Here is something with technological parallels to potential operations at Hay Mountain.

https://www.mining.com/web/newcrest-eyes-deeper-mines-smaller-footprint-with-block-caving/

This is precisely what happened at Superior, Arizona where a massive and high-grade copper deposit was discovered far below the closed Mammoth mine there, and wherein two of the world's largest miners are partners in Resolution Copper which is under development.

By the way, the copper at Superior, Arizona is along the same fissure and mineral trend as is found at Johnson Camp, Tombstone, Bisbee, and on into Mexico. 

VP