The Willyama Supergroup in the Broken Hill and Euriowie Blocks, New South Wales

Precambrian Research, 40/41 (1988) 297-327

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Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE WlLLYAMA SUPERGROUP IN THE BROKEN HILL AND EURIOWlE BLOCKS, NEW SOUTH WALES B.P.J. S T E V E N S 1, R.G. B A R N E S 2, R.E. B R O W N 2, W.J. S T R O U D 2 a n d I.L. W I L L I S 2 1Geological Survey of New South Wales, 32 Sulphide Street, Broken Hill, N.S. W. 2880 (Australia) 2Geological Survey of New South Wales, W.J. McCarthy House, Faulkner Street, Armidale, N.S. W. 2350 (Australia) (Received October 25, 1986; revision accepted September 12, 1987)

Abstract Stevens, B.P.J., Barnes, R.G., Brown, R.E., Stroud, W.J. and Willis, I.L., 1988. The Willyama Supergroup in the Broken Hill and Euriowie Blocks, New South Wales. Precambrian Res., 40/41: 297-327. The Willyama Supergroup consists of highly deformed metasedimentary schists and gneisses with abundant quartzofeldspathic gneisses, lesser basic gneisses and minor 'lode' rocks. Prograde metamorphism ranged from andalusite through sillimanite to granulite grade. Radiometric dating in the Broken Hill Block indicates separation of the source of the quartzo-feldspathic gneisses from the mantle at 2300-2100 Ma. A Rb-Sr model source age of 1820 +__60 Ma has been interpreted as the maximum age of deposition. Deformation (D~ and/or D2) was coincident with high-grade metamorphism at 1660 _+10 Ma. Minor Mundi Mundi-type granitoids and some pegmatites were intruded at 1490 _+20 Ma, minor pyroxenites at 561 _+7 Ma, dolerite dykes soon after, and zoned pegmatites at ~ 500 Ma, in association with the Delamerian Orogeny which reactivated retrograde schist zones at 520 _+40 Ma. Subsequent history mostly comprises faulting, weathering, uplift and erosion. The Willyama Supergroup has an estimated total thickness of 7-9 km, with neither top nor basement exposed. No unconformities have been recognized in the sequence. The Supergroup was deposited in a deepening environment, passing upward from sandy facies to more shaly and fine sandy facies, and from interpreted bimodal acid/basic volcanics, to non-volcanic facies. The metasediments have been interpreted as shelf sediments overlain by deeper water turbidites and minor contourites. Recently they have been reinterpreted as shallow marine, with the quartzo-feldspathic gneisses interpreted as fluvio-deltaic arkoses. In this paper the sequence to the top of Thackaringa Group is interpreted as mainly fluvio-deltaic and lacustrine. The Broken Hill Group and Sundown Group are interpreted as shallow marine. The Paragon Group may have comprised shelf muds overlain by delta-front and lacustrine sediments, in turn overlain by deeper-water fine-grained turbidites. The Broken Hill orebody, near the top of Broken Hill Group, consists of a series of stacked lenses. Much recent work has concluded that the deposit is volcanic exhalative in origin, probably formed from seawater convected through the sequence, and driven by heat from high-level magma chambers. However, some features of the deposit are more appropriate to a sedimentary exhalative origin, and models have been proposed involving'seismic pumping' of connate water, with ore deposition on or below the seafloor. Devolatilization of the mantle has been proposed as a source of ore constituents. Deposition may have occurred in a narrow NE-SW trending seafloor depression. The Willyama Supergroup was probably deposited in a rift zone with thin crust. The basic gneisses are interpreted as Fe-rich tholeiites similar to basalts from some oceanic spreading ridges. The intense folding, post-folding leucogranitic intrusions and extensive granulite/amphibolite facies metamorphism could have resulted from a collision following subduction. However, there is no direct evidence of oceanic crust or subduction, and the low-pressure metamorphic path does not suit a collision model. The rocks were probably deposited in a continental rift, with some similarities to those interpreted for Proterozoic fold belts in northern Australia.

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Introduction

The purpose of this paper is to provide an upto-date account of the Willyama Supergroup (Stevens et al., 1983; Willis et al., 1983a) in Western New South Wales (Figs. 1, 2). These rocks are high-grade (andalusite to granulite grade) metamorphics which are highly deformed, having undergone three major generations of folding and widespread later deformation along retrograde schist zones. A large volume of literature exists on features of the Willyama Supergroup. This is a direct and indirect result of the presence of the Broken Hill orebody, which before mining comprised ~250 million tonnes of Pb-Zn-Ag mineralization. The long history of research has involved development and testing of concepts from many aspects of geology, particularly in structural, metamorphic, ore deposit and isotope geology. The history of Broken Hill research is a fascinating study; however, the aim of this paper is to present the current end-product of this research, rather than mull over the previous stages. Consequently, emphasis is placed on work which reflects 'the state of the art', and on new work which is not incorporated in previous review articles. References to older work can be found in references cited in this paper. In this paper we have attempted to emphasize those features which are of importance in development and testing of tectonic models, and in understanding the origin and geological environment of mineralization. P r o t e r o z o i c r o c k s in N e w South Wales

Proterozoic rocks crop out over only a small portion of New South Wales {Fig. 1 ). The only outcropping rocks of established early-middle Proterozoic age occur in the Broken Hill and Euriowie Blocks and in the small Mt Woowoo-

lahra and Nardoo Inliers nearby (Fig. 2). The Willyama Supergroup in the Broken Hill Block was deposited at about 1820+60 Ma (Shaw, 1968), metamorphosed at 1660 + 10 Ma (Harrison and McDougall, 1981; Gulson, 1984), and intruded by granite at 1490 + 20 Ma (Pidgeon, 1967; Harrison and McDougall, 1981). Good stratigraphic correlation has been established between the Euriowie and Broken Hill Blocks (Tuckwell, 1978; Brown, 1985; Brown et al., in preparation) and the Nardoo and Mt Woowoolahra Inliers are sufficiently similar in lithology and metamorphism to be correlated with the Willyama Supergroup of the Broken Hill and Euriowie Blocks (Cooper et al., 1978; B. Stevens, unpublished field observations). Adjacent to and unconformably overlying these rocks are late Proterozoic (Adelaidean) rocks, comprising epicratonic continental and shallow marine sediments with minor alkali basalt (Cooper et al., 1978). From outcrop, drilling and geophysical data, the Willyama Supergroup appears to be bounded (at depth) on the SE by a N E - S W trending lineament south of the Redan Fault, and to the NE (at depth) by the Nundooka Fault (see Figs. 1, 2). It extends west into the Mutooroo and Olary areas of South Australia and beneath Cretaceous-Cainozoic rocks in part of the Lake Frome Embayment (Tucker, 1983 ). The Mount Painter Block in South Australia also comprises Willyama Supergroup (Willis et al., 1983a). The portion of the Wonominta beds which occupy a N N W - S S E trending belt west of the Koonenberry Fault, may also be of Proterozoic age, although Wonominta beds east of the Fault are probably Cambrian (Webby, 1984; Stevens, 1985). Leitch et al. (1985) suggested that two parts of the Wonominta beds can be lithologically correlated with the Willyama Supergroup and the Adelaidean sequence respectively. However, there is no established stratigraphic

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sequence within the Wonominta beds and there are no radiometric data, so the correlation must be regarded as dubious. The only other Proterozoic dates recorded in New South Wales are from zircon in the Ber-

ridale Batholith and adjacent metasediments (Williams et al., 1983 ), and Sm, Nd, and Sr isotope studies of the Berridale and Kosciusko Batholiths (McCulloch and Chappell, 1982). All of these rocks were deposited or emplaced

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Fig. 2. Extent of Willyama Supergroup in western New South Wales and eastern South Australia.

in the Palaeozoic. Phases of the Berridale Batholith contain some inherited zircons at least 820 + 50 Ma old and others have model ages of 1220 + 10 Ma. Zircon in the adjacent Adaminaby beds is at least 1823 + 10 Ma old. This may be detrital zircon derived from the Willyama Supergroup or more local equivalents. Nd and Sr isotope studies indicate an age of ~ 1400 Ma for the metasedimentary source of S-type granitoids in the Kosciusko and Berridale Batholiths, and ages ranging from 1400 to 380 Ma for the igneous source of the I-type granitoids. Hence there is no evidence at present that rocks as old as the Willyama Supergroup extend further east than the Nundooka Fault and the N E - S W lineament south of the Redan Fault. Therefore it is possible that the Willyama Supergroup was deposited on a continental margin, or that a continental margin formed by rifting after deposition of the Supergroup. Alkaline igneous activity (typical of

continental rifting) occurred in Adelaidean times (the Wilangee Basalt, tentatively correlated with volcanics of 1075 Ma age) and in the Cambrian (nepheline pyroxenite and related intrusions at ~ 560 Ma). The Willyama Supergroup is similar in age and lithology to rocks in the Soldiers Cap area near Mt Isa (Laing and Beardsmore, 1986) and to rocks in the Jervois Range area (eastern end of Arunta Block, Northern Territory). These may all be remnants of a single extensive fold belt. The fragments of this fold belt may show as a series of narrow gravity highs and lows (Fig. 1 ).

