Flagstaff, AZ – Earth Notes: Grand Canyon Uranium
The colorful rock layers of the Colorado Plateau hide many economically important secrets. One of the most valuable, and controversial, is uranium.
Much of the uranium on the Colorado Plateau accumulated around decaying organic material in ancient stream sediments. But by far the richest concentrations occur in structures called breccia (pron. breh-chee-uh, with the emphasis on the first syllable) pipes that are especially common in the Grand Canyon region.
These formations began millions of years ago as caverns in the porous Redwall Limestone. When a cavern's roof failed, the collapse continued into higher layers, often for many hundreds of feet. The result is a vertical cylinder, filled with jagged debris known as breccia.
Some geologists suggest that uranium-rich groundwater came from granite deposits to the south. The water flowed north under hydrostatic pressure, and entered the pipes from below.
Others think that groundwater worked its way down the pipes after running through higher, uranium-rich sediments like the Chinle Formation.
In either case, uranium precipitated throughout the breccia, often in extremely rich concentrations. The average concentration in the Earth's crust is about two parts per million. Only an estimated one to two per cent of the breccia pipes in Grand Canyon contain uranium, but some have been tested at five hundred to a thousand parts per million. They are the most concentrated conventional deposits in the United States.
Given these rich concentrations, and the relative ease of mining a breccia pipe, it is no surprise that the Grand Canyon region continues to be eyed as an important source of uranium.