Skarn Copper (Gold) Metallogeny And Metallogenic Regularities In The West Section Of The Western Qinling Orogen

Skarn deposit is one of the most important ore deposit types.As an important part of the Qinling metallogenic belt,there are abundanct copper,iron or gold skarn deposits related to the Indosinian intermedium-acid intrusion in the eastern Tongren-Zeku?TZ?and western Ela Mountain?EM?of the Tongren-Elashan ore field,in Qinghai Province.The Shuangpengxi copper-gold deposit from the TZ ore field,the Saishitang and Tongyugou copper deposits from the AL ore field were selected as the typical deposits to discuss the ore-forming process and metallogenic regularity of the skarn copper?gold or orpolymetallic ore?mineralization in the TEQP.The main conclusions were listed as follows:?1?Zircons from Shuangpengxi granodiorite sample yield a U-Pb age of 233.9±2.8 Ma.During the middle Triassic,the tectonic setting may in the transition stage from compression to extension environment.Zircons of Saishitang quartz-diorite samples field yield a U-Pb age of 221.7 to 223.5 Ma.The intrusion was resulted from the partial melting of the Mesoproterozoic continental crust during late Triassic post-collisional extension setting.Both granodiorite and quartz-diorite belong to the high-K calc-alkaline series and I type granite.Based on the geological,geochemical and isotopic data,it is suggested that the middle-late Indosinian ore-related intrusive rocks in the Tongren-Ela area were originated from the partial melting of the lower crust triggered by mantle upwelling due to the delamination of the thicken lower crust in the extentional setting.?2?The whole rock geochemistry analysises show that the granodiorites in the TZ ore field characterized by high SiO₂ and K2 O contents,and by low Al₂O₃ and MgO contents.Their Mg^# value are ranged from 33.61 to 35.16.They are enriched in LREEs and LILEs,but depleted in HFSEs and display slightly negative Eu anomalies.They have higher ratio values of La/Ta?27.3-138.1?and La/Sm?4.9-17.9?but lower ratio values of Nb/U?1.2-9.3?and Ce/Pb?3.0¹1.6?.Sr-Nd-Pb isotopic compositions,with ISr ranging from 0.708299⁰.708748,?Nd?t?from-12.62^-9.47,²⁰⁶Pb/²⁰⁴Pb from 18.623¹8.965,207Pb/204 Pb from 15.610¹5.652 and ²⁰⁸Pb/²⁰⁴Pb from 39.124⁴0.732.?3?The Saishitang Quartz-diorite in the EM ore field is characterized by high Al₂O₃,K2 O,Rb,Ba,Th and Sr contents and Sr/Y and La/Yb ratios,but low MgO,Y,Yb and Ti,as well as depletion of Nb,Ta and Ti,suggesting derived from lower continental crust but had been influenced by lithospheric delamination fluids in a post-collisional setting.The quartz diorite show weekly negative to moderately negative ?Hf?t?values?-15.45^-0.56 and-11.18⁰.02?and Two-stage Hf model ages?TDM2?range from 1266²209 Ma and 1231¹769 Ma,respectively.?4?The metallogenic stages of the Indosinian epoch copper?gold or polymetallic?deposits in both of the QZ and AL ore-concentrated area can be divided into skarn???,quartz-sulfide???and sulfide-bearing quartz-calcite vein stage???.Ore-forming fluids in the early stage are characterized by high temperature?349⁵13??,high salinity?37.86%⁶1.27%?,daughter-mineral bearing fluid inclusions,which were compared to the magmatic water.Formation and meteoric water were involved in hydrothermal system in late stage.Formation of the skarn type copper?gold or polymetallic?deposits in the TEQP are associated with the mechanisms including metasomatism between intermediate-acid magma and the carbonate rock,fluid cooling,boiling and mixing.The decompression and associated boiling,due to the pressure transformation from lithostatic to hydrostatic caused by regional partial local extension,eventually lead to the precipitation and enrichment of ore minerals.?5?The concentrations of trace elements and rare earth elements?REEs?of the rocks and ores from the skarn stage???are similar.The total content of rare earth elements,especially LREEs,decrease gradually in the quartz sulfide stage???.The S(?³⁴S=+2.2‰⁺5.57‰),Pb(²⁰⁸Pb/²⁰⁴Pb = 37.834³8.672,207Pb/204 Pb = 15.442¹6.643,²⁰⁶Pb/²⁰⁴Pb = 17.867²0.056)and H-O isotopic compositions show that ore-forming materials and fluids are mainly derived from the deep magma.?6?The ore-forming processes of the skarn type copper?gold or polymetallic?deposits in the study area were controlled by sedimentary formation,fracture structure and magmatic rock,etc.The ore deposits,the mineralization and metallogenic elements display parallelly zonal distribution.The metallogenic regions are distributed in the NWW or EW trend.Ore deposits show a checkerboard or equidistant distributions in space.