American Mineralogist, Volume 71, pages 406427, 1986

Mineralogy and geochemicalevolution of the Little Three pegmatite-aplite layered intrusive, Ramona,California

L. A. SrnnN,t G. E. BnowN, Jn., D. K. Brno, R. H. J*rNs2 Department of Geology, Stanford University, Stanford, California 94305 E. E. Foono Branch of Central Mineral Resources,U.S. Geological Survey, Denver, Colorado 80225 J. E. Snrcr,nv ResearchDepartment, Gemological Institute of America, 1660 Stewart Street,Santa Monica, California 90404 L. B. Sp.c,uLDrNG" JR. P.O. Box 807, Ramona,California 92065

AssrRAcr Severallayered pegmatite-aplite intrusives exposedat the Little Three mine, Ramona, California, U.S.A., display closelyassociated fine-grained to giant-texturedmineral assem- blageswhich are believed to have co-evolved from a hydrous aluminosilicate residual melt with an exsolved supercriticalvapor phase.The asymmetrically zoned intrusive known as the Little Three main dike consists of a basal sodic aplite with overlying quartz-albite- perthite pegmatite and quartz-perthite graphic pegmatite. Muscovite, spessartine,and schorl are subordinate but stable phasesdistributed through both the aplitic footwall and peg- matitic hanging wall. Although the bulk composition of the intrusive lies near the haplo- granite minimum, centrally located pockets concentratethe rarer alkalis (Li, Rb, Cs) and metals (Mn, Nb, Ta, Bi, Ti) of the system, and commonly host a giant-textured suite of minerals including quartz, alkali feldspars, muscovite or F-rich lepidolite, moderately F-rich topaz, and Mn-rich elbaite. Less commonly, pockets contain apatite, microlite- uranmicrolite, and stibio-bismuto-columbite-tantalite.Several ofthe largerand more richly mineralized pockets of the intrusive, which yield particularly high concentrationsof F, B, and Li within the pocket-mineral assemblages,display a marked internal mineral segre- gation and major alkali partitioning which is curiously inconsistent with the overall alkali partitioning of the system. Calculationsof phaserelations betweenthe major pegmatite-aplitemineral assemblages and supercritical aqueous fluid were made assuming equilibrium and closed-systembe- havior as a first-order model, although these assumptions may not be wholly correct. Isobaric phase diagrams were generatedand are used to constrain the observed mineral assemblagesin the system KrO-NarO-AlrO3-SiOr-HrO as a function of temperature and cation-activity ratios of the coexisting fluid phase. Log activity-temperature plots of the stoichiometric assemblagemicrocline-albite-quartz-muscovite at 2 kbar show that log a**r", of the fluid phaseremains essentiallyconstant throughout crystallization, while log QN"*nr1 rises slightly with falling temperatures.Modeling muscovite nonstoichiometry by decreas- ing activity of the pure component from unity to 0.5 markedly shifts the upper thermal stability limit of the assemblagemicrocline-albite-quartz-muscovite from approximately 630'C upward to 710'C. In the pegmatite-aplite system exposedat the Little Three main dike, crystallization temperaturesare thought to have ranged from approximately 700t downward toward 540'C at the central pocket zone; muscovite nonstoichiometry caused by F, Li, and Mn substitution may have contributed to the stability of this assemblageat the outer zonesof the system,where stoichiometric muscovite is not predicted to be stable.

INrnorucrroN Diego County, California, is famous for its past produc- The Litfle Three layered pegmatite-apliteintrusive, lo- tion of gem-quality tourmaline and topaz, as well as for cated in the Ramona pegmatite district of central San its yield of exceptionalpocket matrix specimens.From a presents 'Presentaddress: U.S. GeologicalSurvey, 345 Middlefield petrologicalviewpoint, this system an interesting Road,Mail Stop977, Menlo Park, California 94025. example of a crystallization sequenceof magmatic origin 2Deceased December 31, 1983. which is believed to have involved an initially water- 0003{04x/86/03044406$02.00 406 STERN ET AL.: LITTLE THREE PEGMATITE-APLITE 407

Little Three Mine, 1983

locofions F'.jl Pocket c ross seclfon ,\ j' loco fions

1 New Spoulding

Fig. l. Presentworkings of the Little Three mine. saturatedgranitic residual fraction with exsolvingand co- et. Equilibrium-phase-relation calculations are also pre- existing supercritical aqueousfluid (Jahnsand Burnham, sentedhere, serving as a first-order model to place some 1969).The Little Three main dike, exposedat the Little constraintson the fluid-solid interactions that took place Three mine, exhibits a marked vertical contrastin texture, during the evolution of this system. These calculations composition, and zoning patterns, such that massive and model the evolution of the pegmatite-apliteintrusive from layeredaplite definesthe footwall ofthe intrusive, overlain coexistingsilicate melt and supercriticalaqueous fluid and by increasinglycoarse-grained pegmatite and graphic peg- are assessedthrough a seriesof isobaric phase diagrams matite alongthe hangingwall. Centrally located"pockets" relating calculated activity ratios ofaqueous speciesand host a giant-texturedsuite ofgranitic mineral assemblages stable mineral assemblagesin the system KrO-NarO- as well as some unusual accessoryminerals. AlrO3-SiOr-HrO. This graphicalapproach models general This project was initiated over nine years ago when a trends among cation-activity ratios and observed stable large, richly mineralized pocket was discovered by L. B. mineral assemblages,efected by changing temperature Spaulding, Jr. A variety of museum-quality specimens and mineral stoichiometry. were excavated from this exceptional pocket, and the structure and contentsof the pocket were documented.A Loc.nrroN AND GENERALFEATURES continuing collaboration between Spaulding and the Stan- Numerous geologistshave documentedvarious features ford workers also permitted documentation of the con- of the Ramona pegmatitedistrict. The location of the San tents of other pocketswithin the main pegmatite dike, as Diego County pegmatite districts is shown by Jahns and well as of the zoning sequenceof this asymmetrically lay- Wright (1951), and the generalgeological features of the ered body. Ramona district are describedby Simpson (1965). Gen- The internal zonation and three-dimensionalnature of eral mineralogical and zonational features of San Diego the Little Three main dike are explored in the present County granitic pegmatitesare describedby Jahns(1955, study, in conjunction with a detailed mineralogic study 1982),Simpson (1962, 1965),Jahns and Tuttle (1963), of the pocket material recovered from the mine since Foord (1976, 1977),Shieley (1982), and Shigleyand Brown 1976,with emphasison the large"New Spaulding" pock- (1985).Analyses of bulk composition and of averagezone 408 STERNET AL.: LITTLE THREEPEGMATITE.APLITE

qz- perthite grophic gronite qz - ol bi fe- pe rlhi t e pegm ot i I e

+ l0cm - loyered op/ite ("line rock" )

mossiveoplife

counlry rock

(mof ic tonolite )

.t A' I

9 cm j

+ljcm- Fig.2a. Cross sectionA-A' acrossmain tunnel (darkly shadedarea), with vertical section showing units. Line rock locally splits and remergesdown dip. compositions of the Ramona district pegmatite-aplitein- pocket pegmatite-aplitebodies are documentedby Taylor trusives are reported by Simpson (1965) and compared et al. (1979). with layered pegmatite-aplite intrusives from other San The Little Three main dike is one of five principal peg- Diego County localities by Jahnsand Tuttle (1963).Fluid- matite dikes on the Little Three property. These dikes, inclusion and stable-isotopestudies of San Diego County including the Little Three main dike, the Hercules-Spes-

B, f, scAG-, Fig. 2b. Cross section B-B' along main tunnel. Darkly shaded area indicates small adit of of main tunnel. STERNET AL.: LITTLE THREEPEGMATITE.APLITE 409

3m membersofthe CretaceousSouthern California batholith. A geologicmap and cross-sectionthrough the central por- tion of the Ramona pegmatitedistrict are given by Simp- son (1965,Figs. I and 2). The segment of the main dike exposed at the Little 2m Three mine averagesa N55"W strike, dipping 10'-65'SW, and transectsa mafic facies of the Green Valley tonalite. The Little Three main dike crops out along hillside ex- posureswhere it is folded on a dip-slope orientation and

1m runs perpendicularto it, presentingan irregularly rolling surfacein cross-sectionalview. A plan-view map of the present workings is shown in Figure l, and the cross- sectionalviews of the layered intrusive illustrated in Fig- ures 2a-2d show the rolling nature ofthe dike as well as the generalconcordancy ofthe internal zoning sequence. Locally the dike partially separatesinto two thinner, con- tiguouslayering sequences, remerging down dip to a single thick sequence(Fig. 2a). The contact betweenthe granitic pegmatite-apliteintrusive and the enclosinggabbroic-ton- Fig. 2c. Crosssection C{' acrossmain tunnel(darkly shaded alitic wall rock is sharply defined, and the host rocks gen' area). erally display little evidence of alteration except where proximal to large pockets. sartine dike, the Spauldingdike, the Sinkankasdike, and Despite similar bulk compositions and asymmetrical the axinite-bearing Hatfield Creek dike, apparently formed zoning featuresamong the dikes on the Little Three prop- aslocally anastomosingbut essentiallyseparate intrusives. erty, pocket-mineral assemblagesvary significantly; each The Little Three dikes occur with other Ramona district dike has its own characteristic pocket mineralogy, sug- and neighboringdistrict pegmatiteintrusives as a network gestingindependent formation. While the Hercules-Spes- of northwest-striking, southwest-dipping granitic dikes sartine "ledge" yields muscovite, microcline, albite, spes- which principally intrude several gabbroic to tonalitic sartine, schorl, qvartz, plus minor Mn-rich axinite as

\";2

\o \ \:

