GEOCHEMICAL AND MINERALOGICAL MAPPING OF HYDROTHERMAL ALTERATION IN THE CENTRAL SNAKE RIVER PLAIN

Thermal waters alter country rock by chemically removing and replacing the original minerals present. 46 sediment and rock samples were collected from a hydrothermally active area in the south-central Snake River Plain to map the extent of contact with thermal waters. Clay mineralogy of the sediment and major oxide geochemistry of country rocks was obtained using x-ray diffraction (XRD) and x-ray fluorescence (XRF), respectively. Igneous samples were prepared for XRF by fusion of powdered rock in a Li-borate flux. Concentration of major oxides was obtained from regression curves of USGS standard whole rock powders W-2, BIR-1, AGV-1, DNC-1, STM-1, G-2, GSP-1. Clay minerals were prepared by removing organic matter and purified via centrifugation to obtain clay size fraction. Oriented mounts were air-dried on glass slides. XRD scans were collected after air-drying and a second time after 24-hour glycolation to identify smectite peaks. Preliminary results indicate that the most prominent peaks between 5-8.5° 2θ correspond to mixed-layered smectite-illite. Smectite-illite fraction in the samples is here interpreted as the result of interaction of igneous country rock with thermal waters at temperatures ranging from 100 to 160°C as reported by Steiner (1953).