2730–2670 Ma rifting triggers sagduction prior to the onset of orogenesis at ca 2650 Ma: implications for gold mineralisation, Eastern Goldfields, Western Australia

Abstract

The dominant structural fabric in the Eastern Goldfields is the steep north- to north-northwest-trending S₂ foliation that is axial planar to upright F₂ folds, developed during intense horizontal east–west shortening (D_2–4 event). These structures consistently overprint D₁ structures, which comprise layer-parallel S₁ foliation, extensional shears, thrusts and recumbent F₁ folds. The D₁ event represents a separate and distinct episode of deformation with a markedly different stress regime (dominant vertical σ₁), compared with the stress regime during the D_2–4 events (horizontal east–west oriented σ₁). There is little evidence for pre-2670 Ma ductile deformation in the Eastern Goldfields, as: (1) the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences, with the first significant angular unconformities observed at the base of the post greenstone late basins; (2) a lack of schist or gneiss clasts in the late basins suggests that the surrounding uplifted sequence was largely undeformed; (3) the layer-parallel S₁ foliation is observed throughout the 2720–2670 Ma greenstone sequence; and (4) the dominant low north- and south-plunges of F₂ folds suggest that the sequence was predominantly flat-lying prior to strong east–west shortening (D_2–4 events). The ca 2670–2655 Ma D₁ event represents a period of sagduction that occurred immediately after cessation of rifting as a result of gravitational instability from deposition of dense, cold, greenstone sequences onto thinned, light, hot felsic crust. Late basins represent depocentres on the sinking greenstones during sagduction. Marked contrasts in the structural style of gold deposits, metallogeny and fluid sources, typically attributed to progressive deformation during orogenesis, could instead reflect temporally distinct mineralising events during changing tectonic regimes. In the East Yilgarn, an early plumbing system dominated by north-northwest-trending basin-controlling structures was likely established during rifting and focused fluids during multiple hydrothermal circulation events.

KEY POINTS

1. ca 2720–2670 Ma greenstone sequences in the Eastern Goldfields were deposited in intracratonic rift basins.

2. Rifting triggered an episode of sagduction that formed the early dome-and-basin geometry of the Eastern Goldfields and the onset of orogenesis at ca 2650 Ma (D_2–4 events) modified the existing dome-and-basin geometry.

3. Late basins represent depocentres on sinking greenstone sequences during sagduction.

4. Gold mineralisation occurred throughout the changing tectonic regimes.

Keywords:

• Yilgarn Craton
• Eastern Goldfields
• sagduction
• gold mineralisation
• deformation
• D₁ event

Introduction

The Yilgarn Craton is richly endowed in gold and other metals and because it is easily accessible, it is one of the most studied cratons on the planet. Yet, decades of research have failed to deliver a consensus on its tectonic evolution.

The structural architecture of the East Yilgarn differs from modern tectonic settings. In this region, the crust is dominated by large granite bodies of the tonalite–trondhjemite–granodiorite (TTG) series that are surrounded by narrow corridors of greenstone belts. Over the entire craton, granite and greenstone belts are strongly elongated in a north–south direction, suggesting a strong homogeneous east–west shortening. There is a lack of large-scale thrust and nappe systems akin to those documented in modern orogens. The greenstone belts are characterised by stratigraphic assemblages dominated by mafic volcanic rocks, including komatiites, that reflect the hotter mantle in the Archean. Abundant pillow lavas indicate emplacement below sea-level (Arndt, 1999; Flament et al., 2008) in shallow basins collecting only minor sedimentary sequences. This set of attributes suggests that the continental lithosphere was hotter and much weaker, and therefore unable to sustain significant topography, making Archean landscapes very flat (Arndt, 1999; Rey & Houseman, 2008).

In the 1970s and early 1980s, the dome-and-basin geometry of granite–greenstones was commonly explained by gravity-driven vertical tectonics. Analogue and mathematical models suggested diapirism of light granitoid rocks into denser mafic rock (Anhaeusser, 1973; Archibald et al., 1981; Hickman, 1984; MacGregor, 1951; Mareschal & West, 1980). The advent of plate tectonics in the late 1980s and early 1990s re-interpreted gravity-driven deformation with models that extrapolated present-day plate tectonic processes to the Archean. Passchier (1995) summarised some of the models to include back-arc or marginal basins in an obliquely convergent plate tectonic model (Barley et al., 1989, 2003; Windley 1984); exotic terrane accretion and the amalgamation of island arcs, back-arc crust and microcontinents (Myers, 1995); ensialic rift systems (Archibald et al., 1978; Hallberg, 1986) and plume tectonics (Campbell & Hill, 1988). The regional dome-and-basin geometries were commonly explained in terms of polyphase folding of sill-like intrusions to form the domes (Myers & Watkins, 1985), or core complex style asymmetric extension (Hammond & Nisbet, 1992; Kusky, 1993; Williams & Whitaker, 1993). From the 1990s onwards, most workers considered the Eastern Goldfields to represent an amalgamation of accreted terranes along north-northwest-trending sutures (Barley et al., 2003; Blewett et al., 2010; Cassidy et al., 2006; Czarnota et al., 2010; Krapež et al., 2000; Krapež & Hand, 2008; Myers, 1995). The Eastern Goldfields Superterrane (EGST) comprises the Kalgoorlie, Kurnalpi, Burtville and Yamarna terranes (Figures 1a and 2a). The 3000–2630 Ma Youanmi Terrane to the west is separated from the EGST by the north-trending Ida Fault. The unconformably overlying late basins were thought to be deposited post accretion, possibly in pull-apart basins developed adjacent to large transpressional shears, owing to their diverse detrital zircon populations (Kositcin et al., 2008; Krapež et al., 2008).

Figure 1. (a) Sm–Nd isotope map of the Yilgarn Craton (Lu et al., 2022), (b) geological map of the EGST terranes and (c) simplified rift geometry for the Eastern Goldfields.

Figure 2. (a) Geological map of the EGST terranes and (b) stratigraphy of the Kalgoorlie Terrane.

