Late Ordovician-Early Devonian Magmatic Evolution And Its Constraints On Copper Mineralization In The Xiemisitai Area,Xinjiang

The Xiemisitai area is located in the northern part of western Junggar,Xinjiang,and is the eastern extension of the Boshchekul–Chingiz volcanic arcs in China.The area is characterized by the exposure of large areas of volcanic rocks and intrusive rocks,and the development of copper?gold?mineralization associated with volcanic rocks,subvolcanic rocks and shallow intrusions.Based on the previous studies,this paper based on detailed field geological surveys,systematic geological,geochemical and zircon U-Pb geochronology studies of different rock units and rock types exposed in the study area,in order to establish a study area.The time-space frame of the tectonics is used to identify the genesis and tectonic setting of each rock unit in different ages,and to explore the deep geodynamic processes of magma evolution and the ore-forming response of related tectono-magmatic events.The paper has achieved the following main findings and results:?1?To clarify the spatial and temporal distribution characteristics of volcanic rocks and intrusive rocks in the Xiemisitai areaThe formation time of volcanic rocks and intrusive rocks exposed in the Xiemisitai area is the Late Ordovician-Early Devonian?452⁴00Ma?.According to the size of the magmatic rock,the type of rock combination and the characteristics of the source area,it is divided into three stages:the first stage is formed in the Late Ordovician?452⁴49Ma?,only a small amount of intrusive rocks are exposed,distributed in the Midwest of the Xiemisitai Mountain,the rock types are diorite and granodiorite,represented by the Yinisal intrusive and the Boertuo diorite xenolith.The second period of formation was the Silurian?436⁴20Ma?,and the volcanic rocks were widely distributed along the Xiemisitai Mountain,and the rock combinations such as basalt-andesite-dacite?dacite-porphyry?-rhyolite?rhyolite-porphyry?rocks,The main body is dacite and rhyolite?accounting for more than 70%?;Intrusive rocks are widely exposed in the Midwest part of the Xiemisitai mountain,and the rock types are quartz-diorite,granodiorite porphyry,monzogranite and a small amount of alkali-feldspathic granite,etc.,represented by the Boertuo intrusive body and the Bulasala intrusive body.The third stage of formation was the Early Devonian?419⁴00Ma?,with less volcanic exposed,Which mainly distributed in the Moate area,represented by Moate mineralized rhyolite.The intrusive rocks are exposed in the east-central of the Xiemisitai Mountain,and the gabbro-granodiorite-?quartz syenite?-?alkali granite?-?alkali-feldspathic granite?are developed.The intermediate-acidic granitoids are the main bodies,represented by the Xiemisisai,Halegaitesai and Wulansala intrusive body.?2?Identification of source characteristics of different periods of magmatite in the Xiemistai areaLate Ordovician,Yinisal granodiorite belongs to the Weak peraluminous high-K calc-alkaline series transitional Undifferentiated I-type granite,formed at 452.0±1.9Ma;The Boertuo diorite-xenolith belong to the high-K-calc-alkaline metaluminous rocks,which are formed at449.6±2.1Ma.Their Sr-Nd isotopes and zircon Hf isotopes show a deficit characteristics.(⁸⁷Sr/⁸⁶Sr)_i=?0.7037⁰.7046?,?_Nd?t?=?-0.62⁺4.89?,?_Hf?t?=+9.7⁺14.1,and the source region is the juvenile crust.In the Silurian,volcanic rocks were mainly formed in 436⁴20Ma,and the ages of the dacite-porphyry and rhyolite-porphyry in the Bulate mineralization were 434.9±2.3Ma and423.2±0.9Ma respectively,which belonged to metaluminous-strongly aluminous high-K calc-alkaline-shoshonite series,with low(⁸⁷Sr/⁸⁶Sr)_i value?0.7022⁰.7043?and positive?