Understanding the 1720–1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis

Precambrian Research 166 (2008) 297–317

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Understanding the 1720–1640 Ma Palaeoproterozoic Willyama Supergroup, Curnamona Province, Southeastern Australia: Implications for tectonics, basin evolution and ore genesis Colin H.H. Conor ∗ , Wolfgang V. Preiss Geological Survey Branch, Primary Industries and Resources, South Australia, GPO Box 1671, Adelaide, South Australia 5001, Australia

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Article history: Received 27 April 2006 Received in revised form 14 August 2007 Accepted 30 August 2007 Keywords: Curnamona Province Palaeoproterozoic Mesoproterozoic Willyama Supergroup Stratigraphy Magmatism Tectonic history Pb–Zn–Ag mineralisation Cu–Au–U mineralisation

a b s t r a c t The ∼1720–1640 Ma Willyama Supergroup was deposited in an epi-continental rift basin accompanied by felsic and mafic magmatism, and hosts the largest known Pb–Zn–Ag accumulation at Broken Hill. The Supergroup occupies the southern portion of the Curnamona Province, a largely buried ∼50,000 km2 crustal block, possibly the southernmost part of the Eastern Australian Pb–Zn belt (which includes Mt Isa). Additionally, the Province is a component of the early Mesoproterozoic IOCG ‘super-province’ that includes the Gawler Craton (Olympic Dam), Eastern Succession of the Mt Isa region (Ernest Henry) and perhaps parts of NW North America (Wernecke Mountains). The Curnamona Province, but a small part of the original basin, is divided into domains on depositional, tectonic and magmatic criteria with variably distributed stratigraphic units reflecting syn-depositional faulting and onlapping relationships resulting from rifting during the earlier extensional stage of basin evolution. Rifting controlled the variable distribution and thickness of the succession. In the Olary Domain to the west, the palaeo-flank exposes the ∼1720–1715 Ma Curnamona Group, but overlying sediment packages are but sporadically developed. To the east and closer to the rift axis, sedimentation in the Broken Hill Domain was more complete, comprising ∼1710–1705 Ma migmatitic-metasediments, and the overlying ∼1705–1695 Ma Thackaringa, ∼1695–1685 Ma Broken Hill and ∼1685–1670 Ma Sundown Groups. The ∼1670–1640 Ma Paragon and Strathearn Groups represent sag-phase deposition with a change in basin configuration and provenance. Earliest syn-depositional magmatism is recorded in the Olary Domain by ∼1720–1710 Ma A-type felsic volcanics and intrusives and restricted basaltic lavas. The locus of magmatism then moved to the Broken Hill Domain with partial melting of either the lower parts of the Willyama Supergroup, or underlying rocks of similar composition, producing S-type granitic sills of the ∼1705–1685 Ma Silver City Suite. Magmatism climaxed in the upper Broken Hill Group with emplacement of Fe-rich tholeiite sills and the ∼1685 Ma volcaniclastic felsic Hores Gneiss hosting the Broken Hill Pb–Zn–Ag orebody. Polyphase deformation and high T, low P metamorphism, ranging from greenschist to lower granulite grade, resulted from the ∼1620–1590 Ma Olarian Orogeny. Early layer-parallel schistosity was deformed progressively, first by large ductile recumbent and sheath-style folds, and later by upright folding. The mainly S-type granites of the ∼1595–1580 Ma Ninnerie Supersuite were intruded late in the deformation history. An extensive network of post-1580 Ma retrograde shear zones dismembered the Willyama Supergroup and was partially reactivated during the ∼500 Ma Delamerian Orogeny. Of the wide range of mineralisation styles in the Curnamona Province, Broken Hill-type Pb–Zn–Ag deposits and epigenetic Cu–Au and U are currently of greatest interest. Broken Hill-type mineralisation probably formed in a shallow depositional environment at or below the sea-floor, with maximum development approximating cessation of magmatic activity. Cu–Au (±Co, Mo) deposits (e.g. Kalkaroo, Portia) are located in chemical and structural traps near a regionally extensive redox boundary. By sharing geological aspects of the eastern Gawler Craton, the Curnamona Province has excellent prospectivity for Olympic Dam-style IOCG mineralisation. Uranium deposits, some economic, are located both in basement rocks and as roll-front concentrations in overlying Tertiary channel sands. © 2008 Elsevier B.V. All rights reserved.

∗ Corresponding author. Tel.: +61 8 8338 0079; fax: +61 8 8463 3089. E-mail address: [email protected] (C.H.H. Conor). 0301-9268/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.precamres.2007.08.020

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1. Introduction The Curnamona Province is a largely buried, ovoid crustal element measuring some 50,000 km2 in eastern South Australia and western New South Wales (Figs. 1 and 2), the outlines of which, clearly visible in geophysical imagery, are a result of Neoproterozoic and Palaeozoic tectonic events. This paper concentrates upon the southern part of the Curnamona Province, describing the remnant of a formerly more extensive late Palaeoproterozoic sedimentary basin in-filled by the moderately to highly deformed and metamorphosed Willyama Supergroup, syn-depositional and syn- to post-orogenic magmatic rocks, and Mesoproterozoic volcanic and sedimentary rocks. Neither the top nor base of the Willyama Supergroup are known from outcrop. The Curnamona Province is now separated from the Gawler Craton by the Neoproterozoic rift complex of the Adelaide Geosyncline (Fig. 2) but, immediately west of the rift, the Wallaroo Group at the eastern edge of the Gawler Craton shows considerable lithological similarity to the Willyama Supergroup, although it is about 30 m.y. older. This, together with the presence of early Mesoproterozoic magmatic rocks in the eastern Gawler Craton and Curnamona Province, suggests crustal continuity at least as far west

as the 1850 Ma Donington Suite intrusives (Fig. 2), and progressive eastward shift of Palaeoproterozoic depocentres. By hosting the Broken Hill Pb–Zn–Ag–(Au) lodes and numerous much smaller Broken Hill-type deposits, the southern Curnamona Province represents the southeastern-most extent of the eastern Australian lead–zinc belt that includes deposits such as McArthur River, HYC, Century, Mt Isa and Cannington. This interpretation is supported by the similarity of ages of syn-depositional magmatic units contained by the Willyama Supergroup to those of the Calvert and Isa Superbasins (Page et al., 2005a) (Fig. 1a). Myers et al. (1996) suggested that the North Australian and South Australian Cratons were amalgamated during the ∼1.2 Ga Musgravian Orogeny. However, the basin containing the Willyama Supergroup, situated at the eastern extremity of the South Australian Craton, and the Calvert and Isa Superbasins occupying a similar position 1000 km to the north on the North Australian Craton (see Fig. 1, in Pirajno and Bagas, this volume), were parts of a once continuous late Palaeoproterozoic sedimentary belt that underwent deformation and granite intrusion in the early Mesoproterozoic (the Diamantina Orogen of Laing, 1996a). Also supporting the North Australian Craton connection, Curnamona hosts Cu–Au–Mo and economic U mineralisation, and thus it is an element of the early Mesoproterozoic IOCG province (Fig. 1b) that includes the Gawler Craton, the Eastern Succession at Mt Isa, and perhaps the Wernecke Mountain region of northwestern North America (Fig. 17). In the southern Curnamona Province there are a number of polymetallic prospects (Zn, Pb, Ag, Cu, Co, W), and the industrial minerals feldspar and garnet are currently being mined. Summary descriptions of mineralisation and exploration in the Willyama Inliers in South Australia are available in Mawson (1912), Campana and King (1958), Yates (1992), Yates and Randell (1994), Ashley et al. (1995), Robertson et al. (1998), McCallum (1998a, 1998b), Conor (2004a, 2006) and Cooper and McGeough (2006). Systematic descriptions of mineral deposits in the New South Wales portion are available in Barnes (1988a, 1988b), Barnes et al. (1983), Stevens et al. (1990, 2003) and Burton (1994, 2000). 1.1. Provinces, domains, subdomains and inliers

Fig. 1. Location of the Curnamona Province within: (a) Proterozoic eastern Australian lead–zinc belt and (b) elements of the early Mesoproterozoic IOCG Cu–Au province.

In this paper the Curnamona Province is divided into domains and subdomains to facilitate discussion of its geological history and tectonic development. However, the boundaries are somewhat transitional and should not block consideration of the Curnamona Province as a whole. The word ‘Province’ is used as a non-genetic term referring to a large crustal element with a coherent geological history and recognisable in various geoscientific datasets, but without the tectonic implications of the term ‘terrane’. Domains are subdivisions of a province, defined by particular lithological, stratigraphic, geophysical or tectonic characteristics. Domains are subdivided into subdomains by similar criteria. In the southern Curnamona Province, Willyama Supergroup rocks crop out as partly unconformity-bounded and partly faultbounded basement inliers through the Neoproterozoic and younger cover. The origin and nomenclature of the Willyama Inliers in the Olary Domain were described by Preiss and Conor (2001). In addition to those in South Australia, the Willyama Inliers include the Broken Hill, Euriowie, Poolamacca, Mt Woowoolahra and Nardoo (Burton, 2006) Inliers in New South Wales while, in the extreme northwest of the Curnamona Province, the Mount Painter and Mount Babbage Inliers are outcropping basement cores of regional anticlines (Fig. 2). The Benagerie Ridge is a shallowly buried basement high separating two depocentres of the Cambrian Arrowie Basin: Moorowie Sub-basin to the west and Yalkalpo Sub-basin to the east (Fig. 3).

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Fig. 2. The Curnamona Province in relation to the Gawler Craton, showing distribution of lithostratigraphic units of the Gawler Craton (2550–2500 Ma Sleaford and Mulgathing Complexes, 2480–2420 Sleafordian Orogeny, 1860–1850 Ma Donington Suite, 1770–1750 Ma Wallaroo Group and Moonabie Formation, and 1730–1700 Ma Kimban Orogeny) and areas of Palaeo–Mesoproterozoic outcrop in inliers of the Curnamona Province.

1.2. Tectonic setting of the Curnamona Province The name ‘Curnamona’ was first applied by Thomson (1970) as ‘Curnamona Cratonic Nucleus’ to a small area in the vicinity of Lake Frome where early mineral exploration drilling had intersected flatlying Cambrian cover, attesting to the region’s cratonic character during the Cambro-Ordovician Delamerian Orogeny. The advent of regional aeromagnetic data revealed that this ‘nucleus’ is part of a much larger ovoid area, to which the name ‘Curnamona Craton’ was then applied (Drexel et al., 1993). However, the peripheral regions of this larger entity were severely modified by deformation and metamorphism during the Delamerian Orogeny, so that the term ‘craton’ is not appropriate. The Curnamona Province (Robertson et al., 1998) has a complex history and its present morphology is the result of the following geological events: • Crustal extension, basin formation and deposition of the Willyama Supergroup and related igneous rocks, between 1720 and 1640 Ma, upon a continental substrate. • Polyphase deformation and metamorphism during the ∼1600 Ma Olarian Orogeny, intrusion of 1595–1580 Ma Mesoproterozoic granites, late-stage uplift and erosion prior to deposition of 1580 Ma extrusives and local sediments in the central part of the Curnamona Province.

