Modes of occurrence of uranium in carbonaceous uranium deposits: Characterization of uranium in a south Texas (U.S.A.) lignite

Abstract

Investigations on the nature of the association between U and carbonaceous material in south Texas lignite-sandstone ore bodies and other uraniferous bioliths have been reviewed. A strong correlation between the organic, in particular, humic material and U enrichment appears to exist. Geochemical evidence indicates that in the case of many deposits the emplacement of U occurred when groundwater containing dissolved uranyl complexes came into contact with the organic material in carbonaceous sediments. Attempts to identify the particular forms in which U occurs have met with infrequent success. The presence of specific, U-containing minerals, e.g. uraninite or coffinite, has been confirmed only in a few cases. In a majority of the cases U is present in a finely dispersed state which makes the discrimination between mineralized uranium and U present as true organic compounds very difficult. New experimental results on the modes of occurrence of U in a Jackson lignite (Karnes County, south Texas) are presented. The distribution of U in the density separates suggests that the mode of occurrence of this element varies. The concentration of U in the humic acids is nearly half that in the parent lignite even after several re-precipitations followed by dialyses. The humic acids have been separated into three molecular weight fractions (110,000, 24,000 and 7,000 daltons) by gel filtration chromatography. They have also been separated into five molecular weight fractions (range: > 100,000 to < 1,000) by ultrafiltration. The distribution of U, Mo, Fe, Ti, Zn, Si, Al, Ca and S in these fractions has been measured by neutron activation analysis or inductively coupled plasma atomic emission spectrometry. In the case of most elements, the highest fractions (57–90%) are recovered in the fraction with the highest molecular weight range (> 100,000). The exceptions are Mo and Al, 60–67% of which are recovered in the M.W. < 1000 fraction. Estimates of the degree of enrichment per humic acid “monomer” reveal no major differences among the elements with the exception of Mo and Al which are enriched in the lowest molecular fraction. Lignite humic components were also found to enhance the extraction of U (VI) from carnotite by DMSO, suggesting a probable role in the mobilization and emplacement of the element.