Polyphased gold enrichment as a key process for high-grade gold formation: Insights from the 10 Moz Jundee-Bogada camp (Yilgarn Craton, Western Australia)

Abstract

High-grade (> 10 g/t) gold mineralization in orogenic gold deposits is of significant economic importance. Understanding the formation of such enriched ore zones is critical for gold exploration success. The world-class Jundee-Bogada gold camp in the Yilgarn Craton of Western Australia comprises both high-grade (avg. > 10 g/t, Jundee deposit) and low-grade (avg. < 3 g/t, Bogada prospect) lodes, despite shared host stratigraphy. The paragenetic framework established for the Jundee gold deposit suggests that the overall gold endowment developed over three deformation events. An early episode of low-grade gold mineralization is associated with colloform-crustiform veins that formed during extensional deformation (D_JB2A). A switch to transtensional deformation (D_JB2B) resulted in brecciation of the colloform-crustiform veins and coeval deposition of native gold. Late reverse faults record evidence for a third mineralization stage resulting from a NE-SW-directed shortening (D_JB3). Mineralization during this late stage was dominantly low-grade, with local occurrences of ultra-high-grade ore zones (> 100 g/t). Each event records transient changes in fluid chemistry during continued hydrothermal activity that spanned local deformation histories. We argue that at the Jundee gold deposit, protracted gold enrichment during three polyphased mineralization episodes resulted in the formation of high-grade gold ores. Whereas the complete metallogenic history is recorded at the Jundee deposit, gold within the Bogada prospect was introduced solely during the late contractional stage (D_JB3), resulting in a bulk low-grade endowment. We hypothesize that gold enrichment in high-grade orogenic gold deposits is a direct consequence of the spatial superimposition of protracted ore-forming events.

Introduction

Conventional models associated with the formation of orogenic gold systems indicate gold transport as aqueous solutions (Stefánsson and Seward 2004; Pokrovski et al. 2014). Mineralizing fluids from orogenic gold deposits range in temperature from ~ 250 – 450 °C, pressure up to 1.5 kb (McCuaig and Kerrich 1998), have low salinities and high CO₂ and H₂S contents at neutral pH (McCuaig and Kerrich 1998; Ridley and Diamond 2000). Under these conditions, gold is interpreted to be transported as gold-hydrosulfide complexes, reaching maximum solubilities of hundreds of ppb in aqueous solutions (Pokrovski et al. 2014). Accordingly, the extreme concentrations of gold (> 100 g/t) observed in certain deposits require gold enrichments of over four to five orders of magnitude, involving substantial fluid flow (Petrella et al. 2022). Such extreme fluid flux has been historically attributed to fault-valve behaviour (Sibson 1990; Cox 2020), suggesting that increased fluid pressure prior to the fault rupture is succeeded by rapid pressure drop during the rupture events. This process facilitates the deposition of quartz and gold from aqueous solutions allowing fault sealing and subsequent re-establishment of the over-pressured fluid reservoir (Sibson et al. 1988). Laminated vein textures, commonly observed in orogenic gold deposits, illustrate repetitive fault rupturing and subsequent sealing events (Robert and Poulsen 2001). Densely laminated veins in certain deposits (e.g., Revenge, St Ives; Cox et al. 2001) have been suggested to indicate several increments of fluid flow resulting in the formation of high-grade (> 10 g/t) gold deposits. However, a common characteristic of several high-grade gold deposits includes native visible gold hosted in narrow (few mm to cm) veins that lack textures indicative of multiple rupturing events (Hastie et al. 2020; Petrella et al. 2022). In such deposits, the formation of high-grade gold intercepts is commonly associated with other chemical and/or physical processes, including gold transport as nanoparticles (Herrington and Wilkinson 1993; Petrella et al. 2020, 2022; Hastie et al. 2021); phase separation (Peterson and Mavrogenes 2014); wall rock alteration mechanisms that can directly or indirectly lead to gold deposition (Evans et al. 2006; Neumayr et al. 2008; Li et al. 2020; Petrella et al. 2021); and lastly, gold enrichment associated with the superimposition of multiple gold episodes, whereby high-grade intercepts result from the addition of new gold or through remobilization and/or redistribution of preexisting gold (Morey et al. 2008; Cook et al. 2009; Sung et al. 2009; Kerr et al. 2018; Hastie et al. 2020; Combes et al. 2023).

In recent years, protracted and polyphased gold mineralization processes have been demonstrated for several highly endowed gold deposits and gold camps worldwide (Bateman and Hagemann 2004; Kolb et al. 2005; Fougerouse et al. 2017; Thébaud et al. 2018; Perret et al. 2020; Sugiono et al. 2021; Herzog et al. 2022; McDivitt et al. 2022; Combes et al. 2023). In certain deposits, the polyphased nature of the mineralization process has been advocated to result in the formation of high-grade (> 10 g/t) gold intercepts. For example, high-grade gold mineralization at the ~ 30 Moz Red Lake gold deposit in the Uchi Subprovince (Superior Craton, Canada) formed in late fractures due to gold remobilization during late deformation associated with the emplacement of regional granitic plutons (Dubé et al. 2004). Similarly, at the Lapa gold deposit in the Abitibi Subprovince (Superior Craton, Canada), the liberation of refractory gold was associated with the recrystallization of gold-bearing arsenopyrite during peak metamorphic conditions and the progressive deposition of native gold during retrograde metamorphism (Simard et al. 2013). An upgrade in gold grade is, however, not always associated with metamorphic fluids. At the Madrid deposit in the Hope Bay Greenstone Belt (Nunavut, Canada), secondary gold enrichment was associated with the late influx of magmatic-hydrothermal fluids, highlighting the role of polyphased fluid introduction in upgrading gold tenor (Kerr et al. 2018).

Multistage gold models rely on detailed paragenetic studies to unravel the existence of multiple hydrothermal events, their relationship to deposit- and camp-scale structural framework, and their relative contribution to the overall gold endowment. In this study, we investigate the ~ 10 Moz Jundee-Bogada gold camp, situated in the Yandal greenstone belt of the Kalgoorlie Terrane within the Yilgarn Craton of Western Australia (Fig. 1). The mining camp comprises the Jundee deposit (9 Moz at c. 5 g/t Au, with an average grade > 10 g/t) and the Bogada gold prospect (1.3 Moz at c. 2.3 g/t Au), which are both hosted in Archean mafic–ultramafic igneous sequences. The high-grade (> 10 g/t) and high tonnage observed at the Jundee gold deposit contrasts with the low-grade (< 3 g/t) and low tonnage of the Bogada gold prospect and provides an excellent comparative case study to gain insights into the formation of high-grade orogenic gold mineralization. We examine the structural style and alteration paragenesis at each deposit and place relative timing constraints on individual events leading to the formation of gold mineralization within the wider camp.

Fig. 1

Geological map of the northern Yandal greenstone belt along with the lithological subdivision of the three stratigraphic sequences. Inset map shows the location of the Yandal greenstone belt within the Yilgarn Craton of Western Australia. The location of the studied deposits is marked on the map (star). The circles mark the location of other gold deposits in the Yandal greenstone belt. The geological map is modified after Geological Survey of Western Australia (2020)

Geological Background

Yandal greenstone belt

The Neoarchean Jundee-Bogada gold camp is located 600 km north of Kalgoorlie in the Yandal greenstone belt of the Eastern Goldfields Superterrane (EGST), Yilgarn Craton (Fig. 1). The Yandal greenstone belt represents a 300 km long and 40 km wide, poorly exposed, NNW- to NW-trending litho-tectonic complex of deformed and metamorphosed Archean igneous and sedimentary rocks, flanked by Archean granitoids on either side (Phillips et al. 1998; Baggott et al. 2005). Metamorphic grade in the Yandal belt records lower greenschist-facies metamorphism, with upper greenschist- and amphibolite-facies metamorphism restricted to the margins of granitic plutons (Vearncombe et al. 2000). The Yandal greenstone belt is characterized by three principal lithostratigraphic domains: lower, middle, and upper greenstone sequences, as identified by Phillips et al. (1998). The lower greenstone sequence is located along the western margin of the belt and is composed of banded iron formations, which are interpreted as the stratigraphic base of the greenstone belt (Phillips et al. 1998). The Jundee and Bogada gold deposits are both hosted in the middle greenstone sequence, comprising mafic and ultramafic volcanic flows, with lesser felsic volcanic and sedimentary rocks. The upper greenstone sequence is referred to as the Lake Violet sequence and is dominated by epiclastic and volcanoclastic rocks of intermediate composition, along with ferruginous cherts and shales (Phillips et al. 1998; Vearncombe et al. 2000).

