Skarner has a mana bar but hardly uses any at all

He burns through mana at a decent pace, he just regenerates it just as quickly thanks to the spires.

Depends a lot on Spire usage. You gain a crazy amount back when you pass over them. If your team has good control of them it's hard to ever run out of mana haha. That adds some counterplay though. A team that tries to take spires would be pretty strong in making Skarner less spammy.

Standing on his passive spires provides mana regeneration.

Also, I think Hunter's Talisman is the most common starting item on him and it also provides mana regeneration.

He does use a lot of mana, actually. You will notice it if you spend a lot of time in a lane/invading/doing Krugs, etc.

But the spire mana regen is strong and keeps him from going OOM while on it. Add to that the blue buff from first clear (where mana is an issue), the jungle item mana regen, and the fact that he builds mana items (Iceborn, BV, etc.), and mana won't be a big issue for Skarner.