Genesis And Ore-Forming Process Of The Langcun W(Mo) Deposit In Anji,Zhejiang Province

South China is an important producing area of tungsten in China.The newlydiscovered porphyry W(Mo)deposit is another important type of W deposit with significant resource.However,the origin of the ore-related magmatism,source,migration and enrichment mechanisms of the ore-forming materials are still controversial.Langcun W-Mo ore deposit,a typical tungsten deposit located in Anji in western Zhejiang Province,is a moderate scale deposit(WO3 resources are of 18,500 tons with an average WO3 grade of 1.25%).Multi-stage granitic intrusive rocks and various mineralization types are found in Langcun mining area,which provides the possibility to study the magma origin and metal enrichment mechanism of W deposit.Based on detailed field observation,geochemical and geochronological studies,we discussed the origin of the ore-related magmatism,source of the ore-forming materials,ore-forming mechanism,and geodynamic setting of the Langcun deposit.This study shows that intrusive rocks in Langcun mining area comprise biotite monzonite granite,aplite,granitic porphyry and lamprophyre.U-Pb dating of zircons by LA-ICP-MS shows that the the three granitoids were formed at 133.6±1.4 Ma,131.7±1.0 Ma and 129.7±1.1 Ma,respectively.The LA-ICP-MS U-Pb age of wolframite is 127.4±4.8 Ma.The crystallization ages of the aplite and the granitic porphyry are consistent within error,indicating that they were products of the same magmatic event.According to the temporal and spatial relationships between the intrusive rocks and the W-Mo mineralization,we hold that the W-Mo mineralization in the Langcun area was mainly related to the aplite and granitic porphyry.There are abundant crystal holes and quartz feldspar capsules in the aplite resulted from the escape of magmatic fluid,indicating that the magma is enriched in volatile and has undergone obvious fluid escape.Geochemical data show that the aplite has lower contents of K2O,Na2O,Rb,Sr,REE and other hydrophilic elements compared to granitic porphyry,indicating that the aplite was probably resulted from rapid cooling of granitic magma due to escape of voluminous fluids.Whole-rock Sr-Nd isotope and zircon Hf isotope of aplite and granitic porphyry indicate that the ore-related magma is characterized by crus-mantle mixingand is dominated by ancient crustal material.Based on mineral assemblages and crosscuting relationships of veins,the oreforming processes in the Langcun ore deposit can be divided into four stages:quartz-potassic feldspar stage(Ⅰ),quartz-sericite-sulfide stage(Ⅱ),quartz-chloritepyrite stage(Ⅲ)and calcite stage(Ⅳ).Tungsten and molybdenum mineralization mainly occurred in stages Ⅰ and Ⅱ.The exsolution of magmatic fluids is directly evidenced by the large scale of quartz and potassic feldspar masses developed in the aplite.The exsolved fluids in early stage is characterized by high-moderate temperature and moderate-low salinity and rich in CO2.H-O isotope data show that the ore-forming fluid was mainly derived from magma,which was mixed with atmospheric water in the later stage,which resulted in the decrease of temperature and salinity of ore-forming fluid.Based on the characteristic of mineralization and alteration,the geochronological and geochemical characteristics of the intrusive rocks,the geochronological of wolframite and fluid inclusion results,we conclude that the Langcun W-Mo deposit was formed during the transition from intracontinental compression to post-orogenic extension of South China in the Early Cretaceous.Due to the influence of heat and fluid brought by the upwelling of asthenosphere along the plate-torn zone,the upper crust underwent partial melting,which resulted in the formation of the W-and Mo-enriched magma.Decompression during the upwelling of volatile-enriched ore-bearing magma resulted in fluid exhumation.Early W-Mo precipitation was induced by CO2 escape from fluid boiling,and mixing of magmatic fluid with atmospheric water resulted in the Mo precipitation in later stage.