Informing Future Decision Making on Uranium Mining in Arizona

The uranium ore bodies found in some of the breccia pipes of northern Arizona contain a complex mixture of dozens of minerals. As a result, the chemistry of these uranium deposits is also complex. Intergrown with the uranium minerals are many metal-bearing minerals, causing enrichments in arsenic, cobalt, copper, lead, molybdenum, nickel, and zinc. These metals, as well as uranium, can have toxic effects on wildlife and humans through various pathways (such as ingestion, absorption from waters or fine particles, and inhalation of dusts).

First Objective

Determine the unique chemical and mineral content of the uranium deposits of northern Arizona before they are excavated and brought to the surface.

This information is critical to determine if the uranium mining operations have been sources of metal contributions to surface environments or groundwaters. Mining activities combined with natural processes (rain, wind, evaporation) can disperse and concentrate uranium and other metals in mine waste materials, soils, dusts, groundwaters, natural spring waters, and mining-related waters on site. For example, uranium, arsenic and the other metals noted above are enriched in the breccia pipe deposits, but these metals typically occur in low concentrations in the rocks and soils of the region ("the background"). Thus, a sample that exhibits elevated concentrations of uranium and (or) arsenic may indicate influences of mining, while lower concentrations suggest a lack of influence.

Several analytical techniques are used to determine uranium deposit metal content and mineralogy.

Analyze rock and soil samples to determine the concentrations of more than five dozen elements, including the metals of biological concern.
Determine mineralogy mainly using a scanning electron microscope equipped with a spectroscopy analyzer that can image and measure the chemistry of mineral particles, which range in size from millimeters down to micrometers in diameter. X-ray diffraction analyses are also used to identify minerals in rock samples, in particular in samples of ore.

Second Objective

Determine the solubility of the ore minerals and their capacity to release metals into waters.

A mineral may contain one or more metals that can be potentially toxic, but the structure of the mineral and the chemistry of the interacting water determine whether the mineral will release its metals. This ability will be tested by the leaching of ore samples in waters and measuring the resulting chemistry of the leach solutions. Characterizing the mineralogy and chemistry of the uranium source rock allows one to explain the metal concentrations that are found in waters, soils, or an organism.

Related Product

Element concentrations in surface soils of the Coconino Plateau, Grand Canyon region, Coconino County, Arizona: U.S. Geological Survey Open-File Report 2016â€“1160