However, recent studies suggest autochthonous development of the Eastern Goldfields with the identification of older greenstone stratigraphy (ca 2800 Ma, Youanmi equivalent age) below the 2730–2670 Ma greenstone sequences in scattered locations throughout the Eastern Goldfields (Baggott, 2006; Gole et al., 2019; Masurel et al., 2022; Mole et al., 2019; Pawley et al. 2012; Schreefel et al., 2024; Van Kranendonk et al., 2012). Isotope maps show that the East Yilgarn Craton is dominated by young felsic crust with the granite–greenstones emplaced between 2730 and 2670 Ma (Champion & Cassidy, 2007; Champion & Huston, 2016; Hartnady & Kirkland, 2022; Lu et al., 2022; Mole et al., 2019). The basement comprises felsic crust isotopically similar to the ca 3000–2630 Ma Youanmi Terrane (Figure 1a). Smithies et al. (2023) show that pre-existing northeast to east-northeast geochemical trends in granite extend across the Yilgarn Craton and across some of the major faults or sutures. Preservation of this early architecture precludes amalgamation of exotic terranes by lateral east–west accretion.

The strong north-northwest trend displayed by the isotopic data is thought to reflect rift geometries, with greenstones deposited in discrete rift basins bounded by major north-northwest-trending faults (Figure 1b, c). Significant facies variations across some of the major domain boundary faults indicate that some represent early structures that controlled basin development (Miller et al., 2010; Standing, 2008; Tripp et al., 2007).

In the Laverton and Leonora districts, the younger ca 2730–2670 Ma sequence lies conformably on the older ca 2800 Ma sequence and indicates that the older sequence has not been tilted prior to deposition of the younger sequence (Jones, 2014). This suggests that there is very little pre-ca 2670 Ma ductile deformation; instead, the first significant angular unconformities are observed at the base of the ca 2665–2655 Ma late basins (Jones et al., 2022).

The late basins provide a crucial clue to the nature of early tectonics in the Eastern Goldfields. They are seen throughout the Eastern Goldfields, and a similar depositional age suggests that they formed in response to a regional tectonic process and that this is not a local phenomenon. Late basin formation is synchronous with D₁ deformation and pre-dates the strong east–west horizontal D₂ compression and peak metamorphism, as all basins (except Wallaby Conglomerate) are tightly folded by upright north-trending F₂ folds and contain the steep axial planar S₂ foliation (Jones et al., 2022; Painter & Groenewald, 2001; Swager et al., 1995).

In this paper, I present field evidence from regional mapping, open pit and underground mapping and structural data from GSWA 1:100 000 and 1:250 000 geological map series. I show that the D₁ event records an episode of sagduction in the Eastern Goldfields, triggered by ca 2720–2670 Ma rifting, prior to the onset of ca 2650 Ma convergence and tectonic inversion under high-geothermal gradients. This paper also considers the implications for gold mineralisation during the evolving tectonic regimes.

Regional geology

Stratigraphy

A broadly coherent stratigraphy is observed in the greenstone sequences across the Eastern Goldfields (Figure 2a), and generally comprises a lower package of komatiite and basalt that is intercalated with sedimentary rocks and dacitic volcano-sedimentary units ranging in age from ca 2720 to 2690 Ma (Figure 2b; Barley et al., 2003; Kositcin et al., 2008; Krapež et al., 2008; Krapež & Hand, 2008). The lower sequence is overlain by deep marine siliciclastic and volcaniclastic sedimentary rocks (ca 2690–2670 Ma; Barley et al., 2003; Hand et al., 2002), and is then unconformably overlain by sedimentary rocks of the ca 2665–2655 Ma late basins (Barley et al., 2003; Dunphy et al., 2003; Kositcin, et al., 2008; Krapež et al., 2000, 2008; Squire et al., 2010; Standing, 2008; Tripp, 2013; Wyche et al., 2012). Although this broad stratigraphic sequence can be observed across the Eastern Goldfields, there are distinctive volcanic centres with differing ages and geochemical characteristics (Barley et al., 2003). A widespread ca 2700 Ma komatiite event is recognised as a regional stratigraphic marker and is observed across the Eastern Goldfields. It was initially mapped out in the southern goldfields (Kositcin et al., 2008; Swager et al., 1995; Woodall, 1965) and is described in the Agnew–Wiluna area (Fiorentini et al., 2005), at Murrin Murrin (Barley et al., 2003) and in the Leonora area with the Sullivans ultramafic unit extending through the Leonora region northward into the Mt Clifford area (Thébaud et al., 2012). An older ca 2800 Ma greenstone sequence is observed in scattered locations or districts in the Eastern Goldfields, such as the Leonora, Laverton, Duketon, and Yamarna districts (Baggott, 2006; Dunphy et al., 2003; Gole et al., 2019; Kositcin et al., 2008; Mole et al., 2019; Pawley et al., 2012). This is similar in age to komatiite units in the Youanmi Terrane to the west (Van Kranendonk et al., 2012). At Leonora, the ca 2700 Ma sequence paraconformably overlies the older ca 2800 Ma sequence with no evidence of tilting of the older sequence (Jones, 2014).

The late basins mark the end of the volcano-sedimentary record, with the youngest basin being the Kurrawang Formation in the Kalgoorlie district, with a maximum deposition age of ca 2655 Ma (Figure 3). Late basin sequences typically grade upwards from polymictic mafic-dominated conglomerates to more siliciclastic compositions with abundant well-rounded granitic clasts (Krapež et al., 2000, 2008; Squire et al., 2010). The clastic sequences record the uplift and exhumation of granite-cored domes, with the eroded detritus deposited into the basins (Squire et al., 2010). The absence of clasts with deformation fabrics (e.g. schist or gneiss) suggests that the uplifted units surrounding the basins were largely undeformed.

Figure 3. Late basin sedimentary rocks: (a) localities of late basins in the Eastern Goldfields; (b) rounded clasts in polymict conglomerate in drill core from Scotty Creek Formation, Agnew district, Wallaby Conglomerate, Laverton district and Merougil Formation, Kambalda; (c, d) basal Scotty Creek Formation in, and adjacent to, the D₁ Emu Shear, Agnew; (e) well-developed upright north-trending S₂ foliation overprints bedded units in Scotty Creek Formation, Agnew; and (f) flattened clasts in strongly foliated polymict Mt Lucky Conglomerate, Laverton district.

The linear trend of most late basins reflects their preservation in tight F₂ synclinal keels. All late basins (except Wallaby Conglomerate) are tightly folded and overprinted by upright north-trending F₂ folds and the axial planar S₂ foliation (Figure 3e, f). Wallaby Conglomerate is not folded, as it is located in a D₂ strain shadow above Mt Margaret Dome. Prior to folding, uplift and erosion, late basin sediments would have been widespread.

