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Resources and Hazards of Hydrothermal Systems in Cascades Volcanoes

Planned products:

Potential applications of the data and models generated during the full project include:

Major products are anticipated to include a series of topical reports on the metallogeny of the Cascades, active hydrothermal systems in the Cascades as modern analogues of ore-forming systems, and hydrothermal alteration of Cascades volcanoes and volcanic hazards. In addition, several syntheses will be written that contain highly integrated quantitative models for both hydrothermal alteration on the surface and mineralization in the interiors of Quaternary Cascade stratovolcanoes and comparative metallogeny of the Tertiary Western Cascades and Quaternary Cascades arcs to other continental-margin arcs worldwide.

There are three tasks associated with this project:

  1. Metallogeny of the Cascades
  2. Active hydrothermal systems and ore-forming environments in the Cascades
  3. Hydrothermal alteration and volcano hazards

Task 1 - Metallogeny of the Cascades

James Hardee silica quarry near Enumclaw, WA. Quarry is developed in vuggy silica alteration related to an approximately 20 Ma hydrothermal system consisting of vuggy silica, quartz-alunite, advanced argillic, and argillic alteration exposed over several tens of square kilometers. Silica is quarried for specialty products
James Hardee silica quarry near Enumclaw, WA

Map showing mineral occurrences (MRDS sites) and major mining districts in the Western Cascades (Eocene to Pliocene rocks) and the relationship between the Western Cascades arc and the Quaternary High Cascades.
Map showing mineral occurrences and major mining districts in the Western Cascades

Seeks to better characterize and understand the metallogeny of the Tertiary and Quaternary Cascades arcs in northern California, Oregon, and Washington. It builds on preliminary results of MRP-funded projects of the early 1990s. It addresses fundamental questions regarding regional metallogeny through a comparative study of the weakly mineralized Quaternary High Cascades to more highly mineralized parts of the Oligocene-early Pliocene arc. Work includes digital compilation of existing data, such as geologic and geophysical maps of the study area, building of district deposit MRDS records, field studies of selected mining districts and igneous centers, and collection of new geochemical and geochronologic data for selected mining districts and igneous centers. The results of these studies will provide knowledge about mineral deposit generation in continental-margin arcs that are broadly applicable to magmatic arcs worldwide.

Proposed work

  • The digital geologic map of the project area will be extended to provide a 100 km "buffer" around the Cascades arc.
  • Samples collected will be prepared and submitted for geochemical analysis and isotopic dating
  • GIS maps will be prepared of age and structural constraints on the structural history of the arc and its associated ore deposits.
  • Results from field work will be used to develop a working model of the structural history of the arc and its along-strike variation.
  • Associations of Cascade ore deposits with features of this space-time structural model will be investigated.
  • District deposit MRDS records will be compiled for the Oregon and northern California parts of the Cascades.
  • Field work in FY03 will be aimed at filling in poorly constrained areas and time periods in the working model and in the district deposit MRDS records, especially in Oregon.
  • Ar-Ar dating of samples collected will continue.
  • Alteration zones associated with mercury mineralized systems in the Cascade arc will be delineated as a function of age, volcanic setting, and geochemistry. A cooperative study with U. of Michigan (Chris Smith) will be initiated to evaluate Hg isotopes in the arc and S isotopes (Rye).
  • Gallium geochemistry, mineralogy, and distribution in advanced argillic alteration zones will be studied and compared to similar epithermal environments in the Great Basin.
  • Comparison of geophysical anomalies and mineral deposits will be undertaken. Northeast-striking gravity anomalies in the Cascade Range have been interpreted as originating from pre-Tertiary basement lying beneath younger volcanic rocks. These anomalies have a spatial association with volcanic 5 Ma and younger. This association suggest that basement structures have had a long-term influence on upper-crustal magmatism, which may be reflected in the distribution of volcanic-hosted mineral deposits.
  • Petrochemical data for the Tertiary Cascades arc will be compiled and new sampling will be undertaken in selected igneous centers.


