Geology and Mineralogy of Rare Earth Element-Bearing Breccia Pipes, Pea Ridge Mine, Missouri

摘要

The Pea Ridge Fe-REE-Au deposit has estimated reserves of 120 million tonnes of 57 percent iron, rare earth element (REE) mineralization and traces of gold. The deposit is associated with a 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. Four rare earth element-bearing breccia pipes are located at contacts between major lithologic zones on the magnetite orebody margins. Emplacement was fault-controlled. They dip steeply and are ovoid in plan view. Horizontal lengths are up to 60 m and widths reach 15 m; maximum known vertical extents are 120 m. Monazite and xenotime are the primary REE-bearing minerals. Grab samples exhibited total REE oxide concentrations as high as 37.8 weight percent, while bulk samples averaged 12 weight percent. Gold is erratically distributed; concentrations seldom exceed 1 ppm, although individual assays are as high as 371 ppm. 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. Late-stage mag-matic-hydrothermal fluids, exsolved from the ore system, were released during a second boiling event, brecciating and fracturing the host rocks and emplacing REE mineralization.