"Ore grades have fallen from around 12 grams per tonne in 1950 to nearer 3 grams in the US, Canada, and Australia. South Africa's output has halved since peaking in 1970."
http://www.telegraph.co.uk/finance/newsbysector/industry/mining/6546579/Barrick-shuts-hedge-book-as-world-gold-supply-runs-out.html
Shepp