Carbonatite Dykes Research Articles

The newly discovered Attu carbonatite of West Greenland: A Mesoproterozoic dyke intrusion enriched in primary Sr-Ba-REE minerals

A newly discovered ca. 1.5 Ga old carbonatite dyke in central West Greenland is characterized by very high contents of rare earth elements (REE), Sr and Ba (up to 12.4 wt% total REE, 14.6 wt% SrO and 12.4 wt% BaO). The dyke occurrence is named ‘Attu carbonatite’ after the nearby village Attu. The carbonatite is primarily composed of carbonate minerals such as Sr-rich calcite (CaCO3) and dolomite (CaMg(CO3)2), huntite (Mg3Ca(CO3)4), strontianite (SrCO3), alstonite (CaBa(CO3)2), burbankite ((Na,Ca)3(Sr,Ba, Ce)3(CO3)5) and daqingshanite ((Sr,Ca,Ba)3(Ce, La)(PO4)(CO3)3−x(OH, F)x). In addition to the wide range of carbonate minerals, the carbonatite contains coarse-grained monazite(Ce), apatite and magnetite. Barite occurs as discrete crystals together with the rock-forming carbonates. Texturally, the carbonatite rock displays abundant intimately intergrown fine- to medium-grained Ca, Sr, Ba and REE carbonate minerals, which exhibit prominent exsolution textures within calcite, burbankite, strontianite and dolomite hosts, as well as in the apatite crystals. Several different exsolution textures are observed: 1) alstonite in calcite; 2) daqingshanite in calcite; 3) daqingshanite in burbankite; 4) MgBa carbonate in strontianite; 5) MgBa carbonate in calcite and strontianite in dolomite; 6) strontianite in apatite; and 7) monazite(Ce) in apatite. The carbonatite dyke is foliated and exsolution textures are observed internally in the foliation-defining minerals indicating that exsolution occurred after the main deformation event.The carbonatite magma intruded into Archaean basement gneisses that had been affected by the Nagssugtoqidian tectono-metamorphic event at approximately 1850 Ma. Magmatic monazite(Ce) crystals from the carbonatite yield a UPb age of 1565 ± 53 Ma, which is the current best estimate of dyke emplacement. Monazite(Ce) that exsolved from apatite and calcite yields a UPb age of 1492 ± 33 Ma, which is within the analytical uncertainty of the primary magmatic monazite UPb age. However, the UPb age determinations suggest that mineral exsolution occurred a few million years after carbonatite magma emplacement, in response to further cooling of the deep-seated dyke (tectonically-induced uplift?). Dyke emplacement may have occurred within an active ductile shear zone, which helps to explain the foliation of the carbonatite rock, predating cooling-related mineral exsolution. Country rock fenitization by fluids that emanated from the carbonatite dyke intrusion is recorded by the increasing abundance of mafic silicates such as Ba-rich phlogopite at the contact zone.

How old is the Eye of Africa? A polyphase history for the igneous Richat Structure, Mauritania

The annular Richat Structure, in Mauritania, is among the most striking geological features on Earth visible from space. Nicknamed “the Eye of Africa”, its intriguing concentric shaping has drawn attention for several decades. Formerly hypothesized as an astrobleme, it is now consensually recognized as a complex igneous intrusion, involving two ring-like gabbroic bodies, a central breccia and carbonatite dykes. However, the coexistence of until now undated tholeiitic gabbros and 99 Ma carbonatites akin to alkaline magmatism remained enigmatic.We provide the first plagioclase 40Ar/39Ar age determination and new geochemical data on the gabbroic bodies. Although no robust age was obtained, numerical modelling of our results suggests that the gabbros were intruded between 230 and 200 Ma. This age bracket is compatible with the ∼200 Ma Central Atlantic Magmatic Province (CAMP). Moreover, the compositions of these tholeiitic gabbros match the two chemical groups of the CAMP the best represented in northwest Africa. Therefore, we argue that the Richat gabbros correspond to two CAMP sills intruded conformably into the Late Proterozoic/Lower Paleozoic sedimentary sequences of the Taoudenni basin. About 100 My later, the gabbros and their country rocks were locally uplifted by an alkaline intrusion (expressed now on surface by the carbonatites), resulting in a circular doming, 40 km in diameter. The subsequent erosion led to the actual flattened concentric structure. Therefore, the Richat Structure represents a two-stage igneous-history, separated by a time lapse of ∼100 My, resulting in two CAMP gabbroic sills mimicking ring-like bodies.

