Coupled Microstructural EBSD and LA-ICP-MS Trace Element Mapping of Pyrite Constrains the Deformation History of Breccia-Hosted IOCG Ore Systems

Abstract

Electron backscatter diffraction (EBSD) methods are used to investigate the presence of microstructures in pyrite from the giant breccia-hosted Olympic Dam iron–oxide copper gold (IOCG) deposit, South Australia. Results include the first evidence for ductile deformation in pyrite from a brecciated deposit. Two stages of ductile behavior are observed, although extensive replacement and recrystallization driven by coupled dissolution–reprecipitation reaction have prevented widespread preservation of the earlier event. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) element maps of pyrite confirm that many pyrite grains display compositional zoning with respect to As, Co, and Ni, but that the zoning is often irregular, patchy, or otherwise disrupted and are readily correlated with observed microstructures. The formation of ductile microstructures in pyrite requires temperatures above ~260 °C, which could potentially be related to heat from radioactive decay and fault displacements during tectonothermal events. Coupling EBSD methods with LA-ICP-MS element mapping allows a comprehensive characterization of pyrite textures and microstructures that are otherwise invisible to conventional reflected light or BSE imaging. Beyond providing new insights into ore genesis and superimposed events, the two techniques enable a detailed understanding of the grain-scale distribution of minor elements. Such information is pivotal for efforts intended to develop new ways to recover value components (precious and critical metals), as well as remove deleterious components of the ore using low-energy, low-waste ore processing methods.

1. Introduction

Pyrite (FeS₂) is the most common sulfide mineral in the Earth’s crust. It is sufficiently refractory to allow the preservation of microstructures that are typically erased in more ductile ore minerals, meaning it can be used to fingerprint sequential stages in the deformational history of a mineral deposit [1]. Pyrite is therefore a valuable indicator of deformation in sulfide ore deposits [2], and it has been widely used to characterise metamorphosed ores and successfully constrain the impact of regional metamorphism and associated deformation on mineralization [3,4,5,6].

Early studies viewed pyrite as a mineral with brittle behavior at most crustal conditions, transitioning to ductile behavior at temperatures above ~425 °C [7,8,9]. The application of electron backscatter diffraction (EBSD) techniques to pyrite underpinned major improvements in understanding pyrite deformation mechanisms [5,10,11,12]. This research culminated in construction of a new deformation mechanism map for pyrite [13] that indicates ductile behavior down to temperatures as low as ~260 °C. Ductile deformation in pyrite is commonly accommodated through low-angle subgrain boundary formation by dislocation creep during dislocation glide with a climb recovery step operated through slip on {100} and, less commonly, {110} systems [3,10,11]. However, deformation style largely depends on the mineralogy and fabric of the surrounding groundmass, whereby a ductile or very fine matrix can allow uninterrupted dislocation glide without grain locking or formation of low-angle boundaries [5].

Pyrite is also an important carrier of minor and trace elements, with a capacity to preserve geochemical signatures and compositional zonation through post-formational overprinting events [14,15,16,17,18]. Trace and minor elements are either incorporated in lattice-bound (solid solution) form or as nanoparticles. Several recent studies [19,20,21,22,23] have documented the importance of crystal defects as traps for trace elements, thus confirming theoretical approaches first put forward more than three decades ago [24]. In addition, the ability of pyrite to respond to strain and the mechanism of deformation may further control not only incorporation but also the retention or release of trace elements [25]. Among common trace elements in pyrite, often heterogeneously distributed within intracrystalline zoning patterns, are gold and several ‘critical metals’, notably cobalt and nickel (e.g., [26]). A microstructural analysis of pyrite can therefore lead to not only a better appreciation of how the ore formed and evolved over geological time but can also represent a valuable tool in efforts to understand how these minor, potential by-product elements are distributed and/or remobilized at the grain-scale.

This contribution focuses on new microstructural data for pyrite from the giant (>10 billion tonne) Olympic Dam hematite-dominant Cu-U-Au-Ag deposit, South Australia [27]. Olympic Dam is the largest uranium deposit and largest iron–oxide copper-gold (IOCG) deposit on Earth [28] and is widely considered an archetype for IOCG mineralization. It is also by far the largest ore system in the major metallogenic belt known as the Olympic Cu-Au Province [29]. High-precision ID-TIMS U-Pb hematite geochronology [30] has provided an unequivocal age of 1591.27 ± 0.89 Ma for mineralization and confirmed a magmatic–hydrothermal origin, with mineralizing fluids sourced from contemporaneous Hiltaba suite granites. Mineralogical, geochronological, and structural studies over the past two decades have recognized the impact of several post-Mesoproterozoic tectonothermal events on the deposit and the broader Olympic Cu-Au Province. The Olympic Dam deposit exhibits a systematic deposit scale zonation expressed by variation in the speciation of Cu-Fe-sulfides and proportions of different gangue minerals [28]. Irrespective of mineral zone, the sulfide assemblage is recognized as being dominantly of hypogene origin.

Contemporary opinion is that the gross architecture of the deposit and its inherent mineralogical zoning is largely a preserved, primary feature. Nevertheless, pervasive fluid-assisted modification of the ore assemblage has taken place over the past 1.6 billion years and may have been assisted by a continuous supply of radiogenic heat. Despite widespread replacement textures, nanoscale investigation of Cu-Fe-sulfides from Olympic Dam [31] has shown evidence of primary textures between bornite and chalcocite group minerals, interpreted as exsolutions along multiple cooling paths from primary bornite solid solution ≤400 °C. The complexity of sulfide assemblages at the smallest scale and significance of overprinting events in reshaping the micron-scale distribution of elements of interest, even when the overall structure of the system is for the most part preserved, is illustrated clearly in a recent nanoscale study of gold-bearing pyrite from Olympic Dam [32]. These authors provide evidence for modification of oscillatory zoning patterns in pyrite by dissolution–reprecipitation processes and the syn-deformational remobilization of trace elements that led to release of lattice-bound gold to form trapped Au- and Au-Ag-Te nanoparticles. Moreover, the presence of pore-attached nanoparticles [32] suggests high percolation rates, the redistribution of Pb and Bi, and possibly a degree of scavenging of Au by Bi-bearing melts. This complexity in pyrite is complemented by recognition of new sulfide growth and development of low-angle slip dislocations, twist-wall boundaries, and deformation-dipole nanostructures.

