Study On The Structurally Controlling On The Micro-disseminated Gold Deposits In The Western Of The Southwest Guizhou Gold Ore Concentration Area

The micro-disseminated gold deposits are also known as the Carlin-type gold deposits or Carlin-like gold deposits.Considering the ore genesis of the Carlin-type gold deposits in China remains controversialand its geological characteristics are different from the Carlin-type gold depositsin Nevada,Prof.Yao Shuzhen?2013?suggested to use the micro-disseminated gold deposits to highlight the ore bearing rocks and reflected their mineralizing features.Guizhou is an important concentrated area for micro-disseminated gold deposits.Structures developed extensively in the region and the gold mineralization is obviously controlled by structures,providing a nature laboratory for studying about the structurallyore-controlling mechanism.This paper chose three typical micro-disseminated deposits,including the Shuiying,Getang and Jiadi gold deposits to summarize the geological features of the micro-disseminated depositsIn addition,structural geochemistry,geophysical,ore deposit geochemistry,structural analysis,and numerical simulation are conducted to determine the geological body related to mineralization,the ore-forming fluid system,ore-controlling structural system and metallogenic structural interfaces.Then,this study analyzed the structural ore-controlling rules,built the structural ore-controlling model,and discussed the structuralore-controlling mechanism based on numerical simulation.The key points summarized in this paper are listed:1.This study summarized the geological features of the micro-disseminated gold deposits in the southwest Guizhou Province.The Middle and Late Permian strata are the most important ore-bearing rocks,which are predominated by impure carbonate rocks and pyroclastic rocks.Structures in this region are dominated by NE-,and EW-NWW-trending folds and faults.Gold orebodies usually occur as bedded,lenticular,and veined-like and can by classified into stratabound type and fault-controlled type.Ores are disseminated,net veined,and brecciated.The medium and low temperature minerals associated with mineralization containing pyrite,arsenopyrite,realgar?orpiment?,and quartz are developed.Silicification,illitization,carbonatization,and pyritization are closely related to gold mineralization.The hydrothermal mineralization are composed by four stages,including pyrite-arsenopyrite stage?I?,pyrite-arsenical pyrite stage?II?,pyrite-quartz stage?III?,and calcite-reaglar-orpiment-fluorite stage?IV?.And the second stage is the major mineralization stage.2.This articleconducted structural geochemistry analysis in the Getang gold field,showing the elements close related to granite and the ore-forming elements has a good spatial coincidence,and enriched obviously in the known ore block.Thestructural geochemistry suggested that gold mineralization in the study area are intimate with the concealed granite.Re-inperpreting of the existed aeromagnetic and gravity data also revealed there are large scale granitic batholiths and plutons,which are right below the known gold fields and deposits,in the deep of the ore concentrated area.Integrated with the in-situ sulfur isotope of arsenical pyrite and concorde age of inherited zircon,this study suggests that concealed granitic rocks formed atYanshanianprovided metals and heat for gold mineralization.3.To determine the ore-forming system of the micro-disseminated gold deposits,studies on fluid inclusions and isotopes were performed,and a lot of previous data were collected.The results indicate that the ore-forming fluids of the two types of orebodies share a similar feature,with middle to low temperature?100²60??,low salinity?0⁸wt.%NaCl?,and wide ranges of density?0.54¹.27 g/cm³?and pressure?6.14³59.2Mpa?,deriving from a single ore-forming system.Petrography of fluid inclusions and histograms of microthermometry data show that the homogenization temperature of the stratabound type?180²40??are relatively higher than the fault-controlled type?120¹80??,without significant unmixing and boiling.LA-ICP-MS analysis suggest that the fluid inclusions from early stage contain high Au(below detection limit³.8±0.5×10^-6),Sb(10±4²0×10^-6),and As(80±30⁹0±20×10^-6),with low Fe under?below detection limit?.Discrimination diagrams of C-O isotopes,H-O isotopes,and rare gas jointly highlight the ore-forming fluids evoluted from the mixing of deep magmatic fluid and shallow meteoric water.4.To trace the source of ore-forming metals,sulfur and lead isotopic components of pyrite from typical gold deposits were detected,and previous data were also collected.The?³⁴S of hydrothermal sulfide are-3.98⁸.4‰,showing a consistent feature with magma from mantle.The in-situ sulfur isotopic data?±5‰?of arsenical pyrite detected by previous researchers are also comparable with hydrothermal sulfide.The??=18.004⁶8.102,??