Neoproterozoic (ca. 830 Ma) carbonatite dykes from Qianlishan in the western North China Craton: Petrogenesis and metallogenic implications

Carbonatite and the associated alkaline intrusive complexes are the primary sources of light rare earth element (REEs). Our field investigations in the western margin of the North China Craton (NCC) have firstly identified over 20 carbonatite dykes and minor coeval carbonate-rich lamprophyre dykes intruding the Paleoproterozoic Qianlishan Group in the southern Qianlishan area. Zircon LA-ICP-MS U-Pb dating on six carbonatite samples yielded crystallization ages ranging from 835.2 ± 3.7 Ma to 828.9 ± 3.8 Ma, indicating emplacement of the carbonatite dykes at ca. 830 Ma during the Neoproterozoic. Geochemical data show that most of the carbonatite dykes are rich in light REEs with total light REEs > 600 ppm to 4932 ppm, and the carbonate-rich lamprophyre dykes are characterized by high Cr (1219–1267 ppm) and Ni concentrations (857–1076 ppm). Petrological and scanning electron microprobe (SEM) analytical results show that the dominant REE-bearing minerals in the carbonatites are monazite, and other REE-bearing minerals are parisite and bastnäsite. Geochemical and carbon–oxygen isotopic results indicate that the ca. 830 Ma carbonatite dykes were produced by low-degree partial melting of a previously metasomatised subcontinental lithospheric mantle with assimilation of crustal material. Emplacement of the ca. 830 Ma carbonatite and carbonate-rich lamprophyre dykes in the southern Qianlishan area and the previously reported ca. 810 Ma mafic dykes in the middle Qianlishan area occurred in a continental rift environment. Simultaneous development of the Neoproterozoic (830 − 810 Ma) continental rifting events in the western margin of the NCC and the eastern margin of the Alxa Block as previously suggested may indicate a connection between the eastern Alxa Block and the western NCC during and before the middle Neoproterozoic.

Geology, geochronology, and genesis of Au mineralization from the Huangshui'an Mo-Au-Pb deposit in Eastern Qinling, China

Huangshui'an deposit, situated in the Xiong'ershan area of Qinling orogenic belt (QOB) in China, is a globally rare carbonatite-type molybdenum polymetallic deposit that contains economic Mo, Au, Pb, and REE mineralization. The molybdenite Re-Os ages (Ca. 213.5–209.2 Ma) indicates that Huangshui'an Mo mineralization formed at Late Triassic. However, it is still unclear whether the Au mineralization is related to carbonatite dykes, and the formation age and genesis mechanism of Au mineralization remain poorly understood at Huangshui'an. This paper presents a comprehensive investigation of the Au mineralization events at the Huangshui'an deposit, including geology, mineralogy, and geochronology. The aim is to provide insights into its formation and mineralization processes and offer a broader understanding of regional Au metallogeny. Based on petrographic investigations, four stages are identified in the Au mineralization as follows: (I) quartz-K-feldspar stage; (II) quartz-pyrite-gold stage, including (IIA) pyrite-native gold sub-stage and (IIB) gold-tellurite-bismuthide sub-stage; (III) quartz-pyrite-polymetallic sulphide-oxide stage; and (IV) calcite-quartz-chlorite stage. The LA-ICP-MS titanite U-Pb dating yields a weighted mean age of 130.8 ± 0.63 Ma which determined that the Au mineralization at Huangshui’an was in the Early Cretaceous and was an independent mineralization event, which may be related to the lithosphere's destructive effect on the North China Craton (NCC). Combined with mineralography by automatic quantitative mineral analysis and testing system (TIMA), we consider that tellurium-bismuth minerals are closely associated with Au mineralization. The sulfur and tellurium fugacity of the ore-forming fluids in each stage were calculated using metal sulfide-oxides and tellurium-bismuth minerals. It was concluded that the precipitation of Au in the Huangshui'an deposit was mainly due to the combined effects of sulfidation, gold collection-redissolution by the low-melting point chalcophile elements (LMCE) melts, and the variation of sulfur fugacity and tellurium fugacity.

Mineralogy and element geochemistry of the Bayan Obo (China) carbonatite dykes: Implications for REE mineralization

The Bayan Obo deposit in China is the world's largest rare earth element (REE) deposit. However, its genesis remains highly debated. It is considered to have a close genetic association with carbonatites because of the presence of nearby carbonatite dykes and the geochemical similarities between these dykes and the orebody. However, the evolution of these dykes and associated REE mineralization remain poorly understood, hindering the origin interpretation of the deposit. This study provides new insights into the petrography, mineralogy, and major- and trace-element geochemistry of these carbonatite dykes and rock-forming minerals to reconstruct the magmatic evolution and REE enrichment of the deposit. The dykes evolved from dolomite- to calcite-dominated (i.e., calcite and fenitized calcite) carbonatites, and their REE content remarkably increased, with the latter developing strong REE mineralization. The dolomites and minor calcites within the dolomite carbonatites exhibit a steep negative slope with high light REE (LREE) enrichment and heavy REE (HREE) depletion, similar to the whole rock. In contrast, the calcites from the calcite-dominated carbonatites exhibit flat REE patterns with HREE enrichment. Combined with previous isotopic data and our petrology, mineralogy, and geochemistry study, we propose that these rocks are derived directly from the low-degree melting of the underlying mantle source. REE mineralization may be related to the progressive crystallization of rock-forming calcites of calcite-dominated carbonatites. Monazite, bastnäsite, parisite, and huanghoite are the dominant REE minerals and exhibit intimate associations with barite and alkaline silicate minerals such as aegirine, biotite, K-feldspar, and albite. This assemblage indicates that the REE were primarily transported via REE–sulfate and REE–alkali complexes, with the latter favoring HREE, and the precipitation of alkaline silicate minerals destabilized these REE complexes, facilitating REE mineralization.