• Mesoproterozoic retrograde shearing, segmentation and minor refolding. • Extension and further segmentation caused by Neoproterozoic rifting, accompanied by Neoproterozoic sedimentation. • Early Cambrian extension and sedimentation. • Middle to Late Cambrian basin inversion, contraction and metamorphism during the Delamerian Orogeny, and incorporation of the periphery of the Curnamona Province in the Delamerian Orogen, while the central portion remained cratonic with flat-lying Neoproterozoic and Cambrian cover. • Formation of partly faulted, partly unconformity-bounded basement-cored anticlines and domes in the folded Adelaidean rocks that when eroded produced the present Willyama Inliers. • Deposition of Mesozoic, Tertiary and Quaternary cover, all strongly influenced by neotectonic processes. The southern part of the Curnamona Province has traditionally been divided into the Broken Hill Domain and Olary Domain (Laing, 1996b), a modification of the original Willyama Block of Thomson (1969) and Olary Block and Broken Hill Block of others. In this paper, we recognise the following subdivisions (Fig. 3): • The Broken Hill Domain is characterised by the lode-bearing stratigraphy of the Broken Hill Group. Much of the western

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Aeromagnetic and gravity data suggest the presence of large granitic and mafic intrusions with possible products of intense iron oxide-forming alteration in the Erudina Domain. Proven Palaeoproterozoic rocks are exposed only in the Olary, Broken Hill and Redan Domains, although Teale (1993) suggested that equivalents are present, along with dated Mesoproterozoic rocks, in the relatively small Mount Painter and Mount Babbage Inliers. In this paper, we interpret the tectonic evolution of the Curnamona Province and its relationship to other geological provinces in Australia, with particular emphasis on the Willyama Supergroup (Fig. 4). As host to the Broken Hill Pb–Zn–Ag(–Au) lodes, the Willyama Supergroup is of prime economic importance, as are the orogenic and magmatic events associated with later IOCG copper–gold–uranium mineralisation. 2. Lithostratigraphy of the Willyama Supergroup

Fig. 3. Subdivision of the Curnamona Province into eight geological domains defined by particular lithological, stratigraphic, geophysical or tectonic characteristics.

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boundary is defined by the Mundi Mundi Fault but, at the southern end of the fault, the boundary diverges in a southwesterly direction (Crooks, 2001). The Broken Hill Domain thus crops out in the Broken Hill, Euriowie, Poolamacca, Mt Woowoolahra and Nardoo Inliers, as well as in the southeastern portion of the Kalabity Inlier. The Redan Domain is a region of high magnetic intensity (Stevens and Corbett, 1993) southeast of the Broken Hill Domain, and it contains the oldest rocks known within the Broken Hill Inlier. The Mulyungarie Domain is characterised by a thick sulphidic succession and is possibly transitional between the Olary and Broken Hill Domains. Almost entirely blanketed by younger sediments, the Mulyungarie Domain has poorly defined boundaries, apart from its eastern margin at the Mundi Mundi Fault. To assist discussion, the Mulyungarie Domain is further subdivided into western and eastern parts that approximate structural domains, with the division at the axial trace of a large N–S—trending anticline five kilometres west of the SA–NSW border. The Olary Domain is recognised by restricted development of Broken Hill Group, and by the presence of the oldest known part of the Willyama Supergroup, the Curnamona Group. The Mudguard Domain is characterised by a sheet of flat-lying early Mesoproterozoic (∼1580 Ma) A-type felsic and mafic volcanics (Giles and Teale, 1979) and minor unmetamorphosed sediments. The domain forms a large part of the shallowly buried Benagerie Ridge, but is inferred from aeromagnetic data to extend to the west below the Moorowie Sub-basin (Burtt and Betts, 2003). The Moolawatana Domain at the northwestern extremity of the Curnamona Province includes basement rocks outcropping in the Mount Painter and Mount Babbage Inliers, and a shallowly buried easterly extension of these rocks. The Erudina and Quinyambie Domains are the deeply buried and poorly known basement beneath the Cambrian Moorowie and Yalkalpo Sub-basins, respectively.

The stratigraphic scheme of Willis et al. (1983a) and Stevens et al. (1983) for the Broken Hill Domain has been shown to be robust when tested by subsequent Broken Hill Exploration Initiative (BHEI) geochronological studies (Page et al., 1998, 2000, 2003, 2005a, 2005b). By contrast, the stratigraphic schemes developed for the Olary Domain have been more rudimentary (Clarke et al., 1986; Forbes and Pitt, 1980; Laing, 1996b; Conor, 2000a, 2000b). New geological mapping and geochronology have allowed the erection of a more comprehensive stratigraphic scheme for the South Australian portion of the Curnamona Province and demonstrated the variable distribution of some stratigraphic units, such as the Broken Hill Group (Figs. 4 and 7, and Conor, 2000a, 2000b, 2004a, 2006, in preparation). The absence of certain stratigraphic units in some sections has been explained as facies changes (Laing, 1996b) or structural excision (Gibson and Nutman, 2004), but the variable distribution is more easily attributed to sedimentary and syn-depositional tectonic processes. Geochronological data have demonstrated that, broadly speaking, there are more breaks in sedimentation in the west, with the sedimentary succession being most continuous in the Broken Hill Domain, in particular where exposed in the Euriowie Inlier. This variation has tectonic implications, and also provides a basis for temporal subdivision of the lithostratigraphy into genetic packages (Fig. 4). 2.1. Syn-depositional tectonic evolution The tectonic setting of Willyama Supergroup sedimentation has long been viewed in terms of a volcanic, intracontinental rift (Scheibner, 1974; Phillips et al., 1985; Willis et al., 1983a, 1983b; Laing, 1996b). Previously, stratigraphic evidence for rifting has been ambiguous, given the lack of coarse clastic sediments or obvious growth faults. However, our new stratigraphic interpretations, based on modern field and geochronological studies, support the rift concept (Fig. 5). The current boundaries of the Curnamona Province are the result of post-Willyama tectonism, because they truncate structural and sedimentary trends within the province. The original limits of the sedimentary basin therefore lay outside the present Curnamona Province. Deposition of the Willyama Supergroup took place over a period of at least 80 m.y. (∼1720–1640 Ma) during which basin configuration may have changed considerably. The Paragon Group–Strathearn Group at least may have been deposited in an extension of the Mt Isa Superbasin, which in the Mt Isa area overlay the Calvert Superbasin (Page et al., 2005b). A tectonic reconstruction by Betts et al. (2002) shows the Curnamona and Mt Isa Provinces closer together in the late Palaeoproterozoic. Semi-quantitative ages for detrital zircons of the Willyama

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Fig. 4. Lithostratigraphy/time diagram for the Willyama Supergroup in the southern Curnamona Province: comparison between the Olary, Mulyungarie and Broken Hill Domains.

Supergroup (data from OZCHRON database) suggest a broad clustering over the intervals ∼2750–2500 Ma and ∼2000–1950 Ma, 1900–1715 Ma, with rare older Archaean grains (∼3700 Ma oldest) (Fig. 6). Within these approximate clusters are apparent peaks at ∼1730 Ma, ∼1790 Ma, ∼1820 Ma, ∼1850 Ma, ∼2000 Ma, ∼2550 Ma and ∼2700 Ma. Payne et al. (2006) showed that the detrital zircon population for the Nawa Domain of the northern Gawler Craton occupied the range ∼1840–1740 Ma, and, citing Barovich (2003), indicated that the sediments of the lower Willyama Supergroup and

the Nawa Domain were isotopically indistinguishable. Barovich and Hand (this volume), Payne et al. (2006) and Page et al. (2000) all propose a northern Australian provenance (Arunta), rather than the nearer Gawler Craton, as the source of the lower Willyama Supergroup. The clustering of detrital zircons in the period 1900–1715 Ma does allow the ∼1860–1850 Ma Donington Suite and volcanics of the eastern Gawler Craton (e.g. Wallaroo Group) as potential sources. However, there are no detrital zircons in the Willyama Supergroup at 2400 Ma, the age of the major Sleafordian event of

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Fig. 5. Conceptual section across the Olary, Mulyungarie and Broken Hill Domains showing rift-related sedimentation being responsible for variation of deposition of the major units of the Willyama Supergroup.

the Gawler Craton, but there is a significant ∼2700 Ma peak that is not recorded in the Gawler Craton. This aspect of the detrital zircon age distribution does not favour a Gawler Craton provenance. Using Nd isotopic evidence, Barovich et al. (2002), Barovich (2003) and Barovich and Hand (this volume), have interpreted a switch in sedimentary source in the uppermost part of the Willyama Supergroup (within the Paragon and Strathearn Groups), units below this change possibly being derived from central Australian crust while those above had more juvenile sources. In the Olary and Mulyungarie Domains, a pronounced regional magnetic gradient highlights the change from the magnetite-rich lower part of the Willyama Supergroup to overlying sediments of low mag-

Fig. 6. Detrital age spectra for the Willyama Supergroup (curve) superimposed upon age ranges of magmatic events of the eastern and western Gawler Craton.

netic intensity (this change approximates the top of the Curnamona Group depicted in Fig. 7), possibly attributable to variation in the oxidation state of the original basin. A similar change in the Broken Hill and Redan Domains appears more transitional. The thickness of the known Willyama Supergroup in the outcropping Willyama Inliers is not great in comparison with known rift successions, being about eight kilometres in the Broken Hill Domain (only ∼6 km if the Paragon Group is not included in the rift sequence). However, the base of the total succession is nowhere exposed, and there may be many kilometres of stratigraphy below the oldest outcropping rocks, as indeed is suggested by the recent Curnamona Seismic Transect (Goleby et al., 2006, Fig. 4-1). In addition, overburden thicknesses varying from approximately 10 km to 20 km above the present level of crustal exposure are required for the pressures recorded by metamorphic assemblages (Phillips, 1980; Phillips and Wall, 1981). Much of this overburden was initially sedimentary, followed by thickening of the pile by early Olarian isoclinal recumbent folding. Major crustal extension was instrumental in the formation of the Willyama Supergroup depositional basin. The distribution and thickness variations of stratigraphic packages imply normal faulting, and evolving syn-depositional magmatism included mafic components, indicating a mantle connection. The Curnamona Group of the Olary Domain is characterised by A-type magmatism (∼1720–1710 Ma), suggestive of mantle-derived heating of lower, depleted continental crust, consistent with lithospheric thinning. The change in felsic magma composition from A-type (mostly Curnamona Group, Olary Domain) to peraluminous S-type (Silver City Suite and Hores Gneiss, Broken Hill Domain) after ∼1705 Ma suggests a change of conditions in the crust during this later extensional episode. Intense ∼1705–1685 Ma extension may have brought upwelling asthenosphere into contact with the base of the Willyama Supergroup, leading to melting of the sediments. The notion of extension and heating in the deep crust during Willyama Supergroup deposition (not to be confused with Gibson