Despite the potential of EBSD to constrain the evolution of mineral textures, only limited EBSD work had been undertaken on Olympic Dam ores prior to the present study. Macmillan et al. [33] used EBSD evidence to interpret compositionally zoned uraninite as the result of a sequence of overprinting. The crystal–plastic deformation of uraninite was found to take place via formation and migration of defects and dislocations into tilt boundaries. The utility of EBSD for depicting superimposed events was illustrated by a map of an apparent single grain of chalcopyrite that was shown to be a fused aggregate with boundary domains harboring elevated concentrations of radiogenic lead (Figure 13 in Rollog et al. [34]).

The present study therefore sets out to expand on the evolution of sulfide assemblages at Olympic Dam and assess the impact that different deformation episodes have had on microscale trace element distributions. The EBSD-driven approach targeting microstructures in pyrite and associated phases is complemented by reflected light microscopy, scanning electron microscope (SEM) imaging and mapping, and quantitative trace element analysis by laser ablation inductively coupled plasma mass spectroscopy (LA-ICP-MS). While recognizing that ore textures, mineral compositions, and microstructures can show multiple overprints, EBSD is better suited for constraining deformation episodes and their sequences in some cases, despite poor preservation and heterogeneous expression of such events across the deposit.

Alongside our primary focus on EBSD analysis, we aim to understand the deformational evolution of pyrite by correlating the observed microstructures with internal compositional patterning defined by the grain-scale distribution of trace and minor elements as mapped by LA-ICP-MS. Linking the two methodologies serves two additional purposes. Firstly, efficient comminution benefits from understanding deformation-induced microstructures that are otherwise invisible. Secondly, accurate tracking of minor ore components of value (Au, Ag), or potential value (e.g., Co) through processing is contingent on a comprehensive appreciation of their distributions at the grain-scale (e.g., [35]), especially considering the critical interplay between pyrite textures and metallurgical performance [36,37].

2. Deposit Geology and Sample Suite

The world-class Olympic Dam Cu-U-Au-Ag deposit is part of the Olympic Cu-Au IOCG province located on the eastern edge of the Archean to Mesoproterozoic Gawler Craton [38] (Figure 1). Mineralization across the province was contemporaneous with formation of the Gawler silicic large igneous province (SLIP) (~1.595–1.586 Ga; [39]), which comprises Gawler Range volcanics (GRV) and the bimodal magmatic Hiltaba Suite (HS) [40,41]. At Olympic Dam, the HS granitoids and GRVs unconformably overlie older basement Hutchinson Group metasedimentary units, Donington suite granitoids (~1.86–1.85 Ga; [42]) and Wallaroo Group Meta(volcanic)sediments (~1.76–1.74 Ga; [43]) (Figure 1).

The orebody is hosted entirely in the Olympic Dam Breccia Complex (ODBC; Figure 2; [44]), in turn located within the Roxby Downs Granite (RDG) from which it is derived. The host RDG becomes increasingly altered and brecciated inwards towards the center of the deposit [45,46,47]. Hematite, the main product of Fe-metasomatism, can be as high as >90% in some areas of the deposit. This includes barren hematite quartz ± barite breccias in the shallow central part of the deposit (HEMQ), which are defined by <0.08 wt% Cu, trace concentrations of sulfide minerals, and a lack of aluminosilicates. The RDG is shown to have been initially emplaced at depths of 6–8 km [45,47]. High-precision ID-TIMS U-Pb hematite geochronology provided an age of 1591.27 ± 0.89 Ma for the earliest mineralization, just ~2 Ma later than the latest magmatic zircon in the RDG (1593.28 ± 0.26 Ma; [30]). Uplift and cupola collapse of the RDG at 2–3 km is proposed to have triggered brecciation and ore formation [48].

The structural evolution of the Olympic Dam region is a response to major tectonic events, notably the breakup of the Nuna supercontinent in the ~300 Ma following initial mineralization, amalgamation, and subsequent break-up of Rodinia (~1.3–1.1 Ga and ~0.85–0.6 Ga, respectively), and then amalgamation to Gondwana (~0.6–0.3 Ga). Unroofing of the deposit occurred during formation of Rodinia prior to ~1425 Ma deposition of the Pandurra Formation. The Olympic Province was further affected by far-field effects from the 1.4–1 Ga Grenville Orogeny [49] and/or the 1.3–1.2 Ga Musgrave Orogeny [50], as well as emplacement of the Gairdner LIP and associated dolerite dykes [51] (Figure 1). The formation of the Adelaide Rift Basin was associated with several stages of rifting, hydrothermal activity and deposition of the Stuart Shelf sedimentary succession that consists of the Heysen and Moralana Supergroups across the Olympic Province [52] (Figure 1). The Delamerian Orogeny across South Australia (514–490 Ma; [53]) resulted in extensive faulting and veining across the region.

At the periphery of the orebody is a weakly altered/brecciated granite called the “outer shell” [49] (Figure 2), which is characterized by the presence of magnetite and pyrite–chalcopyrite assemblages that formed during early-stage alteration. This pyrite–chalcopyrite zone develops systematically inwards and upwards to zones defined by bornite–chalcopyrite and bornite–chalcocite mineralogy, into the HEMQ [28,54] (Figure 2). There is a close association between Cu mineralization and Fe-oxides across the NW lobe, core margin and SE lobe of the orebody, whereas Cu mineralization is sporadic in the Fe-oxide rich core. Across the NW lobe and center of Olympic Dam mineralization is observed to depths of ~1000 m, while in the SE lobe south of the Masher’s fault (Figure 2), Cu-Fe-sulfides extend vertically to depths of at least ~2500 m. Pockets of mineralization occur outside the main resource perimeter within less altered RDG (Figure 2). This ’Distal satellite’ mineralization displays a pyrite–chalcopyrite sulfide assemblage resembling that in the outer shell.

