Genetic links between porphyry Mo and peripheral quartz vein Mo–Cu mineralization in the Baituyingzi district, eastern Inner Mongolia, NE China

© 2018Although there are important implications for mineral exploration, the genetic links between porphyry Mo and peripheral Mo–Cu vein deposits remain unclear. To constrain their possible relationships, this paper studied hydrothermal veins, Ar–Ar geochronology, and O–H–S–Pb isotopes from three Mo–Cu deposits in the Baituyingzi district, NE China. Mineralization in the Baituyingzi district occurs as quartz stockworks (e.g., the Baituyingzi porphyry Mo deposit) surrounding the Baituyingzi monzogranite porphyry and 0.1–1.5 m wide quartz veins (e.g., the Kulitu and Baimashigou Mo–Cu deposits) along regional faults. Apart from the quartz unidirectional solidification textures (stage 1) observed at Baituyingzi, the sequence of hydrothermal activity at each deposit was similar (although with minor differences) and includes: stage 2, sulfide-barren and Cu-bearing quartz veins related to potassic alteration (e.g., K-feldspar, biotite, hematite); stage 3, Mo-bearing quartz veins with no obvious alteration; stage 4, pyrite-rich quartz veins related to quartz-muscovite-illite alteration; and stage 5, calcite-quartz veins representing late hydrothermal activity. 40Ar/39Ar plateau ages of muscovite from stage 4 demonstrated that hydrothermal activities in Baituyingzi (241.4 ± 1.4 Ma) and Kulitu (239.4 ± 1.2 Ma) were contemporaneous, consistent with earlier Re–Os ages, indicating that the three deposits were co-genetic in the Early Triassic. Moreover, similar fluid inclusion assemblages, homogenization temperature ranges (up to 500 °C) and O–H isotopic ratios (δ18Ofluid = 4.14‰ to 7.89‰ and δD = −97‰ to −90‰ indicative of magmatic origin) of stage 2 veins, and δ34S values (0–3‰) and radiogenic lead isotope ratios of sulfide samples were identified from each deposit, further confirming that the three deposits belong to individual mineralized centers. Integrated with previous data, we consider that: (1) the Baituyingzi hydrothermal system formed during the intrusion of the ∼248 Ma Baituyingzi monzogranite porphyry, and (2) the different stress regimes (e.g., regional structures and local fluid pressures), wall rock compositions, and fluid characteristics (e.g., salinity) of initial fluid branches resulted in the formation of three hydrothermal deposits with different vein attitudes and Cu/Mo ratios. Thus, our study reveals that the Mo–Cu vein deposits can constitute individual mineralization centers of a porphyry system as a result of different fluid branches that evolved along individual T–P–wall rock routes. This finding is a novel supplement to the magmatic hydrothermal mineralization model and can provide guidance for the exploration of similar Mo–Cu vein deposits.