The origin of fractures in plutons has been ascribed to differential stress resulting from regional tectonics, magmatic emplacement, crystallization, and cooling. The evolution of stress, and hence the evolution of fractures in plutons, controls the timing and spatial distribution of fluid flow and alteration. Quantitative data on fracture orientation, density, vein width, and alteration systematically obtained from the 70-m.y.-old Diamond Joe stock, west-central Arizona, were used to determine the evolution of stress and fractures within the stock during its crystallization. The dome-shaped chamber of the predominantly quartz monzonite stock is 8 km in diameter. Regional east-northeast compression produced the north-northwest crustal dilation necessary for the ascent and emplacement of the stock. Two-dimensional models using analytic solutions for stress in elastic media indicate that the prominent radial fractures within the stock developed in response to magma pressure, contraction, and regional stress. Most fractures formed by tensile failure during cooling, although magma pressure led to shear failure near the center. Deflection of radial fractures away from the east-northeast axis of the pluton at increasing distances from the center indicates north-northwest and east-northeast orientation of regional maximum and minimum principal stress, respectively, during crystallization. Apparently, north-northwest-trending uplift in the vicinity of the pluton led to a shallow local reversal of principal stress after magma emplacement. Theoretical strain estimates show a correlation with high fracture densities and abundant alteration along the north-northwest axis and the pluton margins. Zones of fracture selvage K-feldspar+quartz, muscovite+K-feldspar+quartz, and muscovite+chlorite+quartz alteration are concentric about the center of the stock and extend a short distance into the surrounding host rocks. Younger sericite+K-feldspar, argillic, and carbonate alteration occurs locally. Mineral equilibria and fluid inclusion data indicate low hydrothermal temperatures ( < 150°C) near the center of the stock and higher temperatures (200°C-400°C) near the margins. Apparently, fractures continued to open and fill as cooling proceeded within the center of the stock, whereas fluid circulation ceased at higher temperatures near the margins.

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Red Butte is located 60 km south of Vale, Oregon, about 20 km west of the Oregon/Idaho border. The butte is within the Owyhee Upland physiographic province of eastern Oregon which lies at the intersection of the Western Snake River Plain, the High Lava Plains, and the Northern Basin and Range provinces. The butte is composed of Miocene to Pliocene lacustrine and fluvial volcaniclastic sediments. The topography of the butte is controlled by silicification of the sandstones and mudstones which cap it. Silicification and hydrothermal alteration are both structurally and stratigraphically controlled. North-trending normal faults dominate the area, and show progressively less offset in younger units. Strong northwest and minor northeast faults also cut the area. Anomalous concentrations of Au, As, Sb, and Hg occur in banded quartz, quartz-adularia veins and rarely in calcite-quartz veins cutting basalts. The silicified mudstones at the butte top are generally barren. An hydrothermal explosion crater exposed on the southeast side of the butte lies at the intersection of north and northwest-trending faults. Banded quartz veins, quartz-adularia veins, and quartz-cemented breccias are exposed in the walls of the explosion crater. There is evidence of multiple brecciation events. A few mudstones exposed in the crater wall have an exhalative texture, contain pyrite des seminated along bedding, and contain trace amounts of Au. The Red Butte system developed as a hot spring venting into a lake, possibly within a caldera. Periodic influxes of air fall tuffs and coarser clastic sediments diluted accumulating fine grained clastic and chemical sediments. As hot, silica saturated fluids rose into cooler lake waters they were cooled and diluted, resulting in silica flocculation near the vents and silicification of the lake sediments as they were deposited. Faults cutting the butte acted as conduits for the hydrothermal fluids. Overpressuring, caused by silica sealing of open fractures, resulted in explosions, generally followed by resealing of the system. Simultaneous faulting, sedimentation, and hot spring activity occurred at different rates throughout the life of the system. Late in the life of the system, the local water table dropped, and the lake was replaced by a high-energy fluvial system. As the water table dropped below the surface, zones of low pH alteration and bleaching were developed above the hydrothermal vents, implying boiling at depth. The large explosion crater and bedded explosion breccia on the southeast side of the butte were formed in these last stages of hydrothermal activity.

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Typescript. Includes vita and abstract. Includes bibliographical references (leaves 155-177). Also available online in Scholars' Bank; and in ProQuest, free to University of Oregon users.