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Fig. 7. Solid geology interpretation of the southern Curnamona Province. (a) Simplified geological map showing distribution of lithostratigraphic units. (b) Table indicating the chronological distribution of lithostratigraphic units in each domain.

and Nutman’s, 2004, model involving high-temperature, extensional shearing and metamorphism of near-surface sediments) is supported by mafic magmatism during Ethiudna Subgroup and Broken Hill Group times. Mafic magma appears to have been generated during deposition of both the upper Curnamona Group and Broken Hill Group, but none is known during the interven-

ing Thackaringa Group time. In the Curnamona Group only minor extrusive pillow basalt and non-pillowed amygdaloidal basalt are known. However, during Broken Hill Group time large volumes of tholeiitic ferro-basalt were emplaced as sills, dykes and plugs. The latter probably indicates a peak in the rate of rifting, with high-level magma reservoirs allowing the fractionation of very

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Fe-rich, Mg-poor basaltic magma. There are no known synWillyama igneous rocks younger than the Hores Gneiss (∼1685 Ma), but sedimentation continued, perhaps with some interruptions, until at least 1640 Ma. The Sundown and Saltbush Groups represent a continuation of the relatively shallow-water Broken Hill Group sedimentation, suggesting that the rift phase continued, but with extension insufficient to allow magmatic intrusion. The Paragon and Strathearn Groups represent a different depositional style, provenance and basin configuration, indicating the beginning of sag-phase sedimentation as the underlying asthenosphere and lower crust cooled. 2.2. Curnamona Group and Redan Gneiss-Farmcote Gneiss, and related igneous rocks: ∼1720–1705 Ma 2.2.1. General setting The oldest known rocks of the Curnamona Province are exposed in the Olary Domain in South Australia and are assigned to the Curnamona Group, comprising the older Wiperaminga Subgroup and the younger Ethiudna Subgroup (Figs. 4 and 7). Curnamona Group may be present at depth below the Broken Hill Domain, but it is not known from outcrop. However, the Redan Gneiss (Redan Domain) may at least partially overlap in age with the upper part of the Curnamona Group (Figs. 3 and 4), and the Redan Domain shows lithologic and geophysical similarities to the Olary Domain. Due largely to poor outcrop the relationship between the Redan Domain and the adjacent Broken Hill Domain remains uncertain, and there are significant lithological differences between the two domains. 2.2.2. Lithostratigraphy (Fig. 4) The Wiperaminga Subgroup is the oldest known lithostratigraphic unit of the Willyama Supergroup, but its base has not been recognised. It is dominantly clastic with the only chemical sediments being quartz-rich and containing iron (as magnetite and locally pyrite), barite or occasionally manganese. Though not widespread, sulphide is found in felsic volcanic units that also have associated magnetite iron formations. The George Mine Formation is characterised by albitic units of varying thickness, some probably being altered psammites. Intervening psammopelitic–pelitic metasediments have commonly been converted to migmatitic gneiss (Fig. 8). The Tommie Wattie Formation in its type area overlies George Mine Formation and is a 1.5 km thick package of interbedded psammite and psammopelite in the lower part, fining upwards into andalusite-rich pelite. In the Walparuta Inlier, a thin metavolcanic layer lies locally at this transition, and a volcaniclastic meta-conglomerate is located near the top in the Tommie Wattie Bore area. Sedimentary structures (cross-

Fig. 8. Interlayered albitic metasediment and migmatitic gneiss, George Mine Formation, Mulga Bore, western Kalabity Inlier, Olary Domain.

Fig. 9. Soft-sediment deformation in meta-sandstone of the Tommie Wattie Formation, Waterfall in northern Walparuta Inlier, Olary Province.

beds, ripples, ball, pillow, etc.) are locally perfectly preserved and indicate rapid deposition of sands in a shallow-water, near-shore environment (Fig. 9). The upper part is apparently more distal from the sediment source, and the psammite–pelite transition may be diachronous. A possibly correlative psammitic unit in the Telechie Valley area is similar but lacks the pelitic top. Although the Tommie Wattie Formation has thus far been included in the Wiperaminga Subgroup following the rock relation concepts of Laing (1996a), Flint (2001) regarded it as part of the younger Ethiudna Subgroup. The overlying Ethiudna Subgroup, 200–400 m thick, is generally psammitic but characterised by significant calcium content, generally present in calc-silicate minerals but locally as calcite, and high iron oxide content. Internally it exhibits rapid vertical and lateral facies changes. Although largely planar laminated, the Ethiudna Subgroup displays occasional sedimentary structures indicative of shallow-water deposition and pseudomorphs after evaporitic minerals (Fig. 10), consistent with the sabkha environment envisaged by Cook and Ashley (1992). The widespread basal Cathedral Rock Formation is best developed where the Ethiudna Subgroup overlies the George Mine Formation. It varies from a single thin quartzite to a package up to 100 m thick containing lenticular quartzite and psammopelitic units. Cross-beds are locally observed and at least one unit is volcaniclastic, the latter hosting the Mount Mulga Mine Ba–Cu–Au deposit. In the northern Walparuta Inlier, the Bewooloo Formation of fine siliciclastic metasediments marks the base of the Ethiudna Subgroup, and includes the earliest recorded mafic magmatism in the Willyama Supergroup (Montstephen Metabasalt Member). This large lenticular body of amygdaloidal basalt (Pointon, 1980) has spectacular lateral facies variations—polymict and monomict conglomerates of disaggregated pillows in a ferro-magnesian carbonate matrix, and associated with a variety of siliceous manganiferous rocks interpreted as chemical ‘exhalites’ (Conor, 2003). Pillow lavas were first recorded by Jones et al. (1962) in the Coppertop Inlier. 2.2.3. Syn-depositional magmatism A-type felsic quartz-eye volcanic units and subvolcanic granite sills (Ashley et al., 1996) intercalated in the Wiperaminga and Ethiudna Subgroups are assigned to the Basso Suite (Conor, 2000a, 2000b) (Figs. 4 and 11) and include pyroclastic and sub-aqueous mass flows (Fig. 12) and porphyries, the latter representing either lava flows or subvolcanic sills. U–Pb zircon dating (Page et al., in preparation) shows that the volcanics are mostly in the range 1719–1715 Ma, but geochronology has not yet resolved any clear age difference between the Wiperaminga and Ethiudna Subgroups. The paucity of Basso Suite-aged detrital zircons in the intervening

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Fig. 10. Small ferruginous pseudomorphs possibly after dolomite crystals in calcalbitite rock from Peryhumuck Formation, Burdens Dam, northern Kalabity Inlier.

sediments indicates rapid and continuous deposition with little or no reworking of the volcanic units. The I-type granite at Poodla Hill (Cook et al., 1994), dated at 1719 ± 3 Ma (Page et al., in preparation), and possibly an S-type meta-granite nearby at the Bimba Mine, are of similar age. A-type granites of the Basso Suite intruded the Wiperaminga Subgroup slightly later at 1713–1711 Ma. The youngest A-type felsic intrusive is the orthogneiss in the Farmcote Gneiss in the Redan Domain, giving ages of 1703 ± 3 Ma, 1705 ± 3 Ma (Page et al., 2005b). 2.2.4. Syn-depositional tectonism The Curnamona Group is predominantly metasedimentary, but associated A-type magmatism (1719–1711 Ma Basso Suite) and subsequent mafic volcanism (Montstephen Metabasalt Member) indicate crustal extension and lithospheric thinning during depo-

Fig. 11. Quartz-eye textured felsic metavolcanic rock, ∼1718 Ma Abminga Subsuite, Basso Suite, southern Walparuta Inlier, Olary Domain.

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Fig. 12. Graded felsic mass-flow volcaniclastic near Cathedral Rock, ∼1718 Ma Abminga Subsuite, Basso Suite, western Kalabity Inlier, Olary Domain.

sition. In view of the paucity of coeval mafic magmatism, the A-type magmas were possibly sourced from older lower crust, previously depleted with respect to minimum melt component. Heat for melting of this depleted lower crust may have been supplied by lithospheric thinning, mantle upwelling and finally mafic magma intrusion. Laing (1996b) observed that there are subdomains characterised by different stratigraphic relationships: in his Bulloo Subdomain, rocks now included in the Ethiudna Subgroup overlie George Mine Formation, while in his Outalpa Subdomain they overlie Tommie Wattie Formation (Fig. 13a). While Laing (1996d) attributed this variation to juxtaposition of different facies belts by nappe-thrusting during the ∼1600 Ma Olarian Orogeny, more recent mapping suggests that these facies belts resulted from syndepositional growth faulting, consistent with crustal extension. In the Outalpa Subdomain, Tommie Wattie Formation grades up into the Ethiudna Subgroup, and its alternative inclusion in the latter (Flint, 2001) has considerable merit both lithologically and genetically. In the Bulloo Subdomain, a well-developed sharpbased quartzite (Cathedral Rock Formation) suggests erosion of the George Mine Formation where it is directly overlain by the Ethiudna Subgroup (Conor, 2000a, 2000b; W. Zang, PIRSA, pers. commun., 2000). Thus, it is argued that the Bulloo Subdomain contains the remnants of a horst, while the Tommie Wattie represents deposition in an adjacent rift basin (Fig. 13b). Extensional tectonics, the introduction of mafic magma, and the high-temperature character of the Basso Suite suggest mantle upwelling. The continuation of A-, I-, S-type felsic and mafic igneous activity into Ethiudna times indicates that the crust was still extending, although magmatic activity apparently shifted eastward as evidenced by ∼1710 Ma tuffaceous metasediment and a ∼1705 Ma A-type igneous sill in the Rantyga Group (name introduced by Stevens, 2006, to include the four formations contained in the Redan Domain) (Fig. 4). This easterly

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Fig. 13. Inferred growth faulting in the Olary Domain. The Outalpa Subdomain is defined by the presence of the Tommie Wattie Formation grading up into the Ethiudna Subgroup. The Tommie Wattie Formation is absent from the Bulloo Subdomain; instead quartzite of the Cathedral Rock Formation is interpreted to have been deposited upon eroded George Mine Formation. (a) Parts of the Outalpa and Bulloo Subdomains in plan. (b) Cross-section illustrating the situation at the time of deposition of the Willyama Supergroup, showing the Tommie Wattie Formation as graben-fill in the Outalpa Subdomain, and the George Mine Formation forming a horst in the Bulloo Subdomain.

temporal shift in magmatic activity continued with S-type intrusives and volcaniclastic units in the later Thackaringa and Broken Hill Groups, the change in composition also indicating evolving conditions in the underlying crust.

