In "Once Upon a Time," why does Gordimer mention her house's location above a mine here?

The fact that Gordimer's house is located above a mine reinforces the irony portrayed in the story. The tale deals with the irrational fears white people have had of other racial groups during, specifically, South Africa's apartheid era, when people were segregated on the basis of colour. Whites were the most privileged class and lived in wealth and security. Laws were promulgated to ensure that they occupied the best areas and that they were protected from crime.

Furthermore, the pass laws and other similar enactments ensured that the movement of black people was restricted and controlled. The Job Reservation Act also ensured that whites were guaranteed the best employment opportunities whilst the majority of other races were forced into semi-skilled and unskilled labor.

This system guaranteed wealth to the whites and perpetuated poverty among other groups. Workers were exploited and their labor ensured that the whites enjoyed lives of privilege. The wealth and privilege that whites therefore enjoyed was built on the back of especially black labor that was exploited and underpaid, ensuring greater profits and, as a result, greater wealth. The symbolism of the house, in this sense, should thus be obvious. It is a dwelling occupied by a privileged person whilst the mine is an apt symbol for black labor.

Gordimer was a white person who was granted the privileges extended to people of her race. As an activist, she fought against the injustices suffered by the disenfranchised in South Africa. 

As already suggested, the mine in Gordimer's story is a symbol for black labor and the wealth it brought. The migrant labor system ensured a constant stream of black mine workers who were forced by the disadvantageous conditions in the rural areas to leave and work in the mines. They believed that they would then create better lives for themselves and their families. Miners, especially, were underpaid, overworked and lived in atrocious conditions in the mine hostels.

Crime was rife in the hostels and other locations where the miners lived, and there were many faction and gang fights in which many died. Residents were exposed to extremely dangerous conditions in the gold and other mines. Many lost their lives. Therein lies even greater irony—the fear and paranoia should have been reversed. The migrants had much more reason to fear those they served than vice versa.

The irony Gordimer deliberately exposes in the symbolic location of the house is that, as a representative of the privileged class, she is paranoid about her safety and suspects an intruder when, in fact, she should be grateful for the sacrifices made so that she can live where she does.

In addition, the family who is the subject of the story should have felt the same gratitude. Both, however, felt threatened by those who created, and then ensured, their lives of privilege. So, instead of welcoming and being thankful to the ones who brought them such advantage, they turn against them and try to shut them out. The family soon suffer the terrible consequences of their paranoia, unlike the author, who discovers that the ominous sounds she hears are harmless movements in the house caused by shifting.

It is, therefore, both ironic and sad that the same mental shift was never made by the family and they became victims of their own irrational apprehension.