ACID-NEUTRALIZING POTENTIAL OF BEDROCK INFERRED FROM WHOLE ROCK CHEMISTRY AND HIGH RESOLUTION AIRBORNE GEOPHYSICAL DATA

Information obtained from a high resolution airborne geophysical survey were combined with geologic data to characterize the amounts of acid-consuming mineral assemblages within specific plutonic and volcanic rock units in the Boulder River watershed, southwest Montana. Physical properties of bedrock expressed in geophysical maps as magnetic and apparent resistivity anomalies are intrinsically related to mineralogical and chemical properties that have the potential to neutralize or produce acidic metal-rich waters. In particular, for nearly all the Cretaceous- and Tertiary-age volcanic and plutonic rocks exposed in the Boulder River watershed, increased amounts of acid-consuming calcic and mafic minerals are associated with increased levels of the mineral magnetite. Magnetite is the main source for magnetic anomalies and, in part, resistivity anomalies. Although magnetite is a minor mineral constituent, it provides an easily measured property that can be positively correlated to measurable percentages of important acid-neutralizing minerals, such as chlorite, biotite, hornblende, tremolite, and plagioclase. Magnetic and resistivity anomaly data from a high resolution geophysical survey over the Boulder River watershed were mathematically transformed to pseudo-mineral maps inferring surface and shallow subsurface distribution of magnetite. The geophysical magnetite maps facilitate direct estimates of the associated amounts of acid-consuming minerals present and varying degrees of acid neutralization and acid generation potential. Results suggest that a sizeable volume of volcanic and plutonic rock lacks appreciable magnetite and therefore lacks minerals to provide neutralization for acidic water. This study demonstrates the utility of geophysical surveying to address issues surrounding water quality, aquatic health, and selection of safe waste-rock repositories using a broader study framework than might be used at a single site. The methods and approaches employed here can be applied to other watersheds with similar plutonic regimes, which have water quality degradation caused by historical mining like that in the Boulder River watershed.