Willyama Supergroup Research Articles

Prolonged high thermal gradient metamorphism in the Curnamona Province, south-central Australia, during the latter stages of Nuna assembly

Granulite-facies metasedimentary rocks from the Broken Hill Block of the Curnamona Province, south-central Australia, record a prolonged period (c. 1630–1550 Ma) of high thermal gradient metamorphism and anatexis. We present monazite and zircon U–Pb geochronological and REE geochemical data from garnet-bearing paragneisses of the Willyama Supergroup, coupled with garnet trace element mapping and mineral equilibria modelling, to develop a petrochronological model for the Palaeo–Mesoproterozoic metamorphic evolution of this region. Our results indicate that the Curnamona Province underwent an early phase of high-temperature–low-pressure metamorphism between c. 1630–1610 Ma. Peak metamorphism involving wholesale anatexis subsequently occurred under an apparent thermal gradient of 150–170 °C/kbar at c. 1600–1580 Ma during regional compression. Monazite U–Pb dates reveal that suprasolidus conditions were maintained for at least 30 Myr, with final melt crystallisation potentially occurring as late as c. 1550 Ma, indicating that the system remained hot for an extended duration. We suggest that high thermal gradient metamorphism in the Willyama Supergroup was driven by elevated levels of radiogenic heat production hosted within the Willyama succession. Temperatures may have been initially boosted by low-to-moderate degrees of crustal extension, although there is a dearth of coeval magmatism and structural evidence to support this claim. Peak metamorphism coincides with the timing of compressional orogenesis and related metamorphism in other Proterozoic terranes that are inferred to have occupied the eastern margin of proto-Australia. Many of these terranes also exhibit prolonged metamorphic histories and elevated levels of radiogenic heat production, suggesting that radiogenic heating may have had a major influence in pre-disposing these terranes to be the locus of metamorphism and deformation during the final assembly of Nuna.

Timing and significance of pre-Olarian metamorphism in the Willyama Supergroup, Broken Hill, Australia

In the Willyama Supergroup of the Curnamona Province, the intensity of granulite facies Olarian metamorphism at c. 1600 Ma has hampered efforts to elucidate the timing and nature of preceding lower-grade metamorphic events. Previous dating of pre-peak monazite inclusions in aluminous metapelites has been complicated by potentially incomplete isotopic ‘shielding’ by their host porphyroblasts, owing to pervasive deformation and recrystallisation of these rocks during the c. 1600 Ma event. Near the Broken Hill Pb-Zn orebody, quartz-feldspar-biotite-garnet (‘Potosi-type’) gneiss proved more resistant to recrystallization during granulite facies metamorphism: new Sensitive High Resolution Ion MicroProbe (SHRIMP) U-Pb analyses of monazite from this rock delineated a population of relatively Y-rich (median 0.96 wt%) cores at 1636.8 ± 4.4 Ma (95% confidence), overprinted by Y-poor (median 0.10 wt%) rims and domains at 1597.3 ± 3.3 Ma. Garnet is a sink for Y in metamorphic rocks, which suggests that Y-rich c. 1637 Ma monazite growth predated garnet formation (and possibly occurred at relatively low metamorphic grade), whereas the Y-poor nature of the c. 1597 Ma rims and domains implies that dissolution and reprecipitation during Olarian metamorphism postdated porphyroblastic garnet growth. Coexisting low-Th/U zircon rims are dominated by a 1600.7 ± 3.6 Ma population, emphasising that the event responsible for c. 1637 Ma monazite growth was largely invisible to zircon. Interlayered mafic granulite 70 m away yielded partially recrystallised zircons with dark-CL internal domains which establish a minimum age of 1639 ± 8 Ma for the igneous precursor. Recrystallisation was accompanied by actinide depletion, with bright-CL, U-poor and low-Th/U domains dated at 1596 ± 8 Ma. Our new 1640–1635 Ma dates constitute robust isotopic evidence for pre-Olarian mafic magmatism and metamorphism within the lower Willyama Supergroup. We infer an extensional environment in the upper mid-crust for this tectonothermal activity, coeval with ongoing sedimentation and deposition of the upper Paragon Group at the surface. Extension of the eastern Curnamona Province at c. 1640 Ma is coeval with sediment-hosted base-metal mineralisation in the Mount Isa Inlier and the McArthur Basin, and is consistent with tectonic regimes prevailing throughout Proterozoic southern, central and eastern Australia at that time.

