Magmatic and post-magmatic geochemistry of phosphorus and rare earth elements in carbonatites.
This study documents the magmatic, hydrothermal and supergene mineralogy and
geochemistry of phosphorus and rare earth elements in carbonatite complexes using
examples from Tundulu (Malawi), Sokli (Finland), Siilinjarvi (Finland) and Kaluwe
(Zambia). In carbonatites, phosphorus averages 1-2% P20S and forms the minerals
fluorapatite and monazite. Hydrothermal and supergene processes enrich fluorapatite in Na
and REEs through vitusite-type exchanges which lead to formation of vitusite, belovite and
britholite; and in CO2 through anti-francolite substitutions.
The highest rare earth element contents are found in late-stage ankerite carbonatites or
similar rocks of low temperature origin (T < S(XtC) and in hydrothermally altered rocks,
where they occur mainly as fluorocarbonates or carbonates. Such minerals are consistent with
the REEs having been transported in form of mixed fluoride-carbonate complexes. The
mineral paragenesis in hydrothermal veins suggests that different fluorocarbonates
precipitated depending on the activity of Ca supplied to the fluid by the wall rocks. The
various minerals are modelled to form by simple combinations of calcite (CaCO:v and
bastnaesite (REEC03F) molecules. A secondary characteristic feature of these reactions is
that extreme heavy rare earth enrichment occurs if the wall rocks are apatite-rich.
Petrogenetic modelling using REEs suggests that carbonatites are unlikely to be derived from
carbonated silicate magmas by fractional crystallisation or liquid immiscibility. These
findings are supported by ex solution temperatures of about 9S0·C recorded using the calcitedolomite
geothermometer for quenched lapilli from the Kaluwe carbonatite.