Ore-Controlling Structure-Magma-Fluid-Metallogenetic Regularity And Prospecting Prediction In The W-Mo-Au Polymetallic Ore Concentration Area In Ningshan-Zhen'an Area,South Qinling

Intense and extensive tectonic-magmatic activity occurred in the South Qinling Mountains during the Early Mesozoic intracontinental orogeny,with which a large number of metallic minerals were formed.The W-Mo-Au polymetallic ore concentration area in the Ning Shan-Zhen'an area of Shaanxi Province is located in the northern part of the South Qinling Tectonic Zone,where tectonic and medium-acidic magmatic rocks are very well developed,and hundreds of tungsten and molybdenum-based polymetallic deposits(points)have been discovered.Although some exploration and research results have been achieved in the study area,there are still gaps or deficiencies overally on research of ore-controlling structure,the source of ore-forming minerals,the age of mineralization,etc..This dissertation is based on the study of the coupling of ore-controlling structure-magma-fluid-mineralization,and conducts on the basis of the previous work to anatomically study typical deposits in the ore concentration area.Carry out large-scale ore-controlling structure-alteration mineralization investigations in the ore concentration area,and collect relevant rock and ore samples for geochemical testing to deeply analyse the ore-controlling structure,magma and mineralization in the W-Mo-Au polymetallic ore concentration area in the Ningshan-Zhen'an area,reveal the characteristics of the polymetallic ore-controlling structure,the petrological and geochemical characteristics of the complex massif,etc.in the ore concentration area,and discuss the coupling mechanism of structure-magma-fluid-mineralization and the geodynamic background.A structure-magmatic interaction-based model of W-Mo-Au polymetallic mineralization in the Ningshan-Zhen'an area is initially established,and the prospecting criteria are summarized.Based on the geological,geophysical and geochemical exploration information,the prospecting prospect area is put forward.Major progress and results have been achieved as follows:1.Two groups of fractures,NW-NWW and NE-NNE,are developed in the ore concentration area and the latter truncates the former to form a"well"shaped structure pattern.Between them,NE-NNE-trending faults and joint fissures are the main ore-controlling structures of quartz-vein-type W-Mo polymetallic ore(mineralization)bodies.A small number of NW or nearly E-W-trending faults control skarn-type W-Mo mineralization.2.The magmatic rocks in the ore concentration area are mainly complex bodies,of which some samples from the Tianwan unit of the Lanbandeng rock body,the Wangjiaping concealed rock mass and the granitic fine-grained dikes represent the magmatic evolution direction in the area,with a high degree of magmatic crystalline differentiation,and with Si-rich and low Mg^#values.The total amount of rare earth elements is low,with a strong negative Eu anomaly and a tetrad effect in the rare earth element partitioning curve.The Zr/Hf and Nb/Ta values are low and the Rb/Sr values are high.Zircon U-Pb dating indicates that the ages of Jiujianwu unit of Lanbandeng and Wangjiaping concealed rock mass are 222.7Ma and 201.9Ma,respectively.The magmatic rocks in the ore concentration area are mainly of calc-alkaline quasi-aluminous-weakly peraluminous type I crust-mantle mixed granite type,with ages of 210 Ma-230 Ma and190 Ma-200 Ma.3.Gas-liquid inclusions are mainly developed in tungsten and molybdenum deposits,and the ore-forming fluids can be roughly classified into four types:(1)High temperature category(peak 355°C to 380°C),represented by the Qipangou skarn type and quartz vein-type tungsten mineralization;(2)Medium to high temperature category(209°C to 327°C),represented by quartz vein type Mo-W mineralization in other typical mining areas;(3)Medium temperature category(197??213?),represented by molybdenum mineralization feldspar quartz pegmatite type;(4)Low temperature category(154??189?),represented by tungsten mineralized quartz-fluorite vein type and tungsten mineralized beryl-bearing