REE Mineralization Research Articles

Garnets in the Yinachang Fe-Cu deposit, Kangdian region, China: Mineralogical and geochemical insights into fluid-rock dominated iron oxide-copper–gold (IOCG) mineralization

Garnet is an important metasomatic mineral in iron oxide-copper–gold (IOCG) ore systems, but its crystallization mechanisms and relationship to fluid evolution in IOCG are poorly understood. In this study, major and trace element analyses were conducted on garnets from the Yinachang IOCG deposit in the Kangdian region, southwest China, in order to reveal their characteristics and forming process.Garnet commonly occurs in the K-Fe ± REE mineralization stage and Cu ± Au mineralization stage of the Yinachang deposit. Three types of garnets are identified from this deposit, which generally have high almandine but low grossular and spessartine contents (Alm65-78Gro5-17Sp5-17). The type 1 garnet (Grt 1) in the ore bodies and the type 2 garnet (Grt 2) in the proximal silicate alteration zones are pink to purple, euhedral to subhedral grains. Grt 1 exhibit zoning pattern, and Grt 2 were altered into chlorite and was mainly crosscut by ore-bearing veins. They are both extremely depleted in LILEs and enriched in HFSEs, with low total rare earth element ∑REE contents but variable LREE vs HREE ratios. The type 3 garnets (Grt 3) in the intermediate garnet-biotite-quartz zones are purple-brown, anhedral grains with high LILE, HFSE, and ∑REE contents, and exhibit LREE-depleted and HREE-enriched patterns, with marked positive Eu anomalies. All three types garnets that formed during alteration and mineralization are hydrothermal, but their composition is controlled by different mechanisms due to different environments. Compositions of Grt 1 and Grt 2 are mainly related to substitution mechanism controlled by crystal chemistry, physicochemical conditions of the closed hydrothermal system, and buffering effects of carbonate wall rocks during slow infiltration metasomatism, whereas the composition of Grt 3 is mainly controlled by surface adsorption and related to the composition of external fluid during later rapid garnet growth. Decreases in temperature, pressure, and fO2 triggered by mixing of fluids could be responsible for the distinct mineralogical and geochemical characteristics of the three types of garnets.Garnets from IOCG deposits show various textual and geochemical characteristics worldwide. Generally, almandine and andradite are dominating end members for IOCGs. Andradite is typical for metasomatic origin. Although almandine garnet is generally considered to form during metamorphism, the occurrence of hydrothermal almandine in the alteration zones of IOCG deposits reveal a metasomatism between Fe-bearing fluid and Al-enriched pelite protolith in a sufficiently reduced environment. Also, it is possible that the metasomatism during mineralization has enriched the protolith with Fe, and the post-ore metamorphism formed almandine by providing high temperature and pressure.

Petrology of Granites of the Tommot Rare-Earth Ore Field (Verkhoyansk–Kolyma Orogenic Belt)

The article presents the results of studying the aegirine–arfvedsonite granites of the Somnitelnyi massif within the Tommot ore field located in the Verkhoyansk–Kolyma orogenic belt (NE Asia). Along with the crustal signatures, the rocks display features of mantle contamination at their origin. Their affinity for A-type granites characteristic of continental rifts and hot spots is shown. The associated Tommot REE deposit is the only one discovered in NE Russia. New data are presented for the previously studied Tommot massif within the same ore field, with a wide compositional range from alkaline-ultrabasic rocks to alkaline syenites. It is established that despite a common geochemical enrichment of both massifs’ rocks with REEs, the Somnitelnyi massif granites cannot be interpreted as the final phase of the Tommot massif emplacement. Specific REE mineralization and high crystallization temperatures (up to 1045 °C) of the Somnitelnyi granites may be explained by the existence within the study area of an undepleted mantle source (“hot spot”), whose maximum activity occurred during the granitic melt generation. The ore bodies of the Tommot deposit consist of fenitized albitites, granite gneisses, and, more rarely, the cross-cutting pegmatite veins. They are confined mostly to exocontacts of the Somnitelnyi massif, are less often in its endocontacts, and are not found in the host rocks and in the inner part of the massif away from the contacts. Principal ore minerals are chevkinite, yttrialite, gadolinite, and fergusonite. Based on the data obtained, the deposit is classified as a metasomatic complex Ce–Y–Nb–Zr deposit associated with the alkaline granites.