C,,

Fig. 2d. Cross section C'-C" along adit dipping within plane of dike. Darkly shadedarea indicates a smaller, secondaryadit. 410 STERN ET AL.: LITTLE THREE PEGMATITE-APLITE principal primary pocket minerals,pockets from the Little flakes. Biotite, however, rarely forms elsewherein the Three main dike have produced elbaite, muscovite or intrusive. fluorine-rich lepidolite, quartz, microcline, albite, and to- The lower massive section grades upward composi- paz, plus minor apatite, microlite-uranmicrolite, stibio- tionally to banded aplite or "line rock," ranging from l0- bismuto-columbite-tantalite, columbite, and native Bi. to 35-cm maximum composite thickness (Figs. 2b, 2d). Severalcrystals of beryl and hambergite have also been The banded aplite consists of 0. l- to 4-cm-thick layers extractedfrom the main Little Three workings. The Sin- which alternate in appearancefrom white to dark gray, kankas dike has produced abundant hambergite, which or less commonly, to orange brown. Varying concentra- has not been noted in the other dikes except for the few tions of fine-grained schorl needlesor spessartinegrains specimensrecovered from the main dike. Yet another sharply define thin, darker or varicolored laminations pegmatite dike on the property, the Hatfield Creek dike, which are usually I to 4 mm thick. Neighboring lighter- produced abundant dark red-brown crystals of axinite. colored bandsthat are poorer in tourmaline or spessartine Currently, the Little Three main dike is mined for tour- tend to be thicker and coarser grained, with grain size maline and topaz, and the adjacent Hercules-Spessartine ran$ng from 0.5 mm up to 4 cm. Coarse-grainedlenses ledgefor spessartineand schorl. While a limited amount and pods locally interupt the layering. These lensesare of mining has been done on the lower, southwestward more potassicthan the surroundingaplite laminations and extension ofthe main dike, the upper portions have proved may contain q\Trlz, schorl, and potassic alkali feldspar far more productive. The most significant find was the megacrystsup to approximately 1.5 cm in length. Schorl New Spaulding pocket, occupying an area of approxi- needlesare widely dispersedat all orientations throughout mately 2.2 m3, from which many exceptional specimens the aplite, but are most commonly oriented near-verti- were removed. cally, perpendicularto the hangingwall. While the layering of the line rock is roughly concordant with the attitude of INrnnNlr, zoNATroN the dike, small-scalefluctuations and undulations tend to Similar to layered pegmatite-aplite intrusives exposed form around coarse-grainedpods and lenses,particularly in neighboring districts, the Little Three main dike dis- thosecontaining alkali feldsparmegacrysts (Fig. 2a, inset). plays a marked variation in texture, composition, and zoning patterns; while the aplitic footwall is rich in Na and relatively depletedin K, the coarse-grainedoverlying Quartz-perthite-albitepegrnatite quartz-perthite-albite pegmatite and, particularly, the The contact above the banded aplite grades upward to quartz-perthite "graphic granite" hanging wall are en- increasinglycoarse-grained, allotriomorphic, quartz-per- riched in K while poorer in Na. A zone of cavities or thite-albite pegmatite. Schorl is subordinate but wide- pockets,partially lined or filled with euhedral crystals,is spreadthroughout this zone, while muscovite aggregates located within the upper-central section of the layered and spessartinecrystals are lessabundant. Coarse-grained intrusive (Fig. 2a). Concentrating the minor and trace albite and intermediate microcline-hosted perthite make elementsof the system,the pocket zone commonly hosts up the bulk of this zone, commonly altering to a sugary a giant-textured assemblageof major as well as minor appearanceon weathered surfaces.Large (up to l0 cm phasesof the system. Despite this vertical contrast, the long), elongate, light-colored, intermediate microcline minerals quartz + K-feldspar + albite plus subordinate perthite crystalsgrowing toward an adjacent pocket will muscovite + schorl * spessartineare stablein nearly all often show an abrupt color changeto darker appearance regionsof the dike, and the bulk composition of the Little within severalcentimeters of the pocket; X-ray analyses Three dike (Simpson, 1965) lies near the thermal low of indicate that this changein color immediately adjacentto the haplogranite system KAlSijOt-NaAlSi3O8-SiOr-HrO a pocket is coincident with a symmetry inversion to the (Jahns and Tuttle, 1963). The dike varies in thickness monoclinic state (Table 1). This changeappears to take from approximately 1.4 lo 2.1 m, and a vertical section place within the single crystal, and the darker-coloredor- is illustrated in Figure 2a. thoclasedoes not appear to form as a rind on the micro- cline perthite. The alkali feldspars encloseor are inter- Massive and layered aplitic footwall grown with anhedral quartz grains which vary in length Rangingfrom 0.5- to 1.0-m maximum thickness,the from 0.3 cm upward lo 2 cm. Schorl occurs as upwardly aplitic footwall comprises one-third to, locally, one-half fanning rods frozen in the quartz-perthite-albite pegma- of the zoned intrusive. The lower massiveaplite varies in tite. Thesecrystals are commonly 0.3 to 2 cm in diameter texture from a true fine-grained sugaryappearance to lenses, and 0.8 to 5 cm in length, but proximal to the pocket zone pods, stringers,or particularly thick sectionswith coarser they may reach 8 to l0 cm in length. Muscovite generally grain size. Albite and quartz arc the predominant con- occursin the pegmatiteas sporadicallydistributed coarse- stituents of the aplite, with lessabundant K-feldspar and grainedmasses, more heavily concentratednear the pock- schorl grains or needles.Muscovite and spessartineare et zone. Deep orange-brown to red-brown, idiomorphic only minor, sparselydistributed phases.Locally, the low- spessartinecrystals increasein abundance as well as in ermost sectionof the aplite in contact with biotite tonalite size toward the pocket line, ranging from I to 8 mm in wall rock may contain fine- to medium-grained biotite diameter. STERNET AL.: LITTLE THREEPEGMATITE-APLITE 4ll

Quartz-perthite graphic granite Table l. Lattice parameters and X-ray data for 'lihe pegmatite-zonefeldspars predominant unit comprising the hanging wall of the Little Three dike is a quartz-perthite "graphic granite" internediate microcl i ne, orthocl ase, (Fig.2a, inset); perthite is the dominant constituent, con- fron pegmati te- adiacent to pocKet pocket pocket orthoclase'" sistingofa maximum microcline host with less-abundant, zone near exsolved, low albite lamellae. The perthite encloses u (i) 8.57e(2) 8.s56(2) 8.554(2 ) graphically oriented plates or rods ofquartz arrangedin b 12.e67(2) r2.e70(3) 12.965(2) parallel to subparallelfashion, which branch and reunite c 7 -2125(11 7 .2r0(2\ 7.2r57 (1 ) along the direction of elongation. The three-dimensional o (. ) e0.338(e) 90. 90. nature of the quartz-feldsparintergrowth has been doc- B il 5.970(8) 1r6.008(e) lr6 031(e) 90. umented by Simpson (1962). r 89.121(6) 90. The coarselytextured graphic granite hangingwall dis- v (i3) 7zr.26 7r9 ll 719.12 playswell-developed quartz and perthite crystalfaces. The d* 90.06 3 quartz-perthite ratio is approximately l:3, and the micro- Y* 90.820 #t, cline-albite ratio of the perthite is approximately 2.5:l o*t.,r (Al) .92+ .04 .90+ .04 .95+.04 rtlo-tln .38 .04 0 0 (Simpson, 1962). Schorl, spessartine,and muscovite are similarly distributed as they are in the underlying peg- ##z .574 .636 matite, increasingin grain sizeand abundancetoward the ##r ('c) 5lo + 20 430t 20 pocket line. The quartz-perthite intergrowth is often em- bayed or etched where it grows adjacent to a pocket. 2oo cu Kot Along the upper contact,a thin selvageof"border rock" Zo1 2t.ol 21.02 21.03 23.56 separatesthe graphic granite from the country rock. This I 30 22.42 23.56 060 41.7 6 4t.76 4t.76 fine-grainedselvage is composedchiefly of quartz, albite, Zo4 50.59 50.62 50.65 and K-feldspar, plus less-abundant schorl needles and muscovite. Locally, the border zone contains appreciable * data fo" light-colored internediate microcline (colunn I ) in amounts of biotite, although biotite is not encounteredin quartz - K feldspar - albite pegnatite zone' and darker (co'lunn 2) adiacent to pockets the graphic granite immediately below. The upper border colored orthoclase *'pocket orthociase fron claim imnediately adiacent to Little zone and graphic pegmatite hanging wall grade downward Three maximum I m giant-textured # through a thicknessof to the Al contents in (t,o + t,m) and (t'o - t'n) sites determined |/l|right pocket quartz-perthite-albite pegmatite from b !!. c and ol w.\,* plots glven bj Stewart and zone and below. (r974)

##z orde"ing paraneter dnd corresponding freezing temperatures The pocket zone deterninei 6y b and c cell parameters, given by Hovis (1974) (Z = 7.6344 - 4.3584(b) + 5.8615(c)) Although lensesand pods partially lined or filled with euhedral crystals are found in all zones of the layered intrusive, the true giant-textured pockets, miarolitic cav- moderatelyF-rich topaz.Other pocket minerals that occur ities, or small "bug holes" are chiefly confined along a more rarely in the Little Three main dike include apatite, pocket line which lies within the upper sxtent of the quartz- schorl, microlite-uranmicrolite, stibio-bismuto-colum- perthite-albite pegmatite zone, most commonly near or bite-tantalite, and columbite. Severalspecimens of pocket along the overlying graphic granite contact. The pocket beryl (goshenite-morganite)and hambergite have also been line is always present,although it locally closesdown to recoveredfrom the Little Three workings, aswell as sparse a thin selvage.Larger pockets commonly extend up into amounts of pucherite. One pocket contained relicts of the graphic granite hanging wall, and the floor of such native Bi on the pocket floor surrounded by gray bismite pockets may reach down into the layered aplite. While and bismutite. the smaller pockets or cavities are commonly ellipsoidal Ofparticularinterest is the New Spaulding pocket, which or ovoid, larger pockets are flatter and more irregularly was the largestpocket yet excavatedfrom the Little Three, oblate shaped,following the generalattitude ofthe dike. yielding many exceptional specimensand exhibiting strong Pockets may be either individually developed or inter- mineral segregationand zonation. A reconstructionofthis connectedby thin fissuresor cracks. Locations of exca- pocket is illustrated in Figure 3, and pocket contents are vated pocketsare shown in Figure l, and briefdescriptions listed in Table 3. This very large pocket of approximately ofpocket contents are listed in Table 2.The pocket zone 2.2 m3 was located at what is presently the entrance to of the pegmatite concentratesminor elementsof the sys- the mine, occupying an irregularly shaped area within a tern including F, Li, B, and Mn, as well as trace amounts zone 3.3 m x 3.3 m averaging 0.25 m in height. The of Nb, Ta, Rb, Cs, Ti, W, Sb, Bi, Sr, Ba, IJ, Ca, Ce, l-a, attitude of the pocket was N45oW, 29'-3I'SW, following Pb, Sn, and Be. Larger pockets typically host giant-tex- a slight roll in the dike. tured assemblagesincluding quartz, maximum micro- The contents of the New Spaulding pocket included cline megacrysts,coarsely bladed cleavelandite (low al- quarlz, tourmaline (elbaite), blue and colorless topaz, bite), Mn-rich elbaite, F-rich lepidolite or muscovite, and maximum microcline "pillars," clear and smoky quartz, 412 STERNET AL.: LITTLE THREEPEGMATITE-APLITE

Table2. Little Threemain dike pocketdescriptions

pocket # description, and/or minerals in additjon to quartz, cleavelandite, K fe'ldspar

I "New Spaulding pocket"; see Figure 4

2 Topaz, lepidolite, snall dark-green tourmaline.

3 Small pocket located approxinately I' above layered aplite; contained tri-colored tourmaline "pencils", pink - light green - olive green,

Series of smal1, connectedpockets along ceiling above maln drift; contained irregularly shaped tourmaline coated with black (Mn?) oxide, snoky quartz, no topaz or lepidolite.

Connected by fractures to 4; contained black-brown snoky quartz, good natrix touroaiine and cleavelandite, several large olive-9reen tournaline crystals, someshorl-capped. Pocket shorl only found here, Several "nail-head" tournaline crystals found here Coarse, abraded lepidolite.

Series of small tourmaline (dark green) pockets along ceiling above drift.

"Pasadenapocket"; uppernost extent of 6. Very little clay, essentiaily "dry". Liqht smoky green tournaline pencils, topaz, floor covered with quartz- Blue apatite found down dip of Pasadena pocket.

8 Large pocket, S' x 9' x l2-24", dipping approximately 450. Floor-lining lepldolite, topaz found groving along floor at upper end of pocket and found loose at lower end; very I arge smoky quartz and K fel dspar megacrysts I ocated at upper endi several bl ue- green tourmaline pencils also found along upper extent of pocket. Also loose tournaline pencils: goid-yel lou bases with emerald-green caps.

