Sierra filed unaudited fourth quarter results yesterday for its 81%-owned subsidiary Corona, which has the Yauricocha mine.
Yauricocha contributed 68% of Sierra's total revenue in 2019.
Sierra said on a 100% basis, Yauricocha's copper-equivalent production was down 20%, from 23.1 million pounds in the December 2019 quarter, to 18.4Mlb.
Zinc-equivalent production was down 14% during the quarter, also due to lower throughput and lower head grades, and lower workforce availability had impacted development.
All-in sustaining costs were up 17% to US$2.47/lb Cu-eq payable in the fourth quarter.
Sierra said revenue was up 7% year-on-year in the December quarter to $45.2 million due to higher metal prices.
However annual revenue was down 6% due to the impact of the 66% increase in treatment and refining costs.
CEO Luis Marchese said he was pleased with Yauricocha's performance in the fourth quarter and overall, the company had performed well and produced solid results given the challenging year dealing with the COVID-19 pandemic.
He said the company expected to release a preliminary feasibility study soon on increasing Yauricocha's throughput to 5,500t per day starting in 2024.
"Management remains optimistic that continued operational efficiencies can be obtained at the Yauricocha mine as well as capitalising on further operational and resource growth opportunities," he said.
Sierra is also looking to expand its 100%-owned Cusi and Bolivar mines in Mexico, although it said last month it was considering options including selling up as part of a strategic process.
It appointed Jose Vizquerra Benavides, the CEO and president of 03 Mining and former head of business development at Peru miner Compania de Minas Buenaventura, as chairman in January.
The company recently said it had produced 118.2Mlb Cu-eq and 16.1 million ounces of silver in 2020, and expected to increase production this year to 130-141Mlb and 16.1-17.5Moz respectively.
Sierra will release its fourth quarter results on March 18 after market close.
Its shares (TSX: SMT) have ranged from C89c-$4.92 over the past year and closed down 10.3% yesterday to $3.76, capitalising it at $612 million (US$482 million).