The Newly Discovered Attu Carbonatite of West Greenland: A Mesoproterozoic Dyke Intrusion Enriched in Sr-Ba-Ree Minerals
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Abstract:
The newly discovered ca. 1.5 Ga old Attu 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 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(-Ce) ((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 grains 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 may exhibit prominent exsolution textures within calcite, burbankite, strontianite and dolomite hosts as well as in the apatite grains. Several different exsolution textures are observed: 1) alstonite in calcite; 2) daqingshanite in calcite; 3) daqingshanite(-Ce) in burbankite; 4) Mg-Ba carbonate in strontianite; 5) Mg-Ba 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 tectonometamorphic event at approximately 1850 Ma. Magmatic monazite(-Ce) crystals from the carbonatite yield a U-Pb age of 1565±53 Ma, which is the current best estimate of dyke emplacement. Monazite(-Ce) that exsolved from apatite and calcite yields a U-Pb age of 1492±33 Ma, which is within the analytical uncertainty of the primary magmatic monazite U-Pb age. However, the U-Pb 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 (uplift?). Dyke emplacement may have occurred within an active ductile shear zone, which would help to explain the foliation of the carbonatite rock, predating cooling-related mineral exsolution. Country rock fenitisation 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.Keywords:
Carbonatite
Carbonatite
Layered intrusion
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Carbonatite
Nepheline syenite
Titanite
Metasomatism
Ilmenite
Rare-earth element
Breccia
Aegirine
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We present in situ major element, trace element, and Sr–Nd isotope data of apatite from an alkaline–carbonatite intrusion in the South Qinling Belt (SQB) to investigate their magma evolution and mantle sources. The Shaxiongdong (SXD) complex consists predominantly of the early Paleozoic hornblendite, nepheline syenite, and subordinate Triassic carbonatite. Apatites from all lithologies are euhedral to subhedral and belong to fluorapatite. Elemental substitution varies from REE3+ + Na+ + Sr2+ ↔ 3Ca2+ in carbonatite and syenite apatite to Si4+ + 2Na+ + 2S6+ + 4REE3+ ↔ 4P5+ + 5Ca2+ in hornblendite apatite. Apatites are characterized by enriched rare earth elements (REEs) and depleted high field strength elements (HFSEs). They record the distinct evolution of their parental magmas. The weak, negative Eu anomaly in hornblendite apatite, together with the lack of Eu anomalies in the bulk rocks, indicates a relatively reduced magma. The Sr–Nd isotope data of the apatite in SXD carbonatite, falling on the East African carbonatite line (EACL) and close to the field of Oldoinyo Lengai carbonatite, indicate that the SXD carbonatite is derived from a mixed mantle source consisting of the HIMU component and subducted sedimentary carbonates. The similarity in Sr and Nd isotopic compositions between the SXD hornblendite and syenite apatites and the early Paleozoic mafic-ultramafic dykes in the SQB suggests that they may share a common metasomatized lithospheric mantle source.
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Rare-earth element
Nepheline syenite
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Columbite
Rare-earth element
Trace element
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Metasomatism
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Carbonatite
Peridotite
Ultramafic rock
Mantle plume
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Radiogenic nuclide
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Metasomatism
Aegirine
Allanite
Phlogopite
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Abstract Mountain Pass is the site of the most economically important rare earth element (REE) deposit in the United States. Mesoproterozoic alkaline intrusions are spatiotemporally associated with a composite carbonatite stock that hosts REE ore. Understanding the genesis of the alkaline and carbonatite magmas is an essential scientific goal for a society in which critical minerals are in high demand and will continue to be so for the foreseeable future. We present an ion microprobe study of zircon crystals in shonkinite and syenite intrusions to establish geochronological and geochemical constraints on the igneous underpinnings of the Mountain Pass REE deposit. Silicate whole-rock compositions occupy a broad spectrum (50–72 wt % SiO2), are ultrapotassic (6–9 wt % K2O; K2O/Na2O = 2–9), and have highly