RENDlCONTI DELL\ SOCIETA ITAUANA DI MINERALOGIA E PETRQLOGIA, 1988. Vol. H. pp. B9-149

Early-alpine ore parageneses in the serpentinitesfrom the Balangero asbestos mine and Lanzo Massif

(Internal Western Alps)

P1ERGIORGIO ROSSETIl, STEFANO ZUCCHETTIDiparlimcnto di Georisorse e Territorio and C.N.R., Centro di Sludio per i Problemi Minerari,

PoIitecnioo di Torina, CorlO Duea degli Abro:tzi 24, 10129 Torino, Italy

ABsnAcr. - The ulutlTlafites of lhe Balangero bodyand the adjacent Lanzo massif (I111ernal Pie<!.monteseZone) have been partly re-equilibratcd by the early.alpinehigh prcssure·low temperature metamorphism intoserpenlinites containing antigorite + diopside: + olivine+ Ti-dinohumilc + magnetile :t: Mg-chlorite. 11x:

petrological dala, mainly dc:ri\'C'd from the paragenesesof the assoc::ialed melabasiles, poinl to a tempualW'e of500-550oC and a minimum preuure of 15 kb for theearly-alpine evenl.

The ore parageneses, mainly formed of the associationof Ni and/or Fe sulPlides + native metals (Ni-Fe and Fe­Co alloys) + magnetite, are clearly linked wilh theserpentinization pnns$ and mostly stable with the early­alpine silicate a$Socialions. Parlkularly, the I..$$OCiationtaenite (- Ni!'e) . IWlmile (Nil"e) . waitawte (­Fc)Coll - troilite (FeS) . magnetite (for T _ "O°C andP. U kb) is accolllp<lnied by /Ol values 4-5 log unitsbelow the FMQ buffer, withJS i - 7·8 log units belowthe pyrite-pyrrothite surface and with log fH2S _ 1.70in the associated fluid. As far IS the Ilte.a1pinefClrogression is concerned, the presence. of aalive ironin late chtysotile veins is evidence cl distiDCl fall inj02and iS2 concomitanl with the lowerin8 of bothtemperalure and presSUfc.

Kry words: serpeminiUllion, native metals, fluid phase,high pressure metamorphism, Western Alp5.

Introduction

The characters and distribution of theopaque minerals in the ophiolitic uhramafitesof the Central-Western Alps have beenstudied in depth (DE QUERVAlN, 1945;ZUCCHETIl, 1967, 1968, 1970: PERETn andZUCCHETTI, 1968). These Authors havedemonstrated the presence of native metal(Ni·Fe alloy) + Fe-Ni sulphides parageneses

and also established their genetic link with theserpentinization process. The many papers onthe silicatic associations of the ophiolitic rockspublished over the last fifteen years have ledto outstanding results, especially with respectto assessment of the physical and chemicalconditions that governed the formation of themetamorphic parageneses during alpineevolution (SANDRONE et al., 1986, with ref.therein). The importance of the part playedby the fluid phase in petrogenesis is nowunanimously recognized. This has served tofoster an interest in the study of opaqueminerals in equilibrium with silicates, as minorphase capable of supplying information on thecomposition of the fluid and its variationsduring the metamorphism.

This paper r~xamines the <:ltt parageneseswithin the serpentinized u1tramafics ofBalangero and Lanzo. Extensive use ofmicroprobe as working tool has led to thediscovery of hitherto unreported phases andexact description of the composition of eachmineral. These data, combined withinformation concerning the p~neses of thesilicates, are used to characterize the fluidphase within the metamorphic Alpineperidotites.

Geological ttaJlJt:work

The Lanzo massif and Balangero body lieNW of Turin (Fig. 1) and belong to theinnermost sector of the Western Alps. The

140 P. ROSSETTI, S. ZUCCHETTI

Fig. 1. - Gcological sketch map of the inner sector ofthe Western Alps in the Lan:ro atel. 1: Quaternarydepo$iu. 2: Piedmom ophiolirc nappe. 3: AntigoriticSttpCntiniles with (a) preserved peridotirc «ltt$ of the­Lanzo massif (LA) and Balangero body (B). 4: Paleowiccrystalline basement of the Gran PilI'adiso (upper left)md Dot. Maira (lower left) massifs. ~: Sesia Zone. 6:South.alpine I:xisemenl.

Lan:ro massif covers an area of more than 200km2 and is in contact with the Sesia Zoneand the Piedmont ophiolite nappe to theWest.