9 Poor tourmali ne.

l0 Good topaz natrix specimens, much tourmaline, very little lepidolite

lt Badly decomposed; soIe topaz, poor tourmallne.

t2 - t3 Series of pockets along trough of large roll in dike; pockets of very large proportjons, now collapsed. Topaz and tournal ine, nostly poorly shaped. 12t several quartz-topaz mat|ix specinens, poor tourmaline. Deep purple lepldolite frozen in and adiacent to pocket. Yel low pucheri te here only.

Pocket locations (colunn 1) refer to nuhbered pockets in Figure l.

cleavelandite,fluorine-rich lepidolite, microlite-uranmi- Virtually all of the pockets excavated from the Little crolite, and stibio-bismuto-columbite-tantalite. Coarse- Three mine were filled with wet, varicolored clays en- grainedbooks of lepidolite irregularly lined the floor, with closing numerous fragments of primary pocket minerals large megacrystsof maximum microcline resting on the as well as some organic matter (seeFoord et al., 1986, for floor and projecting upward with well-developed faces a description ofpocket clays).The large pockets have all alongthe exposedsurfaces (Fig. 3). Severalsmaller K-feld- been found to contain surficial clays, but severalpockets sparcrystals were also removed from the roof of the pock- were found "dry." One such pocket was found immedi- et. Elbaite crystals were generally found attached to the ately west of the New Spauldingpocket. This pocket con- roof within a massof coarselybladed cleavelandite.Only tained only primary minerals with a fine white coating of a small amount of cleavelandite(-200lo) was found rooted K-feldspar and muscovite. Such pockets offer a view of to the floor ofthe pocket, and no lepidolite was attached pocket formation prior to low temperature and pressure to the roof, except for severallepidolite crystals attached surficial processes. to elbaite specimens.Large blue topaz crystals were re- There hasbeen sufrcient fracturing of the dikes to allow moved from the wall of the pocket, and others were at- entrance of surficial waters and organic material. Thin tached to both cleavelandite and lepidolite. A mass of cracks are occasionally filled with flattened plant roots. intergrown topaz lined the back floor of the pocket, but Roots and organic material are particularly concentrated was not obviously attached.Small colorlesstopaz crystals near pockets,due to the increasedfracturing ofthe dikes grew on the elbaite faces along the roof of the pocket. in such areas.Within the New Spauldingpocket, numer- Abundant fragmentsof topaz were also removed from the ous live, intertwining roots were removed from the clay pocket clay. Quartz crystals were less commonly found contents. along the floor ofthe pocket than projecting down from the ceiling, and only those growing along the floor were AN.llvtrclr, METHoDS coated with a fine-grained mixture claylike of K-feldspar Chemicalanalyses of alkali feldspars,tourmaline, topaz, and plus muscovite and/or lepidolite. The majority of the sti- lepidolitefrom the New Spaulding pocket, and of muscovitefrom bio-bismuto-columbite-tantalite found in the pocket was the quartz-perthite-albitepegmatite zone, were obtained on a attached to lepidolite and topaz along the floor of the rEor-733A electron microprobe using a beamcurrent of 15 nA pocket. and an acceleratingvoltage of I 5 kV. Eachanalysis represents a STERNET AL.: LITTLE THREEPEGMATITE.APLITE 413

Fig. 3. Reconstruction ofthe New Spauldingpocket.

30-s counting interval or 0.30lostandard deviation, with a beam period. Peak locations were determined with the aid of Rigaku diameter of l0 pm. Silicate mineral standards were used for peak-frnding software. Least-squarerefinements of cell dimen- analysis of Si, Al, I! Na, Ca, Mn, Mg, Fe, Ti, Sr, and Ba. A sions were calculatedafter the patterns were indexed with the synthetic fluor-phlogopite standardwas used for F analysis,and aid ofcalculatedpatterns (Borg and Smith, 1969)for eachmineral syntheticglass standards were usedfor Rb and Cs analysis.Data exceptlepidolite. Powder patternsfor lepidolite were indexed on were corrected using the Benceand Albee (1968) procedure. the basis ofindexed patterns reported by Bailey (1980). Chemical analysesof the stibio-bismuto-columbite-tantalite wereobtained on a ARLEMx-sM electron microprobe usinga beam current of30 nA and an acceleratingvoltage of 15 kV, with a Pocxnr MTNERALocY 5-pm beamdiameter. Standards used were Si-quartz, Fe-FerOr, M- BaSrNboO,r,Ta- MnTarO6, Bi- BirO3, W - MnWOo, Ti - Quartz TiOr, Sb-SbrOr. Data were correctedusing the program MAGrc Within the pocket zone, quartz may be clear, various smoky ru (Colby, 1968). shades,or distinctly yellowish. Giant euhedral quartz crystals The boron and lithium contents of pocket tourmaline were growing in large pockets tended to be rooted to the pocket ceiling, measuredby inductively coupled plasma emission spectroscopy. projecting downward to the center ofthe pocket, although smaller Tourmaline sampleswere crushedto 200 mesh and dissolved in quartz crystals were also found rooted to the pocket floor. In sodium carbonate. smaller pockets,however, quartz was not preferentially situated Cell-dimension data were obtained by powder X-ray diffrac- in any specific area within the pocket. Solid inclusions within tion using an automated Rigaku ouex powder difractometer, pocket quartz are minimal, but fluid inclusionsare prevalent and CuKa radiation, and operating conditions of 35 kV and 15 mA. often cast a milky appearanceto the crystal. Several analyses of Powderedsamples were mixed to an approximate 5:1 ratio with fluid inclusions from Little Three pocket quartz are given by an internal Si metal standard for precise calibration, and all sam- Taylor et al. (1979). ples were scannedwith a 0.01" 20 step width and 4.0-s counting Within the New Spaulding pocket and smaller pockets periph- 414 STERNET AL.: LITTLE THREEPEGMATITE-APLITE

Table3. Specimensrecovered from the New Spauldingpocket

mat|ix specinen description

t4l 3 large dark-green tournaline crystals in cleavelandite natrix: (f) 4 x 5 x 6.5 cm long; (2) 3.5 x 4.5 x 3.5 cm long; (3) 3.5 x 2.0 x 3.5 cm long with smalI topaz crystals on the rins of 2. Medium-sized colorless quartz (12 x 12 x 15 nm). Lepidolite books on 2 of the tourmalines.

2 large dark-green tourmaline crystals in cleavelandite matrix: (1) 6 x 5 x 7 cm long; (2) 4x 5x6cn wi th large quartz and K fetdspar (20 x 15 x !0 cm).

Large euhedral K feldspar crystal (9 x l0 x l4 cn) with some cleavelandite on iides. Also large dark-green tourmaline (4 x 5 x 7 cm)' smaller dark- green tournal ine, and quartz.

Very large smoky quartz (8 x l0 x 17.5 cm) with perfect ternination; also large daik-green tournaline (4.5 x 4.5 x 8 cm) with topaz on ternination and on side.

Large mil ky quartz crystal (6 x 7 x 9 cm); 2 larqe tournal ine crystal s (4 i 5 x 6'cni, 4 x 3 x 3 cm); I colorless, euhedral topaz crystal (l x 1.5 x 2 cm tall); set in cleavelandite natrix.

3 laroe.- dark-qreen tourmalines set in cleavelandite matrix: (l) 4,s x s,s x l0 cn tall, with small, color'less topaz on rim; iZ\ q.S x 5 x 8 cn tall, with several topaz crystals on top and Sldes; (3) 5.5 x 5.5 x 6 cn, small color'less topaz on rim. Also large, snoky quartz crystal (5 x l0 x l4 cm) and snall, colorless topaz'

LI K feldspar (microcline) "pillar" (25 x 25 x 35 cm) resting on floor.

l2 Coarse books of lepidol ite, irregularly I ining floor.

* matrix specinens in col unn I refer to Figure 3

eral to it, excellent overgrowths on quartz substrates were ob- Microprobe analyses, X-ray ditrraction data, and cell-dimen- served,and fractured or broken piecesofquartz \ilerecommonly sion calculations of the maximum microcline structure are listed rehealedand overgrown. "Snow on the roof'texture is common in Table 4. Microprobe analysesof pocket microcline yield an on floor-attachedquartz crystals,such that the substrateis cov- averagecomposition ofOr*Ab, and do not indicate any apparent ered with a fine white coating of K-feldspar plus muscovite and/ zoning features. Minor amounts of Sr are preferentially incor- or lepidolite, and in turn is dusted or covered by quartz. On porated into the microcline structure relative to the albite struc- severalcrystals, a multiple-stagegrowh history is shown, pro- ture, but the microcline is essentiallypure KAISi3OE.AVSi dis- ducing agatelike banding. The overgrowth is present on all faces tribution betweenthe T,O and T,m sites was determined from ofthe quartz crystals,but is particularly developedon thosefaces plots given by Stewart and wright (1974) relating D and c cell first coveredby the white K-feldspar plus mica coating. On some edgesand a* and 7* reciprocallattice parameters;nearly complete crystals which have developed overgrowths on completely clear ordering is indicated (Table 4). X-ray diffraction analysisof max- substrates,color differencesreveal multiple-stagegfowth. imum microcline-hosted perthite from the graphic granite hang- ing wall are also listed in Table 4 for comparison. Potassiun feldspar Adjacent to the pocket zone, light-colored intermediate mi- crocline-hosted perthite from the quartz-perthite-albite zone Like quartz, potassium feldspar is ubiquitous throughout all commonly shifts abruptly to a darker-colored orthoclase. Cell zonesofthe layeredintrusive. Within the pocket zone,potassium parameters determined from powder X-ray diffraction analyses feldspar commonly grows as large, single-phase,mardmum mi- of thesefeldspars are given in Table I . Glassy pocket orthoclase crocline penhite megacrysts,ranging in color from light butrtan collectedfrom an exposureadjacent to the Little Three mine was through darker shadesoftan brolvn. Pocket feldsparcrystals are analyzedand is also listed in Table l. euhedral and predominantly exhibit prism and pinacoid faces. Within the New Spaulding pocket, upward-growing megacrysts Albite ofapproximately 30 cm x 30 cm x 25 cm appearto have served as "pillars" helping to prevent collapseof the pocket. Randomly Pocket albite occurs primarily as the coarsely bladed cleave- oriented blades ofcleavelandite are occasionallypresent within landite variety and less abundantly as lathlike inclusions within the K-feldspar blocks. The material from the New Spaulding tourmaline or as exsolved lamellae within maximurn microcline pocket rarely, but on occasion,displays visibly developedover- perthitic crystals.Pocket cleavelanditeis commonly quite coarse $owths. A peculiarly rough, sawtoothlike surfaceis commonly grained, with individual bladed crystals ranging up to 2 cm across, exhibited on the feldspar surfaces.Where two crystals grow to- 0.5 cm thick. The plates are clear to milky white, with no dis- gether, this serrated appearanceis not present. The rough surfaces cernible overgrowth. Cleavelandite in the New Spaulding pocket may be original primary $owth features; some jagged sawtooth grew abundantly along the ceiling and on some of the tourmaline faceshave the fine-grainedwhite "snow on the roof' covering, crystals rooted to the ceiling. Cleavelandite was noticeably less thereby antedating the late-stage stagnant deposition of K-feld- abundant, however, along the floor of the pocket. In smaller spar and muscovite. pockets, cleavelandite was not preferentially located. STERNET AL.: LITTLE THREEPEGMATITE.APLITE 415

