I think if you use fiat to buy tokens to give to an entity for the express purpose of getting a return (whether over time or at some future time) that is greater than what you invest, then convert that back to fiat, it would be hard to claim it is free from taxes. But I believe you are asking about the case where you have accumulated tokens (in this context any cryptocurrency) with no clear traceability to fiat, invest those tokens, receive a greater amount in return, and retain the value in tokens without converting it to fiat. At this time, I do not believe there is any accounting mechanism available to a government that could assess you with a tax liability.

Assuming that such an ability does or soon will exist, it begs the question of where does mining crypto fall in to this, especially when it takes the form we have on steem (leomm, palm, etc). That is much harder to clearly track!