Mineralogy chemistry and hydrothermal evolution of the Pea Ridge Fe-oxide-REE deposit, Missouri, U.S.A.

摘要

The Pea Ridge Fe-oxide deposit is hosted by a sequence of 1.45 Ga to 1.48 Ga rhyolite tuffs. Mineralization comprises early actinolite-apatite-magnetite alteration and a magnetite-apatite orebody REE in these stages are concentrated in monazite inclusions within apatite. Hematite replaces magnetite around the margins of the orebody Subsequent alteration resulted in the sequential deposition of quartz, andalusite, muscovite, and K-feldspar, indicating that an acidic fluid at temperatures above 341 deg C increased in pH and decreased in temperature during alteration. A late monazite-hematite assemblage represents REE enrichment in these rocks. REE mineralization occurs in two types of late-stage, hydrothermal, heterolithic breccia pipes. Type II is characterized by K-feldspar and barite clasts in a matrix of monazite, xenotime, rutile, thorite, hematite, and quartz that can comprise up to 90 percent monazite + xenotime. Type I is distinguished by the absence of K-feldspar and barite and contains a late-stage allanite-apatite-fluorocarbonate assemblage. Monazite-xenotime geothermometry yields variable temperatures and indicates that Type I breccia pipes formed at lower temperatures than Type II pipes. Chlorite geothermometry yields similar temperatures (100 deg C to 220 deg C) for all lithologies suggesting that late-stage chlorite-hematite alteration is related to a regional event. The presence of hydrothermal breccias is taken to indicate that the deposit formed at less than 1 kbar.