The Broken Hill Block, a new structure and mineral exploration target areas

The Broken Hill Block contains the Paleoproterozoic Willyama Supergroup, host to the world-renowned lead–zinc–silver deposits known as the Line of Lode. A new large-scale geological structural interpretation has been developed based on the construction of 11 regional-scale cross-sections and one longitudinal section. Over an extended period of major folding during the Olarian Orogeny, two distinct sets of regional folds were produced. The longitudinal folds are of similar-type and mostly trend northeast. The cross-folds are subtle, mostly trend at right angles to the longitudinal folds and are open-style. Longitudinal folds in or near the Line of Lode have been reinterpreted. A newly interpreted major slump fold, central to the Line of Lode, allows correlation of rock units from east to west. The well-known Western Anticline and Eastern Syncline, which host the main Line of Lode, are normal drag folds, formed during the initial folding with their axes folded by the later slump folding. The lead lodes are older than the zinc lodes. We interpret previously mapped longitudinal folds, as formed by successive pulses within the one tectonic event. This resulted in increased folding complexity on a local scale, but not a regional scale, and with no substantive impact on the regional structure. Pitch and plunge data were used to identify 23 cross-folds, some more than 50 km long. Away from the Line of Lode, the new structure is simpler than previous interpretations and with no tectonic inversion of the stratigraphy, other than gravity slumping, and no regional-scale nappe structures. The combination of cross-folds and longitudinal folds has produced regional-scale domes and troughs throughout the block. The intersection of longitudinal fold axes with cross-fold crests are proposed to be particularly prospective for metallic sulfides with 25 named and prioritised mineral exploration target areas identified. The cross-folding has caused the ore bodies to plunge steeply to great depths north of the Line of Lode. Further to the northeast, the plunge is reversed bringing strata, containing potential northern extensions of the Line of Lode, closer to the surface. The retrograde schist zones, which are regional-scale faults or shears that have undergone retrograde metamorphism, postdate the longitudinal and cross-folding and are not considered to be related to thrust structures. Displacement of strata and structures is evident across the zones and is generally well understood. KEY POINTS A new set of regional folds, termed cross-folds, has been identified. In the Line of Lode, large-sale longitudinal folds include a major slump fold. The stratigraphic sequence is not inverted; nappes are for the most part not present. Broken Hill-type lead–zinc–silver deposits are preferentially found close to the intersection of longitudinal and cross-fold axes.

The interplay of structure and metamorphism at Broken Hill, NSW, Australia

Downward younging of stratigraphy in several large F2 fold structures in the Paleoproterozoic Willyama Supergroup in the Broken Hill area has previously been explained by very large-scale F1 nappe folds followed by relatively upright F2 and F3 folds. In three major structures, the Mt Robe–East Eldee Structure, the Paps Synform and the Bijerkerno Synform, the andalusite-sillimanite metamorphic isograd parallels stratigraphy for tens of kilometres. Given the conformability of these segments of isograd in plan view, it is reasonable to suggest that in two of the structures, sillimanite-grade rocks rest on andalusite-grade rocks. In all cases the younger stratigraphic units are at lower metamorphic grade than the older stratigraphic units, whether the stratigraphy is upright, as in the Bijerkerno Synform, or inverted, as in the Mt Robe-East Eldee Structure and the Paps Synform. Major bodies (mini-batholiths) of pegmatite are confined stratigraphically to below the Sundown Group, whether the stratigraphy is upright or overturned. In the D1 nappe model the andalusite-sillimanite isograd and the emplacement of pegmatite mini-batholiths must both predate the D1 nappe folding. There is evidence that andalusite preceded D1, but not sillimanite, which is a defining mineral in the first-generation schistosity (S1). The large-scale nappe model is also not compatible with calculated metamorphic pressures. An alternative model involving extensional D1, followed by D2 sheath folds, explains downward-facing metamorphic facies but is not viable. Large-scale fold structures in the Mt Robe Block can be explained by recumbent folding at a moderate scale, followed by inclined sheath folding. The downward-facing stratigraphy and metamorphic facies in the Mt Robe and Paps synforms are interpreted to lie on the underside of inclined F2 sheath-like folds. KEY POINTS In major fold structures in the Willyama Supergroup, upper stratigraphic units are at a lower metamorphic grade than lower stratigraphic units, whether the fold is upward-facing or downward-facing. In two large F2 folds sillimanite-grade rocks showing melting are situated above andalusite-grade rocks. Pegmatitic mini-batholiths show semi-regional stratigraphic control below the top of the Broken Hill Group, whether they are in upward or downward-facing fold limbs. A new structural model with D1 recumbent folding, followed by D2 sheath-like folding, is based on structures in the Mt Robe Block.

Using amphibolite chemistry to assist structural and stratigraphic interpretation at Broken Hill, Australia

Multiple samples taken across the strike of conformable syn-depositional amphibolite (meta-dolerite) bodies in the Willyama Supergroup, Broken Hill, showed that all exhibit chemical variation, mostly consistent with igneous fractionation. The fractionation trends are characterised by very significant reduction in MgO, Ni and Cr from base to top of individual sills, increase in Zr, and less regular variation in Fe and Ti contents. This asymmetric chemical zonation can be used to determine the original ‘way up’ in amphibolite sills as thin as 12 m. Given that the amphibolite sills were emplaced prior to folding of the enclosing sequence, the way up direction of the sills is also the younging direction of the enclosing metasediments, making this a useful tool in stratigraphic and structural studies of folded rocks. Outcropping conformable amphibolite bodies were selected for across-strike sampling in variably well-interpreted stratigraphic positions. Despite uncertainties related to overly wide-spaced sampling and possible internal structural complexity, in most cases the fractionation trend agreed with the prior stratigraphic interpretation, providing an independent stratigraphic ‘way up’ or ‘younging’ direction and demonstrating the potential usefulness of the technique. Subsequent sampling and analysis of drill core through amphibolite sills in the Broken Hill mine area tested alternative interpretations of the enclosing stratigraphy and structure. Fractionation patterns in three intersections of the amphibolite immediately west of the orebody clearly show younging towards the orebody. Intersections of the thicker amphibolite immediately east of the orebody reveal complex and conflicting fractionation patterns indicating internal structural complexity, consistent with this amphibolite being involved in a major thrust zone. The results in the mine area are in agreement with those of a previous study based on graded-bedding observations in the enclosing metasediments. The identification of igneous fractionation patterns in the amphibolites confirms the interpretation that the amphibolite bodies were in fact sills rather than lavas. The extremely high iron contents of some of the amphibolite sills are the result of extreme fractionation of a tholeiitic magma, not metasomatism. KEY POINTS Most Broken Hill amphibolites retain relatively simple igneous fractionation patterns. The fractionation patterns can indicate the stratigraphic younging direction. The stratigraphy west of the Broken Hill orebody youngs eastward towards the orebody. The amphibolite east of the orebody shows complex deformation.

