Date of Award
Master of Science (MS)
The Judith Mountains are a low-elevation “island” mountain range in east-central Montana composed of a series of laccoliths and stocks cut by coarse-grained alkali granite and tinguaite dikes. The geology of the Judith Mountains 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.
This study is the conclusion of a multiple-year project in the Judiths, quantifying the relationship between stream chemistry and local geology in three streams sourced from the Red Mountain porphyry. Chicago Gulch (aka Fords Creek) is the primary focus of this project. Although Collar Gulch and Armells Creek have been previously analyzed, some data are included in this thesis to compare to Chicago Gulch. The local Bureau of Land Management (BLM) office plans to use this thesis to guide decisions regarding future land use issues in the Judith Mountains.
Results from ICP-AES, ICP-MS and IC analysis of synoptic water samples quantify trends in metal concentrations and loads, pH, and distance downstream. White Al-hydroxide flocs are actively forming where the stream pH transitions from < 5 to >5. This white precipitate is rich in trace metals, including Cu, Pb and Zn. Precipitates in headwater reaches with pH < 4 have higher iron content (approximately 25% Fe), with abundant pre-modern ferricrete deposits next to the stream. Alluvial 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. Lead concentrations are especially high in the headwaters of Chicago and Collar Gulches, indicating the possible presence of a weathered lead-sulfide deposit in the subsurface. Concentrations and loads of all metals decrease in all the streams once pH exceeds 5.5, and the lower reaches support a small population of trout. Prediction of pre-modern pH using the methods of Nimick et al. (2009) indicates that current pH regime is broadly similar to conditions when ancient ferricretes were deposited.
Edinberg, Sara, "NATURAL ACID ROCK DRAINAGE AND FERRICRETE DEPOSIT CHEMISTRY OF THE JUDITH MOUNTAINS, MONTANA" (2016). Graduate Theses & Non-Theses. 73.