EVIDENCE OF MAGMATIC CASSITERITE MINERALIZATION AT ...

739

Canadion MineralogistVol. 30, pp.739-761 (1992)

EVIDENCE OF MAGMATIC CASSITERITE MINERALIZATIONAT THE NONG SUA APLITE-PEGMATITE COMPLEX, THAILAND

ROBERT L. LINNENI, ANTHONY E. WILLIAMS-JONES AND ROBERT F. MARTINDepartment of Geologicol Sciences, McGill University, 3450 University Street, Montreal, Quebec H3A 2A7

ABSTRACT

The Nong Sua intrusive complex, in peninsular Thailand, is a boron-rich, rhythmically layered aplite-pegmatitebody that hosts Sn-W mineralization. Aplites and fins:grained pegmatite layers have granitic compositions, whereascoarse-grained plagioclase-tourmaline-rich and K-feldspar-rich pegmatite layers have compositions that trend arrayfrom, and loward the Or apex, respectively, in the system Q-Ab-OI-H2O. Whole-rock concentrations of tin innonmineralized aplite and pegmatite correlate positively with K-feldspar abundances and are inversely proportionalto primary muscovite and tourmaline contents. These relationships suggest that K-feldspar crystallization was relativelylate, which is also supported by textural data, and that tin behaved as an incompatible element during the evolutionof the magna. Nonmineralized K-feldspar-rich pegmatite contains up to 42 ppm Sn and,. using the availablemineral-melt partition coefficients, is estimated to have been in equilibrium with a melt that contained on the orderof ?00 ppm Sn. This value is close to experimentally determined levels of saturation for cassiterite in granitic melts.With further crystallization or changes of intensive parameters, such as a increase of f(O), cassiterite crystallizeddirectly from the pegmatite-forming melt. The onset of cassiterite crystallization is estimated to have occurred atapproximately 9590 crystallization, calculated by Rayleigh fractionation. Cassiterite disseminated in pegmatite hasplanar or embayed contacts with plagioclase and myrmekite (both An5 to An13) and perthite, and feldspars at embayedcontacts contain inclusions of cassiterite. These textures suggest that cassiterite crystallized at, or prior to, vaporsaturation. Irrespective of the timing of vapor saturation, the orthomagmatic fluid had a low salinity; in light ofvapor-melt partition data, at least some of the cassiterite must have crystallized from melt. Vapor saturation alsogave rise to cassiterite and wolframite mineralization hosted by quartz-tourmaline veins in aplite that terminate abruptlyat aplite-pegmatite contacts. The latter relationship suggests that the hydrothermal mineralization also formed whilethe pegmatite was above solidus conditions. The presence of residual disorder in the feldspars and the persistence ofoligoclase indicate that this aplite-pegmatite complex did not undergo protracted fluid-mediated recrystallization.

Keywords: aplite, pegmatite, petrogenesis, tin, tungsten, cassiterite, mineralogy, feldspar, geochemistry, Thailand.

SOMMAIRE

Le complexe intrusif de Nong Sua, en Tharlande p6ninsulaire, contient des niveaux rythmiquement interlites d'apliteet de pegmatite borifdres et une min6ralisation en Sn et W. Les niveaux aplitiques et pegmatitiques d granulom6triefine possddent une composition granitique, tandis que les niveaux pegmatitiques d granulom6trie plus glossibre, enrichisen plagioclase + tourmaline ou en feldspath potassique, ont des cornpositions qui s'6loignent et se rapprochent,respectivement, du p6le Or dans le systbme Q-Ab-Or-H2O. La concentration en 6tain des 6chantillons d'aplite nonmin6ralis6s et de pegmatite montrent une corr6lation positive avec la proportion de feldspath potassique, et n6gativeavec la teneur en muscovite primaire et en tourmaline. Ces relations font penser que la cristallisation du feldspathpotassique a 6t€ relativement tardive, comme l'indiquent les textures, et que l'€tain s'est comporte comme 6l6mentincompatible au cours de l'dvolution du magma. La pegmatite non min6ralis6e riche en feldspath potassique contientjusqu'i 42 ppm de Sn; d'aprOs les coefficients de partage disponibles entre mindraux et liquide silicat6, le magma en6quilibre avec une telle roche en contiendrait de l'ordre de 7@ ppm. Cette valeur est proche des niveaux de saturationde la cassit€rite dans un liquide granitique, tels que d6termin6s exp€rimentalement. Au cours de la cristallisation, ouavec un changement dans les paramdtres intensifs, une augmentation enflO), par exemple, la cassitdrite a cristallisddirectement du magma qui a produit les pegmatites, aprds 9590 de cristallisation, selon le moddle de fractionnementde Rayleigh. La cassit6rite dissdmin€e dans la pegmatite montre des contacts i aspect planaire ou "corrod6" avec leplagioclase et la myrmdkite (An5 i An12 dans les deux cas) et la perthite, e1 les deux feldspaths contiennent desinclusions de cassitdrite prbs desiontacts d aspect "corrod€". Ces textures font penser que la cassit6rite a cristallis€avant ou au cours de la saturation du magma en phase volatile. Quel que soit le point de saturation en phase volatile,le fluide orthomagmatique 6tait de faible salinit6; d la lumidre des donn6es sur le partage de l'€tain entre phase volatileet magma, au moins une partie de la cassit6rite a d0 cristalliser d partir du magma. La saturation en phase volatilea aussi donn6 lieu i la formation de cassit6rite et de wolframite dans des veines i quartz + tourmaline dans I'aplite;celles-ci sont abruptement terminees arx contacts entre aplite et pegmatite. Le stade hydrothermal de la mineralisationse serait donc form6 quand la pegmatite se trouvait toujours au dessus du solidus. La pr6sence de d€sordre r6sidueldans les feldspaths et la persistance de I'oligoclase montrent que ce complexe d'aplites et de pegmatites a refroidi sansrecristallisation poussde par interaction prolong6e avec la phase fluide.

(Traduit par la Rddaction)

Mots-clds; aplite, pegmatite, p6trogenbse, 6tain, tungstbne, cassitdrite, min€ralogie, feldspath, g6ochimie, Thaiilande.

rPresent address: C.R.S.C.M. - C.N.R.S., lA, rue de la F6rollerie,4507l Orl|ans Cedex 2, France.

740 THE CANADIAN MINERALOGIST

INTRoDUCTIoN

Although tin mineralization is typically as-sociated with granitic rocks, the occurrence ofcassiterite as a magmatic mineral has generally beenviewed with caution (cl, Taylor 1979). Such cautionappears to have beg4 justified by experimentaldata, sumarized by Stemprok (1990), which showthat"tin is relatively soluble in granitic melts. Thisled Stemprok (1990) to suggest that the degree offractionation required to achieve saturation incassiterite in a natural granitic melt is unlikely tooccur. However, the solubility of tin in graniticmelts is a complex function of a variety ofparameters, including T, .f(Oz), K,/Na ratio,alkali:aluminum ratio, and the composition of thecoexisting aqueous fluid (Taylor 1988, Taylor &Wall 1992). The possibility that the magma willbecome saturated in cassiterite is particularlyenhanced if a low-salinity aqueous fluid is exsolved(Taylor & Wall 1992). Thus cassiterire in someenvironments may, in fact, be primary. In penin-sular Thailand, cassiterite mineralization is com-monly hosted by boron-rich granitic pegmatites(Charusiri 1989), and some authors have linked themineralization in these pegmatites to magmaticprocesses. For example, Manning (1986) suggestedthat the cassiterite may have crystallized from anorthomagmatic fluid. However, there has been nosystematic investigation of the genesis of tinmineralization in Thailand, and the possibility thatsome of this mineralization may be magmaticremains untested. The object of this paper is toreport the results of a study of the unusually freshaplite-pegmatite complex at Nong Sua, which hostscassiterite and wolframite mineralization. Geolog!cal and petrographic relationships and observedassemblages of minerals at this deposit indicate thatsome cassiterite could have formed at magmaticconditions. This hypothesis is tested using major-and trace-element data, together with mineral-meltand fluid-melt partition calculations, and evidenceis presented that direct magmatic crystallization ofcassiterite is a predictable consequence of theevolution of the magma.

REGIONAL Gsolocy

The Nong Sua tin-tungsten deposit is hosted bya rhythmically layered pegmatite-aplite complexlocated l0 km west of Prachuab Khiri Khan,approxinately 180 km south of Bangkok (Fig. l).The aplite-pegmatite forms a small hill surroundedby a plain of Quaternary sediments. Biotite granite,which forms part of the Cretaceous-Tertiarywestern Thai granite belt, outcrops less than 5 kmwest of Nong Sua (c/. Cobbing et al. 1986), andexposures of metamorphosed pebbly mudstone and

sandstone of the Carboniferous to Permian KaengKrachan Group occur at roughly the same distancenorth of Nong Sua (Fig. l). Minor hornblendegranite also is exposed 10 to 15 km southwest ofNong Sua, but its relationship to the biotite granitehas not been established. The Kaeng KrachanGroup adjacent to the western Thai granite belt inthe Nong Sua area has been metamorphosed to thebiotite facies, and the grade of metamorphismdecreases away from the granites to subgreenschistfacies near the Kaeng Krachan dam. A muscovitesample from the Nong Sua pegmatite yielded a totalfusion $Arl3eAr date of 63.9 t 0.6 Ma, and an4oArl3eAr plateau age of 53.2 + 0.7 Ma wasdeterhined for biotite separated from a biotitegranite located approximately 35 km southwest ofNong Sua (Fie. l; Charusiri 1989). Whether thepegmatite is truly older than the biotite granite isbeyond the scope of this study. However, the grosssimilarity of the ages indicates that the Nong Suaaplite-pegmatite and the biotite granite both belongto the western Thai granite province, but the agedifference underlines the fact that the relationshipsbetween granite and pegmatite are poorly con-strained.