  • Partly digital, published paper maps of the Oregon and Washington Cascades arc (USGS I-2005 and I-2569) were combined into a single digital coverage with unpublished digital maps for the California Cascades arc and with the Washington Dept. of Natural Resources 100K-scale digital geologic maps to produce a digital geologic map of the entire Cascades arc to the Canadian border
  • Preliminary analysis of aeromagnetic and gravity anomalies, Enumclaw area, WA, were made. The spatial association of intense acid-sulfate alteration observed near Enumclaw, Washington, and a magnetic anomaly observed in aeromagnetic data suggest that the anomaly is caused by the destruction of magnetite during alteration of volcanic rocks. The source of the anomaly underlies an area of approximately 25 km2, extending north and northwest of the main exposures of intense alteration. A small gravity anomaly lies directly above the main exposure of acid sulfate alteration and may reflect a deeper intrusion that facilitated the alteration
  • Ground-based geophysical studies, Enumclaw area, WA, were undertaken. Detailed magnetic studies delineated the lateral extent of an aeromagnetic low associated with intense acid-sulfate alteration.
  • Compilation of existing mineral occurrence data (MRDS, DMEA, company and consultant records, published literature) of known mineral deposits in Washington were made to define geographically associated mineral deposit systems (mining districts) and to characterize their geologic attributes (age, deposit character, elemental associations, structural control, igneous association, etc.)
  • Field studies of epithermal districts in west-central Oregon and southwest Washington were undertaken to better characterize epithermal and pluton-related deposits and to collect samples for isotopic dating and for geochemical characterization. Geophysical maps were constructed for the fieldwork in western Oregon
  • Analysis of MRDS records and limited field studies indicate that mercury deposits and occurrences in the Cascade arc are the most abundant deposit type. There are two distinct types of Hg deposits, high sulfidation quartz-alunite, and low sulfidation hot spring type. These two types of deposits occur in both the fore- and back-arc regions as well as in the present day active arc. An abstract on Hg deposits in the Cascades arc was prepared for the GSA meeting in Nov. 2003
  • A guide book to the White River, WA, silica districts and acid-sulfate alteration was prepared for a Society of Economic Geologists field trip for Nov. 2003. An abstract on this district was prepared for a poster at the GSA meeting in Nov. 2003 and the Northwest Mining Association meeting in Dec. 2003
  • Structural data were collected in various mining districts in the Oregon and Washington Western Cascades arc. An abstract comparing late Tertiary paleostress regimes in these parts of the arc was prepared for the GSA meeting in Nov. 2003.

Task 2 - Active hydrothermal systems and ore-forming environments in the Cascades

Young (<600 ybp) rock fall on Tahoma Glacier, Mount Rainier National Park. Rock fall from Sunset Amphitheater near crest of Mount Rainier contains abundant clasts of pyrite-rich hydrothermally-altered Quaternary lava flows. Clasts contain well-developed oxidation rinds 1-5 mm thick. Rock fall is only a few meters thick and sits on ice near the toe of Tahoma Glacier at head of South Puyallup River
Young rock fall on Tahoma Glacier, Mount Rainier National Park

Tephra from Granite Park on northeast side of Mount Rainier. Thin dark-colored layer above orange limonite-rich layer contains abundant fine-grained pyrite that apparently represents hydrothermal alteration minerals incorporated into the tephra during eruption.
Tephra from Granite Park on northeast side of Mount Rainier

Bubbling mudpots containing Fe-sulfide scum, Bumpass Hell, Lassen Volcanic National Park, CA
Bubbling mudpots containing Fe-sulfide scum, Bumpass Hell, Lassen Volcanic National Park, CA

Studying active and dormant hydrothermal systems on Cascade volcanoes to better characterize hydrothermal alteration and mineralization in the volcanoes. It combines results and interpretations of field mapping and sampling, mineralogy, stable isotope, chemical leach analysis, and chemical modeling studies. The goal will be to understand better the geochemical and hydrogeochemical processes active in the volcanoes, and the role these processes play in mineralizing systems, volcanic hazards (such as slope stability in altered areas), and environmental issues (such as natural acid-rock drainage).