Sedimentology in metamorphic rocks, the Willyama Supergroup, Broken Hill, Australia

ABSTRACTIn the upper greenschist to granulite grade rocks of the Willyama Supergroup at Broken Hill, Australia, earlier recognition of metamorphosed graded bedding in siliciclastic metasedimentary rocks led to interpretations of these rocks as deep-water turbidites. However, graded beds can also be deposited in shelfal environments below storm wave base. This study identified other tempestite features including wave oscillation ripples, hummocky cross-stratification and swaley cross-stratification indicating that deposition took place above the wave base of the larger storms.Albitised metasedimentary rocks of the upper Thackaringa Group show structures such as swaley cross-stratification typical of shallow-water conditions above fair-weather wave base. Deposition of the Broken Hill Group commenced with muddy Allendale Metasediments conformable on the Thackaringa Group. The Ettlewood Calc-Silicate Member, originally a dolomitic, siliceous sediment, is interpreted as coastal sabkha indicating onset of a marine transgression. The Parnell Gneiss represents a volcanic or volcaniclastic interruption, heralding gradually increasing input of sand in the Freyers Metasediments reaching a maximum in middle Freyers Metasediments, followed by an abrupt reversion to mud, still influenced by wave action. An open marine shelf is interpreted, possibly 30 m deep (no more than 100 m) in the final stage of a developing rift. The Broken Hill Group terminated with the massive Hores Gneiss volcanic unit.Sedimentation of the siliciclastic Sundown Group took place in similar conditions, commencing with a muddy interval overlying the Hores Gneiss. The shallowing produced by ∼90 m thickness of volcanic/volcaniclastic Hores Gneiss was compensated by subsidence.Paragon Group deposition commenced with substantial black mud, resulting from isolation from the sand supply and probably isolation from the sea. A fresh connection to the sea led first to the deposition of dolomitic carbonate (King Gunnia Calc-Silicate Member), then to deposition of parallel-laminated fine sand below wave base (upper Cartwrights Creek Metasediments), followed by ripple cross-laminated sand above wave base (Bijerkerno Metasediments). The Dalnit Bore Metasediments show abundant very thin graded silt–mud units possibly deposited below storm wave base, and thicker units of stacked wave oscillation ripples deposited above the wave base of larger storms.The Broken Hill orebody is hosted by altered Broken Hill Group metasedimentary rocks deposited at water depths of ∼30 m. Unless the ore fluid temperature was less than 150°C, it is likely that the orebody formed below the seafloor: at such shallow-water depths, the confining pressure would be inadequate to suppress boiling of hotter rising hydrothermal fluids.

The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China

The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60Ga. U–Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649±14Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900Ma to 1850–1800Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan–Dahongshan–Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan–Dongchuan–Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia.

Influence of Preexisting Low Metamorphic Grade Sedimentary Successions on the Distribution of Iron Oxide Copper-Gold Mineralization in the Olympic Cu-Au Province, Gawler Craton

The primary driver for mineralization in the Olympic Cu-Au province of the eastern Gawler craton, South Australia, was a major early Mesoproterozoic thermal event, partly manifest as extensive A-type, I-type, and mafic magmatism. Magmatism of this age is present across the Gawler craton and the Curnamona province to the east, yet hematite-rich, iron oxide copper-gold (IOCG) deposits are apparently restricted to a north-south belt along the eastern margin of the Gawler craton. Whereas geochemical differences between the early Mesoproterozoic granites in the eastern versus western Gawler craton appear to influence the type of mineralization in these regions, there are also stark differences in the nature of the rocks into which these magmas intruded. Metamorphic data from the Gawler craton and Curnamona province show that most of the western, southern, and northern Gawler craton underwent amphibolite- to granulite-facies metamorphism, either during a ca. 2465 to 2410 Ma event (Sleafordian orogeny), a ca. 1730 to 1690 Ma event (Kimban orogeny), or both. In contrast, across the eastern Gawler craton and Curnamona province, potentially evaporate-bearing sedimentary successions including the ca. 1750 Ma Wallaroo Group and ca. 1710 to 1640 Ma Willyama Supergroup, were largely unmetamorphosed prior to the early Mesoproterozoic magmatic and metallogenic event. Thus, a clear spatial partitioning of Paleoproterozoic orogenesis exists in the western, southern, and northern Gawler craton, including a broad spatial correlation between rocks of low metamorphic grade and the region of IOCG mineralization. Multiple fluid sources (e.g., sedimentary, metamorphic, and magmatic) were important in generating the chemistry suitable to precipitate Cu-Au-U-REE–bearing minerals in the Olympic Cu-Au province. Therefore, we suggest that the preservation of rocks at low metamorphic grade containing formation waters and porosity provided a fertile geochemical environment into which high-temperature, felsic and mafic magmas, and their associated hydrothermal cells were emplaced and developed. This example from the eastern Gawler craton and Curnamona province suggests that prior orogenic events can operate as a first-order control on the location of large mineral provinces.

