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Geochemistry of a volcanic hydrothermal system at Mount Spurr, Alaska
Garchar, Laura
Garchar, Laura
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2012
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2012
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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 fluid pathways in the system. Three spatially distinct water compositions have been identified: 1) moderate temperature (around 50 degrees C), acidic, bicarbonate-poor, d18O and dD-enriched waters from Crater Lake; 2) low temperature (14-40 degrees C), neutral, bicarbonate and Mg-rich, slightly d18O and dD-enriched peripheral waters from Crater Canyon, Kid Canyon, and South Spurr; and 3) cold (less than 9 degrees C), 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 per mil fractionation happened over around 30 degrees C. 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 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.
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