ALTERATION CHARACTERISTICS AND AUTHIGENIC MINERALOGY OF BASALTS IN BOREHOLE MIDDLE 1823, EASTERN SNAKE RIVER PLAIN, SE IDAHO

In this study we document and interpret primary and diagenetic alteration features of late Pliocene to Quaternary olivine tholeiite basalts from near the effective “base” of the eastern Snake River Plain (ESRP) aquifer system. This study focuses on a core from Middle 1823, a 1653-foot-deep borehole located at the Idaho National Engineering and Environmental Laboratory (INEEL). It is similar to other deep boreholes at the INEEL because there is a rapid downward transition from grey, unaltered basalt to greenish, altered basalt with depth (beginning at 1120 feet below land surface (fbls)), which approximately correlates with a sharp downward increase in temperature gradient (1025 fbls). Many flows near the base of the borehole (between 1205 and 1630 fbls) overlie lacustrine sedimentary units and exhibit peperitic textures, indicating strong interaction between the original lava flows and water-rich sediment. Those, as well as shallower (to 1120 fbls) non-peperitic flows exhibit heterogeneous weak to strong alteration. The lack of peperitic features in some of the altered basalt flows suggests that the alteration is due to secondary basalt-water interactions. Altered regions typically display partial to complete replacement of intersertal glass by clay minerals. Olivine alteration varies from a rim of secondary clays along grain boundaries and fractures to a complete replacement of olivine by clay minerals, calcite, and zeolite. Plagioclase crystals are weakly altered along grain boundaries and fractures, and clinopyroxene crystals are typically unaltered. Accompanying the alteration is an increase in green to black diagenetic clay, drusy calcite, chabazite, and unidentified zeolite-like minerals that line and often fill fractures, vesicles, and diktytaxitic cavities. The paragenetic sequence of authigenic mineral precipitation typically begins with clay followed by clay ± zeolite ± calcite. Clay minerals occur as radial aggregates that appear to have grown inward from the walls of vesicles, suggesting an authigenic origin. X-ray diffraction analysis of a representative amygdule from 1288 fbls yields a high proportion of inter-layered smectite/illite clay (>85% Fe-rich dioctahedral smectite, <15 % illite) with lesser amounts of primary illite, chlorite, kaolinite, and accessory quartz and calcite.