Magnetite-bearing zones in metasediments at Broken Hill, Australia: signatures of black smokers?

High-quality aeromagnetic data show linear magnetic zones within generally non-magnetic metasediments of the Broken Hill and Sundown Groups in the medium- to high-grade metamorphic Willyama Supergroup at Broken Hill, Australia. On-ground investigations show that these anomalies are generated by small amounts of magnetite within the metasediments. The magnetite concentrations are controlled by bedding and transgressed by schistosity. At the outcrop scale, the bedding-parallel nature of the magnetite concentrations is illustrated by decimetre-scale mapping with a magnetic susceptibility meter. At a 1:25 000 scale, there are areas where the relationship between magnetic anomalies and bedding is either unclear or apparently transgressive. Resolution of such situations requires an order of magnitude more detail in both the magnetic data and the geological mapping. This was provided by ground magnetometer surveying, magnetic susceptibility meter surveying and detailed lithological and structural mapping. In the areas studied, no evidence was found for genuinely transgressive relationships between magnetite concentrations and bedding. On a microscopic scale, the magnetite grains are independent of the other main Fe-bearing minerals with no replacement of garnet or biotite by magnetite. Thus, the magnetite did not result from influx of an oxidising fluid during or after high-grade metamorphism. Given that the magnetite concentrations are bedding-parallel and that the magnetite grains are not the result of replacement of ferrous iron-bearing minerals, it is concluded that the magnetite (or its ferric iron precursor) was an original component of the metasediments in these zones. The magnetite does not form clusters as expected with detrital magnetite. The magnetite-bearing zones can be either pelitic or psammitic, so it appears there was not a porosity/permeability control. The most likely origin for the magnetite or its precursor is a rain of particles onto the seafloor from an overlying oxidised water column. Such particles may have been derived from distal plumes resulting from spent orebody-forming fluids. The close relationship in places, between banded iron formation and magnetite-bearing metasediment zones supports this suggestion. Magnetite in metasediments close to the base of Sundown Group may have resulted from spent fluids related to formation of the Broken Hill orebody. However, there are magnetite-bearing metasediment zones within the Broken Hill Group and also stratigraphically higher in the Sundown Group that may be related to as yet undiscovered orebodies. The detailed lithological and structural mapping resulted in some new insights into the structural behaviour of Willyama Supergroup metasediments. In the Yanco Glen southeast area, the distribution of lithologies is largely controlled by D2 shears, producing a series of slices that resemble bedding on aerial photographs. In the Monuments area, D2 shears also control the distribution of lithologies, but extensive melting of psammites and flow parallel to S2 is an added feature.

Lead and Nd isotopic evidence for a crustal Pb source of the giant Broken Hill Pb–Zn–Ag deposit, New South Wales, Australia

Major element, trace element, Nd isotopic data and U–Pb zircon ages of quartz±Fe-oxide±garnet±accessory mineral-bearing rocks (hereafter referred to as QFeGA), as well as Pb isotope data from ca. 1685Ma high Fe–Ti meta-tholeiites and the Rasp Ridge Gneiss are presented. These rocks are found in the Proterozoic Willyama Supergroup, Broken Hill Domain, southeastern Australia. U–Pb zircon dating of the QFeGA rocks yielded ages of ca. 1600Ma, comparable to the accepted age of the Olarian Orogeny, with no evidence of inherited zircons from a detrital component. Despite the U/Pb age, the lack of detrital zircons in combination of bulk rock major and trace element geochemistry, suggests that the QFeGA rocks formed during the early stages of the geological evolution of the Broken Hill Domain (ca. 1710–1685Ma) in a progressively deepening ensialic rift environment, due to seafloor hydrothermal activity similar to hydrothermal sediments at present day spreading centres, rather than during the Olarian Orogeny. The Nd isotope compositions of the QFeGA rocks (εNd=−2.2 to −6.6) are comparable to those of other garnet-rich rocks associated with the Broken Hill deposit and with the metasedimentary rocks of the Willyama Supergroup. This suggests that the hydrothermal fluids responsible for the deposition of the QFeGA protoliths were in equilibrium with the Willyama supracrustal sequence.Lead isotope data from the high Fe–Ti meta-tholeiites and the Rasp Ridge Gneiss lie within error on the same 1685Ma 207Pb/206Pb reference isochron, with an initial Pb isotope composition similar to that of the Broken Hill orebody (galena). This is inconsistent with independent major and trace element and Nd isotopic evidence showing that the mafic and felsic melts were derived from end-member mantle and crustal sources respectively. Hence a common source for the Pb of the mafic and felsic rocks is likely. It is inferred that this Pb source reflects a period of regional Pb isotope homogenisation due to pervasive supracrustal hydrothermal flux at the time of emplacement of the mafic and felsic rocks (ca. 1685Ma) and coincident with formation of the QFeGA rocks and the Broken Hill deposit. It is suggested that the Pb (and other metals) scavenged by this hydrothermal system from the Willyama Supergroup metasedimentary sequences provided the metals for the Broken Hill ore body.