Ree minerals in black shales of the paleoproterozoic Mikhailovka formation (Baikal-Patom Highland, Siberia)

he paper characterizes REE mineralization from carbonaceous metapelites of the Paleoproterozoic Mikhailovka Formation, which is the most ancient gold-bearing horizon of the Lena province (Bodaibo district, Irkutsk region). The conditions of metamorphism of the studied samples do not exceed those of chlorite-muscovite subfacies of greenschist facies (ilmenite-pyrrhotite isograde). The metamorphic allanite is a main REE host, which crystallized before the last stage of plastic deformation and folding. The matter source for its formation is related to REE and Th absorbed on organic matter and clay minerals, as well as the detrital monazite. Late hydrothermal-metasomatic processes resulted in its replacement by hydroxycarbonates (hydroxybastnaesite, ancylite) and hydrous phosphates of light REEs (rhabdophane?), while Th precipitated as a hydrous silicate. Findings of low-temperature metamorphic monazite and xenotime are also described.

SHRIMP U-Pb zircon geochronology of the carbonatite-hosted REE deposit of Kamthai, Late Cretaceous polychronous Sarnu Dandali alkaline complex, NW India: Links to plume-related metallogeny and CO2 outgassing at the K-Pg boundary

The carbonatite plug at Kamthai in the Late Cretaceous polychronous (66–89 Ma) Sarnu- Dandali alkaline complex (SDAC), NW India, is India’s only known carbonatite-hosted rare- earth-element deposit. The SDAC intrudes the Neoproterozoic (∼750 Ma) Malani silicic igneous suite in the Aravalli raton. The Kamthai carbonatite is a calcio-carbonatite dominated by calcite along with barite, strontianite, quartz, zircon and REE-bearing minerals. Textural features suggest an open-system exchange of the magma chamber at varying depths of crystallization. REE patterns are smooth and characterized by strong LREE enrichment, as typical of carbonatites. SHRIMP dating of 12 spots on three magmatic zircons from the carbonatite yielded a concordia age of 66.1 ± 1.4 Ma and an indistinguishable average 206Pb/238U mean age of 68.4 ± 1.8 Ma. The obtained U-Pb ages unambiguously link the carbonatite emplacement to the Deccan Large Igneous Province, i.e. the Réunion hotspot near the K-Pg (Cretaceous-Paleogene) boundary in the polychronous set-up of the SDAC. These ages are also indistinguishable from the 68.53 ± 0.16 Ma magmatic pulse recorded from the nearby Mundwara polychronous alkaline complex and the 69.7 ± 0.2 Ma alkaline sills from the Tethyan suture zone in Pakistan, which are regarded to be earliest manifestations of the Réunion hotspot in the Indian sub-continent. Our results in the context of REE mineralization in the Kamthai deposit have also implications with respect to CO2 outgassing, induced by at least four near-synchronous carbonatitic eruptions in NW India, and to triggering the mass extinction near the K-Pg boundary.

Allanite in Singhbhum Shear Zone, India and Its Implications on the Genesis of REE Mineralization

Abstract Allanite, being one of the most common REE bearing mineral phases in the multimetallic mineralized domain of Singhbhum Shear Zone, has always attracted attention from researchers. The present paper outlines a detailed petromineralogical study on various textural types of allanites. The allanite is categorized into 6 different types falling under three major categories on the basis of deformation, metamictisation and darkening of pleochroic halos. On the basis of mineral assemblage and probe analysis of allanite it follows that the optical and textural variations are an implication of not only multiple generations of their formation but rather significant variation in nature of hydrothermal fluids as evidenced by the differences in co-crystallized assemblages. They hint at relatively reducing hydrothermal conditions in Singhbhum Shear Zone responsible for REE-U-Th mineralization overprinting earlier event of oxidizing hydrothermal mineralization in which the well-known Th-poor uraninite, crystallized. Majority of the allanite appeared post or at the most syn-tectonic in their formation. The available geochronological data of allanite suggests them to be formed around 1.8 Ga. Considering textural criteria as seen in the present study, it appears that the majority of allanite owing to their undeformed morphology must have formed later to or at the most during waning stages of shearing and hence the minimum age of shearing in SSZ could be considered around 1.8 Ga instead of 1.6 Ga.

Reappraisal of 75 Years of Exploration for Atomic Minerals in India and the Way Forward