EBSD work was undertaken on five samples containing assemblages considered representative of pyrite in assemblages within the outer shell (three drillholes along a ~3.5 km-long transect and the Distal satellite; Figure 2). Irrespective of the sample’s placement across the deposit, all are representative of the deep pyrite–chalcopyrite zone in the “outer shell”. The transect crosses the NW lobe from which two samples (CLC6 and CLC23) were studied, across the SE lobe (MV37) and extends to the SE lobe outer shell (MV18B) (

Table 1. Samples investigated in this study, locations, and brief descriptions.

Deposit Area	Drillhole	Length along Drillhole (M)	Sample	Analysis	Description
Middle (core margin)	RU65-7976a (magnetite drillcore)	90.7	CLC6	EBSD, LA-ICP-MS	Disseminated pyrite as large aggregates or fragments in breccias with hematite clasts and as fine-grained matrix. The latter also comprises minor chalcopyrite and gangue minerals.
		431.4	CLC23	EBSD
SE lobe-outer shell	RD2366 (nodular Fe-oxide granite drillcore)	458.6	MV18B	EBSD, LA-ICP-MS	Pyrite and chalcopyrite as fine stringers in rock flour, siderite-rich, hematite breccia. Further explanation is given in [48].
SE lobe	RD2786A	2349.3	MV37	EBSD, LA-ICP-MS	Fine-grained pyrite within fractures crosscutting quartz and chlorite domains showing slight deformation in a hematite breccia.
Distal satellite	RD2316	798.1	MV65	EBSD	Coarse pyrite grains and/or aggregates along fractures in altered granite; minor chalcopyrite and hematite present.

; Figure 2). A sample from the Distal satellite (MV65) was also chosen for this study. A spectrum of breccias with variable proportions of Fe-oxides and matrix were included: (i) dominant hematite/magnetite (CLC 6 and CLC23) (Figure 3A); (ii) siderite-rich, milled rock flour breccia (MV18B) (Figure 3B); (iii) quartz–chlorite, sulfide-poor breccia (MV37) (Figure 3C); and (iv) altered granite with pyrite veins (MV65) (Figure 3D) and are further described in Table 1.

3. Analytical Methodology

Assemblages containing pyrite grains were identified and texturally characterized at the micron-scale in reflected light and imaged on a FEI Quanta 450 scanning electron microscope (SEM) operated in backscattered electron (BSE) mode. This reconnaissance work also enabled selection of suitable grains for subsequent EBSD and trace element analysis by LA-ICP-MS.

EBSD data was collected on two SEM platforms: a FEI Helios NanoLab “Dualbeam” FIB/SEM platform; and a Hitachi SU1510, both housed at Adelaide Microscopy, The University of Adelaide. The two instruments utilize Oxford Symmetry S3 detectors (Oxford Instruments, Abingdon, UK). Prior to analysis, each sample was polished in a suspension of colloidal silica for 1 h and a thin (~2.5 nm) coat of carbon was applied. Energy-dispersive X-ray spectroscopy (EDS) was also used alongside EBSD data collection using an Oxford Ultim Max Large Area SSD EDS detector (Oxford Instruments) operating at an accelerating voltage of 20 kV and beam current of 2.7 nA. Orientation contrast (OC) data was processed in AZtecCrystal (Oxford Instruments) to produce band contrast (BC), inverse pole figure (IPF) denoted X-, Y-, or Z-based on crystallographic orientation, texture component (TC), and grain rotation orientation deviation (GROD) angle maps. Post-processing was used to reduce noise by applying a “wildspike” correction, gentle clean-up steps and appropriate pseudosymmetry corrections.

LA-ICP-MS trace element mapping was conducted on three pyrite grains that were previously studied by EBSD (Table 1). Over the grains, sets of parallel lines were ablated using a RESOlution-SE 193 nm Excimer laser coupled to an Agilent 8900x quadrupole ICP-MS (Agilent Technologies Australia. Mulgrave, VIC, Australia) also housed at Adelaide Microscopy. Laser spot size varied between 7 and 10 μm, depending on grain size, and operated at a pulse repetition rate of 5 Hz and fluence of 3 J/cm². Twenty-three isotopes were measured: ²⁷Al, ³⁴S, ⁴⁹Ti, ⁵¹V, ⁵⁵Mn, ⁵⁷Fe, ⁵⁹Co, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁷⁵As, ⁷⁷Se, ¹⁰⁷Ag, ¹¹¹Cd, ¹¹⁸Sn, ¹²¹Sb, ¹²⁵Te, ¹⁸²W, ¹⁹⁷Au, ²⁰⁶Pb, ²⁰⁷Pb, ²⁰⁸Pb, ²⁰⁹Bi. The external reference material STD-GL3 [55] was analyzed before, during, and after pyrite ablation. Collected ablation profiles formed a series of parallel line rasters across the grain that were compiled and processed using ‘Iolite’ software (v.3; Elemental Scientific, Melbourne, VIC, Australia) [56] to generate trace element intensity maps. Mapped intensities are scaled independently for each element to enhance different features; unless stated otherwise in figure captions, the scale used is logarithmic (10ⁿ). ⁵⁷Fe was selected as the internal standard, assuming a stoichiometric FeS₂ composition.

4. Results

4.1. Pyrite Textures and EBSD Mapping

4.1.1. Distal Satellite

Pyrite from sample MV65 is characteristically coarse (up to several mm across) and fractured, sometimes displaying a “porphyroblast”-like morphology enclosed within a rigid matrix of quartz and K-feldspar (Figure 3D). The selected pyrite features a dense fracture network, hundreds of µm in width, infilled with chalcopyrite and gangue minerals, mostly sericite (Figure 4A). Fine-grained hematite is clustered outside pyrite within the same vein. At the pyrite–pyrite grain impingement (Figure 4A), these fractures radiate from the interface at which pyrite has been pulverized and milled into a fine rock flour. Pyrite contains rounded inclusions comprising coexisting chalcopyrite and pyrrhotite, and rarely cattierite, CoS₂, (with Co/Ni ~4.3) (Figure 4B,C).

