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

Lithos 54 Ž2000. 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 b

a DiÕision of Earth Sciences, UniÕersity of New England, Armidale, NSW, 2351, Australia 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 SO42y. 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

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Corresponding author. Present address: Danish Lithosphere Centre, Øster Volgade 10, 1350 Copenhagen K, Denmark. Fax: q45-38-14-2667. E-mail address: [email protected] ŽA.J.R. Kent.. 1 Fax: q61-2-6773-3300. 2 Fax: q61-2-6773-3300. 3 Fax: q61-2-6249-4835. 0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 4 - 4 9 3 7 Ž 0 0 . 0 0 0 2 1 - 9

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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 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., 1998.. 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 ŽAgue, 1997.. 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 ŽFig. 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, 1999.. 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, 1998.. 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 ŽFig. 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-

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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 Žandalusite–chloritoid. and IIB Žandalusite–sillimanite. of Clarke et al. Ž1987.. 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 composi-

tions, Sm–Nd isotopic composition and fluid inclusions are given in Appendix A. 2.1. 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

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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. 2.2. 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 Ž1992.. 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 Ž1969.. These calculations are for the pure NaCl–H 2 O system and given the chemically complex nature of the fluids responsible for the formation of garnet–epidote metasomatic zones Žsee 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 Že.g. Ca. must be present ŽRoedder, 1984..

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 Ndr150 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. Ž1993.. 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 ŽFig. 1.. The geology of the Olary Domain has been summarised by Clarke et al.

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 approximately 95–98%. In order to avoid the incorporation of older Žpre-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 occur, they consist of other metasomatic minerals Že.g.

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

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Ž1986, 1987., Cook and Ashley Ž1992., Flint and Parker Ž1993., Robertson et al. Ž1998. and Ashley et al. Ž1998a.. The Olary Domain sequence ŽFig. 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, 1999.. The Willyama Supergroup has been interpreted to represent a failed Palaeoproterozoic rift ŽWillis et al., 1983. 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 ŽCook and Ashley, 1992.. 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 ; 1710–1700 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 ; 1710– 1650 Ma, although the younger age limit is not well-constrained ŽAshley et al., 1998a; Page et al., 1998.. The Olary Domain sequence has been intruded by several suites of plutonic rocks as well as having

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been subject to at least five deformation and metamorphic events ŽClarke et al., 1986, 1987; Flint and Parker, 1993.. Temporal relationships between intrusive, metamorphic and deformational episodes have been investigated by field studies and by zircon U–Pb and muscovite 40Ar– 39Ar 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, 1993. have ascribed the first three deformation events in the Olary Domain ŽOD1 –OD 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 ; 1640–1630 Ma ŽAshley et al., 1998a; cf. Nutman and Ehlers, 1998.; however, for this study we will continue to use the OD1 –OD 3 notation of Flint and Parker Ž1993.. Two later deformation events ŽDD1 , DD 2 . are related to Delamerian orogeny Ž; 500–450 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 ; 1710–1700 Ma ŽAshley et al., 1996.. Recent observations ŽAshley 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 ; 1640–1630 Ma. A major episode of deformation ŽOD1 and OD 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 OD 2 and studies of pelitic rocks by Clarke et al. Ž1987. 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 550–6508C ŽFlint and Parker, 1993.. Peak metamorphic conditions decrease to the north to lower amphibolite and greenschist facies ŽClarke et al., 1987;

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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 OD 2 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 ŽClarke et al., 1986.. Further thermal perturbations also occurred during the Musgravian Orogeny at ; 1200–1100 Ma ŽLu et al., 1996.. Mafic dyke emplacement at ; 820 Ma was a precursor to development of the Adelaide Geosyncline in the region Žcf. 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 Orogeny ŽClarke et al., 1986; Flint and Parker, 1993.. Episodes of fluid flow accompanied most of these later thermal events Že.g. Bierlein et al., 1995; Lu et al., 1996..

3.1. 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, 1999.. The effects of metasomatism in the Olary Domain are evident on a variety of spatial scales, ranging from regional-scale Žkilometres. alteration zones in sediments, felsic volcanic and intrusive rocks through to localised examples of fluid flow such as breccia zones and local fracture systems Že.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 ŽTable 1.. In general, metasomatism has resulted in enrichment of

Fig. 3. Examples of metasomatic alteration in the Olary Domain. ŽA. Brecciated calc-silicate from Cathedral Rock. Breccia consists of angular bleached albite-rich clasts in a dark matrix of diopside and actinolite Žpen shown for scale is 14 cm long.. ŽB. Altered and bleached laminated calc-silicate at Mindamereeka Hill intruded and cut by thin parallel dykes of pegmatite. ŽC. Actinoliterich veins surrounded by bleached albite-rich alteration selvages in altered I-type granite ŽTonga Hill.. ŽD. Fracture-controlled alteration in laminated psammopelitic sediments ŽWhite Rock..

Fe, Na– ŽFe., Ca– ŽNa–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

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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 Žbleaching. ŽFig. 3D.

Replacement of aluminosilicates, micas and feldspars by albite

Addition of Na; loss of K, Rb ŽBa.

Quartzo-feldspathic rocks

Extensive albitisation Žbleaching., minor development of magnetite " hematite, sulfides in veins and disseminations. Local biotiteq magnetite.

Replacement of plagioclase, Kfeldspar and biotite by albite, minor magnetite, hematite, pyrite, chalcopyrite. Local development of biotiteq magnetite

Addition of Na ŽFe,S,Cu,K.; loss of K,Rb ŽCa,Sr.

Calc-silicate and calcalbitite rocks

Pervasive bleaching grading to massive clinopyroxene- and actinolite-matrix breccias andror massive garnet–epidote alteration zones ŽFig. 3A,B.

Destruction of clinopyroxene, titanite, K-feldspar and scapolite; formation of secondary Na–Fe 3q clinopyroxene, amphibole, albite, quartz, andraditic garnet, epidote

Addition of Ca, Fe 3q, Mn, Ž"Cu,Pb,Zn,U.; loss of Na, Fe 2q, Mg, Rb, Ba

Albitisation Žbleaching., minor Fe oxide veining and disseminations Localised albitisation Žbleaching. where late dykes crosscut calcsilicate breccia and garnet–epidote alteration zones Extensive areas of fracture controlled and pervasive bleaching with minor brecciation ŽFig. 3C.