The decidely smaller Balangero I:xx:ly (about7 km2

) is also in tectonic contact with theSesia Zone, and at present level of exposureis separated from the Lanzo massif by a bandof quaternary ttttains sevual kilometres wide.

The Lanzo massif is a large body oftectonitic peridotite, mainly lherzolite withsubordinate han:burgite and dunite, that alsocontains various kinds of gabbros and basaltdykes (NlcoLAS et al., 1972). It is only in theperipheral areas of the body and along somefoliation bands that the peridotites aretransformed into antigoritic serpentinites.

An12lytic121 me/hods

A significant sampling was made of thefresh peridotites and serpentinites. Thesesamples were examined in the form ofpolished thin sections to enable determinationof the metal and accompanying silicateparageneses to be carried out at the same time.All the phase were analysed in an ARL SEMQwavelength·dispersive instrument. Theanalytical procedures and the mineralchemistry data are described and discussed ina separate note (ROSSETTI and ZUCCHETTI,1987, in press) and only the essential analyticaldata will be mentioned here.

The Balangero body is mostly transformedinto antigorite serpentinite, locally displayinga thick network of chrysotile veins. Peridotitessimilar to those of the Lanzo massif are locallypreserved inside this body, suggesting a dosegenetic relationship between the Balangeroand Lanzo massifs (SANDRONE andCOMPAGNONl, 1986).

The structural position of the massifperidotites continues to be the centre of anextensive debate (see POGNANTE et al., 1985).What is certain, is that the original spinetperidotites, following their partialtransformation to plagioclase peridotitesduring a prealpine evolution, were involvedin the alpine tectonic and metamorphicevolution from the start, in the same way asthe inner sectors of the Piedmontese and SesiaZones (SANDRONE and COMPAGNONI, 1983).Studies of the parageneses of the silicates inthe mafic and u1tramafic rocks show that theyhave been mostly re-equilibrated during theearly-alpine event in T:o 500-550°C andp .. 15 kb conditions (SANDRONE andCOMPAGNOl'.'1, 1983).

The chrysotile veins of the Balangerodeposit, on the other hand, belong to asubsequent, late-alpine stage (CO.MPAGNONI etal., 1980).

Nomenclature and chemistry of the metalphases

Apace from the spinel, which will not bedealt with here, the Lanzo and Balangero

..N

TORINOO10 Km,

CB

(\ ~:(\Ri:t~..".;~ ..,,""'"::';'~...\,~.

++

++

+ +++ +

EARlY....LPlNE ORE PARACENESES IN THE SERPENTINITES FROM TIlE BAUoNGERO ASBESTOS ETC. 141

s

2.12 and 2.44. A similar gap was found, byBoTro and MORRISON (1976), in pebbles fromjosephine County. Moreover, in our rockssome Co (up to 6%) is often present, with itsabundance being higher in the alloys with alower Ni/Fe ratio.

As suggested by BOTIO and MORRlsON

Ni

A

Hz

B

Ta Aw•

Ta Aw

Co

Pn-Tr

We

Fe

Fe NiFig. 2. _ S·Fe-Ni and Co-Fe-Ni {at.%j plots of theopaque phases fram the Lanw massif and Balangerobody. Magnetite and 5pinel compositions are notincluded. Points stand for the corresponding averagecompositions (see text fot symbols). Among thesulphides, only pentlandite is reported. in Fig. 2B, inorder to show its compostional range in the Co-Fe-Nisystem.

fresh peridotites are almost devoid of oreminerals. These, on the other hand, appearin by no means negligible quantities in theserpentinized ultramafites.

The fonowing metal phases were detectedand analysed:

- native metals: native copper (Cu),native iron (Fe), awaruite (Nile), taenite(- NiEe), wairauite (- Fe,Co)j

- sulphides: troilite (FeS), pendandite((Fe, Ni),SJ, heazlewoOOlte (Ni1S)j

- oxides: magnetite (Fe,O,J.Of the metal alloys, native Cu, native Fe,

taenite and wairauite had not been previouslyreported in this sector of the Alps and the lastthree have only been occasionally observedin terrestrial ultramafic rocks (CHALUS andLoNG, 1964; CHAMBERLAIN et al., 196';Bono and MORRISON, 1976; HARDING et al.,1982). Awaruite and sulphides, on the otherhand, have already been described for theBalangero deposit and elsewhere in theWestern Alps (ZUCCHETII, 1967, 1970).

