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Science for Watershed Decisions on Abandoned Mine Lands: Review of Preliminary Results, Denver, Colorado, February 4-5, 1998

Digital Geologic Compilations of the Upper Animas River and Boulder River Watersheds: Geographic Information Systems Technology Used as a Scientific Interpretation Tool

By D.B. Yager,¹ K. Lund,² R.G. Luedke,³ D.J. Bove,⁴ M.J. O'Neill,⁵ and T.C. Sole⁶

Digital geologic maps were developed from previously existing geologic maps for the upper Animas River, Colorado, and Boulder River, Montana, watersheds as geographic-information-system (GIS) base coverages for simultaneous visualization of the spatial relationships between geology and diverse abandoned-mine-lands (AML) data sets. These maps provide a multilayered base coverage capability for use in an interdisciplinary approach to a cost effective and timely evaluation of these geologically complex watersheds. Digital geologic information, such as maps of rock types, structures, and veins, is combined with rock-chemical-composition, rock-alteration, and stream-chemistry data to: (1) aid in the interpretation of alteration processes and their distribution; (2) determine how fault and mineralized-vein density and patterns may influence water quality; and (3) delineate rocks with acid-buffering capacity or acid-generating potential. Derivative maps are readily generated from the digital geology to aid in this process-oriented reinterpretation of existing geologic maps. One such derivative map of the upper Animas River watershed shows the distribution of the Burns and Henson Formations, which commonly contain calcite-bearing, altered lava flows that can significantly neutralize acid mine drainage and acidic streams and ground water that intersect these units. Use of this GIS-derivative technique provides watershed-scale data on the natural potential of AML host rocks to neutralize acidic and metal-laden waters, especially when these derivative products are combined with additional mapping of alteration and of formations with different buffering capacities, as guided in part by the application of AVIRIS remote-sensing technology. Mine-waste-pile mitigation may directly benefit from information gained from the derivative maps because waste piles that are known producers of metals and acidity can be geographically placed in context with rock units that have either natural acid-buffering potential that have mineral assemblages that contribute to water degradation. Mitigation efforts on waste piles that occur near rock units with high buffering potential may not need to be as intensive as mitigation efforts on waste piles that occur in rock units with low buffering capacity.

Observations of field relationships in the Boulder River watershed were compiled for a new process-oriented geologic interpretation of the existing descriptive geologic map. One goal of the Boulder River project is to produce, from the digitally compiled geology, a three-dimensional model that depicts the spatial relationship between igneous activity and mineralization. The newly modified, interpretive digital geologic map will be draped on the digital topographic model to provide a visualization tool that may help refine our understanding of the three-dimensional relationships between plutonism, mineralization, alteration, and fracture distribution. Such a view of the geology will test our present hypotheses about the geometry of magmatic cooling phases of the Boulder batholith, the relationship between structural and mineralization patterns, and the dimensions of acid-buffering rocks.

¹U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dyager@usgs.gov)

²U.S. Geological Survey, MS 905, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (klund@usgs.gov)

³U.S. Geological Survey, MS 955, 12201 Sunrise Valley Drive, Reston, VA 20192 (bluedke@usgs.gov)

⁴U.S. Geological Survey, MS 905, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (dbove@usgs.gov)

⁵U.S. Geological Survey, MS 964, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (jmoneill@usgs.gov)

⁶U.S. Geological Survey, MS 973, P.O. Box 25046, Denver Federal Center, Denver, CO 80225 (tsole@usgs.gov)

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