"What about volcanoes that spew lead to replenish these supposed rivers of lead?"

I think you meant to reply to Nick's post.
But in a vacuum and with as high of volcanic heat as we have on Earth's volcanoes, the lead might tend to vaporize more rather than remain a liquid.

But with Mercury's high density- which it's higher than Earth if you remove effect of Earth's additional density due compression from it's higher gravity- and Mercury's thinner crust of the lighter material, one might have higher possibility of more heavy metals being in lava flows.

But with that said, I don't know of any high lead content lava flows on Earth, and so I would question the abundant lead rivers caused by volcanic activity on Mercury. And don't know if Mercury at present as any known volcanic activity.
But would guess that Mercury has had much more volcanic activity, than compared to the Moon or Mars and we will find signs of recent volcanic activity on Mercury when Messenger surveys the planet.