A few excerpts from a good article:
Since 1998, gold grades of the world's top ten operations have fallen from 4.6 g/t gold in 1998 to 1.1 g/t gold in 2012.
Suddenly, grade seems to be recovering its crown. It remains to be seen whether more high-grade discoveries can actually be made, or whether Peak Gold is actually behind us.
http://www.caseyresearch.com/cdd/peak-gold