Rio Tinto Group said it’s close to a breakthrough in a landmark deal to unlock the world’s biggest untapped iron ore mine.

Efforts to develop the massive Simandou project in Guinea have been stymied for years by a litany of disputes over ownership and infrastructure, and by political changes in Guinea. Negotiations between investors and the Guinean government to build a railway for exporting ore may now be nearing a resolution, said Rio Chief Executive Officer Jakob Stausholm.