ELECTROMAGNETIC DATA FOR SUBSURFACE IMAGING OF ARRASTRA GULCH, SAN JUAN COUNTY, COLORADO: IMPLICATIONS FOR LAND-USE MANAGEMENT DECISIONS

Airborne electromagnetic and magnetotelluric geophysical surveys were conducted from 2018 to 2019 in Arrastra Gulch along the southern margins of the San Juan and Silverton calderas. These data characterized resistivities of subsurface lithologies, hydrothermal plumbing of mineralized vein structures and faults, and groundwater near legacy mines slated for remediation. The airborne electromagnetic survey measured resistivity of lithologies from 0 to ~350 meters (m); the magnetotelluric survey extended resistivity depth of investigation from 0 to 10 kilometers (km). The Silverton caldera ring fault zone from 0 to 300 m has low to moderate resistivity (50-140 ohm-m), indicating it may be permeable to fluids. Mineralized radial vein structures south of the ring fault zone have moderate bulk resistivity that may be attributed to interconnected sulfides or clays adjacent to the veins. Caldera ring fault intrusives and possible plutonic roots of the calderas that were part of the ancient magmatic plumbing, where unaltered are resistive (2000 - >3000 Ω-m) and were imaged from surface outcrops to 8 km depth. Permeable, Quaternary deposits blanketing Arrastra Gulch have low resistivity (10 – 100 Ω-m) possibly caused by low to moderate specific conductance pore water. Quaternary sediments and volcanics have low or moderate bulk resistivities in the vicinity of legacy mine sites (Aspen, and Shenandoah-Dives mines, and Mayflower mill tailings). 3-D data integration combining electromagnetic data, geologic and historical mine mapping, and high-resolution drone imagery is useful for communicating study results to land managers tasked with characterizing and remediating legacy mine sites in a watershed context.