THE PETROGENESIS AND GEOCHEMISTRY OF THE ZANDKOPSDRIFT CARBONATITE COMPLEX, NAMAQUALAND, SOUTH AFRICA

Petrologic and geochemical data for carbonatites and associated alkaline igneous rocks are presented for the Zandkopsdrift Carbonatite Complex, Namaqualand. The samples included in this study are relatively fresh, collected by coring at depths of >70 m below the weathered cap zone. The Zandkopsdrift complex is the only locality in the province known to contain significant carbonatite. The carbonatites studied are calico-, ferro- and silico- carbonatites, based on mineralogy, texture, and major element composition. They have low to moderate Mg-numbers (35-65), variable MgO contents (1.2-8.50 wt.%) and high atomic Ca/Ca+Mg (0.73-0.97), indicating that they are not likely simple mantle melts. The carbonatites contain significant apatite, magnetite, pyrochlore and phlogopite. Zandkopsdrift also contains significant amounts of aillikite and olivine melilitite. These rocks have relatively low SiO₂ (25-31 wt.%) and Al₂O₃ (5.3- 6.1 wt.%), high K₂O (6-6.3 wt.%) and TiO₂ (5.6-9.5 wt.%) and moderate Mg numbers (51-58). δ¹⁸O and δ¹³C isotopes were measured for carbonatites and aillikites. δ¹³C_PDB values are close to those expected for mantle-derived carbonatites (-3.9 to -8.83‰), while the δ¹⁸O_SMOW values are significantly higher (+13. 25 to 21.84‰). The high δ¹⁸O value observed in carbonatites and aillikites is most likely attributable to secondary alteration by hydrous/hydrothermal fluids. This supports the inference that the Zandkopsdrift carbonatite is magmatic in origin but was later affected by secondary alteration which resulted in the elevated O stable isotopes. The ‘mantle-like’ δ¹³C is inconsistent with significant assimilation of C-bearing crustal rocks. Chondrite-normalised REE contents in the carbonatites are 2400 to 10,600 for La and 36 to 170 for Lu. The high REE contents of the carbonatites are most likely due to a combination of a source metasomatised by a highly LREE-enriched agent, as well as significant magmatic differentiation. The relatively fractionated composition of the Zandkopsdrift aillikites and melilitites is also consistent with this hypothesis. We propose that the Zandkopsdrift carbonatites were most likely formed by either immiscible liquid separation from or fractional crystallization of a moderately fractionated, carbonate-rich silicate parental magma.