Friedland is chairman and CEO of HPX.
A signing ceremony was held in Conakry yesterday, attended by Guinea president Alpha Condé and former South African president Kgalema Motlanthe.
Motlanthe and Ivanhoe Mines director Guy de Selliers de Moranville had agreed to become co-chairmen of the HPX subsidiary that will hold the Nimba asset and both have joined the board of Société des Mines de Fer de Guinée (SMFG), the Guinean company operating the project.
The terms of the acquisition were not disclosed. BHP tried to sell its 42.8% stake to ArcelorMittal in 2014 but the deal fell through.
Nimba is estimated to contain at least 1 billion tonnes of high-grade, low-impurity iron ore.
"I am delighted that HPX will work with the government and people of Guinea to bring the Nimba deposit into production and to help His Excellency president Condé realise his long-standing vision of seeing Guinea become a world-class producer of iron ore," HPX president Eric Finlayson said.
"With local procurement of goods and services, local recruitment and training of the workforce, the upgrading of transport routes and infrastructure, and the resulting economic multiplier effects, we anticipate that major social and economic benefits will flow from the Nimba mine."
HPX is planning a starter mine of 1-5 million tonnes per annum as soon as possible and will carry out studies into a 20Mtpa operation.
The government of Guinea and HPX have agreed on terms for the updated Nimba Mining Convention, including a 15% free-carried government interest in SMFG, as well as tax and royalty arrangements and a development timetable.
In April, Ivanhoe loaned HPX US$50 million and signed a collaboration agreement. HPX said Ivanhoe was keen to collaborate on Nimba and negotiations will be held to allow Ivanhoe to provide technical support on an arms-length, cost-recovery basis.
HPX is privately held, domiciled in the US and based in Vancouver.