If a mill was built where it would service several mines, and we all know that the mills wanted to keep busy all the time, how was mine X ore kept separate from mine Y as the batches were milled?
I know that mine X ore would be processed as a batch, that's common sense, but at the end of the batch I'd bet that mine Y ore would be dumped at the top for processing. There's bound to be some mixing of ores along the way.
Or was that just a "fact of life" and the mixing wasn't that great to be worried about.