I was reading this Gold Report interview that explains this concept well. It's more about gold than copper but is an interesting read:

TGR: Grade is very important in the overall scope of a deposit. But is it the most important thing when you're looking at drill results?

AR: Grade is king in a deposit. But, in reality, it's the relationship between grade, tons, geometry and depth of the mineralization that indicate if it could potentially be extracted economically. It may well have a high-grade intercept, but it may be so narrow that it's not going to make a deposit, or there may be insufficient tons to justify the capital of a plant and other infrastructure.

TGR: When it comes to gold deposits, what are some mineralized systems you favor?

AR: There are several different types of mineralized systems for gold. The higher-grade ones often tend to be epithermal deposits, known normally as high- or low-sulfur epithermal deposits. Both types can be extremely high grade, but they tend to be narrow zones that can be mined favorably underground. Porphyry systems can have significant amounts of gold, in some instances more gold than copper. They can have billions of tons and still be economic even if it's only 0.5 g/t.

--http://www.theaureport.com/pub/na/13853