Abstract In India, Atomic Minerals Directorate for Exploration and Research (AMD), the oldest unit under the Department of Atomic Energy (DAE) is mandated with the exploration and augmentation of atomic mineral(s) resources of the country to support the Nuclear Power Programme (NPP) of India. AMD, till date have established four (04) generations of uranium metallogeny in India, distributed in 46 deposits and several other occurrences, major share of which are contributed by the globally unique, stratabound carbonate hosted, syn-diagenetic Tummalapalle Group of deposit (~59%), metamorphite type deposits along the Singhbhum Shear Zone (~21%), unconformity related deposits in northern part of Cuddapah Basin (~5%) and sandstone type deposits in Cretaceous Mahadek Basin (~6%). Besides these, metasomatite type deposits along the North Delhi Fold Belt (4%) and granite related-structurally controlled, high grade deposits in Bhima Basin (2%) along with some small hydrothermal/metamorphite/migmatite type deposits in Aravalli, Kotri-Dongargarh, Higher Himalaya belts and Chhotanagpur Granite Gneiss Complex contribute to the national inventory which stands at 3,76,000 tonne uranium oxide. AMD have stockpiled substantial quantities of Nb-Ta, Be and Li minerals needed for the NPP of India from prospective pegmatite belts in the country. AMD have established immense potential for REE mineralization in carbonatite complexes of Ambadungar in Gujarat and per-alkaline granite-rhyolite of Siwana Ring Complex, Barmer district, Rajasthan. Exploration along the coastal and riverine placers of the country has established 130 heavy mineral placer deposits with substantial thorium and REE resources. The progressive advancements by AMD through adaptation of integrated multidisciplinary exploration techniques coupled with major thrust in airborne/ground geophysics and expansion of state-of-the-art analytical facilities have facilitated systematic planning for future augmentation of atomic mineral(s) resources. Exploration inputs are envisaged to be intensified in the brownfield target areas for resource augmentation while R&D and phase-wise exploration inputs will be focused for developing the identified greenfield areas following a planned roadmap to ensure selfsufficiency in atomic mineral resources for sustained growth of the NPP to reach the target of net zero carbon emissions by 2070.

Formation of the Rock Canyon Creek carbonate-hosted REE–F–Ba deposit, British Columbia, Canada: Constraints from Mg-Sr isotopes of dolomite, calcite, and fluorite

The Rock Canyon Creek REE–F–Ba deposit is a carbonate-hosted REE deposit located in the Alkaline Province of British Columbia, Canada, which hosts most REE mineralization. It shares tectonic, stratigraphic and structural similarities with nearby Mississippi Valley-type deposits. To constrain the dolomitization and mineralization processes, the δ26Mg, 87Sr/86Sr, and elemental compositions of dolomite, calcite, and fluorite were determined. The δ26Mg values of dolomite from the unmineralized zone (−1.83 ‰ and −1.67 ‰) are consistent with marine evaporative dolomite, but those from the mineralized zone (−0.97 ‰ to −0.17 ‰) are higher and more variable. The linear 87Sr/86Sr–Sr/Mg relationships, both in the dolomite and calcite from the unmineralized zone, suggest dolomitization in an evaporative marine environment. The high δ26Mg values with large variation and the non-stoichiometric Ca/Mg ratios in the dolomite from the mineralized zone suggest that this dolomite interacted with a relatively low-temperature hydrothermal fluid (<200 °C). Considering the Na, Ca, Sr, Ba, and LREE-enrichment and low 87Sr/86Sr signature in the dolomite from the mineralized zone as well as mineral paragenesis in the deposit, the most plausible scenario for the formation of the deposit is that marine-evaporitic dolomite re-equilibrated with basinal fluids along permeable zones, and at a later stage, carbohydrothermal REE-bearing fluids infiltrated into the Rock Canyon Creek brecciated zone and deposited the REE mineralization. Late-stage, post-mineralization calcite veins and iron oxides formed in the upper part of the deposit under supergene conditions, as suggested by combination of textural observations, REE patterns, and differences in isotopic compositions between dolomite and calcite from the mineralized zone. This study demonstrates that δ26Mg, 87Sr/86Sr, and element/Mg ratios in carbonate minerals are useful for characterizing mineralizing fluids in carbonate-hosted deposits, and discriminating mineralization-related dolomite from marine evaporative dolomite and unmineralized hydrothermal dolomite.

REE and Th minerals in the metasandstones of the Udokan basin (Russia)

Based on ICP-MS, optical and electron microscopy, and electron back-scatter diffraction data, the paper considers the distribution and mode of occurrence of REE and Th in metasandstones of the Lower Proterozoic Sakukan Formation, which host mineralization of the Udokan and Unkur copper sandstone deposits. The REE and Th contents increase in the layers enriched in iron oxides and minerals of natural heavy concentrate relatively to the background metasandstones. Detrital allanite is the major REE mineral, which occurs in layers enriched in iron oxides together with authigenic allanite, monazite, and unidentifed Ca and REE carbonates. Thorium is concentrated in an authigenic silicate (thorite or thorogummite), which contains a signifcant amount of Fe, Ca, Al, and Cu and microinclusions of Cu and Pb sulfdes and barite. The diagenetic formation of authigenic REE and Th minerals is suggested as a result of desorption of these elements from siliceous-ferruginous gel-like sediments formed in river deltas and the wave-cut zone of the Proterozoic Sea.

