Implications of ^(18)O/^(16)O and D/H Data on Hydrothermally-Altered Terranes to the Deep-Level and Long-Term Characteristics of Caldera·Related Hydrothermal Systems

Abstract

Eroded hydrothermally-altered terranes provide much information about the inaccessible deep levels and long-term characteristics of modern geothermal systems. Detailed isotopic maps of large (12 to 60 km dia.) Challis (Geology, 12, 331-334) and San Juan volcanic fields (J. Volc. Geotherm. Res., 30, 47-82) prove that I) the ground- water circulation pattern was affected over lateral distances of 50 km or more; 2) vertical I so gradients were produced in the crust, with fluid penetrating to depths of at least 5 to 10 km; 3) fluid temperatures were mostly 150° to 350°C; 4) isotopic disequilibrium between coexisting minerals was ubiquitous, consistent with system lifetimes of 10^5 to 10^6 years; This page may be freely copied. 5) regional fluid/rock ratios were typically -1, such that a large ^(18)O shift of the deep fluid occurs; and 6) the highest fluid-rock ratios (>>1), as integrated over the lifetimes of the systems, occurred along the ring fracture zones and adjacent to resurgent intrusions. Analogous zones in modern systems are clearly excellent drilling targets for geothermal resources.