My first guess would be some sort of chemical poisoning from the mine. Natural uranium is effectively inert in terms of radiation, but it is still a heavy metal

My first guess would be some sort of chemical poisoning from the mine. Natural uranium is effectively inert in terms of radiation, but it is still a heavy metal and is pretty chemically toxic (in metallic form, I don't know how toxic the ore is). Or some leftover chemical from the mining. Or some gas leaking from underground. Or other radiation sources that are found near uranium (e.g. radon). But the article says they tested all that. Weird weird weird.