Metallogenic Mechanism And Prognosis Based On Comprehensive Information For Gold Deposits In The Asiha-walega Area,Eastern Segment Of The East Kunlun Orogen

The East Kunlun is an important gold polymetallic metallogenic belt in China,producing many large and medium gold fields/deposits.They include the Wulonggou,Kaihuangbei,Tuoketuo,Dachang,Dongdatan,Guoluolongwa,Asiha,Walega,and Kengdenongshe gold deposits.In recent years,a prospecting breakthrough has been made in the Asiha-Walega area,eastern segment of the East Kunlun.About 47 gold belts have been discovered and verified,and the accumulative reserves of gold resources in the area have been identified to exceed 70 tons.However,the genetic research and mineral exploration of gold deposits in the area need to be further studied.Therefore,the dissertation takes the Asiha and Walega gold deposits as the research objects.Based on systematic studies of the high-precision chronology,mineral micro-geochemistry,and rock geochemistry,the metallogenic age,source of ore-forming materials,enrichment mechanism as well as tectonic background of the gold mineralization are discussed.In addition,metallogenic prognosis based on comprehensive information for gold deposits in the Asiha-Walega area are carried out to provide reference for subsequent prospecting and exploration in the region.The main results and insights achieved are as follows:（1）The Asiha and Walega gold deposits,two typical orogenic gold deposits in the eastern segment of the East Kunlun,are strictly controlled by structures with orebodies hosted in the intermediate-acidic igneous rocks.According to the geochronological study,the hydrothermal sericite in the main ore-forming stage at Asiha yield an Ar-Ar age of235±1 Ma,indicating a Late Triassic gold mineralization.The age is consistent with those of the other gold deposits in the eastern segment of the East Kunlun and its periphery,indicating that these gold deposits are mainly formed in the Middle to Late Triassic.Therefore,the Walega gold deposit should be a product of the same period of gold mineralization.（2）Mineragraphy and pyrite LA-ICP-MS trace elements show that the gold-bearing pyrites at Asiha and Walega can be divided into six generations,showing pulsating mineralization characteristics.The second pyrite generation（Py2）in the Asiha deposit has a high gold content（1.31–17.2 ppm）,whereas pyrites from the third and fourth generations are featured by Au-As-poor cores（Py3-1 and Py4-1）and Au-As-rich rims（Py3-2 and Py4-2）.At Asiha,gold in pyrites occurs as both visible gold（gold inclusion,intergranular gold,or gold in the fracture）and invisible gold（lattice-bound solid solutions or nanoparticles）.In addition,all pyrites from the first to the third stages of the Valega deposit are characterized by the alternations of Au-As poor core and Au-As rich rim.Different from the Asiha deposit,gold in pyrites at Valega exists mainly in the form of lattice-bound solid solutions（Au⁺）.The studies show that well-defined positive correlations exist between Au and As in both deposits with As substituting for S,resulting in the lattice distortion of the pyrite which facilitates the incorporation of gold into the pyrite in the form of solid solutions.Physicochemical analysis of the ore-forming fluids indicates that gold is mainly transported as Au（HS）2^–and that fluid immiscibility is the main mechanism for gold precipitation.The fluids underwent decompression and cooling during their upward migration,which caused the desorption and precipitation of gold-sulfur complexes,and CO2 loss was a common feature of the ore-forming fluids.In situ sulfur isotopic studies of Asiha and Walega show that they have similarδ³⁴S values to those of typical gold deposits in the eastern segment of the East Kunlun,indicating that they have a magmatic-hydrothermal origin.Theδ³⁴S and Au-As contents in the rims of each pyrite generation are significantly higher than those in the core,which may be related to the pulsatile incorporation of Au-As-andδ³⁴S-rich ore-forming fluids.（3）The diagenetic chronology shows that the zircon U-Pb age of the Asha granodiorite is 240±2 Ma,formed in the Middle Triassic.They show moderate silica（57.90～65.20 wt.%）and high-K calc-alkalic signature,with high Al2O3 and Mg O（Mg^#=48.9～51.2）and low P2O5 and Ti O2 contents.They are enriched in large ion lithophile elements（e.g.,Rb,Th,U and Pb）and depleted in high field strength elements（e.g.,Nb,P and Ti）,with evident decoupling of Nb and Ta.In addition,the samples are characterized by high Ba（>960 ppm）,Sr（>900 ppm）and low Rb（<200 ppm）contents,which can be classified as high Ba-Sr granites.The Sr-Nd-Hf isotopes show that they have enriched Sr-Nd（εNd（t）=-5.7 to-5.5）and transitional Hf isotopes（εHf（t）=-3.6～-1.0）.The above geochemical features suggest that the Asiha granodiorite might be originated from mixing between the basic-lower crustal and enriched lithospheric mantle-derived magmas.（4）The gold mineralization and diagenesis in the Asiha-Walega area are concentrated in the Middle-Late Triassic,formed in a transition setting from the collision to the post-collision.In addition,they are closely related to the extensively developed deep magmatic activity during the period.The ore-forming fluids migrated towards the shallow parts along the Middle Kunlun Fault and its secondary faults,and experienced pulsating upwelling within a short period controlled by tectonic activities.The fluid immiscibility occurred during the migration.The gold-sulfur complexes carried by the fluids underwent decomplexation reaction,resulting in the precipitation of visible gold（native gold and electrum）with sulfides or the incorporation of invisible gold（solid solutions and nanoparticles）into the pyrite lattice,eventually forming the Asiha and Walega deposits.（5）The main ore-controlling factors are summarized in the Gouli area.Gold deposits in the area are often associated with the Precambrian-Early Paleozoic strata and are often distributed near the intrusions of the Caledonian and Indosinian periods or near geological boundaries.Most gold polymetallic deposits are distributed in the north of the Middle Kunlun Fault,and are located near the ore-field-scale structures.The ore bodies are controlled by the secondary faults of the structures.Based on the 1:50,000 high-precision magnetic measurement,1:25,000-1:50,000 stream sediments survey,and 1:10,000 high-precision remote sensing interpretation,fifteen types of prediction information in the Gouli area were extracted.The metallogenic prediction was carried out by the Fuzzy Logic Method with seven first-level prospecting areas and five second-level prospecting areas delineated.（6）According to the results of this prediction,our team conducted verification work in Seri,Mailong,and Dareer areas,and achieved a breakthrough in prospecting.Four new mineralized belts were discovered in the Mailong gold deposit,and more than ten gold ore bodies were delineated,with the highest gold grade reaching 27.8 g/t.In Seri gold deposit,three mineralized belts and three ore bodies were newly discovered,of which the highest gold grade was 5.53 g/t.In Dareer gold deposit,two mineralized belts were discovered and expanded,and two ore bodies were delineated,with the highest gold grade reaching 17.0 g/t.The above results show that the prospecting area delineated by the metallogenic prediction has a good prospecting prospect.