Mineralogy, chemistry and hydrothermal evolution of the Pea Ridge iron-oxide-rare earth element deposit, Missouri, United States of America

The Pea Ridge Fe-oxide deposit is hosted by a sequence of 1.45--1.48 Ga rhyolite tuffs of the St. Francois terrane of S.E. Missouri. Mineralization comprises early actinolite-apatite-magnetite alteration and a magnetite-apatite orebody. Rare earth elements (REE) in these stages are concentrated in monazite inclusions within apatite. Hematite replaces magnetite around the margins of the orebody. Subsequent alteration of the host rhyolites resulted in the sequential deposition of quartz, andalusite, muscovite and K-feldspar, indicating that an acidic fluid at temperatures above 341°C increased in pH and decreased in temperature during alteration.;Comparisons previously made with the Olympic Dam deposit are justified based on the presence of hematite-REE breccias and quartz-sericite alteration. However important differences exist, most notably the presence of andalusite and alkali feldspar, abundant apatite and monazite (rather than bastnasite), actinolite rather than chlorite, negative rather than positive Eu anomalies, higher Th/U ratios and much lower Cu. (Abstract shortened by UMI.)...