(Adds union statement about detained workers and suspension of talks)

SANTIAGO, June 22 (Reuters) - Workers at Chilean state-owned mining giant Codelco, the world's largest copper producer, launched a major strike on Wednesday to protest the closure of a smelter over environmental issues, though the government downplayed the impact on operations.

The Federation of Copper Workers (FTC), an umbrella group of Codelco's unions, said 50,000 workers were expected to strike, including staff and contractors after the Ventanas smelter was shuttered despite calls for investment to keep it open.

"We already have all divisions stopped today," Amador Pantoja, the union's president told Reuters. "We're going to wait calmly in the morning to see if there's a chance to talk."

The government and the company, however, moved quickly to dispel concerns over an impact to operations.

"There is no stoppage of mining sites," Finance Minister Mario Marcel said at a news conference, hours after the strike started. "Therefore the impact on financial income is basically zero."

Andre Sougarret, Codelco's interim chief executive, said there has been "some discontinuity" regarding operations, but the company was able to take steps to resume them.

"Since this was announced, we took steps to, first of all, guarantee people's safety and, on the other hand, continue operations," Sougarret said.

Later in the day, the FTC said protest leaders had been detained by police forces while protesting outside of the company's El Teniente mine.

"Due to these regretful incidents, we're suspending any attempt at talks with Codelco's upper administration and actively continue with our National Strike," the FTC said in a statement.

Codelco's board of directors approved the closure of the Ventanas smelter last week after it had been suspended for maintenance after dozens in the region fell ill. The decision was later backed by Chilean President Gabriel Boric.

The facility also operates a copper refinery, which will not be affected by the measure.

Workers had warned last week they would strike if investments to upgrade the facility were not approved and insisted on Tuesday the company should upgrade the smelter.

The area the smelter is located in is saturated with industrial operations that environmental activists have described as a "sacrifice zone" due to pollution incidents. (Reporting by Fabian Cambero and Natalia Ramos; Writing by Steven Grattan and Alexander Villegas; Editing by Chizu Nomiyama, Jonathan Oatis and Marguerita Choy)