PRELIMINARY CLAY MINERAL ANALYSES OF MOUNTAIN HOME MH-2B DEEP BOREHOLE

The MH-2B hole was one of three holes completed as a part of HOTSPOT: The Continental Geothermal Drilling Project. MH-2B was drilled on the Mountain Home Air Force Base southeast of Boise, Idaho to evaluate the potential for development of geothermal energy on the base. Artesian hot water was encountered at 1750m, and the measured borehole temperature at the bottom of the well was over 160°C at the time of the well’s completion.

The lithology of the MH-2B core materials includes lake beds above 730m, basaltic volcanic and volcaniclastic rocks interbedded with lake beds to 850m, and volcanic and volcaniclastic rocks to the bottom at 1790m. Alteration minerals visible in veins and fractures in hand specimen include calcite, zeolites (stilbite? or laumontite?), and grey-green clay minerals. Preliminary X-ray diffraction (XRD) analysis of clay minerals suggests a transition with depth from poorly crystalline smectite at around 760m to ordered R1 chlorite:smectite (corrensite) at 1790m. Further work will focus on the smectite to corrensite transition, and will combine XRD analysis with thin section and microprobe study to document the texture of clay mineral occurrences, the mineral composition of their respective occurrences, and will attempt to correlate the conditions of the clay mineral transformations with measured (and paleo) temperatures and fluid compositions.