Ordovician Macquarie Arc Research Articles

3D modelling of subsurface ancient volcanic successions: a workflow developed during regional reconstructions of the Cowal Igneous Complex in the Ordovician Macquarie Arc of NSW, Australia

Mineral deposits are commonly associated with and hosted in volcanic successions. Volcanic facies analysis as applied in mineral exploration is powerful for characterising host stratigraphy to understand volcanological and eruption processes, depositional setting, and tectonic processes before, during and after ore-forming events. However, the specialist experience required in studying ancient, deformed successions impedes the uptake of facies analysis as a routinely applied technique in mineral exploration. This challenge is reflected in the literature with a paucity of examples where 3D models are developed directly from facies analysis and is worth addressing, as stratigraphic approaches are commonly inadequate for understanding and modelling the complexities within ancient volcanic terrains. We address this gap by demonstrating a technique that uses digitised graphic logs to construct correlation charts in a graphics editor to interpret ‘restored’ volcanic successions within fault blocks. This approach is validated with concurrent cross-sections that depict the structural-stratigraphic framework. The work is iteratively revised until the interpretation is internally consistent across both the correlation chart and the cross-section. This technique is presented within a four-phase workflow that guides the reader from initial scoping, through to graphic logging, interpretation of volcanic successions by locality (or fault block) and finally a synthesis and 3D model. This workflow was developed from our experiences in the buried and deformed Ordovician Cowal Igneous Complex that hosts world-class gold deposits that are currently being mined at E42 (open pit) and GRE46 (underground mine). A case study of the Cowal Igneous Complex is presented to demonstrate the value and real-world application of this workflow.

Assessment of magmatic fertility using pXRF on altered rocks from the Ordovician Macquarie Arc, New South Wales

This research presents a new method for assessing magmatic fertility using the portable X-ray fluorescence spectrometer (pXRF) in altered rocks from porphyry terranes. A study of Northparkes and global Cu--Au porphyries highlighted low Zr and Y trends associated with mineralising intrusions. Zircon saturation occurs at low Zr concentrations owing to low-temperature crystallisation depressed by high dissolved H2O in the melt. Yttrium depletion indicates hydrous magmatic conditions that cause hornblende to crystallise early. Previously geochemical differentiation of hydrous, potentially ore-forming (fertile) porphyries has relied on ratios of Sr/Y and Sr/MnO from geochemical and/or pXRF analysis of least altered rocks. Strontium is highly mobile and limits its use in altered terranes. Finding samples in porphyry terranes where Sr can be demonstrated to be retained is difficult given the ubiquity of alteration associated with porphyry emplacement. This precludes the widespread use of pXRF, as quantitative assessment of alteration is beyond the capabilities of pXRF technology. Using immobile elements overcomes issues with alteration and provides more reliable indicators of magmatic fertility. Using a systematic pXRF workflow, plots of Zr vs Y and Zr*Y vs Al2O3/TiO2 were successful in identifying 100% of the mineralising intrusions in the Northparkes intrusive complex and provide an indication of magmatic fertility and a proxy for melt hydration state. The ratios of Zr/Nb vs Y/Nb are insensitive to alteration including weathering and are indicative of evolution and dissolved H2O content of magmatic sources. Moving from bivariate plots to ratio plots shifts the data into a real-number space and provides a linear function to assess against magmatic fertility. Application of the fertility indicators to a regional dataset from New South Wales highlights areas of known porphyry mineralisation indicating that the method has the potential to be a useful indicator of fertility beyond the scope of the pilot study. KEY POINTS The fertility of porphyry intrusions is directly related to their dissolved H2O content and oxidation state. High dissolved H2O content in magmas causes early crystallisation of hornblende, and temperature depression. As a result Zr saturation occurs at low Zr concentrations that results in depletion of Y and the MREE in hydrous rocks compared with normal arc rocks. Magmatic fertility can be rapidly and cheaply assessed in the field using Zr and Y concentrations from pXRF analyses. The use of immobile elements facilitates magmatic fertility assessment in altered rock, which was previously impossible owing to the use of mobile elements Sr and MnO.

