Friedland, founder and executive chairman of Ivanhoe Mines, has a long history of exploration and development of remote mineral deposits in countries including Mongolia and Democratic Republic of Congo.
HPX is to take over stakes in Nimba previously owned by BHP, Newmont Goldcorp, and French nuclear group Orano, the company said in a statement.
The Guinean government will also hold a stake in Société des Mines de Fer de Guinée (SMFG), the Guinean company operating the project, and two seats on its board, a document seen by Reuters showed. Financial terms of the deal were not disclosed.
"We are confident this project will be developed fast and well," Mines Minister Abdoulaye Magassouba told Reuters.
"We have a world-class partner committed to developing this exceptional deposit while taking into account the environmental sensitivity of the site."
Sources told Reuters late last month that Friedland was the front-runner to take over BHP's stake in Nimba.
HPX and BHP did not respond to requests for immediate comment.
The deal did not eliminate the possibility that some of the iron ore produced at Nimba, in southeast Guinea, could be taken to port through Liberia - a much shorter route than the government's preferred route through Guinea to the west coast.
The deal terms did, however, set an extra tax on any iron ore taken through Liberia - of between $0.825 and $2 per tonne.
Guinea's government has separately insisted that the future developer of the Simandou iron ore deposit would have to export through Guinea, building a "trans-Guinean" railway to the west coast - a distance of around 650 km.
(Reporting by Saliou Samb; writing by Helen Reid; editing by Joe Bavier and Jason Neely)
By Saliou Samb