GUANGHAN, China—In July 1986, workers at a brick factory in southwestern China were digging for clay when they hit upon something unexpected: dozens of dazzling pieces of jade.The otherworldly beasts and enigmatic masks of Sanxingdui still enthrall China, decades after the unearthing of ceremonial pits.The chance discovery led to one of the greatest archaeological finds in modern history—one that continues to thrill China with its ornate gold masks, towering humanoid figures, intricate 12-foot-tall bronze “sacred trees,” sculptures of fantastical beasts and hundreds of elephant tusks.The artifacts from Sanxingdui, as the site is known, stirred questions that still perplex experts four…