UTILIZING GEOLOGIC MAPPING TO INVESTIGATE HYDROGEOLOGY AND ASSESS NATURAL HAZARDS IN THE HISTORIC VIRGINIA CITY MINING DISTRICT, SOUTHWEST MONTANA

Virginia City is one of southwest Montana’s oldest gold mining districts whose colorful history now draws over 300,000 tourists annually. Residents have become concerned that growing tourism and development will negatively impact the town’s springs, which are the only developed water source. In 2017, the Montana Bureau of Mines and Geology initiated a study to characterize the hydrogeology of local springs, identify potential supplemental water sources, and assess landslide hazards. This natural resource and natural hazard evaluation was accomplished through the integration of STATEMAP geologic mapping (1:24,000 scale), ground and surface water monitoring, LiDAR elevation data, radiometric dating, geochemistry, and geophysical surveys.

The oldest rocks in the Virginia City 7.5’ quadrangle are Archean quartzofeldspathic gneiss (~2.7–1.7 Ga) layered with subordinate amphibolite, marble, quartzite, and small bodies of ultramafic rock, all of which were intruded by Proterozoic pegmatite and diabase dikes and sills. These units are low yield bedrock aquifers, and are unlikely to be sufficient for public water supply. Late Eocene mafic-to-intermediate lava flows (47–61 wt. % SiO₂; 34.4–32.9 Ma) and intercalated rhyolitic tuff deposits (41.2–34.0 Ma) rest on an unconformity truncating the older Archean-Proterozoic units; small exposures of intrusive dacite porphyry (60–68 wt. % SiO₂; ~50 Ma) occur locally. Lava flows commonly exhibit primary jointing, vesiculation, and brecciation favorable to groundwater flow; however, the lavas mostly occur above the water table and are not likely targets for municipal wells. The tuff deposits generally have low primary permeability and yield only modest flows in zones of extensive fracturing. The volcanic units are generally susceptible to mass wasting, and a large landslide complex encompassing Virginia City is likely the groundwater recharge area for several local springs, including the current municipal water source. Extensive Quaternary gravels mined for placer gold in Alder Gulch are up to ~30 m thick, and may be a viable supplemental water source.

Results from this study will help Virginia City manage its water resources and mitigate natural hazards, demonstrating the value of modern geologic maps to address practical earth science problems.