Gsol-ocrcet- SprrrNc

The Nong Sua complex consists of alternatingand laterally continuous horizons of pegmatite,aplite and metasedimentary rocks (Fig. 2). Aplitesare weakly layered to massive and, on the basis ofminerals present, are divided into garnet- andtourmaline-bearing aplite units. Much of thepegmatite ii strongly layered and thus has beendesignated banded pegmatite. A second pegmatiticunit forms massive tourmaline-rich layers that areconformable to layering in the banded pegmatite,and this unit has been designated tourmalinepegmatite. The attitude of the layering in the apliteand pegmatite is consistent, with a strike of215" t l0o and a dip of 35' + 5o to the northwest.Owing to the shallow dip of the aplite-pegmatite,the plan view is of limited use in showing thedistribution of the different units. Nine verticalsections were therefore measured along the lengthof the complex, and a longitudinal section wasconstructed to show this distribution (Fig. 2).

ApLrrss

The garnet and tourmaline aplites are distin-guished by the respective presence or absence ofgarnet. Both units are generally massive, butcontain minor pegmatite horizons less than l0 cmthick that impart a weak layering to the rock. Thelateral continuity of individual layers of apliteranges up to 100 m along strike (Fig. 2), and the

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MAGMATIC CASSITERITE. NONG SUA COMPLEX, THAILAND

Frc. l. Geology of the Prachuab Khiri Khan area, modified after Silpalit et at, (1985).40{r/3eAr ages of muscovite(M) and biotite (B) are from Charusiri (1989).

741

contacts between aplites and pegmatites are verysharp and parallel to layering in the pegmatite. Acomplete lack of contact irregularities, such asemba)'rnents or inclusions of one unit in another,suggests that both aplite and pegmatite crystallizedfrom a single batch of magma.

The aplites are composed of quartz, K-feldspar

and plagioclase t muscovite t tourmaline +garnet t apatite. The grain size is variable,typically <0.5 to 2 mm, with some larger anhedralgrains of K-feldspar up to 2 cm across. Garnetforms equant subhedral grains that are locallyconcentrated in laminae parallel to, or at a shallowangle to, the principal layering. More commonly,

Longltudinal Eection B

%' -q57

r=-Bru-ffiffiI-.i

Tournnaline pegmatite

Bmded pegmair

Tournaline rylite

Camet rulite

Metasedimentssrike & dioof layering'vein (schematic)Verthal sectio, pta vieulonci0dinal section

Plan Weut


FIc. 2. A plan view and longitudinal section showing the geology of the Nong Sua aplite-pegmatite complex.

garnet is disseminated as clusters, up to 0.5 cmacross, of anhedral grains. Tourmaline is presentas disseminated grains up to 5 mm across that aretypically subhedral and poikilitic, with inclusionsof anhedral qtrartz.

Primary plagioclase generally occurs as sub-hedral to euhedral grains in sharp contact with allother phases; its composition (determined byelectron-microprobe analysis) typically ranges fromAn5 to An13 (Fig. 3). Plagioclase is typically whiteor cream-colored; locally, some coarse grains aregrey, and more translucent. Myrmekite containsplagioclase that is compositionally similar toprimary plagioclase and is locally observed as areplacement of K-feldspar, but myrmekite is muchmore widespread in pegmatite. The significance ofthis will be discussed below. Plagioclase also is

found as exsolved lamellae, as inclusions, and asdiscontinuous veinlets in perthite, generally with acomposition of Ane to An3.

K-feldspar is anhedral, in contrast to plagioclase.Coarse-grained perthite crystals are irregularlydistributed in a fine-grained matrix composedmainly of plagioclase, quartz and microcline.K-feldspar is commonly white, but the occurrenceof coarse-grained, grey, translucent K-feldspar ismore widespread than that of grey plagioclase. Thepresence of euhedral crystals of plagioclase thatextend from the matrix into the perthite (Fig. 4a)suggests that the crystallization of K-feldsparpostdated much of that of plagioclase; this issupported by the presence of euhedral plagioclaseinclusions in K-feldspar. Fine-grained K-feldspar iscommonly present as cross-hatched twinned

Plagioclase

n mineralized apliter barren aplite

Myrmekite


Frc. 3. Plagioclase and myrmekite compositions from barren and mineralized apliteand pegmatite and from mineralized veins. Each point represents the averagecomposition determined by electron-microprobe analysis of several crystals in asingle sample. The field of Nong Sua plagioclase compositions (shaded black)and the 650' (solid lines) and 750'C (dashed line) feldspar compositions at Iand 5 kbar from Seck (1971) are shown on the offset Ab-An-Or diagram.

743

microcline in the groundmass, without visibleperthite exsolution. The origin of the fine-grainedK-feldspar is uncertain; however, its interstitialnature may indicate that it also is late in theparagenetic sequence. The Or content of theK-feldspar portion of perthite and of the fine-grained microcline (determined by electronmicroprobe analysis) generally ranges from 0.90 to0.96, although the most sodic compositions mayreflect sampling of Ab-rich domains (see section ondetails of feldspar mineralogy, below). Quartz andK-feldspar, and rarely quartz and plagioclase, arepresent as dendritic micrographic intergrowths but,like myrmekife, these textures are much morecommon in pegmatite and are discussed below.

Undulose extinction in quartz and feldspars, andfractured garnet and tourmaline crystals, are locallyobserved in aplite, similar to the deformation-in-duced features observed in pegmatite, discussedbelow.

PEGMATITE

The pegmatite units are distinguished texturally;the tourmaline pegmatite is massive, whereas thebanded pegmatite is strongly layered. However, itshould be noted that only tourmaline pegmatitelayers greater than 2 m thick are represented onFigure 2; there are additional layers of tourmalinepegmatite within the banded pegmatite unit that

744

Ftc. 4. Photomicrographs offeldspar textures. a) Euhedralplagioclase @l) enclosed bycoarse-grained anhedral per-thite (Ksp). Some cross-hatched twins, related to un-dulose extinction, are visiblein the upper left-hand corner.The bar scale represents 0.5mm. b) Bulbous myrmekite(M) between K-feldspar (Ksp)and plagioclase @l). Albitetwins are destroyed in themyrmekite domain. The barscale represents 0.5 mm. c)GraphicJike myrmekite (M)replacement of K-feldspar.The bar scale represents 0.5mm.

are too thin to be represented at this scale. Bothunits are composed of quartz, plagioclase, K-feldspar, tourmaline, muscovite, and trace apatite;minor garnet is locally present in the bandedpegmatite. Layering in the banded pegmatite isrecognized by changes in grain size, changes in themodal proportions of minerals, or both. Variationin the local abundance of tourmaline helps definelight and dark horizons in which tourmaline orfeldspar crystals are commonly in sharp contactwith other primary minerals, and are orientedperpendicular to the layering. These features areinterpreted to reflect a magmatic origin for thelayering.

The banded pegmatite consists of the followingfive lithologies and gradations between them: l)fine-grained tourmaline-rich pegmatite, 2) fine-grained aplite, 3) coarse-grained K-feldspar-richpegmatite, 4) coarse-grained graphic qtrartz -K-feldspar pegmatite, and 5) coarse-grained tour-maline-rich pegmatite. Thin (<10 cm) qvartz -K-feldspar - plagioclase - tourmaline + muscovitedykes also cross-cut the pegmatite and aplite unitslocally. The grain size of the banded pegmatitevaries from 2 to 5 mm in the fine-grainedtourmaline-rich pegmatite and aplite layers togreater than 10 cm in the coarse-grained K-feldspar-rich and graphic pegmatite layers. The coarse-grained tourmaline-rich pegmatite typically con-tains crystals 2 to 5 cm (rarely up to 15 cm) long.Fine-grained tourmaline-rich layers are volumetri-cally the most important lithology in the bandedpegmatite. They are composed of coarse-grainedanhedral perthite (up to 5 cm across) set in afiner-grained matrix of plagioclase, quartz, tour-maline, muscovite and K-feldspar (locally contain-ing visible exsolved perthite and cross-hatchedtwins).

The ratio of K-feldspar to plagioclase in thedifferent units of banded pegmatite is highlyvariable, but much of the fine-grained tourmaline-rich unit has a granitic composition (discussedbelow). Coarse-grained tourmaline-rich pegmatitetypically contains very little K-feldspar, but sub-hedral to euhedral primary muscovite grains up to3 cm across are common. K-feldspar-rich pegmatiteis composed of either graphic quartz-perthiteintergrowths, best developed immediately above thegarnet aplite contact (near section 7, Fig, 2), orperthite-quartz layers in which K-feldspar crystalsare euhedral. A trace of metasomatic K-feldsparoccurs along fractures in plagioclase, but there isno petrographic evidence that suggests widespreadpotassic or sodic metasomatism. Both white andgrey varieties of plagioclase and K-feldspar arepresent in pegmatite, and as in the case for theaplites, grey translucent feldspars are typicallycoarse-grained, and grey K-feldspar is more

745

common than grey plagioclase. The importance ofthe variation in color is discussed below.