A SIMS U-Pb (zircon) and Re-Os (molybdenite) isotope study of the early Paleozoic Macquarie Arc, southeastern Australia: Implications for the tectono-magmatic evolution of the paleo-Pacific Gondwana margin

New SIMS U-Pb (zircon) data for intrusive rocks of the Macquarie Arc and adjacent granitic batholiths of the Lachlan Orogen (southeastern Australia) provide insight into the magmatic and tectonic evolution of the paleo-Pacific Gondwana margin in the early Paleozoic. These data are augmented by Re-Os dates on molybdenite from four Cu-Au mineralised porphyry systems to place minimum age constraints on igneous crystallization. The simplicity of the zircon age distributions, and absence of older inheritance, stands in contrast to previous geochronological studies. The earliest magmatism within the Macquarie Arc is registered by a ca. 503 Ma gabbro from the Monza igneous complex, whereas a monzodiorite from the same drillhole records the youngest (ca. 432 Ma). Igneous activity in the Macquarie Arc thus overlapped deformation and magmatism in the craton-proximal Delamerian Orogen to the west, and the emplacement of the Lachlan granitic batholiths at 435–430 Ma; the thermal pulse associated with the latter may have triggered the formation of richly mineralised Silurian porphyries in the Macquarie Arc. The juvenile Hf isotope signature of the Monza Gabbro, together with the lack of zircon inheritance and the radiogenic Hf-Nd isotope systematics of Ordovician Macquarie Arc rocks, is consistent with early development of the arc, or a precursor magmatic belt, in an oceanic setting remote from continental influences, and with the arc being built on primitive Cambrian mafic crust. Outboard arc magmatism in the Cambrian may have initiated in response to convergent Delamerian orogenesis adjacent the Gondwana margin. Overlapping radiogenic isotope-time trends are consistent with the evolution of the Macquarie Arc and the Gondwana continental margin being linked from the Cambrian to the Silurian. These data provide further evidence for the growth of continental crust along the southeastern Australian segment of this margin being related to the dynamics of an extensional accretionary orogenic system.

What is underneath the juvenile Ordovician Macquarie Arc (eastern Australia)? A question resolved using Silurian intrusions to sample the lower crust

The Ordovician intra-oceanic Macquarie Arc of eastern Australia collided with the eastern Gondwanan margin at ~440 Ma. However, the deep crustal architecture resulting from this assembly is poorly known. This is addressed here by a zircon U-Pb-Hf study of the post-assembly Silurian Browns Creek Intrusive Complex and Davies Creek Granite dykes that intrude into the arc, and not adjacent Gondwanan sedimentary sequences. Zircon UPb dating integrated with CL imagery indicate two igneous phases at 430–437 Ma and 420–426 Ma and a zircon recrystallisation phase at 395–396 Ma attributed to a late thermal event. The magmatic zircon initial ɛHf values vary from −5.1 to +4.7. This signature indicates the source of these granitic rocks is strongly influenced by typical pre-Silurian Gondwanan material. Granitic rock and zircon compositions demonstrate that at the likely temperature of the Silurian granitic magma, especially the Davies Creek Granite dykes, inherited source zircons were mostly dissolved, explaining the absence of pre-Ordovician xenocrysts within the zircon population. The unradiogenic Hf isotopic signatures preserved in the Silurian magmatic zircons demonstrate the contribution of Gondwanan crustal material to the magma source region. These results support the interpretation of the Macquarie Arc as an intra-Panthalassa ocean allochthon, emplaced and resting over the edge of Gondwanan crystalline basement, possibly including the continent-derived sedimentary rocks of the Adaminaby Group.

Inception and early evolution of the Ordovician Macquarie Arc of Eastern Gondwana margin: Zircon U-Pb-Hf evidence from the Molong Volcanic Belt, Lachlan Orogen

The Ordovician Macquarie Arc is faulted against and surrounded by coeval Gondwana-derived quartz turbidites, in the Lachlan Orogen of southeastern Australia. How these juvenile, island arc rocks were emplaced and structurally interleaved with continental margin sequences of eastern Gondwana is an ongoing debate. Understanding the inception of this arc is critical in building an accurate Early Paleozoic reconstruction for eastern Gondwana. However, the arc's inception and early evolution are poorly-constrained due to the lack of reliable geochronological data. Integrated field observations, zircon U-Pb-Hf, mineral and whole rock geochemistry are presented here for the oldest units of the Molong Volcanic belt, which is a central strand of the dissected Macquarie arc. Geochemistry indicates that these are calc-alkaline rocks with high-K (locally shoshonitic) to medium-K affinities. The stratigraphically-lowest mafic volcanic and volcano-sedimentary rocks of the Mitchell Formation have a unimodal earliest Ordovician zircon UPb age of 479.8 ± 3.8 Ma, with uniform depleted mantle like initial zircon εHf values of +12 to +13. The lack of pre-Ordovician zircons and the uniform positive initial εHf value indicate that the Macquarie arc was initiated in an intra-oceanic setting, far from the influence of eastern Gondwana. The stratigraphically overlying Fairbridge Volcanics includes some more evolved volcano-sedimentary components and has a polymodal, but mostly Ordovician, zircon UPb age population, with a youngest component at 444.3 ± 2.4 Ma. Overall, its Ordovician grains show a greater spread in initial εHf values from +14 down to +8 than the Mitchell Formation, which suggests some continental influence, at least 35 million years after the arc was initiated. The lack of any significant Gondwanan inheritance in Early Ordovician volcaniclastic rocks of the Macquarie Arc along with geochemical comparisons with modern island arcs, suggest that the arc evolved outboard of Gondwana probably as a result of steep, easterly-directed subduction. As the Macquarie arc approached eastern Gondwana there would have been increasing quantities of continent derived sediment subducted beneath the arc which may account for the decreasing and more variable zircon initial εHf values and the appearance of sparse Precambrian continental detritus at the arc's moribund stage. The collision of the Macquarie island arc with Gondwana initiated the Benambran Orogeny and reflects an important mechanism of episodic continental growth involving the addition of juvenile oceanic crust to a continental margin, which contrasts and alternates with periods of purely accretionary growth.

