Lead (Pb) isotope evidences for mobility of trace metals during weathering of black shales in central Hunan, China

This study applied Pb isotope analysis to understand trace metal mobility during black shale weathering in a profile and acid rock drainage (ARD) from a black shale outcrop in Hunan (China). Results showed that black shales were enriched in sulfophile metals (Pb, Ni, Cu, Zn, Cd, and Mo) and lithophile metals (Ba, V, and U). These trace metals were less enriched in weathered black shales along the profile. Vertical concentration profiles, enrichment factors (EF) and mass change (τi,j) values of trace metals suggest that they were released from black shales, with an abrupt release where the fresh substrate was first exposed to weathering. Fresh black shales were characterized by varible Pb isotope compositions, while Pb isotope compositions of weathered black shales were less variable, with Pb isotope ratios higher than that of pyrite grains. The linear relationship of 206Pb/207Pb versus 208Pb/206Pb ratios of pyrite grains, profile rocks, fresh black shales, and ARD from the outcrop illustrated that black shale weathering led to congruent release of unradigogenic Pb isotopes in sulfide minerals and more radiogenic Pb isotopes (206Pb, 207Pb) in accessory minerals and organic matter. Lead isotope release resulted in modifying Pb isotope compositions of fresh black shales, and thus Pb isotope compositions of profile rocks were homogenised. The release of Pb isotopes corresponded with abrupt release of sulfophile and lithophile trace metals during weathering. This suggests that the congruent release occurred during the incipial stage of weathering, and the trace metal release resulted from a combination of different geochemical processes including aerobic oxidation of sulfides, oxidation of organic matter, dissolution of accessory minerals, and dissolution of carbonate and silicate minerals. As this incipial stage of black shale weathering plays a critical role in determining tmobility patterns of trace metals during weathering, it represents the critical stage of black shale weathering.