The key role of volatile-rich magma in the formation of porphyry molybdenum deposits: A case study of the Chalukou deposit, China

Volatile element abundance is a crucial factor triggering mineralization and controlling the enrichment of molybdenum in porphyry deposits. The Chalukou deposit is the largest porphyry Mo deposit in NE China, with an average grade of 0.087% Mo. In this contribution, we present petrology, bulk geochemistry, zircon U–Pb geochronology and Lu–Hf isotopes, and apatite geochemistry for the Chalukou deposit. The intrusive rocks include pre-mineralization monzogranite, syn-mineralization quartz porphyry, fine-grained porphyry and granite porphyry, and post-mineralization monzogranite and feldspar porphyry. These granitoids are metaluminous to peraluminous, and belong to alkali-calcic to calc-alkalic series. Zircon εHf(t) values of granitoids range from +0.2 to +5.7 with corresponding Hf crustal model ages of 805–1170 Ma, indicating a Mesoproterozoic juvenile crustal source modified by mantle-derived magma. They are characterized by logf(O₂) of −16.1 to −9.4, with an average of −12.5, ΔFMQ of +0.8 to +7.9, with an average of +4.5, showing a relatively high oxidation state for the syn-mineralization magma. The apatites in quartz porphyry have the highest F contents, ranging from 3.61 to 4.62 wt%. At Chalukou, F-rich magma is conducive to the convection of magma and promotes the efficient separation of differentiation magma, making the magmatic-hydrothermal fluid circulate many times, eventually resulting in the accumulation of ore-forming materials on the top of the magma chamber and gradually aggravating mineral enrichment processes.