Abstract
Secondary mineralogy in the 1912 vent region at Novarupta illustrates an alteration history that evolved from high‐temperature degassing of ejecta through surficial argillic alteration to near‐neutral, wet‐steam alteration as the posteruptive volcanic system cooled. The locallized distribution of surficial argillization appears to have been controlled mainly by fractures related to slumping and compaction in the near‐surface fallback ejecta. The fractures guiding the surviving warm, near‐neutral fumarolic fluids, however, must be connected with deeper vent structures that reach a subsurface heat source, perhaps the boiling zone of a hydrothermal system. Trace metals, including Mo, Pb, Cu, Cd, and Zn, concentrated by high‐temperature, vapor‐phase mobilization, have subsequently been partly removed by argillization processes; in contrast, Hg, Sb, Tl, Bi, and As remain high even though the fumaroles now emit only low‐temperature, near‐neutral wet steam.
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