The Chemistry of Magnetite from the Pea Ridge Iron Oxide Deposit, Missouri, U.S.A.

• Main Author: Korfeh, Daniel
• Format: Article in Journal/Newspaper
• Language: English
• Published: My University 2016
• Subjects: Geological Sciences; Science; Kiruna
• Online Access: https://dx.doi.org/10.7302/3985; http://deepblue.lib.umich.edu/handle/2027.42/171473

Thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Geology or Earth and Environmental Sciences, Department of Earth and Environmental Sciences : The Pea Ridge (PR) deposit is one of several 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. The deposit 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 Pea Ridge. Type 1 contains ( 1 sigma) 8.41 1.24 wt. % Ti (range 5.7-12.4 wt. %), 790 360 ppm V (range 120-1400 ppm), 290 130 ppm Al (range 140-955 ppm), and 170 70 ppm Mn (range 80-430 ppm). Type 2 magnetite contains ( 1 sigma) 0.456 0.3 wt. % Ti (range 0.038-1.569 wt. %), 680 220 ppm V (range 280-1030 ppm), 444 500 ppm Al (range 130-2710 ppm) and 310 190 ppm Mn (range 80-640 ppm). Type 3, the modally dominant generation of magnetite, contains ( 1 sigma) 390 680 ppm Ti (range 60-6730 ppm), 486 130 ppm V (range 90-1070 ppm), 520 850 ppm Al (range 130-8980 ppm), and 440 150 ppm Mn (range 80-910 ppm). 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. It is important to note that the stable Fe and O isotope ratios for the same magnetite samples from Pea Ridge (i.e., aggregates of magnetite grains that include Types 1, 2 and 3) are unequivocally magmatic in origin. In this thesis, it is suggested 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 post-mineralized hydrothermal alteration is evinced by Oisotope data from zircon and quartz. The new data presented here support magmatic and magmatic-hydrothermal models for Pea Ridge.