite, cookeite, and , ,and or of crosscuttingrelationships. The crystal-rich inclusions rynersonite (Ca-Ta oxide). In addition, most inclusions in spodumeneand beryl generallypossess the highest av- contain other nonsilicate minerals that are not yet fully eragevalues and narrowest spread of Zn LV (temperature identified. One such prismatic phase shows only phos- of liquid-vapor homogenization; Figs. 3A, 3B). In a few phorus by EDS and, hence, appears to be a lithium or spodumenecrystals, I LV and the crystaVfluid ratio of phosphateor crystalline phosphoric acid (Fig. inclusionsdecrease from crystalbase to tip. In spodumene 4). An unidentified hairlike acicular phase is present in and beryl, the younger crystal-poor or crystal-absentin- LONDON: TOURMALINE-RICH GEM POCKETS 399

d!$

{) B

t.f &,J

to

o c) 'b. oe ! t?,ff ol#n o e o aj oo. ql -.1 G

Fig. 4. Fluid inclusions in spodumene(A4), beryl (DG), and quartz (H-I) from the Kulam pegmatite,Afghanistan. In spodu- mene, inclusions at the basesof some crystals contain a higher proportion of daughter minerals (A) than inclusions dt the crystal tip (B); in A, the inclusion shapehas changedslightly from a rounded surfaceagainst which daughter minerals were precipitated to a more regularnegative-crystal form (arrow). Daughter minerals in the srv image (C) and in the unopenedinclusions are (a) cookeite, (b) pollucite, (c) , (d) quartz, (e) a birefringent light-element (Li, Be, or H) phosphate,(f) an unidentified hairlike phase,and G, h) cassiteriteand microlite or rynersonite. Most crystal-rich inclusions in spodumenecontain both Sn- and Ta-oxides, but the identity of the individual grains cannot be ascertainedwithin unopenedinclusions. The daughter-mineralassemblage in inclusions near the tips of spodumenecrystals (B) is comparable to that of crystal-rich inclusions in beryl (DE). In E, the (001) plane that contains the large, crystal-rich inclusion is cut by a that contains crystal-poor or crystal-absentinclusions such as those in F and G; note the variability of liquid/vapor ratios (at 25'C) of inclusions in F. Most inclusions in quartz have irregular forms and variable liquid/vapor ratios at 25'C (H); rarely, healedfractures contain two-phaseaqueous inclusions with regular negative-crystal forms, comparatively high salinity, and uniform liquid/vapor ratios (I). Dense clusters of low-salinity inclusions (H) are abundant toward the basesofquartz crystals,whereas the high-salinity inclusions along obvious fractures(I) occur only toward the tips. Scale bar in all photos is approximately 30 pm. clusions display extremely variable I LV (ranging from (15 wto/oNaCl equivalent) and a low but nalrow range of inclusions that contain no vapor bubble at 25C to those i", LV : 217 + | (s.d.)'C(n: 42) (Figs.3, 4L). in the samecluster that decrepitateat temperaturesup to 400"C prior to 7i LV) (Fig. aD. Interpretation of inclusion data Tourmaline and quartz. In contrast to spodumeneand The overall uniformity of the crystalline assemblage beryl, inclusions in tourmaline and quartz generallycon- within inclusionshosted by spodumeneand beryl suggests tain no crystalline phasesand have irregular shapesand that most of these phasesare daughter minerals rather variable size (e.g., Fig. aK). Fluid compositions and ap- than accidentallyentrapped solids. Phase assemblages and parent salinities are similar to those in spodumene and crystal-fluid ratios do vary slightly between inclusions beryl; however, like the later-stageinclusions in spodu- within a given cluster (e.g.,Figs. 2A, D). Necking down mene and especiallyberyl, the liquid-vapor ratios of in- of largeinclusions during or after deposition of crystalline clusionsin tourmaline and quartz are highly variable (Fig. solids may explain the variations. Evidence of posten- 4K) and precludeany meaningful determination of ?"nLV. trapment modification is manifest by inclusions such as Some samplesof quartz contain healed fractures with large, the one shown in Fig. 48; the rounded edge of included euhedral inclusions that possessunusually high salinity solids illustrates that the original shape of the inclusion 400 LONDON: TOURMALINE-RICHGEM POCKETS

