A bit of text taken from a generic article on Stok Head,...

A bit of text taken from a generic article on Stok Head, detailing industry grades for different minerals...

"One of Australia’s largest gold/copper projects, Newcrest’s (ASX:NCM) Cadia, is currently extracting less than 0.4% copper equivalent, plus gold at grades of less than 1 gram of gold per tonne (g/t), with probable reserves of 1,400Mt gold and 1,400Mt copper, illustrating how below average grades can be profitable for a large project.

Australia’s second biggest porphyry copper mine after Cadia, CMOC’s Northparkes, has returned above average grades of 0.6% copper and 0.2g/t gold with probable reserves at the project of 103.26Mt gold and 103.26Mt copper."

A really minimal amount of research gives me an idea that something around 0.4% is easily commercial, particularly with Copper costs going up.