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Pyromorphite (Pb-5[PO4](3)Cl) is an abundant mineral in oxidized zones of lead- bearing ore deposits and due to its very low solubility product effectively binds Pb during supergene alteration of galena (PbS). The capacity of a soil or near- surface fluid to immobilize dissolved Pb depends critically on the availability of phosphate in this soil or fluid. Potential phosphorus sources in soil include (i) release during biological processes, i. e. leaching from litter/lysis of microbial cells (after intracellular enzyme activity) in soil and hydrolysis from soil organic matter by extracellular enzymes and (ii) inorganic phosphate from the dissolution of apatite in the adjacent basement rocks. Intracellular enzyme activity in plants/microorganisms associated with kinetic fractionation produces an oxygen isotope composition distinctly different from inorganic processes in soil.

This study presents the first oxygen isotope data for phosphate (delta(OP)-O-18) in pyromorphite and a comprehensive data set for apatite from crystalline rocks. We investigated 38 pyromorphites from 26 localities in the Schwarzwald (Southwest Germany) and five samples from localities outside the Schwarzwald in addition to 12 apatite separates from gneissic and granitic host rocks. Pyromorphites had delta(OP)-O-18 values between + 10 parts per thousand and +19 parts per thousand, comparable to literature data on delta(OP)-O-18 in the readily available P fraction in soil (resin-extractable P) from which minerals potentially precipitate in soils. delta(OP)-O-18 values below the range of equilibrium isotope fractionation can be attributed either to apatites that formed geochemically (delta(OP)-O-18 of apatites:+6 parts per thousand to +9 parts per thousand) or less likely to biological processes (extracellular enzyme activity). However, for most of our samples isotopic equilibrium with ambient water was indicated, which suggests biological activity. Therefore, we conclude that the majority of pyromorphites in oxidized zones of ore bodies formed from biologically cycled phosphate.

This study highlights that biological activity and Pb mobilization are intimately connected: in humid regions with high biological activity in soil, Pb might be precipitated rapidly due to biologically-released phosphate, whereas Pb will be released to the environment from ore deposits or mine dumps much more easily in arid regions with low biological activity, because pyromorphite cannot form due to limited supply of phosphorus. (C) 2013 Elsevier Ltd. All rights reserved.