REE fractionation,mineral speciation,and supergene enrichment of the Bear Lodge carbonatites,Wyoming, USA

The Eocene (ca. 55–38 Ma) Bear Lodge alkaline complex in the northern Black Hills region of northeastern Wyoming (USA) is host to stockwork-style carbonatite dikes and veins with high concentrations of rare earth elements (e.g., La: 4140–21000 ppm, Ce: 9220–35800 ppm, Nd: 4800–13900 ppm). The central carbonatite dike swarm is characterized by zones of variable REE content, with peripheral zones enriched in HREE including yttrium. The principle REE-bearing phases in unoxidized carbonatite are ancylite and carbocernaite, with subordinate monazite, fluorapatite, burbankite, and Ca-REE fluorocarbonates. In oxidized carbonatite, REE are hosted primarily by Ca-REE fluorocarbonates (bastnäsite, parisite, synchysite, and mixed varieties), with lesser REE phosphates (rhabdophane and monazite), fluorapatite, and cerianite. REE abundances were substantially upgraded (e.g., La: 54500–66800 ppm, Ce: 11500–92100 ppm, Nd: 4740–31200 ppm) in carbonatite that was altered by oxidizing hydrothermal and supergene processes. Vertical, near surface increases in REE concentrations correlate with replacement of REE(±Sr,Ca,Na,Ba) carbonate minerals by Ca-REE fluorocarbonate minerals, dissolution of matrix calcite, development of Fe- and Mn-rich gossan, crystallization of cerianite and accompanying negative Ce anomalies in secondary fluorocarbonates and phosphates, and increasing δ¹⁸O values. These vertical changes demonstrate the importance of oxidizing meteoric water during the most recent modifications to the carbonatite stockwork. Scanning electron microscopy, energy dispersive spectroscopy, and electron probe microanalysis were used to investigate variations in mineral chemistry controlling the lateral complex-wide geochemical heterogeneity. HREE-enrichment in some peripheral zones can be attributed to an increase in the abundance of secondary REE phosphates (rhabdophane group, monazite, and fluorapatite), while HREE-enrichment in other zones is a result of HREE substitution in the otherwise LREE-selective fluorocarbonate minerals. Microprobe analyses show that HREE substitution is most pronounced in Ca-rich fluorocarbonates (parisite, synchysite, and mixed syntaxial varieties). Peripheral, late-stage HREE-enrichment is attributed to: 1) fractionation during early crystallization of LREE selective minerals, such as ancylite, carbocernaite, and Ca-REE fluorocarbonates in the central Bull Hill dike swarm, 2) REE liberated during breakdown of primary calcite and apatite with higher HREE/LREE ratios, and 3) differential transport of REE in fluids with higher PO₄³⁻/CO₃²⁻ and F⁻/CO₃²⁻ ratios, leading to phosphate and pseudomorphic fluorocarbonate mineralization. Supergene weathering processes were important at the stratigraphically highest peripheral REE occurrence, which consists of fine, acicular monazite, jarosite, rutile/pseudorutile, barite, and plumbopyrochlore, an assemblage mineralogically similar to carbonatite laterites in tropical regions.     

Mineralogical and geochemical studies of brecciated ores in the Dalucao REE deposit,Sichuan Province,southwestern China

The Dalucao deposit, located in western Sichuan Province, southwestern China, in the western part of the Yangtze Craton, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE belt. Moreover, the Dalucao deposit is the only deposit identified in the southern part of the belt. The Dalucao deposit contains the No. 1, 2, and 3 orebodies; the No. 1 and 3 orebodies are both hosted in two breccia pipes, located in syenite–carbonatite host rocks. Both pipes have elliptical cross-sections at the surface, with long-axis diameters of 200–400 m and short-axis diameters of 180–200 m; the pipes extend downwards for > 450 m. No. 1 and No. 3 have total thickness varying between 55 and 175 m and 14 to 58 m respectively. The REE mineralization is associated with four brecciation events, which are recorded in each of the pipes. The ore grades in the No. 1 and 3 orebodies are similar, and consist of 1.0%–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a Type I mineral assemblage (fluorite + barite + celestite + bastnäsite), whereas the No. 3 orebody is characterized by a Type II assemblage (fluorite + celestite + pyrite + muscovite + bastnäsite + strontianite). Argon (⁴⁰Ar/³⁹Ar) dating of hydrothermal muscovite intergrown with REE minerals in typical ores from the No. 1 and 3 orebodies yielded similar ages of 12.69 ± 0.13 and 12.23 ± 0.21 Ma, respectively, which suggest that both mineral assemblages formed coevally, rather than in paragenetic stages. Both ages are also similar to the timing of intrusion of the syenite–carbonatite complex (12.13 ± 0.19 Ma). The ore-mineral assemblages occur in breccias, veinlets, and in narrow veins. The ore veinlets, which usually show a transition to mineralized breccia or brecciated ores, are commonly enveloped by narrow veins and stringer zones with comparable mineral assemblages. The brecciated ores form 95% of the volume of the deposit, whereas brecciated ores are only a minor constituent of other deposits in the Mianning–Dechang REE belt. The carbonatite in the syenite–carbonatite complexes contains high concentrations of S (0.07–2.32 wt.%), Sr (16,500–20,700 ppm), Ba (3600–8400 ppm), and light REEs (LREE) (2848–10,768 ppm), but is depleted in high-field-strength elements (HFSE) (Nb, Ta, P, Zr, Hf, and Ti). The syenite is moderately enriched in large-ion lithophile elements (LILE), Sr (155–277 ppm), and Ba (440–755 ppm). The mineralized, altered, and fresh syenites and carbonatites exhibit similar trace element compositions and REE patterns. Brecciation events, and the Dalucao Fault and its secondary faults around the deposit, contributed to the REE mineralization by facilitating the circulation of ore-forming fluids and providing space for REE precipitation. Some hydrothermal veins composed of coarse-grained fluorite and quartz are distributed in the syenite–carbonatite complex. The oxygen isotope compositions of ore-forming fluids in equilibrium with quartz at 215 °C are − 4.95‰ to − 7.45‰, and the hydrogen isotope compositions of fluid inclusions in coarse-grained quartz are − 88.4‰ to − 105.1‰. The syenite–carbonatite complex and carbonatite are main contributors to the mineralization in the geological occurrence. Thus, the main components of the ore-forming fluids were magmatic water, meteoric water, and CO₂ derived from the decarbonation of carbonatite. According to the petrographic studies, bastnäsite mineralization developed during later stages of hydrothermal evolution and overprinted the formation of the brecciated fluorite–quartz hydrothermal veins. As low-temperature isotope exchange between carbonates of the carbonatite and water-rich magmatic fluids will lead to positive shifts in δ¹⁸O values of the carbonates, C–O isotopic compositions from the bulk primary carbonatite to hydrothermal calcite and bastnäsite changed (δ¹⁸O_V-SMOW from 8.0‰ to 11.6‰, and δ¹³C _V-PDB from − 6.1 to − 8.7‰). According to the chemical composition of syenite and carbonatite, REE chloride species are the primary complexes for the transport of the REEs in the hydrothermal fluids, and the presence of bastnäsite and parisite means the REE were precipitated as fluorocarbonates. High contents of Sr, Ba and S in the syenite–carbonatite complex led to the deposition of large amount of barite and celestite.     

