The quote below is from Rick Mills:

"The metal content of copper ore has been falling since the mid 1990s. A miner now has to dig up an extra 50 percent of ore to get the same amount of copper. As grade drops the amount of rock that must be moved and processed per tonne of produced copper rises dramatically - all the while using more energy that costs several times more than it use to. With the lower grades of ores now being mined energy becomes more and more of a factor when considering economics."

Full article: http://www.mineweb.com/mineweb/view/mineweb/en/page31?oid=151728&sn=Detail&pid=31