Canadian mining company Turquoise Hill Resources Ltd is locked in a feud with its largest shareholder, Rio Tinto Plc, over the underground expansion of its massive Oyu Tolgoi copper mine in Mongolia.

Tensions between mine operator Rio and Turquoise Hill’s management and minority shareholders have spilled into the open in recent months. Rio owns 51% of Turquoise Hill, which in turn owns 66% of the mine.