Faults are an important part of creating the dilutional areas...

Faults are an important part of creating the dilutional areas (weakness and lower pressure) where carbonatite magma can find its way to the surface (ie moving from high pressure to low pressure as do all fluids). It's not surpriosing that carbs form at or around crustal scale faults that run deep enough to provide a pathway to reach the surface. Carbs usually intrude to the surface from very deep in areas of tectonic extension, where the crust is pulling way from itself as opposed to areas of compression where they are pushing up against itself. Carbs may start off deep rising up major faults zones but then push there way up in-between faults nearer the surface, so carbs don't have to be on or even near the surface expression of major faults necesarily, but often are (like west Arunta).


The 'enrichment' in valuable volatiles like REO and Nb is the tricky part. The highest enrichment (thus economic) in carbs is usually due to late-stage smaller carb intrusions within larger intrusions, and/or hydrothermal enrichment in late-stage events within a carb complex, often multiple times. Maybe events scavenge REO and Nb from the pre-excising carb and deposits those elements higher up in the carb creating even higher concentrations. All sorts, and in the end no geo can predict which carbs will be enriched with what elements, to what concentrations, where. It's a lucky dip...