The source of phosphate in the oxidation zone of ore deposits: Evidence from oxygen isotope compositions of pyromorphite
The source of phosphate in the oxidation zone of ore deposits: Evidence from oxygen isotope compositions of pyromorphite
复制标题
DOI：
10.1016/j.gca.2013.07.042
复制
发表时间：
2013-12
影响因子：
5
通讯作者：
F. Burmann;Maximilian F. Keim;Y. Oelmann;Holger Teiber;M. Marks;G. Markl
中科院分区：
文献类型：
--
作者：
F. Burmann;Maximilian F. Keim;Y. Oelmann;Holger Teiber;M. Marks;G. Markl
磷氯铅矿（Pb_5 [PO_4]_3Cl）是铅矿床氧化带中的一种重要矿物，由于其溶解度极低，在方铅矿（PbS）表生蚀变过程中能有效地与铅结合。土壤或近地表流体对溶解铅的吸附能力主要取决于土壤或流体中磷酸盐的有效性。土壤中潜在的磷源包括（i）生物过程中的释放，即来自土壤中的凋落物/微生物细胞裂解（细胞内酶活性后）的沥滤和土壤有机物通过细胞外酶的水解，以及（ii）来自邻近基底岩石中磷灰石溶解的无机磷酸盐。植物/微生物的细胞内酶活性与动力学分馏产生的氧同位素组成与土壤中的无机过程明显不同。本研究首次提供了磷氯铅矿中磷酸盐（δ 18 OP）的氧同位素数据和结晶岩中磷灰石的全面数据。我们调查了38个火榴石从26个地点在黑森林（德国西南部）和5个样品从黑森林以外的地方，除了12磷灰石分离片麻状和花岗岩寄主岩石。焦绿石的δ 18 OP值在+10‰和+19‰之间，与土壤中易有效磷部分（树脂可提取磷）的δ 18 OP值的文献数据相当，矿物可能从土壤中沉淀出来。低于平衡同位素分馏范围的δ 18 OP值可归因于地球化学形成的磷灰石（磷灰石的δ 18 OP：+6‰ ~+9‰）或不太可能是生物过程（胞外酶活性）。然而，对于我们的大多数样品，同位素与环境水平衡，这表明生物活性。因此，我们得出结论，矿体氧化带中的大多数焦绿石是由生物循环的磷酸盐形成的。这项研究强调了生物活性和Pb活化密切相关：在土壤生物活性高的湿润地区，由于生物释放的磷酸盐，铅可能迅速沉淀，而在生物活性较低的干旱地区，由于磷的供应有限，磷氯铅矿不能形成，铅更容易从矿床或矿山废弃物中释放到环境中。
Pyromorphite (Pb5[PO4]3Cl) 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 (δ18OP) 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 δ18OPvalues between +10‰ and +19‰, comparable to literature data on δ18OPin the readily available P fraction in soil (resin-extractable P) from which minerals potentially precipitate in soils. δ18OPvalues below the range of equilibrium isotope fractionation can be attributed either to apatites that formed geochemically (δ18OPof apatites:+6‰ to +9‰) 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.