1 

Preliminary mineralogic and stable isotope studies of altered summit 

and flank rocks and Osceola Mudflow deposits on Mount Rainier, 

Washington 

By Robert O. Rye1, George N. Breit1 and David R. Zimbelman2 

1

U.S. Geological Survey, Box 25046, MS 973, Denver, CO 80225

2

G.O.Logic, PO Box 1878, White Salmon, WA 98672 

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial 

standards or with the North America Stratigraphic code 

Any use of trade, firm, or product names is for descriptive purposes only and does not imply endorsement 

by the U.S. Government 

U.G. Geological Survey Open-File Report 03-464 

U.S. Department of the Interior 

U.S. Geological Survey 
2 

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 km2

. 

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

34S and δ18OSO4 

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 SO2

, the supergene oxidation of 

hydrothermal pyrite and possible oxidation of H2S to the parent aqueous sulfate. The 18OSO4 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, H2S, SO2 

and other gases from venting magmas migrated upward and condensed into the meteoric 

water. Disproportionation of SO2 into aqueous sulfate and H2S resulted in acid-sulfate 

(alunite + kaolinite + pyrite) and related argillic and propylitic alteration envelopes in a 

magmatic hydrothermal environment. At shallow levels H2S 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 H2S was oxidized by atmospheric O2

 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 H2S, are subject to continuous solution and redeposition. 

Introduction 

In 1985, Nevado del Ruiz volcano (Colombia) had a very small eruption near its 

summit. This eruption triggered a mudflow that resulted in more than 22,000 deaths 

(Lopez and Williams, 1993; Voight, 1990; 1996). More recently, a relatively small 

portion of Casita volcano (Nicaragua) suddenly collapsed along an area of 

hydrothermally altered rock, leading to the largest single cause of death during 1998’s 

Hurricane Mitch (van Wyk de Vries and others, 2000; Sheridan and others, 1999). The 

realization that catastrophic edifice collapse can occur without an attendant eruption 

coupled with the knowledge that edifice collapse at some volcanoes is directly related to 

hydrothermal alteration emphasizes the need for increased understanding of the geologic 
3 

and geochemical causes of alteration at volcanoes. This realization has become especially 

evident at Mount Rainier, where frequent and far-reaching debris flows and debris 

avalanches constitute the greatest volcanic hazard in the Cascade Range (Figure 1, 

Crandell and Mullineaux, 1967; Hoblitt and others, 1998). Because of these hazards, 

Mount Rainier was designated a Decade Volcano and a science plan for its study was 

issued by the U.S. National Research Council (1994) in conjunction with the United 

Nations International Decade for Natural Disaster Reduction (UN Resolution 

42/169/1987). 

Figure 1. Index location map of Mount Rainier (gray) and surrounding communities. 

For more than 100 years, geologists have noted that Mount Rainier must have 

been much higher in its geologically recent past (Russell, 1898). Massive collapse of this 

former summit material transformed into the Osceola Mudflow and Paradise Lahar about 

5600 years before present (Crandell and Waldron, 1956; Crandell, 1963; Crandell, 1971). 

The Osceola is one of the largest (> 3.8 km3

) and farthest traveled (>120 km) mudflows 

in the world (Crandell and Waldron, 1956; Crandell and Mullineaux, 1967; Crandell, 

1973; Dragovich and others, 1994; Vallance and Scott, 1997). The mudflow filled 

valleys of the White River system north and northeast of the volcano to depths of more 

than 100 m before spreading into the Puget Lowland where it covered an area of at least 

505 km2

, including sites now occupied by towns such as Orting, Sumner, Puyallup, Fife, 

Auburn, Kent, Enumclaw, and Buckley (Crandell, 1963; Crandell, 1971; Dragovich and 

others, 1994). The Osceola is one of at least six lahars that have been documented to 

have traveled from Mount Rainier into the Puget Lowlands in the past 6800 years (Scott 

and others, 1995). In this paper the mineralogy and stable isotope geochemistry of 

alteration minerals in the matrix and small clasts of some of the Osceola Mudflow 
4 

deposits are compared with alteration minerals remaining on the flanks and summit of the 

present volcanic edifice. These observations are used to make preliminary reconstruction 

of the hydrothermal environments and conditions that produced the alteration of the 

edifice prior to collapse. 

Geologic framework 

Detailed geologic mapping and radiometric age dating indicate that the present 

Quaternary edifice of Mount Rainier began to form on Tertiary bedrock at about 500 ka 

(Sisson and Lanphere, 1999). Edifice construction occurred during two extended periods 

of magmatic activity, each lasting about 100 kyr, between about 500 and 420 ka and 280 

and 180 ka (Sisson and Lanphere, 1999). Most units were deposited as flows, either 

massive or brecciated, with occasional interbedded tephra layers and most units were 

erupted from the same central vent occupied by the present summit cone, which is 

currently covered by an extensive glacial mantle. Unaltered Mount Rainier lavas are 

mostly geochemically uniform, plagioclase-phyric, two-pyroxene, magnetite-bearing 

andesites (Coombs, 1936; Fiske and others, 1963). 

Mount Rainier’s summit cone represents partial rebuilding of a formerly higher 

summit that was present prior to collapse and onset of the Osceola Mudflow (Crandell, 

1976). The current cone is at least 300 m high, 2 km wide, and consists of two 

overlapping craters, aligned east-west. Both craters are snow-free along their rims and 

flanks, contain caves beneath their central ice caps, and are hydrothermally altered. The 

alteration of these cones has been described in Zimbelman and others (2000). The snowfree zones and snow caves are related to active fumaroles. Some of the fumaroles attain 

near-boiling-point temperatures and show large spikes of CO2

 and traces of H2S but no 

SO2 (Zimbelman and others, 2000). The large amounts of CO2 indicate the gas chemistry 

of the fumaroles is influenced by mild degassing of the underlying magma (Zimbelman 

and others, 2000). 

