937

Canadion MineralogistVol. 30, pp.937-954 (1992)

MAGMATIC HYDROTHERMAL FLUIDS AND THE ORIGIN OFOUARTZ _ TOURMALINE ORBICULES IN THE SEAGULL BATHOLITH,

YUKON TERRITORY"

IAIN M. SAMSONDeportment of Geology, University of Windsor, Windsor, Ontario NgB 3P4

W. DAVID SINCLAIRGeological Suney of Canado, 601 Booth Street, Ottawa, Ontario KIA OE8

ABSTRACT

The mid-Cretaceous Seagull batholith in the Yukon Territory consists of meta- to peraluminous granite in whichtourmaline generally occurs as an accessory phase. The roof zone of the granite is, however, characterized by thepresence of quartz-tounnaline orbicules that constitute up to l0 to l5q0 of the granite. The orbicules are typically 6to 8 cm in diameter, and contain up to 40q0 tourmaline, the remainder being chiefly quartz, feldspar, and minorfluorite. Miarolitic cavities lined with quartz, tourmaline, and rare topzv occur in the center of some orbicules. Theorbicules crystallized from a mixture of residual borosilicate melt and hydrothermal fluid. Quartz, tourmaline andfluorite contain primary aqueous fluid inclusions. Salinities range from 8 to 42 equiv. wt.9o NaCl + CaCl2. As theorbicules crystallized as closed systems, these inclusions contain unevolved orthomagmatic hydrothermal fluids. Largevariations in salinity of fluids associated with epizonal plutons thus do not have to be explained by boiling or mixingwith meteoric water. Isochoric projections indicate that crystallization took place between 600 and 3@'C, and possiblyas low as 150'C. Solids associated with some of these inclusions demonstrate that the fluids were in equilibrium withmuscovite, indicating that they would cause phyllic alteration during migration through previously crystallized granite.Elevated concentrations of tin in the orbicules and the presence of quartz-tourmaline-cassiterite veins in the graniteindicate that, on occasion, the fluids migrated away from the magma and were responsible for the tin mineralizationassociated rr/ith the granite.

Keywords: magmatic hydrorhermal fluid, granite, quartz-tourmaline orbicules, tin mineralization, Seagull batholith,Yukon.

Soutrtunr

Le batholite de Seagull, situ€ dans le Territoire du Yukon, et d'age cr6tac6 moyen, est forme d'un granite m6ta-i hyperalumineux dans lequel la tourmaline forme en g6n6ral une phase accessoire. Cependant, dans la coupole, desorbicules e quartz + tourmaline peuvent constituer jusqu'i 10-1590 du granite. Les orbicules ont typiquement 6 el 8cm de diambtre et contiennent jusqu'i 4090 de tourmaline, le reste 6tant constitue essentiellement de quartz, feldspathet fluorite en quantitd mineure. Des cavitds miarolitiques, renfermant quartz, tourmaline, fluorite en quantit€ mineureet r:ue topaze, existent au centre de certaines orbicules. Les orbicules se sont fonndes A partir d'un mdlange de magmar€siduel borosilicatd et de fluide hydrothermal. Quartz, tourmaline et fluorite contiennent des inclusions fluidesaqueuses primaires, avec des salinit€s allant de 8 d 42t/o dquivalents en poids de NaCl + CaCl2. Les orbicules ontcristallis6 en systCme ferm6; ces inclusions contiennent donc un 6chantillon de la phase fluide orthomagmatique non6volude. Les variations importantes en salinit6 qui caract6risent les plutons dpizonaux ne n6cessitent donc pas uneexplication faisant appel i une dbullition ou i un mdlange avec de I'eau m6t6orique. Les projections isochores indiquentune cristallisation entre 600 et 300oC, et mOme peut Ctre jusqu'd 150'C. En outre, les inclusions solides associ6esmontrent que dans certains cas, le fluide 6tait en dquilibre avec la muscovite, ce qui fait penser qu'il pourrait causerune altdration phyllique s'il traversait le granite cristallis€. La richesse en 6tain des orbicules et la pr6sence dans legranite de fissures remplies de quartz + tourmaline + cassit6rite montrent que ce fluide a pu parfois migrer en dehorsdu magma et induire la min€ralisation en 6tain associ€e au granite.

(Traduit par la R6daction)

Mots-cbsi fluide hydrothermal orthomagmatique, granite, orbicules i quartz + tourmaline, mindralisation en 6tain,batholite de Seagull, Yukon.

"Geological Survey of Canada contribution number 58390.

938 THE CANADIAN MINERALOGIST

INTRODUCTIoN

The roof zone of the mid-Cretaceous Seagullbatholith, located near the southern border of theYukon Territory, is characterized by the presenceof quartz-tourmaline orbicules. These have beenattributed to a borosilicate fluid transitional incomposition between silicate melt and hydrother-

mal fluid, that evolved during the late stages ofcrystallization ofthe granite (Sinclair & RichardsonL992). Elevated tin concentrations (up to 90 ppm)in the orbicules suggest that these late-stage fiuidsalso were responsible for the tin mineralization inand around the batholith. This paper communi-cates the results of a study of fluid inclusionsinitiated in order to characterize the fluids from

E seasut t Bathot i th

f zone of quartz- tourmal ine

a

o

orb i cu les

Tin-bear ing ve in

Tin-bear ing skarn

Sample local i tyo

aa lPo

8 0 - 7 8-'a8 2 - 1 6 6

8 6 - 4 5

82 - 1 65

K i l ome t res

6 0 0 0 0 '1 3 1 0 3 0 '

.l.;.\

d- / ( ' \

Nor thwes l*

; ' ) Terr i tor ies

/ v r ron L., ' )

i Fisure t \'-.

;) ir -\r- .-1^qlBri t ish cotumbia

Frc. l. Distribution of quartz-tourmaline orbicules and tin-bearing veins and skarns associated with the Seagullbatholith, Yukon Territory. Modified from Mato et al. (1983) and Sinclair (1986).

MAGMATIC FLUIDS, SEAGULL BATHOLITH

which the orbicules crystallized, to elucidate theirorigin, and to provide information on anyorthomagmatic hydrothermal fluids that may haveplayed a role in tin mineralization.

