Abstract The structural geology and tectonic setting of hydrothermal gold deposits are paramount for understanding their genesis and for their exploration. Strong structural control on mineralization is one of the defining features of these deposits and arises because the permeabilities of crustal rocks are too low to allow the formation of hydrothermal deposits on realistic timescales unless rocks are deformed. Deformation zones and networks of deformation zones are the fundamental structures that control mineralization. Systematically analyzing deposit geometry, kinematics, and dynamics leads to the most thorough comprehension of a deposit. Geometric analysis relates orebody shape to controlling structures, and networks of deformation zones can be analyzed using topology to understand their connectivity and mineralizing potential. Kinematic analysis determines the location of permeability creation and mineralization. New views of shear zone kinematics allow for variable ratios of pure to simple shear, which change likely directions of mineralization. Multiple orientations of mineralized deformation zones may form simultaneously and symmetrically about the principal strain axes. Dynamic analysis is necessary for a mechanical understanding of deformation, fluid flow, and mineralization and can be achieved through numerical modeling. The relationship between deformation (kinematics) and stress (dynamics) constitutes the rheology; rheological contrasts are critical for the localization of many deposits. Numerous gold deposits, especially the largest, have evidence for multiple mineralizing events that may be separated by tens to hundreds of millions of years. In these cases, reactivation of structures is common, and a range of orientations of preexisting structures are predicted to be reactivated, given that they are weaker than intact rock. Physical and chemical processes of mineralization can be integrated using a nonequilibrium thermodynamics approach. Hydrothermal gold deposits form in contractional, strike-slip, and extensional tectonic settings. However, there may be great variation in the spatial scale over which the tectonic setting applies, and tectonic settings may also change on rapid timescales, so that it is inadvisable to infer local tectonics from deposit-scale patterns, and vice versa. It is essential to place mineralizing events within a complete geologic history in order to distinguish pre- and postmineralizing structures from synmineralization deformation features.

Abstract Many ore-producing hydrothermal systems form within intrinsically low permeability host rocks during fracture-controlled flow in overpressured fluid regimes. The generation and localization of fracture-controlled fluid pathways in these systems involves dynamic coupling between fluid flow, fluid pressures, stress states, and deformation processes. In high fluid flux settings, fracture-controlled permeability enhancement is driven largely by fluid pressurization rather than by tectonic loading. The orientation of the stress field plays a critical role in governing the orientations of activated fractures. Permeability destruction by fracture sealing and cementation of fragmented rock is rapid relative to the lifetimes of hydrothermal systems. Accordingly, repeated regeneration of permeability is necessary to sustain the high fluid fluxes required for ore formation. The evolution of permeability is thus controlled by a dynamic competition between permeability enhancement processes and permeability destruction processes. During fluid pressurization, the failure modes, and hence growth of fluid pathways, are particularly sensitive to differential stress and the relative cohesive strengths of faults and intact rock. The fluid pressure, stress regimes, and mechanical properties of host rocks thus influence whether deposit styles are dominated by extension veins, fault-fill lodes in optimally oriented or unfavorably oriented faults, or lode development in viscous shear zones. Many fracture-controlled hydrothermal systems in intrinsically low permeability host rocks form at very low differential stresses and near-lithostatic fluid pressure regimes. Large-scale fluid injection experiments and contemporary seismicity in fluid-active settings indicate that the characteristic response to injection of large volumes of overpressured fluids into fault zones in low-permeability host rocks is earthquake swarm seismicity. Injection-driven swarm sequences enhance permeability via thousands of microseismic slip events over periods of days to many weeks. The accumulation of net slip in ore-hosting faults involves up to thousands of separate swarm sequences. Injection-driven earthquake swarms provide a very dynamic hydrothermal environment for ore formation. Incremental growth of ore deposits occurs during short bursts of high fluid flux during swarm sequences that are separated by long intervening