Abstract

The geochemical behavior and genesis of rare earth element (REE) ores remains poorly understood. In addition, the processes responsible for many features observed in REE ore deposits are not easily determined. Importantly, the distinction between igneous versus post-magmatic genesis, or the nature of the mineralizing and modifying fluids or melts, remains in dispute. Nolans Bore is a P-REE-Th deposit in the Northern Territory, Australia, hosting REE mineralization in fluorapatite and its alteration products. These mineralization textures are imaged in 3D by X-ray computed tomography and in 2D by electron microscopy, cathodoluminescence, and X-ray mapping. Primary igneous REE-rich fluorapatite was replaced in two stages. The first, at high temperature, was to endmember fluorapatite-britholite symplectite. The second, at lower temperatures, was to hydrothermal veins and patches comprised of REE-Th phosphate, silicate, and carbonate minerals. Quadrivalent Ce occurs together with Th, suggesting oxidized, low-temperature conditions. Strikingly similar textures are observed at Hoidas Lake, Saskatchewan, Canada, where igneous immiscibility or a pegmatitic boundary layer have been implicated in their formation. Our textural and chemical investigations establish that these symplectites and other similar textures are not primary igneous textures, but formed by subsequent cooling and alteration. Understanding the processes that formed these symplectic and vein textures is key to their theoretical and experimental modeling and should lead to a better understanding of “hydrothermal” REE deposits globally. The decoupling of Ce from the rest of the REE and the mineralogical preferences of the light REE versus the heavy REE should be considered when evaluating similar ore deposits for their economic value and mineral processing.

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