Below the summit cone, scarps formed by the Osceola collapse are exposed in 

only a few places, including Liberty Cap, Russell Cliffs, and Gibraltar Rock. In other 

areas, glaciers, post-collapse lava flows, or the post-collapse summit cone cover the 

Osceola scarp. Accessible summit scarps, such as Sunset Amphitheater, that are related 

to later collapse events, contain alteration minerals that can be considered to be 

representative of the type of alteration present on the summit prior to the Osceola 

collapse (e.g., Crandell, 1971; Scott and others, 1995). 

The east flank of the volcano was partially denuded during the Osceola collapse 

event and has subsequently been partially covered by glaciers and post-Osceola volcanic 

flows and deposits. Where the Osceola collapse surface is exposed, two types of 

alteration are recognized: (1) widespread oxidation, silicification and argillization of 

thick block-and-ash flow units, and (2) more localized, acid-sulfate, advanced argillic, 

and argillic alteration of the block-and-ash flow and other units adjacent to dikes and 

fractures. The former has not been studied but is believed to be related to interactions of 

cooling volcanic units with ambient ground water and volcanic gases shortly after 

emplacement. The latter alteration is a result of episodic processes that began with the 

introduction of H2S-rich volcanic gases along fractures during magma-degassing 

episodes. 
5 

Methods 

Sampling 

The locations of samples from the summit scarps, flanks and Osceola mudflow 

are shown in Figure 2. Samples representative of alteration near the summit of Mount 

Rainier were collected at Sunset Amphitheater and Liberty Cap. Alteration on the eastern 

flanks of the volcano was represented by samples collected at Glacier Basin and Little 

Tahoma Peak. The Osceola deposits were sampled at four sites described by Crandell 

(1969; 1971) and Pringle and others (1994). In the area of the Osceola source material, 

samples 9822, 9823, 9827, and 9828 are from thin veneers (few cm thick) of the mudflow 

preserved along ridge tops in the Steamboat Prow and Glacier Basin areas. Seven 

kilometers downstream samples 9806 to 9819 were spaced at approximately 5 m vertical 

intervals, from bottom to top, through a relatively thick stratigraphic section (> 20 m) 

deposited in the Inter Fork drainage upstream of the White River Campground. The 

sampled outcrop is steep and some of samples, particularly those near the base, may 

represent slough. Distal samples 9801, 9802, and 9803 were collected 25 km 

downstream from the source in a gravel pit near The Dalles campground along the White 

River. 

Mineralogy 

The mineralogy of altered rocks from the edifice and Osceola is summarized in 

Table 1. Most alteration phases are very fine grained and identification was by X-ray 

diffraction (XRD) analyses of bulk rock and mineral separates. Clay sized material 

separated from some samples was used to prepare oriented mounts that were analyzed by 

XRD as air-dried, saturated with ethylene glycol, and after heating at 550° C for one 

hour. A scanning electron microscopy (SEM) equipped with an energy dispersive x-ray 

spectrometer provided information on the chemical composition of some alteration 

phases. 

Stable Isotopes 

Sulfur, oxygen, and hydrogen isotope data were obtained by standard techniques 

using both conventional and continuous flow mass spectrometer systems. Stable isotope 

data for minerals from altered rocks on a summit scarp (Sunset Amphitheater), an old 

erosion surface (Steamboat Prow), east flank (Glacier Basin), and the three sites 

containing the Osceola Mudflow are summarized in Table 2 and Figures 3-5. Alunite 

and jarosite were largely separated and processed according to techniques described in 

Wasserman and others (1992). Sulfate in jarosite from the matrix of the Osceola 

mudflow was also extracted by first removing soluble sulfates with water and then 

heating the matrix in 2 N Na2CO3 overnight. This extraction precipitates the iron as ferric 

hydroxide and leaves sulfate in solution which was titrated to pH 3.5 and sulfate was 

precipitated by adding a barium chloride solution. Soluble sulfates were water leached 

and precipitated as BaSO4 with barium chloride. The isotopic composition of water in 

smectite-forming fluids were calculated from δD and δ

18O data on smectite assuming a 

formation temperature of 100-175°C and the equations of Gilg and Sheppard, 
6 

Figure 2. Map of central part of Mount Rainier volcano, including: sample locations 

(rocks, solid circles; debris deposits, open circles); generalized geology, including 

Tertiary rocks (dark gray), Quaternary rocks and deposits (light gray), glaciers (white), 

dikes (heavy black, solid lines), hydrothermally altered rocks (heavy black polygons), 

and east-west structural zone (heavy black, dashed lines; EWSZ). Distribution of dikes 

(Sisson and others, 2001). 
Table 1. Results of X-ray diffraction analysis of bulk rock samples. Shaded areas indicate phase was detected. 

Kaol., kaolinite; Anhy., anhydrite) 

FLANK 

94105a 

Location Sample Type Smec. Kaol. Silica Alunite Jarosite Gypsum Anhy. OtheSUMMIT Little Tahoma Peak 93011a Rock outcrop Little Tahoma Peak 93011b Rock outcrop Sunset Amphitheater 93020a Rock outcrop Sunset Amphitheater 93021a Rock outcrop Sunset Amphitheater 93023b Rock outcrop Liberty Cap 94146c Rock outcrop Liberty Cap 94146b Rock outcrop Liberty Cap 94146c Rock outcrop Mowich Glacier Rock outcrop pota

Glacier Basin 94170a Rock outcrop OSCEOLA MUDFLOW Steamboat Prow 94167a Debris flow Steamboat Prow 94167b Debris flow Steamboat Prow 94168 Debris flow Steamboat Prow 94169c Debris flow Steamboat Prow 98022a Debris flow copi

Steamboat Prow 98023a Debris flow Glacier Basin 94170a Debris flow Glacier Basin 94170b Debris flow Glacier Basin 98027a Debris flow Glacier Basin 98028a Debris flow copi

