FLUID EVOLUTION IN THE GEYSERS GEOTHERMAL SYSTEM, CALIFORNIA: EVIDENCE FROM FLUID INCLUSION AND SECONDARY ION MASS SPECTROMETRY (SIMS) OXYGEN ISOTOPE ANALYSES OF VEIN QUARTZ

The Geysers is the largest geothermal system in the world and one of the few vapor-dominated resources. The evolution of its hydrothermal fluids has been characterized by spatially coupled fluid inclusion and SIMS oxygen isotope analyses of multiple generations of vein quartz from samples that traverse the deep portions of the system. The heat source is a shallow (>700 m depth), large (>300 km³) and young (2.5 to 0.05 Ma) granitic pluton.

Vein quartz contains two-phase vapor-rich inclusions and liquid-rich inclusions containing up to 13 solid phases at room temperature. Calculated salinities range from 0 to ~47 weight percent NaCl equivalent based on ice-melting or halite dissolution temperatures. Total homogenization temperatures range from 220 to 385^oC. In general, both temperature and salinity decrease with time through successively later generations of vein quartz. The paleo liquid-dominated reservoir was highly stratified with regards to salinity with hypersaline fluid inclusions occurring only at depth, within the pluton and surrounding hornfels. Coeval hypersaline fluid inclusions and vapor-rich fluid inclusions in vein quartz intergrown with high-temperature minerals (biotite, amphibole, tourmaline) suggest that fluid immiscibility occurred early in the evolution of the system and continued as the system evolved.

Vein quartz δ¹⁸O ranges from +21.4‰ to +4.3‰. Individual core samples exhibit variations of up to 16.9‰, and up to 8.9‰ in single crystals. Overall, maximum δ¹⁸O of vein quartz decreases as the pluton is approached. Within the system as a whole, and within individual crystals, quartz δ¹⁸O values decrease with time.

The spatial correlation of fluid inclusion and SIMS data allows calculation of the δ¹⁸O values of the water from which the vein quartz precipitated. Calculated δ¹⁸O values for the early deep hypersaline waters are as high as +10‰, consistent with either formation waters or extensively boiled magmatic fluids. Paragenetically later, lower temperature and less saline fluids have δ¹⁸O(H₂O) values as low as -1‰. The decrease in δ¹⁸O of the fluid, coupled with decreases in homogenization temperatures and salinities of the inclusions over time, is interpreted as the result of the progressive incursion of meteoric water into the paleo liquid-dominated reservoir.