Sulfide Mineralogy, Textures, and Sulfur Isotope Evidence for the Origin of Subseafloor Replacement-Type Volcanogenic Massive Sulfide Mineralization in the Wolverine Deposit, Yukon, Canada

Preservation of subseafloor replacement-style mineralization in the Wolverine volcanogenic massive sulfide (VMS) deposit illustrates the importance of ambient sedimentary environment and mud replacement in the formation of replacement-style VMS mineralization. Prior to deposit formation, framboidal pyrite, and likely bacterial sulfate reduction (BSR)-generated H2S in pore fluids, formed during diagenesis in host sediments in association with BSR of seawater sulfate. Diagenesis was succeeded by the first stage (stage 1) of low-temperature (<300°C) Zn-rich mineralization that utilized the preexisting framboids (and likely water and H2S in the sedimentary pore fluids) and involved both macro- and microscale replacement of mud and framboids and the deposition of a Zn-Pb-(Ag-Se-Sn)–rich assemblage dominated by sphalerite-galena; colloform, spongy, and euhedral pyrite; tetrahedrite; arsenopyrite; stannite; cassiterite; clausthalite; and naumannite. The stage 1 mineralization was replaced by a Cu-rich assemblage during stage 2 mineralization dominated by chalcopyrite, pyrrhotite, and rounded pyrite. This study interprets that Cu-rich fluids dissolved stage 1 Zn-rich sulfides, which resulted in remobilization of Zn into solution, precipitation of Cu-rich sulfides in their place, and subsequent reprecipitation of Zn at the stratigraphically overlying mud-sulfide interface. It is also interpreted that this zone-refining and Zn-redistribution process was important for the high Zn grades of the Wolverine deposit. In situ sulfur isotopes from various pyrite textural types, galena, pyrrhotite, and chalcopyrite show extreme variability in the diagenetic and stage 1 mineralization with δ34S values ranging from –27.4 to 12.1‰, whereas stage 2 pyrite, pyrrhotite, and chalcopyrite have restricted δ34S values ranging from 10.1 to 14.3‰. These δ34S variations reflect varying contributions of H2S derived from BSR, inheritance of BSR-derived sulfur from framboids during replacement, and hydrothermal H2S derived from thermochemical sulfate reduction (TSR) of seawater sulfate. The process of nucleation and replacement of VMS sulfides on diagenetic framboidal pyrite, mixing of VMS fluids with BSR-derived H2S in sedimentary pore fluids, and zone refining are likely important in many replacement-style VMS deposits and sediment-rich deposits globally (e.g., mafic-siliciclastic VMS, Irish-type and clastic-dominated Zn-Pb, and sediment-hosted Cu deposits).