Caroline Tiddy and David Giles
Mineral Exploration Cooperative Research Centre (MinEx CRC) and Future Industries Institute, University of South Australia
Download this article as a PDF (756 KB); cite as MESA Journal 93, pages 32–46
Published October 2020
Introduction
The geological history of eastern Australia from 1.60 to 1.57 Ga is a period of continent-scale deformation and metamorphism, voluminous magmatism and generation of large iron oxide – copper–gold (IOCG) mineralisation including the Olympic Dam deposit (Figs 1–3). Published tectonic models for this period range from intracratonic (e.g. Wyborn et al. 1987) to suprasubduction zone settings (e.g. Betts and Giles 2006; Hand et al. 2007; Forbes et al. 2008), with variable importance placed on the significance of spatially and temporally coincident magmatism and its role in ore genesis (e.g. Williams et al. 2005; Groves et al. 2010; Reid 2019). Compelling similarities exist between eastern Proterozoic Australia and the Cretaceous suprasubduction zone IOCG province of the Coastal Cordillera of Chile between 21° and 30° S. These include a relationship with previously metasomatised ‘fertile’ subcontinental lithospheric mantle (SCLM); spatially and temporally coincident, voluminous magmatism; derivation of ore fluids from devolatisation of mantle-derived magmas; and the role of regional, lithospheric-scale structures as fluid pathways (e.g. Groves et al. 2010; Sillitoe 2003; Curtis and Thiel 2019; Wise and Thiel 2020; Williams et al. 2005; Skirrow et al. 2007). In addition, similar models of slab rollback and back-arc extension followed by a switch to shortening and back-arc basin inversion have been proposed for both Proterozoic Australia and Cretaceous Chile (e.g. Sillitoe 2003; Forbes et al. 2008).
Here we summarise the alternative tectonic model for eastern Proterozoic Australia from c. 1.67–1.58 Ga as proposed by Tiddy and Giles (2020). The model places Proterozoic Australia in the context of a suprasubduction zone and provides a mechanism for generation of the F-rich, anhydrous ‘A-type’ magmatism that is associated with IOCG mineralisation in the Gawler Craton, and has implications for the formation of IOCG (and porphyry-epithermal) mineralisation.
c. 1.60–1.57 Ga activity in Australia
The geological features of eastern Proterozoic Australia leading into and following the c. 1.60–1.57 Ga timeslice are shown in Figures 1, 2 and 3 and described in Tiddy and Giles (2020) and references therein. At least nine key geological features need to be incorporated into a tectonic model for this time period.
- Switching between intense shortening and extensional tectonic regimes.
- A mechanism for extensive high-temperature – low-pressure metamorphism and contemporaneous deformation followed by exhumation of terranes in the South Australian Craton (e.g. Mount Woods Inlier and western Gawler Craton).
- A trigger for voluminous magmatism that changes from older A-type to younger I-type magmatism (Hiltaba Suite) in the northern Olympic Domain to the southwestern Gawler Craton at 1.60–1.57 Ga, respectively.
- Extrusion of voluminous hot (~950–1,100 °C), A-type, F-rich lavas of the Gawler Range Volcanics at c. 1.595–1.587 Ga.
- Transition in Nd isotopic values from crustal (e.g. c. 1.74 Ga McGregor Volcanics) to mantle (c. 1.65–1.60 Ga St Peter Suite) compositions/signatures.
- Metasomatism of the SCLM beneath the Gawler Craton.
- A mechanism for generation of extensive IOCG mineralisation (e.g. Olympic Dam, Prominent Hill, Carrapateena) paired with Au-only (e.g. Tunkillia, Tarcoola) and base metal mineralisation (e.g. Menninnie Dam, Paris) linked to melt and volatiles derived from a metasomatised mantle.
- The spatial links between mineralisation and magmatism in restricted sections of the inferred plate boundary.
- Consistency with tectonic settings leading into and following c. 1.60–1.57 Ga.
Suprasubduction zone model
Published models of Betts et al. (2009) and Skirrow et al. (2018) encapsulate two end-member models for the 1.60–1.57 Ga timeslice: mantle plume interaction with a subduction zone (Betts et al. 2009); and foundering/delamination of fertile, metasomatised lithospheric mantle in a post-subduction setting (Skirrow et al. 2018). Both models account for several factors required to explain the 1.60–1.57 Ga period; however, as discussed by Tiddy and Giles (2020) both have limitations. Tiddy and Giles (2020) draw on aspects of both models to propose the alternative model (Fig 4) summarised below.
c. 1.67–1.604 Ga
From c. 1.67–1.604 Ga, a north-dipping, rollback subduction zone is interpreted at the southern margin of the Gawler Craton. The evolution of this subduction zone is evidenced by a shift towards positive εNd values of felsic magmatic rocks in the central and southwest Gawler Craton between 1.74 and 1.60 Ga (see also figure 7 in Tiddy and Giles 2020). Magmatic rocks emplaced prior to the c. 1.73–1.69 Ga Kimban Orogeny preserve negative εNd values indicating a dominantly crustal component (e.g. McGregor Volcanics, εNd(1740 Ma) = −3.4 to −0.6; Turner et al. 1993). Post-Kimban magmatic rocks preserve transitional (e.g. Tunkillia Suite, εNd(1680 Ma) = −6.3 to 2.6; Payne et al. 2010) to more positive εNd values (e.g. St Peter Suite, εNd(1620 Ma) = −3.7 to 3.7; Swain et al. 2008; Reid et al. 2019). It is noted that the tectonic setting of the c. 1.69–1.67 Ga Tunkillia Suite magmatism is conjectural (e.g. Betts and Giles 2006 compared to Payne et al. 2010) and does not uniquely fingerprint a subduction zone setting (e.g. Payne et al. 2010). However, the calc-alkaline nature and juvenile isotopic composition of the c. 1.65–1.604 Ga St Peter Suite (Symington et al. 2014; Reid et al. 2019) is compelling evidence for a suprasubduction zone setting for the Gawler Craton at this time. Rollback of the subduction zone resulted in extension and lithospheric thinning in the overriding plate, development of back-arc basins and elevated crustal thermal gradients (e.g. Forbes et al. 2008; Raveggi et al. 2007; Figs 1, 2, 4a–b).
