Nuggets are rarely better than 20 to 22 Karat. New York Spot price for gold was $668.50 and theres 31.1034768 grams in a Troy Ounce so, that equals about $21.49 per gram. Current eBay selling prices for nuggets are about 80% to 90% of spot price. Larger, or unusually shaped nuggets can sell for more, but small nuggets and flake gold would have to have the impurities removed, to make them 24K, if you hope to get spot price for them.
HI: Right on function, for some reason it is assumed that all gold (Au) in Nature is !00%. At the "Lluvia de Oro" mine in Chihuahua, of which I once had 30%, sniff, it ran only 45 -60%.
Some of the nuggets I've seen on eBay might be from a mine like you mentioned. There are some that have a very noticeable red tint to the gold, likely due to iron and others that seem to have an unusually light "gold" color, possibly alloyed with silver, or platinum. I do find that the prices on eBay are rather low right now and it might have something to do with the person I heard about on the news a couple of weeks back, who was selling fake gold bars. Either that, or people just don't think the price of gold is going to stay up there for much longer. Personally, I think it will stay above $670 U.S. and maybe go higher into the low $700's by the end of the year.