Two sulphide liquids are required to produce the Oktyabr'sky PGE-Cu-Ni deposit,Noril’sk region,Russia

摘要

The largest ore-body of the Oktyabr'sky sulphide deposit is composed of pyrrhotite ores at the base passing upwards and inwards into cubanite-chalcopyrite-mooihoekite ores.It is commonly attributed to bottom-up fractional crystallization of a single pool of sulphide liquid.New mineralogical and geochemical data reveal,however,a prominent compositional break between pyrrhotite and cubanite-chalcopyrite-mooihoekite ores that is marked by two- to three-fold increases in all compatible components (e.g.Os,Ir,Ru and Rh).In addition,these two types of sulphide ores are characterized by distinctly opposite patterns of chemical zoning.The pyrrhotite ores show a systematic downward decrease in compatible elements (e.g.Ru,Rh,Os,Ir) and an increase in incompatible elements (e.g.Cu,Pd,Pt),indicating their bottom-up crystallization.In contrast,the cubanite-chalcopyrite-mooihoekite ores reveal an overall upward decrease in the compatible elements and an increase in the incompatible elements,indicating their bottom-up crystallization.These observations indicate the formation of the Oktyabr'sky sulphide deposit from two compositionally different sulphide liquids that gave rise to two separate portions of pyrrhotite and cubanite-chalcopyrite-mooihoekite composition.