Metasomatic Alteration Associated with Regional Metamorphism: an Example from the Willyama Supergroup, South Australia

Lithos 54Ž. 2000 33±62 www.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, Australia b 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 Australia and western New South Wales, Australia, is an excellent example of geological region that has been significantly altered by metasomatic mass-transfer processes associated with regional metamorphism. Examples of metasomatically altered rocks in the Olary Domain are ubiquitous and include garnet±epidote-rich alteration zones, clinopyroxene- and actinolite-matrix breccias, 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 of oxidised mineral assemblages containing one or more of the following: quartz, albite, actinolite±hornblende, andradite-rich garnet, epidote, magnetite, hematite and aegerine-bearing clinopyroxene. Detailed study of one widespread style of metasomatic alteration, garnet±epidote-rich alteration zones in calc-silicate host rocks, provides detailed information on the timing of metasomatism, the conditions under which alteration occurred, and the nature 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 with formation at locally high fluid pressures and fluidrrock ratios. Metasomatism of the host calc-silicate rocks occurred at temperatures between ;4008C and 6508C, and involved loss of Na, Mg, Rb and Fe2q, gain of Ca, Mn, Cu and Fe3q and mild 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, were 3q 2y hypersaline, oxidised, and chemically complex, containing Na, Ca, Fe , Cl, and SO4 . Sm±Nd geochronology indicates that the majority of garnet±epidote alteration occurred at 1575"26 Ma, consistent with field and petrographic observations that suggest that metasomatism occurred during the retrograde stages of a major amphibolite-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, éster Volgade 10, 1350 Copenhagen K, Denmark. Fax: q45-38-14-2667. E-mail address: [email protected]Ž. A.J.R. Kent . 1 Fax: q61-2-6773-3300. 2 Fax: q61-2-6773-3300. 3 Fax: q61-2-6249-4835.

1600"20 Ma. The fluids responsible for metasomatism within the Olary Domain are inferred to have been derived from devolatilisation of a rift-related volcano-sedimentary sequence, perhaps containing oxidised and evaporitic source rocks at deeper 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 examples of regional and local scale metasomatic alteration phenomenon are numerous and widespread Rocks that have experienced metamorphism com- Že.g. Cook and Ashley, 1992; Ashley et al. 1998a,b; prise a large proportion of the continental regions Skirrow and Ashley, 1999. . The metasomatic fea- and thus an understanding of the changes that are tures of rocks from the Olary Domain also have associated with metamorphic activity is critical for strong analogies with alteration phenomena that have gauging the chemical and mineralogical evolution of been documented in other Proterozoic terranesŽ both the continental crust. Traditional studies of metamor- elsewhere in Australia and in other parts of the phic phenomena have emphasised the isochemical world. , some of which are associated with Cu, Au, mineralogical changes caused by metamorphic re- Fe and U mineral depositsŽ Kalsbeek, 1992; Frietsch equilibration under differing pressure and tempera- et al., 1997; Oliver et al., 1998; Williams, 1998. . ture regimes. However, metasomatic mass-transfer of In this study, the major styles of metasomatic chemical components is increasingly recognised as alteration in the Olary Domain are documented and an important process accompanying regional meta- described; to do this we both present new informa- morphismŽ e.g., Chinner, 1967; Yardley and Bal- tion and review results of earlier studies in the tatzis, 1985; Ferry, 1992; Ague, 1994a,b, 1997; region. Further, in order to constrain the timing and Oliver et al., 1998. . Metasomatic redistribution of nature of metasomatic alteration, and to investigate volatile and fluid-mobile non-volatile chemical com- the composition of the responsible fluids, a detailed ponents during the prograde and retrograde phases of study has been undertaken on a specific type of regional metamorphism can profoundly influence the metasomatic rock, viz. skarn-like garnet±epidote- final chemical and mineralogical status of a meta- bearing alteration zones within laminated calc-sili- morphosed terraneŽ. Ague, 1997 . Such changes must cate rocks. This style of metasomatic alteration, be quantified in order to understand the effects that which occurs throughout the Olary DomainŽ. Fig. 1 , metamorphic and related metasomatic processes can is a manifestation of intense mineralogical and chem- produce on rock masses. ical change resulting from focused fluid passage, and In this study we have investigated the role of therefore provides an opportunity to investigate the metasomatism in the formation and evolution of nature, origin and effects of the metasomatising flu- rocks from the Proterozoic Willyama Supergroup in ids. In addition, as these rocks are suitable for Sm±Nd the Olary Domain of eastern South Australia. The isotopic dating studies, they allow important con- Olary Domain, part of the Curnamona Province, a straints to be placed on the timing of metasomatic major Proterozoic terrane located within eastern activity. South Australia and western New South Wales, Aus- Directly after the attainment of peak regional traliaŽ. Fig. 1 , provides an excellent example of a metamorphic conditions, the Olary Domain experi- geological terrane that has been significantly effected enced regionally extensive episodes of the passage of by metasomatic processesŽ e.g. Cook and Ashley, hot, saline and oxidised aqueous fluids. The fluids 1992; Ashley et al., 1998a,b. . Within the Olary responsible for metasomatic alteration were probably Domain, the chemical and mineralogical composi- derived from metamorphic devolatilisation of crustal tions of rocks within the Willyama Supergroup have rocks, largely a sedimentaryŽ. ±felsic volcanic se- been strongly altered by metasomatic processes, and 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 and clinopyroxene- and actinolite-matrix breccias. Bold dashed line represents the location of the boundary between metamorphic zones IIA Ž.Ž.Ž.andalusite±chloritoid and IIB andalusite±sillimanite of Clarke et al. 1987 . The approximate position of 1600"20 Ma granitoids is also shown. sion of granitoid rocks may have been an important tions, Sm±Nd isotopic composition and fluid inclu- factor in promoting devolatilisation reactions in the sions are given in Appendix A. surrounding wallrocks, there is no clear evidence for the direct contribution of water derived from crys- 2.1. Rock and mineral analysis tallising granitoids to the metasomatising fluids. Samples of altered and unaltered calc-silicate rocks were analysed for major and trace elements by 2. Analytical methods X-ray fluorescence at the University of Melbourne and University of New England, Armidale, Aus- Descriptions and locations for all samples anal- tralia, using Siemens SRS-300 instruments. Mineral ysed for whole rock and mineral chemical composi- compositions were measured using a JEOL 5800 36 A.J.R. Kent et al.rLithos 54() 2000 33±62 scanning electron microscope run in EDS mode at a epidote, garnet, actinolite and quartz. , and not older beam current of 25 nA at the University of New metamorphic mineralsŽ see discussion below on the England and using a range of natural standards for effect of this on isochron calculations. . Samples for calibration of X-ray intensities. Mineral composi- analysis were weighed into dissolution vessels, spiked tions, for phases containing Fe2q and Fe3q were with a mixed 146 Ndr150Sm solution and dissolved calculated assuming stoichiometry. using HF±HNO3 ±HCl acid digestion. Sm and Nd were separated and purified using 3g cation ex- change and HDEHP-teflon columns using the proce- 2.2. Fluid inclusions dure outlined in Bennett et al.Ž. 1993 . Samples were loaded onto the Ta side of a double Re±Ta filament Fluid inclusion heating and cooling determina- and analysed using a FinneganrMAT 261 multicol- tions were performed using a modified USGS heat- lector mass spectrometer in static mode at the Re- ing±cooling stage. Repetition of measurements indi- search School of Earth Sciences, Australian National cated that individual determinations were repro- University. ducible at the 1±28C level. For two-phase inclusions, salinities were estimated via the depression of the freezing point and using the calibration of Bodnar 3. Geological setting Ž.1992 . For halite-bearing three-phase inclusions, salinity estimates were derived from the melting The Olary Domain constitutes one of the inliers of point of halite and the phase relations outlined by the Palaeoproterozoic Willyama Supergroup that oc- Sourirajan and KennedyŽ. 1969 . These calculations cur in northeastern South Australia and western New are for the pure NaCl±H2 O system and given the South Wales, AustraliaŽ. Fig. 1 . The geology of the chemically complex nature of the fluids responsible Olary Domain has been summarised by Clarke et al. for the formation of garnet±epidote metasomatic zonesŽ. see discussion below , can only be considered estimates. This is especially relevant for freezing point depression measurements, where in several examples the measured melting points of inclusions were below the eutectic point of the NaCl±H2 O system, indicating that other cationsŽ. e.g. Ca must be presentŽ. Roedder, 1984 .

2.3. Sm±Nd isotopic analysis

Mineral separates from garnet and epidote-bearing rocks were prepared using standard heavy liquid separation techniques and were purified by magnetic separation and hand-picking. Most samples were prepared to better than an estimated 98% purity, although some mineral separates contained inclusions and composite grains; in these purity was approxi- mately 95±98%. In order to avoid the incorporation of olderŽ. pre-metasomatism REE-rich minerals in mineral separates, we selected the most intensely altered and coarsest-grainsize samples for mineral separation. In samples where mineral inclusions oc- Fig. 2. Olary Domain sequenceŽ modified from Ashley et al., cur, they consist of other metasomatic mineralsŽ e.g. 1996. . Abif B denotes bonded iron formation. A.J.R. Kent et al.rLithos 54() 2000 33±62 37

Ž.1986, 1987 , Cook and Ashley Ž. 1992 , Flint and been subject to at least five deformation and meta- ParkerŽ. 1993 , Robertson et al. Ž. 1998 and Ashley et morphic eventsŽ Clarke et al., 1986, 1987; Flint and al.Ž. 1998a . The Olary Domain sequence Ž Fig. 2 .Parker, 1993. . Temporal relationships between intru- displays broad regional correlations with the sive, metamorphic and deformational episodes have Willyama Supergroup in the adjacent Broken Hill been investigated by field studies and by zircon Block, although there are differences in detailŽ Cook U±Pb and muscovite 40Ar±39Ar geochronology and Ashley, 1992; Preiss, 1999. . The Willyama Su- ŽClarke et al., 1986, 1987; Flint and Parker, 1993; pergroup has been interpreted to represent a failed Cook et al., 1994; Bierlein et al., 1995; Lu et al., Palaeoproterozoic riftŽ. Willis et al., 1983 and the 1996; Ashley et al., 1996; 1998a; Page et al., 1998. , Olary Domain is considered to represent a marginal and the following summary is taken from these portion of this rift, possibly involving a continental studies. Note that previous interpretationsŽ e.g. Flint lacustrine and sabkha setting grading upwards into a and Parker, 1993. have ascribed the first three defor- marine environmentŽ. Cook and Ashley, 1992 . mation events in the Olary DomainŽ. OD13 ±OD to The lower part of the Olary Domain sequence is the Olarian Orogeny, a major episode of deformation occupied by the Quartzofeldspathic Suite, compris- and metamorphism that occurred between ;1600 ing quartzofeldspathic and psammopelitic composite and 1500 Ma. More recent field and geochronologi- gneiss grading into regionally coherent units includ- cal studies imply that an earlier deformation event ing the ALower AlbiteB, dominated by ;1710±1700 occurred prior to ;1640±1630 MaŽ Ashley et al., Ma A-type metagranitoids and co-magmatic felsic 1998a; cf. Nutman and Ehlers, 1998. ; however, for metavolcanic rocksŽ Ashley et al., 1996; Page et al., this study we will continue to use the OD13 ±OD 1998. , the AMiddle SchistB, composed of psam- notation of Flint and ParkerŽ. 1993 . Two later defor- A mopelitic schist and composite gneiss, and the Up- mation eventsŽ. DD12 , DD are related to Delamerian per AlbiteB, dominated by finely laminated albitite, orogeny Ž.;500±450 Ma . as well as minor amounts of iron formation, locally Initial deposition of the Willyama Supergroup grading into barite-rich rock. The Quartzofeldspathic sequence in the Olary Domain commenced at ;1700 Suite grades up-sequence into the Calcsilicate Suite, Ma, A-type granitoids were intruded and co-mag- typified by laminated calcalbitites and minor Mn-rich matic rhyolitic volcanic rocks were erupted at ; calc-silicate rocks. We use the term Acalc-silicateB to 1710±1700 MaŽ. Ashley et al., 1996 . Recent obser- describe a metamorphic rock containing more than vationsŽ. Ashley et al., 1998a suggest that the 25 modal% calc-silicate mineralsŽ typically amphi- Willyama Supergroup was then deformed prior to bole, clinopyroxene, plagioclase, garnet, and intrusion of several mafic igneous masses and small epidote. , whereas the term AcalcalbititeB refers to a I-type granitoid bodies into the central part of the quartz±albite rock with up to 25 modal% calc-sili- Olary Domain at ;1640±1630 Ma. A major episode cate minerals. . The Calcsilicate Suite displays up-se- of deformationŽ. OD12 and OD and amphibolite quence transition into the Bimba Suite, dominated by grade metamorphism affected much of the Olary calc-silicate rocks and marble, locally with Fe±Ž Cu± Domain at ;1600 Ma. This resulted in formation of

