Lüneburger Salz was a salt of extreme purity and for centuries exclusively used around the Baltic sea for conserving fish. From the beginnings on it was constantly mined.
The production was stopped at the end of the second millennium A.D. If it must be a fine German salt you may use Bad Reichenhaller Salz instead which is mined in the Alps. But any fine grained salt will do, I think.

Wilfried