Bitcoins that you mine are immediately taxable as ordinary income at that day's conversion rate, regardless of whether you immediately take them out as dollars, or leave them in your wallet. That conversion value that you pay tax on becomes your cost basis -- functionally the same as if you bought it at that price. Then, when you take them out of your wallet as dollars, you have a second taxable transaction, either a capital gain or a capital loss, depending on whether the coins went up or down in value since you mined them--and either long term or short term gain or loss depending on how long you held the coins. (By the way, keep excellent records of each coin, because when you sell one out of your wallet, you will need to prove your cost basis. If you are audited and can't prove your basis, the IRS can assign you a basis of zero and make the entire amount taxable again.)
Now, the concern is that you bought the contract with BTC. Every BTC you mine will be taxable income, but you have no way to deduct the value of the "input" BTC as a business expense unless you report your mining activities as self-employment on schedule C. And that will mean that you will pay an extra 15% self employment tax on the value of each coin when created. If you do use a schedule C, then the amount you report as a business expense to buy the mining contract is the original purchase price of the BTC (your basis), not the increased value. Reporting the input coin as a business expense will reduce your net profit subject to SE tax that you get from the daily value of mined coins. But any profit from mining after that deduction will be taxed both as ordinary income tax rates and as self-employment tax.
If you don't use schedule C and don't report mining as a business activity,, then you won't pay SE tax on your profits, but I don't see how you can get a deduction for the BTC you invested to get the contract.
