BEIJING -- Police in South China's Guangdong province said Tuesday that they have closed nine illegal rare earth mines in Longchuan county, a major rare earth reserve, over the past six months.

Police have arrested 45 people and seized over 100 metric tons of rare earth from the mines, said the police bureau of Heyuan city, which administers Longchuan.

Rare earth reserves have continued to decline in major mining areas in China, while excessive mining has resulted in pollution-related disasters and even major natural disasters in some places.

To control environmental damage and protect the non-renewable resources, China has implemented multiple policies, including production caps, export quotas, stricter emission standards and higher resource taxes.

Over the last few years, Longchuan has closed over 110 illegal rare earth mines.

However, driven by huge economic gains, underground rare earth mining has started to show signs of revival this year, the police bureau of Longchuan county said.

The county is closely monitoring the closed mines and restoring vegetation near the mines.

Rare earth metals, a group of 17 metals, are vital for manufacturing high-tech products ranging from smart phones and wind turbines to electric car batteries. The process of extracting the metals from the Earth is harmful to the environment.

China supplies more than 90 percent of the world's demand for rare earth metals, although its reserves account for just 23 percent of the world's total.