RESIDENCE OF URANIUM IN ROLL-FRONT DEPOSITS: A CASE STUDY

Abstract

Despite generally low uranium grades, roll-front uranium deposits are attractive exploration targets as these deposits are amenable to in-situ leaching (ISL) techniques. At the Lost Creek Project, a currently operating uranium mine in Wyoming, head grades were five times higher than pre-operational estimates. Similar mines, such as the Crow Butte mine in Nebraska, have experienced head grades that are in good agreement with initial estimates. Ore samples from the Lost Creek Project and the Three Crow Expansion Area, a satellite deposit to the Crow Butte mine, were analyzed in order to understand their mineralogy and to understand the discrepancy between head grades and pre-operational estimates at the Lost Creek Project. Fission track mapping, quantitative trace element mapping of selected areas using EPMA and high-resolution BSE imaging revealed that the uranium phases at the Lost Creek Project are contained in small (<1 µm) wispy phases. Using SEM and TEM techniques, the U bearing phases were identified as a fine intergrowth of kaolinite with becquerelite (Ca(UO2)6O4(OH)6·8(H2O)) and uranophane (Ca(H3O)2(UO2)2(SiO4)2·3H2O). Initial head grades at Lost Creek of 211 ppm are in good agreement with solubility experiments for highly soluble becquerelite (250 ppm), whereas uranophane solubility peaks at 75 ppm in a bicarbonate concentration of 2.0x10-2 mol/L, which is in good agreement with bicarbonate concentrations used at the Lost Creek Project and with current head grades of 42 ppm. The large discrepancy of estimates and initial head grades at the Lost Creek Project is partly due to the different gamma ray intensity of uranophane versus primary U ore minerals usually found in U roll-front deposits. Uranophane has a gamma ray response of almost half of both uraninite and coffinite leading to an underestimated resource estimate. Moreover, the mobile nature of the U6+ ion allowed uranium mineralization of secondary minerals to occur away from the primary ore zone leading to a less well defined, larger ore zone. Ore mineralogy at the Three Crow Expansion area was identified to be primarily coffinite (U(SiO4)1−x(OH)4x) and is in good agreement with previous work and the ore mineralogy at the Crow Butte mine. In contrast, the Lost Creek Project is highly oxidized and primary U ore minerals have been replaced by secondary becqurelite and uranophane.