Zn. sulfides, graphitic pelite and albitite. The Bimba two sub-parallel planar deformation fabricsŽ OS1 and Suite is overlain by the Pelite Suite, composed of OS2 . and development of tight to open, upright to pelitic and psammopelitic schist, with local graphitic steeply inclined folds related to OD2 . Peak regional facies, psammite, calc-silicate rock, tourmalinite and metamorphic conditions were also attained during manganiferous iron formation. It is interpreted that OD12 and OD and studies of pelitic rocks by Clarke the Willyama Supergroup sequence in the Olary et al.Ž. 1987 indicated that grades were highest in the Domain was largely deposited between ;1710± southern and central portions of the Olary Domain, 1650 Ma, although the younger age limit is not reaching upper amphibolite facies, with estimated well-constrainedŽ Ashley et al., 1998a; Page et al., maximum pressures of 4±6 kb and temperatures of 1998. . 550±6508CŽ. Flint and Parker, 1993 . Peak metamor- The Olary Domain sequence has been intruded by phic conditions decrease to the north to lower amphi- several suites of plutonic rocks as well as having bolite and greenschist faciesŽ Clarke et al., 1987; 38 A.J.R. Kent et al.rLithos 54() 2000 33±62

Fig. 1. . Peak metamorphic conditions were followed by widespread emplacement of voluminous S-type granitoids and associated pegmatite bodies at ;1600"20 Ma. S-type granitoids range from massive to foliated and are considered to be late-syn- tectonic; intruded at the end of OD2 event. Retrograde metamorphism and alteration, including a retrograde deformation event, OD3 , continued ; ; episodically between 1580 and 1500 Ma. OD3 deformation was largely restricted to discrete shear zones along which greenschist facies assemblages were developedŽ. Clarke et al., 1986 . Further thermal perturbations also occurred during the Musgravian Orogeny at ;1200±1100 MaŽ. Lu et al., 1996 . Mafic dyke emplacement at ;820 Ma was a precur- sor to development of the Adelaide Geosyncline in the regionŽ. cf. Wingate et al., 1998 and at least two episodes of localised low grade metamorphism and deformation occurred between ;500 and 450 Ma during the Delamerian OrogenyŽ Clarke et al., 1986; Flint and Parker, 1993. . Episodes of fluid flow accompanied most of these later thermal eventsŽ e.g. Bierlein et al., 1995; Lu et al., 1996. .

Fig. 3. Examples of metasomatic alteration in the Olary Domain. Ž.A Brecciated calc-silicate from Cathedral Rock. Breccia consists 3.1. Regional and local scale metasomatic alteration of angular bleached albite-rich clasts in a dark matrix of diopside in the Olary Domain and actinoliteŽ.Ž. pen shown for scale is 14 cm long . B Altered and bleached laminated calc-silicate at Mindamereeka Hill intruded and cut by thin parallel dykes of pegmatite.Ž. C Actinolite- In addition to the magmatic, metamorphic and rich veins surrounded by bleached albite-rich alteration selvages deformational history outlined above, the rocks of in altered I-type graniteŽ.Ž. Tonga Hill . D Fracture-controlled the Olary Domain have experienced a long history of alteration in laminated psammopelitic sedimentsŽ. White Rock . fluid±rock interaction, metasomatism and hydrothermal alterationŽ e.g. Cook and Ashley, 1992; Ashley et al., 1998a,b; Skirrow and Ashley, 1999. . The Fe, Na±Ž. Fe , Ca± Ž Na±Fe . or, less commonly, Fe±K effects of metasomatism in the Olary Domain are and is most commonly evident in quartzofeldspathic evident on a variety of spatial scales, ranging from rocks, granitoids, calc-silicates and calcalbitites, regional-scaleŽ. kilometres alteration zones in sedi- marbles and iron-formations. Metasomatic assem- ments, felsic volcanic and intrusive rocks through to blages are typically more oxidised than original as- localised examples of fluid flow such as breccia semblages with a variety of Fe3q minerals present zones and local fracture systemsŽ. e.g. Fig. 3 . Al- Že.g. epidote, magnetite, hematite, andradite-rich gar- though the manifestations of metasomatism are het- net and aegirine-bearing clinopyroxene. . The typical erogeneously distributed and both lithologic and styles of metasomatic alteration evident in the Olary structural controls are apparent, examples of fluid± Domain are summarised in Table 1 and several rock interactions are so numerous that it is clear that examples are shown in Fig. 3. metasomatic processes have been an intrinsic part of The timing of regional-scale metasomatic activity the development of the Olary DomainŽ. Table 1 . In is constrained by field relations and geochronology. general, metasomatism has resulted in enrichment of Regional-scale alteration zones occur in, and thus A.J.R. Kent et al.rLithos 54() 2000 33±62 39

Table 1 Summary of the common alteration styles and accompanying mineralogical and chemical changes found in different rock types in the Olary Domain Rock type Common alteration styles Mineralogical changes Chemical changes Pelites and Psammopelites Extensive albitisationŽ bleach- Replacement of aluminosilicates, Addition of Na; loss of K, Rb ing.Ž Fig. 3D . micas and feldspars by albite Ž Ba . Quartzo-feldspathic rocks Extensive albitisationŽŽ. bleach- Replacement of plagioclase, K- Addition of Na Fe,S,Cu,K ; loss ing.Ž. , 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 Calc-silicate and calcalbitite Pervasive bleaching grading to Destruction of clinopyroxene, ti- Addition of Ca, Fe3q , Mn, rocks massive clinopyroxene- and acti- tanite, K-feldspar and scapolite; Ž."Cu,Pb,Zn,U ; loss of Na, nolite-matrix breccias andror formation of secondary Na±Fe3q Fe2q , Mg, Rb, Ba massive garnet±epidote alter- clinopyroxene, amphibole, albite, ation zonesŽ. Fig. 3A,B quartz, andraditic garnet, epidote Granitoids: A-types AlbitisationŽ. bleaching , minor Destruction of plagioclase, K- Addition of Na Ž. Fe ; loss of Fe oxide veining and dissemina- feldsparŽ. biotite and formation K,Rb tions of albite Ž." magnetite, quartz S-types Localised albitisationŽ. bleaching Destruction of igneous feldspar Local addition of Na Ž. Ca,Fe ; where late dykes crosscut calc- and biotite; albitisation; local loss of K,Rb silicate breccia and garnet±epi- formation of amphibole, garnet, dote alteration zones epidote, titanite I-types Extensive areas of fracture con- Destruction of igneous feldspar Addition of NaŽ. Ca,Fe ; loss of trolled and pervasive bleaching and biotite; pervasive albitisa- K,Rb with minor brecciationŽ. Fig. 3C tion; deposition of quartz, amphibole and titanite on fractures Iron-formations Fe oxide enrichment; destruction Local loss of quartz; growth of Addition of FeŽ 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 volcanics gressed within OD3 deformation zones, indicating emplaced at 1710±1700 Ma and I-type granites em- that the majority of metasomatic alteration occurred placed at 1640±1630 Ma. In addition, in all locations prior to formation of these zones at ;1500 Ma. observed, metasomatic mineral assemblages ret- However, 40Ar±39Ar ages on metasediments and peg- rogress peak metamorphic assemblagesŽ e.g. Ashley matite muscovite also suggest that fluid movement et al., 1998a,b. and indicate that metasomatic activ- continued episodically along OD13 ±OD structures ity occurred after the metamorphic peak and the for several hundred million years after granite intru- development of OD2 deformation textures. As dis- sionŽ. Bierlein et al., 1995; Lu et al., 1996 . Reactiva- cussed below, metasomatic alteration zones located tion of structures during the Delamerian Orogeny is in calc-silicate rocks are also cut by S-type granites indicated by ;470 Ma 40Ar±39Ar ages of mus- and related pegmatites at several localities demon- covites from pegmatites and OD3 shear zonesŽ Lu et strating that the majority of alteration occurred prior al., 1996.Ž. . Bierlein et al. 1995 also demonstrated to S-type granitoid emplacement at ;1600"20 that at least some of the epigenetic sulfide minerali-

Ma. sation within OD3 shear zones occurred between In several locationsŽ. e.g. Cathedral Rock , meta- ;480 and 450 Ma during retrograde fluid move- somatically altered rocks are deformed and retro- ment along older structures. 40 A.J.R. Kent et al.rLithos 54() 2000 33±62