Bulk composition and O–C isotope constraints on the petrogenesis of Richat carbonatites (Mauritania)

In the northwestern Mauritanian Taoudeni Basin, the famous Richat structure is the only occurrence known of carbonatite. Despite several studies on this unique geological feature, the mineralogy and petrogenesis of the Richat dome carbonatites are underdocumented.Major, trace element whole rocks and carbon-oxygen isotope data are reported for Cretaceous carbonatites from Richat dome in Mauritania. More than 45 carbonatite dykes have been mapped, they are oriented N°15-N°80. Based on color, four groups have been identified, dark, gray, red breccia, and brown carbonatites. They exhibit different mineralogical and texture characteristics. The studied carbonatites are mainly composed of dolomite, calcite, and ankerite, with lesser amounts of apatite and quartz. Accessory minerals such as pyrochlore, spinel, pyrite, chalcopyrite, magnetite, hematite, baryte, phlogopite, and zircon are also present. Other REE-bearing minerals (britholite, monazite, and bastnaesite) were revealed by X-ray diffraction.Three types of carbonatites are identified: magnesiocarbonatites (dominants), ferrocarbonatites and calciocarbonatite. The presence of spinel and pseudomorphed olivine, as well as high contents of Ni (380 ppm), Co (34 ppm), and Cr (840 ppm) in the dark and brown carbonatites, suggests that these rocks are the result of the low-degree partial melting of a spinel-bearing peridotite. Furthermore, the Zr/Hf (48–80) and Nb/Ta (26–48) ratios show significantly higher values than those observed in the primitive mantle, they are interpreted as formed by partial melting of the metasomatized mantle. The high values of δ18O in addition to the presence of baryte, ankerite, and quartz in the Richat carbonatites suggest that they are the result of secondary processes involving a low-temperature fluid-rock interaction.The carbonatites from the Richat dome are enriched in LREE (La = 258–1070 ppm) and LILE as Sr, Ba, Th, U, and depleted in HREE (Yb = 2–14 ppm) and HFSE such as Zr, Hf, Ti. These characteristics are like those of Cretaceous peri-atlantic alkaline carbonatites such as Barra do Itapirapua in Brazil, Ondurakorume and Kalkfeld in Namibian, lupangola in Angola, and Twihinate in Morocco. Reactivation of a deep pan-African NNE-SSW lineament would have been a fundamental factor in the formation of the Richat carbonatites.

When carbonatite met granite: A carbonatite magma- wall rock reaction origin for the Paleoproterozoic pyroxenite and syenite in Fengzhen, North China

A set of carbonatite dykes intruding into Paleoproterozoic granites are exposed in Fengzhen area, where the Trans-North China Orogen joined with the Khondalite Belt. Some pyroxenites and syenites were also developed close to the carbonatite dykes. But the genetic relationship between these rocks and carbonatite dykes has not been solved. The pyroxenites show low SiO2 (40–49 wt%), high CaO (>20 wt%), and are enriched in light rare earth elements, strongly depleted in high field strength elements (Nb, Ta, Zr, Hf), similar to the characteristics of carbonatite dykes. The zircon UPb ages of pyroxenites and syenites are 1789 ± 13 Ma and 1793 ± 14 Ma, respectively. The wall-rock granite yields a metamorphic age of 1839 ± 19 Ma, which is close to the magmatic age of the pyroxenites and syenites. The whole-rock εNd(t) values (−4.4 to −5.1) and εHf(t) values (−3.8 to −6.9) of the pyroxenites are both negative, which are similar to those of the syenites (−3.7 to −4.7 and − 4.1 to −5.3) but slightly lower than those of wall-rock granites (−3.5 and 1.9). The δ18O values of clinopyroxene in the pyroxenites fall in a range of 6.6–11.3‰, which are much higher than the mantle value. The calcite of the carbonatites and pyroxenites gave overlapped δ18O value ranges of 11.8–14.8‰ and 9.3–15.0‰, respectively. They are also higher than typical primary carbonatites. These lines of geochemical evidence indicate that the pyroxenites are not mantle xenolith or derived directly from mafic magma, but closely related to the carbonatite dykes and the wall-rock granites. Since the Ba/La and Ba/Mn ratios of the syenites are much higher than those of the carbonatite, an immiscibility origin is excluded. Mineral chemistry of clinopyroxene, olivine, calcite and phlogopite from the carbonatite, pyroxenite and syenite all suggest they are formed by different degrees of reaction between carbonatite magma and felsic crustal rocks. Modeling shows that the Nd isotope compositions and trace element compositions of the pyroxenites and syenites fall on the mixing trends between the carbonatite dykes and wall-rock granites. These results demonstrate that the pyroxenites and syenites in the Fengzhen area were formed when carbonatite magma intruded and reacted with felsic crustal rocks. This provides a canonical case for the formation of pyroxenite and syenite by carbonatite magma-wall rock reaction.