General history of the Willyama Supergroup The Willyama Supergroup (Stevens et al., 1983; Willis et al., 1983a) is a highly deformed and metamorphosed sequence of sedimentary and igneous rocks. The sequence was interpreted by Johnson and Klingner (1975), Stan-

301

ton (1976a) and Willis et al. (1983a) as consisting originally of sediments with substantial amounts of intercalated acidic and basic volcanics in the lower and middle levels. Many of the rocks interpreted as acidic volcanics by the above authors have been reinterpreted by Haydon and McConachy (1986, 1987) and Wright et al. (1987), as sediments, with or without volcanic detritus. This reinterpretation is discussed later. A considerable amount of radiometric dating is available, but some notable uncertainties remain, as reviewed by Stevens (1986). The most notable uncertainty is the depositional age of the sequence. Hopefully U-Pb dating of zircons now in progress by R.W. Page (Bureau of Mineral Resources), will provide some answers. McCulloch and Hensel (1984) and McCulloch (in press) interpreted from Sm-Nd dating, that the quartzo-feldspathic rocks in the Willyama Supergroup were derived from material which separated from the mantle at 2300-2100 Ma. The age of deposition of the Willyama Supergroup has been estimated by Shaw (1968) at 1820 _+60 Ma, from initial Sr ratios. High-grade metamorphism occurred at 1660 + 10 Ma (from * Rb-Sr, 2°Tpb-2°~Pb data, adjusted and summarized by Harrison and McDougall, 1981, and U - T h - P b in zircon by Gulson, 1984) closely associated with the first two deformations in the Olarian Orogeny. A third deformation occurred during cooling, and retrograde schist zones were initiated at this time or slightly later. Mundi Mundi-type granitoids and some muscovite pegmatites were intruded at 1490_+20 Ma (Rb-Sr isotopes; Pidgeon, 1967, adjusted by Harrison and McDougall, 1981 ). Cooper and Ludwig (1985) dated zircons ( U - T h - P b method) from the Brewery Well pluton, a Mundi Mundi-type granitoid. The zircons "revealed something inherited and of an age probably greater than 2 Ga". They concluded that the zircons were either inherited from the magma source or were *Decay constant from Steiger and Jiiger (1977).

accidental inclusions from the wall rocks. By ~1100 Ma the Willyama Supergroup rocks and Mundi Mundi-type granitoids had been deeply eroded (Stevens, 1986) and deposition of Adelaidean rocks had commenced. At 561 _+7 Ma (Harrison and McDougall, 1981) a nepheline pyroxenite plug was emplaced. Many other basic/ultrabasic plugs and dykes were emplaced at this time or soon after (Stroud et al., 1983; Stevens, 1986). The Delamerian Orogeny produced folds in the Adelaidean sediments, and reactivation of retrograde schist zones in the Willyama Supergroup at about 520 _+40 Ma (Rb-Sr, K-Ar dating; reviewed in Harrison and McDougall, 1981; Stevens, 1986). The subsequent history mostly comprises faulting, weathering, uplift and erosion. Specific events include probable glacial erosion in the Late Carboniferous-Early Permian (Stevens, 1986; and regional field data in Dun, 1898; Flint et al., 1980; O'Brien, 1986); uplift and erosion in the Eocene (deduced by Callen (1977) from sedimentation history in the Lake Frome Embayment); silcrete and ferricrete formation in the Oligocene (correlation with the Tibooburra area; Morton, 1982); and subsequent weathering, uplift and erosion.

Deposition of the Willyama Supergroup The following interpretation is taken from Stevens and Stroud (1983), Stevens et al. (1983), Willis et al. (1983a), Brown (1985) and Stevens (in preparation). These papers incorporate work by Archibald (1978) and Laing et al. (1978). Subsequent to the interpretation below, reinterpretation by Haydon and McConachy (1987) and Wright et al. (1987) is discussed, and further interpretation presented.

Generalfeatures The Willyama Supergroup is interpreted as comprising originally detrital sediments with very minor carbonate sediments, and consid-

302 erable quantities of intercalated rhyolitic/dacitic and basaltic volcanics and subvolcanic intrusives. The metavolcanics are confined to the lower and middle part of the sequence (in and below the Broken Hill Group). The sequence is almost entirely devoid of clastics or pyroclastics coarser than sand grain size (see Stevens and Stroud (1983) for minor exceptions). The stratigraphic thickness of the Willyama Supergroup is ~ 7 km, based on a total of median present thicknesses of each stratigraphic unit (Willis et al., 1983a). The newly identified lower units in the SE (Redan Gneiss, Ednas Gneiss and Mulculca Formation; Stevens, in preparation) together with the unusually thick Lady Brassey Formation in that area, could increase the total stratigraphic thickness to ~ 9 km. The palaeo-environment is interpreted as deepening, commencing with predominantly feldspathic (possibly tuffaceous) sand-rich intervals (Thackaringa Group and below), changing to more chemically mature, relatively quartz-rich sands and shales in the Broken Hill and Sundown Groups, then carbonaceous shales and fine-grained feldspathic sands and silts in the Paragon Group. Willis et al. (1983a) interpreted a progression from "a relatively shallow, shelf-type seafloor, medial to an active volcanic source", through a more active volcanic-sedimentary shelf environment, a turbiditic deepershelf environment, to a continental slope environment. Haydon and McConachy (1986) interpreted shallower environments for Broken Hill Group and below, but agreed with the overall deepening. At present the Willyama Supergroup extends over a width of 200 km or more. Deformation has produced severe shortening, so the original width was much greater. Obviously the area of deposition was very large, probably several thousand square kilometres. No unconformities have been identified within the Willyama Supergroup, nor has a basement been recognized.

DALNIT BORE METASEDIMENTS BIJERKERNO METASEDIMENTS CARTWRIGHTS CREEK METASEDIMENTS I KING GUNNIA CALC-SILICATE MEMBER

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The lowest formations Some of the lowest stratigraphic units (Fig. 3) are restricted to the SE part of the Broken Hill Block (Stevens, in preparation ). All of the units in this part of the block are rich in albite and relatively rich in magnetite. The Redan Gneiss consists largely of albite-hornblendequartz _+clinopyroxene + magnetite rocks and Na-plagioclase-quartz _+magnetite rocks (Corbett, 1981 ). Both rock types are finely layered and were probably deposited as sediments, or as volcanic ash which underwent diagenetic alteration. They could represent metamorphosed analcite-chalcedony-Fe-oxide + dolomite/ankerite rocks. Minor calc-silicate rocks contain scapolite. Ednas Gneiss (Fig. 3) is characterized by abundant quartz-albite-magnetite gneiss (essentially a very magnetite-rich version of the Na-plagioclase-quartz rocks present in this and other stratigraphic units). The stratigraphically overlying Mulculca Formation contains considerable quantities of Na-plagioclase-quartz _+magnetite rock, but is richer in metasedimentary composite gneiss (i.e., par-

303 tially melted metasediment; see Stevens and Willis, 1983). It also contains a substantial number of thin horizons of quartz-magnetite and quartz-secondary Fe-oxide (after sulphide) rocks. The Lady Brassey Formation in this area is much thicker than elsewhere and consists of intercalated units of Na-plagioclase-quartz + magnetite rock, leucocratic quartzo-feldspathic gneiss, quartz-feldspar + magnetite rock, thick bodies of amphibolite/ basic granulite, minor metasediment or composite gneiss. This formation may represent a pile of rhyolitic volcanics (partly melted during metamorphism ), analcitized acid volcanics, basaltic volcanics/subvolcanic intrusives and minor metasediments. In the Mount Darling Range area (Fig. 2 ) the lowest exposed stratigraphic unit is the Clevedale Migmatite. This consists of migmatitic metasediments containing fine layers (mostly 1-30 cm) of Na-plagioclase-quartz rock. The Clevedale Migmatite is interpreted as an interval of detrital sediments probably containing volcanic detritus, with fine layers of airfall tuff. This formation is overlain by the more widespread Thorndale Composite Gneiss consisting of metasedimentary composite gneiss with occasional lenticular bodies of Na-plagioclasequartz rock and various amounts of amphibolite/basic granulite. Minor magnetite-secondary Fe-oxide (after sulphide )-quartz rocks are also present.

metamorphosed to albite-quartz. These rocks were deposited contemporaneously with various amounts of tholeiitic basalts, constituting bimodal volcanism, typical of rifts in thin continental crust (Miyashiro et al., 1982). The Alders Tank Formation is very minor and consists of metasedimentary and quartzo-feldspathic composite gneiss. Cues Formation consists of metasedimentary composite gneiss and metasediments with an association of leucocratic gneiss, garnet-biotite-rich quartzo-feldspathic gneiss and basic gneiss ( _ minor calcsilicate) widespread in the central part of the formation. The Kyong Formation defined by Brown (1985) in the Euriowie Block is largely equivalent to Cues Formation, but contains considerable proportions of quartzo-feldspathic gneiss. Fe-rich stratiform lodes are widespread in Cues Formation and also occur in the Kyong Formation. They include quartzmagnetite, quartz-Fe-rich garnet, quartz-Fesulphide, magnetite-garnet and magnetite-sillimanite rocks. Cu and Zn are minor constituents in some. Quartz-magnetite lodes also occur in the Himalaya Formation, some containing minor Cu and Co. Leyh and Larsen (1983) have described the various facies of some 'iron-formations'. Disseminated to massive pyrite with minor Co is common in Himalaya Formation (e.g., in the Thackaringa area; Plimer, 1977).

