The magmatic to post-magmatic evolution of the Nooitgedacht Carbonatite Complex, South Africa

Abstract

Carbonatites are unusual, mantle-derived igneous rocks characterized by distinctive mineralogical and geochemical fingerprints that may decipher complex processes during their multi-stage evolution. A systematic petrographic, whole rock geochemistry, apatite and pyrochlore mineral chemistry, U-Pb geochronology, and C-O stable isotope study was performed on the carbonatites of the Nooitgedacht Carbonatite Complex in the Republic of South Africa in order to understand their genesis and evolution including the significant differences in the nature of their HFSE-REE endowment. Early carbonatite intrusion is reflected by a plug-like calcite carbonatite. A second stage carbonatite intrusion formed a ring-shaped, quartz-Fe-dolomite-calcite carbonatite comprising apatite and monazite. The third stage of carbonatite intrusion formed dolomite carbonatite dikes. The calcite carbonatite exhibits a well-developed magmatic stage characterized by the presence of oscillatory zoned, F-rich pyrochlore, pill-shaped apatite, magnetite and forsterite, while the hydrothermal stage is indicated by the presence of quartz, baryte, massive apatite, bastnäsite, and monazite. Although the second carbonatite generation is very similar to the first generation in terms of mineralogy, the hydrothermal overprint in particular is much more pronounced in the later. The third carbonatite generation, however, differs significantly from its two predecessors mineralogically and does not show any hydrothermal overprint. Whole rock ΣREE concentrations in early stage carbonatite range between 380 and 2200 ppm, the quartz-Fe-dolomite-calcite carbonatite shows a higher ΣREE concentrations between 3010 and 5900 ppm, while the third stage dolomite carbonatite has ΣREE concentrations between 574 and 2239 ppm. This trend follows a positive correlation with the degree of alteration, as REE mineralization occurs mainly in the hydrothermal post-magmatic stage. Pyrochlore observed in early calcite carbonatite shows oscillatory zoning overgrowing U-Ta rich resorbed cores (entrained from associated syenite). Pyrochlores observed in quartz-Fe-dolomite-calcite carbonatites exhibit relicts of fluorcalciopyrochlore (magmatic stage) overgrown by hydroxycalciopyrochlore (hydrothermal stage: characterized by patchy zonation) and hydropyrochlore (supergene stage: mainly characterized by narrow rims around the pyrochlore grains). Textural and compositional variations in apatite further allow tracking the evolution of Nooitgedacht carbonatites. Pill and ovoidal shaped apatites from calcite carbonatite and quartz-Fe-dolomite-calcite carbonatite, respectively, with marginal higher Si and less F concentrations shows a magmatic origin, while irregular apatite masses from calcite carbonatite and quartz-Fe-dolomite-calcite carbonatite (with higher Sr and F concentrations) suggest a hydrothermal origin. However, the dolomite carbonatite (third intrusion stage) lack both, apatite and pyrochlore. The in-situ U-Pb geochronology in calcite carbonatite (apatite and pyrochlore ~1330 Ma) and quartz-Fe-dolomite-calcite carbonatite (apatite ~1280 Ma) suggested two different magmatic events. The two different ages of carbonatite intrusion, furthermore, suggest the reuse of the same path way system during the carbonatite melt ascent. Stable isotope compositions for early stage carbonatite plot within the primary igneous carbonatite field, while the compositions for quartz-Fe-dolomite-calcite carbonatite and dolomite carbonatite follows a trend from lower to higher values either due to contamination/wall rock interaction during ascent or due to Rayleigh fractionation. Based on the above data, we proposed a magmatic to post-magmatic (hydrothermal/supergene) evolution (including a ~50 Ma gap between the early and the later carbonatite) with enrichment of REE in the later stage carbonatite due to hydrothermal activity.