Inter Fork 9806a Debris flow Inter Fork 9807a Debris flow Inter Fork 9808a Debris flow Inter Fork 9809a Debris flow Inter Fork 9810a Debris flow 
Table 1 (continued) Location Sample Type Smec. Kaol. Silica Alunite Jarosite Gypsum Anhy. OtheOSCEOLA MUDFLOW (continued) Inter Fork 9811a Debris flow Inter Fork 9812a Debris flow Inter Fork 9813a Debris flow Inter Fork 9815a Debris flow Inter Fork 9816a Debris flow Inter Fork 9817a Debris flow Inter Fork 9819a Debris flow White River 98001a Debris flow White River 98002a Debris flow White River 98003a Debris flow WEST SIDE DEBRIS FLOWS Puyallup River 94121 Debris flow silhyPuyallup Glacier 94136a Debris flow Puyallup Glacier 94136c Debris flow Puyallup River 98004a Debris flow 
9 

Table 2. Summary of stable isotope data on alteration minerals from Mt. Rainier edifice 

and flank localities and Osceola Mudflow deposits. (soluble – refers to phases extracted 

by water). 

Location Number Mineral δ

34S δ18OSO4 δ18O δD 

EDIFICE SUMMIT 

Sunset Amphitheater 21B Alunite 7.5 9.5 -121 

Sunset Amphitheater 20 Gypsum 7.1 4.2 

Sunset Amphitheater 23A Jarosite 3.1 -9.0 

Sunset Amphitheater 23B Jarosite 3.3 -4.3 -146 

Sunset Amphitheater 24A Jarosite 1.3 

Sunset Amphitheater 24D Jarosite -1.2 

Sunset Amphitheater 24B Smectite 8.4 -137 

Puyallup River 124 Gypsum 10.7 7.3 

Camp Muir 03AH Smectite 3.7 -131 

Summit 182 Smectite -2.3 -136 

Summit FNK-1 Gypsum 6.9 -1.6 

Mowich Face 114 Smectite 8.5 -111 

FLANKS 

Steamboat Prow 169A 

Steamboat Prow 169C 

Steamboat Prow 170B 

Steamboat Prow 167 

Steamboat Prow 196 

Steamboat Prow 167B 

Little Tahoma Peak 101 

Little Tahoma Peak 11B 

Glacier Basin 27B 

Glacier Basin 173 

Glacier Basin 174 

Glacier Basin 175B 

Glacier Basin MRVB17 

Glacier Basin MRVB15 

Takaloo Rock 130 

Alunite 17.3 15.0 -85 

Jarosite 1.5 -12.3 -135 

Jarosite 2.2 -10.8 -174 

Gypsum 7.5 

Gypsum 3.6 -4.0 

Smectite 13.9 -121 

Smectite 9.2 

Smectite 8.5 -148 

Smectite -5.8 -177 

Smectite 7.9 -77 

Smectite 7.7 -73 

Smectite 7.4 -119 

Jarosite 4.6 

Jarosite 2.7 

Smectite 10.7 -114 
10 

Table 2. (continued). 

Location Number Mineral δ

34S δ18OSO4 δ18O δD 

OSCEOLA MUDFLOW 

Steamboat Prow MR9822 Jarosite 4.0 

Glacier Basin MR9828 Jarosite 3.3 

Inter Fork MR9806 Jarosite 6.2* -5.0 

Inter Fork MR9806 Soluble 1.7 -3.9 

Inter Fork MR9807 Jarosite 5.9 

Inter Fork MR9807 Soluble 0.6 

Inter Fork MR9808 Jarosite 6.5 

Inter Fork MR9808 Soluble 1.7 

Inter Fork MR9808 Pyrite -0.5 

Inter Fork MR9809 Jarosite 5.6 

Inter Fork MR9810 Jarosite 5.2 

Inter Fork MR9810 Pyrite 0.9 

Inter Fork MR9816 Jarosite 7.5* -8.2 

Inter Fork MR9817 Jarosite 5.0 

Dalles Campgound MR9801 Jarosite 3.8 

Dalles Campgound MR9803 Jarosite 5.1 

Dalles Campgound MR9803 Soluble 3.7 -0.9 

* probably contains alunite 
11 

Figure 3. Summary of δD and δ

18OSO4 data for alunite and jarosite and δD and δ18O data 

for undifferentiated clays (kaolinite and smectite). Also shown are calculated range of 

δD and δ

18OH2O values for clay fluids assuming a temperature of 100-175°C and using 

equation of Sheppard and Gilg (1996) and for alunite fluids assuming a temperature of 

200-300°C and using equation of Stoffregen and others (1994). Low δ

18OSO4 values for 

jarosite require a supergene origin (Rye and Alpers, 1997). The δ

18OH2O values of 

supergene jarosite fluids cannot be calculated because the δ

18OOH values of jarosite could 

not be determined. Also shown is the range of values for firn on Mount Rainier (Frank, 

1995) and possible mixing trends with magmatic water (dashed faded arrows). 