elevated concentrations of REEs (La 500–1,100× chondritic). Zircon concordia 206Pb/238U-207Pb/235U ages determined for shonkinite and syenite units are 1409 ± 8, 1409 ± 12, 1410 ± 8, and 1415 ± 6 Ma (2σ). Most shonkinite dikes are dominated by inherited Paleoproterozoic xenocrysts, but there are sparse primary zircons with 207Pb/206Pb ages of 1390–1380 ± 15 Ma for the youngest grains. Our new zircon U-Pb ages for shonkinite and syenite units overlap published monazite Th-Pb ages for the carbonatite orebody and a smaller carbonatite dike. Inherited zircons in shonkinite and syenite units are ubiquitous and have a multimodal distribution of 207Pb/206Pb ages that cluster in the range of 1785–1600 ± 10–30 Ma. Primary zircons have generally lower Hf (<11,000 ppm) and higher Eu/Eu* (>0.6), Th (>300 ppm), Th/U (>1), and Ti-in-zircon temperatures (>800°C) than inherited zircons. Oxygen isotope data reveals a large range in δ18O values for primary zircons, from mantle (5–5.5‰) to crustal and supracrustal (7–9‰). A couple of low-δ18O outliers (2‰) point to a component of shallow crust altered by meteoric water. The δ18O range of inherited zircons (5–10‰) overlaps that of the primary zircons. Our study supports a model in which alkaline and carbonatite magmatism occurred over tens of millions of years, repeatedly tapping a metasomatized mantle source, which endowed magmas with elevated REEs and other diagnostic components (e.g., F, Ba). Though this metasomatized mantle region existed for the duration of Mountain Pass magmatism, it probably did not predate magmatism by substantial geologic time (>100 m.y.), based on the similarity of 1500 Ma zircons with the dominantly 1800–1600 Ma inherited zircons, as opposed to the 1450–1350 Ma primary zircons. Mountain Pass magmas had diverse crustal inputs from assimilation of Paleoproterozoic and Mesoproterozoic igneous, metaigneous, and metasedimentary rocks. Crustal assimilation is only apparent from high spatial resolution zircon analyses and underscores the need for mineral-scale approaches in understanding the genesis of the Mountain Pass system.
Carbonatite
Dike
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Carbonatite
Baddeleyite
Metasomatism
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In Mongolia, rare earth element (REE) mineralization of economic significance is related either to the Mesozoic carbonatites or to the Paleozoic peralkaline granitoid rocks. Carbonatites occur as part of alkaline silicate-carbonatite complexes, which are composed mainly of nepheline syenites and equivalent volcanic rocks. The complexes were emplaced in the Gobi-Tien Shan rift zone in southern Mongolia where carbonatites usually form dikes, plugs or intruded into brecciated rocks. In mineralized carbonatites, REE occur mainly as fluorocarbonates (bastnäsite, synchysite, parisite) and apatite. Apatite is also present in the carbonatite-hosted apatite-magnetite (mostly altered to hematite) bodies. Alkaline silicate rocks and carbonatites show common geochemical features such as enrichment of light REE but relative depletion of Ti, Zr, Nb, Ta and Hf and similar Sr and Nd isotopic characteristics suggesting the involvement of the heterogeneous lithospheric mantle in the formation of both carbonatites and associated silicate rocks. Hydrothermal fluids of magmatic origin played an important role in the genesis of the carbonatite-hosted REE deposits. The REE mineralization associated with peralkaline felsic rocks (peralkaline granites, syenites and pegmatites) mainly occurs in Mongolian Altai in northwestern Mongolia. The mineralization is largely hosted in accessory minerals (mainly elpidite, monazite, xenotime, fluorocarbonates), which can reach percentage levels in mineralized zones. These rocks are the results of protracted fractional crystallization of the magma that led to an enrichment of REE, especially in the late stages of magma evolution. The primary magmatic mineralization was overprinted (remobilized and enriched) by late magmatic to hydrothermal fluids. The mineralization associated with peralkaline granitic rocks also contains significant concentrations of Zr, Nb, Th and U. There are promising occurrences of both types of rare earth mineralization in Mongolia and at present, three of them have already established significant economic potential. They are mineralization related to Mesozoic Mushgai Khudag and Khotgor carbonatites in southern Mongolia and to the Devonian Khalzan Buregtei peralkaline granites in northwestern Mongolia.
Carbonatite
Peralkaline rock
Nepheline syenite
Rare-earth element
Petrogenesis
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