The location of the ore minerals (with theexception of magnetite and native Cu) in theS-Fe·Ni and Co-Fe-Ni diagrams is indicatedin Fig. 2. The points refer to the averagecomposition of these phases from theBalangero and Lanzo rocks.

The fonowing points should be noted withregard to the subsequent discussion:

- native Fe and native Cu are regardedas pure phases (> 99 per cent Fe and Curespectively);

- wairauite (Wa, a Fe and Co alloy) fromthe Balangero body has the general formulaFe}Co2, but its iron content range isbetween 57 and 63 wt%. It does not containappreciable amounts of other components.Such a Fe-Co alloy composition differs fromthose reported from other localities (CHAWSand LoNG, 1964j CHAMBE.R.LAIN et al., 196';Bono and MORRlSON, 1976), which arecharacterized by formula close to CaFe; onthe other hand, the composition rangementioned above is similar to that reportedby LEAVEU. (l983)j

- the Ni-Fe alloys show a large range inthe NifFe ratios. The ratio varies between3.15 and 1.70 (at. %), but is not continuous.Rather, it shows a compositionaI gap between

142 P. ROSSETTl, S. ZUCCHETTI

(1976), our data point to the presence of twodiHerent Ni-Fe alloys:

a) awaruite (Aw), nickel-rich orderedphase of the Fe-Ni system, with an averageformula clo~ to Nile;

b) taenite (Ta), nickel-poor disorderedphase, with an average formula close toNi Fe.

This conclusion is substantiated by the factthat the disordered phase (i.e. taenite)contains more Co than does the ordered phase(i.e. awaruitel

- troilite (Tr, with Fe:S =0 1:1) andheazlewoodite (Hz) are regarded as purephases;

- pentlandite (Po) is shown on the S-Fe­Ni diagram (Fig. 2A) as an area instead of apoint on account to its very variablecomposition, ranging the FefNi ratio from 0.8to 2.10 (at.%), The FefNi ratio is higher whenpentlandite occurs in association with troiliteand/or Ni-Fe alloys; conversdy, it is lower inheazlewoodite·bearing assemblages, inagreement with the data reported by MISRAand FLEET (1973). The Co content is usuallylow (Co < 1 wt.%). Sometimes a more Co­rich pentlandite (Co up to 12 wt. %) occursin the Lanzo massif (Fig. 2B).

In the Fe-Ni-Co·S system (experimentallystudied by IUl'I.'EDA et al., 1986, at P =500 bars) our compositional data wo~fallwithin the pentlandite solid-solution field,close to the Fe-Ni binary, at T ea 500°C.Despite of the higher pressures estimated inthe Lanzo and Balangero ultramafics, the widerange of the Fe/Ni ratio strongly suggestschanges of T and/or fS z conditions (KANEDAet al., 1986; see discussion below);

- magnetite (Mt) is practically pute.

Ore parageneses

The definition of the equilibrium in oreassociations requires some more discussion.In fact, opaque minerals occur as minorphases, irregularly disseminated in the rockand are rarely in contact. On the other hand,the serpentinization process itself is generallyconsidered a non-equilibrium process. Thus,when the ore minerals are not in contact, thefollowing criteria for the definition of

equilibrium association were adoptated (seeECKSTRAND, 1975):

- minerals are in the same set:tion, orbetter in small domains - generally at acentimetric scale - where the silicates are inequilibrium;

- there are not textural indications ofdisequilibrium.

The most frequent and typical associationsof early-alpine age (i.e., not including late­alpine veins) are the following:

(1) Ta - Aw - Wa - Tr . Mt(2) Ta - Aw . Tr . Mt(3) Ta - Wa - Tr - Mt(4) Ta . T, . Mt(5) Aw - Tr . Mt(6) Aw . T, . Pn . Mt(7) Aw . Pn . Mt(8) Tr - Pn • Mt(9) Aw - Pn . Hz - Mt(10) Aw . H, . Mt

It may be argued that at least theheazlewoodite-bearing assemblages, althoughnot inside the late-alpine veins, could reflectthe effect of a retrogressive stage (i.e.,greenschist stage). The following facts should,however, be considered:

- in this sector of the Western Alps theearly-alpine assemblages are normallyextremely well preserved, the effect ofretrogression being mostly confined to shearedzones;

- the petrography of silicates in theseultramafics does not suggest the occurrenceof post-eclogitk recrystallization;

- particularly, in mafk dykes cutting theLanzo peridotite the early-alpine assemblagesare extremely well preserved (POGNANTE andKIENAST, 1987), suggesting that most of themassif did not suffer the effect ofretrogression.