consistentwith the observationof Rossovskii et al. (1978) that beryl encrustskunzite crystalsand henceformed later than the spodumene.The crystallization of spodumene early in pocket development(Rossovskii et al., 1978)may explain why inclusions in this phasecontain a higher pro- portion of daughter minerals. There is no textural evi- dence that any of the simple liquid-vapor inclusions in any phaseare primary. As pointed out by Roedder(1981), however, the fact that pocket minerals are euhedral, smooth-faced,and project into voids implies that the final medium from which such crystals grew must have been a fairly aluminosilicate-poor fluid. Although the simple aqueous inclusions are extensively modified, the fluids contained by them probably are indigenous to the peg- matite. Their compositions are distinctly different from the saline, COr-rich inclusions that are typical of most // metamorphic rocks (e.g.,Hollister and Bumrss, 1976; Touret, l98l). The changesin daughter-mineral assemblagesin se- Tr'G quential populationsofinclusions in spodumeneand beryl in Fig. 5. Estimatedconditions ofpocket formation in miarolitic may be used to evaluate the crystallization sequence pegmatitesof Kulam, Afghanistan (arrow), based on isochores the Kulam pegmatitepockets. From the sequenceof crys- (4 LV, partial homogenization)for spodumeneand beryl con- tal-rich to crystal-poorinclusions in spodumene,the phas- structed from microthermometric data in Fig. 3. The isochore es quartz, albite, microlite/rynersonite, and cassiteriteap- for inclusions in quartz is derived from the microthermometric pear to have crystallized first, followed by pollucite and properties ofthe inclusions shown in Figure 4I. Phaseabbrevi- finally cookeitewith severalother nonsilicateminerals (of ationsare @cr), spodumene (Spd), (Pet), quartz which one is a light-element phosphate).Crystal-rich in- (Qtz), low-salinity (5 wto/oNaCl) aqueousliquid (L) plus vapor generally little or no quartz, al- (v). clusions in beryl contain bite, and oxides. This appearsto confirm the observation of Rossovskii et al. (1978) that beryl crystallized after spodumenebut with some temporal overlap. Rossovskii has changedto a more regular negative-crystalform. The et al. (1978) did not identify light-elementphosphates and variations in daughter-mineralassemblages, however, are other comparatively soluble nonsilicatesfrom the Kulam greaterbetween samples than within a singlecrystal. One pocket assemblages.Roedder (198 l), however, has noted possibleexplanation for thesedifferences is that individ- that solublephases that may have onceoccupied miarolit- ual crystals may have come from different pockets. Iso- ic pocketsmay have beendissolved by circulation ofcon- lation of residual pegmatitic magma in a compositionally nate or meteoric water after pocket formation. The de- heterogeneousand largely crystallized pegmatite could tailed mineralogical investigation by Foord et al. (1986) to significant differencesin pocket zone composition and identified borates and carbonateswithin pockets of the . Rossovskii et al. (1978) reported that the San Diego pegmatites. mineralogy is highly variable from pocket to pocket. Al- A phase that optically and morphologically resembles ternatively, the variability of pocket-mineral assemblages LirB4OT(see London, 1986) is present in some crystal- could be interpreted to reflect the segregatedgrowth of rich inclusions in the Kulam pocket minerals, but such a different mineral phasesin portions ofinterconnected re- light-elementphase has not been observedin the serraand sidual pegmatitic fluid. eosanalysis ofopened inclusions. It is doubtful that LirBoO' The crystal-rich inclusions are distinctly different from would be presentin the crystal-rich inclusions in the Ku- accidentallytrapped solids. For example, fringes of tour- lam spodumenefrom the pocket zones,because the fluids maline needlesthat surround the margins of some beryl in thesepockets are equilibrated with tourmaline, whereas crystals are randomly oriented on the former beryl sur- those in the spodumenefrom Tanco were not (London, facesand contain no associatedfluids. l 986). The crystal-rich aqueousinclusions in spodumeneand Isochoresconstructed from microthermometric prop- beryl are regardedas modified but possibly primary in- erties (partial homogenization) of the crystal-rich inclu- clusions that reflect entrapment of an alkali aluminosili- sionsin spodumeneproject into the stability field of spod- cate-rich fluid. These inclusions are the earliest set that umene + qltartz (Fig. 5). Theseinclusions, which appear was entrapped, becausethey are crosscut by planes of to be the earliest trapped, may reflect the conditions at crystal-poor or simple liquid-vapor inclusions. Isochores the onset of pocket formation. From the experimental for crystal-rich inclusions in beryl (4LV, partial homog- homogenization of aluminosilicate daughter minerals enization) reflect lower temperaturesof entrapment than within spodumene-hostedinclusions, pocket formation similar inclusions in spodumene(Figs. 4 and 5), which is may have begun at temperaturesof approximately 475- LONDON: TOURMALINE-RICH GEM POCKETS 401

