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 The Barrick Golden Sunlight Mine (GSM) in Whitehall, Montana, is an industry leader in safe, responsible resource extraction. With more than 3 million ounces of gold poured since 1983, and current proven and probable reserves of 318,000 ounces of gold, GSM is the largest gold producer in Montana. The gold-silver deposit is localized in a hydrothermal breccia pipe related to Late Cretaceous latite porphyry magmatism hosted by the Mesoproterozoic Belt Supergroup, and is influenced by younger cross-cutting faults and fracture systems. The deposit has been mined by both underground and open pit methods, and the current open pit operation was recently permitted for expansion. The mill and tailings operations practice efficient and environmentally responsible resource recovery by processing ore from historical tailings and dumps from around the state in addition to ore from the Golden Sunlight property. This trip will explore the complex geologic and tectonic controls on mineralization and review how GSM has addressed the technical challenges of mining, milling, and reclamation.

Abstract The Anaconda and Bitterroot metamorphic core complexes are located in western Montana, along the eastern edge of the Cordilleran hinterland. This multi-tiered extensional terrain contains exceptional exposures that collectively exhibit a crustal cross section through orogenic continental crust (i.e., middle through upper crust). The core complex footwall rocks consist of Late Cretaceous arc-related plutons and Eocene granitic plutons intruded into deformed and metamorphosed Midproterozoic Belt Supergroup and Paleozoic to Cretaceous shelf-platform strata. Late Cretaceous shear zones and folds dominate footwall structure, representing significant thinning of the stratigraphic section. Eocene detachments, mylonites, and plutonic suites distinctly overprint the Late Cretaceous structures. A stark example of this Eocene overprint is the Anaconda detachment, which resulted in eastward translation of the Late Cretaceous, arc-related Boulder batholith. This field trip will cover a transect through the Anaconda core complex from the Philipsburg valley to Butte, Montana. Field trip participants will examine key locations that clarify the distinction between the timing and structural style of Late Cretaceous crustal thickening and/or collapse features versus those related to Eocene core complex development.

The late Eocene to early Miocene Renova Formation records initial post-Laramide sediment accumulation in the intermontane basin province of southwest Montana. Recent studies that postulate deposition of the Renova Formation were restricted to a broad, low-relief, tectonically quiescent basin on the eastern shoulder of an active rift zone vastly differ from traditional models in which the Renova Formation was deposited in individual intermontane basins separated by basin-bounding uplands. This study utilizes detrital zircon geochronology to resolve the paleogeography of the Renova Formation. Detrital zircon was selected as a detrital tracer that can be used to differentiate between multiple potential sources of similar mineralogy but with distinctly different U-Pb ages. Laser ablation-multicollector-inductively coupled plasma mass spectrometry (LA-MC-ICPMS) U-Pb detrital zircon ages were determined for 11 sandstones from the Eocene-Oligocene Renova Formation exposed in the Sage Creek, Beaverhead, Frying Pan, Upper Jefferson, Melrose, and Divide basins. Detrital zircon ages, lithofacies, paleoflow, and petrography indicate that provenance of the Renova Formation includes Paleogene volcanics (Dillon volcanics and Lowland Creek volcanics), Late Cretaceous igneous intrusions (Boulder batholith, Pioneer batholith, McCartney Mountain pluton), Mesozoic strata (Blackleaf Formation, Beaverhead Group), Belt Supergroup strata, and Archean basement. The oldest deposits of the Renova are assigned Bridgerian to Uintan North American Land Mammal (NALM) ages and contain detrital zircons derived from volcanic, sedimentary, and metamorphic rocks constituting the “cover strata” to uplift-cored Late Cretaceous plutonic bodies. Regional unroofing trends are manifested by a decreased percentage of cover strata–sourced zircon and an increased percentage of pluton-sourced zircon as Renova deposits became younger. Zircon derived from Late Cretaceous plutonic bodies indicate that initial unroofing of the McCartney Mountain pluton, Pioneer batholith, and Boulder batholith occurred during Duchesnean time. Facies assemblages, including alluvial fan, trunk fluvial, and paludal-lacustrine lithofacies, are integrated with detrital zircon populations to reveal a complex Paleogene paleotopography in the study area. The “Renova basin” was dissected by paleo-uplands that shed detritus into individual intervening basins. Areas of paleo-relief include ancestral expressions of the Pioneer Range, McCartney Mountain, Boulder batholith–Highland Range, and Tobacco Root Range. First-order alluvial distributary systems fed sediment to two noncontiguous regional-trunk fluvial systems during the Chadronian. A “Western fluvial system” drained the area west of the Boulder batholith, and an “Eastern fluvial system” drained the area east of the Boulder batholith. Chadronian paleodrainages parallel the regional Sevier-Laramide structural grain and may exhibit possible inheritance from Late Cretaceous fluvial systems. Detrital zircons of the Renova Formation can be confidently attributed to local sources exposed in highlands that bound the Divide, Melrose, Beaverhead, Frying Pan, Upper Jefferson, and Sage Creek basins. The data presented in this study do not require an Idaho batholith provenance for the Renova Formation.