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

Abstract

Abstract The oxidation state of sulfur (S) plays a critical role in the formation of igneous and hydrothermal mineral systems. Constraining the oxidation state of S during mineralization and alteration is a valuable tool for understanding primary versus secondary processes that affect mineral systems. Recent experimental studies demonstrate that apatite, which is a ubiquitous accessory mineral in igneous and hydrothermal ore-forming systems, structurally incorporates multiple oxidation states of S (i.e., S6+, S4+, S2−), and that the abundance of reduced versus oxidized S in apatite is systematically related to oxygen fugacity. 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 (Putnam County, New York). 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. Significant compositional and textural variations within the apatite were observed by electron probe micro-analysis (EPMA), wavelength dispersive (WDS) spectroscopy element mapping, and cathodoluminescence (CL) imaging, and used in combination with the μ-XANES data to discuss the formation of the Phillips mine apatite. The μ-XANES analyses reveal that apatite contains variable proportions of S6+, S4+, S1− and S2−, with corresponding peak absorption energies of 2481.7\u202f±\u202f0.3\u202feV, 2477.9\u202f±\u202f0.4\u202feV, 2471.8\u202f±\u202f0.1\u202feV, and 2469.8\u202f±\u202f0.04\u202feV, respectively. Notably, this marks the first observation of reduced S species (S2−, S1−) in terrestrial apatite. Peak areas ratios (S6+/∑S) demonstrate systematic variations in the oxidation state of S within the apatite grains. Elevated S6+/∑S peak area ratios typically coincide with higher concentrations of S and rare earth elements within the apatite grains. Several observations, including the presence of multiple oxidation states of S in apatite, and monazite inclusions that record secondary, fluid-mediated dissolution-reprecipitation of apatite, indicate differences in the oxidation of S, thus oxygen fugacity, during primary mineralization and secondary alteration (i.e., metasomatism). We propose a model for the formation of the Phillips mine apatite wherein the primary apatite grains crystallized from a reduced, S-bearing hydrothermal fluid characterized by a low SO2/H2S ratio. Subsequently, metasomatism of apatite in the presence of an oxidized fluid, which contained an elevated SO2/H2S ratio, resulted in the exsolution of rare earth elements from apatite and concomitant growth of monazite, and the structural incorporation of oxidized S (S6+ and S4+) in apatite. This study demonstrates that the oxidation states of S in apatite provide valuable geochemical information regarding the redox evolution of mineralized systems.