The structural evolution of the Yandal greenstone belt is proposed to include five deformation phases, representing protracted reactivation of earlier structures (Vearncombe et al. 2000). Early extensional structures (De; Table 1) have been identified along the granite-greenstone contacts on the western margin of the belt by Hammond and Nisbet (1992). Evidence for D₁ structures is cryptic in the region and is assumed to be associated with early-formed tight folds (Vearncombe et al. 2000). Subsequently, an episode of NNW-striking upright folding and associated axial planar foliation has been recognized in the southern part of the belt and proposed to be associated with D₂ ENE-WSW bulk crustal shortening (Phillips et al. 1998; Vearncombe et al. 2000). Late development of the Moilers and Celia shear zones is correlated with the D₃ event that resulted from a progressive E-W bulk crustal shortening (Table 1). These shear zones bound the Yandal greenstone belt to the west and east, respectively (Fig. 1), and are characterized by protomylonites and foliated schists (Vearncombe et al. 2000). D₄ brittle-ductile faults crosscut previously enumerated structural elements and form a fault network at high angles to both folded greenstone sequences and large-scale D₃ shear zones. The D₄ cross-faults form a conjugate pattern that is interpreted to have formed under E-W bulk crustal shortening (Phillips et al. 1998; Vearncombe 1998; Vearncombe et al. 2000).

Table 1 Structural scheme outlined for the Jundee gold deposit (Baggott et al. 2005) and the Yandal greenstone belt (Vearncombe et al. 2000; Kohler and Phillips 2003)

Jundee gold deposit

At the deposit scale, the ~ 9 Moz Jundee gold deposit is hosted in a mafic igneous rock sequence that strikes SSE and dips moderately (45 – 55°) towards the WSW (Fig. 2a, b, c; Yeats et al. 2001). The host rocks are dominated by tholeiitic basalts metamorphosed to lower greenschist facies and have undergone low-strain deformation characterized by poorly developed or the lack of regionally recognized NNW-trending planar fabric (Messenger 2000; Kohler et al. 2003). The Fisher Basalt and Lyons Basalt, which have an approximate thickness of 600 m and 1200 m, respectively, are separated by a 100 m thick horizon of sedimentary rocks comprising carbonaceous shales, cherts and volcanoclastic rocks (Kohler et al. 2000). This sedimentary unit has been intruded by three strike-continuous differentiated micro-gabbroic (dolerite) sills (Barton, Hughes and Lyons Dolerite; Fig. 2a), which host the bulk of the ore. A deformed succession of dacitic to andesitic volcanic and volcanoclastic rocks assigned to the Lake Violet sequence occurs towards the western edge of the mine (Phillips et al. 1998; Messenger 2000). The host sequence at the Jundee gold deposit is intruded by suites of dacitic porphyry dikes, granodiorite porphyry dikes and mafic dikes, including lamprophyres, which range from pre- to post-mineralization in age (Yeats et al. 1999, 2001; Baggott et al. 2005).

Fig. 2

a. Stratigraphic column depicting the mafic-dominated Jundee mine sequence intruded by pre- and post-Au mineralization intrusives. Age constraints for the intrusives are provided by (1) Baggott et al. (2005) and (2) Yeats et al. (2001). b. Deposit-scale geological map of the Jundee gold deposit with the outline of the open pit mines produced from Northern Star Resources Limited mapping and compilation. Position of the cross section (c) A - B is indicated on the map. c. Schematic cross section of the southern end of the Jundee gold deposit. The cross section highlights the SW-dipping geometry of the stratigraphic sequence, intruded by pre- and post-gold intrusives. Geometries of the four major mineralized trends (Kryten, Nim3, Barton and Hughes) are also marked on the cross section. Mineralized zones (red) are cut by the granodiorite porphyry dikes and lamprophyric intrusives. Lithological contacts and mineralized structures as shown in the cross section were interpreted from 3-D modelling Leapfrog software based on drill core datasets

Gold mineralization at the Jundee gold deposit is hosted by multiple sets of moderately to steeply dipping, narrow (< 2 m) brittle faults and fractures (Vearncombe 1998; Yeats et al. 2001; Baggott et al. 2005). The mineralized lode swarms have been grouped into four discrete orientations with variable dips (Fig. 2c). These include: 1) Nim 3 trending 000 – 020° and dipping to the E with dextral-normal kinematics, 2) Kryten trending 040° and dipping to the SE with dextral-normal kinematics (3) Hughes trending 070 – 110° and dipping to the S with sinistral-normal kinematics and (4) Barton trending 115 – 150° and dipping to the SW with an oblique sinistral-normal or sinistral-reverse offset (Kohler and Phillips 2003; Baggott et al. 2005). Mineralized structures at the Jundee deposit are typically marked by zones of hydrothermal breccia and quartz-carbonate veins that range in length from tens of metres to several hundred metres. High-grade ore zones (> 10 g/t) are restricted to the intersection of two or more structures (Yeats et al. 2001). Yet, no structural model confidently accounts for the variable orientations observed in the mine.

Bogada gold prospect

The ~ 1.3 Moz Bogada gold prospect is located 3 km NE of the Jundee deposit (Fig. 3). Bogada currently represents an exploration prospect accessible through limited drilling. Gold mineralization at the Bogada prospect is hosted in the mafic sequence, similar to the Jundee deposit but lower in the stratigraphic column (Cooper et al. 2005). The host lithology is dominated by SSE-striking ultramafic–mafic rocks, including high-Mg basalts, tholeiitic basalts and micro-gabbroic sills (Cooper et al. 2005). Analogous to Jundee, the volcanic sequence at Bogada is intruded by dacitic porphyry dikes, granodiorite porphyry dikes and lamprophyres (Cooper et al. 2005). Limited published information exists about the style of gold mineralization and associated structures at the Bogada prospect.

Fig. 3

a. Deposit-scale geological map of the Bogada gold prospect produced from Northern Star Resources Limited and M G Creasy mapping and compilation. The map shows the location of the two prospects marked by yellow dots; Bogada Bore (BB) and Plover Bore (PB) which together form the Bogada gold prospect. b. Stratigraphic column depicting the location of the Bogada prospect. c. Schematic cross section across the Bogada prospect representing the major orientation of gold lodes

Methods and methodology

Field mapping, drill core logging, and sampling

Structural mapping was carried out underground at the Jundee gold mine. Underground drives were selected based on discussion with the mine geology team on site as well as interpretation of resource wireframes and drill core database on 3D modelling Leapfrog Geo® software. Due to the structural complexity previously recognized at the Jundee mine, underground exposures were selected to encompass a variety of lode trends and lodes with variable gold grades. Mapping of underground exposures was carried out through the ‘backs-mapping’ technique using geotechnical surveys as base maps (McKinstry et al. 1948). Structural orientations were recorded in true north coordinates and are reported as ‘dip/dip-direction’ for planar features and as ‘plunge-trend’ for linear measurements. The structural data were complemented by section logging of diamond drill cores. Gold grades are assigned in grams/tonnes (g/t) based on fire assay analysis. High-grade ore zones are defined as > 10 g/t and low-grade ores are defined as < 3 g/t based on sampling of mineralized zones either from underground exposures or from drill cores. Structural and lithological associations at the Bogada gold prospect were all described from drill core logging. Diamond drill cores at Bogada were selected using 3D modelling Leapfrog Geo® software and in conjunction with the exploration team. Structural elements and events are suffixed by “JX, BX or JBX” subscripts respectively, where JB represents the initials of the Jundee-Bogada camp and ‘X’ represents the relative timing of the deformation event.

