Why is the city of Erdenet in Mongolia called "treasured"?

Erdenet (Mongolian: Эрдэнэт, literally "with treasure") is the third-largest city in Mongolia, with a 2018 population of 98,057, and the capital of the aimag (province) of Orkhon. Located in the northern part of the country, it lies in a valley between the Selenge and Orkhon rivers about 240 km (149 mi) (as the crow flies) northwest of Ulaanbaatar, Darkhan, the capital. The road length between Ulaanbaatar and Erdenet is about 370 km (230 mi).

The city hosts the fourth largest copper mine in the world. The Erdenet Mining Corporation is a joint Mongolian-Russian venture, and accounts for a majority of Mongolia's hard currency income. Erdenet mines 22.23 million tons of ore per year, producing 126,700 tons of copper and 1,954 tons of molybdenum. The mine accounts for 13.5% of Mongolia's GDP and 7% of tax revenue. About 8,000 people are employed in the mine.