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 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.

The Early to early Late Jurassic magmatic arc of the lower Colorado River region of southern California and southwest Arizona spanned ∼30 m.y., from ca. 190 to 158 Ma. The arc-type volcanic and plutonic rocks interacted extensively with the Proterozoic Mojave Province crust and show evidence for geographic-based age and compositional changes. The region lies adjacent to an amagmatic gap in the Jurassic arc of the southwest United States, near the western terminus of proposed Late Jurassic basins formed in conjunction with the opening of the Gulf of Mexico, and near, but to the north of, the projection of the trace of the sinistral Late Jurassic Mojave-Sonora megashear where it crossed from northern Mexico into the United States. Quartz-phyric dacitic to rhyolitic pyroclastic and locally hypabyssal rocks of the Dome Rock sequence were emplaced in two broad time periods, one between 190 and 185 Ma and the second between 173 and 158 Ma. Three compositionally expanded pluton units constituting the Kitt Peak–Trigo Peaks superunit were emplaced in the mid- to upper crust between 173 and 158 Ma, broadly contemporaneous with the younger phase of explosive volcanism. The compositionally expanded plutonic rocks consist of three informally named temporally and compositionally distinct magmatic units, from oldest to youngest, the Araz Wash diorite, the Middle Camp porphyritic granodiorite, and the Gold Rock Ranch granite. Each unit was emplaced over 4–6 m.y. periods of time. Dioritic rocks dominate the older Araz Wash diorite unit (173–169 Ma), granodiorite dominates the Middle Camp porphyritic granodiorite unit (167–163 Ma), and granite dominates the Gold Rock Ranch granite unit (163–158 Ma). Shortening and regional metamorphism throughout the lower Colorado River region accompanied emplacement of the Gold Rock Ranch granite unit. A Late Jurassic(?) mafic-felsic dike swarm forms the youngest magmatic unit in the region. The Jurassic magmatic history in the lower Colorado River region ended in the early Late Jurassic at ca. 158 Ma. Termination of magmatism in the Late Jurassic in the lower Colorado River region is distinct from adjacent parts of the arc to northwest in the Mojave Desert region or to the southeast in southern Arizona, where Late Jurassic magmatism continued to at least 146 Ma. At this time in the Late Jurassic, the Mojave-Sonora megashear had cut through the arc to the south of the lower Colorado River region, where degradation of the arc is recorded in sedimentary rocks now composing the lower parts of the McCoy Mountains Formation, the Winter-haven Formation, and informally named rocks of Slumgullion.

Abstract Late Cretaceous to Middle Eocene calc-alkaline to alkaline magmatic rocks emplaced within the southeastern Anatolian orogenic belt, the most extensive magmatic belt in Turkey, result from the complex collision between the Afro-Arabian and Eurasian plates and the subduction of the southern and northern Neotethyan oceanic basins beneath the Eurasian continental margin during the Alpine–Himalayan orogeny. In a transect in east-central Turkey extending from Baskil (Elazig) to Divrigi (Sivas) to the north, and from Copler (Erzincan) to Horozkoy (Nigde) to the SW, these magmatic rocks vary in time, spatial distribution, and composition. 40 Ar/ 39 Ar ages supplemented by a few U–Pb ages geochronology from major plutons demonstrate a general younging of magmatism in the transect from c . 83 Ma in the south (Baskil) to c . 69 Ma in the north (Divrigi-Keban), followed by a c . 44 Ma scattered magmatic complex now found along a NE trending arcuate belt between Copler and Horoz. In general, trace element and rare earth element (REE) geochemistry in the magmatic rocks suggest two main sources for the melts: (1) a mantle-wedge and subducted oceanic lithosphere producing arc-type magma; and (2) metasomatized lithospheric mantle modified by subduction producing magmatic rocks with more metasomatized mantle and within plate signatures. The combination of geochemical and geochronological data presented herein provides a basis to reconstruct the temporal and spatial transition from subduction-related to post-collision and to late-orogenic magmatism in the eastern Mediterranean region. Subduction-related magmatism is rooted to closure of the Neo-Tethyan Ocean whereas post-collision and late orogenic-within plate-related magmatism is driven by the collision of a northern promontory of the SE Anatolian orogenic belt with northerly derived ophiolitic rocks. The magmatic transition occurs regionally in northerly to northwesterly trending belts in the southeastern Anatolian orogenic belt. The magmatism exhibit a clear shift from deep seated arc-type to late-orogenic from south (Baskil) to more deeply eroded mid-crustal plutons at the north (Divrigi), then to magmatism related to incipient slab-rupture from northeast (Copler, Kabatas, Bizmisen-Calti) to SW (Karamadazi and Horoz). The age progression follows a south-to-north geochemical trend of decreasing crustal input into mantle-derived magmas, and is explained as a consequence of slab roll-back after the collision/obduction of northerly ophiolites followed by slab steepening and incipient rupture leading to transtensional block faulting and subsidence, and thus to the preservation of near-surface magmatic products along a NE trending belt.