Destruction of plagioclase, Kfeldspar Žbiotite. and formation of albite Ž" magnetite, quartz. Destruction of igneous feldspar and biotite; albitisation; local formation of amphibole, garnet, epidote, titanite Destruction of igneous feldspar and biotite; pervasive albitisation; deposition of quartz, amphibole and titanite on fractures

Addition of Na ŽFe.; loss of K,Rb

Fe oxide enrichment; destruction of laminated texture

Local loss of quartz; growth of magnetite, hematite, local pyrite, trace chalcopyrite

Addition of FeŽCu,Au,U,V,Y, Zn,S.; loss of Si

Granitoids: A-types

S-types

I-types

Iron-formations

predate, A-type intrusives and associated volcanics emplaced at 1710–1700 Ma and I-type granites emplaced at 1640–1630 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 OD 2 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 Že.g. Cathedral Rock., metasomatically altered rocks are deformed and retro-

Local addition of Na ŽCa,Fe.; loss of K,Rb

Addition of Na ŽCa,Fe.; loss of K,Rb

gressed within OD 3 deformation zones, indicating that the majority of metasomatic alteration occurred prior to formation of these zones at ; 1500 Ma. However, 40Ar– 39Ar ages on metasediments and pegmatite muscovite also suggest that fluid movement continued episodically along OD1 –OD 3 structures for several hundred million years after granite intrusion ŽBierlein et al., 1995; Lu et al., 1996.. Reactivation of structures during the Delamerian Orogeny is indicated by ; 470 Ma 40Ar– 39Ar ages of muscovites from pegmatites and OD 3 shear zones ŽLu et al., 1996.. Bierlein et al. Ž1995. also demonstrated that at least some of the epigenetic sulfide mineralisation within OD 3 shear zones occurred between ; 480 and 450 Ma during retrograde fluid movement along older structures.

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3.2. 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 Žsee 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 Ž1992. and Yang and Ashley Ž1994.. 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 2 ŽYang and Ashley, 1994.. 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 3 shear zones Že.g. Cathedral Rock.. 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 ŽFig. 4A., as well as formation of primary actinolite" hematite " quartz " titanite Že.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 ŽFig. 1.. This mirrors the patterns evident in metamorphic isograds ŽFig. 1. and thus most probably reflect regional gradients in temperature during breccia formation, with the clinopyroxene representing higher temperature regions Žsee 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 ; 15–46 equivalent wt.% NaCl ŽA.J.R. Kent and P.M. Ashley, unpublished data.. Clinopyroxene from breccias generally contains higher Na–Fe 3q contents Žup to 33 mol% aegirine. than clinopyroxene in the unaltered calc-silicate rocks ŽFig. 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 Žpiemontite-bearing. calc-silicate rocks ŽAshley,

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Fig. 5. Na vs. Fe 3qrFe 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. Ž1998b.. Clinopyroxene compositions from unaltered calc-silicates are from Cook Ž1993. 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, 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. ŽA. Cross-polarised light photo of clinopyroxenematrix breccia from Cathedral Rock Žsample CR-5. showing retrogression of clinopyroxene to fibrous and massive actinolite adjacent to a crosscutting quartz vein. ŽB. Plane-polarised light photo of garnet–epidote alteration zone from Boolcoomatta Žsample BC-3. showing garnet, epidote and quartz intergrowth with partial granoblastic textures. ŽC. Plane-polarised light view of garnet–epidote alteration zone from Bulloo Well Žsample 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 ŽFig. 1., although observations were also made at several other locations. The alteration types evident

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A.J.R. Kent et al.r Lithos 54 (2000) 33–62

Fig. 6. Handspecimen photos of samples from garnet–epidote alteration zones. ŽA. Brecciated and altered calc-silicate from Boolcoomatta Žsample BC-8.. Bleached and albitised angular fragments of the original calc-silicate are held within an epidote Žactinolite–garnet. matrix. Although taken from a epidote–garnet alteration zone, this sample has similar textures to those observed in calc-silicate breccias ŽFig. 3A.. ŽB. Altered calc-silicate from Bulloo Well Žsample 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.. ŽC. 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. ŽD. Massive garnet-dominated alteration of laminated calc-silicate from Sylvester Bore Žsample 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 Že.g. Figs. 3B and 6.. The size of the metasomatised regions varies substantially, with alteration zones ranging from thin isolated veinlets Žcentimetre scale. to massive lensoid stratabound replacements up to 200–300 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 Žanalogous 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 OD 2 deformation, and in several locations Že.g. White Dam North. the strongest alteration appears to be focused into OD 2 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 2. 4.1.1. 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

A.J.R. Kent et al.r Lithos 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

Disseminated regions of garnet andror epidote, from - 1 cm up to 10 cm across

Boolcoomatta, Bulloo damereeka Hill

Massive layer parallel replacement of calc-silicate minerals by garnet andror epidote Ž"quartz, albite, K-feldspar, amphibole.

Boolcoomatta, Bulloo Well, Sylvester Bore, Mindamereeka Hill, Sampson Dam, White Dam North and other sites

4B,C and 5

Fracture controlled replacement of calc-silicates by garnet andror epidote Ž"quartz, albite, K-feldspar, amphibole.. May combine with layer-parallel replacement of more reactive layers to produce net-like textures.

Boolcoomatta, Bulloo damereeka Hill

Bore,

Min-

5

Massive replacement of calc-silicate by garnet andror epidote Ž"quartz, albite, K-feldspar, amphibole.. These 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.

Boolcoomatta, Bulloo Well, Sylvester Bore, damereeka Hill, Sampson Dam, White Dam North

Min-

4D and 5

Late near-monomineralic veins of garnet, epidote, quartz and local K-feldspar.

Boolcoomatta, Bulloo damereeka Hill

Min-

5

Coarse euhedral garnet crystals, filling open spaces or associated with late quartz filling.

Bulloo Well, Sylvester Bore, Mindamereeka Hill, Sampson Dam, White Dam North

5

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 ŽCook, 1993.. Calc-silicate rocks are dominated by clinopyroxene, albite, quartz, Kfeldspar 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 ŽCook and Ashley, 1992; Cook, 1993..

Figure Well,

Well,

Well,

Sylvester

Sylvester

Sylvester

Bore,

Bore,

Min-

5

4.1.2. 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.

4.1.3. 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-

44

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

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.

4.1.4. 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. ŽFig. 4B.. Where alteration is most intense massive replacement of feldspar-rich layers is also evident ŽFig. 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 ŽFig. 4C.. Within altered laminae, garnet generally occurs as smaller Žmostly 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 Že.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.

4.1.5. 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.

4.2. 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 OD 2 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 Že.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 ŽFig. 6B., and alteration is often controlled by fracture sets associated with the OD 2 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 ŽFig. 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-

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

somatic fluids Žpossibly contained within fluid inclusions. causing further alteration.

45

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 Smr144 Nd vs. 143 Ndr144 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 ŽBW-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-

4.2.1. 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 ŽFig. 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 Ndr144 Nd ratio, and that all minerals formed at the same time. If this is the case, then

Table 3 Sm–Nd analyses of garnet and epidote from metasomatic rocks from the Olary Domain wEp — epidote, Gt — garnetx. Sample locations and descriptions given in Table 6 Sample Bulloo Well BW-1 ŽEp. BW-1 ŽGt.

