MEXICO CITY, July 19 (Reuters) - Heavily armed masked attackers held up a gold mine in northern Mexico for the second time this year, making off with at least $500,000 in gold bars, officials said on Thursday.

The Cerro Colorado mine is in the desert of Sonora state, a mineral-rich region where drug-trafficking gangs are strong.

Toronto-based mine owner Sierra Minerals said the attackers entered the mine before dawn on Wednesday and stole two weeks worth of production. Nobody was injured in the raid, the second at the mine in recent months.

"We don't know if it was the same people who did it," Sierra Minerals' chief executive Martin Walker told Reuters.

He said about 750 ounces (21 kg) of gold worth $500,000 was stolen from the mine some 85 miles (140 km) from the U.S. border.

Four or five masked attackers armed with automatic weapons drove up to the mine dressed in military-style fatigues and forced employees to open a safe, a spokesman for the state attorney general said. He said the haul was even higher at around 1,160 ounces (33 kg).

Cerro Colorado is expected to produce about 20,000 ounces of gold this year, Walker said by telephone. He said the company would not leave the region despite the attacks.

"We will be beefing up security," he said.

In May, more than 20 people were killed in the region when members of Mexico's infamous Gulf Cartel of drug traffickers fought a running battle with police and troops supported by helicopters.

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