Geology, Petrology, and Geochemistry

Understanding the basic geology controlling the flow of thermal water is the basic foundation for developing a conceptual model of the geothermal system.  Geologic mapping as identified a facture network dominated by a fault that runs through the Chena hot springs.  Rocks on opposing sides of this fault have pulled apart allowing hot water to quickly rise to the surface from depths of several thousand feet.  Near the surface the hot water mixes with the cold shallow groundwater system flowing on and through gravel deposits in the valley.

Amanda Kolker, a geology graduate student at the University of Alaska Fairbanks has been responsible for the geologic mapping of the Chena site, under the supervision of UAF researcher Dr. Jess Larson.  You can contact Amanda by e-mail at ftamk1@uaf.edu.

Geologic field mapping and laboratory studies were conducted May 2005 through May 2006. Data is still being reduced as of summer 2006. Study objectives are: 1) To characterize surface geology in the immediate hot springs area by field mapping and analysis of surface samples (petrology, mineralogy, geochemistry); 2) To characterize borehole geology by lithologic logging of cuttings from geothermal wells and analysis of selected samples (petrology, mineralogy, geochemistry); 3) To characterize fluid-rock interactions in the geothermal system by hydrothermal alteration studies and fluid inclusion studies; 4) To develop a hypothesis for the ultimate heat source of Chena Hot Springs.

BACKGROUND: The ultimate heat source driving the geothermal activity in Interior Alaska has not been established. All of the Interior Alaska hot springs areas are in or near granitoid plutons of Mesozoic to early Tertiary age. Chena Hot Springs lies in the center of an enclosing 5x40 km² pluton. Geothermal activity at Chena Hot Springs may be from deep circulation on back-arc faults, radioactive decay of elements (U, Th) within the granite, and/or a concealed magma body.

RESULTS TO DATE: The junction between two concealed, previously unmapped faults appears to be the main upwelling zone for the hydrothermal fluids. The Chena Hot Springs area has likely experienced at least two different intrusive events, one in the late Cretaceous and another in the early Tertiary, followed by mafic dike emplacement. Mineral alteration assemblages record at least three distinct episodes of hydrothermal activity in the area. The oldest assemblage indicates a high- temperature hydrothermal regime and is unrelated to the present-day system. A second assemblage is related to the present system but indicates slightly higher temperature conditions. A third assemblage reflects the present system. All three assemblages are associated with rock microbrecciation, indicating that faults are and have been main hydrothermal conduits for an extended period of time. Fluid inclusion data, chemical geothermometry, and alteration mineralogy all indicate that the CHS geothermal reservoir has temperatures of ~120-130^oC. Near the surface, hot water mixes with a cold shallow groundwater aquifer. While there is no evidence for anomalous concentrations of U or Th in any of the analyzed samples to date, this does not completely refute the radioactive decay hypothesis for the heat source of the Interior Alaska hot springs.

Amanda Kolker and Dr. Jessica Larsen