periods in which there is little or no flow. Rapidly recurring slip events during swarms drive repeated and rapid changes in fluid pressures, flow rates, and stresses. If injection-driven growth of a fracture network breaches a hydrologic barrier between differently pressurized regimes, ensuing rapid depressurization can be a key driver of ore deposition. Although shear failure is an inherently dilatant process that increases permeability by up to many orders of magnitude, permeability distribution in fault zones is extremely heterogeneous. Permeability enhancement in active fault zones is favored by the presence of relatively competent host rocks. Permeability is particularly enhanced within some types of fault stepovers and bends. Fracture damage around rupture termination zones, fault branch lines, and fault intersections may also generate high fluid flux pathways. Directions of flow anisotropy along predominantly linear, high-dilation damage zones in faults are strongly influenced by fault kinematics. Permeability, fluid pressures, and flow rates evolve dynamically during injection-driven rupture sequences. Changes in flow rates and fluid pressures during the lead-up to a swarm, during rupture sequences themselves, and immediately after cessation of a swarm have impacts on ore deposition processes such as gradient reactions, fluid-rock reaction, phase separation, and fluid mixing. Fluid pressurization in the lead-up to a rupture sequence enhances within-fault permeability and may promote aseismic growth of extension fracture arrays. Repeated microseismic slip events dramatically and locally enhance permeability, cause sudden fluid pressure drops in the rupture zone, and transiently disrupt flow patterns. Rupture propagation is associated with coseismic dynamic fracture damage that further enhances permeability, especially in fault sidewalls and near rupture terminations. Immediate postrupture permeability enhancement can be associated with implosion processes and stress relaxation around rupture terminations. Major loss of permeability is associated with fracture sealing during rapid depressurization in the immediate aftermath of swarms. During successive rupture sequences, changes in permeability distributions in faults are expected to lead to complex changes in flow paths. Within individual faults, the highest fluid fluxes tend to be localized within long-lived fracture damage sites that are repeatedly reactivated over a substantial part of the lifetime of a hydrothermal system.

Abstract Epithermal deposits form in tectonically active arc settings and magmatic belts at shallow crustal levels as the products of focused hydrothermal fluid flow above, or lateral to, magmatic thermal and fluid sources. At a belt scale, their morphology, geometry, style of mineralization, and controls by major structural features are sensitive to variations in subduction dynamics and convergence angle in arc and postsubduction settings. These conditions dictate the local kinematics of associated faults, influence the style of associated volcanic activity, and may evolve temporally during the lifetime of hydrothermal systems. Extensional arc settings are frequently associated with arc-parallel low- to intermediate-sulfidation fault-fill and extensional vein systems, whereas a diversity of deposit types including intermediate-sulfidation, high-sulfidation, and porphyry deposits occur in contractional and transtensional arc settings. Extensional rift and postsubduction settings are frequently associated with rift-parallel low-sulfidation vein deposits and intermediate- and high-sulfidation systems, respectively. At a district scale, epithermal vein systems are typically associated with hydrothermal centers along regional fault networks, often coexistent with late fault-controlled felsic or intermediate-composition volcanic flow domes and dikes. Some districts form elliptical areas of parallel or branching extensional and fault-hosted veins that are not obviously associated with regional faults, although veins may parallel regional fault orientations. In regional strike-slip fault settings, dilational jogs and stepovers and fault terminations often control locations of epithermal vein districts, but individual deposits or ore zones are usually localized by normal and normal-oblique fault sets and extensional veins that are kinematically linked to the regional faults. Faults with greatest lateral extent and displacement magnitude within a district often contain the largest relative precious metal endowments, but displacement even on the most continuous ore-hosting faults in large epithermal vein districts seldom exceeds more than several hundred meters and is minimal in some districts that are dominated by extensional veins. Veins in epithermal districts typically form late in the displacement history of the host faults, when the faults have achieved maximum connectivity and structural