Ore-forming event and geodynamic setting of molybdenum deposit at Wenquan in Gansu Province, Western Qinling

Abstract

ARTICLES GEOLOGY Citation: Zhu L M, Ding Z J, Yao S Z, et al. Ore-forming event and geodynamic setting of molybdenum deposit at Wenquan in Gansu Province, Western Qinling.The Wenquan molybdenum deposit is associated with a Triassic granite in this area. The Wenquan granite is enriched in LILE and LREE, poor in HFSE, and has significantly higher contents of alkali (K 2 O+Na 2 O) and Sr, Ba than those of the island arc volcanic rocks. These geochemical characteristics are similar to post-collisional granites in high K calc-alkaline series. Studies of major elements, trace elements, REEs and chronology of the Wenquan pluton show that, in the geodynamic transition stage of continent-continent convergence to extension, the partial melting of the enriched lithospheric mantle generated the basaltic magma and triggered the partial melting of the thickened lower crust which produceded the acidic magma, and the Wenquan pluton was formed by mixing of the two magmas. Molybdenite Re-Os isotopic dating gave Os model ages of 212.7±2.6 Ma to 215.1±2.6 Ma with a weighted mean of 214.1±1.1 Ma, and an Re-Os isochron age of 214.4±7.1 Ma. These ages are close to K-Ar ages (223 to 226 Ma) and a SHRIMP zircon U-Pb age (223±7 Ma) for the Wenquan granite within the error range, but relatively younger. This implies that the Mo mineralization occurred in a late stage of the magmatic intrusion, and the metallogenesis took place in the transition stage from syn-collision to post-collision in the tectonic setting of the Qinling Orogenic Belt (QOB) after continental collision between the North China Block (NCB) and the South China Block (SCB). This process is also corresponding to the geological events of metamorphism and deformation in South Qinling, closure of the Mian-Lue oceanic basin, and exhumation of the Dabie-Sulu ultrahigh-pressure metamorphic rocks. The large-scale continent-continent collision between NCB and SCB in the middle Triassic triggered significant crustal thickening and exhumation of subducted slab. In the late Triassic, the tectonic setting was transformed to the transition stage from collision to extension. Materials from the asthenospheric mantle would ascend into the root of the lower crust, which could induce partial melting of the lower crust and generate Mo-enriched granitic magma. The ore-forming elements enriched in the fluid derived from the condensation and fraction of the magma resulted in the Mo mineralization. The Mo deposits in the QOB are mainly formed in two episodes, namely 220± Ma and 140± Ma. The two episodes of metallogenesis were developed in the tectonic transition settings from compression to extension, but they were in the different stages of the tectonic evolution. The occurrence of the Wenquan Mo-bearing pluton indicates that the Triassic tectonic-magmatic belt of Western Qinling is another favorable region for

Formation of the world's largest molybdenum metallogenic belt: a plate-tectonic perspective on the Qinling molybdenum deposits

“…The Wenquan Mo deposit is a porphyry-type ore deposit related to the Indosinian Wenquan granite (molybdenite Re-Os ages of 212.7 ± 2.6 to 215.1 ± 2.6 million years) , with current Mo reserves of 0.1 Mt (Song et al 2008;Zhu et al 2009). The Wenquan granite is located in the Wenquan Town area of Wushan County, Gansu Province, to the south of the W-extending part of the Shangdan suture (Figure 1).…”

“…The Wenquan granite is located in the Wenquan Town area of Wushan County, Gansu Province, to the south of the W-extending part of the Shangdan suture (Figure 1). It is hosted in the Devonian sedimentary series (Song et al 2008;Zhu et al 2009) (Figure 9). As a multiphased intrusive pluton, from the early to the late stage, the Wenquan intrusion consists of biotite granite, porphyritic biotite-monzonitic granite, porphyry-like hornblende monzonitic granite, porphyrylike macro-to giant-porphyritic biotite-monzonitic granite, and porphyry-like orthoclase granite (Li et al 2003b;Song et al 2008;Zhu et al 2009) in which monzonitic granite is the main rock type.…”

“…As a multiphased intrusive pluton, from the early to the late stage, the Wenquan intrusion consists of biotite granite, porphyritic biotite-monzonitic granite, porphyry-like hornblende monzonitic granite, porphyrylike macro-to giant-porphyritic biotite-monzonitic granite, and porphyry-like orthoclase granite (Li et al 2003b;Song et al 2008;Zhu et al 2009) in which monzonitic granite is the main rock type. Some mafic microgranular enclaves and small quantities of granodiorites are developed in the intrusion (Li et al 2003b;Zhu et al 2009). The ore deposit is not uniform and distributes mainly in the porphyry-like biotite-monzonitic granites and granitic porphyries (Song et al 2008;Zhu et al 2009).…”

“…Some mafic microgranular enclaves and small quantities of granodiorites are developed in the intrusion (Li et al 2003b;Zhu et al 2009). The ore deposit is not uniform and distributes mainly in the porphyry-like biotite-monzonitic granites and granitic porphyries (Song et al 2008;Zhu et al 2009). The controlled ore belt is 800 m long from north to south and 200 m wide from west to east, with thicknesses ranging from 37.5 to 445 m ).…”

“…The country-rock alteration is a lateral distribution including potassic alteration, silicification, kaolinization, epidotization with a little of pyritization, malachitization, and zeolitization. The Mo mineralization is closely involved in potassic alteration and silicification (Song et al 2008;Zhu et al 2009). …”