SHALLOW TO DEEP RESISTIVITY INVESTIGATIONS FOR GROUNDWATER AND MINERAL DEPOSIT SYSTEMS IN THE SAN JUAN AND SILVERTON CALDERA COMPLEX, COLORADO

Magnetotelluric (MT) geophysical methods are being used along with airborne electromagnetic methods (AEM) to investigate the San Juan and Silverton caldera complex located in the Southern Rocky Mountains Volcanic Field. The MT method uses Earth’s naturally occurring electromagnetic energy sources to model subsurface resistivity structure to depths of 10s of kilometers, compared to AEM data which typically have depths of investigation of 100s of meters. MT has been used previously in caldera settings for geothermal and groundwater resource applications. Four MT profiles cross mineralized and barren caldera-related faults and vein structures to: 1) investigate deep groundwater flow paths and 2) model the resistivity structure of Silverton area mineral deposit systems (acid sulfate, epithermal polymetallic vein, and Cu-Mo porphyry). Existing USGS geologic maps facilitated subsurface interpretations of MT profiles. A 2-D resistivity model crossing the northwest caldera structural margin reveals resistive offset blocks of inferred andesitic lavas (~2000 ohm-meters) and a relatively conductive zone (20 – 50 ohm-meters) beneath an intrusion and an area of mineralized sulfide-rich breccia pipe ores. In upper Cement Creek near the vicinity of the Gold King Mine and in the core of the calderas, overlapping AEM and MT profiles show similar resistivity at shallow depths (50 meters) with correlation of resistive lavas and a shallow conductive feature interpreted as groundwater containing high total dissolved solids. MT shows a conductivity high at ~500-m depth west of the Gold King Mine consistent with argillic alteration or possible interconnected sulfides. In upper Eureka Gulch, an MT profile shows apparent resistivity highs on either side of the faults that bound the Eureka Graben, formed during caldera resurgence. A conductive zone at ~800-m depth near former lake Emma is consistent with possible high sulfide contents. MT and AEM data will be integrated into a 3-D geophysical model of the caldera complex. The model will allow resistivity-based interpretations of the hydrothermal plumbing system responsible for mineralization and a better understanding of modern groundwater flow paths that may intersect mine tunnel areas and sites scheduled for mine cleanup.