The work is expected to commence in the March quarter, ahead of the mine's planned initial borate production in the third quarter of this year.
The operation's Phase 1A target rate is 20,000 tonnes per year of potassium sulfate, or SOP, and 9,000t/y of boric acid.
"This is another important step in the fulfillment of our aspiration to become a globally significant producer of borates and specialty fertilisers," American Pacific CEO and MD Mike Schlumpberger said.
Fort Cady, located in southeastern San Bernardino County, California, has been called the largest known contained borate occurrence in the world that is not owned by borate majors Rio Tinto or Turkey's Eti Maden. Its JORC mineral estimate of resource is 120.4 million tonnes.
Subsequent phases of the project will ultimately increase production to 400,000t/y SOP and 450,000t/y of boric acid.