| A series of Miocene large and super-large porphyry copper deposits formed in the post-collisional environment have been discovered in the Gangdese metallogenic belt,southern Tibet.Previous studies suggested that the ore-forming porphyry originates from the partial melting of the juvenile lower crust and mixed with mantle-derived magma,but the research for magma mixing process and its contribution to the porphyry mineralization is poverty.In this paper,we selectet the adakitic porphyry with dark microgranular enclaves and high Mg dioritic porphyry in the Zhunuo copper deposit as the research objects,via detailed petrographic observation,electron microprobe analysis(EMPA)and laser ablation plasma mass spectrometry(LA-ICP-MS)component analysis of plagioclase and amphibole to reveal the deep mineralization process in the Zhunuo area,which provides an accurately data to promote the understanding of the relationship between Miocene magmatic mixing and porphyry mineralization in Gangdese belt.There are a large number of mafic microgranular enclaves derived from enriched lithospheric mantle in the Zhunuo ore-forming porphyry.The plagioclase in the porphyry has the characteristics of rhythmic zoning.From the core to the margin of the plagioclase zone,the contents of An and Fe show a synergistic change,and there is abrupt change zone with ?An?15mol% and rhythm zone with ?An<15 mol %.Amphibole in porphyry can be divided into two types: euhedral amphibole(type I)and amphibole with erosion and poikilitic texture(type ?).Compared with type ? amphibole,type I amphibole has the characteristics of high Si,Mg and low Al,the crystallization temperature of type I amphibole are 704?791?,the pressure is0.2?1.5kbar,the corresponding depth is about 0.6?5.5km,and the oxygen fugacity is ?FMQ =+2.3?+3.0;the crystallization temperature of type ? amphibole are 690?795?,the pressure is0.3?2.6kbar,the corresponding depth is about 1.0?10.0km,and the oxygen fugacity is ?FMQ =+1.7?+3.2.Type ? amphibole is mainly developed in Zhunuo mafic enclave,with crystallization temperature of 739?845?,the pressure of 1.0?4.4kbar,the corresponding depth of 3.8 ? 16.5km,and the oxygen fugacity of ?FMQ = +1.4 ? +2.9.Type I amphibole and type ? amphibole are developed in the Zhunuo diorite porphyrite,the crystallization temperature of type I amphibole is703 ? 725?,the pressure is less than 0.5kbar,the corresponding depth is less than 2km,and the oxygen fugacity is ?FMQ = +2.8?+2.9;The crystallization temperature of type ? amphibole is720 ? 827?,the pressure is 0.5 ? 3.1kbar,the corresponding depth is 1.9 ? 11.7km,and the oxygen fugacity is ?FMQ = +1.8?+3.0.These phenomena indicate that the Zhunuo metallogenic magma mixed with mantle-derived mafic magma at different depths of the crust during the evolution precess.Two magma mixing events corresponding to the inclusions and diorite porphyrite with ore forming porphyrite occurred at the depth of 16?13km and 12?9km,respectively.At this time,the amphibole composition showed that the mixed magma oxygen fugidity evolved from +1.4 ? +1.8 in the deep to +2.3 ? +3.2 in the shallow.It shows not only that the mantle derived magma has a high oxygen fugacity,but also the oxygen fugacity of the mixed magma increases gradually in the process of crystallization and differentiation in the crust,which is conducive to the porphyry magma carrying higher metal Cu and avoiding the loss of copper by entering the sulfide phase too early.Therefore,the mantle-derived magma with high oxygen fugacity,the process of mantle-derived magma mixing with the juvenile lower crust melt,and magma crystallization differentiation in the crust are favorable factors for the formation of post collisional porphyry deposits. |
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