The salt mine is an occidental mine building that produces brine, which is a raw material needed by the salt works, along with coal, to produce salt.
Salt is required by the tannery, butcher's shop, and furrier's workshop.
The cost for a salt mine is eight hundred gold coins, eleven wood, and four tools. Salt mines must be built on brine deposits.
One salt mine produces four tons of brine a minute. That's enough brine to supply one salt works.