Nb–Ta mineralization in Ti-oxide minerals from the Bagolyhegy Metarhyolite Formation (Bükk Mountains, NE Hungary)

Abstract The foliated low-grade metamorphic rocks of the Triassic Bagolyhegy Metarhyolite Formation, mainly of pyroclastic origin, host post-metamorphic quartz-albite veins containing abundant tourmaline and occasionally rutile/ilmenite. The study of the Ti-oxide-mineralized veins with SEM-EDX revealed an unusual mineral assemblage comprising fine-grained Nb–Ta-bearing oxides (columbite-tantalite series, fluorcalciomicrolite and other Nb–Ti–Y–Fe-REE-oxide minerals) intergrown with Nb-rich polymorphs of TiO2 (anatase, rutile), ilmenite and zircon enriched with hafnium. This high field strength elements (HFSE)-bearing paragenesis is unexpected in this lithology, and was not described from any formation in the Paleozoic-Mesozoic rock suite of the Bükk Mountains (NE Hungary) before. The host metavolcanics are significantly depleted in all HFSE compared to the typical concentrations in felsic volcanics and the mineralized quartz-albite veins have even lower Ti–Nb–Ta concentration than the host rock, so the mineralization does not mean any enrichment. From proximal outcrops of the Triassic Szentistvánhegy Metavolcanics, potassic metasomatized lenses with albite-quartz vein fillings containing rutile/ilmenite are known. We studied them for comparison, but they only contain REE mineralization (allanite-monazite-xenotime); the Nb–Ta-content of Ti-oxide minerals is undetectably low. LA-ICP-MS measurements for U–Pb dating of Hf-rich zircon of the Nb–Ta-rich mineral assemblage gave 71.5 ± 5.9 Ma as lower intercept age while dating of allanite of the REE mineralized quartz-albite veins gave 113 ± 11 Ma as lower intercept age. The REE-bearing vein fillings formed during a separate mineralization phase in the Early Cretaceous, while the Nb–Ta mineralization was formed by post-metamorphic alkaline fluids in the Late Cretaceous., controlled by fault zones and fractures.

Granitic magma evolution to magmatic-hydrothermal processes vital to the generation of HREEs ion-adsorption deposits: Constraints from zircon texture, U-Pb geochronology, and geochemistry

The key point for further prospecting of heavy rare earth elements (HREE) ion-adsorption deposit is to figure out the granites that could generate HREE ion-adsorption mineralization in weathering processes. In this study, we present a detailed study of zircons from granites associated with Zudong, Dabu, and Xinfeng HREE ion-adsorption deposits in South China. The zircons were studied with regards to their texture, crystallinity, U-Pb dating, and geochemistry. The zircons from these granites all can be subdivided into two types. The type-1 zircons show oscillatory zonation and have Th/U and Zr/Hf mass ratios of 0.4–1.0 and 30–50, respectively. These textural and geochemical features indicate crystallization in a fractionated magma. The type-2 zircons are unzoned, occasionally porous, and have a low crystallinity. They occasionally rim type-1 zircons. The type-2 zircons show significantly higher F, P, Hf, Th, U, and REE contents, but display lower ZrO2 and SiO2 contents and lower Th/U, Zr/Hf, La/Yb ratios than those of the type-1 zircons. These geochemical features are consistent with zircon formation in a volatile-HREE-rich magmatic-hydrothermal transition stage. Under these conditions, the HREEs were also hosted in volatile-rich REE mineral phases including synchysite-(Y), aeschynite-(Y), calcybeborosilite-(Y), and atelisite-(Y), which have been observed in these HREE deposits. These volatile-rich REE-mineral phases can easily be dissolved during weathering and release HREE3+ to generate ion-adsorption HREE deposits. Therefore, we conclude that the granitic magma progression to a volatile-HREE-rich magmatic-hydrothermal system is vital for the generation of HREE ion-adsorption deposits. The long-term Mesozoic extension of the South China favors the generation of highly fractionated granites and is thus important for the generation of HREE ion-adsorption deposits. Furthermore, the zircons generated in a volatile-rich environment could be used to determine the HREE ion-adsorption mineralization potential of granites.

