How did vast salt deposits develop in the Sahara?

Since the Middle Ages, camel caravans have navigated north from the fabled city of Timbuktu, in present-day Mali, West Africa, in search of the gold of the Sahara desert—salt.
Traveling across the windswept sand dunes, caravans often numbering more than hundred have journeyed to the salt mines of Taudenni, 500 miles (800 kilometers) north of Timbuktu. A human necessity and source of commerce, salt has been in high demand in West Africa since the 12th century when it was first found in the sand dunes of the desert.