Tectonic Evolution And Gold Mineralization In The Southwest Tethyan Ailaoshan Belt

The Ailaoshan belt is the Tethyan suture zone,lateral collision zone of India and Eurasian continents,and also orogenic gold belt formed in the setting of continental collision.In this study,the detrial source of Silurian–Triassic sedimentary rocks,petrogenesis of the Xin'anzhai pluton,and metallogenesis of typical gold deposits?Daping and Chang'an?are researched with the aim to clarify the tectonic evolution,analysis the fluid feature,ore–forming process and material source of typical gold deposits,and probe the metallogenic genesis of orogenic gold deposits formed in collision setting.The Early Silurian–Early Devonian sedimentary rocks in the Mojiang and Xinanzhai areas have the diagnosed detrial zircon U–Pb age peak of⁴50 Ma,which,together with the immature texture of sedimentary rocks,zircon morphology and Hf isotope,indicate that these sedimentary rocks were formed in the setting of continental margin and the detrital source was derived from island arc magmatic rocks in the western margin of Simao block.During the subduction of Paleo–Tethyan Ocean,the high–K calc–alkali I–type Xin'anzhai plution?²57 Ma?was produced.After the closure of Paleo–Tethyan Ocean,the Late Triassic sedimentary rocks were deposited in the extension setting.The euhedral zircon morphology,Hf isotope and close distance with the arc suggest that the island arc igneous rocks provided the detrital source of Late Triassic sedimentary rocks.The host rocks in Daping gold deposit are Neo–Proterozic diorite and Silurian dolomite,both of which host the ore veins controlled by NW-trending ductile–brittle shear zone,while the host rock in Chang'an gold deposit is Ordovician sandstone and develops the dissiminated mineralization controlled by NW-trending brittle faults.The homogenization temperatures of fluid inclusion in the quartz of metallogenic epoch in the Daping and Chang'an gold deposits are 240³80°C and 160²70°C,respectively,which correspond to the salinity of 1.4¹6.9 NaCl eq and 2¹4.2 NaCl eq,indicating that the ore–forming fluid in the Daping and Chang'an gold deposits are moderate and low–temperature low salinity H₂O–NaCl–CO₂ system,respectively.Trace elements and in situ S isotope of pyrites of different generations show that the sedimentary pyrite?py1?in the Daping gold deposit bear the Au,As,Te,Pb,etc.and their?³⁴S are 5.20¹3.00‰?mean 9.40‰?.The Au–bearing pyrites?invisible gold?in the dolomite have similar trace elements with sedimentary pyrite but more enriched Au and As,indicating that ore-forming materials derived partly from py1.The early and late pyrites bearing gold inclusions in the diorite are characterized by low other trace elements overall and enrichment of Co and Ni,respectively,which respond to mean?³⁴S values of 7.15‰and 4.23‰.These signatures,together with the occurrence of barite,indicate that ore-forming materials of early pyrites derived from magma fluid with the mechanism of redox,and late pyrites are produced by fluid-rock interation?magma fluid and diorite?.The sedimentary pyrite?py1a?in the Chang'an gold deposit contain Au,As,Pb,Se,Mo,etc.with?³⁴S of 18.15³6.20‰?mean 26.72‰?.The Au–bearing pyrites?invisible gold?have the similar trace elements with py1a and mean?³⁴S of 1.39‰,implying that ore-forming materials derived partly from py1.The C–O and He–Ar isotope systems suggest that the Daping and Chang'an gold deposits have the characterisitic of mixing crust and mantle sources.It means that the expect the contribution of wallrock,the mantle magma has provided part ore-forming materials.By contrasting the features of other gold deposits in the Ailaoshan belt,two processes of mineralization in the Ailaoshan belt are summarized as follows.The Paleo–Tethyan sedimentation in the continental margin formed initial ore–source layer enriching the trace elements of Au,As,Co,Ni,Pb,Se,Mo,Tl,Bi.The detrital source was derived from island arc igneous rocks.During Cenozoic,the Ailaoshan zone happened the large–scale strike–slip movement and the lithosphere delamination and reactivation.The mantle fluid invaded along the large–scale faults and leached the wallrocks?ore–source layer?to produce the gold deposit.