As I understand it, every time they roll the dice for the discovery, they roll it again to see "is this the last discovery"
You got really lucky. In my current game, they discovered accessibility increases on mars-- from 0.1 to 0.8 for duranium...-- so a little corundium mine became a functionally infinite source of corundium, duranium, with plenty of other bonus minerals to keep my industry happy for the first two game decades.