The Jurassic Magmatism And Its Mineralization Potentiality In The Southern Lhasa Subterrane

The Late Devonian to Miocene magmatism in the Gangdese metallogenic belt recorded the dynamic processes of the evolution of the Paleo-Tethyan Ocean,the subduction of the Neo-Tethyan Ocean,the India-Asia collision and post-collision extension,and accompanied by three major metallogenic events of the Jurassic,Paleocene-Eocene and Miocene.The Jurassic magmatism distributed in the southern Lhasa subterrane has been recognized as a record of the early-stage evolution of the Neo-Tethys Ocean,extending about 1000 km from east to west.However,only a few porphyry copper?gold?deposits formed in the Xiongcun ore concentration area have been discovered.The Xiongcun ore concentration area is identified as a giant magmatic-hydrothermal centre with>1 Mt Cu and>200 t Au,which is confirmed that there is a huge accumulation of Cu and Au metallogenic elements in the Jurassic magmatic belt distributed in the southern Lhasa subterrane.Finding out the mechanisms triggering the emplacement of Jurassic magmatism and establishing the indicators of fertile magmatic suites are of great significance for improving the early-stage evolution of the Neo-Tethys Ocean and realizing the prospecting breakthrough of subduction metallogenesis.In this paper,both Jurassic infertile and fertile magmatic suites in the middle segment of southern Lhasa subterrane are studied.The zircon U-Pb Geochronology,petrogeochemistry,Hf-O isotope geochemistry,and major and trace elements of zircon,apatite,titanite,and rutile are identified by means of back scattering,cathodoluminescence,electron probe analysis,LA-ICP-MS,SHRIMP II?XRF?ICP-MS.The purpose is to further determine the petrogenesis and dynamic mechanisms,reconstruct the early-stage evolution of the Neo-Tethys ocean,identify magma source,crystallization history and crystallization conditions,evaluate the metallogenic potential of Jurassic magmatic rocks,and also to establish a criterion for fertile magmatic suites based on petrogeochemistry and accessory minerals.The research results are as follows:?1?The petrography,zircon U-Pb chronology,petrogeochemistry and Lu-Hf isotopes show that the Jurassic magmatic suites in the southern Lhasa subterrane are a series of continuously evolved magmatic rocks composed of basaltic-dioritic-granitic magmatic rocks.The peak diagenetic age is concentrated during 192-156 Ma.These granitoids are metaluminous and minor peraluminous calc-alkaline I-type granite.The sources of granitoids are juvenile underplated basaltic lower crustal rocks.Pyroxene is the main residual mineral in its magmatic source.The parent magma originated from the depleted mantle that was metasomatised by slab-derived melts±fluids±sediments.Those studies combination with previously published data favour the trigger mechanisms of Jurassic magmatic rocks were controlled by the“double subduction”model of coexistence of the flat subduction of intra-oceanic arc and steep subduction of continental margin arc.The fertile porphyries were related to the flat subduction of intra-oceanic arc.?2?In combined with saturation temperature,Sr,REE,Delta Eu,Cl,F of apatite,saturation temperature,crystallization temperature,?¹⁸O,Hf,REE,Th,U,Pb,Nb,Th/U,Ce/Sm,Yb/Gd of zircon,and crystallization temperature,REE,?Eu,Sr of titanite can effectively identify magma source,crystallization sequence of accessory minerals,crystallization conditions and dissolution of early fluids,which can be used to tracing the different petrogenesis of magmatic rocks.?3?The Jurassic magmatic oxygen fugacity,sulfur,water,copper,and volatiles were evaluated by using the major and trace elements of apatite,zircon,and titanite,combined with petrogeochemistry.The oxygen fugacity of coexisting melt increases gradually from the crystallization of apatite and titanite in the early stage of magmatic evolution to the crystallization of zircon in the later stage.The coexisting melts of fertile magmatic rocks are characterised by enrichment in H₂O and S contents.The fertile magmatic melts have higher Cu and Cl contents in comparison to those of barren magmatic rocks.The above conditions are the key factors leading to the accumulation of mineralization material in late stage of magmatic crystallization.?4?Based on petrogeochemistry and prospecting mineralogy of accessory minerals,the metallogenic indicators for Jurassic fertile magmatic suites of island-arc porphyry copper-gold deposit in the southern Lhasa subterrane were established.The parent magmas of fertile porphyries have much higher Cu,and lower MnO,Y contents.Apatite grains from fertile magmatic suites have higher Cl,MnO,Pb,and lower LREE/HREE?U/Yb?La/Ce?La/Sm.Titanite grains from fertile magmatic suites have higher TiO₂.Rutile grains from fertile magmatic suites have higher Zr,Nb,Sb,Sc,V,Hf,Ta,W,and lower Mo,Cr,Sr,Fe,Pb,Th,?REE.The coordinated variation of Cu and Au in rutile can also be used as a proxy for porphyry Cu-Au mineralization related to the Jurassic porphyry magmatic rocks in the Gangdese metallogenic belt.