Slang for a anal penetration using one's tongue to "mine" for "gold" (I'll let you guess what that means). The practice is commonly done between heterosexual men, but can be done between both sexes.
Guy #1: "Dude, Me and Guy #3 were mining for gold last night." Guy #2: "Ew... wtf." Guy #3: "Bro, my cavern is completely empty now." Guy #2: "Omfg..."