Hydrothermal Alteration and Geochemistry in Core Hole BC12-31: Implications for Segregation of Transient Flow Regimes in the Long Valley Geothermal System

摘要

BC 12-31 is a 2040' (610 m) deep exploration core hole, located between Casa Diablo and Shady Rest, drilled in Long Valley Caldera by Mammoth Pacific in September 2002 (Figure 1). Downhole temperatures in 12-31 increase steadily until ca. 850'(260 m and ca. 185°C), where the well becomes nearly isothermal-reaching a maximum of 191°C at 1410' (430 m)-and then decreases to a bottom hole temperature of 152°C. The upper part of the well penetrates mainly Early Rhyolite and the lower 580' (177 m) ends in Bishop Tuff. Between ca. 1260' (384 m) and 1430' (436 m), the core is composed of a distinctive "Megabreccia" containing clasts of Paleozoic meta-sedimentary and plutonic rocks correlative with those in the Mount Morrison roof pendant of the Sierra Nevada.