A significant proportion of the perthite, includ-ing that in graphic quartz-perthite intergrowths,has been converted to microcline. Domains ofcross-hatched twinning are spatially associated withzones of undulose extinction and are thereforeconsidered to have been stress-induced. A similarassociation between visible exsolution lamellae inperthite and undulose extinction may indicate thatcoarsening of exsolved albite also was stress-related. Most of the K-feldspar in perthite andcross-hatched twinned microcline have composi-tions ranging from Orr to Orru and contain anegligible anorthite component, i.e. they arecompositionally similar to aplite-hosted perthite.As in that case, and in view of compositions derivedfrom X-ray-diffraction data (see below), the sodicend of this range is due to sampling of albitedomains by the electron beam during analysis.

Plagioclase, which displays prirnary textures,generally has a composition ranging from Att, toAnt3; within most crystals and amongst crystals inthe same sample, the variation is less than An1 (alsothe case in aplite). The Or content is typically <2q0(Fig. 3). A comparison of the plagioclase composi-tions with those on the 650' and 750oC solvi ofSeck (1971) in Figure 3 indicates that the plagioclasehas undergone re-equilibration Qoss of potassiumto a fluid phase). This is not surprising sincere-equilibration of feldspars is a pervasive featurein granitic rocks (y' Parsons & Brown 1984).

A conspicuous feature of the pegmatites and, toa lesser extent, the aplites, is the occurrence ofmyrmekite at perthite-plagioclase grain boun-daries. Myrmekite generally forms anhedral massesthat separate the subhedral to euhedral plagioclasefrom perthite. The myrmekite forms part of theplagioclase crystal, and albite twins, visible else-where in the plagioclase, are commonly destroyedin the domain occupied by the myrmekite.Myrmekite contacts are convex into the perthiteand are bulbous or lobate (Fie. 4b; c/. Phillips1974, Hibbard 1979). These textures suggest thatthe myrmekite represents the product of a reactionbetween perthite and plagioclase. Less commonly,myrmekite contacts are irregular, with numerousre-entrants in perthite, or myrmekite zones cross-cut single grains of perthite. These features supporta replacement origin for the myrmekite.

The plagioclase in myrmekite generally iscompositionally indistinguishable from non-myr-mekitic plagioclase in the same sample. In bothcases, there is a track of compositional zonation,and the An contents are 5 to 13 molVo, dependingon the sample (Fig. 3). Some myrmekite grains arezoned, being most sodic at K-feldspar contacts; themost calcic myrmekite has the same An-Ab-Or

MAGMATIC CASSITERITE, NONG SUA COMPLEX, THAILAND

746

.E

.Eoo(Uo

c

x

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Frc. 5. Plagioclase compositions along the zoned myrmekite grain on Figure 7b; the dotted lines represent the locationsof the embayed contacts with cassiterite and perthite.

X(t

TdCLg,

E!t

proportions as non-myrmekitic plagioclase in thesame sample. An example of this zoning isillustrated in Figure 5. Finally, the myrmekite atNong Sua is unusual in that the "wormy quartz"is more commonly rod-shaped, and the quartz rodswithin myrmekite grains are optically continuous(Fig. 4c); hence, the myrmekite is texturallytransitional to graphic quartz-plagioclase, but ishere considered as myrmekite, because it isinvariably related to replacement of K-feldspar.

True graphic intergrowths are much less com-mon than myrmekite, and are developed mainlybetween quartz and perthite. Rare graphic inter-growths of plagioclase and quartz are observed inaplite. These grains have a plumose structure, withopticaly continuous quartz radiating outward.Perthite that is graphically intergrown with quartzis otherwise indistinguishable from that describedabove. It may contain plagioclase inclusions and iscommonly rimmed by plagioclase and myrmekite.In some cases, the rods of graphic quartzintergrown with perthite extend into the myrmekite,indicating that the latter replaced pre-existingperthite.

Deformation features are variably developed inthe pegmatite. These include: l) crystals oftourmaline broken perpendicular to the c axis, 2)kinked muscovite, 3) quartz, plagioclase andK-feldspar with undulose extinction, and 4)plagioclase with displaced twins. Quartz and

Fm

feldspar are unstrained in some samples, and highlydeformed in others.

MEtaseprNasNTARY RocKS

Tabular blocks of massive metasandstone of theKaeng Krachan Group composed of quartz, biotite,muscovite, plagioclase, rutile and, locally,hornblende, are contained within the aplite-peg-matite complex (Fig. 2). The blocks are orientedparallel or subparallel to layering in the pegmatite;pegmatite dykes rarely cross-cut individualmetasedimentary blocks. Paucity of outcropprecludes the establishment of the aplite-pegmatiteas a large dyke cutting the Kaeng Krachan Groupor of the blocks of metasedimentary rocks as roofpendants. Within l0 cm of aplite or pegmatite, themetasandstone is generally altered to a tourmaline-quartz assemblage. .

MINERALIZATIoN

Three modes of mineralization are recognized atthe Nong Sua deposit. and these have beendistinguished as vein, disseminated and greisenstyles. Cassiterite is present in all the styles, whereaswolframite is almost exclusively vein-hosted. Dis-seminated cassiterite also contains significant con-centrations of Ti-M-Ta oxide minerals. Thetiming of each style of mineralization is constrainedby textural and structural relationships with the

.85

MAGMATIC CASSITERITE, NONG SUA COMPLEX, THAILAND 747

host aplite and pegmatite, and thus the crystal-lization of cassiterite and wolframite can be relatedto the evolution of the pegmatitic system. Therelative economic importance of the different stylesof mineralization cannot be ascertained, as themine is not currently in production, and records ofpast production are not available.

Vein minerolizqtion

Mineralized veihs are hosted by aplite; they arediscordant to layering, less than I m thick, less thanl0 m long, and generally lack alteration haloes.Contacts between veins and aplite are usually verysharp, with the mineralization being restricted tothe vein. Rarely, vein contacts are diffuse, andquartz has replaced feldspars in the aplite. At suchcontacts, the mineralization may be disseminatedin the wallrock immediately adjacent to the vein.Veins that host mineralization are composed largelyof quartz and tourmaline, with accessory K-feldspar (perthite), plagioclase, cassiterite andwolframite, and trace apatite. Wolframite, cas-siterite, tourmaline and, to a lesser extent, feldsparare intergrown in the veins. Cassiterite forms darkred, subhedral to anhedral crystals that are typicallyI cm and exceptionally 4 cm across, and wolframiteoccurs as coarse-grained euhedral blades up to 4cm long. Tourmaline crystals in the veins attain alength of 4 cm, and are typically broken perpen-dicular to the c axis, with quartz t feldsparcementing the fragments. The quartz is unstrained

in some veins and highly deformed in others, as isthe case in aplite and pegmatite. Muscovite is themost common secondary mineral in these veins,typically occurring as an incipient alteration of thefeldspars, and with minor scheelite along fracturesas a replacement of wolframite. Trace amounts ofscheelite, and rare chalcopyrite, arsenopyrite, andpyrite, are observed along fractures in quartz andthus also postdate vein formation.

The feldspar mineralogy of the veins is compara-tively simple. The plagioclase is typically untwinnedalbite with compositions ranging from An6 to An3(Fig. 3), and Or contents of less than I molVo. TheK-feldspar is perthitic, but contains fine-grainedstring exsolution of albite, rather than the coarse-grained lamellae that characterize much of theperthite in aplite and pegmatite. Intergrowths ofvein-hosted cassiterite and K-feldspar or cassiteriteand albite are defined by crystallographic faces,suggesting that the cassiterite and the feldspars werecontemporaneously deposited from aqueous fluids.

An important feature of the veins is that theyare discordant, contained within the aplite units,and terminate abruptly at the contacts betweenaplite and banded pegmatite (Fie. 6). As statedabove, aplites and pegmatites are in sharp contact,and there is no evidence to suggest that differentialshearing occurred along the contacts. The termina-tion of veins at the aplite-pegmatite contactstherefore suggests that the aplite was at subsolidusconditions but that the pegmatite had not cooledsufficiently to sustain brittle fracture, l'.e., the

Frc. 6. Quartz-tourmaline veins cutting garnet aplite that terminate abruptly at the aplite-pegmatite contact. Theabbreviations are: A aplite, M metasedimentary rocks, P pegmatite, V vein. The field of view is approximately 3m across.


pegmatite was above or near solidus conditionsduring vein formation. A tourmaline layer in aplite,that continues through a qtJartz vein, is observednear section 9 (Fig. 2), in support of thisinterpretation. This feature is similar to the"phantom layering" through perthite crystalsdescribed by Jahns & Tuttle (1963). We interpretthe phantom layering at Nong Sua similarly, andsuggest that the aplite was locally at or abovesolidus conditions during vein formation, and thatthe tourmaline layer and quartz vein formedsynchronously. In the same area, a late dyke ofpegmatite cross-cuts a vein, indicating that therewas at Ieast some magmatic activity after veinformation. We therefore conclude that the veinsformed from aqueous fluids liberated as thepegmatite-forming melt was achieving saturation invapor, but before it had completely crystallized.

