STRUCTURE AND COMPOSITION OF PRIMARY IGNEOUS AND CLAY MINERALS IN ALTERED BASALT FROM THE PROJECT HOTSPOT MH-2 BOREHOLE, MOUNTAIN HOME AIR FORCE BASE, IDAHO, USA

Clay composition and alteration were studied in Pliocene basalt core samples extracted from geothermal well MH-2B in the Snake River Plane, ID as part of project HOTSPOT. Previous research on the core showed a transition in clay minerals from smectite to corrensite in the zone surrounding the discovery at 1745 m of heated water under artesian pressure. This study characterizes the composition and variability of clay minerals in the core and attempts to determine the relationship between primary igneous mineral composition, hydrothermal alteration, and the formation of clay minerals.

Ten core sample thin-sections from 1653 m to 1808 m were texturally and chemically characterized for alteration of feldspar, olivine, pyroxene, and clay. Textural analysis showed significant changes in relative abundance and increased alteration of primary minerals in the corrensite zone compared to samples from both above and below, which were relatively unaltered.

Chemical analyses were performed with a SEM using energy-dispersive X-ray spectroscopy to analyze oxide concentration in samples both inside and outside the corrensite zone. Structural calculations based on oxides were done to characterize clay composition, mineralogical variability, and alteration progression, with statistical analysis used to validate results. Clay composition and structure were found to be highly variable, including dioctahedral smectite, trioctahedral smectite, and corrensite within individual samples. Relative composition of clays was also found to vary between samples in relation to the degree of hydrothermal exposure. Additionally, alteration of primary minerals was observed, including metasomatism of anorthite to alkali feldspar, and the disappearance of olivine in corrensite zone samples.

These results link clay formation and mineral alteration to specific chemical, structural, and textural changes in igneous rocks, and provide additional tools through which the sources and products of hydrothermal alteration can be distinguished.