Geochemical and mineralogical characteristics of weathered ore in the Dalucao REE deposit,Mianning–Dechang REE Belt,western Sichuan Province,southwestern China

The Dalucao deposit in western Sichuan Province, southwest China, is one of the largest and most extensive rare earth element (REE) deposits in the Himalayan Mianning–Dechang REE Belt. Moreover, this is the only deposit identified in the southern part of the belt. The deposit contains the No. 1, 2, and 3 orebodies. The No. 1 and 3 orebodies are hosted in two breccia pipes within syenite–carbonatite rocks that intrude a Proterozoic quartz–diorite pluton. Both breccia pipes have elliptical horizontal cross-sections at the surface, being 200–400 m long, 180–200 m wide, and extending to > 450 m depth. The No. 1 and No. 3 orebodies have total thicknesses of 55–175 m and 14–58 m, respectively. REE mineralization is associated with four brecciation events that are recorded in both pipes. The ore grades in the No. 1 and 3 orebodies are similar, with the rocks containing 1.0–4.5% rare earth oxides (REOs). The No. 1 orebody is characterized by a mineral assemblage comprising fluorite + barite + celestite + bastnäsite (i.e., Type I), whereas the No. 3 orebody is characterized by an assemblage comprising fluorite + celestite + pyrite + muscovite + bastnäsite + strontianite (i.e., Type II). Significant amounts of weathered high-grade REE ore (up to 60 wt.% of the rock mass) is mainly present in the No. 1 orebody. This is the main ore-type targeted for exploration within the Dalucao deposit, but is rarely present in other deposits in the Mianning–Dechang REE Belt.Faulting and cryptoexplosive breccia events, possibly linked to movement on the Panxi Fault, were more common in the No. 1 orebody than in the No. 3 orebody. This facilitated the introduction of ore-forming hydrothermal fluids and provided space for the precipitation of REE minerals. Based on the present results, we infer that the Dalucao deposit was the product of multiple stages of ore formation. REE minerals formed in envelopes around, or fractures within, quartz, fluorite, calcite, barite, and celestite in the brecciated ores. The main REE minerals were deposited from hydrothermal fluids within cryptoexplosive breccia, followed by weathering that increased the ore grade. Petrographic studies and X-ray powder diffraction (XRD) analyses indicate that the weathered ore contains 5–60% REE minerals (including bastnäsite, parisite, and monazite), together with gangue (quartz, barite, celestite, and fluorite), large amount of clay minerals (smectite, illite, kaolinite, and sepiolite), and relict igneous minerals (quartz, albite, and K-feldspar). The weathered samples are strongly enriched in La (up to 92,390 ppm), Ce (up to 103,500 ppm), Pr (up to 8006 ppm), and Nd (up to 16,690 ppm) compared with the unweathered brecciated ores. Conversely, Sr concentrations are significantly more enriched in the brecciated ores (up to 256,500 ppm) than in the weathered ores (generally less than 2671 ppm with one exception of 37,850 ppm) due to less celestite. Calcite is largely absent from the weathered ores (except one sample with up to 30% mode), which contrasts with the brecciated ores that contain up to 75% calcite. The effects of weathering, oxidation, loss of ions, and hydration on the brecciated ores led to the refertilization of the REEs and an increase in the grade of the ore deposit.     