References are the primary magmatic water (PMW; Taylor, 1979) and volcanic water 

(VW; Giggenbach, 1997), meteoric water line (Craig, 1961) and kaolinite (Savin and 

Epstein, 1970). The starting δDH2O values of magmatic fluids in a degassing volcano can 

vary considerably due to hydrogen isotope fractionation between melt and fluid and 

Rayleigh distillation effects during the exsolution of fluids from different batches of 

magma (Taylor, 1986). 
12 

Figure 4. Summary of δ

34S and δ18OSO4 data of alunite, jarosite and gypsum from Mount 

Rainier summit scarps and jarosite and soluble Fe-Al hydroxysulfate from Osceola 

mudflow deposits. Also shown are fields of values expected for magmatic hydrothermal 

alunite for a fluid having a δ

34SΣS of 0 ‰ and a δ18OH2O of 6‰ and for steam-heated or 

supergene alunite or jarosite for a fluid having the δ

18OSO4 of its parent aqueous sulfate 

controlled by the δ

18O of air or the δ18O of meteoric water (See Rye and others, 1992; 

Rye and Alpers, 1997, for details). The general trend indicates parent aqueous sulfate was 

derived largely from the supergene oxidation of pyrite and the disproportionation of SO2 

from magmatic sources. Over time and space magmatic hydrothermal, steam-heated and 

supergene oxidation processes have probably overlapped as shown in Figure 6. 
13 

Figure 5. Summary of δ

34S data of jarosite from summit scarp, flank and, four Osceola 

mudflow deposits arraigned by elevation. Also shown is data from the soluble sulfate in 

the largest Osceola deposit (Inter Fork). This difference of the δ

34S values for jarosite and 

soluble sulfate in the Inter Fork deposit indicates that they do not share a common origin. 

The difference in δ

34S values for jarosite for different Osceola deposits probably reflects 

the different degrees of exchange between streaming H2S and aqueous sulfate formed 

from oxidation of H2S prior to the precipitation of jarosite on the pre-collapse edifice. 
14 

1996); the composition of the alunite-forming fluids was calculated from δD and δ

18OSO4 

data on alunite assuming a formation temperature of 200-300° C and the equations of 

Stoffregen and others (1994). 

Results 

Mineralogy 

Rocks in the uppermost parts of the pre-Osceola cone, such as Liberty Cap and 

Sunset Amphitheater, are partly to pervasively replaced by fine-grained smectite ± 

kaolinite. Locally, alunite/natroalunite ± kaolinite occur. Pyrite is rarely visible but 

traces are detected by XRD of samples from Sunset Amphitheater and other summit sites 

(Zimbelman, 1996). Patches and veinlets of jarosite/natrojarosite + smectite are also 

present along fractures in the summit scarps. Minor amounts of isomorphous analogs to 

alunite and jarosite, such as ammoniojarosite, florencite, and zunyite may be present 

based on the bulk-sample XRD. Highly soluble iron and aluminum hydrosulfate and 

hydrophosphate minerals, typically observed in acid mine drainage (e.g., Bigham and 

Nordstrom, 2000), were also detected by XRD. These include alunogen, metaalunogen, 

basaluminite, crandallite and salammoniac. Some of the phases likely are related to the 

oxidation of pyrite, but others may have formed as fumarolic precipitates. 

The coarsest grains identified in the massive replacement alteration at Sunset 

Amphitheater are 10-micrometer, compositionally zoned alunite crystals. 

Semiquantitative analysis of these crystals in a polished thin section using a SEM defined 

micron-scale zoning of S, P, Na, Ca, K, and Fe. (Zimbelman, 1996). These crystals 

contain a Na-rich core (ca. 10 μm), surrounded by a more potassic zone that is then 

successively rimmed by a thin (ca. 1 μm) zone rich in Ca and P, and an outer Na-rich 

zone (ca. 2 μm) that lacks P. Surrounding these zoned sulfate crystals is a poorlycrystalline, interstitial Fe-sulfate rich in sodium. 

Anhydrite/gypsum occurs as veins up to 2 meters thick in the summit scarp at 

Sunset Amphitheater. These constitute the only coarse-grained alteration minerals 

observed (Zimbelman (1996). Euhedral barite grains (0.5 to 1.5 mm), which occur in 

fracture-filling gypsum ± anhydrite contain fluid inclusions that are generally 1 to 10 μm 

in size and are irregularly distributed (not aligned along fractures). Populations of 

primary fluid inclusions contain both vapor-rich and liquid-rich inclusions suggesting that 

boiling may have been important during their trapping. As described by Zimbelman ° ° (1996) fluid inclusion trapping temperatures vary between 204 and 266 C, salinities of 

liquid-rich inclusions are relatively high, ranging from 5.7 to >24.6 equivalent wt.% ° ° NaCl. The low eutectic temperatures (< -35 to -45 C) require salts other than NaCl or 

KCl in the trapped fluids (Crawford, 1981). Halite-bearing fluid inclusions typical of 

magmatic brines have not been identified. The low eutectics are likely related to CaCl2 as 

would be consistent with the presence of gypsum and anhydrite in the veins. 

Large areas of relatively unaltered volcanic units exposed on the flanks of the 

volcano have been locally oxidized and (or) vapor phase altered, presumably during 

emplacement. This alteration imparts a distinct orangish/reddish color to the outcrops. 

Bruce (1998) and Zimbelman (1996) noted widespread, but discontinuous, local 

alteration of the matrix in block-and-ash flow units in the lower parts of the volcano 

where smectite, locally present as veins, is the main alteration mineral. XRD analyses 

detected silica phases including cristobalite, tridymite, opal and quartz. Secondary iron 

oxide minerals include hematite, goethite, and maghemite. Smectite is the most 

prominent alteration mineral in volcanic rocks along dikes at Glacier Basin on the east 
15 

flank. Jarosite at Glacier Basin is limited to a relatively small area that includes both the 

block and ash units and the radial dike. Hydrothermal alteration minerals at Little 

Tahoma Peak also include jarosite and smectite with less common kaolin clays. Traces 

of soluble sulfates, including basaluminite, occur locally. Hydrothermally altered rocks 

in isolated areas lower on the volcano's west flank contain jarosite, smectite, and 

kaolinite. 

The clay-sized fraction of the Osceola Mudflow ranges between about 2 and 15% 

and averages about 7% (Crandell, 1971; Scott and others, 1995; Franz and Voight, 1995). 