Late Triassic Au-Mo mineralization in the Xiaoqinling region and a genetic connection to carbonatitic magmatism

The southern margin of the North China craton hosts a number of quartz vein-style deposits of the unusual Au-Mo ore spectrum, including the Dahu deposit in the Xiaoqinling region. Pristine euhedral monazite and rutile crystals from the Dahu Au-Mo mineralized veins yield LA-ICP-MS weighted mean 206Pb/238U ages of 212.1 ± 1.7 Ma (1 s, MSWD = 1.13) and 209.7 ± 2.7 Ma (2 s, MSWD = 1.7), respectively. These ages point to a Late Triassic gold mineralization event in the Xiaoqinling region, aside from the regionally well-recognized Early Cretaceous gold mineralization event. Tectonic setting, mineral association, elemental and isotopic signatures, and fluid characteristics of the Dahu deposit suggest that the Late Triassic Au-Mo mineralization represents a carbonatite-associated hydrothermal system, with ore fluids exsolved from a late-stage carbonatite system. Mineralogically, the Dahu deposit is enriched in hydrothermal minerals common to carbonatite systems, such as carbonates, sulfates, and REE minerals. Trace element and isotope compositions of vein minerals also show similarities to those of carbonatites. Monazite displays a pronounced LREE-enriched and smooth pattern (no Eu anomaly), with εNd(t) values ranging from −14.9 to −13.3. Rutile is characterized by high Nb concentrations (6 to 18 wt%) and extremely high Nb/Ta ratios of 236 to 2044. Fluid inclusions in the Dahu Au-Mo mineralized veins and the regionally distributed coeval carbonatites are all dominated by low to moderate salinity H2O-CO2 fluids that contain sulfates as trapped solids/daughter minerals. Our study suggests that the southern margin of the North China craton has potential for the exploration of polymetallic hydrothermal mineralization related to hidden carbonatites.

Comment on “EPMA geochemistry and LA-ICPMS dating of allanite, epidote, monazite, florencite and titanite from the Jaduguda uranium deposit, Singhbhum Shear Zone, eastern India: Implications for REE mineralisation vis-a-vis tectonothermal events in the Proterozoic Mobile Belt” by Sinha. [(PR 359, 2021), 1–15

Comment on “EPMA geochemistry and LA-ICPMS dating of allanite, epidote, monazite, florencite and titanite from the Jaduguda uranium deposit, Singhbhum Shear Zone, eastern India: Implications for REE mineralisation vis-a-vis tectonothermal events in the Proterozoic Mobile Belt” by Sinha. [(PR 359, 2021), 1–15

Gadolinium-dominant monazite and xenotime: Selective hydrothermal enrichment of middle REE during low-temperature alteration of uraninite, brannerite, and fluorapatite (the Zimná Voda REE-U-Au quartz vein, Western Carpathians, Slovakia)

AbstractA hydrothermal quartz vein with REE-U-Au mineralization in the Zimná Voda (Gemeric Unit, Western Carpathians, Slovakia) is associated with contact metamorphism between Permian granites and host phyllites and metaquartzites. It contains unique REE minerals of the monazite and xenotime groups. Monazite-(Ce), monazite-(Nd), monazite-(Sm), and Gd-dominant monazite [“monazite-(Gd)”], along with xenotime-(Y) to Gd-dominant xenotime [“xenotime-(Gd)”] and Gd-rich hingganite-(Y) show heterogeneous compositions and reflect a strong fractionation trend toward the enrichment of MREE (Sm to Dy), particularly Gd. Here, the gadolinium abundance reported in “monazite-(Gd)” (≤23.4 wt% Gd2O3) and Gd-rich xenotime-(Y) to “xenotime-(Gd)” (≤28.7 wt% Gd2O3) and accompanied by Gd-rich hingganite-(Y) (≤15.8 wt% Gd2O3), is among the highest Gd concentrations ever reported in natural minerals. The Gd-richest compositions show the following formulas: (Gd0.31Sm0.24 Nd0.15Ce0.10La0.05Dy0.03Y0.03…)0.98PO4 [“monazite-(Gd)”], (Gd0.36Y0.32Dy0.13Sm0.08Tb0.05…)0.98 (P0.96As0.04)1.00O4 [“xenotime-(Gd)”] and (Y0.71Gd0.43Dy0.23Sm0.22Tb0.06Er0.04Nd0.06…Ca0.06)1.96 (☐0.87Fe0.132+)1.00(Be1.82B0.18)2.00(Si1.90As0.10)2.00O8(OH1.70O0.30)2.00 [hingganite-(Y)]. The MREE-rich monazites, xenotimes, and hingganite-(Y) precipitated in response to the alteration of primary uraninite, brannerite, and fluorapatite by low-temperature hydrothermal fluids of heterogeneous compositions on a microscale. These are responsible for the strong enrichment of individual MREE, especially Gd in the secondary minerals. This is accompanied by the advancing development of the W-type tetrad effect on REE through monazite species. The substantial incorporation of Gd into both REE-selective monazite and xenotime structures that are accompanied by LREE vs. HREE segregation indicates the possibility of differently sized REE3+ miscibility in REEPO4 solid solutions, as well as the stabilization of the Gd-rich orthophosphate structure by substitution of the remaining A-site cations with smaller HREE+Y in the xenotime-type, and/or larger LREE in the monazite-type structure.

