metasomatic alteration associated with regional...

Ž .Lithos 54 2000 33–62www.elsevier.nlrlocaterlithos

Metasomatic alteration associated with regional metamorphism:an example from the Willyama Supergroup, South Australia

A.J.R. Kent a,) ,1, P.M. Ashley a,2, C.M. Fanning b,3

a DiÕision of Earth Sciences, UniÕersity of New England, Armidale, NSW, 2351, Australiab Research School of Earth Sciences, The Australian National UniÕersity, Canberra, ACT, 0200, Australia

Received 20 October 1998; accepted 12 May 2000

Abstract

The Olary Domain, part of the Curnamona Province, a major Proterozoic terrane located within eastern South Australiaand western New South Wales, Australia, is an excellent example of geological region that has been significantly altered bymetasomatic mass-transfer processes associated with regional metamorphism. Examples of metasomatically altered rocks inthe Olary Domain are ubiquitous and include garnet–epidote-rich alteration zones, clinopyroxene- and actinolite-matrixbreccias, replacement ironstones and albite-rich alteration zones in quartzofeldspathic metasediments and intrusive rocks.Metasomatism is typically associated with formation of calcic, sodic andror iron-rich alteration zones and development ofoxidised mineral assemblages containing one or more of the following: quartz, albite, actinolite–hornblende, andradite-richgarnet, epidote, magnetite, hematite and aegerine-bearing clinopyroxene.

Detailed study of one widespread style of metasomatic alteration, garnet–epidote-rich alteration zones in calc-silicate hostrocks, provides detailed information on the timing of metasomatism, the conditions under which alteration occurred, and thenature and origin of the metasomatic fluids. Garnet–epidote-bearing zones exhibit features such as breccias, veins,fracture-controlled alteration, open space fillings and massive replacement of pre-existing calc-silicate rock consistent withformation at locally high fluid pressures and fluidrrock ratios. Metasomatism of the host calc-silicate rocks occurred attemperatures between ;4008C and 6508C, and involved loss of Na, Mg, Rb and Fe2q, gain of Ca, Mn, Cu and Fe3q andmild enrichment of Pb, Zn and U. The hydrothermal fluids responsible for the formation of garnet–epidote-rich assemblages,as well as those involved in the formation of other examples of metasomatic alteration in the Olary Domain, werehypersaline, oxidised, and chemically complex, containing Na, Ca, Fe3q, Cl, and SO2y.4

Sm–Nd geochronology indicates that the majority of garnet–epidote alteration occurred at 1575"26 Ma, consistent withfield and petrographic observations that suggest that metasomatism occurred during the retrograde stages of a majoramphibolite-grade regional metamorphic event, and prior to the latter stages of regional-scale intrusion of S-type granites at

) Corresponding author. Present address: Danish Lithosphere Centre, Øster Volgade 10, 1350 Copenhagen K, Denmark. Fax:q45-38-14-2667.

Ž .E-mail address: [email protected] A.J.R. Kent .1 Fax: q61-2-6773-3300.2 Fax: q61-2-6773-3300.3 Fax: q61-2-6249-4835.

0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S0024-4937 00 00021-9

( )A.J.R. Kent et al.rLithos 54 2000 33–6234

1600"20 Ma. The fluids responsible for metasomatism within the Olary Domain are inferred to have been derived fromdevolatilisation of a rift-related volcano-sedimentary sequence, perhaps containing oxidised and evaporitic source rocks atdeeper structural levels, during regional metamorphism, deformation and intrusion of granites. At the present structural level,there is no unequivocal evidence for the fluids to have been directly sourced from granites. q 2000 Elsevier Science B.V.All rights reserved.

Keywords: Proterozoic; Willyama Supergroup; Calc-silicate; Metasomatism; Geochemistry; Sm–Nd dating

1. Introduction

Rocks that have experienced metamorphism com-prise a large proportion of the continental regionsand thus an understanding of the changes that areassociated with metamorphic activity is critical forgauging the chemical and mineralogical evolution ofthe continental crust. Traditional studies of metamor-phic phenomena have emphasised the isochemicalmineralogical changes caused by metamorphic re-equilibration under differing pressure and tempera-ture regimes. However, metasomatic mass-transfer ofchemical components is increasingly recognised asan important process accompanying regional meta-

Žmorphism e.g., Chinner, 1967; Yardley and Bal-tatzis, 1985; Ferry, 1992; Ague, 1994a,b, 1997;

.Oliver et al., 1998 . Metasomatic redistribution ofvolatile and fluid-mobile non-volatile chemical com-ponents during the prograde and retrograde phases ofregional metamorphism can profoundly influence thefinal chemical and mineralogical status of a meta-

Ž .morphosed terrane Ague, 1997 . Such changes mustbe quantified in order to understand the effects thatmetamorphic and related metasomatic processes canproduce on rock masses.

In this study we have investigated the role ofmetasomatism in the formation and evolution ofrocks from the Proterozoic Willyama Supergroup inthe Olary Domain of eastern South Australia. TheOlary Domain, part of the Curnamona Province, amajor Proterozoic terrane located within easternSouth Australia and western New South Wales, Aus-

Ž .tralia Fig. 1 , provides an excellent example of ageological terrane that has been significantly effected

Žby metasomatic processes e.g. Cook and Ashley,.1992; Ashley et al., 1998a,b . Within the Olary

Domain, the chemical and mineralogical composi-tions of rocks within the Willyama Supergroup havebeen strongly altered by metasomatic processes, and

examples of regional and local scale metasomaticalteration phenomenon are numerous and widespreadŽe.g. Cook and Ashley, 1992; Ashley et al. 1998a,b;

.Skirrow and Ashley, 1999 . The metasomatic fea-tures of rocks from the Olary Domain also havestrong analogies with alteration phenomena that have

Žbeen documented in other Proterozoic terranes bothelsewhere in Australia and in other parts of the

.world , some of which are associated with Cu, Au,ŽFe and U mineral deposits Kalsbeek, 1992; Frietsch

.et al., 1997; Oliver et al., 1998; Williams, 1998 .In this study, the major styles of metasomatic

alteration in the Olary Domain are documented anddescribed; to do this we both present new informa-tion and review results of earlier studies in theregion. Further, in order to constrain the timing andnature of metasomatic alteration, and to investigatethe composition of the responsible fluids, a detailedstudy has been undertaken on a specific type ofmetasomatic rock, viz. skarn-like garnet–epidote-bearing alteration zones within laminated calc-sili-cate rocks. This style of metasomatic alteration,

Ž .which occurs throughout the Olary Domain Fig. 1 ,is a manifestation of intense mineralogical and chem-ical change resulting from focused fluid passage, andtherefore provides an opportunity to investigate thenature, origin and effects of the metasomatising flu-ids. In addition, as these rocks are suitable for Sm–Ndisotopic dating studies, they allow important con-straints to be placed on the timing of metasomaticactivity.

Directly after the attainment of peak regionalmetamorphic conditions, the Olary Domain experi-enced regionally extensive episodes of the passage ofhot, saline and oxidised aqueous fluids. The fluidsresponsible for metasomatic alteration were probablyderived from metamorphic devolatilisation of crustal

Ž .rocks, largely a sedimentary –felsic volcanic se-quence. Importantly, although we suggest that intru-

( )A.J.R. Kent et al.rLithos 54 2000 33–62 35

Fig. 1. Map of the Olary Domain showing locations mentioned in the text and locations of garnet–epidote replacement zones andclinopyroxene- and actinolite-matrix breccias. Bold dashed line represents the location of the boundary between metamorphic zones IIAŽ . Ž . Ž .andalusite–chloritoid and IIB andalusite–sillimanite of Clarke et al. 1987 . The approximate position of 1600"20 Ma granitoids is alsoshown.

sion of granitoid rocks may have been an importantfactor in promoting devolatilisation reactions in thesurrounding wallrocks, there is no clear evidence forthe direct contribution of water derived from crys-tallising granitoids to the metasomatising fluids.

2. Analytical methods

Descriptions and locations for all samples anal-ysed for whole rock and mineral chemical composi-

tions, Sm–Nd isotopic composition and fluid inclu-sions are given in Appendix A.

2.1. Rock and mineral analysis

Samples of altered and unaltered calc-silicaterocks were analysed for major and trace elements byX-ray fluorescence at the University of Melbourneand University of New England, Armidale, Aus-tralia, using Siemens SRS-300 instruments. Mineralcompositions were measured using a JEOL 5800


scanning electron microscope run in EDS mode at abeam current of 25 nA at the University of NewEngland and using a range of natural standards forcalibration of X-ray intensities. Mineral composi-tions, for phases containing Fe2q and Fe3q werecalculated assuming stoichiometry.

2.2. Fluid inclusions

Fluid inclusion heating and cooling determina-tions were performed using a modified USGS heat-ing–cooling stage. Repetition of measurements indi-cated that individual determinations were repro-ducible at the 1–28C level. For two-phase inclusions,salinities were estimated via the depression of thefreezing point and using the calibration of BodnarŽ .1992 . For halite-bearing three-phase inclusions,salinity estimates were derived from the meltingpoint of halite and the phase relations outlined by

Ž .Sourirajan and Kennedy 1969 . These calculationsare for the pure NaCl–H O system and given the2

chemically complex nature of the fluids responsiblefor the formation of garnet–epidote metasomatic

Ž .zones see discussion below , can only be consideredestimates. This is especially relevant for freezingpoint depression measurements, where in severalexamples the measured melting points of inclusionswere below the eutectic point of the NaCl–H O2

Ž .system, indicating that other cations e.g. Ca mustŽ .be present Roedder, 1984 .

2.3. Sm–Nd isotopic analysis

Mineral separates from garnet and epidote-bearingrocks were prepared using standard heavy liquidseparation techniques and were purified by magneticseparation and hand-picking. Most samples were pre-pared to better than an estimated 98% purity, al-though some mineral separates contained inclusionsand composite grains; in these purity was approxi-mately 95–98%. In order to avoid the incorporation

Ž .of older pre-metasomatism REE-rich minerals inmineral separates, we selected the most intenselyaltered and coarsest-grainsize samples for mineralseparation. In samples where mineral inclusions oc-

Žcur, they consist of other metasomatic minerals e.g.

.epidote, garnet, actinolite and quartz , and not olderŽmetamorphic minerals see discussion below on the

.effect of this on isochron calculations . Samples foranalysis were weighed into dissolution vessels, spikedwith a mixed 146 Ndr150Sm solution and dissolvedusing HF–HNO –HCl acid digestion. Sm and Nd3

were separated and purified using 3g cation ex-change and HDEHP-teflon columns using the proce-

Ž .dure outlined in Bennett et al. 1993 . Samples wereloaded onto the Ta side of a double Re–Ta filamentand analysed using a FinneganrMAT 261 multicol-lector mass spectrometer in static mode at the Re-search School of Earth Sciences, Australian NationalUniversity.

3. Geological setting

The Olary Domain constitutes one of the inliers ofthe Palaeoproterozoic Willyama Supergroup that oc-cur in northeastern South Australia and western New

Ž .South Wales, Australia Fig. 1 . The geology of theOlary Domain has been summarised by Clarke et al.

ŽFig. 2. Olary Domain sequence modified from Ashley et al.,.1996 . Abif B denotes bonded iron formation.


