Nuelle, and 3 co-authors, reported in 1989 that breccia pipes containing rare-earth elements (REE) and gold are a potentially significant economic target in the Pea Ridge iron ore mine, Washington County, Missouri. The Pea Ridge deposit is one of eight known volcanic-hosted iron ore deposits in the Middle Proterozoic St. Francois terrane that are similar to the Olympic Dam type of deposits of Australia.
The iron orebody is steeply dipping and tabular shaped, and it cuts across the host rhyolite. The footwall of the deposit is zoned from the massive apatite-bearing magnetite core outward to heterolithic breccia, pseudobreccia, a specular hematite zone, and an extensive silicified zone. The hanging wall is zoned
from the massive magnetite core outward to heterolithic breccia, pseudobreccia, and an amphibole skarn zone.
The REE-rich breccia pipes cut through the various rock types along the footwall at or near contacts. They are typically about 60 m long and 15 m wide, and they extend at least 120 m vertically. The pipes consist of fragments of rhyolite host rock and zones of massive magnetite, specular hematite, and silicified rocks supported in a matrix of rock flour, barite, feldspar, monazite, apatite, chlorite, xenotime, and, locally, gold.
Total REE oxide content of samples of the groundmass material, which are not diluted with lithic fragments, average about 20 weight percent. Grades from working faces in the mine are lower, averaging about 12 weight percent. Gold distribution is erratic, but concentrations are as high as 371 parts per million.
The Pea Ridge iron deposit represents an intrusion-type deposit in which the magnetite orebody sloped upward into the host rhyolite and created the various breccia zones around the margins of the deposit. The REE-bearing breccia pipes probably formed from late-stage magmatic-hydrothermal fluids that evolved from the main magnetite orebody.
Sanidine phenocrysts (or xenocrysts) in the breccia pipes confirm a magmatic component for their origin. The magmatic-hydrothermal fluids underwent second boiling and decompression and caused crystallization and release of a volatile phase. Fluid-inclusion evidence of boiling includes a mixed population of vapor-rich fluid inclusions coexisting with liquid-rich inclusions in quartz from the REE-bearing breccia pipes. The concomitant volume increase associated with boiling released significant mechanical energy and caused fracturing and brecciation of the wallrock. The REE-bearing fluid and crystal mixture streamed upward along zones of weakness (faults and contacts), entrained and milled wallrock fragments, and formed the breccia pipes.
The REE- and, locally, gold-bearing breccia pipes associated with the Pea Ridge iron deposit are attractive targets in the midcontinent of the United States for exploration of large deposits of iron, copper, REE, gold, and uranium similar to the Olympic Dam deposit in Australia.
[For additional information on the Pea Ridge deposit: https://mrdata.usgs.gov/deposit/show-deposit.php?type=site&id=REE-MO00012 ]
[For geophysical information, see: https://www.sciencebase.gov/catalog/item/5730a3a0e4b0dae0d5db1eab]
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