Temperatures within the main vapor-dominated steam reservoir at The Geysers geothermal field generally are in the range 238°C to 244°C. A few deep wells in the northwestern part of the field have penetrated beneath this reservoir into a second vapor-dominated reservoir where temperatures are >315°C, while vapor pressure remains nearly constant at about 35.9 bars (Walters et al., 1992). Vapor-dominated reservoirs generally are thought to operate like heat pipes in which steam formed near the base of the system convects upward (along with other gases, such as CC>2 and H2S), while liquid that has condensed from steam near the top of the reservoir counterflows downward (White et al., 1971). To the extent that this steam condensate carries H2S in solution, it may dissolve gold from the surrounding rock during the counterflow. Re-evaporation of the down-flowing condensate and precipitation of dissolved material might occur at the base of the upper reservoir where there is a relatively sharp increase in temperature while vapor pressure remains nearly constant. In addition, brine that once was present throughout the system (Moore, 1992) may have deposited a variety of ore minerals when and where boiling was vigorous during the transition from previous hot water-dominated to present-day vapor-dominated conditions. The investigation reported here was a geochemical reconnaissance survey looking for evidence of accumulation of Au and other metals in the transition zone between the two reservoirs. The petrology of the cuttings was not examined as part of the investigation.
|Title||Trace metals and major and rare earth elements in cuttings from five high-temperature wells in the northwest region of The Geysers, California, vapor-dominated geothermal system|
|Authors||Robert O. Fournier, Joseph N. Moore|
|Publication Subtype||USGS Numbered Series|
|Series Title||Open-File Report|
|Record Source||USGS Publications Warehouse|
|USGS Organization||Volcano Hazards Program|