RENDlCONTI DElLA sOCIETA ITALlANA DI MINERALOGIA E PETROLOGIA. 1988. Vol. 43. pp. 1~·24

Brines and Metasomatism

VOLKMAR TROMMSOORFFInstitut fur Mineralogie und Petrographie, ETH-Zentrum, 8092 Zurich, Switzerland

GEORGE SKIPPENDepartment of Geology, Carle:ton University, Colonel By Drive, Onawa, KIS, 5B6, Canada

AaSTRACT. - The frequent observation of saline fluidsin deep seated groundwaters of metamorphic terrainsas well as in fluid inclusions of metamorphic mineralsindicates that brines are common parricipants inmetamorphism and meusomatism. The most abundantcomponents in these brines are H20, COl and NaCl.Available experimental data permit a quantitative P·Tproj~tion to 900°C and 2Kbar for the syStem,C01·H10.NaCI. Extrapolation of this diagram tohigher pressures shows that at even moderate saltcontents, two fluids, i.e. an aqueous brine and aCOrri<:h vapour, <:oexist over a wide range ofmetamorphic conditions. Prograde or retrogrademetamorphk rea<:!ions un <:ause an originallyhomogenous supercriti<:al brine to evolve into a two·phase fluid that may eventually rea<:h saturation in salt.As an example, metasomatic intera<:tion of a <:arbonaterock mass with an aqueous brine is diSl:Usscd. 1be processof evolution towards a two.phase fluid and finally to asalt saturated fluid may be facilitated by boiling.

Salt saturation can be demonstrated if primary solidinclusions of salt are found. Criteria that prove theprimary nature of solid salt inclusions are discussed bymeans of an example from the Central Alps.

Metasomatic brine·rock intera<:tion may result inoonsiderable variation of reaction rates, which mayexplain some of the <:haracteristic textures observed inmetasomatic rocks. Metasomati<: brine·rock interactionmay also result in the predpitation of dissolvedcomponents, thus leading to the formation of mineraldeposits.

Introduction

Saline solutions are commonly present inmetamorphic and metasomatic rocks. A directindication of the abundance of brines is givenby recent studies of deep seated groundwatersin metamorphic rocks of the Canadian Shield(FRAPE et al., 1984) and of the Fennoscandian

Shield (NOROSTROM, 1983) and also by theanalysis of deep-seated fluids from the Kolaborehole (KozLOWSKY, 1985). Evidence of thesaline character of many fluids duringmetamorphism has been found in fluidinclusions from metamorphic rocks (POTY,STALDER and WEISBROO, 1974; TOURET,1977; SCHUILlNG and KREULEN, 1979;KREULEN, 1980; HENDEL and HOLusTER,1981; SISSON et aI., 1981; MERCOLU, 1982;WALTHER, 1983; YARDLEY et al., 1983).

The origin of these saline solutions may inpart be due to the release from magmaticrocks. A considerable part of ROEDDER'S(1984) recent book is on fluid inclusions ofmagmatic origin. The saline nature of manyformation waters of sediments is also a wellknown fact and in many cases themetamorphism of evaporitic rocks must havecontributed to the presence of saline solutionsduring metamorphism (RICH, 1979; MERcoll..let al., 1986; TROMMSDORFF et al., 1985).

Processes leading to the concentration ofsaline solutions in metamorphic rocks are onlypartly understood. Metamorphic reactionsthat consume H 20, fluid unmixing, and thesegregation of two fluids (boiling), areprocesses that concentrate salt components inthe liquid phase (TROMMSDORFF and SKlPPEN,19861.

Under metamorphic conditions at evenmoderate salt contents, brines are capable ofdissolving and transporting chemical species.Recent investigations (BALLHAUS and

16 V. TROMMSDORFF, G. SKIPPEN

STUMPFL, 1986; CHRYSSOULlS andWlLKlNSON, 1983; DIAMOND, 1987) show thecapability of brine to transport even so-calledimmobile elements. This has also been shownthrough the analysis of fluid inclusions by thedecrepitation Iep method (CRYSSOUUS,1983). A direct experimental indication of thecorrosive nature of CO2-H20-NaCl solutionswas given by GEJ-lRIG (1980). Not anJy thesaphirc= window of the apparatus but also thenoble metals of the reaction vessels wefeattacked by these solutions at 3000 bars and550 C. It is therefore not astonishing thatsaline solutions are considered responsible for

the formation of many types of mineraldeposits (EUGSTER. 1986).

