Ore spawn

It seems to me that the place of gold ores should be iron ores. There is a lot of gold ores on the surface.

What I expected to happen was...:
When I just went down into the mine, I hoped to immediately find iron ores.

(I found it after 30 minutes DEEP in the mine)

What actually happened was...:
After I went down into the mine, I found a pile of gold ores (coal was less than gold). Only when I went DEEP into the mine did I find 7 iron ores.

Steps to Reproduce:
1. find an abandoned mine. (height about 69 - 54)
2.go into it.
3. go crazy.