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

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1 Ž. Lithos 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 Fe 2q, gain of Ca, Mn, Cu and Fe 3q 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 hypersaline, oxidised, and chemically complex, containing Na, Ca, Fe 3q, Cl, and SO4 2y. 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, Øster Volgade 10, 1350 Copenhagen K, Denmark. Fax: q address: ajrk@dlc.ku.dk Ž A.J.R. Kent.. 1 Fax: q Fax: q Fax: q r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. Ž. PII: S

2 34 ( ) A.J.R. Kent et al.rlithos " 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 Rocks that have experienced metamorphism comprise a large proportion of the continental regions and thus an understanding of the changes that are associated with metamorphic activity is critical for gauging the chemical and mineralogical evolution of the continental crust. Traditional studies of metamorphic phenomena have emphasised the isochemical mineralogical changes caused by metamorphic reequilibration under differing pressure and temperature regimes. However, metasomatic mass-transfer of chemical components is increasingly recognised as an important process accompanying regional metamorphism Že.g., Chinner, 1967; Yardley and Baltatzis, 1985; Ferry, 1992; Ague, 1994a,b, 1997; Oliver et al., Metasomatic redistribution of volatile and fluid-mobile non-volatile chemical components during the prograde and retrograde phases of regional metamorphism can profoundly influence the final chemical and mineralogical status of a metamorphosed terrane Ž Ague, Such changes must be quantified in order to understand the effects that metamorphic and related metasomatic processes can produce on rock masses. In this study we have investigated the role of metasomatism in the formation and evolution of rocks from the Proterozoic Willyama Supergroup in the Olary Domain of eastern South Australia. The Olary Domain, part of the Curnamona Province, a major Proterozoic terrane located within eastern South Australia and western New South Wales, Australia Ž Fig. 1., provides an excellent example of a geological terrane that has been significantly effected by metasomatic processes Že.g. Cook and Ashley, 1992; Ashley et al., 1998a,b.. Within the Olary Domain, the chemical and mineralogical compositions of rocks within the Willyama Supergroup have been strongly altered by metasomatic processes, and examples of regional and local scale metasomatic alteration phenomenon are numerous and widespread Že.g. Cook and Ashley, 1992; Ashley et al. 1998a,b; Skirrow and Ashley, The metasomatic features of rocks from the Olary Domain also have strong analogies with alteration phenomena that have been documented in other Proterozoic terranes Žboth elsewhere in Australia and in other parts of the world., some of which are associated with Cu, Au, Fe and U mineral deposits ŽKalsbeek, 1992; Frietsch et al., 1997; Oliver et al., 1998; Williams, In this study, the major styles of metasomatic alteration in the Olary Domain are documented and described; to do this we both present new information and review results of earlier studies in the region. Further, in order to constrain the timing and nature of metasomatic alteration, and to investigate the composition of the responsible fluids, a detailed study has been undertaken on a specific type of metasomatic rock, viz. skarn-like garnet epidotebearing alteration zones within laminated calc-silicate rocks. This style of metasomatic alteration, which occurs throughout the Olary Domain Ž Fig. 1., is a manifestation of intense mineralogical and chemical change resulting from focused fluid passage, and therefore provides an opportunity to investigate the nature, origin and effects of the metasomatising fluids. In addition, as these rocks are suitable for Sm Nd isotopic dating studies, they allow important constraints to be placed on the timing of metasomatic activity. Directly after the attainment of peak regional metamorphic conditions, the Olary Domain experienced regionally extensive episodes of the passage of hot, saline and oxidised aqueous fluids. The fluids responsible for metasomatic alteration were probably derived from metamorphic devolatilisation of crustal rocks, largely a sedimentary Ž felsic volcanic. sequence. Importantly, although we suggest that intru-

3 ( ) A.J.R. Kent et al.rlithos 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 andalusite chloritoid. and IIB Ž andalusite sillimanite. of Clarke et al. Ž The approximate position of 1600" 20 Ma granitoids is also shown. sion of granitoid rocks may have been an important factor in promoting devolatilisation reactions in the surrounding wallrocks, there is no clear evidence for the direct contribution of water derived from crystallising granitoids to the metasomatising fluids. 2. Analytical methods Descriptions and locations for all samples analysed for whole rock and mineral chemical compositions, Sm Nd isotopic composition and fluid inclusions are given in Appendix A Rock and mineral analysis Samples of altered and unaltered calc-silicate rocks were analysed for major and trace elements by X-ray fluorescence at the University of Melbourne and University of New England, Armidale, Australia, using Siemens SRS-300 instruments. Mineral compositions were measured using a JEOL 5800

4 36 ( ) A.J.R. Kent et al.rlithos scanning electron microscope run in EDS mode at a beam current of 25 na at the University of New England and using a range of natural standards for calibration of X-ray intensities. Mineral compositions, for phases containing Fe 2q and Fe 3q were calculated assuming stoichiometry Fluid inclusions Fluid inclusion heating and cooling determinations were performed using a modified USGS heating cooling stage. Repetition of measurements indicated that individual determinations were reproducible 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 Ž For halite-bearing three-phase inclusions, salinity estimates were derived from the melting point of halite and the phase relations outlined by Sourirajan and Kennedy Ž These calculations are for the pure NaCl H 2O system and given the chemically complex nature of the fluids responsible for the formation of garnet epidote metasomatic zones Ž 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 H 2 O system, indicating that other cations Ž e.g. Ca. must be present Ž Roedder, epidote, garnet, actinolite and quartz., and not older metamorphic minerals Žsee discussion below on the effect of this on isochron calculations.. Samples for analysis were weighed into dissolution vessels, spiked with a mixed 146 Ndr 150 Sm solution and dissolved using HF HNO 3 HCl acid digestion. Sm and Nd were separated and purified using 3g cation exchange and HDEHP-teflon columns using the procedure outlined in Bennett et al. Ž Samples were loaded onto the Ta side of a double Re Ta filament and analysed using a FinneganrMAT 261 multicollector mass spectrometer in static mode at the Research School of Earth Sciences, Australian National University. 3. Geological setting The Olary Domain constitutes one of the inliers of the Palaeoproterozoic Willyama Supergroup that occur in northeastern South Australia and western New South Wales, Australia Ž Fig. 1.. The geology of the Olary Domain has been summarised by Clarke et al 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 approximately 95 98%. In order to avoid the incorporation of older Ž 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- Ž cur, they consist of other metasomatic minerals e.g. Fig. 2. Olary Domain sequence Žmodified from Ashley et al., Abif B denotes bonded iron formation.

