Geochemistry, geochronology and Fe-Mg-S isotopic composition of the Liaoshang gold deposit, Jiaodong Peninsula, China: Implications for ore-forming processes and mineral exploration

Abstract

The Liaoshang Au deposit in North China is distinguished by gold occurrence in pyrite-dolomite veins and characterized by large Au endowment (about 69 t Au @ 3.3 g/t), representing a novel gold deposit type within the Jiaodong gold province. To date, the mineralization age and metallogenic mechanism remain controversial, limiting the in-depth understanding of ore-forming processes. The Liaoshang Au mineralization differs from classic orogenic Au metallogeny in close relationships with the occurrence of ore-stage pyrite and dolomite. Here, we use syn-ore stage monazite U-Pb geochronology to constrain the ore-forming age. We also measured Fe and Mg isotope compositions in pyrite and dolomite associated with gold mineralization, combined with in situ LA-(MC)-ICP-MS elemental mapping and S isotopes in pyrite to decipher the iron, magnesium, and sulfur source(s), aiming to provide new insights into the ore-forming processes. Petrographic observation shows that gold in pyrite-dolomite veins mainly occurs as micro-grains (20–100 μm) within pyrite. Elemental mapping and in situ S isotope measurements for pyrite indicate that the main Au mineralization is associated with elevated concentrations of Cu, Ag, As, and Bi, as well as heavy sulfur isotope values (δ³⁴S ~13.6 ‰, compared to ~8.0 ‰ in the early stage). This is attributed to the addition of sulfur with high δ³⁴S from the basement Jingshan Group. The δ⁵⁶Fe values of ankerite and pyrite present a narrow range with an average of −0.21 ‰ and +0.56 ‰, respectively. The inverse enrichment of iron isotopes between ankerite and pyrite may be attributed to thermodynamic equilibrium fractionation in an open system. The δ²⁶Mg values of ankerite yields a wide range from −3.02 ‰ to −0.80 ‰. By comparing the Fe and Mg isotopic signatures of ores with those of local granites and basement rocks, we argue that the Jingshan Group contributed Fe and Mg to form auriferous pyrite and dolomite. The U-Pb dating of monazite in Au-bearing pyrite yields an age of 118.8 ± 1.7 Ma, which may represent the mineralization age of the deposit. The ore-forming fluid sourced from metasomatized mantle lithosphere leached Fe, Mg, and some heavier sulfur from the Jingshan Group to precipitate pyrite and dolomite in Liaoshang, forming pyrite‑carbonate veins. This is a critical gold mineralization mechanism for forming the Liaoshang-type gold deposits in the Jiaodong Peninsula, North China, which is different from that of the Jiaojia-type (disseminated stockwork-altered wall-rock type) and Linglong-type (quartz-sulfide vein type) gold deposits.