Metamorphism,Deformation And Element Remobilization Mechanism Of Hongtoushan Massive Sulfide Deposit,Liaoning Province

The Hongtoushan massive sulphide(VMS)deposit,located in the northern margin of North China craton,is the largest Archean massive sulphide deposit known in China.The main orebody and surrounding rock of the deposit have undergone metamorphism and deformation of high amphibolite facies and superimposed hydrothermal transformation in later stage.Strong ductile deformation zones are often developed in the interior of the main orebody or the contact area between the ore body and the surrounding rock.On the basis of previous work,through field observation and laboratory identification,the ore bodies and their associated ductile deformation zones are divided and represented by the following four types of ores.(?)Massive ore is the main body of Hongtoushan deposit.The ore is layered structure and massive structure,equigranular and transgranular textures,sulfide assemblage and proportion are the same as the overall composition of the deposit;(?)Coarse-grained ore occur between medium-sized massive ore and ore mylonite,characterized by lenticular and massive structures,giant porphyry and interstitial textures.Ore minerals consist mainly of giant pyrite and pyrrhotite porphyry,and the content of other sulphides(chalcopyrite,sphalerite)is very low when it is filled between the variegated crystals or in the crystal fissures;(?)Mylonitized sulfide ore is also called mylonite,and is found in the ductile shear of the ore body in the transition zone between the coarse-grained sulfide ore and the physical and chemical rock-rock transition zone.The ore is in the form of lenticular and crumpled structures,the fine-grained and recrystallized textures,and the ore minerals are mainly chalcopyrite and sphalerite,the content of pyrite and pyrrhotite is low;(?)Copper-rich sulphide ore,mainly occurring in surrounding rock slices or massive ore fissures near mylonite,is veined and laminated structures,metasomatic residue and emulsion textures.Ore minerals are long-striped and filamentous chalcopyrite and part of sphalerite.Ore deformation is remarkable,local chalcopyrite content can exceed 90%,so it is also called"copper bar".Based on the mineralogical analysis of the mineral assemblages,combining with the previous research results and the macro-development characteristics of the ores,symbiotic relations and deformation characteristics of the above four kinds of ores,the genesis and formation process of the above four types of ores are analyzed in this paper.The deformation and fluid activity of massive ore is not obvious,which is the product of regional metamorphism and recrystallization of primary VMS ore.The coarse-grained ores are characterized by low deformation degree,development of porphyry and extremely low content of chalcopyrite,and sphalerite,representing the residual phase after strongly metamorphic recrystallization and remobilisation.The ductile deformation of mylonite is the strongest.Chalcopyrite,sphalerite and galena are obviously higher than massive ores,which represent sulfide ores undergoing ductile deformation and remobilisation.The ductile deformation and metasomatism structure of copper bar is developed.The sulphide of it is mainly chalcopyrite,followed by sphalerite,and a small amount of carrollite indicating low temperature origin,almost without other sulphide ore and pyrrhotite.It is the product of mechanical reactivation and metamorphism hydrothermal precipitation.The change of the content of the main and trace elements of the sulfides show that the sulfides underwent a chemical remobilization process with the fluid metasomatic re-precipitation as the main mechanism in the late-stage metamorphic deformation.The change of the content of the main and trace elements of sphalerite(Fe,Zn,Pb,Ag,Hg,In)is the most significant in the sulfides,indicates the effect of temperature,stress,sulfur fugacity and migration evolution of fluid on the later remobilization of the deposit to some extent.The in-situ sulfur isotope analysis of the micro-area indicates that the metamorphic fluid may be mainly from the metamorphic water in the interior of the ore body and the surrounding rock.In the late stage of the metamorphic remobilization process,the foreign fluid may mixed into initial fluid due to the longer migration distance.The trace elements in sphalerite show strong activity in the later-stage metamorphism and remobilization,which has certain indicative significance for the geochemical conditions of the ore-forming fluid in the later-stage metamorphism and remobilization.So the content of trace elements in sphalerite can be used as an important index to study the deformation process of the late stage of the sulfide deposit.In this paper,the iron isotopes of different sulfides in different types of ores have been calibrated by situ sampling of microdrilling and iron isotope analysis by solution method for the first time.The results show that there are obvious iron isotopic fractionation between most sulfides in the same type of ore at different stages of late metamorphism and reactivation,but there is no significant difference in the fractionation values among pyrite,chalcopyrite and pyrotite in different types of ores.Equilibrium fractionation may have been achieved among these sulphides;The iron isotopic fractionation value between sphalerite and other sulfides varies greatly,which may be related to the change of physicochemical conditions of ore-forming fluid during late metamorphism and reactivation.