Thus, the occurrence of heazlewoodite­bearing metamorphic assemblages in theLanzo massif confirms their genetic link withthe early-alpine stage. The analogousassemblages within the more transformedBalangero body are likely to belong to theearly-alpine stage as well. However, we cannotexclude that part of them could represent theproduct of a retrogression stage (although any

EARlYAlJ'INE ORE PARAGENESES IN THE SERPENTINITES FROM TIlE BALANGERO ASBESlOS ETC. 143

evidence of it is lacking), as they surely arestable also at lower temperature and pressure.

The relations among the opaque phases canbe ben shown on a Fe-Ni-S chemographicprojection from oxygen (Fig. 3)(1). On sucha projection the stability of assemblages wilhmagnetite (the only oxide in the assemblages)will expand with increasing 10

2, Thus an

infinite number of such projections arepossible, with each one showing tie-lines stableat different oxygen fugacity. For topologic

All these topologies au present in theBalangero and Lanzo ultrama6cs (heavy tie­lines in Fig. 3). It is worth to note that in allthe parageneses magnetite is stable, and thatthe Ni-Fe alloys cover a small part of thecompositional range shown in the differenttOpologies. Particularly, taenite is always Ni­rich, while awaruite tends to be Fe-rich.

The assemblages (l) to (5) belong to thetopology 3a. In such assemblages, awaruiteshows generally a lower Ni/Fe ratio than

a

Mt Ta Aw

Fig. J. - Rdations amonglhe opaque phases on a Fe·Ni·S chemographic projection fwm oxygen, with increasing/01 from (a) to (c). Symbols in Figg. (h) and (c) are the same indicated in Fig. (a). Heavier tie·lines correspondto the elll"ly.alpine assemblages from the Lamo and Balangero rocks. Ta is stable only in topology (a); the Ni contentin Aw increases fwm (a) to (c) (see text fCl" discussion). The dashed lines in lhe Figg. (h) and (c) show the chemographicrelations in the 10w-varilll"lCC assemblage (6) and (9), respectivdy, where the activities of oxygen and sulphur are fixed.

purposes, however, only three projections arenecessary. one at relatively low f0

2(Fig. 3a).

another at intermediate values (Fig. 3b). andone SE relatively high IO} (Fig. 3c). Atrelatively low f02 there JS an extensivestability range of iron-nickel alloys, with bothtaenite and awaruite stable with troilite. Withincreasing oxygen fugacity, the alloycoexisting with magnetite moves to more Ni­rich compositions. The topology shown in Fig.3b is established when the oxygen fugacityis high enough to stabilize the assemblagemagnetite-pentlandite by the reaction:awaruite + troilite + 02 = magnetite +pentlandite.

Further~ in oxygen fugacity restrictthe alloy to even more Ni compositions andonce the heazlewoodite-magnetite assemblageis stabilized, the topology shown in Fig. 3cis established.

(I) We have chosen 10 plot fll;Hll oxygCll (instead frommagnelite) in order 10 clelll"ly draw tlx tie·lines betweenmagnetile and the other phases in the most commonearly.alpine assemblages.

observed in topologies 3b and 3c.The assemblages (6) io (8) are instead

referred to the topology 3b, while thoseindicated as (9) and (10) belong to thetopology 3c. The nickd content increases inthe Ni-Fe alloys from the first topology to~tlllid.

As for is the distribution of theseassemblages, the parageneses belonging totopology 3a are the most common and mainlyoccur in partially serpentinized peridotites,containing relics of primary olivine. Taeniteand Fe-Co alloy are exclusively associated tothese parageneses.

Parageneses belonging to the topologies 3band 3c mostly occur in more serpentinizedperidotites. or in any case in antigoriticdomains. Alternativdy. heazlewoodite ­magnetite parageneses (with or withoutawaruite and pentlandite), which belong totopology 3c. occur sometimes in poorlyserpentinized peridotites. Only in this casenative Cu is present. These parageneses aremostly inside primary. non metamorphic

144 P. ROSSETTI, S. ZUCCHEm

minerals and are at times in association withrelics of primary spine!.