500'C. At 475oC,the pressureswithin pocketswould have in behavior is not known. Spodumene(and phasessuch been approximately 2600 bars (within the stability field as beryl and ) may be more refractory or insoluble ofspodumene + quartz; Fig. 5). From 475'C and 2600 than quartz, and hence preserve inclusions trapped at bars, the successivepopulations of inclusions in spodu- higher P and 7.. There is no evidence from any of the mene and beryl define an interval from approximately pegmatitesstudied that quartz ever entrappedthe silicate- 4'l5 to 340"c over which pocketswere generated.On the rich fluids that are representedby inclusions in spodu- basis of inclusions in quartz, tourmaline, and beryl, the mene and beryl from the Kulam pegmatites (i.e., solid aluminosilicate componentsof the primary pocket-form- inclusions that may once have beendaughter minerals are ing fluids were depletedby 35G-400'C. The successionof not found in quartz). inclusions from spodumene to beryl to quartz and the The prevalenceofliquid-vapor inclusionsin tourmaline crosscuttingrelations of secondaryinclusions in spodu- may reflect a secondaryorigin, as in quartz, or may be mene and beryl record a continuous changefrom silicate- related to the local generationofan aqueousfluid during rich to silicate-poor aqueousfluid during pocket forma- the formation of tourmaline. Individual tourmaline crys- tion. The limited microthermometric data for secondary, tals may act as centersfor production ofan aqueous-fluid solute-poor liquid-vapor inclusions in quartz (Fig. 3) are phasebecause of the large quantities of HrO liberated by suggestiveof extensivefracturing and rehealingof pocket tourmaline-forming reactions. Development of an aqueous minerals at clearly subsolidusconditions. boundary fluid would most likely occur along the surfaces The proposedP- Zinterval for formation of pocket min- perpendicularto [000 I ], the zone offastest growth in tour- eralsin the Kulam, Afghanistan,pegmatites may be com- maline. As in the San Diego pegmatites (Foord, 1976), pared to the conditions of late-stagecrystallization in the modified fluid inclusions in the Afghanistan tourmaline Li-rich Tanco pegmatite, Manitoba (London, 1986). At lie mostly in planes perpendicularto [0001]. Tanco, a hydrous borosilicate fluid evolved through crys- tallization to a comparatively low-, solute-poor aqueousliquid over the rangesof 470 to 420C and 2700 S,lN DrBco CouNrv eEGMATTTES,C.q,llronNra to 2600 bars. The crystalline products of this fluid in- Taylor et al. (1979) combined studiesof fluid inclusions cluded spodumene,quartz, albite, Li-micas, tourmaline, and -, -, and carbon-isotopesystematics beryl, pollucite, and Ta-Sn oxides. This is essentiallythe to deduce the conditions of formation for three of the samemineralogical assemblage that is reported from mia- best-known gem-bearing pegmatites of the San Diego rolitic pockets in the Kulam pegmatites and elsewhere County field, the Himalaya, the Tourmaline Queen, and (e.g.,Jahns and Wright, I 95 I ; Cameronet al., I 954; Foord, the Stewartdikes (Foord, 1976; T aylor et al., I 979). From 1976,1977:,Jahns, 1979, 1982; Cassedanne and Casse- the isotope systematics,they concluded that the pegma- danne, l98l). It appearsthat (l) the bulk compositions titic magmaswere emplaced at temperaturesof 70G730{, and liquid lines of descentof late-stagefluids in the mas- with pocket formation at subsolidus conditions in the sive Tanco pegmatite and the miarolitic Kulam pegma- rangeof 525-565"C.An estimateof emplacementpressure tites were similar; (2)the P-T conditions over which these was calculatedby determination of the pressurerequired fluids crystallized and evolved an aqueous-fluid phasewere to correct a liquid-vapor homogenization temperaturein virtually identical; and (3) the changesin crystal-fluid aqueousfluid inclusions to the temperatureof pocket for- phasesover this interval mark the transition from mag- mation as definedby the isotope systematics(see Roedder matic to hydrothermal conditions in massive and mia- and Bodnar, 1980, for details of methodology). For this rolitic pegmatiteswith similar Li- and B-rich bulk com- purpose,Taylor et al. (1979) choseto basethe pressure positions. correction on the microthermometric propertiesof a fluid Some of the most important observations from this inclusion with the