Further insights into the geochemical evolution of fumarolic alteration, Valley of Ten Thousand Smokes, Alaska

Abstract

Factor analysis of geochemical data from nine fossil fumaroles and four warm ground argillic alteration sites indicates that the fossil and extant fumarolic alteration of the 1912 ash-flow sheet in the Valley of Ten Thousand Smokes (VTTS) integrate a complex overprinting of primary and secondary alteration events. The five-factor solution model explains 77% of the complete data set variance and a large proportion (60–92%) of the individual element data variance. These data support halide-species (e.g., chlorides and fluorides) vapor-phase element transport having occurred during the cooling of the fumaroles and that this process was significant for As, Sb, B and Bi, and of lesser importance for Cu and Zn. Similarities in geochemical element covariance between the extant argillic alteration and fossil fumarolic encrustations suggest that many of the fossil fumaroles experienced low-temperature argillic alteration events. The combination of the results of this work with those of Kodosky and Keith (1993) enables a general history of the VTTS fumaroles to be reconstructed. During the higher-temperature stages of fumarolic activity, substantial halide-species vapor-phase element transport likely occurred. The high-temperature gases heavily altered the ejecta glass and mineral phases adjacent to the fumarolic conduit. As the fumaroles cooled, Fe-rich acidic condensate leached the ejecta and primary fumarolic deposits (Kodosky and Keith, 1993); aqueous chloride complexes were the primary mechanism of element transport. With further declines in temperature, many of the fumaroles developed moderate- to low-temperature argillic alteration. Low- to ambient-temperature leaching and ongoing hydration reactions subsequently produced abundant hydrated amorphous phases (Kodosky and Keith, 1993). Although relict mineralogical evidence of argillization remains at some of the fossil fumaroles, at many of the sites the secondary alteration reactions have removed or obscured the record of prior argillization events.