GEOCHEMICAL ANALYSIS OF ANTHROPOGENIC FERRICRETES AND STREAM GEOCHEMISTRY, JUDITH MOUNTAINS, MT

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 Judiths in the vicinity of Red Mountain and Judith Peak is conducive to the creation of headwater streams that are naturally acidic. 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, related to a change in geology from mineralized porphyry intrusions at Red Mountain to Paleozoic and Cretaceous sediments in the foothills. This study is the conclusion of a multiple year project in the Judiths examining the geochemistry of three streams that drain opposing sides of the Red Mountain porphyry.

Results from ICP-AES, ICP-MS and IC analysis of synoptic water samples quantify spatial trends in pH and metal concentrations and loads. 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 As, Pb and Zn. Sample sites with pH < 4 have higher iron content, 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 outside the stream channel where Fe²⁺-rich groundwater emerges as springs and is oxidized to ferrihydrite, schwertmannite, jarosite, and/or goethite., depending on the environmental conditions. All water samples collected in the upper reaches of Chicago Gulch exceed Montana water quality standards for aquatic life for lead, cadmium, zinc, and copper, as well as human health standards for thallium. Dissolved lead concentrations are especially high in the headwaters of Chicago Gulch, indicating the possible presence of a weathered lead-sulfide deposit in the subsurface. Lab experiments interacting deionized water with weathered porphyry rock from Red Mountain produce acidic leachates that have elevated metals and sulfate concentrations. Metal concentration data are being compiled to establish metal load “rating curves”. All streams show a positive linear relationship when metal load is plotted against streamflow for most of the trace metals of interest.