Primary and secondary pyrite textures from the Parys Mountain Volcanogenic Massive Sulphide Deposit, Wales: an electron backscatter diffraction investigation
Michael A. Salter Department of Earth and Ocean Sciences, University of Liverpool, Liverpool, L69 3GP, UK. E-mail: [email protected]
Abstract
Colloform textures develop in a sequential manner and have the potential to record changing ore-forming conditions. Current understanding of colloform growth controls is limited.
Recent studies invoke possible links between crystal morphology, degree of supersaturation, and development of crystallographic preferred orientations (CPOs) with common axes that switch across layer interfaces. Another possible factor is trace element sequestration. Further development of these ideas here utilises forescatter orientation contrast (OC) imaging and electron backscatter diffraction (EBSD) to investigate crystal morphology and CPOs in colloforms from the Parys Mountain polymetallic sulphide deposit. Results from two colloforms indicate development of CPO is commonly about <100> and only rarely about
<110> and <111>. Both colloforms show weak CPOs consistent with coarse grain size and stronger CPOs consistent with small grain size, thereby demonstrating a possible link between crystal nucleation rate, degree of supersaturation, and strength of CPO development. In contrast to results of previous studies, common axes frequently lie parallel to nucleation surface and normal to growth direction. This is difficult to reconcile with ore-forming fluid and trace element growth controls and is most easily explained by syntaxial relationship with nucleation surfaces. Abrupt switching of common axis orientation across layer interfaces appears to eliminate this as a possible growth control, but a syntaxial relationship is demonstrated across at least one layer interface.
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Similar techniques are employed in the second part of this work to investigate the nature of deformation of pyrite in low-grade metamorphic terranes. Plastic deformation by dislocation creep is demonstrated at a likely maximum of ~260 °C, representing a new lower limit for such mechanisms. Slip of ~5-20° is demonstrated to be common about <100> directions and occurs more rarely about <110>. Individual grains commonly show multiple rotations about different axes.
Introduction
Understanding the primary and secondary mineralogical textures, and the mechanisms by which minerals deform, is central to comprehending the geological evolution of sulphide ore deposits (Gilligan & Marshall, 1987). Many ore deposits form at plate margins (Allen et al.,
2002) and are commonly subject to metamorphism and/or deformation in subsequent orogenic events during basin closure. Most of the common economic sulphide minerals (e.g. sphalerite, chalcopyrite, and galena) are soft, and undergo intense recrystallisation and grain coarsening in all but the lowest metamorphic grades. Furthermore, they often only preserve deformation textures from retrograde metamorphic conditions (Vokes, 1969). Although not of economic importance, the refractory nature of pyrite, stable under a wide range of conditions and fS2, means it is capable of preserving primary and deformational textures produced prior to peak metamorphism (Craig and Vokes, 1993; Barrie et al., 2007), and is therefore important in understanding the genesis and deformational evolution of ore bodies, thus aiding their successful exploitation.
Pyrite in ore deposits commonly takes the form of equant cubes, framboids, and colloforms.
The latter two textures are regarded as the result of primary precipitation in open spaces in rocks, such as fissures or vugs (Craig, 2001). Colloform textures record conditions during precipitation of primary ore minerals, and their discrete layers provide sequential information
Page | 2 about the evolution of chemical and physical conditions during ore formation (Foley et al.,
2001; Barrie, 2009). They are considered to develop by direct crystallisation due to a high degree of supersaturation (Roedder, 1968) rather than an earlier postulated process invoking the accretion of colloidal gels (Rogers, 1917). Although widely reported as present in ore deposits (e.g. McClay, 1991), only recent work (Freitag et al., 2004; Barrie et al., 2009a; b) has investigated the mechanisms of colloform growth and what they reveal about ore forming environments.
Freitag et al. (2004) use scanning electron microscope (SEM)-based electron backscatter diffraction (EBSD) facilities to analyse discrete colloform pyrite layers from Greens Creek,
Alaska. They show a crystallographic preferred orientation (CPO) initially developed about a
<100>, then a <110>, crystallographic axis associated with a change in grain size. CPO development and switching is considered a primary feature, conjectured to be controlled by changing redox conditions during precipitation, by analogy with models that explain grain size and ordering relationships in framboidal pyrite (Ohfuji et al., 2005). The significance of such a relationship in colloform texture is greater than in framboidal texture, as an evolutionary sequence of conditions is preserved rather than a single snapshot.
Barrie et al. (2009a) find that colloform pyrite textures from Greens Creek, Alaska, and Ezuri,
Japan, exhibit initially random crystal orientation followed by CPOs developed about <100>,
<110>, and <111> crystallographic axes in subsequent layers. Similarity occurs despite δ34S isotope data that suggests quite different sulphur sources at these locations. Barrie et al.
(2009b) find a similar style of switching between orientation axes in colloform sphalerite textures from Galmoy, Ireland. Such CPO changes are interpreted as the effect of changing temperature and degree of supersaturation during two-fluid mixing. Other possible controls include trace element sequestration, in situ bacteria-induced mineralisation, redox conditions,
Page | 3 and sulphur source, although the results from Greens Creek and Ezuri imply that the latter is of low significance (Barrie et al., 2009a).
Early studies of pyrite (e.g. Gill 1969; Graf and Skinner 1970) suggest that it is a very hard mineral that deforms by cataclasis over a large P-T field. Later experimental investigations reveal it is capable of deforming in a ductile manner and indicates a brittle–ductile transition at ~425 °C (Cox et al., 1981; Graf et al., 1981; McClay and Ellis, 1983). Cox et al. (1981) suggest that {100}<001> and possibly {100}<011> are major slip systems. {110} dislocation glide may also be important, but its critical resolved shear stress (CRSS) is several times higher than {100} glide, so it is probably less important. Barrie et al. (2007) demonstrate dislocation creep occurring principally as a lattice rotation about a single <100> axis, and less commonly about a single <110> axis. Further experimental studies (Barrie et al., 2008) involving shortening parallel to <100> and <110> confirm the dominant slip system is
{001}<100>, with slip in this plane activated when <100> is oriented at >~5-15° to shortening direction. Slip on {110} planes is suggested to occur only when <100> is oriented parallel to shortening direction.
Deformation mechanism maps for pyrite have been produced on the basis of experimental results (McClay and Ellis, 1983). These are of great value in determining the deformational behaviour of different phases in ore bodies that have experienced a known set of stress, strain, and temperature conditions (Siemes et al., 1991), and it is therefore important that they present accurate information. However, experimental deformation is limited by high strain- rates relative to those in nature, and thus yields temperatures that likely represent an upper limit for transition to the dislocation field of deformation. Recent work on naturally deformed pyrite (Freitag et al., 2004) indicates that dislocation glide and creep can take place at lower
Page | 4 greenschist facies conditions, much lower temperature than indicated in experimental studies.
A revision of the pyrite deformation mechanism map is therefore necessary.
