Crustal contamination and rare earth element enrichment in the Nooitgedacht Volcano, South Africa

Carbonatites are igneous rocks with significant concentrations of rare earth elements (REEs) and other important metals. Despite their economic potential, their origin and evolution are poorly understood. Crustal contamination can significantly modify REE abundance in carbonatites. The current study provides new petrographic, geochemical, and stable isotopic insight into the processes that led to the formation of high field strength element (HFSE) bearing carbonatites i.e. pyrochlore-calcite carbonatite, and rare earth element (REE) bearing carbonatite i.e. quartz-ankerite-calcite carbonatite in the Nooitgedacht Volcano.

The Nooitgedacht Volcano is an oval-shaped body with approximately a 3 km diameter situated in the Kaapvaal Craton in South Africa. It comprises mostly calcite carbonatite with enclosures of dolomite carbonatite. Petrographically, calcite carbonatite is divided into pyrochlore-calcite carbonatite and quartz-ankerite-calcite carbonatite. The mineral assemblage of pyrochlore, apatite, magnetite, and forsterite indicate an ortho-magmatic origin while the mineral association of monazite, ankerite, and quartz represents a post-magmatic/ hydrothermal origin for the quartz-ankerite calcite carbonatite. Stable isotope data (δ¹³C = -5 to -4) and (δ¹⁸O = +7 to +16) indicate a primary mantle source for the carbonatite with subsequent crustal contamination.

In summary, petrographic, mineralogical, geochemical, and isotopic data provide significant insight into the evolution of the Nooitgedacht Volcano, highlighting a complex history of magmatic differentiation, mineral fractionation, and crustal contamination.