Grolocv

The Seagull batholith (Fie. l), which hosts thequartz-tourmaline orbicules, is a leucocraticgranite of mid-Cretaceous age (Mato et al. 1983),exposed over about 300 km2 in the CassiarMountains in southeastern Yukon Territory. It wasemplaced in the Yukon Cataclastic Complex, atectonic mdlonge of sedimentary, volcanic andintrusive rocks deformed during the Early Mesozoiccollision between the ancient North Americancraton and an allochthonous island arc terrane(Tempelman-Kluir 1979, Abbon 1981). Thebatholith has a high initial ETSr/E6Sr value of 0.712(Mato el al. 1983) and likely crystallized from amagma derived by partial melting of crustal rocksrelated to the Early Mesozoic collision.

Mineralogical and chemical aspects of theSeagull batholith have been described by Gower(1952), Poole (1956), Mato et ol. (1983) and Sinclair& Richardson (1992). The batholith consistsentfuely of leucocratic granite; medium- to coarse-erained seriate granite forms approximately 9090of the exposures, and fine-grained, commonlyporphyritic granite, about 1090. The coarser-grained seriate granite occurs mainly in the centralpart of the batholith, whereas the finer-grainedporphyritic granite is more common near themargin. Contacts between the two varieties are

typically irregular and gradational. The twovarieties of granite are similar mineralogically, andaverage 380/o quartz, 33t/o K-feldspar, 25Voplagioclase and 4t/o biotite. Accessory mineralsinclude fluorite (0.5-l9o) and trace amounts ofihnenite, rutile, monazite and topaz.

A relatively shallow level of emplacement of thegranite (L <P <2 kbar?) is indicated by sharpcontacts with country rocks, the presence ofmiarolitic cavities, and the fine-grained, porphyritictexture of the granite near its margin.

Chemical compositions presented by Mato et al.(1983) and Sinclair & Richardson (1992) show thatthe Seagull granite is slightly srelaluminqus fsperaluminous (all compositions are characterizedby normative corundum), siliceous Q4-77V0 SiO),alkali-rich (8-990 KzO+Na2O), rich in fluorine(0.15-0.6590 F) and poor in TiO2 (0.09-0.1390),FqO, (0.1-0.490), FeO (0.6-1.2c/o), CaO (0.39-0.71s/o) and MgO (0.01-0.l8Vo). The Seagullgranite also has relatively high contents oflithophile elements such as Li (36-176 ppm), Be(6.1-14 ppm), M (57-77 ppm), Y (87-220 ppm),Rb (326-668 ppm) and Ga Q3.3-2i7.4 ppm), butonly minor Ba (30-M ppm) and Sr (9-28 ppm),compared to average granitic rocks. The chemicalcomposition of the Seagull granite is typical ofhighly differentiated, "specialized" granites (cl,Tischendorf 1977).

Although the tin content of the Seagull granite(<3-10 ppm Sn) is not anomalous, tin-bearingveins and skarns are associated with the batholithand probably are genetically related (Fig. l).Aspects of the tin deposits have been described by

FIG. 2. Quartz-tourmaline orbicules in the Seagull granite.

9N THE CANADIAN MINERALOGIST

MAGMATIC FLUIDS. SEACULL BATHOLITH 941

Dick (1980), Mato et ol. (1983), Layne & Spooner(1986, l99l) and Sinclair (1986).

Except in the highest levels of the exposedbatholith, the Seagull granite generally containsonly trace amounts of tourmaline. The upper partof the batholith, however, contains orbicules thatare characterized by abundant tourmaline andquartz (Fig. 2). The orbicules are roughly sphericalto ovoid in shape (Fig. 3A), but may be joined inplaces to form larger, more irregular bodies.Spherical orbicules are typically 6 to 8 cm indiameter, but range from 2 to l0 cm or more.Tourmaline-bearing fractures and quafiz-tour-maline veins also are present in the upper part ofthe batholith. Some orbicules are cut by tour-maline-bearing veins, but most orbicules arespatially unrelated to such features. The orbiculesand veins occur in both the seriate and fine-grainedgranites.

The orbicules are concentrated at the top of thebatholith, where they constitute, in places, 10 to159o of the granite. They extend to several hundredmeters below the upper contact, gradually decreas-ing in abundance with depth. They are mostabundant in the southeastern exposures of thebatholith, particularly along the eastern andnortheastern margins and beneath roof pendants(Fie. l).

MINERALOGY

The orbicules consist mainly of tourmaline andquaxlz, with lesser amounts of microcline, albiteand minor amounts of fluorite. They commonlyexhibit concentric zonation based on the proportionof the constituent minerals (Fig. 3A). From hostgranite inward, this zonation consists of aleucocratic halo of biotite-free granite I to 2 cmwide that surrounds each orbicule, a dark outerzone I to 5 cm wide that consists mainly of quartzand tourmaline, and a slightly more leucocraticinner zone with a radius of I to 2 cm and composedof quartz, microcline, albite and tourmaline. Inmany orbicules, the dark outer zone extends to thecenter of the orbicule, and the leucocratic innerzone is absent. Some orbicules have a central

miarolitic cavity lined with crystals of quartz,tourmaline and, in a few places, topaz.

The leucocratic halo that surrounds each or-bicule consists of host granite that is characterizedby the absence of biotite. The lack of any obviousalteration-related features suggests that the absenceof biotite is a primary feature and is not'due toalteration of pre-existing biotite.

The transition from the leucocratic halo to thedark outer zone is marked by the abrupt ap-pearance of dark brown to black tourmaline, whichconstitutes 25 to 40Vo of the zone (by volume). Thetourmaline is generally fine grained and anhedral.Quartz (35-5090), albite (10-3090) and perthiticmicrocline (0-1590) €re the other principalminerals. Minor amounts of fluorite (0-170) andtrace amounts of muscovite, rutile, zircon, thorite,ilmenite, pyrite, arsenopyrite, wolframite, monaziteand topaz also are present. Tin in the orbicules isprincipally concentrated in rutile (Sinclair &Richardson 1992).

