Preliminary numerical analysis of the magma-hydrothermal history of The Geysers geothermal system, California, USA

Numerical approximations to the thermal history of a representative 2-D semi-infinite section through a cooling pluton equivalent in size, configuration, and composition to the Quaternary-age Geysers plutonic complex (generally known as the felsite) provide new insights into the evolution of the associated, pre-vapor-dominated, magmatic-hydrothermal system. A reference state model demonstrates that 300oC peak paleotemperatures preserved in metagraywacke 1.4 km above the felsite (in core from scientific drillhole SB-15-D) could not have been attained by pure conductive heat transfer. The paleotemperatures are readily matched, however, by a convective-cooling model in which fluids above the igneous heat source can migrate within a permeable (0.05 millidarcy, or 5 × 10−17 m2) reservoir the size and geometry of the modern vapor-dominated regime intermittently open to the surface within a narrow vertical conduit proxying the Big Sulphur Creek strike-slip fault zone. Under these conditions, the thermal maxima are achieved within only 55–70 ky post-intrusion. Model temperatures thereafter cool to 235°C (the modern reservoir value for SB-15-D) within 650 ky and to 130°C within 1 My. The latter value is close to the 1.2–1.1 Ma calculated crystallization age for explored portions of the felsite. These relationships suggest that a 1.2–1.1 Ma felsite-heated hydrothermal system, at the levels penetrated by SB-15-D, should have cooled to the currently prevailing temperature by about 0.5 Ma. This clearly did not happen, and in the simplest analysis (in accordance with previous investigations), the large temporal discrepancy points to a more complex intrusive history for the felsite than age-dating has thus far revealed.