Band contrast and Y-IPF mapping reveal no clear microstructures in the pyrite (Figure 5A,B) although, SEM-BSE imaging does reveal trails of µm-sized galena blebs close to the grain margin (inset Figure 5A). EBSD pole-figure analysis on pyrite outlined on the Y-IPF map does however display an apparent strong rotation (~16°) about a single <100> labelled D1 on Figure 5C. The grains associated with this rotation (light/dark blue in outlined pyrite Figure 5B) are isolated from the pyrite by chalcopyrite and sericite infill (Figure 4A) and consequently register as separate grains on orientation contrast mapping. In Figure 5C, from the {100} positions of D1, a shift of all <100> axes in the same orientation are preserved and labelled D2. Pole figure analysis on pyrite grains external to outlined pyrite that exceed 1000 pixels on Figure 5B, thus removing any randomly oriented finer grains, show an approximate mutual orientation close to the termination of the D2 shift in the ‘outlined pyrite’ (Figure 5D).

4.1.2. NW-SE Transect

One of the coarser pyrite grains within siderite-rich breccias was studied from sample MV18B (Figure 3B). Pyrite is mantled by fine-grained hematite and chalcopyrite, with embayment features characteristic of replacement by chalcopyrite (Figure 4D). Pyrite is characteristically zoned with respect to the distribution of Co, As and Ni (see Section 4.2), in a pattern that transitions from oscillatory near the grain margin to more patchy textures towards the grain center (Figure 4E). Fine structures that display Co enrichment around trails of chalcopyrite and siderite are also observed (Figure 4E).

In band contrast and GROD angle maps these structures are shown to have inflicted a small degree (2–5°) of misorientation to pyrite (Figure 6A,B). GROD angle mapping further reveals domains within pyrite that display different responses to deformation, separated by the major microstructure annotated ‘1’. Pyrite on the left of this microstructure shows subtle lattice distortion at the margins and a lenticular ‘fish-shaped’ region lined by fine trails that displays an 8° misorientation about a single <110> axis with surrounding pyrite (Figure 6B,C). The reflected light image shows this domain is at the margin of the Co-enriched zonation (Figure 6D). Pole figures from across the microstructure annotated ‘1’ display a shift of all <100> axes (Figure 6E) that can only be attributed to fracture rotation. On the right of this structure, subtle lattice distortion is widespread and shows no correlation with the pyrite on the left (Figure 6B,F). In areas of pyrite ‘bottlenecking’ by surrounding gangue, this subtle misorientation locks into dislocations (Figure 6B,F) formed by distortion about multiple <100> axes, as shown in pole figures corresponding to ‘2’ and ‘3’ (Figure 6E), attributed to subgrain-boundary formation. There is also a close mutual orientation between pyrite and surrounding chalcopyrite (~1.5° misorientation), with chalcopyrite being uncharacteristically homogenous in this sample. The morphological and chemical heterogeneity of chalcopyrite and other Cu-Fe-sulfides across the deposit, and their response to deformation will be discussed in a subsequent paper.

Another fine-grained pyrite was analyzed from sample MV37 (Figure 3C inset). Pyrite displays varying degrees of brecciation, although some grains appear less impacted than others (e.g., the left grain in Figure 4F). In contrast to the aforementioned styles of pyrite texture, shear-induced deformation is suggested by pyrite domains comprising sets of low-angle, oblique fractures with fine-fragments of angular pyrite at the contact to the breccia matrix (Figure 4F). These fractures contain gangue and are associated with lower-order splintered fractures.

The BC map displays these finer microfractures well and confirms the presence of microstructures associated with an inclusion at the margin of the intact grain (Figure 7A). SEM-BSE imaging reveals that the pyrite ‘inclusion’ is bound by a zone with enhanced porosity; sub-µm-scale galena inclusions are observed along the microstructure (Figure 7B). The IPF map categorizes the microstructures as low- (2–5°) to mid-angle (5–8°) subgrain boundaries (Figure 7C), with the pole figure showing this lattice rotation occurred about a single <100> (Figure 7D). The pole figure also displays two distinct orientations that allow the inference of two original grains with a close mutual orientation. This is shown by the blue and red shades on the TC map (Figure 7E), which reveals an apparent embayment with no clear deformational response. BSE imaging at this contact (Figure 7F) does, however, show dense trails of quartz and galena, a feature sporadically observed elsewhere in the grain. The embayed fragment is severed from the bulk ‘red’ pyrite on the left of Figure 7E by gangue but exhibits a lower degree of misorientation than the proximal subgrains.

Pyrite grains of variable size (~50 µm and ~750 µm) and texture from two samples were studied, located 340 m apart along the drillhole RU65-7976 from the NW lobe. In the first (CLC 6), pyrite is commonly rounded and sutured, featuring embayed margins against the surrounding matrix and annealing has obliterated any microscale evidence for deformation (Figure 3A). In rare grains where relict structures can be inferred, as shown in Figure 4G, extensive brittle brecciation and milling, marked by trails of the surrounding gangue, are preserved (Figure 4G). The retention of a (sub)-idiomorphic morphology that records partial deformation is evidence that this is an aggregate of pyrite.

BC and Z-IPF mapping of the aggregate confirms this and displays misorientation spatially associated with the relict trails (Figure 8A,B). Domains of gradual misorientation fabrics close to grain margins are seen clearly in the GROD angle map (Figure 8C). Pole figures in Figure 8D depict two distinct grain orientations, labelled as Py1 and Py2 on the Z-IPF map (Figure 8B). The two Py2 sub-grains share identical orientations and lie on either side of Py1 with sharp margins to the matrix. Py2 contains trails that partially correspond with those in Py1; however, the structures associated with large misorientation in Py1 (Figure 8B,E—blue transect) do not extend into Py2. This area of progressive stepwise misorientation viced between the Py2 grains preserve a shift on all <100> axis (Figure 8D) whereas the area of internal misorientation intersected by the red transect (Figure 8B,E) seems to display a rotation about a single <100> axis (Figure 8D). Gradual misorientation in Py2 exhibits a rotation about at least two <100> axis (Figure 8D).

The pyrite depicted in Figure 9 is from a shallower level in the same drillhole (sample CLC23) and has the morphology of an angular fragment with embayed margins. This grain is located in a very fine matrix of magnetite, hematite, chalcopyrite, and gangue minerals, all of which also occur as inclusions alongside coarse, elongated pores (Figure 9A). The BC map shows no obvious orientation contrast while the GROD angle map displays gradual misorientation across the grain (Figure 9B). As in the previous sample (CLC6), the subtle marginal misorientation fabric is indicative of plastic deformation. A profile taken vertically across the grain shows an almost constant rate of misorientation (~0.1°/50 μm) that culminates at 2° (Figure 9D). Inclusions close to the top margin seem to have disrupted the lattice deformation response to some degree, but this was insufficient for dislocation formation.

