Production had begun at ATO in April 2020 and the resource will be used for an upcoming feasibility study, looking to more than double output from about 60,000oz to 150,000oz a year.
"This dramatic increase in the ATO resource further underpins the long life attributes of the project," president and CEO Bataa Tumur-Ochir said.
The updated resource comprises a measured and indicated 1.39Moz gold and 20.48Moz silver, or 2.24Moz Au-eq, and an inferred 208,000oz Au-eq.
The boost was mainly due to drilling at the ATO 4 deposit and the initial resource for the Mungu deposit.
Tumur-Ochir said the update supported the phase two expansion project and showed ample resources for a 2.5 million tonne per annum fresh ore development.
"The bankable feasibility study remains on schedule to be released Q2 2021 and we are optimistic that this study will bear out a robust economic result and a further 10 plus years of production at 150,000oz Au-eq with a focus on the fresh rock ores," he said.
ATO had produced 25,731oz of gold and 11,494oz of silver in the six months to September 30.
Executive chairman Matthew Wood said Steppe planned to ramp up exploration this year and had an internal target of 3-5Moz across its licences.
Steppe is planning a secondary listing on the Mongolian Stock Exchange, with proceeds of a proposed placement expected to be used for the phase two expansion.
Its Toronto-listed shares (TSX: STGO) have ranged between C63c-$3.14 over the past year and closed up 3.3% yesterday to $2.48, valuing it at $156 million (US$125 million).
