THE CHEMISTRY OF MAGNETITE FROM THE PEA RIDGE IRON DEPOSIT, MISSOURI, USA

The Pea Ridge (PR) deposit is one of three major iron oxide deposits hosted by a sequence of 1.45-1.48 Ga rhyolite tuffs of the St. Francois Mountain terrane in southeastern Missouri, USA, and is characterized dominantly by magnetite and overprinted by secondary hematite. There is no consensus on how the deposit formed, with working hypotheses that include circulating basinal brines, meteoric fluids, magmatic-hydrothermal fluids, and immiscible iron oxide melt. In this study, we quantified the abundances of Ti, V, Ca, Al, and Mn for three generations of magnetite at PR. Type-1 contains 8.41 ± 1.24 wt% Ti (range 5.7-12.45 wt%), 0.078 ± 0.036 wt% V (range 0.012-0.139 wt%), 0.0031 ± 0.003 wt% Ca (range 0.0001-0.016 wt%), 0.029 ± 0.013 wt% Al (range 0.014-0.09 wt%), and 0.016 ± 0.003 wt% Mn (range 0.0013-0.0432 wt%). Type-2 magnetite contains 0.436±.0.356 wt% Ti (range 0.111-1.569 wt%), 0.065 ± 0.026 wt% V (range 0.0346-0.103 wt%), 0.003 ± 0.0028 wt% Ca (range 0.0001-0.0126 wt%), 0.027 ± 0.018 wt% Al (range 0.0016-0.07 wt%) and 0.012 ±0.009 wt% Mn (range 0.0002-0.036 wt%). Type-3, the modally dominant generation of magnetite, contains 0.012 ± 0.019 wt% Ti (range 0.0002-0.0914 wt%), 0.046 ± 013 wt% V (range 0.0017-0.107 wt%), 0.0085 ± 0.01 wt% Ca (range 0.0001-0.214 wt%), 0.0399 ± 0.075 wt% Al (range 0.0009-0.897 wt%), and 0.036 ± 0.021 wt% Mn (range 0.0002-0.0914 wt%). When plotted on the [Ca+Al+Mn] vs. [Ti+V] discriminant diagram, Type-1 magnetite is comparable to igneous magnetite, whereas Type-2 and Type-3 scatter across the BIF, IOCG, and Kiruna fields. We highlight that the stable Fe and O isotope ratios for the same magnetite samples from PR (i.e., aggregates of magnetite grains that include Types 1, 2 and 3; see Childress et al., this meeting) are unequivocally magmatic in origin. We suggest that Type 1 magnetite represents the original magnetite mineralization, which retains the magmatic Fe- and O-isotope signatures, and that post-mineralization dissolution and reprecipitation resulted in Types-2 and -3 magnetite with lower Ti concentrations and wide scatter in V, Ca, Al, and Mn. Evidence for a hydrothermal alteration event is evinced by O-isotope data from zircon and quartz (cf. King et al., 2008, Precambrian Research, 165, 49-60). The data support magmatic and magmatic-hydrothermal models for PR.