5 ( ) A.J.R. Kent et al.rlithos Ž 1986, 1987., Cook and Ashley Ž 1992., Flint and Parker Ž 1993., Robertson et al. Ž and Ashley et al. Ž 1998a.. The Olary Domain sequence Ž Fig. 2. displays broad regional correlations with the Willyama Supergroup in the adjacent Broken Hill Block, although there are differences in detail ŽCook and Ashley, 1992; Preiss, The Willyama Supergroup has been interpreted to represent a failed Palaeoproterozoic rift Ž Willis et al., and the Olary Domain is considered to represent a marginal portion of this rift, possibly involving a continental lacustrine and sabkha setting grading upwards into a marine environment Ž Cook and Ashley, The lower part of the Olary Domain sequence is occupied by the Quartzofeldspathic Suite, comprising quartzofeldspathic and psammopelitic composite gneiss grading into regionally coherent units including the ALower AlbiteB, dominated by ; Ma A-type metagranitoids and co-magmatic felsic metavolcanic rocks ŽAshley et al., 1996; Page et al., 1998., the AMiddle SchistB, composed of psammopelitic schist and composite gneiss, and the AUpper AlbiteB, dominated by finely laminated albitite, as well as minor amounts of iron formation, locally grading into barite-rich rock. The Quartzofeldspathic Suite grades up-sequence into the Calcsilicate Suite, typified by laminated calcalbitites and minor Mn-rich calc-silicate rocks. We use the term Acalc-silicateB to describe a metamorphic rock containing more than 25 modal% calc-silicate minerals Žtypically amphibole, clinopyroxene, plagioclase, garnet, and epidote., whereas the term AcalcalbititeB refers to a quartz albite rock with up to 25 modal% calc-silicate minerals.. The Calcsilicate Suite displays up-sequence transition into the Bimba Suite, dominated by calc-silicate rocks and marble, locally with Fe ŽCu Zn. sulfides, graphitic pelite and albitite. The Bimba Suite is overlain by the Pelite Suite, composed of pelitic and psammopelitic schist, with local graphitic facies, psammite, calc-silicate rock, tourmalinite and manganiferous iron formation. It is interpreted that the Willyama Supergroup sequence in the Olary Domain was largely deposited between ; Ma, although the younger age limit is not well-constrained ŽAshley et al., 1998a; Page et al., The Olary Domain sequence has been intruded by several suites of plutonic rocks as well as having been subject to at least five deformation and metamorphic events ŽClarke et al., 1986, 1987; Flint and Parker, Temporal relationships between intrusive, metamorphic and deformational episodes have been investigated by field studies and by zircon U Pb and muscovite 40 Ar 39 Ar geochronology ŽClarke et al., 1986, 1987; Flint and Parker, 1993; Cook et al., 1994; Bierlein et al., 1995; Lu et al., 1996; Ashley et al., 1996; 1998a; Page et al., 1998., and the following summary is taken from these studies. Note that previous interpretations Že.g. Flint and Parker, have ascribed the first three deformation events in the Olary Domain Ž OD OD. 1 3 to the Olarian Orogeny, a major episode of deformation and metamorphism that occurred between ; 1600 and 1500 Ma. More recent field and geochronological studies imply that an earlier deformation event occurred prior to ; Ma ŽAshley et al., 1998a; cf. Nutman and Ehlers, 1998.; however, for this study we will continue to use the OD 1 OD3 notation of Flint and Parker Ž Two later deformation events Ž DD, DD. 1 2 are related to Delamerian orogeny Ž ; Ma.. Initial deposition of the Willyama Supergroup sequence in the Olary Domain commenced at ;1700 Ma, A-type granitoids were intruded and co-magmatic rhyolitic volcanic rocks were erupted at ; Ma Ž Ashley et al., Recent observations Ž Ashley et al., 1998a. suggest that the Willyama Supergroup was then deformed prior to intrusion of several mafic igneous masses and small I-type granitoid bodies into the central part of the Olary Domain at ; Ma. A major episode of deformation Ž OD and OD. 1 2 and amphibolite grade metamorphism affected much of the Olary Domain at ;1600 Ma. This resulted in formation of two sub-parallel planar deformation fabrics ŽOS1 and OS. 2 and development of tight to open, upright to steeply inclined folds related to OD 2. Peak regional metamorphic conditions were also attained during OD1 and OD2 and studies of pelitic rocks by Clarke et al. Ž indicated that grades were highest in the southern and central portions of the Olary Domain, reaching upper amphibolite facies, with estimated maximum pressures of 4 6 kb and temperatures of C Ž Flint and Parker, Peak metamorphic conditions decrease to the north to lower amphibolite and greenschist facies ŽClarke et al., 1987;