Petrographic study

A detailed petrographic study was carried out on structurally and stratigraphically constrained polished thin sections to determine the ore mineralogy and alteration associated with each deformation stage, including vein composition and unaltered host rocks. Polished thin sections (30 µm) were prepared at The University of Western Australia (UWA) and polished thick sections (60 µm) were prepared at Vancouver Petrographics Ltd. All samples were studied using optical microscopy at UWA and scanning electron microscopy (SEM) at the Centre for Microscopy, Characterisation and Analysis (CMCA) at UWA. Backscattered electron (BSE) images were generated using the TESCAN VEGA3 XM SEM. Energy dispersive x-ray spectra (EDS) were obtained using an Oxford Instruments® X-Max 50 silicon drift detector with AZtec software fitted on the TESCAN VEGA3. BSE-EDS investigations were performed (i) to confirm observations made using optical microscopy and (ii) for semi-quantitative analyses of ore mineral chemistry. Operating parameters for the SEM-EDS included an accelerating voltage of 20 kV, a working distance of 15 mm, a beam current of 1.5 nA, and a detector process time of 4 s. The analytical software used to collect semi-quantitative microanalyses was the Oxford Instruments® INCA analytical suite on the TESCAN VEGA3. Detailed petrographic observations are provided in supplementary material (ESM1).

Structures, veins, and gold mineralization at the Jundee gold deposit

Five sets of veins were distinguished at the Jundee gold deposit based on their structural, kinematic, and hydrothermal characteristics. Their timing and crosscutting relationships are presented below as a basis for establishing a paragenetic framework for gold mineralization.

V_J1 calcite-chlorite veins

Thin (< 5 cm) calcite-chlorite-pyrite fault-fill veins provide evidence for the first signs of hydrothermal activity across the Jundee deposit and are termed V_J1. These V_J1 veins are widespread across the deposit, intersect dacitic porphyry dikes, and earlier anastomosing veinlets associated with sea-floor alteration and are intruded by mafic dikes (Fig. 4a, b, c). These V_J1 veins consist of laminated, fault-fill veins that developed along lithological contacts (Fig. 4c). The majority of the V_J1 veins are trending NW and dipping 40 to 60° to the SW, with a conjugate set trending N-S and dipping 50 to 70° to the east (Fig. 4b, d). V_J1 veins preserve narrow (< 1 cm) dark-green zones of fine-grained chlorite selvedge that mark the boundary with the host unit (Fig. 4e). These V_J1 veins are barren and lack significant alteration in wall rocks.

Fig. 4

Structural context of the V_J1 veins. a and b. E and SW-dipping V_J1 chlorite-calcite veins being crosscut by a pre-mineralization mafic dike. The E-dipping veins display normal kinematics associated with them. c. Photo of a drill core displaying a V_J1 vein along the contact between the dacitic porphyry dike and a micro gabbro and cutting across early veins related to sea-floor alteration. d. Stereonet plot of V_J1 veins suggesting a conjugate geometry. e. Transmitted light photomicrograph of a V_J1 vein showing a calcite dominated composition along with chlorite selvedge

V_J2A colloform crustiform veins

Au-bearing V_J2A veins are ubiquitous in the Jundee deposit and represent the first stage of gold mineralization. These veins are characterized by open-space growth textures such as colloform-crustiform textures and cockade-textured hydrothermal breccia with clasts of altered host rock (Fig. 5a, b). These white to cream-colored veins composed of ferroan dolomite are predominantly developed in the Lyons Basalt but also occur in micro gabbro, dacitic porphyry and mafic dikes. The V_J2A veins are commonly ~ 0.5 m wide (up to 2 m wide) and are associated with pervasive sericite alteration haloes extending up to 5 m away from the vein margins. V_J2A colloform-crustiform veins consistently trend NW and dip to the SW, parallel to stratigraphy and are occasionally associated with subparallel, vertical extensional veins (Fig. 5c, e). Locally, the V_J2A veins display conjugate geometries with mutual crosscutting relationships, where opposite-dipping faults trend towards the NW (Fig. 5d, f). Displacement along mineralized V_J2A veins is often less than 1 m and consistently demonstrates hanging wall down movement (Fig. 5f).

Fig. 5

Structural context of V_J2A veins. a. Close-up of the colloform vein displayed in (e). The colloform veins display individual cm-scale white to grey colored bands that also show crustiform textures. b. Close-up on the V_J2A vein in (f) displaying cockade breccia with altered basalt clasts surrounded by carbonate rims. c. Stereographic projections of the V_J2A veins in (e). d. Stereographic project of V_J2A veins in (f) displaying a conjugate geometry. e. NW-trending and SW-dipping V_J2A colloform vein present along the micro gabbro-dacitic porphyry dike contact. The V_J2A colloform veins are associated with NW-trending vertical extensional veins. f. V_J2A veins display a conjugate geometry. The SW-dipping vein is associated with normal kinematics as observed from the displacement of the NE-dipping vein

V_J2B hydrothermal breccia and shear veins

V_J2B faults represent the most ubiquitous gold-hosting structures in the Jundee deposit. These structures form a complex fault system with multiple orientations of gold-bearing shear zones that parallel and crosscut the stratigraphy. V_J2B faults are characterized by tabular zones of brecciated, veined and rarely foliated, hydrothermally altered host rocks hosting native gold mineralization. V_J2B faults consistently overprint V_J2A veins (Fig. 6a). Mineralized V_J2B faults are often surrounded by narrow (5 – 10 cm wide) alteration haloes marked by carbonation, silicification and sulfidation of the host units (Fig. 6b). The V_J2B faults are typically narrow (commonly 10 – 15 cm wide) and extend along strike over a few tens to several hundreds of metres. Although variable in orientation, the mineralized V_J2B faults can be categorized into two main discrete trends that include:

Fig. 6

Structures associated with the V_J2B event. a. V_J2B hydrothermal breccia overprinting the V_J2A colloform breccia. Clasts of V_J2A colloform vein can be observed in V_J2B hydrothermal breccia. b. V_J2B breccia vein in micro gabbro hosting native-Au mineralization associated with a sulfidic alteration halo. c. V_J2B shear veins displaying a conjugate geometry with mutually crosscutting relationship. Both veins display minor (< 5 cm) normal displacement associated with them. Striation lineation associated with the ENE-trending V_J2B veins suggests dextral-normal kinematics. d. NW-trending and NE-dipping V_J2B shear vein in micro-gabbro associated with ENE-trending vertical extension veins. Striation lineation associated with the V_J2B shear veins suggests consistent sinistral-normal kinematics. e. ESE-striking V_J2B shear vein cutting across and displacing the paragenetically earlier V_J1 vein with normal kinematics. Striation lineation associated with the ESE-striking V_J2B shear vein plunges moderately down-dip, consistent with the observed normal kinematics. f. Moderately plunging striation lineation and ore-related step-fibres associated with SE-striking V_J2B structures. Ore-related step fibres suggest normal movement associated with the V_J2B shear veins

NW- to WNW-trending faults represent the dominant mineralized trend in the deposit (Fig. 6c, d, e, f). These structures display opposing dips, with both SW- and NE-dipping lodes (Fig. 6c, d, e, f). Structures along this trend consistently display minor (up to 2 m) apparent normal displacements (Fig. 6c, e) and are occasionally associated with ENE-trending vertical tension veins (Fig. 6d). Fault planes display well-developed striation lineation along with slickenlines, plunging 20° to 30° to the SE for SW-dipping faults (Fig. 6f), moderately (~ 40°) SSW to SSE for S-dipping faults (Fig. 6e) and to the NW for the NE-dipping faults (Fig. 6d). The step fibres display hanging wall down movement indicating normal kinematics along with locally observed minor component of apparent sinistral strike-slip (Fig. 6f).