4.2. Interpretation of EBSD Textures

EBSD results show some pole figures that display shifts of all axes can be attributed to fracture rotation. Fracture-associated rotation is observed where there is obvious cataclasis (Figure 5C), as well as across some trails of gangue mineral inclusions (Figure 6B and Figure 8C). We therefore infer that these gangue trails represent healed fractures (hereafter referred to as such), which were infilled by gangue and superposed by pyrite recrystallization. EBSD analysis also reveals domains that show rotation about {100} (Figure 5C, Figure 6E, Figure 7D and Figure 8D) and rarely {110} (Figure 6C) axes. Lattice distortions associated with these rotations either display subtle degrees of misorientation over hundreds of μm (Figure 6B, Figure 8C and Figure 9B), or sudden shifts of up to 8° misorientation marked on orientation contrast maps by low- and mid-angle subgrain boundaries (Figure 6B and Figure 7C). These match descriptions of the ductile deformation mechanisms dislocation glide and dislocation creep respectively (e.g., [4,5,57]).

In the Distal satellite, pyrite displays no direct microstructures indicative of ductile deformation, though the pole figures do reveal an apparent axis rotation about a single <100> (Figure 5C—D1). The pole figure rotation histories can be traced back to a single orientation (Figure 5C—orange spots) that displays all grains depicted in the Figure 3D inset originally belonged to a single phenocryst. The axis rotation (Figure 5C—D1) is the first preserved deformation in this pyrite. A second deformation (D2) is recognized from a shift of all axes (Figure 5C), corresponding to brittle fracture rotation subsequent to the D1 event that progresses into randomly orientated fine grains during milling.

All pyrites, except the example from Distal satellite, display ductile microstructures irrespective of the breccia matrices in which they are hosted, i.e., (i) hematite-dominant (Figure 3A), (ii) siderite-rich (Figure 3B and Figure 4D), and (iii) veins across quartz–chlorite domains (Figure 3C). In assemblages (i) and (ii) that are characterized by a very fine matrix and/or surrounding ductile phases (Figure 6A, Figure 8A and Figure 9A), marginal ductile deformation in pyrite operated primarily through dislocation glide. Dislocation creep is present in pyrite from siderite-dominated (Figure 6B) and quartz–chlorite (Figure 7C) assemblages (ii and iii above). In assemblage (ii), dislocation creep develops as a continuation of dislocation glide that, in areas of pyrite ‘bottlenecking’, is interrupted, favoring dislocation locking and the formation of low-angle subgrain boundaries (Figure 6B). Assemblage (iii) displays sparse marginal low- to mid-angle subgrain boundaries alongside a pulverized grain containing intact fractures (Figure 7C). This assemblage is mostly composed of coarse rigid quartz (Figure 3C) and is, thus, mechanically distinct from the hematite-dominant or siderite rich breccias.

4.3. Pyrite Trace Element Mapping

LA-ICP-MS element maps were collected on three selected pyrite grains in samples from the ODBC outer shell that had been previously mapped by EBSD.

Element mapping of the pyrite grain in sample MV18B (Figure 3B and Figure 6) reveals the grain core is relatively enriched in Ni, Co, and Te, which is enclosed by a patchy zonation in terms of Ni, Co, and As, in which Co and Ni display an antithetic relationship (Figure 10). The patchy, imperfect zoning patterns may reflect sub-solidus diffusion in the core domain. The patchy domain is enclosed by a Co-As-rich reaction rim, which in turn displays an idiomorphic overgrowth decorated by patchy enrichment in Bi, Pb Au, and Te that is further enclosed by an outermost zone deficient in all trace elements. The medium angle subgrain that displayed rotation about a <110> (Figure 6C) is shown to be at the idiomorphic margin and displays local enrichment in Au, Ag, Te, Bi, and Pb (Figure 10). Other subgrain boundaries (Figure 6B,E) display a comparable enrichment in Au, Ag, Te, Bi, and Pb (Figure 10). Healed microfractures across the grain, including microstructure 1, show a high concentration of mobile elements: Co, As, Te, Au, Pb, Bi, and Ag. Pyrite zonation patterns are offset by microstructure 1 and, to the right, display an apparent irregular depletion in Ni spatial to observed marginal dislocation glide (Figure 6B).

Pyrite from sample MV37 (Figure 3C and Figure 7) displays patchy zoning and overgrowths in which Ni and Co have a correlative relationship that often trends antithetical to As (Figure 11). As expected, Au also displays an association with As-rich domains but shows no correlation with Ag, Te, or Bi that would suggest inclusions containing these elements. Importantly, the low- to medium-angle subgrain boundaries have clearly offset the compositional zonation and exhibit a relative enrichment in Ni and Co (Figure 11). Other trace elements contained in the pyrite show no apparent relationship with the dislocations, while Bi and Pb, however, appear strongly enriched (Figure 11).

Lastly, the pyrite from sample CLC6 (Figure 3A and Figure 8) is rich in trails of gangue mineral inclusions that can be readily seen on the element maps from enhanced ²⁷Al (Figure 12). These trails display a relative enrichment of Au, Bi, Ag, Sb, Zn, Pb, Se, Ni, and As. The same elements also appear at greater concentrations in adjacent healed fractures. In parts of the pyrite grain with fewer or no inclusions, a Ni-rich preserved core is observed, in which Co and As are relatively depleted. This grain core is enclosed by oscillatory-zoned pyrite featuring bands, ~20 µm in thickness, with alternating high and low concentrations of Ni, Co, As, Te, and Se (Figure 12). The entire grain is surrounded by an irregular reaction rim that is relatively enriched in Se and As but depleted in Te, Ni, and Co (Figure 12). Marginal dislocation glide in this grain (Figure 8C) closely correlates with the Se-As rim. We draw attention to the rounded pyrite grain at the top left of the LA-ICP-MS maps. This grain displays contrasting zonation patterns in that the core is relatively enriched in Co, As, and Se, while the rim contains higher Ni.

