Date of Award
Brian St. Clair
The Barker-Hughesville Mining District in the Little Belt Mountains is home to a heavily mining impacted watershed called Galena Creek which has been the subject of remediation and environmental restoration due to the high levels of metals (including Cd, Cu, Pb, and Zn) which have negatively impacted aquatic life in the drainage. Galena Creek was designated a superfund site in the late 1990’s/early 2000’s, and since then several constructional efforts have removed mine waste from direct contact with the stream. Despite these efforts, numerous mine water discharges still enter the creek, and the water quality is still impaired. It is possible that the unique geology of the district is contributing a certain amount of background loading of metals and acidity to the stream. However, because the area was mined in the early 1900’s, no premining baseline water quality samples were ever collected. This thesis used several geochemical techniques to assess whether natural weathering of mineralized bedrock influences the water quality of Galena Creek. A detailed synoptic sampling of the stream and all measurable inflows was conducted in mid-summer baseflow conditions. Both filtered and non-filtered samples were taken, along with samples of mineral crusts and instream precipitates. Samples were collected for O- and H-isotope analysis of water, O- and Sisotope analysis of dissolved sulfate, and S-isotope analysis of fresh sulfide minerals collected from mine dumps. In addition, representative bulk samples of each of the major rock types in the watershed were collected for laboratory leachate studies. The leachate tests included samples of hydrothermally altered and pyrite-mineralized rock that is thought to comprise up to 20% of the outcrop area of the Hughesville Stock: the main host rock of the area. Results of the synoptic sampling investigation show that most of the loads of metals and dissolved sulfate in Galena Creek during baseflow conditions can be accounted for from the known mine discharges. The S- and O-isotope composition of sulfate in the stream is similar to that of sulfate in the mine discharges, and the S-isotope composition of sulfate is similar to that of sulfides on the mine dumps. The hydrothermally-altered Hughesville Stock produced leachates with very poor water quality, whereas the unaltered stock and other bedrock units in the watershed produced leachates with much lower concentrations of metals and sulfate. By mixing the leachate water chemistry from each rock type, scaled to the percentage of the total watershed underlain by each rock type, a first pass approximation of the pre-mining water quality of Galena Creek was obtained. Although this type of calculation rests on several assumptions, the results suggest that Galena Creek could have had elevated concentrations of metals and sulfate from natural weathering prior to mining disturbance. Lastly, the concentrations of several metals of interest in Galena Creek were compared to concentrations in Chicago Gulch, a small stream with natural acidity draining an unmined, but hydrothermally altered, stock in the central Judith Mountains. The range in concentrations in the two drainages overlap. Whereas Galena Creek on average has higher concentrations of Mn, Zn and Cd, Chicago Gulch has higher concentrations of Pb and Al. In summary, although the present chemistry of Galena Creek is obviously impacted by the legacy mines, some metals and acidity would have entered the creek due to natural weathering prior to mining.
Neaville, Austin, "POST-RECLAMATION AND PRE-MINING GEOCHEMISTRY OF THE GALENA CREEK WATERSHED IN THE BARKER-HUGHESVILLE MINING DISTRICT, JUDITH BASIN COUNTY, MONTANA" (2021). Graduate Theses & Non-Theses. 264.