Porphyry Au-Cu mineralization controlled by reactivation of an arc-transverse volcanosedimentary subbasin

Located in the accreted remnants of the Ordovician Macquarie Arc (Australia), Cadia East is the largest alkalic porphyry Au-Cu deposit currently known. The deposit is centered on a 2-km-long, northwest-trending volcanosedimentary subbasin, which was identified by mapping variations in the distribution and thickness of faulted sedimentary marker horizons. The normal faults that define the subbasin predate porphyry mineralization and are oriented parallel to a major arc-transverse lineament. Porphyry Au-Cu mineralization was controlled by reactivation of the subbasin during postorogenic extension as the Macquarie Arc was accreted to the Gondwana margin in the early Silurian. Dilation of the northwest-trending subbasin faults facilitated emplacement of alkalic porphyry dikes and associated sheeted quartz-sulfide veins that define the Cadia East orebody. During the middle Silurian, north-trending fault-bound marine basins buried Cadia East, significantly enhancing preservation of the ore system. These north-trending faults were optimally oriented for reactivation during Devonian east-directed compression, leaving the mineralized Cadia East volcanosedimentary subbasin largely intact. We identify the significance of reactivated premineralization structures within an arc-transverse lineament in controlling porphyry Au-Cu mineralization in a postsubduction setting. At Cadia East, the manifestation of such premineralization structures can be observed at regional, district, and deposit scales, and highlights the influence of preexisting crustal architecture on the structural character, geometry, and preservation potential of porphyry deposits throughout the arc life cycle.

Ordovician marginal basin evolution near the palaeo-Pacific east Gondwana margin, Australia

Although the Ordovician Macquarie Arc, part of the eastern Lachlan Orogen of southeastern Australia, has long been considered to be an intra-oceanic arc within an accretionary orogen, key characteristics contrast with more typical examples of accreted arcs. Significantly, multiple stacked phases of mafic to intermediate volcanic rocks, with suprasubduction-zone chemistry, are flanked to the east and west by extensive, coeval continental margin turbidite, chert and black shale sequences. By analysing stratigraphic and contact relationships within and between the volcanic rocks, ophiolitic components ( sensu lato ) and turbidite sequences, constrained by precise biostratigraphy, we document a repeated cycle of uplift, upper-plate extension and collapse common to all sequences. This cycle is interpreted as resulting from localized extension (rifting or hyper-extension and lower-crustal delamination) within a continent-margin sequence developed upon an already established marginal or back-arc basin of probable middle to late Cambrian age. Our interpretation provides a counter-example to prevailing arc accretion models by inferring extensional tectonics at the palaeo-Pacific east Gondwana margin during the Ordovician with development of alkalic and calc-alkalic Cu–Au porphyry deposits away from an active arc system.

Long-wavelength magnetic anomalies as a guide to the deep crustal composition and structure of eastern Australia

Aeromagnetic data over eastern Australia reveal a pattern of domains defined by systematic regional highs and lows, emphasised by low-pass filtering, over which are superimposed shorter (<20 km) wavelength anomalies related to mappable geology and its inferred subsurface continuation. Long-baseline levelling by Geoscience Australia has clarified the definition of these magnetic domains, and confirmed that they are not an artefact of grid merging. Geothermal and teleseismic data indicate that neither variation in Curie depth nor upper mantle magnetisation can produce the long-wavelength pattern. Hence, domain-wide variations in magnetisation at the middle to lower crustal level are presumably the cause of these long-wavelength features. Although reversed polarity remanence could contribute to deeply sourced negative magnetic anomalies, the correspondence of magnetic low domains with the Proterozoic Curnamona Craton and the Ordovician Macquarie Arc, and of a high domain with the western Lachlan Orogen floored by Cambrian ocean crust, suggests that the control may be simply stark contrasts in lower to middle crustal susceptibility. Moho thickness determined by the AusMoho model mimics the pattern of susceptibility domains, suggesting a relationship between tectonic history and mid to lower crustal composition. Implicit in this analysis is a division of the domains into continental and oceanic basement, with implications for the tectonic evolution of the Tasmanides and the distribution of mineral systems in eastern Australia. The mid to lower crust below the Macquarie Arc appears to be continental, and the Thomson Orogen is a compound feature, comprising both attenuated continental/arc crust and accreted oceanic crust.

