Complex sentence: First explored in the 1960s, Hyesan Mine produces 10,000 tons of copper concentrates annually, which is 80% of all North Korean copper, and the North had estimated that it will be able to continue mining copper there for the next forty years.
Split into simpler sentences: First explored in the 1960s, Hyesan Mine produces 10,000 tons of copper concentrates annually. This area has 80% of North Korea's available copper, and the North had estimated that it will be able to continue mining copper there for the next forty years.