5. Discussion

5.1. Pyrite Deformation Textures

As expected in a breccia-hosted deposit, the most conspicuous and widely reported deformational texture in pyrite across Olympic Dam is cataclasis [28,32]. The cataclastic textures observed are comparable with that described from many sulfide ore deposits [2,58] and display pyrite grain pulverization and fracture offset with subsequent particle size reduction through milling and rock flour formation. This infers prolonged brittle failure that continually acted to lessen bulk strain across the deposit.

Ductile microstructures pertaining to dislocation glide and creep are also shown here in all assemblages from the NW-SE transect, at depths ranging from ~500 m to as deep as 1850 m. Ehrig et al. [32] reported nanoscale structures (low-angle slip dislocations, twist walls, and deformation-dipole nanostructures) and used these to infer brittle-to-ductile deformation of pyrite from depths of ~1800 m in the orebody. P-T conditions were estimated at 0.5–1 kbar and 300–400 °C, in agreement with models for the mineralizing event at Olympic Dam [30,48]. The present data confirm such an interpretation as being a much wider phenomenon and is documented here for the first time at the micron-scale, in a breccia-hosted IOCG deposit.

Dislocation glide is the fundamental ductile mechanism allowing dislocations to move unhindered along a crystal lattice or glide plane in response to stress(es) (e.g., [59]). This is shown to be the dominant mechanism in the studied pyrite (Figure 6B, Figure 8C and Figure 9B), in which the surrounding material allows uninterrupted glide without grain locking or strain hardening which has been shown in prior studies on pyrite [4,59]. However, when ‘obstacles’ or ‘blockages’ hinder glide planes, generation of low-angle (>2°) boundaries are produced by dislocation climb [60], as we have observed in two of the four cases with ductile deformation (e.g., Figure 6B and Figure 7C). When dislocation glide and climb combine, the phenomenon is known as dislocation creep [57].

The relatively rigid quartz from the quartz–chlorite breccia (Figure 3C) represents a mechanically different matrix compared to the other pyrites, facilitating marginal dislocation creep in response to imposition against the larger quartz (Figure 7C), a mechanism discussed for regionally metamorphosed pyrite [4,5]. This dislocation creep can be associated with the shear-induced deformation of pyrite in Figure 4F, implying ductile deformation during a fracturing event superimposed onto a pre-existing breccia. In the absence of a rigid matrix, ‘bottlenecking’ of a developing dislocation glide system also led to dislocation creep (Figure 6B) in pyrite from siderite-rich breccia (Figure 3B and Figure 4D). EBSD studies of both natural and experimentally deformed pyrite have shown that dislocation creep is common down to the brittle/ductile transition at ~260 °C [3,11,13].

5.2. Pyrite Geochemistry and Syn-Deformational Geochemical Remobilization

Dmitrijeva et al. [61] used multivariate statistics to investigate geochemical trends in a relatively large dataset for pyrite from Olympic Dam. Pyrite is characterized by concentrations of As, Co, and Ni that vary across several orders of magnitude and readily attributable to grain-scale zonation. LA-ICP-MS element maps of pyrite (Figure 10, Figure 11 and Figure 12) confirm the zonation with respect to As, Co, and Ni, while noting that the zoning is often irregular, patchy, or otherwise disrupted and overprinted, echoing prior, largely unpublished element maps (e.g., [62]). Cobalt is the most abundant trace component, ~10⁵–10⁴ ppm, a feature typical of hydrothermal pyrite (e.g., [18,63,64]). Cobalt, Ni, and As display only weak relationships with one another (Figure 10 and Figure 12), and Ni and Co often show an antithetical correlation (Figure 11). In sample MV37, most As-rich domains are depleted in Co and Ni yet contain greater concentrations of Au (Figure 11). These relationships reflect a rapidly evolving ore fluid physiochemistry during formation feasibly tied to fluctuation in temperature and fluid compositions (e.g., [64]). LA-ICP-MS element mapping of pyrite from sample MV18B (Figure 10) and CLC 6 (Figure 12) displays Ni-rich relict cores. CLC 6, in particular, is most distinct, because the Ni core is also highly depleted in As, Co, Se, and Te.

The marked correlation between Ag-Bi-Pb-(Au) in Olympic Dam pyrite has been attributed to inclusions of Ag-(Au)-, Pb-, and Pb-Bi-tellurides [61]. Ehrig et al. [32] provided nanoscale evidence for such inclusions, concluding that the Ag-Bi-Pb-(Au) is likely to have been released from the pyrite lattice and remobilized on the scale of microns to millimeters. Mapped pyrites in the present study feature signatures attributable to telluride inclusions, but these are mainly found along healed microfractures with As also present (Figure 10 and Figure 12). Hydrothermal pyrite that is poor in these metals is often invoked to have undergone episodes of reworking and recrystallization (e.g., [26,65,66]), with released trace elements either forming discrete minerals or being partitioned into other coexisting sulfides such as chalcopyrite [67,68]. Redeposition and fixing of these mobile elements in healed microfractures suggest that the reworking of pyrite was syn-deformational. Healed microfractures are largely invisible in reflected light or in BSE images but are readily seen in both EBSD and element maps. In fact, healed microfractures are ubiquitous in Olympic Dam pyrite (Figure 4D,G, Figure 10 and Figure 12), and probably contribute more to the overall Ag-Bi-Pb-(Au) signature of pyrite than mineral inclusions unrelated to fractures. The widespread presence of elevated radiogenic Pb along microfractures is in response to uranium decay and migration of daughter radionuclides [34].

Ductile microstructures like low- to high-angle slip dislocations and subgrain boundary domains are widely shown to accommodate a wide range of trace elements [19,21,25,26]. The micron- to nanoscale study of Ehrig et al. [32] on kin structures in pyrite from elsewhere at Olympic Dam demonstrated that Bi-Pb sulfotellurides and Ag-Au tellurides are hosted as nanoparticles alongside pore-attached nanoparticles indicative of high fluid percolation. In Figure 10 we show similar low- to medium-angle slip dislocations (Figure 6B) with Ag-Au-Pb-Bi-Te enrichment. This chemistry is analogous to that described in Ehrig et al. [32] and thus supports the same syn-deformational ductile trace element (re)mobilization model for these dislocations. The same microstructures in Figure 11 display concentrations of Pb and Bi, and minor Ni and Co remobilization despite visible Au enrichment in pyrite, suggestive of more limited release and remobilization. The LA-ICP-MS trace element maps therefore indicate a response to syn-deformation remobilization that is highly localized and varies in intensity between samples and even among different grains in the same sample. We also note depletion of Te across the grain (Figure 11) if compared to the example in Figure 10. The lack of Ag and Au (re)mobilized into these dislocations alongside the Te depletion in pyrite, may infer the importance of Te held within pyrite in the formation of nanostructural Au-Ag-tellurides, as proposed previously [32].