One of the notable features of the clay-sized fraction of the Osceola Mudflow is the 

widespread presence of a few percent jarosite, resulting in a characteristic ochre color to 

some of the deposits. Morphologically, jarosite occurs as dipyramids and elongate 

prisms, 0.5 to 1 μm thick. Alunite was not detected in the matrix of the Osceola deposits 

by XRD. Preliminary studies of small clasts indicate that about 30 percent contain pyrite. 

Alunite, dickite, and barite, are also present in some clasts. 

Osceola Mudflow deposits from the Inter Fork deposit display systematic changes 

in water extractable phases with depth. Water slurried with the samples (50g solid/300 

mL water) had increasing concentrations of dissolved sulfate and total dissolved solids 

and decreasing pH (6.4 to 3.5) from the upper to the lower samples. These trends are best 

explained by increasing contents of soluble aluminum and iron sulfates that were not 

sufficiently abundant to be detected by bulk sample XRD. Their isotopic composition 

(described below) is consistent with in situ oxidation of pyrite. Their abundance in 

stratigraphically lower samples, which may represent slough, could be a product of 

migration of aqueous sulfate in the deposit. 

Stable isotopes 

Alunite, jarosite, and clay minerals from all locations, display a large range in δD 

and δ

18O values (Figure 3). This range is attributed to mixing of magmatic and meteoric 

waters that have variously exchanged oxygen isotopes with wall rocks. Details of fluid 

history are difficult to determine, because hydrogen isotopes in fine-grained clay minerals 

probably are susceptible to retrograde exchange with later fluids in volcanic 

environments (O’Neil and Karaka, 1976) and the relationship between the alteration 

along faults and block and ash units is not currently understood. The isotope composition 

of the fluids that formed alunite is consistent with the largest relative content of magmatic 

water. In contrast, jarosite formed from fluids that contained predominantly meteoric 

water as indicated by the δ

18OSO4 values that are the lowest recorded anywhere for jarosite 

(Rye and Alpers, 1997).Smectite formed from fluids that were a mixture of magmatic and 

meteoric water. The large range for calculated water compositions reflects the 

uncertainty in formation temperatures. 

The range of δD values observed by Frank (1995) for firn and precipitation on 

Mount Rainier defines a –65‰/km gradient for modern precipitation on Mount Rainier. 

The projected δD in Figure 3 for the isotopically lightest meteoric water component that 

formed the alteration phases is close to that observed for glacial ice on the summit today. 

The presumed range of δDH2O of magmatic water may reflect isotope variations during 

degassing of magma (Taylor, 1986). 

Bruce (1998) found that δD values of hydrothermal smectite along a well exposed 

dike from St Elmo’s Pass to Steamboat Prow at Glacier Basin range from about –75 to – 

145‰, whereas δ

 18O values are much more uniform, from about 5.7 to 7.9‰. The δ18O 
16 

values indicate low water:rock ratios that buffered δ

18OH2O values of fluids. The δD 

values do not correlate with elevation but become larger (less negative) away from the 

dike. The range of δD values is consistent with alteration by meteoric water that 

originated from a range of elevations on the volcano above Glacier Basin. 

The δ

34S and δ18O values of alunite, gypsum and jarosite are summarized in Figure 

δ

4. The data in Figure 4 can be compared with characteristic isotope values for alunite 

and jarosite that form in magmatic hydrothermal, steam-heated, and supergene acidsulfate environments from a given fluid composition, as discussed by Rye and others 

(1992) and Rye and Alpers (1997). The δ

 34S values of all sulfate minerals range from 1.7 

to 17.3‰. The largest value is for alunite at Steamboat Prow and the lowest is for jarosite 

at Sunset Amphitheater. The δ

 18OSO4 values of all sulfates range from -12.3 to 15‰ and 

show a positive correlation with δ

 34S values. Alunite with the largest δ 34S and δ 18OSO4 

values also has the least negative δD value, a combination that requires a magmatichydrothermal origin in which H2

SO4 that formed the alunite was derived from the 

disproportionation of magmatic SO2 at a temperature less than 400°C (Rye and others, 

1992). In contrast, jarosite has the lowest δ

34S and δ18OSO4 values consistent with parent 

aqueous sulfate formation largely from the oxidation of pyrite (Rye and Alpers, 1997). 

The δ

 34S values of jarosite from the upper edifice range from 1.5 to 3.3‰; δ34S values 

from the flanks at Glacier Basin range from 2.7 to 4.6‰; values of jarosite from the 

Osceola deposits range from 3.3 to 6.5‰. The distal samples are 3.8 and 5.0‰; proximal 

samples from the mudflow deposits at different elevations are 3.3 and 4.0 ‰; the jarosite 

in the Inter Fork ranges from 5.0 to 6.5 ‰. The δ

 18OSO4 values for four samples of upper 

edifice jarosite range from -4.3 to -12.3‰, while the δ

 18OSO4 values of two samples of 

Osceola jarosite range are -6.9 and -8.2‰. Vein anhydrite/ gypsum have intermediate 18O and δ

34S values that display a good correlation, as do the δ18O and δ34S values for the 

soluble sulfates from the Osceola deposits (Figure. 3). 

Sulfur isotope data for jarosite, soluble sulfates and pyrite from the Osceola 

Mudflow deposits are further compared in Figure 5. The δ

 34S values for jarosite for all of 

the Osceola deposits range from 3.3 to 6.5 ‰ with a spatial variation for individual 

deposits. These values are greater than those in altered rock at Sunset Amphitheater (1.0 

to 3.3‰) but overlap with values for altered rock near St Elmo’s Pass (2.5 to 4.7‰). The 

distinct sulfur isotope composition of the soluble sulfates (near 1.0 ‰) and jarosite (3.3 to 

6.5‰) in the Osceola preclude the soluble sulfates being precursors to jarosite. Pyrite 

from small clasts in the Osceola deposits has δ

 34S values of about 1 ‰. 