Boron- and phosphate-rich rocks in the Larsemann Hills, Prydz Bay, East Antarctica: tectonic implications

Abstract Granulite-facies paragneisses enriched in boron and phosphorus are exposed over c. 15×5 km 2 in the Larsemann Hills, Antarctica. The most widespread are biotite gneisses containing centimetre-sized prismatine crystals, but tourmaline metaquartzite and borosilicate gneisses are richest in B (676–19 700 µg/g or 0.22–6.34 wt%; B 2 O 3 ). Chondrite-normalized rare-earth element (REE) patterns give two groups: (1) La N >150, Eu*/Eu<0.4, which comprises most apatite-bearing metaquartzite and metapelite, tourmaline metaquartzite, and Fe-rich rocks (up to 2.3 wt%; P 2 O 5 ); (2) La N <150, Eu*/Eu > 0.4, which comprises most borosilicate and sodic leucogneisses (2.5–7.4wt%; Na 2 O). Enrichment in boron and phosphorus is attributed to premetamorphic hydrothermal alteration, either in a rifted, most likely marine basin or in a mud volcanic system located inboard of a c. 1000 Ma continental arc that was active along the leading edge of the Indo-Antarctic craton. This margin developed before collision with the Australo-Antarctic craton ( c. 530 Ma) merged these rocks into Gondwana and sutured them into their present position in Antarctica. Rocks lithologically similar to those in the Larsemann Hills include prismatine-bearing granulites in the Windmill Islands, Wilkes Land, and tourmaline–quartz rocks, sodic gneisses and apatitic iron formation in the Willyama Supergroup, Broken Hill, Australia.

Paleoproterozoic–earliest Mesoproterozoic basin evolution in the Mount Isa region, northern Australia and implications for reconstructions of the Nuna and Rodinia supercontinents

Paleoproterozoic-earliest Mesoproterozoic sequences in the Mount Isa region of northern Australia preserve a 200 Myr record (1800-1600 Ma) of intracontinental rifting, culminating in crustal thinning, elevated heat flow and establishment of a North American Basin and Range-style crustal architecture in which basin evolution was linked at depth to bimodal magmatism, high temperature-low pressure metamorphism and the formation of extensional shear zones. Rifting initiated in crystalline basement ≥1840 Ma old and produced three stacked sedimentary basins (1800-1750 Ma Leichhardt, 1730-1670 Ma Calvert and 1670-1575 Ma Isa superbasins) separated by major unconformities and in which depositional conditions progressively changed from fluviatilelacustrine to fully marine. By 1685 Ma, a deep marine, turbidite-dominated basin existed in the E and basaltic magmas had evolved in composition from continental to oceanic tholeiites as the crust became increasingly thinned and attenuated. Except for an episode of minor deformation and basin inversion at c. 1640 Ma, sedimentation continued across the region until onset of the Isan Orogeny at 1600 Ma. A near-identical record of crustal thinning and basaltic magmatism accompanied basin formation (lower Willyama Supergroup) in the formerly contiguous Broken Hill region from 1730-1670 Ma. This was followed by further extension and a second phase of basin development that lasted until at least 1640 Ma. Modern-day rifted continental margins preserve a comparable record of crustal thinning and near-continuous basin formation over 100-200 Myr timescales, supporting suggestions that the late Paleoproterozoic-early Mesoproterozoic rift basins of Mount Isa and Broken Hill similarly evolved to continental breakup and formed part of a continental margin sequence no later than 1640 Ma and possibly as early as 1670 Ma. This rifted margin predates assembly and breakup of the Neoproterozoic Rodinia supercontinent to which Australia once belonged and best accords with a pre-Rodinia, SWEAT-like super-continent (Nuna) that matches the E-facing late Paleoproterozoic-early Mesoproterozoic rift sequences of eastern Australia against rocks of comparable age in western Canada. Reconstructions of Rodinia (AUSWUS) based on the distribution of Grenville-age orogenic belts that coincidently position the continental rift sequences of Broken Hill along strike from more juvenile 1700-1650 Ma accreted terranes in the SW United States (Yavapai and Mazatzal provinces) are only possible if the proposed alignment of terranes is not original but an artefact of Neoproterozoic supercontinent assembly. The SWEAT hypothesis avoids this complication but, like AUSWUS, presupposes that eastern Australia and western Laurentia remained juxtaposed throughout the Mesoproterozoic until onset of Rodinia breakup after 830 Ma.

