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Geochemistry of a Volcanic Hydrothermal System at Mount Spurr, Alaska

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Geochemistry of a Volcanic Hydrothermal System at Mount Spurr, Alaska GEOCHEMISTRY OF A VOLCANIC HYDROTHERMAL SYSTEM AT MOUNT SPURR, ALASKA by Laura Garchar A thesis submitted to the Faculty and the Board of Trustees of the Colorado School of Mines in partial fulfillment of the requirements for the degree of Masters of Science (Geology). Golden, Colorado Date Signed: Laura Garchar Signed: Richard F. Wendlandt, PhD Thesis Advisor Golden, Colorado Date Signed: John D. Humphrey, PhD Associate Professor and Department Head Department of Geology and Geological Engineering ii ABSTRACT Mount Spurr is an ice and snow-covered andesitic volcano located at the northern extent of the Aleutian arc in south central Alaska. Previous workers have identified a prospective geothermal resource on the volcano’s south side. This research aims to characterize more fully the hydrothermal system and builds on published geological mapping, geophysical surveys, soil sampling, exploration drilling, and water sampling. Aqueous and mineralogical geochemical investigations at a field area on the south flank of Mt. Spurr focus on the possible extent of the hydrothermal system, its temperature at depth, the origin of waters, and location of fluid pathways in the system. Three spatially distinct water compositions have been identified: 1) moderate temperature (˜50), acidic, bicarbonate-poor, δ18O and δD-enriched waters from Crater Lake; 2) low temperature (14-40), neutral, bicarbonate and Mg-rich, slightly δ18O and δD-enriched peripheral waters from Crater Canyon, Kid Canyon, and South Spurr; and 3) cold (<9), neutral, sulfate-rich, dilute/meteoric waters from Chaka Ridge and South Spurr. Geochemical modeling and stable hydrogen, oxygen, and carbon isotopes have confirmed that water chemistry cannot be explained solely by meteoric water-andesite interaction, or by mixing of Crater Lake and meteorically derived waters. Contributions to chemistry must come at least in part from a magmatic system. No alteration minerals were observed at the surface near springs, but kaolinite likely lies along the flow path of waters in the subsurface. Spring and seep waters are derived primarily from shallowly circulating meteoric water that has variously interacted with rock, incorporated condensed steam, in some cases mixed with andesitic water, and been diluted by rain and snowmelt overall. Montmorillonite-rich conglomerates and sandstones of the West Foreland Formation were encountered in exploration core holes located in the eastern field area. Post-depositional changes including the formation of montmorillonite from tuff, the presence of replacement-style calcite, as well as compaction and faulting processes are observed in hand samples and thin sections of 26-11 core. Stable carbon and oxygen isotopes of calcite samples are similar to published calcite isotopes from Cook Inlet sedimentary rocks. If all the calcite examined in this research formed from a fluid of the same isotopic composition, a 15‰ fractionation happened over ˜30. The calcite samples are interpreted to be of meteoric-dominated origin and to have formed at low temperature diagenetic conditions. The sedimentary rocks of the West Foreland Formation have low visual porosity, and no evidence of recent hydrothermal alteration has been observed. Surface discharges do not seem to represent rapidly upwelling, deep, hydrothermal fluid. It appears that meteoric water doesn’t circulate to depth and that deep water doesn’t make it to the surface, except locally. The transport of hydrothermal fluids to the surface seems to be controlled by faults. However, not all iii mapped faults are conduits for deep fluid flow. The montmorillonite of the West Foreland Formation could be acting as a barrier to deep circulation of meteoric water, and could prevent upwelling of deeply sourced hydrothermal