The geochronology, geochemistry, and petrogenesis of the Leimengou porphyries, Qinling Orogen: A case study of intrusions associated with a Dabie-type porphyry Mo deposit

Abstract

The Leimengou giant Mo deposit is a porphyry system in the northeastern Qinling Orogen along the southern margin of the North China Craton, comprising orebodies hosted mainly in a multiphase porphyritic granitic to monzonitic pluton emplaced in the Taihua Supergroup. Zircons from the syn-ore porphyritic monzogranite (PMG) and the porphyritic K-feldspar granite (PKG) yielded U − Pb ages of 131.3 ± 1.0 Ma and 130.6 ± 0.9 Ma, respectively. The post-ore, quartz monzonite porphyry (QMP), has a weighted mean 206Pb/238U age of 124.6 ± 1.0 Ma (n = 14, 1σ, MSWD = 0.13). These ages constrain the Leimengou porphyry system to have formed between 132 and 124 Ma. The PMG and PKG have similar geochemical characteristics, i.e., high-K calc-alkaline affinity, negative εHf(t) values of − 25.1 to − 22.0 and − 25.1 to − 19.2, and TDM2 ages of 2.77–2.57 Ga and 2.76–2.40 Ga, respectively. The PKG has lower concentrations for Sr, Ba, and negative anomalies of δEu, and also has higher total REE values compared to the PMG. These anomalies are interpreted to reflect fractional crystallization involving plagioclase and biotite. The characteristics of the PKG support its derivation from an evolved magma, which likely facilitated the enrichment of Mo. Additionally, the Hf isotope data suggest that the Leimengou porphyry system had a composite source, with inputs from the North Qinling Accretion Belt, the North China Craton, and the mantle. Compared to the syn-ore PMG, the post-ore QMP exhibits lower (La/Yb)N ratios of 20.5–22.8, higher zircon εHf(t) values of − 19.7 to − 9.0, and younger TDM2 ages of 2.42–1.75 Ga. This suggests that the Leimengou porphyry Mo system formed during thinning of an overthickened orogenic crust during the tectonic transition from collisional compression to post-collisional extension. The productive intrusions at Leimengou and other porphyry Mo deposits of Dabie-type have average Nb/Zr ratios that are typically below 1. Moreover, the Rb/Sr ratios range from 0.1 to 1, further distinguishing these intrusions from the intrusions associated with Climax- and Endako-type Mo deposits.