highest liquid-vapor homogenization study pertain to the properties and interpretation offluid temperature (340"C from an inclusion in a - inclusionsin pegmatiteminerals. The examination of fluid garnet).On this basis, the pressureat crystal- inclusions in pocket minerals from the Kulam pegmatites Iization was estimated at 210C-2200 bars. The bulk of facilitates an important comparison with observations inclusionsin pocketquartz, tourmaline, beryl, , and from the Tanco pegmatite (London, 1986). In both de- , however, possessedlower averageliquid- posits, inclusions in spodumeneare crystal rich and pos- vaporhomogenizationtemperatures ([ LV) sf 285-295C. sibly primary, whereas those in quartz arc crystal poor The abundantcrystalline contents ofthe inclusions (which and are secondary.This disparity in the compositions of were regarded as daughter minerals) were not homoge- inclusionsin spodumeneversus quartz appearsto be char- nized, and their contribution to fluid chemistry was not acteristic of other rare-element pegmatites, such as the considered. Bikitadeposit, @. London, unpub. data, 1985). In Figure 6, it is apparentthat the previously estimated These observations reinforce the contention that peg- P-I conditions for the San Diego pegmatiteslie well in matitic quartz generally does not trap primary fluid in- the petalite + quarlz field, approximately 150"Cor 1500 clusions, and that the examination of inclusions hosted bars beyond the limits of the spodumene * quartz field. by quartz only may lead to an elToneousinterpretation Severalof the San Diego pegmatites,including the Hima- of fluid compositions and P-T conditions of pegmatite laya, Stewart,and six other dikes, contain spodumene * formation (London, 1985).The reasonfor this difference quartz, but no petalite (Jahns, 1979). Either the isotopic 402 LONDON: TOURMALINE-RICH GEM POCKETS

P,kbar extensivein pocket zones.In their calculation ofisotopic equilibrium temperatures,Taylor et al. (1979) omitted a P-T estimates number of points from the linear regressionof the data, San Diego pegmatites partly on the basis that thesevalues gave temperaturesof equilibration that were regardedas "unreasonably" low for pocket formation (e.g., Taylor et al., 1979, Fig. 68). Ifthe temperaturesof formation of tourmaline-rich pock- etsare reviseddownward to 425-47 5'C, then correspond- ingly, pressuresin the range of 2400-2800 bars would satisfy the constraints of the lithium aluminosilicate as- semblage,the bulk of Foord's (1976) fluid-inclusion data, and the approximate pressures of regional metamor- phism. The lower temperaturesfor primary pocket for- mation also provide continuity with the conditions of ensuing hydrothermal alteration, estimated by Foord et al. (1986)to have spannedthe interval of 150-400'C.

Fig. 6. Pressure-temperatureestimates for the San Diego DrscussroN Countypegmatites, California. The estimatedP-T conditionsof P-? conditions of pocket forrnation Tayloret al.,(1979) are represented by the box. A revisedesti- Kulam mate (arrow)is basedon isochores(dashed lines) constructed The available fluid inclusion data from the and from microthermomeatricdata in Foord (1976)and Taylor et San Diego pegmatites,coupled with the ubiquitous as- al. (1979)and is consistentwith lithium aluminosilicateassem- semblagespodumene + qvartz, constrain the conditions blagesin thesepegmatites-as constrained by the lithium alu- of pocket formation to the approximate ranges of 425- minosilicatephase diagram of London (1984a;solid reaction 475C and 2400-2800 bars, which is consistent with all lines)-andwith P-Testimatesof peakregional metamorphism. types and sourcesofdata exceptthe isotopic temperature Phaseabbreviations are eucryptite (Ecr), spodumene (Spd), petal- reportedby Taylor et al. (1979).This P-Zrange for late- ite (Pet),quartz (Qtz), low-salinity (5 wto/oNaCl) aqueous liquid stageprimary crystallization is comparable to that of the (L) plusvapor (Y). Tanco pegmatite,Manitoba (London, 1983, 1986) and signifiesthat compositionally similar miarolitic and mas- sive rare-elementpegmatites may crystallizeunder similar temperature or the interpretation of the fluid-inclusion P-I conditions. The development of miarolitic pockets data (or both) appearsto be in error. From the reported in pegmatitesis contingenton factors other than P-Icon- salinity of 4-5 wt0/0NaCl equivalent (Foord, 1976;Taylor ditions of emplacement. et al., 1979) and the average T'LV : 290"C for most pocket minerals, isochoresfor the aqueouscomponent of Causesof pocket formation fluid inclusions from