Mines and the ore that is in them.

So I have taken over a left behind mine that was full of copper boulders in the small square entrance area, not much had been mined and no visible veins were to be seen. I cleared out the copper nuggets and then started to mine, when I found a vein, it was iron ore not copper. So my question is, do mines have different ore veins within the same shaft? Was there copper to start and now iron? I was excited that I had found a copper mine, but now I wonder.