The three kilometre tunnel will connect the Sigma mill with the 405m level of the underground workings from where the Triangle deposit is mined.
The company estimates the cost at US$24 million and expects completion by the first half of 2022.
Eldorado expects the tunnel to reduce costs by eliminating surface re-handling and haulage from the Triangle mine to the Sigma mill and by lowering energy requirements for mine ventilation.
The tunnel will also increase safety by creating a secondary egress and ventilation to the Triangle mine.
It will create underground access for lower cost exploration of the area between Triangle and the historic Sigma and Lamaque mines as well as enable increased production from Triangle and mining of the Parallel deposit.
Eldorado is also exploring adding an underground crushing and conveying system and a mill expansion, with an update expected in the December quarter this year.