Magmatic-Hydrothermal Processes Associated with Rare Earth Element Enrichment in the Kangankunde Carbonatite Complex, Malawi

Frances Chikanda,Takaomi D Yokoyama,Yoko Ohtomo,Tsubasa Otake,Akane Ito,Tsutomu Sato

doi:10.3390/min9070442

Abstract

Carbonatites undergo various magmatic-hydrothermal processes during their evolution that are important for the enrichment of rare earth elements (REE). This geochemical, petrographic, and multi-isotope study on the Kangankunde carbonatite, the largest light REE resource in the Chilwa Alkaline Province in Malawi, clarifies the critical stages of REE mineralization in this deposit. The δ56Fe values of most of the carbonatite lies within the magmatic field despite variations in the proportions of monazite, ankerite, and ferroan dolomite. Exsolution of a hydrothermal fluid from the carbonatite melts is evident based on the higher δ56Fe of the fenites, as well as the textural and compositional zoning in monazite. Field and petrographic observations, combined with geochemical data (REE patterns, and Fe, C, and O isotopes), suggest that the key stage of REE mineralization in the Kangankunde carbonatite was the late magmatic stage with an influence of carbothermal fluids i.e. magmatic–hydrothermal stage, when large (~200 µm), well-developed monazite crystals grew. The C and O isotope compositions of the carbonatite suggest a post-magmatic alteration by hydrothermal fluids, probably after the main REE mineralization stage, as the alteration occurs throughout the carbonatite but particularly in the dark carbonatites.

Highlights

Rare earth elements (REE) are critical and strategic metals [1] and are becoming increasingly important for industrial use [2], warranting further exploration
By integrating detailed field and petrographic observations with isotope variations (C, O, and Fe), this study aims to (i) characterize the geological–geochemical processes that caused REE enrichment in the Kangankunde Carbonatite Complex, (ii) utilize Fe, C and O isotopes to clarify the stages in the carbonatite evolution and their roles in REE enrichment, and (iii) identify the stage that was most important for REE enrichment
The magmatic and post-magmatic stages that were critical in the enrichment of REE in the Kangankunde carbonatites have been characterized by integrating field and petrographic observations with geochemical and isotopic data

Summary

Introduction

Rare earth elements (REE) are critical and strategic metals [1] and are becoming increasingly important for industrial use [2], warranting further exploration. Among the main types of REE deposits, carbonatites are important exploration targets because of their enrichment in these elements as well as their size and, generally, amenable mineralogy for extraction [3,4]. Minerals 2019, 9, 442 roles in concentrating the critical elements [4,12], Chakhmouradian and Wall [2] suggest that REE enrichment in carbonatites typically occurs during the final stages of the carbonatite’s evolution. During this evolution, overprinting by later magmatic or hydrothermal fluids can cause further

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