Occurrence of niobium in biotite-type Fe-REE-Nb ore in the Bayan Obo deposit

The Bayan Obo REE-Nb-Fe deposit, which reserves the current largest REE resources globally, also hosts 63.4% of China's Nb resources. Previous studies mainly focus on the isotopes and petrographic mineralogy of carbonatite dyke, ore-host dolomite and rare earth ore in Bayan Obo deposit, niobium mineralogy is comparatively insufficient. In order to promote the utilization of niobium resources in the Bayan Obo deposit, we focus on the petrological and mineralogical study on the biotite-type Fe-REE-Nb ore of West Mine, by scanning electron microscope (SEM), X-ray energy spectrum (EDX) imaging and micro-XRF scan. From the analysis, we observe that 1) this type of ore shows relative enrichment in niobium, and the size of niobium-bearing minerals up to 1 mm; 2) the niobium mineral composition are extremely complex, with main niobium minerals including aeschynite, fergusonite and columbite and minor niobium minerals ilmenorutile and baotite; 3) REEs are mainly distributed in monazite, bastnaesite and aeschynite and the particle size of the first two is relatively fine; 4) aeschynite occurring as large aggregates maybe the hydrothermal in origin and fluid–rock interaction leads to the aggregation of biotite and the precipitation of niobium-bearing minerals. This study on biotite type Fe-REE-Nb ore has important insights for mineral processing and directive significance on niobium prospecting in the Bayan Obo deposit.

The eruption interface between carbonatitic dykes and diatremes – The Gross Brukkaros volcanic field Namibia

Carbonatites exist as intrusive and extrusive rocks, with the former dominating the rock record. The geochemical link between intrusive and extrusive equivalents and the processes during eruption of carbonatite melt are essentially unknown. This contribution aims at providing new insights into the transition from intrusive to extrusive carbonatites with special emphasis on the trace element budget. The Gross Brukkaros natural laboratory in central Namibia was chosen to study the interface between fine-grained dolomite‑carbonatite dykes and associated diatremes. Whole rock geochemistry combined with micro-scale petrography via BSE imaging and microXRF mapping provide evidence that the carbonatite dykes contain significant amounts of silica and aluminium, which is inherited from crustal xenoliths by resorption and leaching. At the transition from carbonatite dyke to diatreme, the CO2 component (likely together with most of the other volatiles e.g. Cl, H2O, F, S) of the carbonatitic brine-melt vaporised and was released into the atmosphere. During this process the Si, Ca, Mg and Fe transported together with a high amount of trace elements became precipitated from a resulting primarily magmatic fluid as a mixture of cryptocrystalline quartz (quartz I) and aegirine-augite (plus minor magnetite) with frequently occurring microcrystalline quartz I grains. This mineral assemblage forms the matrix in the diatreme breccia and precipitated by rapid temperature drop in the course of decompression from the hydrothermal fluid. In the post-eruption stage the influx of meteoric waters (into the system), which mixed with the remaining magmatic fluid, caused precipitation of a second quartz generation (quartz II; euhedral grains), which is depleted in trace elements. All measured trace elements show significantly higher contents in quartz I compared to quartz II (with exception of Li). The application of the TitaniQ geothermometer indicates that Ti incorporation in quartz is kinetically controlled for quartz I. During quartz II formation the system thermodynamically equilibrates and ends up with realistic precipitation temperatures of 290–350 °C. Therefore, this study shed light on the trace element behaviour at the interface between feeder dykes and diatreme roots of a strongly contaminated carbonatite melt.

Occurrence of high-grade LREE allanite-pegmatites and calcite carbonatite dykes in the Ediacaran complex of Aghracha, Oulad Dlim massif (South Morocco)

High-grade REE-bearing granitic pegmatites and carbonatite dykes were recently discovered in the Aghracha area, South Morocco. They intruded the northern part of the Ediacaran complex, mainly composed of gneiss. The geochemistry of the basement samples displays compositions ranging from monzonitic to alkaline and sub-alkaline granites with prevailing positive Eu anomalies in chondrite-normalized patterns. These have been attributed to a magma mixing likely derived from the re-melting of underplated mafic rocks and a small amount of crustal rocks. The Aghracha pegmatites are rich in epidote and occur in the Awhifrit prospect in the western part of the basement. The analyzed samples display REE contents up to 17 wt%, mainly related to allanite-(Ce) (up to 13 cm in size), minor monazite-(Ce), and thorianite. The Aghracha pegmatite is referred to here as “allanite-pegmatite” because it exceeds the compositional fields of the classical LCT- or NYF-type classification. Calcite carbonatite dykes occur in the Laachariyat area in the western part of the Ediacaran complex. In addition to calcite, they also contain abundant strontianite, magnetite, barite, and apatite. The main REE-host mineral is monazite-(Ce), although minor bastnaesite and allanite also occur. Both pegmatites and carbonatite dykes strike in an NNW direction, likely associated with an extensional tectonic setting which is typical for the REE-bearing deposits and the Ediacaran complex of the Oulad Dlim massif.

