Clay Mineralogy of the MH-2 Core, Snake River Plain, Idaho

The MH-2B hole was one of three holes completed as a part of HOTSPOT: The Continental Scientific Drilling Project. MH-2B was drilled to a depth of 1821m on the Mountain Home Air Force Base southeast of Boise, Idaho to evaluate the potential for development for geothermal energy on the base. Water under artesian pressure was encountered at a depth of 1745 m. We analyzed pieces of the core to study clay mineralization with depth. We took 22 samples from the bottom half of the core, focusing on around 1790m, where preliminary analysis indicated the presence of corrensite (R1 ordered smectite/chlorite). X-ray diffraction (XRD) of these powders showed a transition from smectite at 762m to corrensite at 1790m and back to smectite at 1807m. More detailed XRD analysis with ethylene-glycol solvated samples confirmed the findings of the powder samples (namely a transition from smectite -> corrensite -> smectite). We modeled smectite and corrensite crystallinity with depth and found an increase in the defect free distance of both minerals. Samples around 1790m showed the presence of smectite, corrensite, and possibly chlorite (or serpentine). We interpret this result to support a discontinuous (stepwise) model for the smectite-chlorite transition. The presence of artesian fluid in this zone is evidence for the importance of high fluid/rock ratios in the smectite- chlorite transition, and the association of high fluid/rock ratios with a discontinuous transition model.