Transformation from Permian to Quaternary bauxite in southwestern South China Block driven by superimposed orogeny: A case study from Sanhe ore deposit

Permian karstic bauxite and its Quaternary derivative, in western Guangxi, southwestern, South China Block, possess a total tonnage greater than 0.5 billion tons. The primary late Permian karstic bauxite formed in reduced environment in the background of Tethyan accretionary orogenesis. And as one consequence of Cenozoic convergence of the Indian and Eurasia continents, the primary orebody was uplifted, eroded and re-sedimented within Quaternary laterite. The geochemical variation and its controls during the ore transformation from Permian to Quaternary remain poorly understood. Quaternary ore blocks comprise an inner zone of fresh ore, and then it gradually transited through a middle zone to a margin with extensive weathering. One such bauxite block was selected and further subdivided into twenty-three samples for geochemical and mineralogical analysis. The inner and middle zones contain similar mineralogical compositions, dominated by diaspore and amesite, with minor illite, anatase, goethite, pyrite, zircon, and rutile. The margin is composed of diaspore, with small amounts of amesite, boehmite, illite, goethite, anatase, kaolinite, zircon, rutile, and barite. Bauxite in all three zones is composed of mainly Al, Si, Fe, and Ti, and high contents of Zr, Cr, Li, F, S, Zn, V, Sr, Nb, Ba, and REE. Variations in Fe²⁺ and Fe³⁺ between the three zones were observed. The elements Si, Al, Fe²⁺, Mg, Ba, Cr, F, Li, Ni, Zn, and REE decrease from the core of the ore block outwards, corresponding to an increase in S and Fe³⁺. Depletions in Si, Al, Fe²⁺, Mg, Ba, and Cr were caused by the dissolution of amesite. Most of the Al and Si in amesite were lost during the weathering, and minor retained to form kaolinite. Depletions in Li, Ni, and Zn resulted from changes in the depositional environment between the late Permian and Quaternary. Dissolution of REE-bearing fluorocarbonates resulted in depletions of REE and F. The enrichment of Fe³⁺ and S was related to the precipitation of goethite, hematite, and barite in an oxidizing environment, while local enrichment of Ce resulted from the redox change of Ce³⁺ → Ce⁴⁺ under the same condition. This shows that the chemical composition of laterite enwrapping the bauxite also took part in Quaternary bauxite transformation. This study shows that the elements migrations during bauxite transformation were influenced by multiple independent factors except for the elemental attributes.     

Mineralogical and geochemical investigations of the Mombi bauxite deposit,Zagros Mountains,Iran

The Mombi bauxite deposit is located in 165 km northwest of Dehdasht city, southwestern Iran. The deposit is situated in the Zagros Simply Fold Belt and developed as discontinuous stratified layers in Upper Cretaceous carbonates (Sarvak Formation). Outcrops of the bauxitic horizons occur in NW-SE trending Bangestan anticline and are situated between the marine neritic limestones of the Ilam and Sarvak Formations. From the bottom to top, the deposit is generally consisting of brown, gray, pink, pisolitic, red, and yellow bauxite horizons. Boehmite, diaspore, kaolinite, and hematite are the major mineral components, while gibbsite, goethite, anatase, rutile, pyrite, chlorite, quartz, as well as feldspar occur to a lesser extent. The Eh–pH conditions during bauxitization in the Mombi bauxite deposit show oxidizing to reducing conditions during the Upper Cretaceous. This feature seems to be general and had a significant effect on the mineral composition of Cretaceous bauxite deposits in the Zagros fold belt. Geochemical data show that Al₂O₃, SiO₂, Fe₂O₃ and TiO₂ are the main components in the bauxite ores at Mombi and immobile elements like Al, Ti, Nb, Zr, Hf, Cr, Ta, Y, and Th were enriched while Rb, Ba, K, Sr, and P were depleted during the bauxitization process. Chondrite-normalized REE pattern in the bauxite ores indicate REE enrichment (ΣREE = 162.8–755.28 ppm, ave. ∼399.36 ppm) relative to argillic limestone (ΣREE = 76.26–84.03 ppm, ave. ∼80.145 ppm) and Sarvak Formation (ΣREE = 40.15 ppm). The REE patterns also reflect enrichment in LREE relative to HREE. Both positive and negative Ce anomalies (0.48–2.0) are observed in the Mombi bauxite horizons. These anomalies are related to the change of oxidation state of Ce (from Ce³⁺ to Ce⁴⁺), ionic potential, and complexation of Ce⁴⁺ with carbonate compounds in the studied horizons. It seems that the variations in the chemistry of ore-forming solutions (e.g., Eh and pH), function of carbonate host rock as a geochemical barrier, and leaching degree of lanthanide-bearing minerals are the most important controlling factors in the distribution and concentration of REEs. Several lines of evidences such as Zr/Hf and Nb/Ta ratios as well as similarity in REE patterns indicate that the underlying marly limestone (Sarvak Formation) could be considered as the source of bauxite horizons. Based on mineralogical and geochemical data, it could be inferred that the Mombi deposit has been formed in a karstic environment during karstification and weathering of the Sarvak limy Formation.     

Geological,geochemical and mineralogical characteristics of REE-bearing Las Mercedes bauxite deposit,Dominican Republic

Bauxite deposits, traditionally the main source of aluminum, have been recently targeted for their remarkable contents in rare earth elements (REE). With ∑REE (lanthanoids + Sc + Y) concentrations systematically higher than ∼1400 ppm (av. = 1530 ppm), the Las Mercedes karstic bauxites in the Dominican Republic rank as one of the REE-richest deposits of its style.The bauxitic ore in the Las Mercedes deposit is mostly unlithified and has a homogeneous-massive lithostructure, with only local cross-stratification and graded bedding. The dominant arenaceous and round-grained texture is composed of bauxite particles and subordinate ooids, pisoids and carbonate clasts. Mineralogically, the bauxite ore is composed mostly of gibbsite and lesser amounts of kaolinite, hematite, boehmite, anatase, goethite, chromian spinel and zircon. Identified REE-minerals include cerianite and monazite-Ce, whose composition accounts for the steady enrichment in light- relative to medium- and heavy-REE of the studied bauxites.Considering the paleo-geomorphology of the study area, we propose that bauxites in the Las Mercedes deposit are the product of the erosion and deposition of lithified bauxites located at higher elevations in the Bahoruco ranges. Based on the available data, we suggest a mixed lithological source for the bauxite deposits at the district scale: bedrock carbonates and an igneous source of likely mafic composition.     

