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-hydrothcl mal system. A reference state model demonstrates that 300 degreesC peak paleotemperatures preserved in meta-graywacke 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 x 10(-17) m(2)) reservoir the size ana 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 degreesC (the modern reservoir value for SB-15-D) within 650 ky and to 130 degreesC 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 fur the felsite than age-dating has thus far revealed. (C) 2001 CNR. Published by Elsevier Science Ltd. All rights reserved.