quartz vein type.The ore-forming fluids associated with the quartz vein type tungsten ore in the Qipangou deposit are medium to high temperature and medium to high salinity fluids,formed in a deeper environment that is reductive in nature.The quartz vein-type mineralizing fluids in the Dongyang,Hetaoping and Yanggou-Diergou deposits are medium-temperature,medium-low salinity and formed in a slightly shallow reductive-oxidizing environment,while the pegmatite-type mineralizing fluids are low-temperature,low-salinity and formed in a shallow oxidizing environment.The formation depth range of W-Mo mineralization is 4.2km?8.4km.Fluid boiling and mixing may be an important mechanism for the formation of tungsten-molybdenum mineralization.4.The?D values of quartz in different Mo-W mineralization types vary from-64.9‰to-80.1‰,with a mean value of-74.4‰,and the?¹⁸O_H2O values range from-1.71‰to 6.42‰,with a mean value of 2.67‰.Skarn-type mineralization is dominated by magmatic water,quartz vein-type mineralization has both magmatic water and meteoric water,and meteoric water is more involved in the mineralization stages of quartz-fluorite vein-type,beryl-bearing quartz vein-type and pegmatite-type mineralization.The?³⁴S of quartz vein-type W-Mo mineralization ranges from 3.6‰to 10.2‰with a mean value of 7.3‰,the?³⁴S of skarn-type mineralization is 6.1‰,pegmatite-type mineralization is 4‰,and the?³⁴S of Daxigou granite-type molybdenum ore is 0.1‰.The magma is the main ore-forming materials source.5.The Re-Os isotope model ages of the Qipangou and Jiangkou deposits are 199.7±3.9Ma and 198.7±3.9 Ma,respectively,and the Ar-Ar isotope plateau ages of the altered phlogopite closely associated with scheelite in the Qipangou deposit are 188.6 Ma and 190.1 Ma,respectively.The age of W-Mo deposits in the ore concentration area is generally concentrated between 190 Ma and 200 Ma,belonging to the Early Jurassic.6.The rare-earth element partitioning curve in the skarn-type scheelite of the Dongyang deposit is characterized by an upward convex"tetrad effect"pattern,showing the characteristics of type II scheelite,and the degree of skarnization in the deposit is relatively weak.The content of rare earth elements in the quartz vein-type scheelite in Qipangou is slightly higher than that of the skarn-type scheelite in the Dongyang deposit,showing a weak positive Eu anomaly,similar to the type I scheelite,with a higher degree of skarnization and tungsten ore formation in a sodium-rich environment.The scheelite in the Hetaoping deposit is of medium I type,with a steep dip to the right of the rare earth distribution curve and strong fractionation,which may be related to the early crystallization of the heavy rare earth elements-rich minerals.The ore-forming fluid in Dongyang and Hetaoping deposits is rich in F,while the ore-forming fluid in Qipangou deposit is rich in Cl.7.The granite bodies with close spatial and temporal relationship with W-Mo polymetallic mineralization in the ore concentration area should be highly differentiated evolutionary rocks and dikes formed between 190 Ma and 200 Ma.The Wangjiaping concealed rock mass is rich in F and other volatile components,which is conducive to the formation of Mo-W and other polymetallic mineralization.The early rocks(210 Ma?230 Ma)exposed on the surface of the ore concentration area should be predominantly pre-metallogenic rocks.The W-Mo-Au polymetallic mineralization is a series of metallogenic assemblages associated with acidic magmatic-hydrothermal activity during the late Indosinian-early Yanshanian intracontinental orogenic extension and collapse evolution stage.8.The abnormal structure-hydrothermal vein dense section distributed in the structure-magmatic rock belt should be the favorable prospecting area.Five tungsten-molybdenum-gold polymetallic metallogenic potential areas have been delineated,namely the Jiangkou prospective area,Yindongwan prospective area,Xunyangba prospective area,Xianggoutai-Yuehetai prospective area and Yanggou-Diergou prospective area,some deposits(such as Heigou-Foyeping and Xianggoutai,etc.)still have great prospecting potential in the deep.