c. 1.604–1.595 Ga
A switch from rollback to flat-slab subduction c. 1.604–1.595 Ga (Figs 4a–c) resulted from the arrival of a buoyant oceanic plateau at the subduction trench at c. 1.604 Ga (Fig 4c). Flat-slab subduction terminated the arc magmatism of the St Peters Suite and produced small volume intrusive magmas manifest as the oldest phases of the Hiltaba Suite (Figs 3a–b, 4c) Compressive stresses were transferred into the continental interior, recognised as periods of intense shortening focused within earlier developed, thermally weakened back-arc basins (e.g. Olarian Orogeny; Forbes et al. 2008) and around the rigid Archean nucleus of the Gawler Craton (e.g. c. 1.60–1.58 Ga deformation in the Coober Pedy Ridge, Mable Creek Ridge, Mount Woods Inlier and Barossa Complex; Hand et al. 2007; Forbes et al. 2012; Morrissey et al. 2013) (Figs 3a–b, 4c).
c. 1.595–1.585 Ga
The flat-subducting slab is inferred to have extended to beneath the present-day Olympic Dam area by 1.595 Ga (Figs 4c–d). Differential sinking of the dense frontal section of typical oceanic lithosphere that was attached to the buoyant posterior oceanic plateau led to a concentration of stress and strain, ultimately leading to slab tearing (and potentially complete decoupling) and asthenospheric upwelling at the leading edge of the buoyant slab (see Hu and Liu 2016; Figs 4c, d).
Maximum differential stresses that led to slab tearing and potentially complete decoupling of the dense frontal slab from the buoyant posterior were focused beneath the present-day Olympic Dam area (Tiddy and Giles 2020). Asthenospheric upwelling resulted in elevated temperatures and partial melting of the SCLM to produce F-rich mafic melts that are now recognised within the Hiltaba Suite (e.g. Wade et al. 2019; Fig 4d). Fluorine enrichment of the SCLM is inferred to have occurred via incorporation of anhydrous, F-rich, S-poor slab-derived melts (rather than slab-derived hydrous fluids that are typically F-poor; Köhler et al. 2009; Straub and Layne 2003). Slab-melt modification of the SCLM is interpreted to have occurred above deeper sections of the slab, inboard of the magmatic arc (Figs 4a, b). A contribution of recycled sediments to SCLM metasomatism is suggested to account for the negative range of ɛNd values yielded for many Hiltaba Suite mafic rocks (ɛNd(1590 Ma) = −5 to 0; Wade et al. 2019).
Ascending F-rich SCLM-derived mafic magmas formed a voluminous lower crustal MASH (melting, assimilation, storage, homogenisation) zone, whereby large volumes of increasingly siliceous melts were generated (Fig 5a). These siliceous melts migrated to mid- to upper-crustal magma chambers (Figs 4d, 5a–b) and evolved into highly fractionated melts enriched in incompatible elements (e.g. F, Cl, REE, U; Stewart and Foden 2003; Agangi et al. 2010, 2012), manifest as the anhydrous, F-rich, A-type magmas of the Hiltaba Suite and Gawler Range Volcanics (e.g. the c. 1.594 Ga Roxby Downs Granite; Cherry et al. 2018) and the mafic to felsic lavas of the c. 1.595–1.589 Ga lower Gawler Range Volcanics (Jagodzinski et al. 2016; McPhie et al. 2020; Fig 4d). In the Olympic Dam area, Hiltaba Suite granites were shallowly emplaced (6–8 km; Creaser 1996) and then rapidly exhumed due to differential uplift and erosion in an environment of transient lithospheric thinning (Figs 4d, 5b). Asthenospheric upwelling caused thermal flux into the upper crust resulting in voluminous magmatism (Figs 4d, 5b). The Hiltaba Suite was overprinted by IOCG mineralisation at ~2 km depth (Oreskes and Einaudi 1990) at c. 1.594–1.590 Ga (Cherry et al. 2018; Courtney-Davies et al. 2019; McPhie et al. 2020).
The A-type lavas and ignimbrites of the c. 1.587 Ga upper Gawler Range Volcanics (Jagodzinski et al. 2016) represent the peak of magmatic activity in the 1.595–1.585 Ga interval in the Gawler Craton. Temporally equivalent felsic lavas of the Benagerie Volcanic Suite were extruded in the Curnamona Province (Wade et al. 2012; Fig 4d). The discrete extrusion age, voluminous nature, and chemical and isotopic homogeneity of the upper Gawler Range Volcanics implies derivation from a mature, well-mixed magma chamber that was efficiently tapped at c. 1.587 Ga (Fig 5c). The combined Gawler Range Volcanics – Benagerie Volcanic Suite and Hiltaba Suite are recognised as the Gawler Siliceous Large Igneous Province (SLIP; e.g. Agangi et al. 2012; Wade et al. 2012; Curtis et al. 2018). Several models have been postulated for the generation of SLIPs (e.g. Pankhurst et al. 1998; Bryan et al. 2000; Ferrari et al. 2002; Decelles et al. 2009). The model of Tiddy and Giles (2020) interprets the Gawler SLIP to have been formed by asthenospheric upwelling and thermal input into a fertile, metasomatised SCLM as the result of tearing or decoupling at the leading edge of a subducted buoyant plateau inboard of the continental margin (Fig 4d).
c. 1.585–1.575 Ga
After c. 1.59 Ga the locus of Hiltaba Suite magmatism migrated towards the central and southwest Gawler Craton, coupled with a change from A-type to I-type granite compositions (Budd 2006; Skirrow et al. 2018; Fig 3b). Tiddy and Giles (2020) interpret these changes to reflect lithospheric heating beneath the central and southwest Gawler Craton due to failure of flat-slab subduction and propagation of hot asthenosphere into a newly created mantle wedge (Fig 4e). Failure of flat subduction is interpreted to result from progressive eclogitisation and densification of the flat slab, resulting in slab steepening. Variations in the complex 3D geometry of the down-going slab (see also Hu and Liu 2016) would allow for upwelling asthenosphere to reach variable parts of the overriding plate. The age and migration of the Hiltaba Suite maps how the flat slab was removed and retreated underneath the Gawler Craton.
