Direct observation of dislocation nucleation in pyrite using combined electron channelling contrast imaging and electron backscatter diffraction

Crystal‐plastic deformation is one of the main mechanisms that can accommodate large amounts of strain within the lithosphere. Despite the requirement of understanding dislocation nucleation and arrangement, the only accepted method for direct observation of dislocations in geological materials so far is transmission electron microscopy. Herein, we present a study using a combination of electron channelling contrast imaging (ECCI) and electron backscatter diffraction (EBSD) to visualize and analyse crystal defects in pyrite deforming close to the crystal plastic to brittle transition zone. Structures in focus include (a) dislocation nucleation at crack‐tips and (b) the reactivation of mode I cracks accompanied by the nucleation of dislocations and crystal‐plastic behaviour resulting in the development of complex dislocation structures and low‐angle grain boundaries. EBSD maps reveal an increase in misorientation towards micro‐cracks, consistent with a greater dislocation density along cracks observed by ECCI.


| INTRODUCTION
In geological materials, the presence and multiplication of dislocations is indispensable to accommodate larger amounts of strain by continuous creep processes, possibly controlling large-scale tectonic processes.
To fully understand the deformation behaviour of the lithosphere, it is essential to analyse the nucleation, structure and arrangement of dislocations during deformation. New developments in electron backscatter diffraction (EBSD), namely high-angular resolution EBSD (HR-EBSD) based on pattern cross-correlation, allow for quantitative analysis of microstructures and dislocation densities down to a resolution of 0.05° ( Wallis, Parsons, & Hansen, 2017;Wheeler et al., 2009). Nevertheless, the direct observation of crystal defects has been so far restricted to transmission electron microscopy (TEM), limiting observations to the size of a thin foil (Hirsch et al., 1960). In this study, we use electron channelling contrast imaging (ECCI), partly under controlled diffraction conditions, to analyse crystal defects in pyrite. ECCI uses backscatter electrons in high-resolution scanning electron microscopy, allowing direct observation of crystal defects with a contrast very similar to scanning transmission electron microscopy (STEM). The combined ECCI and EBSD approach allows the adjustment of proper diffraction conditions and a crystallographic interpretation of the defect contrast (Miyajima, Abeykoon, & Heidelbach, 2018;Zaefferer & Elhami, 2014).
Pyrite is a ubiquitous sulphide phase in ore deposits, and is of great interest for its incorporation of Au. Due to its crystallography, it is a relatively strong mineral assumed to mainly deform in a brittle manner. Nevertheless, crystal-plastic deformation of pyrite has been observed and has became the focus of various studies over the last years as it has been linked to the remobilization of Au (Boyle, Prior, Banham, & Timms, 1998;Dubosq, Lawley, Rogowitz, Schneider, & Jackson, 2018;Fougerouse, Micklethwaite et al., 2016;Fougerouse, Reddy et al., 2016). Dubosq et al. (2018) observed Au enrichment in the interaction of micro-cracking, dislocation nucleation and the onset of plastic deformation in pyrite samples from the Detour Lake mine (Canada) using a combination of EBSD and ECCI.

| GEOLOGICAL SETTING
The Detour Lake deposit is a Neoarchaean orogenic gold deposit located in the northernmost region of the Abitibi subprovince in Canada along the Sunday Lake Deformation Zone (SLDZ), which has undergone four major regional-scale deformation events (D 1 -D 4 ; Oliver et al., 2011). Herein, we focus on structures related to D 2 . Its penetrative foliation fabric, S 2 , is defined by actinolite and biotite, which make up part of the lower amphibolite metamorphic mineral assemblage, suggesting peak metamorphic conditions of around 550°C and 3.3 kbar during D 2 (Marmont, 1987;Oliver et al., 2011).
Inclusions of prograde metamorphic minerals in pre-to syn-D 2 mineralized veins are consistent with peak metamorphic conditions during D 2 (Dubosq et al., 2018;Oliver et al., 2011).
Although the exact hydrothermal events or processes that caused veining at the Detour Lake deposit remain poorly understood, we assume they are consistent with the orogenic gold deposit forming model proposed by Bleeker (2015). In this model, Bleeker suggests that the near-surface environment where Au deposition is concentrated has a high pressure and temperature gradient and a low confining pressure, which allow for the dilation and opening of vein systems. The inversion of the main fault zones, in this case, the SLDZ, from extensional to thrust creates an ideal deep-reaching fluid conduit for the advection of Au-bearing hydrothermal fluids (Bleeker, 2015).

| ME TH ODS
Microstructures ranging from brittle to crystal-plastic behaviour have been investigated using a combination of EBSD and ECCI. Quantitative analysis of crystal distortion has been performed by EBSD mapping on a FEI Quanta 3D FEG instrument. The scanning electron microscope (SEM) was operated at a 15 kV accelerating voltage, 4 nA probe current, a working distance of 14 mm and a sample tilt angle of 70°. Crystal defects causing this distortion have been directly visualized using a Zeiss Merlin scanning electron microscope with a Gemini-type field emission gun electron column and a Bruker e-Flash HR-EBSD detector. The SEM was operated at 30 kV accelerating voltage, a 2-4 nA probe current and working distances between 6 and 8 mm. A theoretical introduction to ECCI as well as detailed information on sample preparation, EBSD and ECCI analyses can be found in the electronic supplementary material.