Table 4. Chemical and mineral data for maximum Table 5. Chemical and mineral data for low albite microcline microprobe anaiyses microprobe analyses albite lanellae formula cleavelandite* in perthite** proportions* wt. Z oxide* fornula proportions lv si02 68 22 68.55 si 3 oo si02 64.42 IV si 3.00 Al^0^ 19 52 t9.32 vI nt r.or A1.0^ 18.07 VI Al .99 Fe0, Fe203 .09 .06 Fe0, Fe .08 Fe <.01 203 Ca0 .05 .0,i 0l Fe < 0l Na20 I I 82 11.54 Na .99 A" K .OI Na AN"oz .15 20 ,l6.39 Kzo .18 Kzo K .98 Ba0 .01 .01 I 00 Sr .02 Sr0 .69 Sr0 02 .06 Ba0 .01 1.02 Total 99.91 99 80 Total 99.92 averagecl eavelandi te composition:(N..99K.0t)Alsi308

(K.g8Nu averagecomposition: 0zS".0z)(A.1.99Si308) celI proportionst (A) 8.r388(6) (") 94.308(4) Cel I proportions a " b r2.7883(9) B il6.58r(4) pocket specimen** graphic graniter c 7.r576(6) 87 560(3) a (A) 8.s7r5(r) 8.s5s(2) dx a6.352 o'z .78 '12.965(2) Y* 90.464 b 12.96414) ##r qso+ Cc) zo nontro+ttn c 7.2198(r) 7.215(2) (Al ) .97! .04 o (') 90.597(6) 90.6']6(9) v (i3)oo+.s: ###t,o-t.,. .98 r .04 B il5.874(7) ils 85(r) Y 87.705(6) 87.59(l) * average of 25 analyses of cleavelandite from New Spaulding v (i3) 721.33 721.l 3 pocket averaqeof l7 analyses of albite lanellae in naximum 90.450 90 437 microiline perthite neqacryst from New Spaulding pocket 92 261 92.276 # latice parameters for pocket cleavelandite ootro+tlm + i.04 .98 .04 .96 ## z orderinq paraneter based on b and c paraneters - ##t,"-trt 99a .04 .99J.04 (Z = -12.623 3.4065(b) + 7.9454(c)).andcorresponding ireezing temperature, given by Hovis (.]974) 2eo cu Ko, ### Al .ont"nt, in (tro + trm) and (tro.- trm) sites deternined plots'9iven'by Stewart and 2ol 21.04 21.04 from b vs. c and o* vs.'y* 'I ilrisht TT974) 3l-r 3t - .80 -.78 204 50.53 50.57

average of 3l analyses from pocket negacryst (New Spaulding pocket) to the pocket zone. Within the New Spaulding pocket and in '*l uti"" parameters for pocket negacryst immediately adjacent smaller pockets, lepidolite was the only # latice parameters for naximumnicrocline from graphic mica to form. Coarselyintergrown flat books oflepidolite covered grani te zone much of the floor of the New Spaulding pocket, but was absent ##Al .ont"nt, in (t,o + trn) and (tro - trn) sites determined from the roof. Lepidotite books in the New Spaulding pocket from b -vs . c and d* vs .yi pl ots gi Ven by'steNart and l'lri ght (r974) grew to a maximum of l0 cm in diameter and 2 cm thick and occasionally exhibited unusual multiple growth fronts defined by minute inclusions of stibio-bismuto-columbite-tantalite,as well as occasionallybeing coated by this mineral' pocket listed in Table 5 show Microprobe analysesof albite Color-zoned red-violet tepidolite from small pockets west of albite in the cleavelandite habit and slight differences between the New Spaulding pocket was of diferent habit than the me- inclusions in tourmaline or pocket microcline. Similar to albite diumJavender lepidolite from the New Spaulding pocket. The the pocket albite is nearly pure in com- the microcline analyses, red-violet lepidolite shows excellent bow-tie structures, rather position (Ab*) and includes only trace amounts of Sr or Ba. AV than the simple flat books associatedwith the New Spaulding (Table pocket Si orderingbased on cell dimensions 5) showsthat pocket. All the muscovite found was flat and terminated within be classifredas low albite. albite can the pocket. Microprobe analysesof lepidolite from the New Spaulding Micas pocket are listed in Table 6; the averagecomposition is (IGr'- Muscovite and lepidolite are both present within the pocket Na"o,Rboo, Csoor) (Lir 86AIr oeMno,, Feoo,)(Si, ut AL 33)Or0 (Fo e3- portion of the Little Three main dike. Although the amount of OHoo.)r. The polytype of the lepidolite is lM, and lattice con- muscovitein the aplitic footwall is quite small, substantialquan- stants are also listed in Table 6. Pocket lepidolite has nearly tities ofmuscovite or lepidolite are present in the pocket zone. completeF replacementfor OH, and microprobe analysesreveal While muscovite is typically greencolored in the nonpocket peg- an unusually high Si content. Microprobe traversesalong coarsely matite portion of the dike, it is commonly colorlessor very light bladed lepidolite, from base to termination, showed slight in- pink in the pocket zone.Peripheral to the New Spauldingpocket, creasesin the Al, Ti, Mn, and Rb contents accompanied by green-zonedpocket muscovite with colorlesscores was found; a systematicdecreases in the Si, K, and Cs contents. The Li and similar color zoning is recordedwithin pocket tourmaline. HrO contents given were calculated by diference and by charge Light to medium-dark lavender-coloredlepidolite is confined balance. 416 STERNET AL.: LITTLE THREEPEGMATITE-APLITE

Table 6. Chemical and mineral data for lepidolite Table 7. Chemical and mineral data for topaz

nicroprobe analyses microprobeanalyses# wt. % ox i de* base** tern, *** fornul a proportions* wt z oxide fornula proportions si02 32.62 tV si02 55.62 57 88 54.88 v si 3 72 st I .00 A1203 55.56 At ^0- l8 .25 17.00 r 9 83 AI 2a Vl Fe0, Fe203 .06 At 2.00 Ti 02 .04 o2 .07 4.00 vl F l6 86 Fe < 0t Fe0, Fe.0^ .19 .19 t9 lt r.r6 '| *Hzo L90 Mn0 2.03 .55 2.53 Li 1.47 F 1.63 l'ln .1 2 !'lS0 0H 38 .01 0l .01 Fe .0t -F Ca0 .01 .01 .01 2.01 ------2.76 NaZ0 .20 l0 .24 A Total 100.00 K2o 10.79 ll.04 r0.43 Na .Oz .t9)z Rb 1.57 1.48 I .80 Rb .07 averagecomposition: Al2Si04(F.8l0H 20 Cs .01 Csz0 .39 .45 F 8.90 a.6-9 2 L6-9.2 I 02 CelI proportions *Li F LB8 20 0H 13 #H .30 a (A) 4.6505(3) e(') 90 20 D 8.7987(6) B 90. -t c 8.3879(6) Y 90. v (i3):q:.rac Total : 100.00

" Average of 6l analyses of topaz from the New j average composit on{ Spaulding pocket. Analyses are fron early- stage topaz inclusicns in tournaline, and from (K.gzNu.ozRb.ozcr.61)(el.urri.zqMn late stage topaz growing on tourmaline rims. oo)z(Al zesi:.zzoro)(F sqoH oo)z No zoning or systenatic chances noted. * by difference

Cell proportions

a (A) sztl(z) o('.1 eo. o 8.e9e(3) B 100.4t(2) c 10.036(4) y 90. v (AJ) 465 79 were examined in order to characterize their general morphology. Two general types were distinguished, based on terminations. average of 46 analyses of lepidol ite from the l{ew The first type is characterizedby well-developed (001) basal- Spaulding pocket, from both coarse-bladed crystals pinacoid lining the floor and from inclusions in tour-maline terminations typical of Russian topaz from the Ural average composition at base of coarse-bl aded crystal s, Mountains. Thesecrystals were bound by large (110) and (120) rooted to pocket fl oor prism faces,and about 5090ofthem had large (021) dome fuces. *** average composition of crystal terninations, extending towards center of pocket The secondtype had roof-shapedterminations made up ofin- F by difference tersecting(011) and/or (021) dome faces,bound by (110) and (120) prism faces.The generalizedsequence ofthe most impor- tant facesin order ofdecreasingrelative areais (1 I 0), (l 20), (0 I 1), (001), and (1ll). This sequencedeviates somewhat from the Topaz predictions of Donnay-Harker (1937) theory, but is reasonably consistentwith the theory that the best-developedfaces have a Topaz is found only in the pocket portion ofthe Little Three high density of intraplane bonds. main dike, occurringas two or more generations.Excellent topaz In ultraviolet light, the basal sectionsoftopaz from the New specimenswere recovered from the New Spaulding pocket; in Spaulding pocket show fossil growh zones outlined by yellow excessof 28 kg of topaz were removed in the form of approxi- fluorescence.One light-blue topaz crystal contained 0.030/oGe; mately 75 nearly completecrystals. The earliesttopaz crystalsto the fluorescencemay be causedby zonation of Ge, or perhaps form are commonly the largest, and are pale to medium blue or by lines offluid inclusions. blue green.Fluid inclusions up to I mm in diameter are present Microprobe analyseslisted in Table 7 reveal a moderately high in theselarger topaz crystals.Early generationtopaz also occurs F content ofpocket topaz, and yield an averagecomposition of as small inclusions within tourmaline. In the New Spaulding AISiO{F' EpHo ,o)r. No significant differences or zonations were pocket, many of the largerblue topaz crystalswere found loosely found betweenthe various generationsoftopaz crystalsanalyzed. scatteredalong the floor, not obviously attached to either the Lattice parameters for pocket topaz are also listed in Table 7. floor or to the ceiling. Severalblue topaz crystalswere, however, observedto be attachedto the wall of the pocket. Much smaller, Tourmoline colorless,flawless topaz crystalswhich grew only to a maximum of several millimeters in diameter, are found perched on most Pocket tourmaline from the dike exhibits a variety of forms. pocket minerals, particularly on the elbaite specimens.Growth Although schorl is present within the aplite footwall of the dike spirals are well developed on the faces of topaz, as is the case aswell asin the nonpocketpegmatite and graphicgranite hanging with tourmaline and quartz. Gem-quality material is only found wall, it has only been found in one pocket of the Little Three within the smaller topaz crystals;the early-formed blue crystals main dike (Table 2, no. 5). Elbaite is prevalent throughout the are virtually all microfractured. pockets, and the bulk of the material removed is ofvarying shades Approximately 65 topaz crystals sampled primarily from the from dark green to yellow green, or more rarely, amber colored New Spaulding pocket, rangrng in weight from 0.06 to 0.6 kg, or pink. While some of the larger pockets contain giant dark- STERN ET AL,: LITTLE THREE PEGMATITE-APLITE 4t7 green tourmaline crystals intergrown with a cleavelandite matrix Table 8. Chemical and mineral data for tourmaline along the roof of the pocket, many pockets have produced tour- maline "pencils" enclosedin pocket clay. In one pocket, "nail- nicroprobe analYses average fornul a head" tourmaline with sharply chiseled terminations was re- conposition* core** r'im** proportionsr moved (Table 2, no. 5). Tourmaline pencils are commonly olive green or light smoky green, but tricolored pink to light-green to si02 36.50 si 5.95 olive-green pencils also have been removed. One large pocket Ti0^ .12 B 2.99 produced pencils gold-amber