Sm Žppm. 2.07 8.30

Nd Žppm. 7.79 17.69

147

Smr

144

Nd

143

Ndr

144

Nd

"a

0.1607 0.2841

0.512072 0.513312

"8 "16

Mindamereeka Hill MH-1 ŽEp. MH-2 ŽEp. MH-3 ŽEp. MH-1 ŽGt. MH-2 ŽGt. MH-3 ŽGt.

35.7 4.59 2.06 34.1 22.9 20.7

171 28.4 14.0 101 76.2 85.7

0.1263 0.0976 0.0885 0.2046 0.1814 0.1462

0.511708 0.511306 0.511206 0.512389 0.512172 0.511742

"11 "12 "9 "8 "13 "8

SylÕester Bore SB-2 ŽEp. SB-3 ŽEp. SB-3r1 ŽGt. SB-3r2 ŽGt.

9.73 11.6 26.3 28.3

33.0 37.4 36.6 37.6

0.1781 0.1870 0.2939 0.2936

0.512137 0.512244 0.513353 0.513321

"9 "8 "13 "8

a

95% confidence interval, error given in the last decimal places.

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

46

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

MSWD a

´ Nd

143

All data ll data without Bulloo Well, MH-1ŽEp., MH-3ŽGt. Bulloo Well Bulloo Well q MH-1 ŽEp. Sylvester Bore Mindamereeka Hill Mindamereeka Hill without MH-1ŽEp., MH-3ŽGt.

83 3.8 – 3.3 3.9 107 2.3

y5.6 y6.0 y4.0 y4.1 y5.9 y6.3 y6.3

0.510309 " 52 0.510292 " 30 0.510457 " 31 0.510434 " 310 0.510308 " 134 0.510339 " 2400 0.510305 " 56

a

Ndr144 Nd i

Age

Points

1577 " 80 1575 " 26 1529 " 25 1543 " 220 1568 " 93 1529 " 250 1556 " 66

12 8 2 3 4 6 4

MSWD ŽAmean 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 ŽTable 4.. Deviation of individual points from the isochron may be the result of several factors, including differences in initial 143 Ndr144 Nd ratios; disturbance of Nd isotope systematics after formation of metasomatic minerals; and incorporation of material which predates metasomatism into the analysed aliquots Že.g. metamorphic clinopyroxene or titanite.. The first possibility is most probable for minerals from Bulloo Well where samples are from a geographi-

Fig. 7. 147 Smr144 Nd versus 143 Ndr144 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-1ŽGt., BW-1ŽEp., MH1ŽEp. and MH-3ŽGt. where it is shown in solid, compared for the two point regression of BW-1ŽGt. and BW-1ŽEp. 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 Ndr144 Nd ratio of 0.510457 " 31. This 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 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 ŽTable 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 Ndr144 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 ŽFig. 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

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

47

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 ŽTable 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. 1710–1700 A-type igneous rocks from the Olary Domain have ´ Nd values Žcalculated at 1700 Ma. that range from y0.1 to 1.0 ŽAshley 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 analyses are given in Table 5. Garnets consist predominantly of andradite– grossular solid solution, with minor spessartine and almandine components ŽTable 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 ŽFig. 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 ŽFig. 8B.. Amphiboles in altered calc-silicate rocks are relatively Fe-rich and include ferrohornblende and actinolite ŽTable 5.. Recrystallised clinopyroxene is relatively magnesian, with a typical composition being Wo 50.0 En 41.3 Fs 8.7 with a small calculated aegirine component ŽTable 5..

Fig. 8. Mineral compositions from garnet–epidote altered calcsilicate rocks in the Olary Domain. Additional data from Rolfe Ž1990., Westaway Ž1992., Cook Ž1993., Eykamp Ž1993., Laffan Ž1994., Pepper Ž1996. and Chubb Ž2000.. ŽA. Garnet compositions from garnet–epidote alteration zones and from unaltered calc-silicate rocks. ŽB. Epidote compositions from garnet–epidote alteration zones.

4.4. 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 Ž1986., 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-

48

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

Garnet SB-3

MH-1

BW-2

R74782

R77358

37.21

37.47

37.62

37.48

37.36

38.18

21.56 16.02

23.04 14.41

21.19 15.93

21.88 15.81

22.53 14.76

22.23 15.96

0.38

0.18

0.14

0.29

0.38

23.32

23.88

23.01

23.87

98.49

98.98

97.89

99.45

SB-3

MH-1

BW-1

R74782

R77351

37.03 0.34 13.23 13.45 0.61 1.38

34.91 0.56 2.92 27.82 0.01 0.59

36.76 0.35 10.13 17.45 0.85 1.13

37.74 0.34 13.19 13.68

0.29

35.83 0.30 6.04 23.03 1.55 2.20

1.21

37.01 0.36 7.61 20.19 1.98 0.84

22.87

22.95

31.06

33.95

33.20

33.33

33.51

32.01

97.90

99.61

100.01

99.99

100.01

100.00

99.67

100.00

25ŽO. Si Al iv Al vi Ti Fe 3q Fe 2q Mn2q Mg Ca Na K Cl S

BC-4

24ŽO.

5.966 0.034 4.040

5.940 0.060 4.246

6.050

5.987 0.013 4.241

6.022

4.016

5.950 0.050 4.043

1.933

1.720

1.927

1.888

1.778

1.894

0.051

0.024

0.019

0.039

0.051

4.007

4.058

3.965

4.060

16.031

16.048

15.977

16.030

Clino-pyroxene

R74782

R74782

R74782

45.30 0.07 8.39

51.15 0.09 4.48

53.64 0.04 0.28

20.92 0.93 8.27 11.06 1.70 1.15 0.19 97.98

17.24 0.91 11.26 11.21 1.04 0.57 0.05 98.00

5.18 0.93 14.88 24.31 0.27

23ŽO.

0.039

5.915 0.085 1.089 0.038 2.860 0.213 0.307

5.862 0.138 2.330 0.041 1.603 0.081 0.185

5.831 0.169 0.407 0.070 3.497 0.001 0.083

5.910 0.090 1.831 0.040 2.112 0.114 0.154

5.992 0.008 2.461 0.041 1.508 0.127 0.163

1.459 0.044 2.481 0.270 0.117

3.926

3.878

5.493

5.760

5.942

5.744

5.701

5.592

15.996

15.966

16.000

16.000

16.000

16.000

16.000

16.000

4.133

Amphibole

6.037

99.53 6ŽO.

