FAULT-FLUID COMPOSITIONS FROM FLUID-INCLUSION OBSERVATIONS AND SOLUBILITIES OF FRACTURE-SEALING MINERALS

Host-rock chemical alteration and syntectonic veins in and near fault zones are evidence for episodic fracturing and fluid transport during faulting. Alteration minerals, vein fillings, and fluid inclusions may be used to estimate fault-fluid chemistry, temperature, and pressure. Fluid inclusions in thrust faults, reverse faults hosting mesothermal gold deposits, and exhumed footwall rocks of normal faults show that fluid components include NaCl, CO2, CH4 and CaCl2 in addition to H2O. Fluid composition, temperature, and pressure are spatially and tempora lly variable on most faults; a typical fault fluid does not exist, NaC l concentrations in fault fluids vary from 0 to 39 wt.%, CaCl2 concent rations range up to 19 wt.% and CO2 concentrations range up to 32 mole % in fluid inclusions, but some inclusions are present that are 100 mo le% CO2. Homogenization temperature measurements and pressure estimate s confirm that these fluids were trapped at elevated pressure at depth on the faults. In CO2-bearing fault fluids, pressures fluctuated, and a range of CO2 contents indicate effervescence. Varying solution dens ities of NaCl-H2O fluids have been interpreted to result from entrapme nt of fluids in inclusions at constant temperature and varying pressur es. Diverse fluid compositions an present on some faults with similar homogenization temperatures and estimated pressures suggesting similar depths on the faults. Pressure, temperature and fluid composition det ermine the solubilities of fracture-filling minerals calcite and quart z and the formation of alteration minerals that are related to the mec hanical behavior of the rock. Quartz may precipitate as a result of co oling or pressure reduction, but calcite solubility increases with coo ling and decreases with decreased P-CO2. Higher salinities increase so lubilities of calcite and quartz and decrease the pH for equilibrium a mong feldspars, muscovite and solution. Mineral assemblages provide ev idence of depressurization of the fluid as fluid moves from higher- to lower-pressured reservoirs. Precipitation of quartz, calcite, and K-f eldspar or albite in fractures may result from fluid depressurization. Fault-zone rocks containing stilbite and laumontite reacted with flui d that contained little CO2 at comparatively low temperature and press ure; kaolinite, prehnite, muscovite, epidote, and chlorite formed from fluids at higher temperature and pressure. Variations in mineralogy a nd fluid-inclusion characteristics on individual faults suggest separa te fluids that differ in chemical composition, temperature, and pressu re. (C) 1998 Elsevier Science B.V. All rights reserved.