Hydrothermal origin of the Naojiao Fe deposit in Lingxiang district, Eastern China: Evidence from in-situ LA-ICP-MS magnetite trace element analysis

Abstract

The Naojiao Fe deposit (Lingxiang district of Daye orefield) in Eastern China is closely associated with the Early Cretaceous Lingxiang diorite intrusion. However, the metallogenic origin of the Fe deposit and its possible magmatic link remains controversial. Here, we present microstructural and trace element features of magnetite grains from the ore-related diorite (type 1), orebodies (type 2), and the orebody-wallrock contact (type 3) at Naojiao. Type 1 magnetite grains have primary automorphic texture, and high P, V, Cr, Ni, and Ga contents but low Mg and Al contents. However, both type 2 and 3 magnetite grains display alteration zoning and oscillatory zoning, and have high Mg, Al, Zn, Zn/V, Co/Ni, Ti/V, and Ni/Cr, but low V, Cr, Ni, and Ga contents. Based the multi-element variation plots, and (Ca+Al+Mn) vs. (Ti+V), Ti vs. Ni/Cr, and (Ti+V) vs. (Al+Mn) discrimination plots, type 1 magnetite could be magmatic and underwent metasomatism, and was formed under relatively high temperature (>500 °C). Type 2 and 3 magnetite grains may have been hydrothermal and closely associated with relatively oxidizing fluids, and formed under relatively low temperature (300–500 °C). Type 3 has wider range of trace element contents, and may have undergone late-stage alteration. Moreover, the Naojiao Fe deposit has similar magnetite microstructures and trace element contents, pyrite δ³⁴S values, and mineralization ages to other skarn deposits in the Daye orefield, and further suggests that it is a skarn deposit.