6.910 1.090 0.425 0.005

7.550 0.450 0.330 0.010

2.665 0.120 1.885 1.810 0.500 0.225 0.050 15.685

2.130 0.110 2.480 1.770 0.300 0.110 0.010 15.250

1.990 0.010 0.002 0.005 0.026 0.175 0.029 0.823 0.966 0.019

4.045

Analysts: A.J.R. Kent, M.A. Pepper, A.J. Chubb. See Appendix A for sample information. All Fe is assumed to be trivalent in epidote, whereas the proportions of Fe 3q and Fe 2q in garnet were calculated assuming stoichiometry. Note: blanks signify values below detection limit.

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O Cl Total

Epidote BC-4

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 Alt

MH-2 Alt

MH-3 Unalt

BC-4 Alt

BC-1 Unalt

53.10 0.47 11.84 6.28 1.29 0.94 1.33 20.39 1.30 0.30 0.41 0.17 1.36 99.18 14 145 47 111 11 8 11 19 14 17 168 41 22 196 62 38

62.30 0.60 14.32 1.59 2.34 0.15 1.87 6.78 2.73 5.55 0.23 0.04 0.23 98.73 15 178 34 287 228 15 2 6 8 19 18 18 24 128 79 26

54.67 0.55 14.06 5.53 2.39 0.18 2.23 13.27 5.27 0.61 0.19 0.04 0.38 99.37 14 138 25 75 20 11 5 3 9 20 39 13 29 134 77 31

56.65 0.56 14.59 4.68 2.19 0.14 2.10 11.56 6.13 0.56 0.22 - 0.01 0.25 99.63 13 149 21 88 12 13 5 4 7 19 37 5 27 102 80 27

66.53 0.38 7.20 10.61 0.53 0.76 0.20 12.65 - 0.01 0.01 0.14 0.01 1.04 100.06 32 131 124 167 2 16 25 6 8 21 124 157 14 168 201 96

61.71 0.51 13.27 0.94 1.46 0.12 4.14 7.58 5.45 3.87 0.16 - 0.01 0.29 99.50 18 189 60 119 254 12 1 1 4 24 27 7 75 84 189 87

356 87 14 1361 13 -2 0.17

4394 137 13 98 15 -2 0.60

85 80 10 497 16 -2 0.30

68 55 5 38 14 -2 0.32

81 27 14 32 146 -2 0.05

2908 49 32 90 24 -2 0.61

R73358 Alt

R73357 Unalt

R74782 Alt

R74780 Alt

R74779 Unalt

R77351 Alt

R77354 Alt

R77353 Unalt

43.21 0.30 8.68 16.18 0.94 1.10 0.21 29.97 0.15 0.35 0.23 - 0.01 0.38 101.70 7 69 15 26 18 6 9 3 18 26 83 31 20 101 42 30 26 467 87 17

60.76 0.61 15.78 2.15 3.72 0.34 1.89 6.38 1.94 5.26 0.23 - 0.01 0.88 99.94 14 148 27 1088 246 18 20 8 -2 18 139 20 35 82 114 46 62 4037 57 16

53.38 0.27 14.40 7.78 2.52 0.31 2.18 15.18 1.32 0.65 0.43 0.02 1.27 99.71 17 116 13 159 29 8 15 3 4 22 102 34 6 42 31 15 17 187 53 10

40.73 0.35 12.66 11.97 2.12 1.19 1.17 27.22 1.03 0.27 0.38 0.05 0.81 99.95 17 91 15 31 9 -3 7 5 7 25 162 17 16 24 16 19 13 195 71 14

66.25 0.57 15.74 0.82 1.14 0.15 1.79 8.21 4.46 0.31 0.09 0.02 0.53 100.08 17 165 31 198 18 23 17 2 12 23 89 3 12 76 71 32 33 156 134 13

39.76 0.29 7.69 18.73 0.90 0.81 0.34 30.23 0.20 0.02 0.29 0.01 0.35 99.62 14 87 30 19 1 4 2 5 19 40 25 16 1 36 29 40 23 72 120 22

52.23 0.29 6.80 25.77 8.55 0.03 0.18 1.37 2.50 0.19 0.07 0.03 1.05 99.06 2 107 8 153 15 5 12 1 10 26 16 85 2 46 3 9 15 88 119 6

67.18 0.58 13.46 1.41 2.50 0.34 1.85 5.92 4.02 1.63 0.18 0.01 0.60 99.68 16 197 73 109 72 16 4 2 10 18 39 65 31 68 180 87 171 442 140 13

0.05

0.63

0.24

0.15

0.58

0.05

0.25

0.64

49

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.r Lithos 54 (2000) 33–62

SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 SO 3 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 FeOrŽFeOqFe 2 O 3 .

SB-1 Unalt

50

A.J.R. Kent et al.r Lithos 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 — 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 ŽFig. 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 FeOrŽFeO q Fe 2 O 3 . ratio evident

in several locations ŽTable 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 SO42y . are present within hypersaline fluid inclusions in the altered rocks Žsee Section 4.5.. The

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

presence of these elements in the metasomatic fluid would assist in complexing and transporting many metals. 4.5. 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 Žliquid–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. ŽA. Primary halite and hematite bearing aqueous liquid–vapour inclusion Ž1. and simple two-phase aqueous liquid–vapour inclusion Ž2. hosted in quartz from Toraminga Hill Žsample KY30.. ŽB. Irregular aqueous liquid–vapour inclusions wŽ1. and Ž2.x on the surface of a growth zone in garnet from Mindamereeka Hill Žsample MH-1..

51

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. CO 2-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- 5–10 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 ŽFig. 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 Žfrom 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 Že.g. Fig. 10B. to those with well-developed negative crystal shapes. Although the majority of garnet-hosted inclusions in BW-6A are two-phase Žliquidq vapour., occasional three-phase Žliquid q vapourq solid. inclusions are also evident. Most inclusions observed in garnet

52

A.J.R. Kent et al.r Lithos 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 paragenesis

n

Homogenisation temperature" 1 s Ž8C.

Salinity" 1 s Žequivalent wt.% NaCl.

BW-6A ŽBulloo Well.

Garnet, primary Garnet, secondary Garnet, two-phase Garnet, three-phase Quartz, primary Quartz, secondary Quartz, two-phase Quartz, three-phase Garnet, primary Garnet, secondary Garnet, two-phase Quartz, primary Quartz, secondary Quartz, two-phase

6 4 8 2 18 5 11 12 7 3 10 10 8 18

319 " 14 318 " 16 319 " 14 316 " 23 247 " 39 267 " 42 254 " 42 248 " 39 248 " 17 255 q 35 250 q 22 191 q 22 196 q 34 193 " 27

23 " 1 23 " 2 23 " 1 23 " 1 29 " 5 29 q 5 25 " 5 33 " 1 17 q 2 17 " 2 17 " 2 18 q 3 20 " 3 19 " 3

KY-8 ŽBoolcoomatta.

