Steven Micklethwaite

Fault-segment rupture, aftershock-zone fluid flow, and mineralization

ABSTRACTWe propose that zones of transient high permeability aroundancient fault systems can be predicted if fault segments and likelylocations for paleo-rupture arrest are identified. Lode gold depositsin the Kalgoorlie terrane, Western Australia, are the products offocused fluid flow through faulted crust. Deposits in the MountPleasant area are clustered on small-displacement structures over;10 km of the .50-km-long Black Flag fault. Field relationshipsand net slip distribution along the fault indicate that the depositsare adjacent to, but not within, a kilometer-scale dilatant jog,where two segments of the fault are linked. On this basis we inferthat the dilatant jog was a long-term rupture-arrest site. The ob-servations are compatible with rupture on segments of the BlackFlag fault changing stress in the surrounding crust and bringingspecific zones closer to failure. By analogy with active seismogenicfault systems, those zones correspond to regions where aftershocksoccur preferentially after failure. Stress-transfer modeling ofthe system helps explain the location of mineralized small-displacement structures around the Black Flag fault and indicatesthat gold deposits in the area are located on structures that becametransiently permeable and localized fluid flow during repeated af-tershock ruptures. Thus, localized through-flow, or mixing of fluidswithin fault systems, is likely to be controlled by the distributionof aftershocks following rupture events; this distribution ispredictable.Keywords: fault zones, aftershocks, fluids, mineral deposits, genesis,permeability, segmentation.INTRODUCTIONIn this paper we explain and predict a link between the nonuni-form distribution of aftershocks around earthquakes and the evidencefor nonuniform permeability around ancient fault systems. When anearthquake occurs on a fault, the rock volume around the rupture un-dergoes changes in stress state. Those volumes that are brought nearerto failure are closely associated with aftershocks (Harris, 1998, andreferences therein). Whether a volume is brought closer to failure canbe estimated through stress-transfer modeling (Stein et al., 1992). Inthis way the distribution of aftershocks around seismogenic fault rup-tures has been successfully modeled on a number of fault systems(King et al., 1994; Toda et al., 1998; Kilb and Rubin, 2002). In theArchean Kalgoorlie terrane, Western Australia, lode gold deposits arethe products of fluid flow through faulted crust and are located withinlarge fault systems (Eisenlohr et al., 1989). Gold deposits tend to occuron low-displacement structures around higher-displacement faults andshear zones (Eisenlohr et al., 1989). The deposits are often clusteredalong restricted parts of the crustal-scale shear network, and althoughcommonly associated with certain rock types, deposits are found in arange of host rocks (Witt, 1993). Observations elsewhere in the worldon fault-vein architecture and fluid-inclusion pressure variations haveled several authors to suggest that similar low-displacement structureshosted aftershock ruptures (e.g., Robert et al., 1995; Henderson andMcCaig, 1996; Cox et al., 2001). Cox and Ruming (2004) tested thishypothesis by applying stress-transfer modeling to explain deposit dis-tribution around a large contractional fault jog at the St. Ives gold field,Western Australia.In this study we apply stress-transfer modeling to the MountPleasant gold field in the Kalgoorlie terrane, where geological mappingand high-resolution aeromagnetic data provide good constraints on theassociated Black Flag fault. This work advances from Cox and Ruming(2004) by using along-fault slip distribution and field observations toclearly identify ancient fault segmentation. We demonstrate thatthrough the control of segmentation on rupture arrest sites and thusaftershocks, distinct zones of elevated permeability can develop re-peatedly. Herein we propose a new conceptual model for seismogenicfaulting and time-dependant fluid flow. We show that stress-transfermodeling has wider application than earthquake risk, with potential asa tool for mineral exploration.FAULT GEOMETRY, SEGMENTATION, ANDMINERALIZATIONThe Black Flag fault is .50 km long. It crosscuts the Booraraand Zuleika shear zones near its northern and southern tips, respec-tively (Fig. 1A). At Mount Pleasant, a large zone of quartz veins andbreccias marks a jog that deviates 228E from the overall trend of thefault. Historically, some small deposits have been mined directly onthe jog, but most deposits occur in subsidiary structures and splays offthe main fault. From the jog zone to the north, the fault is hostedpredominantly in units of mafic composition, i.e., the Mount Pleasant

Damage and permeability around faults: Implications for mineralization

Mineral deposits are commonly hosted by small-displacement structures around jogs in major faults, but they are rarely hosted by the major fault itself. This relationship may be explained by time-dependent fracturing and healing in and around major faults and associated permeability evolution. A damage mechanics formulation is used here to explore the spatial-temporal evolution of damage in and around a fault following a fault-slip event. We show that regions of increased damage rate correspond to the location of mineral deposits and that these areas correspond to areas of aftershocks predicted by stress-transfer modeling. The fault itself enters a healing regime following the slip event; hence, it is expected to become less permeable than the fracture network outside the fault. Our results support the hypothesis that mineralization occurs in a fracture network associated with aftershocks; this may be due to the higher time-integrated permeability of the fracture network relative to the main fault.

