Porphyry Mo mineralization related to adakite-like magmas in an intra-continental extensional setting at the Aolunhua deposit, northeastern China

Abstract Most known porphyry Mo deposits in intra-continental extensional settings are associated with highly evolved intrusions. In this contribution, we report an unusual porphyry Mo deposit (contains 0.09 Mt Mo but no recoverable Cu), discovered in an extensional setting and intimately associated with differentiated adakite-like rocks, at Aolunhua in northeastern China. Crosscutting relationships between intrusions and offset veins indicate that the Mo-related intrusions at Aolunhua comprise at least seven units, which were emplaced during (syn-mineralization monzogranite porphyry, diorite, granodiorite porphyry, aplite, and granite porphyry) and near the end of (post-mineralization dacite porphyry and quartz porphyry) alteration and mineralization events. The majority of the ore is associated with quartz-molybdenite ± pyrite and molybdenite veins rimmed by chlorite-sericite-epidote-pyrite alteration, which are primarily hosted in the monzogranite porphyry with minor occurrences in the diorite, granodiorite porphyry, aplite, granite porphyry and hornfels. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb zircon geochronology, together with available isotope dilution–inductively coupled plasma–mass spectrometry (ID-ICP-MS) molybdenite Re-Os data, indicate the time of magmatism and mineralization in the Aolunhua deposit was between 131.6 ± 1.4 Ma (2σ) and 124.4 ± 0.8 Ma (2σ, including analytical and decay constant uncertainties). This age range is coeval with the timing of exhumation of metamorphic core complexes and development of graben basins and large-scale normal faults/detachments throughout the eastern segment of the Central Asian Orogenic Belt, indicating the Aolunhua porphyry Mo deposit formed in an intra-continental extensional setting. Whole-rock major and trace elements and Sr-Nd-Hf isotopic compositions suggest the syn-mineralization monzogranite porphyry (SiO2 = 68.2–70.2 wt%) at Aolunhua is adakite-like (Sr/Y = 57–82) with depleted mantle– or juvenile crust–derived isotopic signatures [87Sr/86Sri = 0.7049–0.7052, εNd(t) = 0.5–1.4, εHf(t) = 3.2–5.4]. Zircon trace element compositions indicate the oxidized nature of the syn-mineralization monzogranite porphyry, as evidenced by its high magmatic oxygen fugacity ([fayalite-magnetite-quartz] FMQ + 2.05 ± 0.30). An oxidized magmatic-hydrothermal system at Aolunhua is also supported by the presence of hematite and anhydrite daughter minerals in fluid inclusions hosted within the quartz phenocrysts of the Aolunhua monzogranite porphyry. The data presented in the results section indicate that the Aolunahua porphyry Mo deposit is a unique porphyry Mo deposit in an intra-continental extensional setting with many geochemical characteristics similar to porphyry Cu-Mo deposits. However, compared to porphyry Cu-Mo deposits, the magmatic apatite grains from the Aolunahua porphyry Mo deposit contain significantly lower Cl (<0.10 wt% versus up to 4.30 wt%) but comparable SO3 (up to 0.41 wt%). Accordingly, we propose that the enrichment in Mo and deficiency in Cu of the Aolunhua deposit implies the Aolunhua magmatic-hydrothermal system was Cl poor. Our results highlight the role of magmatic Cl contents in modulating the Cu/Mo of oxidized porphyry systems. Additionally, we propose that magmatic apatite Cl content can potentially be used to discriminate porphyry Mo–related intrusions from those associated with porphyry Cu-Mo systems.