A second dominant trend includes the ENE-striking faults dipping 40° to 60° towards the SE (Fig. 6c). The ENE-striking and WNW-striking faults often form conjugate geometries and display mutually cross-cutting relationships with consistent apparent normal displacements (Fig. 6c). The ENE-striking faults are associated with a ~ 20° SW-plunging striation lineation which indicates normal-dextral kinematics.

V_J3 gold rich veins

The V_J2B structures are locally overprinted and crosscut by thin (< 2 cm) NNW-striking V_J3 faults (Fig. 7a, b). The V_J3 faults are defined by dark-colored fault-fill veins (Fig. 7c), comprising a calcite- and chlorite-dominated mineral association along with minor quartz. The V_J3 veins are often associated with thin (< 1 cm) sulfide selvedge present as stringers that run along the vein margin (Fig. 7c). These V_J3 veins host extensive native gold that can account for ~ 10% of the vein material (Fig. 7d). The thin V_J3 veins obliquely cut across V_J2B veins (Fig. 7a, c). Local intersections of the V_J3 veins with V_J2B structures are commonly associated with ultra-high-grade gold mineralization (> 100 g/t; Fig. 7a). However, when not intersecting existing mineralized lodes (V_J2B), V_J3 veins only account for low grade (< 3 g/t) gold mineralization (Fig. 7a). The V_J3 veins are continuous over a strike length of a few tens of metres and are associated with reverse kinematics (Fig. 7a, b). This reverse faulting accounts for local centimetric to metric scale displacement of earlier structures (Fig. 7a).

Fig. 7

Structural context of the V_J3 veins. a and b. NNW-striking V_J3 veins cut across and displace the high-grade V_J2B veins with consistent reverse kinematics associated with them. The intersection of the V_J3 and V_J2B veins is associated with localized ultra-high-grade gold mineralization (> 100 g/t). V_J3 veins assayed away from the high-grade V_J2B veins (top-right in a.) are associated with low-grade (< 3 g/t) mineralization. c. Narrow (< 5 mm) chlorite-calcite dominated V_J3 veins cutting across high-grade V_J2B breccia and associated with visible gold mineralization. d. Narrow (< 5 mm) chlorite-calcite dominated V_J3 vein hosting extensive ultra-high-grade gold mineralization

V_J4 post-Au veins

Au-bearing V_J2A and V_J2B structures are cut and truncated by a prominent set of NE-trending granodiorite porphyry dikes (Yeats et al. 2001). Au-bearing mineralized structures and post-Au granodiorite porphyry dikes are cut across by SW-striking normal faults (V_J4) and associated NE-trending vertical extensional veins (Fig. 8a, b). The V_J4 fault domains are defined by massive calcite-dominated veins and locally occurring fault gouges marked by incohesive rock material (Fig. 8c, d, and e). V_J4 fault gouges overprint V_J2B structures and account for sporadic gold mineralization associated with preserved fragments of previously mineralized lodes (Fig. 8d, e). V_J4 faults consistently show normal movement associated with offset on both pre-Au dacitic and post-Au granodiorite porphyry dikes (Fig. 8a).

Fig. 8

Post-Au mineralization structures. a. Post-Au granodioritic porphyry dike is cut and displaced by a NE-trending V_J4 massive calcite vein (blue) associated with normal kinematics and vertical extension veins (white in a. and black-dashed in b.). b. Stereonet projection of V_J4 veins displaying their consistent NE-trending geometry and associated fault-parallel extension veins. c. Mineralized V_J4 fault gauge cutting across and displacing a pre-mineralization dacitic porphyry dike with normal kinematics. d. Close-up on fault gauge in c. displaying loose unconsolidated material. e. Close-up on d. showing fragments of V_J2B mineralized vein preserved in the fault gauge

Structural setting of the gold mineralization at the Bogada prospect

Mineralized zones defined by brittle-ductile shear zones at the Bogada prospect are characterized by alternating bands of white-colored chlorite-calcite shear veins (V_B1) and a brownish cleavage (S_1B) exhibited by fine-grained biotite and plagioclase, hosted in basalt (Fig. 9a, b). Foliated mineralized zones form tabular bodies that extend over hundreds of metres in length and depth and can be up to 5 m wide. The S_1B shear foliation and V_B1 shear veins consistently strike towards the SE, parallel to stratigraphy, dipping 20° to 80° to the SW (Fig. 9a, b, and d). Reverse-thrust kinematics associated with the mineralization event can be inferred from S-C fabrics associated with the S_1B foliation (Fig. 9a). Striations related to the S_1B foliation plunge 20° to 30° towards the NW and display consistent sinistral-reverse step fibers (Fig. 9c, d). The SE-striking V_B1 shear veins are locally boudinaged with boudin necks plunging ~ 10° towards the SE (Fig. 9 b, d). Flat-lying extension veins (V_BExt) strike towards the NW and dips 15° to 25° to the NE. These extension veins appear to be locally crenulated and exhibit shallow (~ 20°), SE-plunging fold axis (Fig. 9b, d).

Fig. 9

Structural context of mineralization at the Bogada gold prospect. a. Mineralization associated shear foliation (S_1B) marked by a dark brown alteration dominated by biotite-plagioclase displays S-C fabric asymmetry suggesting reverse kinematics associated with the mineralization event. b and d. Gold mineralization marked by alternating shear foliation (S_1B; red in d.) and shear veins (V_B1; red in d.). Both shear veins and shear foliation display a consistent SE-striking orientation. The shear veins (V_B1) are locally boudinaged with subhorizontal boudin necks. Mineralized zones are also marked by NE-dipping, flat-lying extension veins (V_BExt; orange-dashed in d.) which are locally crenulated. Fold axes of the crenulated extensional veins (V_BExt) are subhorizontal and parallel to the boudin necks. c and d. Mineralization-related shear foliation (S_1B) display consistent NW-plunging striation lineation. Ore-related step-fibres display reverse kinematics associated with the mineralization at Bogada

Ore mineralogy and alteration

As described in the previous section, gold within the Jundee-Bogada camp is hosted by distinct vein types that exhibit systematic crosscutting relationships. Ore mineralogy and alteration associated with each identified veining stage are summarized in Table 2. Detailed petrological descriptions and illustrations are provided in supplementary material (ESM1) and are summarized here.