The younger, dominantly I-type Hiltaba Suite plutons in the central and southwest Gawler Craton are inferred to have been produced by partial melting of relatively juvenile, hydrated and S-rich lithosphere at the location of earlier c. 1.65–1.605 Ga arc magmatism (Fig 4e). This contrasts with the generation of anhydrous, F-rich, A-type magmatism generated by slab melt-modified lithosphere underneath the Olympic Dam area (Figs 4c and 4d compared with 4e). Although only limited mineralisation ages are available, they broadly follow the magmatic ages. Mineralisation is generally older in the northern Olympic Domain (c. 1.594–1.590 Ga: Cherry et al. 2018; Courtney-Davies et al. 2019; McPhie et al. 2020), slightly younger in the Mount Woods Inlier (c. 1.585 Ga; Belperio et al. 2007) and Yorke Peninsula (1.585 Ga; Gregory et al. 2011) and youngest in the Central Gawler Gold Province (c. 1.58 Ga; Budd and Fraser 2004; Nicolson et al. 2017). Limited older c. 1.59 Ga mineralisation ages have been recognised in the central Gawler Craton (e.g. Nankivel – Nicolson et al. 2017; and Menninnie Dam – Roache and Fanning 1994; Figs 3a–b).
Post c. 1.575 Ga
The end of flat-slab subduction is marked by the emplacement of the youngest Hiltaba Suite plutons at c. 1.575 Ga. Several scenarios may have evolved to cease flat-slab subduction: eclogitisation, steepening and eventual detachment of the entire flat-slab section (see also figure 12 in Skirrow et al. 2018); decoupling of the dense frontal slab from the buoyant oceanic plateau, which rebounded and accreted to the base of the lithosphere (Figs 4e, f); or the dense apron of normal oceanic lithosphere was removed and subducted, and the buoyant oceanic plateau accreted to the base of the lithosphere.
In all cases, convergent margin activity shifted away from the Gawler Craton after c. 1.575 Ga. Younger, episodic magmatism with subduction-related, arc-type geochemistry has been recognised in the Musgrave Province (e.g. c. 1.60–1.54 Ga mafic and felsic rocks – Wade et al. 2006; c. 1.35–1.29 Ga Wankanki Supersuite – Smithies et al. 2010), Coompana Province (e.g. c. 1.53 Ga Bunburra Suite – Wise et al. 2018) and the Madura Province (e.g. c. 1.42–1.39 Ga Haig Cave Supersuite – Smithies et al. 2015) (Fig 1). These magmatic rocks indicate that convergent margin activity progressively shifted away from the Gawler Craton post c. 1.575 Ga (e.g. Betts et al. 2016).
Evidence of geological activity in the Gawler Craton after the intense c. 1.60–1.57 Ga event is relatively limited. Episodic deformation, magmatism and gradual exhumation of marginal terranes of the Gawler Craton (e.g. Stewart and Betts 2010; Morrissey et al. 2019 and references therein) and the Curnamona Province (e.g. Armit et al. 2012) have been recognised.
Phanerozoic comparisons
Laramide flat-slab subduction and retreat in western North America
A flat-slab subduction system has been proposed for the c. 80–50 Ma Laramide Orogeny in the western United States (e.g. Liu and Currie 2016 and references therein). The first stage of the Laramide Orogeny involved an early period of eastward propagating deformation and associated exhumation and uplift from c. 71–58 Ma, associated with flat subduction of the Farallon Plate (e.g. Liu and Currie 2016). The change from normal to flat-slab subduction was triggered by a combination of variations in plate motion and subduction of a thick, buoyant oceanic plateau over 15 million years. Deformation was transferred up to 1,500 km inboard from the subduction margin (e.g. Liu and Currie 2016).
Development of metamorphic core complexes, half-graben basins and low-angle extensional faults in the late stage and following the Laramide Orogeny to c. 20 Ma resulted from the westward migration of arc magmatism and widespread extension in western North America (Humphreys 1995; Fan and Carrapa 2014). Retreat of the subducting Farallon slab has been associated with steepening and rollback (Fan and Carrapa 2014) and/or eclogitisation, densification and subsequent removal of the previously buoyant oceanic plateau (Liu et al. 2010; Liu and Gurnis 2010).
Retreat of the Farallon Slab provides a useful analogy to the model of Tiddy and Giles (2020). Compelling similarities between the Phanerozoic western North America and Proterozoic Australia settings include:
- shutting down of a pre-existing magmatic arc and far-field transfer of compressional stresses inboard of the subduction zone margin, >1,000 km in western North America and ~500 km in Proterozoic Australia, resulting from flat-slab subduction of buoyant oceanic lithosphere.
- lithospheric melting triggered by asthenospheric upwelling associated with rollback and/or removal of the down-going slab.
- magmatism associated with lithospheric extension, normal faulting and the formation of metamorphic core complexes (see Tiddy et al. 2020 for evidence of a c. 1.60–1.57 Ga metamorphic core complex in the Mount Woods Inlier).
The Cretaceous Coastal Cordillera of Chile
Convergent margin IOCG terranes such as the Cretaceous Coastal Cordillera of Chile between 21° and 30° S (Sillitoe 2003; Chen et al. 2013) are also analogous with the model of Tiddy and Giles (2020). In both regions:
- IOCG mineralisation developed during transient periods of extension within the convergent margin (Sillitoe 2003; Belperio et al. 2007; Stewart and Betts 2010; Skirrow et al. 2018)
- a temporal and spatial association between mineralisation and voluminous magmatism exists (e.g. Richards et al. 2017; Williams et al. 2005; Groves et al. 2010; Reid 2019)
- regional-scale fluid pathways facilitated the transport of melts and components of ore fluid derived from devolatisation of mantle-derived magmas (e.g. Sillitoe 2003; Groves et al. 2010)
- metal sources are postulated to have a mix of mantle and crustal components (e.g. Skirrow et al. 2007 and references therein)
- structural controls of ore deposits are hosted within a wide range of rock types which may significantly pre-date mineralisation (e.g. Williams et al. 2005)
- a tectonic setting involving slab rollback with extension switching to compression in the overriding plate and associated with magmatism and mineralisation (e.g. Sillitoe 2003; Reid 2019 and references therein).