Geology,mineralogy and fluid inclusion data of the Kizilcaören fluorite–barite–REE deposit,Eskisehir, Turkey

The Kizilcaören fluorite–barite–Rare Earth Element (REE) deposit occurs as epithermal veins and breccia fillings in altered Triassic metasandstones and Oligocene–Miocene pyroclastics adjacent to alkaline porphyritic trachyte and phonolite. This deposit is the only commercial source of REE and thorium in Turkey. Most of the fluorite–barite–REE mineralisation at Kizilcaören has been formed by hydrothermal solutions, which are thought to be genetically associated with alkaline volcanism. The occurrence of the ore minerals in vuggy cavities and veins of massive and vuggy silica indicate that the ore stage postdates hydrothermal alteration. The deposit contains evidence of at least three periods of hypogene mineralisation separated by two periods of faulting. The mineral assemblage includes fluorite, barite, quartz, calcite, bastnäsite, phlogopite, pyrolusite and hematite as well as minor amounts of plagioclase feldspar, pyrite, psilomelane, braunite, monazite, fluocerite, brockite, goethite, and rutile. Fluid inclusion microthermometry indicates that the barite formed from low salinity (0.4–9.2 equiv. wt% NaCl) fluids at low temperatures, between 105 and 230 °C, but fluorite formed from slightly higher salinity (<12.4 equiv. wt% NaCl) fluids at low and moderate temperatures, between 135–354 °C. The depositional temperature of bastnäsite is between 143–286 °C. The local coexistence of liquid- and vapour-rich inclusions suggests boiling conditions. Many relatively low-salinity (<10.0 equiv. wt% NaCl), low and moderate temperature (200–300 °C) inclusions might be the result of episodic mixing of deep-saline brines with low-salinity meteoric fluids. The narrow range of δ³⁴S (pyrite and barite) values (2.89–6.92‰ CDT)suggests that the sulphur source of the hydrothermal fluids are the same and compatible with a volcanogenic sulphate field derived from a magmatic sulphur source.     

REE mineralogy and geochemistry of the Western Keivy peralkaline granite massif,Kola Peninsula,Russia

The authors have studied the geology, geochemistry, petrology and mineralogy of the rare earth elements (REE) occurring in the Western Keivy peralkaline granite massif (Kola Peninsula, NW Russia) aged 2674 ± 6 Ma. The massif hosts Zr- and REE-rich areas with economic potential (e.g. the Yumperuaiv and Large Pedestal Zr-REE deposits), where 25% of ΣREE are represented by heavy REE (HREE). The main REE minerals are: chevkinite-(Ce), britholite-(Y) and products of their metamict decay, bastnäsite-(Ce), allanite-(Ce), fergusonite-(Y), monazite-(Ce), and others. The areas contain also significant quantities of zircon reaching potentially economic levels. We have discovered that behavior of REE and Zr is controlled by alkalinity of melt/solution, which, in turn, is controlled by crystallization of alkaline pyroxenes (predominantly aegirine) and amphiboles (predominantly arfvedsonite) at a late magmatic stage. Crystallization of mafic minerals leads to a sharp increase of K₂O content and decrease of SiO₂ content that cause a decrease of melt viscosity and REE and Zr solubility in the liquid. Therefore, REE and zirconium immediately precipitate as zircon and REE-minerals. There are numerous pod- and lens-like granitic pegmatites within the massif. Pegmatites in the REE-rich areas are also enriched in REE, but HREE prevails over light REE (LREE), about 88% of REE sum.     

A review of the genesis of the world class Bayan Obo Fe–REE–Nb deposits,Inner Mongolia,China: Multistage processes and outstanding questions

The Bayan Obo Fe–REE–Nb deposit is the world's largest rare earth element (REE) resource and with the increasing focus on critical metal resources has become a focus of global interest. The deposit is hosted in the Palaeoproterozoic Bayan Obo Group, mainly concentrated in the H8 dolomite marble. The ores consist of light REE enriched monazite and bastnäsite, with a wide array of other REE minerals. Niobium mineralisation is hosted primarily in aeschynite and pyrochlore, although there are a wide range of other Nb-minerals. The origin of the host dolomite and ore bodies has been a subject of intense debate. The host dolomite has been proposed to be both of sedimentary origin and an igneous carbonatite. Carbonatite dykes do occur widely in the area, and consideration of the textural, geochemical and isotopic composition of the dolomite suggests an origin via intrusion of magmatic carbonatite into meta-sedimentary marble, accompanied by metasomatism. The origin of the ore bodies is complex, indicated most strongly by an ~ 1 Ga range in radiometric age determinations. Compilation of available data suggests that the ores were originally formed around 1.3 Ga (Sm–Nd isochron ages; Th–Pb ages of zircon), close in time to the intrusion of the carbonatite dykes. The ores were subsequently subjected to several stages of deformation and hydrothermal overprint, culminating in deformation, metamorphism and fluid flow related to the Caledonian subduction of the Mongolian Plate under the North China Craton from ~ 450 to 420 Ma (Th–Pb ages of monazite). This stage resulted in the formation of the strong foliation (‘banding’) of the ore. The presence of undeformed veins with alkali mineral fills, and the overprinting of the foliation by Nb minerals suggest that secondary fluid flow events may also have contributed to the metal endowment of the deposits, as well as remobilising the original Fe and REE mineralisation. The alteration mineralogy and geochemistry of the ores are comparable to those of many REE mineralised carbonatites. Initial Nd isotope ratios at 450 Ma, however, suggest crustal sources for the metals. These conflicting lines of evidence can be reconciled if a (at least) two stage isotopic evolution is accepted for the deposits, with an original mantle-sourced, carbonatite-related metal accumulation forming around 1.3 Ga with εNd close to 0. The ore was remobilised, with associated re-equilibration of Th–Pb isotope systematics during deformation at ~ 450 Ma. A further stage of alkaline hydrothermal fluid was responsible for Nb mineralisation at this stage. The complex geological history, with multiple stages of alkaline, high field strength element-rich, metasomatic fluid flow, is probably the main reason for the exceptional metal endowment of the Bayan Obo area.     