Ž . Ž .1986, 1987 , Cook and Ashley 1992 , Flint andŽ . Ž .Parker 1993 , Robertson et al. 1998 and Ashley et

Ž . Ž .al. 1998a . The Olary Domain sequence Fig. 2displays broad regional correlations with theWillyama Supergroup in the adjacent Broken Hill

ŽBlock, although there are differences in detail Cook.and Ashley, 1992; Preiss, 1999 . The Willyama Su-

pergroup has been interpreted to represent a failedŽ .Palaeoproterozoic rift Willis et al., 1983 and the

Olary Domain is considered to represent a marginalportion of this rift, possibly involving a continentallacustrine and sabkha setting grading upwards into a

Ž .marine environment Cook and Ashley, 1992 .The lower part of the Olary Domain sequence is

occupied by the Quartzofeldspathic Suite, compris-ing quartzofeldspathic and psammopelitic compositegneiss grading into regionally coherent units includ-ing the ALower AlbiteB, dominated by ;1710–1700Ma A-type metagranitoids and co-magmatic felsic

Žmetavolcanic rocks Ashley et al., 1996; Page et al.,.1998 , the AMiddle SchistB, composed of psam-

mopelitic schist and composite gneiss, and the AUp-per AlbiteB, dominated by finely laminated albitite,as well as minor amounts of iron formation, locallygrading into barite-rich rock. The QuartzofeldspathicSuite grades up-sequence into the Calcsilicate Suite,typified by laminated calcalbitites and minor Mn-richcalc-silicate rocks. We use the term Acalc-silicateB todescribe a metamorphic rock containing more than

Ž25 modal% calc-silicate minerals typically amphi-bole, clinopyroxene, plagioclase, garnet, and

.epidote , whereas the term AcalcalbititeB refers to aquartz–albite rock with up to 25 modal% calc-sili-

.cate minerals . The Calcsilicate Suite displays up-se-quence transition into the Bimba Suite, dominated by

Žcalc-silicate rocks and marble, locally with Fe– Cu–.Zn sulfides, graphitic pelite and albitite. The Bimba

Suite is overlain by the Pelite Suite, composed ofpelitic and psammopelitic schist, with local graphiticfacies, psammite, calc-silicate rock, tourmalinite andmanganiferous iron formation. It is interpreted thatthe Willyama Supergroup sequence in the OlaryDomain was largely deposited between ;1710–1650 Ma, although the younger age limit is not

Žwell-constrained Ashley et al., 1998a; Page et al.,.1998 .

The Olary Domain sequence has been intruded byseveral suites of plutonic rocks as well as having

been subject to at least five deformation and meta-Žmorphic events Clarke et al., 1986, 1987; Flint and

.Parker, 1993 . Temporal relationships between intru-sive, metamorphic and deformational episodes havebeen investigated by field studies and by zirconU–Pb and muscovite 40Ar–39Ar geochronologyŽClarke et al., 1986, 1987; Flint and Parker, 1993;Cook et al., 1994; Bierlein et al., 1995; Lu et al.,

.1996; Ashley et al., 1996; 1998a; Page et al., 1998 ,and the following summary is taken from these

Žstudies. Note that previous interpretations e.g. Flint.and Parker, 1993 have ascribed the first three defor-

Ž .mation events in the Olary Domain OD –OD to1 3

the Olarian Orogeny, a major episode of deformationand metamorphism that occurred between ;1600and 1500 Ma. More recent field and geochronologi-cal studies imply that an earlier deformation event

Žoccurred prior to ;1640–1630 Ma Ashley et al.,.1998a; cf. Nutman and Ehlers, 1998 ; however, for

this study we will continue to use the OD –OD1 3Ž .notation of Flint and Parker 1993 . Two later defor-

Ž .mation events DD , DD are related to Delamerian1 2Ž .orogeny ;500–450 Ma .

Initial deposition of the Willyama Supergroupsequence in the Olary Domain commenced at ;1700Ma, A-type granitoids were intruded and co-mag-matic rhyolitic volcanic rocks were erupted at ;

Ž .1710–1700 Ma Ashley et al., 1996 . Recent obser-Ž .vations Ashley et al., 1998a suggest that the

Willyama Supergroup was then deformed prior tointrusion of several mafic igneous masses and smallI-type granitoid bodies into the central part of theOlary Domain at ;1640–1630 Ma. A major episode

Ž .of deformation OD and OD and amphibolite1 2

grade metamorphism affected much of the OlaryDomain at ;1600 Ma. This resulted in formation of

Žtwo sub-parallel planar deformation fabrics OS and1.OS and development of tight to open, upright to2

steeply inclined folds related to OD . Peak regional2

metamorphic conditions were also attained duringOD and OD and studies of pelitic rocks by Clarke1 2

Ž .et al. 1987 indicated that grades were highest in thesouthern and central portions of the Olary Domain,reaching upper amphibolite facies, with estimatedmaximum pressures of 4–6 kb and temperatures of

Ž .550–6508C Flint and Parker, 1993 . Peak metamor-phic conditions decrease to the north to lower amphi-

Žbolite and greenschist facies Clarke et al., 1987;


.Fig. 1 . Peak metamorphic conditions were followedby widespread emplacement of voluminous S-typegranitoids and associated pegmatite bodies at;1600"20 Ma. S-type granitoids range from mas-sive to foliated and are considered to be late-syn-tectonic; intruded at the end of OD event.2

Retrograde metamorphism and alteration, includ-ing a retrograde deformation event, OD , continued3

episodically between ;1580 and ;1500 Ma. OD3

deformation was largely restricted to discrete shearzones along which greenschist facies assemblages

Ž .were developed Clarke et al., 1986 . Further thermalperturbations also occurred during the Musgravian

Ž .Orogeny at ;1200–1100 Ma Lu et al., 1996 .Mafic dyke emplacement at ;820 Ma was a precur-sor to development of the Adelaide Geosyncline in

Ž .the region cf. Wingate et al., 1998 and at least twoepisodes of localised low grade metamorphism anddeformation occurred between ;500 and 450 Ma

Žduring the Delamerian Orogeny Clarke et al., 1986;.Flint and Parker, 1993 . Episodes of fluid flow ac-

Žcompanied most of these later thermal events e.g..Bierlein et al., 1995; Lu et al., 1996 .

3.1. Regional and local scale metasomatic alterationin the Olary Domain

In addition to the magmatic, metamorphic anddeformational history outlined above, the rocks ofthe Olary Domain have experienced a long history offluid–rock interaction, metasomatism and hydrother-

Žmal alteration e.g. Cook and Ashley, 1992; Ashley.et al., 1998a,b; Skirrow and Ashley, 1999 . The

effects of metasomatism in the Olary Domain areevident on a variety of spatial scales, ranging from

Ž .regional-scale kilometres alteration zones in sedi-ments, felsic volcanic and intrusive rocks through tolocalised examples of fluid flow such as breccia

Ž .zones and local fracture systems e.g. Fig. 3 . Al-though the manifestations of metasomatism are het-erogeneously distributed and both lithologic andstructural controls are apparent, examples of fluid–rock interactions are so numerous that it is clear thatmetasomatic processes have been an intrinsic part of

Ž .the development of the Olary Domain Table 1 . Ingeneral, metasomatism has resulted in enrichment of

Fig. 3. Examples of metasomatic alteration in the Olary Domain.Ž .A Brecciated calc-silicate from Cathedral Rock. Breccia consistsof angular bleached albite-rich clasts in a dark matrix of diopside

Ž . Ž .and actinolite pen shown for scale is 14 cm long . B Alteredand bleached laminated calc-silicate at Mindamereeka Hill in-

Ž .truded and cut by thin parallel dykes of pegmatite. C Actinolite-rich veins surrounded by bleached albite-rich alteration selvages

Ž . Ž .in altered I-type granite Tonga Hill . D Fracture-controlledŽ .alteration in laminated psammopelitic sediments White Rock .

Ž . Ž .Fe, Na– Fe , Ca– Na–Fe or, less commonly, Fe–Kand is most commonly evident in quartzofeldspathicrocks, granitoids, calc-silicates and calcalbitites,marbles and iron-formations. Metasomatic assem-blages are typically more oxidised than original as-semblages with a variety of Fe3q minerals presentŽe.g. epidote, magnetite, hematite, andradite-rich gar-

.net and aegirine-bearing clinopyroxene . The typicalstyles of metasomatic alteration evident in the OlaryDomain are summarised in Table 1 and severalexamples are shown in Fig. 3.

The timing of regional-scale metasomatic activityis constrained by field relations and geochronology.Regional-scale alteration zones occur in, and thus


Table 1Summary of the common alteration styles and accompanying mineralogical and chemical changes found in different rock types in the OlaryDomain

Rock type Common alteration styles Mineralogical changes Chemical changes

ŽPelites and Psammopelites Extensive albitisation bleach- Replacement of aluminosilicates, Addition of Na; loss of K, Rb. Ž . Ž .ing Fig. 3D micas and feldspars by albite Ba

Ž Ž .Quartzo-feldspathic rocks Extensive albitisation bleach- Replacement of plagioclase, K- Addition of Na Fe,S,Cu,K ; loss. Ž .ing , minor development of mag- feldspar and biotite by albite, of K,Rb Ca,Sr

netite " hematite, sulfides in minor magnetite, hematite,veins and disseminations. Local pyrite, chalcopyrite. Local devel-biotiteqmagnetite. opment of biotiteqmagnetite

3qCalc-silicate and calcalbitite Pervasive bleaching grading to Destruction of clinopyroxene, ti- Addition of Ca, Fe , Mn,Ž .rocks massive clinopyroxene- and acti- tanite, K-feldspar and scapolite; "Cu,Pb,Zn,U ; loss of Na,

3q 2qnolite-matrix breccias andror formation of secondary Na–Fe Fe , Mg, Rb, Bamassive garnet–epidote alter- clinopyroxene, amphibole, albite,

Ž .ation zones Fig. 3A,B quartz, andraditic garnet, epidote

Granitoids:Ž . Ž .A-types Albitisation bleaching , minor Destruction of plagioclase, K- Addition of Na Fe ; loss of

Ž .Fe oxide veining and dissemina- feldspar biotite and formation K,RbŽ .tions of albite " magnetite, quartz

Ž . Ž .S-types Localised albitisation bleaching Destruction of igneous feldspar Local addition of Na Ca,Fe ;where late dykes crosscut calc- and biotite; albitisation; local loss of K,Rbsilicate breccia and garnet–epi- formation of amphibole, garnet,dote alteration zones epidote, titanite

Ž .I-types Extensive areas of fracture con- Destruction of igneous feldspar Addition of Na Ca,Fe ; loss oftrolled and pervasive bleaching and biotite; pervasive albitisa- K,Rb

Ž .with minor brecciation Fig. 3C tion; deposition of quartz, am-phibole and titanite on fractures

ŽIron-formations Fe oxide enrichment; destruction Local loss of quartz; growth of Addition of Fe Cu,Au,U,V,Y,.of laminated texture magnetite, hematite, local pyrite, Zn,S ; loss of Si

trace chalcopyrite

predate, A-type intrusives and associated volcanicsemplaced at 1710–1700 Ma and I-type granites em-placed at 1640–1630 Ma. In addition, in all locationsobserved, metasomatic mineral assemblages ret-

Žrogress peak metamorphic assemblages e.g. Ashley.et al., 1998a,b and indicate that metasomatic activ-

ity occurred after the metamorphic peak and thedevelopment of OD deformation textures. As dis-2

cussed below, metasomatic alteration zones locatedin calc-silicate rocks are also cut by S-type granitesand related pegmatites at several localities demon-strating that the majority of alteration occurred priorto S-type granitoid emplacement at ;1600"20Ma.

Ž .In several locations e.g. Cathedral Rock , meta-somatically altered rocks are deformed and retro-

gressed within OD deformation zones, indicating3

that the majority of metasomatic alteration occurredprior to formation of these zones at ;1500 Ma.However, 40Ar–39Ar ages on metasediments and peg-matite muscovite also suggest that fluid movementcontinued episodically along OD –OD structures1 3

for several hundred million years after granite intru-Ž .sion Bierlein et al., 1995; Lu et al., 1996 . Reactiva-

tion of structures during the Delamerian Orogeny isindicated by ;470 Ma 40Ar–39Ar ages of mus-

Žcovites from pegmatites and OD shear zones Lu et3. Ž .al., 1996 . Bierlein et al. 1995 also demonstrated

that at least some of the epigenetic sulfide minerali-sation within OD shear zones occurred between3

;480 and 450 Ma during retrograde fluid move-ment along older structures.


3.2. Metasomatism in calc-silicate rocks

Although many different rock types have beeneffected by metasomatism within the Olary Domain,fluid–rock interaction appears to have been espe-cially intense in calc-silicate and associated rocks.The two most common alteration styles evident are

Žcalc-silicate-matrix breccia zones consisting of both.clinopyroxene- and actinolite-matrix breccias and

garnet–epidote-rich alteration zones. Although thesetwo styles of metasomatic alteration occur in similarhost rocks, and are often spatially and temporally

Ž .associated see below , they are associated with dis-tinctly different styles of metasomatic alteration andwill be treated differently for purposes of descriptionand discussion.

