LEACHABILITY OF URANIUM ORE AND MINE WASTES FROM THE GRAND CANYON AREA, NORTH RIM MINING DISTRICT, NORTHERN ARIZONA

Mining of breccia pipe uranium (U) deposits in the Grand Canyon area during 1980s mining operations exposed uraniferous ore and waste rock to enhanced alteration and weathering due, in part, to episodic wetting events. This occurred in a variety of mine-related settings. These included: 1) un-reclaimed surface exposures of waste rock piles, ore piles and settling ponds, 2) erosion of engineered barriers designed to remediate waste rock, 3) flood- or wind-mobilized U-bearing particles, and 4) former underground mine openings now backfilled with mining waste. To understand the potential for future availability, bioavailability and mobility of U and other trace metals, samples of variably-weathered U ore, mine-site weathered waste materials, U-affected soils (physically-transported fines), pond sludges, surficial salt crusts and background soils were subjected to leaching by synthetic rainfall and carbonate-rich solutions. As the contact time increased from 1 to 24 hours, the concentrations of U in the leachates generally increased for all samples. Most samples had a small component (<3%) of U that was leached from the solids during 24-hour contact with synthetic rainwater. Rainwater-leached percentages of U increased in the following order: background soils (0.16 to 0.19%), un-weathered ore (0.2 to 0.33%), wind–transported fines (0.5 to 1.66%), weathered ore and waste rock (0.89 to 2.76%), and pond sludge and surficial salts (0.57 to 17.06%). The carbonate leach solutions mobilized greater percentages of total U in background soils (1-2%), un-weathered ore (3-5%), wind–transported fines (7-13%), weathered ore and waste rock (14-20%), and pond sludge and surficial salts (18-91%). A greater percentage of U was leached from the weathered waste samples found on site than in the un-weathered high-grade ore specimens yet, in general, the total concentration of U in solution was higher in the ore sample leachates. Although the pond sludge and surficial salt samples contained lower total U than the un-weathered samples, they likely formed through chemical redistribution and alteration resulting in phases (such as salts and sorbates) that are more labile than the primary mineral, and therefore more susceptible to leaching upon contact with rain or groundwater.