Bastnäsite U-Th-Pb age, sulfur isotope and trace elements of the Huangshui’an deposit: Implications for carbonatite-hosted Mo-Pb-REE mineralization in the Qinling Orogenic Belt, China

The Huangshui’an deposit is hosted in carbonatite dykes and located in the southern margin of the North China Craton which forms part of the Qinling Orogenic Belt. The genesis of Mo-Pb-REE mineralization associated with carbonatitic magmatism and hydrothermal activity remain poorly understood. Here we present results from field investigation, petrography, in situ sulfur isotope and trace elements of pyrite, and bastnäsite U-Th-Pb age with a view to understand the age and genesis of the Huangshui’an deposit. Three types of pyrite are identified in the Huangshui’an deposit as follows: (i) Py1a is composed of irregular pyrite in stage Ia carbonatite dykes; (ii) the euhedral Py1b coexists with sphalerite and galena in the margin of stage Ia carbonatite dykes; and (iii) Py2 occurs as irregular and anhedral grains in the fluorite + quartz veins. The highly variable Co (Py1a = 0.032–3230 ppm; Py1b = 0.053–779 ppm; Py2 = 0.98–164 ppm) and Ni values (Py1a = 2.1–418 ppm; Py1b = 0.78–28.8 ppm; Py2 = 8.6–373 ppm) indicate heterogeneous material sources. The δ34S values of the various pyrite types range between −9.6‰ and −3.5‰, and the obviously negative values resulted from sulfur isotope fractionation between sulfide and sulfate minerals. The total δ34S value calculated from co-genetic galena and barite is −0.11‰, in combination with previous Sr-Nd-Pb-He-Ar isotopes, indicate a material source dominantly from enriched mantle (EMI) with an additional contribution through partial melting of the thickened lower crust. In situ bastnäsite U-Th-Pb dating results yield mean ages of 211.7 ± 3.1 Ma and 209.6 ± 2.1 Ma, representing the timing of Mo-Pb-REE mineralization in the Huangshui’an deposit. The Late Triassic carbonatite magmatism and associated mineralization were controlled by the fertile source from the enriched mantle and the thickened lower crust, and the post-collisional extension setting facilitated the partial melting and fast propagation of carbonatite magma from the mantle to shallow crustal level.

Burbankite and pseudomorphs from the Main Intrusion calcite carbonatite, Lofdal, Namibia: association, mineral composition, Raman spectroscopy

AbstractThe Neoproterozoic Lofdal alkaline carbonatite complex consists of a swarm of carbonatite dykes and two plugs of calcite carbonatite known as the ‘Main’ and ‘Emanya’ carbonatite intrusions, with associated dykes and plugs of phonolite, syenite, rare gabbro, anorthosite and quartz-feldspar porphyry. In the unaltered Main Intrusion calcite carbonatite the principal rare-earth host is burbankite. As burbankite typically forms in a magmatic environment, close to the carbohydrothermal transition, this has considerable petrogenetic significance. Compositional and textural features of Lofdal calcite carbonatites indicate that burbankite formed syngenetically with the host calcite at the magmatic stage of carbonatite evolution. The early crystallisation of burbankite provides evidence that the carbonatitic magma was enriched in Na, Sr, Ba and light rare earth elements. In common with other carbonatites, the Lofdal burbankite was variably affected by alteration to produce a complex secondary mineral assemblage. Different stages of burbankite alteration are observed, from completely fresh blebs and hexagonal crystals through to complete pseudomorphs, consisting of carbocernaite, ancylite, cordylite, strontianite, celestine, parisite and baryte. Although most research and exploration at Lofdal has focused on xenotime-bearing carbonatite dykes and wall-rock alteration, this complex also contains a more typical calcite carbonatite enriched in light rare earth elements and their alteration products.

The Patterson Lake corridor of Saskatchewan, Canada: defining crystalline rocks in a deep-seated structure that hosts a giant, high-grade Proterozoic unconformity uranium system