| Sample petrography
The sample in focus shows a pinch and swell-like structure of a pyrite layer located at the margin of a sulphidized quartz vein within a       (Figures 3a,b and 4).

| (C) Reactivated intracrystalline micro-cracks
ECC images reveal a variation in crystal-lattice orientation associated with a grey-scale variation (Figure 3e-g).

| (D) Non-reactivated intracrystalline microcracks
Again, the presence of a crack is accompanied by a minor variation in crystal orientation on each side of the crack (Figure 5 and 6). In contrast to reactivated cracks, no localized zones with increased misorientation are visible; instead, we observe a slight increase in the KAM values in the crack vicinity and its tip (Figure 5a and 6a).

| Implications for crystal defect analysis by ECCI in geological materials
Optimizing imaging conditions is essential for obtaining quality ECC images. Three conditions need to be considered: (a) the structure factor of the appropriate reflector must be high to obtain a strong signal; (b) the Bragg angle must be large enough to avoid overlap of the diffraction profiles from the diffraction vectors g and -g; and (c) the diffraction profile must not be too sharp to cope with the convergence angle of the primary beam to avoid the overlap of channelling and backscattering contrast. On pyrite only a few lattice planes, that is, "reflectors" are well suited to obtain good ECC where defects usually show a sharp white, often oscillating contrast (Zaefferer & Elhami, 2014), the defects in pyrite mostly show a distinct black-white contrast and appear blurry. Similar distinct blackwhite contrasts have been observed for other multi-atom crystals (Kamaladasa, Jiang, & Picard, 2011)

| Brittle-crystal-plastic deformation of pyrite
The observed jigsaw-puzzle-like pattern of the fractured pyrite is consistent with hydraulic fracturing (Jébrak, 1997). Previously reported cross-cutting vein types argue for several high-fluid-pressure events (Dubosq et al., 2018). While mineralization is linked to veining, fractures and cracks sealed by fluid-inclusion-rich quartz, calcite and chlorite indicate that the investigated sample has been affected by at least two increased fluid-pressure events. Fluidassisted brecciation by hydraulic or critical fracturing (Jébrak, 1997 (Cox, Etheridge, & Hobbs, 1981;Graf et al., 1981). The quartz textures and occurrence of chlorite suggest reactivation took place at greenschist facies conditions within the wide range of proposed temperatures. The lack of localized dislocation zones or increased dislocation densities next to non-reactivated micro-cracks ( Figure 5 and 6) indicates that the observed misorientation is related to elastic rather than plastic strain (Ohr, 1985;Pollard & Segall, 1987). Similar patterns can be observed along relay structures showing a slight lattice bending without additional dislocation activity ( Figure 4). The presence of the crack seems to allow for minor reorientation and elastic bending of the crystal lattice during crack initiation (Ohr, 1985). However, stress concentration at the micro-crack tip appears to result in the emission of dislocations in the immediate vicinity of the ct (Figures 5e,f and 6g; Rice & Thomson, 1974). These dislocations appear random in line direction and travel between 0.5 and 2 μm deep into the crystal, indicating significant crystal plasticity at the ct. Furthermore, dislocations in proximity to the ct seem to be well arranged, following the traces of the (100) and (010) planes (Figure 5f; ES 3.3.3b) suggesting they lie within these planes. The Burgers vector cannot be determined from this one tilt experiment, yet it is evident that it is not [001] since these dislocations would be invisible with g = (2 -1 0). We argue that continuous deformation and local stress concentration at the ct facilitate the nucleation of dislocations, possibly supporting the onset of crystal plasticity at a later stage. A similar introduction of dislocations into the crystal lattice is likely to occur in other 'strong' minerals (Griffiths et al., 2014) but has so far been mainly reported from material sciences (Anderson, 2005;Ohr, 1985;Ugˇuz & Martin, 1996). The combination of fluid-assisted fracturing and dislocation activity is of special interest for pyrite as dislocations and substructures such as micro-cracks and low-angle grain boundaries might serve as highdiffusivity pathways (Piazolo et al., 2016;Fougerouse et al., 2018) helping to incorporate Au from hydrothermal fluids into the crystal lattice or acting as Au-traps in pyrite (Dubosq et al., 2018). The herein-observed deformation behaviour of pyrite suggests that sulphidized deposits that deformed close to the brittle-crystal-plastic transition for pyrite are likely to be highly enriched in Au.

ACKNOWLEDG EMENTS
We thank the Austrian Science Foundation (FWF) grant number: P 29539-N29 for funding. Gerlinde Habler is thanked for assistance with EBSD analysis. Discussions with Bernhard Grasemann, Michel Bestmann and David Schneider are appreciated. The manuscript benefitted from reviews by D. Fougerouse, C.D. Barrie and an anonymous reviewer. Georges Calas is thanked for careful and quick editorial handling.