Table 2 Gold mineralized vein generations in the Jundee-Bogada gold camp. The sulfides associated with the veins occur both within vein and in host rock alteration

In the Jundee gold deposit, the carbonate-dominated V_J2A colloform-crustiform veins host an early low-grade (< 3 g/t) gold mineralization. This mineralization is associated with an ore assemblage comprising pervasive sericite and ferroan dolomite, along with pyrite, sphalerite, galena, tennantite and rare arsenopyrite (Table 2; ESM1). The absence of native gold during this stage indicates that gold is predominantly lattice-bound in the associated sulfides. The overprinting V_J2B structures mark the second and the main stage of gold mineralization at the Jundee gold deposit and are characterized by the deposition of high-grade (> 10 g/t) native gold (Fig. 6b; Table 2). Native gold mineralization in V_J2B structures is associated with carbonate-dominated assemblages in micro-gabbroic hosts and carbonate-poor assemblages in basaltic host units (Table 2; ESM1). Native gold is closely associated with sulfides comprising predominantly pyrite (Py_VJ2B), characterized by complex textural zoning (ESM1). Thin (< 2 cm) calcite-chlorite V_J3 veins hosting localized ultra-high-grade (> 100 g/t) ore zones comprising extensive native gold (Fig. 7c) mark the third stage of gold mineralization at Jundee (Table 2). EDS analysis of native gold associated with the V_J3 veins suggests high Au contents (> 95% Au). Low-grade (< 3 g/t) zones within V_J3 veins are characterized by the lack of native gold and are associated with texturally zoned pyrite (Py_VJ3; ESM1) along with chalcopyrite, tetrahedrite, stibnite and galena (Table 2). At the Bogada prospect, the ore mineral assemblage is characterized by shear veins (V_B1) associated with a biotite-plagioclase-dominated assemblage along with pyrite (Py_B), pyrrhotite and arsenopyrite hosting low-grade (< 3 g/t), invisible gold mineralization (Table 2; ESM1).

Discussion

Structural framework for the Jundee-Bogada gold camp

Assessment of the paragenetic features documented at the Jundee deposit and Bogada gold prospect reveals a complex deformation history where gold mineralization occurred during at least three distinct deformation events (Fig. 10). The deformation events identified in this study are compared to those documented by Vearncombe et al. (2000), Kohler and Phillips (2003) and Baggott et al. (2005). There is no evidence of any major rotation post-dating bulk ore formation in the camp. Accordingly, one can assume that the present orientation of veins reflects that of their emplacement. This is a critical parameter when undertaking local paleostress reconstructions during multistage gold addition in the Jundee-Bogada system. Detailed paleostress reconstructions are provided in the supplementary material (ESM2) and are summarized hereafter.

Fig. 10

Structural paragenetic framework for the Jundee-Bogada gold camp highlighting the polyphased formation of gold mineralization over multiple deformation events

Cryptic D_JB1

The earliest evidence of hydrothermal activity is recorded by the formation of V_J1 chlorite-calcite-pyrite veins at the Jundee deposit. These barren V_J1 shear veins exhibit a consistent conjugate geometry (McClay 1991) between SE-striking and N-striking veins (Fig. 10). According to Anderson's (1951) theory of faulting, the intersection of conjugate faults provides the intermediate principal stress axis σ2, while the acute bisector can be used to infer the maximum principal stress σ1 orientation. Accordingly, the geometry of the V_J1 veins indicates their formation under NE-SW extension (D_JB1; Fig. ESM2_1). V_J1 veins are often cut across and displaced by Au-bearing faults (Fig. 6e), implying a pre-gold timing of emplacement for such veins.

D_JB2A

Early gold mineralization preserved at the Jundee deposit is linked to the formation of Au-bearing V_J2A colloform-crustiform veins and cockade breccias (Fig. 5a, b). The V_J2A colloform veins overprint the dacitic porphyry and mafic dikes and locally form conjugate geometries (Fig. 5e, d, f). The subhorizontal intersection between the V_J2A conjugate veins indicates vertical shortening and NE-SW extension (D_JB2A in Fig. 10; Fig. ESM2_2). Evidence for extensional deformation is further supported by spatially related, NW-SE-trending vertical extensional veins and normal kinematics recorded on the V_J2A colloform veins (Fig. 5c, e, and f). Previous studies interpreted colloform-crustiform veins to be barren, where gold mineralization post-dated the formation of these veins (Maclean et al. 2000; Kohler and Phillips 2003). However, the consistently low grades associated with these veins mark the earliest gold mineralization stage at the Jundee gold deposit (Fig. 5e).

D_JB2B

The formation of V_J2B shear veins and native gold mineralization, along with the brecciation of the V_J2A colloform structures, characterizes the 'D_JB2B' deformation event. This event marks the second and main stage of gold mineralization at the Jundee deposit (Fig. 10). Locally preserved fragments of colloform veins as clasts in V_J2B breccia also suggest that this event succeeded D_JB2A extensional deformation (Fig. 6a). Extension veins are a key indicator for paleostress orientation (Cox 2020). For example, the intersection between shear veins and extension veins provides intermediate principal stress (σ2), while σ1 lies in the extension vein, perpendicular to σ2 (Cox et al. 2001). Similar crosscutting relationships can be observed between the NE-striking shear veins (V_J2B) and the ENE-trending extensional veins (Fig. 6d), indicating σ1 plunging obliquely (~ 60°) towards the WSW. Such subvertical shortening would result in an oblique-slip displacement consistent with the moderately plunging striation lineation typically associated with V_J2B faults (Fig. 6c, d, e, and f). Locally, the V_J2B faults also display conjugate relationships between NW-striking and ENE-striking faults (Fig. 6c). The subhorizontal intersection between the conjugate faults similarly indicates their formation under a subvertical shortening deformation (σ1 70° towards the west). The minor discrepancy between the principal shortening direction (σ1) might be associated with local strain refraction within rock units of contrasted rheology (Treagus 1988; Cox 2020). The steeply plunging, yet oblique, maximum principal stress (σ1; Fig. ESM2_3 and Fig. ESM2_4) orientation observed during this stage is characteristic of transtensional deformation (Fossen et al. 1994). Transtensional deformation is expressed by oblique-slip structures with both extensional and strike-slip components (Murphy and Copeland 2005). Systematic documentation of moderately plunging striation lineation and kinematic indicators such as step-fibers indicating normal-sinistral movement (Fig. 6c, e, f) further support a local transtensional strain regime (D_JB2B; Fig. 10).

The two veining episodes (V_J2A and V_J2B) have been previously documented as a single deformation event resulting in high-grade gold mineralization at the Jundee deposit (Maclean et al. 2000; Vearncombe 2000; Kohler and Phillips 2003). Past studies strictly documented sinistral-normal kinematics for NW-trending faults and dextral-normal kinematics for NE-trending faults associated with D_JB2B (Vearncombe et al. 2000; Kohler and Phillips 2003). These studies suggested a ‘complex conjugate cyclic compression’ model for this deformation event, inferring a maximum compressive stress (σ1) direction of 55° → 065° (Vearncombe et al. 2000). The investigation of mineralized lodes conducted in this study, indicate that mineralized V_J2B faults (D_JB2B) consistently re-activate V_J2A colloform veins (D_JB2A; Fig. 6a). This systematic overprinting relationship results in a complex structural arrangement leading to the formation of ore shoots with contrasted orientations. The switch from extensional to transtensional deformation (D_JB2A → D_JB2B) might indicate a progressive deformation (Fossen et al. 2019) based on their compatible kinematic record and intimate spatial association.

D_JB3

The third mineralization stage is associated with the formation of thrust faults in both the Jundee deposit and the Bogada gold prospect and marks the onset of D_JB3 contractional deformation (Fig. 10). At Jundee, these structures are characterized by V_J3 veins that locally host ultra-high-grade gold mineralization where NNW-striking V_J3 veins cut across and displace the dominant D_JB2B structures (Fig. 7a, b). The rarely observed but systematic reverse kinematics associated with these structures are tentatively correlated with an ~ E-W-directed horizontal shortening (D_JB3; Fig. 10). At the Bogada prospect, reverse kinematics recorded by S-C fabrics associated with mineralized shear foliation (S_1B) characterize the D_JB3 deformation (Fig. 9a). Ore-related step-fibers further provide evidence for oblique, sinistral-reverse kinematics, that would have developed under a NE-SW shortening deformation (Fig. 9c, d). The occurrence of flat-lying extension veins (V_BExt; Fig. 9b) associated with mineralized shear zones further suggests that the third episode of gold mineralization formed during contractional deformation (Cox 2020). The extension veins are locally crenulated with fold axes plunging 20° to the SE (Fig. 9b, d), whereas the shear veins (V_B1) display boudinage with subhorizontal boudin necks (Fig. 9b). Both fold axis and boudin necks are parallel to the intermediate principal stress σ2 that display a subhorizontal attitude, compatible with contractional deformation (Anderson 1951; Strömgård 1973). Based on the consistent reverse kinematics observed at the Jundee (V_J3) and Bogada deposits (S_1B and V_B1), we conclude that the third mineralization stage was associated with local E-W to NE-SW shortening (D_JB3 in Fig. 10; Fig.  ESM2_5). Post-Au veins composed primarily of calcite-quartz-chlorite have been previously identified in the Jundee deposit (stage 8 in Baggott et al. 2005). These veins are compositionally and paragenetically comparable to the V_J3 veins at Jundee, yet D_JB3 has not been formally documented in previous studies.

