The simplistic answer is that things melt at times and then the...

The simplistic answer is that things melt at times and then the melted substances (iron, gold, silver ...) travel through the rock in fluid form. They obviously cannot travel through solid rock so travel through fractures. Where the fluids intersect shear zones they can go everywhere and cool as they get exposed to more air or simply increase their surface area and cool. Everything cools at different rates so eventually you get concentrations of metals in areas. We happen to be in a region where some gold cooled and was concentrated. It may have traveled very short distances and been melted for a short time or could be brought in by paleo channels from further away.

I am sure real geos could go into far more detail and tell me why this answer is wrong in this case but I did say it was a simplistic answer.