POSCO will pay A8.2c per share in its Black Rock investment.
The investment was expected after the two companies signed a strategic alliance back in June.
POSCO has now secured a right to life of mine offtake supply for 100% of the graphite fines from the first stage of Black Rock's Mahenge development.
According to Blackrock, POSCO has "indicated a preparedness to provide between US$10 million to $20 million as a prepayment facility".
A formal agreement on this is being targeted by the end of March.
Black Rock needs about $116 million for its first stage development.
Known primarily as a steel producer, POSCO is also reported to be one of the world's largest producers of anode feedstock and a major participant in the global lithium ion battery industry.
Tanzanian approvals are also required, with the East African country proving a difficult jurisdiction for miners in recent years as it restructures its business rules.
Black Rock started the current quarter with A$1.7 million.
Shares in Black Rock were unchanged at 9.5c in late morning trade, capitalising the company at $67 million.