Provenance of late Paleoproterozoic cover sequences in the central Gawler Craton: exploring stratigraphic correlations in eastern Proterozoic Australia using detrital zircon ages, Hf and Nd isotopic data

Provenance data from Paleoproterozoic and possible Archean sedimentary units in the central eastern Gawler Craton in southern Australia form part of a growing dataset suggesting that the Gawler Craton shares important basin formation and tectonic time lines with the adjacent Curnamona Province and the Isan Inlier in northern Australia. U–Pb dating of detrital zircons from the Eba Formation, previously mapped as the Paleoproterozoic Tarcoola Formation, yields exclusively Archean ages (ca 3300–2530 Ma), which are consistent with evolved whole-rock Nd and zircon Hf isotopic data. The absence of Paleoproterozoic detrital grains in a number of sequences (including the Eba Formation), despite the proximity of voluminous Paleoproterozoic rock units, suggests that the Eba Formation may be part of a Neoarchean or early Paleoproterozoic cover sequence derived from erosion of a multi-aged Archean source region. The ca 1715 Ma Labyrinth Formation, unconformably overlying the Eba Formation, shares similar depositional timing with other basin systems in the Gawler Craton and the adjacent Curnamona Province. Detrital zircon ages in the Labyrinth Formation range from Neoarchean to Paleoproterozoic, and are consistent with derivation from >1715 Ma components of the Gawler Craton. Zircon Hf and whole-rock Nd isotopic data also suggest a source region with a mixed crustal evolution (εNd –6 to –4.5), consistent with what is known about the Gawler Craton. Compared with the lower Willyama Supergroup in the adjacent Curnamona Province, the Labyrinth Formation has a source more obviously reconcilable with the Gawler Craton. Stratigraphically overlying the Eba and Labyrinth Formations is the 1656 Ma Tarcoola Formation. Zircon Hf and whole-rock Nd isotopic data indicate that the Tarcoola Formation was sourced from comparatively juvenile rocks (εNd –4.1 to + 0.5). The timing of Tarcoola Formation deposition is similar to the juvenile upper Willyama Supergroup, further strengthening the stratigraphic links between the Gawler and Curnamona domains. Additionally, the Tarcoola Formation is similar in age to extensive units in the Mt Isa and Georgetown regions in northern Australia, also shown to be isotopically juvenile. These juvenile sedimentary rocks contrast with the evolved underlying sequences and hint at the existence of a large-scale ca 1650 Ma juvenile basin system in eastern Proterozoic Australia.

The Mt. Mulga barite–magnetite–copper–gold mineralisation, Olary Domain, South Australia

The Mt. Mulga barite Cu–Au deposit in the central part of the Olary Domain is hosted by the Palaeoproterozoic Willyama Supergroup of the Curnamona Craton. The lenticular, stratiform ore bodies are hosted by the Ethiudna Subgroup at and near the contact to the underlying Wiperaminga Subgroup. The ore bodies consist of massive, coarse crystalline barite up to 6 m thick interspersed with granular (< 2 cm) magnetite and quartz. In the southern section the massive barite contains Cu–Au mineralisation consisting of chalcopyrite, bornite and native gold. Along strike the mineralised unit grades into a stratiform quartz–magnetite layer which can be traced for several kms. Several smaller (< 1 m thickness) barite lenses occur about 1.5 km north of main barite body in the upper parts of the Ethiudna Subgroup close to the contact to the Saltbush Subgroup. A pronounced positive Eu-anomaly of the mineralisation hosting unit indicates a reducing character of the mineralising fluids and is characteristic for exhalative sediments. 87Sr/ 86Sr-isotope ratios of barite fall into a narrow range of 0.708 to 0.713 suggesting an origin of the mineralising fluids from the underlying Lower Willyama sequence. The δ 34S values of barite from the main ore body range from + 13 and + 16‰, while the barites of the other ore bodies range from + 6 and + 11‰. Chalcopyrite revealed δ 34S values of − 1.3 to + 4.5‰. Barite is suggested to be formed by mixing of a hydrothermal, reduced, hydrothermal Ba-rich fluid with oxidised, sulphate-rich water of a non-marine basin. Although an epigenetic formation of the Cu–Au mineralisation during the Olarian Orogeny is possible, we suggest a cogenetic formation with the barite–iron-oxide mineralisation.

Decoding Mesoproterozoic and Cambrian metamorphic events in Willyama Complex metapelites through the application of Sm–Nd garnet geochronology and P–T pseudosection analysis

The pressure–temperature ( P–T) path for the Palaeoproterozoic Willyama Supergroup rocks in the Olary Domain, South Australia has been reconstructed through detailed petrographic observations, in conjunction with calculation of compositionally specific P–T pseudosections of metapelitic rock units and Sm–Nd garnet geochronology. The P–T path for the Willyama Complex has historically been interpreted to follow a single anticlockwise path, however the results of this study demonstrate that this path can be better described by two metamorphic events (M 1 and M 2) separated by 1100 Ma. The M 1 event occurred at c. 1600 Ma and was associated with high temperature–low pressure metamorphism. Sm–Nd garnet geochronology constrains the timing of garnet growth at c. 1585 Ma. The growth of large andalusite porphroblasts during M 1 exerted a first order control on the bulk composition during the subsequent M 2 event. Mineral chemistries coupled with quantitative phase diagrams constrain peak conditions to be in the order of c. 550°C and 5.5 kbar during M 2. The identification of two metamorphic events calls into question interpretations of metamorphic core complex formation and the single anticlockwise P–T paths being associated with the early stages of the tectonic evolution of the terrain.

