(Rewrites throughout after responses from companies and unions)

LIMA, May 18 (Reuters) - Peru's production of copper and gold was largely unaffected by a national strike on Monday as unionized workers declined to down tools for fear of losing their jobs and companies used replacements.

Walk-outs at some mines, however, might have curbed silver, tin and iron output, according to unions in Peru, the world's third biggest copper, silver, zinc and tin producer and the seventh-ranked gold producer.

The strike, organized by the National Mining Federation that represents about 20,000 workers, aimed to press the government to tighten restrictions on firings and the use of contract workers.

But plans for an ambitious stoppage across Peru were upended after the government declared the strike unfounded and companies threatened to dismiss strikers or ordered contract workers to fill in, said federation head Ricardo Juarez.

Copper output from Peru's four top producers, Antamina, Southern Copper, Cerro Verde and Antapaccay, was normal, union bosses at the mines said. The mines together produced about a million tonnes of the red metal last year, or more than three quarters of Peru's total copper output.

Likewise, production at Peru's top gold deposits, Yanacocha and Misquichilca, which together churned out 1.6 million ounces of gold last year, continued unaffected, union leaders said.

But silver output from Uchucchacua was partially hurt by a strike that began earlier in May, said Carlos Galvez, the chief financial officer of operator Buenaventura . Galvez declined to specify by how much. The head of the Uchucchacua union said 50 percent of the mine's total workforce remained active.

Operations were normal at other leading silver producers, including Volcan and Antamina, said union leaders there.

Workers at the Peruvian unit of Shougang Group Co Ltd downed tools to back the national strike.

The company and the union disagreed over the impact of the labor action. Managing Director Raul Vera said production continued at 100 percent while union boss Julio Ortiz said it had stopped.

Tin output from Minsur likely slipped as unionized workers went on strike, said union boss Marco Sarca. Minsur did not respond to requests for comment. The mine produced 23,100 tonnes of tin last year.

The union and management at Peru's second biggest zinc and first biggest lead miner, Milpo, did not answer requests for comment on Monday. Antamina and Volcan are Peru's first and third biggest zinc producers, respectively.

(Reporting By Mitra Taj and Marco Aquino; Editing by Richard Pullin and Grant McCool)