Structurally bound S2−, S1−, S4+, S6+ in terrestrial apatite: The redox evolution of hydrothermal fluids at the Phillips mine, New York, USA

•Four oxidation states of Sulfur found in natural apatite (S2−, S1−, S4+, S6+).•Sulfur contents in apatite correlate positively with abundance of oxidized S.•Monazite and REE patterns fingerprint fluid-induced dissolution-reprecipitation processes.•Sulfur oxidation states in apatite elucidate redox evolution of hydrothermal systems.