Depth of rich ore bodies are going to vary over the 'ROF' and the deep one's are going to be hard to find and expensive to trace/outline the deposit once found. Hard to blame the players for testing the shallow targets first.

Love to hear from someone who is more of an expert than me as to the difficulty in identifying 'deep' mineralization. Must be harder to make a 'hit' with drill as well even on wide veins or layers.