NATURAL ACID ROCK DRAINAGE CHEMISTRY AND FERRICRETE DEPOSITS OF THE JUDITH MOUNTAINS, MONTANA

The Judith Mountains are a low-elevation “island” mountain range in east-central Montana with composite intrusive centers cut by coarse grained alkali granite and tinguaite dikes. The area is part of the Central Montana Alkalic Province. The geology of the Judiths in the vicinity of Red Mountain and Judith Peak, two of the tallest mountains in the range, is conducive to the creation of headwater streams that are naturally acidic. Extensive limonite staining on Red Mountain attests to the widespread presence of disseminated pyrite in hydrothermally altered porphyry rock. However, no major historical mining operations are known to the area. The acidic streams undergo neutralization in pH as they travel downstream due to the influx of alkaline groundwater and tributary streams. The change in character of the water is related to a change in geology from mineralized porphyry intrusions at the summit of the range to Paleozoic and Cretaceous sediments, including the Madison Limestone, in the foothills.

The current study is focusing on one stream of interest, Chicago Gulch (aka Fords Creek). Results from ICP-AES and ICP-MS analysis of synoptic water samples quantify trends in metal concentrations and loads, pH, and distance downstream. Sample sites with pH < 4 have higher iron content, with abundant pre-modern ferricrete deposits next to the stream. Alluvial (in-stream) ferricrete forms where pH transitions from < 3.5 to > 3.5, whereas broad ferricrete terraces form where Fe²⁺-rich groundwater emerges as springs and is oxidized to ferric hydroxide. All water samples collected in the upper reaches of Chicago Gulch exceed Montana water quality standards for protection of aquatic life for lead, cadmium, zinc, and copper, and human health standards for thallium. White Al-hydroxide flocs (tentatively identified as hydrobasaluminite) are actively forming where the stream pH transitions from < 5 to >5. This white precipitate is very rich in trace metals, including As, Pb and Zn. Concentrations and loads of all metals drop drastically in lower Chicago Gulch once pH exceeds 5.5, and the lower reaches support a small population of trout. Data from this study is being used by the local Bureau of Land Management (BLM) office to guide decisions regarding future land use issues in the central Judith Mountains.