In Section 4 we describe the petrological, chemi-cal and mineralogical features of garnet–epidote-richalteration zones in detail. These zones are the focusof our research as they appear to have formed atrelatively high fluidrrock ratios in areas of focussedfluid flow and thus provide an excellent opportunityto assess the nature of metasomatic fluids and thechemical changes associated with metasomatism.However, as the clinopyroxene- and actinolite-matrixbreccias are also observed to be spatially and tempo-rally related to the formation of garnet–epidote-richalteration zones, and appear to have formed fromsimilar composition metasomatic fluids, we believethat understanding the relation between these meta-somatic breccias and garnet–epidote-rich metaso-matic alteration zones provides important insightsinto the nature of metasomatism within the OlaryDomain. To this end, both clinopyroxene- and acti-nolite-matrix breccias are briefly described in theremainder of this section.

Clinopyroxene and actinolite-matrix breccias formmany spectacular outcrops in the Olary Domain,Že.g. Cathedral Rock, Toraminga Hill, Telechie Val-

.ley; Figs. 1 and 3A and have been discussed byŽ .Cook and Ashley 1992 and Yang and Ashley

Ž .1994 . Calc-silicate-matrix breccias are commonlystratabound, range from irregular and locally tran-gressive bodies up to tens of metres across down tonarrow piercement masses and are associated withzones of hydrothermal alteration, involving albitisa-

Žtion white AbleachingB and local pink hematitic.pigmentation in the host calcalbitite. Field relations

imply that breccias formed during deformation asthey contain rare folded fragments and appear tohave been injected into fractures in fold hinges inter-

Žpreted to be temporally linked to OD Yang and2.Ashley, 1994 . In several locations, breccias have

also been intruded by S-type granite and pegmatiteŽrelated to the ;1600"20 Ma episode e.g. Cathe-

.dral Rock, Toraminga Hill and have been deformedŽ .by OD shear zones e.g. Cathedral Rock .3

Breccias consist of angular altered rock fragmentsin a medium to coarse grained matrix dominated byclinopyroxene andror actinolite, with minor quartz,albite, hematite, titanite and epidote. All gradationsoccur between bleached, altered calcalbitite contain-ing minor clinopyroxene andror albite veins and

Žmassive clast and matrix-supported breccias e.g..Fig. 3A . In general, the early phases of breccia

formation are associated with aegirine-bearingclinopyroxenes as the dominant matrix mineral. Laterstages of breccia evolution involve retrograde re-

Ž .placement of clinopyroxene by actinolite Fig. 4A ,as well as formation of primary actinolite"hematite

Ž ."quartz" titanite e.g. Toraminga Hill . Clinopy-roxene-matrix breccias are most common in the cen-tral part of the Olary Domain whereas amphibole-dominated matrix breccias occur in the central north-

Ž .ern and northern portions Fig. 1 . This mirrors theŽ .patterns evident in metamorphic isograds Fig. 1

and thus most probably reflect regional gradients intemperature during breccia formation, with theclinopyroxene representing higher temperature re-

Ž .gions see discussion below . Fluid inclusions inclinopyroxene and quartz associated with brecciasare commonly hypersaline, and measurements ofquartz-hosted inclusions from clinopyroxene- andactinolite-matrix breccias display fluid salinities be-

Žtween ;15–46 equivalent wt.% NaCl A.J.R. Kent.and P.M. Ashley, unpublished data . Clinopyroxene

from breccias generally contains higher Na–Fe3q

Ž .contents up to 33 mol% aegirine than clinopyrox-Ž .ene in the unaltered calc-silicate rocks Fig. 5 and

this, coupled with the presence of hematite in actino-lite-matrix breccias and as a daughter mineral phasein fluid inclusions, indicates that breccia formationoccurred under oxidizing conditions.

A third type of metasomatic alteration in calc-silicate rocks, found locally in laminated Mn-richŽ . Žpiemontite-bearing calc-silicate rocks Ashley,


Fig. 4. Photomicrographs from altered calc-silicate rocks from theŽ .Olary Domain. A Cross-polarised light photo of clinopyroxene-

Ž .matrix breccia from Cathedral Rock sample CR-5 showingretrogression of clinopyroxene to fibrous and massive actinolite

Ž .adjacent to a crosscutting quartz vein. B Plane-polarised lightŽphoto of garnet–epidote alteration zone from Boolcoomatta sam-

.ple BC-3 showing garnet, epidote and quartz intergrowth withŽ .partial granoblastic textures. C Plane-polarised light view of

Ž .garnet–epidote alteration zone from Bulloo Well sample BW-3 .Euhedral and subhedral zoned garnets are surrounded by laterquartz and contain irregular inclusions of epidote. Abbreviations:Cpx. — clinopyroxene, Act. — actinolite, Gt. — garnet, Ep. —epidote, Qtz. — quartz.

Fig. 5. Na vs. Fe3qrFe2q plot for clinopyroxenes from calc-sili-cate rocks and ironstones from the Olary Domain. The starrepresents the average of 26 clinopyroxene analyses from alteredironstones from Mindamereeka Hill taken from Ashley et al.Ž .1998b . Clinopyroxene compositions from unaltered calc-silicates

Ž .are from Cook 1993 and those from clinopyroxene-matrix brec-.cias from Yang and Ashley, 1994 . The composition of recrys-

tallised clinopyroxene within the garnet–epidote-rich alterationzone at White Dam North is also shown.

.1984 is a variant of garnet–epidote alteration, andcontains coarse grained assemblages of one or more

Žof piemontite, quartz, garnet andradite- and spessar-.tine-rich , hematite, manganoan tremolite and brau-

nite. These rocks will not be described further in thispaper.

4. Garnet–epidote-rich metasomatic alterationzones

4.1. Field setting and description of alteration phe-nomenon

Garnet–epidote-rich alteration zones are best de-veloped in calc-silicate-bearing rocks of the Calcsili-cate and Bimba Suites, but also occur rarely inquartzofeldspathic rocks of the QuartzofeldspathicSuite. For this study, samples from garnet–epidote-rich alteration zones and associated calc-silicate rockswere examined in detail from six locations, termedBulloo Well, Boolcoomatta, Sylvester Bore, Min-damereeka Hill, Sampson Dam and White Dam NorthŽ .Fig. 1 , although observations were also made atseveral other locations. The alteration types evident


Fig. 6. Handspecimen photos of samples from garnet–epidoteŽ .alteration zones. A Brecciated and altered calc-silicate from

Ž .Boolcoomatta sample BC-8 . Bleached and albitised angularfragments of the original calc-silicate are held within an epidoteŽ .actinolite–garnet matrix. Although taken from a epidote–garnetalteration zone, this sample has similar textures to those observed

Ž . Ž .in calc-silicate breccias Fig. 3A . B Altered calc-silicate fromŽ .Bulloo Well sample BW-2 . Original calc-silicate has been largely

replaced by massive epidote with subsidiary quartz and garnet.ŽSmall dark residual laminae of clinopyroxene partially altered to

.actinolite are also apparent. Alteration has been largely controlledby the composition of individual laminae and the folded structure

Žof the calc-silicate has been preserved folding may represent. Ž .soft-sediment deformation of the original calc-silicate . C Par-

Žtially altered laminated calc-silicate from Sylvester Bore sample. ŽSB-2 . Individual laminations have been replaced by garnet the

largest garnet-replaced laminae has a small epidote-rich region inthe centre and is bordered by a thin pale zone of quartz–albite

.alteration . The remaining calc-silicate has been recrystallised andmuch of the original clinopyroxene has been altered to actinolite.Ž .D Massive garnet-dominated alteration of laminated calc-silicate

Ž .from Sylvester Bore sample SB-3 . The primary laminated tex-ture is partially destroyed by massive regions of garnet andquartz–albite alteration.

at each location are essentially the same and areŽdescribed below and illustrated in Figs. 3B, 4B,C

.and 6 .In outcrop, garnet–epidote-rich zones occur as

dark brown, black and green masses showing partialto complete replacement of laminated calc-silicaterock, with local replacement controlled by former

Ž .bedding and fractures e.g. Figs. 3B and 6 . The sizeof the metasomatised regions varies substantially,with alteration zones ranging from thin isolated vein-

Ž .lets centimetre scale to massive lensoid strataboundŽreplacements up to 200–300 m across e.g. White

.Dam North, Bulloo Well . Alteration can also oftenbe traced for tens of metres along specific laminae,resulting in distinctive Anet-typeB textures where re-placement occurs along both reactive bedding layers

Žand along fractures at high angles to bedding analo-gous to the texture shown in altered psammopelitic

.sediments in Fig. 3D . Bleached quartz–albite-richlayers and zones are also common, and breccias withepidote–garnet matrix cementing bleached albite-rich

Žfragments occur at the Boolcoomatta locality Fig..6A . In addition to garnet and epidote, quartz is

common, occurring in veins, open space fillings andin intergrowths with garnet and epidote. Other min-erals are present in minor quantities and includealbite, actinolite, clinopyroxene, K-feldspar, hematite,magnetite, carbonate and tiny traces of chalcopyriteand pyrite. Metasomatism commonly follows frac-ture sets that appear to be related to OD deforma-2

Ž .tion, and in several locations e.g. White Dam Norththe strongest alteration appears to be focused intoOD fold hinge zones, perhaps suggesting that these2

acted as fluid conduits.On the outcrop, hand specimen and microscopic

Žscales’ five categories of alteration phenomena with.progressive alteration intensity have been recog-

nised, ranging from unaltered calc-silicate through toincipient disseminated and fracture-controlled alter-ation to total replacement of the pre-existing calc-silicate rocks and late monomineralic veining. Thealteration styles are described below and summarisedin Table 2.

4.1.1. Unaltered laminated calc-silicate rockŽ .These commonly crop out as thin -20 m lenses

with strike continuity of less than a kilometre, inter-calated with pelitic, psammopelitic and laminated


Table 2Typical modes of occurrence of altered calc-silicate rocks in the Olary Domain

Mode of occurrence Locations observed Figure

Disseminated regions of garnet andror epidote, from -1 Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 5cm up to 10 cm across damereeka Hill

Massive layer parallel replacement of calc-silicate minerals Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 4B,C and 5Žby garnet andror epidote "quartz, albite, K-feldspar, damereeka Hill, Sampson Dam, White Dam North and

.amphibole other sites

Fracture controlled replacement of calc-silicates by garnet Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 5Ž .andror epidote "quartz, albite, K-feldspar, amphibole . damereeka Hill

May combine with layer-parallel replacement of morereactive layers to produce net-like textures.

Massive replacement of calc-silicate by garnet andror Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 4D and 5Ž .epidote "quartz, albite, K-feldspar, amphibole . These damereeka Hill, Sampson Dam, White Dam North

areas commonly show quartz-rich zones with euhedralgarnet crystals. Zones of epidote and garnet replacementmay be surrounded by a AbleachedB quartz androralbite-rich zone. Pseudomorphous replacement of the origi-nal rock is locally evident with garnet and epidote formingnear monomineralic layers.

Late near-monomineralic veins of garnet, epidote, quartz Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 5and local K-feldspar. damereeka Hill

Coarse euhedral garnet crystals, filling open spaces or Bulloo Well, Sylvester Bore, Mindamereeka Hill, Sampson 5associated with late quartz filling. Dam, White Dam North

Epidote cementing brecciated fragments of albitised rock Boolcoomatta 4A

Garnet–quartz veins in calcalbitite South Burden’s Dam

albitic rocks. These rocks are typically well-laminated, commonly defined by alternating ferro-magnesian and quartzofeldspathic layers. Individualcompositional laminae are from 1 mm to 10 cm inthickness and are interpreted as a primary deposi-

Ž .tional characteristic Cook, 1993 . Calc-silicate rocksare dominated by clinopyroxene, albite, quartz, K-

Žfeldspar and amphibole hornblende andror actino-.lite , with variable, but generally minor amounts of

scapolite, garnet, epidote and titanite. Actinolite andhornblende occur as disseminated retrogression prod-ucts of clinopyroxene and as discrete grains. Scapo-lite is found erratically in granoblastic aggregates inferromagnesian and quartzofeldspathic layers. Thecalc-silicate rocks of the Olary Domain have beeninterpreted as the result of clastic sedimentation offelsic detrital material and interaction of sedimentswith evaporative brines, as well as contemporaneousevaporitic and exhalative chemical sedimentationŽ .Cook and Ashley, 1992; Cook, 1993 .