3.2. Metasomatism in calc-silicate rocks imply that breccias formed during deformation as they contain rare folded fragments and appear to Although many different rock types have been have been injected into fractures in fold hinges inter- effected by metasomatism within the Olary Domain, preted to be temporally linked to OD2 Ž Yang and fluid±rock interaction appears to have been espe- Ashley, 1994. . In several locations, breccias have cially intense in calc-silicate and associated rocks. also been intruded by S-type granite and pegmatite The two most common alteration styles evident are related to the ;1600"20 Ma episodeŽ e.g. Cathe- calc-silicate-matrix breccia zonesŽ consisting of both dral Rock, Toraminga Hill. and have been deformed clinopyroxene- and actinolite-matrix breccias. and by OD3 shear zonesŽ. e.g. Cathedral Rock . garnet±epidote-rich alteration zones. Although these Breccias consist of angular altered rock fragments two styles of metasomatic alteration occur in similar in a medium to coarse grained matrix dominated by host rocks, and are often spatially and temporally clinopyroxene andror actinolite, with minor quartz, associatedŽ. see below , they are associated with dis- albite, hematite, titanite and epidote. All gradations tinctly different styles of metasomatic alteration and occur between bleached, altered calcalbitite contain- will be treated differently for purposes of description ing minor clinopyroxene andror albite veins and and discussion. massive clast and matrix-supported brecciasŽ e.g. In Section 4 we describe the petrological, chemi- Fig. 3A. . In general, the early phases of breccia cal and mineralogical features of garnet±epidote-rich formation are associated with aegirine-bearing alteration zones in detail. These zones are the focus clinopyroxenes as the dominant matrix mineral. Later of our research as they appear to have formed at stages of breccia evolution involve retrograde re- relatively high fluidrrock ratios in areas of focussed placement of clinopyroxene by actinoliteŽ. Fig. 4A , fluid flow and thus provide an excellent opportunity as well as formation of primary actinolite"hematite to assess the nature of metasomatic fluids and the "quartz"titaniteŽ. e.g. Toraminga Hill . Clinopy- chemical changes associated with metasomatism. roxene-matrix breccias are most common in the cen- However, as the clinopyroxene- and actinolite-matrix tral part of the Olary Domain whereas amphibole- breccias are also observed to be spatially and tempo- dominated matrix breccias occur in the central north- rally related to the formation of garnet±epidote-rich ern and northern portionsŽ. Fig. 1 . This mirrors the alteration zones, and appear to have formed from patterns evident in metamorphic isogradsŽ. Fig. 1 similar composition metasomatic fluids, we believe and thus most probably reflect regional gradients in that understanding the relation between these meta- temperature during breccia formation, with the somatic breccias and garnet±epidote-rich metaso- clinopyroxene representing higher temperature re- matic alteration zones provides important insights gionsŽ. see discussion below . Fluid inclusions in into the nature of metasomatism within the Olary clinopyroxene and quartz associated with breccias Domain. To this end, both clinopyroxene- and acti- are commonly hypersaline, and measurements of nolite-matrix breccias are briefly described in the quartz-hosted inclusions from clinopyroxene- and remainder of this section. actinolite-matrix breccias display fluid salinities be- Clinopyroxene and actinolite-matrix breccias form tween ;15±46 equivalent wt.% NaClŽ A.J.R. Kent many spectacular outcrops in the Olary Domain, and P.M. Ashley, unpublished data. . Clinopyroxene Že.g. Cathedral Rock, Toraminga Hill, Telechie Val- from breccias generally contains higher Na±Fe3q ley; Figs. 1 and 3A. and have been discussed by contentsŽ. up to 33 mol% aegirine than clinopyrox- Cook and AshleyŽ. 1992 and Yang and Ashley ene in the unaltered calc-silicate rocksŽ. Fig. 5 and Ž.1994 . Calc-silicate-matrix breccias are commonly this, coupled with the presence of hematite in actino- stratabound, range from irregular and locally tran- lite-matrix breccias and as a daughter mineral phase gressive bodies up to tens of metres across down to in fluid inclusions, indicates that breccia formation narrow piercement masses and are associated with occurred under oxidizing conditions. zones of hydrothermal alteration, involving albitisa- A third type of metasomatic alteration in calc- tionŽ white AbleachingB and local pink hematitic silicate rocks, found locally in laminated Mn-rich pigmentation. in the host calcalbitite. Field relations Ž.piemontite-bearing calc-silicate rocks Ž Ashley, A.J.R. Kent et al.rLithos 54() 2000 33±62 41

Fig. 5. Na vs. Fe3qrFe2q plot for clinopyroxenes from calc-silicate rocks and ironstones from the Olary Domain. The star represents the average of 26 clinopyroxene analyses from altered ironstones from Mindamereeka Hill taken from Ashley et al. Ž.1998b . Clinopyroxene compositions from unaltered calc-silicates are from CookŽ. 1993 and those from clinopyroxene-matrix breccias from Yang and Ashley, 1994. . The composition of recrystallised clinopyroxene within the garnet±epidote-rich alteration zone at White Dam North is also shown.

1984. is a variant of garnet±epidote alteration, and contains coarse grained assemblages of one or more of piemontite, quartz, garnetŽ andradite- and spessartine-rich. , hematite, manganoan tremolite and brau- nite. These rocks will not be described further in this paper.

4. Garnet±epidote-rich metasomatic alteration zones

4.1. Field setting and description of alteration phenomenon

Fig. 4. Photomicrographs from altered calc-silicate rocks from the Garnet±epidote-rich alteration zones are best de- Olary Domain.Ž. A Cross-polarised light photo of clinopyroxene- veloped in calc-silicate-bearing rocks of the Calcsili- matrix breccia from Cathedral RockŽ. sample CR-5 showing cate and Bimba Suites, but also occur rarely in retrogression of clinopyroxene to fibrous and massive actinolite adjacent to a crosscutting quartz vein.Ž. B Plane-polarised light quartzofeldspathic rocks of the Quartzofeldspathic photo of garnet±epidote alteration zone from BoolcoomattaŽ sam- Suite. For this study, samples from garnet±epidote- ple BC-3. showing garnet, epidote and quartz intergrowth with rich alteration zones and associated calc-silicate rocks partial granoblastic textures.Ž. C Plane-polarised light view of were examined in detail from six locations, termed garnet±epidote alteration zone from Bulloo WellŽ. sample BW-3 . Bulloo Well, Boolcoomatta, Sylvester Bore, Min- Euhedral and subhedral zoned garnets are surrounded by later quartz and contain irregular inclusions of epidote. Abbreviations: damereeka Hill, Sampson Dam and White Dam North Cpx. Ð clinopyroxene, Act. Ð actinolite, Gt. Ð garnet, Ep. Ð Ž.Fig. 1 , although observations were also made at epidote, Qtz. Ð quartz. several other locations. The alteration types evident 42 A.J.R. Kent et al.rLithos 54() 2000 33±62

at each location are essentially the same and are described below and illustrated inŽ Figs. 3B, 4B,C and 6. . In outcrop, garnet±epidote-rich zones occur as dark brown, black and green masses showing partial to complete replacement of laminated calc-silicate rock, with local replacement controlled by former bedding and fracturesŽ. e.g. Figs. 3B and 6 . The size of the metasomatised regions varies substantially, with alteration zones ranging from thin isolated vein- letsŽ. centimetre scale to massive lensoid stratabound replacements up to 200±300 m acrossŽ e.g. White Dam North, Bulloo Well. . Alteration can also often be traced for tens of metres along specific laminae, resulting in distinctive Anet-typeB textures where replacement occurs along both reactive bedding layers and along fractures at high angles to beddingŽ analo- gous to the texture shown in altered psammopelitic sediments in Fig. 3D. . Bleached quartz±albite-rich layers and zones are also common, and breccias with epidote±garnet matrix cementing bleached albite-rich fragments occur at the Boolcoomatta localityŽ Fig. 6A. . In addition to garnet and epidote, quartz is common, occurring in veins, open space fillings and in intergrowths with garnet and epidote. Other minerals are present in minor quantities and include albite, actinolite, clinopyroxene, K-feldspar, hematite, magnetite, carbonate and tiny traces of chalcopyrite Fig. 6. Handspecimen photos of samples from garnet±epidote and pyrite. Metasomatism commonly follows frac- alteration zones.Ž. A Brecciated and altered calc-silicate from ture sets that appear to be related to OD2 deforma- BoolcoomattaŽ. sample BC-8 . Bleached and albitised angular Ž. fragments of the original calc-silicate are held within an epidote tion, and in several locations e.g. White Dam North Ž.actinolite±garnet matrix. Although taken from a epidote±garnet the strongest alteration appears to be focused into alteration zone, this sample has similar textures to those observed OD2 fold hinge zones, perhaps suggesting that these in calc-silicate brecciasŽ.Ž. Fig. 3A . B Altered calc-silicate from acted as fluid conduits. Bulloo WellŽ. sample BW-2 . Original calc-silicate has been largely On the outcrop, hand specimen and microscopic replaced by massive epidote with subsidiary quartz and garnet. Ž Small dark residual laminae of clinopyroxeneŽ partially altered to scales' five categories of alteration phenomena with actinolite. are also apparent. Alteration has been largely controlled progressive alteration intensity. have been recog- by the composition of individual laminae and the folded structure nised, ranging from unaltered calc-silicate through to of the calc-silicate has been preservedŽ folding may represent incipient disseminated and fracture-controlled alter- soft-sediment deformation of the original calc-silicate.Ž. . C Par- ation to total replacement of the pre-existing calc- tially altered laminated calc-silicate from Sylvester BoreŽ sample SB-2.Ž . Individual laminations have been replaced by garnet the silicate rocks and late monomineralic veining. The largest garnet-replaced laminae has a small epidote-rich region in alteration styles are described below and summarised the centre and is bordered by a thin pale zone of quartz±albite in Table 2. alteration. . The remaining calc-silicate has been recrystallised and much of the original clinopyroxene has been altered to actinolite. 4.1.1. Unaltered laminated calc-silicate rock Ž.D Massive garnet-dominated alteration of laminated calc-silicate Ž.- from Sylvester BoreŽ. sample SB-3 . The primary laminated tex- These commonly crop out as thin 20 m lenses ture is partially destroyed by massive regions of garnet and with strike continuity of less than a kilometre, inter- quartz±albite alteration. calated with pelitic, psammopelitic and laminated A.J.R. Kent et al.rLithos 54() 2000 33±62 43