The Patterson Lake corridor in the Athabasca Basin region of Saskatchewan, Canada, hosts a large-scale uranium system with two major deposits already delineated. The corridor developed in crystalline rocks of the SW Rae Province, which host all of the known uranium endowment. Orthogneisses along with voluminous pegmatites are the hosts of the uranium mineralization. These rocks, however, underwent significant open-system metasomatic–hydrothermal modification. Principal amongst these alterations is early and pervasive quartz flooding that resulted in the development of widespread secondary quartzites and associated rock types. These secondary quartzites and their altered host rocks suffered ductile deformation, typically focused at silicification fronts. Late carbonatite dykes exploited the associated shear zones. Semi-brittle deformation zones nucleated near the previously developed ductile high-strain zones. Graphite and associated iron-sulfides precipitated in a semi-brittle structural regime. These graphitized zones provided the necessary structural architecture to focus the uranium system, which exploited the conduit hundreds of millions of years later at c. 1.425 Ga. Host rocks of the Patterson Lake corridor prove that metasedimentary rocks are not a requirement for the development of giant Proterozoic unconformity uranium deposits. Crustal-scale fault zones with access to the mantle (i.e. carbonatites) should be considered a key parameter in the exploration model for Proterozoic unconformity uranium deposits. Given the similarity of the mineral assemblages in the crystalline basement rocks of the main exploration corridor in the eastern Athabasca Basin region, it is likely that a similar, cryptic geological boundary focused the giant uranium system in that region. Thematic collection: This article is part of the Uranium Fluid Pathways collection available at: https://www.lyellcollection.org/cc/uranium-fluid-pathways

Stratigraphy and U–Pb Zircon–Titanite Geochronology of the Aley Carbonatite Complex, Northeastern British Columbia: Evidence for Antler-Aged Orogenesis in the Foreland Belt of the Canadian Cordillera

The tectonic significance and age of carbonatite intrusions in the central Foreland Belt of the Canadian Cordillera are poorly constrained. Recent work has demonstrated that one of these carbonatite intrusions, the Aley carbonatite, was emplaced as a syn-kinematic sill, coeval with a major nappe-forming tectonic event. Determining the age of the Aley carbonatite thus provides a means of directly dating syn-tectonic magmatism. Attempts at dating carbonatite units failed due to low U–Pb content in sampled zircon; however, a U–Pb titanite age of 365.9 ± 2.1 Ma was obtained from the Ospika pipe, an ultramafic diatreme spatially and genetically related to the carbonatite. This U–Pb titanite age is further supported by respective 40Ar/39Ar phlogopite ages of 359.4 ± 3.4 Ma and 353.3 ± 3.6 Ma for the pipe and a spatially associated lamprophyre dyke. We interpret the Late Devonian U–Pb titanite age of the Ospikapipe to be the minimum possible age of the carbonatite and syn-magmatic nappe-forming tectonic event. The maximum possible age of the carbonatite is constrained by the Early Devonian age of the Road River Group, the youngest strata intruded by carbonatite dykes and involved in the nappe-forming event. Our dating results for the Aley carbonatite closely correlate with U–Pb zircon and perovskite ages obtained for the Ice River carbonatite complex in the central Foreland Belt of the southern Canadian Cordillera, and support the interpretation of carbonatite intrusions of the western Foreland Belt as genetically linked components of an alkaline-carbonatitic magmatic province. Structural, stratigraphic, and geochronological data from the Aley area indicate that deformation was similar in style to, and coeval with, structures attributable to the Antler orogeny, and are consistent with the Antler orogen having extended the length of the Cordilleran margin from the southern United States to Alaska.

Post-Magmatic Fluids Dominate the Mineralization of Dolomite Carbonatitic Dykes Next to the Giant Bayan Obo REE Deposit, Northern China

The Bayan Obo rare earth element (REE) deposit in Inner Mongolia, northern China, is the largest REE deposit in the world, whose mineralization process remains controversial. There are dozens of carbonatite dykes that are tightly related to the deposit. Here we report the petrological and mineralogical characteristics of a typical dolomite carbonatite dyke near the deposit. The dolomite within the dyke experienced intense post-emplacement fluids metasomatism as evidenced by the widespread hydrothermal REE-bearing minerals occurring along the carbonate mineral grains. REE contents of bulk rocks and constituent dolomite minerals (>90 vol.%) are 1407–4184 ppm and 63–152 ppm, respectively, indicating that dolomite is not the dominant mineral controlling the REE budgets of the dyke. There are three types of apatite in the dyke: Type 1 apatite is the primary apatite and contains REE2O3 at 2.35–4.20 wt.% and SrO at 1.75–2.19 wt.%; Type 2 and Type 3 apatites are the products of replacement of primary apatite. The REE2O3 (6.10–8.21 wt.%) and SrO (2.83–3.63 wt.%) contents of Type 2 apatite are significantly elevated for overprinting of REE and Sr-rich fluids derived from the carbonatite. Conversely, Type 3 apatite has decreased REE2O3 (1.17–2.35 wt.%) and SrO (1.51–1.99 wt.%) contents, resulting from infiltration of fluids with low REE and Na concentrations. Our results on the dyke suggest that post-magmatic fluids expelled from the carbonatitic melts dominated the REE mineralization of the Bayan Obo deposit, and a significant fluid disturbance occurred but probably provided no extra REEs to the deposit.