Among supracrustal sequences of the Jurassic magmatic arc of the southwestern Cordillera, the Middle Jurassic Topawa Group, Baboquivari Mountains, south-central Arizona, is remarkable for its lithologic diversity and substantial stratigraphic thickness, ≈8 km. The Topawa Group comprises four units (in order of decreasing age): (1) Ali Molina Formation—largely pyroclastic rhyolite with interlayered eolian and fluvial arenite, and overlying conglomerate and sandstone; (2) Pitoikam Formation—conglomerate, sedimentary breccia, and sandstone overlain by interbedded silt-stone and sandstone; (3) Mulberry Wash Formation—rhyolite lava flows, flow breccias, and mass-flow breccias, with intercalated intraformational conglomerate, sedimentary breccia, and sandstone, plus sparse within-plate alkali basalt and comendite in the upper part; and (4) Tinaja Spring Porphyry—intrusive rhyolite. The Mulberry Wash alkali basalt and comendite are genetically unrelated to the dominant calcalkaline rhyolite. U-Pb isotopic analyses of zircon from volcanic and intrusive rocks indicate the Topawa Group, despite its considerable thickness, represents only several million years of Middle Jurassic time, between approximately 170 and 165 Ma. Sedimentary rocks of the Topawa Group record mixing of detritus from a minimum of three sources: a dominant local source of porphyritic silicic volcanic and subvolcanic rocks, identical or similar to those of the Topawa Group itself; Meso-proterozoic or Cambrian conglomerates in central or southeast Arizona, which contributed well-rounded, highly durable, polycyclic quartzite pebbles; and eolian sand fields, related to Middle Jurassic ergs that lay to the north of the magmatic arc and are now preserved on the Colorado Plateau. As the Topawa Group evidently represents only a relatively short interval of time, it does not record long-term evolution of the Jurassic magmatic arc, but rather represents a Middle Jurassic “stratigraphic snapshot” of the arc. This particular view of the arc has been preserved primarily because the Topawa Group accumulated in deep intra-arc basins. These nonmarine basins were fundamentally tectonic and extensional, rather than volcano-tectonic, in origin. Evidence from the Topawa Group supports two previous paleogeographic inferences: the Middle Jurassic magmatic arc in southern Arizona was relatively low standing, and externally derived sediment was introduced into the arc from the continent (northeast) side, without appreciable travel along the arc. We speculate that because the Topawa Group intra-arc basins were deep and rapidly subsiding, they became the locus of a major (though probably intermittent) fluvial system, which flowed into the low-standing magmatic arc from its northeast flank.

