Resource Mines are spread all over the map. There are mines for each of the 5 main resources - Food, Petroleum, Alloys, Composites and Gold. The mines are split in levels as the higher the level is, the more resources the mine holds. Gathering happens by sending units towards it. Each unit has a different collection capacity so some units may be better than other for collecting resources. The more you watch towards the center of the map, the higher level mines are there.

Always keep in mind that:

• An indication will be shown on mines that are occupied by another player. Sending an army to a mine that is already occupied will result in a battle between you and the army on that mine. (if it's not an alliance or a coalition member of course)
  
• Whenever you gather resources from a mine in a territory, that is held by your alliance or an alliance that you're in coalition with, you will receive a bonus to the gathering speed.
• Whenever you gather resources from a mine in a territory, held by an alliance that has set you to the enemy status, you will receive a penalty to the gathering speed.
• You can increase your Resource gathering speed (except for Gold Mines) by using the Boosters in your HQ.

The base gather speed for Regional Mines can be found here