Pore-fluid chemistry and chemical reactions on the Wasatch normal fault, Utah

Abstract

Abstract Mineral assemblages and fluid inclusions (F.I.) in hydrothermally altered and tectonically deformed Oligocene quartz monzonite in the footwall of the active Wasatch normal fault, Utah have been used to estimate fluid pressure, temperature, chemical composition, and chemical reactions associated with progressive displacement of the fault. Vein filling and pervasive alteration mineralogy includes (earliest to latest) biotite-K-feldspar, chlorite-epidote-sericite, and laumontite-prehnite-clay. Secondary F.I. in quartz, associated with chlorite-epidote-sericite alteration, consist of CO 2 and salt solution; the homogenization temperature mode is 285°C, X CO 2 is 0.03–0.32, and salinity ranges from 4.5–17.3 wt.% NaCl equivalent; estimated minimum entrapment pressures vary from 600–2950 bars. The homogenization temperature mode of secondary F.I. associated with the laumontite-prehnite assemblage is 100°C, salinity ranges from 2.0–16.0 wt.% NaCl equivalent, and no CO 2 was detected; entrapment pressures are presumed to have been hydrostatic and below 460 bars. Fluid pressure and temperature evolved along a path from lithostatic to hydrostatic with continued displacement of the footwall relative to the hanging wall. Age constraints are provided by a 17.6 ± 0.6 m.y. K-Ar age of hydrothermal seriate from a sample with mean T h of 309°C, and 7.3 to 9.6 m.y. fission track ages of apatite (closure temperature 120 ± 15° C ). Calculated equilibrium phase diagrams illustrate the systematic variation in cation activities, CO 2 fugacity, and alteration minerals as fluid temperature and pressure decrease. The ratios of a Ca 2+ (a H + ) 2 and a Mg 2+ (a H + ) 2 increase and the ratio a K + / a H + decreases by more than two orders of magnitude each as temperature decreases and alteration mineralogy changes from K-feldspar and muscovite to muscovite and clay. The hot aqueous fluids affect the mechanical behavior of the rock during faulting. Crack sealing by alteration product minerals reduces permeability, and the effective stress is decreased by pore fluid pressures substantially greater than hydrostatic in the early alteration.