Zircon trace elements and magma fertility: insights from porphyry (-skarn) Mo deposits in NE China

It has been widely accepted that magmas genetically linked to porphyry (-skarn) Cu (Mo) deposits are commonly oxidized. Recently, using laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques, zircon Ce4+/Ce3+, CeN/CeN*, EuN/EuN*, and Ce/Nd ratios, and magma ΔFMQ values (departure from the fayalite–magnetite–quartz oxygen buffer) based on zircon trace element compositions, have been used as proxies to quantify magma oxidation state. Here we present the zircon trace element compositions of 13 Mesozoic porphyry (-skarn) Mo deposits in NE China of various sizes to examine the relationship between magma Mo fertility and magma oxidation state. Generally, the studied deposits with > 0.3 Mt Mo have Ce4+/Ce3+ > 100, CeN/CeN* > 100, Ce/Nd > 10, and EuN/EuN* > 0.3, whereas those containing < 0.3 Mt Mo have Ce4+/Ce3+ < 100, CeN/CeN* < 100, Ce/Nd < 10, and EuN/EuN* < 0.3. The calculated magma ΔFMQ values do not show significant correlation with metal tonnage, probably due to the large uncertainties of the estimated ΔFMQ data. Among these proxies, Ce4+/Ce3+ and CeN/CeN* ratios show the strongest correlation with Mo tonnage, followed by Ce/Nd and EuN/EuN*. The above results confirm the previous proposal that zircon Ce and Eu anomalies can represent an intrusion’s oxidation state and indicate that the Mo endowment of magmatic-hydrothermal deposits is positively correlated with the magma oxidation state. Compared with Mo-bearing intrusions, the trends for Cu-bearing intrusions are similar but are more complicated, especially for those deposits with > 10 Mt Cu. The findings in this study can be used to evaluate an intrusion’s potential to produce Mo mineralization.