the San Diego pegmatitesintersect Miarolitic pockets provide incontrovertible evidence the isotopic equilibration temperature of 525-560'C at for the exsolution of a comparatively low-density aqueous minimum pressuresof 2800-3300 bars. This P-7 con- fluid from silicate melt during the late stagesof pegmatite dition lies outside of or maryinally within the spodu- genesis.Crystallined cavities suchas thosecited by Jahns mene + quartz field. At someof the SanDiego pegmatites (1979,1982) are primary (although subsolidusalteration (e.g.,the Stewart dike), as in the miarolitic pegmatitesof is common). In view of the conclusionthat miarolitic and Afghanistan, Brazil, and the Rumford-Newry district of massivepegmatites crystallize at similar P-Zconditions, Maine, primary spodumenecrystallized prior to pocket the presenceor absenceofpegmatitic pockets appearsto development; thus, these pegmatites should have been be controlled by the timing and extent of volatile exso- well into the stability field of spodumene+ quartz at the lution from silicate melt. time of pocket formation. In addition, the resultant pres- Fluid-inclusion data from this and previous studies (e.9., sure estimates for the San Diego pegmatites are higher Taylor et al., I 979; London, I 986) revealthat the salinities than those for the Afghanistan pegmatitesand are at the (i.e., chloride content) of typical rare-elementpegmatites upper boundary for the inferred peak metamorphic pres- are low. Exsolvedlate-stage aqueous fluids generallycon- sures for the western portion of the Peninsular Ranges tain lessthan 4 wto/oNaCl equivalent. Low-salinity aqueous batholith and included metamorphic rocks (which reflect fluids are incapableoftransporting largequantities ofdis- greenschist-amphibolitefacies conditions, but with an- solved aluminosilicate components (e.g., Anderson and dalusite-cordieriteassemblages in some metapelites;e.g., Burnham, 1967; Burnham, 1967). In particular, the sol- Todd and Shaw, 1979). ubility of Al is markedly suppressedin chloride-bearing The oxygen-isotopefractionation betweenquartz-feld- solutions (Anderson and Burnham,1967; Burnham and spar, quartz-, quartz-garnet, and garnet-mus- Nekvasil, 1986), yet pocket-zone minerals (e.g., beryl, covite show considerablescatter that is especially great tourmaline, spodumene)are distinctly Al-rich. The pre- for pocket-zoneminerals. Taylor et al. (1979) attributed cipitation of primary pocket silicatesfrom a low-salinity thesevariations to isotopic re-equilibration that was more aqueousfluid requires either a large water/rock ratio or LONDON: TOURMALINE-RICH GEM POCKETS 403 extensivefluid recirculationduring pocket formation. Evi- in which "hab" and "py" representhydrous alkali borate dencecited by London ( I 985)argues against either of these and components of the melt. "Elbaite" (al- conditions during pegmatiteconsolidation (seealso Jahns, uminous, -free tourmaline), albite, and quarlz are 1982;Norton, 1983;Watker, 1985).It appearsthat pock- crystallization products. In this model reaction, 8 mol of et-zonealuminosilicate minerals crystallizefrom hydrous HrO are liberated for every mole of tourmaline produced. silicate-richfluids that may be in contact with an exsolving The pocket shown in Figure lA yielded approximately aqueous phase, but that the solute-poor aqueous fluid 2l.2kg of tourmaline (E. E. Foord, 1985,pers. comm.). (representedby inclusions in pocket quartz) doesnot con- By the above reaction, the crystallization of this quantity stitute the bulk composition of the medium from which of tourmaline (at 460C and 2500 bars) would have lib- pocket minerals crystallize. erated approximately 4250 cm3 of (pure) aqueous fluid The abundance of tourmaline in pegmatitic miaroles (volumetric data for pure HrO from Burnham et al., 1969). (Fig. l) reflectshigh concentrationsofboron in late-stage With a specificvolume for elbaite : 0.33 cm3/g,every 3 pegmatitic fluids and provides an important clue for un- cm3 of tourmaline crystallized by the above reaction at derstandingpocketformation. Although variable in abun- 460"Cand 2500 bars createsapproximately 2cm3 of fluid- dance,tourmaline may form more than 50 volo/oof pock- filled void space. ets (e.g., see Fig. lA); thus, pockets contain a large To a largedegree, the crystallization of tourmaline may proportion ofthe that was originally presentin the govern the timing and extent of HrO exsolution. Early pegmatitic magmas,and that boron was retained in fluids and continuous crystallization of tourmaline buffers the until the late stagesof pegmatite