Proposed work:

  • Mineralogical, stable isotope, and geochronological characterization of hydrothermally altered rocks from Cascade volcanoes will continue. Samples from both the edifices and their associated debris flows will be collected and characterized. Work will continue to focus on Mt Rainier and Mt Adams, two of the most highly altered Cascades volcanoes. In subsequent years, other less altered volcanoes such as Mt Hood and Mt Shasta will be evaluated as a comparison. Field studies and laboratory analyses will be completed on samples from Mt Rainier and its associated mudflow depostis. Results will be interpreted and reports initiated. Detailed field sampling and laboratory characterization studies will be initiated on Mt Adams. Reconnaissance studies have identified abundant jarosite, which we suspsect is derived from the oxidation of pyrite. Based on data collected from these studies, the chemical conditions and processes responsible for the mineral assemblages (include both volcanic-hydrothermal alteration processes and lower-temperature weathering processes) will be interpreted.
  • Geochemical modeling of Cascade hydrothermal and ground-water processes will be initiated to help in the interpretation of the results of the mineralogical, stable isotope, and chemical studies and data compilations mentioned in items 1 above. Chemical speciation and inverse chemical modeling (using PHREEQEC) will be used to understand the magnitude and nature of chemical reactions (such as amounts of minerals precipitated or dissolved, and gases absorbed or exsolved) that produced measured water compositions. Forward chemical reaction path modeling (using SOLVEQ, CHILLER, and Geochemist's Work Bench) will be used to refine geochemical models of water-rock-gas interactions that produce rock alteration and hydrothermal mineralization on the volcanoes. The use of coupled hydrologic and chemical modeling routines to evaluate gechemical and hydrologic processes on the Cascades volcanoes will also be evaluated.
  • Smectites and kaolin-group minerals are major products of alteration on the Cascade volcanoes. Relatively limited work has been done to test the ability of these phases to record conditions of formation in the context of the hydrothermal evolution. A reconnaissance of a few selected samples will attempt to prepare relatively pure separates of the clay minerals to evaluate their mineralogy, crystallinity, exchangable ions, as well as chemical and isotopic composition. Targeted efforts will compare in-place rock of varying degrees of alteration, debris flows, and tephra deposits.
  • Reconnaissance studies in the Lassen Peak area have identified substantial amounts of mercury in and gold mineralization in the active geothermal areas and in the older volcanic centers. More detailed work will be carried out to define the extent of and processes that formed this mineralization. We will determine the flux of Hg and As being released in effluent from the active geothermal sytems in the Lassen Peak area, and the potential for Lassen as a source of Hg to the Sacramento River watershed.


