[gcb]

> (Mount Weld) has high concentrations of rare earths, is a huge site, and has low levels of thorium contamination.

just a guess, the thorium was not what they were extracting there -- otherwise they wouldn't have left it behind. Maybe it was moved there to be used to extract(?) some of the other more valuable heavy metals? ...and then discarded along with several other by-products the as cheaper as possible.