2.2.5. Depositional environment The shallow-water, possibly locally emergent, depositional environment of much of the Curnamona Group is consistent with shelf sedimentation, perhaps on a rift shoulder, as illustrated by

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Laing (1996b, Fig. 9) and Conor and Page (2003) (Fig. 5). The high sodium (as metasomatic albite) and iron oxide contents are possibly attributable to original oxidised saline basinal conditions, consistent with a shallow, restricted rift basin developed above an attenuated lower crust in an intracontinental setting. The Redan Gneiss of the Redan Domain, which is lithologically similar to the Ethiudna Subgroup, may represent continued evaporitic sedimentation in the main rift to the east after ∼1715 Ma when, in the Olary Domain, deposition ceased and erosion may have set in. 2.2.6. Syngenetic mineralisation Basso Suite volcanic units are of metallogenic significance because they are commonly iron and sulphur rich. Magnetite is present as disseminations, veins and associated iron formations, the latter presumably representing chemical sedimentation or diagenetic replacement. Disseminated pyrite (and rarely chalcopyrite) is most common in volcanic units. Styles of mineralisation are varied, for example: • The East Doughboy Mine on Bimbowrie Station represents a similar setting to the later Broken Hill Pb–Zn–Ag mineralisation: a fluorite and gahnite-bearing metasedimentary package contains lenses of manganiferous quartz–garnet–magnetite rock within a 400 m thick felsic volcanic succession. • The Mount Mulga Mine Ba–Cu–Au deposit is located within the basal volcaniclastic quartzite of the Cathedral Rock Formation, possibly onlapping a disconformity surface. • The White Dam Cu–Au deposit is hosted by psammopelitic migmatitic gneiss, probably upper George Mine Formation (Cooke, 2003). The contact with the nearby Ethiudna Subgroup is faulted. • The widespread nature of Cu or Cu–Au mineralisation within the Ethiudna Subgroup suggests syngenetic enrichment, although significant concentrations of sulphide are generally structurally controlled, e.g. Waukaloo and Mount Howden mines. 2.3. Clevedale Migmatite, Thorndale Composite Gneiss, Thackaringa Group and Portia Formation: ∼1710–1700 Ma 2.3.1. General setting Although the Olary Domain records very little deposition between about 1715 Ma (top of Ethiudna Subgroup) and 1693 Ma (Plumbago Formation), in the Redan and Broken Hill Domains, the Rantyga Group, much or all of the Thackaringa Group and possibly the Thorndale Composite Gneiss and Clevedale Migmatite were deposited in this time interval (Fig. 4). The minimum age of metasediments up to lower Thackaringa Group is set by the intrusive 1704 ± 3 Ma Alma Granite Gneiss (Page et al., 2005b), while the overlying Cues Formation is dated by Stevens et al. (2008) at 1700 ± 4 Ma on a leucogneiss interpreted as a metavolcanic (Willis, 1982). The age of the Himalaya Formation (uppermost Thackaringa Group) is between that of the Cues Formation and the 1693 ± 4 Ma age (Page et al., 2005b) on metasiltstone immediately overlying the Ettlewood Calc-Silicate Member in the overlying Broken Hill Group (Fig. 4). Sedimentation also continued in the Mulyungarie Domain as evidenced by age determinations of a tuff unit in the Portia Formation (see later discussion). 2.3.2. Lithostratigraphy Both the Clevedale Migmatite and Thorndale Composite Gneiss are partially melted clastic metasediments, the former being distinguished by thin layers or laminae of albitised metasediment. The Thorndale Composite Gneiss was a relatively thinly bedded, sandy sediment. The Clevedale Migmatite and the lower part of the Thorndale Composite Gneiss are magnetite-bearing. Mafic dykes,

Fig. 14. Cross-bed sets in albitised meta-sandstone from the Himalaya Formation, Spring Creek, near Allendale, northern Broken Hill Domain.

sills and pods in both units were probably emplaced slightly later during Broken Hill Group time. The Thackaringa Group, the oldest unit in the Broken Hill Domain that preserves sedimentary structures (Fig. 14), is characterised by thick albitic, relatively psammitic facies of the Himalaya and Lady Brassey Formations, and psammopelitic metasediments with non-manganiferous iron formations and thin quartzo-feldspathic volcanogenic gneisses of the Cues Formation (Willis et al., 1983a, 1983b). Conor (2006) suggested the name ‘Portia Formation’ for a ∼250 m thick assemblage of metasediments in the Mulyungarie Domain (Figs. 3 and 4), characterised by the presence of carbonate, calc-silicate minerals, alkali feldspar, possible pseudomorphs after evaporite minerals (Fig. 15), sulphide and anomalous syngenetic Zn-Pb mineralisation (Teale, 2000). A thin tuff is dated at 1705–1703 Ma (Teale, 2000; Jagodzinski et al., 2006). The base of the Portia Formation is defined by a change from iron sulphide (commonly massive) to magnetite in the underlying thick psammitic succession of albitic laminated, flaser-bedded or cross-bedded metasiltsone, which is locally epidotitic and hence similar to parts of the Redan Gneiss or Ethiudna Subgroup. 2.3.3. Syn-depositional magmatism and tectonism Orthogneiss units in the Cues Formation are interpreted to result from syn-depositional volcanism (Page et al., 2005b). Other orthogneisses in the Thackaringa Group, together with associated mafic gneisses, are now considered to represent intrusives (Vassallo and Vernon, 2000; Page et al., 2005b). Although not part of the stratigraphic sequence, the intrusives do record magmatism soon after deposition. The ∼1705–1703 Ma tuff of the Portia Formation of the Mulyungarie Domain approximates the age of the ∼1704 Ma Alma Gneiss, an early representative of the Silver City Suite (Stevens et al., 2008). The moderately thick metasedimentary successions of this time interval in the Broken Hill and Mulyungarie Domains have no time equivalent in the flanking Olary Domain, supporting the concept that these sediments were confined to a graben or half-graben bounded by one or more extensional growth faults (Fig. 5). Continued lithospheric extension at this time in the Broken Hill Domain led to mantle upwelling and melting of metasediments in the mid-crust, possibly the oldest parts of the Willyama Supergroup or other sediments of similar composition. This produced S-type magma intruded as the ∼ Alma Granite Gneiss (see below)

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that in the Curnamona Group much of it grew or was redistributed during the Olarian Orogeny. However, the great extent of the regional magnetic gradient suggests variation in the original basin chemistry. The change from high to low magnetic susceptibility varies from sharp to transitional. In the Mulyungarie Domain the change is so abrupt that it defines the base of the Portia Formation. In the Broken Hill Domain the apparent change is more gradational, approximating the base of the Thackaringa Group, but in the Olary Domain the magnetic boundary falls at the disconformity between the 1715 Ma Ethiudna Subgroup and the ∼1700 Ma base of the Saltbush Group. From the available geochronology, it appears that the redox boundary is constrained to a narrow time interval. It is speculated that a basin-wide change in oxidation state, either synchronous or slightly diachronous within that time interval, may have been ultimately responsible. The physical manifestation of oxidising conditions (the presence of iron oxide) would be controlled by local facies, e.g. the sandy, feldspathic rocks may have originally been redbeds in which the primary haematite has been recrystallised to magnetite during metamorphism. In this case, the change in basin conditions may have occurred at about 1705 Ma.

Fig. 15. Carbonate-rich unit with pseudomorphs, possibly after evaporite minerals, Portia Formation, North Portia Prospect (drillhole BEN 603, 350.5 m, Scale block is 1.0 Cm).

and extruded as minor felsic volcanic/volcaniclastic rocks in the Thackaringa Group and Portia Formation. 2.3.4. Depositional environment Relatively sandy, shallow-water sediments were deposited in the Broken Hill Domain, coincident with possible emergence in the Olary Domain. Depositional conditions were probably shallow marine for the Thorndale Composite Gneiss, Cues Formation and sediments below the Portia Formation, but may have been hypersaline for the Clevedale Migmatite, Lady Brassey, Himalaya, and Portia Formations. These last three units contain abundant bedded alkali feldspar (generally albite)-rich rocks. Albitisation may have occurred very soon after deposition, most likely from interaction with hypersaline groundwater, as unalbitised mafic sills and dykes of presumed Broken Hill Group age are seen to intrude already albitised metasediments in the Broken Hill Domain. Support for non-marine conditions comes from boron isotope analyses of tourmaline from the Thackaringa, Broken Hill and Sundown Groups; Slack et al. (1989) found that boron isotope values from tourmalinites in these groups and from tourmaline laminae in albite rocks of the Himalaya Formation conform only with values obtained from non-marine evaporites. The upper parts of the Broken Hill Group and the Sundown Group show no other characteristics of evaporites, so it was deduced that the non-marine boron originated in the Thackaringa Group. An important feature of this part of the Willyama Supergroup is the presence of a regional magnetic gradient, interpreted as a redox boundary (Leyh and Conor, 2000) that separates magnetite-rich feldspathic units of the lower Willyama Supergroup from overlying relatively non-magnetic metasediments. In the Olary Domain, this change approximates the top of the Curnamona Group (Fig. 7). Distribution of magnetite is variable, and field observation suggests