4.1.2. Recrystallisation of calc-silicate minerals ad-jacent to alteration zones

Clinopyroxene, titanite, scapolite and feldspar arerecrystallised adjacent to alteration zones. This iscommonly shown by an increase in grainsize, betterdevelopment of granoblastic texture and decreasedabundance of feldspars. Recrystallised clinopyroxeneis paler in colour and more Mg-rich, compared to thegreen, more Fe-rich compositions evident in unre-crystallised clinopyroxenes. Actinolite retrogressionof clinopyroxene is also more common in recrys-tallised zones.

4.1.3. Incipient formation of garnet–epidote bearingassemblages

Minor to major development of epidote, garnetand local quartz in clinopyroxene-bearing lamina-tions and along fractures. K-feldspar is altered toalbite. Subhedral garnet and epidote occur as individ-


ual crystals or small aggregates. Scapolite and titan-ite are less common, and generally only occur inzones where relict clinopyroxene remains. Actinolitealteration of clinopyroxene is also common.

4.1.4. Total alteration of calc-silicate rockThese zones consist of intense alteration along

fractures and laminae and total replacement ofclinopyroxene-bearing laminae by garnet, epidote and

Ž .quartz "minor K-feldspar, actinolite and albiteŽ .Fig. 4B . Where alteration is most intense massivereplacement of feldspar-rich layers is also evidentŽ .Fig. 6 . Although epidote- and quartz-rich zonesoccur, garnet is commonly dominant and in manyplaces is the only significant constituent. Along frac-tures and in open space fillings, garnet, and to a

Žlesser extent quartz and epidote, occur as large up to.several cm subhedral crystals, with garnets com-

Ž .monly showing oscillatory zoning Fig. 4C . Withinaltered laminae, garnet generally occurs as smallerŽ .mostly less than 2–3 mm crystals with granoblastictexture and is strongly poikilitic, containing inclu-sions of quartz, epidote and minor feldspar. Epidoteoccurs as subhedral to anhedral crystals in aggre-gates up to several centimetres across associatedwith garnet or as a matrix to bleached and brecciated

Ž .calc-silicate rock e.g. Fig. 6 . There is no consistenttextural relationship between garnet and epidote; insome samples, quartz and epidote form late crys-talline aggregates around euhedral garnet and inother samples occur as inclusions with poikiliticgarnet. This is interpreted to indicate that garnet andepidote crystallised coevally. The observed texturalrelations are probably the result of local variations inthe relative time and rate of growth of either mineral.At the White Dam North location, intense develop-

Ž .ment of garnet –epidote–quartz rock is locallycored in a synformal hinge zone by magnetite–quartzŽ .–albite rock.

4.1.5. Late ÕeinsAt most altered calc-silicate rock locations, nar-

Ž .row - 10 mm late veins of garnet, epidoteŽ ."quartz, K-feldspar crosscut all other assem-blages.

4.2. Timing of formation of garnet–epidote-rich al-teration zones

Field and petrographic observations indicate thatmetasomatism occurred after development of peakmetamorphic mineral assemblages and associatedOD and OD deformation events. At all localities1 2

Ž .investigated, the garnet–epidote–quartz –actinolitemetasomatic mineral assemblages overprint the pri-mary metamorphic assemblages in the host calc-

Ž .silicate rocks e.g. Fig. 3 . Further, mineral fabrics inŽaltered rocks are not foliated e.g. Figs. 3B, 4B, C,

.6 , deformed calc-silicate rock at Boolcoomatta isoverprinted and pseudomorphed by granoblastic-tex-

Ž .tured epidote–garnet–albite Fig. 6B , and alterationis often controlled by fracture sets associated withthe OD deformation event. The common presence2

of actinolite, rather than clinopyroxene, in garnet–epidote-rich zones is consistent with a retrogradeorigin.

Timing relations between metamorphism, metaso-matic formation of both clinopyroxene-matrix brec-cias and garnet–epidote-rich alteration zones andintrusion of S-type granites are particularly clear atMindamereeka Hill, where laminated calc-silicate

Žrocks have been altered to garnet–epidote –quartz–.actinolite–albite"hematite assemblages along frac-

tures and laminae. Several small lenses of clinopy-roxene-matrix breccias also occur at this location andare crosscut by veins of garnet andror epidote,indicating that garnet–epidote-rich alteration zonesformed after clinopyroxene breccias. Leucocratictwo-mica S-type granite and associated pegmatitedykes cut both garnet–epidote-altered calc-silicate

Ž .rocks Fig. 3B and clinopyroxene-matrix breccias,and granite intrusion is interpreted to have postdatedformation of both clinopyroxene-matrix breccias andthe bulk of garnet–epidote replacement of calc-sili-cate rock. However, we note that pegmatite dykesadjacent to garnet–epidote-rich zones also containirregular veins and clots of garnet"quartz"epidote"hematite, and where granite has intruded alteredcalc-silicate rocks, it has been altered to a bleached

Žalbiteqquartz" titanite assemblage e.g. at Min-.damereeka and Toraminga Hills; Fig. 1 . We suggest

that intrusion of these dykes either occurred duringthe waning stages of metasomatic alteration or thatthe heat associated with intrusion remobilised meta-


Žsomatic fluids possibly contained within fluid inclu-.sions causing further alteration.

4.2.1. Sm–Nd datingIn order to determine the time of formation of

garnet–epidote-rich alteration zones, garnet and epi-dote in samples from Sylvester Bore, Mindamereeka

Ž .Hill and Bulloo Well Fig. 1 were analysed forSm–Nd isotopic composition and concentration. Re-sults are shown in Table 3.

Variations in Sm and Nd compositions and iso-topic ratios between analyses of the same mineralsfrom the same locations are evident at SylvesterBore and Mindamereeka Hill. This could be due tocontamination of separates by variable amounts of a

Žphase with different Sm and Nd concentration i.e..contamination of garnet with epidote and vice versa

or may reflect compositional variation within anal-ysed minerals. Electron microprobe analyses andoptical observations show that minerals are composi-tionally zoned, and this may also be the case for Smand Nd concentrations and the SmrNd ratio. It isimportant to note, however, that variation in Sm andNd composition will not effect isochron calculations,provided that all minerals analysed had the sameinitial 143Ndr144 Nd ratio, and that all mineralsformed at the same time. If this is the case, then

Žmixing of two minerals with different SmrNd ra-.tios will move samples along the isochron line, not

away from it. For a metasomatic rock, these assump-tions are probably justified assuming that localisedequilibrium existed between the fluid and reactingcalc-silicate during metasomatism.

Results from the regression of data from garnet–epidote alteration zones are given in Table 4, and areplotted on a 147Smr144 Nd vs. 143Ndr144 Nd isochrondiagram in Fig. 7. Regression of all data correspondsto an age of 1577"80 Ma, with a high MSWD of

Ž83 see footnotes for Table 4 for explanation of this.term . Examination of Fig. 7 shows several points

which lie off the isochron. Both garnet and epidoteŽ .aliquots from the one sample BW-1 from Bulloo

Well and epidote from sample MH-1 lie well abovethe best-fit line. Removal of these from the regres-sion improves the MSWD to a more acceptable 3.8,equivalent to an age of 1575"26 Ma. Subject toappropriate justification for removal of these points,this is interpreted to be the age of formation ofgarnet–epidote-rich zones at Mindamereeka Hill andSylvester Bore. Ages from individual regression ofdata for the Sylvester Bore and Mindamereeka Hilllocalities are within error of the age derived fromregression of all data. Uncertainties for these agesare higher, and this probably reflects the lower num-

Table 3w xSm–Nd analyses of garnet and epidote from metasomatic rocks from the Olary Domain Ep — epidote, Gt — garnet . Sample locations and

descriptions given in Table 6147 144 143 144 aŽ . Ž .Sample Sm ppm Nd ppm Smr Nd Ndr Nd "

Bulloo WellŽ .BW-1 Ep 2.07 7.79 0.1607 0.512072 "8Ž .BW-1 Gt 8.30 17.69 0.2841 0.513312 "16

Mindamereeka HillŽ .MH-1 Ep 35.7 171 0.1263 0.511708 "11Ž .MH-2 Ep 4.59 28.4 0.0976 0.511306 "12Ž .MH-3 Ep 2.06 14.0 0.0885 0.511206 "9Ž .MH-1 Gt 34.1 101 0.2046 0.512389 "8Ž .MH-2 Gt 22.9 76.2 0.1814 0.512172 "13Ž .MH-3 Gt 20.7 85.7 0.1462 0.511742 "8

SylÕester BoreŽ .SB-2 Ep 9.73 33.0 0.1781 0.512137 "9Ž .SB-3 Ep 11.6 37.4 0.1870 0.512244 "8

Ž .SB-3r1 Gt 26.3 36.6 0.2939 0.513353 "13Ž .SB-3r2 Gt 28.3 37.6 0.2936 0.513321 "8

a95% confidence interval, error given in the last decimal places.


Table 4Regression data for Sm–Nd analyses of garnet and epidote from altered calc-silicates. Abbreviations as for Table 3

143 144aRegression MSWD ´ Ndr Nd Age PointsNd i

All data 83 y5.6 0.510309"52 1577"80 12Ž . Ž .ll data without Bulloo Well, MH-1 Ep , MH-3 Gt 3.8 y6.0 0.510292"30 1575"26 8

Bulloo Well – y4.0 0.510457"31 1529"25 2Ž .Bulloo WellqMH-1 Ep 3.3 y4.1 0.510434"310 1543"220 3

Sylvester Bore 3.9 y5.9 0.510308"134 1568"93 4Mindamereeka Hill 107 y6.3 0.510339"2400 1529"250 6

Ž . Ž .Mindamereeka Hill without MH-1 Ep , MH-3 Gt 2.3 y6.3 0.510305"56 1556"66 4

a Ž .MSWD Amean squares weighted deviatesB is a measure of the quality of the isochron fit. Ideally the MSWD should be close to one.

ber of data points contributing to the regressionsŽ .Table 4 .

Deviation of individual points from the isochronmay be the result of several factors, including differ-ences in initial 143Ndr144 Nd ratios; disturbance ofNd isotope systematics after formation of metaso-matic minerals; and incorporation of material whichpredates metasomatism into the analysed aliquotsŽ .e.g. metamorphic clinopyroxene or titanite . Thefirst possibility is most probable for minerals fromBulloo Well where samples are from a geographi-

Fig. 7. 147Smr144 Nd versus 143Ndr144 Nd isochron plot for garnetand epidote from garnet–epidote alteration zones. Individual datapoints are labeled. Abbreviations as for Fig. 4 and: BW — BullooWell; MH — Mindamereeka Hill; SB — Sylvester Bore. The

Ž . Ž .regression line is for all data except BW-1 Gt , BW-1 Ep , MH-Ž . Ž .1 Ep and MH-3 Gt where it is shown in solid, compared for the

Ž . Ž .two point regression of BW-1 Gt and BW-1 Ep where it isshown in dashed.

cally different location. Differences in the Nd isotopecomposition of calc-silicates, metasomatic fluidandror the fluidrrock ratio could produce variationsin the initial Nd isotope composition of metasomaticminerals from different locations. Both samples fromBulloo Well appear to lie on a separate isochron thanthat defined by the remainder of the data. The twopoint isochron defined by Bulloo Well samples cor-responds to an age of 1529"25 Ma and has aninitial 143Ndr144 Nd ratio of 0.510457"31. Thisvalue is different, outside the given 95% confidencelimit, from the initial ratio of 0.510292"30 fromthe regression of data combined from Mindamereeka

ŽHill and Sylvester Bore and from the regression ofdata from both these localities regressed separately;

.Table 4 . This is consistent with an interpretationthat the fluid responsible for metasomatism at BullooWell had an initial Nd isotope composition differentfrom that responsible for metasomatism at the othertwo locations studied. However, at present it is notpossible to distinguish whether metasomatism at Bul-loo Well occurred at a different time to other loca-tions as the ages from regression of the Bulloo Wellsamples and the combined data from MindamereekaHill and Sylvester Bore are within error at 95%

Ž .confidence limits Table 4 . Further, the age forBulloo Well is not definitive as it is only based on atwo-point regression.

