The Toronto-headquartered company said surveying and planning issues at its flagship Platosa mine, in Durango state, Mexico, wes to blame for the poor performance.
CEO Brendan Cahill said production during the third quarter was "unacceptably below expectations" and not representative of the improvements the company had made at Platosa in recent quarters.
"We took immediate steps to improve management and realign development with the orebody. We now have multiple ore faces available, are realising improved development rates into key production areas and taking further steps to increase production into 2019," he said.
For the quarter, silver output fell 24% year-on-year to 171,227 ounces, lead production fell by half to 800,000 pounds and zinc output fell 54.5% to 1Mlbs, for total output of 300,766oz silver-equivalent.
The surveying and planning issues resulted in the company mining peripheral and lower-grade ore instead of the high-grade block model ore.
However, after rectifying the problem, Excellon said it was now once more accessing the core of the orebody and had from four to six ore faces available for production. Development accelerated 36% versus the prior quarter, with increased mechanisation and headings to access more key areas shortly, for future production.
Excellon said it expected to achieve a steady production rate going forward.
The company continued to process low-grade historical stockpiles and sump material, with minimal associated mining cost. This ore is blended with mined ore to improve payability and recoveries, particularly for lead and zinc, as well as being cash flow positive.
Excellon expects to release third quarter financial results at market open on November 1.