Table 9. Chemical data for stibio-bismuto-columbite-tantalite were analyzed(Foord, 1982a, 1982b) and yielded unique com- positions with highly variable Sb, Bi, Nb, and Ta contents(Table microprobe analyses 9). Of additional interest is the presenceof W and Ti, which are wt X oxi de* range formul a proportions* uncommon for either bismutotantalite or stibiotantalite-stibio- columbite. The calculated average structural formula for this sb203 37.23 28 73 - 44.93 sb 084 mineral, on the basisof four oxygenatoms, is (SborpL rexl{b06r- - 8i203 r 3.55 7 27 21.67 Bi 0 t9 Ta" rrWoor)POo. '| ti02 .27 0.40 - 2.24 Ti 0 05 Columbite si02 0.01 5i

Nb203 24.15 tt 63 - 37.02 Nb 0 6r One, flattened, tabular crystal of columbite (1 cm x 3 cm x pocket la^u- 19.44 5.93 - 27.96 0.29 0.75 cm) from a in the main dike west of the New Spauld- ing pocket was analyzed by emission spectrographyand found H0. 229 I 10 - 4.28 }l 0.03 to contain major Nb and Mn, as well as 7o/oFe, 0.3o/oTi, 0. 150/o - Fe0 0 06 0.0 0 l2 Fe Zr. and l.5VoTa.

Total : 98.6C K-feldspar * muscovite coating

avera9e conposi (sb ti on B4Bi.t9)(Nb.6tTa.29il.03)04 Within the New Spaulding pocket, minerals and fragments that had fallen to the floor were coveredwith a veneerofwhite, hard, *aue"age of 15 analyses of s tibio-bishuto-col unbi te- porcelanouscoating resembli4gmontmorillonite, but identified tantal i te fron the New Spaul di ng pocket as fine-grainedK-feldspar plus muscovite. Included within the coating, which may be as thick as I cm, are fragments of topaz, albite, and tourmaline, and the coatingis commonly intercalated with fine-grained quartz. significantly larger than those calculated for pocket elbaite, as Many of the primary pocket minerals within the New Spauld- expectedfrom compositional differences. ing pocket show some faces with this dusting or coating. The pieces possessed while Apatite matrix intact along the ceiling the least, the material at the bottom was completely coated except for No large crystals of apatite were found within the New Spauld- wherethe coatinghad broken off. This "snow on the roof'coating ing pocket, but one large crystal of K-feldspar was coated with appearsto reflect nearly stagnant precipitation ofthe fine-grained tiny, colorlessapatite, topaz, and stibio-bismuto-columbite-tan- assemblage.Pockets in other locations within the dike occasion- talite. Several clear blue and pink fragments were found in one ally exhibit floor-rooted crystals that are coated preferentially on pocket west ofthe New Spaulding;both varieties ffuoresceyel- only halfofthe exposedfaces, possibly indicating unidirectional low-yellow orangeunder SW UV light. No well-formed crystals flow within the late-stage pocket fluids. were encountered,and the fragments are l-2 cm in maximum diameter, with hopper faces. Cnvsrnr.rzATroN oF THELrrrr.B'I.nnrr MAIN DIKE Microlite-uranmicrolite Despite the curious variation in texture, composition, Specimensof this mineral seriesare rare and have not been and zoning patterns observed at the Little Three mine, found larger than approximately I mm in diameter, but the crys- the closely associatedfine-grained to giant-textured min- tals are always euhedral and intact. Basedon textural relations, eral assemblagessuggest common parageneticevolution this late-stagepocket mineral formed coevally with pink tour- from system. The genetic model proposed by maline, second-generationtopz, and stibio-bismuto-columbite- a single granitic pegmatites tantalite. Microprobe spectralscans ofone grain included within Jahnsand Burnham (1969)for igneous a doubly terminated tourmaline crystal from a small pocket up credibly accountsfor most featuresobserved at the Little dip and adjacentto the New Spauldingshowed Ta, Nb (suchthat Three. The framework of their model postulatesclosed- Ta > Nb), Ti, Bi, Ca, U, Na, and F to be major elements,and system crystallization in the presence,successively, of a Mn, Ce, ta, Pb, Sb, and Sn as minor elementsin addition to O hydrous silicatemelt, of melt and coexistingaqueous fluid, and H. Radiation damageis presentaround grains ofuraniferous and finally of the supercritical fluid phase alone. This microlite. model, basedupon experimentalstudies as well as on field observationsof many layeredpegmatite-aplite dikes, em- phasizesthe role ofwater both as a dissolved constituent This late-stagepocket mineral is black in hand specimen,red in the granitic derivative magma and as the major com- brown in thin section, and forms as euhedral crystals rangrng ponent of the fluid phase. Formation of the San Diego from less than 0.01 mm to 2 cm in length. An estimated 500 pegmatite-aplite dikes from a magmatic origin with an crystals weighing a total of approximately I kg were recovered exsolvingaqueous-fluid phase is alsosupported by isotopic from the New Spauldingpocket. Polysynthetictwinning is com- data presentedby Taylor et al. (1979). monly present, and several specimens display inegular color boundaries.Complete details are given by Foord (1982a). The apparently early-stagealkali partitioning observed Compositions of this mineral were determined to be inter- at the Little Three dike strongly supportsearly vapor sat- mediate between stibiotantalite-stibiocolumbite and bismuto- uration and its subsequentseparation as a medium through tantalite-bismutocolumbite (not known naturally). Four mark- which K and other constituentsare preferentiallyremoved edly inhomogeneouscrystals of stibio-bismuto-columbite-anralite from the footwall melt and transported toward the hang- STERN ET AL.: LITTLE THREE PEGMATITE-APLITE 419 ing wall. Experimental data reported by Jahns and Burn- no discernible overgrowths, however' The abrasion of ham (1969) indicate that in a water-saturatedgranitic sys- pocket minerals and mineral fragments occurred during tem with coexisting supercritical aqueous fluid, K is both pocket rupture and postrupture aqueous flow. The removed from the melt by the vapor phasein preference lepidolite on the floor of the New Spaulding pocket was over Na and can travel rapidly to the hanging wall via rounded and abraded on the terminations which had no the vapor phase in responseto a temperature gradient. porcelainous coating on them. It appears that abrasion With a starting composition near the thermal minimum occurred after formation of the white coating, which in for the confining pressure,loss of K plus other volatiles turn postdatedpocket ruPture. could result in the formation of the fine-grained sodic constraints footwall primarily from residual melt, with K transferred Temperature to the crystallizing K-rich hanging wall by the volatile Results of experiments conducted by Jahns and Burn- water-rich fraction. The composition of the giant-textured ham (1969) on the water-saturatedhaplogranite system graphic granite doesnot correlate to either the minimum indicate that crystallization of San Diego-type granitic or to any cotectic of the NaAlSi3O8-KAlSi3O8-SiO2-HrO pegmatites,following exhaustion of the silicate melt, can systemas determinedby Tuttle and Bowen (1958).Recent be expectedto persist down to temperaturesof approxi- experimental studies of graphic granites by Fenn (1986) mately 600-550"C under confining pressuresofapproxi- indicate that graphic intergrowths of quartz and K-feld- mately 2kbar,where the presenceof such componentsas spar are produced by nonequTlibrium crystallization in HF, B2O3,and LirO suppressesthe final crystallization pegrnatitic melts with water contents ranging from un- temperature toward the lower limit given. Oxygen and dersaturatedto saturated. These textures were not pro- deuterium isotopic data given by Taylor et al' (1979) on duced in water-supersaturatedruns, which instead pro- minerals, whole-rock samples,and fluid inclusions from duced separateidiomorphic crystals.This evidencesuggests pocket quartz from a suite of San Diego gem-bearingpeg- that the graphic granites present in the hanging wall of matite-aplite dikes support the experimental temperature pegmatiteslike the Little Three main dike formed from and pressurerange determined by Jahns and Burnham, a water-rich melt rather than primarily from a supercrit- althoughlower temperaturesfor formation of tourmaline- ical vapor phase(cf. Martin, 1982,vs. Simpson, 1962), bearingpockets are suggestedby London (1986b)' Taylor with K enrichment enhanced by the presence of an et al. (1979) concludedfrom their data that the intrusives exsolvedvapor phase.Small cavities distributed through- were emplaced at temperatures of approximately 700- out the hanging wall also support the presenceof a fluid 730'C, followed by supersoliduscrystallization of the low- phaseduring the formation of the graphic pegmatitehang- er aplite and upper border zones persisting to tempera- ing wall. tures down through approximately 540t. Formation of Pocket material presumably formed in the near or vir- the graphic granite hanging wall occurred over a temper- tual absenceof silicatemelt, after the melt had beenlargely ature range of approximately 630-540{ (Taylor et al.' exhaustedby crystallization of the anhydroussolid phases 1979),indicating that graphic granite crystallization took predominant in the aplitic and pegmatitic portions of the place at lower temperaturesthan the aplitic footwall and dike. The rarer alkalis and metals of the system became did not crystallize until after a significant portion of the sufficiently concentrated in these pocket subsystemsto basalaplite had formed (Taylor etal.,1979; Foord' 1976; allow crystallizationof minerals containing theseelements Jahns and Burnham, 1969).These temperature determi- as major constituents.Such minerals include fluorine-rich nations arebased in part on isotopic equilibration between : lepidolite, elbaite, topaz, microlite-uranmicrolite, stibio- hydrousminerals and a coexistingaqueous fluid with DD bismuto-columbite-tantalite, and columbite. -50V*, which is consistent with dD values obtained on Pocket rupture is believed to occur when the internal fluid inclusions from Little Three pocket quartz reported pressureof the pocket exceedsthe confining strength of by Taylor et al. (1979). the surrounding dike material. A certain oversteppingof Subsolidusformation of the pocket zone is believed to pressureis necessaryto activate the rupture process;the have persistedover an averagetemperature range of40- continued build-up of the proportion of aqueous-vapor 50', from approximately 565 to 520C, or lower, under a phaseexsolved from the silicate melt, perhaps enhanced confrning pressureof approximately 2 kbar, which may by the crystallization of tourmaline (London, 1986a),is have increasedas pocket formation progressed(Taylor et believed to be the mechanismby which the internal pres- al., 1979). This pocket-crystallization temperature range sureis increased.Depending on the geometryofthe pocket is partly basedon an averageA'8O of 0.457- betweenbase and the rate of escapeof containedfluid, the rupture event and termination of pocket quartz sampledfrom SanDiego may be minor to catastrophic. In the case of the New pegrnatite-aplitedikes (Taylor et al., 1979).The data which Spaulding pocket, the available evidence suggeststhat they reportedfor quartz overgrowthson Little Three pock- rupture was violent, and pocket minerals were broken off et quartz, however, yield a At8O differenceof l.87oocom- and fractured extensively, producing doubly terminated pared to core values, suggestingthat either deposition of crystals and repaired fragments of crystals. Subsequent the overgrowths persisted through considerably lower rupture events and later through-flowing aqueous solu- temperaturesthan core formation, or that there was an tions resulted in additional breakage.Such material has abrupt increasein the isotopic composition of the fluid. 420 STERNET AL.: LITTLE THREEPEGMATITE.APLITE

