Heavy lateritic weathering over long time oxidises and leaches...

Heavy lateritic weathering over long time oxidises and leaches minerals in the carb to new minerals, and water percolating down into ground removes lots of dissolved elements like Ca, Mg etc. When weathering is heavy enough, it will even dissolve the resistant minerals like pyrochlore which host most of the niobium in the fresh carb rock. So the upper part of oxidised weathered profile is often barren of Nb because it has been leached away also...

Deeper down the pyrochlore has not yet been leached away, so Nb grades are higher. Added to this is the enrichment effect of increasing the grade of Nb in deeper weathered clays vs fresh rock because the Ca, Mg etc has still been dissolved so the remaining Nb-pyrochlore is higher in grade by weight in the remaining ore. Add to that Nb that was dissolved up in the leached zone that precipitates back out into new Nb-minerals at some 'redox front' deeper down towards the semi=fresh rock and water-table, and the Nb grades will be even higher than fresh carb rock again (ie supergene enrichment).

'Recovery' of Nb to a concentrate that can be processed into saleable Nb-products is determined by the Nb-minerals and their size ultimately. Many high grade Nb deposits are uneconomic because Nb is hosted in the wrong Nb-minerals at too small size to be practically or economically recovered into a concentrate...

Fresh rock carb will have completely different minerals to oxidised supergene ore, which means different floatation reagents and processes to concentrate the primary Nb minerals. On the upside, primary carb minerals are usually larger, simpler, and thus more easily and economically recovered. The downside is that Nb grades are always lower in bulk than the upper supergene oxide ore...

GLTAH