Trace element evolution of magnetite in iron oxide-apatite deposits: Case study of Daling deposit, Eastern China

Abstract

In recent years, magnetite trace elements have increasingly been applied in studies into ore deposits. The magnetite trace element characteristics of different deposit types have been summarized, but to date no satisfactory explanation has been obtained as to why magnetite samples from some iron oxide-apatite (IOA) deposits exhibit similar trace element characteristics to those in iron-oxide copper gold (IOCG) deposits, or even skarn deposits. The Daling IOA deposit is located in the Luzong volcanic basin, Eastern China. Multi-stage magnetite have developed within the deposit; which provides a good opportunity to study the evolution process of magnetite trace elements in an IOA deposit. Here, sensitive high-resolution ion microprobe (SHRIMP) in situ sulfur isotope analysis was carried out for pyrites at different depths in the deposit. Pyrites at different locations exhibited similar sulfur isotope compositions, falling within the magmatic sulfur range (0.1‰–5.2‰), indicating that the ore-forming fluid of the Daling deposit was not affected by the Triassic evaporite, and that the trace element characteristics of its magnetite were mainly controlled by its magmatic-hydrothermal evolution process. Magnetite within the deposit can be divided into three sub-stages. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis revealed that magnetite (Mag-a) in vertical veinlets exhibits the highest Ti, V, Mn, and Zn contents; magnetite (Mag-b), which is associated with diopside and tremolite in coarse veinlets, exhibits the highest Mg, Al, Co, and Ni contents and brecciated magnetite (Mag-c) exhibits the lowest V content, the highest Mn and Zn contents, and similar Mg, Al, Ti, and Co contents to those of Mag-b. Comparing the trace elements of the three types of magnetite revealed that hydrothermal fluid can assimilate and absorb albite-diopside altered rock that forms in the early stage, resulting in hydrothermal fluid that is rich in Mg, Al and Si. Thus, magnetite samples in IOA deposits can exhibit similar features to those in skarn deposits through multi-stage, long-distance fluid evolution. Therefore, before using magnetite trace elements to discuss the characteristics of hydrothermal fluid, it is important to carefully investigate the basic geological characteristics of a given deposit and the mineralogical characteristics of its magnetite.