6 38 ( ) A.J.R. Kent et al.rlithos 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-syntectonic; intruded at the end of OD2 event. Retrograde metamorphism and alteration, including a retrograde deformation event, OD 3, continued episodically between ;1580 and ;1500 Ma. OD 3 deformation was largely restricted to discrete shear zones along which greenschist facies assemblages were developed Ž Clarke et al., Further thermal perturbations also occurred during the Musgravian Orogeny at ; Ma Ž Lu et al., Mafic dyke emplacement at ;820 Ma was a precursor to development of the Adelaide Geosyncline in the region Ž cf. Wingate et al., and at least two episodes of localised low grade metamorphism and deformation occurred between ; 500 and 450 Ma during the Delamerian Orogeny ŽClarke et al., 1986; Flint and Parker, Episodes of fluid flow accompanied most of these later thermal events Že.g. Bierlein et al., 1995; Lu et al., Regional and local scale metasomatic alteration in the Olary Domain In addition to the magmatic, metamorphic and deformational history outlined above, the rocks of the Olary Domain have experienced a long history of fluid rock interaction, metasomatism and hydrothermal alteration Že.g. Cook and Ashley, 1992; Ashley et al., 1998a,b; Skirrow and Ashley, The effects of metasomatism in the Olary Domain are evident on a variety of spatial scales, ranging from regional-scale Ž kilometres. alteration zones in sediments, felsic volcanic and intrusive rocks through to localised examples of fluid flow such as breccia zones and local fracture systems Ž e.g. Fig. 3.. Although the manifestations of metasomatism are heterogeneously distributed and both lithologic and structural controls are apparent, examples of fluid rock interactions are so numerous that it is clear that metasomatic processes have been an intrinsic part of the development of the Olary Domain Ž Table 1.. In general, metasomatism has resulted in enrichment of Fig. 3. Examples of metasomatic alteration in the Olary Domain. Ž A. Brecciated calc-silicate from Cathedral Rock. Breccia consists of angular bleached albite-rich clasts in a dark matrix of diopside and actinolite Ž pen shown for scale is 14 cm long.. Ž B. Altered and bleached laminated calc-silicate at Mindamereeka Hill intruded and cut by thin parallel dykes of pegmatite. Ž C. Actinoliterich veins surrounded by bleached albite-rich alteration selvages in altered I-type granite Ž Tonga Hill.. Ž D. Fracture-controlled alteration in laminated psammopelitic sediments Ž White Rock.. Fe, Na Ž Fe., Ca Ž Na Fe. or, less commonly, Fe K and is most commonly evident in quartzofeldspathic rocks, granitoids, calc-silicates and calcalbitites, marbles and iron-formations. Metasomatic assemblages are typically more oxidised than original assemblages with a variety of Fe 3q minerals present Že.g. epidote, magnetite, hematite, andradite-rich garnet and aegirine-bearing clinopyroxene.. The typical styles of metasomatic alteration evident in the Olary Domain are summarised in Table 1 and several examples are shown in Fig. 3. The timing of regional-scale metasomatic activity is constrained by field relations and geochronology. Regional-scale alteration zones occur in, and thus

7 ( ) A.J.R. Kent et al.rlithos 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 albitisation Žbleach- 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 albitisation Žbleach- Replacement of plagioclase, K- Addition of Na Ž Fe,S,Cu,K.; loss ing., minor development of mag- feldspar and biotite by albite, of K,Rb Ž Ca,Sr. netite " hematite, sulfides in minor magnetite, hematite, veins and disseminations. Local pyrite, chalcopyrite. Local develbiotiteqmagnetite. opment of biotiteqmagnetite Calc-silicate and calcalbitite Pervasive bleaching grading to Destruction of clinopyroxene, ti- Addition of Ca, Fe, Mn, rocks massive clinopyroxene- and acti- tanite, K-feldspar and scapolite; Ž "Cu,Pb,Zn,U.; loss of Na, 3q 2q nolite-matrix breccias andror formation of secondary Na Fe Fe, Mg, Rb, Ba massive garnet epidote alter- clinopyroxene, amphibole, albite, ation zones Ž Fig. 3A,B. quartz, andraditic garnet, epidote Granitoids: A-types Albitisation Ž bleaching., minor Destruction of plagioclase, K- Addition of Na Ž Fe.; loss of Fe oxide veining and dissemina- feldspar Ž biotite. and formation K,Rb tions of albite Ž " magnetite, quartz. S-types Localised albitisation Ž 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 Na Ž Ca,Fe.; loss of trolled and pervasive bleaching and biotite; pervasive albitisa- K,Rb with minor brecciation Ž 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 FeŽCu,Au,U,V,Y, of laminated texture magnetite, hematite, local pyrite, Zn,S.; loss of Si trace chalcopyrite 3q predate, A-type intrusives and associated volcanics emplaced at Ma and I-type granites emplaced at Ma. In addition, in all locations observed, metasomatic mineral assemblages retrogress peak metamorphic assemblages Že.g. Ashley et al., 1998a,b. and indicate that metasomatic activity occurred after the metamorphic peak and the development of OD2 deformation textures. As discussed below, metasomatic alteration zones located in calc-silicate rocks are also cut by S-type granites and related pegmatites at several localities demonstrating that the majority of alteration occurred prior to S-type granitoid emplacement at ; 1600" 20 Ma. In several locations Ž e.g. Cathedral Rock., metasomatically altered rocks are deformed and retrogressed within OD3 deformation zones, indicating that the majority of metasomatic alteration occurred prior to formation of these zones at ; 1500 Ma. However, 40 Ar 39 Ar ages on metasediments and pegmatite muscovite also suggest that fluid movement continued episodically along OD 1 OD3 structures for several hundred million years after granite intrusion Ž Bierlein et al., 1995; Lu et al., Reactivation of structures during the Delamerian Orogeny is indicated by ;470 Ma 40 Ar 39 Ar ages of muscovites from pegmatites and OD shear zones Ž 3 Lu et al., Bierlein et al. Ž also demonstrated that at least some of the epigenetic sulfide mineralisation within OD3 shear zones occurred between ; 480 and 450 Ma during retrograde fluid movement along older structures.