Abstract Abstract:Carlin-type Au deposits in Nevada have huge Au endowments that have made the state, and the United States, one of the leading Au producers in the world. Forty years of mining and numerous studies have provided a detailed geologic picture of the deposits, yeta comprehensive and widely accepted genetic model remains elusive. The genesis of the deposits has been difficult to determine owing to difficulties in identifying and analyzing the fine-grained, volumetrically minor, and common ore and gangue minerals, and because of postore weathering and oxidation. In addition, other approximately contemporaneous precious metal deposits have overprinted, or are overprinted by, Carlin-type mineralization. Recent geochronological studies have led to a consensus that the Nevada deposits formed ~42 to 36 m..y ago, and the deposits can now be evaluated in the context of their tectonic setting. Continental rifting and deposition of a passive margin sequence followed by compressional orogenies established a premineral architecture of steeply dipping fluid conduits, shallow, low dipping “traps” and reactive calcareous host rocks. Sedimentary rock sequences that formed following continental margin rifting or in a foreland basin ahead of an advancing thrust front contain reactive pyritic and carbonaceous silty limestones, the primary host rocks in almost every deposit. The largest deposits now lie in the lower plate to the Devonian to Mississippian Roberts Mountain thrust, which placed nonreactive, fine-grained siliciclastic rocks with less inherent rock permeability, above more permeable carbonate stratigraphy, forming a regional aquitard. North-northwest- and west-northwest-striking basement and Paleozoic normal faults were inverted during postrifting compressional events and formed structural culminations (anticlines and domes) that served as depositional sites for auriferous fluids in the Eocene. These culminations are now exposed as erosional windows through the siliciclastic rocks of the Antler allochthon. During the Eocene, northwesterly to westerly extension reopened favorably oriented older structures as strike-slip, oblique-slip, and normal-slips faults. Fluid flow and mineral deposition appear to have been fairly passive as there is minimal evidence for overpressured hydrothermal fluids, complicated multistage vein dila-tancy, or significant synmineralization slip. Geologic reconstructions and fluid inclusions indicate that deposits formed within a few kilometers of the surface. Ore fluids were moderate temperature (~180°-240°C), low salinity (~2-3 wt % NaCl equiv), CO 2 bearing (<4 mol %), and CH 4 poor (<0.4 mol %), with sufficient H2S (10 –1 –10– 2 m) to transport Au. Ore fluids decarbonatized, argillized, and locally silicified wall rocks, and deposited disseminated pyrite containing submicron Au as Fe liberated from wall rock reacted with reduced S in the ore fluid. Isotopic studies indicate multiple sources for ore fluids and components and require either different models for different districts or call upon meteoric waters to overwhelm a deep ore-fluid signal in most districts. Oxygen and H isotope ratios of minerals and fluid inclusions indicate a deep magmatic or metamorphic fluid source at the Getchell deposit; however, most similar studies in other districts have identified meteoric water. A large range in S isotopes in ore pyrite from all districts suggests derivation from a sedimentary source; yet studies at Getchell and a few studies in the northern Carlin trend are consistent with a magmatic S source. As a result of these inconsistencies, current models relate deposits to (1) metal leaching and transport by convecting meteoric water, (2) epizonal intrusions, and (3) deep metamorphic and/or magmatic fluids. With the exception of the isotopic studies, compiled data from all Nevada trends and districts indicate compelling similarities, suggesting that all Nevada Carlin-type deposits formed in response to similar geologic processes. We propose a model in which removal of the Farallon slab promoted deep crustal melting that led to prograde metamorphism and devolatilization, thus generating deep, primitive fluids. Such fluids were likely incorporated in deep crustal melts that rose buoyantly and ultimately exsolved hydrothermal fluids, possibly containing Au. Metamorphism at midcrustal levels may have contributed fluids, all of which were collected into basement-penetrating rift faults, where they continued to rise and scavenge various components, evolving in composition to become ore fluids. North-northwest–trending paleo-normal faults and northeast-trending paleo-transform faults, preferentially dilated during Eocene extension, controlled the regional position, orientation, and alignment of the deposits. Eventually the ore fluids accumulated in areas of reduced mean effective stress, particularly boundaries of older Jurassic and Cretaceous stocks and structural culminations. The ore fluids were diluted by exchanged meteoric water as extension increased fault permeability in the upper crust. Within a few kilometers of the surface, fluids were diverted by structural and stratigraphic aquitards into reactive host rocks, where they sulfidized host rock iron and deposited Au. Sedimentary rock-hosted disseminated Au deposits in other parts of the world exhibit many similarities to Nevada Carlin-type Au deposits, yet no district has been discovered anywhere else that approaches Nevada’s Au productivity. The deposits found in other parts of the world are products of diverse, well-recognized, hydrothermal systems (e.g., low-sulfidation epithermal, porphyry Cu-Mo-Au, reduced intrusion-related epizonal orogenic, and sedimentary exhalative or sedex). Of these, the deposits in southern China are remarkably similar to Nevada Carlin-type deposits and are interpreted to have formed where metamorphic fluids reacted with wall rocks and local meteoric water.