permeability. While varying by district, common unidirectional vein-filling sequences in low- and intermediate-sulfidation veins comprise sulfide-bearing colloform-crustiform vein-fill, cockade, and layered breccia-fill stages, often with decreasing sulfide-sulfosalt ± selenide abundance, and finally late carbonate-fill; voluminous early pre-ore barren quartz ± sulfide fill is present in some districts. These textural phases record cycles associated with transient episodes of fluid flow triggered by fault rupture. The textural and structural features preserved in epithermal systems allow for a field-based evaluation of the kinematic evolution of the veins and controlling fault systems. This can be achieved by utilizing observations of (1) fault kinematic indicators, such as oblique cataclastic foliations and Riedel shear fractures, where they are preserved in silicified fault rock on vein margins, (2) lateral and vertical variations in structural style of veins based on their extensional, fault-dominated, or transitional character, (3) extensional vein sets with preferred orientations that form in the damage zones peripheral to, between, or at tips of fault-hosted veins, and (4) the influence of fault orientation and host-rock rheology and permeability on vein geometry and character. Collectively, these factors allow the prediction of structural settings with high fracture permeability and dilatancy, aiding in exploration targeting. Favorable structural settings for the development of ore shoots occur at geometric irregularities, orientation changes, and vein bifurcations formed early in the propagation history of the hosting fault networks. These sites include dilational, and locally contractional, steps and bends in strike-slip settings. In extensional settings, relay zones formed through the linkage of lateral fault tips, fault intersections, and dilational jogs associated with rheologically induced fault refraction across lithologic contacts are common ore shoot controls. Upward steepening, dilation, and horsetailing of extensional and oblique-extensional fault-hosted vein systems in near-surface environments are common and reflect decreasing lithostatic load and lower differential stress near surface. In these latter settings, the inflection line and intersections with branching parts of the vein system intersect in the σ 2 paleostress orientation, forming gently plunging linear zones of high structural permeability that coincide with areas of cyclical dilation at optimal boiling levels to enhance gangue and ore precipitation. The rheological character of pre- and syn-ore alteration also influences the structural character, morphology, and position of mineralized zones. Adularia-quartz-illite–dominant alteration, common to higher-temperature upflow zones central to intermediate- and low-sulfidation epithermal vein deposits, behaves as a brittle, competent medium enabling maintenance of fracture permeability. Lateral to and above these upflow zones, lower-temperature argillic alteration assemblages are less permeable and aid formation of fault gouge that further focuses fluid flow in higher-temperature upflow zones. Fault character varies spatially, from entirely breccia and gouge distally through progressively more hydrothermally lithified fault rocks and increasing vein abundance and diminishing fault-rock abundance proximal to ore shoots. In poorly lithified volcaniclastic rocks or phreatic breccia with high primary permeability, fault displacement may dissipate into broader fracture networks, resulting in more dispersed fluid flow that promotes the formation of disseminated deposits with low degrees of structural control. In disseminated styles of epithermal deposits, mineralization is often associated with synvolcanic growth faults or exploits dikes and phreatic breccia bodies, feeding tabular zones of advanced argillic and silicic alteration that form stratabound replacement mineralized zones. In lithocap environments common to high-sulfidation districts, early, laterally continuous, near-surface barren zones of advanced argillic alteration and silicification form near the paleowater table above magmatic-hydrothermal systems. In many high-sulfidation deposits, these serve as aquitards beneath which later hydrothermal fluids may localize mineralization zones within permeable stratigraphic horizons, although deeper mineralization may also be present within or emanating from faults unrelated to lithocap influence. Silicified lithocaps may contain zones with high secondary structural permeability that localize ore through the formation of zones of vuggy residual quartz and/or elevated fracture densities in the rheologically competent silicified base of the lithocap, often along or emanating laterally outward from ore-controlling faults. Syn-ore faults in such settings may form tabular, intensely silicified zones that extend downward below the lithocap.