Metasomatic veins resulting from theinteraction of brines with rocks havecommonly been observed in metamorphicterranes. Classical examples of such veins havebeen described from the Alps, for exampleAdamello (DRESCHER-KAOEN, 1969), Bergell(BUCHER-NuRMINEN, 1981), Campolungo,Central Alps (MERCOUJ, 1982; WALTHER,1983; MERCOUJ et al., 1987). The study ofsuch metasomatic veins and accompanyingsolutions deserves more attention than so farattained.

2

00 200 400 600 800TEMPERATURE C

Fig. l. - PrasW'e-lcmperatU«' protection for the ternary systcm COr H10·NaCI. Unary curves with a thinsignature binary C\1f'W:S with an intcrmediatc thickness, a portion of the tcrnary supcrcrilical~ wilh a heavysignalure. 1bc cxperimentally meas~ partS of the curves an: solid and eslimllcd C\1f'W:S an: dashed. No scaleis indiC'lled above 9QOOC and 6 kilobars IS the posilion of curves 11 these conditions is UfICeI"tain. All three curvescxtrapolllcd above 9OQ°C meet It !he cxperi~tally undetermined crilical point foe NaCI. 1bosc c;urves IlbekdQ an: coincident It a PngIe point It I hi&h presSU«' beyond the diagram. This single poinl and point P an: criticalendpoinu in the binary COfNaCl that limit the letnary critical curve. O1emographic rdationships in CO{"Hp­NaCl an: shown schcmuicaJ.ly wilh Ihrtt chcmographies cnlarged !hal were used to construct Fig. 3. L _liquid,V _ Vlpour, SCF _ supttcritical fluid, H _ haJite, K _critical poim, T _ uipk point.

BRINES AND METASOMATlSM 17

COz·HzO.NaCI brines

SKIPPEN and TRoMMsDORFF (1986) usedavailable experimental data (ADAMS, 1931;KEEVIL, 1942; SOURIRAJAN and KENNEDY,1962; GRYOTHElM et al., 1962; TOEDHEIDEand FRANK, 1963; TAKENOUCHI and KENNEOY,1964; AKELLA et al., 1969; HILBERT, 1979;GEHRIG, 1980; CHOU, 1982; GUNTER et al.,1983; BODNAR, 1985) co construct aschematic, geologically relevant P-T projectionof the system, CO -HzO-NaCl (Fig. 1).Many of the phase eiements of the system(GEHRIG, 1980) which plot outside the poTrange of most metamorphic rocks have beenomitted. Schematic chemographic projectionscorresponding to the various P-T regimes ofFig. 1 have been add~. Thin lines representunary equilibria between phases of constantcomposition. The composition of fluid phasesvaries continuously along the phase­boundaries of binary subsystems (lines ofmedium thickness). A part of the ternary

critical curve (PQ) has been added to thediagtamm using a heavy signature. The phaserelations in che ternary composition space canbe read from the chemographies. Two-phasefields have been shown by schematic tie-linesconnecting coexisting liquid and vapour;chree-phase triangles and single-phase fieldsare left blank. Three geologically impertaO[chemographies (a, b, c) are enlarged.

To illustrate the phase relations, thechemography b is shown separately fot atempetature of 500 C and a pressure of 2KB.A three-phase triangle for coexisting HzO­rich liquid (L), COz-rich vapour (V), andhalite (H), is surrounded by three two·phasefields, H + L, H + V, and L + V. Thecomposition of coexisting liquid and vapourphases is given by the schematic tie-lines onthe L + V field. At point C, the consolutepoint, the tie-line is teduced to a pointand the liquid and vapour becomeindistinguishable. The field along the

NoCI

SCFC

L+V

H+l+V

2 KB500 C

C02

Fig. 2. - Composition section at .500oC, 2 kilobars for the ternary syst~m, COr H20-NaCI. H .. halit~, L .. liquid,V .. vapour, SCF .. supercritical fluid. C represents the consolute point at which the: tie-Iin~ between liquid andvapour is reduced to a point.

