Animals, including us, have always been attracted to extra minerals and salt. http://en.wikipedia.org/wiki/Mineral_lick

Also here http://en.wikipedia.org/wiki/Salt

"...A very ancient saltworks operation has been discovered at the Poiana Slatinei archaeological site next to a salt spring in Lunca, Neamţ County, Romania. Evidence indicates that Neolithic people of the Precucuteni Culture were boiling the salt-laden spring water through the process of briquetage to extract the salt as far back as 6050 BC.[9] The salt extracted from this operation may have had a direct correlation to the rapid growth of this society's population soon after its initial production began.[10] The harvest of salt from the surface of Xiechi Lake near Yuncheng in Shanxi, China dates back to at least 6000 BC, making it one of the oldest verifiable saltworks.[11]:18–19..."