滇西金宝山富铂钯岩体岩浆源区研究:铂族元素和Sr-Nd同位素约束

The source of primitive magma for the Jinbaoshan PGE-rich intrusion,western Yunnan:constraints from PGE contents and Sr-Nd isotopes

金宝山杂岩体位于扬子板块西缘,毗邻哀牢山造山带北段,岩体中蕴含丰富的铂钯资源,是峨眉山大火成岩省(ELIP)中大型岩浆型铂族元素矿床。岩体的主要组成为辉石橄榄岩,矿体以似层状、透镜状产出于辉石橄榄岩中。地幔是镍矿床和铂族元素矿床最重要的源区,因而对Ni、Cu及PGE等元素行为的研究,是剖析岩浆型Ni-Cu-PGE矿床源区特征的一个重要研究方向。该研究获得金宝山岩体中辉橄岩铂族元素及Sr-Nd同位素数据,结合前人对ELIP中不同类型岩石系列PGE成分研究及Ni、Cu、PGE等元素在岩浆和硫化物中的分配系数,半定量模拟得到金宝山原始岩浆的形成源于地幔中高度部分熔融(25%-40%)形成的富PGE岩浆(含12.8×10^-9 Pd,9.8×10^-9 Pt,0.6×10^-9 Rh和0.7×10^-9 Ir),其铂族元素成分与ELIP苦橄岩成分相当,并且岩浆在演化的过程中遭受了10%-20%地壳混染作用。利用批式部分熔融公式及各铂族元素在硅酸盐矿物和熔体之间的分配系数反演计算得到产生金宝山熔体的地幔约含有5.3×10^-9 Pd,7.5×10^-9 Pt,0.75×10^-9 Rh和1.5×10^-9 Ir,相比原始地幔而言并没有表现出明显富集PGE的特征。这表明地幔中高度部分熔融+大量岩浆与硫化物的充分反应是形成大型岩浆型铂族元素矿床的一种可具备的条件。

The Jinbaoshan complex is located in the west margin of the Yangtze Plate,adjacent to the northern part of the Ailaoshan orogenic belt.It contains abundant Pt-Pd resources and is a large magmatic PGE deposit in the Emeishan Large Igneous Province (ELIP).The complex is mainly composed of wehrlites with ore body presented as stratoid or lentoid in wehrlites.Mantle is the most important source of forming the Ni and PGE deposits,thus the study of Ni,Cu and PGE geochemical behavior is important for understanding the genesis of magmatic Ni-Cu-PGE deposits.In this work we determined the PGE and Sr-Nd isotope contents in wehrlites from the Jinbaoshan intrusion.Combined with PGE studies on different series of rocks in the ELIP and research on the Dsul/sil for Ni,Cu and PGE by previous researchers,we suggest that the primitive magma is formed by moderate to high degrees of melting (25%-40%),resulting in 12.8 ppb Pd,9.8 ppb Pt,0.6 ppb Rh,and 0.7 ppb Ir,close to the contents of the ELIP picrite.The Jinbaoshan intrusion has experienced 10%-20% crustal contamination from the Yangtze upper crust during magma evolution.Based on the batch partial melting formula and Dmineral/melt parameters for the PGE,the PGE contents of Jinbaoshan mantle were calculated to be 5.3×10-9 Pd,7.5×10-9 Pt,0.75×10-9 Rh,and 1.5×10-9 Ir.Compared to primitive mantle,the mantle of the Jinbaoshan intrusion does not show obvious PGE enrichment.Therefore we may conclude that a large magmatic PGE deposit could be formed by moderate to high degrees of mantle melting accompanied by large scale mixing of magma and sulfides.