Mining ceased on March 8th and processing activities have now stopped following the exhaustion of surface stockpiles.
Cerro Negro produced 489,000 ounces in 2018 at an all-in sustaining cost of US$535 per ounce.
Goldcorp said it is working with the union and government authorities to work toward resolution and restart the operation.
The company gave no indication of why the union downed tools.
Jefferies analyst Christopher LaFemina said there would likely be a quick resolution, because of Goldcorp's desire to avoid negative attention before the Newmont vote next month, as well as its contribution the annual production.