Table 2 Typical 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- 5 cm up to 10 cm across damereeka Hill Massive layer parallel replacement of calc-silicate minerals Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 4B,C and 5 by garnet andror epidote Ž"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- 5 andror epidote Ž."quartz, albite, K-feldspar, amphibole . damereeka Hill May combine with layer-parallel replacement of more reactive layers to produce net-like textures. Massive replacement of calc-silicate by garnet andror Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 4D and 5 epidote Ž."quartz, albite, K-feldspar, amphibole . These damereeka Hill, Sampson Dam, White Dam North areas commonly show quartz-rich zones with euhedral garnet crystals. Zones of epidote and garnet replacement may be surrounded by a AbleachedB quartz andror albite-rich zone. Pseudomorphous replacement of the original rock is locally evident with garnet and epidote forming near monomineralic layers. Late near-monomineralic veins of garnet, epidote, quartz Boolcoomatta, Bulloo Well, Sylvester Bore, Min- 5 and local K-feldspar. damereeka Hill Coarse euhedral garnet crystals, filling open spaces or Bulloo Well, Sylvester Bore, Mindamereeka Hill, Sampson 5 associated 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- 4.1.2. Recrystallisation of calc-silicate minerals ad- laminated, commonly defined by alternating ferro- jacent to alteration zones magnesian and quartzofeldspathic layers. Individual Clinopyroxene, titanite, scapolite and feldspar are compositional laminae are from 1 mm to 10 cm in recrystallised adjacent to alteration zones. This is thickness and are interpreted as a primary deposi- commonly shown by an increase in grainsize, better tional characteristicŽ. Cook, 1993 . Calc-silicate rocks development of granoblastic texture and decreased are dominated by clinopyroxene, albite, quartz, K- abundance of feldspars. Recrystallised clinopyroxene feldspar and amphiboleŽ hornblende andror actino- is paler in colour and more Mg-rich, compared to the lite. , with variable, but generally minor amounts of green, more Fe-rich compositions evident in unre- scapolite, garnet, epidote and titanite. Actinolite and crystallised clinopyroxenes. Actinolite retrogression hornblende occur as disseminated retrogression prod- of clinopyroxene is also more common in recrys- ucts of clinopyroxene and as discrete grains. Scapo- tallised zones. lite is found erratically in granoblastic aggregates in ferromagnesian and quartzofeldspathic layers. The calc-silicate rocks of the Olary Domain have been 4.1.3. Incipient formation of garnet±epidote bearing interpreted as the result of clastic sedimentation of assemblages felsic detrital material and interaction of sediments Minor to major development of epidote, garnet with evaporative brines, as well as contemporaneous and local quartz in clinopyroxene-bearing lamina- evaporitic and exhalative chemical sedimentation tions and along fractures. K-feldspar is altered to Ž.Cook and Ashley, 1992; Cook, 1993 . albite. Subhedral garnet and epidote occur as individ- 44 A.J.R. Kent et al.rLithos 54() 2000 33±62 ual crystals or small aggregates. Scapolite and titan- 4.2. Timing of formation of garnet±epidote-rich al- ite are less common, and generally only occur in teration zones zones where relict clinopyroxene remains. Actinolite alteration of clinopyroxene is also common. Field and petrographic observations indicate that metasomatism occurred after development of peak metamorphic mineral assemblages and associated OD and OD deformation events. At all localities 4.1.4. Total alteration of calc-silicate rock 12 investigated, the garnet±epidote±quartzŽ. ±actinolite These zones consist of intense alteration along metasomatic mineral assemblages overprint the pri- fractures and laminae and total replacement of mary metamorphic assemblages in the host calc- clinopyroxene-bearing laminae by garnet, epidote and silicate rocksŽ. e.g. Fig. 3 . Further, mineral fabrics in quartz Ž."minor K-feldspar, actinolite and albite altered rocks are not foliatedŽ e.g. Figs. 3B, 4B, C, Ž.Fig. 4B . Where alteration is most intense massive 6. , deformed calc-silicate rock at Boolcoomatta is replacement of feldspar-rich layers is also evident overprinted and pseudomorphed by granoblastic-tex- Ž.Fig. 6 . Although epidote- and quartz-rich zones tured epidote±garnet±albiteŽ. Fig. 6B , and alteration occur, garnet is commonly dominant and in many is often controlled by fracture sets associated with places is the only significant constituent. Along frac- the OD deformation event. The common presence tures and in open space fillings, garnet, and to a 2 of actinolite, rather than clinopyroxene, in garnet± lesser extent quartz and epidote, occur as largeŽ up to epidote-rich zones is consistent with a retrograde several cm. subhedral crystals, with garnets com- origin. monly showing oscillatory zoningŽ. Fig. 4C . Within Timing relations between metamorphism, metaso- altered laminae, garnet generally occurs as smaller matic formation of both clinopyroxene-matrix brec- Ž.mostly less than 2±3 mm crystals with granoblastic cias and garnet±epidote-rich alteration zones and texture and is strongly poikilitic, containing inclu- intrusion of S-type granites are particularly clear at sions of quartz, epidote and minor feldspar. Epidote Mindamereeka Hill, where laminated calc-silicate occurs as subhedral to anhedral crystals in aggre- rocks have been altered to garnet±epidoteŽ ±quartz± gates up to several centimetres across associated actinolite±albite"hematite. assemblages along frac- with garnet or as a matrix to bleached and brecciated tures and laminae. Several small lenses of clinopy- calc-silicate rockŽ. e.g. Fig. 6 . There is no consistent roxene-matrix breccias also occur at this location and textural relationship between garnet and epidote; in are crosscut by veins of garnet andror epidote, some samples, quartz and epidote form late crys- indicating that garnet±epidote-rich alteration zones talline aggregates around euhedral garnet and in formed after clinopyroxene breccias. Leucocratic other samples occur as inclusions with poikilitic two-mica S-type granite and associated pegmatite garnet. This is interpreted to indicate that garnet and dykes cut both garnet±epidote-altered calc-silicate epidote crystallised coevally. The observed textural rocksŽ. Fig. 3B and clinopyroxene-matrix breccias, relations are probably the result of local variations in and granite intrusion is interpreted to have postdated the relative time and rate of growth of either mineral. formation of both clinopyroxene-matrix breccias and At the White Dam North location, intense develop- the bulk of garnet±epidote replacement of calc-sili- ment of garnetŽ. ±epidote±quartz rock is locally cate rock. However, we note that pegmatite dykes cored in a synformal hinge zone by magnetite±quartz adjacent to garnet±epidote-rich zones also contain Ž.±albite rock. irregular veins and clots of garnet"quartz"epidote "hematite, and where granite has intruded altered calc-silicate rocks, it has been altered to a bleached 4.1.5. Late Õeins albiteqquartz"titanite assemblageŽ e.g. at Min- At most altered calc-silicate rock locations, nar- damereeka and Toraminga Hills; Fig. 1. . We suggest row Ž.-10 mm late veins of garnet, epidote that intrusion of these dykes either occurred during Ž."quartz, K-feldspar crosscut all other assem- the waning stages of metasomatic alteration or that blages. the heat associated with intrusion remobilised meta- A.J.R. Kent et al.rLithos 54() 2000 33±62 45 somatic fluidsŽ possibly contained within fluid inclu- mixing of two mineralsŽ with different SmrNd ra- sions. causing further alteration. tios. will move samples along the isochron line, not away from it. For a metasomatic rock, these assump- 4.2.1. Sm±Nd dating tions are probably justified assuming that localised In order to determine the time of formation of equilibrium existed between the fluid and reacting garnet±epidote-rich alteration zones, garnet and epi- calc-silicate during metasomatism. dote in samples from Sylvester Bore, Mindamereeka Results from the regression of data from garnet± Hill and Bulloo WellŽ. Fig. 1 were analysed for epidote alteration zones are given in Table 4, and are Sm±Nd isotopic composition and concentration. Re- plotted on a 147Smr144 Nd vs. 143 Ndr144 Nd isochron sults are shown in Table 3. diagram in Fig. 7. Regression of all data corresponds Variations in Sm and Nd compositions and iso- to an age of 1577"80 Ma, with a high MSWD of topic ratios between analyses of the same minerals 83Ž see footnotes for Table 4 for explanation of this from the same locations are evident at Sylvester term. . Examination of Fig. 7 shows several points Bore and Mindamereeka Hill. This could be due to which lie off the isochron. Both garnet and epidote contamination of separates by variable amounts of a aliquots from the one sampleŽ. BW-1 from Bulloo phase with different Sm and Nd concentrationŽ i.e. Well and epidote from sample MH-1 lie well above contamination of garnet with epidote and vice versa. the best-fit line. Removal of these from the regres- or may reflect compositional variation within anal- sion improves the MSWD to a more acceptable 3.8, ysed minerals. Electron microprobe analyses and equivalent to an age of 1575"26 Ma. Subject to optical observations show that minerals are composi- appropriate justification for removal of these points, tionally zoned, and this may also be the case for Sm this is interpreted to be the age of formation of and Nd concentrations and the SmrNd ratio. It is garnet±epidote-rich zones at Mindamereeka Hill and important to note, however, that variation in Sm and Sylvester Bore. Ages from individual regression of Nd composition will not effect isochron calculations, data for the Sylvester Bore and Mindamereeka Hill provided that all minerals analysed had the same localities are within error of the age derived from initial 143 Ndr144 Nd ratio, and that all minerals regression of all data. Uncertainties for these ages formed at the same time. If this is the case, then are higher, and this probably reflects the lower num-

Table 3 Sm±Nd analyses of garnet and epidote from metasomatic rocks from the Olary Domainwx Ep Ð epidote, Gt Ð garnet . Sample locations and descriptions given in Table 6 Sample SmŽ. ppm Nd Ž. ppm147 Smr 144Nd 143 Ndr 144Nd " a Bulloo Well BW-1Ž. Ep 2.07 7.79 0.1607 0.512072 "8 BW-1Ž. Gt 8.30 17.69 0.2841 0.513312 "16

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

SylÕester Bore SB-2Ž. Ep 9.73 33.0 0.1781 0.512137 "9 SB-3Ž. Ep 11.6 37.4 0.1870 0.512244 "8 SB-3r1Ž. Gt 26.3 36.6 0.2939 0.513353 "13 SB-3r2Ž. Gt 28.3 37.6 0.2936 0.513321 "8

a 95% confidence interval, error given in the last decimal places. 46 A.J.R. Kent et al.rLithos 54() 2000 33±62

Table 4 Regression data for Sm±Nd analyses of garnet and epidote from altered calc-silicates. Abbreviations as for Table 3 a 143r 144 Regression MSWD ´ Nd Nd Ndi Age Points All data 83 y5.6 0.510309"52 1577"80 12 ll data without Bulloo Well, MH-1Ž. Ep , MH-3 Ž. Gt 3.8 y6.0 0.510292"30 1575"26 8 Bulloo Well ± y4.0 0.510457"31 1529"25 2 Bulloo WellqMH-1Ž. Ep 3.3 y4.1 0.510434"310 1543"220 3 Sylvester Bore 3.9 y5.9 0.510308"134 1568"93 4 Mindamereeka Hill 107 y6.3 0.510339"2400 1529"250 6 Mindamereeka Hill without MH-1Ž. Ep , MH-3 Ž. Gt 2.3 y6.3 0.510305"56 1556"66 4

a MSWD Ž.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 regressions cally different location. Differences in the Nd isotope Ž.Table 4 . composition of calc-silicates, metasomatic fluid Deviation of individual points from the isochron andror the fluidrrock ratio could produce variations may be the result of several factors, including differ- in the initial Nd isotope composition of metasomatic ences in initial 143 Ndr144 Nd ratios; disturbance of minerals from different locations. Both samples from Nd isotope systematics after formation of metaso- Bulloo Well appear to lie on a separate isochron than matic minerals; and incorporation of material which that defined by the remainder of the data. The two predates metasomatism into the analysed aliquots point isochron defined by Bulloo Well samples cor- Ž.e.g. metamorphic clinopyroxene or titanite . The responds to an age of 1529"25 Ma and has an first possibility is most probable for minerals from initial 143 Ndr144 Nd ratio of 0.510457"31. This Bulloo Well where samples are from a geographi- value is different, outside the given 95% confidence limit, from the initial ratio of 0.510292"30 from the regression of data combined from Mindamereeka Hill and Sylvester BoreŽ and from the regression of data from both these localities regressed separately; Table 4. . This is consistent with an interpretation that the fluid responsible for metasomatism at Bulloo Well had an initial Nd isotope composition different from that responsible for metasomatism at the other two locations studied. However, at present it is not possible to distinguish whether metasomatism at Bul- loo Well occurred at a different time to other locations as the ages from regression of the Bulloo Well samples and the combined data from Mindamereeka Hill and Sylvester Bore are within error at 95% confidence limitsŽ. Table 4 . Further, the age for Bulloo Well is not definitive as it is only based on a two-point regression. The explanation for the samples from Min- damereeka Hill which lie off the isochron is not Fig. 7. 147Smr144 Nd versus 143 Ndr144 Nd isochron plot for garnet clear. Epidote from sample MH-1 may have a similar 143 144 and epidote from garnet±epidote alteration zones. Individual data initial Ndr Nd ratio to that defined by the two points are labeled. Abbreviations as for Fig. 4 and: BW Ð Bulloo samples from Bulloo Well, as it lies close to the Well; MH Ð Mindamereeka Hill; SB Ð Sylvester Bore. The two-point regression line defined by theseŽ. Fig. 7 . It regression line is for all data except BW-1Ž. Gt , BW-1 Ž. Ep , MH- 1Ž. Ep and MH-3 Ž. Gt where it is shown in solid, compared for the is possible that paragenetically late epidote veinlets two point regression of BW-1Ž. Gt and BW-1 Ž. Ep where it is observed in this sample formed from a fluid with shown in dashed. initial Nd isotope composition slightly different from A.J.R. Kent et al.rLithos 54() 2000 33±62 47 that which was responsible for the majority of alteration at Mindamereeka Hill. y The ´ Nd value of 6.0 calculated from regression of the combined data from Mindamereeka Hill and Sylvester Bore and the value of y4.0 calculated from the Bulloo Well dataŽ. Table 4 are consistent with the formation of these rocks via the action of LREE-enriched, crustally derived, fluid. This does not 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 sequence are crustally derived and thus would be expected to be LREE-enriched. However, these ´ Nd values limit the direct contributions of REE to the metasomatic fluid from LREE-depleted mafic rocks. In addition the ca. 1710±1700 A-type igneous rocks from the Olary Domain have ´ Nd valuesŽ calculated at 1700 Ma.Ž that range from y0.1 to 1.0 Ashley et al., 1996; Page et al., 1998. and thus are also unlikely to have contributed REE to the metasomatic fluid.