18 V. TROMMSDORFF, G. SKIPPEN

H20

CO2-H20 side of the projection ischaracterized by the occurrence of a singlesupercrirical fluid phase. Fig. 2 shows that byfar the largest area of the CO2-H O-NaClcomposition space is characterized by thecoexistence of two fluid phases (fields L + Vand H + L + V). This has importantconsequences for equilibria and reactions ofmetamorphic mineral assemblages involvingfluid components.

The behaviour of the COl·HlO-NaClsystem between 200°C and 800°C and from1 to 6KB can be read from the ternary prism,Fig 3. The five dimensions needed to fullyrepresent the relationships between P, T,XCOl' Xmo and XNlS:;1 have been reduced byprojecting along a 1'-T gradient. This poTgradient includes the chemographies of theenlarged triangles (a, b, c) from Fig. 1. In thenine sections shown on Fig. 3, compositionsthat comprise the coexistence of two fluidphases, L + V, are hatched. Shadingcorresponds to the three-phase field,H + L + V. In contrast to the subsystemCOl-HlO that has a solvus at only relativelylow temperatures, the ternary P.T,XNcl"XC02·XH20 space is characterize9 by thecoexistence of two fluid phases, L + V, overa wide range of conditions.

Metasomatk reactions inC01·H20-NaCI solutions

In order to construct isobaric, isothermalequilibriurm lines for dehydration,decarbonation and mixed volatile reactions onthe XC9fXH?0-XNaQ s?rfac:, it is necessaryto conSider phase relations 10 tLmo-tLc02-tLNoClspace (SKIPPEN and TROMMSDORFF, 1986).Fig. 4 schematicalIy illustrates the equilibriumboundary of a typical hydration.decarbonatioreaction:5CaMg (CO})? + 8Si01 + H20 =

dolomite quartz fluidCa2Mg~Si8022 (OH)2 + 3CaCO} + 7C02 (1)

tremolite calcite fluidat three different temperatures. Fig. 4a, 4band 4c show isothermal isobaric triangularsections of the p.T,Xco2-Xmo·XNaCl spacecorresponding to the topology of Fig. 2.Several of the stability fields on the triangles

C

F~500

KB

Fig. 3. - T~mll.ry composition section in the systemCOrH10·NaCI plotted along a temperature-pressuregradient. The binary phase element, H + L, in thesySlem, H~O-NaCI, is shown from the base of theprism to the melting temperature of pure NaC!. Twobinary elements in the system COrNaCI are shown onthe ldt back of the prism at high pressure; these areliquid in «juilibrium with vapour and liquid inequilibrium with halite. The latter curve terminates atthe melting point of pure NaGl. These binary elementsconverge to a point with de<:reasing pressure andtemperature where they encounter a ternary phaseelement representing liquids in equilibrium with vapour.The corresponding ternary curve for vapour inequilibrium with liquid is shown on the opposite sideof the prism. The ternary consolute point curve beginsat low temperature in the CO2,H20. binary and endson the CO2·NaCI binary at temperatures and pressuresabove those illustrated. In the chemographic sections theliquid plus vapour fields are indicated by a hatching andthe liquid plus vapour plus halite triangles by shalding.

1"':::::-:--'::"i~t:;"l200NaCI C02

Noel

BRINES AND METASOMATlSM

Nael Nael

19

I I-? <:,;""

, ,., , ,.,,.' ,.0H,O Xeo2'- 00, H,O Xe02'- 00, H,O Xe02'- 00,

Fig. 4. - IsothermllJ·isobaric composition section as shown in Fig. 2 with a part of the binary T-Xco~ sectionadded below. Sections a, b, and c refer to increasing temperatures, each with a schematic isothenn for reaction(I) added (heavy line). H • halite, L '" liquid, V _ vapour, SCF _ supercritical fluid, Co _ consolute point, C _ calcite,D _ dolomite, Q. quartz, T. tremolite.

have been enlarged for clarity. Below thetriangles, a part of the adjacent T-XCOl-XIl20space is shown. Fig. 4a corresponds torelatively lower, Fig. 4b to medium and Fig.4c to relatively higher temperature. In theT.Xco~ sections the typical shape of thehydratlon-<ieearbonisation equilibrium (l)namely a curve of the inflection type(GREENWOOD, 1962) is shown by a heavy line.In the adjacent isothermal ternarycomposition space of Fig. 4a the equilibriumboundary continues through the fields SCF,L + V, SCF and L + H towards the NaCl­apex. Because of the equality of the chemicalpotentials in coexisting phases the equilibriumline for reaction (1) must run parallel to thetielines within the two-phase fields. In Fig.4b the equilibrium (1) coexists wichL + V + H, the three-phase region, and in Fig.4c with V + H.

