Using Isotopic Alteration Modeling to Explore the Natural State of the Geysers Geothermal System, USA

Abstract

Oxygen isotope alteration of host rocks is a ubiquitous feature of hydrothermal/geothermal systems. This alteration can be used as a quantitative constraint in numerical models of geothermal systems, and is especially useful in determining system-wide pre-development characteristics. Fluid, heat and oxygen isotope transport models of the natural state of The Geysers system were made to better-understand its development and guide its long-term management. The unusual distribution of maximum oxygen isotope alteration low on the flanks of the main intrusive (felsite) at The Geysers requires caprock integrity to have been maintained throughout most of the systemís lifetime. This served to limit hydrothermal circulation and isotopic alteration above the apex of the intrusion. In contrast, many fossil hydrothermal systems exhibit concentrated alteration above the apex. The lateral extensiveness of alteration along the top of the felsite requires good horizontal connectivity of permeable zones at depth to allow penetration of 18O-depleted fluids. A maximum hydrothermal lifetime for the system is 0.5 million years (Ma), while the youngest dated intrusive is around 1.2 Ma. This supports the concept that as-yet concealed younger intrusions have continued to provide thermal input to the system. Petrologic evidence for high paleo-fluid temperatures (300 ?C) within 1 km of the surface are difficult to reconcile with subdued 18O alteration at depth in the same location. These vein occurrences may represent very limited penetration of the caprock during the liquid-dominated phase of The Geysers.. An alternative hypothesis is that boiling occurred early in the development of the system, halting rock isotopic alteration. Internet-accessible animations of model results are available at http://www.utdallas.edu/~brikowi/Publications/Geysers.