Stephen F. Cox

Faulting processes at high fluid pressures; an example of fault valve behavior from the Wattle Gully Fault, Victoria, Australia

The internal structures of the Wattle Gully Fault provide insights about the mechanics and dynamics of fault systems exhibiting fault valve behavior in high fluid pressure regimes. This small, high-angle reverse fault zone developed at temperatures near 300°C in the upper crust, late during mid-Devonian regional crustal shortening in central Victoria, Australia. The Wattle Gully Fault forms part of a network of faults that focused upward migration of fluids generated by metamorphism and devolatilisation at deeper crustal levels. The fault has a length of around 800 m and a maximum displacement of 50 m and was oriented at 60° to 80° to the maximum principal stress during faulting. The structure was therefore severely misoriented for frictional reactivation. This factor, together with the widespread development of steeply dipping fault fill quartz veins and associated subhorizontal extension veins within the fault zone, indicates that faulting occurred at low shear stresses and in a near-lithostatic fluid pressure regime. The internal structures of these veins, and overprinting relationships between veins and faults, indicate that vein development was intimately associated with faulting and involved numerous episodes of fault dilatation and hydrothermal sealing and slip, together with repeated hydraulic extension fracturing adjacent to slip surfaces. The geometries, distribution and internal structures of veins in the Wattle Gully Fault Zone are related to variations in shear stress, fluid pressure, and near-field principal stress orientations during faulting. Vein opening is interpreted to have been controlled by repeated fluid pressure fluctuations associated with cyclic, deformation-induced changes in fault permeability during fault valve behavior. Rates of recovery of shear stress and fluid pressure after rupture events are interpreted to be important factors controlling time dependence of fault shear strength and slip recurrence. Fluctuations in shear stress and transient rotations of near-field principal stresses, indicated by vein geometries, are interpreted to indicate at least local near-total relief of shear stress during some rupture events. Fault valve behavior has important effects on the dynamics of fluid migration around active faults that are sites of focused fluid migration. In particular, fault valve action is expected to lead to distinctly different fluid migration patterns adjacent to faults before, and immediately after, rupture. These fluid migration patterns have important differences with those predicted by models for dilatancy-diffusion effects and for poroelastic responses around reverse faults.

Deformational and metamorphic processes in the formation of mesothermal vein-hosted gold deposits-examples from the Lachlan Fold Belt in central Victoria, Australia

Abstract Gold-bearing quartz vein systems in metamorphic terranes are one of the most important types of lode gold resource. Major vein-type Au mineralisation of this style in central Victoria is restricted to narrow, structurally-controlled domains in a low grade metamorphosed quartz-rich turbidite sequence. Vein systems in these domains have developed in fault-related and fold-related dilatant fractures which were generated at supralithostatic fluid pressures during regional deformation and metamorphism. The timing of mineralisation, the nature of associated hydrothermal alteration, and the isotopic and chemical compositions of the fluids point to ore genesis involving large volumes of COH metamorphic fluids whose flow has been channelised along high-permeability fault zones. The development of auriferous vein systems has been controlled by the coincident development of structural and geochemical traps. The major structural traps are dilatant jogs on reverse faults, extension fracture arrays adjacent to faults, and saddle reefs and related structures in fold hinges. Gold precipitation is ascribed largely to structurally controlled mixing of secondary CH4-bearing fluids with more oxidised primary gold-bearing fluids traversing the dilational fracture systems. The CH4-bearing fluids have been produced by interaction of the primary fluid with graphitic slates adjacent to fault zones. Cyclic fluctuations in fluid pressure and shear stress accompanying episodes of fault motion are shown to control repeated episodes of fracture opening, fluid mixing, and mineralisation.

Crack-seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures

Abstract An analysis of crack-seal fibre growth mechanisms has shown two important implications of the crack-seal process for the interpretation of microstructures in low-grade metamorphic rocks: 1. (1) The ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms. 2. (2) Fibrous crystals may develop cristallographic preferred orientations in certain crack-seal situations. The microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate (001) and grain shape preferred orientation can develop during crack-seal deformation by oriented growth mechanisms. During successive crack-seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history, resulting in preferential rejoining, by syntaxial overgrowth, of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack. Preferred orientation may also be developed and enhanced in crack-seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls. In this case the crystallographic preferred orientation need not be simply related to the displacement history during crack-seal fibre growth. Since crack-seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock, such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes.

The role of fluids in syntectonic mass transport, and the localization of metamorphic vein-type ore deposists

Abstract In low- to medium-grade metamorphic environments, the mobilization of species is closely associated with the operation of solution-precipitation creep processes during foliation development. Such deformation processes involve dissolution of material from source sites which are usually discrete dissolution surfaces having orientations controlled by the applied stress and displacement fields. Mass transfer occurs via solute diffusion in an essentially static fluid network, or by transport of dissolved species in a migrating fluid. Precipitation of material removed from dissolution sites occurs in a variety of sink sites including pore surfaces, grain-scale microcracks, and veins. Sink sites may be close to, or distantly removed from source sites. The transport of material during deformation involving solution-precipitation creep is largely controlled by the mobility of metamorphic fluids. In regions of high fluid mobility, significant mass transfer may occur over large distances. Fluid flow, mass-transfer paths and the development of sink sites, in such cases, are constrained by the structurally controlled development of high fluid-pressure domains and the consequent mechanical enhancement of permeability by hydraulic-fracture processes and the development of grain-scale dilatancy. These processes may result in focussed fluid flow and precipitation of mobilized species in restricted regions. A proportion of syntectonically mobilized material is precipitated to form vein-type deposits in hydraulic-fracture arrays which develop in high fluid-pressure environments. The microstructures developed in minerals deposited in these sites reflect the interaction of a range of nucleation and growth processes which usually involve multiple generations of fracture growth and mineral crystallization. The formation of syntectonic gold-quartz vein deposits in central Victoria (Australia) illustrates the importance of structural controls on both the large-scale migration of metamorphic fluids, and the development of sites suitable for the precipitation of metamorphically mobilized species.

