My guess is it wasn't pyrite you were finding that is coppery looking. It was chalcopyrite which is copper iron sulphide. Just my thoughts.
Certainly an interesting looking piece of equipment you were using. I never seen one like that before.
Went down along the Utah border last weekend. Visited an old gold/silver mine looking for pyrite nuggets. Ran the Sweep Jig (patent pending) dry to see if it could recover the pyrite. Obviously, it did OK.
I'm wondering about the "coppery" color. If anybody knows what that is, I'd appreciate the info. The GMT doesn't detect the nuggets, but easily "sees" the coppery sand.