Epithermal Gold Mineralization And Magmatism Process Constraints In Yanbian Area

Yanbian area is one of the important epithermal-porphyry Au-Cu depositsconcentration areas related to the late Mesozoic continental volcanism in the eastern continental margin of China,characterized by the epithermal Au deposits,as well as the epithermal Au-Cu deposits and gold-rich porphyry Cu deposits.On the bases of existing researches and scientific disputes of epithermal deposits in Yanbian area,We have selected Ciweigou,Ganhegou,Wufeng-Wuxingshan deposits to carry out systematic deposit geology,fluid microthermometry,chronology,whole-rock element,Sr-Nd-Hf isotope,thermodynamic numerical simulation,mineral in-situ S isotope,and have inverted diagenesis and mineralization processes as well as evolution and succession of physical-chemical conditions of ore-forming fluids,established the dynamic model and ideal mineralization model in Yanbian area,evaluated the depth of mineralization and denuded,providing a theoretical basis for subsequent ore prospecting and mineral exploration.The progress and achievements are as follows:1.The deposit geology studies reveals that their genetic types are low-sulfide gold deposits,characterized by vein type ore bodies.At present,the exploration reveals that the Wufeng is dominated by calcite veins,and quartz and calcite complex veins appear downward;the Wuxingshan is dominated by quartz veins;the upper part of Ciweigou is gold-bearing calcite veins,and the lower part is gold-bearing quartz veins;Ganhegou is characterized by gold-bearing quartz veins.The whole spatial distribution has obvious zoning in vertical:the calcite vein in the upper part→the gold-bearing quartz-calcite veins in the middle part→the gold-bearing gray quartz veins in the middle to deep part→the gold-bearing gray-white quartz veins in the deep part from the upper to the bottom,and the quartz-adularia network veins and gold-bearing breccia on both sides of the quartz veins.The wall rock alteration near the ore body changes from silicification-adularization to sericitization-illiteization and distal propylitization-carbonation.According to the cross-cutting relationship of veins and mineral paragenesis,the ideal LS type mineralization can be divided into quartz-pyrite-hematite-arsenopyrite stage（I）,white（chalcedony）quartz stage（II）,quartz-calcite stage（III）,[gray quartz vein stage（IV）],quartz-adularia stage（V）and late carbonate-quartz stage（VI）.Au is mainly unloaded at II-IV stage.2.The sulfide content of the ore is low（mainly pyrite<3%）.Combined with the scanning electron microscope,the main gold-bearing minerals are determined to be hessite,argentite,silver,electrum,küstelite and gold.3.The fluid inclusion,microthermometry and Laser Raman show that they generally develop liquid-rich（WL-type）and vapor-rich（WV-type）inclusions,and only daughter crystallized silicate-bearing inclusions（S-type）and CO₂-containing three-phase inclusions（C type）are found in the Wuxingshan;The ore-forming fluid belongs to a medium-low temperature（160～350℃）,low salinity（less than 12 wt.%Na Cl）,near-neutral,reducing H₂O-Na Cl-CO₂system,characterized by mixing of magmatic fluid and meteoric water.The mineralizing temperature and pressure conditions of quartz veins and gold-rich quartz-calcite veins in Wufeng are 481±11～564±19K,19.0±4.5～77.4±21.2 bar,and the estimated paleodepth is about～460±140m,similar to the the gold-rich quartz-calcite veins in Ciweigou（527±24 K,45.1±12.0 bar,～430±120 m）.Smoke-grey quartz vein has lower mineralizing temperature（440±8～478±13 K）,but is located at the lower part of quartz calcite vein in Ciweigou,which may be the product of secondary or multiple fluid pulses.The mineralizing temperature and pressure conditions of quartz veins in Wuxingshan are 539±14～591±31 K,98.5±21.3 bar,and the estimated paleodepth is about～1000±210m,similar to the the gold-rich quartz veins in Ganhegou（502±26～573±21 K;27±8.8～88.3±18.6bar;700±110～950±190 m）.Based on the terrain and the outcropping of the hypabyssal