Abstract The Jerónimo sedimentary rock-hosted disseminated Au deposit is located within the Potrerillos district of the Atacama region of northern Chile, east of the Potrerillos porphyry Cu-Mo and El Hueso high-sulfidation Au deposits. Prior to development, the Jerónimo deposit contained a resource of approximately 16.5 million metric tons (Mt) at 6.0 g/t Au. Production began in the oxidized, nonrefractory portion of the deposit in 1997 and terminated in 2002. During that time, approximately 1.5 Mt at 6.8 g/t Au was mined by underground room-and-pillar methods, from which a total of approximately 220,000 oz of Au was recovered by heap-leach cyanidation. Jerónimo mineralization occurs as irregular strata-bound lenses within particular bioclastic limestone units of the Jurassic Asientos Formation. The manto-shaped mineralized zone extends over an area of approximately 2.0 by 1.3 km and averages 6 m in thickness. Mineralization and alteration are focused along subvertical fractures and joints within the bioclastic units. Alteration involved decarbonatization followed by the formation of the following assemblages: (1) intense, pervasive, replacement-style silicification; (2) carbonate, mainly restricted to vugs, consisting of Mn carbonate (rhodochrosite and kutnohorite) in the center of the orebody and calcite-dolomite on the margins; and (3) argillization, consisting of illite as widespread disseminations and veinlets and kaolinite as vug fillings in the center of the deposit. Other common alteration minerals include apatite, rutile, monazite, and barite. The ore mineral suite consists of pyrite, arsenopyrite, sphalerite, lead sulfosalts, orpiment, and realgar, with minor coloradoite, altaite, cinnabar, and cassiterite. Gold is present dominantly as submicron-sized grains, ranging from 140 nm to 1.13 μ m, that are encapsulated in pyrite, arsenopyrite, quartz, and realgar and also occur within vugs in the silicified matrix. Lead isotope results of the main-stage sulfide and sulfosalt minerals ( 206 Pb/ 204 Pb: 18.564–18.644; 207 Pb/ 204 Pb: 15.592–15.662; 208 Pb/ 204 Pb: 38.536–38.638) indicate that lead in the ore fluids was dominantly from a Tertiary magmatic source, with input from a more radiogenic source—igneous Carboniferous to Triassic basement rocks and/or the overlying Jurassic limestone and sandstone. Carbon and oxygen isotope compositions of ore zone rhodochrosite and kutnohorite, ranging from δ 18 O of 16.65 to 22.52 per mil (VSMOW) and δ 13 C of −2.84 to −1.3 per mil (PDB), suggest contributions from both magmatic and Jurassic limestone wall-rock sources. The critical features that define the style of mineralization at Jerónimo include lithological and structural control, enrichment in Au-As-Mn-Zn-Pb-Ag-Hg, silicification and carbonate alteration, the presence of native Au grains, the close spatial association with porphyry and related styles of mineralization, and isotopic evidence for a magmatic contribution to metals and hydrothermal fluids. These characteristics are more similar to carbonate-replacement deposits than to typical Carlin-type sediment-hosted Au deposits. Structural and isotopic data suggest that Jerónimo is late Eocene-early Oligocene in age, but the precise temporal and genetic relationships of Jerónimo to other magmatic-hydrothermal systems in the district are unknown.

Abstract The Cretaceous Tambogrande volcanogenic massive sulfide (VMS) deposits of northwestern Peru represent some of the largest Cu-Zn-Au-Ag bimodal-mafic VMS deposits in the world. There are currently three known deposits each with approximately 100 million metric tons (Mt) of massive pyrite-rich sulfide. The deposits are intimately associated with dacite lava dome complexes and were deposited within steep-sided basins on the sea floor. Reconstructed sea-floor paleogeomorphic models suggest that sulfide deposition was concentrated in the deepest parts of the basins. Sulfide deposition accompanied synvolcanic faulting and episodic dacitic and basaltic eruptions. A series of time-stratigraphic horizons are defined at the TG1 and TG3 deposits and mark stages in the development of the volcanic complex and massive sulfide bodies. There is only limited evidence for replacement of host rocks during formation of the Tambogrande deposits, in contrast to many other large massive sulfide deposits. The deposits at Tambogrande resulted from focused hydrothermal fluid flow along synvolcanic faults with deposition of sulfide minerals within deep and restricted basins. These depressions, the results of the structural and volcanologic setting, acted as efficient traps for sulfide deposition and were also important for the preservation of the sulfide masses as they acted to shield them from submarine oxidation and weathering. Steep basins and episodic bimodal lava eruptions are key geologic attributes of the depositional setting at Tambogrande and may be necessary for the formation of anomalously large VMS deposits in a volcanic-rock–dominated environment.

Full article available in PDF version.

Full article available in PDF version.

Full article available in PDF version.

