I hadn’t really given it a thought, but here’s two ideas that just came to mind:
Make it so that the player has to somehow “transfer” the precious metal from the area. This could be as simple as forcing there to be one unit in the zone, or somehow involve a shipping convoy.
Another thing I was thinking that might help with the same old boredom in Africa, is perhaps gold or silver can be used to recruit more African locals and deploy infantry in the colonies.