8 40 ( ) A.J.R. Kent et al.rlithos Metasomatism in calc-silicate rocks Although many different rock types have been effected by metasomatism within the Olary Domain, fluid rock interaction appears to have been especially intense in calc-silicate and associated rocks. The two most common alteration styles evident are calc-silicate-matrix breccia zones Žconsisting of both clinopyroxene- and actinolite-matrix breccias. and garnet epidote-rich alteration zones. Although these two styles of metasomatic alteration occur in similar host rocks, and are often spatially and temporally associated Ž see below., they are associated with distinctly different styles of metasomatic alteration and will be treated differently for purposes of description and discussion. In Section 4 we describe the petrological, chemical and mineralogical features of garnet epidote-rich alteration zones in detail. These zones are the focus of our research as they appear to have formed at relatively high fluidrrock ratios in areas of focussed fluid flow and thus provide an excellent opportunity to assess the nature of metasomatic fluids and the chemical changes associated with metasomatism. However, as the clinopyroxene- and actinolite-matrix breccias are also observed to be spatially and temporally related to the formation of garnet epidote-rich alteration zones, and appear to have formed from similar composition metasomatic fluids, we believe that understanding the relation between these metasomatic breccias and garnet epidote-rich metasomatic alteration zones provides important insights into the nature of metasomatism within the Olary Domain. To this end, both clinopyroxene- and actinolite-matrix breccias are briefly described in the remainder of this section. Clinopyroxene and actinolite-matrix breccias form many spectacular outcrops in the Olary Domain, Že.g. Cathedral Rock, Toraminga Hill, Telechie Valley; Figs. 1 and 3A. and have been discussed by Cook and Ashley Ž and Yang and Ashley Ž Calc-silicate-matrix breccias are commonly stratabound, range from irregular and locally trangressive bodies up to tens of metres across down to narrow piercement masses and are associated with zones of hydrothermal alteration, involving albitisation Žwhite AbleachingB and local pink hematitic pigmentation. in the host calcalbitite. Field relations imply that breccias formed during deformation as they contain rare folded fragments and appear to have been injected into fractures in fold hinges interpreted to be temporally linked to OD Ž 2 Yang and Ashley, In several locations, breccias have also been intruded by S-type granite and pegmatite related to the ;1600"20 Ma episode Že.g. Cathedral Rock, Toraminga Hill. and have been deformed by OD shear zones Ž e.g. Cathedral Rock. 3. Breccias consist of angular altered rock fragments in a medium to coarse grained matrix dominated by clinopyroxene andror actinolite, with minor quartz, albite, hematite, titanite and epidote. All gradations occur between bleached, altered calcalbitite containing minor clinopyroxene andror albite veins and massive clast and matrix-supported breccias Že.g. Fig. 3A.. In general, the early phases of breccia formation are associated with aegirine-bearing clinopyroxenes as the dominant matrix mineral. Later stages of breccia evolution involve retrograde replacement of clinopyroxene by actinolite Ž Fig. 4A., as well as formation of primary actinolite" hematite " quartz" titanite Ž e.g. Toraminga Hill.. Clinopyroxene-matrix breccias are most common in the central part of the Olary Domain whereas amphiboledominated matrix breccias occur in the central northern and northern portions Ž Fig. 1.. This mirrors the patterns evident in metamorphic isograds Ž Fig. 1. and thus most probably reflect regional gradients in temperature during breccia formation, with the clinopyroxene representing higher temperature regions Ž see discussion below.. Fluid inclusions in clinopyroxene and quartz associated with breccias are commonly hypersaline, and measurements of quartz-hosted inclusions from clinopyroxene- and actinolite-matrix breccias display fluid salinities between ; equivalent wt.% NaCl ŽA.J.R. Kent and P.M. Ashley, unpublished data.. Clinopyroxene from breccias generally contains higher Na Fe 3q contents Ž up to 33 mol% aegirine. than clinopyroxene in the unaltered calc-silicate rocks Ž Fig. 5. and this, coupled with the presence of hematite in actinolite-matrix breccias and as a daughter mineral phase in fluid inclusions, indicates that breccia formation occurred under oxidizing conditions. A third type of metasomatic alteration in calcsilicate rocks, found locally in laminated Mn-rich Ž. Ž piemontite-bearing calc-silicate rocks Ashley,

9 ( ) A.J.R. Kent et al.rlithos Fig. 5. Na vs. Fe 3q rfe 2q 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 Cook Ž and those from clinopyroxene-matrix breccias from Yang and Ashley, The composition of recrystallised clinopyroxene within the garnet epidote-rich alteration zone at White Dam North is also shown is a variant of garnet epidote alteration, and contains coarse grained assemblages of one or more of piemontite, quartz, garnet Žandradite- and spessartine-rich., hematite, manganoan tremolite and braunite. 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 Olary Domain. Ž A. Cross-polarised light photo of clinopyroxenematrix breccia from Cathedral Rock Ž sample CR-5. showing retrogression of clinopyroxene to fibrous and massive actinolite adjacent to a crosscutting quartz vein. Ž B. Plane-polarised light photo of garnet epidote alteration zone from Boolcoomatta Žsam- ple BC-3. showing garnet, epidote and quartz intergrowth with partial granoblastic textures. Ž C. Plane-polarised light view of garnet epidote alteration zone from Bulloo Well Ž sample BW-3.. Euhedral and subhedral zoned garnets are surrounded by later quartz and contain irregular inclusions of epidote. Abbreviations: Cpx. clinopyroxene, Act. actinolite, Gt. garnet, Ep. epidote, Qtz. quartz. Garnet epidote-rich alteration zones are best developed in calc-silicate-bearing rocks of the Calcsilicate and Bimba Suites, but also occur rarely in quartzofeldspathic rocks of the Quartzofeldspathic Suite. For this study, samples from garnet epidoterich alteration zones and associated calc-silicate rocks were examined in detail from six locations, termed Bulloo Well, Boolcoomatta, Sylvester Bore, Mindamereeka Hill, Sampson Dam and White Dam North Ž Fig. 1., although observations were also made at several other locations. The alteration types evident