Implications for Cu–Au mineralisation
Despite differences between Cretaceous Andean and Proterozoic Australian IOCG provinces, Tiddy and Giles (2020) draw on knowledge of modern IOCG systems to inform three key features required to generate ancient IOCG deposits:
- Metasomatised or melt-modified SCLM is recognised as the most likely source of elevated high field strength and light rare earth elements (and F in the case of the Gawler Range Volcanics – Hiltaba Suite magmatism) in the mafic melts associated with IOCG mineralisation. The SCLM may also be the source of Cu and Au (e.g. Skirrow et al. 2007; Groves et al. 2010). Therefore, generation and preservation of metasomatised/melt-modified SCLM beneath the future site of mineralisation is critical.
- A trigger to induce partial melting of the SCLM. This trigger may be thermal or mechanical (decompression). Melting will produce volatile- and/or metal-rich magmas that migrate into the overriding crust (e.g. Begg et al. 2012; Griffin et al. 2013).
- Deeply penetrating faults linking the upper crust to the mantle via mid- to lower-crustal magma chambers, thereby forming a lithospheric-scale plumbing system along which melts and fluids can be transported from depth (e.g. Begg et al. 2012; Reid 2019).
Tectonic setting of SCLM metasomatism
Preservation of metasomatised SCLM beneath the present-day Archean nucleus of the Gawler Craton and the Olympic Domain has been evidenced from geophysics (Thiel and Heinson 2013; Skirrow et al. 2018). Constraining the absolute timing of SCLM metasomatism beneath the Gawler Craton is difficult due to its complex and protracted geological history and conjecture in tectonic setting of the region (e.g. Betts and Giles 2006; Wade et al. 2006; Payne et al. 2009). Regional SCLM metasomatism in suprasubduction settings is suggested to be caused by interaction with fluids or melts expelled from the down-going slab (e.g. Köhler et al. 2009); however, may also occur by interaction with metasomatic fluids derived from a mantle plume independent of subduction processes (e.g. Howarth et al. 2014).
There is potential that SCLM metasomatism was associated with mantle plume activity at c. 2.52–2.00 Ga (Wade et al. 2019) and/or with subduction-related settings at c. 2.46–2.41 Ga (Sleafordian Orogeny; Reid et al. 2014) and/or 1.73–1.69 Ga (Kimban Orogeny; e.g. Hand et al. 2007). The temporally closest major tectonic event to the c. 1.60–1.57 Ga event is the Kimban Orogeny. During this period the Archean nucleus of the Gawler Craton is inferred to have been accreted to the North Australian Craton along a north-dipping subduction zone (Betts et al. 2016 and references therein). However, in this geometry, regional subduction-induced metasomatism of the SCLM beneath the Gawler Craton would not have been possible because the Gawler Craton was located on the down-going plate rather than in a suprasubduction setting.
Following accretion during the Kimban Orogeny, a new north-dipping subduction zone initiated to the west of the Gawler Craton (Fig 4a). Regional subduction-induced metasomatism of the Gawler Craton SCLM in such a suprasubduction setting is interpreted here to have occurred leading into the period of flat-slab subduction at c. 1.604–1.595 Ga. This is analogous with the Laramide Orogeny where dehydration of the Farallon flat slab metasomatised the SCLM beneath western North America ~1,000 km inboard from the subduction margin (e.g. Lee 2005).
Mechanism of SCLM metasomatism
Tiddy and Giles (2020) suggest that the geometry of the subducting slab beneath the Gawler Craton influenced the composition of SCLM metasomatism at various distances inboard from the subduction margin, which subsequently dictated the style of magmatism and mineralisation in the craton from c. 1.60 to 1.57 Ga. This is now reflected as S-poor, F-rich IOCG deposits and A-type magmatism in the Olympic Domain compared to Au-only epithermal deposits and I-type magmatism in the central Gawler Craton (Central Gawler Gold Province).
IOCG mineralising systems are S-poor (e.g. Williams et al. 2005; Groves et al. 2010), therefore SCLM metasomatism processes are also likely to be S-poor. However, the role of halogens, particularly F, is less apparent in the Gawler Craton. Within the Olympic Dam deposit itself, F is postulated to have been sourced from Gawler Range Volcanics – Hiltaba Suite magmatism and transported to the site of mineralisation via hydrothermal fluids (e.g. McPhie et al. 2012 and references therein). Following from Wade et al. (2019), Tiddy and Giles (2020) suggest that as the metasomatised SCLM-derived mafic rocks of the Gawler Range Volcanics – Hiltaba Suite are F-rich, the elevated F in the these rocks reflects the chemistry of their metasomatised SCLM source (Figs 4c–4d, 5). Enrichment of the lithospheric mantle in F (F-apatite and/or F-phlogopite) is considered as being typical of subduction-related metasomatism (e.g. volcanic rocks in the Andes; Köhler et al. 2009).
The geometry of the down-going slab will determine whether the slab will dehydrate or melt, and in effect control F enrichment in the metasomatised SCLM. Slab dehydration during normal subduction of oceanic crust will release oxidised, sulfate-rich fluids during breakdown of hydrous phases (typically occurring at slab depths of ~100–120 km) and result in an oxidised S-rich mantle wedge (Zellmer et al. 2015; Pons et al. 2016). During slab dehydration, F will be retained in the slab and transported to greater depths within F-bearing mica and amphibole end members (Straub and Layne 2003; Kendrick et al. 2014). However, during slab melting, F will be released and partition into melt phases (e.g. Köhler et al. 2009; Straub and Layne 2003).