The large granitic complexes that separate the greenstone belts in the EGST are typically composite bodies comprising multiple intrusions that range in age from ca 2800 to 2640 Ma. Champion and Sheraton (1997) subdivided granites in the Leonora–Laverton region into five main groups and was extended to the entire Eastern Goldfields by Cassidy and Champion (2004). Ranked by granite volume, these are: high-Ca (∼60%), low-Ca (25%), high HFSE (5%), mafic (5%) and syenitic (1%). Changes in the nature of granitic magmatism over time appear to be broadly coherent, with high-Ca and lesser mafic granites (known as the tonalite–trondhjemite–granodiorite TTG series) dominant between 2720 and 2680 Ma, and transitional high-Ca, mafic and syenitic magmatism between 2675 and 2665 Ma, and low-Ca and lesser syenitic magmatism after 2655 Ma (Figure 2b).

Metamorphism

Metamorphic facies display a coherent pattern throughout the Eastern Goldfields, with prehnite–pumpellyite and lower to mid-greenschist facies rocks in the centre of the greenstone belts, increasing to upper greenschist to amphibolite facies near the contacts with the granitic complexes and within shear zones (Binns et al., 1976; Czarnota et al., 2010; Goscombe et al., 2009; Mikucki & Roberts, 2004; Ridley, 1993). Late basins are located in the middle of the greenstones, typically above the lowest metamorphic facies in the greenstones (Figure 4).

Figure 4. Regional pattern of metamorphic facies across the EGT (modified from Binns et al., 1976).

Ridley (1993) related these metamorphic patterns to exhumation of the granite-gneiss bodies relative to the greenstone belts, rather than the effect of contact metamorphism, as the metamorphic isograds are zonal around granite-gneiss bodies rather than individual plutons (Figure 4). Goscombe et al. (2009) also emphasised that the centres of the granitic complexes have higher-pressure mineral assemblages, thus representing a lower crustal position. Alternatively, there could be a component of contact metamorphism in the overlying greenstone sequences if the granitoid bodies were emplaced as sills, with later exhumation creating the domal architecture (Collins et al., 1998; Van Kranendonk et al., 2004). In contrast, Archibald et al. (1981) and Swager and Nelson (1997) suggested an extensional event to explain the final positions of the high-grade granite-gneiss domes relative to the lower-grade greenstone belts, syn- to post-main granitoid emplacement at ca 2660 Ma. In a sagduction context, it is expected that the core of the sinking keels would register the lowest metamorphic grades, and that the core of granitic domes would record the highest P and T metamorphic conditions.

Structural geology

There are two main phases of ductile deformation in the Eastern Goldfields with early D₁ deformation overprinted by strong east–west D_2–4 compression (Figure 5). Most deformation schemes in the Eastern Goldfields attribute the onset of strong east–west horizontal compression to the regional D₂ event (e.g. Jones, 2014; Miller, 2006; Platt et al., 1978; Passchier, 1994; Swager, 1997; Weinberg et al., 2003) or the D_4b event (Blewett et al., 2010; Czarnota et al., 2010). Early faulting and brittle deformation associated with basin development during rifting are assigned to the D_e event (Figure 5). Localities discussed in the next two sections are shown in Figure 6.

Figure 5. Summary of deformation schemes for the Eastern Goldfields.

Figure 6. Locality map for images in following Figures 7–9.

Although D₁ fabrics (bedding-parallel S₁ foliation, recumbent F₁ folds, D₁ thrusts and D₁ extensional shears) are observed throughout the Eastern Goldfields, the pre-D₂ deformation history is generally poorly understood. The D₁ event has been variously attributed to north–south directed compression (Miller, 2006; Swager, 1997; Weinberg et al., 2003) or extension associated with uplift and exhumation of granitic complexes (Blewett et al., 2010; Czarnota et al., 2010; Jones et al., 2022; Nelson, 1997; Passchier, 1994; Platt et al., 1978; Swager & Nelson, 1997; Weinberg & van der Borgh, 2008; Williams & Currie, 1993). The markedly different interpretations likely reflect the lack of good exposure, as well as the strong overprinting effects of D_2–4 compression.

Early deformation (D₁ event)

D₁ structures are well preserved in D₂ strain shadows such as the northern and southern ends of the large granite domes; in places, the flat-lying layer-parallel S₁ foliation is the dominant fabric, and recumbent F₁ folds are common (Figure 7a–d). Throughout the rest of the Eastern Goldfields, the dominant fabric is the steep north-trending S₂ foliation that is axial planar to abundant north-trending upright F₂ folds (Figure 7e–h).

Figure 7. D₁ vs D₂ structures (localities shown in Figure 6): (a) flat-lying S₁ foliation, Trump pit, Leonora; (b) flat-lying S₁ foliation close to granite contact, Agnew district; (c) recumbent F₁ fold, Jasper Hill, Leonora district; (d) recumbent F₁ fold, southeast Yilgarn; (e, f) typical outcrops of upright north-trending S₂ foliation, Kalgoorlie district; (g) upright north-plunging F₂ folds, southeast Yilgarn; and (h) upright north-plunging F₂ folds, Jasper Hill, Leonora district.

The layer-parallel S₁ foliation is variably developed through the entire greenstone package but is more intense in the deeper/older parts of the sequence (e.g. ca 2800 Ma sequence at Leonora) and the low-angle tectonic fabric, which is well preserved in the 2750 Ma BIF-basalt sequence of the Mt Ida greenstone belt on the western flank of the Ida Fault (Zibra et al., 2022). Fine-grained sedimentary units commonly contain a composite bedding-S₁ fabric.

Recumbent F₁ drag folds are commonly associated with the large extensional D₁ shears that wrap around granite domes such as the Sons of Gwalia Shear, Leonora (Figure 8a, b; Jones, 2014; Vearncombe, 1992) and Emu Shear, Agnew (Jones et al., 2022). Extensional kinematics are indicated by foliation drag, recumbent F₁ drag folds and down-dip L₁ stretching lineations (Figure 8c, d). Large-scale F₁ recumbent folds are also found above D₁ thrusts that are developed within the greenstone sequences at a high angle to the S₂ foliation. For example, in the Kambalda area, there are large east-southeast-trending recumbent F₁ folds located above the Feysville Thrust at Carnilya Hill and above the Foster Thrust (Swager & Griffin, 1990). The thrusts and F₁ folds have been produced by local north-northeast–south-southwest contraction during the D₁ event. These structures are overprinted by upright north-trending F₂ folds and the axial planar S₂ foliation.

