滇西金宝山铂钯矿床元素地球化学

金宝山铂钯矿床位于扬子板块西缘红河断裂东侧,宁蒗-弥渡镁铁-超镁铁岩带内,矿体呈似层状、透镜状产于辉石橄榄岩中。辉石橄榄岩和铂钯矿石均富集LREE,具有弱的Eu负异常和较强的Sr、Ba负异常;与N-MORB相比,辉石橄榄岩具有较低的(Nb/Th)PM比值和较高的(Th/Yb)PM比值,表明金宝山岩体受到了地壳物质混染;通过(Th/Yb)PM-(Nb/Yb)PM图解估算得到地壳混染程度在55%~70%之间,强烈地壳混染表明岩浆中的S达到饱和并使得硫化物发生大规模熔离。而利用硅酸盐岩浆/硫化物的质量比值(R因子)方程进行模拟计算,得到金宝山矿床R因子集中于5000~1000之间,明显大于金川、图拉尔根、白马寨等典型岩浆硫化物矿床,说明金宝山岩体形成时岩浆中的硫化物熔离程度较低。辉石橄榄岩和铂钯矿石的S/Se和Cu/Pd比值也同样反映了硫化物低程度熔离的特征。与Nb、Th等元素含量相对稳定的高场强元素相比,S、Se、Pd等元素在硫化物部分熔解以及热液作用过程中更容易发生迁移。类似于River Valley和Platreef矿床等大型层状PGE矿床,金宝山铂钯矿床的形成是一个两阶段的过程,早阶段在岩浆通道或深部岩浆房中,地壳混染使得硫化物发生强烈熔离并在有限的空间内大量聚集,产生富PGE岩浆;后由于硫化物的部分熔解,岩浆中硫化物熔体富集Se、Pd,亏损S、Fe,岩浆中的S由饱和变为不饱和。晚阶段在浅部岩浆房,少量地壳S的加入并未使得S饱和从而发生硫化物大规模熔离。金宝山岩体具有较低的Cu/Pd、Cu/Pt比值,即出现Cu/Pd比值较低的岩体也可能有较大的成矿潜力,这与传统意义上所认为的具有较高Cu/Pd、Cu/Pt比值的矿体不同。

Elements geochemistry of Jinbaoshan Pt-Pd deposit, western Yunnan, China

The Jinbaoshan Pt-Pd deposit is located in the west side of Yangtze plate and the east side of the Red River faults, inside the Ninglang-Midu mafic-ultramafic intrusion zone. The ore body is situated in wehrlites, presenting as a stratoid or lentoid body. The ores and wehrlites enrich LREE and have a characteristic of a weak and marked negative anomalies for Eu and Sr, Ba. Compared with N-MORB, wehrlites have a lower (Nb/Th)_PM and a higher (Th/Yb)_PM ratio, which means Jinbaoshan magma experienced crustal contamination. By estimation, the magma experienced 55%~70% crustal contamination using (Nb/Yb)_PM-(Nb/Th)_PM scattergram. This strong crustal contamination indicates the magma has undergone S-saturated and sulfide segregation took place. Using the equation of R factor (magma/sulfide, mass ratio) we know R factor of Jinbaoshan Pt-Pd deposit vary from ~5000 to ~1000, which is larger than some typical Cu-Ni-PGE deposits such as Jinchuan, Tulargen and Baimazhai. It means little sulfide segregation happened when the Jinbaoshan intrusion formed. The ratio of S/Se and Cu/Pd for ores and wehrlites can also reflect this signature. Compared with HFS such as Nb and Ta, elements like S, Se and Pd are more mobile in the process of alteration or partial dissolution. Analogous to the River Valley and Platreef deposits, Jinbaoshan PGE-fertile intrusion forms from a multistage-dissolution upgrading process, with an early stage of crustal contamination in a deep chamber triggering the formation of the main sulfide liquid that became enriched in precious metals. The process of multistage-dissolution upgrading can gain Se, Pd and lose Fe, S. The later stage of minor in situ contamination which does not make the magma become S-saturated and only causes little sulfide segregation. Jinbaoshan intrusion has the characteristics of low Cu/Pd, Cu/Pt ratio. It is different with the opinion that the Cu-Ni-PGE ore body will have a high Cu/Pd and Cu/Pt ratio. Therefore, the use of Cu/Pd ratios in exploration must be taken with caution and the intrusion with a low Cu/Pd ratio still has the potential to form a large PGE deposit.