BEIJING - China has closed 40 mining companies which failed to meet environmental standards in Tibet since 2010, a national research institute said in a report on Wednesday.

"Mineral resources exploration in Tibet only involves survey, mining and dressing, but not smelting," said the report on environment change on the Tibetan plateau organized by the Institute of Tibetan Plateau Research of the Chinese Academy of Sciences (CAS).

"Prominent problems" propped up in the past decades, however, which had negatively impacted environment, the report said, adding that it was because the sparsely-populated areas with high altitudes and poor transportation conditions had made environmental monitoring very difficult.

Aiming to protect the fragile ecological environment of the Tibetan plateau, the Tibet autonomous regional government banned mining of gold dust on Jan. 1, 2006, or exploitation of iron sand on Jan. 1, 2008.

The government started to restore environment and renovate mines with prominent problems from 2003.

Fifty-six renovation projects had been carried out by 2010, in areas with a total size of 77.11 square kilometers.

Tibet has 102 varieties of underground minerals.

The regional government has tightened controls on exploration of mining resources. By 2010, the areas with registered mining rights have covered 749.62 square kilometers, less than 0.1 percent of total areas of the autonomous region.

The CAS institute describes the Tibetan plateau in its report as the areas mainly in southwest China's Tibet Autonomous Region, with an average altitude of over 4,500 meters.