SIMS discovers invisible gold in pyrite from the high-grade seafloor hydrothermal deposits in the Higashi-Aogashima knoll caldera, Izu-Ogasawara arc, Japan

Seafloor hydrothermal deposits with extremely high concentrations of gold were recently discovered in the Higashi-Aogashima knoll caldera hydrothermal field, Izu-Ogasawara arc Japan. We report here the discovery of ultra-high concentrations of “invisible gold” (up to 1.9 wt%) in pyrite from the sulfide mounds and a black smoker chimney using a secondary ion mass spectrometry (SIMS). A notable feature of this study is that quantitative gold concentrations were obtained from all analyzed pyrites due to the high sensitivity (7 ppb) and wide measurement range of the SIMS analysis. The concentrations of gold and arsenic in pyrite are higher than those in the whole rock on any hydrothermal sites within the caldera, suggesting that gold and arsenic are concentrated in pyrite. The gold concentrations in pyrite vary greatly depending on the site of the deposit and morphology of pyrite. The extremely high gold concentrations are found from colloform pyrites with high lead and/or copper concentrations in addition to arsenic concentration in pyrite, which might have induced the high gold concentration in pyrite. No gold nanoparticles were observed in high spatial resolution SIMS depth profiles of pyrite, and it is believed that gold and other elements are contained within the pyrite crystal structure.