=6.592⁷4.006 of hydrothermal sulfide also suggest a magmatic source from mantle.To analyze the occurrence state of gold in arsenical pyrite and arsenopyrite,electron imaging by EPMA and trace elements detecting are used,which indicated that most gold are combined as Au¹⁺in the crystal lattice of arsenical pyrite and arsenopyrite,with rare native gold.Water-rock reaction is the most important metallogenic process,and the logarithmic isocon plots show the Fe₂O₃ is immobile component and mainly extracted from wall rocks.The correlation diagram between the iron and total sulfur suggest sulfidation to be a critical metallogenic mechanism.5.According to structural analysis,the trends of ore-controlling structures are dominated by NE and EW-NWW in the southwest Guizhou gold concentrated area.Taking the line from Xiongwu through Nibao to Laowanchang as the boundary,the western region from the boundary are occupied by NE-trending structures,including the Lianhuashan,Xiongwu,Nibao,and Dachang anticlines,while the eastern region from the region are dominated byEW-NWW-trending structures,such as the Dayakou,Getang,and Huijiabao anticlines.The metallogenic interfaces are mainly faults,favorable rocks,and interlayer fractures.The faults occurred in the Huijiabao orefield,the favorable rocks are represented by the impure carbonate rocks in the Upper Permian Longtan Formation,and the interlayer fractures developed along the second segment of the Ermeishan basalt series and the unconformity interface between the Middle Permian and the Upper Permian.The shallow faults and thedeep layered structures jointly constructed a“Shallow Faults and Deep Layers”structural mode.The joints analysis and finite strain measurement showed that the Getang Orefield experienced two tectonic stresses in directions of NNE-SSW and NW-SE,respectively,and the tectonic activity compressed in NNE-SSW should be stronger than the other one.The tectonic regime transferred from compressive to tensional,and mainly performing as performing as a tensional environmentduring gold mineralization.6.Some special structural maps were compiled to summarize the regional structural ore-controlling rules:the structural framework of the southwest Guizhou gold concentrated area is featured by interphase distribution of strong and weak strain blocks.Taking the line from Xiongwu through Nibao to Laowanchang as the boundary,micro-disseminated gold deposits in the eastern region from the boundary usually occur in the weak strain blocks and controlled by EW-NWW trending anticlines;while micro-disseminated gold deposits in the western region from the boundary usually occur in the strong strain blocks,or the transitions from strong strain block to weak strain block,and controlled by NE trending anticlines.To analyze the Orefield-scale structural ore-controlling rules,the trend surface analysis,cumulative frequency method,and structural geochemistry were performed.The results indicated that gold mineralization in the Huijiabao Orefield should be occurred during two periods,are controlled by the Huijiabao anticline and mainly distributed along structural transition zones;the gold orebodies in the Getang Orefield formed by multiperiod and out of sync with the formation of SBT,the orebodies plunged to the southeast controlled by the Getang anticline,and distributed in rows and columns from northwest to southeast;the gold orebodies in the Lianhuashan Orefield predominantly occurred along the transition zones of superposed structures on the southeastern lamb of the Lianhuashan anticline,and the interlayer fractures and SBT are two major ore structures.7.To study the regional migration rule of ore-forming fluids,this paper collected the mode ranges of homogenization temperature and in-situ sulfur isotopic data of arsenian pyrite in the southwest Guizhou gold concentrated area.A comprehensive map was compiled with the collected data to show the lateral migration of ore-forming fluids in the concentrated are insignificant but may be effective in the Huijiabao anticline.Besides,the Sb,Hg,Tl,and Au deposits could be products of a single metallogenic system.This paper selected a representative cross section from the Shuiyindong gold deposit to study the deposit-scale migration rule of ore-forming fluids.The spatial distribution features of trace elements on the section suggest that trapping of anticline core are critical factor for the formation of large scale orebodies in the SBT,the fractures along the anticline core channel the ore-forming fluids upward migrate to upper Longtan Formation,replacement of iron bearing impure carbonate along favorable rocks by ore-forming fluids resulted in bedded orebodies in the Longtan Formation.8.To discuss the structural ore-controlling mechanism,a geological model was constructed based on the above research findings.Numerical simulation of ore-forming fluid flow and mass transport driven by tectonic deformation was conducted by using the FEM simulation software COMSOL.The simulation indicated that faults usually provide conduits and space for the migration and enrichment of ore-forming fluids and ore-forming material,and the critical factor is permeability difference between structures and wall rocks;the trapping of anticline causes the ore-forming fluids converge into the anticline core;fractures along the anticline core control the vertical migration of ore-forming fluids;the trapping of argillaceous rocks guides the metallogenic material migrate along the impure carbonate with high permeability and form sulfidation in this impure carbonate rocks.The ore-controlling mechanism of tectonic activities presented as deformation of geological blocks caused by tectonic stress.Extensional tectonic activity is an indispensable condition for the explosive formation of large-scale ore concentrated area?such as the southwest Guizhou gold concentrated area?in a short period.Finally,we discuss some insight about mineral exploration in the study area based on above conclusions.