Geological Characteristics And Genesis Of Copper-Gold Deposits In Maanshan-Wuhu Area Of Ningwu Basin

The Ningwu basin is an important part of the metallogenic belt in the middle and lower reaches of the Yangtze River.A series of hydrothermal vein-type copper-gold deposits?spot?related to volcanic-intrusive processes are developed in this area,compared with other basins in the metallogenic belt.The research degree of copper and gold deposits?points?in this basin is not high.Moreover,the previous research mainly focuses on the northern part of the basin,while the Maundefinedanshan-Wuhu area in the middle and south of the basin lacks some research,which restricts the comprehensive understanding of the copper and gold deposits in the whole Ningwu basin.Based on the 2016 public welfare geological work project of Anhui Province,"investigation and Evaluation of Copper and Gold deposits in Maanshan-Wuhu area",on the basis of collecting a large number of geological data of copper and gold deposits?spot?in Ningwu Basin,this paper is based on the investigation and evaluation of copper and gold deposits in Maanshan-Wuhu area.The copper-well copper-gold deposits in the central and southern parts of the Ningwu basin and the Longwangshan copper ore deposit are selected as the typical ore deposits,combined with the Dashan gold deposit,the Qilijian working area,the Lianzhushan gold deposit,the Sishan copper-gold deposit and the Xiao Liu-fang gold deposit.The geological characteristics,chronological and geochemical characteristics and fluid characteristics of copper-gold deposits in Maanshan-Wuhu area are systematically summarized through detailed field geology,laboratory petrological identification and experimental analysis.On this basis,the Genesis of copper and gold deposits?spot?in the study area is preliminarily discussed.The main understanding and achievements of this paper are as follows:?1?There are Dashan,Tongjing,Longwangshan,Lianzhu Mountain,Sishan and Xiaoliufang copper gold deposits closely related to volcanic hydrothermal fluid in Maanshan-Wuhu area of Ningwu Basin.The ore structures of copper and gold deposits in this area are mainly vein structure,reticulated vein structure,breccia structure and disseminated structure.The ore structure is mainly semi-self-shaped grain structure,interlaced structure and metasomatic residual structure.The main mineralization types are pyrite mineralization,brass mineralization,occasionally copper blue and limonite in the oxidation zone,and the alteration of surrounding rock is mainly silicified,chlorite,kaolinized,carbonate and sericite.It has the characteristics of volcanic hydrothermal copper deposit.The further study on Tongjing gold and copper deposit and Longwangshan copper deposit shows that the metallogenic period can be divided into hydrothermal metallogenic period and epigenetic period.The hydrothermal stage is divided into Shiying siderite sulphide stage and quartz-porphyry copper stage as well as carbonate stage or quartz-sulphide stage and carbonate stage.?2?The U-Pb dating results of LA-ICP-MS zircon show that the age of potash feldspar granite in Dashan gold deposit is 127.9±2.1 Ma.the age of monzonitic granite in Qilijian working area is 129.3±1.9 Ma,and the age of potash feldspar granite in Qilijian working area is 129.3±1.9 Ma.The age of feldspar in Tongjing gold-copper deposit is134.8±3.7 Ma,the age of diorite in Longwangshan copper-gold deposit is 135.9±5.6Ma,the age of trachyandesite is 129.1±2.6 Ma,and the age of crude ampere volcanic breccia is 131.2±3.6 Ma,and the age of feldspar in Longwangshan copper and gold deposit is 135.9±5.6 Ma,129.1±2.6 Ma and 131.2±3.6 Ma,respectively.The age of quartz diorite in Xiaoliufang gold deposit is 129.2±2.4 Ma.At the same time,the Rb-Sr dating of pyrite shows that the metallogenic ages of Tongjing gold-copper deposit and Longwangshan copper deposit are 119.4±2.1 Ma and 129.2±2.3 Ma,respectively,and the diagenetic mineralization ages are all early Cretaceous.It is the product of the second stage diagenesis and the transition from the second to the third stage of the metallogenic belt in the middle and lower reaches of the Yangtze River.?3?The whole rock geochemical analysis shows that the magmatic rocks of each copper-gold deposit in Maanshan-Wuhu area are intermediate acid rocks,and may belong to the same magma evolution system.At the same time,during the process of magma evolution,the magmatic rocks gradually evolve towards the direction of rich potassium.The magmatic source area of copper and gold deposits in the study area may be enriched mantle by the subduction and metasomatism of the paleo-Pacific plate,and the magma is mixed with crustal materials during the process of rise and evolution.?4?The tectonic discrimination diagram shows that the diagenetic mineralization of other copper and gold deposits in the study area is in volcanic arc environment except Dashan gold deposit,indicating that it was formed in the tectonic background of compression-extension.?5?The study of fluid inclusions shows that the ore-forming fluid at the point of copper-gold deposit in Maanshan-Wuhu area is characterized by medium and low temperature and low salinity system,and the ore-forming fluid may come from the mixture of magmatic water and atmospheric precipitation.At the same time,the inclusion composition analysis indicates that the initial ore-forming fluid of the Cu-au deposit is a CO₂-poor H₂O-NaCl system.?6?Through endoscopic observation and electron probe scanning,In the Tongjing gold-copper deposit and Longwangshan copper deposit in Maanshan-Wuhu area,the independent minerals of gold are found to be in the shape of other-shaped and semi-self-shaped particles embedded in pyrite particles of hydrothermal metallogenic period,with a particle size of about tens of microns.The energy spectrum shows natural gold or silver gold deposits.?7?The ore-bearing strata of copper and gold deposits in Maanshan-Wuhu area are mainly Longwangshan formation,Dawangshan formation,Niangniangshan formation volcanic rocks and Huangmaqing formation sandstone,among which Longwangshan formation volcanic rocks are more prominent.Faults are the main ore-controlling and ore-bearing structures in the basin,and most of them form structural fragmentation zones,which creates conditions for ore tolerance.The formation of copper-gold deposits is related to Gushan and Niangnangshan volcanic cycles,and granitoids may provide heat source and some metallogenic materials for mineralization at the same time.Therefore,strata,structures,magmatic rocks and other factors are closely related to the formation of the deposits.?8?The metallogenic mechanism of copper and gold deposits in Maanshan-Wuhu area is consistent with that of late copper-gold mineralization in northern Ningwu,which is manifested in the late Gushan-Niangnangshan cycle.Under the combined action of post-volcanic hydrothermal solution and granitic magma in the same period,a large number of Cu,Au and other elements are activated and migrated from surrounding rock and mixed with atmospheric precipitation in the later stage.With the change of physical and chemical conditions,ore-bearing hydrothermal fluid rises along the pressure-torsion fault and fills into the early volcanic rocks or basement strata fractures to precipitate and form ore-forming.