I have found all kinds of stuff attached to gold. It could be part of the mother rock or a stain from something that it was next to for a million years. I have also saw the same color on gold after it was put in Nitric Acid to get the mercury off of it. You can put it in a plastic jar with vinegar and salt and shake it to see if it comes off….Art
Probably limonite staining, Iron oxide, Sometimes nuggets can have bits of copper, I have seen nuggets with bits of copper, lead, and even coated with palladium.
It appears to be copper.Natural gold is not pure,as you will find out when you try to sell it.I have an area that all the gold has that reddish orange hue to it,there is a copper mine right adjacent,its cooper in the gold,nitric acid will remove it.I have another area that produces a light,sheenish type gold,that is silver in the gold.