Paper No. 4
Presentation Time: 9:00 AM


ALEINIKOFF, John N.1, SELBY, David2, DAY, Warren C.3, SLACK, John F.4, PILLERS, Renee M.1, SEEGER, Cheryl5 and SAMSON, Iain M.6, (1)US Geological Survey, MS 963, Denver, CO 80225, (2)Department of Earth Sciences, University of Durham, Science Labs, Durham, DH1 3LE, United Kingdom, (3)US Geological Survey, MS 911, Denver, CO 80225, (4)US Geological Survey, National Center, MS 954, 12201 Sunrise Valley Drive, Reston, VA 20192, (5)Missouri Dept. of Natural Resources, P.O. Box 250, Rolla, MO 65402, (6)Department of Earth and Environmental Science, University of Windsor, 401 Sunset Ave, Windsor, ON N9B 3P4, Canada,

Rare Earth Element (REE)-rich breccia pipes (600,000 t @ 12% REO) occur along the margins of the 136-Mt Pea Ridge magnetite-apatite deposit, within ~1.47 Ga volcanic-plutonic rocks of the St. Francois Mountains terrane of southeastern Missouri. Previous underground mapping has shown that the breccia pipes (ca. 50 x 200 m in cross-section) cut the host rhyolite, magnetite ore, and alteration zones associated with the iron-ore system. Two varieties of breccia pipes are recognized: “soft” (SBP; n=3) and “hard” (HBP; n=1).

Grains of monazite and xenotime were extracted from samples collected from both SBP and HBP for SHRIMP U-Pb geochronology; both minerals were also dated in a polished thin section from the HBP. Separated grains of monazite occur in two morphologies: (1) coarse granular grains composed of numerous small (<50 µm) crystallites, together with rare xenotime, thorite, apatite, and magnetite; and (2) coarse glassy, bright yellow grains similar to typical igneous and metamorphic monazite. In addition, zircon grains from two samples of rhyolite and two late, pre-breccia pipe, aplite dikes collected underground at Pea Ridge were dated.

Ages obtained by SHRIMP U-Pb analysis are: (1) SBP, pale yellow granular monazite—1464.9 ± 3.3 Ma; (2) HBP, reddish granular monazite—1461.2 ± 3.8 Ma, associated xenotime—1453 ± 11 Ma; (3) HBP, glassy yellow monazite—1464.8 ± 2.7, 1461.9 ± 2.5 Ma, and a few grains at 1447 ± 6 Ma; (4) HBP matrix monazite (in situ) 1462.9 ± 3.5 Ma, matrix xenotime (in situ) 1466 ± 13 Ma. The younger generation of glassy monazite coincides with a new Re-Os age of 1440.6 ± 9.2 Ma for fine-grained molybdenite that occurs within quartz and allanite in the HBP. Zircon from the two host rhyolite samples have SHRIMP U-Pb ages of 1473.6 ± 8.0 and 1472.7 ± 5.6 Ma, whereas the late aplites yield ages of 1480 ± 8 and 1477.1 ± 6.6 Ma, establishing the temporal framework of local igneous activity.

We propose that the granular grains of monazite and xenotime are fragments of REE-rich mineralization that predated brecciation, whereas the older of the two glassy monazite populations may have formed as new growth during emplacement of the breccia pipes. A second growth episode of glassy monazite (plus molybdenite) occurred at ca. 1443 Ma, possibly during faulting and fluid flow that rebrecciated the pipes.