NATURALLY OCCURRING ACID ROCK DRAINAGE IN THE ANACONDA-PINTLER MOUNTAIN RANGE, MONTANA: PRELIMINARY GEOCHEMISTRY OF TWO STREAMS FLOWING FROM MT. EVANS

Mt. Evans is located in the Anaconda-Pintler mountain range of southwest Montana approximately 22 km southwest of Anaconda, Montana. Mt. Evans appears a reddish orange on Google Earth amongst the rest of the mountain range which is a grey white color, indicating the presence of limonite staining and probable sulfide mineralization. In addition, two streams draining opposite sides of the mountain have conspicuous white coatings on boulders that are clearly visible with Google Earth. During a mineral resource assessment in the 1980s, the USGS suggested that the Mt. Evans area could be the top of a porphyry Cu-Mo system. However, this is not an area with historic mining and no evidence has been found to indicate exploration for economic deposits was ever done. The purpose of the present study is to find the source and causes of the white precipitates in the streambeds, and to characterize the chemistry of the streams draining Mt. Evans.

Field work completed in the Fall of 2014 confirmed the presence of natural acidic drainage in the headwaters of both streams. The stream pH’s range from about 3.8 in their headwaters to about 5.5 in the lower reaches, and locally exceed Montana water quality standards (chronic aquatic life) for copper, zinc, and nickel. Orange-red hydrous Fe-oxide coats the streambed at pH < 4.5 whereas white, hydrous Al-oxide is abundant at pH > 5. Based on portable XRF analysis, the white precipitate is probably hydro-basaluminite, Al₄(SO₄)(OH)₁₀·5(H₂0). The secondary precipitates are rich in other trace elements, such as As, Co, Cu, Mn, Pb, and Zn. A pre-modern ferricrete deposit near the headwaters of one of the drainages has a similar trace element chemistry to the precipitates forming today in the nearby stream. The acid rock drainage in these streams is clearly related to the limonite-rich color anomaly on Mt. Evans. The nature of the mineralization at the source is currently being investigated in a parallel study.