Because of the high mineralization, several mines have opened in the belt.

The route was designated in 1964, shortly after the mine opened.

The original underground mine opened in 1921, and continued until the mid-1950s.

These mines opened in 1919 about the time the game was developed.

The first mines opened in this area about 1852.

Over the next 10 years five more mines opened in the area, each constructing houses near their own locations.

The open-cast mine opened in 1976 and at one time was face with closure.

The region's first mine opened and within a year 40 tons had been exported.

As the mine opened the New Village was built to house the workers and their families.

What value of N makes it cheaper to the state to keep the mine open?