The Thackaringa Group

The Broken Hill Group is very widespread, and mainly comprises metasediments and metasedimentary composite gneiss, but also contains various quantities of quartz-feldsparbiotite-garnet ('Potosi'-type) gneiss, basic gneiss (commonly Fe-rich garnet amphibolite) and minor quartz-gahnite rock and Mn-rich garnet-quartz rock. The Allendale Metasediments, at the base of the Broken Hill Group, represent a transition in sedimentation. They include feldspar-rich psammitic metasediments similar to those in Thackaringa Group, and more quartz-rich

The Thackaringa Group is very widespread and is characterized by quartzo-feldspathic gneiss (Rasp Ridge and Alma Gneiss) and Naplagioclase-quartz rocks (mostly in Lady Brassey and Himalaya Formations). The quartzofeldspathic gneisses are interpreted as metamorphosed rhyodacitic lava flows and/or ashflow tuffs, while the Na-plagioclase-quartz rocks are interpreted as metamorphosed Na volcanics or airfall tufts altered by alkaline lake water or seawater to analcite-chalcedony then

The Broken Hill and Sundown Groups

304

psammitic metasediments and pelitic metasediments typical of Broken Hill Group. They also host the Ettlewood Calc-Silicate Member, a discontinuous, generally thin, metamorphosed carbonate unit rich in diopside, edidote and/or vesuvianite, and containing thin (1 cm or less) quartz laminations and sporadic W, Zn, Ag, and/or Pb mineralization. The rock was deposited as a bedded impure dolomitic carbonate, over an extensive area, but probably as discontinuous lenses. The sporadic metal concentrations and quartz laminae could represent an exhalative component from hot springs (Stroud et al., 1983). However, the quartz laminae could alternatively represent chert deposited by normal sedimentary processes. The Parnell Formation is characterized by the association of amphibolite/basic granulite, garnetiferous quartzo-feldspathic gneiss and quartz-gahnite rock/garnet-quartz rock. This is interpreted as the product of bimodal rhyodacite-tholeiitic basalt volcanism, with associated Zn- and/or Mn-rich exhalites. The volcanic association is similar to that in the Thackaringa Group, but the Broken Hill Group volcanics tend to be more unusual in composition, notably more Fe-rich (Stroud et al., 1983 ). This Fe-enrichment is interpreted by James et al. (1987) as an igneous fractionation trend and by Phillips et al. (1985) as a result of pre-metamorphic hydrothermal alteration. The Parnell Formation contains numerous small Broken Hill-type stratiform Pb-Ag-Zn sulphide deposits, and a number of small scheelite deposits are associated with irregular lenses of calc-silicate rock (Barnes, in press). Freyers Metasediments separates the lithologically distinctive Parnell Formation and Hores Gneiss/Silver King Formation (Fig. 2). Hores Gneiss ranges from a single continuous body of garnetiferous quartzo-feldspathic ('Potosi'-type) gneiss, to large lenses of gneiss intercalated with metasediments. The formation is best developed in the northern part of the

Broken Hill Block, and is weakly developed or absent elsewhere. Current interpretation (Laing, 1980; Main et al., 1983; Laing et al., 1984; Haydon and McConachy, 1987) places the Broken Hill orebody in metasediments within Hores Gneiss. In the NW part of the Broken Hill Block the Silver King Formation takes the place of Hores Gneiss, and consists of massive amphibolites intercalated with metasediments. Silver King type stratabound Pb-Zn-Cu-AgAu-W deposits are associated with the amphibolites (Barnes, 1980). Very small to extensive, thin (up to ~2 m), stratiform tourmaline-quartz rocks (tourmalinites; Slack et al., 1984), and some scheelitewolframite mineralization occur in or adjacent to the Hores Gneiss and Silver King Formation, and in lower parts of the Broken Hill Group, and the lower section of the Sundown Group. Some of the tourmalinites display bedding, crossbedding and/or graded bedding. The Sundown Group mainly comprises metasediments similar in character to those in the Broken Hill Group. In both the Sundown and Broken Hill Groups, calc-silicate nodules (0.21 m across) are common, and are interpreted as metamorphosed diagenetic carbonate concretions, indicating an original minor carbonate component in the sediments, and probably a marine depositional environment. Graded beds are relatively common in the Broken Hill and Sundown Groups. These have been interpreted as indicating turbidite deposition, probably in deep water (Wall et al., 1976; Plimer, 1978; Laing, 1980). Willis et al. (1983b) proposed a turbidite fan model for the Sundown Group, with sandy channels, sandy/shaly overbank and/or reworked deposits, and shaly interchannel outer fan, basin plain deposits. However, they recognized that the graded beds were simply graded and not Bouma sequences. Such simple graded beds can also occur in deltaic sequences (e.g., crevasse splay and frontal splay deposits) and lacustrine deposits.

305

The Paragon Group The Paragon Group is entirely metasedimentary and is characterized by graphitic pelitic schists and fine-grained psammites. The psammites are more Na-rich and less Fe-rich than those in the Sundown and Broken Hill Groups, and are more akin to those of the Thackaringa Group. Abundant sedimentary structures in the phyllitic Dalnit Bore Metasediments are very similar to those found in fine-grained turbidites (Stow and Shanmugam, 1980; Willis et al., 1983b). Structures in the underlying Bijerkerno Metasediments may indicate contourite deposition (Stow and Lovell, 1979), or perhaps much shallower storm wave or storm current deposition.

Pegmatitic quartzo-feldspathic rocks Large bodies of leucocratic quartzo-feldspathic rock, comprising various proportions of pegmatite, granitoid and gneissic rock, occur in the Thackaringa and Broken Hill Groups in parts of the Broken Hill Block. Smaller bodies occur in the Sundown Group. The rocks exhibit folded and refolded layering, and the larger bodies show some stratigraphic control. They may represent partial melts of original stratigraphic units (Stevens, 1978a), but they are locally transgressive and of uncertain origin, and are not considered in the published stratigraphic sequence (Willis et al., 1983a).

Re-interpretation of the depositional environment Haydon and McConachy (1987) and Wright et al. (1987) have radically reinterpreted the nature of the Thackaringa and Broken Hill Groups. In their interpretation there are no acid volcanics (except perhaps some detritus). The quartzo-feldspathic gneisses (granitic gneiss, 'Potosi'-type gneiss and quartz-albite rock ) are all interpreted as arkosic sediments deposited as a series of fluvio-deltaic wedges interfinger-

ing with marine transgressive cycles. The Broken Hill orebody is hosted by shallow marine sands. The Broken Hill Group was deposited as shelf muds and silts with occasional sheet sand bodies, including possible offshore tidal or storm-generated sand bars or ridges, near-shore bars, barriers or shorefaces. Thin graded sandy units are interpreted as storm-surge layers. We do not accept some aspects of the above model. Specifically, we do not believe that the acid volcanic model for the quartzo-feld~pathic gneisses has been refuted, and we believe that much of the silica in the 'shallow marine sand' associated with the orebodies was deposited as a chemical precipitate and that the quartz-garnet, garnet 'sandstone' and quartz-gahnite rocks were chemical sediments (discussed elsewhere ). Nevertheless, most of us have reservations concerning some aspects of our previous interpretation (Willis et al., 1983a). These reservations are as follows: ( 1 ) The most likely explanation for the highNa rocks (Na-plagioclase-quartz rocks) in the Thackaringa Group and below, is alteration of glassy volcanics in an alkaline, sodic environment (Coombs, 1965; Plimer, 1977; Stevens et al., 1980; Willis, 1984). In younger and modern environments these conditions can be found under special circumstances in deep marine conditions, in an enclosed marine basin, or in a continental rift with carbonate volcanism. But the most common alkaline, sodic environments occur in sabkha and playa lake evaporitic settings. (2) Other features consistent with evaporitic conditions (also suggested by Herriman, 1980, and personal communication, 1981 ) include the presence of scapolite (a halide-bearing mineral) in the Redan Gneiss, the abundance of B and halides (in the form of tourmaline ) in parts of the sequence, and the deposition of dolomite interbedded with chert (Ettlewood Calc-Silicate Member). (3) Ferric Fe is abundant in the lower part of

306 the sequence up to the top of the Thackaringa Group (Tucker, 1983), both in stratiform lodes and as disseminations. Magnetite-rich lodes can form in deep-sea conditions (as with Cyprustype deposits). However, disseminated ferric Fe minerals require pervasive oxidizing conditions, most readily available in very shallow marine or supratidal environments. The minor magnetite-sillimanite rocks and possibly more widespread quartz-sillimanite-magnetite-rich schists~may be products of extreme leaching, as in lateritic profiles. (4) The Redan Gneiss combines some of the above features. The fine, planar layering and albite-hornblende-quartz + clinopyroxene + magnetite composition of the main rock type are best explained if the rock was deposited in a non-marine evaporitic environment and consisted of analcite-dolomite/ankerite-chalcedony-Fe-oxide in the form of a sediment or altered airfall tuff. (5) Although graded beds are abundant in the Broken Hill and Sundown Groups relative to underlying units, most beds in these groups are not graded. Graded beds are most readily observed in drill core, but even here W.R. Leyh (North Broken Hill Ltd, personal communication, 1986) estimates that no more than 5% of beds are graded. Inspection of many exposures with very well-preserved bedding, but little grading, has led to the conclusion that tectonic destruction of graded beds does not adequately explain their scarcity. The Broken Hill and Sundown Groups were deposited as predominantly non-graded, planar beds of sand, silt and clay, with sporadic graded beds or groups of graded beds. An alternative depositional model is proposed as follows. The sequence up to and including the Lady Brassey Formation/Alma Gneiss was deposited in a fluvio-deltaic and lacustrine or coastal sabkha environment, in a low-relief landscape. The major fluvio-deltaic deposit is the feldspathic sand represented by metasedimentary

composite gneiss of the Thorndale Composite Gneiss. The Na-plagioclase-quartz rocks of the Lady Brassey Formation and other similar rocks represent lacustrine or coastal sabkha deposits, probably altered airfall tuff. The Cues Formation is similar in character to the Broken Hill Group, and may represent a marine incursion. However, the greater abundance of magnetite in the Cues Formation suggests oxidizing conditions, possibly lacustrine. The Himalaya Formation represents the return of evaporitic, lacustrine conditions. Both the Alma Gneiss and Rasp Ridge Gneiss may represent massive piles of rhyodacitic lava/ashflow tuff, laterally equivalent to the altered airfall tufts in the Lady Brassey and Himalaya Formation. The psammitic composite gneiss found in parts of the Thackaringa Group represents a continuation of fluvio-deltaic feldspathic sand deposition, as in the Thorndale Composite Gneiss. This continued into parts of the Allendale Metasediments, then gave way to a more shaly marine sequence containing more mature sands, which continued through the Broken Hill and Sundown Groups. Wright et al. (1987) have presented a very plausible model for the sedimentary part of the Broken Hill Group, up to the top of Freyers Metasediments. They interpret a basin-wide marine transgression, with much of the Allendale Metasediments, Parnell Formation and lowest part of the Freyers Metasediments being deposited as shelf muds and silts, with occasional thin turbidite sands deposited as storm-surge layers. The remainder of the Freyers Metasediments contains thicker sheet sand bodies interpreted as prograded storm- or tide-dominated offshore bars, linear sand ridges, and/or sandwaves. Above this were deposited the precursors of 'Potosi'-type gneisses within the Hores Gneiss which we interpret as mass-flow acid volcanics. We interpret the psammopelites within the Hores Gneiss as shelf muds/silts. Wright et al. (1987) interpret most of the Sundown Group as deep water (> 1000 m ?),