An examination of thin sections shows that thetourmaline in the outer zone has replaced micro-cline, albite and quartz to varying degrees. Incipientstages of tourmalinization consist of stringers andirregular networks of tourmaline partly replacingperthitic microcline (Fig. 3B). Advanced stages ofthis alteration are characterized by nearly completeto complete replacement of microcline and as-sociated albite intergrowths, partial replacement ofdiscrete albite grains, and replacement of quartzalong grain boundaries. Tourmaline replacingmicrocline commonly contains inclusions of quartzand remnants of microcline and albite. Rutile,zircon, thorite and monazite also occur as in-clusions in tourmaline. In some orbicules, rr'inoramounts of muscovite occur as an alterationproduct of microcline, particularly in associationwith toirrmaline, and in cases interstitially toquartz, feldspar and tourmaline.

The more leucocratic inner zone of the orbiculesconsists of fine-grained quartz (30-4090), rour-maline (20-400/o), microcline (20-25c/o), albile(15-200/o) and fluorite (0.5-190). In this zone,tourmaline typically occurs as discrete, anhedral,interstitial grains that generally show no widenceof replacement of the feldspars (Figs. 3C,D).

Frc. 3. A. Quaflz-tourmaline orbicule showing the leucocratic halo in granite (outlined by dashed line), dark outerzone, and slightly more leucocratic inner zone. B. Photomicrograph showing K-feldspar partly replaced bytourmaline (dark grey), albite relatively unreplaced, and quartz. C. Photomicrograph showing tourmaline, in partreplacing K-feldspar (cloudy grains) and in part infilling cavity (note euhedral terminations on quartz gxainsprojecting into tourmaline), D, Photomicrograph of zoned tourmaline in core of orbicule; tourmaline fills cavitybounded by K-feldspar, which is extensively replaced by tourmaline. Scale bar = 0.5 mm.

942 THE CANADIAN MINERALOGIST

t-v

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o

Jo

A

wI=

o

o

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e

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@ @g @

ogSLL

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Ftc. 4. Drawings of fluid inclusions from the quartz-tourmaline orbicules. A. Growth zone in quartz containing type-latrd type-6 inclusions with unknown rhombic solid. B. Type-6 inclusions (muscovite) with and without arrays oftype-l inclusions. C. Isolated array of type-6 inclusion (muscovite) surrounded by a cluster of type-l inclusions.P.ltirnuw type-l and type-S inclusions in tourmaline. E. Primary type-S inclusions in quartz. F. Secondary type-3inclusions in quartz. l, type-l inclusion; L, liquid; V, vapor; S, solid; H, halite. Scale bar : 20 pm.

MAGMATIC FLUIDS, SEAGULL BATHOLITH 943

Tourmaline also occurs, less commonly, as ir-regular grains within granophyric intergrowths ofquartz and K-feldspar.

Representative orbicules from four differentlocalities in the Seagull batholith were used in thissrudy (Fis. l).

ANALYTIcAL TncHNIqurs

Microthermometric analyses were carried out ona Linkam TH600 heating-freezing stage calibratedwith various organic and inorganic liquids andsolids, H2O and melting-point standards. Ramansp€ctra were collected at the University of Windsoron an Instruments S.A., Ramanor U-1000 spec-trometer fitted with a Spectra Physics Ar ion laser(514.5 nm line) and a Nachet microscope (X80objective), using a qounting time of I second anda stepping interval of I cm-r.

TYpBs oT. FLUID INCLUSIONS

In the orbicules, fluid inclusions are present inquartz, tourmaline, and fluorite. Inclusions appeilto be less abundant in tourmaline than in quartz;however, inclusions in tourmaline are generallyobscured by the dark color and high indices ofrefraction of the host. Fluorite is rare in the samplesstudied; inclusions were only observed in fluoritefrom sample 80-78.

Six types of fluid inclusions are present in theseminerals (Fig. 4), distinguished on the basis of ratioof included phases at room temperature. Generally,only groups of inclusions with apparently consis-tent ratios of phases have been described andmeasured. Measurements on trapped phases werenot attempted. All results are reported as theaverage and standard deviation for a single,cogenetic group, except in those cases whereanalytical difficulties allowed only one result to beobtained from a group, or the rare situation wherea single, isolated inclusion was measured. The sixtypes of inclusion are:

l. Two-phase, liquid-vapor aqueous inclusions(Fig. 4C) that homogenize to the liquid phase.These inclusions may contain trapped, generallybirefringent, solids.

2. One-phase, liquid-only aqueous inclusions.These are generally associated with type-l in-clusions with high degrees of fill, and eitherrepresent the same fluid in which a vapor bubblehas failed to nucleate, or part of a necked inclusion.

3. Two-phase, vapor-liquid (vapor-rich) in-clusions (Fig. 4F).

4. One-phase, vapor-only inclusions.5. Aqueous liquid-vapor-solid inclusions. In a

majority of these inclusions, the only daughtermineral pre$ent is a subhedral to euhedral square

(cubic) solid, interpreted as halite (see later section)(Fie. E). Some inclusions contain additionalsolids, which, in most cases' are small and notpresent in all inclusions (trapped). In some, rare,secondary groups, these additional solids are large,are present in all inclusions in a group' andtherefore may be daughter minerals.

6. Generally large (20 - l0 pm), solid + vaport liquid inclusions. In most inclusions, the solidsexhibit strong birefringence and can be anhedral oreuhedral. The vapor phase varies from a smallsphere to a large irregular mass @igs. 44, B, C).In a few examples, these inclusions contain twovapor bubbles, one on either side of the solid (Fig.5E). In some inclusions, a volumetrically minorliquid phase may is present.

Primary inclusions

Primary inclusions in quartz occur in a varietyof forms: in growth zones, as small, isolatedclusters, and as dispersed, three-dimensional ar-rays. Growth zones may comprise type-6, type-I,or a mixture of both types of inclusion (Ftg. 4A).Large, isolated, type-6 inclusions are common andare also interpreted as primary (Fig. B).

