Abstract The Kalgoorlie gold camp in the Yilgarn craton of Western Australia comprises the supergiant Golden Mile and the smaller Mt. Charlotte, Mt. Percy, and Hidden Secret deposits. Since the camp’s discovery in 1893, ~1,950 metric tons (t) of Au have been produced from a total estimated endowment of ~2,300 t. The camp is located within Neoarchean rocks of the Kalgoorlie terrane, within the Eastern Goldfields superterrane of the eastern Yilgarn craton. Gold mineralization is distributed along an 8- × 2-km, NNW-trending corridor, which corresponds to the Boulder Lefroy-Golden Mile fault system. The host stratigraphic sequence, dated at ca. 2710 to 2660 Ma, comprises lower ultramafic and mafic lava flow rocks, and upper felsic to intermediate volcaniclastic, epiclastic, and lava flow rocks intruded by highly differentiated dolerite sills such as the ca. 2685 Ma Golden Mile Dolerite. Multiple sets of NNW-trending, steeply dipping porphyry dikes intruded this sequence from ca. 2675 to 2640 Ma. From ca. 2685 to 2640 Ma, rocks of the Kalgoorlie gold camp were subjected to multiple deformation increments and metamorphism. Early D 1 deformation from ca. 2685 to 2675 Ma generated the Golden Mile fault and F 1 folds. Prolonged sinistral transpression from ca. 2675 to 2655 Ma produced overprinting, NNW-trending sets of D 2 -D 3 folds and faults. The last deformation stage (D 4 ; < ca. 2650 Ma) is recorded by N- to NNE-trending, dextral faults which offset earlier structures. The main mineralization type in the Golden Mile comprises Fimiston lodes: steeply dipping, WNW- to NNW-striking, gold- and telluride-bearing carbonate-quartz veins with banded, colloform, and crustiform textures surrounded by sericite-carbonate-quartz-pyrite-telluride alteration zones. These lodes were emplaced during the earlier stages of regional sinistral transpression (D 2 ) as Riedel shear-type structures. During a later stage of regional sinistral transpression (D 3 ), exceptionally high grade Oroya-type mineralization developed as shallowly plunging ore shoots with “Green Leader” quartz-sericite-carbonate-pyrite-telluride alteration typified by vanadium-bearing muscovite. In the Hidden Secret orebody, ~3 km north-northwest of the Golden Mile, lode mineralization is a silver-rich variety characterized by increased abundance of hessite and petzite and decreased abundance of calaverite. At the adjacent Mt. Charlotte deposit, the gold-, silver-, and telluride-bearing lodes become subordinate to the Mt. Charlotte-type stockwork veins. The stockwork veins occur as planar, 2- to 50-cm thick, auriferous quartz-carbonate-sulfide veins that define steeply NW- to SE-dipping and shallowly N-dipping sets broadly coeval with D 4 deformation. Despite extensive research, there is no consensus on critical features of ore formation in the camp. Models suggest either (1) distinct periods of mineralization over a protracted, ca. 2.68 to 2.64 Ga orogenic history; or (2) broadly synchronous formation of the different types of mineralization at ca. 2.64 Ga. The nature of fluids, metal sources, and mineralizing processes remain debated, with both metamorphic and magmatic models proposed. There is strong evidence for multiple gold mineralization events over the course of the ca. 2.68 to 2.64 orogenic window, differing in genesis and contributions from either magmatic or metamorphic ore-forming processes. However, reconciling these models with field relationships and available geochemical and geochronological constraints remains difficult and is the subject of ongoing research.

Abstract Neoarchean greenstone-hosted gold deposits in the Eastern Goldfields Superterrane of the Yilgarn craton of Western Australia are diverse in style, timing with respect to magmatic activity, structural environment, host rocks, and geochemical character. Geologic constraints for the range of gold deposits indicate deposit formation synchronous with volcanism, synchronous with syn- and postvolcanic intrusion, synchronous with postvolcanic deformation in faults and shear zones, or some combination of superposed events over time. The gold deposits are distributed as clusters along linear belt-parallel fault zones internal to greenstone belts but show no association with major terrane boundary faults. World-class gold districts are associated with the thickest, internal parts of the greenstone belts identified by stratigraphic preservation and low metamorphic grades. Ore-proximal faults in those regions are more commonly associated with syn- and postvolcanic structures related to greenstone construction and deformation rather than major terrane amalgamation. Using the Kalgoorlie district as a template, the gold deposits show a predictable regional association with thicker greenstone rocks overlain unconformably by coarse clastic rock sequences in the uppermost units of the greenstone stratigraphy. At a camp scale, major gold deposits show a spatial association with unconformable epiclastic and volcaniclastic rocks located above an unconformity internal to the Black Flag Group. Distinct episodes of gold deposition in coincident locations suggest fundamental crustal structural controls provided by the fault architecture. Late penetrative deformation and metamorphism overprinted the greenstone rocks and the older components of many gold deposits and were accompanied by major gold deposition in late quartz-carbonate veins localized in crustal shear zones or their higher order fault splays.

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 Neoarchean rocks of the Tropicana Zone, including granites with subduction-zone affinities, formed in a terrane adjacent to, or on the margin of, the Yilgarn Craton at the commencement of a long-lived, amphibolite to granulite facies event – the 2722–2554 Ma Atlantis Event. Early stages of this event overlap with extensive komatiite emplacement within the Eastern Goldfields Superterrane (Yilgarn Craton), suggestive of a plume-related rift environment, which was followed by 2660–2630 Ma greenschist facies, orogenic gold mineralization. This indicates differences in the tectonic evolution of the Tropicana Zone compared with within the craton, although isotopic data show similarities in crustal sources. At c. 2520 Ma, the Tropicana Zone was retrogressed to greenschist facies as it was thrust onto the Yamarna Terrane (Yilgarn Craton), forming a northwesterly directed fold-and-thrust belt above the flat-lying Plumridge Detachment. This fold-and-thrust belt is host to the c. 2520 Ma, Tropicana gold deposit. The Plumridge Detachment may extend north to the Yamarna greenstone belt, linking to the Yamarna Shear Zone – the boundary between the Burtville and Yamarna Terranes. The fertility of the Tropicana Zone is related to its Neoarchean geodynamic setting within a continental arc environment, implying that deformed margins of Archean cratons may be prospective for Neoarchean Au deposits.