307 but Sundown Group metasediments appear to us no different from those in the Broken Hill Group. We interpret a continued deposition of shelf muds and silts, occasional storm-surge turbidite sands, and thick, shallow marine sheet sands. The above interpretation places the Ettlewood Calc-Silicate Member close to the transition from fluvio-deltaic feldspathic sands, to shelf muds and silts. The calc-silicate could represent intertidal dolomitic sediments with chert laminae (as in the McNamara Group, Mt Isa; Derrick et al., 1981), or a shallow-marine deposit. The banded iron formations (garnet-magnetite-quartz-apatite rocks) and some finegrained garnet-quartz rocks, in the Freyers Metasediments and Hores Gneiss, may have formed as chamositic ironstones (Stanton, 1976b). Literature research by Byrnes et al. (1985) (see also Porrenga, 1967) shows that chamosite is most commonly deposited in shallow-marine to estuarine conditions, consistent with the environment interpreted for the Broken Hill Group. In the Paragon Group the Cartwrights Creek and Bijerkerno Metasediments were interpreted by Willis et al. (1983b) as deep water, but may well be shallow-water deposits. The Cartwrights Creek Metasediments may represent organic-rich (probably algal) shelf muds, overlain by delta-front (interdistributary estuarine) sands and silts, with minor intercalated dolomitic marl. The Bijerkerno Metasediments include cleaner, well-sorted fine sand, rich in albite and with abundant crossbeds and scours. This unit could be shallow lacustrine. The uppermost unit, the Dalnit Bore Metasediments appears to represent substantial deepening, with deposition of fine-grained (silty to muddy) turbidites. Deposition of the Willyama Supergroup is interpreted to have occurred in a continental rift hundreds of kilometres in width (see later discussion). Hence the term 'shelf' should be read as 'shallow marine', and such terms as

'continental slope' are inappropriate. Basin floor geometry was probably determined by basement faulting and build-up of piles of sediments and volcanics. Deltaic deposits interpreted in the lower part of the sequence may have been deposited from deltas discharging into lakes, or deposited in the upper delta plain (s) of a delta or overlapping deltas emptying into a shallow sea. Sediments comprising the Broken Hill and Sundown Groups were probably deposited in the lower delta plain and prodelta parts of a large delta or overlapping deltas emptying into the encroaching sea. The rift may have been similar in morphology to the East African rift or to the Salton Sea/Gulf of California, with one end potentially open to the sea. It is also possible that the so far unidentified eastern margin of the rift was submerged at times, permitting marine transgression. The almost complete absence of conglomerates or pebbles indicates a low-relief landscape, with river gradients insufficient to transport coarse detritus, a n d / o r with rivers draining a largely unconsolidated sediment source. S m / N d isotope data and chemical similarities between sediments and contemporaneous volcanics suggest that most of the sediments were derived from within the rift, by reworking of volcanic ash a n d / o r erosion of volcanics.

Geochemistry of the metavolcanic rocks Geochemical data for most rock types in the Willyama Supergroup were presented in Main et al. ( 1983 ) and in Stevens and Stroud (1983). Probably the most important geochemical problem pertains to the origin of, and the relationships between, the acid and basic metavolcanics (and whether or not the acid rocks really were volcanics). The crux of the problem is the composition of the quartzo-feldspathic gneisses and basic gneisses in the Thackaringa Group and Broken Hill Group. James et al. (1987) concluded from the compositions of the amphibolites/basic granulites that they originated by partial melting of a

308 slightly incompatible-element-enriched MORB source followed by fractional crystallization along a trend of strong Fe enrichment coupled with some crustal assimilation. They also concluded that at least some of the intermediateacid rocks were derived from fractionation of the basic rocks, but that crustal material was also important. A crustal source for much of the acidic material is suggested by the 2300-2100 Ma S m - N d model ages of separation from the mantle (McCulloch and Hensel, 1984), compared with the estimated 1820_+ 60 Ma depositional age (Shaw, 1968). This implies a crustal residence time of between 200 and 500 Ma, before deposition in the WiUyama Supergroup. Wall et al. (1976) and Phillips et al. (1985) interpreted the Fe-rich nature of the basic gneisses as resulting from the differentiation of an originally tholeiitic magma, producing Feoxide contents of up to 15 wt.% FeO (total), overprinted near the Broken Hill lode by premetamorphic alteration, producing up to 25 wt.% FeO (total). This interpretation is not compatible with the fact that TiO2 correlates strongly with Fe203 (total) up to very high values of Fe2Q (total) (Fig. 4). This is a typically igneous differentiation trend (Miyashiro, 1975 ). Fe and Ti have very different mobilities in a hydrothermal system, and should not correlate so well in altered rocks. James et al. (1987) interpret the very high Fe contents essentially as a result of igneous fractionation. They and Stevens (1978b) suggest a genetic connection between the basic and acid gneisses within the Broken Hill Group. James et al. ( 1987 ) suggest that the basic gneisses were developed in a propagating continental rift, where small, high-level magma chambers separated from the magma source. High Fe/Mg ratios in basalts typically develop at shallow depths, in areas of high heat flow (see Chayes, 1969; Clague and Bunch, 1976; Byerly et al., 1976). L. Wyborn (personal communication, 1985) suggested that some of the quartzo-feldspathic gneisses of the Willyama Supergroup are chem-

28 × X ×

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Fig. 4. Plot of Fe203 (total) and Ti02 for basic gneisses fromthe WillyamaSupergroup.Most of the data are from the Broken Hill Group. The diagramshows strong correlation betweenFe and Ti, up to very high values of Fe203 and Ti02. Data fromBarron and Stroud (1986). ically very similar to extensive areas of granitoids of similar age in northern Australia. She has proposed a two-stage origin (Wyborn, 1985; Etheridge et al., 1987) for these rocks, involving underplating of the thinned continental crust by masses of mafic magma, followed by partial melting of the mafic rock to form acid magmas. The whole process took ~ 200 Ma, and with this model there should be no compositional relationship between the acid rocks and any contemporaneous basic rocks, since basic rocks are derived directly from the mantle, while the source for the acid rocks was derived from the mantle 200 Ma or more earlier. Obviously, further research is required to test for a genetic link between quartzo-feldspathic and basic gneisses at Broken Hill, and to compare ~the quartzo-feldspathic gneisses with northern Australia granitoids and volcanics. Intrusive rocks

Intrusive rocks are relatively minor as a proportion of outcrop area of the Willyama Super-

309

group. Small parts of the metamorphosed sequence could represent metamorphosed intrusions, and some rocks formed by anatectic melting during metamorphism could also be considered intrusive. Several types of minor intrusive were emplaced after the prograde metamorphism and before or during the Delamerian Orogeny. The intrusive rocks were described in Stevens and Stroud (1983) and their ages of intrusion were discussed by Stevens (1986).

Pre-metamorphic and syn-metamorphic intrusions The earliest possible intrusions are some of the basic gneisses and acid gneisses included within the Willyama Supergroup. Some of the quartzo-feldspathic gneisses in the Thackaringa Group are lenticular masses which could represent deformed intrusions. However, the very strong stratigraphic control shown by these bodies indicates that such an origin is unlikely. In contrast, grossly transgressive bodies of leucocratic gneiss occur to the north of Yanco Glen (Felton, 1975; Brown et al., 1983). These gneisses comprise quartz-microcline-oligoclase-minor biotite, and exhibit a well-developed folded gneissosity. A similar gneissic granitoid has been described by Tuckwell (1975) in the northern part of the Euriowie Block. These rocks could have been intruded during deposition of the supergroup or up to the time of the first deformation event. Basic gneisses are abundant in the Willyama Supergroup, in and below the Broken Hill Group. Some of these are relatively coarse grained, some exhibit sub-ophitic textures interpreted as relict igneous, and some appear to be transgressive (although competence differences with adjacent rocks can produce apparent transgressions during deformation). If some of these basic gneisses were intrusive they formed as shallow sub-volcanic intrusions, since virtually none appear above the top of the Broken Hill Group. Some of the many small and large bodies of

pegmatite and mixed pegmatite/leucocratic granitoid/leucocratic gneiss show intrusive relationships with enclosing rocks. However, an overall stratigraphic control of the larger bodies was recognized by Stevens (1978a). These bodies are located mainly within the Thackaringa and Broken Hill Groups and to a lesser extent within the Sundown Group. These rocks are very leucocratic, silica- and K-rich, and contain structures which indicate they have undergone the high-grade deformations. The rocks have either intruded during the first deformation or have formed by anatectic melting and local mobilization of a suitable lithology in the stratigraphic sequence. Stevens (1978a) suggested that the parent rock may have been a leucorhyolite. J. Downes (personal communication, 1987) suggests that the rocks have been generated by partial melting of metasediments and quartzo-feldspathic gneisses during D1.

