Structural controls and chemical characterization of brecciation and uranium vanadium mineralization in the northern Bighorn Basin

Abstract

The goals of this research were to determine if the mode of mineralization and the geology of two abandoned uranium and vanadium mining districts that border the Crow Reservation might be a source for contaminants in the Bighorn River and a source of elevated uranium in home water wells on the Reservation. Surface and spring waters of the Crow Reservation have always been greatly respected by the Crow people, valued as a source of life and health and relied upon for drinking water. Upon learning that the Bighorn River has an EPA 303d impaired water listing due to elevated lead and mercury and that mercury has been detected in the fish from rivers of the Crow Reservation this study was implemented. Watersheds from both mining districts contribute to the Bighorn River that flows through the Crow Reservation. Initial research used the National Uranium Resource Evaluation database to analyze available geochemistry for the study areas using GIS. The data showed elevated concentrations of lead in drainages related to the mining areas. The data also showed elevated uranium in many of the surface waters and wells that were tested as a part of the study on the Crow Reservation. The author attended meetings and presented results of the National Uranium Resource Evaluation data analyses to the Crow Environmental Health Steering Committee. Thus, both uranium and lead were added to the list of elements that were being tested in home water wells as part of a community based participatory research project addressing many issues of water quality on the Crow Reservation. Results from home wells tested on the reservation did show elevated uranium. Rock samples were collected in the study areas and geochemically analyzed. The results of the analyses support a Permian Phosphoria Formation oil source of metals in the two mining districts. Structural data support fracturing accompanied by tectonic hydrothermal brecciation as a process that introduced oil and brines from the Bighorn Basin into the deposits where the uranium vanadium deposits later formed.