Origin of the ore par.geneses

In the f~sh periclotites the opaque mineralsare only present in small amounts, thus onlya some proportion of the opaque assemblagescould possibily derive from «in situ_transformation of pre-alpine ores. Conversely,since the firsl stages of scopentinizationseveral small grains of opaque minerals(principally Ni-Fe alloys) tend to contour therdics of prima7 olivine. These facts provethe existen~ 0 a close connection betw~n

the genesis of the metal ph~s and theserpentinization process, as now generallyaccepted (NICKEL, 1959; ICRISHNARAO, 1964;CHAMBERLAIN et al., 1965; RAMOOHR, 1967;ZUCCliETIl, 1970; ECKSTRAND, 1975; FROST,1985). In particular, most of nickel in theopaque minerals is conside~ to be releasedby primary olivine during serpeminization.

The petrography of the ultramafitesindicates that serpentinization has primaryoccurred during the regional alpinemetamorphism and has produced parageneseswith amigorite + diopside + olivine + Ti­clinohumite + magnetite == Mg-ehlorite. Thedata for the silicatic parageneses in theultramafic and mafic rocks (SANDRONE andCOMPAGNONI, 1983) point to a temperatureand pressure combination (T = 500-550°C,p"" 15 kb) (2) attributable to the early-alpinestage of the orogeny. The effect of thesubsequent late-alpine retrocession isnegligible here and confined to the latechrysotile veins.

Most of the ore associations are inequilibrium with the early-alpine silicaticparageneses and the same T and P conditionscan be postulated for them.

If, as is certain, the ore minerals are linked

{lJ Recently, lower T·P values (T. 4'O·'OO°C andP_.12·13 kh) ha~ been suggested, by PoGNANTE.ndKiENAST (1987), fa the dykes of Fe-Ti metagr1lbbrowhich cros5Cl.lt t~laUte lhenolitesin the Unzo manif.However, the$e differences in !he T-P estimations willhave only nqligible eff~ls in the following discussion.

with the serpentinization process, then amobilization of both oxygen and sulphur inthe rock must be taken into account for theirorigin. Thus, assessment of significance of theore parageneses requires the consideration, atfixed T and P, of oxygen and sulfur fugacity(/02 and fS 2) (ECKSTRAND, 1975). In otherwords, under certain temperature and pressureconditions the stability of the metalparageneses depends on the composition ofthe associated fluid.

The composition of the fluid phase duringthe early-alpine metamorphism wasdetermined on the association Ta - Aw - Wa_Te - Mt. The criteria underlying the choiceof this paragenesis were:

_ the certainty that this association isstable with the early-alpine silicateparageneses. This certainty rests on thepetrographical and minerographical data andis confirmed by the presence of taenite, the.high temperature» disordered Ni-Fe alloy(see FROST, 1985);

_ the existence of the albeit scantythermodynamic data for the mineralsmemioned above;

_ the fact that this paragenesis belongsto the topology 3a, the most common in ourrocks;

_ the fact that it is a low varianceassemblage.

Analysis of f01 and fS1

In the calculation of /01 and 1S2 valuesfor the association mentioned above themethod described by FROST (1985) wasadopted, using the equilibria listed in Table1. The equilibrium constant for each reactionwas determined from the relation (FROST,

1979L

log K = AfT + B+ CIP-OfTwhere A = -aH°/2.303 R

B = t;5°/2.JOJ RC = -",vo/2.JOJ RT = temperature in degrees KelvinP = total fluid phase pressure in bars.

The water fugacity coefficient (yH20 =3.0) in reaction 5 was determined using theMODRK, namely a computer program kindly

EARLY.ALPINE ORE PA!UGENESES IN THE SERPENTlNITES FROM THE BALANGERO ASBESTOS ETC.. 145

TABLE 1List of the equilibri4 used in the cakuliJlion of

the composition of the fluid pluzse

• • ,~ '.,0. ' "0, - ~ '.z"O. ." 02727'.1 lJ.~ .-'d. ·~·'-'''2 = .• _7.OU -.~

" . .s "•• '0$ 8267.2 -,.". -.-.~ ",0•• '.s ,•• 0z ~.. '0.0IIe .d'2 Ha' .0.-2HZO••• = .• -.-supplied by R.B. Frost and described in FROST(1979).

