For context, I'm in the UK, but I suspect the answer will apply regardless of the jurisdiction.

I'm of the understanding that if you buy BTC and then sell for profit at a later date, you're liable for Capital Gains Tax. But what's the tax situation if you're mining coins, and then selling them on your local exchange? My intuition is that it can't be CGT, because you didn't buy the coins in the first place - in the extreme case, you bought a mining rig, which was then used to produce coins, which you later sold. But I find it impossible to believe that you don't have to declare income gained like this. So what does it fall under?