_Nd?t?value?+2.64⁺5.78?;intrusive rocks,quartz-diorite and monzogranite of Boertuo intrusive body were formed in 428.9±2.6Ma and 429.2±2.4Ma respectively,which belong to the high-K calc-alkaline I-type granite.They have low(⁸⁷Sr/⁸⁶Sr)_i value?0.7039⁰.7052?,Positive?_Nd?t?value?+0.19⁺1.88?and young t_DM?0.70⁰.85Ga?.Dark corpuscular inclusions Which are discovered in quartz-diorite formed in 431.2±2.0Ma,431.5±2.6Ma.It was formed simultaneously with the host rock and has similar geochemical characteristics.The Sr-Nd isotope shows low(⁸⁷Sr/⁸⁶Sr)_i value?0.7042⁰.7043?,positive?_Nd?t?value?+4.25⁺4.81?and young t_DM?0.81⁰.88Ga?;Bulasala granodiorite-porphyry were formed 430.5±1.9Ma,belonging to the Undifferentiated I-type granite,Its zircon?_Hf?t?value is+8.2⁺13.7.The alkali-feldspathic granite of Wulansala intrusive body was formed at 422.7±2.0Ma,with low(⁸⁷Sr/⁸⁶Sr)_i value?0.7017⁰.7038?,positive?_Nd?t?value?+4.49⁺6.58?and high zircon?_Hf?t?Value?+10.0⁺14.2?.The Silurian volcanic rocks and intrusive rocks have the characteristics of depleted Sr-Nd isotopes,indicating that the magma is mainly from the partial melting of the juvenile crust,with the addition of mantle source materialIn the Early Devonian,volcanic rocks were mainly formed in 419⁴00Ma,and Moate mineralized rhyolite was formed at 419⁴11Ma,with low(⁸⁷Sr/⁸⁶Sr)_i value?0.7022⁰.7045?,positive?_Nd?t?value?+0.19⁺1.88?.Intrusive rocks,the Wulansala granodiorite was formed at 411.7±1.7Ma,belonging to high-k calc-alkaline metaluminous I-type granitoid,which with positive?_Nd?t?value?+1.66⁺3.87?,low(⁸⁷Sr/⁸⁶Sr)_i value?0.7041⁰.7046?and relatively young t_DM1?0.83¹.02Ga?isotope characteristics;Hategailesai gabbro was formed at417.9±3.6Ma,with low(⁸⁷Sr/86^Sr)_i value?0.7039⁰.7052?,positive?_Nd?t?value?+5.20⁺6.54?,young t_DM?0.59⁰.69Ga?,and zircon?_Hf?t?value+10.1⁺15.3.The alkali granite and alkali-feldspathic granite of Hategailesai intrusive body were formed at411.9±2.1Ma and 408.9±1.8Ma respectively,which belonging to high-k calc-alkaline A₂-type granite,and they have positive?_Nd?t?value?+4.17⁺4.65?)and the young t_DM1?0.74⁰.85Ga?;The quartz-syenite of the Xiemisisai intrusive body was formed at 411.1±2.2Ma,belonging to the alkaline rock,with a low(⁸⁷Sr/86^Sr)value?0.7051⁰.7058?,high?_Nd?t?value?+5.55⁺5.95?,young t_DM?0.72⁰.77Ga?.The Sr-Nd isotopic signature of the invading rocks in the Early Devonian are characterized by depleted,indicating that the source area is mainly the juvenile lower crust.?3?Exploring the tectonic setting and evolution process of the Late Ordovician-Early Devonian magmatism in the Xiemisitai areaThe Late Ordovician-Early Devonian in the Xiemisitai area is in a continental marginal arc environment with a subduction background.The tectonic magmatism of the research area should be product of the Palaeo-Asian Ocean which subducted from north to south.The start time of the Palaeo-Asian Ocean southward in the Xiemisitai area is no later than the late Ordovician?452Ma?.The evolution process is:Late Ordovician?452⁴49Ma?is the initial stage of magmatic activity.The Palaeo-Asian Ocean slab subducts to the Junggar block,the fluid originating from the subduction slab acts on the mantle wedge,and the mantle wedge partial melting.The formed mafic magma invaded the juvenile lower crust and partial melting the the juvenile lower crust,forming calc-alkaline-high-K calc-alkaline series of granodiorite and diorite.During this period,the magma activity was relatively weak.The Silurian?436⁴20Ma?is the peak period of magmatic activity.With the continuous subduction of the Palaeo-Asian Ocean,due to low angle and gravity,the subduction slab roll-back,the asthenosphere material were upwelling,and mantle-derived magmatic magmatic