Greisen mineralization

A very small proportion of the mineralizationoccurs as cassiterite disseminated in pegmatite atthe contacts with metasedimentary rocks and isclassified as greisen. This cassiterite is in closespatial association with coarse-grained secondarymuscovite and beryl. The muscovite has replacedthe feldspars, and defines funnel-shaped zones thatspread out along the contacts, but do not extendinto the metasedimentary rocks. This is interpretedto reflect a hydrothermal process in which themetasedimentary rocks acted as an impermeableboundary and forced fluids to flow laterally.

D isse m in a t ed m in e ra I i zat io n

Disseminated cassiterite in the banded pegmatite

unit, and less commonly in the garnet aplite unit,occurs in small patches (<0.5 m across) that haveno discernible orientation. Mineralized pegmatitesand aplites contain the highest grades of tinencountered in this study, but are otherwise similarto those barren of mineralization, i.e., composedof quartz, K-feldspar, plagioclase + tourmaline tmuscovite. The cassiterite is dark red, subhedral toeuhedral, typically less than I cm across, and isintergrown with all pegmatite minerals and, inparticular, tourmaline. Opaque inclusions arecommon in disseminated cassiterite but are ex-tremely rare in vein-hosted cassiterite. The in-clusions have been identified from results ofelectron-microprobe analyses as niobian-tantalianrutile, ilmenite, columbite and ixiolite. Theseminerals also are common to other pegmatite-hosted tin deposits in Thailand (Praditwan 1988).The opaque inclusions generally have irregularshapes; their abundance is highly variable amongstcassiterite crystals. These features suggest that theoxide inclusions in cassiterite did not originate byexsolution.

The contacts between disseminated cassiteriteand plagioclase generally are sharp, defined bycrystal faces. Locally, however, cassiterite seems tohave been resorbed, and is embayed by plagioclase(Fig. 7a). The plagioclase in Figure 7a is oligoclase(An1s) and contains inclusions of cassiterite in theembayment. Cassiterite more commonly displaysembayed contacts with perthite and myrmekite(Fig. 7b), and the feldspars at the embayed contactsalso contarn fine-grained inclusions of cassiterite.The myrmekite in Figure 7b is zoned (c/. Fig. 5),contains inclusions of cassiterite, and at the contactwith the embayed crystal of cassiterite has acomposition of An13 (the same as primary

g rlS{,"

Ftc. 7. Cassiterite-feldspar textures. a) Cassiterite (Cas) embayed by An16 plagioclase (Pl) (plane-polarized light). Thebar scale represents 0.1 mm. b) Myrmekite (M) embayment of cassiterite (Cas) and perthite (Ksp); tourmaline (T)is also intergrown with cassiterite (cross-polarized light). The bar scale represents 0.25 mm.

749

TASLE 1. INDICATOBS OF COMPOSITION AND DEGREE OF AI-SI ORDEB

IN K-RICU AND Na-RlCg FELDSPABS IN BEPRESBNTATIW SAMPLES OF

TIIE NONC SUA APLITE.PECMATITE COMPLET

plagioclase elsewhere in the same sample). Thesefeatures suggest that cassiterite crystallization waspenecontemporaneous with, but lo:ally preceded,the crystallization of some of the primaryplagioclase, and was followed by the formation ofperthite and myrmekite.

STRUcTURAL AND CoMPoSITIoNAL DATAoN THE cOBXrSrnC FgtOSpans

Ten representative specimens of aplite, pegmatiteand vein were selected for further characterizationof feldspar mineralogy. Various fractions weresampled individually for characterization by X-raydiffraction (powder method, Guinier-Hiiggcamera, synthetic spinel internal standard, CuKolradiation). Refinement of the unit-cell parametersin twenty-two subsamples was carried out usingcorrected and indexed diffraction-maxima, and theprogr€rm of Appleman & Evans (1973), as modifiedby Garvey (1986). A listing of unit-cell dimensionsof the coexisting feldspars in these ten samples isavailable from The Depository of UnpublishedData, CISTI, National Research Council, Ottawa,Ontario KIA 0S2. The unit-cell parameters havebeen used to obtain information of the compositionand degree of Al-Si order of the feldspars (Tablel ) .

In all samples, the K-rich feldspar is microcline,as inferred from the presence of cross-hatchedtwinned domains. Whereas this finding is notunexpected for a body of granitic pegmatite,especially one subjected to deformation (Martin1982), the degree of Al-Si order attained typicallyfalls short of the perfect state of order that isexpected. The value of r,0 is low (as low as 0.92)in the bulk of the megacrysts of grey andtranslucent perthite (lr0 is 1.00 in fully orderedmicrocline). Interestingly, the strongest peaks oforthoclase or of structurally intermediatemicrocline also are present in five of the sevensubsamples of grey, translucent megacrystic per-thite examined. Both intergrain heterogeneity indegree of Al-Si order attained in the microclineand the presence of more disordered phases pointto a relqtive lack of low-temperature, fluid-mediated recrystallization in this suite. White,opaque portions of the microcline perthite grains,which are volumetrically minor, line cracks in andrim the deformed megacrysts; the white areastypically contain a more fully ordered microcline.The finer-grained assemblages (aplite, fine-grainedtourmaline pegmatite) also contain the same rangeof No, and 110 values for the microcline as themegacrysts. The suite contains microcline rangingin obliquity from 0.89 to 0.99 (Table l).

The range of microcline compositions (95 < No,< 99: Table l) reflects temperatures of final

MAGMATIC CASSITERITE. NONG SUA COMPLEX. THAILAND

K-ftch feldBpa!

N- t,o a An

Na-dch feldsta

!e lro A131

PK-2a grey Kls" g4.8

PK-2a fath€ry Nfs A7 .3PK-3b gfty Kfs 96.2PK-3b whlte Kfs 96.6PK-3b @td:PK-sb qclave

PK-3c grJr trfs 98.2PK-3c whtlo Kls 95.4PK-3c lpltte 98.3PK-17a gnpblc qEdte 96.?PK-17a1 grey trls 98.7PK-l?d wbtte trfs S5.0PK-lga grey Kf8' 95.9PK-19a wblt6 Kfe 96.8PK-33 grey-sblto Kfs 90.5PK-3:l(:l@PlPK-46f rhtte Nfs S6.5PK-46f qtz-Tu v€d!PK-64 qtz-Tu.velE 98.9PK-58 gFJr trfs 9?.3Ptr-st wbtto dm SA.2PK-58 aptt€ 94.9

0.95 0.920.99 0.95o.s2 0 .921.@ 0.9?

0.98 0.800,99 0.9?0.98 0.911.@ 0.s80.s8 0.s50.s9 0.s80.98 0.970.98 0.950.99 0.98

1.00 0.99

0.s80.E90.970.93

0.3 0.98 1.1401.143

0.0 0.98 1.1:l l1.t721.3671.636

0.4 0.96 1.1480.? 0 .s9 1 .134

0.4 0.s8 1.1382. t 0 .90 1 .1170.0 0 .98 1 .1130.0 0.96 1.0690.0 1 .00 1 .1100,5 0.97 1.084

1.3090.3 0.97 1.1&l0.6 0.98 1.131

1.0190.9 0,90 1.103

1.1821.38'

I20

t323

10-orI

11r

0.900.950.990.98

CoEportto! Nft ls e{gEsed b @fE $ O!, ed q8 @l@lated udng the

squttou of fFIf & Blbb (1983) rcladlg u!it-@ll vol@ to NG fo

feldspq @EpodttoE that @ sinrctelly o!d€!€d (!9:' thqt bsloDg to

tlre loF dcmlt@ - lor slbtto 6dt6). The deg@ d Al-sl ordo!,

sfr@s€d by tto' @ @Epqtod udng th€ squtloE of Bl'd (19?7). Tho

€m r! !o ed lto t6 botieved to bo !0.015 ll1 @t (Er6. rhs obuqutty

A of a d.cellae ts squal to 12.5(gBl - 4l3 ) t tt shoqfd havs a valus of,

1.00 f,olfirlly oldsr€dEl@Una. Tho A131 hdt@to! of a plsgtal@18 tho

@lqlatod egula! spa$tl@ of !h6 131 ed I 3 I atltrlaotloD @d@' ,r o2O

(cu5gr ndtatton). ^For a plagt@l,ae @ €Ldo the AEt, E tlf€ned by

@ldlmb h th€ B' - t- dlag@ d Sdt.h (19?a ' ng. 7-44) , vallE of No,ed lro (ruot bg tdgrt€d qdrg tho ildt@toB bas€d o! ths low blo!@ue- lowalldte ed6. - Eu.trg!@ of Fsidualdlerdelltlplagt@lE€' b€aedoB

@r!dhat6 t! tho B'- t'rrlot. '

rh. "t-oogot

tEts of olthel6o also @p@t ,! the twdo! 1nttm. " Ths stlong6t p@ls of a slluotuElly

,ltsmsdtat€ dc!@fre abo e p@t ,! tbe twalo tEtten. CEy @d

whlts sbsEt lss @ taks of tstbits @gpcNryst eBples Pf-3b, PK-3c,

PK-l?al, Pf-19a dal PK-68. SaEple6 ssl*t€df,ot etualyt PK-2a tou!@Iht

ap[te, PK-gb baldedlFg@ttte@ta6€dl@t @t8st, PK-& grct adlts,

PK-l?a (@gEhod gnpbic qudtz-K-fqLlstE, PK-Ud @gnl&d

K-fsI't8pa! (euhsdFl tFltbtto) , PK-lea o@gEtusd gnphtc qurytt-K-

fold6ls, PK-gil c(nrrygdaea tomllulfchpog@tlto, PK-46f q|rytz-

t@!@ll@-foldsBsF@sltolitemlfladto v€|!, PK-fl quartt-toq@Ilr

feldsprwolbedte vdu, PK-58 ga@t aD[ts.

equilibration between approximately 350" and150oC or less. Values of Nor closer to 95Vo aremore prevalent in microcline from aplite andpegmatite (Table l), which reinforces the inferencemade on structural grounds that this aplite-peg-matite system was not characterized by pervasivefluid-mediated recrystallization following deforma-tion and accompanying cooling. The presence ofperthite in the vein in which the low microcline isalmost pure (N6, = 99, Table l) is consistent withcontinued hydrothermal re-equilibration of a high-temperature hydrothermally deposited orthoclase,at least in the case of vein PK-54.