The explanation for the samples from Min-damereeka Hill which lie off the isochron is notclear. Epidote from sample MH-1 may have a similarinitial 143Ndr144 Nd ratio to that defined by the twosamples from Bulloo Well, as it lies close to the

Ž .two-point regression line defined by these Fig. 7 . Itis possible that paragenetically late epidote veinletsobserved in this sample formed from a fluid withinitial Nd isotope composition slightly different from


that which was responsible for the majority of alter-ation at Mindamereeka Hill.

The ´ value of y6.0 calculated from regres-Nd

sion of the combined data from Mindamereeka Hilland Sylvester Bore and the value of y4.0 calculated

Ž .from the Bulloo Well data Table 4 are consistentwith the formation of these rocks via the action ofLREE-enriched, crustally derived, fluid. This doesnot imply a specific rock type from the Olary Do-main sequence as the source of REE in the garnet–epidote alteration zones, as most rocks in the se-quence are crustally derived and thus would beexpected to be LREE-enriched. However, these ´ Nd

values limit the direct contributions of REE to themetasomatic fluid from LREE-depleted mafic rocks.In addition the ca. 1710–1700 A-type igneous rocks

Žfrom the Olary Domain have ´ values calculatedNd. Žat 1700 Ma that range from y0.1 to 1.0 Ashley et

.al., 1996; Page et al., 1998 and thus are also un-likely to have contributed REE to the metasomaticfluid.

4.3. Mineral chemistry

The results of electron microprobe analysis of thecomposition of epidote, garnet, amphibole andclinopyroxene from garnet–epidote-rich alterationzones are shown in Fig. 8 and representative mineralanalyses are given in Table 5.

Garnets consist predominantly of andradite–grossular solid solution, with minor spessartine and

Ž .almandine components Table 5, Fig. 8A ; composi-tions extend to 95 mol% andradite. Variations areexpressed largely as differences in the andradite–grossular ratio between different localities and be-tween samples from the same locality. Garnets fromaltered calc-silicates have lower almandine qspessartine components than those from largely unal-

Ž .tered calc-silicates Fig. 8A .Epidote from altered calc-silicate rocks has rela-

tively Fe-rich compositions, ranging between 8% and32% pistacite end-member, and with low piemontite

Ž .contents Fig. 8B . Amphiboles in altered calc-sili-cate rocks are relatively Fe-rich and include ferro-

Ž .hornblende and actinolite Table 5 . Recrystallisedclinopyroxene is relatively magnesian, with a typicalcomposition being Wo En Fs with a small50.0 41.3 8.7

Ž .calculated aegirine component Table 5 .

Fig. 8. Mineral compositions from garnet–epidote altered calc-silicate rocks in the Olary Domain. Additional data from RolfeŽ . Ž . Ž . Ž .1990 , Westaway 1992 , Cook 1993 , Eykamp 1993 , LaffanŽ . Ž . Ž . Ž .1994 , Pepper 1996 and Chubb 2000 . A Garnet composi-tions from garnet–epidote alteration zones and from unaltered

Ž .calc-silicate rocks. B Epidote compositions from garnet–epidotealteration zones.

4.4. Geochemistry

Samples of garnet–epidote-rich altered calc-sili-cate and unaltered calc-silicate rocks from Bool-coomatta, Mindamereeka Hill, Sylvester Bore, Bul-loo Well, Sampson Dam and White Dam North wereanalysed for major and trace elements in order toassess the chemical changes resulting from metaso-matic alteration. Analyses are presented in Table 6.Changes in major element and selected trace elementcompositions were evaluated using the isocon calcu-

Ž .lation procedure outlined in Grant 1986 , with re-sults summarised in Fig. 9. This method assumesthat the composition of the unaltered rock is repre-sentative of the protolith of the altered rock. Al-

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Table 5Representative electron microprobe analyses of epidote, garnet, amphibole and clinopyroxene from altered calcsilicate rocks from the Olary Domain

Sample Epidote Garnet Amphibole Clino-pyroxene

BC-4 SB-3 MH-1 BW-2 R74782 R77358 BC-4 SB-3 MH-1 BW-1 R74782 R77351 R74782 R74782 R74782

SiO 37.21 37.47 37.62 37.48 37.36 38.18 35.83 37.03 34.91 36.76 37.74 37.01 45.30 51.15 53.642

TiO 0.30 0.34 0.56 0.35 0.34 0.36 0.07 0.09 0.042

Al O 21.56 23.04 21.19 21.88 22.53 22.23 6.04 13.23 2.92 10.13 13.19 7.61 8.39 4.48 0.282 3

Fe O 16.02 14.41 15.93 15.81 14.76 15.96 23.03 13.45 27.82 17.45 13.68 20.192 3

FeO 1.55 0.61 0.01 0.85 1.98 20.92 17.24 5.18MnO 0.38 0.18 0.14 0.29 0.38 0.29 2.20 1.38 0.59 1.13 1.21 0.84 0.93 0.91 0.93MgO 8.27 11.26 14.88CaO 23.32 23.88 23.01 23.87 22.87 22.95 31.06 33.95 33.20 33.33 33.51 32.01 11.06 11.21 24.31Na O 1.70 1.04 0.272

K O 1.15 0.572

Cl 0.19 0.05Total 98.49 98.98 97.89 99.45 97.90 99.61 100.01 99.99 100.01 100.00 99.67 100.00 97.98 98.00 99.53

Ž . Ž . Ž . Ž .25 O 24 O 23 O 6 O

Si 5.966 5.940 6.050 5.950 5.987 6.022 5.915 5.862 5.831 5.910 5.992 6.037 6.910 7.550 1.990ivAl 0.034 0.060 0.050 0.013 0.085 0.138 0.169 0.090 0.008 1.090 0.450 0.010viAl 4.040 4.246 4.016 4.043 4.241 4.133 1.089 2.330 0.407 1.831 2.461 1.459 0.425 0.330 0.002

Ti 0.038 0.041 0.070 0.040 0.041 0.044 0.005 0.010 0.0053qFe 1.933 1.720 1.927 1.888 1.778 1.894 2.860 1.603 3.497 2.112 1.508 2.481 0.0262qFe 0.213 0.081 0.001 0.114 0.127 0.270 2.665 2.130 0.175

2qMn 0.051 0.024 0.019 0.039 0.051 0.039 0.307 0.185 0.083 0.154 0.163 0.117 0.120 0.110 0.029Mg 1.885 2.480 0.823Ca 4.007 4.058 3.965 4.060 3.926 3.878 5.493 5.760 5.942 5.744 5.701 5.592 1.810 1.770 0.966Na 0.500 0.300 0.019K 0.225 0.110Cl 0.050 0.010S 16.031 16.048 15.977 16.030 15.996 15.966 16.000 16.000 16.000 16.000 16.000 16.000 15.685 15.250 4.045

Analysts: A.J.R. Kent, M.A. Pepper, A.J. Chubb. See Appendix A for sample information. All Fe is assumed to be trivalent in epidote, whereas the proportions of Fe3q andFe2q in garnet were calculated assuming stoichiometry. Note: blanks signify values below detection limit.

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Table 6Chemical composition of altered and unaltered calcsilicate rocks from the Olary Domain. Sample locations and descriptions are given in Appendix A

SB-3 SB-1 MH-2 MH-3 BC-4 BC-1 R73358 R73357 R74782 R74780 R74779 R77351 R77354 R77353Alt Unalt Alt Unalt Alt Unalt Alt Unalt Alt Alt Unalt Alt Alt Unalt

SiO 53.10 62.30 54.67 56.65 66.53 61.71 43.21 60.76 53.38 40.73 66.25 39.76 52.23 67.182

TiO 0.47 0.60 0.55 0.56 0.38 0.51 0.30 0.61 0.27 0.35 0.57 0.29 0.29 0.582

Al O 11.84 14.32 14.06 14.59 7.20 13.27 8.68 15.78 14.40 12.66 15.74 7.69 6.80 13.462 3

Fe O 6.28 1.59 5.53 4.68 10.61 0.94 16.18 2.15 7.78 11.97 0.82 18.73 25.77 1.412 3

FeO 1.29 2.34 2.39 2.19 0.53 1.46 0.94 3.72 2.52 2.12 1.14 0.90 8.55 2.50MnO 0.94 0.15 0.18 0.14 0.76 0.12 1.10 0.34 0.31 1.19 0.15 0.81 0.03 0.34MgO 1.33 1.87 2.23 2.10 0.20 4.14 0.21 1.89 2.18 1.17 1.79 0.34 0.18 1.85CaO 20.39 6.78 13.27 11.56 12.65 7.58 29.97 6.38 15.18 27.22 8.21 30.23 1.37 5.92Na O 1.30 2.73 5.27 6.13 -0.01 5.45 0.15 1.94 1.32 1.03 4.46 0.20 2.50 4.022

K O 0.30 5.55 0.61 0.56 0.01 3.87 0.35 5.26 0.65 0.27 0.31 0.02 0.19 1.632

P O 0.41 0.23 0.19 0.22 0.14 0.16 0.23 0.23 0.43 0.38 0.09 0.29 0.07 0.182 5

SO 0.17 0.04 0.04 -0.01 0.01 -0.01 -0.01 -0.01 0.02 0.05 0.02 0.01 0.03 0.013

LOI 1.36 0.23 0.38 0.25 1.04 0.29 0.38 0.88 1.27 0.81 0.53 0.35 1.05 0.60Total 99.18 98.73 99.37 99.63 100.06 99.50 101.70 99.94 99.71 99.95 100.08 99.62 99.06 99.68Nb 14 15 14 13 32 18 7 14 17 17 17 14 2 16Zr 145 178 138 149 131 189 69 148 116 91 165 87 107 197Y 47 34 25 21 124 60 15 27 13 15 31 30 8 73Sr 111 287 75 88 167 119 26 1088 159 31 198 19 153 109Rb 11 228 20 12 2 254 18 246 29 9 18 1 15 72Th 8 15 11 13 16 12 6 18 8 -3 23 4 5 16Pb 11 2 5 5 25 1 9 20 15 7 17 2 12 4As 19 6 3 4 6 1 3 8 3 5 2 5 1 2U 14 8 9 7 8 4 18 -2 4 7 12 19 10 10Ga 17 19 20 19 21 24 26 18 22 25 23 40 26 18Zn 168 18 39 37 124 27 83 139 102 162 89 25 16 39Cu 41 18 13 5 157 7 31 20 34 17 3 16 85 65Ni 22 24 29 27 14 75 20 35 6 16 12 1 2 31Cr 196 128 134 102 168 84 101 82 42 24 76 36 46 68Ce 62 79 77 80 201 189 42 114 31 16 71 29 3 180Nd 38 26 31 27 96 87 30 46 15 19 32 40 9 87La 26 62 17 13 33 23 15 171Ba 356 4394 85 68 81 2908 467 4037 187 195 156 72 88 442V 87 137 80 55 27 49 87 57 53 71 134 120 119 140Sc 14 13 10 5 14 32 17 16 10 14 13 22 6 13Cl 1361 98 497 38 32 90Co 13 15 16 14 146 24Mo -2 -2 -2 -2 -2 -2

Ž .FeOr FeOqFe O 0.17 0.60 0.30 0.32 0.05 0.61 0.05 0.63 0.24 0.15 0.58 0.05 0.25 0.642 3

Blanks: not determined. For sample information see Appendix A. Alt. — Altered calcsilicate rocks containing garnet"epidote"quartz-bearing assemblages, R77354 being richin magnetiteqquartz, Unalt. — unaltered calcsilicate rocks.


