STRUCTURE AND HYDROTHERMAL ALTERATION OF THE DIAMOND JOE STOCK, MOHAVE COUNTY, ARIZONA (FELDSPAR, QUARTZ, SERICITE).

Abstract

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.