J. McL. Miller

Terminal suturing of Gondwana and the onset of the Ross-Delamerian Orogeny : the cause and effect of an Early Cambrian reconfiguration of plate motions

Abstract A review of U–Pb age data from intrusive and volcanic rocks for Antarctica and Australia has highlighted the remarkable synchronicity in the shift from passive margin or continental arc tectonism, to convergent orogenesis along the Pacific margin of Gondwana. This event, the Ross–Delamerian Orogeny, marks a distinct and rapid shift in the style of deformation, rate of uplift, volume of magmatism and syn-orogenic sedimentation along the entire 4000+ km length of the orogen. The Ross–Delamerian Orogeny is constrained to have begun at 515±5 Ma, an interval that also encompasses the youngest collisional events that lead to the suturing of Gondwana. We propose that these geographically separate events are intimately linked and represent the cause and effect of a major tectonic reorganisation in the motion of the Earth’s plates. We suggest the mechanism controlling this Early to Middle Cambrian plate reorganisation was the rapid reduction in relative motion, driven by continental collision, between the final two components of Gondwana. Due to the need to maintain a zero sum for all plate motions, the change in the rate of convergence at this boundary resulted in compensatory changes elsewhere in the global plate circuit. One such change was an increase in the rate and perhaps direction of convergence along the Pacific margin of Gondwana.

Evolution of a reworked orogenic zone: The boundary between the delamerian and lachlan fold belts, southeastern Australia *

40Ar/39Ar age data from the boundary between the Delamerian and Lachlan Fold Belts identify the Moornambool Metamorphic Complex as a Cambrian metamorphic belt in the western Stawell Zone of the Palaeozoic Tasmanide System of southeastern Australia. A reworked orogenic zone exists between the Lachlan and Delamerian Fold Belts that contains the eastern section of the Cambrian Delamerian Fold Belt and the western limit of orogenesis associated with the formation of an Ordovician to Silurian accretionary wedge (Lachlan Fold Belt). Delamerian thrusting is craton-verging and occurred at the same time as the final consolidation of Gondwana. 40Ar/39Ar age data indicate rapid cooling of the Moornambool Metamorphic Complex at about 500 Ma at a rate of 20 – 30°C per million years, temporally associated with calc-alkaline volcanism followed by clastic sedimentation. Extension in the overriding plate of a subduction zone is interpreted to have exhumed the metamorphic rocks within the Moornambool Metamorphic Complex. The Delamerian system varies from a high geothermal gradient with syntectonic plutonism in the west to lower geothermal gradients in the east (no syntectonic plutonism). This metamorphic zonation is consistent with a west-dipping subduction zone. Contrary to some previous models involving a reversal in subduction polarity, the Ross and Delamerian systems of Antarctica and Australia are inferred to reflect deformation processes associated with a Cambrian subduction zone that dipped towards the Gondwana supercontinent. Western Lachlan Fold Belt orogenesis occurred about 40 million years after the Delamerian Orogeny and deformed older, colder, and denser oceanic crust, with metamorphism indicative of a low geothermal gradient. This orogenesis closed a marginal ocean basin by west-directed underthrusting of oceanic crust that produced an accretionary wedge with west-dipping faults that verge away from the major craton. The western Lachlan Fold Belt was not associated with arc-related volcanism and plutonism occurred 40 – 60 million years after initial deformation. The revised orogenic boundaries have implications for the location of world-class 440 Ma orogenic gold deposits. The structural complexity of the 440 Ma Stawell gold deposit reflects its location in a reworked part of the Cambrian Delamerian Fold Belt, while the structurally simpler 440 Ma Bendigo deposit is hosted by younger Ordovician turbidites solely deformed by Lachlan orogenesis.

Stawell gold deposit: a key to unravelling the cambrian to early devonian structural evolution of the western Victorian goldfields

