Probably minerals and/or salts.

Butterflies commonly feed on scat from all manner of different animals. More info here:

https://thefriendlybug.weebly.com/blog/bugs-on-poop-part-1-butterflies

Yes, I work at a doggy daycare and it's very common to see both moths and butterflies feeding on the dog poop. It's generally males that do it.

It’s called “puddling.” I helped my brother chase Tiger Swallowtails across Michigan, when he was working on his PhD in entomology, and large groups of males were often together feeding on the ground in muddy gravel driveways or near rotting fish on sandy beaches. Sometimes the numbers were huge, maybe 15 at a time.

I learn something every day!