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

Fig. 11. Histograms of measured salinity data Žin equivalent wt.% NaCl. from fluid inclusions in garnet and quartz. ŽA. KY-8 Boolcoomatta, ŽB. BW-6A Bulloo Well, ŽC. quartz-hosted fluid inclusions from clinopyroxene- and actinolite-matrix breccia zones ŽA.J.R. Kent and P.M. Ashley, unpublished data..

are also observed. Inclusions in quartz occur as 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 inclusions Žup 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 Žwith 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 fluid Žc.f. De Jong and Williams, 1995.. After initial freezing Žrequiring supercooling to temperatures of y808C to y508C. development of a mottled brown appearance at temperatures down to ; y35 8C probably reflects the presence of CaCl 2 in inclusions ŽShepherd et al., 1979.. In addition, the observed depression of the freezing point of

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

two-phase inclusions below the eutectic point for the pure NaCl–H 2 O system during freezing-point determinations Žfirst melting temperatures were generally closer to the ca. y528C eutectic of the CaCl 2 – NaCl–KCl–H 2 O system, e.g. Shepherd et al., 1979. also suggests the presence of Ca in trapped fluids. Final melting temperatures, marked by the disappearance of clear rounded cubes of ice, ranged between ; y108C to y458C and temperatures of halite dissolution upon heating ranged between 1708C and 2408C. These results indicate that the fluids involved in metasomatism and quartz and garnet formation were hypersaline, with ; 15–35 equiv. wt.% NaCl ŽFig. 11.. Homogenisation temperatures Žuncorrected for pressure of formation. ranged between ; 1608C and 3408C, although for individual inclusions no correlation was observed between salinity and homogenisation temperature. In both samples, the garnet-hosted inclusions have slightly higher average homogenisation temperature than quartz-hosted inclusions ŽTable 7.. This implies that the fluids present during garnet deposition were at slightly higher temperatures than those present during quartz formation, consistent with the mineral textures showing that quartz postdated garnet deposition Žsimilar relations are shown for sample BW-3 in Fig. 4B.. Both quartz and garnet-hosted inclusions from sample KY-8 have similar estimated salinities, whereas quartz-hosted inclusions from BW-6A have slightly higher salinities than garnet-hosted. Two- and threephase inclusions in garnet and quartz from BW-6A Žthe only sample for which measurements were made on both two- and three-phase inclusions. also show similar range of homogenisation temperatures, although salinities in quartz-hosted three-phase inclusions are slightly higher than in two-phase in BW-6A ŽTable 7.. In both samples, both primary and secondary fluid inclusions show similar ranges of homogenisation temperature and estimated salinity ŽTable 7.. This suggests that metasomatic fluids of the same approximate composition as those responsible for metasomatism continued to circulate after mineral growth. Given the uncertainties in the ambient pressure during metasomatism and fluid inclusion trapping, and in the overall bulk composition of the metasomatic fluids, we have made no rigorous attempt to use the homogenisation temperatures to constrain the

53

temperature of metasomatism. Peak metamorphic pressures in the Olary Domain are estimated at 4–6 kbar ŽClarke et al., 1986; Flint and Parker, 1993.. This corresponds to a temperature correction of ; 200–3008C for homogenisation temperatures of fluids in the H 2 O–NaCl system ŽPotter, 1977.. With such a correction applied, the estimated fluid trapping temperatures range between 4008C and 6508C, and this is broadly consistent with the interpretation that metasomatism occurred slightly after the peak of metamorphism Žwhich occurred at ; 550–6508C; Flint and Parker, 1993.. 5. Discussion 5.1. Formation of garnet–epidote-rich alteration zones Garnet–epidote-rich zones exhibit features that are characteristic of a metasomatic origin. These include hydrothermal textures such as vein and replacement textures, open-space fillings and breccias, and an abundance of fluid inclusions. Open space cavities and breccias also imply that fluid pressures may have been locally higher than lithostatic pressure, and thus metasomatic alteration zones probably also represent zones of focussed fluid flow. The nature of the fluid responsible for metasomatism can be deduced from fluid inclusion properties, and the mineralogical and chemical changes which accompanied alteration. Fluid inclusions show that the metasomatic fluids were hypersaline Ž; 15–35 equiv. wt.% NaCl., and the freezing behaviour of inclusions and the array of daughter minerals present Žhalite, hematite, anhydrite and several unidentified phases. indicate that the fluids were chemically complex, and contained Na, Ca, Fe 3q, Cl and SO42y Žand probably several other species.. The changes in the bulk chemistry Žincreased Fe 3qrFe 2q ., the presence of hematite as a daughter phase in fluid inclusions, and formation of Fe 3q-bearing minerals Žandraditerich garnet and epidote. in metasomatically altered rocks also suggest that the fluid was substantially more oxidised than the pre-existing calc-silicate Žmetasomatism probably occurred at oxygen fugacities equal to or greater than those of the hematite– magnetite buffer.. Comparisons of the bulk chemical composition of altered and unaltered calc-silicates

54

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

show that metasomatic fluid were capable of mobilising many different elements Žagain consistent with a chemically complex fluid., and metasomatism was accompanied by substantial changes in chemical compositions ŽFig. 9.. In general altered calc-silicate rocks are enriched in Fe 3q, Ca, Mn, U and Cu, and depleted in Fe 2q, Na, Mg, K, and Rb and several altered rocks are also enriched in Pb, Zn, S and Cl. It is also possible to estimate the general conditions under which metasomatism occurred. The presence of actinolite within garnet–epidote alteration zones and the retrogressive replacement of clinopyroxene by actinolite associated with formation of garnet–epidote zones provides some constraint on the temperature–pressure conditions of metasomatism. Although the stability fields of garnet, epidote and actinolite are not well known under oxidising conditions ŽLiou, 1972, 1974., at lower oxygen fugacities Žfayalite–magnetite–quartz buffer. the breakdown of clinopyroxene to actinolite, at pressures ) 2–3 kbar, occurs with decreasing temperature at ; 500–6008C ŽGilbert et al., 1982.. We suggest that these are the approximate conditions of metasomatism within garnet–epidote alteration zones. This estimate is in broad agreement with both the range of corrected homogenisation temperatures from fluid inclusions Ž400–6508C. and observations that suggest metasomatism occurring after peak metamorphic conditions Ž550–6508C, 4–6 kbar; Flint and Parker, 1993.. Field and petrographic observations and Sm–Nd dating allow us to place the formation of garnet–epidote-rich metasomatic zones within the known sequence of geological development of the Olary Domain. Sm–Nd dating suggests that the majority of garnet–epidote-rich alteration zones formed at 1575 " 26 Ma Žalthough metasomatism at Bulloo Well may have occurred slightly later than this.. This is consistent with observations which show that garnet–epidote-rich metasomatic alteration occurred during the retrograde phases of amphibolite-grade regional metamorphism and after the deformation of the Olary sequence by the OD1 and OD 2 events. Garnet–epidote-rich metasomatic alteration zones also postdate formation of clinopyroxene-matrix breccias, but predate dykes associated with intrusion of the regional suite of S-type granitoids at 1600 " 20 Ma.