bottom invasion crust source of felsic magma,and a strong magmatic mixing occurred,forming intrusive rocks such as quartz-diorite,granodiorite porphyry,monzogranite and basalt,andesite,dacite,rhyolite and other volcanic rock combinations.The Early Devonian?419⁴00Ma?magmatism was a continuation of the Silurian and began to weaken.Under the continuous subduction of the slab,the subduction slab was thickened at the top and thinned in the middle,followed by the slab break-off,the asthenosphere material were upwelling,and caused partial melting of the lithospheric mantle and the juvenile lower crust,forming gabbro-granodiorite-?quartz syenite?-?alkali granite?-?alkali-feldspathic granite?rocks combination and a small number of rhyolites.The magmatic rocks in the area show the evolutionary characteristics from the west to the east from the west to the east,which is the result of the zipper closure of the Palaeo-Asian Ocean from west to east.?4?Defining the ore-forming response of tectonic magmat events in the Xiemisitai areaThe continental marginal arc environment under the background of subduction makes the Late Ordovician-Early Devonian crust in the Shemistai area strong,which is conducive to the exchange of crust and mantle,providing important mineralization for copper-gold mineralization in the area.Dynamic background,thermodynamic conditions,mineral source conditions.The magmatic activity in the area restricts mineralization from the aspects of metallogenic dynamics background,source area,oxygen fugacity and rock combination.The Silurian magmatism?436⁴20Ma?is strong,the magma rock is exposed in large scale,and the high-k calc-alkaline to shoshonite series is dominated by medium-acid rocks,and its magmatic oxygen fugacity is high,and crust-mantle is strong,which is the most promising magmatic stage.The ore-forming effects in the area are mainly divided into three types:?1?porphyry Cu-Au mineralization related to the Middle-Silurian granodiorite porphyry?430.5±1.9Ma?,and which was represented by Bulansala porphyry Cu-Au deposit;?2?porphyry Cu mineralization associated with the Silurian subvolcanic rocks——dacite porphyries?434.9±2.3Ma?and rhyolite porphyry?423.2±0.9Ma?,and which was represented by the Bulate porphyry Cu deposit;?3?the volcanic hydrothermal Cu-Ag mineralization related to the Silurian and basic volcanic rocks,which was represented by the natural copper mineralization of the Bulate volcanic rock type.?5?Analysis of metallogenic potential in the Xiemisitai areaThe Boshchekul-Chingiz volcanic arc in Kazakhstan to the Xemistai-Yamaquan-Qiongheba volcanic arc in the northern margin of Junggar developed the Paleozoic tectonic-magmatic-mineralization belt whcih started from the west to the east.The fault structure is developed in the area,the magmatic activity is strong,and the fluid activity is frequent,which provides the necessary structure,magma,provenance and fluid conditions for mineralization/mineralization,suggesting that there is great potential for prospecting in the area.The intermediate-acidic porphyry of the Middle Silurian in the Shemistai area is the target geological body for the porphyry Cu-Au mineralization;the Silurian intermediate-acidic subvolcanic rock development region is a favorable area for finding porphyry Cu mineralization.In the deep of the subvolcanic rock,there should be better prospecting potential for porphyry Cu deposits;medium and basic volcanic rocks with strong veins and alterations are favorable areas for finding natural copper mineralization of volcanic rocks.The porphyry Cu?Au?mineralization associated with early Paleozoic magmatism may be the main mineralization type for subsequent prospecting breakthroughs in the Xiemistai area.