Characteristics Of Rock-mineral Assemblages Of Water-rock Reactions During The Metallogenic Process Of Dashui Gold Ore In The Western Qinling Orogenic Belt

Gold mine is an important strategic resource to economic development.Dashui gold deposit is super large gold deposit,which is located in the Xiqin mountain metallogenic belt of Western Qinling.The mineralization type is special,the ore is red and is mainly composed of silicified hematite calcite veins,silicified hematite dolomitic limestone,altered granodiorite veins,silicified hematite breccia and metasomatic jade rock.The ore body occurs in the silicified carbonate rock.Since its discovery in the 1990s,considerable progress has been made in prospecting and exploration,but the genesis of the deposit is still quite controversial,and there are still many problems in the study of mineral assemblages formed by water-rock reaction during hydrothermal mineralization.Through the study of the characteristics of water-rock reaction mineral assemblage in the ore-forming process,not only the prospecting criteria can be determined,but also the ore-forming process can be inverted and mineralogical evidence can be provided for the genesis of the deposit.Based on this,this paper takes the Au20-2 orebody,Geerkuohe rock mass and the tailings in the southern margin of Maqu Dashui gold deposit in the West Qinling as the research objects to study the characteristics and formation stages of water-rock reaction mineral assemblages in the ore-forming process by means of thin section,XRD,SEM and energy spectrum analysis,and the following understanding was obtained:(1)by observing the content,distribution,metasomatism and structure of hydrothermal minerals,the hydrothermal alteration can be divided into three stages:(?)amphibolite-chlorite stage,(?)quartz-kaolinite stage,and(?)hematite-quartz-calcite stage.The mineral assemblage of the amphibole-chlorite stage(?)consists of amphibole,chlorite,potash feldspar,calcite,sericite and magnetite.Quartz-kaolinite stage(?)mineral assemblages include quartz,kaolinite,rutile,calcite,vermiculite,biotite,etc.The hematite-quartz-calcite stage(?)is the main mineralization stage,and the mineral assemblages are hematite,quartz,calcite,pistil,realgar,cinerar,etc.,which can be divided into(?a)hematite-quartz stage,(?b)quartz stage,and(?c)calcite-hematite/limonite stage.In the hematite-quartz stage,the natural gold is distributed in the fissure of quartz-hematite veins and the gap between hematite and quartz in vein-like.In the quartz stage,the natural gold is distributed in the quartz gap formed after the silication of calcite.Natural gold in the calcite-hematite/limonite stage is mainly distributed near the calcite hematite/limonite vein.(2)By studying the crystallization characteristics of minerals at different stages,it is found that the crystal cell parameters(a₀,c₀,and crystal cell volume)of quartz and calcite in the main ore-forming stage(hematite-quartz-calcite stage)are larger,which may be related to the allographic Si⁴⁺and Ca²⁺of impurity elements Al,Fe,Ba and Sr entering the crystal lattice of quartz and calcite during the main ore-forming stage.In the Au20-2 profile,quartz crystallinity,kaolinite crystallinity and Au content increase synchronically.The high crystallinity of quartz and kaolinite may be used as a mineralogical indicator for prospecting.(3)The mineral symbiotic association shows that the metallogenic hydrothermal evolution process is from alkaline?acidic?alkaline.In the stage of amphibole-chlorite and quartz-kaolinite,the silicification of feldspar and fading of dark minerals release magnesium and iron ions and ore-forming materials,which may be the main stage of activation and migration of ore-forming materials The stage of hematite-quartz-calcite is the main precipitation stage of gold,the precipitation of gold may be mainly caused by pressure sag,water-rock reaction.Hematite,silicification,and carbonation are closely related to gold mineralization.