The albite that coexists with microcline in the


grey, translucent perthite megacrysts (Ane to An2and less than Or,) presumably formed largely byexsolution at a temperature above the orthoclase-to-microcline transition, i. e., abov e approximately450'C. In some cases, albite, both as exsolution inperthite and as discrete crystals in veins, ischaracterized by residual Al-Si disorder (Table l).It is also significant that much of the aplite- andpegmatite-hosted plagioclase is oligoclase (dis-cussed above in light of electron-microprobemeasurements; see also Table l). In bodies ofgranitic pegmatite that have been subjected"to animportant internal circulation of aqueous fluidbelow about 400"C, such plagioclase would beexpected to have been converted to a greenschist-facies assemblage of Ca-free albite + epidote. Thepersistence of oligoclase and of residual disorderin the albite is consistent with rapid coolingfollowing emplacement, deformation and uplift,and general lack of low-temperature recrystalliza-tion.

INTERPRETATToN oF Fer-ospanAND CASSrrEnlre TsxtuREs

The fine grain-size of aplites has commonly beenattributed to vapor loss (e.g., Jahns & Tuttle 1963);in support ofthis interpretation, these textures havebeen experimentally reproduced in vapor-saturatedgranitic systems (e.9., London et al. 1988). It thusseems likely that the aplites at Nong Sua crystallizedlargely under vapor-saturated conditions. There ismuch less consensus concerning the timing of vaporsaturation in magmas that produce pegmatitictextures, although high contents of dissolved H2Oin these melts are generally considered to befundamental. Of particular importance to theinterpretation of the textures at Nong Sua are theexperiments of Fenn (1986), London et al. (1989)and Maclellan & Trembath (1991), where graphicintergrowths of quartz and K-feldspar have beeninterpreted to reflect nonequilibrium growth (su-persaturation) in melts that are vapor-under-saturated and water-bearing. Much of the coarse-grained K-feldspar at Nong Sua, including thatgraphically intergrown with quartz, formed aftercrystallization of oligoclase or calcic albite. If thegraphic intergrowths at Nong Sua also formed atvapor-undersaturated conditions, most of thefeldspar in the pegmatite units probably crystallizedat vapor-undersaturated conditions. Additionalsupport for this comes from the fact thatmyrmekite, including that graphically intergrownwith quartz, formed after K-feldspar crystal-lization. This myrmekite cannot be metamorphic inorigin, since evidence for a postmagmatic high-temperature metamorphic event is completelylacking in the surrounding metasedimentary rocks;

on the other hand, the replacement of K-feldsparby myrmekite indicates the involvement of a fluidphase but the preservation of intermediate struc-tural states in the K-feldspar suggests that the fluidevent must have been relatively shortlived. lnaddition, the compositional similarity of myrmekiteto magmatic plagioclase at Nong Sua suggests thatboth crystallized at comparable conditions. Wetherefore conclude that myrmekite replacement ofK-feldspar reflects the vapor saturation of the melt,which is supported by the "graphicJike" nature ofthe myrmekite. This interpretation is not unique,as myrmekite in similar environments is alsointerpreted as having originated during late vapor-saturation of a magma (Hibbard 1979, Hopson &Ramseyer 1990).

The planarity of cassiterite-oligoclase contactssuggests penecontemporaneous growth, and theembayment of cassiterite by oligoclase (Fig. 7a)implies that some cassiterite crystallized before theend of plagioclase crystallization. The texturalevidence therefore indicates that disseminatedcassiterite crystallized directly from the melt. Analternative explanation is that the disseminatedcassiterite precipitated from an aqueous fluid in avapor-saturated melt (if vapor and melt areseparated, the chemical potentials of tin in meltand fluid are not necessarily equal). The fact thatdisseminated cassiterite predates myrmekite (Fig.7b) indicates that cassiterite probably began tocrystallize at or near conditions of vapor satura-tion.

WHOLE-ROCK CHEMISTRY

If, as the textures suggest, the disseminatedcassiterite indeed crystallized from the melt,evidence of such crystallization should be recordedby the petrochemistry of the aplite-pegmatite, inparticular, by the distribution of tin. Four types ofsamples were therefore analyzed for major andtrace elements: l) garnet aplite, 2) fine-grainedtourmaline-rich banded pegmatite, 3) coarse-grained K-feldspar-rich and graphic pegmatite, and4) coarse-grained tourmaline-rich pegmatite. Asingle samplb of plagioclase-rich aplite, from thebanded pegmatite unit, also was analyzed. Averagecompositions for each of the sample types are listedin Table 2. Sample sizes ranged from I b 2 kg,and are probably representative of the aplite andfine-grained pegmatite. Compositions of thecoarse-grained pegmatite units may not be repre-sentative; however, they represent, at worst, amixture of lithologies and are still useful inassessing the behavior of tin during the evolution,and any subsequent alteration, of the pegmatiticrocks.

TABLE 2. AVERAGE MIOLE-ROCK COMPOSITIONS OF APLITEAND PECMATITE

Apl. Ksp. Tdr. Apl.BP. BP.9 ? 4 7 4

74,95 ',t4.43 77.88 76.?0 78.370.04 0 .02 0 .08 0 .02 0 .06

14.88 14.36 13.?4 14.85 14.490.50 0 .1? 1 .25 0 .21 0 .?90.08 0 .01 0 .03 0 .01 0 .o2

<0.01 <0.01 0.03 <0.01 <0.010.72 0 . 0 .?6 1 .39 0 .783.97 2 .85 4 .60 6 .3? 4 .424.28 ? .80 1 .16 0 .42 2 .870.05 0 .05 0 .03 0 .05 0 .040.59 0 .25 0 .50 0 .29 0 .45

100.10 100.01 100.05 [email protected] 100.28

751

are peraluminous. This is also reflected bynorrnative corundum, the values of which rangefrom 0.79 to 5.08 wt.9o. Figure 8 shows aplite andpegmatite compositions expressed in terms ofnormative quartz-ofihoclase-albite and anorthite-orthoclase-albite. Most of the aplite and fine-grained tourmaline-rich pegmatite samples aresimilar in composition to the water-saturatedminimum at I to 2 kbar in the haplogranite system.Other samples are either enriched or depleted inK-feldspar. In this respect, the compositions arecomparable to those of other pegmatites (c/. Jahns& Tuttle 1963). Considering the coarse grain-size,the degree of scatter is relatively small. The absenceof compositional shifts toward the Q or Ab cornerson Figure 8 is consistent with the lack ofpetrographic evidence for significant greisenizationor sodic metasomatism. It is difficult to assesspotassic alteration using Figure 8, as pegmatitecompositions trend toward the Or apex, but thelack of widespread replacement of plagioclase byK-feldspar suggests that potassic alteration was oflimited importance in controlling whole-rockchemistry.

Most of the samples contain significant calciumand boron, both of which shift the composition ofthe granite minimum; the normative anorthitecomponent is typically 3 to 4 wt.9o, and B contentsnearly attain I w.9o for tourmaline-rich pegmatite(average 4000 ppm, Table 2). The addilion ofanorthite as a component to the haplogranitesystem shifts minimum-melt compositions towardthe quartz-orthoclase tie-line (James & Hamilton1969), and excess aluminum, toward the quartzapex (Holtz et ol. l99l), whereas the addition ofboron shifts compositions toward the albite apex@ichavant 1987). A comparison of the Nong Suacompositions with experimentally determinedgranite minimum-melt compositions therefore doesnot provide an estimate of pressure.

The dependence of proportions of major ele-ments on the K-feldspar-to-plagioclase ratio isapparent if the major elements are plotted as oxidesas a function of SiOr. Figure 9 shows that K2O andAl2O3 vary inversely, whereas Na2O and CaO varypositively with increasing SiO2. In terms of thealkali-to-aluminum ratio lexpressed as molar(K+Na)/(K+Na+Al)1, the most stronglyperaluminous rocks are plagioclase-rich, almostdevoid of K-feldspar. This is reflected, mineralogi-cally, by the presence of primary muscovite andtourmaline in these rocks. Rocks having highercontents of K-feldspar have less stronglyperaluminous compositions; the coarse-grainedK-feldspar-rich units are metaluminous (Fig. lOa)'The lack of widespread alteration is confirmed bythe consistent variation in Rb/Sr with the alkali-to-aluminum ratio (Fig. l0b).