The aims of the present study, based on samples from the Parys Mountain polymetallic sulphide deposit, Anglesey, North Wales, are two-fold. The first part examines discrete layering in colloform pyrite textures, in order to offer comparison of colloform growth styles and mechanisms with those of Greens Creek, Ezuri (Freitag et al., 2004; Barrie et al., 2009 a), and Galmoy (Barrie et al., 2009b). The second part extends the investigation of naturally occurring plastic deformation of individual pyrite grains into lower anchizone metamorphic conditions (Merriman, 2006), as such representing a new low temperature constraint for initiation of plastic deformation. Both parts utilise EBSD facilities in combination with forescatter orientation contrast (OC) imaging. The results are coupled with energy dispersive
X-ray (EDX) chemical mapping to explore the possibility of a link between microstructures and element distribution.
Geological Setting
Parys Mountain forms a low profile hill in the northeast of Anglesey, northwest Wales.
Anglesey comprises a thin sequence of Lower Palaeozoic Welsh Basin strata that discontinuously and unconformably overlie a Precambrian basement of schists and gneisses
(Barrett et al., 1999). Locally, sedimentation in the Ordovician and Silurian is considered to have been tectonically controlled (Bates, 1966), with the Carmel Head Thrust (fig 1) probably playing an important role. At Parys Mountain this resulted in the deposition of a sequence of shales that are interrupted by a series of rhyolites erupted dominantly as pyroclastic flows
(Pointon and Ixer, 1980). Basalt and an unusual quartz-rich ‘white rock’ are also present in minor amounts (Barrett et al., 1999).
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Figure 1. Location and geological map of Parys Mountain and the north-eastern part of Anglesey showing the outcrop relationship of rhyolites, Ordovician shales, and Silurian shales. Modified after Barrett et al. (1999).
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Figure 2. N-S oriented geological cross-section of Parys Mountain, showing synclinal geometry and typical locations of massive and semi-massive polymetallic sulphide lenses. The rhyolite is split into sub-groups A-D based on lithogeochemical relations. See figure 1 for key and cross-section transect. Modified after Barrett et al. (1999).
Volcanic deposits are of Lower Silurian age (Parrish, 1999), and consist of five distinct types of rhyolite based on lithogeochemical relationships (A, B, C, D1, and D2). Several unpublished studies attempt to reconcile the sequence of volcanic events and their relationship to mineralisation (e.g. Barrett et al., 2001). Volcanic deposits are interpreted as being tapped by basement faults and emplaced in a shallow marine environment during intra-plate rifting
(Pointon and Ixer, 1980; Barrett et al., 2001).
Figure 1 is a geological map that shows the outcrop pattern of these lithologies. Silurian shales are bound to the north and south by the east-west trending northern and southern limbs of rhyolite, which are bound to the north and south by Ordovician shales. Age of the shales is determined on palaeontological evidence (Bates, 1966). Most authors (e.g. Pointon and Ixer,
1980; Barrett et al., 2001; Barrett, 2009) agree that the outcrop pattern is the manifestation of large scale folding that forms an asymmetric syncline, its northern limb overturned to the south (fig 2). The southern and northern units are therefore interpreted as stratigraphically
Page | 7 equivalent, and Silurian shales (stratigraphically higher than the rhyolites) occupy the fold hinge zone.
Deformation is associated with the Caledonian Orogeny, and previous studies of the ore deposit suggest that “soft” sulphides, such as galena, chalcopyrite, and sphalerite were remobilised at this time (Pointon and Ixer, 1980). Analysis of clay minerals in local shales indicates that deformation was associated with metamorphism that attained a pelitic zone of lower anchizone (Merriman, 2006). There appears to be an absence of fluid inclusion data from Parys Mountain to indicate peak metamorphic temperature (and temperature of the ore- forming fluid), thus local lower anchizone shales provide the best constraint. Several attempts at correlating Kübler Index (used to determine metamorphic pelitic zone) with temperature
(e.g. Kisch, 1987; Potel et al., 2006) yield similar results, suggesting that lower anchizone in shales represents a temperature range of ~200-260 °C (fig 3).
Figure 3. Correlation of the Kübler Index for anchizone metamorphic grade with temperature, indicating an upper metamorphic temperature limit at Parys Mountain of ~260 °C. The grey bar in A encompasses the range of results from different studies. Modified after A, Potel et al. (2006); and B, Kisch (1987).
Polymetallic stratiform sulphide lenses hosted mainly in rhyolites, and cp-py-qtz vein systems hosted in Ordovician shales and rhyolites, are the dominant styles of mineralisation (Barrett et al., 1999; Barrett, 2009), with the main occurrences near the contact between Ordovician shales and rhyolite B. Early workers (e.g. Greenly, 1919) proposed an epigenetic model for mineralisation. Conversely, most recent authors (e.g. Pointon and Ixer, 1980; Barrett et al.,
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2001) favour a syngenetic origin of Upper Ordovician and Lower Silurian age, emplaced as a
Kuroko-type volcanogenic massive sulphide (VMS) deposit associated with eruption of the rhyolites. Most sulphide bodies are inferred to have accumulated immediately before, and during, the earliest eruptions, with cp-py-qtz vein systems interpreted as stockwork to the main polymetallic lenses. Presence of ore-bearing vein systems in Silurian shales suggests hydrothermal activity persisted long after volcanism ceased (Barrett et al., 1999).
Pointon and Ixer (1980) note that mineralogy and paragenesis of the sulphides is consistent throughout the deposit, suggesting a uniform ore-forming fluid. In contrast, Barrett (2009) notes that locally occurring intervals of massive sulphide have relatively low copper content, reflecting lower ore-forming fluid temperatures in these zones.
Methods and Materials
For the present investigation, twenty samples from various depths in seventeen boreholes have been studied. Samples were prepared as 25 mm diameter, 10 mm thick resin mounted polished blocks. Initial observations using standard reflected light techniques facilitated the identification of sulphide phases and microtextures, and allowed areas requiring further investigation using Scanning Electron Microscope (SEM) techniques to be established. SEM data was collected using the Philips XL30 microscope at the University of Liverpool, and involved backscatter-electron (BSE) and forescatter orientation contrast (OC) imaging.
BSE images were collected with the polished surface positioned normal to the electron beam, which was fired from a tungsten filament, and operated with a spot size of 5.0 or 5.5, accelerating voltage of 20.0 kV, and working distance of 13mm. Prior to imaging samples were cleaned using acetone, and a 15 nm carbon coat was applied in order to minimise charging effects. Chemical analyses were carried out using an Oxford Instruments INCA energy dispersive X ray (EDX) analyser and software system. Calibration was against natural Page | 9 and synthetic standards while run-time calibration was achieved using a cobalt standard mounted in the specimen holder.
Forescatter OC images were obtained using detectors mounted below the specimen. The specimen was tilted at 20° to the incident electron beam, which operated with a spot size of
6.0-6.5, accelerating voltage of 20.0 kV, and working distance of 22.9-26.4 mm. Images were tilt-corrected and adjusted for dynamic focus using the Philips XL30 operating software.
Chemical-mechanical polishing was performed to remove any surface damage caused by mechanical polishing. This was achieved using a suspension of 0.05 µm colloidal silica
(SYTON™) on a polyurethane lap for ~2-4 hours. Specimens were then given a very thin carbon coat to minimise charging effects while maintaining a strong crystallographic signal.
