Mines require a resource (salt, copper, silver, gold) to be build. In the campaign file, for example Early.TXT, you should set the resources for all the provinces in which you want to be able to build mines:

//SetResources:: ID_REGIONNAME RESOURCE1 RESOURCE2

Where Resource1/2 can be either salt, copper, silver, gold or iron (if you want multiple resources you have to do it like this, but you don't need to). Only the first 4 resources are for mines, iron is a requirement for weaponsmiths.

To start a campaign with mines in the provinces with resources you need to add the following in the campaign file:

MakeBuilding:: ID_REGIONNAME RESOURCE_MINE

Where RESOURCE can be SALT, COPPER, SILVER or GOLD.

Happy modding,
Duke John