Abstract Fluid pathways between metal sources and sites of ore deposition in hydrothermal systems are governed by fluid pressure gradients, buoyancy effects, and the permeability distribution. Structural controls on ore formation in many epigenetic systems derive largely from the role that deformation processes and fluid pressures play in generating and maintaining permeability within active faults, shear zones, associated fracture networks, and various other structures at all crustal levels. In hydrothermal systems with low intergranular porosity, pore connectivity is low, and fluid flow is typically controlled by fracture permeability. Deformation-induced fractures develop on scales from microns to greater than hundreds of meters. Because mineral sealing of fractures can be rapid relative to the lifetimes of hydrothermal systems, sustained fluid flow occurs only in active structures where permeability is repeatedly renewed. In the brittle upper crust, deformation-induced permeability is associated with macroscopic fracture arrays and damage products produced in episodically slipping (seismogenic) and aseismically creeping faults, growing folds, and related structures. In the more ductile mid- to lower crust, permeability enhancement is associated with grain-scale dilatancy (especially in active shear zones), as well as with macroscopic hydraulic fracture arrays. Below the seismic–aseismic transition, steady state creep leads to steady state permeability and continuous fluid flow in actively deforming structures. In contrast, in the seismogenic regime, large cyclic changes in permeability lead to episodic fluid flow in faults and associated fractures. The geometry and distribution of fracture permeability is controlled fundamentally by stress and fluid pressure states, but may also be influenced by preexisting mechanical anisotropies in the rock mass. Fracture growth is favored in high pore fluid factor regimes, which develop especially where fluids discharge from faults or shear zones beneath low-permeability flow barriers. Flow localization within faults and shear zones occurs in areas of highest fracture aperture and fracture density, such as damage zones associated with fault jogs, bends, and splays. Positive feedback between deformation, fluid flow, and fluid pressure promotes fluid-driven growth of hydraulically linked networks of faults, fractures, and shear zones. Evolution of fluid pathways on scales linking fluid reservoirs and ore deposits is influenced by the relative proportions of backbone, dangling, and isolated structures in the network. Modeling of the growth of networks indicates that fracture systems reach the percolation threshold at low bulk strains. Just above the percolation threshold, flow is concentrated along a small proportion of the total fracture population, and favors localized ore deposition. At higher strains, flow is distributed more widely throughout the fracture population and, accordingly, the potential for localized, high-grade ore deposition may be reduced.

Abstract Fault motion in the upper continental crust is accommodated principally by earthquake rupturing within a seismogenic zone whose base, depending on composition, generally lies in the 300° to 450°C temperature range. Rupture initiation, propagation, and termination within this zone are affected by structural and rheological irregularities. Sawtooth accumulation and release of shear stress on the seis-mogenic structures leads to cycling of both shear and mean stress (affecting fluid content) throughout the surrounding rock mass, with significant fluid redistribution throughout the aftershock phase following large earthquakes. Structural permeability in such regions is intrinsically dynamic: episodic creation of permeability accompanying seismic slip and fracturing is counteracted by the development of low-permeability fault gouge and hydrothermal cementation, so that flow systems are modulated by intercoupled stress and permeability cycling. Because criteria for all modes of brittle failure and fault reshear depend on fluid pressure as well as tectonic stress, a variety of mechanisms may link fluid redistribution to episodic faulting and fracturing. Stress changes accompanying large-scale rupturing on established faults redistribute fluids through subsidiary fracture networks during aftershock periods, but packages of over-pressured fluid migrating through stressed crust may also create new structural permeability by distributed brittle failure, generating earthquake swarms. The fluid pressure state at different crustal levels is critical to the formation and preservation of void space. Fluid overpressuring above hydrostatic values is generally easier to sustain in compressional tectonic regimes, but maximum sustainable overpressure in any particular setting depends not only on the intrinsic permeability of the rock mass but also on the tectonic stress state and existing fault architecture. Large-scale hydrothermal flow through low-permeability rocks is often channeled within dilatant mesh structures of interlinked shear and extension fractures. These fault-fracture meshes can form and reactivate only under low effective stress (o 3 ’ <0, or P f > o 3 ) in the absence of throughgoing low-cohesion faults that are well oriented for frictional reactivation. High-flux flow of this kind can, therefore, occur only under special structural circumstances. in extensional-transtensional tectonic regimes, dilatant meshes can be maintained under hydrostatic fluid pressures in the shallow crust to depths dependent on rock tensile strength, defining the epizonal environment for mineralization. However, at all depths within compressional-transpressional regimes, development of fault-fracture meshes involves hydrothermal fluids overpressured to near-lithostatic values. in particular, mesozonal lode mineralization requires the accumulation and intermittent high-flux discharge of strongly overpressured fluids in the midcrust, most commonly around the base of the continental seismogenic zone. important precipitation mechanisms linked to intermittent seismic slip include the suction-pump mechanism arising from rapid slip transfer across dilational fault jogs and bends, and various forms of fault-valve action where ruptures transect boundaries to overpressured portions of the crust. These mechanisms induce abrupt localized reduction in fluid pressure at specific structural sites, triggering phase separation and hydrothermal precipitation throughout the postseismic period of readjustment (aftershock phase). However, renewal of fault-fracture permeability may also lead to episodic mixing of fluids derived from different sources. For example, each fault-valve discharge may promote precipitation through the mixing of originally deep, hot, overpressured fluids of metamorphic and/or magmatic origin with colder fluids circulating in the near-surface hydrostatic regime. Regional episodes of fluid redistribution are likely to accompany major tectonic transitions (e.g., tectonic inversion) because of changes in the stress state and sustainable levels of fluid overpressure, and an inherited architecture of faults poorly oriented for slip in the new stress field. Evidence of structural channeling in such settings reveals interesting comparisons between the flow paths of hydrothermal and hydrocarbon fluids.