TYP.nSiO2 wttfiozAlzorF"ZOgMuoMgoCaON%orqoPeosLOITotal

Ba ppmNbZtvSrRbDfi

ThF

TaBSB

Nomqo!abm

tlhemep

hyBt

13 15 1527 10 522 3 3 1 82 0 5 1 4t2 11 15

399 Aze 96s2 4g 16

8 5 8384 10? 48322 10 288 4 1 3

258 2r7 413377 22 14

34.09 28.v 45.0825.27 44. 6.8733.56 U. t4 38 .913.24 0 .99 3 .592 , U 1 . 1 0 3 . 8 10.08 0 .02 0 .070.43 0 .1? 1 .250.72 0 .10 0 .070.01 0 .01 0 .040.04 0 .02 0 .080. l t

38 1635 3928 1323 118 1 5

3't8 2332/l 2l

d l l

140 385t4 1810 74

389 1951t2 18

35.20 38.142.48 16.96

53.90 37.406.55 3 .50L.52 2 .430.02 0 .050.27 0 .?9o. t2 0 .090.01 0 .040.02 0.03

Apl. leps@ta gamet apllte, Ksp. K-feLlsl)rrlcb tpg@dto, Tmr.tourmllne-rlch lFgmdts, Apl.BP. ap[te @ntalned wit]ll bsndsd

md BP. ftne-gmlned toumllno-dch banded Fgaattte.llajor elsB€lts md BaO reE datomlnsd fEm fuasd polloxs by X-Ey-fluoresen@ strEtm8@py, wlth a detsctlon lldt of 0.01 wtt' ed 10ppm, FslFctiv€ly. Total lron ls Flprted I Fe2O3. Most ssEplss@ntain <0.01 wtb MgO; tberEfoE redlaD valu6 @ glve!. For Tlo2'n/ho ed P2Oj, longp! mtlng tlM ree uaed to lnpmve thsdet@doD Unit to 0.001 wtt. Excgpt for Sn, B, F, Ta ed Ll, tlre@D@ntmdoc of tm@ elerelts seF d€tomltrod fFn pEsed Ipwdertpllets by X-my fluoru@€, wlth dEteclion lmlis ol3IrpB for Nb, Z!ed Sr, ed 5 ppm fo! ths mt. Co!@ntmtlon of Sn w dstemlnsd byNH4I fslon md atomlc abslptlo! anslysls, wlth a detmdo! llntt of 1ppE. Tbro apllle mple onxeln vlslbla @sltedte (53, 95, ud 106ppm sD) ed m excluded fFm ths avemg€. CoD@tEdon of B rudetemlred by Na2O2 fulon ed alalysts by |lrdu@d @uplsd plas@,with a detscdo! Untt of 2 ppm. Con@tFdo! of F ru dgterulned byNaOg fusioD ed aualysls by slFclftc lo! elsctmde, wlt!' a det@tlonllnit of 10 ppe. Con@ltEdoN of Ta ed Ll reF detemhed bylndu@d @upled pl,aa@ uElysla afts 0.5 g of saBple w dfueeted wlth10 EL of HC1q-HNO3-BF at 2O0oC ed dtluled wtth l0 Et of aqu Egl,B.The dot4don llmit of Ta ud Il ie 3 ppa. Aleo sught, but b€lowdetecdon lleits, re V ((10 ppm), Nt ((10 ppm), Cr (<5 pp!), md U((5 ppn).

Major- and trace-element relationships

The presence of primary muscovite, garnet andtourmaline indicates that the aplites and pegmatites

752

Boron concentration is inversely proportional tothe alkali-to-aluminum ratio (Fig. l0c), whichdemonstrates the petrographic association of tour-maline with plagioclase-rich rocks. Figures l0d andlOe show similar trends for TiO, and Nb, whichsuggest that Ti and Nb were controlled largely bytourmaline abundance (oxide minerals are rare innon-mineralized aplite and pegmatite). In addition,strong positive correlations between B and Fe, Ta,F and Li concentrations indicate that these elementsalso were probably controlled dominantly bytourmaline (tourmaline pegmatite also containsprimary muscovite). However, the Tal(Ta+Nb)ratio displays the opposite trend, increasing withincreasing alkali-to-aluminum value (Fie. l0f). Thisratio, used as a measure of fractionation (Cernf elql. 1985, Jolliff er al. 1992), suggests that thetourmaline-rich units may be less evolved.

. aplite

o pegmatite

Distribution of tin

It is possible to evaluate the behavior of tinduring granite evolution and the effects ofsubsolidus alteration on its remobilization byplotting the concentration of Sn ve,",rlts parameterssuch as TiO2 and Rb/Sr, which can in turn berelated to the degree of fractionation (Lehmann1990). Although cassiterite-feldspar textures sug-gest that the pegmatite-forming melt was saturatedin cassiterite, the tin contents of cassiterite-bearingsamples are difficult to interpret owing to theinhomogeneous distribution of cassiterite and therelated problem of sample size. The relationshipsbetween tin abundances and compositions in termsof major elements, discussed below, are thereforebased only on samples that are free of cassiterite;data on three samples of aplite that contain trace

!

A

THE CANADIAN MINERALOCIST

Granite MinimaJames & Hamilton (1969)I kbar, An5Luth (1976)0.5, 2.0 & 3.0 kbarPichavant (1987)1.0 kbar, t.0 & 4.5.ttt Vo B2O3

An An

Ftc. 8. Compositions of aplite and pegmatite expressed in terms of normative quartz(Q), albite (Ab), orthoclase (Or), and anorthite (An), and water-saturated graniteminima for various pressures and compositions.

l .O Vo'82O3 O

uJrtr

"EO.

MAGMATIC CASSITERITE. NONG SUA COMPLEX. THAILAND 753

be#

3

oc\l

Y

sH

3(f)

o_s{

4

12

10

I6

berF,

=

oN

(dz

be+t

B

oCUO

otr9.dtr#

trE trtrtr

68 70 767472 78 80 80760L68

disseminated cassiterite are excluded (in terms ofmajor- and trace-element chemistry, these samplesare otherwise similar to cassiterite-free aplite).

Most granitic suites contain progressively less Srand Ba and more Rb as a result of fractionation.At Nong Sua, Rb contents vary from 37 lo 995ppm, Sr from 7 to 18 ppm, and Ba from < l0 to48 ppm. The melt-feldspar partition coefficientsfor Rb are in favor of K-feldspar over plagioclase(Smith 1974), which is reflected by the strongcorrelation between Rb and KzO (Fig. lla). Thiscorrelation, and that between log Sr and log Rb/Sr,further suggest that the Rb-Sr distribution has notbeen affected by alteration. The wide range of logRb/Sr and restricted range of log Sr values (Fig.llb) undoubtedly reflect differences in theplagioclase to K-feldspar ratios among the samplesanalyzed. This is not unusual, since a near-constantand low abundance of Sr with variable Rb

70 72 74 78

Sio2wt %

68 72 76 80

SiO2 wt "/"

concentrations is interpreted elsewhere as beingindicative of highly fractionated melts (Walker eral. 1986), and similar Rb-Sr relationsbips arereported for pegmatites elsewhere (e.g.,aerny etal. 1984, Trumbull l99l).

The behavior of Sn in granitic suites is dependenton theflO, of the melt. In oxidized melts, tin, inthe 4+ valence, substitutes in minerals such asmagnetite and is not concentrated by fractionation.By contrast, in reduced granites, tin is dominantlydivalent, and its abundance increases with frac-tionation (c/. Lehmann 1990). The values of thelog Sn concentration of the Nong Sua sanrplescorrelate positively with those of log Rb/Sr andnegatively with those of log TiO2, similar to thefractionation trends proposed by Lehmann &Mahawat (1989) for western Thai granites, butat lower abundances of Sn @gs. llc, d). Thesmall degree of scatter suggests that significant

68

SiO2wt %

72 76 8070 7874707874

otrd

.so# "tr

Sio2wt %Fto. 9. Compositions of aplile (solid circles) and pegmatite (open squares) expressed in terms of weight percent K2O,

Na2O, Al2O3, and CaO rersas SiO2.


Eo-o-

U'bE

EEctz

4.4

3.5

3.0

2.5

2.0

1 .5

1 .0

0.5

100 ;e0f80 f70 f60 fs0 f40 f30 f20 ftoL0.35

0.15

- 0.10

5ol

o o.o5F

o.70.0

0.5

0.3

0.1

Eo-o.

dl

90008000700060005000/10(X)

300020001000

(Na+K)/(Na+K+Al) mol.

l c

trtr

- r-ut-lUEI O

^Grlr rfl

(Na+K)/(Na+K+Al) mol.

d

%ffi0.35 0.40 0.35 0.40

(Na+K)(Na+K+Al) mol.

f mul

0.500./t5

1009080

e70a60g50

-o4ozso

n1 0

(Na+K)(Na+K+Al) mol.

e

JfuF.0.4

hydrothermal remobilization of tin did not occur,and that the assumption that these rocks arecassiterite-free is probably valid. Since tin behaves

as an incompatible element in suites of unalteredS-type granite, it is also likely that tin behaves asan incompatible element during the internal

aB

otr

%tr!

eo-g

-oz+

.fitI

-atf

0.35 0./10 0.45 0.50 0.35 0./t5 0.50

(Na+K)/(Na+K+Al) mol. (Na+K)l(Na+K+Al) mol.

