The roles of emplacement depth, magma volume and local geologic conditions in the formation of the giant Yulong copper deposit, Eastern Tibet

Abstract

The relative importance of the oxidation state, water content, and copper and sulfur content of the causative magma in the formation of porphyry Cu deposits is hotly debated. The giant Yulong Cu deposit in Eastern Tibet, with proven reserves of 6.5 Mt Cu at 0.62%, is an example of an economically important porphyry deposit. We investigate the relative roles of factors and different processes controlling the formation of the ores. Zircon U-Pb age data and whole rock, amphibole and apatite major and trace element geochemistry are reported together with previously published data. The results indicate a focused sub-alkaline magma event at 41 Ma in the North Yulong district and contemporaneous mineralization between 41.0 and 40.9 Ma at Yulong. Among the Yulong intrusive complex, the causative K-feldspar granite porphyry with high K2O (up to 7 wt%) and SiO2 (∼70 wt%) contents and elevated Sr/Y ratios (70–90), is interpreted to be a product of fractional crystallization of the evolved sub-alkaline magma. When the Yulong deposit is compared with the global porphyry Cu deposits with similar metal endowment and coeval sub-economic intrusions and barren igneous rocks in the belt, we find that high oxidation states (ΔFMQ + 1.0∼+2.5) and water contents (∼4–6 wt%) coupled with elevated background Cu contents (∼40–120 ppm) in the causative magma are essential in the formation of porphyry Cu deposits. However, the porphyry Cu deposits have magmatic S concentrations (∼10–100 ppm; estimated from apatite SO3 contents) comparable to that of the barren igneous rocks, independent of the relative timing of apatite crystallization to fluid exsolution. Overall, these conditions are necessary but not sufficient alone to form a porphyry deposit. The economic porphyries commonly have shallower emplacement depths (∼4–5 km) which favor extensive fluid exsolution and larger magma volume (∼50–250 km3 at Yulong) which provide enough storage of ore elements than the barren igneous rocks. In the North Yulong district, extensive fluid exsolution is absent from the two sub-economic intrusions indicated by their relatively high and fixed oxygen fugacity and H2O and Cl contents in the magma evolution. In contrast, at Yulong, a local anticlinal structure provided a trap in which exsolved fluid from the magma was concentrated, and carbonate wall rocks triggered the efficient precipitation of metals during intense fluid-rock interactions and skarn alteration. Both are also critical processes in the formation of the giant Yulong Cu deposit.