Nanoscale gold clusters in arsenopyrite controlled by growth rate not concentration: Evidence from atom probe microscopy

Abstract Auriferous sulfides, most notably pyrite (FeS2) and arsenopyrite (FeAsS), are among the most important economic minerals on Earth because they can host large quantities of gold in many of the world’s major gold deposits. Here we present the first atom probe study of gold distribution in arsenopyrite to characterize the three-dimensional (3D) distribution of gold at the nanoscale and provide data to discriminate among competing models for gold incorporation in refractory ores. In contrast to models that link gold distribution to gold concentration, gold incorporation in arsenopyrite is shown to be controlled by the rate of crystal growth, with slow growth rate promoting the formation of gold clusters and rapid growth rate leading to homogeneous gold distribution. This study yields new information on the controls of gold distribution and incorporation in sulfides that has important implications for ore deposit formation. More broadly this study reveals new information about crystal-fluid interface dynamics that determine trace element incorporation into growing mineral phases.

Optimally oriented “fault‐valve” thrusts: Evidence for aftershock‐related fluid pressure pulses?

A thrust-vein network from the Triumph gold deposit, Western Australia, is explained in terms of an extremely high rate of fluid-pressure increase, prior to failure, relative to the rate of stress increase. The thrust fault is a small-displacement fault characterized by a thick, fault-parallel shear vein, plus multiple low-angle extension veins, with orientations that demonstrate the thrust was optimally oriented relative to the locally imposed crustal stresses. Large extension veins have irregular margins, are dominantly composed of coarse milky quartz with no obvious laminations or solid inclusion trails, and are regularly spaced along the thrust (1–2 m). The fault-vein geometries indicate the Triumph thrust is a rare candidate for ‘‘fault-valve’’ failure of an optimally oriented thrust, and it is possible the structure formed in a small number of failure events, during load weakening of the thrust. An analysis using the Coulomb criterion shows that load weakening of a thrust occurs when fluid pressure increases relative to tectonic stress by a factor dependent on the orientation of the thrust. Thrust and reverse faults in dry crust load strengthen prior to failure, but the poroelastic behavior of sealed, fluid-saturated crust is enough to induce load weakening in compressive environments; thus poroelastic load weakening is expected to be an important failure mechanism in hydrothermal environments. However, in the case of the Triumph thrust, dilatant shear failure necessitates a fluid pressure increase which is an order of magnitude larger still. The observations and results are consistent with a pulse of high fluid pressure migrating up through fault or fracture networks that have elevated permeability relative to the wall rock, under conditions of transiently low differential stress. Fluid pressure differences resulted between the fault and wall rock, leading to extension fracture and fault failure. Such conditions may occur when adjacent large earthquakes induce damage, breach overpressured fluid reservoirs, and generate fluid-pressure driven aftershocks.

An interactive image segmentation method for lithological boundary detection

Large volumes of images are collected by geoscientists using remote sensing platforms. Manual analysis of these images is a time consuming task and there is a need for fast and robust image interpretation tools. In particular the reliable mapping of lithological boundaries is a critical step for geological interpretation. In this contribution we developed an interactive image segmentation algorithm that harnesses the geologists input and exploits automated image analysis to provide a practical tool for lithology boundary detection, using photographic images of rock surfaces.In the proposed method, the user is expected to draw rough markings to indicate the locations of different geological units in the image. Image segmentation is performed by segmenting regions based on their homogeneity in colour. This results in a high density of segmented regions which are then iteratively merged based on the colour of different geological units and the user input. Finally, a post-processing step allows the user to edit the boundaries.An experiment was conducted using photographic rock surface images collected by a UAV and a handheld digital camera. The proposed technique was applied to detect lithology boundaries. It was found that the proposed method reduced the interpretation time by a factor of four relative to manual segmentation, while achieving more than 96% similarity in boundary detection. As a result the proposed method has the potential to provide practical support for interpreting large volume of complex geological images. Interactive segmentation based on colour similarity.Boundary editing step is used to improve the accuracy of the results for complex images.Works as a multi-label image segmentation algorithm.Effectively detects the lithological boundaries of geological images.

