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

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Primary Contact:  John Aleinikoff, jaleinikoff@usgs.gov


Images of grains separated from REE-bearing breccia pipes:


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.

Our 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. We will 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. We will date ore samples from core, in situ (in polished thin section). We wiil extract zircon from the plutonic rock samples. To better understand the genesis of the rare earth element minerals, we will also analyze dated zones for trace elements.

Images of grains separated from REE-bearing breccia pipes:

Hand-picked grains

Images below: SEM-BSE (Scanning Electron Microscope-backscattered electron) images of monazite grains extracted from rare earth element-bearing breccia pipe at the Pea Ridge mine, southeast Missouri (J.N. Aleinikoff, USGS, written communication, 2013)

monazite grain A

monazite grain B

monazite grain C


Polished grains in epoxy mount for SHRIMP U-Pb geochronology, below

Images below: BSE (backscattered electron) images of large, reddish granular grain from sample 2275-15 ("hard" breccia pipe)
A high contrast/low brightness image showing showing faint zoning in monazite (gray)
B low contrast/high brightness image image showing crystallintes of monazite (white) and xenotime (gray)

monazite grains


Image below: BSE (backscattered electron) image of pale yellow granular grains from sample PR-33C ("soft" breccia pipe). Both large grains are composed almost entirely of monazite.

monazite grains


Images below: BSE (backscattered electron) images of two varieties of monazite (large orangish granular; small yellow adamantine) from sample PR-182 ("hard" breccia pipe).
A high contrast/low brightness image showing faint zoning in monazite (gray); bright white grains are thorite
B low contrast/high brightness image showing crystallites of monazite (light gray), xenotime (medium gray), and thorite (white)
C two grains of yellow adamantine monazite showing very faint zoning

monazite grains monazite grains


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