Geochemistry of Garnet-Rich Rocks in the Southern Curnamona Province, Australia, and Their Genetic Relationship to Broken Hill-Type Pb-Zn-Ag Mineralization

Garnet-rich rocks are locally associated with the giant Paleoproterozoic Broken Hill Pb-Zn-Ag deposit and hundreds of smaller Broken Hill-type occurrences in the southern Curnamona Province, Australia; however, others are present unrelated to sulfides. Mn-bearing garnet-rich rocks are conformable to bedding in metasedimentary rocks of the Paleoproterozoic Willyama Supergroup and, together with sulfide-bearing zones, are spatially related to amphibolite and/or felsic volcanic rocks or shallow granite intrusions. The stratiform garnet-rich rocks can be subdivided into two groups: Mn-rich varieties typical of the stratigraphic hanging wall of the Broken Hill deposit and Mn-poor and Fe-rich varieties that are typical of the footwall and are regionally widespread. These garnet-rich rocks formed on the floor of a rifted basin as a mixture of chemical precipitates and detrital sediments and were subsequently metamorphosed to the amphibolite or granulite facies during the Olarian orogeny. A plot of Fe/Ti versus Al/(Al + Fe + Mn) ratios of these rocks suggests equal proportions of hydrothermal and detrital components in the precursor phases. Low Co + Cu + Ni contents are consistent with the input of a small hydrogenous component. Quartz garnetite and garnet-gahnite rocks in the stratigraphic footwall of the Broken Hill deposit (Zn-rich, Cu-poor B and C lodes), and garnet-rich rocks elsewhere in the southern Curnamona Province that are Mn poor and Fe rich, exhibit chondrite-normalized rare earth element (REE) patterns with negative Eu anomalies, which indicate that the precursor minerals formed from hydrothermal fluids at <300°C and under relatively low f O2 conditions. Garnetite and hedenbergite-, bustamite-, and rhodonite-bearing garnet-rich rocks from the stratigraphic hanging wall of the Broken Hill deposit (Pb-rich 2 and 3 lenses and the A lode) are Mn and Ca rich, Fe poor, and have positive Eu anomalies. Precursors to these Mn-rich rocks formed from relatively cool hydrothermal fluids (~250°C) at f O2 conditions higher than those that formed Mn-poor, Fe-rich garnet- and gahnite-rich rocks at Broken Hill. Correlations among various elements show that the presence of positive or negative Eu anomalies in garnet-rich rocks depends on the composition of the precipitate (Fe, Mn, and trace metal content) as well as the chemical, crystallographic, or sorption control that the precursor Fe-Mn oxyhydroxides/oxides/carbonates had upon the incorporation of Eu. Garnet-rich rocks from the Broken Hill deposit have higher average Mn, K, Rb, Eu, Ga, Cs, Cu, Pb, Zn, As, Cd, Sb, Ag, W, and Au contents than those from other locations in the southern Curnamona Province, and these are more enriched in Fe, Mg, Ni, P, V, Co, Sc, Sr, Y, and U, which indicates a larger hydrogenetic component to their precursor minerals. Enrichments in Mn, S, Ga, Eu, Cu, Pb, Zn, As, Cd, Sb, Ag, and Au, and a positive Eu anomaly in garnetite can be used as exploration guides in the search for Broken Hill-type deposits by indicating close proximity to mineralization.

Source and significance of the felsic magmatism in the Paleoproterozoic to Mesoproterozoic Broken Hill Block, New South Wales

Major, trace, rare-earth elements and isotopic (Sm – Nd and Rb – Sr) data from the ca 1704 – 1685 Ma Alma, Farmcote, Rasp Ridge and Hores-Potosi felsic magmatic gneisses and the ca 1600 Ma syn-orogenic felsic intrusions of the Willyama Supergroup in the Broken Hill Block of western New South Wales are documented. The ca 1704 – 1685 Ma felsic melts were generated by anatexis of the Willyama Supergroup metasediments with variable degrees of mixing of a juvenile, mantle-derived component represented by coeval high Fe – Ti metatholeiitic rocks. This is consistent with previous interpretations for bimodal magmatism occurring in an extensional environment with an elevated geothermal gradient driven by lithospheric thinning and mafic magmatism. Interpretations involving the presence of a mafic underplate as a source for the ca 1704 – 1685 Ma felsic melts are not supported by these data. The ca 1600 Ma syn-orogenic felsic intrusions are a direct product of partial melting of the sedimentary sequences of the Willyama Supergroup as a response to the high-temperature, low-pressure amphibolite- to granulite-facies metamorphic event accompanying the Olarian Orogeny. The presence of an Archean basement for the Willyama Supergroup remains unclear, although if this basement exists it was not sampled during the period of felsic magmatic activity that took place in the Broken Hill Block between ca 1710 and 1600 Ma.

