Search

The rare-earth elements (REEs, La–Lu, Y) are essential for the development of renewable technologies. Bastnäsite (REECO3F) is a common REE ore mineral that is often subject to hydrothermal alteration at all crustal levels. Mechanisms of hydrothermal bastnäsite alteration therefore govern the evolution of REE deposits, though these mechanisms remain poorly understood. This experimental work investigates the hydrothermal replacement of bastnäsite by rhabdophane (REEPO4∙xH2O, x = 0–1) and monazite (REEPO4) in phosphatic fluids. Two temperature-dependent alteration pathways were identified; both follow the coupled dissolution-reprecipitation (CDR) mechanism. At 90 °C, bastnäsite was replaced by highly-porous metastable rhabdophane which was then replaced by monazite, forming an inner layer of rhabdophane and an outer layer of monazite. At 220 °C, bastnäsite was replaced directly by monazite. Although replacement initiated more quickly at 220 °C, greater overall replacement occurred at 90 °C (~ 61 wt.% after 500 h, compared to ~ 13 wt.% at 220 °C) due to surface passivation by monazite at 220 °C. Geochemical analyses showed REE fractionation during bastnäsite alteration. At 90 °C, rhabdophane was enriched in heavy REEs (Eu–Lu, Y), likely due to the evolving fluid chemistry, while at 220 °C secondary monazite was enriched in Sm and Ho compared to bastnäsite. These results indicate that: 1) the hydrothermal alteration of bastnäsite by rhabdophane and monazite in ore deposits leads to REE immobilisation, with little net loss of REEs to solution; 2) rhabdophane is metastable relative to monazite at 90 °C, and; 3) variable temperatures can cause different mineral textures and REE fractionation trends during hydrothermal alteration and mineral replacement. [ABSTRACT FROM AUTHOR]

Fluocerite has been shown experimentally and by thermodynamic modeling to be an important phase for the evolution of magmatic-hydrothermal REE mineralizing systems, but one that is rarely observed in nature. The reason for the latter is poorly understood. The newly discovered Taipingzhen REE deposit in Henan province, Central China, is unique for containing abundant fluocerite and thus provides a great opportunity to explore the role of fluocerite in REE mineralization. Fluocerite was the earliest REE mineral to crystallize in this deposit and was replaced extensively by bastnäsite. It has a higher La/Nd ratio than the later-depositing REE minerals, e.g., bastnäsite and monazite. This decrease in the La/Nd ratio with time is interpreted to have resulted from the fluocerite-mediated fractionation of REE (fluocerite prefers the lightest REE during its deposition from a high temperature, sulfate-rich hydrothermal fluid, in which the REE were transported as sulfate complexes). This study provides compelling evidence that fluocerite is, indeed, an important, early crystallizing mineral in magmatic-hydrothermal REE systems, which elsewhere was replaced entirely by bastnäsite during subsequent fluid evolution. Moreover, the study shows that the bastnäsite, which replaced fluocerite, inherited its high La/Nd ratio, suggesting that this ratio could be used as a tool with which to identify the former presence of fluocerite. The preservation of fluocerite in the Taipingzhen deposit is interpreted to have been due to the lack of significant cooling until a late stage of mineralization, at which time most of the fluid was lost from the system. [ABSTRACT FROM AUTHOR]

Rare earth element (REE) fluorcarbonates are found for the first time in the ore-hosting sequence of the Saf’yanovka Cu–Zn massive sulfide deposit, Middle Urals: bastnäsite, synchysite-(Ce), and synchysite-(Y) in an assemblage with fluorite, calcite, barite, pyrite, and sphalerite. Samples with fluorcarbonates were found at a depth of 262 m in quartz veinlets of an altered zone of pyrite–carbonate–kaolinite–sericite–quartz rocks at the contact with massive pyrite–chalcopyrite and pyrite–sphalerite ores. It is suggested that the REE fluorcarbonates formed from a hydrothermal fluid synchronously with fluorite and calcite. This event can be related to hydrothermal activity at the postcollision stage of the evolution of the Urals fold system. [ABSTRACT FROM AUTHOR]