Hydrothermal genesis of Nb mineralization in the giant Bayan Obo REE-Nb-Fe deposit (China): Implicated by petrography and geochemistry of Nb-bearing minerals

The Bayan Obo REE-Nb-Fe deposit, which reserves the current largest REE resources globally, also hosts over 70% of China’s Nb resources. Unlike many world-class carbonatite-related Nb deposits (e.g. Morro dos Seis Lagos and Araxá, Brazil) with igneous or secondary origin, Nb was mainly stored in Nb-bearing minerals (aeschynite, ilmenorutile, baotite, fergusonite etc.) of hydrothermal origin at Bayan Obo, supported by evidence from petrography, element and isotopic geochemistry. Although igneous fersmite and columbite were occasionally discovered in local carbonatite dykes, the Mesoproterozoic and Paleozoic hydrothermal metasomatism occurred in the ore-hosting dolomite, related to carbonatite intrusion and the closure of Paleo-Asian Ocean respectively, has played a more significant role during the ultimate Nb enrichment. REE, however, was significantly enriched during both the carbonatite-related magmatic and hydrothermal processes. Consequently, there was differentiated mineralization between REE and Nb in the carbonatite dykes and the ores. Niobium mineralization at Bayan Obo is rather limited in Mesoproterozoic carbonatite, whereas more extensive in the metasomatized ore-hosting dolomite, and generally postdating the REE mineralization at the same stage. According to mineral geochemistry, Bayan Obo aeschynite was classified into 3 groups: aeschynite-(Nd) with convex REE patterns (Group 1); aeschynite-(Ce) (Group 2) and nioboaeschynite (Group 3) with nearly flat REE patterns. Aeschynite (Group 1), ilmenorutile and fergusonite precipitated from Paleozoic hydrothermal fluids with advanced fractionation of Ce-rich REE minerals. The Mesoproterozoic hydrothermal Nb mineralization, represented by aeschynite (Group 3) and baotite, occurred postdating REE mineralization at same stage. Besides, fersmite and aeschynite (Group 2) precipitated from the Mesoproterozoic REE-unfractionated melt and hydrothermal fluids, respectively. All above Nb-bearing minerals exhibit extreme Nb-Ta fractionation as a primary geochemical characteristic of mantle-derived carbonatite. The forming age of the aeschynite megacrysts (Group 1) has not been accurately determined. However, the potential age was constrained to ~430 Ma or alternatively ~270–280 Ma subjected to subduction and granite activity, respectively. These aeschynite crystals inherited REEs from multiphase former REE mineralization, with an intermediate apparent Sm-Nd isochron age between the Mesoproterozoic and the Paleozoic REE mineralization events.

Carbonatite dyke and related REE mineralization in the Bayan Obo REE ore field, North China: Evidence from geochemistry, C[sbnd]O isotopes and Rb[sbnd]Sr dating

The Bayan Obo deposit in North China is famous for its enormous rare-earth element (REE) resources in the world. The carbonatite dykes occurring around the orebodies are considered to be closely related to the REE mineralization. Here we present new geochemical and CO isotopic data of the ‘Wu’ carbonatite dyke, together with integrated studies of fluid inclusion preserved in the quartzite spatially associated with the dyke and RbSr dating to reveal the dyke genesis and metallogenic process. The dyke has a geochemical signature and C isotopic composition similar to those of typical magmatic carbonatite, suggesting an igneous origin. It is also characterized by significant enrichment in REEs with extreme differentiation between light and heavy REEs, and negative Sr, Zr, and Ti anomalies, indicating that the magma evolved through fractional crystallization. The quartzite surrounding the dyke underwent strong fenitization. The types, microthermometric data and chemical compositions of the fluid inclusions suggest that the fluids preserved in the quartzite have great similarity with carbonatitic fluids, and RbSr dating of these fluid inclusions from the quartzite yielded an isochron age of 368 ± 43 Ma, indicating that there was an early Paleozoic activity of carbonatitic fluids caused by intrusion of the carbonatite dyke. During the early Paleozoic, the Bayan Obo deposit was under a subduction setting. The REE- and CO2-rich fluids/melts were released from subducted sediments during the subduction, and metasomatized and overprinted the pre-existing mineralization hosted in the dolomite, resulting in large-scale REE enrichment and mineralization.

Timing of carbonatite-hosted U-polymetallic mineralization in the supergiant Huayangchuan deposit, Qinling Orogen: Constraints from titanite U–Pb and molybdenite Re–Os dating

