Nor did they know what the materials they carried would be used for. So in addition to the physical and environmental consequences of uranium mining and transportation there were mental and spiritual consequences as well. The Dene did not learn that the uranium mined from their land and carried by their men was dropped on Hiroshima until approximately 1990.
Likewise, from a blog post titled The Hibakusha who Apologized for Hiroshima and Nagasaki
Until the 1990s, because of their isolation and neglect by the Canadian government, they had little understanding of where those “money rocks” had gone, and little awareness of the rocks’ connection to numerous deaths among them from strange new illnesses. But then journalists, academics and filmmakers began to appear with questions about the past and information about the causes of those illnesses. The Dene were dismayed by the neglect they had suffered, but were equally burdened by the new awareness of what they had helped to bring upon Japanese people. Their sense of responsibility knew nothing of the civilized impulse toward self-exculpation. They felt responsible for not having asked questions about what they had agreed to work on, for not having made every effort to understand the implications of their participation. That’s an ethical standard that few people could live up to.
I'm not skeptical of the mining of ore being secret during the war, or that they worked in unsafe conditions. However, I'm skeptical that an indigenous people who had contact with westerners took half a century to be aware of a connection between what they did and one of the most well-known incidents in World War II. (I'm also skeptical that cancer wasn't previously known about by indigenous people, but that's a separate question)
Were the Dene people unaware they had mined ore for the atomic bombing of Hiroshima and Nagasaki until the 1990s?