It is in Azerbaijan and dates to the bronze age.
ScienceDaily (Nov. 27, 2010) — CNRS (1) archeologists have recently provided proof that the Duzdagi salt deposits, situated in the Araxes Valley in Azerbaijan, were already being exploited from the second half of the 5th millennium BC. It is therefore the most ancient exploitation of rock salt attested to date. And, to the researchers' surprise, intensive salt production was carried out in this mine at least as early as 3500 BC.
This work, conducted in collaboration with the Azerbaijan National Academy of Sciences and published on 1st December 2010 in the journal TÜBA-AR, should help to elucidate how the first complex civilizations, which emerged between 4500 BC and 3500 BC in the Caucasus, were organized.
The economic and symbolic importance of salt in ancient and medieval times is well known. Recent discoveries have shown that salt most probably played a predominant role in protohistoric societies, in other words those that preceded the appearance of writing. How is salt obtained? The two most widely used techniques are based on the extraction of rock salt, in other words a sedimentary deposit containing a high concentration of edible salt (2), and the collection of sun-dried salt in salt marshes, for example. Knowledge of the techniques used in former times to exploit raw materials such as salt, obsidian (3) or copper enables archeologists to deduce essential information on the needs and the level of complexity of ancient societies. In the Caucasus, the first traces of... Read more here.