Yahua, one of China's largest lithium hydroxide and carbonate producers, was in line to buy 50,000 tonnes per annum from the emerging operation will now be expanded to 75,000tpa of lithium concentrate grading 5.5%.
Core said Yahua's support further derisked its rapid development plans, and provided additional clarity on project funding as it moves towards a near-term final investment decision on development of the Grants and BP33 deposits.
The agreement covers about 40% of the expected mine life for the initial two deposits.
Yahua also has gained a first right of refusal over lithium concentrate offtake produced from the two deposits up to the greater of 300,000t produced prior to November 2023, or 50% of the forecast production for any calendar year.
The final definitive feasibility study for the estimated $55 million project is due for release this month.
Core aims to fast-track development of the initial two deposits, which are part of a total resource of 8.9 million tonnes grading 1.3% lithium oxide spread across multiple pegmatite orebodies.
It hopes to produce its first spodumene concentrate in late 2019.
The plant will be built adjacent to the Grants deposit, just 25km from the Darwin port and close to road, rail and power infrastructure.
Shares in the junior were up 3.7% in afternoon trade to 5.6c, valuing the company at $39 million.