The Yeniutan granodiorite in Sunan County, Gansu Province, China: Petrological features, geological settig and relationship to tungsten mineralization

Abstract

The 460 Ma-old Yeniutan granodiorite intrusion is spatially and temporally associated with the Ta'ergou tungsten deposit, one of the recently explored large tungsten resources in northwest China. It is located in the North Qilian Caledonian-aged belt and consists of a granodiorite main phase cut by dikes of biotite granite. The country rock is the Early Proterozoic Beidahe Group and Ordovician volcanic rocks which are overprinted by a 500~800 m wide thermal metamorphic aureole. The granodiorite has a coarse-grained and pseudoporphyritic texture and is composed of variable amounts of amphibole, pyroxene, biotite, plagioclase, quartz and alkali feldspar perthite. Accessory minerals are sphene, magnetite, zircon, apatite and allanite. The biotite granite is fine- to medium-grained, and is composed of plagioclase, K-feldspar, quartz and biotite with accessory apatite, zircon and thorite. The SiO2 content of the granodiorite ranges from 57 to 63 wt%, the Al index (Na+K/Al) varies from 0.43 to 0.58. The SiO2 content of the biotite granite ranges from 65 to 66 wt% and its Al index varies from 0.43 to 0.58. The two granitic rocks share similar characteristics of trace elements and rare earth elements. Their contents of compatible components such as V, Ba, Co, Ni, Cr and Sr are high, the Rb content ranges from 20 to 188 ppm, and their Rb/Sr ratios are mainly between 0.3 and 0.6. The REE content of the granodiorite ranges from 141 to 241 ppm, the La/Yb ratio varies from 10 to 31, δEu from 0.6 to 0.8. The total REE amount of the biotite granite is about 160 ppm, the La/Yb ratio varies from 19 to 36, and δEu from 0.5 to 0.9. All geochemical indicators reflect an evolution from pseudoporphyritic granodiorite to biotite granite at a generally low degree offractionation. Sr and O isotope data indicate a high degree of contamination and assimilation in the upper crust. Unlike most tungsten granite suites in which the latest intrusive unit is genetically associated with tungsten mineralization, the main-phase granodiorite of the Yeniutan intrusion is related to tungsten mineralization. With the regional geological evolution as the background and in light of geochemical data, zircon U-Pb and other dating, the Yeniutan granodiorite and its related tungsten mineralization are deduced to have formed in a pre-collisional rather than a collisional setting.