EVIDENCE FOR OXYGEN-ISOTOPE DISEQUILIBRIUM IN SELECTED GEOTHERMAL AND HYDROTHERMAL ORE DEPOSIT SYSTEMS

There is ample evidence from geothermal systems that temperatures estimated from the oxygen isotope fractionation between alteration phases and coexisting aquifer fluids agree closely (+/-25 degrees C) with measured bore-hole temperatures. Conversely, many hydrothermal systems exhibit varying degrees of apparent fluid-rock oxygen isotope disequilibrium (measured alpha(rock-water) not equal equilibrium alpha(rock-water)). A comparison of data from a variety of hydrothermal systems indicates that an increase in the degree of isotopic equilibration can be related, in a general way, to an increase in the temperature, salinity, and intensity of fracturing. This relationship is entirely consistent with calculations carried out for a simple closed-system model that estimates the fraction of isotope exchange between rock and fluid as a function of temperature, rock type, salinity of the fluid, grain size, water/rock ratio, and the rate of isotope exchange. The fact that many silicate-fluid reactions and their isotopic counterparts are fast leads us to conclude that observed variations in the degree of exchange may have resulted from local self-sealing of the fracture network prior to equilibration. The ability for fractures to remain open or to propagate, allowing continued fluid flow, may be the deciding factor in the attainment of isotopic equilibration.