Pyrite Rb-Sr, Sm-Nd and Fe isotopic constraints on the age and genesis of the Qingchengzi Pb-Zn deposits, northeastern China

Abstract

Sr-Nd-Fe isotopes are reported for pyrites and whole rocks of ores and associated rocks for the Qingchengzi Pb-Zn deposits in the northeasern China, to investigate the age, sources and processes of the Pb-Zn mineralization. Rb-Sr leaching isotopic dating of pyrites that crystallized coeval with galena and sphalerite from five subsamples of ores yields isochron ages of 143 ± 12 Ma to 159 ± 12 Ma with a weighted mean age of 151.8 ± 5.2 Ma. This age can be interpreted to be the timing of Pb-Zn mineralization, coeval with the emplacement of Late Jurassic granites in the region. The pyrites have negative εNd (152 Ma) values of −17.6 to –22.0, among the Neoarchean TTG rocks, Paleoproterozoic meta-sedimetary rocks (i.e., wall rocks) and granites, and Mesozoic mafic dykes and granites. However, the initial 87Sr/86Sr ratio of 0.7188 ± 0.0004 is similar to those of Late Jurassic granites and Paleoproterozoic rocks (0.7082–0.7183), distinct from those of mafic dykes (0.7069–0.7110), indicating the ore-forming materials (i.e., Pb and Zn) were mainly derived from wall rocks and Late Jurassic granites. Combined with the previously-published H-O isotopes, the new age and Sr-Nd isotopic data indicate that the mineralizaing fluids were mainly derived from dehydration of granitic magmas, interacted with meteoric water that extracted Pb and Zn from the wall rocks. The δ56Fe values of pyrites in the Pb-Zn ores and Paleoproterozoic meta-sedimentary rocks range from +0.23‰ to +1.09‰ and −0.49‰ to +0.17‰, respectively, distinct from those of Paleoproterozoic basement rocks and Mesozoic rocks (−0.03‰ to +0.28‰). Combined with Nd isotopes, the large range of δ56Fe values for pyrites would be the result of rapid precipitation of pyrites in the wall rocks and contemperal deposition of pyrites in the residual fluids during interaction between wall rocks and ore-forming fluids. Collectively, our new Sr-Nd-Fe isotopes of pyrites not only constrain the age and sources of Pb and Zn mineralization, but also reveal the detailed mineralization processes, providing a new method to constrain the genesis of hydrothermal Pb-Zn deposits.