Holk and 4 co-authors reported in 2003 that lead isotope ratios and associated trace element concentrations (U, Th and Pb) extracted by partial-leaching with 2% nitric acid from Proterozoic sandstones and basement rocks reveal much about the fluid evolution of sedimentary basins hosting unconformity-type uranium deposits. In addition, these techniques have great potential as a guide for exploration of uranium and other types of deposits in basins of any age.
Isotope ratios of Pb in Proterozoic sandstones from basins known to contain high-grade uranium deposits are radiogenic at key geological localities and settings distal to known mineralization and particularly in altered zones proximal to mineralization. Sandstones completely cemented by quartz overgrowths typically have non-radiogenic Pb isotope ratios, indicating early closure of porosity and isolation of these rocks from later fluid events.
Alternatively, the unconformity served as both a source of uranium and radiogenic Pb as well as an avenue for late-stage (< 250–900 Ma) fluid flow. The mafic volcanic units, which are relatively reducing lithologies and therefore have removed uranium from basinal brines, have uranium-supported radiogenic Pb isotope ratios. Comparison of 238U/206Pb and 206Pb/204Pb ratios is useful in determining the timing and nature of U and Pb migration before, during and after mineralization in these basins.
This comparison can be used to delineate the presence of radiogenic Pb isotope ratios that are not internally supported by uranium and thorium in rocks, eventually providing the explorationist with geochemical vectors that point toward sites of high potential for economic uranium mineralization.
The authors concluded that studies of Pb isotopes and mobile trace elements from weak acid leaching of basement rocks and basin sediments provide insight into the late-stage fluid history of Proterozoic sedimentary basins that host unconformity-type uranium deposits. Early isolation of silicified zones from basinal fluids preclude these lithologies as being useful for determining modes of post-mineralization element transport. Radiogenic Pb in mafic volcanic rocks indicate these served as local sinks of U during diagenesis. The presence of such volcanic rocks in a basin hinders the formation of large deposits at the unconformity due to the scavenging of uranium from the basinal fluids prior to their reaching the unconformity.
Late structures, such as fractures and faults, were the most likely avenues of late-stage element migration in these basins; thus, these domains within a basin have the greatest utility for determining whether anomalous concentrations of uranium are present along the upstream path of these structures.
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