2.3.5. Syngenetic mineralisation Relatively scarce mineralisation in the Clevedale Migmatite and Thorndale Composite Gneiss takes the form of magnetite-rich lodes with minor base metals but the Thackaringa Group contains significant mineralisation. The Broken Hill-type Pb–Zn–Ag deposit at the Pinnacles Mine occurs in Cues Formation, with some of the Pb emplaced ∼10 m.y. earlier than that in the Broken Hill orebody (Parr et al., 2005). Cues Formation contains various styles of iron formation: almandine garnet–quartz (±magnetite) rocks, quartz–iron oxide/sulphide rocks, quartz–magnetite rocks, and minor base metal mineralisation is associated with some of these (Barnes, 1988a). The Himalaya Formation hosts stratiform cobaltian pyrite, probable epigenetic Diamond Jubilee-type Au–Cu deposits (similar to the White Dam deposit in the George Mine Formation: Stevens et al., 2003), and the epigenetic Copper Blow magnetite–hematite–Cu(–U) deposit (Burton, 1994). Mineral exploration in the Mulyungarie Domain has successfully focused on the observed regional aeromagnetic gradient between the sulphidic Portia Formation and the underlying magnetite-rich metasiltstone. Multi-element mineralisation includes syngenetic Zn, Pb, Ag and epigenetic Cu, Au, Mo hosted by the ∼1705 Ma Portia Formation at the Portia Prospect, and by probable equivalents at Kalkaroo, Hunters Dam and Polygonum (Fig. 7). 2.4. Broken Hill and Sundown Groups (Broken Hill Domain), and Saltbush Group (Olary and Mulyungarie Domains): 1700–1670 Ma 2.4.1. General setting The Broken Hill Group is best developed in the Broken Hill Domain (thickness 300–2000 m, mostly 1000–1500 m); its equivalents in the Olary and Mulyungarie Domains are either much thinner and less complete or, more commonly, missing (Figs. 4 and 7). This thickness variation favours the concept of the Broken Hill Group in the Broken Hill Domain representing rift fill, with the Olary Domain being on the rift flank. The Broken Hill Group is characteristically manganiferous, with chemical sediments, felsic volcanics and metamorphosed high-Fe tholeiitic mafic sills. It hosts the 300 mt Pb–Zn–Ag lodes and is widely mineralised. Though of similar sedimentary style to the Broken Hill Group, the overlying Sundown Group, and its interpreted equivalent in the Olary Domain (Walparuta Formation) are distinguished by the absence of syn-depositional igneous units.

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2.4.2. Lithostratigraphy The basal unit of the Broken Hill Group is the lowest part of the Allendale Metasediments, restricted to the Broken Hill Domain. It is pelitic with very little psammite, and has an apparently transitional contact with the uppermost albitic rocks of the underlying Thackaringa Group. The overlying Ettlewood Calc-Silicate Member is developed only in the western part of the Broken Hill Domain, but continues into the Olary Domain as the Bimba Formation where it is the basal unit of the Saltbush Group interpreted to rest disconformably upon the Ethiudna Subgroup. The Bimba-Ettlewood is an important unit, varying in thickness from less than 1 m to 50 m. It is characterised by carbonate and calc-silicate-rich parts showing obvious bedding defined by irregular quartzo-feldspathic or quartz-rich layers separated by diopside, amphibole or epidoterich layers. Intervening metasediment is variably metasiltstone and fine-grained psammite, which in the Olary Domain locally contains scattered pebbles. Typically the Bimba Formation is pyrite–pyrrhotite-rich, locally with massive sulphide lenses up to a few metres thick, which give rise to gossanous outcrop or trains of float. The unit is base-metal anomalous. In places in the Olary Domain a clean quartzite unit, marking the base, is the only obvious sign of the ∼20 m.y. break between the Bimba Formation and the Ethiudna Subgroup. The Bimba-Ettlewood is lithologically similar to the Portia Formation, and previously these units have been correlated. However, recent geochronological results suggest that either the two are not of the same age (the tuff in Portia Formation being dated at ∼1705 Ma, and the Bimba-Ettlewood being younger than the ∼1700 Ma volcanic units of the Cues Formation) or they form part of a single diachronous lithofacies. Graphitic metasiltstone of the Plumbago Formation immediately above the Bimba Formation (Conor, 2004b) and equivalent metasiltstone overlying and interfingering with the Ettlewood Calc-Silicate Member (Mladek, 1988) contain abundant volcanic zircon grains dated at 1693 ± 3 Ma in the Olary Domain (Page et al., 2005a). This supports the long-held supposition that the Bimba and Ettlewood are equivalent and provides a tie-point for the two domains (Fig. 4). The Plumbago Formation is interpreted to represent an ash fall punctuating a period of sediment starvation with accumulated organic matter. The upper part of the Allendale Metasediments is dominantly a siliciclastic facies, typically with finely bedded pelites and minor thin psammites, but also including chemical sediments, and extends up to the base of the Parnell Gneiss. The Purnamoota Subgroup of Stevens et al. (1983), the uppermost part of the Broken Hill Group (Fig. 4), is made up of two “Potosi-type” gneiss units enclosing the Freyers Metasediments. Typically, the Purnamoota Subgroup is manganese-rich, including manganiferous iron formations, and also hosts mafic gneisses now interpreted as sub-seafloor sills (Stevens, 1998a). The lowermost unit is the Parnell Gneiss (formerly part of the Parnell Formation), the contact of which with the underlying Allendale Metasediments is now defined as the base of the lower “Potosi-type” gneiss unit, a dominantly garnet and biotite-rich felsic rock interpreted as a volcaniclastic sediment. Similarly, the Hores Gneiss represents an extensive and thick sheetlike complex of felsic extrusive material, volcaniclastic and clastic metasediment, and importantly hosts the Broken Hill Pb–Zn–Ag lodes (Page and Laing, 1992; Laing, 1996c; Laing et al., 1984; Stevens and Barron, 2002). The age of the Broken Hill Group is effectively constrained by the ages of the Plumbago Formation and Hores Gneiss (1693 ± 3 Ma and 1685 ± 3 Ma, respectively, Page et al., 2005a). In the Olary Domain the interpreted equivalent of the Purnamoota Subgroup is the Raven Hill Subgroup (Fig. 4), preserved only within synclinal cores in the ‘Old Boolcoomata’ area of the Kalabity Inlier. It comprises the psammite-dominated Black Maria Formation, separated by a graphite-rich layer from the overly-

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Fig. 16. Psammite interbed in sillimanite-bearing pelite, Saltbush Group, near Mulga Bore, western Kalabity Inlier, Olary Domain.

ing more pelitic Oonartra Creek Formation. The latter contains manganiferous banded quartz–magnetite–grunerite–garnet iron formations and quartz–garnetite (coticule) rocks, and occasional mafic sills, all of which are diagnostic of the Broken Hill Group. Interpreted Broken Hill Group equivalents in the Mulyungarie Domain are best developed in the eastern part, but locally absent towards the west. In the Polygonum Prospect area of New South Wales (Fig. 7), siltstone and sulphidic carbonates interpreted as Portia Formation are abruptly capped by 150–200 m of graphitic siltstone that grades upwards into andalusite pelite. Above this is a 400 m section of thinly interlayered psammite and psammopelite characterised by fine-grained spessartine garnet, sphalerite and galena, which is intruded by a thick amphibolite sill. The pelite, the possible equivalent of the Allendale Metasediments, and manganiferous psammopelite are interpreted to be Broken Hill Group (W. Leyh, Eaglehawk Consulting, pers. commun., 2000). The zinc-bearing package possibly extends in the western limb of a NS-trending regional-scale anticline into South Australia, but further to the west is either very thin or not present. The Sundown Group (Fig. 4), a few hundred metres to 1500 m thick, overlying the Broken Hill Group, typically consists of schistose interlayered psammopelite and pelite with variably thin to very thick (to ∼30 m) psammite layers, some of which exhibit grading. Some psammitic layers contain calc-silicate nodules (metamorphosed concretions: Stevens, 1998b) similar to those seen in parts of the underlying Broken Hill Group. The stratigraphic equivalent in the Olary Domain, the Walparuta Formation (Fig. 16), is generally restricted to synclinal cores in the Outalpa, Walparuta and Plumbago Inliers. 2.4.3. Depositional environment The transition from psammites of the Thackaringa Group to the pelite-dominated Allendale Metasediments suggests marine transgression. In the Olary Domain, the Bimba Formation rests with inferred disconformity on the Ethiudna Subgroup, and thus may represent the later stages of transgression onto a tilted block (Conor and Page, 2003) (Fig. 5). Plimer (1994) and Dyson (2003) have argued for an evaporitic depositional environment for the calcareous Ettlewood Calc-Silicate Member, which also indicates a much-reduced clastic supply. The Bimba Formation, comprising lower siliciclastic and upper carbonate facies, overlies an apparently 20 m.y. disconformity and is interpreted as transgressive (Conor and Page, 2003). The widespread preservation from subaerial exposure and consequent erosion of the thin sediment package comprising the

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Bimba-Ettlewood, Plumbago Formation volcaniclastic sheet and overlying graphitic pelite suggests a maximum flooding surface at the top of the Plumbago Formation. Younger units have progressively onlapped onto this surface: upper Allendale Metasediments in the Broken Hill Domain, Saltbush Group in the southern Olary Domain, and Strathearn Group in the northern Olary and Mulyungarie Domains (Fig. 5). Previously the clastic metasediments of the Broken Hill and Sundown Groups have been interpreted as deep-water turbidites (Wall et al., 1976; Plimer, 1986; Laing, 1980; Willis et al., 1983b). However, Wright et al. (1987), Haydon and McConachy (1987) and Stevens et al. (1988) reinterpreted these as shallow marine shelf sediments with occasional storm beds. A recent sedimentological study of key sections has supported this reinterpretation (Dyson, 2003), for example in the Mt Gipps H.S. area (northern Broken Hill Domain) the upper Allendale Metasediments grade upwards “into a 60 m thick, upward-sanding succession comprising thinly interbedded to interlaminated psammite and pelite” (Dyson, 2003, p. 8). The same author described the psammite beds as up to 15 cm thick, sharp-based, with sharp and undulose tops, concluding that: “These beds typically display the undulose style of lamination of QPL (quasi-planar lamination) and are interpreted as shoreface sands.” Evidence for rapid deepening at several levels (e.g. at the tops of the following units: Thackaringa Group, Bimba-Ettlewood, the very psammitic intervals in the Freyers Metasediments, and the middle Sundown Group), strongly suggests episodes of increased tectonic subsidence. This is also consistent with Dyson’s (2003) interpretation that volcaniclastics of both the Parnell Gneiss and Hores Gneiss were deposited during transgressive phases of sedimentation, indicating that volcanic pulses coincided with episodes of normal faulting. 2.4.4. Syn-depositional magmatism A critical characteristic of the Broken Hill Group depositional period is synchronous bimodal magmatism manifested by felsic volcanism and felsic and mafic intrusive activity. The 1693 Ma detritus of the Plumbago Formation was deposited close to the base the Broken Hill Group and the ∼1685 Ma volcaniclastic Hores Gneiss, where present, marks its top. The 1705–1685 Ma Silver City Suite (Fig. 4) of S-type gneissic granites form several structurally concordant sheets with similar characteristics and field relationships (Stevens et al., 2008). Granite intrusion commenced with the Alma Granite Gneiss during Thackaringa Group time, and continued throughout the depositional time range of the Broken Hill Group. Importantly, the geochronological data indicate that all of these bodies were emplaced at a very high level under little cover. The S-type composition and isotopic characteristics of the magmas implies derivation by partial melting of either the more deeply buried parts of the Willyama Supergroup, or underlying crust of similar composition (Barovich and Hand, 2004). Mafic gneisses intrude and are therefore younger than the granites of the Silver City Suite but none intrude stratigraphically higher than the top of the Broken Hill Group. Similarly in the Olary Domain, the differentiated amphibolite sills of the 1685 ± 4 Ma Lady Louise Suite (Conor and Fanning, 2001) (Fig. 4) are of similar age and intrude up to but no higher than the Raven Hill Subgroup. The only precise SHRIMP zircon U–Pb date from a mafic gneiss in the Broken Hill Domain is 1683 ± 5 Ma (Nutman and Ehlers, 1998b). 2.4.5. Syn-depositional tectonism The maximum rate of crustal extension is registered in the Broken Hill Domain by the climax of magmatic activity, evidenced by the deposition of the extensive ∼1685 Ma Hores Gneiss volcaniclastic sheet(s) and the emplacement of high Fe-tholeiitic sills and