Mineralogy of high-field-strength elements (Y,Nb, REE) in the world-class Vergenoeg fluorite deposit,South Africa

The Vergenoeg fluorite deposit in the Bushveld Complex in South Africa is hosted by a volcanic pipe-like body. The distribution characteristics, composition and formation conditions of high-field-strength element (HFSE)-rich minerals in different lithological units of the deposit were investigated by optical and cathodoluminescence microscopy, scanning electron microscopy, X-ray fluorescence, inductively-coupled plasma mass-spectrometry and electron-probe microanalysis. The Vergenoeg host rocks comprise a diverse silica-undersaturated assemblage of fayalite–magnetite–fluorite with variably subordinate apatite and mineral phases enriched in rare-earth elements (REEs). The Sm–Nd isotope systematics of the fluorite from the various lithological units of the pipe support the model that the HFSE budget of the Vergenoeg pipe was likely derived from a Lebowa-type granitic magma. Isotopically, there is no evidence for other REE sources. Formation of the pipe, including development of the fluorite mineralization, occurred within the same time frame as the emplacement of other magmatic rock units of the Bushveld Complex (Sm–Nd isochron age for fluorite separates: 2040 ± 46 Ma). Hydrothermal alteration is manifested in strongly disturbed Rb–Sr isotope systematics of the Vergenoeg deposit, but did not affect its HFSE and REE budget. Whole-rock chondrite-normalized REE + Y distribution patterns of two types were observed: (i) flat patterns characteristic of magnetite–fluorite unit, gossan, metallurgical-grade fluorite (“metspar”) plugs and siderite lenses, and (ii) U-shaped patterns showing enrichment towards the heaviest REE (Tm–Lu) observed in the fayalite-rich units. Common HFSE minerals are complex Nb-rich oxides (samarskite, fergusonite), REE phosphates and fluorocarbonates. Additionally, fluocerite and REE silicates, whose identification requires further work, were found. Most of the HFSE-rich minerals are spatially associated with Fe-rich phases (e.g., pyrite, magnetite, greenalite and hematite). To a smaller extent, they are found finely disseminated or healing micro-fractures in fluorite. The whole-rock REE + Y distribution patterns of the individual lithological units are mainly controlled by the distribution of Yb-rich and Y-rich xenotime in these rocks. The common occurrence of bastnäsite-(Ce) in the gossan, “metspar” plugs and especially in the rhyolitic carapace at the pipe–wall-rock contact, controls the REE + Y distribution patterns of these rocks. HFSE minerals in the Vergenoeg pipe rocks have formed in several stages. Samarskite and coarse fluorapatite belong to the primary mineral assemblage. Fergusonite and Yb-rich xenotime formed during high- to moderate-temperature hydrothermal activity. Significant remobilization of the HFSE from the early-crystallized minerals (breakdown of fluorapatite and possibly allanite with release of REE + Y) and subsequent partial redistribution of these elements into near surface rocks are inferred. The late-stage assemblages are characterized by the presence of fine-grained REE fluorocarbonates, monazite-(Ce), monazite-(La) and xenotime-(Y).     

Discovery of the REE minerals and its geological significance in the Quyang bauxite deposit,West Guangxi,China

Most of the karstic bauxite deposits in China are enriched in REE, and the REE has commonly been considered to be adsorbed on the surface of clay or diaspore minerals as ion state. However, occurrence of REE minerals in the bauxite deposits has not been reported by far. In the Quyang bauxite deposit from Guangxi province, we find REE minerals. The minerals are parisite and churchite. Parisite is widely distributed in the ooids and matrix, inducing positive Ce anomaly of the ores; whereas, churchite is rare and occurred in the core of ooid. The compositions of the REE minerals show the Eu anomaly can change gently during the weathering, and the Eu anomaly analyses suggest that the REE in the ores are mostly derived from the underlying carbonates. The parisite is formed in an alkaline condition, while churchite in an acidic situation; the co-existence of the two minerals in the ores suggests the Permian environment for bauxite formation is complex with great change of pH value. Moreover, parisite is inferred to be formed earlier than the churchite.     

Content and mode of occurrences of rare earth elements in the Zagrad karstic bauxite deposit (Nikšić area,Montenegro)

Three vertical sections through the Zagrad deposit of Jurassic karst bauxite in central Montenegro have provided knowledge of the vertical distribution of major and some selected trace elements, including rare earth elements (REE). Variations in the mineralogy, particularly those hosting REE, have been studied. This has revealed the presence of authigenic mineral phases such as xenotime, mottramite and monazite (best proved using Raman microprobe analysis) as well as residual phases such as zircon, titanite and monazite. The mobility of the elements during bauxitization processes has been studied to show that the REE minerals ensure progressive concentration of these elements during removal of major elements through weathering. The similarity of normalized REE in the bauxite to the typical Post-Archean Australian Shales (PAAS) and Upper Continental Crust (UCC) profile, and the preserved Eu anomaly, are evidence that the bauxite was not derived from carbonates and represents alteration of shale, marly limestone and volcanogenic or proximal igneous sourced detritus that accumulated in the original karst landform. Mass change during bauxitization, using Ti as “index” element and compared to PAAS composition, revealed almost 100% depletion of Si and weak enrichment in Al. Deeper parts of the deposit with authigenic minerals exhibit very strong enrichment in all REE. The bauxite ores have high ΣREE contents (693.5–6953.4 ppm), especially ΣLREE contents (582.8–4882.9 ppm), while ΣHREE contents (106.6–2070.5 ppm) are much lower.     