Figure 8. D₁ extensional shears (localities shown in Figure 6): (a) extensional shears in the carapace of Raeside Batholith, Trump pit, Leonora; (b) Sons of Gwalia (SOG) Shear, Gwalia pit, Leonora; (c) extensional kinematics in SOG shear, Harbour Lights pit, Leonora; and (d) extensional shearing on Emu Shear, Waroonga mine, Agnew.

D₁ shears range from 1 to 2 m wide (e.g. Emu Shear, Agnew) to 500 m wide [e.g. Sons of Gwalia (SOG) Shear, Leonora, Thets Shear, Wallaby, Laverton; Jones et al., 2022]. The amplitude of the associated F₁ recumbent folds also varies from metre-scale, for example, folds in the Emu Shear, Waroonga Mine, Agnew (Jones et al., 2022), to kilometre-scale with regional fold interference patterns such as Carnilya Hill, Kambalda; Mt Clifford area, Leonora; the area northeast of the Wallaby deposit; Relief Well northwest of Kirgella Dome, Laverton district (Kneeshaw, 2002), and Dingo Range (Liu & Chen, 1998).

Later deformation—convergence (D_2–4 events)

The onset of intense east–west horizontal compression produced the steep north- to north-northwest-trending S₂ foliation that is axial planar to abundant north- and north-northwest-trending upright F₂ folds (Figure 7e–h). The initial phase of folding was followed by continued east–west shortening and development of major transpressional D₃ shears such as the Ida, Ockerburry, Laverton-Hootunui and Yamarna fault systems (Champion & Cassidy, 2007; Liu et al., 2000; Pawley et al., 2012; Swager, 1997; Swager et al., 1995). In places, the S₂ foliation becomes a composite fabric with a local S₃ crenulation cleavage developed. A slight shift in the stress axes during the D₄ event reactivates the existing fault framework and most commonly produces dextral movement on northeast-trending faults.

It is likely that the original rift architecture (D_e faults) is reactivated by both D₁ and D_2–4 deformation phases. Marked facies variations across some of the major shears indicate that D_e faults represent early growth faults, suggesting that the early rift architecture is preserved in places (Miller et al., 2010; Standing, 2008; Tripp et al., 2007).

Orogenic collapse (D₅ event)

A late phase of orogenic collapse is observed throughout the Eastern Goldfields and is associated with abundant sets of equally spaced steep planar normal faults with down-dip slickenfibres. These D₅ faults are particularly evident in open pit and underground mines, and commonly cause issues for ground stability.

Overprinting relationships and timing of deformation

D₁ structures are consistently overprinted by structures and fabrics associated with the intense horizontal east–west D_2–4 shortening. This overprinting relationship is observed in outcrops and in regional map patterns. In outcrop, the early layer-parallel S₁ foliation is commonly folded by upright north-trending F₂ folds (Figure 9a, b). At Jasper Hill, Leonora, the trace of S₂ can be seen cutting across the L₁ stretching lineation that is developed on the bedding-S₁ surface (Figure 9c). In the southeast Yilgarn, L₁ stretching lineations are folded by upright north-trending F₂ folds (Figure 9d), and at Agnew, the flat-lying S₁ foliation in basalt is folded by the upright F₂ folds (Figure 9e, f). Some D₁ shear fabrics are also overprinted by mineral growth associated with peak metamorphism (e.g. Emu Shear, Waroonga deposit, Jones, Waters, & Ashley, 2019; Thets Shear, Wallaby deposit, Jones et al., 2022).

Figure 9. D₁ fabrics overprinted by D₂ fabrics (localities shown in Figure 5): (a) upright F₂ fold in sedimentary unit, southeast Yilgarn; (b) oriented petrographic sample from previous outcrop shows the early bedding-parallel S₁ foliation is folded by the upright F₂ folds; (c) L_0–2 intersection lineation trace of S₂ overprints L₁ lineations on the S₀ surface, Jasper Hill, Leonora; (d) L₁ lineations are folded by upright F₂ folds, southeast Yilgarn; and (e, f) flat-lying S₁ foliation in basalt is folded by upright F₂ folds, Songvang pit, Agnew.

Foliation trajectory maps show the overprinting relationship between D₂ fabrics and the early D₁ fabrics in an area of relatively low D₂ strain (Leonora region) and an area that is more strongly affected by D₂ strain (Kalgoorlie region, Figure 10). To construct the maps, structural data were collected from GSWA 1:250 000 and 1:100 000 geological map series, my own data, and various publications (Harris et al., 1997; Kneeshaw, 2002; Standing, 2008; Tripp, 2013; Williams & Whitaker, 1993; Zibra et al., 2022). Foliation and cleavage are assigned to two main groups based on the orientation of the fabric. Most measurements can be assigned to the regional north-northwest-trending steep S₂ fabric (group 2), but some areas are dominated by layer-parallel S₁ (group 1). The maps also show F₁ fold axes that are typically at a high angle to the S₂ foliation. Younging directions in the overlying greenstone sequences generally face away from the granites, and in most areas the youngest rocks are located in the central parts of the greenstone belts.

Figure 10. Foliation trajectory maps of the Leonora–Laverton and Kalgoorlie districts show that the north- to north-northwest-trending S₂ foliation is the dominant fabric, particularly in the Kalgoorlie district. However, S₁ foliation and D₁ shears are well preserved in D₂ strain shadows around the major granite bodies such as the area east of the Raeside Batholith, above the Mt Margaret Dome, around the Bali Dome and south of the Siberia, Owen and Scotia domes. Younging directions typically face away from the granite domes.