4.3. Mineral chemistry

The results of electron microprobe analysis of the composition of epidote, garnet, amphibole and clinopyroxene from garnet±epidote-rich alteration zones are shown in Fig. 8 and representative mineral Fig. 8. Mineral compositions from garnet±epidote altered calc- silicate rocks in the Olary Domain. Additional data from Rolfe analyses are given in Table 5. Ž.1990 , Westaway Ž. 1992 , Cook Ž. 1993 , Eykamp Ž. 1993 , Laffan Garnets consist predominantly of andradite± Ž.1994 , Pepper Ž. 1996 and Chubb Ž.Ž. 2000 . A Garnet composi- grossular solid solution, with minor spessartine and tions from garnet±epidote alteration zones and from unaltered almandine componentsŽ. Table 5, Fig. 8A ; composi- calc-silicate rocks.Ž. B Epidote compositions from garnet±epidote tions extend to 95 mol% andradite. Variations are alteration zones. expressed largely as differences in the andradite± 4.4. Geochemistry grossular ratio between different localities and between samples from the same locality. Garnets from Samples of garnet±epidote-rich altered calc-sili- altered calc-silicates have lower almandine q cate and unaltered calc-silicate rocks from Bool- spessartine components than those from largely unal- coomatta, Mindamereeka Hill, Sylvester Bore, Bul- tered calc-silicatesŽ. Fig. 8A . loo Well, Sampson Dam and White Dam North were Epidote from altered calc-silicate rocks has rela- analysed for major and trace elements in order to tively Fe-rich compositions, ranging between 8% and assess the chemical changes resulting from metaso- 32% pistacite end-member, and with low piemontite matic alteration. Analyses are presented in Table 6. contentsŽ. Fig. 8B . Amphiboles in altered calc-sili- Changes in major element and selected trace element cate rocks are relatively Fe-rich and include ferro- compositions were evaluated using the isocon calcu- hornblende and actinoliteŽ. Table 5 . Recrystallised lation procedure outlined in GrantŽ. 1986 , with re- clinopyroxene is relatively magnesian, with a typical sults summarised in Fig. 9. This method assumes composition being Wo50.0 En 41.3 Fs 8.7 with a small that the composition of the unaltered rock is repre- calculated aegirine componentŽ. Table 5 . sentative of the protolith of the altered rock. Al- 48 A.J.R. Kent et al.rLithos 54() 2000 33±62 and q 3 tions of Fe n 8.27 11.261.701.15 14.88 0.19 1.04 0.57 0.05 0.27 1.885 2.4800.5000.225 0.823 0.3000.050 0.110 0.019 0.010 Ž. Ž. Ž. Ž. BC-4 SB-3 MH-1 BW-2 R74782 R77358 BC-4 SB-3 MH-1 BW-1 R74782 R77351 R74782 R74782 R74782 25 O16.031 16.048 15.977 16.030 15.996 24 15.966 O 16.000 16.000 16.000 16.000 16.000 16.000 15.685 15.250 4.045 23 O 6 O in garnet were calculated assuming stoichiometry. Note: blanks signify values below detection limit. q 2 2 2 q q q 2 23 vi 23 iv 3 2 2 2 Table 5 Representative electron microprobe analyses of epidote,Sample garnet, amphibole and Epidote clinopyroxene from altered calcsilicate rocks from theSiO Olary Domai TiOAl 37.21 OFe O 37.47FeO 21.56 37.62MnO 16.02 23.04MgO 37.48 14.41 21.19 0.38 37.36 15.93 21.88 0.18KO 15.81 38.18 22.53Cl 14.76 0.14 22.23 35.83 15.96 0.29 37.03 6.04 0.38 23.03Si 34.91 13.23Al Garnet 13.45 0.29Al 36.76 2.92Ti 27.82 5.966 37.74Fe 0.034 2.20 5.940 10.13 17.45Fe 4.040 0.060 37.01 6.050Mn 13.19 1.38 13.68 4.246 45.30Mg 1.933 5.950 4.016 0.59 1.720 20.19 7.61 0.051 51.15 5.987 4.043 0.30 1.927 0.050 0.024 53.64 8.39 6.022 1.13 4.241 0.34 1.888 0.013 0.019 1.55 4.133 1.21 4.48 5.915 1.778 0.039 0.56Analysts: 0.61 5.862 A.J.R. 0.051 1.894 Kent, 1.089 0.28 M.A. 0.84 Pepper, A.J. 0.35 Chubb. 5.831 2.330 See 0.039 0.01 Appendix A 2.860 for 0.93 sample 0.085 0.34 information. All 5.910 0.407 Fe 1.603 is 0.307 0.85 assumed 0.138 to 0.91 be 5.992 trivalent 1.831 in 0.185 3.497 0.36 epidote, Amphibole whereas 0.169 the propor 2.461 0.93 6.037 0.083 2.112 0.07 0.090 6.910 1.508 1.459 0.154 Clino-pyroxene 0.008 0.09 1.98 7.550 0.425 0.163 2.481 20.92 0.04 1.990 0.330 0.038 0.117 17.24 0.041 0.002 0.120 0.213 1.090 5.18 0.070 0.081 0.110 0.450 0.040 0.001 0.029 0.010 0.041 0.114 0.026 0.127 0.044 0.005 0.270 0.010 2.665 0.005 2.130 0.175 CaONa O 23.32 23.88Total 23.01 98.49 23.87 98.98 22.87 97.89 22.95 99.45 97.90 31.06 33.95 99.61 33.20 100.01Ca 99.99 33.33Na 100.01K 33.51 4.007Cl 100.00S 4.058 32.01 99.67 3.965 11.06 100.00 4.060 11.21 97.98 3.926 24.31 98.00 3.878 99.53 5.493 5.760 5.942 5.744 5.701 5.592 1.810 1.770 0.966 Fe A.J.R. Kent et al.rLithos 54() 2000 33±62 49 quartz-bearing assemblages, R77354 being rich " 3234516 - epidote " 24 712191010 0.01 0.02 0.05 0.02 0.01 0.03 0.01 - - 0.01 26 62 17 13 33 23 15 171 - 0.01 2 - - 0.01 5.45 0.15 1.94 1.32 1.03 4.46 0.20 2.50 4.02 2 - - 0.01 0.01 2 - - 2 - 2 - 2 - SB-3Alt SB-1 Unalt MH-2 MH-3 Alt BC-4 Unalt BC-1 Alt R73358 R73357 Unalt R74782 R74780 Alt R74779 R77351 Unalt R77354 R77353 Alt Alt Unalt Alt Alt Unalt 23 quartz, Unalt. Ð unaltered calcsilicate rocks. Fe 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 0.64 q q FeO Ž. r 2 2 3 2 23 23 2 25 in magnetite GaZnCuNiCrCeNdLa 17Ba 168V 41Sc 19 22Cl 18 196Co 18 62Mo 20 38 24 39 128 13 356 79 19 134 29 37 26 87 4394 14 5 1361 77 102 21 124 27 31 137 13 85 157 168 80 13 98 24 27 14 27 80 68 15 201 497 84 10 7 26 83 96 75 55 81 16 189 101 38 31 5 139 18 20 87 2908 27 14 42 32 82 102 14 20 22 467 30 146 35 49 114 42 90 162 4037 34 32 25 87 46 24 6 31 187 24 89 17 17 23 15 16 57 16 195 76 25 16 40 3 19 53 12 156 71 36 16 10 26 16 32 71 72 29 1 46 39 14 18 85 134 40 88 3 2 68 13 120 65 442 9 180 31 22 119 87 140 6 13 Pb 13As 83 5 2 5 1U 2 196346 418 148978 11 2 5 5 25 1 9 20 15 7 17 2 12 4 Table 6 Chemical composition of altered and unaltered calcsilicate rocks from the Olary Domain. Sample locations andSiO descriptions are givenTiO in Appendix A Al OFe OFeOMnOMgOCaO 53.10Na O 0.47K 62.30 11.84 OP O 6.28 54.67 188 0.60SO 14.32 1.29LOI 56.65 0.94 14.06 1.59 0.55Total 1.33 66.53 2.34Nb 20.39 14.59 0.15 5.53 0.56Zr 1.30 1.87 61.71 2.39Y 7.20 6.78 0.18 4.68 0.38 0.30Sr 43.21 2.23 2.73 2.19 13.27 13.27 0.41Rb 10.61 0.14 0.17Th 0.51 5.55 60.76 2.10 11.56 1.36 5.27 0.53 8.68 99.18 0.23 0.76 0.94 53.38 0.30 12.65 0.61 0.04 0.20 14 6.13 98.73 0.23 1.46 15.78 8 16.18 0.19 15111316126 145 40.73 0.12 0.56 0.04 7.58 99.37 14.40 0.61 4.14 0.38 0.94 47 0.22 66.25 15 111 1.10 2.15 29.97 99.63 178 0.01 12.66 0.27 0.21 11 0.25 100.06 0.14 39.76 3.72 7.78 34 287 15.74 14 138 0.34 3.87 6.38 0.35 1.04 99.50 52.23 1.89 228 2.52 11.97 0.16 15.18 0.31 101.70 0.35 7.69 149 25 75 13 67.18 0.57 2.18 0.29 0.23 0.82 2.12 27.22 20 99.94 1.19 131 6.80 5.26 0.29 0.38 21 88 32 18.73 1.17 99.71 1.14 8.21 0.23 13.46 12 0.15 0.65 189 25.77 0.29 124 167 1.79 99.95 0.88 30.23 0.43 0.90 18 0.81 100.08 0.27 1.41 2 0.58 1.27 69 0.34 119 1.37 0.38 60 8.55 99.62 0.03 0.31 7 0.81 254 0.18 5.92 Blanks: 0.09 148 not 2.50 26 99.06 determined. 15 For sample 0.34 0.02 information see Appendix 0.53 A. 1.85 14 99.68 Alt. 116 0.29 18 Ð 1088 Altered calcsilicate rocks 0.19 containing garnet 27 0.35 0.07 17 159 246 91 1.63 1.05 13 0.18 165 29 17 31 0.60 15 198 87 17 9 31 107 19 14 18 30 197 153 2 1 109 8 16 15 73 72 FeO 50 A.J.R. Kent et al.rLithos 54() 2000 33±62