Discussion 

Two types of hydrothermal alteration are recognized on Mount Rainier 

(Zimbelman, 1996): 1) alteration of block and ash units on the flanks of the volcano that 

presumably occurred during and shortly after their eruption, and 2) alteration resulting 

from the introduction of acid volcanic gases into the meteoric water in fractured rocks on 

the summit and along lower flanks, within the EWSZ. The latter alteration was sampled 

in place on the summit and flanks of the volcano and also in the Osceola debris flow, 

which was derived from presumably altered summit rock that collapsed. Different types 

of alteration are the result of the range of physical and chemical conditions present in 

volcanoes and these conditions are inferred from the mineralogic and stable isotope data 

(Rye and others, 1992). 
17 

Stratovolcano hydrothermal model 

According to Moran and others (2000) a magma chamber underlies Mount 

Rainier at a depth of about 5-6 km. Fluids degasssed from the chamber produced 

hydrothermal alteration that is schematically described in Figure 6. Further discussion of 

the model leading to Figure 6 is presented in Rye (in press). The general features of this 

model have been developed from the study of ore deposits that formed in stratovolcanoes 

(or in silicic domes in continental terranes) and are based on the work of many people 

(e.g., Henley and Ellis, 1983; Rye and others 1992; Rye, 1993; Hedenquist and 

Lowenstern, 1994; Fournier, 1999). A fundamental spatial reference in this model is the 

brittle-ductile transition that occurs above a crystallizing magma at about 400 °C. Below 

this transition, rocks behave quasiplastically and brittle fracturing is restricted, 

penetration by meteoric water is minimized, and lithostatic pressures dominate. Above 

the transition, meteoric water can be present in open fractures and interact with a 

condensing magmatic vapor phase from degassing magmas. Because of the depth of the 

magmas and possible equilibration of magmatic fluids with crystalline calc-alkaline 

crystalline rocks below the brittle-ductile transition, condensing vapor fluids will 

typically have H2S/SO2 ratios >1 (Rye, 1993; in press). During times of high volcanic gas 

flux fluids may be more SO2 rich. (Heald and others, 1963;). Residual saline liquids from 

boiling of magmatic fluids may become the source of Cu±Mo±Au porphyry 

mineralization at higher levels. As the vapor plume condenses beginning at about 400 

°C, SO2 disproportionates according to the reaction 4SO2 + 4H2O = 3H2SO4 + H2S. The 

H2SO4 leads to extreme base leaching and the formation of various zoned silica +alunite + 

kaolinite + pyrite assemblage and argillic and propylitic envelopes in a magmatic 

hydrothermal environment. The H2

S in these fluids normally reacts with iron in the rocks 

to produce pyrite with excess H2S either venting to the surface or being oxidized by 

reaction with atmospheric oxygen. The H2

S, released by a magma or the boiling of 

deeper fluids, may be oxidized by atmospheric oxygen in steam-heatedenvironment. 

Where atmospheric oxygen is unavailable in magmatic hydrothermal or steam-heated 

environment H2

S may form pyrite by the sulfidation of andesites such as at Marysvale, 

Utah (Cunningham and others, 1984; Cunningham and others, in press). Alunite often 

forms in steam-heated environments, but jarosite may develop instead of alunite under 

exceptionally low pH and high Fe3+ activity conditions (Stoffregen, 1993; Rye and 

Alpers, 1997). Pyrite is susceptible to oxidation when exposed to atmospheric oxygen in 

a supergene environment. The acid-sulfate alteration produced by oxidation of pyrite in a 

supergene environment can be isotopically and texturally similar to that produced by the 

oxidation of H2S in a steam-heated environment (Rye and others, 1992). 

Mount Rainier meteoric water hydrothermal system 

Previous work (Frank, 1995; Zimbelman and others, 2000) identified an active 

meteoric water hydrothermal system on Mount Rainier in which water from glacial melt 

recharges underlying fractures at the summit edifice and is discharged at flanks and base 
18 

Figure 6. Schematic model of hydrothermal system on Mount Rainier (showing glacier 

cover) based on general models for such systems in stratovolcanoes. Model is modified 

from Henley and Ellis (1983); Rye and others (1992); Rye (1993); Rye (in press). 

Magmatic hydrothermal (grey), steam-heated and supergene environments (grey–black 

outline) are shown as overlapping but are normally separate at a given point in time. 

Diagonal cross pattern represents pyrite-rich areas which can occur 1) coexisting with 

alunite in the magmatic hydrothermal alteration, 2) in the surrounding intermediate 

argillic and propylitic alteration, or 3) below the unoxidized zone of a steam-heated 

environment. Also shown schematically relative to the magmatic hydrothermal and 

steam-heated acid-sulfate and supergene environments is approximate location of 

Osceola edifice collapse surface. Diagram is unscaled but Moran and others (2000) 

suggest that it is 5-6 km to the active magma underlying Mount Rainier. 
19 

of the volcano. The system has a deep water table. This meteoric water “scrubs” 

(Symonds and others 2001) volcanic gases such as SO2, HF, and HCl and also serves as a 

source of H2S to overlying vadose or shallow vapor dominated zones as proposed by 

Ingebritsen and Sorey (1988). Gases in active fumaroles in summit cones contain spikes 

of CO2

 up to 10,000 ppm and traces of H2S but no SO2 (Zimbelman and others, 2000). 

These fumaroles have been more active in the past, possibly with higher SO2

 contents, as 

indicated by the occurrence of alunite-type minerals, such as miamiite, on the surface 

near the dormant vents (Zimbelman and others, 2000). 

The positive correlation between δ

 34S and δ 18O values for all sulfate minerals 

including soluble sulfates in the Osceola deposits (Figure 4) is similar to relationships 

observed for sulfate minerals in many hydrothermal ore deposits (Seal and others, 2000). 