The newly-discovered supergiant Huayangchuan uranium (U)-polymetallic (Sr, Se, REEs, Ba, Nb and Pb) deposit is located in the Qinling Orogen, central China. The deposit underwent multistage mineralization, with the main carbonatite ore stage being the most important for the U, Nb, REE, Sr and Ba endowments. According to the mineral assemblages, the main carbonatite ore stage can be divided into three substages, i.e., sulfate (Ba–Sr), alkali-rich U and REE-U mineralization.Main-stage titanite from the Huayangchuan igneous carbonatite are rich in high field strength elements (HFSEs, e.g., Zr, Nb and REEs), and show clear elemental substitutions (e.g., Ti vs. Nb ​+ ​Fe ​+ ​Al and Ca ​+ ​Ti vs. Fe ​+ ​Al ​+ ​REE). High-precision LA-ICP-MS titanite dating yielded a U–Pb age of 209.0 ​± ​2.9 ​Ma, which represents the main-stage mineralization age at Huayangchuan, and is coeval with the local carbonatite dyke intrusion. This mineralization age is further constrained by the Re–Os dating of molybdenite from the Huayangchuan carbonatite, which yielded a weighted mean age of 196.8 ​± ​2.4 ​Ma. Molybdenite Re contents (337.55–392.75 ​ppm) and C-O-Sr-Nd-Pb isotopic evidence of the Huayangchuan carbonatite both suggest a mantle origin for the carbonatite. Our study supports that the Late Triassic carbonatite magmatism was responsible for the world-class U-Mo-REE mineralization in the Qinling Orogen, and that the regional magmatism and ore formation was likely caused by the closure of the Mianlue ocean and the subsequent North China-South China continent-continent collision.

Use of QEMSCAN® to characterize oxidized REE ore from the Bear Lodge carbonatite, Wyoming, USA

• Automated mineralogical analysis (AMA) of a complex REE deposit has characterized variations in ore types due to hydrothermal and supergene processes. • The complexity of the deposit necessitated a mineralogical survey to establish a realistic database of minerals to reference for AMA. • Resulting modes from AMA reconcile well with independent chemical assay data. • When correctly conducted, AMA provides useful data in developing mineral processing flowsheets for metallurgy, even on complex deposits with multiple ore phases. The Bear Lodge REE deposit is located in northeastern Wyoming. The Eocene carbonatite dyke and stockwork system intrudes trachytic-phonolitic rocks that contain multiple diatremes. The original magmatic characteristics of the REE-enriched carbonatite complex are strongly overprinted by carbo-hydrothermal and later supergene fluids. These fluids redistributed the REEs and created high variability in the ore mineral assemblage, as well as compositional variability within individual REE mineral species. The REE ore minerals at Bear Lodge can be classified into four types: 1. fluorocarbonates (bastnaesite, parisite, synchysite), 2. phosphates (monazite, xenotime, florencite, rhabdophane, churchite), 3. cerianite, and 4. ancylite. These minerals vary greatly in abundance, grain size, and morphology. REE distribution is heterogeneous throughout the deposit. Variations within a given REE mineral in terms of Ce depletion, Th content, degree of heavier REE enrichment, etc., create difficulties in the initial definition of discrete mineral species (i.e., by X-ray spectra) and their resultant species identification protocols for use in automated mineralogy (QEMSCAN Ⓡ in this study). Prevalent submicron-scale supergene mineralization result in hybrid spectra from multiple phases. Iterative work in reconciling QEMSCAN Ⓡ data interpretation with bulk assay, XRD, SEM, and optical petrography data allows for refinement of the protocols to quantify for both ore and gangue minerals. Use of automated mineralogy in the development of complex deposits requires rigorous review of these identification criteria in order to achieve results that can be applied with confidence to resolve mineral processing issues.

The role of sulfate-rich fluids in heavy rare earth enrichment at the Dashigou carbonatite deposit, Huanglongpu, China

AbstractThe Huanglongpu carbonatites are located in the north-western part of the Qinling orogenic belt in central China. Calcite carbonatite dykes at the Dashigou open pit are unusual due to their enrichment in heavy rare earth elements (HREE) relative to light rare earth elements (LREE), leading to a flatREEpattern, and in that the majority of dykes have a quartz core. They also host economic concentrations of molybdenite. The calcite carbonatite dykes show two styles of mineralogy according to the degree of hydrothermal reworking, and these are reflected inREEdistribution and concentration. TheREEin the little-altered calcite carbonatite occur mostly in magmaticREEminerals, mainly monazite-(Ce), and typically have ΣLREE/(HREE+Y) ratios from 9.9 to 17. In hydrothermally altered calcite carbonatites, magmatic monazite-(Ce) is partially replaced to fully replaced byHREE-enriched secondary phases and the rocks have ΣLREE/(HREE+Y) ratios from 1.1 to 3.8. The fluid responsible for hydrothermalREEredistribution is preserved in fluid inclusions in the quartz lenses. The bulk of the quartz lacks fluid inclusions but is cut by two later hydrothermal quartz generations, both containing sulfate-rich fluid inclusions with sulfate typically present as multiple trapped solids, as well as in solution. The estimated total sulfate content of fluid inclusions ranges from 6 to >33 wt.% K2SO4equivalent. We interpret these heterogeneous fluid inclusions to be the result of reaction of sulfate-rich fluids with the calcite carbonatite dykes. The finalHREEenrichment is due to a combination of factors: (1) the progressiveHREEenrichment of later magmatic calcite created aHREE-enriched source; (2)REE–SO42–complexing allowed theREEto be redistributed without fractionation; and (3) secondaryREEmineralisation was dominated by minerals such asHREE-enriched fluorocarbonates, xenotime-(Y) and churchite-(Y) whose crystal structures tends to favourHREE.