Ann: Ammonium Sulphate Test confirms presence of IAC REE, page-67

Firstly, I have to say that I would not pass the CP test for REEs so if other geos have a different view point, please comment. Clay hosted REE deposits are formed by weathering of a body of rock, in most cases this is a felsic, in BBXs case, this is a rhyolite. So the REEs are already contained within the rhyolite, then during weathering, the rhyolite breaks down allowing the REE to concentrate and bond to the clays within the regolith/laterite profile. Weathering isn't uniform, particularly in tropical regions so its possible that this is the cause of the variance in recoverable REE, although I'd have thought going from <1% to 50% would be odd.

So your question about it being a different mineralisation style is slightly off target, clay hosted REE deposits aren't an overprinting like a classic gold deposit. A difference might be that its a different rhyolite, hence a different source rock, but thats also unlikely given the usual deposition of a rhyolite.