Paper No. 11
Presentation Time: 10:55 AM
GEOCHEMISTRY OF NATURAL ACID ROCK DRAINAGE IN THE JUDITH MOUNTAINS, CENTRAL MONTANA, USA
GAMMONS, Christopher H., Geological Engineering, Montana Tech, Butte, MT 59701, WILLIAMS, George P., Chemistry and Geochemistry, Montana Tech, Butte, MT 59701 and PARKER, Stephen R., Chemistry and Geochemistry, Montana Tech of The University of Montana, 1300 West Park St, Butte, MT 59701, cgammons@mtech.edu
The Judith Mountains of central Montana are cored by a cluster of alkaline plutons intruded into Paleozoic and Mesozoic sedimentary rock. Three streams near the center of the range form a radial drainage pattern around a central stock of pyrite-rich porphyry, termed Red Mountain. Each stream is highly acidic in its headwaters (pH 2 to 3.5) and transitions to neutral pH with distance downstream over several km due to alkaline tributaries and groundwater inflows. Concentrations of Al, Cu, F, Fe, Mn, Pb, Zn and thallium (Tl) locally exceed regulatory standards for aquatic life and/or human health. Seasonal sampling of surface water is being conducted at over 20 locations, along with streamflow measurement. The study also involves detailed synoptic sampling of each watershed using the continuous tracer (KBr) injection method, diel sampling to examine 24-h changes in solute concentration, and examination of the chemistry and mineralogy of ancient ferricrete, alcrete, and Mn-crete deposits.
Although the Judith Mountains contain a number of historic precious metal mines, the watersheds of interest in this study, including Red Mountain itself, have been minimally impacted by mining. A comparison of the trace metal concentration in a longitudinal set of ferricrete samples in one of the watersheds with actively forming FeOx precipitates in the adjacent stream suggests that the acidity and trace metal loads witnessed today are broadly similar to conditions prior to settlement in the late 1800s. Although the ferricrete deposits in the headwaters of each stream have not been dated, their considerable size and the lack of Quaternary glaciation in the Judith Mountains suggest that they could be hundreds of thousands if not millions of years old. The study area is a potential reference site to help constrain pre-mining water quality in nearby mining districts with similar geology but more severe mining disturbance, such as the Kendall and Zortman-Landusky gold-mining districts.