5.2. Widespread metasomatism within the Olary Domain Garnet–epidote alteration zones represent one example of metasomatic alteration in rocks of the Olary Domain; however, as already described many other styles of metasomatic alteration are also evident ŽTable 1.. Although the detailed nature of metasomatic alteration varies between different host rocks, there are many consistencies, summarised below, between the chemical and mineralogical changes that are observed in different host rocks ŽTable 1., the timing of metasomatism, and the inferred composition of the metasomatic fluids. Collective data indicate that the majority of metasomatism in the Olary Domain occurred after peak metamorphism and development of the fabrics related to the OD1 and OD 2 deformational events and prior to the intrusion of S-type granitoids, although further studies are required to firmly establish the timing of formation of each style of metasomatic alteration. For the examples listed in Table 1, metasomatic assemblages overprint metamorphic minerals and textures and in several cases alteration phenomenon appears to be controlled by pre-existing OD 2 structures ŽYang and Ashley, 1994; Ashley et al., 1998a.. Crosscutting field relations at Cathedral Rock and Toraminga Hill indicate that formation of clinopyroxene- and actinolite-matrix breccias occurred prior to intrusion of adjacent S-type granitoids and associated pegmatite dykes Že.g. Fig. 3B.. In addition, A- and I-type granitoids Žintruded at ; 1710–1700 and ; 1640–1630 Ma, respectively. and associated rocks are often extensively affected by metasomatism. The later S-type intrusives Ž1600 " 20 Ma. are only altered in localised zones where dykes crosscut previously metasomatised calc-silicate rocks, suggesting that the majority of regional metasomatism predates intrusion of S-type granitoids. Localised episodes of fluid flow and metasomatism probably continued for several hundred million years ŽBierlein et al., 1995; Lu et al., 1996.; however, the vast majority of metasomatic activity appears to have occurred directly after peak regional metamorphism. In general, the formation of metasomatic alteration zones in the Olary Domain involved widespread development of Fe-, Ca- and Na-bearing mineral assemblages, involving formation of one or more of

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

the following minerals: andradite-rich garnet, epidote, hematite, magnetite, aegerine-bearing clinopyroxene, actinolite and albite. These mineral assemblages are typically more oxidised Ži.e. alteration involves an increase in the Fe 3qrFe 2q ratios and alteration assemblages contain an array of Fe 3q-rich minerals. than their un-metasomatised equivalents. Metasomatic clinopyroxenes from clinopyroxenematrix breccias and metasomatically altered ironstones also have higher Fe 3qrFe 2q ratios and Na 2 O contents than clinopyroxenes from laminated calcsilicate rocks ŽFig. 5., and clinopyroxene- and actinolite-matrix breccias contain fluid inclusions that have hematite as a daughter phase. The chemical changes associated with different styles of metasomatic alteration typically involve addition of Fe, Ca and Na and loss of K, Rb and Mg ŽTable 1. and metasomatic fluids appear to have been highly saline and chemically complex. Hypersaline fluid inclusions with an array of daughter mineral phases, similar to those observed in garnet– epidote-rich alteration zones, have been documented from a number of different metasomatic rocks, including intense zones of albite Ž –quartz–actinolite. alteration in quartzofeldspathic rocks, clinopyroxeneand actinolite-matrix breccias Že.g. Fig. 11. and from epigenetic ironstones ŽYang and Ashley, 1994; Ashley et al., 1998b; A.J.R. Kent and P.M. Ashley, unpublished data.. The consistencies in the timing, alteration style and metasomatic fluid associated with different styles of metasomatic alteration suggests that the Olary Domain experienced a major episode of metasomatic alteration, involving the action of saline, oxidised and chemically complex fluids, during the retrograde stages of amphibolite-grade regional metamorphism. However, evidence suggests that metasomatic activity was not simply restricted to a single fluid pulse, but probably occurred during a succession of fluid flow events as the terrane cooled from peak metamorphic temperatures. The best evidence for this are the crosscutting and petrological relationships observed between the different styles of metasomatic alteration that occur in calc-silicate host rocks. Metasomatic calc-silicate-matrix breccias in the southern and central Olary Domain are dominated by clinopyroxene. Although clinopyroxene-matrix breccias clearly postdate the formation of peak metamor-

55

phic assemblages in host calc-silicate rock, the presence of clinopyroxene as the dominant matrix mineral implies that it formed at pressure–temperature conditions close to those of the primary metamorphic assemblages. Primary actinolite-matrix breccias within the northern portion of the Olary Domain formed in areas that experienced lower peak metamorphic temperatures ŽFig. 1.. At many locations, however, clinopyroxene-matrix breccias are variably retrogressed to actinolite ŽFig. 4A.. Given that the reaction of clinopyroxene to form actinolite occurs with decreasing temperatures ŽGilbert et al., 1982. actinolite retrogression of clinopyroxene-matrix breccias appears to represent continued metasomatic activity at lower temperatures. In addition, garnet– epidote-rich alteration zones are observed to overprint clinopyroxene-matrix breccias at Mindamereeka Hill, and garnet–epidote alteration zones in calc-silicate rocks contain accessory actinolite Žand are associated with retrogression of metamorphic clinopyroxene to actinolite.. This suggests that this style of alteration also represents a later episode of fluid movement that occurred at slightly lower ambient temperatures than formation of the clinopyroxene-matrix breccias. We therefore suggest that the Olary Domain provides an excellent example of a terrane that has experienced widespread metasomatic activity during the retrograde stages of a major regional metamorphic event. Metasomatism resulted in significant changes to the mineralogical and chemical constitution of the terrane and continued for some time as terrane cooled following metamorphism. 5.3. Source of metasomatising fluids Two primary possibilities exist for the origin of the hypersaline and oxidised fluids responsible for metasomatic alteration: Ži. fluids may have derived from the crystallisation of one or more types of granitoids; or Žii. fluids could have been derived from within the Olary Domain sequence, or from similar crustal rocks located at deeper structural levels, by metamorphic devolatilisation reactions. Field relations and radiometric dating in the Olary Domain show that a local temporal association between the emplacement of S-type granitoids and associated pegmatites and some occurrences of meta-

