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Genetic Links Between Extensional Tectonomagmatism and the Formation of Low Sulfidation Epithermal Au-Ag Deposits: Defining a Mineral System
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AuthorMilliard, Justin Brent
AdvisorMuntean, John L.
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The relationship between the geologic processes of extensional tectonics, magmatism, and hydrothermal activity that controls when and where low sulfidation epithermal deposits form is not well established. However, many researchers have noted the similar characteristics of deposits suggests a common theme for the principle processes dictating their formation. To determine the principle processes influencing deposit formation, this study focuses on the formation of a genetically related series of low sulfidation epithermal deposits that coincide in space and time with the formation of the northern Nevada rift. The northern Nevada rift is an ancient rift system, the predecessor to the modern Basin and Range, composed of extensional basins formed along a common long axis that coevally was filled by volcanics and sediments during the Late-Eocene and Mid-Miocene. The processes by which the northern Nevada rift’s development influenced when and where low sulfidation epithermal deposits formed has not been established, although a simple model has been proposed for Mule Canyon. Questions being addressed include: (1) Are there common structural frameworks controlling deposit location within an extensional setting? (2) Does magmatic or tectonic activity influence deposit formation? (3) Does basin architecture influence hydrothermal fluid pathways? This project documents the structural evolution of discrete portions of segmented basins where exposure or mining data allows. By understanding the kinematics of individual project areas, through the integration of multiple scales of kinematic observations, a cohesive regional model is constructed for the basin’s architectural evolution. Field relationships combined with analytical data (i.e., kinematic, forward, and fault geometry modeling, and microscopy of syntectonic vein textures, SEM-CL, & SEM-BSE, and 40Ar/39Ar dating) indicate the Fire Creek mine and Mule Canyon mine formed during a transition in basin architecture resultant of asymmetric strain partitioning across the rift. Field mapping indicates the deposits formed in analogous structural settings, at the apex of rider blocks where strain partitioning was transiently focused during a discrete stage of basin evolution. Mineralization is hosted within secondary, west-dipping faults that are antithetic to principal, east-dipping, basin-forming faults on the rift’s western shoulder. The secondary faults served to form a pathway short-circuiting the buoyant ascent of the hydrothermal fluid through the upper crust.Further, the use of 2D kinematic forward modeling indicates that the vertical propagation of growth faults during fault rupture creates and maintains structural permeability immediately prior to or during the ascent of hydrothermal fluids. Results of the 2D kinematic model are supported by field observations of vein material emplaced during micro-slip events on small-scale structures, manifested as growth like geometries of colloform quartz bands. Analogous spatiotemporal relationships between seismotectonics and fluid flow is well-documented in seismologic and hydrologic literature. However, this research presents the first well-constrained evidence in the geologic record relating low sulfidation epithermal deposit formation to coeval fault rupturing at multiple scales. Cross-cutting relationships, observed at the basin scale, deposit scale, and individual vein scale, suggests mineralizing events were influenced by fault rupture events. These multi-scale relationships indicate that deposit formation is genetically linked to tectonomagmatic activity. In addition, observed sluice-box and fractal dendrite textures along with isotopic analysis of 65Cu in electrum and Pb isotopic compositions in electrum, adularia, and basalt suggest a component of Au-Ag may have been transported via nanoparticles from deep, mafic magmatic sources. An interpretation supported by the coeval eruption of the Grande Ronde basalts on the same lithosphere scale architecture, which account for the vast volumetric majority of the Columbia River flood basalts and record open communication between the mantle and Earth’s surface. The rapid ascent of these nanoparticles is interpreted to be facilitated by the transient, coseismic enhancement of structural permeability by large rupture events on faults hydrologically linked to lithosphere scale architecture.The evolutionary process of architectural development leading to basin-forming faults capable of producing bonanza deposits is a multi-stage process. The process initiates as a broad zone of deformation characterized by low displacement faults and equally partitioned strain. Eventually, the strain is partitioned asymmetrically as structures coalesce, increasing the faults dimensions as greater degrees of regional strain are partitioned onto individual basin-forming faults. The ongoing growth of basin-forming faults eventually leads to fault dimensions (length and width) capable of rupturing through the brittle-crust. At this scale, seismically-enhanced permeability enables the transient ascent of deeply sourced, magmatic fluids to be introduced into the epithermal environment by exploiting lithosphere scale permeability pathways. These unique conditions signal the establishment of a self-organizing critical system, where the critical elements of lithosphere architecture, fertility and geodynamic processes overlap to form an ephemeral mineralizing system. Prior to reaching these suitable fault dimensions, it is possible and very likely that shallow, circulating meteoric and/or hydrothermal waters will be compartmentalized and channelized by adolescent faults to form low sulfidation epithermal veins. Albeit, these fluids will lack the critical component of magmatic water containing elevated concentrations of precious metals that is necessary to contribute to the high precious metal concentrations required to form a deposit. Thus, results from this study demonstrate the existence of a genetic link between the geologic processes of extensional tectonics, magmatism, and hydrothermal activity that is necessary to form low sulfidation epithermal deposits. New knowledge and perspectives gained through this study provide geologic reasoning to determine the geologic framework and history that is ideal to form a low sulfidation epithermal deposit.