Deep fluid circulation and isotopic alteration in The Geysers geothermal system: profile models

Rock alteration patterns related to the large felsic intrusive complex (felsite) beneath The Geysers steam field (California, USA) are important indicators of the origins of the modern geothermal system. Metagreywacke host rocks for the system show widespread moderate oxygen isotope alteration in the permeable steam reservoir above the felsite, with concentrated alteration low on the flanks of the intrusion. Numerical models of fluid, heat and oxygen isotope transport in the pre-development (natural state) Geysers system demonstrate that an unbroken caprock is required throughout the liquid-dominated lifetime of the system to produce this pattern. The widespread moderate alteration throughout the steam reservoir suggests a long-lived liquid system, and probably required upwardly increasing permeability in the steam reservoir. The models indicate that the maximum hydrothermal lifetime for the system is 0.5 million years (Myr), whereas the youngest dated large intrusive is 1.2 Myr. Combined, these factors indicate repeated igneous intrusions at The Geysers, up to at least 0.5–0.6 Myr ago, and development of a stable liquid-dominated system after that, the evolution of which was truncated by a relatively recent transition to vapor-dominated conditions. Observed chemical compartmentalization of fluids in The Geysers steam reservoir is inconsistent with the lateral extensiveness of alteration at depth, since the latter requires good horizontal connectivity of deep permeable zones to allow penetration of 18O-depleted fluids. This compartmentalization is probably recent, developing as a consequence of vapor-dominated conditions introducing relative permeability effects. Petrologic evidence for high paleo-fluid temperatures (300°C) within 1 km of the surface is difficult to reconcile with subdued 18O alteration beneath these locations. These peak paleotemperatures are likely to indicate small, short-lived penetrations of the caprock. Natural-state models allow the combined influence of these factors on the evolution of The Geysers to be analyzed quantitatively. Internet-accessible graphical animations of these results are available at http://www.utdallas.edu/not, vert, similarbrikowi/Research/Geysers.