Marginal dislocation glide (sample MV18B; Figure 6B) correlates closely with an irregular depletion in Ni that is not reproduced by other elements (Figure 10). To the best of our knowledge, no other study has reported trace element distributions over domains of dislocation glide. Although our interpretations are preliminary and demand corroboration in the future, they suggest that dislocation glide processes have the capacity to disrupt and remobilize trace elements in pyrite, causing an order of magnitude difference in concentrations of Ni in this case. This mechanism most closely resembles dislocation-impurity pair diffusion [69], in which a gliding dislocation attracts and disrupts solid state impurities within its strain field, or ‘Cottrell atmosphere’ [70,71]. Domains depleted in trace elements have been attributed to Cotrell atmospheres in several prior studies [22,72,73], although dislocation glide has never been observed across the same domain.

5.3. Pyrite Deformation History—Early Stages

Textural observations in the Distal satellite are in marked contrast to those from samples selected along the NW-SE transect (e.g., Figure 6, Figure 7, Figure 8 and Figure 9) that clearly record pervasive recrystallization and replacement that has resulted in wholesale obliteration of textural histories. Poor preservation of early textures and fabrics in the central ODBC is a consequence of the iron-metasomatism of initial RDG, involving pervasive replacement of igneous minerals by hematite, sericite, and chlorite [28]. The identity of the precursor granite and superimposed alteration reactions can only be resolved from the evidence preserved in RDG well outside or away from the deposit [46], where alteration is markedly weaker [74].

It is therefore implicit that the microstructural evolution of the deposit may well be better preserved in samples such as those from the Distal satellite, emphasizing the utility of such samples for constraining the early stages of IOCG evolution. Indeed, although the brecciated granite (sample MV65, Figure 3D) appears only as an initial alteration stage, pyrite records two distinct rotations (D1-2; Figure 5C,D) implying the Distal satellite was exposed to at least two deformation events. The two-phase chalcopyrite–pyrrhotite assemblage is indicative of unmixing from a higher temperature intermediate solid solution (iss; [75,76]) that is preserved as rounded inclusions (Figure 4B,C), and thus suggests that the pyrite was exposed to temperatures in excess of 557 °C [76]. This links initial pyrite mineralization to during the magmatic–hydrothermal transition [30,48]. The D1 rotation would allow inference of ductile behavior at this early stage of deposit formation, although no direct microstructures are observed to unequivocally support such a hypothesis and moreover, the ‘original’ orientation, prior to the two rotations (Figure 5C—orange spots) could pertain to a later replacement or recrystallization stage rather than primary pyrite. Despite this, D1 is likely a ductile rotation taking place at high temperature reached during the early magmatic to hydrothermal transition [30,48] whereas D2 is controlled by fracture rotation at an undefined later stage.

5.4. Were All Microstructures Obliterated by Coupled Dissolution Reprecipitation Reactions?

At Olympic Dam, semi-continuous coupled dissolution reprecipitation reactions (CDRR) are invoked from the widespread observation of replacement, remobilization, and recrystallization phenomena in numerous mineral groups. Previous work on pyrite from elsewhere [4,11,77] suggests that recrystallization driven by processes such as CDRR struggles to completely erase dislocations from grains compared to recrystallization/recovery processes. As shown from Olympic Dam [78,79,80], however, pervasive CDRR can be sufficient to account for wholesale grain replacement and recrystallization due to the likely long-lived nature of fluid flow. EBSD mapping of pyrite grains from diverse samples during reconnaissance work for this study emphasized the lack of microstructures that could directly corroborate such an interpretation, however encouraging the macroscale textures appeared. Nonetheless, the representative pyrite assemblages from across the outer shell illustrated here as Figure 6, Figure 7, Figure 8 and Figure 9 represent compelling evidence for pervasive CDRR, even if they are compiled from a select group of relatively rare examples in which annealing, and recrystallization were insufficient to erase all evidence of applied deformation. This is particularly relevant in the context of studies that recognize multiple intense fluid percolation events across diverse mineral groups from Olympic Dam and other IOCG systems in the region [32,74,81].

Despite pervasive CDR-driven replacement reactions, a succession of structural events can be postulated based on the ductile microstructures (as dislocation glide and/or creep at grain margins; Figure 6B, Figure 7C, Figure 8C and Figure 9B) alongside growth textures and crosscutting fractures from pyrites along the NW-SE transect. Based on the discontinuity of dislocation glide across fractures (Figure 6B, Figure 8C and Figure 9B) and the association of dislocation creep relative to fragmented grains (Figure 7E), we can infer that these instances of ductile deformation are the latest preserved deformation in our selected grains and supersede fracturing.

However, early ductile deformation is evidenced by internal domains rotated about <100> or <110> axis relative to the host grain (Figure 6B,C and Figure 8B). Such features cannot be attributed to late recrystallisation because intra-domain rotation is accommodated by the {110} slip system in pyrite (Figure 6C). This slip system is rarely activated in experimentally deformed pyrite due to its much higher critical resolved shear stress compared to the {100} [8,59,82] but can occur by grain impingement in natural pyrite [10]. Such impingement (<110> domain location at the margin of internal zonation and its bent lenticular morphology) is observed for shallow pyrite in the ‘Nodular Fe-oxide granite’ drillcore (Figure 6C). Furthermore, the impinged {110} domain is separated from the later marginal dislocation glide superposed by crosscutting fractures (Figure 6B), providing evidence for multi-stage, brittle-to-ductile transition events recorded in pyrite. Such interpretation is concordant with an environment that experienced periodic moderate- to high-temperature fluid flow during CDR-driven replacement reactions, recognized in minerals including pyrite at Olympic Dam.

