Date of Award
Master of Science (MS)
Mount Evans is located in the Anaconda-Pintler mountain range of southwest Montana approximately 22 km southwest of Anaconda, Montana. Mount Evans appears 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, and either side of the continental divide 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 Mount 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 characterize the chemistry of the two streams draining Mount Evans: 1) East Fork Twin Lakes Creek (EFTLC), on the north; and 2) Sullivan Creek, on the south.
The hydro-geochemical trends in EFTLC and Sullivan Creek are similar. The stream pH values range from about 4.0 in their headwaters to near-neutral in their lower reaches, and locally exceed Montana water quality standards (chronic aquatic life) for cadmium, copper, zinc, and nickel. Orange-red hydrous Fe-oxides coat the streambed at pH < 4.5 whereas white, hydrous Al-oxide is abundant at pH > 5. Based on bulk digestion, portable XRF analysis, and geochemical modeling, the white precipitate is most likely a mixture of hydro-basaluminite, Al4(SO4)(OH)10·5(H20) and amorphous Al(OH)3. The secondary precipitates are rich in other trace elements, such as As, Co, Cu, Mn, Ni, Pb, and Zn. A cold spring with heavy, white Al coatings near the headwaters of EFTLC is thought to be emanating from the toe of a rock glacier. This flow has higher SC and trace metal concentrations than the rest of the field area, and also has abnormally high concentrations of the rare earth elements. A pre-modern ferricrete deposit near the headwaters of Sullivan Creek has similar trace element chemistry to the precipitates forming today in the nearby stream. This suggests that the hydro-geochemical conditions in Sullivan Creek have been similar for a long time, possibly since the end of the Pleistocene. Overall, the Mount Evans area is an excellent example of natural acid rock drainage in a pristine, mountainous setting.
Doolittle, Margaret, "Naturally Occurring Acid Rock Drainage in the Anaconda-Pintler Mountain Range, Montana: A Case Study of Geochemistry in Two Streams Flowing from Mount Evans" (2017). Graduate Theses & Non-Theses. 117.