Couple of things caught my eye in this report: https://www.bcgperspectives.com/content/articles/metals-mining-tackling-crisis-mineral-exploration/?chapter=2

"The number of giant discoveries, which historically have accounted for the vast majority of the world’s metal supplies, has tapered off to only one or two per year despite the massive increase in exploration spending from 2010 through 2012. The number of major discoveries has also dropped sharply, underscoring the worsening of exploration performance around the world. Even if some recent major discoveries ultimately mature to giant status, major discoveries on the whole will still have declined significantly."

"In addition, throughout the industry it has been widely believed that most of the easy pickings—the world’s outcropping and shallow ore bodies—have already been discovered." - Recalls to mind JB's excitement with regard to Hay Mountain's gossans.