REE Redistribution Textures in Altered Fluorapatite: Symplectites, Veins, and Phosphate-Silicate-Carbonate Assemblages from the Nolans Bore P-REE-Th Deposit, Northern Territory, Australia

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.

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

“…Presumably, thorite or huttonite (complementary to monazite) is the product of the selective concentration of elements (Th and HREE) released during apatite decomposition. A similar selective redistribution of LREE and Th-HREE has been described in carbonatites of Nolans Bore (Australia) [8].…”

“…Therefore, rare earth carbonatites are a major focus of critical metal research. Over the past five years, several dozens of papers have been published on the subject of rare earth carbonatites (RE carbonatites) in Angola [5,6], Australia [7][8][9], Brazil [10,11], Canada [12][13][14][15][16][17], China [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], Finland [38], India [39][40][41][42], Italy [43,44], Malawi [45][46][47][48][49][50][51], Mongolia [52,53], Namibia…”

Open Apatite Sr Isotopic System in Low‐Temperature Hydrous Regimes

“…However, previous experimental results have indicated that apatite could be altered or metasomatized by moderate to high‐temperature (300 to 900°C) fluids, such as H 2 O‐CO 2 fluids, acidic and alkalic fluids (Betkowski et al, ; Harlov & Förster, ; Harlov et al, ; Migdisov & Williams‐Jones, ; Migdisov et al, ). Altered apatite, with significant variations in elemental content, is widely reported in rocks affacted by moderate to high‐temperature (>300°C) hydrothermal fluids, especially in mineral deposits (Anenburg et al, ; Bouzari et al, ; Li et al, ; Li & Zhou, ; Palma et al, ; Zeng et al, ). In addition, modifications of the Sr, Nd, and O isotopic systems in apatite have been reported in mineralized rocks that have been altered by moderate to high‐temperature hydrothermal fluids (Li et al, ; Zeng et al, ).…”

Rare earth element mobility in and around carbonatites controlled by sodium, potassium, and silica

“…Although REE are generally considered immobile in most natural systems, the occurrence of REE-rich minerals of unmistakably hydrothermal genesis attests to their mobility (8). In addition, P and F are considered to immobilize REE in hydrothermal systems (39)(40)(41)(43)(44)(45). However, many carbonatite-associated hydrothermal veins contain P-and F-bearing minerals such as fluorapatite, monazite, xenotime, and REE fluorcarbonates (39,41), indicating that REE, P, and F were enigmatically transported together.…”