Fig. 9. Semi-quantitative summary of the chemical changes during garnet–epidote alteration of samples from Boolcoomatta, Bulloo Well,Sylvester Bore and Sampson Dam. Sample MH-2 from Mindamereeka Hill shown in Table 5 is only slightly altered and has not been usedto compile this summary. The categories are defined as follows: Astrongly depletedB and Astrongly enrichedB mean concentration in alteredrock is greater or less than five times that in unaltered rock; AdepletedB and AenrichedB mean that the element is between two and five timesdepleted or enriched in altered over unaltered rock. Note: a LOI — Loss on ignition.

though this assumption may be tenuous in variablyŽ .laminated calc-silicate rocks, large ;2 kg samples

were analysed and thus primary heterogeneity prob-lems were probably satisfied to the degree requiredto demonstrate broad changes in chemical composi-tion.

Altered calc-silicate rocks are commonly enrichedin Fe3q, Ca, Mn, U and Cu, and depleted in Fe2q,

Ž .Na, Mg, K, and Rb Fig. 9 , and several alteredrocks are also enriched in Pb, Zn, S and Cl. Alter-ation is accompanied by strong oxidation, with large

Ž .decreases in the FeOr FeOqFe O ratio evident2 3

Ž .in several locations Table 6 . Many of these changesare in accord with alteration of a clinopyroxene–feldspar-bearing assemblage to an andradite-rich gar-net–epidote assemblage where Mg, Na, K and Rbare lost during clinopyroxene and feldspar destruc-tion and Ca, Fe3q and Mn are fixed by the formationof garnet and epidote. In unaltered calc-silicates, Sand Cl are hosted in scapolite, which is destroyedduring alteration; however, daughter minerals in fluid

Žinclusions indicate that appreciable Cl and S as2y.SO are present within hypersaline fluid inclu-4

Ž .sions in the altered rocks see Section 4.5 . The


presence of these elements in the metasomatic fluidwould assist in complexing and transporting manymetals.

4.5. Fluid inclusions

Fluid inclusions are abundant in minerals frommetasomatic garnet–epidote alteration zones. For thisstudy, an investigation of the petrographic featuresand examination of the simple physical properties ofinclusions was performed to constrain the nature ofthe metasomatising fluid. In garnet–epidote-richrocks, fluid inclusions occur predominantly withingarnet and quartz. Inclusions are also observed inepidote, but are too small for physical measurements.

Ž . ŽBoth two-phase liquid–vapour and three- and. Žmulti- phase inclusions containing one or more

solid phases coexisting with liquid and vapour; e.g..Fig. 10 are present in varying proportions in sam-

Ž .Fig. 10. Fluid inclusions from altered calc-silicate rocks. APrimary halite and hematite bearing aqueous liquid–vapour inclu-

Ž .sion 1 and simple two-phase aqueous liquid–vapour inclusionŽ . Ž . Ž .2 hosted in quartz from Toraminga Hill sample KY30 . B

wŽ . Ž .xIrregular aqueous liquid–vapour inclusions 1 and 2 on thesurface of a growth zone in garnet from Mindamereeka HillŽ .sample MH-1 .

ples from all localities. Estimated liquid–vapour ra-tios typically vary between 2:1 and 9:1, although rarevapour-dominated inclusions with liquid–vapour ra-tios less than 1:2 were also observed. CO -bearing2

inclusions were not observed in any samples. Inclu-sions range in morphology from irregular to anhedraland euhedral inclusions showing partial to full devel-opment of negative crystal shapes. In addition, manygarnet-hosted inclusions have intricate semi-rectan-gular shapes that commonly define surfaces parallel

Žto zonation surfaces within large garnet crystals Fig..10B . Both primary and secondary inclusion habits

are evident, with the primary inclusions occurring asisolated inclusions whereas the secondary types are

Ž .generally small -5–10 mm and occur along healedfractures in the host mineral. Primary inclusions aretypically in the size range 1–20 mm, although forpractical reasons physical measurements were re-stricted to inclusions greater than 5 mm across.

Multi-phase inclusions may contain up to foursolid phases, although the majority contain two.Halite is always present and other daughter phases

Ž .include tiny platelets of hematite Fig. 10A and anelongate birefringent mineral with straight extinction,probably anhydrite. Several other types of daughterminerals were noted but not identified. No system-atic relationship was apparent between two-, three-and multiphase inclusions and primary and sec-ondary inclusion habits.

Homogenisation temperatures and salinities for 61fluid inclusions from two samples, BW-6A fromBulloo Well and KY-8 from Boolcoomatta, wereestimated using the methods outlined above. Inclu-

Žsions hosted in both quartz and garnet both two- and.three-phase and with primary and secondary parage-

nesis were analysed. Results are summarised in Table7 and Fig. 11. Sample BW-6A consists of coarselycrystalline garnet and quartz. Garnets are osci-

Ž .llatory-zoned from honey brown to dark browneuhedral crystals and occur within a matrix of para-genetically late quartz. Primary inclusions withingarnet are up to 20 mm across, have a range of

Ž .morphologies, from irregular e.g. Fig. 10B to thosewith well-developed negative crystal shapes. Al-though the majority of garnet-hosted inclusions in

Ž .BW-6A are two-phase liquidqvapour , occasionalŽ .three-phase liquidqvapourqsolid inclusions are

also evident. Most inclusions observed in garnet


Table 7Summary of homogenisation temperatures and estimated salinities for inclusions from samples BW-6A and KY-8

Sample Host mineral and n Homogenisation Salinity"1s

Ž . Ž .paragenesis temperature"1s 8C equivalent wt.% NaCl

Ž .BW-6A Bulloo Well Garnet, primary 6 319"14 23"1Garnet, secondary 4 318"16 23"2Garnet, two-phase 8 319"14 23"1Garnet, three-phase 2 316"23 23"1Quartz, primary 18 247"39 29"5Quartz, secondary 5 267"42 29q5Quartz, two-phase 11 254"42 25"5Quartz, three-phase 12 248"39 33"1

Ž .KY-8 Boolcoomatta Garnet, primary 7 248"17 17q2Garnet, secondary 3 255q35 17"2Garnet, two-phase 10 250q22 17"2Quartz, primary 10 191q22 18q3Quartz, secondary 8 196q34 20"3Quartz, two-phase 18 193"27 19"3

from this sample are primary, although secondaryinclusions along growth zones and healed fractures

ŽFig. 11. Histograms of measured salinity data in equivalent wt.%. Ž .NaCl from fluid inclusions in garnet and quartz. A KY-8

Ž . Ž .Boolcoomatta, B BW-6A Bulloo Well, C quartz-hosted fluidinclusions from clinopyroxene- and actinolite-matrix breccia zonesŽ .A.J.R. Kent and P.M. Ashley, unpublished data .

are also observed. Inclusions in quartz occur asanhedral and euhedral two- and three-phase types.Solid phases observed in three-phase inclusions in-clude halite and anhydrite as well as several uniden-tified species. Numerous irregular secondary inclu-

Ž .sions up to 20 mm across also occur along healedfractures in quartz.

Sample KY-8 from Boolcoomatta consists of eu-hedral zoned garnet and granular to subhedral epi-dote in a quartz matrix. Garnet contains two- andlesser three-phase primary fluid inclusions that rangefrom 10–25 mm in size and from irregular to nega-tive crystals in shape. In places, large secondaryinclusions occur along healed fractures in garnet.Inclusions hosted in quartz range from smooth-walledŽ .with some negative crystal faces to partially irregu-lar-shaped primary inclusions, 5–20 mm in size.Both two- and three-phase inclusions are apparent inquartz, with halite, hematite and possible anhydriteoccurring as daughter phases. Thin trails of smallersecondary inclusions along healed fractures are alsoapparent in quartz.

In general, the melting behaviour of frozen fluidinclusions is consistent with melting of a saline and

Žchemically complex fluid c.f. De Jong and Williams,. Ž1995 . After initial freezing requiring supercooling

.to temperatures of y808C to y508C developmentof a mottled brown appearance at temperatures downto ;y35 8C probably reflects the presence of

Ž .CaCl in inclusions Shepherd et al., 1979 . In addi-2

tion, the observed depression of the freezing point of


two-phase inclusions below the eutectic point for thepure NaCl–H O system during freezing-point deter-2

Žminations first melting temperatures were generallycloser to the ca. y528C eutectic of the CaCl –2

.NaCl–KCl–H O system, e.g. Shepherd et al., 19792

also suggests the presence of Ca in trapped fluids.Final melting temperatures, marked by the disappear-ance of clear rounded cubes of ice, ranged between;y108C to y458C and temperatures of halite dis-solution upon heating ranged between 1708C and2408C. These results indicate that the fluids involvedin metasomatism and quartz and garnet formationwere hypersaline, with ;15–35 equiv. wt.% NaClŽ . ŽFig. 11 . Homogenisation temperatures uncorrected

.for pressure of formation ranged between ;1608Cand 3408C, although for individual inclusions nocorrelation was observed between salinity and ho-mogenisation temperature. In both samples, the gar-net-hosted inclusions have slightly higher averagehomogenisation temperature than quartz-hosted in-

Ž .clusions Table 7 . This implies that the fluids pre-sent during garnet deposition were at slightly highertemperatures than those present during quartz forma-tion, consistent with the mineral textures showing

Žthat quartz postdated garnet deposition similar rela-.tions are shown for sample BW-3 in Fig. 4B . Both

quartz and garnet-hosted inclusions from sampleKY-8 have similar estimated salinities, whereasquartz-hosted inclusions from BW-6A have slightlyhigher salinities than garnet-hosted. Two- and three-phase inclusions in garnet and quartz from BW-6AŽthe only sample for which measurements were made

.on both two- and three-phase inclusions also showsimilar range of homogenisation temperatures, al-though salinities in quartz-hosted three-phase inclu-sions are slightly higher than in two-phase in BW-6AŽ .Table 7 . In both samples, both primary and sec-ondary fluid inclusions show similar ranges of ho-

Žmogenisation temperature and estimated salinity Ta-.ble 7 . This suggests that metasomatic fluids of the

same approximate composition as those responsiblefor metasomatism continued to circulate after min-eral growth.

Given the uncertainties in the ambient pressureduring metasomatism and fluid inclusion trapping,and in the overall bulk composition of the metaso-matic fluids, we have made no rigorous attempt touse the homogenisation temperatures to constrain the

temperature of metasomatism. Peak metamorphicpressures in the Olary Domain are estimated at 4–6

Ž .kbar Clarke et al., 1986; Flint and Parker, 1993 .This corresponds to a temperature correction of ;

200–3008C for homogenisation temperatures of flu-Ž .ids in the H O–NaCl system Potter, 1977 . With2

such a correction applied, the estimated fluid trap-ping temperatures range between 4008C and 6508C,and this is broadly consistent with the interpretationthat metasomatism occurred slightly after the peak of

Žmetamorphism which occurred at ;550–6508C;.Flint and Parker, 1993 .

5. Discussion

5.1. Formation of garnet–epidote-rich alterationzones

Garnet–epidote-rich zones exhibit features thatare characteristic of a metasomatic origin. Theseinclude hydrothermal textures such as vein and re-placement textures, open-space fillings and breccias,and an abundance of fluid inclusions. Open spacecavities and breccias also imply that fluid pressuresmay have been locally higher than lithostatic pres-sure, and thus metasomatic alteration zones probablyalso represent zones of focussed fluid flow.

The nature of the fluid responsible for metasoma-tism can be deduced from fluid inclusion properties,and the mineralogical and chemical changes whichaccompanied alteration. Fluid inclusions show that

Žthe metasomatic fluids were hypersaline ;15–35.equiv. wt.% NaCl , and the freezing behaviour of

inclusions and the array of daughter minerals presentŽhalite, hematite, anhydrite and several unidentified

.phases indicate that the fluids were chemically com-3q 2y Žplex, and contained Na, Ca, Fe , Cl and SO and4.probably several other species . The changes in the

Ž 3q 2q.bulk chemistry increased Fe rFe , the presenceof hematite as a daughter phase in fluid inclusions,

3q Žand formation of Fe -bearing minerals andradite-.rich garnet and epidote in metasomatically altered

rocks also suggest that the fluid was substantiallymore oxidised than the pre-existing calc-silicateŽmetasomatism probably occurred at oxygen fugaci-ties equal to or greater than those of the hematite–

.magnetite buffer . Comparisons of the bulk chemicalcomposition of altered and unaltered calc-silicates


show that metasomatic fluid were capable of mobil-Žising many different elements again consistent with

.a chemically complex fluid , and metasomatism wasaccompanied by substantial changes in chemical

Ž .compositions Fig. 9 . In general altered calc-silicaterocks are enriched in Fe3q, Ca, Mn, U and Cu, anddepleted in Fe2q, Na, Mg, K, and Rb and severalaltered rocks are also enriched in Pb, Zn, S and Cl.

