Metallogeny of the Lietinggang-Leqingla Fe–Cu–(Mo)–Pb–Zn polymetallic deposit, evidence from Geochronology, Petrogenesis, and Magmatic oxidation state, Lhasa terrane

Abstract

Lietinggang-Leqingla deposit consists of Lietinggang Fe–Cu–(Mo) ore block and Leqingla Pb–Zn–(Fe–Cu) ore block, locating in the southern margin of the central Lhasa subterrane. The ore occurs as laminated or lenticular units in skarns, distributed in the contact zone between the Jubuzhari complex intrusions and carbonate strata of the Mengla Formation. Intrusions associated with the two ore blocks are granodiorite, granodiorite porphyry, and granite porphyry of 62.4 ± 1.2 Ma, 62.6 ± 0.6 Ma, and 63.7 ± 1.0 Ma by zircon U–Pb age dating. Molybdenite Re–Os analyses yield an isochron age of 59.0 ± 1.7 Ma, with a MSWD of 1.6. Comparable isotopic ages between the intrusions and mineralization indicate that they are genetically related, and occurred during or shortly before the main stage of the Indian–Asian continental collision. Granodiorites have SiO2 content of 62.0–69.1 wt%; Al2O3 content of 15.1–15.3 wt%; total alkali concentration (K2O + Na2O) of 6.63–7.46 wt% and vary in terms of the differentiation index (DI = 67–78). Rocks from the granodiorite porphyry and granite porphyry are characterized by high SiO2 contents (72.3–74.0 wt%; 74.8–75.8 wt%) and DI (89–90; 93–94). Contents of CaO, Al2O3, MgO, Fe2O3, P2O5, TiO2 of granodiorite, granodiorite porphyry and granite porphyry show negative correlations with SiO2. Samples from the granite porphyry have the most negative Eu anomalies (δEu = 0.35–0.36), which are lower than the granodiorite porphyry and granodiorite (δEu = 0.46–0.55 and 0.46–0.66). All the rocks are enriched in large-ion lithophile elements (Rb, Ba, Pb, K and Th) and depleted in high-field-strength elements (Ta, Ti and Nb). The granodiorites have zircon ƐHf(t) values from +2.5 to +8.4, while ƐHf(t) values of the granodiorite porphyries and granite porphyries range from −6.3 to +6.8, and +1.8 to +6.9, respectively. These data suggest that the three types of intrusions are genetically related, and the less evolved granodiorites are I-type granites that were derived from a mantle-crust mixed source. The granodiorite porphyries and granite porphyries were strongly fractionated, and most likely derived from the melts represented by granodiorites through fractional crystallization of plagioclase, hornblende, biotite, ilmenite, titanite, apatite, and zircon. Zircon trace elements (average ΔFMQ values of granodiorites, granodiorite porphyries, granite porphyries are 0.15, −0.07 and 0.25, respectively) indicate that the Lietinggang-Leqingla magmas were weakly oxidized to moderately reduced. The involvement of mantle materials and medium oxygen fugacity controlled the Fe-Cu mineralization, while the contribution of ancient crustal end-member and strongly fractional crystallization caused the Pb–Zn mineralization of the Lietinggang-Leqingla deposit. Combined with previously published data, we suggest that the Fe–Cu–Pb–Zn fertilities of magmas in the central Lhasa subterrane and their metal series were not only influenced by magma sources, but also by the evolution degree (fractional crystallization) and oxidation state of the magmas.