@naaman ... "Those aren't the percentages. SUM is in Paraiba."...

@naaman ... "Those aren't the percentages. SUM is in Paraiba." ??? What are you going on about? I quoted off the document YOU provided...

The only mention that I could find of approximate 'grade' was in this bit .....

"About 1 kg of concentrate per cubic meter is recovered from weathered rock that may contain as much as 68 percent cassiterite, 15 percent tantalite-columbite and 10 percent microlite (djalmaite). "
pg 28
https://pubs.usgs.gov/of/1975/0029/report.pdf

If you think my percentages are 'out', then please explain where with some numbers. It's very clear they took 1kg of concentrate from 1 cubic metre of weathered rock. 1 cubic metre of weathered rock is likely to be a lot heavier than 1 cubic metre of water (1000kg). The text is then very clear that of the concentrate 15% is tantalite-columbite.

I suggest everyone reads this for themselves!!

BTW it's a very interesting read on the Niobium and Tantalum Resources of Brazil, so thanks for the link..