Major 440 Ma orogenic-gold deposits in the western Victorian goldfields formed during east – west shortening but have markedly different structural complexity. These deposits occur in: (i) a Cambrian Delamerian basement block that was substantially reworked and reactivated during the Lachlan Orogeny (Stawell); and (ii) Ordovician turbidites deformed solely by Lachlan-aged deformation (Bendigo, Ballarat, Castlemaine). This produced different structural histories prior to mineralisation, although gold deposits have been localised at the top of regional domal culminations. At Stawell, the 440 Ma gold event reactivated a strike-change along a pre-existing Cambrian fault system above a major lithospheric boundary. In the Bendigo Zone, 440 Ma orogenic-gold deposits have a trend oblique to the dominant structural grain and parallel to the western edge of an inferred crystalline basement block (the Selwyn Block). This gold trend is parallel to metamorphic field gradients and pluton age boundaries, which suggests an underlying basement control on the localisation of these orogenic-gold deposits, even though the exact basement architecture is still unresolved. Major variations in the regional stress fields occurred between 425 and 370 Ma, with large gold deposits [>62 t (2 million ounces) endowments] forming at ca 380 – 370 Ma. These events are not deposit-scale structural anomalies as they also regionally affect overlying cover sequences (e.g. the Grampians Group). Gold deposits that formed in the 425 – 400 Ma period have small endowments, but introduce a marked amount of structural and mineralogical complexity to the gold province. The 425 – 400 Ma period preserved at Stawell records sinistral wrenching associated with gold mineralisation, southeast-directed faulting, intrusion-related gold mineralisation and extensive high-level Early Devonian plutonism.

Timing of gold mineralisation in the western Lachlan Orogen, SE Australia: A critical overview

The western sub-province of the Paleozoic Lachlan Orogen in Victoria is dominated by thick turbidite sequences overlying Cambrian basement volcanics. The region was subjected to multiple Cambrian to Late Devonian regional deformation events, followed by extensive post-tectonic granitic magmatism. The western Lachlan Orogen is considered a typical ‘orogenic’ gold province and hosts a large number of goldfields, including the world-class Bendigo–Ballarat goldfields. A variety of geochronological methods (e.g. U–Pb zircon; Re–Os sulfide; 40Ar/39Ar whole-rock, mica) have been used to constrain the timing of gold mineralisation, and the relationship to metamorphism/deformation/magmatism. Regional granitic magmatism is relatively well constrained from U–Pb zircon dating, with the timing of deformation/metamorphism and gold mineralisation reliant largely on 40Ar/39Ar dating results. Owing to inconsistencies in the available 40Ar/39Ar data and recent revisions to 40Ar/39Ar monitor ages and decay constants, we recalculate and re-evaluate all existing 40Ar/39Ar age results. These revisions confirm that the western Lachlan Orogen is characterised by multiple deformation/metamorphism events, with the Stawell structural zone deformed during the ca 500 Ma Delamarian and ca 445 Ma Benambran orogenies, the Bendigo Zone deformed during the Benambran orogeny (with minor Tabberabberan overprinting), and the Melbourne Zone affected by the ca 380–370 Ma Tabberabberan orogeny. Post-tectonic granitic magmatism occurred in two main time intervals, the Early Devonian (ca 400 Ma) and the Late Devonian (ca 380–370 Ma), with the former limited to the Stawell and northwest Bendigo Zones, and the latter distributed throughout the Bendigo and Melbourne Zones and southeast Stawell Zone. Gold mineralisation occurred in two main episodes at ca 445 Ma and ca 380–370 Ma, with another possible (minor) event at ca 410–400 Ma. The ca 445 Ma event is prevalent across the Stawell and Bendigo Zones, with Late Devonian gold mineralisation restricted to the Melbourne and eastern Bendigo Zones. The timing of the two main events is supported by geological constraints, the reproducibility of 40Ar/39Ar results and, in the case of the Bendigo goldfield, coincidence with Re–Os data. Suggestions of a single Devonian age gold mineralisation event are not supported by the available data. The two main gold mineralisation episodes (ca 445 Ma; ca 380–370 Ma) coincide with the waning stages of the Benambran and Tabberabberan orogenies, respectively. Crustal thickening and consequent metamorphic devolatilisation during the Benambran orogeny may have been the main cause of fluid flow related to gold mineralisation at ca 445 Ma. In contrast, crustal anatexis is considered responsible for metamorphic fluid generation and Early Devonian gold mineralisation.

Structural and metamorphic evolution of the Moornambool Metamorphic Complex, western Lachlan Fold Belt, southeastern Australia

