Mineralogy, chemistry, and fluid-aided evolution of the Pea Ridge Fe oxide-(Y + REE) deposit, southeast Missouri, USA

Economic Geology
By: , and 

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Abstract

The Kiruna-type Pea Ridge iron oxide-apatite (IOA) deposit is hosted by a sequence of 1.47 Ga rhyolite tuffs of the St. Francois Mountains, southeast Missouri, USA. It consists of a series of altered zones composed mainly of amphibole, magnetite, hematite, and quartz, together with the presence of several rare earth element (Y + REE)-rich breccia pipes. In many cases, the fluorapatite within these zones is rich in inclusions of monazite, iron oxide, and quartz inclusions, plus minor xenotime. Monazite and minor xenotime are also found intergrown as inclusions in the fluorapatite, as well as in surrounding recrystallized magnetite and hematite in the magnetite ore. Monazite and xenotime typically occur as inclusions within both oxides. Monazite-(Ce) and xenotime-(Y) are both relatively poor (<2 wt %) in ThO2 and UO2. No significant compositional differences exist in the (Y + REE) chemistry between monazite and xenotime inclusions in fluorapatite compared to grains intergrown with magnetite and hematite, suggesting that these two REE-rich minerals are cogenetic. Monazite-xenotime geothermometry and geochronology of monazite inclusions in fluorapatite provide evidence that formation/remobilization of the (Y + REE) phosphates took place at ca. 50° to 400°C, approximately 5 to 10 m.y. after emplacement of the main iron oxide-phosphate orebody. Evidence from field relationships and fluid inclusion chemistry, together with the massive recrystallization and remobilization of fluorapatite, monazite, xenotime, and iron oxides at Pea Ridge, suggest a subvolcanic origin coupled with a strong metasomatic reworking of the IOA deposit.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Mineralogy, chemistry, and fluid-aided evolution of the Pea Ridge Fe oxide-(Y + REE) deposit, southeast Missouri, USA
Series title Economic Geology
DOI 10.2113/econgeo.111.8.1963
Volume 111
Issue 8
Year Published 2016
Language English
Publisher Society of Economic Geologists
Contributing office(s) Central Mineral and Environmental Resources Science Center
Description 22 p.
First page 1963
Last page 1984
Country United States
State Missouri