Abstract Ancient volcanogenic massive sulfide (VMS) deposits formed in rifted arc, back-arc, and other extensional geodynamic environments and were deformed during later convergent collisional and/or accretionary events. Primary features of deposits influenced the development of tectonic structures. Except for pyrite, common sulfides in VMS deposits are much weaker than their volcanic host rocks. During deformation, strain is taken by the weak sericitic and chloritic alteration envelope surrounding the deposits and by the sulfide bodies themselves, which act as shear zones, undergo hinge thickening and limb attenuation during regional folding, and are deformed into elongate bodies parallel to regional fold hinges and stretching lineations. A tectonic foliation forms as a sulfide banding in the interior of VMS lenses due to shearing and flattening of primary textural and compositional heterogeneities and as a banded silicate-sulfide tectonic foliation along the margins of the VMS lenses due to transposition and shearing of primary silicate (exhalites)-sulfide layers. Other characteristic structures, such as cusps, piercement cusps, piercement veins, and durchbewegung structures (sulfide breccias), formed as a result of the strong competency contrast between the massive sulfide deposits and their host volcanic rocks. Some features of VMS deposits may have both primary and tectonic components, requiring careful mapping of volcanic lithofacies and primary and tectonic structures to assess the nature of these features. One example is the vertical stacking of VMS lenses. The stacking may be primary, due to the rapid burial of lenses by volcanic or sedimentary deposits as the upward flow of hydrothermal fluids continued and precipitated new lenses above the earlier formed lenses. Or it may be tectonic, due to thrusting or isoclinal folding and transposition of the VMS lenses. Metal zoning (Cu/Cu + Zn), produced by zone refining at the seafloor or subseafloor, is refractory to deformation and metamorphism and can be used to delineate hydrothermal fluid upflow zones and, together with stratigraphic mapping, determine if the stacking is primary, tectonic, or both. Similarly, the elongation of VMS lenses may have a primary component due to the deposition and coalescence of sulfide lenses along linear synvolcanic faults or fissures, as well as a tectonic component due to mechanical remobilization of sulfides parallel to linear structural features in the host volcanic rocks. Structural mapping of VMS deposits is hampered by low-temperature recrystallization of sulfides, which masks the effects of deformation, by discontinuous and abrupt lithofacies changes in the volcanic host rocks, and by the weak development of tectonic fabrics and strong strain partitioning in volcanic rocks. To mitigate these issues, mapping of volcanic lithofacies should be done concurrently with structural mapping to delineate repeated stratigraphic panels across reactivated faults and to identify, in the absence of well-developed fabrics, regional folds characterized by abrupt changes in strata orientation from limbs to hinge. Where well-layered sedimentary rocks are intercalated with volcanic rocks, structures should be mapped in the sedimentary rocks and then correlated with those in volcanic rocks to alleviate difficulties in mapping structures in volcanic rocks and defining the sequence of deformation events that affected the volcanic rocks and their VMS deposits.