D_JB4

Post-gold deformation is marked by the SW-striking V_J4 normal faults (Fig. 10) that cut across and displace the post-Au granodiorite porphyry dikes in the Jundee deposit (Fig. 8a). The presence of vertical extension veins, striking parallel to the fault (Fig. 8a, b), along with consistent normal kinematics (Fig. 8a, c), indicate that V_J4 veins formed during vertical shortening deformation (extensional; D_JB4; Fig. 10). Local occurrences of gold hosted in the V_J4 fault gouge where overprinting previously mineralized veins (V_J2B; Fig. 8c, d, e) indicate that gold was physically remobilized during this deformation. Normal faults characterized by carbonate veins lacking sulfide and gold have been previously documented to cut across mineralized faults at the Jundee gold deposit (Kohler and Phillips 2003). Such veins were inferred to be part of the post-gold hydrothermal event that affected the granodiorite porphyry dikes and Proterozoic gabbroic dikes (Kohler and Phillips 2003). Locally occurring calcite-altered fault gouges were also documented during this deformation event (Kohler and Phillips 2003).

The discrepancy in the structural framework outlined in this study compared to previous work (Vearncombe 2000; Kohler and Phillips 2003; Baggott et al. 2005) is attributed to complexity resulting from multiple overprinting deformation phases. Spatial overlap of distinct deformation events has resulted in various ore shoot orientations, altered kinematic records, overprinting assemblages, remobilization and/or redistribution of early gold into superimposed structures. The established framework, although complex, clearly outlines the protracted introduction and remobilization of gold during multiple stages linked to polyphased deformation (Fig. 10).

Vein textures, hydrothermal alteration and polyphased gold mineralization

The structural framework established in the previous section highlights that gold mineralization in the Jundee-Bogada camp developed over at least three deformation phases that account for the formation of both low-grade invisible gold and high-grade native gold. By clearly establishing the diachronous emplacement of gold, the key question remains whether the three episodes record a singular or punctuated hydrothermal history. Mineral paragenesis associated with each stage is detailed in Table 3.

Table 3 Hydrothermal and ore paragenesis for mineralized vein styles at the Jundee-Bogada gold camp. The thickness of the line corresponds to the relative abundance of minerals, whereas the dashed lines correspond to the frequency of occurrence of particular minerals

At the Jundee deposit, early low-grade, invisible gold mineralization developed in colloform-crustiform veins (V_J2A) that formed during extensional deformation (D_JB2A). Open-spaced colloform-crustiform vein textures are indicative of shallow, near-surface (< 5 km) crustal environments (Dong et al. 1995). The sequential deposition of monomineralic ferroan dolomitic bands in the V_J2A veins indicates episodic fluid flow that might be associated with repeated fault-rupturing episodes, as has been proposed for gold hydrothermal systems (Cox 2020). Gold grades are often associated with pervasive sericite and ferroan dolomite-dominated wall rock alteration (Table 3; Fig. ESM1_1 a, b, c). The abundance of ferroan dolomite and muscovite suggests that mineralizing fluid resulted in carbonation and K-metasomatism of host rocks. Silicification and sulfidation were limited during this episode, as evidenced by the lack of quartz and sulfides associated with the V_J2A colloform-crustiform veins (Table 2). A CO₂-dominated fluid composition is inferred for the early mineralization stage based on the carbonate-dominated vein composition. This CO₂-rich fluid might have reacted with the Fe-/Mg-rich silicates in the mafic host rocks to produce ferroan dolomitic carbonates and release Al, which thereupon might react with K-rich fluids to form sericite/muscovite (Kerrich and Fyfe 1981). The source of such CO₂-rich fluids is highly debated, with both metamorphic and magmatic origins and is atypical of orogenic-style gold deposits (Kerrich and Fyfe 1981; Lowenstern 2001; Phillips and Evans 2004; Goldfarb and Pitcairn 2023). CO₂ solubility in hydrothermal fluids increases as a function of pressure such that fluids from deep crustal environments tend to have high CO₂/H₂O ratios (Baker 2002). Such conditions are typically associated with degassing of CO₂ from deep magmatic systems and even mantle (Lowenstern 2001; Goldfarb and Pitcairn 2023). In such magmatic-hydrothermal systems, the initial fluid composition is CO₂-dominated and evolves to later CO₂-H₂O composition (Lowenstern 2001; Baker 2002). A similar evolution can be observed in the changing vein mineralogy at Jundee, where the early V_J2A colloform veins are carbonate-dominated and are overprinted by later quartz-carbonate V_J2B veins. Early carbonation related to CO₂-rich fluids has been recorded to predate gold mineralization elsewhere in the Yilgarn Craton and is suggested to be a critical ‘ground-preparation’ stage before the main gold event (Voute and Thébaud 2015). Furthermore, the formation of epizonal textures in extensional settings complemented by sulfide mineralogy dominated by pyrite, sphalerite, galena and Cu-sulfosalts (Table 3) is atypical of classic orogenic gold deposits (Groves et al. 1998) and more characteristic of low-sulfidation epithermal deposits (Albinson et al. 2001; Corbett 2002). Such shallow-crustal vein textures are uncommon in Archean orogenic deposits but have been previously documented in a number of deposits, including Golden Mile (Clout et al. 1990), Wiluna (Hagemann et al. 1992), Kanowna Belle (Tripp et al. 2020) and Racetrack (Gebre-Mariam et al. 1993) in the Yilgarn Craton and the world-class Campbell Red Lake, Dome and Kirkland Lake deposits in the Superior Craton (Dubé et al. 2004; Stromberg et al. 2018). For example, at the Au-Ag Racetrack deposit, shallow crustal vein textures, including comb, rosette and banded, that host atypical metal associations (including tellurides, Ag-rich sulfosalts, stibnite and chalcedony) were interpreted as epithermal in origin (Gebre-Mariam et al. 1993). Similarly, colloform-crustiform veins hosting Fimiston-style ore at the Golden Mile deposit are associated with a magmatic-derived fluid source (McDivitt et al. 2022). The presented vein compositions, textures, and associated wall rock alteration assemblages for the Jundee deposit collectively suggest that the earliest gold mineralization at Jundee has epithermal affinities (Hedenquist et al. 2000).