For the temperature range considered onFig. 4 the following phase assemblages arerepresented (D", dolomite, Q "" quartz,T '" tremolite, C = calcite): DQTCSCF,DQTCLV, DQTCLH, DQTCHV, DQTCH,DQTCLVH. In the three-dimensionalisobaric T.XC02-XH20-XN.CI space, theseequilibria correspond to surfaces which shareedges with each other. Fig. 5 schematicallyshows equilibrium (1) in T-XC02-X1I20-XN.clspace. The three sections labelecl, a, b, c,correspond to those of Fig. 4a, 4b, 4c

respectively. In Fig. 5 the equilibrium surfaceof reaction (l) rises from the NaCl-H~O sideof the projection towards the NacI·C02side. At the temperature corresponding tosection b, the XCOl values in the ternary

+T

OOTC+SCF

H20

Fig. 5. - Ternary composition section in the system,COr H20-NaCl plotted along a temperature gradient.a), bl and cl refer to the sections a), b), and c) from Fig.2. The equilibrium plane for reaction (1) is shown withinthe prism with the individual phase elementS labeled.The typical inflection type curve of reaction (1) is visibleon the binary, H20.CO~. H. halite, L. liquid,V • vapour, SCF. supercritical fluid, C. calcite,D. dolomite, Q _ quartz, T. tremolite.

20 V. TROMMSDORFF, G. SKIPPEN

composition space representing equilibrium (l)vary from 0.1 to O.9.lt is evident therefore,that the addition of NaCl to H O~CO

fluids can significantly change the stability otmineral assemblages in fluid composition space(SKIPPEN and TROMMSDORFF, 1986).

Fluid evolution in COz"H10.NaCI space

The evolution of fluids as a consequenceof metamorphic reactions was discussed byTROMMSDORFF and SKIPPEN (1986). Fluidevolution is a function of initial composition,reaction stoichiometry and reaction progress.The theoretical endpoint of fluid evolutionis defined by reaction stoichiometry. In theternary system, CO

2-H

10-NaCl, an

originaUy homogeneous supercritical fluid canevolve as a result of mineral reaction to a two­phase fluid and finally to salt-saturation of thefluids.

As an example, fluid evolution inCO2-H20-NaCl is shown for reaction (1) inFig. 6a and Fig. 6b. All lines of fluid evolutionconverge at a theoretical endpoint, point Pin Fig. 6a with the coordinatesXCQ2 =0 1.167, XH20 =0 -0.167 and XNaCr =0 0as defined by the stoichiometry of reaction

NaGI

(1). Fig. 6b corresponds to Fig. 4c andcontains a schematic topology for the systemCO -H20-NaCl together with a dashedequnibrium line for reaction (1). The area inwhich reaction (1) proceeds to the right isshaded; the area in which reaction (1) procedsto the left is blank. At the boundary of thetwo areas, products and reactants are inmutual equilibrium and reaction progress iszero. Fluid evolution as a consequence ofpositive progress in reaction (1) is indicatedby the thick arrows. An initial fluidcomposition I, corresponding to a supercrilicalaqueous chloride solution, evolves towardspoint P. At point A, which is at theintersection of the fluid evolution curve IPand the boundary of the two-phase region,L + V, an infinetesimal amount of a vapourphase of composition A' in equilibrium withthe liquid A is formed. Further reactionprogress creates liquids along curve AB andcoexJsting vapours along curve A'B'. At pointBand B' the system reaches salt-saturation.

