Adam C. Simon Warren C. Day Craig C. Lundstrom Ilya N. Bindeman Tristan Childress 2016 <p><span>New O and Fe stable isotope ratios are reported for magnetite samples from high-grade massive magnetite of the Mesoproterozoic Pea Ridge and Pilot Knob magnetite-apatite ore deposits and these results are compared with data for other iron oxide-apatite deposits to shed light on the origin of the southeast Missouri deposits. The </span><i>δ</i>¹⁸<span>O values of magnetite from Pea Ridge (</span><i>n</i><span> = 12) and Pilot Knob (</span><i>n</i><span> = 3) range from 1.0 to 7.0 and 3.3 to 6.7‰, respectively. The </span><i>δ</i>⁵⁶<span>Fe values of magnetite from Pea Ridge (</span><i>n</i><span> = 10) and Pilot Knob (</span><i>n</i><span> = 6) are 0.03 to 0.35 and 0.06 to 0.27‰, respectively. These </span><i>δ</i>¹⁸<span>O and the </span><i>δ</i>⁵⁶<span>Fe values suggest that magnetite crystallized from a silicate melt (typical igneous </span><i>δ</i>⁵⁶<span>Fe ranges 0.06–0.49‰) and grew in equilibrium with a magmatic-hydrothermal aqueous fluid. We propose that the </span><i>δ</i>¹⁸<span>O and </span><i>δ</i>⁵⁶<span>Fe data for the Pea Ridge and Pilot Knob magnetite-apatite deposits are consistent with the flotation model recently proposed by </span><span id="xref-ref-36-1" class="xref-bibr">Knipping et al. (2015a)</span><span>, which invokes flotation of a magmatic magnetite-fluid suspension and offers a plausible explanation for the igneous (i.e., up to ~15.9 wt % TiO</span>₂<span> in magnetite) and hydrothermal features of the deposits.</span></p> application/pdf 10.2113/econgeo.111.8.2033 en Society of Economic Geologists Iron and oxygen isotope signatures of the Pea Ridge and Pilot Knob magnetite-apatite deposits, southeast Missouri, USA article