Intrusions post-dating prograde metamorphism Very coarse-grained, zoned pegrnatites (Lishmund, 1982) (some containing cassiterite, beryl, Li minerals, etc.) provide a range of age relationships and radiometric dates. Some are folded (by F2 or F3 folds), others are axial planar to the same folds, many others transgress all previous structures. Radiometric dates range from 1490+20 Ma to ~500 Ma (Pidgeon, 1967"). Three types of post-folding granitic intrusive were identified by Brown et al. (1983). The Mundi Mundi-type granites consist of quartz, microcline, albite-oligoclase, minor biotite and (primary) muscovite. They have porphyritic microcline, are clearly transgressive and are undeformed except for minor retrograde schistosity. They have been dated at 1490__ 20 Ma (Pidgeon, 1967"). The Champion and Umberumberka type granitoids are similar, but more *Dates adjusted by Harrison and McDougall (1981) using decay constants of Steiger and Jiiger ( 1977 ).

310 leucocratic and non-porphyritic, and the Champion types are less potassic. Some of the Umberumberka type are not clearly transgressive and may have been derived by anatexis of nearby parts of the Willyama Supergroup. Nepheline pyroxenite stocks and pyroxenite and hornblendite dykes were intruded at ~ 561 Ma (Harrison and McDougall, 1981). Dolerite dykes were intruded between 561 and ~ 500 Ma. A small number of ultramafic plugs and dykes have intruded the Willyama Supergroup after the prograde metamorphism. Some are strongly altered to serpentine, talc, carbonate, chlorite, etc. Giles (1974) interpreted the primary mineralogy as essentially orthopyroxene and olivine, and concluded that the rocks were cumulate products. The rocks may have been altered and deformed during the Delamerian Orogeny, but they have not been dated. A few acid to intermediate altered feldspar porphyry dykes of possible Palaeozoic age have been recognized in the Euriowie Block (R. Brown, personal communication, 1986). Mineralization The Willyama Supergroup is host to a wide variety of types of mineral deposits. The preeminent deposit is the 8-km-long Broken Hill orebody, which prior to erosion and mining contained ~ 250 million tonnes of high-grade Pb-Ag-Zn ore. In addition to the Broken Hill orebody, the Supergroup hosts a multitude of medium to mainly very small metalliferous deposits and occurrences mainly containing Pb, Ag, Zn, Cu, W and/or pyrite (see Barnes, 1986).

The range of deposit types Metalliferous deposits of the Broken Hill Block have been systematically classified and their stratigraphic positions discussed by Barnes ( 1980, 1983a, b, 1986, in press ). The nature of the stratiform and stratabound mineralization varies within the stratigraphic sequence as follows:

(1) The lower part of the sequence, particularly the Thackaringa Group, is dominated by deposits containing Fe, Cu, Co with minor PbAg-Zn and very minor Au and U. (2) The Broken Hill Group contains the Broken Hill orebody, an abundance of small stratiform Pb-Ag-Zn deposits, a considerable number of W deposits of various types, and several stratabound Pb-Zn-Cu-Ag-Au-W deposits. (3) The Sundown and Paragon Groups are mostly barren, except in the Euriowie, Yanco Glen and Kantappa areas where they host pegmatitic cassiterite deposits. The pegmatites are broadly stratabound and might be derived by mobilization of Sn from within the sediments (Barnes, 1987). Vein-type mineralization is widespread. Vein types include Pb-Ag or Cu-rich siderite-quartz veins (Thackaringa-type), and various quartz veins containing Cu, Pb, Au and F mineralization. Other styles of mineralization include magnetite-pyrite veins and breccias associated with some anatectic granites; Pt-Cu-Ni deposits in serpentinized ultrabasic intrusions in the eastern Broken Hill Block; and U- and Th-bearing pegmatites.

Broken Hill-type deposits In the Broken Hill Block, there are > 100 small Broken Hill-type Pb-Ag-Zn deposits, but very few approach the size of even the satellite bodies (mentioned below) near the Broken Hill lode. The deposits occupy a range of stratigraphic positions. The Pinnacles deposit ( ~ 0.5 million tonnes), the Stirling Hill deposit and several other small deposits are interpreted as occurring in the Cues Formation (Barnes, 1983a; Willis et al., 1983a). Almost all other Broken Hill-type deposits occur within the Broken Hill Group. The greatest number are in the Parnell Formation, a few are in the upper part of the Allendale Metasediments, and several occur within the Freyers Metasediments,

311 Hores Gneiss and Silver King Formation. Laing (1980) and Haydon et al. (1983) placed the Broken Hill orebody in a stratigraphic position defined by Willis et al. (1983a) as Hores Gneiss. The small deposits occur as single or multiple lenses, generally in or associated with quartzgahnite rock a n d / o r fine-grained Mn garnetquartz rock (Fig. 5). Mn and Ca-Mn silicates (apart from spessartine) and calcite are rare in these small deposits. None of the deposits away from the Broken Hill lode and its satellites are known to contain more than a few tens of thousand tonnes of mineralization (with the exception of the 0.5 million tonne Pinnacles deposit).

The Broken Hill orebody Most exploration in the Willyama Supergroup has been directed towards the discovery of another large Broken Hill-type deposit. To this end, considerable effort has been directed towards identifying factors which may have localized the Broken Hill orebody. The orebody itself has been described by numerous authors. Recent review papers include Johnson and Klingner (1975), Both and Rutland (1976) and Plimer (1979, 1984 ). Some lithological, mineralogical and assay data are presented in Table I, and the stratigraphic position of the orebody is discussed in the previous section. The Broken Hill orebody consists of seven sulphide-bearing lenses, stacked one above the other, each having distinctive chemical and mineralogical characteristics (Johnson and Klingner, 1975; Plimer, 1979, 1984). There is a general increase in P b / Z n ratio and Ag values stratigraphically upwards, though each lens is relatively homogeneous in metal ratios. The mineralogy of the ore lenses is complex, with ~ 300 primary and secondary mineral species identified (Worner and Mitchell, 1982). The main minerals in each lens are listed in Table I. We have given each lens simple lithological names to show its general character, for comparison with rocks found elsewhere in the district.

The stratigraphically lower lenses ('zinc' lodes) tend to have gradational boundaries with host rocks and are enclosed in garnet-quartzrich metasediments, which may represent simple metamorphosed sandy sediments, but more probably included a substantial chemical Si02 and Mn component. A rock composed almost wholly of fine-grained, Mn-rich garnet ('garnet-sandstone') is associated with the stratigraphically upper lenses ('lead' lodes), but does not extend far from the orebody. The 'lead' lodes tend to be high in grade and have sharp boundaries with the host pelitic and psammitic metasediments. Outcrops of quartz-gahnite rock extend NE and SSW more or less along strike from the Broken Hill lode, for a total distance of about 25 km. This rock probably represents an exhalite laterally equivalent to the 'zinc' lodes. Associated with this exhalite are small Broken Hill-type Pb-Ag-Zn deposits, containing up to 600,000 tonnes of sulphide mineralization. Downdip from the Broken Hill orebody, drilling has identified two more sulphide-bearing lenses (or groups of lenses): the Western Mineralization and the Centenary Mineralization. These contain respectively an inferred resource of 15 and 9 million tonnes of about 2% Pb, 3% Zn, 30 g t - ' Ag (Haydon and McConachy, 1987 ).

Origin of the Broken Hill orebody The origin of the Broken Hill orebody has been the subject of many theories (see Stevens, 1975, for review). In recent times there has been a consensus of opinion that the orebody is premetamorphic and syngenetic in origin, probably having formed as a result of exhalations onto the seafloor. The close association of the orebody with metavolcanics led Stanton (1972) to classify the deposit as volcanic exhalative. However, some features of the orebody are more suggestive of a sediment-hosted exhalative deposit (e.g., Gustafson and Williams, 1981; Phillips et al., 1985 ). These features include the huge

312 i 141°15 '

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Fig. 5. Distribution of Broken Hill Group rocks and quartz-gahnite rocks and other 'lode horizon' rock types in the Broken Hill Block. size, the absence of a definite footwall feeder zone, the abundant Pb, Zn, and Ag, with low Cu values, and the metasedimentary nature of the immediate host rocks. Haydon and McConachy (1987) and Wright et al. (1987) have rejected the syngenetic

models, and propose an origin related to compactive expulsion of metal-bearing brines during accumulation of the sedimentary pile, and deposition in pore spaces and by diagenetic replacement of shallow-marine sands. They confuse this diagenetic 'inhalative' model within the

313 TABLE I Lithology, mineralogy and metal contents of ore lenses constituting the Broken Hill lode (in part after Plimer, 1984, and Mackenzie, personal communication, 1986) Ore lens

No. 3 lens northern

Rock types

Sulphide-quartz-Mn-silicatefluorite rock, rhodonite-sulphide rock, fine-grained garnet rock (adjacent).

Minerals

Abundant: Minor:

Trace:

'Typical' mining grade Pb (%)

Ag (gt -1 )

Zn (%)

galena, sphalerite, rhodonite, fluorite, quartz, garnet chalcopyrite, pyrrhotite, loellingite, arsenopyrite, gahnite, apatite, pyroxmangite, amphiboles, calcite, feldspars sillimanite, staurolite, chloritoid, sulphosalts, bustamite, pyrosmalite

15

300

13

No. 3 lens southern

Sulphide-quartz-Mn-silicatefluorite rock, fine-grained garnet rock (adjacent).

Abundant:

sphalerite, galena, quartz, fluorite, rhodonite, garnet

11

200

15

No. 2 lens

Sulphide-calcite-CaMn-silicate rock, fine-grained garnet rock (adjacent).