10 42 ( ) A.J.R. Kent et al.rlithos Fig. 6. Handspecimen photos of samples from garnet epidote alteration zones. Ž A. Brecciated and altered calc-silicate from Boolcoomatta Ž sample BC-8.. Bleached and albitised angular fragments of the original calc-silicate are held within an epidote Ž actinolite garnet. matrix. Although taken from a epidote garnet alteration zone, this sample has similar textures to those observed in calc-silicate breccias Ž Fig. 3A.. Ž B. Altered calc-silicate from Bulloo Well Ž sample BW-2.. Original calc-silicate has been largely replaced by massive epidote with subsidiary quartz and garnet. Small dark residual laminae of clinopyroxene Žpartially altered to actinolite. are also apparent. Alteration has been largely controlled by the composition of individual laminae and the folded structure of the calc-silicate has been preserved Žfolding may represent soft-sediment deformation of the original calc-silicate.. Ž C. Partially altered laminated calc-silicate from Sylvester Bore Žsample SB-2.. Individual laminations have been replaced by garnet Žthe largest garnet-replaced laminae has a small epidote-rich region in the centre and is bordered by a thin pale zone of quartz albite alteration.. The remaining calc-silicate has been recrystallised and much of the original clinopyroxene has been altered to actinolite. Ž D. Massive garnet-dominated alteration of laminated calc-silicate from Sylvester Bore Ž sample SB-3.. The primary laminated texture is partially destroyed by massive regions of garnet and quartz albite alteration. at each location are essentially the same and are described below and illustrated in ŽFigs. 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 fractures Ž e.g. Figs. 3B and 6.. The size of the metasomatised regions varies substantially, with alteration zones ranging from thin isolated veinlets Ž centimetre scale. to massive lensoid stratabound replacements up to m across Že.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 bedding Žanalo- 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 locality ŽFig. 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 and pyrite. Metasomatism commonly follows fracture sets that appear to be related to OD2 deformation, and in several locations Ž e.g. White Dam North. the strongest alteration appears to be focused into OD2 fold hinge zones, perhaps suggesting that these acted as fluid conduits. On the outcrop, hand specimen and microscopic scales five categories of alteration phenomena Žwith progressive alteration intensity. have been recognised, ranging from unaltered calc-silicate through to incipient disseminated and fracture-controlled alteration to total replacement of the pre-existing calcsilicate rocks and late monomineralic veining. The alteration styles are described below and summarised in Table Unaltered laminated calc-silicate rock These commonly crop out as thin Ž - 20 m. lenses with strike continuity of less than a kilometre, intercalated with pelitic, psammopelitic and laminated

11 ( ) A.J.R. Kent et al.rlithos 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 welllaminated, commonly defined by alternating ferromagnesian and quartzofeldspathic layers. Individual compositional laminae are from 1 mm to 10 cm in thickness and are interpreted as a primary depositional characteristic Ž Cook, Calc-silicate rocks are dominated by clinopyroxene, albite, quartz, K- feldspar and amphibole Žhornblende andror actinolite., with variable, but generally minor amounts of scapolite, garnet, epidote and titanite. Actinolite and hornblende occur as disseminated retrogression products of clinopyroxene and as discrete grains. Scapolite is found erratically in granoblastic aggregates in ferromagnesian and quartzofeldspathic layers. The calc-silicate rocks of the Olary Domain have been interpreted as the result of clastic sedimentation of felsic detrital material and interaction of sediments with evaporative brines, as well as contemporaneous evaporitic and exhalative chemical sedimentation Ž. Cook and Ashley, 1992; Cook, Recrystallisation of calc-silicate minerals adjacent to alteration zones Clinopyroxene, titanite, scapolite and feldspar are recrystallised adjacent to alteration zones. This is commonly shown by an increase in grainsize, better development of granoblastic texture and decreased abundance of feldspars. Recrystallised clinopyroxene is paler in colour and more Mg-rich, compared to the green, more Fe-rich compositions evident in unrecrystallised clinopyroxenes. Actinolite retrogression of clinopyroxene is also more common in recrystallised zones Incipient formation of garnet epidote bearing assemblages Minor to major development of epidote, garnet and local quartz in clinopyroxene-bearing laminations and along fractures. K-feldspar is altered to albite. Subhedral garnet and epidote occur as individ-

