The Fluorapatite P–REE–Th Vein Deposit at Nolans Bore: Genesis by Carbonatite Metasomatism

Abstract

Nolans Bore is a 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 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 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 due 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 crystallise 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 homogenous initial 87Sr/86Sr≈0.7054 and εNd≈-4 at 1525 Ma, the best age estimate of the mineralisation. 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 lead to the formation of the three deposits. The REE-rich compositions of the mid-crustal Nolans Bore fluorapatite are the cumulates hypothesised to cause REE depletion in some unmineralised 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.

Rare earth element mobility in and around carbonatites controlled by sodium, potassium, and silica

“…Therefore, while both LREE and HREE are expected in sodic fenites, we expect potassic fenites to contain the highest HREE/LREE ratios, consistent with observations from nature (22,48,56). Carbonatites intruding siliceous rocks can assimilate SiO 2 , which negates this ore-forming potential by partitioning some (but not all) REEs into early-crystallizing apatite [e.g., at Nolans Bore, Australia (78)] and by loss of alkalis into early-crystallizing silicate minerals (73). Therefore, REE mineralization is confined to the carbonatite body itself (54), and valuable REEs such as Nd and Dy are diluted with the LREE La and Ce.…”

Rare earth mineral diversity controlled by REE pattern shapes

“…This difference becomes pronounced on small scales such as those of a mineral grain, leading to strong decoupling of Y and the lanthanides, and Y no longer forms a coherent part of the REE (e.g. Anenburg et al , 2020b).…”

The Syenite–Carbonatite Complex of Ihouhaouene (Western Hoggar, Algeria): Interplay Between Alkaline Magma Differentiation and Hybridization of Cumulus Crystal Mushes

“…A primary, igneous origin of minerals in the Ihouhaouene carbonatites is further attested by their trace element composition. The chondritic-to-near-chondritic Y/Ho ratio in clinopyroxene (23-30), wollastonite (27-34), apatite (27-32), and calcite (28-40) in both carbonatites and syenites strongly suggests a magmatic origin at high temperature and crystallization over a temperature range evident by the increasing Y/Ho ratios in calcite (e.g., Chakhmouradian et al, 2017;Anenburg et al, 2020). However, while the Si-rich carbonatites contain hedenbergitediopside (Mg# 0.42-0.77-Figure 7) associated with Si-rich apatite and wollastonite (Figures 9,11), the Si-poor carbonatites contain diopsides (Mg# 0.63-0.72) associated with Si-poor apatite.…”

“…Extreme fractional crystallization of alkaline magmas may also culminate in the segregation of carbohydrothermal carbonatites crystallized at subsolidus temperatures from a mixed CO 2 -H 2 O fluid (Woolley and Kjarsgaard, 2008;Mitchell, 2009). In addition, carbonate melts (igneous or unmixed carbonate melt) may be partly counterbalanced by wall-rock assimilation or "antiskarn metasomatism" at high temperature and high pressure, resulting in substantial silica enrichment of carbonatites (Chakhmouradian et al, 2008;Giebel et al, 2019;Anenburg et al, 2020).…”