McEwen said its insurance policies will not be enough to cover the losses.
Its shares were down about 6.7 per cent in early trading Wednesday.
McEwen said no one was injured and mining operations were not affected, but did not provide any more details about the robbery which it said is under investigation by Mexican authorities
McEwen said the theft occurred Tuesday at the El Gallo 1 mine and involved 900 kilograms of gold-bearing concentrate, which requires further processing until it becomes gold metal.
Besides Mexico, McEwen also has operations in Nevada and Argentina.
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