| The Dunhuang block is an important part of the southernmost edge of the Central Asian Orogenic Belt,which is located on the eastern edge of the Tarim Plate.It connects the Tianshan-Beishan orogenic belt to the north,the Altun strike-slip fault to the south,and has a stable basement of the Precambrian metamorphic craton,and goes through the complex ocean basin expansion,plate subduction,collisional orogenic and post-orogenic ocean-to-land conversion and intraplate activities.This paper selects magmatic rocks in the southern margin of the Dunhuang block for petrology,mineralogy,geochemistry,LA-ICP-MS zircon U-Pb geochronology,and Sr-Nd-O isotopic geochemistry to determine the diagenetic ages of magmatic rocks,petrogenesis,magma source area,tectonic magma evolution processes,etc.The Dobagou gold deposit and Yanlongshan scandium deposit were selected as typical deposits.Pb-S-O-H isotopes,fluid inclusions and gold occurrence states of the ore were studied,and the sources and genesis of ore-forming materials were discussed.Analyzed 39 kinds of elemental data of 1:200,000 geochemical exploration in Dunhuang block,using gold as predictive mineral,carried out quantitative selection of gold prospecting target area based on magmatic rock information,providing reliable evidences for gold ore prospecting in the area.Based on the study of 9 granites and 3 basic-ultrabasic rock masses on the southern edge of the Dunhuang block,it is believed that the tectono-magmatic evolution can be divided into four stages.?1?Large-scale cracking of the Columbian supercontinent in the late Paleoproterozoic,and the Dahongshan granite on the southern edge of the Dunhuang block responded to this stage.The diagenetic age of the Dahongshan granite is 1732.3±7.4Ma,and it is rich in silicon and alkali,with high ratios of Fe*/Mg,?K+Na?/Al,K/Na,and Ga/Al and F,Zr,Nb,Ga,Y and REE?except Eu?,etc.,while Mg,Ca,Al,Cr,Sr and Ni elements are lower,LREEs are strongly enriched,and the lower?18OV-SMOW value?7.6‰8.6‰?is associated with A2 type granites.Compared with lower Sr content and higher Yb content,it is typical A1 type granite,reflecting that the source material is dominated by low maturity crust,which is related to Paleoproterozoic?Nd model age:1.96Ga2.29 Ga?and the characteristics of the crust are the same and formed in the thin crust background.The heat from upwelling of the asthenospheric mantle causes partial melting of the lower crust,indicating that the high-temperature events happened in the lower pressure environment in the Lower Paleoproterozoic Dunhuang block occurred in the Dahongshan-Sanwei area.?2?The main evidences of early Paleozoic ocean basin closure,plate collision,subduction,and tectonic evolution are as follows:1)Kyziletag East pyroxenite-gabbro is generally characterized by quasi-aluminous and alkaline series,enriched with large-ionic lithophilic elements Rb,Ba,Sr and relative high loss field strength.The elements Ta,Nb,Zr and Hf have intervening terrestrial components and fluids,suggesting that gabbro and pyroxenite are the products of the tectonic stress release phase of the active continental margin;2)the Xiaoduogou ultramafic rocks are high MgO,Ni,Cr,and very low CaO,TiO2,Zr,Hf,Sr,Nb,etc.,especially with a very low total content of rare earth elements,suggest that ultramafic rocks may be refractory remnants after high-basic basaltic melt extraction.The mantle-derived magma were affected by the early subduction of the oceanic crust,or was contaminated by the crust during the ascending emplacement;3)The age of the diorite diagenesis at the Duobagou gold deposit was 497±13 Ma.The diagenesis age of the tonalite was 435.9±6.6 Ma and 432.8±7.2 Ma.There was a time difference in emplacement,but the characteristics of trace elements and rare earth elements showed the homologous magma,and the enrichment of Rb,Cs,K,Ba,U and Pb was obvious,relatively depleted Nb,Ta,Ti,Sr,which is mainly a product of partial melting of the crustal material.The content of Sr and Yb elements in rocks further reveals that the formation of diorite to tonalite has undergone a process of evolution from thicker crust to thinned decompression.LA-ICP-MS zircon U-Pb age was 422.2±5.6 Ma of dacite in Haizi Bay,and dacites were strongly enriched in active elements such as Rb,Cs,Th,K and Pb,indicating that this set of rocks formed in the transitional stage of the syn-collision,uplift after collision,and late orogeny,and may suffer a late stage transformation of fluids.The Dunhuang block at this stage was affected not only by the collision-orogenic activity in the southern Central Orogenic Belt,but also by the orogeny of the Central Asian Orogenic Belt on the northern margin.?3?In the Late Devonian-Early Permian,the Dunhuang block was expanded by the back-arc basin in the southern part of the Paleo-Asian Ocean and was in the process of dilation-dive-collision of the micro-oceanic basin.The Qingshigou quartz diorite?335 Ma?,Xiaohongshan granodiorite?289.6 Ma?and Dahongshan North granodiorite?272.8 Ma?with adakitic geochemistry