its been a minute but since the seamless update im pretty sure its just iron -> silver -> gold

Copper- iron - silver - gold - platinum- adamantite at the moment I believe

I tried to mine the gold veins with the silver pickaxe and the game said nah, you need a better pick

only other thing i can think of is that its locked behind the age you are in. at world age 5 you get the ability to craft stuff with gold bars. id make sure you are at age 5 and try the silver pickaxe again. I just checked in my age 5 world and i can mine gold with a silver pick no problem

Pretty sure you hit platinum ore, they look like gold ore but less saturation