12 44 ( ) A.J.R. Kent et al.rlithos ual crystals or small aggregates. Scapolite and titanite are less common, and generally only occur in zones where relict clinopyroxene remains. Actinolite alteration of clinopyroxene is also common Total alteration of calc-silicate rock These zones consist of intense alteration along fractures and laminae and total replacement of clinopyroxene-bearing laminae by garnet, epidote and quartz Ž "minor K-feldspar, actinolite and albite. Ž Fig. 4B.. Where alteration is most intense massive replacement of feldspar-rich layers is also evident Ž Fig. 6.. Although epidote- and quartz-rich zones occur, garnet is commonly dominant and in many places is the only significant constituent. Along fractures and in open space fillings, garnet, and to a lesser extent quartz and epidote, occur as large Žup to several cm. subhedral crystals, with garnets commonly showing oscillatory zoning Ž Fig. 4C.. Within altered laminae, garnet generally occurs as smaller Ž mostly less than 2 3 mm. crystals with granoblastic texture and is strongly poikilitic, containing inclusions of quartz, epidote and minor feldspar. Epidote occurs as subhedral to anhedral crystals in aggregates up to several centimetres across associated with garnet or as a matrix to bleached and brecciated calc-silicate rock Ž e.g. Fig. 6.. There is no consistent textural relationship between garnet and epidote; in some samples, quartz and epidote form late crystalline aggregates around euhedral garnet and in other samples occur as inclusions with poikilitic garnet. This is interpreted to indicate that garnet and epidote crystallised coevally. The observed textural relations are probably the result of local variations in the relative time and rate of growth of either mineral. At the White Dam North location, intense development of garnet Ž epidote quartz. rock is locally cored in a synformal hinge zone by magnetite quartz Ž albite. rock Late Õeins At most altered calc-silicate rock locations, narrow Ž - 10 mm. late veins of garnet, epidote Ž "quartz, K-feldspar. crosscut all other assemblages Timing of formation of garnet epidote-rich alteration zones Field and petrographic observations indicate that metasomatism occurred after development of peak metamorphic mineral assemblages and associated OD1 and OD2 deformation events. At all localities investigated, the garnet epidote quartz Ž actinolite. metasomatic mineral assemblages overprint the primary metamorphic assemblages in the host calcsilicate rocks Ž e.g. Fig. 3.. Further, mineral fabrics in altered rocks are not foliated Že.g. Figs. 3B, 4B, C, 6., deformed calc-silicate rock at Boolcoomatta is overprinted and pseudomorphed by granoblastic-textured epidote garnet albite Ž Fig. 6B., and alteration is often controlled by fracture sets associated with the OD2 deformation event. The common presence of actinolite, rather than clinopyroxene, in garnet epidote-rich zones is consistent with a retrograde origin. Timing relations between metamorphism, metasomatic formation of both clinopyroxene-matrix breccias and garnet epidote-rich alteration zones and intrusion of S-type granites are particularly clear at Mindamereeka Hill, where laminated calc-silicate rocks have been altered to garnet epidote Ž quartz actinolite albite" hematite. assemblages along fractures and laminae. Several small lenses of clinopyroxene-matrix breccias also occur at this location and are crosscut by veins of garnet andror epidote, indicating that garnet epidote-rich alteration zones formed after clinopyroxene breccias. Leucocratic two-mica S-type granite and associated pegmatite dykes cut both garnet epidote-altered calc-silicate rocks Ž Fig. 3B. and clinopyroxene-matrix breccias, and granite intrusion is interpreted to have postdated formation of both clinopyroxene-matrix breccias and the bulk of garnet epidote replacement of calc-silicate rock. However, we note that pegmatite dykes adjacent to garnet epidote-rich zones also contain 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 albite q quartz" titanite assemblage Že.g. at Mindamereeka and Toraminga Hills; Fig. 1.. We suggest that intrusion of these dykes either occurred during the waning stages of metasomatic alteration or that the heat associated with intrusion remobilised meta-

13 ( ) A.J.R. Kent et al.rlithos somatic fluids Žpossibly contained within fluid inclusions. causing further alteration Sm Nd dating In order to determine the time of formation of garnet epidote-rich alteration zones, garnet and epidote in samples from Sylvester Bore, Mindamereeka Hill and Bulloo Well Ž Fig. 1. were analysed for Sm Nd isotopic composition and concentration. Results are shown in Table 3. Variations in Sm and Nd compositions and isotopic ratios between analyses of the same minerals from the same locations are evident at Sylvester Bore and Mindamereeka Hill. This could be due to contamination of separates by variable amounts of a phase with different Sm and Nd concentration Ži.e. contamination of garnet with epidote and vice versa. or may reflect compositional variation within analysed minerals. Electron microprobe analyses and optical observations show that minerals are compositionally zoned, and this may also be the case for Sm and Nd concentrations and the SmrNd ratio. It is important to note, however, that variation in Sm and Nd composition will not effect isochron calculations, provided that all minerals analysed had the same initial 143 Ndr 144 Nd ratio, and that all minerals formed at the same time. If this is the case, then mixing of two minerals Žwith different SmrNd ratios. will move samples along the isochron line, not away from it. For a metasomatic rock, these assumptions are probably justified assuming that localised equilibrium existed between the fluid and reacting calc-silicate during metasomatism. Results from the regression of data from garnet epidote alteration zones are given in Table 4, and are plotted on a 147 Smr 144 Nd vs. 143 Ndr 144 Nd isochron diagram in Fig. 7. Regression of all data corresponds to an age of 1577"80 Ma, with a high MSWD of 83 Žsee footnotes for Table 4 for explanation of this term.. Examination of Fig. 7 shows several points which lie off the isochron. Both garnet and epidote aliquots from the one sample Ž BW-1. from Bulloo Well and epidote from sample MH-1 lie well above the best-fit line. Removal of these from the regression improves the MSWD to a more acceptable 3.8, equivalent to an age of 1575"26 Ma. Subject to appropriate justification for removal of these points, this is interpreted to be the age of formation of garnet epidote-rich zones at Mindamereeka Hill and Sylvester Bore. Ages from individual regression of data for the Sylvester Bore and Mindamereeka Hill localities are within error of the age derived from regression of all data. Uncertainties for these ages are higher, and this probably reflects the lower num- Table 3 Sm Nd analyses of garnet and epidote from metasomatic rocks from the Olary Domain wep epidote, Gt garnet x. Sample locations and descriptions given in Table 6 Sample Sm Ž ppm. Nd Ž ppm. Smr Nd Ndr Nd " a Bulloo Well BW-1 Ž Ep "8 BW-1 Ž Gt "16 Mindamereeka Hill MH-1 Ž Ep "11 MH-2 Ž Ep "12 MH-3 Ž Ep "9 MH-1 Ž Gt "8 MH-2 Ž Gt "13 MH-3 Ž Gt "8 SylÕester Bore SB-2 Ž Ep "9 SB-3 Ž Ep "8 SB-3r1 Ž Gt "13 SB-3r2 Ž Gt "8 a 95% confidence interval, error given in the last decimal places.

