The GMB claim half of the staff working at the Glensanda Quarry, north of Oban, face the sack from a new contractor.
The union fears Spanish-owned Oban Earth Works will cut 50% of staff when it takes over excavation and haulage operations at the quarry.
That work had been carried out by the Hargreaves company on behalf of Aggregate Industries.
A spokeswoman for Hargreaves said their contract ended in May, but they would remain during a handover period until the end of July.
They say staff have been transferred across to the new company.
A spokesman for Aggregate Industries said: "As with any commercial operation, it's important for Aggregate Industries to review and renew our partnerships, to ensure we can offer the best service available.
"We are switching load and haul provider at our plant in Glensanda, and as part of that process there are individuals who will be transferred from our current provider Hargreaves, to our new one (FERPI). Unfortunately, due to increasing production efficiencies on site, there are some employees at risk of being made redundant by FERPI.
"We understand and appreciate that this is a potentially difficult time for those individuals affected and have been in close contact with FERPI to ensure employees are offered all the support they need throughout the consultation process."
The BBC has been unable to speak to anyone from Oban Earth or Aggregate Industries.

The computer, about the same size as a credit card, was first released in 2012 and is widely used as an educational tool for programming.
However, it can also be used for many practical purposes such as streaming music to several devices in a house.
A new starter kit for Raspberry Pi, including a keyboard and mouse, has been released to celebrate the success.
The kit also includes an SD storage card, official case, power supply, HDMI cable, mouse, keyboard and guidebook - it costs Â£99 plus VAT and will be available in the coming weeks.
The Pi, which is manufactured in Wales, has been adopted by pupils, programmers and inventors around the world.
Many have received versions for free via the Raspberry Pi Foundation, a charity.
British astronaut Tim Peake even took two to the International Space Station.
"We thought our lifetime volumes might amount to 10,000 units - if we were lucky," wrote founder Eben Upton in a blog.
"There was no expectation that adults would use Raspberry Pi, no expectation of commercial success and certainly no expectation that four years later we would be manufacturing tens of thousands of units a day in the UK and exporting Raspberry Pi all over the world."
The latest sales figures were "fantastic", said tech industry analyst Chris Green - who owns four of the computers himself - at business consultancy Lewis.
"It's more than just a geeky coding tool - it's increasingly proving itself as a useful computer," he told the BBC.
The Raspberry Pi Zero, a version of the computer that costs Â£4, was ideal for use in developing countries, he added.
"They can run off batteries so where there isn't an abundance of electricity you still have a computer that can be used for teaching."
Although the Raspberry Pi has been used in many schools, it was more complicated for children to get to grips with than the BBC Micro Bit, argued Bethany Koby, co-founder of education-focused start-up Technology Will Save Us, which helped design the BBC's device.
"[The Raspberry Pis are] incredibly powerful but they're hard to use," she told the BBC. "That being said, the resources are amazing."
One million BBC Micro Bit computers were delivered free to every Year Seven student in England and Wales earlier this year.
Up to 25 jobs are under threat at Europe's biggest granite quarry in Argyll, according to union leaders.
[NEXT_CONCEPT]
The Raspberry Pi has sold 10 million units - continuing its success as the most popular British computer ever.