fluids. Montmorillonite could also be the source of resistivity anomalies in the field area. Exploration drilling has not penetrated the entire thickness of the West Foreland Formation, and what lies below is unknown. If a fractured granite basement that contains hydrothermal fluids exists beneath the West Foreland Formation, it could be a viable geothermal reservoir. Precise mapping of fault structures that act as conduits to fluid flow and deeper drilling, especially near Crater Canyon, will enhance the understanding of a possible geothermal resource at Mt. Spurr. iv TABLE OF CONTENTS ABSTRACT . iii LIST OF FIGURES . viii LIST OF TABLES . xiii ACKNOWLEDGMENTS . xiv CHAPTER 1 MOTIVATION . 1 1.1 Geothermal Resources of Alaska . 1 1.2 Geothermal Resource at Mt. Spurr . 5 CHAPTER 2 BACKGROUND . 6 2.1 Regional Geologic History . 6 2.1.1 Paleozoic . 8 2.1.2 Mesozoic . 8 2.1.3 Cenozoic . 9 2.1.4 Recent Faulting . 10 2.2 Eruptive History of Mt. Spurr . 11 2.3 Previous Geothermal Investigations . 12 CHAPTER 3 METHODS . 20 3.1 FieldSampling .............................................20 3.2 Analytical Chemistry . 23 3.3 Petrography of Drilled Core . 26 3.3.1 Clay Mineral Characterization . 26 3.3.2 Calcite Mineral Characterization . 27 CHAPTER 4 RESULTS . 30 4.1 Aqueous Geochemistry . 30 4.2 Petrography of Conglomerate and Sandstone Unit in Drilled Core . 30 4.3 Whole Rock and Clay Mineral Characterization . 33 v 4.4 Isotopes of Calcite in Veins, Rims, and Cements . 35 CHAPTER 5 INTERPRETATION: AQUEOUS AND GAS GEOCHEMISTRY . 46 5.1 Dissolved Chemical Species . 46 5.1.1 Cl-HCO3-SO4 ..........................................48 5.1.2 Na-K-Mg . 50 5.1.3 Cl-Li-B . 52 5.1.4 Chloride vs. Enthalpy . 53 5.1.5 Silica Saturation . 54 5.2 Stable Isotopic Composition . 55 5.2.1 Origin of δ18O and δDinMt.SpurrWaters.........................56 5.2.1.1 Hot Water Fraction of Crater Canyon Discharges . 57 5.2.1.2 Elevation and Chloride Concentration Effects . 59 5.2.2 Origin of δ13CinMt.SpurrWaters .............................60 5.2.3 Origin of δ34S in Mt. Spurr Waters . 61 5.3 Gas Geochemistry from Crater Peak Fumaroles . 63 5.3.1 N2-He-Ar . 63 5.3.2 CO2-CH4-N2 ..........................................65 5.4 Geothermometry Calculations . 65 5.4.1 Solute Geothermometer Calculations . 65 5.4.2 Isotope Gethermometer Calculations . 67 5.4.3 Gas Geothermometer Calculations . 67 5.5 Summary of Aqueous and Gas Geochemistry . 69 CHAPTER 6 INTERPRETATIONS: MINERALS AND MINERAL RELATIONSHIPS IN DRILLED CORE . 73 6.1 Unit Identification . 73 6.2 Diagenetic Changes . 74 6.2.1 Ductile Deformation . 74 6.2.2 Authigenic Clay . 74 vi 6.2.2.1 Montmorillonite . 74 6.2.2.2 Chlorite . 75 6.2.2.3 Laumontite . 75 6.2.3 Calcite . 76 6.2.4 Porosity and Permeability . 77 CHAPTER 7 DISCUSSION . 80 7.1 Water-Rock Interaction . 80 7.1.1 Models Involving Crater Lake Water, Glacial Melt, and Andesite . 80 7.1.2 Models Involving Na-K-Ca-Mg Aluminosilicates and Water at Various Temperatures . 84 7.2 Conceptual Cross-Sections . 89 7.2.1 Temperature Gradient . 92 7.3 Comparison to Producing Geothermal Systems in Similar Settings . 92 7.3.1 Southeast Asia . 93 7.3.2 Central America . 94 7.3.3 Summary . 95 CHAPTER 8 CONCLUSIONS AND FUTURE WORK . 96 8.1 Possible Geothermal System . 96 8.1.1 Extent of Hydrothermal System . 96 8.1.2 Origin of Waters . 96 8.1.3 FluidPathways.........................................97 8.1.4 Deep temperature and Reservoir Characteristics . 97 8.2 Implications for Future Geothermal Exploration and Development . 97 REFERENCES CITED . 99 vii LIST OF FIGURES Figure 1.1 Geothermal resources of Alaska are indicated by areas of red shading. Nearly all of the Aleutian arc volcanoes fall into this category. Data from U.S. Department of Energy, figure modified from INEL. 2 Figure 1.2 Mt. Spurr is located in south-central Alaska in the Tordrillo Mountains on the northwest side of the Cook Inlet about 100km west of Anchorage. Regional faults and geographic features are labeled. ..
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