Abstract Consideration of the role of rock property variations is crucial in any analysis of the effects of deformation on fluid flow and mineralization. An empirical analysis of any mineralized terrain should consider this factor, in addition to those used in any other analysis of geometry and kinematics, such as orientation, evolution of the stress and strain fields, and the known distribution of veins, shear zones, breccias, and alteration. The conceptual models that arise from such an analysis can be enhanced by computer models. The models shown here are finite difference models that simulate fluid flow in deforming rock masses, one for fluid flow along predefined rock boundaries (Universal Distinct Element Code, UDEC), and another for fluid flow through deforming porous media (Fast Lagrangian Analysis of Continua, FLAC). UDEC modeling of the perturbed stress field around stronger, lower permeability meta-intrusive rocks in the Mary Kathleen district and the Hilton mine of the Mount Isa district, northwest Queensland, reproduces the observed location of the most intense veining and alteration. FLAC modeling of the Mary Kathleen U-REE orebody reproduces the location and geometry of ore shoots and provides an explanation for focusing of regional fluid towards the ore deposition sites. FLAC models of the giant Mount Isa copper deposit reveal that the effect of the rheological heterogeneity on fluid flow and solute transport is amplified if consideration is made of whether or not the rocks are contractant or dilatant, with increasing strain. Multiple working hypotheses can be evaluated quickly by such modeling; therefore, the models can be used in exploration and orebody extension studies. Furthermore, it is suggested from our work that the size and spacing of epigenetic, structurally controlled ore deposits is related in a fairly systematic way to the scale and degree of rock property variations, at least for a given strain history. For giant ore deposits to form, it requires that the gradients in pore pressure generated at local scales by heterogeneous rock packages must be subordinate to those operating at broader scales.

Abstract Interacting fractures enhance and localize permeability in the Earth's crust and are, therefore, important phenomena in localizing magmatic and hydrothermal systems. The ability to identify where such interactions are present is useful in evaluating likely areas of mineralized rock, particularly in covered terrains. Regardless of map scale, the interpretation of gravity and magnetic data can define deep-seated crustal fractures and faults that may have guided emplacement of igneous rocks and large ore deposits. Here we emphasize recurring regional-scale structural relationships mainly from the western United States based on the interpretation of potential-field data, which can elucidate areas of past and present fluid flow in the crust. In particular, we explore the utility of regional gravity and magnetic data to aid in understanding the distribution of large Mesozoic and Cenozoic ore deposits (primarily epithermal and pluton-related precious and base metal deposits, and sediment-hosted gold deposits) in the western United States cordillera. On the broadest scale, most ore deposits lie within areas characterized by low magnetization. The Mesozoic Mother Lode gold belt displays characteristic geophysical signatures (regional gravity high, regional low-to-moderate background magnetic field anomaly, long curvilinear magnetic highs) that might serve as an exploration guide. Geophysical lineaments characterize the Idaho-Montana porphyry belt and the La Caridad-Mineral Park belt (from northern Mexico to western Arizona) and, thus, indicate deep-seated control for these mineral belts. At a more local scale, in Nevada, geophysical data define deep-rooted faults and magmatic zones that correspond to patterns of epithermal precious-metal deposits, and that may relate to the Carlin gold trend and the Battle Mountain-Eureka mineral belt. One recurring structural model evolving from this study is that mineralization in the western United States may be localized along strike-slip fault zones where pull-apart basins or releasing bends provided the increased fracture permeability for the migrating ore-forming fluids (e.g., the Butte, Tombstone, Bagdad, and Battle Mountain districts). Many deposits discussed in the paper appear, at least in part, to be associated with reactivated older faults as well as with faulting contemporaneous with ore deposition. We conclude that at a local scale, structural elements work together to localize mineral deposits within regional zones or belts. Perhaps the greatest utility of regional geophysical data is the identification of structural relationships that help narrow the study area, where more intensive multidisciplinary team studies can be carried out in a concerted effort to evaluate the mineral potential.

