Geochemistry and origin of high-Sr carbonatite from the Nuba Mountains, Arabian-Nubian Shield, Sudan

Abstract

Abstract Carbonatite from the Arabian-Nubian Shield of Sudan occurs as dykes in the Nuba Mountains. It is composed of calcite with some feldspars, quartz and fluorite. CaO is the major constituent in this carbonatite and accordingly, it is classified as calico-carbonatite. The studied carbonatite shows exceptionally high concentrations of SrO (4.4 to 5.9 wt%). Ba, Pb and Y occur in relatively higher concentrations compared to other trace elements. Concentration of rare earth elements (ΣREEs) is relatively low (average 1550 ppm) compared to many primary igneous carbonatites. The chondrite-normalized REE patterns display higher light rare earth elements (LREEs) compared to heavy rare earth elements (HREEs) with slight negative Ce/Ce* and Eu/Eu* anomalies. The δ18OV-SMOW values range between 7.48 and 10.05‰, while δ13CV-PDB values vary from −6.24 to −7.38‰, which is close to the primary carbonatites values. Occurrence of carbonatite as dykes with cumulate and triple junction textures, plot of the carbonatite in the true carbonatite fields of the Ba-Sr and Ba + Sr-REE + Y diagrams, igneous-derived δ13CV-PDB and δ18OV-SMOW values and high (La/Yb)N ratios indicate its primary igneous origin. The strong positive correlation between REEs and Sr suggests the occurrence of these elements as secondary strontianite, which was confirmed by SEM and EDX analyses. This might indicate that the enrichment of REEs and Sr in the studied carbonatite is not from the primary magma and most probably took place during a sub-solidus metasomatic process after the carbonatite emplacement.