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As has been mentioned before, they are investing a lower % of there money here and are spending less on exploration.
Compare Anglos holdings from back in 2000 to 2012 and see what you find.
The Oppenheimers just are gatvol of SA, that's why they sold their stake.
As has been mentioned the lower %age invested is largely due to the operating costs involved with deep mining compared to other countries where the resources are closer to the surface. That is a normal investment decision.
The Oppenheimers were not gatvol of South Africa. They simply lost control of De Beers when the cross holding was unbundled. And I imagine they felt it was better to have control of whatever device their billions are in.
But your narrative of South Africa being the problem seems to work better for you.