Study On The Three-dimensional Structural Geochemistry And Structural Ore-control Mechanism Of The Lannigou Gold Deposit In Guizho

The Lannigou gold deposit in Guizhou province is located at the junction of two tectonic units,Yangtze block and Cathaysia block.It is a typical super-large fault-controlled gold deposit in the "Golden Triangle" ore concentration area of Yunnan,Guizhou and Guangxi.For a long time,scholars at home and abroad have conducted a large number of studies on carlin-type gold deposits in this region by various means,and obtained corresponding results in the genesis of the deposit,the source of ore-forming materials,and the chronology of ore-forming materials.However,most of the previous studies focused on the microscopic and"two-dimensional" level.There is still a lack of research on the structural framework,the source of ore-forming fluid,the spatial distribution characteristics and regularities of ore bodies in the three dimensional space.Based on this,the article from the three dimensional space mining tectonic system,distribution of ore-forming elements and fluid inclusions temperature field and so on has carried on the system research,discusses the Lannigou gold deposits of guizhou source of ore-forming materials,ore-forming fluid properties,evolution and enrichment regularity,metallogenic tectonic ore-controlling factors and metallogenic conditions,combined with the dynamic background.To reveal the process of gold mineralization and its migration,enrichment and precipitation mechanism,and establish the metallogenic model of the deposit.The main insights of this research work are as follows:（1）The structural framework system of Lannigou gold deposit is systematically identified by means of wide area electromagnetic method,seismic and other geophysical exploration results and drilling verification,and the "three-level structure" framwork system is proposed.The Qiaoluo fault that cuts through the basement is a first-order ore guide fault,the boundary faults F1 and F2 that connect with the Qiaoluo fault are second-order ore guide faults,and F3,F2 and F6 are third-order ore host faults.The fault structure is the key controlling factor for the formation of super-large gold deposit in Lannigou gold deposit.It is found that the connection between the concealed fault Qiaoluo fault and F2 fault,and the source channel of ore-forming fluid in Lannigou gold deposit is explained.The 3D ore control structure model and 3D rock quality index（RQD）model were established by using 3D mine software Surpac,and the spatial distribution characteristics and statistical analysis were carried out.Lannigou gold ore bodies are strictly controlled by fault,related to the degree of rock rupture,the broken rock mass on the whole,the trend of the enrichment of the ore bodies,when the RQD value between 5～30%,the highest grade of gold ore,peaked at around 18%,then decrease with the higher grade of RQD value,shows that the ore-forming fluid more enrichment in broken rock mass.By using the test method of paleotectonic stress and in-situ stress,it is found that the strata in the mining area have undergone four tectonic stresses,one of which is obviously lower than the other three,which is inferred to be tectonic stresses under lithosphere extension and stretching environment.The existing maximum tectonic stress direction is 187°,which is basically consistent with the stress direction of tectonic interpretation.Combined with the regional geological background and geological characteristics of the mining area,it is inferred that there are five tectonic evolution stages in the mining area,including basin rift stage（D₂-T₂）,syngenetic compression stage（T₂）,orogenic compression stage（T₃/T₂）,collision orogenic compression stage（T₃-J1）,lithosphere extension stage（J₂-K）.The tectonism of early Jurassic and late Triassic may be related to gold mineralization.（2）Statistical software SPSS was used for statistical analysis of 9364 mineralized samples.The element histogram and Q-Q diagram show that the ore-forming elements（Au,As,Hg,Sb,S）are normally distributed.The correlation coefficient between Au and As,Hg and Sb is higher than 0.8,and the correlation between Au and Ag,Mo and W is also higher（>0.3）,but the correlation between Au and Co,Cu,Ni,Sn and Zn is weak.As,Hg,Sb,Ag were strongly correlated with W.The three-dimensional modeling and analysis of mineralizing related elements（Au,As,Hg,Sb,S）from 67770 groups of laboratory data of 215 boreholes were carried out by using the 3D mine software Surpac.The spatial distribution of mineralizing related elements was strictly limited by the ore-controlling structure.Au is most enriched at the intersection of F2 and F3 faults with the highest grade of 91.7g/t,forming thick and rich ore bodies.The intersection of secondary faults and main ore bodies has similar enrichment characteristics.The spatial distribution of As,Hg,Sb and S is similar to that of Au,but the contents of As,Hg and Sb are the highest in 200～400m elevation section,which is related to the intersection of F3 and F2 faults.The temperature measurement results of fluid inclusions in the mining area show that the homogenization temperature is normally distributed,with a maximum value of 394℃and a minimum value of 111℃,mainly concentrated between 170℃ and 260℃,and the average temperature is 228℃.The three-dimensional spatial distribution characteristics of thermal motion of fluid inclusions show that the mineralization temperature is the highest at the intersection of F2 and F3 faults,and gradually decreases to the periphery,which is consistent with the structural research results and the spatial distribution of ore-related elements,indicating that F2 is a ore-guiding structure.Combined with the RQD index and the spatial enrichment rule of ore-forming elements,it shows that the favorable ore-hosting structure spatial superposition and favorable lithologic combination（RQD index indicates the degree of fragmentation and is related to structure and lithology）are the key ore-controlling mechanism of Lannigou gold deposit.