The Mesozoic Era In The Southwest Of Gakyi County In The Western Part Of Gangdise Magmatic Activity And Metallogenic Characteristics

The Mamu district in Gakyi County is a Pb-Zn skarn mineralized zone discovered in recent years.The district is located on the southern margin of the western Central Gangdese metallogenic belt(or the Central Lhasa Terrane).Mineralization occurred in the contact zone between granite porphyry and limestone.Metallic minerals include galena,sphalerite,magnetite,specularite,and pyrite,whilst non-metallic ones include garnet,diopside,rhodonite,actinolite,epidote,quartz,feldspar,and calcite.Alteration styles include silicic,argillic,marl,and chlorite.The mineralization is hosted in the Jiega Formation(Fm.)limestone.The granite porphyry is LA-ICP-MS zircon U-Pb dated to be 117.6±0.4 Ma,which shows that ore-causative porphyry was emplaced in the Early Cretaceous.The age of crystal lithic tuff is 119.7±0.5 Ma,and the tuff should thus belong to the Zenong Group(J₃K₁Z)instead of the Dianzhong Formation(E₁d)in the 1:250,000Yagra geologic map sheet.These two sets of magmatic rocks are high-potassium calc-alkaline I-type granites with the characteristics of rich in Si,rich in K and rich in Al.The normalized distribution curve of trace elements in the original mantle is very similar to that of chondrite,and the rare earth elements show obvious negative Eu anomalies.The trace elements are characterized by the strong enrichment of large ion lithophile elements such as Rb,Ba,Th,U and Pb,while the strong depletion of high field strength elements such as Nb,Ta,Ti and P,and the negative anomaly of Sr elements,indicating that granite porphyry and crystal clastic tuff may be arc volcanic rocks.The average values of zircon?_Hf(t)are-7.48 and-7.66 respectively,and the main body is a negative value with a small range of variation,suggesting that the magmatic source area of the numb study area may be dominated by the ancient basement of the middle Lhasa terrane.The crystallization temperature of zircon from granitic porphyry(MM19-9)is between 567.85 and 676.35?,The zircon crystallization temperature of MM19-218 is between 587.11 and 705.97?.The crystallization temperatures of zircons in the two sets of magmatic rocks are very close,and almost all of them are lower than 700?,indicating that the magma experienced melting under the condition of near water saturation.In the process of plate subduction,the rotation of the oceanic crust lithosphere plate causes the asthenosphere material to rise and cause the ancient basement material to remelt.Between 120?116Ma,magma intruded along the fault to form shallow magma chamber,one part erupted to form volcanic rock strata of Nong Group,and the other part emplaced and permeated,and strongly contacted and metasomatism with the limestone of Jiega formation,surrounding rock to form chalcopyrite,galena and sphalerite.The characteristics of the ore-forming rock bodies in the Maimu study area are consistent with those of the Pb-Zn deposit in the most major collision period,showing a good Pb-Zn metallogenic potential.