Ore Genesis,Metallogenic Geological Mode And Deep Metallogenic Prediction Of The Yongxin Large Au Deposit Based On 3D Digital Model In The Northwestern Lesser Xing'an Range

The study region is located at the eastern part of the Central Asian Orogenic Belt?CAOB?.It is in the Heihe area on the northwestern segment of the Lesser Xing'an Range,which is an important gold metallogenic belt in NE China.It was selected as a pilot area for deep three-dimensional?3D?metallogenic prediction in the Duobaoshan-Daxintun National mineral exploration belt in 2014.In this thesis,we choose the typical Yongxin gold deposit to carry out systematic studies on the geological characteristics,ore-forming fluids,magmatic and metallogenic geochronology and isotope geochemistry of the deposit,and further discuss the ore genesis,magmatic and metallogenic ages and the dynamic background of the Yongxin gold deposit.Based on these studies,the metallogenic characteristics and prospecting criteria are summarized,and the"trinity"comprehensive prospecting prediction model of the Yongxin gold deposit is established.Combined with the deep geophysical prospecting?such as audio-frequency magnetotelluric sounding,gravity and magnetic profiling survey?results,the 3D geological model and orebody model of the Yongxin gold deposit are established by using Creatar software.The peripheral and deep metallogenic prediction of studied area are carried out,and the deep prospecting target area is delineated.Further the verification and resources prediction are carried out for deep prospecting target area,whereas further prospecting suggestions were made,and the target area was positioned,quantitatively optimized and evaluated.The main achievements are as follows:1.The geological characteristics of the deposit reveal that the orebodies are mainly hosted in the hydrothermal breccias that are distributed in the contact zones between the Late Carboniferous syenogranite and granitic mylonite.The orebodies occurs as lenticular and veins,and along the direction or tendency appears branching phenomenon,thinning out and reappearing.The ore structures mainly include brecciated,disseminated,stockwork,comb,and geode or druse ones.The ore textures mainly include euhedral-subhedral granular,anhedral granular,metasomatie-relict and cataclastic.The main ore minerals include native gold,pyrite,sphalerite,galena,and minor chalcopyrite,among which the native gold is mainly divided into visible gold and invisible gold.Visible gold often occurs in quartz and pyrite as microfracture and inclusion gold,whereas the invisible gold mainly occurs as nanometer-sized particles Au⁰ and solid solution Au⁺in the pyrite.Gangue minerals mainly include quartz,K-feldspar,sericite,chlorite,calcite and epidote.The wall-rock alteration mainly includes potassic alteration,silicification,sericitization,kaolinization,chloritization and carbonation,of which the silicification alteration is closely related to gold mineralization.The ore-forming process is divided into four mineralization stages:quartz-K-feldspar stage?I?,gray quartz-pyrite stage?II?,black gray quartz-polymetallic sulfide stage?III?,and late quartz-calcite fine vein stage?IV?.2.LA-ICP-MS zircon U-Pb dating of volcanic-subvolcanic rocks closely related to mineralization reveal that the volcanic eruption occurred at 120¹12Ma,which can be divided into two stages:Longjiang?¹20 Ma?and Guanghua?¹12 Ma?stage.The mineralization age of the Yongxin Au deposit occurred at 107±4 Ma,as revealed by the Rb-Sr isotope dating of gold-bearing pyrites in the main mineralization stage,indicating that the metallogenic age of the Yongxin gold deposit formed in the Early Cretaceous,which is related to the volcanism of Guanghua Period.The magmatic and metallogenic thermal events extended about⁵Ma.3.The studies of geology,petrography,geochemistry and Sr-Nd-Pb-Hf isotope analyses of volcanic-subvolcanic rocks reveal these volcanic-subvolcanic rocks closely related to mineralization belong to calc-to high-K calc-alkaline series rocks,and are generally enriched in light rare earth elements and large ion lithophile elements,but depleted in high-field-strength elements.Among these igneous rocks only the granite porphyry has obvious negative Eu anomalies,whereas the rest has no or weak negative Eu anomalies.The whole-rock Sr-Nd-Pb and zircon Hf isotope studies show that the volcanic-subvolcanic rocks closely related to mineralization are formed in a volcanic arc setting on the active continental margin.The magma sources may have been affected by the subduction of the Paleo-Pacific Oceanic Plate during the Early Cretaceous.The magmas were likely derived from deep-formed magmas by the partial melting of the overlying mantle wedge by the fluids released through the dehydration of subduction Pacific Oceanic Plate,and then mixed with the juvenile lower crust-derived melts.Such mixing melts then experienced different degrees of fractional crystallization and weak crustal contamination,which should be responsible for the Au mineralization.4.The studies of S-Pb-H-O isotope,fluid inclusion,and the REE characteristics of gold-bearing pyrite and comparison with wall rocks reveal that the ore-forming fluid at the Yongxin gold deposit was a weakly reduced fluid with a low salinity;The gas phase composition for fluid inclusions is mainly H₂O,with minor CO₂,but do not contain detectable CH₄ and H₂;The ore-forming fluids could be dominantly of meteoric water with obvious water-rock reaction between meteoric water and wall rocks.The ore-forming materials could be genetically related to the volcanic-subvolcanic rocks.5.On the basis of the above studies and the comparative analysis of regional deposits,the Yongxin gold deposit is a typical low-sulfidation epithermal Au deposit,similar to many other epithermal gold deposits in Lesser Xing'an Range,which could have formed in an extensional background as a consequence of the rollback of the subducted Paleo-Pacific Oceanic Plate.Finally,combined with the results of the above studies,the"trinity"comprehensive prospecting prediction model of the Yongxin gold deposit has been established.6.Based on the research results of typical deposits,combined with regional geological,geophysical,geochemical,remote sensing survey and deep geophysical exploration comprehensive method to reveal the deep geological conditions of the study area,and uses the multi-method phase constraints and mutual correction interpretation to construct the 3D geological model and orebody model of the Yongxin gold deposit by using Creatar software.The transparency of the geological structure within the above the depth of-1500m in study area and visualization of the geology and ore body of Yongxin gold deposit have been realized,which provides a model basis for further deep prospecting prediction and target delineation.7.In this work,the metallogenic prediction theory and method combining shallow and deep,2D and 3D modeling is used.Based on the research results of typical deposits and 3D geological modeling,and guided by the prospecting prediction model,the ore-controlling favorable factors are extracted,and the shallow and deep prediction models are established by using the weights-of-evidence method.Seven shallow prospecting targets and eight deep prospecting targets have been delineated,which provided exploration basis for the further exploration in the peripheral and deep prospecting of the area.8.Based on the comparative study of shallow and deep prospecting target,and combined with the comprehensive analysis of the ore-forming geological conditions in the deep prospecting target area,eight deep prospecting target area were optimized and evaluated,and the G is the best first-level prospecting target area,the H is the second-level prospecting target area,the B,C,D is the third-level prospecting target area,whereas the A,E,F is the fourth-level target area.9.In this work,the verification and resources prediction work of the deep prospecting target area was carried out.Among them,several mineralization information were found in the deep part of deep prospecting target area in C,especially at 750⁷66m,showing strong mineralization.This result is basically consistent with the depth of the deep prediction,which verifies the accuracy and reliability of this prediction.It can be judged that the deep prospecting target areas of other surface gold bodies that have not been found are the key areas for further deep prospecting and exploration.In this work,the resource estimation of the deep prospecting target areas are carried out and the total predicted mineral resource is17879.10 kg Au.This work has also realized the positioning and quantitative evaluation of deep metallogenic prediction target area.