Let us consider the association:Ta(- Ni,je) - Aw(Nile) - Wa(- Fe)CoJ- Tr (FeS) . Mt (Fe)OJ. In this associationat equilibrium, the Fe activity values (aF)

must be: the same for all three alloys. Thismeans that the activity-compositionrelationship in the alloys surely is not ideal;in f~ct, assuming at'. X~, the resultingaj'A;: the allo~s w~uld be ve~ ~~££erent(XF( .24-.32, X~>• .47-.44, XFc •.58).Most authors indicate a large negative 41Hmixat high temperature for the Ni-Fe alloys(DALVI and 5RIDHAR, 1976; RAMMENSEE andFRAsER, 1981); therefore, ~c in the Ni-Fealloys should be: lower than .47.

On the other hand, also for the Co-Fe alloysthe few experimental data indicate an activitycoefficient lowtt than unity (RAMMENSEE andFRASER, 1981).

Unfortunatdy, all the: e:xpe:rimental data are:referred to T and P conditions very differentfrom those supposed for our association. Thefollowing assumptions seem, however, to bereasonable:

- aol~. < .6. An af • value higher than.6 would mean positive 41Hmix for both Fe­Co and Ni-Fe alloys, contrarly to allexperimental data;

- a~ > .25. An afc value of .25corresponds to Xfe in awaruite, the lowtemperature phase in the Ni-Fe system; alower value would mean an extreme deviationfrom ideality for both taenite and wairauite.

In consequence, we decided to calculate the/02 and fS 2 values in the whole rangebetween aFc =.25 and aFc =.6. Troilite andmagnetite are practically pure phases. a~:s

and ~:'Ol are therefore taken as equal to 1.

Once defined the range of at"'", the /02value can be calculated considering thataw.aruite, taenite and wairauite are inequilibrium with magnetite. Thus, thefollowing equilibrium reaction can be written:.5 Fe,O.,. 1.5 Fe.Jlqr + 02

The log /02 values obtained vary (atT = 550°C and P = 15 kb) between -23.72(if aF• = .25) and -24.29 (if aFc ••6). Thesevalues are greatly influenced by T and onlyto a subordinate degree by P (see Table 2),and lie between 3.88 and 4.45 hg units belowthe surface represented by the FMQ buffer(Fig. 4 and following discussion).

The fS2 value can be calculated in thesame way, being the Ni-Fe and Fe-Co alloysin equilibrium with troilite, through thefoDowing equilibrium reaction:Fe + .5 52 = FeS

*e corresponding log fS2 values at thesame T and P conditions are -11.14 and

TABLE 2List 0/ /0, and fS2 values at different P·T

conditions

lot. t02 .alot. t02

lot. (5:2 01.. tSz,., - -n.n ~.~ -10.1. _1.!loO

"" " ~.~ .....5 _10.90 ~.~

_2'0.08 ~.~ _10.&0 _1.98

"" " -2•.65 ~.~ _1I.60 ~.~

_25.96 ~.'" -1I.30 _1.62

"" " _12.06 ~.M_26.S3 ....65

~." ~.~ _12.05 -1.99

"" " -26.92 -4 .•5 _l2.81 -$.15

_26.14 ~.'" -l2.1lO -8.35

"'",

_21.31 ....25 -l3.56 _9.B

_32.01 -4.21 _15.35 -8."~

,.".~ _4.83 _16.ll -9.l1

.".~ -4.13 -15.81 ~.'"

"'" • ~.~ _16.63 _9."2-32.91

-,..., ~.n -20.01 ~.'"

"",

_.~

~.~ ->0.n ~.~

.-.o.2l -4.51 -20.6l _9.31

"",

_.~

-5.l3 -21.31 _10.13

For each T·P combination, the fin! value refers 10

ate"" •.25, the K<:'Ond to at"' •.6. TIle conditionssuggested for the early.a1pine .ssembl.ges areT.500-550°C .nd P _ 15 kb: calcul.tions for otherT·P values are only given in order 10 ~w 'h<dependence of ft:Jz and fS2 on T .nd P (sce text).

146 P. ROSSETTl. S. Zl/CCHE1TI

-11.90 respectively. These, too, are stronglyinfluenced by T and only subordinately by P(see Table 2). If the pyrite·pyrrothite surfaceis used as reference, log /SI values are7.54-8.30 log units below it (see discussion).