Integrative geophysical approach for assessing the prospectivity of the Idlewilde intrusion, NSW

The Idlewilde Intrusion (IWI) sits below ~150 m-thick sediment overburden and consists of a Palaeozoic basement of presumed Ordovician Macquarie Arc volcanics and the Nyngan Intrusive Complex. The geological setting is similar to the gold-rich Cadia and Northparkes porphyry-style deposits. Geophysical data include ground gravity, airborne magnetic and electromagnetic.Gravity data define a negative ovoid with an elevated gravity inner zone. Magnetic data show a central high surrounded by a circular low zone as part of a broader regional low. Electromagnetic data outline: (a) ?150 m- thick dual conductor corresponding to the overburden; (b) a medium conductor associated with the volcanics; (c) a resistor linked to the intrusion and (d) a slightly conductive annulus around the resistor interpreted as alteration, but may instead reflect the lack of sensitivity below the conductive overburden. The central part of the intrusion associated to gravity and magnetic highs could conceivably correspond to: (1) a magnetite-altered potassic core characteristic of gold-rich porphyry, (2) eroded granite with volcanics close to the surface, (3) roof pendant volcanics above a barely-eroded granite, and (4) a late monzonite pulse.From an exploration point of view, gravity data delineate the extent of the IWI. The regional low magnetisation suggests that the intrusion may be a later intrusive phase. Electromagnetic data identify the footprint of a granitic body which cannot be clearly associated with an alteration system because of conductive overburden. A mineralised system is yet to be identified, and one of the first company priorities is now to resolve the inner part of the IWI.

U–Pb and Hf isotope data from zircons in the Macquarie Arc, Lachlan Orogen: Implications for arc evolution and Ordovician palaeogeography along part of the east Gondwana margin

The Ordovician Macquarie Arc in the eastern subprovince of the Lachlan Orogen, southeastern Australia, is an unusual arc that evolved in four vertically stacked volcanic phases over ~ 37 million years, and which is flanked by coeval, craton-derived, passive margin sedimentary terranes dominated by detrital quartz grains. Although these two terranes are marked by a general absence of provenance mixing, LA-ICPMS analysis of U–Pb and Lu–Hf contents in zircon grains in volcaniclastic rocks from 3 phases of the arc demonstrates the same age populations of detrital grains inherited from the Gondwana margin as those that characterise the flanking quartz-rich Ordovician turbidites. Magmatic Phase 1 is older, ~ 480 Ma, and is characterised by detrital zircons grains with ages of ~ 490–540 with negative ε Hf from 0 to mainly –7.78, 550–625 Ma ages with negative ε Hf from 0 to −26.6 and 970–1250 Ma (Grenvillian) with ε Hf from + 6.47 to −6.44. We have not as yet identified any magmatic zircons related to Phase 1 volcanism. Small amounts of detrital zircons also occur in Phase 2 (~ 468–455 Ma), hiatus 1 and Phase 4 (~ 449–443 Ma), all of which are dominated by Ordovician magmatic zircons with positive ε Hf values, indicating derivation from unevolved mantle-derived magmas, consistent with formation in an intraoceanic island arc. Because of the previously obtained positive whole rock ε Nd values from Phase 1 lavas, we rule out contamination from substrate or subducted sediments. Instead, we suggest that during Phase 1, the Macquarie Arc lay close enough to the Gondwana margin so that volcaniclastic rocks were heavily contaminated by detrital zircon grains shed from granites and Grenvillian mafic rocks mainly from Antarctica (Ross Orogen and East Antarctica) and/or the Delamerian margin of Australia. The reduced nature of a Gondwana population in Phase 2, hiatus 1 and Phase 4 is attributed to opening of a marginal basin between the Gondwana margin and the Macquarie Arc that put it out of reach of all but rare turbiditic currents.

