Cenozoic Tectonic Evolution And Jinchang Gold Mineralization In The Ailaoshan Belt

Ailaoshan belt,a characteristic orogenic gold belt,was formed under the background of continental collision.This study focused on the plutons of Fenshuiling and Mengha within the southwestern section of Ailaoshan belt and the precise research of typical orogenic gold deposits(Jinchang gold-nickel deposit).The results revealed the metallogenic model of the Jinchang gold-nickel deposit and the relation between the Cenozoic tectonic evolution and gold mineralization,which had certain instruction meaning for restoring the paleogeographic pattern,determining the suture zone of Yangtze and Simao block,and perfecting the Ailaoshan orogenic gold system.In this contribution,the plutons of Fenshuiling and Mengha were examined on the basis of petrography,geochronology,major and trace elemental and Sr-Nd-Hf isotopic compositions.Zircon U-Pb isotopic dating of Fenshuiling and Mengha plutons showed they were formed contemporaneously at 35±0.5 Ma,which were coeval with mafic rocks and granitic plutons within the western margin of the Yangtze block.The plutons of Fenshuiling,Mengha and mafic rocks in western Yunnan shared a comagmatic origin,the metasomatized lithospheric mantle.The Hf isotopes and two-stage Hf mantle model ages of magmatic zircons from Fenshuiling and Mengha corresponded with the Meso-Neoproterozoic(～1000 Ma)lasting northeastward oceanic subduction beneath the southwestern margin of the Yangtze block,supporting a scenario that the Ailaoshan suture should be located on the west of Ailaoshan shear zone.The Fenshuiling and Mengha granitic plutons separated from those within the western margin of the Yangtze block were attributed to Cenozoic sinistral shearing events.A unique orogenic gold deposit,Jinchang,was selected as a case study in Ailaoshan belt,and also produced considerable valuable nickel.Gold mainly occurs in ultramafic rocks and meta-sandstone in the form of the invisible gold within the arsenopyrite,and nickel occurs in the form of gersdorffite in ultramafic rocks.The ore bodies are mainly controlled by lithologic boundaries.The trace elements and S isotopes of the sulfide carried out in our study manifested the pyrite of multistage.The hydrothermal fluids of Py3 and Py4 represented the major metallogenic events of gold and nickel.Based on previous PGE and isotopic studies,it was obvious that the hydrothermal fluids of Jinchang deposit owned the characteristics of significant contribution of mantle fluid.The negative δ34S values of Py3 and Py4 may be caused by the reaction between mantle fluid and oxidized magnetite-bearing ultramafic rocks,and the negative values represented the degree of oxidation to some extent.When the fluid-rock reaction reduced the δ34S values,it also introduced Ni of ultramafic rocks into the fluid.The positive correlation between Au and As in Py4 and Apy reflected the coupling of As-Au into pyrite in the hydrothermal metallogenic system.With the decrease of temperature and the gradual rise of the fluid-rock ratio,As-Au were increasingly concentrated in Apy.Jinchang deposit and the typical orogenic gold deposits(Zhenyuan,Chang’an,etc.)in the Ailaoshan belt share the same tectonic background.The geochemical properties of the Jinchang deposit indicated that the main material source was still the enriched lithospheric mantle.Therefore,the metasomatic lithospheric mantle caused by the Neoproterozoic subduction event along the western margin of the Yangtze block may not only be the source of the Fenshuiling and Mengha plutons,but also the source of mantle fluid in the Jinchang deposit.However,potassic magmatism did not provide gold and fluid.