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 used to compile this summary. The categories are defined as follows: Astrongly depletedB and Astrongly enrichedB mean concentration in altered rock is greater or less than five times that in unaltered rock; AdepletedB and AenrichedB mean that the element is between two and five times depleted or enriched in altered over unaltered rock. Note: a LOI Ð Loss on ignition. though this assumption may be tenuous in variably in several locationsŽ. Table 6 . Many of these changes laminated calc-silicate rocks, large Ž.;2 kg samples are in accord with alteration of a clinopyroxene± were analysed and thus primary heterogeneity prob- feldspar-bearing assemblage to an andradite-rich gar- lems were probably satisfied to the degree required net±epidote assemblage where Mg, Na, K and Rb to demonstrate broad changes in chemical composi- are lost during clinopyroxene and feldspar destruction. tion and Ca, Fe3q and Mn are fixed by the formation Altered calc-silicate rocks are commonly enriched of garnet and epidote. In unaltered calc-silicates, S in Fe3q, Ca, Mn, U and Cu, and depleted in Fe2q, and Cl are hosted in scapolite, which is destroyed Na, Mg, K, and RbŽ. Fig. 9 , and several altered during alteration; however, daughter minerals in fluid rocks are also enriched in Pb, Zn, S and Cl. Alter- inclusions indicate that appreciable Cl and SŽ as 2y ation is accompanied by strong oxidation, with large SO4 . are present within hypersaline fluid inclu- r q decreases in the FeO Ž.FeO Fe23 O ratio evident sions in the altered rocksŽ. see Section 4.5 . The A.J.R. Kent et al.rLithos 54() 2000 33±62 51 presence of these elements in the metasomatic fluid ples from all localities. Estimated liquid±vapour ra- would assist in complexing and transporting many tios typically vary between 2:1 and 9:1, although rare metals. vapour-dominated inclusions with liquid±vapour ra-

tios less than 1:2 were also observed. CO2 -bearing 4.5. Fluid inclusions inclusions were not observed in any samples. Inclu- sions range in morphology from irregular to anhedral Fluid inclusions are abundant in minerals from and euhedral inclusions showing partial to full devel- metasomatic garnet±epidote alteration zones. For this opment of negative crystal shapes. In addition, many study, an investigation of the petrographic features garnet-hosted inclusions have intricate semi-rectan- and examination of the simple physical properties of gular shapes that commonly define surfaces parallel inclusions was performed to constrain the nature of to zonation surfaces within large garnet crystalsŽ Fig. the metasomatising fluid. In garnet±epidote-rich 10B. . Both primary and secondary inclusion habits rocks, fluid inclusions occur predominantly within are evident, with the primary inclusions occurring as garnet and quartz. Inclusions are also observed in isolated inclusions whereas the secondary types are epidote, but are too small for physical measurements. generally small Ž.-5±10 mm and occur along healed Both two-phaseŽ. liquid±vapour and three- Ž and fractures in the host mineral. Primary inclusions are multi-.Ž phase inclusions containing one or more typically in the size range 1±20 mm, although for solid phases coexisting with liquid and vapour; e.g. practical reasons physical measurements were re- Fig. 10. are present in varying proportions in sam- stricted to inclusions greater than 5 mm across. Multi-phase inclusions may contain up to four solid phases, although the majority contain two. Halite is always present and other daughter phases include tiny platelets of hematiteŽ. Fig. 10A and an elongate birefringent mineral with straight extinction, probably anhydrite. Several other types of daughter minerals were noted but not identified. No system- atic relationship was apparent between two-, three- and multiphase inclusions and primary and secondary inclusion habits. Homogenisation temperatures and salinities for 61 fluid inclusions from two samples, BW-6A from Bulloo Well and KY-8 from Boolcoomatta, were estimated using the methods outlined above. Inclu- sions hosted in both quartz and garnetŽ both two- and three-phase. and with primary and secondary paragenesis were analysed. Results are summarised in Table 7 and Fig. 11. Sample BW-6A consists of coarsely crystalline garnet and quartz. Garnets are oscillatory-zonedŽ. from honey brown to dark brown euhedral crystals and occur within a matrix of paragenetically late quartz. Primary inclusions within garnet are up to 20 mm across, have a range of Fig. 10. Fluid inclusions from altered calc-silicate rocks.Ž. A morphologies, from irregularŽ. e.g. Fig. 10B to those Primary halite and hematite bearing aqueous liquid±vapour inclu- with well-developed negative crystal shapes. Al- sionŽ. 1 and simple two-phase aqueous liquid±vapour inclusion though the majority of garnet-hosted inclusions in Ž.2 hosted in quartz from Toraminga Hill Ž sample KY30 . . Ž B . Ž.q wŽ. Ž.x BW-6A are two-phase liquid vapour , occasional Irregular aqueous liquid±vapour inclusions 1 and 2 on the q q surface of a growth zone in garnet from Mindamereeka Hill three-phaseŽ. liquid vapour solid inclusions are Ž.sample MH-1 . also evident. Most inclusions observed in garnet 52 A.J.R. Kent et al.rLithos 54() 2000 33±62

