Pyrite Textures, Trace Element Chemistries, and Isotopic Signatures (S, Pb) Suggest Hybrid Ore-Forming Processes in the World-Class Metasediment-Hosted Liba Au Deposit, West Qinling Orogen, Central China

The Liba Au deposit (109.8 t Au at 1.99 g/t) in the West Qinling orogen (central China) is hosted in Middle Devonian greenschist facies metamorphosed clastic rocks and carbonates and has a close spatio-temporal relationship with Late Triassic granitoid complex and mafic to felsic dikes. The Au mineralization is localized within west-northwest to east-west fracture zones and characterized by pyrite disseminations and subordinate auriferous quartz veins. Based on field and textural relationships, the following four paragenetic stages are defined: (1) pre-ore stage marked by deformed pyrite, (2) early ore stage comprising quartz-pyrite and quartz-sericite-pyrite ores, (3) late ore stage composed of quartz-polymetallic sulfide veins, and (4) post-ore stage of barren calcite. Pre-ore stage pyrite (Au ≤ 3.0 ppm) has δ34S values of 5.0 to 6.8‰, which are lower than those for orestage pyrites (6.8–10.4‰). This S isotope difference discounts the possibility that some of the ore-forming S was sourced from the local metasedimentary host rocks. Pyrite from the early ore stage displays core-mantlerim zoned textures and hosts fine-grained native gold grains and has relatively high contents of invisible Au (≤600 ppm) in its mantle zones. The uniform δ34S values (mostly 8.5–10.4‰) and 206Pb/204Pb ratios (18.3918–18.6362) for these pyrites suggest a deep-seated homogenized source for ore fluids, most likely derived from metamorphic devolatilization of Paleozoic metasedimentary rocks. Pyrite of the late ore stage, closely associated with abundant native gold, can be subdivided into a texturally early Au-bearing generation (Au ≤ 8.9 ppm; δ34S of 8.6–9.3‰; 206Pb/204Pb of 18.9885–18.9959), which is overprinted by a texturally later Au-poor generation (Au ≤ 2.0 ppm; δ34S of 8.8–9.6‰) and Au-bearing generation (Au ≤ 3.9 ppm; δ34S of 6.8–8.5‰; 206Pb/204Pb of 18.9501–18.9890). The lower δ34S values and more radiogenic Pb isotope signature, combined with enrichment in Cu, Zn, Ag, Sn, Sb, and Pb in this latest Au-bearing pyrite, indicate a different fluid source, which is suggested to originate from magmatic-hydrothermal fluids that interacted with the enveloping Devonian metasedimentary rocks. Integration of our new data with previous studies suggests the Liba Au deposit is hybrid in nature and involved multiple fluid sources and multiple Au deposition stages—a finding that has also been documented in other orogenic Au deposit settings worldwide. In addition, systematic in situ chemical and isotopic analyses of texturally well-constrained pyrite provide the means to geochemically characterize the entire fluid evolution history, as done at Liba, and fingerprint the fluid reservoirs and processes implicated in orogenic Au systems.