210 Pb and 210 Po in Geological and Related Anthropogenic Materials: Implications for Their Mineralogical Distribution in Base Metal Ores

摘要

The distributions of 210 Pb and 210 Po, short half-life products of 238 U decay, in geological and related anthropogenic materials are reviewed, with emphasis on their geochemical behaviours and likely mineral hosts. Concentrations of natural 210 Pb and 210 Po in igneous and related hydrothermal environments are governed by release from crustal reservoirs. 210 Po may undergo volatilisation, inducing disequilibrium in magmatic systems. In sedimentary environments (marine, lacustrine, deltaic and fluvial), as in soils, concentrations of 210 Pb and 210 Po are commonly derived from a combination of natural and anthropogenic sources. Enhanced concentrations of both radionuclides are reported in media from a variety of industrial operations, including uranium mill tailings, waste from phosphoric acid production, oil and gas exploitation and energy production from coals, as well as in residues from the mining and smelting of uranium-bearing copper ores. Although the mineral hosts of the two radionuclides in most solid media are readily defined as those containing parent 238 U and 226 Ra, their distributions in some hydrothermal U-bearing ores and the products of processing those ores are much less well constrained. Much of the present understanding of these radionuclides is based on indirect data rather than direct observation and potential hosts are likely to be diverse, with deportments depending on the local geochemical environment. Some predictions can nevertheless be made based on the geochemical properties of 210 Pb and 210 Po and those of the intermediate products of 238 U decay, including isotopes of Ra and Rn. Alongside all U-bearing minerals, the potential hosts of 210 Pb and 210 Po may include Pb-bearing chalcogenides such as galena, as well as a range of sulphates, carbonates, and Fe-oxides. 210 Pb and 210 Po are also likely to occur as nanoparticles adsorbed onto the surface of other minerals, such as clays, Fe-(hydr)oxides and possibly also carbonates. In rocks, unsupported 210 Pb- and/or 210 Po-bearing nanoparticles may also be present within micro-fractures in minerals and at the interfaces of mineral grains. Despite forming under very limited and special conditions, the local-scale isotopic disequilibrium they infer is highly relevant for understanding their distributions in mineralized rocks and processing products.