  • Substantial progress was made in the mineralogic, isotopic, and chemical characterization of hydrothermal alteration minerals from Mount Rainier. Samples collected from various debris flow deposits and tephra deposits were analyzed using petrographic, X-ray diffraction, scanning electron microscope, and stable isotope analytical techniques, which have provided significant information on the minerals present, as well as the isotopic composition and chemical composition of the minerals. A wide variety of alteration minerals have been identified, including silica minerals (opaline silica, quartz), acid-sulfate alteration minerals (alunite, jarosite, kaolinite, pyrophyllite, and others), clay minerals (smectites, saponites, others), native sulfur, sulfides (primarily pyrite), and sulfate minerals (gypsum, anhydrite, and a variety of soluble metal sulfates). Many of the minerals have quite characteristic isotopic compositions that provide insights into their origins.Chemical modeling studies were initiated that model the chemical interactions of volcanic gases with ground waters and volcanic rocks on Mount Rainier. The calculations use as input a range of volcanic gas compositions measured on a variety of active volcanoes, and the andesite rock compositions from Rainier. The calculations model gas condensation into varying amounts of ground waters, and the progressive chemical reaction of the gas-groundwater mixtures with andesite rocks; the calculations predict the changes in fluid chemistry and the amounts of minerals precipitated or dissolved as a result of these chemical processes. The model results are then compared to the actual alteration mineral assemblages observed on Mount Rainier, including the abundances of different minerals and zoning of minerals spatially within hand samples and across the volcano. For example, observed alteration assemblages grading from acid-sulfate alteration minerals into clay+ pyrite assemblages are consistent with the progressive reaction of a highly acidic gas condensate with andesite rock. These studies provide new insights into the processes that produced hydrothermal alteration on Mount Rainier, as well as the 3-dimensional distribution of alteration types within the volcano. Five abstracts summarizing the results to date have been prepared and submitted to the Nov. 2003 GSA national meeting for presentation at the special session on hydrothermal alteration in Cascades volcanoes.
  • Field investigations of the Lassen Peak area began. Field work was concentrated on (1) the Maidu volcanic center, where several square kilometers of intense acid-sulfate alteration is exposed; (2) the core of Brokeoff Volcano, which is dissected by glaciation and landslides exposing areas of intense acid-sulfate alteration; and (3) the active hydrothermal systems at Bumpass Hell and Little Hot Springs Valley. Samples were collected for petrographic, geochemical, isotopic, and geochronologic studies. These samples also are being used to calibrate AVIRIS remote sensing imagery of the Lassen Peak area.

Task 3 - Hydrothermal alteration and volcano hazards

Map showing area (black) inundated by lahars or associated floods from Mount Rainier in the last 6000 years. The Osceola Mudflow flowed north and northwest down the White River to the Puget Sound lowlands 5600 years ago. From Sisson and others (2001, EOS, v. 82, no. 9, p. 113)
Map showing area (black) inundated by lahars or associated floods from Mount Rainier in the last 6000 years

Simplified geologic map of upper Mount Rainier showing mapped dikes (red) and fractures and areas of intense hydrothermal alteration (yellow). Cross pattern shows area underlain by post-Oceola Mudflow volcanic rocks. Glaciers heading on the altered western flank drain into the Puyallup River system. From Sisson et al., 2001, EOS, v. 82, no. 9, p. 113.
Simplified geologic map of upper Mount Rainier showing mapped dikes and fractures and areas of intense hydrothermal alteration

Osceola Mudflow exposed on the banks of the White River near Greenwater, WA, about 50 km downstream from the summit of Mount Rainier. Mudflow deposits are about 8 m thick and contain abundant hydrothermal clay minerals in the matrix and clasts of hydrothermally altered rocks. Pale yellow color due to oxidizing pyrite. Unoxidized matrix contains several percent fine-grained pyrite
Osceola Mudflow exposed on the banks of the White River near Greenwater, WA, about 50 km downstream from the summit of Mount Rainier

Aims at improving information on the distribution and origins of hydrothermal alteration on active Cascades volcanoes to support improved hazards evaluations. This task is working closely with experts on individual volcanoes from the Volcano Hazards Program. Work includes geologic mapping, mineralogical characterization, remote sensing, and geophysical studies of hydrothermal alteration on selected volcanoes (Mounts Rainier, Adams, Baker, and Shasta, and Lassen Peak), and integration of these data with existing data on the geology and eruptive histories of the volcanoes to model the potential role of hydrothermal alteration in volcanic hazards. Hazards evaluations, assessments, and pronouncements will remain the responsibility of the Volcano Hazards Program.

