The parent company of Tara Mines in Co Meath has announced that production and exploration is to cease temporarily at the facility, resulting in the temporary laying off of 650 workers.
Operations at the mine will be temporarily suspended within the next four weeks.
The company described the zinc mine, which is Europe's largest, as a high-cost mine.
Local politicians have expressed their shock and disappointment at the news.
In a statement, Boliden Tara Mines said that the decision was taken to safeguard the long-term future of the company "in response to significant and unsustainable financial losses".
The company described the decision to suspend operations as "unavoidable".
It said that the business was "cash-flow negative" and that losses had been brought about by a combination of factors, including a decline in the price of zinc, high energy prices and general cost inflation.
Gunnar Nyström, General Manager of Boliden Tara Mines, said it was "in the long-term interest" of the mines.
"We are acutely aware of the difficulty and disruption that this decision will cause for our employees, and the wider community in Navan, where the mine has been operating since 1977.
"This was not a decision we made lightly, but we simply have to stem the unsustainable cash outflow that we are currently experiencing, in order to safeguard the long-term future of the mine."
Mr Nystrom said the company could not be definitive about how long this period of suspension would last, but that company believed it is temporary.
"We will keep the situation under continuous review. We will remain in dialogue with our employees and stakeholders throughout this period."
A statement from the company said the management team will review further supports available to employees during the temporary layoff and will update employees.
It said it would work with local social welfare authorities and other agencies to assist employees about the supports available to them.
A small number of employees will continue to work on site during this period in order to care for and maintain the mine.
A statement from Tara Mines earlier said: "We are holding a meeting with colleagues tonight to inform them about the impact this decision will have, and will issue a statement after the meeting."
Fianna Fáil Senator Shane Cassells said it was "extremely worrying for the town of Navan".
He said that Tara Mines employs 650 people but 3,000 other jobs are supported in the economy through the mines.
Mr Cassells said it is essential to ensure that this is only a temporary situation and said he will be raising it in the Seanad with the Minister for the Environment.
"This is the largest zinc mine in Europe and it simply cannot fail," he said.
Local Sinn Féin TD Johnny Guirke described the news as "very disappointing and worrying".
He said that Government needed to step in as soon as possible to try to make sure that these jobs are saved.
Aontú TD Peadar Toíbín said the announcement was "shocking news" and that it would have a big impact on the people of Navan and Co Meath.
He said that Tara Mines was the "backbone" of the area and he called on the Minister for Enterprise to meet workers and management as soon as possible.
SIPTU has said that workers at the mines are "shocked and disappointed" following the announcement.
Adrian Kane, Divisional Organiser said: "SIPTU members are deeply disappointed at the news that 650 workers are to be laid off within the next few weeks and the manner by which it was communicated to them this evening."
Mr Kane said that members were concerned that they were not informed in advance of the announcement of temporary lay-offs, without pay, by the company.
"Tara Mines is a critical part of both the local and national economy and we will be engaging with management to discuss how the effects of these lay-offs can be mitigated," he said.
Supports
In a joint statement, Minister for Enterprise Trade and Employment Simon Coveney and Minister for Social Protection Heather Humphreys, said that supports are being put in place immediately to assist Tara Mines workers.
Minister Coveney described the announcement as a shock and said the Government would continue to engage with "Irish management and the parent company to try reverse the closure".
He said the board of the company "is convinced this is a temporary closure".
"My department will immediately send in State agencies to assist workers, especially apprentices who are in the middle of their training," he said.
Minister Humphreys said that a dedicated team from the Department of Social Protection is being established to provide supports to the impacted workers and their families.
The team will provide information on income, redundancy entitlements and employment supports for those affected by the announcement.
Ms Humphreys said: "My immediate thoughts tonight are with the workers and their families who are dealing with this extremely difficult news.
"My Department of Social Protection is on standby to help in whatever way we can."
Additional reporting Laura Hogan