Positioning these deformation events in time is, however, impossible at present given the uncertainties surrounding the broader structural evolution at Olympic Dam. Unravelling the evolution and assigning individual microstructures to specific events would require substantial additional study, potentially involving an integrated microstructural-geochronological approach for pyrite and co-existing minerals (e.g., magnetite, hematite, quartz, chalcopyrite, and albite). Despite this, the radioisotope record and recognition of several post ~1.6 Ga tectono-thermal events across the Olympic Cu-Au Province (Figure 1), allow us to offer some speculative scenarios.

A first consideration is South Australia’s enrichment in heat producing elements (HPE) which is shown to generate anomalously elevated geothermal gradients and heat flows across the region [83]. This has produced the South Australian Heat Flow Anomaly in which heat flow is two to three times above average and can drive high temperature–low pressure metamorphism (e.g., [83,84,85]). In the Yorke Peninsula, SE Gawler Craton, Bockmann et al. [85] found evidence for a metamorphic event between 1.56–1.5 Ga, thermally driven by this HPE heat anomaly at P-T conditions of 660–750 °C and 3.6–5.5 kbar. They established a geothermal gradient of 45–50 °C/km, which resembles present day measurements of the surrounding Stuart Shelf [86]. Olympic Dam is the largest single uranium deposit on Earth and consequently displays a major thermal anomaly linked to uranium decay [87]. At Olympic Dam, an average present day heat flow of 125 mW/m² and steep temperature gradient of 83.1 (±5.5) °C/km was recorded [86], which is ~63% higher than the surrounding South Australian Heat Flow Anomaly. If this is considered alongside a (semi-)continual supply of radiogenic heat and the semi-continuous fluid flow, as argued by Owen et al. [88], this may have allowed for long-lived rejuvenation of economic metals across the mineral system [85]. Evidence presented for secondary U enrichment at Olympic Dam [89] and the isotope systematics of carbonates [90] also indicate multiple stages of hydrothermal activity after 1 Ga.

The recognition of ductile deformation at Olympic Dam is novel for IOCG deposits and would require temperatures above ~260 °C [13], more typical of orogenic gold [6,20,21] or metamorphosed massive sulfide deposits [4,5,10]. At Olympic Dam, the primary Woodall fault of the Discovery Fault Series records >2.5–3 km of normal displacement, with later superimposed faults preserving 1 km of sinistral strike slip and an undetermined oblique aspect [49]. These structures are used to infer multiple burial and unroofing events at Olympic Dam [29,91]. The anomalous present day geothermal gradient of ~83 °C/km at Olympic Dam is sustained by radioactive decay [86,87]. Higher heat production likely existed earlier in the evolution of the deposit. If accompanied by large fault displacements and associated tectonothermal events like emplacement of the Gairdner LIP [51], sudden transition between brittle to ductile regimes would be plausible and correlates with our observation of alternating ductile and brittle deformation microstructures in pyrite. The proposed waxing and waning, effectively semi-continuous fluid flow at Olympic Dam [88] would assist this, with even small changes in burial depth generating high-temperature metamorphic fluid systems [85].

6. Conclusions

Brittle microfractures and ductile microstructures such as low- to medium-angle boundaries and subtle misorientation from dislocation glide were identified in pyrite using EBSD. Dislocation glide textures are more frequently observed and are attributable to the abundant enclosure of pyrite within softer phases (commonly chalcopyrite or siderite) or mechanically similar fine brecciated matrices. LA-ICP-MS element mapping across these microstructures displayed: (i) relative enrichment in Ag-Au-Pb-Bi-Te in low- to medium-angle boundaries; (ii) a similar enrichment along healed microfractures; and (iii) depletion of Ni, in this case, by an order of magnitude, correlating spatially with dislocation glide. All the above styles of chemical (re)mobilization are interpreted as syn-deformational in origin. Variation in chemistry between microstructures and their correspondence with trace element zoning visualized on LA-ICP-MS maps indicate that the response to deformation is highly localized and varies in intensity between grains.

This work presents the first evidence for ductile deformation in pyrite from a breccia-hosted IOCG deposit. Two stages of ductile behavior are observed in specimens from the Olympic Dam deposit. However, extensive replacement and recrystallization driven by CDRR have prevented widespread preservation of the earlier event. A combination of radioactive decay heat and adjacent tectonothermal events are invoked to account for an anomalously high geothermal gradient within and surrounding the deposit that reached temperatures of ~260 °C to enable ductile behavior in pyrite. (Re)activation of faults with large displacement in the elevated geothermal gradient supported multiple alternations between brittle to ductile regimes. The coupling of EBSD methods with LA-ICP-MS element mapping therefore provides a comprehensive characterization of pyrite textures and microstructures that are otherwise invisible on conventional reflected light or BSE images. This data is shown to provide new insights into ore genesis and superimposed events.

7. Implications and Outlook

A radioactive heat source has operated at Olympic Dam over the past 1.6 million years and must therefore have been integral for the post-initial evolution of many mineral groups, and especially for Cu-Fe-sulfides, which can remain reactive under even mild hydrothermal conditions (e.g., [92,93,94]). Models of secondary evolution for Olympic Dam should therefore be updated to account for these findings. Although this study has revealed ductile deformation in pyrite and invoked mechanisms to attain the required temperature, how these events fit into the deposits structural evolution remains unclear. Future EBSD work should be directed towards grains of time-resolved pyrite and associated minerals to constrain the character and timing of individual structural brittle/ductile events. As ductile behavior in pyrite is intimately linked to fault (re)activation, findings would directly relate to tectonism across the deposit.

This study also displayed the grain-scale distribution of minor elements and how these distributions relate to textures and microstructures. A detailed understanding of this is essential to efforts aimed at recovery of value components (precious and critical metals), as well as removal of deleterious components (e.g., [35,36,37]). The development of low-energy, low-waste methods of ore processing, coupled with recovery of all contained elements of value, is not only linked to mineralogy and rock features at the hand specimen scale, but also to the hidden features that impact on and control rock fragmentation, separation, and recovery. Considering the long-term value of such information to the resource industry of the future, EBSD methods appear underutilized by researchers despite their demonstrated effectiveness (e.g., [6,19,25,26]).