Magmatism,Hydrothermalism And Metallogenic Mechanism Of The Tongchanggou Porphyry Mo(Cu) Deposit In Northwestern Yunnan

The Tongchanggou Mo（Cu）deposit is the largest intracontinental porphyry molybdenum（copper）deposit in the Zhongdian region,mainly producing porphyry type Mo ore bodies and a small amount of skarn type Cu-Mo ore bodies.The mineralization is closely related to the late Yanshan magma activity.The exposed area of its mineralized rock mass is less than 0.1 km2,and it is basically in a concealed state.Currently,there is relatively few research on the magma-fluid process and mineralization of the deposit.Therefore,on the basis of previous research results,through field investigation and indoor observation of the geological characteristics,alteration zoning,hydrothermal veins and mineral assemblages at different stages of the Tongchanggou porphyry molybdenum（copper）deposit,we systematically studied the geochemical,age isotope,fluid inclusion,stable isotope and other characteristics of metallogenic intrusive rock and deposit,and expounded the evolution process of metallogenic fluid and the law of element migration.Further,we have identified the relationship between magma oxygen fugacity and differential mineralization,revealed the magma-fluid interaction,and established a diagenetic and mineralization model,providing new references for the study of intracontinental porphyry deposits.It also has important guiding and practical value in regional mineralization and exploration prediction.The main conclusions are drawn below:（1）The Late Cretaceous magmatic intrusion event occurred in the Tongchanggou mining area.The granodiorite porphyry is a molybdenum hosting and metallogenic porphyry body,and a small amount of quartz monzonite porphyry is developed.Quartz diorite veins are occasionally seen,and the magma activity time is 87～83 Ma.With a high silicon alkali content,the rock is an Ⅰ-type granite and possess the properties of adakite rocks,i.e.,having high Sr/Y and La/Yb ratios,low Yb and Y,and unobvious Eu anomalies.The whole rock has a relatively high(⁸⁷Sr/⁸⁶Sr)_i,while the values ofε_Nd（t）andε_Hf（t）are negative.The content of Cr,Ni,Mg O,and Mg^#is significantly lower.The isotopic model age ranges from 1.2 to 1.4 Ga,indicating that the main source area is partial melting of the thickened ancient lower crust.（2）Dark inclusions are developed in the host rock granodiorite porphyry,and the whole-rock trace and rare earth elements,as well as its Sr-Nd-Pb isotopes,are similar to those of the host rock,indicating a strong degree of magma mixing.There are three sets of zircon ages in the dark inclusions,which are 81.3～85.3 Ma,235.7～243.6 Ma,and 949.8 Ma.Among them,zircons from age groups of 235.7～243.6 Ma has a positive high value forε_Hf（t）and the Hf isotope model age(t _DM)ranges from 400 to 448 Ma,indicating the involvement of young and newly formed components in magmatism and mineralization.The Tongchanggou magma is a mixture of a large amount of ancient lower crust partially melted magma and a small amount of newly formed lower crust partially melted magma,providing the necessary Mo and Cu for mineralization.（3）Using a zircon oxygen fugacity meter,the magma oxygen fugacity of the porphyry deposit in the Zhongdian ore concentration area was studied.Research suggests that there is a high requirement for oxygen fugacity in the mineralization process of porphyry type copper deposits,but high oxygen fugacity is not a sufficient condition for their mineralization,while porphyry type molybdenum（tungsten）deposits have relatively low requirements for oxygen fugacity.Further,oxygen fugacity plays an important controlling factor in the differential mineralization of porphyry molybdenum and tungsten deposits.High oxygen fugacity is beneficial for the formation of molybdenum deposits,while low oxygen fugacity is beneficial for the formation of tungsten deposits.