Minimum viable depends on many factors, but in simplistic terms...

Minimum viable depends on many factors, but in simplistic terms the minimal viable grade for an ionic clay is driven by the basket price, which is the composition of LREE and HREE which make up the basket.

Looking at the average distribution based on todays ann:




Dy, and particularly Tb are your high value add elements. NdPr fetches a fraction.

The biggest problem all these explorers face is not really the grade nor the composition, it's the cost associated with leaching these elements out (because most are not true ionic clays).

The IXR DFS is probably worth a skim through to get an idea of a 'lower' grade ionic clay resource (where 'lower' grade compared to Serra Verde, MEI, Aclara). SF2TH has done some calcs on the threads over there which discuss the minimum viable level for IXR specifically to be profitable based on rare earth prices.