The original article translated from Italian puts it this way:

>the results indicate that the colossal statue is most likely an elaborate reassembly of what was initially a zhènmùshòu (镇墓兽 "keeper of tombs") fused in the Tang period (609-907 AD) with copper from the mines of the lower basin of the Yang-tze River, the Blue River in southern China. This is confirmed by accurate analyses of lead isotopes, which leave in the bronze unmistakable traces of the original mines from which the copper was extracted.

The implication is that the mines themselves have different isotope signatures that have been established in previous archaeological studies.