Spatial evolution of Zn-Fe-Pb isotopes of sphalerite within a single ore body: A case study from the Dongshengmiao ore deposit, Inner Mongolia, China

Analyses of sphalerite minerals from the characteristic brecciated Zn-Pb ores of the main ore body in the giant Dongshengmiao deposit have revealed variations in delta Zn-66 from 0.17 to 0.40aEuro degrees and in delta Fe-56 from -1.78 to -0.35aEuro degrees. Further, the investigated pyrrhotite samples have iron that is isotopically similar to that of associated sphalerite minerals. The most distinctive pattern revealed by the zinc and iron isotope data is the lateral trend of increasing delta Zn-66 and delta Fe-56 values from southwest to northeast within the main ore body. The lead isotopic homogeneity of ore sulfides from the main ore body suggests that there is only one significant source for metal, thus precluding the mixing of multiple metal sources as the key factor controlling spatial variations of zinc and iron isotopes. The most likely control on spatial variations is Rayleigh fractionation during hydrothermal fluid flow, with lighter Zn and Fe isotopes preferentially incorporated into the earliest sulfides to precipitate from fluids. Precipitations of sphalerite and pyrrhotite have played vital roles in the Zn and Fe isotopic variations, respectively, of the ore-forming system. Accordingly, the larger isotopic variability for Fe than Zn within the same hydrothermal system perhaps resulted from a larger proportion of precipitation for pyrrhotite than for sphalerite. The lateral trend pattern revealed by the zinc and iron isotope data is consistent with the occurrence of a cystic-shaped breccia zone, which is characterized by marked elevation in Cu. The results further confirm that Zn and Fe isotopes can be used as a vectoring tool for mineral prospecting.