56

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

somatically altered rocks. This association could also suggest that metasomatic fluids are also derived from crystallising S-type plutons. However, the following evidence argues against a direct contribution to metasomatic fluids from crystallising regional S-type granites. Ži. Metasomatic fluids in the alteration zones are highly oxidised ŽG hematite–magnetite buffer., whereas the regional S-type granitoids in the Olary district are relatively reduced and ilmenite-bearing. Žii. Although there is a temporal association between some metasomatic rocks and rocks related to the S-type granitoids, there is no consistent spatial relationship between S-type granitoids and metasomatised rocks in the Olary district ŽFig. 1.. Many metasomatic rocks occur away from known outcrops of S-type granitoids and many large areas of granitoid and adjacent rocks are devoid of metasomatic alteration. Žiii. S-type granitoids do not show evidence of strong sub-solidus fluid accumulation Že.g. miarolitic cavities, fracture-controlled or pervasive alteration of plutons or adjacent country rocks, potassic alteration, or greisen development.. Živ. Preliminary O-isotope studies of metasomatic rocks Žsee below. show little evidence for the contribution of magmatic fluids to the metasomatising fluids. We also believe that metasomatic fluids in the Olary Domain are unlikely to be related to I-type intrusive rocks. Regional-scale alteration zones in other Australian Proterozoic regions, such as the Mt. Isa eastern succession in northwest Queensland and the Gawler Craton Žto the west of the Olary Domain. have been suggested to, at least partially, be related to fluids released by I-type granitoids ŽDe Jong and Williams, 1995; Oliver, 1995; Conor, 1998; Davidson, 1998; Williams, 1998.. However, in the Olary Domain, the only known I-type granitoids were emplaced at ; 1640–1630 Ma Žsome 70–80 Ma prior to metasomatism; Ashley et al., 1998a.; these intrusions are clearly pre-alteration, are spatially restricted and have no relation to the regional-scale occurrence of altered rocks. It may be inferred that later Že.g. Mesoproterozoic. I-type granitoids could occur in the Olary Domain, at depth, or under younger cover sequences, and have a relationship to alteration zones and to Cu–Au mineralisation. To

date, however, magnetic, gravity and exploration drilling results have failed to confirm the hypothesis. We suggest that the saline and oxidised metasomatic fluids responsible for garnet–epidote alteration zones and other metasomatic alteration types were most likely derived from metamorphic devolatilisation of crustal rocks, dominated by metasediments in the Olary Domain sequence. Some of these rocks may have originally contained oxidised sequences Že.g. red beds. and evaporites, and are now manifest as hematite- and magnetite-bearing laminated albitites, calcalbitites and certain calc-silicate-rich rocks of the Willyama Supergroup ŽCook and Ashley, 1992.. During peak metamorphism, the breakdown of hydrous and volatile-bearing phases may have released large volumes of fluids and devolatilisation may have also been facilitated by the thermal effects of intrusion of S-type granitoids. Preliminary results from oxygen isotopic studies on regional and local scale Na–Fe–Ca alteration zones Žincluding garnet–epidote alteration zones. are consistent with fluids being derived from crustal rocks ŽR. Skirrow and P.M. Ashley, unpublished data.. These may be metamorphic waters that equilibrated with the Olary Domain sequence; however, direct input from evaporitic brines or magmatic fluids were evidently minimal. The post-metamorphic timing of metasomatic alteration phenomenon may indicate that peak metamorphic conditions were attained at structurally deeper levels at slightly later times than they were attained in rocks currently exposed at the surface. We also note that, although our observations rule out the direct contribution of metasomatic fluids from crystallising granitoids, it is harder at present to constrain the contribution of metasomatic fluids released by crystallising plutons at deep structural levels and subsequently heavily modified by interaction with crustal rocks. Studies of porphyry copper deposits Že.g. Cline and Bodnar, 1991. have shown that at high pressures Ž) 2 kbar. fluids evolved from crystallising granitoids can be highly saline Žup to 60 wt.% NaCl.. Interaction of such fluids with crustal sequences at depth could substantially modify the oxygen fugacity and O-isotope signature of these fluids and render them difficult to discern from fluids related to devolatilisation of crustal rocks. In addition, control of metasomatic circulation by preexisting crustal structures could supplant spatial as-

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

sociations between deep plutons and the sites of metasomatic alteration. 5.4. Similarities to other Proterozoic regions and implications for ore deposition The widespread metasomatism involving saline oxidised fluids in the Olary Domain is similar to that observed in other Proterozoic regions in Australia Že.g. Davidson, 1994, 1998; Williams, 1994, 1998; De Jong and Williams, 1995; Oliver, 1995; Oliver et al., 1998; Conor, 1998. and elsewhere in the world Že.g. Kalsbeek, 1992; Barton and Johnson, 1996; Frietsch et al., 1997.. Barton and Johnson Ž1996. have proposed that a worldwide link exists between Proterozoic and Phanerozoic Fe-rich ŽREE–Cu–Au– U-bearing. hydrothermal deposits and evaporitic source rocks, whereby devolatilisation associated with igneous intrusions forms the oxidised S-poor brines responsible for the observed hydrothermal mineralisation. In the Mt. Isa Block Eastern Succession in northwestern Queensland, examples of widespread Na, Na–Ca and Fe-metasomatism related to saline and chemically complex fluids are recorded, both regionally Že.g. Oliver and Wall, 1987; Oliver, 1995; Williams, 1998. and on more localised scales ŽDavidson, 1994, 1998; Williams, 1994; De Jong and Williams, 1995.. It has been proposed that certain types of epigenetic Cu–Au and U–REE mineralisation in this region are spatially and genetically related to alteration Že.g. Davidson and Large, 1994; Williams, 1994, 1998; Oliver, 1995; Adshead, 1995; Davidson, 1998.. Although differences are evident in the style and nature of alteration in the Mt Isa Block Eastern Succession and the Olary Domain, these are probably due to localised factors, such as host rocks, fluid histories, and P–T conditions of metasomatism. A common theme is the action of hypersaline Žgenerally Na–Ca–K–Fe-bearing. and locally oxidised fluids. It is also possible that the saline fluids responsible for the Mt. Isa Block Eastern Succession Na–Ca alteration Žand locally associated Cu–Au mineralisation. were at least partly evolved from a former evaporitic-bearing sequence Že.g. Oliver and Wall, 1987; Oliver, 1995; De Jong and Williams, 1995., although recent models infer that magmatic