Further progress in reaction (1) leads toconsumption of the liquid and simultaneousprecipitation of salt. The endpoint of theevolution of the system lies at poit E, theintersection of the fluid evolution curve IP

NaGI

a)VH~ __I

VC02 .. 7

VNaCI .. 0

p

Fig. 6. - a} Fluid evolution for reaction (I) in the system, H20-COr NaCI. All fluid evolution curves for positiveprogress in reaction (1) tend towards the endpoint, P, as, indicated by arrows.b) Schematic phase relations in CO2·HI P.NaCI with an equilibrium line for reaction (1), dashed. In the shadedarea progress of reaction (1) is positive in the white area it is negative. A fluid of starting composition I evolvesthrough positive progress in reaction (I} towards the two phase region and reaches it at point A. Liquids evolvingwithin the two--phase region are defined by curve AB and coexisting vapours by A'B'. At poin! A and B' halitesaturalion is reached. Further fluid evolution ends at point E, the intersccdon of the fluid evolution line JP andthe equilibrium line for reaction (1). At point E halite coexists with a vapour of composition D. H .. halite, L .. liquid,V .. vapour, SCF .. supercritical fluid, C .. consolute point.

BRINES AND METASOMATISM 21

Fig. 7. - &hematic sketches of a [Juid inclusion withdaughter crystals Oeh) oflhe system CO2-H20.KCl alroom lempcralurc and of solid inclusions (K, NI.) Cl(right). The four crystals 10 the tight ill show lhe samefcarura i.e. unmiring from an original solid soJution ofhalile + sylvite. Thc c!qhtet crySlW within theinclusion are halile and sylvitll'

easilly recognizable on basis of theirdendritic growth and will not be furtherconsidered here.

2. Dauhter crystals within fluid inclusions.3. Primary inclusions of salt which existed

before the growth of the enclosing hostmineral, mostly quartz.

The distinction between daughter crystalsand primary inclusions is not possibile on basisof their shape and, if pun: NaCl, alsoimpossibile on the basis of their chemistry.

(No,K)CINoCIKCI

a)

Solid inclusions, however, tend to occur ingroups of several crystals with' the samecharacteristics whereas daughter minerals ofNaCI almost invariably occur as one singlecrystal per inclusion (5« also ROEDDER, 1984,p. .53). This distinction, however, is notunambiguos.

For crystals of NaCl and KCl, as inthe case of the Campolungo occurrence(TRoMMsDORPF et a!', 1985) the distinctionbetween daughter minerals and solidinclusions becomes easier (Fig. 7). Fluidinclusions that are saturated with respect toKCI and NaCl at room temperature containtwo daughter crystals of different compositioni.e. a halite and a sylvite cube. In contrast tosuch daughter minerals, the solid inclusionsin the Campolungo quartz crystallized as KCI­NaCI solid solutions. During cooling theyunmixed in a manner similar to that discussedby HEIDE and BauECKNER (1967) for theexperimental work of NACKEN (1918).

and the equilibrium line for reaction (1). Atpoint E, a vapour with composition D coexistswith halite and the product and reactantminerals of reaction (1).

Fluid evolution is more efficient if boilingoccurs during the reaction and the vapour canleave the system. In this case the residualliquid evolves as in the foregoing case frompoint A to saturation at point B. Furtherreaction progress accompanied by boiling leadsto the consumption of the liquid andsimultaneous precipitation of salt. Theresidual bulk compositions within CO

2-H

20­

NaCl then evolve along the boundary betweenthe areas H + Land H + L + V until theyreach the field H (halite) near the NaCl-apex.

Primary salt indusions in metasomatic rocks

The recognition of primary solid inclusionsof salt is of special interest for theinterpretation of brine-rock interaction. Suchinclusions are taken to indicate the presenceof a salt-saturated fluid phase duringrnetasomatism. In addition, primary inclusionsof (Na, K) Cl are a useful geothermometerbecause of the temperatutt dependency of themixing of KCI and NaCl. The bulkcomposition of these inclusions is preservedduring cooling if no fluid surrounds the solidinclusion.

