You'll find this more often than you expect, don't be confused with the jade. An easy way to guarantee Gold ore is at the end of every tomb you'll find two sets of 6 gold ore either side of the Stele.
The other (and most frequent) way to gather gold ore is the gold veins in walls, they look like a jagged crack, they can be gold or jade. They are often found on bare rock and very often underwater. There is some early on Kuwaq Yaqu underwater. There are often quite a few seams in tombs and crypts as you work your way to the exit.
You can also get this as part of Monolith rich rewards but when you solve the clues and dig (so in theory you can start getting this from the 1st monolith before the bridge puzzle to Kuwaq Yaqu.)
You can start getting this from very early on, the 1st gold ore is right after the 1st set of bells and pushing down the cart to get weight on one of them. If you turn 180 before jumping onto the next set you'll see a small roof behind you which you can jump on.