Carbonatitic versus hydrothermal origin for fluorapatite REE-Th deposits: Experimental study of REE transport and crustal “antiskarn” metasomatism

Abstract

Nolans-type ore deposits contain REE and Th mineralization hosted in fluorapatite veins. These veins intrude granulite facies rocks and are surrounded by a diopside selvage. Nolans-type deposits are thought to form by REE, F and P-rich hydrothermal fluids derived from alkali or carbonatitic intrusions. However, REE are not effectively transported in F and P-rich systems. REE ore deposits are commonly hydrothermally overprinted, possibly obscuring the igneous nature of the primary mineralization. We conducted a series of piston cylinder "sandwich" experiments, testing the hydrothermal fluid hypothesis, and a newly suggested process of carbonatite metasomatism. Our results confirm theoretical predictions that REE are hydrothermally immobile in these systems and the experimental phase assemblage is not compatible with the natural rocks. Our results show that fluorapatite can only host several weight percent levels of REE at temperatures higher than ϳ600°C. Below that temperature, a miscibility gap exists between REE-poor fluorapatite and REE-rich silicates such as britholite or cerite. In contrast, experiments reacting P and REE-rich carbonatite with silicate rock above 700°C closely resemble natural rocks from Nolans-type deposits. Selvage mineralogy is sensitive to the MgO content of the carbonatite. A diopside selvage formed at carbonatite MgO/(CaO؉MgO) Ϸ 0.2 while wollastonite and forsterite formed at lower and higher ratios, respectively. Phosphate solubility in carbonatites decreases with decreasing MgO contents. As diopside formed, REE-rich fluorapatite preferentially crystallized from the selvage inwards. Thus, carbonatites are effective at simultaneously mobilizing REE, F and P to the site of deposition. Nolans-type deposits are the cumulate residue of this reaction, with the carbonatite liquid migrating elsewhere. At temperatures below 700°C the carbonatitesilicate reaction additionally formed monticellite, cuspidine and magnesioferrite, resembling a skarn assemblage. Whereas skarns form by infiltration of silicate magmas or related fluids to carbonate rocks, our experiments are the opposite: intrusion of carbonatite into silicate rock. These mid-crustal skarn-like rocks may host elevated ore elements of carbonatitic affinity, such as F, P, Y, REE, Th, Ba, Sr, and Nb. We propose the term "antiskarn" to describe such systems, and suggest they trace the migration of carbonatite liquids through the crust. Hydrothermal reworking, retrogression, or metamorphism of antiskarns may obscure the carbonatitic genesis of the rocks. These metasomatic zones are the crustal equivalent of wehrlites that form by peridotitecarbonatite reaction at mantle depths.

The Petyayan-Vara Carbonatite-Hosted Rare Earth Deposit (Vuoriyarvi, NW Russia): Mineralogy and Geochemistry

“…Moreover, the crystallization of alkaline feldspars, which are widespread in high-Ti carbonatites, requires abnormally high (for carbonatite melts) SiO 2 activity [9,127]. This high SiO 2 activity is explained by assimilation and/or by superimposed hydrothermal alteration [9,127,128]. This is consistent with the data on Sr and Nd isotope systems in silicocarbonatites [129].…”

“…According to the results of the study of natural materials [122] and the experimental data [123][124][125][126], the low contents of the indicated elements are caused by their partitioning into the silicate part during immiscible separation of carbonatite and silicate melt. Moreover, the crystallization of alkaline feldspars, which are widespread in high-Ti carbonatites, requires abnormally high (for carbonatite melts) SiO 2 activity [9,127]. This high SiO 2 activity is explained by assimilation and/or by superimposed hydrothermal alteration [9,127,128].…”

“…Therefore, rare earth carbonatites are a major focus of critical metal research. Over the past five years, several dozens of papers have been published on the subject of rare earth carbonatites (RE carbonatites) in Angola [5,6], Australia [7][8][9], Brazil [10,11], Canada [12][13][14][15][16][17], China [18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33][34][35][36][37], Finland [38], India [39][40][41][42], Italy [43,44], Malawi [45][46][47][48][49][50][51], Mongolia [52,53], Namibia…”

REE Redistribution Textures in Altered Fluorapatite: Symplectites, Veins, and Phosphate-Silicate-Carbonate Assemblages from the Nolans Bore P-REE-Th Deposit, Northern Territory, Australia

“…We are not aware of any mineral that qualitatively fits this composition and suspect it is a new, unknown mineral. Possibly, it is the britholite-cerite-like mineral reported by Holtstam & Andersson (2007) and synthesized in the hydrothermal experiments of Anenburg & Mavrogenes (2018). Even though some phases seem to be part of the symplectite, their composition is different from britholite.…”

“…The Nolans project is being developed by Arafura Resources for production of REE and P. It was thought to have formed from phosphate-rich hydrothermal fluids or from evolved carbonatitic or alkali associated melts, but no cognate source has been identified (Huston et al 2016). Instead, it has recently been reconsidered as an igneous cumulate that crystallized directly from carbonatitic melt (Anenburg & Mavrogenes 2018). In this scenario, Nolans Bore was initially composed of REE-rich fluorapatite (via britholite substitution: Ca 2þ þ P 5þ REE 3þ þ Si 4þ ), which experienced several episodes of hydrothermal alteration (Schoneveld et al 2015, Huston et al 2016.…”