14 46 ( ) A.J.R. Kent et al.rlithos Table 4 Regression data for Sm Nd analyses of garnet and epidote from altered calc-silicates. Abbreviations as for Table 3 a Regression MSWD Ndr Nd Age Points All data 83 y " " ll data without Bulloo Well, MH-1Ž Ep., MH-3Ž Gt. 3.8 y " " 26 8 Bulloo Well y " " 25 2 Bulloo WellqMH-1 Ž Ep. 3.3 y " " Sylvester Bore 3.9 y " " 93 4 Mindamereeka Hill 107 y " " Mindamereeka Hill without MH-1Ž Ep., MH-3Ž Gt. 2.3 y " " 66 4 a Ž. MSWD Amean squares weighted deviatesb is a measure of the quality of the isochron fit. Ideally the MSWD should be close to one. Nd i ber of data points contributing to the regressions Ž Table 4.. Deviation of individual points from the isochron may be the result of several factors, including differences in initial 143 Ndr 144 Nd ratios; disturbance of Nd isotope systematics after formation of metasomatic minerals; and incorporation of material which predates metasomatism into the analysed aliquots Ž e.g. metamorphic clinopyroxene or titanite.. The first possibility is most probable for minerals from Bulloo Well where samples are from a geographi- Fig Smr 144 Nd versus 143 Ndr 144 Nd isochron plot for garnet and epidote from garnet epidote alteration zones. Individual data points are labeled. Abbreviations as for Fig. 4 and: BW Bulloo Well; MH Mindamereeka Hill; SB Sylvester Bore. The regression line is for all data except BW-1Ž Gt., BW-1Ž Ep., MH- 1Ž Ep. and MH-3Ž Gt. where it is shown in solid, compared for the two point regression of BW-1Ž Gt. and BW-1Ž Ep. where it is shown in dashed. cally different location. Differences in the Nd isotope composition of calc-silicates, metasomatic fluid andror the fluidrrock ratio could produce variations in the initial Nd isotope composition of metasomatic minerals from different locations. Both samples from Bulloo Well appear to lie on a separate isochron than that defined by the remainder of the data. The two point isochron defined by Bulloo Well samples corresponds to an age of 1529" 25 Ma and has an initial 143 Ndr 144 Nd ratio of "31. This value is different, outside the given 95% confidence limit, from the initial ratio of "30 from the regression of data combined from Mindamereeka Hill and Sylvester Bore Žand 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 Bulloo 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 limits Ž 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 Mindamereeka Hill which lie off the isochron is not clear. Epidote from sample MH-1 may have a similar initial 143 Ndr 144 Nd ratio to that defined by the two samples from Bulloo Well, as it lies close to the two-point regression line defined by these Ž Fig. 7.. It is possible that paragenetically late epidote veinlets observed in this sample formed from a fluid with initial Nd isotope composition slightly different from

15 that which was responsible for the majority of alteration at Mindamereeka Hill. The Nd value of y6.0 calculated from regression of the combined data from Mindamereeka Hill and Sylvester Bore and the value of y4.0 calculated from the Bulloo Well data Ž 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 Domain 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 A-type igneous rocks from the Olary Domain have values Ž Nd calculated at 1700 Ma. that range from y0.1 to 1.0 ŽAshley et al., 1996; Page et al., and thus are also unlikely to have contributed REE to the metasomatic fluid Mineral chemistry ( ) A.J.R. Kent et al.rlithos 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 analyses are given in Table 5. Garnets consist predominantly of andradite grossular solid solution, with minor spessartine and almandine components Ž Table 5, Fig. 8A.; compositions extend to 95 mol% andradite. Variations are expressed largely as differences in the andradite grossular ratio between different localities and between samples from the same locality. Garnets from altered calc-silicates have lower almandine q spessartine components than those from largely unaltered calc-silicates Ž Fig. 8A.. Epidote from altered calc-silicate rocks has relatively Fe-rich compositions, ranging between 8% and 32% pistacite end-member, and with low piemontite contents Ž Fig. 8B.. Amphiboles in altered calc-silicate rocks are relatively Fe-rich and include ferrohornblende and actinolite Ž Table 5.. Recrystallised clinopyroxene is relatively magnesian, with a typical composition being Wo50.0En 41.3Fs8.7 with a small calculated aegirine component Ž Table 5.. Fig. 8. Mineral compositions from garnet epidote altered calcsilicate rocks in the Olary Domain. Additional data from Rolfe Ž 1990., Westaway Ž 1992., Cook Ž 1993., Eykamp Ž 1993., Laffan Ž 1994., Pepper Ž and Chubb Ž Ž A. Garnet compositions from garnet epidote alteration zones and from unaltered calc-silicate rocks. Ž B. Epidote compositions from garnet epidote alteration zones Geochemistry Samples of garnet epidote-rich altered calc-silicate and unaltered calc-silicate rocks from Boolcoomatta, Mindamereeka Hill, Sylvester Bore, Bulloo Well, Sampson Dam and White Dam North were analysed for major and trace elements in order to assess the chemical changes resulting from metasomatic alteration. Analyses are presented in Table 6. Changes in major element and selected trace element compositions were evaluated using the isocon calculation procedure outlined in Grant Ž 1986., with results summarised in Fig. 9. This method assumes that the composition of the unaltered rock is representative of the protolith of the altered rock. Al-

