Age, origin and metallogenic mechanism of the Yilanlike Nb-Zr deposit in the northern margin of Tarim-South Tianshan, NW China: Evidence from zircon U-Pb dating, trace element, and Hf isotope

• Three episodes of zircon were identified in pegmatite veins at Yilanlike deposit. • Two mineralization types were identified at Yilanlike deposit. • The 3rd zircon generation were formed from 273.7 Ma to 275.7 Ma, which are related to Nb mineralization. The peralkaline rocks have the property of deep source and shallow formation, and are thought to form from mantle-derived magmas that undergo differentiation at shallow crustal levels. Alkaline-peralkaline rock complex often develops deposits rich in Nb, Ta and REE, and is also one of the most important sources of Nb and REE resources in the world. An alkaline igneous rock belt was formed between the northern margin of Tarim and the southern Tianshan in the early Permian, and many Nb-Ta-Zr-REE deposits have been developed. The Yilanlike deposit is a rare metal deposit discovered in the alkaline igneous rock belt. Yilanlike Nb-Zr rare metal mineralization mainly occurs in the Paleoproterozoic Xingditage Formation and the Upper Silurian Keketiekedaban Formation. The alkaline pegmatites in the mining area are mainly nepheline pegmatites related to alkaline syenite, and can be divided into two categories according to mineral assemblages. The first is a combination of albite, potassium feldspar, nepheline, biotite, magnetism, hematite and ilmenite. Minerals related to mineralization are mainly zircon, pyrochlore, ilmenite and monazite (YL-L4-B2). The second is a combination of albite, nepheline, aegirine, and magnetite. The minerals related to mineralization are mainly zircon, pyrochlore and columbite (HYS-07). For HYS-07, we obtained three generations of zircon U-Pb ages (Zr-1, Zr-2 and Zr-3) of 294.0 Ma, 285.4 Ma and 273.7 Ma, respectively. In addition, we obtained a zircon U-Pb age of 275.7 Ma from the sample of YL-L4-B2, which is consistent with the age of Zr-3 from HYS-07. Zr-1 is LREE-depleted and HREE-enriched, with strong positive Ce and weak negative Eu anomalies, showing a magmatic zircon–like REE pattern. Its T DM2 ages (1363 to 1454 Ma) are older than the U–Pb age, and negative εHf(t) values (−2.33 to − 0.87) indicate it has the characteristics of crust-mantle mixed source. Zr-2 also displays LREE depletion, HREE enrichment, positive Ce, and positive to weakly negative Eu anomalies. While, most of Zr-2 have low Th/U ratios (mostly < 0.1), broadly similar to hydrothermal zircon. The T DM2 ages (1412 to 1529 Ma) again exceed U–Pb ages, while εHf(t) values (−3.63 to − 1.77) are slightly lower, suggesting an origin of crust-mantle mixed source. Samples YL-L4-B2 and Zr-3 (HYS-07) share similar REE features, with HREE enrichment, LREE depletion, positive Ce, weak Eu anomalies and low Th/U ratios (mostly < 0.1), which are also interpreted as hydrothermal genesis. Both yield lower Hf isotopic values than Zr-1 and Zr-2, and their T DM2 (1418 to 1510 Ma and 1445 to 1483 Ma) ages also exceed U–Pb ages, further supporting derivation mainly from the partial melting of enriched mantle and crustal components. Therefore, we believe that complex magmatic-hydrothermal activities occurred in the Yilanlike deposit. The parental magma crystallized at about 294.0 Ma, while two hydrothermal activities at 285.4 Ma and 273.7 Ma metasomatized zircon megacrysts and recrystallized Zr-2 and Zr-3. The latest hydrothermal activity resulted in the enrichment of Nb in hydrothermal zircons, indicating that the last Nb-enriched mineralization age of the Yilanlike deposit is about 273.7 Ma.