In carbonate-free metaperidotites thedominant species in the fluid will be H 0and HIS (FROST, 1985); the !HIS in t~efluid depends on theJO] andfS2 values and,if these ace known, it can be calculated. Thelog }HIS value in the fluid phase can beobtained considering the followingequilibrium between fluid species:

2 HZS+02 =2 HZO+S1Such a value is comprised between 1.77

(aF ,. .25) and 1.67 (aFt"" .6) (for T. 5500e,na p. 15 kb).

Discussion

The widespread occurrence of nativemetals-sulphides-magnetite assemblages in theserpentinizec:l peridotites corresponds to low/02 and /52 values. Such highly reducingconditions are related to the transformationof oLivine into serpentine + magnetite, since

4

6

'" 2.P

Ant Fo• •

Bru H2OTr

Ant •• FoOi •MQ HoD

F

-W!'- - /-- - - ---.........~\

I

&" ..... ...-::;" II

- .%' /;%' /

1,-"'/ ,~ / // -

600400

roc200

-4

o

-8

-2

-6

Clo<l

Fig. 4. - ~og f02-T projection $howing equilibrium relations In $crpcntinites at PH10 + Pm- l' kb.Ant .. lIf1tigoritc; BTU" brucite; Di .. wOp$idc; Fo .. fomcritc; Tr _ tn:molitc. FMQ and IM arc the fayalitc ­magnclitc - quaru and the iron-magnetitc buHCI", respcnively. Thc dtihcd curves dlow the stability rangc of thcNi-Fc allO)'$ and the portions of the dehydration equilibria where they may be w$placed due to the presence ofH1 in the fluid. 'The ball; between the twO dehydration reactions shows the)Ol conditions of the cvly-a1pinca»emblage Ta· Aw - Wa -Tt _Mt in the: Lanzo and Balllf18CJ"O rocks. 'The doued-dashed CUNC is the iron-magnetitebuHcr at PH10 + PH1 .. 2 kb. 'The open boll estimates the /02 conditions for the latc alpine: veins 15 infernd bythc occurrcnce of nativc iron.

EARl.y....LPINE ORE PARltGENESES IN THE SERPENTINITES FROM THE BAl.ANGERO ASBESIOS En::. 147

the production of magnetite induces reductionof the associated fluid phase (~FROST, 1985,with ref. therein).

Our observations on the ore parageneses inthe Lanzo and Balangero rocks s~m tosuggest that at T = 550°C and P = 15 kb thesystem is of much the same type as thatobserved at lower temperatures (ECK-STRAND,

1975; FROST, 1985).The composition of the fluid phase

associated ro the early-alpine taenite . aWanllre- wairauite - troilite . magnetite assemblage,however, is marked by log /02 and log /S2values appreciably higher than thosecalculated by FROST (1985) for similarassociations in serpentinized peridoritesformed at lower temperatures and pressures.

The apparent discrepancy is due to thestrong dependence of the log/02 and log/S2values on temperature, since they increase byabout 6 and 3 log units respectively per100°C. Their changes due to pressure are lessmarked. (about 0.5 and 0.8 log units per 5 kb,see Table 2), although not negligible becauseof the very high pressure values estimated. forthese ulrramafics.

In order to lower the influence oftemperature, /0 and}S2 have bttn referredto the FMQ b~er and to the Py-Po surfacerespectively (Fig. 4). Our values arecomparable with those provided by FROST(1985, Fig. 4) on native metals + sulphidesassociations at lower T and P, though thecorresponding absolute values are verydifferent.

The study of the Ta· Aw - Wa . Tr. Mtparagenesis gives values of /02 and}S2 thatcan be extrapolated, excepted for smalldifferences, to all the associations occurringin the topology shown in Fig. 3a.

Calculations of the corresponding values of/0 and}S2 for the two last topologies aredifficult, because of the paucity ofthermodynamic data on all the compositionalrange of pendandite and be:ause of theuncertainly d~ to the choice of the af~

values in the alloys. However, in topology 3cthe association of awaruite with bothheazlewoodite and magnetite suggests /02and}S2 values 0.5 and 0.3 log units hjgher,respectively, than those referred to the

topology 3a. Higher /S2 values for topology3c are also suggested, on the other hand, bythe low FefNi ratio in pentlandite associatedwith heazlewoocl..ite (see KANEDA et al., 1986).