Abstract Veins are common components of greenstone gold deposits. Their analysis is one key aspect in understanding the sequence of events leading to the formation or deformation of gold deposits. This analysis is essential for the determination of controls on mineralization and ore-forming processes, and for the prediction of the geometry and plunges of deposits and orebodies. Many greenstone gold districts have experienced a common structural evolution: D 1 thin skin-style shortening and D 2 thick skin-style shortening are largely responsible for the structural trend and penetrative fabrics in a district, whereas D 3 and D 4 transcurrent deformation are largely focused along preexisting major fault zones. A majority of greenstone gold deposits consists of quartz-carbonate veins in or adjacent to high-angle reverse, and less commonly transcurrent, shear zones, viewed as splays or subsidiaries of major, complex, belt-scale fault zones. In other deposits, veins simply overprint gold mineralization and provide important information about the postore deformation history. Three main types of veins occur in greenstone gold deposits and each records small increments of bulk strain. Laminated fault-fill veins form by slip along the central parts of active shear zones in low-angle di-lational bends, or less commonly by extensional opening of foliation planes. Extensional and oblique-extension veins form within or adjacent to shear zones, at high angles to foliation and elongation lineation. They represent opening and filling of extensional and hybrid extensional-shear fractures, respectively. In more competent host rocks, extensional veins can form arrays of en echelon planar or sigmoidal veins, or of stacked planar veins, and can also combine into multiple sets to form stockwork and breccia bodies. Multiple types and sets of auriferous veins commonly combine to form variably complex vein networks, especially in large deposits. These vein networks record deposit-scale bulk incremental strain, with axes of elongation and shortening that can be compared with those of the main deformation increments in the district as a further way of constraining their timing of formation. The formation of vein networks in many districts is compatible with D 2 , and in a number of others with D 3 , reflecting their formation in contractional or transcurrent deformation regimes, likely involving subhorizontal compressional stress under high fluid pressures. Veins in many districts also systematically display evidence of overprinting deformation, in the form of folds, boudins, striated vein margins, and a number of internal vein textures such as recrystallized quartz and stylolites. Overprinting deformation is a natural consequence of vein formation in active shear zones, but it can also result from overprinting of veins by a younger increment of regional deformation. This can lead to local shear zone reactivation or wholesale folding or boudinage of a deposit. The confident determination of the structural timing of veins in deposits is critical but challenging, and is at the center of divergences of interpretation of the origin of many greenstone gold deposits. A number of guidelines are offered to help distinguish pre-orogenic veins and deposits from those with syn- to postorogenic timing.

Abstract Porphyry Cu ± Mo ± Au deposits require the coincidence and positive interaction of a series of individually commonplace geological processes. They, and all their genetically associated deposits, are a natural consequence of convergent margin magmatism, and reflect the dynamic interplay between magmatic, hy-drothermal, and tectonic processes. Magmas generated during subduction rise into the upper crust, commonly along zones of lithospheric weakness, where they pond in tabular magma chambers at depths of 6 km or deeper. The chambers grow laterally by chamber floor depression (cantilever mechanism) and some roof lifting (piston mechanism). Apophyses rise from the parental magma chamber and intrude to within 1 to 3 km of the surface, where they may undergo volatile exsolution and crystallization as por-phyritic stocks. Emplacement of porphyry stocks is facilitated by structural anisotropy in the roof rocks. Ascending hydrothermal fluids exsolved from the porphyry stocks and the underlying parental magma chamber are focused into the cupola, taking advantage of vertical structural and rheological anisotropies introduced either before or during porphyry emplacement. From a structural standpoint, three recurrent processes enhance permeability in the form of fracture or breccia networks through which hydrothermal fluids pass and precipitate minerals. Fracture-producing events are related to intrusion of pre-, syn-, and post-mineral porphyry stocks or dikes to near-surface depths (1-3 km), phase separation and volume expansion of a hydrothermal fluid through a variety of mechanisms, and tectonically induced failure. Concentric and radial fracture patterns reflect magmatic processes whereas more linear arrays of veins reflect tectonic influences. The resulting different vein arrays are commonly vertically and temporally distributed in the porphyry system; concentric and radial arrays are more common above or in the upper parts of the stocks, whereas linear arrays dominate at depth, forming as the system cools and the pluton solidifies. Orthogonal and conjugate arrays of veins characterize all scales and all parts of porphyry systems. Veins from a particular paragenetic stage do not have unique orientations, but rather occur with all orientations typical of that system. The common conjugate to orthogonal inter-vein relationships in porphyry Cu deposits requires repetitive exchange of principal stress orientations, events that are facilitated by conditions of low differential horizontal stress. Such stress conditions indicate that many porphyry Cu deposits form in specific environments where the magmatic arc is under a near-neutral stress state. These conditions occur either in areas removed from active deformation, or during periods of stress relaxation and low strain in the magmatic arc. Achievement of these conditions in time and space is likely to be infrequent and transitory during the life of a convergent margin, which may explain the spatial and temporal clustering of deposits in large porphyry districts.

