The data presented herein is from an airborne electromagnetic-resistivity-survey conducted by Dighem Limited of Toronto Canada for the U.S. Geological Survey. The area surveyed is located in the western San Juan Mountains near Lake City, Colorado. The general area covered is between 37°45' and 38° latitude north and 107°15' and 107°35' longitude west. The survey flying was confined to nine valleys which surround the Lake City caldera. Four blocks were surveyed from October 22 to October 27, 1979 for a total of 535 line - kilometers. The survey was flown as part of a mineral appraisal study conducted in cooperation with Bureau of Land Management (BLM). The survey was done to detect massive mineralization, mainly copper, and to locate conductive faults that may be suitable sites for uranium mineralization.
To be useful, airborne electromagnetic measurements must be made within a few hundred feet of the surface. The area that was flown is so rugged that a helicopter rather than a fixed-wing aircraft was used.
Fraser and Dvorak summarize the data of the survey areas as follows: in the east and south, the ground resistivities varied over a broad range—, from 20 to 1000 ohm-m. The western and northern parts of the area were characterized by resistivities mostly in the 300 to 1000 ohm-m range. Several weak to moderate EM anomalies were located that may warrant ground follow-up work.