Reply to the comments of D.K. Sinha on “EPMA geochemisty and LA-ICPMS dating of allanite, epidote, monazite, florencite and titanite from the Jaduguda uranium deposit, Singhbum Shear Zone, Eastern India:Implications for REE mineralisation vis-a-vis tectonothermal events in the Proterozoic Mobile Belt”

Reply to the comments of D.K. Sinha on “EPMA geochemisty and LA-ICPMS dating of allanite, epidote, monazite, florencite and titanite from the Jaduguda uranium deposit, Singhbum Shear Zone, Eastern India:Implications for REE mineralisation vis-a-vis tectonothermal events in the Proterozoic Mobile Belt”

Unmixing of REE-Nb enriched carbonatites after incremental fractionation of alkaline magmas in the Shaxiongdong complex, Central China

Carbonatites are often temporally and spatially associated with alkaline silicate rocks. However, petrogenetic models of formation of alkaline‑carbonatite complex and the related rare metal element enrichment processes are still in a matter of debate. In this study, we present detailed petrographic observations and geochemical data for the Shaxiongdong complex, consisting of meta-pyroxenite, syenite, and carbonatite in the South Qinling Belt, Central China. Meta-pyroxenite crops out as a marginal face of the complex and has higher Mg, Fe, Ti, Ca and P contents relative to syenite, consistent with a cumulate of pyroxene, magnetite, ilmenite, titanite and apatite. Syenite is the main central phase of the complex and can cut the meta-pyroxenite as dykes or veins. Both meta-pyroxenite and syenite show various degrees of fenitization, in which the residual syenite looks like breccia cemented by fenite. Carbonatite locally occurs as dykes of dozens of centimeters in width within syenite and meta-pyroxenite and hosts the most Nb and REE of the complex. Nb-rich minerals, e.g., pyrochlore, crystallized coeval with aegirine and apatite in carbonatite, implying a magmatic origin. In contrast, most REE minerals are spatially associated with barite and are interstitial to calcite, implying they were mainly crystallized from residual carbonatite magmas or magmatic-hydrothermal fluids. Carbonatite samples with REE mineralization have oxygen isotopes slightly higher than those of unmineralized and Nb mineralized samples, supporting the involvement of hydrothermal fluids in REE mineralization. UPb dating of titanite and zircon from meta-pyroxenite, syenite and the associated fenite yielded identical ages of ~440 Ma. Carbonatite, syenite, and meta-pyroxenite all have nearly identical SrNd isotopes ((87Sr/86Sr)i: 0.70298 to 0.70367; εNd(t):+3.6 to +4.6), suggesting that all of these rocks were derived from the same mantle source. Fractionation modeling suggests the syenites evolve from a parental alkali basalt magma by separation of cumulate pyroxenite in the complex and finally lead to the unmixing of Nb-REE enriched carbonatites. The subsequent fractionation of carbonatite melts led to Nb and REE mineralization in the Shaxiongdong carbonatite. Niobium minerals formed at early magmatic stage, while most REE minerals likely crystallized from the late magma residuals or precipitated from magmatic-hydrothermal fluids. This genetic model can be widely applicable to other alkaline‑carbonatite complexes and provides a reasonable explanation for extreme enrichment of rare metals in such rocks.

Formation and evolution of Th–REE mineralizing fluids at the Kiruna‐type Choghart iron oxide–apatite deposit, Central Iran: Insights from fluid inclusions and H‐C‐O isotopes

The Choghart iron oxide–apatite (IOA) deposit is one of the largest iron mines of the Bafq district in Central Iran. This deposit is hosted by the Early Cambrian rhyolites, and diabase dikes crosscut both host rocks and the ore bodies. The Choghart rhyolites erupted in a continental arc setting, while the Choghart diabase dikes formed in a back‐arc basin environment. Thorite, minor titanite, and REE‐oxide are the main hosts of Th and REEs. The mineralogy and geochemistry support that the Th–REE mineralization formed at relatively reduced conditions. The presence of calcite accompanied by thorite and titanite suggests that Th and REE probably migrated as carbonate complexes in the mineralizing hydrothermal fluids. Microthermometric data of calcite associated with thorite indicate that the salinity of the ore‐forming fluids varies from 20 to 30 wt% NaCl equivalent with temperature estimates between 300 and 370°C. The narrow range of homogenization temperature and low salinities of the fluid inclusions imply the presence of only one fluid phase in the Th–REE mineralization. The calcite δ13CPDB (−3.9 to −4.1‰) and δ18OSMOW (6.6–7.0‰) support a magmatic source for the ore‐forming fluids. Actinolite δD (5.55–6.72‰) and δ18O (−100.8 to −82.4‰) also suggest a magmatic source of the ore‐forming fluids. Microthermometric data imply that fluid–rock interaction and cooling were critical factors triggering the thorite precipitation at Choghart IOA deposit. Stable isotopic data indicate that post‐magmatic fluids derived from rhyolitic magmas played a significant role in the Th–REE mineralization.