Abundant:

galena, sphalerite, calcite, bustamite, manganoan hedenbergite, rhodonite chalcopyrite, p~rrhotite, loellingite, arsenopyrite, garnet, quartz, Mn-olivines, feldspars, micas, amphiboles sillimanite, staurolite, cubanite, ilvaite, vesuvianite, johannsenite, sulphosalts

14

100

11

sphalerite, galena, quartz, calcite feldspars, bustamite, manganoan wollastonite apatite, gahnite, fluorite, chalcopyrite, pyrrhotite sillimanite, staurolite, micas, amphibole

8

50

20

sphalerite, rhodonite, manganoan hedenbergite, quartz, galena garnet, calcite, gahnite, loellingite, arsenopyrite, chalcopyrite, pyrrhotite, apatite, feldspar sillimanite, amphibole, pyroxene, staurolite, micas, sulphides, sulphosalts

4

40

10

quartz, sphalerite, galena chalcopyrite, garnet, pyrrhotite, apatite, feldspars, gahnite rhodonite, calcite, manganoan hedenbergite, sillimanite, micas, staurolite, amphibole

5

40

17

2.5 20 (not mined)

5

Minor:

Trace:

No. 1 lens

Sulphide-quartz-calcite rock

Abundant: Minor:

Trace: A lode

Sulphide-MnCa-silicate-quartz rock, quartz-garnet rock.

Abundant: Minor: Trace:

B lode

Quartz-sulphide rock

Abundant: Minor: Trace:

C lode

Quartz-garnet-gahnite-sulphide rock.

Abundant: Minor:

quartz, garnet, gahnite, sphalerite feldspar, galena, chalcopyrite

depositional basin, with the geologically and isotopically very different Laisvall model. We find it difficult to accept a diagenetic origin for the orebody. There is an indisputable relationship between the Broken Hill orebody (and the many other Broken Hill-type deposits), and extensive horizons of quartz-gahnite and/or garnet-

rich rocks. These siliceous lode rocks are texturally and chemically distinct from the more normal bodies of psammitic metasediment found in the sequence, and are unlikely to have originated as sand beds. The coarse, irregular grainsize of the quartz, and the high concentrations of Zn, Pb,Mn and/or Fe contained in these

314

rocks more probably developed from a metalbearing cherty chemical sediment. Similarly, the siliceous metasediments enclosing parts (not all) of the Broken Hill lode are texturally dissimilar to the 'normal' psammitic metasediments, and probably contain a significant component of chemically deposited silica. Without knowing the proportion of chemically deposited silica, it is not valid to interpret the host rocks as 'shallow-marine sands'. In fact, parts of the orebody are enclosed by pelitic metasediment, as are most of the small Broken Hill-type deposits and quartz-gahnite 'lode horizons' in the district (Barnes, in press). If it is accepted that the orebody formed on the seafloor, as a product of hot spring exhalations, the next steps in understanding its genesis are to determine the source of the orebearing fluids and their metallic constituents, the mechanism of transport from the source to the seafloor, and the factors which constrained deposition of over 250 million tonnes of sulphide-rich precipitate. The immediate source of the metals could be either mantle or crust. Plimer (1985) used the 'primordial' S and Pb isotopic values of the Broken Hill orebody, the abundance of carbonate, fluorite, fluorapatite, LIL-, K- and Rb-rich pre-metamorphic alteration assemblages, Sr isotopes and S/Se ratios to suggest that the orebody formed by devolatilization of the mantle, as a result of propagation of deep fractures related to rifting. Phillips et al. (1985) and James et al. (1987) suggested that the metals were leached out of the sequence by seawater convection cells, driven by heat from high-level magma chambers. This is not necessarily contrary to Plimer's proposition, since the metals could have been brought from the mantle, into the sequence, with mantle-derived volcanics. James et al. (1987) proposed that the basic gneisses in the sequence were fractionated tholeiitic basaltic rocks (mantle derived), with some crustal assimilation. They also proposed that some of the quartzo-feldspathic rocks in Broken Hill and

Thackaringa Groups were derived by fractionation of the basaltic rocks, and incorporated a crustal component. W.J. Stroud (personal communication, 1986) reports that the garnetbiotite quartzo-feldspathic gneisses of the Broken Hill Group, and the basic gneisses, are variably rich in base metals, while the quartzofeldspathic gneisses of the Thackaringa Group do not show such enrichments (Table II). It is possible that the base metals have been brought to the surface with mantle-derived tholeiitic magma and its more siliceous fractionation product, leading to a background enrichment of the Broken Hill Group. The metasediments also show variably high Pb and Zn contents, and it has been suggested (Stevens, 1978b) that the metasediments of the Broken Hill Group may have been derived largely from volcanic detritus within the sequence. This explanation, however, only accounts for a general metal enrichment in the Broken Hill Group. Why is such a huge amount of metal concentrated in the Broken Hill orebody, while so little is concentrated in other known Broken Hill-type deposits? One answer could be that the metal-bearing solutions which formed the orebody came up fractures directly from their source (the mantle? ), and were deposited onto the seafloor, so that there was no need to concentrate dispersed metals from the Broken Hill Group. Another possibility is that the metals were concentrated from dispersions in the Broken Hill Group, and that a particularly efficient concentration mechanism existed near the Broken Hill orebody. Both Phillips et al. (1985) and James et al. (1987) suggest that seawater convection cells may have been driven by highlevel basaltic magma chambers. This may explain the base metal concentrations in Parnell Formation, but the Broken Hill orebody postdates the basic gneisses in that part of the Broken Hill Block. The Broken Hill orebody is very close spatially and stratigraphically, to lenses of quartz-feldspar-biotite-garnet ('Potosi'type) gneiss within the Hores Gneiss. The Po-

315 TABLE II Base metal statisticsa for some Broken Hill rock types (analyses from various sources) Metasediments

'Potosi'-type gneiss

'Granitic' gneiss

Basic gneiss

Pb

Low (ppm) Medium (ppm) High (ppm) Number of samples

83 150 275 318

17 72 300 188

9 17 34 60

5 18 62 171

Zn

Low (ppm) Medium (ppm) High (ppm) Number of samples

150 270 480 320

83 228 600 192

20 40 80 60

78 214 600 170

Cu

Low (ppm) Medium (ppm) High (ppm) Number of samples

24 65 170 236

15 39 100 179

9 16 29 49

20 72 250 170

"The statistics quoted are based on log-normal distributions. The value quoted as 'medium' corresponds to the log-normal mean. 'Low' corresponds to log-normal mean minus one log-normal standard deviation. 'High' corresponds to log-normal mean plus one log-normal standard deviation. (Ordinary means and standard deviations are not very meaningful with trace element data.) Note: The metasediments are mostly from the Broken Hill and Sundown Groups, the 'Potosi'-type gneisses are all from the Broken Hill Group, the 'granitic'-gneisses are all from the Thackaringa Group, and the basic gneisses are mostly from the Broken Hill Group. There is considerable sampling bias towards the area close to the Broken Hill mines.

tosi-type gneiss has been interpreted as metamorphosed rhyodacitic ashflow tuff by Stanton (1976b), Brown et al. (1983), Laing et al. (1984). However, Laing et al. (1984) and Barnes (in press) have suggested that the source of the ashflows was close to rocks which now lie in the Yanco Glen area, and that Broken Hill was relatively distal from the source of the volcanics. A heat source for convection cells in the Broken Hill Group could conceivably lie in rocks classed as Thackaringa Group. This would depend on one of the rocks interpreted by Willis et al. (1983a) as volcanics, being reinterpreted as an intrusive, contemporaneous with Broken Hill Group deposition. The only possible contenders are the quartzo-feldspathic gneisses (Rasp Ridge Gneiss a n d / o r Alma Gneiss) or some of the basic gneisses. The quartzo-feldspathic gneisses seem unlikely intrusives in view of their strong stratigraphic control and lateral equivalence with bedded Na-plagioclase-quartz

rocks. Some of the basic gneisses in the Thackaringa Group are well layered, and could represent sill-like intrusives with cumulate layering. However, they are not very thick, and it is doubtful whether they could have generated sufficient heat to drive large convection systems. In the absence of major intrusive heat sources for convection cells, basin-scale flows of connate water should be considered. Sawkins (1984) proposed such a model for conformable, sediment-hosted Pb-Zn deposits, including Broken Hill. In his model, metal-bearing brines were expelled episodically from the sedimentary pile, and ore deposits formed where these brines emerged on the seafloor. Wright et al. (1987) proposed a similar model of brine transport as a result of compactive expulsion, with solutions utilizing growth faults to move up -~ ward in the sequence and precipitate within buried sand units. Their model could be readily adapted to seafloor deposition. Important fea-

316 tures in these connate water flow models are permeability systems, basin geometry, stratigraphic configurations, and fracture systems. Whether the metals were leached out of the Broken Hill Group, or came direct from a deeper source, a suitable depositional environment was required. Johnson and Klingner (1975) and Barnes (in press) suggested that the Broken Hill orebody formed in a local basin or seafloor depression (probably fault-bounded)up to 30 km long, and relatively narrow. The orebody formed close to the source of fluids, with some fluids spreading away to the NE and SW to form small satellite deposits. The Broken Hill and Mt Gipps 1:25 000 Geological Sheets (Bradley, 1984; Brown, 1984) show facies changes across the closures of N E - S W trending F2 folds near Broken Hill, possibly reflecting original N E SW trending irregularities in the depositional environment.

Isotope studies of mineralization Reynolds (1971) and Gulson et al. (1985) analysed Pb isotopes from stratiform and stratabound mineralization in the Broken Hill Block. Their data demonstrated small but significant differences between various styles of mineralization, but showed that particular deposit types have almost identical values at differing stratigraphic positions and widely spaced locations. The Pb isotope values for the stratiform and stratabound mineralization suggest formation from a homogeneous source area of mixed crustal and mantle material (Doe and Zartman, 1979). Small differences in the processes of formation of the deposits and minor variations caused by metamorphic remobilization may account for the small variations observed between deposit types. S isotope studies by Spry (1987) indicate that the S in the Broken Hill orebody was derived from either: (a) inorganic reduction of seawater sulphate, mixed with magmatic S, or (b) low-temperature biological reduction of sulphate from contemporaneous seawater.

Studies of S, C, 0 and Pb isotopes in veintype deposits (Reynolds, 1971; Both and Smith, 1975; Dong et al., 1987) indicate that the Thackaringa-type Pb-Ag veins could be derived from disseminated mineralization in the sequence, mixed with other components possibly derived from meteoric water or seawater.