consolidation. activity ofalkali borate to low values in hydrous silicate Addition of boron to aluminosilicate melts (1) signifi- melts (Pichavant,1981, 1983;Benard et al., 1985).Un- cantly enhancesthe miscibility of silicate melt and HrO der these conditions, the crystallization of tourmaline (e.g.,Chorlton and Martin,19781,Pichavant, 1981, 1983; should lead to steadydevolatilization of silicatemelt, with London, 1984b, 1986),(2) lowersmelt viscosityand in- the result that an aqueousfluid may escapefrom the large- creasesthe solubility ofincompatible traceelements (Bon- ly unconsolidatedpegmatite. Observations presented here niaud et al., I 978;L,ondon, I 986),and (3) promotesrapid and elsewhere(e.g., London, 1984b, 1986),however, in- crystalgrowth (London, 1986).Fluid inclusions in spodu- dicate that tourmaline commonly does not crystallize mene from the Tanco pegmatite provide direct evidence throughout much of the consolidation of pegmatitic sys- forhigh concentrationsofboron and rare elementsin late- tems, leading to high alkali borate contents in late-stage stagehydrous silicate fluids (London, 1984b, 1986).In fluids. The causesof tourmaline instability are not fully the San Diego County pegmatites, Foord et al. (1986) known but may be functions of temperature,tourmaline recognizedadditional evidencefor high boron concentra- crystal chemistry, the proportion of IVB/IIIBcoordination tions in residual liquids: (l) tourmalinization of wall rock in pegrnatitic fluids, and the generalabsence offerromag- adjacent to pockets (in some cases,a direct relation to nesiancomponents in highly fractionated pegmatites(see aplitic dikes that emanate from pockets) and (2) anoma- the discussionin London, 1986). The accumulationof lously high boron contents (2-3 wto/oboron) of aplitic boron through fractional crystallization in the absenceof feldsparsin pocket zones. tourmaline enhancesthe solubility (retention) of HrO in The role of boron and tourmaline as elucidatedby Lon- Iate-stagefluids. The ultimate crystallization of abundant don (1986) in the massiveTanco pegmatiteappears to be Al-rich and consequentexsolution of HrO applicable to miarolitic pegmatitesas well. The crystal- frorn such late-stagefluids provides favorable conditions lization of tourmaline consumesthe fluxing alkali borate for pocket formation. component (lithium or tetraborate) of late-stage pegnatitic melts, which to a consequentrise in the CoNcr-usroN solidus temperatureof the residual aluminosilicate fluid, Miarolitic and massive rare-element pegnatites crys- resultantprecipitation of aluminosilicate and oxide-form- tallize under similar P-7 conditions. The magmatic-hy- ing minerals (e.g.,NLTa-Sn oxides), and exsolution of drothermal transition in miarolitic pegmatitesoccurs in HrO. For example,LirBoOr-bearing fluid inclusions from the approximateranges of425475{ atd240f2800 bars. the Tanco pegmatitecontain approximately 2 mol of HrO Theseconditions are consistentwith the observedlithium per mole of aluminosilicate component (normalized to aluminosilicateassemblages, fl uid-inclusion data from the albite), and 2 mol of HrO per mole of alkali borate com- Kulam and San Diego pegmatites, and the peak meta- ponent (seeLondon, 1986,Table 2). A model reaction for morphic conditions attained by the host rocks of both the crystallization of tourmaline from such a fluid may pegmatite districts. Whether or not pockets form in peg- be expressedas matites may be controlled largelyby the activities of alkali borate melt componentsand, hence,by the timing oftour- 3 NaB.O,(OH)4 + 17 Al,Si4Oe(OH)4 .,hab" ..py" maline crystallization. In miarolitic rare-element peg- matites such as those described in this paper, the crys- : a Na(trAlrXBO3)3Si6O,r(OH)o tallization of tourmaline does not occur until the late stages "elbaite" of pegmatite consolidation. Crystallization of tourmaline + 2 NaAlSi3Or+ 38 SiO, + 32H2O, consumesan alkali borate fluxing component, which re- albite qvarrz V sults in the deposition of other alkali aluminosilicate and 404 LONDON: TOURMALINE-RICH GEM POCKETS oxide minerals, and the consequent exsolution of large Chorlton,L.B., and Martin, R.F. (1978)The effectof boron on amounts of HrO. The deposition of tourmaline contrib- the solidus. Canadian Mineralogist, 16,239-244. Fisher, J.R. (1976) The volumetric propertiesof H'O-A graph- utes directly to pocket formation by promoting the exso- ical portrayal. U.S. Geological Survey Journal ofResearch, 4 lution of an aqueous fluid phase. Vesiculation (pocket (2), r89-193. formation) is not restricted