Significant slab melting can occur in a number of subduction geometries including stalling of a slab in a subduction zone, flat slab subduction, or failed flat-slab subduction where slab rollback causes the slab to steepen into a more normal subduction angle (Mungall 2002). Tiddy and Giles (2020) propose a combination of these processes contributed to significant slab melting of the down-going slab in the subduction zone of eastern Proterozoic Australia. Slab melting occurred possibly from as early as c. 1.67 Ga and through to the period of flat-slab subduction and subsequent failure of the flat slab at c. 1.595–1.585 Ga (Fig 4). The slab would have been dehydrated and depleted in S and water beneath the magmatic arc, resulting in metasomatism of the SCLM under the present-day Olympic Domain by anhydrous, S-poor and F-rich slab melts.
Mantle controls on mineralisation style
The composition of partial melts derived from a metasomatised SCLM are influenced by the composition of the SCLM. Subsequently, properties of the hydrothermal fluids exsolved from those partial melts (e.g. oxidation state, volatile and complexing agent content and metal carrying capacity; Richards et al. 2017), are also linked to the SCLM composition. Oxidised mantle-derived melts are considered important for the transport of Cu and Au in IOCG and Cu porphyry systems (e.g. Mungall 2002). However, Richards et al. (2017) demonstrated the S content of mantle-derived partial melts can influence the distribution of Cu porphyry and IOCG deposits in the same tectonic setting (e.g. Mesozoic IOCG and Cu porphyry deposits in the Chilean Coastal Cordillera). Copper porphyry mineralisation is associated with oxidised, hydrous, S-rich, calc-alkaline magmas produced from the partial melting of a metasomatised asthenospheric wedge proximal to the subduction zone margin. Conversely, IOCG mineralisation is associated with S-poor, relatively primitive, oxidised asthenospheric magmas that develop distal from the subduction zone margin (e.g. within a back-arc extensional environment).
The model of Richards et al. (2017) is an appealing analogy for eastern Proterozoic Australia as a spatial and temporal relationship exists between IOCG and porphyry-epithermal mineral systems in the Gawler Craton, which is similar to those seen for Mesozoic IOCG and Cu porphyry deposits in the Andes. The Mesozoic Andean and Proterozoic Australian mineral belts also share a broadly similar tectonic setting including evolving continental margins with transient periods of extension. Mineral deposits within the central Gawler Craton (e.g. Tarcoola, Tunkillia) are sulfide-rich (where Au is associated with pyrite, chalcopyrite and arsenopyrite; Budd and Skirrow 2007), whereas IOCG deposits are dominated by Fe-oxides (Groves et al. 2010). The transition from older A-type affinity rocks in the Olympic Domain to younger I-type affinity rocks in the central and southwest Gawler Craton (Fig 3a) suggests a spatial variation in the composition of the SCLM/SCLM-derived melts exists, which may be related to the chemistry of the fluids/melts derived from the subducting slab. Spatial and temporal variations in c. 1.60–1.57 Ga magmatism and mineralisation are therefore linked to the evolution of the subducting slab beneath the Gawler Craton (Tiddy and Giles 2020).
Tiddy and Giles (2020) note two variations between their model for the Gawler Craton and that of Richards et al. (2017). First, the S-poor magmatic-hydrothermal system related to IOCG mineralisation was F-rich and sourced from metasomatised SCLM. Second, IOCG mineralisation is spatially distinct (towards the continental interior) from the S-rich epithermal system (towards the subduction zone).
Mineral genesis
In accordance with the alternative tectonic model proposed by Tiddy and Giles (2020), Cu–Au mineralisation in the Gawler Craton is linked to mechanisms by which metasomatised SCLM and partial melts favourable for Cu–Au mineralisation can be generated. As summarised above, a switch from rollback to flat-slab subduction zone geometry occurred c. 1.604 Ga, which is proposed as the initiation of mineralisation (Figs 4a–c). As the flat slab steepened and rolled back, it was progressively removed and retreated, allowing upwelling of hot asthenosphere (Fig 4c–d). Associated heating and melting (as opposed to dehydration) of the down-going oceanic slab generated anhydrous, F-rich, S-poor melts that interacted with the fertile, metasomatised SCLM. Initial upwelling of hot asthenosphere was located underneath the present-day Olympic Domain at c. 1.595 Ga (Figs 4d, 5a). Mafic melts produced were mostly concentrated into a lower crustal magma chamber that evolved through processes of assimilation-fractional crystallisation (Figs 5a–b), where assimilation of approximately 30% crustal material occurred (Stewart and Foden 2003; Chapman et al. 2019). Asthenospheric upwelling and sudden heat input also caused localised upper crustal extension in the Olympic Dam area and generation of the highly fractionated, F-rich Gawler SLIP. Metal-bearing fluids rich in incompatible elements and complexing agents (e.g. F, Cl, REE and U) were exsolved from evolved and highly fractionated magmas that stalled in the upper crust (Fig 5b). Regional-scale structures (e.g. lithospheric faults, including structures identified beneath the Olympic Dam deposit; Heinson et al. 2018; Skirrow et al. 2018; Curtis and Thiel 2019; Motta et al. 2019) facilitated melt and fluid transport from depth to the upper crustal levels. The metal-bearing fluids were then variably mixed with meteoric- and/or basinal- derived fluids (e.g. Williams et al. 2005; Bastrakov et al. 2007), forming IOCG deposits in the Olympic Domain (Figs 2, 3a–b, 4d, 5b–c).
The migration of magmatism maps the progressive retreat of the down-going slab. By c. 1.575 Ga the under-riding plate had retreated to the present-day central Gawler (Central Gawler Gold Province; Fig 2). The composition of mantle-derived partial melts produced at this time was influenced by hydrous, S-rich, F-poor fluids exsolved from the down-going slab during slab dehydration (Fig 4e). Lithospheric-scale structures in the central Gawler Craton region transported the hydrous, S-rich, F-poor fluids to the upper crust. Resultant mineralisation shows similarities to sulfide-rich, relatively Fe-depleted Cu-porphyry and epithermal style deposits (see also Richards et al. 2017).