Equilibration of pocket minerals at lower temperatures imentally shown to be secondarymetasomatic products is also supported by Z ordering parameters determined generatedby the action of late F-bearing aqueous fluids for albite and orthoclasefound within or proximal to the on early lithi]'m aluminosilicatesand K-feldspars (Haw- New Spaulding pocket; Z values were calculated from thorne and Cernj', 1982; Stewart, 1978, 1963; Munoz, equationsgiven by Hovis (1974) and Thompson et al. l97l). Pocket micas and feldsparsfrom the Little Three (1974) relating b and c cell parametersto Al/Si ordering. show no evidenceof replacingpre-existing minerals, how- The resultsofthese calculationsare listed in Tables I and ever, and appear to be primary phases. 5 and yield temperaturesfrom 500"Cdown toward 420t. Experimental studies on the roles of F and B in water- While thesecalculated temperatures are not presumedto saturatedgranitic melts reveal several important factors be absolute,they do suggestequilibration to significantly that may bear on pocket formation. Wyllie and Tuttle lower temperaturesthan those for primary pocket crys- (1961)noted that the increasein F contentin the system tallization. servesto depressliquidus temperatures,enhances crys- tallization from the vapor phase,and depolymerizesthe Pocrnr EvoLUTroN: melt into which F preferentially partitions relative to the Ror-n or voLATTLE coNSTITUENTS vapor phase. Experimental data reported by Manning Crystallization of phasesin large pockets such as the ( I 98 I ) on the role ofF on the water-saturatedhaplogranite New Spaulding pocket, with unusually high hyperfusible system at I kbar concur with the findings of Wyllie and concentrations,was probably strongly influenced by the Tuttle (1961) and support the conclusion that F serves volatile constituents. The New Spaulding pocket, unlike to increasethe silicate solute content ofthe vapor phase. smaller miarolitic cavities, displays strong mineral seg- Manning also noted that increasedF content in the melt regation; K-rich minerals such as fluorine-rich lepidolite displacesthe quartz-K-feldspar-albite minimum liquidus and microcline crystalsare preferentiallydistributed along position away from that for the F-free systemand toward the floor of the pocket, while Na- and B-bearing minerals the albite apex. such as cleavelanditeand elbaite form the matrix lining Experimentaldata discussedby Pichavant ( I 98 I, I 983) the roof. This alkali segregationis the reverseofthat ob- on the role of BrO, on the water-saturatedgranite system servedthroughout the bulk of the layeredintrusive, where indicate that B serves as a network-forming cation, in- the basalsection is sodic while the hangingwall is potassic. creasingthe solubility of SiO, in the aqueousfluid, and Most Mn is concentratedin elbaite, although Mn is also strongly modifies the distribution of Na, K, Si, and Al presentin the floorJining lepidolite. Most quartz crystals between melt and vapor phases.While the vapor phase project down from the roof, suggestingthat Si activity is in the experiments was found to become enriched in B, also increased roofward. Fluorine-bearing minerals are Si, and Na, the melt was enriched with K and Al. The distributed betweenboth roofand floor, but floor-lining partition coefficient of BrO, between silicate melt and a fluorine-rich lepidolite has nearly complete F for OH re- coexistingvapor phasewas experimentallydetermined by placement while only partial replacementis found in el- Pichavant(1983) to be approximately0.33 + 0.02 at I baite. Topaz with moderately high F contents is ubiqui- kbar. While this panition coefficient favors the vapor tous. Li and Al distributions appear to be approximately phase,significant amounts of BrO, are still able to dissolve uniform throughout the pocket. in the silicate melt. The behavior of granitic melts with excessHrO, as well The implications of the experimentalwork with respect as the rangeof crystallization and solidus temperaturesof to pocket formation suggestthat the F plus B contents of suchsystems, has been shown experimentallyto vary con- Iarge pockets serve not only to depressliquidus temper- siderablywith F content (Wyllie and Tuttle, l96l; Bailey, atures,but also to strongly modify the distribution of the 1977;Manning, 1981;Dingwell et al., 1985),B content alkalis and to ultimately enhancecrystallization from the (Chorltonand Martin, I 978; Pichavant,198 l, I 983),and vapor phaseby increasingthe silicate solute capability of Li content (Stewart, 1978; Burnham and Jahns, 1962; the aqueousfluid. Assuming that some residual melt was Wyllie and Tuttle, 1964). The high concentrations of F, present during pocket formation and was distributed B, and to a lesserextent Li, may be largely responsible floorward, then the experimentally determined effectsof for the internal zoning oflarge pockets such as the New high B and F concentrations may account for the floor- Spaulding.That F activity dominates over Li activity at ward distribution of K-rich minerals (lepidolite, K-feld- the Little Three is indicated by the abundanceofF-bearing spar), while Na-, B-, and Si-rich minerals (cleavelandite, tourmaline in addition to high-F topaz and virtually F-end- elbaite, quartz) are preferentially distributed roofivard in member lepidolite. Li is present in the mica and tour- severalofthe larger pockets.Formation ofthe largepock- maline structure, but the common lithium aluminosili- ets may have been largely influenced by the F-, B-, and catesassociated with Li-rich pegmatites,such as petalite, Li-rich aqueous-vaporphase; crystallization occurredpri- spodumene,and eucryptite (London and Burt, 1982), are marily from the volatile-rich vapor phasein the presence noticeably absent from the Little Three. Lithium alumi- of nominal amounts of residual silicate melt, with sub- nosilicateshave beenobserved to exhibit pseudomorphic sequent crystallization from the vapor phase only, and replacementby albite and micas (London and Burt, 1982), persistedto considerablylower temperaturesthan the F-, and some lithium micas have been observed and exper- B-, and Li-deficient pockets. STERN ET AL,: LITTLE THREE PEGMATITE-APLITE 421

Prusn-nqurLIBRIA cALcuLATroNS AND the constraint of quartz saturation used here. The ther- PREDICTIONS rnodynamic properties of reactions involving minerals and at temperaturesless than The geochemicalevolution of the Little Three pegma- water, as well as aqueousspecies the Fortran program supcnr from tite-aplite dike during crystallization can be approxi- 550t, arecomputed by given by Helgeson and mately modeled with phase-equilibriacalculations relat- equations, data, and coefficients (197 Walther and Helgeson(1977), and ing the observedmineral assemblagesand a supercritical Kirkham 4, l9'l6), (1978, l98l). Thermodynamicproperties aqueousfluid. Theserelations are evaluatedin the present Helgesonet al. greaterthan 550"C at study through isobaric phase diagrams, which constrain ofaqueous speciesat temperatures and data reported phaseboundaries as a function oftemperature and activity 2 kbar are computed from equations (1980) Mckenzieand Helgeson(1984). ratios of aqueous-electrolytespecies in the systemNarO- by Mckenzie and given by Barton et K'O-AlrO3-SiOr-HrO. The mineral assemblageK-feld- Thermodynamic data for fluor-topaz (1982) incorporated into the data base and spar * albite + quartz + muscovite is of primary interest al. were also The standard state adopted here, owing to the stability of this assemblagethroughout are used in the calculations. for unit activity of the all zonesof the system.A similar approach for modeling for minerals and liquid HrO calls pure pressureand temperature,and that stableassemblages observed at the SprucePine pegmatite substanceat any to unit activity ofa is discussedby McTigue(1981). Not only doesthis graph- for aqueouselectrolytes corresponds referencedto infinite di- ical approach allow relationships between stable mineral hypothetical one molal solution pressure assemblagesand calculatedactivity ratios ofaqueouselec- lution at any and temperature. at crystallization temperatures,but trolytes to be assessed Equilibrium phaserelations this method also indicates the sensitivity of reactions to data to the supcnr fluctuations in temperature, cation activity, and mineral The addition ofthe aqueous-species permits pegrnatite-aplitephase relations to stoichiometry. Becausethe bulk composition of the Little data base the temperaturesof interest, at Three dike lies near the thermal minimum of the hap- be evaluated at the elevated most readily by plotting logranitesystem (Simpson, 1965;Jahns and Tuttle,1963), 2 kbar. This can be achieved quartz-bearing as a function of tem- this procedure provides a limiting-case example of the stable assemblages perature ratios of the electrolytes evolution of the composite pegmatite-aplite sequence; and logarithmic activity within the system KAlSi3O8- however, these phase-equilibria calculations model the in solution. Reactions were balanced conserving Al3*, water-rich granite system and are not meant to apply to NaAlSirOr-SiOr-HrO ratios of log 46vHr and log 4ipa*zH*y the high F-, B-, and Li-bearing pocket subsystems.Similar thereby allowing the mass-action equations for modeling of such pockets is not currently possible owing to be evaluated. Generating permits the activity to insufficient thermodynamic data for pocket minerals reactionswithin the two subsystems log arp,vu*;to be solved for in terms and aqueousspecies bearing F, B, and Li. ratios log 46vn*; arrd calculatedby suPcRr,as well The system observed at the Little Three is assumed ofthe equilibrium constants minerals if nonstoichiometric. Because here to have maintained closed-systemconditions, which as the activity of for the isobaric reactions con- is a reasonableassumption concluded by Taylor et al. the equilibrium constants the mass-action (1979)from the isotopic disparity betweenSan Diego peg- sideredhere are temperaturedependent, given reaction constrainssta- matite-aplite intrusives and the wall rocks into which they equation correspondingto a function of temperature, are emplaced.The calculationspresented here assumeno bility field boundaries as a nonstoichiometric min- significant CO, present in the fluid phase, supported by aqueous-electrolyteactivity, and fluid-inclusion data from Little Three pocket quartz given eral or HrO activity. graphs in Figures4a and 4b plot the stable, by Taylor et al. (1979), and assumeunit activity of HrO. The shown assemblagesat qtarlz saturation The assumptionof equilibrium in thesecalculations is not stoichiometric mineral for the two subsystemsKrO- as well founded, and crystallization of certain portions of from 30 to 900"C, at2kbar, NarO-AlrOr-SiOr-HrO, respective- the dike system (e.g., graphic granite) may be best de- AlrO3-SiOr-HrO and 4a that within the crystallization-tem- scribed as nonequilibrium; however, these approxima- ly. Figure shows perature 700-540oC,muscovite will not appear as tions provide a reasonablemodel for the actual system range phasewith K-feldspar and quartz until temper- and serve as a useful point of departure for initial calcu- a stable with a correspondinglog ar**,^, lations to illustrate general trends in the supercritical ature falls below 630'C, 4.00. Because aqueousphase. of approximately -LH?/RT2: (d ln a,**,^,r/07), Thermodynamicdata base =Q Prediction of equilibrium phaserelations between a fluid for the K-feldspar-muscovite reaction at crystallization phaseand the observedpegmatite-aplite mineral assem- temperatures,log a6*n*1remains essentiallyconstant as blagesrequires calculations of equilibrium constantsfor temperaturefalls toward 540'C, producing the nearly hor- only thosereactions in the systemNarO-KrO-Al2O3-SiOr- izontal topology of the K-feldspar-muscovite boundary. HrO involving quartz. The presenceof quartz in all as- The phaserelations depictedin Figure 4a also suggestthat semblagesobserved in the Little Three main dike justifies formation of muscovite at 630'C takes place in response 422 STERNET AL.: LITTLE THREEPEGMATITE-APLITE

