Abstract:

The mafic-ultramafic intrusive rock body I is the largest for Baimazhai Cu-Ni deposit distributed in the Ailaoshan-Jinshajiang tectonic zone. It is a ringlike body consisting of orebearing peridotite, mineralized pyroxenite and gabbro phase zones from the center to margin. Accordingly, the contents of SiO₂,TiO₂,Al₂O₃,CaO,Na₂O and K₂O are increasing gradually, and that of TFe and MgO are decreasing as well; the concentrations of ∑REEs, and LREE/HREE ratios are increasing gradually from the central phase to outer phase. Along the same direction, δEu shows negative to positive anomalies; the abundance of the large ion lithophile elements (LILE) and high field strength elements varies increasingly, but the contents of magmatophile elements have a reversal evolutive trend. The PGE distributive patterns are the PtPd type, the Pt/Pd ratios of 0.32-0.68 are between the values of enriched sulfide tholeiite (0.38) and primitive upper mantle (1.36), and the similarity of partition patterns of chondritenormalized REE, MORB-normalized trace element and primitive mantle normalized PGE for the rocks of the three phase zones, indicate the Baimazhai intrusive body I formed by the evolution, liquation, intrusion and diagenesismetallogenesis of basaltic magma. The fact that stable sedimentary succession outcropped in the deposit district, intrusive contact relationships between the sediments and the intrusion and the Zr-Y-Nb diagram of the ring rock body shows the Baimazhai intrusion body formed in an intracontinent. The lower Ti content, strongly Th positive anomaly, and the Th/Ta ratios, Th/Nb ratios, Pd/Pt ratios and the primitive mantle normalized Ir-group patterns for Baimazhai intrusive body I, which are different from those of the Emeishan basalt and typical mantle plume basalts, but similar to those of basalts from a continental rift, suggest that the Baimazhai intrusive body I are probably formed in the tectonic settings of continental rift.

Key words: PGE；geochemistry, continental rift, Baimazhai；CuNi sulfide deposit, Ailaoshan-Jinshajiang tectonic zone