Question

Why does Gordimer mention her house being above a mine in the story Once Upon a Time by Nadine Gordimer

Answer 1

Gordimer was granted the privileges extended to people of her race: white. She fought against the terribly unfair treatments in the South. The mine in Gordimer's story is a symbol for black labor. The traveling worker labor system made sure of a constant stream of colored mine workers who were forced to work in bad conditions in areas away from cities and work in the mines. 

Simpler answer: The house sitting on top of the gold mine represents the white culture that was built on the work and labor of the colored servant class from South Africa.

An extra fact is that Gordimer say's her house settles and makes creaking noises even when noone is present as a symbol and foreshadowing.