Nb/Ta, Zr/Hf and REE fractionation in exotic pegmatite from the Keivy province, NW Russia, with implications for rare-metal mineralization in alkali feldspar granite systems

The White Tundra pegmatites are associated with the Keivy alkali feldspar granite complex in the Kola Peninsula, NW Russia. The host granites are aegirine-arfvedsonite, biotite-arfvedsonite and biotite-ferrohastingsite varieties, ranging compositionally from peralkaline through metaluminous to peraluminous. The pegmatites are remarkable for the diversity of the rare-metal mineralization. The inferred crystallization sequence of the main REE, HFSE and Ti minerals in the pegmatite is: zircon – fergusonite-(Y) – monazite-(Ce) – gadolinite-hingganite series – britholite-(Y) – astrophyllite – titanite – allanite-(Ce) –– chevkinite-(Ce) – ilmenite – kainosite-(Y) – REE carbonates. The pegmatites show unusual parageneses indicating a transition of the mineral associations from agpaitic to miaskitic in the same body (e.g. zirconosilicate → zircon; astrophyllite → titanite). A number of minerals have anomalous minor element contents. Astrophyllite and some titanites have low (even < 1) Nb/Ta and Zr/Hf ratios that contrast with the overall geochemical environment (ratios ≫ 1); titanite with very high Y and HREE contents; elevated Sn, W and V contents in some titanosilicates; allanite-(Ce) with very high Ti contents coupled with low (less than stoichiometric) values of Al, suggesting a Ti → Al substitution. Whole-rock compositions of the host granite show a steady decrease of Nb/Ta and Zr/Hf ratios in peralkaline through metaluminous to peraluminous types. Compositions of Ti minerals suggest that further fractionation of Nb-Ta, Zr-Hf and REE in the White Tundra exotic pegmatite and the crystallization of Ti minerals with anomalous compositions are due to a combination of several factors: assimilation by the primary magma (enriched in Zr, Nb, REE) of upper crustal lithologies enriched in Ta, Hf, LREE, W, Sn; consumption of significant amounts of Nb and Zr by early-crystallized pegmatitic fergusonite and zircon with Nb/Ta and Zr/Hf ≫ 1; transition of HFSE mineral assemblages in pegmatite from agpaitic to miaskitic with consequent lowering of Nb/Ta and Zr/Hf; crystallization of minerals in a low-temperature (hydrothermal) environment with changing F contents, that promoted a sporadic further decrease of Nb/Ta and Zr/Hf (to values < 1) and increase of Y + HREE. The implications of the fractionation mechanisms for ore-forming processes related to rare-metal granites are discussed.

Genesis of the Carboniferous karstic bauxites in Qingzhen region, central Guizhou, southwest China

In addition to the major source of aluminum, karstic bauxite also contains high concentrations of trace elements such as Li, Ga, V, Sc, Co, Nb, Zr, Ni, U and REE. At present, more than 33.9 million tons of early Carboniferous bauxite overlies the karstic surface of the Cambrian carbonate rocks in Qingzhen region, central Guizhou, southwest China. Here, new mineralogical and geochemical evidence of the Lindai bauxite in Qingzhen region have been provided, aiming to identify parental affinity, discuss the provenance differences between the Qingzhen and Xiuwen areas, illustrate the mineral phase transformation, analyze the factors affecting the distribution of critical elements, and reveal the ore-forming process. Mineralogically, the bauxite ores are dominated by diaspore, boehmite, illite, kaolinite and anatase, with small amount of zircon, hematite, pyrite, cassiterite and medium-low temperature hydrothermal minerals such as barite, chalcopyrite, galena, arsenopyrite. Textural characteristics are dominated by matrix-supported clastic texture, followed by pseudo-breccias, recrystallizational and ooidal textures. Clastic grains with various sizes and shapes are often mixed together without obvious sorting and directional structures, indicating that bauxite ores have undergone short-distance transportation in the form of gravity flow. Provenance analysis indicates that the protoliths of the Lindai bauxite were a mixed lithology including underlying Cambrian Shilengshui Formation dolomite, hydromica claystone and purple-red mudstone rather than single dolomite considered by previous studies. Particularly, the presence of hydromica claystone and purple-red mudstone has a significant contribution to high concentrations of trace elements and REE in the Jiujialu Formation. Studies of mineral genesis have shown that the formation of diaspore includes supergene crystallization and metamorphism. R-mode factor analysis revealed that (i) these elements including Al, Ti, Ga and HFSE are relatively stable and immobile during weathering process, and tend to be enriched in the karstic bauxite, (ii) the distributions of HREE are controlled by the REE minerals and Fe oxide-hydroxides, (iii) vanadate minerals have crucial roles in the distribution of LREE in addition to the phosphate minerals, and (iv) the enrichment of Ni is related to the adsorption of hydroxo complex of uranium. The studies reported in this paper provide new insights into the provenance and mineral phase transformation of the Carboniferous karstic bauxite in central Guizhou area.

