Young rock fall on Tahoma Glacier, Mount Rainier National Park
Tephra from Granite Park on northeast side of Mount Rainier
Bubbling mudpots containing Fe-sulfide scum, Bumpass Hell, Lassen Volcanic National Park, CA
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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.
Accomplishments
- 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.
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