It is also possible to estimate the general condi-tions under which metasomatism occurred. The pres-ence of actinolite within garnet–epidote alterationzones and the retrogressive replacement of clinopy-roxene by actinolite associated with formation ofgarnet–epidote zones provides some constraint onthe temperature–pressure conditions of metasoma-tism. Although the stability fields of garnet, epidoteand actinolite are not well known under oxidising

Ž .conditions Liou, 1972, 1974 , at lower oxygen fu-Ž .gacities fayalite–magnetite–quartz buffer the

breakdown of clinopyroxene to actinolite, at pres-sures )2–3 kbar, occurs with decreasing tempera-

Ž .ture at ;500–6008C Gilbert et al., 1982 . Wesuggest that these are the approximate conditions ofmetasomatism within garnet–epidote alterationzones. This estimate is in broad agreement with boththe range of corrected homogenisation temperatures

Ž .from fluid inclusions 400–6508C and observationsthat suggest metasomatism occurring after peak

Žmetamorphic conditions 550–6508C, 4–6 kbar; Flint.and Parker, 1993 .

Field and petrographic observations and Sm–Nddating allow us to place the formation of garnet–epi-dote-rich metasomatic zones within the known se-quence of geological development of the Olary Do-main. Sm–Nd dating suggests that the majority ofgarnet–epidote-rich alteration zones formed at 1575

Ž"26 Ma although metasomatism at Bulloo Well.may have occurred slightly later than this . This is

consistent with observations which show that gar-net–epidote-rich metasomatic alteration occurredduring the retrograde phases of amphibolite-graderegional metamorphism and after the deformation ofthe Olary sequence by the OD and OD events.1 2

Garnet–epidote-rich metasomatic alteration zonesalso postdate formation of clinopyroxene-matrixbreccias, but predate dykes associated with intrusionof the regional suite of S-type granitoids at 1600"20Ma.

5.2. Widespread metasomatism within the Olary Do-main

Garnet–epidote alteration zones represent one ex-ample of metasomatic alteration in rocks of theOlary Domain; however, as already described manyother styles of metasomatic alteration are also evi-

Ž .dent Table 1 . Although the detailed nature of meta-somatic alteration varies between different host rocks,there are many consistencies, summarised below,between the chemical and mineralogical changes that

Ž .are observed in different host rocks Table 1 , thetiming of metasomatism, and the inferred composi-tion of the metasomatic fluids.

Collective data indicate that the majority of meta-somatism in the Olary Domain occurred after peakmetamorphism and development of the fabrics re-lated to the OD and OD deformational events and1 2

prior to the intrusion of S-type granitoids, althoughfurther studies are required to firmly establish thetiming of formation of each style of metasomaticalteration. For the examples listed in Table 1, meta-somatic assemblages overprint metamorphic miner-als and textures and in several cases alteration phe-nomenon appears to be controlled by pre-existing

ŽOD structures Yang and Ashley, 1994; Ashley et2.al., 1998a . Crosscutting field relations at Cathedral

Rock and Toraminga Hill indicate that formation ofclinopyroxene- and actinolite-matrix breccias oc-curred prior to intrusion of adjacent S-type granitoids

Ž .and associated pegmatite dykes e.g. Fig. 3B . InŽaddition, A- and I-type granitoids intruded at ;

.1710–1700 and ;1640–1630 Ma, respectively andassociated rocks are often extensively affected by

Žmetasomatism. The later S-type intrusives 1600"20.Ma are only altered in localised zones where dykes

crosscut previously metasomatised calc-silicate rocks,suggesting that the majority of regional metasoma-tism predates intrusion of S-type granitoids. Lo-calised episodes of fluid flow and metasomatismprobably continued for several hundred million yearsŽ .Bierlein et al., 1995; Lu et al., 1996 ; however, thevast majority of metasomatic activity appears to haveoccurred directly after peak regional metamorphism.

In general, the formation of metasomatic alter-ation zones in the Olary Domain involved widespreaddevelopment of Fe-, Ca- and Na-bearing mineralassemblages, involving formation of one or more of


the following minerals: andradite-rich garnet, epi-dote, hematite, magnetite, aegerine-bearing clinopy-roxene, actinolite and albite. These mineral assem-

Žblages are typically more oxidised i.e. alterationinvolves an increase in the Fe3qrFe2q ratios andalteration assemblages contain an array of Fe3q-rich

.minerals than their un-metasomatised equivalents.Metasomatic clinopyroxenes from clinopyroxene-matrix breccias and metasomatically altered iron-stones also have higher Fe3qrFe2q ratios and Na O2

contents than clinopyroxenes from laminated calc-Ž .silicate rocks Fig. 5 , and clinopyroxene- and acti-

nolite-matrix breccias contain fluid inclusions thathave hematite as a daughter phase.

The chemical changes associated with differentstyles of metasomatic alteration typically involveaddition of Fe, Ca and Na and loss of K, Rb and MgŽ .Table 1 and metasomatic fluids appear to havebeen highly saline and chemically complex. Hyper-saline fluid inclusions with an array of daughtermineral phases, similar to those observed in garnet–epidote-rich alteration zones, have been documentedfrom a number of different metasomatic rocks, in-

Ž .cluding intense zones of albite –quartz–actinolitealteration in quartzofeldspathic rocks, clinopyroxene-

Ž .and actinolite-matrix breccias e.g. Fig. 11 and fromŽepigenetic ironstones Yang and Ashley, 1994; Ash-

ley et al., 1998b; A.J.R. Kent and P.M. Ashley,.unpublished data .

The consistencies in the timing, alteration styleand metasomatic fluid associated with different stylesof metasomatic alteration suggests that the OlaryDomain experienced a major episode of metasomaticalteration, involving the action of saline, oxidisedand chemically complex fluids, during the retrogradestages of amphibolite-grade regional metamorphism.However, evidence suggests that metasomatic activ-ity was not simply restricted to a single fluid pulse,but probably occurred during a succession of fluidflow events as the terrane cooled from peak meta-morphic temperatures. The best evidence for this arethe crosscutting and petrological relationships ob-served between the different styles of metasomaticalteration that occur in calc-silicate host rocks.

Metasomatic calc-silicate-matrix breccias in thesouthern and central Olary Domain are dominated byclinopyroxene. Although clinopyroxene-matrix brec-cias clearly postdate the formation of peak metamor-

phic assemblages in host calc-silicate rock, the pres-ence of clinopyroxene as the dominant matrix min-eral implies that it formed at pressure–temperatureconditions close to those of the primary metamorphicassemblages. Primary actinolite-matrix brecciaswithin the northern portion of the Olary Domainformed in areas that experienced lower peak meta-

Ž .morphic temperatures Fig. 1 . At many locations,however, clinopyroxene-matrix breccias are variably

Ž .retrogressed to actinolite Fig. 4A . Given that thereaction of clinopyroxene to form actinolite occurs

Ž .with decreasing temperatures Gilbert et al., 1982actinolite retrogression of clinopyroxene-matrixbreccias appears to represent continued metasomaticactivity at lower temperatures. In addition, garnet–epidote-rich alteration zones are observed to over-print clinopyroxene-matrix breccias at Mindame-reeka Hill, and garnet–epidote alteration zones in

Žcalc-silicate rocks contain accessory actinolite andare associated with retrogression of metamorphic

.clinopyroxene to actinolite . This suggests that thisstyle of alteration also represents a later episode offluid movement that occurred at slightly lower ambi-ent temperatures than formation of the clinopyrox-ene-matrix breccias.

We therefore suggest that the Olary Domain pro-vides an excellent example of a terrane that hasexperienced widespread metasomatic activity duringthe retrograde stages of a major regional metamor-phic event. Metasomatism resulted in significantchanges to the mineralogical and chemical constitu-tion of the terrane and continued for some time asterrane cooled following metamorphism.

5.3. Source of metasomatising fluids

Two primary possibilities exist for the origin ofthe hypersaline and oxidised fluids responsible for

Ž .metasomatic alteration: i fluids may have derivedfrom the crystallisation of one or more types of

Ž .granitoids; or ii fluids could have been derivedfrom within the Olary Domain sequence, or fromsimilar crustal rocks located at deeper structural lev-els, by metamorphic devolatilisation reactions.

Field relations and radiometric dating in the OlaryDomain show that a local temporal association be-tween the emplacement of S-type granitoids andassociated pegmatites and some occurrences of meta-


somatically altered rocks. This association could alsosuggest that metasomatic fluids are also derived fromcrystallising S-type plutons. However, the followingevidence argues against a direct contribution tometasomatic fluids from crystallising regional S-typegranites.

Ž .i Metasomatic fluids in the alteration zones areŽ .highly oxidised G hematite–magnetite buffer ,

whereas the regional S-type granitoids in the Olarydistrict are relatively reduced and ilmenite-bearing.

Ž .ii Although there is a temporal association be-tween some metasomatic rocks and rocks related tothe S-type granitoids, there is no consistent spatialrelationship between S-type granitoids and metaso-

Ž .matised rocks in the Olary district Fig. 1 . Manymetasomatic rocks occur away from known outcropsof S-type granitoids and many large areas of grani-toid and adjacent rocks are devoid of metasomaticalteration.

Ž .iii S-type granitoids do not show evidence ofŽstrong sub-solidus fluid accumulation e.g. miarolitic

cavities, fracture-controlled or pervasive alteration ofplutons or adjacent country rocks, potassic alteration,

.or greisen development .Ž .iv Preliminary O-isotope studies of metasomatic

Ž .rocks see below show little evidence for the contri-bution of magmatic fluids to the metasomatisingfluids.

We also believe that metasomatic fluids in theOlary Domain are unlikely to be related to I-typeintrusive rocks. Regional-scale alteration zones inother Australian Proterozoic regions, such as the Mt.Isa eastern succession in northwest Queensland and

Ž .the Gawler Craton to the west of the Olary Domainhave been suggested to, at least partially, be related

Žto fluids released by I-type granitoids De Jong andWilliams, 1995; Oliver, 1995; Conor, 1998; David-

.son, 1998; Williams, 1998 . However, in the OlaryDomain, the only known I-type granitoids were em-

Žplaced at ;1640–1630 Ma some 70–80 Ma prior.to metasomatism; Ashley et al., 1998a ; these intru-

sions are clearly pre-alteration, are spatially re-stricted and have no relation to the regional-scaleoccurrence of altered rocks. It may be inferred that

Ž .later e.g. Mesoproterozoic I-type granitoids couldoccur in the Olary Domain, at depth, or underyounger cover sequences, and have a relationship toalteration zones and to Cu–Au mineralisation. To

date, however, magnetic, gravity and explorationdrilling results have failed to confirm the hypothesis.

We suggest that the saline and oxidised metaso-matic fluids responsible for garnet–epidote alterationzones and other metasomatic alteration types weremost likely derived from metamorphic devolatilisa-tion of crustal rocks, dominated by metasediments inthe Olary Domain sequence. Some of these rocksmay have originally contained oxidised sequencesŽ .e.g. red beds and evaporites, and are now manifestas hematite- and magnetite-bearing laminated al-bitites, calcalbitites and certain calc-silicate-rich

Žrocks of the Willyama Supergroup Cook and Ash-.ley, 1992 . During peak metamorphism, the break-

down of hydrous and volatile-bearing phases mayhave released large volumes of fluids and devolatili-sation may have also been facilitated by the thermaleffects of intrusion of S-type granitoids. Preliminaryresults from oxygen isotopic studies on regional and

Žlocal scale Na–Fe–Ca alteration zones including.garnet–epidote alteration zones are consistent with

Žfluids being derived from crustal rocks R. Skirrow.and P.M. Ashley, unpublished data . These may be

metamorphic waters that equilibrated with the OlaryDomain sequence; however, direct input from evap-oritic brines or magmatic fluids were evidently mini-mal. The post-metamorphic timing of metasomaticalteration phenomenon may indicate that peak meta-morphic conditions were attained at structurallydeeper levels at slightly later times than they wereattained in rocks currently exposed at the surface.

