Not exactly a mine, but this was how early Hawaiian settlers got their salt. Suitably cupped rocks were filled with saltwater. The hot sun evaporated the water leaving behind salt crusts on the rocks. In this instance, the water in the bowls is probably rainwater, hence the lack of any salt residue.
These rocks were at Lapakahi State Historical Park, which contains the remains of an old Hawaiian fishing village.
Graham's Island 
Great bit of living history 🙂
LikeLiked by 1 person
It’s an interesting place. I’ve enjoyed walking down there more of late.
LikeLiked by 1 person
It just takes a bit of ingenuity. I bet they could fry eggs on the rocks too.
LikeLiked by 2 people
It does get pretty hot down there on the coast!
LikeLiked by 1 person