Isolated type-6 inclusions commonly have anarray of small inclusions associated with them@igs. 48, C, 5E). These arrays generally comprisetype-I, with or without type-2, inclusions; onearray contains type-S inclusions. The arrays are, inmost cases, isolated from other groups of in-clusions: some crystals have numerous, large,type-6 inclusions, each with their own envelope oftype-l inclusions. Most of the arrays appear to beplanar, although some are distributed around thecentral type-6 inclusion in a three-dimensionalcluster. In rare cases, several large type-6 inclusionsmay lie within a single type-l array. In manycrystals, type-l inclusions are only found associatedwith type-6 inclusions. It is clear that there is agenetic link between the type-6 inclusions and theassociated type-l arrays. The arrays could repre-sent: l) pseudosecondary planes, 2) decrepitationclusters, or 3) growth surfaces, where the type-linclusions were trapped at the same time as thetype-6 inclusions. If the arrays represent pseudo-secondary planes, the solid (type-6 inclusion) wouldhave to have been precipitated in the fracture, asmost are probably too large (commonly 50 to 100pm) to have been trapped along with the fluid. Insuch a model, it is difficult to explain why the solidalways occurs at, or close to, the center of thearray. We therefore do not consider these to bepseudosecondary inclusions.

Planar arrays of small inclusions around a largercentral inclusion may also be produced throughdecrepitation as a result of internal overpressure

94

A.

B.

THE CANADIAN MINERALOGIST

c.

D. €co o e alE=iff intOoo o -

negative crystals, and the arrays are not alwaysrestricted to the immediate vicinity of the type-6inclusions, both of which would argue against anorigin through decrepitation. Therefore, we ijlter-pret these arrays to be primary growth-relatedfeatures, analogous to the growth zones in Figure4A., but where inclusions have formed over a morerestricted part of the growth surface. Note that eachsection of the growth zone in Figure 4A comprisesa type-6 inclusion surrounded by type-l inclusions.In this interpretation, the solid is central to theformation of the type-l array, both literally andfiguratively. Our interpretation is that the solid,precipitated on the surface of the growing quartzcrystal, acted as a discontinuity in the growth ofthe quartz. This resulted in the trapping of somefluid in a restricted grofih-zone immediatelyaround the solid, which subsequently healed toform a planar to three-dimensional array of type-land type-2 inclusions, with a type-6 inclusion at itscenter @ig. 5A-D). Isolation of fluid on either sideof the solid and subsequent healing also explain thepresence of two vapor bubbles, on opposite sidesof the solid, in some type-6 inclusions (Fig. 5B).

In sample 80-78, primary type-5 inclusions occuras a a three-dimensional array (Fig. 4E). Thissample is free of any other type of primaryinclusions.

Many tourmaline crystals contain abundantelongate or tubular inclusions that are alignedparallel to the Z axis of the crystal; these areinterpreted as primary inclusions (Fig. 4D). Thecontents of these inclusions, particularly the solidphases, are commonly difficult to resolve opticallyowing to the color and indices of refraction of thetourmaline. These inclusions are either type-l ortype-S. Some crystals appear to have mixtures ofthe two types of inclusion (Fig. 4D), but this maybe a problem in resolving halite in these darkinclusions.

The only inclusions in fluorite that may beprimary are large, isolated, type-5 inclusions insample 80-78.

Pseudoseco nd ary inc lus io w

Type-l and type-5 inclusions that form short,isolated planes have been interpreted as pseudo-secondary, although, by analogy with the arraysdiscussed above, they could represent surfaces ofrestricted growth that formed without the aid of asolid.

Secondary inclusions

Secondary planes are cornmon in all samples andcontain all the types of inclusion described, withthe exception of type 6. Inclusions of types 3 and

0 .

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Frc. 5. A to D. Sequence of sketches illustrating a proposedmodel for the formation of the l,/6 arrays of fluidinclusions on a growth surface of quartz (Qtz), asinduced by the precipitation of muscovite (Ms) (or anyother mineral). This sketch includes the formation oftwo vapor bubbles on either side of the muscovitecrystal. E. Drawing of an actuil l/6 array showing twovapor bubbles in the type-6 inclusion. Scale bar : l0pm.

exerted by the central inclusion (P6cher 1981,Sterner & Bodnar 1989). Inclusions that re-equi-librate to a lower external pressure tend to becomenegative crystals (Gratier & Jenatton 1984, Sterner& Bodnar 1989), and may also decrepitate to forma plane of uniformly small inclusions immediatelysurrounding the original inclusion (P6cher l98l).Sterner & Bodnar (1989) have, however, cast somedoubt on the frequency with which decrepitationclusters form. In their bxperiments, they found thatthe formation of a decrepitation cluster was ,.the

rare exception rather than the rule" (p. 251), andsuggested that'many of the arrays described byother investigators from experimental studies mayin fact be unrelated to decrepitation.

The central inclusions in the Seagull arrays ofinclusions tend to be irregularly shaped, rather than

MAGMATIC FLUIDS. SEAGULL BATHOLITH

TABLE I . SUMMAFT OF THERMOMETBY DATA FOR INCLUSION TYPES 1 AND 6

945

SAMP MIN TYPE ORIG * T€ave 9.o.

TmICE SALavg 6.d.

Tdvav6 6.d.

7A787A7A165a78l66045a45a45a787A/tsa45a45a/tsa1660/tsa45a165a.tsc7A165a45a,f5078166brtsb45b165artsa

7a7A45a

45crtSo

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oo

oaooooooTTT

? 2 € , . os 3 35.0s 29 t? 3 -45.0? 5 -n,o? 1Pl I €4.0P11 1 -35.0P 1 ? 5 -PS 6 -23.0PS 1 €5.0

PS|/S 2 -4p'.os 4s 2 .35.0s 4 40.0s 2 -25.0s 4 -25.0s 4 -25.0S 6 -40.0s 2s 29 3s 6 €5.OS 5 -47.0s 4P 5 2 -P5 5 -27.OP 5 1 -

