Control on the size of porphyry copper reserves in the North Balkhash–West Junggar Metallogenic Belt

Abstract

The North Balkhash–West Junggar Metallogenic Belt is one of the most important areas within the Central Asian Metallogenic Domain. The belt comprises the North Balkhash and West Junggar regions, which have significantly different Cu reserves. We present mineral geochemistry, whole-rock major and trace element chemistry, zircon Hf and O isotope compositions, and zircon trace element chemistry, on the basis of which we conclude that porphyry copper deposits within the belt formed by different processes. We interpret ore-related magmas in the West Junggar region to have formed in an immature arc setting by the partial melting of lower crust (30–40 km), with some mantle input and sediments contamination. In contrast, ore-related magmas within the North Balkhash region are interpreted to have formed in a mature arc setting by the partial melting of lower crust (~40 km), with mantle input and possible upper-crustal contamination. The oxygen fugacities of ore-related intrusions within the North Balkhash region are typically higher than those within the West Junggar region. A high magmatic water content is considered to be important, but not sufficient, for the formation of large porphyry copper deposits. Thicker crust and high oxygen fugacity are the two main reasons for the higher Cu reserves in the North Balkhash region compared with the West Junggar region.