Hydrothermal alteration and geochemistry in core hole BC12-31: Implications for segregation of transient flow regimes in the Long Valley geothermal system

The results of fluid inclusion, intra-crystalline mineral, and stable isotope thermometric analyses suggest up to three periods of vein- and pore-filling mineralization: (1) shortly after eruption of the Bishop Tuff and deposition of the Megabreccia, when chlorite and possibly carbonate formed at temperatures in excess of 200°C; (2) after deposition of the Early Rhyolite, when quartz, carbonate, and some chlorite formed at temperatures closer to the present day boiling curve; and (3) most recently, at temperatures similar to present day measured downhole temperatures in BC 12-31. The microtextures of the Megabreccia, specifically the extremely low secondary porosity and fine-grained, chloritic matrix that appears to have formed shortly after the deposition of the Bishop Tuff, suggest that the Megabreccia might serve as an effective hydrologic barrier separating mixing of hotter fluids above and cooler fluids below. (Graph Presented).