GRC Transactions, Vol. 35, 2011 785 Keywords Long Valley Caldera, Megabreccia, Core BC 12-31, fluid in- clusions, chlorite geothermometry, oxygen isotopes, carbon isotopes Introduction 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 Pacifc in September 2002 (Fig- ure 1). Downhole temperatures in 12-31 increase steadily until ca. 850'(260 m and ca. 185°C), where the well becomes nearly iso- thermal — 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. Suemnicht et al. (2007) have argued that the Megabreccia — identifed in cuttings and core from other wells in the southern moat of the Long Valley Caldera (LVC) — serves as a hydrologic barrier that isolates overlying geothermal fuids from cooler meteoric fuids below. The purpose of this study was to: (1) to assess the spatial and temporal relations of alteration mineralogy and stable isotope geochemistry in this core hole; and (2) to determine if the Megabreccia exhibits the petrophysical attributes that would serve as an effective hydrologic barrier. Petrography The BC 12-31 core samples we examined in thin section and using back-scattered elec- tron (BSE) imaging exhibit variable degrees of hydrothermal alteration (Figures 2a through 2f). In samples from the Early Rhyolite, we see evidence for an early silicifcation event, followed by carbonate veining and subsequent illitization; dissolution of primary pumice and phenocrysts has resulted in signifcant secondary porosity partially flled with hydrothermal quartz, potassium feldspar, and illite (Figure 2a). The Megabrec- cia contains clasts of metamorphic and (mainly plutonic) igneous lithologies, consistent with its suggested origin as a debris ava- lanche from Sierran basement (Suemnicht et al., 2007). A single altered pumice clast was seen in thin section, but clear evidence for incorporation of clasts or phenocrysts from the Bishop Tuff was lacking. The matrix of the Megabreccia appears in plane polarized light (Figure 2b) to be fnely comminuted fragments of the larger clasts with quartzo-feldspathic material, carbonate, chlo- rite, illite and semi-opaque material. Although the fne-grained matrix appears to be generally similar to the clasts, it appears that carbonate, illite and chlorite are relatively more abundant. However, there are locally developed patches in the matrix that are dominantly chloritic, with less abundant quartzo-feldspathic and illitic material. It is not clear if these patches had a more mafc initial composition or if there has been heterogeneous metasomatic Hydrothermal Alteration and Geochemistry in Core Hole BC12-31: Implications for Segregation of Transient Flow Regimes in the Long Valley Geothermal System Andrew Fowler 1 , Maya Wildgoose 1 , Robert Zierenberg 1 , Peter Schiffman 1 , and Gene Suemnicht 2 1 Department of Geology, University of California, Davis CA 2 EGS, Inc., Santa Rosa CA Figure 1. Study area location,