Right, you can't. Saltpeter is what i think you are referring to.
To use a stragetic resource, you just need a road to it. Mining is not required. This will allow you to use a resource that is outside your city radius'. If you have one inside your radius', you can work the tile and recieve bonus' to it. But the resource must be connected by a road to a city inside your cultural borders in order to be used. And every other city must be connected to that city via roads/harbors/airports in order to use it as well.
If it is outside your cultural borders, you'll need to have a worker make a road to it, and then build a colony. This button will show up when the worker can build it, and it looks like a square with three dots around it. The worker will morph into the colony, and you will not be able to use the worker anymore.
To use a stragetic resource, you just need a road to it. Mining is not required. This will allow you to use a resource that is outside your city radius'. If you have one inside your radius', you can work the tile and recieve bonus' to it. But the resource must be connected by a road to a city inside your cultural borders in order to be used. And every other city must be connected to that city via roads/harbors/airports in order to use it as well.
If it is outside your cultural borders, you'll need to have a worker make a road to it, and then build a colony. This button will show up when the worker can build it, and it looks like a square with three dots around it. The worker will morph into the colony, and you will not be able to use the worker anymore.