- 4.1 0.0- €.3 0.0- -4.3 0.1- 3.3- -23.0- €.0 3.3

1.4 €.6 0.3- -14.9- -10.3 0.0

4.0 -15.3 0.1- -t3.0- €.E- €.6 0.4- {.8

0.0 €.3 0.7- -2.7 0.0

8.6 €.3 0,30.0 42 02

- 4.7 0.0- -7.1 02- €.3- €.7

0.0 €.8 0.1- -7.1 0.0- -2,8 0.4- -4.6- €0.0 1.0- -11.0- -16.0- -7:t 0,3- €.5 0.0- -5.8 1.0- -13.7 0.4- -10.8- -2..O

- - l t .o- €.0 02- 4,2- -18.5 0.7

- -t0.4 0.4- -7,5 02- -12.2 0.0- €0.5 0.6

- -23.0 3.0- -25.0- -a.2- €.3 0.5- -25.0 0.0- -7.6- -7.7

7,1 2€B Fa.7 260 47,3 276 15.9 160 10

2 7 7 6 1E.3

- 1/()127 1A 2.182 31214.4 378 1E18.5 I 6517.s12p 97 0127 375 712€ 113 41 2 l 1 1 6 1 14.9

12-4 130 512.3 145 136.5 1A 10

11 .05.9 285 416.5 l3l' 4

12.9 136 Ut11.0 12. l t5.1 153 27.6 111 70

21.3 14{t 12.O15.1 2d'19.0 A7 211.7ls.o ' r lo io9.3 126 30

17.3 127 1314.9 1982.20.3 101 1

15.18.3 1E1 15

12.3 114 A20.5 1lo I0.3 127 16

14.5 115 t311 .5 t t 4 1316.1 127 22'1.5 175 59

- 124 t9274.52,.713.4 356 323.5 77 711.5 370 /1911.7 g

166b O 1(6) Pl 6 -.15o O 1(6) R! 7 <41660 O 1(0) P6 7 €0.0166c O 1 (6 ) Po 1€0 .0165a O 1(6) P6 1 <-4o155a O 1(6) P6 2 -'t66o O 1(6) P6 t -

o 1(6) P6 3 -

o 0s) P?JPS 4 45.0o 10s) P2lPs 4 .45.0o l ( E ? s

Q 1 5 ? 1 4 7 . 0o ' t s / l ? 1 - 2 6 . 0o lb I 12 -30.0

l6sa O 1(6) Po 3 -'166c O 1(6) P6 2 €0.016sa O l(6) P6? 3 -165a a 1(6) PS/P3 6 40.0rtso A l(6) PS/Pg? 4 €5.0165a O 1(61) P6? 2 <€0

Q r / 1 S ? 3 - 2 5 . 0106a A 112 S? 2 -1660 Q 1/6 F6 2 -

165a Q ls PS/P2 2 -5o.0

Q 6 P ? , 3 .Q 6 P 3 1 -

MIN: Mlneral: Q, QuarE; F, Fluorne; T, TourmallneTYPE: Type of Incluslon as defned In text (phase ass€mblage)ORIG: Orlgln of Incluslon group: P, Prlmary (Pl, growlh zone;

P2, 3D anay; P3, 1/6 anry; P4, lsolated lncl.; P5, tubular lncl intourmallne); PS, Pseudosecondaly; S, Secondary

#: Number of lncluslons ln group used for averags (a\reJ andstandard devlatlon (s.d.)

Srilj Sallnlty ln equlv. wt. o/o NaCl + CaCl2All temperdure measurements ln degrees c

946 THE CANADIAN MINERALOGIST

4 were found only along secondary planes (Fig.4F).

CouposTrToNS oF INCLUSIONSaNo PHnss BEHAvToR

Tables l, 2 and 3 contain a summary of themicrothermometric results obtained on the fluid

inclusions. Results are given as the average valuefor a cogenetic group, along with the standarddeviation for that group. Data points on diagramsalso represent average values.

Type-I inclusions

Type-l inclusions homogenize to the liquid phase

TABLE 2. SUMMAFY OF fiERMOMETFY DATA FOR INCLUSION TYPES 3 AND 4

SAtvlP MINTYPEORIG # TmC@ave e.d.

TmCTHavo

ThPave s.d.

ThT

4 5 a C l 4 S1 6 6 c O g S1 6 6 c O 3 S78 0 Sl4 S?7 8 0 3 S1 6 6 d O 3 S

2 -59.03 €8.35 {9.22 S8.31 €9.25 €8.8

o.7o.; :o.2 6

- - 7- s

0.1 5

-1.'t

4

3d).{203tx!410

35G450

ThT: Total homogenlzatlon of lhe Incluslon

TABLE 3. SUMMAFT OF THEBMOMEIITT DATA FOB INCLUSION TYPE 5

SAMP MIN TYPE OFlc rf Teavig g.d.

TmICE SALavo a.d.

TdV Td MUTEavg o.d. ave 9.d.

7A7A7A7878787a787A787A7e78166b78787A'165a

7A1680

P4n,

PS/PFS?PS?ssssssssIP4P5P6P5P5P1

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MAGMATIC FLUIDS. SEAGULL BATHOLITH 947

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Frc. 6. Temperatures of vapor disappearance and icemelting for type-l, type-5 and type-6 primary, pseudo-secondary and secondary inclusions. Type 1(6) are thosetype-l inclusions associated with a type-6 inclusion(Figs. 48, C, 5E). For type-l inclusions, TdV is thetemperatures of homogenization, Each point representsthe average for a cogenetic group. Error bars representstandard deviations for each group. Points in boxesadjacent to top and left-hand axes represent groups ofinclusions where either TmICE or TdV is known.

at temperatures of between 69 and 392'C. Eutectictemperatures (Te) were generally found to bedifficult to measure, but where observed, rangefrom -50 to -20oC. Final melting of ice (TnICE)occurred between -25 and -2.7"C (Fig. 6). Becauseof the low eutectic values and TmICE values below-20.8oC, salinities have been calculated in terms ofthe system H2O-NaCl-CaCl2 and represent themedian salinity for a given TmICE in that system(oakes et al.1990).

Primary and pseudosecondary inclusions haveoverall lower TnICE values (higher salinities) thansecondary inclusions; salinities of primary andpseudosecondary type-l inclusions range from 6.6to 24.3 equiv. wt.9o NaCl + CaCl2. There are nosecondary inclusions with TnICE less than -iOoC(4.9 to 13.4 equiv. wt.9o NaCl + CaCl) (Table l,Fie. 6).

