The miners were collecting stones in the jade-rich Hpakant area of Kachin state when the "muddy wave" crashed onto them, after heavy rain, the fire service department said in a Facebook post.
Rescue workers recovered 113 bodies, the department said, but more were missing.
“Other bodies are in the mud,” Tar Lin Maung, a local official with the information ministry, told Reuters by phone, “The numbers are going to rise.”
Deadly landslides and other accidents are common in the poorly regulated mines of Hpakant, which draw impoverished workers from across Myanmar, but this is the worst in more than five years.
About 100 people were killed in a collapse in 2015, which strengthened calls to regulate the industry.
Media have reported scores of people killed in the area in recent years, many of them freelance "jade pickers" who scour tailings - the residue from mining - for stones that have been missed by larger operators.
Video footage on social media showed frantic miners racing uphill to escape as a towering pile of black waste cascaded into a turquoise lake, churning up a tsunami-like wave of mud.
Photos showed rows of dead bodies laid out on a hill, covered by tarpaulin.
Maung Khaing, a 38-year-old miner from the area who witnessed the accident, said he was about to take a picture of the precarious waste mound that looked set to collapse when people began shouting "run, run!".
“Within a minute, all the people at the bottom (of the hill) just disappeared,” he told Reuters by phone. “I feel empty in my heart. I still have goose bumps ... There were people stuck in the mud shouting for help but no one could help them.”
Than Hlaing, a member of a local civil society group helping in the aftermath of the disaster, said those killed on Thursday were freelancers scavenging the waste left by a larger mining company. She said about 100 people were still missing and 30 had been hospitalized.
The government of Nobel laureate Aung San Suu Kyi pledged to clean up the industry when it took power in 2016, but activists say little has changed.
Official sales of jade in Myanmar were worth 671 million euros ($750 million) in 2016-17, according to data published by the government as part of an Extractive Industries Transparency Initiative.
But experts believe the true value of the industry, which mainly exports to China, is much larger.
Than Hlaing said a local official had warned people not to go to the mine on Thursday because of the bad weather.
“There’s no hope for the families to get compensation as they were freelance miners,” she said, “I don’t see any route to escape this kind of cycle. People take risks, go into landfills, as they have no choice.”