Wall rock contamination and mineralogical modifications in carbonatite dykes of the Palabora Complex, South Africa

Contamination of carbonatite melts is often neglected due to a fast magma ascent and low liquidus temperatures. However, increased silicate mineral formation observed in numerous carbonatite occurrences world-wide requires an external Si introduction. Our study demonstrates that carbonatite dykes penetrating different lithologies of Palabora (South Africa) shows different modes of mineralogical modification. In particular Al and Si-rich lithologies show the most significant effects. Besides silicate mineral formation Si introduction may cause directly and indirectly variations of the REE mineralization at different stages of the carbonatite emplacement. While Si introduction during apatite formation causes an increased REE incorporation into apatite due to the britholite substitution accompanied by an early consumption of REE from the melt, an REE enrichment in the melt and related specific REE mineral formation in late magmatic stages become inhibited. A Ti-rich carbonatite magma additionally experiences the formation of titanite at the expense of ilmenite. Although REE consumption by titanite is less important as for apatite, specific REE consumption can influences REE patterns of subsequent mineralizations. On the other hand, magma wall rock interactions in a carbonatitic systems may furthermore directly influence the type of REE mineralization reflected by discrete REE minerals. In this way contamination can directly control the formation of either allanite, britholite, chevkenite or monazite and hence influences the economic processibility of a REE deposit. Furthermore, the stability of HFSE minerals such as baddeleyite or thorianite can be suppressed by the predominance of their Si-bearing counterparts (e.g., zircon and thorite).