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Setting and Origin of Iron Oxide-Copper-Cobalt-Gold-Rare Earth Element Deposits of Southeast Missouri : SHRIMP Uranium-Lead Geochronology

Primary Contact:  John Aleinikoff, jaleinikoff@usgs.gov

We will research the age and origin of Pea Ridge Rare Earth Element mineralization. Currently, the age of the Pea Ridge Rare Earth Element deposits is thought to be about 1.4 Ga, based on TIMS analysis of one xenotime grain. However, previous geochemical analyses of xenotime indicates multiple compositions, suggesting several episodes of xenotime formation. The previous TIMS age data probably represent a mixture of these age components. We will use SHRIMP spot analysis of xenotime in situ (to preserve textural context(s) of ore deposition) to decipher the complex history of the Pea Ridge deposit.

The objective here is to determine the ages of formation of uraniferous rare-earth-element-bearing phosphate minerals (xenotime and monazite) that occur at Pea Ridge. The plan is to obtain ore samples from existing cores for uranium-lead geochronology. In addition, we intend to collect outcrop samples of Mesoproterozoic plutonic rocks of the St. Francois Mountains that host the rare earth element deposits or that are related to the Mesoproterozoic magmatic activity of the region. Ore samples from core will be dated in situ (in polished thin section). Zircon will be extracted from the plutonic rock samples. To better understand the genesis of the rare earth element minerals, dated zones will also be analyzed for trace elements.

Images below: SEM-BSE images of monazite grains extracted from rare earth element-bearing breccia pipe at the Pea Ridge mine, SE Missouri. (J.N. Aleinikoff, USGS, written communication, 2013.) (A) Composite grains of monazite (white) and apatite (black); (B) Granular grains of mostly monazite; few dark crystallites are xenotime; (C) Adamantine yellow monazite.

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