Factors controlling the formation of large porphyry Cu deposits: A case study from the Jiurui ore district of Middle-Lower Yangtze River Metallogenic Belt using in situ zircon and apatite chemistry from syn-mineralization intrusions

Abstract

The geological processes leading to the formation of porphyry Cu deposits have been well studied and documented. However, the major factors controlling the scale of mineralization resulting in giant ore deposits, remain poorly understood. In this study, we evaluate the compositions of zircon and apatite from syn-mineralization intrusions associated with two porphyry Cu deposits (Chengmenshan and Xiangutai) in the Jiurui ore district of the Middle-Lower Yangtze River Metallogenic Belt, in an attempt to reveal the possible role of mantle-contribution to ore-forming materials in controlling metal endowment in porphyry systems. The Chengmenshan and Xiangutai deposits share similar characteristics including age of formation and type of host-rocks but with different Cu endowment. Chengmenshan (2.6 Mt at grade 0.8%) is classified as well-endowed whereas Xiangutai (280,000 t at grade 0.5%) is poorly-endowed. New laser ablation ICP-MS U-Pb ages of zircon from the Chengmenshan intrusions are between 144.8 ± 1.4 Ma and 147.3 ± 1.2 Ma. These ages are similar to the U-Pb ages for the Xiangutai intrusions (146.1 ± 1.3 Ma to 149.3 ± 1.1 Ma). Both Chengmenshan (average 699 °C) and Xiangutai (average 692 °C) intrusions have similar calculated Ti-in-zircon temperatures for magma crystallization and both are inferred to have crystallized from highly oxidized magmas on the basis of calculated values of Ce/Ce*, Eu/Eu* and Ce(IV)/Ce(III) in zircon and Mn in apatite. Water contents of magma are both high and similar based on zircon trace element ratios. However, the average εHf(t) value (−3.2) of zircon from the Chengmenshan intrusions is much higher than the Xiangutai (−13.2). Furthermore, the Chengmenshan apatites show lower 87Sr/86Sr ratios (0.70780 to 0.70909, average 0.70842) than those from the Xiangutai intrusion (0.70964 to 0.71061, average 0.71017). Intrusions associated with both deposits share a similar age and physio-chemical characteristics (e.g. log (fO2), T, water content) but have different magmatic sources. This suggests that the contribution of mantle material is a controlling factor in the formation of large porphyry Cu deposits in the Jiurui ore district and even in the whole metallogenic belt.