Overburden chemistry and mineralogy at Hanging Woman Creek, Big Horn County, Montana, and recommendations for sampling at similar sites

Cored overburden rock of the Fort Union Formation at the Hanging Woman Creek potential coal mine site was classified in hand specimen on the basis of color and grain size into 3 types: sandstone. siltstone-plus-shale, and dark shale. The samples were analyzed for bulk chemistry and mineralogy. In general the samples of sandstone contain the lowest concentrations of elements of environmental concern (arsenic, beryllium, mercury. molybdenum, selenium and sulfur), and samples of the dark shale contain the highest.

Comparison of samples within each rock type shows that chemical composition does not change systematically across the study site (a distance of 5 km), and almost as much information about overburden chemistry could have been obtained from a single hole drilled anywhere on the site.

Overburden data from other sites in the region show that rocks at the Hanging Woman Creek site are chemically similar to those of the region as a whole, which suggests that the rock chemistry at other sites in the region could also be estimated from a low-density drilling and sampling pattern.