Zones of intense S₂ foliation are observed around the major north and north-northwest-trending transpressional shears such as the Keith-Kilkenny, Celia and Barnicoat shears in the Leonora–Laverton district and the Kunanalling, Zuleika, Boulder-Lefroy and Roe Hills shears in the Kalgoorlie region (Figure 10). Although strongly overprinted, D₁ shears, S₁ foliation and F₁ folds are well preserved in both areas, particularly in low-D2-strain domains around the large granite domes. Extensional D₁ shears wrap around and dip away from the granite domes, and although many are located on the contact with the granite domes (e.g. SOG shear around the Raeside Batholith; or the shears around the Siberia, Scotia, Owen, and Bali domes), many are located higher in the sequence. For example, in the Laverton district, the southeast-dipping Thets Shear at Wallaby sits well above the Mt Margaret Dome, and the northwest-dipping Sunrise Shear is located well above the Kirgella Dome (Jones et al., 2022). In the Kalgoorlie area, the Fitzroy Shear is higher in the sequence above the Scotia Dome (Davis et al., 2010; Tripp, 2013). Local D₁ thrusts are located within the greenstone sequences and are typically oriented at a high angle to the S₂ foliation. These structures are folded by north-trending F₂ folds (e.g. Foster, Feysville and Tramways thrusts, St Ives, Kambalda; Swager & Griffin, 1990).

Lineation data show the marked difference in kinematics between the D₁ event and later deformation (Figure 11). L₁ lineations in this study represent stretching lineations measured on S₁ and bedding surfaces, while L₂ stretching lineations are observed on the steep S₂ foliation, and intersection lineations are excluded. L₂ lineations consistently trend north or north-northwest and are best developed in the higher D₂ strain zones adjacent to or around the major transpressional shears, whereas L₁ lineations display more random orientations and typically plunge away from the granite domes, sometimes with a radial pattern (e.g. Leonora area, and around the Mt Margaret Dome, Laverton). The L₁ lineation trends are mostly at a high angle to L₂ lineations and the S₂ foliation. They are best preserved in D₂ strain shadows such as the northern and southern ends of the large granite domes. There are zones dominated by north–south-trending L₁ lineations, such as the area adjacent to the Ida Fault, west of Leonora and the Agnew district, where L₁ lineations plunge gently to the north on extensional shears that wrap around the granite dome. North-trending L₁ lineations are also observed in the eastern part of the Laverton district, on flat-lying D₁ shears on the contact of Hanns Camp syenite (Jones et al., 2022) and on the Sunrise Shear, where Miller and Nugus (2006) note early normal-dextral movement based on north-plunging L₁ lineations on moderately northwest-dipping D₁ shears.

Figure 11. L₁ and L₂ lineation trends for the Eastern Goldfields. L₁ lineations in this study represent stretching lineations measured on S₁ and bedding surfaces, while L₂ stretching lineations are observed on the steep S₂ foliation and intersection lineations are excluded. L₂ lineations typically trend north-northwest or south-southeast and are best developed in high-D₂-strain zones adjacent to the major shears. In contrast, L₁ lineations display more random trends and are not preserved in the high-D₂-strain zones.

The timing of the D₁ event across the Eastern Goldfields is between ca 2670 and 2655 Ma. This is based on: (1) the SHRIMP U–Pb ages of zircons in porphyry intrusions overprinted by D₁ fabrics (Jones et al., 2022 and references herein); (2) direct dating of D₁ fabrics, for example, the SHRIMP U–Pb age of titanite (2661 ± 8 Ma) that is aligned parallel with L₁ stretching lineations on the north-dipping extensional Songvang Shear that extends over the granite dome (Thébaud et al., 2018); and (3) the layer-parallel S₁ foliation is observed through the entire greenstone sequence and indicates that D₁ deformation occurred after 2670 Ma. For example, S₁ foliation is observed in the uppermost ca 2680–2670 Ma Black Flag Group in the Invincible Deposit, St Ives, Kambalda (Jones, Doutch, & Lutter, 2019) and stratigraphic equivalents such as the Mt White Sequence at Agnew (Jones et al., 2022). The layer-parallel S₁ foliation is more pervasive in the older, deeper greenstones such as the ca 2800 Ma sequence at Leonora, Jones (2014), and the ≥2750 Ma BIF-basalt sequence in the Ida greenstone belt (Zibra et al., 2022). The D₁ event overlapped with the formation of late basins.

There is little evidence for early (pre-2670 Ma) ductile deformation in the Eastern Goldfields, as the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences. The older sequence has not been tilted prior to deposition of the younger sequence; instead, the first significant angular unconformities are at the base of the late basins. The lack of gneiss or schist clasts in the late basin sequences suggests that the surrounding uplifted sequence was largely undeformed. In addition, the dominant shallow north or south plunges of F₂ folds suggest that the sequence was predominantly flat-lying prior to the intense horizontal east–west shortening (D₂ event). Steeply plunging F₂ folds are only observed where the sequence has been previously affected by D₁ deformation, and in these areas fold interference patterns are common.

The orientation, morphology and timing of D₁ structures make it difficult to attribute them to progressive deformation during an early phase of orogenesis (horizontal east–west shortening). The D₁ event represents a separate and distinct episode of deformation with a markedly different stress regime (dominant vertical σ₁), compared with the D_2–4 events (dominant horizontal east–west oriented σ₁). The D₁ event was therefore most likely dominantly extensional and occurred after ca 2730–2670 Ma rifting and deposition of the greenstone sequences but prior to the onset of convergence at ca 2650 Ma. D_2–4 contraction occurs after deposition of the ca 2665–2655 Ma late basins.

Gravity-driven sagduction (granite-up, greenstone-down tectonics)

The similar timing of the D₁ event and late basin formation suggests that this deformation is associated with the development of the late basins. A mechanism is required to explain the uplift and exhumation of the granite-cored domes to supply granitic clasts into the late basins. Exhumation was previously attributed to contractional tectonics during amalgamation of the EGST terranes, prior to formation of the late basins (Barley et al., 2003; Krapež et al., 2008; Standing, 2008). However, exhumation is more easily produced by sagduction in response to density inversion, with colder, heavier, volcanic rocks sitting above light, hot, TTG gneisses (Bouhallier et al., 1995; Chardon et al., 1998; Dewey et al., 2021; François et al., 2014; Lin & Beakhouse, 2013; MacGregor, 1951; Thébaud & Rey, 2013; Van Kranendonk et al., 2004).

Sagduction allows exhumation of granite domes without significant deformation in the overlying greenstone sequences and the preservation of younging directions away from the granites. Recumbent F₁ folds form within local tectonic slides around the rising granite bodies. Whitney et al. (2004) described these as cascading recumbent folds that form during vertical flow-dominated systems. The lack of strong deformation in the mafic–ultramafic sequences during the D₁ event could explain the distinct lack of clasts with internal deformation fabrics (e.g. schist or gneiss) in the late basin sequences.

