Geochemistry Of The Abitibi Greenstone-type Gold Deposit In Quebec,Canada

The Abitibi greenstone belt,located in the south of Superior Archaean Craton,is the largest and best-preserved Archean greenstone terrane in the world and one of the most economically valuable metamorphic rock terranes,with numerous metal deposits.The main content of this paper is to study the occurrence of gold and geochemistry of the greenstone,granitic intrusion and gold deposit of Joe Mann gold deposit in Chibougamau area,northeast Abitibi greenstone belt.In order to better understand the occurrence status of gold in the Joe Mann gold deposit,the collected ore was made into probe pieces for BSE and energy spectrum analysis,and the ore minerals including chalcopyrite,pyrite and pyrrhotite were analyzed by electron probe analysis(EPMA),while pyrite was analyzed by in situ sulfur isotope analysis.When the ore is observed under microscope,visible gold can be found.The results of electron probe analysis of metal sulfide showed that high Au content was detected in chalcopyrite,pyrite and pyrrhotite.In addition,according to the empirical diagram of Au saturation in pyrite,Au content in pyrite is above the saturation boundary and exists in the form of fine gold particle inclusions(Au0).In situ sulfur isotope analysis of pyrite shows that its sulfur isotope composition δ34Sv-CDT ranges from 0.03 to 3.12‰,indicating that mantle or ultrabasic rocks and mafic or magmatic rocks may be potential sources of sulfur.The main body of the study conducted whole-rock principal and trace element analysis of Joe Mann greenstone and granite respectively,and conducted U-Pb isotopic dating of zircon from granite and zircon trace element analysis.Combined with the Joe Mann greenstone formation,the formation age is estimated to be between 2.73 and 2.76 Ga.Zircon U-Pb dating of Joe Mann granite is 2720±36 Ma,and it was formed in Neoarchean.Zircon has been calculated to have a high temperature(754～885℃,average 830℃)and high oxygen fugidity(-4.68～-11.13,average 8.12,relative to FMQ buffer pairs+3.69～8.80,average+6.24)conducive to mineralization.It can be seen from the geochemical characteristics that the protolith of Joe Mann greenstone is subalkaline basalt,which is further divided into ferrotholite.The REE content is low(34.25～100.75ppm),the REE elements show a flat partition pattern,the large ion lithophile elements are enriched,and the high field strength elements are relatively deficient.Combined with the trace element diagram,it can be inferred that the magma originated from the depleted mantle,which was subjected to the subduction plate derived fluid or melt metasomatism,and experienced the separation and crystallization of monoclinic pyroxene in the process of magmatic evolution.Joe Mann granite has the characteristics of Archaean TTG geochemistry,that is,rich in Al2O3 and Na2O,low contents of MgO and TiO2 and K2O/Na2O ratio,high Sr/Y(16.03～104.19),(La/Yb)N ratio(14.23～30.49)and light rare earth content.Low Y(3.10～11.70ppm),Yb(1.53～7.12ppm)and heavy rare earth content,rich in large ion lithophile elements.According to Harker diagram,the Joe Mann granite and the regional Archaean TTG have the same elemental characteristics.Combined with further analysis,it is inferred that the process of separation crystallization and partial melting occurred during the magmatic evolution.Combined with its tectonic setting,it is speculated that the granitic magma was formed by partial melting of subduction oceanic crust.The Abitibi greenstone belt in Canada represents the collision of the oceanic island arc and evolved the stage of island arc formation and fragmentation over a period of approximately 65 Ma(2735～2670Ma).The formation of the plutonic pluton associated with this tectonomagmatic history lasted for approximately 95 Ma(2735～2640Ma).