Forescatter OC images are produced in grey scale, such that grey scale variations represent misorientations of a crystal lattice in an individual grain. However, while these images can be used to qualify misorientations, electron backscatter diffraction (EBSD) analysis is required to quantify them. EBSD data were collected using the same system configuration as with forescatter OC imaging, but with the electron beam fixed on the point of interest. Electron backscatter patterns (EBSPs) were collected on a phosphor screen and indexed using the
Flamenco utility of the CHANNEL 5 software (HKL Technologies Ltd., Denmark). Flamenco also enabled the indexing step size to be set at 1 µm for plastically deformed pyrite, 2 µm for colloform 2 and plastically deformed sphalerite, and 4 µm for colloform 1. Further details of the operational settings are presented in Prior et al. (1996; 1999). EDX chemical mapping was completed in unison with EBSD runs in order to distinguish between different cubic minerals.
Post-processing of the data was carried out in the Tango utility of the CHANNEL 5 software package. In five EBSD runs, 11-50% of points were successfully indexed. Tango was used to systematically interpolate existing data in order to minimise the number of zero solution
Page | 10 points. Pyrite has a pseudosymmetry that causes a small proportion (<5%) of points to be indexed incorrectly, rotated 90° about <100> from the actual orientation. The Tango program enables systematic correction of at least 95% of misindexed points (Barrie et al. 2007).
Processed data sets were then used to detect grains, each defined as having >5° misorientation with respect to its neighbours. Pole figures were plotted using the Mambo utility of the
CHANNEL 5 software package. For colloform textures, where individual grains showed no evidence of plastic deformation, pole figures were plotted using one point per grain, thus removing the effect of the high density of points in large grains, and generating results representative of all grains in each layer.
Results
Colloform textures
Two colloform pyrite textures are investigated. They occur within ~4 mm of each other in a single sample taken from a massive sulphide lens at a depth of 6.20 m in borehole PM26b.
Colloform 1 has a core-to-rim distance of ~1.8 mm, and colloform 2 has a core-to-rim distance of at least 0.4-0.8 mm. The rim of colloform 2 is not well-defined, and the core-to- rim distance may be greater than 0.8 mm. The three-dimensional morphology of the colloforms (i.e. tubular, spherical, etc.), and their in situ position relative to large-scale deformational structures, is not known.
Colloform mineralogy was identified under reflected light observation and confirmed using the EDX system in the SEM. Both colloforms comprise discrete concentric layers of pyrite and occasional seams and inclusions of chalcopyrite, galena, and a phase that falls approximately halfway between end-members of the tennantite-tetrahedrite series (hereafter referred to as fahlore). Discrete layers are recognised by textural variation, with sub-layers determined by variations in trace element sequestration and/or abundance of inclusions.
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Trace Element Results
EDX spot analyses were performed on both colloforms, yielding similar results. With the exception of two analyses in colloform 2 that reveal an absence of trace elements, all pyrite sampled contains trace amounts of As between 2.26 and 5.65 wt%. Variability in As abundance does not appear to be related to proximity to chalcopyrite and fahlore. Laterally persistent pale seams parallel to the traces of discrete textural layers are revealed in pyrite in
BSE images. Relatively pale shades in BSE images reflect the presence of relatively heavy elements, thus pale seams indicate that the pyrite comprises seams rich in a trace element heavier than sulphur or iron. EDX linescans reveal small As peaks that appear to correlate with pale bands (fig 4).
Figure 4. EDX linescan results across laterally persistent layer-parallel seams (in colloform 1, layer 6) that appear pale in BSE images. Linescan position is indicated by the yellow line on the BSE image, with measurement from top left to bottom right. Results for arsenic reveal distinct peaks at ~50 and ~65 μm along the transect line that correspond with pale bands. Note that both peaks correspond with distinct lows in iron content, as shown by the graph overlain on the BSE image.
Fahlore is of similar composition in both colloforms. Sb ranges from 4.63 to 8.14 at.%; As from 6.24 to 10.22 at.%; and Ag from 0 to 0.64 at.%. A zone of strongly acicular pyrite adjacent to the thickest layer of fahlore in colloform 2 is chemically unique within the colloforms (Fig 5). Immediately adjacent to the rim-ward edge of the fahlore is a micron-scale seam of pyrite that contains 0.86 wt.% Cu. Rim-ward of this, most of the layer contains
Page | 12 similar amounts of Cu as well as 4.92 wt.% As and 1.25 wt.% Sb. Only at the edge closest to the rim does the composition of the layer change again, with a seam of 4.82 wt.% Cu, 4.68 wt.% As, and 2.77 wt.% Sb. Lateral variation within the layer occurs only as thin seams of fahlore and chalcopyrite oriented normal to the trace of layering.
Figure 5. BSE image showing fahlore and a chemically distinct layer of acicular pyrite in colloform 2. Acicular pyrite is rich in trace elements, with highest concentration at the rim-ward limit of the layer. py = pyrite, fa = fahlore, cp = chalcopyrite.
Colloform 1
Petrographic results
Colloform 1 is divided into seven zones (core plus six pyrite layers) based on textural variation (fig 6). The inner core comprises a single equant pyrite crystal surrounded by a 200
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μm wide zone of chalcopyrite. As a relatively soft sulphide, chalcopyrite polishes less well than pyrite, and it is difficult to identify individual grains in any of the images presented.
Outside the core, layer 1 is a pyrite layer divided into three lateral zones and five sub-layers.
Sub-layers are defined by laterally persistent (i.e. zone-crossing) grey-scale variations in BSE images, suggesting possible changes in trace element sequestration. Sub-layer 1c comprises crystals that show distinct chemical zonation. Zoning is not completely concentric, but terminates on the rim-ward surface of 1b (fig 6). Lateral variation is defined by textural variation revealed in forescatter OC images. Sub-zone A comprises a small area of acicular crystals 50-100 μm long and a few microns wide. Adjacent to this, sub-zone B is characterised by equant crystals of ≤50 μm diameter, while sub-zone C comprises equant crystals that are ≥50 μm. The boundary between B and C is coincident with a brittle fracture that persists (normal to trace of layering) throughout layer 1. An offset in the thickest fahlore seam in layer 2 appears to represent a continuation of this fracture. However, the trace of a brittle fracture parallel to that in layer 1 extends through layers 5 and 6, but is not aligned with the fahlore offset, suggesting that a single brittle fracture has affected layers 1, 5, and 6, but the fahlore offset is unrelated. Furthermore, the fracture does not result in offset (in the plane of the image) of layers 1, 5, and 6.
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Figure 6. Colloform 1 layers and sub-layers based on textural and trace element variation. Layers are indicated by solid lines and sub-layers are indicated by coarsely dashed lines. Finely dashed lines indicate limits of lateral variation in layer 1. EBSD results for each textural domain are plotted as lower hemisphere <100> or <110> pole figures. Common CPOs are about <100> axes (indicated by circles/ellipses) and show variability across layer interfaces. Results from the chalcopyrite core (a replacement texture, thus results may be invalid), and layers 2 and 4, show random or “uniform” CPO. Although layer 6 shows an apparent weak <100> CPO, closer inspection reveals 5 moderate to strong <100> CPOs (fig. 7). In the reflected light image, py = pyrite, fa = fahlore, cp = chalcopyrite, and ga = galena.
