## Breadcrumb

-  Publications 
-  Articles 

#           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      

      January 7, 2020    

Voluminous silicic magmatism was coeval with iron ore mineralization in the St.Francois Mountains terrane in southeast Missouri, part of the broader MesoproterozoicGranite-Rhyolite province along the eastern margin of Laurentia. Some of the irondeposits contain extraordinary endowments of critical elements, such as the Pea Ridgeiron oxide-apatite (IOA) deposit, which has an average grade of ~12 wt% rare earthoxides in breccia pipes that flank the ore body. To assess the role of silicic magmatism inthe genesis of the Pea Ridge deposit, we present a high-spatial resolution study of zirconhostedmelt inclusions from rhyolitic ash-flow tuffs. Melt inclusion data are combinedwith textural, geochemical, and geochronological analyses of zircon hosts to elucidate themagmatic-hydrothermal evolution of the Pea Ridge system. Two contemporaneous silicicigneous centers in the St. Francois Mountains terrane, Bourbon and Eminence, werestudied for comparison. Pea Ridge melt inclusions are trachydacitic to rhyolitic (~63-79wt% SiO2, ~5.6-11.7 wt% Na2O+K2O) with very high Cl in the least-evolved and mostalkaline melt inclusions (~2,000-5,000 ppm Cl). Rare earth elements (REE) in meltinclusions have identical chondrite-normalized patterns to the mineralized breccia pipes,but with systematically lower absolute concentrations. Haplogranite ternary pressuresrange from ~0.5-10 kbar, with an average of ~2-3 kbar (7-12 km depth), and liquidustemperatures are ~850-950 °C, with an average of ~920 °C. Silicate and phosphatemineral inclusions have compositions that overlap minerals from the iron ore body andbreccia pipes, recording a transition from igneous to hydrothermal zircon growth.Igneous iron oxide inclusions have compositions that indicate Pea Ridge magmas werereduced to moderately oxidized (log fO2 of -0.8 to -1.84 NNO). Zircons from two PeaRidge samples have 207Pb/206Pb concordia ages of 1456 ± 9 Ma and 1467 ± 13 Ma thatoverlap 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 thatcorrespond to high Fe, U, and REE concentrations, consistent with the uniquegeochemical attributes of the IOA-REE ore body. Inherited cores in Pea Ridge andBourbon zircons have concordant 207Pb/206Pb dates of 1550-1618 Ma, providing directevidence of cratonic basement beneath these centers. Oxygen isotope data for inheritedand 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 weremixed with cratonic and accreted crustal material in a back-arc or rifted segment of avolcanic arc, with ore fluids derived from Cl-rich melts to transport Fe and REE in along-lived (tens of Myr), pulsed, magmatic-hydrothermal system. Bourbon, which alsopossesses IOA mineralization, shares key petrologic similarities with the Pea Ridgesystem, whereas Eminence, which is not mineralized, has disparate geochemical andisotopic signatures that indicate it formed in a different crustal setting. The location ofPea Ridge and Bourbon along a cratonic margin may have been important in focusingsilicic melts and mineralization in the upper crust, serving as a guide for futureexploration efforts.

## Citation Information

## Related Content

                  -              Data           
                  -              Connect           

January 9, 2020

####                 Geochemistry, geochronology, and isotope geochemistry data for zircons and zircon-hosted melt and mineral inclusions in the St. Francois Mountains terrane, Missouri              

Geology, Minerals, Energy, and Geophysics Science Center

#### Kathryn Watts

##### Research Geologist

Geology, Minerals, Energy, and Geophysics Science Center

kwatts@usgs.gov

509-368-3108

#### Celestine Mercer

##### Research Geologist

Geology, Geophysics, and Geochemistry Science Center

cmercer@usgs.gov

303-236-1866

## Related Content

January 9, 2020

####                 Geochemistry, geochronology, and isotope geochemistry data for zircons and zircon-hosted melt and mineral inclusions in the St. Francois Mountains terrane, Missouri              

Geology, Minerals, Energy, and Geophysics Science Center

#### Kathryn Watts

##### Research Geologist

Geology, Minerals, Energy, and Geophysics Science Center

kwatts@usgs.gov

509-368-3108

#### Celestine Mercer

##### Research Geologist

Geology, Geophysics, and Geochemistry Science Center

cmercer@usgs.gov

303-236-1866

### Study Area

### Contact

####               Geology, Minerals, Energy, and Geophysics Science Center          

Building 19, 350 N. Akron Rd.P.O. Box 158Moffett Field, CA94035United States

### Explore Search

                  - United States
                  - missouri

View All **