The wedge‐shaped Moornambool Metamorphic Complex is bounded by the Coongee Fault to the east and the Moyston Fault to the west. This complex was juxtaposed between stable Delamerian crust to the west and the eastward migrating deformation that occurred in the western Lachlan Fold Belt during the Ordovician and Silurian. The complex comprises Cambrian turbidites and mafic volcanics and is subdivided into a lower greenschist eastern zone and a higher grade amphibolite facies western zone, with sub‐greenschist rocks occurring on either side of the complex. The boundary between the two zones is defined by steeply dipping L‐S tectonites of the Mt Ararat ductile high‐strain zone. Deformation reflects marked structural thickening that produced garnet‐bearing amphibolites followed by exhumation via ductile shearing and brittle faulting. Pressure‐temperature estimates on garnet‐bearing amphibolites in the western zone suggest metamorphic pressures of ∼0.7–0.8 GPa and temperatures of ∼540–590°C. Metamorphic grade variations suggest that between 15 and 20 km of vertical offset occurs across the east‐dipping Moyston Fault. Bounding fault structures show evidence for early ductile deformation followed by later brittle deformation/reactivation. Ductile deformation within the complex is initially marked by early bedding‐parallel cleavages. Later deformation produced tight to isoclinal D2 folds and steeply dipping ductile high‐strain zones. The S2 foliation is the dominant fabric in the complex and is shallowly west‐dipping to flat‐lying in the western zone and steeply west‐dipping in the eastern zone. Peak metamorphism is pre‐ to syn‐D2. Later ductile deformation reoriented the S2 foliation, produced S3 crenulation cleavages across both zones and localised S4 fabrics. The transition to brittle deformation is defined by the development of east‐ and west‐dipping reverse faults that produce a neutral vergence and not the predominant east‐vergent transport observed throughout the rest of the western Lachlan Fold Belt. Later north‐dipping thrusts overprint these fault structures. The majority of fault transport along ductile and brittle structures occurred prior to the intrusion of the Early Devonian Ararat Granodiorite. Late west‐ and east‐dipping faults represent the final stages of major brittle deformation: these are post plutonism.

Variable hangingwall palaeotransport during Silurian and Devonian thrusting in the western Lachlan Fold Belt: Missing gold lodes, synchronous Melbourne Trough sedimentation and Grampians Group fold interference

The western margin of the Lachlan Fold Belt contains early ductile and brittle structures that formed during northeast‐southwest and east‐west compression, followed by reactivation related to sinistral wrenching. At Stawell all of these structural features (and the associated gold lodes) are dismembered by a complex array of later northwest‐, north‐ and northeast‐dipping faults. Detailed underground structural analysis has identified northwest‐trending mid‐Devonian thrusts (Tabberabberan) that post‐date Early Devonian plutonism and have a top‐to‐the‐southwest transport. Deformation associated with the initial stages of dismemberment occurred along an earlier array of faults that trend southwest‐northeast (or east‐west) and dip to the northwest (or north). The initial transport of the units in the hangingwall of these fault structures was top‐to‐the‐southeast. ‘Missing’ gold lodes were discovered beneath the Magdala orebody by reconstructing a displacement history that involved a combination of transport vectors (top‐to‐the‐southeast and top‐to‐the‐southwest). Fold interference structures in the adjacent Silurian Grampians Group provide further evidence for at least two almost orthogonal shortening regimes, post the mid‐Silurian. Overprinting relationships, and correlation with synchronous sedimentation in the Melbourne Trough, indicates that the early fault structures are mid‐ to late‐Silurian in age (Ludlow: ca 420–414 Ma). These atypical southeast‐vergent structures have regional extent and separate significant northeast‐southwest shortening that occurred in the mid‐Devonian (‘Tabberabberan orogeny’) and Late Ordovician (‘Benambran orogeny’).

3D structural modelling and implications for targeting gold mineralisation in western Victoria

A 150×150 km area of western Victoria has been modelled in three dimensions to a depth of ∼20 km. This was constructed through integrated analysis and serial cross-sections of geological and geophysical datasets, utilising mapped positions of major faults, intrusive bodies and lithostratigraphic packages as primary inputs. These are extrapolated to depth and under cover through interpretation of upward continued multiscale wavelet edges of aeromagnetic and gravity data, inversion of the gravity field and the positions of acoustic boundaries. The objective was to develop an understanding of crustal structure in the context of gold mineralisation potential. The upper crustal structure is modelled as comprising a planar array of northwest-trending, steep to moderate inclined mainly east-dipping faults (Moyston, Pleasant Creek). These are interpreted to merge with a basal detachment (Western Fault). This elongate dome-shaped detachment overlies a buried wedge of inferred Proterozoic basement. We suggest that gold distribution in the upper crust may be influenced by the position in the mid-crust of the leading edge of the wedge and its interface with the mafic substrate that largely encloses it. The Coongee Fault appears to be a first-order regional-scale control on the localisation of gold deposits adjacent to basaltic dome prospects in the Stawell corridor. It represents a backthrust, with superimposed sinistral transpression, and is interpreted to have developed above the mid-crustal ramp detachment. Cross-faults that intersect the Coongee Fault may have high exploration potential for localising both orogenic- and intrusion-related gold.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.