Conclusion
This article summarises an alternative model placing eastern Proterozoic Australia into the context of a modern-day suprasubduction zone at c. 1.60–1.575 Ga at a time of IOCG (and porphyry-epithermal) mineralisation as proposed by Tiddy and Giles (2020). This new model builds on previously proposed tectonic models of eastern Proterozoic Australia that include the c. 1.60–1.575 Ga timeslice and draws on aspects from modern analogues of the Late Cretaceous to Early Eocene Laramide Orogeny in western North America, present-day slab geometries of the South American Andes and formation of Cretaceous IOCG and Cu-porphyry deposits in the Chilean Coastal Cordillera. In this model, the temporal and spatial distribution of magmatic and mineralisation styles in the Gawler Craton are dictated by the geometry of the subducting slab and its respective melts and fluids interacting with a metasomatised SCLM. Of note, Tiddy and Giles (2020) state that ‘it is not the type of magmatism that was important to the generation of IOCG versus porphyry-epithermal deposits, but rather the type of partial melt produced in the mantle – oxidised, S-poor (IOCG) or S-rich (porphyry-epithermal) and could carry metals – that interacted with the metasomatised SCLM’.
Acknowledgements
We are thankful to those involved in long-term discussions as well as reviews of the manuscript by Tiddy and Giles (2020). This work has been supported by the Mineral Exploration Cooperative Research Centre (MinEx CRC) which is funded by the Australian Government’s Cooperative Research Centre Program. This is MinEx CRC Document 2020/41.
References
Agangi A, Kamenetsky VS and McPhie J 2010. The role of fluorine in the concentration and transport of lithophile trace elements in felsic magmas: insights from the Gawler Range Volcanics, South Australia. Chemical Geology 273:314–325. doi:10.1016/j.chemgeo.2010.03.008.
Agangi A, Kamenetsky VS and McPhie J 2012. Evolution and emplacement of high fluorine rhyolites in the Mesoproterozoic Gawler Silicic Large Igneous Province, South Australia. Precambrian Research 208–211:124–144. doi:10.1016/j.precamres.2012.03.011.
Armit RJ, Betts PG, Schaefer BF and Aillères L 2012. Constraints on long-lived Mesoproterozoic and Palaeozoic deformational events and crustal architecture in the northern Mount Painter Province, Australia. Gondwana Research 22:207–226. doi:10.1016/j.gr.2011.11.003.
Bastrakov EN, Skirrow RG and Davidson GJ 2007. Fluid evolution and origins of iron oxide Cu-Au prospects in the Olympic Dam district, Gawler Craton, South Australia. Economic Geology 102:1415–1440. doi:10.2113/gsecongeo.102.8.1415.
Begg GC, Griffin WL, O'Reilly SY and Natapov LM 2012. The lithosphere, geodynamics and Archean mineral systems. International Geological Congress, 34th, Brisbane, Australia, 2012, Abstracts, p 1870.
Belperio A, Flint R and Freeman H 2007. Prominent Hill: a hematite-dominated, iron oxide copper-gold system. Economic Geology 102:1499–1510. doi:10.2113/gsecongeo.102.8.1499.
Betts PG and Giles D 2006. The 1800-1100 Ma tectonic evolution of Australia. Precambrian Research 144:92–125. doi:10.1016/j.precamres.2005.11.006.
Betts PG, Armit RJ, Stewart J, Aitken ARA, Aillères L, Donchak P, Hutton L, Withnall I and Giles D 2016. Australia and Nuna. In Li, ZX, Evans DAD and Murphy JB eds, Supercontinent cycles through earth history, Geological Society Special Publication 424. Geological Society of London, pp 47–81. doi:10.1144/SP424.2.
Bryan SE, Ewart A, Stephens CJ, Parianos J and Downes PJ 2000. The Whitsunday Volcanic Province, central Queensland, Australia: lithological and stratigraphic investigations of a silicic-dominated large igneous province. Journal of Volcanology and Geothermal Research 99:55–78. doi:10.1016/S0377-0273(00)00157-8.
Budd AR 2006. The Tarcoola Goldfield of the Central Gawler Gold Province, and the Hiltaba Association Granites, Gawler Craton, South Australia. PhD thesis, Australian National University, Canberra.
Budd AR and Skirrow RG 2007. The nature and origin of gold deposits of the Tarcoola Goldfield and implications for the Central Gawler Gold Province, South Australia. Economic Geology 102:1541–1563. doi:10.2113/gsecongeo.102.8.1541.
Chapman ND, Ferguson M, Meffre SJ, Stepanov A, Maas R and Ehrig KJ 2019. Pb-isotopic constraints on the source of A-type suites: insights from the Hiltaba Suite - Gawler Range Volcanics Magmatic Event, Gawler Craton, South Australia. Lithos 346–347:105156. doi:10.1016/j.lithos.2019.105156.
Cherry AR, Ehrig K, Kamenetsky VS, McPhie J, Crowley JL and Kamenetsky MB 2018. Precise geochronological constraints on the origin, setting and incorporation of c. 1.59 Ga surficial facies into the Olympic Dam Breccia Complex, South Australia. Precambrian Research 315:162–178. doi:10.1016/j.precamres.2018.07.012.
Courtney-Davies L, Tapster SR, Ciobanu CL, Cook NJ, Verdugo-Ihl MR, Ehrig KJ, Kennedy AK, Gilbert SE, Condon DJ and Wade BJ 2019. A multi-technique evaluation of hydrothermal hematite U-Pb isotope systematics: implications for ore deposit geochronology. Chemical Geology 513:54–72. doi:10.1016/j.chemgeo.2019.03.005.
Creaser RA 1996. Petrogenesis of a Mesoproterozoic quartz latite-granitoid suite from the Roxby Downs area, South Australia. Precambrian Research 79:371–394. doi:10.1016/S0301-9268(96)00002-2.
DeCelles PG, Ducea MN, Kapp P and Zandt G 2009. Cyclicity in Cordilleran orogenic systems. Nature Geoscience 2:251–257. doi:10.1038/ngeo469.
