Magma Evolution Of Paleozoic Intermediate Acidity Magmatic Rocks And Metallogenetic Genetic Of Porphyry Cu-Au-Mo Deposits In South Mongolia

Mongolia is located in the middle-east area of the Paleo-Asian Ocean metallogenic domain.The Mandalovoo-Gurvansaikhan island arc is one of the seventeen tectonic-stratigraphic units in southern Mongolia,and one of the most important porphyry copper metallogenic belts in the world.A series of porphyry-type Cu-(Mo)-(Au)deposits are found in this area,including the giant world-class deposits Oyu Tolgoi,giant deposits Kharmagtai,large deposits Tsagaan Suvarga and so on.In recent decades,many significant progresses have been made in the exploration of porphyry copper deposits in this area,so it has become a hot area in the exploration and metallogenic theory research of porphyry copper deposits.There are many calc-alkaline intermediate-basic intrusive rocks related to island arc magmatism in the Mandalovoo-Gurvansaikhan island arc terrane,and highly differentiated granite and alkaline granite formed in the post-subduction extensional environment.The boundaries of the terrane are defined mainly by three regional deep faults in the north,southwest,and southeast.The NE-NEE trending faults or broken zones in the terrane may be related to the tectonic activity of the regional left strike-slip faults,and control the distribution of regional strata and magma body.The ore-forming magma in the Mandalovoo-Gurbansaihan island arc experienced a certain degree of crystallization and differentiation of basic minerals in the early stage,mainly clinopyroxene and amphibole,and then experienced the crystallization and differentiation of plagioclase feldspar.The magma evolution degree is relatively low.The trace elements of the granite during the metallogenic period indicate that the rock related to the mineralization of the porphyry copper deposit has obvious characteristics of subduction zone magmatism,the diagenesis and mineralization occurred in a typical island arc environment,and the diagenetic material source is from a typical mantle source metasomatized by subduction-zone fluids.The zircon LA-ICP-MS U-Pb ages show that the diagenetic age of the quartz monzodiorite porphyrite and granodiorite porphyry related to the mineralization of the Kharmagtai copper-gold deposit is between 332 and 324 Ma.The diagenetic age of the monzogranite porphyry of the host rock of the Tsagaan Suvarga deposit is 372±3 Ma.The diagenetic age of the diorite related to the mineralization of the Bronze Fox deposit is 333±4 Ma.The diagenetic age of the andesite and granodiorite complexes related to the mineralization of the Oyut Ulaan deposit is from 338 to 332 Ma.Combined with the published data,it can be determined that the magmatic activity in the Mandalovoo-Gurbansaihan island arc porphyry deposits area is mainly divided into three stages:383 to 369 Ma,367to 363 Ma and 338 to 321 Ma.The diagenetic ages of the ore-hosting plutons related to porphyry deposit mineralization are concentrated in the Late Devonian(375 to 369 Ma)and the Early Carboniferous(338 to 328 Ma).The porphyry Cu deposits formed in the two periods of metallogenic eruption have obvious zoning characteristics.Late Devonian deposits are located in the south belt,and Early Carboniferous deposits are located in the north belt.These data also show that mineralization in theMandalovoo-Gurbansaihan island arc has undergone a gradual south-to-north migration process,possibly indicating a gradual south-to-north migration of subduction.The zircon Hf isotope results of the Kharmagtai,Oyut Ulan,Bronze Fox,and Tsagaan Suvarga plutons show that the?_Hf(t)is from+7.85 to+16.14,which is consistent with the previous data of Oyu Tolgoi(+11.6 to+14.5).These data are partly near the chondrite evolution line,and also have a certain distribution range,indicating that the diagenetic magma is the product of partial melting of the newly formed crust by Paleozoic subduction,but all of them were contaminated by mature crust materials.The two-stage Hf model age t _DM2 is mainly concentrated in the Neoproterozoic to the Late Paleozoic,which may represent the time of important crustal accretion events in this area.The LA-ICP-MS Sr isotope data reflects a relatively uniform composition of apatite.The average value of initial ⁸⁷Sr/⁸⁶Sr for granodiorite from the Bronze Fox deposit,monzonite and quartz monzonite from the Tsagaan Suvarga deposit,and Khan Bogd alkaline pluton are very consistent with the mantle composition.Sr is mainly from the mantle and weakly contaminated by crustal components.The maximum LA-ICP-MS Nd isotopic values are close to the depleted mantle evolution line,which shows the contamination by the mature crustal material.The magma fractionated weakly in the major porphyry Cu deposit and is similar to the island arc magma of the large-scale porphyry Cu deposits in the southwest Pacific metallogenic belt.The mineralization of Oyu Tolgoi and Tsagaan Suvarga have the characteristics of short-time and high-intensity.The rapid formation process from island arcs to intra-ocean arcs with a certain thickness represents the transfer of massive materials from the mantle to the crust,which is also the precondition for the mineralization of porphyry Cu deposits in Southern Mongolia.The magma has a relatively higher initial water content in the magma at the beginning and in the process of the island arc formation,so it is likely that the magma is saturated with water at a relatively low level of evolution,which is the key to the large-scale mineralization of ore-forming elements with fluid exsolution.