A similar correlation is observed for all sulfate minerals at all Cascade volcanoes studied 

to date (Zimbelman and others, in press). At a fundamental level the correlation reflects 

the mixing of isotopically heavy sulfate derived from magmatic fluids with isotopically 

light sulfate derived from the shallow oxidation of H2S or pyrite (Rye and others, 1992; 

Seal and others, 2000; Rye in press). However, as shown in Figure 4 the possibility of 

sulfur isotope exchange between aqueous sulfate and H2S prior to precipitation of sulfate 

minerals may complicate detailed interpretation of the isotope data. 

Magmatic hydrothermal environment 

The mineralogic and stable isotope data on summit and flank alunites require that 

a magmatic hydrothermal environment has been active at Mount Rainier. The alunite in 

an outcrop of altered rock at Steamboat Prow exposed in an old erosion surface has 

sulfur, oxygen and hydrogen isotope signatures (Figures 3 and 4) characteristic of the 

magmatic hydrothermal environment (Rye and others, 1992). The alteration is at least 

200 ka years old (Sisson and Lanphere, 1999). Pyrite was not recovered at Steamboat 

Prow. Pyrite in the Osceola has an average δ

34S value of about 1 ‰. The sulfur isotope 

fractionation (17 ‰) between alunite and pyrite gives a calculated (Ohmoto and Lasaga, 

1982) equilibrium temperature of 360 °C, typical of magmatic hydrothermal alteration 

(Rye and others, 1992). Alunite from near the present summit at Sunset Amphitheater 

has intermediate δ

34S and δ18O values similar to those observed in other Cascade 

volcanoes (Zimbelman and others, in press). Isotope equilibrium involving aqueous 

sulfur species in hydrothermal fluids is kinetically inhibited at low temperatures, even in 

low pH fluids (Ohmoto and Lasaga, 1982). These intermediate isotope values are 

consistent with aqueous sulfate that formed during the disproportionation of magmatic 

SO2

 that was scrubbed out (Symonds and others, 2001) by low temperature meteoric 

water. This scrubbing appears to be going on today as the fumaroles on the present 

summit contain no SO2 and only traces of H2S (Zimbelman and others, 2000). 

The fact that the alunite at the Sunset Amphitheater has a magmatic hydrothermal 

origin is supported by the presence of: (1) zunyite, a phosphate alunite analog, and 

florencite, a fluorine-rich alunite analog, (2) micron-scale, compositionally zoned alunites 

with highly variable K, Na and P contents, and (3) veins of anhydrite/gypsum containing 

barite crystals. Barite and anhydrite readily precipitate by mixing (Holland and Malinin, 

1979), and it is likely that they precipitated as saline fluids mixed with meteoric water 

containing sulfate from the shallow oxidation of H2S or pyrite. Hydrothermal pyrite was 

detected in only a few samples from the summit scraps (Zimbelman, 1996), because it 

quickly oxidizes to aqueous sulfate believed to form jarosite that is abundant along 

fractures. Pyrite detected in clasts from the Osceola Mudflow presumably came from 
20 

deeper within the mountain, beyond zones of extensive oxidation. This pyrite is likely the 

intermediate argillic or propylitic fringe of a magmatic hydrothermal environment. 

Steam-heated environment 

Jarosite + smectite is a characteristic of the matrix of the Osceola Mudflow 

deposits. Presumably, this assemblage was derived from original pyrite + smectite 

hydrothermal alteration. Acid-sulfate alteration in a classical steam-heated environment 

results from the oxidation of H2S derived from the magma or deeper boiling fluids and 

typically results in formation of alunite + kaolinite (Rye and others, 1992). To what 

extent the pyrite + smectite alteration should be considered part of the low temperature 

intermediate argillic or propylitic fringe of a magmatic hydrothermal environment or the 

unoxidized part of a steam-heated environment is not clear. 

Jarosite in the Osceola Mudflow has δ

34S values different from pyrite in clasts and 

from pyrite currently exposed on a summit scarp (Sunset Amphitheater) . Therefore, 

jarosite in the Osceola did not form from the simple in situ oxidation of pyrite in the 

clasts or from pyrite near the current summit. The large range of δ

34S values could reflect 

variations in the δ

34S of magmatic H2

S over time (Taylor, 1986) or, more likely, exchange 

of aqueous sulfate that formed from the oxidation of pyrite with volcanic H2S before the 

precipitation of jarosite. Such exchange between aqueous sulfate and H2S has been 

interpreted to be relatively common in studies of the steam-heated environment (Ebert 

and Rye, 1997). 

Supergene environment 

Disseminated soluble iron and aluminum sulfates in the Osceola are attributed to 

the oxidation of pyrite. This interpretation is supported by the fact that the δ

34S values of 

the soluble sulfates are similar to those of the pyrite preserved in clasts (Figure. 4, Table 

1). The soluble Fe and Al hydroxysulfates that are observed in both the Osceola deposits 

and on summit scarps typically form in very low pH environments, which could include 

alteration zones resulting from surficial oxidation of pyrite. These sulfates are typically 

observed in acid-mine waters where they go through many cycles of solution and redeposition (Bigham and Nordstrom, 2000). An active stratovolcano offers a unique 

environment where both supergene oxidation of pyrite and oxidation of H2S could 

simultaneously occur at a given place. 

Jarosite + smectite alteration is recognized on the flanks of the volcano at Glacier 

Basin below St Elmo’s Pass. This occurrence is a circular -shaped alteration zone that 

includes both the block and ash units and a dike along the EWSZ. It probably represents 

the trace of a hydrothermal plume. The ochre colored zone is easily visible from Glacier 

Basin campground. The well-defined columnar jointing in the dike indicates that the dike 

was probably intruded underneath a glacier and that the hydrothermal plume that 

produced the circular alteration was close (probably within 50 meters, T.W. Sisson, 

personal communication, 2000) of the rock paleosurface. Within a zone of smectite 

alteration, the dike contains fine-grained pyrite. SEM examination and isotope analyses 

indicate that jarosite in the area is supergene and formed from aqueous sulfate derived 

from the oxidation of pyrite. 