Abstract Fluid pathways between metal sources and sites of ore deposition in hydrothermal systems are governed by fluid pressure gradients, buoyancy effects, and the permeability distribution. Structural controls on ore formation in many epigenetic systems derive largely from the role that deformation processes and fluid pressures play in generating and maintaining permeability within active faults, shear zones, associated fracture networks, and various other structures at all crustal levels. In hydrothermal systems with low intergranular porosity, pore connectivity is low, and fluid flow is typically controlled by fracture permeability. Deformation-induced fractures develop on scales from microns to greater than hundreds of meters. Because mineral sealing of fractures can be rapid relative to the lifetimes of hydrothermal systems, sustained fluid flow occurs only in active structures where permeability is repeatedly renewed. In the brittle upper crust, deformation-induced permeability is associated with macroscopic fracture arrays and damage products produced in episodically slipping (seismogenic) and aseismically creeping faults, growing folds, and related structures. In the more ductile mid- to lower crust, permeability enhancement is associated with grain-scale dilatancy (especially in active shear zones), as well as with macroscopic hydraulic fracture arrays. Below the seismic–aseismic transition, steady state creep leads to steady state permeability and continuous fluid flow in actively deforming structures. In contrast, in the seismogenic regime, large cyclic changes in permeability lead to episodic fluid flow in faults and associated fractures. The geometry and distribution of fracture permeability is controlled fundamentally by stress and fluid pressure states, but may also be influenced by preexisting mechanical anisotropies in the rock mass. Fracture growth is favored in high pore fluid factor regimes, which develop especially where fluids discharge from faults or shear zones beneath low-permeability flow barriers. Flow localization within faults and shear zones occurs in areas of highest fracture aperture and fracture density, such as damage zones associated with fault jogs, bends, and splays. Positive feedback between deformation, fluid flow, and fluid pressure promotes fluid-driven growth of hydraulically linked networks of faults, fractures, and shear zones. Evolution of fluid pathways on scales linking fluid reservoirs and ore deposits is influenced by the relative proportions of backbone, dangling, and isolated structures in the network. Modeling of the growth of networks indicates that fracture systems reach the percolation threshold at low bulk strains. Just above the percolation threshold, flow is concentrated along a small proportion of the total fracture population, and favors localized ore deposition. At higher strains, flow is distributed more widely throughout the fracture population and, accordingly, the potential for localized, high-grade ore deposition may be reduced.

Abstract Lead (Pb) isotope compositions of sulfide minerals coupled with rocks associated with an ore deposit provide critical constraints on the source of metals and fluid pathways in a fossil hydrothermal system (Heyl et al., 1966; Stacey et al., 1968; Gulson, 1986; Sanford, 1992). Lead isotope compositions of sulfide minerals also provide chronologic information, either absolute or relative, for ore deposition (for example, Carr et al., 1995) and can also be used as an exploration tool during prospect evaluation (Gulson, 1986; Young, 1995). These varied applications of Pb isotopes to achieve an understanding of the ore genesis process are too diverse to be adequately discussed in a single overview chapter. Instead, this chapter focuses attention on what Pb isotopes tell us about (1) the sources of Pb and other metals in ore deposits, (2) the interaction between hydrothermal fluids and wall rocks, (3) the influence of basement rocks and tectonic setting on Pb sources in ore deposits in magmatic arcs, and (4) the application of crustal-scale Pb isotope variations to an understanding of regional controls on ore deposition. Before Pb isotopes pertinent to understanding ore genesis can be examined, we must review some basic principles of Pb isotope geochemistry (Fig. 1). Elegant discussions of U-Th-Pb geochemistry are presented by Doe (1970), Faure (1977), Zartman and Haines (1988), Gariépy and Dupré (1991), and Dickin (1995). The following discussion is simplified from these sources. Three isotopes, 208Pb, 207Pb, and 206Pb, are partly the radiogenic daughter products from the radioactive decay of one isotope of thorium