Multiple-Stage Mineralization in the Huayangchuan U\u2212REE\u2212Mo\u2212Cu\u2212Fe Ore Belt of the Qinling Orogen, Central China: Geological and Re\u2212Os Geochronological Constraints

The Huayangchuan ore belt is located in the western segment of Xiaoqinling Orogen in the southern margin of the North China Craton (NCC), and hosts voluminous magmatism and significant U−REE−Mo−Cu−Fe polymetallic mineralization. However, geochronological framework of the various mineralization phases in this region is poorly understood. Here, we present new Re−Os isochron ages on magnetite from the Caotan Fe deposit (2 675 ± 410 Ma, MSWD = 0.55), and on pyrite from the Jialu REE deposit (2 127 ± 280 Ma, MSWD = 1.9) and Yuejiawa Cu deposit (418 ± 23 Ma, MSWD = 11.5), and Re−Os weighted average model age on pyrite from the Taoyuan Mo−U deposit (235 ± 14 Ma, MSWD = 0.17). These ages, combined with regional geology and mineralization ages from other deposits, suggest that mineralization in the Huayangchuan ore belt lasted from the Neoarchean to the Late Mesozoic. The mineralization corresponds to regional tectono-magmatic events, including the Neoar-chean alkali magmatism (REE mineralization), Paleoproterozoic plagioclase-amphibolite emplacement (Fe mineralization), Paleoproterozoic pegmatite magmatism (U mineralization), Paleozoic Shangdan oceanic slab subduction-related arc magmatism (Cu mineralization), Early Mesozoic Paleo-Tethys Ocean subduction-related arc magmatism (Mo−U mineralization), and Late Mesozoic Paleo-Pacific oceanic plate subduction direction change-related Mo(-Pb) mineralization. We proposed that the Huayang-chuan ore belt has undergone prolonged metallogenic evolution, and the magmatism and associated mineralization were controlled by regional geodynamic events.

Investigation of Sensor-Based sorting and selective comminution for pre-concentration of an unusual parisite-rich REE ore, South Namxe, Vietnam

The South Namxe deposit in Vietnam contains unusually high quantities of parisite, in addition to minor quantities of other fluorocarbonates, such as synchysite and bastnaesite. Together these minerals constitute the main REE-bearing minerals. This carbonatite-related deposit presents tabular (dykes) or lens-form ore bodies, up to several tens of meters wide and tens to hundreds of meters in length, that are hosted in Permian limestone and Triassic basalts. The main gangue minerals in the ore bodies are from the barite-celestine solid solution series, ankerite and calcite. It was considered necessary to assess the different grain characteristics presented by the presence of parisite intergrowths and its association with minerals of the barite-celestine solid solution series to structure a probable initial beneficiation strategy. Mineral liberation analysis revealed that an optimal liberation particle size between 45 µm and 90 µm could be achieved without overgrinding the material. Additionally, it showed a strong mineral association between fluorocarbonates, baryto-celestine, and calcite. Grinding below 100 µm is necessary. One substantial energy saving potential is removing the host rock prior to grinding. The potential for pre-concentration with sensor-based sorting and selective comminution is investigated in a pre-study. Sensor-based sorting using X-ray transmission resulted in an enrichment of REE by a factor of 4, a reduction of the mass stream down to approximately 30%, and a loss of rare earths below 2%. Selective comminution using a pin mill, at a circumferential speed of 20 m/s, yielded a recovery of 96.5% of REE minerals and allowed rejection of 27% of barren material. This shows that both selective comminution and sensor-based sorting can be considered as complementary beneficiation steps when processing these REE ores.