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Figure 7. BSE image of colloform 1, showing crystallographic zoning of pyrite in sub-layer 1c. Note terminations (circled) at the interface between 1b and 1c, indicating that the surface of 1b acted as a crystal nucleation site, and that growth of crystals was towards the colloform rim.
Layer 2 differs from layer 1 by comprising micron-scale crystals. At least three sub-layers of
~20 μm thickness are defined by seams of fahlore, galena, and chalcopyrite, but are too narrow to analyse individually at the scale used here. Immediately overlying the thickest fahlore seam of layer 2, layer 3 is a thin layer of pyrite that is laterally restricted, defined on the basis of abundant, slightly elongate silicate inclusions. Layer 4 also lies immediately above layer 2, and is also laterally restricted. It forms a dome-like shape and consists of equant crystals of ~25 μm diameter that appear to coarsen rim-ward.
Layer 5 overlies layers 2, 3, and 4, and comprises crystals that coarsen rapidly away from the core to become dominantly elongate, 100-200 μm in length and up to 100 μm in diameter.
Sub-layers are defined by laterally persistent grey-scale variations revealed in BSE images (as in layer 1). Sub-layer 5a is also characterised by abundant micron-scale inclusions (likely silicates) and occasional ~10 μm-scale chalcopyrite inclusions. Forescatter OC imaging reveals the upper surface of sub-layer 5a to be an apparent nucleation surface for several new crystals in sub-layers 5b and 5c.
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Layer 6 shows an abrupt change from the coarse crystals of layer 5 to fine acicular crystals up to 50 μm long and a few microns wide, many of which thicken towards the colloform rim. As in layers 1 and 5, sub-layers are defined by grey-scale variations in BSE images. Another similarity with layer 5 is the presence of abundant micron-scale inclusions in sub-layer 6a
(probably silicates) and occasional ~10 μm-scale chalcopyrite inclusions. Both inclusion types are near-absent in overlying sub-layers. Crystals in layers 5 and 6 exhibit a radial relationship with respect to the core.
Crystallographic results
EBSD results for each textural domain are presented as lower hemisphere pole figures (fig 6).
CPOs in most layers are laterally consistent, although layers 1 and 6 display some variation.
Where present, common CPOs are arranged about <100> directions that commonly change orientation across layer interfaces. However, the CPOs of some coarse crystals in the upper part of layer 5 are similar to those of fine acicular crystals immediately rim-ward of them in layer 6 (fig 6b), and the lateral variation of CPO in layer 6 appears to be a result of this. There are no significant changes in CPO across sub-layers. Misorientation profiles across individual grains show that they are not internally deformed.
EBSD software was programmed only to index pyrite in this run. The tetragonal system of chalcopyrite is very close to being cubic, and CHANNEL 5 commonly indexed chalcopyrite- rich areas (confirmed by EDX results) as pyrite. Despite being incorrectly indexed, it is likely that crystallographic orientation data for chalcopyrite is accurate owing to the similarity of pyrite and chalcopyrite crystallographic systems. The chalcopyrite core shows a “uniform” alignment, as do layers 2 and 4. The significance of this in layer 2 is ambiguous as it may be an artefact of resolution problems.
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Figure 6 EBSD results for colloform 1, layer 6. Layers are presented as an image of EBSD indexing results, where colour variation represents change in orientation. Crystallographic orientation results are plotted as lower hemisphere <110> pole figures. They show CPOs about several different <100> axes that are divided into lateral domains of fine crystals in layer 6. Domains exhibit a shared CPO with the coarse crystals of layer 5 that they are in contact with. Note the ~20° deviation of CPOs that is commonly in a sub-vertical plane.
Layer 1 shows two distinct moderate CPOs about <100> axes. CPO variation appears to correlate with lateral variation in crystallographic texture: one CPO corresponds to sub-zone
B; the other to sub-zones A and C. Layers 3 and 5, where coarse crystals dominate, share a
CPO about a <100> axis that is different to those in layer 1. Layer 6 comprises several lateral domains with CPOs about different <100> axes, all differing from the layer 5 CPO. Four domains have CPOs sub-parallel to trace of layering, the common axis of each exhibiting approximately 20° of variation in a sub-vertical plane, but little variation in the horizontal plane (fig 8). Although CPOs of the domains are different to the dominant CPO in layer 5,
Page | 18 each domain is located immediately at the rim-ward end of a crystal (or group of crystals) in layer 5 that displays the same CPO.
Where CPOs are present they are generally weak, except in the fine acicular crystals of layer
6, where they are quite strong. <110> and <111> always plot as high-angle arcs about the common <100> directions. The common axes are never vertical, and in layers 1 and 6 they are sub-parallel to the trace of layering.
Colloform 2
Petrographic results
Colloform 2 comprises six discrete layers of pyrite (fig 9). Chalcopyrite occurs in greater amounts than in colloform 1, appearing to commonly replace pyrite. A large area of chalcopyrite shows a discordant relationship with colloform layering, and thus identification of discrete pyrite layers is not straightforward. It is necessary to extrapolate the trace of some layering in order to correlate individual layers across zones of chalcopyrite. The suite of textures in colloform 2 is generally quite different to that of colloform 1.
Layer 1 is circular in the plane of the polished section and forms an apparent core of the colloform. Individual crystals appear to be elongate, up to 40 μm long, radiating from a central point. The outer limit of this core is made apparent by a discontinuous seam of fahlore.
Partially encompassing the core, layer 2 is up to 70 μm thick and comprises elongate pyrite crystals up to 70 μm long radiating away from centre.
Layer 3 is complex, and is dissected into four sub-zones. Left of the core (fig 9), it is up to
250 μm wide and consists of equant crystals ≥50 μm in diameter. Grain boundaries are occasionally rimmed by thin seams of fahlore. The layer is further divided into sub-layers 3a and 3b by apparent chemical variation shown in BSE images. Below the core, layer 3 narrows
Page | 19 to ~120 μm in width and comprises two distinct packages. Immediately rim-ward of layer 2 is a laterally restricted ~70 μm thick band of fahlore. Localised on the rim-ward surface of this is a 50 μm thick layer of strongly acicular pyrite (sub-layer 3c), with crystals ≤50 μm long and a few microns wide. This layer bears trace elements that make it chemically distinct from other pyrite layers, as documented above. The trace element-rich band at the top of this layer
(fig 5) persists laterally a little way along the top of 3b.
Separating much of layers 4 and 5 from layer 3 is a seam of chalcopyrite and fahlore that is mostly concordant with layering, except on the left-hand side of the image, where it protrudes into and through the outer layers, appearing to dissect layers 4 and 5. The layers appear texturally identical on either side of the chalcopyrite.
