Structurally bound S2-, S1-, S4+, S6+ in apatite: The redox evolution of ore fluids at the Phillips Mine ore deposit, New York, USA

Abstract

The oxidation state of S plays a critical role in the formation of igneous and magmatic-hydrothermal ore deposits so constraining the oxidation state of S in these systems can be a valuable tool for understanding mineralizing processes. Apatite, commonly Ca10[PO4]6[F,Cl, OH]2, is a prevalent accessory mineral in igneous and magmatic-hydrothermal ore-forming systems, and can incorporate redox sensitive elements such as Fe, Mn, and S. Recent experimental studies demonstrate that the behavior of S (e.g., oxidation states of S, S content) in apatite is sensitive to oxygen fugacity. However, there is an overall lack of data with respect to the redox behavior of S in magmatic-hydrothermal systems. In this study, we used micro X-ray absorption near edge structure (μ-XANES) spectroscopy at the S K-edge to measure the oxidation states of S in natural apatite from the Phillips Mine magnetite-sulfide mineral deposit in Putnam County, New York. Here, the data are used to test whether the oxidation state of S in apatite from natural systems 2 can be used to assess possible fluctuations of oxygen fugacity during primary growth of apatite, and subsequent secondary alteration. Micro-XANES transects were collected within two apatite grains, starting near the edge of (1) a pyrrhotite inclusion, and (2) an inclusion assemblage consisting of pyrite, ferroan carbonate, pyroxene, and magnetite. Transects were conducted moving away from the inclusions and into the apatite host. Electron probe micro-analysis (EPMA) transects were performed parallel to the μ-XANES transects to correlate changes in the oxidation state of S in apatite with changes in the S concentration of apatite. The XANES analyses reveal that apatite contains variable proportions of S6+, S4+, S1- and S2-, with corresponding peak absorption energies of 2481.8 0.3 eV, 2477.9 0.4 eV, 2471.8 0.1 eV, and 2469.8 0.04 eV, respectively. Peak areas determined for the different oxidation states of S in apatite demonstrate systematic variations in S6+/⅀S, where elevated S6+/⅀S ratios typically coincide with higher concentrations of S and rare earth elements (REEs) in apatite. The observation of multiple oxidation states of S, and the presence of monazite inclusions that record secondary, fluid-mediated dissolutionreprecipitation of apatite, indicate differences in S and oxygen fugacity during primary mineralization and secondary metasomatism. We propose that the apatite grains crystallized from hydrothermal conditions where reduced S, i.e., sulfide (or H2S in the fluid; HS-; S2-), was the dominant stable S species. Subsequently, metasomatism of apatite in the presence of an oxidized fluid; e.g., elevated SO2/H2S, resulted in the exsolution and growth of monazite, and the incorporation of oxidized S (S6+ and S4+) in apatite. This study demonstrates that the oxidation states of S in apatite can provide valuable geochemical information regarding the redox evolution of magmatichydrothermal systems.