In-situ trace element characteristics of pyrite in the Tengjia gold deposit at Northwestern Jiaodong Peninsula: Implications for the ore-forming process

ObjectiveThe Tengjia gold deposit is a newly-discovered altered rock-type giant lode gold deposit located within the illustrious gold-rich Zhaoping fault of northwestern Jiaodong peninsula. Despite pyrite being a main carrier of Au, little is known about microstructure and trace element characteristics of pyrite in the Tengjia Au deposit, which limits our understanding of the ore-forming process.MethodsIn this study, trace element composition of pyrite in the Tengjia gold deposit was analyzed by using LA-ICP-MS, with the aim of investigating the occurrence state, migration and precipitation mechanism of gold, as well as constraining the source of mineralization fluids and the ore-forming process.ResultsHigh grade gold mineralization mostly occurred in the quartz-polymetallic sulfide stage, with a large amount of visible gold appearing concurrently with polymetallic sulfides. Time resolved depth profiles for single spot LA-ICP-MS analyses demonstrate the existence of sub-micron gold-bearing minerals (Au-Ag-Te) within some pyrites of high grade gold ore, whereas most pyrites have significantly low Au(<1×10^-6), suggesting predominant of visible gold in the Tengjia gold deposit. Since pyrites in Tengjia gold deposits are mostly Au-poor, remobilizaiton of lattice Au is unlikely to be the key process generating visible gold. Alternatively, visible gold should be directly precipitated from ore-forming fluids. The variable As content and Co/Ni ratio of pyrite in the Tengjia gold deposit indicate varying degrees of water-rock reaction. The decoupled Au and As geochemistry of pyrite from the Tengjia gold deposit implies that As-rich fluids generated from water-rock reaction is not necessarily Au-rich, and that a initial Au-rich ore-forming fluid combined with rapid visible gold precipitation is more important for high grade gold mineralization.ConclusionsCombined with the fact that Tengjia gold deposit occurred in the lower plate of Zhaoping fault, it could be inferred that variable fluid-rock reaction between deep-sourced ore-forming fluids and wall rocks within a small space is the main ore-forming mechanism for Tengjia gold deposit.