In situ sulfur isotope analysis of pyrite from the Ezuri Kuroko‐type volcanogenic massive sulfide deposit, northeastern Japan: Contribution of microbial sulfate reduction to initial sulfide mineralization

Abstract Volcanogenic massive sulfide (VMS) deposits are ancient analogues of seafloor massive sulfide (SMS) deposits. The importance and contribution of microbial activity during initial mineralization has recently been recognized in both VMS and SMS deposits. Here, we report in situ sulfur isotope compositions (δ 34 S) of pyrite from the Ezuri Kuroko‐type VMS deposit in northeastern Japan as determined by secondary ion mass spectrometry. During the evolutionary process of sulfide mineralization, pyrite textures changed from framboidal to colloform to euhedral. Initial framboidal pyrite had highly negative δ 34 S values down to −31.8‰ (average ± 1SD = −18.8‰ ± 13.0‰; n = 21), banded colloform pyrite exhibited medium δ 34 S values (−11.7‰ ± 10.4‰; n = 8), and euhedral pyrite displayed the highest average δ 34 S value of +2.7‰ ± 1.6‰ ( n = 5); thus, δ 34 S varied as different textures of pyrite were produced during mineralization. The maximum isotopic fractionation between framboidal pyrite and past seawater sulfate (δ 34 S ca. +20‰) is −52‰; such values can be produced only by microbial sulfate reduction (MSR) in an open system. Framboidal pyrite with a low δ 34 S value is observed at the centers of sulfide‐rich areas within polished sections and has often been replaced by later sulfide minerals (sphalerite, galena, and chalcopyrite); thus, our S isotope data from the Ezuri pyrite reveal that sulfur derived from MSR induced and acted as a nucleation point for later sulfide mineral growth. Combined with previously reported data, our results endorse the importance and universality of MSR‐derived sulfur during the initial mineralization stage of both VMS and SMS deposits.