Immediately rim-ward of layer 3, layer 4 comprises very fine (~10 μm) weakly acicular crystals that coarsen slightly towards the rim. Layer 5 is defined by an abrupt change to equant, and occasionally weakly acicular, crystals of ~100-200 μm diameter. Rim-ward of layer 5 much of the colloform structure is lost due to replacement by chalcopyrite. Layer 6 is only partially preserved, and grains appear equant and coarse (similar to those in layer 5) in
OC images. Laterally persistent grey-scale variations in BSE images suggest crystallographic chemical zoning in this layer.
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Figure 7. Colloform 2 layers and sub-layers based on textural and trace element variation. Layers are indicated by solid lines and sub-layers are indicated by coarsely dashed lines. Finely dashed lines indicate inferred continuation of layer boundaries. EBSD results for each textural domain are plotted as lower hemisphere <100> or <110> pole figures. Common CPOs are about <100> axes (indicated by solid black circles/ellipses), except in layers 3a, 5a, and 6. 3a has a common CPO about <110> (indicated by a solid red ellipse), while 5a and 6 have common CPOs about <111> axes (indicated by solid red circles). A <110> pole figure for the pale grey area in layer 3a (fahlore) suggests constituent crystals have a common CPO about a <100> axis. py = pyrite, fa = fahlore, cp = chalcopyrite, and sp = sphalerite.
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Crystallographic results
As with colloform 1, CPOs within each layer of colloform 2 are commonly arranged about a
<100> axis that switches its orientation across layer interfaces. However, there is also one example of a CPO about <110>, and two about <111>. None of the layers shows a “uniform”
CPO. Many of the common axes are oriented normal, or sub-normal, to the plane of the image, sub-parallel to traces of layering (fig 9), and the high angle of arcs about these axes suggests that pyrite orientations develop rotated about a common axis, e.g. a crystal in a layer might share a common <100> axis with other crystals of the same layer, but it can grow at any rotation about this axis such that <110> and <111> are different to the other crystals.
Therefore, crystal growth necessarily rotates on a cube edge where the common axis is sub- parallel to traces of layering.
Layers 1 and 2 show strong CPOs about different <100> axis. The change to equant grains in layer 3a is accompanied by a change to a weak <110> CPO, but this reverts to strong <100>
CPOs in layers 3b and c. The fahlore core-ward from 3c is indexed in CHANNEL 5 as pyrite, but its cubic crystallographic system means the CPO data it yields bears significance.
However, like chalcopyrite, fahlore polishes poorly, meaning that a high percentage of indexing points returned a zero solution. Results are based on 11 grains that suggest a <100>
CPO. Arcs about this axis are small to absent, suggesting little variation in <110> and <111> axis orientations. This may be a real feature of the fahlore, but could also be an artefact of small sample size.
In the bottom part of the images (fig 9), layers 4 and 5 share a moderate <100> CPO. On the opposite side of the discordant chalcopyrite, layer 4 has a CPO about a different <100> axis, while layer 5 has a moderate <111> CPO. Layer 6 shows a similar, but weaker, <111> CPO.
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Plastically deformed grains
Pyrite deformation
A sample taken from a massive sulphide with shale inclusions at a depth of 267.7 m in borehole PM109 shows abundant evidence for strain, such as the presence of sphalerite and galena “tails” adjacent to pyrite grains. Pyrite-pyrite grain boundaries are rare in the section plane of the sample. Forescatter OC images display grey-scale variations within individual
100 micron-scale pyrite grains (fig. 10C), indicating that some plastic deformation has taken place. However, these are rather indistinct and do not display obvious banding or sharp contacts. EBSD results, presented as lower hemisphere pole figures in figures 10 and 11, indicate lattice rotations ranging from ~5 to 20° about <100> and less commonly at small angles about <110> and possibly <111> axes.
Much of grain 3 is shows little evidence of deformation, but its edges show a single invariant
<100> axis with remaining <100> axes forming a great circle about it. <110> and <111> axes lie on small circles to the invariant <100> direction. Deformation is therefore best described as a lattice rotation of ~7° about a single <100> axis. The remaining pyrite grains show more complex deformation histories, requiring description as multiple rotations about different crystallographic axes. The largest grain (grain 2) shows rotation up to ~20° about a single
<100> axis, and a second component of rotation up to 14° about a different <100> axis. Grain
1 exhibits lattice rotation of ~5° about a single <100> axis. Another rotational component of
~5° is about an axis that may be <110> or <111>. Grain 4 shows a ~7° rotation about two separate <100> axes, and additionally a ~5° rotation about <110>.
Page | 23
Figure 8. EDX and EBSD results for plastically deformed pyrite grains. A) EDX results: Green areas represent iron-rich grains (pyrite); pink areas represent zinc-rich grains (sphalerite). Sphalerite occurs as “tails” in pressure shadows associated with pyrite; B) EBSD indexing results (interpolated) of the same grains; C) Forescatter OC image: grey-scale variations demonstrate that some plastic deformation of pyrite has taken place. EBSD results for grain 1 are plotted as lower hemisphere <100>, <110>, and <111> pole figures. Whole grain results indicate slip has occurred on different systems. Component rotations are isolated and shown below the whole grain pole figure. Rotation axes are circled. They suggest lattice rotations of ~5° about a single <100> axis and either a <110> or <111> axis.
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Figure 9. EBSD results for pyrite grains 2, 3, and 4 (shown in fig. 10) plotted as lower hemisphere <100>, <110>, and <111> pole figures. Whole grain results indicate slip about a single <100> direction in grain 3. Grains 2 and 4 show multiple rotation components. Grain 2 shows ~14-20° of rotation about two different <100> directions. Grain 4 shows ~7° of rotation about two different <100> directions, and ~5° about a single <110> direction.
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Sphalerite deformation
A different sample from a massive sulphide lens at 182.5 m depth in borehole CZ2 is rich in sphalerite, chalcopyrite, galena, and silicate phases. Abundant evidence of shearing includes a moderately developed foliation in silicate phases; foliation-parallel elongation of sphalerite, chalcopyrite, and galena; and presence of galena in sphalerite pressure shadows. However, pyrite cubes and framboids are locally abundant, and occasional small colloform pyrite textures are preserved, indicating that pyrite in this sample has not deformed in a ductile manner.
Indeed, EBSD results for the area shown in figure 12 display no evidence of plastically deformed pyrite. However, sphalerite from the same EBSD run yields results that, when plotted as lower hemisphere pole figures, suggest preservation of significant plastic deformation. Lattice rotation of ~20° occurs about a <110> direction, and may also occur at
~10° about a <111> direction.
Figure 10. Reflected light and BSE images of pyrite, sphalerite, galena, and silicate phases (dark areas) from borehole CZ2.EBSD results show that pyrite is not plastically deformed. Lower hemisphere pole figures display EBSD results for the large sphalerite crystal at the top left of the images (highlighted by the dashed line). They indicate lattice rotation of ~20° about a <110> direction (upper pole figures), and ~10° about what appears to be a <111> direction.
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Discussion
Colloform samples
As shown in previous colloform studies, the colloform samples show textural variation and significant CPO changes across layers. However, CPOs of the Parys Mountain colloforms, particularly colloform 2, are commonly arranged about axes that lie sub-parallel to the trace of layering (and normal to the plane of the images), considerably different to those found in previous studies, where CPOs are arranged about axes sub-parallel to core-to-rim (growth) direction and normal to trace of layering (Freitag et al., 2004; Barrie et al., 2009a; Barrie et al., 2009b). Before discussing the implications of this on colloform development processes, it is important to assess the validity of the data and the order in which layers were precipitated.