Petrological and geochemical characteristics of Zhaibei granites in Nanling region,Southeast China: Implications for REE mineralization

Mineralization with ion adsorption rare earth elements (REEs) in the weathering profile of granitoid rocks from Nanling region of Southeast China is an important REE resource, especially for heavy REE (HREE) and Y. However, the Jurassic granites in Zhaibei which host the ion adsorption light REE (LREE) ores are rare. It is of peraluminous and high K calc-alkaline composition, which has similar geochemical features of high K₂O + Na₂O and Zr + Nb + Ce + Y contents and Ga/Al ratio to A-type granite. Based on the chemical discrimination criteria of Eby [Geology 20 (1992) 641], the Zhaibei granite belongs to A1-type and has similar source to ocean island basalts. The rock is enriched in LREE and contains abundant REE minerals including LREE-phosphates and halides. Minor LREE was also determined in the feldspar and biotite, which shows negligible and negative Eu anomalies, respectively. This indicates that the Zhaibei granite was generated by extreme differentiation of basaltic parent magmas. In contrast, granites associated with ion adsorption HREE ores contain amounts of HREE minerals, and show similar geochemical characteristics with fractionated felsic granites. Note that most Jurassic granitoids in the Nanling region contain no REE minerals and cannot produce REE mineralization. They belong to unfractionated M-, I- and S-type granites. Therefore, accumulation of REE in the weathering profile is controlled by primary REE mineral compositions in the granitoids. Intense fractional crystallization plays a role on REE enrichment in the Nanling granitoid rocks.     

Evolution of rare-earth mineralization in the Bear Lodge carbonatite,Wyoming: Mineralogical and isotopic evidence

The Bear Lodge alkaline complex in northeastern Wyoming (USA) is host to potentially economic rare-earth mineralization in carbonatite and carbonatite-related veins and dikes that intrude heterolithic diatreme breccias in the Bull Hill area of the Bear Lodge Mountains. The deposit is zoned and consists of pervasively oxidized material at and near the surface, which passes through a thin transitional zone at a depth of ~ 120–183 m, and grades into unaltered carbonatites at depths greater than ~ 183–190 m. Carbonatites in the unoxidized zone consist of coarse and fine-grained calcite that is Sr-, Mn- and inclusion-rich and are characterized by the presence of primary burbankite, early-stage parisite and synchysite with minor bastnäsite that have high (La/Nd)cn and (La/Ce)_cn values. The early minerals are replaced with polycrystalline pseudomorphs consisting of secondary rare-earth fluorocarbonates and ancylite with minor monazite. Different secondary parageneses can be distinguished on the basis of the relative abundances and composition of individual minerals. Variations in key element ratios, such as (La/Nd)_cn, and chondrite-normalized profiles of the rare-earth minerals and calcite record multiple stages of hydrothermal deposition involving fluids of different chemistry. A single sample of primary calcite shows mantle-like δ¹⁸O_V-SMOW and δ¹³C_V-PDB values, whereas most other samples are somewhat depleted in ¹³C (δ¹³C_V-PDB ≈ − 8 to − 10‰) and show a small positive shift in δ¹⁸O_V-SMOW due to degassing and wall-rock interaction. Isotopic re-equilibration is more pronounced in the transitional and oxidized zones; large shifts in δ¹⁸O_V-SMOW (to ~ 18‰) reflect the input of meteoric water during pervasive hydrothermal reworking and supergene oxidation. The textural relations, mineral chemistry and C and O stable-isotopic variations record a polygenetic sequence of rare-earth mineralization in the deposit. With the exception of one Pb-poor sample showing an appreciable positive shift in ²⁰⁸Pb/²⁰⁴Pb value (~ 39.2), the Bear Lodge carbonatites are remarkably uniform in their Nd, Sr and Pb isotopic composition: ¹⁴³Nd/¹⁴⁴Nd_t = 0.512591–0.512608; εNd_t = 0.2–0.6; ⁸⁷Sr/⁸⁶Sr_t = 0.704555–0.704639; εSr_t = − 1.5–2.7; ²⁰⁶Pb/²⁰⁴Pb_t = 18.071–18.320; ²⁰⁷Pb/²⁰⁴Pb_t = 15.543–15.593; and ²⁰⁸Pb/²⁰⁴Pb_t = 38.045–39.165. These isotopic characteristics indicate that the source of the carbonatitic magma was in the subcontinental lithospheric mantle, and modified by subduction-related metasomatism. Carbonatites are interpreted to be generated from small degrees of partial melt that may have been produced via interaction of upwelling asthenosphere giving a small depleted MORB component, with an EM1 component likely derived from subducted Farallon crust.     

