The Superimposed Mineralization of the Dongguashan Cu Deposit in Tongling Area,Anhui Province: Evidence from the Ore Texture

The massive sulfide deposits in the Middle-Lower region of Yangtze River have undergone extensive superimposition and reformation by Yanshanian magma.Taking the Dongguashan Cu deposit in Tongling as a study case,this paper discusses the metallogenic mechanism of these Cu deposits.Dongguashan deposit comprises mainly of a large stratiform sulfide orebody with some subordinate skarn and porphyry type orebodies.Field and microscopic investigation show that the sulfide ores at Dongguashan deposit underwent intensive thermal metamorphism and hydrothermal replacement.The prograde ore textures were formed through the phase conversions from pyrite to pyrrhotite and magnetite by the heating of Yanshanian granite magma.Retrograde metamorphism is characterized by presence of annealing textures in pyrrhotite,porphyroblastic growth of pyrite and formation of pure hexagonal pyrrhotite.The hydrothermal replacement textures were represented by the intergrowths of hexagonal and monoclinic pyrrhotites and replacement of pyrrhotites and pyrite by chalcopyrite.The chalcopyrite in the stratiform orebody was formed by replacement of pyrite and pyrrhotite,which have experienced contact metamorphism related to the Yanshanian intrusions.This suggests that chalcopyrite in the stratiform orebody was mainly overprinted from the Yanshanian magmatic fluids.The pre-existing pyrrhotite and pyrite have important significance for the precipitation of chalcopyrite from magmatic fluids.Base on the characteristics of ore textures,the formation of Dongguashan Cu deposit can be described as three stages: 1) During the Carboniferous period,the massive sulfide deposit was formed by the submarine exhalative and sedimentation,the compositions of which were dominantly S and Fe;2) The heat from Yanshanian granite intrusions has resulted in phase changes from the pyrite to pyrrhotites and magnetite,and then the stratiform sulfide orebody has underwent a slow cooling process;3) The overprinting of magmatic hydrothermal fluids lead to enrichment of large amounts of copper in the stratiform orebody,and formed new skarn and porphyry type orebodies.