characteristics are mainly developed,and isotope geochemistry and further reveal that the source area is part of Archaeozoic-Proterozoic Dunhuang Rocks.The Dahongshan North granite has relatively lower Sr,Y and Yb than the Xiaohongshan granite,but the Sr/Y ratio of the Dahongshan North granite is higher,indicating that the garnet content in the magmatic source area is high and the crust is thicker.?4?For the late Late Permian-early Early Triassic,the Dunhuang block was in a phase of rapid transition from intraplate extension to late collisional orogeny.Duobagou rock?252.1 Ma?,Jiashan rock?250.5 Ma?and Balonggou rock?247.5 Ma?show continuous changes in regional tectonic stress.The high-silicon,potassium,low-calcium,and low-magnesium of the Dobagou rock mass belong to the quasi-aluminum-peraluminous high-potassium calc-alkaline rock series.The LILEs are relatively rich,the HFSEs,Nb,Ta,Sr,P,and Ti are are depleted,which are similar to the geochemical characteristics of I-type granites,associated with magmatic activities in the active continental margin subduction zone.The Jiashan and Balonggou granites are silicon-rich,alkali-rich,with high TFe/Mg,?K+Na?/Al.,K/Na,Ga/Al ratios and F,Zr,Nb,Ga,REE?except Eu?,Y,etc.,while Mg,Ca,Al,Cr,Sr,and Ni are lower,and the“Seagulls Style”REE curves show the characteristics of A-type granite.Duobagou granite?I-type?,Jiashan granite?I-type to A-type?,and Balonggou granite?A-type?show transitional characteristics from I-type granite to A-type granite,that filled the blank of the tectono-magmatic evolution records of the Dunhuang block in the Late Permian to Early Triassic.The Duobagou structural altered rock type gold deposit and Yanlongshan hornblende type scandium ore deposit were discovered and evaluated.The Duobagou gold deposit is closely related to the tonalite and bare and semi-naked natural gold is the main form of gold deposit.The characteristics of H-O stable isotopic in quartz and S stable isotopic in pyrite show that the sulfur source is magmatic sulfur and the data of?DV-SMOW and?18OH2O show that the ore-forming fluid is involved in magmatic water and surface water.The magmatic water is mainly provided by the tonalite in the mining area and the Pb isotope indicates that the formation of the Dobagou gold deposit is closely related to the magmatic activity during the orogenic period,which they indicate the minerals are derived from the crust.The average homogenization temperature of the fluid inclusions in the deposits is 326.6°C,which is a high-temperature interval and may represent hydrothermal alterations when the magma is in place.The diagenesis and metallogenic ages are similar,and they are in the Silurian.The lithology of the Yanlongshan scandium deposit is hornblendite and its diagenetic age is 273.6±4.8 Ma,which is consistent with the mineralization age.The large amount of water required for diagenesis and mineralization may be melted by the subduction plate and subducted.Fluids are provided,and their formation should be related to the Permian large-scale basic-ultrabasic magmatism in the Tarim and Beishan areas.It is clearly pointed out that the mantle magmatism below the Dunhuang block was strong involvement of the subducted plate melt and subduction fluids and crust-mantle interaction process.The average grade of Sc in the Yanlongshan scandium ore mining area is 69.85×10-6.It is estimated that the total amount of the scandium ore resources is 65.196 million tons,and the total amount of base scandium metal resources is 4553.78 tons,reaching a very large scale.A multivariate statistical method was used to construct a quantitative prediction model for prospecting gold deposits based on geophysical prospecting data in Dunhuang block and a quantitative optimization model for gold prospecting target areas based on magmatic rock information.123 gold prospecting target areas were determined from geochemical information using magmatic rock information,and the prospecting target areas were optimized and a total of 27 gold prospecting targets were selected.81.5%of the target areas were less than 20 square kilometers,which provided convenience for further mineral inspection work.The rate of increase of ore-bearing rate based on the magmatic rock-optimized target area is relatively large,and the ore-producing rate of the Class I prospecting target area is increased from 38.8%to 58.2%,and increased by 1.5 times;the ore-gathering rate of the?-level prospecting target areas is determined by 22.2%increase to 33.3%,a 1.5-fold increase;the ore-seeking rate in the?-level prospecting target areas increased from 7.4%to 14.8%,which is a two-fold increase,indicating that the magmatic rock information prospecting technology was used in the Dunhuang block.The ore prospecting target areas have a preferable effect,which indicate that the gold mineralization in the area is closely related to some magmatic rocks,providing a reliable basis for the prospecting of gold deposits in the Dunhuang block. |
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