The first-stage development will lead to a pit 4.2km long, 1.6km wide and 90m deep with a 28m average of overburden.
Firestone said there was only a 2.7:1 waste-to-saleable product ratio and expected capital expenditure for the development to be $A400 million.
The Perth-based company noted that coal extended beyond the pit boundaries in all directions.
The prefeasibility study results will help support the planned bankable feasibility study, while Firestone aims to lodge a mining right application to South Africa’s Department of Mineral Resources by the end of March.
The Smitspan property hosts 97Mt of measured, 500Mt of indicated and 734Mt of inferred resources.
Firestone recently entered into a third joint venture agreement with South Africa’s Sekoko Coal to acquire the Swanepoelpan and Duikerfontein properties to the west of Smitspan.
Shares in Firestone closed down 3.2% to 3c yesterday and are unchanged this morning.