Here is a part of Roulston's article from the followjg link:

http://www.kitco.com/ind/Resopp/20120504.html

"In the 1960s, the average grade of a gold mine was 12 grams per tonne. Today, it is 1.3 grams per tonne. That means that nearly 10 times as much rock has to be processed to get the same amount of gold.

These changes have a profound effect on the industry, yet only a small number of people really appreciate the extent of the changes.

Here is an example of the rapid pace of change: 2 years ago, people thought that a gold deposit had to be well over a gram per tonne to have any value. Last year, a mid-tier producer paid a half billion dollars to buy a sub-1-gram per tonne gold deposit in British Columbia. That takeover price was 10 times the amount the company traded at just a year before."