Data handling and section-cut orientation
The crystallographic results of this study suggest CPOs in colloform layers are arranged about axes rotated ~90° with respect to those of previous studies. It is possible that such a difference can be an artefact of the way in which CHANNEL 5 software handles x, y, and z coordinate systems. This occurs when the sample coordinate system is rotated relative to the coordinate system employed for pole figure presentation. Data presented here are based on identical coordinate systems in the sample and pole figures, such that the images are in the same plane as pole figures, thus removing the possibility of misrepresentative pole figures.
Barrie et al. (2009b) suggest that pole figures similar to those produced for colloform 2 might result when the orientation of section-cut is normal to crystal growth direction (fig 13a). Near- concentric layering around an apparent core suggests that the polished section is effectively cut through the centre of both colloforms. With this in mind, if crystals that make up the colloforms grew radially from a central point in a core-to-rim direction, the section-cut must be parallel to crystal growth direction regardless of its orientation (fig 13). The only way to
Page | 27 conceive of a situation where the polished section is cut through the colloform core, and normal to crystal growth direction, is if all crystals have grown in a single direction and the section-cut is normal to this direction (fig 13). This would imply that the images shown here represent a plan view relative to crystal growth direction. If crystal growth was normal to this plane, any chemical zoning in crystals should be observed as complete, approximately concentric, zones. However, zoning within crystals of colloform 1 (sub-layer 1c) is consistently terminated on the rim-ward surface of sub-layer (1b) (fig 7), indicating nucleation on an earlier surface, and radial growth outward from the centre. Furthermore, acicular crystals are arranged in a radial pattern about the core, suggesting growth was either toward or away from it, and not normal to images planes. Given that colloforms 1 and 2 share similar morphology and radial pattern of crystals, it is likely that colloform 2 developed in a similar manner to colloform 1. Thus it is likely that the section is cut parallel or sub-parallel to growth direction in both colloforms.
Figure 11. a) Possible relationships between cut planes and crystal growth directions (1), and how such relationships might be represented on pole figures (2). Modified after Barrie et al. (2009b). b) Two possible models for colloform growth style. Arrows indicate crystal growth direction. Dashed lines represent possible cut planes. In the upper image, where crystal growth is radial, all cut planes that pass through the colloform core are parallel to growth direction, thus are in accord with scenario A (a). The lower image shows unidirectional crystal growth, and cut planes that pass through the colloform core may therefore fall into scenario A, B, or C (a) depending on their orientation. Crystallographic textures suggest radial growth, thus growth-parallel polished surface cut.
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Stratigraphic order of layers
In attempting to resolve growth mechanisms, the correct sequence stratigraphy in a colloform should be established. A widely held view is that outer layers are youngest (e.g. Falconer et al., 2006). However, Roedder (1968) demonstrates how growth may occur from edge-to-core, and Barrie et al. (2009 b) show that later replacement due to fluid influx may contribute to a more complex sequence.
The colloforms studied here show systematic layering about a central core. Apart from the chalcopyrite in colloform 2, which is interpreted as a replacement texture, relationships between layers are generally conformable, suggesting that growth was either in a core-to-rim or rim-to-core direction.
In colloform 1, termination of pyrite crystallographic chemical zoning in layer 1c suggests crystal nucleation occurred on the upper surface of 1b. Thus 1b necessarily precipitated earlier than 1c, implying core-to-rim growth direction. In support of this is rim-ward grain coarsening in layers 5 and 6. Crystals developed in vugs or veins characteristically coarsen away from the initial nucleation surface (Passchier and Trouw, 2005). Therefore the
Coarsening in layers 5 and 6 therefore indicate growth towards the rim. Crystal coarsening in layer 4 of colloform 2 implies that it also probably grew in a core-to-rim direction.
Three possibilities exist for the nature of emplacement of fahlore seams and layers: (i) they were precipitated out of hydrothermal ore-forming fluid during colloform growth; (ii) they have replaced a pre-existing phase; or (iii) the pyrite layer interfaces they occur at represent inherent weaknesses, facilitating the accumulation of more mobile phases (e.g. fahlore and chalcopyrite) during later deformation.
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If the fahlore was injected as a mobile phase, precipitation of crystals would have taken place either on the walls of an open fracture, or in a crack-seal fracture propagation scenario. In either case crystals should have developed drusy texture, but fahlore crystal morphology is not discernable in reflected light, BSE, or OC images. Late-stage injection is unlikely though, as the apparent textural relationship between the thickest fahlore and immediately overlying acicular pyrite in colloform 2 suggests that either the fahlore was present before growth of the pyrite, and exerted some control over its development, or that a distinct pre-existing feature
(now replaced by fahlore) had some control over pyrite growth.
Replacement textures commonly display irregular outlines, yet fahlore in both colloforms is generally concordant with traces of pyrite layering. This suggests that the fahlore was probably precipitated as a primary feature. The lateral restriction exhibited in the thickest layer of fahlore in colloform 2 is best explained as a product of blister-like precipitation or partial dissolution of a layer (fig 14). Its conformable relationship with pyrite layers makes the inclusion of another structure an unlikely scenario.
Chalcopyrite commonly occurs concordant with layering, often in the presence of fahlore.
However, in places it protrudes into, or cross-cuts, pyrite layers in an irregular fashion. This suggests that chalcopyrite commonly replaces fahlore, inheriting a conformable relationship with colloform layering, but occasionally replaces pyrite in less conformable manner. Thus the crystallographic orientation data presented here for chalcopyrite may be spurious, unless it inherited crystallographic orientations from pre-existing phases.
Lateral variation within layers
Lateral variation in colloform 1, layer 6, has a different origin to that of layer 1 and is discussed below. In layer 1, the coincidence of one side of sub-zone B with a brittle fracture suggests it might have moved slightly relative to sub-zone C. This explains the misorientation
Page | 30 between B and C, but falls short of explaining misorientation at the other end of sub-zone B, where no fracture exists. Other possibilities include (i) blister-like precipitation, with gaps filled by later precipitation that generates a different CPO; (ii) precipitation of a layer followed by partial dissolution, with gaps filled in a similar manner to (i); or (iii) incorporation of a different structure (fig 14). Any of these scenarios might have resulted in an inherent weakness that facilitated propagation of the brittle fracture. It is also likely that layer 4 (appearing to reside in layer 5) could be a result of one of these scenarios.
Figure 12. Schematic diagrams showing possible explanations for lateral variation of common CPO in colloform 2, layer 1. A) blister-like precipitation style at t3, leaving a micro-topography to be filled at t4, when a different common CPO is dominant. B) palaeotopography on which colloforms nucleate is conducive to an intergrowth of colloforms. If colloform X for some reason grows at a higher rate than colloform Y, the latter becomes engulfed by the former and can no longer continue to grow. If a polished section cut is made as indicated, part of colloform Y would appear to constitute part of Colloform X at t3, resulting in an apparent lateral variation of common CPO at t3. C) a complete layer is precipitated at t3. Between t3 and t4, the ore fluid becomes undersaturated, resulting in patchy dissolution of the t3 layer. The resulting voids are infilled by precipitation at t4.
