T207 Evolution of REE-Enriched Carbonatite-Alkalic Rock Systems: In Honor of Daniel R. Shawe

Tuesday, 29 October 2013
8:00 AM - 12:00 PM
Room 304

U-PB AND RE-OS GEOCHRONOLOGY OF REE-RICH BRECCIA PIPES FROM THE MESOPROTEROZOIC PEA RIDGE FE-REE-AU DEPOSIT, ST. FRANCOIS MOUNTAINS, MISSOURI

Paper number: T207-5
Program: Technical Sessions
Time: 9:00 AM – 9:15 AM

Authors

Abstract

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.