The IRS issued specific guidance detailing the tax treatment associated with mining cryptocurrency back in 2014 with Notice 2014-21 (https://www.irs.gov/pub/irs-drop/n-14-21.pdf). In Q-8 through Q-9, the IRS specifically clarifies that a coin is taxable at it's fair market value at the moment it is mined as well as the fact that a taxpayer will be subject to self-employment taxes to the extent that the taxpayer is not acting as an employee. Along with this treatment, a taxpayer will be able to deduct any expenses associated with mining the cryptocurrency.
The character of the income recognized on the mining of a coin will be ordinary. If, however, the taxpayer decides to sell the cryptocurrency at a later date after it has appreciated in value, the taxpayer's basis in the asset will be equal to the income initially recognized and the character of the gain will be capital.
Disclaimer
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