Evolution of strength recovery and permeability during fluid–rock reaction in experimental fault zones

Abstract Physical and chemical fluid–rock interactions are implicated in controlling earthquake nucleation and recurrence. In particular, interseismic compaction, sealing and healing of fractured fault rocks can lead to strength recovery and stabilisation of fault zones. In contrast, these same processes can also assist increases in pore fluid pressures and consequent destabilisation of faults. Here, we present high-temperature, hydrothermal experiments designed to assess the evolution of strength of fault zones in previously intact rock, and also characterise the associated changes to porosity and permeability. Cores of Fontainebleau sandstone were initially loaded to failure in a high-pressure gas–medium apparatus. The failed specimens were then hydrothermally reacted at 927°C for variable duration under isostatic conditions, and subsequently re-fractured to determine the ‘interseismic’ strength recovery. In the most extreme case, hydrothermally induced gouge compaction, cementation and crack healing resulted in 75% strength recovery after reaction for 6 h. Isostatic hydrothermal treatment also resulted in dramatic reduction in porosity and permeability. Strength of the fault zone following hydrothermal reaction appears to be closely correlated to porosity, consistent with previous studies on brittle failure of porous aggregates. The experimental results show how hydrothermal reactions in fault zones may lead to two competing, time-dependent effects; fault strengthening due to increased cohesion in the fault zone and fault weakening arising from elevated pore pressures within a well cemented, low-permeability gouge layer.

Coupled grain-scale dilatancy and mass transfer during deformation at high fluid pressures: examples from Mount Lyell, Tasmania

Abstract High fluid pressures have promoted the coupled operation of grain-scale dilatancy and solution-precipitation processes as dominant deformation mechanisms during cleavage development in a sequence of Cambrian silicic volcanics in the Mount Lyell area, Tasmania. Episodic opening of pervasive microcracks has localized the precipitation of material removed from dissolution sites, and has led to the growth of oriented crack-seal microstructures which are a major element of the overall deformation microfabric. It is argued that transient, dilatancy-driven fluid pressure gradients, and consequent fluid migration within the fluid-containing grain-boundary and microcrack network, can play an important role in contributing to mass transfer between dissolution sites and dilatant microfracture sites. The enhancement of grain-scale microfracture processes and coupled solution-precipitation processes, which accompanies the development of near-lithostatic fluid pressures, is expected to lead to high fluid pressure crustal regimes becoming substantially weaker than otherwise similar low fluid pressure regimes. The onset of such weakening processes in response to rising fluid pressures is probably a significant factor triggering pervasive regional deformation of the upper-crust.

Antitaxial crack-seal vein microstructures and their relationship to displacement paths

Abstract The microstructures developed in an example of a syntectonic crack-seal vein indicate that antitaxial fibres do not necessarily track the incremental opening history during crack-seal vein growth. This observation serves as a caution against the uncritical use of fibrous microstructures to make inferences about displacement histories during fibre formation. It is demonstrated that crack-seal processes can lead to the development of both irregular and laminated vein microstructures, as well as fibrous microstructures. The types of microstructures which develop in crack-seal veins, and the extent to which they reflect the opening paths of veins depend on a range of factors. The most important of these include the nucleation and growth kinetics of phases precipitated from fluids in opening fractures, and the location of sites of material accretion during successive crack-seal increments.

Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at not, vert, similar0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zones major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnasite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO32−, PO42−, and SO42−ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.

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

The influence of room temperature deformation on porosity and permeability in calcite aggregates

Changes in permeability and porosity during shortening deformation of Carrara marble and hot-pressed calcite aggregates were measured under high pressure at room temperature using argon as pore fluid. At effective pressures of 30 and 50 MPa, the permeability of Carrara marble increased by up to 2 orders of magnitude with less than 2% strain during which the connected porosity increased by only 0.005. The permeability increased more slowly with further strain up to 18%, during which the connected porosity increased by a further 0.05 to 0.06. At effective pressures of 100 MPa to 200 MPa, these effects were much less marked. In hot-pressed calcite aggregates, deformed at an effective pressure of 50 MPa, the permeability increased by about 2 orders of magnitude after about 12% strain and an increase in connected porosity of about 0.03. Microstructural studies indicate that, in the coarse-grained Carrara marble specimens, both transgranular and grain boundary cracks are present after room temperature deformation. For a given strain, the average length and the linear density of transgranular cracks decrease with increasing effective pressure. In fine-grained, hot-pressed calcite aggregates, dilatancy is mainly due to opening of grain boundary cracks. The very marked increase in permeability with small strain at low effective pressure can be correlated with the proliferation of connected microcracks of relatively large apertures, deduced on the basis of theoretical models.