Petrological, geochemical and mineralogical characteristics of Wigu Hill carbonatite, Uluguru Mountains, Tanzania: insights into carbonatite evolution and REE mineralization

The Wigu Hill Carbonatite, located south of Uluguru Mountain, is amongst the REE-endowed carbonatites in Tanzania. The carbonatite comprises apatite dolomite carbonatite that has been locally brecciated and intruded by small bodies of mica dolomite carbonatite. These early carbonatites are fine to medium grained, poorly enriched in REE₂O₃ (< 0.4 wt%), and show elevated Nb (> 200 ppm). The early carbonatites are crosscut by REE-bearing carbonatite dikes that host pegmatitic, well-preserved pseudomorphs after burbankite. The REE-bearing carbonatites are characterised by high REE₂O₃ (6–10 wt%), and pseudomorphs that vary in colour and mineralogy, reflecting the dissolution of primary burbankite through reaction with evolving carbothermal fluids in two major phases; (1) early altered burbankite formed yellow-colored pseudomorphs typified by an assemblage of synchysite-(Ce) with high (La/Ce)_N - (La/Nd)_N ratios, barite, Ca-strontianite, calcite, and quartz; and (2) subsequent late alterations by highly evolved fluids resulted into green and pink-colored pseudomorphs consisting of synchysite-(Ce) with low (La/Ce)_N - (La/Nd)_N ratios, barite, Ca-strontianite, fluorite, calcite, quartz, apatite, monazite, and Al-REE-phosphates. The stable C-O and Mg isotopes signatures of dolomite across Wigu Hill indicate a pristine mantle source (δ¹³C_VPDB -4.1‰ to -6.2‰; δ¹⁸O_VSMOW +6.5‰ to + 7.31‰ and δ²⁶Mg -0.44 to + 0.19‰), and are locally modified by surface processes which resulted in bastnaesite enriched zones with up to 15 wt% REE₂O₃. Textural, geochemical, and stable isotope data tracks a polygenetic evolution of Wigu Hill with the main REE mineralization phase occurring at the end of magmatic phase as a result of magmatic fractionations. Reworking by carbothermal fluids and locally by surface process has resulted in REE enrichment.