Alacer said Copler's sulphide plant achieved commercial production criteria on May 31, averaging 80% throughput.
For May the plant recorded gold recovery of 93%, producing 22,000oz gold, which the company said were in line with expectations.
"We continue to tune and optimise the plant and expect quarter-on-quarter improvements over the remainder of 2019. Performance to date provides us with the confidence to reaffirm sulphide plant production and cost guidance for 2019," CEO Rod Antal said.
Alacer last year completed construction of the US$660 million project $84 million under budget, with operations starting in the September-quarter.
The sulphide expansion complements the initial oxide plant and is expected to add 3.5Moz to Alacer's production profile for the 20-year life of the operation.
The plant is expected to produce gold at an all-in sustaining cost of $645/oz.
Alacer said expected oxide output was now 125,000-145,000oz versus 90,000-110,000oz previously. Exploration success in-pit, positive reconciliation of oxide-reserve ounces and better recoveries from blended ore have increased oxide ore volumes available for processing.
The oxide AISC guidance is forecast to be $50/oz lower at between $650-$700/oz.
Alacer shares jumped 15% or C56c higher Thursday to $3.72, capitalising the company at $1.2 billion.