High Spatial Resolution Mapping of Dykes Using Unmanned Aerial Vehicle (UAV) Photogrammetry: New Insights On Emplacement Processes

understanding of the geometry of dykes and dyke swarms on Earth, Venus and Mars (West and Ernst, 1991; Mege and Masson, 1995; Ernst et al., 2005). Since the 1970’s traditional aerial photography has been supplemented by satellite-based passive (e.g., Landsat, SPOT, WorldView) and active (e.g., synthetic aperture radar SAR) imaging systems, with spatial resolutions varying from ~30 m to ~0.3 m. Similar systems have been deployed on a range of planetary remote sensing missions. Since the 1980’s advances in instrumentation, survey design and digital data processing have steadily improved the quality and resolution of aeromagnetic survey data for mapping of dykes and dyke swarms at spatial resolutions ranging from a few hundred meters to tens of meters. Despite these advances, very high-resolution (i.e., cm accuracy) remote sensing imagery of dykes is limited to low altitude aerial photography surveys. This in turn creates a critical gap in the observation scale of dyke studies from < 1 mm at the outcrop and thin section scale to the 10’s of cm to 100’s of meters scale provided by conventional remote sensing and geophysics. Fortunately, the emerging capability of unmanned aerial vehicle (UAV) photogrammetry fills this gap. Here we describe a photogrammetric workflow applied to detailed structural studies of dykes exposed on coastal outcrops in SE and SW Australia (Bemis et al. 2014; Vollgger & Cruden, 2016). We have surveyed these locations using a variety of downward looking digital cameras mounted on multirotor UAVs (quadcopters and hexacopters). The survey design (flight height and speed, flight line spacing, camera focal length, shutter interval, image overlap, etc.) is optimised to minimise motion blur and to maximise the spatial resolution of the resulting photogrammetric models and ortho-images (Vollgger & Cruden, 2016). The use of ground control points using RTK-GPS allows us to produce geo-referenced 3D point clouds from hundreds to thousands of digital images, which in turn results in orthoimages covering <1,000 – >15,000 m at spatial resolutions < 10 mm per pixel, and cm-scale location accuracy. The 3D point cloud can be used to extract 3D structural data (e.g., strike and dip of fractures) while the ortho-image provides a high-resolution base map for structural analysis. We are using UAV photogrammetry as one component of field and theoretical research on dyke interaction with host rock structures, dyke-sill-dyke transitions, and how magma transport networks are built and self-organise. The broader context of this work is to understand how magma flow is channelized in such networks and how sulphide liquids become trapped in channels to form magmatic sulphide deposits. The high spatial, textural and colour fidelity of 3D point clouds and ortho-images derived from UAV photogrammetry allow us to measure geometrical attributes of dykes and dyke networks. These include the spacing of jogs in relation to dyke width, angles between main dyke trends and structures that deviate into steps, and how often steps occur in relation to host-rock fracture frequency and orientation. We can also document locations within networks where conduit widening occurs and relate this to host rock structure. These measurements will enable us to define the geometrical scaling relationships necessary to provide constraints for theoretical and laboratory modelling on the influence of pre-existing structures on stair-stepping magma network development. The results will underpin and refine 3D models of (unexposed) mineralisation associated with jogs in stair-stepping networks (e.g., plunging dyke-sill-dyke intersections at Voisey’s Bay, and the funnel-shaped Jinchuan and Eagle intrusions; Saumur et al., 2015; Barnes et al., 2016). Our ultimate objective is to define a set of rules to better predict the subsurface Alexander CRUDEN, Stefan VOLLGGER, Greg DERING and Steven MICKLETHWAITE, 2016. High Spatial Resolution Mapping of Dykes Using Unmanned Aerial Vehicle (UAV) Photogrammetry: New Insights On Emplacement Processes. Acta Geologica Sinica (English Edition), 90(supp. 1): 52-53.

Exploring backscattered imaging in low voltage FE-SEM

Contrast levels in backscattered SEM images were investigated, utilising stage deceleration for low voltage imaging and also electron energy filtering. Image contrast variations are explained via use of Monte Carlo simulations which can predict the optimum accelerating and filter voltages for imaging complex sample mixtures.

An elevated perspective: dyke-related fracture networks analysed with UAV photogrammetry