Forbes CJ, Betts PG, Giles D and Weinberg R 2008. Reinterpretation of the tectonic context of high-temperature metamorphism in the Broken Hill Block, NSW, and implications on the Palaeo- to Meso-Proterozoic evolution. Precambrian Research 166:338–349. doi:10.1016/j.precamres.2006.12.017.
Forbes CJ, Giles D, Jourdan F, Sato K, Omori S and Bunch M 2012. Cooling and exhumation history of the northeastern Gawler Craton, South Australia. Precambrian Research 200–203:209–238. doi:10.1016/j.precamres.2011.11.003.
Griffin WL, Begg GC and O’Reilly SY 2013. Continental-root control on the genesis of magmatic ore deposits. Nature Geoscience 6:905–910. doi:10.1038/ngeo1954.
Groves DI, Bierlein FP, Meinert LD and Hitzman MW 2010. Iron oxide copper-gold (IOCG) deposits through earth history: implications for origin, lithospheric setting, and distinction from other epigenetic iron oxide deposits. Economic Geology 105(3):641–654. doi:10.2113/gsecongeo.105.3.641.
Hand M, Reid A and Jagodzinski L 2007. Tectonic framework and evolution of the Gawler Craton, South Australia. Economic Geology 102(8):1377–1395. doi:10.2113/gsecongeo.102.8.1377.
Howarth GH, Barry PH, Pernet-Fisher JF, Baziotis IP, Pokhilenko N, Pokhilenko LN, Bodnar RJ, Taylor LA and Agashev AM 2014. Superplume metasomatism: evidence from Siberian mantle xenoliths. Lithos 184–187:209–224. doi:10.1016/j.lithos.2013.09.006.
Hu J and Liu L 2016. Abnormal seismological and magmatic processes controlled by the tearing South American flat slabs. Earth and Planetary Science Letters 450:40–51. doi:10.1016/j.epsl.2016.06.019.
Humphreys E 1995. Post-Laramide removal of the Farallon Slab, western United-States. Geology 23:987–990. doi:10.1130/0091-7613(1995)023<0987:PLROTF>2.3.CO;2.
Kendrick MA, Jackson MG, Kent AJR, Hauri EH, Wallace PJ and Woodhead J 2014. Contrasting behaviours of CO2, S, H2O and halogens (F, Cl, Br, and I) in enriched-mantle melts from Pitcairn and Society seamounts. Chemical Geology 370:69–81. doi:10.1016/j.chemgeo.2014.01.019.
Köhler J, Schönenberger J, Upton B and Mark lG 2009. Halogen and trace-element chemistry in the Gardar Province, South Greenland: subduction-related mantle metasomatism and fluid exsolution from alkalic melts. Lithos 113(3–4):731–747. doi:10.1016/j.lithos.2009.07.004.
Lee C-TA 2005. Trace element evidence for hydrous metasomatism at the base of the North American lithosphere and possible association with Laramide low-angle subduction. The Journal of Geology 113:673–685. doi:10.1086/449327.
Liu L and Gurnis M 2010. Dynamic subsidence and uplift of the Colorado Plateau. Geology 38(7):663–666. doi:10.1130/G30624.1.
Liu L, Gurnis M, Seton M, Saleeby J, Müller RD and Jackson JM 2010. The role of oceanic plateau subduction in the Laramide orogeny. Nature Geoscience 3:353–357. doi:10.1038/ngeo829.
Liu S and Currie CA 2016. Farallon plate dynamics prior to the Laramide Orogeny: numerical models of flat subduction. Tectonophysics 666:33–47. doi:10.1016/j.tecto.2015.10.010.
McPhie J, Ehrig KJ, Kamenetsky MB, Crowley JL and Kamenetsky VS 2020. Geology of the Acropolis prospect, South Australia, constrained by high-precision CA-TIMS ages. Australian Journal of Earth Sciences 67:699–716. doi:10.1080/08120099.2020.1717617.
Morrissey LJ, Hand M, Wade BP and Szpunar M 2013. Early Mesoproterozoic metamorphism in the Barossa Complex, South Australia: links with the eastern margin of Proterozoic Australia. Australian Journal of Earth Sciences 60:769–795. doi:10.1080/08120099.2013.860623.
Mungall JE 2002. Roasting the mantle: slab melting and the genesis of major Au and Au-rich Cu deposits. Geology 30(10):915–918. doi:10.1130/0091-7613(2002)030<0915:RTMSMA>2.0.CO;2.
Oreskes N and Einaudi MT 1990. Origin of rare earth element-enriched hematite breccias at the Olympic Dam Cu-U-Au-Ag deposit, Roxby Downs, South Australia. Economic Geology 85(1):1–28. doi:10.2113/gsecongeo.85.1.1.
Pankhurst RJ, Leat PT, Sruoga P, Rapela CW, Marquez M, Storey BC and Riley TR 1998. The Chon Aike Province of Patagonia and related rocks in West Antarctica: a silicic large igneous province. Journal of Volcanology and Geothermal Research 81:113–136. doi:10.1016/S0377-0273(97)00070-X.
Payne JL, Hand M, Barovich KM, Reid A and Evans DAD 2009. Correlations and reconstruction models for the 2500-1500 Ma evolution of the Mawson Continent. In Reddy SM, Mazumder R, Evans DAD and Collins AS eds, Palaeoproterozoic supercontinents and global evolution, Geological Society Special Publication 323. Geological Society of London, pp 319–355. doi:10.1144/SP323.16.
Payne JL, Ferris G, Barovich KM and Hand M 2010. Pitfalls of classifying ancient magmatic suites with tectonic discrimination diagrams: an example from the Paleoproterozoic Tunkillia Suite, southern Australia. Precambrian Research 177:227–240. doi:10.1016/j.precamres.2009.12.005.
Raveggi M, Giles D, Foden J and Raetz M 2007. High Fe-Ti mafic magmatism and tectonic setting of the Palaeoproterozoic Broken Hill Block, NSW, Australia. Precambrian Research 156:55–84. doi:10.1016/j.precamres.2007.02.006.
