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U.S. Geological Survey research diamond-drill hole Y-11 in the Mud Volcano area of Yellowstone National Park was drilled to a depth of 105.7 m. The hole penetrated about 2.1 m of Holocene fluvial sedimentary deposits, 13.1 m of glaciosedimentary deposits associated with the Pinedale Glaciation (≥45,000-14,000 yr B.P.), approximately 4.3 m of tuff beds of the Upper Basin Member of the Plateau Rhyolite (~265,000 yr B.P.), and 86.2 m of a 600,000 year-old ash-flow tuff (the Lava Creek Tuff of the Yellowstone Group). Cavities in the welded ash-flow tuff contain abundant sandine and tridymite from vapor-phase crystallization during cooling of the flow. The rhyolitic glassy groundmass had undergone extensive devitrification to form alkali feldspar and alpha-cristobalite before hydrothermal alteration.
Hydrothermal alteration in drill core Y-11 was produced by an early chloriderich near-neutral hot-water system and a later vapor-dominated system. The hotwater system precipitated minerals such as chalcedony, quartz, septechlorite, calcite, rhodochrosite, bastnaesite, mordenite, fluorite, and some beta-cristobalite, pyrite, and montmorillonite. These minerals cannot have been deposited from the steam and bicarbonate-sulfate condensate found in the present geothermal system. Later minerals related to the current vapor-dominated regime include opal, kaolinite, halloysite, alunite, and some beta-cristobalite, pyrite, and montomorillionite. These younger minerals are, in part, superimposed on the earlier formed minerals. Relations between the two generations of hydrothermal minerals are best observed in the drill core within the later cross-cutting fractures at 74.6-77.6 m, where later fracture fillings of kaolinite and pyrite cut across earlier chalcedony fracture fillings.
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