The Fluorapatite P–REE–Th Vein Deposit at Nolans Bore: Genesis by Carbonatite Metasomatism
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Résumé
Abstract Nolans Bore is a rare earth element (REE) ore deposit in the Reynolds Range, Aileron Province, Northern Territory, Australia. It consists primarily of fluorapatite and alteration products thereof, surrounded by a diopside-dominated selvage. Previously considered to form via hydrothermal fluids, we now suggest that the deposit formed by a metasomatic reaction between a mantle-derived carbonatite and granulite-facies felsic host rocks, after peak metamorphism. REE patterns of fluorapatite are strongly light REE (LREE) enriched, convex with maxima at Ce to Nd, and contain a weak negative Eu anomaly. Textural and geochemical properties of the fluorapatite are consistent with its formation from a carbonatite liquid. Sinusoidal REE patterns in diopside along with strong Yb–Lu enrichment relative to coexisting titanite are suggestive of derivation from a Ca-rich carbonatite. Likewise, hyalophane present in the selvages forms by reaction of a BaCO3 component in the carbonatite with K-feldspar in the silicate host rocks. The overall morphology of Nolans Bore is consistent with carbonatite–silicate reaction experiments, with the carbonatite itself migrating elsewhere owing to the open-system nature of Nolans Bore. Ekanite veins in massive fluorapatite zones and allanite–epidote crusts on fluorapatite in contact with the diopside selvages formed by hydrothermal fluids exsolved from the carbonatite. Minor interstitial calcite was not igneous but was the last mineral to crystallize from the carbonatite-exsolved fluid. Y/Ho ratios qualitatively trace the transition from mantle-dominated igneous minerals to later low-temperature hydrothermal minerals. Rb–Sr and Sm–Nd analyses of unaltered minerals (fluorapatite, allanite, calcite) show that the carbonatite had homogeneous initial 87Sr/86Sr ≈ 0·7054 and εNd ≈ –4 at 1525 Ma, the best age estimate of the mineralization. Fluorapatite–allanite Sm–Nd dating results in an age of 1446 ± 140 Ma, consistent with forming soon after the end of the Chewings Orogeny. Neodymium depleted mantle model ages are older than 2 Ga, indicating the presence of recycled crustal material within the source. We suggest that the carbonatite was sourced from a mantle enriched by subduction of LREE-rich oceanic crustal rocks, marine sediments, and phosphorites, potentially from the south, or the Mount Isa area to the east. Nolans Bore represents the root zone of a now-eroded carbonatite. Other Nolans-type deposits (Hoidas Lake, Canada and Kasipatnam, India) are similarly hosted within siliceous granulite-facies rocks in regions with a long tectonic history, suggesting common processes that led to the formation of all three deposits. The REE-rich compositions of the mid-crustal Nolans Bore fluorapatite are the cumulates hypothesized to cause REE depletion in some unmineralized carbonatites. The rocks at Nolans Bore demonstrate that carbonatites, previously thought to be mostly unreactive, can undergo modification and modify the composition of the silicate rocks which they encounter, forming an ‘antiskarn’. At igneous temperatures, the resulting mineral assemblage (other than fluorapatite) consists of diopside and titanite, both of which are common in granulite-facies rocks. Therefore, carbonatite metasomatism can remain unnoticed if the resulting assemblage does not contain distinctively carbonatitic minerals.
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