Mineralogy,Petrogeochemistry And Ni-Cu Mineralization For The Mafic-Ultramafic Intrusions From The Hongqiling Area,Jilin Province

The Hongqiling magmatic Ni-Cu sulfide deposit,situated on the southern margin of the eastern Central Asian Orogenic Belt?CAOB?,is composed of numerous mafic-ultramafic intrusions.In recent years,there has not been much progress in prospecting in the area.The studies of magmatic evolution,sulfide segregation mechanism,and systematic comparison with other magmatic Cu-Ni sulfide deposits are weak.In this study,we chose the Hongqiling ore-bearing mafic-ultramafic intrusions?Nos.1,2,3 and 7?as our research object.The mineral composition,whole-rock major and trace elements,platinum-group elements?PGE?,and S isotopic compositions were analyzed.Based on the above obtained data,we have discussed the nature and composition of the parent magma as well as the mineralization and tectonic setting of these ore-bearing mafic-ultramafic intrusions.The Hongqiling ore-bearing intrusions composed mainly of harzburgite,lherzolite,websterite,orthopyroxenite,and norite?gabbro?.Their constituent minerals are olivine,diopside,bronzite,calcic-hornblende,and plagioclase.These rocks show high MgO?24.80 wt.%?and low TiO₂?0.57 wt.%?contents,and are characterized by enrichment of large ion lithophile element?LILE?,light rare earth element?LREE?and PPGE,and depletion of high field strength elements?HFSE?,heavy rare earth element?HREE?and IPGE.All samples show similar trace and PGE patterns,suggesting that they have the same origin.The content of trace and minor elements of the Hongqiling ore-bearing mafic-ultramafic intrusions suggest that they have undergone crustal contamination to some degree.However,the?³⁴S values of the mafic-ultramafic rocks?-4.70².13‰,-0.42‰in average?are low,indicating that the S came mainly from the mantle.The data suggest that their parent magmas were contaminated by very small amount of crustal material.We have summarized the similarities and differences between Xiarihamu,Huangshandong and Hongqiling No.7 magmatic Cu-Ni sulfide deposits,and found that they are all small intrusions,with peridotite and pyroxenite as the ore-bearing rocks.The Ni in olivine and the PGE in whole-rocks are all depleted.However,there are significant differences in S isotopes and surrounding rocks,suggesting that contamination by crust-derived S may not be the only source in these ore deposits.Thus,we consider that sulfide saturation may not be a fixed mechanism for these magmatic Cu-Ni deposits in orogenic belts.The parent magma shows high-temperature magma?1300°C?estimated by the compositions of spinel and olivine.The mineral compositions?such as the spinel,olivine,and pyroxene?indicate that the parent magma of the Hongqiling intrusions is not a komatiitic magma,but most likely a high-Mg tholeiitic magma.In addition,we have calculated the composition of the parental magma based on olivine-liquid equilibrium and mass balance,and found that the parental magma is high-Mg basaltic,with MgO of 14.2wt.%,TiO₂ content of 0.9 wt.%and FeO content of 8.8 wt.%.Both the Hongqiling ore-bearing mafic-ultramafic intrusions and the calculated“trapped liquids”for these olivine-orthopyroxene cumulates rocks are enriched in LILE and depleted in HFSE,which is similar to that of the Huangshanxi deposit in the western segment of the CAOB,suggesting that they may have the same tectonic setting.The Cu/Zr,Cu/Pd,and the Fo value of olivine verses Ni content show that sulfide segregation occurred in the Hongqiling intrusions.The sulfide segregation was almost simultaneous with the olivine crystallization with a mass olivine/sulfide ratio of 10:1.Based on the simulation of PGE composition of the whole rock,we infer that the Hongqiling parental magma may have achieved sulfide saturation and experienced approximately 0.01⁰.02%sulfide segregation with R factors between 10 and 100.Thus,the depletion of PGE(4.96×10^-9)in the Hongqiling deposit is most likely caused by sulfide segregation from the parental magma.The ore-bearing mafic-ultramafic intrusions in the Hongqiling area contain primary hydrous mineral?such as hornblende?,indicating that it is a H₂O-bearing magma.In addition,both the mineral and the whole-rock compositions?such as olivine with low Ca content<1000 ppm,the Nb-Ta depletion,and Th enrichment?show the features of island-arc magma.Combined with trace element features and regional tectonic evolution history,we infer that the mantle source of Hongqiling intrusions experienced fluid metasomatism.Based on the morphology,mineral assemblage,mineral composition,and geochemical characteristics of the Hongqiling mafic-ultramafic intrusions,we consider that these intrusions have no obvious relationship with mantle plume and island arc environment?Alaska-type intrusion?.Considering the tectonic evolution history of the eastern CAOB,the Hongqiling ore-bearing rocks are more likely formed in a post-collision extensional environment during the late Triassic.As a result of the sinking of the lithosphere and the upwelling of asthenosphere,the mantle was partially melted to form the basaltic magma with high-Mg and high-temperature features.Then,they intruded upwards along faults or cracks,formed the Hongqiling mafic-ultramafic intrusions.