Flc. 10. Alkali-to-aluminum values [(Na+K),/(Na+K+Al)] of aplite (solid circles) and pegmatite (open squares)expressed in terms of a) K/Na, b) Rb/Sr, c) B, d) TiO2, e) Nb, and f) Ta,z(Ta+Nb).


0.4 0.8 1.2 1.6

KzO wt % log Rb/Sr2.8

1 .6

1 .2

0.8

0.4

0.4 0.8 1.2 1.6 2 2.4 2.8 -2 -1.6 -1 .2 -0.8 -0.4 0

log Rb/Sr

755

2.8

p€,

-oE.

.05

development of peraluminous pegmatites, i.e,, themost evolved rocks at Nong Sua contain the highestconcentrations of tin.

Figure 12 shows the distribution of Sn yerszsconcentrations of the major elements, expressed asoxides. Tin is positively correlated with Al2O, andK2O, and is negatively correlated with SiO2. Similardiagrams were constructed for B and F; theseelements do not correlate with concentrations oftin and thus did not affect its distribution. In termsof the alkali-to-aluminum ratio, the mostperaluminous rocks have the lowest concentrationsof Sn, and the metaluminous rocks K-rich) have

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log TiOz (wt %)Frc. ll. Compositions of nonmineralized aplite (solid circles) and pegmatite (open squares), The reference lines of

Lehmann & Mahawat (1989) are regressed from compositions of granites in western Thailand; the tin-barren versastin-bearing fields are also from that study. a) Rb verszs K2O. b) Log Sr versas log Rb/Sr. The reference lineindicates the change of granite composition, s/ith log Rb/Sr increasing with fractionation. Nong Sua compositionsreflect highly fractionated melts and do not follow this trend. c) Log Sn versus log Rb/Sr and d) log Sn versaslog TiO2. Nong Sua compositions in c and d delineate trends similar to the granite trends of Lehmann & Mahawat(1989). In the latter study, increasing abundance of tin was interpreted to reflect increasing fractionation.

the highest abundances. The metaluminous K-richsamples are mainly composed of K-feldspar andquarrz, and lack tourmaline and muscovite. Despitethe fact that the concentration of tourmaline andmuscovite might be expected to control whole-rockabundance of tin (see below), the distribution oftin in nonmineralized aplite-pegmatite is apparent-ly related to K-feldspar abundance.

Apr-trs-Psctr4artrs EvoLurIoN ANDCessnsnrug CnvsraluzertoN

A growing body of natural and experimental

band Mahawat (1989)

cLehmann and Mahawat (

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THE CANADIAN MINERALOGIST

a ntrn

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data suggests that the origin of layering in granitesand pegmatites is a consequence of supersaturationand rapid growh of crystals (e.g., Rockhold et al.1987, London et al. 1989). These models offer analternative to the Jahns & Burnham (1969) model,which invokes vapor transport, in particular thatof potassium, to explain pegmatite layering. In allof these models, the pegmatite units can beconsidered to represenl cumulates; this makespartition-coefficient data, especially for trace ele-ments, extremely important. Although pafiitiondata are sparse, they are nevertheless sufficient topermit rough interpretation of the whole-rock data.The aplites at Nong Sua consistently have composi-tions that are intermediate between those of K-richand K-poor pegmatites. The close chemicalrelationship of aplite and pegmatite supports theconclusion, based on field relations, that aplite andpegmatite both crystallized from a single batch of

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magma. In general, if a granitic melt has evolvedto a minimum-melt composition, further crystal-lization will have little effect on the major-elementcomposition. However, trace-element compositioncan vary significantly with continued crystal-lization, particularly if the melt structure (andhence partition coefficients) change with evoludon.The major-element compositions of the Nong Suaaplites can therefore be considered to representminimum-melt compositions (1.e., liquids), whereasthe abundances of trace elements may also reflectthe effects of mineral accumulation (i.e., the melt'scomposition can only be calculated from partitioncoefficients).

The timing of crystallization of the differentpegmatite units at Nong Sua is poorly constrained,except that K-feldspar is commonly late. Sincegraphic q\arlz - K-feldspar and discrete grains ofK-feldspar within units were late in the paragenetic

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Kzo (wt Y") (Na+K)/(Na+K+Al) molFrc. 12. Distribution of tin in nonmineralized aplite (solid circles) and pegmatite (open squares) in terms of a) Al2O3,

b) SiO2, c) K2O, and d) the molar (Na+K)/(Na+K+Al) ratio.

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sequence, the graphic and coarse-grained K-feldspar units probably also were late, although thebanded nature of the pegmatite suggests that thesequence of crystallization may have been repeated.It thus follows that the most alkaline(metaluminous, K-rich) units at Nong Sua were thelast to crystallize. This evolution is similar to the"liquid line of descent" proposed by London et al,(1989), wherein pegmatite evolution was modeledby decreasing SiO2 and increasing alkali elements.Such an evolution for the Nong Sua pegmatite notonly explains the textural relationships, but also thelower TiO2 and the higher Sn and Tal(Ta+Nb)ratios observed in the K-rich units.

Solubility of tin

The above discussion suggests that tin behavedas an incompatible element in the pegmatite-form-ing melts at Nong Sua. The systematic variation oftin in aplite and pegmatite also indicates thatsignificant tin was not remobilized as a result ofalteration of silicates or oxides. Although magmaticprocesses clearly controlled the distribution of tinin the nonmineralized aplite and pegmatite, itremains to be shown whether or not the tinabundances at Nong Sua are consistent withcassiterite saturation, in light of the available dataon the solubility of tin in granitic magmas. Itshould be noted that, since the samples examinedare nonmineralized, the highest estimates of tincontents in melt should reflect a $light undersatura-tion of c,assiterite. In a summary of experimentalwork, Stemprok (1990) concluded that thedominant control of Sn solubility in granitic meltsis temperature. In contrast, Naski & Hess (1985)and Taylor & Wall (1992) concluded that thedominant controls of tin solubility in granitic meltsare the (Na+K)/Al and K/Na ratios and /(O).The latter authors also suggested that Sn may occuras an alkali complex, of the type that has beenproposed for other high-field-strength elementssuch as Ti (Dickinson & Hess 1985). Although theeffect of melt structure on mineral-melt partitioncoefficients for tin is not known, the strongcorrelation between K and Sn at Nong Sua indicatesthat' such compledng may be important atsupersolidus conditions.

According to the solubility data of Stemprok(1990), approximately 850 and 1900 ppm Sn arerequired to saturate a melt in cassiterite at 600.and 700"C, respectively. The experimental data ofTaylor & Wall (1992) predict somewhat differentsolubilities of cassiterite. At 700' to 800.C, 2 to 3kbar, and/(O) at FMQ to FMQ + 1.5 log units,cassiterite saturation occurs between 400 and 2500ppm Sn. Tin solubility in these experiments wasfound to increase with increasine (Na + K)/Al and

t ) t

Na,/K ratios and with decreasing /(Or). All thevalues of cassiterite saturation reported above aremuch higher than the Sn contents of aplites andpegmatites at Nong Sua (Table 2). However, sincetin was concentrated by fractionation, the bulkdistribution coefficient was less than l, and theconcentration of tin in the rock is less than that inthe melt from which it crvstallized.

Partitioning of tin

Experimental studies of mineral-melt partitioncoefficients for tin are unfortunately lacking.However, Kovalenko et al. (1988) proposed partition coefficients based on empirical data. For theminerals observed at Nong Sua, the partitioncoefficients areKS"u I I > Kfffgi,"u* ) Kilr"ro,p-> K!tur,r. Using these partitioncoefficients andwhole-rock composilions, it is possible to estimatethe concentration of tin in the melt at Nong Sua.The pegmatites with the highest tin contents arecomposed of >9890 perthite + quartz (Fig. 12)and contain, on average, 28 wt.olo normativequartz. The average tin content ofthese pegmatitesis 22 ppm (Table 2), and that of individual samplesattains 42 ppm. The tin content of the melt wascalculated using normative quartz and perthite(Or + An + Ab) and their respective partition coef-ficients at 600o and 700oC (Kovalenko et ol. 1988);we assumed that all the tin is contained in quartzand K-feldspar (as indicated by the K2O-Sncorrelation). Concentrations of Sn estimated tohave been in the melt arc391 arrd253 ppm at 600oand 700oC, respectively, for the average K-feldspar-rich pegmatite. These concentrations arebelow the cassiterite saturation values of Taylor &Wall (1992) and Stemprok (1990), which suggeststhat during much of the pegmatite's evolution,cassiterite had not achieved saturation. The samplewith the highest content of tin (42 ppm; l3Vonormative quartz) is estimated to have been inequilibrium with a melt containing 667 ppm Sn at600'C and 444 ppm at 700oC. These values aie inthe range of saturation values quoted by.Taylor &Wall (1992) and are close to those of Stemprok(1990). Since these values were obtained fromnonmineralized samples, we conclude that furthercrystallization, or change of intensive parameters,particularly an increase in /(O), resulted incrystallization of cassiterite from the melt.

