Multi-stage Mineralization in the Giant Erdaokan Ag-Pb-Zn Deposit, Northeastern China: Evidence from Magnetite EPMA and LA-ICPMS Geochemistry

Although previous researchers have attempted to decipher ore genesis and mineralization in the Erdaokan Ag-Pb-Zn deposit, some uncertainties regarding the mineralization process and evolution of both ore-forming fluids and magnetite types still need to be addressed. In this study, we obtained new EPMA, LA-ICP-MS, and in situ Fe isotope data from magnetite from the Erdaokan deposit, in order to better understand the mineralization mechanism and evolution of both magnetite and the ore-forming fluids. Our results identified seven types of magnetite at Erdaokan: disseminated magnetite (Mag1), coarse-grained magnetite (Mag2a), radial magnetite (Mag2b), fragmented fine-grained magnetite (Mag2c), vermicular gel magnetite (Mag3a1 and Mag3a2), colloidal magnetite (Mag3b) and dark gray magnetite (Mag4). All of the magnetite types were hydrothermal in origin and generally low in Ti (<400 ppm) and Ni (<800 ppm), while being enriched in light Fe isotopes (δ⁵⁶Fe ranging from –1.54‰ to –0.06‰). However, they exhibit different geochemical signatures and are thus classified into high-manganese magnetite (Mag1, MnO > 5 wt%), low-silicon magnetite (Mag2a-c, SiO₂ < 1 wt%), high-silicon magnetite (Mag3a-b, SiO₂ from 1 to 7 wt%) and high-silicon-manganese magnetite (Mag4, SiO₂ > 1 wt%, MnO > 0.2 wt%), each being formed within distinct hydrothermal environments. Based on mineralogy, elemental geochemistry, Fe isotopes, temperature trends, T_Mg-mag and (Ti + V) vs. (Al + Mn) diagrams, we propose that the Erdaokan Ag-Pb-Zn deposit underwent multi-stage mineralization, which can be broken down into four stages and nine sub-stages. Mag1, Mag2a-c, Mag3a-b and Mag4 were formed during the first sub-stage of each of the four stages, respectively. Additionally, fluid mixing, cooling and depressurization boiling were identified as the main mechanisms for mineral precipitation. The enrichment of Ag was significantly enhanced by the superposition of multi-stage ore-forming hydrothermal fluids in the Erdaokan Ag-Pb-Zn deposit.