I'll have to look up Archean deposits. I confess I learn my geology as I go along.

As I understand it, different types of deposits are found in different types of surrounding rocks. This can have a big effect on the amount it costs to process/ get the gold out. Some strata, for example, are really amenable to being heap-leached with acid. Low grade porphyries are popular because they can be mined easily, even though the concentrations of gold or moly are usually low.

With some rocks, it is virtually impossible to extract the metal. For example, last fall Barrick Gold was offering a multimillion dollar prize to any scientist worldwide who could find an economical way to extract the silver contained in one of their gold mines in South America.

I suspect that if one type of deposit is less favored, it may traditionally be more expensive to mine. But then, mining processes change and make what was uneconomical 20 years ago suddenly a viable mine.