CPO origin: primary or secondary?
There are two possible origins for the development of CPOs in colloforms: (i) they are developed as a primary feature during precipitation at the initial colloform development phase; or (ii) they are a secondary feature resulting from recrystallisation during deformation and metamorphism. Several observations from the Parys Mountain colloforms make the latter a difficult concept to explain.
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CPO switching across layers is a common feature, yet if developed as a secondary texture one would anticipate a single CPO for an entire colloform. Furthermore, the external morphology of both colloforms, and the absence of internal deformation within individual crystals suggests they are undeformed. Individual pyrite crystals from borehole PM109 commonly display ~5-20° of misorientation, indicating that metamorphic temperatures were high enough to facilitate plastic deformation in areas that experienced sufficient strain. Plastic deformation of pyrite is commonly enhanced by pyrite-pyrite grain indentation (Boyle et al., 1998). While the possibility remains that this process might have been responsible for plastic deformation in the PM109 samples (see further discussion), there is no direct evidence for it, and it is equally possible that deformation occurred in the absence of pyrite-pyrite grain indentation.
Either way, the potential for pyrite-pyrite grain indentation-induced deformation in colloform pyrite layers is high due to an abundance of pyrite; yet individual crystals are undeformed, suggesting that the colloform samples are from a zone that experienced low strain during deformation. It is hard to conceive of a way in which a CPO would develop during recrystallisation in a low strain regime.
It is reasonable to suppose that the coarse grains in some layers might have developed in response to recrystallisation during recovery, although it is unlikely given the relatively low temperature metamorphic setting. If recrystallisation did occur, it is likely to have inherited a precursor CPO (Freitag et al., 2004). Thus regardless of whether crystallographic texture is primary or secondary, it is likely that the development of CPOs was primary.
Implications for crystal growth
In geology, CPOs are commonly developed in response to applied stresses during deformation (Passchier and Trouw, 2005). The primary origin of CPO textures in Parys
Mountain colloforms suggests that other factors must have been responsible for their
Page | 32 development. One possibility is that crystal growth was syntaxial or epitaxial with respect to the crystal upon which it nucleated. While this can explain common CPOs within discrete layers, it fails to account for common CPO switching across layer interfaces. This leads to a second possibility: textural differences that define layers likely relate to changing ore-forming conditions (e.g. temperature and degree of supersaturation) that might be responsible for development of different crystallographic orientations.
Another possibility is changing trace element sequestration. Barrie et al., (2009a) demonstrate a possible relationship in which Sb sequestration tends to correlate with a <100> CPO, while
As sequestration appears to favour <110> or <111> CPOs. However, they also show that this is not an exclusive relationship, and further work is required to better understand it. Arsenic in pyrite attaches to {111} faces and is suggested to promote growth in the <111> direction
(Sunagawa and Takahashi, 1955; Chouinard, 2005), although it is noted that its effects are often sub-ordinate, easily overcome by other factors. Spot analyses reveal trace amounts of
As (up to 5.65 wt%) commonly present within colloform pyrite, while Sb is rare, except in acicular pyrite above the thick fahlore layer in colloform 2, where it occurs at up to 2.77 wt%.
Given the relative abundance of As, it might be anticipated that CPOs are developed about
<111> axes. However, most layers have <100> CPOs, suggesting that As has not played a significant role their development.
Experimentally grown pyrite crystals are used to suggest degree of supersaturation is an important factor controlling crystal shape, increasing through development of acicular, bladed, cubic, pyritohedral, to spherulitic morphologies (Murrowchick and Barnes, 1987;
Alonso-Azcárate et al., 2001). Grain size may also be an indicator of degree of supersaturation, increasing with decreasing nucleation rates and possibly decreasing degree of supersaturation (Murrowchick and Barnes, 1987).
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Crystallographic texture in colloform 1 suggests an increase in nucleation rate between layers
1 and 2, followed by a decrease to produce the large grains of layer 5. Fine grain size of layer
6 indicates high nucleation rate, thus possibly high degree of supersaturation. However, strongly acicular morphology in this layer should indicate low degree of supersaturation
(Alonso-Azcárate et al., 2001). Acicular crystals precipitating in high supersaturation conditions can be explained by high nucleation rate resulting in many crystals growing outward inter-competitively, therefore being restricted in all growth directions except outwards. Such growth constraint inevitably yields acicular crystals.
Each change in nucleation rate is possibly related to change in degree of supersaturation.
Disregarding the chalcopyrite core, and layers 2 and 4 (which may each exhibit misleading data), and incorporating layer 3 with layer 5 (they show very similar CPOs), a pattern emerges that possibly relates CPO to degree of supersaturation. In relative terms, degree of supersaturation possibly varies from moderate to low to high in layers 1, 5, and 6 respectively.
This correlates with CPOs that are moderate to weak to strong in the same order. Thus high degree of supersaturation may lead to well-developed CPOs. However, the close relationship between layer 6 and crystals in the upper part of layer 5 implies that layer 6 crystals have developed with a syntaxial relationship to layer 5, thus indicating that at least some level of control on CPO development is exerted by substrate. That CPOs are inherited across a potentially significant change in degree of supersaturation indicates that substrate-controlled
CPO development is more important than degree of supersaturation, at least in the case of the layer 5/layer 6 transition.
A general trend in colloform 2 is one of coarsening grains becoming more equant through layers 1 to 3a. A similar trend then occurs through layers 3c to 6. These trends show a correlation, albeit weak, with CPOs, whereby strong CPOs correspond to finer grain sizes and
Page | 34 acicular morphology. It is also apparent that coarse equant grains correspond to CPOs about
<110> or <111> (with exception of layer 5b), similar to the findings of Barrie et al. (2009a).
The layer of pyrite stratigraphically above the thickest fahlore layer shows a particularly strong CPO developed about a <100> direction parallel to substrate, strongly acicular crystals, and unique trace element chemistry. The latter may linked to the waning of trace elements in the ore-forming fluid after precipitation of the fahlore. It is therefore likely that either the fahlore substrate or the trace element chemistry exerted control on growth. A <100> CPO in the fahlore layer is different to that of the pyrite, therefore seemingly eliminating inheritance as a CPO development control.
CPOs about axes aligned parallel to layer interfaces are difficult to reconcile. If precipitating fluids or trace elements control CPO development, it is likely that crystal nuclei will form at random orientations when growth of a new layer is initiated. These will be overcome by crystals growing in a certain orientation favoured by the precipitating fluid or attachment of trace elements to certain crystal faces, producing a CPO in the direction of growth, as previously found in colloform pyrite and sphalerite (Barrie et al., 2009a; b). The only way to explain CPOs developed parallel to substrate is to invoke inheritance of crystallographic orientation from the substrate. However, this is only obvious in one instance in colloform 1.