Hydrothermal alteration and edifice failure 

As shown in Figure 6, magmatic hydrothermal, steam-heated and supergene acidsulfate environments were developed on Mount Rainier prior to Osceola edifice collapse. 
21 

The deep-seated magmatic hydrothermal environment can be sampled only because of 

exposure by deep erosion at Steamboat Prow and edifice collapse at Sunset 

Amphitheater. The extent to which the magmatic hydrothermal environment is 

developing at depth on the volcano today is unknown. 

Judging from the presence of pyrite and soluble hydroxysulfates observed in 

samples from summit scarps (Zimbelman, 1996), the supergene oxidation of pyrite and 

attendant jarosite precipitation in rocks containing smectite is occurring near the summit. 

The soluble sulfates may also form as sublimates around fumaroles and from the 

evaporation of acid waters. These sulfates should be quickly flushed from the rocks 

when they are exposed to dilute surface waters. Their presence in old altered rocks may 

imply the presence of ongoing pyrite oxidation. Recent acid-sulfate alteration is 

observed in the rocks in snow caves and on flank fumaroles (Zimbelman and others, 

2000). Judging from mineral assemblages around dormant fumaroles, fluid flow appears 

to have been more vigorous in the past (probably immediately after the formation of the 

summit cones) than at present. 

The spatial and isotopic relationships suggest that during edifice collapse the rock 

broke up along fractures containing supergene jarosite, such as observed today on summit 

scarps. The preservation of a unique range of δ

34S values for jarosite in the individual 

Osceola deposits (Figure 5) indicates that a crude edifice stratigraphy may be preserved 

in the proximal deposits, perhaps similar to that preserved in the proximal deposits of the 

1980 eruption of Mount St. Helens (Glicken and others, 1980; Glicken, 1996). 

Jarosite, regardless of origin, forms above the water table (Stoffregen, 1993; Rye 

and Alpers, 1997) whereas alunite forms at or below the water table (Rye and others, 

1992). Alteration was localized along the EWSZ flank fractures and probably in the 

permeable core of the pre-Osceola portion of the edifice. Within the large wedge of rock 

defined by these fractures basal zones of volcanic rock are variously altered to smectite. 

Areas at Mount Rainier which contain large volumes of such altered rock have been the 

source to many post-Osceola collapse events, especially from the amphitheater directly 

west of Liberty Cap, but not the north, Willis Wall, side where altered rock is generally 

lacking. The area containing such alteration was largely removed during the Osceola 

collapse on the eastside of the volcano (Finn and others, 2001). However, a smaller area 

containing similar jarosite + smectite bearing rocks remains on the west-side of the 

volcano (Finn and others, 2001). As recently modeled by Reid and others (2001), this 

area represents the most likely source of future edifice failure. 

Conclusions 

Geologic, mineralogic and stable isotope data of Osceola Mudflow deposits, 

summit scarp and flank outcrops on Mount Rainier can constrain the processes leading to 

the alteration on the edifice of Mount Rainier prior to collapse 5600 years ago. Probably 

the most important feature in the development of the conditions that led to the Osceola 

edifice collapse and subsequent Osceola Mudflow on Mount Rainier was the EWSZ 

fracture system, which bisects the volcano through its summit. This fracture system has 

apparently existed for most of the volcano’s history, ca. >500 ka, and has served as a 

conduit for magmas whose eruption built the volcanic edifice during two major periods 

(Sisson and Lanphere, 1999). It has also served as the plumbing system for meteoric 

water that was recharged at the summit from glacial melt and discharged at various levels 

along the flank of the volcano (Frank, 1995). Just as it does today, the fracture system on 

Mount Rainier focused acid gases from degassing magmas. The meteoric water fluids in 
22 

this fracture system served as a sink for acid volcanic gases such as H2S, SO2, HF and 

HCl at deeper levels and, after modification of gas chemistry, a source of H2S to shallow 

levels. The introduction of magmatic gases into this system led to the development of a 

magmatic hydrothermal environment that involved the disporportionation of SO2 to 

produce acid-sulfate alteration at depth. Reaction of H2S with reduced iron in the 

andesite at shallow levels led to the formation of pyrite. The extent of oxidation of H2

S 

and the formation of a steam-heated environment at shallow levels is not clear. 

Judging from the amount of pyrite+ smectite alteration in small clasts in the 

Osceola Mudflow deposits and jarosite + smectite alteration and soluble Fe and Al 

hydroxysulfates in summit scarps, the sulfidation of andesite related to hydrothermal 

processes appears to have been substantial. In the hydrothermal environment of an active 

stratovolcano, where wet and dry periods will be common (Symonds, 2001), this pyrite is 

susceptible to supergene oxidation with attendant acid-sulfate alteration. 

Volcanic degassing led to formation of extensive hydrothermal pyrite and 

extensive alteration of rocks in the volcano’s upper edifice and along EWSZ fractures. 

These structural zones may have been strengthened by the intrusion of dikes, many of 

which are relatively unaltered. However, altered rocks adjacent to these structures may 

have provided structurally weakened zones that localized collapse of triangular wedges of 

relatively stronger rock with possible local basal altered zones. The ages and relative role 

of magmatic hydrothermal, steam-heated, and supergene acid-sulfate alteration in edifice 

collapse needs to be assessed from further studies of debris flows from Mount Rainier. 

Acknowledgements 

Portions of this study were the basis of a Ph.D. dissertation at the University of 

Colorado while David Zimbelman was employed or contracted by the US Geological 

Survey. This work was supported by the USGS Mineral Resources Program and the 

National Park Service. Additional work was supported by the Minerals Resources 

Program. We wish to thank Carol Gent and Cyndi Kester for help with the stable isotope 

analyses. 
23 

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