Geochemical characteristics and genesis of the Zhubu mafic-ultramafic intrusion,Yunnan Province

The mineralized complex which host Zhubu medium-sized Cu-Ni-PGE deposit is a mafic-ultramafic intrusion,intruded the schists and granitic-gneisses of Yuanmou Group.This intrusion is vertically well differentiated with peridotite,olivine pyroxenite,pyroxenite and gabbros from bottom to the top.The ore bodies are mainly composed of edge ores,a few suspended which occur as lenses located at the upper part.This paper provided new test results on major elements,trace elements,platinum-group elements(PGE) and Sr-Nd isotopes for some rocks and ores in the intrusion.The major and REE elements of the rocks show a tholeiitic evolution trend.The samples enrich in LREE and indicate a marked and weak negative anomalies for Sr and Nb,respectively.The patterns of REE and trace elements are similar to those of picrites in Emeishan Large Igneous Province(ELIP),suggesting a genetic link between the Zhubu intrusion and picrites in ELIP.Primitive mantle normalized chalcophiles elements patterns of the Zhubu sulfide ores show enrichments in Pt and Pd compared to other elements.The higher(87Sr/86Sr)i(0.7096~0.7107) and the lower eNd(t)(3.1~ 2.3) show Zhubu magma experienced variable degree of crustal contamination.The inversion of magma evolution show that the primary magma is a picritic melt.By estimation,the magma experienced 3%~20% crustal contamination.When the R factor(the mass ratio between silicate magma and sulfide liquid) fall into the range of 1000 to 5000,it is close to the actual measured value of the sulfide in Zhubu samples,indicate that it maybe a magma conduit in an open system.Successive pulses of new magma passing through the conduit and upgraded the previously segregated sulfide in PGE by a multistage upgrading process.The mineral cumulus and magma evolution also exit in the chamber,formed peridotite at the bottom and gabbros on the top,respectively,with pyroxenite and olivine-pyroxenite in between.