Formation of Rare Earth Deposits in Carbonatites

Michael Anenburg,Wei Chen,Sam Broom-Fendley

doi:10.2138/gselements.17.5.327

Michael Anenburg, Wei Chen + Show 1 more

Open Access

https://doi.org/10.2138/gselements.17.5.327

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Abstract

Carbonatites and related rocks are the premier source for light rare earth element (LREE) deposits. Here, we outline an ore formation model for LREE-mineralised carbonatites, reconciling field and petrological observations with recent experimental and isotopic advances. The LREEs can strongly partition to carbonatite melts, which are either directly mantle-derived or immiscible from silicate melts. As carbonatite melts evolve, alkalis and LREEs concentrate in the residual melt due to their incompatibility in early crystal-lising minerals. In most carbonatites, additional fractionation of calcite or ferroan dolomite leads to evolution of the residual liquid into a mobile alkaline “brine-melt” from which primary alkali REE carbonates can form. These primary carbonates are rarely preserved owing to dissolution by later fluids, and are replaced in-situ by monazite and alkali-free REE-(fluor)carbonates.

Highlights

Most carbonatites are dominated by the four lightest rare earth elements (REEs), of which Nd and Pr are marketable
The Mianning-Dechang REE deposits in China are derived from previously metasomatised subcontinental lithospheric mantle: prior to carbonatite formation, this mantle domain was enriched by REE-rich and CO2-rich fluids that were themselves derived from subducted marine sediments, leading to a final radiogenic isotope signature intermediate between the enriched mantle type 1 (EM1) and enriched mantle type 2 (EM2) end members (Hou et al 2015)
Degassing of solute-poor H2O–CO2 Si activity low, fluid leads to brecciation REEs, Na, K retained in melt Late light rare earth element (LREE) carbonates

Summary

INTRODUCTION

Carbonatites are one of the most prospective rock types on Earth. Almost 10% of carbonatite occurrences host an active or former mine and a further 10% are defined as a mineral resource (Simandl and Paradis 2018). It has been suggested that North American carbonatites, such as Bear Lodge (Wyoming, USA), derive from an asthenospheric mantle plume that was composed of depleted mantle refertilised by a subduction-derived EM1 component (Moore et al 2015) In another example, the Mianning-Dechang REE deposits in China are derived from previously metasomatised subcontinental lithospheric mantle: prior to carbonatite formation, this mantle domain was enriched by REE-rich and CO2-rich fluids that were themselves derived from subducted marine sediments, leading to a final radiogenic isotope signature intermediate between the EM1 and EM2 end members (Hou et al 2015). In a high-Si melt, alkalis are sequestered in silicate minerals, and the REEs predominantly substitute into apatite plus minor REE-fluorcarbonate or monazite crystallisation These high-Si carbonatites tend to have a greater proportion of calcite to dolomite and ankerite, as Mg and.

C Qz Cal

Findings

C Subducting slab