铂族元素组成和铂族矿物组合对镁铁-超镁铁质岩浆成岩成矿作用的指示OA

The compositional variations of platinum-group elements(PGE)and the assemblages of platinum-group minerals(PGMs)in mafic-ultramafic rocks and related magmatic deposits result from diverse petrogenetic processes,such as mantle partial melting,fractional crystallization of mafic-ultramafic magmas,and the segregation and migration of sulfide melts.Therefore,PGE signatures and PGM occurrences can be used to reveal the processes of magmatic differentiation and ore formation.This study presents some case studies showing how PGE partitioning patterns and PGM assemblages can constrain the petrogenesis and evolution of ophiolitic peridotite,podiform chromitite,and magmatic sulfide deposits.First,in the harzburgites of the Kızıldağ ophiolite(Türkiye),the total PGE content ranges from 18.15×10-9 to 39.65×10-9,with narrow ranges of Os,Ir,and Ru,contrasted by more variable concentrations of Pt and Pd that result in high Pd/Ir and Pt/Ir ratios.All harzburgite samples have consistent,relatively flat,primitive-mantle-normalized PGE patterns.Based on PGE and trace element compositions integrated with quantitative models,we conclude that the harzburgites of the Kızıldağ ophiolite are the products of～20％partial melting of the primitive mantle followed by melt-rock interaction.Secondly,the PGM assemblages vary distinctly across the different types of chromitite from the Kızıldağ ophiolite.In all chromitite samples,primary laurite and Os-Ir alloy are typically enclosed within chromite,suggesting that chromite crystallized at temperatures of 1100～1200℃and lgfS2 values of-2 to-1.However,in nodular ores,an assemblage of Os-Ru nanoparticles,OsRu3 nanoalloy,awaruite(FeNi3),and trevorite(NiFe2O4)occurs in chromite intergranular spaces,indicating that the nodular chromitite underwent weak serpentinization under low water/rock ratios(<～1)and low fS2 and fO2.On the other hand,in banded and massive chromitite,Os-rich laurite+Os-Ir(Ru)alloy/oxide+pentlandite+millerite(NiS)occupy chromite intergranular spaces,reflecting high water/rock ratios and elevated fS2 and fO2 during serpentinization.Third,in the J-M reef of the Stillwater complex(Montana,USA),we discovered a new mineral,wangyanite.Wangyanite is a Pd end-member mineral of the pentlandite group,containing 9.64％～10.59％Pd.Based on the textural features and previous experimental data from the Pd-Fe-Ni-S phase system,wangyanite could form by peritectic reaction between braggite,pentlandite,and sulfide liquid,rather than via exsolution from monosulfide solid solution(MSS)or intermediate solid solution(ISS).To sum up,the study of PGE compositions and PGM assemblages can provide new insights into the formation processes and genetic mechanisms of magmatic ore deposits.