Trace element characteristics of magnetite: Constraints on the genesis of the Lengshuikeng Ag-Pb-Zn deposit, China

Magnetite occurs as an ore mineral in many types of deposits and its trace element characteristics can be used to fingerprint various types of mineral deposits and distinguish different ore forming processes. The Lengshuikeng Ag-Pb-Zn deposit (LSKD) is one of the largest silver deposits in China, but the ore forming processes involved in its formation are still unclear. In this study, magnetites from six representative samples of different mineralization types were examined. Their trace element contents were analyzed using in situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to better understand the genesis and evolution of the ore forming fluids responsible for Ag-Pb-Zn mineralization. The studied magnetites were divided into two types based on their spatial occurrence, associated minerals and distinctive textures. Type A magnetites are large, display subhedral to euhedral forms and appear inclusion-free. They coexist with siderites and formed early, preceding the formation of surrounding sulphides. Type B magnetites are small, exhibit irregular and anhedral forms that feature as wall-rock alterations, and were formed during the main mineralization stage. They coexist with main-stage sulphides and Fe-Mn-bearing carbonate minerals, in which Mn content increases corresponding to the evolution of the mineralization process. Generally, magnetites from the LSKD contain low amounts of Ti and V, and widely variable Al and Mn contents, which resulted from multiple influxes of low-temperature hydrothermal fluid. Type A magnetites are inferred to have formed paragenetically early at relatively high temperatures without coeval precipitation of base-metals. Conversely, Type B magnetites are interpreted to have formed during the early (B1, stage 1) and main (B2, stage 2) stages of Ag-Pb-Zn mineralization under alteration and dissolution-reprecipitation processes (DRP) from the later ore-forming hydrothermal fluids. Trace element characteristics of magnetite suggest the late hydrothermal fluid was characterized by low temperatures, and was enriched of Cl and Mn. The wide range of contents of trace elements (e.g. Ga, Mo and Sn) in magnetite that underwent DRP can be explained by different wall-rock types and water/rock ratios. Characteristics of the late hydrothermal fluids, including those from detailed microscopic observations, indicate that the ingress of Ag, Pb and Zn occurred as metal-Cl complexes, and that dissolution of early Fe, Pb, and Zn sulphides supplied the S required for the final precipitation of silver.

Automated summary

This study analyzed magnetite trace element contents from different mineralization types of the Lengshuikeng Ag-Pb-Zn deposit to understand the ore forming fluids responsible for mineralization. Two types of magnetites were identified: Type A formed early without base-metals precipitation, while Type B formed during main mineralization stages under dissolution-reprecipitation processes. Characteristics of late hydrothermal fluids indicate the ingress of Ag, Pb, and Zn occurred as metal-Cl complexes.