The gold mines are a great source of income. Make sure, though, that you colonize them until they become a city (700+ people)...that is how you will get the most money out of them. However, you will probably also need to promote the Bailiff and the Chief Legal Counsel in order to get the tax income up enough to build soldiers there. Also, if you have enemies in the area and need soldiers stationed at these cities, build walls because otherwise the attrition rates will be so high that your soldiers will all die.