Structure and m e t a m o r p h i s m

Structural history The Willyama Supergroup in the Broken Hill Block shows evidence of three major episodes of folding, with minor subsequent folding, and further major deformation associated with retrograde schist (shear) zones and faulting (Marjoribanks et al., 1980; Hobbs et al., 1984). The Euriowie Block shows a comparable history of folding and schist zone deformation, but is more strongly faulted than the Broken Hill Block (Brown et al., in preparation). The first deformation (D1) produced very large, reclined to recumbent isoclinal folds, with amplitudes of tens of kilometres. Very few small F1 folds have been identified, but a prominent high-grade $1 schistosity parallels bedding in most areas. Downward-facing F2 and F3 folds over substantial areas of the Willyama Supergroup have formed as a result of refolding of overturned limbs of F1 folds. The Broken Hill orebody occurs in one of these areas of downward-facing folds. The second deformation (D2) produced large and small folds, the largest being several kilometres in amplitude. The F2 folds tend to be tight to isoclinal, with open hinges and axial planes dipping steeply to the NW. The $2 axial plane schistosity is high grade, and very similar in grade to $1. The third deformation (D3) produced abundant small folds and some larger folds of significance on a regional scale. F3 folds generally have near-vertical axial planes and a variably developed axial planar retrograde schistosity ($3). Initial development of

317

retrograde schist zones may have coincided with F3 folding (Rutland and Etheridge, 1975). However, the common association of staurolite and kyanite with the schist zones and the lack of these minerals in the $3 schistosity may suggest that the schist zones formed as a response to a separate and probably later phase of deformation. Biotite analysed from several retrograde schist zones produced Delamerian Orogeny dates of 520 + 20 Ma (Pidgeon, 1967; Harrison and McDougall, 1971 ) and 460 Ma (Etheridge and Cooper, 1981), indicating extensive reactivation at that time. This agrees with field evidence of schist zone deformation of the Adelaidean unconformity, Mundi Mundi-type granitoids and basic/ultrabasic dykes (Stevens, 1986 ). Retrograde schist zones are steeply dipping, planar or curviplanar zones, with a well-developed, generally intense schistosity, and a steeply pitching lineation, defined by retrograde minerals, notably micas and chlorite. They form an anastomosing network throughout the Broken Hill Block. In these zones, bedding in metasediments is strongly deformed and may be transposed or even destroyed. Many schist zones perform a similar structural function to a fault, with offset of units across the schist zone. However, deformation within a schist zone is largely ductile rather than brittle. Faulting occurred within or at one margin of some schist zones in the Broken Hill Block (some of these faults contain Thackaringa-type Pb-Ag veins). Faults independent of schist zones are rare. An exception is the area north of Mt Robe, where faults are relatively abundant and produced significant displacements. The Euriowie Block and Poolamacca Inlier (between the Broken Hill and Euriowie Blocks) are extensively faulted (Brown, 1985; Brown et al., in preparation). It is possible that these areas and the northern part of the Broken Hill Block were at a higher level in the crust than the main part of the Broken Hill Block, during the Delamerian Orogeny.

Structural configuration Despite the impressive history of structural studies on the Broken Hill Block, the three-dimensional shape of most large folds is poorly understood. Axial plane traces of individual folds are interpreted by different authors as having different positions, and recent authors attribute individual folds to different deformations (Marjoribanks et al., 1980; Willis et al., 1983a; Hobbs et al., 1984). The shapes and orientations of F1 folds in particular are poorly understood. Marjoribanks et al. (1980) proposed a zone of upright F1 folds in the Mount Darling Range area SE of Broken Hill, forming a 'root' zone between two areas of nappe-like F1 folds. This model involves major tectonic transport, with different nappes travelling different distances in one or other of two directions. This should have produced a fairly chaotic reshuffling of original facies relationships. Laing and Barnes (1986) identified five alternate limbs of 'recumbent nappes'. After 'unfolding', the nappe limbs "reflect their present relative distribution, with the Broken Hill limb at the base of an en echelon nappe pile whose upper parts root from the west". This agrees with observations from detailed Geological Survey of New South Wales mapping (summarized in Stevens, 1980, and Willis et al., 1983a) that sedimentary/volcanic facies vary in a regular manner across the Broken Hill Block, so that the Marjoribanks at al. (1980) model for F1 folds is unlikely to be correct. Hobbs et al. (1984) presented a block diagram for part of the Broken Hill Block, but stated that insufficient information is available to construct the diagram accurately. They concluded that the first deformation produced an inclined to recumbent F~ fold pile, verging to the NW. Presumably this means that the F~ axial planes are dipping east (in an overall sense ), whereas Laing and Barnes (1986) "root from the west" suggests an overall westerly dip for F1 axial planes. In the Broken Hill mine area, which is the only area in which the three-di-

318 mensional configuration is well documented, the axial plane of the F, Nine Mile Fold (Willis et al., 1983a) must dip eastwards below the Hanging Wall and Broken Hill Synforms. This may be only a local feature, however.

Metamorphism The prograde metamorphism has been been described by Phillips (1980) and Phillips and Wall ( 1981 ), superseding work by Binns ( 1963, 1964). Prograde partial melting has been described by Downes and Wall (1984) and Downes (in preparation) and retrograde metamorphism was described by Chenhall (1973) and Corbett and Phillips (1981). The prograde metamorphic path has been deduced from mapping of zones of different grade (Fig. 6, Table III). It is interpreted as a progression from andalusite + muscovite (lowest grade recognized) through a narrow interval of sillimanite+muscovite, to sillimanite + K-feldspar, to orthopyroxene + clinopyroxene + hornblende (i.e. hornblendegranulite grade ). Within the granulite zone, sillimanite and biotite are orientated parallel to $1 and $2 (Marjoribanks et al., 1980) and it is concluded that these schistosities were formed during highest-grade metamorphism. Laing (1977) suggested that $2 formed at a slightly higher pressure (~0.5 kbar higher) than $1. Glen et al. (1977) suggested that prograde andalusite pre-dated $1, but this was disputed (Corbett, 1979; Corbett and Phillips, 1981 ). $3 is a retrograde schistosity and is generally defined by muscovite, although sillimanite is present in some areas (Stevens, 1978b; Marjoribanks et al., 1980). Retrograde schist zones and the extensive regional retrogression are characterized by mineral assemblages ranging from medium-pressure amphibolite to greenschist grade (Table IV). In the SW of the Broken Hill Block (Fig. 6) kyanite, staurolite, garnet, and biotite are common retrograde minerals, chloritoid occurs in the

northern and central areas and muscovite, sericite and chlorite are widespread. Retrograde andalusite has been recorded from the central part of the Broken Hill Block (Stevens, 1978b ), and some of the andalusite in the NW may also be retrograde (B. Stevens, unpublished field observations). Phillips and Wall (1981) concluded that there was a low-intermediate pressure prograde P-T path and nearly constant pressure retrograde path (Tables III and IV). This indicates that the heat source could not be related to tectonic thickening alone, but required an external heat source, such as deep crustal mafic magma. One problem with the metamorphic path interpretation is that it has not been conclusively demonstrated whether the staurolite- and kyanitebearing assemblages are truly retrograde (i.e., formed during decline in temperature following prograde metamorphism), or formed during a later, separate metamorphic event (see discussion under 'Structural History'). Studies on element partitioning in poikiloblastic minerals such as garnet are needed to resolve these uncertainties and provide accurate detail of the high-grade P - T p a t h (see, e.g., St Onge, 1987). Garnet grew at various stages of prograde metamorphism, and some additional growth occurred during retrograde metamorphism. One important conclusion from metamorphic and geochemical studies is the largely isochemical nature of the prograde metamorphism. Stanton (1976b) and Stanton and Williams (1978) concluded that during metamorphism movement of components was restricted to a few millimetres or less. Most other recent Broken Hill researchers have assumed and/or concluded that the prograde metamorphism was largely isochemical, and this is implicit in interpretations of original rock types. James et al. (1987) concluded that allochemical metamorphic processes affected some major and trace elements, including the group CaO-K20-Rb. From our field observations it is obvious that there has been at least local redistribution of

319

Andalusite

+

muscovite z o n e

Sillimanite

+

muscovite z o n e

Sillimani~e

+

K-feldspar

zone

2- pyro~ene

I

zone

SCALE

25

0 I

I

kiiometres

I 14777

Fig. 6. Prograde metamorphic zones in the Broken Hill Block (modified after Phillips, 1980) and Euriowie Block, overprinted by the approximate distribution of retrograde kyanite and staurolite (from Geological Survey of New South Wales mapping).

TABLE III Physical conditions during prograde metamorphism (largely after Phillips, 1980) Zone

Distinctive assemblage

T ( °C )

Ptot~l(kbar)

aH2o

1 2 3 4

Andalusite + muscovite Sillimanite + muscovite Sillimanite + K-feldspar Orthopyroxene + clinopyroxene

500-580? 580?-680 680-760 760-800

3? 4? 4.5 _+0.6 5.2-6

1.0 1.0 0.5-0.8 0.3-0.6

components during high-grade metamorphism in s o m e rocks: t h o s e w h i c h c o n t a i n a b u n d a n t q u a r t z o - f e l d s p a t h i c veining, g a r n e t p o i k i l o b lasts a n d / o r g a r n e t - q u a r t z i n t e r g r o w t h s ( u p to

30 c m a c r o s s in s o m e m e t a s e d i m e n t a r y c o m posite gneisses). Several authors have invoked mechanisms o t h e r t h a n p r o g r a d e m e t a m o r p h i c a l t e r a t i o n to

320 TABLE IV Retrograde metamorphic conditions (physical conditions estimated from data in Corbett and Phillips, 1981, and Miyashiro, 1973) Area

Probable assemblage

T ( ° C)

Ptot~l(kbar)

aH~o

Southwest

Kyanite + staurolite + almandine + biotite + quartz + muscovite ( + rare sillimanite) Staurolite + almandine + biotite + quartz + muscovite ( + rare andalusite) Chloritoid + muscovite + biotite (?) + chlorite (?) ÷ quartz ( + almandine?) Muscovite + chlorite + quartz ( + almandine?)

