Iron and magnesium isotope systematics from the Shuangwang gold deposit in the Qinling Orogen, central China

Iron and Magnesium isotopes have been increasingly widely applied to trace mineralization processes of hydrothermal ore systems. In this study, Fe and Mg isotope compositions of whole-rocks, Fe-bearing minerals (pyrite and ankerite) and Mg-bearing mineral (ankerite) from the Shuangwang gold deposits are investigated to evaluate Fe and Mg isotope behaviors in a gold deposit hydrothermal system and provide new constraints on ore-forming processes. The results showed that the magmatic rocks in the Shuangwang gold deposit (mainly granodiorite , monzonitic granite and lamprophyre) yielded δ 56Fe values of 0.05–0.10‰ and δ26 Mg values concentrated in a narrow range from −0.23‰ to −0.21‰. The gold orebodies are hosted by a NW-trending breccia belt. The wall rock samples (mainly argillaceous limestone, carbonaceous slate and albitite) have higher δ 56Fe values of 0.12–0.29‰ and a wide δ26 Mg range from −2.54‰ to −0.35‰. Pyrite mineral separates show larger δ 56 Fe variation from −0.44‰ to 0.16‰, whereas ankerite has a small δ 56Fe range from −0.15‰ to −0.06 and a δ26Mg range from −0.66‰ to −0.35‰. The crystallization of magmatic rocks incorporated preferentially heavy Fe isotopes, resulting in enrichment of light Fe isotopes in the residual fluid exsolved from magma . The interactions between the wall rocks and the fluids resulted in increase of δ 56Fe value in fluids and resulted in decrease of δ56Fe value in the wall rocks. However, the fluid still had much lighter Fe isotope composition. Thus, Fe in the main ore-forming stages is regarded as derived from the magmatic stock. The opposite enrichment in Fe isotopes between pyrite and ankerite from stages II and III may be explained by equilibrium Fe isotope fractionation in an open system. In addition, the negative δ56Fe value in pyrite from stage IV is indicative of the equilibrium between FeS and dissolved Fe in a closed system. The exsolved fluid from magma has Mg isotope composition similar to magmatic rock, and the water–rock reaction led to the enrichment of heavy Fe isotopes in fluid. The evolutions of Mg isotopes in stages II and III precipitation matched well with the prediction of Rayleigh fractionation model. The Mg from wall rocks contributed to the composition of hydrothermal magnesium isotopes during the formation of the Shuangwang gold deposit. A multi-origin is supposed whereby the main stage ore-forming fluids were derived from metamorphic water and were affected by the influx of magmatic water.