Thermodynamic calculations on the impact of carbonate–bicarbonate fluids cooling from 500 to 100°C on a monazite + calcite association with a simultaneous decrease in pressure from 2000 to 125 bar are performed. It is shown that the REE carbonate–bicarbonate complexes gain importance only at low temperatures. In this case, the total concentration of lanthanides in the fluid turns out to be rather low. This indicates that, in the presence of even insignificant amounts of fluorine and calcium in the hydrothermal system, carbonate–bicarbonate fluids do not lead to the removal of REEs but contribute to their accumulation in the form of precipitating fluorocarbonates. Thus, the role of carbonate-bicarbonate hydrothermal fluids in the transport and deposition of rare earth elements has been assessed for the first time for a wide range of TP-parameters and for the entire series of lanthanides. [ABSTRACT FROM AUTHOR]

Bastnäsite is widespread in alkali granite, carbonatite and REE ore deposits. The U-Th-Pb ages of bastnäsite can constrain the mineralization age. Currently, the lack of suitable in situ analysis reference material is the major obstacle of bastnäsite U-Th-Pb geochronology. In this study, the matrix effects of U-Pb and Th-Pb age determination in bastnäsite using glass NIST 610 as the external calibration reference were evaluated with the 193-nm excimer LA-ICP-MS analysis in normal and additional gases laser ablation modes. The obtained U-Th-Pb ages of bastnäsite were approximately 7–11% lower than their reference values in the normal analytical mode (without additional gas). Fortunately, the determined systematic age biases in U-Th-Pb dating were significantly reduced (less than 1–2%) with the addition of water vapor within the ablation chamber. This reduction may be attributed to the observed significant suppression of 206Pb/238U and 208Pb/232Th downhole fractionation in both NIST 610 glass and bastnäsite by introducing a small quantity of water vapor in the ablation chamber. In addition, the obtained 206Pb/238U and 208Pb/232Th ages of bastnäsite K-9, LZ1384, and MAD809 showed great consistency with their corresponding recommended values. The obtained results show that accurate and precise U-Th-Pb ages of bastnäsite can be simultaneously obtained by using glass NIST 610 as the primary calibration standard in 193-nm LA-ICP-MS with the water vapor-assisted method. This simple and effective water vapor-assisted non-matrix-matched laser ablation method helps to promote the geological application of bastnäsite U-Th-Pb geochronology. [ABSTRACT FROM AUTHOR]

The test results of the automated mineral identification and characterization system (AMICS), including the mineral composition, particle size distribution, dissemination state and degree of liberation of the target minerals, could be used to improve the beneficiation process. Taking the Dalucao rare earth ore located in Dechang, Sichuan Province, China (with an average content of 2.40 wt%) as the research object in this paper, the chemical composition, phase composition and dissemination state of the minerals were tested by AMICS, and the minerals of different fineness were ground. The concentrate yield, grade and recovery rate of the minerals of different fineness were compared through flotation tests. When the grinding lasted for 5 min and 82.60% of mineral grains passed through the − 74-μm sieve, the yield, grade and recovery rate could reach 20.19%, 8.75% and 73.64%, respectively (as the best grinding fineness), under the same flotation conditions. [ABSTRACT FROM AUTHOR]