Plate-driven extension and convergence along the East Gondwana active margin: Late Silurian–Middle Devonian tectonics of the Lachlan Fold Belt, southeastern Australia

The Lachlan Fold Belt of southeastern Australia developed along the Panthalassan margin of East Gondwana. Major silicic igneous activity and active tectonics with extensional, strike-slip and contractional deformation have been related to a continental backarc setting with a convergent margin to the east. In the Early Silurian (Benambran Orogeny), tectonic development was controlled by one or more subduction zones involved in collision and accretion of the Ordovician Macquarie Arc. Thermal instability in the Late Silurian to Middle Devonian interval was promoted by the presence of one or more shallow subducted slabs in the upper mantle and resulted in widespread silicic igneous activity. Extension dominated the Late Silurian in New South Wales and parts of eastern Victoria and led to formation of several sedimentary basins. Alternating episodes of contraction and extension, along with dispersed strike-slip faulting particularly in eastern Victoria, occurred in the Early Devonian culminating in the Middle Devonian contractional Tabberabberan Orogeny. Contractional deformation in modern systems, such as the central Andes, is driven by advance of the overriding plate, with highest strain developed at locations distant from plate edges. In the Ordovician to Early Devonian, it is inferred that East Gondwana was advancing towards Panthalassa. Extensional activity in the Lachlan backarc, although minor in comparison with backarc basins in the western Pacific Ocean, was driven by limited but continuous rollback of the subduction hinge. Alternation of contraction and extension reflects the delicate balance between plate motions with rollback being overtaken by advance of the upper plate intermittently in the Early to Middle Devonian resulting in contractional deformation in an otherwise dominantly extensional regime. A modern system that shows comparable behaviour is East Asia where rollback is considered responsible for widespread sedimentary basin development and basin inversion reflects advance of blocks driven by compression related to the Indian collision.

Styles of Cenozoic collisions in the western and southwestern Pacific and their applications to Palaeozoic collisions in the Tasmanides of eastern Australia

The western and southwestern Pacific preserve evidence of Cenozoic collisions that guide our understanding of processes and geometries involved in collisions in ancient orogens, in particular in this case, the Palaeozoic Tasmanides of southeastern Australia. Although several styles of collisions are present in the Pacific, ranging from arc–arc collision to arc–plateau collision, the dominant two are oblique and strike-slip collisions between island arcs and rifted continental fragments, and collisions between forearc lithosphere and continental fragments. The 58 Ma collision along the northern margin of the Australian plate in New Guinea, the 44–34 Ma collision preserved in New Caledonia and the 26–25 Ma collision in the North Island of New Zealand may be parts of a single plate boundary collision that migrated southwards along the plate boundary. They characterize the main style of deformation in which a collision between forearc crust and continental fragment produces subduction flip or rollback, thereby avoiding a classic arc–continent collision. Processes involved in, and geometries that have developed from, SW and W Pacific style collisions have been applied to the interpretation of the evolution of the Delamerian Orogen and Lachlan Orogen in the southern Tasmanides with varying degrees of success. The ophiolite obduction model has been successfully applied to the western Tasmania part of the Delamerian Orogen, although there is discussion about its applicability to the mainland. The best example of an arc accretion, that of the Ordovician Macquarie Arc in the eastern Lachlan Orogen, developed from rare geometry in the western Pacific wherein (with the constraint that no forearc or subduction complex has been identified) the arc lies on the continental plate, above a continental-dipping subduction zone. The multiple subduction zone model of Halmahera has been widely applied to the back arc of the Lachlan Orogen, but evidence for clear subduction zones or arcs is not present.

Tectonic evolution of the Ordovician Macquarie Arc, central New South Wales: arguments for subduction polarity and anticlockwise rotation

The Ordovician Macquarie Arc is most widely exposed in the Lachlan Fold Belt of central New South Wales. Complex relationships between the arc and the Ordovician turbidite mega-fan are partly explained by anticlockwise rotation of the arc during the Ordovician. Thus, initially two lobes of the mega-fan formed to the north and south of the east–west-trending arc, using present-day coordinates. The arc consists of the western Goonumbla–Trangie Volcanic Belt, replacing the inappropriate term Junee–Narromine Volcanic Belt, and an eastern composite of the Molong, Rockley–Gulgong and Kiandra Volcanic Belts. These two major segments of the arc are separated by Ordovician quartz turbidites of the Kirribilli Formation, and it is probable that the arc has been duplicated by a sinistral strike-slip fault. Eastonian paleogeographic reconstruction of the eastern segment of the arc highlights a prominent limestone platform in the western Molong Volcanic Belt that grades eastwards into a realm of mainly deep-marine sedimentation and volcanic activity. By analogy with Guam in the western Pacific Ocean, the limestone platform is equated to a frontal arc ridge. This implies that the associated subduction zone was along the western side of the arc and not to the east, as in previous reconstructions. A wide zone of deformed Ordovician quartz turbidites, making up the Girilambone and Wagga–Omeo Zones west of the Macquarie Arc, is interpreted as a subduction complex that formed rapidly in the Late Ordovician. Flipping of the subduction zone was a relatively long event, inferred to have occurred during the latest Ordovician to early Silurian Benambran Orogeny. This was driven by collision of the subduction complex with northern continuations of the Stawell and Bendigo Zones, with a new west-dipping subduction zone forming to the east.

