But they were discovered in the late 1400s, resulting in a change of interest. Nations invested in New World colonies with similar goals as their trade with China. It would follow that trade with China decreased as resources were directed elsewhere.
Silver was the item that the chinese intensively desired for monetary reasons as multiple experiments with paper money failed due to runaway inflation and they lacked precious metals of their own. Almost 40% of the silver mined in the late 16th century ended up in china.