Underground operations at Mt Morgans are sourced from the Westralia mine area which comprises the Beresford and Allanson deposits. The deposits are ~15km from the Mt Morgans treatment plant.
The deposits occur over a horizontal strike distance of 2.8km and lies within 700m of the surface. The 2016 Resource estimate indicates the deposits have an endowment level of around 2,200 ounces per vertical metre, making it a highly mineralised gold deposit of significance in the West Australian gold fields. Previous mining and on-site treatment of the deposit has demonstrated that the gold is free milling with recoveries of 91%-93% achieved historically from conventional site-based CIP/CIL processing.
The gold at Westralia occurs within a well-defined banded iron formation (BIF) unit from which approximately 900,000 ounces at 4.5 g/t gold was produced up to 1998; with the majority being sourced from the Westralia open pit and historic underground workings. When added to the current 2016 Resource estimate, the pre-mined resource at Westralia is over 2.5 million ounces at a grade of 5.0 g/t Au.
Underground mining at Westralia is by Long-Hole Stoping methods (LHS), retreating to three separate declines – two at Beresford and one at Allanson. Combined, the underground infrastructure supports a 3,000 tpd run-rate. The November 2016 Feasibility Study demonstrated a maiden Ore Reserve of 3.3Mt @ 4.6g/t for 492,000oz at a forecasted AISC of A$837/oz. A parallel Pre-Feasibility Study highlighted the potential expansion scenario at Westralia (i.e. including inferred material) to 938,000oz at a forecasted AISC of A$795-805/oz.