Based on previous U.S. Geological Survey studies that measured 3 to 5 parts per billion dissolved uranium being carried by the Colorado River, AZGS senior geologist Jon Spencer and geological consultant Karen Wenrich calculated that with river discharge of about 3.6 cubic miles annually, this amounts to an average transport load of 40 - 80 tonnes (metric tons, or an average of 132,000 pounds -66 tons/60 tonnes - of uranium). Jon presented the results yesterday at the Arizona Hydrological Society Annual Symposium in Tucson. [right, breccia pipe exposed in canyon wall, northern Arizona]
To evaluate some of concerns about the environment impacts of mining of uranium from breccia pipes in northern Arizona, Jon and Karen constructed a hypothetical model involving a haul truck loaded with 10 metric tons of uranium ore -- with an ore grade of 1 percent, equivalent to the high-grade ore of breccia pipes -- swept into Kanab Creek during a flash flood.
In this scenario, the ore is pulverized by river action and added to the dissolved uranium content of the river. The result: a one-year increase of 220 pounds of dissolved uranium, which is the equivalent of less than one-fifth of one percent (0.17 percent) increase in the river’s uranium content. The authors conclude this added amount would be undetectable given the much higher natural concentrations of uranium, the natural variability of the concentrations, and the difficulty of determining such a small change in concentration with modern analytical techniques.
In this limited study, Jon and Karen did not attempt to model how the dissolved uranium would be deposited in river sediments, or examine potential impacts of mining on groundwater. Those issues require additional study.
Jon and Karen did offer a perspective on the origin of the uranium ores in breccia pipes are being tied to the development of Mississippi Valley-style ore deposits in other parts of the country. In response to an audience question, Jon speculated that the long distance migration of vast amounts of mobilizing (oxidizing) fluids over a long geologic period and large area could have concentrated normal background amounts of uranium into the rich deposits we see today.
Talk title: The Grand Canyon breccia‐pipe uranium province, northwestern Arizona