Geochronological, isotopic and mineral geochemical constraints on the genesis of the Diyanqinamu Mo deposit, Inner Mongolia, China

Abstract

The Diyanqinamu Mo deposit, located in central Inner Mongolia in China, is a newly discovered Mo deposit. Due to the lack of detailed deposit-scale studies, the metallogenesis of the deposit has been a matter of debate. In this study, petrography, zircon U–Pb geochronology, Sr, Nd and Pb isotopes, and apatite mineral geochemistry have been used to constrain the genesis of the Diyanqinamu Mo deposit.Petrographic evidence indicates that the granites in the Diyanqinamu deposit have experienced fluid exsolution, as demonstrated by the corroded quartz in the porphyritic granites, skeletal quartz in the aplitic granites, and the unidirectional solidification texture (UST) in the apex of the aplitic granites. Sr and Nd isotopic data of the purple fluorite from the fluorite±molybdenite veins, showing the (87Sr/86Sr)i (0.7046–0.7052, avg. 0.7048) and εNd(t) values (2.4–3.8, avg. 3.3) are similar to those of the porphyritic- and aplitic granites. This similarity implies that the Mo ore-forming fluids may have been derived from the granitic magmas. The hypothesis is further supported by the fluorine content (3.79–5.48wt.%, avg. 4.53wt.%) in apatite from the porphyritic granites and the widely distributed fluorite in the hydrothermal alteration mineral assemblages, which indicate that the porphyritic granites contain high F content and the hydrothermal fluids are also F-rich. Petrographic observation also shows that magnetite occurs commonly in the hydrothermal mineral assemblages, indicating that the hydrothermal fluids are characterized by high oxygen fugacity (fO2). It is important to note that the intruding granites are essentially barren, and the Mo is mainly hosted in the andesite and volcaniclastic rocks. In comparison, Pb isotopic compositions of molybdenite are similar to those of both the andesite and K-feldspar from the porphyritic granites, which suggests that both the intruding granites and ore-hosting andesite have contributed to the ore-forming materials. This hypothesis is further supported by the abnormally high Mo for the andesite.Our new evidence suggests that the Diyanqinamu Mo deposit may have formed in a post-collisional extensional environment. Ore-forming materials may have been derived mainly from the granites and minimally from the host rocks. The ore fluids are characterized by high fO2 and being F-rich, and the trigger for the Mo mineralization has been attributed to the reduction of fO2 and/or temperature for the ore fluids.