dykes and the younger intrusives of the Silver City Suite at or near the end of Broken Hill Group time. Derivation of the Silver City Suite by melting of sediments is also consistent with crustal attenuation. The distribution of lithostratigraphic packages also suggests continued crustal extension during the time interval 1700–1670 Ma. In the Broken Hill Domain, the Thackaringa, Broken Hill and Sundown Groups form an apparently conformable succession but, in the Olary Domain, only restricted and thin equivalents of this succession are preserved, i.e. Bimba, Plumbago, Black Maria, Oonartra Creek and Walparuta Formations. This reduced stratigraphic thickness and completeness is consistent with the Olary Domain being on the uplifted rift shoulder during post-Ethiudna to Sundown Group times, while the Broken Hill Domain occupied the actual rift (Laing, 1996c, Fig. 9; Conor and Page, 2003; Conor, 2004b) (see also Fig. 5). The Bimba-Ettlewood, together with the overlying volcaniclastic Plumbago Formation, is a thin couplet deposited uniformly across a large area of the Olary Domain and into the western part of the Broken Hill Domain, perhaps indicating an interval of tectonic quiescence between the rift phases of the underlying Curnamona and Thackaringa Groups and the renewed crustal extension represented by the upper Broken Hill Group and its associated magmatism. The Bimba Formation covered the Ethiudna Subgroup of the Olary Domain following a 20 m.y. hiatus, suggesting marine transgression over a peneplained surface. The Ettlewood Calc-Silicate Member within the psammopelitic Allendale Metasediments is not present in the eastern part of the Broken Hill Domain, perhaps due to flooding by clastic detritus, a change in water chemistry (e.g. a gradation from restricted circulation to open sea), or a change in water depth (from shallow evaporitic to deeper water), all of which fit the model of deposition in an eastwarddeepening rifted basin. The top of the Plumbago Formation forms an onlap surface for successively overlapping younger units from east to west, i.e. upper part of the Allendale Metasediments, Black Maria Formation, Walparuta Formation, and Alconie Formation (see below) (Conor and Page, 2003; Conor, 2004b) (Fig. 5). The survival of this thin (<20 m thick) friable unit suggests that it was protected from erosion, probably by at least partial submergence throughout Saltbush Group and into Strathearn Group times, and thus indicates significant water depth above the rift flank. The restricted distribution of the Raven Hill Subgroup (upper Broken Hill Group equivalent) and parts of Walparuta Formation suggests deposition in a subsidiary graben on the Olary Domain flank of the main rift. 2.4.6. Syngenetic mineralisation The ∼300 Mt Broken Hill Pb–Zn–Ag orebody is hosted by siliciclastic metasediments enclosed within two thick volcaniclastic units of the Hores Gneiss at the top of the Broken Hill Group (Fig. 5), which also hosts a range of other smaller mineral deposits (Barnes, 1988a), from middle Allendale Metasediments up to Hores Gneiss, the latter also hosting scheelite and wolframite. Corruga-type W ± base metal deposits are in calc-silicate altered amphibolite, metasediment or garnetiferous gneiss in middle Broken Hill Group. Earlier studies concluded that the Broken Hill Pb–Zn–Ag orebody was formed on the seafloor from relatively high temperature exhalations (e.g. Stanton, 1976a). However, the reinterpreted shallow-water depositional environment is of great significance for understanding its origin. Stevens (2006) suggested that ore fluids had boiled under the low confining pressures, depositing sulphides within the sub-seafloor sediments (Parr et al., 2005). Anomalously high Pb content of the tholeiite sills is generally greatest in the Broken Hill Domain (Rutherford et al., 2006a, 2006b) and has been attributed to enrichment during fractionation (Crawford, 2006). The Mutooroo Cu–Co deposit in South Australia is a granular quartz–pyrite lode adjacent to amphibolite (Campana and King,

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1958), considered by A.F. Crooks (PIRSA, pers. commun., 2005) to be most likely hosted by Broken Hill Group, although the mineralisation style is more typical of Cues Formation (Thackaringa Group). Multi-element mineralisation (Cu, Pb, Zn, Co, As, Ag, Au, Mn, Ba, W, Mo, Bi and U) in the Bimba Formation—Ettlewood Calc-Silicate Member is interpreted to consist of both syngenetic (e.g. Fe–Zn ± Cu–Co, Ag, W) and epigenetic syn- to post-metamorphic (e.g. Cu–As–Co–Mo–Au) components. The most significant syngenetic mineralisation is represented by fine laminae and stratabound/stratiform accumulations of Fe–Zn–(Pb) sulphides, typically associated with calc-silicates and/or carbonates. Sulphide mineralogy of the Bimba Formation is dominated by pyrite or pyrrhotite, present as disseminations, laminae, veins and less commonly as massive lenses (e.g. Blue Dam, Mt Howden). 2.5. Paragon and Strathearn Groups: ∼1670–1640 Ma 2.5.1. General setting The Paragon Group of the Broken Hill Domain and the Strathearn Group of the Olary Domain (Figs. 4 and 7) are broadly stratigraphically equivalent and, despite variations in thickness, their distribution and lithological similarity suggests that they were widespread. Lithological correlation was confirmed by the geochronologic results reported by Page et al. (2005a, 2005b), which showed that these upper Willyama Supergroup rocks were of basin-wide extent, and were the temporal equivalent of parts of the Mt Isa Group of northern Australia. No syn-depositional magmatism is recorded at this time, and it is proposed that the Paragon and Strathearn Groups represent sag phase sedimentation postdating the climax of rifting. Barovich et al. (2002) and Barovich and Hand (this volume) have provided isotopic evidence for a shift in the source of sediment supply during this time interval. 2.5.2. Lithostratigraphy The lower part of the succession, i.e. the Cartwrights Creek Metasediments of the Broken Hill Domain and the Alconie Formation of the Olary Domain comprise thickly interlayered graphitic metasiltstone and aluminous pelite (generally chiastolite-bearing). These grade upwards into the albitic, rippled and cross-bedded psammites of the Bijerkeno Metasediments and Mooleulooloo Formation. The ages obtained from the Bijerkeno Metasediments (1655 ± 4 Ma) and Mooleulooloo Formation (1651 ± 7 Ma) provide the critical chronostratigraphic ties not only of the Broken Hill and Olary Domains (Fig. 4), but also of the Curnamona Province and northern Australia (Page et al., 2005a, in preparation). 2.5.3. Depositional environment Large variations in thickness of the Cartwrights Creek Metasediments and Alconie Formation attest to continuing local variations in subsidence rate during this period of slow deposition. The Cartwrights Creek Metasediments are ∼500–1000 m thick in the Mundi Mundi Creek and Bijerkerno areas, but less than 300 m thick in the Campbells Creek area; the Alconie Formation is over 250 m thick at Alconie Hill, but only 40 m in the southeastern limb of the syncline at Mount Howden in the Olary Domain. Deposition commenced with hundreds of metres of organic-rich mud and silt, with minor sand interbeds, followed by the thin planar-bedded silty carbonate of the King-Gunnia Calc-Silicate Member, and then a return to carbonaceous fine-grained sedimentation. The depositional environment is interpreted as either deep-water or shallow restricted and starved of sediment, the calc-silicate units representing maximum flooding (Dyson, 2003). The Cartwrights Creek Metasediments and Alconie Formation grade up with increasing sand interbeds into thick units of well-sorted, albitic siltstone and fine sandstone—the Bijerk-

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erno Metasediments (700–1000 m) and Mooleulooloo Formation (20–400 m). This gradation shows a change from deeper shelf sediments, possibly storm deposits (tempestites), to shallow-water deposits, partly with shoreface characteristics (Dyson, 2003). The overlying Dalnit Bore Metasediments (700 m+), and their possible equivalent, the Dayana Formation, may represent fine-grained turbidites. 2.5.4. Syngenetic mineralisation Leyh and Conor (2000) report stratiform-stratabound Zndominated mineralisation in the King Gunnia Calc-Silicate Member below Cainozoic sediments of the Mundi Mundi Plain (drill hole PO16), and suggest the potential for McArthur River, Mt Isa and Dugald River styles of mineralisation. Such prospectivity is enhanced by the chronostratigraphic correlation of the Curnamona Province with northern Australia (Page et al., 2005a). 3. Late Palaeoproterozoic to early Mesoproterozoic The period ∼1620 Ma to ∼1580 Ma encompassed the Palaeo–Mesoproterozoic Olarian Orogeny (Thomson, 1969), the ∼1595–1580 Ma intrusion of syn- and post-tectonic dominantly S-type granites and, in the Mudguard Domain, the extrusion of ∼1580 Ma A-type and mafic lavas that are similar to the Gawler Range Volcanics on the Gawler Craton (Giles and Teale, 1979). The later phases of the Olarian Orogeny overlap in time with a more extensive Mesoproterozoic tectonothermal event, the products of which are visible in the Gawler Craton, the Mt Isa and Georgetown Inliers and the Wernecke Mountain region of North America. During the period ∼1600–1500 Ma, these regions were affected variously by intense alkali-iron oxide hydrothermal alteration and endowed with Cu–Au mineralisation. The eastern Gawler Craton experienced the voluminous magmatism of the Hiltaba Suite and Gawler Range Volcanics, accompanied by localised deformation, and the Mt Isa and Georgetown Inliers underwent the Isan Orogeny. According to several continental reconstructions, these regions may all have lain in close proximity at this time (e.g. Thorkelson et al., 2001) (Fig. 17), and mantle plume activity has been suggested as a cause for the tectonothermal event (Giles, 1988; Flint, 1993; Conor, 2003). The earlier onset of the Olarian Orogeny at ∼1620–1600 Ma (Forbes et al., 2005b; Rutherford et al., 2007b) invites the question as to whether it was a manifestation of the early stages of the more extensive event, or incidental to it.