In the early stage of D₁ extensional shears and recumbent drag, folds developed around the rising granite domes (Figure 12). Radial L₁ stretching lineations developed around the granite domes and younging directions in the overlying sequences are typically away from the domes. Low strain is observed in the centres of the granite domes, with increasing flattening and plane strain at the edges of the bodies as described by Bouhaller et al. (1995) and Dewey et al. (2021). Compressional structures develop in the down-going greenstones to accommodate strain. The pattern of metamorphic facies throughout the Eastern Goldfields is consistent with uplift of the domes by buoyant rising, rather than by compression. Compressional features formed within the sinking greenstones to accommodate strain in the constricting area (e.g. D₁ Foster Thrust, Kambalda).

Figure 12. Schematic 3D cartoons illustrate the development of D₁ event (sagduction) and the effects of later horizontal east–west compression (D₂ event).

As the process continued, the late basins formed as depocentres above the sinking greenstones, leading to the first significant angular unconformities. A similar mechanism is invoked for the Timiskaming-type sediments in the Superior Province of Canada (Lin & Beakhouse, 2013). Subsequent horizontal east–west compression then folded the entire stratigraphic sequence, with remnants of the late basins now preserved in tight synclinal keels. The major pulse of magmatism at this time (including syenite, lamprophyre and granites) was likely related to decompression melting in the rising felsic crust during sagduction.

During sagduction, the late basins would have formed over an extensive area across the greenstone sequences. Coarse clastic material would be deposited proximal to the areas of greatest uplift, while more distal parts of the basin would be dominated by finer-grained material. For example, the ca 2665 Ma Mount Belches Formation comprises sandstone and interbedded mudstone and represents the more distal finer-grained equivalent of the Penny Dam Conglomerate to the north (Painter & Groenewald, 2001).

The dominance of well-rounded clasts with no internal deformation fabrics (i.e. schist or gneiss) in the late basins suggests that there is minimal deformation in the uplifted sequences around the basins. As exhumation continued, deeper sheared and foliated units would be exhumed, with schist clasts deposited into the basins. However, granite and other undeformed clasts would likely be preferentially preserved in such a high-­energy environment. Today, only the basal parts of the late basins are now preserved in the keels of tight F_2–3 synclines, with schist and gneiss clasts quasi-absent.

Rare well-rounded granite clasts are observed in the uppermost units of the Black Flag Group in the Invincible Deposit, St Ives near Kambalda. They are also observed in the upper part of the stratigraphic equivalent, Mt White Formation, in the Agnew district. These areas are close to the western margin of the rift basin, and the likely source for these well-rounded and well-travelled granite clasts is the rift shoulder to the west.

A post-rift sagduction model for the East Yilgarn

The timing of the ca 2670–2655 Ma sagduction (D₁ event) immediately after cessation of rifting suggests that sagduction is a direct result of the rifting and emplacement of volcanic rocks. Sagduction started where the crust was thinned and weakened over a relatively short period (2670–2655 Ma).

Sagduction is described in Mesoarchean rocks in Archean cratons such as the Pilbara (Collins et al., 1998; François et al., 2014; Sandiford et al., 2004; Thébaud & Rey, 2013; Van Kranendonk et al., 2007). However, many studies suggest that modern plate tectonics, with large lateral plate motions and subduction, were dominant in the Neoarchean and possibly earlier in the Mesoarchean (Kloppenburg et al., 2001; Kusky, 2020; Smithies et al., 2007; Turner et al., 2020; Van Kranendonk et al., 2007; Windley et al., 2021; Zibra et al., 2017; Zibra et al., 2022).

In the East Yilgarn, Zibra et al. (2022) attributed deep burial and exhumation of 2720–2690 Ma greenstone sequences to transpressional tectonics during orogenesis. This model is based on the presence of horizontal to gently north-plunging mineral lineations in sheared ca 2670–2660 Ma units along the western edge of the Eastern Goldfields, including the ca 2660 Ma Waroonga Shear and a ca 2670–2660 Ma linear belt of granitic gneiss (Figure 13). Zibra et al. (2022) suggested that intense horizontal east–west shortening and transpression produce granite domes in antiformal zones, with deep burial of greenstones in adjacent synclinal keels followed by rapid uplift of the greenstones during continued transpression. Variations on this process have been used to explain Archean dome-and-keel geometries elsewhere (Cagnard et al., 2006; Chardon et al., 2009; Davis et al., 2010; Gapais et al., 2014; Harris et al., 2012). However, the area along the western edge of the Eastern Goldfields is the only place where most L₁ stretching lineations plunge gently north and south. Elsewhere, L₁ lineations display more random patterns, typically at a high angle to north–south-trending L₂ stretching lineations and D₃ shears. In addition, compressional features are absent on the western and eastern sides of the granite bodies in the Agnew and Leonora districts (Figure 6). Instead, extensional kinematics are observed with down-dip L₁ stretching lineations, S₁ foliation drag and recumbent F₁ folds on steep shears along the western side of the granite dome at Agnew (Jones, Waters, & Ashley, 2019) and on the eastern side of the granite dome (Raeside Batholith) at Leonora (Jones, 2014).

Figure 13. A linear belt of granitic gneiss with shallow stretching lineations observed along the western margin of the Eastern Goldfields, extending up to the Waroonga Shear at Agnew. Direct dating of these gneissic fabrics indicates an age range from 2670 to 2660 Ma (Zibra et al., 2022). Identical shallow north-plunging L₁ stretching lineations are observed on the D₁ Songvang Shear at Agnew and have recently been dated at 2661 ± 8 Ma (Thébaud et al., 2018).

The ca 2660 Ma age of the transpressional fabrics in the linear belt of granitic gneiss is similar to that of the D₁ structures throughout the Eastern Goldfields and pre-dates transpression associated with horizontal east–west contraction at ca 2650 Ma. The linear belt of granitic gneiss is unusual in the Eastern Goldfields and appears to be constrained to the western edge of the Eastern Goldfields. This suggests that a tectonic model based on these local features cannot be used to explain the development of the East Yilgarn.

