It was probably gilded, not plated. The Lambayeque culture was very advanced in metallurgical knowledge, like their neighbors the Moche. The Moche probably were more advanced but there appears to have been extensive exchange of metal-working technology in that entire region.
This is from a scientific paper on the Moche:
"Metallurgical studies of a group of gilt copper objects from Loma Negra have shown that the gilding was achieved by an electrochemical replacement plating process in which gold and silver are dissolved in an aqueous solution of corrosive minerals. The precious metals are then plated from solution onto the copper objects. Moche metal craftsmen can now be credited with having developed the two most sophisticated of Andean gilding procedures: depletion gilding and electrochemical replacement plating."
Those people were damned smart. I've seen an exhibition of their gold and silver work and it was amazing. I'm a silver and gold-smith so I'm not exactly unfamiliar with the topic.
All you'd need to do is heat the copper to the point where gold flour could be brushed across it.
Some of the gold would stick to the copper. Then, using maybe NaCl or stomach acid you could etch any stray copper off the surface leaving behind nothing but a very thin gold layer.
I can see this done with little more than a standard stone circle fire place. Should have been well within the range of technological feats of even the more primitive North American tribes.
NOTE: back in the late Neolithic 'they' began to acquire gold flour through some very simple but time consuming means ~ it makes a wonderful material for decoration.
Right now I could use a couple of pounds of gold flour.