Key Factors Of Intracontinental Porphyry Mineralization

The Jinshajiang-Ailaoshan metallogenic belt is located on the eastern margin of the Qinghai-Tibet Plateau.A series of alkali-rich porphyry of 43?35 Ma and related porphyry-skarn Cu-Mo(Au)deposits developed in this belt,making it a natural laboratory to study the key factors that controls the intracontinental porphyry mineralization.Based on the systematic study of the metallogenic characteristics of alkali-rich porphyries in the Jinshajiang-Ailaoshan metallogenic belt,we focused on the fertile Narigongma,Yulong,Beiya and barren Jianchun,Shigu,Liuhe granite porphyry,to compare the geochemical and isotopic characteristics of causative intrusions and evaluate the P-T-H2O-fO2 conditions for their formation.The results unraveled that the intracontinental porphyry mineralization depends on the distinctive magmatic source affinities,water content and oxygen fugacitof their parental magmasAlthough the fertile and barren granite porphyries in the metallogenic belt have similar water content,the emplacement pressure of the barren granite porphyries(142.3?238.0 MPa)is significantly higher than that of the fertile granite porphyries(31.5?142.6 MPa),indicating that both the magmatic water content and the emplacement pressure restrict the fluid dissolution.Compared with the barren granite porphyries(?FMQ+0.6),the fertile porphyries in the Jinshajiang-Ailaoshan metallogenic belt have a higher oxygen fugacity(?FMQ+3.9),suggesting that higher oxygen fugacity is conductive to the porphyry mineralization.Moreover,high(87Sr/86Sr)i ratios(0.70500?0.70920),low bulk-rock ?Nd(t)values(-5.3 to-0.7),MgO(<1.36 wt.%)and Cr(<28.50 ppm),and radiogenic Pb isotopic compositions suggest that the granite porphyries,both fertile and barren,were most likely generated by partial melting of thickened lower crust with the addition of mantle-derived magmas.However,the fertile Narigongma,Yulong and Beiya granite porphyries have low Y(7.8?16.0 ppm)and Yb(0.60?1.49 ppm)contents,high(La/Yb)N ratios(31.29?98.86)and overall steeper REE profiles,reflecting residual garnet during partial melting of thickened lower crust,whereas the barren Jianchuan,Shigu and Binchuan granite porphyries have high Y(12.3?18.6 ppm)and Yb(1.08?1.63 ppm)contents,low(La/Yb)N ratios(30.48?38.66)and flat HREE patterns,indicating a garnet-poor magma source.The potassic intrusions associated with Eocene-Oligocene porphyry Cu-Mo mineralization are characterized by positive zircon ?Hf(t)values(+0.5?+8.1),young TDM2 ages(0.4?0.7 Ga)and mantle-like zircon ?18O values between 5.5 and 6.4‰,indicating these intrusions were probably derived from the partial melting of a Neoproterozoic lower crust hybridized with a metasedimentary component.However,the potassic intrusions associated with Eocene-Oligocene porphyry-skarn Cu-Au mineralization show variable zircon EHf(t)(-5.0?+4.3)and old TDM2 ages(0.8?1.4 Ga)and zircon ?18O values of 6.63 to 7.83‰,suggesting they are the product of the partial melting of juvenile thickened lower crust with the addition of mantle-derived magmas.Therefore,the nature and compositions of the crust control the mineralization diversity and the formation and development of the metallogenic system,and the crustal architecture controls the localization of the deposit.