Type-3 and type-4 inclusions

In some type-3 and type4 inclusions, no phasechanges were observed during the cooling runs;however, in many, a CO2 melting event wasobserved (TmCO), and in some, clathrate melting(TdCTH) and partial L-V homogenization (ThP)(of the nonaqueous portion of the inclusion)occurred. It is possible that those inclusions thatexhibit no phase changes contain very low con-centrations of gases other than H2O; however, theiropacity may have obscured any phase changes.

Values of TmCO2 of type-3 inclusions fall withina narrow range from -59,2 to -58.2'C (Table 2).The only set of type-4 inclusions that yielded dataEave a similar value of -59.0"C. Final melting ofclathrate ranged from -7 to 8oC, with most valuesbetween 5 and 8oC (Table 2). Partial homogeniza-tion only wa$ observed in two inclusions, at -l.l

and 4oC, to the vapor phase. In some inclusions,the vapor portion of the COr-rich phase was seento expand on warming from approximately -56oC,

but final homogenization could not be seen.The lowered CO, melting point in these in-

clusions indicates the presence of miscible gasessuch as CHa or N2, but the lack of reliable data onclathrates or homogenization precludes quantifica-tion of their concentrations.

Type-S inclusions

On cooling type-5 inclusions, the cubic solidgenerally does not change, and freezing is accom-panied by the shrinkage and, in cases, disap-pearance, of the vapor bubble. On warming, theinclusions appear to start to melt at temperaturesranging from -95 to -60oC, followed by ice meltingat temperatures up to approximately -25oC. Inmost inclusions, after the ice has melted, the cubic

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948 THE CANADIAN MINERALOGIST

rdv fc)Ftc. 7. Temperatures of vapor disappearance (TdV) and halite dissolution

(TdHALITE) for type-s inclusions. As in Figure 6, points represenr averagesand standard deviations.

400Oo-

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solid reacts with the liquid, diminishes in size, anda new solid forms around the original solid. Insome cases, a new solid is not visible. This reactionindicates that the cubic solid is halite (reacting toform hydrohalite). Where such behavior takesplace, ice melting occurs under disequilibriumconditions, as halite is present. In such cases, anequilibrium TmICE was obtained by refreezing theinclusion after the hydrohalite had formed (abovethe original TmICE value) and before halite hadreformed. Subsequent melting of ice was thereforemeasured in the presence of hydrohalite only.Equilibrium TmICE values range from -54 to-28'C. In some inclusions, no freezing or iceformation was observed, and on warming from lessthan -100"C, the first event was the reaction ofliquid and halite.

On further warming, the hydrohalite meltsincongruently to reform halite at temperaturesbetween -0.6 and 23'C (Table 3). Reactiontemperatures above OoC must represent disequi-librium conditions in the inclusions, as hydrohaliteis not stable above OoC (Roedder 1984).

As noted above, those inclusions that containsolids in addition to halite are rare (only observedin a few planes in sample 80-78). The other solidscommonly are birefringent, generally anhedral, andhave not been identified. Some such inclusions

exhibit complex low-temperature behavior; newsolids form at the expense of halite and thebirefringent solids. Ice was not seen to form inthese inclusions.

Average disappearance temperatures of vapor(TdV) and halite (TdHALITE) in type-5 inclusionsrange from 95 to 480'C (vapor) and from 165 to452'C (halite) (Fig. 7, Table 3). In most inclusions,TdHALITE was greater than TdV (Fig. 7). Someinclusions decrepitated prior to total homogeni-zation.

The very low TmICE and Te values for type-5inclusions preclude interpretation of the above datain terms of the HrO-NaCl or H2O-NaCl-KClsystems, which have eutectics at -20.8 and -22.9"C,respectively. The dissolved salts are most likely tobe a mixture of NaCl, KCl, CaCl, and FeCl2(Roedder 1984, Whitney et al. 1985). Given a lackof phase-equilibrium data in FeClr-bearing systems,we have modeled the composition of type-5inclusions in terms of the system H2O-NaCl-CaClr.The bulk composition of liquid-vapor-halite in-clusions in this system may be estimated usingTnICE and TdHALITE (Williams-Jones & Sam-son 1990) (Fig. 8). Type-5 inclusions have modeledsalinities of 34 to 52.5 wt,olo NaCl + CaCL andmodeled NaCl,zCaCl, ratios (by weight) of 6-.5 to0.74 (Fie. 8).

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Ftc. 8. Modeled compositions of type-5 inclusions in the system NaCl-CaCl2-H2O,Fields represent composition of aqueous phase in equilibrium with solid indicated.H, halite; I, ice; Hh, hydrohalite; Ant, antarcticite. Phase boundaries fromYanatieva (l9ul6).

949

Type-6 inclusions

Three minerals are present in type-6 inclusions:l) tourmaline (rare); 2) muscovite (very common)(Figs. 48, C, 5B); and 3) a highly birefringentrhombic solid (Fig. 4A).

The muscovite is anhedral to lath-shaped andhighly birefringent. The identification of thesecrystals as muscovite was confirmed with Ramanspectroscopy. Muscovite commonly replacesfeldspar in the orbicules, and in a few examplesappears to be interstitial to the other minerals. TheRaman spectra of inclusion solids and of muscovitefrom the orbicules are shown in Figure 9A.. Thebands at 100, 260, and 700 cm-l correspond to thoseof the muscovite, and agree with published values(Loh 1973). Superimposed on the spectra are theRaman bands for quartz. Quartz and muscoviteboth have bands at approximately 260 cm-r;however, in the inclusion spectra, the intensity ofthis band is too high, relative to the other quartzbands, to be attributed only to quartz, whichindicates that it is a composite muscovite-quartzband.

The identity of the rhombic mineral has not yetbeen established. Its optical properties and formsuggest that it could be a carbonate; however, the

Raman spectra obtained from this mineral clearlyshow that it is not (Fig. 9B).

