Arsenopyrite geochemistry and sulfur isotopes reveal the genesis of the Liba Au deposit, West Qinling, China

Understanding the genetic type of gold (Au) deposits is crucial for elucidating ore-forming process and guiding mineral exploration. The Liba Au deposit, a representative supergiant deposit (>150 t Au) in the West Qinling Orogen, China, remains controversial regarding its genetic type, Au occurrence state, and metal sources, hindering deep-target exploration. This study employed in-situ microanalytical techniques, including electron probe microanalysis (EPMA) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), to investigate the major and trace element geochemistry and sulfur isotopes of Au-bearing arsenopyrite of the deposit, with major objective to provide novel insights into its genesis. EPMA results show that arsenopyrite major elements vary from 27.04–36.25 wt% Fe, 17.74–21.97 wt% S, and 41.22–48.09 wt% As, corresponding to formation temperatures of ca. 290–400 °C, indicative of meso- to low-temperature ore-forming conditions. After normalization to atoms per formula unit (apfu), a strong negative correlation between Fe and Co + Ni (apfu) along the theoretical Fe+(Co + Ni) = 1 substitution line provides direct crystallochemical evidence for Fe 2+ replacement by (Co,Ni) 2+ in the arsenopyrite lattice. According to LA-ICP-MS analysis, arsenopyrite has Th/U ratios greater than 1.25, signifying strongly reducing fluid conditions. Correlation analysis of Fe-Au-Ag metals and ionic radius mismatches indicates minimal lattice-bound Au substitution. Instead, Au mainly occurs as nanoscale inclusions of native Au within arsenopyrite, with negligible electrum. LA-MC-ICP-MS sulfur isotopes of arsenopyrite (δ 34 S = +5.49‰ to + 8.83‰, mean + 7.44‰) indicate a mixed sulfur source derived from magmatic-hydrothermal fluids and metasedimentary rocks. The decoupling of this crystallographic Co-Ni substitution from As-S variations reveals independent controls by fluid composition and physicochemical conditions, respectively. Arsenopyrite Co/Ni ratios show a bimodal distribution (mostly < 1, with some values between 1 and 10), indicating contributions from both the sedimentary protolith and syn -mineralization hydrothermal fluid input. When integrated with the geological setting (shear zone-hosted orebodies, metasedimentary host rocks, and the absence of direct magmatic affinity), we classify the Liba deposit as a typical orogenic Au deposit. Gold mineralization is interpreted to have occurred through fluid migration along ductile shear zones, with Au precipitating as nanoparticles during fluid-rock interactions under reduced conditions. This study also proposes a multi-scale targeting framework for Au exploration in the West Qinling Orogen, providing robust criteria for regional mineral prospectivity assessment.