（3）The structural geochemistry of the structural fracture zone and surrounding rock of Lannigou gold mine has been studied systematically,including petrochemistry,petrography,mineral composition,pyrite composition and morphology.The results show that SO₃,Au,As,Sb,Hg,T1 and Ag are obviously enriched in the tectonic fracture zone.In addition,the contents of Mo,W and SiO₂ also increase in the tectonic fracture zone,but CaO,MgO and Na₂O decrease with the increase of Au content.The main mineral compositions in surrounding rock are quartz（59.153vol.%）,illite（11.3103vol.%）,calcite（13.1533 Vol.%）,albitite（9.4583vol.%）,andolomite（4.2753vol.%）and siderite（1.0933vol.%）.Quartz（79.361 vol.%）,illite（15.117 vol.%）,dolomite（2.596 vol.%）and pyrite（1.275 vol.%）are the main components in the structural fracture zone.Pyl,Py2 and Py3 are sedimentary or diagenetic pyrite,which are mainly found in surrounding rock,while Py4 and OPy are the core and edge of ring banded pyrite respectively,which are only found in broken zone.The content of Fe in the crushing zone usually does not increase,while the content of S increases significantly,indicating that the hydrothermal pyrite in the crushing zone is formed by sulfidation,and the ferrite and siderite in the crushing zone provide iron for the formation of the hydrothermal pyrite.（4）The systematic isotope study reveals the origin and evolution of ore-forming fluid in Shamudigou gold deposit.H-O isotope analysis of quartz shows that theδDV-SMOW of hydrothermal quartz ranges from-79‰ to-67.4‰ with an average of-73.75‰,and the oxygen isotope composition of fluid inclusion in quartz ranges from-6.97‰ to 2.53‰ with an average of-2.01‰,showing a trend of evolution from primary magmatic water to rain water.It reflects the mixing of tianshui ore-bearing hydrothermal in different ore-forming stages.The 813C（PDB）of hydrothermal calcite in the ore ranges from-5.4‰ to 1‰,with an average of-2.85‰,and δ18OV-SMOW ranges from 9.4%‰ to 24.4‰,with an average of 19.96‰.C-O isotope analysis suggests that the ore-forming fluid may come from deep magma or basement.During the upward migration of ore-forming fluid along the deep fault,isotope exchange with surrounding rock and a small amount of meteoric water were carried out,which showed the characteristics of ore-forming fluid mixing.The δ34S in Py1,Py2 and Py3 vary from-5.1‰ to 35.4‰,with an average of 15.4‰.The δ34S values of Py4 ranged from 7.6‰ to 13.3‰ with an average of 9.1‰,while those of OPy ranged from 9.4‰to 14.1‰ with an average of 12.2‰.The sulfur isotopic compositions of Py4 and OPy indicate that the deep magma may have mixed into the basin fluids during the upward migration along the deep fault.The Pb isotopic compositions of sulfide in the rich orebodies are in the range of related magmatism in the subduction zone,indicating that the ore-forming fluid related to the magmatism in the subduction zone is the main fluid source of the Lannigou gold deposit.To sum up,the results show that the ore-forming fluid of Lannigou gold deposit has the characteristics of magmatic origin,and there are obvious basin fluid and meteoric water in the migration process along the deep fault,thus forming the ore-forming fluid with the characteristics of mixed contamination.（5）Based on the study of mineral composition,in-situ isotope and mineralogy of altered minerals and pyrite,combined with regional geological characteristics and metallogenic age,it is suggested that the Lannigou gold deposit experienced two stages of hydrothermal process.The first stage of hydrothermal process was about 200～230Ma,which was related to the indosinian collisional orogeny,forming Py4 in the ring banded pyrite core.The second stage of hydrothermal process is about 130～150Ma,which is related to lithosphere extension formed by regional tectonic process（Pacific plate subduction to the west）during yanshanian period.The upwelling of ore-forming hydrothermal and iron-bearing carbonate rocks formed rim pyrite.The premetallogenic hydrothermal pyrite may promote the rapid non-equilibrium crystallization of pyrite in the metallogenic period,thus facilitating the effective transfer of gold from the ore-forming fluid to the ore-forming pyrite.Local low-grade gold orebodies may be formed in premineralization hydrothermal events.This multi-stage hydrothermal activity may be beneficial to the formation of super-large Carlin-type gold deposits in Youjiang Basin.（6）Under the action of high pressure heat source or tectonic stress,deep magmatism related ore-forming fluid upwelled to the secondary ore guide structure F2 along the deep and large fault Qiaoluo fault,and enriched at the intersection of F2 and F3 faults,diffused and permeated into the broken surrounding rock.The ore-forming fluid rich in H₂S and the Fe-rich carbonate rocks in the fault fracture zone have water-rock reaction,leading to the gold deposit in favorable structural space precipitation and enrichment,thus forming the Lannigou super-large gold deposit.