Petrogenesis of granitoids in the Dewulu skarn copper deposit: implications for the evolution of the Paleotethys ocean and mineralization in Western Qinling,China

The Tethyan tectonic domain hosts numerous world-class mineral deposits. Among these, the Dewulu skarn copper deposit in Western Qinling, China belongs to the Paleotethys ore belt. The skarn and orebodies here occur as stratoids or lenses at the contact between the Triassic Dewulu intrusive complex and Permian marine clastic and carbonates. Alteration minerals include prograde skarns (garnet, diopside, wollastonite), plagioclase, hornblende, actinolite, tremolite, epidote, chlorite, calcite, quartz and sericite. The main ore types include early disseminated skarn-type replacement orebodies and late-stage quartz-sulfide veins. Chalcopyrite is the major ore mineral, along with pyrite, bornite and sphalerite. The Dewulu intrusive complex comprises quartz diorite, quartz diorite porphyry and dioritic mafic microgranular enclaves (MME). The MMEs are spheroidal in shape, and have igneous mineral assemblages, acicular apatites, complex oscillatory zoned plagioclase and quartz megacrysts surrounded by mafic minerals. The MMEs are metaluminous and calc-alkaline to high-K calc-alkaline, and possess relatively high Ni, Cr and MgO contents and Mg^# values. They display sub-parallel patterns in trace element spider diagrams and rare earth element (REE) plots. They are also characterized by the enrichment of Rb, U and Th, depletion of Ba, Sr, Nb and Ta and negative Eu anomaly. Zircon LA-ICP-MS U–Pb dating of the dioritic MME yields an age of 247.0 ± 2.2 Ma, coeval with the host quartz diorite, quartz diorite porphyry and ore-related sericite ⁴⁰Ar/³⁹Ar plateau ages within analytical uncertainties. Oxygen fugacity estimated from trace element compositions of zircons from the dioritic MME shows FMQ ± 3.3. The zircons have negative ε_Hf(t) values in a range of − 8.0 to − 3.3, corresponding to two-stage model ages ranging from 1.48 to 1.78 Ga. The integrated data from petrology, geochronology and bulk geochemistry suggest that the Early Triassic granitoids associated with Cu skarn mineralization at Dewulu were products of arc magmatism and involved magma mixing in an active continental margin setting. The magma was sourced through partial melting of enriched sub-continental lithospheric mantle that had been previously modified by slab-derived melt during the continuous northward subduction of the Paleotethys oceanic slab.     

REE and their relevance to the development of the Kupferschiefer copper deposit in Poland

Over one hundred samples, representing mainly clayey-organic- and carbonate-rich shales (Kupferschiefer) but also other members of different ore sections, including hangingwall dolomites (Z1 Werra) and footwall Weissliegend sandstone (Lower Permian), have been collected in different mines of the Lubin–Głogów mining district, mainly near the contact (transitional zone) between the copper-mineralized zone and secondarily oxidized (Rote Fäule = RF) zone. In general, the Polish Kupferschiefer shales are enriched in MREE in comparison to NASC. In a typical copper-rich ore section the REE amounts and patterns depend on lithologies, being generally similar in shales and dolomite. ∑REE vary among sandstones, shales and dolomites (average 73, 143 and 85 ppm, respectively), probably reflecting mainly their clay contents. Sandstones have strongly convex REE patterns with positive Eu and negative Gd anomalies and depletion in LREE and enrichment in MREE relative to HREE. The REE patterns of shale and dolomite are similar to one another and rather flat, with strong negative Eu anomalies, and a small positive Gd anomaly in the case of shales.The REE patterns of shales from the mineralized Cu-zone are generally convex (MREE enriched) and have negative Eu anomalies. However, in a section with Cu-, Zn- and Pb-shales the REE pattern of Pb-bearing shales shows a positive Eu anomaly, in contrast to other shales and overlying dolomite. More oxidizing conditions of deposition can be assumed for Pb-shales.No significant differences between REE distributions in transitional and oxidized zones have been observed. Their REE patterns are more convex and are much higher (av. 247 ppm) than those in the mineralized zone and they do not show Eu anomalies. The strongly convex pattern may suggest either enrichment in MREE or relative depletion in LREE due to localized precipitation of light REE minerals, both in shales and in the uppermost part of the sandstones.Two unique sections, one Cu-rich and one Cu-lean (partly oxidized), comprising three shale beds interbedded with dolomites have been compared. Generally ∑REE contents are similar in these two sections. Also similar are changes in contents of REE between beds in both sections, which decrease significantly upwards (from 157–171 ppm to 54–60 ppm). The REE patterns of the lowermost beds (directly overlying sandstones) are ramp-like, with LREE enrichments. The upper beds have concave REE patterns. Comparison between sections shows generally stronger negative Eu and positive Gd anomalies in the highly-mineralized section.There is a highly significant positive relationship between Cu and ∑ REE contents in Cu-rich shales and slightly less significant negative relationship for their concentration in oxidized and transitional shales. There is a moderate significant positive correlation between P₂O₅ and ∑ REE contents in Cu-rich shales.The observed differences in REE contents cannot be provenance dependent but have been caused by diagenetic processes, possibly related to mineralization and oxidation processes. Europium anomalies, generally reflecting different Eh conditions in the deposit, can be eliminated by the prolonged oxidation. Strong enrichment of the RF zones in REE may result from their desorption from large volumes of oxidizing, including mineralizing, solutions which probably emerged from the underlying molasse lithologies into the Rote Fäule areas. Higher contents of REE in the lowermost shales suggest upward movement of solutions from the underlying sandstones also far away from the RF areas.     