Recent exploration revealed a number of rare earth element (REE) deposits that are distributed along the Qinling Orogenic Belt, Central China. These deposits have an estimated total reserve of about 2 Mt. REE2O3, thus making this belt a world-class REE metallogenic province. Understanding the metallogenesis of the belt requires direct dating of REE minerals. In this study, LA-ICP-MS U-Th-Pb dating on bastnäsite and monazite from the Huangshuian, Taipingzhen, and Miaoya deposits in different units of the belt was used to precisely determine the timing of the REE mineralization. The Huangshuian deposit in the north is a carbonatite-related Mo-(REE) deposit in which the REE minerals are closely associated with molybdenite. After correction for common Pb and excessed 206Pb decayed from 230Th, a weighted average 206Pb/238U age of 207 ± 4 Ma (n = 17; MSWD = 1.9) is obtained for bastnäsite grains from this deposit. Such an age is slightly younger than that of the final collision (peak at 230–220 Ma) of the Qinling Orogenic Belt, thus indicating that the Mo-REE mineralization is likely related to a post-collisional extension setting. The Taipingzhen REE deposit in the middle contains sheet-like ore bodies composed of veins where bastnäsite, the dominant REE mineral, is closely associated with quartz, fluorite, and barite. In situ bastnäsite U-Th-Pb dating shows that the REE mineralization in this deposit has formed at 421 ± 7 Ma (n = 17; MSWD = 1.5), synchronous with extension-related magmatism in the region. The Miaoya deposit in the south is the largest one in the belt, and it is essentially a REE-mineralized syenite-carbonatite complex. In this deposit, the monazite grains are closely associated with major minerals of the syenites or carbonatites (e.g., K-feldspar or calcite), but commonly exhibit complex internal textures. Different domains of monazite yield two groups of U-Pb ages at 414 ± 11 Ma (n = 5; MSWD = 0.91) and 231 ± 2 Ma (n = 21; MSWD = 3.1), whereas the bastnäsite has an age of 206 ± 4 Ma (n = 14; MSWD = 1.5). The early age of 414 Ma is obtained from homogenous monazite grains and in good agreement with zircon U-Pb ages of the Miaoya syenite-carbonatite complex, and thus is considered to represent the timing of the major REE mineralization in the deposit. The younger ages of 231–206 Ma are obtained from monazite grains with complex internal textures and bastnäsite in late veinlets, thus recording secondary, consecutive REE remobilization events likely related to the compression process during formation of the Qinling Orogenic Belt. Our new U-Th-Pb ages, in combination with previously geochronological data, demonstrate that there are two episodes of REE mineralization at 440–410 Ma and 220–200 Ma in the Qinling Orogenic Belt. [ABSTRACT FROM AUTHOR]

The French Creek Granite, New Zealand, is an alkaline intrusion with enrichment in rare earth elements (REE). Petrography and whole-rock geochemical signatures demonstrate that the c. 82–84-Ma French Creek Granite is a composite granitoid, dominated by a ferroan, weakly peraluminous biotite alkali feldspar granite to syenogranite, with subordinate quartz-alkali feldspar (QAF) syenite. Maximum total REE + Y contents are higher in the granite and felsic dikes relative to the marginal QAF syenite, cogenetic mafic Hohonu Dike Swarm, and pegmatites. Our findings suggest that REE and high-field-strength element enrichment in the unaltered French Creek Granite is a function of partial melting from an enriched HIMU-like lithospheric mantle source, minor crustal assimilation, and extensive magmatic differentiation. Primary (magmatic) REE enrichment is defined by the occurrence of allanite, zircon, apatite, fergusonite, monazite, perrierite, and loparite, which are commonly associated with interstitial biotite. French Creek Granite samples from a phyllic-sericitic alteration zone in the Eastern Hohonu River are strongly elevated in REE relative to unaltered French Creek Granite, indicating remobilization and secondary REE enrichment by hydrothermal fluids. The REE are hosted in bastnäsite group minerals, monazite, xenotime, and zircon. Quartz protuberances, dilatational microfractures, and dike emplacement indicate that this alteration is structurally controlled. The δ13C and δ18O values of secondary carbonate veinlets are consistent with mixed low-temperature (~ 250–260 °C) magmatic hydrothermal fluids containing mantle-derived carbon. These fluids were likely part of a late-stage porphyry-type system operating during the same mantle degassing and extensional episode that was associated with the emplacement of the French Creek Granite and lamprophyric dikes of the Hohonu Dike Swarm, and initial Tasman Sea spreading. Results from this study provide an important insight into the complex processes responsible for REE enrichment in alkaline igneous systems. [ABSTRACT FROM AUTHOR]