16 48 Table 5 Representative electron microprobe analyses of epidote, garnet, amphibole and clinopyroxene from altered calcsilicate rocks from the Olary Domain Sample Epidote Garnet Amphibole Clino-pyroxene BC-4 SB-3 MH-1 BW-2 R74782 R77358 BC-4 SB-3 MH-1 BW-1 R74782 R77351 R74782 R74782 R74782 SiO TiO Al O Fe2O FeO MnO MgO CaO Na 2O K2O Cl Total Ž. Ž. Ž. Ž. 25 O 24 O 23 O 6 O Si iv Al vi Al Ti q Fe q Fe q Mn Mg Ca Na K Cl S A.J.R. Kent et al.rlithos 54 ( 2000 ) Analysts: A.J.R. Kent, M.A. Pepper, A.J. Chubb. See Appendix A for sample information. All Fe is assumed to be trivalent in epidote, whereas the proportions of Fe 3q and Fe 2q in garnet were calculated assuming stoichiometry. Note: blanks signify values below detection limit.

17 Table 6 Chemical composition of altered and unaltered calcsilicate rocks from the Olary Domain. Sample locations and descriptions are given in Appendix A SB-3 SB-1 MH-2 MH-3 BC-4 BC-1 R73358 R73357 R74782 R74780 R74779 R77351 R77354 R77353 Alt Unalt Alt Unalt Alt Unalt Alt Unalt Alt Alt Unalt Alt Alt Unalt SiO TiO Al O Fe2O FeO MnO MgO CaO Na O K O P O SO LOI Total Nb Zr Y Sr Rb Th Pb As U Ga Zn Cu Ni Cr Ce Nd La Ba V Sc Cl Co Mo FeOrŽ FeOqFe O Blanks: not determined. For sample information see Appendix A. Alt. Altered calcsilicate rocks containing garnet"epidote"quartz-bearing assemblages, R77354 being rich in magnetiteqquartz, Unalt. unaltered calcsilicate rocks. A.J.R. Kent et al.rlithos 54 ( 2000 )

18 50 ( ) A.J.R. Kent et al.rlithos 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 Loss on ignition. though this assumption may be tenuous in variably laminated calc-silicate rocks, large Ž ; 2 kg. samples were analysed and thus primary heterogeneity problems were probably satisfied to the degree required to demonstrate broad changes in chemical composition. Altered calc-silicate rocks are commonly enriched in Fe 3q, Ca, Mn, U and Cu, and depleted in Fe 2q, Na, Mg, K, and Rb Ž Fig. 9., and several altered rocks are also enriched in Pb, Zn, S and Cl. Alteration is accompanied by strong oxidation, with large decreases in the FeOrŽ FeOqFe O. ratio evident 2 3 in several locations Ž Table 6.. Many of these changes are in accord with alteration of a clinopyroxene feldspar-bearing assemblage to an andradite-rich garnet epidote assemblage where Mg, Na, K and Rb are lost during clinopyroxene and feldspar destruction and Ca, Fe 3q and Mn are fixed by the formation of garnet and epidote. In unaltered calc-silicates, S and Cl are hosted in scapolite, which is destroyed during alteration; however, daughter minerals in fluid inclusions indicate that appreciable Cl and S Žas 2y SO. 4 are present within hypersaline fluid inclusions in the altered rocks Ž see Section The

19 ( ) A.J.R. Kent et al.rlithos presence of these elements in the metasomatic fluid would assist in complexing and transporting many metals Fluid inclusions Fluid inclusions are abundant in minerals from metasomatic garnet epidote alteration zones. For this study, an investigation of the petrographic features and examination of the simple physical properties of inclusions was performed to constrain the nature of the metasomatising fluid. In garnet epidote-rich rocks, fluid inclusions occur predominantly within garnet and quartz. Inclusions are also observed in epidote, but are too small for physical measurements. Both two-phase Ž liquid vapour. and three- Žand multi-. phase inclusions Žcontaining one or more solid phases coexisting with liquid and vapour; e.g. Fig. 10. are present in varying proportions in sam- Fig. 10. Fluid inclusions from altered calc-silicate rocks. Ž A. Primary halite and hematite bearing aqueous liquid vapour inclusion Ž. 1 and simple two-phase aqueous liquid vapour inclusion Ž. 2 hosted in quartz from Toraminga Hill Ž sample KY30.. Ž B. Irregular aqueous liquid vapour inclusions wž 1. and Ž.x 2 on the surface of a growth zone in garnet from Mindamereeka Hill Ž sample MH-1.. ples from all localities. Estimated liquid vapour ratios typically vary between 2:1 and 9:1, although rare vapour-dominated inclusions with liquid vapour ratios less than 1:2 were also observed. CO2-bearing inclusions were not observed in any samples. Inclusions range in morphology from irregular to anhedral and euhedral inclusions showing partial to full development of negative crystal shapes. In addition, many garnet-hosted inclusions have intricate semi-rectangular shapes that commonly define surfaces parallel to zonation surfaces within large garnet crystals ŽFig. 10B.. Both primary and secondary inclusion habits are evident, with the primary inclusions occurring as isolated inclusions whereas the secondary types are generally small Ž mm. and occur along healed fractures in the host mineral. Primary inclusions are typically in the size range 1 20 mm, although for practical reasons physical measurements were restricted 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 hematite Ž Fig. 10A. and an elongate birefringent mineral with straight extinction, probably anhydrite. Several other types of daughter minerals were noted but not identified. No systematic relationship was apparent between two-, threeand 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. Inclusions hosted in both quartz and garnet Žboth 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-zoned Ž 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 morphologies, from irregular Ž e.g. Fig. 10B. to those with well-developed negative crystal shapes. Although the majority of garnet-hosted inclusions in BW-6A are two-phase Ž liquidq vapour., occasional three-phase Ž liquid q vapourq solid. inclusions are also evident. Most inclusions observed in garnet