When that one mine runs out of salt? It will be closed. We as a humanity will not run out of salt, some places have the opposite problem: https://en.wikipedia.org/wiki/Monte_Kali
"According to the Werra Potash Mining Museum in Heringen, Monte Kali has been in operation since 1976; as of August 2016, it covered 98 hectares (240 acres) and contained approximately 201 million tonnes of salt, with another 900 tonnes being added every hour and 7.2 million tonnes a year."
It is apparently contaminated/not pure enough and refining it isn't financially viable compared to mining it from more pure deposits (according to the german wikipedia page)
"According to the Werra Potash Mining Museum in Heringen, Monte Kali has been in operation since 1976; as of August 2016, it covered 98 hectares (240 acres) and contained approximately 201 million tonnes of salt, with another 900 tonnes being added every hour and 7.2 million tonnes a year."