to the site of tourmaline crys- Foord, E.E. (1976) Mineralogy and petrogenesisoflayered peg- tallization but can occur anywhere in the pegmatitic fluid matite- dikes in the Mesa Grande district, San Diego Ph.D. thesis, Stanford University, Stan- where the activity of alkali borate is diminished. De- County, California. ford, California. pending on the structural environment and the extent of - (1977) Famous mineral localities: The Himalaya dike pegmatite consolidation, the liberated aqueous fluid may system,Mesa Grande district, San Diego County, California. form pockets or may be lost along fractures to react with Mineralogical Record, 8, 461-474. crystallized pegmatite or host rocks. Field relations from Foord, 8.8., Starkey,H.C., and Taggart,J.E., Jr. (1986)Miner- alogy and paragenesisof "pocket" clays and associatedmin- miarolitic pegmatites (e.g., Foord et al., 1986) attest to a erals in complex granitic pegmatites,San Diego, California. genetic link between pocket formation, late-stage boron- American Mineralogist, 7 I, 428439. rich albitic liquids, and adjacent tourmalinization of wall Hollister, L.S., and Bumrss, R.C. (1976) Phaseequilibria in fluid rocks. inclusionsfrom the Khtada Lake metamorphic complex. Geo- chimica et Cosmochimica Acta, 40, 163-175. AcxNowr,nncMENTS Jahns,R.H. (1979) Gem-bearingpegmatites in SanDiego Coun- ty. In P.L. Abbott and V.R. Todd, Eds. Mesozoic crystalline GeorgeB. Morgan VI and Margaret A. Walsh assistedin the rocks: PeninsularRanges batholith and pegmatites,Point Sal fluid-inclusion study of the Afghanistanpegmatites. Mike J. Hol- ophiolite, 3-38. Guidebook to field trips for the Geological daway performed the hydrothermal runs by which daughter-min- Society of America Annual Meeting, published by the De- eral reactionswere monitored. The manuscript was carefully re- partment of Geological Sciences,San Diego State University, viewed by EugeneE. Foord and an anonymousreviewer. Mineral San Diego, California. (1982) Internal evolution of pegmatite bodies. In Petr samplesfrom thesepegmatites were provided by John S. White, - Cernf, Ed. Granitic pegmatitesin scienceand industry, 293- Pete J. Dunn, and GeorgeE. Harlow. 327. MineralogicalAssociation of CanadaShort CourseHand- This researchwas supported by a grant from the Oklahoma book 8. Mining and Mineral ResourcesResearch Institute (Robert H. Jahns, R.H., and Burnham, C.W. (1969) Experimental studies Arndt, director) of the U.S. Bureau of Mines (Allotment Grant of pegmatite genesis.L A model for the derivation and crys- G-l154r40). tallization of granitic pegmatites.,64, 843- 864. (1963) pegmatite-aplite RrrnnnNcns Jahns,R.H., Tuttle, O.F. Layered intru- sives. Mineralogical Society of America SpecialPaper, no. 1, Anderson,G.M., and Burnham, C.W. (1967) Reactionsof quartz 78-92. and corundum with aqueouschloride and solutions Jahns,R.H., and Wright, L.A. (1951)Gem- and lithium-bearing at high temperaturesand pressures.American Journal ofSci- pegmatitesof the Pala district, San Diego County, California. ence,265,12-27. California Division of Mines SpecialReport, no. 7-A. Appleman, D.8., and Stewart,D.B. (1968) Crystal chemistry of Jollifl B.L., Papike,J.J., Shearer,C.K., and Laul, J.C. (1986) spodumene-typepyroxenes. (abs.) Geological Society of Amer- Tourmaline asa recorderof pegmatiteevolution: Bob Ingersoll ica SpecialPaper l0l, 5-6. pegmatite,Black Hills, . American Mineralogist, Benard, F., Moutou, P., and Pichavant, Michel. (1985) Phase 71,472-500. relations of tourmaline leucogranitesand the significanceof London, David. (1983) The magmatic-hydrothermaltransition tourmaline in silicic magmas. Journal of Geology, 93, 271- in rare-metal pegmatites:Fluid inclusion evidence from the 29r. , Manitoba. (abs.) EOS (American Geophysical Bodnar,R.J. (1983)A methodofcalculating fluid inclusionvol- Union Transactions),64, 549. umesbased on vapor bubble diametersand the P-V-T-X prop- - (1984a) Experimental phase equilibria in the system erties of inclusion fluids. Economic Geology, 78, 535-542. LiAlSiO4-SiOr-H,O: A petrogeneticgrid for lithium-rich peg- Bonniaud, R., Redon, A., and Sombret, C. (1978) Application matites. American Mineralogist, 69, 995-1004. ofborate glassesand various boron bearingglasses to the man- - (1984b) The role oflithium and boron in fluid evolution agementof Frenchradioactive wastes.In L.D. Pye, V.D. Fren- and depositionin rare-metalpegmatites. Geological Society chette, and N.J. Kreidl, Eds. Borate :Structure, prop- of America Abstractswith Programs,16,578. erties, applications.Materials ScienceResearch, 