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

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 km 2 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.

Tectonic setting and provenance of the Paleoproterozoic Willyama Supergroup, Curnamona Province, Australia: Geochemical and Nd isotopic constraints on contrasting source terrain components

The Curnamona Province Paleoproterozoic Willyama Supergroup was deposited in one of a series of basins developed along the eastern margin of the Proterozoic Australian continent. Linked tectonic evolutionary paths have previously been drawn between the northern and southern Australian cratons, based in part on similar depositional ages and stratigraphy, and tectonic evolutions of the sedimentary basins of this region. Detailed geochemical and Nd isotopic analyses from the Paleoproterozoic Willyama Supergroup suggest an intracratonic source for the lower parts of the sequence, with introduction of a significantly more juvenile source region for the ca. ≤1650 Ma uppermost part, marking a significant change in tectonic regime. The geochemical and Nd isotopic signature of both mature quartz-rich and pelitic sedimentary rocks of the lower part of the Willyama Supergroup support a dominant intracratonic central and northern Australian provenance, marked by a recycled and felsic upper crustal composition unusually enriched in REE and incompatible elements. Sedimentary REE abundance patterns double those of the Post-Archean Australian Shale average. Initial ɛ Nd values between −7 and −4 suggest old upper crustal recycled source basement. In contrast, the uppermost part of the Willyama sedimentary section has REE patterns much more like Post-Archean upper crustal shale average, and is characterised by more primitive initial ɛ Nd values of around −3 to 0. Significant ca. 1650 Ma felsic source terranes with such a juvenile Nd isotope signature are unknown in Australia. The data suggest proximity of a relatively juvenile as yet unidentified source to the present-day east of the basin. Some pre-Rodinian reconstruction models place eastern Australia proximal to southwestern Laurentia, which contains rocks of appropriate geochemical and isotopic signatures and could have delivered juvenile felsic sediment into the Willyama basin.

Geochronology of Willyama Supergroup metavolcanics, metasediments and contemporaneous intrusions, Broken Hill, Australia*

New SHRIMP U – Pb dating of the rocks from the Willyama Supergroup clarifies the geochronology of the lower to middle parts of the sequence and contemporaneous intrusions, and confirms that the prospective upper part of the Broken Hill Group extends into South Australia. In the Rantyga Group that constitutes the Redan geophysical zone in the southeastern part of the Broken Hill Domain, the relative stratigraphic positions of the Redan Gneiss (lowermost) and (newly named) Farmcote Gneiss (uppermost) are confirmed. An age of 1710 ± 4 Ma on a probable metavolcanic gneiss in the Redan Gneiss confirms this formation as the oldest dated unit in the Rantyga Group and in the Broken Hill Inlier. Metasediments in the Farmcote Gneiss contain approximately contemporaneous zircons dated at 1705 ± 5 Ma. The Rantyga Group is considered part of the Willyama Supergroup, rather than Archean or Early Paleoproterozoic basement as was suggested earlier. Previous attempts to date the Thackaringa Group concentrated on albitised metasediments, producing a range of detrital dates, with no definitely contemporaneous zircon. In the present study, three probable metavolcanic rocks from the Cues Formation yielded a consistent age of ca 1700 Ma, slightly younger than the underlying Alma Granite Gneiss (metagranite). The metagranite was intruded under very shallow cover. Three other concordant metagranites were dated. The 1685 ± 5 Ma Wondervale Well Granite Gneiss is indistinguishable in age from the 1683 ± 3 Ma Rasp Ridge Granite Gneiss. However, the Stephens Creek Granite Gneiss appears to be slightly older in that the stratigraphically lower part was dated at 1689 ± 5 Ma, while the upper part was dated at 1686 ± 4 Ma. Previous correlations of the lower and upper parts, with the Alma and Rasp Ridge Granite Gneisses, respectively, are therefore invalid. The 1695 ± 4 Ma Georges Bore Granite Gneiss correlates with neither the Alma Granite Gneiss nor the Rasp Ridge Granite Gneiss. All three granite gneisses dated in this study have intruded the upper Thackaringa Group and/or lower Broken Hill Group, indicating very shallow levels of emplacement, probably <1 km of current stratigraphic thickness. Very shallow emplacement of sill-like granites more or less coincides with deposition of Broken Hill-type mineralisation, and may have influenced ore formation. Samples from two critical parts of the Broken Hill Group were analysed. A sample of suspected Hores Gneiss from near Mingary in South Australia gives an interpreted age of 1684 ± 6 Ma, indistinguishable from Hores Gneiss in the type area north of Broken Hill. An age of 1693 ± 4 Ma on Potosi-type gneiss from the Parnell Formation north of Broken Hill confirms earlier dating from the same location. The new geochronological information shows that the time interval between the oldest stratigraphic unit (Redan Gneiss) and the top of the Broken Hill Group (Hores Gneiss) is only ∼25 million years, leaving little space for major time breaks. More specifically, the time interval between the middle Thackaringa Group (Cues Formation) and the lower Broken Hill Group (Ettlewood Calc-Silicate Member) is only ∼5 million years, suggesting continuous or near-continuous deposition.