The development of ca 2660 Ma subhorizontal north–south lineations in granitic gneiss along the western margin of the Eastern Goldfields suggests lateral movement or ‘escape’ structures parallel to the rift margins in response to local east–west contraction during sagduction (Figure 14). The original rift architecture appears to constrain the area affected by sagduction and may contribute to the elongate shape of the large granitic bodies throughout the Eastern Goldfields. The early north-northwest-trending basin-controlling structures are reactivated during subsequent extensional and compressional deformation episodes.

Figure 14. Development of dome-and-basin geometry in the Eastern Goldfields as a result of sagduction within the ca 2730–2670 Ma rift zone: (a) during the initial phase of sagduction, the felsic crust rises, forming granite domes while greenstones sink with a radial pattern of L₁ lineations developing around the rising domes; and (b) at a later phase of sagduction, depocentres develop on the sinking greenstones with deposition of widespread late basin sequences. Zones of strong north–south L₁ lineations on the edges of the original rift suggest development of ‘escape’ structures as a result of increasing east–west constriction from the rising domes.

A degree of lateral movement is suggested by the rifting, prior to sagduction, with renewed plate motion at ca 2650 Ma during orogenesis. A 10–15 My period of sagduction is very similar to the time frames in recent numerical modelling by Rey (2023), suggesting that sagduction is a realistic process to explain the deformation fabrics and overprinting structural relationships in the East Yilgarn.

A transitional change from early stagnant lid tectonics to mobile lid tectonics is widely accepted to have occurred sometime during the late Archean or early Proterozoic (Bédard, 2018; Brown et al., 2020; Cawood et al., 2022; Condie, 2018; Dewey et al., 2021; Laurent et al., 2014; Nebel et al., 2018; Palin & Santosh, 2021; Piper, 2013; Rey et al., 2014; Van Kranendonk et al., 2007). Key evidence for this transition is the change from Archean sodic granitoid intrusions of the tonalite–trondhjemite–granodiorite (TTG) series to potassic K-granite suites that are similar to I-type granites in modern subduction zones (Cawood & Hawkesworth, 2019; Laurent et al., 2014; Moyen & Laurent, 2018; Nebel et al., 2018). In the East Yilgarn, this transition is observed over a ca 30 My period and coincides with the D₁ event, the end of the sedimentary record and the onset of orogenesis (Blewett et al., 2010; Champion & Cassidy, 2007; Champion & Sheraton, 1997; Czarnota et al. 2010; Jones et al., 2022).

Implications for gold mineralisation

Geochronological studies show that the timing of gold deposition in the Eastern Goldfields ranges from ca 2670 Ma to 2635 Ma (Blewett et al., 2010; Czarnota et al., 2010), extending through the change in tectonic regimes (sagduction to orogenesis), and indicates that some gold deposits formed during the ca 2670–2655 Ma phase of sagduction (Figure 15).

Figure 15. Timeline for the changing tectonic regimes in the Eastern Goldfields, from rifting, sagduction and later orogenesis. Granite and gold age ranges from Blewett et al. (2010) and Czarnota et al. (2010).

Marked contrasts in the structural style of gold deposits are usually attributed to progressive deformation during orogenesis (e.g. Cox, 1995; Groves et al., 2000). However, there is also evidence for gold deposition in temporally unrelated mineralising events with markedly different structural settings and fluid sources (Bateman et al., 2001; Bateman & Hagemann, 2004; Bucci et al., 2002; Cerda et al., 2020; Jones, 2014; Jones, Waters, & Ashley, 2019; Jones, Doutch, & Lutter, 2019; McDivitt et al., 2022; Meffre et al., 2016; Nichols & Hagemann, 2014; Thébaud et al., 2008, 2018, Zametzer et al., 2022). Multi-stage gold deposition through changing tectonic regimes is observed in the Shanggong Deposit (Tang et al., 2019). Here, early gold is deposited during a collisional orogeny, while later gold is associated with extensional structures in a post-collision extensional regime, resulting in diverse fluid, metal and sulfur sources.

In the Eastern Goldfields, the different structural styles of some gold deposits appear to reflect their formation in different stress regimes. Synchronous vertical and horizontal tectonism is observed during sagduction with a dominant vertical σ₁ and local areas of contraction (horizontal σ₁) within the down-going greenstone sequences (e.g. Ibrahim et al., 2024; Lin & Beakhouse, 2013). However, early D₁ structures formed under local contraction (e.g. Fosters, Feysville, Tramways, Republican thrusts) are overprinted by subsequent D_2–4 structures and do not appear to be mineralised. Overprinting relationships between deformation fabrics and mineralisation show that gold deposits in the Eastern Goldfields are associated with temporally distinct deformation episodes.

For example, in the Leonora district, the Gwalia, Tower Hill and Harbour Lights deposits are hosted in the 500 m-wide extensional D₁ SOG Shear (Figure 16a–f, modified from Jones, 2014). The shear wraps around the Raeside Batholith and large recumbent folds are commonly developed in the hanging wall (Figure 15e). Extensional kinematics on the shear are indicated by foliation drag, recumbent F₁ drag folds and down-dip L₁ stretching lineations (Figure 16b, c, f; Jones, 2014; Vearncombe, 1992). At Gwalia, gold is hosted in tightly folded veins within the SOG shear (Figure 16e). The large extensional shear developed during sagduction (D₁ event). In contrast, gold lodes in the Tarmoola-King of the Hills deposit, about 25 km to the north, are associated with sinistral-reverse shears that developed in the deformed carapace of a large granite body during strong horizontal east–west horizontal compression (Duuring et al., 2001; Jones, 2014). Flat-lying oblique sinistral shears are observed on the upper contact of the granite (Figure 16g, i) while steep sinistral shears and breccia veins are observed along the steep eastern side of the granite body (Figure 16h, j, k).

Figure 16. Contrasting structural styles of gold deposits in the Leonora area: (a) location map; (b) Sons of Gwalia (SOG) shear in Gwalia pit; (c) foliation drag indicates extensional displacement on the SOG shear in Gwalia pit; (d) tightly folded lodes of the Gwalia deposit within the SOG shear; (e) large recumbent F₁ folds in the hanging wall of the SOG shear, Harbour Lights pit; (f) foliation drag indicates extensional movement on the SOG shear, Harbour Lights pit; and (g–k) gold is hosted in veins and shears in the deformed carapace of a granite body. Sinistral movement is observed on flat-lying lodes developed on the top of the granite body, and on the steep lodes and breccia veins along the eastern side of the granite body.