Fig. 17. Interpreted early Mesoproterozoic continental reconstruction showing possible dimension of the IOCG province (after Thorkelson et al., 2001).

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3.1. Olarian Orogeny: deformation and metamorphism of the Willyama Supergroup The effects of the polyphase Olarian Orogeny are highly variable across the Curnamona Province, being generally most intense in the southeast and least in the central zone. There is evidence of extreme shortening in much of the Broken Hill and Olary Domains, involving early recumbent folds with overturned limbs extending over many tens of kilometres and later upright buckle folds and reverse faults. Metamorphic grade decreases from granulite facies in the southern part of the Broken Hill Domain and adjacent Redan Domain to lower amphibolite facies in the northern Broken Hill Domain. The Olary Domain also shows a general northward decrease in grade, from upper amphibolite facies in the south to greenschist facies in the north, but there are local deviations from this overall trend (Webb and Crooks, 2005). Metasediments in the northernmost Olary Domain and Mulyungarie Domain display open folding associated with a series of east-dipping thrusts imaged in the Curnamona Seismic Transect (Goleby et al., 2006). The deformational sequence has been much debated and several structural schemes have been proposed involving up to four discrete Olarian events (e.g. Wiltshire, 1975; Berry et al., 1978; Hobbs et al., 1984; Clarke et al., 1986; Laing et al., 1978; Marjoribanks et al., 1980; Gibson and Nutman, 2004; Gibson et al., 2004). In addition, there is clear evidence of overprinting by further deformation and metamorphism during the ∼0.5 Ga Delamerian Orogeny (Talbot, 1967; Berry et al., 1978; Dutch et al., 2005; Rutherford et al., 2006a, 2006b). The structural schemes have been devised on the basis of local overprinting relationships, but the paucity of hard evidence for correlating these from region to region, or even from outcrop to outcrop, has led to as yet unresolved inconsistency of the numbering schemes. Despite these uncertainties, some elements are common to most observations: • Near-ubiquitous layer-parallel foliation. • Evidence of very early heating in the form of migmatitic veining parallel to this foliation, and early pegmatites. • Relatively early isoclinal recumbent folds of markedly different orientations and vergences, and the formation of near-regional scale overturned limbs. • Relatively later upright folds that refold the isoclinal folds. • Granite intruded late in the structural sequence. • Retrograde shear zones cutting across all the earlier structures and affecting the late granites. 3.1.1. Layer-parallel foliation Foliation parallel to bedding is characteristic of all Willyama Supergroup metasediments, with the possible exception of the youngest parts of the Paragon Group, and is also represented in syn-depositional intrusives such as Basso Suite and Silver City Suite metagranites. The origin of this foliation has been much debated: Laing et al. (1978) and Marjoribanks et al. (1980) regarded it as an axial-plane foliation to the major, early recumbent folds first recognised by them, but Gibson and Nutman (2004) and Noble (2003) postulated that it resulted from extensional deformation and highgrade metamorphism as a metamorphic core complex at ∼1680 Ma, i.e. during deposition of the Willyama Supergroup. The latter ideas were refuted by Conor et al. (2005), Stevens (2006) and Rutherford et al. (2007b), partly because Gibson and Nutman (2004) relied on elusive high-temperature shear zones, incorrect identification of critical stratigraphic units, but in particular because the proposed timing is inconsistent with geochronological data (Page et al., 2005a, 2005b).

The concept of extensional deformation immediately prior to the onset of compressive deformation (e.g. one of the options suggested by Forbes et al., 2005a) is not inconsistent with field and geochronological data, since this permits the whole of the Willyama Supergroup to be affected. However, we do not find the evidence for such extension compelling and prefer one or both of two alternative models: (a) Sedimentary loading and heating of the stratigraphic pile before the onset of folding. (b) Horizontal foliations formed and then folded in a continuous and progressive process at mid-crustal depths. The observation that parts of the uppermost Paragon Group lack layer-parallel foliation is consistent with both these models. 3.1.2. Early heating Stevens (2006) indicated that in the Broken Hill Domain the earliest known folding deformed pegmatite sheets (partial melt) and metasediments that had already been subjected to moderately high-grade metamorphism. Similarly, in the Olary Domain, migmatised metasediments with layer-parallel foliation are found in the cores of the earliest recumbent folds. These observations imply early heating of the sedimentary pile at or before the onset of folding, and are consistent with the very ductile nature of much of the deformation. It is argued that heat generated by radioactive decay (Stevens, 2006), perhaps with the addition of heat from deeper sources (e.g. crustal delamination or mantle plume) and gravitational load were responsible for the earliest foliation and melting. A thickness of 10–15 km of sedimentary overburden was necessary to account for the pressure required to form the earliest developed metamorphic assemblages (Stevens, 2006; Forbes et al., 2005a). Such sediment cover is likely to have been pelite-dominated, like the uppermost known Willyama Supergroup, and therefore had the potential to act as an insulating blanket. Therefore, it is considered likely that sedimentation in the southern Curnamona Province continued well after 1642 ± 5 Ma, the youngest recorded detrital zircon age from the Paragon Group (Page et al., 2005b). Forbes et al. (2005b) recorded the first growth of metamorphic monazite, armoured by early-formed metamorphic minerals, at ∼1620 Ma in the Broken Hill Domain. While the relationship of these monazite grains to the earliest deformation is debatable, it can be argued that they grew before peak metamorphic conditions were attained, as recorded by the common 1600 ± 10 Ma ages of metamorphic zircon (Page et al., 2005b), especially in the highestgrade, and perhaps most deeply buried, regions of the Curnamona Province. Rutherford et al. (2007b) found that metamorphic monazites in the Olary Domain fall mainly in the same age range as the zircons. 3.1.3. Recumbent folding The earliest folding is of isoclinal, recumbent style (Fig. 13) and regional extent, with subparallel normal and overturned limbs in both Broken Hill and Olary Domains. Evidence for closure of such folds is rare at best, hence the uncertainty as to whether the earliest layer-parallel foliation is axial planar to the folds (e.g. Laing et al., 1978 and Marjoribanks et al., 1980, who assigned them to D1), or folded by them (e.g. Gibson and Nutman, who assumed that they relate to mappable meso-scale folds that deform the layer-parallel foliation and therefore regarded them as D2). If deformation is viewed as continuous and progressive, then discovery of the very earliest folds (i.e. showing first-generation axial-planar foliation) would be extremely improbable (Preiss, 2006). The observations by Flint (1981) and Forbes et al. (2005a) that the layer-parallel foliation can itself be composite is consistent with a model involving continuous and progressive deformation.

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developed during the later stages of the Olarian Orogeny. Clark et al. (2005) have demonstrated complex alteration chemistry involving leaching and transport of metals, and tracking of fluid pathways has the potential for identifying sites of metal precipitation. 3.2. Syn- and post-Olarian granites

Fig. 18. Calc-silicate breccia, Cathedral Rock, western Kalabity Inlier, Olary Domain.

There is great variation in the orientations of meso-scale isoclinal folds across the Curnamona Province, while the plunges of the regional recumbent structures are poorly constrained. Part of this variation is due to refolding, but much of it is probably due to the non-cylindrical nature of the early folds, in places approaching true sheath fold morphology (e.g. Hills et al., 2001; Forbes et al., 2004). It is likely that crustal thickening resulting from isoclinal folding led to the higher temperatures and pressures associated with peak metamorphism, and is therefore probably dated by the common zircon growth at ∼1600 Ma. 3.1.4. Upright folding Upright folds with large amplitudes and steep limbs overprint layer-parallel foliations and early recumbent folds throughout the Broken Hill and Olary Domains, where they display an arcuate pattern of axial traces. In the Mulyungarie Domain, these folds are more open and are associated with east-dipping thrusts imaged on the Curnamona Seismic Transect (Goleby et al., 2006). In the Olary Domain, folds display a general northwesterly vergence, and northeasterly fold trends are common, but these swing abruptly to north–south in the Mulyungarie Domain (Fig. 7). A similar change in trend occurs in the Broken Hill Domain north of Broken Hill and in the Euriowie Inlier. Stevens (2004) reconstructed folds with slight southeasterly vergence in the Broken Hill area. The folds commonly display an axial-plane schistosity, mostly a crenulation cleavage but locally so intense as to obliterate earlier fabrics. Spaced cleavage is common in more brittle lithologies such as calc-albitite and, in some cases, late stage albitisation and brecciation are developed along anticlinal crests. 3.1.5. Hydrothermal brecciation and alteration Breccias on a centimetric to kilometric scale are widely developed in the Olary Domain, e.g. Cathedral Rock (Fig. 18) and northern Walparuta Inlier. The most spectacular breccias are seen in brittle rocks such as calc-albitite of the Ethiudna Subgroup, but mafic rocks and parts of the Wiperaminga Subgroup of the Olary Domain and Portia Formation in the western Mulyungarie Domain are also brecciated. Breccia-styles include in situ crackle breccia, matrixsupported breccia resulting from spalling into dilatant zones, and intrusive milled polymict breccia. Matrix compositions broadly relate to the host (calc-silicate minerals in calc-albitites, muscovite in quartzo-feldspathic metasediments, biotite in granites at Poodla Hill and Antro). Breccia bodies and associated intense alkali-feldspar (albite > microcline), calc-silicate (amphibole, epidote, diopside) and iron oxide (magnetite > haematite) alteration of clasts, wall-rock and matrix tend to occupy the cores of late Olarian folds, in places proximal to major shear zones. Great quantities of hydrothermal fluids were evolved and focussed by structures

