Contrasting geochemical signatures between Upper Triassic Mo-hosting and barren granitoids in the central segment of the South Qinling orogenic belt, central China: Implications for Mo exploration

Abstract

As a world-class molybdenum province, the Qinling orogenic belt is characterized by voluminous Mesozoic granitoids, which are attributed to the closure processes of the Paleo-Tethys and the collision between the South China and North China blocks. The middle segment of the South Qinling orogenic belt is marked by widespread Triassic granitoids. Several small intrusions, occurring as porphyry stocks or apophyses, are associated with Mo mineralization that occurred at ca. 200 Ma, as constrained by molybdenite Re−Os dating. To delineate the differences between ore-hosting and barren granitic intrusions, in this contribution, we report new laser ablation inductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U–Pb ages, and element and Sr-Nd-Hf isotope data. The granitoids were emplaced episodically from 215 to 203 Ma, with the lithologies evolving from quartz diorites through granodiorites to monzogranites. They are metaluminous to peraluminous high-K calc-alkaline rocks transitional to shoshonites, showing arc-affinity geochemical traits, such as enrichment in LILEs and depletion in Nb, Ta and Ti. The ore-hosting intrusions are monzogranites with high SiO2 contents (>68 wt.%), whereas the barren ones are quartz diorites and granodiorites with low SiO2 contents (60.7–64.6 wt.%). Compared to the barren quartz diorites and granodiorites, the ore-hosting monzogranites show much lower Mg#, lower contents of transition metals (e.g., Sc, Ti, Co and V), and much stronger depletion of Sr, Ba, Eu, Ti and P. The isotopic ratios of the ore-hosting monzogranites (Isr = 0.7012 – 0.7060, εNd(t) = -3.32 − -4.58, εHf(t) = -0.81 − 2.30) and the barren granitoids (Isr = 0.7058 – 0.7060, εNd(t) = -1.24 − -5.80, εHf(t) = -3.47 − -5.24) are similar, but the former show slightly lower Isr, higher εNd(t) and higher εHf(t). This suggests that the ore-hosting magmas originated from a slightly more juvenile source, but were much more fractionated, which is also supported by their younger ages compared to the barrens. Therefore, the prolonged fractional crystallization is a feasible mechanism for the formation of the ore-hosting monzogranites. In addition, the U−Th correlation and Zr/Hf ratios indicate that the fluid process was responsible for the formation of quartz veins in both the barren and ore-hosting intrusions. Consequently, a combination of fractional crystallization and fluid action may have played a crucial role in the development of the Mo mineralization.