Y-1 penetrated an active hot-spring system to 215 feet (65.5 m), where the temperature was 171°C. The core is sinter to 12 feet (3.7 m), sandstone, conglomerate, and siltstone composed of rhyolite detritus to 211 feet (64.3 m), and bedrock rhyolite.

Hydrothermal minerals replace obsidian and fill open spaces. Original plagioclase and alkali feldspar are unaltered. Clinoptilolite, mordenite, opal, and relic a-cristobalite occur in the less altered core. In the more intensely altered core analcime is the sole zeolite, hydrothermal quartz is abundant, and a-cristobalite has been converted to quartz. Mont-morillonite and celadonite occur throughout most of the core; calcite, pyrite, muscovite, kaolinite, erionite, and aegirine are sporadic. Hydrothermal alkali feldspar occurs in only one sample.

Associated hot-spring fluids are dilute, slightly alkaline, and contain mainly Na, Cl, HCO3, and SiO2.

Major factors controlling formation of hydrothermal minerals in Y-1 are 1) nature of starting material, 2) elevated temperature, and 3) fluid composition. Parallel distribution patterns of zeolites and silica species suggest that variation in SiO2 activity is important.

Hydrothermal alteration in Y-1 has produced mineral assemblages similar to those produced during low-temperature diagenesis of sedimentary rocks rich in volcanic detritus

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