Fluid inclusion gas compositions from an active magmatic–hydrothermal system: a case study of The Geysers geothermal field, USA

Hydrothermal alteration and the active vapor-dominated geothermal system at The Geysers, CA are related to a composite hypabyssal granitic pluton emplaced beneath the field 1.1 to 1.2 million years ago. Deep drill holes provide a complete transect across the thermal system and samples of the modern-day steam. The hydrothermal system was liquid-dominated prior to formation of the modern vapor-dominated regime at 0.25 to 0.28 Ma. Maximum temperatures and salinities ranged from 440°C and 44 wt.% NaCl equivalent in the biotite hornfels adjacent to the pluton to 305°C and 5 wt.% NaCl equivalent at distances of 1730 m from the intrusive contact. The major, minor, and noble gas compositions of fluid inclusions in the hydrothermally altered rocks were integrated with microthermometric and mineralogic data to determine their sources and the effects of mixing and boiling. Major and minor gaseous species were released from the inclusions by crushing or thermal decrepitation; noble gases were released by crushing. The samples were analyzed by mass spectrometry. The analyses document the presence of magmatic, crustal, and meteoric components in the trapped fluids. Hydrothermal fluids present during the liquid-dominated phase of the system contained gaseous species derived mainly from crustal and magmatic sources. At The Geysers, N2/Ar ratios greater than 525 and 3He/4He ratios of 6–10.7 Ra are diagnostic of a magmatic component. Crustal gas has CO2/CH4 ratios less than 4, N2/Ar ratios between 45 and 525, and low 3He/4He ratios (0.5 Ra). Meteoric fluids have CO2/CH4 ratios greater than 4 and N2/Ar ratios between 38 (air-saturated water) and 84 (air). However, N2/Ar ratios between 15 and 110 can result from boiling. Ratios less than 15 reflect the incorporation of N2 into NH3-bearing clay minerals. In the central Geysers, the incursion of meteoric fluids occurred during the transition from the liquid- to vapor-dominated regime. Variations in the relative CH4, CO2, and H2 contents of the gas analyses demonstrate that boiling took place under open-system conditions. The gas data indicate that the inclusions have remained closed to the diffusion of He and H2 since their formation.