An axample of primary solid inclusions ofsalt has been described form the classicmineral locality of Campolungo! Central Alps,Switzerland (TROMMSOORFF et al., 198.5).Boudins and nodules of preexisting quartzveins reacted with sorrounding dolomite rockto from tremolite plus calcite according toreaction (l) (WALTHER, 1983). There isagreement among the various authors whoworked on these veins (MERCOLLI, 1982;WALniER, 1983; MERCOLU et al., 1987) thatthe fluid necessary to produce the tremoliteof the veins was an aqueous brine introducedfrom outside into the dolomite layercontaining the quartz nodules.

At least three kinds of salt have beenrecognized withn quartz crystals fromCampolungo (Ltpontine Alps, Switzerland):1. Salt particles crystallized from residues of

decrepitated fluid inclusions. They an:

22 v. TROMMSOORFF, G. SKIPPEN

Unmixing phenomena in the Campolungoexample were visible in a group of fouradjacent salt crystals each of about 10 P.ffidiameter and were analyzed by ~ans of theelectron microprobe (TROMMSDORFF et al.,1985, Fig. 2d-O. A temperature of about500°C was determined from the bulk analysesof [he~ crystals using the salvus calculatedby THOMPSON and WALD8AUM (1969) for thehalite-sylvite system. This tem~ture agreeswell whith those: determined by MERcou.I(1982) and WALTHER (1983) for theCampolungo veins using calcite-dolomitegeothermometry .

Phenomena as described for tM primarysolid inclusions of salt from Campolungo arealso possibile for daughter crystals thatb«ome sepanlted from the enclosing fluid by«necking down. processes. But in this C~the temperatures resulting from the halite­sylvire geothermometer should fall belowthose independently determined for the cocksystem.

Independem evidence for the existence ofsaturated brines at Campolungo results fromsalt to liquid homogenization tempttatures influid inclusions from the same sample thatcomains solid inclusions of salt. In this samplehomogenization temperatures weredetermined by MERCOLU as 49.5-498°C forseveral inclusions, which is well in agreementwith the .500oC determined on calcite­dolomite pairs for the formation of themetasomatic veins. The inclusions of H~O­rich saline liquid at Campolungo invariablycoexist with very CO2-rich inclusions(MERCOUJ, 1982, p. 28100). It is thereforepossible that the reacting salt saturated brinesat Campolungo were two-phase fluids thatevolved as discussed in the foregoing chapter.

Solid inclusions of salt were also observedin monticellite from Comone di Blumone, andin diopside from Val Caffaro, Adamdlo, Italy(TROMMSDORFF et al., 198.5) as well as incordierite from the Cemral Alps (IROUSCHEK,1984). Proof for the primary nature of theinclusions in these occurrences has not beenobtained so far; the inclusions do confirm thesaline nature of fluids attending metasomatismin these rocks.

Conclusion

It has been shown in the foregoing chaptersthat metasomatic reaction during brine-rockimeraction may lead to the formation of two­phase, salt-saturated fluids. Reaction progressduring such brine-rock interaction may varyconsiderably, depending upon the availabilityof heat to drive the reaction and of fluid topermit the reaction to go on. MERCOLU et al.(1987) have shown that the heat stored in thesurrounding rock mass is sufficiem to drivereaction (1) during the formation ofmetasomatic veins caused by introducedbrines at Campolungo. During reaction, fluidevolution across ascending isotherms of theSCF field and of lh< L + V field (Fig. 4)requires considerable superheating of the rockmass surrounding the veins. On the otherhand, the only heat that is necessary to drivethe reaction at salt-saturation is the heat ofreaction.

If the vapour phase produced duringongoing reaction can leave the system, therates of reaction may be relatively fastcompared to rocks that from in a «closed.system. Different textures could result fromthe two cases: whereas in the Campolungorock mass tremolite shows well-developedgcanoblastic textures, it radiates from a pointsource in the veins (see also photos inTROMMSDORFF and SKlPPEN, 1986). Weanticipate much higher rates for reaction (1)in the veins as compared with the rock. Asystematic study of textures in metasomaticrocks that formed by brine-rock imeractionmay help to solve this problem.

Acknowledgement. - We thank the Sodet~ ltaliana diMineralogia e Petrologia for the invitation to presentthis conference at their Siena meeting. We alsoacknowledge greatly the hospitality of Professors C.A.Ricd, 1. Memmi and C. Ghezw at the Dipartimentod.i Scienze ddla Terra of the Siena University duringa visit in the summer 1986.

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