At the gates of Glencore’s La Jagua mine

The complex is an hour and half south of Valledupar, capital of northern Colombia’s Cesar state, down a straight road flanked by coconut palms in the shadow of Andean peaks, small wooden eateries and the invisible presence of paramilitaries and guerrillas—made all the more tangible by the gun on the driver’s lap.

Many of La Jagua’s population of 20,000 live in small brick houses with corrugated iron roofs that struggle to ward off torrential rains currently plaguing much of Colombia. It is a town where the horse is still an everyday form of transport, trotting alongside cars with motorbikes weaving through them.

La Jagua welcomes visitors with small mines that, as you drive towards its centre, grow into the some of the country’s largest. Glencore took over the mining complex at La Jagua in 2005. While appearing open, with wide, sandy roads in and out, it is guarded by private security clad entirely in black save for a white facemask and helmet.

Strikes are common along the main road into La Jagua, where mine entrances carve up the landscape. Around 50 staff dressed conspicuously in the bright orange overalls of Colombian energy company Kilbury stand in protest at the entrance to their own mine just down the road from Glencore’s La Jagua complex.

In its prospectus for potential investors, Glencore raises concerns about strike action. “Prodeco has been impacted by some industrial activity in recent years, including a 38-day strike by employees at one of La Jagua's concessions in 2010,” it reads.

It continues to add that the “level of union membership amongst Prodeco’s employees … is generally low”. This may be down to the $7m Colombian peso (£2,385) bonus that is paid to staff who do not join unions. [I] have seen two letters from Prodeco to staff explaining this bonus to them, to be paid in two instalments over the year.

There are two options for staff in Colombia when it comes to unionisation. They may be part of a pacto collectivo, a pact between the company and employee with no union involvement. Contracts are often unilaterally imposed by employers on the workers and restrict the right to bargain or strike. Or staff may be part of a sindicato, a union. The $7m is paid by Prodeco to those who choose the former.

Since 2005, workers have been on strike five times. “In twenty years of work beforehand, there was never a strike,” says Ricardo Machado, 40, president of Sintramienerge La Jagua, the Mining and Energy Industry Workers' Union in Colombia, who has been working with Glencore’s wholly owned CDJ subsidiary for a decade.

The miners at La Jagua express anger at Glencore’s attitude towards its local staff. “When a worker anywhere else in the world has an accident, the company helps. In our case, it is the opposite; the company sacks him,” Machado adds.

With its flotation on Monday, Glencore hopes to increase production at its La Jagua mines by 48% to 7.1 million tonnes this year from 4.8 million tones last year. “The flotation will have no benefit for us here. The salary, benefits and contract will be the same,” says the clearly frustrated Richard Rangel Verges, a 35-year-old miner working for Glencore’s CMU for two years.