Yuan-Can Ying

Carbonatites are primary sources of LREE, yet the mechanisms of HREE enrichment in carbonatite systems remains poorly understood. Here we investigate REE mineralization in the Jialu deposit, Qinling orogenic belt, using U–Pb dating, chemical compositions, and Nd isotope analyses of parisite, bastnäsite, and monazite. Three ore types are recognized: primary carbonatite veins, brecciated ore, and banded ore from potassic alteration zones. Primary carbonatite has the highest REE contents, but discrete REE minerals are scarce, with nearly all REE hosted in calcite. U–Pb dating reveals three generations of REE minerals: primary parisite (231.4 ± 7.2 Ma) and relict parisite in brecciated ore (234.7 ± 6.5 Ma) constrain early magmatic REE mineralization at ca. 234–231 Ma; secondary bastnäsite (226.3 ± 4.6 Ma) records post-magmatic fluid activity and REE remobilization; and hydrothermal monazite (211.5 ± 6.7 Ma) documents the latest fluid–rock interaction event. This ca. 234–211 Ma age spectrum indicates a protracted multistage magmatic–hydrothermal history. Compositionally, the fluorocarbonate minerals define a systematic trend of increasing LREE/HREE fractionation and decreasing Sr and Y contents from primary parisite [(La/Yb)ₙ = 1035–2532] to secondary bastnäsite [(La/Yb)ₙ = 4256–39508]. They show homogeneous εNd(t) values (−4.91 to −4.00) and define a Sm–Nd isochron age of 235 ± 38 Ma, indicating an EMI-type enriched lithospheric mantle source and internal fluid derivation during brecciation. In contrast, monazite yields distinctly lower εNd(t) values (−5.67 to −4.83), requiring externally derived, crustally enriched fluids that interacted extensively with the Taihua Group gneiss. We conclude that HREE enrichment at Jialu involved two stages: early magmatic calcite acted as the principal HREE reservoir, and subsequent hydrothermal brecciation (ca. 226 Ma) and potassic alteration (ca. 211 Ma) progressively liberated HREE and precipitated xenotime. These results demonstrate that REE mineralization in the Jialu carbonatite is the cumulative product of multistage magmatic and hydrothermal processes involving two genetically distinct fluid systems, and highlight hydrothermal brecciation and fluid–rock interaction zones as favorable targets for HREE exploration in orogenic carbonatites.

The Huangliangou Nb-Ta-Be deposit in the Baoshan Block of western Yunnan hosts two distinct generations of columbite–tantalite group minerals (CGMs) and tapiolite, which record the evolution of a highly fractionated rare metal pegmatite. To investigate the relationship between Huangliangou pegmatite differentiation and Nb-Ta mineralization, we conducted an integrated study combining petrography with mineral chemistry and geochronology. Electron probe microanalysis (EPMA) was used to determine the compositions of two CGMs and tapiolite. LA-ICP-MS U-Pb dating of these Nb-Ta oxides yields weighted mean ages of 60.25 ± 0.75 Ma for CGM-1 and 59.4 ± 1.1 Ma for CGM-2, indicating their synchronous formation in the early Paleocene. LA-ICP-MS trace element analysis of muscovite reveals a trend of decreasing Nb/Ta and K/Rb ratios with increasing Cs content from two-mica to garnet-bearing pegmatites. This chemical evolution in muscovite parallels the mineralogical transition from magmatic CGM-1 to metasomatic CGM-2 and tapiolite, confirming that late-stage hydrothermal fluids were characterized by volatile enrichment and Ta accumulation. The textural and chemical evolution reflects a late-stage, fluid-assisted autometasomatism within a highly fractionated melt. These results identify the northern garnet-bearing pegmatite dikes as a high-priority target for Ta exploration and provide a chrono-lithological framework for prospecting Paleocene pegmatite-type Nb-Ta deposits in western Yunnan and comparable Tethyan settings.

《地球科学》(ISSN 1000-2383;CN42-1874/P) (原刊名《地球科学——中国地质大学学报》ISSN 1000-2383;CN42-1233/P)，创刊于1957年，是由教育部主管、中国地质大学主办的综合性地学学术理论期刊，月刊，2015年改名为《地球科学》,她以反映中国地质大学教学与科研最新的、高水平的基础地质、应用地质、资源与环境地质及地学工程技术研究成果为主要任务。

The Mushgai Khudag complex consists of numerous silicate volcanic-plutonic rocks including melanephelinites, theralites, trachytes, shonkinites, and syenites and also hosts numerous dykes and stocks of magnetite-apatite-enriched rocks and carbonatites. It hosts the second largest REE–Fe–P–F–Sr–Ba deposit in Mongolia, with REE mineralization associated with magnetite-apatite-enriched rocks and carbonatites. The bulk rock REE content of these two rock types varies from 21,929 to 70,852 ppm, which is much higher than that of syenites (716 ± 241 ppm). Among these, the altered magnetite-apatite-enriched rocks are characterized by the greatest level of REE enrichment (58,036 ± 13,313 ppm). Magmatic apatite from magnetite-apatite-enriched rocks is commonly euhedral with purple luminescence, and altered apatite displays variable purple to blue luminescence and shows fissures and hollows with deposition of fine-grained monazite aggregates. Most magmatic apatite within syenite is prismatic and displays oscillatory zoning with variable purple to yellow luminescence. Both magmatic and altered apatite from magnetite-apatite-enriched rocks were dated using in situ U–Pb dating and found to have ages of 139.7 ± 2.6 and 138.0 ± 1.3 Ma, respectively, which supports the presence of late Mesozoic alkaline magmatism. In situ ⁸⁷Sr/⁸⁶Sr ratios obtained for all types of apatite and calcite within carbonatite show limited variation (0.70572–0.70648), which indicates derivation from a common mantle source. All apatite displays steeply fractionated chondrite-normalized REE trends with significant LREE enrichment (46,066 ± 71,391 ppm) and high (La/Yb)_N ratios ranging from 72.7 to 256. REE contents and (La/Yb)_N values are highly variable among different apatite groups, even within the same apatite grains. The variable REE contents and patterns recorded by magmatic apatite from the core to the rim can be explained by the occurrence of melt differentiation and accompanying fractional crystallization. The Y/Ho ratios of altered apatite deviate from the chondritic values, which reflects alteration by hydrothermal fluids. Altered apatite contains a high level of REE (63,912 ± 31,785 ppm), which are coupled with increased sulfur and/or silica contents, suggesting that sulfate contributes to the mobility and incorporation of REEs into apatite during alteration. Moreover, altered apatite is characterized by higher Zr/Hf, Nb/Ta, and (La/Yb)_N ratios (179 ± 48, 19.4 ± 10.3, 241 ± 40, respectively) and a lack of negative Eu anomalies compared with magmatic apatite. The distinct chemical features combined with consistent Sr isotopes and ages for magmatic and altered apatite suggest that pervasive hydrothermal alterations at Mushgai Khudag are most probably being induced by carbonatite-evolved fluids almost simultaneously after the alkaline magmatism.