The nail did have a cracked, hammered head, and it had a round shift (unlike old forged nails which traditionally had a square or rectangular cross section I think. It had a 5 degree bend in it so thats probably why it was discarded. It didn't have a twist or spiral, but maybe it was used as a ground probe to determine soil depth down to bedrock.

Ive suspected that maybe the wall was part of a sluice or race used for washing materials.

Ive fossicked in the dry creek bed for crystals, gemstones and gold. There is hardly any gold, but heaps of crystal.

I reckon you're probably right about it being a prospectors camp.

Id love to take a metal detector out there one day to look for more iron, but alas I'm a broke boy.