Proposed work:

  • Analysis, modeling and interpretation of the Mt. Adams and Mt. Baker helicopter magnetic and electromagnetic data will be completed. Carol Finn, assisted by Eric Anderson (GIS tech), will analyze the magnetic data; the EM analysis will be done by Maryla Descz-Pan. Interpretation of these data will be done with Tom Sisson, David John, and Bob Rye and integrated with the results from Mt. Rainier. Preparation of 1-2 journal articles will begin.
  • ASTER and available AVIRIS data of several eroded Cascade stratovolcanoes will be analyzed to help explore the mineralogy and structural controls on deeper alteration zones within volcanic edifices. Samples will be analyzed by using spectroscopy, X-ray diffraction, and possibly, stable isotopic, and fluid inclusion methods to determine mineralogy, gas and fluid compositions, and P/T regimes. Laboratory measurements will also be made to characterize sample geophysical properties, particularly magnetic properties. The geophysical property information will be pertinent to the interpretation of 3-D geophysical models being developed for some Cascade volcanoes based on aeromagnetic and electromagnetic data. For example, studies of eroded stratovolcanoes may help to understand recent geophysical results at Mount Rainier, which suggest that only limited alteration exists beneath the two young cones that comprise the post-Osceola summit.
  • New AVIRIS data for Mount Adams and Mount Hood will be analyzed to produce alteration mineral maps that are more detailed than those currently available. The alteration maps will also be used in conjunction with digital elevation data to identify steep altered rock masses that may be prone to slope failure.
  • Hydrothermal alteration products in the Osceola Mudflow and other lahars will be characterized by David John in conjunction with Jim Vallance (USGS-WRD) and George Breit and others on task 2.
  • A Society of Economic Geologists-sponsored theme session on Hydrothermal alteration and volcano hazards for the 2003 GSA annual meeting in Seattle was organized (John and Sisson).


  • Field investigations of the summit and flanks of Mt. Adams were conducted. The Mt. Adams summit expedition collected geophysical data to ground-truth remote sensing, aeromagnetic, and resistivity maps and samples for petrographic, geochemical, and geochronologic studies
  • Preliminary modeling of the electromagnetic and magnetic data from Mt. Adams was completed. At Mt. Adams, the depth of penetration of the lowest frequency electromagnetic data was greater than at Mt. Rainer. Within the low resistivity, altered zones, penetration was ~100 m; outside the zones it was several hundred meters. Total-field magnetic data can detect magnetization variations to several thousand meters depth. Limited outcrop of altered rock occurs primarily in headwalls of the Avalanche and Adams Glaciers and in Roosevelt Cliff. Cores through the ice cap north of the summit suggest an additional area of 0.5 to 1 km2 of variably altered rock concealed beneath glaciers. The part of this region north of the summit correlates with low magnetizations and resistivities that extend more than 250 m below the surface. The lowest resistivities just north of the summit suggest the presence of water-saturated, clay-rich hydrothermally altered rock. Higher resistivities and magnetizations over the Roosevelt Cliff area indicate that alteration there is not as intense.
  • Geologic maps were digitized and overlain on geophysical maps and the DEM for Mt. Adams.
  • Magnetic and topographic data from Mt. Shasta, Mt. Hood and Mt. St. Helens (pre- and post-1980) were retrieved from archives.
  • A paper comparing remote sensing data for Mount Shasta accepted for publication in Remote Sensing of Environment.
  • NASA funding was obtained for collection of MASTER data of Mount Adams and Mount Hood. Initial data analysis was performed.
  • An initial ASTER alteration map of Mount Hood was constructed with B. Hubbard (EMR). Hubbard is in the early stages of work with T. Pierson in an effort to link edifice alteration with debris flow mineralogy in the Mount Hood area.
  • A summary abstract on remote sensing surveys of Cascade volcano alteration was written for the GSA meeting in Nov. 2003.
  • A theme session on hydrothermal alteration on active volcanoes was organized for GSA annual meeting in Seattle in Nov. 2003.
  • Andy Calvert (VHZ) produced the first 40Ar/39Ar measurements of Quaternary alunite and jarosite from the Cascades. These results provide limiting ages of < 100 - 130 kyrs ago for alteration of rocks in the Holocene Osceola Mudflow and Paradise Lahar from Mt. Rainier.
  • Field Guide to Hydrothermal Alteration in the White River Altered Area and in the Osceola Mudflow, Washington by David A. John, James J. Rytuba, Roger P. Ashley, Richard J. Blakely, James W. Vallance, Grant R. Newport, and Gary R. Heinemeyer

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