57

fluids from evolved I-type granitoids were also involved Že.g. Pollard et al., 1998; Williams, 1998.. Metasomatism within the Olary Domain also has important implications for the metallogenic status of this region. We have shown that the formation of garnet–epidote alteration zones and other metasomatic rocks in the Olary Domain involved oxidised saline fluids. These fluids would have been capable of transporting significant quantities of metals Že.g. Fe, Cu, Au, Mo, Zn, Pb, Ag, REE, U and Mn. in solution at the temperatures implied from fluid inclusion results Že.g. Hemley et al., 1992; Seward and Barnes, 1997.. Although garnet–epidote-rich rocks from metasomatic alteration zones contain only modest enrichments of Cu, Zn, Pb and U ŽFig. 9., there may be a spatial and genetic link between these rocks and sites where significant Ži.e. ore grade. metal deposition could occur. The strongly oxidised nature of the garnet–epidote rocks may not be conducive to sulfide deposition, but in rock types in which metasomatism would involve major redox changes Že.g. graphitic pelite, psammopelite, calcsilicate-bearing pelite., or in reactive rock types Žmarble, iron-formations and mafic rocks. substantial metal deposition could occur. In the Olary Domain, several historic mineral prospects, as well as new discoveries, display a characteristic Cu–Au–Mo association Žin places with anomalous Co, Zn, As, U, Ba, REE. ŽAshley et al., 1998a; Skirrow and Ashley, 1999.. These deposits all occur within the above lithological settings, in places mediated by fracture systems, but with no substantiated genetically associated granitoids. As the solubility of Zn and Pb remains high at temperatures ) 3008C, in saline fluids with high ClrS and low reduced S Žcf. Hemley et al., 1992., it is probable that mineralisation associated with metasomatic fluids may be limited to Fe–Cu sulfides" molybdenite" gold.

6. Conclusions The Proterozoic Olary Domain is an excellent example of a terrane that has been significantly altered by metasomatic mass-transfer processes associated with regional metamorphism. Metasomatically altered rocks in the Olary Domain are ubiquitous and include garnet–epidote-rich alteration zones,

58

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

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 Ca, Na andror Fe-bearing oxidised mineral assemblages. Detailed study of garnet–epidote-rich alteration zones in calc-silicate host rocks, one common manifestation of intense metasomatic alteration, provides 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 Na, Mg, Rb and Fe 2q, gain of Ca, Mn, Cu and Fe 3q and mild enrichment of Pb, Zn and U. Fluid inclusions show that the hydrothermal fluids responsible for the formation of garnet–epidote-rich assemblages were chemically complex Žcontaining Na, Ca, Fe 3q, Cl, and SO42y ., hypersaline and oxidised Žat or above the hematite–magnetite buffer.. Sm–Nd isotopic analyses show that garnet–epidote-rich alteration zones formed at 1575 " 26 Ma, consistent with field and petrographic observations that suggest that metasomatism occurred prior to the latter stages of regional-scale intrusion of S-type granites at 1600 " 20 Ma. Evidence suggests that the majority of metasomatic alteration throughout the Olary Domain was broadly contemporaneous and involved the action of oxidised hypersaline fluids. We suggest that widespread episodes of fluid flow and metasomatic alteration affected the Olary Domain during the retrograde phases of a major regional metamorphic event. The fluids responsible for metasomatism

within the Olary Domain probably derived from devolatilisation of a rift-related volcano-sedimentary sequence, perhaps containing oxidised and evaporitic source rocks at deeper structural levels, during regional metamorphism and deformation. Although metasomatic rocks are temporally associated with S-type granitoid intrusive rocks, there is no evidence that the metasomatic fluids have been directly sourced from granites.

Acknowledgements We wish to acknowledge funding for this work from the Australian Research Council, Primary Industries and Resources South Australia and a consortium of Australian mineral exploration companies who have supported the Olary Mapping Project. We have also drawn on work by the following honours students from the Universities of New England and Melbourne: James Anderson, Andrew Chubb, Will Eykamp, Michael Fechner, Mark Kent, Maree Laffan, Mark Pepper, Gary Rolfe and Jane Westaway. Assistance with analytical work was provided by Rick Porter, Peter Garlick, John Bedford and Gael Watson. Discussions with colleagues Frank Bierlein, Colin Conor, Nick Cook, Dave Lawie, Bernd Lottermoser, Ian Plimer, Roger Skirrow and Kai Yang also contributed to this work. In addition, AJRK would like to especially thank Professor M. Tattersall and the staff of RPAH, Sydney. Reviews by J.L.R. Touret and P.J. Williams improved this manuscript considerably.

Appendix A. Description and locations of rocksamples used for this study AMG — Australian Map Grid coordinates Location

Sample

Description

Bulloo Well wAMG 442900E 6498450Nx

BW-1

Altered calc-silicate: Massive garnet– Žquartz–epidote.. Altered calc-silicate: Garnet–epidote Žactinolite–quartz–albite.. Alteration pseudomorphs folded laminae in original calc-silicate.

BW-2

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

BW-6A R73358 R73357

Sylvester Bore wAMG 427500E 6437500Nx

SB-1

SB-3 Mindamereeka Hill wAMG 396100E, 6453200Nx

MH-1

MH-2

MH-3

Boolcoomatta wAMG 455200E 6462800Nx

BC-1

BC-4

BC-8

KY-8

Sampson Dam wAMG 445680E 6450640Nx

R74779

R74780

Altered calc-silicate: Massive garnet–quartz Žepidote.. Altered calc-silicate: Massive garnet Ž –quartz–epidote–hornblende.. Unaltered calc-silicate: Laminated clinopyroxene–K-feldspar–albite–quartz– hornblende–epidote Ž –titanite.. Unaltered calc-silicate: Laminated clinopyroxene–quartz–K-feldspar–albite Ž –actinolite–titanite. with minor actinolite alteration of clinopyroxene. Altered calc-silicate: Massive garnet Ž –epidote–quartz–albite.. Altered calc-silicate: Laminated clinopyroxene–quartz– albite Ž –scapolite–K-feldspar–titanite. grading to bleached albite–quartz-rich rock and massive coarse garnet Žup to 3 cm.. 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. Relatively unaltered calc-silicate: Laminated clinopyroxene–albite quartz Ž –actinolite–K-feldspar–titanite. with minor alteration to garnet–epidote– quartz along fractures. Relatively unaltered calc-silicate: Laminated clinopyroxene–albite–K-feldspar–quartz Ž –titanite. with small zones of epidote alteration. Altered calc-silicate: Massive garnet–epidote– quartz Ž –hematite. with relicts clinopyroxene partly replaced by actinolite. Brecciated and altered calc-silicate: Bleached quartz–albite fragments in a epidote Ž –actinolite–garnet. matrix. Altered calc-silicate: Garnet–quartz Ž –epidote–actinolite–clinopyroxene.. Relict clinopyroxene largely altered to actinolite. Unaltered calc-silicate: Laminated quartz– clinopyroxene–albite–actinolite–epidote Ž –titanite. Altered calc-silicate: Massive garnet Ž –quartz–epidote–hornblende–albite.

59

60

A.J.R. Kent et al.r Lithos 54 (2000) 33–62

R74782 White Dam North wAMG 453400E 6451520Nx

R77351 R77354

R77353

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