The second mineralization episode is linked to D_JB2B transtensional deformation, silicification and brecciation of preexisting low-grade colloform-crustiform veins (V_J2A) along with the formation of new structures (V_J2B). Gold mineralization during this episode is distinguished by the ubiquitous presence of native gold hosted in V_J2B faults (Fig. 6b). This episode of gold mineralization is further characterized by the pervasive silicification of host units (Table 3; Fig. ESM1_1 e, f) as compared to the dominant carbonation and K-metasomatism observed during the first episode (V_J2A). CO₂ addition is marked by the presence of calcite-chlorite-dominated vein mineralogy and wall rock assemblage in the gabbroic host unit. Contrastingly, in the basaltic hosts, V_J2B veins display a relative abundance of quartz over calcite. The lack of calcite in V_J2B veins hosted in basaltic rocks is possibly related to the removal of Ca during the former colloform veining event (V_J2A), which preferentially developed in basalts as compared to the gabbroic units. This inference is further strengthened by the relative abundance of Ca-bearing phases such as actinolite and titanite in gabbroic host units when compared to basalt during the second mineralization episode (V_J2B; Table 3). Native gold mineralization occurred alongside intense sulfidation marked by the widespread sulfide phases present within and along V_J2B faults (Table 3; Fig. ESM1_1 h, i). The Fe-poor nature of the carbonates in this episode might be associated with the amplified sulfidation as Fe would preferentially fractionate into sulfides phases as compared to Fe-carbonate phases such as ferroan dolomite observed during the V_J2A veining episode. Native gold mineralization during this episode (D_JB2B) is intimately associated with texturally zoned pyrite and Cu-rich phases, including chalcopyrite and tennantite (Table 3; Fig. ESM1_1 i). Similar textural associations between native gold and zoned pyrites have been previously documented in several orogenic gold deposits and are typically related to remobilization processes (Morey et al. 2008; Cook et al. 2009; Velásquez et al. 2014; Fougerouse et al. 2016; Hastie et al. 2020; Sugiono et al. 2022). The role of such processes during the D_JB2B deformation stage, however, remains currently cryptic and requires additional geochemical-microstructural work that falls beyond the scope of this paper. Local occurrences of hematite and sulfate phases, including anhydrite, might indicate an oxidized ore fluid (Mikucki and Ridley 1993). Petrographic investigation of V_J2B veins and associated alteration suggest that the second episode of gold mineralization resulted from significant Si, CO_2, and S addition along with carbonation, which differs from the predominant K-metasomatism and carbonation associated with the colloform veining episode.

A third episode of gold mineralization is recorded by the paragenetically late calcite-chlorite V_J3 veins that formed at the Jundee gold deposit during the D_JB3 contractional deformation (Fig. 10). Gold mineralization during this stage is characterized by locally occurring ultra-high-grade (> 100 g/t) zones (Fig. 7 d). The rare occurrences of bonanza grades within V_J3 veins are restricted to intersections with the former V_J2B faults (Fig. 7 a). Solitarily, these veins only account for low-grade mineralization (< 3 g/t; Fig. 7 a). The exhibited evidence argues against a distinct ultra-high-grade gold event associated with the V_J3 veins. Instead, we propose that some (if not all) of the gold in the V_J3 veins was locally remobilized and redistributed spatially from earlier stages. Remobilized gold is often purer than the precursor stages due to the higher solubility of Ag (Hough et al. 2011). Such inferences are consistent with the semi-quantitative assessment (EDS) of native gold in the V_J3 veins that indicate higher Au contents (> 95% Au) when compared to native gold in V_J2B veins (~ 88 – 95% Au). The narrow (< 2 cm) outline of the V_J3 veins along with the weakly associated wall rock alteration assemblage, are characteristics of low fluid-flux faults (Cox 2020). Mineralizing fluids during this episode resulted in an addition/recycling of CO₂, Si, S and Sb. However, the intensity of this metasomatism was minor. The abundance of Sb-rich phases, including tetrahedrite and stibnite present in Py_VJ3 along with chalcopyrite and galena (Table 3; Fig. ESM1_2 c, d) is typical of gold sourced from metamorphic origins (Pitcairn et al. 2021).

Contemporaneous to the third gold episode at the Jundee deposit, mineralization at the Bogada prospect is distinguished by low-grade invisible gold hosted in ductile shear zones that developed during contractional deformation (D_JB3). This mineralization stage is best exemplified in the Bogada prospect. There, the gold-related biotite-plagioclase-pyrrhotite-dominated alteration assemblage (Table 3; Fig. ESM1_2 e, f, g, and h) is characteristic of orogenic gold deposits forming in upper-greenschist to lower-amphibolite metamorphosed mafic hosts (Eilu et al. 1999). Gold mineralization at Bogada resulted from significant enrichment of CO₂, K, Ca and S and minor input of Si and Na. Carbonation and Ca-addition are characterized by the calcite-chlorite-dominated composition of shear veins (V_B1; Table 2). Ca addition is further marked by the abundance of Ca-bearing phases in the ore mineralogy, including Ca-rich plagioclase, titanite, actinolite and apatite (Table 3). K-metasomatism resulted in the biotite-dominated alteration of host units. S-addition marked by the pyrite-pyrrhotite dominated assemblage that reflects reduced conditions. The variation in the alteration assemblage of the late gold event (D_JB3) between Jundee and Bogada likely resulted from a difference in the metamorphic grade of the host lithologies (Eilu et al. 1999). This difference conforms with previous descriptions of an increase in the metamorphic grade of the greenstone sequence towards the granitoid margin in the Yandal greenstone belt (Vearncombe et al. 2000; Kohler et al. 2003).

Cross-cutting relationships coupled with overprinting ore assemblages establish that gold mineralization in the Jundee-Bogada camp developed periodically over three episodes that spanned local deformation history. Distinctive alteration styles and metal associations reflect transient changes in fluid chemistry between individual episodes. Metal associations and vein textures associated with early low-grade mineralization at the Jundee deposit (V_J2A) conflict with those described for classic metamorphic-sourced orogenic-style gold deposits (Groves et al. 1998; Phillips and Powell 2009). In contrast, Ore assemblages such as those observed in the late mineralization (D_JB3) at Jundee and Bogada are typical of the classic orogenic-style deposits documented ubiquitously across the Yilgarn Craton (Groves et al. 1998; Eilu et al. 1999).

Insights into the episodic formation of high-grade gold mineralization

The markedly contrasting style of gold mineralization documented between the Jundee and Bogada gold deposits provides a unique opportunity to examine primary controls influencing the development of high-grade ore zones in orogenic gold deposits. Historically, several processes have been proposed as potential drivers for forming high-grade gold mineralization. Contrasting gold grades have been associated with variation in host rock composition, whereby chemically distinct lithologies can either increase or decrease the efficiency of the fluid-rock interaction processes, leading to gold deposition (Klein 2014; Petrella et al. 2021). In such scenarios, a single mineralizing fluid can lead to both high and low-grade mineralization, hosted in chemically distinct host rocks (Kříbek et al. 2015; Petrella et al. 2021). At the Jundee-Bogada gold camp, both high and low-grade gold mineralization occurs within basalt and gabbroic host rocks, suggesting that processes such as fluid-rock interaction had a minimal contribution to the formation of high-grade ore zones. Furthermore, the structural and paragenetic framework established at the two deposits indicates multiple episodes of gold introduction associated with the polyphased deformation history.

Evidence for punctuated timing of gold mineralization where the collective gold endowment accumulated over multiple discrete deformation stages has been documented in several orogenic gold deposits across the Yilgarn Craton (Bateman and Hagemann 2004; Jones 2014; Thébaud et al. 2018; Jones et al. 2019; Sugiono et al. 2021; McDivitt et al. 2022). Yet, not all these deposits account for high-grade gold mineralization, as evidenced at the Jundee deposit. For instance, in the Agnew gold camp, multiple diachronous gold events sourced from distinct reservoirs developed during a progressive shortening deformation where the ultra-high-grade gold mineralization is spatially extremely restricted (Thébaud et al. 2018). Similarly, at the 80 Moz Golden Mile deposit, early Fimiston-style mineralization formed during transtensional deformation and is overprinted by later Oroya-type mineralization that accounts for exceptionally high gold grades (up to 1000 g/t; McDivitt et al. 2021b). However, the last gold stage characterized by Mt. Charlotte stockwork veins is typically low grade (< 3 g/t; McDivitt et al. 2021b) and lacks evidence for high-grade gold mineralization (Mueller 2015; McDivitt et al. 2021b). In both deposits, high-grade gold intercepts are spatially restricted to areas where polyphased gold events overprint each other. Examples of such areas include Oroya-type mineralization at Golden Mile (Mueller and Muhling 2020; McDivitt et al. 2021b) and the Waroonga deposit in the Agnew gold camp (Jones 2014; Thébaud et al. 2018). Contrastingly, where polyphased gold events are spatially disjointed, gold mineralization typically tends to be of relatively lower grade (Mt. Charlotte; McDivitt et al. 2021b).

