A Synthetic Study Of Veins Structure And Ore-forming Fluid Of Maocaoping Vein Cu Deposit In Lanping Basin,western Yunnan

The veindeposit is one of the important type of deposits developed in orogenic belt with great significance for indicating the structural-fluidic evolution and mineralization.There occurs a vein Cu ore belt with hundreds of kilometers long along Lanping basin's western border in western Yunnan.Much previous research has been operated,but the forming environment of the deposit,the coupling relationship between tectonic deformation and mineralization,the source of the ore-forming fluid and the genesis of the deposit are still being disputed.This paper mainly studies Maocaoping deposit,a deposit newly found in the south of the ore belt.With deep understanding of the geological features,observation by outcrop and microscope of ore bearing veins,hydrothermal muscovite 40Ar/39Ar dating of the metallogenic period,analysis of fluid inclusion,polyelement and isotope and combining regional comparison,this paper reveals the forming environment of the deposit as well as the coupling relationship between tectonic deformation and mineralization,and determines the nature and source of the fluid,which can provide new information for defining the genesis of the vein Cu deposit in western Yunnan.Maocaoping deposit,of which the wall rocks are Jurassic schist and marble experienced shear metamorphism,is located in Chongshan shear zone.The ore veins are composed of serried chalcopyrite-bearing quartz,carbonate and tourmaline veins paralleling to shear foliations.The essential minerals are chalcopyrite,tourmaline,quartz,calcite,dolomite,muscovite and some pyrite and magnetite.The minerals occur in different patterns from early to late:?1?alteration halos,occurring in the wall rocks next to the ore veins,consist of tourmaline and quartz.The minerals are well orientated arranged.The long axis of the minerals are parallel to the shear foliations,which indicates strong press-shearing and ductile deformation;?2?A1 vein,paralleling to the shear foliations,is formed a littler than alteration halos.Its main minerals are tourmaline,quartz,calcite,dolomite,muscovite,chalcopyrite,etc.The minerals are slithtly orientated arranged,and the long axis are slightly parallel to the shear foliations,which indicates a weak press-shearing and a brittle-ductile transition;?3?A2 vein,crosscutting the shear foliations,has the similar mineral association like A1 vein but lesser muscovite and more dolomite.The extensionally filling minerals implies that shear deformation is over,and such veins are formed by squeezing action and brittle deformation.It reflects that the deposit is formed by a waning shearing action and is a product of final stage of shearing.The deposit occurs at the depth of ductile-brittle transition zone,which is deeper than other vein Cu deposit like Jinman deposit.Muscovite ⁴⁰Ar/³⁹Ar dating of the metallogenic period of ore veins gives a forming age of 20.34±0.19 Ma,which is the end stage of Chongshan shear zone's strike-slip shearing.In contrast,vein Cu deposits of Jinman and Liancheng may be formed before that shearing action.The fluid inclusion data shows:Fluid inclusions in the ore-bearing quartz vein of the earlier?A1 vein?or the later?A2 vein?period are both CO²-bearing saline aqueous inclusions,which can be subdivided into halite-bearing inclusions,liquid-riched?aqueous phase?inclusions and vapor-riched?CO² phase?inclusions.In accordance with the symbiotic characteristics,fluid inclusions can be divided into three groups of assemblages.?1?group I,appearing in A1 vein,which is enriched in liquid?type I-L?,vapor?type I-V?and contains halite?type I-S?inclusions;?2?group?,appearing in A1 vein as well,is a paragenetic association enriched in liquid?type?-L?and vapor?type?-V?inclusions;?3?group?,appearing in A2 vein,is enriched in liquid?type?-L?and vapor?type?-V?inclusions.Micro-thermometric data of the fluid inclusions shows:Homogenization temperatures of type I-S range from 293 to 370?with salinity of 30.1³9.8 wt.%NaCl;Homogenization temperatures of type?-L range from 302to 490?with salinity of 1.2¹8.6 wt.%NaCl;Homogenization temperatures of type?-L range from 263 to 400?with salinity of 1.2¹1.3 wt.%NaCl.Within the same group,the vapor/liquid ratio varies greatly,which implies an inhomogeneity of the trapping fluid.In this case,the lowest value of the homogenization temperature of the fluid inclusion represents the real temperature of trapping.Therefore,inclusions are captured in temperature ranging from280 to 320??group I and??and from 260 to 280??group??.Combining the salinity data,it suggests the boiling of the initial fluid.With the halite precipitation,salinity of fluid decreased and then so did the temperature.The value of hydrothermal quartz?D_V-SMOW-SMOW is56.2‰^-94.6‰,and the fluid?¹⁸O_V-SMOW-SMOW value is+8.1‰⁺9.6‰.D isotope value of hydroxyl in muscovite is-136.5‰^-120‰,and the calculated fluid?D value is-88.32‰^-71.82‰.Such results indicate that fluid is derived from degassed magmatic water in open system,with no contribution from basinal or meteoric water;The?¹³C_V-PDF-PDF and?¹⁸O_V-SMOW of hydrothermal carbonate calcite are respectively-8.3‰to-2.4‰and 14.46‰to 16.63‰,which are similar to other vein Cu deposits in regional area.But values of?¹³C_V-PDF-PDF of this deposit are lower than the vein Pb-Zn deposits'in the region?carbon mainly from carbonate rock of basin fluid dissolved carbon?;The value of?³⁴S_V-CDT-CDT of chalcopyrite is-6.4‰to-3.9‰that is within the corresponding interval of other vein Cu deposits,but differs from sediment-hosted stratiform Cu deposits?sulfur from sulfate reduction bacteria?.Therefore,it indicates that the carbon and sulfur can be derived from?degassed?magma.The research of Maocaoping deposit declares that all the vein Cu deposit of western Yunnan are developed in the continent-continent collisional setting of India-Asia continent.The deposits may take shape before or after the regional strike-slip shearing action.The former pattern is mainly developed by expansion of axial-plane cleavage and fluid-filled mineralization?such as Jinman deposit?.The latter is formed by ductile-brittle transition and brittle deformation at the end period of the press-shearing action.The vein Cu deposits of western Yunnan occurs at the depth of ductile-brittle transition zone to brittle deformation zone.Presently,most deposits are found to occur at the depth where brittle deformation structure controls the mineralization,so deeper environment has a metallogenetic potentiality as well;The ore-forming fluids of the vein Cu deposits in western Yunnan may mainly come from multi-stage crustal magmatic fluids?before the shearing and homochromous shearing?in the western border of the basin rather than from basin fluids like previous views.