Nakhlak carbonate-hosted Pb(Ag) deposit,Isfahan province,Iran: a geological,mineralogical, geochemical,fluid inclusion,and sulfur isotope study

The Upper Cretaceous Nakhlak epigenetic vein-type Pb(Ag) deposit is located 55 km northeast of the town of Anarak in Isfahan Province, Iran. The deposit contains 7 Mt of galena-barite ore with an average grade of 8.33% Pb, 0.38% Zn, and 72 ppm Ag. The ore mineralization occurs as stratabound, epigenetic, steeply dipping, east-west–trending veins in faulted- or fracture-controlled Upper Cretaceous Sadar carbonates. Galena and barite are the primary minerals. Minor sphalerite, tennantite-tetrahedrite, pyrite, and chalcopyrite occur as inclusions in galena. Cerussite with minor amounts of anglesite and plattnerite formed in the oxidized supergene zone. The ore and ore-related minerals were deposited in the hydrothermally dolomitized carbonate host rock containing saddle-shaped dolomite. Geochemically, the dolomitized carbonate host rocks are enriched in MgO, Fe₂O₃, MnO, Pb, Zn, and Ba, but depleted in CaO. The galena concentrate contains high values of Ag (932 ppm), Sb (342 ppm), Cu (422 ppm), As (91 ppm), and Zn (296 ppm); the presence of these trace elements indicates a low-temperature type of galena mineralization. This interpretation is corroborated by fluid inclusions containing 12.98 wt.% NaCl equivalent salinity; the inclusions homogenize at the low temperature of about 152.1 °C. The similarity between δ³⁴S_(V-CDT) values in Nakhlak barite and Permian–Triassic δ³⁴S marine sulfate values indicates that the Nakhlak sulfur was probably provided from evaporates of Permian–Triassic age. The δ³⁴S_(V-CDT) values of galena and barite samples occupy the ranges of − 1.04‰ to + 8.62‰ and + 10.95‰ to + 13.71‰, respectively, and are similar to Mississippi Valley–type (MVT) deposits. The low-temperature basinal fluids, evaporate-originated sulfur, and fault- or fracture-controlled galena-rich veins in the Nakhlak deposit resemble the type of geological features documented in Pb-rich MVT deposits.     

REE and isotope (Sr,S, and Pb) geochemistry to constrain the genesis and timing of the F–(Ba–Pb–Zn) ores of the Zaghouan District (NE Tunisia)

The F–(Ba–Pb–Zn) ore deposits of the Zaghouan District, located in NE Tunisia, occur as open space fillings or stratabound orebodies, hosted in Jurassic, Cretaceous and Tertiary layers. The chondrite-normalized rare earth element (REE) patterns may be split into three groups: (i) “Normal marine” patterns characterizing the wallrock carbonates; (ii) light REE (LREE) enriched (slide-shaped) patterns with respect to heavy REE (HREE), with small negative Ce and Eu anomalies, characteristic of the early ore stages; (iii) Bell-shaped REE patterns displaying LREE depletion, as well as weak negative Ce and Eu anomalies, characterizing residual fluids of subsequent stages. The ⁸⁷Sr/⁸⁶Sr ratios (0.707654–0.708127 ± 8), show that the Sr of the epigenetic carbonates (dolomite, calcite) and ore minerals (fluorite, celestite) are more radiogenic than those of the country (Triassic, Jurassic, Cretaceous, lower Miocene) sedimentary rocks. The uniformity of this ratio, throughout the District, provides evidence for the isotopic homogeneity and, consequently, the identity of the source of the mineralizing fluids. This signature strongly suggests that the radiogenic Sr is carried by Upper Paleozoic basinal fluids.The δ³⁴S values of barite, associated to mineralizations, are close to those of the Triassic sea water (17‰). The δ³⁴S values of sulfide minerals range from − 13.6‰ to + 11.4‰, suggesting two sulfur-reduced end members (BSR/TSR) with a dominant BSR process.Taking account of the homogeneity in the Pb-isotope composition of galenas (18.833–18.954 ± 0.001, 15.679–15.700 ± 0.001 and 38.690–38.880 ± 0.004, for the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios respectively), a single upper crustal source for base-metals is accepted. The Late Paleozoic basement seems to be the more plausible source for F–Pb–Zn concentrated in the deposits. The genesis of the Zaghouan District ore deposits is considered as the result of the Zaghouan Fault reactivation during the Late Miocene period.     

The sources of ore-forming material in the low-sulfidation epithermal Wulaga gold deposit,NE China: Constraints from S,Pb isotopes and REE pattern

The Wulaga gold deposit, located in Heilongjiang province, NE China, is a subvolcanic rock-hosted, low-sulfidation epithermal gold deposit, and has an Au reserve of about 84 tons. The gold mineralization occurs in a crypto-explosive breccia, and is spatially and temporally associated with an Early Cretaceous granodioritic porphyry. Three individual stages of mineralization have been identified in the Wulaga gold deposit: an early white quartz-euhedral vein stage, a fine-grained pyrite–marcasite–stibnite–chalcedony stage, and a late calcite–pyrite stage. The sulfur isotopic values of sulfide minerals vary in a wide range from − 4 to 4.9‰, but are concentrated in the range of − 3 to 0‰, implying that sulfur in the hydrothermal fluids was derived from magmatic volatiles. Lead isotopic results of the granodioritic porphyry (²⁰⁶Pb/²⁰⁴Pb = 18.341–18.395, ²⁰⁷Pb/²⁰⁴Pb = 15.507–15.523, ²⁰⁸Pb/²⁰⁴Pb = 38.174–38.251) and sulfide minerals (²⁰⁶Pb/²⁰⁴Pb = 18.172–18.378, ²⁰⁷Pb/²⁰⁴Pb = 15.536–15.600, ²⁰⁸Pb/²⁰⁴Pb = 38.172–38.339) are comparatively consistent and clustered together between the orogenic and upper mantle lines, indicating the lead in the ores is closely related to the parent magma of the granodioritic porphyry. The REE patterns of fluid inclusions trapped in sulfides are similar to those of the granodioritic porphyry, which confirms the magmatic origin of the REE in the hydrothermal fluids. The characteristics of S and Pb isotopes and REE suggest that the ore-forming materials of the Wulaga gold deposit are partly magmatic in origin, and related to a high-level hydrous granodioritic magma.