Abstract Porphyry Cu deposits, the major source of many metals currently utilized by modern civilization, form via the interplay between magmatism, tectonism, and hydrothermal circulation at depths ranging from about 2 to as much as 10 km. These crustal-scale features require the deep crustal formation of a hydrous and oxidized magma, magma ascent along extant permeability fabrics to create an upper crustal convecting magma chamber, volatile saturation of the magma chamber, and finally the episodic escape of an ore-forming hydrothermal fluid and a phenocryst-rich magma into the shallow crustal environment. Three general fluid regimes are involved in the formation of porphyry Cu deposits. These include the deep magma ± volatile zone at lithostatic pressure, an overlying zone of transiently ascending magmatic-hydrothermal fluids that breaches ductile rock at temperatures ~700° to 400°C, and an upper brittle zone at temperatures <400°C characterized by hydrostatically pressured nonmagmatic and magmatic fluids. Critical structural steps include the formation of the magma chamber, magmatic vapor exsolution and collection of a hydrothermal fluid in cupola(s), and episodic hydrofracturing of the chamber roof in order to create the permeability that allows a hydrothermal fluid to rise along with a phenocryst-bearing magma. The interplay between stress produced by far-field tectonics and stress produced by buoyant magma and magmatic hydrothermal fluid creates the fracture permeability that extends from the cupola through an overlying ductile zone where temperatures exceed ~400°C into an overlying brittle zone where temperatures are less than ~400°C. As a consequence, during each fluid escape and magma intrusion event, the rising hydrothermal fluid ascends, depressurizes, cools, reacts with wall rocks, and precipitates quartz plus sulfide minerals, which seal the permeability fabric. A consistent vein geometry present in porphyry Cu deposits worldwide is formed by steeply dipping veins that have mutually crosscutting orientations. Two general orientations are common. The principal vein orientation generally consists of closely spaced sheeted veins with orientations reflecting the far-field stress. Subsidiary veins may be orthogonal to the main vein orientation as radial or concentric veins that reflect magma expansion and extensional strain in the wall rocks as they are stretched by ascent of the buoyant magma and fluids. Episodic magmatic-hydrothermal fluid-driven hydrofracturing creates permeability that is commonly destroyed, as well as locally enhanced, by vein and wall-rock mineral precipitation or dissolution and by wall-rock hydrothermal alteration, depending upon fluid and host-rock compositions. The pulsing character of porphyry Cu magmatic-hydrothermal systems, in part produced by permeability creation and destruction, creates polyphase overprinted intrusive complexes, associated vein networks, and alteration mineralogy that reflects temporal temperature fluctuations beginning at magma temperatures but continuing to low temperatures. Temperature oscillations locally allow external nonmagmatic fluids to access principally the marginal areas but also in some cases the center of the porphyry Cu ore zone at ~<400°C between porphyry dike emplacement events. Over time, the upper part of the source magma chamber at depth cools and crystallizes downward and is accompanied by diminishing magmatic fluid input upward, leading to cooling and isothermal collapse of the porphyry system. Cooling permits the access of external circulating groundwater into the waning magmatic-hydrothermal plume. Magmatic-hydrothermal fluids dominate at temperatures >400°C at pressures transient between lithostatic and superhydrostatic. The external, nonmagmatic saline formation waters or meteoric waters dominate the surrounding and overlying brittle crust at temperatures <400°C at hydrostatic pressures, except where they may mix with buoyantly rising magmatic-derived fluids. Exhumation requires substantial topographic relief, precipitation, and time (typically >1 m.y.) and may enhance overprinted relationships and telescope low-temperature on high-temperature hydrothermal alteration assemblages. Synmineral propagation of faults into or out of a porphyry Cu hydrothermal system in the brittle regime at <400°C can provide an escape channel through which a metalliferous fluid may depart, potentially to form lateral quartz-pyrite veins, overprinted polymetallic Cordilleran lode veins, or an epithermal precious metal-bearing deposit at shallow crustal depths.

Abstract Pore fluid pressure and differential stress are among the most important controls on the mechanical behavior of mineralizing systems. Their separate influences can be readily identified on failure mode diagrams, which show failure envelopes for pore fluid factor or pore fluid pressure at failure against differential stress. The effect of the intermediate principal stress can be shown on such diagrams by using a failure criterion that includes all three principal stresses, such as the Murrell extension to the Griffith criterion. The effect is apparent from the significant variation of the position of the failure envelope as a function of the ratio between the three principal stresses, which is therefore another important control on failure. Characteristic regimes for different gold deposit types occur as distinctive fields on failure mode diagrams. Carlin, epithermal, and volcanogenic massive sulfide (VMS) deposit types have low absolute pore fluid pressures. Iron oxide copper-gold (IOCG), intrusion-related gold, and porphyry deposits encompass low to intermediate values of pore fluid pressure, while the field of lode gold deposits may extend to the highest pore fluid pressures. Lode gold, IOCG, and epithermal deposit types may have the largest values of differential stress. Carlin and VMS deposits are associated with normal stress regimes; the other deposit types may have structures that that formed in either normal or reverse stress regimes. Exploring the effects of the stress ratio and refining these currently broadly defined regimes for the mechanics of mineralization are important future directions for research.