Table 10. Chemical data for muscovite \ 'u1rrt microprobe analyses* \ wt. Z oxide formula proportions I IV 14uscovite si02 45.87 si 3.04 \ Al 87 Al .96 I 203 36 ,\ Ti02 .07 4 .00 \- z Fe0, Fe203 1 -21 VI Al I .92 o2 Itln0 .08 Fe .06 a- Ti <.01 I 1"190 0.0 l,4n < .01 Pyro- Anda ,slte I I imani te 00 Ca0 1.99 phylI i I Na20 05 r0.28 Na .l4 Kzo K .86 F .96 I .00 400 600 orro 4 .00 F .20 trmptnATUnroc -F = 0 .40 0H 1.77 I .97 Fig.4a. Phaserelations for thesystem KrO-AlrO3-SiOr-HrO Total I 00.00 plottedas a functionof temperatureand log a(<+s+r,for peg- average conposition: matite-aplitemineral assemblages in equilibriumwith a super- (K.86Na. (Al ( F.t criticalaqueous fluid. P, : Pszo: 2kbar; ar*: l. Solidlines t4) .geF".o3Ti oosiln.oos)z(Al.sesi :.oq0t o) o0H.gg)z denote stability field boundariescalculated for stoichiometric mineralsat qvafiz saturation.Dashed lines contour decreasing * averaEeof I 2 anaiysesof non-pocketnuscovi te, 4ur26si3yo1e1or12.As activity of the pure muscovitecomponent from the pegmatite zone decreasesfrom I downto 0.5, the upperthermal stability limit t H20 by di fference of the assemblageK-feldspar (maximum microcline) + musco- vite + quartzincreases from 640t up to 710"C.

first approximation made concerning mineral stoichi- Becausethe K-feldspar-muscovite boundary ris- to, and at the expenseof, the breakdown of sillimanite. ometry. es only slightly with decreasingtemperatures, minor sub- Yet the absenceof both sillimanite or andalusitefrom all into the K-feldspar or muscovite structure observedassemblages in the pegmatite-aplitedike, as well stitutions either producesa substantialshift in the upper thermal stability as the presenceof muscovite at the outer zones of the limit K-feldspar + muscovite + quartz. system,indicates that stoichiometric mineral assemblages of the assemblage Figure 4a illustrates the significant expansion ofthe sta- plotted in the temperaturerange 700-540"C are not con- sistent with the observedsequences. bility field for muscovite with increasingdeviation from As muscovite is reduced The occurrenceof muscovite in the upper hangingwall stoichiometry. activity to 0.8, stability range is boosted from 630 to as well as in the lower aplite, where early-stagecrystalli- the upper thermal 655"C with the correspondinglog aK*/H*\ratio increasing zation temperatureswere presumablygreater than 630t, from 4.00 to 4.05. Further reduction of the activity of the suggeststhat the likely sourceofinconsistency lies in the pure muscovite component to 0.5 expandsthe muscovite stability field to 710'C, with log dK*/Hr)of the coexisting fluid phaseincreasing to 4.2. Nonstoichiometric muscovite in the Little Three dike may account for the common occurrenceof the assem- blage K-feldspar * albite * muscovite + quartz in most regionsof the pegmatite-aplitedike, including early-stage crystallization areas where stoichiometric muscovite is not predicted to be stable.Microprobe analysesof pocket feldspar and micas from the Little Three main dike show

Andalusite that pocket lepidolite contains nearly complete F substi- tution into the hydroxyl site, as well as significant Li sub- phyll'ite stitution and Mn substitution for octahedral Al. Alkali feldsparanalyses, however, yield nearly pure end-member compositions.These data suggestthat the minor elements 200 400 600 800 ofthe systemare preferentiallyincorporated into the mica tttr,tpERATuREoc structure relative to the alkali feldspar structures,serving Fig. 4b. Phaserelations for the systemNarO-AlrOr-SiOr-H, to significantly expand the muscovite stability field, and plotted as a function of temperatureand log ac{a+a+).Pr: Pap thereby stabilizing the assemblageK-feldspar + albite + : 2 kbar; dnzo: l.Solid lines show stability field boundaries muscovite + quartz at outer zones of the system where calculated for stoichiometric minerals at qtartz saturation. the stoichiometric assemblageis not predictedto be stable. STERNET AL.: LITTLE THREEPEGMATITE-APLITE 423

2 kb

o.5 a(,Vtusc): auzo):1'o Y Ol '/' a1tu"d:'8 o o.o i-l.---'-,,''"'--.'a" l,-ur"1=.a / :.-1--'-,"--<\\. , a. .=.8 Y Imuscl ef ,ect ol ,7 i decrcasing.IHrO) -0.5 rr' "iuzol='st/ a Inrc,

- 1.O 500 600 700 900 Temperature "C

Fig. 5. The reactionKA1,(AISi3O,0XOH), (muscovite) + SiO, (quartz): KAtSi3O, (maximum microcline) + A1'SiO' (sillimanite) + HrO (iq.) plotted as a function of -log K of the reaction and temperature, at 2 kbar. All phasesinvolved in the reaction are stableat unit activity at the point where - log K : 0. Decreasing- log K from 0 along the solid curve illustratesthe effectiveincreased thermal stability limit of the assemblagemuscovite + quartz + maximum microcline + sillimanite * HrO when activities of all phasesexcept for muscovite are held constant at l, such that -1og K : log a-"*o",,".The dotted curvesillustrate the effectivedecrease of the thermal stability limit of the assemblageas 4;,o decreasesfrom 1.

An approximation of the activity of nonpocket mus- quartz observedthroughout most zonesof the pegmatite- covite was calculatedfrom the averageof 10 microprobe aplite dike. analysesof muscovite sampled from the pegmatite zone Deviation from unit activity of HrO in the coexisting (Table l0). This calculation was computed for the ther- fluid phase,however, would promote the reverseeffect of modynamic component KAlr(AlSi3)O,o(OH),in solid so- decreasingmuscovite activity. These opposite efects are lution with NaAlr(AlSi3)O,o(OH)r,where the standard- illustrated in Figure 5, which graphs the reaction statethermodynamic data for thesecomponents are given muscovite + quartz: sillimanite * microcline + HrO by Helgesonet al. ( I 978) and refer to a completelyordered -log tetrahedral-sitedistribution ofAl3* and Sia*,discussed by at2-kbar pressure,as a function of temperatureand - Bird and Norton ( 198l). The correspondingactivity-com- K of the reaction. Figure 5 showsthat where log K : 0, position relation for the pure muscovite component,given activity of all phasesis unity, and the correspondingtem- by Bird and Norton (1981),is perature of 630"C representsthe upper thermal stability limit where all stoichiometric phasesare stable together. a : (X"..")(X ^:*,.2)2(Xo1r.,tr6X&'.-,t,-X& ie.,a2)2(X6s)2. Following the solid curve in Figure 5 to increasing tem- peraturesillustrates the effective expansion of the mus- This relation was derived from Equations 46 through 48 covite stability field resulting from depressingmuscovite ofHelgeson et al. (1978) and equationsand data reported activity, ifthe activities ofall other phasesare held con- by Helgesonand Aagaard (1985) and agreeswith ordered stant at unity such that -log K: log 4m*o,ie.The dotted standard-statesite distributions, equations,and data giv- curvesproject the reactionwith decreasedactivityofwater en by Helgesonet al. (1978)for muscovite. from unity down through 0.8, illustrating how decreased The analysesof nonpocket muscovite from the peg- activity of HrO in the fluid phase servesto depressthe matite zone yield an averagecomposition of (&rNao,o)- upper theflnal stability limit of the mineral assemblage (Alo Feo Ti=o Mn.o (Alo Si3 (OHo Fo nu o, o, o,), ru oo)O,o r* ,o)r. muscovite + quartz + microcline. Applyrng this composition to the above activity-compo- The effectsof depressingaHzo in the fluid phaseof the sition relation allows an approximate calculation for ac- Little Three system,however, is presumedto be negligible. pure muscovite component to be made based tivity of the Fluid-inclusion data reported by Taylor et al. (1979) from primarily on K, Al, and OH distribution: Little Three pocket material yield extremely low X.o, val- ues (-0.001), and data from neighboring district pocket o = (XK-.^)(XN*.uz)2(X*t.tro)(Xo")' = 0.6 material yield averageequivalent NaCl salinities of 4 to which shifts the upper thermal stability limit of the as- 5 wto/o(< I molal). The effectsof such low salinities on semblagemicrocline * muscovite + quaftz + HrO to the activity of HrO are shown by Helgeson(1981, Fig. 690t. This approximate value of a*u""o*it": 0.6 supports 7.31) to be extremely low; at 2kbar, over the temperature modeling the evolution of the Little Three system with range 0-500"C, a I molal NaCl solution is shown to de- nonstoichiometric muscovite to account for the common pressHrO activity from unity to approximately 0.98. This occurrenceof the assemblagemuscovite * K-feldspar * decreasefrom unit activity is insignificant on the loga- 424 STERNET AL.: LITTLE THREEPEGMATITE-APLITE

Paragonite + Muscovi + te + Quartz + z

Andalus i te Sillimanite * I Pyro- I + phylliteI Muscovite K-fel dspar + * r Quartz Quartz Quartz I

truprnntune,oc Fig. 6. Phaserelations for the systemNarO-KrO-AlrO3-H2O plotted as a function of log a^"-,r-, and temperature,for pegmatite- aplite mineral assemblagesat quartz saturation, in equilibrium with a supercritical fluid phase.P, : P^ro : 2 kba4 a^ro: 1. Solid lines denote stability field boundaries calculated for stoichiometric minerals. Dot-dashed lines project the K-feldspar-muscovite boundary, contoured for decreasing4musodte. Stippled lines contour log ao*,r*, of the fluid phase. rithmic scaleand thereforeis not consideredan important dicted to remain essentiallyconstant within the range of factor in the phase-relationcalculations here. 4.10 (cf. Fig. 4a), while log 46N6*7H.;rises gradually several To trace the path along which muscovite + K-feldspar + tenthsof a log unit from approximately4.55 to 4.75.The albite + qttartz + aqueous solution all coexist requires exact values at any given temperature, however, are de- evaluation of the equation pendent on deviation from mineral stoichiometry or unit activity of HrO. NaAlSi3O8+ 2KAlSi3Or + 2H*: KAlr(AlSi3O'oXOH), Figure 7 plots isobaric isothermal activity diagrams + K+ + Na* * 6SiO, adding H* and F- to the systemKrO-AlrO3-SiOr-HrO, at probable primary pocket-forming temperatures for that simultaneouslyrelates all four solid phasesas well as F-bearing pockets containing topaz. These figures show the electrolytesof interest.The correspondingmass-action the systematicincrease sf log a1H*xr-rand nearly constant equation is log a,**r"*,of the fluid phase,with decreasingtemperatures, log K: log c,".r"-, * log a""r"*, necessaryto stabilize the pocket assemblageK-feldspar + quartz + OH-muscovite + F-topaz + HrO. + log[(a!*) (a^u""o,r..) / (alr.o,u"*) (arou")], which can be solved for either log ar**,r*,or log c^"r"*, in Suvrulnv AND CoNCLUSToNS terms of the other. With the use of this mass-actionequa- Field relations coupled with experimental and isotopic tion, combined with the known log a,**r"*yvalues at a given data from the literature supportcrystallization ofthe Little temperature for the K-feldspar-muscovite reaction Three pocket pegmatite-aplite dike from a magmatic sys- boundary from the K2O-AlrO3-SiOr-HrO system, con- tem involving an initially hydrous aluminosilicate melt tours of log d,**,"*, and the K-feldspar * muscovite re- with coexistingsupercritical aqueous fluid. Closed-system action line were projected onto the albite field of Figure fractional crystallization of the basal aplite largely from 6. Dashed lines also trace the K-feldspar + muscovite + the melt presumably beganshortly after emplacementof albite + quartz path with muscovite activity reduced from the dike at approximately 73G-700oC,with subsequent unity to 0.8 and 0.5. As crystallization progresseswith crystallizationof the associatedgraphic pegmatitehanging falling temperatures,log d,**,"*,of the fluid phase is pre- wall from water-saturatedmelt in the presenceof a su- STERNET AL.: LITTLE THREEPEGMATITE-APLITE 425

