This is relevant to Tropico Paradise Island but may also apply to other Tropico versions.
In order for mines to be productive the mine has to be built close to, but not on top of, the yellow area for the minerals. But the mine also has to be built BELOW it. Why? Because the miners move half as fast when they're carrying minerals, speed is a lot slower when moving uphill (and makes them tired quickly) so it follows they will be a lot less productive when carrying minerals uphill.
You know that a mine's not going to be profitable anymore when they start going below the mine for minerals, although gold sells for so much that it can run for maybe ten or fifteen years after that.
Mines typically are profitable for about 25 years. And if you have a huuge mining plot you should build mines on the outside then tear them down when they're not profitable, rebuilding them working your way in. On flat islands only gold mines are worthwhile, and they don't last long.