Petrology and hydrothermal mineralogy of U.S. geological survey Newberry: 2. Drill core from Newberry Caldera, Oregon

Abstract

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.

Multiple origins of obsidian pyroclasts and implications for changes in the dynamics of the 1300 B.P. eruption of Newberry Volcano, USA

“…Drilling in Newberry Caldera approximately 2 km north of the 1300 B.P. vent shows that hydrothermal calcite is common in the intracaldera volcanic and sedimentary facies, at least to the base of drilling at 932 m depth (Bargar and Keith 1999;Keith and Bargar 1988). In samples with CO 2 gradients, the water content usually increases with distance from the domains containing xenoliths.…”

Genesis of alteration of Quaternary basalts within a portion of the eastern Snake River Plain Aquifer

“…Keith and Bargar (1988) documented low-tem- Figure 2. Temperature profiles of six deep wells discussed in this study.…”

Field-trip guide to the geologic highlights of Newberry Volcano, Oregon

“…In 1978, the U.S. Geological Survey began research drilling in the center of the caldera (Sammel and others, 1988;Keith and Bargar, 1988) and encountered a temperature of 265 °C at 3,058 feet (ft) (932 m). Geothermal drilling by private industry (Swanberg and others, 1988) followed and has continued intermittently (Frone and others, 2014).…”

“…A 1981 U.S. Geological Survey drill hole in the center of the caldera encountered a temperature of 265 °C at 3,051 ft (930 m) (Sammel and others, 1988). Chemistry of rocks and hydrothermal minerals encountered in this drill hole are discussed in Keith and Bargar (1988). Subsequent geothermal exploration involved drilling core holes on the flanks of the volcano.…”