Reid AJ, Jagodzinski EA, Fraser GL and Pawley MJ 2014. SHRIMP U–Pb zircon age constraints on the tectonics of the Neoarchean to early Paleoproterozoic transition within the Mulgathing Complex, Gawler Craton, South Australia. Precambrian Research 250:27–49. doi:10.1016/j.precamres.2014.05.013.
Roache MW and Fanning CM 1994. Timing of mineralization at the Menninnie Dam Pb-Zn-Ag deposit, Eyre Peninsula, South Australia. 12th Australian Geological Convention, 26-30th September, 1994, The University of Western Australia, Perth, Abstracts 37. Geological Society of Australia, pp 376–377.
Sillitoe RH 2003. Iron oxide-copper-gold deposits: an Andean view. Mineralium Deposita 38:787–812. doi:10.1007/s00126-003-0379-7.
Skirrow RG, Bastrakov EN, Barovich K, Fraser GL, Creaser RA, Fanning CM, Raymond OL and Davidson GJ 2007. Timing of iron oxide Cu-Au-(U) hydrothermal activity and Nd isotope constraints on metal sources in the Gawler Craton, South Australia. Economic Geology 102:1441–1470. doi:10.2113/gsecongeo.102.8.1441.
Smithies RH, Spaggiari CV, Kirkland CL, Wingate MTD and England RN 2015. Madura Province: geochemistry and petrogenesis. In Spaggiari CV and Smithies RH eds, Eucla basement stratigraphic drilling results release workshop: extended abstracts, Record 2015/10. Geological Survey of Western Australia, Perth, pp 17–28.
Smithies RH, Howard HM, Evins PM, Kirkland CL, Kelsey DE, Hand M, Wingate MTD, Collins AS, Belousova E and Allchurch S 2010. Geochemistry, geochronology and petrogenesis of Mesoproterozoic felsic rocks in the western Musgrave Province of central Australia, and implication for the Mesoproterozoic tectonic evolution of the region, Report 106. Geological Survey of Western Australia, Perth.
Stewart JR and Betts PG 2010. Implications for Proterozoic plate margin evolution from geophysical analysis and crustal-scale modelling within the western Gawler Craton, Australia. Tectonophysics 483:151–177. doi:10.1016/j.tecto.2009.11.016.
Straub SM and Layne GD 2003. The systematics of chlorine, fluorine, and water in Izu arc front volcanic rocks: implications for volatile recycling in subduction zones. Geochimica et Cosmochimica Acta 67:4179–4203. doi:10.1016/S0016-7037(03)00307-7.
Swain G, Barovich K, Hand M, Ferris G and Schwarz M 2008. Petrogenesis of the St Peter Suite, southern Australia: arc magmatism and Proterozoic crustal growth of the South Australian Craton. Precambrian Research 166:283–296. doi:10.1016/j.precamres.2007.07.028.
Symington NJ, Weinberg RF, Hasalová P, Wolfram LC, Raveggi M and Armstrong RA 2014. Multiple intrusions and remelting-remobilization events in a magmatic arc: the St. Peter Suite, South Australia. Geological Society of America Bulletin 126:1200–1218. doi:10.1130/B30975.1.
Tiddy CJ and Giles D 2020. Suprasubduction zone model for metal endowment at 1.60-1.57 Ga in eastern Australia. Ore Geology Reviews 122:103483. doi:10.1016/j.oregeorev.2020.103483.
Tiddy CJ, Betts PG, Neumann MR, Murphy FC, Stewart J, Giles D, Sawyer M, Freeman H and Jourdan F 2020. Interpretation of a ca. 1600–1580 Ma metamorphic core complex in the northern Gawler Craton, Australia. Gondwana Research 85:263–290. doi:10.1016/j.gr.2020.04.008.
Turner S, Foden J, Sandiford M and Bruce D 1993. Sm-Nd isotopic evidence for the provenance of sediments from the Adelaide Fold Belt and southeastern Australia with implications for episodic crustal addition. Geochimica et Cosmochimica Acta 57:1837–1856. doi:10.1016/0016-7037(93)90116-E.
Wade BP, Barovich KM, Hand M, Scrimgeour IR and Close DF 2006. Evidence for early Mesoproterozoic arc magmatism in the Musgrave Block, central Australia: implications for Proterozoic crustal growth and tectonic reconstructions of Australia. Journal of Geology 114:43–63. doi:10.1086/498099.
Wade CE, Reid AJ, Wingate MTD, Jagodzinski EA and Barovich K 2012. Geochemistry and geochronology of the c. 1585 Ma Benagerie Volcanic Suite, southern Australia: relationship to the Gawler Range Volcanics and implications for the petrogenesis of a Mesoproterozoic silicic large igneous province. Precambrian Research 206–207:17–35. doi:10.1016/j.precamres.2012.02.020.
Wade CE, Payne JL, Barovich KM and Reid AJ 2019. Heterogeneity of the sub-continental lithospheric mantle and ‘non-juvenile’ mantle additions to a Proterozoic silicic large igneous province. Lithos 340–341:87–107. doi:10.1016/j.lithos.2019.05.005.
Williams PJ, Barton MD, Jonson DA, Fontboté L, de Haller A, Mark G, Oliver NHS and Marschik R 2005. Iron oxide copper-gold deposits: geology, space-time distribution, and possible modes of origin. Economic Geology 100:371–405. doi:10.5382/AV100.13.
Wyborn LAI, Page RW and Parker AJ 1987. Geochemical and geochronological signatures in Australian Proterozoic igneous rocks. In Pharaoh TC, Bechinsale RD and Rickard D eds, Geochemistry and mineralisation of Proterozoic volcanic suites, Geological Society Special Publication 33. Geological Society of London, pp 377–394. doi:10.1144/GSL.SP.1987.033.01.26.
Zellmer GF, Edmonds M and Straub SM 2015. Volatiles in subduction zone magmatism. In Zellmer GF, Edmonds M and Straub SM eds, The role of volatiles in the genesis, evolution and eruption of arc magmas, Geological Society Special Publication 410. Geological Society of London, pp 1–17. doi:10.1144/SP410.13.