12, 597-616. - (1985) Origin and significanceofinclusions in quartz: A Plenum Press.New York. cautonaryexample from the Tanco pegmatite,Manitoba: Eco- Burnham, C.W. (1967) Hydrothermal fluids at the magmatic nomic Geology,80, 1988-1995. stage.In H.L. Barnes,Ed. Geochemistry of hydrothermal ore - (1986) Magmatic-hydrothermal transition in the Tanco deposits, first edition, 34-76. Holt, Rinehart, and Winston, rare-element pegmatite: evidence from fluid inclusions and New York. phase-equilibrium experiments. American Mineralogist, 71, Burnham,C.W., Holloway, J.R., and Davis, N.F. (1969)Ther- 376-395. modynarnic properties of water to 1,000"Cand 10,000 bars. London, David, and Burt, D.M. (1982) Lithium minerals in peg- GeologicalSociety of America SpecialPaper 132. matites. In Petr Cernf, Ed. Granitic pegmatitesin scienceand Burnham, C.W., and Nekvasil, Hanna. ( 1986).Equilibrium prop- industry, 99-133. Mineralogical Association of Short erties of granite pegmatite magmas. American Mineralogist, Course Handbook 8. 71,239-263. Norton, J.J. (1983) Sequenceof mineral assemblagesin differ- Cameron,E.N., and others.( 1954)Pegmatite investigations, 1942- entiatedgranitic pegmatites.Economic Geology, 78, 854-874. 45, . U.S. Geological Survey ProfessionalPaper Pichavant, Michel. (1981) An experimentalstudy of the effectof 255. boron on a water-saturatedhaplogranite at I kbar vapour pres- Cassedanne,J.P., and Cassedanne,J.O. (1981)The Urubu peg- sure.Contributions to Mineralogy and Petrology,7 6, 43U439. matite and vicinity. Mineralogical Record, 12,73-77. - (1983) Melt-fluid interaction deduced from studies of LONDON: TOURMALINE-RICH GEM POCKETS 405

silicate-BrOr-HrO systemsat I kbar. Bulletin de Min6ralogie, 109 (transl. Soviet Geology and Geophysics, 19 (l L), 82-87, 106.201-211. l 978). Porter,R.W., II, Clynne,M.A., and Brown,D.L. (1978)Freezing Rossovskii,L.N., and Shmakin,B.M. (1978)Unique exampleof point depressionof aqueoussodium chloride solutions. Eco- vertical geochemicalzoning in pegmatitesof the Hindu Kush, nomic Geology,'13, 284-285. Afghanistan(in Russian).Akademii Nauk SSSRDoklady, 240 (2),448451(transl. Roedder,Edwin. (1981) Natural occurrenceand significanceof Reportsof the Academyof Science,USSR, fluids indicating high pressureand temperature. Physics and Earth ScienceSections, 240, 204-206, 1978). Chemistry of the Earth, 13,9-39. Taylor, B.E., Foord, E.E., and Friedrichsen, H. (1979) Stable isotopeand fluid inclusion studiesofgem-bearing granitic peg- Roedder,Edwin, and Bodnar, R.J. (1980) Geologic pressurede- matite-aplite dikes, San Diego County, California. Contribu- terminations from fluid inclusion studies.Annual Reviews of tions to Mineralogy and Petrology, 68, 187-205. Earth and PlanetarySciences, 8, 263-301. Todd, V.R., and Shaw,S.E. (1979) Structural,metamorphic, and Rossovskii,L.N. (1977) First of pollucite and its crystals intrusive framework of the Peninsular Ranges batholith in in Afghanistan (in Russian).Akademii Nauk SSSRDoklady, southern San Diego County, California. In P.L. Abbott and 236 (I), 216-219 (transl. Reports of the Academy of Science, V.R. Todd, Eds.Mesozoic crystalline rocks: PeninsularRanges USSR,Earth ScienceSections, 236 (l),157-160, 1977). batholith and pegmatites,Point Salophiolite, 177-23I . Guide- - (1981) Rare-elementpegmatites with preciousstones and book to field trips for the GeologicalSociety of America Annual conditions of their formation (Hindu Kush) (in Russian).Zap- Meeting, published by the Department of GeologicalSciences, iski VsesoyuznogoMineralogicheskogo Obshchestva, I 09, 30l- San Diego StateUniversity, San Diego, California. (transl. 311 International Geology Review, 23, 1312-1320, Touret, Jaques(1981) Fluid inclusions in high-grademetamor- r 98l). phic rocks. In L.S. Hollister and M.L. Crawford, Eds. Fluid Rossovskii,L.N., Chmyrev,W.M., and Salakh,A.S. (1975)New inclusions: Applications to petrology, 182-208. Mineralogical finds and belts of rare-metal pegmatitesin the Hindu Kush Association of Canada Short Course Handbook 6. (EasternAfghanistan) (in Russian).Geologiya Rudnykh Mes- Walker, R.J. (1985) The origin of the Mountain pegmatite, torozhdeniy, 17 (5) 102-106 (transl. International GeologyRe- Black Hills, South Dakota. Ph.D. thesis,South Dakota School view, 18, 1339-1342,1976). of Mines, Rapid City. Rossovskii,L.N., Makagon,V.M., and Kuz'mina, T.M. (1978) Characteristicsof the formation of a kunzite deposit in Af- Maxuscnrpr RECETvEDJurv 19, 1985 ghanistan(in Russian). Geologiya i Geofizika, 19 (1 1), 102- MaNuscnrpr AccEprEDOcrospn 21, 1985