I'm a newbie having the same problem. I too assumed it was set aside for mining fees, but the amount is unusually high. In my case, I deposited 0.0062 as a test a couple of days ago and only 0.0042 is spendable.
I'm glad I deposited a test first! Hopefully, support will eventually respond and I can move to a different wallet without any loss.
I'm glad that I'm not the only person who have that problem. Jaxx is actually a good wallet. Simple GUI and it also supports other altcoins. Really want to keep using this wallet but this problem must be addressed by the devs ASAP.