A TRIPLE OXYGEN ISOTOPE SEARCH FOR EVAPORITE-DERIVED OXYGEN IN MAGNETITE APATITE DEPOSITS

The formation of some magnetite - apatite deposits was possibly associated with intrusions of silicate magmas into sulfate-rich evaporites [e.g., 1-3]. Studying triple oxygen isotope ratios (¹⁷O/¹⁶O and ¹⁸O/¹⁶O ratios) can be a powerful method to trace the role of evaporites in ore genesis processes in general, because sulfate in evaporites can carry an unusual, mass-independently fractionated oxygen isotope signal (MIF-O) that was ultimately derived from atmospheric O₂. We studied the triple oxygen isotope compositions of magnetite and to lesser extent of apatite from several major magnetite - apatite deposits. Samples from magnetite - apatite deposits of Proterozoic ages typically contain a MIF-O component in magnetite and, in the few cases in which apatite samples were studied as well, also in apatite. These MIF-O components were likely derived from evaporitic sulfate – the only material that can trap atmospheric O₂ with a MIF-O composition in the rock record in large quantities. The MIF-O component is less pronounced in samples from Cretaceous and Quaternary magnetite - apatite deposits; an observation that we interpret to reflect the lower MIF-O signal in atmospheric O₂ during these time periods compared to the Proterozoic. The MIF-O components in our samples are typically complemented by “normal” oxygen (i.e., mass-dependently fractionated oxygen) that is in apparent equilibrium with typical compositions for silicate magmas and high-temperature magmatic fluids. Co-existing magnetite and apatite from samples of some magnetite - apatite deposits record Δ¹⁸O_{magnetite-apatite} values that are expected for magmatic temperatures, but are too low values for equilibrium oxygen isotope fractionation between magnetite and apatite at hydrothermal temperatures. We conclude that the triple oxygen isotope data for those magnetite - apatite deposits that we studied are consistent with formation scenarios for the deposits involving high-temperature magmatic fluids that had interacted with evaporite rocks [1], and/or sulfate melts that had formed by anatectic melting of evaporites [2,3].

[1] Peters et al. (2020) Geology 48 3 p. 211-215. [2] Bain et al. (2020) Nature Geoscience 13 11 p. 751-757 [3] Bain et al. (2021) Geology 49