Stable lead isotope characteristics of lead ore deposits of environmental significance

Abstract

Stable Pb isotopes are increasingly used in environmental science as tracers of natural and anthropogenic Pb sources. This review provides a summary of the recent geological literature concerning Pb isotopes in global Pb ore deposits. The isotopic characteristics of 151 Pb deposits, including 78 20th-century producing mines, have been summarized using the 204Pb-based ratios common to geological science and the 206Pb- and 207Pb-based ratios (i.e., excluding 204Pb) more often employed in environmental studies. A number of current mines, including those exploiting several Australian, Scandinavian, and U.S.A. deposits, have extreme isotopic compositions that provide unique signatures. However, a majority of mines (and unproductive deposits) fall within a relatively narrow range:206Pb/207Pb of 1.15-1.22 and 208Pb/207Pb of 2.42-2.50. In some contexts, unequivocal identification of a source exhibiting one of these common signatures would be difficult, especially with the relatively low precision (ca. 0.2-0.5% RSD) of quadrupole inductively coupled plasma - mass spectrometry (ICP-MS) which has been the most common instrument for environmental Pb isotope measurements. In settings with disparate industrial and natural Pb signatures (i.e., sources withisotopic ratios differing by about 2% or more), ICP-MS precision is adequate for source discrimination. Statistical analyses suggested that while 204Pb is critical for identifying a small proportion of environmental Pb sources, about 86% of the source discrimination power is due to the 206Pb, 207Pb, and 208Pb isotopes. Thus, the requisite analytical precision, rather than a lack of 204Pb data, is the most critical issue with respect to unequivocal identification of Pb sources in most cases. Several factors, especially the increasing dominance of recycling in global Pb production and the international transportation of ore concentrate and refined Pb, may cause unpredictable changes in the isotopic signatures of industrial sources,with a long-term trend towards homogenization. More frequent,comprehensive, and high-precision isotopic characterization of possible point and non-point Pb emitters such as gasoline, smelters, and battery-recycling plants, together with increased efforts to document the origin of constituent leads in industrial sources, would help to address these concerns.Key words: lead isotopes; lead pollution; source identification; lead ore deposits.