Norris Geyser Basin, normally shortened to Norris Basin, is adjacent to the north rim of the Yellowstone caldera at the common intersection of the caldera rim and the Norris-Mammoth Corridor, a zone of faults, volcanic vents, and thermal activity that strikes north from the caldera rim to Mammoth Hot Springs. An east-west fault zone terminates the Gallatin Range at its southern end and extends from Hebgen Lake, west of the park, to Norris Basin. No local evidence exists at the surface in Norris Basin for the two oldest Yellowstone volcanic caldera cycles ("-'2.0 and 1.3 m.y.B.P.). The third and youngest cycle formed the Yellowstone caldera, which erupted the 600,000-year-old Lava Creek Tuff. No evidence is preserved of hydrothermal activity near Norris Basin during the first 300,000.years after the caldera collapse. Glaciation probably removed most of the early evidence, but erratics of hot-spring sinter that had been converted diagenetically to extremely hard, resistant chalcedonic sinter are present as cobbles in and on some moraines and till from the last two glacial stages, here correlated with the early and late stages of the Pinedale glaciation «150,000 years B.P.). Indirect evidence for the oldest hydrothermal system at Norris Basin indicates an age probably older than both stages of Pinedale glaciation. Stream deposits consisting mainly of rounded quartz phenocrysts of the Lava Creek Tuff were subaerial, perhaps in part windblown and redeposited by streams. A few small rounded pebbles are interpreted as chalcedonic sinter of a still older cycle. None of these are precisely dated but are unlikely to be more than 150,000 to 200,000 years old. The dominant quartz sand is hydrothermally cemented by chalcedony and is extremely hard, thereby justifying the term hydrothermal quartzite. Comparison with other sinter-depositing hot-spring systems indicates that the upper part of this fossil western Norris system was completely stripped by glacial erosion, leaving its chalcedonized roots but no sinter. Hydrothermal K-feldspar (adularia) is present locally, indicating channels of former upflowing spring water. Comparison with other drilled sinter-depositing systems suggests that at least 10 m of chalcedonic sinter has been eroded. Some combination of time (10,000 years or more) and high temperature (above surface boiling, perhaps "-'120 DC) was necessary to reconstitute the initial amorphous opal to chalcedony; the same general depths and temperatures are also essential to form adularia. The Al content of this hydrothermal feldspar is probably derived from volcanic feldspars or glass. The fundamental water type in Norris Basin is nearly neutral in pH and high in CI and Si0 2. Its Si0 2 content depends on its temperature of equilibration with respect to quartz, which is abundant in the under-IV.S. National Park Service, Yellowstone National Park. lying rhyolitic rocks, and on how much Si0 2 was precipitated enroute to the surface. Temperatures immediately below Norris are probably near 270 DC, a temperature that would...

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U.S. Geological Survey Newberry 2 was drilled to a depth of 932 m within Newberry caldera. The bottom‐hole temperature of 265°C is the highest reported temperature of any drill hole in the Cascades region of the United States. The upper part of the stratigraphic section penetrated by Newberry 2 consists of caldera fill below which are increasingly more mafic lavas ranging from rhyodacite at 501 m to basalt at 932 m. Measured temperatures shallower than 300 m are less than 35°C, and rock alteration consists of hydration of glass and local palagonitization of basaltic tuffs. Incipient zeolitization and partial smectite replacement of ash and pumice occurred throughout the pumiceous lithic tuffs from 300 to 500 m. Higher‐temperature alteration of the tuffs to chlorite and mordenite occurs adjacent to a rhyodacite sill at 460–470 m; alteration minerals within the sill consist of pyrrhotite, pyrite, quartz, calcite, and siderite. Below 697 m the rocks are progressively more altered with depth mainly because of increased temperature along a conductive gradient from 100°C at 697 m to 265°C at 930 m. Fluid inclusions in quartz and calcite indicate that temperatures in the past have been higher than at present, most likely due to local confining pressures between impermeable lava flows. Flow breccias are more altered than the adjacent dense massive lava flows, regardless of composition, because of their much higher permeability. Hydrothermal minerals in this zone are mainly mixed‐layer chlorite‐smectite, quartz, calcite, and pyrite. Chlorite becomes more abundant than mixed‐layer clays near the bottom of the hole. In the lowest two lava flows, epidote, anhydrite, and scarce hematite occur locally. Alteration and leaching in the basal 2 m are unique and have led to the postulation of a localized two‐phase fluid zone consisting mainly of steam and CO2. The hydrothermal system of Newberry 2 is a simple evolving system associated with the evolution of Newberry Volcano. Only a few localized highly altered intervals where fracturing controlled fluid access occur in the core. There are no crosscutting fractures to indicate multiple hydrothermal systems. Chemical analyses of altered rocks and equivalent fresh rocks indicate that little chemical migration has taken place.

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