| Huangshandong Ni-Cu sulfide deposit,located in the Huangshan-Jing'erquan ductile shear zone,belongs to the Jueluotage island arc belt of the Eastern Tianshan area,Central Asian Orogenic Belt.The Huangshandong mafic-ultramafic pluton,in shape of fusiform,intruded in the Later Carboniferous volcanic rocks with intensive foliation near the margin of the pluton which is consistent with the strike of the regional ductile shear zone.Some orebodies in the deposit also show intensive ductile deformation.The No.17 orebody was totally controlled and hosted by the ductile shear zone.The rocks and ores from the No.17 orebody show intensive deformation and alteration and could be called as ore mylonites.Considerable amounts of network and veinlet copper-rich carbonite-sulfide veins can be also observed in this ductile shear zone.In this contribution,detailed microscopy petrography and mineralography observation were carried out on different types of ores from the Huangshandong deposit.Combined with field geological investigation and ore fabric observation,three different types of ores were distinguished,that is,magmatic ores,deformed ores and fluid-overprinted ores.Magmatic ores with sideronitic fabric are barely altered and deformed,while sulfide minerals in it only show some brittle fractures.Deformed ores develop intensive deformation fabric in silicate and sulfide minerals,such as the kink bands,wavy extinction of uralite,domino structure of phlogopite and the elongation of pyrrhotite.Fluid-overprinted ores are characterized by strongly deformation,flattened grains and recrystallization of pyrrhotites,while annealing of pyrrhotites could be observed frequently.Besides,several transitional types of ores between these three types of ores could also be founded in Huangshandong Ni-Cu deposit.Intruding of Huangshandong pluton is coeval with ductile deformation and regional Huangshan-Jing'erquan ductile shearing.During the intruding and cooling of Huangshandong pluton,intensive regional ductile shearing induced ductile deformation of sulfide ores in the deposit and then ore mylonites formed.Gangue minerals in the deformed orebodies were crushed to fine-grained minerals and alterated.In these processes,fluid and metallogenic elements were released and overprinted into the deformed rocks and ores,forming network and veinlet hydrothermal overprinted ores.In-situ trace element analysis of pyrrhotite,pentlandite,and chalcopyrite showed high Ni and Co concentration of pyrrhotite,higher Ni concentration of pentlandite,and high Ag and Cd concentration of chalcopyrite from fluid-overprinted ores.We interpret that Ni and Co sources for fluid overprinted ores were derived mainly from the alteration of olivine,and the dissolution of primary chalcopyrite should serve as source of copper in chalcopyrite-rich fluid-overprinted ores.Contained higher iron in original sulfide solid solution,the magmatic Ni-Cu sulfide ores could only exsolved hexagonal pyrrhotite without associated monoclinic pyrrhotite and pyrite during the crystallization process of sulfide solid solution.During hydrothermal overprinting process,fluid overprinting occurred along margins and fractures of early stage magmatic hexagonal pyrrhotite,formed monoclinic pyrrhotite reactional rims.Our studies argued that monoclinic pyrrhotite can also be formed during hydrothermal precipitation processes.In ths process,monoclinic pyrrhotites can exsolved from hexagonal grains in high oxygen and sulfur fugacity environments,as the cooling of such hydrothermal fluids... |
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