- percritical aqueous phase. Pocket pegmatite evidently ;"o-;-----l formed last, in the near-absenceof silicate melt, at tem- z-ro- I (+ Hr0, quartz) peraturesof approximately 565-525'C. Crystallization of | the large pockets may have persistedto even lower tem- K fel dspar (max. microclin peratures,where unusually high concentrations of F, B, and Li are believed to have strongly affectedthe behavior -a(musc1=l + 'o of the melt and vapor phase. Otl- nuscou;te a(mrsc)='8 Electron-microprobe analysesofminerals from the New f, Spauldingpocket yield a pocket assemblagethat includes elbaite with an uncommonly high Mn content; lepidolite F - topaz with nearly completereplacement of F or OH; moderately 3 F-rich topaz; quart4' and nearly pure alkali feldsparsin- andalusite cluding maximum microcline (Oror) perthite megacrysts and low albite (Abrr) occurring as cleavelandite. Minor phasesfound in pocketsfrom the Little Three main dike include microlite-uranmicrolite, stibio-bismuto-colum- -10 -9 bite-tantalite, and columbite, plus very rare beryl, apatite, Lo9 a(H+)(F-) and hambergite. X-ray analysesof samples from other zones of the pegmatite-aplite dike indicate that the per- thite composition of the graphic hangrngwall is a maxi- mum microcline host with exsolved low albite lamellae. Elongateperthite crystals sampled from the central quartz- perthite-albite pegmatite zone, however, yield an inter- mediate microcline structure and commonly show an abrupt light- to dark-colored transition adjacent to the pocket zone; the dark-colored material was determined + rlt 0H - muscovite by X-ray diffraction methods to be orthoclase.No pocket '(musc)-' L orthoclasewas recovered from the Little Three main dike, lmusc)='8 although one specimenfrom a pegmatite exposureadja- ;; '" nusc) cent to the Little Three main dike was found. 3 Large pockets such as the New Spaulding pocket show F- topaz extensivemineral segregationwith alkali partitioning par- andalusite tially or markedly reversed relative to that observed throughout the bulk of the system.Minerals from the New rl Spauldingpocket are distributed such that the K-bearing -10 -9 minerals including lepidolite and microcline are largely Los a(H+)(F-) distributed floorward, whereas the Na-, B-, and Si-rich minerals incuding cleavelandite, elbaite, and quartz tend to form along the ceiling. Fluorine-bearing phaseswere distributed ubiquitously, although the F-rich lepidolite along the floor yields proportionately the highest fluorine K fe1dspar replacement of the hydroxyl site of all hydrous phases (max. microcl i ne) present in the pockets. Al and Si were also ubiquitous, although large quartz crystals were more commonly found ^4 + projecting down from the roof, and abundant topaz was La(rrsc;=1'o found intergrown along the floor, but not obviously at- muscovite a1rur.;='8 a(,nusc)='5 (- 3 Fig. 7. Phase relations for the system llrO-AI2O3-H2O- - !opaz HF plotted as a function of log a,"*^-, and log a6,ye-'y,dt 2-kbar pressure,at probable crystallization temperatures forhigh F-bear- ing pockets. These relations illustrate the systematic increase of phase,with temperatures, necessary andal usite a*-r-, in the fluid decreasing 2 to stabilize the assemblagemadmum microcline + muscovite + l1 -lo -9 quartz + fluor-topaz. Solid lines denote stability field boundaries Los a(H+)(F-) calculated for unit activity ofHtO and unit activity ofall minerals except muscovite; pocket muscovite is modeled by contouring 4n.*ntc from I down through 0.5. 426 STERNET AL.: LITTLE THREEPEGMATITE-APLITE tached.Experimental data reported on high F and B con- Barton, M.D., Haselton, H.T., Hemingway, B.S., Kleppa, O.J., centrationsin the systemKAlSi3Os-NaAlSi3OB-SiOr-HrO and Robie, R.A. (l 982) The thermodynamic propertiesof fluor American Mineralogist, 67, 350-355. show that thesevolatile constituentsmay topaz. serveto modifu Bence,A.E., and Albee, A.L. (1968) Empirical correction factors the behavior of alkalis in a similar manner to that ob- for the electron microanalysisofsilicates and oxides. Journal servedin the New Spauldingpocket, to lower the liquidus of Geology, 76, 382-403. temperatureof the residual silicate melt, to promote crys- Bird, D.K., and Norton, D.L. (1981)Theoretical predictions of phase minerals: tal growth from the vapor phase,and to increasethe silica relations among aqueoussolutions and Salton Seageothermal system. Geochimica et Cosmochimica Acta, solute capability ofthe vapor phase. 45, t479-1493. Predictions from calculations ofequilibrium phasere- Borg, I.Y., and Smith, D.K. (1969) Calculated X-ray powder lationsamong stoichiometric'minerals in the systemNarO- patterns for silicate minerals. Geological Society of America KrO-AlrO3-SiOr-HrO are not entirely consistentwith the Memoir 122. Burnham, and R.H. (1962) A method for deter- observed mineral assemblages C.W., Jahns, at the Little Three mine; mining the solubility of water in silicate melts. American Jour- however, the approximation of mineral stoichiometry is nal of Science,260, 72l-7 45. useful as a basis for predicting general trends among Chorlton, L.B., and Martin, R.F. (1978) The effect of boron on aqueous-electrolyteratios, effectedby changing temper- the granite solidus. Canadian Mineralogist, 16,239-244. ature and mineral assemblages.Solution-mineral-equilib- Colby, J.W. (1968) Quantitative analysisof thin insulating films. In Newkirk, G. Mallett, and H. Pfeiffer, Eds. Advances in phases J. ria calculationsfor stable in the systemKrO-NarO- X-ray analysis,Volume 11. Plenum Press,New York. AlrO3-SiOr-HrO are particularly useful for modeling the Dietrich, R.V. (1985) The tourmaline goup. Van Nostrand- nonpocketportions of the pegmatite-aplitesystem, where Rinehold Co.. New York. the bulk composition lies near the thermal minimum of Dingwell,D.B., Scarfe,C.M., and Cronin, D.J. (1985)The effect viscositiesin the systemNarO-AlrOr-SiOr; im- the haplogranite system. This graphical of fluorine on approach indi- plications for phonolites, trachytes, and rhyolites. American catesthe virtual temperature independenceof log a,**r"*, Mineralogist, 70, 80-87. and log 4qq"*zu*ygiven the stableassemblage K-feldspar + Donnay, J.D.H., and Harker, D. (1937) A new law of crystal albite + qtaftz + muscovite. Microprobe analyses of morphology extending the law of Bravais. American Miner- pocket phasessuggest that while alkali feldspar compo- alogjst, 22, 446-467 . Fenn, P.M. (1986) On the origin of graphic granite. American pure sitions are virtually end members with minor im- Mineralogist, 7 l, 325-330. purities, the mica structuresmay host major substitutions. Foord, E.E. (1976) Mineralogy and petrogenesisoflayered peg- Modeling muscovite nonstoichiometry by decreasingthe matite-aplite dikes in the Mesa Grande district, San Diego activity of the pure component from unity down to 0.5 County, California. Ph.D. dissertation, Stanford University, significantly increases Stanford, California. the thermal stability limit of the - (1977) The Himalaya dike system,Mesa Grande district, assemblageK-feldspar + albite + quartz + muscovite at San Diego County, California. Mineralogical Record, 8, 461- 2 kbar from 625'C upward to 700"C. Substitutions into 474. the muscovite structure may, therefore, help to stabilize - (1982a) Bismuthian stibiocolumbite-tantalite and other this assemblageat the outer zonesofthe dike, whereinitial minerals from the Little Three mine, Ramona, California. Fourth annual MSA-FM Symposium,February 1981,Tucson, temperaturesof formation are expectedto have been ap- Arizona. In G.E. Brown, Jr., Ed. The mineralogyofpegmatites. proximately 700"C. American Mineralogist, 67, l8l-182. - (1982b)Minerals of tin,litanium, niobium, and tantalum Acxrowr,BocMENTs in granitic pegmatites.In P. Cernf, Ed. Granitic pegmatitesin industry, 187-238. Mineralogical Association of We wish to thank Consulof the AnalyticalBranch, scienceand J. U.S. CanadaShort Course Handbook 8. GeologicalSurvey, Menlo Park,California, for determinationof Foord, E.8., Martin, R.F., and Long, P.E. (1979) Potassicfeld- B and Li contentsof tourmalinesamples; N. Conklin of the sparsfrom pocket pegmatites,San Diego County, California. Branchof AnalyticalLaboratories, U.S. Geological Survey, Den- Geological Society of America Abstracts with Programs, I l, ver,Colorado, for emissionspectrographic analyses; E. Winter- All bum andM. Hochellaof StanfordUniversity for their helpwith Foord, E.E.,Starkey, H.C., and Taggart,J.E., Jr. (1986)Miner- X-ray diftaction analysesand cell-dimensiondeterminations. alogy and paragenesisof "pocket clays" and associatedmin- We alsowish to thankG. Lumpkin,P. Modreski,and P. Cern! erals in complex granitic pegmatites,San Diego County, Cal- for reviewingan earlierversion ofthe manuscriptand acknowl- ifornia. 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(1977) Fluorine in graniticrock and melts: A review. relations among silicates and aqueous solutions. I. Thermo- ChemicalGeology, 19, l-42. dynamics of intrasite mixing and substantialorder/disorder in Bailey,S.W. (1980)Structure of layer silicates.Chapter l. In minerals. American Journal of Science,28 5, 7 69-844. Crystalstructures of clayminerals and their X-ray identifica- Helgeson,H.C., and Kirkham, D.H. (1974) Theoretical predic- tion. MineralogicalSociety, I-ondon. Monograph 5, l-123. tion ofthe thermodynamic behavior ofaqueous electrolytesat STERNET AL.: LITTLE THREEPEGMATITE-APLITE 427

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