By James Regan

August 18, 2017 - Coal mine workers from seven Glencore collieries in Australia returned to work on Thursday after a four-day work stoppage, with many voting to keep up pressure on the company over wages and job security.

More than 1,400 workers met in the mining hub of Singleton, New South Wales, about 200 km (125 miles) north of Sydney, this week and voted to continue industrial action that began in June against the Swiss-based company, according to a statement from the Construction, Forestry, Mining and Energy Union.

The latest stoppages targeted the Bulga and Ulan underground mines and five open cut mines across the Hunter Valley coal region.

The open-cut mines together produce around 31 million tonnes annually of thermal and coking coal, with the underground operations contributing a further 3 million tonnes a year.

Five of the mines are working under expired employment agreements, leaving the company exposed to the risk of ongoing industrial action until matters are resolved.

Glencore declined to comment.

The threat to production comes as spot cargo prices for Australian thermal coal from country's main coal export port of Newcastle have jumped by over 40 percent from their 2017 lows. Coal topped $100 in early August and last closed at $101.25 per tonne.

Glencore's first-half production report released on July 27, shows overall coal production of 61.1 million tonnes was up 4 percent on the year-ago period, mainly reflecting planned increases from its Australian coal mines.

In another protracted dispute between workers and Glencore 190 employees have been locked out at the Oaky North metallurgical coal mine in neighbouring Queensland state in a dispute over benefits, the union said.