Petrogenesis Of Indosinian Granites And Gold Metallogeny Of Yangshan Gold Belt In Western Qinling

Qinling orogen has experienced complex geological evolution along with large number of magmatism.The Indosinian intrusions wildly spread at western Qinling,including Zhongchuan,Baijiazhuang,Jiaochangba,Lvjing,Luchuba,Wenquan,Geerkuohe intrusions and all kinds of dikes.Meanwhile,western Qinling,the most important a part of Qinling gold-polymetallic orogen,has found medium to large Liba,Wenquan,Dashui and Yangshan gold deposits,and hundreds of metallic ore spots,which has a closely spatial relationship with hosted intrusions and/or dikes.In this study,the Indosinian granitic dikes from Yangshan gold deposit in western Qinling has been sampled.We have discussed the relationship between Indosinian granitic dikes and gold mineralization,and built metallogenic model,further supervising ore prospecting.The western Qinling Yangshan gold ore belt is located in the Mianlue tectonic belt.This belt experienced complicated geological evolution,including subduction,collision and intracontinental orogenesis,which was accompanied by multi-stage magma activities.These complex geological processes resulted in the formation of many large and super large gold deposits,among which the Yangshan gold deposit is the biggest one.Voluminous igneous rocks occurred in the Yangshan area,which were dominated by fine-grained granite porphyrite,granite and granite aplite.These rocks have the similar geochemical features.They show high contents of SiO₂?>70wt%?,Al₂O₃ and K₂O and lower concentrations of MgO and TFeO,with K₂O/Na₂O ratios and A/CNK values higher than 1 and 1.1,respectively.Generally,they can be ascribed to peraluminous,calc-alkaline to high K calc-alkaline granites.All of them are characterized by the enrichment of Cs?U?K?Pb?Hf and Rb and the depletion of Ba?Nb?La?Ce?Sr and Ti,consistent with a crust origin.REE patterns show that they are depleted in HREEs and display lightly negative Eu anomalies,which is similar to that of the S-type granites in the western Qinling orogenic belt.These granites exhibit large variations of(⁸⁷Sr/⁸⁶Sr)i ratios?0.709184⁰.726174?and negative?_{Nd?t=220 Ma?}values,suggesting that the magma was derived from crust and/or significantly contaminated by the crust during ascent.In summary,it is inferred that the granites in Yangshan area were derived from the partial melting of lower crust and subsequently experienced the contamination of upper crust.LA-ICP-MS zircon U-Pb datings reveal that they were mainly formed during 190²28Ma.A minor group of Cretaceous zircons have also been observed.In addition,a large population of inherit zircons have been identified in the samples,which were considered as the large-grained zircons?>50¹00?m?surviving from the partial melting of the crust substances.Based on the geochemical and Sr-Nd-Pb isotopic features of granites,chlorite schist of Bikou Group and Devonian Sanhekou Formation and the regional tectonic history,it is concluded that the granites in the Yangshan area were derived from the partial melting of the thickened crust substances-Bikou Group.During the ascent,the magma experienced the contamination of the Devonian strata.The quartz H-O isotopic compositions of Bikou Group,Sanhekou Formation in the periphery of Yangshan gold deposit and early and main mineralization stages do not show large variations,with?D and?¹⁸O_H2O2O ranging from-92.4‰^-35.7‰and 5.83‰¹4.53‰,respectively.They are plotted in the right of the primary magmatic water and under the metamorphic water.Compared with the quartz from mineralization stages,the?¹⁸O_H2O2O values of Bikou Group and Sanhekou Formation are a bit lower,whereas there is no significantly difference in?D values,which is similar to that of other gold deposits in the western Qinling.Sulfur isotopic compositons of pyrites from the same sample show evident discrepancy.?³⁴S values of Pyrites from Bikou Group and Devonian strata vary from 5.6‰to 7.6‰and 8.9‰to 30.5‰,respectively,indicative of seawater sulphate.The sulfur isotopes of the pyrites from main mineralization stage range from-8.5 to 2.1‰,averaged at-1.4‰,suggesting the magmatic origin.During the Late Triassic extension following the collision between the North China and Yangtze Cratons,magma in the Yangshan area was formed through partial melting of Bikou Group which was enriched in Au.During its ascent,the magma was contaminated by Au enriched Devonian Sanhekou Formation,which made the magma possess higher contents of gold.In the course of the further upward migration,lots of volatiles escaped from the magma due to the decreasing of pressure and mineral crystallization,which induced the precipitation of gold and formed the gold ore bodies in the granite porphyrites and the Devonian strata.