TmICE values were obtained from three type-6inclusions only (Table l); they correspond to theTmICE values obtained from the associated type-linclusions. On heating, the vapor bubbles in mosttype-6 inclusions showed no change to temperaturesof 50 to l00oC above the homogenization tempera-tures ofassociated type-l inclusions. Some isolatedtype-6 inclusions in sample 45c, that have noassociated type-l inclusions, and that appear tohave consistent ratios of phases, gave TdV valuesof between 300 and 4fi)'C (Table l, Fig.6).

DISCUSSIoN

Nature of the primary fluids

Texturally, the orbicules partly resemble the hostgranite, which suggests that they in part crystallizedfrom a silicate melt. However, the presence ofprimary, aqueous, type-l and type-5 inclusions inquartz, tourmaline and fluorite provides unequivo-cal evidence that these minerals are partlyhydrothermal in origin. This is supported bytextural features in the orbicules: tourmalinecommonly occurs as a replacement of feldspar andas microveinlets, and fluorite is invariably intersti-

950 THE CANADIAN MINERALOGIST

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260 500 750 1@

A IYAVENIiMBER (cm-l

FIc. 9. A. Raman spectra for muscovite, quartz and twosolid inclusions. Peaks at lffi,260, and 700 cm-r arefor muscovite; all others pertain to quartz. B. Ramanspectrum of rhombic solid inclusion in quartz. Alsoshown are the principal bands of carbonate minerals,as illustrated by the strongest bands for calcite (C) anddolomite (D).

tial to the other minerals, including quanz, whichcommonly has bipyramidal terminations. Theaqueous fluids responsible for the precipitation ofthese minerals had salinities ranging from 8 to 42equiv. wt.9o NaCl + CaCl2 (Fig. l0), and areconsidered to represent orthomagmatic hydrother-mal fluids that evolved from the Seagull graniticmagma. The lack of tourmaline alteration in thesurrounding granite and the general absence ofquartz-tourmaline veins extending from the or-bicules indicate that the orbicules acted as closedsystems during their crystallization and that, ingeneral, the fluids did not escape. The conclusionthat the orbicules were closed systems during theircrystallization also requires that they in part

Flc, 10. Ranges of salinity and homogenization tempera-tures for type-l and type-5 inclusions.

crystallized from a silicate melt, as the requiredamount of material could not be dissolved in anequivalent volume of hydrothermal fluid.

The presence of muscovite and tourmalirre intype-6 inclusions indicates that the hydrothermalfluid was in equilibrium with the assemblagequartz-muscovite-tourmaline, and may thereforebe the cause of silicification, tourmalinization orphyllic alteration if it migrated away from itssource. The occurrence of quanz-tourmaline veinsand the association of phyllic alteration with someof the mineralized veins in the batholith (Mato etal. 1983) indicate that fluids similar to those thatwere present in the orbicules flowed through thebatholith, and may have been responsible for thetin mineralization.

Large variations in the salinity of hydrothermalfluids associated with granitic intrusive bodies havegenerally been attributed to the involvement andmixing of orthomagmatic and meteoric waters orto boiling. However, recent calculations by Bodnar& Cline (1990) show that the salinity of orthomag-matic fluids generated from a single magma mayrange from almost pure water to greater than 50wt.9o NaCl, depending on the pressure of crystal-lization. As the fluid inclusions trapped in theorbicules represent essentially unevolved orthomag-matic fluids, the data collected from the orbiculesare consistent with, and support, the calculationsof Bodnar & Cline (1990). We cannot rule outboiling as a cause of some of the variations insalinity. Boiling is suggested by, and would explain,the presence of the CO2-rich, type-3 and 4inclusions; however, they are all secondary, so thattheir relationship to the other fluids is unknown.

For the type-S inclusions (mostly from sample80-78), a positive correlation exists between the

SAUNITY (oq. wt. % NaCl + CaCl2)

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947

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MAGMATIC FLUIDS. SEACULL BATHOLITH 95r

modeled salinities and NaCl/CaCl2 ratios (Fie. 8).There are several possible mechanisms that couldcause salinity to covary with NaCl/CaCl2 ratio:differential partitioning of NaCl and CaCl2 be-tween melt and aqueous fluid during exsolution,differential partitioning of these components be-tween liquid and vapor during boiling, mixing oftwo fluids with different salinities and NaCl/CaCl,ratios, and halite crystallizatio\.

It is unlikely that the observed trend wasproduced through mixing, as most of the inclusionsthat define the trend are from one orbicule. AsCaCl2 partitions more strongly than NaCl into theliquid phase during boiling (Zhane & Frantz 1989),this trend could not have been produced throughopen-system boiling. There are insufficient data onpartition coefficients for high-salinity liquids to testa model of closed-system boiling. Because thepartition coefficient for CaCl2 between a vaporphase and a granitic melt increases more rapidlywith salinity than the coefficient for NaCl(Holland 1972), differential partitioning duringcrystallization cannot explain the observed trend.The relationship shown on Figure 8 is reminiscentof the halite trend (Cloke & Kesler 1979); however,the extension of the trend on Figure 8 projects toa point on the HrO-NaCl binary, and thereforecannot have been caused by halite crystallization.Further interpretation of the trend on Figure 8 willrequire more complete compositional data on theseinclusions and more data on phase equilibria in theappropriate chemical systems.

P-T conditions of entrapment

As indicated freviously, a relatively shallow levelof emplacement of the Seagull batholith is inferredfrom field observations, such as the porphyriticnature of the marginal phases and the presence ofmiarolitic cavities. The absence of a diagnosticmineral assemblage in the contact aureole precludesa more precise estimate of the P-T conditions. Itis likely that the batholith was emplaced at I to 2kbar, or less, which is consistent with the pressureestimate of approximately I kbar for emplacementmade from stratigraphic reconstructions by Layne& Spooner (1991).

