Its management discussion and analysis for the year ended December 31, 2018 said the company uncovered the scheme to steal concentrate by criminal elements when it investigated security threats to employees.
Excellon said the amount and value of concentrate stolen remained uncertain, but it estimated the thefts impacted its revenues, "by approximately 10% per year during the affected period".
The company reported revenues of US$24.3 million in 2018 and $21.2 million in 2017.
Investigations indicated three employees were involved in facilitating and concealing the theft and their employment was subsequently terminated.
Excellon has retained security consultants to assist with investigations, and to develop and implement a plan to enhance security at its Platosa silver mine and Miguel Auza plant.
Shares in Excellon (TSXV:EXN) are trading at C95c, valuing the company at $95 million. Its share price has increased 20% so far this year.