Sulfur and He-Ar isotopic constraints on the origin of alkalic-type epithermal Au-Ag-Te deposits: Insights from the Golden Sunlight deposit, Montana, USA

• The Golden Sunlight breccia hill links shallow epithermal and deeper porphyry systems. • Mantle-derived fluids mixed with crustal components contributed ore formation. • Pyrite acts as a physical host and chemical monitor for Au-Ag-Te mineralization. Alkalic-type epithermal Au-Ag-Te deposits are economically significant but remain genetically controversial. Debates center on whether ore-forming fluids and sulfur are primarily magmatic or involve substantial contributions from meteoric water or crustal materials. The Golden Sunlight Au-Ag-Te deposit in southwest Montana, USA, is unusual because the mineralization is concentrated within and around the hydrothermal Mineral Hill breccia pipe, which is hosted in the lower Mesoproterozoic Belt Supergroup and is spatially and genetically associated with a Late Cretaceous alkaline to subalkaline porphyry system. The mineralization, which constitutes a gold reserve >100 t, occurs either as disseminated or as structurally controlled veins within faults and joints, predominantly in the sulfide-rich matrix of the breccia pipe, the latite porphyry, and in Proterozoic sedimentary rocks. Alteration in and around the breccia pipe is principally composed of quartz and sericite, and is associated with four discrete stages of hypogene mineralization, in which stage I, represented by a quartz-pyrite-precious metal assemblage, is the most critical and economic stage. Throughout these mineralization stages, pyrite plays a pivotal role in the formation and stabilization of Au-Ag-Te minerals, and its association with native Au and tellurides suggests that pyrite not only acted as a physical host for the Au-Ag-Te mineralization, but also provided a favorable chemical environment for mineralization. Here, we combine the S isotope composition of pyrite, determined in-situ and on separates from micro-drilling, with its noble gas isotope composition to constrain the S and fluid sources, and the evolution of ore fluids during magmatic-hydrothermal processes. The light δ 34 S values of pyrite from the different mineralization stages, ranging from −13.57 to +0.30 ‰, along with the presence of oxides in stage I, suggest that mineralization is originated from oxidized-alkaline magmatic-hydrothermal fluids. By contrast, pyrite from the surrounding sedimentary rocks yielded significantly heavier δ 34 S values (+3.73 to +10.95 ‰), indicating a limited contribution of S from the Proterozoic sedimentary rocks to the magmatic-hydrothermal fluids. The obtained pyrite He-Ar isotopic compositions, specifically the 3 He/ 4 He ratios that range from 0.23 to 1.01 Ra, revealed a mixing trend between mantle (up to 13 %) and crustal components, confirming the contribution of mantle-derived fluids to the shallow low-temperature ore system. Our study emphasizes that combined S and He-Ar isotope analyses of pyrite within a robust geological and mineralogical framework could constrain the sources of ore-forming fluids, provide insights into the processes of mineralizing events, and offer valuable clues for deep mineral exploration.