扬子地块北缘毕机沟层状岩体亲铜元素地球化学特征及其对岩浆演化和硫化物熔离过程的制约

毕机沟层状岩体是扬子地块北缘汉南杂岩中最重要的镁铁-超镁铁质侵入体之一, 主要由下部带超镁铁质岩、中部带辉长岩和上部带闪长岩组成。本文对该岩体下部带橄长岩、橄榄辉长岩和橄榄辉长苏长岩以及中部带粗粒辉长岩、磁铁辉长岩和角闪辉长岩进行全岩主微量元素及亲铜元素分析, 全岩(La/Sm)_N(0.43~2.89)和(Tb/Yb)_N (1.08~1.52)比值以及La/Yb-Sm/Yb图解均表明毕机沟层状岩体源区属于尖晶石二辉橄榄岩。Ni/Cu-Pd/Ir图解显示毕机沟层状岩体母岩浆主要为高镁玄武岩, 模拟计算及高的S/Se比值显示其母岩浆经历了~5%的地壳混染并引进了外界硫, 导致毕机沟母岩浆在深部发生早期硫化物熔离, 造成其极高的Cu/Pd比值(5.21×10³~1.67×10⁶)和低的PGE含量。毕机沟母岩浆侵位到浅部岩浆房后, 下部带极少量的硫化物熔离进一步导致残余岩浆亏损PGE, 但S和Cu含量相对升高; 中部带从下部带残余岩浆中结晶分异, 具有更低的PGE含量和相对较高的S以及Cu含量。根据毕机沟层状岩体岩浆演化及硫化物熔离过程, 推测岩体深部及附近具有寻找Cu-Ni-PGE矿床的潜力, 查明其岩浆通道系统及岩浆运移方向对找矿勘查具有重要意义。

Chalcophile element geochemistry of the Bijigou layered intrusion,northern margin of the Yangtze Block: Implications for the processes of magma evolution and sulfide segregation

The Bijigou layered intrusion is one of the most important mafic-ultramafic intrusions of the Hannan complex in the northern margin of the Yangtze Block, and it is composed of ultramafic rocks in the lower zone, gabbro in the middle zone and diorite in the upper zone. This study focuses on the troctolite, olivine gabbro, olivine gabbro norite in the lower zone and coarse-grained gabbro, magnetite gabbro, hornblende gabbro in the middle zone. The (La/Sm)N (0.43~2.89) and (Tb/Yb)N (1.08~1.52) ratios and La/Yb-Sm/Yb diagram of the whole rock samples indicate that the Bijigou layered intrusion was derived from spinel lherzolite. The parental magma of the Bijigou intrusion is mainly high-magnesium basalt according to the Ni/Cu-Pd/Ir diagram. The simulation calculation and high S/Se ratios show~5% crustal contamination of the parental magma and the introduction of the external sulfur, which leads to the prior sulfide segregation in the deep, extremely high Cu/Pd (5.21×103~1.67×106) and low PGE contents of the parental magma. When the parental magma of the Bijigou intrusion was emplaced into the shallow chamber, the segregation of a small amount of sulfide in the lower zone resulted in further depletion of the PGE, but relatively increasing of the S and Cu contents in the residual magma. Thus, the middle zone crystallized from the residual magma of the lower zone showed even lower PGE content and relatively higher S and Cu contents. According to the magmatic evolution and sulfide segregation process of the Bijigou layered intrusion, it is speculated that there is potential to search for Cu-Ni-PGE deposit in the deep and nearby part of the intrusion. It is of great significance to find out its magmatic channel system and magmatic migration direction of the Bijigou intrusion for ore prospecting and exploration.