Preliminary Mineralogic and Stable Isotope Studies of Altered Summit and Flank Rocks and Osceola Mudflow Deposits on Mount Rainier, Washington

Abstract

About 5600 years ago part of Mount Rainier's edifice collapsed with the resultant Osceola Mudflow traveling more than 120 km and covering an area of at least 505 km 2 . Mineralogic and stable isotope studies were conducted on altered rocks from outcrops near the summit and east flank of the volcano and samples of clasts and matrix from the Osceola Mudflow. Results of these analyses are used to constrain processes responsible for pre-collapse alteration and provide insight into the role of alteration in edifice instability prior to the Osceola collapse event.Jarosite, pyrite, alunite, and kaolinite occur in hydrothermally altered rock exposed in summit scarps formed by edifice collapse events and in altered rock within the east-west structural zone (EWSZ) of the volcano's east flank. Deposits of the Osceola Mudflow contain clasts of variably altered and unaltered andesite within a clay-rich matrix. Minerals detected in samples from the edifice are also present in many of the clasts. The matrix includes abundant smectite, kaolinite and variably abundant jarosite.δ Hydrothermal fluid compositions calculated from hydrogen and oxygen isotope data of alunite, and smectite on Mount Rainier reflect mixing of magmatic and meteoric waters. The range in the δD values of modern meteoric water on the volcano (-85 to 155‰) reflect the influence of elevation on the δD of precipitation. The δ 34 S and δ 18 O SO4 values of alunite, gypsum and jarosite are distinct but together range from 1.7 to 17.6‰ and -12.3 to 15.0‰, respectively; both parameters increase from jarosite to gypsum to alunite. The variations in sulfur isotope composition are attributed to the varying contributions of disproportionation of magmatic SO 2 , the supergene oxidation of hydrothermal pyrite and possible oxidation of H 2 S to the parent aqueous sulfate. The 18 O SO4 values of jarosite are the lowest recorded for the mineral, consistent with a supergene origin.The mineralogy and isotope composition of alteration minerals define two and possibly three environments of alteration. At deeper levels magmatic vapor, H 2 S, SO 2 and other gases from venting magmas migrated upward and condensed into the meteoric water. Disproportionation of SO 2 into aqueous sulfate and H 2 S resulted in acid-sulfate (alunite + kaolinite + pyrite) and related argillic and propylitic alteration envelopes in a magmatic hydrothermal environment. At shallow levels H 2 S reacted with andesite to form pyrite that is associated with smectite along fractures on both the flanks and upper edifice. It is not clear to what extent H 2 S was oxidized by atmospheric O 2 to form aqueous sulfate in a steam-heated environment. Near the ground surface, pyrite is oxidized by atmospheric oxygen resulting in soluble iron-and aluminumhydroxysulfates. These supergene hydroxysulfates, which may also form around fumaroles from the oxidation of H 2 S, are subject to continuous solution and redeposition.

Origin of secondary sulfate minerals on active andesitic stratovolcanoes

“…The volcano's summit cone represents partial rebuilding of a formerly higher summit that collapsed to form the Osceola Mudflow~5600 years ago (Crandell, 1976). Recent studies by the authors (Rye et al, 2003) indicate that jarosite is abundant in the matrix and pyrite is a common minor mineral in clasts of Osceola Mudflow. Large (up to 10 m) veins of gypsum occur on the exposed scarps created during edifice collapse (Zimbelman, 1996).…”

“…Recent studies (Rye et al, 2003) of Mount Rainier debris-flow deposits indicate that a significant amount of pre-collapse jarosite is present in the matrix, and that pyrite is present in about 30% of the small clasts. The formation of some of the Fe-and Al hydroxysulfates and gypsum has been attributed to the recent oxidation of this pyrite.…”

“…Barite and anhydrite precipitation can occur during the mixing of sulfate-rich saline and dilute fluids (Holland and Malinin, 1979 (Rye and Alpers, 1997), indicating that jarosite formed from aqueous sulfate derived from the oxidation of pyrite in meteoric water. The authors' (Rye et al, 2003) recent studies of the Osceola mudflow deposit indicate that a high percentage of coarse fragments contain a pyritic assemblage undergoing oxidation to soluble sulfates and gypsum. Although the relative importance of the oxidation of H 2 S versus pyrite in the formation of jarosite needs further study, much of the jarosite on Mount Rainier may have been derived from the oxidation of pyrite.…”

A review of the stable-isotope geochemistry of sulfate minerals in selected igneous environments and related hydrothermal systems

“…Thus, almost all secondary sulfur-bearing minerals on stratovolcanoes are derived from sulfur that is introduced from degassing magmas. Recent studies on Mount Rainier (Rye et al, 2003) indicate that a surprising amount of shallow hydrothermal pyrite formed by the sulfidation of andesite by H 2 S. This pyrite is susceptible to oxidation by atmospheric oxygen entrained in either meteoric water or magmatic vapor. Oxidation of the pyrite and subsequent reactions by the resulting acidic solutions produces soluble Al and Fe hydroxysulfates and jarosite (Zimbelman et al, this volume).…”

“…The predicted meteoric water compositions for stratovolcano summits are based on a decrease of 65x km À1 in the yD of precipitation with elevation on stratovolcanoes (Frank, 1995). Data from Zimbelman et al (this volume) and Rye et al (2003). PMW-primary magmatic water and VW-volcanic water are reference fields of Taylor (1979) and Giggenbach (1997), respectively.…”