That is simply oxides of lead and tin- nothing more. Its quite common to see when casting lead, and even more common if you are using scrap lead you picked up off the range, which often has tin and antimony, and other trace minerals.
What you see is NOT GOLD! Colors vary from silver, to purple, to gold with the lead oxides, depending on temperature of your furnace, and trace minerals in the lead. If you skim off the oxide, the surface of the molten lead will have its usual "silver" color to it.
Expect to skim the surface of the molten lead periodically during a session of molding balls or bullets, and particularly after you clean the lead using beeswax, or some other substance to draw the foreign matter( dross) to the surface so that it can be skimmed off with a spoon.
I recommend dedicating a specific spoon for this purpose, so you don't use that same spoon for eating food! My father found a large, stainless steel serving spoon at a church bazarre that he bought for $.10. He used that for more than 35 years for casting duties. I am sure that you would not have to visit many garage sales to find a spoon that would fill this need.
