Zircon-hosted melt inclusion record of silicic magmatism in the Mesoproterozoic St. Francois Mountains terrane, Missouri: Origin of the Pea Ridge iron oxide-apatite rare earth element deposit and implications for regional crustal pathways of mineralizatio

Voluminous silicic magmatism was coeval with iron ore mineralization in the St.
Francois Mountains terrane in southeast Missouri, part of the broader Mesoproterozoic
Granite-Rhyolite province along the eastern margin of Laurentia. Some of the iron
deposits contain extraordinary endowments of critical elements, such as the Pea Ridge
iron oxide-apatite (IOA) deposit, which has an average grade of ~12 wt% rare earth
oxides in breccia pipes that flank the ore body. To assess the role of silicic magmatism in
the genesis of the Pea Ridge deposit, we present a high-spatial resolution study of zirconhosted
melt inclusions from rhyolitic ash-flow tuffs. Melt inclusion data are combined
with textural, geochemical, and geochronological analyses of zircon hosts to elucidate the
magmatic-hydrothermal evolution of the Pea Ridge system. Two contemporaneous silicic
igneous centers in the St. Francois Mountains terrane, Bourbon and Eminence, were
studied for comparison. Pea Ridge melt inclusions are trachydacitic to rhyolitic (~63-79
wt% SiO2, ~5.6-11.7 wt% Na2O+K2O) with very high Cl in the least-evolved and most
alkaline melt inclusions (~2,000-5,000 ppm Cl). Rare earth elements (REE) in melt
inclusions have identical chondrite-normalized patterns to the mineralized breccia pipes,
but with systematically lower absolute concentrations. Haplogranite ternary pressures
range from ~0.5-10 kbar, with an average of ~2-3 kbar (7-12 km depth), and liquidus
temperatures are ~850-950 °C, with an average of ~920 °C. Silicate and phosphate
mineral inclusions have compositions that overlap minerals from the iron ore body and
breccia pipes, recording a transition from igneous to hydrothermal zircon growth.
Igneous iron oxide inclusions have compositions that indicate Pea Ridge magmas were
reduced to moderately oxidized (log fO2 of -0.8 to -1.84 NNO). Zircons from two Pea
Ridge samples have 207Pb/206Pb concordia ages of 1456 ± 9 Ma and 1467 ± 13 Ma that
overlap published ages for the breccia pipes and iron ore zones of the Pea Ridge deposit.
A population of texturally and chemically disrupted zircons have discordant domains that
correspond to high Fe, U, and REE concentrations, consistent with the unique
geochemical attributes of the IOA-REE ore body. Inherited cores in Pea Ridge and
Bourbon zircons have concordant 207Pb/206Pb dates of 1550-1618 Ma, providing direct
evidence of cratonic basement beneath these centers. Oxygen isotope data for inherited
and autocrystic igneous zircons span from mantle to crustal values (18Ozircon=5.5-7.9‰).
Our data are consistent with a model in which metasomatized mantle components were
mixed with cratonic and accreted crustal material in a back-arc or rifted segment of a
volcanic arc, with ore fluids derived from Cl-rich melts to transport Fe and REE in a
long-lived (tens of Myr), pulsed, magmatic-hydrothermal system. Bourbon, which also
possesses IOA mineralization, shares key petrologic similarities with the Pea Ridge
system, whereas Eminence, which is not mineralized, has disparate geochemical and
isotopic signatures that indicate it formed in a different crustal setting. The location of
Pea Ridge and Bourbon along a cratonic margin may have been important in focusing
silicic melts and mineralization in the upper crust, serving as a guide for future
exploration efforts.