It is important to note that the lowest abundan-ces of tin occur in the most strongly peralumiuousrocks, those containing tourmaline and muscovite.In light of the relative order of the abovemineral-melt partition coefficients, the per-aluminous rocks must have crystallized from a meltwith much lower Sn concentrations, in accord withthe interpretation that the peraluminous units



formed early and were followed by metaluminousunits. The degree of fractionation required toaccount for a concentration of 700 ppm Sn in themelt can be estimated using the Rayleigh fractiona-tion equation:

C1 = Pto- t )ag1where C1 is the concentration of tin in thefractionated melt, Cl is the concentration of tin inthe initial melt, D is the bulk solid-melt distributioncoefficient, and F is the proportion of meltremaining. Bulk distribution coefficients of 0.1 to0.3 probably are reasonable (c!. Coetzee & Twist1989); since the aplites of Nong Sua have anaverage tin content of 17 ppm (Table 2), the initialmelt is estimated to have had 50 (D : 0.1) to 100(D = 0.3) ppm Sn. At a Ci of 50 ppm Sn, 95 to9890 fractionation is required to attain 700 ppm Snin the melt, or at a Ci of 100 ppm Sn, 88 to 94Vofractionation is required (0.1 < D < 0.3). Ittherefore follows that the crystallization of cas-siterite at Nong Sua probably occurred after morethan 90qo of the crystallization of the aplite-peg-matite was completed.

Effect of vapor soturation

The above calculations do not consider theeffects of vapor saturation, which potentiallychanged the bulk distribution coefficient. Theoccurrence of vein-hosted and greisen styles ofmineralization are evidence that the hydrothermalfluids contained significant concentrations of tin.The lack of alteration of aplite and pegmatiteindicates that these fluids did not leach tin fromthe wallrock, which implies that tin was partitionedinto the orthomagmatic fluid upon vapor satura-tion. However, even if Kiioid-..Ir was less than 1.0,hundreds of ppm Sn could still have been insolution, given that the melt contained on the orderof 700 ppm Sn. Taylor (1988) found that thepartition coefficient for tin between vapor andhaplogranitic melt (Ksrfu'o-."), derived from ex-perimental studies, is proportional to the square ofthe aqueous chloride molality and that Kituid-.* isgreater than 1.0 if the chloride concentration isgreater than approximately I molar. Durasova e/al. (1986) also found that Ksr"hjd-m"t, is proportionalto chloride concentration, but estimated that theK?luio-..rt for a I N Cl solution coexisting withhaplogranite is 0.2, at 750'C and 1.5 kbar, at theNi-NiO buffer. Keppler & Wyllie (1991) estimateda K?luio-."rt of less than 0.1 for haplogranite at750oC, 2 kbars, at the Ni-NiO buffer. Discrepan-cies among the experimental resirlts may, in part,have resulted from high solubilities of tin in thenoble metals used as capsules. However, there issufficient agreement to conclude that, forchlorinities of less than I M, Klidd-me1, is less than

1.0 in granitic systems. At Nong Sua, theorthomagmatic fluid is considered to have had alow salinity (<l M; Linnen & Williams-Jonesl99l). Tin, therefore, was not strongly partitionedinto this fluid. Since the mass of fluid at vaporsaturation was small compared to the mas$ of melt,and Klfu;6-."1 was less than 1.0, vapor saturationwould have had only a minor effect on the bulkdistribution coefficient and thus on the tin contentof the melt (calculated above). In addition, the factthat Kituid-*elr was less than 1.0 implies that somecassiterite must have crystallized directly from thelate-stage granitic melt at Nong Sua, regardless ofthe timing of vapor $aturation.

Lers-Srecr EvolurtoN oF PEGMATITE

It was demonstrated above that samples ofnonmineralized K-rich pegmatite are close to beingsaturated with cassiterite. This, however, does notexplain why disseminated cassiterite is associatedwith pegmatite containing both K-feldspar andplagioclase, why disseminated cassiterite is typicallyassociated with tourmaline, and why Ti-Nb-Taoxide minerals are abundant as inclusions indisseminated cassiterite but are absent in veincassiterite. The saturation of the granitic melt invapor potentially explains these characteristics.Lowering the water content of the melt by vaporsaturation raises the solidus temperature andpromotes crystallization. If boron is partitionedinto the vapor phase, the solubility of water in theremaining melt is further reduced (Pichavant 1987);thus water saturation of a boron-rich melt is aneffective mechanism of quenching. Although theboron content of Nong Sua melts could not havebeen extraordinarily high, if buffered by thecrystallization of tourmaline, vapor saturationnevertheless could have been an effectivemechanism of quenching. The K-rich units areinterpreted as cumulates, resulting from crystalgrowth at supersaturated conditions. The melt fromwhich this unit crystallized also contained Na andB, for example, that are not reflected by thecumulate composition since plagioclase and tour-maline were not crystallizing. But, upon quenching,a composition closer to the granite minimumshould have resulted (i,e,, a composition closer tothat of the liquid, than that of the K-rich cumulate)as is the case for the host to the disseminatedmineralization. The release of boron into aqueousfluids is evident by the formation of quartz-lour-maline veins and by the presence of tourmaline asan alteration mineral in metasedimentary rocks atthe pegmatite contacts. Tourmaline intergrownwith disseminated cassiterite may have had eithera magmatic or hydrothermal origin; fluctuationsbetween saturation and undersaturation of cas-

siterite at vapor-saturated melt conditions mayexplain the etching (embayment) of cassiterite,which was later filled by plagioclase or perthite (thefeldspars also potentially crystallized either fromsilicate melt or aqueous fluids). Furthermore, Ti,Ta and Nb in the residual melt must have eitherbeen forced to crystallize as minerals or partitioninto the aqueous phase. The lack of Ti-Ta-Moxide minerals in vein cassiterite (or as accessoryminerals in the quartz-tourmaline veins) indicatesthat these elements rpere not highly mobile. Theinclusions of niobian-tantalian rutile, ixiolite andcolumbite in disseminated cassiterite are thereforeinterpreted to reflect the quenching process.

We propose that the quenching also resulted influid overpressure in the latest magma. This causedbrittle failure in the aplite, and the formation ofquartz-tourmaline + cassiterite a wolframiteveins. The pegmatite was, at least locally, super-solidus and thus did not deform by brittle failure.However, the features of brittle and ductiledeformation, developed locally in aplite, pegmatiteand veins, also are interpreted to have formed atthis stage, or during the early subsolidus evolution(indicated by stress-related perthite exsolutionabove approximately 450'C, the temperature of theorthoclase-to-microcline transition). A fluid-as-sisted tectonic overprint of the aplite-pegmatitecomplex can be ruled out on the basis of thepreservation of intermediate structural states inK-feldspar (K-feldspar would have equilibrated tomaximum microcline during a tectonic overprint)and of oligoclase.

Given that the veins and pegmatite crystallizedat similar P-T conditions, it remains to beexplained why the vein plagioclase is albite (An6 toAnr), whereas oligoclase is observed in pegmatite.The composition of plagioclase crystallizing fromthe aqueous fluids was dependent on T, P and, inparticular, fluid composition. In general, theCa,/Na ratio of fluid coexisting with melt is stronglydependent on chlorinity, and for the low-salinityfluids at Nong Sua (cl, Linnen & Williams-Jones1991), the Ca./Na ratio of the fluid would havebeen much lower than that of the melt (cl, Candela1989). Similarly, plagioclase that crystallized fromthe orthomagmatic fluid would have had a veryIow Ca,/Na ratio (Schliestedt & Johannes 1990).Therefore, the albite observed in veins that hostcassiterite at Nong Sua could have formed fromhydrothermal fluid while the final granitic magmawas above or near solidus conditions.

CoNcLUsIoNs

The crystallization of the Nong Sua complexcommenced with the formation of aplite with agranitic composition. Pegmatite crystallized later to

759

form units with strongly contrasting compositions.The K-feldspar-rich units are interpreted to be late,relative to the plagioclase-tourmaline-rich units.Tin abundance increases with potassium content,suggesting that the tin concentration of the meltincreased with fractionation. The textural relation-ships between cassiterite, plagioclase and myr-mekite imply that cassiterite crystallized prior to orat vapor-saturated melt conditions. This interpreta-tion has been tested by comparing the calculatedabundance of tin in the melt with experimentallydetermined limits of tin solubility; we conclude thatthe melt contained on the order of 700 ppm Sn,which is consistent with the crystallizatiorr ofprimary cassiterite. Mineralization continued aftervapor saturation to form vein-hosted cassiterite andwolframite, and greisen cassiterite. In the case ofthe former, the timing of mineralization isconstrained, by cross-cutting relationships, to havebeen at or near solidus conditions of the pegmatite.The hydrothermal stage at Nong Sua was relativelyshort-lived, as indicated by the preservation ofintermediate structural states in K-feldspar and thepresence of oligoclase. However, as a consequenceof fluid overpressure, brittle failure occurred in theaplite units, resulting in the formation of veins, andfeatures of brittle and ductile deformation weredeveloped locally in aplite, pegmatite and veins.

AcKNowLEDGEMENTS

This research was carried out with the aid of agrant from the International Development Re-search Centre, Ottawa. We thank S. Bunopas, N.Jungwsuk, S. Khositanont, and P. Putthapiban ofthe Geological Survey Division, Department ofMineral Resources of Thailand for their supportduring the course of this project. The manuscriptwas improved by comments from D. Baker, J.Clark, B. Lehmann, R.P. Taylor and ananonymous referee, and from discussions with F.Holtz and M. Pichavant.

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MAGMATIC CASSITERITE. NONG SUA COMPLEX. THAILAND

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Received Juty 10, 1991, revised manuscript occeptedSeptember 15, 1992.

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