Geochemistry and age of magmatic rocks in the unexposed Narromine, Cowal and Fairholme Igneous Complexes in the Ordovician Macquarie Arc, New South Wales

Much of the northern and southern sections of the Junee – Narromine Volcanic Belt of the Ordovician Macquarie Arc in central-western New South Wales are buried beneath the sediment cover of the Great Artesian Basin. Exploration drilling of these aeromagnetically defined blocks has provided important new material to assess the temporal and magmatic affinities of the rocks in the Narromine Igneous Complex (northern end) and the Cowal and Fairholme Igneous Complexes (southern end). Basement rocks are representative of Phase 1 magmatism in the Macquarie Arc, and consist of Lower Ordovician basalts and andesites and common volcaniclastic rocks, all with high-K calc-alkaline affinities. These are very similar compositionally to the Lower Ordovician Nelungaloo Volcanics that outcrop in the central part of the Junee – Narromine Volcanic Belt west of Parkes. Intruding the basement volcanic – volcaniclastic package are three distinct igneous suites. The oldest of these, dated at ca 466 – 460 Ma (Middle Ordovician) and representative of Phase 2 magmatism in the Macquarie Arc, consists of stocks of monzogabbro, monzodiorite and monzonite with high-K calc-alkaline affinities, and is well represented in the Narromine and Cowal Igneous Complexes. Phase 2 suite rocks were, in turn, intruded in the Bolindian (445 ± 5 Ma) by kilometre-size stocks, narrow sheets and dykes made up of hornblende gabbro, diorite and granodiorite, and including quartz + plagioclase + hornblende-phyric dacitic porphyries in the Narromine Igneous Complex. These medium-K calc-alkaline rocks, assigned to the Phase 3 Copper Hill Suite that also occurs in the Molong and Rockley – Gulgong Volcanic Belts further east, crystallised from andesitic or more evolved magmas, probably derived via partial melting of low-K rocks in the arc basement, or amphibolites in the subducting slab. The youngest rocks in these igneous complexes are believed to be the high-level shoshonitic intrusives in the Cowal and Fairholme Igneous Complexes, which are correlated on petrographic and compositional grounds with the Macquarie Arc Phase 4 (Bolindian and Llandovery) magmatic products best represented by the Northparkes Igneous Complex in the central part of the Junee – Narromine Volcanic Belt.

Arc and mantle detritus in the post-collisional, Lower Silurian Kabadah Formation, Lachlan Orogen, New South Wales

The Kabadah Formation outcrops in central New South Wales as a thrust package 66 km long, interleaved with Lower Silurian Canowindra Volcanics and situated between the Junee – Narromine and Molong Volcanic Belts of the Ordovician Macquarie Arc. The Kabadah Formation contains Early Silurian corals and Llandovery graptolites. Its provenance is complex, with detrital fragments of mafic – intermediate volcanic rocks, free crystals of pyroxene, chromite and ultramafic clasts, detrital volcanic quartz, garnet, and clasts of welded S-type rhyolitic volcanic rocks; and rare clasts from uplifted fold-belt rocks (granite and metamorphosed and deformed sediments). The variety of these clasts suggests that the Kabadah Formation records the Benambran collision of the Macquarie Arc with Ordovician quartz-rich sedimentary rocks, with detritus also derived from coeval Early Silurian mafic and felsic magmatism. The major source of detritus was from the short-lived emergent Fifield arc that formed from the subduction of an older backarc basin. The Kabadah Formation accumulated in an upward-shallowing Early Silurian marine basin between phases of the Benambran Orogeny.