Abstract Structural data is vital for the understanding of the geometry and evolution of a deposit and feeds into geologic, structural, resource, and geotechnical models. Accurate models are critical for targeting, resource estimation, and geotechnical design and, if rapidly available, support real-time decisions on drilling and grade control. Structural drill core data add a high-resolution data set to traditional data from mapping or the structural interpretation of remote sensing and geophysical data and, therefore, add indispensable information to any integrated model. In this paper we propose standardized workflows for data collection, review technological advances and quality control processes accelerating structural data collection from both oriented drill core and televiewer techniques, and provide an overview of structures that may be observed in drill core and discuss their significance to record for the geometry of the deposit. Critical to the data collection process is an interpretative process that recognizes and identifies domain-based structures that ultimately are fundamental to developing 3-D structural models.

Abstract As the functionality and speed of 3-D geologic modeling software have improved over the last 30 years, it has become a core tool for identifying, understanding, and modeling the structural controls on ore deposits. This chapter attempts to summarize some of the key considerations involved in the 3-D modeling of structurally controlled ore deposits and establishes a basic three-step workflow that can be applied to almost any deposit style: establish a geologic framework through field work and 3-D visualization, model the project-scale geology, and finally identify, model, and understand the controls on ore shoots. Importantly, the geologic understanding of a project is not a static concept. Each step in the modeling process should add to it, highlighting which aspects of the model fit the current geologic understanding, and thus increase confidence, and which require further review and possible modification. This chapter also provides guidance on preparing data for 3-D modeling, basic 3-D visualization techniques, selecting a modeling approach, and model validation, as well as commentary on some of the more common pitfalls encountered in 3-D modeling. Finally, case studies of the Tuzon gold deposit in Liberia and the Yalea gold deposit in Mali are provided as examples of the process involved in building a 3-D geologic model, from field work to final model.

Abstract The integrated interpretation of aeromagnetic data is a key exploration tool to define the concealed, potentially prospective geology that we plan to explore. It helps define the district-scale morphology of structural networks and predict which structures may be associated with the formation of mineral deposits. Aeromagnetic data is particularly useful in guiding geologic mapping, exploration targeting, and strategy because the data available is usually broad, geologic processes and features are normally well imaged in the data, and it is relatively cheap to acquire and process. A foundation to the interpretation of the geophysical data is that the interpreter should be a geoscientist familiar with the geology of the area in question, maximizing the integration of geologic knowledge of the area into the interpretation product. The interpreter must think geologically when building the interpretation, drawing on the parallels between aeromagnetic interpretation and geologic mapping/air photo interpretation. Geologic mapping observations have direct parallels in aeromagnetic interpretation (e.g., lithology, structure, alteration). The interpretation process is outlined using the Lake Lefroy region, Western Australia, including form line construction, identification of magnetic rock units, domain definition, data set integration, definition of structural elements, lithological definition, interpretation of the structural framework, and evaluation of the interpretation. Case studies are then provided at a range of scales from the Pine Creek inlier in northern Australia, the Superior province in eastern Canada, and the Zambian Copperbelt Northwest province to illustrate the connection between the interpretations and exploration targeting. The final integrated interpretation is a supplement to outcrop maps, not a competitor. The purpose is to generate a structural and lithological framework that combines the geophysical data of different types with the mapped geology, which can be interrogated by mineralization models over a much wider area than can be achieved if structural elements and lithology are restricted to areas of outcrop.