The Occurrence Of Platinum Group Elements In Magmatic Cu-Ni Sulfide Deposits And Its Genetic Significance

Platinum group elements（PGE）include six metal elements:Os,Ir,Ru,Rh,Pt,and Pd.PGE resources are strategic and key metal mineral resources,mainly produced in magmatic Cu-Ni-（PGE）sulfide deposits in China.PGE in ores are bimodally hosted by base metal sulfides（BMS）and platinum-group minerals（PGM）.However,there are differences in the occurrence of PGE in ores in different deposits.The mechanism of different PGE occurrence has always been one of the main concerns in the study of magmatic Cu-Ni-（PGE）sulfide deposits.In this study,We choose the Jinchuan Ni-Cu-（PGE）sulfide deposit and Kalatongke Cu-Ni-（PGE）sulfide deposit as the main research objects.At the same time,We choose Merensky Reef（MR）,which is a homogeneous layered mineralization of Bushveld complex in South Africa,as a comparative object,in order to find out the occurrence of PGE in three different types of ores.Different sulfide ores from these deposits were selected to identify PGM by the scanning electron microscope（SEM）and analyzed by the in-situ laser-ablation inductively coupled-plasma mass spectrometry（LA-ICP-MS）to obtain the major and trace elements in BMS.In this way,We find out the origin of micro-and nano-scale PGMs and the behavior and distribution of PGE,so as to further understand the occurrence of PGE in different types of ores.（1）The paragenesis and formation conditions of PGM formed in the magmatic and hydrothermal stages in the Jinchuan Ni-Cu-（PGE）sulfide deposit were identified.The Jinchuan Ni-Cu-（PGE）sulfide deposit hosts the major resource of PGE in China.Its ore has undergone strong hydrothermal alteration and produces a variety of PGMs.The major PGM species include PGE-bearing arsenide（irarsite and sperrylite）and Pd-bearing bismuthide,telluride and selenide,as well as a few other types of PGM.Irarsite（Ir As S）is enclosed within pentlandite,pyrrhotite and chalcopyrite,indicating that irarsite may have crystallized earlier from the As-rich sulfide melt.During the fractionation of sulfide melts,the irarsite can be trapped in the initial monosulfide solid solution（MSS）or residual intermediate solid solution（ISS）.Most of the Pdbismuthide（PdBi）grains enclosed within chalcopyrite show obvious emulsion texture,indicating that they were exsolved from the ISS during subsolidus cooling.Some irregular Pdbismuthide（PdBi₂）grains are associated with secondary magnetite veins along micro-fractures in the ores.With the increase of the degree of ore alteration,the Pdbismuthide grains have chemical compositions varying from PdBi to PdBi₂.It indicates that some Pdbismuthide（PdBi₂）grains were likely produced by late hydrothermal alteration.Michenerite（PdBi Te）and Pdselenides[PdSe（Bi Te）]grains mainly occur in the fractures throughout pentlandite and are associated with secondary magnetite,indicating a hydrothermal origin.Abundant Pdselenides in the ores indicates that the fluids containing Cl-are acidic,saline and highly oxidized,and may play a key role in the Pdenrichment.Therefore,sperrylite,irarsite and Pdbismuthide（PdBi）grains are magmatic minerals,while most of the Pd-bearing bismuthide,telluride and selenide are the products of post-magmatic hydrothermal alteration.（2）The reasons for the enrichment of Pdin PGM and pentlandite in the Cu-rich ores of the Kalatongke deposit were identifiedKalatongke Cu-Ni-（PGE）sulfide deposit has undergone sulfide fractionation during ore formation.Os,Ir,Ru,Rh and Pt contents in sulfides are generally below or close to the detection limit,whereas Pdis concentrated in pentlandite in the range of0.1 to 3.6 ppm.Three generations of pentlandite are present in the massive ore from the Kalatongke intrusion.Granular pentlandite has the highest Pdconcentrations（1.4 to 3.6ppm）,and then the loop-texture pentlandite（0.8 to 1.7 ppm Pd）,and pentlandite that associated with myrmekitic magnetite is poorest in Pd（0.1 to 0.5 ppm Pb）.We believe that the variation of Pdcontent in pentlandite is related to the sulfide fractionation of sulfide melts.When the sulfide melt is cooling and monosulfide solid solution（MSS）fractionation,since Pdis incompatible with MSS,Pdis preferentially concentrated into the intermediate solid solution（ISS）during the fractionation of sulfide liquid.Pdtends to enter the pentlandite lattice to replace Ni,once pentlandite crystalizes from the MSS,Pdwould fast diffuse from the Pd-rich ISS into the MSS and pentlandite.Therefore,the Pdcontent of pentlandite is only related to its paragenesis in MSS,the early crystallized granular pentlandite contains the highest concentration of Pd,and then the loop-textured pentlandite and pentlandite associated with myrmekitic magnetite.LA-ICP-MS elemental mapping show that TABS are not enriched in the Cu-rich domains of massive ores.Most of the PGM（n=285）are euhedral to subhedral Ni-merenskyite and enclosed by pyrrhotite.These results indicates that the distribution of TABS and formation of PGM are not controlled by the sulfide fractionation.Te and Bi may occur as associations with Pdand Pt in the sulfide liquid and are randomly distributed in the MSS and ISS during sulfide fractionation,eventually forming the Ni-Merenskyite during cooling.Therefore,the PGM in the massive ore formed before MSS fractionation,and the Pddiffused from ISS to MSS,resulting in the enrichment of Pdin pentlandite.（3）Comparing the distribution characteristics of PGE in Jinchuan,Kalatongke and Merensky Reef deposits,we found that TABS controls the occurrence of PGE and the formation of PGM.By comparing the trace element compositions of sulfides in Jinchuan,Kalatongke and Merensky Reef,we found that the content of TABS in sulfide melts can affect the occurrence of PGE in ores.During crystallization of the sulfide liguid,if the content of As in the early melt is high,Pt and Ir tend to combine with As to form As-PGM nanoparticles,resulting in the differentiation of PGE;when Te and Bi exist in the melt,they can combine with Pdand Pt,forming Pt-Pd-Te-Bi nanoparticles and randomly distributing in MSS and ISS,resulting in inhomogeneous distribution of PGE in sulfides.During hydrothermal alteration,Pdin sulfide or Pd-PGM can be activated and migrated,and combined with Bi and Se to form new PGM.Therefore,the TABS content in the sulfide melt is one of the key factors affecting the enrichment of PGE to form PGM.TABS can combine with PGE to form PGM nanoparticles,thereby affecting the partitioning behavior of PGE.Source rocks containing high TABS and hybridization of surrounding rock may cause TABS concentrations in the melt,promote the formation of PGM,and reduce the content of PGE in the sulfide.