Volcanology, geochemistry and structure of the Ordovician Cargo Volcanics in the Cargo – Walli region, central New South Wales

The Middle to lower Upper Ordovician Cargo Volcanics occur in a structural outlier immediately west of the contiguous Molong Volcanic Belt of the Ordovician Macquarie Arc. The sequence of basaltic to dacitic lavas, lava breccias and associated volcaniclastic rocks with medium-K calc-alkaline affinities has been interpreted as the subaqueous portion of a major intra-oceanic arc stratovolcano. The oldest part of the succession consists of massive to pillowed, poorly vesicular, aphyric to moderately plagioclase + clinopyxroxene-phyric basalt and andesite lavas and hypabyssal sills. The main part of the interpreted volcanic edifice is composed of massive and pillowed vesicular andesite and dacite, flanked by lenses of hyaloclastite, pillow breccias and interbedded with debris-flow and turbiditic deposits of crystal-rich and pebbly volcanic sandstones, siltstone and minor conglomerate. Uplift and erosion of the edifice prior to deposition of the overlying Eastonian Barrajin Group limestones is indicated by local aprons of thickly bedded subaqueous volcanic conglomerate and pebbly sandstone which appear to be separated from both the underlying andesite-dominant pile and overlying limestones by angular unconformities. The Cargo Volcanics, and the neighbouring Walli Volcanics further south, are clearly distinguished from other Macquarie Arc lavas by their high (>25) Zr/Nb values. However, the Walli Volcanics are readily differentiated from the Cargo Volcanics by their higher P2O5 contents and likely high-K calc-alkaline to shoshonitic affinities. The Cargo Volcanics are intruded by Cu – Au mineralised, plagioclase + hornblende + quartz-phyric dacites with medium-K calc-alkaline affinities. These dacites have lower TiO2 and higher MgO contents at any SiO2 level compared to the Cargo Volcanics, and are compositionally similar to Late Ordovician to Early Silurian (453 – 441 Ma) dacites at Copper Hill and in the Narromine Complex. However, at Cargo, clasts derived from the dacites are locally abundant in volcaniclastic conglomerate near the top of the Cargo Volcanics, indicating that the dacites were intruded and exhumed prior to deposition of the Barrajin Group limestones, which commenced at ca 454 Ma in this area. The dacites at Cargo are intruded by small, apparently unmineralised monzonitic intrusions with shoshonitic affinities, which also appear to pre-date deposition of the Barrajin Group. LA-ICPMS U – Pb dating of detrital zircons from the Ranch Member at the base of the Daylesford Limestone (basal Barrajin Group) revealed a dominant population with an average age of 453.0 ± 4.1 Ma (identical within error to the mid-Ea1 age for the host sediments based on fossil assemblages) and subordinate populations with average ages of 480 Ma and 505 Ma. The 453 Ma zircons were most probably derived from either the intrusive dacites or monzonites, suggesting that the Cargo region was rapidly exhumed following emplacement of the felsic intrusions. The angular unconformities near and at the top of the Cargo Volcanics and the broadly coincident change in magma chemistry suggest that this sector of the Macquarie Arc underwent major tectonic upheaval commencing in the latest Gisbornian – early Eastonian. The Cargo Volcanics are interpreted to have undergone major eastward translation at this time (from an original position now buried beneath the Cowra Trough). Their present juxtaposition with the contiguous Molong Volcanic Belt probably did not occur until the late Early Silurian during the final stages of the Benambran Orogeny.

Alkalic porphyry Au – Cu and associated mineral deposits of the Ordovician to Early Silurian Macquarie Arc, New South Wales

Twenty-one alkalic porphyry deposits of Late Ordovician to Early Silurian age occur in two mineral districts in New South Wales. The Cadia and Northparkes districts formed in shoshonitic volcanic centres where a major basement structure (the Lachlan Transverse Zone) cut the Ordovician Macquarie Arc obliquely. Processes of mineralisation in both districts were centred in and around quartz monzonite porphyry complexes that intruded the volcanic centres. These composite intrusive complexes comprise pipes, dykes and stocks. Hydrothermal alteration in and around the intrusions produced a complex sequence of potassic, calc-potassic, sodic, propylitic and late stage, typically fault- and fracture-controlled phyllic assemblages. Hematite dusting was a common alteration product giving the intrusions and the altered volcano-sedimentary host sequences a distinctive pink-orange coloration. Several of the deposits have bornite-rich cores, chalcopyrite-dominant annuli and pyritic outer haloes. Gold is well correlated with bornite in most of the deposits, and with chalcopyrite at Cadia Hill. The mineralising intrusions have Sr and Nd isotopic compositions consistent with derivation from a depleted mantle source regime, although the Nd data are permissive of limited crustal contamination. Epidote peripheral to the porphyry deposits has Sr isotopic compositions indistinguishable from the host intrusions, precluding the involvement of external seawater in the mineralising processes. In contrast, slightly elevated initial Sr values have been detected in epidote from nearby skarn deposits, indicative of incorporation of a minor component of Sr from limestone dissolution or seawater mixing. The alkalic porphyry deposits are difficult exploration targets because intensely developed hydrothermal alteration zones are restricted to within a few hundred metres of the monzonite complexes.