The polyphased nature of gold mineralization in orogenic gold deposits, similar to the one observed in the Jundee-Bogada gold camp and other deposits in the Yilgarn Craton has also been well documented in other Archean and Proterozoic greenstone belts worldwide (Couture et al. 1994; Kolb et al. 2005; Robert et al. 2005; Sarma et al. 2008; Simard et al. 2013; Peterson and Mavrogenes 2014; Velásquez et al. 2014; Fougerouse et al. 2016; Le Mignot et al. 2017; Kerr et al. 2018; Stromberg et al. 2018; Wu et al. 2019; Dubé and Mercier-Langevin 2020; Hastie et al. 2020; Thébaud et al. 2020; Herzog et al. 2022; Combes et al. 2023; Lawley et al. 2023; Perret et al. 2023). Several deposits documented in these studies are characterized by the early formation of low-grade gold mineralization followed by the development of high-grade native gold mineralization during later stages (Simard et al. 2013; Velásquez et al. 2014; Fougerouse et al. 2016; Kerr et al. 2018; Hastie et al. 2020; Combes et al. 2023; Perret et al. 2023). For example, at the 62 Moz Obuasi gold deposit in West Africa, the formation of the early refractory-style mineralization characterized by gold-bearing arsenopyrite was coetaneous with the formation of quartz veins, isoclinal folding and foliation development during the main NW-SE shortening deformation. Late refolding and fracturing of the quartz veins during N-S shortening resulted in the localized formation of high-grade ore zones (up to 2000 g/t) attributed to remobilization from the precursor stage (Fougerouse et al. 2016, 2017). A similar study from the ~ 3 Moz Galat Sufar South deposit in the Nubian Shield, Sudan suggests that the main gold mineralization event occurred during the early ductile deformation characterized by the development of main shear foliation, strong albitization and localized silicification (Perret et al. 2020). At Galat Sufar, the localized formation of high-grade ore shoots occurred during the late brittle fracturing of the silicified units and has been linked to rheological control induced by alteration associated with former hydrothermal stages (Perret et al. 2020). In the Neoarchean Hutti gold deposit in the Dharwar Craton, gold mineralization developed over polyphased structural episodes under different metamorphic conditions (Kolb et al. 2005). Early low-grade gold mineralization at Hutti is associated with disseminated pyrite-arsenopyrite alteration that developed during ductile shearing at upper greenschist to lower amphibolite facies conditions. Late strike-slip reactivation of gold-bearing shear zones during low-greenschist facies conditions resulted in the formation of laminated quartz veins hosting native gold mineralization (Kolb et al. 2005).

In several deposits, the latter formation of native gold mineralization is often associated with remobilization processes during late deformation stages (Kolb et al. 2005; Simard et al. 2013; Velásquez et al. 2014; Fougerouse et al. 2016; Hastie et al. 2020; Combes et al. 2023; Perret et al. 2023). Evidence for gold remobilization is often invoked based on the textural and chemical analyses of gold-bearing sulfides, typically comprising pyrite and arsenopyrite (Morey et al. 2008; Sung et al. 2009; Peterson and Mavrogenes 2014; Velásquez et al. 2014; Fougerouse et al. 2016; Wu et al. 2019; Hastie et al. 2020; Combes et al. 2023; Perret et al. 2023). Sulfides associated with remobilized native gold often exhibit textural zoning, resembling those described for the D_JB2B (Py_VJ2B; Fig. ESM1_1 h) and D_JB3 (Py_VJ3/ Py_B; Fig. ESM1_2 d, i) stages and might suggest the involvement of such processes in upgrading gold tenor. This interpretation, however, relies solely on textural observations and requires further confirmation through in-situ geochemical analyses of pyrite.

Although concrete evidence for gold remobilization is lacking, the prevailing multiphase ore model established at the Jundee-Bogada gold camp emphasizes the significance of polyphase gold mineralization in forming high-grade ore zones and the cumulative high tonnage as observed for the 9 Moz Jundee gold deposit. The initial formation of high-grade (> 10 g/t) ore zones at Jundee occurred during the transtensional deformation (D_JB2B). This stage marks the second and main mineralization episode at the Jundee deposit, characterized by the localized brecciation of the low-grade colloform-crustiform veins (Fig. 6 a). A subsequent upgrade in gold tenor is associated with the localized formation of ultra-high-grade (> 100 g/t) gold veins at Jundee (V_J3), where late contractional deformation (D_JB3) overprints the precursor high-grade veins (V_J3 overprinting V_J2B; Fig. 7 a). Although this stage records the highest gold intercepts (up to 15 wt. % Au), their occurrence is localized and indicates only a minor contribution to the overall gold endowment at the Jundee deposit. This late mineralization stage (D_JB3), although evidenced at the Bogada prospect, lacks the bonanza grades documented at Jundee. The absence of such high-grade ore zones at Bogada is attributed to the lack of multistage gold additions, such as those exhibited at Jundee. These findings highlight the critical role of the progressive superimposition of polyphased gold events, as observed at Jundee, in forming high-grade gold mineralization in orogenic gold deposits.

Conclusions

A simple yet detailed paragenetic evaluation of the structurally complex Jundee-Bogada gold camp has revealed that gold mineralization resulted from multiple episodes spanning a protracted deformation history. The findings presented in this study, along with worldwide examples of other orogenic gold deposits, provide substantiation for the progressive development of such deposits. High-grade gold mineralization (> 10 g/t) occurring at the Jundee gold deposit resulted from an episodic enrichment over three deformation phases. Early extension-related colloform-crustiform veins (V_J2A) associated with atypical mineral associations of sphalerite, galena and tennantite host a low-grade invisible gold mineralization. Epizonal textures such as those preserved by V_J2A veins, along with the potassic metasomatism and base metal associations are more characteristics of epithermal systems than typical orogenic Au deposits (Albinson et al. 2001; Corbett 2002). A switch to transtensional deformation (D_JB2B) resulted in the brecciation (V_J2B) of colloform-crustiform veins and consequent formation of native gold mineralization. Native gold mineralization was accompanied by intense sulfidation and silicification that overprinted the earlier colloform veining event. Late contractional deformation (D_JB3) and associated thrusting was coeval with the last mineralization episode. This late stage was characterized by the localized formation of ultra-high-grade (> 100 g/t) ore zones within V_J3 veins where crosscutting the high-grade V_J2B breccia at the Jundee deposit. Coincidently, low-grade invisible gold mineralization hosted in shear zones (V_JB1) developed at the Bogada prospect. Our results demonstrate that the high-grade and high-tonnage endowment of the Jundee deposit resulted from the spatial overprint of multiple mineralizing episodes as opposed to the singularly sourced low-grade Bogada prospect.

A fundamental characteristic of high-grade gold systems discussed in this study requires multiple overprinting gold events superimposed in space. This episodic enrichment witnessed in several high-grade deposits worldwide is often associated with the multistage introduction of hydrothermal fluids during polyphased deformation history. The work presented in this paper emphasizes the importance of comprehensively documenting the paragenetic evolution of orogenic gold deposits to decipher individual mineralization episodes and by extension, gain a better understanding of the processes resulting in the formation of bonanza gold deposits like Jundee.