Hi Hans,
Since you have already inquartated your gold, simply complete the
process by leaching out the silver, copper, zinc, etc in warm nitric
acid. You are probably familiar with the procedure, roll out your
alloy as thin as possible, cut it up, and cover it with acid. You
will ultimately be left with nearly pure gold, which you can then
melt and re-alloy. It doesn’t sound like your ratios are critical,
so this should be sufficient for your purposes. Good luck!
Best regards,
John