Geochemistry and Petrology of Kangankunde Carbonatite, Chilwa Alkaline Province, Malawi: Implications to Rare Earth Elements Mineralization

ABSTRACT We integrated drill core observations, petrography, whole‐rock geochemistry, and C and O stable isotope analyses of the late Cretaceous Kangankunde Carbonatite to understand its magmatic evolution with respect to its significance for rare earth element (REE) mineralization. The Kangankunde Carbonatite was emplaced as a ring structure and comprises four carbonatite types: beforsite, magnesio‐carbonatite, ferro‐carbonatite, and ferruginous calcio‐carbonatite. The carbonatite is depleted in Nb and Zr relative to mantle values. The abundance of dolomite and apatite in the Kangankunde Carbonatite suggests a Mg‐ and P‐rich initial magma. The bulk SrO contents decrease from magnesio‐carbonatite (0.7–8.8 wt.%), ferro‐carbonatite (0.7–6.9 wt.%), to ferruginous calcio‐carbonatite (0.4–2.0 wt.%). On the other hand, the MnO contents increase from magnesio‐carbonatite (1.8–6.0 wt.%) to ferro‐carbonatite (5.0–10.1 wt.%), while in ferruginous calcio‐carbonatite it ranges from 2.2 to 4.1 wt.%. We suggest two trends of differentiation of carbonatite magma. The first differentiation trend was from beforsite through magnesio‐carbonatite to ferro‐carbonatite. The second trend was possibly from beforsite to ferruginous calcio‐carbonatite. The two identified differentiation trends are consistent with the observed mineral paragenetic sequence. The δ 13 C VPDB and δ 18 O VSMOW values of ankerite and calcite from ferruginous calcio‐carbonatite range from −0.5‰ to −4.0‰ and 14.8‰ to 26.7‰, while those of strontianite from magnesio‐carbonatite are −4.3‰ and 12.0‰, respectively. Assuming that the effect of weathering was minor, the wide variation of stable isotopes is suggestive of the differentiation of carbonatite magma from magnesio‐carbonatite to ferruginous calcio‐carbonate. The main REE‐bearing mineral in Kangankunde is monazite. Minor REE‐bearing minerals include apatite, synchysite, goyazite, and daqingshanite. The REE mineralization in the Kangankunde Carbonatite occurs as dissemination, wispy veinlets, and patchy forms within almost all identified carbonatite rocks, as well as acicular forms in magnesio‐carbonatite, and is classified into primary magmatic and late hydrothermal stages. The primary REE mineralization occurred during magmatic differentiation, evidenced by the occurrence of monazite in interstitial spaces between subhedral dolomite and monazite inclusions in dolomite. The late hydrothermal mineralization is considered localized and is characterized by wispy veins of monazite with spherulitic texture, monazite overprinting dolomite or quartz, and strontianite enveloping primary monazite. Geochemical data show decreasing La/Yb ratios normalized to chondrite from magnesio‐carbonatite (6020), ferro‐carbonatite (5139), to ferruginous calcio‐carbonatite (846). The total REE contents decrease from more evolved magnesio‐carbonatite, which appears relatively hydrothermally altered, to ferro‐carbonatite with similar characteristics, to the less evolved ferruginous calcio‐carbonatite. These petrography and geochemical data suggest that magmatic processes played a significant role in REE enrichment at Kangankunde Carbonatite, while hydrothermal processes and re‐equilibration of REE were possibly localized.