Most layer interfaces correspond with CPO axis switching. It is possible that sub-microscopic crystals may be responsible for imparting CPOs in subsequent layers, but further studies are required to develop this idea. In colloform 2, layers 2, 3c, 4, and 5 display CPOs of varying strength about similar axes parallel to layer interfaces, possibly indicating inheritance from substrate.
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Plastically deformed grains
Lattice rotation in pyrite indicates that plastic deformation has taken place in the dislocation field. In some of the examples here (e.g. grain 2) deformation is difficult to reconcile by dislocation glide alone. Such a large degree of rotation (up to 20°) sometimes exhibited about
<100> axes likely necessitates the involvement of dislocation climb, thus invoking dislocation creep as a deformation mechanism.
These results, combined with the lower anchizone metamorphic setting of Parys Mountain, imply that plastic deformation of pyrite occurred at an upper temperature limit of ~260 °C.
Although better constraint on peak temperature could be obtained from fluid inclusion studies, metamorphic textures of the deposit are in accord with low metamorphic temperatures. Preservation of lattice rotation in sphalerite is not anticipated where metamorphic temperatures exceed ~450 °C, as recrystallisation occurs above this temperature
(McClay, 1983) and destroys pre-existing crystallographic features. That lattice rotations in sphalerite are preserved indicates that deformation took place at <~450 °C.
Plastic deformation of pyrite at ~260 °C is somewhat lower than the limits of deformation mechanism map for pyrite of similar grain size (McClay and Ellis, 1983), which indicates a requirement of ~400 °C to initiate dislocation, and ~500 °C to facilitate dislocation creep (fig
15). Such a finding is not entirely unexpected, as recent studies utilising similar techniques have identified temperature plastic deformation as low as ~300 °C (Freitag et al., 2004; A.P.
Boyle, pers. comm.). However, large degrees of lattice rotation are noteworthy, especially given the apparent absence of pyrite-pyrite grain contacts. Boyle et al. (1998) show that plastic deformation of pyrite is enhanced by pyrite-pyrite grain indentation. The possibility remains that pyrite-pyrite grain contacts exist in the third dimension relative to the image
Page | 36 plane, or that the grains examined were in contact when they deformed but have since been displaced by top-to-right shearing, as implied by the nature of the sphalerite “tails” (fig. 10).
Figure 13. Pyrite deformation mechanism map for grain size of ~100 μm. Contours indicate strain rate, and are labelled as 10-n s-1. Fields indicated by black dashed lines are experimentally derived (McClay and Ellis, 1983). Dislocation glide and creep is demonstrated by Freitag et al. (2004) to occur naturally in lower temperature and/or strain rate regimes. The current study indicates that these deformation mechanisms occur naturally in pyrite down to temperatures of ≤260 °C. Modified after McClay and Ellis (1983).
Most lattice deformation in the pyrite investigated occurs as rotation about <100> axes. This concurs with experimental studies that suggest {001}<100> are the easiest slip systems on which to initiate glide (Cox et al., 1981). Other slip systems are only required when principal shortening is parallel to <100>, and the most likely is considered to be about <110> directions
(Cox, 1987). Indeed, evidence for slip on any other plane is yet to be demonstrated in nature or experimentally. At least one example of rotation about <110> is shown here (grain 4), with the implication being that the grain must have been crystallographically oriented with <100> parallel to shortening. Confirmation of such conjecture requires that all grains displaying
Page | 37 rotation about <110> should be similarly oriented. In this study grain 1 shows possible rotation about <110>, but it does not appear oriented similar to grain 4. The other possibility is a rotation about <111>, in which case the necessity for a common grain orientation is eliminated. However the likelihood of this must be remote given the absence of previous demonstration of slip about <111>. Perhaps more likely is that rotation occurred about <110> when grains 1 and 4 shared a common orientation, but subsequent top-to-right shearing has rotated the grains relative to each other.
Conclusions
1. Crystal morphologies displayed in Parys Mountain colloform pyrites are similar to
those observed in Ezuri and Greens Creek colloforms. However, EBSD results reveal
marked differences in CPO trends, with most developed about <100> directions, as
opposed to more common <110> and <111> directions determined in earlier studies.
Furthermore, common crystallographic directions are never growth-parallel, normal to
layer interfaces, as in earlier work. Instead they are commonly sub-parallel to the
surfaces of preceding layers (i.e. substrate), with crystal growth able to occur in any
rotation on a crystal edge parallel dominantly to <100>.
2. Changes in orientation of common direction across layer interfaces occur frequently,
indicating that there is no simple inheritance control on CPO. It is possible that these
switches are controlled by changes in degree of supersaturation or trace element
abundance, but control by these factors alone is difficult to reconcile with common
axes sub-parallel to substrate. Arsenic is widespread as a trace element in the pyrite. It
is widely invoked to attach to {111} faces and promote <111> growth (e.g. Chouinard,
2005). <111> CPOs appear only twice in the two colloform samples investigated. The
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absence of a common <111> directions in the presence of arsenic suggests that arsenic
has only weak, if any, effect on colloform pyrite growth mechanisms.
3. Common axes sub-parallel to substrate are perhaps best explained by CPO inheritance
from nucleation surface. At least one example of CPO inheritance is demonstrated in
one colloform, while it is possible that the other (colloform 2) displays a weak
inheritance control in several layers. If inheritance is the major control on all CPOs
observed, axis orientation switching across layer interfaces might be best explained by
sub-microscopic crystals at these interfaces.
4. These new results cannot easily be explained from the data collected here. Further
work is required to develop the suggestion that substrate inheritance is important. This
will be best approached in additional, higher resolution studies either in Transmission
Electron Microscope studies or EBSD studies utilising a field emission gun.
5. At least one sample from Parys Mountain that exhibits abundant strain fabrics
comprises individual pyrite grains that are deformed plastically by dislocation creep.
Given a metamorphic zone of lower anchizone, this represents peak metamorphic
temperature of ~200-260 °C. This work therefore finds plastic deformation of pyrite at
the lowest temperature yet recorded, and much lower than temperatures indicated on
the pyrite deformation mechanism map. A revision of the deformation mechanism map
is necessary to provide practical parameters for naturally deformed pyrite.
6. Lattice rotation is commonly about <100>, which has the lowest critical resolved shear
stress in pyrite. However, it is also evident about <110> in at least one grain. Recent
experimental results (Barrie, 2008) suggest that for such lattice rotation to take place
principal shortening should be parallel to <100>. This implies that grains showing
rotation about <110> in nature should develop common orientations with respect to
Page | 39
shortening direction. Sample size in this study is too small to comment on the validity
of this relationship.
7. Future work on low temperature plastic deformation in pyrite might include fluid
inclusion analysis to better constrain the metamorphic temperature limits at which
pyrite deformed.
Acknowledgments
I thank Dr. A.P. Boyle for helpful supervision and discussion throughout this research, and for technical assistance in SEM work and use of the Channel 5 software package at the
University of Liverpool. I also acknowledge Mark Pearce, Carmel Pinnington, and Daniel
Tatham for further SEM assistance. I thank Western Metals ltd. for provision of polished sections, and Anglesey Mining plc for granting permission for this work, and for providing useful unpublished work on the ore deposit.
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