The government is next month to decide whether to greenlight the controversial project led by UK firm Beowulf, which has promised to create 250 to 300 jobs in the area.
But the Sami, an estimated 20,000 to 40,000 of whom live in Sweden, say the plan will prevent reindeer herding, disrupt hunting and fishing, and destroy the environment in their homeland.
“We believe that the climate, the environment, clean air, water, reindeer herding, indigenous rights and the future of humanity should be prioritised above the short-term profit of a company,” Thunberg said in an English-language video message.
“The Swedish government needs to stop the colonisation of Sapmi,” she added, using a term for Sami land.
Stina Lanta, a young resident of the village of Jahkagasska near the proposed Kallak mine, said she was worried.
“A mine would have a permanent negative impact on our grazing lands,” she said.
The European Union’s only indigenous population, an estimated 100,000 Sami live across the vast Arctic wilderness of northernmost Finland, Norway and Sweden as well as Russia’s Kola peninsula.
For much of the 20th century, governments denounced the indigenous people and their culture as uncivilised and inferior.
In the last five years, Finland, Norway and Sweden have stepped up moves to atone for past injustices, setting up truth and reconciliation commissions and repatriating stolen Sami artefacts.
But the Sami argue that their rights continue to go unrecognised, pointing for example to government plans to open up parts of their mineral-rich homeland to mining companies.