Geophysical and geological interpretation of the Junee – Narromine Volcanic Belt

The Junee-Narromine Volcanic Belt is the westernmost and most poorly outcropping belt of Ordovician rocks that represent the dispersed remnants of the Ordovician Macquarie Arc. The northern part is under deep cover of Surat Basin. In areas of poor or nonexistent outcrop such as these, most of the knowledge of the geometry and components of the geology comes from the interpretation and modelling of gravity and aeromagnetic data - interpolation between outcrops and extrapolation into areas of deep cover. Most of the shape of the Junee-Narromine Volcanic Belt is a reflection of meridional bounding faults of the Tullamore Trend. Individual complexes are elongated N-S or NNW and deformation is generally low, except in the high strain zones. From north to south, Ordovician complexes with approximately meridional Tullamore trends become rotated into the Gilmore Trend and then become aligned along splay faults as thrust sheets (Gidginbung Volcanics) before being truncated by the Gilmore Fault.

Tectonic and economic implications of trace element, 40Ar/ 39Ar and Sm–Nd data from mafic dykes associated with orogenic gold minerals in central Victoria, Australia: reply

The western and southwestern Pacific preserve evidence of Cenozoic collisions that guide our understanding of processes and geometries involved in collisions in ancient orogens, in particular in this case, the Palaeozoic Tasmanides of southeastern Australia. Although several styles of collisions are present in the Pacific, ranging from arc–arc collision to arc–plateau collision, the dominant two are oblique and strike-slip collisions between island arcs and rifted continental fragments, and collisions between forearc lithosphere and continental fragments. The 58 Ma collision along the northern margin of the Australian plate in New Guinea, the 44–34 Ma collision preserved in New Caledonia and the 26–25 Ma collision in the North Island of New Zealand may be parts of a single plate boundary collision that migrated southwards along the plate boundary. They characterize the main style of deformation in which a collision between forearc crust and continental fragment produces subduction flip or rollback, thereby avoiding a classic arc–continent collision. Processes involved in, and geometries that have developed from, SW and W Pacific style collisions have been applied to the interpretation of the evolution of the Delamerian Orogen and Lachlan Orogen in the southern Tasmanides with varying degrees of success. The ophiolite obduction model has been successfully applied to the western Tasmania part of the Delamerian Orogen, although there is discussion about its applicability to the mainland. The best example of an arc accretion, that of the Ordovician Macquarie Arc in the eastern Lachlan Orogen, developed from rare geometry in the western Pacific wherein (with the constraint that no forearc or subduction complex has been identified) the arc lies on the continental plate, above a continental-dipping subduction zone. The multiple subduction zone model of Halmahera has been widely applied to the back arc of the Lachlan Orogen, but evidence for clear subduction zones or arcs is not present.

Crustal structure of the Ordovician Macquarie Arc, Eastern Lachlan Orogen, based on seismic‐reflection profiling

In the Eastern Lachlan Orogen, the mineralised Molong and Junee‐Narromine Volcanic Belts are two structural belts that once formed part of the Ordovician Macquarie Arc, but are now separated by younger Silurian‐Devonian strata as well as by Ordovician quartz‐rich turbidites. Interpretation of deep seismic reflection and refraction data across and along these belts provides answers to some of the key questions in understanding the evolution of the Eastern Lachlan Orogen—the relationship between coeval Ordovician volcanics and quartz‐rich turbidites, and the relationship between separate belts of Ordovician volcanics and the intervening strata. In particular, the data provide evidence for major thrust juxtaposition of the arc rocks and Ordovician quartz‐rich turbidites, with Wagga Belt rocks thrust eastward over the arc rocks of the Junee‐Narromine Volcanic Belt, and the Adaminaby Group thrust north over arc rocks in the southern part of the Molong Volcanic Belt. The seismic data also provide evidence for regional contraction, especially for crustal‐scale deformation in the western part of the Junee‐Narromine Volcanic Belt. The data further suggest that this belt and the Ordovician quartz‐rich turbidites to the east (Kirribilli Formation) were together thrust over ?Cambrian‐Ordovician rocks of the Jindalee Group and associated rocks along west‐dipping inferred faults that belong to a set that characterises the middle crust of the Eastern Lachlan Orogen. The Macquarie Arc was subsequently rifted apart in the Silurian‐Devonian, with Ordovician volcanics preserved under the younger troughs and shelves (e.g. Hill End Trough). The Molong Volcanic Belt, in particular, was reworked by major down‐to‐the‐east normal faults that were thrust‐reactivated with younger‐on‐older geometries in the late Early ‐ Middle Devonian and again in the Carboniferous.