The Gold Mines yield Gold. Upgrade them to increase your Gold production rate and storage.
Gold Mines, as their name suggests, mine Gold. They provide a steady stream of gold even when the player is offline. Generated gold is stored in the mine, and requires the user to "collect" manually by taping on each mine, which is then transferred to the Gold Vault.
Other than Gold Vaults, Gold Mines are another prime target during a Resources Raid, especially those bases that contains high level Gold Mines. In fact, Gold Mines are often the better target than Gold Vaults as the amount of total Gold which can be looted from Gold Vaults is capped at 25% or 190,000, whichever is less. Gold Mines give up 50% of their loot to raiders and have no cap. Also, in most bases, Gold Mines are usually less protected than Gold Vaults, which makes them the best target for fast gold raids.
The amount of Gold can be raided from a Gold Mine is 50% of its currently stored gold. There is no cap limit on the amount that can be raided.
Although gold mine does generate free gold, its rate is relatively slow if compared to the upgrade cost of buildings on the same respective levels. In other words, trying to gather gold to upgrade building by solely dependent on the gold mine generation might take forever. This leads some player to simply deprioritize the upgrade of gold mine at early game and instead just focus on Resources Raid whenever gold is needed.
One of the Quests requires the player to upgrade their Gold Mine. However, the rewards are just Gold and EXP.