When Zhang Xiaojun was young, he worked at a gold mine digging holes underground, a job others envied because such work could provide a stable income at the time.

Zhang is a villager at Qipanliang village in Chongli district, Zhangjiakou, in Hebei province. The district, which used to be a county, is about 150 kilometers northwest of Beijing.

Since the 1980s, Chongli has developed its economy mainly by relying on locally abundant resources of minerals such as gold, coal and iron.

When Zhang turned 50 this year, he began to worry more about his health.

"After working there for several years, I began to cough a lot. My family said it must have been caused by the pollutants at mines," Zhang recalled.

He said dust accompanied him every day on the job.

"My mask was clean when I started working each day and very dirty when I finished," he said.

Zhang quit the job for his own health.

Chongli wasn't always known for its mines.

The former county－covering about 2,300 square kilometers, more than 80 percent of which is mountains－had no big industries to support its economy, relying only on some traditional agricultural products. But the mountainous terrain and lengthy snow seasons restricted the development of large-scale agriculture.

The situation changed in 1985 when a villager found two pieces of ore with high gold content while he was herding sheep.

The discovery led to a series of geological surveys and explorations by local authorities, who found rich mineral resources in the county.

Since then, mines have sprung up all over the area.

By 2006, Chongli had 10 gold mining companies and 34 iron ore enterprises, which contributed as much as 75 percent of the county's annual revenue, according to the local government.

However, the mining business started to fade about 10 years later when the government realized exploring minerals would not be a sustainable method of development.

Though the mining industry had greatly contributed to the district's economy, it also brought in high energy consuming and high polluting companies, said Guo Yanwei, deputy head of the district's development and reform bureau.

People's lives improved, but the mountains and vegetation had been destroyed. Moreover, the businesses caused other problems including ground subsidence, slag accumulation and dust pollution.

"If it continued to go like this, what would our future generations rely on? We must transform and make ecology the top priority while finding a high-quality development method," Guo said.