AIRBORNE AND GROUND GEOPHYSICAL SURVEYS OF THE UPPER ANIMAS RIVER WATERSHED; APPLICATION TO ABANDONED MINE LAND STUDIES

A helicopter electromagnetic and magnetic (HEM) survey, ground follow-up geophysical surveys, and digital geologic map compilation were utilized in an abandoned mine land study. An important purpose of this study is to identify possible ground water flow paths in the surface and subsurface (bedrock). The apparent electrical conductivity and reduced to the pole total field magnetic maps from the HEM survey are used to identify geologic features that can influence ground water flow. The most easily resolved ground water flow paths are near surface streams and flow through alluvial or colluvial deposits. The HEM data suggests areas where less resolved bedrock structure (fractures and faults) might control ground flow. Though the upper Animas River is perhaps controlled by deep crustal structure, two other major tributaries (Cement and Mineral Creek) are not associated with geophysical responses that indicate deep structures. High conductivities near one mine waste pile suggest near surface flow paths and a source for high dissolved solids. The high sulfide mill tailings at this site were removed after the HEM survey. Apparent electrical conductivity maps from different HEM frequencies suggest a previously unrecognized shallow northwest-trending structural zone, in part along Cement creek, that may control ground water flow. Geophysical, mineralogical, and geomorphologic attributes of geologic structures were quantified through a statistical modeling approach. This analysis shows that high resistivities and moderate magnetization responses have the highest probability of association with mapped faults and veins. Such geophysical signatures are consistent with silicic mineralogy of veins emplaced along major structural trends and with silica-rich phases of the Silverton volcanic rocks. Total field magnetic data suggest northwest-trending structures that may cross the Silverton Caldera ring fracture system. The youngest dacite-rhyolite volcanic rocks show different types of magnetic and electrical properties that may have implications for the occurrence of acid-generating lithologies.