porphyry in the mining district,this paper speculates that the Wuxingshan and Ganhegou gold deposit may have 200～300m uplift and erosion than the former two,which objectively explains the vertical zoning and the vacancy of the ore body.4.Detailed volcanic-subvolcanic rocks and hydrothermal mineral isotope dating reveal that the volcanic eruptions of the mineralization system are at least five times,which are equivalent to Tuntianying I cycle（Barremian stage,130-125 Ma[I]）,Tuntianying II cycle（Aptian stage,122-114 Ma[II]）,Jingouling I cycle（Albian stage I,112-109 Ma[III]）Jingouling stage II cycle（Albian stage II,108～105 Ma[IV]）and hedgehog stage cycle（Cenomanian stage,102-93 Ma[V]）.LS epithermal large-scale gold mineralization occurred at 105～102 Ma,which took place in the process of volcanic eruption（108～105Ma）and emplacement of subvolcanic rocks（108～106 Ma）of Jingouling II cycle in Yanbian area.5.The elements and Sr-Nd isotopic geochemical characteristics of volcanic rocks related to LS mineralization indicate that the volcanic-subvolcanic rocks of Jingouling II cycle with relatively high magmatic differentiation are genetically related to LS epithermal gold deposits in time and space.They are obviously enriched in LREEs and LILES,and depleted in HREEs and HFSE,which are the products of calc-alkaline arc magma eruption and hypabyssal emplacement.Combined with the depleted whole rock Sr-Nd and zircon Hf isotope results,the volcanic-subvolcanic rocks of the Jingouling p I-II cycle and the Ciweigou cycle are derived from the source attribute of the enriched mantle formed by the metasomatism of the plate exhaled fluids.They aer the product of the multiple magmatic crystallization differentiation or extraction of the basaltic（andesitic）magma chamber in the shallow crust.6.Thermodynamic numerical simulation and in-situ S isotope revealed that the overall oxygen fugacity logf O₂value of Wuxingshan→Ganhegou→Wufeng→Ciweigou decreased（in order:-37.7～-27.3→-43.0～-29.3→-46.0～-34.8→-52.5～-34.8）.With the evolution of the mineralization stage,the p H of the ore-forming fluid increased slightly,while the logf S₂value decreased.In-situ sulfur isotopes of the three deposits are mainly affected by temperature and oxygen fugacity（f O₂）fractionation,andδ³⁴S value decreases as the temperature decreases,showing the reducing fluid characteristics.S and Pb isotopes show that the ore-forming materials are derived from calc-alkaline volcanic magma.In the process of mineralization,under the influence of fluid boiling and fluid mixing,the decrease of logf O₂,logf S₂and the increase of p H are the main reasons to promote the decrease of solubility of Au and base metal complexes(Au HS _aq、Au[HS]₂^-、Cu Cl₂^-、Zn Cl_{2 aq}and Pb Cl_{2 aq})and to aggravate the deposition of gold and base metal sulfides.7.Combining chronology,magma and mineralization,it is concluded that the epithermal Au mineralization in Yanbian area should be suitable for the extensional setting of the high angle subduction of the Paleo-Pacific Plate to Eurasian Plate at112-102Ma in the late Early Cretaceous.Underplating mantle-derived basaltic magma heats the juvenile lower crust with enriched mantle attributes,resulting in partial melting.The calc-alkaline basaltic andesitic magma experienced a small amount of crust contamination in the process of upwelling to the shallow crust,and three cycles of volcanic eruptions occurred during the process of multiple magmatic crystallization differentiation or extraction.The highly differentiated Jingouling II cycle magma upwelling and emplacement along the volcanic faults or channels.At the end of volcanic eruption and subvolcanic hypabyssal emplacement,the ore-bearing fluids dissolved from the residual magma migrated to the shallow surface along the volcanic fault system and interacted with circulating meteoric water.Fluid boiling leads to the changes of physical and chemical conditions in fluid system,resulting in the deposition of Au and base metal sulfides and the formation of calcite and quartz veins.