A similar contrast in lode style and timing of gold mineralisation is observed in the Agnew district (Figure 17). The Waroonga deposit is hosted in the layer-parallel D₁ extensional Emu Shear that extends along the steep western side of the Agnew granite dome (Jones, Waters, & Ashley, 2019). Gold is hosted in bedding-S₁ parallel shear veins that commonly display recumbent F₁ drag folds consistent with west-side-down displacement (Figure 17c, d). A markedly different structural setting is observed at the New Holland deposit, located 2 km to the west at the same elevation (Figure 17a). Here, gold is hosted in flat-lying extension veins that cut across vertical north-trending beds in the Scotty Creek Formation (late basin). Sinistral movement is indicated by abundant wing veins extending off the lodes (Figure 17e). At depth, open upright folds develop in the lodes indicating progressive deformation during continued horizontal east–west compression (Figure 17f). The Waroonga deposit formed during the D₁ event (sagduction), while the New Holland deposit formed at a late stage of the D₃ event, after tight F₂ folding of the late basin sedimentary units (Jones, Waters, & Ashley, 2019; Jones et al., 2022).

Figure 17. Contrasting structural styles of gold deposits in the Agnew area: (a) location of Waroonga and New Holland deposits in the Scotty Creek basin; (b) cross-section illustrates the location and very different structural setting of lodes at Waroonga vs New Holland; (c, d) at Waroonga, gold is hosted in the layer-parallel extensional D₁ Emu Shear with recumbent F1 drag folds indicating west-side-down displacement; (e) at New Holland, gold is hosted in flat-lying lodes that cut across steep to sub-vertical bedding; and (f) at depth in the New Holland mine, open upright folds develop in the lodes, indicating progressive deformation during prolonged east–west horizontal compression.

Thermo-mechanical numerical modelling by Thébaud and Rey (2013) suggests that sagduction is a plausible process to produce gold mineralisation in the Warrawoona greenstone belt in the Pilbara. During sagduction, the downward advection of cold rocks in the sinking greenstones and upward advection of hot felsic crust into the rising domes can produce crustal-scale horizontal thermal anomalies greater than 500 °C, over a distance of 30 km leading to temperature gradients up to 26 °C/km. These temperature gradients and large volumes of water, including seawater above the greenstones, hydrated ultramafic rocks (e.g. Hartnady et al., 2022), metamorphic and magmatic fluids (a large pulse of magmatism occurs as a result of decompression melting in the rising TTG domes), could power crustal-scale hydrothermal systems and associated gold deposition.

In the East Yilgarn, an early deep plumbing system may have been established during the initial rifting event, dominated by early north-northwest-trending basin-controlling structures. Gold-bearing fluids could be focused on these pre-existing deep-seated structures during multiple hydrothermal circulation events and changing tectonic regimes (e.g. Zuleika Shear, Zametzer et al., 2022). Sagduction provides a mechanism to form significant gold deposits on extensional shears and to introduce widespread magmatism and associated alteration (±gold) to the upper crust, which can be upgraded by later overprinting orogenic mineralising events.

Conclusions

The ca 2670–2655 Ma D₁ event represents an episode of gravity-driven sagduction that was triggered by the deposition of dense, cold, greenstones on thinned, weak, hot, TTG crust within an intracratonic rift system. Sagduction produced a dome-and-basin geometry within the rift zone, which was later modified by intense horizontal east–west compression during orogenesis (D_2–4 events). Late basins, observed throughout the Eastern Goldfields, represent depocentres on the sinking greenstones during sagduction.

There is little evidence for early (pre-2670 Ma) ductile deformation in the Eastern Goldfields, as: (i) the ca 2730–2670 Ma greenstone sequences are deposited conformably on the older ca 2800 Ma greenstone sequences, with the first significant angular unconformities observed at the base of the late basins; (ii) a lack of schist or gneiss clasts in the late basins suggests that the surrounding uplifted sequence was largely undeformed; (iii) the layer-parallel S₁ foliation is observed throughout the 2720–2670 Ma greenstone sequence; and (iv) the dominant low north and south plunges of F₂ folds suggest that the sequence was predominantly flat-lying prior to the strong horizontal east–west shortening (D₂ event).

D₁ deformation fabrics, comprising flat-lying layer-parallel S₁ foliation, extensional shears, thrusts and recumbent F₁ folds cannot be attributed to an early phase of progressive deformation during orogenesis. The D₁ event represents a separate and distinct episode of ductile deformation with a markedly different stress regime (dominant vertical σ₁), compared with the stress regime during the D_2–4 events (dominant horizontal east–west-oriented σ₁). The timing of the ca 2670–2655 Ma D₁ event is based on the presence of the S₁ layer-parallel foliation in units older than 2670 Ma, the age of porphyry intrusions containing D₁ fabrics and direct dating of D₁ shears. The intense horizontal east–west shortening (D_2–4 events) postdates deposition of the ca 2665–2655 Ma late basins.

Gold deposition in the Eastern Goldfields ranges from ca 2670 Ma to 2635 Ma, indicating that some gold deposits formed during ca 2670–2655 Ma sagduction. Marked contrasts in the structural style of gold deposits, metallogeny and fluid sources, typically attributed to progressive deformation during orogenesis, may instead reflect temporally distinct mineralising events related to changing tectonic regimes. Early deep-seated plumbing systems established during rifting could then focus hydrothermal fluids during multiple mineralising events. Sagduction provides a mechanism to form significant gold deposits on extensional shears and to introduce widespread magmatism and associated alteration (±gold) to the upper crust to be later upgraded by orogenic mineralising events.

Acknowledgements

Thorough reviews by Patrice Rey and Brett Davis are much appreciated and greatly improved this paper. Thanks also to Hugh Smithies and Alicia Verbeeten who reviewed an early version of the manuscript. Ongoing discussions and field visits with the mining, resource and exploration geologists at Gold Fields Ltd have also greatly added to the ideas in this study.

Disclosure statement

No potential conflict of interest was reported by the author.

Data availability statement

Much of the raw data that support the findings of the study were generated at the Geological Survey of Western Australia and are available in GeoVIEW.WA software or in the 1:100 000 and 1:250 000 map sheet series in the eBookshop.