Granite, largely derived from the melting of Willyama Supergroup metasediments, or from underlying rocks of similar composition, was intruded late in the Olarian Orogeny over wide areas of the Curnamona Province (Fig. 7). These dominantly felsic intrusives are grouped as the Ninnerie Supersuite (Fricke, 2006), with ages in the range ∼1590–1580 Ma. In the Olary Domain, the Ninnerie Supersuite includes the dominantly potassic S-type twomica granites of the Bimbowrie Suite (Stewart and Foden, 2003) and the dominantly biotite-rich and sodic varieties of the Crockers Well Suite, which include trondhjemite, alaskite, and more mafic I-type granodiorite and diorite (Barovich and Ashley, 2002; Knaak, 2002). In the Broken Hill Domain, the Mundi Mundi granites (Brown et al., 1983) are of similar age and composition to the Bimbowrie Suite. The Billeroo alkaline magmatic complex in the northwestern Olary Domain is also considered to be of similar age, comprising syenite and ijolite veined by lamprophyre (Rutherford et al., 2002, 2007a). Granite bodies vary from small masses to large intrusions several kilometres across, locally with migmatitic aureoles. The granites generally exhibit sharp contacts with metasediments displaying structural relationships consistent with intrusion late in the deformation cycle, either following along or cutting across the preexisting upright fabric of the metasediments. Intrusion therefore occurred after peak metamorphism although, in the Broken Hill Domain, the 1596 ± 3 Ma Cusin Creek Granite has been interpreted to have intruded prior to the upright fabric (Page et al., 2005b). However, in general the granites of the Ninnerie Supersuite are not foliated except where near late shear zones. The Mudguard Domain (Figs. 3 and 7) is characterised by a flat-lying sheet of ∼1580 Ma Mesoproterozoic Benagerie Volcanics, of both felsic A-type and mafic composition (Giles and Teale, 1979, 1981), apparently unconformably overlying folded Willyama Supergroup. Teale (2000), Burtt and Betts (2003) and Burtt et al. (2004) suggest that from aeromagnetic interpretation and limited drill intersections high level granites and diorite intrude the volcanics. 3.3. Late Olarian deformation (shearing) Imposed upon the regional arcuate fabric of the upright fold-set is an extensive network of ramifying late Olarian retrograde shear zones of various orientations but dominated by northeasterly and W–E to northwesterly trends (Fig. 7). In the Broken Hill Domain, the northeast-trending shear zones generally parallel the upright fold trend, while the W–E to northwesterly subset cross-cuts these folds. At Broken Hill, the Pb–Zn lodes are severely disrupted by shear zones such as the Globe-Vauxhall Shear Zone. In the Olary Domain the second set is dominant and in some cases, where measurable, displacement is large, e.g. the pre-Neoproterozoic lateral component of apparent dextral displacement of the Walter-Outalpa Shear Zone in the Walparuta Inlier is greater than three kilometres. Determining the age range of shearing is difficult. Attempts so far at direct dating via the Sm–Nd system on garnet grown within shear zones records only later tectonic reworking during the Delamerian Orogeny. The ∼1595–1580 Ma Ninnerie Supersuite does provide useful constraints on the timing of shearing relative to intrusion. Some non-foliated lenticular granite bodies are within shear zones and locally are folded, suggesting possible intrusion during shearing, while others show a foliation that increases

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in intensity towards shear zones that cut them, and so provide evidence for shearing later than intrusion, e.g. the Cu–Au mineralised Walter-Outalpa Shear Zone and the one hosting the Mount Victoria uranium mineralisation. Large retrograde W–E-trending shear zones in the Ameroo Hill and Mount Mulga areas parallel the vertical axial planes of very large open folds, internally defined by buckling of bedding and crenulation of pre-existing cleavages. These folds are interpreted as either being rotated upright folds, or having developed contemporaneously with shearing. In one case, in the Morialpa Inlier, such folds and kinks overprint an already retrograde steep fabric containing flattened sericitised andalusite, and are truncated by the basal unconformity of the Neoproterozoic cover. It is possible that this late folding was responsible for bending the earlier regional northwest-verging upright fold set into its current arcuate shape. The arc is not a smooth curve but, west of the Mundi Mundi Fault, appears from the solid geology interpretation of Burtt and Betts (2003) to be kinked about a W–E axis at approximately 31◦ 40 S (Fig. 7). In the Mudguard Domain, aeromagnetic data backed up by limited drilling show an early Mesoproterozoic volcanic sheet overlying a basement of greenschist facies Willyama Supergroup that displays upright Olarian folds. In contrast, in the Olary Domain, granites of similar age were intruded at depth. The inference is that the low grade basement of the Mudguard Domain had been exhumed and exposed by erosion during the short period of time between upright folding, and deposition of the ∼1580 Ma volcanic sheet and interdigitated sediments. The high-grade rocks of the Broken Hill, Redan and Olary Domains are now at a similar crustal level to the low-grade rocks of the Mudguard Domain, and it is possible that further uplift took place in the southern areas via the retrograde shear zones. Exhumation must have been completed before deposition of the very thick Neoproterozoic cover. 3.4. Syn-Olarian and related mineralisation 3.4.1. Epigenetic iron oxide-associated Cu–Au–U mineralisation Cu–Au ± Mo ± U mineralisation in the Olary Domain is generally considered to be epigenetic, related to the Olarian Orogeny or emplacement of the Ninnerie Supersuite. It is assumed that metals were precipitated from transient fluids at chemical traps, such as the small Cu–Au ± Mo deposits associated with ironstone bodies in the Curnamona Group, e.g. Wilkins Prospect (Lottermoser and Ashley, 1996). Apart from low-level gold mineralisation associated with the Broken Hill lodes, the region containing the greatest quantity of known Cu–Au mineralisation is the Mulyungarie Domain. The majority of Cu–Au prospects (e.g. Portia, Kalkaroo, Polygonum Prospects: Fig. 7) in the region cluster near the laterally extensive, but relatively abrupt magnetic gradient that separates the magnetite-rich lower Willyama Supergroup from the upper, less oxidised units; this chemical redox front is interpreted to have controlled the distribution of Cu–Au–Mo mineralisation (Leyh and Conor, 2000). Two areas of recently concentrated exploration activity, the Kalkaroo and Portia sets of prospects are located on the same N–S-trending anticline, but on adjoining domes that were created by interference from west–east-trending structures (Teale, 2003, 2006). The domes were eroded during the Mesozoic and Tertiary, thus the mineralisation at both sites is located at the intersection of the unconformity and the folded Portia Formation. An important component of the Portia Formation is its sulphide content, mainly pyrite and pyrrhotite, but locally also sphalerite and galena. Incipient Cu, Au and Mo mineralisation is visible as replacement of the earlier syn-depositional or diagenetic sulphide, but higher-grade zones are of dilatant vein or breccia styles. A-type ∼1580 Ma volcanics, granites and breccias are hallmarks of the Olympic Dam Cu–Au–U–REE deposit 300 km to the west on

the eastern Gawler Craton. Significantly, drilling has demonstrated both iron-oxide alkali metasomatism and sporadic Cu–Au mineralisation associated with the Benagerie Volcanics, thus enforcing the prospectivity of the Mudguard Domain for Olympic Dam-style iron oxide copper–gold mineralisation (Burtt et al., 2004). 3.4.2. Uranium mineralisation The Palaeo–Mesoproterozoic provinces of southern Australia are uranium-rich. Uranium has been mined from Palaeo–Mesoproterozoic host rocks of the Curnamona Province, e.g. Radium Hill and small low-grade deposits in the Crockers Well region. Much of this mineralisation is related to early Mesoproterozoic syn- to post-Olarian magmatism and hydrothermal activity. At Mount Painter, uranium has been mined from Palaeozoic hydrothermal breccias, but uraniferous Mesoproterozoic granites may have been the ultimate source. Some of the uranium from the Mesoproterozoic and Palaeozoic granites and breccias, and perhaps from carbonaceous metasediments of the Willyama Supergroup, has been eroded and/or leached out to form significant sediment-hosted roll-front uranium deposits in buried Tertiary fluviatile sands, e.g. Honeymoon, Goulds Dam and Four Mile. Uranium from the Beverley deposit is being extracted by in situ leaching, while exploration for other similar deposits continues (Cooper and McGeough, 2006). Acknowledgements The authors are indebted to both Barney Stevens and Rod Page, the former for sharing his knowledge of Broken Hill geology and the latter for his dedication to the production of high-quality zircon dating. Without the contributions of both, and their many coworkers, to the Broken Hill Exploration Initiative programme, the greatly improved chronological framework and geological interpretations of the southern Curnamona Province would not have possible. In addition, Barney Stevens has provided the authors with much previously unpublished factual data and many new ideas, but responsibility for the conclusions rests solely with the authors. Also we offer our thanks to Bob Johnson and Chris Giles of Havilah Resources NL, Graham Teale, and Wolfgang Leyh and Bob Richardson of Platsearch NL for access to drill core and discussion about the geology of the Mulyungarie Domain. This paper is published with the permission of the Executive Director, Minerals and Energy Resources, Primary Industries and Resources South Australia. References Ashley, P.M., Cook, N.D.J., Lawie, D.C., Lottermoser, B.G., Plimer, I.R., 1995. Olary Block geology and field guide to 1995 excursion stops. South Australia. Department of Mines and Energy. Report Book 95/13. Ashley, P.M., Cook, N.D.C., Fanning, C.M., 1996. Geochemistry and age of metamorphosed felsic igneous rocks with A-type affinities in the Willyama Supergroup, Olary Block, South Australia and implications for minerals exploration. Lithos 38, 167–184. Barnes, R.G., 1988a. Metallogenic studies of the Broken Hill and Euriowie Blocks, New South Wales. 1. Styles of Mineralization in the Broken Hill Block. New South Wales Geol. Surv. Bull. 32 (1), 115. Barnes, R.G., 1988b. Metallogenic studies of the Broken Hill and Euriowie Blocks, New South Wales. 2. Mineral deposits of the southwestern Broken Hill Block. New South Wales Geol. Surv. Bull. 32 (2), 117–250. Barnes, R.G., Stevens, B.P.J., Stroud, W.J., Brown, R.E., Willis, I.L., Bradley, G.M., 1983. 5. Zinc, manganese and iron-rich rocks and various minor rock types, pp. 127–226. In: Stevens, B.P.J., Stroud, W.J. (Eds.), Rocks of the Broken Hill Block: their classification, nature, stratigraphic distribution and origin. New South Wales Geol. Surv. Record 21 (1), 323 pp. Barovich, K.M., Ashley, P.M., 2002. Peraluminous trondhjemites in the Curnamona Province, Olary Domain, SA: magmatic or metasomatic? 16th Australian Geological Convention, Adelaide, July 2002. Geol. Soc. Aust. Abstracts 67, p. 199. Barovich, K.M., 2003. Geochemical and Nd isotopic evidence for sedimentary source changes in the Willyama basin, Curnamona Province. In: Peljo, M. (Compiler),

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