RIO DE JANEIRO (AP) — It was dusk on April 14 when Francisco Kuruaya heard a boat approaching along the river near his village in Brazil's Amazon rainforest. He assumed it was the regular delivery boat bringing gasoline for generators and outboard motors to remote settlements like his. Instead, what Kuruaya found was a barge dredging his people's pristine river in search of gold.

Kuruaya had never seen a dredge operating in this area of the Xipaia people's territory, let alone one this massive; it resembled a floating factory.

Kuruaya, 47, motored out to the barge, boarded it and confronted th...