Table 7 Summary of homogenisation temperatures and estimated salinities for inclusions from samples BW-6A and KY-8 Sample Host mineral and n Homogenisation Salinity"1s paragenesis temperature"1s Ž.8C Ž equivalent wt.% NaCl . BW-6AŽ. Bulloo Well Garnet, primary 6 319"14 23"1 Garnet, secondary 4 318"16 23"2 Garnet, two-phase 8 319"14 23"1 Garnet, three-phase 2 316"23 23"1 Quartz, primary 18 247"39 29"5 Quartz, secondary 5 267"42 29q5 Quartz, two-phase 11 254"42 25"5 Quartz, three-phase 12 248"39 33"1 KY-8Ž. Boolcoomatta Garnet, primary 7 248"17 17q2 Garnet, secondary 3 255q35 17"2 Garnet, two-phase 10 250q22 17"2 Quartz, primary 10 191q22 18q3 Quartz, secondary 8 196q34 20"3 Quartz, two-phase 18 193"27 19"3 from this sample are primary, although secondary are also observed. Inclusions in quartz occur as inclusions along growth zones and healed fractures anhedral and euhedral two- and three-phase types. Solid phases observed in three-phase inclusions include halite and anhydrite as well as several unidentified species. Numerous irregular secondary inclu- sionsŽ. up to 20 mm across also occur along healed fractures in quartz. Sample KY-8 from Boolcoomatta consists of euhedral zoned garnet and granular to subhedral epidote in a quartz matrix. Garnet contains two- and lesser three-phase primary fluid inclusions that range from 10±25 mm in size and from irregular to negative crystals in shape. In places, large secondary inclusions occur along healed fractures in garnet. Inclusions hosted in quartz range from smooth-walled Ž.with some negative crystal faces to partially irregular-shaped primary inclusions, 5±20 mm in size. Both two- and three-phase inclusions are apparent in quartz, with halite, hematite and possible anhydrite occurring as daughter phases. Thin trails of smaller secondary inclusions along healed fractures are also apparent in quartz. In general, the melting behaviour of frozen fluid inclusions is consistent with melting of a saline and chemically complex fluidŽ c.f. De Jong and Williams, 1995.Ž . After initial freezing requiring supercooling to temperatures of y808Ctoy508C. development Fig. 11. Histograms of measured salinity dataŽ in equivalent wt.% of a mottled brown appearance at temperatures down NaCl.Ž. from fluid inclusions in garnet and quartz. A KY-8 ;y 8 Boolcoomatta,Ž. B BW-6A Bulloo Well, Ž. C quartz-hosted fluid to 35 C probably reflects the presence of inclusions from clinopyroxene- and actinolite-matrix breccia zones CaCl2 in inclusionsŽ. Shepherd et al., 1979 . In addi- Ž.A.J.R. Kent and P.M. Ashley, unpublished data . tion, the observed depression of the freezing point of A.J.R. Kent et al.rLithos 54() 2000 33±62 53 two-phase inclusions below the eutectic point for the temperature of metasomatism. Peak metamorphic pure NaCl±H2 O system during freezing-point deter- pressures in the Olary Domain are estimated at 4±6 minationsŽ first melting temperatures were generally kbarŽ. Clarke et al., 1986; Flint and Parker, 1993 . y ; closer to the ca. 528C eutectic of the CaCl2 ± This corresponds to a temperature correction of NaCl±KCl±H2 O system, e.g. Shepherd et al., 1979. 200±3008C for homogenisation temperatures of flu- also suggests the presence of Ca in trapped fluids. ids in the H2 O±NaCl systemŽ. Potter, 1977 . With Final melting temperatures, marked by the disappear- such a correction applied, the estimated fluid trap- ance of clear rounded cubes of ice, ranged between ping temperatures range between 4008C and 6508C, ;y108Ctoy458C and temperatures of halite dis- and this is broadly consistent with the interpretation solution upon heating ranged between 1708C and that metasomatism occurred slightly after the peak of 2408C. These results indicate that the fluids involved metamorphismŽ which occurred at ;550±6508C; in metasomatism and quartz and garnet formation Flint and Parker, 1993. . were hypersaline, with ;15±35 equiv. wt.% NaCl Ž.Fig. 11 . Homogenisation temperatures Ž uncorrected for pressure of formation. ranged between ;1608C 5. Discussion 8 and 340 C, although for individual inclusions no 5.1. Formation of garnet±epidote-rich alteration correlation was observed between salinity and ho- zones mogenisation temperature. In both samples, the garnet-hosted inclusions have slightly higher average Garnet±epidote-rich zones exhibit features that homogenisation temperature than quartz-hosted in- are characteristic of a metasomatic origin. These clusionsŽ. Table 7 . This implies that the fluids pre- include hydrothermal textures such as vein and re- sent during garnet deposition were at slightly higher placement textures, open-space fillings and breccias, temperatures than those present during quartz forma- and an abundance of fluid inclusions. Open space tion, consistent with the mineral textures showing cavities and breccias also imply that fluid pressures that quartz postdated garnet depositionŽ similar rela- may have been locally higher than lithostatic pres- tions are shown for sample BW-3 in Fig. 4B. . Both sure, and thus metasomatic alteration zones probably quartz and garnet-hosted inclusions from sample also represent zones of focussed fluid flow. KY-8 have similar estimated salinities, whereas The nature of the fluid responsible for metasoma- quartz-hosted inclusions from BW-6A have slightly tism can be deduced from fluid inclusion properties, higher salinities than garnet-hosted. Two- and three- and the mineralogical and chemical changes which phase inclusions in garnet and quartz from BW-6A accompanied alteration. Fluid inclusions show that Žthe only sample for which measurements were made the metasomatic fluids were hypersaline Ž;15±35 on both two- and three-phase inclusions. also show equiv. wt.% NaCl. , and the freezing behaviour of similar range of homogenisation temperatures, al- inclusions and the array of daughter minerals present though salinities in quartz-hosted three-phase inclu- Žhalite, hematite, anhydrite and several unidentified sions are slightly higher than in two-phase in BW-6A phases. indicate that the fluids were chemically com- 3q 2y Ž.Table 7 . In both samples, both primary and sec- plex, and contained Na, Ca, Fe , Cl and SO4 Ž and ondary fluid inclusions show similar ranges of ho- probably several other species. . The changes in the mogenisation temperature and estimated salinityŽ Ta- bulk chemistryŽ increased Fe3qrFe2q. , the presence ble 7. . This suggests that metasomatic fluids of the of hematite as a daughter phase in fluid inclusions, same approximate composition as those responsible and formation of Fe3q -bearing mineralsŽ andradite- for metasomatism continued to circulate after min- rich garnet and epidote. in metasomatically altered eral growth. rocks also suggest that the fluid was substantially Given the uncertainties in the ambient pressure more oxidised than the pre-existing calc-silicate during metasomatism and fluid inclusion trapping, Žmetasomatism probably occurred at oxygen fugaci- and in the overall bulk composition of the metaso- ties equal to or greater than those of the hematite± matic fluids, we have made no rigorous attempt to magnetite buffer. . Comparisons of the bulk chemical use the homogenisation temperatures to constrain the composition of altered and unaltered calc-silicates 54 A.J.R. Kent et al.rLithos 54() 2000 33±62 show that metasomatic fluid were capable of mobil- 5.2. Widespread metasomatism within the Olary Do- ising many different elementsŽ again consistent with main a chemically complex fluid. , and metasomatism was accompanied by substantial changes in chemical Garnet±epidote alteration zones represent one ex- compositionsŽ. Fig. 9 . In general altered calc-silicate ample of metasomatic alteration in rocks of the rocks are enriched in Fe3q, Ca, Mn, U and Cu, and Olary Domain; however, as already described many depleted in Fe2q, Na, Mg, K, and Rb and several other styles of metasomatic alteration are also evi- altered rocks are also enriched in Pb, Zn, S and Cl. dentŽ. Table 1 . Although the detailed nature of meta- It is also possible to estimate the general condi- somatic alteration varies between different host rocks, tions under which metasomatism occurred. The pres- there are many consistencies, summarised below, ence of actinolite within garnet±epidote alteration between the chemical and mineralogical changes that zones and the retrogressive replacement of clinopy- are observed in different host rocksŽ. Table 1 , the roxene by actinolite associated with formation of timing of metasomatism, and the inferred composi- garnet±epidote zones provides some constraint on tion of the metasomatic fluids. the temperature±pressure conditions of metasoma- Collective data indicate that the majority of meta- tism. Although the stability fields of garnet, epidote somatism in the Olary Domain occurred after peak and actinolite are not well known under oxidising metamorphism and development of the fabrics re- conditionsŽ. Liou, 1972, 1974 , at lower oxygen fu- lated to the OD12 and OD deformational events and gacitiesŽ. fayalite±magnetite±quartz buffer the prior to the intrusion of S-type granitoids, although breakdown of clinopyroxene to actinolite, at pres- further studies are required to firmly establish the sures )2±3 kbar, occurs with decreasing tempera- timing of formation of each style of metasomatic ture at ;500±6008CŽ. Gilbert et al., 1982 . We alteration. For the examples listed in Table 1, meta- suggest that these are the approximate conditions of somatic assemblages overprint metamorphic miner- metasomatism within garnet±epidote alteration als and textures and in several cases alteration phe- zones. This estimate is in broad agreement with both nomenon appears to be controlled by pre-existing the range of corrected homogenisation temperatures OD2 structuresŽ Yang and Ashley, 1994; Ashley et from fluid inclusionsŽ. 400±6508C and observations al., 1998a. . Crosscutting field relations at Cathedral that suggest metasomatism occurring after peak Rock and Toraminga Hill indicate that formation of metamorphic conditionsŽ 550±6508C, 4±6 kbar; Flint clinopyroxene- and actinolite-matrix breccias oc- and Parker, 1993. . curred prior to intrusion of adjacent S-type granitoids Field and petrographic observations and Sm±Nd and associated pegmatite dykesŽ. e.g. Fig. 3B . In dating allow us to place the formation of garnet±epi- addition, A- and I-type granitoidsŽ intruded at ; dote-rich metasomatic zones within the known se- 1710±1700 and ;1640±1630 Ma, respectively. and quence of geological development of the Olary Do- associated rocks are often extensively affected by main. Sm±Nd dating suggests that the majority of metasomatism. The later S-type intrusivesŽ 1600"20 garnet±epidote-rich alteration zones formed at 1575 Ma. are only altered in localised zones where dykes "26 MaŽ although metasomatism at Bulloo Well crosscut previously metasomatised calc-silicate rocks, may have occurred slightly later than this. . This is suggesting that the majority of regional metasoma- consistent with observations which show that gar- tism predates intrusion of S-type granitoids. Lo- net±epidote-rich metasomatic alteration occurred calised episodes of fluid flow and metasomatism during the retrograde phases of amphibolite-grade probably continued for several hundred million years regional metamorphism and after the deformation of Ž.Bierlein et al., 1995; Lu et al., 1996 ; however, the the Olary sequence by the OD12 and OD events. vast majority of metasomatic activity appears to have Garnet±epidote-rich metasomatic alteration zones occurred directly after peak regional metamorphism. also postdate formation of clinopyroxene-matrix In general, the formation of metasomatic alter- breccias, but predate dykes associated with intrusion ation zones in the Olary Domain involved widespread of the regional suite of S-type granitoids at 1600"20 development of Fe-, Ca- and Na-bearing mineral Ma. assemblages, involving formation of one or more of A.J.R. Kent et al.rLithos 54() 2000 33±62 55 the following minerals: andradite-rich garnet, epi- phic assemblages in host calc-silicate rock, the pres- dote, hematite, magnetite, aegerine-bearing clinopy- ence of clinopyroxene as the dominant matrix min- roxene, actinolite and albite. These mineral assem- eral implies that it formed at pressure±temperature blages are typically more oxidisedŽ i.e. alteration conditions close to those of the primary metamorphic involves an increase in the Fe3qrFe2q ratios and assemblages. Primary actinolite-matrix breccias alteration assemblages contain an array of Fe3q-rich within the northern portion of the Olary Domain minerals. than their un-metasomatised equivalents. formed in areas that experienced lower peak meta- Metasomatic clinopyroxenes from clinopyroxene- morphic temperaturesŽ. Fig. 1 . At many locations, matrix breccias and metasomatically altered iron- however, clinopyroxene-matrix breccias are variably 3qr 2q stones also have higher Fe Fe ratios and Na2 O retrogressed to actinoliteŽ. Fig. 4A . Given that the contents than clinopyroxenes from laminated calc- reaction of clinopyroxene to form actinolite occurs silicate rocksŽ. Fig. 5 , and clinopyroxene- and acti- with decreasing temperaturesŽ. Gilbert et al., 1982 nolite-matrix breccias contain fluid inclusions that actinolite retrogression of clinopyroxene-matrix have hematite as a daughter phase. breccias appears to represent continued metasomatic The chemical changes associated with different activity at lower temperatures. In addition, garnet± styles of metasomatic alteration typically involve epidote-rich alteration zones are observed to over- addition of Fe, Ca and Na and loss of K, Rb and Mg print clinopyroxene-matrix breccias at Mindame- Ž.Table 1 and metasomatic fluids appear to have reeka Hill, and garnet±epidote alteration zones in been highly saline and chemically complex. Hyper- calc-silicate rocks contain accessory actinoliteŽ and saline fluid inclusions with an array of daughter are associated with retrogression of metamorphic mineral phases, similar to those observed in garnet± clinopyroxene to actinolite. . This suggests that this epidote-rich alteration zones, have been documented style of alteration also represents a later episode of from a number of different metasomatic rocks, in- fluid movement that occurred at slightly lower ambi- cluding intense zones of albiteŽ. ±quartz±actinolite ent temperatures than formation of the clinopyrox- alteration in quartzofeldspathic rocks, clinopyroxene- ene-matrix breccias. and actinolite-matrix brecciasŽ. e.g. Fig. 11 and from We therefore suggest that the Olary Domain pro- epigenetic ironstonesŽ Yang and Ashley, 1994; Ash- vides an excellent example of a terrane that has ley et al., 1998b; A.J.R. Kent and P.M. Ashley, experienced widespread metasomatic activity during unpublished data. . the retrograde stages of a major regional metamor- The consistencies in the timing, alteration style phic event. Metasomatism resulted in significant and metasomatic fluid associated with different styles changes to the mineralogical and chemical constitu- of metasomatic alteration suggests that the Olary tion of the terrane and continued for some time as Domain experienced a major episode of metasomatic terrane cooled following metamorphism. alteration, involving the action of saline, oxidised and chemically complex fluids, during the retrograde 5.3. Source of metasomatising fluids stages of amphibolite-grade regional metamorphism. However, evidence suggests that metasomatic activ- Two primary possibilities exist for the origin of ity was not simply restricted to a single fluid pulse, the hypersaline and oxidised fluids responsible for but probably occurred during a succession of fluid metasomatic alteration:Ž. i fluids may have derived flow events as the terrane cooled from peak meta- from the crystallisation of one or more types of morphic temperatures. The best evidence for this are granitoids; orŽ. ii fluids could have been derived the crosscutting and petrological relationships ob- from within the Olary Domain sequence, or from served between the different styles of metasomatic similar crustal rocks located at deeper structural lev- alteration that occur in calc-silicate host rocks. els, by metamorphic devolatilisation reactions. Metasomatic calc-silicate-matrix breccias in the Field relations and radiometric dating in the Olary southern and central Olary Domain are dominated by Domain show that a local temporal association be- clinopyroxene. Although clinopyroxene-matrix brec- tween the emplacement of S-type granitoids and cias clearly postdate the formation of peak metamor- associated pegmatites and some occurrences of meta- 56 A.J.R. Kent et al.rLithos 54() 2000 33±62 somatically altered rocks. This association could also date, however, magnetic, gravity and exploration suggest that metasomatic fluids are also derived from drilling results have failed to confirm the hypothesis. crystallising S-type plutons. However, the following We suggest that the saline and oxidised metaso- evidence argues against a direct contribution to matic fluids responsible for garnet±epidote alteration metasomatic fluids from crystallising regional S-type zones and other metasomatic alteration types were granites. most likely derived from metamorphic devolatilisa- Ž.i Metasomatic fluids in the alteration zones are tion of crustal rocks, dominated by metasediments in highly oxidised Ž.G hematite±magnetite buffer , the Olary Domain sequence. Some of these rocks whereas the regional S-type granitoids in the Olary may have originally contained oxidised sequences district are relatively reduced and ilmenite-bearing. Ž.e.g. red beds and evaporites, and are now manifest Ž.ii Although there is a temporal association be- as hematite- and magnetite-bearing laminated al- tween some metasomatic rocks and rocks related to bitites, calcalbitites and certain calc-silicate-rich the S-type granitoids, there is no consistent spatial rocks of the Willyama SupergroupŽ Cook and Ash- relationship between S-type granitoids and metaso- ley, 1992. . During peak metamorphism, the break- matised rocks in the Olary districtŽ. Fig. 1 . Many down of hydrous and volatile-bearing phases may metasomatic rocks occur away from known outcrops have released large volumes of fluids and devolatili- of S-type granitoids and many large areas of grani- sation may have also been facilitated by the thermal toid and adjacent rocks are devoid of metasomatic effects of intrusion of S-type granitoids. Preliminary alteration. results from oxygen isotopic studies on regional and Ž.iii S-type granitoids do not show evidence of local scale Na±Fe±Ca alteration zonesŽ including strong sub-solidus fluid accumulationŽ e.g. miarolitic garnet±epidote alteration zones. are consistent with cavities, fracture-controlled or pervasive alteration of fluids being derived from crustal rocksŽ R. Skirrow plutons or adjacent country rocks, potassic alteration, and P.M. Ashley, unpublished data. . These may be or greisen development. . metamorphic waters that equilibrated with the Olary Ž.iv Preliminary O-isotope studies of metasomatic Domain sequence; however, direct input from evap- rocksŽ. see below show little evidence for the contri- oritic brines or magmatic fluids were evidently mini- bution of magmatic fluids to the metasomatising mal. The post-metamorphic timing of metasomatic fluids. alteration phenomenon may indicate that peak meta- We also believe that metasomatic fluids in the morphic conditions were attained at structurally Olary Domain are unlikely to be related to I-type deeper levels at slightly later times than they were intrusive rocks. Regional-scale alteration zones in attained in rocks currently exposed at the surface. other Australian Proterozoic regions, such as the Mt. We also note that, although our observations rule Isa eastern succession in northwest Queensland and out the direct contribution of metasomatic fluids the Gawler CratonŽ. to the west of the Olary Domain from crystallising granitoids, it is harder at present to have been suggested to, at least partially, be related constrain the contribution of metasomatic fluids re- to fluids released by I-type granitoidsŽ De Jong and leased by crystallising plutons at deep structural Williams, 1995; Oliver, 1995; Conor, 1998; David- levels and subsequently heavily modified by interac- son, 1998; Williams, 1998. . However, in the Olary tion with crustal rocks. Studies of porphyry copper Domain, the only known I-type granitoids were em- depositsŽ. e.g. Cline and Bodnar, 1991 have shown placed at ;1640±1630 MaŽ some 70±80 Ma prior that at high pressures Ž.)2 kbar fluids evolved from to metasomatism; Ashley et al., 1998a. ; these intru- crystallising granitoids can be highly salineŽ up to 60 sions are clearly pre-alteration, are spatially re- wt.% NaCl. . Interaction of such fluids with crustal stricted and have no relation to the regional-scale sequences at depth could substantially modify the occurrence of altered rocks. It may be inferred that oxygen fugacity and O-isotope signature of these laterŽ. e.g. Mesoproterozoic I-type granitoids could fluids and render them difficult to discern from occur in the Olary Domain, at depth, or under fluids related to devolatilisation of crustal rocks. In younger cover sequences, and have a relationship to addition, control of metasomatic circulation by pre- alteration zones and to Cu±Au mineralisation. To existing crustal structures could supplant spatial as- A.J.R. Kent et al.rLithos 54() 2000 33±62 57 sociations between deep plutons and the sites of fluids from evolved I-type granitoids were also in- metasomatic alteration. volvedŽ. e.g. Pollard et al., 1998; Williams, 1998 . Metasomatism within the Olary Domain also has important implications for the metallogenic status of 5.4. Similarities to other Proterozoic regions and this region. We have shown that the formation of implications for ore deposition garnet±epidote alteration zones and other metasomatic rocks in the Olary Domain involved oxidised The widespread metasomatism involving saline saline fluids. These fluids would have been capable oxidised fluids in the Olary Domain is similar to that of transporting significant quantities of metalsŽ e.g. observed in other Proterozoic regions in Australia Fe, Cu, Au, Mo, Zn, Pb, Ag, REE, U and Mn. in Že.g. Davidson, 1994, 1998; Williams, 1994, 1998; solution at the temperatures implied from fluid inclu- De Jong and Williams, 1995; Oliver, 1995; Oliver et sion resultsŽ e.g. Hemley et al., 1992; Seward and al., 1998; Conor, 1998. and elsewhere in the world Barnes, 1997. . Although garnet±epidote-rich rocks Že.g. Kalsbeek, 1992; Barton and Johnson, 1996; from metasomatic alteration zones contain only mod- Frietsch et al., 1997.Ž. . Barton and Johnson 1996 est enrichments of Cu, Zn, Pb and UŽ. Fig. 9 , there have proposed that a worldwide link exists between may be a spatial and genetic link between these Proterozoic and Phanerozoic Fe-richŽ REE±Cu±Au± rocks and sites where significantŽ. i.e. ore grade U-bearing. hydrothermal deposits and evaporitic metal deposition could occur. The strongly oxidised source rocks, whereby devolatilisation associated nature of the garnet±epidote rocks may not be con- with igneous intrusions forms the oxidised S-poor ducive to sulfide deposition, but in rock types in brines responsible for the observed hydrothermal which metasomatism would involve major redox mineralisation. changesŽ e.g. graphitic pelite, psammopelite, calc- In the Mt. Isa Block Eastern Succession in north- silicate-bearing pelite. , or in reactive rock types western Queensland, examples of widespread Na, Ž.marble, iron-formations and mafic rocks substantial Na±Ca and Fe-metasomatism related to saline and metal deposition could occur. In the Olary Domain, chemically complex fluids are recorded, both region- several historic mineral prospects, as well as new allyŽ e.g. Oliver and Wall, 1987; Oliver, 1995; discoveries, display a characteristic Cu±Au±Mo as- Williams, 1998. and on more localised scales sociationŽ in places with anomalous Co, Zn, As, U, ŽDavidson, 1994, 1998; Williams, 1994; De Jong Ba, REE.Ž Ashley et al., 1998a; Skirrow and Ashley, and Williams, 1995. . It has been proposed that cer- 1999. . These deposits all occur within the above tain types of epigenetic Cu±Au and U±REE mineral- lithological settings, in places mediated by fracture isation in this region are spatially and genetically systems, but with no substantiated genetically associ- related to alterationŽ e.g. Davidson and Large, 1994; ated granitoids. As the solubility of Zn and Pb Williams, 1994, 1998; Oliver, 1995; Adshead, 1995; remains high at temperatures )3008C, in saline Davidson, 1998. . Although differences are evident in fluids with high ClrS and low reduced SŽ cf. Hem- the style and nature of alteration in the Mt Isa Block ley et al., 1992. , it is probable that mineralisation Eastern Succession and the Olary Domain, these are associated with metasomatic fluids may be limited to probably due to localised factors, such as host rocks, Fe±Cu sulfides"molybdenite"gold. fluid histories, and P±T conditions of metasomatism. A common theme is the action of hypersaline Ž.generally Na±Ca±K±Fe-bearing and locally oxi- 6. Conclusions dised fluids. It is also possible that the saline fluids responsible for the Mt. Isa Block Eastern Succession The Proterozoic Olary Domain is an excellent Na±Ca alterationŽ and locally associated Cu±Au example of a terrane that has been significantly mineralisation. were at least partly evolved from a altered by metasomatic mass-transfer processes asso- former evaporitic-bearing sequenceŽ e.g. Oliver and ciated with regional metamorphism. Metasomatically Wall, 1987; Oliver, 1995; De Jong and Williams, altered rocks in the Olary Domain are ubiquitous 1995. , although recent models infer that magmatic and include garnet±epidote-rich alteration zones, 58 A.J.R. Kent et al.rLithos 54() 2000 33±62 clinopyroxene- and actinolite-matrix breccias, re- within the Olary Domain probably derived from placement ironstones and albite-rich alteration zones devolatilisation of a rift-related volcano-sedimentary in quartzofeldspathic metasediments and intrusive sequence, perhaps containing oxidised and evaporitic rocks. Metasomatism is typically associated with source rocks at deeper structural levels, during re- formation of Ca, Na andror Fe-bearing oxidised gional metamorphism and deformation. Although mineral assemblages. metasomatic rocks are temporally associated with Detailed study of garnet±epidote-rich alteration S-type granitoid intrusive rocks, there is no evidence zones in calc-silicate host rocks, one common mani- that the metasomatic fluids have been directly sourced festation of intense metasomatic alteration, provides from granites. detailed information on the nature and timing of metasomatism, and the composition of the responsible fluids. Metasomatism occurred at temperatures between ;4008C and 6508C, and involved loss of Acknowledgements Na, Mg, Rb and Fe2q, gain of Ca, Mn, Cu and Fe3q and mild enrichment of Pb, Zn and U. Fluid inclu- We wish to acknowledge funding for this work sions show that the hydrothermal fluids responsible from the Australian Research Council, Primary In- for the formation of garnet±epidote-rich assemblages dustries and Resources South Australia and a consor- were chemically complexŽ containing Na, Ca, Fe3q , tium of Australian mineral exploration companies 2y Cl, and SO4 .Ž , hypersaline and oxidised at or above who have supported the Olary Mapping Project. We the hematite±magnetite buffer. . Sm±Nd isotopic have also drawn on work by the following honours analyses show that garnet±epidote-rich alteration students from the Universities of New England and zones formed at 1575"26 Ma, consistent with field Melbourne: James Anderson, Andrew Chubb, Will and petrographic observations that suggest that meta- Eykamp, Michael Fechner, Mark Kent, Maree Laf- somatism occurred prior to the latter stages of re- fan, Mark Pepper, Gary Rolfe and Jane Westaway. gional-scale intrusion of S-type granites at 1600"20 Assistance with analytical work was provided by Ma. Rick Porter, Peter Garlick, John Bedford and Gael Evidence suggests that the majority of metaso- Watson. Discussions with colleagues Frank Bierlein, matic alteration throughout the Olary Domain was Colin Conor, Nick Cook, Dave Lawie, Bernd Lotter- broadly contemporaneous and involved the action of moser, Ian Plimer, Roger Skirrow and Kai Yang also oxidised hypersaline fluids. We suggest that contributed to this work. In addition, AJRK would widespread episodes of fluid flow and metasomatic like to especially thank Professor M. Tattersall and alteration affected the Olary Domain during the ret- the staff of RPAH, Sydney. Reviews by J.L.R. Touret rograde phases of a major regional metamorphic and P.J. Williams improved this manuscript consid- event. The fluids responsible for metasomatism erably.