We also note that, although our observations ruleout the direct contribution of metasomatic fluidsfrom crystallising granitoids, it is harder at present toconstrain the contribution of metasomatic fluids re-leased by crystallising plutons at deep structurallevels and subsequently heavily modified by interac-tion with crustal rocks. Studies of porphyry copper

Ž .deposits e.g. Cline and Bodnar, 1991 have shownŽ .that at high pressures )2 kbar fluids evolved from

Žcrystallising granitoids can be highly saline up to 60.wt.% NaCl . Interaction of such fluids with crustal

sequences at depth could substantially modify theoxygen fugacity and O-isotope signature of thesefluids and render them difficult to discern fromfluids related to devolatilisation of crustal rocks. Inaddition, control of metasomatic circulation by pre-existing crustal structures could supplant spatial as-


sociations between deep plutons and the sites ofmetasomatic alteration.

5.4. Similarities to other Proterozoic regions andimplications for ore deposition

The widespread metasomatism involving salineoxidised fluids in the Olary Domain is similar to thatobserved in other Proterozoic regions in AustraliaŽe.g. Davidson, 1994, 1998; Williams, 1994, 1998;De Jong and Williams, 1995; Oliver, 1995; Oliver et

.al., 1998; Conor, 1998 and elsewhere in the worldŽe.g. Kalsbeek, 1992; Barton and Johnson, 1996;

. Ž .Frietsch et al., 1997 . Barton and Johnson 1996have proposed that a worldwide link exists between

ŽProterozoic and Phanerozoic Fe-rich REE–Cu–Au–.U-bearing hydrothermal deposits and evaporitic

source rocks, whereby devolatilisation associatedwith igneous intrusions forms the oxidised S-poorbrines responsible for the observed hydrothermalmineralisation.

In the Mt. Isa Block Eastern Succession in north-western Queensland, examples of widespread Na,Na–Ca and Fe-metasomatism related to saline andchemically complex fluids are recorded, both region-

Žally e.g. Oliver and Wall, 1987; Oliver, 1995;.Williams, 1998 and on more localised scales

ŽDavidson, 1994, 1998; Williams, 1994; De Jong.and Williams, 1995 . It has been proposed that cer-

tain types of epigenetic Cu–Au and U–REE mineral-isation in this region are spatially and genetically

Žrelated to alteration e.g. Davidson and Large, 1994;Williams, 1994, 1998; Oliver, 1995; Adshead, 1995;

.Davidson, 1998 . Although differences are evident inthe style and nature of alteration in the Mt Isa BlockEastern Succession and the Olary Domain, these areprobably due to localised factors, such as host rocks,fluid histories, and P–T conditions of metasoma-tism. A common theme is the action of hypersalineŽ .generally Na–Ca–K–Fe-bearing and locally oxi-dised fluids. It is also possible that the saline fluidsresponsible for the Mt. Isa Block Eastern Succession

ŽNa–Ca alteration and locally associated Cu–Au.mineralisation were at least partly evolved from a

Žformer evaporitic-bearing sequence e.g. Oliver andWall, 1987; Oliver, 1995; De Jong and Williams,

.1995 , although recent models infer that magmatic

fluids from evolved I-type granitoids were also in-Ž .volved e.g. Pollard et al., 1998; Williams, 1998 .

Metasomatism within the Olary Domain also hasimportant implications for the metallogenic status ofthis region. We have shown that the formation ofgarnet–epidote alteration zones and other metaso-matic rocks in the Olary Domain involved oxidisedsaline fluids. These fluids would have been capable

Žof transporting significant quantities of metals e.g..Fe, Cu, Au, Mo, Zn, Pb, Ag, REE, U and Mn in

solution at the temperatures implied from fluid inclu-Žsion results e.g. Hemley et al., 1992; Seward and.Barnes, 1997 . Although garnet–epidote-rich rocks

from metasomatic alteration zones contain only mod-Ž .est enrichments of Cu, Zn, Pb and U Fig. 9 , there

may be a spatial and genetic link between theseŽ .rocks and sites where significant i.e. ore grade

metal deposition could occur. The strongly oxidisednature of the garnet–epidote rocks may not be con-ducive to sulfide deposition, but in rock types inwhich metasomatism would involve major redox

Žchanges e.g. graphitic pelite, psammopelite, calc-.silicate-bearing pelite , or in reactive rock types

Ž .marble, iron-formations and mafic rocks substantialmetal deposition could occur. In the Olary Domain,several historic mineral prospects, as well as newdiscoveries, display a characteristic Cu–Au–Mo as-

Žsociation in places with anomalous Co, Zn, As, U,. ŽBa, REE Ashley et al., 1998a; Skirrow and Ashley,

.1999 . These deposits all occur within the abovelithological settings, in places mediated by fracturesystems, but with no substantiated genetically associ-ated granitoids. As the solubility of Zn and Pbremains high at temperatures )3008C, in saline

Žfluids with high ClrS and low reduced S cf. Hem-.ley et al., 1992 , it is probable that mineralisation

associated with metasomatic fluids may be limited toFe–Cu sulfides"molybdenite"gold.

6. Conclusions

The Proterozoic Olary Domain is an excellentexample of a terrane that has been significantlyaltered by metasomatic mass-transfer processes asso-ciated with regional metamorphism. Metasomaticallyaltered rocks in the Olary Domain are ubiquitousand include garnet–epidote-rich alteration zones,


clinopyroxene- and actinolite-matrix breccias, re-placement ironstones and albite-rich alteration zonesin quartzofeldspathic metasediments and intrusiverocks. Metasomatism is typically associated withformation of Ca, Na andror Fe-bearing oxidisedmineral assemblages.

Detailed study of garnet–epidote-rich alterationzones in calc-silicate host rocks, one common mani-festation of intense metasomatic alteration, providesdetailed information on the nature and timing ofmetasomatism, and the composition of the responsi-ble fluids. Metasomatism occurred at temperaturesbetween ;4008C and 6508C, and involved loss ofNa, Mg, Rb and Fe2q, gain of Ca, Mn, Cu and Fe3q

and mild enrichment of Pb, Zn and U. Fluid inclu-sions show that the hydrothermal fluids responsiblefor the formation of garnet–epidote-rich assemblages

Ž 3qwere chemically complex containing Na, Ca, Fe ,2y. ŽCl, and SO , hypersaline and oxidised at or above4

.the hematite–magnetite buffer . Sm–Nd isotopicanalyses show that garnet–epidote-rich alterationzones formed at 1575"26 Ma, consistent with fieldand petrographic observations that suggest that meta-somatism occurred prior to the latter stages of re-gional-scale intrusion of S-type granites at 1600"20Ma.

Evidence suggests that the majority of metaso-matic alteration throughout the Olary Domain wasbroadly contemporaneous and involved the action ofoxidised hypersaline fluids. We suggest thatwidespread episodes of fluid flow and metasomaticalteration affected the Olary Domain during the ret-rograde phases of a major regional metamorphicevent. The fluids responsible for metasomatism

within the Olary Domain probably derived fromdevolatilisation of a rift-related volcano-sedimentarysequence, perhaps containing oxidised and evaporiticsource rocks at deeper structural levels, during re-gional metamorphism and deformation. Althoughmetasomatic rocks are temporally associated withS-type granitoid intrusive rocks, there is no evidencethat the metasomatic fluids have been directly sourcedfrom granites.

Acknowledgements

We wish to acknowledge funding for this workfrom the Australian Research Council, Primary In-dustries and Resources South Australia and a consor-tium of Australian mineral exploration companieswho have supported the Olary Mapping Project. Wehave also drawn on work by the following honoursstudents from the Universities of New England andMelbourne: James Anderson, Andrew Chubb, WillEykamp, Michael Fechner, Mark Kent, Maree Laf-fan, Mark Pepper, Gary Rolfe and Jane Westaway.Assistance with analytical work was provided byRick Porter, Peter Garlick, John Bedford and GaelWatson. Discussions with colleagues Frank Bierlein,Colin Conor, Nick Cook, Dave Lawie, Bernd Lotter-moser, Ian Plimer, Roger Skirrow and Kai Yang alsocontributed to this work. In addition, AJRK wouldlike to especially thank Professor M. Tattersall andthe staff of RPAH, Sydney. Reviews by J.L.R. Touretand P.J. Williams improved this manuscript consid-erably.

Appendix A. Description and locations of rocksamples used for this study

AMG — Australian Map Grid coordinates

Location Sample Description

Bulloo Well BW-1 Altered calc-silicate: Massivew x Ž .AMG 442900E 6498450N garnet– quartz–epidote .

BW-2 Altered calc-silicate: Garnet–epidoteŽ .actinolite–quartz–albite .Alteration pseudomorphs folded laminaein original calc-silicate.


BW-6A Altered calc-silicate: MassiveŽ .garnet–quartz epidote .

R73358 Altered calc-silicate: Massive garnetŽ .–quartz–epidote–hornblende .

R73357 Unaltered calc-silicate: Laminatedclinopyroxene–K-feldspar–albite–quartz–

Ž .hornblende–epidote –titanite .Sylvester Bore SB-1 Unaltered calc-silicate: Laminatedw xAMG 427500E 6437500N clinopyroxene–quartz–K-feldspar–albite

Ž .–actinolite–titanite with minoractinolite alteration of clinopyroxene.

SB-3 Altered calc-silicate: Massive garnetŽ .–epidote–quartz–albite .

Mindamereeka Hill MH-1 Altered calc-silicate: Laminated clinopyroxene–quartz–w x Ž .AMG 396100E, 6453200N albite –scapolite–K-feldspar–titanite

grading to bleached albite–quartz-richŽ .rock and massive coarse garnet up to 3 cm .

MH-2 Partially altered calc-silicate: Laminatedclinopyroxene–quartz–albiteŽ .–scapolite–K-feldspar–titanite withpatchy alteration of clinopyroxene to actinoliteand extensive replacement along laminations andfractures by garnet–epidote–quartz.

MH-3 Relatively unaltered calc-silicate: Laminatedclinopyroxene–albite quartzŽ .–actinolite–K-feldspar–titanitewith minor alteration to garnet–epidote–quartz along fractures.

Boolcoomatta BC-1 Relatively unaltered calc-silicate: Laminatedw xAMG 455200E 6462800N clinopyroxene–albite–K-feldspar–quartz

Ž .–titanite with small zones of epidotealteration.

BC-4 Altered calc-silicate: Massive garnet–epidote–Ž .quartz –hematite with relicts clinopyroxene

partly replaced by actinolite.BC-8 Brecciated and altered calc-silicate: Bleached

quartz–albite fragments in a epidoteŽ .–actinolite–garnet matrix.

KY-8 Altered calc-silicate: Garnet–quartzŽ .–epidote–actinolite–clinopyroxene .Relict clinopyroxene largely altered toactinolite.

Sampson Dam R74779 Unaltered calc-silicate: Laminated quartz–w xAMG 445680E 6450640N clinopyroxene–albite–actinolite–epidote

Ž .–titaniteR74780 Altered calc-silicate: Massive garnet

Ž .–quartz–epidote–hornblende–albite


R74782 Altered calc-silicate: MassiveŽ .garnet–epidote–quartz –albite

White Dam North R77351 Altered calc-silicate: Massive coarse grainedw x Ž .AMG 453400E 6451520N garnet –epidote–quartz

R77354 Altered calc-silicate: Massive coarse grainedŽ .magnetite–quartz –albite rock enclosed

in garnet-rich alteration zone.R77353 Relatively unaltered calc-silicate: Laminated

clinopyroxene–quartz–albiteŽ .–K-feldspar–titanite with minoralteration to actinolite and epidote.

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