Such diHerences, at the same T and Pconditions, can be explained in several ways:

- the equilibrium controlling oxygenfugacity:12 Fe SiO (in olivine) + 12 H20 + 02 •6 Fe}~20,tOH)4 (in serpentine) + 2 Fe)04is dependent on changes in minor elementsconcentration. Changes in MgfFe in thesilkates, Al, Cr, F content of serpentine, andCr content of magnetite could all causevariations in oxygen fugacity from rock torock (FROST, 1986, pers. comm.);

- during the same metamorphic event,the degree of serpentinization was notuniform. As shown by FROST (1985) andconfirmed in our study, at the same T andP conditions we must expect lower fOzvalues in tlon completely serpeminizedperidotites, which still contain relics ofolivine;

- on the contrary, mineral assemblagesreferred to the topology 3c occur sometimesin poorly serpentinized peridorites, where,however, magnetite contains some Cr, andheazlewoodite some Fe and/or Cu. Theseparticular compositions, as mentioned above,could explain such an occurrence. Moreover,textural relationships suggest that at least partof these assemblages grew on older sites (pre­alpine, perhaps primary blebs?), as it could beconfirmed. by the local abundance of Cu,which is always diffused in the rock duringthe following stage. Elsewhere, heazlewooclite- magnetite parageneses grew on a primaryspineJ. Thus, the higher /02 assemblages inthe poorly serpentinized peridotites could berelated to the presence of minor elements inthe ore phases, and/or to the derivation fromolder associations perhaps partially re­equilibrated during the alpine metamorphism.

The 10gjHzS value for the fluid associatedwith the paragenesis studied in detail mayappear excessively high. The explanation liesin the fact that it is accompanied by a low/02' In a hydrated environment with low/02' such as that considered, a hypotheticalreaction as:

148 P. ROSSETIl. S. ZUCCHETTl

Feollqy + HzS + .5 O2 ,,, FeS + H20would proceed to the left, promoting thestability of the native metals and releasingHzS as product. The presence of localconcentrations of late sulphides in some areasof the Balangero body (PEREITI andZUCCHETTl, 1968) may indeed be ascribableto migration of such a H)).rich fluid toareas favorable to deposition.

Although the study of transformationslinked to late alpine retrogression has not beenexamined in this paper, the discovery of nativeiron in late chrysotiIe veins is infomative inthis respect. These veins are attributed to alate alpine stage, at low temperature andpressure. Native iron has 1>«0 alreadyuported, although rarely, from partiaUyserpentinized peridotites (CHAMBf.RI.AIN et al.,1965; HARDING et al., 1982). The occurrenceof native iron suggests that a marked fall in~~ by at least 1 log unit with respect to theFMQ buffer during the retrogression occurred(Fig. 4). However, these extremely reducingconditions are not likely to reJlect equilibrium,but rather are due to partial retrogression ofolivine-bearing assemblages.

Conclusions

This study of the panially serpentinizedperidotites in the Lanzo massif and theBalangero body has elicited a compositionalspectrum for the metal parage~, especiall}'for the native metals, that is among the widestso far observed in rocks of this kind.

Opaque mineral associations, attributableto the early-alpine event (T = 500·550°C andP = 15 kb) in the light of the silicateparageneses, suggest that the topologies in theFe-Ni-S·O system are not verydifferem fromthose reported at lower temperatures andpressures (ECKSTR.AND, 1975; FROST, 1985).

These same associations, however, arestable for much higher /02 and jS values.On the mher hand, the relative shifts of /OJandjS2 with respect to the FMQ buffer anpyrite-pyrrothite reference surface arecomparable with the low temperature data.

The composition of the fluid phase in thesetpentinized peridotites of BaJangero during

the early-alpine stage, as shown by the lowvariance association taenite - awaruite .wairauite - troiIite . magnetite, was markedby /02 values about 4·5 log units below theFMQ buffer and by fS2 valuescorresponding to 7.5-8.3 log units below thepyrite·pyrrothite surface, with logjH S. 1.70.

in these ranges, higher /02 mineralassociations could reflect different degrees ofserpentinization, the distribution of minorelements and, possibly, the partial re­equilibration of pre·alpine ore assemblages.

In the last retrogression stage following theearly-alpine, at least locally, extremelyreducing conditions - with production ofnative iron - were reached. They are likelyto reflect adi~brium process, linked withthe partial transformation of olivine-bearingassemblages.

AcJmowkdgnnClu. - We would like 10 thank R. Froufor making us more fl.IDiliar wilh the role of the fluidphase in metamorphic ulu2IDafics. He also reviewedearlier venions ot the manuscript. Suggestions .ndcommems by D. Cl5telli I/o'crc mut:h appreciated.

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