monitoring has provided new insight into dyke propagation and emplacement mechanisms. These studies show that faulting and fracturing is part of the magma emplacement process, preceding and accompanying intrusion on timescales of hours and days. Unfortunately, the precision of earthquake hypocentre locations is typically limited to tens or hundreds of meters, which cannot resolve whether the hypocentres relate to strain of wall rock fragments within the dykes, in a process zone around the intrusion or peripherally in the country rock. To better understand the distribution and role of brittle deformation associated with dyke emplacement we examine an exceptionally well exposed swarm of 19 dolerite dykes, along a swath of coastline near the town of Albany, Western Australia. The dykes are vertical and emplaced in Neoproterozoic monzogranite of the AlbanyFraser orogen. The age of the dykes is poorly constrained, but probably post-dates the onset of regional exhumation of the monzogranite at 1.1 Ga (Scibiorski et al, 2015). Faults and fractures cross cut foliation in the monzogranite. The fault rocks are cataclasites containing granitic host rock fragments, and no mafic material. An early dyke within the swarm is faulted, whereas other dykes have solidified against the faults. This suggests faulting was on-going during the earliest phase of dyking, but preceded the bulk of magma emplacement. We use Structure-from-Motion photogrammetry and an unmanned aerial vehicle (UAV) for accurate, high resolution 3D reconstruction of outcrop and extraction of structural data. The model is constructed from 1099 images collected from a digital camera mounted to the body of a small quadcopter, flying semi-autonomously over a survey area of ~10,000 m. Commercial photogrammetry software (Agisoft Photoscan Pro) was used to construct a dense point cloud. From the point cloud, a ground resolution cell size of 3.5 mm was achieved by construction of an orthorectified image mosaicked from the field images, draped on a digital elevation model (DEM) of the same resolution. Internal model accuracy is constrained in 3D by the use of ground control points surveyed with a total station (30-90 mm measurement precision). The locations and orientation of faults, fractures, and dyke margins were sampled along a digital scanline oriented orthogonal to the dyke swarm. Planes were fit to the vertices of 3D fault and fracture traces using a Random Sample and Consensus (RANSAC) algorithm and least squares regression analysis implemented in Java (Thiele et al., 2015). The cumulative thickness of the 19 dyke segments is ~35 m (average aperture 1.8 m) emplaced over a distance of 105 m, measured perpendicular to strike. The first critical observation is that dyke emplacement is accommodated by mode one extension but the faults and fractures are parallel with the dykes, with a total dispersion of <20°. Secondly, the number of faults/ fractures increase into the dyke swarm, which has 2.2 ± 0.7 more fractures, per unit length of scanline, in host rocks intruded by dykes relative to the background value. This suggests a broad damage zone developed around the dyke swarm. However, thirdly, within the swarm fractures are heterogeneously distributed such that there is no measurable systematic distribution of faults and fractures relative to individual dyke segments. Instead, shear failure Gregory DERING, Steven MICKLETHWAITE, Stephen J. BARNES, Marco FIORENTINI, Alexander CRUDEN and Eric TOHVER, 2016. An Elevated Perspective: Dyke-Related Fracture Networks Analysed with Uav Photogrammetry. Acta Geologica Sinica (English Edition), 90(supp. 1): 54-55.

Obituary: Sandy Island (1876–2012)

In October 2012, scientists investigating the tectonic evolution of the eastern Coral Sea aboard the R/V Southern Surveyor uncovered a quirky discrepancy in maps of seafloor topography during their 25-day voyage. While on a transit leg between dredge sites, the ship passed near a purported island between the Chesterfield Islands and Nereus Reef that appeared in numerous scientific data sets and in Google Earth™ with the label “Sandy Island.” However, this 25-kilometer-long and 5-kilometer-wide feature was absent from the hydrographic charts used by the master onboard the ship for navigation.

Magma Plumbing Systems: A Geophysical Perspective

Over the last few decades, significant advances in using geophysical techniques to image the structure of magma plumbing systems have enabled the identification of zones of melt accumulation, crystal mush development, and magma migration. Combining advanced geophysical observations with petrological and geochemical data has arguably revolutionised our understanding of, and afforded exciting new insights into, the development of entire magma plumbing systems. However, divisions between the scales and physical settings over which these geophysical, petrological, and geochemical methods are applied still remain. To characterise some of these differences and promote the benefits of further integration between these methodologies, we provide a review of geophysical techniques and discuss how they can be utilised to provide a structural context for and place physical limits on the chemical evolution of magma plumbing systems. For example, we examine how Interferometric Synthetic Aperture Radar (InSAR), coupled with Global Positioning System (GPS) and Global Navigation Satellite System (GNSS) data, and seismicity may be used to track magma migration in near real-time. We also discuss how seismic imaging, gravimetry and electromagnetic data can identify contemporary melt zones, magma reservoirs and/or crystal mushes. These techniques complement seismic reflection data and rock magnetic analyses that delimit the structure and emplacement of ancient magma plumbing systems. For each of these techniques, with the addition of full-waveform inversion (FWI), the use of Unmanned Aerial Vehicles (UAVs) and the integration of geophysics with numerical modelling, we discuss potential future directions. We show that approaching problems concerning magma plumbing systems from an integrated petrological, geochemical, and geophysical perspective will undoubtedly yield important scientific advances, providing exciting future opportunities for the volcanological community.