（4）The mineralization and alteration in the Tongchanggou mining area are characterized by potassium silicate mineralization,sericitization,and propylitization from the center outwards,with skarnization developed in carbonates.Mass balance calculations indicate that the sericite mineralization zone is more closely related to Mo mineralization,suggesting that Mo mainly migrates in the form of chloride complexes,and its precipitation may be controlled by"acid-base conversion barriers".Mo in the ore-forming hydrothermal solution may originate more from the continuous injection of the lower magma chamber.（5）The porphyry-type mineralization stages of the Tongchanggou are divided into three main stages:potassium feldspar-quartz stage（Ⅰ）,quartz-sulfide stage（Ⅱ）,and quartz-gypsum stage（Ⅲ）.Stage Ⅱ is the main precipitation stage of Mo.The early stage of the ore-forming fluid is a high-temperature（300-390℃）and medium-salinity(8～16wt%Na Cl._eqv)H₂O-Na Cl-CO₂ system,the sulfide precipitation period is medium/low-temperature（180-300℃）and medium/low-salinity(6～12wt%Na Cl._eqv),and the late stage is a low/medium-temperature（150-250℃）and low-salinity(2～8wt%Na Cl._eqv)H₂O-Na Cl system.During the fluid evolution process,the overall pressure changes from（ultra）static rock pressure to static water pressure,and the pressure changes in Stage II are large and unstable;Hydrogen and oxygen isotopes show that Stage I is magmatic water,Stage II is magmatic water and a small amount of atmospheric water,reaching a maximum in the late stage.During the mineralization process,cooling,depressurization,and CO₂ escape and immiscibility are the main factors contributing to Mo precipitation and mineralization.The S-Pb isotope characteristics of ore metal sulfides show that S is deep-seated sulfur（-2.70‰～+2.02‰）,and lead mainly comes from the lower crust of the orogenic belt.（6）The geochemical characteristics of porphyry mineralized altered minerals constrain the evolution of fluid physicochemical state.Specifically,there is Cl-rich and F-poor fluid hydrothermal solution which has a high temperature（301℃～423℃）,alkali-rich silicon,and high oxygen fugacity（between NNO and HM）in the potassium silicate salinization period.During the propylitization and alteration,the fluid evolved into a medium-high temperature（260℃～340℃）,low oxygen fugacity（<NNO）,and low p H state.The change of Mo content in scheelite indicates that the oxidizability decreases during the evolution of ore-forming hydrothermal fluid,and Mo⁶⁺changes to Mo⁴⁺.The trace elements of metal sulfides such as pyrite and molybdenite indicate that the hydrothermal system is obviously cooled and the oxygen fugacity decreases when sulfide is precipitated in large quantities;The typomorphic characteristics of late vein sphalerite reflect that it was formed in a low-temperature environment.Overall,it is shown that when the fluid is an alkali-rich fluid with high temperature and oxygen fugacity in the early stage,it is most favorable for the precipitation of Mo when it evolves into reduction and medium-temperature conditions.（7）The skarn type Cu-Mo mineralization in the Tongchanggou deposit mainly occurs in the contact zone between granite porphyry and carbonate rocks,as well as in nearby fault zones.Metal mineralization is closely related to degraded altered minerals;There is mainly calcium skarn with a small amount of magnesium skarn.During the dry skarn stage,the appearance of garnet and magnesia indicates that the early fluids were high-temperature,oxidized and water-rich,and moderately weak alkaline.The magnetite stage is a high-medium temperature environment with moderately alkaline and relatively high oxygen fugacity and sulfur fugacity.During the quartz-sulfide stage,pyrite and molybdenite precipitated in a medium-high temperature and low oxygen fugacity environment.At the end,the appearance of calcite veins indicates that the hydrothermal solution has changed to a low-temperature state.The skarn-related fluids underwent an evolution process of oxidation,reduction,high-temperature,and low-temperature at different stages.（8）In this paper,the physical and chemical characteristics of minerals and ore-forming fluids in intrusive rock,alteration zones,hydrothermal mineralization stages of the Tongchanggou deposit have been analyzed in detail,and the magma-fluid evolution and diagenetic and metallogenic models of the Tongchanggou deposit have been summarized and established.