Mineral sands processor CFM Minerales has increased its annual offtake by 1000t to 5000t of premium zircon, with its initial agreement signed back in October 2017.
The 5000tpa accounts for about 8% of stage one volumes planned at Thunderbird.
Significantly, Sheffield said the latest sale means "our marketing team has now secured offtake for approximately 100% of all stage one zircon and ilmenite production volumes at Thunderbird".
News of the new sale wasn't accompanied by any update on the company's hunt for a strategic partner at Thunderbird, with A$143 million of equity seen as required to develop the $478 million project.
Shares in Sheffield were unchanged at 47.5c in morning trade, capitalising the company at $121 million.
The stock is down from levels of more than $1 last October.