Glittering and mysterious mines give you gold nuggets; after you collect enough, you’ll be able to convert them to gold.
In addition, they give you mysterious ore that you can refine for more ore on top of the regular ore you’ll get. You need a Collier Noble to gather resources in the Mysterious Ore Mines. They must also have the Tier 4 talent, "Automations IV" to make mining possible. For the glittering, you’ll need a Prospector noble with Tier 4 talent.