no idea how to cost these (so probably overpowered) but its the concept i'm going for. not playtested. in games using Copper Mine, include Silver and Gold Mines, putting them beside the Copper Mine Supply pile.
COPPER MINE, $2, Treasure
While you have this set aside, you may gain 1 Copper putting it on top of your deck at the start of Clean-up this turn.
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When you gain this, set it aside. Put it in your deck at the end of the game.
SILVER MINE, $5, Treasure
While you have this set aside, you may gain 1 Silver putting it on top of your deck at the start of Clean-up this turn.
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You can't gain this unless you have a Copper Mine set aside. When you gain this, set it aside and trash your Copper Mine. Put it in your deck at the end of the game.
GOLD MINE, $8, Treasure
While you have this set aside, you may gain 1 Gold putting it on top of your deck at the start of Clean-up this turn.
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You can't gain this unless you have a Silver Mine set aside. When you gain this, set it aside and trash your Silver Mine. Put it in your deck at the end of the game.