550-600

5-5.5

1.0?

550?

3-5?

1.0?

350-530

3-4?

1.0

300-500

9

1.0

Central

North and east General

explain unusual compositions of some of the rocks. Coombs (1965), Wall et al. (1976), Plimer (1977) and Phillips et al. (1985) invoked premetamorphic diagenetic/hydrothermal alteration. Changes in bulk composition have been documented in retrograde schist zones by Chenhall (1973), Plimer (1975), Corbett and Phillips (1981) and Etheridge and Cooper (1981).

Tectonic interpretation The Willyama Supergroup was deposited, deformed and metamorphosed in an early to middle Proterozoic mobile zone. Willis et al. (1983a) summarized the evidence for deposition in a rift zone with thin crust. This includes the abundant bimodal acid-basic volcanics, the high-Fe tholeiites, the progressive deepening of the environment and presence of the stratiform Broken Hill P b - Z n - A g deposit. The amount of extension and crustal thinning must have been considerable, to accommodate a sequence 7-9 km thick. The question must arise whether extension proceeded to the stage of formation of oceanic crust, and whether the later deformation was a result of collision involving subduction of that crust.

Was the WiUyama Supergroup deposited in an arc-trench system? The points in favour of an arc-trench system are: ( 1 ) the Fe-enriched tholeiitic composition of the basic gneisses is similar to basalts found on oceanic spreading ridges over hot spots (Clague and Bunch, 1976; Byerly et al., 1976). Considered in isolation this could be taken as geochemical evidence of an oceanic environment. (2) The intense folding, including large recumbent folds, could be readily explained by collision between an arc, microcontinent or continent to the east, and the Australian Archaean craton to the west. Morgan and Burke (1985) suggested that extensive areas (typically > 500 000 km 2) of 8 kbar metamorphism in granulite facies may represent deep levels of ancient collisional plateaus. Maximum pressure calculated for the Willyama Supergroup is 6 kbar (Phillips and Wall, 1981 ), and the area of exposed granulite facies is only about 4000 km 2. However, extensive areas of amphibolite facies Willyama Supergroup and possible equivalents occur in the Olary, Mt Painter, Jervois Range and Soldiers Cap areas. These could represent a dismembered collisional plateau. (3) The post-folding Mundi Mundi-, Cham-

321

pion- and Umberumberka-type granitoids are all leucocratic quartz-alkali feldspar-rich granitoids. Morgan and Burke (1985) state that such minimum melt granitoids are common in collisional plateaux• J. Pearce (personal communication, 1986) suggested that post-folding granitoids form after the collision is completed, as the collisional plateau begins to relax under the force of gravity. Lateral extension occurs, the thick pile of continental crust is thinned, and the temperature of some of the crustal rocks is increased as new thermal gradients are established. Melting of crustal rocks can then occur. The relaxation and extension phase, soon after F3, could have initiated the retrograde schist zones observed in the Willyama Supergroup. A simple model with the Willyama Supergroup deposited on oceanic crust, then scraped off during subduction and deformed during a collision, is difficult to accept. The tholeiitic rocks are a minor component in a sequence of siliceous metasediments and abundant probable acid volcanics. This is not a typical oceanic sequence. Also, S m / N d data (McCulloch and Hensel, 1984; McCulloch, in press) indicate that the material from which the quartzo-feldspathic rocks were derived, separated from the mantle between 2300 and 2100 Ma. Assuming the quartzo-feldspathic rocks were volcanics, this requires underlying continental crust. Fe-Rich tholeiites are also known from rift environments with thin continental crust, and bimodal acid-basic volcanism is common in rifts. Such a rift environment could fit an arctrench model, if the rift formed in an andesitic island arc or in an Andean-type arc above a subduction zone. In this model, the 2300-2100 Ma data would be the date at which subduction caused melting in the overlying mantle wedge, with intrusives lodging in the crust below the arc. The quartzo-feldspathic rocks formed from partial melting of these intrusives at ~ 1860 Ma. Further subduction is required between 1860 and 1660 Ma, leading to a collision. In the above model, subduction is required before, after and probably during Willyama de-

position. However, as in all Australian Proterozoic fold belts, there is no direct evidence of subduction, i.e., no ophiolites, andesites, blue schists or melanges. Possible (but perhaps not probable) explanations are: (1) that subduction was continuous, with no obduction; (2) that the andesitic arcs were systematically buried or subsided during development of later rifting with massive volcanism and sedimentation; (3) that trench complexes were buried during continental collisions. Perhaps blueschists were not formed in the early to middle Proterozoic, owing to global higher heat flow at that time (Grambling, 1981). Another objection to the above model is the low-pressure metamorphic path, which is inappropriate for a collision regime, and probably required addition of heat from mafic magma or mantle upwelling (Phillips and Wall, 1981 ). A further objection is the lack of a suture. On the contrary, tentative stratigraphic correlations have been established throughout the Broken Hill, Euriowie, Olary and Mt Painter Blocks (Willis et al., 1983a). Alternative models involving no oceanic crust

As stated above, there are no recorded ophiolites, andesites, blueschists or melanges in the Australian early-middle Proterozoic fold belts. These belts are characterized by paired acid-basic volcanism, or just basic volcanism. Most models developed for these fold belts involve intra-continental rifting. Etheridge et al. (1985, 1987) proposed a tectonic model for the development of early-middle Proterozoic fold belts in northern Australia. In this model Australia is compared with modern Africa. The Archaean craton was stationary with respect to the mantle; a polygonal pattern of convection cells in the mantle led to a pattern of rifts around a number of Archaean blocks. These rifts became the Proterozoic fold belts. No oceanic crust was formed and no subduction occurred

322

within the presently exposed area of the continent. Orogenesis was not related to plate collision, but may have been due to sudden change in the mantle convection pattern. It was characterized by high heat flow and absence of significant crustal overthickening. The data from Broken Hill fit some aspects of the northern Australian model, but the overall fit is incomplete. The S m - N d model age of 2300-2100 Ma for the Willyama Supergroup corresponds to the time at which basic magma separated from the mantle and underplated thinned Archaean crust, in the northern Australian model. In northern Australia there were two episodes of crustal stretching and deposition, separated by an orogeny at about 18801850 Ma. At Broken Hill there is only one continuous depositional sequence, the Willyama Supergroup, and as yet no evidence for the 18801850 Ma orogeny or associated I-type granitoids. The depositional history of the Willyama Supergroup has some similarity with the depositional cycles in northern Australia. These contain a lower 'rift' sequence, generally with bimodal volcanics, then a 'sag' phase, poor in volcanics, which is overlain by a turbidite phase containing abundant volcanogenic detritus. At Broken Hill the sequence up to the top of the Broken Hill Group could equate to the 'rift' phase, the Sundown Group could represent the 'sag' phase, and the Paragon Group the turbidite phase (the albitic psammites in the Bijerkerno Metasediments are possibly volcaniclastic). The deformation age of 1660 + 10 Ma at Broken Hill fits the second orogenic period in northern Australia, as does the style of deformation and the nature of the metamorphism. Although the Willyama Supergroup does not entirely fit the northern Australian model, the general interpretation of an ensialic rift environment has been supported by Willis et al. (1983a), and Laing and Barnes (1986). The latter proposed that the rocks were deposited in a rift, and that during deformation "the nappe pile perhaps formed a drape over closing 'failed rift' basement".

Wickham and Oxburgh (1985) proposed continental rifts as a general environment for all high-T/low-P metamorphism, using the Hercynian of the Pyrenees as a model. They described flat-lying isoclinal folds in association with the high-T/low-P metamorphism, and also formation of synmetamorphic leucogranite pods (analogies of which can be found at Broken Hill). The stratigraphic sequence in the Pyrenees extends from Cambrian to Carboniferous, and deformation/metamorphism occurred during the Carboniferous. Wickham and Oxburgh (1985) did not provide a mechanism for the formation of flat-lying folds, and the high thermal gradients were not convincingly explained. However, their model may provide a basis for further study of Australian early-middle Proterozoic fold belts, including the deformed Willyama Supergroup.

Summary The Willyama Supergroup can be placed in a plate tectonic model with oceanic crust and an island arc only with great difficulty, since there is no remnant of oceanic crust, andesitic arc, or trench complex and no suture identified. Also the high-T/low-P metamorphism and metamorphic path are not consistent with a subduction-related collision. An intra-continental rift environment appears more likely, but this presents some problems including: (1) the lack of a convincing mechanism for very strong compression, and formation of nappe-like folds, coincident with intrusion of basic magma or upwelling of mantle (to produce high- T / l o w - P metamorphism); (2) the lack of information as to what happened to underlying continental crust during the tremendous shortening experienced by the Willyama Supergroup during folding; (3) from a general consideration of heat flow (as expressed by P. Hoffman at the 1985 Dar-

323

win Conference), it appears anomalous that subduction was not involved in formation of any of the early-middle Proterozoic fold belts in a continent the size of Australia. The basis of this argument is that seafloor spreading is now by far the most efficient mechanism known for removing heat from the mantle, and no other mechanism of similar efficiency has been identified in the geological record.

Acknowledgements The manuscript benefited considerably from comments by Drs W. Laing, B. Hobbs and L. Wyborn, although these reviewers take no responsibility for the views expressed. Mrs G. Carruthers and Mrs C. Tester are thanked for typing the manuscript. Diagrams were drafted in Cartography Section, Geological Survey of New South Wales. The paper is published with the permission of the Secretary, Department of Mineral Resources.

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