THE PEA RIDGE IRON-RARE EARTH ELEMENT DEPOSIT, WASHINGTON COUNTY, MISSOURI

The Pea Ridge Iron-Rare Earth Element deposit, part of the Southeast Missouri Iron Metallogenic Province, has estimated reserves of 120 million tonnes of 57 percent iron, rare earth element (REE) mineralization and traces of gold and cobalt. The deposit is associated with a Mesoproterozoic rhyolite high in a downfaulted megabreccia block near the center of a collapsed caldera. Host rocks are rhyolitic lava flows and ash-flow tuffs. The deposit was mined for magnetite, and produced road metal from waste rock and apatite for phosphorous as byproducts.

Tectonic breccias related to either ring complex development or caldera collapse and resurgence controlled deposit emplacement. Magmatic fluids, related to nearby trachytic intrusions, developed shallow magma chambers in overlying volcanic rocks. An iron-rich immiscible fluid developed in the chamber and stoped upward into the host rocks, forming the magnetite ore. Ore emplacement was preceded, accompanied, and followed by extensive hydrothermal alteration of the rhyolite host rocks.

The deposit exhibits both hydrothermal replacement and magmatic textures, with hydrothermal processes dominating the deposit margins. Extensive potassium metasomatism permeates the surrounding host rhyolites, and is notable in rhyolite clasts within the magnetite. Actinolite with accompanying quartz is indicative of skarn-like alteration on the hanging wall of the deposit. Successively increasing hydrothermal replacement of the host rock by iron oxide along fractures is a marked characteristic of deposit margins.

Late-stage magmatic-hydrothermal fluids, exsolved from the ore system, were released during a second boiling event, brecciating and fracturing the host rocks and emplacing REE mineralization. This event is also marked by alteration of magnetite to hematite, and by barite and calcite open space fill, both found as halos surrounding the breccia pipes.

Increased understanding of the fluid systems involved in mineralization at Pea Ridge and other deposits in this Province will be an important factor in future understanding of critical mineral resource potential in the United States.