So what happens when you sell? You have accumulated coins over the months, but how do you know which particular coins you are selling? Maybe you're selling 5% of the coins you mined at $600, 2% of the coins mined at $630, 6% of the coins you mined at $510
Do you see where I'm going with this?
Those amounts are regular income taxed at regular income rates. Your mining pool, if complying with US law, is required to report these transactions to the IRS if they exceed $600 during the year.
The profits that you gain by holding onto them are taxed at a lower capital gain rate. Your exchange, if complying with US law, will have to report the sale of your of your bitcoin if it exceeds $600 during the year.
If the IRS audits you, you will have to come up with documentation that explains the reported transactions.