Isochores for primary type-l and type-5 fluidinclusions are shown in Figure ll. These werecalculated using the equation of state of Brown &Lamb (1989) and the computer program FLINCOR(Brown 1989). For isochore calculations, theinclusions have been modeled on the systemH2O-NaCl, as CaCl2 contents are not known forthe type-l inclusions. By ignoring CaCl2, thecalculated isochore for a given Th will be slightlyshallower than if CaCl2 were included (Zhang &Frantz 1987). Primary inclusions have a wide range

0 2 0 0 { t I , @

IEMPMAIURE fC)

Frc. ll. Isochores for primary inclusions oftypes l, 5 and6, Horizontal lines at I and 2 kbar represent likelyminimum and maximum pressures of entrapment'Points A, B and C are discussed in the text.

of homogenization temperatures, from aP:proximately 100 to 500'C (Figs. 6, l0). Withtrapping pressures of I to 2 kbar, the highest-temperature inclusions will have been trapped at orclose to liquidus temperatures (500 to 600'C) (Fig.ll), given that late-stage granitic melts may existat temperatures as low as 450 to 500oC (Manning1981, Manning & Pichavant 1984, London 1989).Those inclusions that homogenized at lowertemperatures must have either cooled significantlyprior to entrapment, in some cases down to aroundl50oc, or their densities have been increasedsubsequent to entrapment. As there appears to havebeen little fluid leakage out of the orbicules, it isreasonable to hypothesize that crystallization oc-curred over a protracted period and, therefore, thatthe fluids may have cooled significantly beforeprecipitating some of the quartz and tourmaline.However, the type-l inclusions that are associated

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952 THE CANADIAN MINERALOGIST

with type-6 inclusions all have low Th values (Figs.6, I l). As these inclusions are commonly associatedwith type-2 inclusions, and with a type-6 inclusionthat commonly contains a large vapor bubble, it isquite likely that these arrays formed at moderatelyhigh temperatures, but underwent necking duringsubsequent cooling. The necking-off of a largevapor bubble attached to the solid (Fig. 5) wouldresult in an increase in the density of thesubsequently formed inclusions, resulting in lowtemperatures of homogenization. This also explainsthe presence of type-2 inclusions in these clusters.In support of this model, the isolated type-6inclusions (no type-l inclusions) that werehomogenized indicate entrapment conditions of 500to 600'c (Fig. ll).

The density of fluid inclusions may also beincreased through re-equilibration during cooling.This occurs where the external pressure greatlyexceeds the pressure within the fluid inclusion (asconstrained by the original isochore for theinclusion), as would occur during isobaric cooling(Sterner & Bodnar 1989). This pressure differentialgenerally results in the formation of an implosionhalo of new fluid inclusions around the originalinclusion (Sterner & Bodnar 1989). Although someof the 6/l arrays resemble the textures describedby Sterner & Bodnar, and it is possible that theSeagull batholith cooled nearly isobarically, thegenerally planar nature of the 6/1 arrays suggeststhat most did not form in this manner.

The low temperatures of homogenization for the6/l anays are further evidence that they are notdecrepitation-induced clusters. Decrepitationresults in a decrease in fluid density and, therefore,an increase in Th. In order to obtain the fluiddensities for the type-l inclusions in these arrays(e.9., point A, Fig. I l), the original inclusions musthave been trapped at temperatures of less than 200to 230'C (poinl B, Fie. ll), asSuming that aminimum overpressure of I kbar is required fordecrepitation. In order also to explain the higher-temperature primary inclusions (e.g., point C, Fig.l1), the uplift path for the batholith would requirealmost isobaric cooling (C * B), followed byalmost isothermal decompression (B - A). Such apath of uplift is highly unlikely.

CoNcr-usroNs

The quartz-tourmaline orbicules that occur inthe roof zone of the Seagull batholith represent atransition from magmatic to hydrothermal proces-ses. The orbicules crystallized from residual bub-bles of a mixture of borosilicate melt andorthomagmatic hydrothermal fluid that formed ina largely crystallized granitic magma. This explainsthe rounded form of the orbicules. High degrees

of crystallization were probably required toproduce a Si-rich residue with the high viscositiesnecessary to prevent coalescence and migration ofthe melt-fluid mixture.

A significant proportion of the quartz andprobably all of the tourmaline and fluorite in theorbicules was precipitated from unevolved or'thomagmatic hydrothermal fluids with salinitiesthat ranged from 8 to 42 equiv. wt.9o NaCl +CaCl2. As the orbicules crystallized as closedsystems, these values suggest that large ranges insalinity of the hydrothermal fluids associated withgranitic intrusive bodies may represent originalvariations in the exsolved fluid rather thanvariations that resulted from boiling or mixing witha meteoric fluid.

Isochoric projections indicate that hydrothermalprecipitation probably occurred between 600 and300oC, and possibly as low as l50oc.

The hydrothermal fluid that precipitated thequartz in the orbicules was in equilibrium withmuscovite, rather than K-feldspar. Therefore, themigration of these fluids through the granite wouldinitially cause phyllic alteration.

Elevated concentrations of tin in the orbicules,relative to the surrounding granite, indicate that tinwas concentrated in these residual melts andpossibly in the orthomagmatic hydrothermal fluids.Quartz-tourmaline veins in the Seagull batholithand its country rocks are evidence that fluidssimilar to those that formed the orbicules escapedfrom the crystallizing melt, and were likelyresponsible for the tin mineralization around theSeagull batholith. However, information on thecharacter of fluids responsible for the vein systemsand for the tin mineralization is required to chartthe evolution of the magmatic fluids and to testthis hypothesis.

AcKNowLEDGEMENTS

This research was supported by an NSERCoperating grant to I.M.S. This paper has beenimproved as a result of reviews by B.E. Taylor, J.Leroy and an anonymous reviewer.

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BnowN, P.E. (1989): FLINCOR: a microcomputer

MAGMATIC FLUIDS, SEAGULL BATHOLITH 953

76(854),progr.rm for the reduction and investigation offluid-inclusion data. Am. Minerol. 74, 1390-1393.

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954 THE cANADIAN MINERALoGIST

H2O using synthetic fluid inclusions. Chem. Geol. &, using synthetic fluid inclusions. Chem. Geol. 14,335-350. 289-308.

& - (1989): Experimental determination ofthe compositional limits of immiscibility in the system Received Februory 4, 1991, revised manuscript occeptedCaCI2-H2O-CO2 at high temperatures and pressures June 26, 1991.