Brent Cook at the Au report has a good way to evaluate this deposit. He says.
Actually, one rule of thumb is the cutoff grade used in estimating the tonnes and grade of a deposit. In theory, that cutoff grade should be what a company or resource estimator projects is the break-even point between making money and losing money. If the cutoff grade is, say, 0.5 grams per tonne (g/t), then I like to see an average grade that is at least twice that, say, over 1 g/t, because above that is where you really make the money, if the cutoff assumption is close. Then you've got to look at metallurgy. This is probably the most important aspect of any deposit once it has been found.
So to be economic the maiden resourse average grade should be at least to times the cutoff grade.
http://www.theaureport.com/pub/na/14904 but he also said ther are no easy numbers