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

AMG Ð Australian Map Grid coordinates Location Sample Description Bulloo Well BW-1 Altered calc-silicate: Massive wxAMG 442900E 6498450N garnet±Ž. quartz±epidote . BW-2 Altered calc-silicate: Garnet±epidote Ž.actinolite±quartz±albite . Alteration pseudomorphs folded laminae in original calc-silicate. A.J.R. Kent et al.rLithos 54() 2000 33±62 59

BW-6A Altered calc-silicate: Massive garnet±quartzŽ. epidote . R73358 Altered calc-silicate: Massive garnet Ž.±quartz±epidote±hornblende . R73357 Unaltered calc-silicate: Laminated clinopyroxene±K-feldspar±albite±quartz± hornblende±epidoteŽ. ±titanite . Sylvester Bore SB-1 Unaltered calc-silicate: Laminated wxAMG 427500E 6437500N clinopyroxene±quartz±K-feldspar±albite Ž.±actinolite±titanite with minor actinolite alteration of clinopyroxene. SB-3 Altered calc-silicate: Massive garnet Ž.±epidote±quartz±albite . Mindamereeka Hill MH-1 Altered calc-silicate: Laminated clinopyroxene±quartz± wxAMG 396100E, 6453200N albiteŽ. ±scapolite±K-feldspar±titanite grading to bleached albite±quartz-rich rock and massive coarse garnetŽ. up to 3 cm . MH-2 Partially altered calc-silicate: Laminated clinopyroxene±quartz±albite Ž.±scapolite±K-feldspar±titanite with patchy alteration of clinopyroxene to actinolite and extensive replacement along laminations and fractures by garnet±epidote±quartz. MH-3 Relatively unaltered calc-silicate: Laminated clinopyroxene±albite quartz Ž.±actinolite±K-feldspar±titanite with minor alteration to garnet±epidote± quartz along fractures. Boolcoomatta BC-1 Relatively unaltered calc-silicate: Laminated wxAMG 455200E 6462800N clinopyroxene±albite±K-feldspar±quartz Ž.±titanite with small zones of epidote alteration. BC-4 Altered calc-silicate: Massive garnet±epidote± quartzŽ. ±hematite with relicts clinopyroxene partly replaced by actinolite. BC-8 Brecciated and altered calc-silicate: Bleached quartz±albite fragments in a epidote Ž.±actinolite±garnet matrix. KY-8 Altered calc-silicate: Garnet±quartz Ž.±epidote±actinolite±clinopyroxene . Relict clinopyroxene largely altered to actinolite. Sampson Dam R74779 Unaltered calc-silicate: Laminated quartz± wxAMG 445680E 6450640N clinopyroxene±albite±actinolite±epidote Ž.±titanite R74780 Altered calc-silicate: Massive garnet Ž.±quartz±epidote±hornblende±albite 60 A.J.R. Kent et al.rLithos 54() 2000 33±62

R74782 Altered calc-silicate: Massive garnet±epidote±quartzŽ. ±albite White Dam North R77351 Altered calc-silicate: Massive coarse grained wxAMG 453400E 6451520N garnetŽ. ±epidote±quartz R77354 Altered calc-silicate: Massive coarse grained magnetite±quartzŽ. ±albite rock enclosed in garnet-rich alteration zone. R77353 Relatively unaltered calc-silicate: Laminated clinopyroxene±quartz±albite Ž.±K-feldspar±titanite with minor alteration to actinolite and epidote.

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